Chemical Engineering SCWW, Vol. 44, No. Ii, pp 2463-2474, 1989 ooo9- 2509.‘89 S3.CMIt0.00 Printed in Great Britain. 1: 1989 Pergamon Press plc THE CONVEX BLADED MIXED FLOW IMPELLER: A MULTIPURPOSE AGITATOR A. B. PANDIT, C. D. RIELLY, K. NIRANJAN and J. F. DAVIDSON’ Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, U.K. (First received 18 July 1988; accepted in rchcd form 2 February 1989) Abstract-A new type of impeller for stirred tanks-the convex bladed mixed flow impeller (CBMF) is described. IL is a compromise between the disc turbine (Rushton) impeller and the marine propeller: the CBMF combines the swept-back blades with a propeller action. The swept-back feature gives lower energy dissipation in the vicinity of the impeller than the disc turbine; the propeller action gives a strong downward liquid motion through the impeller and hence good circulation in the stirred tank. Such a compromise is necessary for a multipurpose mixing vessel. The CBMF impeller was tested comprehensively in comparison with the disc turbine, the marine propeller and the pitched bladed turbine; the comparison includes published data. The tests included measurements of: (1) power consumption in water. glycerol and aqueous CMC solutions, (2) minimum power for gas dispersion, (3) 95% mixing time for a tracer in water, (4) gas hold up in water, (5) k,a for oxygen transfer from pure oxygen to water and CMC solutions and (6) minimum power and speed for solids suspension. The CBMF is not better than the above mentioned impellers in all tests but it does appear to have advantages in respect of: (1) lower power consumption, (2) shorter mixing time, (3) good mass transfer and (4) lower minimum power for solids suspension. 1. INTRODUCTION Mechanically agitated contactors are commonly used as reactors in the chemical industry. The ease with which the liquid phase residence time may be changed makes them suitable for a variety of heat and mass transfer operations as well as processes involving liquid mixing, gas-liquid dispersion, solids suspension and chemical reaction. The behaviour of two- and three-phase reactors has been discussed in detail by Chapman et al. (1983a-d) and Joshi et al. (1982). The purpose of this paper is to present data on a new design of impeller which is believed to be an improve- ment on currently used configurations. Many small scale chemical plants work in cam- paigns which last only a few months and involve a variety of processes using the same basic reactor vessel but requiring different agitators and drives. The stan- dard types of impeller used in the chemical industry are only efficient under specilic process conditions, e.g. the marine propeller is most efficient, in terms of power consumption, for blending operations; the disc turbine is good for gas dispersion; the pitched bladed impeller is good for solids suspension (Chudacek, 1985). Changing the impeller and drive for each new process is expensive and time-consuming and there is a clear need for a multi-purpose impeller which per- forms well under a variety of process conditions. This type of impeller should also be useful for three-phase reactors where solids suspension and gas dispersion are required simultaneously. The objective of this paper is to demonstrate that this multi-purpose func- tion is fulfilled by a new design of impeller termed the convex bladed mixed flow impeller (CBMF): Fig. 1 ‘Author to whom correspondence should be addressed. shows the form of the new impeller, for which there is a provisional patent (Pandit et al., 1987). There is still considerable uncertainty regarding the scale-up of stirred tank reactors (Mersmann and Laufhuette, 1985). Although the use of equivalent specific power input, equal tip speed and geometric similarity have had some success, no unified design method has so far emerged. Any new impeller should undergo comparative testing against other standard impeller types in the same experimental set-up so that details such as impeller blade and disc thickness and baffle width, spacing and thickness are the same for all impellers. These geometric parameters have been shown to have a significant effect on the power consumption and performance of an impeller (Nienow and Miles, 1971). In the present work, the CBMF impeller has been compared with three other impeller types operating under a variety of process conditions. The impellers considered here (see Fig. 2) were: (1) the disc-turbine or Rushton turbine, (2) the marine propeller pumping downwards and (3) the pitched bladed or mixed flow impeller, pumping downwards; unless otherwise indi- cated the CBMF was used in the downward pumping mode, i.e. rotated so as to move liquid downwards through the impeller. There is a variety of designs of marine propcllcr; the data used in this work were taken from Pandit and Joshi (1983). The process operations examined were: (1) gas dispersion, (2) liquid mixing, (3) solids suspension and (4) gas-liquid mass transfer. Experiments were carried out in water, glycerol solutions and CMC solutions to investigate the performance of the impeller in liquids of differing viscosities and liquids showing non-Newtonian be- haviour. CES 44:11-D 2463