The phase and interfacial properties of azeotropic refrigerants: the prediction of aneotropes from molecular theory

Abstract

The use of hydrofluorocarbons (HFCs) as alternative non-ozone depleting refrigerants for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) has grown during the last couple of decades. Owing to their considerable global warming potential, a global deal has been reached recently to limit the production and consumption of HFCs. For rational design of new refrigerants that are environmentally friendlier, the thermodynamics of current ones need to be well understood first. In this work, we examine the phase behavior of azeotropic refrigerants obtained by mixing HFCs with normal alkanes. The vapor-liquid equilibria (VLE) of these binary systems exhibit positive deviation from Raoult's law in the bulk, and a negative deviation from surface ideality (aneotrope) at the interface. The phase equilibria, second order thermodynamic derivative properties and interfacial properties of these complex systems were studied here using a modified version of the Statistical Associating Fluid Theory (SAFT) combined with Density Gradient Theory (DGT). The model was able to accurately capture the azeotropic nature of the phase equilibria and predict their composition and pressure at temperatures where experimental data are limited. In addition, accurate descriptions of the interfacial tensions were also obtained when compared with available experimental data, predicting the minimum found in surface tension as a function of composition. The molecular-based theory allowed the calculation of interfacial properties for which there is no experimental data available yet. Predictions show that the aneotrope occurs at a lower HFC composition for R-152a and R-134a systems in comparison to R-143a and R-125 systems. According to the calculated density profiles, HFC molecules appear to be preferentially adsorbed at the interface causing the surface tension of the n-alkane rich phase to decrease at low HFC concentrations. At high HFC concentrations, the phenomenon is inverted and n-alkane molecules are preferentially adsorbed causing the surface tension of the HFC rich phase to decrease. Consequently, the aneotrope point can be defined as the state at which the surface activity of both molecules is identical, or the relative adsorption of one component versus the other at the interface becomes zero.

Next generation of low global warming potential refrigerants: Thermodynamic properties molecular modeling

“…The parameter c ij for the components i and j , or simply c , is known as the influence parameter . In this work, the influence parameter required by the theory was fitted to pure component surface tension data and then used to predict the interfacial properties of refrigerant blends; their values are provided in Table , where the values of c used for the other refrigerants as taken from published works in a transferable manner are also provided.…”

“…In a previous publication, it was shown that 1,1‐difluoroethane (HFC‐152a) and 1,1,1,2‐tetrafluoroethane (HFC‐134a) share great similarities in terms of their thermophysical properties. The same applies for 1,1,1‐trifluoroethane (HFC‐143a) and 1,1,1,2,2‐pentafluoroethane (HFC‐125).…”

“…Following our previous work on HFCs, we have investigated the effect of fluorine number on the phase behavior of the low GWP HFO‐1234yf/HFC blends. The binary interaction parameters used to correlate the VLE data are summarized in Table .…”

“…The remaining molecular parameters ( m , σ , and ɛ/k B ) needed by the equation of state were fitted to saturated liquid density and vapor pressure data. Pure component parameters for the HFCs and HCs were obtained from the work of Fouad and Vega and Gross and Sadowski, respectively, and are provided in Table for completeness.…”

“…This work belongs to a long‐term project on using molecular modeling tools for the rational design of new low GWP refrigerants, first focused on thermophysical properties. The Polar PC‐SAFT Equation of State (EoS) was used in our recent publication to study the phase and interfacial properties of HFC‐containing binary mixtures as a step toward understanding their performance based on their molecular structure. It was shown that HFC + HC binary systems exhibit strong deviations from solution and surface ideality.…”