Gravity-induced density and concentration profiles in binary mixtures near gas–liquid critical lines

Abstract

We have calculated gravity-induced density and concentration gradients using scaled equations of state fashioned after that of Leung and Griffiths for binary mixtures near gas–liquid critical lines. The mixtures considered here are those of helium-3 and helium-4 and of carbon dioxide and ethane. Our calculations show that the density profiles for both mixtures in any proportion of the components are similar to those of pure fluids. The concentration gradients in the helium mixture have the same appearance as the density gradients. In the carbon dioxide–ethane system, however, the form of the concentration profile varies greatly, depending on the overall composition. Moreover, the temperature at which a mixture separates into two phases is slightly different from that expected for the mixture in the absence of gravity. We have also examined the case where a mixture is subjected to a large gravitational field such as can be generated in a centrifuge and found that, although the density gradient in all the mixtures is like that in pure fluids, the concentration gradients in the mixtures of carbon dioxide and ethane have complex features related to the presence of critical azeotropy.