Agitated vessels efficiency

Flynn and Treybal [Am. Inst. Chem. Eng. J., I,. 324 (1955)]. Continuous extraction of benzoic acid from toluene and kerosine into water baffled vessels, turbine agitators. Stage efficiency is correlated with agitator energy per unit of liquid treated. 

Agitated vessels (liquid-solid systems) Below the off-bottom particle suspension state, the total solid-liquid interfacial area is not completely or efficiently utilized. Thus, the mass transfer coefficient strongly depends on the rotational speed below the critical rotational speed needed for complete suspension, and weakly depends on rotational speed above the critical value. With respect to solid-liquid reactions, the rate of the reaction increases only slowly for rotational speed above the critical value for two-phase systems where the sohd-liquid mass transfer controls the whole rate. When the reaction is the ratecontrolling step, the overall rate does not increase at all beyond this critical speed, i.e. when all the surface area is available to reaction. The same holds for gas-liquid-solid systems and the corresponding critical rotational speed. 

Before these matters are discussed, it will be well to define the terms carefully as they apply both to batch and to continuous systems. This is especially important since, even in the meager literature of the subject now available, at least three different types of stage efficiencies have already been used to describe agitated vessels. 

In a continuous stirred tank reactor, a cmstant flow of reacliai substrates is fed to the reactor, where the immobilized lipase is suspoided in an agitated vessel. The main character of this type of reactitm is that Ihae is no tanperature or concentration gradients due to efficient mixing that promotes intimate contact of the reaction mixture with the immobilized lipase. Like batchwise reactors, immobilized lipase can be retained within the bioreactor by filtration. This is known to have lower construction cost. However, it requires larger volumes than a PER to achieve the same reaction. Commonly, a microfilter is provided at the bioreactor outlet to prevent immobilized lipase from leaving the reactor. 

Figure 14-2 Typical impellers for mechanically agitated vessels (a) six-blade disk impeller (6BD) or Rushton turbine (b) four-blade flat impeller (4BF) (c) four-blade pitched impeller (4BP) (d) helical ribbon impeller (e) anchor impeller (f) high efficiency turbine impeller.

For more fibrous solids, such as sugar cane, which is leached with water to remove the sugar, it has been shown [35] that leaching is generally more efficient in a thoroughly agitated vessel than by percolation, probably because the large amount of static liquid holdup (see Chap. 6) makes important amounts of solute unavailable. 

The kinetics of suspension polymerization is similar to that of bulk or solution polymerization. The equations derived for polymerization rate and polymer molecular weight in bulk/ solution apply to suspension with the lower reactor volume efficiency. The trade-off for the ease of agitation, heat removal, and product separation is the bulk reactoTs volume efficiency. A real challenge in modeling suspension polymerization, as well as dispersion polymerization, is the PSD of the products, which is not well developed in an agitated vessel. The mechanisms of drop breakup and coalescence are not well understood. 

Bulk polymerizations that are stirred have been used for various commercial polymers. Some, such as free-radical-initiated polyethylene, are carried out to low conversion so that the unreacted monomer acts as a diluent. These are, in effect, solution polymerizations and are treated in Section 5.3. Equipment that will handle liquids progressively from monomer ( 0.01 poise) to polymer ( 10 poise) with efficient heat removal is usually designed specifically for a given installation. Conventional turbine- or propeller-agitated vessels can handle a limited degree of conversion. With low-molecular-weight condensation polymers, the completed polymer melt may be transferable by gear piunps or merely extruded from the reactor by application of moderate pressme. 

A useful stirrer—sometimes termed a Hershberg stirrer— Fig. 11,7,5. for efficient agitation in round-bottomed vessels, even of... 

Topics that acquire special importance on the industrial scale are the quality of mixing in tanks and the residence time distribution in vessels where plug flow may be the goal. The information about agitation in tanks described for gas/liquid and slurry reactions is largely apphcable here. The relation between heat transfer and agitation also is discussed elsewhere in this Handbook. Residence time distribution is covered at length under Reactor Efficiency. A special case is that of laminar and related flow distributions characteristic of non-Newtonian fluids, which often occiu s in polymerization reactors. 

High turbulence is required for efficient mixing this is created by the vortex field which forms behind the blades. For all the gas to flow through this region it must enter the vessel close to and preferably underneath the disk hence it is recommended that spargers should always be nearer, about a distance of DJ2 below the agitator, where D( is the impeller diameter. 

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