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Modeling Gas Hydrates Containing Carbon Dioxide, Hydrogen and Methane Using Simplified Pc-Saft

Saravanan Swaminathan1, Michael L. Michelsen2, Georgios M. Kontogeorgis2, and Nicolas von Solms2. (1) IVC-SEP, Department of Chemical and Biochemical Engineering, Technical University of Denmark, Søltofts Plads, Building 229, room 254, Kgs. Lyngby, 2800, Denmark, (2) Department of Chemical and Biochemical Engineering, Technical University of Denmark, Kgs. Lyngby, 2800, Denmark

Gas Hydrates are ice-like crystalline solids formed with a water host-lattice that encages guest-gas atoms or molecules. Under the right conditions of temperature (low) and pressure (high), water molecules form lattice structures on hydrogen bonding, with cavities large enough to incorporate small gas molecules, thereby, providing a potential method for CO2 capture and sequestration. The other interest to studying these gas hydrates is to provide more insights in using the vast source of methane (natural gas) hydrates available in the subsurface. The added motivation to understand hydrates is the oil and gas industry, as a lot of money is spent in order to prevent the formation of natural gas hydrates as they clog and block the flowlines leading to disastrous economic ramifications.

The basis of modeling hydrate phase behavior is to use the van der Waals Platteeuw (vdW-P) model1 for the hydrate phase and to use an equation of state (EOS) (and/or activity coefficient model) to model the fluid phases (vapor and liquid). The vdW-P model was first published almost five decades ago, and generalized by Parrish and Prausnitz2 in the early 1970s. Various thermodynamic models have been used to describe the liquid and vapor phases depending on the authors. One of the important parts of modeling the hydrate phase behavior is describing the fluid phases accurately; the predictive ability of the over-all model is constrained by EOS used. Due to the nature of compounds present in these systems such as water, CO2, alcohols etc., it is essential to choose thermodynamic models that can describe the mixture very well and have good predictive nature. The statistical associating fluid theory (SAFT) developed based on the Wertheim's perturbation theory has been successful in modeling various associating systems with water, alcohols etc. Hence, it should be able to provide excellent representation of the thermodynamics of the guest gases. However, there have been very few attempts to use molecular-based equations of state such as the Statistical Associating Fluid theory (and its variants)3-5. One of the variants of SAFT, the Perturbed-Chain SAFT has been very successful in modeling various highly asymmetric and associating mixtures. A modification to PC-SAFT was proposed by von Solms et.al6 that reduced the computational times markedly for associating systems (which would be the case in hydrate systems). Hence, in this work, we use the Simplified PC-SAFT6 with the vdW-P to model the hydrate phase behavior with a better degree of predictability.

1. J.H. van der Waals and J.C. Platteeuw, Adv. Chem. Phys. 22-57 (1959)

2. W.R. Parrish and J.M. Prausnitz, Ind. Eng. Chem. Process. Des. Develop. 11 26-35 (1972)

3. X. Li, H. Wu, Y. Li, Z. Feng, L. Tang, and S. Fan, J. Chem. Therm., 39, 417- 425 (2007)

4. X. Li, H. Wu, and P. Englezos, Ind. Eng. Chem. Res., 45, 2131-2137 (2006).

5. A. Haslam, A. Galindo and G. Jackson, ‘Modelling gas clathrate hydrates: incorporating an advanced thermodynamic description of the fluid' presented at the Eleventh PPEPPD Conference in Crete, Greece (2007)

6. N. von Solms, M.L. Michelsen and G.M. Kontogeorgis, Ind. Eng. Chem. Res. 42, 1098-1105 (2003)