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SIMULATION OF LIQUID-GAS FOR CPO-METHANOL MIXING PROCESS IN BUBBLE COLUMN REACTOR MOHD SAIFUL BIN SALEH A project report submitted in partial fulfillment of the requirement for the award of the Degree of Master of Mechanical Engineering Faculty of Mechanical and Manufacturing Engineering Universiti Tun Hussein Onn Malaysia JULY 2015

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SIMULATION OF LIQUID-GAS FOR CPO-METHANOL MIXING PROCESS IN

BUBBLE COLUMN REACTOR

MOHD SAIFUL BIN SALEH

A project report submitted in partial

fulfillment of the requirement for the award of the

Degree of Master of Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

JULY 2015

v

ABSTRACT

The computational fluid dynamics (CFD) highly concern in understanding the

complexity hydrodynamics of bubble column. Research on superheated methanol

very important in investigate influence towards biodiesel production. The aim of

simulation is to find the effect of various size of holes distribution and velocities

superheated methanol towards crude palm oil (CPO) in bubble column reactor to

predict the effect towards reaction rate between this materials. Superheated methanol

gas was dispersed through bubble column reactor contains liquid crude palm oil. This

process occurs at atmospheric pressure and temperature of 290°C. This analysis used

ANSYS FLUENT fluid flow to simulate and compute the mathematical problem.

According to the result from turbulent transient analysis of Volume of Fluid model

(VOF), the value contact surface area and gas holdup is highly indicated at model C

which is this model provide an open area ratio A=4.76% and at 6 m/s of velocity.

Thus, even the percentage of an open area ratio was higher than others model, but

with systematic arrangement of distance holes distribution, it will evade the bubble

coalescence further increase the value of contact surface area and gas holdup.

However, the investigated result also show that the maximum value of turbulent

dissipation rate was indicated at model B compare with others model which is

indicate higher contact time between methanol and oil. Further research with variety

of parameters and others method is capable in detail prediction of superheated

methanol hydrodynamics behaviour.

vi

ABSTRAK

Computational Fluid Dynamic (CFD) amat penting dalam memahami hidrodinamik

yang kompleks di dalam bubble column reactor. Kajian berkaitan gas methanol

panas lampau amat penting bagi menyelidiki kesan terhadap pengeluaran biodiesel.

Tujuan simulasi ini adalah untuk mencari kesan daripada saiz lubang masukkan dan

halaju gas methanol yang berbeza terhadap minyak mentah kelapa sawit di dalam

reaktor bagi mengesan kesan terhadap nisbah reaksi diantara dua bahan ini. Gas

panas lampau methanol disalurkan ke dalam reactor yang mengandungi minyak

mentah kelapa sawit. Proses ini dilaksanakan pada tekanan atmosfera dan pada suhu

290°C. Analisis ini menggunakan ANSYS FLUENT bagi membuat simulasi serta

menyelesaikan masalah berdasarkan penyelesaiaan matematik secara berkomputer.

Keputusan yang diperolehi dengan menggunakan “transient analysis” bagi “Volume

of Fluid model (VOF)”, Nilai paling tinggi bagi “contact surface area” dan “gas

holdup” diperolehi pada model C dengan nisbah luas kawasan yang terbuka,

A=4.76%, pada halaju 6 m/s. Walaupun peratusan nisbah luas kawasan terbuka bagi

model ini lebih tinggi berbanding model lain, namum dengan susunan lubang yang

sistematik, ianya dapat mengelak buih bergabung seterusnya meningkatkan nilai

“contact surface area” dan “gas holdup”. Walau bagaimanapun, keputusan juga

menunjukkan bahawa nilai maksimum bagi “turbulent dissipation rate” diperolehi

pada model B berbanding dengan model lain dimana ia menyatakan bahawa masa

sentuhan diantara methanol dan minyak adalah tinggi. Kajian lebih lanjut dengan

pelbagai parameter dan kaedah lain yang digunakan mampu dalam menyelidiki

kelakuan hidrodinamik gas methanol yang panas lampau dengan lebih mendalam.

vii

CONTENTS

TITLE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF SYMBOLS AND ABBREVIATIONS

