Oil waste waters can be used for the production of biodiesel. Dmytryshyn et al. (2004) conducted the transesterification of four vegetable oils (canola oil greenseed canola oil heat-damaged seeds, processed waste fryer grease and unprocessed waste fryer grease) using methanol and KOH as catalyst. The methyl esters of the corresponding oils were separated from the crude glycerol, purified and characterized by various methods to evaluate their densities, viscosities, acid numbers, fatty acid and lipid compositions, lubricity properties and thermal properties. The fatty acid composition suggests that 80-85% of the ester was from unsaturated acids. A substantial decrease in density and viscosity of the methyl esters compared with their corresponding oils suggested that the oils were in mono- or di-glyceride form. The lubricity of the methyl esters, when blended 1 vol% treat rate with ISOPAR M reference fuel, showed that the canola ester enhanced the fuel's lubricity number. From the analyses performed, it was determined that the ester with the most potential for being an additive or substitute for diesel fuel is the canola methyl ester, whose physical and chemical characteristics are similar to diesel fuel.
Most of methods for biodiesel production use an alkaline catalyst in a batch-type processing, followed by additional effort to remove the catalyst and saponified products from free fatty acids. Recently, there has been a strong interest in developing a flow-type transesterification of vegetable oil in an effort to find a process more applicable for use on a commercial scale. Kudsiana and Saka (2001a) developed a continuous, simpler process in the absence of alkali catalyst for biodiesel fuel production. Kudsiana and Saka (2001a) conducted a series of experiments to study the optimum condition for transesterification of vegetable oil in supercritical methanol to biodiesel fuel. The effect of temperature and pressure was quantified in terms of reaction rate constants. Results indicated that the course of the reaction is both temperature and pressure dependent. Free fatty acids are effectively converted to their fatty acid methyl esters through a methyl esterification reaction in supercritical methanol.
The basic idea of supercritical treatment is a relationship between pressure and temperature, and the thermophysical properties of the solvent (methanol) - such as dielectric constant, viscosity, specific weight and polarity. Therefore, a set of experiments was carried out to study the effect of reaction temperature and pressure on methyl ester formation. It was revealed that supercritical treatment at 350 °C, 30 MPa and 240 s in molar ratio of 42 in methanol was the best condition for transesterification of rapeseed oil to biodiesel fuel. In addition, the methyl esters produced were similar to those produced by the common catalysed process. At a sub-critical state of methanol (<239 °C, <8.1 MPa), reaction rate was low and gradually increased as either pressure or temperature rose. Furthermore, at the transition state between sub-critical and supercritical, a relatively low rate constant is apparent. The reaction rate was increased by a factor of 85 at 350 °C and 30 MPa. In addition, a considerable change in the rate constant can be seen for the pressure above 20 MPa. From this result, it is further concluded that the rate constant of the reaction was corresponding linearly to both temperature and pressure.
In the common catalysed method, direct use of crude vegetable oil as a raw material for transesterification results in an incomplete reaction because the presence of free fatty acids leads to catalyst destruction. Therefore, it is suggested that vegetable oil is refined to have the free fatty acids content as low as possible, below 0.5%. Kudsiana and Saka (2001b) reported that free fatty acids could be converted to their fatty acid methyl esters through a methyl esterification reaction. Unsaturated fatty acids (oleic, linoleic and linolenic acids) are converted effectively at the lower temperature, while for saturated fatty acids (palmitic and stearic acids), a relatively higher reaction temperature is necessary to allow the methyl esterification reaction to take place. On average over 75% of free fatty acids were converted to their fatty acid methyl esters. The optimum condition for free fatty acid conversion is similar to that of transesterification, i.e. a temperature of 350 °C. These facts suggest that free fatty acids that become wastes as saponified products in the common catalysed method can be available as biodiesel fuel.
The authors expanded the experiment to other vegetable oils. The results showed that the optimum condition for the reaction is different for different vegetable oils. For high-saturated vegetable oils such as coconut and cottonseed oils, a relatively longer treatment is needed to achieve a high conversion, whereas for soybean and corn oils, the optimum condition is similar to that of rapeseed oil, because of their similarity in fatty acid composition. A highly complete conversion of methyl esters was obtained for all of those samples. These observations suggest that supercritical methanol has a high potential for both transesterification of tryglicerides and methyl esterifi-cation of free fatty acids to methyl esters for diesel fuel substitute production.
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