Forward Osmosis Desalination Process With …epubs.surrey.ac.uk/807218/1/Final Maryam Aryafar PhD... · Web viewThe results presented in this project demonstrate that the proposed

A NOVEL FORWARD OSMOSIS DESALINATION PROCESS

WITH THERMAL-DEPRESSION

REGENERATION

By

MARYAM ARYAFAR

A Thesis for the Degree of Doctor of Philosophy

Centre of Osmosis Researcher Applications (CORA)

Department of Chemical Process Engineering

Faculty of Engineering and Physical Sciences

University of Surrey

Guildford, Surrey GU2 7XH, United Kingdom

February 2015

Supervisors:

Professor Adel Sharif

Dr Mohammed Sanduk

Co-Supervisor:

Dr Sami Al-Aibi

A Novel Forward Osmosis Desalination Process With Thermal-Depression Regeneration

DisclaimerI hereby declare this thesis has been composed by myself and has not been presented or

accepted in any previous application for a degree. The work of which this thesis is a record, has

been carried out by myself unless otherwise stated and, where the work is mine, it reflects

personal views and values. All quotations have been distinguished by quotation marks and all

sources of information have been acknowledged by means of references, including of the

internet.

Maryam Aryafar

January 2014


AbstractIn this project, the concept of employing liquefied gas compounds as draw agent has

been investigated among 137 gaseous compounds by determining their high solubility in water,

the resulting osmotic pressure and their re-generation through thermal-depression methods. The

screening process resulted in an organic liquefied gas draw solution suitable for Forward

Osmosis desalination process. This is a polar, non-ideal with partially miscibility under 4 bars

external pressure generates an osmotic pressure at maximum solubility (34% weight

percentage) of 220 bars which is seven times more than seawater osmotic pressure. In addition,

there is a significant reduction in solubility of the liqefied gas in water when the external

pressure on draw solution is reduced from 4 bars to atmospheric pressure. This suggests that the

liquefied gas draw agent could be separated from the solution by depression – thermal

processes such as gas striping or atmospheric-vacuume flash methods. The performance of FO

process using the novel liquefied gas draw solution was simulated using Excel software to

achieve optimum operating conditions including operating temperature, cross flow rate and

draw solution concentration. The results showed that the draw solution side should be kept

under a pressure of maximum 10 bars. This depends on the operating temperature to dissolved

the liquefied gas in water as much as possible. However, the operating pressure of the feed side

could vary to cover a range 1 bar to 10 bar.

Furthermore, the feasibility of the integrated Forward Osmosis process and depression -

compression methods for seawater desalination was investigated in terms of estimating the

specific energy consumption (SEC) using HYSYS 7.2 simulation software. The specific energy

consumption (SEC) was predicted at optimum operating conditions resulting from FO process

simulation based on the production of 1m3/h of potable water from seawater at a recovery rate

of 50%. The electrical energy requirement of the process was calculated and the result of

simulation was compared to the energy requirement of current desalination technologies.

Energy saving of the novel FO desalination process is projected to range from 30% to 60%.

The estimated SEC of the present FO desalination process was 2.7kWh/m3 and could be

decreased to 0.5kWh/m3 when a heat recovery process is used.

I


The results presented in this project demonstrate that the proposed novel forward osmosis

desalination process with thermal-depression regeneration using the liquefied gas draw solution

is a feasible and cost-effective desalination method. The novel draw agent produces high

osmotic pressure and can be easily separated from the product clean water by using low-

pressure steam with temperature input less than 150°C. While the feed water recovery in the

FO process is higher than other desalination methods, the specific energy consumption of this

novel FO desalination process is significantly low.

The future works should focus on experimental tests to measure the osmotic pressure,

permeated water flux, reverse draw agent flux and energy consumption in a bench scale or a

pilot unit studies.

A patent application, based on the present process, has recently been filled at the UK patent

Office and the application number is GB1321711.2 (Adel Sharif and Maryam Aryafar, A novel

Forward Osmosis Desalination process, GB1321711.2, 2013).

II


AcknowledgementFirst thanks to GOD for all the gifts, He has given me. Among them is the kind, helpful

and trustable people in my life. Thanks GOD for giving me the opportunity to study, research

and work on water treatment to prepare clean water for people. The reference to the difference

between salty and fresh water in the Quran is in chapter Forghan part 53, ‘And GOD it is Who

has made two seas to flow freely, the one sweet that subdues thirst by its sweetness, and the

other salt that burns by its saltiness; and between the two GOD has made a barrier and

inviolable obstruction.’

Secondly, I would like to express my very deep appreciation to the two great people, my late

father, Ahmad, and my mother Khadijeh, who changed and made a new future for their

children. However, they did not have opportunity of education, and both were illiterate, they

brought up seven children who all have university education. Thank you for teaching us

kindness, love, trusts, honestly, hardworking, and being helpful.

Next, I would like to offer my special thanks and deep gratitude to my supervisor Professor

Adel Sharif, Director of the Centre for Osmosis Research and Applications at the University of

Surrey, for introducing me to this project and to work on his novel concept. Thank you for

introducing me to Forward Osmosis, as I did not know anything about FO when I started doing

my PhD project. I sincerely thank him for his enthusiastic, encouragement, patience, guidance

and the support. He taught me that: ‘Progress has little to do with speed but much to do with

direction’. I am proud to be his student.

I would like to thank my sister, Manijeh and my brother, Dr Mostafa for their financial support.

My brother Mojtaba and his wife Sanam, for all official coordination in Iran because of the

closure of Iranian Embassy in London during my studying here. Thanks to my young brother,

Dr Reza to live with me here in Guildford and not be an alone. Thanks to the rest of my kind

and warm family, Marjan, Morteza, their family Maryam, Dr Arezoo, Arduino and my sweet

III


nieces Armina, Ava, Teresa and nephews Arvin and Armin for giving me positive energy, by

calling, sending gifts, greeting cards and paints, during the past two years.

I would like to express my deep appreciation to Dr Safekordi at Sharif University of

Technology, Iran and the late, Dr Farhad Farhadpour for introducing me to Professor Adel

Sharif.

I would like to thank from the funders and all my colleagues in KEC-KITC Company in Iran-

Tehran for preparing the opportunity for me during the past 15 years to train and work in 20

EPC (engineering, procurement and construction) water treatment and utility supply projects

including Reverse Osmosis (RO), Ion Exchange and conventional pre-treatment processes.

I would like to thank from my second supervisor Dr Mohammed Sanduk for his patience and

enthusiastic, and thanks from Dr Sami Al-Aibi the co-supervisor in my project for his support

in risk assessment procedure, checking the drawings, attending on meetings to get the approval

and permission to do experimental test.

I would like to express my thanks to Hilary Mitchell for her always help and perfect

coordination at the Chemical and Process Engineering Department. Also, thank to David

Hawkins to preparing the experimental rig to test the membrane.

I would like to extend my thanks to the following technical engineers and academic researchers

for replying to all my questions through emails and send me helpful information to complete

my thesis accordingly:

1- Dr. Ali Farsi, from University of Aalborg, Denmark for preparing the ceramic

membrane.

2- Professor Kazuyuki Oshita, from Kyoto University, for information about using the

liquefied gas in dewatering.

IV


3- Dr. Roland Smeink, from Akzonobel Company, for sending me all curves and tables

mentioned in Appendix B about the liquefied gas daw agent.

4- Dr. Antonin Chapoy, from University of Edinburgh, for sending me the curves and

information about the liquefied gas freezing point depression were approached in their

research.

5- The technical service of BOC Company, for arranging a technical meeting and all

phone discussion about the liquefied gas experimental test rig.

6- Mr Kevin Joyce at University of Surrey for all our meetings to check, complete and

approve the risk assessment of experimental test.

7- Resnova Company to construct the vessel and accessories for experimental test.

8- My colleague Eng. Mahmood Alizadeh from EIED Company for his helpful

information about the liquefied gas production process.

9- Dr. Nadir Hossain, research fellow at INRS (Institute national de la recherché

scientifique) for his helpful information about modelling.

10- Alireza Abbas, PhD student at University of Surrey for his useful help to proof reading

some parts of my thesis.

Thanks GOD for you All.

V


Table of Contents

Description Page

Abstract I

Acknowledgement III

Table of contents VI

Nomenclature XII

Chapter one

Introduction and Literature Review1

1.1. Introduction 2

1.2. Challenging in Draw Agents and Hybrid Desalination by Forward Osmosis 5

1.2.1. Forward Osmosis Batch System 6

1.2.2. Forward Osmosis Continuous System 7

1.2.2.1. Volatile Draw Solution 7

1.2.2.2. Sugar as Draw Solution 8

1.2.2.3. Nanoparticles Draw Solution 8

1.2.2.4. Polymeric Draw Solution 8

1.2.2.5. Inorganic Draw Solution 9

1.3. Membrane Development 13

VI


Description Page

1.3.1. Membrane Morphology 13

1.3.2. Membrane Orientation 20

1.3.3. Membrane Fouling 20

1.3.4. Concentration Polarization in Membrane 22

1.4. Summary 26

Chapter two

A novel Draw Solution Concept27

2.1.The Prior Art 28

2.2. A Novel Draw Agent for FO Process 29

2.3.Dimethyl Ether (DME) Background Applications 34

2.4.Dimethyl Ether (DME) - Water Solution as a Novel Draw Agent 36

2.5.Summary 41

Chapter three

Osmotic Pressure, Physical Properties Behavior &Experimental Results and Data

Reduction

42

3.1. Introduction 43

3.2. Osmotic Pressure Behavior 43

3.2.1. Osmotic Pressure Methodology 43

VII


Description Page

3.2.2. Osmotic Pressure Determination Methods 45

3.2.3. Freezing Point Depression 47

3.2.4. Vapour Pressure Osmometers 48

3.3. Model for Calculating the Physical Properties 50

3.4. Experimental Results and Data Reductions 51

3.4.1. Binary Vapour- Liquid Equilibrium of DME 52

3.4.2. Models for the Excess Gibbs Energy 56

3.5. Predicted Osmotic Pressure of DME-Water Solution 58

3.5.1. Freezing Point Depression Results 58

3.5.2. Vapour pressure Lowering Results 63

3.5.3. Osmotic Pressure Prediction Results and Discussion: 65

3.6. Membrane Osmometer 66

3.7. Experimental Data for Calculating the Physical Properties 66

3.8. Summary 72

Chapter four

Forward Osmosis Modelling Comprehensive Review73

4.1. Introduction 74

VIII


Description Page

4.2. Modelling the Effect of ICP and ECP on Water Flux in FO Process 75

4.3. Solute Reverse Diffusion Flux in Modelling Water Flux in FO Process 82

4.4. Numerical Simulation and Performance Analysis of Forward Osmosis Process 84

4.5. Summary 85

Chapter Five

Forward Osmosis Process Design Criteria and Simulation Results and Discussion86

5.1. Introduction on FO Process Design Criteria 87

5.1.1. Modified ECP Model Considering Effect of Suction/Dilution Parameter 88

5.1.2. Revised ICP Model Considering a Variable Diffusivity 93

5.1.3. The Combined Modified ECP and ICP model for Flux Prediction of FO

Process in PRO Mode96

5.2. FO Process Simulation Results and Discussion 101

5.2.1. RO Experiments, Bench-Scale System and Membrane Coefficients 101

5.2.2. Osmotic Pressure and Diffusion Coefficient as a Function of

Concentration104

5.2.3. Methodology of Water Flux Prediction in FO Process 108

5.2.3.1 The Effect of Changing the Osmotic Pressure Difference 109

5.2.3.2. Effect of Changing the Operating Temperature 114

5.2.3.3. The Effect of Reverse Draw Solute Flux 116

IX


Description Page

5.2.3.4. The Effect of Changing the Cross- Flow Velocity 118

5.2.4. Forward Osmosis Unit Mass Balance 122

5.3. Summary 125

Chapter six

FO Desalination Process with Regeneration Method Design Criteria and Simulation

Results and Discussion

127

6.1. Introduction to an Integrated Forward Osmosis and Decompression Method 128

6.2. Principle of DME Separating Method 128

6.3. DME Separating Process Simulation Methodology 132

6.3.1. Forward Osmosis (FO) Regenerating Unit Mass Balance Relations 132

6.3.2. DME Compression Unit for Recycling DME Draw Solution 137

6.4. Specific Energy Consumption (SEC) of Distillation Column Thermal-Depression

Regenerating of DME Draw Solution 140

6.5. Comparison of Energy Requirements of Current Seawater Desalination

Technologies to the Proposed Forward Osmosis Desalination Process with

Depression Regeneration Method

143

6.6. Summary 145

Chapter seven

Conclusions and Future Works146

X


Description Page

7.1. Conclusions 147

7.2. Future Work 151

7.2.1. Membrane Osmometer 151

7.3. Bench Scale Demonstration of the DME Forward Osmosis Desalination Process 156

7.4. Modified Depression-Thermal Regeneration Method 156

References 159

Appendixes 170

Appendix A: Table A-1 Solubility of Selected Gases Compounds in Water 170

Appendix B- DME Solubility in Water Versus Pressure 181

XI


NomenclatureAbbreviations

CTA Cellulose Triacetate

CP Concentration Polarization

DA Draw Agent

DI Di-ionized water

DS Draw Solution

ED Electrodialysis

ECP External Concentration Polarization

FO Forward Osmosis

FW Feed Water

GOR Gained output ratio

H Enthalpy of steam

HFF Hollow Fine Fibre Membrane

ICP Internal Concentration Polarization

MD Membrane Distillation

MED Multi Effect Distillation

MSF Multi Stage Flash

MOD Manipulated Osmosis Desalination

MF Microfiltration

MVC Mechanical vapor compression

NF Nano Filtration

PBI Polybenzimidazole

PE polyethylene

PES Polyethersulfone

PP polypropylene

PRO Pressure Retarded Osmosis

PTFE polytetrafluoroethylene

XII


PVDF polyvinylidenedifluoride

PA Polyamide

RO Reverse Osmosis

UF Ultra filtration

VC Vapour Compression

XIII


Symbols

a Activity

A Pure water permeability coefficient (ms-1bar-1)

Am Membrane Area, m2

Aij Energy parameter characteristic of i-j interaction in NRTL equation

B Draw solute permeability coefficient (ms-1)

V Cross flow velocity (m3/m2s)

Cf Molar concentration of the feed solution (mol/l)

Cfb Molar concentration at feed bulk (mol/l)

Cfw Molar concentration of feed at membrane surface (mol/l)

CD Molar concentration of the draw solution (mol/l)

CDb Molar concentration of draw solute in the bulk solution (mol/l)

CDw Molar concentration of draw solution at membrane surface (mol/l)

dh Hydraulic diameter (m)

D Solute Diffusion Coefficient (m²/s)

En/Fn/G Constants Associated with diffusivity Coefficients

gE Excess Gibbs energy in NRTL equation

h Total waters of hydration per mole solute

∆Hfus Solvent molar enthalpy of fusion

i Van’t Hoff index

Jw Water flux (m3/ m²s)

Js Solute reverses diffusion flux (kg/ m²s)

k Mass transfer coefficient (m/s)

Kf Cryoscopy constant

kf Mass transfer coefficient in feed solution stream (m/s)

km Mass transfer coefficient in support layer (m/s)

kD Mass transfer coefficient in draw solution stream (m/s)

kc Mean mass transfer coefficient (m/s)

XIV


K Solute resistivity(sm-1)

K* Solute resistance coefficient independent of diffusivity (m)

Ks Solute specific resistivity coefficient independent of diffusivity (m)

L Length of channel (m)

M Molality of DS solution (mol/kg)

m Mass flow rate (kg/s)

MW Molecular weight (g/mol)

n Number of dissolved species created by draw solute

P Operating pressure (bar)

Peδ Peclet Number in the external boundary layer

Pes Peclet Number in support layer

R Membrane solute rejection

Rg Ideal gas constant

Re Reynolds number

Rec Critical Reynolds number at L

ReL Reynolds number at L

Rey Local Reynolds number

Ret Transition Reynolds number

S Membrane structural parameter

Sc Schmidt number

Sh Sherwood number

T Absolute temperature (ºC, K)

∆Tf Deviation in the freezing point

Tfb Feed bulk temperature (ºC, K)

TDb Draw solution bulk temperature (ºC, K)

t Thickness of membrane (mm)

Vm Molecular volume

W Width of channel (m)

W% Weight Percent

XV


x Mole fraction

X Association parameter

Z Cation/anion charge

XVI


Greek letters

ρ fluid density (kg/m3)

μ fluid viscosity (Pa s)

δ Boundary layer thickness (m)

τ Membrane tortuosity

ε Membrane porosity

λ Degree of interaction between solute and membrane material

ƞ Dimensionless variable

π Osmotic pressure (bar)

∆π osmotic pressure difference across membrane

∆πeff Effective osmotic pressure difference across membrane

γ Activity coefficient

Subscripts

b Bulk

d Draw solution

f Feed solution

m Membrane

p Permeate

s salt

w Membrane wall

XVII


CHAPTER ONE

INTRODUCTION AND LITERATURE REVIEW

1


1.1. IntroductionShortage of fresh water is one of the greatest challenges, particularly in developing

countries, where the population figures rise exponentially. Most Middle Eastern countries, for

example, have less than 500 m3 per capita water requirement of renewable natural water

sources, while the minimum limit set by the UN is 1700 m3 [1]. Keeping up with the

requirements for proper sanitation and water treatment in these countries is found to be

increasingly complicated. Freshwater can be produced from seawater, brackish water, or

wastewater, using different desalination technologies. As the cost of desalination is determined

by water salinity, feed water containing the minimum amount of impurities is most favourable

for use in the desalination processes leading to lower costs.

Seawater and brackish water are desalinated by various methods such as pressure-driven

membrane separation processes (including RO), thermal distillation and Electro Dialysis (ED)

while all these methods involve high operating and capital costs. The high operating cost of the

membrane-based methods such as Reverse Osmosis (RO) are due to essential pre-treatment,

scaling, bio-fouling and high-energy consumption while scaling and low thermal efficiency are

the main constraints of the thermal distillation technique such as Multi Stage-Flash distillation

(MSF). The proposed process would overcome many of the practical problems associated with

these conventional processes hence reduce the operating cost.

The Reverse Osmosis (RO) process, which was commercialised in 1960, became more

competitive with the historical thermal desalination techniques in the eighties, and since the

mid-nineties the worldwide installed capacity of RO plants has been exceeding that of thermal

plants [2]. A recent review by the US National Research Council [3] strongly recommended the

support of further research and development in the application of novel membrane based

technologies to reduce energy and capital costs and brine disposal. The review states that the

most optimistic limit of achievement is a 50 to 80% capital and operating cost reduction,

coupled with a similar increase in energy efficiency using the application of new “break-

through” technologies over the next twenty years. By the year 2020, the review states that

desalination and water purification technologies will contribute significantly to ensuring a safe,

sustainable, affordable, and adequate water supply.

For current state-of-the art seawater RO systems, the most optimistic reduction is 20% that

represents the process thermodynamic limit of 1.77kWh/m3 for a 50% recovery rate (54 bar

2


osmotic pressure in concentrated side or 65 bar required hydraulic pressure due to ICP effect)

and a 100% energy recovery in seawater applications. The review states that to obtain further

reductions in energy consumption, a different desalination approach is recommended [3]. There

have been numerous attempts during the past few decades to apply Forward Osmosis (FO) as a

method of desalting saline water. The number of papers published on FO has seen a very

significant increase over the last three years (24 in 2012) [4], indicating the increasing level of

academic interest [5]. Forward Osmosis (FO) process is one of the recent developments and the

most promising desalination technique which has the potential to provide a reliable and cost-

effective method for producing fresh water with low energy consumption if the draw solution

(DS) regeneration process limitations are addressed and the membrane technology is further

developed. Forward Osmosis desalination has faced two main challenges, which include a)

selecting a suitable draw agent with a sustainable regeneration method and b) the structure and

material of the appropriate membrane. All studies on these two challenges in Forward Osmosis

process during the past few decades show that most of the research effort has been focused on

the improvement of membranes; while only a few studies have been carried out to improve the

draw solution and the energy efficiencies of the draw agent regeneration methods. The

selection of a suitable draw solution as the main source of the driving force as well as achieving

high flux is a question to design of an optimal FO process. Investigating an easy and cost-

effective regeneration method to separate and recycle draw solution relating its physical and

chemical properties is another question of an optimum performance in FO desalination system.

The present study introduces a novel desalination process based on Forward Osmosis system

and an integrated thermal-depression regeneration method. This research focuses on applying

gas compounds as draw agent with high solubility in water in FO process, while the significant

reduction of solubility in water happen in the regeneration step by changing operating

temperature or pressure to optimize the energy consumption and achieve the highest quality

and quantity of clean water. The osmotic pressure of possible candidates as gas draw agent is

first predicted with Van’t Hoff ideal law. Then osmotic pressure of the screened gas draw agent

is calculated accurately using three different theoretical models. In this project, desalination

process involves Forward Osmosis (FO) system in the first step that naturally drives out the

fresh water from feed water (FW) through osmotically driven process using the novel liquefied

gas draw solution (DS). The second step involves a depression-thermal separation process to

regenerate the osmotic agent and produce clean water. A key process simulation is conducted

3


to calculate the specific energy consumption of the proposed novel hybrid FO desalination

process with the regeneration method.

The aims and objectives of this investigation are summarized in four steps as follows:

To study the suitability of the selected gas compounds as draw agent in terms of high

osmotic pressure and significant drop in their solubility in water by thermal-depression

process.

Simulate FO process using the success previous theories with the selected membrane

properties to investigate the effect of operating conditions such as draw solution

concentration, operating temperature and cross flow rates of feed/draw solutions on

water flux through the membrane.

Design and simulate a reliable and cost effective regeneration process for draw

solution, according to physical/chemical properties of the selected draw agent.

Calculate the specific energy consumption of the regenerating method and compare

with the current desalination methods to find an economical hybrid FO desalination

process to decrease energy cost.

The specification of draw solution is critical in providing both the sustainability and cost-

effective clean water from salty feed water. The main features in selecting an osmotic agent

(OA) or draw solution are high solubility in water producing high osmotic pressure accordingly

and easy and inexpensive separation of the draw solution in the subsequent regeneration

process to yield clean water without itself being consumed. Initial investigation were conducted

to predict whether the selected gas draw solutions produce high osmotic pressure in solution

describing in detail in chapters 2 and 3.

Concentration polarization (CP) plays a key role in hindering the performance of membrane

based desalination process. The previous research on modelling Forward Osmosis desalination

process considering CP is reviewed in chapter 4, and then the reliable modified models are

applied to calculate the water flux in our project.

In chapter 5, the effect of increasing temperature, varying the DS/FW concentration and flow

rates on CP and water flux is investigated to evaluate the selected model and predict the

optimum operating condition for FO desalination process.

Finally, the results of simulation the thermal-depression regeneration system to separate the DS

and produce clean water are discussed in chapter 6. Specific energy consumption (SEC) of the

simulated regeneration process for the DS is calculated to compare with the current

desalination methods and evaluate whether the presented novel FO desalination process with

4


thermal-depression regeneration system is economical for commercializing and producing

cheap clean water.

A summary of most significant previous FO efforts for desalination is reviewed in the

following sections. The main challenges that FO desalination has faced include suitable draw

agent with reliable regenerating method, the structure and material of membrane, membrane

fouling and concentration polarization in membrane. The review begins with a classification of

different types of DS used so far in batch and continuous FO desalination process and a review

on DS regeneration methods is proposed as well. Then recent membrane development in FO

desalination system is presented and summarised in tables and charts. The aim of this literature

review is to identify the scientific gaps between the previous research and our study and present

how they are addressed in this project successfully.

1.2. Challenging in Draw agents and Hybrid Desalination by Forward

Osmosis The FO system is driven by the natural osmosis process without external mechanical

pressure and hence lower energy consumption. In the Forward Osmosis (FO) system, the

differential osmotic pressure between salty feed solution and a highly concentrated solution

“draw solution” (DS) is used as the driving force to transfer water across a semi-permeable

membrane from feed into draw solution. In a simple description, in FO process, pure water

flows out of seawater or any impure water that has lower osmotic pressure, across a selective

permeable membrane to dilute the draw solution with a higher osmotic pressure. Then the

diluted draw solution goes to the regeneration unit in order to separate and recycle draw agent

for reusing and extract fresh water as the product. Figure 1-1 shows the FO desalination process

integrating with membrane based regenerating method.

5


Figure 1-1: Manipulated Forward Osmosis process [6]

A review of the background of draw solution separation and recovery methods proved that FO

desalination processes have been developed in batch and continuous modes. The initial works

focused on investigating application of FO in desalination and the diluted draw solution used

directly for drinking without the regeneration process while the recent research activities

attempt to develop continuous FO process involving regeneration and recovery of diluted draw

solution in an efficient and sustainable energy cycle [7].

1.2.1 Forward Osmosis Batch System

Kessler and Moody [8], Stache [9] and Kravath and Davis [10] applied mixture of

dissolved glucose in seawater, concentrated fructose and concentrated solution of glucose and

fructose as draw solution for producing drinkable emergency water in lifeboats, in natural

disasters and nutrient drinks. Later on inorganic mixtures including MgCl2, CaCl2 and NaCl and

sugar (Glucose and Sucrose) were examined as draw solutions and were immersed in brackish

water for producing potable water in emergency without the power source by Wallace et al.

[11]. Recently HTI used sugar as draw solution in the same method and manufactured

6


hydration bags for supplying emergency water to military, the public and humanitarian water

desalination filters [12]. Although all these studies developed batch FO desalination process for

producing clean water in emergency conditions, supplying sustainable drinking water requires

continuous FO process.

1.2.2. Forward Osmosis Continuous System

Along with the batch FO desalination process, continuous FO process comprising the

regeneration of draw solution has been investigated using different thermal and membrane

based separation methods. A summary of tested draw solutions and the regeneration methods

for continuous FO desalination is described on the following subsets.

1.2.2.1 Volatile Draw Solution

Batchelder [13] employed volatile solute sulphur dioxide as draw solution for

demineralising salty water. The volatile solute was separated by heating gas stripping once the

DS was sufficiently diluted. Glew [14] further continued on this idea using a similar mixture of

water and sulphur dioxide and an aliphatic alcohol recovering by heating/cooling gas stripping

process. It was anticipated that the obtained mixture would lower the ionic mobility of the

solution to allow a net flow of potable water to be absorbed from seawater. Recently,

McCutcheon et al. [15] examined a high-concentration solution of NH3 and CO2 as draw

solution in FO desalination process and recovered the diluted mixture through distillation

method. High solubility and osmotic pressure of ammonium bicarbonate (NH4HCO3) produced

high permeate water flux and feed water recovery as well. Later on, Hancock and Cath [16]

reported a high reverse diffusion of NH4HCO3 due to its low MW, which may limit its field of

use without addressing this problem. Furthermore, Ng et al. [17] found that ammonium

bicarbonate is not stable and decomposes under temperatures higher than 30 °C and the

required heat for the regeneration of the mixture must be a waste heat; otherwise, the overall

energy consumption would not be competitive with the conventional RO process. McGinnis

[18] tested both KNO3 and SO2 draw solutions in two-stage FO desalination process

respectively where the former has solubility directly dependent on temperature and the latter

has solubility inversely dependent on temperature. The separation method for the mentioned

draw solutions in each step was precipitation by changing the operating temperature; therefore,

this method needs a considerable amount of energy for cooling and heating processes.

McCormick and co-workers [19] chose ethanol aqueous solution as draw solution. Due to the

considerable difference in boiling point with water, this makes it easier to separate ethanol from

7


water. However, this group examined ethanol in a hybrid FO-pervaporation desalination

process; they focused on the loss of DS through different membranes and found that membrane

selectivity for water versus ethanol may not be economically feasible.

1.2.2.2 Sugar as Draw Solution

Yaeli [20] investigated again the idea of using sugar as draw solution but in a

continuous process with the idea of combining FO and a low-pressure reverse osmosis process.

Due to the relatively high osmotic pressure and solubility of sucrose, the recovery rate was

restricted and the Specific Energy Consumption (SEC) was higher than other desalination

methods.

1.2.2.3 Nanoparticles Draw Solution

Yen and their colleagues [21] tested four tailored-design charged and neutral

compounds of 2-methyllimidazole-based draw solution in a hybrid Forward Osmosis

desalination process. The results indicated that the charged and large molecules have higher

solubility and water flux and less reverse solute diffusion flux than the others. Membrane

Distillation (MD) was integrated with Forward Osmosis process to demonstrate the potential of

application and recycling of the designed draw agent. However, the result reported that the

water flux using 2-methyllimidazole-based draw solution is lower than NaCl draw solution in

the same condition; the water flux differences between both draw solutions were rather small in

low concentration applications. In addition, the reverse solute flux of 2-methyllimidazole-based

draw agent was considerably lower than NaCl draw solution.

1.2.2.4 Polymeric Draw Solution

Lyer [22] tested polyethylene/polypropylene glycol with molecular weight between

300–800 Da as the cloud point solutes draw solutions in Forward Osmosis desalination process.

Subsequently, draw agent was recovered through cloud point precipitation and Ultra/Nano

Filtration systems. The results illustrated that this method can reduce the operating costs of

desalination by a factor of three to five over existing desalination technologies although the

water flow rate through the membrane was still not reasonable.

Ling and Chung [23] examined Magnetic Nanoparticles capping with polyacrylic acid as a

novel draw solution in Forward Osmosis desalination. The effect of surface hydrophilic and

particle size on performance of FO process was investigated and the result demonstrated that

8


modification of surface chemistry and diameter of magnetic particles of DS increased the water

flux in FO process. Despite the aggregation problem of magnetic particles, high Osmolality and

easy-efficient separation in a magnetic field and recovering of draw solution make them

attractive for studying in future works. Yen et al. [21], Ling and Chung [23] looked into two

hybrids FO-MD and FO-UF desalination methods using Nanoparticles draw agents

respectively. However, aggregation problem of magnetic Nanoparticles during recovery

process was decreased in hybrid FO-MD and FO-UF processes, these novel draw solutions are

considerably expensive and their syntheses are still very complex to operate.

Ge et al. [24, 25] examined polyelectrolyte draw solution due to good solubility in water, high

osmotic pressure, various molecular weights and their structure, which caused easy separation

from water and low reverse salt diffusion. The results among three examined samples,

polyacrylic acid sodium (PAA-Na 1200, 1800 and 5000) claimed acceptable water flux,

osmotic pressure and less salt leakage but the satisfying performance ratio in seawater

desalination may need a significantly developed semi-permeable membrane in the future.

Polymer hydrogel was used as draw agent in FO desalination by Li et al. [26, 27] and the

results showed that hydrogel polymers absorbed large volumes of water through the membrane

and fresh water could be extracted by a combination of hydraulic pressure and thermal stimuli

on dewatering the polymer. In a more recent study, they reported a new composite polymer

hydrogel with light-absorbing carbon particles as draw solution dewatering with sunlight

irradiation for FO desalination process. However, more than 98% of produced water can be

recovered after 1 hour with 1Kw/m2 irradiation intensity, a practical challenge for solid form

DS is required for more investigation [25].

1.2.2.5 Inorganic Draw Solution

Later in 1972, Frank [28] examined aluminium sulphate as a draw agent in FO

desalination process. Aluminum sulphate has good solubility in water and high osmotic

pressure. Freshwater extracted from DS after precipitating aluminium sulphate ions by adding

calcium hydroxide while the DS is consumed during the process and cannot be regenerated,

which increases the operating cost. Recently, Zhao et al. [29] examined sodium sulphate

Na2SO4 draw solution for brackish water desalination using hybrid FO-NF system. The result

was compared with a pressure driven desalination RO and showed more advantages including

lower operating pressure, less flux decline, no chemical cleaning consumption, no pre-

treatment requirement and higher permeate quality in hybrid FO process.

9


Tan and Ng [30] proposed Forward Osmosis-Nano Filtration (FO-NF) process for seawater

desalination. They used seven inorganic draw solutions including NaCl, KCl, CaCl2, MgCl2,

Na2SO4, MgSO4 and C6H12O6 in FO process. They found that the quality of produced water

meets the recommended total dissolved solids (TDS) guideline from the world Health

Organisation (WHO) when MgSO4 and NaSO4 were applied as draw solutions in FO-NF

process with two-pass NF regeneration system.

Recently Sharif and co-workers [31] developed the Manipulated Osmosis Desalination (MOD)

process as a potential replacement of the conventional RO process. The manipulated Forward

Osmosis approach is based on the manipulation of the osmotic potential between two solutions

to allow pure water to diffuse in the preferred direction. This approach differs from previous

techniques in the ability to use tailor-made (and selected) osmotic agents (OA). These OAs

give the highest separation and operational efficiencies using a membrane separation method in

comparison with conventional RO process. This makes the process more energy efficient,

environmentally friendly and economically viable. Figure 1-2 shows schematically the current

MOD pilot plant, which has been installed at the University of Surrey’s Centre for Osmosis

Research and Applications (CORA). A pilot plant study at CORA, supported by the UK Royal

Society Brian Mercer Award for Innovation [32], has shown that significant savings can be

achieved in both energy consumption and capital cost in comparison with the conventional RO

desalination process for similar throughput, water recovery rate and salt rejection [33].

Additional benefits include minimal chemical treatment and concentrate (brine) disposal, which

are strong measures of this environmentally sustainable process. The results are very promising

and support the need for scaling up to large-scale commercial applications. The technical

breakthroughs by CORA have been commercialised by Surrey Aqua Technology, the

University of Surrey spin- out company, which is a wholly owned subsidiary of Modern Water

Plc. [34, 35]. The first industrial-scale installation of the CORA Manipulated Osmosis

Desalination Technology (18 m3/day) was commissioned by Modern Water in Gibraltar in

September 2008 delivering water to the local drinking water system since 1 May 2009 [34].

This was followed by a much larger plant (100 m3/day) located at the Public Authority for

Electricity and Water’s site at Al Khaluf in Oman in November 2009 [34]. These plants use the

Manipulated Osmosis technology. The plant has operated without any chemical cleaning,

changing of membranes and decline in productivity since November 2009. This demonstrates

significant advantages in lower energy consumption as well as particularly low fouling.

10


Modern Water is the first company successfully commercialise Manipulated Forward Osmosis

on a large scale for desalination applications. Modern Water was granted an EPC

(Engineering, Procurement and Construction) contract for a 200 m3/day MOD plant at Al

Najdah again in Oman [34].

Figure 1-2: Schematic representation for the MOD pilot plant at the University of Surrey.

The aforementioned studies were summarised in table 1-1 including the applied draw solutions

and related regeneration methods with the research groups and the membranes used.

