Ettouney 2004 Paper

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Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation

between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European

Desalination Society and Office National de lEau Potable, Marrakech, Morocco, 30 May2 June, 2004.

0011-9164/04/$ See front matter 2004 Elsevier B.V. All rights reserved

Desalination 165 (2004) 393408

Visual basic computer package for thermal and membranedesalination processes

Hisham EttouneyDepartment of Chemical Engineering, College of Engineering and Petroleum, Kuwait University,

PO Box 5969, Safat 13060, Kuwait

Fax: +965 483-9498; email: [email protected]

Received 23 February 2004; accepted 3 March 2004

Abstract

A visual basic computer package was developed for the design and analysis of thermal and membrane desalinationprocesses. The package includes conventional processes, i.e., reverse osmosis, single-effect mechanical vaporcompression, multiple-effect evaporation with/without thermal or mechanical vapor compression, and multi-stage flashevaporation. The models for these systems provide detailed design data that include flow rates, stream salinity,temperatures, heat transfer or membrane area, ejector dimensions, and bundle dimensions. The model predictions are

based on detailed energy and material balances and well tested correlations for the heat transfer coefficient, thermo-dynamic losses, and physical properties of the seawater and water vapor. The visual basic interface provides displaysfor profiles of system variables across the effects, stages, or membrane modules, which may include salinity, flow rates,etc. Also, displays for the process flow diagram and design results are generated simultaneously. The design resultsinclude the unit product cost, process capital, performance ratio or specific power consumption, the flow rate of coolingwater, the heat transfer or the membrane area, and a number of thermodynamic losses.

Keywords: Desalination; Modeling; Computer simulation; Thermal desalination; Membrane separation; Economics

1. Introduction

In arid and semi-arid regions around the globe,the desalination industry has proved to be the

most viable solution to provide a sustainable

source for water. This is because natural sources

of fresh water are not evenly distributed. It is

common to have massive annual floods in oneregion and prolonged draughts in others. At

present more than half the worlds population is

experiencing water shortages. Moreover, many


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H. Ettouney / Desalination 165 (2004) 393408394

conventional water resources have limited capa-city and are difficult and expensive to expand.

This is manifested in the cost of water trans-

portation over long distances, destructive effects

of water dams on the delicate environment up and

down river bodies, and limited resources of

ground water [1].

Desalination processes have developed rapidly

since its start where the unit production capacity

was limited to 100 m3/d. Also, all processes were

thermally based and had a submerged tube con-

figuration. Subsequently, new and more efficientprocesses emerged, especially the multistage flash

(MSF) evaporation, single- and multiple-effect

evaporation (MEE), and reverse osmosis (RO)

membrane desalination. The size of these units

increased rapidly to reach much larger capacity,

close to 100,000 m3/d [2]. In many arid regions

around the world, desalination of sea and brack-

ish water is the main source of fresh water for

urban and industrial applications. Examples

include the Gulf States, several Mediterranean

and Caribbean Islands, Spain, and southern Italy.

In the US desalination of low-salinity water using

RO is found on a very large scale to produce

high-purity water for boiler houses and the

electronics industry.

The desalination industry is expanding con-

stantly. In this regard, large producers areincreasing their desalination capacity to meet theever-increasing demands. Several new countries

are also adopting desalination as the mostpractical solution for water shortages. During theperiod 19962002, the desalination capacity in

Spain has doubled. As a result, Spain became theleading producer in Europe with more than a 30%share of the installed desalination capacity on the

continent. Currently, the desalination capacity inSpain is approaching 1.5106m3/d [3,4]. Anotherexample is found in Saudi Arabia, which

currently stands at 5106 m3/d and is expected todouble this capacity by the year 2020. Similarscenarios are also found in Oman, Kuwait, andUnited Arab Emirates [5].

Modeling, simulation, and costing of thermaland membrane desalination processes are essen-

tial for better understanding, efficient and

accurate process design, troubleshooting of

operational difficulties, performance analysis,

process control, and cost estimation. Several

studies in the literature can be cited for modeling

of various thermal and membrane desalination

processes. A summary of most of these studies

can be found in El-Dessouky and Ettouney [5].

