Biodiesel from Vegetable Oils by Supercritical Methanol

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Presently, alkaline catalyst method is applied commercially to produce biodiesel. However, the process is not simple and not applicable to wastes of oils and fats. Therefore, supercritical methanol method (hereafter, called SS process) was developed as a non-catalytic process in our laboratory, which found that even wastes of oils and fats containing water and free fatty acids can be converted by this method successfully to biodiesel in a high yield. However, the reaction conditions by this SS process were high to be 350℃ and over 20MPa, thus special alloy such as hastelloy etc. was required for the reaction vessel. Additionally, the biodiesel produced was thermally deteriorated.Therefore, to realize the even milder reaction condition for biodiesel production, the process by hydrolysis of oils or fats by subcritical water and subsequent methyl esterification of the obtained fatty acids by supercritical methanol (hereafter, called SS-DK process) was developed. A subsequent comparison between the SS and SS-DK processes has clearly revealed that, in the SS process, transesterification was reduced in its reaction rate constant below the temperature of 300ーC, while in the SS-DK process, reaction rate constants in hydrolysis and esterification were not reduced but larger than that in transesterification. Consequently, the SS-DK process was concluded to produce the biodiesel in high yield at a very mild conditions of 270℃ and less than 10MPa. Furthermore, in the SS-DK process, glycerol produced as a by-product after hydrolysis could be successfully isolated to have the total glycerol content to be 0.15 wt% in the biodiesel fuel which could satisfy the requirements in EU and US standards of the biodiesel fuel.