Study of isolated mixing regions using Newtonian fluids in mechanically stirred vessels

Laminar or slow mixing is required in many process industries such as pharmaceutical, food and polymer to avoid over-shearing of products which cause substantial material and financial losses. The problem of inefficient mixing in stirred tanks is associated with the formation of isolated mixing regions between low to moderate Reynolds numbers. Recently, several researchers have investigated the presence, location and size of IMR structures and proposed various solutions to eliminate these structures. However, the knowledge on the phenomena of IMRs in complex fluids and the practical techniques required to destroy them is lacking. Therefore the present work aims to investigate the effect of impeller speed modulation on the behavior of IMRs and also the influence of tank impeller geometries on IMRs behavior by varying the tank size, baffles, and impeller type. <br><br>Experiments were carried out in two cylindrical tanks of 0.39 and 0.19 m diameters using glycerol (a Newtonian liquid) as the liquid phase. A standard 6-bladed Rushton turbine was used in the 0.39 m tank whereas a number of axial and radial flow impellers were used in the 0.19 m tank. Standard wall baffles and modified baffles were employed only in the 0.19 m tank. An acid-base reaction was used to observe the formation and degeneration of IMRs with a fluorescent green dye as a passive tracer to enable flow visualization of IMRs. Experiments were carried out using steady state impeller speed and different impeller speed modulation. Two wave forms, namely square and sine were used in speed modulation. Both amplitudes (<i>Re</i>_max and <i>Re</i>_min) and the wave period in the speed modulation protocols were varied to investigate the effects of unsteady stirring on mixing performance. The size and the volume of IMRs were determined using digital video images captured during the mixing process with respect to the dimensionless mixing time <i>Nt</i>_m at various time intervals.<br><br>Experimental results show that laminar mixing under <i>Re</i> < 100 could be enhanced using impeller speed modulation by changing both amplitude and wave period appropriately. The IMR structures disappear in less than 15 minutes when the impeller speed is modulated using square wave form with larger amplitude and shorter wave period. Especially, using shorter wave period or frequency modulation yields better results than that obtained with mere amplitude fluctuation. Speed modulation using sine wave protocol also helps to decrease the volume of IMRs. However, IMRs do not disappear completely under this protocol due to relatively slow diffusion mechanism even after several hours. <br><br>The study carried out to investigate the effects of impeller and baffle types in 0.19 m tank shows that the pitched blade impeller with 4 alternating pitch blades leads to smaller IMR structure volume and shorter mixing time as compared to those obtained with 4-blade disc turbine. The use of wall baffles under steady-state operating conditions only helps to reduce the volume of the IMRs (up to 95% of total tank volume) even after several hours of mixing. However, the use of modified baffles effectively prevents the formation of the IMR structures.