Chun-Gang Xu

Research progress of hydrate-based CO2 separation and capture from gas mixtures

Hydrate-based CO2 separation and capture from gas mixtures containing CO2 has gained growing attention as a new technology for gas separation, and it is of significance for reducing anthropogenic CO2 emissions. Previous studies of the technology include the thermodynamics and kinetics of hydrate formation/dissociation, hydrate formation additives, analytical methods, separation and capture progress, equipment and applications. Presently, the technology is still in the experimental research stages, and there are few reports of industrial application. This review examines research progress in the hydrate formation process and analytical methods with a special focus on laboratory studies, including the knowledge developed in analog computation, laboratory experiments, and industrial simulation. By comparing the various studies, we propose original comments and suggestions on further developing hydrate-based CO2 separation and capture technology.

Research progress on methane production from natural gas hydrates

Due to the consumption of fossil fuels, an alternative energy source is necessary for the world’s continuous development. Methane hydrates, a vast energy resource that exists in deep-ocean or permafrost sediments containing approximately 10 000 Gt of carbon, are a potential energy source for the future. However, economically and safely producing methane from gas hydrate deposits is still not on the drawing board. The main reasons include (1) low methane production efficiency, (2) low methane production, (3) poor production sustainability. Thus, it is pressing to develop methane production technology and/or approaches to improve methane production efficiency. In this paper, we comprehensively review the research on methane production from gas hydrates, including the research on the characteristics of gas hydrate reservoirs, production methods, numerical simulations and field production tests. The different investigations are analyzed and relevant comments and suggestions are proposed accordingly.

Hydrate-based carbon dioxide capture from simulated integrated gasification combined cycle gas

Abstract The equilibrium hydrate formation conditions for CO 2 /H 2 gas mixtures with different CO 2 concentrations in 0.29 mol% TBAB aqueous solution are firstly measured. The results illustrate that the equilibrium hydrate formation pressure increases remarkably with the decrease of CO 2 concentration in the gas mixture. Based on the phase equilibrium data, a three stages hydrate CO 2 separation from integrated gasification combined cycle (IGCC) synthesis gas is investigated. Because the separation efficiency is quite low for the third hydrate separation, a hybrid CO 2 separation process of two hydrate stages in conjunction with one chemical absorption process (absorption with MEA) is proposed and studied. The experimental results show H 2 concentration in the final residual gas released from the three stages hydrate CO 2 separation process was approximately 95.0 mol% while that released from the hybrid CO 2 separation process was approximately 99.4 mol%. Thus, the hybrid process is possible to be a promising technology for the industrial application in the future.

Molecular dynamics simulation of methane hydrate dissociation by depressurisation

Methane (CH4) hydrate dissociation and the mechanism by depressurisation are investigated by molecular dynamics (MD) simulation. The hydrate decomposition processes are studied by the ‘vacuum removal method’ and the normal method. It is found that the hydrate decomposition is promoted by depressurisation. The quasi-liquid layer is formed in the hydrate surface layer. The driving force of dissociation is found to be controlled by the concentration gradient between the H2O molecules of the hydrate surface layer and the H2O molecules of the hydrate inner layer. The clathrates collapse gradually, and the hydrate decomposes layer by layer. Relative to our previous MD simulation results, this study shows that the rate of the hydrate dissociation by depressurisation is slower than that by the thermal stimulation and the inhibitor injection. This study illustrated that MD simulation can play a significant role in investigating the hydrate decomposition mechanisms.

Effect of temperature fluctuation on hydrate-based CO2 separation from fuel gas

A new method of temperature fluctuation is proposed to promote the process of hydrate-based CO2 separation from fuel gas in this work according to the dual nature of CO2 solubility in hydrate forming and non-hydrate forming regions [1]. The temperature fluctuation operated in the process of hydrate formation improves the formation of gas hydrate observably. The amount of the gas consumed with temperature fluctuation is approximately 35% more than that without temperature fluctuation. It is found that only the temperature fluctuation operated in the period of forming hydrate leads to a good effect on CO2 separation. Meanwhile, with the proceeding of hydrate formation, the effect of temperature fluctuation on the gas hydrate gradually reduces, and little effect is left in the completion term. The CO2 separation efficiencies in the separation processes with the effective temperature fluctuations are improved remarkably.

