International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:12 No:06
41
Process Analysis for Esterification and Two-step
Transesterification in the Biodiesel Production
Plant
Winardi Sani1 , Khalid Hasnan2 , Mohd Zainal Md Yusof3
Abstract—
Esterification and transesterification reacting
vessels are the core unit operations of typical industrial biodiesel
production plants. Feedstock with a high free fatty acid is
esterified first in an acid condition before continuing to the
transesterification under presence of an alkaline catalyst. Process
analysis is an important tool to a plant engineer in the biodiesel
plant operation to estimate the conversion of the palm oil to
biodiesel and the yield. This paper describes the process analysis
for the methanolysis of crude palm oil through the esterification
and the subsequent two-step transesterification in the biodiesel
production plant with a capacity of 1000 kg per batch. Physical
pretreatment of the crude palm oil (CPO) is necessary to remove
the unsaponifiable and other undesired trace components to
become bleached palm oil (BPO). Conversion at 85% (w/w) of
free fatty acid (FFA) to biodiesel has been achieved in the
esterification of BPO with methanol under acid catalyst reaction.
The first transesterification is able to produce up to 88% (w/w)
conversion of triglycerides (TG) to biodiesel. The remaining TG
is carried out in the second step of the transesterification to
complete the reaction toward achieving a high methyl ester
content. Analytical method using gas chromatography is used for
validation against the theoretical results. GC analysis results
conforms the conversion estimated by the process analyses based
on the material balance, especially in the esterification and firststep transesterification, 81% and 88%, respectively. After one
hour retention time of the second-step transesterification, 95%
conversion of TG to biodiesel has been achieved. The process
analysis applied at the equilibrium states shows consequently in
accordance with the GC analysis results. Therefore, it offers a
useful compendium to a plant engineer for better understanding
of the biodiesel processes.
Index
Term—
Biodiesel,
Esterification,
transesterification, Material balance.
Two-step
I.
INTRODUCTION
Biodiesel has attracted the attention of many researchers and
engineers more than two decades worldwide to prolong the
lifetime of the fossil-based fuel. Issues in the environments,
the limited reserves of petroleum, and the high cost of
1
[Winardi Sani is with the Department of Mechanical Engineering T echnology
(Plant), Faculty of Engineering T echnology, Universiti T un Hussein Onn
Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia
winardi@uthm.edu,my]
2
3
biodiesel as well as the oil price fluctuation in the market,
among other things, are the major driving forces in conducting
research and development in the renewable energy sector
either in the lab scale or the industrial one. The pilot plant in
UTHM with a capacity at one metric ton (MT) and operated
in batch mode under a supervisory control and data acquisition
(SCADA) system, has been constructed to strengthen a
promising research in area of renewable energy. Crude palm
oil (CPO) is chosen as the dominant feedstock due to the
abundance of this crop in the State of Johor which is also the
biggest producer of palm oil in the Peninsular of Malaysia
with around 0.7 Mha of the plantation area for the palm trees.
Owing to the grandness of the palm oil to the community and
to sustain the inherently local strength, UTHM has taken a
prudent initiative to explore the potential niche area in the
refinery of the biodiesel production. The block flow diagram
(BFD) of the biodiesel plant is depicted in Figure 1.
[Assoc. Prof. Dr. Khalid Hasnan, khalid@uthm.edu,my]
Fig. 1. BFD of the biodiesel production plant
II.
PROCESS DESCRIPTION
The CPO stored at a temperature 40 \celsius~is fed into the
degumming and bleaching vessel of the pretreatment plant.
Phosphoric acid is used to remove the phospholipids due to
their strong emulsifying action [7]. The operating conditions
are kept under vacuum at a temperature of 90 – 110 o C to
make the CPO free of moisture. The dried oil is treated with
bleaching earth or clay to adsorb the residual colour. The
[Prof. Emiritus Ir. Mohd Zainal Md Yusof : mdzainal@uthm.edu,my]
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mixture of oil is the passed through to the 10 µm filter for
separation of the spent earth from the oil. The obtained oil
refined in the pretreatment plant is a bleached, degummed, dry
crude oil and yellow-reddish in colour. Researchers [4,5,6]
reported that the esterification process is required if the
feedstock has more than 0.5% by weight of free fatty acid.
