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A Cyclic electrodialysis process : investigation of closed systems Bass, Dieter

Abstract

The cyclic electrodialysis process combines two concepts. The idea of flow reversal from parametric pumping is applied to an electrically driven absorption-desorption process operating with a stack of three-layer, ion-selective membranes. The cyclic process was investigated for the demineraliza-tion of aqueous NaCl solutions in a closed system. The standard parametric pumping operation was found to be generally very inefficient because of the finite rates of mass transfer. Flow pauses after each polarity reversal substantially improved both the rate and the limit of separation. Two designs of the electrodialysis cell were studied. Ten system parameters were analysed on a simple bench scale cell. Final separations were limited by large axial dispersion and ranged between 1 and 40. A second electrodialysis cell consisted of up to eight single stages (15 [cm] channel length) which were usually operated in series hydraulically and in parallel electrically. Final separation factors ranged between 2 and ~6000. Large separation factors were achieved for long channels, long pause times, and high applied potentials. The initial rate of separation appeared to be a maximum for a channel length of approximately one meter and pause times of about ten seconds. During the first few cycles the separation factor could be approximated by an exponential function of time. The potential of the process for continuous separation in open systems was demonstrated. Models of the closed systems are presented and are used to discuss the experimental results.

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