Optimization of Cottonseed Oil Ethanolysis to Produce Biodiesel High in Gossypol Content

Transesterification of cottonseed oil was carried out using ethanol and potassium hydroxide (KOH). A central composite design with six center and six axial points was used to study the effect of catalyst concentration, molar ratio of ethanol to cottonseed oil and reaction temperature for percentage yield (% yield) and percentage initial absorbance (%A _385nm) of the biodiesel. Catalyst concentration and molar ratio of ethanol to cottonseed oil were the most significant variables affecting percentage conversion and %A _385nm. Maximum predicted % yield of 98% was obtained at a catalyst concentration of 1.07% (wt/wt) and ethanol to cottonseed oil molar ratio of 20:1 at reaction temperature of 25 °C. Maximum predicted %A _385nm of more than 80% was obtained at 0.5% (wt/wt) catalyst concentration and molar ratio of 3:1 at 25 °C. The response surfaces that described % yield and %A _385nm were inversely related. Gossypol concentration (% wt), oxidative stability and %A _385nm of biodiesel were found to be highly correlated with each other. Hence, color %A _385nm is a measure of the amount of pigments present in biodiesel fuels that have not yet been subjected to autoxidation. High gossypol concentration also corresponds to a fuel with high oxidative stability. The fatty acid ethyl esters (FAEE) produced from cottonseed oil had superior oxidative stability to fatty acid methyl esters (FAME) produced from cottonseed oil.