A surface diffusion transport model considering multilayer adsorption behavior of gas

Key words:  multilayer adsorption  surface diffusion model  nanopore  temperature effect  pressure effect

Abstract:

Gas mainly exists as adsorbed phase in nanopores, so the contribution of surface diffusion of adsorbed gas to the total gas flow cannot be ignored. Due to Langmuir monolayer adsorption cannot effectively describe the characteristics of gas adsorption under high temperature and high pressure, a new theoretical formula about surface diffusion coefficient of adsorbed gas was developed on the basis of monolayer adsorption coverage concept. And, its rationality and accuracy were verified by the relevant experimental and theoretical data. Further, a surface diffusion model about multilayer adsorption in pore wall was constructed, and the effects of pressure and temperature on surface diffusion transport were analyzed. The results show that the proposed formula about surface diffusion coefficient is more advantageous compared to the monolayer adsorption, which effectively reflects the multilayer adsorption characteristics of gas molecules. Based on multilayer adsorption theory, the new surface diffusion model that coupling temperature and pressure is more accurate and reliable than the traditional surface diffusion model. Pressure and temperature are two important aspects that affect the transport of surface diffusion. The effect of pressure on surface diffusion flux is significant in the nanopores dominated by surface diffusion transport (the pressure increases 9 MPa, the flux increases two orders of magnitude). The effect of temperature on surface diffusion flux is weak (the average temperature increases by 1 ℃, and the flux decreases by 1.13％).