ARTICLE pubs.acs.org/EF
Kinetic Studies of Base-Catalyzed Transesterification Reactions of Non-edible Oils To Prepare Biodiesel: The Effect of Co-solvent and Temperature Gobbaka Ravi Kumar,† R. Ravi,*,† and A. Chadha‡,§ †
Department of Chemical Engineering, ‡Department of Biotechnology, and §National Center for Catalysis Research, Indian Institute of Technology Madras, Chennai 600 036, India ABSTRACT: The non-edible oils of mahua and jatropha were transesterified using methanol and 1 wt % KOH as the catalyst. The effect of co-solvent and the kinetic study of the transesterification of mahua oil is being reported here for the first time. Kinetics, modeled as a single-step reaction, revealed that the order of the reaction is 2 with respect to the triglyceride concentration and 1 with respect to the methanol concentration in both oils. In the presence of co-solvent, tetrahydrofuran (THF), methanolysis of mahua oil resulted in the increase of the rate constants from 0.08 to 1.17 L2 mol2 min1 at 28 °C and from 0.43 to 3.18 L2 mol2 min1 at 45 °C. The corresponding values for jatropha oil were found to be 0.50 and 2.76 L2 mol2 min1 at 28 °C and 1.26 and 4.56 L2 mol2 min1 at 45 °C.
1. INTRODUCTION Transesterification, the reaction of vegetable oil or animal fat with an alcohol to form esters, is one of the most common ways of producing biodiesel.1 It can be carried out using an acid, base, or enzyme as a catalyst or without a catalyst under supercritical conditions.2 Methanol is the most commonly used alcohol, although ethanol and butanol have also been tried.3 Biodiesel is gradually gaining importance as an alternate fuel because of its superior properties, such as high flash point, nontoxicity, and less emissions compared to conventional diesel.1,2 The importance of kinetic parameters is clear from the work by Fonseca et al.4 and Klofutar et al.,5 in which the authors designed industrial-scale continuous reactors using data obtained from laboratory-scale batch reactors. However, despite the increasing number of studies devoted to the study of transesterification over the years, kinetic parameters, e.g., rate constants, are not always reported.6 Kinetics critically depends upon the type of oil used [its fatty acid profile as well as the relative amounts of triglycerides (TG), diglycerides (DG), and monoglycerides (MG) present], nature and amount of catalyst used, temperature, level of mixing, and co-solvent. In India, non-edible oils, such as Pongamia pinnata,7 Jatropha curcas,8 and Madhuca indica,9 among others, are the most appropriate feedstock for biodiesel preparation. Kinetics of base-catalyzed transesterification of MG10 and TG11 from Pongamia oil has been studied. In the case of jatropha oil, there are three reported studies1214 on the kinetics of transesterification, but none of these studies reports the effect of co-solvent on the rate of transesterification. Biodiesel from mahua oil has been studied for its emission characteristics,15 performance of the compression engine,16 and storage stability of biodiesel.17 The kinetics of transesterification of mahua oil is reported here for the first time. Base-catalyzed transesterification results in higher yields at shorter reaction times compared to acid catalysts.18 Kinetics of transesterification using base catalysts has been studied for many r 2011 American Chemical Society
other edible oils, such as soybean,19,20 sunflower,21 palm,22 and canola,23 and the rate constants vary with the oil used.5 The mechanisms used to model the kinetics of transesterification are commonly (a) a three-step reaction mechanism11,19 and (b) a single-step reaction mechanism.12,13 The transesterification reaction is biphasic in nature because oil and methanol are immiscible, and hence, mass-transfer resistance could inhibit the progress of the reaction. To improve interphase mass transfer, the following strategies have been employed: high mixing intensities,20,24 addition of co-solvent,23,25 and membrane reactors.26 Among these, the addition of co-solvent, mainly tetrahydrofuran (THF), is attractive because it has been shown to result in a large increase in the magnitudes of the rate constants in the case of both soybean27 and canola23 oils. However, Karmee et al.11 in their studies on pongamia oil did not observe any dramatic increase in transesterification rates on the addition of THF. Given that mahua and jatropha oils are important feedstocks for biodiesel production and the effect of co-solvent on their transesterification has not been reported thus far, this study was undertaken to quantify this effect.
2. EXPERIMENTAL SECTION 2.1. Materials. Crude mahua and jatropha oils were obtained from local sources in Tamilnadu, India. High-purity standards, viz. triolein, diloein, monoolein, and methyl esters of palmitic, oleic, linoleic, and stearic acids were purchased from Sigma-Aldrich Co., St. Louis, MO. All solvents and reagents were of analytical grade. 1H nuclear magnetic resonance (NMR) was recorded on a Bruker 400 MHz instrument using CDCl3, with tetramethylsilane (TMS) as an internal standard to characterize the triglyceride and methyl esters. Quantitative analysis of samples was carried out with a Perkin-Elmer Clarus 600 series model gas chromatograph equipped with a Zebron ZB-5HT fused silica capillary Received: March 28, 2011 Revised: May 30, 2011 Published: May 31, 2011 2826
dx.doi.org/10.1021/ef200469u | Energy Fuels 2011, 25, 2826–2832
Energy & Fuels
ARTICLE
column (15 m 0.32 mm 0.1 μm) and flame ionization detector. A gradient mode was used for heating the oven with an initial temperature of 50 °C and final temperature of 330 °C. Both injector and detector temperatures were maintained at 360 °C.28
2.2. Determination of Free Fatty Acids (FFAs), TG, DG, and MG. The FFA content in crude mahua and jatropha oils was determined using the American Oil Chemists’ Society (AOCS) official method.29 The amounts of TG, DG, and MG in the crude oil were determined by column chromatography10 using hexane and ethyl acetate as the eluent. The TG fraction was characterized by 1H NMR. The average molecular weight of TG used in the study was calculated on the basis of the composition of the fatty acids present in the oil. Because the fatty acid composition of oils varies with the geographical location, an average molecular weight was used for each of the two oils, i.e., mahua oil3032 and jatropha oil.3337 2.3. Base-Catalyzed Transesterification. The FFA content in crude jatropha and mahua oils was reduced to less than 1% using saturated sodium bicarbonate solution. Initially, ethyl acetate (50 mL) and saturated sodium bicarbonate solution (100 mL) were added to the oil (50 mL) taken in a 500 mL separating funnel. The mixture was shaken and then kept for settling for 45 min to obtain two layers (organic and aqueous). The aqueous layer was discarded, and this process was repeated until the FFA content reduced to 80% within 5 min in the case of jatropha methanolysis), experiments were redesigned to collect aliquots at short times, such as 0.3, 0.6, 1, 2, 3, and 4 min.
2.4. Transesterification Reaction Using Co-solvent, THF. To the above oil/KOH/methanol mixture, freshly dried THF in a 1.25:1 (v/v) ratio of THF/methanol was added. This proportion of THF has been used in the transesterification of soybean oil,25,27 and the same proportion has been employed in this work to make a comparative assessment of the effect of co-solvent. Aliquots at different time intervals were worked up as described above. Conversion of TG at different time intervals was estimated using gas chromatography. Reactions with THF were carried out at two different temperatures, i.e., 28 and 45 °C, using a 1:6 molar ratio of oil/methanol and checked for reproducibility.
3. RESULTS AND DISCUSSION Crude oils of mahua and jatropha contained 3.4 and 7.9 wt % FFA, respectively. The FFA was reduced to
