STANDARDIZATION OF TRANSESTERIFICATIO PROCESSES OF
SOYBEAN ETHYL ESTERAND ITS EFFECT ON VISCOSITY
By:
Mekalilie Benjamin Bol and Hassan I. Mohammed1
1- Department of Agricultural Engineering, College of Agriculture Upper Nile University Po Box 1660 – Khartoum – Sudan.
2- Department of Agricultural Engineering, College of Agricultural Studies, Sudan University of Science and Technology, P. O. Box 72,
Shambat.
KEYWORDS: Soybean
oil, Ethanol, Transesterification and Renewable fuel
ABSTRACT
To aid the development of chemical composition models for diesel fuel
substitutes on vegetable oils and their derivatives, the transesterification of refined
soybean oil with alcohol as a renewable fuel to improve engine performance were
analyzed. The transesterification of refined soybean oil with ethanol was carried out at
6:1 and 9:1 molar ratio in the presence of 1 % Na OH to produce soybean ethyl esters.
The effect of process parameters such as molar ratio, preheating time, reaction
temperature, and settling time were studied to standardize the transesterification
process for estimating the highest recovery of ester with the lowest possible kinematic
viscosity. It was found that molar ratio; preheating time and reaction temperature was
significantly (P < 0.05) affecting the recovery of ester, whereas settling time had no
significant effect on the recovery. However, all the selected process parameters and
their interactions had significant effect on kinematic viscosity. Consequently, to
reduce the problem of the high viscosity in soybean oil that hinders its use as an
environmentally friendly biodiesel the study recommended adopting a specific
chemical model to perform the etserification process.
‫ألجل المساعدة في إنشاء نماذج لتركيبات كيميائية للزيوت النباتية ونواتجها الستخدامها كبديل لوقود‬
:‫الملخص‬
.‫الجازولين تم تحليل عمليات أسترة زيت فول الصويا النقي بوجود الكحول كوقود متجدد لتحسين أداء الماكينات‬
‫ في وجود واحد‬1 : 9‫ و‬1 : 6 ‫أجريت أسترة زيت فول الصويا النقي مع كحول اإليثانول بنسب موالرية‬
.‫بالمائة هايدروكسيد صوديوم إلنتاج أيستر إثيلي فول الصويا‬
‫تمت دراسة أسلوب تأثير المتغيرات الداخلة في عمليات األسترة والتي تشمل نسبة الموالرية وفترة ما قبل‬
‫التسخين ودرجة ح اررة التفاعل وفترة االستقرار لتوحيد قياس عمليات األسترة وألغراض تقدير أكبر نسبة‬
‫ وجد أن نسبة الموالرية وفترة ما قبل التسخين ودرجة ح اررة التفاعل‬.‫استخالص لاليستر ذو أقل لزوجة كينماتيكية‬
‫) على نسبة اإليستر المستخلص بينما لم تؤثر فترة االستقرار بصورة معنوية على‬P < 0.05( ً‫تؤثر معنويا‬
‫ لكل المتغيرات المختارة وللتداخل بينها تأثير معنوي على اللزوجة وتبعاً لهذا‬،‫ على كل حال‬.‫المستخلص‬
‫ أوصت‬،‫وألغراض خفض مشكلة لزوجة إيستر زيت الصويا حتى يمكن استخدامه كوقود حيوي صديق للبيئة‬
.‫الدراسة باستخدام نموذج كيميائي محدد إلجراء عمليات أسترة إيثلي زيت فول الصويا‬
INTRODUCTION
The use of modified vegetable oils as substitute to diesel fuel may provide a
short term source emergency fuel in time of fossil fuel shortages. Common vegetable
oils such as soybean oil and peanut oils can be used in diesel engines for short period
of time but eventually the high viscosity of these oils will cause deterioration of engine
performance (Geller, 2003). The primary problems associated with using straight
soybean oil as fuel in a compression internal combustion engine are caused by high
fuel viscosity attributed to the long fatty acid chain present triglycerides comprising
the oil. Transesterification of refined soybean oil with an alcohol provides a
significant reduction in viscosity, thereby enhancing the physical properties of the
renewable fuel to improve engine performance (Clark et al., 1984). However, the
transesterification is governed by process parameters such as molar ratio, catalyst
used, preheating time, reaction temperature and setting time. (Canakci and Van
Gerpen 1999) studied the effect of catalyst amount on methyl ester conversion of
soybean oil, using 1, 3 and 5 % sulphuric acid as catalyst. The reaction with each
amount was continued for 48h at 600C with a 6:1 molar ratio of methanol to oil. The
ester conversion for each catalyst was 72.7, 77.8 and 95.5 respectively. It was also
reported that acid catalyst transesterification is much slower than an alkali catalyzed
process. (Freedman and Pryde 1981) showed that molar ratio of alcohol to vegetable
oils was one of the most important variables affecting the yield of ester. The
stoichiometry of alcoholysis reaction requires three moles of alcohol for every one
mole of triglycerides to yield three moles of fatty ester and a mole of glycerol. A
minimum 4:1 molar ratio had been suggested for esterifictation of sunflower and
soybean oil for adequate conversions.