The transesterification of the oil is used to convert the
remaining oil completely into biodiesel. These two chemical
reactions are the core processes in the biodiesel production.
The downstream processes are employed for the purification
of the crude biodiesel, the recovery of the methanol, and
neutralization of the glycerol byproduct, along with the
treatment of the waste water.treatment of the waste water.
Esterification refers to the alcoholysis of triglycerides or
renewable oil under presence of an acid catalyst, and if an
alkaline catalyst is employed instead of the acid counterpart,
the process is called transesterification [2,9]. The overal
chemical reaction of the transesterification of palm oil is
described by the following stoichiometric equation:
⇔
(1)
where PO, M, G, and E, each represents palm oil, methanol,
glycerol, and methyl ester or biodiesel, respectively.
Theoretically, under the appropriate conditions of pressure and
temperature, in the presence of a catalyst, each mole of palm
oil requires three moles of methanol to produce three moles of
biodiesel and one mole of undesired glycerol. Since the
reaction is reversible, the forward direction is in favour toward
the desired product. The esterification process hereby takes
place in one hour with water as the by product. However, this
process also yields the desired biodiesel at a certain extent of
conversion and glycerol as the byproduct. Water and glycerol
resulted from the reaction must be discharged after
completion. The subsequent transesterification is done in two
steps. Removal of glycerol by manually phase separation is
done before proceeding to the second step. At the end of the
transesterification, hot water at 5% (w/w) is introduced gently
to the vessel to capture the remaining glycerol and a vacuum
flashing follows thereafter to ensure the crude biodiesel free of
water. The operating conditions for both processes are at 65 o C
and 2 bar to ascertain the reacting mixture being in liquid
phase. This higher pressure is established by introducing
nitrogen gas to the mechanical-agitated vessels.The
esterification process hereby takes place in one hour with
water as the by product. However, this process also yields the
desired biodiesel at a certain extent of conversion and glycerol
as the byproduct. Water and glycerol resulted from the
reaction must be discharged after completion. The subsequent
transesterification is done in two steps. Removal of glycerol
by manually phase separation is done before proceeding to the
second step. At the end of the transesterification, hot water at
5 % (w/w) is introduced gently to the vessel to capture the
remaining glycerol and a vacuum flashing follows thereafter
to ensure the crude biodiesel being free of water. The
operating conditions for both processes are at 65 o C and 2 bar
to ascertain the reacting mixture being in liquid phase. This
42
higher pressure is established by introducing nitrogen gas to
the mechanical-agitated vessels.
III.
PROCESS SPECIFICATION
This quantifies the amount of the reacting components
necessary for the entire processes of producing biodiesel from
palm oil. The process spefication is indicated in Table 1.
In the esterification of the bleached, degummed palm oil. para
toluenesulfonate, C7 H8 O3 S, abbreviated with PTSA, is
employed for the acid catalyst. Sodium methoxide (NaOCH 3 )
30% acts as the alkaline catalyst in the first and second
transesterification. This specification makes the plant operator
convenient in preparing the chemical materials. Each reaction
occurs in nitrogen blanket which ensures the inherently safe
condition.
T ABLE I
REACT ION SPECIFICAT ION /1000 KG OF OIL
Ratio To Oil
Reaction
Esterification
First
Transesterification
Second
Transesterification
IV.
M eOH [mol]
PTSA[wt %]
3
0.3
NaOM e
[kg]
-
1.25
17.7
1.25
5
MATERIAL AND MODEL
The crude palm oil (CPO) with a food grade is purchased
from the local palm oil refinery. The FFA level as palmitic is
at 3.4 % (w/w) , and the moisture content of 0.2 % (w/w) by
measurement. It means the oil contains 94.6 % (w/w) of
triglycerides and other trace components. [1] reported that the
triglycerides comprises of five different fatty acids, seeTable
2, the average molecular mass M=848.24 kg/kmol. To enable
analysing the mass balance in the chemical reactions, the palm
oil is modeled as tripalmitic due to the major contribution in
the composition. Methyl palmitate or palmitic acid methyl
ester is therefore the biodiesel under this study.
The general chemical reaction in Eq. (1) becomes therefore a
methanolysis of tripalmitin as stated in Figure 2.
