Search

The adsorption and mass transport of CO 2 and CH 4 in CHA zeolite were studied experimentally. First, large and well-defined CHA crystals with varying Si/Al ratios and morphologies ideal for adsorption studies were prepared. Then, adsorption isotherms were measured, and adsorption parameters were estimated from the data. In the next step, permeation experiments for pure components and mixtures were conducted for a defect-free CHA membrane with a Si/Al ratio of 80 and a thickness of 600 nm over a wide temperature range. A maximum selectivity of 243 in combination with a CO 2 permeance of 70 × 10−7 mol/(m2 s Pa) was observed for a feed of an equimolar CO 2 /CH 4 mixture at 273 K and 5.5 bar. Finally, a simple model accounting for adsorption and diffusion through the surface barriers and the interior of the pores of the membrane was fitted to the permeation data. The fitted model indicated that the surface barrier was a surface diffusion process at the pore mouth with higher activation energy than the diffusion process within the pores. The model also showed that the highly selective mass transport in the membrane was mostly a result of a selective surface barrier and, to a lesser extent, a result of adsorption selectivity. [Display omitted] • CHA crystals with various Si/Al ratios were studied for gas adsorption. • The adsorption were performed in a wide temperature range of 150–350 K. • Single gas and CO 2 /CH 4 permeation were measured over an ultra-thin CHA membrane. • Adsorption parameters were used to model mass transport through the CHA membrane. • High CO 2 /CH 4 selectivity is mostly due to high surface permeability selectivity. [ABSTRACT FROM AUTHOR]

Abstract: A magnesium-based metal organic framework (MOF), also known as Mg-MOF-74, was successfully synthesized, characterized, and evaluated for adsorption equilibria and kinetics of CO2 and CH4. The Mg-MOF-74 crystals were characterized with scanning electron microscopy for crystal structure, powder X-ray diffraction for phase structure, and nitrogen adsorption for pore textural properties. Adsorption equilibrium and kinetics of CO2 and CH4 on the Mg-MOF-74 adsorbent were measured in a volumetric adsorption unit at 278, 298, and 318K and pressures up to 1bar. It was found that the Mg-MOF-74 adsorbent prepared in this work has a median pore width of 10.2Å, a BET specific surface area of 1174m2/g, CO2 and CH4 adsorption capacities of 8.61mmolg−1 (37.8wt.%) and 1.05mmolg−1 (1.7wt.%), respectively, at 298K and 1bar. Both CO2 and CH4 adsorption capacities are significantly higher than those of zeolite 13X under similar conditions. The pressure-dependent equilibrium selectivity of CO2 over CH4 (q CO2/q CH4) in the Mg-MOF-74 adsorbent showed a trend similar to that of zeolite 13X and the intrinsic selectivity of Mg-MOF-74 at zero adsorption loading is 283 at 298K. The initial heats of adsorption of CO2 and CH4 on the Mg-MOF-74 adsorbent were found to be 73.0 and 18.5kJmol−1, respectively. The adsorption kinetic measurements suggest that the diffusivities of CO2 and CH4 on Mg-MOF-74 were comparable to those on zeolite 13X. CH4 showed relatively faster adsorption kinetics than CO2 in both adsorbents. The diffusion time constants of CO2 and CH4 in the Mg-MOF-74 adsorbent at 298K were estimated to be 8.11×10−3 and 4.05×10−2 s−1, respectively, showing a modest kinetic selectivity of about 5 for the separation CH4 from CO2. [Copyright &y& Elsevier]

Abstract: Separation of carbon dioxide and methane is an important issue in upgrading low-quality natural gas. Adsorption equilibria and kinetics of CO2 and CH4 on a copper metal–organic framework (MOF), Cu(hfipbb)(H2hfipbb)0.5 [H2hfipbb=4,4′-(hexafluoroisopropylidene) bis(benzoic acid)], were investigated to evaluate the feasibility of removing CO2 from CH4 in a pressure swing adsorption process using this new MOF adsorbent. The heat of adsorption of CO2 on the Cu–MOF at zero-coverage (29.7kJ/mol) is much lower than those on a carbon molecular sieve and a zeolite 5A adsorbent; and the heat of adsorption of CH4 on the Cu–MOF (21.4kJ/mol) is similar to that on the zeolite 5A adsorbent and smaller than that on a carbon molecular sieve. The Cu–MOF being investigated has apertures of (∼3.5×3.5Å), which favors the kinetically controlled separation of CO2 and CH4. The kinetic selectivity is found to be 26 at 298K, and the overall selectivity (combining the equilibrium and kinetic effects) is about 25 for an adsorption separation process. These results suggest that the Cu–MOF adsorbent is an attractive alternative adsorbent for the CO2/CH4 separation. [Copyright &y& Elsevier]