Prediction Of Natural Gas Hydrate Phase Equilibria Based On Different Combination Methods

Natual gas hydrates,whose reserves are very abundant,are expected to a kind of potential energy.Moreover,the natural gas hydrates are more environmentally friendly than the traditional energy such as coal and oil in engineering applications.The prediction of the phase equilibria of natural gas hydrates,which is the research that calculating the phase equilibrium temperature and pressure by describing the mechanism of natural gas hydrates formation and decomposition,is the basics of the experimental and exploitation researches.In this paper,a new research approach is introduced for the prediction of natural gas hydrates phase equilibria.Different from the previous research approaches for the prediction of gas hydrates phase equilibria,the research approach introduced in this paper comprehensively evaluates all the possible methods’ combinations of the methods of the component fugacities and the methods of the Langmuir constant,and considers the influence of the types of gas hydrates and the conditions of temperature or pressure.Based on the Van der Waals-Platteeuw model,five kinds of candidate equations of state(EoSs)including the RK EoS,SRK EoS,PR EoS,PRSV EoS and BWRS EoS are adopted to calculate the gas component fugacity in non-hydrate phases.Four methods including the Parrish-Prausnitz model(is improved by Munck),the Parrish-Prausnitz model(is improved by Moradi),the Du-Guo model and the explicit pressure-dependent Langmuir constant(Hsieh’s model)are used to calculate the Langmuir constant.We get twenty methods’ combinations by selecting one EoS from the four candidate EoSs and one model from the three candidate models calculating Langmuir constant.In order to illustrate the research approach introduced in the paper,the phase equilibrium conditions of two different gas hydrate systems are predicted.For the first gas hydrate systems,the twenty methods’combinations are applied to calculate the phase equilibrium pressures of pure gas hydrates(CH4,C2H6,C3H8 and CO2)at different temperature ranges.For the other gas hydrate systems,the twenty methods’ combinations are used to calculate the phase equilibrium pressures of binary gas hydrates(CH4-C2H6,CHa-C3H8 and CH4-CO2)under dfferent gas ratioes and at different temperature ranges.Then the performance of these twenty methods combinations is evaluated for each gas hyardte over different specific temperature ranges.The comparison between the phase equilibrium pressures calculated by these twenty methods’ combinations and the experimental data demonstrates the phenomenon that the accuracies of these methods’combinations not only depend on the types of gas hydrates(and the ratioes of binary guest gases),but also depend on the conditions of temperature or pressure.Different from the previous knowledge,we cannot expect to use one methods’ combination to predict the phase equilibria of all the gas hydrates at different temperature ranges with the most accuracy.By analyzing the most accurate methods’ combinations obtained for different gas hydrates at different temperature ranges,we find out that all the best combination of CH4,C2H6,CO2,CH4-C2H6 and CH4-CO2(when the ratio of CH4 is less than 56.4%)gas hydrates contain the Du-Guo model,while all the best combination of C3H8 gas hydrates contain the Hsieh’s model.The accuracies of methods,combinations for the prediction of binary gas hydrates phase equilibria is not the sum of each accuracy of the pure gas hydrate.Moreover,for the improvement of the prediction accuracy of gas hydrates phase equilibria,the research approach points out an effective approach that we can focus on improving the accuracies of these most accurate methods’ combinations.