11


Table1-1 Overview of tested draw agents and regenerating methods for FO desalination process [7, 36, 37]

Year Draw Solute/Solution Membrane Regenerating

MethodResearch

Group References

1965 Volatile solutes (SO2)

Cellulosic membrane, Hollow carrot root, Submersible

Heating or air stripping Batchelder 13

1965 Alcohols, SO2 NA Distillation Glew 14

1972 Al2SO4 Flat sheet Cellulose acetate (CA)Doped Ca(OH)2 to

precipitateCaSO4 and Al(OH)2

Frank 28

1975 Glucose Hollow Fibre Cellulose acetate (CA) Direct Application Kravath &

Davis 10

1976 Nutrient Solution NA Direct Application Kessler & Moody 8

1989 Fructose NA Direct Application Stache 91992 Glucose/sucrose NA RO process Yaeli 201997 MgCl2 Flat sheet Cellulose acetate (CA) Direct Application Loeb et al. 38

2002 KNO3 and SO2 NASO2 is removed through standard

MeansMcGinnis 18

2004-2005

Gas NH3 and CO2, NH4HCO3

Flat sheet Cellulose acetate (CA)-HTI

Heating NH4HCO3, decompose intoNH3 and CO2

McCutcheon et al. 15

2006 NH4HCO3Flat sheet- Cellulose acetate (CA)-

HTI - Ng et al. 17

2007 NaCl FO: Flat sheet- Cellulose acetate (CA)

Distillation/ RO process Cath et al. 5

2008 Ethanol Flat sheet Nafilon 117, Selemion AMV & CMV, Poly vinyl alcohol

FO-Distillation & Pervaporation

McCormick et al. 19

2008 NaCl FO: Flat sheet Polyamide SW30- Filmtec FO-RO Seok et al. 39

20092-Methylimidazole-

basedsolutes

FO: cellulose triacetate (CTA)MD: while Durapore HVHP FO–MD Yen et al. 21

2009 Refined sea salt (maximum 100 g/l)

FO: spiral wound cellulose triacetate (CTA)-HTI R.O Bamaga et

al. 40,41

2009NaCl& Na2SO4 & MgCl2 & KCl & MgSO4&C6H12O6

FO: Hollow Fibre Cellulose acetate , NF: spiral wound cellulose triacetate (CTA)

FO-NF Sharif et al. 6,31

2011 Polyethylene glycol group

FO: Flat sheet cellulose triacetate (CTA) FO-NF Lyer 22

2011 Stimuli-responsive polymer hydrogel polymer hydrogel

Deswelling with hydraulic pressure

and heatingLi et al. 26

2011 Stimuli-responsive polymer hydrogel polymer hydrogel Deswelling with

Sunlight Irradiation Li et al. 27

2011 Hydrophilic Nano Particles

Flat sheet- Cellulose acetate (CA)-HTI FO-UF Ling &

Chung 23

2011 PolyelectrolyteFO: Flat sheet cellulose triacetate

(CTA)-HTI & Hollow Fibre Cellulose acetate (CA)

FO-UF Ge et al. 24,25

2011 Na2SO4

FO: Flat sheet cellulose triacetate (CTA)-HTI

NF: NF-270 & BW30LEFO-NF Zhao et al. 29

2012NaCl& Na2SO4 & MgCl2 & KCl & MgSO4&C6H12O6

FO: Flat sheet cellulose triacetate (CTA)- HTI

NF: TFC-HL-GEFO-NF Tan & Ng 30

12


However, Sharif et al. [6, 31, 32, 34, 35] developed the first hybrid FO-RO process from lab to

the market and operated the first FO desalination plant in Oman; most of those studies have not

been commercialized yet. The advantages and limitation of the draw solutions and regeneration

methods reported in the literature indicate the requirement for developing a suitable draw

solution for desalination to produce potable water with following characteristics:

- An osmotic pressure higher than seawater in addition low cost and non-toxicity.

- Minimum reverse diffusion flux to maintain the effective osmotic driving force.

- Easy and complete separation from the product water so that energy consumption is

affordable.

Beside the best choice of draw solution, FO membrane with suitable semi permeable

characteristics has a key role in the performance of FO desalination process. In section 1.3, a

review of the recent challenges to produce an ideal membrane for FO desalination process is

introduced.

1.3. Membrane DevelopmentThe other critical challenge in Forward Osmosis desalination is membrane

development, which has been investigated by researching groups in various aspects such as

material, facing orientation, fouling and structure relating to concentration polarization. Here

the preceding and recent studied have been reviewed in four following subsets.

1.3.1. Membrane Morphology

The material, thickness, hydrophilicity and structure of activated and support layer of

membrane are the aspects of challenges in FO process have been investigated since 1960.

Cellulose acetate and polyamide composite membranes, which have been used successfully in

the commercialised NF and RO membranes due to the reasonable low cost, available material,

good mechanical strength, low fouling and high water flux, was tested in FO process. Ng et al.

[42] compared the performance of one FO type membrane with two flat sheet RO membranes

and found that FO membrane could achieve higher water flux than RO membranes. In the

recent studies, Wang et al. [43,44] investigated polybenzimidazole (PBI) nanofiltration hollow

Fibre membrane cross linking by p-xylene dichloride in FO process and the results

demonstrated the water permeation around 32.4 L/m2 h for Di-ionized (DI) feed water and

magnesium chloride draw solution. The developed dual layer polybenzimidazole-

13


polyethersulfone (PBI-PES) nanofiltration hollow Fibre membrane was examined by Yang et

al. [45,46, 47] in FO desalination and achieved water flux 33.8 L/m2h and a salt flux less than 1

g/m2h using MgCl2 as draw solution. Chou et al. [48] applied a new thin film PES hollow Fibre

membrane and got the water flux of 42.6 L/m2h using NaCl and DI water as draw and feed

solution respectively. Yu et al. [49] developed a high performance nanoporous

polyethersulfone (PES) membrane for FO application and compared the results with two

commercial cellulose tri-acetate CTA and polyamide PA membranes which showed the

polyethersulfone PES-FO membrane achieved the water flux nearly twice as high as the

commercial type and a decreased reverse solute flux twice that of commercial membrane.

The thickness, hydrophilicity and structure of activated and support layer of membrane were

studied and modified in parallel of synthesizing the material of FO membranes. Tiraferri et al.

[50] studied influence of the structure of support layer of thin-film composite membrane on FO

performance. The support layer casting conditions systematically was varied by polymer

concentration and amount of different solvents, which produced the different structure of

support layer. The water fluxes ranged from 4 to 25 L/m2h using 1M NaCl as draw solution and

DI feed water with high salt rejection about higher than 95.5 % consistently. The results using

RO membranes and FO membrane showed that the performance of CA dense selective layer is

1.5 times higher than FO membrane due to a thinner membrane has lower internal and external

concentration polarization in FO process.

Tan and Ng [51] investigated the influence of membrane structure on the performance of FO

process. They tested a support layer properly peeled off CA membrane in a laboratory scale

unit. The constant water flux 8 l/m2hr during the test showed the potential of using forward

osmosis to concentrate brine. McCutcheon and Elimelech [52] demonstrated that the

hydrophilic and wetted support layer could make higher water flux across semi-permeable

asymmetric membrane; therefore, the wetting mechanisms and the chemistry of the support

layer could be considered to increase the water flux in osmotically driven membrane processes.

The heat-treated membrane in a short time showed high salt rejection due to shrinking pore size

to 0.3 nm on membrane surface also favourable FO performance. However, the water fluxes

through this hollow Fibre membrane decreased by increasing the salinity of feed water due to

rising internal concentration polarization in the FO process. Zhang et al. [53] used double dense

layer of cellulose acetate membrane to minimize internal concentration polarization and

improved the performance in FO process. The result showed good resistance to fouling, low

14


salt leakage and much less internal concentration polarization (ICP) for FO seawater

desalination compared to previous works.

Sairam et al. [54] developed a method to prepare flat sheet thin layer of cellulose acetate

composite membrane for FO desalination and used MgSO4 as draw solution. The water

permeability and salt rejection of the fabricated membrane were found to be dependent on pore

formation agent and annealing temperature which zinc chloride as a pore agent gave high flux,

good permeability of 0.27 l/m2h bar and NaCl rejection of 95% at 70C annealed temperature.

Wei et al. [55] synthesized flat sheet thin film TFC composite FO membrane was with porous

polysulfone support layer. The achieved water flux of 54 L/m2h and low solute reverse

diffusion with 2M NaCl draw solution compared with the commercial FO and RO membranes

exhibited the importance of presented structure of support layer which was made straight

finger-like pore over spongy pore structure to reduce the internal concentration polarization.

Qiu et al. [56] fabricated layer-by-layer chemical cross-linked xLbL and non-crosslinking LbL

of poly allylamine hydrochloride PAH and poly sodium 4-styrene-sulfonate PSS on a porous

polyacrylonitrile PAN substrate which both type had high water permeability. The result

showed an excellent water flux about 100 L/m2h for 3M MgSO4 as draw solution which

demonstrated the potential of these types membrane for high flux FO applications. Wang et al.

[57] introduced the double-skinned FO membrane for reducing the internal concentration

polarization and an analytical model is developed and verified experimentally for this type

membrane by Tang et al. [58]. The prototype double-skinned cellulose acetate membrane

which was fabricated by Wang et al. displayed a water flux about 48.2 L/m2h in a FO process

using 5M MgCl2 as draw solution with DI feed water.

Tang et al. [58] demonstrate that feed skin in double-skinned membrane should be NF type

with high mass transfer coefficient to minimize the overall hydraulic resistant and reduce ICP

simultaneously. Setiawan et al. [59] fabricated a novel hollow Fibre positive charged

nanofiltration membranes, which were using polyamide-imide PAI microporous, hollow Fibres

as the porous substrate followed by polyethyleneimine PEI for developing positive dense

selective layer. The reported water flux of 9.74 L/m2h using 0.5M NaCl as draw solution and

DI feed water in FO process indicated that the positive charged FO membrane could provide an

effective access to make suitable hollow Fibre membrane for FO application.

15


Yin Yip et al. [60] presented a thin-film composite fabricated membrane for FO desalination

application consisted of the selective polyamide active layer on top of the polysulfone support

layer which prepared a high FO performance compared with the commercial membranes due to

thickness, porosity, tortuosity and pore structure of the support layer. The reported data

indicated the water flux of 18 L/m2h using 1.5M NaCl draw solution and pure water as feed

with salt rejection more than 97% consistently. Although the second skin layer may induce

additional water transferring resistance and decrease the water flux, the ICP effect can be

mitigated significantly and also the porous support structure of membrane needs future

improvement to increase the water flux and salt rejection.

Table 2 shows the overall view of the recent researches on FO membranes with the reached

water flux and salt rejection in FO process.

16


Table 2-2: Overview of the used membranes in different researches on FO process [43 to 69]

Feed Draw Solution Membrane Orientation

Water Flux

(L/m2.h)

Salt Rejection (g/m2.h)

Temp (°C)

Group work / Reference

DI water 5.0 M MgCl2

CA double-selective layer membrane & Hollow Fibre

membrane

Bottom & PRO

48.2 & 11.2 6.5 22 ± 0.5 Wang et al. [57]

DI water 1.0 M NaCl

FO flat sheet membrane, HTI. FO 16.8 21.8 23 ±1 Achilli et al.

[61]

DI water 0.5 M NaCl

FO flat sheet membrane, HTI. PRO 18.6 7.4 22 ± 1.5 Gray et al. [62]

DI water 1.5 M NaCl

cellulosic & Polyamide RO

membrane & FO flat sheet with the

fabric layer removed, GE

Osmonics, Dow Filmtec & HTI

PRO36.0 & 8.1 & 43.2

17 20.0McCutcheon

and Elimelech [52]

DI water 3 M NaCl

Commercial Polyamine ,CTA

and polyethersulfone

(PES) membranes

FO 30 5.6 20.0 Yu et al. [49]

1 M NaCl 6 M Fructose

polyamide and Cellulose acetate RO membrane &

FO

PRO 9.1 5 50 Tan and Ng [51]


dual-layer (PBI-PES/PVP)

nanofiltration hollow Fibre membrane

PRO 33.8 / 45.6 0.55

23 ± 0.5/ 38.5 Yang et al.

[46,47]


Cellulose acetate membrane PRO & FO 17 & 10 1.2 & 0.8 20 Zhang et al. [53]

DI water 1.5 M MgSO4

TS80 NF TFC membrane,

TriSepFO 1.1 0.04 20 ± 2 Cornelissen et

al. [69]

NaCl 4.0 M NaCl

FO flat sheet membrane, HTI. PRO 37.8 - 20 ± 1 Mi, and

Elimelech [68]

0.6 M NaCl 1.5

MgSO4

Cellulose acetate membrane FO flat sheet membrane,

HTI

FO 6.5 20 Sairam et al. [54]

10mM NaCl

0.5M NaCl

TFC flat-sheet Polyamide–

polysulfone & (CTA-HW)Cellulose triacetate

PRO 20.5 & 15.4 5.9 & 9.4

23Wei et al. [55]

DI water 2-3 M MgCl2

Layer by layer poly(allylamine hydrochloride)

(PAH) and

PRO 105 23 Qiu et al. [56]

17



Water Flux

(L/m2.h)


Temp (°C)


poly(sodium 4-styrene-sulfonate)

(PSS)

DI water 1.0M NaCl

TFC flat-sheet membranes Polyamide–polysulfone

FO 25.0±4.1 - 25±0.5 Tiraferri et al. [50]

500 ppm (8.6 mM)

NaCl& 3.5 wt.% (0.59

M)NaCl

0.5M & 2M NaCl

TFC hollow Fibre Polyamide–

polyethersulfonePRO 32.9 &

12.4 2.9 20∼25 Chou et al. [48]

DI Water 1.5 M NaCl

polyamide FO membrane FO 18.6 25 Yin et al. [60]

DI water 1.5M MgCl2

Positively charged hollow Fibre

Poly(amideimide)–

Polyethyleneimine

FO / PRO 11.7 / 17.2 3.9 / 37.7 23 Setiawan et al.

[59]

DI water 2.0M MgCl2

Asymmetric hollow Fibre

Cellulose acetatePRO / FO 7.3 / 5.02 - - Jincai et al. [67]

DI water

2.0M NaCl/2.0

M MgSO4//

2.0M Na2SO4/

2.0M MgCl2


Polybenzimidazole

PRO3.84/5.65/7.74/9.0

2- 22.5 Wang et al. [44]

0.1 g/L lysozymeaqueous solution

3.125M MgCl2

Dual-layer hollow Fibre

Polybenzimidazole–

polyethersulfone/polyvinylpyrrolid

one

PRO / FO 17.1 / 12.7 - - Yang et al. [46]

DI water 5.0M MgCl2


Polybenzimidazole

PRO 36.5 - 23 Wang et al. [43,44]

Permasep B-10 hollow Fibre,

DuPontPRO 2.8 - 25 Mehta and Loeb

[66]

Sea water Glucose solution

CA hollow Fibre, Dow FO - - Kravath and

Davis [10]

3.5% NaCl

45% Na2HPO

4

Cellulose triacetate ROmembrane, Osmotek

PRO

14.5 (degrade

dat pH 9)

- 69.0 Miller and Evans [65]

18



Water Flux

(L/m2.h)


Temp (°C)


water

98 g/L anonymo

usosmotic agent

FO membrane, Hydration

Technologies- 24.0 -

ambientCath et al. [64]

0.5 M NaCl

4 M NH4HC

O3


TechnologiesPRO 11.0 - 50.0 Ng et al. [42]

digester centrate

70 g/L NaCl


TechnologiesFO 16.4 - 25.0 Holloway et al.

[63]

0.5 M NaCl

5 M fructose


TechnologiesPRO 19.5 - 50.0 Tan et al. [58]

19


1.3.2. Membrane Orientation

The orientation of membrane can be done in two positions including Forward Osmosis

(FO) and Pressure Retarded Osmosis (PRO) modes, which are named reverse and normal

orientation respectively. In normal or PRO mode membrane active layer is faced the draw

agent solution and reversely facing the feed solution in FO mode. Gray et al. [62] studied the

effect of membrane orientation on internal concentration polarization (ICP) phenomena and the

water flux through cellulose acetate membrane was tested for three draw agents comprising

NaCl, dextrose and sucrose. The membrane orientation has a significant effect on FO

performance so that the measured water flux in PRO mode in different osmotic pressure

difference was higher than FO mode. In addition, solutes with larger molecular weight have

lower water flux due to more severe ICP when the porous support layer faced DS in FO mode.

Recently Zhao et al. [29] investigated the role of membrane orientation on performance of FO

desalination. The result showed that the water flux in FO mode is more favourable when the

salinity and fouling of feed water is considerable such as seawater desalination however PRO

mode is preferable for feed water with low salinities and fouling tendencies which could be

found in brackish water desalination.

1.3.3. Membrane Fouling

Fouling is a major problem in all membrane applications. The extent to which the

membrane fouls depends on the type of membrane and the concentration of contaminants in the

feed water. Several studies have proposed methods to reduce fouling of Nano-filtration (NF)

membranes; which it is still a major factor in using NF membranes in desalination. Pre-

treatment processes are needed to remove substances that would interfere with the desalting

process. Algae and bacteria can grow in both Reverse Osmosis (RO) and distillation plants, so

a biocide (usually less than 1 mg/l chlorine) is required to clean the system. Some RO

membranes cannot tolerate chlorine, so de-chlorination techniques are required. Ozone or

ultraviolet light may also be used to remove marine organisms. If ozone is used, it must be

removed with chemicals before reaching the membranes. Mi and Elimelech [68] investigated

the physical and chemical aspects of different organic foulant on FO membrane and found that

both chemical (i.e. calcium binding) and hydrodynamic interactions such as hydrodynamic

shear and permeation drag controlled FO fouling and the rate and extent of organic fouling

could be determined by foulant-foulant interaction and intermolecular adhesion force. As the

20


proposed FO desalination process is not reliant on mechanical pressure, suspended particles are

not forced into the membrane pores, thus any fouling at the FO step should be reversible. It is

also expected in the FO step that membrane compaction effect due to high pressure, which

results in reducing the flux, would be minimal as compared to a conventional RO process [70].

In 2010, Lee et al. [71] investigated systematically comparison of fouling behaviour in FO and

RO methods with different type of organic foulant and various particle sizes of silica colloids

foulant at common hydrodynamic operating condition and feed water chemistries. They

reported structure of fouling layer (i.e. thickness and compactness) is quite different in FO and

RO and organic fouling in FO could be controlled by optimizing the hydrodynamics without

adding chemical cleaning. FO flux declined dramatically according to the type of foulant, size

of particles and the type of DS. Furthermore, the cake enhanced osmotic pressure within the

fouling layer due to the reverse salt diffusion from DS to feed. Therefore, by selecting the

proper DS and improving the properties of membrane such as higher selectivity, the fouling in

FO method could be minimized. Backwash may be enough to remove the deposited particles;

allowing Clean-In-Place (CIP) online operation. This would obviously depend on the type of

the fouling, but it may be true with non-biological fouling. It is also the aim to produce draw

solution creating minimal fouling or scaling at the second stage of the NF separation step. Mi

and Elimelech [68] studied the model for organic foulant in FO membrane. The result showed

that the recovery of permeate water flux after rinsing by water and without using any chemical

was more than 98%. In addition, the flux recovery in FO mode was much higher than RO mode

in a similar cleaning conditions although the flux decline rates in FO were as same as RO

mode. Furthermore, the adhesive sites on FO membrane had an important role in increasing the

fouling and decreasing the cleaning efficiency.

In the proposed process, the CIP of the FO unit could be done in a number of ways. By either

replacing the draw solution by pure water or reducing the DS concentration, such that water

will pass in the opposite direction and thus clean the membrane. Alternatively, similar results

could be achieved by increasing the salt concentration in the feed solution, either by fluctuating

the operating pressure (increasing and reducing the pressure) or by using salt dissolve shocks to

allow water to back diffuse from the DS side to the feed solution side [72].

21


1.3.4. Concentration Polarization in Membrane

In pressure-driven membrane separation processes, such as Reverse Osmosis (RO) and

Nano-filtration (NF), solutes and particles in the feed solution are transported with the solvent

to the membrane surface. The accumulation of solutes and particles close to the membrane is

known as concentration polarization (CP). RO membranes are designed to have a thin and

dense separating layer called the active layer, which is supported by multiple porous layers.

The purpose of the active layer is to reject the salts while the function of the supporting layer is

to provide mechanical stability to the membrane during the pressure-driven water flow. The salt

rejection takes place near the membrane surface where a region of increased salt concentration

or diluted draw solution forms; this is known as concentrative and dilutive external

concentration polarization (ECP) respectively.

Internal concentration polarization (ICP) occurs exclusively in FO process when feed or draw

solution is placed against the support layer. Since the solute cannot pass through the active

layer of membrane easily, it will concentrate in the internal structure of the support layer of the

membrane. The concentrative or dilutive internal concentration polarization forms when the

support layer is faced feed or draw solution side respectively. The effective driving force is

reduced severely due to internal concentration polarization in FO process. Determining the

effect of internal and external concentration polarization (ICP and ECP) on FO processes has

had increased attention in the recent studies. Gray et al. [62] examined the effect of various DS

on concentration polarization considering forward osmosis (FO) and pressure retarded osmosis

(PRO) membrane orientation. The results showed a significant impact on the quality and

quantity of produced water. The external concentration polarization (ECP) can be reduced by

altering the hydrodynamic conditions around the membrane. The proposed techniques include

increasing the cross flow rate, looking at turbulent promoters, impulse methods and agitating

methods. Possible alternatives to reducing the effect of ECP are gas sparging techniques, flow

reversal or mechanical methods [73].

The studies on concentration polarization identified that internal concentration polarization

(ICP) as a key performance-limiting phenomenon that may capable of reducing water flux by

more than 80% [74]. The reduction of concentration polarization in osmotic driven membrane

process can increase the water flux and reduce the effect of fouling. Tang et al. [74]

investigated and examined systematically the effect of both fouling and internal concentration

22


polarization (ICP) on FO water flux behaviour through different concentration of draw solution

and membrane orientation. The observations which agreed as well with ICP model showed that

the water flux increased in higher level of the concentration of DS; however the ICP was

enhanced especially when activated layer of membrane was faced with feed water.

Furthermore, the membrane surface was covered greatly by fouling in PRO orientation whereas

FO mode was more stable against membrane fouling. The results showed that the surface

coverage percentage was constant up to a critical flux and after that it enhanced dramatically by

increasing the water flux. Wang et al. [57] investigated the effect of the feed spacers on water

flux through the membrane and the result reported a strong positive effect on FO flux although

particle precipitation was observed near to spacer. Figure 1-3 illustrates an asymmetric

membrane with firstly the dense active layer against the draw solution that the profile shows

concentrative internal concentration polarization (ICP) and dilutive external concentration

polarization (ECP). Secondly, the porous support layer against the DS giving rises to dilutive

internal concentration polarization (ICP) and concentrative external concentration polarization

(ECP).

23


Figure 1-3 Schematic representation of concentration profile across two types of membranes

orientations in FO process using two different solutes and one solvent [75].

The mentioned challenges, which closely related to each other in FO desalination, are

summarized in figure 1-4. In fact, the thin porous support layer and a high selective active layer

reduce the impact of internal concentration polarization (ICP) and reverse solute diffusion

respectively. The minimized reverse solute diffusion can further decline the fouling of

membrane. On the other hand, small ion/molecule size of DS makes low internal concentration

polarization (ICP) however; both the reverse solute diffusion and membrane fouling are

increased considerably. Further internal concentration polarization (ICP) and membrane fouling

24


have adversely affected on the water flux in FO desalination. In addition, the effects of both

direct diffusion of water and reverse solute diffusion through the membrane on the performance

of FO desalination have not been investigated fundamentally yet and could be considered at

future studies.

Figure 1-4: Schematic representation of relationship between membrane structure, DS ion/molecule

size, ICP, membrane fouling and reverse solute diffusion and their effects on water flux and the

performance of FO desalination system.

25


1.4 Summary

As reviewed above, Forward Osmosis desalination process has faced two main

challenges including appropriate draw solution can be separated from produced water with a

cost effective regeneration process and design of high performance FO membrane lowering

ICP effect and reverse draw solution diffusion.. Since the effects of draw solute and membrane

on FO performance are closely linked together, both aspects should be considered in the design

strategy of FO process. The preferred draw solute must have high osmotic pressure, reasonable

molecular size and low viscosity whereas FO membrane should have characteristics of high

permeability with small structure parameter to eliminate the effect of ICP phenomenon. The

advantages and limitation of tested draw solutions and regeneration methods mentioned in the

literature indicates obviously the requirement of selecting a suitable draw agent with easy

separation method in FO desalination process. In this project two objectives has been followed

in FO desalination process including introducing a novel draw agent with high osmotic

pressure, nontoxic and low cost suitable to producing potable water and simulate a reliable and

low cost regeneration method appropriate for the novel draw solution according to its

physical/chemical properties.

26


CHAPTER TWO

A NOVEL DRAW SOLUTION CONCEPT

27


2.1.The Prior ArtForward Osmosis (FO) or Manipulated Osmosis Desalination (MOD) is one of the

evolving membrane technology that has been proven to consumes less energy than traditional

processes for desalination. The novelty of FO process lies in using natural osmosis as a driving

force for water alone to move through a semi-pearmeable membrane from a solution of low

osmotic pressure (less solute concnetration) to a solution of high osmotic pressure (high solute

concentration). In the MOD process the common NaCl salt in seawater as well as other

nondesrirable salts are replaced, using natural omosis, by other dissolved ions (organic or non-

organic) termed osmotic agents or draw solutions (DS), where their separation from water is

much easier and less energy intensive than the removal of NaCl in a desalination process, using

Reverse Osmosis or other efficient separation processes. Forward Osmosis desalination has the

potential to provide a reliable and cost effective technology for producing fresh water if the

draw solution (DS) regeneration process limitations are addressed and the membrane

technology is further developed. One of the key challenges of FO process is in developing a

suitable DS that has high osmotic pressure than feed solution and also high solubility in water

in order to produce high water flux through the membrane, but most importantley that it can be

regenerated in a practical and an energy efficient way. One of the effective factors to improve

FO method is the selection of a draw agent with high osmotic pressure. All compounds include

volatile, nutrient, inorganic salts, organic salts and synthetic materials such as magnetic

nanoparticles can be used as a draw solution to extract water from a feed solution through FO

membrane. Additional criterions in the selection of a draw agent can be summarized in

minimum reverse diffusion to feed solution, economical cost, safe handling, nontoxic, low risk

of scaling in high concentration and easy to regenerate for reusing in the FO process.

The concept of employing volatile compounds as a draw agent has been developed by different

researchers using ammonium bicarbonate, mixing of ammonium hydroxide- ammonium

bicarbonate, sulfur dioxide, mixture of water-sulfur dioxide- aliphatic alcohols, combination of

potassium nitrate- sulfur dioxide and finally ethanol since 1964 [76]. Volatile draw agents can

be separated from the produced water with thermal regeneration processes such as distillation

or heating method and recycled to FO process by dissolving the volatile gases back into water.

28


2.2. A Novel Draw Agent for FO Process The selection of the suitable draw agents with high osmotic pressure, non-toxic, low cost

and easy separating from clean water is addressed as the first research question in this project.

Gas compounds obtaining high solubility in water could be of great interest as draw agent in

FO process in terms of separating from water by changing operating temperature or pressure.

The cost-effective FO process could be achieved, where the fresh water is extracted from the

liquiefied gas draw solution if the solubility of draw agent in water is changed considerably

with varying operating pressure or temperature. Figure 2-1 shows the diagram of the suggested

FO process with depresion regeneration method using liquefied gas as draw solution. Fresh

water is drived out from seawater (120) to liquefied gas draw solution (130) carring out by

manipulating the osmotic pressure differential across the semi-permeable membrane (120). The

operating pressure of the diluted draw solution may be decreased by a regulating valve (160).

Then the solution is introduced into a vacuume or atmospheric column (140), where it is

subjected to a pressure below vapour pressure of liquefied gas draw solution. The sudden

reduction in pressure causes gas stripping to occur. The separated gas can be liquefied and

compressed again by a compressor (150) and recycled to the Forward Osmosis unit (100-110-

120) as a concentrated draw solution.

Figure 2-1 Block flow diagram of depression regeneration method

29


In this project, all 137 gaseous compounds from gas encyclopedia [77] were initially

considered as draw agent then were evaluated by detemining their solubility in water resulting

in the osmotic pressure and their relative easy regenration through a depression method. The

osmostic pressure of all candidates were calculated using Van’t Hoff [78] equation for ideal

gas:

Π = MRT (2-1)

Where M is the molarity of the solute which is equal to the ratio of the number of solute moles

(n) to the volume of the solution (V), R is the gas constant of 8.3145 JK-1mol-1and T is the

absolute temperature. The potential of a draw solution to generate relevant osmostic pressure

higher than seawater osmotic prssure 27 bar have been considered to select the appropriate

draw agents. The full list of the solubility and the predicted osmotic pressure of all 137 gas

compounds in water were tabulated in appendix A1.

At the end of the screen process, only four gases including Monomethylamine, Dimethyl ether,

Ammonia and Sulfur dioxide are listed in Table 2-1 were selceted as suitable draw agents due

to their high solubility in water and relative osmotic pressure particular higher than seawater

osmotic pressure. .

.

30


Table 2-1 Estimated Osmotic Pressure of the selected gases as draw agent in FO process

No Name FormulaMW

(g/mole)

Operating

Temperature

(°C)

Operating

Pressure

(bar)

Solubility

(g/l)

Osmotic

Pressure

(bar)

1

Mono-

methyl

amine

CH5N 31.057 20 1 1080.00 836.02

2

Dimethyl

ether

(DME)

C2H6O 46.069 20 4 340 196

3 Ammonia NH3 17 20 1 454.94 643.37

4Sulphur

DioxideSO2 64 20 1 99.98 38.2

Although sulfur dioxide (SO2) was used as a draw solution by McGinnis [18] , its special

physicochemical properties such as corrosive, acidic and unstable solution needs careful

operations in FO process and subsequent post treatment increasing capital and operating cost.

Therefore SO2 was not considered and was deleted from the list of condidated draw agents in

this project. In addition, monomethyl amine is toxic and harmful if it is swalloed reporting in

material safety data sheet (MSDS) by suppliers [123]. Therefore monomethylamine was

eliminated from the list due to the osmotic agent employed in FO process should ideally be

non-toxic.

Draw solutions from the rest two selected gas compounds were then evaluated by determining

their easy and cost-effective separation from clean water with thermal-depression regeneration

process. This was involved investigating the variation of osmotic pressure of Ammonia

solution at saturated concentration in water versus increasing temperature using OLI stream

analyser software package [OLI System INC,2005]. Figure 2-2 shows that by increasing

temperature, the osmotic pressure of Ammonia in water is stable before 70°C while it decreases

moderately after 70°C . Furthermore, Ammonia gas can be expelled in diluted draw solution at

water boling temperature, 100°C. In addition, the osmotic pressure of Ammonia draw solution

was not changed with varying hydraulic pressure resulting in OLI software output. In

conclusion, the results indicate that thermal separation method may not be a cost-effective

31


regeneration process using Ammonia as draw solution; therefore Ammonia was omitted from

the candidated gas draw agents in this study.

20 30 40 50 60 70 80 90 100 1100

25

50

75

100

125

150

175

200

NH3 Osmotic Pressure,atm V.S Temperature,°C, at 5.62 Molarity

Temperature, °C

Osmo

tic Pr

essure

, atm

Figure 2-2 Ammonia osmotic pressure Vs. operating temperature at saturated concentration (5.62

Molarity)

The last screened liquefied gas draw agent is Dimethyl ether (DME). Two solubility data of

Dimethyl ether (DME) in water at two different operating pressures 4 bars and atmospheric

were reported in gas Encyclopedia book [77]. It shows the solubility of DME in water is

enhanced by increasing the external pressure from atmospheric to 4 bars. Therefore liquefied

gas DME could be separated from the DME-water solution by depression method such as

atmospheric or vacuum gas striping in FO regenearting process. The solubility of DME in

water at 4 bars and atmospheric pressure with the estimated osmotic pressure are illustrated in

table 2-2 as:

32


Table 2-2 DME solubility in water at 4 bars and atmospheric pressure with Estimated Osmotic Pressure

[77]

NO. Name FormulaMW

(g/mole)

Temperatur

e

(°C)

Pressure

(bar)

Solubility

(vol/vol)

Osmotic

Pressure

(bar)

1

Dimethyl

ether

DME)

C2H6O 46.069 20 1 35 11

2

Dimethyl

ether

DME)

C2H6O 46.069 20 4 197 196

The solubility of DME in water at pressure range between 0.7 to 4 bars was reported by

AckzoNobel [79] shows the increase of solubility of DME in water by increasing the external

pressure on DME-water solution as well. The graph is illustrated in Appendix A2.

The aformentioned results indicate that the liquefied gas Dimethyl ether (DME) could be a

suitable draw agent obtaining high solubility in water, significant osmotic pressure higher than

seawater, non-toxic and easy separating from water by depression method. In the next section

more information is introduced about physical- chemical properties of DME and its application.

33


2.3.Dimethyl Ether (DME) Background Applications Dimethyl ether (DME) is the simplest ether expressed by the chemical formula,

CH3OCH3. DME is a colorless, chemically stable liquid and gas, with boiling point -25.1°C at

atmospheric pressure. It is typically stored as a liquid at 6 bars in standard vessels. DME is a

polar gas when liquefied under pressure is partially miscible with water [80]. One DME

molecule has two lone pairs of electrons, and it easily forms hydrogen bounds with water

molecules as shown in figure 2-3. It is widely used as a propellant for aerosol sprays,

particularly for its polar solvent capacity and it has drawn attention as a promising clean

burning alternative fuel [81].

In addition, it causes no greenhouse effect and does not affect the ozone layer; thus its effect on

ecological systems is very small. DME is a synthetic material produced from natural gas, coal-

bed gas, biomass and similar substances. DME has attracted attention as a next-generation fuel

in place of LPG because it is harmless and naturally decomposable, contains very samll

amounts of metal, nitrogen, sulphur, halogen and other substances of concern, and is

combustible. DME and CO2 have been used as refrigerants and mixtures of CO2 and DME can

also be used to give a high performance refrigerant that operates at moderate pressures [80]. It

is also possible to use liquid DME as a solvent for extraction of lipids from a range of feed

stocks [79]. DME has a relatively high vapour pressure compared to liquid organic solvents and

can be easily and more completely about 99% removed from extracted products [79]. DME has

been used for extraction of a wide range of natural and polar lipids, including extraction

aqueous systems. DME can be mixed with CO2 to extract polar compounds and give a solvent

with increased polarity [81]. DME is highly flammable in air although it does not form

peroxides. Mixture of DME and CO2 have a reduced flammability due to the critical

temperature of the mixture is reduced then adding CO2 may make DME based processing more

practical to carry out [80].

34


Figure 2-3 Electrostatic potential surface for ethanol (left) and DME (right) [82].

