Volume 7, Issue 1 (January 2010)
Lipase-Catalyzed Production of Biodiesel from Tallow
The use of enzymes (lipases) to convert triglycerides to acylesters (biodiesel) holds potential as an alternative to the conventional base catalyzed processes. Advantages include the ability to utilize impure feedstocks, less downstream processing, and potentially overcoming the requirement to dedicate expensive agricultural land for raw oil production. Lipid sources such as oleaginous microorganisms along with processing wastes may provide renewable substrates requiring minimal pre-processing. Transesterification of vegetable oils and restaurant grease has been studied using immobilized enzymes as catalysts with an optimal temperature of around 35°C. This study investigates ester formation from unrefined tallow at 50°C employing similar enzymes used for vegetable oils. Novozyme 435 (a commercially available immobilized enzyme) proved best at a loading of 2 % w/w based on tallow reaching 98 % conversion after 96 h using methanol as the chosen alcohol, with immobilized lipase from Pseudomonas cepacia (PS-30) (69 % conversion) proving superior to free PS-30 (11 % conversion), and the other lipases barely registering any activity. Immobilized PS-30 was chosen to investigate the effect of lipase level, alcohol reactant, water content of the tallow, grade of tallow, and influence of solvent or a solvent-free system. Results showed that water content of up to 0.6 % and the tallow grade had no effect on ester formation, while enzyme loadings of at least 4 % were needed to achieve a good conversion that plateaued at around 6–8 % water content. The primary alcohols ethanol, propanol, and butanol reacted equally efficiently with immobilized PS-30, reaching 96–99 % conversion, while methanol performed less well along with 2-propanol and 2-butanol where conversion was only in the range of 10–40 %. Transesterification reactions worked best in a non-polar environment with hexane as solvent (95 % conversion), with a higher polarity solvent, acetonitrile, also proving an efficient medium (80 % conversion), while solvents intermediate to these had little comparative activity, suggesting that solubility of reactants and products is a key driver in the conversion. Interestingly, petro-diesel proved to be a better solvent than using either a solvent-free system or a diethylether environment.