LIST OF APPENDICES

i

ii

iii

iv

v

vi

vii

x

xi

xiii

xiv

CHAPTER 1 INTRODUCTION

1.1 Research background

1.2 Problem statement

1.3 Objective

1.4 Scope of study

1.5 Significant of study

1

2

3

3

3

4

CHAPTER 2 LITERATURE REVIEW

2.1 Flow structure and liquid circulation

pattern

2.2 Hydrodynamics regime

2.3 Gas hold up

2.3.1 Effect of superficial gas velocity

towards gas hold up

2.4 Mixing time

2.5 Sparger design

2.6 Bubble column geometry

5

5

6

9

9

11

13

15

viii

2.6.1 Effect of H/D ratio

2.7 Mixing characteristics factors (W) and

interfacial area

2.8 Crude palm oil

2.9 Methanol

2.10 Non-catalytic method

2.11 Supercritical liquid

15

16

17

18

19

20

CHAPTER 3 METHODOLOGY

3.1 Flow chart

3.2 Pre-processor

3.2.1 Model geometry

3.2.2 Parameter for analysis

3.2.3 Meshing

3.3 Solution setup

3.3.1 Boundary conditions

3.4 Solver

3.5 Post-processing

21

22

24

24

26

26

30

30

32

32

CHAPTER 4 RESULT AND DISCUSSION

4.1 Result validation

4.2 Contact Surface Area

4.2.1 Prediction contact surface area at

velocity 2 m/s

4.2.2 Prediction contact surface area at

velocity 4 m/s

4.2.3 Prediction contact surface area at

velocity 6 m/s

4.3 Gas holdup

4.3.1 Effect of velocity towards gas

holdup

4.3.1.1 Prediction gas holdup

design A

4.3.1.2 Prediction gas holdup

design B

34

34

35

36

37

38

39

39

40

40

ix

4.3.1.3 Prediction gas holdup

design C

4.3.2 Effect of open area ratio and

distributor holes size

4.4 Turbulent dissipation rate

4.5 Hydrodynamics behavior and flow pattern

4.5.1 Volume of fraction vapor methanol

4.5.2 Facet average phase volume

fraction methanol

4.5.3 Velocity magnitude

41

43

46

47

47

49

50

CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1 Conclusion

4.2 Recommendation

54

54

55

REFERENCES 56

x

LIST OF TABLES

2.1

3.1

3.2

3.3

3.4

3.5

3.6

4.1

4.2

Properties of palm oil

Geometry of bubble column

Geometry of spargers

Material and thermal properties of bubble

column

Meshing statistics

Mesh parameters

Summary of boundary conditions

Turbulent dissipation rate

Facet average phase volume fraction methanol

18

25

25

26

27

29

31

46

49

xi

LIST OF FIGURES

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

2.10

2.11

2.12

3.1

3.2

3.3

3.4

3.5

Liquid circulation patterns in bubble columns

Flow regime map for bubble columns

The flow regime observed in gas-liquid bubble

column reactors

Definition sketch of flow regimes in bubble

column

Overall gas holdup (εg) versus superficial gas

velocity (Usg)

Effect of superficial gas velocity on gas hold up

in air-water system

Mixing time (tm) versus superficial gas velocity

(Usg) with H/D ratio as a parameter

Axial dispersion coefficient (Dax) versus

superficial gas velocity (Usg) with H/D ratio as a

parameter

Gas distributor perforated plates

Gas holdup and flow regime transition using

different aeration plates in an air-water system

Overall gas holdup (εg) versus superficial gas

velocity (Usg) with H/D ratio as a parameter

Variation of mixing characteristic factor with

phase velocities

CFD modeling overview

Flow chart of methodology

Cylindrical bubble column

Type of holes arrangement

Location of face sizing

6

7

8

8

10

11

12

12

14

14

16

17

22

23

25

26

28

xii

3.6

3.7

3.8

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

Mesh distribution of bubble column and sparger

Initial condition for vapor (red) and oil (blue)

Plot of residual simulation progress

Gas holdup versus time at velocity 4 m/s

Result of gas holdup at 4 m/s previous study

Contact surface area versus time at velocity 2

m/s

Contact surface area versus time at velocity 4

m/s

Contact surface area versus time at velocity 6

m/s

Gas holdup versus time for design A

Gas holdup versus time for design B

Gas holdup versus time for design C

Gas holdup versus time at velocity 2 m/s

Gas holdup versus time at velocity 4 m/s

Gas holdup versus time at velocity 6 m/s

Volume fraction vapor

The contour of volume fraction vapor

The contour of velocity magnitude mixture

Velocity magnitude at 2 second

Vectors velocity magnitude at 0.5 second

28

33

33

35

35

37

38

39

40

41

42

44

45

45

48

49

51

52

53

xiii

LIST OF SYMBOLS AND ABBREVIATIONS

3D

A

CAD

CFD

CPO

D,d

Dax

ƐG

FAME

H

HD

k-ε

m

s, sec

tm

Usg

VG

VL

VOF

W

Three Dimensional

Open area ratio

Computer Aided Design

Computational Fluid Dynamics

Crude Palm Oil

Diameter

Axial dispersion coefficient

Gas holdup

Fatty Acid Methyl Ester

Height

Height of Dispersion

k-epsilon

meter

second

Mixing time

Superficial gas velocity

Volume of Gas

Volume of Liquid

Volume of Fluid

Mixing characteristic factor

xiv

LIST OF APPENDICES

APPENDIX

A

B

C

D

E

F

G

H

TITLE

Table contact surface area design A

Table contact surface area design B

Table contact surface area design C

Table gas holdup design A

Table gas holdup design B

Table gas holdup design C

Gantt chart project 1

Gantt chart project 2

PAGE

58

61

64

67

70

73

76

77

CHAPTER 1

INTRODUCTION

The understanding of complexity fluid dynamics in bubble column reactor is very

important due to increasing of application in biodiesel process engineering include

on demand to produce biofuel from crude palm oil. Currently, research in biodiesel

process mainly use bubble column as device in analyses chemical reaction by

experimentally or computationally to obtain the best result in producing new energy

(biodiesel fuel) for industrial and mankind application. Regarding to Mahajan,

(2010), the capability of bubble columns as gas–liquid contactors and as reactors

enable the bubble column to use widely in a lot of application such as in chemical,

petrochemical and biochemical industries. Recently, highly increasing of extensive

studies based on demand of bubble column especially in field of biochemical

engineering, biotechnology as well as in waste water treatment also in syngas

conversion process.

