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Process Optimization for Biodiesel Production from Waste
CookingPalm Oil (Elaeis guineensis) Using Response Surface
Methodology
L. H. Chin, B. H. Hameed,* and A. L. Ahmad
School of Chemical Engineering, Engineering Campus, UniVersity
of Science Malaysia,14300 Nibong Tebal, Penang, Malaysia
ReceiVed September 19, 2008. ReVised Manuscript ReceiVed
NoVember 8, 2008
A central composite rotatable design was used to study the
effect of methanol to oil ratio, reaction time,catalyst amount, and
temperature on the transesterification of waste cooking palm oil
using oil palm ash as acatalyst. The reaction was carried out at 10
bar. All of the variables except reaction time significantly
affectedthe biodiesel yield, amount of catalyst and reaction
temperature being the most effective, followed by methanolto oil
ratio. Using response surface methodology, a quadratic polynomial
equation was obtained for biodieselyield by multiple regression
analysis. The optimum conditions for transesterification of waste
cooking palmoil to biodiesel were found as follows: amount of
catalyst of 5.35 wt% (based on oil weight), temperature of60 C,
methanol to oil ratio of 18.0 and reaction time of 0.5 h. The
predicted and experimental biodieselyields were found to be 60.07%
(wt) and 71.74% (wt), respectively.
1. Introduction
The high demand for energy in the industrialized world andthe
pollution problems caused by the use of fossil fuels hasmade
necessary the development of alternative renewable energysources.
One of the particular alternatives considered is the useof
biodiesel (transesterification from vegetable oil). Biodieselis a
well known alternative, renewable fuel which producesfewer harmful
emissions than conventional fossil-based dieselfuel.1 Moreover, due
to its biodegradability and nontoxicity, theproduction of biodiesel
is considered to be an advantage to thatof fossil fuels. Its use
leads to a decrease of the carbon dioxide,sulfur dioxide, unburned
hydrocarbon, and particulate matteremissions generated in the
combustion process.2 However, inspite of the favorable impact, the
economic aspect of biodieselproduction is still a barrier, as the
cost of biodiesel productionis highly dependent on the cost of
feedstock, which affects thecost of the finished product by up to
60-75%.3 Currently,partially or fully refined and edible-grade
vegetable oils, suchas soybean, rapeseed, and sunflower, are the
predominantfeedstocks for biodiesel production,4 which obviously
resultsin the high price of biodiesel. Therefore, exploring ways
toreduce the cost of the raw material is of great interest. In
thepresent work, waste cooking palm oil was chosen as the
rawmaterial to produce biodiesel.
Currently, the main process for the synthesis of biodiesel isthe
transesterification of vegetable oils using a strong base asthe
homogeneous catalyst. However, this process presents
somedisadvantages, as it requires the use of high amounts of
catalyst(which cannot be recovered), the production of different
streams
which might be treated (neutralization step and wash step),
andthe purification of glycerine to reuse it. These aspects also
playimportant roles in the economy of the process.5
The use of heterogeneous catalysts is related to the
develop-ment of an environmentally benign process and the
reductionof the production cost.6 Recently, several studies on
thetransesterification of triglycerides have been conducted
usingheterogeneous catalysts such as supported CaO,7
calciumethoxide,8 MgO-functionalized mesoporous catalyst,9
KF/Eu2O3,10 MgO loaded with KOH,11 and KF/Hydrotalcite.12
The aim of this work was to study the performance of emptyfruit
palm ash, a waste of the palm oil industry, as a catalyst inthe
transesterification of waste cooking palm oil. Most of thestudies
on the transesterification changed one separate factor ata time.
However, a reaction system simultaneously influencedby more than
one factor can be poorly understood with thechange one separate
factor at a time approach.13 Therefore, theexperiments were
performed according to central compositedesign (CCD) and respond
surface methodology (RSM) tounderstand the relationship between the
factor and yield tobiodiesel, and to determine the optimum
conditions for produc-tion of biodiesel.
