Preparation of Biodiesel by Enzyme Method

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Biodiesel is a kind of fatty acid methyl ester produced by the transesterification reaction of low carbon alcohols of animal and vegetable oils under the action of the catalyst. As a new environmentally friendly renewable alternative fuel, biodiesel has become the research object of scholars in recent decades. Compared with fossil fuels, biodiesel has the advantages of low flash point, low sulfur content and zero net greenhouse gas emissions, etc. It can be used cleanly and safely instead of ordinary diesel and has great development prospects.

The synthesis of biodiesel requires the participation of catalysts, which are divided into acid, alkali and enzyme catalysts. The acid catalysis method has higher requirements on raw material, would produce a large quantity of waste acid, and the catalyst is difficult to be reused. Alkali catalysis method requires the acid value of raw materials to be less than 1, the water content less than 0.5%. The production process is complex and it’s easy to saponify. However, the enzyme catalysis method has the advantages of mild reaction conditions, basically no requirements for the quality of raw oil, easy separation of reaction products, easy collection and no pollutant discharge, etc., so it is widely used. Lipase is usually the appropriate enzyme catalyst for the transesterification of fats and alcohol.

Lipase is a kind of biocatalyst with great potential and wide application range. It’s a general term for the group of enzymes that hydrolyze fats, widely used in food, chemical, medical and health, energy development and other fields. In the field of energy development, biodiesel produced by lipase catalysis is a kind of renewable clean energy, which has been paid more and more attention because of its good environmental effect.

Lipase is a kind of biocatalyst that can catalyze a specific reaction. Compared with other catalysts, the enzyme-catalyzed reaction has the characteristics of tenderness, high efficiency, and specificity. Lipase can catalyze the production of biodiesel under mild conditions, and lipase is easy to biodegrade, and will not cause pollution to the environment, with incomparable advantages of chemical catalysis. The production of biodiesel by using enzyme technology is a pollution-free “green” technology, which has the advantages that other methods do not have. It’s the development direction of industrial production. At present, there are many kinds of commercial lipase. Different lipase sources often lead to different reaction processes.

The production of biodiesel by enzymatic catalysis is the transesterification reaction between oil and short-chain alcohol with the catalysis of lipase. The enzyme catalysis method has low requirements on raw materials, free fatty acid can be directly esterified by lipase, and the by-product glycerol separation is simple, which reduces production process requirements and production costs, and improves product quality.

The factors that directly affect the conversion rate of enzymatic biodiesel preparation include oil-alcohol ratio, enzyme type, enzyme dosage, reaction temperature, and water content. The reaction time does not directly affect the conversion rate of biodiesel prepared by the enzymatic method, but directly affects the reaction balance degree and reactant saturation, etc. With the extension of reaction time, the conversion rate of biodiesel tends to a certain value. The enzyme activity was directly affected by the methanol content in the reaction system. When the substrate is a mixture, the methanol content in the reaction system can be appropriately increased. To avoid enzyme inactivation, methanol can be added in several batches. Sheng mei et al. found that the activity and stability of immobilized lipase could be significantly improved by the use of an organic solvent when studying the immobilized enzyme catalyzed the reaction of rapeseed oil and methanol to biodiesel. At the same time, the batch addition of methanol can avoid the inhibition of excessive methanol on the activity of immobilized lipase. Lillian et al. used tert-but me as the reaction medium to prepare biodiesel through methanol glycolysis by using high-quality materials catalyzed by an immobilized enzyme, eliminating the negative effects of methanol and glycerol on the enzyme and prolonging the service life of the enzyme. In the study of immobilized enzyme Novozym435 catalyzing transesterification of high acid waste oil with methyl acetate to prepare biodiesel, Chen zhi-fei et al. found that the main reason leading to the significant decrease of enzymatic exchange reaction rate and methyl ester yield was that acetic acid, the by-product produced by the reaction of high content-free fatty acid with methyl acetate in waste oil, inhibited the enzyme. In addition, it was proved that adding the proper amount of organic alkali in the reaction system could not only greatly improve the transesterification reaction rate and methyl ester yield, but also significantly improve the operation stability of the immobilized enzyme.

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