Note: The surface for ethanol clearly shows the polar O-H bond. The O atom in DME has a

partial negative charge. Color coding is: Red indicates a region of largest negative charge.

Colors from yellow to green indicate increasing positive charge (or decreasing negative

charge). Blue indicates a region of partial positive charge.

Recently Kanda [83] developed an energy-saving dewatering process for high-moisture coal

using liquefied DME as the water extracting agent. The liquefied DME under moderate

pressure, 6-8 bars, at room temperature can be effectively used to extract water from coal. The

principle underlying conventional dewatering methods is evaporation of the water content by

heating the fuels to a high temperature, but this approach consumes a considerable amount of

energy. The DME dewatering method is also being studied by Oshita et al. [84,85] to see

whether it can be used for the treatment of sewage sludge. Sewage sludge has a water content

of 80% even after mechanical dewatering and a huge amount of energy is required to evaporate

the remaining water.

In both dewatering methods, first the high-moisture coal or sludge packed and liquefied DME

are mixed. Approximately 7-8 g of water is extracted when 100 g of liquefied DME is used.

The liquid mixture of liquefied DME and water is then separated from the dewatered coal or

sludge packed. By decompressing the mixture at normal temperature, only the DME is

evaporated. The water then can be extracted in liquid form. The evaporated DME is liquefied

by compressing or cooling and can be reused. This method can be operated at ordinery

temperatures at all stages. The amonut of energy required for dewatering has a significant

reduction in relative to that employed in conventional methods. High amount of thermal

energy provided by an external source involves vaporising the water content and drying in

35


convectional method.. Kenda [83] reported that the amount of energy consumed by dewatering

method using liquefied DME is half the amount of energy consumed by conventional drying.

Figure 2-3 shows the result of dewatering of a sample biosolid presented by Kenda et al. in 7 th

Asian DME conference on November 2011 [86].

Figure 2-4 Deodorization and Dewatering of Biosolids using liquefied DME [86].

2.4.Dimethyl Ether (DME) - Water Solution as a Novel Draw Agent The first predicted osmotic pressure of novel liquefied gas draw agent, Di-methyl Ether

(DME), indicates that it could serve as a suitable osmotic agent in a Forward Osmosis process.

DME is a low-boiling point (-25.1°C) temperature solvent and easy extraction agent. Hence the

proposed draw agent may provid up to 50% energy saving as compared to other osmotic

agents regeneration processes.

With reference to Avogadro’s law, Vant Hoff [87] demonstrated that there is a deep analogy

between dilute solution exerting osmotic pressure and the gases under ordinary atmospheric

pressure [87]. This analogy will show that if Henry’s law be taken into consideration, the

osmotic pressure in solution is absolutely equal to the gaseous pressure, under similar

conditions of temperature and concentration. On the other hand, under equal osmotic pressure

and at the same temperature, equal volumes of all solutions contain equal numbers of

molecules. As shown in table 2-2, the solubility of liquefied DME gas in water may increase by

rising with the pressure , thus the osmotic pressure of the DME-water solution can be

calculated at a reliable pressure and temperatue, using the above mentioned analogy by Vant

Hoff [87]. The results illustrated in table 2-2, shows the high osmotic pressure around 200 bars

may be generated by DME in water under 4 bars external pressure and at the defined DME

concentration. Preferably, the gaseous draw agent has an osmotic pressure which is more than

36


twice the osmotic pressure of the seawater solution being 27 bars. Therefore the osmotic

pressure of the draw solution is preferably more than 30 bars.

The predicted osmotic pressure of DME-water solution under 6 bars pressure shows that DME

could be a suitable choice of an osmotic agent for a Forward Osmosis process. Furthermore,

with reference to table 2-2, if the external pressure on DME-water solution is decreased to

atmospheric pressure, a significant decline would be occured on the solubility of DME in

water. It seems that DME draw agent could be separated from the solution by depression

process such as gas striping or atmospheric-vacuume flash methods. By taking into

consideration of coal and sludge dewatering process using liquefied DME solution, the novel

Forward Osmosis desalination process with depression-thermal regeneration using DME draw

agent is designed and simulated in this project. A summary of the whole FO desalination

process is described in this chapter and the detail design criteria and simulation results is

discussed in chapters 5 and 6 respectively.

Figure 2-5 illustrates the flow diagram of the novel Forward Osmosis desalination using

depression regeneration process.

The process of the present invention may be a continuous or a batch process. The process of the

present invention may comprise a pre-treatment step including removing suspended solid and

biological matter from the feed water or adjusting pH or controling scaling by inhibitors in

desalination or waste water treatment purposes. Where seawater is applied deep well seawater

on beach is rather used as usually includes fewer suspended solid and less biological matter

than seawater attained from the surface of the ocean using intake facilities.

Process comprising by placing a slective membrane (120) between the feed solution (100) and

the novel draw solution (110) of disolved gas (111) having a higher Osmotic pressure than feed

solution, such that water from feed solution passes through the membrane to dilute the novel

draw agent solution, where clean water is subsequently extraced from the diluted draw agent

solution (112) using a gas striping process (140 & 170) . In the process of the present invention,

the feed solution (101) is placed on one side of a selective membrane (100) and the novel draw

agent solution (111) having a higher osmotic potential is positioned on the opposite side of the

membrane (110) . As a result of the difference in osmotic pressure between the feed solution

and the draw agent solution, water passes across the membrane from the side of low osmotic

potential to the side of high osmotic potential. Thus the draw agent solution will become

37


increasingly diluted (112) and the feed solution increasingly concentrated (102). The diluted

draw solution (112) may be recovered from the interior of the tubes, end port of housing or

output port of flat frame in tubular, spiral wound or flat sheet type membranes respectively.

Both side of membrane may be kept under low or medium pressure (P-101 & 150) to induce

high solubility of darw agent, enhance the water flow rate cross the membrane and increase the

specific energy generation in desalination and power generation processes. The gas draw agent

DME (141) is dissolved in water (143) using a compressor (150) for increasing the pressure for

instant to 6 bars (111) and nitrogen gas flow could be applied to keep the DME-water draw

solution under pressure in FO process. A circulating pump ( P-101) is employed to flow sea

water through feed side under the required pressure. The size of the pores within the semi-

permeable membrane is small enough to prevent the movement of gaseous draw agent into the

feed chamber of the Forward Osmosis apparatus. In embodiments where the draw agent is

Dimethyl ether, the size of the pores within the semi-permeable layer may be between 5-10

Angstrom, preferably 3-5Ao. Although the solute species in the feed solution may be

adequately small to pass through the membrane pores, they are prohibited from doing so due to

high osmotic potential of the draw solution on the other side of membrane.

The method may comprise extracting water from the diluted draw solution using a depression

(170) or thermal method (140), or a combination of these two methods. Thermal methods

preferably include thermal flashing and pressure methods preferably include vacuum gas

stripping or atmospheric gas stripping (140). These methods may be employed in a single

apparatus. Alternatively, separate apparatus may be employed for each method. The discharged

liquefied DME solution (111) from compressor (150) would be warm due to compression

process and could be passed through a heat exchanger (H.E) to prepare the required heat in

thermal flash tank (140). The flow of water through the membrane is generally influenced by

thermal condition. Thus, the feed solution may be heated while the draw agent solution

warming during compression may be cooled in a heat exchanger, if desired. The feed solution

may be heated to temperatures of 30 to 50 οC, in parallel the draw agent solution may be cooled

to 20 to 40 οC. The heating or cooling may be carried out on each solution independently. The

whole FO unit may be maintained at a hydrostatic pressure of 3 to 10 bar. The invention may

reduce energy usage (130) by about 50% compared to other seawater desalination system

including thermal systems such as MSF, MED or membrane based Reverse Osmosis method

operating with 60 bars hydraulic pressure. Furthermore lower energy consumption for DME-

38


water depression regeneration than other osmotic agent regeneration processes, such as the

regeneration of NaCl or other organic or non-organic osmotic agents.

In the FO desalination process, the feed water contains saline solution that some solutes thoes

are sufficiently small to pass through the membrane, may be included in the draw agent

solution. These probably diffused species along the flow of water from the saline feed solution

to draw soultion may be extracted from the bottom stream (142) of the separating column (140)

by an extra or a combined of two of extraction techniques (160) such as low pressure reverse

osmosis, adsorption or membrane distillation using low grade heat to produce drinking water

(161). In such case, the pressure or heat required to extract water (161) from the bottom stream

(142) of the separation column by reverse osmosis or membrane distllation (160) is generally

less than the pressure required to extract water from seawater, brackish water or waste water by

reverse osmosis using the first saline feed water directly. However the capital cost of this

hybrid FO-RO system is around one and half times more than seawater RO system, the

operating cost of energy in FO-low pressure RO is significantly lower than RO unit. The

separated DME gas in the extra extraction unit (160) is combined with DME gas stream(141) in

the thermal-depression system (140).

39


Figure 2-5 the novel FO desalination process using depression regeneration method

40


2.5 Summary

The first investigation on the concept of using liquefied gas as a draw agent in FO

process shows that Di-methyl Ether (DME), has high solubility in water; therefore it would

generate high osmotic pressure and would serve as a suitable osmotic agent in a Forward

Osmosis process. According to the existing experimental data, the osmotic pressure of the

liquefied DME-water solution at 4 bar external pressure was determined 200 bar. The

predicted osmotic pressure of DME-water solution under 4 bars pressure could be changed if

the external pressure on DME-water solution is decreased to atmospheric pressure, due to a

significant decline in the solubility of DME in water would occur. Hence DME draw agent

could be separated from the solution by depression process such as gas striping or atmospheric-

vacuume flash methods. By taking into consideration of aformentioned potential of employing

liqufied gas DME as draw solution in FO desalination process, the osmotic pressure of DME

draw solution is determeined accurately using available experimentla data in the next section.

In chapter 4, 5 and 6, the novel Forward Osmosis desalination process with depression-thermal

regeneration system is simulated and designed in detail to evaluate the whole system in terms

of energy consumption and fresh water product in this project.

41


CHAPTER THREE

OSMOTIC PRESSURE, PHYSICAL PROPERTIES BEHAVIOR

&EXPERIMENTAL RESULTS AND DATA REDUCTION

42


3.1. Introduction Ideally, a draw agent should be soluble and generate an aqueous solution with an osmotic

pressure that is high enough to draw pure water out of the feed solution through a semi-

permeable membrane and into the draw solution. The osmotic pressure is a function of solute

concentration and temperature, and it is therefore a colligative property and relates to other

colligative properties such as relative lowering vapour pressure, elevation of boiling point and

depression of freezing point since they all depend on the number of solute particles and not the

type of chemical species presents [78,87]. In this chapter the criteria of different methods for

estimating the osmotic pressure and physical properties of the defined concentration of

dissolved solute in water is described then the osmotic pressure, viscosity, density and diffusion

coefficient of liquefied DME aqueous solution is calculated using the experimental data were

reported by different research groups. The osmotic pressure of the liquefied DME in water is

estimated with three different methods including depression of freezing point, lowering vapour

pressure and accounting for waters of hydration. Chapoy et al. [89], Holldrff and Knapp [90]

and Miller et al. [91] achieved experimental data for depression of freezing point, lowering

vapour pressure and waters of hydration for DME respectively. These experimental results are

applied to estimate osmotic pressure of the liquefied DME. Furthermore, an experimental

method providing a direct measurement of osmotic pressure with semi-permeable membrane,

suitable for liquefied DME draw agent is introduced as the future work.

3.2. Osmotic Pressure Behavior

3.2.1. Osmotic Pressure MethodologyThe analogy was introduced by Van’t Hoff [87] describes the pressure characteristics of

gases also come into play for solution in the form of osmotic pressure. The pressure (P) of

gases is due to the impacts of gas-molecules on the vessel wall and osmotic pressure (π) of

solution is the impacts of the dissolved-molecules on the semi permeable membrane as

illustrates in fig 3-1. Osmotic arises when two solutions at different concentration are

separated with a semi-permeable membrane. Then solvent mainly moves from a solution with

low-concentration into a solution with high concentration through a membrane to balance the

chemical potential of both solutions and equilibrium is occurred. The osmotic pressure is the

hydraulic pressure must be exerted to prevent the net flow of solvent across a membrane. In

figure 3-1, a liquefied gas is dissolved in a solvent e.g., water and store in a cylinder equipped

43


with a free movement piston on one side and a semi-permeable membrane on the other side.

The cylinder is installed into a container of a pure solvent e.g., water. A pure solvent moves

from container into a liquefied gas solution through a membrane due to osmosis phenomena.

The volume of gas solution is expanded and the piston goes up consequently. The pressure in

the cylinder is due to the impact of gas molecules on the vessel wall that is described as

osmotic pressure (π) of gas solution. On another point of view, the hydraulic pressure must be

exerted on the piston to prevent solvent movement from container into the cylinder through a

membrane is described as osmotic pressure (π) of gas solution.

Figure 3-1 (Top) expansion of a compressed gas (dark stars), Pressure of gas and (Bottom) dilution of a

concentrated solute such as liquefied gas dissolved in water (white stars) through solute un-permeable

membrane (white dashed line), osmotic pressure of solution [88]

Avogadro’s law [87] provided the conception of osmotic pressure for gaseous pressure that at

the same temperature containing the same number of molecules in unit volume of a solution,

the osmotic pressure of the solution is thus seen to be equal to the pressure of the gas. [89].

The application of Avogadro’s law in solutions was investigated experimentally by Van’t Hoff

[87] to confirm the determining of osmotic pressure with the following methods:

Direct determination of the osmotic pressure in a given volume of all solutions contain

an equal number of molecules, exhibit equal osmotic pressure

44


Molecular lowering of vapour pressure namely that the isotonic of the solutions in the

same solvent determines equality of vapour pressure therefore the molecular lowering

of vapour pressure relates proportionally with the molecular weight of the dissolved

substance.

Molecular depression of freezing point connecting with osmotic pressure of a solution may be

stated that solutions in the same solvent which have the same freezing point are isotonic at that

temperature. Therefore the depressions of vapour pressure and of freezing point can be

connected with osmotic pressure and measured instead of osmotic pressure consequently.

However the deviation from Avogadro’s law should be observed to indicate that the

composition into ions does not occur. In the next section different methods to measure

indirectly the osmotic pressure of the dissolved solute in water such as liquefied DME solution

is described.

3.2.2. Osmotic Pressure Determination Methods

There are three commercialized methods [92] to measure osmotic pressure each

leveraging a particular colligative property to achieve the analytical result as follows:

Freezing point depression: determine the osmotic strength of solution by utilizing

freezing point depression.

Vapor pressure Osmometers: determine the concentration of osmotically active

particles that reduce the vapor pressure of the solution.

Membrane Osmometers: measure the osmotic pressure of a solution by a semi-

permeable membrane.

Advantages and disadvantages of the different methods are explained in table 3-1.

Table 3-1 Advantages and disadvantages of the osmotic pressure measuring methods [92]

45


No. Measuring Method Advantages/ Disadvantage

1 Freezing point

Accurate result, Performed rapidly, inexpensive, simple,

reliable, small sample size and suitable for most aqueous and

low viscosity applications but not for colloidal and high

Molality solutions.

2 Vapor pressure

Performed fast, inexpensive, small sample size and suitable

for most aqueous applications, less accurate than freezing

point method and cannot be used for volatile or other organic

solvents, not ideally for high Molality and colloidal solutions.

3Semi-permeable

Membrane

Provide a direct measurement of osmotic pressure, suitable

for high Molality and colloidal samples, but takes long

analysis time and large samples volume. For small molecule

and aggressive solvent is not applicable due to membrane

porosity.

46


3.2.3. Freezing Point DepressionThe freezing point (Tf) of pure water freezing at 0.010 °C is lowered by the addition of

a solute due to a fewer molecules of solvent are available to freeze [93]. The deviation in the

freezing point is related to the Molality of the solute through the equation (3-1) [87, 93]:

ΔT f=−i . K f .C (3-1)

Where ∆Tf is the deviation in the freezing point, i is the Van’t Hoff index, Kf is the cryoscopy

constant equal to 1.858 (°C Kg/mol) for the freezing point of water and C is the Molality

(mol/Kg) of the solute. The deviation in the freezing point, the Molality and cryoscopy constant

can be used to calculate the Van’t Hoff index. A common method for measuring freezing point

depression is through super-cooling a solution to several degrees below its known freezing

point followed by mechanical agitation to induce crystal formation. The formation of crystal

releases the heat of fusion returning the solution to a stable liquid/solid equilibrium

temperature, which is readily measured. In practice, the ratio of measured freezing points for a

series of concentration, per cryoscopy constant (∆Tf / Kf) is plotted against Molality and fitted

with a regression line. The slop of the regression line is an averaged value of the Van’t Hoff

index (i) while the linearity of the line provides additional insight. A positive deviation from

linearity at high concentrations suggests the influence of waters of hydration while a negative

deviation suggest higher order ion pairing processes. The depression of freezing point is useful

for estimating osmotic pressure near a solvent’s freezing point. The osmotic pressure as a

colligative property is proportional to the molar concentration of solute particles C (mole/lit)

and absolute temperature of solution T(K) by the following equation:

π=i .C . RT (3-2)

Where π is osmotic pressure (bar), i is Van’t Hoff index, R is molar gas constant (here is 0.082

atm.lit/g mole K), C is the Molality (mol/Kg) of the solute and T is absolute temperature of

solution (K). The average deviation between the measured osmotic pressure and osmotic

pressure calculated from freezing point depression is less than 1% at 25°C and less than 2% at

30°C for cane sugar [88].

47


3.2.4. Vapour Pressure OsmometersRaoult’s law described that if a non-volatile solute is added to a solvent such as water,

the vapour pressure of the solvent is decreased according to a linear relationship to the mole

fraction of the solvent. The addition of solute to pure solvent lowers the activity of the solvent

as reflected alterations in its colligative properties. If the pure solvent and the dissolved solute

in solvent foreseen on both side of a semi-permeable membrane, then water moves from pure

solvent side to the solution phase side to raise the activity and balance the chemical potential,

due to the chemical potential of both sides must be equal at equilibrium. The external pressure,

which is exerted on the solution side to increase the activity of water and equalize the chemical

potential, is called osmotic pressure. Based on Raoult’s law [87] and using the activity of water,

the theoretical representation of osmotic pressure can be involved as follows:

μ(T , p)liquid =μ(T , P+π )

solution(3-3)

Where μ(T , p)liquid

is standard chemical potential (J/mol) of pure solvent (water), T is temperature

(K) and μ(T ,P+ π )solution

is the chemical potential (J/mol) of water in the solution at equilibrium. The

chemical potential of water in the solution is decreased can be rewritten:

μ(T , P )liquid =μ(T , p+π )

liquid +RT ln(aw ) (3-4)

Where μ(T , p+π )liquid

is standard chemical potential (J/mol) of water in solution and μ(T , P )liquid

is the

standard chemical potential (J/mol) of water, R is gas constant (8.314 J/mol K), T is

temperature (K) and aw is water activity in solution. The amount of RT ln( aw ) indicates the

reduction of water activity by added solute in the solution. Furthermore, the amount of external

pressure that must be exerted on the solution side to raise the activity of the water by the

amount RT ln( aw )at constant temperature can be determined by:

μ(T , p+π )liquid =μ(T , p )

liquid + ∫p

p+ π

V m dp(3-5)

48


Where Vm is the molar volume of water (m3/mol) and P is pressure upon the solution (Pa).

Equations (3-4) and (3-5) can be combined to give the following relationship:

−RT ln ( aw )= ∫p

p+π

V mdp=πV m(3-6)

Where, π is the osmotic pressure (Pa) of the solution. The negative sign refers the direction of

water flow from low concentration or pure into high concentration solution. Finally, the

osmotic pressure can be determined through the following equation:

π=−RT ln (am )/V m (3-7)

Where, π is osmotic pressure (Pa), Vm is water molecular volume (m3/mol), aw is activity of

water, R is gas constant (8.314 J/mol K) and T is temperature (K) respectively. The empirical

activity coefficient values, which vary with temperature and concentration, can be used to

determine the activity of water by the written equation as:

aw=γ w xw (3-8)

Here γw and xw are the activity coefficient and the mole fraction of water respectively. Margules,

Van Laar and NRTL equations have been proposed for the relation between activity coefficient

and mole fraction however NRTL equation often provides a good representation of

experimental data for strongly non-ideal mixtures and especially for partially immiscible

system [94]. The NRTL equation for activity coefficient in a binary system is:

ln γ1=x22[ τ21(

G21

x1+x2G21)2+

τ12G12

( x2+x1 G12)2 ]

(3-9)

ln γ2=x12[ τ12(

G12

x2+x1G12)2+

τ21G21

( x1+x2G21)2 ]

(3-10)

τ12=A12−A22

RT∧τ21=

A21−A11

RT (3-11)

G12=exp (−α12 τ12) & G21=exp (−α12 τ21) (3-12)

49


Where Aij is an energy parameter characteristic of i-j interaction, α ij is related to the no

randomness in the mixture and varies from 0.2 to 0.47 for a large number of binary systems, γ ij

is the activity coefficient, xi is mol fraction, R is gas constant (8.314 J/mol K) and T is

temperature of solution.

3.3. Model for Calculating the Physical PropertiesPhysical properties such as viscosity, density and diffusion coefficient of liquid binary

systems are very important in engineering calculations involved in the process design for FO

desalination system. The transport properties of DME-water binary system are calculated using

the thermodynamic properties of pure liquefied DME, which were predicted with molecular

simulation by Guevara et al. [95] and Wang et al. [96] and pure water respectively.

The general form of the mixing rule [97] is used to calculate the density of DME-water solution

at different mole fraction as follow:

ρm=∑ xi ρi (3-13)

Here ρm is density of mixture (g/l) xi and ρi are the mole fraction and density of component i

(g/l) respectively.

Laliberte [97] referenced the modified form of the Arrhenius mixing rule by Irving, using mass

fractions instead of mole fraction, and claimed that generally gives better result. The model is

applied in the calculation of viscosity of DME-water solution as:

ln μm=W m ln ( μw )+W DME ln (μDME ) (3-14)

Here Ww is water mass fraction WDME is mass fraction of DME, μw is water viscosity (cp) and

μDME viscosity of DME (cp) .

The Wike-Chang correlation [98] for diffusion in associated solvents making hydrogen

bounding is used for the calculation of diffusion of DME in water solution is written in

equation 3-16 as:

50


D=7 . 4×10−8 ( xM )1/2T

ηV 0 . 6 (3-15)

Where μ is the viscosity (cp) of water, V is the molal volume (m3/mol) of solute (DME) at

normal boiling point, M is the molecular weight of solvent, T is the temperature (K) and x is

association parameter. The association parameter x is introduced to define the effective

molecular weight of the solvent with respect to the diffusion process and for water x = 2.6.

Cussler [99] reported the average error about 10% for the calculated diffusion coefficient of

oxygen in water from the empirical value.

3.4. Experimental Results and Data ReductionsThe solubility data shows that liquefied DME is soluble in water and generates an

aqueous draw solution however the miscibility limits are indicated. DME-water solution as a

draw agent must have an osmotic pressure which is more than twice the osmotic pressure of

seawater, brackish water or waste water. For example, for seawater, the osmotic pressure of the

draw solution is preferably more than 30 bar.

Therefore, the osmotic pressure of liquefied DME dissolved in water is calculated based on the

aforementioned methods using the experimental liquid-liquid equilibrium data reductions in the

following sections.

51


3.4.1. Binary Vapour- Liquid Equilibrium of DME Mutual solubility of liquid Dimethyl ether and water was investigated by Holldroff and

Knapp [90] in a static equilibrium kit at various temperatures between 250 K to 350 K and

pressure up to 10 bars applying the synthetic method. In synthetic method, one pure substance

was filled in the cell, and the other substance was added until a small amount of second liquid

phase appeared. The overall composition was determined by weighting and the correction was

made accounting the quantity contained in the vapour phase. By heating and cooling the

system, the disappearance and reappearance of the second phase was investigated. The

experimental result of their study including mutual solubility of liquid Dimethyl ether in water

(LLE) is presented in table 3-2. Where, X1 is mole fraction of DME in water and T (K) is

temperature of liquid mixture.

Table 3-2 Experimental mutual liquid solubility of DME (1) in water system (LLE) at different

temperature by Holldroff and Knapp [90]

Water Phase

T (K) X1 (mol/mol)

259.3 0.2081

265.8 0.2050

270.4 0.2027

272.5 0.2021

278.1 0.1967

279.5 0.1961

280.4 0.1972

282.5 0.1940

284 0.1915

289.2 0.1884

289.9 0.1815

295.7 0.1795

299.2 0.1735

306.4 0.1635

314.1 0.1528

319.6 0.1462

319.9 0.1451

52


The measured vapour pressure at different temperature for a known composition of liquid

mixture was introduced in table 3-3. Where, T is temperature, P (kPa) is experimental vapour

pressure and X1 is the composition of Dimethyl ether in water.

Table 3-3 Experimental PLV (T) of DME in water system at constant composition of liquid by

Holldroff and Knapp [90]

T (K) P (kPa) X1(mol/mol)

273.04 16.73 0.0083

282.94 27.02 0.0083

292.94 42.52 0.0082

302.86 64.10 0.0082

312.78 93.64 0.0081

322.89 133.11 0.0081

332.88 182.41 0.0080

342.89 242.6 0.0080

272.91 30.81 0.0153

273.05 31.17 0.0153

282.87 49.38 0.0153

292.92 76.43 0.0153

302.92 114.09 0.0152

312.68 163.49 0.0151

312.68 163.73 0.0151

322.62 228.47 0.0151

332.72 310.56 0.0150

342.61 408.00 0.0149

342.62 408.04 0.0149

273 59.54 0.0285

282.84 92.00 0.0285

292.77 138.25 0.0284

292.78 138.56 0.0284

53


T (K) P (kPa) X1(mol/mol)

292.78 138.76 0.0284

302.70 202.44 0.0283

312.60 285.85 0.0282

312.62 285.98 0.0282

322.53 393.38 0.0282

332.62 526.95 0.0281

263.20 114.88 0.1007

273.14 174.87 0.1006

282.88 255.69 0.1005

292.82 365.39 0.1004

302.72 507.84 0.1002

312.70 689.83 0.1000

312.70 689.83 0.1000

322.57 912.86 0.0998

327.53 1042.60 0.0997

258.32 124.86 0.1449

268.17 184.84 0.1448

272.93 221.42 0.1447

282.83 315.14 0.1446

292.78 438.97 0.1444

302.74 597.13 0.1442

312.64 792.92 0.1441

317.61 907.75 0.1440

The experimental studies of vapour- liquid equilibrium of DME-water system at temperature

between 50 to 220°C and pressure to 50.9 MPa was investigated by Pozo and Streett [100]

indicating the same results with Holldorff and Knapp at 50°C which tabulated in table 3-4.

Where, P (MPa) is pressure, X1 is the mol fraction of DME in water in liquid phase and Y1 is

DME mol fraction in water in vapour phase.

54


Table 3-4 Experimental equilibrium compositions of DME-water in Vapour– liquid region by Pozo and

Streett [100] at T = 50°C

P (MPa) X1 Y1 P (MPa) X1 Y1

0.012 0.000 0.000 0.407 0.041 0.956

0.138 0.000 0.894 0.448 0.045 0.957

0.152 0.000 0.900 0.552 0.051 0.964

0.193 0.000 0.922 0.765 0.085 0.965

0.248 0.000 0.934 0.834 0.098 0.967

0.310 0.028 0.945 1.027 0.155 0.977

Chapoy et al. [89] investigated the mutual solubility of DME- water and compared their results

with the data were available in literature. Figure 3-2 shows the solubility of DME in water at

three temperatures 0°C, 20°C and 50°C resulting from their introduced modelling [89]. In

addition, data from Pozo and Streett [100], Holldroff and Knapp [90], Dahlhoff et al. [124] and

Kono [125] were fitted on the solubility data showing well represented the modelling [89].

Figure 3-2 Mutual solubility of DME and Water [89]

Note: Pozo and Streett [100], Holldroff and Knapp [90], Dahlhoff et al. [124] and Kono

[125]

The experimental data were fitted on two models suggested DME a non-polar (dash line) or a

polar (straight-line) compound by Chapoy et al. [89]. The plotted data in figure 3-2 indicates

that the DME solubility data are well represented by the model assuming DME as a polar

55


compound. The developed model by Chapoy et al. [89] also used to predict the pressure-

composition equilibrium of DME-water at three isotherms equilibrium showing in figure 3-3.

Figure 3-3 P-x-y diagram of DME-water at 0, 20 and 50°C [89]

Note: Pozo and Streett [100], Holldroff and Knapp [90], Dahlhoff et al. [124] and Kono

[125]

The miscibility limits in this figure shows the maximum solubility of 34% by weight DME in

water and a maximum of 6% by weight water is miscible with DME. Furthermore, there are

two liquid phases between these borderlines. The experimental data reported by the mentioned

four research groups in figure 3-2 were fitted on the developed model as indicated in figure 3-3.

3.4.2. Models for the Excess Gibbs EnergyHolldorff and Knapp [90] used the experimental data resulting in their study to fit

parameters of several excess Gibbs energy (gE) models and checked the reproducibility of the

data with the gE –models by cross-predicting VLE and LLE consequently.. First, the binary

parameters A12 and A21 were estimated at several temperatures from isothermal mutual

solubility data, and used to predict the vapour pressure of the corresponding isotherm. The

parameters (A12 and A21) for binary LLE data for DME-water system and average of the

deviation (δ (∆P) in (KPa)) between calculated vapour pressure and experimental data are listed

in table 3-5.

56


Table 3-5 gE model parameters estimated from binary LLE data of DME-water system at one

temperature and average deviation of the vapour pressure of the corresponding isotherm by Holldorff

and Knapp [90]

ModelTemperatur

e (K)

Parameter

A12

Parameter

A21

Deviation

δ (∆P)

(KPa)

Margules

273.15 2.4127 0.2048 6.59

293.15 2.4075 0.0712 17.30

323.15 2.4206 -0.1296 48.37

Van Laar

273.15 2.2300 2.6328 6.25

293.15 2.3388 2.4810 17.7

323.15 2.5568 2.2995 47.50

NRTL α = 0.24 273.15 4180.7 2582.5 5.43

NRTL α = 0.34 293.15 4323.8 3929.4 8.39

NRTL α = 0.38 323.15 4714.8 5340.6 9.98

UNIQUAC

273.15 4014.7 51.517 7.9

293.15 3821.5 218.12 7.8

323.15 3395.3 634.58 27.27

Note: The third NRTL parameter, α, was adjusted to give the accurate representation of the

vapour pressure data. Parameter α is related to the no randomness in the mixture and varies

from about 0.2 to 0.47.

Then, model parameters were fitted to each set of isothermal VLE data of DME-water and

mutual solubility predicted. The non-idealises of vapour phase were considered as well. The

model parameters A12 and A21 for binary VLE data of DME-water system and average

deviations (δ (∆P) in (KPa)) between experimental and calculated values of vapour pressure are

listed in table 3-6.

57


Table 3-6 Model parameters estimated for binary VLE of DME-water system by Holldorff and Knapp [90]

ModelTemperatur

e (K)

Parameter

A12

Parameter

A21

Deviation

δ (∆P)

(KPa)

Margules

273.15 2.5142 0.3059 3.98

293.15 2.4894 0.0677 8.2

323.15 2.6836 0.0025 62.6

Van Laar

273.15 2.2035 2.9980 3.49

293.15 2.4042 2.6111 8

323.15 2.4976 2.8312 8.24

NRTL α = 0.3

273.15 5981.1 2460 2.24

293.15 5036.6 3316.8 3.83

323.15 4935.4 4326.3 5.48

UNIQUAC

273.15 5437.1 -239.08 4.16

293.15 4706.5 34.073 1.84

323.15 4305.6 407.50 4.28

Note: Note: The third NRTL parameter, α, was adjusted to give the accurate representation of

the vapour pressure data. Parameter α is related to the non-randomness in the mixture and

varies from about 0.2 to 0.47.

3.5. Predicted Osmotic Pressure of DME-Water SolutionWith reference to, the aforementioned experimental data the osmotic pressure of the

DME-water draw solution is calculated at different temperatures and DME concentration using

the methods were described in sections 3.2.3 and 3.2.4.

3.5.1. Freezing Point Depression Results Water and DME have a relatively high degree of mutual solubility. Furthermore, DME is

known to form a type-II solid clathrate hydrate while the hydrate dissociation point is - 23°C [91].

Chapoy et al. [89] presented the results of an experimental and theoretical study on the hydrate

stability zone, ice melting point and water content of DME-water system at temperature and

pressure conditions appropriate to cavern storage conditions of DME. The model predicted with

58


reasonable accuracy the freezing point of DME aqueous solution and the good agreement between

model, experimental data and the data of Miller et al. [91] demonstrated the reliability of the

developed model and the generated experimental data [90]. Figure 3-4 illustrates the freezing

point depression of DME aqueous solution with data from Chapoy et al.[89] and Miller et al. [91].

Figure 3-4 Freezing point depression in °C, Chapoy et al.[89] & Miller et al. [91

According to the experimental data for freezing point depression in DME-water system, the

Van’t Hoff index, Kf, is calculated using the equation 3-1. In practice, the ratio of measured

freezing points for a series of concentration, per cryoscopy constant (∆Tf / Kf) is plotted versus

molality and fitted with a regression line. The slop of the regression line is an averaged value of

the Van’t Hoff index (i) while the linearity of the line provides additional insight. Table 3-7 and

figure 3-5 show the calculation of Van’t Hoff index, where Tf is freezing point temperature (K),

∆Tf is the deviation in the freezing point, Kf is the cryoscopy constant equal to 1.858 (°C

Kg/mol) , C is Molarity (mol/Kg).

59


Table 3-7 calculated freezing point depression per cryoscopy constant for predicting Van’t Hoff index

DME Weight

Percent %

Molality, C

(mole/kg)

Freezing

Temperature Tf

(C)

∆Tf /Kf

34 7.39 -18.80 10.12

30 6.52 -16.80 9.05

20 4.35 -10.25 5.52

10 2.17 -4.50 2.43

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.000.00

2.00

4.00

6.00

8.00

10.00

12.00

f(x) = 1.49813254002474 x − 0.873363258190216R² = 0.998798890519859

Vant Hoff Constant

Molality mol/Kg

∆Tf /

Kf

Figure 3-5 Van’t Hoff Index diagram

Then osmotic pressure is determined using equation 3.2 Where i and R are Van’t Hoff index and

molar gas constant which is 0.082 (atm.lit/gmole.K) respectively.

Wilson and Stewart [88] developed the equation 3-16 to calculate the osmotic pressures of draw

solutes accounting water of hydration based on the draw solute water hydrates content.

