| Membrane separation processes are low-energy consuming and high efficient.Being widely used in many fields,such as chemical industry, food production,environmental protection,and pharmaceuticals, membrane separation is an important technology for solving the problems on energy sources and environmental pollution.Compared with polymer membranes,inorganic membranes are thermal-,high pressure-,chemical corrosion-,and high mechanical- resistant.As a type of inorganic membranes,nanoporous carbon membranes display uniform porosity distribution and controllable pore size,which make them promising candidates for gas separation processes.In this thesis,molecular simulations were carried out for investigating the adsorption,diffusion, and separation of gases in nanoporous carbon membranes.The main contents and findings are summarized as follows. 1.The non-equilibrium molecular dynamics(NEMD) method which was carried out by former researchers was improved in this thesis.A slit-like pore model with entrance and exit was adopted to represent carbon membrane.Importantly,buffer regions were employed between the control volumes,which are located at both the two sides of the membrane,and carbon membrane to eliminate the discontinuity of the flux at the entrance and exit of a membrane pore,so as to take into account their effects on gas permeation and separation.In contrast,this has to be done in simulation without the buffer regions by adopting a streaming velocity method.Compared with the streaming velocity method,the buffer region method proposed here is more natural and reasonable from a physical point of view.2.The H2 and CO mixture is called syngas in industry.By the separation of syngas,H2 and CO can be produced as an ideal clean energy carrier and the material to synthesis acetic acid,respectively.More importantly,after adjusting its compositions,syngas can be used to prepare many chemical products for the Fischer-Tropsch synthesis method,such as hydrocarbons,olefins,and alcohols.In this thesis,grand canonical ensemble Monte Carlo(GCMC) and NEMD simulations were carried out to investigate systematically the equilibrium adsorption separation and the non-equilibrium diffusive separation of H2/CO in nanoporous carbon membranes,respectively.Beginning with the microscopic phenomena obtained in simulations,the separation mechanisms were analyzed in detail.Moreover,the effects of the width of the slit pore,temperature,pressure,feed gas composition and membrane thickness on separation were discussed.Based on the simulation results, suitable operation conditions were proposed from a molecular simulation point of view for H2/CO separation by carbon membranes.For the equilibrium adsorption separation,an optimum pore width of 0.74 nm is recommended.In this case,the equilibrium separation factor,adsorption amount of H2 and CO for the equimolar mixture are 6.5,2.0 mmol/g and 12.9 mmol/g,respectively,at 300 K and 1.0 MPa.Low temperature is preferred for high adsorption amount and equilibrium separation factor. Low pressure operation can improve the equilibrium separation factor, but is unfavorable for adsorption amount.Moreover,the equilibrium separation factor increases with the mole fraction of H2 in the feed gas, indicating adsorption separation is particularly suitable for gas mixtures with high H2 mole fraction.For the non-equilibrium diffusive separation process,the effect of pore width on separation is quite obvious.Three categories of separation mechanisms can be summarized according to the variation of pore width.For the pores between 0.50 and 0.64 nm,carbon membranes behave as molecular sieves,and the dynamic separation factor reaches 52.88 at 300 K and 0.5 MPa in this case.For the pores between 0.64 and 0.84 nm,the separation is governed by the combing effect of molecular sieving and adsorption and diffusion properties of the two gases.For the pores between 0.84 and 2.01 nm,the separation process is dominated by the difference of the adsorption and diffusion properties of the two gases.In this case,CO molecules adsorbed densely on the pore walls and diffuse through the membrane by surface flow, while H2 molecules go through the centre of a membrane pore.High temperature is preferred for the non-equilibrium separation processes and a range of temperature 330～350 K is recommended.High pressure can provide larger flux,but decreases the dynamic separation factor. Moreover,the dynamic separation factor hardly changes with the composition of the feed gas.The effect of membrane thickness on membranes with various pore widths is quite different,for a small pore of 0.64 nm based on molecular sieving mechanism,the dynamic separation factor increases with membrane thickness,while for a large pore of 1.01 nm dominated by the adsorption and diffusion properties of diffusing gases,the result is just the opposite.3.By simulating the diffusion of H2,CO,N2,O2,and CH4,it is found that the Fick's law is obeyed.Based on this point,the diffusive flux, transport diffusivity,and diffusive activation energy for various temperatures and pore widths of the five gases were calculated.The achieved transport diffusivities are comparable with those had been reported in the literature.Therefore,since transport diffusivity is always expensive and hardly measured by experimental methods,the simulation method presented in this thesis can be considered as a candidate.It is expected that this simulation method can be helpful for the design and application of porous materials.4.Since the entrance and exit of a membrane greatly affect the non-equilibrium separation process,and a real carbon membrane is always consisted of many small graphite crystals,different simulation boxes were adopted to investigate the transport resistance distribution for CH4 diffusing through carbon membranes with different pore widths and thicknesses.It can be referenced for the design and development of carbon membranes.
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