On the other hand, attempts to develop a simu-

lation package have ben rather limited. Simu-lating desalination plants using commercial

packages for a simulation of chemical process

plants is rather difficult and might not produce

accurate results. This is because of a lack of

models for thermodynamic losses or specific heat

transfer functions for seawater.

Attempts to develop simulation packages forthermal and membrane desalination processes

include the studies by Ettouney et al. [6], Herreroet al. [7], and Jernqvist et al. [8]. The simulator byJernqvist et al. [8] is modular and includes basic

modules forming thermal desalination processesincluding vapor compressors, evaporators, con-densers, and preheaters. The simulator also

includes a database for physical properties ofwater as a function of temperature and salinity.Other features include a specialized correlation

for the heat transfer coefficient on different sur-faces as well as thermodynamic losses and atemperature drop caused by demisters, trans-

mission lines, and condensers.The simulator by Uche et al. [7] focused on

the design and analysis of dual-purpose powerand desalination plants. The developed software

allows for the graphical design of plant layout,

calculation of the heat and mass balances,

thermo-economic analysis, and a parametric

analysis. This software remains under develop-

ment by Uche et al. [8].

The DEEP economic simulator focuses on

evaluation of the unit product cost of MSF, MEE,

or RO combined with various types of co-


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H. Ettouney / Desalination 165 (2004) 393408 395

generation power plants, which include nuclear aswell as fossil fuel [9]. The DEEP simulator does

not perform simulation of the desalination plant;

instead, it utilizes input data provided by the user

such as the performance ratio, capacity, and other

parameters for the power plant to determine the

required process capital, unit product cost, and

other economic parameters.

The simulator presented in this study was

previously developed by Ettouney et al. [6]. The

previous simulator focused on an analysis of a

conventional thermal desalination process inaddition to a number of novel configuration, i.e.,

absorption and adsorption single-effect evapo-

ration, MSF with vapor compression. The

development presented in this study focuses on

conventional thermal and membrane desalination

processes. The new additions to the simulator

include calculations of the unit product cost,

number of tubes and evaporator dimensions, and

detailed design of the steam jet ejector. Also, new

displays are added to the simulator, including

system profiles, cost analysis, performance results

and a flow chart.

Another important addition to the simulator

presented in this study is the design and analysis

of the RO process. Several RO simulators are

available for downloading from sites of RO

manufacturing companies. These commercial

simulators focus on specific system design, which

is based on the membranes produced by the

manufacturing company. The RO simulator

developed in the package presented here is more

general and allows the user to define the mem-

brane characteristics, i.e., salt rejection andrecovery. In addition, cost calculations determine

the unit product cost and required capital.

The following sections include a description

of conventional thermal and membrane desali-

nation processes, features of the simulation pack-

age, a case study of the MEE process, a model of

the MEE process, cost analysis model, and

package results.

2. Conventional desalination process

Conventional thermal and membrane desali-

nation processes (Fig. 1) include the following:

C single-effect mechanical vapor compression

(MVC)

C multiple-effect evaporation with/without ther-

mal vapor compression (MEE and MEE-TVC)

C multiple-effect evaporation with/without

mechanical vapor compression (MEE and

MEE-MVC)

C once-through multi-stage flashing (MSF-OT)

C brine circulation multi-stage flashing (MSF)C reverse osmosis (RO) with options for single

stage, two stages, and two passes.

The thermal desalination processes (MVC,

MEE, MEE-MVC, MEE-TVC, and MSF-OT, and

MSF) operate only on seawater feed. On the other

hand, the RO process operates on low-salinity

river water, brackish water, and seawater. Market

share among these processes differs considerably.

As for seawater desalination, the MSF process

accounts for more than 60%, while the RO

market share may exceed 30%; other thermal

desalination process accounts for less than 10%.

These shares differ when desalination of low-

salinity and brackish water is taken into con-

sideration. In this case, the RO market share is

almost identical to the MSF process, and both

processes account for a total of 94%, while other

thermal desalination processes account for 6%

[10].

Schematics of the four conventional desali-

nation processes are shown in Fig. 2, which

include MVC (a), MEE (b), MSF (c), and RO (d).The MVC process is characterized by being

driven solely by electric current, which is used to

drive the mechanical vapor compressor. Start-up

of the MVC process requires use of an external

heating source, i.e., heating steam. The MVC pro-

cess presents a viable choice for water desali-

nation in remote areas and for small populations.