2. Experimental SectionWaste cooking palm oil with a kinematic
viscosity of 38.37
mm2s-1 was collected from the canteen of the Engineering
Campus,
* To whom correspondence should be addressed. Tel: +604-599
6422.Fax: +604-594 1013. E-mail: [email protected].
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K.; Bakhshi,N. N. Chem. Eng. J. 2008, 140, 7785.
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C.;Ramrez, A. I. Bioresour. Technol. 2002, 83, 111114.
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A .Appl. Catal., A: General 2008, 346, 7985.
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D.-K.; Lee,J.-S.; Lee, K.-Y. Catal. Today 2004, 93-95, 315320.
(7) Yan, S.; Lu, H.; Liang, B. Energy Fuels 2008, 22, 646651.(8)
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Sun, H.; Hu, K.; Lou, H.; Zheng, X. Energy Fuels 2008, 22, 2756
2760.(11) Ilgen, O.; Akin, A. N., Energy Fuels 2008,
doi:10.1021/ef800345u.(12) Gao, L.; Xu, B.; Xiao, G.; Lv, J.,
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ef800340w.(13) Yuan, X.; Liu, J.; Zeng, G.; Shi, J.; Tong, J.;
Huang, G. Renew.
Energy 2008, 33, 16781684.
Energy & Fuels 2009, 23, 104010441040
10.1021/ef8007954 CCC: $40.75 2009 American Chemical
SocietyPublished on Web 01/06/2009
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University of Science Malaysia, Penang. It was heated to 120 Cto
remove excess water before use. Methanol (g99.9%, HPLCGradient
grade) was purchased from Merck (Malaysia) and refer-ence
standards, such as methyl stearate (g99.5%), methyl
palmitate(g99.5%), methyl myristate (g99.5%), methyl oleate
(g99.5%),methyl linoleate (g99.5%) were employed, while methyl
hepta-decanoate (g99.5%) was used as an internal standard and
waspurchased from Sigma-Aldrich (Malaysia) for gas
chromatographicanalysis. N-hexane (g96%) was used as a solvent for
GC analysisand was purchased from Merck (Malaysia). All of the
chemicalsused were of analytical reagent grade.
2.1. Catalyst. Oil palm ash was obtained from an oil palm millat
Jawi, Penang, Malaysia. The precursor of the oil palm ash wasempty
fruit bunches consisting of fibers which were combusted at800 C to
generate energy for a boiler in the mill.14 The oil palmash
produced was observably coarse in nature. Large and
unburnedresidues were manually discarded. The ash was sieved and
driedin an oven overnight prior to use as a catalyst.
2.2. Experimental Design. The synthesis of biodiesel from
palmoil transesterification using oil palm ash as a catalyst was
developedand optimized using the Central Composite Design (CCD)
andResponse Surface Methodology (RSM). CCD helps in
investigatinglinear, quadratic, cubic, and cross-product effects of
the four reactioncondition variables on the biodiesel yield. The
four independentvariables studied were reaction time, methanol to
oil molar ratio,temperature, and amount of catalyst. Table 1 lists
the range andlevels of the four independent variables studied.
Selection of thelevels was carried out on the basis of results
obtained in apreliminary study, considering limits for the
experiment set-up andworking conditions for each chemical species.
The value of R forthis CCD was fixed at 2.15 The complete design
matrix of theexperiments employed and their results are given in
Table 2. Allvariables at the zero level constitute the center
points and thecombination of each of the variables at either its
lowest (-2.0)level or highest (+2.0) level with the order variables
at zero levelconstitute the axial points. The experiment sequence
was random-ized to minimize the effects of the uncontrolled
factors.
