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A Cyclic electrodialysis process : investigation of closed systems

University of British Columbia

Cyclic electrodialysis is a novel separation process in which a modified membrane stack is operated in a periodic unsteady-state manner. Repeated reversals of polarity could avoid the main problems encountered in conventional electrodialysis; fouling and scale formation on the membranes. In cyclic electrodialysis the standard electrodialysis stack is converted into an adsorption-desorption stack with only one set of flow channels, the other set being replaced by storage compartments. Thesr compartments are in the form of three-layer membranes consisting of an anion and a cation selective membrane enclosing a core of non- selective material. The depleted and enriched products are produced successively in the single set of channels instead of simultaneously in adjacent channels. The process is potentially applicable for commercial desalination of brackish water to make it potable, to remove harmful ions from discharge waters, or to concentrate ionic solutions for recovery of valuable materials. Previously reported experiments with aqueous NaCl solutions in a closed (batch) system showed that a large separation factor could be obtained in cyclic electrodialysis. Batch operation is somewhat analogous to total reflux in distillation. The present work extends the earlier work to potentially more useful operating conditions in which feed is supplied and product removed. A constant-rate model has been developed for the process and used extensively throughout the work as a simple and efficient tool to compare various operating cycles and modes of operation. Scattered articles in the literature on the resistance of an electrodialysis stack have been compiled to develop a stack resistance model. Good agreement was obtained between the model predictions and measured values of resistance. Experimental apparatus is described and the effects of the following eight system parameters are reported: (i) Demineralizing path length (ii) Production rate (iii) Pause time (iv) Applied voltage (v) Initial concentration (vi) No-pause operation (vii) Pure-pause operation (vii) Semi-symmetric operation Large separations were achieved for asymmetrical paused operation with long demineralizing path, long pause time, high applied voltage, low feed concentration and small production rate. Despite the strong trade-off between production rate and separation, a separation factor as high as 50 was obtained at the highest production rate used. This value is higher than that obtained in commercial plants currently in use. The process looks promising and is worth further consideration.

Text

2010-02-21

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http://hdl.handle.net/2429/20638

Chemical and Biological Engineering

University of British Columbia

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