CHAPTER 5 TWO-STEP ACID ALKALI CATALYZED...
Transcript of CHAPTER 5 TWO-STEP ACID ALKALI CATALYZED...
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CHAPTER 5
TWO-STEP ACID ALKALI CATALYZED TRANSESTERIFICATION OF
C. pentandra SEED OIL FOR BIODIESEL PRODUCTION
5.1 INTRODUCTION
Environmental concerns of fossil fuel depletion and fluctuating oil price has
intensified the search for alternate fuel from renewable resources. Vegetable oil and
animal fats are found to be the best alternate energy source that can be used directly
in the existing engine. Their direct use is limited due to two main reasons, that is
high viscosity and low volatility (Knothe and Steidley 2005). Transesterification
technique has been widely used to reduce the viscosity of oils and fats.
Transesterification is nothing but displacement of alcohol from an ester by another
alcohol (Srivastava and Prasad 2000). The advantages of biodiesel as diesel fuel is
its portability, ready availability, renewability, higher combustion efficiency, lower
sulfur and aromatic content, higher cetane number and higher biodegradability
(Demirbas 2009a).
The use of edible vegetable oils and animal fats for biodiesel production has
received great concern because they compete with food materials (Kalam et al
2008). The demand for vegetable oils for food has increased tremendously in recent
years. It is impossible to justify the use of these oils for fuel purposes such as
biodiesel production. Moreover, these oils could be more expensive to use as fuel
(Demirbas 2009b). The uses of non-edible plant oil sources are keeping competition
with food edible oil for biodiesel feed stock. Hence, the contribution of non-edible
oil from C. pentandra will be significant source for biodiesel production.
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5.1.1 Description of the Plant
C. pentandra is a tall, deciduous tree bearing short, sharp prickles all along
the trunk, branches and supported by pronounced buttresses at the base. It has a
light crown and is leafless for a long period. Leaves are alternate with slender green
petioles. There are usually 5 leaflets in a mature form. Great quantities of flowers
are in lateral clusters near the ends of the twigs. Fruit is leathery, ellipsoid,
pendulous capsule, 10-30 cm long and usually tapering at both ends. White, pale
yellow or grey floss originates from the inside wall of the fruit. Seed capsules split
open along 5 lines. Each capsule releases 120-175 seeds rounded black seeds
embedded in a mass of grey woolly hairs. Seeds are in dark brown color. The
generic name comes from a local South American word. The specific name,
‘pentandra’, is Latin for ‘five-stemmed’ from the Greek word ‘penta’ (five) and
‘andron’ (male).
5.1.2 Economic Benefits
The pressed cake is cattle feed containing about 26% protein. Sheep, goats
and cattle relish the foliage. The fiber from the inner wall of the fruit is unique in
that it combines springiness and resilience to make it ideal for stuffing pillows,
mattresses and cushions, life jackets and lifeboats. It is an excellent material for
insulating iceboxes, refrigerators, cold-storage plants, offices, theatres and
aeroplanes.
C. pentandra seed contains 20 to 25% non-drying oil, and is used as a
lubricant, in soap manufacturing and in cooking. Medicine: Compressed fresh
leaves are used against dizziness, decoction of the boiled roots is used to treat
oedema, gum is eaten to relieve stomach upset, tender shoot decoction is a
contraceptive and leaf infusion is taken orally against cough and hoarse throat.
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5.1.3 Origin of Plant
It is believed that this tree originated in Central America. It has been
cultivated for a long time and can be found pantropically between 16 degrees north
and 16 degrees south.
It is native to India, Indonesia and United States of America. In Australia,
Cambodia, Eritrea, Ethiopia, Gambia, Ghana, Kenya, South Africa, Tanzania,
Thailand, Uganda and Zanziba are the region where it grows exotically In India, it
is found usually in southern parts of India. C. pentandra is grown around villages
and temples in Tamil Nadu, India, as an ornamental tree.
At present the C. pentandra oil has only limited application and the natural
production of seeds remain under utilized. Literature shows that no work has been
established so far on the production of biodiesel using C. pentandra oil. In this
present investigation, C. pentandra oil has been used as a potential source for
biodiesel production. The reaction conditions have been investigated to optimize the
process variables that lead to higher yield of biodiesel and to develop a simple
kinetic model for extraction process.
