Exploration of high pressure equilibrium separations of nitrogen and carbon dioxide

• Equilibrium separations of nitrogen and carbon dioxide at high pressures correspond with density driven concentration gradients.
• Equilibrium separation of nitrogen and carbon dioxide increases with increasing pressure and decreasing temperature.
• High Grashof numbers correspond with effective separations in cases of natural convection.

The separation of a supercritical mixture of nitrogen and carbon dioxide is explored at pressures from 10 to 31 MPa and temperatures of −5 °C and 23 °C. An “ideal interface” like that between water and air at ambient conditions does not form. Rather, a concentration gradient is observed. Nitrogen concentration increases toward the top of the separation vessel while carbon dioxide concentration decreases. In this way, the mole fraction of nitrogen at the top of the mixture is close to one and the concentration of carbon dioxide at the bottom of the mixture also approaches one. This concentration gradient is the basis of an effective separation. As the system pressure increases and/or the system temperature decreases, this density driven concentration gradient becomes less pronounced while consequently, the separation effectiveness is increased. The time to achieve maximum separation is quantified at different conditions, and found to decrease with increasing pressure. At the conditions studied, nitrogen is a supercritical fluid as a pure component while carbon dioxide is a liquid as a pure component, however the mixture is considered supercritical. The observation of a concentration gradient, and thus separation, is due to nonhomogeneities in the supercritical fluid mixture with density driven concentration gradients. This kind of separation has applications in continuous processing of pressurized gasification products.

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