The Cubic – Plus – Association (CPA) equation of State [1] combines the well known SRK EoS for describing the physical interactions with the Wertheim's association term (typically applied in SAFT type model's), which explicitly accounts for hydrogen bonding. The fact that the CPA EoS reduces to the classical SRK EoS for mixtures of hydrocarbons, makes the model desirable for industrial applications. In this work CPA EoS has been applied to two different families of phase equilibria which are of importance to the oil and gas industry. First, we consider liquid – liquid equilibria of binary and ternary systems containing glycol/aromatic hydrocarbons and water, which are of importance when drying the gas in dehydration units. New experimental data over an extended temperature range are obtained using gas-chromatography for analyzing the traces of glycols and hydrocarbons and Karl-Fisher titration for the traces of water. The modeling part validates the approach recently presented by Folas et.al.[2], in order to account for the solvation between glycols, water and the electronegative ring of aromatic HC's. The results with the CPA EoS are compared to those obtained with conventional engineering tools (EoS/GE models), which were previously found successful for applications to mixtures with methanol as hydrate inhibitor [3]. Then, we focus on the prediction of gas phase water content in mixtures of high pressure methane, nitrogen and natural gas mixtures. A new approach of modeling the hypothetical empty hydrate phase as a solid phase is implemented within a thermodynamically consistent framework, where the CPA EoS is used to obtain the properties of both the liquid and vapor phase. It is shown that this approach enables accurate water content predictions between the gas phase and the stable solid phase (liquid, hydrate or ice), similar and occasionally better to those obtained using the ISO-standard GERG model [4]. Furthermore, satisfactory predictions of gas hydrate formation conditions are obtained for single hydrates or mixtures with inhibitors.

References

1. G.M. Kontogeorgis, E.C. Voutsas, I.V. Yakoumis, D.P.Tassios, Ind. Eng. Chem. Res. 35, (1996), 4310-4318. 2. G.K. Folas, M.L. Michelsen, G.M. Kontogeorgis, E.H. Stenby, Ind. Eng. Chem. Res. 45, (2006), 1527 – 1538. 3. Pedersen, KS.; Michelsen, M.L.; Fredheim, A.O. Phase equilibrium calculations for unprocessed well streams containing hydrate inhibitors. Fluid Phase Equilib. 1996, 126, 13 4. ISO 18453, Natural Gas – Correlation between water content and water dew point.