Study on Hydrate Phase Equilibrium Diagram of Methane Containing System Based on Thermodynamic Model

Hao Liang,Na Wei,Jun Pei,Yonggang Duan

doi:10.3389/fenrg.2021.743296

Abstract

Natural gas hydrate is a potential energy source in the future, which widely occurs in nature and industrial activities, and its formation and decomposition are identified by phase equilibrium. The calculation of multicomponent gas phase equilibrium is more complex than that of single component gas, which depends on the accurate model characterized by enthalpy and free energy. Based on the Kvamme-Tanaka statistical thermodynamic model, theoretical and experimental methods were used to predict and verify the phase equilibrium of pure methane hydrate and carbon dioxide hydrate in the temperature range of 273.17–289.05 K. The phase equilibrium curves of methane-containing gases such as CH4+CO2,CH4+C2H6,CH4+H2S and CH4+CO2+H2S under different mole fractions were drawn and analyzed, and the decomposition or formation enthalpy and free energy of hydrate were calculated. The results show that, the phase equilibrium curves of the methane containing systems is mainly related to the guest molecule type and the composition of gas. The evolution law of phase equilibrium pressure of different gases varies with composition and temperature, and the phase splitting of CO2at the quadruple point affects the phase equilibrium conditions. Due to the consideration of the interaction between the motion of guest molecules and the vibration of crystal lattice, the model exhibits a good performance, which is quantified in terms of mean square error (MSE) with respect to the experimental data. The magnitudes of MSE percent are respectively 1.2, 4.8, 15.12 and 9.20 MPa2for CH4+CO2, CH4+C2H6, CH4+H2S and CH4+CO2+H2S systems, and the values are as low as 3.57 and 1.32 MPa2for pure methane and carbon dioxide, respectively. This study provides engineers and researchers who want to consult the diagrams at any time with some new and accurate experimental data, calculated results and phase equilibrium curves. The research results are of great significance to the development and utilization of gas hydrate and the flow safety prediction of gas gathering and transportation.

Highlights

Natural gas hydrate (NGH) is a solid clathrate crystal material composed of water cages, which contain molecules such as hydrocarbons, carbon dioxide, hydrogen sulfide and other molecules, in which methane is the dominant gas (Vedachalam et al, 2015)
Taking CH4 and CO2 as examples, the phase equilibria of sI hydrate formed by pure gas were calculated by using the thermodynamic model mentioned above
Note that the black and blue lines respectively represent the equilibrium curves of CH4 hydrate and CO2 hydrate predicted in this paper, and the black and blue dots are the reference values of the phase equilibrium of CH4 hydrate and CO2 hydrate respectively (De Roo et al, 1983; Ohgaki et al, 1993; Mei et al, 1996; Fan and Guo, 1999; Seo et al, 2001; Yang et al, 2001; Sun et al, 2002; Mohammadi et al, 2005; Mu and von Solms, 2018a)

Summary

Introduction

Natural gas hydrate (NGH) is a solid clathrate crystal material composed of water cages, which contain molecules such as hydrocarbons, carbon dioxide, hydrogen sulfide and other molecules, in which methane is the dominant gas (Vedachalam et al, 2015). Collett et al estimate that the amount of natural gas stored in hydrate reservoirs in the world is between 2.8×105 and 8 × 108 m3 at standard conditions, which is a fairly high value (Collett, 2009). It has attracted great attention in Japan, China, South Korea, India and other countries with relatively scarce resources, where researchers are more interested in hydrates than in the United States, Canada and Europe in recent years (Zhao et al, 2019). The academia pay close attention to the phase equilibrium research of NGH, and the related work has been applied in many fields such as energy, chemical industry, bioengineering and environmental protection (Qorbani, 2017)

Methods

Results

Conclusion