(543h) Esterification of Organic Acids with Pervaporation

In recent years, membrane technology has emerged as a viable separation process in an effort to combine downstream/upstream separation with reaction to improve process performance. Since membranes allow selective permeation of a component from a multi-component mixture, these 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. Pervaporation is used to separate a liquid mixture by partly vaporizing it through a porous or a nonporous membrane. The permeate is enriched in the more rapidly permeating component of the feed mixture, whereas the retentate is depleted in this component. Removal of water from mixtures (dehydration) is the most widely used application of pervaporation. Pervaporation is ideal for enhancing conversion in reversible condensation reactions, such as esterification of carboxylic acids and alcohols, generating water as a product. Substantial acceleration of these reactions can be achieved using appropriate commercially available solid catalysts.

Lactic and succinic acids are important intermediates, which play a significant role in glucose metabolism in most living systems. Diethyl succinate and ethyl lactate are important chemical intermediates. Esterification of each acid with ethanol/methanol is a reversible reaction and hence thermodynamically limited. A detailed analysis of reversible reactions where one of the products is removed by membrane separation is presented here. The reactions of interest are condensation reactions, with one of the products, water, to be removed by pervaporation. The reactor-separator configurations considered are: (1) a semi-batch perfectly mixed reactor with complete recycle of pervaporation retentate, (2) a plug flow reactor (PFR) with in situ pervaporation (tubular membrane reactor), (3) a continuous stirred tank reactor (CSTR) with in situ pervaporation, (4) a CSTR/PFR with external pervaporation and recycle. The analysis is supplemented by appropriate numerical illustrations based on esterification of lactic acid with ethanol. The effects of resistances for reaction and membrane separation on the performance of the reaction-pervaporation systems are studied. Performance of membranes with finite resistance for mass transfer is compared with that of perfect membranes, membranes with negligible mass transfer resistance.