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Simulations of Binary Mixture Adsorption of Carbon Dioxide and Methane in Carbon Nanotubes

Liang-Liang Huang1, Xin Feng1, Qing Shao1, Ling-Hong Lu1, and Xiao-Hua Lu2. (1) Nanjing University of Technology, Nanjing, 210009, China, (2) College of Chemical Engineering, Nanjing University of Technology, Nanjing, 210009, China

There is a serious concern that long term use of increasing amount of fossil fuels will cause significant climate changes. This concern arises from the inevitable production of CO2, which traditionally has been disposed of by direct dilution in air. In response, a variety of schemes for collection and disposal of CO2 have been investigated recently. Given the very large volumes of CO2 produced and the fact that storage would have to be permanent, the geological sequestration of CO2 is considered to be a better choice. While on the other hand, the static and dynamic properties of CO2 in porous geological media, such as the deep saline aquifers, depleted hydrocarbon reservoirs, or deep coal beds, are not fully understood, especially at the extreme conditions of high temperature and high pressure. In this work, Grand canonical Monte Carlo (GCMC) simulations were performed to investigate the adsorption behavior of equimolar CO2/CH4 mixture in carbon nanotubes (CNTs). Five CNTs [(6,6), (7,7), (8,8), (9,9) and (10,10)] with diameters varying from 0.678nm to 1.356nm, seven temperatures (283, 293, 303, 313, 323, 333, 343K) and seven pressures (1, 5, 10, 15, 20, 25, 30MPa) were chosen to investigate the effect of temperature, pressure and pore size on the adsorption behavior. The results show that the CNTs have a preferential adsorption of CO2 in the binary CO2/CH4 mixture. For pore size effect on the adsorption behavior, we found that the adsorption of CO2 is much larger than that of CH4 in the same CNT, and that the CO2 adsorption in the CNTs increases dramatically with an increase of CNT's diameter, whereas the adsorbed CH4 hardly changes with CNT's pore size. For the temperature and pressure effects, we observed that in the CNTs with diameters less than 1.1nm, the temperature and pressure have little effect on the adsorption behavior of the binary mixture, while in larger CNTs the adsorption behavior changes with temperature and pressure significantly. In addition, the CNTs demonstrated a higher selectivity of CO2 than other materials (activated carbons, zeolites 13X and mental-organic frameworks) reported in literature. For example, in (6, 6) CNT at 303K and 1MPa, the selectivity reaches 20.8, five times larger than in activated carbon. The selectivity in narrow CNTs (<1nm in diameter) is fluctuating with temperature and pressure, but it remains almost the same in larger CNTs.