Further developments in the high temperature electrodialysis

The high temperature electrodialysis method being developed in the Negev Institute for Arid Zone Research utilizes the Joule heat evolved in the process to increase the temperature of operation thus reducing the energy consumption of the process. Initial calculations indicated that by suitable increase of temperature the energy consumption may be reduced as much as 60-70%. The economics of the process seem sufficiently favourable to justify its application to sea water desalination. During the first period of the research it was shown that commercially available membranes preserve their electrochemical properties at elevated temperatures, and their functional groups withstand high temperature conditions. Deterioration of membranes, if it occurs, is due to disintegration of the matrix of the membranes. More extensive investigation of the changes in membrane resistance with temperature has shown that it decreases much more markedly below the range 70-80°C than above it. It was also found that electro-osmotic water transfer through membranes increases, especially in the higher temperature region (70-100°C). This behavior of membranes was reflected in the results obtained from laboratory scale high temperature electrodialysis experiments. The saving of energy consumption by increasing the temperature of operation from 30°C to 70°C was 60-70%, with different types of membranes. Further increase in temperature contributed less substantially to the decrease in energy consumption. During these experiments it became evident that electrodialysis apparatus built from conventional materials of construction using general purpose ion-exchange membranes may be successfully operated for prolonged periods, up to at least 70°C.