Читать книгу Biodiesel Technology and Applications - Группа авторов - Страница 26

Glycerol is another product obtained along with TAG in transesterification reaction for biodiesel production when alcohols are used as an acyl acceptor. Like methanol inhibition effect, glycerol also hinders enzymatic transesterification. Production of glycerol cause reversal of reaction equilibrium and thus opposes biodiesel production. Glycerol insolubility in the reaction system makes it to accumulate in the system and increases the viscosity of the reaction mixture. Not only it just increases viscosity but because of its hydrophilic nature, it surrounds the lipase and hence prevents substrate to interact with enzyme and binding at enzyme catalytic site. This effect is more prominent when enzyme is in immobilized state. These things all together impacts negatively on the transesterification reaction [93]. According to a research study, rapeseed oil was transesterified using ethanol as an acyl acceptor with different immobilized enzymes such as Novozym 435, Lipozyme TL HC (immobilized on polymeric resin) and Lipozyme TL IM (immobilized on silica). Among all reactions, glycerol became more accumulated and hindered reaction when silica was used for immobilization purpose because it had large number of micropores that helped in accumulation [94].

A lot of methods have been devised to overcome and tolerate glycerol inhibition problem such as continuous removal of glycerol, use of solvents, and use of acyl acceptors other than alcohols. Biodiesel production in PBR is effective for continuous production and also to tolerate glycerol inhibition because it allows continuous removal of glycerol from it [95]. Bélafi-Bakó [96] showed that 97% conversion yield was obtained by methanolysis due to continuous removal of glycerol. Use of solvents is another strategy to resolve glycerol inhibition problem. Solvents, for example, tert-butanol and ionic liquids dissolve glycerol and thus reduce the glycerol inhibition problem. Moreover, lipases also perform better in the presence of solvents [97]. Azócar et al. [4] inferred that inhibition effect was eliminated when tert-butanol was used as solvent to convert soybean oil into biodiesel production in a continuous way because it is an excellent solvent for methanol and glycerol to dissolve in it and, thus, reduces inhibitory effects of both methanol and glycerol. These methods have shown promising work but these are not as good for industrial scale production.

There is another novel method in which instead of alcohols other compounds like methyl acetate, ethyl acetate, and DMC are in use. Use of methyl acetate or ethyl acetate does not produce glycerol as a product along with the main product instead it leads to produce triacetylglycerol that does not inhibit any enzymatic or reaction activity and further downstream processes are not halted [98]. According to Zhang et al. [99], transesterification of palm oil was done using Novozyme 435 as catalyst and DMC as acyl acceptor with reaction conditions were 10:1 DMC to oil ratio, 55°C reaction temperature and 20% lipase in a solvent-free system. In addition, 90.5% conversion yield, i.e., FAME was obtained, and after eight reaction cycles, no reduction in enzyme activity and loss of yield was observed. It was just because glycerol was not produced instead glycerol dicarbonate was formed because of DMC as acyl acceptor for the reaction. After discussing all negativity about glycerol, it seems glycerol does not have any positive effect but it’s not true. Glycerol if purified absolutely from the transesterification, as mostly huge biodiesel producing companies do, has vast number of uses in diverse industrial fields. Moreover, 99.7% purified glycerol can be used as a raw material for various types of fields such as paints, toiletries, animal feed, emulsifiers, pharmaceuticals, textiles, drugs, tobacco, cosmetics, toothpaste, leather, plasticizers, paper, food, and for different chemicals production [100, 101].