Behavior of ethylene and ethane within single-walled carbon nanotubes. 1-Adsorption and equilibrium properties
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Behavior of ethylene and ethane within single-walled carbon nanotubes. 1-Adsorption and equilibrium properties Fernando J.A.L. Cruz · Erich A. Müller
Received: 20 March 2008 / Revised: 13 January 2009 / Accepted: 9 February 2009 / Published online: 28 February 2009 © Springer Science+Business Media, LLC 2009
Abstract Endohedral adsorption properties of ethylene and ethane onto single-walled carbon nanotubes were investigated using a united atom (2CLJQ) and a fully atomistic (AA-OPLS) force fields, by Grand Canonical Monte Carlo and Molecular Dynamics techniques. Pure fluids were studied at room temperature, T = 300 K, and in the pressure ranges 4 × 10−4 < p < 47.1 bar (C2 H4 ) and 4 × 10−4 < p < 37.9 bar (C2 H6 ). In the low pressure region, isotherms differ quantitatively depending on the intermolecular potential used, but show the same qualitative features. Both potentials predict that ethane is preferentially adsorbed at low pressures, and the opposite behavior was observed at high loadings. Isosteric heats of adsorption and estimates of low pressure Henry’s constants, confirmed that ethane adsorption is the thermodynamically favored process at low pressures. Binary mixtures of C2 H4 /C2 H6 were studied under several (p, T ) conditions and the corresponding selectivities towards ethane, S, were evaluated. Small values of S < 4 were found in all cases studied. Nanotube geometry plays a minor role on the adsorption properties, which seem to be driven at lower pressures primarily by the larger affinity of the alkane towards the carbon surface and at higher pressures by molecular volume and packing effects. The fact that the selectivity towards ethane is similar to that found earlier on carbon slit pores and larger diameter nanotubes points to the fact that the peculiar 1-D geometry of the nanotubes provides no particular incentive for the adsorption of either species.
F.J.A.L. Cruz · E.A. Müller () Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK e-mail: [email protected]
Keywords Adsorption · Molecular simulation · Carbon nanotubes · Ethylene · Ethane · Grand canonical Monte Carlo · Molecular dynamics
1 Introduction Separations of close boiling point mixtures, such as the ethylene/ethane mixture, are amongst the most energyintensive unit operations in chemical and petrochemical plants, where they usually involve low-temperature distillation techniques. For this type of separations, encompassing structurally and/or energetically similar molecules, alternative techniques are actively sought, particularly for small-scale applications such as those used in the pharmaceutical, biomedical and aerospace industry (Noble and Agrawal 2005). While adsorption using nanoporous materials is posed to be considered as an option, our current molecular-level knowledge of the physical phenomena involved in the adsorption and diffusion of fluids in confined nanospaces still needs improvement. Molecular simulation has played a key role in understanding the intricate det
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