Phase behavior of clathrate hydrates: a model for single and multiple gas component hydrates

Presented here is a model that accurately predicts equilibrium pressures as a function of temperature of hydrates with CH4, C2H6, C3H8, N-2, H-2, and CO2 and their mixtures as guests. The model parameters fit to a subset of the equilibrium pressure data for single guest hydrates allow the prediction of phase behavior in mixed guest hydrates. For single guest hydrates, our model improves upon the van der Waals and Platteeuw (vdWP) model with a percent absolute average deviation, (%AAD) from all equilibrium pressure data of 5.7% compared to 15.1% for the vdWP model. Predictions of equilibrium pressures for all available mixed guest hydrates result in a 11.6%AAD with our fugacity-based model compared to 18.6% for the vdWP model. Also, our model leads to a prediction of the structure change of the methane-ethane hydrate within 5% of its known equilibrium composition in the vapor phase without any adjustment of its parameters. We have also found that at temperatures above 300 K, double occupancy of nitrogen in the large cavity of structure II hydrate is important for the prediction of accurate equilibrium pressures. (C) 2002 Elsevier Science Ltd. All rights reserved.