Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 163

The main factors affecting transesterification are the molar ratio of glycerides to alcohol, catalyst, reaction temperature and pressure, reaction time, and the contents of free fatty acids and water in oils.

The free fatty acids and moisture content are key parameters for determining the viability of the vegetable oil transesterification process. In the transesterification, free fatty acids and water always produce negative effects, since the presence of free fatty acids and water causes soap formation, consumes catalyst, and reduces catalyst effectiveness, all of which result in a low conversion. These free fatty acids react with the alkaline catalyst to produce soaps that inhibit the separation of the biodiesel, glycerin, and wash water. To carry the base catalyzed reaction to completion, a free fatty acid value lower than 3% is needed.

The presence of water has a greater negative effect on transesterification than that of the free fatty acids. In the transesterification of beef tallow catalyzed by sodium hydroxide (NaOH) in presence of free fatty acids and water, the water and free fatty acid contents must be maintained at specified levels.

The effect of reaction temperature on production of propyl oleate was examined at the temperature range from 40°C to 70°C with free P. fluorescens lipase (Iso et al., 2001). The conversion ratio to propyl oleate was observed highest at 60°C (140°F), whereas the activity highly decreased at 70°C (158°F).

The conversion rate increases with reaction time. The transesterification of rice bran oil with methanol was studied at molar ratios of 4:1, 5:1, and 6:1. At molar ratios of 4:1 and 5:1, there was significant increase in yield when the reaction time was increased from 4 to 6 h. Among the three molar ratios studied, ratio 6:1 gave the best results.

One of the most important factors that affect the yield of ester is the molar ratio of alcohol to triglyceride. Although the stoichiometric molar ratio of methanol to triglyceride for transesterification is 3:1, higher molar ratios are used to enhance the solubility and to increase the contact between the triglyceride and alcohol molecules. In addition, investigation of the effect of molar ratio on the transesterification of sunflower oil with methanol showed that when the molar ratio varied from 6:1 to 1:1 and concluded that 98% conversion to ester was obtained at a molar ratio of 6.1. Another important variable affecting the yield of methyl ester is the type of alcohol to triglyceride. In general, short chain alcohols such as methanol, ethanol, propanol, and butanol can be used in the transesterification reaction to obtain high methyl ester yields.

Catalysts used for the transesterification of triglycerides are classified as alkali, acid, and enzyme. Alkali-catalyzed transesterification is much faster than acid-catalyzed transesterification and is most often used commercially and, quite often, for the base-catalyzed transesterification, the best yields were obtained when the catalyst was used in small concentration, i.e., 0.5% wt/wt of oil. On the other hand, during the production of free and bound ethyl ester (FAEE) from castor oil, hydrochloric acid is much more effective than sodium hydroxide