| Porous media is a kind of material with network structure composed of interconnected or closed holes,which generally has the advantages of low relative density,high specific strength,high specific surface area and high pore volume.Therefore,porous media is often used as the chemical reaction and storage medium of gas.In general,the pore size in porous media can reach the nanometer scale,in which the gas will spontaneously adsorb and diffuse.In nano pores,the adsorption layer on the pore surface will occupy a considerable pore volume,and the diffusion movement of gas molecules in the adsorption layer will affect the storage of gas.In addition,when there are more than two kinds of gases in the porous medium,due to the different interaction forces between gases and the pore surface,the replacement of one gas by another will occur in the adsorption layer.Therefore,studying the competitive adsorption and diffusion process of mixed gas in nano pores is of great significance to reveal the storage,transportation and replacement mechanism of mixed gas in porous media.In this study,molecular dynamics method and grand canonical Monte Carlo method were used to simulate the competitive adsorption and diffusion process of gas mixture in nano pores.Based on the classical mass transfer theory,a mathematical model was proposed to describe the competitive adsorption and diffusion process of mixed gas.The conclusions of this study are summarized as follows:(1)Methane and nitrogen showed similar adsorption characteristics on the surface of kerogen slit,while helium did not significantly adsorb in kerogen.Oxygen is monolayer adsorbed on Pt(111)surface,and water vapor is multilayer adsorbed on Pt(111)surface.At 3-30 MPa,the thickness of adsorption layer of methane(T=360K)and nitrogen(T= 300K)on the surface of kerogen slit is 0.77 nm and 0.786 nm respectively.At 0.1-0.3MPa and 413 k,the thickness of oxygen adsorption layer on Pt(111)surface is 0.651 nm,and the thickness of water vapor adsorption layer on Pt(111)surface is1.155 nm,1.855 nm and 2.185 nm respectively.(2)In the confined slit space,the diffusion ability of the free gas is stronger than that of the adsorbed gas.The diffusion coefficient of gas in the slit is negatively correlated with pressure and density,and the gas density is the key factor to determine the diffusion rate;The diffusion motion will also be constrained by the finite height of the slit,but the constraint will weaken with the increase of the slit height.Under the condition of no slit constraint,the mutual diffusion coefficient of mixed gas is less than that of pure gas.(3)For the problem of methane dissolution and diffusion,the mass transfer model proposed in this study used chemical potential driving instead of density driving to solve the problem of mass flux between different regions,which coupled the gas adsorption and diffusion with the solid solution process.The fitting between the numerical calculation results and the simulation results has achieved good results.For the competitive adsorption and diffusion of nitrogen and helium,the mass transfer model was also driven by chemical potential.The solid solution process of gases was separated from the mass transfer process.The numerical results were basically consistent with the simulation results.For the competitive adsorption and diffusion of oxygen and water vapor,the mass transfer model was driven by density,in which ignored the mass transfer flux between free gas and adsorption layer.The mass transfer process can be described accurately by the mass transfer model.(4)During the dissolution and diffusion of methane into kerogen,the amount of methane dissolved in kerogen increased with the increase of pressure and slit height;In the process of competitive adsorption and diffusion of mixed gas,the equilibrium state of gas under competitive adsorption will deviate from the adsorption equilibrium of single component gas,which can result in part of adsorbed gas desorbed into free gas. |
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