U.S. patent application number 10/900868 was filed with the patent office on 2006-04-27 for system and method for nanoparticle and nanoagglomerate fluidization.
Invention is credited to Rajesh N. Dave, Guangliang Liu, Caroline H. Nam, Robert Pfeffer, Jose A. Quevedo, Qun Yu, Chao Zhu.
Application Number | 20060086834 10/900868 |
Document ID | / |
Family ID | 34278464 |
Filed Date | 2006-04-27 |
United States Patent
Application |
20060086834 |
Kind Code |
A1 |
Pfeffer; Robert ; et
al. |
April 27, 2006 |
System and method for nanoparticle and nanoagglomerate
fluidization
Abstract
With the coupling of an external field and aeration (or a flow
of another gas), nanoparticles can be smoothly and vigorously
fluidized. Multiple external fields and/or pre-treatment may be
employed with the fluidizing gas: sieving, magnetic assistance,
vibration, acoustic/sound or rotational/centrifugal forces. Any of
these forces, either alone or in combination, when coupled with a
fluidizing medium, provide excellent means for achieving homogenous
nanofluidization. The additional force(s) help to break channels as
well as provide enough energy to disrupt the strong interparticle
forces, thereby establishing an advantageous agglomerate size
distribution. Enhanced fluidization is reflected by at least one of
the following performance-related attributes: reduced levels of
bubbles within the fluidized system, reduced gas bypass relative to
the fluidized bed, smooth fluidization behavior, reduced
elutriation, a high level of bed expansion, reduced gas velocity
levels to achieve desired fluidization performance, and/or enhanced
control of agglomerate size/distribution. The fluidized
nanoparticles may be coated, surface-treated and/or
surface-modified in the fluidized state. In addition, the fluidized
nanoparticles may participate in a reaction, either as a reactant
or a catalyst, while in the fluidized state.
Inventors: |
Pfeffer; Robert; (Fort Lee,
NJ) ; Nam; Caroline H.; (Budd Lake, NJ) ;
Dave; Rajesh N.; (Short Hills, NJ) ; Liu;
Guangliang; (Wilmington, DE) ; Quevedo; Jose A.;
(Brick, NJ) ; Yu; Qun; (South Bound Brook, NJ)
; Zhu; Chao; (Edison, NJ) |
Correspondence
Address: |
McCARTER & ENGLISH, LLP;Attn: Anita Lomartra
City Place I
185 Asylum Street
Hartford
CT
06103
US
|
Family ID: |
34278464 |
Appl. No.: |
10/900868 |
Filed: |
July 27, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60490912 |
Jul 29, 2003 |
|
|
|
60568131 |
May 4, 2004 |
|
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Current U.S.
Class: |
241/5 |
Current CPC
Class: |
B01J 8/1872 20130101;
B01J 8/42 20130101; B82Y 15/00 20130101; B01F 11/0266 20130101;
B01F 13/0255 20130101; B01J 2208/00681 20130101; B01J 8/40
20130101; B01J 19/10 20130101; B01F 13/0809 20130101 |
Class at
Publication: |
241/005 |
International
Class: |
B02C 19/06 20060101
B02C019/06 |
Claims
1. A method for fluidizing nanoparticles comprising the steps of:
(a) providing a nanoparticle feedstock having an initial
agglomerate size distribution; (b) exposing said nanoparticle
feedstock to a flow of fluidizing gas and at least one additional
force or a pre-treatment selected from the group consisting of (i)
sieving, (ii) a vibration force; (iii) a magnetic force, (iv) an
acoustic force, (v) a rotational force, and (vi) a combination of
two or more of said forces; wherein exposure of said nanoparticle
feedstock to said flow of fluidizing gas and said at least one
additional force is effective to modify said initial agglomerate
size distribution from said initial agglomerate size distribution
to a second, reduced agglomerate size distribution; and (c)
establishing an expanded fluidized bed with said nanoparticle
feedstock in a substantially fluidized state, wherein the
agglomerate size distribution of said nanoparticle feedstock in
said fluidized state is in dynamic equilibrium and is substantially
equivalent to said second, reduced agglomerate size
distribution.
