Riser fluidizing velocity

Increasing gas velocity or gas throughput in a reactor eventually leads to a high production rate with transition of flow regimes to turbulent or riser fluidization. Some designs and modifications have been implemented on bubbling bed reactors to operate as turbulent bed reactors ... 

Fig. 7. Axial density profiles in the (—) bubbling, (------) turbulent, and (----) fast and ( ) riser circulating fluidization regimes. Typical gas velocities for...

Direct measurement of particle velocity and velocity fluctuations in fluidized beds or riser reactors is necessary for validating multiphase models. Dudukovic [14] and Roy and Dudukovic [28] have used computer-automated radioactive particle tracking (CARPT) to foUow particles in a riser reactor. From their measurements, it was possible to calculate axial and radial solids diffusion as well as the granular temperature from a multiphase KTGF model. Figure 15.10 shows one such measurement... 

A variant on the fluidized bed is the riser reactor. In this reactor the flow velocity is so high that the solids are entrained in the flowing fluid and move with nearly the same velocity as the fluid. The solids are then separated trom the effluent gases at the top of the reactor by a cyclone, and the solids are returned to the reactor as shown in Figure 7-4. The FCC reactor is an example where the catalyst is carried into the regenerator, where carbon is burned off and the catalyst is heated before returning to the reactor. 

The flow behavior in the riser varies with gas velocity, solids circulation rate, and system geometry. On the basis of the flow behavior, the fast fluidization regime can be distinguished from neighboring regimes. 

Figure 10.2. Variations of pressure drop per unit riser length with solids circulation rate and gas velocity for various fluidization regimes (after Bai et al., 1993).

The transport velocity can also be evaluated from the variations of the local pressure drop per unit length (Ap/Az) with respect to the gas velocity and the solids circulation rate, Jp. An example of such a relationship is shown in Fig. 10.4. It is seen in the figure that, along the curve AB, the solids circulation rates are lower than the saturation carrying capacity of the flow. Particles with low particle terminal velocities are carried over from the riser, while others remain at the bottom of the riser. With increasing solids circulation rate, more particles accumulate at the bottom. At point B in the curve, the solids fed into the riser are balanced by the saturated carrying capacity. A slight increase in the solids circulation rate yields a sharp increase in the pressure drop (see curve BC in Fig. 10.4). This behavior reflects the collapse of the solid particles into a dense-phase fluidized bed. When the gas... 

The Fischer-Tropsch synthesis of hydrocarbons is used on a large scale for fuel production in South Africa [78, 79]. Synthesis gas generated from coal in Lurgi fixed-bed gasifiers enters the Synthol reactor (Fig 18), where it is reacted over an iron catalyst at 340°C. The reactor works on the principle of the circulating fluidized bed. The mean porosity in the riser is 85%, and the gas velocity varies between 3 and 12ms1 [2]. Reaction heat is removed by way of heat-exchanger tube bundles placed inside the riser. 

Katoh, Y., Kaneko, S., and Miyamoto, M. Flow Patterns and Velocity Distributions on Fluidized Particles in a Riser of Circulating Fluidized Beds, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 332-337. Somerset, Pennsylvania (1993). 

---