Abstract:
A process for the production of fatty acid lower alkyl esters comprising the steps of: 
     A) treating at least one fatty acid ester of glycerol with from about 0.3 to about 3% by weight, based on the weight of ester, of an acid at an elevated temperature; 
     B) removing said acid from the treated fatty acid ester; and 
     C) reacting the treated fatty acid ester with at least one lower aliphatic alcohol to transesterify the fatty acid ester.

Description:
BACKGROUND OF THE INVENTION 
     This application is a 371 of PCT/EP94/03576 filed Oct. 31, 1994. 
     1. Field of the Invention 
     This invention relates to a process for the production of fatty acid lower alkyl esters by transesterification of fatty acid glycerides with lower aliphatic alcohols. 
     2. Statement of Related Art 
     Fatty acid methyl esters are important industrial raw materials for the production of a range of products, for example lubricants and surfactants. The esters are normally produced from natural fats and oils, i.e. full esters or partial esters of glycerol with fatty acids, which are transesterified with methanol in the presence of catalysts. Processes relating to the transesterification of fats and oils are described in a number of publications, cf. for example the synoptic article in Seifen-Ole-Fette-Wachse, 114, 595 (1988). 
     If the transesterification reaction is to be carried out with satisfactory conversions in an economically reasonable time, catalysts have to be added. Suitable catalysts are, in particular, heavy metal compounds such as, for example, zinc oxide (GB 712,747) or zinc silicate (U.S. Pat. No. 2,727,049). Unfortunately, processes such as these are attended by the disadvantage that, after the transesterification, the catalysts cannot remain in the product for toxicological reasons, but instead have to be removed which involves considerable outlay on equipment. 
     The transesterification reaction may also be carried out in the presence of mineral acids which merely have to be neutralized on termination of the reaction. Although the problem of catalyst removal does not arise in this case, the volume/time yields of transesterification products are distinctly poorer by comparison with heavy metal catalysis which limits the application of the process on an industrial scale. A yield comparable with that obtained with the heavy metal compounds mentioned above is achieved by carrying out the transesterification reaction in the presence of free fatty acids. A corresponding process is disclosed in WO 93/1263 (Henkel). 
     In the catalytic transesterification processes mentioned above, the natural fatty acid glycerides used as starting compounds have to be treated with bleaching earth (&#34;fullering&#34;) in order to remove shell remains and mucins. This pretreatment is necessary to obtain a substantially odorless and light-colored product. After removal of the bleaching earth, the transesterification reaction may be carried out in the presence of the catalysts described above. However, the shell remains and mucins are not completely removed by the treatment with bleaching earth so that the product obtained from the transesterification of fatty acid glycerides with lower aliphatic alcohols is generally attended by an unpleasant odor and is discolored. 
     Accordingly, the problem addressed by the present invention was to provide an economic process for the transesterification of fatty acid glycerides which would provide a fatty acid lower alkyl ester free from unpleasant odors and discoloration. 
     DESCRIPTION OF THE INVENTION 
     The present invention relates to a process for the production of fatty acid lower alkyl esters, in which full and/or partial esters of glycerol with fatty acids corresponding to formula (I): 
     
         R.sup.1 --COOH                                             (I) 
    