The system capacity remains limited below


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Fig. 1. Conventional thermal and membrane desalination processes.

5000 m3/d for single-effect configurations. The

system is operated at low temperatures, below

70C, and is characterized by low specific power

consumption, which may range between 5

8 kWh/m3 [11]. However, actual field data report

a much higher specific power consumption of

14 kWh/m3 [12]. Further details of the MVC

system, field data, and model can be found in thestudy by Ettouney et al. [13].

The MEE system, which is shown in Fig. 2b,

includes three main configurations. The first is

MEE without vapor compression. The second and

third are those for thermal or mechanical vapor

compression. The stand-alone system, evapo-

ration is driven in the first effect by low-

temperature heating steam. This results in

formation of a smaller amount of vapor, which is

used to drive evaporation in the second effect.

This process continues throughout all subsequenteffects, which may vary from two effects up to

12. The vapor formed in the last effect is then

condensed in the down condenser against the feed

and cooling seawater stream. System operation in

a stand-alone mode provides a performance ratio

of 8 for a 12-effect system. Operation in the

thermal vapor compression mode increases the

performance ratio to a range of 1416. As for the

MEE system combined with mechanical vapor

compression, its specific power consumption

remains the same as single-effect mechanical

vapor compression where it will vary over a

range of 58 kWh/m3. Use of the MEE-MVC

system is thought to increase the production capa-

city of the system rather than to reduce the

specific power consumption [14].

The MSF system shown in Fig. 2c is the work-horse of seawater desalination and, in particular,

the desalination industry in the Gulf States. The

MSF process dates back to the 1950s. Since then

the process has progressed considerably and a

large amount of field experience has been accu-

mulated in system design, construction, commis-

sioning, operation, maintenance, and cleaning.

Currently, MSF operation has been continuous

for periods varying from 2 to 5 years. This is

achieved in part by developments in antiscalents,

adoption of an on-line ball cleaning system,frequent acid cleaning, and progress in material

selection [15].

The RO process accounts for more than 45%

of the entire desalination market, which includes

low-salinity river water, brackish, and seawater.

The RO process requires an increase of the feed

pressure to 60 bars for the case of seawater

desalination; however, desalination of low-

salinity river water requires operation at much


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Fig. 2. Schematics of conventional desalination processes. (a) MVC, (b) MEE, (c) MSF, and (d) RO.


6/16


lower feed pressures that may not exceed 10 bars.The RO process requires extensive feed pretreat-

ment, which is necessary to prevent scaling or

fouling of the membrane surface. Failure to

operate the feed pretreatment process properly

may have adverse effects on the membrane. This

might result in an increase of the operating cost

expressed in terms of down-time as well as

increased frequency of membrane replacement

and cleaning. Large-scale RO seawater desali-

nation plants are becoming more visible.

Examples include the plants: Trinidad, with acapacity of 135,000 m3/d [16]; Cyprus, with a

capacity of 45,000 m3/d [16]; and Florida, USA,

with a capacity of 94,625 m3/d [2]. Adoption of

such large capacities is thought to reduce the unit

product cost, which is currently reported at a

range of $0.5/m3.

3. Features of the computer package

Development of a comprehensive computer

package seeks ease of use, flexibility, and accu-racy of the results. Features of the visual basic

desalination computer package include the

following:

C Ability to design and perform cost estimate for

conventional desalination processes including

single-effect mechanical VC, MEE with/with-

out VC, MSF and RO.

C Ability to select and adjust the design and cost

parameters used in the calculations.

C The computer codes check and limit the value

of input parameters within practical ranges.C Several displays are used to present the design

and cost results. These displays include per-

formance results, profiles, flow diagram, and

cost results.

C Availability of help and tutorial files.

C Capability to print forms and results data file.

C Capability for handling of various errors.

The process selection feature includes six

choices: MVC; MEE and MEETVC; MEE and

MEE-MVC; MSF-OT; MSF; and RO with op-tions for single stage, two stages, and two passes.

