Pervaporation-assisted esterification of lactic and succinic acids with downstream ester recovery

Reactors coupled with membrane separation, such as pervaporation, can help enhance the conversion of reactants for thermodynamically or kinetically limited reactions via selective removal of one or more product species from the reaction mixture. Esterification of lactic acid (C3H6O3) and succinic acid (C4H6O4) with ethanol (C2H5OH) to generate ethyl lactate (C5H10O3) and diethyl succinate (C8H14O4), respectively, is studied in well-mixed reactors with solid catalysts (Amberlyst XN-1010 and Nafion NR50) and two pervaporation membranes (GFT-1005 and T1-b) in this article. The results of catalyst compatibility and membrane stability studies are discussed. Experiments with a closed-loop system of a “batch” catalytic reactor and a pervaporation unit employing GFT-1005 reveal that for esterification of both lactic and succinic acids, fractional conversions of the two reactants exceeding the corresponding maximum values in a reaction-only operation and close to unity are obtained by stripping water, a reaction product. The kinetics of pervaporation is studied to obtain a working correlation for the flux of water in terms of temperature and water concentration on the feed side of the pervaporator. The efficacy of pervaporation-aided esterification is illustrated by attainment of near total utilization of the stoichiometrically limiting reactant within a reasonable time. Protocols for recovery of ethyl lactate and diethyl succinate from pervaporation retentate are discussed and simultaneous esterification of lactic and succinic acids, which is an attractive and novel concept, is proposed.