π=iρ (ns

M w−hns MW W)RT

(3-16)

Where π is osmotic pressure (bar), i is Van’t Hoff index, ρ is density (g/l), n is number of solute

moles, Mw is mass solute (g), MWw is solute molecular weight (g/mol), R is gas constant, 0.082

(atm.lit/gmole.K) and h is total waters of hydration per mole solute. Water content in

60


equilibrium with the liquid DME and hydrate at 6 bars were measured by Chapoy et al. [89]

from 1.1°C down to -29.7°C. The experimental data from their work was showed in figure 3-6

illustrating the good agreement with the accepted values in the DME community from

AkzoNobel Industrial Chemicals B.V [79]. Miller et al. [91] studied the structure of the

clathrate hydrates II of DME at - 24°C and found that the 16-hedra is nearly fully occupied in a

structure II hydrate.

Figure 3-6 Water content in liquid DME at 6 bars (red line) and 20 bar (dashed blue line) modeled and

fitted on previous experimental data by Chapoy et al. [89].

Note: Literature studies include: Miller et al. [91], Pozo and Streett [100], Holldorff and Knapp [

90], Dahlhoff et al. [124], Akzo Nobel Industrial Chemicals B.V [79], Catchpole et al. [126],

Park et al. [127], Laursen et al. [128] and Naicker et al. [129].

The experimental data of waters of hydration were applied in equations 3-2 and 3-16 to

determine the osmotic pressure of DME-water. The results are listed in table 3-8 and illustrated

in figure 3-7.

61


Table 3-8 Estimated Osmotic Pressure of DME-water using experimental Data in this work

Van’t Hoff constant, i (figure 3-4) 1.4981 1.4981 1.4981 1.4981

Molality, C (mole/kg) 7.39 6.52 4.35 2.17

DME Concentration (g/lit) 340 300 200 100

Gas Constant, R (atm.lit/gmole.K) 0.082 0.082 0.082 0.082

Temperature, T (K) -18.80 -16.80 -10.25 -4.50

Density, ρ (Kg/l) 0.955 0.960 0.973 0.987

Osmotic Pressure, (bar)

(Using Van’t Hoff equation 3.2)220.40 197.09 136.61 70.75


(Using Miller et al. [91] data for water

hydrates in equation 3.16)

220.87 197.46 136.78 70.80


(Using Chapoy et al. [89] data for water

hydrates in equation 3.16)

222.47 198.72 137.37 70.95

50 100 150 200 250 300 350 4000

50

100

150

200

250

Estimated Osmotic Pressure of DME-water us-ing experimental Data

DME Concentration (g/lit)

Osm

otic P

ress

ure

(bar

)

Figure 3-7 Estimated Osmotic Pressure of DME-water using experimental Data

Van’t Hoff equation (3-2), Miller et al. [91] data (equation 3-16), Chapoy et al. data (equation

3-16)

62


As shown in figure 3-7 and tabulated results in table 3-9, the calculated osmotic pressures of

DME –water draw solution applying equations 3-2 and 3-16 and the experimental data from

studies by Chapoy et al. [89] and Miller [91], are in reasonable agreement with each other.

Furthermore, the osmotic pressure of DME-water solution at maximum solubility

concentration, which is seven times more than seawater osmotic pressure, approves the concept

of using DME as a novel draw agent in FO desalination process.

3.5.2. Vapour Pressure Lowering ResultsDME is of polar, water miscible nature. The miscibility limits of DME- water solution

shows the maximum solubility of DME in water about 34% by weight and a maximum of 6

percent water is miscible with DME. In between these borderlines, the blend forms liquid

phases. As mentioned in section 3.2.4 NRTL equation often provides a good representation of

experimental data for non-ideal mixtures and especially for partially immiscible system [94].

With reference to the experimental binary parameters of NRTL model for the excess Gibbs

energy, which was introduced by Holldorff and Knapp [90], the osmotic pressure is determined

using equations 3-7, 3-8 and 3-9 and the results are shown in table 3-9.

63


Table 3-9 Estimated Osmotic Pressure of DME-water using experimental Data in NRTL model

Temperature

(K)

Pressure

(bar)

DME

mol

fraction

X

Water

mol

fraction

(1-X)

DME

Concentratio

n (g/l)

NRTL

Parameter

(A12)

NRTL

Parameter

(A21)

292.78 4.38 0.1444 0.8556 302 4323.8 3929.4

292.78 3.65 0.1004 0.8996 222 4323.8 3929.4

292.78 1.38 0.0283 0.9717 74. 4323.8 3929.4

292.78 0.76 0.0153 0.9847 39. 4323.8 3929.4

292.78 0.42 0.0082 0.9918 21 4323.8 3929.4

323.00 10.27 0.1550 0.8450 320 4714.8 5340.6

323.00 8.34 0.0980 0.9020 217 4714.8 5340.6

323.00 7.65 0.0850 0.9150 198 4714.8 5340.6

323.00 5.52 0.0510 0.9490 121 4714.8 5340.6

323.00 4.48 0.0450 0.9550 109 4714.8 5340.6

Continue Table 3-9

Paramete

r in

NRTL

Model

τ12

Parameter

in NRTL

Model

τ21

Paramete

r in

NRTL

Model

G12

Parameter

in NRTL

Model

G21

Water

Activity

Coefficien

t γ2

Water

Activity

(Eq. 3-8)

Osmotic

Pressure

(bar)

(Eq. 3-7)

0.16 -0.16 0.95 1.06 0.9998 -0.16 208.24

0.16 -0.16 0.95 1.06 0.9999 -0.11 141.23

0.16 -0.16 0.95 1.06 1.0000 -0.03 38.30

0.16 -0.16 0.95 1.06 1.0000 -0.02 20.57

0.16 -0.16 0.95 1.06 1.0000 -0.01 10.98

-0.23 0.23 1.09 0.92 0.9994 -0.17 248.63

-0.23 0.23 1.09 0.92 0.9998 -0.10 152.09

-0.23 0.23 1.09 0.92 0.9998 -0.09 130.96

-0.23 0.23 1.09 0.92 0.9999 -0.05 77.11

64


-0.23 0.23 1.09 0.92 1.0000 -0.05 67.82

3.5.3. Osmotic Pressure Prediction Results and DiscussionThe predicted osmotic pressure results using experimental data of freezing point

depression were compared to NRTL model results in figure 3-8. Both determined osmotic

pressure values show that the liquefied DME-water as a draw solution has high osmotic

pressure at maximum solubility concentration of 340 g/l, which is suitable for use in FO

desalination process. DME draw agent is soluble in water and generates an aqueous draw

solution which has a maximum osmotic pressure of about 220 bars, which is more than seven

times of the seawater osmotic pressure of about 28 bars. For example, for seawater, the osmotic

pressure of the draw solution is preferably more than 30 bars.

Freezing point depression model: πDME = 220.4 bar at C = 340 g/l

NRTL model: πDME = 244 bar (at 20°C) at C = 340 g/l

Furthermore, for comparing the prediction made with experimental data of freezing point

depression versus NRTL model, the 17% variation between the osmotic pressure indicates little

difference in the predictions of the experimental data between the two models. While for non-

ideal solutions, and especially for partial immiscible systems, the NRTL equation often

provides a good representation of the experimental data, the osmotic pressure can be predicted

using the freezing point depression more accurately. Figure 3-3 shows that DME-water solution

is a partial immiscible system; therefore the DME- water solution is strongly non-ideal system.

Non-ideality is considered in NRTL equations by Aij that is an energy parameter characteristic

of the DME and water molecules interaction.

65


50 100 150 200 250 300 350 4000

50

100

150

200

250

300

DME Osmotic Pressure using NRTL Model at 20°C & Freezing Point Depression Data

NRTL

F.P.D

Concentration, g/li

Osm

otic P

ressu

re , b

ar

Figure 3-8 Plot of Estimated Osmotic Pressure versus concentration of DME-water based on

experimental data of Freezing Point Depression & NRTL model

3.6. Membrane OsmometerThe osmotic pressure of DME-water solution could be tested and measured by a semi-

permeable membrane. Membrane Osmometer provides a direct measurement of osmotic

pressure especially for high molality solution. In order to measure the osmotic pressure of

DME-water solution the procedure were prepared for future experimental works on this project

mentioned in chapter 7 section 7.2 recommendation for future works.

3.7. Experimental Data for Calculating the Physical PropertiesLiquid properties such as viscosity, density and diffusion coefficient of Dimethyl ether

were predicted from molecular dynamics simulation by Wang et al. [96] and Carrion et al. [95]

in parallel. In the recent years molecular modelling and simulation has been used as a powerful

engineering tool to predict the thermodynamic properties of fluids. The calculated physical

properties of DME were compared with experimental data and showed a good agreement

between prediction and experiments according to the report of both researcher groups [95, 96].

66


The calculated density and viscosity of DME is illustrated in table 3-10 and the self-diffusion

coefficient of liquid DME at 50, 100 and 200 MPa is shown in figure 3-9.

Table 3-10 calculated density and viscosity at temperature 160-350K by Wang et al. [96]

Figure 3-9 Self-diffusion coefficient of liquid DME at 50( ), 100( ) and 200 ( ) MPa [95]

67


According to experimental data of physical properties of pure liquid DME, pure water and

seawater, the physical properties of DME-water solution in different concentration have been

calculated using equations 3-14, 3-15 and 3-15 and the results are shown in tables 3-11, 3-12,

3-13 and 3-14. All calculated physical properties are used in chapter five in simulation of FO

desalination process.

68


Table 3-11 the calculated density of DME-water solution based on experimental data

Temperatur

e

(K)

Pressure

(bar)

DME

mol

fraction

X

Water

mol

fraction

(1-X)

DME

Concentration

(g/l)

DME

Density

ρ (g/cm3)

Water

Density

ρ

(g/cm3)

DME-

Water

Density

ρ(g/cm3)

292.78 4.38 0.1444 0.8556 302 0.664 0.998 0.950

292.78 3.65 0.1004 0.8996 222 0.664 0.998 0.965

292.78 1.38 0.0283 0.9717 74 0.664 0.998 0.989

292.78 0.76 0.0153 0.9847 39 0.664 0.998 0.993

292.78 0.42 0.0082 0.9918 21 0.664 0.998 0.995

302.74 5.97 0.1442 0.8558 302 0.648 0.996 0.945

302.74 5.07 0.1002 0.8998 222 0.648 0.996 0.961

302.74 2.02 0.0283 0.9717 74 0.648 0.996 0.986

302.74 1.14 0.0152 0.9848 39 0.648 0.996 0.990

302.74 0.64 0.0082 0.9918 21 0.648 0.996 0.993

312.64 7.92 0.1441 0.8559 302 0.631 0.992 0.940

312.64 6.89 0.1000 0.9000 222 0.631 0.992 0.956

312.64 2.85 0.0282 0.9718 74 0.631 0.992 0.982

312.64 1.63 0.0151 0.9849 39 0.631 0.992 0.987

312.64 0.93 0.0081 0.9919 21 0.631 0.992 0.989

323.00 10.27 0.1550 0.8450 320 0.613 0.988 0.930

323.00 8.34 0.0980 0.9020 217 0.613 0.988 0.951

323.00 7.65 0.0850 0.9150 198 0.613 0.988 0.956

323.00 5.52 0.0510 0.9490 121 0.613 0.988 0.969

323.00 4.48 0.0450 0.9550 109 0.613 0.988 0.971

323.00 3.10 0.0280 0.9720 74 0.613 0.988 0.978

69


Table 3-12 Calculated Viscosity of DME-water solution based on experimental data

Temperature

(K)

Pressure

(bar)

DME

mol

fraction

X

Water

mol

fraction

(1-X)

DME

Concentration

(g/l)

DME

Viscosity

µ (cp)

Water

Viscosity

µ (cp)

DME-

Water

Viscosity

µ (cp)

292.78 4.38 0.1444 0.8556 302 0.128 1.0020 0.5589

292.78 3.65 0.1004 0.8996 222 0.128 1.0020 0.6347

292.78 1.38 0.0283 0.9717 74 0.128 1.0020 0.8598

292.78 0.76 0.0153 0.9847 39 0.128 1.0020 0.9234

292.78 0.42 0.0082 0.9918 21 0.128 1.0020 0.9594

302.74 5.97 0.1442 0.8558 302 0.118 0.7980 0.4480

302.74 5.07 0.1002 0.8998 222 0.118 0.7980 0.5217

302.74 2.02 0.0283 0.9717 74 0.118 0.7980 0.6920

302.74 1.14 0.0152 0.9848 39 0.118 0.7980 0.7400

302.74 0.64 0.0082 0.9918 21 0.118 0.7980 0.7664

312.64 7.92 0.1441 0.8559 302 0.108 0.6530 0.3793

312.64 6.89 0.1000 0.9000 222 0.108 0.6530 0.4377

312.64 2.85 0.0282 0.9718 74 0.108 0.6530 0.5713

312.64 1.63 0.0151 0.9849 39 0.108 0.6530 0.6085

312.64 0.93 0.0081 0.9919 21 0.108 0.6530 0.6289

323.00 10.27 0.1550 0.8450 320 0.099 0.5470 0.3165

323.00 8.34 0.0980 0.9020 217 0.099 0.5470 0.3774

323.00 7.65 0.0850 0.9150 198 0.099 0.5470 0.3899

323.00 5.52 0.0510 0.9490 121 0.099 0.5470 0.4448

323.00 4.48 0.0450 0.9550 109 0.099 0.5470 0.4540

323.00 3.10 0.0280 0.9720 74 0.099 0.5470 0.4823

70


Table 3-13 calculated diffusion coefficient of DME-water solution based on experimental data

Temperatu

re

(K)

Pressur

e

(bar)

DME

mol

fractio

n

X

Water

mol

fraction

(1-X)

DME

Concentrati

on (g/l)

DME Molar

Volume

ν

(cm3/mol)

Draw

Solution

Diffusion

Coefficient

D (m2/s)

292.78 4.38 0.1444 0.8556 302 61.70 2.16E-09

292.78 3.65 0.1004 0.8996 222 61.70 1.84E-09

292.78 1.38 0.0283 0.9717 74 61.70 1.35E-09

292.78 0.76 0.0153 0.9847 39 61.70 1.26E-09

292.78 0.42 0.0082 0.9918 21 61.70 1.21E-09

302.74 5.97 0.1442 0.8558 302 61.70 2.69E-09

302.74 5.07 0.1002 0.8998 222 61.70 2.31E-09

302.74 2.02 0.0283 0.9717 74 61.70 1.74E-09

302.74 1.14 0.0152 0.9848 39 61.70 1.63E-09

302.74 0.64 0.0082 0.9918 21 61.70 1.57E-09

312.64 7.92 0.1441 0.8559 302 61.70 3.28E-09

312.64 6.89 0.1000 0.9000 222 61.70 2.84E-09

312.64 2.85 0.0282 0.9718 74 61.70 2.18E-09

312.64 1.63 0.0151 0.9849 39 61.70 2.04E-09

312.64 0.93 0.0081 0.9919 21 61.70 1.98E-09

323.00 10.27 0.1550 0.8450 320 61.70 4.06E-09

323.00 8.34 0.0980 0.9020 217 61.70 3.40E-09

323.00 7.65 0.0850 0.9150 198 61.70 3.30E-09

323.00 5.52 0.0510 0.9490 121 61.70 2.89E-09

323.00 4.48 0.0450 0.9550 109 61.70 2.83E-09

71


Table 3-14 calculated diffusion coefficient of seawater solution based on experimental data

Temperatu

re

(K)

Pressur

e

(bar)

DME

mol

fractio

n

X

DME

Concentrati

on (g/l)

Seawater

Viscosity

µ (cp)

DME-

Seawater

Viscosity

µ (cp)

Feed

Solution

Diffusion

Coefficie

nt

D (m2/s)

292.78 0.42 0.0082 21.1288 1.0579 1.01181.15E-

09

302.74 0.64 0.0082 21.1288 0.8483 0.81361.48E-

09

312.64 0.93 0.0081 20.8690 0.6992 0.67241.85E-

09

322.89 1.33 0.0081 20.8690 0.5879 0.56652.27E-

09

3.8. SummaryIn this chapter, first the osmotic pressure of the liquefied DME-water solution was

predicted using freezing point depression and activity coefficient NRTL models based on

experimental data reported by different research groups. The results show that DME-water

solution is polar, non-ideal with partially miscibility and generates an osmotic pressure at a

maximum solubility around seven times more than seawater osmotic pressure. Therefore, the

liquefied DME-water solution could be used as a draw agent for FO desalination process. Then

the physical properties including viscosity, density and diffusion coefficient of liquefied DME

aqueous solution were estimated using the experimental data based on molecular simulation of

the liquefied DME in literature. Furthermore, the designed experimental method providing a

direct measurement of osmotic pressure with semi-permeable membrane, suitable for liquefied

DME draw agent is recommended as the future work.

In the next chapter an comprehensive review of modelling of FO process is introduced, then the

compatible models with suitable boundary conditions are applied to simulate FO desalination

process. The water flux at different operating condition using DME-water draw solution is

calculated to find the optimum operating condition.

72


73


CHAPTER FOUR

FORWARD OSMOSIS MODELLING COMPREHENSIVE

REVIEW

74


4.1. IntroductionRecent studies have identified Forward Osmosis process as a potential method for

desalination and purification of wastewater. Forward Osmosis is an osmotically driven

membrane separation process addressing the limitation of the current membrane based

processes by taking advantage of low energy consumption and unique transport characteristics

such as high water recovery, low fouling tendency and high solute rejection. A concentrated

draw solution is used in FO process to generate an osmotic pressure gradient as the driving

force through a semi-permeable membrane. Water is diffused across the membrane from a feed

solution with lower osmotic pressure to a draw solution with higher osmotic pressure. Then the

feed solution is concentrated and the draw solution is diluted during FO process. The water

recovery, permeate water flux and solute rejection are considered to evaluate the performance

of FO process in comparison to the current pressure driven membrane methods such as RO.

Despite the high driving force generates by draw solution in FO process, the experimental

water flux is lower than theoretical calculated flux due to concentration polarization (CP)

occurring on both sides of the membrane. External concentration polarization (ECP) occurs in

the liquid boundary layers on both side of the membrane, whereas internal concentration

polarization (ICP) takes place in the porous support layer. Furthermore, solute diffusion from

draw solution into the feed and vice versa, may pose to decrease the effective driving force and

increase the impact of concentration polarization consequently.

The modelling of membrane transport in FO process could be an important tool to predict water

flux through different membranes, feed and draw solutions and operating conditions without

actually conducting an experimental test. In FO process modelling, the external concentration

polarization (ECP), internal concentration polarization (ICP) effect, reverse solute diffusion and

the permeability of membrane should be considered to allow for accurate water flux prediction

across membrane. The challenge of predicting the water flux effectively has been the selection

of an accurate model suitable for a wide range of testing conditions in FO process. In this

chapter, a review on the developed models have been investigated in previous and recent

studied and validated with experimental results is introduced to find the suitable modified

model applying for simulation of the novel FO desalination process using DME-water draw

solution in this project.

75


4.2 Modelling the Effect of ICP and ECP on Water Flux in FO ProcessLee et al. [101] developed a model for obtaining the performance of different Reverse

Osmosis (RO) membranes considering internal concentration polarization in pressure retarded

osmosis (PRO) mode in direct osmosis process. The external concentration effect was

considered zero by stirring the feed and draw solutions on both sides of the membrane. The

model was validated with experimental data obtained with a variety of Reverse Osmosis (RO)

membranes and showed that the water flux is markedly decreased due to internal concentration

polarization within the porous support layer under pressure retarded osmosis (PRO) conditions.

The water flux across a semi-permeable membrane on pressure retarded osmosis (PRO) mode

was introduced as:

JW=A [π Dw

1−C fb

Cdwexp (J w K )

1+ BJ w

[ exp(J w K )−1 ] ](4-1)

C fw

Cdw=

B exp[(J w K )−1 ]+Jw

C fb

Cdwexp (J w K )

B[ exp( Jw K )−1 ]+J w (4-2)

Where, Jw is water flux (cm3/cm2sec) across a semi-permeable membrane, A is water

permeation coefficient (cm3/cm2-sec-atm) of a semi-permeable membrane, Cfb, Cfw and Cdw are

the bulk feed solution concentration (g/cm3), feed and draw solution concentration (g/cm3) on

surface of the membrane respectively. Parameter B is salt permeation coefficient (cm/sec) of a

semi-permeable membrane and K is porous substrate resistance to salt diffusion (sec/cm) which

is given as:

K= tτDε (4-3)

Where, t is the thickness of porous substrate (cm), τ is the tortuosity (dimensionless), є is

porosity of the porous support layer (dimensionless) and D is diffusion coefficient of solute

through support layer (cm2/sec).

76


Later on, the models accounted the coupled effect of external and internal concentration

polarization on water flux occurring in both the forward osmosis (FO) and pressure retarded

osmosis (PRO) modes of FO membrane was developed by McCutcheon and Elimelech [70].

These presented models were used to predict the water flux at different temperatures using

NaCl as draw solution, and were found to closely match with the experimental data collected

from FO tests. The flux performance improved by increasing operating temperature however

increases was limited due to the severity of internal concentration polarization (ICP) and

external concentration polarization (ECP) effects at higher fluxes. Furthermore the flux

reduction exhibited in both the pressure retarded osmosis (PRO) and forward osmosis (FO)

modes due to sever ICP whereas external concentration polarization impacted negatively on the

water flux in the pressure retarded osmosis (PRO) mode though its effect is small in the FO

mode. The reverse solute flux and any salt passage in the direction of water flux were ignored

in their models. The developed models to predict the water flux in FO and pressure retarded

osmosis (PRO) modes were introduced as follows:

FO Mode: JW =A [ πDb exp (J W K )−π Fb exp(

J w

K F)]

(4-4)

PRO Mode: JW =A [πDb exp (

J W

K D)−π Fb exp(J W K )]

(4-5)

Where, Jw is water flux (cm3/cm2sec) across a semi-permeable membrane, A is water

permeation coefficient (cm3/cm2-sec-atm) and parameter B is salt permeation coefficient

(cm/sec) of a semi-permeable membrane. In addition, K is porous substrate resistance to salt

diffusion (sec/cm), π Db is bulk osmotic pressure (atm) of draw solution, π Fb is bulk osmotic

pressure (atm) of feed solution. In addition, KF and KD present mass transfer coefficient (m/s) in

feed and draw solution streams respectively. Mass transfer coefficient of each stream relate to

Sherwood number for the appropriate flow regime by:

k=Sh∗Ddh (4-6)

77


Where dh is the hydraulic diameter (m), k is mass transfer coefficient (m/s) of stream, D is

solute diffusion coefficient (m2/s) and Sh is dimensionless Sherwood number. They continued

the investigation on modelling water flux in FO process under a range of feed, draw solution

concentrations and membrane structural properties in both the forward osmosis (FO), and

pressure retarded osmosis (PRO) modes [102]. The negative impacts of both internal

concentration polarization (ICP) and external concentration polarization (ECP) on osmotic

driving force were quantified for asymmetric and symmetric membranes for modelling the

permeate water flux through membrane at a set of specified experimental conditions. It was

determined that the water flux decreased sharply when the feed contained solutes in pressure

retarded osmosis (PRO) mode due to the concentrative internal concentration polarization

(ICP) reduced significantly the effective osmotic driving force even in dilutive feed solution. In

contrast, in the forward osmosis (FO) mode, dilutive internal concentration polarization (ICP)

had a dramatic negative impact on the driving force. They found with modelling the water flux

through asymmetric membrane that smaller value of the solute resistance to diffusion, K,

yielded better water flux due to decreasing internal concentration polarization (ICP) impact in

either pressure retarded osmosis (PRO) or forward osmosis (FO) mode however the

improvement of flux performance was limited by dilutive ECP as the value of K became small.

The models considered only external concentration polarization (ECP) or internal concentration

polarization (ICP) effect to predict the water flux were presented in the following equations as

modelling flux with external concentration polarization (ECP) effect and modelling flux with

internal concentration polarization (ICP) effect:

Modified with effect of ECP: JW=A [πDb exp (−

J W

k D)−π Fb exp(

J W

kF)]

(4-7)

Modified with effect of ICP: K= 1

JWln [ B+ Aπ Dm−J W

B+ Aπ Fb ](4-8)

Here, B is the salt permeability coefficient (cm/s) of membrane, A is water permeability

coefficient (cm3/cm2-sec-atm) of membrane and Jw is water flux (cm3/cm2-sec) across the

membrane. In addition, π Dm is osmotic pressure (atm) of draw solution on membrane surface,

π Db is bulk osmotic pressure (atm) of draw solution and π Fb is bulk osmotic pressure (atm) of

78


feed solution. K is the solute resistivity (cm/s) for diffusion through the porous support layer,

kD and kF is mass transfer of draw/feed solution streams. Overall, reducing K indicated the

possibility of higher recovery of feed water as well. Therefore, by reducing the support layer

thickness or making more porous layer in the new design of membrane, the better water flux

performance, and higher feed water recovery could be achieved in osmotically driven

membrane processes.

Tan and Ng [75] analyzed the external concentration polarization (ECP) layer by using the

mass transfer coefficient, k, which calculated by Sherwood relations were developed in the

boundary layer concept under laminar and turbulence flow regimes in models based on the film

theory. Furthermore the governing convective-diffusion equations were derived considering the

variation of the diffusion coefficient for the feed, draw solutes within the internal concentration

polarization (ICP) layer and then water fluxes were predicted with both external concentration

polarization (ECP) and internal concentration polarization (ICP) correlation, and verified

against a set of experimental data. The predicted water fluxes using the developed models were

compared with the previous models, as well as with experimental water fluxes and the results

showed that the improved models considering both external concentration polarization (ECP)

and internal concentration polarization (ICP) phenomenon can predict the water flux more

accurately at high draw solution concentration. Previous models overestimated the water flux

by as much as 15% of the experimental flux. They introduced that in order to minimize the

effect of external concentration polarization (ECP), both operating temperature and cross flow

velocity should be increased and the spacers could improve the water flux through membrane.

The same suggestions were proposed to reduce the internal concentration polarization (ICP)

effect on the structure properties of membrane including the thickness of support layer must

coincide with the proper amount for FO process and then the tortuosity and porosity of porous

layer should be enhanced and improved in the new design of FO membrane. The developed

models presented in the following equations:

k c=0 .664 D(Ret )

1/2( Sc )1/3+0.0365 D( Sc )1/3[(Re L )4/5−( Ret )4 /5 ]

L (4-9)

Cdw

Cdb=exp (−

JW

kc)

(4-10)

79


Where kc is mean mass transfer coefficient (m/sec), D is solute diffusion coefficient (m2/sec), L is

length of channel (m) and JW is water flux (m3/m2-sec) through membrane. Dimensionless

numbers include Ret is transition Reynolds number ReL is Reynolds number at L and Sc is

Schmidt number. Molar concentration (M) of draw solution on membrane wall and at bulk are

presented withCdw and Cdb respectively.

Tan and Ng [75] suggested that the solute resistivity for diffusion within the porous support layer

(K) might not be constant due to diffusion coefficient (D) is not constant especially at large

solute concentration difference. They developed solute resistance coefficient independent of

diffusivity for both membrane orientations including forward osmosis (FO) and pressure retarded

osmosis (PRO) modes written as:

PRO mode: (4-11)

K ¿=[ En

J w(Cdb−Cdw )+

En−1

Jw(Cdb

2 −Cdw2 )+.. .+

E1

J w(Cdb

n −Cdwn )]+ E

Jwln [ B(Cdw−C fw )+Jw Cdb

B(Cdw−C fw )+J w Cdw ]FO mode: (4-12)

K ¿=[ En

J w(Cdw−Cdb )+

En−1

Jw(Cdw

2 −Cdb2 )+.. .+

E1

J w(Cdw

n −Cdbn )]+ E

Jwln [ B(C fw−Cdw)+J w Cdw

B(C fw−Cdw)+Jw Cdb ]Where Ei is are constants associated with the empirical correlation of diffusion coefficient of

solute solution andK ¿ is solute resistance coefficient independent of diffusion coefficient. All

parameter including Jw and B have the same description in equation 4-8. ParametersC fw , Cdw ,

Cdb present molar feed and draw solution concentration (M) on membrane wall and bulk molar

draw solution concentration (M) respectively. An iteration procedure using the mentioned

equations was introduced by applying mathematical software to solve for water flux [75]. Tan

and Ng [103] developed their previous modified external concentration polarization (ECP)

model to improve accuracy for using other draw solution such as MgSO4, MgCl2, CaCl2 and

glucose whereas the previous model could only accurately account for the external

concentration polarization (ECP) effect with NaCl and KCl draw solutions in FO process. In

80


order to improve the water prediction accuracy, the effect of dilution/suction and diffusivity

variation was included in the revised external concentration polarization (ECP) model. In

addition, the modified internal concentration polarization (ICP) model was improved by

considering the specific solute resistivity constant, K, of membrane for each draw solute used

due to different degree of interactions between the porous support layer material and solutes.

K S=t τλε (4-13)

Where Ks is solute specific resistivity independent of diffusivity (m) and Parameter λ involves

the interaction of the porous support layer material with any specific draw solutes. The

definition of parameters t, τ and є are same with equation 4-3. The modified average Sherwood

number relations for calculating mass transfer coefficient and then estimating the concentration

of feed/draw solutes on surface of membrane were presented in the following equations as:

PRO mode:

Shave=1 . 849(Re . Sc .dh

L )1

3(0 . 0319 Q+0 .0003 Q2−0 . 001Q3 ) (4-14)

Cdw

Cdb= k

J w+k (4-15)

FO mode:


L )1

3(0 .997+0 . 0315 Q+0 .022 Q2−0 . 008Q3 )(4-16)

Cdw

Cdb= k

k−J w (4-17)

Here, Shave is average Sherwood number, Re and Sc introduce Reynolds and Schmidt numbers, dh

is hydraulic diameter (m) and L is length of channel for feed or draw solution sides streams. Q is

a lumped parameter varies from very low values up to 10 and implies the effect of water flux

81


dilution on the external concentration polarization (ECP) layer. The parameters Cdw, Cdb, k and Jw

indicate molar concentration of draw solution, on membrane wall and bulk solution, mass

transfer coefficient and water flux across membrane respectively.

82


The water flux was predicted with the given concentration for both the bulk feed/draw

solutions, cross-flow velocities, Ks, water and salt permeability of membrane and physical

properties of solutions with the revised external concentration polarization (ECP) and internal

concentration polarization (ICP) models. The predicted water flux using the revised external

concentration polarization (ECP) and internal concentration polarization (ICP) models was

verified by experimental data and the appropriate FO models could be selected based on the

feed and draw solutes under both pressure retarded osmosis (PRO) and forward osmosis (FO)

modes in FO process when different draw solutions were considered.

Recently, Xiao et al. [104] and Li et al. [105] developed a mathematical modelling for studying

the effect of support structure on internal concentration polarization (ICP) and optimizing the

design of hollow Fibre membrane in FO process. The model development was based on a one-

dimensional coordinate system, and flux model accounting for both internal concentration

polarization (ICP) and external concentration polarization (ECP) was developed on their

investigation is given as:

Jw=(

1k f

+1

k m+

1k D

)−1 ln [ Aπ D+B−J w exp( J w /kD )Aπ f +B ]

(4-18)

Jw=( 1k f

+ 1k m

+ 1k D

)−1 ln [ AπD+BAπ f+B+J w exp(−(J w /k f )) ] (4-19)

Where Jw is water flux (m/s) through membrane, km is dimensionless mass transfer coefficient

of the support layer; kf and kD are the mass transfer coefficients (m/s) in feed and draw solution

streams respectively. Two parameters πf and πD indicate the osmotic pressure (atm) of feed and

draw solutions. Parameter A is water permeability coefficient (cm3/cm2-sec-atm) and B is the

salt permeability coefficient (cm/s) of membrane.

83


4.3. Solute Reverse Diffusion Flux in Modelling Water Flux in FO

ProcessThe effective operation of Forward Osmosis process for wastewater treatment and

desalination requires the minimum reverse permeation of draw solution into the feed solution.

Phillip et al. [106] developed a model to describe the reverse diffused solute flux from draw

solution to feed solution in FO process. The experiments were carried out to validate the model

predictions using NaCl draw solution with cellulose acetate membrane designed for FO

process. The reverse draw solute flux was introduced in terms of the experimentally accessible

bulk draw solution concentration in the following equation:

J S=J

WCD

1−(1+J

W

B)exp(

JW

t s τ

Dε)

(4-20)

Where, Js is reverse draw solute flux (mol/m2hr), Jw is water flux (m3/m2hr), CD, indicates draw

solution concentration (mol) and t/τ/є are membrane structural parameters. Bulk diffusion

coefficient of solute (m2/s) is specified with D and salt permeability of membrane (m/s) is

presented with B parameters.

The strong agreement between the experimental results and the model prediction was observed

and showed that the ratio of the permeated water flux to the reverse solute flux is a key

parameter being independent of the draw solution concentration and the structure of support

layer gives as:

J w

J s= A

BnRgT

(4-21)

Where A (cm3/cm2-sec-atm) and B (m/s) are the permeability of membrane for water and salt

respectively, n is number of dissolved species created by draw solute, Rg is ideal gas constant

and T (K) is absolute temperature in FO process. Jw is water flux (m3/m2hr) and Js is reverse

draw solute flux (mol/m2hr). The key parameter introduced as the reverse flux selectivity can

be used with other design parameters to help optimize a FO process.

84


They continued their investigation to derive a model for predicting reverse fluxes of three

neutral draw solutes including Urea, ethylene glycol and glucose through asymmetric

membrane in FO process [107]. The equation 4-22 was developed to express the reverse flux of

solute in terms of experimentally accessible quantities.

J S=J

WB(CF exp (Pe s +Peδ )−CD )

(B exp (Peδ)+JW)exp( Peδ)−B (4-22)

Here Peδ is the Peclet number of the boundary layer and Pes is Peclet number in support layer of

membrane. Js is total flux (mol/m2 hr) of draw solute, Jw is water flux (l/m2hr) through

membrane, and B is solute permeability coefficient (l/m2 hr). Molar concentration of solute in

the bulk feed and draw solutions are presented with CF and CD respectively. The experimental

results showed an additional resistance mass transfer developed due to external concentration

polarization on the feed side of membrane, which was ignored in their previous modelling.

Therefore, a reflection coefficient was introduced in the model to indicate the coupling between

the permeate water flux and reverse solute flux within the active layer is given as:

J w

J s= A

Bσ nRgT

(4-23)

Parameter σ is reflection coefficient that characterizes the ability of active layer of membrane

preferentially allowing solvent permeation over solute permeation. All parameters A, B, Jw, Rg,

T and Js were described in equation 4-21.

