Separation of fermentation products by membrane techniques. II. Conversion of lactate to lactic acid by electrodialysis

The yield of an electrodialysis unit operating for the conversion of lactate to lactic acid (LA) is depressed by low conductivity of the lactic acid solution. Experiments performed with a laboratory electrodialysis unit (ED) designed to establish the limiting current density i(lim), for the whole unit and for each of the membrane/solution elements revealed high voltage drops for membranes in contact with the lactic acid solution. By filling the LA compartment with a cation-exchange resin as the conducting turbulence promoter, it was possible to increase i(lim), from 2.7 +/- 0.3 to 18.3 +/- 2.0 mA/cm(2). With a resin suspension the rate of lactic acid formation was 6.0 mol.m(-2).h(-1), the current efficiency amounted to 92%, and the energy consumption was 3.3 kW.h.kg(-1). A hypothesis for the similar to 7-fold increase of i(lim) by filling the LA compartment with a grained conducting resin is presented.