Types of agitators

The reader may consult the reference section of this chapter for citations dealing with these types of agitator configurations and specific applications. 

The commonly used types of mixing equipment can be placed in the broad categories (1) mechanical agitators, (2) inline motionless mixers, (3) tank jet mixers, and (4) miscellaneous. The nature and type of agitator used depends upon the scale and type of mixing and upon the fluids being mixed. The broad classes of impellers are ... 

The eonstant K and the exponents b and d must be determined for the partieular type of agitator, its size and loeation in the tank, the dimensions of the tank, and the depth of the liquid. 

The value of n is based on theoretieal and empirieal eonsidera-tions and depends on the type of agitation problem. 

The values of eonstant C and the exponents a, b, and e depend on the type of agitator, whether baffles are used and their type, and whether the transfer is via the vessel wall or to eoils. Baffles are normally used in most applieations, and the values of a, b and e in the literamre are 2/3, 1/3, and 0.14 respeetively. Tables 7-14 and 7-15 give typieal eorrelations for various agitator types. 

A software package (MIXER) was developed to determine the heat transfer coefficient for any type of agitator and surface using the value in Table 7-16, fluid physical properties, agitator speed, and diameter. 

Figure 5-32. Scale-up exponent characterizes the desired type of agitation in order to determine speed-volume ratios. By permission, Rautzen, R. R., et a/., Chem. Engr., Oct. 25,1976, p. 119 [32].

There are four types of agitator commonly used in the bioreactors ... 

These types of agitator are used in low-viscosity systems (ji < 50 kg m 1 s-1) with high rotational speed. The typical tip speed velocity for turbine and intermig is in the region of 3 m s 1 a propeller rotates faster. These impellers are classified as remote clearance type, having diameters in the range 25-67% of the tank diameter. 

The most common type of agitator is turbine. It consists of several short blades mounted on a central shaft. The diameter of a turbine is normally 35 15% of the tank diameter. There are four to six blades for perfect mixing. Turbines with flat blades give radial flow. This is good for gas dispersion in the media, where the gas is introduced just below the impeller, is drawn up to the blades and broken up into uniform fine bubbles. 

The most common type of agitator used is the four-bladed disk turbine. However, research on the hydrodynamics of the system has shown that other disk turbine agitators with 12, 18 or concave blades have advantages. 

In bubbling, the control of the bubble diameter is a little easier. In these methods bubbles are made at an orifice or a multitude of orifices. If there is only one orifice, of radius r, and if bubble formation is slow and undisturbed, the greatest possible bubble volume is 27rry/gp] y is the surface tension of the liquid, p the difference between the densities of liquid and gas (practically equal to the density of the liquid), and g is acceleration due to gravity. Every type of agitation lowers the real bubble size. On the other hand, if there are many orifices near enough to each other, the actual bubble may be much larger than predicted by the above expression. 

The values of constant C and the indices a, b and c depend on the type of agitator, the use of baffles, and whether the transfer is to the vessel wall or to coils. Some typical correlations are given below. 

Finally, the reactor vessel has been assumed to be perfectly mixed. Imperfect mixing and a flow pattern created by different types of agitators, baffles, feed locations and other reactor vessel configurations will cause the performance to be below that indicated by perfect mixing. 

Types of agitators to use for fluids of various viscosities 13 Propeller agitators — fluids below 3,000 cp Turbine agitators — fluids at 3,000-50,000 cp Paddle agitators — fluids at 50,000-90,000 cp Modified paddle agitators — fluids at 90,000-1,000,000 cp... 

Some examples in which this agitation effect is more likely to occur are reactions during which the viscosity changes significantly, such as in polymerizations, and reactions with suspensions. Equipment dimensions, type of agitators, and type of solvents and coolants used affect the heat transfer as well [174],... 

The factors that can affect the rate of heat transfer within a reactor are the speed and type of agitation, the type of heat transfer surface (coil or jacket), the nature of the reaction fluids (Newtonian or non-Newtonian), and the geometry of the vessel. Baffles are essential in agitated batch or semi-batch reactors to increase turbulence which affects the heat transfer rate as well as the reaction rates. For Reynolds numbers less than 1000, the presence of baffles may increase the heat transfer rate up to 35% [180]. 

The type of agitator and tank geometry required to achieve a particular process result, is determined from pilot plant experiments. The desired process result may be the dispersion or emulsification of immiscible liquids, the completion of a chemical reaction, the suspension of solids in a liquid or any one of a number of other processes [Holland and Chapman (1966)]. 

The rate of mass transfer is a funetion, among other variables, of the drop size distribution or interfaeial area between the phases. The drop size is governed by the surface tension, and densities of the two phases and the type of agitation and design of the eontaetor. Up to a point, the smaller the drop, the greater the rate of mass transfer. 

Common to all or most solvent extraction operations in the mining industry is the problem of stable formation of cruds. The crud can constitute a major solvent loss to a circuit and thereby adversely alfect the operating costs. Because there can be many causes of crud formation, each plant may have a crud problem unique to that operation. Factors such as ore type, solution composition, solvent composition, presence of other organic constituents, design and type of agitation all can adversely alfect the chemical and physical operation of the solvent extraction circuit and result in crud formation [32-34]. 

AGITATOR APPLICATION. TYPE OF AGITATION FOAMING TENDENCY ... 

The sketch (Fig. 36) shows a battery of agitators driven by an electric motor and employing different types of agitators. By changing the driving belt to different pairs of pulleys, different speeds of agitator can be obtained. It is always wise to have some idea of the speed of agitation. 

The parameters a, b, and m depend on the type of agitator and the presence or absence of baffles. For example, the values of these parameters for a flat-blade agitator with baffles are a = 0.74, b =, and m = 0.14. A similar equation can be used for a CSTR with an internal cooling coil (see footnote 1). 

---