5.2 METHODOLOGY
Pods of C. pentandra were collected from local villages near Chennai, Tamil
Nadu, India during the month of July. Sample was identified as C. pentandra and
authenticated at Centre for Advanced Studies in Botany, University of Madras,
Chennai, Tamil Nadu, India.
5.2.1 Extraction
The C. pentandra pods were disrupted and seeds were removed manually
from the fiber. The collected seeds were dried under sun, ground to powder, passed
through 60 mesh and then the seed powder was dried at 105 C until a constant
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weight was obtained. The C. pentandra seed powder was mixed with one forth
weight of diatomaceous earth for better solvent flow through sample. The mixture
was packed inside a thimble and extracted as prescribed in Chapter 3. The oil yields
obtained was expressed in terms of weight percentage of the samples.
5.2.2 Oil Characterization
The acid, saponification and iodine values were determined by titrimetry
(Sadasivam and Manickam 2004). Water content was determined using a KF
titrator. The unsaponifiable fractions of the extracted oils were analyed in duplicate
and the results are presented as mean values (Leon-Camacho et al 2004).
5.2.3 Biodiesel Production and Characterization
The transesterification reaction was carried out in a system as described in
Chapter 3. The effect of different parameters like catalyst concentration, methanol
to oil molar ratio, reaction temperature and reaction time were optimized. The
stirring rate of 600 rpm was kept constant throughout the process to get sufficient
mixing.
5.2.3.1 Acid catalyzed pre-esterification process
The alkali catalyzed reaction is reported to be very sensitive to the content of
FFAs, which should not exceed a certain limit to avoid deactivation of catalyst by
formation of soaps and emulsion (Van Gerpen 2005). Therefore, FFAs were first
converted to respective esters in a pretreatment process with methanol using an acid
catalyst (H2SO4). It was reviewed from the literature and found that the product
having acid value < 2 mg KOH g-1 is used for alkali catalyzed reaction (Sharma et
al 2008).
The acid catalyzed esterification is a pretreatment process employed to
decrease the acid value of the feedstock below 2 mg KOH g-1. Based on the results
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of Chongkhong and Tongurai (2007) esterification reaction was performed by
employing methanol to oil ratio as 8:1 at 65°C with 1.834 wt% H2SO4 as a catalyst.
The FFA level of the mixture was checked at different time intervals. When the
required FFA level was reached, the mixture was cooled to room temperature and
transferred to a separating funnel without agitation, leading to separation of two
phases. Finally the acid value of the product separated at the bottom was
determined.
5.2.3.2 Alkali-catalyzed transesterification process
Alkali-catalyzed transesterification is the most effective in the
transesterification processes and is used in the commercial production of biodiesel.
Even at ambient temperature, the alkali-catalyzed reaction proceeds rapidly usually
reaching 95% conversion. It is noted that the parameters like catalyst concentration,
methanol to oil molar ratio, reaction temperature and reaction time play an
important role in production of biodiesel (Pilar et al 2004). The effect on varying
these parameters such as catalyst concentration (0.25, 0.50, 0.75, 1.0 and 1.25
wt%), methanol to oil molar ratio (3:1, 6:1, 9:1 and 12:1), reaction temperature (45,
50, 55, 60 and 65°C), reaction time (15, 30, 45 and 60 min) on the biodiesel yield
was studied.
The H1NMR spectra of bio-diesel were recorded. as per Knothe and Kenar
(2004), Gelbard et al (1995). Error bars shown in the figures represent the standard
deviations of experiments that had been done in triplicates.
To study the fuel properties, two 200 mL batches of biodiesel were produced
at optimized condition. The obtained dried methyl ester was properly stored in an
airtight brown glass container for characterization studies. Biodiesel fuel properties
were determined by ASTM test methods (ASTM 1998) and compared with ASTM
D6751 standards.