Fig. 3 shows a flow diagram of the computer

package, which includes help files, process selec-

tion, adjustment of input design data, calcula-

tions, view of various displays, and print of forms

or output results.

4. Multiple effect evaporation: a case study

The following case study illustrates the mainfeatures of the simulation package through the

analysis of the MEE system. The illustration

includes model assumption, model equations,

process economics, and results of the simulation

package. Details of other processes will be

presented in subsequent publications.

4.1. Mathematical model

The MEE mathematical model includes the

following assumptions and features:

C Steady-state operation, which is valid for theentire operating regime except for start-up,

shut-down, or change of the operating con-

ditions to a new set. The latter condition is

caused by variations in the production capa-

city as dictated by product demand.

C There is no temperature gradient within

various phases in each effect. Irrespective of

this, temperature differences between vapor

and liquid, which are caused by boiling point

elevation, non-equilibrium allowance, and

other thermodynamic loss, are included in themodel.

C All physical properties of the vapor stream are

evaluated as a function of the stream temp-

erature. Also, all physical properties of the

liquid stream are evaluated as a function of

water temperature and salinity.

C The heat transfer coefficients for seawater

flowing inside the tubes, the falling film of

seawater on the outside surface of the evapo-


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Fig. 3. Elements of computer package.

rator tubes, or the condensing vapor inside or

outside the tubes are obtained from well testedcorrelations. The correlations depend on the

stream salinity and temperature as well as the

stream physical properties, which include

specific heat at constant pressure, viscosity,

thermal conductivity, and density.

C The heat transfer area is the same in all

effects. This is standard practice in the indus-

try, which reduces the cost of spare parts,

initial construction, and maintenance.

The mathematical model of the MEE system

includes the following set of system variables:C Brine flow rates in effects 1 through n, which

are defined byB1,B2, ,Bn!1,Bn. This gives

n unknowns.

C Feed flow rates in effects 1 through n, which

are definedF1,F2, ,Fn!1,Fn. This gives n

unknowns.

C Distillate flow rate due to brine evaporation in

effects 1 through n, which are defined byD1,

D2, ,Dn!1,Dn. This gives n unknowns.

C Distillate flow rate due to brine flashing in

effects 2 through n, which are defined by d2,d3, .. , dn!1, dn. This gives n!1 unknowns.

C Distillate flow rate due to distillate flashing in

effects 2 through n, which are defined by

. This gives n!1 unknowns.

C Temperature of evaporating brine in effects 1

through n!1, which are defined by T1, T2, ,

Tn!1. This gives n!1 unknowns.

C The flow rate of the heating steam, which is

defined byMs.

C The flow rate of the cooling seawater, which

is defined byMcw.C The heat transfer area in each evaporation

effect, which is defined byAe.

C The condenser heat transfer area of the con-

denser, which is defined byAc.

This gives a total of (6n+1) variables, which

requires simultaneous solution of (6n+1) equa-

tions. These equations include the following:

C Total mass balance for each effect, which

gives n equations.


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(5)

C Salt balance for each effect, which gives nequations.

C Energy balance for each evaporator, which

gives n equations.

C Heat transfer rate for each evaporator, which

gives n equations.

C Energy balance for brine flashing in effects 2

to n, which gives n!1 equations.

C Energy balance for distillate flashing in effects

2 to n, which gives n!1 equations.

C Energy balance and heat transfer rate for the

condenser, which gives two equations.C Constraint on the total distillate flow rate,

which gives one equation.

A summary of the system model is given

below:

C Total mass balance in the first effect:

F1 =D1 +B1 (1)

C Total mass balance in effects 2 to n:

Fj

+ Bj!

