Reactor counter-current configuration

F. Gallucci, M. De Falco, S. Tosti, L. Marrelli and A. Basile, Co-current and counter-current configurations for ethanol steam reforming in a dense Pd-Ag membrane reactor, Int. J. Hydrogen Energy, 2008, 33, 6165-6171. 

Packed bed reactors Reactor in which iiquid fiows over immobiiized soiid materiai aiso known as packing materiai, Soiids cannot be present in inputor product. Large units possible usually counter current configuration which is limited by flooding... 

Figure 3.6 Co-current configuration (a) and counter-current configuration (b) for membrane reactor...

Figures. Hydrogen partial pressure in co-current configuration (a) and counter-current configuration (b) for ethanol reforming in packed bed membrane reactors. ("Source Reprinted from Ref 22 with permission of Elsevier)...

In Fig. 14.2, the direction of the hot gas is the same as the retentate, leading to a co-current configuration of the heating stream. In a counter-current configuration, hot gases would move from the left to the right side of the reactor represented in the figure. 

Temperature profiles, methane conversion, HRF and permeated flow are shown in Fig. 14.6. Figure 14.7 shows the membrane temperature profile and the permeation driving force along the reactor. Table 14.4 shows the permeation results and the product outcome, outlining that total hydrogen permeated is lower (26%) for the counter-current configuration. The calculated HRF and methane conversion ( CH4 ) in the counter-current flow... 

In a simple membrane reactor, basically the membrane divides the reactor into two compartments the feed and the permeate sides. The geometries of the membrane and the reaction vessel can vary. The feed may be introduced at the entrance to the reactor or at intermediate locations and the exiting retentate stream, for process economics, may be recycled back to the reactor. Furthermore, the flow directions of the feed and the sweep (including permeate) streams can be co-current or counter-current or some combinations. It is obvious that there are numerous possible process and equipment configurations even for a geometrically simple membrane reactor. 

Applying an isothermal and plug>flow membrane reactor (on both sides of the membrane) to the above reactions, Itoh and Xu [1991] concluded that (1) the packed-bed inert membrane reactor gives conversions higher than the equilibrium limits and also performs better than a conventional plug-flow reactor without the use of a permselective membrane and (2) the co-current and counter-current flow configurations give essentially the same conversion. 

Itoh and Govind [1989b] further analyzed an isothermal packed-bed inert membrane reactor but under a counter-current flow configuration. Under the conditions studied, the authors found that the counter-current flow configuration provides a much greater conversion than the co-current flow mode. 

The two most common flow directions of the permeate/sweep stream relative to the feed stream are co-current and counter-current As expected, these two flow configurations produce different reactor performances. They will be treated in the next subsection. 

Co-current versus counter-current flows. It is noted that in the operation of a separation system, a counter-current flow has always given a larger average concentration gradient than a co-current flow. Thus, it is expected that counter-current flow configuration is preferred between the two in a membrane unit. In a membrane reactor, however, an additional factor needs to be considered. To obtain a high conversion of a reversible reaction, it is necessary to maintain a high forward reaction rate. 

As will become evident later, the counter-current flow conflguration appears to provide a clear-cut advantage over the co-current flow configuration with respect to the reaction conversion in a dense membrane reactor. 

Three phase reactions comprise gas-liquid-solid and gas-liquid-liquid reactions. Gas-liquid reactions using solid catalysts represent a very important class of reactions. They may be carried out in either slurry (such as bubble column, stirred tank and gas-liquid fluidized configurations) or fixed-bed reactors (trickle bed with co-current-downflow or co-current-upflow, segmented bed and counter-current gas-liquid arrangements). 

Industrial fixed-bed catalytic reactors have a wide range of different configurations. The configuration of the reactor may give rise to multiplicity of the steady states when other sources are not sufficient to produce the phenomenon. The most well known is the case of catalytic reactors where the gas phase is in plug flow and all diffusional resistances are negligible however, the reaction is exothermic and is counter-currently cooled (68). One of the typical examples for this case is the TVA-type ammonia converter. 

In phot-scale DMTO fluidized bed reactor, the regenerated catalyst normally has very low coke content. As discussed above, such catalyst may not favor the selectivity to light olefins. Therefore, a counter-current fluidized bed configuration is adopted. In this configuration, the regenerated catalyst is injected into the reactor via catalyst distributor from the top of the dense bed, and the coked catalyst is taken from the draw-off bin beneath the gas distributor at the bottom. Thus, the methanol feed from the gas distributor first contacts the coked catalyst, by which a higher selectivity to light... 

Figure 2 Variations of fluidized bed reactor configurations, (a) Baffled bed, (b) multi-stage bed, (c) multi-stage with counter-current solid flow, (d) multi-stage with external solid circulation, (e) fluidized bed with online solid exchange, (f) multi-cell bubbling bed, (g) CFB, (h) centrifugal fluidized bed, (i) downer, (j) spouted bed.

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