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5.3 RESULTS AND DISCUSSION
5.3.1 Effect of Different Solvents on Extraction of Oil
The selection of the solvent system for oil extraction from C. pentandra seed
is an important factor. Solvent selection for extraction of oil at the initial step would
allow cost-effective for fuel production without further expense required for the
purification of the product. The solvent chosen should have good extraction
capacity and low viscosity to enhance the free circulation. An efficient extraction
requires the penetration of solvent into the seed and to match the polarity of the
targeted compounds. An organic solvent has a higher solubility with oil, this solvent
system was used further to degrade the cell walls of the seed and to dissolve the oil
to enhance the oil yield.
The percent oil yield values for the different solvents at 60°C are shown in
Table 5.1 for oil extraction. The solvent required for extraction was selected on the
basis of oil yield and umsaponifiable matter content. Higher amount of
unsaponifiable matter requires intensified pre-treatment for oil to be used for
biodiesel production. The oil yields peaked for THF at 27.2 wt% with 3.56 wt%.
This higher amount of unsaponifiable matter was undesirable. Methanol extract
yields was poor due high polar in nature and having high percentage of
unsaponifiable matter. Solvent hexane yields high oil (26.4 wt%) with less amount
of unsaponifiable matter (1.98 wt%). Hence, it was chosen for as a solvent for
extraction.
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Table 5.1 Effect of different solvents on extraction of C. pentandra oil
Solvents Yield (wt%) Unsaponifiable matter wt%
Hexane 26.4 1.98
Petroleum ether 27 2.81
Tetra hydro furan 27.2 3.56
Methanol 14 5
Chloroform 20.4 2.9
5.3.2 Effect of Solvent Ratios on Oil Extraction
The effect of seed to hexane weight ratios on the oil extraction is shown in Figure
5.1. The experiments were studied under batch condition at 250 rpm, 65°C for 2 h
in a temperature controlled shaker. The influence of seed to hexane ratio from 1:4 to
1:12 on oil extraction was studied. As the seed to solvent ratio increased from 1:4 to
1:10, the oil yield was found to be increased from 11.9 wt% to 26.1 wt%. The trend
was continued with increase in seed to hexane ratio up to 1:10. Further increase
above 1:10 did not show much improvement in the oil extraction. Therefore the
ratio of 1:10 was found to be an optimum ratio for the further study.
0
5
10
15
20
25
30
1:4 1:6 1:8 1:10 1:12 Seed to Hexane weight ratio
Figure 5.1 Effect of oil yield on weight of seed to solvent different ratio
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5.3.3 Kinetic and Thermodynamic Studies on Oil Extraction
Extraction was performed in batch mode at different time intervals 20, 40, 60
80 and 100 min. The percentage oil yields at various temperatures are given in
Table 5.2. From the analysis of the data, the oil yield was found to be increased
with increase in extraction time. The yield was also analyzed with respect to the
extraction time at constant temperature ranging from 30 to 60°C.
Table 5.2 Percent oil yield at various extraction temperature
Temperature (°C) Time (min) 30 40 50 60
20 8.92 10.62 12.43 14.6
40 9.58 11.49 13.52 15.92
60 10.29 12.46 14.72 17.39
80 11.06 13.46 15.97 18.97
100 11.89 14.6 17.4 20.7
Using the values in Table 5.2 and applying the differential method, plots of
ln Y versus ln (dY/dt) at different temperatures with optimum conditions were
established. A first-order kinetic model was fitted well with average regression
coefficient (R2) value obtained as 0.936 (Figure 5.2). The reaction rate constants
and the order of the reaction were determined using the intercept and slope of the
liner plot (Table 5.3).
