Membrane reactor concept

TABLE X Catalytic Membrane Reactor Concepts for Chemical Synthesis... 

The reasoning behind developing different membrane reactor concepts is based on the realization of selective transport processes. Typically, certain components should be brought into - or removed selectively from - a reaction zone. Thus, an essential requirement for the successful operation of membrane reactors is to understand, and quantify, these transport processes correctly. 

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 B takes place and no permeate is withdrawn, the concentration profiles are identical to those in a catalyst slab (Fig. 29a).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],... 

One major disadvantage of catalytic membrane reactors is the fact that so far no convincing large scale concepts have been proposed. This concerns both the implementation of large membrane areas necessary for the production of bulk chemicals within a chemical reactor and its combination with devices for the addition or removal of the required heat of reaction. Membrane reactor concepts are therefore presently limited to lab scale investigations while the above mentioned sorptive methods seem closer to a large scale realization. 

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]. 

In addition to this, that an interesting novel emulsion membrane reactor concept overcomes the difficulties of the large solvent volume otherwise required for the reduction of poorly soluble ketones [30]. 2-Octanone was reduced by a carbonyl reductase from Candida parapsilosis to (S)-2-octanol with > 99.5 % ee and total turnover number of 124 - the 9-fold value of that obtained in a classical enzyme reactor. 

Cataly tica Study Division, Catalytic Membrane Reactors Concepts and Applications, Catalyt-ica Study No. 4187, Mountain View, California (1988). 

Struis RPWJ and Stuck S. Verification of the membrane reactor concept for the methanol synthesis. Appl Catal A Gen 2001 216(1-2) 117-129. 

For some of them, the use of membrane reactors for their recovery or application in continuously operated reactors has been demonstrated. Examples include the use of dendrimer-bound nickel catalysts for the Kharasch addition [54, 59] and dendritic palladium catalysts for an allylic substitution [73, 60]. The membrane reactor concept has also been transferred to reactions at higher pressure, as shown for the hydrovinylation of styrene (cf. Section 3.3.3) [75]. Modem ultra-and nanofiltration membranes allow an effective recovery of the homogeneously soluble catalyst. However, in some cases the long-term stability of the catalyst under operating conditions has to be improved. 

On behalf of KTI an experimental programme on these reactor concepts has been started at the University of Southern California (USC). Some of the experimental results, concerning the use of Knudsen diffusion membranes are available in the literature [32,40]. These data have been used to calculate the economics of an isothermal propane dehydrogenation membrane reactor concept and are compared with the commercial Oleflex and Catofin processes, based on an adiabatic concept. The experimental circumstances of these lab-scale experiments, especially residence time, pressures and gas composition are not the same as in commercial, large-scale processes. However, we do not expect these differences to have a great influence on the results of the work presented here. 

Following the results of the adiabatic reactor concept it is expected that high selective membranes will further improve the economics. However, it should be recognised that the process conditions in an isothermal concept are more severe than in an adiabatic concept. In particular, decoking conditions can be a problem in using high selective membranes. Detailed calculations on the isothermal membrane reactor concept are being performed and will be reported in future. 

Microporous carbon membranes have been developed [59] but their possibilities in high temperature hydrogen separation are still unclear, although it is believed that there are opportunities. Scaling-up of these membranes seems possible from a technical point of view. All these membrane types are potentially suitable for application in the WGS membrane reactor concept, provided their endurance is sufficient. 

The SEMR combines the membrane reactor concept with the working principle of a... 

The membrane reactor concept was demonstrated in laboratory scale a decade ago by Butterworth et al. (15) and by Chose and Kostick (16) in studies on the hydrolysis of starch and cellulose, respectively. Later on several publications have appeared describing the analogous, continuous conversion of various proteins into peptides intended for human nutrition (17-22). Among these works only that of laccobucci et al. (18) presents a quantitative model of the membrane reactor in continuous protein hydrolysis, and it is also the only demonstration of the practical feasibility of the concept in pilot plant scale. 

The catalytic dehydrogenation of light alkanes is, potentially, an important process for the production of alkenes, which are valuable starting chemical materials for a variety of applications. This reaction is endothermic and is, therefore, performed at relatively high temperatures, to improve the yield to alkenes, which is limited, at lower temperatures, by the thermodynamic equilibrium. Operation at high temperatures, however, results in catalyst deactivation (thus, requiring frequent reactivation), and in the production of undesired by-products. For these reasons, this reaction has been from the beginning of the membrane reactor field the most obvious choice for the application of the catalytic membrane reactor concept, and one of the most commonly studied reaction systems. 

Figure 3.19. The membrane reactor concept of Chemseddine and Audinos. Adapted from Chemseddine and Audinos [3.94].

Fig. 28. Permselective membrane reactor concept for dehydrogenation of ethyl benzene to produce styrene.

Figure 26 Schematic of the two-membrane reactor concept for the oxidative coupling of methane...

Taking advantage of the enormous differences in molecular weight (and hydro-dynamic volume) between the catalyst (an enzyme) and the substrates or the products, it is possible to confine the catalyst in a membrane reactor while the reagents and products can be selectively extracted from the reaction mixture without modifying the catalytic active intermediate. Remarkably, the Chemzyme membrane reactor concept allows a productivity per gram of catalyst which could be higher than in the case of enzymes because the polymers can be multifunctionalized with several active centers [1]. 

Verification of the Membrane Reactor Concept for the Methanol Synthesis. 

Catalytica (1988) Catalytic membrane reactors concepts and applications, Catalytica... 

As a result of continuous improvements in membrane science, ultra-thin, highly permeable and highly selective H2 membranes have recently become available, which has triggered the development of novel, improved membrane reactor concepts for the production of ultra-pure hydrogen. 

Figure 10.9 Schematic representation of the membrane reactor concept with bubble increasing in size.

Membrane Reactor Technologies Ltd (MRT) has experimentally verified the permeative-stage membrane reactor concept. With the membranes outside the reaetor, operation at more favorable conditions for both reaction (750 °C) and membrane separation (450 °C or lower) is possible. A decrease in the metal cost of palladium-based membranes by 86.5% and membrane area by >70% to aehieve equal hydrogen production capacity was reported. The volume of reformer decreases accordingly, thus, the costs of both the reactor and membrane module are reduced. 

Figure 11.4 Schematic representation of the two fluidized membrane reactor concepts for autothermal methane reforming with integrated CO2 capture (a) Methane combustion configuration (b) Hydrogen combustion configuration, after Patil et al.

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