Other Types of Mixers

Such an arrangement may well be suitable when it is necessary to prevent tine particles from settling out at the bottom of the tank. 

The mixing problems considered so far have related to batch systems in which two materials are mixed together and uniformity is maintained by continued operation of the 

Frequently, stirred tanks are used with a continuous flow of material in on one side of the tank and with a continuous outflow from the other. A particular application is the use of the tank as a continuous stirred-tank reactor (CSTR). Inevitably, there will be a vety wide range of residence times for elements of fluid in the tank. Even if the mixing is so rapid that the contents of the tank are always virtually uniform in composition, some elements of fluid will almost immediately flow to the outlet point and others will continue circulating in the tank for a very long period before leaving. The mean residence time of fluid in the tank is given by  

In a completely mixed system, the composition of the outlet stream will be equal to the composition in the tank. 

The variation of time for which fluid elements remain with the tank is expressed as a residence time distribution and this can be calculated from a simple material balance if mixing is complete. For incomplete mixing, the calculation presents difficulties. 

Only a selection of commercially available mixing equipment has been described here. Indeed, the devices described all exist in a variety of configurations. In a ition, there are many items of equipment based on altogether different principles typical examples include jet mixers, in-line dynamic mixers, mills, valve homogenisers, ultrasonic homogenisers, etc. These, as well as many other types, have been discussed by Harnby et alp-K Oldshue and Nagata  

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Anchor mixers may be used in combination with other types of mixers, such as turbine mixers, high-shear mixers, or rotor-stator mixers, which were described in the previous subsection. Such mixers can be placed on a vertical shaft midway between the anchor shaft and blade. A secondary mixer can promote top-to-bottom motion and also limit bulk rotation of the fluid. A stationary baffle is sometimes placed between the anchor shaft and rotating blade to limit fluid rotation and enhance shear. 

Many types of multishaft mixers do not require planetary motion. Instead the mixers rely on an anchor-style impeller to move and shear material near the tank wall, while another mixer provides a different type of mixing. The second or third mixer shafts may have a pitched-blade turbine, hydrofoil impeller, high-shear blade, rotor-stator mixer, or other type of mixer. The combination of multiple impeller types adds to the flexibility of the total mixer. Many batch processes involve different types of mixing over a range of viscosities. Some mixer types provide the top-to-bottom motion that is missing from the anchor impeller alone. 

For the purpose of this chapter, the discussion will be centred on gas-liquid contacting in mixers, with emphasis on the mixing vessels commonly used in the chemical industry. Other types of mixer are used (for example motionless mixers in pipes, jets, ejectors, beaters) and of course some degree of mixing does occur in other gas-liquid devices (bubble columns, plate columns, spray towers) not designated as mixers . 

Thus we see that for each type of mixer there is a minimum speed that will give the greatest adhesion. For the first type of mixer, only 3% of the particles remain in the solution for other types of mixers, this fraction may be different. 

Other Types of Mixers Apart from the methods described above, there are several different ways in which microfluidic flows can be driven [5]. Several of these methods have been used for microfluidic mixing including... 

Many other types of mixers exist, the commonest of which is probably the two-roll mill in which mixing takes place by repeatedly forcing the polymers through the gap between two rotating rollers. These machines are also commonly used for converting bulk polymer into a sheet which is easier to handle. 

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