Determination of the coefficient of heat transfer at the bed wall boundary of an externally heated fluidized bed

Abstract

Fluidization, a relatively new unit operation by which a solid and gas or liquid may be contacted, is being widely developed in the field of catalytic cracking of petroleum because of its characteristic reduction of temperature gradients within the reaction mass. Basic research of the heat transfer properties of fluidized systems has lagged far behind industrial applications. It was the purpose of this investigation to evaluate the effect of temperature driving force and mass superficial air velocity on the coefficient of heat transfer at the bed wall of an externally heated, fluidized bed of Ottawa sand at wall temperature of 200, 400, and 600 °F , and average mass superficial air velocities of 82.5, 123.2, 170.3, and 217.5 pounds per hour-square foot.

The tests were carried out under steady state conditions of air flow and bed wall temperature. A complete heat and material balance, including evaluation of heat losses, was made for each test. The boundary coefficients based on the internal area and temperature of the pipe wall were calculated. The effects of mass superficial air velocity and wall temperature on the boundary coefficient of heat transfer and on bed and bed wall temperature gradients were studied.

From observations made it was noted that the fluidization range of the Ottowa sand bed began at a mass superficial air velocity of 91.0 pounds per hour-square foot and ended at 210.0 pounds per hour-square foot, the velocity at which slugging occurred throughout the bed.

The horizontal temperature gradient across the bed increased with increasing bed wall temperature, increasing from a minimum of 0 °F at 200 °F wall temperature to 6 °F at 600 °F . The rate of heat flux to the air stream passing through the fluidized bed increased with mass air flow rate at constant bed wall temperature. The minimum heat flux was 84 Btu per hour and occurred at 200 °F and 82.5 pounds per hour-square foot, while the maximum was 1172 Btu per hour and occurred at 600 °F and 217.5 pounds per hour-square foot. The coefficient of heat transfer increased with bed wall temperature, reaching maximum values of 9.55, 13.40, 13.31, and 13.30 Btu per hour-square foot-°F at 600 °F and at mass superficial air velocities of 82.5, 123.2, 170.3, and 217.5 pounds per hour-square foot, respectively.