KINETICS OF TRANSESTERIFICATION OF ESTERIFIED CRUDE COTTON (Gossypium hirsutum) SEED OIL

Transcription

1 KINETICS OF TRANSESTERIFICATION OF ESTERIFIED CRUDE COTTON (Gossypium hirsutum) SEED OIL 1* Lebnebiso, J. S., 2 Aberuagba, F., 1 Kareem, S. A. and 1 Cornelius, J. 1 Department of Chemical Engineering, Moddibo Adama University of Technology, Yola, Nigeria. 2 Department of Chemical Engineering, Federal University of Technology, Minna, Nigeria. Correspondence Address: . jlebnebiso@yahoo.com, lebnebisob@gmail.com * Correspondent Author ABSTRACT Kinetics of transesterification of crude cottonseed oil with Free Fatty Acid (FFA) content of 5.2% was carriedout by a two-step process, using 250ml Spherical round bottom flask as a batch reactor. The first step was acid pretreatment which reduces the FFA content to 0.9%. The esterified crude cottonseed oil obtained during the first step was then investigated at various temperatures. The composition of the methylesters was analysed by High Performances Liquid Chromatograph (HPLC). The experimental data obtained appear to fit a pseudo second-order kinetic model for the initial stages of the transesterification reaction. Reaction rate constants for Triglycerides (TG), Diglycerides (DG) and Monoglycerides (MG) hydrolysis reactions were in the range of (wt%-min) -1 and were higher at higher temperatures. Activation energies were , and kj/mol for the TG, DG and MG hydrolysis reactions respectively. Key Words: Esterification, Transesterification, Kinetics, Crude cottonseed oil and Esterified crude cottonseed oil. INTRODUCTION The pace of research into renewable energy sources have increased over the years, due to increasing environmental consciousness. Vegetable oils are among the important sources of renewable energy, it can be chemically converted to biodiesel, which is an alternative fuel for diesel engines. Although Biodiesel production can replace only a small percentage of the global diesel supply, when blended with Ultra Low Sulfur Diesel (ULSD) provides a near-zero sulphur content biodegradable additive that improves the fuel s lubricity (Kotrba, 2006), reduce green house gas emissions and ultimately creates markets for fats and oils. The most common reaction schemes include the use of alkali catalysts to reduce reaction times to a few minutes at atmospheric pressure and methanol reflux temperatures, but require refined, low Free Fatty Acid (FFA) oils to obtained high conversion(caye et al, 2008, Fan, 2008). Acid catalysts are also effective especially for high Free Fatty Acid (FFA) oils, but typically require higher temperatures and/or longer reaction times (Crabbe et al, 2001; Zullaikah et al, 2005; Williams et al, 2007; Thiruvengadarui et al, 2009), or enzyme catalysed (Ranganathan et al, 2008). The greatest hurdle in the commercialisation of biodiesel is the cost of production, which include raw material costs and costs of product purification. This has kept the retail price of biodiesel too high for it to be an option for many users. Raw materials generally consist of simple alcohols and high-quality vegetable oils and fats. Although there are still many problems with using crude feedstock (Ma and Hanna, 1998), utilization of less expensive feedstocks such as crude vegetable oils and used vegetable oils will be a crucial determinant to producing a competitive biodiesel to consumers. Quite a number of studies have been conducted on the kinetics of transesterification of crude vegetable oils and animal fats (Ma and Hanna, 1999; Haas et al, 2004; Leung and Guo, 2006) and reported conversions of between 50 to 75%. It has also been established that transesterification depends on several basic variables, namely, catalyst type, alcohol type, catalyst-to-oil ratio, alcohol-to-oil ratio, reaction 87

