A theoretical and experimental investigation of spouted beds with draft tubes.

A spouted bed with a draft tube insert and separate gas supplies into the annular region and central core at the bottom of the bed was built. The influence of the central gas feed flow rates, annular gas feed flow rate, and the bottom gap height on the solids circulation rate, gas bypassing rate through the bottom gap beneath the draft tube, and pressure drop across the bed were investigated. The experimental results show that within the experimental range: (1) an increase in the central gas feed flow rate increases the solids circulation rate and gas bypassing flow rate from the central core to the annular region through the bottom gap; (2) an increase in the annular gas feed rate increases the solids circulation rate and reduces the gas bypassing rate from the central core to the annular region; (3) the pressure drop across the bed is almost constant with changes of annular gas feed flow rate and central gas feed flow rate; (4) the solids circulation rate increases with increasing, the bottom gap height within the operating range; and (5) among the operating parameters, the most sensitive parameter to the solids circulation rate is the bottom gap height. A new theoretical model was developed based on only fundamentally theoretical analysis, using only physically meaningful and controllable variables as the input variables without adjusted parameters. By using mass and momentum balances of gas and solids in the four regions of the bed and equating the variables at the boundaries between neighboring regions, a system of differential and algebraic equations were presented and solved. The input variables to the model are operating parameters, bed geometry, and properties of the gas and solids. The model can predict the hydrodynamic behavior of spouted beds with draft tubes, such as solids circulation rate, profiles of solids velocity, gas velocity, voidage, and pressure in the central core, gas redistribution between the central core and the annular region, and mass distribution of solids among all the different regions. The model agrees fairly well with the experimental data within the current operating range