Optimisation of biodiesel production from waste cooking oil under supercritical conditions

Consumption of energy from fossil fuels has become inevitable for people life. In recent years, enormous increase occurred as a result of increase in the demand of fossil fuel. This increasing in demands of fossil fuels increases greenhouse gases in the atmosphere and there by affects the environment. (Datta A. and Mandal B.K., 2016) Biodiesel has been recognized as one of the effective green, renewable and sustainable fuels. It is a clean burning fuel derived from renewable living resources and can be derived from either animal oils or vegetable oils. The unique quality of biodiesel is less pollutant when compared to the conventional fossil diesel. The combustion exhaust of biodiesel is free of sulphur and aromatics unlike the conventional fossil diesel. (Ghoreishi S.M and Moein P., 2013) The syntheses of biodiesel without the use of catalyst but within the supercritical conditions of methanol are at the heart of every researcher. The use of supercritical transesterification facilitates simple recovery of the products and high conversion of oil within short time reaction. Moreover, supercritical transesterification tolerates feed stock with high free fatty acid and water content. (Wen et al, 2009) In this study, biodiesel production in the supercritical conditions of reactants was analysed with studying the main factors affecting the biodiesel yield. Experimental design via Box Behnken Design (BBD) was used to evaluate the influence of four independent variables (molar ratio of methanol to oil, temperature, pressure and time) on the yield of biodiesel. A quadratic polynomial model was concluded by analysing the results using Response Surface Methodology (RSM). The numerical optimisation technique concluded that the maximum yield that can be reached with minimum reaction temperature, pressure and time was 91% at methanol to oil ratio of 37:1, reaction temperature of 253.5oC, reaction pressure of 198.5 bar and time of 14.8 minutes. In addition, kinetics of the reaction was studied at the optimum conditions concluding an irreversible first order reaction with reaction rate constant of 0.0006s-1. Moreover, thermodynamics of the reaction was studied within the temperature range from 240 to 270 oC resulting frequency factor and activation energy of 4.05 s-1 and 50.5 kJ/mol respectively. Furthermore, a kinetic reactor was simulated on HYSYS using the concluded experimental kinetic data with the achieved optimum conditions resulting in conversion of 91.7% with 0.7% relative error from the experimental results. Finally, a complete plant process simulation design using HYSYS program was developed. The process includes the developed kinetic reactor, methanol recovery and product separation units. References: • Datta A. and Mandal B.K. A comprehensive review of biodiesel as an alternative fuel for compression ignition engine. Renewable and Sustainable Energy Reviews 57 (2016) 799–821. • Ghoreishi S.M and Moein P. Biodiesel synthesis from waste vegetable oil via transesterification reaction in supercritical methanol. Journal of Supercritical Fluids 76 (2013) 24– 31. • Wen D., Jian H., Zhang K. Supercritical fluids technology for clean biofuel production. Progress in Natural Science 19 (2009) 273–284