Isosteric heats of adsorption (q st ) of propane and butane adsorbed on carbon were studied by numerical differentiation of nonlocal density functional theory (NDFT) isotherms. The q st values of both adsorbates in slit-shaped pores were a weak function of temperature, and decreased with increasing pore width. Using the calculated pore size distribution (PSD) of an activated carbon, it was further determined that the q st for both propane and butane decreased with increasing loading, consistent with a heterogeneous adsorbent. The NDFT results utilizing the PSD were also in fair agreement with those obtained from the classical approach using experimental isotherms fitted to a model and applied to the Clausius-Clapeyron-type equation; both models predicted q st of butane to be ∼10 kJ/mol higher than that of propane at the same loading. On a model homogeneous carbon, the q st of both adsorbates increased with reduced surface coverage up to ∼0.5, then they dropped rapidly. The reduced surface coverages corresponding to monolayer completion were 0.61 for propane and 0.65 for butane, in agreement with published experimental results.
Classical density functional theory (DFT) was used to predict the adsorption of nine different binary gas mixtures in a heterogeneous BPL activated carbon with a known pore size distribution (PSD) and in single, homogeneous, slit-shaped carbon pores of different sizes. By comparing the heterogeneous results with those obtained from the ideal adsorbed solution theory and with those obtained in the homogeneous carbon, it was determined that adsorption nonideality and adsorption azeotropes are caused by the coupled effects of differences in the molecular size of the components in a gas mixture and only slight differences in the pore sizes of a heterogeneous adsorbent. For many binary gas mixtures, selectivity was found to be a strong function of pore size. As the width of a homogeneous pore increases slightly, the selectivity for two different sized adsorbates may change from being greater than unity to less than unity. This change in selectivity can be accompanied by the formation of an adsorption azeotrope when this same binary mixture is adsorbed in a heterogeneous adsorbent with a PSD, like in BPL activated carbon. These results also showed that the selectivity exhibited by a heterogeneous adsorbent can be dominated by a small number of pores that are very selective toward one of the components in the gas mixture, leading to adsorption azeotrope formation in extreme cases.
The nonlocal density functional theory of Kierlik and Rosinberg was used to predict isosteric heats of adsorption (qi) of three binary gas mixtures: (1) CO2-C2H4, (2) CH4-C2H6, and (3) CH4-C3H8 in homogeneous and heterogeneous carbons at 350 K and 1 bar. The qi's of the components in the mixture were different from their pure state qi°'s and showed complex behavior for the nonideal systems (2 and 3). Adsorbent heterogeneity also played an important role in determining the behavior of the qi's compared with the qi°'s. The differences were attributed to effects caused by the loading of the opposing component and by differences in the intermolecular forces between the adsorbate molecules.
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