As explained earlier, the cost of lipase production is the main hurdle to the commercialization of the enzymatic process. Therefore, the reuse of lipase is essential from the economic point of view, which can be achieved by using the lipase in immobilized form. The operational stability of the catalyst in a continuous process plays a vital role. Further details about stability and possible ways of enhancing it is found in Sections 6.6 and 6.8. Shimada et al. (2002) achieved 93% conversion of SO in absence of organic solvents in a series of three continuous packed-bed bioreactors at a rate of 6.0 ml h-1. The productivity relative to the total mass of enzyme used was however lower than when t-butanol was added to the continuous reactor (Royon et al., 2007). The necessity of solvent recovery can be a drawback to such a process. However, the relatively low optimum t-butanol concentration, and low boiling point, allows easy separation, and hence the energy expense required for its recovery is usually acceptable. Chen and Wu (2003) achieved 70% conversion in continuous packed-bed bioreactor in the absence of organic solvent, but with periodical regeneration of the immobilized lipase with t-butanol washing. Nie et al. (2006) used lipase immobilized on cheap cotton fibers in a series of three packed-bed bioreactors with stepwise addition of methanol to produce biodiesel from SO and WO and achieved 93% and 92% conversions, respectively. A hydrocyclone was used on-line to separate glycerol. The operational stability of the immobilized lipase was more than 20 days at input flow rate of 15 L h-1 of substrate and ether solvent in a volume ratio of 2:3.

On the other hand, the use of membrane bioreactors for the enzymatic processing of fats and oils is increasingly becoming more attractive to substitute conventional stirred tanks or packed-bed reactors (Basheer et al., 1994). As the reaction proceeds, glycerol is generated and physically mixes with the alcohol to form a second liquid phase that is not completely miscible with the oil. This second polar organic phase serves to extract alcohol from the oil phase, thereby decreasing the concentration of this substrate in the reaction medium and causing a concomitant decrease in the conversion achieved in a fixed amount of time. In addition, glycerol is adsorbed on the surface of the immobilized lipase, and blocks the substrate from reaching the active sites. Consequently, conversions will be enhanced if glycerol is removed from the substrate mixture as the reaction proceeds. To achieve this, membrane reactors with immobilized lipase are proposed, which may take either a flat sheet (Isono et al., 1998) or hollow fiber form (Hilal et al., 2004; Shamel et al., 2005). Membrane reactors enhance efficiencies by combining in one unit a reactor that generates a biodiesel and a separator that separates it from the other products. Removal of a product drives equilibrium-limited reactions towards completion and prevents product inhibition.

152 Handbook of biofuels production