Cyclones efficiency

Rosin et al. (1932) derived a simple expression for cyclone efficiency for a fixed cyclone diameter based on a force balance on the particle at the cyclone inlet, i.e., 

This expression indicates that cyclone efficiency should not change with gas density because (pp - pg) is insensitive to changes in pressure. However, cyclone efficiency should decrease with gas viscosity because it is harder for a particle to travel to the cyclone wall in a more viscous fluid. 

There have been relatively few literature articles reporting on the observed effects of temperature and pressure on cyclone efficiency. 

Parker et al. (1981) conducted tests with a 5 cm diameter cyclone at temperatures up to 700°C with 20 micron fly ash. They found that cyclone efficiencies decreased with temperature as expected. Patterson and Munz (1989) investigated the effect of temperature on cyclone efficiency up to 1700°C in a 10-cm-diameter cyclone, and also found that cyclone efficiency decreased with temperature. However, this effect was only seen at very small particle sizes (below about 12 microns) where cyclones become relatively inefficient. 

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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). 

Practically all cyclone performance data have been related to a present cyclone set of geometric ratios. One model for cyclone grade-efficiency curves has been tested against reported commercial cyclone efficiencies (159). A good fit was obtained. 

Other problems affecting cyclone efficiency are usually caused by abuse or poor maintenance. Problems may arise from temperature warpage, rough interior surfaces, overlapping plates and rough welds, or misalignment of parts, such as an uncentered (or cocked) vortex oudet in the barrel. 

Cyclone Efficiency The methods described below for pressure drop and efficiency calculations were given by Zenz in Manual on Dis-po.sal of Refinery Wa.ste.s—Atmo.spheric Emls.sion.s, chap. 11 (1975), American Petrofeum Institute Publ. 931 and improved by the Particulate Sohd Research Inc. (PSRl), Chicago. 

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). 

Since cyclones rely on centrifugal force to separate particulates from the air or gas stream, particle mass is the dominant factor that controls efficiency. For particulates with high densities (e.g., ferrous oxides), cyclones can achieve 99 per cent or better removal efficiencies, regardless of particle size. Lighter particles (e.g., tow or flake) dramatically reduce cyclone efficiency. 

The fluidization characteristics of an FCC catalyst largely depend on the unit s mechanical configuration. The percentage of less than 40 microns in the circulating inventory is a function of cyclone efficiency. In units with good catalyst circulation, it may be economical to minim the fraction of less than 40-micron particles. This is because after a few cycles, most of the 0-40 microns will escape the unit via the cyclones. 

PSD is an important indicator of the fluidization characteristics of the catalyst, cyclone performance, and the attrition resistance of the catalyst. A drop in fines content indicates the loss of cyclone efficiency. This can be confirmed by the particle size of fines collected downstream of the cyclones. An increase in fines content of the E-cat indicates increased catalyst attrition. This can be due to changes in fresh catalyst binder quality, steam leaks, and/or internal mechanical problems, such as those involving the air distributor or slide vah es. 

The cyclone efficiency increases with VY up to about 1.25 VK, after which reentrainment results in a decrease in efficiency. 

Tube diameter Jet nozzle diameter Average particle size Cyclone efficiency... 

Hugi and Reh [Chem. Eng. Technol. 21(9) 716-719 (1998)] have reported that (at high solids loadings) enhanced cyclone efficiency occurs when the solids form a coherent, stable strand at the entrance to a cyclone. The formation of such a strand is dependent upon several factors. They reported a higher cyclone efficiency for smaller = 40 rm) solids than for larger solids (dp,50 = 125 xm). This is not what theory would predict. However, they also found that the smaller particles formed coherent, stable strands more readily than the larger particles, which explained the reason for the apparent discrepancy. 

Cyclone Roughness Large weld beads, etc., can also reduce cyclone efficiency. If the solids flow along the wall of a cyclone encounters a large protuberance such as a weld bead, the weld bead acts as a type of ski jump and causes the solids to be deflected farther into the center of the cyclone, where they can be thrown into the vortex and carried out of the cyclone. In small pilot or research cyclones, this is especially common, because the distance between the wall of the cyclone and the vortex tube is very small. Because of their... 

With very small laboratory or pilot cyclones, some solids (large polymer beads, spherical particles, etc.) can sometimes bounce off the cyclone wall immediately across from the cyclone inlet and be deflected into the vortex. Very large particles can be found in the gas outlet stream of the cyclone with these very small cyclones and with particles that bounce. To increase cyclone efficiency with these types of solids, the cyclone barrel diameter can be increased. This increases the distance between the cyclone vortex and the wall and prevents most of the solids from bouncing back into the vortex. 

Hoffman et al. [AIChE J. 47(11) 2452-2460 (2001)] studied the effect of cyclone length on cyclone efficiency and showed that the efficiency of a cyclone increases with length. However, they also found that after a certain length, cyclone efficiency decreased. They reported that cyclone efficiency suddenly decreased after a certain cyclone length, which in their cyclone was at a length/diameter ratio of 5.65. 

Most processes operate at high temperatures and/or high pressures. Therefore, it is important to know how cyclones operate at these conditions. Efficient cyclones are able to collect very small particles sizes. Therefore, cyclone efficiency is proportional to 1/Dpth. 

The effects of temperature and pressure manifest themselves in how they affect the gas density and gas viscosity From the equation above, it can be seen that cyclone efficiency is theoretically related to gas density and gas viscosity as... 

As pressure is increased, gas density will increase. However, the term pp - pg does not change with increases in gas density because particle density is so much greater than the gas density (typically about 2000 kg/m3 versus approximately 20 kg/m3 at high pressure) that it dominates this term. Therefore, it is expected that gas density would have little or no effect on cyclone efficiency. Conversely, cyclone efficiency would be expected to decrease with system temperature because gas viscosity increases with increasing pressure. 

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. 

Note that a higher particle density, not necessarily catalyst ABD, raises cyclone efficiency due to increased centrifngal force as well as rednced solids entrainment to... 

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