Solubilities and selectivities in supercritical fluid mixtures near critical end points

Abstract

Solubilities and selectivities were measured for hydrocarbon solids in pure and mixed supercritical fluids near multicomponent upper critical end points (UCEPs). Densities of both the mixed solvents and saturated solutions were also measured in order to interpret the results. Selectivities do not change significantly with pressure or temperature, even in the vicinity of the UCEP, for the naphthalene-2,6-dimethylnaphthalene-CO 2 system. In solid- cosolvent-CO 2 systems, solubilities are enhanced and highly adjustable with small changes in pressure near the UCEP. The logarithm of the solubility is linear in density, even in the complex UCEP region, which provides a means for data correlation. The cosolvent offers a potential benefit in that the UCEP pressure is reduced by 50%, for example,with the addition of 8 mol% n-pentane to the naphthalene-CO 2 system. At moderate pressures, the slopes of solubility versus pressure isotherms can be increased in some cases with a cosolvent owing to this pressure reduction in the UCEP. This increase is explained in terms of the measured values of the apparent solute molar volume. In the 2-naphthol-phenanthrene-methanol-CO 2 system, the selectivity varies by 50% as the pressure changes by 200 bar, whereas this variation is much smaller for systems that are further away from the UCEP. The operation of a separation process in the UCEP region leads to higher yields at lower pressures, and for particular systems may also lead to more selective extraction or product recovery.