Recently Suh and Lee [108] developed a model for reverse draw solute flux considering

internal concentration polarization (ICP) and significant external concentration polarization

(ECP) on both sides of membrane in FO process. They verified the model using the existing

experimental data to investigate the effect of operating conditions such as concentration of feed

and draw solution and cross-flow velocities on both sides of membrane. The results showed

that the effects of external concentration polarization and reverse solute flux on predicting the

water flux is significant particularly in high concentration of feed solution. Furthermore, the

simulation results confirmed that the diffused solute on the surface of membrane was enhanced

directly by increasing cross-flow velocity that showed the effect of cross-flow on internal

85


concentration polarization (ICP), external concentration polarization (ECP) and water flux

could simultaneously be simulated by the developed model.

J s=B[ CDb+(J s

J w)

exp (J w K )exp (J w /kD )−(CFb+(

J s

Jw))exp(

J W

kF)]

(4-24)

Here Js and Jw introduce reverse solute flux (mol/m2 hr) and water flux (l/m2hr) within

membrane. In addition, K is solute resistivity (s/m) for diffusion across support layer; kD and kF

are mass transfer coefficient in draw and feed solution streams. Parameters CDb, CFb and B

indicate draw, feed concentration in the bulk and solute resistance (m/s) respectively.

The numerical analysis method was used to solve the implicit equation. The previous models

introduced by McCutcheon and Elimelech [70,102] and Tan and Ng [103] for predicting water

flux were mentioned in section 4.2 did not considered the simulation of the reverse solute flux

and external concentration polarization (ECP) on draw solution side in calculating the

concentration of solute on the interface between the active and support layers (CDb = CDw).

Moreover the previous model developed in [106,107] involved the reverse solute flux in

modelling but ignored external concentration polarization (ECP) on feed and draw solutions on

both sides of membrane in FO process (CDb = CDw and CFb = CFw). Therefore the current

developed model is suitable for simulating the reverse solute flux and both external

concentration polarizations (ECP) phenomenon compare the previous FO models which

neglected these effects in modelling.

4.4. Numerical Simulation and Performance Analysis of Forward

Osmosis ProcessRecently numerical simulation to develop models such as 2D finite element analysis

(FEM) and computational fluid dynamic (CDF) yielded new insights for optimizing the

performance of Forward Osmosis process. The analytical models were used as quantitative

tools for optimizing FO process by analysing the effects of various parameters on maximizing

water recovery and minimizing reverse solute flux from draw to feed solutions. Sagive and

Semiat [109] developed a steady state finite element (FEM) 2D model of FO process using

NaCl draw solution to quantify theoretically each individual effect of FO system on water flux.

86


Jung et al. used mathematical model to investigate the effect of system parameters including

concentration polarization, membrane orientation, flow direction, flow rate and solute

resistivity on FO membrane performance. Reverse solute flux and fouling were not considered

in their modelling. Gruber et al. [110] exhibited a computational fluid dynamic (CDF) model

competent of simulation of flow and concentration polarization in FO membrane systems. They

used computational fluid dynamic (CFD) model to resolve simultaneously the effects of cross-

flow velocity, physical fluid properties, bulk osmotic pressure difference and slip velocity on

concentration polarization profile and water flux.

By reviewing the required operating conditions for DME–water draw solution which were

introduced in Chapter 3, including using pressure retarded osmosis (PRO) mode due to keeping

DS under pressure higher than feed solution, it seems that the models introduced by Lee and

Tan, and by Ng could be suitable for the calculation of the water flux in FO process which is

described in chapter 5.

4.5. SummaryIn order to find the appropriate model for the simulation of the proposed FO

desalination process using DME-water draw solution, comprehensive review of previous and

recent models were carried-out. These models were used to determine water flux and the

performance of FO process. The models by Lee [101], Tan and Ng [75, 103] and Li et al. [105]

seem to be more complete and compatible with the operating conditions of this project than

others. The operating conditions of our FO desalination process such as pressure retarded

osmosis (PRO) mode, tubular ceramic membrane, using an organic draw solution are

considered in FO process and the iteration procedure for selecting the appropriate model is

described in the next chapter in detail.

87


CHAPTER FIVE

FORWARD OSMOSIS PROCESS DESIGN CRITERIA AND

SIMULATION RESULTS AND DISCUSSION

88


5.1. Introduction on FO Process Design Criteria

The modelling of membrane transport in Forward Osmosis supports an FO operation to

predict water flux with different operating conditions, type of membrane, feed and draw

solutions. This is useful for providing indicative data of the proposed novel draw solution FO

process without having to conduct physical experiments. In FO process, convective water flow

causes the buildup of solute concentration polarization or external concentration polarization

(ECP) at both sides of membrane. Since water permeates from feed to draw solution due to the

difference in osmotic pressure, concentrative and dilutive ECP are occurred at the feed and the

draw solution sides of the semi-permeable membrane. Combination of these two ECP

phenomenon reduces the effective osmotic driving force and the expected water flux

consequently. Furthermore, a polarized layer is established due to the solute diffusion and

reverse solute flux inside of the porous support layer referring to internal concentration

polarization (ICP). The concentrative and dilutive ICP can occur depending on membrane

orientation including pressure retarded osmosis (PRO), the support layer is faced the feed

solution, or forward osmosis (FO), the porous layer is faced the draw solution respectively.

Similarly, the internal concentration polarization phenomenon significantly decreases the net

osmotic driving force, leading to decrease in the water flux. In order to model and simulate the

performance of FO process accurately, the effects of external concentration polarization (ECP)

on both sides of feed and draw solution, internal concentration polarization (ICP) in membrane

support layer, membrane permeability and reverse draw solute flux must be considered in a

combined model. In this chapter, the simulation of Forward Osmosis desalination process is

investigated based on the following criteria:

Firstly, the DME draw solution is placed against the active layer as being required to

resist the pressurization of DS stream, and the feed solution against the porous support

layer. This orientation of the membrane is referred to PRO mode.

Secondly, the recent modified dilutive ECP model [101], which considered the effect

of dilution (injection) parameters and diffusivity variation, is applied to calculate the

mass transfer coefficients, k, and the solute concentration, Cdw, on the activated layer

of the membrane wall in the draw solution side respectively.

Next, the developed ICP model [75,103] using the concept of variable diffusion

coefficient , D, and solute resistivity, K, due to a large solute concentration difference

within the membrane support layer, is used to calculate the feed solution concentration

Cfw on the support layer membrane wall in the feed side.

89


Finally, the water flux for the FO process in PRO mode is predicted with the modified

ECP and ICP models when combined with the osmotic pressure model. Figure 5-3

illustrates the flow chart of the iterative steps.

5.1.1. Modified ECP Model Considering Effect of Suction/Dilution

Parameter The water flux in FO process can be determined by applying the overall effective osmotic

driving force in osmotic pressure model in equation 5-1.

J w =A (π dw−π fw ) (5-1)

Where, Jw is water flux (m3/m2sec) across a semi-permeable membrane, A is pure water

permeability coefficient (m/sec-atm) and πdw-πfw is effective osmotic pressure differential of

draw and feed solution on membrane wall.

Therefore the concentration of feed and draw solution must be determined at the membrane

surface due to the effects of concentrative and dilutive ECP phenomenon. In order to calculate

the concentration of the solute at the membrane interface, the modified film theory boundary

layer concept is applied in the FO process. Since mass transfer coefficient, k strongly depends

on the hydrodynamics of the process, the Sherwood number is used to calculate k coefficient.

The effect of suction and dilution (injection) parameters comes from water flux through the

membrane should contribute to concentration of the feed solution and dilution of the draw

solution within the ECP layer in FO process. De and Bhattacharya [111] studied the effect of

suction on the mass transfer coefficient and developed the theoretical Sherwood number

relation considering the aforementioned effect for the ECP model in RO process. The results

showed the enhancement in mass transfer from the surface to the bulk and the stability of the

laminar- flow condition by shifting the critical Reynolds number from 2100 to 4000 [111].

Based on previous modelling work by De and Bhattacharya [111] relating to suction effect on

ECP in UF and RO process, Tan and Ng [103] developed the dilutive ECP model in a

rectangular channel for the FO process as illustrated in figure 5-1.

90


Figure 5-1 – The formation of the dilutive external concentration polarization (ECP) effect [103]

When FO process is operated in the pressure retarded osmosis (PRO) mode, water is passed

through the membrane and injected into the ECP layer on the draw solution side. Permeated

water can contribute to dilute draw solution within the layer; therefore the dilution effect may

be significant. On the other hand, a suction effect is inferred when forward osmosis (FO) mode

is used in FO process, water is transferred across the membrane from the ECP layer on the feed

solution side that may affect the concentrative ECP effect. Tan and Ng [103] developed a

theoretical Sherwood number for use in FO modelling following to the previous study by De

and Bhattacharya [111], considering the effect of suction on mass transfer coefficient and

Reynolds number. The development of the dilutive ECP layer (δ) resulting by water

permeation through the membrane into the rectangular channel is shown in figure 5-1. Based

on the dilution effect of the ECP layer, the solute mass-balance equation (5-2) for flow through

rectangular draw solution channel in figure 5-1 is written as [103]:

D ∂2C∂ y2 =u ∂ C

∂ x+J w

∂C∂ y (5-2)

Where Jw is water flux through the membrane, D is the average solute diffusion coefficient

(m2/s) within the membrane and bulk solution interface layer, u is the axial velocity (m/s) and

C is molar concentration (M) of the solute. The concentration variation was considered in two

axial (x) and vertical (y) dimensions. The axial velocity profile within the channel was foreseen

as:

u=3u0 y

h (5-3)

91

Draw Solution

Feed Solution


Here, u is axial velocity (m/s), u0 is axial velocity (m/s) at the entrance point (x0, y0), y is

vertical distance (m) from the membrane and h is half of channel height (m). The equation (5-

2) can be written considering the boundary condition at the edge of the ECP layer (y = 0) with

the draw solution concentration on membrane (Cdw) wall as:

C = Cdw &

∂C∂ y = 0 at y = 0

D ∂2 C∂ y2 =u ∂ C

∂ x (5-4a)

And then equation (5-4a) can obtain the following solution as:

δ=(D hx3u0 )

13

(5-4b)

Where, δ is boundary layer thickness (m), D is average solute diffusivity (m2/s), h is half of

channel height (m), x is axial direction (m) of membrane and u0 is axial velocity at channel

entrance. By defining a dimensionless variable η, the equation (5-3) can be solved along with

the boundary condition at the forming edge boundary layer (y = δ), and the average flux across

the whole membrane length can be derived from the solution as:

C = Cdb at y = δ

η= y (3u0

Dhx )1 /3

(5-5)

J w =32

M (3u0 D2

hL)1/3

(5-6)

Given that:

M=0 . 2912×Q (5-7)

Where Q is a lumped parameter shown as:

92


Q=Jw dh

D(Re .Sc .dh / L)1/3(5-8)

Where, Cdb is the bulk draw solution molar concentration (M) at the forming edge of ECP layer,

J w is average water flux (m3/m2s) across membrane, dh is hydraulic diameter (m), D is solute

diffusivity (m2/s) and L is length (m) of channel. Dimensionless numbers Re introduces

Reynolds number and Sc presents Schmidt number. In all above mentioned equations,

parameters u0, h, D and δ are axial velocity (m/s), half of channel diameter (m), average solute

diffusivity (m2/s) and boundary layer thickness (m) respectively.

The term relating the dilution effect of water flux through the membrane into the ECP layer can

be expressed as:

P=∫0

∞

e−( η3

9−0 .2912Qη)

dη(5-9)

Then average Sherwood number for dilutive ECP considering average solute diffusivity and

dilution effect over the whole membrane length can be obtained by [103]:

Sh=3 . 434(Re . Sc .dh / L)1/3

P (5-10)

Where dimensionless numbers are described as:

Re=ρudh

μ (5-11)

Sc= μρD (5-12)

93


Where ρ, μ , D are density (g/l), viscosity (cp) and diffusivity coefficient (m2/s) of the solution

respectively, dh is hydraulic diameter (m), L is length (m) of channel, Q is a lumped parameter

vary from very low values up to 10 [111] and the dilution effect is integrated in P parameter.

The behaviour of 1/p in equation (5-10) and Sherwood number depend on variations of Q

indicating the effect of water flux dilution on the ECP layer [103]. Tan and Ng [103] predicted

the final Sherwood equation for dilutive ECP layer by using the best -fitted polynomial

correlation obtained from plotting the variation of 1/p with Q as:


L )1

3(0 . 0319 Q+0 .0003 Q2−0 . 001Q3 ) (5-13)

Where two dimensionless numbers Re and Sc were defined in equations (5-11) and (5-12), dh is

hydraulic diameter (m) of channel and L is channel length (m).

In order to obtain the water flux considering dilutive ECP effect under pressure retarded

osmosis (PRO) mode in FO process, the solute mass-balance on the membrane active layer

surface at the draw solution side and the average water flux through the membrane are written

as:

Jw Cdw=k (Cdb−Cdw )=D( ∂ C∂ y

) y=0 (5-14)

And give:

Cdw

Cdb= k

J w+k (5-15)

Where mass transfer coefficient can be obtained from modified Sherwood number as:

Shave=kdh

D

(5-16)

94


In addition, the dilutive draw solution concentration on the membrane wall (Cdw)can be

calculated when the average water flux is known in experimental test by using modified

average Sherwood number and mass transfer coefficient due to dilutive ECP effect under PRO

mode in FO process with equations (5-8), (5-11), (5-12),(5-13), (5-15) and (5-16).

In this study, pressure retarded osmosis (PRO) mode is used in FO process simulation due to

operating pressure in the draw solution side is higher than feed side to keep DME liquefied in

water. The Sherwood number and mass transfer coefficient in concentrative ECP modified

model in forward osmosis (FO) mode was expressed by Tan and Ng [103] by fitting the

obtained polynomial correlation from the plot as follows:

Shave=1 . 849(Re . Sc .

dh

L )1

3(0 . 997+0 . 0315 Q+0 .022 Q2−0 . 008Q3 ) (5-17)

Cdw

Cdb= k

k−J w (5-18)

5.1.2. Revised ICP Model Considering a Variable Diffusivity The recent studies on FO modelling supported the concept of using variable diffusion

coefficient, D, under different solute concentration for ICP modelling. Tan and Ng [75, 103]

modified the developed model by Lee et.al [101] using the diffusion coefficient as a function of

the concentration of solute in the governing convective-diffusion equation in FO process under

both PRO and FO modes.

In the simulation of FO process in this project, the equation was developed by Lee et.al [101] is

used for the calculation of the water flux in PRO mode considering the empirical correlation of

diffusion coefficient with the solution concentration. The governing convective – diffusion

equation can be written for the solute flux, Js, across the porous support layer of membrane as:

−J s=εdD c( x )C( x )

dx−J w C ( x )

(5-19)

Where Js is the solute reverse flux (g/cm2s) through the membrane support layer, Dc(x) is the

solute diffusion coefficient with the empirical correlation of the solution concentration, c(x) is

95


solute concentration (g/cm3) at distance x from the dilute solution-membrane substrate interface,

є is the porosity of the support layer dimensionless and Jw is the water flux (cm3/cm2s) through

membrane. The solute reverse flux across the membrane can be written as:

−J s=B (Cdw−C fw ) (5-20)

Here, B is the solute permeation coefficient (cm/s), Cdw and Cfw are draw and feed solution

concentration (g/cm3) on membrane wall respectively. The diffusion coefficient of the solute is

considered as a function of concentration of solute as follows:

DC ( X )=E1+E2 Cx+E3Cx2+. .. .+E n Cx

n(5-21)

Where, Cx is the solute concentration (g/cm3) at distance x away from the membrane active layer

measured within the membrane support layer, En are constants resulting from the empirical

correlation of diffusion coefficient with solution concentration and Dc(x) (cm2/s) is diffusion

coefficient of solute in solution as a function of solute concentration. By combining equations (5-

19) and (5-20) as:

B (Cdw−C fw )=εdDc ( x )C( x )

dx−J w C( x )

(5-22)

The appropriate boundary conditions for PRO mode is shown in figure 5-2 and are defined as:

C ( x )=C fb at x = 0

C ( x )=C fw at x=tτ

Where t is the thickness, τ is the tortuosity of the porous support layer, Cfb and Cfw is the bulk

feed solution concentration and the feed concentration on membrane active layer wall

respectively.

96


Figure 5-2 – Transport Phenomena in FO process in PRO mode [75]

The equation (5-22) can be integrated with the mentioned boundary conditions by Lee et al.

[101] to give as:

C fw

Cdw=


C fb

Cdwexp (J w K )

B[ exp( Jw K )−1 ]+J w (5-23)

Where B is solute permeability coefficient (cm/s) through the membrane, Jw is water flux

(cm3/cm2s) across the membrane, Cfw, Cdw are feed and draw solution concentration (g/cm3) on

membrane wall, Cfb and Cdb are bulk feed and draw solution concentration (g/cm3) respectively.

K is the solute resistivity (s/m) for diffusion within the porous support layer of membrane

which is given as:

K= tτDε (5-24)

Here, D is solute diffusion coefficient (cm2/s) within the membrane support layer, t is thickness

(cm) of the membrane support layer, τ and є are both dimensionless parameter present tortuosity

and porosity of the membrane support layer.

97

Solute fluxX


The solute resistivity K relates to the physical morphology of the porous support layer of

membrane and the variable diffusion coefficient, D describing in equation (5-21). Tan and Ng

[103] solved the equation (5-22) for concentrative ICP in the form independent of diffusion

coefficient as:

K¿=[En

J w(Cdw−Cdb)+

E3

J w(C

2dw−C2db)+. ..

E1

J w(C

ndw−Cndb )]+ E

Jwln(

B(C fw−Cdw )+Jw Cdw

B(C fw−Cdw)+Jw Cdb)

(5-25)

Where Jw is water flux (m3/m2s) across the membrane, Cdw, Cfw are draw and feed solution

concentration on the membrane wall, Cfb and Cdb are bulk feed and draw solution concentration,

En are constants associated with diffusivity coefficient and K* can be written as :

K ¿= tτε (5-26)

The K* is a constant specification for each membrane which is determined experimentally and

is not affected by other process conditions.

The feed solution concentration on support layer of membrane wall in concentrative ICP layer

in PRO mode can be calculated using equations (5-23) and (5-24) or (5-25) and (5-26) when

the solute draw concentration has been calculated with the modified ECP model in section 5.1.1

and the water flux is foreseen as a first assumption. The flow chart of the iterative steps is

illustrated in figure 5-3.

5.1.3. The Combined Modified ECP and ICP Model for Flux Prediction

of FO Process in PRO ModeAs illustrated in figure 5-2, the effects of ICP and ECP reduce the osmotic pressure

difference across the membrane to an effective value calling effective differential osmotic

pressure, ∆πeff. The water flux, Jw, of the FO process is based on the differential flux across the

active layer of membrane and is represented with the osmotic pressure model was mentioned in

equation (5-1) as:

98


Jw=A (π dw−π fw ) (5-1)

Where A is pure water permeability coefficient (m3/m2s atm) and is measured experimentally.

The (πdw-πfw) is the effective osmotic pressure difference across the active layer of membrane in

FO process whereas πdw and πfw are the osmotic pressure of feed and draw solution on the

membrane wall respectively. In this study, the water flux is predicted by estimating the

effective osmotic pressure differential in the simulation of FO process in pressure retarded

osmosis (PRO) mode. The effective osmotic pressure difference is calculated with the

concentration of the solute draw and feed solution on the membrane wall using the modified

ICP and ECP models. Recently Tan et.al [104,105] developed a model for optimizing the

design of FO process using hollow Fibre modules. They developed one dimensional governing

differential mass balance equation for three sub-domains comprising the boundary layer on a

membrane activated layer side, overall support layer and on the side of a membrane support

layer. The one dimensional governing differential mass balance equations for three sub-

domains were written as follows:

−D d2Cdz2 +J v

dCdz

=0(Sub-domains on both sides of membrane) (5-27)

−Dmd2 Cdz2 +J v

dCdz

=0(Sub-domain on support layer) (5-28)

−B (Cdw−C fw )=−D

dCdb

dz+J vC=−Dm

dC fb

dz+J v C

(5-29)

Where Jv is the water flux (m3/m2s) through membrane, Cdw and Cfw are draw and feed solutions

concentration (mol/m3) on membrane wall respectively. In addition, z is one-dimensional

coordinate system (m), D is diffusion coefficient (m2/s) of the solute in the solution and Dm is

solute diffusion coefficient (m2/s) in the membrane support structure. By combining equations

5-27, 5-28 and the scalar form of the equation 5-29, the water flux model in PRO and FO

modes, accounting for both ICP and ECP was employed and was rewritten in equations 5-30

and 5-31 respectively as:

99


Jw=(1k f

+1

k m+

1k D

)−1 ln [ Aπ D+B−J w exp( J w/k D )Aπ f+B ]

(5-30)

Jw=( 1k f

+ 1k m

+ 1k D

)−1 ln [ Aπ D+BAπ f+B+J w exp(−(J w /k f )) ] (5-31)

Where, km is the mass transfer coefficient (m/s) of solute in the support layer, kf and kD are the

mass transfer coefficients (m/s) of solute in the streams of feed and draw solution respectively.

Furthermore, Jw is the water flux (m3/m2s) across the membrane, A is water permeation

coefficient (m3/m2 s atm) through the membrane, B is solute permeation coefficient within the

membrane, πD and πf are osmotic pressure of draw and feed solutions respectively. The

equation 5-30 is used for a tubular ceramic membrane as a single hollow fibre module to

predict the water flux in simulation of the FO process in pressure retarded osmosis (PRO) mode

in this project.

Lipnizki and Field [112] developed a model to estimate the mass transfer coefficient in hollow

fibre modules for laminar flow. They employed a combination of three correlations for

different regimes of concentration and hydrodynamic profiles based on the dimensionless

numbers such as Sherwood, Reynolds and Schmidt numbers. The average Sherwood number

accounting all three regimes including both fully developed hydrodynamic and concentration

profiles, neither concentration nor hydrodynamic profile is developed or hydrodynamic profile

is developed, whereas concentration profile is still developing can be given as:

Sh=1 .615 (1+0. 14√1−ε )−0 . 5 3√Re Scdh

L fiber (5-32)

ε=1−N fibers(do, fiber

Di ,mod ule)2

(5-33)

Where N is number of Fibres in module, Di is internal diameter (m) of module, Do is diameter

(m) of one Fibre, L is length (m) of Fibre, dh is the hydraulic diameter of the fluid channel and

є is the void fraction among the fibres and for one fibre it would be zero; therefore Sherwood

number can be modified for one fibre as follow:

100


Sh=1 .615 3√Re Scdh

Lfiber (5-34)

Similarly, the average Sherwood number in equation 5-34 was described by Leveque solution

referencing by Gekas and Hallstrom [113] for a non-porous tube module. De and Bhattacharya

[111] considered the dependence of the Sherwood number on the suction effect fitting the

obtained polynomial correlation from the plot for a tubular or one hollow fibre module as

follow:


L )1

3 (1. 0+0. 37 Q+0 .03Q2−0 . 00105 Q3 )(5-35)

In this project, equation 5-36 is applied in the simulation of FO process using the ceramic

tubular membrane. The flow chart of the iterative steps in figure 5-3 illustrates the procedure in

the simulation of FO process in this study.

101


Figure 5-3 – The introduced procedure by Tan and Ng [3] for prediction of the Water Flux of FO

process in PRO mode using excel software in this project [75, 103]

102

Input membrane morphology properties

such as water permeability constant A, salt permeability constant B, Tortuosity τ, porosity є

and thickness t.

Calculate Q, Re, Sc and Sh using Equations 5-8, 5-11, 5-12 & 5-36

Calculate mass transfer coefficient k & Cdw using equations 5-15 & 5-

16

Use the calculated

Jw and repeat the

loop procedure

Calculate solute resistivity K & Cfw using equations 5-23 & 5-24

or 5-25 & 5-26

Calculate the water flux Jw using equation 5-30 or 5-31

Check for convergence the calculated Jw & the initial value

NO YES

Exit Loop

Input cross flow rates and Molarity of the bulk feed and draw solution and assume the initial value of water flux Jw

Input the physical properties of feed and

draw solution at different temperatures such as

Osmotic pressure, Diffusivity, viscosity,

molar volume, etc.


5.2. FO Process Simulation Results and DiscussionTo demonstrate the practicability of using DME as draw solution in Forward Osmosis

process, the FO process was simulated to estimate the water flux cross the ceramic tubular ( or

one hollow Fibre) membrane. The modified ECP and ICP models are used to predict the water

flux across the membrane with the given bulk concentrations of feed and draw solutions, cross

flow rate of both feed and draw solutions, membrane water permeability coefficient (A), salt

permeability (B) and membrane structure parameters. The water permeability coefficient, A, is

determined by reverse osmosis testing at the appropriate temperature. The mass transfer

coefficient, k, is calculated with the modified ECP models and the solute resistivity, K, can be

determined using the supplier’s data for thickness, tortuosity and porosity and the calculated

diffusion coefficient of DME in chapter 3.

5.2.1. RO Experiments, Bench-Scale System and Membrane

Coefficients One tubular ceramic membrane supplied by Department of Biotechnology, Chemistry

and Environmental Technology at Aalborg University was used in RO test unit to determine

the membrane permeability coefficients for water, A, and for salt B. The physical properties of

the membrane are illustrated in table 5-1.

Table 5-1 – Proposed membranes for investigation and their properties

Membrane Material Pore Size Membrane type Dimension(L x D x t)

Silica on Alumina Coating 4 °A- 6 °A Tubular 25 cm x 1 cm x 3 mm

This membrane is a silica membrane coated on alumina with pore size 0.5 nm. The test unit has

a half-inch channel on the feed side of the membrane to allow the feed solution to flow inside

the tube of the tubular membrane. The tubular membrane is 25 cm long, 1 cm outer diameter

and 3 mm thickness; it has an effective membrane area of 55 cm2. The feed solution is

contained in a 9L reservoir. A high-pressure positive displacement pump with capacity 3 l/min

(Totton Pump Ltd., AD4/90) is used to recirculate the feed solution at 1.17 – 1.4 L/min (0.3

m/s). The operating pressure before and after membrane module is recorded with two pressure

indicators (0-10 bar). The permeate water is collected in a 1L graduated cylinder placed on an

analytical balance. Flux through the membrane is calculated based on the change of weight of

103


water in the graduated cylinder at the measured time. Figure 5-4 shows the ceramic membrane

and RO test unit.

Figure 5-4 Ceramic membrane module and RO test unit

RO tests are conducted with the dense layer of the membrane facing the feed solution. The first

set of experiments is performed to determine A using DI water as the feed solution. The most

commonly used model to determine water flux through the membrane in reverse osmosis (RO)

process named solution diffusion model [130] is introduced in equation (5-36) written as:

J w =A ( Δπ−ΔP ) (5-36)

Where Jw (l/m2hr) is water flux across the ceramic membrane, A (l/m2hr bar) is water

permeability of the ceramic membrane, ∆π (bar) and ∆P (bar) are osmotic pressure and

hydraulic pressure differential between feed and permeate sides respectively. During the test,

hydraulic pressure is increased at 4 bars increments from 4 to 8 bars for duration of 2.5 h (each

30 minutes). The temperature is held constant at 20◦C. Referring to equation (5-36) the water

flux (Jw) was plotted versus applied hydraulic pressure (∆P) during the RO experiment

performed to determine permeability coefficient A of ceramic membrane from the slop of

plotted water flux as a function of applied hydraulic pressure. The results are shown in table 5-

2 and figure 5-5 illustrates the plot.

104


Table 5-2 – Water permeability coefficient A from RO test results

Pressure (bar) Water Flux (L/m2.hr)

4 30.03

5 35.49

6 41.49

7 44.77

8 50.23

3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.50.00

10.00

20.00

30.00

40.00

50.00

60.00

f(x) = 4.96815286624219 x + 10.5914467697895R² = 0.992925659472422

Ceramic Membrane Permeability Coefficient A

Pressure (bar)

Wat

er F

lux

(L/m

2.hr

)

Figure 5-5 – Water flux (Jw) as a function of applied hydraulic pressure (∆P) in RO test

The water permeability coefficient, A, for ceramic membrane was calculated based on

determining slop of the plotted line in figure 5-5, resulting 1.72507 E-07 m/s bar.

The solute permeability coefficient, B, across the ceramic membrane was predicted with

considering the following assumptions:

Feed solution is a mixture of 2 g/l NaCl (TDS = 2000 ppm) and reversed DME solutes

at 0.015 mol/mol concentration.

The permeate side is in atmospheric condition and diffused DME solutes

concentration can be 0.008 mol/mol which was reported as DME concentration at

atmospheric pressure [90].

Reverse Osmosis process operating pressure equals one and half times of the osmotic

pressure of the feed solution osmotic pressure.

105


With reference to the mentioned assumptions, the salt permeability, B, of ceramic membrane

can be determined using equations 5-37 and 5-38 as follows:

B=A (1−R ) ( Δπ−ΔP )/R (5-37)

R=1−CP

CF (5-38)

Where B is solute permeability coefficient (mol/m2s) through the membrane, A is water

permeability coefficient (m3/m2 s atm) across the membrane, R is solute rejection dimensionless

number within the membrane and CP is the salt concentration in the permeate solution and CF is

the salt concentration in the feed solution. The result is listed in table 5-3 given as:

Table 5-3 – Predicted Salt permeability coefficient B for ceramic membrane

Reverse Osmosis

System

DME

Concentration

(g/l)

Hydraulic

Pressure

(bar)

Osmotic

Pressure

(bar)

Feed Side, CF

0.015 30

19.67

Permeate Side, Cp 0.008 1.0 12.7

Solute

Permeability, B

1.47863 e-07

5.2.2. Osmotic Pressure and Diffusion Coefficient as a Function of

ConcentrationThe various physical properties of the feed and draw solutions used in this project are

osmotic pressure, diffusivity, density and dynamic viscosity which were calculated at different

solute concentrations and temperature in chapter 3. Holldorff and Knapp [90] presented the

experimental binary parameters of NRTL model for calculation of the osmotic pressure for

DME-water solution at 20°C and 50°C experimentally. The osmotic pressure and diffusion

coefficient were determined at various solute concentrations at temperatures 20°C and 50°C

according to the available experimental data. Then the polynomial equations were fitted to each

set of the predicted values of osmotic pressure and diffusion coefficient to simplify the

106


calculations of these properties in the revised ECP and ICP models. The results are listed in

table 5-4 and plotted in figures 5-6, 5-7 and 5-8.

Table 5-4 – physical Properties of Feed and Draw solutions

T

(°C)

P

(bar)

CDME

(g/l)ρMixture

(g/cm3)

µMixture

(cp)

DDraw

(m2/s)

DFeed

(m2/s)

πDraw

(bar)

19.78 4.38 301 0.950 0.5389 2.16E-09 1.15E-09 208.24

3.65 222 0.965 0.6347 1.84E-09 1.15E-09 141.23

1.38 74 0.989 0.8598 1.35E-09 1.15E-09 38.30

0.76 39 0.993 0.9234 1.26E-09 1.15E-09 20.57

0.42 21 0.995 0.9594 1.21E-09 1.15E-09 10.98

50 10.27 320 0.930 0.3165 4.06E-09 2.27E-09 248.63

8.34 217 0.951 0.3774 3.40E-09 2.27E-09 152.09

7.65 198 0.956 0.3899 3.30E-09 2.27E-09 130.96

5.52 121 0.969 0.4448 2.89E-09 2.27E-09 77.11

4.48 109 0.971 0.4540 2.83E-09 2.27E-09 67.82

Note: T: Temperature (°C), P: Pressure (bar), CDME: Concentration (g/l) of DME-Water draw

solution, ρMixture: Density (g/cm3) of DME-Water draw solution, µMixture: Viscosity (cp) of DME-

Water draw solution, DDME: Diffusion coefficient (m2/s) of DME-Water draw solution, DFeed:

Diffusion coefficient of feed solution and πDraw: Osmotic pressures of DME-Water draw

solution.

107


50 100 150 200 250 300 3500.00

50.00

100.00

150.00

200.00

250.00

300.00

Osmotic Pressure of DME with NRTL Parameter at T=19.78°C, 50°C, P= 6 bar

DME Concentration (gr/li)

Osm

otic P

ress

ure

(bar

)

Figure 5-6 Osmotic pressure variations with DME concentration at 20°C and 50°C

2.00 3.00 4.00 5.00 6.00 7.00 8.000.00E+00

5.00E-10

1.00E-09

1.50E-09

2.00E-09

2.50E-09

3.00E-09

3.50E-09

4.00E-09

4.50E-09

f(x) = 1.04948304316144E-11 x² + 1.70163232278802E-10 x + 2.3689318861953E-09R² = 0.999999653044976

f(x) = 8.06819604857818E-12 x² + 9.67426673259796E-11 x + 1.18089678270601E-09R² = 0.999999407019249

Diffusion Coefficient of DME solution at 20°C &50°C

D 20CPolynomial (D 20C)D 50Polynomial (D 50)

DME concentration M

Diffu

sivity

, m2/

s

c

Figure 5-7 Diffusivity variations with DME concentration at 20°C and 50°C.

108


Referring to figures 5-6 and 5-7, the correlated polynomial for osmotic pressure and diffusion

coefficient of DME solution at 20°C and 50°C can be written as:

π Db(20 ° C)=0 . 0009C2+0.383 C+9. 6379 (5-39)

π Db(50 ° C)=0 .001C2+0 .4296 C+9 .6891 (5-40)

DDs(50 ° C)=1 E(−11)C2+2 E (−10 )C+2 E(−09 ) (5-41)

DDs(20 ° C)=8 E(−12)C2+1 E (−10 )C+1 E(−09 ) (5-42)

The osmotic pressure of seawater as feed solution can be estimated from the polynomial

correlation reported by Tan and Ng [75] as given in the equation 5-39 being shown in figure 5-

8.

π Fb=6 .2971C2+40. 714 C (5-43)

Figure 5-8 Osmotic pressure variations with NaCl concentration. Data obtained from OLI software [75].

109


5.2.3. Methodology of Water Flux Prediction in FO Process In order to determine feasibility of using DME as draw solution under pressure retarded

osmosis (PRO) mode in terms of the water flux across the tubular ceramic membrane, FO

process is simulated in this project. The simulation can help to predict how water flux will

change with varying system conditions or membrane type and therefore help to optimize

performance without time consuming or expensive experimentation. The methodology can be

summarized as follows:

Firstly the water flux is predicted based on the estimated physical properties of feed

and draw solution such as osmotic pressure, diffusivity and viscosity at temperature

between 20 to 50°C following to the available experimental data in chapter 3. The

cross flow velocity of feed/draw solution is considered constant and the equations 5-3,

5-11, 5-12, 5-15, 5-16, 5-23, 5-24, 5-30, 5-35, 5-39, 5-40, 5-41, 5-42 and 5-43 are

applied in simulation of FO process in pressure retarded osmosis (PRO) mode. Then

the effect of changing temperature and concentration of DME draw solution on water

flux is investigated in FO process.

