Membrane reactors challenges

Whilst the basic process for generation and conversion of syngas is well established, production from biomass poses several challenges. These centre on the co-production of tars and hydrocarbons during the biomass gasification process, which is typically carried out at 800 °C. Recent advances in the production of more robust catalysts and catalytic membrane reactors should overcome many of these challenges. 

One of the main challenges for zeolite membrane reactors is that optimal reactor operation requires that the membrane flux be in balance with the reaction rate.. Whether acting in an inert or catalytic capacity, the extractive ability of the membrane needs to keep up with the production of the species being removed in order to fully participate in improving the reaction yield [44]. 

The present study investigates a different approach. The membrane is used to allow the desired intermediate product to escape from the reaction zone before it is consumed by further reaction. This use of a membrane reactor was first suggested by Michaels [15]. The partial oxidation of methane, which is a challenging reaction of the type propos for this application of membrane reactors, has been analyzed herein. There is no thermodynamic limitation for the production of carbon dioxide and water, actually these products are favored. It is desired to remove any partial oxidation product, for example formaldehyde, before it has a chance to be further oxidized. 

The results obtained for microporous silica membranes in the membrane steam-reforming project, described in this thesis, provide favourable perspectives to realise a Th-permselective membrane reactor for the dehydrogenation of H2S. Realisation of such a reactor, however, imposes significant scientific and technical challenges. 

Sorptive reactor concepts where periodic operation is used to temporarily store or remove educts or products in the fixed bed can be considered close to industrial realization, whereas membrane reactor concepts with permselective inert or catalytically active microporous membranes are still at the laboratory stage. They promise the highest potential for a further improvement of catalytic reactor technology and present the biggest challenges [54]. 

Production technology of p>orous inorganic membranes is progressing toward smaller pore sizes to meet more challenging requirements for such applications as gas separation and membrane reactors. Research in this area has intensified in recent years. The trend of exploring new combinations of materials and processes for finer pore membranes can be highlighted in Table 3.5 based on the literature. 

The combination of high temperature and chemical exposure poses a very challenging material problem that is quite common in high-temperature membrane reactor applications. The consequence of structural degradation as a result of such a combination not only affects the permeability and permselectivity but also leads to physical integrity or mechanical properties. These issues apply to both metal and ceramic membranes. 

Corrosive reaction streams. In some application environments, the reactive or corrosive nature of one or more of the reaction components in a membrane reactor can pose a great technical challenge to the selection as well as the design of the membrane element Feed streams often contain some Impurities that may significantly affect the performance of the membrane. Therefore, attention should also be paid to the response of the selected membrane material to certain impurities in the reactant or product streams. Care should be taken to pretreat the feed streams to remove the key contaminants as far as the membrane is concerned in these cases. For example, palladium alloy membranes can not withstand sulfur- or carbon-containing compounds at a temperature higher than, say, 500 C [Kamcyama et al., 1981]. Even at lOO C, the rate of hydrogen absorption (and, therefore, permeation) in a pure palladium disk is... 

Catalytic membrane reactors represent the most compact and yet challenging membrane reactor design. The membrane material may be inherently catalytic or rendered catalytic by impregnating a catalyst on the surface of the membrane itself or the pores inside the membrane/support matrix. When the inner tube of a shell-and-tul reactor is a permselective and also catalytic membrane, the reactor is called catalytic membrane tubular reactor. Under this special circumstance, ibj = 0 = kf for Equations (10-36) to (10-37) and (10-44) to (10-45), assuming plug flows on both the tube and shell sides. The transport equations for the membrane zone. Equations (10-5) to (10-6), hold. 

It is often desirable to operate the reactor and the catalyst under isothermal conditions to achieve high reactor performance. Heat requirement of an endothermic reaction in a membrane reactor to maintain an isothermal condition can be challenging as in most of the dehydrogenation reactions such as conversions of ethylbenzene to styrene and prc pane to propylene. Maintaining an isothermal condition implies that some means must be provided to make the adequate heat input (e.g., from a burner) that is longitudinally dependent It is not trivial to make the temperature independent of the longitudinal position because the permeate flow also varies with the location in the axial direction. 

By using palladium or other inorganic H2-selective WGS membrane reactors, many researchers have achieved high CO conversion values beyond the equilibrium ones or close to 100%.35-40 However, the difficulty to prepare thin, flawless, and durable membranes is still the remaining challenge for the commercial application of this type of membrane reactor.41... 

Some of the major challenges facing the area of catalytic membrane reactors are of a more conventional nature. We have briefly discussed the need for standardization in the membrane preparation techniques and for better quality control the preparation of inexpensive good quality macroporous supports still remains a challenge as are reliable chemically inert and cost effective seals (an excellent discussion on the latter topic can be found in the review paper by Saracco and Specchia [14]). For highly exothermic (endothermic) reactions... 

There are a number of membrane reactor systems, which have been studied experimentally, that fall outside the scope of this model, however, including reactors utilizing macroporous non-permselective membranes, multi-layer asymmetric membranes, etc. Models that have been developed to describe such reactors will be discussed throughout this chapter. In the membrane bioreactor literature, in particular, but also for some of the proposed large-scale catalytic membrane reactor systems (e.g., synthesis gas production) the experimental systems utilized are often very complex, in terms of their configuration, geometry, and, of course, reaction and transport characteristics. Completely effective models to describe these reactors have yet to be published, and the development of such models still remains an important technical challenge. 

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