Page 76 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

Aerosols 299

strength per unit mass, high temperature stability, chem- triboelectric effects due to transport through a duct, flame
ical uncertainties, and cost. Fabrics include cotton, wool, ionization, or other processes. These charges are gener-
paper, nylon, glass, and asbestos. ally too small to provide effective precipitation, and in all
The application of external electrical fields can enhance industrial precipitators charging is accomplished by the
filtration efficiency beyond a simple system. Bipolar elec- attachment of electrical charges produced by an electri-
trostatic charge between the fabric and the particles can cal corona. A corona discharge producing negative ions is
induce migration to the filter surface and particle agglom- normally used in precipitators.
eration in the aerosol. Ideally, electrical precipitators generally achieve col-
lection efficiencies of more than 99% for a full range of
particle size. The efficiency depends on the ratio of the
5. Electrostatic Precipitation
collector surface area particle size and dielectric proper-
A particle removal method commonly used in industry is ties and the volumetric gas flow rate times the charged
electrostatic precipitation. Industrial interest in this very particle migration speed induced by the applied electrical
efficient scheme can be traced back to 1911 with the inves- field.
tigation of F. Cottrell. His pioneering studies of sulfuric The removal of electrostatic precipitators can be im-
acid mist removal from copper smelter effluents led to the proved by combining electrical collection with filters ei-
production of the Cottrell precipitator. ther with conventional design in series or via integration
Success in the nonferrous metals industry was followed of the two technologies. Hybrid systems of this type are
by the application of precipitators to the collection of being introduced in some industries where very high effi-
dust from cement kilns. From these beginnings, the use ciencies are needed.
of precipitators has expanded to include a wide variety of
forms including unique boilers for electric power genera-
SEE ALSO THE FOLLOWING ARTICLES
tion. The principal uses of precipitators today are in gas-
cleaning applications in which high collection efficiencies
• ATMOSPHERIC DIFFUSION MODELING • CLOUD PHYSICS
of small particles are required for processes that emit large
• COAL STRUCTURE AND REACTIVITY • COMBUSTION
gas volumes. Since the separation force in a precipitator
is applied to the particle itself, the energy required for gas • FIRE DYNAMICS • LIQUID ROCKET PROPELLANTS •
PARTICLE SIZE ANALYSIS • PLANETARY ATMOSPHERES
cleaning is less than that for equipment in which energy is
• POLLUTION,AIR • POLLUTION,CONTROL • VOLCA-
applied to the entire gas stream. This unique characteris-
NOLOGY
tic of precipitators results in lower gas pressure drops and
usually lower operating costs than other methods of gas
cleaning. BIBLIOGRAPHY
The precipitation process requires: (1) a method of pro-
viding an electrical charge on a particle, (2) a means of
Friedlander, S. K. (1977). “Smoke, Dust and Haze,” Wiley-Interscience,
establishing and maintaining an electrical field, and (3) a New York.
method of removing the particle from the precipitator. Hidy, G. M. (1984). “Aerosols: Industrial and Environmental Science,”
The process of electrically charging a particle involves Academic Press, New York.
the addition of electrons to or removal of electrons from Reist, P. (1984). “Introduction to Aerosol Science,” Macmillan, New
York.
the material or the attachment of ionized gas molecules to
Seinfeld, J., and Pandis, S. (1998). “Atmospheric Chemistry and Physics:
the particle. Almost all small particles in nature acquire From Air Pollution to Climate Change,” Wiley-Interscience, New
some charge as a result of naturally occurring radiation, York.