Secondly, the effect of reverse solute diffusion from draw solution side to feed side on

water flux is investigated in simulation ignoring solute flux (B=0) and using equation

5-1 for predicting the water flux.

Next the cross flow velocity of feed and draw solution are changed respectively at the

optimum operating temperature to find the sustainable retention time in both feed and

draw solution sides in FO process.

Finally, the optimum operating conditions is determined by calculating the required

membrane area in FO process

Table 5-5 summarises the process simulation conditions and the equations applied in

calculation.

110


Table 5-5 FO process simulation conditions and applied equations

Simulation Conditions Value Units Equation

Seawater feed & Draw Solution cross

flow velocity, QF, QD

0.055–0.111 &

0.222

m3/m2hr Equation 5-11

Draw solution Osmotic pressure, πD 208 to 56 bar Equations 5-39 & 5-

40

Seawater Feed Osmotic Pressure, πf 27 bar Equation 5-43

Seawater feed TDS concentration, CFb 35,000 mg/l

Membrane water Permeability

Coefficient, A

1.725 E-07 m/s.bar Table 5-2

Membrane salt Permeability

Coefficient, B

1.4786 E-07 m/s Table 5-3

Membrane structural parameters,

thickness, t, porosity, є, tortuosity, τ

3/40/1.75 mm Supplier Ref.

Tubular membrane dimensions, D x L 7 x 250 mm Supplier Ref.

5.2.3.1. The Effect of Changing the Osmotic Pressure DifferenceThe concentration of salt and solute of feed and draw solution at the membrane-solution

interfaces are different from the bulk solution concentration due to water flux across the

membrane from feed to draw solution side. The concentration profile produces significant

external concentration polarization (ECP) on membrane active layer in draw solution side and

internal concentration polarization (ICP) in porous membrane support layer in feed solution

side. Both external and internal concentration polarization (ECP and ICP) effect on water flux

decline extremely. The flow of the solute and water through the membrane is in opposite

directions. The water flow dilutes draw solution concentration due to reducing the solute

concentration at the membrane interface compare with the bulk solute solution and increasing

the feed salt concentration at the water – side interface. Therefore the osmotic pressure

difference across the membrane is lower than the apparent value corresponding to the bulk feed

and draw solutions. The reduced osmotic pressure difference is denoted effective osmotic

pressure difference ∆πeff, which is calculated considering developed external and internal

111


concentration polarization (ECP and ICP) models here. Aside from the inherent concentration

polarization phenomenon, hollow Fibre membrane is coupled with the dilution effect on water

flux in this study due to being one of the membrane configurations widely used for large-scale

water treatment and the relatively large packing density. The following equations are used

consequently with the first guess of water flux across the tubular membrane, which is foreseen

as one hollow Fibre membrane was fabricated that active layer is on inner surface given as:

PRO Mode:

Re=ρudh

μ ,Sc= μ

ρD , K= tτ

Dε

Q=Jw dh

D(Re . Sc . dh / L)1/3

PRO Mode: Shave=1 . 62(Re . Sc .

dh

L )1

3 (1. 0+0. 37 Q+0 .03Q2−0 . 00105 Q3 )

k =

Shave . Ddh

Cdw

Cdb= k

J w+k Cdw

C fw

Cdw=


C fb

Cdwexp (J w K )

B[ exp( Jw K )−1 ]+J w Cfw, πD and πf

Jw=(

1k f

+1k m

+1k D

)−1 ln [ Aπ D+B−J w exp( J w /kD )Aπ f +B ]

New Jw

Check new Jw with the first Guess

The overall osmotic pressure πFw of feed solution at membrane feed wall side is assumed to be

comprising of osmotic pressure of seawater (35 g/l) and reverse diffused DME concentration at

atmospheric or lower operating pressure which is 0.008 mol/mol.

The effect of changing the bulk and effective osmotic pressure difference on water flux in FO

process was simulated using the typically represented osmotic-pressure model (I), equation (5-

112

First Guess for Jw

Jw=A (π dw−π fw )

Bulk concentration

Cdb & Cfb, Ddb


1) and the modified model (R) considering both internal and external concentration polarization

(ECP & ICP) effects. Figure 5-9 illustrates the water flux against the concentration of DME as

draw solution considering bulk and effective osmotic pressure difference in the calculation of

water flux for cross flow rate 0.222 m3/m2hr; pressure retarded osmosis (PRO) mode and 20°C

operates temperature in FO process. As expected, water flux increased by increasing osmotic

pressure difference. However, the water flux (R) considering the effective osmotic pressure

difference is much lower than would be predicted on the basis of bulk osmotic pressure

difference (I) due to the effect of external and internal concentration polarization (ECP and

ICP) phenomenon at higher concentration of the DME draw solution is more severe. As shown

in table 5-6 the proportion of water flux drop by the effect of internal concentration polarization

(ICP) is around 80% whereas the contributions of external concentration polarization (ECP)

effect to the osmotic pressure drop is around 50%.

Table 5-6 Water flux vs. DME concentration for T=292.78 K & Q=0.222 m3/m2hr

Cdb Cfb

πDb

(bar)

πFb

(bar)

Jw(I)

(L/m2.hr)

DDS

(m2/s)Sc(DS) Re(DS)

Q(DS) Shave

6.55 0.62 208 27 112 2.16E-09 262 2732 0.5 43.35

4.83 0.62 141 27 71 1.84E-09 358 2362 0.39 46.05

4.72 0.62 137 27 68 1.82E-09 366 2340 0.38 46.22

4.30 0.62 118 27 56 1.75E-09 394 2260 0.35 46.87

2.63 0.62 70 27 27 1.49E-09 534 1952 0.18 49.68

2.13 0.62 56 27 18 1.42E-09 585 1869 0.11 50.58

113


Continue Table 5-6

kD

(m/s)

Cdw

(mol/l)

Ddw

(m2/s)

Km

(s/m)

Cfw

(mol/mol)

πdw

(bar)

πfw

(bar)

Jw(R)

(L/m2.hr)

1.34E-05 4.80 3.19E-09 1.31E-04 0.74 138 45.72 15.08

1.21E-05 3.78 2.90E-09 1.31E-04 0.71 103.5 44.13 10.66

1.20E-05 3.71 2.88E-09 1.31E-04 0.71 101 44.02 10.37

1.17E-05 3.46 2.81E-09 1.31E-04 0.70 93.4 43.57 9.26

1.06E-05 2.35 2.52E-09 1.31E-04 0.66 61.6 41.36 4.31

1.03E-05 1.98 2.44E-09 1.31E-04 0.64 52.18 40.51 2.62

Note: Jw(I) = Ideal Water Flux (l/m2hr), equation (5-1)

πDb = Osmotic pressure (bar) of bulk draw solution, equation (5-39)

πFb = Osmotic pressure (bar) of bulk feed solution, equation (5-43)

DDs = Draw solution diffusion coefficient (m2/s), equation (3-15)

Sc(DS) = Schmidt number of draw solution stream, equation (5-12)

Re(DS) = Reynolds number of draw solution stream, equation (5-11)

Q(Ds) = A lumped parameter indicated the dependence of average Sherwood number on

the effect of dilutive ECP layer, equation (5-8)

Shave = Sherwood number of draw solution stream, equation (5-35)

kD = Mass transfer coefficient in draw solution stream, equation (5-16)

Cdw = Draw solution concentration on membrane wall, equation (5-15)

Ddw = Diffusion coefficient of drag solution on membrane wall, equation (5-42)

Km = Support layer resistance to draw solute diffusion, equation (5-24)

Cfw = Feed solution concentration on membrane wall, equation (5-23)

πDw = Osmotic pressure (bar) of draw solution on membrane wall, equation (5-39)

πFw = Osmotic pressure (bar) of feed solution on membrane wall, equation (5-43)

Jw(R) = Real Water Flux (l/m2hr) considering the effect of ECP-ICP, equation (5-30)

114


The second noteworthy feature of the simulated results is the distinct non-linearity of the water

flux versus DME concentration plotted. This of course is a direct result of ECP and ICP effects

and is noted that increasing draw solution concentration does not produce a proportional

increase in water flux.

The results also show that the terminated concentration of DME draw solution to produce

positive flux could be higher than 1.62 M or 74 g/l at 20°C due to the effective osmotic

pressure difference would be negative at this concentration. This observation suggested the

lowering dissolved DME concentration in water by decreasing the operating pressure and

therefore, the rapid decreasing of osmotic pressure difference. Therefore the DME draw

solution concentration in FO process to predict the water flux across the membrane should be

ranged between 2.13 M to 6.55 M (0.04 to 0.155 mol/mol) or 98 to 301 g/l at operating

pressure 4 bars at 20°C.

Figure 5-9 Plots of water flux against bulk and effective DME concentrations at 20°C.

115


5.2.3.2. Effect of Changing the Operating TemperatureThe water flux was calculated at 20, 30, 40 and 50°C in order to determine the effect of

operating temperature on water flux across the membrane in FO process. The water flux

expected to increase with increasing temperature due to a decreased viscosity of water which

increases the diffusion rate of water through the membrane. By increasing diffusion coefficient

for aqueous solutions, the mass transfer coefficient is raised and external concentration

polarization (ECP) impact will be decreased consequently. The similar effect will happen for

internal concentration polarization (ICP) module, where an increased diffusion coefficient

reduces solute resistivity. Figure 5-10 presents water flux through the ceramic membrane for

FO process in the PRO mode as a function of DME concentration (or osmotic pressure

indirectly) at different temperatures.

0.04 0.06 0.08 0.10 0.12 0.14 0.160

5

10

15

20

25

30

Water FLUX against DME DS concentration at 20°C, 30°C, 40°C & 50 °C

20

DS Concentration (mol/mol)

Flux

, Jw

, L/

m2.

hr

Figure 5-10 water flux Vs. DME concentration at 20°C , 30°C , 40°C & 50°C temperatures

116


The results notice the significant increase in water flux with an increase in DME draw

temperature from 20°C to 50°C and indicate that FO process conducted at 50°C will be able to

gain much higher water flux per unit heat energy input than FO process operated at 20°C. The

calculated results for water flux at 50°C are tabulated in table 5-7.

Table 5-7 Water flux vs. DME concentration for hollow Fibre membrane at T=323 K & Q=0.222 m3/m2hr

CDb CFb DDS, m2/s Sc(DS) Re(DS)Q(DS) Shave kD

(m/s)

Cdw

6.96 0.62 4.06E-09 84 4565 0.54 35 2.04E-05 4.95

4.72 0.62 3.4E-09 117 3916 0.39 37.5 1.82E-05 3.7

4.3 0.62 3.30E-09 124 3811 0.36 38 1.79E-05 3.44

2.63 0.62 2.89E-09 159 3385 0.19 40 1.64E-05 2.33

2.37 0.62 2.83E-09 165 3324 0.16 40.15 1.62E-05 2.15

Continue Table 5-7

Ddw Cfw Km

(s/m)πdw(bar) πfw(bar)

Jw(R )

L/m2.hr

3.23E-09 0.83 1.31E-04 159 49.6 24.54

2.88E-09 0.76 1.31E-04 111.6 46 15.63

2.81E-09 0.75 1.31E-04 103 45.5 13.91

2.52E-09 0.68 1.31E-04 67 42 6.52

2.48E-09 0.67 1.31E-04 62 41.6 5.26

Note: Referring to note in table (5-6)

117


The water flux dependency on temperature is shown on figure 5-11. The water flux increases

by increasing temperature due to increasing diffusion coefficient of DME draw solution,

reducing both values of mass transfer coefficient k and solute resistivity K, and reducing the

ECP and ICP effect consequently. Therefore these results indicate that rising the temperature

plays a positive role in reducing the effect of concentration polarization (CP) at higher draw

solution concentration.

15 20 25 30 35 40 45 500

5

10

15

20

25

30

f(x) = 0.02 x² − 0.899999999999999 x + 25R² = 1

Water FLUX VS. Temperature at Max-imum Solubility of DME DS in water

Temperature °C

Flux

, Jw

, L/

m2.

hr

Figure 5-11 water flux vs. temperatures at 0.144 mol/mol DME concentration in water

5.2.3.3. The Effect of Reverse Draw Solute Flux The solute in draw solution diffuses into the feed solution in the opposite direction of

the water flux due to the concentration gradient within the membrane that is named reverse

solute flux. The diffused solute inside the porous support layer of membrane in pressure

retarded osmosis (PRO) mode causes a decrease in the osmotic pressure difference across the

active layer, which is known as internal concentration polarization (ICP). Furthermore, any

draw solute leakage to the feed solution could result in the loss of draw agent, and increases the

operating cost. In addition, the reverse solute flux could increase cake-enhanced on the

membrane and reduce the water flux considerably. In this section, solute flux in reverse

118


direction of the water flux is ignored and solute permeability, B, is foreseen negligible to

estimate the water flux across the membrane. Table 5-8 shows the predicted water flux in terms

of ignoring reverse solute diffusion flux.

Table 5-8 Water flux vs. DME concentration without solute passage B=0, PRO mode at T=323 K &

Q=0.222 m3/m2hr.

CDb CFb DDS, m2/s Sc(DS) Re(DS)

Q(DS) Shave kD

(m/s)

Cdw

6.96 0.62 4.06E-09 84 4565 0.54 35 2.04E-05 4.95

4.72 0.62 3.4E-09 117 3916 0.39 37.5 1.82E-05 3.7

4.3 0.62 3.30E-09 124 3811 0.36 38 1.79E-05 3.44

2.63 0.62 2.89E-09 159 3385 0.19 40 1.64E-05 2.33

2.37 0.62 2.83E-09 165 3324 0.16 40.15 1.62E-05 2.15

Continue Table 5-8

Ddw Cfw Km

(s/m)πdw(bar) πfw(bar)

Jw(R )

L/m2.hr

3.23E-09 0.83 1.31E-04 159 49.6 67.8

2.88E-09 0.76 1.31E-04 111.6 46 40.67

2.81E-09 0.75 1.31E-04 103 45.5 35.65

2.52E-09 0.68 1.31E-04 67 42 15.5

2.48E-09 0.67 1.31E-04 62 41.6 12.65

Note: Referring to note in table (5-6)

The water flux without reverse solute flux is clearly higher than water flux considering solute

permeable through the membrane because the effect of ICP in support layer. Both estimated

water flux with and without ignoring solute permeability are compared and illustrated in figure

5-12.

119


2.00 3.00 4.00 5.00 6.00 7.00 8.000

10

20

30

40

50

60

70

80

Water FLUX With & Without ignoring Solute Reverse Flux Vs. DME DS con-

centration at 50 °C

DS Concentration (mol/mol)

Flux

, Jw

, L/

m2.

hr

Figure 5-12 Predicted water flux vs. DME Draw Solution Concentration considering solute reverse

diffusion and ignoring at 50°C operating Temperature.

5.2.3.4. The Effect of Changing the Cross- Flow VelocityIt is generally known that water flux increases as cross-flow velocity gets increases.

That is the reason for the reduction in the ECP effect. The effect of varying cross flow velocity

of both the feed and draw solution on the performance of FO process is investigated in this

section. The equal cross-flow velocities including 0.055, 0.111 and 0.222 m/s is first employed

on both the draw and feed solution sides to find the optimum cross flow rate. Next the optimum

cross flow rate resulting from the first step is applied on feed solution side, while draw solution

flow rate is changed from 0.055, 0.111, and 0.222 m/s to find the effect of changing DS cross

flow rate on FO performance. Finally, by considering the optimum cross flow rate on the draw

solution side, the feed solution flow rate is altered from 0.055 to 0.222 on the same steps, to

study the performance of FO process affects with changing feed solution cross flow rate. The

results of the calculation of the water flux through the membrane as the factor to evaluate FO

performance are shown in table 5-9 and plotted in figure 5-13.

120


Table 5-9 Effect of the inlet flow rates of both feed and draw solutions on the Water flux through

Hollow Fibre membrane in PRO mode at T=323 K.

CDME

Jw (L/m2.hr)

at Q= 0.055 (m3/m2h)

Jw (L/m2.hr)

at Q= 0.111 (m3/m2h)

Jw (L/m2.hr)

at Q= 0.222 (m3/m2h)

6.96 17.57 20.89 24.544.72 11.31 13.37 15.634.30 10.11 11.93 13.912.63 4.84 5.66 6.522.37 3.91 4.58 5.26

CDME

Jw (L/m2.hr)

at QDs= 0.222 (m3/m2h);

QFs= 0.055 (m3/m2h)

Jw (L/m2.hr)

at QDs= 0.222 (m3/m2h);

QFs=0.111 (m3/m2h)

6.96 22.36 23.564.72 14.14 14.944.30 12.58 13.302.63 5.88 0.922.37 4.72 5.02

CDME

Jw (L/m2.hr)

at QDs= 0.055 (m3/m2h);

QFs= 0.222 (m3/m2h)

Jw (L/m2.hr)

at QDs= 0.111 (m3/m2h); QFs=

0.222 (m3/m2h)

6.96 18.88 21.684.72 12.25 13.914.30 10.95 12.432.63 5.28 5.902.37 4.27 4.78

Note: Jw = water flux (l/m2hr) across the ceramic membrane

QDs = Cross flow velocity (m3/m2hr) of draw solution stream

QFs = Cross flow velocity (m3/m2hr) of feed solution stream

Q = Cross flow velocity (m3/m2hr) of feed and draw solution stream

121


0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.220

5

10

15

20

25

30

Water Flux Vs. DME DS concentration, Vary-ing DS & FS Cross Flow Velocities,

at constant DS Molarity 6.96M &50 °C

Varying FWLinear (Varying FW)Varying DSPolyno-mial (Vary-ing DS)Equal FW=DS

DS Concentration Molarity , M (mol/l)

Flux

, Jw

, L/

m2.

hr

Figure 5-13 Influence of cross flow direction on the water flux across Hollow Fibre membrane on PRO

mode at 50°C,

Equal DS &FS cross flow rates: 0.055, 0.111, and 0.222 m/s,

Constant DS: 0.222 m/s, Varying FS cross flow rates: 0.055, 0.111and 0.222 m/s,

Constant FS: 0.222m/s, Varying DS cross flow rates: 0.055, 0.111and 0.222 m/s.

The results in figure 5-13 indicates that water flux increases when FO unit is operated at higher

cross flow rate on both draw solution and the feed solution sides due to reduced concentrative

external concentration polarization (ECP) on DS and FS sides of the membrane. According to

film theory, altering the solution flow rates changes the thickness of the mass transfer boundary

layer at the surface of the membrane. At higher flow rates, the boundary layer is thinner

resulting in higher rate of mass transfer and, consequently, reduced dilutive external

concentration polarization (ECP). Furthermore, the water flux can be enhanced when the cross

flow rate of draw solution stream QDS is more than feed solution cross flow rate QFS due to the

concentrative internal concentration polarization (ICP) is hardly affected by cross flows on

pressure retarded osmosis (PRO) mode. In addition, the results in table 5-9 clearly show that

the water flux is significantly increased when higher inlet flow rate of the draw solution is

applied. This is because of the fact that more sever dilution of the draw solution occurs with

increasing the amount of the fresh draw solution.

122


Noticed that Reynolds number generally is higher than 4000 and therefore turbulent flow on

DS and FS flows region at cross flow rate 0.222 m/s. Therefore, a trade-off between pressure

drop cross hollow Fibre module and higher water flow rate should be considered in detail

design of FO process.

123

FeedBrine

Draw Solution Outlet

Draw Solution Inlet


5.2.4. Forward Osmosis Unit Mass balance

To demonstrate the effect of all aforementioned operating conditions mutually on FO

desalination unit, the FO process is simulated for producing 1m3/h potable water to estimate the

required membrane area as an economical factor to compare FO process under different

operating conditions. The system recovery is considered 50% and the operating pressure of

feed water is foreseen one bar to keep the reverse diffusion of DME from draw solution to feed

side at lower concentration. Also the concentration of feed and draw solutions on membrane

wall are employed in the calculation of the water flux using the modified external and internal

concentration polarization (ECP and ICP) models results in the previous sections. Figure 5-14

shows the schematic diagram of the FO unit and the mass balance of the unit are illustrated in

equations 5-44 and 5-45. Table 5-10 summarized the assumed process conditions, the mass

balance calculations and the required membrane area.

Figure 5-14 Schematic diagram of the FO unit

QFS , iFO −QFS ,o

FO +QDS, iFO −QDS , o

FO =0 (5-44)

QFS

FO =QFS , iFO −QFS ,o

FO

QFS

FO =Am

FO ¿ JwFO

JwFO=A

W

FO ¿ [( PFS , avFO −PDS ,av

FO )−(π FS ,avFO −π DS , av

FO ) ] (5-45)

Where, QFS, QDS, are cross flow rate (m3/m2hr) of feed and draw solution streams, Aw is water

permeability coefficient (m3/m2hr bar) across the membrane and Jw is water flux through the

membrane respectively. In addition, πFS and πDS present average osmotic pressure (bar) of feed

and draw solutions. Here, FO presents Forward osmosis process as well.

124


Table 5-10 Forward Osmosis Desalination Process Simulation conditions & Results For 1m3/h Fresh

Water Capacity and 50% Feed Water Recovery at 30°C feed water temperature

TDS

(°C)

PDS

(bar)

CDS

(M)

∆πeff

(bar)

Jw

(l/m2hr)

A

(m2)

20 4.5 6.55 79.44 15.14 66

4 4.83 44.54 10.66 94

4 4.72 42.4 10.37 96

4 4.3 34.7 9.26 108

30 6 6.55 80.1 16 63

5.5 4.83 45.1 11 92

5.5 4.72 43 10.6 95

5 4.3 35.2 9.3 107

40 8 6.55 79.9 21.1 48

7 4.83 45.3 15.1 67

7 4.72 43.2 14.7 68

6.5 4.3 36.8 13.2 76

50 10.5 6.96 87 24.5 41

9 4.72 42.1 15.6 64

8 4.3 34.1 13.9 72

6 2.63 3.3 6.5 153

Note: TDS = Operating temperature (°C) of draw solution input stream

PDS = Operating pressure (bar) of Draw solution input stream

CDS = Concentration (M) of draw solution input stream

∆πeff= Effective differential osmotic pressure (bar) between draw and feed solution

streams

Jw = Water flux (l/m2hr) across the ceramic membrane

A = Required ceramic membrane area (m2)

125


The required membrane area is resulted from mass balance calculations shows that the

optimum operating temperature of DME draw solution should be between 40 to 50°C while the

operating temperature of feed side is assumed to be 30°C to prevent using extra energy for

heating. However, the capital cost would decrease due to lowering the membrane area by

increasing the operating temperature; the operating cost may be increased because the operating

pressure is increased accordingly. Therefore, a trade-off between membrane area and operating

pressure should be considered for achieving the optimum operating condition. Figure 5-15

illustrates the trade-off between the water flux, required membrane area and hydraulic

operating pressure varying operating temperature at constant DME concentration in draw

solution.

15 20 25 30 35 40 45 50 550

20

40

60

80

100

120

The Water Flux & Required Membrane Area VS. Temperature at 4.7 M of DME

Draw Solution AreaPolynomial (Area)FluxPolynomial (Flux)PressureLinear (Pressure)

Temperature, °CWat

er F

lux

l/m

2h /

Mem

bran

e Ar

ea,

m2

Figure 5-15 water flux, required membrane area and hydraulic operating pressure vs. different operating

temperature at constant DME draw solution concentration at 4.7M.

The results in figure 5-15 show that the optimum operating temperature could be between 40°C

to 50°C due to the required membrane area, which decreases significantly, whereas the

operating pressure increases moderately with the temperature rising.

On the other hand, an increase in the draw solution temperature also leads to elevation the

solution pressure, which raises the energy requirement of the FO process and the recycling

operating pressure consequently. The plotted curve of saturated vapour pressure versus

temperature of DME was mentioned in Appendix B.

126


In the next chapter, the specific energy consumption of the integrated FO unit and the

depression-thermal regeneration process is estimated to find the optimum operating condition

such as temperature and pressure and investigate whether this project is a viable economical

desalination system compare with the current methods.

5.3. Summary The performance of FO process using a novel DME-water draw solution was simulated to

achieve optimum operating conditions including operating temperature, hydraulic pressure,

cross flow rate. According to the current simulation results, the following comments can be

made:

The water flux in the proposed FO desalination process is estimated ranged between

10 to 15 l/m2hr at 20°C as well as the current membrane based desalination systems

such as RO system has an average permeate flux in the range of 11-15 l/m2hr [122].

The developed model by Lee and Baker [101], and Tan and Ng [75, 103] considering

both external and internal concentration polarization (ECP and ICP) effects on FO

process, were applied in the simulation of FO process. The results showed that the

proportion of the water flux drop by internal concentration polarization (ICP) is

around 80% whereas the contribution of external concentration polarization (ECP) to

the osmotic pressure drop is around 50%.

The water flux across the membrane trends to zero at the terminated concentration of

DME-water draw solution, because the effective differential osmotic pressure ∆πeff

would be zero. The osmotic pressure of the diluted DME-Water draw solution is

decreased while osmotic pressure of the concentrated feed water is increased lowering

the effective differential osmotic pressure ∆πeff to zero. The terminated concentration

of diluted DME-Water draw solution at 50°C is around 1.62 M. Furthermore,

theterminated concentration of the diluted DME-water draw solution depends on the

feed water recovery and it would be increased with elevating the feed water recovery.

For instant at 75% recovery, the terminated concentration of DME-water solution

could be 4.3M.

The water flux increases by increasing operating temperature due to increasing

diffusion coefficient of DME draw solution, reducing the mass transfer coefficient, k,

and the value of solute resistivity, K. In addition, the effect of external and internal

concentration polarization (ECP and ICP) phenomenon is reduced consequently. The

127


required membrane area resulted from mass balance calculations shows that the

optimum operating temperature of DME draw solution could be between 40 to 50°C

while the operating temperature of feed side is considered to be 30 °C to prevent using

extra energy for heating.

The water flux increases when FO unit is operated at higher cross flow rate on both the

draw solution and the feed solution sides due to reducing the concentrative external

concentration polarization (ECP) on DS and FS sides of the membrane. However, the

effect of increasing the draw solution cross flow rate on raising the water flux is

significantly more than increasing the feed solution cross flow rate.

The operating pressure of the feed side should be considered as much as less than 1

bar although the DME draw solution must be kept under pressure of maximum 10

bars. The solubility of DME in water decreases with lowering the operating pressure;

therefore any reverse diffusion of DME from the draw solution to the feed side can be

purged and recycled to the draw solution side by decreasing the operating pressure of

feed solution.

The draw solution side should be kept under pressure to dissolve DME in water as

much as possible at a defined temperature. Therefore, the draw solution is against the

active layer of the membrane, which is referred to as pressure retarded osmosis (PRO)

mode.

The Forward Osmosis process being operated at 50°C temperature has a higher water

flux and lower membrane area than the three others operating temperature of 20, 30

and 40 °C respectively.

However, operating FO process under high temperature could increase the water flux

and reduce the required membrane area. Increasing the draw solution temperature

leads to elevating the required hydraulic pressure of the draw solution due to rising the

vapour pressure of DME in water, and this raises the energy requirement of DME

recycling into the FO process consequently. A trade-off between energy consumption

of the integrated FO desalination process with depression-thermal system and the

operating temperature-pressure is illustrated in the next chapter using Hysys chemical

process simulation software.

128


CHAPTER SIX

FO DESALINATION PROCESS WITH REGENERATION

METHOD DESIGN CRITERIA AND SIMULATION RESULTS

AND DISCUSSION

129


6.1. Introduction to an Integrated Forward Osmosis and

Decompression Method Several characteristics of the DME Forward Osmosis desalination process were described

including water flux, terminated concentration of DME in draw solution, impact of temperature

and cross flow velocity of draw solution on water flux in chapter 5. In a continuous Forward

Osmosis process, the DME draw solution is diluted by the permeated water from the feed

solution due to the osmotic pressure differential between the DS and the feed solution. Then the

diluted draw solution is regenerated using decompression. The regeneration process separates

the diluted DME draw solution into the desired product water and a concentrated draw solution

that is recycled to the Forward Osmosis unit. The DME draw solution is used in the FO process

liquefies at ordinary temperatures under the influence of a moderate pressure less than 10 bars.

After the FO unit, the diluted DME draw solution can be depressurized and subsequently

vaporized, thereby leaving the extracted water. The feasibility of an integrated FO and

depressurizing - compressing method for seawater desalination is investigated in this chapter.

In this combined process, FO uses the natural tendency of water to flow in the direction of

higher osmotic pressure, to the DME draw solution from the seawater feed stream. The

purpose of this chapter is to examine the energy requirement of the DME- water FO

desalination process, and compare the specific energy consumption to other current

desalination methods.

6.2. Principle of DME Separating MethodDME is produced from coal in large-scale plants that is rapidly increasing in China

[83]. Therefore, massive quantities of low-priced DME are expected to flow into the market.

Although this DME exists in a gaseous form at ordinary temperatures and pressure, it liquefies

under pressure of 5 to 7 bars, even at room temperature. Liquefied DME demonstrates the

outstanding capability of absorbing water. It is also readily mixed with water and easy to

compress. Kanda and Makino [114] developed dewatering method using liquefied DME that is

capable of efficiently extracting the water from coal or sewage sludge at ordinary temperature.

In their new developed dewatering method, the solid with high moisture content is mixed with

liquefied DME to absorb the moisture by DME. Then the solid is separated from the moisture-

containing DME and water, as well as the water separated from the latter even at room

temperature by slightly reducing the pressure. The resulting evaporated DME is then liquefied

under pressure and re-used for dewatering. The procedure is shown in figure 6-1.

130


Figure 6-1 Schematic of bench-scale equipment used for coal dewatering using DME [114]

As shown in figure 6-1, the bench- scale equipment consists of a compressor, condenser, two

DME buffer tanks, a DME supply pump, a dewatering column, a decompression valve and a

flash distillation tower connected in series to form a closed loop. In 2008, their first small-scale

prototype DME sewage dewatering treatment plant with 10 l/batch installed in Yokosuka

region is shown in figure 6-2.

131


Figure 6-2 the prototype DME dewatering of sewage sludge plant. The arrows and numbers indicate the

circulation route of DME [83].

In Forward Osmosis desalination using the DME draw solution, firstly the developed method

by Kanda [83] for recycling DME is applied for DME regeneration at downstream of the FO

unit. The integrated FO and depression process for DME regeneration is illustrated in Figure 6-

3.

132


Figure 6-3 Process Flow Diagram (PFD) of FO-DTR desalination process for seawater or brackish

water

Reference is now made to figure 6-3 of the drawing, which depicts the diagram of FO

desalination of seawater, brackish water to produce clean water for potable water and or

irrigation or industrial or polished water using Dimethyl ether (DME) draw agent and gas

striping regeneration process. The proposed FO system comprises a feed chamber 100 and a

draw agent solution (DS) chamber 110 separate by a semi-pearmeable membrane 120. In this

method, feed water line 101 is introduced into one side of the membrane via a pump P-101 and

the compressed draw solution stream 111 is introduced to the other side of the membrane.

Since the osmotic pressure of the draw agent solution is higher than feed solution, water flows

through the membrane due to natural osmosis. The concentrated feed solution 102 leaves FO

unit while the diluted draw agent solution 112 flows to the regeneration unit. The feed side is

maintained at atmospheric or minimum hydrostatic pressure less than 1 bar while the DME

draw solution is operated under hydraulic pressure between 4 to 10 bar. The draw agent

solution that is at an elevated pressure in addition of the operating pressure due to the flow of

the water from the feed solution into the draw agent solution occures along a concentration

gradiant. This pressure may be applied by a pressure exchanging 130 apparatus to supplement

the compression of the separated draw agent by compressor 150 or is declined by passing

through the regulator 170. The draw agent solution would be separated from the water by a gas

depression or striping unit 140. As the solubility of the novel presented draw agent changes by

altering the operating pressure; therefore the criteria of separating the DS from water in column

133


140 could be decreasing the pressure up to atmospheric pressure. The upper stream 141 in the

separating process preferably would include the concentrated pure gas of DME draw agent

solution whereas the bottom stream 142 would be pure or comprise some species of the draw

agent or probably passed salt species from feed solution (e.g. sodium chloride or potasium

chloride in seawater or brackish feed water). The extracted water will be divided into two

portions. The first portion 142 including the required amount of clean water may be treated in

unit 160 by thermal, vacuum gas striping methods to produces the potable water according to

WHO standard. The second portion 143 would be used to dissolved the concentrated gas draw

agent by a compressor 150. The upper stream of the separating column 160 going back to the

separated draw agent stream 141 and the bottom stream 161 is used for drinking water or clean

water for irrigation.

The proposed FO desalination using DME depression regenerating method was simulated by

the use of chemical process simulation software HYSYS 7.2 . The regenerating process is

simulated using single flash column at atmospheric pressure to separate DME draw solution

from the product water by depressurizing method. The diluted DME-water draw solution is

heated before the flash tank due to acheive compelete separation . The separated DME gas will

be liquefied using a compressor 150 to recycle within the FO system. The thermal-vacuum

flash process might be used after the first depression unit to separate DME from clean water

compeletely. Electrical energy with any thermal energy requirements of the process are

calculated, as well as a combined term for equivalent electrical work. The detail description of

the calculation and the results of the simulation are explained in the next section.

6.3. DME Separating Process Simulation Methodology

6.3.1. Forward Osmosis Regenerating Unit Mass Balance Relations

Partial or total flash vaporization occurs when a saturated liquid stream goes through a

reduction in pressure by passing through a throttling device. If the throttling device is located at

the entrance into a pressure vessel, the flash evaporation happens within the flash vessel. If the

saturated liquid is a multi-component liquid (for example, a mixture of DME and water), a part

of the liquid will also immediately flash into a vapor and the flashed vapor will be richer in the

more volatile component here is DME than is the remaining liquid. Figure 6-4 illustrates a

schematic of flash evaporation column.

134

http://chemengineering.wikispaces.com/pressure


Figure 6-4 Schematic diagram of flash drum opeartion [115]

For a multi-component liquid, calculating the amounts of flashed vapour and residual liquid in

equilibrium involves solving the following Rachford-Rice equation [115] at a given

temperature and pressure and requires a trial-and-error iterative solution:

∑i

zi (K i−1 )1+β (K i−1 )

=0

(6-1)

y i=K i xi (6-2)

Where zi is the mole fraction of component i in the feed liquid; β is the fraction of feed that is

vaporised; Ki is the equilibrium constant of component i, xi is the mole fraction of

component i in liquid phase and yi is the mole fraction of component i in gas phase. Figure 6-5

shows a flow diagram of these equilibrium streams.