Biodiesel is defined as the mono-alkyl esters of long chain fatty acids derived

from renewable bio-lipids through the process of trans-esterification of triglycerides

or esterification of free fatty acids. It involved low molecular weight alcohols such as

methanol and ethanol in the presence of a catalyst to produce methyl or ethyl esters

and glycerin (Tio, 2010). Trans-esterification reaction is a reverse reaction of the oil

consists of triglycerides with alcohol to form fatty acid alkyl ester and glycerol.

Glycerol was separated from the Fatty Acid Methyl Esters (FAME) with both

products subsequently purified. The purified FAME was known as biodiesel while

the purified glycerol was sold as a co-product.

Trans-esterification reaction resulted in exchange of the original organic

group of an ester with the organic group of alcohol to produce alkyl ester and

2

glycerin. The reaction occurs through three consecutive reversible reactions. First

reaction occurs was di-glyceride produced from triglyceride. It were continuous with

second reaction where mono-glyceride is produced from di-glyceride and finally

glycerin produced as a reaction of mono-glycerides in third reaction. Ester molecules

produced in these three reactions (Alsoudy et al., 2012). Thus, at the end of the

reaction, the three alkyl ester molecules and one molecule of glycerin obtained.

1.1 Research background

Superheated methanol known as non-catalytic method to produce biodiesel. In

addition, by using this superheated methanol trans-esterification, purification of

biodiesel has become easier because there is no soap and catalyst in the system (Asri

et al., 2013). Both trans-esterification and esterification reaction occurs

simultaneously in the non-catalytic method, therefore no cleaning process before and

after the reaction. The limitation of this method is the reaction rate of biodiesel

production is very low.

This method has advantages such as environmentally friendly because there

is no danger of waste. This method was simple without having FFA (free fatty acids)

oil refining process before the reaction and no saponification reaction occurs. The

reaction rate was influenced by temperature, the flow of methanol, gas hold up and

the contact surface between methanol and oils (Wulandani et al., 2015).

Design of obstacle or perforated plate and diameter holes was important in

order to improve contact surface area between methanol and oil. Previous studies by

(Wulandani et al., 2015) have provided evidence that the design of obstacles and

holes diameter produce towards increased response rates for biodiesel production. In

the previous research such as by (Saka et al., 2006 and Wulandani et al., 2015),

several configuration of perforated plate were installed in the cylinder obstacle and

have been simulated by computational fluid dynamics (CFD) method in order to

improve reaction rate for produce biodiesel.

The combination of methanol and crude palm oil were important as the main

material in production of biodiesel by using this method. Previous research by (Nagi

et al., 2008 and Lim et al., 2002) were stated that crude palm oil, consisting of long

chain triglycerides, have the potential to substitute for liquid fuels, due to their

environmental benefits and renewability. Moreover, the capability of methanol

3

completely soluble in both FAME and glycerol was become as important properties

in order to produce biodiesel.

1.2 Problem statement

The superheated methanol vapor in bubble column is one of Non-catalytic method in

biodiesel production and this approach is highly concern. However, this method has

certain limitation due to the low rate of reaction of methanol and oil in biodiesel

production. The reaction rate basically influenced by methanol flow rate,

temperature, gas hold up and contact surface contact between methanol and oil.

1.3 Objective

1. To simulate a superheated methanol vapour in bubble column reactor by using

Non-Catalytic method.

2. To predicting the main parameters of hydrodynamic, such as gas velocity

magnitude, surface contact area and gas hold up of bubble column towards

reaction rate of methanol and crude palm oil.

3. To determine the performance of numerical modelling in simulation of

superheated methanol vapour in bubble column reactor under different operating

conditions.

4. To compare the simulated results with previous experimental or simulated result

in order to test the validity of the present models.

1.4 Scope of study

The system used in the study is a cylindrical column of 55 mm diameter and 200 mm

height. The simulation focuses at three velocities as 2 m/s, 4 m/s and 6 m/s. The

column is equipped with perforated plate sparger. The designs were differentiate

with three different holes sizes which are 2 mm, 3 mm and 4 mm respectively. Each

design constructed with nine distributor holes arrangement. The data were obtained

within 2 second of flow. The standard k-ε mixture turbulence, implicit model,

transient time and Volume of Fluid method (VOF) will be used to account the effect

of turbulence. Ansys Fluent software package will be used to simulate the system for

4

various hydrodynamics parameters. The results were compared to the simulated

results from the previous research.

