Catalysts catalytic PBMR

Based on matenal considerations, membrane reactors can be classified into (1) organic-membrane reactors, and (2) inorgamc-membrane reactors, with the latter class subdivided into dense (metals) membrane reactors and porous-membrane reactors Based on membrane type and mode of operation, Tsotsis et al. [15] classified membrane reactors as shown in Table 3. A CMR is a reactor whose permselective membrane is the catalytic type or has a catalyst deposited in or on it. A CNMR contains a catalytic membrane that reactants penetrate from both sides. PBMR and FBMR contain a permselective membrane that is not catalytic the catalyst is present in the form of a packed or a fluidized bed PBCMR and FBCMR differ from the foregoing reactors in that membranes are catalytic. 

Before proceeding further it would be appropriate for our readers to familiarize themselves with the few additional acronyms that will be used in this chapter and which are listed in Table 11.1. They are used to describe some of the most common membrane reactor configurations that have been studied in the technical literature. By far the most commonly referred to reactor is the PBMR, in which the reaction function is provided by a packed bed of catalysts in contact with the membrane. The membrane is not itself catalytic at least not intentionally so. Some of the commonly utilized inorganic and metal membranes, on the other hand, are intrinsically catal) ically active. The PBMR clcissification, therefore, should be assigned with caution. When the packed bed... 

The most commonly utilized catalytic membrane reactor is the PBMR, in which the membrane provides only the separation function. The reaction function is provided (in catalytic applications) by a packed-bed of catalyst particles placed in the interior or exterior membrane volumes. In the CMR configuration the membrane provides simultaneously the separation and reaction functions. To accomplish this, one could use either an intrinsically catalytic membrane (e.g., zeolite or metallic membrane) or a membrane that has been made catalytic through activation, by introducing catalytic sites by either impregnation or ion exchange. This process concept is finding wider acceptance in the membrane bioreactor area, rather than with the high temperature catalytic reactors. In the latter case, the potential for the catalytic membrane to deactivate and, as a result, to require sub-... 

Figure 1.5. Different MR configurations. 1 tubeside, 2 catalytic membrane, 3 inert membrane, 4 catalyst bed, 5 shellside. a) CMR, CNMR, b) PBMR, FBMR, c) PBCMR, FBCMR.

Non-isothermal 1-D models for adiabatic PBMR and FBMR reactors utilizing Pd tubular membranes have been developed by Elnashaie et al [5.35], and applied to the catalytic ethylbenzene dehydrogenation reaction. In contrast to many other modelling studies their model takes into account intraparticle diffusional limitations. The catalyst particles... 

In Fig. 5.18 IMRCF (s=l) corresponds to the PBMR with catalyst pellets with a non-uniform catalyst distribution, while CMR ( =1) is the CMR with the catalyst placed non-uniformly on the membrane surface in contact with the catalytic reactor feed. IMRCF (a(s)=I) and CMR (a( )=I) correspond to the PBMR and CMR with uniform catalyst distributions. The conventional packed-bed reactor (FBR in Figure 5.18) exhibits conversions, which are below the equilibrium conversion, and for large residence times are lower than those exhibited by the CMR and the PBMR. The highest conversions are obtained with the non-uniform activity (Dirac delta case) profiles. This result was explained on the basis that the access of the reactants to the active catalytic sites was not limited by diffusion. When the catalyst is uniformly distributed the PBMR exhibits better performances than the CMR. It is interesting to note that at low residence times the packed-bed reactor conversion is higher than that of the PBMR with a uniformly distributed catalyst this is because in this case for the PBMR the reactants are only partially in contact with the catalyst due to diffusional limitations. 

Langhendries et al [5.74] analyzed the liquid phase catalytic oxidation of cyclohexane in a PBMR, using a simple tank-in-series approximate model for the PBMR. In their -reactor the liquid hydrocarbon was fed in the tubeside, where a packed bed of a zeolite supported iron-pthalocyanine catalysts was placed. The oxidant (aqueous butyl-hydroperoxide) was fed in the shellside from were it was extracted continuously to the tubeside by a microporous membrane. The simulation results show that the PBMR is more efficient than a co-feed PBR in terms of conversion but only at low space times (shorter reactors). A significant enhancement of the organic peroxide efficiency, defined as the amount of oxidant used for the conversion of cyclohexane to the total oxidant converted, was also observed for the PBMR. It was explained to be the result of the controlled addition of the peroxide, which gives low and nearly uniform concentration along the reactor length. 

The catalyst may be deposited within the membrane, including the case where the membrane itself is intrinsically catalytically active (CMR), catalyst particles may be packed inside or outside a membrane tube (PBMR) or both (PBCMR). These configurations are illustrated in Figure 1. 

An interesting point regarding OCM is that the membrane material itself may act as a total oxidation catalyst, thus unintentionally turning the PBMR into a PBCMR. Lu et pre-treated their dense membrane with an OCM catalyst to prevent contact between hydrocarbons and the membrane oxide material. Coronas et al. carried out experiments to estimate the contribution from the membrane and used it to modify their model of OCM in a porous membrane. They found that the predicted advantage of the membrane reactor was decreased if the catalytic activity of the membrane was taken into account, and suggested the development of inert membrane reactor materials, and more active OCM catalysts, as possible remedies. 

TS) and outer side (shell side, SS) of the membrane. The membrane itself is considered to be catalytically inert. The reactants are converted on the alternatively also catalyst particles, positioned in the tube side of the membrane (e.g., catalyst can be placed in the shell side). The dead-end reactor configuration shown in the figure (i.e. closed shell-side outlet) allows dosing reactants through the membrane in a controlled manner, with predefined flow rates. All reactants dosed have to permeate through the membrane. The products and the unconverted reactants leave the reactor at the tube side outlet. The PBMR shown in Fig. 

Oxidative coupling of methane using a catalytic-membrane reactor (CMR), catalyst packed-bed reactor (PBR), and catalyst packed-bed membrane reactor (PBMR), have been also compared. The authors conclude that the catalytic activity of PBR and PBMR (using Na-WMn/Si02) were lower than that observed for CMR (with a yield of 34.7%). 

PBMRs. This configuration is applied mostly in practical use of dense ceramic MRs. The reactions take place in the catalyst bed while the membrane functions mainly as an oxygen distributor or extractor. Since the catalyst is separated physically from the membrane, the separation function of the membrane and the catalytic properties of catalysts can be modulated separately so that the MR performance will be optimized. 

Figure 11.1 illustrates how the catalyst and the membrane unit can be combined in various ways. According to Miachon and Dalmon (2004), general overall analysis is almost impossible for classifying the combination of a catalyst and a membrane as there are so many various possibilities. The MR with a catalyst can be either a (catalytic) packed-bed membrane reactor ((C)PBMR), a (catalytic) fluidised-bed membrane reactor ((QFBMR) (where the catalyst is part of the set-up inside the free space reactor), or a catalytic membrane reactor (CMR) (where the catalytic material is part of the membrane-based wall) (Dudukovic, 1999 GaUucci et al, 2011). This chapter mainly concentrates on the catalytic materials used for hydrogen production from hydrocarbons and alcohols in MRs. 

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