Thermodynamic Models Describing Systems with Mixed Gas Hydrate Formation
Final Report Abstract
In the course of this project, a recently developed model for hydrates of pure gases relevant mostly for CCS applications has successfully been extended in a way that allows for modeling of mixed hydrates. As was the case for the pure hydrates model, the model for mixed hydrates is compatible with highly-accurate multi-parameter equations of state for phases other than hydrate. A favorable consequence of the application of fundamental equations of state is that the parameters of the hydrate model, which were fitted primarily to phase equilibrium data, are to a much lesser extent affected by the uncertainties of the equations of state for non-hydrate phases. Since the model for gas hydrates directly depends on the fugacities obtained from the fluid or pure solid phase models, a more accurate description of phases other than hydrate has a positive impact on the accuracy of the description of the hydrate phase. The fitted parameters can thus be closer to the values given by their physical meanings and, consequently, the predictions for non-fitted properties become more plausible. Additionally, the model of this work was extended to represent high-pressure phase equilibria and hydrate composition data for which double cage occupancy occurs. Similar to other approaches in the literature, a term was added to the model that represents the influence of two guest molecules occupying one cavity. The model is simplified with the assumption that the distance between two guest molecules in one cavity is constant. By preserving the physical relation of the parameters of the hydrate model, it is possible to predict whether or not a certain type of hydrate cavities is double occupied. The pure hydrate models obtained in this way can be combined with the help of a simple mixing rule for the hydrate volume. No additional mixing parameters are required in the new model and consequently no model parameters have been fitted to mixed hydrate data. This is a big advantage for modeling mixed hydrates: In case new hydrate forming substances are appended to the model, the parameters representing the new substances have to be adjusted to experimental data of pure hydrates only. For most of the ternary mixtures, the new model is capable of predicting various types of phase equilibria over wide temperature and pressure ranges in good agreement with the experimental phase equilibrium data. For application of the new hydrate model, new phase equilibrium algorithms were developed allowing for the calculation of complex equilibria of up to four phases for multi-component systems forming gas hydrates and pure solid phases. A new stability analysis algorithm based on the search of the minimum in the Gibbs energy has been developed in order to model structure changes of the gas hydrates. The new model for mixed hydrates is implemented in the most recent version of the thermodynamic property software package TREND 4.0. For future research, there is potential for improvement with regard to the fluid phase equations, the hydrate model, and the phase equilibrium algorithms: The consideration of inhibitors is important for the simulation of technical applications, e.g., in flow assurance of pipelines. Recently, the Gibbs energy equation of state for seawater has been successfully implemented in a Helmholtz mixture model to represent the interaction of brines with CCS-relevant mixtures by Semrau et al. This model could be used as a good starting point for the investigation of the influence of electrolytes as inhibitors. - There are property models available for substances in the fluid phases, which are known to enter hydrate cavities and are not implemented in the hydrate model, e.g., hydrogen. This can cause confusions when using the hydrate model. An extension of the model to additional substances is recommended. - The lattice parameter of “the other” hydrate structure needs to be fitted to mixed hydrate data in order to improve the representation of mixtures with substances that are known to form different crystal structures for pure hydrates. This step requires a modification of the optimization algorithm in order to consider mixed hydrate data.
Publications
- Progress in modeling gas hydrates relevant for CCS using reference equations of state and extension of the model for mixed hydrates. IAPWS Annual Meeting, Dresden (2016)
S. Hielscher, A. Jäger, V. Vinš, R. Span, J. Hrubý, C. Breitkopf
- A New Approach to Model Mixed Hydrates Consistent with Multiparameter Equations of State. European Conference on Thermophysical Properties, Graz (2017)
S. Hielscher, A. Jäger, V. Vinš, C. Breitkopf, J. Hrubý, R. Span
- Model for gas hydrates applied to CCS systems part II. Fitting of parameters for models of hydrates of pure gases. Fluid Phase Equilibria 435, 104-117 (2017)
V. Vinš, A. Jäger, J. Hrubý, R. Span
(See online at https://doi.org/10.1016/j.fluid.2016.12.010) - Temperature and pressure correlation for volume of gas hydrates with crystal structures sI and sII, EPJ Web Conf. 143 (2017) 02141
V. Vinš, A. Jäger, S. Hielscher, R. Span, J. Hrubý, C. Breitkopf
(See online at https://doi.org/10.1051/epjconf/201714302141) - A new approach to model mixed hydrates, Fluid Phase Equilibria. 459 (2018) 170–185
S. Hielscher, V. Vinš, A. Jäger, J. Hrubý, C. Breitkopf, R. Span
(See online at https://doi.org/10.1016/j.fluid.2017.12.015) - A new Model for Mixed Hydrates Consistent with Multiparameter Equations of State. 20th Symposium on Thermophysical Properties, Boulder (2018)
S. Hielscher, A. Jäger, V. Vinš, C. Breitkopf, J. Hrubý, R. Span
- Modification of a model for mixed hydrates to represent double cage occupancy, Fluid Phase Equilibria. 490 (2019) 48–60
S. Hielscher, B. Semrau, A. Jäger, V. Vinš, C. Breitkopf, J. Hrubý, R. Span
(See online at https://doi.org/10.1016/j.fluid.2019.02.019)
