Analysis of parameters and interaction between parameters of the microwave-assisted continuous transesterification process of Jatropha oil using response surface methodology

https://doi.org/10.1016/j.cherd.2011.06.002Get rights and content

Abstract

A simple continuous process was designed for the transesterification of Jatropha curcas (J. curcas) oil to alkyl esters using microwave-assisted method. The product with purity above 96.5% of alkyl ester is called the biodiesel fuel. Using response surface methodology, a series of experiments with three reaction factors at three levels were carried out to investigate the transesterification reaction in a microwave and conversion of alkyl ester from J. curcas oil with NaOH as the catalyst. The results showed that the ratio of methanol to oil, amount of catalyst and flow rate have significant effects on the transesterification and conversion of alkyl ester. Based on the response surface methodology using the selected operating conditions, the optimal ratio of methanol to oil, amount of catalyst and flow rate of transesterification process were 10.74, 1.26 wt% and 1.62 mL/min, respectively. The largest predicted and experimental conversions of alkyl esters (biodiesel) under the optimal conditions are 99.63% and 99.36%, respectively. Our findings confirmed the successful development of a two-step process for the transesterification reaction of Jatropha oil by microwave-assisted heating, which is effective and time-saving for alkyl ester production.

Highlights

► We designed a microwave-assisted process for transesterification of Jatropha oil. ► The methanol/oil ratio, amount of catalyst and flow rate have significant effects. ► Optimal operating parameters in this transesterification process were obtained.

Introduction

The depletion of energy sources and environmental issues are the two critical problems faced with the increase in consumption of fossil fuels. The desire to overcome these problems leads to a widespread interest to develop biofuels from renewable and eco-friendly energy source. Biodiesel, known as fatty acid mono-alkyl esters or alkyl esters (FAAE), is a technically feasible, environmentally acceptable and good alternative energy to fossil fuels due to its renewability, biodegradability and non-toxicity. Its use can minimize the emissions of carbon dioxide, sulfur oxides and hydrocarbon pollutants compared to those of fossil fuel (Chen et al., 2007, Prafulla and Patil, 2009). Thus, there are many experts who strongly advocate that the use of biofuel replace those of fossil fuel in the future.

Different fats or oils from animal or plant sources may be transesterified with acidic and basic catalysts to generate biodiesel, or may also be directly used in the diesel engine (Hameed et al., 2009, Yong et al., 2006). The current challenge with the use of plant oil as a diesel is to reduce its viscosity in order for it to be used directly in diesel engines. Common methods used to reduce viscosity of plant oil include dilution, pyrolysis (thermal decomposition), microemulsion (fine emulsification) and transesterification (Srivathsan et al., 2008). Nowadays, the transesterification technique is the most common and considered to be the best way to convert plant oils into transportation fuels. The property of transesterified oil (biodiesel) is similar to fossil fuels, which allows its direct use in diesel engines without requiring any change in the engine design or can be used as an additive to diesel fuels (Dennis et al., 2010).

Edible vegetable oils such as rapeseed oil, soybean oil, sunflower oil, corn oil and other non-edible vegetable oils have been utilized as raw materials for the production of biodiesel and were found to be good substitutes to diesel fuel. However, these oils are consumed as food giving an advantage to non-edible oils as better source of fuel alternatives. The most popular non-edible oil is Jatropha curcas oil. It has a free fatty acid (FFA) level of 8.17% that is several times higher than the acceptable amount (1%) that can be transesterified using a basic catalyst (Alok et al., 2007). Therefore, an optimized condition to convert J. curcas oil to biodiesel is highly recommended in order to get an acceptable biodiesel conversion (Ting et al., 2008).

Transesterification can be performed either in a batch or continuous mode and can be catalytic or noncatalytic using different heating systems (Nezihe and Aysegul, 2008, Clayton et al., 2008, Park et al., 2008, Ayhan, 2006). In general, the alkali-based transesterification requires that the raw materials must contain a moisture content of less than 0.06%, while the free fatty acid of oil must be less than 1% (Ghadge and Raheman, 2005). On the contrary, the J. curcas oil used in this study has a high moisture content of 9% as well as a high free fatty acid (acid value 16.33 mg KOH/g).

In the study of Hanny, a two-step catalytic transesterification process was performed (Hanny and Shizuko, 2008). The first step was acid esterification, which is mainly a pretreatment process to reduce the FFA in oil. The second step was alkali-based catalyzed transesterification to produce the alkyl esters. The two-step method can produce high biodiesel conversion but is obviously time consuming. The present work applies response surface methodology to a continuous microwave process in order to study the effect of the three selected factors, namely: methanol to oil molar ratio, amount of catalyst and flow rate. This work shows that microwave-assisted continuous reactor system can solve the above-mentioned problems.

According to literature, microwave irradiation can be used for the production of alternative energy source (Yuan et al., 2008). It activates the smallest degree of variance of polar molecules and ions, such as alcohol, with the continuously changing magnetic field. The changing electric field, which interacts with the molecular dipoles and charged ion, causes these molecules or ions to have a rapid rotation, generating heat due to molecular friction. Hence, microwave irradiation accelerates the chemical reaction and high product conversion is achieved within a short time, with a reduced quantity of by-products.

In the present study, a continuous microwave irradiation reactor was developed to convert J. curcas oil to alkyl ester by NaOH. This reduces the reaction time which is usually prolonged by the effects of saponification. The microwave-assisted continuous transesterification of J. curcas oil was compared with traditional reactors. By applying RSM (Wang et al., 2006), the interaction between the reaction variables (methanol/oil molar ratio, amount of catalyst, flow rate (mL/min)) and response (conversion %) was explored, and the optimal condition for biodiesel synthesis was obtained.

Section snippets

Materials and methods

Crude J. curcas oil was obtained from Dawn Exports, India. Methanol was in ACS grade. The sodium hydroxide pellets were used as a base catalyst for transesterification reaction sulfuric acid was used for the pretreatment step. A household 800 W and 2450 Hz, microwave (SAMPO, Taiwan) was modified as a biodiesel reactor for continuous transesterification of J. curcas oil (as shown in Fig. 1).

Experimental procedure and analysis

The crude unrefined J. curcas oil used was dark greenish yellow in color. The free fatty acid content of the

Results and discussion

In order to solve the problems of saponification phenomenon, J. curcas oil must be pretreated by acid esterification prior to the actual base transesterification process to get the biodiesel (or alkyl ester). In the acid pretreatment process, different catalyst concentrations (1–4 vol.%, sulfuric acid/J. curcas oil) and methanol/J. curcas oil ratios (16–40 vol.%) were used to investigate their influence on the reduction of acid value by microwave system at 80 W with a flow rate of 5 mL/min.

At a

Conclusions

In this experiment, J. curcas oil was used as the raw material for biodiesel (or alkyl ester) production. A two-step process including an acid-catalyzed esterification step and a base-transesterification step was employed, which was operated in a continuous mode microwave assisted heating. Response surface methodology was used to design the experiment and obtain the maximum possible conversion of biodiesel in the microwave-assisted reaction. At the first step, the acid value of J. curcas oil

Acknowledgements

The authors wish to express their sincere gratitude to the Center-of-Excellence (COE) Program on Membrane Technology from the Ministry of Education (MOE), ROC and the project Toward Sustainable Green Technology in the Chung Yuan Christian University, ROC, under grant CYCU-98-CR-CE.

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