Minimum suspension

FIG. 6-33 Durand factor for minimum suspension velocity. (From Govier and Aziz, The Flow of Complex Mixtures in Pipes, Van Nostrand Reinhold, New York, 1972.)... 

The exponents were found to be independent of the impeller type, vessel size, impeller clearance and impeller to tank diameter ratio. The dimensionless constant Ci accounted for variations in the system geometry (e.g. on dc/di). This would indicate that the basic mechanism leading to minimum suspension may be the same for rather different stirrer geometries. Table 2, which is an update of the one given by Nienow [19], indicates different exponents found in a few other investigations. Baldi et al. [26] made a relatively successful theoretical approach by assuming that the suspension of particles is mainly due to eddies of a certain critical scale comparable to the particle size. From an energy balance it follows that... 

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. 

Provided the particle settling velocities vt are known, this equation allows the calculation of )e,s Usually, experiments at non-zero liquid rates are used to evaluate t , and )e,s separately. A similar concentration profile might occur in practice if slurry column reactors are operated close to the conditions given by the minimum suspension criterium. In this case, reactor calculations should take the solids concentration profiles into account. A recommended correlation for the solids dispersion coefficient for small particles is given by Kato et al. [15] ... 

Calculate the minimum suspension speed and the power requirement for the following ... 

Given that the impeller is a glass-coated retreat-curve type, the approximate calculation for the minimum suspension speed is 115 rpm. A recommended design speed would be 130-150 rpm. This would guarantee more than minimum suspension conditions. The power at 150 rpm based on a power number of 0.6 would be 0.323 kW ( 0.5 hp). 

Under the conditions of this experiment [308], the minimum suspension concentration, corresponding to the maximum adhesion, is reached at a speed of 5 rev/sec for the laboratory mixer. 

In sparged yegsels with a stagnant liquid medium a minimum suspension gas velocity can be defined at which all solid particles are just suspended. Three different experimental techniques have been used to obtain an operational definition of minimum suspension gas velocities ... 

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. 

Ap = pd —Pc. The magnitude of the constants C20 and as, given in the Table 12-5, are a measure of the ease of suspension formation. Low C20 values indicate that dispersions are formed at low speeds. Large C20 values (single impellers) suggest that higher speeds are required for minimum suspension. Turbines at the o/w interface require lower speed than in other locations. Radial flat-blade turbines placed in the light phase appear to be inefficient. 

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