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Energy & Fuels 2009, 23, 1040–1044
Process Optimization for Biodiesel Production from Waste Cooking Palm 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 a catalyst. The reaction was carried out at 10 bar. All of the variables except reaction time significantly affected the biodiesel yield, amount of catalyst and reaction temperature being the most effective, followed by methanol to oil ratio. Using response surface methodology, a quadratic polynomial equation was obtained for biodiesel yield by multiple regression analysis. The optimum conditions for transesterification of waste cooking palm oil to biodiesel were found as follows: amount of catalyst of 5.35 wt% (based on oil weight), temperature of 60 °C, methanol to oil ratio of 18.0 and reaction time of 0.5 h. The predicted and experimental biodiesel yields were found to be 60.07% (wt) and 71.74% (wt), respectively.
1. Introduction The high demand for energy in the industrialized world and the pollution problems caused by the use of fossil fuels has made necessary the development of alternative renewable energy sources. One of the particular alternatives considered is the use of biodiesel (transesterification from vegetable oil). Biodiesel is a well known alternative, renewable fuel which produces fewer harmful emissions than conventional fossil-based diesel fuel.1 Moreover, due to its biodegradability and nontoxicity, the production of biodiesel is considered to be an advantage to that of fossil fuels. Its use leads to a decrease of the carbon dioxide, sulfur dioxide, unburned hydrocarbon, and particulate matter emissions generated in the combustion process.2 However, in spite of the favorable impact, the economic aspect of biodiesel production is still a barrier, as the cost of biodiesel production is highly dependent on the cost of feedstock, which affects the cost of the finished product by up to 60-75%.3 Currently, partially or fully refined and edible-grade vegetable oils, such as soybean, rapeseed, and sunflower, are the predominant feedstocks for biodiesel production,4 which obviously results in the high price of biodiesel. Therefore, exploring ways to reduce the cost of the raw material is of great interest. In the present work, waste cooking palm oil was chosen as the raw material to produce biodiesel. Currently, the main process for the synthesis of biodiesel is the transesterification of vegetable oils using a strong base as the homogeneous catalyst. However, this process presents some disadvantages, as it requires the use of high amounts of catalyst (which cannot be recovered), the production of different streams * To whom correspondence should be addressed. Tel: +604-599 6422. Fax: +604-594 1013. E-mail:
[email protected]. (1) Issariyakul, T.; Kulkarni, M. G.; Meher, L. C.; Dalai, A. K.; Bakhshi, N. N. Chem. Eng. J. 2008, 140, 77–85. (2) Antolı´n, G.; Tinaut, F. V.; Bricen˜o, Y.; Castan˜o, V.; Pe´rez, C.; Ramı´rez, A. I. Bioresour. Technol. 2002, 83, 111–114. (3) Cetinkaya, M.; Karaosmanoglu, F. Energy Fuels 2004, 18, 1888– 1895. (4) Haas, M. J. Fuel Process. Technol. 2005, 86, 1087–1096.
