Permeation Through 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. 

For high-quality MFI-type zeolite membranes either Knudsen or configurational diffusion is representative of the flow. Knudsen diffusion occurs in pores where 

The other component of molecular transport through zeoHte membranes is the equilibrium sorption strength between the molecule and zeoHte. In general, this information is reported in the form of adsorption isotherms, which are tabulated for many species and zeoHte structures [22]. For example, Langmuir isotherm commonly describes the adsorpHon behavior of molecules transporhng through MFI-type zeoHte membranes and can be represented by the foUowing equahons  

Equation (10.6) and can be directly substituted into Eq. (10.1) resulting in the expression for surface flux, Jz. 

In this case, it is assumed that D - is independent of q. Equation (10.9) is vahd at low temperatures where surface diffusion is dominant and transport occurs through molecules jumping from adsorption site to adsorption site. 

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Section 6 will deal with theory and practise of permeation through zeolite membranes, and finally examples will be given of the use of zeolites in membrane reactors and catalytic membranes. 

In this chapter examples will be given of permeation and separation characteristics of reported membranes. Accurate models for the description of permeation through zeolitic membranes are indispensable for the engineering implementation of these membranes in... 

The above-mentioned studies reveal several features that determine the permeation through zeolitic membranes as well as their selectivity. Apart from size exclusion due to molecular sieving, both the affinity of the membranes for a given component and the mobility of that component in the pore network of the zeolite play a major role. In this section the importance of these features is shown on the basis of several examples. The emphasis will be on inorganic zeolitic membranes. 

III. THE MODELING OF PERMEATION THROUGH ZEOLITIC MEMBRANES A. Introduction... 

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 of the Maxwell-Stefan equations to permeation through zeolitic membranes was done by Kapteijn et al. [50,56] and Krishna and van den Broeke [57]. Kapteijn showed that both the temperature and occupancy dependence of the steady-state /i-butane flux can accurately be described by Eqs. (20), (24), and (25) [56]. The advantage of using the Maxwell-Stefan description is that it is able to describe both occupancy and temperature... 

Chen and Sholl presented a detailed model for the permeation of CH4/H2 mixtures through membranes constructed from closely packed bundles of single walled carbon nanotubes [13]. Combination of atomically detailed and continuum models that has proven effective in previous treatments of mixture permeation through zeolite membranes was apphed. 

Lito P F, Santiago A S, Cardoso S P, Figueiredo B R and Silva C M (2011), New expressions for single and binary permeation through zeolite membranes for different isotherm models ,7 Membrane Sci, 367,21-32. 

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. 

A theory of gas diffusion and permeation has recently been proposed [56] for the interpretation of experimental data concerning molecular-sieve porous glass membranes. Other researchers [57,58], on the basis of experimental evidences, pointed out that a Stefan-Maxwell approach has to be preferred over a simple Pick one for the modeling of mass transfer through zeolite membranes. 

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. 

The transport mechanisms through zeolite membranes depend on different variables such as operation conditions (especially temperature and pressure), membrane pore size distribution, characteristics of the pore surface of the zeohtic-channel network (hydrophilicity/hydrophobicity ratio), as well as the characteristics of the crystal boundaries and the characteristics of the permeating molecules (kinetic diameter, molecular weight, vapor pressure, heat of adsorption), and their interactions in the mixture. 

The results reported here are restricted to experiments undertaken with silicalite-1 membranes. To ensure that the developed membranes were defect free, gas relative permeability experiments were conducted. In these experiments the membrane was initially strongly equilibrated by a strongly adsorbed gas (CO2) and subsequently a non-adsorbable gas such as He permeated through the membrane. It was found that as the pre-adsorbed amount of CO2 increased there was a sharp drop in He permeability, compared to the corresponding value on a clean zeolite membrane. At a certain partial pressure of CO2, He could no longer permeate... 

The free aperture of the main 100 channels in Y-type zeolite is 0.74 nm [7] and is much larger than the diameter of CO2 and N2 molecules. If the concentrations of CO2 and N2 in the micropores of the Y-type zeolite membrane are equal to those in the outside gas phase, these molecules permeate through the membrane at a low CO2/N2 selectivity. However, this was not the case. Carbon dioxide molecules adsorbed on the outside of the membrane migrate into micropores by surface diffusion. Nitrogen molecules, which are not adsorptive, penetrate into micropores by translation-collision mechanism from the outside gas phase. 

Molecular movement under non-equilibrium conditions (i.e. under the influence of differences in the overall concentration) is associated with a macroscopic particle transfer and is generally referred to as transport diffusion. Transport diffusion may be measured under both steady-state conditions (e.g., by studying the permeation rates through zeolite membranes [187-... 

Single-gas permeation of different gases through 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. 

Two types of gas permeation measurements are generally performed through zeolite membranes to identify their mass transport properties the pressure drop and the concentration gradient methods. 

There are two main separation techniques to extract hydrogen membranes and adsorption. Studies indicate that separation of hydrogen through zeolite membranes is ineffective for FCV applications, since the toluene content in the permeate is too high (>2000 ppm). When toluene was present at high concentrations, the diffusion of hydrogen was hindered due to a strong adsorption of toluene in the membranes. Palladium membranes are... 

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