Environmentally benign biodiesel production by heterogeneous catalysis
thesisposted on 30.09.2013, 15:32 by Kathleen F. Haigh
Process options to minimise the environmental impact and improve the efficiency of biodiesel production have been investigated. The process options considered include the use of heterogeneous catalysts and used cooking oil (UCO). An esterification pre-treatment reaction was investigated using an ion-exchange resin (Purolite D5082) and an immobilised enzyme (Novozyme 435). Another immobilised enzyme (Amano Lipase PS-IM) was investigated for transesterification. The fresh and used catalysts have been characterised. The catalytic activity of Purolite D5082, Novozyme 435 and Amano Lipase PS-IM have been investigated using a jacketed batch reactor with a reflux condenser. Purolite D5082 has been developed for the esterification pre-treatment process and is not commercially available. Novozyme 435 has been shown to be an effective esterification catalyst for materials with high concentrations of free fatty acid but it has not been investigated for the esterification pre-treatment reaction. It was found that a high conversion was possible with both catalysts. The optimum reaction conditions identified for Purolite D5081 were a temperature of 60 C, a methanol to free fatty acid (FFA) mole ratio of 62:1, a catalyst loading of 5 wt% resulting in a FFAs conversion of 88% after 8 h of reaction time. The optimum conditions identified for Novozyme 435 were a temperature of 50 C, a methanol to FFA mole ratio of 6.2:1 and a catalyst loading of 1 wt% resulting in a conversion of 90% after 8 h of reaction time. These catalysts were compared to previously investigated Purolite D5081 and it was found that the highest conversion of 97% was achieved using Purolite D5081, however there were benefits to using Novozyme 435 because the reaction could be carried out using a much lower mole ratio, at a lower temperature and in much shorter reaction time. During the Novozyme 435 catalysed esterification pre-treatment reactions it was found that the amount of free fatty acid methyl esters (FAME) formed during the reaction was greater than the amount of FFAs consumed. In order to investigate further an ultra-performance liquid chromatography mass spectrometry (UPLC-MS) method was developed to monitor the monogclyeride (MG), diglyceride (DG) and triglyceride (TG) concentrations. This analytical method was used to show that Novozyme 435 would catalyse the esterification of FFAs as well as the transesterification of MGs and DGs typically found in UCO. With the UPLC-MS method it was possible to separate the 1, 2 and 1, 3 DG positional isomers and from this it could be seen that the 1, 3 isomer reacted more readily than the 1, 2 isomer. The results from the UPLC-MS method were combined with a kinetic model to investigate the reaction mechanism. The kinetic model indicated that the reaction progressed with the sequential hydrolysis esterification reactions in parallel with transesterification. Commercially available Amano Lipase PS-IM was investigated for the transesterification reaction. Enzymes are not affected by FFAs and as a result the optimisation was carried out with UCO as the raw material. An optimisation study for the transesterification of UCO with Amano Lipase PS-IM has not previously been reported. The conditions identified for the Amano Lipase PS-IM catalysed transesterification step are addition of 5 vol% water, a temperature of 30 C, a methanol to UCO mole ratio of 3:1 and a catalyst loading of 0.789 wt% resulting in a TG conversion of 43%. An overall enzyme catalysed process was proposed consisting of Amano Lipase PS-IM catalysed transesterification (stage 1) followed by Novozyme 435 catalysed esterification (stage 2). The previously identified optimum conditions identified for each catalyst were used for above stages. It was found that when the oil layer from stage 1 was dried the final TG conversion was 55%.
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