Selection of Agitators

Intended use for reactor, dissolver, mixer, melter, etc. Factors to be considered for selection of agitator  

Composition of medium to be agitated—the proportion of liquid and solid Properties of liquid—density and viscosity at different temperatures, vapour pressure, corrosive and toxic nature, boiling point, melting point and inflammable nature of vapour, tendency for foaming 

Properties of solid—density, solnbility, melting point, abrasive natnre, corrosive and toxic nature 

Rate of withdrawal from the vessel—continnons or after every batch or intermittent 

Point of withdrawal from the vessel— from bottom or from overflow nozzle Point of addition of liqitid and solid to the vessel—at top of hqitid siuface or through a pipe dipping in the hqitid along wall of vessel or in centre of vessel Level of liquid to be maintained inside the vessel 

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Selection of agitator drive and speed reducer Mechanical assessment of shaft. Vibrations Mass transfer at bubble/liquid, and liquid/solid... 

POLLARD, G. I. (1984) A Guide to the Design and Selection of Agitator Drives , FMP Report 002. BHR Group Ltd, Cranfield, Bedford, February. 

Typically, indirect dryers are used for small- or medium-size production. The product from such a unit has a higher bulk density than the same material processed in direct dryers. Particle size degradation usually can be minimized by proper selection of agitator speed or design. The common indirect heated dryers are tubular dryers (with or without vacuum), drum dryers (atmospheric, vacuum, horizontal or rotary vacuum, and others), hollow disk dryers, paddle... 

The thermal duty here is the opposite of solidification operations. The indirect heat-transfer equipment suitable for one operation is not suitable for the other because of the material-handling rather than the thermal aspects. Whether the temperature of transformation is a definite or a ranging one is of little importance in the selection of equipment for fusion. The burden is much agitated, but the beds are deep. 

Figure 7.20 shows some of the impellers which are frequently used. Propellers, turbines, paddles, anchors, helical ribbons and screws are usually mounted on a central vertical shaft in a cylindrical tank, and they are selected for a particular duty largely on the basis of liquid viscosity. By and large, it is necessary to move from a propeller to a turbine and then, in order, to a paddle, to an anchor and then to a helical ribbon and finally to a screw as the viscosity of the fluids to be mixed increases. In so doing the speed of agitation or rotation decreases. 

This is an important consideration in the selection of an optimum polymerization diluent, which is very easily neglected in laboratory investigations. Also, since little is known cd>out particle coalescence in the presence of mechanical agitation, extreme care must be taken in mixing scale-up. 

The selection chart given in Figure 10.57, which has been adapted from a similar chart given by Penney (1970), can be used to make a preliminary selection of the agitator type, based on the liquid viscosity and tank volume. 

Selection of the most appropriate design of agitator is a specialized subject outside of the scope of this text (see, for example, Hamby, Nienow and Edwards5). However, it is important to note that the selectivity of the system can be effected by agitation when competing reactions are involved that produce byproducts. For example, consider the reaction system ... 

Recently, it has been shown that ultrasonic agitation during hydrogenation reactions over skeletal nickel can slow catalyst deactivation [122-124], Furthermore, ultrasonic waves can also significantly increase the reaction rate and selectivity of these reactions [123,124], Cavitations form in the liquid reaction medium because of the ultrasonic agitation, and subsequently collapse with intense localized temperature and pressure. It is these extreme conditions that affect the chemical reactions. Various reactions have been tested over skeletal catalysts, including xylose to xylitol, citral to citronellal and citronellol, cinnamaldehyde to benzenepropanol, and the enantioselective hydrogenation of 1-phenyl-1,2-propanedione. Ultrasound supported catalysis has been known for some time and is not peculiar to skeletal catalysts [125] however, research with skeletal catalysts is relatively recent and an active area. 

Lower original potential in grinding is beneficial to flotation selectivity of galena. Thus, collectors should be added into the mill directly. Table 10.12 is the results of flotation separation of Beishan concentrator of PbS-ZnS ore for collector addition in different places. It shows that the recovery and grade of galena concentrate are improved obviously when collectors are added in mill compared to in agitator. 

Equipment selection is important, as is also the proper operation of the contacting devices. It is generally recognized that high shear is the primary cause of droplet haze and subsequent emulsion and crud formation. Thus, the type and amount of agitation (shear) must be optimized for mass transfer while minimizing emulsion and crud formation. Table 7.5 indicates items of information that may be required in analysis of crud formation problems in a plant [32]. 

Selection of placebo powder. The chosen material must be easily sterilizable, dispersible, or dissolvable in the chosen medium. The principal sterile placebo materials (irradiated in a final container) are irradiated lactose, mannitol, polyethylene glycol 6000, and sodium chloride. The material should pass the solubility testing at the desired concentration with suitable amount and time of agitation. 

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