FBMRs

Comparison of performance of fluidized bed membrane reactor (FBMR), fluidized bed reactor (FBR) and continuous stirred tank reactor (CSTR)... 

The choice between membrane elements and modules for use in a FBMR is pdmadly determined by balancing the considerations of having a higher packing of bare membrane elements on one hand, and protecting the membrane elements from severe... 

Horizontal versus vertical membrane tubes or modules. Two general types of fluidized-bed membrane reactors have been tested. The first type places the membrane elements or modules perpendicular to the general direction of the fluidizing reaction gases (see Figures 10.14a and 10.14b). In the second type of FBMR, the membrane elements or modules are essentially parallel to the fluid flow direction inside the reactor, as schematically shown in Figure 11.50. It appears that the vertical type exhibits more advantages for practical implementation. 

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. 

CMRH catalytic membrane reactor high conversion FBCMR fluidized bed catalytic membrane reactor FBMR fluidized bed membrane reactor... 

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... 

Similar terminology is used if the membrane is a flat plate, and by extension, if the membrane is used with a fluidized bed then FBMR and FBCMR are used for the two possible configurations. 

The PBMR and PBCMR reactors have their counterparts in fluidized bed systems. These have been studied by Adds and co-workers, both theoretically and experimentally. The steam reforming of methane was the system studied, which appeared promising since the FBMR addressed problems of heat transfer and equilibrium limitations simultaneously. 

Recently, the fluidized bed membrane reactor (FBMR) has also been examined from the scale-up and practical points of view. Key factors affecting the performance of a commercial FBMR were analysed and compared to corresponding factors in the PBMR. Challenges to the commercial viability of the FBMR were identified. A very important design parameter was determined to be the distribution of membrane area between the dense bed and the dilute phase. Key areas for commercial viability were mechanical stability of reactor internals, the durability of the membrane material, and the effect of gas withdrawal on fluidization. Thermal uniformity was identified as an advantageous property of the FBMR. 

Different types of membrane reactors for hydrogen production have been proposed in the literature. Most of the previous work has been performed in packed bed membrane reactors (PBMRs) however, there is an increasing interest in novel configurations such as fluidized bed membrane reactors (FBMRs) and membrane micro-reactors (MMRs), especially because better heat management and decreased mass transfer limitations can be obtained in these novel reactor configurations. 

Figure 10.6 A schematic representation of the two-phase fluidized bed reactor model (FBMR) (E = emulsion phase, B = bubble phase).

For example, during steam reforming in a 60 cm high FBMR with inserted 10 dead-end membranes, Gallucci et al. concluded that for the predicted membrane fluxes matched reasonably well with the experimental measured fluxes when both the bubble and emulsion phases are considered in plug flow. 

Conversion reached for a given area in case of mass transfer limitations for different stages (FBMR). Reprinted from Gallucci et al with permission of Professor T. Nejat Veziroglu. 

The combination of these drawbacks has driven the research towards new reactor concepts such as MMRs or FBMRs, as discussed in the following sections. 

Even though Rahimpour and co-workers often used FBMRs for distributive hydrogen feeding in methanol reactors [35-37], most of the literature has focussed on pure hydrogen production through Pd-based membranes (see among others [38-41]) and on autothermal reforming reactions (see a.o. Ref. [29,42-44]). 

The model was applied in order to investigate the influence of various parameters on the performance of FBMR with oxygen addition. Although the results showed that autothermal operation can be achieved by using approximately 0.3 O2/CH4 feed ratio, the interaction between the different parameters is quite complex. For instance, in methane reformers an important parameter is the steam/carbon ratio. However, when feeding oxygen, the steam becomes also a product of the oxidation reaction and this makes the prediction of the reactor behaviour a bit more complicated. Furthermore, an important conclusion of the work is that oxygen addition reduces the coke formation and consequently the catalyst deactivation. 

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