Experimentally measured shear stress in the standpipe of a circulating fluidized bed

Abstract

AbstractShear stress measurements were obtained in the standpipe of a circulating fluidized bed (CFB) for 230‐μm coke breeze particles under a variety of flow conditions. These data were combined with incremental gas‐phase pressure drop readings, and an estimate of the bed weight to predict the solids‐phase pressure drop. It was determined that the wall shear stress and solids‐phase pressure drop are significant portions of the momentum balance and cannot be neglected when modeling frictional flow behavior of Geldart type B powders in a standpipe using the conservation of linear momentum. The combined magnitude of the axial solids‐phase pressure drop and wall shear stress exceeded 48% of the total forces in the mixture momentum balance for a standpipe. However, tests indicated that the influence of the wall shear stress and axial solids pressure in the mixture momentum balance may be assumed to be negligible once minimum fluidization has been exceeded. Under packed standpipe flow conditions most literature applications treat the wall shear stress as being directly proportional to the axial solids pressure divided by a proportionality factor that does not change with solids flow rate. This assumed constant is the product of the Janssen coefficient and coefficient of friction or the stress ratio. However, shear stress measurements indicated that this proportionality factor experienced a large monotonic decrease from conditions of incipient flow, where it was at its maximum, to conditions of high volumetric fluxes, where it approached a much smaller constant value. Two packed‐bed flow regimes were identified. They are the smooth flow regime, characterized as spanning from moderate to large solid volumetric flow rates, and a stick‐slip regime characterized as spanning from incipient to moderate solid volumetric fluxes. © 2005 American Institute of Chemical Engineers AIChE J, 2005