Recently, the mixture of carbon dioxide (CO₂) and dimethyl ether (DME) has been considered as promising alternative working fluid for heat pumps and refrigeration systems. Pure CO₂ has gotten a lot of attention as natural, nontoxic, and inflammable working fluid, but heat pumps using CO₂ need to be operated at a very high pressure (usually over 10 MPa). The mixing with DME can reduce the high operating pressure. Pure DME has also found commercial use as a refrigerant. By mixing with CO₂, the flammability of DME would be reduced.

This work deals with the modeling of the vapor-liquid equilibrium (VLE) of the CO₂ - DME mixture. Three different approaches are formulated and compared. The first two approaches are based on semi-empirical Peng-Robinson-Stryjek-Vera (PRSV) equation of state [1]. The first approach applies the simple mixing rule with only one interaction parameter, and the second incorporates the Wong-Sandler mixing rule [2] coupled with the NRTL active coefficient model [3]. The final approach uses a mixture model based on empirical Helmholtz free energy equations of state for pure CO₂ and DME [4,5]. A comparison of these three methods shows the superiority of the PRSV equation of state with the WS mixing rule and the NRTL model in predicting the VLE of the mixture up to very high temperatures and pressures.

[1] R. Stryjek and J. H. Vera, Can. J. Chem. Eng., 64 323 (1986).

[2] D. S. Wong and S. I. Sandler, AIChE J., 38 671 (1992).

[3] H. Renon and J. M. Prausnitz, AIChE J., 14 135 (1968).

[4] R. Span and W. Wagner, J. Phys. Chem. Ref. Data, 25 1509 (1996).

[5] E. C. Ihmels and E. W. Lemmon, Fluid Phase Equilib., 260 36 (2007).