Gas-liquid dispersion in stirred tanks: scale-up and agitator comparison

This thesis is a description of an experimental study of gas-liquid dispersion in mechanically agitated vessels. The objectives of the project were to investigate scale-up and to compare different agitator types. Three vessels up to 2.67m in diameter were available for test work. The work consisted largely of the measurement of the following parameters : (i) Impeller power demand under gassed and ungassed conditions. (ii) Overall gas hold-up. (iii) Mass transfer coefficient using a dynamic response technique. (iv) Flooding point, especially for Rushton turbine impellers. The influence of scale effects on mass transfer has been discussed in some detail from a theoretical standpoint, especially the influence of liquid depth on the concentration driving force available for mass transfer. The use of specific power input and superficial gas velocity was adequate to correlate both hold-up and mass transfer coefficient, but some second order scale effects appeared to be present which made the correlations in each vessel statistically significantly different from each other. The Rushton turbine showed little difference in dispersion efficiency from the concave turbine. Differences were, however, discovered between pitched blade turbines and Rushton turbines, especially when the pitched blade turbine was pumping upwards. The economics of mixer design has also been considered, and the potential economic value of academic research into energy efficiency has been clearly demonstrated.