2.3. Statistical Analysis. The experimental data obtained
byfollowing the above procedure were analyzed by the respond
surfacemethodology using the following second-order polynomial
equation:
y) 0 +i)1
n
Bixi +i)1
n
Biixi2 +
i
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calculated from their content in the biodiesel as analyzed by
gaschromatography. The yield was defined as a ratio of the weight
ofmethyl esters, determined by gas chromatography, to the weightof
oil used.16
2.6. Characterization of the Catalyst. Scanning
electronmicroscopy (SEM) analysis was carried out on the catalyst
beforethe transesterification reaction to study its surface
morphologiesusing field-emission scanning electron microscope
(FESEM) system(Philips XL30S model). The sample was placed in a
sample gridand coated with gold-palladium for electron reflection
andvacuumed before analysis. Energy dispersive X-ray (EDX) was
usedto determine the elemental composition of oil palm ash by
analyzingthe microscopic image under EDX instrument (Philips
XL30Smodel). Fourier transform infrared (FTIR) analysis was applied
onthe same catalyst to determine the surface functional groups
usingFTIR spectroscopy (FTIR-2000, Perkin-Elmer). The spectra
wererecorded from 4000 to 400 cm-1.
3. Results and Discussion
3.1. Response Surface Methodology (RSM). A centralcomposite
design (CCD) was used to develop a correlationbetween the
transesterification condition variables to the biodie-sel yield.
The complete design matrix and biodiesel yield atvarious
transesterification condition variables are listed in Table2. The
biodiesel yields obtained were in the range from 23.14to 52.96 wt%.
Runs 25 to 30 at the center point of the designwere used to
determine the experimental error. The normal plotof residuals for
biodiesel yield (Figure 1) was normallydistributed and showed no
deviation of the variance. Besides,the model was also tested for
any transformation that could havebeen applied, but the Box-Cox
plot did not suggest anytransformation for the response.
3.2. Regression Analysis. A regression analysis was per-formed
to fit the response function and predict the outcome ofbiodiesel
yield with a simple equation. The model is expressedby Eq. (2)
which takes their coded value.Biodiesel yield) 47.68+ 1.09 A- 1.84
B- 3.28 C-
3.49 D- 1.00 B2 - 3.50 D2 - 3.66 B C-2.16 B D- 1.99 C D (2)
The summary of the analysis of variance (ANOVA) result isshown
in Table 3. The regressors or term incorporated in themodel are
those statistically tested to be significant. The Prob> F value
indicates that probability equals the proportion of
the area under the curve of the F-distribution that lies
beyondthe observer F value. The small probability values called for
therejection of the null hypothesis, in other words, the
particularterm significantly affected the measured response of the
system.In these cases, the Prob > F less than 0.05 indicated
theparticular term was statistically significant. The analysis
con-firmed the significant terms of B, C, D, D2, BC, BD, and CDfor
biodiesel yield. The term A was found not to be significantand null
hypothesis in the biodiesel yield but was included intothe analysis
for the sake of maintaining the hierarchical structureof the model
terms. The coefficient of determination, R2 forthe model was
90.56%. This indicates that only 9.44% of thetotal variability was
not explained by the regressors in the model.The high R2 value
specifies that the model obtained will be ableto give a
convincingly good estimate of response of the systemin the range
studied. The lack of fit test, which is not significantfor the
model developed, shows that the model satisfactorilyfits the data.
Figure 2 shows the predicted values versus actualvalues for
biodiesel yield. As can be seen, the predicted valuesobtained were
quite close to the experimental values, indicatingthat the model
developed was successful in capturing thecorrelation between the
transesterification condition variablesto the biodiesel yield.
3.3. Model Analysis. The result of regression analysisseemed to
suggest that biodiesel yield was only affected by themain factor of
methanol to oil molar ratio (B), temperature (C),amount of catalyst
(D), and their respective higher-order term(D2). Significant
interactions terms were found to exist betweenthe main factors (BC,
BD, and CD). Figure 3 shows the three-dimensional response surface
that was constructed to show theeffects of the transesterification
condition variables (methanol
(16) Jitputti, J.; Kitiyanan, B.; Rangsunvigit, P.; Bunyakiat,
K.; Attana-tho, L.; Jenvanitpanjakul, P. Chem. Eng. J. 2006, 116,
6166.
Figure 1. Normal plot of residual for biodiesel yield.
