57
CHAPTER V CONTINUOUS PRODUCTION OF PALM METHYL ESTERS
IN A BUBBLE COLUMN REACTOR
Introduction
The bubble column reactors have been widely used for conducting gas- liquid reactions in numerous industrial applications. Yamasaki et al [14] and
Joelianingsih et al [63, 65] have been reported a catalyst-free biodiesel production method using a BCR at atmospheric pressure. Transesterification of triglycerides
experiments have been made in a semi-batch mode reactor in which bubbles of superheated MeOH vapour were blown into the oil containing reactor. However, a
continuous-flow BCR for biodisel production by non-catalytic transesterification has not yet been revealed. Continuous transesterification processes are preferred
over batch processes in large-capacity commercial production due to the consideration of consistent product quality and low costs of capital and operation
per unit of product. Continuous transesterification of vegetable oils to mono-alkyl esters was proposed as early as in the 1940s [66, 67] and studied until recent years
[36, 43, 44, 68, 69]. In this study, the effects of the MeOH feed flow rate 1.5, 2.5, 3.0, 4.0 and 6.0 mLmin and reaction temperatures 250, 270, and 290
o
C on the productivity and methyl esters ME content in the product were conducted on
a laboratory-scale continuous-flow BCR system with a fixed liquid palm oil volume in the reactor of 200 mL. Palm oil and methanol were pumped
continuously to the reactor and product in the gas phase was removed continuously from the reactor. This study is focused on determining the optimal
residence time for maximum production of biodiesel from palm oil with a good quality.
Materials and Methods Materials
Refined palm oil was obtained from Spectrum Chemical Mfg. Corp., Gardena, New Brunswick, with the following characteristic: Iodine Value 50-55,
free fatty acid as oleic 0.1 ww, myristic acid, 0.5-5.9 ww; palmitic acid, 32-47 ww; stearic acid, 2-8 ww; oleic acid, 34-44 ww; linoleic acid, 7-
58 12 ww. Methanol p.a Pro analysis of min. 99.8 purity was purchased from
Merck, Germany. Benzene and hexane all HPLC grade used in the TLCFID analysis were
purchased from Wako Pure Chemical Industries, Ltd., Japan. Benzene was used as developing solution, and hexane as solvent. Squalane C
30
H
62
as internal standard used in the TLCFID analysis was purchased from Sigma-Aldrich Japan. Triolein,
diolein, monoolein, oleic acid and metlyl oleate used as standards solution in the TLCFID analysis were obtained from Sigma Chemical, St. Louis, MO.
The Continuous-Flow BCR System
Schematic flow diagram of the laboratory-scale continuous-flow BCR system used in the experiment is shown in Fig. 19. The photograph of equipment
is shown in Appendix 7.
Fig.19 Schematic flow diagram of the continuous-flow BCR system. The system consisted of a vaporizer VR, a superheater SH, a bubble column
reactor R1, a level controller B1, a condenser Cd1 and a glass container for sample collector F1. To control of the flow process, the system was equipped
with 8 valves V1– V8. The R1 is a bubble column with a draft inner tube installed with two
perforated plate in the bottom and the top of the inner tube. Each of the perforated plate had 30 holes with the diameter of one hole was 4 mm. The inner tube had a
VR SH
H5
59 diameter of 43 mm and a height of 90 mm. The inside and outside diameter of the
column were 55 mm and 71 mm, respectively and height of the column was 210 mm. The nominal liquid hold-up in the column was about 200 mL. The material
of the column and inner tube was 316 stainless-steel 316SS. The VR, SH and R1 were equipped with 5 electrical heaters H1–H5 and 5 temperature controllers.
The B1 was equipped with an inner pipe O3. The F1 was equipped with one pipe in the bottom O2 and one pipe in the top O1.
