Many studies have appeared in the literature dealing axial solids distribution along CFB risers (e.g. Yerushalmi et al, 1976: Kwauk et al, 1986: Arena et al 1991, Bai et al 1992). It is most often represented as either a plot of the radially averaged solids suspension density or voidage versus height. This profile is experimentally determined from the pressure distribution along the riser, recognizing that the suspended solids constitute the major contributor to the pressure drop.
The experimental work of Yerushalmi et al (1976) is generally considered as the pioneering academic study of the axial solids distribution in circulating fluidized bed riser. They discussed the densification at the riser base and proposed several advantages of operating in the fast fluidization regime.
[...] Axial profiles of solids volume fraction ) are directly measured by means of quick closing valves and compound to those estimated from pressure drops. Bed blow out experiments are carried out by suddenly turning off solids control valve to find out instantaneous solid mass flux. They have also provided a model by taking into account of solids concentration profiles ) as the result of diffusion/segregation mechanism with uniform particle cluster size through out the columns to determine the derivatives of S-shaped axial voidage profiles at the theory of elutriation, explicitly takes into account the effect of the size of the column, the flux of circulating solids and the solids particle densities. [...]
[...] Values for the limiting voidage and the axial location of the interface were determined from previously published correlations; Fujima et al (1991) modeled the axial voidage profiles as due to “chain growth” or cluster formation. Choi et al (1991) developed empirical correlations to predict the average axial voidage profiles. Rhodes et al (1991) studies particle motion at the wall of a riser in a cold model CFB using both high-speed video and optic fiber probes. They observed that the particles at the wall were descending in the form of sworms having voidages in the range of 0.8 to The sworms resulted from the transport of solids from the core of the riser. [...]
[...] The Kunii and Levenspiel exponential-decay model for the axial voidage profiles has been modified to consider a freeboard height lower than the transport disengaging height developing some equations to obtain the axial voidage profiles at height lower than the TDH. Two equations have been proposed to calculate the K constant and the decay constant ‘a' as a function of the operating conditions. Mandal S. et al (1995) reported the experimental data on the voidage profile in the riser of a shallow circulating fluidized bed. [...]
[...] The interface between the regions (the middle of the is found to be a region of relatively low axial solids momentum throughout the riser cross-section. Schlichthaerle.P et al (1999) measured the solids concentration distribution in the bottom zone of a cold model CFB unit with fiber-optical probes and by -ray absorption. A higher solids concentration at the wall and a lower concentration in the center of the riser was found. Starting at the distributor, the time and cross-sectional average solids volume concentration increases slightly or remains constant with height up to a height Hb from where it drops off. [...]
[...] The clusters are conjectured to be fragments of the relatively dense particle film falling near the wall: this film is shown to have features in common with a film of ordinary liquid flowing down a vertical surface. Sung Won Kim. et al (2004) determined the flow structures in a circulating fluidized bed (CFB) riser ( 0.203 m i.d.x 5.9 m high) of FCC particles (dp=70 =1700 kg/m3). A momentum probe was used to measure radial momentum flux profiles at several levels and to distinguish between upward and downward flow regions. [...]
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