MEEF Recycling - Plastic Recycling - Cyclones

Plastic Flakes - Paper Labels & Dust Separation

Definition:

Cyclone separators are devices that utilize centrifugal forces and low pressure caused by spinning motion to separate materials of differing density, size, and shape. Gas cyclones are widely used in industry for the separation of particles from gas and air streams (Coker, 1993), while water cyclones, also known as hydrocyclones, are used for the separation of fluids of differing densities (Svarovsky, 1984). Cyclones are popular because they are simple and inexpensive to manufacture, require little maintenance, contain no moving parts, and have the ability to operate at high temperatures and pressures (Coker, 1993).

Operating Principles:

Figure one illustrates the basic working principles of a cyclone separator. The gas or liquid stream is injected at high velocities through the inlet pipe, which is positioned tangentially to the body of the cyclone (Seinfeld, 1975). The shape of the cone induces the stream to spin, creating a vortex. Larger or more dense particles are forced outward to the walls of the cyclone where the drag of the spinning air as well as the force of gravity causes them to fall down the sides of the cone into an outlet (Seinfeld, 1975; Svarovsky, 1984). Meanwhile the lighter and/or less dense particles as well as the gas or liquid medium exit through the top of the cyclone via the low pressure center.
The separation process in cyclones requires a steady flow, free of fluctuations or short term variations in the flow rate. Cyclone separators are customarily operated with the top and bottom open to the atmosphere so that there is no pressure difference between the two (Singh and Eckhoff, 1995). According to Singh and Eckhoff (1995), if one of the outlets has to be operated with a back pressure, it is best to have both outlets at the same pressure.

Applications:

There are many uses for cyclone separators in the agricultural industry. One area where cyclone separators are widely used is in the removal of dust particles from emissions from cotton gins, grain elevators, tractors, grain mixers, and other agricultural machinery. Most cyclone separators are not efficient at removing particles less than ten micrometers (Columbus, 1993). However, now that the Environmental Protection Agency is limiting the allowable amount of PM-10 (< 10 µm) particles, researchers are having some success in efficiently separating PM-10 particles by combining 2 or more cyclone separators of varying sizes (Columbus, 1993).

Cyclone separators are also used in the food industry for the separation of agglomerated particles and for the separation of starch and protein. Some specific examples where cyclone separators are used in the food industry are in the separation of sand from sugar cane juice and in the cleaning of wash water in potato processing (Svarovsky, 1984). Cyclone separators are also used outside of the agricultural industry in the separation of silt from well water, the separation of petroleum sludge, and in the collection of carbon (Svarovsky, 1984).

Sizing and Selection:

There are many sizes and types of cyclone separators available. The two main types of cyclones are axial and tangential (Stern et al, 1973). They both operate on the same principles, however, in the axial flow cyclones the material enters from the top of the cyclone and is forced to move tangentially by a grate at the top. In tangential cyclones the material enters from an inlet on the side which is positioned tangentially to the body (see Figure 2). According to Heumann (1991), axial flow cyclones are the most commonly used.

The sections of cyclone separators are manufactured in varying proportions of the body diameter (Figure 3). Some commonly used cyclones mentioned by Columbus (1993) are LD, 1D3D (body length = diameter, cone length = 3*diameter), and 2D2D (body length and cone length = 2*diameter). The efficiency of cyclone separators is dependent upon the cyclone diameter and the pressure drop between the inlet and outlet of the cyclone (Clift et al, 1991). According to Svarovsky (1994), if you want to increase the efficiency of a cyclone you can:

reduce the cyclone diameter,
reduce the outlet diameter,
reduce the cone angle,
or increase the body length.

However, if you want to increase the capacity of a cyclone, you can:

increase the cyclone diameter,
increase the inlet diameter,
increase the outlet diameter,
or increase the body length.

Increasing the pressure drop will also result in:

an increase in separation efficiency
higher capacity
decrease the underflow to throughput ratio
more concentrated underflow
cleaner overflow.

Another method to improve efficiency with high capacity loads is to have sequential cyclone separators of varying size; the first cyclone may separate larger particles and the second separator may be used to separate smaller particles. For example, Columbus (1993) found that the combination of a 2D2D cyclone followed by a 1D3D cyclone was very efficient in reducing total emissions as well as PM 10 emissions.

NB: Figures not available.

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