Membrane reactors permeability

Membrane filtration Membrane module Membrane permeability Membrane process Membrane processes Membrane reactor Membrane roofing Membranes... 

Membrane reactors are known on the macro scale for combining reaction and separation, with additional profits for the whole process as compared with the same separate functions. Microstructured reactors with permeable membranes are used in the same way, e.g. to increase conversion above the equilibrium limit of sole reaction [8, 10, 11, 83]. One way to achieve this is by preparing thin membranes over the pores of a mesh, e.g. by thin-fihn deposition techniques, separating reactant and product streams [11]. 

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

Carbon molecular sieve membranes Resistant to contaminants Intermediate hydrogen flux and selectivity Intermediate hydrogen flux and selectivity High water permeability Pilot-scale testing in low temperature WGS membrane reactor application Need demonstration of long-term stability and durability in practical applications... 

Uemiya, S., N. Sato, H. Ando, T. Matsuda, and E. Kikuchi, Steam reforming of methane in a hydrogen permeable membrane reactor, Appl. Catal., 67,223-230,1991c. 

V-Ti-Ni alloys and Fe- /Co-Based metallic glasses have been evaluated with respect to hydrogen permeability for potential use in hydrogen purification membrane reactor application. Microstructural characterization of the V-Ti-Ni alloy using SEM has shown similar microstructural features to a previously evaluated Nb-Ti-Ni alloy namely, the occurrence of a primary phase surrounded by interdendritic eutectic. 

The wall of the small intestine is permeable to water and to small molecules such as the amino acids produced by protein breakdown and sugars produced by carbohydrate breakdown so this system is a reactor-separator combination, a membrane reactor. Finally the undigested food passes into the large intestine, where more water is removed through the permeable wall before exiting the reactor. 

Chemical species can transfer between phases, and this represents the coupling between the mass-balance equations. This geometry looks like a membrane reactor in which a permeable area A (dashed lines) separates the phases, but all multiphase reactors can be described by this notation. 

One example of membrane reactors is oxidation, in which oxygen from one phase diffuses from one side of an oxygen-permeable membrane to react with a fuel on the other side of the membrane. This avoids a high concentration of O2 on the fuel side, which would be flammable. A catalyst on the fuel side of the membrane oxidizes the fuel to partial oxidation products. One important process using a membrane reactor is the reaction to oxidize methane to form syngas,... 

In all cases studied, the membrane reactor offered a lower yield of formaldehyde than a plug flow reactor if all species were constrained to Knudsen diffusivities. Thus the conclusion reached by Agarwalla and Lund for a series reaction network appears to be true for series-parallel networks, too. That is, the membrane reactor will outperform a plug flow reactor only when the membrane offers enhanced permeability of the desired intermediate product. Therefore, the relative permeability of HCHO was varied to determine how much enhancement of permeability is needed. From Figure 2 it is evident that a large permselectivity is not needed, usually on the order of two to four times as permeable as the methane. An asymptotically approached upper limit of... 

The Damkdhler-Peclet product also had an impact on performance the optimal value ranged from is 1.0 x 10 at 773K, to 1.0 x 10 at 873K. Little or no improvement was observed when the pressure in the tube was larger than the pressure in the shell, and no improvement was seen when the shell pressure exceeded the tube pressure. When the inert gas sweep rate was increased, the membrane reactor improved until the amount of sweep gas to reactant gas was approximately one hundred as seen in Figure 3. Once again there was an asymptotic limit to the amount of enhancement seen. There was no improvement when the permeabilities of any other component were increased over the permeability of methane. 

In order for a membrane reactor to produce yields of HCHO greater than in a plug flow reactor, the membrane must be permselective for this species. The more permselective the membrane is to formaldehyde the better the membrane reactor performs until the formaldehyde is approximately one thousand times more permeable than methane. At this limit, the concentration of HCHO is essentially equal on both sides of the membrane at all times. No further improvement is possible by increasing the diffusivity of the formaldehyde further because there is... 

Membrane reactors, using semi-permeable membranes, usually of sheet or hollow fiber type... 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The construction of a commercial reactor, however, is difficult, and a sweep gas is required to shift the product composition away from equilibrium values. The achievable conversion also depends on the permeability of the membrane. Figure 7 shows the equilibrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equilibrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

Kurungot et al. [48] developed a novel membrane material and a catalytic membrane reactor for the partial oxidation of methane. The driver of the development was the fact that rates of reforming reactions are much higher compared with the low permeability of conventional palladium membranes [49], Silica was previously recognized as a low-cost alternative to palladium [50], Additionally, the conventional... 

Membrane reactors (MRs) are an interesting alternative to traditional reactors (TRs) owing to their characteristic of product separation during the reaction progress. The simultaneous separation shows some advantages related to the process of both permeate and retentate downstreams and on the reaction (rate) itself. In fact, the load of the downstream separation is significantly lower because both (permeate and retentate) streams leaving the MR are concentrated in more and fewer permeable species, respectively. In addition, separation/purification is not required in the special case of pure permeate. 

To improve process economics, an integrated process shown conceptually in Fig. 27 has been proposed. A per-vaporation subsystem is equipped with a membrane selectively permeable to water and ammonia, but rejects ethanol and ethyl lactate. The retentate stream carrying these reactants may be returned to the reactor to help drive the reaction toward completion. 

The Van Koten group has developed an interesting approach to the assessment of the permeability of nanofiltration membranes for the application of metallodendrimer catalysts in membrane reactors. They have selectively grafted dendrons to organometallic pincers with sensory properties and have used these as dyes in a colorimetric monitoring procedure. 

Membrane reactors tu. continuous operation Ho permeation sluggish > permeability vs. permselectivity thermal driving force not feasible no unwanted catalytic activity t sealing tu, operating life... 

Note that, in particular, aspect (1) is relevant for the evaluation of the potential of membrane reactors aiming to withdraw products via permeable walls. 

Highly selective and still sufficiently permeable membranes are required for the efficient use of extractor-type membrane reactors. 

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