2. The method of claim 1, wherein said fluidizing gas is selected
from the group consisting of: air, nitrogen, helium, argon, oxygen
and mixtures thereof.
3. The method of claim 1, wherein said nanoparticle feedstock in
said fluidized state forms highly porous agglomerates in a size
range of about 50 microns to about 1000 microns.
4. The method of claim 1, further comprising a pre-screening step
wherein said nanoparticle feedstock is sieved to remove
nanoparticle agglomerates that exceed a predetermined threshold
size.
5. The method of claim 4, wherein said predetermined threshold size
is about 500 .mu.m.
6. The method of claim 1, wherein said at least one additional
force is sufficient to disrupt interparticle forces between
nanoparticle agglomerates, thereby reducing the initial particle
size distribution of said nanoparticle feedstock.
7. The method of claim 6, wherein said at least one force is a
magnetic force and said magnetic force is imparted by magnetic
particles that are independent of said nanoparticle feedstock.
8. The method of claim 7, wherein said magnetic particles are not
fluidized by said flow of fluidizing gas.
9. The method of claim 7, wherein said magnetic particles are
energized by a force of at least 100 Gauss.
10. The method of claim 6, wherein said at least one force is a
vibratory force.
11. The method of claim 10, wherein said vibratory force is
generated by vibrational energy of at least 1.5 g.
12. The method of claim 6, wherein said at least one force is an
acoustic force.
13. The method of claim 12, wherein said acoustic force is
generated by acoustic energy of at least 90 dB.
14. The method of claim 6, wherein said at least one force is a
rotational force.
15. The method of claim 14, wherein said rotational force is
generated by centrifugal forces of at least 5 g.
16. The method of claim 1, further comprising introducing a coating
material such that said coating material coats said nanoparticle
feedstock in said substantially fluidized state.
17. The method of claim 1, wherein said nanoparticle feedstock
includes a first reactant, and further comprising introducing at
least one additional reactant, such that a reaction occurs between
said first reactant and said at least one additional reactant when
said nanoparticle feedstock is in said substantially fluidized
state.
18. The method of claim 1, wherein said exposure of said
nanoparticle feedstock to said flow of fluidizing gas and said at
least one additional force or pre-treatment is effective to achieve
at least one of the following performance attributes: a reduction
in bubble level within the fluidized system, a reduction in gas
bypass relative to the fluidized bed, smooth fluidization behavior,
a reduction in elutriation, a high level of bed expansion, a
reduction in gas velocity levels to achieve a desired fluidization
performance, enhanced control of agglomerate size or agglomerate
distribution, and a combination of the foregoing performance
attributes.
19. An apparatus for use in fluidizing a nanoparticle feedstock,
comprising: at least one gas inlet, at least one distributor, a
fluidization chamber, and at least one vent; at least one ancillary
energy source communicating with said fluidization chamber, said at
least one ancillary energy source effective to provide sufficient
energy to a nanoparticle feedstock within said fluidization chamber
to reduce the agglomerate size distribution of said nanoparticle
feedstock by an amount effective to facilitate fluidization
thereof, said at least one ancillary energy source being selected
from the group consisting of: (i) a source of vibration force; (ii)
a source of magnetic force, (iii) a source of acoustic force, and
(iv) a source of rotational force.
20. The apparatus of claim 19, wherein said source of magnetic
force is an electric field generator coil operatively connected to
one or more electric power supplies surrounding a portion of said
fluidization chamber.
21. The apparatus of claim 19, wherein said fluidization chamber
has a substantially cylindrical geometry.
22. The apparatus of claim 19, wherein said at least one ancillary
energy source is a source of vibrational force.
23. The apparatus of claim 22, wherein said source of vibrational
force includes a mechanical, electromagnetic or piezoelectric
component that is caused to oscillate by an input current, voltage
or drive signal from a power amplifier to impart said vibrational
force.