1.5 Significant of study

The significant of study is to identify the impact of different size of distribution holes

in sparger and various gas velocity in bubble column towards hydrodynamics

behaviour in bubble column between methanol and crude palm oil. The sizing of

distributor holes indirectly will produce variety of gas bubble holdup in the bubble

column. Therefore, it’s capable in prediting the affect towards gas holdup and

surface contact area between gas methanol and liquid crude palm oil.

CHAPTER 2

LITERATURE REVIEW

Bubble column widely uses in biodiesel analysis by experimentally or simulation in

producing fuel for using by mankind or industrial. In bubble column process, there

are a lot of variety parameters has been studied by researchers to predict behavior of

hydrodynamics in biodiesel production. Superheated methanol is Non-catalytic

method widely used in order to increase reaction rate of biodiesel production. One of

the important parameters can be studied to predict reaction rate in bubble column is

surface contact of gas-liquid contactor.

The flow dynamics in bubble column has been studied extensively in past

decades by using techniques such as experimental, modeling, and simulation. This

studied focusing on approach that probably affects the reaction of gas-liquid in

bubble columns such as influence by hydrodynamics regime, bubble column

geometry, gas hold up profile, gas velocity, surface contact and sparger. This chapter

wills presents the previous contributions made by some researchers for further

understanding of fundamentals of bubble columns hydrodynamics to support the

methodology of research.

2.1 Flow structure and liquid circulation pattern

In the bubble columns, the process involved by gas dispersed with some range of

velocity. This process will influence the liquid circulation motion upwards and

downwards. Therefore, a larger degree of circulation of both discrete and continuous

phases occurs in many multiphase contactors such as gas-liquid contactor, gas-solid

contactor, and liquid-liquid contactor also in gas-liquid-solid contactor. The

6

circulation behaviour will producing a good degree of mixing and indirectly will

enhance heat and mass transfer between fluid and walls.

Based on this behaviour, the circulation of the liquid can be the one of major

indicator to observer in predicting of mass or heat transfer coefficient and mixing

capability. This phenomenon occurs due to factor such as bubble size, bubble

dynamic and holdup. Thus, these factors are very crucial in determining the

efficiency of liquid-liquid mixing in bubble columns. Figure 2.1 show the three basic

types of liquid circulation pattern occur in bubble column reactor. For ‘a’ types of

circulation called as large-scale circulation, ‘b’ types called as donut-model of Joshi

and Sharma (1979) and ‘c’ types called as circulation cells according to Zehner

(1986).

Figure 2.1: Liquid circulation patterns in bubble columns.

2.2 Hydrodynamics regime

According to Kantarci et al., (2005), there are three flow regimes occur in bubble

column. The review of flow regimes in bubble columns are homogeneous regime

(bubbly flow), heterogeneous regime (churn-turbulent) and slug flow regime.

Regarding to the Figure 2.2, the bubbly flow regime or homogeneous regime is

obtained at low superficial gas velocities approximately less than 0.05 m/s. This

flow regime is characterized by uniform small sizes of bubbles dispersed by gas rise

7

velocities below 0.05 m/s. In practically, there is no bubble coalescence or break-up

on this regime based on sparger design and system properties.

The second flow regime called as heterogeneous regime or churn turbulent

regime occurs at superficial gas velocities greater than 0.05 m/s. The characterizing

of this regime based on the gas bubble behavior. This behavior occurs caused by the

disturbed form of the homogeneous gas–liquid system due to enhanced turbulent

motion of gas bubbles and liquid recirculation. The coalescence formed by unsteady

flow patterns and large bubbles with short residence times due to high gas velocities

dispersed.

A slug flow regime has been only observed in small diameter laboratory

columns at high gas flow rates. Practically, slug regime is highly unstable and it can

be found when the superficial gas velocity is increased further. This regime based on

the formation of bubble slugs when larger bubbles are stabilized by the column wall

Figure 2.2: Flow regime map for bubble columns (Kantarci et al., 2005)

8

Figure 2.3: The flow regime observed in gas-liquid bubble column reactors

(Mahajan, 2010)

Based on Figure 2.4, the study by Naji (2010) also stated that another regime

occur called as transition region where gas holdup may go through a maximum. This

region located between homogeneous and heterogeneous region. In the transition

region, the fast rising bubbles will be produce by coalescence of bubbles based on

gas velocity. Automatically it will enhances liquid circulation because the existing an

onset of upward liquid circulation in the column center and downward liquid

circulation near the column wall.

Figure 2.4: Definition sketch of flow regimes in bubble column (Naji, 2010)

Bubbly Flow Churn-Turbulent Slug Flow

9

2.3 Gas hold up

One of the important parameter should be considering in bubble column design is

overall gas hold up in order to characterize the hydrodynamics of bubble column.

