Framework for clathrate hydrate flash calculations and implications on the crystal structure and final equilibrium of mixed hydrates

Clathrate hydrates, usually called gas hydrates, are compounds of great interest in oil industry, as well as in gas separation and storage, water purification etc ... Like many other compounds that phase change, in this case from liquid water to non stoichiometric crystalline compound, modeling is required to understand and optimize the processes that involve them. Therefore, the classic thermodynamic equilibrium model is combined with mass balance calculations during gas hydrate crystallization. Two frameworks for performing clathrate hydrate thermodynamic flash calculations at constant volume are presented and compared to experimental results at low crystallization rate. The inputs are the quantity of mass (water and gas molecules), and the volume. The variable is temperature (three phases thermodynamic flash at a given temperature), while the volume is kept constant. The first framework suggests that the hydrate phase is growing at local thermodynamic equilibrium, without any reorganization of its content of the occupancy of the cavities. In the second framework, the hydrate phase can reorganize itself during growth (locally or completely). These frameworks are investigated, as well as the impact of the Kihara parameters uncertainties. These frameworks calculate well the final pressure, hydrate composition. In addition, the hydrate volume and mole amount in each phase is provided with reasonable accuracy. Note: uncertainties on the final pressure and hydrate volume are below 5%. Moreover, the results are quite sensitives to the value of the Kihara parameters, demonstrating the importance of their values for a given computer code. This work provides a more reallistic and comprehensive view of gas hydrate crystallization. (C) 2015 Elsevier B.V. All rights reserved.