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Emerging Pollution Control Technologies 457

2
ψ= C ρ Vd / 9 µ D (19)
p g c
where V is the particle velocity relative to the target and D is the collector diameter
c
(consider the larger droplet as the collector).
For impactor devices in which particles pass through a slit or opening and are captured
by a plate-type collector,

2
ψ= (C ρ Vd / 18 µ D ) 1/2 (20)
p g c
where V is the particle velocity relative to the target and D is the slit width.
c
Efficiency of impactors can then be expressed for individual size particles as a function
of the impaction parameter. Calvert (11) showed that the fractional collection efficiency
η for spherical drops collected by droplets when ψ > 0.2 is
p
η ≅ [ψ / (ψ+ 0.7)] 2 (21)
p
No impaction occurs on spheres when ψ < 0.083.
In contrast, particulate removal efficiencies can be predicted on an overall basis for
impaction devices using empirical data. For example, Hesketh (12) showed that overall
collection efficiency by weight percent E for a Venturi scrubber is
o
E = L + (1 − 3.47 ∆P −1.43 )F (22)
o
where L is the percentage of particles larger than 3 µm, F is the percentage of particles
smaller than 3 µm, and ∆P is the Venturi scrubber pressure drop (inches of water).
Furthermore, orifice scrubbers follow Eq. (22) if the orifice scrubber pressure force
drop is divided by 2 to obtain the value of ∆P for use in the Venturi equation (22).
Equation (22) is applicable to open-throat Venturi systems in which the gases do not
exceed 600ºF and the particulates are somewhat wettable (i.e., they are not hydropho-
bic). The equation is also applicable for a wide range of materials because the data were
obtained from flue gas, lime kiln, black liquor recovery, sinter furnaces, blast furnaces,
foundry cupola, and terephthalic acid processing operations. The equation is based on
the fact that all particles with diameter greater than 3 µm are captured according to
Hesketh (13) with a penetration (one minus efficiency fraction) of
C / C = 3.47 ∆P −1.43 (23)
o i
where C / C is the ratio of concentration out to concentration in.
o i
The pressure drop of the Venturi scrubbing system can be estimated using
∆P = V ρ A 0.133 (L') 0.78 /1,270 (24)
t g
where ∆P is the Venturi pressure drop (inches of water), L' is the liquid-to-gas ratio
3
(gal/1000 actual ft wet gas leaving throat), ρ is the gas density downstream from the
g
3
Venturi throat (lb/ft ), and V is the throat gas velocity based on wet gas (in actual
t
2
ft/min) downstream throat (ft/s), and A is the throat cross-section area (ft ).
4.3.2. Phoretic Forces
Phoretic or radiometric forces include diffusiophoresis, Stephan flow, photoporesis,
and thermophoresis, with diffusiophoresis being the most significant. These forces are
exerted by a gas on particles in the gas because of nonuniformity of gas molecule energy
and they are only effective on small, submicron-sized particles.