Evaluation of the gas-solid suspension density in CFB risers with exit effects

In order to model the gas-solid hydrodynamic flow structure in circulating fluidized bed (CFB) riser reactors, there is a need to predict the average and local suspension density and flow direction as functions of both axial and radial coordinates, gas and solids physical properties, and operating conditions (gas superficial velocity and imposed solids circulation rate). Existing hydrodynamic models fail to properly and reliably describe the flow characteristics typical of CFB catalytic reactors operating at high superficial gas velocities and solids mass fluxes. In the present work, we have compiled an extensive experimental database to encompass published laboratory, pilot, and available industrial scale hydrodynamic data in a concise and useable form. Thorough analysis of the database followed by an in-depth multiple regression analysis yielded a new correlation for calculating the "slip factor", defined as the ratio of the interstitial gas velocity to the particle velocity, to describe Geldart Group A powder systems in the fully developed flow (FDF) region of the riser. The correlation allows the evaluation of the gas-solid suspension density in the riser in both the fast fluidization (FF) and pneumatic transport (PTR) flow regimes. Tn addition, a comparative study has been carried out on the database to investigate the effects of the geometrical configuration of the top exit of the riser on the suspension density and flow structure. Two reflection coefficient correlations are proposed to account for exit effects for both Groups A and B solids systems. The slip factor and reflection coefficient correlations have been further compared to existing correlations available in the literature for discussion and validation and shown to successfully characterize the large majority of data available in the literature. (C) 2000 Elsevier Science S.A. All rights reserved.