Operable Fluidization Regimes

The riser cannot be considered as an isolated entity in the CFB loop. When no particles are introduced into or discharged from the loop, the solids mass flow rate in the riser is equal to that in the downcomer. For a given quantity of solids in the loop, the presence of fewer solids in the riser implies the presence of more solids in the downcomer. Likewise, the pressure drop across the riser must be balanced by that imposed by the flow through its accompanying components such as the downcomer and the recirculation device. Furthermore, the flow characteristics of the riser can be significantly affected by the behavior of the accompanying components in the loop. 

Typical pressure profiles in a CFB loop with a nonmechanical valve (see Chapter 8) are shown in Fig. 10.6, In this figure, line a-b-c-d represents the pressure drop across the riser, 

For the gas flow in a riser, energy in the gas phase is partially transferred into the solids phase through gas-particle interactions and is partially dissipated as a result of friction. Under most operating conditions, gravitational effects dominate overall gas phase energy consumption. Thus, neglecting the particle acceleration effects, the pressure drop in the 

The pressure drop through a cyclone has been analyzed in Chapter 7. Empirically, the pressure drop can be expressed by 

Maintaining the downcomer or a standpipe under a moving bed condition allows a large pressure buildup along the downcomer. For a given quantity of particles in the downcomer, the pressure at the bottom of the downcomer is closely associated with the relative velocity between the gas and particles. The pressure drop rises with the relative velocity as the particles are in a moving packed state. The maximum pressure drop in the downcomer is established when particles are fluidized, a state which can be expressed in terms of the pressure drop under the minimum fluidization condition as 

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The first commercial fluidized bed polyeth)4eue plant was constructed by Union Carbide in 1968. Modern units operate at 100°C and 32 MPa (300 psig). The bed is fluidized with ethylene at about 0.5 m/s and probably operates near the turbulent fluidization regime. The excellent mixing provided by the fluidized bed is necessary to prevent hot spots, since the unit is operated near the melting point of the product. A model of the reactor (Fig. 17-25) that coupes Iduetics to the hydrodynamics was given by Choi and Ray, Chem. Eng. ScL, 40, 2261, 1985. 

As noted earlier, increasing gas velocity for any given fluidized bed beyond the terminal velocity of bed particles leads to upward entrainment of particles out of the bed. To maintain solid concentration in the fluidized bed, an equal flux of solid particles must be injected at the bottom of the bed as makeup. Operation in this regime, with balanced injection of particles into the bed and entrainment of particles out of the bed, may be termed fast fluidization, FFB. Figure 10 presents an approximate map of this fast fluidization regime, in terms of a dimensionless gas velocity and dimensionless particle diameter. 

Fluidization Regime. As for traditional fluidization applications, the fluidization regime—dispersed bubble, coalesced bubble, or slugging—in which a three-phase fluidized bioreactor operates depends strongly on the system parameters and operating conditions. Generally, desirable fluidization is considered to be characterized by stable operation with uniform phase holdups, typical of the dispersed bubble regime. It would be useful to be able to predict what conditions will produce such behavior. 

The operation of fluidized-bed reactors can be seen as the transition region between con-tinuous-stined tank and packed-bed reactors. In a fluidized bed, a bed of solid particles is fluidized by the upward flow of the gas or liquid stream, which may be inert or contain material relevant to the reaction. The several fluidization regimes are shown in Figure 3.51. 

For a bed with Group A particles, bubbles do not form when the gas velocity reaches Umf. The bed enters the particulate fluidization regime under this condition. The operation under the particulate fluidization regime is characterized by a smooth bed expansion with an apparent uniform bed structure for Umf < U < Umb, where Umb is the superficial gas velocity at the minimum bubbling condition. The height of the bed expansion in terms of a can be estimated by [Abrahamsen and Geldart, 1980a]... 

The gas velocity for the upper bound of the particulate fluidization regime is Umb. The operational range of the particulate fluidization regime is quite narrow, as shown in Fig. 9.6. It is seen that for U > Umf, Hf monotonically increases with U until the minimum bubbling point is reached. Further increase in the gas velocity leads to a decrease in the bed height as bubbling fluidization begins. 

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