The gas hold up can be defined as the fraction of the reactor dynamic volume

occupied by the gas (Pirdashti & Kompany, 2009). Moreover, the characterizes

retention of the bubble under the liquid depending on gas holdup in a bubble column.

As mentioned by Pirdashti et al., (2009), gas hold up depends mainly on the gas

velocity, physical properties of the liquid and type of gas sparger. Based on Shintaro

Furusaki et al., (2002), gas holdup is defined as the percentage of volume of the gas

phase in the total volume of the dispersion:

At high flow rates the gas holdup decreases in concurrent systems because

gas bubbles pass through the column more quickly. In contrast, the gas holdup rises

in countercurrent systems. Increasing the viscosity of the liquid phase leads to

increased bubble coalescence and thus a decrease in gas holdup. In the simulation

analysis, the gas holdup predicted based on ratio of volume methanol gas to the total

volume mixture in the column system (Wulandani, Ilham, & Hagiwara, 2012). The

determination of gas holdup in superheated methanol focusing in obtaining the effect

towards reaction rate.

2.3.1 Effect of superficial gas velocity towards gas hold up.

The gas holds up increase with enhancement of superficial gas velocity. Figure 2.5

show the result from analysis by Pirdashti et al., (2009). Based on result, the

increasing of gas velocity will influence towards increasing of overall gas hold up.

The increasing of gas hold up also depending on two regime called as homogeneous

(bubbly flow) and heterogeneous (churn turbulent) regimes.

10

Figure 2.5: Overall gas holdup (εg) versus superficial gas velocity (Usg) (Pirdashti &

Kompany, 2009).

Based on study by Moshtari et al.,(2009), Figure 2.6 show homogeneous flow

regime occurs at low gas flow with range between 0.025 m/s and 0.1 m/s which is

the gas holdup increase smoothly by increasing of superficial gas velocity. Then,

when the gas superficial velocity increases more than 0.1 m/s, it will turn the flow

regime into transition regime and heterogeneous flow regime. It can be conclude

that the velocity of gas will affect directly towards the changing of the flow regime in

bubble column. At low superficial gas velocity, the bubble will appear in small size

and travel upward in uniform condition and a helical path without any major

collision or coalescence. However, there are some different statement by (Kantarci et

al., 2005), which is the increasing of gas velocity in churn-turbulent region affected

towards increasing of gas holdup due to large bubble rise upward.

However, when the superficial gas velocity increases, it will influence

towards the instances of bubble coalescence (more than about 9 cm/s). This

phenomenon will create bubbles in large size. The large bubbles have higher rise

velocity than small bubbles therefore residence time of large bubbles will be

decrease and cause to decrease rate of increasing gas hold up.

11

Figure 2.6: Effect of superficial gas velocity on gas hold up in air-water system

(Moshtari et al., 2009)

2.4 Mixing time

Mixing time is the important indicator of mixing performance in bubble column for

mixing process. Typically, mixing time is used in considering of time required for

the tracer concentration to reach the desired degree of homogeneity (Pirdashti et al,

2009). Based on the result analysis by Pirdashti et al., (2009); the mixing time

decreases with an increase of superficial gas velocity. Figure 2.7 show the result with

different of H/D.

The result state the phenomena can be happened when more gas velocity

dispersed in bubble column. This phenomenon will influence the increasing of axial

dispersion coefficient as shown in Figure 2.8. Indirectly, it will cause the time

needed to reach the desired degree of homogeneity will reduced. It means the mixing

time (tm) for homogeneity condition will be shorter. Axial dispersion arises from

convective motion of fluid caused by the following main factors such as relative

movement of the gas and liquid phases, bubble coalescence and break up, the carry

forward of fluid in wakes behind the rising gas bubbles and the consequent return

flow generated for maintaining mass balance, and also turbulence generated by any

superimposed flow of liquid. In contact of gas-liquid contactor, contact time is

12

important to generated reaction rate between gas-liquid especially in biodiesel

production. Increasing contact time will give capability of gas-liquid to react in

longer time for producing biodiesel. Contact time causes by turbulence appear in

bubble column due to collision among the bubble. Turbulence causes the backflow

appear which is rise the contact time between methanol and oil in biodiesel

production (Wulandani et al., 2015), thus provide prolonging reaction between

methanol and oil. The author also stated that the increasing of turbulent dissipation

rate influence towards increasing the collision rate.

Figure 2.7: Mixing time (tm) versus superficial gas velocity (Usg) with H/D ratio as a

parameter (Pirdashti & Kompany, 2009)

Figure 2.8: Axial dispersion coefficient (Dax) versus superficial gas velocity (Usg)

with H/D ratio as a parameter (Pirdashti & Kompany, 2009)

13

2.5 Sparger design

Gas Sparger is important parameters to indicate the characteristic of bubble and

indirectly affects gas hold up value. The sparger used to determines the bubble sizes

observed in bubble columns. Basically, small orifices of diameter plate enable the

formation of small bubble sizes. The common used gas sparger types in recent study

are perforated plate, ring type, and porous plate. Regarding to the literature study by

Kantarci et al., (2005), stated the smaller sizes of bubble will influencing towards the

greater value of the gas hold up. It can be concluded that with small gas distributors

their gas hold up values were higher. Literature study of Kantarci et al., (2005) also

stated that gas holdup strongly affected by the type of gas distributor. Moreover, the

type design of sparger directly will contribute towards small or large sizes of bubble

by gas velocity dispersed in bubble column. Practically, the probability size of both

small and large bubbles to gas velocity was appearing with ring sparger is lower

compared to the perforated plate.