Transesterification Reaction Procedure and Conditions
The R1 was initially charged with about 200 mL of the refined palm oil via B1 and V5. Before and during charging of the oil, nitrogen gas N
2
was flowed
to fill oil pipe with N
2
and to prevent the backflow to methanol area
. The N
2
was taken out via O1. The oil in the R1 heated to the desired temperature. Liquid MeOH was
pumped out of the storage glass at various flow rates 1.5, 2.5, 3.0, 4.0 and 6.0 mLmin to the vaporizer for vaporization flow of N
2
was stopped. First, the effect of the MeOH feed flow rate on the productivity was studied at the same
reaction temperature 290
o
C. Then, the effect of reaction temperature 250, 270 and 290
o
C on the productivity was studied at the same MeOH feed flow rate. The MeOH vapor was taken through a superheater and the reaction started by
blowing the bubbles of superheated MeOH 0.1 MPa, 250, 270 and 290
o
C continuously into the reactor. Palm oil was pumped to the reactor continuously via
V3 to maintain the level of oil in the reactor. The excess of oil will be removed via O3. Reacted products in gas phase were condensed and collected in the F1.
The reaction products were taken from the F1 via O2 every 30 min and then weighed samples A. During the reaction course 300 min, 10 samples were
collected.
Analysis
TLCFID was used to analyze contents of TG, DG, MG, free fatty acids FFA, and ME in the samples of reaction products without MeOH [12]. Methanol
contents in the samples A were evaporated using a rotary evaporator and then the biodiesel products were obtained samples B. Each of samples B was weighed
and its composition was analyzed using TLCFID. Analyses were performed with
60 an Iatroscan MK-5 Analyzer Iatron Laboratories, Inc., Japan. The FID used
hydrogen and air with flow rates of 160 mL and 2000 mLmin, respectively. Type SIII Chromarods were used as thin layer. Before being spotted, rods were scanned
as blank on the instrument to obtain the proper degree of hydration. The samples B 20-30 mg were diluted with 1 mL solvent 250 mg squalane in 50 mL hexane
and 1 µL of the solution was spotted on each rod. Five replicates were used for each solution. The rods were developed for 30 min they were stored in a glass
chamber in which the atmosphere was saturated with benzene vapor, oven dried at 110-130
o
C for 5 min and then analyzed on the Iatroscan. The mass fraction of TG, DG, MG, FFA and ME in the samples B was calculated based on the
concentration of internal standard.
Results and Discussion
Generally speaking, the operation of a BCR reactor for biodiesel production by non-catalytic transeseterification of TG is complicated because consists of
several processes such as diffusion of MeOH gas into liquid oil, transesterification reaction of TG in the liquid phase and distillation. Studies of
the mechanism and kinetics of transesetrification have shown that this reaction consists of a number of consecutive, reversible reactions as presented in Eqs. 3,
4 and 5. In this case, MeOH was not only as reactant, but also as a carrier gas that has a same role with steam in the steam distillation process. Methyl ester and
GL as the reaction products were extracted by MeOH vapor and separated from the reaction zone liquid phase; this implies a shift of the reaction to the right and
an increase in conversion rate of the reaction. Therefore, the completion of the reaction depends on multiple parameters, including the MeOH feed flow rate,
temperature, reaction time, and properties of the feedstock and all of component in the reaction mixture. The optimum operating conditions are determined as the
result of systematic investigations of all operating parameters. However, the complexity of the BCR system is minimized when applied to the biodiesel
production for two reasons. First, the difference between the boiling temperatures of reaction product GL FAME and TG is so large that the separation of the
reaction product from the product mixture becomes easy Table 6. Second, the
61 transesterification reaction occurs in the liquid phase only. The column below the
feeding point of oil determines the reaction zone. The reaction time is then established by the total liquid hold-up and the feeding rate of the oil.
Effect of the MeOH Feed Flow Rate on the Productivity
Productivity refers to the amount of GL or FAME kg produced per unit of liquid volume in the reactor L per unit time h. Figure 20 shows mass of the
biodiesel product every 30 min reaction time at the various MeOH feed flow rate and 290
o
C reaction temperature. As presented in Fig. 20, productivity increased with the reaction time and to
be constant after close to 300 min reaction time. Productivity increased with MeOH feed flow rate, except at the 4.0 mLmin MeOH feed flow rate. In addition,
only at 2.5 and 3.0 mLmin MeOH flow rate, the products consisted of two layers; FAME in the upper layer and GL in the bottom layer after 300 and 270 min
reaction time respectively. At the 1.5, 4.0 and 6.0 mLmin MeOH feed flow rate, the product consisted of only one FAME layer.
