Co-ion fluxes of simple inorganic ions in electrodialysis metathesis and conventional electrodialysis

Abstract

a b s t r a c tThe main functionality of ion-exchange membranes is to selectively permeate either cations or anions. However, co-ions may permeate as well to some extent through these membranes. For the production of chemicals with electrodialysis metathesis and conventional electrodialysis, these co-ion fluxes are a major source of impurities in the produced chemicals. The present paper studies the co-ion fluxes and the influence of operating parameters. The results clearly indicate that co-ion fluxes are predominately determined by the difference in ion concentration across the membrane. A more than linear increase in co-ion flux with respect to the ion concentration was observed. The current density has no significant influence on the co-ion fluxes. Therefore, a high current density leads to a smaller transport number of co-ions and improves the product purity in electrodialysis metathesis. Moreover, the co-ion flux can be decreased by the use of bulky counter-ions. In a conventional electrodialysis configuration, cation impurities are mainly caused by a diffusional flux. This flux is approximately three times larger than the co-ion flux. In contrast, the anion impurities are predominantly initiated by a co-ion flux and in magnitude smaller than the cation impurities.

Electrodialytic Processes: Market Overview, Membrane Phenomena, Recent Developments and Sustainable Strategies

“…Hence, it was reported that high voltage difference across the stack, sufficiently high feed concentration, equimolar ratio of reactant salts, high membrane transport number and moderate flow rate are critical parameters to obtain ideal operating conditions [ 144 , 145 , 146 , 147 , 148 , 149 ]. The difference in concentration across the membrane is also of particular importance, since it may result in co-ion fluxes leading to impurities in the final product; monovalent cations are especially subject to this phenomenon [ 150 ]. On another note, IEM characteristics strongly influence EDM performance.…”

Modeling of Potassium Sulfate Production from Potassium Chloride by Electrodialytic Ion Substitution

“…To our knowledge, co-ions can theoretically be completely excluded from the polymer matrix in an ideal ion-exchange membrane. However, as a matter of fact, the selectivity of the ion exchange membrane cannot attain 100%, and some co-ions still exist in the membrane phase. Hence, the quantity of electric charge across the membrane includes that carried by counterions and co-ions.…”

“…Generally, membrane counterion transport number T represents the fraction of the current that is carried by counterions . The relationship between the membrane counterion transport number and ion transport number in Figure can be expressed as where T A and T S are the membrane counterion transport numbers of AEM and MAEM, respectively, ε K A is the K + transport number through AEM, and ε NH 4 S is the NH 4 + transport number through MAEM.…”

Ion-exchange membranes in chemical synthesis – a review

“…The rate of product formation can be controlled by applied current density which makes ED-M a safe route of synthesis. Concentration of products is limited by the rate of water transport, and product purity is limited by permselectivity of membranes and back diffusion, analogously to ED [22,23]. ED-M has the potential to be an energy and cost-effective intermediate synthesis step and reevaluation tool for highly soluble ionic compounds.…”

“…The concentration of products was, however, limited by inevitable water transport, caused mostly by electroosmosis [81,82]. Purity of product is crucial for the application of final product and depends mostly on selectivity of membranes and composition of raw materials Unauthenticated Download Date | 5/11/18 5:27 AM [22,23]. The main advantage of applying ED-M to synthesis or conversion of inorganic salts is its non-equilibrium character which allows for using substrates that will lead to the co-precipitation of undesired products or formation of double salts when components are simply mixed together, as in conventional metathesis.…”