2 temperature, reaction time, agitation rate, FFA, and water content of oils. Free fatty acid and moisture contents have significant effects on the transesterification of glycerides with alcohol using alkali catalyst. The high free fatty acid content (>1% w/w) will help in soap formation and the separation of products will be exceedingly difficult, as a result, low biodiesel yield will be achieved. Acid-catalysed esterification of the oil is an alternative, but it is much slower than the base-catalysed transesterification reaction. Therefore, an alternative process known as a two-step process used by (Ghadge and Raheman, 2005; Veljkovic et al., 2006; Berchmans and Hirata 2008) for high FFA feedstock for biodiesel production was adapted, to study the kinetics of estarified crude cottonseed oil transesterification. The first step was the FFA reduction process, while the second step was the transesterification process. MATERIALS AND METHOD Crude cottonseed oil obtained from expeller (i.e. screw pressed cottonseed oil) was purchased from Yola Oil mill, Yola, Adamawa state, Nigeria and used without further purification in this study. Methanol (ML), Sulfuric acid and all other reagents were of analytical grade obtained from an Agent of BDH Chemicals Limited Poole England. Esterification and Transesterification reactions were carried out in a 250 ml spherical round bottom flask as a batch reactor, The flask was immersed inside a water bath (Morphy Richards, 45470) equipped with temperature controller to maintain the temperature of the water and in turn the temperature of the reactants at a desired value. The spherical flask consists of three openings sealed tightly with silicon rubber caps that retained any vapour mixture. The central opening of the flask was used for inserting the stirrer into the reactor. The speed controller motor (Fair lawn, SL1200) propelled the stirrer at a desired speed. A thermometer was immersed inside the second opening to continuously monitor the temperature of the reaction. Samples were withdrawn during the reaction through the third opening. Esterification Esterification of the FFA in the crude cottonseed oil was carried out with methanol and concentrated sulphuric acid (H 2 SO 4 ) as the catalyst. The reaction was conducted in a laboratory scale setup described above. The oil fed into the reactor was preheated to 55 o C before the catalyst and the alcohol mixture were added. Agitation was provided with a speed controller mechanical stirrer, at a constant speed of 464rpm, to overcome mass transfer limitations. On completion of the esterification reaction, the reaction products were poured into a separating funnel to settle over night. Water was removed as by-product at the bottom. Transesterification Transesterification of the esterified oil using an alkali catalyst was carried out at different reaction temperatures between 40 o C and 60 o C to study the effect of temperature on the transesterification of the esterified crude cottonseed oil. Analysis Samples (0.5ml each) were removed from the reaction mixture at initially 2 min intervals, and later at 5 min intervals during the 30 min reaction time chosen, as a result of the fast nature of transesterification processes. Samples removed were immediately quenched with two drops of 0.6N hydrochloric acid, thus stopping further reaction. High Performance Liquid Chromatography (HPLC) (Shimadzu, LC-10AT) was used to determine the triglyceride content, the free fatty acid content and the methylester content of both the crude and the esterified crude cottonseed oil. Later, the sample s Triglyceride (TG), Diglyceride (DG), Monoglyceride (MG), methyl esters(me), and glycerol(gl) contents for the Transesterification reaction were also determined. The equipment consists of a column (Capcell Pak C18, 25 cm in length and 4.6 mm in inner diameter) and ultraviolet detector at 220 nm (Tosoh, UV-8024) operated at 30 o C with 0.7 ml/min flow rate of 80% acetonitrile solution containing 20% of 0.1% H 3 PO 4 as a carrier solvent. The sample volume was 20 µl and the percentage of a compound in the sample was determined by comparing it with a standard compound. 88

