Prediction of Structural Changes in Gas Hydrate for Methane and Ethane Mixture by Using Tangent Plane Distance Minimization

In this study, the change in the crystalline structure of gas hydrate was predicted for ternary mixture of methane-ethane-water. For this purpose, the tangent plane distance (TPD) minimization method was used. First, the calculations were performed for the binary mixtures of methane-water and ethane-water as the gas and liquid phases. The results show that for a binary mixture of these components in the gas phase, at the temperatures of 274 K and 275 K, for a wide range of pressure, the phase remains stable; however, the liquid phase is unstable and it splits into two or more new phases. The method was used for any possible new phases and the stable new phases determined by TPD minimization. By using this method for the ternary mixture at 20 different points (pressure and composition) at the temperature of 277.6 K, the results for stable phases are found to be consistent with the literature; further, a structural change from sI to sII is observed at methane mole fraction of 0.74, and another structural change from sII to sI is observed at methane mole fraction of 0.99 at 277.6 K. This method can be used for the stability analysis of more points to predict the boundary of the phase diagram and stable zone for different phases. In this study, the genetic algorithm was used for minimization of TPD function.