PREDICTION OF CO2 FREEZE-OUT CONDITIONS AND CO2 HYDRATE FORMATION CONDITIONS WITH A SINGLE EQUATION OF STATE

Abstract

A recent study by Eggeman and Chaffin (2005), which showed large discrepancies in CO2 freeze-out conditions as predicted by several commercial simulators, prompted a reexamination of using the TBS equation of state for phase equilibrium calculations involving solids. Salim and Trebble (1994) had previously presented a methodology for extending the Trebble-Bishnoi-Salim (TBS) equation of state (Salim, 1990) to calculations involving a solid phase. In this study, the CO2 freeze-out conditions in CO2/CH4 and CO2/C2H6 mixtures are calculated from the TBS equation of state, and it is shown that they provide a better data fit than the traditional Poynting correction method. Furthermore, since the use of an equation of state in SLE/SVE calculations does not require the explicit assumption of a pure solid phase, the model was assessed for its ability to correlate CO2 gas hydrate equilibrium conditions. Gas hydrates were simply treated as an impure solid phase, and it was seen that the predictions of gas hydrate equilibrium were in very good agreement with the experimental data. Computationally, the use of the TBS equation of state has the advantage, over the model of Yokozeki (2005), that it does not require a modifying factor (cb) in the repulsive term to handle the presence of hydrates; they are instead handled using a unique binary interaction parameter for the hydrate phase.