Abstract
The development of industrial software, the decreasing cost of computing time, and the availability of well-tested forcefields make molecular simulation increasingly attractive for chemical engineers. We present here several applications of Monte-Carlo simulation techniques, applied to the adsorption of fluids in microporous solids such as zeolites and model carbons (pores < 2 nm). Adsorption was computed in the Grand Canonical ensemble with the MedeA<sup>®<sup/>-GIBBS software, using energy grids to decrease computing time. MedeA<sup>®<sup/>-GIBBS has been used for simulations in the NVT or NPT ensembles to obtain the density and fugacities of fluid phases. Simulation results are compared with experimental pure component isotherms in zeolites (hydrocarbon gases, water, alkanes, aromatics, ethanethiol, etc.), and mixtures (methane-ethane, <i>n<i/>-hexane-benzene), over a large range of temperatures. Hexane/benzene selectivity inversions between silicalite and Na-faujasites are well predicted with published forcefields, providing an insight on the underlying mechanisms. Also, the adsorption isotherms in Na-faujasites for light gases or ethane-thiol are well described. Regarding organic adsorbents, models of mature kerogen or coal were built in agreement with known chemistry of these systems. Obtaining realistic kerogen densities with the simple relaxation approach considered here is encouraging for the investigation of other organic systems. Computing excess sorption curves in qualitative agreement with those recently measured on dry samples of gas shale is also favorable. Although still preliminary, such applications illustrate the strength of molecular modeling in understanding complex systems in conditions where experiments are difficult.
Highlights
Adsorption processes are important in chemical engineering: separation of aromatic isomers, drying, removal of VOC (Volatile Organic Chemicals) from air or industrial gases, hydrogen separation from flue gas, oxygen separation from air, are largely based on the properties of industrial adsorbents to adsorb selectively key compounds that would not separate efficiently by distillation
Care must be taken that the principle of Grand Canonical Monte-Carlo (GCMC) is thermodynamic equilibrium, and it does not account for possible kinetic limitations
We use mainly the following all-atoms forcefields: 1. the Polymer Consistent ForceField (PCFF) forcefield was the result of research conducted by the biosym potential energy functions and polymer consortia [6]
Summary
Adsorption processes are important in chemical engineering: separation of aromatic isomers, drying, removal of VOC (Volatile Organic Chemicals) from air or industrial gases, hydrogen separation from flue gas, oxygen separation from air, are largely based on the properties of industrial adsorbents to adsorb selectively key compounds that would not separate efficiently by distillation. The implementation of adsorption processes in the industry is often limited by the cost and delay of experimental data acquisition, and by the lack of simple methods for predicting adsorption properties In this context, several factors make molecular modeling 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 parameterization of well-tested forcefields. The organic matter in gas shales is not an industrial adsorbent, its study is typical of the insight provided by molecular level modeling, in a context where the exact structure is not known.
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