Membranes transport, zeolite

Defect-free membranes comprising zeolites and amorphous glassy perfluoropolymers can be prepared by modifying the surface of the filler. The pure gas permeation experiments of a series of Teflon AF 1600 membranes with various amounts of 80 and 350nm silicalite-1 crystals cannot be interpreted on the basis of the Maxwell model, but are compatible with a model in which a barrier to transport exists on the zeolite surface and a lower density polymer layer surrounds the crystals. With a small zeolite size (80nm) the low density layers around the crystals may coalesce and form percolation paths of lesser resistance and less selectivity. Silicalite-1 crystals improve the CO2/CH4 selectivity of Hyflon AD60X, and drive the N2/CH4 selectivity beyond the Robeson s upper bound. It also turns out that the presence of silicaUte-l crystals, like fumed silica, promote the inversion of the methane/butane selectivity of Teflon AF2400 in mixed gas experiments. 

In dense stmctures there is no intentional void space with dimensions much larger than the atomic building blocks. Zeolites that may have a large stmctural void space in their crystal stmcture are generally not considered dense stmctures. Dense membranes generally have an ideal selectivity in which only one species is transported. The separative layer in supported polymer membranes is also considered dense, even though it may not be 100% selective for one component. In dense inorganic membrane transport, molecules such as H2 and O2 are converted at the surface into (smaller) atomic, ionic, and/or... 

Transport of heat and mass across a zeolite membrane 643 Enhancing membrane transport... 

In order to design a zeoHte membrane-based process a good model description of the multicomponent mass transport properties is required. Moreover, this will reduce the amount of practical work required in the development of zeolite membranes and MRs. Concerning intracrystaUine mass transport, a decent continuum approach is available within a Maxwell-Stefan framework for mass transport [98-100]. The well-defined geometry of zeoHtes, however, gives rise to microscopic effects, like specific adsorption sites and nonisotropic diffusion, which become manifested at the macroscale. It remains challenging to incorporate these microscopic effects into a generalized model and to obtain an accurate multicomponent prediction of a real membrane. 

This paper describes the morphological and transport properties of a composite zeolite (silicalite) - alumina membrane. Some advantages obtained in combining the membrane with a conventional fixed-bed catalyst are also reported. 

Vroon, Z.A.E.P., Keizer, K., Gilde, M.J., Verweij, H., and Burggraaf, A.J. (1995) Transport properties of alkanes through ceramic thin zeolite MEI membranes. 

The present review of zeolite membrane technology covers synthesis and characterization methods as well as the theoretical aspects of transport and separation mechanisms. Special attention is focused on the performance of zeolite membranes in a variety of applications including liquid-liquid, gas/vapor and reactive... 

Transport Theory and Separation Capability of Zeolite Membranes... 

For single-component gas permeation through a microporous membrane, the flux (J) can be described by Eq. (10.1), where p is the density of the membrane, ris the thermodynamic correction factor which describes the equilibrium relationship between the concentration in the membrane and partial pressure of the permeating gas (adsorption isotherm), q is the concentration of the permeating species in zeolite and x is the position in the permeating direction in the membrane. Dc is the diffusivity corrected for the interaction between the transporting species and the membrane and is described by Eq. (10.2), where Ed is the diffusion activation energy, R is the ideal gas constant and T is the absolute temperature. 

In this paper, we consider a zeolite membrane comprised of a single zeolite crystal. While real zeolite membranes are polycrystalline, this approximation is useful because many experimental studies aim to characterize transport that occurs purely through zeolite pores [6], The steady state flux, J, through this membrane is... 

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