Kinetics of Non-Catalytic Esterification of Free Fatty Acids Present in Jatropha Oil

Journal of Oleo Science Copyright 2016 by Japan Oil Chemists Society doi : 10.5650/jos.ess15255 Kinetics of Non-Catalytic Esterification of Free Fatty Acids Present in Jatropha Oil Karna Narayana Prasanna Rani 1*, Tulasi Sri Venkata Ramana Neeharika 1, Thella Prathap Kumar 2, Bankupalli Satyavathi 2 and Chintha Sailu 3 1 Centre for Lipid Research, Indian Institute of Chemical Technology 2 Chemical Engineering Division, Indian Institute of Chemical Technology, Tarnaka, Uppal Road, Hyderabad 500007, INDIA 3 University College of Technology, Osmania University Hyderabad 500007, INDIA Abstract: Non-catalytic esterfication of free fatty acids (FFA) present in vegetable oils is an alternative pretreatment process for the biodiesel production. Biodiesel, consists of long-chain fatty acid methyl esters (FAME) and is obtained from renewable sources such as vegetable oils or animal fat. This study presents kinetics of thermal esterification of free fatty acids present in jatropha oil with methanol. The effect of process parameters like reaction time (1-5 h), temperature (170-190 C) and oil to methanol ratio (1:3-1:5) at constant pressure was investigated. The optimal conditions were found to be oil to methanol ratio of 1:4, 190 C, at 27.1 bar and 5 h which gave a maximum conversion of 95.1%. A second order kinetic model for both forward and backward reactions was proposed to study the reaction system. A good agreement was observed between the experimental data and the model values. The activation energy for forward reaction and the heat of reaction were found to be 36.364 Kcal/mol and 1.74 Kcal/mol respectively. Key words: jatropha oil, non-catalytic esterification, free fatty acids, kinetics, biodiesel 1 INTRODUCTION Non-catalytic processes, typically at higher temperatures and pressure are among those being investigated and preferred when compared with conventional acid catalyzed esterification process. During thermal esterification free fatty acids present in the oil convert into esters within shorter period with no neutralization process. Literature shows that non-catalytic esterification of fatty acids with alcohols has been carried out mostly at supercritical conditions 1, 2. A bubble column reactor was developed to produce fatty acids methyl esters by blowing superheated methanol continuously at 4 g/min into triglycerides or fatty acids without using any catalysts at atmospheric pressure by Joelianingsiha et al. 3. The reactivity of palm fatty acids was studied at 533 K in a semi-batch reactor system to produce variety of fatty acid methyl esters namely methyl myristate, methyl palmitate, methyl stearate, methyl oleate and methyl linoleate. The rate constant and conversions were reported after 60 min of reaction time. Hong et al. 4 proposed a model representing the noncatalytic biodiesel production reaction. In the modeling of the reaction, a nonlinear programming scheme to estimate reaction kinetic parameters which minimize a specified objective function was employed. The behavior of the methanol during the reaction was investigated both experimentally and numerically. Non-catalytic esterification of palm fatty acid distillate was investigated by Cho et al. 5 at high temperature 250 enough to be above boiling point of water and at pressure 0.85 1.20 MPa without any catalyst. The effects of temperature, methanol feed rate and pressure on a semi-batch reaction were investigated and the optimal values of these variables were found to be 290, pressure: 0.85 MPa, feed rate: 2.4 g/min. The acid value was reduced from 191.4 to 0.36 mg KOH/g in 180 min. The activation energy was found to be 17.74 kj/mol. Vijayalakshmi et al. 6 provide a process for simultaneous conversion of carboxylic acid and their glycerides to the alkyl esters. Joelianingsih et al. 7 studied the effects