Methods, processes, apparatus, equipment and systems are disclosed for converting carboxylic acids into esters by esterification with alcohol and a dual catalyst. The method combines a homogenous and heterogeneous catalyst in one or more reactors to increase the conversion of carboxylic acids compared to using a homogenous and/or heterogeneous catalyst independently. The invention converts free fatty acids into esters by esterification with alcohol and a dual catalyst such that the reaction mixture contains sufficiently low free fatty acids and the amount of free fatty acids in the product stream leaving the process remains stable over time.
