Cyclone separators proportions

Figure 4-41. Cyclone separator proportions—dust systems. By permission, Perry, J. H., Chemical Engineers Handbook, 3rd Ed., McGraw-Hill Company, 1950.

For a free vortex, it is shown in Volume 1, Chapter 2, that the product of the tangential velocity and the radius of rotation is a constant. Because of fluid friction effects, this relation does not hold exactly in a cyclone separator where it is found experimentally that the tangential velocity is more nearly inversely proportional to the square root of radius,... 

Various modifications may be made to improve the operation of the cyclone separator in special cases. If there is a large proportion of fine material present, a bag filter may be attached to the clean gas outlet. Alternatively, the smaller particles may be removed by means of a spray of water which is injected into the separator. In some cases, the removal of the solid material is facilitated by running a stream of water down the walls and this also reduces the risk of the particles becoming re-entrained in the gas stream. The main difficulty lies in wetting the particles with the liquid. 

The solids are thrown outwards against the wall of the vessel and then move away from the gas inlet and are collected in the conical base of the equipment. This separation process is very effective unless the gas contains a large proportion of solid particles less than 10 p in diameter and is equally effective when used with dust or mist-laden gases. The cyclone separators are obviously rubber lined to withstand wear at the velocity of gas and corrosion and are the most commonly used general purpose separator in the chemical process industry. 

Cyclone separators Can be designed to separate very small particles and to give good mass segregation. Particle cutoff size depends on inlet flow rate. Instrument size is proportional to l/D",/, ( Dmin = minimum particle diameter). 

A ratio of in the range 5 to 10 will usually be more than sufficient to ensure a well-mixed product. As equation (4.14) shows, short turnover times in deep beds require high gas velocities, and this may result in excessive entrainment of the finer particles. Procedures for calculating entrainment are beyond the scope of this chapter, and the reader should consult Reference 7, in which a worked example is given. Entrainment is roughly proportional to the gas velocity to the fourth power and can be drastically reduced by increasing the cross-sectional area of the column above the bed level. Alternatively, blow-back filters or gas cyclone separators can be fitted to the system, inside or outside the column, and the entrained material returned to the bed. 

Cyclone separators (Fig. 5) these rely on the action of centrifugal forces on the dust particles carried along in the swirling stream of gas. The particles are thus flung radially outwards to the wall of the cyclone, from where they fall into the dust hopper. The centrifugal force which determines the collection efficiently is directly proportional to the mass of the particles and to the square of the circumferential velocity, but inversely proportional to the radius of the cyclone ... 

The solution or suspension to be dried is sprayed into a hot air stream and circulated through a chamber. The dried product may be carried out to cyclone or bag separators or may fall to the bottom of the drying chamber and be expelled through a valve. The chambers are normally cylindrical with a conical bottom although proportions vary widely. The process can be divided into four sections atomization of the fluid, mixing of the droplets, drying, and finally removal and collection of the dry particles. 

Production of ZrCl4. Zirconium oxide from the hafnium-separation step was mixed with carbon black, dextrin, and water in proportions 142 Zr02, 142 C, 8 dextrin, and 8 water. The mixture was pressed into small briquettes (3.8 X 2.5 X 1.9 cm) and dried at 120°C in a tray drier. The oxide briquettes were charged to the reaction zone of a vertical-shaft chlorinator lined with silica brick. The charge was first heated by carbon resistance strips until it became conductive. During production, the bed temperature was maintained at 600 to 800 C by an electric current passed directly through the bed. After steady conditions were reached, a reactor 66 cm in diameter produced about 25 kg ZrCLt/h. The ZrCU was condensed from the reaction products in two cyclone-shaped aftercondensers in series, and the chlorine off-gas was removed in a water scrubbing tower. 

An important aspect of a fluidized-bed combustor (Figure 14.21) is that only a small concentration of coal (ca. 5%) is necessary to sustain combustion and at typical operating conditions the concentration is less than 1%. The coal feed will, of course, contain a proportion of fine material and inevitably some of these fines will be elutriated before they can be completely burned, but unbumed material leaving the furnace is collected by cyclones and, if necessary, can be burned by refiring to the original bed or a separate one. 

There has been a lot published about the relative proportions of the cyclone dimensions and their effect on separation efficiency and pressure drop. A user need not be concerned with this aspect except for two observations. Firstly, there is a rule by which every measure which increases resistance to flow improves solids recovery, and vice versa. This applies to all proportions of the cyclone body, within certain reasonable limits, except for the length of the cyclone. Thus for example, a cyclone with relatively small inlet and outlet openings is expected to give higher mass recovery but will offer higher resistance to flow and therefore have lower capacity. Several optimum or recommended designs have been published and, as these have been well tested and enough is known about their performance, they may be adopted if needed. 

The proportion of each particle size fraction which is precipitated in a cyclone of particular shape and rating is called the fractional dust collection efficiency and is plotted as a percentage against the particle size diameter (in microns). The curve thus obtained is called a separation curve. It is an important criterion for comparing different cyclones with one another. Table6 indicates, by way of example, the overall collecting performance of a cyclone for dust with known particle size distribution. 

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