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

Abstract

Given the knowledge gap which exists in our understanding of impeller induced gas dispersion in highly viscous liquids, this paper investigates gas hold-up characteristics of six impellers in a highly viscous Newtonian liquid (Castor oil, μ=0.76Pas): four disc-type impellers (conventional Rushton turbine and three other modified designs of disc turbines which are perceived to overcome its drawbacks), and two types of pitched bladed turbines (upflow – PTU and downflow – PTD). Gas hold-up is mainly compared on the basis of specific power dissipation (PG/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. The relative performance of these impellers is evaluated by comparing: (a) the final total gas hold-up, (b) the hold-up of tiny bubbles and (c) the parameter describing time dependency of gas hold-up. The key inferences drawn from 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 specific power required to initiate gas dispersion throughout vessel (PGT/V); it decreases progressively at higher powers. As a consequence, it is suggested that such impellers are operated at a power dissipation level just greater than the threshold value.3.And finally, modified disc-type impellers show a considerable potential for enhanced performance over Rushton turbine, while pitched bladed turbines are, in general, inferior to disc-type impellers.