Optimisation of biodiesel production from Croton Gratissimus oil

Abstract

Consumption of liquid energy products, primarily fossil-based fuels, by the transportation industry, is high and has caused an escalation of the energy crisis facing global communities. This protracted use of fossil fuels has inadvertently resulted in an increased concentration of CO2 and other greenhouse gases (GHG) in the atmosphere, leading to environmental degradation. An environmentally friendly alternative fuel source, in the form of biofuels, has been found. These biofuels are biodegradable, boasting reduced levels of particulate matter (PM), carbon monoxide (CO), obnoxious sulphur (SOx) and nitrogen compounds (NOx) in their combustion products.

In African countries, particularly the Republic of South Africa (RSA), the urgency for the establishment of a viable biodiesel industry is driven by the vulnerability of crude oil prices, high unemployment, climate change concerns and the need for the continent’s growing economies to use their resources in a sustainable manner. In order to address these concerns, this investigation focused on the extraction of non-edible oil from the seeds of the indigenous Croton gratissimus plant, the catalytic synthesis of biodiesel and the optimisation of the developed biodiesel production process.

In this optimisation study, biodiesel was produced from oil extracted from Croton gratissimus seeds using synthesised monoclinic sulphated zirconia (SO42–/ZrO2) and KOH as catalysts. Low oil extraction yields (29.35%) obtained for this crop were attributed to its low unsaturated fatty acid content of 25.4%. From the model developed for the esterification of Croton 2–

gratissimus oil, the concentration of SO4

/ZrO2 catalyst had the most significant effect in the

reduction of the Acid Value of oil. This was substantiated by flat response surfaces observed on the RSM surface plots when all other design factors were varied whilst keeping catalyst concentration constant. The operating conditions for the esterification process that could give an optimum Acid Value of 2.693 mg KOH/g of oil were therefore found to be; 10.96 mass % SO42–/ZrO2 catalyst concentration, 27.60 methanol-to-oil ratio and 64 0C reaction temperature.

In the optimisation of the transesterification process, the model showed that catalyst concentration, methanol-to-oil ratio, reaction temperature, and their interactions were all significant model terms. But catalyst concentration and methanol-to-oil ratio, were the terms found to have the most influence on the percentage fatty acid methyl ester (FAME) yield and

percentage FAME purity. It was established from the combined model that optimum responses of 84.51% FAME yield and 90.66% FAME purity could be achieved when operating the transesterification process at 1.439 mass % KOH catalyst concentration, 7.472 methanol-to-oil ratio and at a temperature of 63.50 0C.

The two-step biodiesel process used in this work, produced biodiesel with a high FAME purity and a relatively high FAME yield. Improvement of the oil extraction process may be possible with polar co-solvent such as ethyl acetate, which may increase the FAME yield in the Croton gratissimus biodiesel production process.