Zeolite membrane profiles

Figure 24 Simulation results for A B in a zeolite membrane CMR (a) occupancy profiles across the membrane (b) fraction of reactant A flux that slips to product side versus Thiele modulus for different occupancy levels...

Synthesis of membranes with high permeability and selectivity, that is, oriented and thin zeolite membranes. Optimal MR operation requires the membrane flux to be in balance with the reaction rate. A large number of factors - such as the support, organic additives, temperature, and profile - have a significant influence on the microstructure and overall quality of the membrane. However, the precise correlation between the synthesis procedure and conditions and the properties of the resultant zeolite membranes is not clear. In contrast, the majority of membranes synthesized so far are MFI-type zeolite membranes that have pore diameters 5 A, which are still too big to separate selectively small gaseous molecules. Zeolite membranes with pores in the 3 A range should be developed for membrane reactors, to separate small gas molecules on the basis of size exclusion. In addition, a method to produce zeolite membranes without non-zeolite pores or defects has to be found. 

Zeolite membranes, due to their microporous nature and hence the low diffusivity coefficients, could work under mass transfer limited regime due to the high thickness of the zeolite layer. When the membrane acts as a reactant distributor the mass transfer should help to obtain the desired reactant profile in the reactor. In a catalytic zeohte membrane reactor, when the intrinsic reaction rate is higher than the diffusion rate, the mass transfer becomes a limiting factor for the conversion. 

In a catalytic zeolite membrane, especially when the zeolite layer is sufficiently thick, the catalyst distribution can be an important factor in determining the reactor performance due to presence of mass transfer limitations which affect the reactant profile. 

To examine the interaction of heat and mass transport across the zeolite membrane, we have chosen to present the profiles of concentration and temperature for the following conditions. Results are reported in tables or... 

In Fig. 18.7 we present the temperature and concentration profile for this case. The temperature profile has a jump across the surfaces on both sides of the membrane. There is also a jump in the chemical potential (not shown). The concentration in the different parts of the system is shown in Fig. 18.7b. The concentration in the gas-phases is low and the concentration in the zeolite membrane is relatively high. 

It is evident that the ceramic membrane, which is represented in the XRD pattern (see Figure 10.6) by the amorphous component of the XRD profile, was covered by the AlP04-5 molecular sieve, since the crystalline component of the obtained XRD pattern fairly well coincides with the standards reported in the literature [107]. Consequently, the porous support was successfully coated with a zeolite layer, which was shaped by the hydrothermal process as previously described. Thus, a composite membrane, that is, an AlP04-5 molecular sieve thin film zeolite-based ceramic, was produced. 

Fig. 3 One-dimensional loading profiles of benzene across a NaX zeolitic, single crystal membrane. The loading is the spatial average over planes perpendicular to the main diffusion direction (three-dimensional simulations are conducted periodic boundary conditions are employed in the transverse direction and Robin at the membrane interfaces exposed to the high- and low-pressure sides). The inset shows a schematic of the membrane. (View this art in color at www. dekker.com.)...

Te Hennepe HJC, Bargeman D, Mulder MHV, Smolders CA. Zeolite filled sihcone rubber membranes experimental determination of concentration profile. J Membr Sci 1994 89(l-2) 185-96. 

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. 

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