(187p) Modeling Phase Equilibrium with Wong-Sandler Mixing Rule for Ternary CO2/H2/C2H6 Hydrates | AIChE

(187p) Modeling Phase Equilibrium with Wong-Sandler Mixing Rule for Ternary CO2/H2/C2H6 Hydrates

Authors 

Thakre, N. - Presenter, Energy and Process Engineering Laboratory, Department of Chemical Engineering, Indian Institute of Technology?Kharagpur, India?721 302
Jana, A. K., IIT Kharagpur
Gas hydrate deposits in marine and permafrost region are believed to meet the worldwide energy demand for several decades. Besides energy production, the gas hydrate technology has plentiful industrial applications, e.g., carbon dioxide capture, desalination, gas storage, separation, transportation and so on. Hydrates are formed by encapsulating the small-sized molecules (guests) into the three-dimensional network of host (water) cages. The structures of the hydrates are specific to the interactions between guest and host molecules. Moreover, in case of mixed hydrates, a structural change is possible at different vapor phase composition (Babu et al., 2013). The presence of polar and asymmetric components may impart nonideality in the fluid phases, for which, the linear mixing rules are inappropriate at high and low density limits. In this regard, we introduce the Gibbs free energy based Wong-Sandler (WS) mixing rule (Wong and Sandler, 1992) in association with the Patel-Teja (PT) equation of state model. The Gibbs free energy is calculated using the non-random two-liquid (NRTL) activity coefficient model. The three phase (vapor-liquid-hydrate) equilibrium is estimated by the van der Waals-Platteeuw model, in which the Kihara cell potential method is chosen to calculate the cage occupancy. The model is validated with a ternary mixture hydrate of carbon dioxide, hydrogen and propane, which forms both sI and sII hydrate at different compositions, 0.8/0.188/0.012 and 0.381/0.594/0.025 mole fractions, respectively (Babu et al., 2013). The binary interaction coefficients are estimated by minimization of the average absolute relative deviation (AARD) in predicted and experimental pressure. The particle swarm optimization technique (particleswarm tool in MATLAB) is used for the same. Analysing the model performance, it is observed that the proposed PT-WS shows a better prediction (4.7% AARD) than the conventional van der Waals mixing rule based model (14% AARD). An accurate prediction of phase equilibrium of multicomponent hydrates using the proposed model can be useful in the hydrate based applications.

References

Babu, P., Yang, T., Veluswamy, H. P., Kumar, R., Linga, P. (2013). Hydrate phase equilibrium of ternary gas mixtures containing carbon dioxide, hydrogen and propane. The Journal of Chemical Thermodynamics, 61, 58-63.

Wong, D. S. H., Sandler, S. I. (1992). A theoretically correct mixing rule for cubic equations of state. AIChE Journal, 38(5), 671-680.

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