Application of natural deep eutectic solvents in bulk liquid membrane system for removal of free glycerol from crude fatty acid methyl ester

Fatty Acid Methyl Ester (FAME) produced from biomass offers several advantages such as renewability and sustainability. The typical production process of FAME is accompanied by various impurities such as alcohol, soap, glycerol, and the spent catalyst. Therefore, the most challenging part of the FAME production is the purification process. In this work, a novel application of bulk liquid membrane (BLM) developed from conventional solvent extraction methods was investigated for the removal of glycerol from FAME. The extraction and stripping processes are combined into a single system, allowing for simultaneous solvent recovery whereby low-cost quaternary ammonium salt-glycerol-based deep eutectic solvent (DES) is used as the membrane phase. Moreover, for the first time, diethyl ether was introduced as a strip phase to strip the free glycerol in the BLM system. The effects of DES composition, DES:FAME ratio, stirring speed and operating time on the extraction and stripping efficiencies were investigated. A kinetic model was proposed for the transportation of free glycerol through the DES-based BLM, where the kinetic parameters (k1, k2, tmax, GMmax) for the extraction and stripping were calculated using the glycerol content in the feed and the strip phase. The performance of the lab-scale BLM system confirms the viability of the proposed system with a best DES molar composition of 1:2 (salt:glycerol), DES:FAME ratio of 1:1, mixing speed of 200 rpm and an extraction time of 4 hr. Glycerol content in the purified FAME meets the international standards such as EN 14214 and ASTM D6759. Under the optimal conditions, the suggested model curves of glycerol content in the feed, membrane, and strip phases showed a good fitting with the experimental results. Finally, the rate constants values of the kinetic model were 0.0037 min⁻¹ for k1 and 0.0066 min⁻¹ for k2.