Bipolar Membrane Process

Figure 10.20 Schematic of a bipolar membrane process to split sodium chloride into sodium hydroxide and hydrochloric acid...

Electrodialysis with Bipolar Membrane Process Costs... 

FIGURE 43 Bipolar membrane processes for (a) S02 removal from stack gases and (b) stainless steel pickling bath waste acid regeneration. 

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 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. 

FIG. 5 Basic operating principles of an electrodialysis process using (A) a couple of monopolar (anionic, a, and cationic, c) membranes or (B) a bipolar membrane. 

A great deal of research work has been carried out to enhance bioreactor productivity (2 14 kg m 3h 1) using cell recycle via membrane processing (Boyaval and Corre, 1987 Boyaval et al., 1994) and recovering propionic acid by monopolar or bipolar ED (Boyaval et al., 1993 Weier et al., 1992 Zhang et al., 1993). 

A two-stage ED process was also proposed to recover succinic acid [HOOC (CH2)2COOH] from sugar- and triptophane-based fermentation media (Glassner and Datta, 1992). The broth was previously concentrated via ED using monopolar membranes and then separated into sodium hydroxide-and free succinic acid-rich streams using bipolar membranes. Further removal of sodium cations and sulfate anions was achieved using weakly acid and -basic IER. 

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. 

Electromembrane processes such as electrolysis and electrodialysis have experienced a steady growth since they made their first appearance in industrial-scale applications about 50 years ago [1-3], Currently desalination of brackish water and chlorine-alkaline electrolysis are still the dominant applications of these processes. But a number of new applications in the chemical and biochemical industry, in the production of high-quality industrial process water and in the treatment of industrial effluents, have been identified more recently [4]. The development of processes such as continuous electrodeionization and the use of bipolar membranes have further extended the range of application of electromembrane processes far beyond their traditional use in water desalination and chlorine-alkaline production. 

In this chapter only electromenbrane separation processes such as electrodialysis, electrodialysis with bipolar membranes and continuous electrodeionization will be discussed. 

Electrodialysis with Bipolar Membrane System and Process Design... 

The design of an electrodialysis process with bipolar membranes is closely related to that of a conventional electrodialysis desalination process. 

Because of the relatively high concentrations of the acid and base as well as the salt solution the limiting current density is in general no problem and a bipolar membrane stack can generally be operated at very high current densities compared to an electrodialysis stack operated in desalination. However, membrane scaling due to precipitation of multivalent ions such as calcium or heavy-metal ions is a severe problem in the base-containing flow stream and must be removed from the feed stream prior to the electrodialysis process with a bipolar membrane. 

The total costs of the electrodialytic water dissociation with bipolar membranes are the sum of fixed charges associated with the amortization of the plant investment costs and of the operating costs which include energy and maintenance costs and all pre- and post-treatment procedures. The total costs are a function of the membrane properties, of the feed-solution composition, the required acid and base concentrations, and several process and equipment design parameters such as stack construction and operating current density. 

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. 

The schematic shown in Fig. 43(a) is a commercial example of this technology. Stack gas is scrubbed with an alkaline solution of sodium hydroxide, sodium sulfite, and sodium sulfate. The sodium sulfite reacts with SO2 in the stack gas to form sodium bisulfite. This salt solution is processed in a bipolar membrane unit [Type (I) shown in Table IX] to generate an alkaline solution and an acidic solution. The alkaline solution contains regenerated caustic soda and sodium sulfite, and can be recycled to the scrubber, while the sulfurous acid can be further processed to sulfur or sulfuric acid for sale. 

A number of relatively new methods are being investigated to improve the recovery of small molecules. These methods include elec-trokinetic separators with bipolar membranes, simulated moving-bed chromatography and supercritical fluid extraction. The latter is practiced for food components. It has also been described for proteins but has not yet found wide acceptance in this field. A fastgrowing field is the production of bioethanol via fermentation processes either from milled com or from recycled biomass. The fermentation and saccharification processes can occur simultaneously in the fermenting tank by means of saccharification enzymes (amylases, cellulases). 

The future for electrodialysis-based wastewater treatment processes appears bright. The dilute concentrations of metals in the waste streams do not degrade or foul the cation or anion exchange membranes. The concentrate streams are recirculated to build up their metal content to a level that is useful for further recovery or direct return to the process stream. Ongoing research in the development of cheaper cation exchange membranes, and stable anion exchange and bipolar membranes will allow electrodialysis-based applications to become more competitive with other treatments. 

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. 

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