| Low-concentration coal-bed methane with the properties of extensive storage and friendly to environment are being the focus of many investigations. Unfortunately, the utilization rate is not optimistic. In this paper, some attempts to enhance utilization for separating low-concentration coal-bed methane have been done. It is extremely significant for realizing the sustainable development of energy sector. Hydrate method that consumed lowly and degraded easily was significantly better than that of traditional methods including pressure swing adsorption, cryogenic distillation and membrane separation in separating low-concentration coal-bed methane; and it is easy to be generated under mild conditions and can be stored stably.In an attempt to reach the separation of coal-bed methane, hydrate formation for low-concentration coal-bed methane has been performed mainly being based on CH、N2、O2which are used for formatting hydrate under different temperature and pressure conditions. Thus, it is particularly crucial to study hydrate phase equilibrium thermodynamics of mixed gas. In this paper, we have mainly conducted on hydrate phase equilibrium of low-concentration coal-bed methane in different systems. The thermodynamic models were analyzed, compared, and combined with various equations of state, which are beneficial to the study of hydrate equilibrium conditions of different components in pure water system. Moreover, the best thermodynamic model was established by comprehensive analysis and evaluation. Interestingly, the gas hydrate equilibrium conditions for additive systems were evaluated to calculate the activity using optimistic thermodynamic model with UNIFAC method. Simultaneously, we have laid basic theoretical foundation to realize industrialized applications of hydrate formation of low-concentration coal-bed methane. Specific research work includes:(1) Three hydrate thermodynamic models (vdW-P model, Chen-Guo model and the fugacity model of water) with different mechanism are combined with varying equations of state (PR EOS,PT EOS and PRSV2EOS), and then utilizing Matlab programming to discuss hydrate equilibrium conditions of single-component (CH、N2、O2) gas and two-component (CH4-N2、N2-O2) gas in pure water system. Besides, the reliability of model was verified by comparison with experimental data, each model was analyzed and compared. We found that the third model prediction was consistent with experimental data; three hydrate thermodynamic models were of quite benefits to the study of hydrate equilibrium conditions of low-concentration coal-bed methane (CH4-N2-O2) in pure water system. The results showed that low-concentration coal-bed methane hydrate formation pressure increased exponentially with an increase in temperature, which also provided a theoretical reference for hydrate formation conditions of low-concentration coal-bed methane in pure water system.(2) Groups of THF-aqueous solution were divided by UNIFAC group contribution method to calculate activity of each composition in liquid phase, and study hydrate equilibrium conditions of single-component (CH、N2、O2) gas and two-component (CH4-N2、N2-O2) gas in the presence of THF using modified fugacity model of water. Lastly, values-calculated were compared with experimental data. The results showed that the model could well speculate hydrate equilibrium conditions in the presence of THF; furthermore, on the basis of the model, hydrate formation conditions of low-concentration coal-bed methane with three various THF concentrations (0.6%,1.8%,5.4%) were analyzed, discussed. The presence of THF could reduce gas hydrate equilibrium pressure obviously. When the temperature is at284K and THF at a concentration of5.4%, hydrate equilibrium pressure of low-concentration coal-bed methane was reduced by more than80%. And with an increase of THF contents in the solution, the amplitude of hydrate equilibrium pressure for low-concentration coal-bed methane decreases. |
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