NEAR-CRITICAL AND SUPERCRITICAL-FLUID DENSITIES OF CO2-SF6 MIXTURES

A recently proposed technique for the separation of the major recyclable thermoplastics incorporates the variable density of CO2-SF6 mixtures at mild temperatures and elevated pressures. Therefore, near-critical liquid and supercritical fluid densities of these mixtures were determined at temperatures between 294 K and 338 K and pressures up to 47 MPa. Mixtures containing SF6 mole fractions from 0.230-0.849 were used. The temperature-pressure conditions required to attain a specified density were determined in a high-pressure, variable-volume cell by isothermally compressing a fluid of known composition until a glass density float remained suspended in the fluid. The densities of the eight floats used in this study ranged from 900.0 to 1449.4 kg M-3. The results were correlated with the modified Lawal-Lake-Silberberg (MLLS) EOS, designed specifically for accurate near-critical liquid and supercritical fluid phase densities, the mean field lattice gas (MFLG) EOS and the Benedict-Webb-Rubin-Starling (BWRS) EOS. Both the MLLS and the MFLG equations had three adjustable parameters for mixture density correlations, while the BWRS had only one. Better results for mixture densities were obtained with the MLLS EOS, which yielded an average absolute pressure deviation (AAPD) of 4.49% at a specified temperature, density and composition. The AAPDs for the MFLG and the BWRS EOS were 7.07% and 7.64%, respectively.