simulation of liquid-gas for cpo-methanol mixing … · or esterification of free fatty acids. it...
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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
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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
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70
73
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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.
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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