Page 67 - 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

290 Aerosols

plasma.Laserablationapplicationscanprovidenon-(ther- yields very small particles after evaporation of the volatile
modynamic) equilibrium vaporization controlling the ma- liquid.
terial stoichiometry, as well as control of particle crys-
talline structure through temperature and concentration
management. V. MEASUREMENT PRINCIPLES
The measurement of the physical and chemical proper-
B. Comminution Processes
ties of particle suspensions has been a central theme of
The disintegration of coarse bulk material into colloids aerosol science since its beginnings. The variety of de-
is accomplished by three main types of devices. The first vices and methods adopted for such purposes represents a
is the air blast or aerodynamic atomizer, in which com- diverse collection of instrumentation designed for specific
pressed gas is ejected at high speed into a liquid stream applications. The reason for this diversity is that no sin-
emerging from a nozzle. This type of breakup is found, for gle technique or group of techniques provides a means of
example, in paint spray guns, venturi atomizers, and other characterizing properties over the extremely wide range
practical sprayers. A second class of atomizer depends on of particle size, shape, and chemical composition found
centrifugal action wherein a liquid is fed into the center of in nature or in the laboratory. The devices range from sim-
a spinning disk, cone, or top and is centrifuged to the outer ple instruments for the measurement of light transmission,
edge. Provided that the flow rate of liquid into the spin- to porous filters for the collection of material in order to
ning device is well controlled, sprays produced in this way determine mass concentration, to sophisticated sensors or
are rather uniform in size, in contrast to those produced by collectors for the characterization of particle size distribu-
other methods of atomization. A third type of atomizer has tion and chemistry.
a hydraulic design in which a liquid is pumped through a To characterize adequately the dynamic properties of a
nozzle and, upon its exit from the orifice, breaks up into chemically reactive aerosol, a very large amount of infor-
droplets. Disintegration here depends largely on the phys- mation is required. However, aerosol properties generally
ical properties of the liquid and the ejection dynamics at aredeterminedinonlyalimitedwaybecauseoflimitations
the nozzle orifice rather than on the intense mixing be- of available techniques. With air pollution monitoring and
tween the liquid and the surrounding gas. Perhaps most the driving force of progress in the development of theory,
well known of these devices is the swirl chamber atomizer, heavy emphasis has been placed on the size distribution
which has been used in agricultural equipment, oil-fired and its moments, as well as the chemical composition of
furnaces, internal combustion engines, and gas turbines. In particles and the suspending gas.
addition to these three main classifications, special meth- The measurement of particles or particle collections
ods are available including the electrostatic atomizer and is achieved by one of two approaches: (1) in situ, or
the acoustic atomizer. The former makes use of a liquid quasi-in situ, continuous observation or (2) collection on
breakup by the action of electrostatic forces, while the lat- a medium and subsequent laboratory investigation of the
ter applies high-intensity sonic or ultrasonic vibrations to accumulated particulate material. No single method pro-
disrupt a liquid. vides a self-consistent, complete physical picture of a
The generation of dust clouds by the dispersal of fine particle suspension. For example, the first approach ba-
powders is a straightfoward method, in principle. All such sically assumes that the particles can be treated as inert
generators depend on blowing apart a bed of finely divided spheres during the measurement process. The second as-
material by aerodynamic forces or by a combination of air sumes the accumulation of material on a medium or sub-
flow and acoustic or electrostatic vibrations. The size of strate without modification of the particles. This is known
particulate suspensions produced in this way is limited by to be a less than satisfactory assumption for particles re-
the minimum size of material ground up by mechanical or acting with the suspensing gas, but no better techniques
other means and the nature of cohesive and adhesive forces have been developed.
acting between particles in agglomerates. Generally, dust If one focuses on the particle size distribution func-
clouds produced by powder dispersal are not less than a tion as a central framework for describing aerosols, one
few micrometers in radius. can conveniently classify the measurement instruments
Aerosols composed of solid particles or nonvolatile liq- according to the properties of the size distribution func-
uids with sizes much less than those attainable by atom- tion. Organization of instrumentation gives perspective
ization of pure liquids or by dispersal of powders may be on the ideal requirements as contrasted with the practi-
produced by atomizing salt solutions. Breakup of suspen- cal limits imposed by current technology. An idealized
sions of a volatile carrier liquid, in which solid particles, an hierarchy was suggested by S. K. Friedlander in 1977. As
immiscible liquid, or a nonvolatile solute are suspended, an ideal, the modern aerosol analyzer gives a continuous