(671d) Comparing Predicted Just Suspended Speed Versus Experimental Just Suspended Speed for Axial Flow Impellers in a Stirred Vessel

Mixing applications that require suspension of solids in a liquid are common in just about every major industry. In many cases, the mixer just needs to keep the solids from settling, in order to allow the slurry to be transported from tank to tank. This is known as the just suspended condition, or the Zwietering criterion, which states that all particles have some vertical motion and do not remain on the bottom of the vessel for more than 1 to 2 seconds. Zwietering studied suspension of particles in stirred tanks to develop a correlation to predict the just suspended speed for a given impeller style (equation 1). This correlation has been an industry standard for decades. One limiting factor for the correlation is the impeller “S” constant, which is specific to the impeller style, impeller diameter to tank diameter ratio (D/T) and the impeller’s location relative to the tank bottom. Thus, a specific “S” value is required for each geometry. Grenville, Mak and Brown developed a new correlation that brings in the impeller geometry (off bottom distance ratio) and categorizes impellers in two categories; pitched blade turbines and hydrofoils, since these are the most commonly used impellers for suspension applications. This correlation (equation 2) has similar elements to the Zwietering correlation [Advances in Industrial Mixing Chapter 10].

For this experimental study, a Lightnin A200 (pitched blade turbine) and a Lightnin A310 (hydrofoil) were tested over three volume scales to experimental determine the just suspended speed condition. Vertical tanks with four anti swirl wall baffles and a torispherical dished bottom were used for the study. The three scales were a 445 mm diameter vessel (90 liters), a 597 mm diameter vessel (220 liters) and a 800 mm diameter vessel (530 liters). A batch aspect ratio of 1.3 was used in each case with a single impeller near the bottom. The impellers were selected to match as closely as possible an existing geometry where an “S” value for the Zwietering correlation existed. Two types of glass beads were tested in each case; one with a particle size of 120 microns and one with a particle size of 300 microns all with a solids concentration of 1% wt. The mixer shafts were connected to strain gauge torque cells to measure shaft torque and optical tachometers to measure shaft speed. Although not a focus of this paper, cloud height was also measured for each condition tested. The Zwietering criterion was used to determine the just suspended speed condition by first increasing the speed of the mixer to achieve this condition and then reducing speed subsequentially to ensure the condition was not overshot.

At the time of writing this abstract the experimental procedure is partially finished. Initial results indicate that the experimentally determined just suspended speed is within +-10% of that predicted by each correlation. The work for the largest scale is not yet considered in this statement as that work is ongoing. A more detailed statistical analysis will be completed and presented.