Transport in Zeolite Membranes

In zeolites, the rate of molecular diffusion depends on the position of charge-compensating cations in the pore network and the structure of the framework [77-81], Since mass transport in micropo-rous media takes place in an adsorbed phase [82,83], this transport can be envisaged as activated molecular hopping between fixed sites [60,82,84] (for more details, see Section 5.9.1). 

The Physical Chemistry of Materials Energy and Environmental Applications 

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Transport in zeolite membranes is a complex process that is governed by adsorption and diffusion. The mechanism depends strongly on pore size, pore network structure, size and shape of diffusing molecules, interac-... 

Theoretical development. Transport in zeolite membranes is a complex process. The multi-component transport and separation behavior through zeolitic and non-zeolitic pathways in the membranes at industrially relevant operating conditions have to be clarified. 

Hardly any research has been performed on ferrierite in zeolite membrane configurations. Matsukada et al. [50,51] prepared a ferrierite-based membrane by the frequently used Vapour-phase Transport Method. By using ethylenediamine, triethylamine and steam (under hydrothermal conditions), a porous alumina support, covered with the proper aluminosilicate gel, was transformed into a alumina supported (30 pm thick) ferrierite layer. No permeation with 1,3,5-triisopropylbenzene coirld be observed, proving the layer to be defect-free. Fluxes of small gases were found in the order of 10" -10 mol.m. s. Pa and decreased in the order H2>He>CH4>N2>02>C02... 

Two important considerations that appear in zeolite membranes should be taken into account for the transport of the molecules through the membrane [113] ... 

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). 

The lack of methods for a fast and reliable assessment of membrane quality is still 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 with a reasonable flux therefore, a system-specific property and a universal membrane quality test do not exist. In general, specihc 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 existence of intercrystalline defects, and amorphous material and permeation fluxes, as well as information about the main transport controlling effect (adsorption or diffusion). 

The current commercial zeolite membranes, developed for pervaporation, are not yet useful in gas separations (H2/CO2 selectivity for NaA membranes of Mitsui and Inocermic are 6 and 5.6, respectively) because of the presence of large inter-crystalline defects. They, furthermore, participate in the separation process. During pervaporation the water fills the intra-crystalline and intercrystalline pathways. However, much effort is in progress to produce defect free zeolitic membrane also for gas separations. In this chapter the application of zeolite membranes in gas separations is reported and deeply discussed. The main strategic methods used for the membrane preparation and mass transport through zeolite membranes are also dealt with. 

MFl has been extensively studied in zeolite membranes preparation due to its pore size suitable for several industrially important separations." " Using MFl supported membranes it was demonstrated that the CO2/N2 separation factor increases with CO2 feed composition because of the higher CO2 adsorption on the zeolite wall, which consequently limits the N2 transport in zeolitic channels." The selectivity of this gas species reaehes the value of 20 at 180 °C when the carbon dioxide composition is higher than 60% in the feed. Other researchers using membranes with the same topology found the same effect of the CO2 feed concentration on its separation from nitrogen." ... 

In this chapter the application of zeolite membranes as membrane reactors will be illustrated. A short overview on the methods to synthesize membranes will be given. Basic concepts on mass transport through zeolite membranes will be also shown. Finally, the application of the zeolite membranes for use as reactors will be discussed in more detail. 

There are different ways to treat this type of membrane system. Equations [18.4]-[18.12] are formulated such that we can calculate fluxes and temperature profiles on the membrane surface. For other formulations, we refer to the work of Kjelstrup and Bedeaux (2(X)8). In this case, we have the resistivities as a function of pressure for the gas-zeolite interface, and the resistivities for transport in the membrane as a function of the local temperature and concentration. The cases of equal temperature difference over the membrane and isobaric transport, can be solved directly by solving Equations [18.4], [18.5] and [18.9]-[18.12]. In addition the total energy balance of Equation [18.2] is used. This leads to a system with eight equations and eight unknowns. The equations can be solved numerically. In this case, we wish to find the fluxes, J, j, j q, and j °. In addition, we obtain the temperature and concentration profiles across the membrane. 

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. 

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... 

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... 

Transport and adsorption processes in microporous materials have been, during the last years, a topic of significant research activity [19,92-94,98], Section 5.9.2 dealt with the phenomenological description of diffusion in zeolites. Applying this methodology to a membrane, it is possible to express the flux of a gas through the zeolite membrane in isothermal conditions as follows [19,70,92-94]... 

Zeolite membranes may play either a passive or an active role in catalytic (organic) conversion reactions and the potential applications of zeolite membrane reactors are quite promising. Both liquid phase and gas phase reactions may advantageously be carried out in a membrane reactor, and transport from the reaction zone is promoted by continuous removal of the permeating molecules. 

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. 

W.J.W. Bakker, G. Zheng, M. Makkee, F. Kapteijn, J.A. Moulijn, E.R. Geus, and H. van Bekkum, Single- and Multi-component transport through metal-supported MR zeolite membranes, in Precision Process Technology (M.P.C. Weijnen and A.A.H. Drinkenbuig, eds.), Kluwer Academic Publishers, Amsterdam, 1993, p. 425. 

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