Thermodynamic and kinetic modelling of mixed CH4-THF hydrate for methane storage application

Solidified natural gas (SNG) is a promising solution to long standing NG storage problem. NG hydrate formation is the most significant step in the application of SNG technology. For improving the hydrate forming conditions, THF has been shown to be effective as a thermodynamic promoter. For optimization and comprehensive evaluation of SNG technology, it is essential to model the hydrate formation accurately. In current study, we undertake CH4-THF uptake modelling by first performing thermodynamic modelling. (L-H-G/V) equilibrium is estimated based on the equality of fugacity of water in aqueous phase and hydrate phase. Non-random two-liquid (NRTL) model is used for the liquid phase and van der Waals and Platteeuw (vdW-P) model for representing the hydrate phase fugacity. The model is then validated for CH 4 -THF and CO2 -THF systems with varying THF concentrations. The model performs with AARD-P of similar to 5% for both the systems. Subsequently, we propose a 2 step CH4-THF hydrate growth model with THF predominantly occupying the large cages of the resultant sII hydrate and CH4 subsequently diffusing into the hydrate phase to occupy the small cages. We propose a mass transfer based model in which CH4 diffuses through the layer of THF hydrate and gets trapped in the cages subsequently. Model is formulated and validated against CH4-THF hydrate formation kinetic experimental data obtained from our research lab's recent study (Veluswamy et al., 2016). Finally, insights from the model are discussed and optimum reactor configuration for SNG application are obtained.