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214 3. Heterogeneous Processes and Reactor Analysis

Mixing of phases in fluidized beds
wer is initially needed, For the fluidization of a bed to occur, a lar b ge amount of po ut once
achieved, further increase in the feed flow rate requires much less enerThe operation . gy
of the fluidized-bed reactor is normally in the region between the packed bed and contin-
uous-stirred tank reactor operation, due to backmixing caused by small linear v elocities of
the fluid. At high flow rates and low reactor diameters, almost ideal plug-flow character-
istics may be achieved. Almost ideal plug-flow behavior can be achieved if high flow rates
and small reactor diameters are used. The exact mode of fluidized-bed reactor operation
depends on its actual design characteristics.

Solids mixing The mixing of solids in fluidized-bed reactors is achie v ed by the entrain-
ubbles,
ment of solids in the lower portion of b as well as by the shedding of these solids
atridge,
ak from the we of the bubble (Rowe and P 1962). Thus, mixing will appear as the
gas rate increases above its minimum value for the onset of fluidization. Especially, at high
gas rates, by far higher than the minimum fluidizing rate, the intense circulation of the
solids from the top to the bottom of the bed leads to rapid mixing of the solids. The parti-
cles that reach the top of the bed displace those found at the bottom of the bed. Practically,
if the ratio of length to diameter of the bed ranges between 0.1 and 4, the solid phase can
be considered to be completely mix ed.
The complete mixing of solids in the emulsion phase is necessary for considering the
various parameters, inolving the mass or volume of solids constant, throughout the reac- v
tor. That is exactly the case in the two-phase model and the Leunii three-phase v enspiel–K
model. This is achieed by circulation of the solids through their entrainment by b v ubbles,
as shown in Figure 3.61. As solids fall from the upper portions of the bed, they follo w

Figure 3.61 Heterogeneous solids distrib ution.