Suspension of Non-Uniform Size Solid Particles in Mixing Tanks
Author:
James Y. Oldshue
This research investigates the suspension of non-uniform solid particles in mixing tanks by taking draw-off samples at various elevations and radial positions in 18- and 30-inch diameter tanks. The study defines different levels of suspension uniformity and compares the performance of axial and radial flow turbines and the effect of baffle width on the required horsepower for visual uniformity, particularly focusing on the distribution of different particle size fractions. Key findings illustrate that achieving complete uniformity requires significantly higher power for certain impellers and that in continuous flow, the tank's composition adjusts so the outlet matches the feed, which can result in non-uniformity throughout the tank, especially for varying particle sizes.
Key Learnings
- Suspension Uniformity Definitions: Different criteria exist to define solid suspension, including Complete Analytical Uniformity (uniform composition throughout the tank, quantifiable by standard deviation of samples) and Complete Visual Uniformity (a visually defined point, useful in transparent tanks).
- Impeller Type and Power: The axial flow propeller and axial flow turbine achieved essentially the same standard deviation (uniformity) at the same power level. Conversely, the spiral backswept turbine required approximately three times the horsepower to achieve complete visual uniformity compared to the axial flow impeller.
- Baffle and Bottom Shape Effects: The required horsepower for visual uniformity is sensitive to mixing parameters like baffle width and tank bottom shape. Changing the baffle width from 1/12 to 1/24 tank diameter had opposite effects on the power needed for axial flow and radial flow turbines.
- Non-Uniform Particle Behavior: With mixtures of particle sizes, different particle size fractions behave differently at power levels below that required for Complete Analytical Uniformity. Studies reported the distribution of individual particle size fractions at various elevations and power levels.
- Sampling Challenges: Obtaining an accurate sample of the slurry composition at a specific point is difficult. The presence, orientation, and withdrawal rate of the sample tube can affect the local flow pattern and particle distribution.
- Continuous vs. Batch Operation: In a continuous flow system, the composition of the slurry at the draw-off point must equal the inlet feed composition. This requirement forces the rest of the tank to adjust, often resulting in a composition profile throughout the tank that is different from the feed, especially when solids are non-uniformly sized.
- Process Performance Criteria: The difficulty in making general statements about mixing parameters highlights that the results of changing variables (like baffle width) depend on the process performance criterion chosen (e.g., Complete Visual Uniformity vs. Complete Off-Bottom Suspension).
