As. Khare et K. Niranjan, An experimental investigation into the effect of impeller design on gas hold-up in a highly viscous Newtonian liquid, CHEM ENG SC, 54(8), 1999, pp. 1093-1100
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.