conditions nearly complete conversion of oil to ester was obtained [138]. Other studies [139] reported the ethanolysis of sunflower oil with lipozyme in a medium totally composed of sunflower oil and ethanol. In this case the factors studied for the conversion of the oil to esters included substrate molar ratio, reaction temperature and time, and enzyme load. Ethyl ester yields, however, did not exceed 85% even under the optimized reaction conditions. These authors also reported that the ester yields could be improved by adding silica to the medium. The positive effect of silica on yield was attributed to the adsorption of the polar glycerol coproduct onto the silica, which reduced glycerol deactivation of the enzyme. The reuse of the enzyme was also investigated, but ester yields decreased significantly with enzyme recycle, even in the presence of added silica.
In other studies [140, 141], mixtures of soybean and rapeseed oils were treated with various immobilized lipase preparations in the presence of methanol. Lipase from Candida antarctica was found to be the most effective in methyl ester formation. To attain high levels of conversion of oil to methyl ester, three equivalents of methanol were needed because this level of methanol resulted in lipase deactivation. It was necessary to add methanol in three separate additions. Under these conditions, >97% conversion of oil to methyl ester was achieved. In another study [142], it was reported that the lipase of Rhizopus oryzae catalyzed the methanolysis of soybean oil in the presence of 4–30% water in the starting materials but was inactive in the absence of water. Methyl ester yields of >90% could be obtained with stepwise additions of methanol to the reaction mixture. Lately, the conversion of soy oil to BD in a continuous batch operation catalyzed by an immobilized lipase of Thermomyces lanuginosus was reported [143]. These instigators also used a stepwise addition of methanol to the reaction, and in this manner complete conversion of oil to ester was achieved. Replicates recycle of the lipase was made possible by removing the bound glycerol by washing with isopropanol. When crude soy oil was used as substrate, a much lower yield of methyl ester was obtained compared with that using refined oil [144]. The reduction in ester yields was directly related to the phospholipids content of the oil, which apparently deactivated the lipase. Maximum esterification activity could be attained by pre‐immersion of the lipase in the crude oil before methanolysis.
During the transesterification of tallow with secondary alcohols, the lipases from C. antarctica (trade name SP435) and Pseudomonas cepacia (PS30) offered the best oil conversions to esters [145]. Reactions, run without the addition of water, were sluggish for both lipases, and conversions of only 60–84% were obtained overnight (16 h). The accumulation of small amounts of water improved the yields. The converse effect was observed in the case of methanolysis, which was extremely sensitive to the presence of water. For the branched‐chain alcohols, isopropanol and 2‐butanol, better ester yields were obtained when the reactions were run without solvent [146]. Reduced yields when using the normal alcohols methanol and ethanol, in solvent‐free reactions were attributed to enzyme deactivation by these more polar alcohols. Similar effects were observed for both the methanolysis and iso‐propanolysis of soybean and rapeseed oils [147]. The enzymatic conversion of lard to methyl and ethyl esters was reported [148] using a three‐step addition of alcohol to the substrate in solvent‐free medium [149]. The conversion of Nigerian palm oil and the lauric oils, palm kernel and coconut, to simple alkyl esters for use as BD fuels was also reported [150]. The best ester yields (>95%) were of ethyl esters.