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Figure 5.2 Plot of ln (dY/dt) versus ln Y at different temperatures ranged from 30
to 60°C for extraction of oil
Table 5.3 Values of the reaction rate constants at different temperature
Temperature (°C) k min-1 R2 value
30 2.9557*10-3 0.9996
40 3.284*10-3 0.9575
50 3.7975*10-3 0.9646
60 4.0533*10-3 0.9949
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5.3.3.1 Calculation of activation energy
The rate constant k increases with increasing temperature, and this trend is
shown in Table 4.3. The changes can be described by the Arrhenius equation
(Levenspiel 2003). A plot of ln k versus 1/T (Figure 5.3) gives a straight line whose
slope represents the activation energy of extraction ( Ea/R) and whose intercept is
the Arrhenius constant (ln A). Thus, the activation energy and the Arrhenius
constant were calculated as Ea= 9.1803 kJ mol-1 and A =0.147 s-1, respectively.
y = -1104.2x - 2.1793 R² = 0.9852
-5.85
-5.8
-5.75
-5.7
-5.65
-5.6
-5.55
-5.5
-5.45
0.00295 0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 T-1 K
Figure 5.3 Plot of ln k versus 1/T for oil extraction
5.3.3.2 Calculation of activation thermodynamic parameters
The activation thermodynamic parameters were calculated according to the
transition state theory (Topallar and Gecgel 2000). The activation entropy ( S ), the
activation enthalpy H and the activation free energy or Gibb's energy G at
different temperatures were shown in Table 5.4 for each temperature.
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Table 5.4 Activation thermodynamic parameters at different temperatures
T (Kelvin) H (kJ mol-1 ) S (J mol-1K-1 ) G (kJ mol-1 )303 6.66 -260.98 85.73
313 6.57 -261.25 88.34
323 6.49 -261.51 90.95
333 6.41 -261.76 93.56
5.3.3.3 Calculation of thermodynamic parameters
Thermodynamic parameters ( H, S and G) for the extraction of . C.
pentandra oil using hexane as solvent was estimated. A plot of ln YT vs. 1/T
(Figure 5.4) at 100 min, gives a straight line whose slope represents the enthalpy
change of extraction, - H. Thus, the enthalpy change was calculated to be H =
0.016 kJ mol-1 for oil extraction. The H value obtained was indicating the physical
and chemical nature of the C. pentandra oil extraction process. Other
thermodynamic parameters ( S and G) and the equilibrium constant values for oil
extraction at 100 min are given in Table 4.5 for each temperature.
Figure 5.4 Plot of ln YT versus 1/T for oil extraction
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According to these results from Table 5.5, the positive value of enthalpy
indicates that the process is endothermic and requires energy during process. In
addition, the negative value of G ( G < 0) at 60°C indicates that there is a
decrease in the free energy. The extraction process using hexane at 60°C is
spontaneous process. The system initially consists of the seeds and hexane, whereas
the oil molecules are extracted from the C. pentandra seeds during the extraction
process and therefore, the entropy of the mixture increases in the course of the
extraction, which is the positive value of entropy change ( S > 0) at 60°C indicates
that the process is irreversible.
Table 5.5 Thermodynamic parameters ( S and G) and equilibrium
constants at different temperatures
T, Kelvin K S, J mol-1K-1 G, kJ mol-1
308 0.8194 5.71 -0.53
318 1.2372 8.32 -0.55
328 1.9333 11.58 -0.57
338 3.6315 17.65 -0.58
5.3.4 Oil Properties and Characteristics of C. pentandra oil
Extracted C. pentandra oil was pale yellow in colour. The oil yield of the
seed was found to contain a mean value of 26.4% (w/w). The physical and chemical
properties of C. pentandra oil are given in Table 5.6. The oil yields were
appreciably higher than soybeans (18 to 22%) and lower than many other oily
vegetables such as palm (40%), rape seed (41%) and sunflower (40%) (Lidefelt
2007). The saponification value of 195 clearly suggests that the oil consist mainly
of medium-chain fatty acids (i.e. C16 and C18). Its initial acid value was
determined as 28.71 mg KOH g-1 corresponding to a FFA level of 14.35%.
Unsaponifiable matter present in the extracted oil was 1.98%, which includes
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tocopherols, sterols, triterpenic alcohols, hydrocarbons, aliphatic alcohols and
waxes (Lidefelt 2007). The iodine value of the oil is 101.1 which was quite high
and it lies within the values of semi-drying.
