Measurement of critical properties for the binary mixture of R744 (carbon dioxide) + R1234ze(E) (trans-1,3,3,3-tetrafluoropropene)

In the context of global warming and demanding alternative refrigerants, the transcritical CO₂ refrigeration cycle was considered a potentially energy-saving and carbon-reducing technology. The use of CO₂ mixtures with higher critical temperatures and lower critical pressures has been demonstrated as an effective way to improve the performance of transcritical CO₂ refrigeration cycles. To achieve this approach, accurate critical locus parameters are needed for establishing cross-over state equations and calculating thermodynamic properties in the near-critical region. Currently, experimental methods are the most effective way to obtain high-precision critical locus parameters of mixtures. In this work, the critical properties including critical temperature, critical pressure, critical density, and mole fraction of the R744 (carbon dioxide) + R1234ze(E) (trans-1,3,3,3-tetrafluoropropene) binary mixture were measured according to a variable-volume method with a metal-bellows volumeter. The critical point was determined by visual observation of the critical opalescence and reappearance of the vapor-liquid meniscus. The combined expanded measurement (relative) uncertainties of the critical temperature, critical pressure, critical density, and mole fraction were within 50 mK, 0.021 MPa, 0.6%, and 0.012 (k = 2, 0.95 level of confidence), respectively. After comparing the fitting results of the Modified Chueh-Prausnitz (MCP) method, the Modified Wilson (MW) method, and the Redlich-Kister (RK) method. We recommended the Modified Wilson method to correlate the critical properties of the binary system of R1234ze(E)+R744. Our research showed that R1234ze(E)+R744 mixture can achieve a higher critical temperature and critical density simultaneously and a lower critical pressure compared with pure CO₂ when the composition of R744 is less than 0.23.