Investigation On Fluidization Behaviors Of Coarse Particles In A Gas-Solid CFB Riser

Gas-solid circulating fluidization, as a high efficient gas-solid contact technology, is widely used in process industries. However, most previous investigations focused on the FCC particles (Geldart A). To study the effects of particle properties on the gas-solid flow behaviors in CFB riser, the paper selected sands as typical coarse particles and carried out a systematic experiment. With the superficial gas velocity ranging from 4.6 to 11.4m/s and the solids circulation rate from 16 to 210kg/m2.s, the distributions of axial pressure gradients and local concentrations of coarse particles were measured in a 10.5m high riser. The results show that the pressure gradients and radial distribution profiles of solids holdups and the effects of operation conditions on them are quite different from those of FCC. For sand particles, different non-uniformities of radial distribution of local concentrations are observed. The local solid concentration will decrease linearly from the centre to the wall of the riser under some operation conditions, unlike the FCC particles for which the local solid concentration always increases from the centre to the wall. No clear similarities for the radial distributions of normalized solids holdups are found along the axial direction, and the core-annulus flow structures are also changed greatly, with the annulus region diminishing and core region extending to the vicinity of the riser wall. By using intermittency index to describe the gas-solid mixing behaviors, it is found that there is not much cluster in the riser and the local flow of sands is steady. For various conditions with the same values of Gs1.2/Gg2.0, the radial distributions of intermittency index become similar on a given cross-section of riser. Because of the effects of particle posterities and gas distributor, there is another particular axial distribution of cross-sectional averaged solids holdups, in which the averaged holdups increase first and then decreases along the riser height.From the experiment, it is also found that the errors between the apparent solids holdups and the actual ones are negligible in high density risers, but in the conditions of low solid-to-air mass ratio Rs and high superficial gas velocity Ug, the errors become significant owing to the friction between the gas-solid suspensions and the riser wall. Taking the friction into account and based on the empirical relation for flow friction in circular tube, the paper developed a correlation for the friction factor of gas-solid flow in riser. It can give the solids holdups from the measured pressure gradients with more accuracyThe results of this thesis are significantly important for the understanding of the effects of solids properties on circulating fluidization flow behavior and for the modeling of gas-solid hydrodynamics in CFB riser.