Particle-Wall Contact Time in Fluidized Beds

Knowledge of the basic mechanisms of heat transfer in gas fluidized beds with immersed heat exchanger surface or bed walls is essential for any optimum design. Convective heat transfer from/to a heat exchanger surface in a gas fluidized bed can be described by the gas and/or particle convection. While many studies have been conducted on this topic, the contribution through particle convection inadequately described, especially in turbulent fluidized beds. The role of hydrodynamics of the dense bed, status of particles in form of cluster and density of clusters near the heat exchanging surface in particle convection is briefly outlined. The existing contact time models in the literature suggest a constant decrease of particle-wall contact time when increasing the gas velocity. However, it has been shown experimentally that the contact time increases in the bubbling regime of fluidization and increases in the turbulent fluidization when increasing the gas velocity. A theoretical stochastic model is developed to represent such a trend and improve agreement with experimental data presented in literature. The comparison of predicted results with latest experimental data from literature confirms the validity of the present model for fluidized bed of sand particles in the dense section of the bed. Key-Words: Contact Time, Fluidization, Heat Transfer, Particle Convection

[1]  H. Martin,et al.  Heat transfer between gas fluidized beds of solid particles and the surfaces of immersed heat exchanger elements, part I , 1984 .

[2]  Towards a generalized model for vertical walls to gas—solid fluidized beds heat transfer—I. Particle convection and gas convection , 1993 .

[3]  H. Mickley,et al.  Mechanism of heat transfer to fluidized beds , 1955 .

[4]  L. B. Koppel,et al.  Effects of Solid Thermal Properties on Heat Transfer to Gas Fluidized Beds , 1964 .

[5]  Jamal Chaouki,et al.  Characterization of dynamic gas–solid distribution in fluidized beds , 2000 .

[6]  J. Chaouki,et al.  On the Axial Movement of Solids in Gas-Solid Fluidized Beds , 2000 .

[7]  Martin J Rhodes,et al.  Determination of particle residence time at the walls of gas fluidized beds by discrete element method simulation , 2003 .

[8]  Particle migration at solid surfaces and heat transfer in bubbling fluidized beds , 1997 .

[9]  M. Schiavetti,et al.  Quantitative estimation of bubble size in PFBC , 1994 .

[10]  Jamal Chaouki,et al.  Flow structure of the solids in gas–solid fluidized beds , 2004 .

[11]  Prabir Basu,et al.  An experimental investigation of burning rate and mass transfer in a turbulent fluidized bed , 1986 .

[12]  Jamal Chaouki,et al.  Experimental investigation of particle contact time at the wall of gas fluidized beds , 2005 .

[13]  A. Burschka,et al.  Particle migration at solid surfaces and heat transfer in bubbling fluidized beds. II: Prediction of heat transfer in bubbling fluidized beds , 1995 .