Dust collection mechanisms

Dust collection mechanisms The various means by which particulate matter is collected, which may be classified as... 

Dust collection mechanisms include aU of the entrapment processes described in Section 1C, with the effect of electrostatic forces also being very important in many systems. Filtration of dusts is achieved by depth filtration - for which the filter pad is very effective - and by the combination of surface and cake filtration that the pleated sheet media do so well. The two materials have vied with one another over the years as to which can achieve the highest degree of separation, a history of success that is largely determined by the development of new materials of one kind or the other. 

Practical separation techniques for hquid particles in gases are discussed. Since gas-borne particulates include both hquid and sohd particles, many devices used for dry-dust collection (discussed in Sec. 17 under Gas-Sohds Separation ) can be adapted to liquid-particle separation. Also, the basic subject of particle mechanics is covered in Sec. 6. Separation of liquid particulates is frequently desirable in chemical processes such as in countercurrent-stage contacting because hquid entrainment with the gas partially reduces true countercurrency. Separation before entering another process step may be needed to prevent corrosion, to prevent yield loss, or to prevent equipment damage or malfunc tion. Separation before the atmospheric release of gases may be necessaiy to prevent environmental problems and for regula-toiy compliance. 

This subsection is concerned with the application of particle mechanics (see Sec. 5, Fluid and Particle Mechanics ) to the design and apphcation of dust-collection systems. It includes wet collectors, or... 

From the standpoint of collector design and performance, the most important size-related property of a dust particfe is its dynamic behavior. Particles larger than 100 [Lm are readily collectible by simple inertial or gravitational methods. For particles under 100 Im, the range of principal difficulty in dust collection, the resistance to motion in a gas is viscous (see Sec. 6, Thud and Particle Mechanics ), and for such particles, the most useful size specification is commonly the Stokes settling diameter, which is the diameter of the spherical particle of the same density that has the same terminal velocity in viscous flow as the particle in question. It is yet more convenient in many circumstances to use the aerodynamic diameter, which is the diameter of the particle of unit density (1 g/cm ) that has the same terminal settling velocity. Use of the aerodynamic diameter permits direct comparisons of the dynamic behavior of particles that are actually of different sizes, shapes, and densities [Raabe, J. Air Pollut. Control As.soc., 26, 856 (1976)]. 

The basic operations in dust collection by any device are (1) separation of the gas-borne particles from the gas stream by deposition on a collecting surface (2) retention of the deposit on the surface and (3) removal of the deposit from the surface for recovery or disposal. The separation step requires (1) application of a force that produces a differential motion of a particle relative to the gas and (2) a gas retention time sufficient for the particle to migrate to the coUecting surface. The principal mechanisms of aerosol deposition that are apphed in dust collectors are (1) gravitational deposition, (2) flow-line interception, (3) inertial deposition, (4) diffusional deposition, and (5) electrostatic deposition. Thermal deposition is only a minor factor in practical dust-collectiou equipment because the thermophoretic force is small. Table 17-2 lists these six mechanisms and presents the characteristic... 

Most forms of dust-collection equipment use more than one of the collection mechanisms, and in some instances the controlhng mechanism may change when the collec tor is operated over a wide range of conditions. Consequently, collectors are most conveniently classified by type rather than according to the underlying mechanisms that may be operating. 

A relationship of power consumption to collection efficiency is charac teristic of all particulate scrubbers. Attaining increased efficiency requires increased power consumption, and the power consumption required to attain a given efficiency increases as the particle size of the dust decreases. Experience generally indicates that the power consumption required to provide a specific efficiency on a given dust does not vary widely even with markedly different devices. The extent to which this generahzation holds true has not been fully explored, but the known extent is sufficient to suggest that the underlying collection mechanism may be essentially the same in all types of particulate scrubbers. 

A venturi scrubber is a venturi-shaped air passage with water introduced just ahead of or into the venturi throat. The liquid-gas contact is at a maximum in the venturi throat. The relative velocity between gas and liquid aerosol droplets is high, with the gas velocities in the range of 50-100 m/s. The particles are conditioned in the throat, and condensation is the important collection mechanism. After the particles in the gas have been deposited on droplets, a comparatively simple device such as a cyclone collector can be used to collect the wetted dust. 

The primary mechanism for conveying dust-laden air to a central collection point is a system of pipes or ductwork that functions as a pneumatic conveyor. This system gathers dust-laden air from various sources within the plant and conveys it to the dust-collection system. 

In this process EAF dust, other zinc-bearing wastes, recycled materials, coke or coal, lime, and silica are mixed and fed to a rotary furnace. The zinc and other volatile nonferrous metals in the feed are entrained in the furnace off-gas and are carried from the furnace to an external dust collection system. The resulting oxide (zinc calcine) is a crude zinc-bearing product that is further refined at zinc smelters. A byproduct of the process is a nonhazardous, iron-rich slag that can be used in road construction. Solidification technologies change the physical form of the waste to produce a solid structure in which the contaminant is mechanically trapped. 

Brown and Bern (26) cinalyzed the elemental composition of four card room dusts using X-ray fluorescence spectroscopy. Two of these were from filter cake material collected in two textile mills from which fine dusts (<20 ym) were separated by mechanical agitation (sonic sifting). The third sample was from filter cake material collected in a textile mill from which dust was removed by hexane washing followed by sonification of the bath, filtration and further sonification. The fourth sample came from dust collected on an electrostatic precipitator in a model card room. Results are shown in Table VI. 

Textile mill dusts investigated include samples collected using a vertical elutriator cotton dust sampler, seunples of fine dust separated mechanically from carding filter cake material, Scimples collected on electrostatic precipitators in a card room, coarse condenser trash from a card room, and weave room dusts. 

Wet scrubbing uses liquid droplets to remove fine dust in a gas stream. In all types of wet scrubbing, the basic cleaning mechanism involves the attachment of particles to the droplets. The function of the droplets in scrubbers is similar to that of spherical fibers in filtration. Likewise, the primary collection mechanisms in scrubbing are similar to those in filtration, i.e., inertial impaction, interception, and diffusion [e.g., Fan, 1989]. Secondary collection mechanisms include thermophoresis due to temperature gradients, coagulation of particles due to particle electrification, and particle growth due to liquid condensation. 

Scrubbers make use of a combination of the particulate collection mechanisms listed in Table 5. It is difficult to classify scrubbers predominantly by any one mechanism but for some systems, inertial impaction and direct interception predominate. Semrau (153,262,268) proposed a contacting power principle for correlation of dust-scrubber efficiency the efficiency of collection is proportional to power expended and more energy is required to capture finer particles. This principle is applicable only when inertial impaction and direct interception are the mechanisms employed. Furthermore, the correlation is not general because different parameters are obtained for differing emissions collected by different devices. However, in many wet scrubber situations for constant particle-size distribution, Semrau s power law principle, roughly applies ... 

---