M i c h i g a n Technological University, Houghton, M i c h i g a n 49931The response of a continuous flow mixing system to a step change in an input variable is discussed from the standpoint of dimensional analysis. This idea is presented as an alternative to using zone models for predicting residence time distributions in systems where imperfect mixing occurs.Mixing experiments were performed in a cylindrical flat bottomed tank geometrically similar to tanks commonly used for industrial processes. The response of the system to a step change in feed concentration was observed. Initially the tank contained a salt solution. At the start of an experiment, a stream of salt-free diluent was introduced a t the top of the tank and a stream of the salt solution was drained from the bottom, keeping the liquid volume i n the tank constant. The salt concentration in the output stream was measured continuously after the start of the experiment.The experimental results are correlated in terms of dimensionless variables, and the variables affecting the mixing process most strongly are determined. The results show that variations in throughput rate or i n impeller shape or rotational speed affect the mixing process much more than does impeller position. The data are also compared with models proposed by other authors, and the model constants are evaluated.Recently, experimenters have been studying stirred-tank systems from the fundamental point of view, using the principles of fluid mechanics, energy transport, and mass transport to predict the behavior of the system. Several authors (1, 2) have reported detailed velocity profiles within the mixing tank itself and have related the profiles Correspondence concerning this article should be addressed to Davis W. Hnbbad. Harish Patel is with Dow Chemical Co., Midland, Michigan 48640.to the impeller speed, fluid properties, and geometric variables. Logical extensions of such work would be to determine temperature and concentration distributions in the stirred tank and correlate the data in a similar manner. Other workers ( 3 to 13) have analyzed the stirred-tank system from a macroscopic viewpoint. Power re uirements this approach, and energy and mass transfer processes have been studied. Many of these studies have been carried out using a continuous flow stirred tank. A schematic flow November, 1971