Zeolitic microporous membranes

Microporous inorganic membranes have pores that can be tuned to the molecular size. This enables zeolite membranes to carry out separations (i.e., the separation of isomer compounds) that are not possible with membranes in which only Knudsen selectivity is possible. Moreover, zeolite microporous membranes can compete with traditional energy costly separation methods, such us distillation of mixmres of close boiling point components, separation of mixtures of low concentration, and azeotropic distillation. 

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]. 

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. 

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

Recent developments demonstrate possibilities for inorganic C02 selective membranes. Microporous membranes with strong C02 adsorption show C02 selectivity if other gas species are hindered in accessing the pores. For instance, at intermediate temperatures, limited C02 selectivity to N2 (to about 400 °C) and H2 (to about 200 °C) is reported for MFI zeolite membranes [96]. Also, at high pressure (10-15 bars) C02 selectivity has been demonstrated in MFI membranes (C02/N2 separation factor ... 

As have been seen above adsorption plays an important role in permeation through microporous membranes. So, single and multicomponent adsorption isotherms are required for a successful modelling of the permeation behaviour. An extensive treatment of the recent state of the art of zeolite permeation modelling is given by Van de Graaf et al. [70]. A shortened treatment follows here. 

Adsorption plays an important role in permeation through microporous membranes. First of all, steps 1 and 5 involve adsorption and desorption processes. Second, the concentration dependence of the diffusion coefficient is often described by the adsorption isotherm. Some data on adsorption in zeolites will be presented in Section III.D. 

J.P. Boom, D. Bargeman, and H. Strathmann, Zeolite filled membranes for gas separation and pervaporation. Zeolites and related microporous materials State of the art 1994 Part B, Proc. 10th Int. Zeol. Conf., Garmisch-Panenkirchen (J. Weitkamp, H.G. Kaige, H. Pfeifer, and W. Holderich, eds.), Elsevier, Amsterdam, 1994, p. 1167. 

E.R. Geus, W.J.W. Bakker, J.A. Moulijn, and H. van Bekkum, High-temperature stainless steel supponed zeolite (MFI) membranes Preparation, module construction and permeation experiments, Microporous Mater. 7 131 (1993). 

FIGURE 6.9 Microporous membrane structures (a) resulting from packing and sintering of ceramic nanoparticles and (b) ultramicroporous channels in the crystalline structure of a zeolite. 

Okamoto et al. [141] studied several water/organic systems that are listed in Table 10.6, and the performance of the zeolite A membrane was excellent for aU the separations. These results could be also compared with the ones obtained using microporous sflica membranes [153]. Sflica membranes, for a water/dioxane (10/90 wt%) mixture at 60°C, showed a separation factor of 125 and a water flux of 2.2 kg/m h. For dymethilformamide, (DMF), the results obtained for a mixture of water/DMF (13.2/86.8 wt%) were 30 and 0.225 kg/m h for the separation factor and water flux, respectively. In both separations, zeohte A outperforms the microporous silica membrane. 

More specifically in the area of this overview, zeolite materials constitute the main group of microporous membranes with regard to their potential membrane-reactor apphcations. The wide variety of existing zeolite structures, together with the possibility of modifying their adsorption and catalytic properties, provides us with a working material of high flexibihty. As a... 

Packings can also be obtained by a packing of plates as shown in Fig 2.2 [2], Note that here the pores have a slit-shaped structure with a limiting pore diameter in only one direction. Because thermostable particles with diameters below 5-6 nm are very difficult to make, microporous membranes with a pore diameter below 2 nm cannot be produced by packings of spherical or plateshaped particles. Packings of fibrillous particles can result in microporous membranes as observed by de Lange et al. [3] with polymeric silica particles (see also the Chapter 8). Finally, zeolite membranes are formed by intergrown... 

Several other interesting methods to obtain microporous membranes are reported in literature. Except for zeolite membranes (see Section 8.2.3) they are probably of less practical or commercial importance and therefore will only be briefly summarised. 

Microporous membrane (pore diameter smaller than 2 nm) synthesis is still in its infancy. Microporous membrane layers of amorphous silica and sUica-ti-tania composites, zeolite, and carbon are reported on supports of (a or y) alumina (for silica and zeolite) or on stainless steel (for zeolite) or on carbon (for carbon or zeolite). Seeding up of the different processes used to obtain larger membrane surface areas have to be demonstrated. 

Overview and separation data of typical supported microporous zeolite (MFI) membranes... 

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. 

Molecular models can considerably impact the chemical process industry. Obviously, numerous problems fall beyond the realm of conventional molecular simulation (see the example above on zeolitic membranes). Examples include dynamics of protein folding, diffusion through microporous membranes and human cells, formation of quantum dots in heteroepitaxial growth of semiconductors, and pattern formation on catalyst surfaces. 

Typical applications of zeolite membranes in reactors include i) conversion enhancement either by equilibrium displacement (product removal) or by removal of catalyst poisons/ inhibitors and ii) selectivity enhancement either by control of residence time or by control of reactant traffic. A large number of examples are reported and discussed in [49,50,52], Several of them are reported in fable 3. The use of a zeolite membrane as a distributor for a reactant has been attempted for the partial oxidation of alkanes such as propane to propene [137], or n-butane to maleic anhydride [138]. Limited performances were obtained because the back-diffusion of the alkane is hardly controllable with this type of microporous membrane [139]. 

Kumakiri I, E. Landrivon, S. Miachon, J.A. Dalmon, In Proc. Ini. Workshop on Zeolitic and Microporous Membranes, IWZMM 2001, Purmerend (The Netherlands), July 1-4, 2001. 

Langhendries et al [5.74] analyzed the liquid phase catalytic oxidation of cyclohexane in a PBMR, using a simple tank-in-series approximate model for the PBMR. In their -reactor the liquid hydrocarbon was fed in the tubeside, where a packed bed of a zeolite supported iron-pthalocyanine catalysts was placed. The oxidant (aqueous butyl-hydroperoxide) was fed in the shellside from were it was extracted continuously to the tubeside by a microporous membrane. The simulation results show that the PBMR is more efficient than a co-feed PBR in terms of conversion but only at low space times (shorter reactors). A significant enhancement of the organic peroxide efficiency, defined as the amount of oxidant used for the conversion of cyclohexane to the total oxidant converted, was also observed for the PBMR. It was explained to be the result of the controlled addition of the peroxide, which gives low and nearly uniform concentration along the reactor length. 

Membrane separators offer the possibility of compact systems that can achieve fuel conversions in excess of equilibrium values by continuously removing the product hydrogen. Many different types of membrane material are available and a choice between them has to be made on the basis of their compatibility with the operational environment, their performance and their cost. Separators may be classified as (i) non-porous membranes, e.g., membranes based on metals, alloys, metal oxides or metal—ceramic composites, and (ii) ordered microporous membranes, e.g., dense silica, zeolites and polymers. For the separation of hot gases, the most promising are ceramic membranes. 

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