Palm Fatty Acid Biodiesel: Process Optimization and Study of Reaction Kinetics

Transcription

1 Journal of Oleo Science Copyright 2010 by Japan Oil Chemists Society Palm Fatty Acid Biodiesel: Process Optimization and Study of Reaction Kinetics Praveen K. S. Yadav 1, Onkar Singh 2 and R. P. Singh 1 1 Department of Oil and Paint Technology, Harcourt Butler Technological Institute (Kanpur, INDIA) 2 Department of Mechanical Engineering, Harcourt Butler Technological Institute (Kanpur, INDIA) Abstract: The relatively high cost of refined oils render the resulting fuels unable to compete with petroleum derived fuel. In this study, biodiesel is prepared from palm fatty acid (PFA) which is a by-product of palm oil refinery. The process conditions were optimized for production of palm fatty acid methyl esters. A maximum conversion of 94.4% was obtained using two step trans-esterification with 1:10 molar ratio of oil to methanol at 65. Sulfuric acid and Sodium hydroxide were used as acid and base catalyst respectively. The composition of fatty acid methyl esters (FAME) obtained was similar to that of palm oil. The biodiesel produced met the established specifications of biodiesel of American Society for Testing and Materials (ASTM). The kinetics of the trans-esterification reaction was also studied and the data reveals that the reaction is of first order in fatty acid and methanol (MeOH) and over all the reaction is of second order. Key words: biodiesel, transesterfication, palm fatty acid (PFA), fatty acid methyl ester (FAME); 1 INTRODUCTION Biodiesel, an alternative to petroleum fuel that can be produced through trans-esterification of vegetable oils and animal fats with alcohol in the presence of a catalyst, will greatly contribute to mitigation of environmental issues such as global warming and air pollution since its feedstock is carbon-neutral and simultaneously low in sulfur content. It significantly reduces regulated exhaust emissions including hydrocarbons, carbon monoxide and particulate matter at all blend levels with petroleum middle distillates. It also reduces polycyclic aromatic hydrocarbon, sulfur dioxide emissions and smoke opacity. It can be stored for long term by using oxidation inhibitors such as butylated hydroxytoluene BHT, butylated hydroxyanisole BHA and propyl gallate PrG 1-5. However, presently as a result of higher cost of feedstock, the production cost of biodiesel is not economically competitive in comparison to the conventional diesel. The production cost of biodiesel can be reduced by using low cost feedstock containing high amount of free fatty acid FFA. The conversion of fatty acids into FAME is done by two step process. The reaction proceeds by conversion of almost all FFA content into FAME by esterification with methanol in the presence of acid catalyst followed by further reduction of FFA by reacting the product with metha- nol in presence of base catalyst. The earlier work done for production of biodiesel from high free fatty acid FFA content feedstock were limited to FFA less than 40 wt. In the present work PFA, a by- product obtained from refining of palm oil having FFA content of 93 wt is used as feedstock. 2 EXPERIMENTAL 2.1 Materials and methods Palm fatty acid was procured from M/s Kanpur Edibles Pvt. Ltd. Kanpur, India. The composition of the palm fatty acid was determined with the help of Gas Liquid Chromatography Fig. 1. The palm fatty acid contains 0.2 Lauric acid, 1.2 Myristic acid, 42.7 Palmitic acid, 0.1 Palmitoleic acid, 4.5 Stearic acid, 38.9 Oleic acid, 11.5 Linoleic acid, 0.7 Linolenic acid, 0.1 Arachdic acid, and 0.1 traces. All chemicals including methanol, sulfuric acid and sodium hydroxide were of analytical grade. The acid value, saponification value and iodine value of palm fatty acid were 184, 198 and 52 respectively. 2.2 Production of biodiesel A two step process was used for conversion of PFA into Correspondence to: Praveen K.S. Yadav, Department of Oil and Paint Technology, Harcourt Butler Technological Institute, Kanpur, INDIA pkyhbti@yahoo.co.in Accepted May 1, 2010 (received for review January 29, 2010) Journal of Oleo Science ISSN print / ISSN online 575

