Purification of FAME by rice hull ash adsorption

Abstract

Rice hull ash (RHA) and silica gel were used for purification of FAME by adsorption of FFA at atmospheric conditions. Studies demonstrated that silica gel is a better adsorbent than RHA in purifying FAME. However, both adsorbents were effective in reducing FFA levels from FAME extracts. The FFA adsorption efficiency increased with high FAME concentration and high adsorbent dosage. But FAME adsorption also increased with larger doses of adsorbent. The adsorption isotherms of FFA were determined with both adsorbents. FFA adsorption displayed Freundlich-type isotherms.Paper no. J10342 in JAOCS 80, 373-376 (April 2003). KEY WORDS:Adsorption, adsorption isotherms, competitive adsorption, fatty acid methyl ester, rice hull ash. FAME derived from vegetable oils and animal fats (biodiesel) have attracted considerable attention during recent years as an alternative to petroleum-based diesel fuel. Biodiesel has environmental advantages, relative to conventional diesel, as a renewable, biodegradable, and nontoxic fuel that does not require engine modification. FAME are produced by methanolysis of fats and oils with alcohols in the presence of an alkaline catalyst. Methyl esters are purified by distillation under vacuum and at high temperatures. The purity level of the diesel has a significant effect on its properties. Contaminating glycerides and FFA can cause serious combustion problems. Therefore, European Union biodiesel standards demand 99% purity (1). Seventy-eight percent of FAME production cost comes from refining the raw material (2). Cheaper raw materials and processing costs would affect profitability positively. If energy costs could be reduced by alternative technology, the importance of FAME would increase. For this reason we reported an in situ esterification method of rice bran oil that was more cost effective than the alternative methods (3-5).Rice hull ash (RHA), obtained by commercial combustion of rice hulls, has been used as a silica-based adsorbent to remove minor lipid components from vegetable oils (6-9). RHA was used to remove lutein, FFA, and phospholipids from soybean oil-hexane miscella (6-9). Proctor et al. (9) also showed that the adsorption of oleic acid onto RHA occurred by surface hydrogen bonding of the carboxylic acid and by the process of FFA adsorption, which followed Freundlich isotherms. Saleh and Adam (10) reported that lauric, myristic, and stearic acids adsorbed on RHA according to a Langmuir isotherm (10). Ooi and Lean found that rice ash could selectively adsorb MG in the presence of DG (11).The objectives of this study were (i) to demonstrate the removal of FFA from FAME by adsorption on RHA at room temperature, (ii) to determine the isotherm behavior of FFA adsorption onto RHA from FAME, and (iii) to compare RHA adsorption with that of commercial silica gel. MATERIALS AND METHODS Preparation of crude methyl esters.Crude methyl esters were prepared by in situ esterification according to the method of Özgül and Türkay (3,5). Rice bran containing approximately 70% FFA was refluxed...

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