Zeolite membrane system

High separation factors can be obtained with microporous membranes with a pore diameter smaller than 2 nm and are realised with carbon, silica and zeolite membrane systems. The description of these systems is still in its infancy. 

The application of zeolite membranes in microreactors is still in an early stage of development, and suffers sometimes from unexpected problems arising from template removal [70]. However, several application examples of zeolite membranes in microstructured devices have been demonstrated yielding similar advantages as were to be expected from experiences on the macroscale. Because of the high surface to volume ratio of microreactors, the application of zeolite membranes in these systems has great potential. 

When using the microporous zeolite membrane (curve 3) the N2 permeance decreases when the pressure increases such a behaviour can be accounted for by activated diffusion mechanisms [21], which are typical of zeolite microporous systems. In such systems the difflisivity depends on the nature and on the concentration of the diffusing molecule which interacts with the surface of the pore. For gases with low activation energies of diffusion, a decrease of the permeability can be observed [22]. 

The more permeable component is called the fast gas, so it is the one enriched in the permeate stream. Permeabihly through polymers is the product of solubility and diffusivity. The diffusivity of a gas in a membrane is inversely proportional to its kinetic diameter, a value determined from zeolite cage exclusion data (see Table 20-26 after Breck, Zeolite Molecular Sieves, Wiley New York, 1974, p. 636). Tables 20-27, 20-28, and 20-29 provide units conversion factors useful for calculations related to gas-separation membrane systems. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Other MFl-type zeolite-sorbate systems are known to exhibit similar behavior. In a recent study, Yu et al. [34] reported that at saturated loadings of -hexane a single MFl-type zeolite unit cell has an overall volume expansion of 2.3%, which can correlate to shrinkage in non-zeoUtic pores up to 7 nm for a 1 tm crystal when isotropic expansion is assumed. It was demonstrated that, even in membranes with large number of defects, the crystallite swelling caused the membrane to achieve significant separation between n-hexane and trimethylbenzene, iso-octane and 2,2-dimethylbutane using pervaporation [34]. 

There are several models to describe intracrystalline diffusion (step 3) in microporous media. Diffusion in zeolites is extensively described in Ref. 30. For the modeling of permeation through zeolitic membranes, such a model should take the concentration dependence of zeolitic diffusion into account. Moreover, it should be easy applicable to multicomponent systems. In Section III.C, several models will be discussed. 

The application of the Maxwell-Stefan theory for diffusion in microporous media to permeation through zeolitic membranes implies that transport is assumed to occur only via the adsorbed phase (surface diffusion). Upon combination of surface diffusion according to the Maxwell-Stefan model (Eq. 20) with activated-gas translational diffusion (Eq. 12) for a one-component system, the temperature dependence of the flux shows a maximum and a minimum for a given set of parameters (Fig. 15). At low temperatures, surface diffusion is the most important diffusion mechanism. This type of diffusion is highly dependent on the concentration of adsorbed species in the membrane, which is calculated from the adsorption isotherm. At high temperatures, activated-gas translational diffusion takes over, causing an increase in the flux until it levels off at still-higher temperatures. 

Application at high temperature requires robust and thermostable systems. Both for ceramically and stainless-steel-supported systems the thermostability has been demonstrated. So, in spite of the different thermal expansion coefficients, the asymmetric membrane remains intact. However, there are no data available on the resistance of zeolitic membranes to thermal stresses, as a result of, for example, large sudden changes in temperature. The siainless-steel-supported system seems the most promising configuration... 

The lack of methods for a fast and reliable assessment of membrane quality is stiU one of the outstanding issues in zeolite membrane development. The usual meaning of the term quality relates to the ability of the membrane to carry out a given separation, therefore, is a system-specific property and the universal membrane quality test does not exist. In general, specific permeation measurements at different temperatures, either of single gases (or vapors) or of multicomponent mixtures in the gas or liquid (pervaporation) phase, provide extremely useful information on the effective pore structure of the membrane, on the... 

Single-gas permeation of different gases on zeolite membranes is frequently used to estimate the molecular sieving ability of a given membrane. From the absence of a clear cutoff it is possible to conclude that the mass transport is not controlled by the zeolite-pore system. 

While, in general, the investigations discussed in this section do not belong to the field of zeolite-membrane reactors, they are mentioned here because they provide interesting clues and ideas for further development of existing systems. 

New Applications of Zeolite-Membrane Reactors Microchemical Systems... 

The efforts and advances during the last 15 years in zeolite membrane and coating research have made it possible to synthesize many zeolitic and related-type materials on a wide variety of supports of different composition, geometry, and structure and also to predict their transport properties. Additionally, the widely exploited adsorption and catalytic properties of zeolites have undoubtedly opened up their scope of application beyond traditional separation and pervaporation processes. As a matter-of-fact, zeolite membranes have already been used in the field of membrane reactors (chemical specialties and commodities) and microchemical systems (microreactors, microseparators, and microsensors). 

A polycrystalline Y-type zeolite membrane was formed by hydrothermal synthesis on the outer surface of a porous a-alumina support tube, which was polished with a finely powdered X-type zeolite for use as seeds. When an equimolar mixture of CO2 and N2 was fed into the feed side, the CO2 permeance was nearly equal to that for the singlecomponent system, and the N2 permeance for the mixture was greatly decreased, especially at lower permeation temperatures. At 30"C, the permeance of CO2 was higher than 10- mol m-2 s- Pa-, and the permselectivity of CO2 to N2 was 20-100. 

Defect-free zeolite membranes have so far only been produced for membranes of the MFI (silicalite type) with thicknesses of about 50 im on stainless steel supports and 3-10 pm on alumina and carbon supports. They are produced by in situ methods of zeolite crystals grown directly on the support system. There are some reports of formation of defective membranes with, e.g., zeolite A. Much more research is needed to widen the range of available zeolite membrane types especially small and wide pore systems. The permeance values of the defect-free membranes is lower than that of the amorphous membranes (see Chapter 6) and to improve this the layer thickness must be decreased together with improving the crystal quality (no impurities, no surface layers, high crystallinity, crystal orientation) and microstructure (grain boundary engineering). 

A schematic picture of different t5q)es of pores is given in Fig. 9.1 and of main types of pore shapes in Fig. 9.2. In single crystal zeolites the pore characteristics are an intrinsic property of the crystalline lattice [3] but in zeolite membranes other pore types also occur. As can be seen from Fig. 9.1, isolated pores and dead ends do not contribute to the permeation under steady conditions. With adsorbing gases, dead end pores can contribute however in transient measurements [1,2,3]. Dead ends do also contribute to the porosity as measured by adsorption techniques but do not contribute to the effective porosity in permeation. Pore shapes are channel-like or slit-shaped. Pore constrictions are important for flow resistance, especially when capillary condensation and surface diffusion phenomena occur in systems with a relatively large internal surface area. 

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