135


Figure 6-5 Schematic diagram of Rachford-Rice equation [116]

The equilibrium constants Ki are in general functions of many parameters. If the Raoult’s law

holds for the process, then Ki depends on pressure and temperature only, and can be calculated

by equation 6-3 as:

K i=Pi

sat

P (6-3)

Where Pisat is the vapour pressure of component i in gas phase and P is the operating pressure in

flash tank. Once the Rachford-Rice equation is solved for β, a reverse relation between the

vaporized feed and equilibrium constant can be seen as follow:

β=

Z i

x i−1

(K i−1 ) (6-4)

Here, Zi, Ki, xi and β are the same parameters described in equation (6-1). Therefore, the

pressure of flash tank should be atmospheric to increase the K value and maximize the fraction

of vaporized feed β. In addition the experimental vapour pressure data of the binary mixture

Dimethyl ether- water (VLLE) shows that the vapour pressure of DME-water solution is

increased by elevating the operating temperature. Figure 6-6 illustrates the T-P diagram of

DME-water solution.

136


0 2 4 6 8 10 120

50

100

150

200

250

300

350

f(x) = − 0.608649311703035 x² + 14.0475143588373 x + 240.141282296631R² = 0.996088207933742

T-P Diagram of DME

Pressure , bar

Tem

pera

ture

°K

Figure 6-6 Experimental vapor pressures vs. temperature data of DME-water binary mixture [100]

In the simulation of thermal-depression regeneration process of diluted DME-water solution in

the atmospheric flash tank, the operating temperature of the diluted draw solution was changed

from 20 to 50°C and the DME concentration in the bottom clean water is recorded to find the

optimum operating temperature. In all cases, the operating basis is the production of 1m3/h

clean water from seawater in FO desalination system with recovery rate 50%. The diluted draw

solution contained 2.6M (0.051 mol/mol) of DME-water which was resulted as output stream

of FO process in table 5-10. The flow diagram of Hysys simulation is given in Figure 6-7.

Furthermore, the operating temperature and pressure of the feed solution is considered 30°C

and one bar following to the achieved optimum results in chapter 5 to save energy consumption

and to decrease the solubility of any reverse diffused DME to the feed side respectively. The

results are shown in table 6-1.

137


Figure 6-7 Flow diagram for simulation of DME depression-thermal separation by Hysys 7.2

Table 6-1 Simulation results for atmospheric thermal - depression regeneration of diluted DME-water

draw solution at constant 2.6M input concentration

Temperature

(°C)

Pressure

(bar)

Residual DME in

clean water,

(mol/mol)

Recovery

Percentage%

20 4 0.033 63

30 6 0.023 74

40 7 0.017 80

50 8 0.013 85

The results showed that the second flash tank working under vacuum should be used to

separate DME from clean water completely. Therefore, the simulation was modified by

considering the second vacuum flash vessel at 0.1 bars in the downstream. Table 6-2 tabulates

the total recovery of DME separating process.

138


Table 6-2 Simulation results for vacuum depression regeneration of diluted DME-water draw solution

Temperature,

(°C)

Pressure

(bar)

Residual DME in

clean water, (mol/mol)

Total Recovery

Percentage%

20 4 0.0029 96.7

30 6 0.0019 97.8

40 7 0.0012 98.6

50 8 0.0007 99.2

The results indicate that there is a direct relationship between the temperature of the dilutive

draw solution entering the solute recovery system and the amount of recovered DME from the

produced clean water in the regeneration process. The quality of the clean water in terms of

DME concentration increased to reach less than 1 ppm by increasing the operating temperature.

The second step in the regeneration process is the compression of DME gas from atmospheric

pressure to the vapour pressure of DME at a defined operating temperature in order to recycle

to the FO process using a gas compressor, which is described in the next section.

6.3.2. DME Compression Unit for Recycling DME Draw Solution

Reciprocating compressor was considered to compress DME gas by a piston moving

backwards and forwards in a cylinder. Valves control the flow of low-pressure gas into the

cylinder and high-pressure gas out of the cylinder. The mechanical work to compress DME gas

is the product of the external force acting on the DME gas and the distance, which the force

moves. The compressor adds energy to the DME gas by doing work. In principle, compression

could be carried out either at constant temperature or adiabatically. Most compression

processes are carried out close to adiabatic conditions. The work required for an ideal adiabatic

(isentropic) compression is introduced by equation 6-5 as:

W S=( γγ−1

)Pin V in

ηIS [1−(Pout

P in)

γ −1γ ]

(6-5)

139


Where, W is work required for DME gas compression, Pin and Pout are inlet and outlet pressure

in (bar), Vin presents inlet gas volume (m3/min), ƞIS indicates isentropic efficiency and γ is heat

capacity ratio Cp/Cv.

In practice, the compression will be neither perfectly adiabatic nor ideal. To allow for this, the

DME gas compression can be assumed to follow a polytropic compression that is neither

adiabatic nor isothermal, but specific to the physical properties of the DME gas and the design

of the compressor. The polytropic coefficient, n, relates to the isentropic compression

efficiency, heat capacity ratio γ and inlet-outlet pressure and can be estimated by equation 6-6

as:

n=

ln ( Pout

Pin)

ln [ ηIS( Pout

Pin)

ηIS−1+( Pout

Pin)

γ−1γ ]

(6-6)

Here, n is polytropic coefficient, ηIS is isentropic efficiency of compressor, y is heat capacity

ratio, Pin and Pout are operating pressure (bar) at input output of compressor. Then the outlet

temperature can be calculated in the real compression using polytropic compression by

equation 6-7 given as:

T out=T in (Pout

P in)

n−1n

(6-7)

Where, n is polytropic coefficient, Tin and Tout present temperature (K) of stream at input and

output of compressor and Pin and Pout introduce pressure (bar). The output temperature of

liquefied DME after compressor can be calculated using the following input data and equations

6-6 and 6-7 respectively.

Input Data into Compressor:

Ratio of specific heats of DME (γ= Cp/Cv) = 1.16

140


Tin = 20, 30, 40 and 50°C

Pin = inlet pressure = 1 bar

Pout = outlet pressure = 4, 6, 8 and 10 bars

ƞIS = isentropic efficiency = 0.85

Vin = inlet gas volume = 3 m3/min

Output Results:

N = polytropic coefficient = 1.19

Tout = 97, 128, 160 and 192 °C

W = Energy required for DME gas compression = 5 to 8 kWh/m3

The results indicate that output temperature of the liquefied DME solution after compression is

higher than the operating temperature in the FO process. Therefore, the hot liquefied DME

solution can be used to heat up the diluted DME- water draw solution before the first flash tank

and the downstream vacuum flash tank to increase the recovery performance of DME from

clean water. In addition, concentrated feed water might be heated by the liquefied DME to

extract any reversed diffused DME to the feed solution.

The resulted work required for DME gas compression from atmospheric pressure to the

appropriate operating pressure showed that there is no significant saving in the operating cost

or energy consumption using the compressor. Fritzmann et al. [131] reported the achievable

energy consumption in RO desalination system applying recovery system has led to as low as

2-4 kWh/m3 whereas energy consumption in this project using compressor is 5-8 kWh/m3

considerably is higher than the current desalination processes such as Reverse Osmosis (RO)

process. Despite key potential advantages of the novel process including high feed water

recovery, minimization of brine discharge and separation of DME draw agent at atmospheric

pressure, the relatively energy consumption in DME compression process is still higher than

current desalination methods. Therefore, the thermal-depression process using flash tank and

compressor is replaced with a distillation column using steam for column reboiler in Hysys

simulation software for more investigation on reduction total energy consumption. The process

and the calculated specific energy consumption are described in the next section.

141


6.4. Specific Energy Consumption (SEC) of Distillation Column,

Thermal- Depression Regenerating of DME Draw Solution The depression- heating and striping of the DME gas might be accomplished in the

distillation column producing as its products the clean water and the re-concentrated DME-

water draw solution for reuse in FO process. The product clean water from this process may be

specified to contain zero ppm of DME as is appropriate for drinking water. The energy required

for this system is almost thermal, with a small amount of electricity power used for transferring

pump. Figure 6-8 shows the flow diagram of the designed process for DME regeneration using

single distillation column. The thermal and electricity requirement for DME regeneration

process were simulated using Hysys software 7.2. The input data for distillation column was

the production of 1000 kg/hr clean water from seawater at a recovery rate of 50%. The

concentrated DME draw solution contained 4.3M (20% wt) to produce the diluted DME draw

solution 2.14M (10% wt) as was resulted from FO process tabulated in table 5-10. This diluted

draw solution is directed as feed to the single distillation column. The operating temperature of

concentrated draw solution was assumed to be ranged between 20°C at 4 bars to 50°C at 8 bars

and seawater temperature was specified at 30°C. The distillation column assumed to work

under pressure less than 4 bars and a reducing pressure device was foreseen before the column

for this purpose. The steam in the reboiler could be at this pressure and the condensate would

be returned to the steam source.

142


Figure 6-8 flow diagram of single distillation column, DME thermal-depression regeneration process.

The results from simulation showed that the operation condition of distillation column is

converged at 131°C and 3.3 bars to produce clean water with zero ppm DME concentration. An

effective method to estimate the specific energy consumption value of this thermal separation

unit involves the calculation of ‘equivalent work’ [122]. In this method, thermal energy is

allocated an electricity energy based on the capacity to generate electricity in a steam turbine as

well as for real world costing of process steam supplied to a desalination plant. The equivalent

work was calculated using the equation 6-8 in units of kWh/m3 [122]:

W Eq=(1000 Kgwaterpproduct÷( H steamused−H steamatcondenser )GOR )×ETurbine×0 . 00277 kWh

kJ (6-8)

Here, H is enthalpy of the steam at the mentioned points and GOR or gained output ratio

introduces the number of kilograms of water produced for each kilogram of steam in the

reboiler. The condenser temperature was assumed to be 35°C, based on seawater cooling

temperature and efficiency of turbine, E, assumed to be 95%. The calculated equivalent work is

added to pump power consumption to result of the total value of energy consumed. The GOR

for FO regeneration process is calculated using the equation 6-9 as:

143


GOR=( H steamused ( kJKg )

Energy . for . FO( MJm3 ))

(6-9)

The calculated of equivalent work for regeneration of 4.3M diluted DME draw solution at

operating temperature 40°C is indicated in table 6-3.

Table 6-3 Energy data of DME regeneration process using single distillation column

Steam

Temperatur

e °C

Steam

Pressure(psi)

Diluted DME

Concentration

M

Heat

Duty

(MJ/m3)

Electricity

Duty

(kWh/m3)

GOR

Equivalent

Work

(kWh/m3)

131 47.13 2.14 158 0.16 18.2 2.7

The estimated total equivalent work showed that the specific energy consumption (SEC) could

be decreased from 5-8 kWh/m3 using flash-compressing process to 2.7 kWh/m3 employing a

distillation column.

The advantages of depression-thermal regeneration process using distillation column compare

with flash tank and the compression method could be summarized as follows:

Significant reduction of the specific energy consumption (SEC) in the regeneration

process.

Applying low grade heat for separating DME from clean water.

Reducing maintenance cost in the system by replacing rotary equipment such as

compressor with fixed equipment such as distillation column.

Increasing the quality of the produced clean water to have no DME.

The results in section 6.3 indicated that there is a direct relationship between the concentrations

of dilutive draw solution entering the solute recovery system and the amount of energy used by

the FO desalination process. The concentration of diluted DME draw solution relates to the

water flux within the membrane and depends on effective osmotic pressure through active layer

144


directly. Therefore, the energy consumption in thermal- depression regeneration process would

be a function of the measured water flux resulting from experimental test.

6.5. Comparison of Energy Requirements of Current Seawater

Desalination Technologies to the Proposed Forward Osmosis

Desalination Process with Depression Regeneration Method

The novel FO desalination process with thermal-depression regeneration method using

DME as draw solution is compared with the several current desalination methods in terms of

energy consumption in this section. The equivalent work could indicate the required energy for

operating the process or specific energy consumption (SEC) in kWh/m3. The several values of

equivalent work (kWh/m3) available in the literature and was listed in table 6-4. The percentage

reductions in equivalent work realized by the use of the novel FO process, relative to the other

processes examined in this table as well.

Table 6-4 Comparison of energy requirement for current desalination methods and the novel FO

desalination process

TechnologyEquivalent Work

(KWh/m3)

Percentage Energy

Saving Using the Present

FO Desalination

Reference

MSF 5.66 62% [117]

MED-TVC 4.05 47% [117]

MED-low Temperature 3.21 33% [117]

RO-Energy recovery 3.02 29% [118]

FO+RO 4.49 52% [120]

FO (NH3-CO2,1.5M) 0.84- 3.69 Same [119]

FO Pilot plant (72 g/l

feed, NH3-CO2 DS)21

There is not the same feed

water TDS[121]

Novel FO- Depression

unit, DME as DS2.77 This work This work

145


The proposed FO desalination process using DME draw solution with thermal- depression

regenerating process, offers significant improvement in energy efficiency and cost over current

desalination technologies. Furthermore, the electricity energy consumption in FO desalination

process is applied for pumping draw solution and is significantly lower than current

desalination methods. In addition, using low grade heat source increases the quality of the

product clean water in FO process relative to current desalination methods due to vaporizing

the draw solution rather than feed water.

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6.6. SummaryThe feasibility of the integrated Forward Osmosis and depressurizing - compressing

method for seawater desalination was investigated by estimating the specific energy

consumption SEC with simulation using HYSYS 7.2 software in this chapter. The operating

basis is the production of 1m3/h potable water, recovered from seawater at a recovery rate 50%.

According to the current simulation results:

There is a direct relationship between the temperature of the dilutive draw solution

entering the solute recovery system and the amount of recovered DME from the

produced clean water in the regeneration process. The quality of clean water in terms

of DME concentration increased to reach less than 1 ppm DME, by increasing the

operating temperature to 50°C. .

The thermal-depression process using a flash tank and a compressor was replaced with

a distillation column using steam for column reboiler due to the high electrical duty of

the compressor as compared with the current desalination processes.

The available low grade heat could be used in distillation column reducing specific

energy consumption (SEC) in regeneration system compare with using electricity in

compressor. Furthermore, maintenance cost in fixed equipment such as columns is

usually less than rotary equipment for instance compressor. In addition, purity of the

product water in bottom stream of distillation column is 100% while it is 85% in

atmospheric flash tank downstream.

The specific energy consumption SEC of the proposed FO desalination process was

estimated to be 2.7 KWh/m3 at the selected operating conditions.

The optimum operating temperature of the FO process could be between 30°C to 40°C

due to lowering the operating hydraulic pressure and the specific energy consumption

consequently.

The optimum concentration of DME-water draw solution could be 4.3 M between

30°C to 40°C draw solution temperature in terms of specific energy consumption in

the integrated FO-depression process.

147


CHAPTER SEVEN

CONCLUSIONS AND FUTURE WORKS

148


7.1. ConclusionsOne of the most promising desalination techniques with the potential to increase process

efficiency, cost-effectiveness with reduced energy consumption and environmental impact, is

Forward Osmosis (FO). Recent investigations on Forward Osmosis desalination process

indicate two main challenges to develop a commercialized FO process including cost-effective

regeneration process for draw solution and design a thinner and more permeable FO membrane.

The concept of employing liquefied gas compounds as draw agent in FO process interms of

changing their solubility in water by varying operating pressure and or temperature to separate

from the product water was investigated in this project. More than 100 gas compounds were

initially considered and the screening process resulted in four draw agents comprising Sulfur

Dioxide, Monomethyl Amine, Ammonia and Dimethyl Ether suitable for FO desalination

application in terms of high soubility in water and relating osmotic pressure. Then the osmostic

pressure of four listed gas compounds was calculated using Van’t Hoff model for ideal gas.

However the osmotic pressure of Sulfur dioxide (SO2) and monomethyl amine (CH5N) are

higher than seawater osmotic pressure, they both were deleted from the list due to corrosivity of

SO2 and toxicidity of CH5N. The solubility of Ammonia gas in water trends to decrease after

80°C while there is not any change by varying operating pressure. With consideration of the

cost associated with draw agent regeneration process, showed that Dimethyl ether (DME)

appeared to be a suitable draw agent in the screened group. DME has high solubility in water

and could generate high osmotic pressure around seven times higher than seawater source. In

addition, DME liquefies at ordinary temperatures under the influence of a moderate pressure of

less than 10 bars. Thus, the diluted DME-water draw solution can be depressurized and

subsequently vaporized, thereby leaving the extracted water. The proposed innovative process

has the potential to lower the specific energy consumption, lower capital cost and lower

environmental impact as compared to traditional desalination and water treatment techniques

such as Reverse Osmosis and thermal desalination. The optimum operating conditions of the

FO process including temperature, pressure and cross flow rate were predicted with the

calculation of water flux using the recent modified ECP and ICP models. Also the physical

properties of the feed and draw solutions such as osmotic pressure, diffusivity, density and

dynamic viscosity were calculated at different solute concentrations and temperature according

to the available experimental data at 20°C, 30°C, 40°C and 50°C. The feasibility of the

integrated FO with thermal - depressurizing method for seawater desalination is simulated

using HYSYS 7.2 software to estimate the specific energy consumption (SEC). The specific

149


energy consumption is estimated at optimum operating conditions, which is based on

production rate of 1m3/h potable water recovered from seawater at a recovery rate of 50%. The

results showed that the DME draw solution side should be kept under pressure of 4 bars at an

ambient temperature between 30°C to 40°C to dissolve DME in water as much as possible.

However, the operating pressure of the feed side should be considered as much as less than 1

bar. Therefore, the draw solution should be against the active layer of the membrane, which is

referred to as PRO mode. The solubility of DME in water decreases with lowering the

operating pressure and increasing the temperature; therefore any reverse diffused DME from

the draw solution to the feed side can be purged and recycle to the draw solution side by

decreasing the operating pressure of the feed solution.

The specific energy consumption, SEC, is predicted for different operating conditions which

were calculated in chapter 5 in table 5-10. In all cases, the operating basis is the production of

1m3l/h potable water, recovered from seawater at a recovery rate 50%. According to the

optimum results in chapter 5, the cross flow rate for both the feed and the draw solution is

foreseen 0.222 m/s in turbulent region to decrease ECP and ICP effects on ceramic one hollow

Fibre membrane. Furthermore the operating pressure and temperature of the feed solution is

considered to be 30°C and under less than one bar.

According to the current simulation results, the following points to be made:

The proposed FO-depression desalination process with DME-water as the draw

solution represents an effective membrane based seawater desalination approach, due

to working at pressure less than 10 bars, which reduces the capital cost of the Forward

Osmosis unit.

The water flux across the ceramic membrane in FO process was predicted ranged

between 10 to 15 l/m2hr at 20°C as well as the current desalination systems such as

RO system has an average permeate flux in the range of 11-15 l/m2hr.

The simulation results of FO process showed that the proportion reduction of the water

flux across the membrane affected by internal concentration polarization (ICP) is

around 80% whereas the contribution of external concentration polarization (ECP) is

around 50%.

150


The terminated concentration of the diluted DME-water draw solution trending the

water flux across the membrane to zero, could be at 0.04 mole fraction of DME-water

draw solution.

The optimum operating temperature of FO process using DME draw solution could be

between 40 to 50°C result the optimum membrane area while the operating

temperature of the feed side should be considered 30°C to reduce heating energy

consumption.

The performance of FO process in terms of the water flux through the membrane

increases when the FO unit is operated at transient region between laminar and

turbulent flow on the feed and draw solution streams. The high cross flow rate on both

draw solution and feed solution sides reduces external concentration polarization

(ECP) extremely.

The operating pressure of the feed side should be considered less than 1 bar however

the DME draw solution must be kept under pressure higher than 4 bars. The solubility

of DME in water decreases with lowering the operating pressure; therefore any reverse

diffused DME from the draw solution to the feed side can be separated and recycled to

the draw solution side.

The draw solution was foreseen to place against the active layer of the membrane or

pressure retarded osmosis (PRO) mode. Because the draw solution side should be kept

under pressure to dissolve the liquefied DME in water as much as possible at the

defined temperature.

The optimum concentration of DME-water draw solution could be 4.3 M in order to

reduce the specific energy consumption at 30 °C operating temperature.

The temperature of the diluted DME-water draw solution should be increased before

entering to the depression regeneration process to decrease the solubility of DME in

the solution and improve the water quality of the produced clean water. The quality of

the product water in terms of DME concentration could be less than 1 ppm DME when

the operating temperature increases to 50°C.

The specific energy consumption (SEC) of thermal depression regeneration method

for DME draw solution could be optimized by replacing flash- compressing process

with distillation column operating at 4 bars and ambient temperature between 30 to

40°C.

151


The advantages of using distillation column as thermal-depression regeneration

process could be summarized as:

o Operating with available low grade heat and significant reduction in energy

consumption,

o High quality of the product water cross complete separation of DME from

water,

o Lowering maintenance cost compare with using rotary equipment such as

compressor.

The specific energy consumption (SEC) of the proposed FO desalination process was

estimated 2.7 KWh/m3 in this project and could be developed to 0.5kWh/m3 when a

heat recovery process is used.

One of the advantages of the proposed FO desalination process is applying low-

pressure steam less than 150°C in DME draw solution thermal-depression

regeneration system. Furthermore, using heat source increases the quality of the

produced clean water in the FO process relative to the current desalination methods,

due to vaporizing the draw solution rather than the feed water.

In addition, the electricity energy consumption in the proposed FO desalination

process is applied for pumping draw solution is significantly lower than current

desalination methods.

152


7.2. Future work

7.2.1. Membrane OsmometerThe osmotic pressure of DME-water solution could be tested and measured by a semi-

permeable membrane as a future work. The design procedure, which was approved by risk

assessment group at the University of Surrey including liquefied DME injection in water,

flowing cross the membrane module and captures the waste DME, is as follow:

(a) start-up and shutdown procedures:

This process was designed to measure the possible permeability of dissolved liquefied

Dimethyl Ether in water through Reverse Osmosis membrane under 6-7 bar pressure. DME is

the simplest ether and exists in the vapor state at normal temperature and pressure. Its normal

boiling point is -25°C and it can be liquefied at 6-7 bars even at normal temperature. The

process is divided into two parts comprising dissolving gaseous DME (solute) in water

(solvent) as part one and measuring the permeability of DME-water solution through the

membrane in part two. All equipment and their arrangement were shown in the attached P&ID

drawing. The startup procedure is as follows:

1- Check installation: Check all equipment, instrument, valves and pipe connections

arrangement according to the attached P&ID.

2- Cycle purging the system with N2 gas: Open operating valves, N2 gas regulator, and push

N2 gas through the system while all the other valves keep close up to the pressure reaches to

7 bars. Close N2 supply valve. Then open vent valves gradually up to pressure reaches to

atmosphere. Close vent valves and repeat the procedure for 5 times to dilute air inside the

system less than 0.05%.

3- Check all isolating valves to be closed including N2 gas regulator.

4- Input water: Open input valve to fill the first vessel VE-201 with 1000 ml water, which

was pre-measured before then close valve.

5- Decrease Temperature: The operating temperature of vessel VE-201 must be kept at -14

°C using 100 g ethanol and 100 g ice in its jacket tank. Check the temperature of vessel VE-

201 with thermocouple indicator is mounted on the vessel.

153


6- Input DME gas: After reaching temperature of vessel VE-201 to -14 °C, record the weight

of DME gas cylinder by balance that would be around 4000g of DME gas. Open input

valves and DME gas cylinder regulator respectively to push gas into the vessel VE-201 until

balance indicates 3925 g (4000-75 g), then close regulator and disconnect DME cylinder

from test rig and return it to the outside cage. Open Nitrogen regulator slowly to keep

pressure inside mixing vessel VE-201 at 2 bars. Open hand hole of jacket tank to drain

ethanol-ice. Increase pressure inside mixing vessel VE-201 up to 7 bars by opening Nitrogen

gas regulator slowly. Liquefied DME- water solution is not flowed into the membrane

module FO-01until the temperature of vessel VE-201 reaches in equilibrium with room

temperature.

7- Test Operation: When temperature indicator shows temperature of vessel VE-201 at

ambient, open valves V-207 and V-208. Check the pressure after the membrane module FO-

01 with monitoring pressure indicator around 7 bar to make sure on pressurizing the line by

DME-water flowing. Keep this situation for 5 minutes and see whether any water passes

through the membrane or not. The predicted osmotic pressure of 75 g/l DME-water solution

is 38 bar therefore it is expected that there would not be any water passage cross the

membrane while the hydraulic pressure is 7 bars in RO process. If water can pass through

the membrane, it seems that the pore size of activated layer membrane is not suitable for

DME-water solution. After finishing the resident time for DME-water solution, open

isolating valves and needle gradually to direct the flow to gathering second vessel VE-202

from first mixing vessel VE-201 completely. Nitrogen gas pushes DME-water solution from

VE-201 into the membrane module FO-01 at a pressure of 6-7 bars and then the

concentrated solution, which mainly contained un-permeated liquefied DME, and water

flows to the gas-liquid gathering vessel VE-202. After all liquid was directed through VE-

202, close operating valves then fill the second jacket tank with ice-ethanol to cool the

vessel at -14 C.

8- Result: The predicted osmotic pressure of 75 g/l DME-water solution is 38 bar therefore it

is expected that there would not be any water passage cross the membrane while the

hydraulic pressure is 7 bars in RO process. If water can pass through the membrane it seems

that the pore size of activated layer membrane is not suitable for DME-water solution. The

probably permeated water obtains from the other side of the membrane cell FO-01 flows

154


through small opening in the side part of the cell and gathers in TK-205. If DME permeates

through membrane, it flows by this line and vents to atmosphere.

9- Test time: Operation time was chosen 5 minutes to measure one pass of DME – water

solution through membrane module FO-01 in a batch process.

10- Shutdown the system and disposal the waste: The system must be depressurized to

atmospheric pressure after finishing the test to shut down completely. Close N2 regulator

valve then by closing operating valves divide the system into two parts for depressurizing.

First part starts from N2 gas regulator and terminates to isolating valve after the first

vessel. The second part initiates from isolating valve after the first vessel and ends the

gathering vessel VE-202 including membrane module FO-01 and tubing. For

depressurizing the first part open isolating vent valve then open needle vent valve slowly to

release the pressure gradually to purge the inside gas comprising N2 and probably

remained DME to atmosphere. Keep isolating vent open for 20 seconds then close it and

repeat this procedure until the pressure indicator on vessel VE-201 indicates atmospheric

pressure. Then leave the vent valves open to discharge pressure including any residual

DME gas from the vessel completely. Drain ethanol and ice mixture from jacket tank allow

reaching room temperature. In depressurizing the second part, the gathered DME-water in

vessel VE-202 must be separated from water and purged from the system. In order to

separate DME from water, by reducing the pressure of the mixture to less than 5 bars or

atmospheric pressure at normal temperature, only DME is evaporated and separated from

water completely. Therefore decrease pressure inside the gathering/recovery vessel VE-

202 to atmospheric by opening the isolating vent valve and needle slowly to release the

pressure gradually. Continue this procedure until atmospheric pressure is reached. DME

causes no greenhouse effect and does not affect the ozone layer, thus, its effect on

ecological systems is very small. Therefore, it can be vented through ventilation system

with 6-14 m3/s capacity at the end of test by opening the vent valves on separation vessel

VE-202.

11- Drain Water: The separated liquid water blows down from gathering tank through drain

line. Before depressurizing VE-202, ethanol-ice mixture take out from jacket tank by

opening the hand hole during depressurizing vessel VE-202.

155


Cycle purging the system with N2 gas: To purge any remained DME gas from system repeat

Cycle purging again at final step. First, make sure all separated liquid water was blow down

from VE-202. Then open operating valves, N2 gas regulator, and push N2 gas through the

system while all the other valves keep close up to the pressure reaches to 7 bars. Then open

vent isolating valves and open needle valves gradually up to pressure reaches to atmosphere.

Close vent valves and repeat the procedure for 3 times to dilute DME inside the system to

0.29% that is less than DME minimum flammability limit 2.7%. Figure 7-1 shows the

experimental set up of RO process for testing DME- water solution in RO membrane mode.

156


Figure 7-1 P&ID of the experimental set up of RO process for testing DME- water solution in RO

membrane mode

157


7.3. Bench scale Demonstration of the DME Forward Osmosis

Desalination ProcessThe DME-water draw solution should be tested in a laboratory bench scale unit to

evaluate the simulation results such as water flux and reverse solute diffusion through the

membrane. Also the data presented in this project by simulation the process should be tested in

a bench scale unit of the forward osmosis desalination process integrated depression-thermal

regeneration method. The proposed process is demonstrated in figure 7-2.

7.4. Modified Depression-Thermal Regeneration MethodThe presented novel FO desalination process integrated depression-thermal regenerating

method could be a state of art current desalination methods if the liquefaction of DME is done

without using compressor. Following to description in section 7.1, DME gas could be liquefied

with using the mixture of ethanol-ice at atmospheric pressure. Therefore, the compressor could

be replaced by a centrifugal pump in DME draw solution regeneration process to lower energy

consumption consequently. The specific energy consumption using centrifugal pump could be

decreased from 2.77kWh/m3 to less than 0.5 kWh/m3 due to the considerable difference

between the density of liquid and gas flow of DME draw solution. The optimum operating

conditions in this method should be investigated as the future work Figure 7-3 shows the

proposed process as future work.

Furthermore, the results in section 6.3 indicated that there is a direct relationship between the

concentrations of dilutive draw solution entering the solute recovery system and the amount of

energy used by the FO desalination process. The concentration of diluted DME draw solution

relates to the water flux within the membrane and depends on effective osmotic pressure

through active layer directly. Therefore, the energy consumption in thermal- depression

regeneration process would be a function of the measured water flux resulting from

experimental test. The following parameters should be measured experimentally in FO process

as input data for downstream draw solution regeneration method:

- The water flux across the ceramic tubular membrane

- Optimum operating condition such as temperature, draw solution concentration and

hydraulic pressure.

- Maximum diluted concentration of draw solution entering thermal-depression

regeneration method.

158


Figure 7-2 DME novel Forward Osmosis desalination with depression regeneration method bench scale

unit

159


Figure 7-3 Novel Forward Osmosis desalination with modified regeneration method

160


References[1] United Nations World Water Development Report, Water for People water for Life,

UNESCO-WWAP, 2003,

http://www.unesco.org/new/en/natural- sciences/environment/water/wwap/wwdr/wwdr1-2003/

[2] Global water Intelligence, The Desalination Market returns, 11 (2010),

http://www.globalwaterintel.com/archive/11/7/market-insight/desalination-market-returns.html

[3] National Research Council of the National Academies, The Desalination and Water

Purification Technology Roadmap, The National Academic Press, Washington D.C. 2004

[4] P.G. Nicoll, Forward Osmosis is not to be ignore, Desalination and water reuse, 22 (2013)

30-33.

[5] T.Y. Cath, A.E. Childress and C.R. Martinetti, 2007, Combined membrane distillation

forward osmosis systems and methods of use, WO 2007/147013.

[6] A.O. Sharif, Novel Manipulated Osmosis water purification and power generation processes

– A pilot plant study, The Royal Society Policy Centre Report, London, 2007.

[7] L. Chekli, SH. Phuntsho, H.K. Shon, S. Vigneswaran, J. Kandasamy and A. Chanan, A

review of draw solutes in Forward Osmosis process and their use in modern applications,

Desalination and water treatment, 43 (2012) 167-184.

[8] J.O. Kessler and C.D. Moody, Drinking water from sea water by Forward Osmosis,

Desalination, 18 (1976) 297–306.

[9] K. Stache, Apparatus for transforming seawater, brackish water, polluted water or the like

into a nutritious drink by means of osmosis, US Patent 4,879,030, 1989.

[10] R.E. Kravath and J.A. Davis, Desalination of seawater by direct osmosis, Desalination, 16

(1975) 151–155

[11] M. Wallace, Z. Cui and N.P. Hankins, A thermodynamic benchmark for assessing an

emergency drinking water device based on Forward Osmosis, Desalination, 227 (2008) 34–45.

[12] HTI Inc., HTI Water Division, 2010,

http://www.htiwater.com/divisions/military_regulatory/index.html and

http://www.htiwater.com/divisions/humanitarian/index.html.

[13] G.W. Batchelder, Process for the Demineralization of Water, US Patent 3,171,799, 1965.

161


[14] D.N. Glew, Process for Liquid Recovery and Solution Concentration, US Patent

3,216,930, 1965.

[15] J.R. McCutcheon , R.L. McGinnis and M. Elimelech, Desalination by ammonia–carbon

dioxide Forward Osmosis: Influence of draw and feed solution concentrations on process

performance, J. Membr. Sci., 278 (2006) 114–123.

[16] N.T. Hancock and T.Y. Cath, Solute coupled diffusion in osmotically driven membrane

processes, Environ. Sci. Technol., 43 (2009) 6769–6775.

[17] H.Y. Ng, W. Tang, and W.S. Wong, Performance of Forward (Direct) Osmosis Process:

Membrane Structure and Transport Phenomenon. Environ. Sci. Technol., 40 (2006) 2408–

2413.

[18] R. McGinnis, Osmotic desalination process: US patent 7,560,029, 2002

[19] P. McCormick, J. Pellegrino, F. Mantovani and G.Sarti, Water, salt, and ethanol diffusion

through membranes for water recovery by forward (direct) osmosis processes. J. Membr. Sci.,

325 (2008) 467–478.

[20] J. Yaeli, Method and apparatus for processing liquid solutions of suspensions particularly

useful in the desalination of saline water, US Patent 509857524, 1992.

[21] S.K.Yen, F.M. Haja, N.M. Su, K.Y. Wang and K.Y. Chung, Study of draw solutes using

2-methylimidazole-based compounds in Forward Osmosis. J. Membr. Sci., 364 (2010) 242–

252.

[22] S. Iyer, Systems and Methods for Forward Osmosis Fluid Purification Using Cloud Point

Extraction, United States Patent: 8021553, 2011.

[23] M.M. Ling and T.S. Chung, Desalination process using super hydrophilic Nanoparticles

via Forward Osmosis integrated with ultra filtration regeneration. Desalination, 278 (2011)

194–202.

[24] Q. Ge, Su. Jincai, T.-S Chung and A. Gary, Hydrophilic Super paramagnetic

Nanoparticles: Synthesis, Characterization, and Performance in Forward Osmosis Processes.

Ind. Eng. Chem. Res. 50 (2010) 382–388.

[25] Q. Ge, Su. Jincai, L.A. Gary and T.S. Chung, Exploration of Polyelectrolytes as Draw

Solutes in Forward Osmosis Processes. Water Research 46 (2012) 1318–1326.

162


[26] D. Li, X. Zhang, J. Yao, G.P. Simon and H. Wang, Stimuli-responsive Polymer

Hydrogels as a New Class of Draw Agent for Forward Osmosis Desalination. Chem.

Commun., 47 (2011) 1710–1712.

[27] D. Li, X. Zhang, J. Yao, Y. Zeng, G.P. Simon, and H. Wang, Composite Polymer

Hydrogels as Draw Agents in Forward Osmosis and Solar Dewatering, Soft Matter, 7 (2011)

10048 –10056.