Table 3. ANOVA for Model Regression
source SSa d.f.ameansquare F-value
probability> F
model 1373.35 9 152.59 21.31
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to oil molar ratio and reaction temperature) on biodiesel
yield.The reaction time and amount of catalyst were fixed at
zerolevel. As can be seen from Figures 3 and 4, biodiesel
yieldincreases with an increase in the methanol to oil molar ratio
atlow temperature and low amount of catalyst. The excessmethanol
can promote the transesterification reaction forwardand also
extract products, such as glycerin and methyl esters,from the
system to renew the surface of the catalyst.7 However,biodiesel
yield decreases at higher reaction temperatures above130 C and at
higher amount of catalyst with increased inmethanol to oil molar
ratio. This may be due to the large amountof methanol diluting the
oil and reducing the biodiesel yield.12
The reaction temperature can influence the reaction rate andthe
biodiesel yield because the intrinsic rate constants are
strongfunctions of temperature. The study of the effect of
temperatureis very important for a catalyzed reaction.17 From
Figures 3and 5, the results show that the biodiesel yield increased
withan increase of reaction temperature at low amount of
catalystand a low methanol to oil molar ratio. Being an
equilibriumreaction, the equilibrium constant is influenced by
temperatureand pressure. In addition, in our experiments, which
were carriedout at higher pressure, both factors affected the
equilibrium
constant. Therefore, as the temperature increased, the
biodieselyield increased. Moreover, because of the solid catalyst
usedin this reaction, the mass transfer effect should be
considered.A high temperature is a benefit to the mass transfer.12
Neverthe-less, the biodiesel yield dropped obviously at high
temperaturefor higher methanol to oil molar ratio and higher
amounts ofcatalyst. This may possibly be due to presence of solid
catalyst,the reaction mixture constitutes a three-phase system,
oil-methanol-catalyst, in which the reaction would be slowed
downbecause of the diffusion resistance between different
phases.18This might be the reason for lower biodiesel yield at
highermethanol to oil molar ratio and higher amounts of
catalyst.
In the case of homogeneous catalysts, it has been revealedthat
the amount of catalyst has a strong influence on theconversion of
vegetable oil to ester.19 Thus, the effect of the oilpalm ash as a
catalyst on the transesterification of waste cookingpalm oil was
studied. From Figures 4 and 5, the biodiesel yieldincreased with an
increase in the amount of catalyst. This resultindicates that with
the addition of more catalyst, there was afaster rate at which the
reaction equilibrium was reached becauseof the increase in the
total number of available active catalyticsites for the reaction.17
Nevertheless, biodiesel yield decreasesafter the amount of catalyst
exceeded 7 wt.% from wastecooking oil used. This may be due to the
presence of the solidcatalyst, the reaction rate is determined by
surface reaction andmass transfer.17 Higher catalyst dosage may
make the reactantmixture more viscous, which will increase the mass
transferresistance in the multiphase system.6 Besides, the decrease
inbiodiesel yield was also observed by Li and Xie20 which is
mostlikely due to a mixing problem involving reactants and the
solidcatalyst.
3.4. Optimization of Biodiesel Yield. The CCD was ableto
function as an optimal design for the desired response of thesystem
based on the model obtained and the input criteria. Theoptimization
of biodiesel yield was carried out based on alltransesterification
variables, which were in the range of experi-mental runs. The
software predicted that optimized conditionsfor biodiesel yield
were obtained when the reaction time,methanol to oil molar ratio,
temperature, and amount of catalystwere at 0.5 h, 18.0 molar ratio,
60 C and 5.35 wt% of catalyst,
(17) Liu, X.; Piao, X.; Wang, Y.; Zhu, S.; He, H. Fuel 2008, 87,
10761082.
(18) Liu, X.; He, H.; Wang, Y.; Zhu, S.; Piao, X. Fuel 2008, 87,
216221.
(19) Arzamendi, G.; Campo, I.; Arguinarena, E.; Sanchez, M.;
Montes,M.; Gandia, L. M. Chem. Eng. J. 2007, 134, 123130.
(20) Li, H.; Xie, W. J. Am. Oil Chem. Soc. 2008, 85, 655662.
