Influence of pressure and fluidization velocity on the bubble behaviour and gas flow distribution in a fluidized bed

Abstract

Measurements of the visible bubble flow rate and the through-flow velocity of gas inside bubbles were made in a pressurized fluidized bed, using capacitance and Pitot-static pressure probes, respectively. The influence of pressure and fluidization velocity on the bubble behaviour and gas flow distribution was studied. The bed, of 0.2 × 0.3 m cross-section, was operated at pressures between 0.1 and 1.6 MPa and at excess gas velocities between 0.1 and 0.6 m/s. The bed material was silica sand with a mean particle diameter of 0.7 mm. The mean bubble frequency, the mean bubble rise velocity, the mean bubble volume fraction and the visible bubble flow rate were found to increase with both increasing pressure and excess gas velocity. The mean pierced length of bubbles increases with increasing excess gas velocity but decreases, after an initial increase, wtih increasing pressure. With both increasing pressure and excess gas velocity, the bubbles tend to redistribute themselves towards the center of the bed cross-section. The pressure probe measurements show that the through-flow velocity of gas inside the bubbles decreases with increasing pressure. Furthermore, gas flow balance calculations show that the ratio of dense phase superficial gas velocity to minimum fluidization velocity increases with both increasing pressure and excess gas velocity. Pressure drop measurements show a decrease in minimum fluidization velocity with increasing pressure, in accordance with predictons from the Ergun equation.