Electrodialysis of itaconic acid: A short-cut model quantifying the electrical resistance in the overlimiting current density region

Abstract

Itaconic acid is an anticipated intermediate chemical in the synthesis of biofuels from cellulosic feed stock. Itaconic acid (IA) can be produced by fermentation and continuous fermentation with continuous product removal is highly desired. Electrodialysis with bipolar membranes (EDBM) has this potential and can be used simultaneously for the pH control of the fermenter. EDBM processes for acidification are operated at overlimiting current densities. The resistance of a diffusion boundary layer and consequently mass transfer of the IA through the anion exchange membrane under these conditions are not fully understood yet. To design the process a short cut method is required to describe the current voltage characteristic of the EDBM stack. In a first modeling approach, a differential resistance was assumed to describe the current–voltage behavior of the anion exchange membrane determining the major resistance. Experiments on a single membrane test cell under stationary conditions were carried out to measure iV -curves for different bulk phase compositions and IA transfer through the AEM. Special attention was given to the influence of pH and ionic strength. The resistance at overlimiting currents follows an exponential law and it depends on pH and ionic strength only with regards to the absolute level of the current. An approximation is presented for the current voltage characteristics above the limiting current density based on an extended Nernst–Plank model having diffusion coefficients as input parameters for the limiting current densities and a single Schmidt Number for the overlimiting currents at any feed composition. The latter acknowledges the hydrodynamic character of electroconvection as contributor to the overlimiting transport.