24. The apparatus of claim 19, wherein said at least one ancillary
energy source is a source of magnetic force, and wherein said
source of magnetic force includes at least one magnetic coil
operatively connected to one or more magnetic field generators.
25. The apparatus of claim 24, wherein said one or more magnetic
field generators are positioned around at least a portion of said
fluidization chamber.
26. The apparatus of claim 24, further comprising magnetic
particles positioned within said fluidization chamber and wherein a
magnetic field generated by said one or more magnetic field
generators causes said magnetic particles to impart exciting force
within said fluidization chamber.
27. The apparatus of claim 26, wherein said magnetic particles are
substantially spherical and include a rough exterior surface.
28. The apparatus of claim 24, wherein said fluidization chamber
defines a plurality of spaced stages, and wherein magnetic
particles are positioned within each of said plurality of spaced
stages.
29. The apparatus of claim 28, wherein a first set of magnetic
particles are confined to a first spaced stage and wherein a second
set of magnetic particles are confined to a second spaced
stage.
30. The apparatus of claim 19, wherein said at least one ancillary
energy source is a source of acoustic force, and wherein said
source of acoustic force includes a function generator, an
amplifier and at least one loudspeaker.
31. The apparatus of claim 19, wherein said at least one ancillary
energy source is a source of rotational force, and wherein said
source of rotational force includes a motor for causing said
fluidization chamber to rotate around its axis or an angle offset
from said axis.
32. The apparatus of claim 31, wherein said motor is adapted to
impart variable rotational speed to said fluidization chamber.
33. The apparatus of claim 19, wherein said at least one ancillary
energy source is a source of vibrational force, and wherein said
source of vibrational force is positioned substantially below said
fluidization chamber and is adapted to impart axially oriented
vibrations to said fluidization chamber.
34. The apparatus of claim 33, wherein said source of vibrational
force is adapted to generate a vibrational force of at least 1.5 g
at a frequency of between about 20 Hz and about 200 Hz.
35. A method for mixing nanoparticles that comprises the steps of:
(a) introducing a first nanoparticle species into a fluidization
chamber; (b) introducing a second nanoparticle species into said
fluidization chamber; said first and second nanoparticle species to
a flow of fluidizing gas and at least one additional force or
pre-treatment selected from the group consisting of (i) sieving;
(ii) a vibration force; (iii) a magnetic force, (iv) an acoustic
force, (v) a rotational force; and (vi) a combination of two or
more of said forces, (c) establishing an expanded fluidized bed
with said first and second nanoparticle species in a substantially
fluidized state, wherein the agglomerate size distribution of said
first and second nanoparticle species in said fluidized state is in
dynamic equilibrium and is substantially equivalent to a reduced
agglomerate size distribution; (d) effecting mixing of said first
and second nanoparticle species within said substantially fluidized
state.
36. A method for treating nanoparticles comprising the steps of:
(a) providing a volume of nanoparticles having an initial
agglomerate size distribution; (b) introducing said volume of
nanoparticles to a fluidization chamber (c) exposing said volume of
nanoparticles to a flow of fluidizing gas and at least one
additional force or pre-treatment selected from the group
consisting of (i) sieving, (ii) a vibration force; (iii) a magnetic
force, (iv) an acoustic force, (v) a rotational force, and (vi) a
combination of two or more of said forces; wherein exposure of said
volume of nanop articles to said flow of fluidizing gas and said at
least one additional force or pre-treatment is effective to modify
said initial agglomerate size distribution from said initial
agglomerate size distribution to a second, reduced agglomerate size
distribution; (d) establishing an expanded fluidized bed with said
volume of nanoparticles in a substantially fluidized state, wherein
the agglomerate size distribution of said nanoparticles in said
fluidized state is in dynamic equilibrium and is substantially
equivalent to said second, reduced agglomerate size distribution;
and (e) effecting a treatment of said volume of nanoparticles in
said substantially fluidized state.
37. The method of claim 36, wherein said treatment includes coating
said volume of nanoparticles with a coating material introduced to
said fluidization chamber.
