Collection Efficiency of Cyclones

The collection efficiency usually depends on the shape of the cyclone, size and density of the particles, and incoming tangential velocity. The collection efficiency of a 

Consider a horizontal cross section of a cyclone, as shown in Fig. 7.9. During a time interval d t, the number of particles collected, AN, within a differential volume dr (r AO) dz is expressed by 

Therefore, the fraction of particles collected within dr is obtained as 

The relationship between dr and dr is determined from the equation for the radial motion of the particle, which is obtained by the force balance between the drag force and the centrifugal force, i.e., 

Assume that the initial radial position of the uncollected particles is at the vortex center where r0 = 0. Substitution of Eqs. (7.20) and (7.21) into Eq. (7.18) yields 

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The collection efficiency of cyclones varies as a function of particle size and cyclone design. Cyclone efficiency generally increases with (1) particle size and/or density, (2) inlet duct velocity, (3) cyclone body length, (4) number of gas revolutions in the cyclone, (5) ratio of cyclone body diameter to gas exit diameter, (6) dust loading, and (7) smoothness of the cyclone inner wall. Cyclone efficiency will decrease with increases in (1) gas viscosity, (2) body diameter, (3) gas exit diameter, (4) gas inlet duct area, and (5) gas density. A common factor contributing to decreased control efficiencies in cyclones is leakage of air into the dust outlet (EPA, 1998). 

A series of theoretical analyses of the fluid and particle collection efficiency of cyclones were performed in the 1970s by Bloor and Ingham " and they have been found to be in good agreement with the experimental data. 

Dietz, P.W., 1981. Collection efficiency of cyclone separators. American Institution of Chemical Engineers Journal, 27(6), 888. 

Hoekstra, A. J., Gas flow field and collection efficiency of cyclone separators , Ph.D. Thesis, Delft University of Technology, Delft, Netherlands (2000). 

Leith, D. and Licht, W. (1972). The Collection Efficiency of Cyclone Type Particle Collectors A New Theoretical Approach. AlChE. Symp. Ser., 68(126), 196. 

D. Laith and W, Licht, The Collection Efficiency of Cyclone Typa Particle Collectors—A New... 

Figure 7.3.16. Comparison between theory and eiqperiment for the second-stage cyclone at Exxon s miniplant (assumed m = 0.7). Reprinted, with permission, from P.W. Dietz, Collection efficiency of cyclones," AlChE]., 27(6), 888(1981). Copyriffit [1981] American Institute of Chemical Engineers (AIChE).

Hoekstra A J 2000 Gas Flow Field and Collection Efficiency of Cyclone Separators PhD thesis Delft University of Technology. 

Cyclone Efficiency. Most cyclone manufacturers provide grade-efficiency curves to predict overall collection efficiency of a dust stream in a particular cyclone. Many investigators have attempted to develop a generalized grade-efficiency curve for cyclones, eg, see (159). One problem is that a cyclone s efficiency is affected by its geometric design. Equation 15 was proposed to calculate the smallest particle size collectable in a cyclone with 100% efficiency (157). 

The emission of volatile trace elements from roasting, smelting, and converting processes is undesirable from both an air pollution and an economic standpoint. Gravity collectors, cyclones, and ESPs are used to attain collection efficiencies of up to 99.7% for dust and fumes. 

Small solid particles, present in dust and grit emissions, have very low settling velocities (Table 4.4) The collection efficiencies of simple cyclones are tlierefore, as shown in Figure 17.3, relatively low. Fabric filters, electrostatic precipitators or wet scrubbers may be required to remove particles <5 pm in size with an acceptable efficiency. Therefore the cost of pollution control inevitably increases when dealing with particle size distributions skewed towards the lower end. 

The collection efficiency curve is usually employed to demonstrate the performance of a cyclone. Figure 13.3 shows a typical collection efficiency curve for a cyclone at a particular airflow rate. The size of particles that have a collection efficiency of 50% is usually employed as a simple indication of the separation efficiency of the cyclone, and is known as the cut-off particle size... 

Therefore, Bloor and Ingham found that the collection efficiency of a cyclone for the p.article size d may be estimated by the following formula ... 

In the KBR system, as with the ABB Lummus design, the riser cyclones are hard-piped to the riser. The diplegs of both the riser cyclone and the upper reactor cyclone are often sealed with catalyst. This minimizes the carry-under of reactor vapors into the reactor housing and maximizes the collection efficiency of the riser cyclones. 

Example A. Suppose it was desired to design a cyclone to collect all particles larger than 97 microns and release no more than 60 lbs/hr from a 6328 ACFM gas stream bearing 80 grains of solids per cubic foot of gas. The particle size distribution and density of the solids is given in Fig. 18 the gas has a density of 0.1 lbs/ft3 and a viscosity of 0.02 centipoise. The pressure drop across the cyclone is not to exceed 10" H20. Note from Fig. 1 of the text that 100% collection efficiency of 97 micron particles would require a Dth of 9.7 microns which would be collected at an average 50%... 

Table 2. Prediction of Collection Efficiency for Cyclone in Example B...

The collection efficiency of a cyclone is 45 per cent over the size range 0-5 xm, 80 per cent over the size range 5-10 (im, and 96 per cent for particles exceeding 10 xm. 

Discharging of the cyclone into the fluidized bed requires some care. It is necessary to seal the bottom of the cyclone so that the collection efficiency of the cyclone will not be impaired by the passage of... 

The effect of temperature on cyclone efficiency was studied by both Parker et al. [/. Environ. Sci. and Technol. 15 (4) 451 (1981)] and Patterson and Munz [Canadian J. Chem. Eng. 67 321 (1989)]. Both studies showed that cyclone efficiency decreased with increasing gas viscosity. As with the studies at high pressure, Patterson and Munz (1989) reported that only the collection efficiency of particles less than about 10 o,m was reduced because of operation at high temperature. 

In these systems, the total collection efficiencies of the dry product are 85 per cent for the drying vessel, 90 per cent for the cyclone collector and 98 per cent for the scrubber-condenser. The net efficiency of the system may be as high as 99.97 per cent if the scrubber effluent is considered as product. All the runs are based on 1.25 kg/s product and 0.75 kg/s evaporation at an elevation of 300 m above sea level. The total air flow is measured at the outlet before the stream is split into the recycle and bleed portions and, for such flows, the design of suitable fans is outlined by Jorgensen164 . The calculations outlined here may be confirmed by the use of psychometric charts, and this procedure has been considered in some detail by Cook and Demount165. ... 

It is noted that the particle-wall interaction in the boundary layer and the electrostatic effect due to the electrostatic charge carried by the particles may strongly affect particle collection and reentrainment in the cyclone and, consequently, affect the collection efficiency. In the presence of electrostatic charges of particles and an external electric field, the collection efficiency of a tangential inlet cyclone with a steep cone is given by [Soo, 1989]... 

Zhou, L. X. and Soo, S. L. (1991). On Boundary Conditions of Particle Phase and Collection Efficiency in Cyclones. Powder Tech., 64,213. 

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