States of Solid Suspension and Distribution

1 On-Bottom Motion or Partial Suspension. This state is characterized by the visual observation of the complete motion of all particles around the bottom of the vessel. It excludes the formation of fillets, a loose aggregation of particles in comers or other parts of the tank bottom. Since particles are in constant contact with the base of the vessel, not all the surface area of particles is available for chemical reaction or mass or heat transfer. On-bottom motion conditions are sufficient for the dissolution of highly soluble solids. 

2 Off-Bottom or Complete Suspension. The state of suspension known as off-bottom or complete suspension is characterized by the complete motion of all particles, with no particle remaining on the base of the vessel for more than 1 to 2 s. This condition is known as the Zwietering criterion. Under this condition, the maximum surface area of the particles is exposed to the fluid for chemical reaction or mass or heat transfer. The just suspended condition refers to the minimum agitation conditions at which all particles attain complete suspension. 

In mechanically agitated vessels, the minimum agitation speed for the just suspended state, Njs, has been the subject of many experimental and theoretical analyses (Nienow, 1985). The pioneering study by Zwietering (1958) covered by far the widest range variables. The resulting correlation is discussed below. 

3 Uniform Suspension. Uniform suspension corresponds to the state of suspension at which particle concentration and particle size distribution are practically uniform throughout the vessel any further increase in agitation speed or power does not appreciably enhance the solids distribution in the fluid. A 

Suspension Critma Speed Ratio Difficult Moderate Easy 

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