Catalyst-free microwave-assisted conversion of free fatty acids in triglyceride feedstocks with high acid content

• Catalyst-free microwave-assisted esterification of free fatty acids (FFA).
• Response surface modelling to predict conversion.
• 90% conversion of FFA after 1 h of reaction at 250 °C.
• Esterification of different FFA feedstocks with different long-chain alcohols (≥C4).

The catalyst-free microwave-assisted conversion of free fatty acids (FFA) to alkyl esters in triglyceride feedstocks with a high acid content was studied. To this end, esterification of FFA with higher alcohols was investigated using the response surface modelling approach (RSM). A model was constructed using stearic acid and 1-butanol. The influence of reaction temperature and the FFA content in the triglyceride feedstock were studied by varying both parameters on 5 levels between 175 °C and 275 °C, and 30 mass% and 100 mass% FFA respectively. Afterwards, the general applicability of the model was verified using other FFA, fatty acid distillates (FAD) from industry and other alcohols. When using other FFA, FAD or primary alcohols other than 1-butanol, the model is able to accurately predict the actual conversion with a typical deviation between actual and predicted data of 2%. After 1 h of reaction at temperatures above 250 °C, conversions over 90% were reached.