Vapor-liquid equilibrium in systems (water + organic solvent + salt) at low water concentrations but high ratios of salt to water: experimental results and modeling

Water-free organic solvents are often required in chemical processes. It is common practice to dewater organic solvents by distillation. The operation of such distillation equipment often shows that the design specifications are not met as for example the amount of energy required to remove water to very small concentrations is much higher than expected from reliable data for the vapor-liquid equilibrium of the binary (water + organic solvent) system at low water concentrations. One of the reasons might be the presence of very small amounts of strong electrolytes-but at a rather high molar ratio of electrolyte to water. Under such conditions the ions might be hydrated resulting in a considerable reduction of the activity coefficient of water in the organic solvent. Neglecting that phenomenon in the design of the distillation equipment might result in serious operational problems, as for example the heat exchanger might not be suited to supply the increased energy consumption and/or the number of theoretical plates provided by the distillation equipment might be too small. A literature review revealed that no reliable experimental information on the influence of small amounts of strong electrolytes on the volatility of water in organic solvents at a comparatively high molar ratio of electrolyte to water is available. Furthermore, among the various methods published for describing the vapor-liquid equilibrium of aqueous-organic solutions of electrolytes only one method could be found that is able to describe the phenomenon qualitatively. Therefore, the described hydration phenomena can only be assumed to be the reason for the inappropriate functioning of some distillation equipment used for the dewatering of organic solvents. A real progress can only be expected when reliable experimental data on the influence of small amounts of strong electrolytes on the volatility of water in some organic solvents under such conditions (i.e. high molar ratios of electrolyte to water, but very low water concentrations) becomes available. The present work is aiming to provide such experimental data, which then can also be used to develop and test methods for correlating and predicting such phenomena.