of reaction temperatures 200, 225, and 250 on the composition of reaction product, conversion of the reaction and the reaction rate constant under a semi-batch mode of operation with oleic acids as free fatty acids. The activation energy and the frequency factor values of the methyl esterification reaction reported were 24.8 kj/mol and 2.9, respectively. * Correspondence to: Karna Narayana Prasanna Rani, Centre for Lipid Research, Indian Institute of Chemical Technology, Tarnaka, Uppal Road, Hyderabad 500007 INDIA E-mail: knpr@iict.res.in Accepted January 20, 2016 (received for review November 6, 2015) Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 online http://www.jstage.jst.go.jp/browse/jos/ http://mc.manusriptcentral.com/jjocs 441

K. N. P. Rani, T. S. V. R. Neeharika and T. P. Kumar et al. Melo Junior et al. 8 investigated the C18 fatty acids esterification under microwave irradiation. The effects of alcohol used methanol or ethanol, temperature 150-225, molar ratio of alcohol/ fatty acid 3.5-20, and total microwave irradiation power on the noncatalytic reaction conversion were evaluated. The results showed conversion up to 60 in 60 min of reaction. The authors claim that the esterification reaction under microwave irradiation yielded similar results to those obtained with the conventional heating but with very fast heating rates. Despite several esterification kinetic studies available for diverse feedstock with high content of FFA, there still remains thermal esterification kinetics study of different feedstock which contains a complex blend of FFA. Thus thermal esterification of the feedstocks with high FFA into biodiesel might address the problem of handling high FFA oils. The objective of this work is to kinetically interpret the thermal esterification of free fatty acids present in jatropha oil. Similar to our earlier work 9, 10, we have determined the kinetics of the reaction. The effect of process parameters such as methanol ratio and temperature for the reaction were studied to determine the reaction rate constants for both forward and backward reactions and also the activation energy. 2 EXPERIMENTAL 2.1 Materials Jatropha oil with acid value 53.43 used in the experiments was purchased from the local market. All chemicals used in the experiments such as methanol, potassium hydroxide pellets were of analytical grade and was procured from M/s. Sd Fine Chem. Pvt. Ltd., Mumbai. 2.2 Analytical methods Acid value of the oil was determined using AOCS Method 11. The degree of conversion was based on the titrimetric analysis of the FFA in the feed samples and in the final product. 2.3 Experimental procedure Esterification reactions were carried out in batch type 100 ml stainless steel autoclave equipped with a impeller, provision to handle high temperature and pressure and with external heating media, and PID device for controlling reactor and sampling device. The system was heated by means of external electrical jacket. Initially the reactor was charged with 10 g of jatropha oil and 40 ml of methanol. The contents were heated to 190, the reaction temperature was maintained for 1 h at constant pressure 27.1 bar and 500 rpm. The reactor was cooled and the product desolventized and dried under reduced pressure. The product was analyzed for its acid value for determination of residual acidity. Initial AV final AV conversion 100 Initial AV AV: Acid Value Different sets of experiments were carried out for the generation of data under different operating conditions to arrive at optimum process conditions. The first set of experiments was carried out at different reaction times ranging from 1 5 h by keeping fixed oil to methanol ratio at 1:4, at a temperature of 190. The second set of experiments was carried out at different oil to methanol ratio of 1:3, 1:4, and 1:5 at 190 temperature for 5 h. The third set of experiments was carried out at different temperatures ranging from 170 to 190 with oil to methanol ratio of 1:4 for 5 h. The initial sample at zero time was taken after the temperature was attained. 