38. The method of claim 36, wherein said treatment includes
effecting a surface modification to said volume of nanoparticles in
said substantially fluidized state.
39. The method of claim 36, wherein said treatment includes
effecting a reaction between said volume of nanoparticles and an
additional reactant introduced to said fluidization chamber.
40. The method of claim 36, wherein said treatment includes a
chemical reaction, and wherein said volume of nanoparticles
functions as a catalyst for said chemical reaction.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims the benefit of the following
co-pending, commonly assigned provisional patent applications: (i)
"Vibrofluidization and Magnetically Assisted Fluidization of
Nanoparticles," filed on Jul. 29, 2003 and assigned Ser. No.
60/490,912, and (ii) "Nanoparticle and Nanoagglomerate Fluidization
System and Method," filed on May 4, 2004 and assigned Ser. No.
60/568,131. The contents of each of the foregoing provisional
patent applications are incorporated herein by reference.
BACKGROUND OF THE DISCLOSURE
[0002] 1. Technical Field
[0003] The present disclosure relates to system(s) and
method(s)/process(es) for fluidizing nanoparticles and
nanoagglomerates. More particularly, the present disclosure is
directed to systems and methods/processes for fluidizing
nanoparticles and nanoagglomerates utilizing a fluidizing gas with
one or more external forces, e.g., a vibration force, a magnetic
force, an acoustic force, a rotational force and combinations
thereof. Advantageous results are achieved, at least in part, by
establishing a desired nanoparticle/nanoagglomerate particle size
distribution within the system and substantially maintaining such
distribution as the system achieves and maintains a fluidized
state.
[0004] 2. Background of Related Art
[0005] Fluidization is a widely used process in several industries
to achieve continuous powder handling ability, particle mixing, and
desirable levels of solid-gas contact. By definition, gas
fluidization is a process in which solid particles are transformed
into a fluid-like state through suspension in a gas. Gas
fluidization is one of the best techniques available to disperse
and process powders belonging to the Geldart group A and B
classifications. Fluidization processes can be used to achieve high
heat and mass transfer and reaction rates. Gas fluidization of
small solid particles has been widely used in a variety of
industrial applications because of its unusual capability of
continuous powder handling, good mixing, large gas-solid contact
area and high rates of heat and mass transfer.
[0006] Extensive research has been done in the area of gas
fluidization, and the fluidization behavior of classical powders in
the size range of 30 to 1000 .mu.m (Geldart group A and B powders)
is relatively well understood. However, the fluidization behavior
of ultrafine particles, including nanoparticles which are in the
extreme low end of Group C particles (<20 microns) in Geldart's
Classification of Powders, is much more complex and has received
relatively little attention in the literature. Nanoparticles are
difficult to fluidize due to their strong interparticle forces. A
bed of nanosized silica, for example, will exhibit plug formation,
channeling, and/or spouting in a conventional fluidized bed. As far
as is known, fluidization of nanoparticles (which are three orders
of magnitude smaller than traditional group C powders) has
heretofore been extremely difficult, if not impossible, to
effectively achieve.
[0007] At least in part based on their very small primary particle
size and very large surface area per unit mass, nanostructured
materials are effective for the manufacture of drugs, cosmetics,
foods, plastics, catalysts, high-strength or corrosion resistant
materials, energetic and bio materials, and in mechatronics and
micro-electro-mechanical systems (MEMS). Based on such uses,
processing technologies which can handle large quantities of
nanosized particles, e.g., mixing, transporting, modifying the
surface properties (coating) and downstream processing of
nanoparticles to form nano-composites, are desirable. But before
processing of nanostructured materials can take place, the
nanosized particles have to be well dispersed.
[0008] Strong interparticle forces exist between nanoparticles,
such as van der Waals, electrostatic and moisture-induced surface
tension forces. Based on such forces, nanoparticles are found to be
in the form of large-sized agglomerates (rather than as individual
nano-sized particles) when packed together in a gaseous medium.
