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Gas-Solids Two Phase Flow Dynamics In Circulating Fluidized Bed Risers

Posted on:2004-01-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:X B QiFull Text:PDF
GTID:1101360095453646Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
The gas-solid two-phase flow in circulating fluidized bed (CFB) risers presents very complicated hydrodynamic aspects. Although numerous research efforts have led to significant advances in the understanding of hydrodynamics from the macro-distributions of gas-solid flow structure to the micro-behaviors of particle clustering, further studies are still needed in order to make clear the inherent mechanisms of various phenomena exhibiting in gas-solid two-phase flow so that more reliable models and scale-up methods of CFB reactors can be developed.This work conducted systematic experiments and studies on the radial distributions of local solids concentration and particle velocities as well as the axial distributions of pressure gradients. The experiments were carried out using FCC and sand and under wide operation conditions. Two CFB riser ser-ups used in the experiment have the same diameter of 100mm, but different heights of 15.1m and 10.5m, respectively. Based on the measured data, the effects of particle properties and geometric structure on hydrodynamics of gas-solid flow in risers and the inherent relationship between local and overall flow structure were intensively investigated.Significant effects of riser height on the distribution of axial pressure gradients were found. Under the same operating condition, the concentrations in whole riser decrease with increasing the height of column, and consequently leading to more uniform distribution of particles in risers. This suggests the effects of riser height must be taken into account when developing of the gas-solid two-phase flow hydrodynamic model and the scale-up method ofthe CFB reactors, because the most of past experimental conclusions and correlations were based on single height of riser and did not consider the influences of riser height.Under certain operating conditions, sand particles (belonging to group B of Geldart's classification) will decelerate to some degree after the effect of gas distributor disappears. Under low gas velocity conditions (?g<5.5m/s), the axial and radial distributions of FCC are much more uniform than those of sand; while under high gas velocity conditions (?>8.0m/s), the axial and radial distributions of sand, however, are much more uniform than those of FCC.Based upon the systematical experimental data obtained in this study and from other literatures, it is found that under the operating conditions with the same modified solids-to-air flow ratio Gs'2/G^? the gas-solid two-phase flows in risers show a very similar macro distribution and micro behaviors. This gives way to develop new model parameters so that the behaviors of gas-solid two-phase flows in CDB riser reactors can be described more exactly.By referring to the flow of fluid in circular pipe, the pressure drop due to the friction between the gas-solid suspension and the wall was quantitatively analyzed, and a model was proposed to predict the pressure drop of gas-solid flow in fully developed section of risers. This model includes particle and gas properties, riser structure parameters, and operating parameters and is in good agreement with the experimental data.Taken into account of the effect of riser height, particle properties, and the friction between the gas-solid suspension and the wall on the solids holdups, a correlation was proposed to predict the solids holdup in fully developed region of CFB risers. The correlation explains the differences among the predicated values of other authors and delineates more clearly the variations of solids holdup in fully developed region with operation parameters, ratio of height to diameter of riser, particle properties, the friction between the gas-solid suspension and the wall.Local solids concentrations, particle velocities, and solids fluxes reach the correspondingcross-sectional average values always at r/R ~ 0.8, and consequently the dimensionless radius of the core region can be defined at r/R=0.8. Preliminary studies show that this dimensionless radius is independent of particle pro...
Keywords/Search Tags:Circulating Fluidized Bed, Riser, Gas-Solid Two-Phase Flow, Hydrodynamics, Particle Velocity, Solids Concentration, Particle Cluster
PDF Full Text Request
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