Bipolar membrane cell

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

The dimensionally slahle characteristic of the metal anode made the development of the membrane chlorine cell possible. These cells arc typically arranged in ail electrolyzer assembly which docs not allow for anodc-ro-cathode gap adjustment alter assembly. Also, very close tolerances are required. The latitude that titanium affords the cell designer has made a wide variety of monopolar and bipolar membrane cell designs possible. 

Fig. 26.13. Expanded view of ELTECH ExL bipolar membrane cell electrolyzer. (Courtesy of ELTECH Systems Corporation.)...

Fig. 8 S implified cross section through a single element of a bipolar membrane cell. The electrodes are supported by corrugated bands, which are in turn supported by contact plates. Compression of the single cells together gives the series connections for the stack [14, p. 442],...

TABLE 5.4. Comparison of Bipolar Membrane Cell Technologies... 

Overcoming these problems was an important part of the development of bipolar membrane cells. From bonded bimetallic sheets, electrode design has advanced to more sophisticated assemblies that prevent titanium embrittlement. As designs have become more successful and as die technology has moved toward higher current densities, bipolar designs have increased their market share. Chapter S illustrates some of the specific designs that are now in use. 

Typical bipolar membrane cell rooms are shown in the following Figures 66 and 67. 

J.F. Walther, Process for production of electrical energy from the neutralization of acid and base in a bipolar membrane cell, U.S. Patent 4,311,771,1982. 

A schematic of the production of acid and base by electrodialytic water dissociation is shown in Fig. 20-84. 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 electrodialysis, many cells may be stacked between the anode and the cathode. 

A bipolar pilot membrane cell with six cell elements each of 1.8 m2 has been installed and run with industrial electrolyte. 

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

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. 

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. 

So-called zero-gap membrane cells in which cathode and chlorine-evolving anodes are touching the cation exchange membrane, which separates the anode from the cathode compartment, are also state of the art (35). Bipolar chlorine electrolyzers have also been developed (36), for instance, at ICI, an achievement that could only be envisaged due to the introduction of RuCL-coated titanium anodes. 

Figure 19.16. Basic designs of electrolytic cells, (a) Basic type of two-compartment cell used when mixing of anolyte and catholyte is to be minimized the partition may be a porous diaphragm or an ion exchange membrane that allows only selected ions to pass, (b) Mercury cell for brine electrolysis. The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they are in parallel at low voltage, (d) Bipolar electrical connections 50 or more cells may be series and may require supply at several hundred volts, (e) Bipolar-connected cells for the Monsanto adiponitrile process. Spacings between electrodes and membrane are 0.8-3.2 mm. (f) New type of cell for the Monsanto adiponitrile process, without partitions the stack consists of 50-200 steel plates with 0.0-0.2 ram coating of Cd. Electrolyte velocity of l-2 m/sec sweeps out generated Oz.

The main advantages of bipolar membranes are no formation of gases at their surfaces or within the bipolar membranes themselves, a power consumption to dissociate water into 02 and H2 about half that used in electrolytic cells, a minimum formation of by-product or waste streams in the case of dilute (< 1 kmol/m3) acids or bases, and reduced downstream purification steps. 

Electrodialysis is by far the largest use of ion exchange membranes, principally to desalt brackish water or (in Japan) to produce concentrated brine. These two processes are both well established, and major technical innovations that will change the competitive position of the industry do not appear likely. Some new applications of electrodialysis exist in the treatment of industrial process streams, food processing and wastewater treatment systems but the total market is small. Long-term major applications for ion exchange membranes may be in the nonseparation areas such as fuel cells, electrochemical reactions and production of acids and alkalis with bipolar membranes. 

Stack design in bipolar membrane electrodialysis The key component is the stack which in general has a sheet-flow spacer arrangement. The main difference between an electrodialysis desalination stack and a stack with bipolar membranes used for the production of acids and bases is the manifold for the distribution of the different flow streams. As indicated in the schematic diagram in Figure 5.10 a repeating cell unit in a stack with bipolar membranes is composed of a bipolar membrane and a cation- and an anion-exchange membrane and three flow streams in between, that is, a salt... 

The required membrane area A refers actually to a unit cell area that contains a bipolar membrane, and a cation- and an anion-exchange membrane. Since in strong acids and bases the useful life of the bipolar membrane as well as the anion-exchange membrane is rather limited, the stack-related investment costs are dominating the total investment costs. 

Here, Epro is the energy for the production of a certain amount of acid and base, I is the current passing through the stack, Nceu is the number of cell units in a stack, A n is the cell unit area, C and C are the concentration and the average concentration in a cell, A is the thickness of the individual cells, and A is the equivalent conductivity, r is the area resistance, , is the current utilization, R is the gas constant, T the absolute temperature, F the Faraday constant, and ApH is the difference in the pH value between the acid and base, the subscript p refers to product and the subscript i refers to salt, acid and base, The superscripts am, cm, and bm refer to the cation-exchange, the anion-exchange, and the bipolar membrane, the superscript out and in refer to cell outlet and inlet, Q is the total flow of the acid or base through the stack and t is the time. 

Continuous electrodeionization is widely used today for the preparation of high-quality deionized water for the preparation of ultrapure water in the electronic industry or in analytical laboratories. The process is described in some detail in the patent literature and company brochures [29]. There are also some variations of the basic design as far as the distribution of the ion-exchange resin is concerned. In some cases the diluate cell is filled with a mixed bed ion-exchange resin, in other cases the cation- and anion-exchange resins are placed in series in the cell. More recently, bipolar membranes are also being used in the process. 

FIGURE 21.8 EDBM cell assembly. (Reprinted with permission from Kemperman, A.J.B., Ed. Handbook of Bipolar Membrane Technology, Twente University Press, Enschede, The Netherlands, 2000. Copyright 2000 Twente University Press, p. 162.)... 

FIGURE 21.12 BPM electroacidification cell configuration, (a) Process for soy protein isolate production and (b) modified process for production of soy protein isolate. BPM, bipolar membrane CEM, cation exchange membrane. 

FIGURE 21.20 Electrodialysis cells for inhibiting enzymatic browning in cloudy apple juice, (a) Electroacidification step and (b) electro-alkalinization step. CEM, cation-exchange membrane BPM, bipolar membrane AEM, anion-exchange membrane. 

---