2 4
6 8
10 12
14 16
18 20
30 60
90 120
150 180
210 240 270
300 330
Reaction time min M
a s
s o
f th
e p
ro d
u c
t g
1.5 mLmin 2.5 mLmin
3.0 mLmin 4.0 mLmin
6.0 mLmin
Fig. 20 Mass of the product at the various MeOH feed flow rate at 290
o
C.
62 Productivity at the 2.5 mLmin = 0.593 kgLh MeOH feed flow rate was 0.006
kg GLLh and 0.058 kg FAMELh after 300 min reaction time with the mass flow rate of oil was 0.06 kgLh. While at the 3.0 mLmin = 0.711 kgLh MeOH
feed flow rate, productivity was 0.014 kg GLLh and 0.128 kg FAME Lh after 270 min reaction time with the mass flow rate of oil was 0.13 kgLh.
As mentioned above, MeOH was not only as reactant, but also as a carrier gas that extracted GL and ME from the liquid phase so that these products were
separated from the reaction zone. If the MeOH feed flow rate was very fast 3.0 mLmin, the contact between MeOH and oil was very short so that the role of
MeOH as a carrier gas was more dominant than as a reactant. As a consequence, the transesterification reaction did not conduct completely, particularly the
reaction of step 3 Eq. 5, namely the MG react with MeOH to form FAME and GL. In addition, formation of FAME and GL from MG is believed as a step which
determines the reaction rate the slowest reaction rate, since MG are the most stable intermediate compound [62].
Effect of the MeOH Feed Flow Rate on the ME Content
The characteristics of biodiesel are similar to diesel fuels, and therefore biodiesel becomes a strong candidate to replace the diesel fuels if the need arises.
The primary criterion for biodiesel quality is adherence to the appropriate standard. The ME content is one of the important parameter in the biodiesel
standard such as in Europe EN 14214 and in Indonesia SNI 04-7182-2006. Generally, the fuel quality of biodiesel can be influenced by several factors,
including the quality of feedstock, the fatty acid composition of the parent vegetable oil or animal fats, the production processes, other materials used in the
process, and postproduction activities. The ME content is a parameter influenced by the production processes namely in the reaction and separation process after
reaction. In the catalytic conventional process, the high ME content can be obtained as the reaction conversion of the reactant TG and the intermediate
products DG and MG to form ME in the reactor conduct completely. Beside that, separation of the FAME from other components after reaction determines the
purity of the FAME in the final product. In the BCR, chemical reactions and
63 product separations occur simultaneously in one unit. Effect of the MeOH feed
flow rate on the ME content in the product at 290
o
C reaction temperature is given in Fig. 21.
50 55
60 65
70 75
80 85
90 95
100
30 60
90 120 150 180 210 240 270 300 330
Reaction time min M
E c
o n
te n
t in
t h
e p
ro d
u c
t w
w
2.5 mLmin 3.0 mLmin
4.0 mLmin
Fig. 21 Effect of MeOH flow rate on the ME content in the product at 290
o
C. The ME content in the product increased with reaction time, but decreased
with the MeOH feed flow rate. As mentioned above if the MeOH feed flow rate was very fast 3 mLmin, the contact between MeOH and oil was very short so
that the role of MeOH as a carrier gas was more dominant than as a reactant. Therefore, the transesterification reaction didn’t conduct completely, especially
the formation of FAME from MG, since MG are the most stable intermediate compound. As a consequence, amount of MG in the reaction mixture increased
with the MeOH feed flow rate and then the MG were extracted in the gas phase, so ME content in the product decreased with the increasing of the MG content.
Figure 22 shows effect of MeOH feed flow rate on the MG content in the product at the 290
o
C.
