An experimental investigation into vapor dispersion and solid suspension in boiling stirred tank reactors

Abstract

Power consumption, gas hold-up, critical impeller speed for just complete off-bottom solid suspension and minimum impeller speed for ultimately homogeneous solid suspension were measured in boiling water using a 0.2 m i.d. stirred tank reactor with three four-pitched blade downflow turbines. Water and three different size glass beads were used as the liquid and solid phases, respectively. At higher vapor generation rates nucleation occurred at the heater, whereas at low vapor generation rates vapor was mainly generated from the impellers instead of the heater. At higher vapor generation rates, the boiling-to-nonboiling mechanical power ratio and the gas hold-up decreased and increased uniformly with increasing impeller rotational speed, respectively. At lower vapor generation rates, however, boiling-to-nonboiling mechanical power ratio exhibited a minimum and gas hold-up went through a maximum with varying impeller rotational speed. The changes in the nucleation site and solid suspension with impeller speed were responsible for these hydrodynamic behaviors. An empirical correlation for the Reynolds number corresponding to the minimum power consumption ratio or corresponding to the maximum gas hold-up in boiling liquids was developed using the present experimental data. The critical impeller speed for just complete off-bottom solid suspension and minimum impeller speed for ultimately homogeneous solid suspension in boiling systems were higher than those in gas-sparging systems. On the whole, the solid suspension in the boiling systems was poor as compared with that in the gas-sparging systems. Empirical correlations for critical impeller speed for just complete off-bottom solid suspension and minimum impeller speed for ultimately homogeneous solid suspension in boiling slurry stirred tank reactors were proposed.