Membranes bipolar

The theoretical potential to achieve the water splitting is 0.83 V at 25°C. The actual potential drop across a bipolar membrane is quite close to this, being in the range of 0.9-1.1 V for current densities between 50 and 1500 A m , which is the general region of practical interest. The energy 

There are only a few known suppliers of membranes and of plant for this technology, e.g. Allied Signals Aquatec system, and Stantech, although other ion exchange membrane manufacturers and research organisations have developed bipolar membranes. The potential applications of bipolar membranes in recycling or effluent control [104] are numerous. Table 11.4 summarises the applications under development or implemented by one company, Aquatec. 

Na2S04 Rayon processing and pulp and paper processing 

NaRCOOH Organic acid purification and concentration (acetic, formic, citric) 

A bipolar membrane consists of a cation-exchange membrane, an anion-exchange membrane and an intermediate layer between the two membranes which are laminated together fFigtire VI - 69). WTien an electrical potential is applied between the cathode and anode the transfer of electrical charge will be earned out by the ions present. If no ions are available, the electrical current will be transferred by the hydroxyl and hydrogen ions formed bv the dissociation of water. 

The current efficiency of acid/base generation and the purity of the acid and base made with bipolar membranes drops off as concentrations increase, because Donnan exclusion diminishes with increasing solution concentrations. Further, the production rate is limited by the rate of diffusion of water into the bipolar membrane. Nevertheless, there are substantial advantages to the process. Since there are no gases evolved at the bipolar membranes, the energy associated with gas evolution is saved, and the power consumption is about half that of electrolytic cells. Compared to the electrodes used in conventional electrolytic cells, the bipolar membranes are inexpensive. Where dilute (e.g., 1 N) acids or bases are needed, bipolar membranes offer the prospect of low cost and minimum unwanted by-products. 

A commercially available ED stack with bipolar, anion and cation membranes was used by the author to generate 1 N HCl and NaOH from NaCl. A current density of 100 mA/cm2 was maintained with an applied potential of 2V/cell. 

The current efficiency was about 80%, and the purity of the acid and base exceeded 98%. 

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The latitude that titanium affords the cell designer has made a wide variety of monopolar and bipolar membrane cell designs possible. 

M. Paleologou, R. M. Berry, and B. I. Fleming, "Bipolar Membrane Electro dialysis A New Solution to the Problems of Chemical Imbalance iu Kraft Mills," 78th Finnual Meeting, Technical Section, Canadian Tulp and Taper Association, Preprints A,Jan. 28—29, 1992, pp. KM—KSl. 

The bipolar membranes are used in a more or less conventional ED stack together with conventional unipolar membranes. Such a stack has many acid—alkah producing membranes between a single pair of end electrodes. The advantages of the process compared to direct electrolysis seem to be that because only end electrodes are required, the cost of the electrodes used in direct electrolysis is avoided, and the energy consumption at such electrodes is also avoided. 

The disadvantages appear to be that the bipolar membranes are comparatively expensive, and the economic life is limited to about one year. Such short lifetime appears to result from the very high (- 10 V/cm) voltage gradients at the interface between the AX and CX regions. Additionally, practical current densities are limited to about 1000 A/m available area. 

Water Splitting A modified electrodi ysis arrangement is used as a means of regenerating an acid and a base from a corresponding salt. For instance, NaCl may be used to produce NaOH and HCl. Water sphtting is a viable alternative to disposal where a salt is produced by neutralization of an acid or base. Other potential applications include the recovery of organic acids from their salts and the treating of effluents from stack gas scrubbers. The new component required is a bipolar membrane, a membrane that sphts water into H and OH". At its simplest, a bipolar membrane may be prepared by... 

A schematic of the production of acid and base by electrodialytic water dissociation is shown in Fig. 22-61. The bipolar membrane is inserted in the ED stack as shown. Salt is fed into the center compartment, and base and acid are produced in the adjacent compartments. The bipolar membrane is placed so that the cations are paired with OH" ions and the anions are paired with H. Neither salt ion penetrates the bipolar membrane. As is true with conventional elec trodialysis, many cehs may be stacked between the anode and the cathode. 

FIG. 22-61 Electrodialysis water dissociation (water splitting) membrane inserted into an ED stack. Starting with a salt, the device generates the corresponding acid and base by supplying and OH" from the dissociation of water in a bipolar membrane. CouHesy Elsevier.)... 

Figure 7. Current-potential curve of a bipolar membrane of hydrous ironflll) oxide in chloride solution. FeCl3(aq)/Fe(HI)(oxideyNaCl(aq)-Na2Mo04(aq).l The system is kept at room temperature. C, cation A, anion. (Reproduced bom M. Sakashita and N. Sato, Corrosion 35,351,1979, Fig. 4 with permission from NACE International.)...

