Gas and solid behavior in cracking circulating fluidized beds

Gas and solid hydrodynamics have been studied in dilute circulating fluidized beds under conditions occurring in catalytic cracking risers. Gas radial velocity profiles and dispersions were established by a tracer technique in a cold set-up. The gas axial dispersion was determined in an industrial riser. The local concentrations of the solid phase were measured by a tomographic technique. This has allowed an assessment of the core—annulus structure of the bed and an estimate of the solid radial and axial dispersions. The axial solid concentration profiles were determined in pilot and industrial scale beds. These show an important accumulation upstream of the abrupt exit. The overall conclusion is that the gas flow can be considered to be plug flow with a radial velocity profile and a radial dispersion; the solid flow is slightly more dispersed due to the core—annulus structure and a high radial mixing.