64
1 2
3 4
5 6
7 8
9 10
30 60
90 120 150 180 210 240 270 300 330
Reaction time min
M G
c o
n te
n t
in t
h e
p ro
d u
c t
w w
2.5 mLmin 3.0 mLmin
4.0 mLmin
Fig. 22 Effect of MeOH flow rate on the MG content in the product at 290
o
C. The ME content in the product at the 2.5 mLmin of MeOH feed flow rate and
290
o
C reaction temperature was 96.7 ww after 300 min reaction time, while at the 3.0 mLmin of MeOH feed flow rate and 290
o
C reaction temperature, the ME content was 95.1 ww after 270 min reaction time. The minimum acceptable
purity of biodiesel ME content is 96.5 ww according to the European Union EU and Indonesia standards for biodiesel.
Effect of Reaction Temperature on the Productivity
According to the experiment results of transesterification in the semi-batch process Chapter III, conversion and yield increased with reaction temperature.
The conversion and yield are two major process parameters used to examine the efficiency of the BCR by semi-batch process, but in the continuous process,
productivity is an important parameter that can be use to measure the efficiency of the BCR. Productivity of FAME in the continuous process is constant after a fixed
reaction time, and at that time, conversion and yield can achieve almost 100. Figure 23 summarizes the experimental results of the effect of variations in the
65 reaction temperature from 250-290
o
C on the mass of the reaction product without MeOH at the same MeOH feed flow rate 3.0 mLmin.
2 4
6 8
10 12
14
30 60 90 120 150 180 210 240 270 300 330 Reaction time min
M a
s s
o f
th e
p ro
d u
c t
g
Fig. 23 Mass of the product at the various reaction temperature: Productivity of FAME calculated from mass of the product in Fig. 23 at
reaction temperature 250, 270 and 290
o
C were 0.010, 0.052 after 300 min reaction time and 0.128 kgLh after 270 min reaction time respectively.
Productivity of GL was not obtained at 250 and 270
o
C, but only obtained at 290
o
C. These results show that at 250 and 270
o
C formation reaction of GL from MG was very slowly so that only FAME that was produced from transesterification
reaction step one and two as given by Eqs. 3 and 4. The normal boiling point of GL is 290
o
C, while the normal boiling points of methyl esters are higher than normal boiling point of GL as shown in Tables 6 and 16, therefore if the methyl
esters can be obtained in the gas phase so GL can be obtained in the gas phase too. Thus, in the liquid phase didn’t not contain GL, but reaction of GL formation was
not conducted. Base on the productivity of FAME and GL, the optimum reaction temperature was 290
o
C.
ο T = 250
o
C ▲T = 270
o
C T = 290
o
C
66
Effect of Reaction Temperature on the ME Content
The reaction temperature influenced the ME content as mentioned in the chapter III, effect of reaction temperature on the ME content in the product at the
same MeOH feed flow rate 3.0 mlmin is presented in Fig. 24. The ME content increased with reaction time at the various of the reaction temperatures. Initially,
the ME content increased with reaction temperature, but after 210 min reaction time decreased with reaction temperature. Increase of reaction temperature, time
needed to achieve a stability of ME content was shorter. Time needed to achieve the ME content 94 ww at 250, 270 and 290
o
C was 240, 180 and 90 min respectively. At the end of reaction time 300 min, the ME content in the product
at 250, 270 and 290
o
C was 97.7, 96.5 and 95.1 ww respectively. These experimental results agree with the principles of reaction kinetics reaction rate
and thermodynamic chemical-reaction equilibrium. In the principle of chemical- reaction equilibrium, the exothermal characteristic of transesterification explained
the reason why this phenomenon occurred. The exothermal reaction has a characteristic that at lower temperature the equilibrium shift to the product [70],
so that the ME content increased with decreasing of reaction temperature.
50 60
70 80
90 100
30 60 90 120 150 180 210 240 270 300 330
Reaction time min M
E c
o n
te n
t in
t h
e p
ro d
u c
t w
w
Fig. 24 Effect of reaction temperature on the ME content in the product
ο T = 250
o
C ▲T = 270
o
C T = 290
o
C
67 In the principle of reaction kinetics, reaction rate increases with reaction
temperature, so that time needed to achieve a stability of ME content was shorter. In addition, the BCR has a same principle with a RD, in the distillation principle,
amount of non-volatile compound such as TG, DG and MG in the gas phase increased with reaction temperature so that the ME content decreased.
