Effect of dissolved inorganic salts on the isothermal vapor-liquid equilibrium of the propionic acid-water mixture

Propionic acid-water is one of the minimum boiling point azeotropic binary mixtures. Alteration of the vapor-liquid equilibrium (VLE) by addition of inorganic salts is of interest, since it allows bypassing the azeotropic point due to salt-in and salt-out effects. The effect of NaCl, NH₄Cl, CaCl₂, and AlCl₃ at salt concentrations of 1 m and the effect of NaCl molality (0.5-3 m) on the VLE of the binary propionic acid-water mixture has been experimentally investigated at 60°C using the headspace gas chromatography (HSGC) technique. The alteration in the VLE of the system was salt type and salt concentration dependent. Salting out of the propionic acid was observed when salts were added and was most obvious in the case of AlCl₃ salt. The salt effect parameter, determined from the Furter's equation was a function of the liquid concentration.