Hence, gas fluidization of nanoparticles generally refers to the
fluidization of nanoparticle agglomerates.
[0009] It is generally possible to fluidize nanoparticles as
relatively large agglomerates when the gas velocity exceeds the
expected minimum fluidization velocity of the agglomerates.
However, there tends to be significant powder loss and non-uniform
fluidization behavior. In addition, large agglomerates can form
near the distributor. Thus, there remains a need for a fluidization
process that minimizes or avoids powder loss and accomplishes a
smoother, more controlled fluidization with good mixing.
[0010] Previous studies of gas fluidization of nanoparticle
agglomerates have found that the minimum fluidization velocity is
relatively high (about several orders of magnitude higher than the
minimum fluidization velocity of primary nanoparticles). The size
of the fluidized nanoparticle agglomerates is typically from about
100 to 700 .mu.m, while the primary particle size ranges from 7 to
500 nm. A typical nanoparticle agglomerate size distribution (by
weight percentage) for a commercially available product
(Aerosil.RTM. R972 silica; Degussa; Dusseldorf, Germany) is shown
in FIG. 1. The data reflected in FIG. 1 was generated by: (i)
randomly sampling a storage bag of commercially available R972
silica (20.0 g), (ii) sieving the sample using ten (10) different
sieve sizes and measuring the weight retained on each such sieve,
(iii) recording sieve size and particle weight, and (iv)
calculating weight percentage for each sieve and plotting results.
As shown on FIG. 1, a typical agglomerate size distribution for a
commercially available nanoparticles products is widely dispersed
and includes a significant weight percentage at larger agglomerate
sizes.
[0011] For some nanoparticles, very smooth fluidization occurs with
extremely high bed expansion, practically no bubbles are observed,
and the velocity as a function of voidage around the fluidized
agglomerates obeys a modified Richardson-Zaki equation. This type
of fluidization of nanoparticle agglomerates has been termed
agglomerate particulate fluidization (APF) by Wang et al [See, Wang
et al., Fluidization and agglomerate structure of SiO.sub.2
nanoparticles, Powder Technology, 124 (2002) 152-159.8]. For other
nanoparticles, fluidization results in a very limited bed
expansion, and large bubbles rise up very quickly through the bed.
This type of fluidization has been termed agglomerate bubbling
fluidization (ABF) by Wang et al. However, even for the
homogeneously fluidized nanoparticles, relatively large powder
elutriation occurs at the high gas velocities required to fluidize
the nanoagglomerates. This loss of particles may hinder the
applicability of fluidization of nanoparticle agglomerates in
industrial processes.
[0012] In addition to conventional gravity-driven fluidization,
nanoparticle agglomerates can also be fluidized in a rotating or
centrifugal fluidized bed [See, Matsuda et al., Particle and bubble
behavior in ultrafine particle fluidization with high G,
Fluidization X, Eng. Found, 2001, 501-508; Matsuda et al., Modeling
for size reduction of agglomerates in nanoparticle fluidization,
AIChE 2002 Annual Meeting, Nov. 3-8, 2002, Indianapolis, Ind.,
138e], where the centrifugal force acting on the agglomerates can
be set much higher than gravity.
[0013] A number of studies dealing with modeling and numerical
simulation of the fluidization of nanoparticle agglomerates can be
found in the literature. These models are based either on force or
energy balances around individual agglomerates, the use of the
Richardson-Zaki equation, or a combination of the Richardson-Zaki
equation with fractal analysis for APF fluidization, or a modified
kinetic theory. Recently, some applications of nanoparticle
agglomerate fluidization were investigated, including the
production of carbon nanotubes, and its application to
photocatalytic NO.sub.x treatment. However, very little
experimental data on the fluidization characteristics and
differences between APF and ABF nanoparticles, such as minimum
fluidization velocity, agglomerate size, hysteresis effects, and
the effect of nanoparticle material properties, are available.