2 P. K.S. Yadav, O. Singh and R. P. Singh 2.4 Analysis of palm fatty acid methyl ester The functional group composition of biodiesel was confirmed by FT-IR spectrum Fig. 2.Sharp band at cm 1 is due to C-H stretching vibrations of methylene groups. A sharp band at cm 1 is attributed to C O stretching frequency. Absorption at 1437 cm 1 and cm 1 is assigned to asymmetric -CH 3 or -CH 2 bending vibrations. Bands at cm 1, cm 1 and cm 1 are due to C-O stretching of ester. The bands obtained at cm 1, cm 1 and cm 1 are due to C-C stretching. 3 RESULTS AND DISCUSSIONS 3.1 Optimization of process conditions The trans- esterification reaction was optimized for the production of biodiesel by varying the molar ratio of PFA to MeOH, studying the effect of temperature and the amount of catalyst Effect of molar ratio of palm fatty acid to methanol The completion of the trans-esterification reaction was Fig. 1 Chromatogram of palm fatty acid methyl ester FAME. The first step of the process was to convert FFA into FAME by esterification with methanol in presence of acid catalyst. In the second step the FFA content was further reduced by reaction with methanol in presence of base catalyst Acid- base catalyzed methanolysis of palm fatty acid Methanolysis process was carried out in 500 ml three neck flask by putting PFA followed by MeOH and H 2 SO 4 into it. The three neck flask was then heated in 1 L heating mantle fitted with stirrer and water condenser. Operating parameters for methanolysis process, including reaction temperatures in the range of 40-65, molar ratios of MeOH to PFA in the range of 12:1, quantity of H 2 SO 4 catalyst in the range of were investigated. In the second step, the product obtained after first step was further reacted with methanol in presence of NaOH catalyst in the range of to form FAME of PFA. The product mixture was then poured into the separating funnel and then allowed to settle into two phases. The FAME was given 3-4 water washings until it is free from soap. The FAME free from MeOH and moisture was analyzed by FT-IR. Fig. 2 FT-IR spectrum of produced biodiesel Fig. 3 Effect of molar ratio of palm fatty acid to methanol on conversion of palm fatty acid to palm fatty acid methyl ester at 65, 180 min and 1wt% of H 2 SO 4 576

3 Palm Fatty Acid Biodiesel decided by the determination of acid value of the reaction product. The decrease in acid value indicates the progress of reaction in forward direction. Figure 3 shows that the maximum conversion was achieved at 1: 10 molar ratio of PFA to MeOH Effect of reaction time Figure 4 shows the effect of reaction time on conversion of PFA to FAME with 1:10 molar ratio of PFA to methanol, 1wt of H 2 SO 4 and 65 temperature. It was observed that the drop in Acid Value was maximum upto 180 minutes and thereafter remained unchanged Effect of acid catalyst amount The amount of acid catalyst H 2 SO 4 was varried from 0.2 to 1.2 wt. It was found that esterification reaction does not proceed without catalyst. Figure 5 shows that the maximum conversion occurred at 1.0 wt of H 2 SO 4 and thereafter on increasing the amount of catalyst there was no incerase in the conversion. 3.2 Kinetic studies The sample was withdrawn from the three neck flask at regular intervals and the acid value was determined for the study of kinetics of reaction for production of biodiesel. It was observed that at lower concentration of MeOH i.e. when molar ratio of PFA: MeOH was 1: 1 the plot of 1/ Acid Value versus Time Fig. 6 was linear with positive slope. At higher concentrations of methanol i.e. under the conditions of MeOH PFA, a plot of log Acid Value versus Fig. 4 Effect of reaction time on conversion of palm fatty acid to palm fatty acid methyl ester at 65, 1wt% of H 2 SO 4 and 1:10 molar ratio of palm fatty acid to methanol Fig. 6 Plot of 1/ (acid value ) versus time at 65 and 1:1 molar ratio of palm fatty acid : methanol Fig. 5 Effect of wt% of acid catalyst (H 2 SO 4 ) on conversion of palm fatty acid to palm fatty acid methyl ester at 65, 180 min and molar ratio of 1:10 of palm fatty acid to methanol. Fig. 7 Plot of log (acid value ) versus time at 65 and 1:10 molar ratio of fatty acid: methanol 577

