Electrodialysis of aqueous NaCl-glycerol solutions: A phenomenological comparison of various ion exchange membranes

Industrial processes such as e.g. oil, fat and resin production and saponification generate large volumes of aqueous NaCl-glycerol containing streams. With typical glycerol concentrations ranging from 5 to 15 % (m/m) and a high salt content, these streams pose a challenge to further processing, both to end-of-pipe treatment as to possible reuse scenarios for organic components and/or salts. Electrodialysis is evaluated as an alternative technology to remove salt from these streams. Salt, glycerol and water transport is studied experimentally through desalination of synthetic solutions for 7 commercial ion exchange membrane pairs, Fumatech (FAB-FKB, FAM-FKM, FAS-FKS), Neosepta (AMX-CMX), PCA (MVA-MVK, SA-MV) and Ralex (AMH-CMH). A phenomenological model is applied i.e. model coefficients are derived from test data through regression analysis to evaluate transport phenomena. Salt transport is found to be related to electromigration only and the NaCl transference number is determined to be highest in AMH-CMH (0.96 +/- 0.04) and FKS-FAS (1.01 +/- 0.03). Limiting current density is highest for FKS-FAS and lowest for FAM-FKM. Water transport is attributed to electroosmosis and osmosis. Water transference numbers found are similar to those in literature (t(w) similar to 7-10). The value of the osmotic coefficients (P-W) differs strongly among the different membrane pairs and is highest for MV-SA Q380 (0.99 x 10(-5) ms(-1)). Glycerol transport is attributed to diffusion and water flux induced cotransport. The diffusion of glycerol differs significantly among all membrane pairs (range 2.06 x 10(-8) ms(-1) FABFKB to 12.4 x 10(-8) ms(-1) MV-SA). Reflection coefficients differ significantly among membrane pairs but fall within a relative narrow range (0.36-0.75 ms(-1)). Numerical simulation of batch desalination with the derived transport model identifies current density as the largest contribution to the overall glycerol transport (38% - 64%) followed by osmotic co-transport (16%-41%) and glycerol diffusion (9%-28%). A low glycerol/ NaCl flux is desirable, this occurs when both initial glycerol/NaCl and NaCl concentrations are low while current density is high.