Читать книгу Biodiesel Technology and Applications - Группа авторов - Страница 27

One of the major problems in enzymatic biodiesel production is enzyme inhibition by short chain alcohols such as methanol and ethanol that are used in the reaction. These alcohols’ insolubility in the reaction system denatures the enzyme and hence reduces yield of biodiesel. So, solvent application plays its role in this regard. Organic solvents are used to solubilize these excessive alcohols so that enzyme denaturation can be prevented. Hence, it stabilizes the enzyme. Solubility of oils and alcohols become increased due to presence of organic solvents, this provides the required environment for substrate to interact with enzyme at its active site. Organic solvents also reduce viscosity of the reaction mixture and enhances mass transfer toward the enzyme that leads to improved reaction rate [101].

Organic solvents also eliminate the need of stepwise addition of alcohol. All these things in combination increase production of biodiesel. Organic solvents that are commonly used include tert-butanol, petroleum ether, hexane, and n-heptane [102]. Some other organic solvents that are used are 2-butanol, cyclohexane, isooctane, acetone, 1,4-dioxane, and chloroform. While considering nature of organic solvents, hydrophobic organic solvents are majorly used. Hydrophobicity of the organic solvents helps in accumulating water molecules around enzyme which is important for enzyme structural stability [103]. Polar or hydrophilic solvents work opposite to hydrophobic organic solvents by playing role in distortion of enzymatic structure. But solvent with little polarity can be beneficial to dissolve oil and alcohol. For example, hydrophilic 1,4-dioxane and tert-butanol have produced some good results by producing high enzymatic transesterification yield [104]. Tert-butanol, having moderate polarity, eliminates glycerol and methanol inhibition problem for enzyme because it can dissolve both in itself. This makes the enzyme more stable and active and then ultimately produce better reaction yield [105]. Tertbutanol is the most common solvent that proved its effectiveness in various cases. According to Royon et al. [84], cottonseed oil was transesterified in the presence of Candida antartica lipase. Methanol was found to be the cause of enzyme inhibition in the reaction but when tert-butanol was used as solvent, reaction yield goes up to 97% with minimal enzyme inhibition. Similarly, in another research experiment, tert-butanol was tested for its effectiveness when rapeseed oil was used as substrate for biodiesel production. In solvent-free system, methyl ester yield was 10% but after utilizing tert-butanol yield was 75%. But under optimum conditions having Lipozyme TL IM and Novozyme 435 both in the reaction system, biodiesel yield reached 95% and the reaction was so stable that enzymes did not lose their activity even after 200 cycles. Reaction was favored and well supported by tert-butanol [86].

Use of solvents provide many benefits but they also come with some disadvantages such as organic solvents do not completely dissolve glycerol, by-product of the reaction, that causes the enzyme to lose its activity and become unstable. Use of solvents also make the process very costly because there is a need of extra purification step to separate out solvent and product from the reaction mixture. Organic solvents are mostly toxic and highly flammable so there are also environmental and health concerns while using them [11]. In order to tackle problems of conventional organic solvents, researchers have suggested some alternatives. Diesel oil was found to be an interesting alternative but the most recent, beneficial, and popular alternatives are super critical carbon dioxide (SC-CO₂) and ionic liquids (ILs). Researchers have also confirmed the positive effect of using SC-CO₂ and ILs in the enzymatic transesterification [106, 107].