This study was carried out with the objective of standardizing transesterif-ication
process for refined soybean oil to use it as fuel blend with diesel.
MATERIAL AND METHODS
Fatty Acid Analysis: The estimation of fatty acids present in the refined soybean oil used
in the experiment was done using a Hewlett Packard make, 5890 series II model gas
chromatograph According to method of (Shingari, 1988).
Transesterification Process: The ethyl esters of refined soybean oil were prepared for the
experiment in place of methyl esters because ethyl esters have higher heat content and
cetane number due to extra carbon atom brought by ethyl alcohol during
transesterification process (Romain et al., 1995). The process was carried out as per
steps described in (Fig.1). The effect of process parameters shown in (Table 1) was
studied to standardize the transesterification process for estimating recovery of ester
as well as recovering ester of lowest possible kinematic viscosity. In all 27 samples of
refined soybean oil-ethyl alcohol having 9:1 molar ratio and another 27 samples
having 6:1 molar ratio were transesterified to study the effect of three levels of selected
preheating time, three levels of selected reaction temperature and three levels of selected
settling time on ester recovery and subsequent measure of their kinematic viscosity.
Each sample was replicated three times thus making 81 experimental units under each
selected molar ratio.
Estimation of Conversion to Ester: The estimation of conversion of vegetable oil to ester
was done by comparing the glycerol content in the vegetable oil and its ester by
saponification of refined soybean oil as well as that of ester using the method reported
by (Work and Work 1972). The Estimation of glycerol in the above solution was also
carried out according to the method of (Work and Work 1972). The estimation of
conversion of refined soybean oil used in the study to ester was done for the ethyl
ester having lowest kinematic viscosity of 5.03 cS. The ethyl ester of 5.03 cS kinematic
viscosity was obtained when transesterification process was carried out at the
conditions shown in (Table 2). A Systronics make, spectrophotometer 169 models was
used to measure the absorbance of standard glycerol having selected micromole,
blank, and glycerol present in oil and its ester. The spectropho-tometer was calibrated
before use as described by (Gupta, 1994). The experiment was statistically analyzed
as 3×3×3×2 asymmetrical factorial experiment in Complete Randomize Design (CRD)
according to the method described by (King 1995). There were 54 treatment with
three replication each thus having an experiment of 162 experimental unit.
Refined Soybean Oil
Ethanol Molar
Ratio 6:1, 9:1
Preheating of Refined Soybean Oil to Selected
Time at Selected Reaction Temperature
Mix
Table (1): Process Parameters Selected for Standardization of Transesterification
S. No.
1
2
3
4
5
Name of Parameter
Molar Ratio (vegetable oil : alcohol)
Preheating Time, (min)
Reaction Temperature, (°C)
Reaction Time, (min)
Settling Time, h
Levels Selected
6:1 and 9:1
20, 30, 40
45, 55, 60
60
24, 36, 48
RESULTS AND DISCUSSION
Fatty Acid Composition of Refined Soybean Oil:
The observed levels of content of different
fatty acids found in the oil sample are shown in (Table 3). It is evident that the output
of fatty acids analysis by gas chromatograph of refined soybean oil used in
experiment had 10.8% palmitic acid, 3.0% stearic acid, 26.5% oleic acid, 47.3%
linoleic acid, 9.0% linolenic acid, 1.4% eicosenoic acid, 1.1% erucic acid and 0.9%
other acids. The above results fall within the range specified by (Codd 1972) who
indicated a range of major fatty acids present in soybean oil at 7-11% palmitic acid, 26 percent stearic acid, 15-33% oleic acid, 43-56% linoleic acid and 5-11% linolenic
acid and with (Ryan et al., 1984) using refined soybean oil found that 10.8% palmitic
acid, 4.8% stearic acid, 23.8% oleic acid, 55.2% linoleic acid, 4.4% linolenic acid.