4 P. K.S. Yadav, O. Singh and R. P. Singh Time was linear with a negative slope up to of the reaction Fig. 7. These results suggests, An overall second order reaction A pseudo- first order dependence of rate with respect to fatty acid under the pseudocondition i.e. when MeOH PFA. The pseudo- first order rate constants in fatty acid K obs at different initial concentrations of catalyst, temperature etc. were evaluated from the slope of straight lines plotted between log Acid Value and time at 1:10 molar ratio of PFA : MeOH. The observed rate constant K obs was reproducible within 5 in replicate kinetic runs. It was observed that maximum conversion of fatty acid to methyl ester was obtained in 180 min. After 180 min there was no appreciable change in the acid value suggesting that almost all conversion of fatty acid to methyl ester has taken place in 180 min. Biodiesel was produced by taking different molar ratios of PFA and MeOH and the values of pseudo- first order rate constant K obs have been determined and given in form of plot of K obs versus molar ratio PFA : MeOH Fig. 8. It is observed from the figure, that on increasing the concentration of MeOH, the K obs increases linearly up to 1:10 molar ratio of PFA : MeOH. However, on further increasing the molar ratio, there was no change in K obs. This result clearly indicate 1:10 as optimum molar ratio of PFA : MeOH. The effect of temperature on the rate of production of biodiesel was studied by carrying out the reaction at different temperatures i.e keeping optimum molar ratio i.e. 1:10 of fatty acid and methanol. The values of Kobs at various temperatures are given in Table 1. The energy of activation for the reaction was also evaluated from the slope of Arrhenius plot Fig. 9 and was found to be KJ mol 1. The effect of catalyst i.e. H 2 SO 4 on the rate of reaction was studied by carrying out the reaction at 65 in presence of different wt of catalyst keeping optimum conditions of molar ratio of PFA and MeOH at 1;10. The results are given in the form of a plot of K obs versus catalyst Fig. 10. It is observed that the observed rate constant increases on increasing the catalyst percentage upto 1 beyond this there was no further increase in the rate constant and also no increase in the yield of product Table 2 Fig. 9 Plot of log (K obs ) versus 1/T Fig. 8 Plot of K obs (min -1 ) versus molar ratio of fatty acid : methanol at 65 Table 1 K obs at different temperatures for the production of biodiesel Temp ( ) (K obs ) 10-2 min Fig. 10 Plot of K obs (min -1 ) versus % catalyst at 1:10 molar ratio of fatty acid : methanol and

5 Palm Fatty Acid Biodiesel Table 2 Yield of product in presence of catalyst at 65 and 1: 10 molar ratio of fatty acid : methanol Catalyst ( Wt % ) Yeild (%) Reaction mechanism Generally, the protonation of PFA takes place in presence of concentrate H 2 SO 4. Due to the transfer of proton to oxygen atom which is double bonded to carbon atom, a positive charge is developed on oxygen atom. The positive charge is delocalized with fair amount of positiveness on carbon atom of the molecule. The rate law 3, suggests a first order dependence of rate with respect to each fatty acid and methanol i.e. overall a second order reaction in presence of acid catalyst. In presence of excess of methanol, the step a and b becomes comparable and we cannot apply equilibrium conditions with respect to step a.however, in such case on applying steady state condition with respect to FA we get, FA 4 And, the rate of disappearance of FA may be given as, d/dt FA 5 At higher MeOH where k MeOH k 2 1, the rate law 5 becomes d/dt FA k FA H 1 6 The rate law 6 suggest a first order dependence of rate with respect to fatty acid in presence of acid catalyst and at higher MeOH. The experimental results are in agreement with the rate law 3 and 6. In the next step, the positive charge on the carbon atom is attacked by one of the lone pair on the oxygen of the methanol molecule giving a water molecule and ester is formed. 4 CONCLUSIONS The process for production of biodiesel from relatively low cost PFA a by-product of refining of crude palm oil has been optimized for the process conditions. The maximum conversion of PFA to FAME was obtained at a molar ratio 1:10 PFA: MeOH, 1wt of H 2 SO 4 at 65 for 180 min. The kinetic results are also in agreement with the above observed optimum conditions. A mechanism involving protonation of fatty acids and its reaction with methanol in the subsequent steps is proposed. The proposed mechanism is consistent with the observed kinetic results. On the basis of above literature and observed experimental results the mechanism of the reaction in general, may be represented as follows: FA H k 1 FA a k 1 FA MeOH k 2 Intermediate b Intermediate k 3 Product c From equilibrium step a FA K FA H 1 1 Now rate of disappearance of FA is given as d/dt FA k FA H 1 k 1 FA k FA 2 MeOH 2 On substituting the value of FA from equation 1 into equation 2, the rate of disappearance of FA becomes, d/dt FA k 2 K FA H 1 MeOH 3 ACKNOWLEDGEMENT Authors are thankful to All India Council for Technical Education, New Delhi, India, for providing the financial support for pursuing this research work at H.B.T.I. Kanpur, U. P., India. References 1 Robert O. Dunn. Effect of antioxidants on the oxidative stability of methyl soyate biodiesel. Fuel Proc. Technol. 86, Nestor, U; Sorian, Jr.; Veronica, P.; Migo; Matsumura, M. Ozonized vegetable oil as pour point depressant for neat biodiesel. Fuel 85, Imahera, H.; Minami, E.; Saka, S. Thermodynamic study on cloud point of biodiesel with its fatty acid 579

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