(661c) 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