FBMRs reactors

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... 

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

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... 

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).

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. 

Recently, a new reactor has been proposed by combining the MR features with the chemical looping features for heat production with inherent CO2 capture. The novel reactor concept called Membrane Assisted Chemical Looping Reforming (MA-CLR) was introduced by Medrano et al. [55]. In this system (Figure 3.16), a FBMR is located in the fuel reactor of a CLR system, where the incorporation of membranes substitutes the WGS and PSA steps of the traditional CLR process. The selective extraction of hydrogen provides a pure H2 stream and also displaces the thermodynamic equilibria. Flence, reaction and... 

However, each configuration, PBMR and FBMR, presents benefits and drawbacks. In particular, PBMR is characterized by a very simple configumtion in which catalyst particles can be packed. The particles dimension plays an important role for the performance of this kind of reactor. Indeed, very small particles can increase pressure drop and, on the contrary, big particles can limit the internal mass transfer. Moreover, other drawbacks can occur by using PBMR, such as the mass transfer limitafion from bed to wall, which negatively influences the hydrogen permeation and remarkable temperature profile along the reactor, with a consequent detrimental effect on catalyst and membrane (Roses et al., 2013). 

Table 2.3 Experimental data taken from the open literature concerning the methane steam reforming (MSR) reaction in packed-hed membrane reactors (PBMRs) and fluidized-hed membrane reactors (FBMRs)... 

Regarding the FBMRs, Adris, Elnashaie, and Hughes (1991) were the first to propose this kind of reactor configuration by mathematical models. In their work, they showed that complete methane conversion could be realized by using typical operating industrial temperatures. 

Successively, many other scientific papers have dealt with FBMRs for performing the MSR, demonstrating the benefits of this configuration. In most of them, a modeling investigation was performed. Indeed, from our best knowledge, few authors have experimentally performed the MSR reaction in FBMRs (Adris et al., 1997 Patil et al., 2006, 2007 Roses et al., 2013), probably because of the difficulties in reactor construction, membrane sealing, and its erosion. For instance, Adris et al. (1997)... 

Pseudo-homogeneous - Plug flow - Membrane completely selective toward H2 permeation FBMR - The reactor is composed of two phases a bubble phase and emulsion phase - Reaction takes place in emulsion phase - The bubble phase gas is assumed to be in plug flow the lower concentration gradients. 

Regarding FBMR modeling, it is useful to refer to the models used for fluidized bed reactors. Two model classes have been proposed based on a pseudo-homogeneous or two-phase approach, respectively (Ho, 2003). 

The studies have proven that both MRs can realize better performance in terms of methane conversion and hydrogen production than the CRs, working also at milder operating conditions. By making a comparison between the two reactor configurations, it has been shown that a PBMR has a very simple configuration whereas an FBMR is typically characterized by enhanced mass and heat transfer rates, which favor more uniform temperature profiles. Nevertheless, possible bubble-to-emulsion mass transfer... 

The so-called fluidized bed membrane reactor (FBMR) is obtained by immersing bundles of hydrogen-selective membranes into a gas—solid fluidized suspension (see Figure 3.7) (Gallucci et al., 2008a, 2008b). 

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