Page 438 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 7 ENVIRONMENTAL AND SAFETY ISSUES WITH NANOPARTICLES

type typical collector shape of collector particle size
settling >20 μm
gravity chamber dusty clean
air air
screen
inertia mist separator >10 μm

cyclone
centrifugal >2 μm
force
nozzle
venturi
scrubbing
scrubber >0.5 μm
spray
all size
range
bag filter
(high conc.)
fabric Figure 7.4.5
filtration Penetration of airborne nanoparticles through a circular tube.
all size
air filter range
Fibrous (low conc.)
layer
discharge HV particle size. For the charged fiber, particle collection
electrostatic electrode
precipitator collection efficiency is very high even for uncharged particle,
electrostatic (EPS) electrode
force >50 nm and the efficiency for charged nanoparticles is
extremely high because of strong Coulombic force
between fiber and particle. The experimental data
plotted in Fig. 7.4.4 are qualitatively in good agree-
Figure 7.4.3 ment with the theoretical prediction following the
Classification of particle separators. particle size dependency on particle migration veloc-
ity (shown in Fig. 7.4.2).
However, as particle size becomes smaller and
comparable with the size of a molecule, particles may
10 0 rebound on a collector surface, and the adhesion
5 probability of particles drops, resulting in a decrease
Interception
in collection efficiency. Fig. 7.4.5 is an example of
10 -1 fiber diameter : 2.5 μm experimental data that confirm the particle rebound
5 velocity : 0.1 m/s [2]. The figure shows the penetration of nanoparticles
through a grounded circular tube. The solid curve is
the theoretical line derived by assuming that particles
particle penetration (−) 10 5 -3 Brownian Uncharged Uncharged Brownian diffusion. It is evident that experimental
Charged state
-2
are deposited from a laminar flow in a tube by
10
Key
Fiber
Particle
penetration deviates from the theoretical line for
particles less than 2 nm. This means that molecular
Charged
Charged
diffusion
behavior begins to appear when the particle size
5
Charged
Charged
becomes as small as 2 nm, and as a result, the collec-
10
It should be noted that considerable amounts of nano-
5 -4 Induced force tion efficiency is reduced.
sized particles are contained in diesel exhaust particles
Coulombic (DEP), possibly penetrating through the honeycomb
force
10 -5 type (tubular channel) diesel particulate filters (DPF).
5
2 References
10 -2 5 10 -1 5 10 0 5 10 1 5 10 2
particle diameter (μm) [1] K. Takahashi: Ouyou Earozorugaku, Youkendou,
p. 144, (1984).
Figure 7.4.4 [2] Y. Otani, H. Emi, S.J. Cho and N. Namiki: Adv. Powder
Particle penetration of electrically charged filter. Technol., 6(4), 271–281 (1995).
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