Elsevier

Fuel Processing Technology

Volume 95, March 2012, Pages 62-66
Fuel Processing Technology

Reaction kinetics of dolomite catalyzed transesterification of canola oil and methanol

https://doi.org/10.1016/j.fuproc.2011.11.015Get rights and content

Abstract

In this study, using a power law approximation, the reaction orders of dolomite catalyzed transesterification of canola oil and methanol were evaluated. The reaction orders for triglyceride and methanol were determined as 1 and − 1.2 respectively, with a 0.9182 correlation coefficient. The rate constant was calculated as 3.9579 × 10 3 (lt 0.2/mol 1.2). (gr. 1 cat. min 1) for this correlation. The reaction order of triglyceride which appeared in nominator of rate equation was considered as driving force of the reaction. The negative reaction order of methanol indicates the adsorption of methanol on the catalyst surface.

Highlights

► Using a power law approximation, the reaction orders were determined. ► Triglyceride concentration acts as driving force for the reaction. ► A good correlation was obtained between experimental and calculated rate values.

Introduction

Biodiesel or fatty acid methyl ester (FAME) is renewable and environmentally friendly fuel which can be obtained by transesterification of vegetable oils or animal fats with methanol in the presence of both homogeneous and heterogeneous catalysts. The transesterification of vegetable oils is usually performed by using sodium hydroxide (NaOH) and potassium hydroxide (KOH) as alkaline catalysts or sulfuric acid (H2SO4) and hydrochloric acid (HCl) as acid catalysts in a homogeneous manner [1], [2]. The homogeneous catalytic systems have some drawbacks. A large amount of waste water is produced during the process of separating and cleaning the catalyst and the products. The replacement of homogeneous catalysts with heterogeneous catalysts eliminates the drawbacks of homogeneous catalyst. Heterogeneous catalysts have received much attention in biodiesel production because the process can be simplified by facilitating separation and purification, and omitting the washing stage. Thus the biodiesel production process becomes more economic and environment friendly [3], [4]. The reusability is the main problem in biodiesel production with heterogeneous catalysts. The leaching of support or active sites causes catalyst loss and thus decreases the FAME yield. After solving this problem, the cleaner biodiesel production will become more competitive with petroleum diesel [5].

The fatty acid composition of every vegetable oil is different. The fuel characteristics of biodiesel depend on the fatty acid composition of the oil used. Canola oil is rich in oleic acid, which contains 18 carbon atoms and one double (unsaturated) bond. Therefore, this property results in superior cold flow property for canola based biodiesel. Also, canola is the preferred feedstock in biodiesel production because of high oil content (42.5% average) [6].

Many compounds have been used as heterogeneous catalyst for the biodiesel synthesis. Alkaline earth oxides, in particular magnesium oxide (MgO) [7], [8], [9] and calcium oxide (CaO) [10], [11] are potential heterogeneous catalysts for use in biodiesel production research and have attracted attention in recent years. CaO has attracted many interests as a heterogeneous catalyst because it can be synthesized from cheap sources like calcium carbonate, calcium acetate and calcium nitrate. It can be used as a simple, safe and non toxic catalyst for the biodiesel production reaction. It also possesses basicity as high as alkali metal oxides while it showed less leaching of active sites into the product stream. It was observed that the CaO existed in the reaction mixture in a form of suspensoid due to its poor mechanical strength, resulting in separation problems of catalyst from biodiesel products. Therefore, supporting CaO onto carriers was proposed to improve the CaO catalysts [12], [13], [14]. MgO which is produced by direct heating of magnesium carbonate or magnesium hydroxide was also investigated on its catalytic activity in transesterification. Di Serio et al. [9] reported that MgO exhibited very low catalytic performance in transesterification of soybean oil at reaction temperature of 100 °C.

