Solids segregation in gas-fluidized beds of large particles

A series of experiments was performed to develop an improved understanding of the fundamental nature of solids segregation in gas-fluidized beds of relatively large particles. Large particles were selected because they are becoming increasingly important in industrial fluidized beds but have not been very thoroughly investigated.

Segregation experiments were conducted with binary systems of large particles in a 10.2 cm (4 in.) diameter bed fluidized with air at ambient temperature and pressure. The particles studied ranged from 1750 pm (0.069 in.) to 6270 pm (0.247 in.) mean diameter and from 0.905 g/cm³ (56.5 Ib/ft³) to 7.50 g/cm³ (468 Ib/ft³) absolute density. Air velocity, particle properties, bed height, relative amounts of jetsam and flotsam, and bed internals were varied to determine their influence on the steady-state segregation pattern. Transient segregation and steady-state bed expansion were also measured.

A mixing index was developed which proved to be extremely useful in quantifying the variation of segregation with bed parameter changes. In correlating this index with air flow, it was discovered that abrupt changes sometimes occurred in the segregation pattern when small changes were made in the air flow. These abrupt changes appeared to be associated with changes in the fluidization mode. Such abrupt changes have not been previously reported.

The steady-state segregation profiles were found to be in good agreement with a modified form of the Gibilaro-Rowe model developed originally for small particle beds. The parameter representing large-scale circulation effects was found to have a similar range as previously reported for small-particle beds. The parameter representing bulk-phase dispersion was found to be appreciably larger than previously reported. The transient segregation phenomena appeared to be simulated reasonably well by a transient form of the Gibilaro-Rowe model. A special limiting case of the Gibilaro-Rowe model was also found to be useful in describing the solids and void distribution in an expanded bed. The Gibilaro-Rowe model was combined with differential mass balances to illustrate the expected performance of fluidized bed reactors which ex hibit this type of segregation.