1

=Dj

+ Bj

(2)

C Salt balance in the first effect:

Xcw F1 =Xbj Bj (3)

C Salt balance in effects 2 to n:

Xcw Fj + Xbj!1Bj!1 =Xbj Bj (4)

C Constraint on the total distillate flow rate:

C Energy balance in the first effect:

Ms8s =F1Cp (Tb1!Tcw) +D18v1 (6)

C Energy balance in the second effect:

D18c1 =F2Cp (Tb2!Tcw) +D28v2 (7)

C Energy balance in the effects 3 to n:

(8)

C Heat transfer rate in the first effect:

Ms8s = U1A (Ts!Tb1) (9)

C Heat transfer rate in the second effect:

D18c1 = U2A (Tc1!Tb2) (10)

C Heat transfer rate in effects 3 to n:

(Dj!1 + dj!1 + ) 8cj!1 = UjA (Tcj!1!Tbj) (11)

C Flow rate of vapor formed by brine flashing ineffects 2 to n

Bj!1Cp (Tbj!1!Tbj) = dj8vj (12)

C Flow rate of vapor formed by distillate flash-

ing in effects 2 to n

(13)

C Evaporation temperature in effects 1 to n:

Tvj = Tbj!BPEj!NEAj (14)

C Condensation temperature in effects 2 to n:

Tcj = Tvj!

)Tpj!

)Ttj!

)Tcj (15)

C Condenser energy balance:

(Mf + Mcw) Cp (Tf!Tcw) =Mu8cn (16)

C Condenser heat transfer rate:

(17)


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(18)

C Mass balance of entrained and un-entrained

vapor by the steam jet ejector in vapor com-

pression mode:

Mev + Mu = (dn + +Dn) (19)

C

Entrainment ratio by the steam jet ejector:

w = Mev/Mm (20)

C Performance ratio:

PR = Md/Mm (21)

C The specific flow rate of cooling water:

sMcw = Mcw/Md (22)

C The conversion ratio:

CR = Md/Mf (23)

C The specific heat transfer area:

sA = (Ac + n Ae)/Md (24)

4.2. Process economics

Calculations of the product unit cost dependon process capacity, site characteristics, and

design features. System capacity specifies the

heat transfer area, size of various pumping units,

and dimensions of the evaporation effects. Site

characteristics considerably affect the process

capital, i.e., construction on a new site is quite

different from construction on a site that has older

desalination units. In the latter case, the new

installation can benefit from a common intake

piping system, discharge lines, and pretreatmentunits. The heating steam temperature dictates the

type of antiscalent used, material of construction,

deaerator capacity, and capacity of a non-con-

densable gas removal system. In a low-temp-

erature ME system where the heating steam

temperature is below 70C, the condenser/

evaporator tubes are constructed from 90/10

Cu/Ni alloys, titanium, or aluminum brass alloys.

Costs include direct capital, indirect capital,

and operations. The direct capital cost includes

land, well construction, process and auxiliaryequipment and buildings. The land cost is usually

greatly reduced because most of the desalination

plants are owned by governments or munici-

palities. The cost of process equipment includes

evaporators, instrumentation, controllers, pipe-

lines, valves, pumps, and treatment equipment.

The auxiliary equipment includes intake lines,

transmission pipes, storage tanks, generators, and

transformers. Buildings include the control

rooms, laboratories, workshop, storage space, and

offices. Indirect capital cost is expressed as

percentage of the total direct capital cost or the

cost of materials and labor. Insurance and con-

tingency may account for up to 15% of the total

direct capital costs. Other indirect capital costs

include construction overhead, which may

account for up to 15% of the material and labor

cost.

Operating costs cover all expenditures in-

curred after plant commissioning and duringactual operation. These include labor, energy,chemical, spare parts, and miscellaneous costs.

Energy costs include heating steam and elec-tricity. Electricity cost varies over a range of$0.040.09/kWh. Estimating the heating steam

cost depends on the features of the co-generationfacility and type of power plant production. Also,demand for electric power affects the estimated

cost of the heating steam, i.e., high vs. lowdemand periods. The maintenance and spare partscosts account for up to 2% of the annual direct

capital cost. Another important operating cost


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item is the chemicals cost, which includes acids,alkali, chlorine, and antiscalent.

The following illustration gives the required

parameters and calculation steps of the unit

product cost. The following system parameters

are used in the calculations:

C Production capacity (Md) is set at 12,000 m3/d

C Plant life (n) is set at 30 years

C Electricity cost (ce) is set at $0.05/kWh

C Steam heating cost (cs) is set at $1.5/MkJ

C Performance ratio (PR) is set at 16 kg product/

kg steamC Latent heat of heating steam at 70C is equal

to 2333.9 kJ/kg

C Specific cost of operating labor (cl) is set at

$0.1/m3

C Interest rate (i) is set at 5%

C Plant availability (f) is set at 0.9.

