W.J.W. Bakker

Permeation characteristics of a metal-supported silicalite-1 zeolite membrane

Abstract Permeation data are presented which give an overview of the permeation and separation characteristics of a metal supported silicalite-1 zeolite membrane over a broad temperature and pressure range. Methane, ethane, ethene, propane, propene, n -butane, i -butane, carbon dioxide, hydrogen, and i -octane are used as probe molecules and helium is used as sweep gas. One-component and binary systems are studied in a temperature range of 193–673 K and a pressure range of 0.05–500 kPa. Large differences have been found between the different one-component permeation fluxes, which amounts up to a factor of about 500 between methane and the bulky iso -butane. The permeation fluxes at 295 K, generally decrease with increasing molecular size. The alkenes permeate faster than their corresponding alkanes. Diffusion coefficients calculated with the Maxwell-Stefan equations are in accordance with the literature. A remarkable temperature dependency has been observed. For the bulky i -butane the permeance increases steadily with temperature. For methane, ethane, and n -butane a maximum in permeation is observed and for methane and ethane also a minimum. This maximum can be explained by the combined temperature dependency of diffusion and adsorption in configurational mass transport. The minimum is explained by the occurrence of a Knudsen-like mass transport at low occupancy and high temperature. In many cases the separation selectivity of a mixture does not reflect the one-component permeation ratio. Besides molecular sieving and difference in diffusivity, difference in adsorption appears to be a key factor in separation selectivity. The permeation of weakly adsorbing molecules (e.g. hydrogen at 295 K) can drop over two orders of magnitude in the presence of strongly adsorbing molecules (e.g. n -butane at 295 K). This results in high separation selectivities favouring the strongest adsorbing component. Typical separation selectivities for hydrogen/ n -butane (at 295 K, 95 kPa/5 kPa), n -butane/ i -butane (at 295 K, 50 kPa/50 kPa), and methane/ i -octane (at 423 K, 25 kPa/5 kPa) mixtures, are 125, 27 and > 300, respectively. An inversion in separation selectivity is observed during a temperature programmed permeation which is explained from the temperature dependence of adsorption. The membrane appears to be very stable upon thermal cycling (193–673 K) and the permeation characteristics have changed less than 10% over the testing period of 1.5 year.

High-temperature stainless steel supported zeolite (MFI) membranes: Preparation, module construction, and permeation experiments

Abstract Continuous layers of MFI (silicalite-1; Si-rich ZSM-5) have been prepared on porous, sintered stainless steel supports. Similar metal supported MFI membranes of ∼ 50 μm thickness have been grown within stainless steel membrane modules in order to perform (high-temperature) permeation experiments. As-synthesized layers are found to be gas-tight even for small molecules such as neon. The supported MFI layers remain thermomechanically stable upon calcination at 400°C in air to remove template ions (tetrapropylammonium). Gas permeation experiments have been performed using neon, methane, n-butane, and isobutane according to the Wicke—Kallenbach principle with helium as a purge gas. The sequence of the pure gas permeabilities at room temperature and 0.3 bar partial pressure difference is methane >n-butane > neon a isobutane, demonstrating that the permeation is based on both adsorption and diffusion. The deviating permeation behaviour between the butane isomers is attributed to the bulkiness of isobutane, which is also reflected in the substantially lower adsorption capacity as compared with n-butane. In experiments using binary mixtures of strongly (butane isomers) and weakly (methane) adsorbing species, the permeation rate of the former is hardly affected, whereas for the latter a drop in permeability of some two orders of magnitude is observed. At higher temperatures (up to 350°C) with a constant feed composition, the methane permeation rate increases as a result of the decreased adsorption of n-butane. The MFI layer retains its separation potential after several heating and cooling cycles.

Permeation and separation behaviour of a silicalite-1 membrane

Abstract The permeation behaviour of single component and binary mixtures of hydrogen, n-butane and carbon dioxide through a silicalite-1 membrane as a function of temperature (steady state) and time (transient) are presented. Multicomponent permeation can be well described by the Generalized Maxwell-Stefan equations.

Temperature- and occupancy-dependent diffusion of n-butane through a silicalite-1 membrane

The permeation flux of n-butane through a silicalite-1 (MFI) membrane as a function of the partial feed pressure at 300 K and as a function of the temperature at 0.5 bar partial pressure is excellently described by a Maxwell-Stefan diffusion model in the temperature range 300–630 K, which takes into account the occupancy dependency. The Maxwell-Stefan diffusivity is independent of the occupancy over the whole temperature range and is consistent with an activated process having an activation energy of nearly 30 kJmol. The Fickian diffusivity increases strongly with increasing occupancy. The obtained diffusivity values compare well with frequency response and square wave values, but deviates from nuclear magnetic resonance (NMR) and quasi-elastic neutron scattering (QENS) values, especially with respect to the stronger temperature dependency.

