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-hexafluoro2-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.