Applications of bipolar membrane electrodialysis

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Since bipolar membranes became available as commercial products in 1977 [5], a very large number of potential applications has been identified and has been studied extensively on a laboratory or pilot plant scale [31]. However, in spite of the obvious technical and economical advantages of the technology, large-scale industrial plants are still quite rare. The main reasons for the reluctant use of bipolar membrane electrodialysis are shortcomings of the available bipolar and monopolar membranes, which result in a short useful membrane life, poor current utilization, and high product contamination. Nevertheless, there are a number of smaller scale applications in the chemical process industry, biotechnology, food processing, and wastewater treatment. Some of the potential applications of electrodialysis are summarized in Table 2.

Table 2 Potential applications of electrodialysis with bipolar membranes, and their state of development and possible advantages and experienced problems

Application

Development state of process

Potential advantages

Problems related to application

Production of mineral acids and bases from salts

Pilot plant operation

Lower energy consumption

Contamination of products, poor membrane stability

Recovering of organic acids from fermentation processes

Commercial plants

Integrated process, lower costs

Unsatisfactory membrane stability and fouling

pH-control in chemical processes

Laboratory tests

Less chemicals and less salt production

Application experience, process costs

Removal of SO2 from flue gas

Extensive pilot plant tests

Decreased salt production

Long-term membrane stability

Recycling of HF and HNO3 from steel pickling solutions

Commercial plants

Recovered acids and decreased salt disposal

Relatively complex process, high investment costs

Ion-exchange resin regeneration

Pilot plant tests

Decreased salt disposal

High investment costs

High-purity water production

Laboratory tests

Better removal of weak acids and bases

No long-term experience

5.2.1 Production of acids and bases by bipolar membrane electrodialysis

The largest potential application of bipolar membrane electrodialysis is the production of acids and bases from the corresponding salts. Presently, caustic soda is produced as a coproduct with chlorine by electrolysis. Utilizing bipolar membrane electrodialysis and producing caustic soda and an acid instead of chlorine from the corresponding salts is an interesting alternative to the conventional chlorine/alkaline electrolysis because of lower energy consumption. However, the process is impaired by poor membrane stability and insufficient permselectivity at high ion concentrations, resulting in substantial product salt contamination, low current utilization, and short membrane life under operating conditions. Problemfree operation of the bipolar membrane electrodialysis also requires a substantial pretreatment of the salt solution. The overall result of extensive laboratory tests is that presently the production of mineral acids and bases by bipolar membrane electrodialysis does not meet the product quality requirements under economic conditions.

However, the situation is quite different when acids or bases must be recovered from salts obtained in chemical reactions or neutralization processes. In these cases, the requirements for the concentration and the purity of the recovered acids or bases are not as stringent as in the production of high-quality commercial products and electrodialysis with bipolar membrane can be applied economically.

5.2.2 Applications of bipolar membranes in wastewater treatment

Recovering acids and bases from their salts generated in neutralization reactions to minimize waste disposal is one of the most promising applications of bipolar membrane electrodialysis. One of the more promising applications is the recovery of acids such as hydrofluoric and nitric acids from an effluent stream containing potassium fluoride and nitrite generated by neutralization of a steel pickling bath. The process is illustrated in the simplified flow diagram of Fig. 20.

The spent pickling acid is neutralized with potassium hydroxide. The solution is then filtered and the precipitated heavy metal hydroxides are removed. The neutral potassium fluoride and nitrite containing solution is fed to the bipolar membrane electrodialysis unit in which the salts are converted to the corresponding acids and potassium hydroxide. Potassium hydroxide is recycled to the neutralization tank and the acids to the pickling bath. The depleted salt solution from the bipolar membrane electrodialysis unit is concentrated in a conventional electrodialysis system and recycled

Base to neutralization

Base to neutralization

Bipolar Electrodialysis System

bipolar membranes

Figure 20 Simplified flow diagram of acid recovery and recycling from steel pickling neutralization bath.

bipolar membranes

Figure 20 Simplified flow diagram of acid recovery and recycling from steel pickling neutralization bath.

Biomass Recycling

Biomass Recycling

Bipolar Electrodialysis System
Figure 21 Simplified flow diagram illustrating the production process with integrated electrodialysis.

directly to the bipolar membrane unit. The diluate is used for rinsing and cleaning of the filter.

The treatment of alkaline or acid scrubbers that are used to remove components, which are harmful to the environment such as NOx, SO2, or NH3 from waste air streams, is another interesting application for bipolar membrane electrodialysis. In alkaline and acid scrubbers, large amounts of acids or bases are consumed and salts are produced, which are often contaminated with toxic materials such as heavy metal ions or organic compounds.

5.2.3 Applications of bipolar membrane electrodialysis in biotechnology

A very promising application of electrodialysis with bipolar membranes is the recovery of organic acids from fermentation processes. The process is illustrated in Fig. 21, which shows the production of lactic acid by continuous fermentation with an integrated product recovery process based on bipolar membrane electrodialysis.

Conventionally, lactic acid is produced in a batch process and the separation and purification of the lactic acid is achieved mainly by ionexchange, resulting in a large volume of wastewater from regeneration salts. In the production process with integrated electrodialysis, which is shown in Fig. 21, a minimum of ion-exchange resin is needed in a final purification step. The concentration of the lactate salt is achieved by conventional electrodialysis and the conversion of the lactate into lactic acid by bipolar membrane electrodialysis. The simultaneously produced base is recycled to the fermenter to control the pH value.

Other typical applications of bipolar membrane electrodialysis in biotechnology are the recovery of gluconic acid from sodium gluconate and the production of ascorbic acid from sodium ascorbate.

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