Removal of Particles from Gases

In this first chapter we start by briefly discussing the removal of particles from gases in general, to put the topic in context. We will then focus on centrifugal devices, which are the most widely used gas cleaning devices in industry today. We will explain qualitatively the principle by which centrifugal separators work, and mention the different types that are in use. 

Particulate pollution is a societal concern and has been recognized as a problem for many decades. Airborne particles manifestly cause increase in in- 

Aside from meeting ambient air quality targets, various types of separation equipment, including knock-out drmns, bag filters, electrofilters/electrostatic precipitators (ESP s), scrubbers and cyclones are also used to captme solid particles for  

Another reason for removing particles from process streams is to protect downstream equipment, such as blowers and turbines, from erosion damage caused by particles impacting on, for instance, rotorblades. 

If the goal of a given gas cleaning operation is to limit emissions or to collect as much product as is practical, the target is often one of not allowing particles in the emitted gas stream to exceed some maximal allowable concentration. If, on the other hand, the goal is to protect downstream equipment, the target can be in terms of a maximal particle size allowed in the cleaned gas stream. 

---

Removal of particles from gases (Acoustic) agglomeration of fine particles in flue gases and other gaseous effluents Flocculation of solid contaminants in waste water and other liquid effluents... 

Slinn,W.G.N.(1977) Some approximations for the wet and dry removal of particles and gases from the atmosphere. Water, Air Soil Pollution, 7, 513—43. 

The efficiency of currently used techniques for the removal of particulates from gases drops off sharply if the particle size becomes smaller than 1 nm (Figure 362). On the other hand, the human pulmonary system is most efficient in absorbing and retaining particles in the micrometer and submicrometer range. These particles are then the primary cause of respiratory ailments. " While such suspended particles in emissions from, for example, stacks of power plants are invisible, measurements have revealed that approximately 50 o of the particulates suspended in the air of urban regions are smaller than 1 fim. 

The mechanisms for the removal of particles from liquids are identified as screening, cake filtration, and depth filtration. As with the removal of particulate matter from gases, screening is the method of retaining the particles that are larger in size than the pore or opening. Cake filtration is based upon the build-up of particles, which have the effect of becoming the filtration structure. 

Extinction Coefficient a measure of the ability of particles or gases to absorb and scatter photons from a beam of light a number that is proportional to the number of photons removed from the sight path per unit length. See absorption. Extinction Cross Section the amount of light scattered and absorbed by a particle divided by its physical cross section. 

Electrostatic precipitators are satisfactory devices for removing small particles from moving gas streams at high collection efficiencies. They have been used almost universally in power plants for removing fly ash from the gases prior to discharge. Electrostatic precipitators have the capability of fine particulate control. Resistivity plays an important role in determining whether a particle can be readily collected in this device. 

The use of cyclone separators for the removal of suspended dust particles from gases is discussed in Section 1.6. By suitable choice of operating conditions, it is also possible to... 

Since the removal of solids from liquids in these ways is discussed elsewhere, this section is concerned only with the removal of suspended particles and liquid mists from gases. 

Bag filters are still extensively used for the removal of dust particles from gases and can be operated either as pressure filters or as suction filters. Their use is discussed in Chapter 1. 

Purification consists of removal of gaseous impurities by means of absorption or adsorption processes (such as by activated charcoal) Refs 1) F. Fraas O.C. Ralston, I EC 32, v 600—04(1940) (Ekectrostatic sepn of solids from gases) 2) H.F. Johnstone M,H. Roberts, IEC 41, 2417(1949) (Deposition of aerosol particles from moving gas streams)... 

These are used for removal of suspended dust particles from gases. The rate of settling of suspended particles in a gas stream may be greatly increased if centrifugal rather than gravitational forces are... 

---