The spectrophotometric analysis revealed that the total glycerol in the oil was 0.210
and free glycerol in the ester was 0.015 thus, making the ester of lowest kinematic
viscosity recovered to be 92.86% (Table 3).
Table (3): Fatty Acid Composition of Refined Soybean Oil used in the Experiment
Sl. No.
1.
2.
3.
4.
5.
6.
7.
8.
Fatty Acid
Notation
Content (%)
Palmitic
C* 16 : 0
10.8
Stearic
C 18 : 0
3.0
Oleic
C 18 : 1
26.5
Linoleic
C 18 : 2
47.3
Linolenic
C 18 : 3
9.0
Eicosenoic
C 20 : 1
1.4
Erucic
C 22 : 1
1.1
Other
–
0.9
Free glycerol
0.015
Total glycerol
0.210
* C denotes carbon, followed by the number of carbons in the chain and then the number of double bonds in the chain separated
by colon.
(Table 4) shows the percent soybean
ethyl ester recovered by transesterification process carried out at different preheating
time, reaction temperature and settling time. It is evident that the recovery of soybean
ethyl ester observed at different selected process parameters varied between 73 to 87
and 85 to 93% for refined soybean oil and ethanol mixtures having molar ratio 9:1
and 6:1 respectively. It is, therefore, seen that higher recovery of 87% of ethyl ester
was obtained at 9:1 molar ratio when the refined soybean oil was preheated at 55°C
for 20 min, reacted in presence of 1 percent of Na OH at 55°C for 1h and then allowed
to settle for 24 h. The results are in lined with (Canakci and Van Gerpen, 1999) who
found
Effect of Process Parameters on Recovery of Ester:
Table (4): Recovery of Soybean Ethyl Ester under Different Transesterification Process Conditions
Preheating
Time
(min)
Reaction
Time
(h)
Reaction
Temperature
(0C)
Settling
Time
(h)
20
20
20
20
20
20
20
20
20
30
30
30
30
30
30
30
30
30
40
40
40
40
40
40
40
40
40
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
45
45
45
55
55
55
60
60
60
45
45
45
55
55
55
60
60
60
45
45
45
55
55
55
60
60
60
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
Ester Recovery
(%)
9:1 Molar Ratio
6:1 Molar Ratio
84
89
81
87
80
88
87
92
87
88
85
90
87
93
86
91
87
92
77
87
77
86
75
86
82
88
87
86
83
89
78
90
84
88
80
90
75
85
74
85
75
85
79
86
78
85
77
86
76
89
73
87
76
89
ester recovery of soybean ethyl ester of 6:1 molar ratio settled for 48 to 96h to be 87.8
to 95.1%. Results also indicate that the highest recovery of 93% of ethyl ester was
obtained at 6:1 molar ratio when the refined soybean oil was preheated at 60°C for 20
min and then reacted with ethanol at 60°C for 1h and then allowed to settle for 24 h.
The higher recovery of soybean ethyl ester ranging between 90 to 92% was possible
when refined soybean oil was preheated at 55 or 60°C for 20 min and then reacted at
55 or 60°C for 1h and allowed to settle between 24 to 48 hours. Based on the observations
of percent recovery of ethyl ester from transesterification of refined soybean oil
selected for the experiment, it may be concluded that the oil be preheated to 60°C for
20 min and reacted with anhydrous ethanol mixed with 1% Na OH by oil weight and
having 6:1 molar ratio for 1h at 60°C temperature and then allowed to settle for 24h.
These findings are in line with that of (Freedman et al., 1984) who mentioned that
transesterification of refined soybean oils to methyl, ethyl or butyl esters was
essentially complete in 1h if the process is carried out with an alkaline catalyst at
60°C or higher temperature with oil-alcohol mixture having molar ratio at least 6:1.
The analysis of variance for the effect of different process parameters on
recovery of ester indicates that molar ratio, preheating and reaction temperature
significantly (P < 0.05) affect the recovery of the ester whereas settling time had no
significant on the recovery. Further, the interactions between molar ratio and
preheating time, molar ratio and reaction temperature and preheating time and
reaction temperature were also found to affect significantly the recovery of ester. The
interactions between the parameters other than those mentioned above were found
insignificant.
