LITMIR.BIZ

Скачать книгу

comprising soybean oil, methanol, and sulfuric acid reduced ester conversion from 95% to below 90% [127]. At a water content of 5%, ester conversion decreased to only 5.6%. It should also be noted that water released during esterification of FFA might inhibit further reaction, so that very acidic raw materials might give moderate conversion even in acid‐catalyzed alcoholysis. Shu et al. investigated the effect of reaction variables such as feed composition, temperature, and rate of mixing on the kinetics of the acid‐catalyzed transesterification of waste frying oils. The optimal yield of 99% was achieved after a 4 h reaction [133].

      For acid‐catalyzed transesterification, the concentrated sulfuric acid is the most frequently used catalytic substance. Its advantages are its low price and its hygroscopicity, which is important for the esterification of FFAs, removing released water from the reaction mixture. Drawbacks include its corrosiveness, its tendency to attack double bonds in unsaturated FAs, and the fact that concentrated H₂SO₄ may cause dark coloring in the ester product [65]. Besides, also the use of various sulfonic acids as homogeneous catalysts is reported. These substances have lower catalytic activity than mineral acids. However, they pose fewer problems in handling and do not attack double bonds within the starting material.

1.8 Enzymatic‐Catalyzed Transesterification

Although at present BD is successfully produced chemically, there are several associated problems such as glycerol recovery and the need to use refined oils and fats as primary feedstocks [127]. The use of lipases from various microorganisms is becoming important in BD production. Lipases are enzymes that catalyze both the hydrolytic cleavage and the synthesis of ester bonds in glycerol esters. The disadvantages of using chemical catalysts can be overcome by using lipases as the catalysts for ester synthesis [134]. Advantages mentioned for lipase catalysis over chemical methods in the production of simple alkyl esters include the ability to esterify both acylglycerol linked and FFA in one step, the production of a glycerol side stream with minimal water content and with little or no inorganic material, and catalyst reuse. Other advantages include the occurrence of transesterification under mild temperature, pressure, and pH conditions; neither the ester product nor the glycerol phase has to be purified from basic catalyst residues or soaps. This means that phase separation is easier, high quality glycerol can be obtained as a by‐product, and environmental problems due to alkaline wastewater are eliminated [135]. Moreover, both the transesterification of triglycerides and the esterification of FFAs occur in one process step. Consequently, also highly acidic fatty materials, such as palm oil or waste oils, can be used without pretreatment [136]. Finally, many lipases show considerable activity in catalyzing transesterifications with long‐ or branched‐chain alcohols, which can hardly be converted to FA esters in the presence of conventional alkaline catalysts.

      Early work on the application of enzymes for BD synthesis was conducted using sunflower oil as the feedstock [137] and various lipases to perform alcoholysis reactions in petroleum ether. From the tested lipases, only three were found to catalyze alcoholysis with an immobilized lipase preparation of a Pseudomonas sp. offering the maximum ester yields. Maximum conversion (99%) was obtained with ethane, and when the reaction was repeated without solvent, only 3% product was produced with methanol as alcohol, whereas with absolute ethanol and 96% ethanol and 1‐butanol, the ester yields were ranged between 70 and 82%, respectively. Reactions by a progression of homologous alcohols showed that reaction rates, with or without the addition of water, increased with increasing chain length of the alcohol. For methanol, the highest conversion was obtained without the addition of water, but for other alcohols the addition of water increased the esterification rate two to five times.

      Pedro et al. reported the lipase‐catalyzed alcoholysis of low erucic acid rapeseed oil without organic solvent in a stirred batch reactor. The best results were obtained with a Candida rugosa lipase, and under optimal 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

Скачать книгу