An experimental investigation into the effect of impeller design on gas hold-up in a highly viscous Newtonian liquid

Given the knowledge gap which exists in our understanding of impeller induc ed gas dispersion in highly viscous liquids, this paper investigates gas ho ld-up characteristics of six impellers in a highly viscous Newtonian liquid (Castor oil, mu = 0.76 Pa s): four disc-type impellers (conventional Rusht on turbine and three other modified designs of disc turbines which are perc eived to overcome its drawbacks), and two types of pitched bladed turbines (upflow - PTU and downflow - PTD). Gas hold-up is mainly compared on the ba sis of specific power dissipation (P-G/V), and analysed in the context of: (i) the formation of large and tiny bubbles, and (ii) its time dependency, both being typical features of gas dispersion in high viscosity liquids. Th e relative performance of these impellers is evaluated by comparing: (a) th e final total gas hold-up, (b) the hold-up of tiny bubbles and (c) the para meter describing time dependency of gas hold-up. The key inferences drawn f rom this study with regard to gas hold-up are as follows: 1. Unlike in low viscosity liquids, gas hold-up in high viscosity Newtonian liquid is strongly influenced by impeller type. 2. In the case of disc-type impellers, gas hold-up peaks at the threshold s pecific power required to initiate gas dispersion throughout vessel (P-GT/V ); it decreases progressively at higher powers. As a consequence, it is sug gested that such impellers are operated at a power dissipation level just g reater than the threshold value. 3. And finally, modified disc-type impellers show a considerable potential for enhanced performance over Rushton turbine, while pitched bladed turbine s are, in general, inferior to disc-type impellers. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.