Preparation of fatty acid methyl ester through temperature gradient driven pervaporation process

Fatty acid esters are useful functional molecules in pharmaceutical, cosmetic and lubricant industries. The conversion of esters may be enhanced by continuous removal of the water produced during the esterification process which is a reversible reaction. Non-porous hydrophilic pervaporation membranes have the ability to remove water using milder conditions compared to conventional technologies. The potential of pervaporation driven by temperature gradient has been investigated in this study for esterification of oleic acid, a long chain (C18) fatty acid, with methanol. Composite polyvinyl alcohol (PVA) membrane was cast on polyether sulfone (PES) ultraporous substrate in view of its feasibility for scale-up in the form of plate and frame modules. Characterization of the membrane was carried out by measuring the thickness of membrane layers, scanning electron microscopy (SEM) studies, thermogravimetric analysis (TGA) and tensile strength testing before and after esterification experiments. The swelling studies of the composite membrane were also carried out in binary mixtures of methanol and water. The water removing efficiency of the membrane was evaluated in terms of flux and separation factor when methanol was spiked with 1–3 wt% water. Separation of water from methanol is a bigger challenge compared to dehydration of higher alcohols due to the polar nature of both molecules. The performance of the same membrane was evaluated as percentage conversion of ester during reaction between oleic acid and methanol, catalyzed by sulfuric acid. Research thrust was to minimize the amount of the corrosive sulfuric acid without compromising on conversion. The reaction conditions such as reactant ratio and temperature were varied and optimized in order to get maximum conversion of ester. Using PVA composite membrane, 99.9% conversion of methyl ester having acid value 0.2 could be obtained using 0.3 wt% sulfuric acid at 65 °C. As per international specification for biodiesel, the acid value of the ester should be below 0.5. Pervaporation aided esterification, driven by temperature gradient eliminated the necessity of using an inert sweep gas or vacuum on the downstream side. The % ester conversion was found to be better than the result in the absence of pervaporation. Results observed with the indigenous membrane were comparable to performance exhibited by commercial membrane.