Piezodialysis

Another membrane process which uses pressure as the driving force is piezodialysis [15-17]. This process is applied with ionic solutes where in contrast to reverse osmosis, the ionic solutes permeate through the membrane rather than the solvent, which is usually water. A schematic drawing of the process is given in figure VI -13. 

If a pressure is applied at one side of the membrane an electromodve force (AE) will be generated which is proportional to the applied pressure difference (AP). The proportionality constant is called the electric osmotic coefficient p. This coefficient is negative for anion-exchange membranes and positive for cation-exchange membranes 

30 atm cm-1. Though the salt is enriched in the low concentration KC1 solution, the enrichment is not significant for concentrations higher than 10-2 moll-1. The Es increases at higher pressures than 30 atm, which is not practical. It was reported to be difficult to desalinate saline water with a high flux with this type of mosaic ion exchange membrane.206 The results should be comparable or superior to those for reverse osmosis membranes. 

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This book provides a general introduction to membrane science and technology. Chapters 2 to 4 cover membrane science, that is, topics that are basic to all membrane processes, such as transport mechanisms, membrane preparation, and boundary layer effects. The next six chapters cover the industrial membrane separation processes, which represent the heart of current membrane technology. Carrier facilitated transport is covered next, followed by a chapter reviewing the medical applications of membranes. The book closes with a chapter that describes various minor or yet-to-be-developed membrane processes, including membrane reactors, membrane contactors and piezodialysis. 

To-be-developed industrial membrane separation technologies Carrier facilitated transport Membrane contactors Piezodialysis, etc. Major problems remain to be solved before industrial systems will be installed on a large scale... 

Three other processes using ion exchange membranes (Donnan dialysis, diffusion dialysis and piezodialysis) are covered in Chapter 13. 

Now the major application of dialysis is the artificial kidney and, as described in Chapter 12, more than 100 million of these devices are used annually. Apart from this one important application, dialysis has essentially been abandoned as a separation technique, because it relies on diffusion, which is inherently unselec-tive and slow, to achieve a separation. Thus, most potential dialysis separations are better handled by ultrafiltration or electrodialysis, in both of which an outside force and more selective membranes provide better, faster separations. The only three exceptions—Donnan dialysis, diffusion dialysis and piezodialysis—are described in the following sections. 

A second potential application is pressure-driven desalination. When a pressure difference is applied across the membrane, the concentrated ionic groups in the ion exchange domains are swept through the membrane, producing a salt-enriched permeate on the low-pressure side. This process, usually called piezodialysis, has a number of conceptual advantages over the alternative, conventional reverse osmosis, because the minor component (salt), not the major component (water), permeates the membrane. 

Figure 13.7 Piezodialysis of 0.02 M potassium chloride solution with block copolymer charge mosaic membranes [14]. Enrichment is calculated using the expression ...

F.B. Leitz, Piezodialysis in Membrane Separation Processes, P. Meares (ed.), Elsevier Sand Publishing Company, Amsterdam, pp. 261-294 (1976). 

Note that latex paints (42) and materials relating to piezodialysis (24) may be composed of one copolymer in the latex and an ionic copolymer in the aqueous phase. (This last class of materials, mixtures of two polymers, was accidentally omitted from Refs. 14 and 15.)... 

Leitz, F. B., Sharr, J., Research on Piezodialysis—Third Report to the... 

One other form of pressure-driven membrane process should be mentioned piezodialysis In this process, a selectiwly salt-permeable membrane is used... 

Donnan dialysis successfully involves mosaic membranes which are an important part of piezodialysis. Their basic advantage is the close juxtapoation of anion- and cation-exchange resins in the same membrane. It is a pity that research on piezodialysis, a desalination process with great promise and hopes, has been abandoned. However, one may expect work on it will be reassumed as soon as some difficulties originated from the deficiency of mosaic membranes, as well as some engineering problems, are overcome. 

T. Yamabe, S. Yoshida and N. Takai, Basic studies on piezodialysis, Kogyou Kagaku Zasshi, 1971, 74, 2410. 

Ion exchange membranes have been used in various industrial fields, and have great potential for use in new fields due to their adaptable polymer membrane. As mentioned in the Introduction, membranes are characterized mainly by ion conductivity, hydrophilicity and the existence of carriers, which originate from the ion exchange groups of the membrane. Table 6.1 shows reported examples of applications of ion exchange membranes and the membrane species used in various fields. Various driving forces are usable for separation electrochemical potential, chemical potential, hydraulic pressure such as piezodialysis and pervaporation, temperature difference (thermo-osmosis), etc. Of these, the main applications of the membrane are to electrodialysis, diffusion dialysis, as a separator for electrolysis and a solid polymer electrolyte such as in fuel cells. 

Diffusion dialysis Donnan dialysis Piezodialysis Neutralization dialysis Pervaporation Facilitated transport... 

Figure 6.36 Piezodialysis of a potassium chloride solution using a mosaic ion exchange membrane (membrane thickness about 200 A, ion exchange capacity of cation and anion exchange parts are almost the same). Salt enrichment Es = [(CujCi) — 1] X 100 (C, concentration of feed solution Cu concentration of permeate). ( ) Membrane prepared by casting dioxane solution of the polymer. ( ) Membrane prepared by casting benzene solution of the polymer. (A) Membrane prepared by casting cyclohexane solution of the polymer.

Leitz, F. B., and Shorr, J. (1972), Research on Piezodialysis— Third Report, Research and Development Progress Report No. 775, Office of Saline Water, U. S. Department of the Interior, May 1972. 

Reverse Osmosis n Dialysis Electrodialysis Piezodialysis Gas Permeation Pervaporation... 

Three engineering uses have been proposed for ionic IPNs (1) cationic/anionic ion exchange resins, (2) piezodialysis membranes, and (3) thermoplastic IPNs. The last item is discussed briefly in Sections 5.7 and 8.7. The first two will be briefly treated here. 

Piezodialysis is a novel desalination technique in which salt is preferentially transported across the membrane and removed from a feed using pressure as the driving force.The theory requires the membrane to consist of two continuous phases, one anionic and one cationic/ A suitable material should have dual-phase continuity, but with a minimum of molecular mixing between the phases. The morphological features attainable with IPN formation fit these theoretical requirements. " ... 

L. H. Sperling, V. A. Forlenza, and J. A. Manson, Interpenetrating Polymer Networks as Piezodialysis Membranes, J. Polym. Sci. Polym. Lett. Ed. 13(12), 713 (1975). Piezodialysis membrane. Cationic/anionic IPNs. PS, sulfonated/poly(vinyl pyridine), quaternized sequential IPNs. 

Figure VI - 13. The transport of ions through a mosaic membrane during piezodialysis...

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