C Production efficiency (g) is set at 0.9.C Maintenance annual cost, expressed as a per-

centage of the direct annual cost (x), is set at

0.01.

C Direct capital cost (cd

) = $27.5106

C Specific consumption of electric power (w) =

3 kWh/m3

C Specific chemicals cost (ck) = $0.025/m3

The results of the calculations are:

C Amortization factor:

C Annual fixed charges:

A1 = (a) (cd) = (0.065051) (27.5106)

= $1788902.5/y

C Annual heating steam cost:

A2 = (cs) (8)(f) (g) (Md) (365)/[(1000) (PR)]

= (1.5) (2333.9) (0.9) (0.9) (12,000) (365)/

[(1000)(16)] = $776,269.7/y

C Annual electric power cost:

A3 = (ce) (w) (f) (g) (Md) (365)

= (0.05) (3) (0.9) (0.9) (12,000) (365)

= $532,170/y

C Annual chemicals cost:

A4 = (ck) (f) (g) (Md) (365)

= (0.025) (0.9) (0.9) (12,000) (365)

= $88,695/y

C Annual labor cost:

A5 = (cl) (f) (g) (Md) (365)

= (0.1) (0.9) (0.9) (12,000) (365)

= $354,780/y

C Annual maintenance cost:

A6 = (x) (a) (cd) = (0.01) (0.065051) (27.506

)= $17,889/y

C Total annual cost:

At =A1 +A2 +A3 +A4 +A5 +A6 = 1,788,902.5

+ 776,269.7 + 532,170 + 88,695 + 354,780

+ 17,889 = $3,558,706.2/y

C Unit product cost:

As = At/[(f) (g) (Md) (365)] = (3558706.2)/( 0.9)

(0.9) (365) (12,000) = $1.003/m3

The above value for the unit product cost is

within limits of the reported field data, which

may vary over a range of $0.8/m3 up to $1.5/m3.

Such variations depend on the plant capacity,

energy cost, labor experience in operation and

maintenance, plant life, efficiency of chemical


11/16


Table 1

Summary of previous economic data for multiple effect evaporation in stand-alone and vapor compression modes

Reference Process Capacity,

m3/d

Capital,

$

Unit capital

cost, $/(m3/d)

Unit energy

cost, $/m3Unit chemical

cost, $/m3Unit product

cost, $/m3

Matz and

Fisher (1981)

MVC 1,000 8.94105 894 0.52 0.02 1.51

Veza (1995) MVC 1,200 1.586106 1322 1.057 3.22

Leitner (1992) MEE 37,850 70.4106 1860 0.08 0.024 1.08

Wade (1993) MEE 32,000 67.2106 2100 1.147 0.207 1.31

Morin (1993) MEE 22,730 35.05106 1562 0.49 0.0606 1.24

Morin (1993) MEE-TVC 22,730 34.65106

1524 0.785 0.0606 1.55

treatment, etc. A summary for some results of the

field data are shown in Table 1. Additional data

and analysis of the economics of thermal and

membrane desalination processes can also be

found in the study by Ettouney et al. [17].

4.3. Package results

Results of the MEE process are shown inFigs. 48. The illustration includes the following:

C design data display, shown in Fig. 4

C results display, shown in Fig. 5

C profiles display, shown in Fig. 6

C flow diagram display, shown in Fig. 7C cost display, shown in Fig. 8.

The design data display (Fig. 4) allows the

user to define the following variables:C number of effects (n), set equal to 12

C compression ratio (Cr), set equal to 4

C pressure of motive steam (Pm), set equal to1500 kPa

C heating steam temperature (Ts), set equal to

70CC rejected brine temperature (Tn), set equal to

40C

C feed salinity (Xf), set equal to 36,000 ppmC feed temperature (Tf), set equal to 30C

C intake seawater temperature (Tcw), set equal to

25C

C plant capacity (Md), set equal to 12,000 m3/d

C brine salinity leaving each effect (Xbj), set

equal to 52,000 ppm.

