Vegetable oils, transesterification

Biodiesel is a fuel derived from renewable natural resources such as soybean and rapeseed and consists of alkyl esters derived from transesterification of triglycerides with methanol. In spite of all the advantages of biodiesel, such as low emissiotts, biodegradability, non-toxicity, and lubricity, the major hurdle in penetration of biodiesel is its high cost because of the expensive food grade refined vegetable oil feedstock. 

Biomass is a renewable resource from which various useful chemicals and fuels can be produced. Glycerol, obtained as a co-product of the transesterification of vegetable oils to produce biodiesel, is a potential building block to be processed in biorefineries (1,2). Attention has been recently paid to the conversion of glycerol to chemicals, such as propanediols (3, 4), acrolein (5, 6), or glyceric acid (7, 8). 

Biodiesel (fatty acid methyl ester (FAME)) production is based on transesterification of vegetable oils and fats through the addition of methanol (or other alcohols) and a catalyst, giving glycerol as a by-product (which can be used for cosmetics, medicines and food). Oil-seed crops include rapeseeds, sunflower seeds, soy beans and palm oil seeds, from which the oil is extracted chemically or mechanically. Biodiesel can be used in 5%-20% blends with conventional diesel, or even in pure form, which requires slight modifications in the vehicle. 

Conversion of vegetable oil into diesel via transesterification of hydrogenolysis [75]. 

Fatty Acid Esters and Fatty Alcohols Fatty acid esters are obtained by transesterification of triglycerides (vegetable oils) or by esterification of fatty acid with alcohol or polyols. Fatty alcohols are obtained by hydrogenation of esters on metal catalysts. Fatty acid esters and fatty alcohols are useful platform molecules to prepare surfactants, emulsifier, lubricants and polymers. 

Since FAS can be produced either from vegetable oil based or petrochemical-based fatty alcohol (Fig. 4.9), both types have been evaluated in a life-cycle analysis with a positive overall result for the natural based product. With vegetable-based fatty alcohol sulfate, the analysis starts with the harvesting of the oil fruits (palm kernels or coconuts) and their processing to isolate the desired plant oil. Subsequent transesterification and hydrogenation of the methyl ester intermediates lead to the fatty alcohols, which are finally sulfated to produce the desired product. Based on this analysis the environmental impact of vegetable oil based fatty alcohol sulfate compared with the petrochemical based product is as follows ... 

Biodiesel is a mixture of methyl esters of fatty acids and is produced from vegetable oils by transesterification with methanol (Fig. 10.1). For every three moles of methyl esters one mole of glycerol is produced as a by-product, which is roughly 10 wt.% of the total product. Transesterification is usually catalyzed with base catalysts but there are also processes with acid catalysts. The base catalysts are the hydroxides and alkoxides of alkaline and alkaline earth metals. The acid catalysts are hydrochloride, sulfuric or sulfonic acid. Some metal-based catalysts can also be exploited, such as titanium alcoholates or oxides of tin, magnesium and zinc. All these catalyst acts as homogeneous catalysts and need to be removed from the product [16, 17]. The advantages of biodiesel as fuel are transportability, heat content (80% of diesel fuel), ready availability and renewability. The... 

Fig. 10.1 Transesterification of vegetable oils to produce biodiesel (methyl esters).

Vegetable oils have the potential to substitute a fraction of petroleum distillates and petroleum-based petrochemicals in the near future. Possible acceptable converting processes of vegetable oils into reusable products are transesterification, solvent extraction, cracking and pyrolysis. Pyrolysis has received a significant amount of interest as this gives products of better quality compared to any other thermochemical process. The liquid fuel produced from vegetable oil pyrolysis has similar chemical components to conventional petroleum diesel fuel. 

Demirbas, A. 2003. Biodiesel fuels from vegetable oils via catalytic and non-catalytic supercritical alcohol transesterifications and other methods a survey. Energy Convers Manage 44 2093-109. 

