Separation of fermentation products by membrane techniques. IV. Electrodialytic conversion of carboxylates to carboxylic acids

For the salt-to-acid conversion of carboxylates by ED technique, the high resistance of weak carboxylic acid solutions Limits a current and increases the energy consumption per unit of a product. Carrying experiments for the conversion of sodium lactate to lactic acid (pK(lactic) = 3.86), we proved that the energy consumption can be lowered by filling the acid compartment in ED unit with the hydrogen conducting filler. In this paper the results of the same procedure applied for the conversion of sodium acetate to acetic acid (pK(acetic) = 4.75) and sodium propionate to propionic acid (PKpropionic = 4.87) are presented. It has been stated that with a filler in a unit the limiting current densities increase from 0.9 mA/cm(2) to 19.5 mA/cm(2) for the production of acetic acid and from 0.75 mA/cm(2) to 22 mA/cm(2) for propionic acid. With these current densities, the intensification factor found for these two conversions was within 30-35, and the energy consumption was in a range 0.3-0.5 kWh/mol. Two phenomena coupled to electrotransport-osmotic dilution and back diffusion of acids-are known to depress the yield of conversion. By experiments performed for acetic, propionic, and lactic acids using three anion exchange membranes Neosepta AM-1, ACM, and AMX (Tokuyama Co., Japan) it has been stated that diffusional leakage of acetic acid is rather high (up to 1 mol/m(2)h), whereas for lactic acid it is much lower (similar to0.1 mol/m(2)h). Of three membranes tested, the Neosepta AMX was the most effective in preventing both effects, thus assisting in getting high-yield conversions.