Encyclopedia of Renewable Energy. James G. Speight
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be converted into biodiesels but to a lot of soap. These free fatty acids react with the alkaline catalyst to produce soaps that inhibit the separation of the biodiesel, glycerin, and wash water. Triglycerides are readily transesterified in a batch operation in the presence of alkaline catalyst at atmospheric pressure and at a temperature of approximately 60 to 70°C (140 to 160°F) with an excess of methanol. It often takes at least several hours to ensure the alkali (NaOH or KOH) catalytic transesterification reaction is complete. Moreover, removal of these catalysts is technically difficult and brings extra cost to the final product. Nevertheless, these methods are a good alternative since they can give the same high conversions of vegetable oils just by increasing the catalyst concentration to 1 or 2 mol%. Alkaline metal alkoxides (as CH3ONa for the methanolysis) are the most active catalysts since they give high yields (> 98%) in short reaction times (30 min) even if they are applied at low molar concentrations (0.5 mol%).
The transesterification process is catalyzed by sulfuric, hydrochloric, and organic sulfonic acids. In general, acid-catalyzed reactions are performed at high alcohol-to-oil molar ratios, low-to-moderate temperatures and pressures, and high acid catalyst concentrations. These catalysts give high yields in alkyl esters, but these reactions are slow, requiring typically temperature above 100°C (212°F) and more than 3 h to complete the conversion. Studies of the acid-catalyzed system have been limited in number. Despite the relatively slow reaction rate, the acid-catalyzed process offers benefits with respect to its independence from free fatty acid content and the consequent absence of a pretreatment step. These advantages favor the use of the acid-catalyzed process when using waste cooking oil as the raw material.
Enzyme-catalyzed reactions (such as lipase-catalyzed reactions) have advantages over traditional chemical-catalyzed reactions: the generation of no by-products, easy product recovery, mild reaction conditions, and catalyst recycling. Also, enzymatic reactions are insensitive to free fatty acids and water content in waste cooking oil. As for the enzyme-catalyzed system, it requires a much longer reaction time than the other two systems. The enzyme reactions are highly specific and chemically clean. Because the alcohol can be inhibitory to the enzyme, a typical strategy is to feed the alcohol into the reactor in three steps of 1:1 mole ratio each. The reactions are slow, with a three-step sequence requiring from 4 to 40 hours, or more. The reaction conditions are modest, from 35 to 45°C (95 to 113°F). The main problem of the enzyme-catalyzed process is the high cost of the lipases used as catalyst.
Synthesis of biodiesel using enzymes such as Candida antarctica, Candida rugasa, Pseudomonas cepacia, immobilized lipase (Lipozyme RMIM), Pseudomonas sp., and Rhizomucor miehei is well reported in the literature. In the previously mentioned work (Shah and Gupta, 2007), the best yield 98% (w/w) was obtained by using Pseudomonas cepacia lipase immobilized on celite at 50°C in the presence of 4–5% (w/w) water in 8 hours.
Biodiesel – Transesterification, Supercritical Methanol
The transesterification of triglycerides by supercritical methanol (SCM), ethanol, propanol, and butanol has proved to be the most promising process. Recently, a catalysts-free method was developed for biodiesel production by employing supercritical methanol (Saka and Kusdiana, 2001). The supercritical treatment at 350°C, 43 MPa, and 240 s with a molar ratio of 42 in methanol is the optimum condition for transesterification of rapeseed oil to biodiesel fuel (Kusdiana and Saka, 2004a).
To achieve more moderate reaction conditions, further effort was made through the two-step preparation. In this method, oils/fats are, first, treated in subcritical water for hydrolysis reaction to produce fatty acids. After hydrolysis, the reaction mixture is separated into oil phase and water phase by decantation. The oil phase (upper portion) is mainly fatty acids, while the water phase (lower portion) contains glycerol in water. The separated oil phase is then mixed with methanol and treated at supercritical condition to produce fatty acid methyl esters (FAMEs) thorough methyl esterification. After removing unreacted methanol and water produced in reaction, fatty acid methyl esters can be obtained as biodiesel. Therefore, in this process, methyl esterification is the main reaction for the formation of fatty acid methyl esters, while in the one-step method, transesterification is the major one.
Reaction by supercritical methanol has some advantages: (i) glycerides and free fatty acids are reacted with equivalent rates, (ii) the homogeneous phase eliminates diffusive problems, (iii) the process tolerates great percentages of water in the feedstock catalytic process and requires the periodical removal of water in the feedstock or in intermediate stage to prevent catalyst deactivation, (iv) the catalyst removal step is eliminated, and (v) if high methanol-to-oil ratios are used, total conversion of the oil can be achieved in a few minutes. Some disadvantages of the one-stage supercritical method are clear: (i) the method operates at high pressure, (ii) the high temperatures bring along proportionally high heating and cooling costs, (iii) high methanol-to-oil ratio (usually set at 42) involves high costs for the evaporation of the unreacted methanol, and (iv) the process as posed to date does not explain how to reduce free glycerol to less than 0.02% as established in the ASTM D6584 or other equivalent international standards.
Biodiesel – Transesterification, Reaction Parameters
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