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Item type:Publication, Access status: with files , Predicting local transport coefficients at solid-gas interfacesThe regular nanoporous structure make zeolite membranes attractive candidates for separating molecules on the basis of differences in transport rates (diffusion). Since improvements in synthesis have led to membranes as thin as several hundred nanometers by now, the slow transport in the boundary layer separating bulk gas and core of the nanoporous membrane is becoming increasingly important. Therefore, we investigate the predictability of the coefficient quantifying this local process – the surface permeability α – by means of a two-scale simulation approach. Methane tracer-release from the one-dimensional nanopores of an AFI-type zeolite is employed. Besides a pitfall in determining α on the basis of tracer exchange, we, importantly, present an accurate prediction of the surface permeability using readily available information from molecular simulations. Moreover, we show that the prediction is strongly influenced by the degree of detail with which the boundary region is modeled. It turns out that not accounting for the fact that molecules aiming to escape the host structure must indeed overcome two boundary regions yields too large a permeability by a factor of 1.7 to 3.3, depending on the temperature. Finally, our results have far-reaching implications for the design of future membrane applications.Publicationtype: Journal ArticleScopus© Citations 36 261 496 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Adsorption and diffusion in zeolites : the pitfall of isotypic crystal structures(Taylor & Francis, 2011-06-27); ; ; ; The influence of isotypic crystal structures on adsorption and diffusion of methane in all-silica LTA, SAS and ITE zeolites is studied. Results obtained with the experimental structures are compared with structure predictions and approximations that are commonly employed. The results indicate that diffusion coefficients are much more affected than Henry coefficients. In fact, orders of magnitude deviations in the diffusivity can be observed and a systematic parameter study finally gives rise to the correlation between structure sensitivity and diffusion-window size.Publicationtype: Journal ArticleScopus© Citations 13 330 473 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the effects of the external surface on the equilibrium transport in zeolite crystalsWith the aid of molecular simulation techniques (molecular dynamics [MD], Grand-Canonical Monte Carlo [GCMC], and reactive flux correlation function [RFCF]), the influence of the external surface on the equilibrium permeation of methane and ethane into and out of an AFI-type zeolite crystal has been studied. In particular, “extended dynamically corrected transition state theory”, that has been proven to describe the transport of tracers in periodic crystals correctly, has been applied to surface problems. The results suggest that the molecules follow paths that are close to the pore wall in the interior, and also at the crystal surface. Moreover, the recrossing rate at the surface turns out to be non-negligible, yet, in contrast to the intracrystalline recrossing rate, remains almost constant over loading which gives indication to diffusive barrier crossing at the crystal surface. As a consequence of very different adsorption and desorption barriers, the corresponding permeabilities are shown to be not equal for one and the same condition (T , p). The critical crystal length, beyond which surface effects can be certainly neglected, is computed on basis of flux densities. Entrance/exit effects, in the present cases, are practically important solely for ethane at low pressures. The influence of the type of external surface on the surface flux is, hereby, rather small, because the transport at the surface is controlled by the slow supply from the gas phase. This has been evidenced by a simplified thermodynamic model that has been derived within this work and which is based on rapidly assessable simulation data. Finally, we propose a procedure for estimating the importance of different factors that have an impact on surface effects.Publicationtype: Journal ArticleScopus© Citations 41 288 551 - Some of the metrics are blocked by yourconsent settings
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