Thermodynamic Models Describing Systems with Mixed Gas Hydrate Formation

本项目旨在描述流体相、纯固相（如干冰、固体水）和混合水合物固相的复杂相平衡。创新之处在于对流体相使用高精度的多参数状态方程，对其他纯固相使用最佳方程。由于处于平衡状态的流体相中水合物形成组分的逸度对水合物的化学势有贡献，因此准确和一致的水合物模型依赖于对流体相的准确描述。最近发表了八种纯天然气水合物形成物的热力学模型。新模型的一个重要优点是它能够准确地模拟各种相平衡，包括在宽温度和压力范围内的纯固相平衡。这个模型只能计算纯水合物。如果混合流体相包含不止一种水合物形成组分，则从混合流体相形成水合物导致混合水合物。该项目旨在将纯水合物模型增强到技术上更重要的混合水合物模型。基于项目第一阶段的良好成果，本项目第二阶段的目标与原提案中描述的概念基本一致。由于将模型扩展到混合水合物需要改变对纯水合物的描述，因此有必要修改参数。在修正模型时将考虑双人入住的结果。在纯水合物体系中不稳定的水合物结构可能出现在形成混合水合物的体系中。必须为此应用程序开发和优化考虑这种行为的稳定性分析。第一阶段开发的初步模型仅对含有CO2和CH4作为水合物形成物质的混合水合物体系进行了测试。将水合物形成组分CO2、CH4、C2H6、C3H8、N2和O2的二元组合扩展到本提案所述的第二个项目阶段。对系统中抑制剂的影响进行了定性准确的预测；预计将在即将到来的第二个项目阶段对这一主题进行更详细的研究。生成的算法和最终模型将在一个名为TREND的软件中实现，该软件易于无经验的用户操作，以确保模型的广泛分布。TREND是开源软件，已经被全世界60多个研究小组和公司使用。

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.