Turbulent to fast fluidization

Transition from turbulent to fast fluidization occurs at the transport velocity, C/tr, where significant numbers of particles are carried out from the top of the column. At the same time, continuous and smooth feeding of solids into the bottom of the riser should be maintained in order to keep the stability of a fast fluidization. Thus a fast-fluidization regime is connected with a circulating fluidized bed (CFB). 

Figure 7.2 Schematic representation of FBMRs for hydrogen production (a) bubbling fluidization regime (b) turbulent fluidization regime (c) fast fluidization regime. U = superficial gas velocity [7 = minimum bubbling velocity U =velocity of transition from bubbling to turbulent fluidization regime U =velocity of transition from turbulent to fast fluidization regime/significant entrainment ROG = reactor off-gas V=reactor volume. Reproduced from [6]. With permission from Elsevier.

This class of reactions, carried out in fluidized beds, involves parallel and series reactions, with reaction intermediates being the desired products. Industrial examples include partial oxidation of n-butane to maleic anhydride and o-xylene to phthalic anhydride. The vigorous solid mixing of fluidized beds is valuable for these reactions because they are highly exothermic. However, gas backmixing must be minimized to avoid extended gas residence times that lead to the formation of products of total combustion (i.e., CO2 and H2O). For this reason, fluidized bed catalytic partial oxidation reactors are operated in the higher velocity regimes of turbulent and fast-fluidization. 

In a circulating fluidized bed, the carbon fines generated due to attrition may be lost through the cyclone. It is essential to consider this in any performance models of high-velocity turbulent or fast fluidized bed. 

Depending on the speed with which catalysts are entrained and move with the fluid reactants, there are different kinds of fluidized bed reactors. They range from incipiendy fluidized bed to bubbling bed and turbulent bed to fast fluidized and finally pneumatic or transport bed. 

In general, nonuniform structures, in both time and space, is widespread in bubbling, turbulent, and fast fluidization regimes. On the one hand, such nonuniformity can enhance the mass and heat transfer of a bed. On the other hand, it decreases the contact efficiency of gas and solids and makes the scale-up rather difficult. Internals are usually introduced not to eliminate the nonuniform flow structure completely but to control its effect on chemical reactions. The function of internals varies in different fluidization regimes, as do the types and parameters of internals. Taking these purposes into consideration, internals may be successfully applied to catalytic reactors with high conversion and selectivity, and some other physical processes. 

The region at the bottom of the riser may operate as a turbulent or bubbling bed, with a gradual transition to fast fluidization, dense suspension upflow, or dilute pneumatic conveying. 

Models similar to those described above for the bubbling regime may also be used to characterize beds operated in the slugging, turbulent and fast fluidization regimes, with appropriate changes in the relationships used to describe the mass transfer and other model parameters. The conditions needed to ensure that the bed is operating in these regimes are presented in Section 2. 

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

The boundary between the turbulent and the fast fluidization regimes has been of some dispute in the fluidization field. However, the choking velocity (Uch) appears to be a practical lower-velocity boundary for this regime (Karri and Knowlton, 1991 Takeuchi et al., 1986). 

As the fluidizing quality of the powder deteriorates from Group A to Group B, however, the range for fast fluidization dwindles, until, for sandy materials, TURBULENT often jumps to TRANSPORT without the intermediate FAST stage. This is shown in Fig. 25 for a titanomagnetite concentrate, which is heavy and comparatively coarse. 

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