Membrane reactors partial pressures

However, in the case of integrated MBRs, with the membrane inside the reactor, partial pressures pB.ret and pB.pem and, often, temperature T change, in general, along the tubular reactor. Therefore, Eq. 1.22 gives the local flux at a given section of the MBR, and Eq. 1.24 must be given in differential form ... 

Fig. 5. Methane conversion and oxygen flux during partial oxidation of methane in a ceramic membrane reactor. Reaction conditions pressure, 1 atm temperature, 1173 K, feed gas molar ratio, CH Ar = 80/20 feed flow rate, 20 mL min-1 (NTP) catalyst mass, 1.5 g membrane surface area, 8.4 cm2 (57).

Fig. 6. Configuration of a ceramic membrane reactor for partial oxidation of methane. The membrane disk was prepared by pressing Bao.5Sro.5Coo.8Feo.2O3-s oxide powder in a stainless steel module (17 mm inside diameter) under a pressure of (1.3-1.9) X 109 Pa. The effective area of the membrane disk exposed to the feed gas (CH4) was 1.0 cm2 (72).

It is evident, that in any membrane reactor operation mode there are important parameters which determine the performance of the process (Shah, Remmen and Chiang 1970). These are (1) the total and partial pressures on both sides of the membrane, (2) the total and partial pressure differences across the membrane, (3) the diffusion mechanism through the support and the membrane layer (membrane structure), (4) the thickness of the membrane, (5) the reactant configuration (i.e. whether the reactants are supplied from the same or from opposite sides of the membrane, in counter or co-current flow) and (6) the catalyst distribution. 

Recent results on isobutane dehydrogenation have been reported, and a conventional reactor has been compared with membrane reactors consisting of a fixed-bed Pt-based catalyst and different types of membrane [51]. In the case of a mesoporous y-AKOi membrane (similar to those used in several studies reported in the literature), the observed increase in conversion could be fully accounted for simply by the decrease in the partial pressures due to the complete mixing of reactants, products and sweep gas. When a permselective ultramicroporous zeolite membrane is used, this mixing is prevented the increase in conversion (% 70%) can be attributed to the selective permeation of hydrogen shifting the equilibrium. 

In addition to a proper membrane, CMRs also need a good catalyst. Due to the specific conditions under which catalysts are placed in CMRs, conventional active phases could behave differently from when under classical conditions. For example, in dehydrogenation reactions, due to the removal of H2, the hydrogen hydrocarbon ratio is smaller in CMRs when compared to other reactors, which will probably affect the stability of the catalyst. The low oxygen partial pressure used in CMRs for selective oxidation (Section A9.3.3.2) could also lead to some changes in catalyst behavior. These aspects could necessitate the specific design of catalysts for CMRs. 

The conditions are substantially more favorable for the microporous catalytic membrane reactor concept. In this case the membrane wall consists of catalyti-cally active, microporous material. If a simple reaction A -> B takes place and no permeate is withdrawn, the concentration profiles are identical to those in a catalyst slab (Fig. 29a). By purging the permeate side with an inert gas or by applying a small total pressure difference, a permeate with a composition similar to that in the center of the catalyst pellet can be obtained (Fig. 29b). In this case almost 100% conversion over a reaction length of only a few millimeters is possible. The advantages are even more pronounced, if a selectivity-limited reaction is considered. This is shown with the simple consecutive reaction A- B- C where B is the desired product. Pore diffusion reduces the yield of B since in a catalyst slab B has to diffuse backwards from the place where it was formed, thereby being partly converted to C (Fig. 29c). This is the reason why in practice rapid consecutive reactions like partial oxidations are often run in pellets composed of a thin shell of active catalyst on an inert support [30],... 

Figure 10 contrasts the conversion obtained with a conventional reactor to that conversion obtained with a permselective reactor when the partial pressure of the hydrogen on the sweep side is 0.02 atm. It can be seen that the difference in conversion between the two systems increases as the temperature increases. An important operational limitation is imposed by the sintering temperature for the membranes. It should be noted that this factor is not considered since the membranes are assumed to be stable. 

A membrane reactor preceded or followed bv a conventional reactor. Consider a typical commercial porous membrane currently available that exhibits a moderate to low gas separation factor and a high gas permeance even for the gas intended to be the retentate. Much of this less permeable gas in the feed stream is lost to the permeate (low pressure) side in the entrance section of the reactor due to its high partial pressure difference across the membrane layer. This leads to the undesirable effect of a low reactant conversion in that section. An effective way of reducing this reactant loss is to have a membrane enclosed section preceded by an impermeable reaction zone. To achieve a maximum total conversion, the impermeable length relative to the membrane length needs to be optimized. 

Pi C02 partial pressure on the high-pressure side of the membrane (atm) reactor (cm)... 

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