Other input design parameters include dimen-

sions and properties of the evaporator or con-

denser tubes as well as the specifications of the

evaporator demister:

C wall thickness of evaporator tubes (te), set

equal to 5 mmC outer diameter of evaporator tubes (deo), set

equal to 31.75 mm

C wall thickness of condenser tubes (tc), set

equal to 5 mm

C outer diameter of condenser tubes (dco), set

equal to 31.75 mm

C thermal conductivity of evaporator tubes (ke),

set equal to 0.042 kW/mC

C thermal conductivity of condenser tubes (kc),

set equal to 0.042 kW/mC

C

fouling resistance in the evaporator (Rfe), setequal to 0.1 m2 C/kW

C fouling resistance in the condenser (Rfc), set

equal to 0.1 m2 C/kW

C velocity of the falling film in the evaporator

(Vf), set equal to 1.5 m/s

C length of the condenser tubes (Lc), set equal to

10 m

C length of the condenser tubes (Le), set equal to

10 m


12/16


Fig. 4. Display of design

data.

Fig. 5. Display of perfor-

mance results.


13/16


Fig. 6. Display of process profiles, which include temperatures, flow rates, heat transfer coefficients, and losses.

Fig. 7. Display of flow chart showing the design data of the steam jet ejector.


14/16


Fig. 8. Display of cost data and results.

C thickness of the falling film in the evaporator

(tfe), set equal to 1 mm

C vapor velocity in the demister (Vp), set equal

to 5 m/sC demister thickness (Lp), set equal to 0.2 m

C demister density (Dp), set equal to 300 kg/m3

Performance results shown in Fig. 5 give the

following main features for the MEE and MEE-

TVC:

C specific heat transfer areas of 767.4 m2/(kg/s)

and 757.9 m2/(kg/s) for the MEE and MEE-

TVC, respectively

C performance ratios of 9.5 and 14.7 for the

MEE and MEE-TVC, respectively

C specific flow rates of cooling water of 7.17

and 3 for the MEE and MEE-TVC, respect-ively

C shell diameter for each evaporator of 3.38 m

and number of tubs in each evaporator, 8706

The system profiles shown in Fig. 6 show the

following behavior:

C Mass flow rate of distillate vapor varies

between 12.9 kg/s to 10.4 kg/s from effect 1 to

effect 12.


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C The overall heat transfer coefficient variesbetween 2.487 kW/m2 C to 2.253 kW/m2 C

from effect 1 to 12.

C The sum of the BPE and the non-equilibrium

allowance from effects 1 to 12 varies between

1.6C and 1.8C.

The flow diagram of the process is shown in

Fig. 7. The flow diagram includes the 12 effects,

the steam jet ejector, the condenser, as well as

blocks for various streams. The display shows the

design results for the steam jet ejector. Thisspecific condition requires use of two ejectors in

series in order to achieve the required vapor

compression. The design data include dimensions

of nozzles and diffuser, flow rates of entrained

vapor, compressed vapor, and motive steam. Each

block in the flow diagram includes design results

for the block, which may include heat transfer

coefficient, area, flow rates, temperatures, or

pressures.

The cost display is shown in Fig. 8, and it

includes the input parameters, which can be

adjusted by the user. The cost parameters include

plant life, plant factor, production efficient,

production capacity, performance ratio, interest

rate, and various cost elements. The plants capa-

city and performance are the same as those used

in the system design. However, the user can

adjust these values. The display also includes the

cost results, which indicate that the unit product

cost is equal to $1.016/m3. This value is con-

sistent with field data shown in Table 1.

5. Conclusions

This paper summarizes the developments in

the visual basic simulation package of thermal

and membrane desalination processes. Several

additions have been made in the packages includ-

ing cost estimation, simulation of the RO process,

calculations of effect dimensions, and a detailed

design of the steam jet ejector for vapor com-

pression. The development process was necessaryto improve the capabilities of the simulator. The

simulator proved to be highly useful in teaching

desalination and engineering training.

The simulator gives the user efficient tools for

system design or simulation. Results give the user

the means to have a better understanding of the

desalination systems. Further modification and

additions are underway, which are based on feed-

back of users in other colleges and the industry.

The package integrates well with other literature

attempts focusing on developing a package forprocess simulation or cost estimation.

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Documents

Ettouney 2004 Paper