Journal of Chemical and Engineering Data, Vol.64, No.8, 3493-3504, 2019
Consistent Enthalpies of the Hydrate Formation and Dissociation Using Residual Thermodynamics
Kinetics of the hydrate phase transition is implicit functions of the free-energy change of the phase transition and the related dynamics of associated mass and heat transport. The enthalpy change of any phase transition is trivially coupled to the free-energy change of the phase transition. We propose a consistent scheme for calculating the free-energy change, as well as the associated enthalpy change, using residual thermodynamics for every component in all phases, this includes the hydrate phase. The proposed method does not have any limitations to pure components, although we focus on pure CH4 and pure CO2 hydrates in this work. One reason is that there are some experimental data for these components. But the main reason is that the work presented here is a part of a project related to the use of CO2 for simultaneous production of CH4 from natural gas hydrates and safe long-term storage of CO2 in the form of hydrate. A fast mechanism for this exchange entails the formation of a new carbon dioxide (CO2) hydrate from the injected CO2 gas. The exothermic heat of formation released from this phase transition will help to dissociate the in situ methane (CH4) hydrate. A brief review of the available experimental data from the open literature reveals the lack of important information in the reported data. Typical missing information are hydrate composition, pressure and/or temperature, as well as details on how the experiments have been conducted. Indirect methods that utilize measured or calculated hydrate pressure temperature equilibrium curves are frequently oversimplified and of limited accuracy. Yet another advantage of the proposed method is that it is not limited to heterogeneous hydrate formation from water and a separate hydrate phase. Enthalpy change related to homogeneous hydrate formation from dissolved hydrate formers in water can also be calculated from the proposed approach.