Page 437 - Book Hosokawa Nanoparticle Technology Handbook

7.4 REMOVAL OF NANOPARTICLES FUNDAMENTALS

Elementary cause Force field Force field Obstacle
and obstacle

¨
Š
•
á
Q
Obstacle Obstacle
áŠQ•¨
Š
á áŠQ•¨ • ¨
Q
F
Form
Collection efficiency low middle high
Pressure drop low middle high
Critical factor • Deposition velocity • Collision efficiency • Pressure drop
for performance • Pressure drop
• Thickner • Venturi scrubber • Filter press
Separator • ESP • Fibrous filter • Bag filter
• Cyclone • Granular bed • Membrane filter

Figure 7.4.1
Basic forms of particle separation.

Therefore, the migration velocities or displacement 10 2
of a particle per second due to the individual forces
gives the basis for the comparison of removal effi- p = 1g • cm −3
ciencies due to each force. In Fig. 7.4.2, migration electrostatic
velocities of particles due to various forces are 10
force V E
depicted against particle diameter at normal temper- (V 0 = 1kV • cm )
−1
3
ature and pressure for particle density of 1g/cm [1].
As seen from the figure, the velocities due to grav- unipolar
ity, centrifugal force, and inertia monotonically 1 diffusion charging
decrease with decreasing particle diameter, suggest- (Nt = 10 7 , ε p = ∞)
ing that the removal of nanoparticles with these unipolar field charging
forces is difficult. On the contrary, the velocities due 10 −3 (E = 5kv, Nt = ∞ , p = ∞)
to Brownian diffusion and electrostatic forces migration velocity (cm • s − 1 ) thermophoresis v r (NaCl)
increase with decreasing particle diameter for parti- k g /k 3 = 0.0041
cles less than 100 nm. This suggests that Brownian dT = 100K • cm −1
dx
diffusion and electrostatic forces are most effective 10 −2 Brownian diffudion V B
in collecting nanoparticles.
gravity V g
Fig. 7.4.3 summarizes typical conventional dust
collectors. Among them, the effective collectors for centrifugal force V c (Z=10)
nanoparticles are ESP and fabric/air filter. However, 10 −1 inertia V i
for the case of ESP, which relies on only electrostatic = 340m • s −1 ) inertia V i
force, nanoparticles ( 10nm) fail to carry even one (u = 1m • s − 1 )
electron resulting in low collection efficiency. In this −4 (u 0
case electrically charged filters are effective because 10 10 -3 10 -2 -1
we can expect the combined effects of electrostatic 10 1 10
forces and Brownian diffusion. particle diameter (μm)
Among charged filters, so-called electret filter,
which consists of permanently polarized fibers, is the Figure 7.4.2
most favorable filter because of its charge stability. Migration velocity of airborne particles under force fields.
Particle penetration data of electret filter are plotted
against particle diameter in Fig. 7.4.4 and compared diameters. For the uncharged fiber, collection effi-
with that of uncharged filter with the same structure. ciency of uncharged particle has a minimum at
For the three combinations of charged states of fiber 100 nm and increases with decreasing particle diam-
and particle there exist the most penetrating particle eter, showing that 100 nm is the most penetrating

411