Defluidized zones in liquid-solid fluidized beds

Defluidized zones often appear on the distributor plates of liquid-solid fl uidized beds. They can lead to hot spots, the formation of undesirable side products or the degradation of products or reactants. In some cases, a sol id residue forms and plugs the distributor. Two different techniques were developed to detect defluidized zones. The fi rst technique uses a specially designed collision probe to monitor local pa rticle motion. The second technique is aimed at the on-line detection of de fluidized zones in industrial bioreactors. It uses local bed conductivity f luctuations. Defluidized zones were measured in beds of 3 or 5 mm diameter glass beads f luidized by an aqueous saline solution. Special experiments established the importance of horizontal liquid flow and distributor plate roughness on th e formation of defluidized zones. A model describes how a defluidized zone can be eliminated. It considers th at a defluidized zone is broken by the drag force on its particles of downw ard and sideways liquid flow. This liquid flow is induced by suction from t he liquid jets issuing from the distributor holes. The resulting drag force is resisted by friction between particles or between particles and the dis tributor surface. (C) 2001 Elsevier Science S.A. All rights reserved.