Measurements of solids concentration and axial solids velocity in gas-solid two-phase flows.

Several techniques reported in the literature for measuring solids concentration and solids velocity in (dense) gas-solid two-phase flow have been briefly reviewed. An optical measuring system, based on detection of light reflected by the suspended particles, has been developed to measure local solids concentration and local axial solids velocity in dense gas-solid two phase flows. This system has been applied to study hydrodynamics of a cold-flow circulating fluidized bed unit operated in the dense flow regime (uD: 7.5¿15 m s¿1 and Gs = 100¿400 kg m¿2 s¿). With increasing solids mass flux, at constant superficial gas velocity, lateral solids segregation became more pronounced (i.e. extent of development of core-annulus structure) while the radial profiles of axial solids velocity hardly changed. A decrease in superficial gas velocity, at constant solids mass flux, also augmented the lateral solids segregation. The axial solids velocity decreased over the entire tube radius, although the shape of the profiles showed no strong dependence with respect to the superficial gas velocity. Average solids mass fluxes calculated from the measured local values of solids concentration and solids velocity exceeded the imposed solids mass flux, a finding which could be explained by the downflow observed visually of solid particles close to the tube wall. In addition, cross-sectional averaged solids concentrations obtained on the basis of the optical measuring system and those obtained from the pressure gradient measurements showed satisfactory agreement

Hydrodynamic modelling of circulating fluidised beds

A one-dimensional model for the riser section of a circulating fluidised bed has been developed which describes the steady-state hydrodynamic key variables in the radial direction for fully developed axisymmetric flow. Both the gas and the solid phase are considered as two continuous media, fully penetrating each other. As a first approximation gas phase turbulence has been incorporated in our hydrodynamic model by applying a slightly modified version of the well-known Prandtl mixing length model. To solve the resulting set or transport equations, the solids distribution along the tube radius is required. Several strategies are given to obtain this information. In addition the effect of clusters on the momentum transfer between both phases has been modelled using an empirical correlation. Theoretically calculated results agree well with reported experimental data of different author

Hydrodynamic modelling of gas-particle flows in riser reactors.

Complex hydrodynamic behavior of circulating fluidized beds makes their scale-up very complicated. In particular, large-scale lateral solids segregation causes a complex two-phase flow pattern which influences significantly their performance. Lateral solids segregation has been attributed to direct collisional interactions between particles as well as to interaction between gas-phase eddies and dispersed particles. However, these phenomena have not been investigated thoroughly. \ud This article discusses an advanced 2-D hydrodynamic model developed for circulating fluidized beds based on the two-fluid concept. Because theory to model the interaction between gas-phase eddies and dispersed particles is not available, turbulence was modeled on a macroscopic scale using a modified Prandtl mixing length model. To model the influence of direct particle-particle collisions the kinetic theory for granular flow was applied based on the Chapman-Enskog theory of dense gases. For model validation purposes, a cold flow circulating fluidized bed was employed in which sand was transported with air as fluidizing agent. The column is equipped with pressure transducers to measure the axial pressure profile and with a reflective optical fiber probe to measure the local solids concentration and axial solids velocity. Theoretically calculated solids concentration and axial solids velocity agree satisfactorily with experiment, especially when one realizes that the model contains no adjustable parameters. In general, however, the model slightly underpredicted the experimentally observed lateral solids segregation and yielded a more peaked velocity profile compared to its experimental counterpar

Bubble formation at a single orifice in gas-fluidised beds

An earlier developed hydrodynamic model describing dense gas-solid two phase flow has been used to study the bubble formation process at a single orifice. A systematic experimental and theoretical study has been conducted to investigate the effect of particle properties (i.e. particle diameter and density) on the bubble growth process for Geldart type B powders. Theoretical results, obtained for both two-dimensional and three-dimensional geometries, have been compared with experimental data and with predictions from approximate models reported in literature. A comparison of the theoretical results and experimental data shows that the advanced hydrodynamic model gives a satisfactory good description of the bubble growth process for several particle types which makes this model a useful tool to study the bubble formation process in fluidised beds. It appears that the influence of particle size and particle density on bubble formation can be related to the effect of the minimum fluidisation velocity on this process. At a constant gas injection rate through the orifice higher minimum fluidisation velocities result in larger bubbles and decreased leakage. Further, it has been found that coarse particles give rise to the formation of relatively elongated bubbles. The detachment times, on the other hand, seem to be independent of the particle size used

Bubble formation at a single orifice in gas-fluidised beds

An earlier developed hydrodynamic model describing dense gas-solid two phase flow has been used to study the bubble formation process at a single orifice. A systematic experimental and theoretical study has been conducted to investigate the effect of particle properties (i.e. particle diameter and density) on the bubble growth process for Geldart type B powders. Theoretical results, obtained for both two-dimensional and three-dimensional geometries, have been compared with experimental data and with predictions from approximate models reported in literature. A comparison of the theoretical results and experimental data shows that the advanced hydrodynamic model gives a satisfactory good description of the bubble growth process for several particle types which makes this model a useful tool to study the bubble formation process in fluidised beds. It appears that the influence of particle size and particle density on bubble formation can be related to the effect of the minimum fluidisation velocity on this process. At a constant gas injection rate through the orifice higher minimum fluidisation velocities result in larger bubbles and decreased leakage. Further, it has been found that coarse particles give rise to the formation of relatively elongated bubbles. The detachment times, on the other hand, seem to be independent of the particle size used

An engineering model for dilute riser flow

To facilitate understanding of the hydrodynamic behaviour of CFBs, a one-dimensional model for the riser tube of a CFB has been developed. The model describes steady state hydrodynamic key variables (i.e. cross-sectional averaged values of pressure, solids concentration and velocities of both phases) for developing axi-symmetrical flow as a function of the axial position in the riser tube. Calculated results have been compared with experimental data obtained from a small scale CFB unit which could be operated at pressures up to 6 bar. Despite the simplicity of the model, it turned out that the model was capable of predicting the effect of changes in operating conditions (i.e. gas velocity, solids mass flux, operating pressure and particle diameter). The model neglects the existence of clustering of particles, lateral solids segregation and solids downflow near the tube wall, which limits the applicability to dilute systems (εs<0.04).\ud \u