Optimum membrane length in electrodialytic desalination of diluted solutions

Abstract

The fixed size of a working membrane, applied in our previous work, caused a serious shortcoming, especially in the case of small desalination degrees (50% and 80%) when 0.02 M NaCl solution was desalinated. 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 only about 10 to 16 cm along the membrane. It was then necessary to work at a lower current density than the permissible 80% of its limiting value, which results in an increase in capital and maintenance, and therefore in total unit costs of desalination. To overcome the shortcoming, the length of working membrane was not fixed in our present work; it was assumed that it is a result of the required degree of desalination, but it should not exceed the value of 200 cm. The 4-cm margins were considered as a gasket. Two types of membranes were considered for calculation: a low price ($40/m 2) with a relatively high surface resistance of 10 Ω/cm 2 and a high price ($200/m2) with a resistance of 2 Ω/cm 2. The cost was calculated for a commercial ED unit with a 40-cm-width working membrane and 0.19–0.75 mm separator thickness range. The optimum ED parameters were: 0.4 mm intermembrane distance, 7.6 cm/s linear velocity, 38 cm effective membrane length and 0.93 V voltage drop per cell pair. The influence of non-fixed membrane length on optimization results was almost negligible. The dependence of cost on intermembrane distance found in this work is, however, very weak, while our previous experiments indicated high costs ofdesalination for small intermembrane distance conditions.