Experimental investigation of critical parameters for the binary mixture of CO₂ + R1336mzz(Z) (cis-1,1,1,4,4,4-hexafluoro-2-butene) and analysis of critical locus for CO₂ + hydrofluoroolefin systems

The transcritical CO₂ power cycle has been recognized as a highly promising technique for reducing carbon emissions in heat utilization. However, this technology is limited because of the critical parameters of pure CO₂ (including low critical temperature and high critical pressure). To overcome these limitations, one potential solution is to introduce a new working fluid consisting of a blend of hydrofluoroolefins (HFOs) and CO₂. Precise knowledge of the critical parameters is essential for accurately evaluating the effectiveness and capacity of CO₂+HFO mixtures. In this study, the critical properties of the CO₂ + R1336mzz(Z) (cis-1,1,1,4,4,4-hexafluoro-2-butene) blend were determined using a metal-bellows variable volumeter. The critical point was identified by visually observing the critical opalescence and the reappearance of the vapor-liquid meniscus. The Modified Wilson method and the Redlich-Kister method were used to fit critical data. Our results reveal that the CO₂ + R1336mzz(Z) mixture can elevate the critical temperature in comparison to pure CO₂. The highest critical pressure is achieved when the mole fraction of CO₂ is approximately 0.82. For the critical temperature, critical pressure, critical density, and mole fraction, the expanded combined uncertainties were below 50 mK, 21 kPa, 0.6 %, and 0.004 (k = 2, 95 %), respectively. Meanwhile, the Modified Extended Chueh-Prausnitz (MECP) method and a simplified MECP method were used to predict critical properties of the CO₂ + R1336mzz(Z) binary mixture. Finally, the critical locus of the CO₂ + R1336mzz(Z) mixture was compared with other CO₂ + HFO mixtures.