Particle collection cyclone separation

Centrifugal Separation Centrifugal force can be utilized to enhance particle collection to several hundredfold that of gravity. The design of cyclone separators for dust removal is treated in detail in Sec. 17 under Gas-Solids Separations, and typical cyclone designs are shown in Fig. 17-43. Dimension ratios for one family of cyclones are given in Fig. 17-36. Cyclones, if carefully designed, can be more efficient on hquids than on solids since liquids coalesce on capture and are easy to drain from the unit. However, some precautions not needed for solid cyclones are necessary to prevent reentrainment. 

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. 

The separation step requires (1) application of a force that produces a differential motion of the particles relative to the gas, and (2) sufficient gas-retention time for the particles to migrate to the collecting surface. Most dust-collections systems are comprised of a pneumatic-conveying system and some device that separates suspended particulate matter from the conveyed air stream. The more common systems use either filter media (e.g., fabric bags) or cyclonic separators to separate the particulate matter from air. 

A widely used type of dust-collection equipment is the cyclone separator. A cyclone is essentially a settling chamber in which gravitational acceleration is replaced by centrifugal acceleration. Dust-laden air or gas enters a cylindrical or conical chamber tangentially at one or more points and leaves through a central opening. The dust particles, by virtue of their inertia, tend to move toward the outside separator wall from where they are... 

Cyclone separators can be designed for 95% collection of 5 pirn particles, but usually only droplets greater than 50 pirn need be removed. 

The efficiency of the cyclone separator is greater for large than for small particles, and it increases with the throughput until the point is reached where excessive turbulence is created. Figure 1.55 shows the efficiency of collection plotted against particle size for an experimental separator for which the theoretical cut occurs at about 10 ttm. It may be noted that an appreciable quantity of fine material is collected, largely as a result of agglomeration, and that some of the coarse material is lost with the result that a sharp cut is not obtained. 

The bed material normally consists initially of an inert material, such as sand or ash, of particle size between 500 and 1500 xm. This gradually becomes replaced by ash from the coal and additives used for sulphur removal. Ash is continuously removed from the bottom of the bed and, in addition, there is a considerable carry-over by elutriation and this flyash must be collected in cyclone separators. Bed depths are usually kept below about 0.6 m in order to limit power requirements. 

Only a small fraction of these particles is included in the impactor sample because the cyclone separator removed most particles larger than about 2 urn. The composition of those large particles which were collected by the impactor is that expected for the ash residue. The major species are the oxides of Si,... 

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. 

Zenz, F. A. and Othmer, D. F. (1960). Fluidization and Fluid-Particle Systems. New York Reinhold. Zhou, L. X. and Soo, S. L. (1990). Gas-Solid Flow and Collection of Solids in a Cyclone Separator. Powder Tech., 63,45. 

The collection of the pyrolysis oils is difficult due to their tendency to form aerosols and also due to the volatile nature of many of the oil constituents. As the aerosols agglomerate into larger droplets, they can be removed by cyclonic separators. However, the submicron aerosols cannot be efficiently collected by cyclonic or inertial techniques, and collection by impact of the aerosols due to their Brownian or random motion must be utilized. A coalescing filter is relatively porous, but it contains a large surface area for the aerosol particles to impact by Brownian motion as they are swept through by the pyrolysis gases. Once the aerosol droplets impact the filter fibers, they are captured and coalesce into large drops that can flow down the fibers and be collected. 

In gas suspensions, where very fine particles have to be removed, US action involves agglomeration of particles in order to increase their size and, consequently, to improve the collection efficiency of conventional filters (e.g. electrostatic precipitators, cyclone separators). These filters, while effective for large particle separation, are inefficient for retaining particles smaller than 2.5 pm. Therefore, acoustic agglomeration provides a means for separating fine particles released from industrial, domestic or vehicle sources, which, analytically, constitutes an excellent method for sampling in environmental analysis. 

Particle size and particle density measurements were made on partially burnt char particles which were collected in a cyclone separator at the exit of the reactor. Their burn-off was evaluated from a knowledge of char feedrate, gas flowrates and gas composition. The particle size of the collected material was determined by sieving particle density was derived from measuring the bulk density of a bed of particles in a manner described by Field ( ). 

Cyclone Separators. The most commonly used equipment for the separation of dust particles from an air/gas stream is the cyclone separator. The literature on design and operation of cyclones has been extensively reviewed by Rietemer and Vetver (1961), Maas (1979), Zenz (1982), and Pell and Dunson (1999). A sketch of a cyclone separator and typical dimensional ratios are found in Figure 20.1(b). The dust-laden gas stream enters near the top of the collection chamber tangentially. The force on the larger particles is greater than the force on the smaller ones because the latter... 

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 collection efficiency of a Cottrell precipitator is much less dependent upon particle size than is the case with a cyclone separator. Thus in one comparison the efficiency only decreased from 99.7% to 98.5% as particle size was varied from 50 microns to 10 microns (288a). 

For sampling in the disengagmcnt zone during gasification, a size selective inlet was located inside the reactor to separate coarse particles and increase sampling time for the sintered filter. The inlet consisted of a stainless steel cyclone with both top and bottom outlets ducted outside the reactor wall. The coarse particle fraction was collected from the bottom outlet, while fine particles were collected on the heated sintered filter connected to the top outlet. The fine particle Alter was again maintained at a temperature equal to or above the gas temperature at the extraction point. Coarse and Ane particles were thus separated without cooling. 

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