Solids suspension is defined as maintaining a slurry of solid particles in a liquid. It usually pertains to systems in which solids would settle to the bottom of the vessel if no agitation were provided. It can also refer to maintaining a slurry of solids that would otherwise float. Suspension or “complete suspension” normally means that no particle will rest more than one second on the floor of the vessel. “Uniform” suspension means that the concentration of solids measured in sample taken at any point in the vessel will be within five percent of the concentration of a sample taken at any other point.
Vessel Design Considerations
The optimum vessel for solids suspension is round and has an aspect ratio of about 1:1. It must either be baffled with the mixer on center or the mixer can be angle-offset mounted. A center mounted mixer in an unbaffled tank will not effectively suspend solids. Offset mixers in unbaffled vessels will leave a significant heal of solids in one sector of the vessel floor. Angle offset mixers are generally restricted to relatively small volumes although we have used them successfully in 25,000 gallon vessels.
It is important to recognize that solids can easily be suspended to a height of about 85% of the vessel diameter with a single impeller. However, even when multiple impellers are used, it is generally impractical to suspend solids higher than about 1.4 times the vessel diameter and it is impossible to suspend them more than 2 times the vessel diameter in a conventional vessel. If these constraints are unacceptable, a draft tube must be installed in the vessel.
When the system contains very small solid particles, their tendency to settle out is exclusively controlled by the viscosity of the liquid. This is called Stokes settling. Most of these systems can be treated as liquid blending applications, and many of the restrictions above do not apply. However, if there is a power failure and the solids are allowed to settle, they may be difficult or impossible to re-suspend if the restrictions above were ignored in the design of the vessel.