Assessment of quality of fluidization using gamma ray transmission technique

Voidage distribution is one of the key hydrodynamic parameters useful for efficient heat and mass transfer in gas fluidized beds. The quality of fluidization strongly depends on the uniformity of distribution of the fluidizing gas and often expressed in terms of time averaged local void fraction distribution. Fluidization of friable solids requires careful attention to avoid formation of fines due to attrition. Accordingly a low operating velocity ratio (u/u_mf) is advisable for processing friable solids in fluidized bed reactor. In most cases the minimum fluidization velocity (u_mf) is estimated using conventional pressure drop method and the operating velocity (u) is chosen little above the u_mf. The pressure drop method does not guarantee complete fluidization of particles throughout the bed. At low u/u_mf the segregation of larger particles may occur when the particles have wide size variation. This affects the quality of fluidization and may eventually lead to defluidization of part of the bed resulting in an uneven distribution of the gas. Gamma ray transmission technique has been explored to identify such defludization phenomena in a gas fluidized bed operating at low u/u_mf. Gamma ray photons were allowed to pass through a laboratory scale fluidized bed of zirconium oxide particles at different angular and axial positions of the bed. It was observed, that gamma ray transmission technique offers better assessment of quality of fluidization compared to conventional bed pressure drop method. Determination of optimum u/u_mf for friable solid using gamma ray transmission technique is also discussed. (author)