Effect of the Operating Condition on the Gas-Liquid Interfacial Area
The prediction value of the diameter bubble
b
d and the gas-liquid
interfacial area A
v
were calculated and the results are presented in Appendix 8. The effects of MeOH feed flow rate and reaction temperature 250, 270 and
290
o
C on the superficial gas velocity
SG
u , the diameter bubble and the gas-
liquid interfacial area are summarized in Table 18. Table 18 Effect of the operating condition on the
b
d and
v
A T
o
C MeOH flow rate
mlmin, liquid
SG
u ms
G
ε m
3
m
3
b
d m
v
A m
2
m
3
250 270
290 290
290 290
290 3.0
3.0 1.5
2.5 3.0
4.0 6.0
0.022 0.023
0.012 0.020
0.024 0.032
0.048 0.0715
0.0694 0.0402
0.0589 0.0676
0.0838 0.1136
0.0019 0.0018
0.0021 0.0018
0.0017 0.0016
0.0015 222.907
228.286 116.762
194.603 233.523
311.364 467.046
Results in the Table 18 show that
SG
u and
v
A increased with the MeOH feed flow
rate and reaction temperature, but the
b
d decreased the MeOH feed flow rate and
reaction temperature. Increase in the MeOH feed flow rate and reaction temperature caused increase in biodiesel productivity, except at 4.0 ml of MeOH
68 feed flow rate. The value of interfacial area for BCR is between 100 and 500
m
2
m
3
[71].
The Prediction of the Specific Energy Consumption
Material and energy balance calculation for 3.0 mL of MeOH feed flow rate and 290
o
C reaction temperature was presented in Appendix 9. Based on this
result, the mass and energy balance was made for 1000 kgh biodiesel product and the biodiesel production process flow diagram was proposed as presented in Fig.
25. The mol ratio of MeOH to oil is 148 molmol. Then the ratio of output to input energy without electricity and the specific energy consumption was
determined based on the values in Fig. 25.
Fig. 25. Biodiesel production process flow diagram
The ratio of output to input energy excluding the energy for electricity such as for pums obtained in this experiment was 26.5 and the specific energy consumption
was 1.5 MJkg biodiesel. The prediction result for the alkaline transesterification
69 shows that the ratio of output to input energy, excluding the energy for electricity
was about 35 and the specific energy consumption was 1.12 MJkg biodiesel. However, the raw materials cost for the non-catalytic process is lower because the
cost of chemical reagent such as acid and alkaline catalyst, neutralization reagent can be removed. In addition, the non-catalytic process is simpler and
environmentally friendly.
Conclusion
1. A laboratory-scale continuous-flow BCR system has been developed and
tested for biodiesel production from refined palm oil by non-catalytic transesterification. Preliminary results showed that the optimum MeOH feed
flow rate was about 2.5-3.0 mLmin at 290
o
C reaction temperature for a BCR with a fixed liquid volume in the reactor of 200 ml. The optimum value was
based on the productivity of FAME and GL and the ME content in the product. Productivity at the 2.5 mLmin = 0.593 kgLh MeOH feed flow
rate was 0.006 kg GLLh and 0.058 kg FAMELh with the ME content was 96.7 ww after 300 min reaction time and the mass flow rate of oil was 0.06
kgLh. While at the 3.0 mLmin = 0.711 kgLh MeOH feed flow rate, productivity was 0.014 kg GLLh and 0.128 kg FAME Lh with the ME
content was 95.1 ww after 270 min reaction time and the mass flow rate of oil was 0.13 kgLh. The ratio of output to input energy excluding energy for
electricity of this experiment was 26.5 and the specific energy consumption was 1.5 MJkg biodiesel at 3.0 mLmin MeOH feed flow rate and 290
o
C the molar ratio of MeOH to oil was 148. The productivity and purity of biodiesel
should be enlarged. It is predicted that increasing the interfacial area and prolonging the residence time of methanol bubble in the liquid phase leads to
an improvement in the productivity and purity of biodiesel.
70
CHAPTER VI FINAL EVALUATION AND DISCUSSIONS
Kategori