Investigating the phase behavior of alcohol/water/salt mixtures at ambient conditions (alcohol: ethanol and isopropanol; salt: Na2CO3 and K2CO3)

Abstract

Alcohol has emerged as a promising alternative to fossil fuels in transportation due to lower CO2 emissions, compatibility with existing engine technologies and that can be produced from renewable resources. However, the separation of alcohol from water using conventional distillation is energy and carbon intensive. Salting-out, a liquid/liquid-based separation, has the potential to reduce the carbon footprint and cost associated with conventional distillation because it can be carried out at lower temperatures. Adding some salt to a mixture of alcohol and water reduces the solubility of the alcohol triggering a liquid/liquid (LL) phase split with one of the phases rich in alcohol. Therefore, understanding the phase behavior of alcohol/water/salt mixtures is essential for developing the salting-out separation technology. The objectives of this study were to map the phase behaviour of mixtures containing alcohol water and salt at ambient conditions. The alcohols tested were ethanol and isopropanol; and, the salts tested were sodium carbonate (Na2CO3) and potassium carbonate (K2CO3). The following phase boundaries were experimentally mapped in this study: liquid↔liquid-liquid (L↔LL) and liquid-liquid↔liquid-liquid-solid (LL↔LLS). The compositions of the two liquids phases at equilibrium within the liquid-liquid region were experimentally measured. The thermodynamic reliability of the collected experimental data (boundaries and liquid-liquid phase compositions) was assessed using an in-house graphical method. Note: all the data collected in this study were tested for thermodynamic reliability in order to ensure that all of it corresponds to actual thermodynamic equilibrium data. A simple modeling approach was developed for the calculation of the L↔LL and the liquid-liquid tie-lines. This model is based on empirical correlations fitted to the partitioning coefficients calculated from the experimental equilibrium compositions. This study is therefore aimed at producing reproducible, reliable and thermodynamically consistent data that can facilitate the modelling of the aforementioned systems for practical engineering applications.