Abstract
Numerous industrial operations involve zeolite adsorbents: separation of aromatics, separation of high-octane branched alkanes, and purification of fuels from sulphur-bearing compounds, among others. A limiting step in searching for appropriate zeolites to improve these processes is the poor capacity of classical thermodynamic models to predict adsorption behavior, thus requiring the exploration of many unsuccessful possibilities by experimental means before significant improvements are found. In order to provide a general answer to this problem, molecular simulation methods have been developed to address a large array of systems. Various types of statistical bias (configurational bias and reservoir bias) have been associated with parallel tempering to provide efficient sampling of all possible configurations, including when cation mobility is considered together with molecular adsorption. Both nonpolar and electrostatic contributions to energy have been considered. These features are available in a single Monte Carlo software, named GIBBS, which may consider either linear, branched, cyclic or more complex flexible molecules. A special effort has also been devoted to the development of a multipurpose force field to evaluate guest-host interactions. The contribution of these methods is illustrated by several examples in which their results are confronted with available experimental data. The first example pertains to the understanding of cation location in faujasites and its interplay with the adsorption of water. The second example pertains to the adsorption of alkanes in faujasites, where the account of polarization energy allows a good transferability of guesthost potential. Lastly, we consider the competitive adsorption of traces of alkanethiols with the other components of a multicomponent natural gas in high-pressure conditions. Although these Monte Carlo methods still merit numerous improvements, they are already providing a very significant contribution to the general understanding of competitive adsorption and to the design of better processes.
Highlights
Separation processes based on adsorption in zeolites have become a key tool in the oil and gas industry
While the diversity of pore sizes and crystalline structures [2] produces a range of possible effects in which entropy plays a large role, the substitution of silicium by aluminium and the associated presence of charge-compensating cations in the micropores [3] produce a variety of other effects, in which the energetic aspects of polar interactions between the zeolite and guest molecules play a large role
Further cations occupy sites III, which are even less favorable than II, I and I’. Finding this sequence as a result of a simple force field is encouraging for more complex applications involving guest molecules
Summary
Separation processes based on adsorption in zeolites have become a key tool in the oil and gas industry. Some simple models are successful in correlating the behavior of known systems, but they are generally unable to provide a reasonable estimate of adsorption isotherms from the molecular structure of the adsorbate and from the crystalline structure of the zeolite They are unable to predict adsorption selectivity in mixtures, the changes in selectivity which result from modifications of the silicon to aluminium ratio or of the charge-compensating cation. The development of adsorption processes has been based on numerous experiments, with an associated duration and cost that limits the development of this technology This is why molecular simulation has been identified as the necessary theoretical tool to understand the basis of adsorption selectivity and to improve the screening of possible adsorbents when a given separation target is assigned [4]. We will show a few applications, in which we will try to illustrate the capacity of available force fields to model consistently the adsorption properties of hydrocarbons as well as of polar species
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