which might be treated (neutralization step and wash step), and the purification of glycerine to reuse it. These aspects also play important roles in the economy of the process.5 The use of heterogeneous catalysts is related to the development of an environmentally benign process and the reduction of the production cost.6 Recently, several studies on the transesterification of triglycerides have been conducted using heterogeneous catalysts such as supported CaO,7 calcium ethoxide,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 empty fruit palm ash, a waste of the palm oil industry, as a catalyst in the transesterification of waste cooking palm oil. Most of the studies on the transesterification changed one separate factor at a time. However, a reaction system simultaneously influenced by more than one factor can be poorly understood with the change one separate factor at a time approach.13 Therefore, the experiments were performed according to central composite design (CCD) and respond surface methodology (RSM) to understand the relationship between the factor and yield to biodiesel, and to determine the optimum conditions for production of biodiesel. 2. Experimental Section Waste cooking palm oil with a kinematic viscosity of 38.37 mm2s-1 was collected from the canteen of the Engineering Campus, ´. (5) Ramos, M. J.; Casas, A.; Rodrı´guez, L.; Romero, R.; P´; erez, A Appl. Catal., A: General 2008, 346, 79–85. (6) Kim, H.-J.; Kang, B.-S.; Kim, M.-J.; Park, Y. M.; Kim, D.-K.; Lee, J.-S.; Lee, K.-Y. Catal. Today 2004, 93-95, 315–320. (7) Yan, S.; Lu, H.; Liang, B. Energy Fuels 2008, 22, 646–651. (8) Liu, X.; Piao, X.; Wang, Y.; Zhu, S. Energy Fuels 2008, 22, 1313– 1317. (9) Li, E.; Rudolph, V. Energy Fuels 2008, 22, 145–149. (10) 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., Energy Fuels 2008, doi:10.1021/ ef800340w. (13) Yuan, X.; Liu, J.; Zeng, G.; Shi, J.; Tong, J.; Huang, G. Renew. Energy 2008, 33, 1678–1684.
10.1021/ef8007954 CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
Biodiesel Production from Waste Cooking Palm Oil
Energy & Fuels, Vol. 23, 2009 1041
Table 1. Levels of the Transesterification Condition Variables levels variable
coding
units
-2
-1
0
+1
+2
reaction time methanol to oil molar ratio temperature amount of catalyst
A B C D
hour
0.50 6 60 1
1.25 9 95 4
2.00 12 130 7
2.75 15 165 10
3.50 18 200 13
°C wt%
University of Science Malaysia, Penang. It was heated to 120 °C to remove excess water before use. Methanol (g99.9%, HPLC Gradient grade) was purchased from Merck (Malaysia) and reference 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 heptadecanoate (g99.5%) was used as an internal standard and was purchased from Sigma-Aldrich (Malaysia) for gas chromatographic analysis. N-hexane (g96%) was used as a solvent for GC analysis and was purchased from Merck (Malaysia). All of the chemicals used were of analytical reagent grade. 2.1. Catalyst. Oil palm ash was obtained from an oil palm mill at Jawi, Penang, Malaysia. The precursor of the oil palm ash was empty fruit bunches consisting of fibers which were combusted at 800 °C to generate energy for a boiler in the mill.14 The oil palm ash produced was observably coarse in nature. Large and unburned residues were manually discarded. The ash was sieved and dried in an oven overnight prior to use as a catalyst. 2.2. Experimental Design. The synthesis of biodiesel from palm oil transesterification using oil palm ash as a catalyst was developed and optimized using the Central Composite Design (CCD) and Response Surface Methodology (RSM). CCD helps in investigating linear, quadratic, cubic, and cross-product effects of the four reaction condition variables on the biodiesel yield. The four independent variables studied were reaction time, methanol to oil molar ratio, temperature, and amount of catalyst. Table 1 lists the range and levels of the four independent variables studied. Selection of the levels was carried out on the basis of results obtained in a preliminary study, considering limits for the experiment set-up and working conditions for each chemical species. The value of R for this CCD was fixed at 2.15 The complete design matrix of the experiments employed and their results are given in Table 2. All variables at the zero level constitute the center points and the combination of each of the variables at either its lowest (-2.0) level or highest (+2.0) level with the order variables at zero level constitute the axial points. The experiment sequence was randomized to minimize the effects of the uncontrolled factors. 2.3. Statistical Analysis. The experimental data obtained by following the above procedure were analyzed by the respond surface methodology using the following second-order polynomial equation: n
y ) β0 +
∑ i)1
n
Bixi +
∑ i)1
n
Biix2i +
∑ ∑ B xx
ij i j
(1)
iF 9 1 1 1 1 1 1 1 1 1 20 15 5
152.59 28.58 81.29 258.40 292.81 28.56 348.58 214.70 74.61 63.56 7.16 6.66 8.66
21.31 3.99 11.35 36.09 40.89 3.99 48.68 29.98 10.42 8.88