Transport and separation properties of a silicalite-1 membrane—I. Operating conditions

Results are reported for the one-component permeation of a number of gases through a silicalite-1 membrane. The effect of several operating conditions, like the temperature, the feed pressure, the sweep gas flow rate, and the orientation of the membrane, on the flux are discussed. The optimal experimental conditions are identified for both steady state and transient permeation. Results for the flux as function of the pressure, 10–900 kPa, and as function of the temperature, 200–700 K, are modelled taking two diffusion mechanisms into account. Permeation results are reported for two different silicalite-1 membranes. A small difference in the permeance is observed, indicating a high reproducibility of the zeolite membrane synthesis. The contribution of the equilibrium isotherm to the flux and the permeance is demonstrated. A comparison is made between results obtain with helium and argon as the sweep gas. Finally, results are reported for the permeation of pure gases through a silicalite-1 membrane modified by silanation.

Permeation and separation of light hydrocarbons through a silicalite-1 membrane: application of the generalized Maxwell-Stefan equations

Abstract Single-component permeation data are given for methane, ethane, propane, ethene and propene through a silicalite-1 membrane of approximately 40 μm thickness at 293 K as a function of their partial pressure. The permeation fluxes generally decrease with increasing molecular size, while the alkenes permeate more rapidly than their corresponding alkanes at identical conditions. In 1:1 mixtures of ethane-ethene and propane-propene (1 bar total pressure) the alkanes permeate faster, yielding selectivity factors of 1.9 and 1.3 respectively. The generalized Maxwell-Stefan (GMS) equations, adapted for surface diffusion, could describe the permeation data well. The unary systems yielded diffusivity data that were fairly constant or varied at most by a factor of 2–3. These diffusivities compare well with literature values obtained with other (transient) techniques that yield transport diffusivities. The binary system permeation data could be quantitatively described by the GMS equations without exchange contributions (“single-file” diffusion) and need only the diffusivity values of the unary permeation experiments.

Single and Multi-Component Transport through Metal-Supported MFI Zeolite Membranes

Continuous zeolite (silicalite; MFI) layers were grown on a porous sintered stainless steel support in an all metal high-temperature membrane reactor. To test the application potentials of these layers, permeation experiments, according the Wicke-Kallenbach method, were performed using noble gases, lower n-alkanes, the butane-isomers, CFC-12 and 2,2,4-trimethylpentane.

Permeation measurements on in situ grown ceramic MFI type films

Continuous MFI type films were grown on macroporous ceramic clay-type supports. Permeation experiments were performed using a Wicke-Kallenbach experimental set-up. Applying helium as an inert carrier gas, the permeation behaviour of strongly (n-butane, isobutane), and weakly adsorbing species (neon, argon, methane) was studied at room temperature for both ’pure’ gases, and binary mixtures. Significant differences in permeation behaviour were found between strongly and weakly adsorbing gases, although steady state permeation rates varied less than one order of magnitude. The lower diffusivity of heavier alkanes is compensated.by the higher sorbate concentration within the zeolite micropores. Accordingly, only low selectivities were found for binary mixtures, where the permeation rate is governed by the slowest moving species. For sorption near or within the Henry region, substantially higher selectivities were obtained. The presence of a macroporous layer on one side of the membrane will lead to reduced permeation rates, especially for strongly adsorbing molecules.

Permeation and separation behaviour of a silicalite (MFI) membrane

Publisher Summary Combining catalytic conversion processes with membrane permeation in-situ, that is, in the reactor configuration, offers in principle many new opportunities such as increased yields of equilibrium limited reactions, increased selectivities in complex reaction networks and coupling of catalytic reactions by mass and/or heat exchange. This requires controlled addition of reactants or separation of products under reaction conditions. Hence, knowledge of permeation and separation characteristics is indispensable for the design and process control of this emerging new type of reactors. The behavior of membranes operating in the molecular- and Knudsen type diffusion region can be predicted on the basis of established theories. If the membrane pores approach the size of molecular dimensions, however, and the so called configurational diffusion and molecular sieving are operative, hardly any theory and data are available to predict permeation and separation properties. This is mainly due to the fact that up to now these zeolite type of membranes are hardly available. Success has been achieved in preparing a silicalite (MFl-type) membrane, which turned out to possess high permeability and interesting and surprising separation properties, on which the chapter reports further with new insights and results.