Page 66 - 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 289

proportional to the particle surface area or radius squared key periods of time, and (3) control of volumetric output.
for spheres. Volume reactions are controlled by particle The generation devices themselves have also been investi-
volume or are proportional to the radius cubed for spheres. gated extensively to verify the physicochemical processes
in particle formation.
The generation of aerosol requires the production of a
3. Collision and Coagulation
colloidal suspension in one of four ways: (1) by condens-
Once particles are present in a volume of gas, they collide ing out small particles from a supersaturated vapor (the
and agglomerate by different processes. The coagulation supersaturation may come from either physical or chem-
process leads to substantial changes in particle size dis- ical transformation); (2) by direct chemical reaction in a
tribution with time. Coagulation may be induced by any medium such as a flame or a plasma; (3) by disrupting
mechanism that involves a relative velocity between par- or breaking up bulk material, including laser ablation; or
ticles. Such processes include Brownian motion, shearing (4) by dispersing fine powders into a gas. In each of these
flow of fluid, turbulent motion, and differential particle broad groupings, a wide variety of ingenious devices have
motion associated with external force fields. The theory been designed, some of which employ hybrids of two or
of particle collisions is quite complicated even if each of more of these groups.
these mechanisms is isolated and treated separately.
The rate of coagulation is considered to be dominated
A. Nucleation and Condensation Processes
by a binary process involving collisions between two par-
ticles. The rate is given by bn i n j , where n i is the number of The means for the production of particles during con-
particles of ith size and b a collision parameter. For colli- densation is well represented by the generator introduced
sionbetweeni-and j-sizedparticlesduringBrownianmo- by V. K. LaMer in the 1940s. This device was specif-
tion, the physicist M. Smoluchowski derived the relation: ically built to produce a laboratory aerosol with con-
trolled physical properties, using a low-volatility liquid
b = 4π(D i + D j )(R i + R j ) such as glycerine or dioctyl phthalate. The device gen-
erated particles from a vapor supersaturated by mixing a

2 kT 1 1 1/3 1/3
= + + warm, moist vapor with a cooler gas. Later, a wide va-
1/3 1/3 i j
3 µ g
i j riety of refinements of the LaMer concept emerged, in-
cluding a series reported by Milton Kerker in the 1970s.
from Einstein’s diffusion theory. This formula essentially
Many generators using the condensation process have ap-
comes from the fact that b is proportional to the sum
peared; some of these achieve vapor supersaturation by
of Brownian diffusion rate of the two particles. Analo-
adiabatic expansion in the vapor, others by the mixing
gous forms for b have been derived for other collision
process. Aerosols have also been formed from condensa-
mechanisms.
tion of a supersaturated vapor produced by chemical reac-
To be derived rigorously, Smoluchowski’s models must
tion. Some examples include reactions in combustion pro-
be corrected for gas slippage around the particles. This
cesses, photochemical processes, and through discharges
adds correction terms in Kn that accelerate the coagulation
between volatile electrodes. An example of a hybrid of
rate over the original estimates. Particles in a gas are often
condensation and breakup or vaporization is an exploding
naturally charged. Bipolar charging increases the expected
metal wire technique.
coagulation rate, while unipolar charging suppresses the
Another process involves molecular aggregation by
rate. Fluid motion relative to the particles and the diffential
action of external forces induce relative motion between means of direct chemical reactions akin to polymeriza-
particles. This motion also increases the coagulation rate tion. The best known example of this is the process of
of particles. carbon particles in a premixed acetylene–oxygen flame.
Evidently particle formation in this case does not involve
condensation from a supersaturated vapor, but proceeds
IV. PRODUCTION OF AEROSOLS directly through the pyrolysis of the acetylene, forming
in the process unstable polyacetylenes as intermediates in
A large amount of effort has gone into the investigation the flame.
and development of aerosol generation devices. Over the Molecular aggregation to produce very small particles
years, a wide variety of methods for the production of can be achieved through synthesis of particles in plasmas
aerosols has emerged; these methods depend on the tech- as well as ablation of bulk material using lasers. Plasma
nological requirements of the aerosol. For many scientific synthesis offers opportunities for high volume through-
applications they include (1) control of the particle size put for a wide range of refractory materials and can pro-
distribution, (2) stability of operational performance for duce high-density particles with rapid quenching of the