Table 5.6 Physical and chemical properties of C. pentandra oil
Properties Values Density 921 kg m-3
Viscosity at 40°C 29.32 mm2 s-1
Water content 0.05 wt%
Acid value 28.71 mg KOH g-1
FFA 14.35% as oleic acid
Saponification value 195
Iodine value 101.9
Average molecular weight of oil 848 g mol-1
Unsaponifiables 1.98 wt%
5.3.4.1 Fatty acid profile
The fatty acid compositions of the C. pentandra oil are given in Table 5.7,
which shows the principal fatty acid profile of linoleic (35.11%), oleic (29.69%),
palmitic (23.20%) and stearic (5.68%) acids. The level of total monounsaturated,
polyunsaturated and saturated fatty acid of the extracted oil was 52.89%, 35.11 %
and 9.44% respectively (Figure 5.5). The high content of unsaturated fatty acid
makes it a stable liquid at room temperature.
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Figure 5.5 Chromatograph of C. pentandra oil fatty acid
Table 5.7 Fatty acid distribution in C. pentandra oil
Fatty acid Composition wt% Myristic 0.11
Palmitic 23.20
Stearic 5.68
Oleic 29.69
Linoleic 35.11
Arachidic 1.89
Behenic 0.25
Lignoceric 1.51
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5.3.5 Acid Catalyzed Esterification Pretreatment
C. pentandra oil shows a high FFA content. It was found that the
pretreatment stage considerably reduced the acid value from 28.71 to below 2 mg
KOH g-1. The effect of acid value on reaction time is shown in Figure 5.6. The
experimental results suggested that the acid catalysis esterification occur in three
stages i.e. fast, slow and stationary. In the fast stage, the rate of the reaction was fast
for which the acid value drop down from 28.71 to 3.6 in 40 min and the reduction in
acid value was 87.47%. After 40 min the rate of the reaction slows down up to 60
min, the reduction in acid value was 94.15%. In the stationary stage the
esterification reached equilibrium after 60 min. The acid value was observed less
than 2 mg KOH g-1 at 50 min, but it was desirable to keep a longer reaction time of
60 min to get a much lesser target of 1.68 mg KOH g-1.
0
5
10
15
20
25
30
0 10 20 30 40 50 60 70
Time(mins)
Figure 5.6 Effect of reaction time on acid catalyzed esterification
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5.3.6 Alkali Catalyzed Transesterification Process
5.3.6.1 Effect of catalyst
In general, from the literature it was observed that alkali catalyst
concentration of less than or equal to 1% is required for a successful conversion of
oils and fats to FAME depending on the type of oil used (Hanh et al 2007, Felizardo
et al 2006, Tomasevic and Siler-Marinkovic 2003). According to the results in
Figure 5.7, the reaction yield was found to be low for the catalyst concentration
0.25 to 0.75%. This is due to the insufficient amount of KOH. It is observed that 1.0
% KOH is optimal enough to get higher conversion of 79%. The addition of excess
amount of catalyst results in the formation of soap, which decreases the yield by
giving rise to emulsification of biodiesel and glycerol phase.
0
20
40
60
80
100
0 0.25 0.5 0.75 1 1.25
Catalyst (wt%)
Figure 5.7 Effect of catalyst on biodiesel production at methanol to oil ratio 6:1, 55°C
reaction temperature, 30 min reaction time and 600 rpm stirrer speed
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5.3.6.2 Effect of methanol to oil molar ratio
The molar ratio of alcohol to oil is one of the most important variable
influencing the conversion into esters. Although the stoichiometric molar ratio of
methanol to triglyceride for transesterification is 3:1, higher molar ratios are used to
enhance biodiesel conversion (Noureddini et al 1998). The effect of methanol to oil
molar ratio was studied in the range of 3, 6, 9 and 12. Figure 5.8 depicts the effect
of methanol to oil molar ratio in the yield of biodiesel. It has been observed that the
yield of the process increases with increase in molar ratio. The optimum conversion
of 86% was obtained at methanol to oil ratio of 9:1 at constant reaction temperature
of 55°C and reaction time of 30 min with catalyst concentration of 1% (Figure 5.8).
With further increase in molar ratio the yield remains more or less same. The excess
methanol was removed in downstream process.
