A study into homogeneously and heterogeneously catalysed transesterification of waste cooking oil for the production of biodiesel.

Abstract

The ever-increasing population around the globe, along with the imbalances in food and fodder supply, the dwindling supply of fossil fuels, and the diminishing availability concerning natural resources have led to the emergence of significant energy challenges on a global scale. To address these issues, it is essential to pursue sustainably and economically viable growth, relying on domestic and renewable energy sources, to reduce the need for imported oil. An established renewable alternative to imported oil is biodiesel. For biodiesel to become market competitive with diesel, it is important to ensure the development of cost-effective processing schemes to optimize production. This research paper aims to determine whether heterogeneously and homogeneously transesterification of waste cooking oil can be used to produce biodiesel efficiently. The chosen homogeneous catalyst was Potassium Hydroxide, and the heterogeneous catalyst was Magnesium Oxide. The selected feedstocks were waste canola oil and ethanol. After a pre-treatment process, the acid value of the waste cooking oil was calculated to be 0.1167 mg KOH/g, indicating that a single step transesterification process can be used. The box-Behnken Design was utilised on Minitab Statistical Software to generate 27 different experiments using variations in the process variables. The process variables considered in this study were Catalyst Loading, Reaction Temperature, Reaction Time, and Alcohol to Oil Molar Ratio. The optimum yield for biodiesel produced from used canola oil utilising potassium hydroxide and magnesium oxide was 91.53% and 96.02%, respectively. From the various models explored, the full quadratic model most accurately fitted both experimental results with the KOH catalyst experimental data obtaining an R2 value of 0.9575 and the MgO catalyst experimental data obtaining an R2 value of 0.9710. Minitab calculated the optimal process variable conditions using the KOH catalyst to have a reaction temperature of 74 oC, a catalyst loading of 0.0673%, a total reaction time set at 84.55 minutes and an alcohol to oil molar ratio of 26:1. Minitab calculated the optimal process variable conditions using the MgO catalyst to have a reaction temperature at 54.41 oC, a catalyst loading of 1.44%, total reaction time set to be 120 minutes and alcohol to oil molar ratio to be 18.18:1. The kinematic viscosity was higher than the ASTM maximum limit of 6 mm2/s for both the pure KOH and MgO Biodiesel samples at 8.29 mm2/s and 15.24 mm2/s, respectively. It was concluded that further modification would be required for direct use in an engine. The research for this study found KOH to be the most suitable catalyst for transesterification of waste cooking oil. This conclusion is drawn from the pure KOH biodiesel having a much lower viscosity than MgO biodiesel; KOH biodiesel also had a higher flash point than pure MgO biodiesel of 103.67 oC. The bio-jet fuel blends were not in accordance with many of the ASTM Standards. The acid values of the jet fuel blends (B10 and B20) from KOH and MgO samples were over the allowable limit set at 0.015 mg KOH/g oil. The pour point for Jet fuels should be approximately -47 oC. All the jet fuel blends (B10 and B20) exceeded the maximum limit. The critical success factors for biodiesel production in South Africa were determined.