Minimum suspension criteria

In stirred vessels the stirrer usually provides most of the energy required to keep the particles suspended. The minimum suspension criteria for a three-phase... 

Minimum suspension criteria for multiimpeller reactors are given by Dutta and Pangarkar [33], Data for venturi-loop reactors are limited [34-35], However, because of continuously sucking the slurry away from the bottom by the recycle pump, possibly not many problems on solids settling occur. 

Minimum suspension criteria can be based on fluid velocity or on energy input in the fluid phases. The energy input per unit volume fluid phase is generally applicable to widely different geometries. 

Minimum suspension criteria for solids in three phase stirred vessels can be related to those of the two phase stirred vessels. Data on three phase systems are relatively scarce, however. The same holds for three phase sparged columns. Moreover, in laboratory equipment with a large value of L/d it may be difficult to obtain a state of homogeneous particle distribution. More data on the influence of scale of operation on this phenomenon is required. 

Off-bottom suspension defines a condition in which individual particles never come to rest for longer than a second at the bottom of the vessel. Zwietering was the first to work with this visual suspension criterion. He determined minimum shaft speeds Wjs (js = just suspended) at which this condition first occurs. The off-bottom suspension point can be determined by methods such as the ultrasonic Doppler anemometer, which do not, however, take into account the uniformity of distribution in the vessel. This means that particularly with partides with high settling velocities, even when the criteria on-bottom motion and off-bottom suspension are fulfilled, there are often clear zones near to the liquid surface containing no particles. In these cases, a further assessment process is employed, the so-called layer thickness criterion, at which the layer of particles in suspension has reached 90% of the height of the liquid. 

Convective bulk transport is also an extremely important factor in the suspension of solids in a stirred tank (this is also responsible for the flow pattern at the tank bottom). P/V cannot be used as a scale-up criterion in this process either. Measurements have shown that the minimum rotational speed, ncrit, of the stirrer which is necessary for the suspension (whirling-up) of particles in the turbulent regime is given by the appropriate Froude number ... 

As homogeneous suspension in nonaerated stirred vessels can hardly be achieved, even with very high stirrer speeds, mainly Nc, needed for complete suspension, is of interest for the design purposes. This value, by definition, is characterized by the just-suspended criterion, i.e., the state where only a small fraction of the solids remains at the bottom of the reactor for one second at maximum (Einenkel, 1979). Zwietering (1958) proposed the following correlation to predict Nc, the minimum rotational speed of agitation required for the complete suspension ... 

A criterion for the prediction of minimum stirrer rotation speeds for the suspension of coarse-grain particles (Archimedes number > 40) is derived by Molerus and Latzel (1987). They showed that the minimum stirrer rotation speeds can be predicted by the evaluation of two diagrams the drag of fluidized particles as a function of concentration, and the pressure-head volumetric flow-rate characteristics of the agitated vessel. The latter can be obtained using the similarity of fluid-kinetic machines and can be expressed as lAav = /([Pg.48]

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

Complete suspension of solids would be satisfactory for most purposes, and the correlations developed to predict conditions for suspension have generally used this criterion. Some of these correlations are discussed here along with guidelines for scaleup. Keep in mind that these correlations give the minimum agitation condition for suspension, and that requirements for dispersion of a gas or good heat transfer to a coil or jacket may indicate higher power inputs for some cases. 

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