Turbulent fluidization regime

Classical bubbles do not exist in the vigorously bubbling, or turbulent fluidization regimes. Rather, bubbles coalesce constantly, and the bed can be treated as a pseudohomogenous reactor. Small bubble size improves heat transfer and conversion, as shown in Figure 5b. Increasing fines levels beyond 30—40% tends to lower heat transfer and conversion as the powder moves into Group C. 

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. 

The contemporary commercial reactors used for sulphide ore roasting, Ficher-Tropsch synthesis and acrylonitrile manufacture were routinely operated in the bubbling and turbulent fluidization regimes [56, 112]. 

Therefore, installing more membranes per unit of volume seems to be the only reasonable way to improve the membrane fluxes. Installing more membrane area (thus more membranes) will drastically reduce the space between membranes where the catalyst is suspended in fluidization. For instance, using planar membranes (Mahecha-Botero et al., 2008) close to each other would result in a small compartment that can be seen as microstructured FBMR as theoretically studied by Wang et al. (2011). Their simulation study has also elucidated that in these small confinements the turbulent fluidization regime (with anticipated improved mass transfer characteristics) can be achieved at lower superficial gas velocities. 

Figure 432 Time-averaged solids holdup distribution (scale [0-0.6]) (A), solids circulation pattern (B), and lateral profiles of the axial solids flux for different heights (C) in the fluidized bed with a constant inflow, for different gas extraction and addition ratios. Turbulent fluidization regime, superficial gas velocity used from the bottom distributor is 2.0 m/s. Reprinted from Dang et al. (2014) with permission from Elsevier.

Fig. 4.34 shows the extent of the stagnant zones as a fimction of the permeation velocity at different fluidization gas velocities in both bubbling fluidization (A) and turbulent fluidization regimes (B). It is pointed out that with the selected threshold value even in the reference case without gas extraction, the presence of small semistagnant zones is found, e.g., for the reference case in the bubbling regime, the extent of the densified zones is approximately 8% of the bed volume. Increasing the permeation gas velocity results in much extended densified zones (that even reach values higher than 30% as shown in Fig. 4.34). However at the same permeation gas velocity, increasing the fluidization gas velocity will decrease the extent of densified zones for example, the densified zones decrease from 32% to 23% for 40% of...

U,turbulent fluidization Pressure fluctuations decrease gradually until turbulent fluidization regime is reached... 

A typical fluidized membrane reactor (or membrane-assisted fluidized bed reactor - MAFBR) consists in a bundle of permselective membranes immersed in a catalytic bed operated in a bubbling or turbulent fluidization regime. The use of fluidized bed membrane reactors not only makes possible the reduction of bed-to-wall mass transfer limitations, but also allows operating the reactor under virtually isothermal conditions (due to the movement of catalyst). This possibility can be used for operating the autothermal reforming of hydrocarbons inside the membrane reactor. In fact, as indicated by Tiemersma et al. [13], the autothermal reforming of methane in a packed bed membrane reactor is quite... 

An iterative procedure is required (2) since H is needed so that the representative height (H/2) can be chosen for evaluation of d and u. This procedure converges quickly. Values of in excess or about 0.4-0.5 are unrealistic, and probably Indicate transition to the turbulent fluidization regime. 

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