Fig. 2. Methaloysis of T ripalmitin
This stoichiometric equation is used to determine the
theoretical yield and the conversion of the triglycerides. The
conversion of the reacting component, $X_i$~is defined as
follows:
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(2)
where n i0 and n i are the moles of the reacting components
before and after reaction in a volume-constant batch reactor V.
V.
A NALYSIS AND RESULTS
For the theoretical analysis purpose, the fatty acid attached in
the glycerol backbone are modeled solely as the
tripalmiticacid. Additionally, GC analysis for methyl ester
content determination of samples during the plant operation is
also applicable for validation. The material balance will be
applied following the operating stage performed in the
esterification
T ABLE II
MOLAR MASS OF PALM OIL
Triglyceride
Chemical
M % [kg/kmol]
structure
Trimyristin
1
C45 H86 O6
7.2316
Tripalmitin
45
C51 H98 O6
363.2940
Tristearin
4
C57 H110 O6
35.6592
Triolein
39
C57 H104 O6
345.3185
Trilinolein
11
C57 H98 O6
96.7323
and transesterification vessels of the actual plant. The process
calculations of this balance is performed based on the mole
unit, and it is however tabulated for convenient in the mass
unit.
Fig. 4. M aterial Streams for Esterification
Comp.[%]
A. Esterification
The FFA level of 3.4% (w/w) needs to be reduced to 0.5%
maximum to avoid saponification problem in the
transesterification. Esterification itself is a chemical reaction
similar to the Figure \ref{chapter4:fig:tripalmitintrans}, with
the difference in the catalyst used. Instead of an alkaline
condition, esterification employs an acid catalyst. Converting
the FFA into the biodiesel governs the stoichiometric as
shown in Figure 2.
Fig. 3. Esterification of FFA
Refering to Figure 3, R stands for palmitic acid,
CH3 (CH2 )14 COOH. The reacting vessel VE 201 in Figure 4
illustrates the material streams at the inlet and outlets for the
esterification.
Lowering the FFA level to 0.5\% yields 85.3\% conversion to
the desired product, or it is equivalent to 30.6 kg of biodiesel,
see Table III.
T ABLE III
MAT ERIAL BALANCE FOR FFA REDUCT ION
Inlet Streams [kg]
Outlet
Streams[kg]
1
2
3
FFA
34.0
5
MeOH
96.0
92.4
PTSA
3.0
3.0
Water
2.0
4
FAME
30.6
TOTAL
135.0
135.0
The theoretical conversion of the FFA (M=256 kg/kmol), X,
into biodiesel analyzed through the following relationship:
Material
(3)
The conversion of FFA to biodiesel is 85.3\%, or in other
word, 5 kg of FFA remains in the oil after esterification. The
conversion of the FFA to biodiesel stops at this level due to
water accumulation that hinders toward completion of the
reaction process. Refering to Table 3, 30.6 kg of crude
biodiesel (Fatty Acid Methyl Ester, FAME) is obtained and
water also generated (4 kg) by converting the FFA level down
to 0.5%. Its function is to accelerates the reaction process
without getting involved in the reaction. During the
esterification of FFA to biodiesel, an acid transesterification of
triglyceride takes also place. The material balance of this
process is formed using the stoichiometric equation in Figure
2 but under presence of an acid catalyst. With ΔmTG = 187.14
kg, it yields 81% conversion. FAME is the desired product in
the esterification along with reducing the FFA content. The
first crude biodiesel produced through the esterification comes
from the FFA conversion and the TG reaction. In other words,
m3,FAME = ΔmFFA + ΔmTG . And the theoretical yield, referring
to the definition in [8] is Φ = 80.7%. The yield, Φ is defined
as the weight percentage of the final product relative to the
CPO weight at the initial stage. Water and glycerol are
discharged to the glycerol neutralization vessel by separation.
The main product is then transfered to vessel VE 202 for
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transesterification reaction.
B. First Transesterification
The acid catalyst resulted from the previous esterification
must be first neutralized. However, the conversion of the TG
to biodiesel takes place at a moderate level. The reaction shall
therefore be completed in the second reaction to th e desired
conversion up to 96.5 % minimum.
