Liquid-phase mixing time in boiling stirred tank reactors with large cross-section impellers

Liquid-phase mixing times in boiling systems were measured in a 0.2 m i.d. stiffed tank reactors with large cross-section impellers, i.e. Maxblend and Fullzone impellers. For reference, a triple-impeller system consisting of three six-flat blade disk turbines was also used. The impeller speed and vapor generation rate were varied from 0.83 to 7.5 s(-1) and 0.011 to 0.045 m/s, respectively. Nucleation occurred at the impeller instead of the heater at higher impeller speeds, whereas vapor was mainly generated from the heater at lower impeller speeds. The mechanical power consumption decreased due to vapor generation. Although the gas hold-ups increased with increasing vapor generation rate, for the large cross-section impellers the gas hold-ups in the boiling systems at higher gas flow rates and lower impeller speeds changed only slightly with increasing impeller speeds and were rather larger than those in the cold systems. This finding for the large cross-section impellers was quite different from the result for the triple-impeller system. The mixing times for larger-scale impellers decreased with increasing vapor generation rate and were almost independent on the impeller speed at low rotational speeds of impeller. At higher impeller speeds, the mixing time decreased with impeller speed. The experimental results for dimensionless mixing times were reasonably correlated with the inverse of the power number. (c) 2005 Elsevier B.V. All rights reserved.