(423b) 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.

The experimental study presented at the 2021 AIChE Annual Conference contained a Lightnin A200 (pitched blade turbine) and a Lightnin A310 (hydrofoil) which were tested over three volume scales to experimentally 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 an 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 147 microns and one with a particle size of 352 microns all with a solids concentration tested at 1.0 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 subsequently to ensure the condition was not overshot.

At the last AIChE conference in 2022, the quantity of impeller diameters tested for the hydrofoils increased and ranged from 5.2” to 8.8” in diameter and varying concentration from 1.0 wt% to 15.0 wt% (in 5.0 wt% increments) for the smallest vessel scale.

Since the last presentation at the AIChE conference, the extended work to be conducted to further this study are completing the two larger vessel scales using the various impeller diameters scaled up using the same D/T ratio as well as varying the solids concentrations from 1.0 wt% to 15.0 wt% (in 5.0 wt% increments). A more detailed statistical analysis will be completed and presented once all the scales are tested. The expected result is to be consistent to what was seen thus far, GMB and Zwietering both predicting similar results to what was experimentally collected for the larger particle size. The observation from last time was that the smaller particle size was the most conservative (easier to suspend than predicted) for both correlations, and this will be examined at these larger scales to see if this trend is consistent.