30
40
50
60
70
80
90
0 3 6 9 12
Methanol oil ratio
Figure 5.8 Effect of methanol to oil molar ratio on biodiesel production at
1% of catalyst, 55°C reaction temperature, 30 min reaction time and
600 rpm stirrer speed
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5.3.6.3 Effect of reaction temperature
Reaction temperature was found to significantly affect the yield of biodiesel.
At minimum reaction duration (30 min), when the transesterification reaction was
carried out at 45°C, the conversion of biodiesel was only 49%. However, the
conversion reaches the maximum at 93 %, when the reaction temperature was
increased to 65°C. In another words, reaction temperature has a more significant
effect on the conversion at higher reaction temperature than lower temperature.
Figure 5.9 shows the effect of reaction temperature on biodiesel conversion. At
lower reaction temperature, there is insufficient energy to promote extensive
collisions among reactant particles. However, at higher reaction temperature, the
possibility of collision among reactant particles became greater and easily obtains
the necessary activation energy (Al-Widyan and Al-Shyoukh 2002).
40
50
60
70
80
90
100
40 45 50 55 60 65Temparture °C
Figure 5.9 Effect of temperature on biodiesel production at 1% of catalyst,
methanol to oil molar ratio 9:1, at 30 min reaction time and 600 rpm
stirrer speed
5.3.6.4 Effect of reaction time
Figure 5.10 shows the effect of reaction time on biodiesel conversion. The
dependency of reaction time was studied at different time intervals ranging from 15
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to 60 min. The optimum conversion of 99.5% was observed at 45 min as shown in
Figure 5.11. The increase in reaction temperature increases the reaction rate and
reduces the reaction time as reported by Antonlin et al (2002). Thus if longer
contact timeis provided, most of the reactants will collide and reacts to give higher
yield of biodiesel (Yee et al 2011).
50
60
70
80
90
100
110
10 20 30 40 50 60
Time (mins)
Figure 5.10 Effect of time on biodiesel production at 1% of catalyst, methanol to oil
ratio 9:1, 65°C reaction temperature and 600 rpm stirrer speed
Figure 5.11 H1NMR spectrum of obtained biodiesel at 99.5% conversion
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5.3.7 Fuel Properties of C. pentandra Biodiesel
Pretreatment followed by alkali-catalyzed transesterification was
successfully used to produce high quality biodiesel. The fuel properties of final
product biodiesel obtained from C. pentandra oil were determined with the aid of
standard methods and were summarized in Table 5.8. The biodiesel properties were
similar to those of ASTM D6751 standard. The exhibited properties also indicate
that the downstream process was adequate.
Table 5.8 Fuel specification of C. pentandra oil compared with ASTM
D6751 standard
Properties Units Test methods Limits C. pentandra oil Biodiesel
Specific gravity --- ASTM D4052 --- 0.876 Flash point oC ASTM D93 130 Min 169 Cloud point oC ASTM D2500 Report 1
Viscosity@40oC mm2 s-1 ASTM D445 1.9-6 4.17
Acid number mg KOH g-1 ASTM D664 0.05 Max 0.036
Carbon residue wt% ASTM D4530 0.05 Max 0.042 Water &
sediments vol% ASTM D2709 0.05 Max 0.031
Copper strip corrosion --- ASTM D130 Number 3
Max 1a
Sulphated ash wt% ASTM D874 0.02 Max 0.01 Phosphorous
content wt% ASTM D4951 0.001 Max 0.0008
Na & K combined Ppm EN 14538 5 Max 4.2 Ca& Mg combined Ppm EN 14538 5 Max 2
Cetane Number --- ASTM D613 47 Min 47
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5.4 CONCLUSION
This study elaborates the utilization of underutilized, non-edible C.
pentandra oil as a source to produce fuel quality biodiesel. A two-stage process
through acid pre-treatment and alkali catalyzed transesterification for the production
was discussed. The effects of different parameters and the kinetics of the alkaline
catalysts transesterification were studied. The potential use of C. pentandra oil
appears to be promising and have possible applications in the biodiesel production.
The production of C. pentandra biodiesel could be an added value to an
underutilized agricultural product.