Molecular Modeling of Selective Adsorption from Mixtures
- PDF / 871,113 Bytes
- 12 Pages / 414.72 x 648 pts Page_size
- 10 Downloads / 200 Views
Department of Chemistry, The City College of New York, New York, NY 10031 §School of Chemical Engineering, Cornell University Ithaca, New York, 14853
¶
Escola Tbcnica Superior d'Enginyeria QuImica, Carretera de Salou, s/n 43006 Tarragona,
Spain ABSTRACT
Molecular simulation methods provide a means for carrying out systematic studies of the factors affecting adsorption phenomena. For selective adsorption, the selectivity is strongly affected by the interaction energy with the pore walls, molecular size and shape, site specific interactions, entropic effects, differences in diffusion rates, and networking effects. Two recent studies of site specific selectivity will be described. The first is an investigation of the effect of oxygenated surface sites on the adsorption of water vapor on activated carbons. Hydrogen-bonding sites are modeled using off-center square well interactions for both water and wall sites; wall sites are placed at the edges of the graphite micro-crystals. New experimental results for water adsorption at low pressures on carefully characterized activated carbons are reported, and are found to be in good agreement with the simulations. In the second application, we consider the separation of alkene/alkane mixtures using aluminas whose surfaces have been doped with metal ions. 7r-complexation between these metal ions and the alkenes can produce a highly selective separation. The simulations are found to be in good agreement with the available experimental data, and have been used to predict separations for other conditions not yet studied in the laboratory. INTRODUCTION The adsorption of water on activated carbons is qualitatively different from that of simple fluids. There are two sources of this dissimilarity: (a) the water-water interaction is very strongly attractive, compared to simple fluids, and (b) the adsorption of water is largely controlled by the formation of H-bonds with oxygenated groups on the surface. For simple fluids, pore filling occurs via the formation of a fluid monolayer on each surface, often followed
by a second and further layers, prior to capillary condensation and pore filling. By contrast, water molecules adsorb onto oxygenated surface sites and these adsorbed molecules provide nucleation sites for the formation of larger water clusters; eventually these clusters connect, and pore filling occurs. When the density of oxygenated sites on the surface is appreciable, the pore filling seems to occur by a continuous filling process without capillary condensation. The modeling of water on graphitic carbons is straightforward. The absence of oxygenated sites on the surfaces permits even simple representations of the system to mimic the observed type V isotherms [1]. In such isotherms, there is very little adsorption at moderate pressures, and pore filling occurs rapidly at high relative pressure. Both point charge and square well models of water reproduce the phase behavior well [2]-[6]; moreover even simple Lennard-Jones models capture the essence of the isotherm, if the fl
Data Loading...