44
the conversion of the FFA into s oap in the alkaline condition.
5 kg of FFA produces 5.43 sodium salt that must be remo ved
after transesterification. The amount of the alkaline catalyst
required for the neutralization of the FFA is calculated as
follows:
(6)
C. Neutrailization
The acid condition of the oil mixture in the previous process
must be neutralized before proceeding to the alkaline
transesterification. The alkaline catalyst is employed to
neutralize the existing PTSA (CH3 C6H4 SO3 H). This process
is described in the following stoiochiometric equation:
Methylsulfuric acid sodium salt (M = 134.09 kg/kmol) formed
in the Eq. (1) is soluble in water. Toluene (normal boiling
point at 110.6 o C) is also produced during this reaction and it
will be removed in the vacuum flashing.
PTSA acts as the limiting reactant and it therefore reacts with
the sodium methoxide completely at the end of the process.
Based on the stoichiometric equation, 1 mole of sodium ion is
required to form the soap. Since the methoxide exists in the
form of sodium methoxide at a concentration of 30 %, 1.336
kg of the alkaline chemical (or 0.95 kg of NaOCH3 ) is
required to thoroughly neutralize 3 kg of the acid catalyst
(PTSA). Table 4 shows the material balance for the reaction
process. 2.34 kg of salt is formed after the neutralization stage,
and it must be removed after the first-step transesterification.
T ABLE IV
MAT ERIAL BALANCE FOR FFA NEUT RALIZAT ION
Inlet Streams [kg]
Material
1
FFA
NaOMe
Salt
Toluene
TOTAL
2
3.00
16.40
19.40
Outlet
Streams[kg]
3
5
15.45
2.34
1.61
19.40
D.Soap Formation by FFA
The dissolution of the sodium methoxide in the methanol
leads to the formation of the methoxide ion and methanol. The
remaining free fatty acid (FFA) resulted from the previous
esterification is then converted by the sodium methoxide to
soap (sodium palmitate, M = 278.41 kg/kmol) according to the
following reaction:
For the material balance calculation, the FFA is used as the
limiting reactant since it must be totally removed and onverted
into soap. The neutralization process produces also methanol.
Table 5gives the results of the material and mass balance for
where the values of the remaining FFA at 0.5 wt. %, M FFA =
256.4 kg/kmol, and M MeOH = 54.02 kg/kmol as well as mTG
=187.43 kg. The sodium ion is present in the sodium metoxide
30% by weight, then:
(7)
In the first step transesterification, the remaining triglycerides
are converted to biodiesel
T ABLE V
MAT ERIAL BALANCE FOR SOAP FORMAT ION
Inlet
Outlet
Streams [kg]
Streams[kg]
1
3
FFA
5.0
NaOMe
15.46
14.41
Soap
0.00
5.43
Methanol
104.00
104.62
TOTAL
124.46
19.40
The following section analyses the biodiesel formation in this
step.
Material
E. Alkaline catalyst and methanol
Sodium methoxide 30\% by weight required to neutralize
the acid catalyst and the FFA has been determined previously.
The methanol content of the alkaline catalyst must be included
when calculating the total methanol needed for the right osing
of the reactants. With 1.25 molar ratio methanol to the initial
TG, the actual molar ratio is above 80 % in methanol excess.
With 1.25 molar ratio methanol to the initial TG, the actual
molar ratio is above 80% in methanol excess. With 187.4251
kg as the remaining triglycerides or it is equivalent to 0.2322
mol (M = 807.3 kmol/kg). The actual amount of methanol in
the reaction is:
(8)
where mTG = 964.0 kg, M TG = 807.0 kg/kmol, and M MeOH =
32.0 kg/kmol. The quantity of the alkaline catalyst is set
through the relationship:
(9)
that is equivalent to 11.333 kg of sodium methoxide 30% (3.4
kg NaOH). The total sodium methoxide necessary is
13.225kg. That is the sum of the catalyst for neutralization and
the actual catalyst in the base condition. The total methanol in
the first transesterification is actually the sum of the methanol
in equation (8) and methanol inherently at 70% in the sodium
methoxide. It corresponds to 57.483 kg or 1.8 mol. The mole
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ratio of methanol to the oil is then 7.74:1, that means, the
methanol excess is 4.34 mole relative to the theoretical
stoichiometrics, as indicated in Figure 2. The neutralization of
the acid catalyst and the remaining FFA contribute to the low
conversion of the oil to biodiesel. The sodium methoxide
becomes less reactive as alkaline catalyst for the biodiesel
production. It acts first as the neutralizing reactant before as
the catalyst. It prevents the transesterification from
completion. The conversion of the oil to biodiesel therefore
reduces significantly. The total conversion of TG to biodiesel
yields XTG = 88.3%. Table 6 clarifies the overall mass balance
of the first transeterification process to produce the second
crude biodiesel. The glycerol produced during the reaction is
at 11.4% of the consumed triglycerides (74.61 kg).