Figure 8. Schematic potential distribution across a bipolar membrane under increased anodic polarization. 15 At the neutral layer, dehydration proceeds in accordance with anodic polarization. and A are the inner potential and membrane potential, respectively. (Reproduced from N. Sato, Corrosion 45,354,1989, Fig. 27 with permission from NACE International.)...

Crystal surface specificity of the potential of zero charge, 152 Current-potential curves for bipolar membranes, 228 of iron dissolution in phosphoric acid,... 

Much more simply, the same result can be attained with bipolar membranes, membranes consisting of an anion- and cation-permeable (an anion- and cation-exchange) membrane laminated together. At such a membrane, when mounted between electrodes so that the cation-exchange layer faces the anode, water is split into and OH ions so that the acidic and alkaline solutions required for regeneration as above are produced at the respective surfaces of the bipolar membrane. When such membranes are suitably integrated into the sequence of membranes in the electrodialysis unit above, gas evolution at the electrodes is not needed the acid-base pair is produced with about half the power. 

In the case of ED, the availability of bipolar membranes allows electrodialitic water dissociation, and NaX can be. separated into NaOH and HX. 

The bipolar membrane is an alternative cell arrangement which can act as a direct source of acid or base for a process stream. 

A bipolar membrane is a sandwich of a cation and an anion exchange membrane which splits H20 to H+ and OH- under a potential of about 0.9 V. The reactor consists of a stack, of bipolar membranes, cation exchange membranes and anion exchange membranes arranged between a single anode and a single cathode with parallel hydraulytic circuits for the salt, acid product and alkali product, Fig. 32. 

The process operates at current densities of about 1 kA/m2 and unit cell voltage of 1.5 V. The specific energy consumption is about 2 kWh/kg NaOH. Under the influence of the electric gradient the H + and OH ions emerge on opposite faces of the membrane. Bipolar membrane electrodialysis is being developed by several companies, e.g. WSI Technologies Inc. [270] and Aquatech Systems [129,275,276], Typical product specification ranges for the ICI electrodialysis process is summarized in Table 19. 

Fig. 32. Bipolar membrane electrodialysis of NaA to HA and NaOH ( salt splitting ) [270]...

Paleologou M, Wong P-Y, Berry RM (1992) A solution to the caustic/chlorine imbalance bipolar membrane electrodialysis, J Pulp and Paper Sci, 18(4) J138 Chem Abstr 117 (1991) 253647u... 

If bipolar membrane electrolysers are installed (Fig. 15.10), such as the ICI BiChlor, then even less floor area is required for the same production capacity. In both membrane cases the space available also depends on the chosen operating current density. Utilising the 150 kA available, nine bipolar electrolysers can operate up to 16.7 kA each. This would require 112 anodes per electrolyser to manufacture the 200 000 tonnes per year of caustic soda, utilising about 360 V of the 450 V available. With bipolar electrolyser centres of 6.5 m, including operator walkways, an area of around 60 m by 14 m or 840 m2 will be required. However, more extensive pipework modifications are required with bipolar arrangements. 

Fig. 15.13 Utilisation of rectifier capacity in bipolar membrane electrolysers.

Replacement of diaphragm cells with bipolar membrane electrolysers requires a different electrical layout (Fig. 15.17) since each bipolar membrane electrolyser can only take about 17 kA of the 150 kA available (for a selected current density). This means that all nine electrolysers need to be installed together. The number of anodes in each bipolar electrolyser can be set depending on the number of diaphragm cells left on load, up to the maximum voltage of the rectifiers. 

Finally ( 5.5), in order to illustrate an alternative asymptotic approach, available for systems with nowhere vanishing fixed charge density, we shall treat, following Please [4], a p — n junction (bipolar membrane). Presentation of this chapter is purely heuristic, based on numerical or formal asymptotic results. For some rigorous results concerning related matters, the reader is referred to [5]—[7]. 

A uniform asymptotics for the nonequilibrium space charge in a bipolar membrane under a steady electric current [4]. It is... 

McCoy. September/October 1988. "Bipolar Membranes Applied to Spent Pickle Liquor Recovery." The Hazardous Waste Consultant. McCoy and Associates, Inc. Lakewood, CO. Pp. 1-18 to 1-21. 

AQUATECH Systems is a state-of-the-art bipolar membrane separation technology which exemplifies "pollution prevention" technology rather than "end of the pipe" regulation compliance. Consistent with SARA s definition of treatment, AQUATECH Systems is a technology "that in whole or in part will result in a permanent and significant decrease in the toxicity, morbidity or volume" of a hazardous waste material. 

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