[0014] Sound waves, in combination with vibration, have been used
to increase fluidization quality in cohesive powders whose sizes
range from submicron to 20 microns. Also, vibration combined with
gas flow has been used to successfully fluidize particles of
smaller size, such as nanoparticles. However, notwithstanding the
benefits associated with these known fluidizing techniques, often a
dense immobile phase forms at a bottom of a fluidizing bed.
[0015] U.S. Pat. No. 4,720,025 to Tatevosian discloses a technique
that utilizes an alternating magnetic field along with magnetic
particles to loosen up material at the bottom of a hopper for
feeding into a certain operation. However, the disclosed technique
does not include loosening up cohesive materials for application in
a fluidized bed. Similarly, U.S. Pat. No. 6,471,096 to Dave
discloses the use of alternating magnetic field along with
permanent magnets to produce controllable discharge of cohesive
powders from a container, but does not provide for fluidization of
nano-powders. U.S. Pat. No. 3,848,363 to Lovness et al. discloses
the use of magnetic force to move particles in a predetermined
area, but again does not provide for any application to
fluidization.
[0016] The idea of using a magnetofluidized bed was proposed in
1960 by Fillipov [see, M. V. Filippov, The effect of a magnetic
field on a ferromagnetic particle suspension bed, Prik. Magnit.
Lat. SSR, 12 (1960) 215] and became popular as a means of
suppressing bubbles in gas fluidized beds for a variety of
industrial applications [see, R. E. Rosensweig, Process concepts
using field stabilized two-phase flow, J. of Electrostatics, 34
(1995)163-187]. Generally, the particles to be fluidized were
either magnetic particles or a mixture of magnetic and non-magnetic
particles, and the magnetic field was usually generated by DC
current [see; V. L. Ganzha, S. C. Saxena, Heat-transfer
characteristics of magnetofluidized beds of pure and admixtures of
magnetic and nonmagnetic particles, Int. Journal of Heat Mass
Transfer, 41(1998) 209-218; J. Arnaldos, J. Casal, A. Lucas, L.
Puigjamer, Magnetically stabilized fluidization: modeling and
application to mixtures, Powder Technology, 44(1985) 57-6224; W. Y.
Wu, A. Navada, S. C. Saxena, Hydrodynamic characteristics of a
magnetically stabilized air fluidized bed of an admixture of
magnetic and non-magnetic particles, Powder Technology, 90(1997)
39-46; W. Y. Wu, K. L. Smith, S. C. Saxena, Rheology of a
magnetically stabilized bed consisting of mixtures of magnetic and
non-magnetic particles, Powder Technology, 91(1997) 181-187; X. Lu,
H. Li, Fluidization of CaCo.sub.3 and Fe.sub.2O.sub.3 particle
mixtures in a transverse rotating magnetic field, Powder
Technology, 107(2000) 66-78], causing magnetic particles to form
chains along the field. For example, Arnaldos et al [see, J.
Arnaldos, J. Casal, A. Lucas, L. Puigjamer, Magnetically stabilized
fluidization: modeling and application to mixtures, Powder
Technology, 44(1985) 57-6224] studied the fluidization behavior of
a mixture of magnetic and non-magnetic particles of several hundred
microns in size, such as sintered nickel-silica, steel-copper and
steel-silica particles. The fluidization of larger particle
mixtures of millimeter size (Geldart group D particles), such as
iron-copper shot of 0.935 to 1.416 mm in diameter is described in
[W. Y. Wu, A. Navada, S. C. Saxena, Hydrodynamic characteristics of
a magnetically stabilized air fluidized bed of an admixture of
magnetic and non-magnetic particles, Powder Technology, 90(1997)
39-46] and [W. Y. Wu, K. L. Smith, S. C. Saxena, Rheology of a
magnetically stabilized bed consisting of mixtures of magnetic and
non-magnetic particles, Powder Technology, 91(1997) 181-187], and
Lu et al [X. Lu, H. Li, Fluidization of CaCo.sub.3 and
Fe.sub.2O.sub.3 particle mixtures in a transverse rotating magnetic
field, Powder Technology, 107(2000) 66-78] studied the fluidization
of very fine (Geldart group C) particle mixtures of
CaCO.sub.3--Fe.sub.2O.sub.3 in a transverse rotating magnetic
field. However, in all of these studies, the magnetic particles
were fluidized along with the non-magnetic particles.
