Experimental study of heat transfer in a co-current downflow fluidized bed (downer)

Local heat transfer was studied in a 9.3 m tall, 100 mm diameter gas-solid concurrent downflow fluidized bed (downer) with FCC particles. A miniature cylindrical heat transfer probe (6.4 mm in diameter and 38 mm in length) was inserted into the bed to measure the local heat transfer coefficients. The radial and axial distribution of heat transfer coefficients between the suspended surface and the gas-particle flow suspension was obtained using a miniature heat transfer probe, under different operating conditions. The solids concentration and particle velocity, which are considered as the two most important parameters, are measured by two optical fibre probes. The results show that the radial and axial distributions of heat transfer in the downer are not the same as those in the riser. The heat transfer rate in the downer is closely related to the hydrodynamics, with bed suspension density being the most influential factor. The average heat transfer coefficient decreases with decreasing solids circulating rate due to the decreased solids holdup. The heat transfer coefficient does not always decrease with increasing gas velocity given the increased importance of gas convective heat transfer under high gas velocities and low solids holdups in the downer.