T ABLE VI
MAT ERIAL BALANCE FOR T HE 1 ST T RANSEST ERIFICAT ION
Outlet
Streams[kg]
3
MeOH
48.61
TG
112.82
FAME
74.97
Glycerol
34
8.5118
TOTAL
244.91
244.91
After the first transesterification, the crude biodiesel
undergoes a phase separation based on a difference in density.
The reacting vessel VE 202 contains solely the crude biodiesel
and the remaining triglycerides.
Material
Inlet
Streams [kg]
1
57.48
187.4251
F. Second Transesterification
In the same vessel, the second transesterification is
accomplished to complete thoroughly the reaction to a higher
conversion of oil to the biodiesel product. With the desired
remaining triglycerides of 0.2 w/w % maximum , as required
in the EN 14214 standard, the remaining oil after the second
transesterification is then: the uncoverted oil ist 1.92 kg and
can be removed partly by means of the water washing and the
vacuum flashing at the end of the process. The methanol in
excess in chosen hereby, and the triglycerides becomes the
limiting reactant. The amount of the alkaline catalyst being
added reduces to 1.5 w/w % or 5.0 kg of NaOCH3 30%, which
is also specified in Table 1. The second step is accordingly the
ultimate transesterification, where the TG conversion to crude
biodiesel must be at the highest point for the complete
reaction. This is accomplished by the high excess of methanol.
The theoretical conversion at 96.5 % minimum shall
accordingly be also achieved in the real plant operation. Table
7 shows the material balance for the second transesterification.
45
T ABLE VII
MAT ERIAL BALANCE FOR T HE SECOND
T RANSEST ERIFICAT ION
Material
MeOH
TG
FAME
Glycerol
TOTAL
Inlet
Streams [kg]
1
95.78
112.82
34.00
208.60
Outlet
Streams[kg]
3
82.59
1.9179
111.43
12.65
208.60
VI.
GC A NALYSIS
GC analysis to determine the methyl ester content (between
C14 } and C24 ) in biodiesel follows EN14103:2003 method [3].
The FAME analysis is conveyed in a split injection into an
analytical column with a polar stationary phase and a flame
ionization detector (FID. The GC configuration used here is
the PerkinElmer Clarus 500, fitted with a capillary
split/splitless injector and FID. In order to determine the
retention times of the fatty acid methyl esters, methyl
heptadecanoate (C17 ) acting as the internal FAME standard
needs to be run. The ester content expressed as a mass fraction
in percent, is calculated using the following formula:
∑
(10)
where A is the total peak area from the ME, A EI is the peak
area corresponding to C17 , VEI the volume of C14 , and m is the
sample weight. The samples have been taken at a certain time
intervals of 10 or 15 minutes after each process which
retention time of each process during the plant operation is set
at one hour. Three hours are needed for the entire production.
The samples are measured first using the TLC method for
ester content determination. The appropriate samples are
hence selectively prepared for the GC analysis. The results of
this analysis is shown graphically in Figure 4. The values of
the ester content after esterification, the first-step
transesterification, and the second-step transesterification are
81 %, 88%, and 95%, respectively. Referring to the graph, the
chemical equilibrium is achieved both at the end of the
esterification and the end of the first transesterification. The
time dependency of the rate of the concentration change of the
reactants is therefore negligible. The conversion of TG to
biodiesel after one hour operation of the second-step
transesterification is at 95% can be understood that the
reaction is actually incomplete. The curve in the last region
indicates that the tendency to the higher conversion is
possible. By slightly increasing the retention time, the reaction
becomes definitely completed and the ultimate target of the
minimum conversion at 96.5\% can be accordingly achieved.
