Fluidization riser

Bolton, L. W. and Davidson, J. F. (1988). Recirculation of Particles in Fast Fluidized Risers. In... 

Slugging Turbulent Dilute phase Bed Fluidization Riser... 

Bolton LW, Davidson JF. Recirculation of particles in fast fluidized risers. In Basu P, Large JF, eds. Circulating Fluidized Bed Technology II. Toronto Pergamon Press, 1988. 

Kraemer, D.W. and de Lasa, H.L, "Modelling Catalytic Cracking in a Fluidized Riser Simulator", Poster Session, International Fluidization Conference, Banff, Alberta (1989). 

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

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Figure 2.3.2 (Kraemer and deLasa 1988) shows this reactor. DeLasa suggested for Riser Simulator a Fluidized Recycle reactor that is essentially an upside down Berty reactor. Kraemer and DeLasa (1988) also described a method to simulate the riser of a fluid catalyst cracking unit in this reactor.

Fluidized-bed catalytic cracking units (FCCUs) are the most common catalytic cracking units. In the fluidized-bed process, oil and oil vapor preheated to 500 to SOOT is contacted with hot catalyst at about 1,300°F either in the reactor itself or in the feed line (called the riser) to the reactor. The catalyst is in a fine, granular form which, when mixed with the vapor, has many of the properties of a fluid. The fluidized catalyst and the reacted hydrocarbon vapor separate mechanically in the reactor and any oil remaining on the catalyst is removed by steam stripping. 

In the DCC unit, the hydroearbon feed is dispersed with steam and eraeked using a hot solid eatalyst in a riser, and enters a fluidized bed reaetor. A known injeetion system is employed to aehieve the desired temperature and eatalyst-to-oil eontaeting. This maximizes the seleetive eatalytie reaetions. The vaporized oil and eatalyst flow up the riser to the reaetor where the reaetion eonditions ean be varied to eomplete the eraeking proeess. The eyelones that are loeated in the top of the reaetor effeet the separation of the eatalyst and the hydroearbon vapor produets. The steam and reaetion produets are diseharged from the reaetor vapor line and enter the main fraetionator where further proeessing ensure the separation of the stream into valuable produets. 

The heat required to evaporate moisture may be obtained by eonveetion from the gas stream, by eonduetion e.g. on heated trays, in pneumatie risers, fluidized beds, spray towers, or by radiation. A typieal fluidized bed drier is depieted in Figure 4.28. The hydrodynamie prineiples of fluidization were summarized in Chapter 2. Within a fluid bed drier, heated air, or other hot... 

High pressure in the riser could also be due to insufficient fluidization gas in the base of the riser. Fluffing gas will vary the catalyst density more fluffing gas lowers the density in the system and the backpressure on the slide valve. 

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... 

Patience, G. S., Chaouki, J., Berruti, F., and Wong, R., Scaling Considerations for Circulating Fluidized Bed Risers, Powder Technol., 72, 31 (1992)... 

Louge, M., Lischer, J., and Chang, H., Measurements of Voidage Near the wall of a Circulating Fluidized bed Riser, Powder Tech., 62 269-276 (1990)... 

In circulating fluidized beds two main attrition sources, namely the riser and the return leg, may be distinguished. Although a lot of information is available about solids flow patterns and flow structures inside the circulating fluidized bed risers, no systematic investigations have been found in the open literature on the influence of riser geometry and flow conditions inside the riser on attrition. With respect to attrition occurring in the return leg, the work of Zenz and Kelleher (1980) on attrition due to free fall may be mentioned (cf. Sec. 4.3). 

Gas-particle flows in fluidized beds and riser reactors are inherently unstable and they manifest inhomogeneous structures over a wide range of length and time scales. There is a substantial body of literature where researchers have sought to capture these fluctuations through numerical simulation of microscopic TFM equations, and it is now clear that TFMs for such flows do reveal unstable modes whose length scale is as small as ten particle diameters (e.g., see Agrawal et al., 2001 Andrews et al., 2005). 

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