| Natural gas has been widely applied as a clean and economical energy. Depending on the location of gas fields, raw natural gas has different levels of CO2, H2 S and other acidic gas impurities. These impurities must be removed to meet the pipeline specifications. The absorption of organic amine solution is a main method for desulfurization and decarbonization process in today’s chemical industry. However, this method has such problems as high energy consumption and environmental pollution. Some new separation methods, such as solid-material adsorption and membrane separation, overcome these shortcomings. New carbon-materials, especially graphene, graphene/nanotube hybrid structure(GNHS) and graphyne, show unique mechanical strength, thermal and chemical stability and have large surface area. They can be used as excellent adsorbent and membrane materials. In this Thesis, Grand canonical Monte Carlo(GCMC) simulations were used to investigate the adsorption mechanism of H2S/CH4 and CO2/CH4 binary mixture in new carbon materials. Effects of presssure, temperature, mixture composition and pre-adsorption some water molecules in adsorbent on adsorption and separation were discussed. Molecular dynamics(MD) simulation were employed to study the kinetic behaviors of all components in the adsorbents, and the influences of charges around graphene pore on the H2S/CH4 mixture separation were examined as well. The main results are as follows:Firstly, the adsorption of equimolar H2S/CH4 mixture on the single graphene surface was investigated by MD simulation. This kind of adsorption can be regarded as a monolayer adsorption. The gas-solid interfacial tension increases with vapor density increasing. When there exist defects in the graphene surface, the adsorption amount of each component around the defects obviously increases while the potential energy of gas molecules reduces.Secondly, GNHS model was established. The adsorption and separation behaviors of H2S/CH4 and CO2/CH4 binary mixture in multilayer graphene nanostructure(MGN), carbon nanotube(CNT) array and GNHS are investigated by GCMC combined with MD simulations. When the GNHS has a large SWCNT distance(20.32 ? in the x direction, 21.98 ? in the y direction), its adsorption capacity and selectivity for H2 S are lower than with the same layer width of MGNs. The loadings of CH4 in two kinds of adsorbents are basically the same. It is found that the separation performance of H2S/CH4 in GNHS can be efficiently improved by reducing the SWCNT distance. The selectivity of H2 S increases to 1366.23 from 471.45 when SWCNT distance just decreases 0.5 nm, while the loading of H2 S only reduces 7.9 %. The reduction of SWCNT distance has an adverse effect on the separation for CO2/CH4 mixture and both loading and selectivity drop down. Besides, the separation capacity of GNHS for mixture decreases with increasing temperature. With increase of fraction of CH4 in gas mixture, the selectivity of H2 S gradually reduces while the selectivity of CO2 increases. From the analysis of kinetic behaviors, it is found that the self-diffusivity of CH4 in GNHS is the highest while the residence time is the lowest. Therefore, CH4 molecules more easily enter into the adsorbent, but adsorbed CH4 molecules are easily replaced by H2 S or CO2 molecules with lower self-diffusivity.In addition, the adsorption of H2 S, CH4 pure gas and their mixture in the multilayer graphene and graphyne structures is investigated. Effects of pressure, temperature, gas composition and pre-adsorption some water molecules in adsorbents on the gas adsorption performances are discussed. For the pure gas, as the number of acetylenic linkage increases, higher pressure is needed to achieve saturation adsorption amount and adsorption isotherms of H2 S changes from typical type-I into type-V. Mass loading of H2 S in graphyne-1 structure with smaller pore volume(0.662 cm3 g-1) and higher density(0.879 g cm-3) is higher than that in graphyne-2 structure(pore volume is 0.933 cm3 g-1 and density is 0.696 g cm-3). For the mixture adsorption, due to the competition between two components, the adsorption isotherm of each component is different from pure gas adsorption. In the adsorbents with odd number of acetylenic linkage(graphyne-1, graphyne-3 and graphyne-5), the loading of CH4 increases under low pressures and in the high pressure range decreases to a stable value. However, the loadings in the graphyne-2 and graphyne-4 keep a rising state in the investigated pressure ranges. As the pressure increases, the selectivity of six adsorbents for H2S/CH4 mixture shows an increase at first and then slightly decrease trend. Among these adsorbent, graphene structure has the highest selectivity while graphyne-1 structure takes the second place. With the rising of temperature, loading of H2 S in all adsorbents falls monotonically and loading of CH4 shows a trend of increasing first and then decreasing, and the maximum loading of CH4 shifts to low temperature with the pore size increases. It has an important effect on the adsorption and separation performance of the gas mixture when pre-adsorbed some water molecules in the adsorbents. As the content of water increases, the loadings of the two components gradually reduce. When the pore volume of the adsorbents is bigger or the content of H2 S in the gas bulk is lower, the selectivity of H2 S will increases with increasing water content.Finally, the effect of charges around pore on the separation of H2S/CH4 mixture using a porous graphene membrane was investigated by MD simulations. The results show that the charges have both positive and negative effects on the mixture separation. The presence of charges leads the potential energy of H2 S around the pore to sharply reducing, and there are more H2 S molecules accumulate, the density is about 9.53 times larger than that of CH4. These effects are beneficial to improve the separation performance of porous graphene membrane for H2S/CH4 mixture. These charges also lead H2 S molecules difficultly to escape out after passing through the pore and increase the probability of H2 S molecules to return back to the reservoir side. The permeability of H2 S is reduced. It has an adverse effect on the separation. |
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