Suspending

In contrast to turbulent mixing, in the laminar range the microblend time is identical with the macroblend time. In the laminar range Ne Re, that is, N R = const applies in the laminar zone (see Equation 13.5), when scaling up with P/V — const. The same rotational frequency n must be used on a commercial scale as on a model scale. This also means, however, that the same blend time is achieved on both scales, and is therefore in agreement with Equation 13.15. 

Suspending of solids is the basic mixing task for any agitated crystallization process. Technical applications cover a wide range of various suspending tasks. Particle size distribution, viscosity of the solvent, and the density difference between solids and solution mainly influence the settling behavior of a suspension. The setfling rates of particles are found mostly between 0.001 and 0.1 m/s, while the soUds content of the suspension can go up to values of 50 wt% [2]. 

Slow sedimentation of particles will occur, for example, in an activated sludge or in fine particle catalyst suspensions. For those lands of systems, a homt eneous distribution of solids is characteristic. Here, the liftoff from the vessel bottom as well as the state of a homogeneous suspension can be achieved with a comparably low power input or only slight movement of the liquid. On the other hand, at higher solids concentrations a pseudoplastic flow characteristic of the suspension can occur. As an example, concentrations of only 6% of fibrous material - typically known from paper industry - can lead to this non-Newtonian behavior Frequently observed in suspensions with high solids concentrations is a Bingham plastic behavior. In this case, if a certain amount of shear is not introduced by agitation, the system behaves like an elastic solid body or a gel. 

In this chapter, solid/liquid systems are treated, where the particle size and the density difference between both phases are large enough and at the same time the liquid viscosity is low enough. Due to gravitation and presumably also due to inertial forces generated by the flow scheme in those systems, both phases tend to separate. The power needed to lift the particles off the bottom and keep them homogeneously suspended has to be introduced by the agitator. 

With on-bottom motion although most of the solids are in motion, large dear (i.e., partide-free) zones can occur in the upper part of the vessel. Temporary deposits of solids are tolerated underneath the impeller and in the corners. 

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Figure 3.3 shows a simple type of classifier. In this device, a large tank is subdivided into several sections. A size range of solid particles suspended in vapor or liquid enters the tank. The larger, faster-settling particles settle to the bottom close to the entrance, and the slower-settling particles settle to the bottom close to the exit. The vertical baffles in the tank allow the collection of several fractions. 

This type of classification device can be used to carry out solid-solid separation in mixtures of different solids. The mixture of particles is first suspended in a fluid and then separated into fractions of different size or density in a device similar to that in Fig. 3.3. 

Filtration. In filtration, suspended solid particles in a liquid or gas are removed by passing the mixture through a porous medium that retains the particles and passes the fluid. The solid can be retained on the surface of the filter medium, which is cake, filtration, or captured within the filter medium, which is depth filtration. The filter medium can be arranged in many ways. 

In situations where a low concentration of suspended solids needs to be separated from a liquid, then cross-flow filtration can be used. The most common design uses a porous tube. The suspension is passed through the tube at high velocity and is concentrated as the liquid flows through the porous medium. The turbulent flow prevents the formation of a filter cake, and the solids are removed as a more concentrated slurry. 

In aerobic processes, the mean sludge residence time is typically 5 to 10 days. The hydraulic residence time is typically 0.2 to 0.3 days. Suspended growth aerobic processes are capable of removing up to 95 percent of BOD. 

Attached growth processes are capable of removing up to 90 percent of BOD and are thus less effective than suspended growth methods. 

Another suspended growth method is the upward-flow anaerobic sludge blanket illustrated in Fig. 11.6a. Here the sludge is contacted by upward flow of the feed at a velocity such that the sludge is not carried out of the top of the digester. 

They are designed to improve effluents from secondary treatment processes by removing suspended material and with it some of the remaining BOD. 

Ultrafiltration. Ultrafiltration was described under pretreatment methods. It is used to remove finely divided suspended solids, and when used as a tertiary treatment, it can remove virtually all the BOD remaining after secondary treatment. 

Aerosol sprays consist of a material dissolved or suspended in a liquid which when pressure is released volatilizes to produce a fine spray. The spray carries the active material. Used in hair lacquers, paints, etc. the propellant should be inert and non-inflammable. Chlorofluorocarbons have been used extensively but are now being replaced. 

It is necessary to keep these materials suspended in the oil to avoid the formation of varnishes on the engine walls and deposits in the crankcase. 

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The RCCA s (32 to 52 assemblies in the core), regulate the neutron flux in the reactor, and are used for emergency shutdown of the reactor activity. In normal working conditions, the RCCA s are suspended above the fuel elements. 

An alternative and probably now more widely used procedure is to raise the liquid level gradually until it just touches the hanging plate suspended from a balance. The increase in weight is then noted. A general equation is... 

Fig. 11-13. Apparatus for measuring the time dependence of interfacial tension (from Ref. 34). The air and aspirator connections allow for establishing the desired level of ftesh surface. IV denotes the Wilhelmy slide, suspended from a Cahn electrobalance with a recorder output.

A modification of the foregoing procedure is to suspend the plate so that it is partly immersed and to determine from the dry and immersed weights the meniscus weight. The procedure is especially useful in the study of surface adsorption or of monolayers, where a change in surface tension is to be measured. This application is discussed in some detail by Gaines [57]. Equation 11-28 also applies to a wire or fiber [58]. 

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