(488e) Modelling of Ion Transport across Anion Exchange Membranes Under Current Reversal Conditions

Lactic acid is an important chemical used in industry as pH regulator, emulsifying agent, animal feed supplement, solvent, electrolyte and it is an important potential feedstock for the production of the biodegradable Poly-Lactic Acid (PLA). The PLA has applications in plastic engineering where it can replace hydrocarbon based polymers and thereby reduce the non-biodegradable waste. Besides, PLA will diminish the dependence on fossil feedstock. The production of Lactic acid by Lactic Acid Bacteria is normally impaired by product inhibition like many other fermentation processes at a certain concentration level of the product. Therefore, continuous removal of lactic acid from the fermenter will result in a higher productivity and product yield. Recently, the application of Reversed Electro-Enhanced Dialysis (REED) has shown promising performance for continuous removal of Lactic Acid during fermentation [1].

The REED module combines elements from Electro-Dialysis Reversal (EDR) and Donnan Dialysis (DD) operations. The REED stack is composed only of anion exchange membranes (AEM). Using this unit, the fluxes obtained using Donnan Dialysis can be enhanced by applying electrical current. In addition, the adverse influence of fouling is reduced when the current density is reversed periodically. Another advantage arises if only anion-exchange membranes are utilized, since scaling problems are avoided because only inorganic anions and negatively charged organic acids and amino acids are extracted [1].

A dynamical model is derived to estimate the fluxes and concentration profiles of some ions commonly found in a fermentation broth. The model is based on first engineering principles for diffusion, migration and convection, resulting in a stiff system of partial differential and algebraic equations [2]. The key idea is to reveal the interactions between operation conditions for the REED module, when it is operated with or without applying electrical current. The model is validated against experimental data for dialytic transport of lactate and other carboxylic acids reported in literature [3]. Furthermore, the model can be potentially applied to optimize the design and operation of the REED unit.

References

[1] Rype, J.U. (2003) Separation and recovery of organic acids using Electrodialysis, Ph.D. Thesis. CAPEC, DTU.

[2] Møllerhøj, M. (2006). Modeling the REED process. M.Sc. Thesis. CAPEC, DTU.

[3] Zheleznov, A, et. al. (1998). Dialytic transport of carboxylic acids through an anion exchange membrane. Journal of Membrane Science. Vol 139, pag. 137-143.