Optimization of electrodialytic desalination in diluted solutions

An optimization of the electrodialysis process and an estimation of its costs by the analysis of experimental results of model natrium chloride solutions was compared to the optimization and cost estimation made by using the convective-diffusion hydrodynamic model. A NaCl 0.02 M solution was desalinated to a degree of 50, 80 and 95%. The cost was calculated for a commercial ED unit with 40x40 cm working membrane, which is 69% of the total membrane area. An appropriate desalination degree was reached in a single-pass mode. The fixed size of the working membrane caused serious deficiencies, especially in the case of small desalination degrees (50 and 80%). It was found that electrodialysis with 0.19 and 0.27 mm intermembrane distances results in a desalination rate so high that its assumed level was reached passing about 10 to 16 cm along the membrane only. It was then necessary to work at a lower current density than permissible 80% of its limiting value, which resulted in an increase in capital and maintenance, and, as a consequence, in total unit costs of desalination. The optimum value of intermembrane distance found, as a result of the experiments, is higher than calculated using the convective-diffusion model. We believe that the reason for this discrepancy is that the aforementioned very high desalination rate, observed in the case of small intermembrane distance was not considered in the calculations using convective-diffusion model. The convective-diffusion model was developed for ED desalination in a membrane without a spacer, which may be the second presumable reason for this disparity.