(225b) Monte Carlo Simulation of Fluid Phase Equilibria and of Adsorption In Microporous Materials : Investigation of Binary and Multicomponent Systems of Industrial Interest

Monte Carlo simulation of fluid phase equilibria and of adsorption in microporous materials : investigation of binary and multicomponent systems of industrial interest.

Marianna Yiannourakou¹, Philippe Ungerer¹, Benoit Leblanc¹ and Paul Saxe²

¹Materials Design Sarl, Montrouge, France

²Materials Design Inc., Angel Fire, USA

Several factors make molecular modelling increasingly attractive for industrial applications : the availability of powerful algorithms for complex systems, the development of industrial software allowing efficient use by chemical engineers, the decreasing cost of computing time, and the parametrization of well-tested force fields. The purpose of the talk is to illustrate this trend in the case of Monte Carlo simulation techniques, applied to the equilibrium properties of fluids  -including adsorption in microporous solids such as zeolites and model carbons-. For organic fluids we use generic force fields based on united atoms such as TraPPE [1] or AUA [2]. Force field parameters for small molecules (H₂, CO₂, etc. ) and for solids are taken from published work, and standard combining rules are selected.

The first type of system investigated is the solubility of gases (H₂, N₂, O₂, CH₄, CO₂, …) in organic liquids (hydrocarbons, ethanol, dimethylether, cresol….). This is an important topic for the conversion of heavy crude oil fractions into high quality fuels and for the co-processing of ligno-cellulosic biomass. We show that biased Widom test insertions allow reliable determination of the Henry constant of gas solubility.  Simulation results predict consistently the influence of all factors : nature of the gas, temperature, and nature of the organic liquid.  These results provide a quantitative explanation for the solubility behavior : very light gases like H₂ do not display sufficient attraction for the solvent, and they are mostly sensitive to the presence of temporary cavities in the liquid. Their entropy-driven solubility increases with temperature, as a result of the decreasing density of the solvent. Heavier gases like CO₂ feel more strongly the attraction for the liquid solvent, and the effect of temperature is therefore opposite. Gibbs ensemble simulations allow a very consistent extrapolation of these results to high pressures, whenever data are available (especially considering alcohols, ethers..). They allow also to model the vapour phase composition at high pressure.  

The second type of systems considered in this work is the adsorption of multicomponent fluids in zeolites, which are often used in the industry for purifications and delicate separations . Grand Canonical ensemble Monte Carlo techniques that are used for this purpose. The assumptions made while building the sodium faujasite models NaY and NaX  (framework structure, location of sodium cations in zeolites, force field parameters…) follow from published  work [3]. Simulation results are compared with experimental adsorption data on pure compounds (water, alkanes, aromatics, ethanethiol), and binary systems, with very good results with some exceptions. Once these validations are made, the behavior of complex multicomponent systems in condensed phases (high pressure natural gas, liquid mixture) can be considered. For this purpose, compressibility factor at high pressure may be used as an indirect test of the gas model, and biased test insertions are used to determine the chemical potential of each molecular type. The adsorption behavior of the multicomponent gas may then be investigated. This is illustrated by the example of natural gases comprising up to 12 components. The sensitivity of adsorption to minute amounts of heavy or polar compounds (such as water, methanethiol, liquid hydrocarbons) is discussed.

The examples treated in this work are systems for which analytical models (equations of state, activity coefficient models,…) need abundant experimental data to be parametrized. Our results support the idea that molecular modeling is now a cost-effective tool to perform difficult extrapolations. Other comparable applications (supercritical fluids, sequestration of CO₂ for coal bed methane recovery) are cited in conclusion.

1.            Maerzke, K.A., et al., TraPPE-UA Force Field for Acrylates and Monte Carlo Simulations for Their Mixtures with Alkanes and Alcohols. Journal of Physical Chemistry B, 2009. 113(18): p. 6415-6425. and references cited.

2.            Ferrando, N., et al., Transferable Force Field for Alcohols and Polyalcohols. Journal of Physical Chemistry B, 2009. 113(17): p. 5985-5995, and references cited.

3.            diLella, A., et al., Molecular simulation studies of water physisorption in zeolites. Phys. Chem. Chem. Phys., 2006. 8: p. 5396-5406.