Improving laminar mixing in a stirred vessel using impeller speed modulations

Mixing of high viscous liquids at low Reynolds numbers often leadas to inefficient mixing due to the presence of isolated mixing regions (IMRs) formed above and below the impeller. Much research effort has gone in recent years into developing better mixing methods to destroy IMRs. This paper investigates the mixing effects of impeller speed modulation in a Newtonian viscous liquid. The impeller speed modulation was carried out using four different wave forms namely, square, sine, triangle and pulse. Glycerol solution was agitated with a standard six-bladed Rushton turbine in an unbaffled mixing vessel. Direct visualization of an acid-base neutralization reaction with fluorescent green dye was used to measure the mixing time. Digital image analysis was performed to determine the volume of the IMRs at different intervals of the mixing process. Results show that the best mixing performance can be achieved by using square wave modulation. Mixing with pulse, sine wave and triangle wave modulations does help in destroying IMRs. IMRs were still visible after a long period of mixing for the sine wave modulation while they were completely destroyed by other types of modulations.