Effect of Process Parameters on Kinematic Viscosity of Recovered Esters: (Table 5) shows the
kinematic viscosity of soybean ethyl esters obtained by transesterification of refined
soybean oil at the selected process conditions. It is evident that, when refined soybean
oil-ethanol mixture having 9:1 molar ratio was transesterified at different selected
preheating time, reaction temperature, and settling time, it yielded ethyl esters whose
kinematic viscosity ranged from 12.11 to 18.55 cS. The soybean ethyl ester of lowest
kinematic viscosity (12.11 cS) was obtained when refined soybean oil was preheated
at 60°C for 20 min and then reacted with ethanol mixture having 9:1 molar ratio in
presences of 1% Na OH at 60°C for 1h and allowed to settle for 24 h.
The results also indicate that the ethyl esters obtained from transester-ification of
refined soybean oil-ethyl alcohol mixture having 6:1 molar ratio, had their kinematic
viscosity ranging from 5.03 to 18.44 cS. The soybean ethyl ester having lowest
kinematic viscosity of 5.03 cS was obtained when refined soybean oil-ethanol having
6:1 molar ratio was preheated for 20 min at 55°C and then reacted at 55°C for 1 h and
allowed to settle for 24 h. this matched the results found by (Sangha et al. 2000) who
found viscosity of soybean oil after esterification at 60 0C to 4.7cS.
Table (5): Statistical Analysis of Factorial Experiment on Ester Recovery ANOVA
Source of Variance
df
SS
MS
Fcal
Ftab at 5%
Remarks
Molar ratio (I)
Preheating time (J)
Reaction temperature (K)
Settling time (L)
I×J
I×K
I×L
J×K
J×L
K×L
I×J×K
I×J×L
I×K×L
J×K×L
I×J×K×L
Error
Total
1
2
2
2
2
2
2
4
4
4
4
4
4
8
8
106
161
0.536
9.982
2.776
28.076
123.6
97.611
30.608
44.691
66.167
782.154
34.971
0.023
0.00958
0.0271
0.0137
6.28
0.77
0.536
4.991
1.388
14.038
61.781
48.805
15.304
11.173
16.542
195.538
0.0087
0.0056
0.0024
0.0034
0.0017
593.09
0.4825
904.02
84.153
23.398
2.367
10.417
8.229
2.58
1.884
2.789
0.033
0.147
0.095
0.404
0.570
0.288
–
–
3.8
3.09
3.09
3.09
3.09
3.09
3.09
2.46
2.46
2.49
2.49
2.49
2.49
2.03
2.03
–
–
*
*
*
NS
*
*
NS
NS
*
NS
NS
NS
NS
NS
NS
–
–
Table (6): Observed Viscosity of Soybean Ethyl Ester Recovered under Different Transesterif-ication
Process Conditions
Preheating
Time (min)
Reaction
Time (h)
Reaction
Temperature (0C)
Settling
Time (h)
20
20
20
20
20
20
20
20
20
30
30
30
30
30
30
30
30
30
40
40
40
40
40
40
40
40
40
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
45
45
45
55
55
55
60
60
60
45
45
45
55
55
55
60
60
60
45
45
45
55
55
55
60
60
60
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
24
36
48
* = Significant at 5%,
Kinematic Viscosity (%)
9:1 Molar Ratio
6:1 Molar Ratio
15.18
14.22
16.71
14.48
13.21
14.55
12.11
16.05
17.69
14.09
14.94
15.25
13.56
14.18
14.40
16.05
15.14
13.99
15.43
15.21
16.20
14.27
13.89
14.71
18.55
17.73
14.93
8.11
6.07
7.53
5.03
6.55
7.70
10.59
12.60
13.68
8.09
6.76
8.26
5.07
7.13
8.30
12.19
15.58
14.65
11.09
6.94
8.92
6.09
7.78
8.75
14.74
18.10
18.44
NS = Non-significant
The analysis of variance for the effect of transesterification process parameters
on the kinematic viscosity of ethyl esters recovered reveals that all the selected
process parameters and their interactions had significant effect on kinematic viscosity
at 5 % level of significance.
CONCLUSION AND RECOMMENDATIONS
On the basis of the above results, it may be concluded that in order to obtain
soybean ethyl ester of lowest possible kinematics viscosity with highest recovery
(92%), oil must be transesterified with ethanol keeping molar ratio 6:1, preheating
time 20 min, reaction time 1h, reaction temperature 55°C and settling time 24 h.
consequently the key factor of reduced viscosity restricting the soybean oil and its
derivatives as environmentally friendly boidiesel fuel can be solved by adopting the
model by optimizing the parameters of transesterification given in this study.
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