[28] B.S. Frank, Desalination of seawater, US Patent 367089720, 1972.

[29] S. Zhao, L. Zou and D. Mulcahy, Brackish Water Desalination by a Hybrid Forward

Osmosis–nanofiltration System Using Divalent Draw Solute. Desalination, 284 (2012) 175-

181.

[30] C.H. Tan and H.Y. Ng, A novel hybrid Forward Osmosis—nanofiltration (FO-NF)

process for seawater desalination: Draw solution selection and system configuration.

Desalination Water Treatment, 13 (2010): 356–361.

[31] A.O.Sharif and A. Al-Mayahi, Solvent Removal Method, US Patent No. US 7,879,243,

Feb. 2011.

[32] Supporting Excellent Scientists, Royal Society Review of the Year 2005/2006,

http://www.royalsoc.ac.uk/publication.asp?id=5347

[33] International Water Desalination Report, 2010, 11(7),

http://www.globalwaterintel.com/archive/11/7/market-insight/desalination-market-returns.html.

[34] N.A. Thompson and P.G. Nicoll, Forward Osmosis desalination: A commercial reality,

Perth: IDAWC, 2011.

[35] P.G. Nicoll, N.A. Thompson and M.R.Bedford, Manipulated Osmosis Applied to

Evaporative Cooling Makeup Water – Revolutionary Technology, Perth: IDAWC, 2011.

[36] Ch. Laura, Sh. Shon, H.K. Phuntsho, S. Vigneswaran, J. Kandasamy, and A. Chanan, A

Review of Draw Solutes in Forward Osmosis Process and Their Use in Modern Applications,

Desalination and Water Treatment, 43 (2012) 167–184.

[37] S. Zhao, L. Zou, C.Y. Tang, and D. Mulcahy, Recent Developments in Forward Osmosis:

Opportunities and Challenges. J. Membr. Sci, 396 (2012) 1–21.

163


[38] S. Loeb, L. Titelman, E. Korngold and J. Freiman, Effect of Porous Support Fabric on

Osmosis through a Loeb-Sourirajan Type Asymmetric Membrane. J. Membr. Sci., 129 (1997)

243–249.

[39] Ch.J.Seok, H.J. Oh, S. Lee, D. R. Yang, and J. H. Kim, Toward a Combined System of

Forward Osmosis and Reverse Osmosis for Seawater Desalination, Desalination, 247 (2009)

239–246.

[40] O.A. Bamaga, A. Yokochi, B. Zabara, and A.S. Babaqi, Hybrid FO/RO Desalination

System: Preliminary Assessment of Osmotic Energy Recovery and Designs of New FO

Membrane Module Configurations, Desalination, 268 (2011) 163–169.

[41] O.A. Bamaga, A. Yokochi, and E.G. Beaudry, Application of Forward Osmosis in

Pretreatment of Seawater for Small Reverse Osmosis Desalination Units, Desalination and

Water Treatment, 5 (2009) 183–191.

[42] H.Y. Ng, W. Tang, and W.S. Wong, Performance of Forward (Direct) Osmosis Process:

Membrane Structure and Transport Phenomenon, Environment Science Technology, 40 (2006)

2408–2413.

[43] K.Y. Wang, T.S. Chung and J.J. Qin, Polybenzimidazole (PBI) Nanofiltration Hollow

Fibre Membranes Applied in Forward Osmosis Process, Journal of Membrane Science, 300

(2007) 6–12.

[44] K.Y. Wang, Q. Yang, T.S. Chung, and R. Rajagopalan, Enhanced Forward Osmosis from

Chemically Modified Polybenzimidazole (PBI) Nanofiltration Hollow Fibre Membranes with a

Thin Wall, Chemical Engineering Science, 64(2009) 1577–1584.

[45] Q. Yang, K.Y. Wang and T.S. Chung, Dual-Layer Hollow Fibres with Enhanced Flux As

Novel Forward Osmosis Membranes for Water Production, Environ. Sci. Technol., 43 (2009)

2800–2805.

[46] Q. Yang, K.Y. Wang and T.S. Chung, A novel dual-layer Forward Osmosis membrane

[47] Q. Yang, K. Y. Wang and T.S. Chung, A novel dual-layer forward osmosis membrane for

protein enrichment and concentration, Separation and Purification Technology, 69 (2009) 269–

274

[48] S. Chou, L. Shi, R. Wang, C.Y. Tang, C. Qiu, and A.G. Fane, Characteristics and Potential

Applications of a Novel Forward Osmosis Hollow Fibre Membrane, Desalination, 261 (2010)

365–372 .

164


[49] Y. Yu, S. Sunkeun, In-Chul Kim, and S. Lee, Nanoporous Polyethersulfone (PES)

Membrane with Enhanced Flux Applied in Forward Osmosis Process, Journal of Membrane

Science, 375 (2011) 63–68.

[50] A. Tiraferri, N.Y. Yip, W.A. Phillip, J.D. Schiffman, and M. Elimelech, Relating

Performance of Thin-film Composite Forward Osmosis Membranes to Support Layer

Formation and Structure, Journal of Membrane Science, 367 (2011) 340–352.

[51] W. Tan and H.Y. Ng, Concentration of Brine by Forward Osmosis: Performance and

Influence of Membrane Structure, Desalination, 224 (2008) 143–153.

[52] J.R. McCutcheon and M. Elimelech, Influence of Membrane Support Layer

Hydrophobicity on Water Flux in Osmotically Driven Membrane Processes, Journal of

Membrane Science, 318 (2008) 458–466.

[53] S. Zhang, K.Y. Wang, T.S. Chung, H. Chen, Y.C. Jean, and G. Amy, Well-constructed

Cellulose Acetate Membranes for Forward Osmosis: Minimized Internal Concentration

Polarization with an Ultra-thin Selective Layer, Journal of Membrane Science, 360 ( 2010)

522–535.

[54] M. Sairam, E. Sereewatthanawut, K. Li, A. Bismarck and A.G. Livingston, Method for the

Preparation of Cellulose Acetate Flat Sheet Composite Membranes for Forward Osmosis

Desalination Using MgSO4 Draw Solution, Desalination, 273 (2011) 299.

[55] J. Wei, C. Qiu, C.Y. Tang, R. Wang and A.G. Fane, Synthesis and Characterization of

Flat-sheet Thin Film Composite Forward Osmosis Membranes, Journal of Membrane Science,

372 (2011) 292–302.

[56] C. Qiu, S. Qi, and C.Y. Tang, Synthesis of High Flux Forward Osmosis Membranes by

Chemically Cross linked Layer-by-layer Polyelectrolytes, Journal of Membrane Science, 381

(2011) 74–80.

[57] K.Y. Wang, R.C. Ong, and T.S. Chung, Double-Skinned Forward Osmosis Membranes for

Reducing Internal Concentration Polarization within the Porous Sub layer, Ind. Eng. Chem.

Res. 49 (2010) 4824–4831

[58] C. Y. Tan, Q. She, W.C.L. Lay, R. Wang, R. Field, and A.G. Fane, Modelling Double-

skinned FO Membranes, Desalination, 283 (2011) 178–186.

165


[59] L. Setiawan, R. Wang, K. Li, and A.G. Fane, Fabrication of Novel Poly(amide–imide)

Forward Osmosis Hollow Fibre Membranes with a Positively Charged Nanofiltration-like

Selective Layer, Journal of Membrane Science 369 (2011) 196–205.

[60] Y. N. Yin, A. Tiraferri, W.A. Phillip, J.D. Schiffman, and M. Elimelech, High

Performance Thin-Film Composite Forward Osmosis Membrane, Environ. Sci. Technol. 44

(2010) 3812–3818.

[61] A. Achilli, Y. T. Cath, A. E. Marchand and A.E. Childress, The Forward Osmosis

Membrane Bioreactor: A Low Fouling Alternative to MBR Processes, Desalination, 239 (2009)

10–21.

[62] G. Gray, J.R. McCutcheon, and M. Elimelech, Internal Concentration Polarization in

Forward Osmosis: Role of Membrane Orientation, Desalination 197 (2006) 1–8.

[63] R.W. Holloway, A.E. Childress, K.E. Dennett, T.Y. Cath, Forward Osmosis for

concentration of anaerobic digester centrate, Water Res., 41 (2007) 4005–4014.

[64] T.Y. Cath, S. Gormly, E.G. Beaudry, M.T. Flynn , V.D. Adams and A.E. Childress,

Membrane contactor processes for wastewater reclamation in space Part-I, Direct osmotic

concentration as a pre-treatment for reverse osmosis, Journal of Membrane Science, 257 (2006)

85-98.

[65] J. E. Miller, L. R. Evans, Forward Osmosis: A New Approach to Water Purification and

Desalination, Sandia National Laboratories Report: Albuquerque, NM, 2006.

[66] G.D. Mehta and S. Loeb, Performance of Permasep B-9 and B-10 membranes in various

osmotic regions and at high osmotic pressures, Journal of Membrane Science, 4 (1979) 335–

349.

[67] S. Jincai, Q. Yang, J. F. Teo, and T.S. Chung, Cellulose Acetate Nanofiltration Hollow

Fibre Membranes for Forward Osmosis Processes, Journal of Membrane Science 355 (2010)

36–44.

[68] Mi. Baoxia, and M. Elimelech, Organic Fouling of Forward Osmosis Membranes: Fouling

Reversibility and Cleaning Without Chemical Reagents, Journal of Membrane Science, 348

(2010) 337–345.

[69] E.R. Cornelissen, D. Harmsen, K.F. de Korte, C.J. Ruiken, J.J. Qin, H. Oo, and L.P.

Wessels, Membrane Fouling and Process Performance of Forward Osmosis Membranes on

Activated Sludge, Journal of Membrane Science 319 (2008) 158–168.

166


[70] J. McCutcheon and M. Elimelech, Influence of concentrative and dilutive internal

concentration polarization on flux behaviour in Forward Osmosis, Journal of Membrane

Science, 284 (2006) 237–247.

[71] S. Lee, C. Boo, M. Elimelech and S.Hong, Comparison of Fouling Behavior in Forward

Osmosis (FO) and Reverse Osmosis (RO), Journal of Membrane Science 365 (2010) 34–39.

[72] B. Liberman and I. Liberman, Replacing membrane CIP by direct osmosis cleaning, Int.

Desalination & Water Reuse, 15 (2005) 28-32

[73] A.G. Fane, A.I. Schafer and T.D. Waite, Nanofiltration Principles and Applications,

Elsevier Science & Technology Books (2005) 67-88.

[74] C.Y. Tan, Q. She, W. C. L Lay, R. Wang, and A.G Fane, Coupled Effects of Internal

Concentration Polarization and Fouling on Flux Behavior of Forward Osmosis Membranes

During Humic Acid Filtration, Journal of Membrane Science, 354 (2010) 123–133.

[75] C.H. Tan and, H.Y. Ng, Modified models to predict flux behavior in Forward Osmosis in

consideration of external and internal concentration polarizations, Journal of membrane

science, 324 (2008) 209-219.

[76] Q. Ge, M. Ling and T.S. Chung, Draw solutions for Forward Osmosis process:

Developments, challenges and prospects for future, Journal of membrane science, 2013.

[77] Gas Encyclopaedia,

http://encyclopedia.airliquide.com/Encyclopedia.asp?LanguageID=11&CountryID=19 &

Formula=C2H4F2&GasID=0&UNNumber=& btnFormula.x =8&btnFormula.y=11

[78] Van’t Hoff, The role of osmotic pressure in the analogy between solutions and gases,

Journal of membrane, 100 (1995) 39-44.

[79]AkzoNobel, http://www.akzonobel.com/ic/products/dimethyl_ether/product_specification/

[80] S. Tallon and K. Fenton, The solubility of water in mixtures of dimethyl ether and carbon

dioxide, Fluid phase Equilibria, 298 (2010) 60-66.

[81] M.M. Mench, H.M. Chance and C.Y. Wang, Direct Dimethyl Ether polymer electrolyte

fuel cells for portable applications, Journal of electrochemical society, 151 (2004) 144-150.

[82] J. C. Kotz, P. M. Treichel and J. R. Townsend, Chemistry and Chemical Reactivity, 7th

Edition, Cengage Learning, 2009 – page 563.

167

http://encyclopedia.airliquide.com/Encyclopedia.asp?LanguageID=11&CountryID=19%20&%20Formula=C2H4F2&GasID=0&UNNumber=&btnFormula.x=8&btnFormula.y=11

http://encyclopedia.airliquide.com/Encyclopedia.asp?LanguageID=11&CountryID=19%20&%20Formula=C2H4F2&GasID=0&UNNumber=&btnFormula.x=8&btnFormula.y=11


[83] H. Kanda, Super-energy-saving dewatering method for high- specific- surface-area fuels

by Dimethyl ether, Adsorption science & technology, Oxford Blackwell scientific, 1984.

[84] K. Oshita, K. Takeda, M. Takaoka, H. Kanda, S. Morisawa, H. Makino and N. Takeda,

Dewatering of electroplating sludge using Dimethyl ether, Doboku Gakkai Ronbunshuu 66

(2010) 96-102.

[85] K. Oshita, M. Takaoka, H. Kanda, S. Morisawa, H. Makino, T. Matsumoto and N. Takeda,

Extraction of PCBs and water from river sediment using liquefied Dimethyl ether as an

extractant, Chemosphere 78 (2010) 1148-1154.

[86] H. Kanda, P. Li and H. Makino, Dewatering and Deoiling Technology, 7th Asian DME

Conference, Japan, 2011.

[87] J. Van’t Hoff, The function of osmotic pressure in the analogy between solutions and

gasses, Philosophical magazine and journal of science, 26 (1888) 81-105,

http://iopscience.iop/1478-7814/9/1/344

[88] A.D. Wilson and F. F. Stewart, Deriving osmotic pressures of draw solutes used in

osmotically driven membrane processes, Journal of membrane science, 431 (2013) 205-211.

[89] A. Chapoy, R. Burgass, B. Tohidi, C. Baudry and A. J. Defossez, Potential DME storage

in underground caverns: investigation of the phase behaviour of the DME-water system at low

temperature, Processing of the 7th international conference on gas hydrates (ICGH 2011)

Edinburgh, Scotland, United Kingdom, 2011.

[90] H. Holldroff and H. Knapp, Binary vapor-liquid- liquid equilibrium of Dimethyl ether-

water and mutual solubility of Methyl chloride and water, Fluid phase equilibrium, 44(1988)

195-209.

[91] S. L. Miller, S. R. Gough and D.W. Davidson, Two clathrate of Dimethyl Ether, The

journal of physical chemistry, 81 (1977) 2154- 2157.

[92] Type of Osmometers, Advanced Instruments, Inc.,

http://www.aicompanies.com/index.cfm/AIUniversity/OsmolarityExplained

[93] T. Engel and P. Reid, Physical Chemistry, Pearson Benjamin Cummings, 2006, P. 205-5.

[94] J. M. Prausnitz, Molecular Thermodynamics of Fluid-Phase Equilibria, - University of

California, Berkeley; Rüdiger N. Lichtenthaler - University of Heidelberg; Edmundo Gomes de

Azevedo - Institute Superior Técnico, Lisbon, Third Edition, 1998,P: 258-292.

168


[95] G. Guevara-Carrion, J. Vrabec and H. Hasse, On the prediction of transport properties of

Monomethylamine, dimethyl amine, dimethyl ether and hydrogen chloride by molecular

simulation, Fluid phase Equilibria, 316 (2012) 46-54.

[96] S.B. Wang, A. H. T. Li and S. D. Chao, Liquid properties of Dimethyl ether from

molecular dynamics simulations using An Initio force fields, Journal of Computational

chemistry, 33 (2012) 998-1003.

[97] M. Laliberte, Model for calculating the viscosity of aqueous solutions, J. Chem. Eng. Data,

52(2007) 321-335

[98] C.R. Wilke and P. Chang, Correlation of diffusion coefficients in dilute solutions,

A.I.Ch.E. Journal, 1 (1955) 264-270.

[99] E. L. Cussler, Diffusion: mass transfer in fluid systems, Cambridge University Press,

1984.

[100] M.E. Pozo and W. B. Streett, Fluid phase Equilibria for the system Dimethyl ether/water

from 50 to 220°C and pressures to 50.9 MPa, J. Chem. Eng. Data, 29 (1984) 324-329.

[101] K.L. Lee, R.W. Baker and H.K. Lonsdale, Membrane for power generation by pressure-

retarded osmosis, Journal of membrane science, 8 (1981) 141-171.

[102] J.R. McCutcheon and M. Elimelech, Modelling water flux in Forward Osmosis:

Implications for improved membrane design, Journal of membrane science, 284 (2006) 237-

247.

[103] C.H. Tan and H.Y. Ng, Revised and internal concentration polarization models to

improve flux prediction in Forward Osmosis process, Desalination, 309 (2013) 125-140.

[104] D. Xiao, W. Li, S. Chou, R. Wang and C. Tan, A modelling investigation on optimizing

the design of forwards osmosis hollow Fibre modules, Journal of membrane science, 392

(2012) 76-87.

[105] W. Li, Y. Gao and C. Y. Tan, Network modelling for studying the effect of support

structure on internal concentration polarization during Forward Osmosis: Model development

and theoretical analysis with FEM, Journal of membrane science, 397 (2011) 307-321.

[106] W.A. Phillip, J.S. Yong and M. Elimelech, Reverse draw solute permeation in Forward

Osmosis: Modelling and experiments, Environment science technology, 44 (2010) 5170-5176.

169


[107] J.S. Young, W.A. Phillip and M. Elimelech, Coupled draw solute permeation and water

flux in Forward Osmosis with natural draw solutes, Journal of membrane science, 392 (2012)

9-17.

[108] C. Suh and S. Lee, Modelling reverse draw solute flux in Forward Osmosis with external

concentration polarization in both sides of the draw and feed solution, Journal of membrane

science, 427 (2013) 365-374.

[109] A. Sagiv and R. Semiat, Finite element analysis of Forward Osmosis process using NaCl

solutions, Journal of membrane science, 379 (2011) 86-96.

[110] M.F. Gruber, C.J. Johnson, C.Y. Tan, M.H. Jensen, L. Yde and C.H. Nielsen,

Computational fluid dynamics simulations of flow and concentration polarization in Forward

Osmosis membrane systems, Journal of membrane science, 397 (2011) 488-495.

[111] S. De and P.K. Bhattacharya, Prediction of mass transfer coefficient with suction in the

applications of reverse osmosis and ultra filtration, Journal of membrane science, 128 (1997)

119- 131

[112] F. Lipnizki and R.W. Field, Mass transfer performance for hollow Fibre modules with

shell-side axial feed flow: using an engineering approach to develop a framework, Journal of

membrane science, 193 (2001) 195-208.

[113] V. Gekas and B. Hallstrom, Mass transfer in the membrane concentration polarization

layer under turbulent cross flow Critical literature review and adaptation of existing Sherwood

correlations to membrane operations, Journal of membrane science, 80 (1987) 153-170.

[114] H. Kanda and H. Makino, Energy-efficient coal de-watering using liquefied Dimethyl

ether, Fuel, 89 (2010) 2104-2109.

[115] P.C. Wankat, Separation Process Engineering,

http://webpages.sdsmt.edu/~ddixon/CBE_417_FlashDrumSizing_Wankat.pdf

[116] R. Smith, Chemical design and Integration, 2005, John Wiley & Sons Ltd

[117] O.J. Morin, Design and operating comparison of MSF and MED systems, Desalination,

93 (1993) 69-109

[118] S.A Avlonitis, K. Kouroumbas and N. Vlachakis, Energy consumption and membrane

replacement cost for seawater RO desalination plants, Desalination, 157 (2003) 151-158.

170


[119] R.L. McGinnis and M. Elimelech, Energy requirements of ammonia-carbon dioxide

Forward Osmosis desalination, Desalination, 207 (2007) 370-382

[120] D.L. Shaffer, N.Y. Yip, J. Gilron and M. Elimelech, Seawater desalination for agriculture

by integrated forward and reverse osmosis: Improved product water quality for potentially less

energy, Journal of membrane science, 415 (2012) 1-8.

[121] R.L. McGinnis, N.T. Hancock, M.S. N. Slepowron and G.D. McGurgan, Pilot

demonstration of the NH3/CO2 Forward Osmosis desalination process on high salinity brines,

Desalination, 312 (2013) 67-74.

[122] M.Wilf and K. Klinko, Optimisation of seawater RO system design, Hydranautics,

http://www.membranes.com/docs/papers/06_optimization.pdf

[123] Monomethylamine material safety data sheet (MSDS), Airgas,

http://www.airgas.com/documents/pdf/001034.pdf.

[124] G. Dahlhoff, A. Pfenning, H. Hammer, M. Van Oorschot, Vapor-liquid Equilibria in

quaternary mixtures of Dimethyl ether+ n-butane + ethanol+ water, J.Chem. Eng. Data, 45

(2000) 887-892.

[125] M. Konno, Personal Communication, 6 Feb 2009.

[126] O. Catchpole, S. Tallon, J. Grey, K. Fenton, K. Fletcher, A. Fletcher, Extraction of lipids

from aqueous protein-rich streams using near-critical dimethyl ether, Chem. Eng. Technol, 30

(2007) 501-510.

[127] S.J. Park, K.J. Han, J. Gmehling, Isothermal phase Equilibria and excess molar enthalpies

for binary systems with dimethyl ether at 323.15 K, J. Chem. Eng. Data, 52 (2007) 1814-1818.

[128] T. Laursen, P. Rasmussen, S. Andersen, VLE and VLLE measurements of dimethyl ether

containing systems, J. Chem. Eng. Data, 47 (2002) 198-202.

[129] P.K. Naicker, S.I. Sandler, S. Reifsnyder, Measurement of the liquid-liquid Equilibria for

mixtures of water + sodium hydroxide + an alkanol or dimethyl ether using near-infrared

spectroscopy, J. Chem. Eng. Data, 47 (2002) 191-194.

[130] R.W. Baker, Membrane Technology and Applications, 2004, West Sussex, John Wiley &

Sons Ltd

[131] C. Fritzmann, J. Löwenberg, T. Wintgens and T. Melin, State-of-the-art of reverse

osmosis desalination, Desalination, 216 (2007) 1–76

171

http://www.airgas.com/documents/pdf/001034.pdf

http://www.membranes.com/docs/papers/06_optimization.pdf


APPENDIXES

Appendix A: Table A-1 Solubility of selected gases compounds in water [77]

NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

1 Air Air 29 -191.4 25 1 0.02

2 Argon Ar 40 -185.86 25 1 0.06

3 Arsine AsH3 78 -62.48 20 13 1

4Boron

TrichlorideBCl3 117 12.5 - 1

Hydrolysed

Immediately

5Boron

TrifluorideBF3 67.8 -100.3 0 1 5

6 Diborane B2H6 27.67 -92.5

Decomposes

at room

temperature

7Bromine

TrifluorideBrF3

136.89

9125.75

Decomposes

at room

temperature

8Bromine

PentafluorideBrF5

174.89

640.5

Decomposes

at room

temperature

9Hydrogen

BromideBrH 80.9 -66.72

High affinity

for water

10 Bromine Br2 159.8 58.75

It is liquid

at room

temperature

11

Bromo

Chlorodifluoro

methane(R12B1)

CBrClF2 165.38 -4

It is very

slightly

soluble

in water.

12 Bromo CBrF3 148.9 -57.75 25 1 0.02

172


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

Trifluoro

methane (R13B1)

13

Dibromo

Difluoro

methane (R12B2)

CBr2F2 209.8 22.79

It is liquid

at room

temperature

& soluble in

Alcohol,

ether.

14

Chloro

Trifluoro

methane (R13)

CClF3 104.47 -81.5 25 1 0.09

15Cyanogen

ChlorideCClN 61.49 12.95

It is very

Slightly

soluble

in water.

16

Dichloro

Difluoro

methane (R12)

CCl2F2 120.93 -29.78 25 1 0.27

17Carbonyl

ChlorideCCl2O 98.9 7.55

Decomposes

in water to

give HCl and

CO2

18

Trichloro

Fluoro

methane (R11)

CCl3F 137.38 23.77

It is liquid

at

atmospheric

conditions

19Carbonyl

FluorideCF2O 66 -83.1

Decomposes

rapidly in

water

20Tetrafluoro

methane (R14)CF4 88 32.4 25 1 0.01

21 Chloro CHClF2 86.48 -40.78 25 1 0.96

173


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

Difluoro

methane (R22)

22Dichloro

Fluoro

methane (R21)

CHCl2F 102.92 8.9 25 1 7.89

23Trifluoro

methane (R23)CHF3 70 -82.1 25 1 1.46

24Hydrogen

CyanideCHN

It is liquid

at room

temperature

25Difluoro

methane (R32)CH2F2

It is soluble

in ethyl

alcohol.

26Methyl

bromide (R40B1)CH3Br 94.94 3.56 25 1 12.84

27Methyl

Chloride (R40)CH3Cl 50.488 -23.76 15 1 8.08

28Methyl

fluorideCH3F 34.033 -78.41 25 1 2.30

29 Methane CH4 16 -161.52 20 1 0.06

30Methyl

mercaptanCH4S 48.1 5.96 25 1 12.87

31Monomethyl

amineCH5N 31.057 -6.33 20 1 1080.00

32Carbon

monoxideCO 28 -191.53 25 1 0.09

33Carbonyl

sulphideCOS 60 -50.23 25 1 1.50

34Carbon

dioxideCO2 44 -57 25 1 2.09

35 Bromo

Trifluoro

C2BrF3 160.92 -2.5 Solubility is

not Available

174


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

ethylene

36

Chloro

Trifluoro

ethylene (R1113)

C2ClF3 116.47 -28.36Solubility is

not Available

37

Chloro

Pentafluoro

ethane (R115)

C2ClF5 154.48 -38 25 1 0.06

38

Dichloro

Tetrafluoro

ethane (R114)

C2Cl2F4 170.93 3.6 25 1 0.13

39Tetrafluoro

ethyleneC2F4 100 -75.62

Solubility is

not Available

40Hexafluoro

ethane (R116)C2F6 138.02 -78.2

It is very

slightly

soluble in

water.

41

Chloro

Difluoro

ethylene (R1122)

C2HClF2 98.48 -18.6Solubility is

not Available

42 Acetylene C2H2 26 -83 25 1 1.61

43Difluoro

ethylene (R1132a)C2H2F2 64 -84 25 1 0.14

44Vinyl

bromideC2H3Br 106.95 15.7

It is very

slightly

soluble in

water.

45Vinyl

chlorideC2H3Cl 62.499 -13.7

It is very

slightly

soluble in

water.

46 Chloro

Difluoro

C2H3ClF2 100.49

5

-9.8 20 1 4.68

175


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

ethane (R142b)

47Vinyl

fluorideC2H3F 46.044 -72.2

It is soluble

in ethyl

alcohol.

48Trifluoro

ethane (R143)C2H3F3 84 -47.6

It is soluble

in ethyl &

chlorine.

49Ethylene

C2H4 28 -103 25 1 0.23

50Difluoro

ethane (R 152a)C2H4F2 66.05 -25 20 1 1.02

51Ethylene

oxideC2H4O 44.053 10.45

Solubility is

not Available

52Ethyl

chloride (R160)C2H5Cl 64.514 12.28 20 1 4.38

53Ethyl

fluoride (R161)C2H5F 48.06 -37.1

Solubility is

not Available

54 Ethane C2H6 30 -88.68 20 1 0.10

55Dimethyl

ether (DME)C2H6O 46.069 -25.1 24 4 340.00

56Dimethyl

telluriumC2H6Te 157.67 355

It is liquid

at room

temperature

57Dimethyl

amineC2H7N 45.084 7.4

It forms an

alkaline

solution.

58Monomethyl

amineC2H7N 45.084 16.6

It forms an

alkaline

solution.

59 Cyanogen C2N2 52.035 -21.15 30 1 17.52

60 Hexa

Fluor propylene

C3F6 150.02

3

-29.6 Solubility is

not Available

176


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

(perfluoropropene

)

61

Hexa

Fluoro

acetone

C3F6O166.02

3-27.5

It forms a

hydrate

which

exhibit

unusual

solvent

properties.

62

Ocafluoro

PropaneC3F8 188 -36.7

Solubility is

not Available

63

Methyl

acetylene

(allylene)

C3H4 40 -23.21 25 1 3.60

64Propadiene

(allene)40 -34.4

65 Propylene C3H6 42.08 -47.72 20 1 0.42

66 Cyclopropane 42.08 -32.8

It is soluble

in alcohol

and ether.

67Methyl

vinyl etherC3H6O 58.08 6 20 1 9.42

68 Propane C3H8 44 -42.045 20 1 0.07

69Tri methyl

aluminiumC3H9Al 72 127.12

It reacts

explosively

on contact

with water.

70 Trimethylgallium C3H9Ga114.82

555.8

It is liquid

at room

temperature

177


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

71Trimethyl

amineC3H9N 59 2.87

It forms an

alkaline

solution.

72Trimethyl

stibineC3H9Sb 166.85 80.6

It is liquid

at room

temperature

73Perfluoro

buteneC4F8 200 1.2

Solubility is

not Available

74

Ocafluoro

Cyclobutane

(RC318)

C4F8 200 -5.99 20 1 0.16

75

Perfluorobutane

(R610)C4F10 238 -1.7

Solubility is

not Available

76 Butadiene C4H6 54.091 -4.5

It is slightly

soluble in

water.

77 Ethyl acetylene C4H6 54.091 8.08 25 1 3.68

78 Butene C4H8 56.107 -6.3 20 1 0.23

79 Cis-Butene C4H8 56.107 3.7

It is soluble

in organic

solvent.

80 Trans- Butene C4H8 56.107 0.88

It is soluble

in organic

solvent.

81 Methylpropene C4H8 56.107 -7.2 20 1 0.44

82 Cyclobutane C4H8 56.107 12.51It is a good

solvent.

83 Butane C4H10 58 -0.5 20 1 0.09

84 Methylpropene C4H10 58 -11.7 20 12.59

178


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

85 Diethyl zinc C4H10Zn 123.5 117.8

It is soluble

in organic

solvent.

86 Nickel carbonyl C4NiO4 170.75 43

It is liquid

at room

temperature

87 Iron carbonyl C5FeO5 195.9 105

It is

insoluble in

water.

88 Methyl butene C5H10 70 20.06Solubility is

not Available

89 Dimethyl propane C5H12 72 9.5

It is soluble

in alcohol

and ether.

90Triethylaluminiu

mC6H15Al 114 186.6

It reacts

explosively

on contact

with water.

91

Triethyl

aluminium

sescuichlorure

C6H15Al2Cl

3247.5 204

It is liquid

at room

temperature

92 Trimethylgallium C6H15Ga 156.9 142.6

It reacts

violently

with water.

93Triisobutyl

aluminiumC12H27Al 198.3 212.4

It reacts

explosively

on contact

with water.

94Chlorine

TrifluorideClF3 92.448 11.75

Solubility is

not Available

95 Chlorine

Pentafluoride

ClF5 130.55 -13.1 It is rapidly

hydrolysed

179


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

in

contact with

water

to produce

chlorine

and

hydroflourid

e acid.

96Hydrogen

chlorideClH 36 -85.1

It has a great

affinity for

water.

97 Chlorine Cl2 70.906 -34.1 20 1 8.38

98 Dichlorosilane Cl2H2Si 101 8.4

It is soluble

in benzene

and ether.

99Phosphorus

trichlorideCl3P 137 74.2

It is liquid

at room

temperature

100

Normal

Deuterium D2 4 -249.58 0 1 0.05

101Hydrogen

fluorideFH 20 19.51

It forms

hydrates at

low

temperature

102 Fluoride F2 37.997 -188.2

It

decomposes

violently

water.

103Oxygen

difluorideF2O 53.996 -145.3 20 1 81.70

180


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

104Sulphuryl

fluorideF2O2S 102 -55 0 1 1.57

105Nitrogen

trifluorideF3N 71 -129 20 1 0.06

106Nitrogen

tetra fluorideF4N2 104 -73

Reacts

slowly with

water

107Sulphur

tetra fluorideF4S 108 -40.4

Is

Hydrolysed

in water

108Iodine

PentafluorideF5I

221.89

6104.48

It is liquid

at room

temperature

109Phosphorus

PentafluorideF5P 125.96 -84.6

It reacts with

water.

110Molybdenum

hexafluorideF6Mo 209.93 35

Is

Hydrolysed

in water

111Sulphur

hexafluorideF6S 146 -63.9 25 1 0.03

112Tellurium

hexafluorideF6Te 241.59 -38.9

Is

Hydrolysed

in water

113Tungsten

hexafluorideF6W 297.84 17.06

Is rapidly

Hydrolysed

in water

114 Germane GeH4 76.62 -88.5Solubility is

not Available

115 Hydrogen

Iodide

HI 127.91 -35.4 It is

dissolved in

water to

form 3

181


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

hydrates.

116Normal

hydrogenH2 2

-

252.76

6

25 1 0.02

117 Para-hydrogen 2

-

252.76

6

118Hydrogen

sulphideH2S 34.08 -60.2 20 1 4.98

119

Hydrogen

Selenide

(selenious

hydrogen)

H2Se 80.97 -41.4 20 1 1.86

120Hydrogen

tellurideH2Te 129.61 -1.3

Rapidly

reacts with

water

121 Ammonia H3N 17 -33.41 20 1 454.94

122 Phosphine H3P 33.997 -87.77 20 1 4.64

123 Monosilane H4Si 32 -111.4Insoluble in

water

124 Disilane H6Si2 62.22 -14.3

Does not

react with

pure water

125 Helium 4 He 4

-

268.92

6

20 1 0.15

126 Krypton Kr 83.3 -153.36 20 1 0.20

127 Nitric oxide NO 30 -151.75 20 1 0.06

128 Nitrogen N2 28 -195.8 25 1 0.07

129 Nitrous oxide N2O 44 -88.47 25 1 1.77

130 Nitrogen

trioxide

N2O3 76 3.5 Reacts with

water and

182


NO

.Name Formula

MW

(g/mole)

B.P

(°C)

Temperature

(°C)

Osmotic

Pressur

e

(bar)

Solubility

(g/l)

decomposes

subsequently

.

131Nitrogen

dioxideN2O4 21

Reacts with

water and

decomposes

subsequently

.

132 Neon Ne 20 -246.05 20 1 0.09

133 Oxygen O2 32 -182.97 25 1 0.13

134 Sulfur dioxide O2S 64 -10 25 1 99.98

135 Ozone O3 48 -111.3 25 1 0.36

136 Sulfur trioxide O3S 80 44.8

It is liquid at

room

temperature

137 Xenon Xe 131.3 -108 20 1 1.06

183


Appendix B- DME solubility in water versus pressure [79]

184


185


186

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