15 - Slugging fluidization

Abstract

Different types of slugging behavior are reported and subjected to empirical study, in particular for the case of gas fluidization. The arguments and analyses presented in this chapter apply predominantly to gas systems having particle/bed diameter ratios in excess of about 1/200, which give rise to the formation of square-nosed fluid slugs. The boundaries separating adjacent fluid and solid slugs in such systems are essentially horizontal, leading to an overall behavior pattern that is essentially one-dimensional, amenable to analysis in terms of the one-dimensional equations of change. Lower particle/bed ratios are associated with the formation of round-nosed, axisymmetric fluid slugs, around which particles from the dense phase above flow downwards, predominately at the bed wall. High fluid velocities lead to progressively less ordered behavior: asymmetric fluid slugs and wall slugs, the latter involving irregular elongated bubbles which travel up the bed wall, effectively signaling an end to truly slugging behavior and the approach to the turbulent fluidization regime. The transition from the bubbling to the slugging regime is accompanied by a marked change in fluidization quality. Bubbles provide short cuts for the fluid, enabling it to bypass sections of dense phase in its passage through the bed.