The effect of perforated plate open area analysis by Su et al., (2005) is shown

in Figure 2.10 has been studied. From the result, the gas hold up increasing with

increasing open area ratio from A=0.57% and A=0.99 %. Based on Figure 2.10, for

both A=0.57% and 0.99%, gas holdup increases with increasing superficial gas

velocity until a maximum gas holdup is reached, and then gas holdup decreases with

increasing superficial gas velocity to a minimum value which indicates the end of the

transitional flow regime. For A=2.14%, no maximum gas holdup is observed, and the

gas holdup continuously increases with superficial gas velocity. It means that the gas

hold up will perform effectively if the perforated plates open area within certain

range (A≤1%). If the open area is beyond this range, it will decrease the effectiveness

of gas hold up.

However, Su et al., (2005) also stated that further increasing open area

beyond a critical value by increasing the number of holes with a constant holes

diameter enhances bubble coalescence near the aeration plate because of a reduced

holes spacing. This phenomenon will leads to a reduction in gas holdup. However,

the analysis by (Su & Heindel, 2005) also stated that the arrangement of holes

spacing also affected towards bubble formation mode and directly influenced

increment gas holdup value, gas distribution and flow region transition.

14

Even the holes size increased and influenced increase open area ratio, with

the systematic holes arrangement can produce homogeneous bubble rise thus

increase gas holdup. Previous study by (Solanki et al. 1992) also stated that at closely

spaced holes arrangement could contributed to a higher probability of bubble

coalescence compared to a large holes spacing. This behaviour also has been

observed by (Xie & B.H. Tan, 2003), with decreasing hole spacing, it will enhanced

bubble-bubble interaction and leading to decreasing gas holdup value.

Figure 2.9: Gas distributor perforated plates by (Su & Heindel, 2005)

Figure 2.10: Gas holdup and flow regime transition using different aeration plates in

an air-water system(Su & Heindel, 2005)

A= 0.57% A= 0.99% A= 2.14%

15

2.6 Bubble column geometry

The bubble columns geometry also important criteria that should be taken

considerable attention in prediction of hydrodynamics and its influence on transport

characteristics. Industrial bubble columns usually operate with a length-to-diameter

ratio, or aspect ratio of at least 5. Regarding to Kantarci et al., (2005), the value of

ratio in biochemical application usually varies between 2 and 5. Even more, when

involved with large gas throughput, the use of large diameter reactors is totally

desired.

Basically there are two types of mode of operation are valid for bubble

columns, namely the semi batch mode and continuous mode. In continuous

operation, the gas and the suspension flow concurrently upward into the column and

the suspension that leaves the column is recycled to the feed tank. In the semi batch

mode, there are zero liquid throughputs and suspension is stationary. In this mode,

the gas dispersed is bubbled upward into the bubble column (Kantarci et al., 2005).

Generally, the design and scale-up of bubble column reactors depend on the

quantification of three main phenomena:

i. Heat and mass transfer characteristics

ii. Mixing characteristics

iii. Chemical kinetics of the reacting system

2.6.1 Effect of H/D ratio

The process of coalescence and dispersion are practically absent in the homogeneous

regime and hence the sizes of bubbles are entirely dictated by the sparger design and

the physical properties of the gas and liquid phases. In contrast, in the heterogeneous

regime, the role of sparger design diminishes depending upon the column height. The

total column height can be divided into two regions. Firstly in the sparger region, the

bubble size changes with respect to height depending upon the coalescence nature of

the liquid phase, the extent of turbulence and the bulk motion. The bubbles attain an

equilibrium size at the end of the sparger region.

Secondly is the bulk motion where the equilibrium is governed by the

breaking forces due to bulk motion (turbulent and viscous stresses) and the retaining

16

force due to surface tension. Result by (Pirdashti & Kompany, 2009) show there was

practically no critical effect of the H/D ratio on gas holdup as shown in Figure 2.11.

The gas holds up value still increase based on increasing of superficial gas velocity.

However, the different of H/D ratio have influence in mixing time as show in Figure

2.7.

Figure 2.11: Overall gas holdup (εg) versus superficial gas velocity (Usg) with H/D

ratio as a parameter (Pirdashti & Kompany, 2009)

2.7 Mixing characteristics factors (W) and interfacial area

The variation comparison of mixing characteristic factor with gas phase velocities

from different author is shown in Figure 2.12. The observation by (Parmar &

Majumder, 2013) show that the mixing characteristic factor (W) increases with

increase in gas velocity. Gas velocity indicates a higher dispersion or higher mixing

in the column at constant increasing of liquid velocity. The increasing of gas velocity

will created the momentum exchange in the column indirectly influence increasing

the internal circulation of the gas phase in the column. This will lead more

enhancement of spreading of tracer molecule and increase contact surface area.