     in which R 1  is an aliphatic hydrocarbon radical containing 5 to 23 carbon atoms and 0 or 1 to 5 double bonds, are reacted with lower aliphatic alcohols containing 1 to 4 carbon atoms at elevated temperature and optionally under elevated pressure, characterized in that, before the transesterification reaction, the full and/or partial esters of glycerol with fatty acids corresponding to formula (I) are treated with 0.3 to 3% by weight, based on the quantity of the full and/or partial esters of glycerol with fatty acids corresponding to formula (I), of an acid at a temperature of 75° C. to 120° C. 
     It has surprisingly been found that fatty acid glycerides can be transesterified in high yields to form odorless and non-discolored products providing the full and/or partial esters of glycerol with fatty acids used are treated with an acid before the transesterification reaction. This process has the advantage that 1. the shell remains and mucins present in the natural starting products are completely removed so that a white odorless product is obtained and 2. an acid value at which the transesterification reaction can be carried out is established by the treatment with an acid so that there is no need for another catalyst to be added. 
     Suitable starting materials for the transesterification reaction on which the process according to the invention is based are both full esters and partial esters of glycerol with fatty acids. Triglycerides are preferably used. Typical examples are glycerol esters of caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, elaidic acid, petroselic acid, linoleic acid, linolenic acid, chaulmoogric acid, ricinoleic acid, arachic acid, gadoleic acid, behenic acid, erucic acid, arachidonic acid and clupanodonic acid. 
     The glycerol esters of the fatty acids mentioned are natural products, more particularly those in which the glycerol is attached to two or three different fatty acids. Glycerol fatty acid esters based on natural products are fats and oils of animal or vegetable origin, for example coconut oil, palm oil, palm kernel oil, peanut oil, cottonseed oil, rapeseed oil, sunflower oil, coriander oil, linseed oil, soybean oil, beef tallow or fish oil. 
     According to the invention, the full and/or partial esters of glycerol used are treated before the transesterification reaction with 0.3 to 3% by weight and preferably 0.8 to 1.5% by weight, based on the quantity of the full and/or partial esters of glycerol, of an acid. Suitable acids are H 2  SO 4 , mineral acids and H 3  PO 4 . H 2  SO 4  is preferably used. The acid treatment is carried out at a temperature of 75° C. to 120° C. and preferably at a temperature of 80° C. to 98° C. The acid is allowed to act on the full and/or partial esters of glycerol used over a period of 3 to 36 hours and preferably over a period of 8 to 16 hours. The acid then has to be completely removed, for example by washing with water, extraction or centrifugation. The acid used should be completely removed to avoid secondary reactions, such as the formation of sulfoesters. The correspondingly treated full and/or partial esters of glycerol with fatty acids corresponding to formula (I) have an acid value of 20 to 70 and preferably in the range from 35 to 70 after the acid treatment and before the transesterification reaction. It is surprising that the transesterification reaction can readily be carried out even at high acid values of up to 70 without being accompanied by any back-reaction into acid and alcohol. There is no need for additional catalysts. 
     Suitable lower aliphatic alcohols which may be replace the glycerol in the fatty acid ester are ethanol, n-propyl alcohol, i-propyl alcohol, n-butanol or tert.-butanol. The transesterification reaction is preferably carried out with methanol. 
     The transesterification of the pretreated glycerol ester may be carried out in known manner at elevated temperature and optionally under elevated pressure, for example at temperatures of 150° C. to 300° C. and more particularly at temperatures of 200° C. to 250° C. and under pressures of 1 to 100 bar and preferably under pressures of 50 to 80 bar. The transesterification reaction may be followed by working up in which the glycerol released and residues of unreacted alcohol are removed and the fatty acid lower alkyl ester obtained is distilled or fractionated. 
     The following Examples are intended to illustrate the invention without limiting it in any way. 
    
    
     EXAMPLES 
     In a stirred reactor, 320 g (0.5 mole) of unrefined coconut oil with the following fatty acid composition: 
     Caproic acid: 0.5% by weight 
     Caprylic acid: 8% by weight 
     Capric acid: 7% by weight 
     Lauric acid: 48% by weight 
     Myristic acid: 17% by weight 
     Palmitic acid: 9% by weight 
     Stearic acid: 2% by weight 
     Oleic acid: 7% by weight 
     Linoleic acid: 1.5% by weight 
     Bound glycerol: 13.8% by weight 
     Acid value: 9.3 
     were treated with 50% sulfuric acid in a ratio by weight of 99:1 over a period of 24 hours with continuous stirring at 95° C. The sulfuric acid used was removed by extraction. After the treatment with sulfuric acid, the following characteristic data were determined for the coconut oil: 
     Bound glycerol: 13.1 
     Acid value: 67 
     The coconut oil thus pretreated was introduced into a 1 liter autoclave with 320 g (10 moles) of methanol (ratio by volume 1:1). The reaction mixture was then kept at a temperature of 280° C. and under a pressure of 65 bar for 180 minutes. After cooling and venting of the reaction mixture, a transesterification product with the following characteristic data was obtained: 
     Bound glycerol: 1.1% by weight 
     Acid value: 2.3.