Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 162

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.