The increasing of internal circulation also wills consequent the more mixing

in the column. Bubbles are relatively smaller and rise uniformly without much

17

interaction with the liquid at low gas velocity compared towards higher gas velocity.

These phenomena occur due to large fast-rising bubbles appear and disrupt the

system contents. Moreover, in the context of biodiesel production, increasing of

methanol feed flow rate wills increased gas-liquid interface area thus increases in

enhanced reaction rate (Sagara, 2007). Generally, contact surface area or interfacial

area can be described as a potential area occurs by reaction between gas and liquids.

In this biodiesel analysis, the contact surface area is measured when high temperature

methanol bubble diffused to oil (Crude Palm Oil). Calculation of contact surface area

in CFD simulation determined is based on calculation when the volume fraction of

methanol and oil is 0.5 respectively (Wulandani et al., 2012.).

Figure 2.12: Variation of mixing characteristic factor with phase velocities (Parmar

& Majumder, 2013)

2.8 Crude palm oil

Palm oil is obtained from the fruit of palm tree, which grows well in hot and humid

countries, the main ones being in Malaysia and Indonesia. Palm oil has very large

yield per hectare and it is the largest vegetable oil in world production. Fresh fruit

branches of palm trees are collected and taken to crude palm oil mill where the fruits

are crushed to produce crude palm oil. Crude palm oil is then usually taken to a

refinery where it is 18 fractionated into lighter liquid fraction called palm oil olein

and more viscous fraction called palm oil stearin. Palm oil can be used as a fuel oil

18

for heat production, but due to its relatively high melting point, it needs to be heated

to reduce its viscosity before combustion(Prof & Esa, 2012).

Palm oil is derived from the pulp of the fruit of the oil palm Elaesisguineesis.

Palm oil is one of the few vegetable oil high saturated fats. It is thus semi-solid at

typical temperate climate room temperatures, though it will more often appear as

liquid in warmer countries. Palm oil is naturally reddish because it contains a high

amount of beta-carotene. Good palm oil must have lower amount of mono and

diglycerides compositions. Palm oil is a common cooking ingredient in Southeast

Asia and the tropical belt of Africa. Its increasing use in the commercial food

industry in other parts of the world is buoyed by its cheaper pricing and the high

oxidative stability of the refined product.

Table 2.1: Properties of palm oil (Fei & Teong, 2008)

Fatty acids Percentage (%)

Lauric, C-12:0 0.1

Myristic, C-14:0 1

Palmitic, C-16:0 42.8

Stearic, C-18:0 4.5

Oleic, C-18:1 40.5

Linoleic, C-18:2 10.1

Linolenic, C-18:3 0.2

Other/Unknown 0.8

2.9 Methanol

Alcohols such as methanol are the most frequently employed especially in trans-

esterification reaction to form methyl ester or glycerol. The process involved the

molar ratio between methanol and oil. Regarding to literature by Garino et al.,

(2013), the molar ratio requires for reaction stoichiometric is 3:1 molar ratio of

alcohol to oil with an excess of alcohol. The low molecular weight of alcohol being

used is ethanol. However, by using methanol, the greater conversion can be

achieved. Based on physical and chemical advantages, methanol is more preferred

19

due to its relatively cost. The advantages of methanol due to capability in provide a

proper viscosity and boiling point and a high cetane number of the produce biodiesel.

The physical condition of methanol is an alcohol and is a colorless, neutral,

polar and flammable liquid. The methanol also can miscible with water, alcohols,

esters and most other organic solvents. The physical of methanol is only slightly

soluble in fats and oils. Methanol as a volatile organic compound also can be present

in contaminated condensates. Other common names for methanol include methyl

alcohol, methyl hydrate, wood spirit, wood alcohol, and methyl hydroxide. Current

implementation process and product produce by methanol are include windshield

washer antifreeze, fuels, waste water treatment and biodiesel production.

2.10 Non-catalytic method

Non-catalytic method is another method to overcome the problem occurs during

catalytic trans-esterification method. This technology has been produced by Saka and

Dadan (Ang, Tan, Lee, & Mohamed, 2014). Both trans-esterification and

esterification reaction occurs simultaneously in the non-catalytic reaction method.

Therefore no purification process before and after the reaction as a catalyst methods

do because existing of catalyst in the process. Trans-esterification is the reaction

between oil and methanol to obtain biodiesel (fatty acid methyl ester (FAME) and

glycerol). FAME can be produce by catalytic method or non-catalytic method.

Currently, several methods have been used in order to produce biodiesel. The

conventional method is using homogeneous base catalysts. However, the purification

steps are required in this process to remove the catalyst and saponified products,

ultimately leading to lower yields of biodiesel (Asri et al., 2013). This process also

involved high production cost and energy consumption.