Figure 3. Three-dimensional response surface plot of biodiesel
yield(effect of temperature and methanol to oil molar ratio,
reaction time )2 h, amount of catalyst ) 7 wt%).
Figure 4. Three-dimensional response surface plot of biodiesel
yield(effect of amount of catalyst and methanol to oil molar ratio,
reactiontime ) 2 h, temperature ) 130 C).
Figure 5. Three-dimensional response surface plot of biodiesel
yield(effect of amount of catalyst and temperature, reaction time )
2 h,methanol to oil molar ratio ) 12).
Biodiesel Production from Waste Cooking Palm Oil Energy &
Fuels, Vol. 23, 2009 1043
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respectively, with predicted biodiesel yield of 60.07% (wt).
Theexperimental biodiesel yield was 71.74% (wt). This means thatthe
experimental value obtained was in good agreement withthe value
calculated from the model.
3.5. Characterization of Oil Palm Ash Catalyst. Figure 6shows
the SEM image of the oil palm ash, which illustrates thespongy and
porous nature of the ash particles. The porous natureof oil palm
ash was also reported by Yin et al. 21 andTangchirapat et al.22
Table 4 shows the elemental compositionof oil palm ash used in this
study. A significant observation isthat oil palm ash contains high
weight percentage of potassium,while aluminum, zinc, and magnesium
weight percentages arecomparatively low. Besides, due to the high
weight percentageof oxygen, it can be postulated that potassium,
magnesium,silicone, zinc, and aluminum may exist for the most part
in oxideform. Furthermore, it was found that the K2O was the cause
ofthe high catalytic activity and basicity of the
catalyst.23-25
Figure 7 shows the FTIR transmission spectrum of oil palmash. A
prominent peak observed at about 3390 cm-1 is assignedto -OH band.
The existence of phenols and alcohol groups aresupported by the
presence of the bands at 1400-1300 cm-1,
attributed to deformation (OH), 1100-1000 cm-1
(stretching(C-O)), and 680-620 cm-1 (out of plane bending
(O-H)).
4. Conclusions
Oil palm ash, a waste from the oil palm industry, was foundto be
suitable catalyst for the transesterification of waste cookingpalm
oil to biodiesel. A central composite design was conductedto study
the effects of methanol to oil ratio, reaction time,catalyst
amount, and temperature on the transesterification ofwaste cooking
palm oil. The predicted and experimentalbiodiesel yields were found
to be 60.07% (wt) and 71.74% (wt),respectively.
Acknowledgment. The authors acknowledge the research
grantprovided by the Ministry of Science, Technology, and
Innovation(MOSTI), Malaysia under Science Fund grant (Project No.
03-01-05-SF0207), that resulted in this work.EF8007954
(21) Yin, C. Y.; Wan Ali, W. S.; Lim, Y. P. J. Hazard. Mater.
2008,150, 413418.
(22) Tangchirapat, W.; Saeting, T.; Jaturapitakkul, C.;
Kiattikomol, K.;Siripanichgorn, A. Waste Manage. 2007, 27,
8188.
(23) Xie, W.; Li, H. J. Mol. Catal. A: Chem. 2006, 255, 19.(24)
Xie, W.; Peng, H.; Chen, L. Appl. Catal., A: General 2006, 300,
6774.(25) Noiroj, K.; Intarapong, P.; Luengnaruemitchai, A.;
Jai-In, S., Renew.
Energy 2008, doi:10.1016/j.renene.2008.06.015.
Figure 6. SEM image of oil palm ash.Figure 7. FTIR transmission
spectrum of oil palm ash.
Table 4. Elemental Compositions of Oil Palm Ash Used in
ThisStudy by EDX
elements weight (%)potassium (K) 40.59oxygen (O) 29.36carbon (C)
14.56silicone (Si) 2.63magnesium (Mg) 0.76phophorus (P)
0.73aluminum (Al) 0.50zinc (Zn) 0.33clorin (Cl) 7.07
1044 Energy & Fuels, Vol. 23, 2009 Chin et al.