[0017] Further, from studies at New Jersey Institute of Technology
(NJIT), it has been shown that a magnetically assisted impaction
coating (MAIC) process may be an effective method for providing the
extra force needed to break up the dense phase or layer of
particles. The MAIC process has been successfully used as a dry
coating method. The MAIC process utilizes an oscillating magnetic
field to accelerate magnetic particles thereby providing collisions
between particles and the walls of the apparatus. Each of the
foregoing techniques are directed to the use of a magnetic field
with magnets for accomplishing certain processes, but none of the
techniques are directed to fluidization of extreme Geldart C
particles, in particular, nano-powders.
[0018] At a low sound frequency, typically from 50 to 400 Hz, and a
high sound pressure level, typically above 110 dB, sound waves have
been shown to improve the fluidization of fine particles, which
otherwise showed intense channeling or slugging rather than
fluidization [Morse, Sonic energy in granular solid fluidization,
Ind. Eng. Chem., 47 (6) (1955) 1170-1175]. Standing waves are
generated in the experimental column and at a fixed sound pressure
level, sound assisted fluidization can only occur within a certain
range of low sound frequencies. Channeling has been found above and
below this frequency range [Russo et al., The influence of the
frequency of acoustic waves on sound-assisted fluidization of beds
offine particles, Powder Technology, 82 (1995) 219-230]. At the
natural frequency of the bed of micron sized particles, high
intensity sound waves have been found to lead to reductions in both
the minimum bubbling velocity and the minimum fluidization
velocities [Levy et al., Effect of an acoustic field on bubbling in
a gas fluidized bed, Powder Technology, 90 (1997) 53-57]. The
literature also shows that an increase in sound pressure level may
also yield a decrease in bed expansion, an increase in bubble
frequency and an increase in bubble size, and that high intensity
sound can also effectively reduce the elutriation of fine particles
[Chirone et al., Bubbling fluidization of a cohesive powder in an
acoustic field, Fluidization VII, 1992, 545-553]. To date, the
reported research has been directed to sound-assisted fluidization
of micron or sub-micron sized particles. No results have been
reported on the effects of sound on the fluidization of
nanoparticle agglomerates.
[0019] Thus, despite efforts to date, a need remains for systems
and methods/processes that provide for effective fluidization of
nanoparticles. A further need remains for systems and processes
that uniformly fluidize a bed of nanoparticles. Also needed are
systems and processes for nanoparticle fluidization that function
without forming a dense layer of agglomerates. Additionally,
fluidization systems and processes that minimize powder loss while
fluidizing nanoparticles are needed. It is a further need to
determine characteristics of nanoparticle agglomerates and to use
such characteristics in enhancing fluidization effectiveness.
SUMMARY OF THE DISCLOSURE
[0020] The present disclosure provides an improved system and
method/process for fluidizing nanoparticles and nanoagglomerates
that includes exposing nanoparticles and nanoagglomerates to a
combined flow of fluidizing gas and at least one additional force.
According to exemplary embodiments of the present disclosure, the
additional force may be supplied from a variety of sources and may
take a variety of forms, e.g., a vibration force, a magnetic force,
an acoustic force, a rotational/centrifugal force and/or a
combination thereof. The disclosed system and method utilizes a
fluidizing gas (e.g., air, N.sub.2, He, Ar, O.sub.2 and/or
combinations thereof or other fluidizing gas or gases) that may be
combined with an appropriate amount of magnetic energy, mechanical
energy, acoustic energy and/or rotational/centrifugal energy to
enhance fluidization by disrupting interparticle forces. The
nanoparticles/nanoagglomerates treated according to the disclosed
system/method can form highly porous agglomerates in the size range
of approximately 200-400 microns.
