Particle removal cyclone collectors

Dust entrained in the exit-gas stream is customarily removed in cyclone cohectors. This dust may be discharged back into the process or separately cohected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube diyer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collec tors in diying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the cohection system. 

Cyclone collectors are popularly used both for particle removal and for particle sampling (Fig. 13.1). The separation process of a cyclone relies on the centrifugal accelerations that are produced when particle-laden fluid experi-... 

As a simple and efficient particle separation device, cyclone collectors can be used for anything from dust removal in a fluid stream to material collection in the fluid conveying system. However, the cyclone is not suitable or economical for the separation of extremely small particles (say, less than 1 /xm), which frequently occur in industrial processes. It is recommended that the size of particles to be separated in an industrial ventilation cyclone be in the region of around 10 to 100 p.m. However, for the purpose of aerosol sampling, the size of particles to be separated may be much less than 10 jxm. 

A cyclone collector is another type of dry collector and is in the form of an inverted cone, but with no box. The incoming air containing contaminants is passed into the cone where it is spun at high speed. The spinning causes the solids to settle out to the periphery and fall into the apex of the cone for removal. Because of the relative densities, it is most effective with larger coarse particles, and not useful for fine particles. 

Emissions of sulfur oxides, nitrogen oxides, and particulates from coal combustion are problems of increasing concern and regulation. Coal combustion contributes about 25 percent of the particulate matter, 25 percent of the sulfur oxides, and 5 percent of the nitrogen oxides emitted to the atmosphere. Much of the particulates are derived from the mineral content of the coal, but some particulates also result from sulfur and nitrogen oxides that react to form various sulfate and nitrate salts. A major concern about particulate matter is that the smallest particles are respirable and may pose a health hazard. Particulate matter is recovered in most power plants by the use of electrostatic precipitators, which have been developed to very high efficiencies (>99%). Other methods of particulate removal include baghouses and cyclone collectors. 

The ash loading and size distribution depend on (I) the extent of grinding and fuel cleaning and (2) the combustion time-temperature history. Regardless of the type of combustion system, much of the ash is in the 1-20 pm size range. Particles down to about 10 pm can be efficiently ranoved by hot cyclone collectors. Particles in the 1-10 pm range can be removed by other advanced methods however, this tends to cause additional pressure drop and heat loss. 

Owing to presence of a reflector the particles, which have been not deduced removed fi om a gas stream under action of centrifugal forces, again are rejected on walls of the chamber. Connection of the cylindrical chamber with an inclination to a cyclone in view of a drain slime provides tap removal of disperse particles in slime collector that also raises increases efficiency dust separation. 

Connection of the cylindrical chamber with an inclination to a cyclone in view of a drain slim also provides tap removal atomizer particles in slim collector. 

Other Centrifugal Collectors. Cyclones and modified centrifugal collectors are often used to remove entrained Hquids from a gas stream. Cyclones for this purpose have been described (167—169). The rotary stream dust separator (170,171), a newer dry centrifugal collector with improved collection efficiency on particles down to 1—2 pm, is considered more expensive and hence has been found less attractive than cyclones unless improved collection in the 2—10-pm particle range is a necessity. A number of inertial centrifugal force devices as well as some others termed dynamic collectors have been described in the Hterature (170). 

Spray Dryers A spray diyer consists of a large cyhndrical and usu ly vertical chamber into which material to be dried is sprayed in the form of small droplets and into which is fed a large volume of hot gas sufficient to supply the heat necessary to complete evaporation of the liquid. Heat transfer and mass transfer are accomphshed by direct contact of the hot gas with the dispersed droplets. After completion of diying, the cooled gas and solids are separated. This may be accomplished partially at the bottom of the diying chamber by classification and separation of the coarse dried particles. Fine particles are separated from the gas in external cyclones or bag collectors. When only the coarse-particle fraction is desired for fini ed product, fines may be recovered in wet scrubbers the scrubber liquid is concentrated and returned as feed to the diyer. Horizontal spray chambers are manufactured with a longitudinal screw conveyor in the bottom of the diying chamber for continuous removal of settled coarse particles. 

Drying and cooling the products of ammonium phosphate production are conventionally achieved in a rotary drum, and a means must be provided to remove the dust particles from the air streams to be exhausted to the atmosphere. At the Minnesota plant, a high-efficiency dry cyclone recovery system followed by a wet scrubber was designed. In this way, material recovered from the dry collector (and recycled to the process) pays for the dry system and minimizes the load and disposal problem in the wet scrubber, because it eliminates the need for a system to recover the wet waste material that is discharged to the gypsum disposal pond for settling. 

A review of Table 8 and Fig. 3-2 indicates that large-diameter particles can be removed with low-energy devices such as settling chambers, cyclones, and spray chambers. Submicron particles must be removed with high-energy units such as bag filters, electrostatic precipitators, and venturi scrubbers. Intermediate particles can be removed with impingement separators or low-energy wet collectors. Obviously, other equipment performance characteristics as noted in Table 8 will also have their influence on the final equipment... 

The fluidized-bed reactor allows catalyst to be regenerated while the unit is in operation by continuously removing a portion of the catalyst from the reactor for regeneration treatment and subsequent flow-back into the reactor. Because there is a tendency for the catalyst particles to be carried over in the product stream, auxiliary units, such as cyclone separators or dust collectors, must be provided for separating out the solid particles or catalyst fines. 

Vatavuk (1990) pointed out that a key dimension in the sizing of a cyclone is the inlet area. Properly designed cyclones can remove nearly every particle in the 20-30 micron range. Typically, cyclone separators have efficiencies in the range of 70-90%. Because of the low efficiency of these units, they are often used as a first stage of dust collection, or are referred to as primary collectors. 

The initial and perhaps most critical design assumption is that both the MgO and the fly ash, which are carried out of the calciner and into the MgO product cyclone, have the same particle size distribution and density. This design (i.e., no possibility of physical separation of MgO and fly ash) is a conservative design assumption and adds complexity to the FGD process. It results in the need to recirculate large quantities of the MgO/fly ash mixture through the spray dryer and the calciner. In order to keep the MgO/ash recycle streams to a reasonable size, the mechanical collectors in the main flue gas ducts upstream from the SO2 absorber, which were used in the initial design because they are relatively inexpensive but yet remove only 80% of the fly ash, had to be replaced with the 95% efficient ESP mentioned earlier. 

The obtained leady oxide with acceptable small particles and phase composition passes through a series of cyclone separators and a dust collector to remove the lead oxide dust from the air stream, and is then transported to a silo. The entire technological process is monitored by sensors and controlled by a computer. 

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