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chemically to an equilibrium in which state the conversion of
96.5% minimum shall definitely be achieved.
A CKNOWLEDGMENT
This research study has been funded by the "Fundamental
Research Grand Scheme" from the Higher Education Ministry
of Malaysia (FRGS Vot 1063).
REFERENCES
[1]
[2]
[3]
[4]
Fig. 4. Ester Content Profile
The sampes have been taken at a certain time intervals, 10 or
15 minutes after each process which retention time of each
process during the plant operation is set at one hour. Three
hours are needed for the entire production. The samples are
measured first using the TLC method for ester content
determination. The appropriate samples are hence selectiv ely
prepared for the GC analysis. The results of this analysis is
shown graphically in Fig. 4. The values of the ester content
after esterification, first-step transesterification, and secondstep transesterification are 81%, 88 %, and 95%, respectively.
Referring to the graph, the chemical equilibrium is achieved
both at the end of the esterification and the end of the first
transesterification. The time dependency of the rate of the
concentration change of the reactants is therefore negligible.
The conversion of TG to biodiesel after one hour operation of
the second-step transesterification is at 95% can be understood
that the reaction is actually incomplete. The curve in the last
region indicates that the tendency to the higher conversion is
probably still possible. By slightly increasing the retention
time, the reaction becomes definitely completed and the
ultimate target of the minimum conversion at 96.5% can be
accordingly achieved.
[5]
[6]
[7]
[8]
[9]
Yusof Basiron, Bailey's Industrial Oil and Fat Products Palm Oil, John
Wiley & Sons Inc. 2005 Vol, 6
Ayhan Demirbas, Biodiesel: a realistic fuel alternative for diesel
engines, Springer-Verlag London Limited, British Library Cataloguing
in Publication Data, 2008
EN14103:2003, Technical Committee CEN/TC 307, Fatty Acid Methyl
Ester (FAME), , Determination of Ester and Linolenic Acid Methyl
Ester Contents, EN14103:2003, CEN, Rue De Stassart 36 B - 1050,
Brussels, Apr 2003
Cheng Sit Foon and Choo Yuen May and Ma Ah Ngan and Chuah
Cheng Hock, Kinetics Study On Transesterfication Of Palm Oil, Journal
of Oil Palm Research, 2004.
Bernard Freedman and Royden O. Butterfield and Everett H. Pryde,
Variables affecting the yields of fatty esters from transesterified
vegetable oils, JAOCS, Vol, 61, pp 163 – 1643, 1984
Jo Van Gerpen, Biodiesel processing and productiontitle, Journal of
Fuel Processing T echnology, ElseVier, 2005, Vol. 86, pp 97 -1107..
In-Chul Kima and Jong-Ho Kim and Kew-Ho Lee and Tae-Moon T ak,
Phospholipids separation (degumming) from crude vegetable oil
by polyimide ultrafiltration membrane, Journal of Membrane Science,
Elsevier, no. 205, pp 113- 123, February 2002.
Octave Levenspiel, Chemical Reaction Engineering, Department of
Chemical Engineering Oregon State University, John Wiley & Sons,
ISBN 0-471-25424-X, 1999.
Jo Van Gerpen and Gerhard Knothe, The Biodiesel
Handbook, Fuel Properties Chapter, AOCS Press, 2005.
VII.
CONCLUSION
The process analysis is the significant tool for a plant engineer
for proper running biodiesel production plant to estimate the
product quality and yield. Starting from the pretreatment of
the CPO to bleached palm oil (BPO), and the subsequent
processes such as esterification, transesterification, and
purification as well as the methanol recovery have been
described to illustrate the important unit operations in the
biodiesel production plants. The conversion of 81% after
esterification and 88 % after first-transesterification measured
in the GC analysis conforms the estimated value resulted from
process analyses based-on material balance. Palmitic acid is
used for TG model since it is the major component in the fatty
acid profile for palm oil. The ultimate target of 96.5%
conversion of TG to biodiesel, can be achieved by a slightly
increasing the retention time of the second-step
transesterification in the actual plant operation. The process
analysis for the last step can be done when the reaction comes
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