Some effects of rheology on the spatial distribution of gas hold-up in a mechanically agitated vessel

Abstract

The effects of rheology on gas hold-up in non-Newtonian circulation flow has been rarely discussed in the literature, especially so far as local gas distribution in mechanically agitated vessels is concerned. This study analyses experimental gas dispersion behaviour for a wide range of complex-rheology conditions. Non-Newtonian aqueous solutions of xanthan gum with concentrations from 0.5 to 5 kg/m 3 and Newtonian glycerol solutions of 450 and 800 kg/m 3 containing also 5% electrolyte are studied. Hold-up is measured conductometrically at 36 spatial points within the stirred vessel mainly around and above the impeller for a conventional geometry (Rushton standard configuration with T=0.2 m and D/ T=0.33). In addition to increasing viscosity, three aspects of rheology impact on gas hold-up and different dominances of component effects upon the overall gas dispersion mechanism are revealed. The results are presented as local and global gas hold-up vs. consistency, pseudoplasticity, and position. By comparing data obtained for Newtonian and non-Newtonian flow conditions and data obtained for the same polymer solution at constant plasticity and variable consistency, an attempt to reveal the net effects of consistency and pseudoplasticity is made. The boundaries of regions with large differences of mixing intensity reflected in terms of local gas availability are quantified.