2.4 Statistical analysis The data was analyzed by a paired Student s t-test to evaluate the level of statistical significance. 3 KINETIC MODEL For the present reaction system, a kinetic model with second-order for both forward and backward reactions was proposed 12. The reaction mechanism for the kinetic model involving the reversible reaction is as follows: A B k 1 k2 C D 1 The reaction rate equation is expressed as: r A dc A k dt 1 C A C B k 2 C C C D 2 where C A, C B, C C and C D denote the concentrations of FFA, methanol, FAME and water formed during reaction respectively. k 1 and k 2 are kinetic rate constants for the forward and backward reactions respectively. As C A C Ao 1 X where X is the conversion of FFA and C Ao is the initial concentration of FFA, as C Ao C Bo i.e., C B C Ao 1 X where C Bo is the initial concentration of alcohol and C C C D C Ao C A C Bo C B C Ao X, substituting these in Eq. 2, we get, dx dt k 1C Ao 1 X 2 k 2 C Ao X 2 3 At equilibrium, dx dt 0 and X X E, and from Eq. 3, we get, Equilibrium constant, X2 E K k 1 k 2 1 X E 2 4 k 2 k 1 1 X E 2 X 2 5 E By substituting k 2 in Eq. 3 and rearranging the terms, we get, 442

Kinetics of Non-Catalytic Esterification of Free Fatty Acids Present in Jatropha Oil dx dt k 1C Ao X 2 2X E 1 X 2 2X 2 EX X 2 E E Integration of Eq. 6 yields, 6 ln X e 2X e 1 X X e X 2k 1C Ao 1 X e 1 t 7 The conversion as a function of time can be deduced from Eq. 7 as follows, X X e e 2k1CAo 1 X e e 2k 1C Ao 1 X e 1 t 1 1 t 2 X e 1 8 4 RESULTS AND DISCUSSION The reactions were carried out in an autoclave employing an autogenous pressure. The experimental data was used to study the effect of different parameters on the rate of reaction and the same is discussed in this section. 4.1 Effect of reaction time The extent of esterification reaction depends on the time of reaction. Figure 1 depicts the reactants conversion with respect to time and shows that conversion increased with the reaction time. During the initial period of 1 hr, it was found that the conversion reached 77.3. A reaction time longer than 5 h did not yield further change in the reaction conversion. A maximum value of 95.1 after 5 h was recorded. This may be due to the fact that the equilibrium was achieved within 5 h reaction time. The time of reaction was thus optimized for 1:4 oil to Methanol ratio at 190. 4.2 Effect of oil to Methanol ratio Oil to Methanol ratio is one of the most important variables affecting the esterification reaction conversion. Esterification of FFA is an equilibrium limited reaction. Therefore, excess alcohol will shift the equilibrium towards Fig. 1 Effect of time of reaction on non-catalytic esterification of FFA present in jatropha oil Reaction temperature 190, Oil to MeOH ratio 1:4. experimental values model values Fig. 2 Effect of oil to MeOH ratio on non-catalytic esterification of FFA present in jatropha oil Reaction temperature 190. 1:4 1:3 1:5 model values the production of more ester and also increases the rate of esterification. The effect of oil to Methanol ratio on conversion was studied at various ratios ranging from 1:3 to 1:5 where pressure of the system varied from 21.14 to 29.92 bar, and is shown in Fig. 2. It is clearly observed from Fig. 2 that the conversion increased from 90.1 to 95.1 as the methanol ratio increased from 1:3 to 1:4, and further increase in methanol ratio to 1:5 increased the conversion to 96.9. So, 1:4 Oil to Methanol ratio was used in all the other experiments as it is the optimum ratio. 