Molecular Dynamics Simulation of the Crystal Nucleation and Growth Behavior of Methane Hydrate in the Presence of the Surface and Nanopores of Porous Sediment

The behavior of hydrate formation in porous sediment has been widely studied because of its importance in the investigation of reservoirs and in the drilling of natural gas hydrate. However, it is difficult to understand the hydrate nucleation and growth mechanism on the surface and in the nanopores of porous media by experimental and numerical simulation methods. In this work, molecular dynamics simulations of the nucleation and growth of CH4 hydrate in the presence of the surface and nanopores of clay are carried out. The molecular configurations and microstructure properties are analyzed for systems containing one H2O hydrate layer (System A), three H2O hydrate layers (System B), and six H2O hydrate layers (System C) in both clay and the bulk solution. It is found that hydrate formation is more complex in porous media than in the pure bulk solution and that there is cooperativity between hydrate growth and molecular diffusion in clay nanopores. The hydroxylated edge sites of the clay surface could serve as a source of CH4 molecules to facilitate hydrate nucleation. The diffusion velocity of molecules is influenced by the growth of the hydrate that forms a block in the throats of the clay nanopore. Comparing hydrate growth in different clay pore sizes reveals that the pore size plays an important role in hydrate growth and molecular diffusion in clay. This simulation study provides the microscopic mechanism of hydrate nucleation and growth in porous media, which can be favorable for the investigation of the formation of natural gas hydrate in sediments.

Molecular dynamics simulation of the intercalation behaviors of methane hydrate in montmorillonite

The formation and mechanism of CH4 hydrate intercalated in montmorillonite are investigated by molecular dynamics (MD) simulation. The formation process of CH4 hydrate in montmorillonite with 1 ~ 8 H2O layers is observed. In the montmorillonite, the “surface H2O” constructs the network by hydrogen bonds with the surface Si-O ring of clay, forming the surface cage. The “interlayer H2O” constructs the network by hydrogen bonds, forming the interlayer cage. CH4 molecules and their surrounding H2O molecules form clathrate hydrates. The cation of montmorillonite has a steric effect on constructing the network and destroying the balance of hydrogen bonds between the H2O molecules, distorting the cage of hydrate in clay. Therefore, the cages are irregular, which is unlike the ideal CH4 clathrate hydrates cage. The pore size of montmorillonite is another impact factor to the hydrate formation. It is quite easier to form CH4 hydrate nucleation in montmorillonite with large pore size than in montmorillonite with small pore. The MD work provides the constructive information to the investigation of the reservoir formation for natural gas hydrate (NGH) in sediments.

Raman Spectroscopic Analysis on the Hydrate Formed in the Hydrate-Based Flue Gas Separation Process in Presence of Sulfur Dioxide and Tetra-n-butyl Ammonium Bromide

In this work, Raman spectroscopic analysis was applied to determine the structures and cage occupancies of the hydrates that formed from the system of flue gas (simulated by carbon dioxide–nitrogen–sulfur dioxide)–sulfur dioxide aqueous solution, and from the system of flue gas–sulfur dioxide containing tetra-n-butyl ammonium bromide (TBAB) aqueous solutions (sulfur dioxide mass concentration 0, 1.0, and 7.0 wt%). Comprehensive TBAB (solid, aqueous, and hydrate) Raman spectra were also obtained. The results reveal that when TBAB is used as promoter, both sulfur dioxide and carbon dioxide are encaged in the hydrate from systems of flue gas-TBAB solution with low sulfur dioxide concentration (0, 1.0 wt%), whereas in the hydrate from the system of flue gas-sulfur dioxide highly concentrated (7.0 wt%) TBAB solution, sulfur dioxide will be the sole gas guest encaged in the semi-clathrate hydrate. This suggests the sulfur dioxide concentration significantly influences the hydrate cage occupancies and separation selectivity of the hydrate-based technology. A two-stage hydrate-based flue gas purification process is proposed: one aims at desulfurization when sulfur dioxide concentrates to a relatively high level with the solutions recycling and in the other we can remove the sulfur dioxide and carbon dioxide simultaneously.

Recovery of methane from coal-bed methane gas mixture via hydrate-based methane separation method by adding anionic surfactants

ABSTRACT To screen a preferable kinetics promoter, the effects of sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS) within tetrahydrofuran (THF) solution on recovering methane (CH4) from simulated coal-bed methane (CBM) were investigated at 279.15 K and 1.50–4.50 MPa. The results show the addition of surfactants can remarkably enhance the hydrate formation rate. However, gas uptake, CH4 split fraction and split factor in THF-SDS solution are superior to those in THF-SDBS solution. Therefore, THF-SDS solution is an appropriately integrated additive for recovering CH4 from coal-bed methane gas mixture.