Fluidized catalyst beds holdup

The turbulent kinematic viscosity vt of the fluidized catalyst bed has been determined, as Eq. (3-3 la), from the use of axial dispersion coefficient This is a natural consequence of the analogy between the bubble column and the fluidized catalyst bed of good fluidity (such as in fluidized catalytic cracking). The mean gas holdup (Fig. 36) and the mean bubble velocity along the bed axis (Fig. 37) are reasonably well predicted by applying Eq. (3-3 la) for the fluidized cracking catalyst bed. 

Figure 45 illustrates the plot of (f/c f mf)/cb versus (C/g Umi) according to Eq. (5-8) for fluidized beds. In the figure, Eq. (5-4) is also shown for slugging beds. The mean gas holdup for the FCC-catalyst bed is taken from Fig. 36. The plot for the FCC bed shows clearly that the bed is fluidized smoothly, without slugging. The plot also shows data for a bed of fluidized glass beads of mean diameter of 287 m. (Data are taken from W13 cf. Fig. 14). The bed behavior is seen to approach that of the slugging bed as(f/c f/mf) increases beyond 20cm/sec. Also, the averagers is approximately 0.8 for Uq < 20 cm/sec, showing the possibility of bulk recirculation of the emulsion.

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

Whereas for bubbling fluidized beds the solids holdup in the upper part of the reactor and the entrainment of catalyst are often negligible, these features become most important in the case of circulating fluidized beds These systems are operated at gas velocities above the terminal settling velocity ux of a major fraction or even all of the catalyst particles used (% 1 m s 1 < umass flow rales to be externally recirculated are high, up to figures of more than 1000 kg m 2s-1... 

The three-phase fluidized-bed reactor with countercurrent mode of operation was used by Pruden and Weber 10 to study the hydrogenation of a-methyl styrene to cumene in the presence of palladium black catalysts. They used low gas velocities so that the gas was dispersed as bubbles in the slurry. They showed that the countercurrent mode of operation was better than the slurry operation (with no liquid flow), due to improved catalyst usage and improved gas holdup characteristics. 

Until recently only a few papers were available on moving beds in cross flow [11-18]. This type of reactor is sometimes a favorable process solution for a selective catalytic process with a moderate catalyst rcsidence time and with a short gas residence time, especially when the process is accompanied by a continuous catalyst regeneration. The use of conventional short-contact-time reactors like fluidized-bed reactors, risers, and fixed-bed reactors does not always yield satisfactory results. This may be explained by problems connected with gas back-mixing, channeling of gas, low catalyst holdup, attrition of the solid catalyst, or difficulties in temperature control. 

In slurry systems, similar to fluidized beds, the overall rate of chemical transformation is governed by a series of reaction and mass-transfer steps that proceed simultaneously. Thus, we have mass transfer from the bubble phase to the gas-liquid interface, transport of the reactant into the bulk liquid and then to the catalyst, possible diffusion within the catalyst pore structure, adsorption and finally reaction. Then all of this goes the other way for product. Similar steps are to be considered for heat transfer, but because of small particle sizes and the heat capacity of the liquid phase, significant temperature gradients are not often encountered in slurry reactors. The most important factors in analysis and design are fluid holdups, interfacial area, bubble and catalyst particle sizes and size distribution, and the state of mixing of the liquid phase. ... 

Reactor/ Stripper Complete feed conversion and remove adsorbed hydrocarbons Bubbling bed reactor with two phases Switches to fluidized-bed reactor model for units with low catalyst holdup... 

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