A Simplified van der Waals-Platteeuw Model of Clathrate Hydrates with Multiple Occupancy of Cavities

In clathrate hydrates formed by small guest molecules such as H-2 or He, hydrate cavities may be occupied by clusters of several guest molecules. Multiple occupancy of cavities is important for applications of clathrate hydrates as gas storage and transportation media due to the increase of storage capacity of the material associated with multiple occupancy. Computational approaches for clathrate hydrates with multiple occupancy such as Grand Canonical Monte Carlo (GCMC) simulations or van der Waals-Platteeuw (vdW-P) models with rigorous calculations of Langmuir adsorption constants are complex and require considerable computational effort. In this work, a simplified vdW-P model for clathrate hydrates with multiple occupancy is presented. In this model, it is assumed that guest molecules inside cavities form clusters in which molecules occupy fixed positions with respect to each other. For validation of this supposition, results obtained with this model have been compared with GCMC simulations of sII He and H-2 hydrates with multiple occupancy. Results of the simplified procedure presented in this work regarding the fractional occupancy of cavities by molecular clusters agree well with GCMC simulations. The simplified vdW-P model presented in this work requires a small computational effort, equivalent to calculations with the standard vdW-P model for hydrates with single occupancy.