Elektrodialysis with bipolar membranes

The essential characteristic of electrodialysis with bipolar membranes (EDBM) is the combination of conventional electrodialysis for salt separation with electrodialysis water splitting for the conversion of a salt into its corresponding acid and base. Bipolar membranes induce the splitting of water into protons and hydroxide ions.

The process can be considered a rival to the electrolytical production of acids and bases. As a result of the compulsory co-production of hydrogen and oxygen, the energy theoretically needed for electrolytical water splitting is more than twice as high.

The potential difference needed for the electrodialytical production of one-molar solutions is 0.83 V which is equivalent to an energy consumption of 22 Wh. In contrast, the electrolytical production of a theoretical drop in potential is equivalent to an energy consumption of 55 Wh. Further advantages of bipolar membrane technology include the comparatively simple apparatus configuration, the option of a stack-like set-up as well as the low investment costs.

Bipolar membranes consist of a cation exchange membrane, an anion exchange membrane and a catalytic intermediate layer to accelerate the splitting of the water into protons and hydroxide ions. The main requirements for a bipolar membrane are:

  • Excellent long-term stability
  • A low passive drop in potential
  • A high rate of water splitting
  • High permselectivity
  • as well as good mechanical stability

The development work for the production of bipolar membranes of the type fumasep® FBM was initiated in 1984, resulting in the first patent registration in 1988. Coinciding with work of Aqualytics in the USA, numerous processes for the production and application of bipolar membranes were tested and patented. In 1999, Aqualytics, together with all trademark rights and more than 200 patent families, was taken over by FUMATECH BWT GmbH and the Aqualytics product range was fully incorporated into production in St. Ingbert. The current basic patents protect the splitting of inorganic and organic acids and amino acids in a two and three chamber arrangement. The production of bipolar single film membranes is also widely protected by the industrial property rights of FUMATECH BWT GmbH and Aqualytics.

The simplest design for bipolar membrane processes is the so-called three-chamber stack. In this case, the recurring membrane unit in the stack is composed of one bipolar membrane, one cation exchange membrane and one anion exchange membrane. In the electric field, the chloride ions migrate through the anion exchange membrane to the anode, are then retained on the cation-selective side of the bipolar membrane and finally form hydrochloric acid together with the protons produced in the bipolar membrane. Similarly, caustic soda solution is formed on the anion-selective side of the bipolar membrane while a diluate is produced in the central chamber for the raw solution.

A large number of different applications for bipolar membrane technology has been tested and developed. The first industrial process was developed by Aqualytics for the recovery of hydrofluoric acid and nitric acid from pickling baths in the steel industry. Nowadays, much interest is paid to the separation and treatment of organic acids such as lactic acid and citric acid from fermentation solutions. The first plants for producing amino acids have been developed not only in the field of biotechnology but also in the chemical and pharmaceutical industries. A bipolar membrane process, suitable for standardisation, can result from the production of ultra-pure water via continuous deionisation using bipolar membranes.

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