Solid-Vapor-Liquid Equilibrium

The use of low-temperature processes has recently received great attention as an effective method for removing CO₂ from flue gases or from natural gas to produce pipeline quality natural gas. These processes are particularly interesting when the concentration of carbon dioxide is high. They present other advantages, including the production of liquid CO₂ which can be readily transported through pipelines or used for EOR. Some of these processes (CO₂ capture by anti-sublimation and the CFZ^TM technology) can be classified as solid-based processes, whereas other processes (the Ryan-Homes process, the Dual Pressure Low-Temperature process) avoid the formation of dry ice.

In all these cases, the prediction of CO₂ freezing boundaries is a key point for process design, and requires to perform equilibrium calculations involving the solid phase. There exist two possible approaches. The Classical Method expresses the fugacity of the freezing component in the solid phase in terms of that in the liquid phase and of the freezing component’s properties, using a cubic equation of state. An alternative solution consists in using an analytical equation of state like the one proposed by Yokozeki (Yokozeki, 2003), which can simultaneously model all three phases (solid, liquid and vapor); it can also be used for gas-hydrates modeling.

The GASP group is currently working on both these approaches for the prediction of CO₂ freezing points in systems containing compounds of interest (in addition to CH₄, H₂S, nitrogen, and others).

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