Dolomite is a mixture of calcium carbonate and magnesium carbonate. During the calcination of dolomite at high temperatures, the carbonate groups of dolomite are decomposed. The resulting products from decomposition of dolomite are MgO and CaO. Ngamcharussrivichai et al. [15] carried out the transesterification of palm kernel oil with methanol in the presence of dolomite. In their study, 98% of methyl ester content was obtained under the reaction conditions of 60 °C, methanol/oil molar ratio of 30, 6 wt.% dolomite calcined at 800 °C, and reaction time of 3 h. In another study, Ngamcharussrivichai et al. [16] studied the transesterification of palm kernel oil with methanol in the presence of dolomite modified with calcium nitrate. They obtained 99.9% of methyl ester content under the reaction conditions of the methanol/oil molar ratio of 15, catalyst amount of 10 wt.%, and reaction time of 3 h. In my previous paper [17], the FAME yield reached over 90% after 850 °C of calcination of dolomite when the reaction was carried out at reflux temperature of methanol, with a molar ratio of methanol/canola oil of 6:1, a catalyst amount of 1.5 wt.%, and after 3 h of reaction time. The catalyst was reused three times without any loss of activity at 6:1 of molar ratio of methanol to oil.

The importance of biodiesel as an alternative fuel has increased in the last years. Understanding the kinetics of the transesterification reaction is very important for the process design. Although homogeneously catalyzed kinetics of transesterification has been investigated in several papers [18], [19], [20], [21], [22], [23], [24], the kinetics of heterogeneously catalyzed reaction has been rarely studied. Biodiesel production reaction with homogeneous catalyst such as NaOH or KOH proceeds via formation of methoxide anions from the reaction of the catalyst and methanol. A heterogeneous catalytic reaction process involves adsorption of reactants onto the catalyst surface, reaction on the catalyst surface and desorption of products. According to Filippes et al. [25] in the presence of solid base catalysts the active species for transesterification reactions are methoxide ions formed upon adsorption of methanol on the catalyst surface. Hattori et al. [26] proposed a five step mechanism for the transesterification of ethyl acetate with different rate determining steps according to the catalyst basicity. Methanol adsorption was assumed to be the rate-determining step with catalysts such as MgO, La2O3 or KF/alumina, while the surface reaction step becomes rate determining with catalysts having a higher basicity, such as BaO, CaO or SrO. Dossin et al. [27] proposed the Eley–Rideal type kinetic model for the transesterification of ethyl acetate with methanol over MgO. The proposed three-step mechanism was based on the reaction between adsorbed methanol on a basic active site of the MgO surface and ethyl acetate from the liquid phase. Methanol adsorption was the rate determining step. The first order kinetic model with respect to triglyceride [28] or methanol [29] was presented for the reaction kinetics of metal oxide catalyzed transesterification of soybean oil and methanol at high temperatures. Kouzu et al. [30] reported that the reaction kinetics varied from the zero order to the first order with respect to triglyceride.

In this study, reaction kinetics of dolomite catalyzed transesterification of canola oil and methanol was evaluated. The aim of this study was to present an alternative approach to calculate the rate constant and reaction orders of the studied transesterification process by modifying the kinetic equations developed by Singh and Fernando [29]. The calculations were performed by using the FAME yields with respect to the reaction time data taken from my previous paper [17].

Section snippets

Kinetic model and determination of reaction kinetics

The overall transesterification reaction of canola oil with methanol is presented as Eq. (1):TG+3Mcatalyst3F+Gwhere TG is the triglyceride, M is methanol, F is FAME and G is glycerin. The reaction is considered irreversible because the excessive presence of M in the reaction. In this study, the reaction kinetic equations developed by Singh and Fernando [29] were used after modification. The general rate equation of the reaction is defined as − dCTG/dt (change in concentration of TG per unit

Results and discussion

Reaction kinetic of the transesterification of canola oil was studied using the optimum reaction conditions obtained in previous study [17]. The optimum reaction conditions were obtained as follows: molar ratio of methanol to canola oil of 6:1, reflux temperature of methanol, 3 h of reaction time and catalyst amount of 3 wt.%. Under the optimum conditions FAME yields of 91.78% were obtained with dolomite. With a further increase in reaction time, FAME yield was almost kept constant at about 90%

Conclusions

The reaction kinetics of dolomite catalyzed transesterification of canola oil and methanol was studied to calculate the rate constant and reaction orders. The kinetic equations developed by Singh and Fernando [29] were used after modification. The reaction orders with respect to TG and M were 1 and − 1.2, respectively, with 3.9579 × 10 3 (lt 0.2/mol 1.2). (gr. 1 cat. min 1) of rate constant at reflux temperature of M. A good correlation was obtained between experimental and calculated reaction

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