4.3 Effect of reaction temperature The reaction temperature affects the reaction rate. The thermal esterification temperatures were arrived at based on literature 13 and modified in order to study the influence of temperature over the reaction. The heat up time calculated is less than one minute therefore the effect of heat up time was not taken into consideration. The effect of temperature was determined at different temperatures ranging from 170 to 190 where pressure of the system varied from 14.13 to 27.1 bar, by keeping other reaction parameters constant, i.e. 1:4 Oil to Methanol ratio, and is shown in Fig. 3. It was found that rate of reaction increased rapidly from 73.76 to 95.1 with increase in temperature from 170 to 190. 4.4 Validation of kinetic model Levenberg Marquardt algorithm was applied for the optimization of the two parameters, X E and k 1 by non-linear regression by fitting the experimental data to the kinetic model. Statistica software was used for the above analysis and was found to be satisfactory for carrying out the non linear estimation. The R-Squared value of 0.99 implies that the model was significant statistically and adequate to represent the relationship between the experimental and theoretical parameters. Experimental values were fitted to the model equation, Eq. 8 to determine X E and k 1 values by 443

K. N. P. Rani, T. S. V. R. Neeharika and T. P. Kumar et al. Fig. 3 Effect of reaction temperature on non-catalytic esterification of FFA in jatropha oil Oil to MeOH ratio 1:4. 190 180 170 model values Fig. 4 Effect of reaction temperature on reaction rate constants. k 1 ; k 2 ; trial and error until the best fit was observed. The rate constants for forward and backward reactions, k 1 and k 2 were calculated from the Eqs. 8 and 5 respectively. The results obtained for equilibrium conversion, reaction rate constants, k 1 and k 2, and equilibrium constant, K, are reported in Table 1. 4.4.1 Effect of temperature on reaction rate constant The effect of temperature on the forward reaction rate constant, k 1, was evaluated by using Arrhenius equation. ΔE k Ae RT 10 and In k 1 ΔE In A 11 RT From the plot of lnk 1 as a function of the reciprocal temperature, for 1:4 Oil to Methanol ratio, as shown in Fig. 4, the frequency factor, A, and the energy of activation, ΔE, for forward reaction were found to be 5.84 10 17 and 36.364 Kcal/mol and 2.0319 10 12 and 34.624 Kcal/mol respectively for backward reaction. 4.4.2 Effect of temperature on equilibrium constant The effect of temperature on equilibrium constant, K, was also evaluated by using van t Hoff equation. In K ΔH const. 12 RT From the plot of lnk and the reciprocal temperature which is shown in Fig. 5, the heat of reaction was found to Fig. 5 Effect of reaction temperature on equilibrium constant. be 1.74 Kcal/mol. So, it can be concluded that thermal esterification reaction is endothermic in nature. As the heat up time is less than one minute the effect of heat up time on the kinetics is assumed to be negligible. Based on the numerical values obtained by kinetic analysis it can be concluded that there is a marginal increase in the equilibrium constant with increase in temperature. Furthermore, the rate constant k 1 and k 2 values obtained confirm that k 1 k 2 under the reaction conditions employed. 4.4.3 Comparison of experimental and predicted conversions The fitting of the experimental data to the proposed model was also assessed by comparing the experimentally obtained conversions with the theoretically predicted con- Table 1 Temperature ( ) Equilibrium conversion, kinetic rate constants and equilibrium constant for thermal esterification of free fatty acids present in jatropha oil. X e k 1 k 2 K R 2 170 0.994991 0.646252 1.63782E-05 3.9458E+04 0.9999 180 0.995145 1.559886 3.71277E-05 4.2014E+04 0.9998 190 0.995199 3.859604 8.98227E-05 4.2969E+04 0.9999 444

