Extractant screening for the separation of dichloroacetic acid from monochloroacetic acid by extractive distillation

Monochloroacetic acid (MCA) is produced via the chlorination of acetic acid, in which a part is overchlorinated to the undesired dichloroacetic acid (DCA). The separation of DCA from MCA by distillation is highly energy intensive, because of the rather low relative volatility of ∼1.05–1.3, depending on the mixture composition. Extractive distillation is a promising alternative and often applied to separate close boiling mixtures. The benchmark solvent sulfolane is known to increase the relative volatility of the MCA/DCA mixture only slightly. By applying basic complexing agents, the large difference in the acid dissociation constant between MCA (pKa = 2.87) and DCA (pKa = 1.25) can be exploited to further enhance the relative volatility of the MCA/DCA mixture. The aim of this study was to select a proper complexing agent. Such a complexing agent should not only enhance the relative volatility more than obtained with sulfolane, but also be stable in the presence of MCA and DCA, and the complexation should be reversible. To study on the relative volatility and the reversibility of complexation, vapor–liquid equilibrium (VLE) and thermal/chemical stability experiments were performed. Many extractants were found that improve the relative volatility more than sulfolane, with relative volatilities up to 5.9. There is, however, a clear trade-off between the effect of the extractant on the relative volatility of the MCA/DCA mixture and the regeneration ability of the extractant. Extractants with a strong effect on the relative volatility of the MCA/DCA mixture appeared difficult to regenerate. Complexation agents from the classes of ethers, ketones, and phosphine oxides, and the benchmark extractant sulfolane were the only extractants that demonstrated to be thermally/chemically stable in the strongly acidic environment. With regard to the relative volatility, the regeneration ability, and the stability of the extractants, it was concluded that glymes, e.g. diethylene glycol dipentyl ether are the most promising extractants, improving the relative volatility of the MCA/DCA system up to 2.1–2.4 at a DCA/extractant molar ratio of 1.