Power consumption and gas dispersion in agitated vessels.

Abstract

An experimental study of the steady-state and dynamic characteristics of gas-liquid mixing in an agitated vessel has been made. The effect of impeller speed and gassing rate on the power consumption, bubble size, gas holdup and specific interfacial area was investigated over a wide range of conditions, in both coalescing and 'non-coalescing' systems. A further extension of this included the effect of continuous liquid flow. The experiments were carried out using a fully baffled tank of 0.2m diameter and three sizes of Rushton turbine with D/T = 0.375 to 0.66. The bubble size in the vessel was measured by a capillary probe technique. In the ungassed state (no sparging of gas) and at sufficiently high Reynolds number, surface aeration plays an important role. Sampling of the entrained bubbles near to the liquid surface and in the impelle region revealed that surface aeration takes place before there is any drop in Power number and at correspondingly much lower impeller speeds A mechanism for surface aeration is proposed and correlations are presented. A new measure of gas dispersion efficiency has been defined. It can be used to identify the different regions of gas-liquid mixing as well as providing estimates of the degree of flooding or gas recirculation. The correlations of gas dispersion obtained from the gassed power measurements enable a more accurate prediction of Pg to be made for any region of gas-liquid mixing. The impeller power response has been studied using pulse and step change in gas flow rate. These dynamical measurements have provided further insight into the cavity formation processes occurring behind the impeller blades and the reverse process of cavity stripping. A simple lumped parameter model has been proposed to describe the process of cavity formation. The physical implications of this model are analysed and estimates of obtained are compared with the steady-state results. Extensive measurements have been made of the bubble size produced in air-water and air-electrolyte dispersions. These are presented in the form of spatial and point distributions, gas-holdup and specific interfacial area. Averaged overall estiamtes of these dispersion properties have been analysed and compared with other results and correlations. The position of the inlet and outlet liquid flows in a continuous-flow system have a very significant effect on the gas-liquid mixing. However, having the liquid inlet at the bottom of the tank and the outlet pipe at the side does not substantially alter the pattern of mixing from that observed in a batch vessel.

Date of Award	1981
Original language	English
Awarding Institution	University of Bath