S. AL-ZUHAIR, UAE University, UAE

Abstract: This chapter discusses the enzymatic production of biodiesel using lipase enzyme as a biocatalyst. It starts by highlighting the advantages and limitations of the enzymatic approach and includes a review on the effects of the source of lipase, type and quality of feedstock, type of acyl acceptor and temperature. The chapter then discusses importance of using the lipase in immobilized form and different immobilization techniques. A kinetic model that is developed from the mechanismic steps of enzymatic transesterification of triglyceride is also presented. The chapter concludes with an exploration of the future advances in enzymatic biodiesel production.

Key words: enzymatic biodiesel production, waste oil feedstock, immobilized lipase, kinetic model.

6.1 Introduction

With the inevitable depletion of the non-renewable resources of fossil fuels, and due to its favorable environmental features, biodiesel promises to be the favorable fuel of tomorrow. Biodiesel is formed from transesterification of vegetable oils or animal fats with methanol (or ethanol) in the presence of a catalyst, as shown in Fig. 6.1. It is a renewable energy source that is non-toxic and biodegradable. Compared to petroleum-based diesel, biodiesel has lower emission levels of carbon monoxide, particulate matter and unburned hydrocarbons (Yusaf et al., 2005). In addition, using biodiesel on large scale will promote plantations of crops used to produce its feedstock, which results in more carbon dioxide recycling, minimizing its impact on the greenhouse effect (Korbitz, 1999; Agarwal and Das, 2001). Furthermore, biodiesel has a relatively high flash point (150°C) that makes it less volatile and safer to transport or handle than petroleum diesel (Krawcsyk, 1996). It provides lubricating properties, which reduce engine wear and extend engine life (Von Wedel, 1999). At the same time, biodiesel has physical properties and energetic content close to those of petroleum diesel, which allows its efficient function in conventional diesel engines without any modification.

The transesterification of triglycerides, being from vegetable oil or animal fat, is conventionally catalyzed chemically by alkaline or acid catalysts. The basic catalysts employed are sodium or potassium hydroxide because they are relatively inexpensive (Freedman et al., 1984; Akoh and Swanson, 1988). Usually, a stoichiometric excess of methanol, in a molar ratio of 6:1 (methanol:vegetable oil), is preferred to increase methyl ester yield, and the reaction can be completed in a few hours at 40-65°C. The alkali-catalyzed processes, however, are sensitive

to moisture and free fatty acids (FFA) content in feedstock. Saponification reaction of the FFA consumes the alkali catalyst and at the same time generates soaps that cause the formation of emulsions, which increase the viscosity and create difficulties in downstream recovery and purification of the biodiesel. Therefore, pre-treatment of the oil is required for commercially viable alkali- catalyzed systems. This requirement is likely to be a significant limitation to the use of low-cost feedstock, and the cost of the highly refined feedstock can account to up to 70-80% of the final cost of the biodiesel (Fukuda et al., 2001). On the other hand, the acid-catalyzed processes are insensitive towards FFA contents. However, they are rarely used because they result in much slower reactions and produce by-products, from alcohol etherification, that also results in difficulties in downstream recovery and purification. In addition, careful removal of catalyst from the biodiesel fuel is essential, since acid-catalyst residues can damage engine parts (Fukuda et al., 2001). Furthermore, acid-catalyzed reactions require higher temperatures of around 55-80°C and higher substrate molar ratios of alcohol of around 30:1 to yield approximately 99% biodiesel in 50 h (Marchetti et al., 2007). The preferred acid catalysts are sulfuric, hydrochloric and sulfonic acids (Freedman et al., 1984).

Biodiesel can also be produced in the absence of any catalyst, using supercritical methanol (Demirbas, 2002). This simple process results in high yield due to the simultaneous transesterification of triacylglycerols and esterification of fatty acids. However, this is an energy intensive process that requires operating at temperatures and pressures above the critical points for methanol, which are 512 K and 8.1 MPa, respectively. Furthermore, operating at these harsh conditions destroys the antioxidant inherently found in the feedstock, which results in reducing the oxidative stability of biodiesel.

Recently, a less energy intensive and environmental friendly procedure has been proposed by using enzymes to catalyze the transesterification of triglycerides. Enzymatic transesterification can overcome the problems facing conventional chemical methods without compromising their advantages. Biodiesel has been successfully produced in lab scale by lipase-catalyzed reactions. Conversions as high as 90% have been reached within short reaction times, provided that the reaction takes place under the appropriate conditions. Nevertheless, there are many obstacles hindering the effective use of enzymes for commercial production

of biodiesel in large scales. The most important challenges and the proposed ways to overcome them are presented in this chapter. In the following Section 6.2, a general introduction to the enzymatic approach is provided. The advantages of the enzymatic catalyzed process over conventional chemically catalyzed ones are also explained in this section. On the other hand, the limitations of enzymatic approach are presented in Section 6.3. In Section 6.4, the effectiveness of lipase, the enzyme to be used in biodiesel production, from different sources is discussed. After that the capacity of lipase to produce biodiesel from various feedstock, with special emphasis on feedstock that does not compete with food stock, is assessed in Section 6.5. This is followed by Section 6.6 that describes the effects of the type and amount of the acyl-acceptor on the enzymatic biodiesel production process and possible ways to overcome the inhibition by short-chain alcohols. In Section 6.7, the thermo-stability and optimum temperatures of lipases from different sources are presented. Section 6.8 discusses the use of immobilized lipase in biodiesel production. This is crucial since the cost of lipase remains the main obstacle facing full exploitation of its potential, the reuse of lipase is essential from the economic point of view, which can be achieved by using the lipase in immobilized form. In Section 6.9, the development of a kinetic model to describe the system, taking into consideration the inhibition effects by both substrates is presented. This chapter concludes with an explanation of the future advances in enzymatic biodiesel production and sources for further information in Sections

6.10 and 6.11, respectively.