2 research outputs found
Evaluating adsorbed-phase activity coefficient models using a 2D-lattice model
<div><p>Despite the wide use of the real adsorbed solution theory to predict multicomponent adsorption equilibrium, the models used for the adsorbed phase activity coefficients are usually borrowed from the gasâliquid phase equilibria. In this work, the accuracy of the Wilson and NRTL models for evaluating adsorbed phase activity coefficients is tested using a 2D-lattice model. An accurate model for adsorbed-phase activity coefficients should have no problem in fitting adsorption data obtained using this simple lattice model. The results, however, show that the commonly used Wilson and NRTL models cannot describe the adsorbed phase activity coefficients for slightly non-ideal to strong non-ideal mixtures. Therefore, until new models for adsorbed phase activity coefficients are developed, we should use existing models for liquids with care. In the second part of this work, the use of Monte Carlo simulations on a segregated 2D-lattice model, for predicting adsorption of mixtures is investigated. The segregated model assumes that the competition for adsorption occurs at isolated adsorption sites, and that the molecules from each adsorption site interact with the bulk phase independently. Two binary mixtures in two adsorbent materials were used as case studies for testing the predictions of the segregated 2D-lattice model: the binary system CO<sub>2</sub>âN<sub>2</sub> in the hypothetical pure silica zeolite PCOD8200029, with isolated adsorption sites and normal preference for adsorption, and the binary system CO<sub>2</sub>âC<sub>3</sub>H<sub>8</sub> in pure silica mordenite (MOR), with isolated adsorption sites and inverse site preference. The segregated 2D-lattice model provides accurate predictions for the system CO<sub>2</sub>âN<sub>2</sub> in PCOD8200029 but fails in predicting the adsorption behaviour of CO<sub>2</sub>âC<sub>3</sub>H<sub>8</sub> in pure silica MOR. The predictions of the segregated ideal adsorbed solution theory model are superior to those of the 2D-lattice model.</p></div
Fick Diffusion Coefficients in Ternary Liquid Systems from Equilibrium Molecular Dynamics Simulations
An approach for computing Fick diffusivities directly
from equilibrium molecular dynamics (MD) simulations is presented
and demonstrated for a ternary chloroformâacetoneâmethanol
liquid mixture. In our approach, Fick diffusivities are calculated
from the MaxwellâStefan (MS) diffusivities and the so-called
matrix of thermodynamic factors. MS diffusivities describe the friction
between different molecular species and can be directly computed from
MD simulations. The thermodynamic factor describes the deviation from
ideal mixing behavior and is difficult to extract from both experiments
and simulations. Here, we show that the thermodynamic factor in ternary
systems can be obtained from density fluctuations in small subsystems
embedded in a larger simulation box. Since the computation uses the
KirkwoodâBuff coefficients, the present approach provides a
general route toward the thermodynamics of the mixture. In experiments,
Fick diffusion coefficients are measured, while previously equilibrium
molecular dynamics simulation only provided MS transport diffusivities.
Our approach provides an efficient and accurate route to predict multicomponent
diffusion coefficients in liquids based on a consistent molecular
picture and therefore bridges the gap between theory and experiment