Mixed operations

R. S. Brodkey, Turbulence in Mixing Operations Academic Press, Inc., New York, 1975. 

In any event, the users of the mixer can issue a mechanical specification aud determine the speed, diameter of an impeller, aud power with in-house expertise. Or they may issue a processes specification which describes the engineering purpose of the mixing operation and the vendor will supply a description of the mixer process performance as well as prepare a mechanical design. 

Properties Affecting Solids Mixing Wide differences among properties such as particle-size distribution, density, shape, and surface characteristics (such as elec trostatic charge) may m e blending very difficult. In fact, the properties of the ingredients dominate the mixing operation. The most commonly observed characteristics of solids are as follows ... 

Power In general, power requirements are not a major consideration in choosing a sohds mixer since other requirements usually predominate. However, sufficient power must be supplied to handle the maximum needs should there he changes during the mixing operation. Also, when a variety of mixes may be required, power must be sufficient for the heaviest bulk-density materials. If the loaded mixer is to be started from rest, there should be sufficient power for this. When speed variation may be desirable, this should be taken into account in planning power requirements. 

Applications for mixers in blending operations vary from small laboratory mixers to large petroleum tanks of several million gallons. Three common mixing operations are ... 

In the previous section, a description of the various types of equipment commonly used for carrying out mixing operations was provided. Emphasis was placed on mechanical agitators since these have the broadest range of application, however there are other important devices worth noting, such as jet, pump, eductor, orifice, and inline mixing devices. Some of these important pieces of equipment are briefly described below. 

Mixer a device for carrying out a mixing operation and which generally inclusively refers to an impeller, driver, baffle, and vessel combination. 

Paddle A paddle is similar to a turbine impeller but typically has only two large blades and operates at lower speeds than a turbine. They are primarily used in high viscosity mixing operations. In European and Japanese literature the term "paddle" also is used to describe the flat blade and pitched blade turbines discussed above. The term "turbine" generally is reserved for disk turbines. 

Three basic fluid contacting patterns describe the majority of gas-liquid mixing operations. These are (1) mixed gas/mixed liquid - a stirred tank with continuous in and out gas and liquid flow (2) mixed gas/batch mixed liquid - a stirred tank with continuous in and out gas flow only (3) concurrent plug flow of gas and liquid - an inline mixer with continuous in and out flow. For these cases the material balance/rate expressions and resulting performance equations can be formalized as ... 

An important mixing operation involves bringing different molecular species together to obtain a chemical reaction. The components may be miscible liquids, immiscible liquids, solid particles and a liquid, a gas and a liquid, a gas and solid particles, or two gases. In some cases, temperature differences exist between an equipment surface and the bulk fluid, or between the suspended particles and the continuous phase fluid. The same mechanisms that enhance mass transfer by reducing the film thickness are used to promote heat transfer by increasing the temperature gradient in the film. These mechanisms are bulk flow, eddy diffusion, and molecular diffusion. The performance of equipment in which heat transfer occurs is expressed in terms of forced convective heat transfer coefficients. 

At high Reynolds, at which most mixing operations are performed, the Power number is constant, that is, Np P/pN D = constant. 

The size and shape of the vessel to be used for the mixing operation is important in achieving the desired mixing results therefore, this aspect of the design must accompany the actual mechanical mixer design/sizc selection. 

Figure 5-5A. A flat blade turbine can handle the majority of all fluid mixing applications when correctly applied. Its high pumping capacity makes it preferable for general mixing operations. It Is well adapted to the application of protective coverings, such as lead, rubber and plastics. Courtesy of Lightnin (formerly Mixing Equipment Co.), a unit of General Signal.

Flow and power numbers each decrease as the Reynolds number increases. In unbaffled tanks, a vortex forms that takes over the flow regime and does not allow the usual relationship to describe the performance of the mixing operation. It is proper and good practice to provide baffles in all vessels (see later description for the physical configurations). 

The application of draft tubes as related to various mixing operations is showm in Figures 5-231 and 5-24A-5-241. The draft tubes are basically a tube or shell around the shaft of the mixer including the usual axial impeller, which allows a special or top-to-bottom fixed flow pattern to be set up in the fluid system. The size and location of the tube are related to both the mechanical and mixing performance characteristics as well as peculiar problems of the system. Usually they are used to ensure a mixing flow pattern that cannot or w ill not develop in the system. Weber gives the followdng points for draft tubes [23] ... 

Entrainment is an important element in the mixing operation and involves incorporation of low velocity fluid into the mass of the fluid stream or jet issuing from a source such as a mixing impeller. The axial flow from a propeller under proper physical conditions serves as a circular cross-section jet to produce mixing by turbulence and entrainment. The flat-blade turbine issues a jet for entrainment at the top and bottom, areas of the ring [2]. It is significant to estimate the relative amount of liquid involved due to entrainment, as this helps to describe the effectiveness of the operation. 

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