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

Power consumption, gas hold-up, critical impeller speed for just complete o ff-bottom solid suspension and minimum impeller speed for ultimately homoge neous solid suspension were measured in boiling water using a 0.2 m i.d. st irred tank reactor with three four-pitched blade downflow turbines. Water a nd three different size glass beads were used as the liquid and solid phase s, respectively. At higher vapor generation rates nucleation occurred at th e 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 decr eased and increased uniformly with increasing impeller rotational speed, re spectively. 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 nucleati on site and solid suspension with impeller speed were responsible for these hydrodynamic behaviors. An empirical correlation for the Reynolds number c orresponding to the minimum power consumption ratio or corresponding to the maximum gas hold-up in boiling liquids was developed using the present exp erimental data. The critical impeller speed for just complete off-bottom so lid suspension and minimum impeller speed for ultimately homogeneous solid suspension in boiling systems were higher than those in gas-sparging system s. On the whole, the solid suspension in the boiling systems was poor as co mpared with that in the gas-sparging systems. Empirical correlations for cr itical impeller speed for just complete off-bottom solid suspension and min imum impeller speed for ultimately homogeneous solid suspension in boiling slurry stirred tank reactors were proposed. (C) 2002 Elsevier Science B.V. All rights reserved.