At low temperatures, the activity of acid catalysts in transesterification is normally fairly low and to obtain a sufficient reaction rate it is necessary to increase the reaction temperature to >170 °C. Therefore, sulfonic acid resins can be used in esterification reactions where they perform well at temperatures <120 °C and particularly in the pretreatment of acidic oils. Under these reaction conditions, acidic resins are stable. Poly(styrenesulfonic add), for example, has been used in the esterification of a by-product of a vegetable oil refinery with a 38.1 wt% acidity at 90-120 ° C and 3-6 atm. It was not deactivated after the first batch and maintained a steady catalytic performance in the next seven batches [22]. 

This catalyst showed good activity in the esterification-transesterification of a number of unrefined vegetable oils (Table 10.4), used oils and non-edible oils at 150 °C with an oil-to-methanol ratio of 1 15. The catalyst performance is not influenced by the water content and it can be reused after separation by centrifugation without any further purification [38, 39]. 

A byproduct of vegetable oil transesterification to make biodiesel fuel, glycerol (GO) has captured our attention for several reasons, in aqueous medium, GO itself can be converted to value-added commodity products such as propylene glycol (PG), lactic acid (LA) and ethylene glycol (EG) in the presence of a metal catalyst at mild conditions (2-7). An array of metals deposited on various supports have been examined as catalysts for the above reaction (6, 7). 

The reader should also be aware that some information provided in this section and in the sections below does not directly address the subject of biodiesel synthesis. Some discussions, for instance, are about the transesterification of simple esters or the production of monoglycerides by transesterification of vegetable oils nevertheless, the information provided is relevant to the topic of biodiesel synthesis since knowledge of catalyst reactivity in these systems is directly applicable to reactions involving TGs and FFAs. 

Biodiesel is made through a ohemioal prooess called transesterification, whereby the glyoerin is separated from the fat or vegetable oil. The process leaves behind two products methyl esters (the chemical name for biodiesel) and glyoerin (a valuable byproduct usually used in soaps and other products). 

We conclude that a commercial immobilized lipase from C. antarctica (Novozym 435) was stable in SCC02 for all experimental conditions investigated. Based on the results obtained here and comparison of them with the results obtained by other investigators, it can be concluded that the magnitude of pressure, temperature, decompression rate, and exposure time needed to inactivate the enzyme strongly depends on the nature and the source of enzyme and, primarily, whether the enzyme is in its native or immobilized form. For the purpose of using this enzyme to catalyze the transesterification reaction of vegetable oils in order to produce esters, the results obtained herein are relevant, because the immobilized lipase can be used with low activity loss at typical conditions of temperature and pressure employed in many biotransformations of raw materials. 

Reaction temperature and time were significant operating parameters, which are closely related to the energy costs, of the biodiesel production process. Figure 7 shows the effect of reaction time on the transesterification of rapeseed oil at a catalyst concentration of 1%, molar ratio of 1 6, and 60°C. Within 5 min, the reaction was rapid. Rapeseed oil was converted to above 85% within 5 min and reached equilibrium state after about 10 min. Several researchers reported that the conversion of vegetable oils to FAME was achieved above 80% within 5 min with a sufficient molar ratio (8,11). For a reaction time of 60 min, linoleic acid methyl ester was produced at a low conversion rate, whereas oleic and linolenic methyl ester were rapidly produced. 

Methyl esterification of fatty acids with methanol is generally conducted in the presence of acid catalyst at an elevated temperature close to the boiling point of methanol. Recently, we found that fatty acids could be successfully methyl esterified in supercritical methanol without the use of a catalyst (10). In addition, from a comparative study between transesterification of vegetable oil and alkyl esterification of fatty acids with supercritical alcohols at 300°C in a batch-type reaction system, the reaction rates of alkyl esterification were found to be faster than those of transesterification (11). An additional finding was that alkyl esterification of fatty acids could be performed at a lower reaction temperature than transesterification. 

Gelbard, G. and Vielfaure-Joly, F. Polynitrogen strong bases as immobibzed catalysts for the transesterification of vegetable oils, Compt. Rend. Acad. Sci., Ser. lie, 2000, 3, 563-567. 

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