Low‐cost lipids, such as waste deep fat fryer grease, usually have relatively high levels of FFA (>8%). The lipases are of particular interest as catalysts to produce fatty esters from such feedstocks because they accept both free and glyceride‐linked FAs as substrates for ester synthesis. On the other hand, BD production from such mixed feedstocks (e.g. spent rapeseed oil) using inorganic catalysts requires multistep processing [141]. To develop these attractive features of lipase catalysis, studies were conducted using a lipase from P. cepacia and recycled restaurant grease with 95% ethanol in batch reactions [151]. Subsequent work showed that methyl and ethyl esters of lard could be obtained by lipase‐catalyzed alcoholysis [152]. The restaurant greases using a series of immobilized lipases from T. lanuginosus, C. antarctica, and P. cepacia in solvent‐free medium utilizing a one‐step addition of alcohol to the reaction system for methanolysis and ethanolysis were reported [153]. The continuous production of ethyl esters of grease using a phyllosilicate sol–gel immobilized lipase from Burkholderia cepacia (IM BS‐30) as catalyst was investigated [154]. Enzymatic transesterification was carried out in a recirculating packed column reactor using 1M BS‐30 as the stationary phase and ethanol and restaurant grease as the substrates, without solvent addition. The bioreactor was operated at temperatures (40–60 °C), flow rates (5–50 ml min−1), and times (8–48 h) to optimize ester production. Under optimum operating conditions (flow rate, 30 ml min−1; temperature, 50 °C; mole ratio of substrates, 4 : 1, ethanol:grease; reaction time, 48 h), the ester yields were >96%.
Nasaruddin et al. devised a two‐step enzymatic protocol for the conversion of acid oils, a mixture of FFA and partial glycerides obtained after acid dilution of soap stock, to fatty esters. In the first step, the lipids in the acid oil were hydrolyzed using Caulerpa cylindracea lipase. In the second step, the high acid oils were esterified to short‐ and long‐chain esters using an immobilized Mucor miehei lipase [155].
Another important aspect of lipase‐catalyzed transesterifications is whether or not to use an organic lipophilic solvent. In general, alcoholysis with long‐chain or branched alcohols proceeds efficiently even in a solvent‐free medium, whereas solvent‐free methanolysis tends to give low ester yields. This may be attributed to the poor solubility of methanol in fats and oils [152]. Depending on the type of lipase used, various solvents for fatty material and methanol have been suggested, including petroleum ether [156], hexane [147], isooctane [142], commercial fossil diesel fuel [157], 1,4‐dioxan [158], and supercritical carbon dioxide [159]. From an economic viewpoint, however, the use of organic solvents is hardly useful [137], even more so as these have to be removed from the ester product by evaporation. Moreover, the toxicity and inflammability of organic solvents is also another issue to be considered. As a consequence, considerable effort has been directed toward conducting lipase‐catalyzed methanolysis reactions in a solvent‐free medium.
Zhang et al. reported regenerating enzyme preparations by using them with 2‐butanol or tert‐butanol [154], which proved successful for mobilized C. antarctica lipase. A recommendation for further treatment with 1‐propanol for immobilized Thermomyces iamgmosa lipase [160]. If the enzyme chosen for transesterification turns out to be particularly sensitive to glycerol released by ester formation, it might make sense to use methyl acetate instead of methanol [161]. The authors claim that triacetylglycerol, which is produced instead of glycerol in this process, has no negative effects on the enzyme activity of immobilized C. antarctica lipase and does not affect the quality of the resulting fuel either.
Finally, BD producers can choose between several methods of preventing enzyme inactivation, which is a phenomenon frequently reported for lipase‐catalyzed methanolysis. Enzymes are easily inactivated by compounds contained in the oil or fat. Quayson et al. found that phospholipids present in crude soybean oil efficiently inhibit methanolysis, as these bind to the immobilized enzyme and interfere in the interaction of lipase and substrate [162]. They concluded that for enzymatic methanolysis, vegetable oils have to be degummed. The enzyme‐catalyzed reactions have the following disadvantages: (i) lose some initial activity due to volume of the oil molecule; (ii) number of support enzyme is not uniform; (iii) biocatalyst is more expensive than the natural enzyme; (iv) inactivation by acyl acceptors, such as methanol, and inactivation by minor components in the crude oil and waste oils; and (v) desorption from immobilization support and fouling in packed bed bioreactors. Due to said disadvantages, the enzymatic catalyzed transesterification reactions are not in common practice for commercial scale BD production.
1.9 Fuel Properties and Quality Specifications for Biodiesel
While BD is produced in reasonably differently scaled plants from vegetable oils of varying origin and quality, it was essential
