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Bipolar Electrodialysis

Electrodialysis is an electrochemical membrane process where charged particles of an electrolyte solution (ions) are separated from a crude solution in an electrical field using ion-selective membranes. Through the combination of the product separation with the electrodialytically forced water dissociation on bipolar membranes, aqueous ion-ogenic solutions are transferred into the respective inorganic or organic acid and base. The advantage is that usage of neutral salts is not needed in this technique. This opens a wide field of applications for using the bipolar membrane technique.

Applications of bipolar membrane technique:

  • Separation and purification of organic acids and amino acids from fermentation broths by combined electro-dialysis
  • Separation of amino acids from racemic mixtures
  • Gentle adjustment of the pH value of aqueous solutions without increasing the ion potential, e.g. flavor enhancing
  • Pasteurization of hydrous solutions at room temperature
  • Partial inversion of sucrose or inversion of potato starch
  • Combined deacidification of fruit juices and recovery of the organic acid.

Bipolar electrodialysis is used for the recovery of lactic acid during the production of lactic acid. A selected strain of lactic acid bacteria produces lactic acid from lactose with high efficiency in the fermentation.

After the fermentation the organisms are removed by cross-flow-filtration (cell recycle). The lactate ions are then separated from the fermentation broth and converted to free lactic acid using the bipolar electrodialysis technique.

 

AEM: Anion Exchange Membrane
CEM: Cation Exchange Membrane
BM: Bipolar Membrane

The outline of the bipolar electrodialysis uses the three-chamber stack as illustrated above. The mechanism behind this transfer reaction can be demonstrated using sodium chloride solution to hydrochloric acid and caustic soda as an example.

The stack consists of a bipolar membrane (both a cation- and anion-exchange membrane), a cation and an anion exchange membrane. The chloride ions move through the AEM to the anode in electrical field. They are retained in the cation selective side of the BM and together with the protons produced by the BM they build up hydrochloric acid.

Similarly caustic soda recombines on the anion selective side of the BM and an ion-depleted diluate remains in the neighboring feed solution chamber.