The trans-esterification with heterogeneous catalysts is another alternative

method to produce biodiesel from crude palm oil. An advantage of this method is

able to overcome the homogeneous base catalyst process. These processes are

expected to be an effective process also provides minimal impact towards

environment (Asri et al., 2013). Disadvantages of this process are required relatively

longer reaction time and high amount of catalyst. Both methods that have been

described are known as catalytic trans-esterification process. Another method is

superheated methanol known as non-catalytic process to produce biodiesel.

20

Superheated liquid refers to a liquid that is heated to a temperature higher

than the boiling point. Production of biodiesel by superheated methanol are involved

the heating process. Methanol liquid is heated to a high temperature above 250°C

above the boiling point (64°C) at atmospheric pressure, 0.1 MPa and become as

vapor or gas (Asri et al., 2013). Methanol gas is introduced from the bottom of the

bubble column reactor into a liquid oil to produce biodiesel by trans-esterification

reaction without catalyst. In addition, by using this superheated methanol trans-

esterification, purification of biodiesel has become easier because there is no soap

and catalyst in the system.

2.11 Supercritical liquid

During supercritical method, the reaction between alcohol and liquid will be

pressurized by heating process until its critical points where this mixture will possess

unique solvating and transport properties. The immiscibility characteristics of

alcohol and oil at room temperature will be disappear and become as a single phase

(Ang et al., 2014). Subsequently, this mixture will be reacting in a short time period

to produce biodiesel. In addition, since the supercritical methanol is more likely to be

a real gas in terms of diffusivity and viscosity, there is no limit on the mass transfer

reaction, allowing the reaction to proceed in a short time and the mixing is not

necessary in supercritical methanol as reactants are in the form of homogeneous

(Saka & Minami, 2006).

CHAPTER 3

METHODOLOGY

Recently, many method has been using by researchers to review dynamic flow

behavior in bubble column. Some of the method has been using by researcher such as

experimental method, finite element method and simulation method by using

computational fluid dynamics. In consideration of costing, the evaluation of

performance in bubble column using approach by computational fluid dynamics is

highly effective compare to experimental setup. The purpose CFD packages use in

this research is ANSYS-FLUENT. These packages are based on finite volume

method and are used to solve fluid flow and mass transfer problems. CFD allow

users to simulate the performance of alternative configurations, eliminating

guesswork that would normally be used to establish equipment geometry and process

conditions. The use of CFD enables users to obtain solutions of problems with

complex geometry and boundary conditions. Advantages of CFD can be summarized

as:

1. It provides the flexibility to change design parameters without the expense of

hardware changes. It therefore costs less than laboratory or field experiments,

allowing users to try more alternative designs than would be feasible otherwise.

2. It has a faster turnaround time than experiments. It guides the users to the root of

problems, and is therefore well suited for trouble-shooting.

3. It provides comprehensive information about a flow field, especially in regions

where measurements are either difficult or impossible to obtain.

22

ANSYS FLUENT analysis is the main step in obtaining the result. Regarding to

ANSYS FLUENT procedure, there are some basic steps need to follow include

sophisticated user interface input problem parameters and to examine the result.

Hence, this chapter explained in detail the methodology step based on Figure 3.1

which were shows the CFD modeling overview as a guide in analysis procedure.

Figure 3.1: CFD modeling overview

3.1 Flow chart

Figure 3.2 show the flow chart of methodology used referring to the planning of

analysis and working methods to ensure that every working step was carried out with

systematic and efficient.

23

Figure 3.2: Flow chart of methodology

24

3.2 Pre-processor

Pre-processing consist inputs of flow problem and also become as operator friendly

interface and subsequent transformation of this input into a suitable form for

application in solver. Pre-processor step performed by using SOLIDWORKS for

design and Meshing Design Modular consist in ANSYS FLUENT packages. Hence,

below are list of step involved in pre-processing stages.

1. Define the geometry of the region for computational domain.

2. Generated grid for subdivision of the domain.

3. Specified appropriate boundary and continuum conditions at cells

Each cells in various nonlinear equation form used to define the solution of flow

problem such as velocity, phase holdup, pressure, and temperature and etc. Influence

of accuracy in CFD solution governed by number of cell in grid of meshing.

Basically, large numbers of cell produce better solution of accuracy.

3.2.1 Model geometry

This analysis used CAD implementation by packages SOLIDWORKS software in

modeling bubble column geometry before export into ANSYS FLUENT. Three types

of bubble column were created in 3D with same in column height, column diameter

and distributor holes arrangement of reactor systems but different holes diameter of

distributor. The references of geometry were referred based on previous studied and

analysis by (Wulandani, Ilham, & Hagiwara, 2012.), (Wulandani et al., 2015) and

(Sagara, 2007). This reference was taken for ease of validation process between

obtained data and previous study. This interactive process is the first stage in pre-

processing before produce meshing. Below are list of table and figure consisting

geometry of bubble column.

56

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