Bed-to-wall mass transfer limitations

On the other hand, with ultra-thin (high permeation flux) membranes, which have recently become available, it has been experimentally shown that the extent of bed-to-wall mass transfer limitations in case of hydrogen purification/ production become prominent, which greatly influences the reactor performance. When these limitations prevail, the hypotheses behind the ID model are no longer valid and more sophisticated 2D models need to be used. 

Let us compute the radial H2 concentration profiles at different axial positions at isothermal conditions. As can be seen in Figure 10.4, radial concentration profiles are present but not very pronounced. It can be concluded that for the membranes used and for small membrane diameters (1 cm in the simulation shown in the figure), the bed-to-wall mass transfer limitations have a negligible influence on the required membrane area. 

With this reactor, the bed-to-wall mass transfer limitations can be circumvented, while the heat required for the reforming reactions (often endothermic equilibrium reaction) is supplied through heat exchange surfaces inserted in the reactor system. In fact, fluidized bed reactors present higher heat exchange coefficients compared to fixed-bed reactors. 

Packed bed membrane reactors have two main limitations (i) the difficult heat management that can be very detrimental for highly exothermic reactions like in perovskite membrane reactors and (ii) the extent of bed-to-wall mass transfer limitations that are more important for extractor-type reactors like Pd-based membrane reactors (due to the high permeation fluxes of membranes). In fact, the bed-to-wall mass transfer limitations would decrease the partial pressure of hydrogen close to the membrane surface and thus decrease the membrane flux. 

A typical fluidized membrane reactor (or membrane-assisted fluidized bed reactor - MAFBR) consists in a bundle of permselective membranes immersed in a catalytic bed operated in a bubbling or turbulent fluidization regime. The use of fluidized bed membrane reactors not only makes possible the reduction of bed-to-wall mass transfer limitations, but also allows operating the reactor under virtually isothermal conditions (due to the movement of catalyst). This possibility can be used for operating the autothermal reforming of hydrocarbons inside the membrane reactor. In fact, as indicated by Tiemersma et al. [13], the autothermal reforming of methane in a packed bed membrane reactor is quite... 

A sequel to this study was presented later by the same group (Freund et al., 2003), this time for the simple first-order reaction A->B in a cylindrical bed of spheres with N — 5. The reaction was again taken to be mass-transfer limited and to occur on the surfaces of the catalyst particles, but at a very low flow rate at Re — 6.5. It was found that concentration peaks occurred near the wall at values close to the inlet value of species A, indicating that channeling was taking place. There were also local peaks of product concentration that indicated areas of high reactivity that could give rise to hotspots in practice. 

For multi-tubular MR configurations, the catalyst-in-tube configuration can be preferred especially for construction reason and for the extent of bed-to-wall mass and heat transfer limitations, which can be very detrimental in the catalyst in shell configuration. 

Immobilization, dehned as the physical confinement or localization of an enzyme into a specihc micro-environment, has been a very common approach to prepare enzymes for aqueous as well as nonaqueous applications. For nonaqueous enzymol-ogy, immobilization improves storage and thermal stability, facilitates enzyme recovery, and enhances enzyme dispersion. In addition, immobilized enzymes are readily incorporated in packed bed bioreactors, allowing for continuous operation of reactions. Moreover, lyophilized enzyme powders often aggregate and attach to reactor walls, particularly when the water activity is moderately high. The major disadvantage of immobilization is low activity, induced by pore diffusion mass transfer limitations and by alteration of protein stmcture. For enzymes in nonaqueous media, the following broad categories of immobilization exist ... 

The superiority of microreactors for reduced deactivation due to less thermal load on the catalyst has been proven several times (e.g. [47]), even benchmarked to a fixed bed [18] or to a foam [19]. However, one should be careful when operating the catalyst in a microreactor at its kinetic limit. The deactivation behavior of a specific catalyst could be more visible than in a conventional fixed bed or even coated foam. The mass of thin wall-coated catalysts operated free from mass transfer limitations could be much less for reaching 100% conversion. However, there is no backup catalyst mass. Hence additional catalyst mass has to be considered for process layout [20]. Also, leaks and the parallelizing of microstructures (concerning their feed distribution and heat distribution) are a challenge that can influence the catalyst stability and thus the operation of a microreactor [40]. 

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). 

Three-phase packed bed reactors generally have a lower specific capacity than slurry reactors, for two reasons Much larger catalyst particles are used, so that for rapid reactions, with diffusion or mass transfer limitations, much larger catalyst volumes are required. Also, the maximum specific gas/liquid interfacial area is generally smaller. On the other hand, the volumetric mass transfer coefficients at the gas/liquid and at the liquid/solid interfaces are of comparable magnitude, so they are better adapted to one another. Heat transfer rates to the walls are quite limited. 

A further modification of the Pall ring was developed by Mass Transfer Limited. Their Cascade (trademark Mass Transfer Ltd.) Mini-Ring also is a cylinder with fingers punched from the wall projecting into the interior of the ring however, the height of the cylinder is only one-third the outside diameter. This shape is said to orient itself preferentially when dumped into a packed bed. 

A commercially available cell " uses this type of particulate turbulence promoter in the form of a fluidized bed of electrochemically inert particles, typically glass spheres. This Chemelec cell is aimed at the secondary recovery of metals and their removal from effluents. Little reliable information is available on mass transfer to spheres in a fluidized bed. Nassif, using a limited current technique, investigated mass transfer to the particles of a fluidized bed as well as to the wall enclosing them. For the latter he derived ... 

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