Kinetics of Non-Catalytic Esterification of Free Fatty Acids Present in Jatropha Oil Fig. 6 Comparison of experimental and predicted conversions. 190 180 170 1:3 1:5 versions calculated using Eq. 6, and is presented in Fig. 6 paired Student s t-test statistical approach has been used to analyse the data, and a p-value of 0.0496 was obtained which was considered significant. So, it can be inferred that the proposed model represented the present reaction system satisfactorily. 5 CONCLUSIONS This study suggests that FFA present in jatropha oil can be converted to fatty acid methyl esters by thermal esterification reaction. The objective of this study was to determine to optimum process conditions like reaction time, temperature and oil to methanol ratio on noncatalytic thermal esterification of FFA present in jatropha oil. The degree of conversion was monitored by acid value. The optimal conditions were found to be oil to methanol ratio of 1:4, 190, and 5 h with constant stirring at 500 rpm gave a maximum conversion of 95.1. A second order kinetic model w.r.t both forward and backward reactions was proposed to fit the experimental data and it explained the present reaction system satisfactorily. The effect of temperature on forward reaction rate constant and equilibrium constant was evaluated by using Arrhenius and van t Hoff equations respectively. The activation energy for forward reaction and the heat of reaction were found to be 36.364 Kcal/mol and 1.74 Kcal/mol respectively, and the esterification reaction was endothermic in nature. REFERENCES: 1 Akaraphol, P.; Duangkamol, Y.; Artiwan, S.; Motonobu, G.; Mitsuru, S. Production Methyl Esters from Palm Fatty Acids in Supercritical Methanol. Chiang Mai J. Sci. 35, 23-28 2008. 2 Pinnarat, T.; Savage, P. E. Noncatalytic Esterification of Oleic Acid in Ethanol. J. Supercrit. Fluids 53, 53-59 2010. 3 Joelianingsih; Tambunan, A. H.; Nabetani, H. Reactivity of Palm Fatty Acids for the Non-Catalytic Esterification in a Bubble Column Reactor at Atmospheric Pressure. Procedia Chem. 9, 182-193 2014. 4 Hong, S. W.; Cho, H. J.; Kim, S. H.; Yeo, Y. K. Modeling of the Non-Catalytic Semi-Batch Esterification of Palm Fatty Acid Distillate PFAD. Korean J. Chem. Eng. 29, 18-24 2012. 5 Cho, H. J.; Kim, S. H.; Hong, S. W.; Yeo, Y. K. A Single Step Non Catalytic Esterification of Palm Fatty Acid Distillate PFAD for Biodiesel Production. Fuel 93, 373-380 2012. 6 Vijayalakshmi, P.; Rao, D. V.; Laxmi, A. A.; Ramalinga, B.; Ali, A. Z.; Kaimal, T. N. B.; Vedanayagam, H.S. A Process for Simultaneous Conversion of Carboxylic Acids and their Glycerides to Alkyl Esters. Indian Patent 242255. 7 Joelianingsih; Nabetani, H.; Hagiwara, S.; Sagara, Y.; Soerawidjaya, T. H.; Tambunan, A. H.; Abdullah, K. Performance of a Bubble Column Reactor for the Non- Catalytic Methyl Esterification of Free Fatty Acids at Atmospheric Pressure. J. Chem. Eng. Jpn. 40, 780-785 2007. 8 Melo-Junior, C. A. R.; Albuquerque, C. E. R.; Fortuny, M.; Dariva, C.; Egues, S.; Santos, A. F.; Ramos, A. L. D. Use of Microwave Irradiation in the Noncatalytic Esterification of C18 Fatty Acids. Energy Fuels 23, 580-585 2009. 9 Prasanna Rani, K. N.; Prathap Kumar, T.; Neeharika, T. S. V. R.; Satyavathi, B.; Prasad. R. B. N. Kinetic Studies on the Esterification of Free Fatty Acids FFA in Jatropha Oil. Eur. J. Lipid Sci. Tech. 115, 691-697 2013. 10 Prasanna Rani, K. N.; Neeharika, T. S. V. R.; Prathap Kumar, T.; Satyavathi, B.; Sailu, Ch.; Prasad, R. B. N. Kinetics of Enzymatic Esterification of Oleic Acid and Decanol for Wax Ester and Evaluation of its Physicochemical Properties. J. Taiwan Inst. Chem. Eng. 55, 12-16 2015. 11 Firestone, D. Ed., AOCS Official Methods and Recommended Practices of the American Oil Chemists Society, 5th Edn., AOCS Press, Champaign 2003, Cd 3d-63. 12 Patil, T. A.; Sen, T. K.; Chattopadhyay. S. Production Kinetics of the Esterification of a Polyol. J. Synthetic Lubric. 19, 119-129 2002. 13 Berrios, M.; Martin, M. A.; Chica, A. F.; Martin, A. Study of esterification and transesterification in biodiesel production from used frying oils in a closed system. Chem. Eng. J. 160, 473-479 2010. 445