Production and purification of fatty acid methyl esters from plant oils of different origin
Biodiesel production from plant oils has been studied since ca. 1900 and is now being widely adopted as a means to reduce carbon emissions in transport applications. Its properties are similar to those of fossil diesel fuel, thus allowing its use in diesel engines both pure and in mixtures with fossil diesel fuel. Biodiesel is obtained from vegetable oils or animal fats by a transesterification reaction whereby the triglycerides, contained in the oil or fat, react with a short chain alcohol in the presence of a catalyst. The products of the reaction are fatty acid methyl esters (biodiesel) and glycerol, obtained as two separate phases. However, current reaction efficiency using a homogeneous catalyst, such as sodium hydroxide, has various drawbacks. These include non-specificity leading to saponification, difficulty in isolating the catalyst from the fatty acid methyl esters, immiscibility of the catalyst with the reactants and incomplete transesterification.\ud \ud The aim of the research reported in this thesis is to provide a full description of the transesterification reaction. According to previously published methodologies, the transesterification of glycerol trioleate with sodium hydroxide should provide 97-98 % (w/w) conversion to the ester. However, the results reported, herein, indicated only 95.2% (w/w) ester content. To understand the differences in the results, the concentrations of sodium hydroxide and methanol were varied by using unrefined rapeseed oil. Results showed that the optimum reaction conditions to produce higher ester content (93.3% w/w) from unrefined rapeseed oil i.e. molar ratio of methanol/oil was 6:1 and 0.015 mol of sodium hydroxide at 60 min. The same results were obtained with various plants oils of different origin under similar reaction conditions, but no increase in the ester content was observed. The data suggested that the results were in accord with biodiesel specifications i.e. EN ISO 12937 and EN 14104. However, EN 14103 standard could not be met, perhaps due to the reversible nature of the reaction, higher acid value in the oil and other competing reactions of the triglycerides. It was not possible to achieve the ester content (%) according to EN 14103 standard if any moisture or free fatty acid was present in the oil or during the reaction.\ud \ud The failure to meet the required EN 14103 standards by using homogeneous catalytic systems paved the way for kinetics studies of the transesterification reaction. Heterogeneous catalysts offered the opportunity to study the reaction kinetics of the system because they can be separated rapidly from the reaction mixture by centrifugation. Various heterogeneous metal oxide catalysts were investigated. Strontium oxide was confirmed to be an effective catalyst but, contrary to expectations, similar catalytic activity was not observed for the other metal oxides. The experimental results obtained, by optimising reaction conditions using a heterogeneous catalyst were found to be 3% (w/w) SrO, 6:1 CH3OH/oil molar ratio at 120 min. Therefore, subsequent reactions were planned to carry out real-time kinetic studies using SrO as a catalyst. The application of refractometry allowed real-time kinetic studies of the transesterification reaction. The ester content obtained after transesterification were determined by gas chromatography and validated the results obtained by the refractometer method. This analytical method helped to improve the reaction conditions from 120 min to 90 min using 3% (w/w) SrO and 6:1 CH3OH/oil molar ratio to achieve 92% (w/w) ester content.\ud \ud During kinetic studies, using a heterogeneous catalyst, solubility issues were observed between oil, methanol and fatty acid methyl esters (FAMEs). Therefore, a phase solubility diagram was plotted to identify the miscible region. The transesterification reaction was conducted in the miscible region of a ternary phase diagram to overcome the phase limitation problems. The ester content obtained was higher than 98% (w/w) within 25-30 min depending on the concentration (% v/v) ratio of the reactants used. These results were encouraging in terms of using a heterogeneous catalyst since its use is limited to lower ester content (%) and longer reaction time.
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