Physicochemical and Thermodynamic Investigation of Ethanolic Solution of Phosphonium-Based Ionic Liquids-Measurements, Correlations, and Application to Absorption Cycles

The main objective of this work is to search for ionic liquid systems as a new generation of working fluids in absorption refrigeration (AR) technology. In this research, physicochemical and thermodynamic properties, including (vapor + liquid) equilibria (VLE), density (rho), and dynamic viscosity (eta), of ethanolic solutions of IL are presented. The proposed working fluids are composed of ethanol as a refrigerant and the following ILs: 1,3-dimethylimidazolium dimethyl phosphate, [C1C1IM][DMP], 1-ethyl-3-methylimidazolium dimethyl phosphate, [C1C2IM][DMP], 1-methylpyridinium dimethyl phosphate, and [C1Py][DMP] as absorbent. The isothermal VLE has been measured by an ebuliometric method within a temperature range of T = (328.15-348.15) K and pressures up to 90 kPa. Experimental VLE data were correlated using the Redlich-Kwong equation of state. Based on the experimental VLE and available literature data for ethanolic solutions of different ILs, the modeling of the theoretical cycles using working fluids proposed here was performed. The calculated values of the coefficient of performance (COP) were compared to other ILs from the available literature and {LiBr + water} system conventionally used on the industrial scale in AR technology. From the point of view of the future application of the proposed systems in a real technological process, the physicochemical characteristics play a significant role. In this work, the density and viscosity of binary solutions were determined as a function of the IL mole fraction over a wide temperature range of T = (293.15-338.15) K and under ambient pressure. The empirical equations were applied for density and viscosity data correlation. Additionally, the synthesis procedure and NMR analysis of 1-methylpyridinium dimethyl phosphate, as well as the thermophysical characterization using the differential scanning calorimetry (DSC) technique of each IL under study, are presented. The work presented here summarizes research on a new generation of working fluids in absorption refrigeration technology based on DMP ionic liquids as absorbers and ethanol as a circulating medium. The discussion concerns the influence of the IL's cation structure on the thermodynamic and physicochemical characteristics of the analyzed systems.