The Confined Impeller Stirred Tank (CIST): A New Mixing Device Designed to Properly Scale-Down Local Mixing Effects in Industrial Applications

The confined impeller stirred tank (CIST): a new mixing device designed to properly scale-down local mixing effects in industrial applications

 

 

Marcio B. Machado, Suzanne M. Kresta

University of Alberta, Edmonton AB T6G 2V4

 

Scaling-down and -up an industrial application remains challenging. The effectiveness of these industrial processes is improved if efficient dispersion and dissolution of the additives is achieved. It is known that current bench scale tests do not always represent the industrial scale behavior. One of the most critical steps for a process scale-down or scale-up is to have a  fully turbulent flow regime in the bench scale mixing vessel, because if the flow falls into transitional flow, the results are not reproducible. This work presents the confined impeller stirred tank, as shown in Figure 1, which is a mixing device designed to provide uniform intensity of mixing over the entire volume of the tank. The CIST is a 1 L tank filled with five impellers. Such configuration confines the flow. The tank has a diameter of  T = 7.6 cm and a height H = 3T. Mean velocity profiles were measured at different axial positions of the tank using a Laser Doppler Velocimeter. Several impeller geometries, impeller diameters and fluids were used in order to check their effect in the flow. This tank is able to sustain fully turbulent flow throught the entire vessel at the Reynolds numbers less than 3000 while conventional stirred tanks require Reynolds numbers up to 300 000 to achieve fully turbulent flow at the surface. Having fully-developed turbulent flow at the surface, where the chemicals are usually added, is ideal for decreasing the dissolution time, avoiding mesomixing effects and providing reproducible bench scale experimental data which can be used to specify local mixing conditions in the industrial plant. These results have implications both for scale-up and for many industrial applications with surface feed or with dip pipes in the top third of the tank. A methodology to scale-down or –up an industrial process based on the total mixing energy provided to the system is also presented.

 

Figure 1: The confined impeller stirred tank (CIST) operating with a set of five Rushton turbines