Production of salicylic acid in a three compartment bipolar membrane electrodialysis configuration

Salicylic acid is nowadays produced by the Kolbe-Schmitt reaction. Using this method, sodium salicylate is converted into salicylic acid by the addition of sulphuric acid. Electrodialysis is proposed for this production process and avoids the use of sulphuric acid and the production of an acidic sodium sulphate waste stream. The low solubility of salicylic acid in water can be overcome by using mixtures of water and a primary alcohol. However, preliminary results have indicated that bipolar membranes of large dimensions cannot be used in direct contact with these solvent mixtures. In this paper, a three compartment configuration is proposed, which resolves this problem by enclosing the solvent mixture with two cation-exchange membranes. An optimization of this configuration was performed by systematically varying different operating parameters. As the co-solvent, the use of methanol, ethanol and 1-propanol was tested. The results indicate that a high salicylate concentration, a low co-solvent concentration of 1-propanol and a low base concentration are the most optimal settings.