3 RESULTS AND DISCUSSION The results obtained from the analysis of the crude cottonseed oil before and after the esterification reaction are presented in Table 1. Table 1: Analysis of Oils Method Crude cottonseed oil Triglyceride (%) HPLC FFA (%) HPLC Methyl ester (%) HPLC Nil 3.1 Esterified crude cottonseed oil Kinetics data were obtained from the different sets of transesterification reactions of the esterified crude cottonseed oil. The best model for the data obtained appears to be a pseudo second-order reaction kinetics at the early stage of the reaction (0-20min reaction time) as reported by Darnoko and Cheryan (2000). To test this hypothesis, a model was developed based on the kinetics of TG hydrolysis (Muniyappa et al, 1996). The mechanism of alkali-catalysed transesterification is as described by (Schuchardt et al., 1997), which takes place in three stages; Stage 1 K 1 TG + ML DG + ME (1a) K 2 Stage 2 K 3 DG + ML MG + ME (1b) K 4 Stage 3 K 5 MG + ML GL + ME (1c) K 6 The second-order reaction rate for TG in Equation 1a would be as follows (Smith, 1981) d [ TG] / dt = k[ TG] 2 (2) Integration of Equation 2 yields 1 /[ TG ] = ktg. t + 1/ [ TG o ] (3) Similarly, the same procedure to Equations 1b and 1c would result in: 1 /[ DG ] = kdg. t + 1/ [ DG o ] (4) 1 /[ MG ] = kmg. t + 1/ [ MG o ] (5) 89

4 /T G (w t% ) oC 45oC oC 55oC oC Reaction time (min) Figure 1: Second-Order Plot of 1/[TG] vs Reaction Time (t) for the five Temperatures where k is the overall pseudo rate constant, t is the reaction time, TG o is the initial triglyceride concentration, DG o is the initial highest diglyceride concentration, and MG o is the initial highest monoglyceride concentration. For the hydrolysis of TG, a plot of 1/[TG] vs reaction time (t) will be a straight line if the model (Equation 3) is valid. Figure 1 shows such plots for the five temperatures, under atmospheric pressure during the 20 min of reaction time. The slope is k TG with the units (wt%.min) -1. Similarly straight lines were obtained for DG and MG hydrolysis (Darnoko, 1999). The values of k and its corresponding correlation coefficient are shown in Table 2. There is an increase in k at higher temperatures. The Arrhenius energy of activation were obtained from a plot of the reaction rate constant (k) vs. the reciprocal of absolute temperature (T) according to the Equation; 90

5 0 1/T (K -1 ) L o g y = x R 2 = Figure 2: A plot of the reaction rate constant (k) vs. the reciprocal of absolute temperature (T) for TG DG log k = ( Ea / 2.303R) / T + C (6) Where E a is the energy of activation, R is the gas constant, and C o is a constant. The plots are shown in Figures 2, 3, and 4, and the activation energies are shown in Table 3. The values of E a are slightly less than those reported by Darnoko and Cheryan (2000) for TG DG, DG MG and MG GL. 91

6 1/T (K -1 ) L o y = x R 2 = Figure 3. A plot of the reaction rate constant (k) vs. the reciprocal of absolute temperature (T) for DG MG Table 2: Reaction rate constant K (wt% min) -1 for Triglyceride (TG), Diglyceride (DG), and Monoglyceride (MG) Hydrolysis at different Temperatures R 2 Glyceride Temperature( o C) Reaction rate constant (wt%-min) -1 TG DG 40 o C o C o C o C o C DG MG 40 o C o C o C o C o C MG GL 40 o C o C o C o C o C

7 Table 3: Activation energy for Hydrolysis of TG, DG, and MG During Transesterification of Esterified Crude Cottonseed Oil Rate constant E a (kj/mol) R 2 TG DG DG MG MG GL /T (K -1 ) Log -0.8 k y = x R 2 = Figure 4. A plot of the reaction rate constant (k) vs. the reciprocal of absolute temperature (T) for MG GL CONCLUSION Estimation of transesterification kinetic parameters of crude cottonseed oil with Free Fatty Acid (FFA) content of 5.2% by a two-step process in a batch reactor has been presented. The experimental data appear to fit a pseudo second-order kinetic model for the initial stages of the transesterification reaction. Reaction rate constants for Triglycerides (TG), Diglycerides (DG) and Monoglycerides (MG) hydrolysis reactions were in the range of (wt%-min) -1 and were higher at higher temperatures. Activation energies were , and kj/mol for the TG, DG and MG hydrolysis reactions respectively. 93

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