Activity coefficient prediction of hydrofluorocarbon + ionic liquid systems by UNIFAC model for working-fluids selection of compression and absorption hybrid cycles


L. Dong1, X. Wu1 and D. Zheng1

1Beijing University of Chemical Technology, P.R. China

Keywords: vapor-liquid equilibria
property: activity coefficient, excess Gibbs function
material: hydrofluorocarbons, ionic liquids

In recent years, ionic liquids (ILs) have attracted considerable attention for their potential to be used as novel absorbent species of the absorption cycle working fluids. Researchers have investigated various refrigerant and IL systems, involving H2O, NH3, various hydrofluorocarbons (HFCs), CO2, 2,2,2-trifluoroethanol and other refrigerants. In order to select appropriate working pairs, the extremum value of the excess Gibbs function and the infinite dilution activity coefficients γi∞ have been using, which can be obtained from vapor-liquid equilibrium (VLE) data. The experimental data only at a limited series of systems are available from the literatures. As well-known, the UNIFAC model is a very helpful tool to predict the γi values for those interested systems rapidly and to reduce the amount of experimental work. In the UNIFAC model, the group parameters and interaction parameters calculated from the VLE data have been obtained and tabulated, so that the unknown solutions, in the absence of experimental data, can still be estimated γi with reasonable accuracy. Lei et al. added the group parameters for 12 main groups and 24 subgroups including ILs, CH2, CH3OH, H2O into the current UNIFAC parameter matrix by means of correlating the γi∞ in ILs at different temperatures. In previous work, we investigated the thermophysical properties of HFC-IL systems. Taking HFCs as refrigerants for compression/ absorption hybrid cycles has been considered suitable. This study aims at a further develop of model of the group contribution type that can predict the solubility data for working fluids of HFC and IL systems even no experimental data are available. The investigation is based on solubility data for 19 systems selected from the combinations of nine different HFCs in eight different ILs. To describe the solubility data for the 19 systems, 22 general parameters were extended the group parameters of ILs in the current UNIFAC parameter matrix with an average relative deviation(ARD) of less than 7.5 %. In prediction work of HFC + IL systems, the experimental data fall close to the illustration obtained by the UNIFAC model. The ARD value of pressures is 2.64 % for the HFC-32 with [Bmim]PF6 system at the temperatures between (283.15 and 348.15) K. And the predicted γ1∞ values of six HFCs in the IL [Bmim]PF6 show that the γ1∞ values of HFCs almost increase in the following order: HFC-32< HFC-152a< HFC-23< HFC-134a< HFC-125< HFC-143a. The result is in accordance with solubility experimental investigations by Shiflett. The γ1∞ values decrease with increasing the solubilities of HFC in IL.

References
  1. Lei Z. G., et al.. Ind. Eng. Chem. Res., 2009, 48, 2697-2704.

  2. Shiflett M. B., et al,. AIChE J., 2006, 52, 1205-1219

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