Thermodynamic Models Describing Systems with Mixed Gas Hydrate Formation

• Applicant: Professor Dr.-Ing. Roland Span
• Subject Area: Technical Thermodynamics
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 273769352

This project aims to describe complex phase equilibria with fluid phases and pure solid phases (e.g., dry ice, solid water) and mixed hydrate solid phases. The innovation is to use highly accurate multiparameter equations of state for fluid phases and the best equations available for other pure solid phases. Since the fugacity of the hydrate forming components in the fluid phases in equilibrium yields a contribution to the chemical potential of the hydrate, accurate and consistent hydrate models depend on an accurate description of fluid phases. A thermodynamic model for eight pure gas hydrate formers has recently been published. An important advantage of the new model is its capability to accurately model a large variety of phase equilibria including those with pure solid phases over wide temperature and pressure ranges. This model enables the calculation of pure hydrates only. The formation of hydrates from a mixed fluid phase leads to mixed hydrates if the fluid phase contains more than just one hydrate forming component. This project addresses the enhancement of the model for pure hydrates to the technically more important mixed hydrates. Based on the promising results of the first project phase, the objectives for the second phase of this project are mostly in line with the concept described in the original proposal.Since the extension of the model to mixed hydrates required changes in the description of pure hydrates, a refit of the parameters is necessary. Results found for double occupancy will be considered in refitting the model.Hydrate structures, which are not stable in pure hydrate systems, may occur in systems forming mixed hydrates. A stability analysis taking this behavior into account has to be developed and optimized for this application. The preliminary model developed in phase one has been tested only for mixed hydrate systems containing CO2 and CH4 as hydrate forming substances. The extension to binary combinations of the hydrate forming components CO2, CH4, C2H6, C3H8, N2, and O2 will be considered in the second project phase addressed in this proposal.The influence of inhibitors in the system is predicted qualitatively correct; a more detailed study on this topic is foreseen for the upcoming second project phase. The resulting algorithms and the final model will be implemented in a software called TREND, which is easy to operate by unexperienced users, to ensure a broad distribution of the model. TREND is available as open source software and is already used by more than 60 research groups and companies all over the world.

• DFG Programme: Research Grants