CO₂/CH₄ Mixed Gases Separation By Carbon Membrane Molecular Simulations

With rapid development of global industrialization, the emissions of greenhouse gases such as CO2 are growing. Increase of these gases in the atmosphere will lead to a series of environmental, economic and social problems at a multi-scale, all-round, multi-level. The new type of clean energy-natural gas becomes a kind of important method to alleviate the greenhouse effect. It can reduce content of CO2 in the gas to improve calorific value and reduce emissions of CO2. Early studies have suggested that carbon membrane of inorganic membrane can be effective to mixed CO2/CH4 separation because of its special structure and properties, but the mechanism of gas separation in the carbon membrane cannot be directly observed through experiments. So it is necessary to explore mechanism of gas adsorption and diffusion process in carbon membrane.A simulation study was conducted in separation of CO2/CH4 gas mixture using Materials Studio and Lammps softwares respectively. Zigzag-type pore model was developed to describe the pore structure, and verified by CO2 adsorption isotherm experimentally determined with home-manufactured carbon membranes. Then, through handling Zigzag-type pore model with different styles of defects, three different defect pore models (random, averaged, entirety) were established in carbon membrane. Adsorption and diffusion behaviors of pure gas and gas mixture within the pores of carbon membrane were investigated under different conditions in an effort to explore separation mechanism.Total separation coefficient of CO2/CH4 adsorption and diffusion was examined in the present study. Results showed that for the zero defect model appropriately low temperature and small pore size were beneficial to achieve separation of CO2/CH4 gas mixture. As the temperature was decreased and the pore size was decreased, the total separation coefficient increased. The total separation coefficient of gas mixture was 18.2 at 298 K of the operating temperature and 0.670 nm of the pore size of membrane, which was consistent with the experimental value. Comparative studies show that, at different temperatures, the total separation coefficients of integral defect model were relatively lower, which suggested that C of partial deleting was more close to the real carbon membrane. At lower temperatures, the total separation coefficients of zero defect model is relatively lower, while at higher temperatures they were greater compared with the other two kinds of defect ones. And the total separation coefficients of random defect model at various temperatures (but 285 K) were greater than the averaged defect model but their gaps were smaller. These suggested the method of random deleting C atom was more reasonable and more closed to the real carbon membrane.In this paper, this fundamental research would provide a basis for optimizing the preparation of carbon membranes and for studying the separation mechanism of CO2/CH4 gas mixture.