[0021] Enhanced fluidization of nanoagglomerate/nanoparticles
systems is achieved according to the systems and methods/processes
of the present disclosure, at least in part, by establishing a
desired nanoparticles/nanoagglomerates particle size distribution
within the system and substantially maintaining such distribution
as the system achieves and maintains a fluidized state. According
to exemplary embodiments of the present disclosure, a desired
particle size distribution is established by introducing an
external energy stimulus at a level effective to overcome the
inter-particle forces associated with
nanoparticles/nanoagglomerates systems and to thereby shift the
particle size distribution into a range that supports and/or
evidences enhanced fluidization. Alternatively, a desired particle
size distribution may be effected through a pre-treatment step,
e.g., a sieving step.
[0022] As used herein, "enhanced fluidization" is reflected by at
least one of the following performance-related attributes: reduced
levels of bubbles within the fluidized system, reduced gas bypass
relative to the fluidized bed, smooth fluidization behavior,
reduced elutriation, a high level of bed expansion, reduced gas
velocity levels to achieve desired fluidization performance, and/or
enhanced control of agglomerate size/distribution.
[0023] According to the present disclosure, modification of an
initial particle size distribution (e.g., an "as received" particle
size distribution) to a desired particle size distribution range
allows the disclosed fluidization system to achieve and maintain
desired fluidization conditions. Through introduction of an
external energy source and/or a pre-treatment step, as described in
greater detail herein, the disclosed fluidization system
advantageously establishes a state of dynamic equilibrium, wherein
nanoagglomerates are formed, broken and randomly reformed, in an
expanded fluidized bed. The dynamic equilibrium established
according to the disclosed system/method offers many advantages,
including facilitating substantially homogenous coating and/or
treatment of nanoparticles/nanoagglomerates. Exemplary fluidization
apparatus according to the present disclosure includes a gas supply
source and at least one energy source for generating and supplying
one or more of the energies disclosed herein, e.g., a vibrating
source, a source for inducing a magnetic field, an acoustic source,
and/or a source of centrifugal and/or rotational force. Other
features that may be associated with the fluidization apparatus of
the present disclosure include a gauge for measuring gas flow, a
fluidization chamber, a distributor, gas dispersion elements (e.g.,
glass beads), filter(s), viewing device(s) and/or a vent.
[0024] According to the present disclosure, advantageous results
are achieved in fluidizing nanoparticles and nanoagglomerates
across a broad range of applications, e.g., applications that
involve the manufacture of drugs, cosmetics, foods, plastics,
catalysts, high-strength or corrosion resistant materials,
energetic and bio materials, and in mechatronics and
micro-electro-mechanical systems. More particularly, effective
dispersion of nanoparticles and nanoagglomerates is achieved
according to the present disclosure, thereby facilitating a host of
nanoparticle-related processing regimens, e.g., mixing,
transporting, surface property modifications (e.g., coating),
and/or downstream processing to form nano-composites. In
particular, by combining or coupling the flow of a fluidizing gas
with one or more external forces, the combined effect is
advantageously sufficient to reliably and effectively fluidize a
chamber or bed of nanosized powders. That is, a bed may be expanded
to more than double its original chamber or bed height with hardly
any elutriation of the nanoparticles.
[0025] In addition, the system and method of the present disclosure
advantageously provides for greater control of the fluidizing
process, despite a high degree of mixing, thereby reducing powder
loss relative to conventional fluidized chambers or beds. In one
aspect of the present disclosure, for example, once the chamber or
bed has been expanded, the supply of energy or force, e.g.,
vibration, may be terminated (or reduced) so that the chamber or
bed remains expanded and fluidized for a considerable duration.
Thus, the supply of energy or force in accordance with one aspect
of the present disclosure may advantageously only be utilized
initially to aid in the break-up of interparticle forces and form
nanoagglomerates, so that the chamber or bed can be effectively
fluidized. Thereafter, such energy/force may be discontinued, as
desired by the system operator or applicable control systems.
[0026] Further, depending on the size distribution of the
nanoagglomerates, some powder beds, under flow of fluidizing gas
and external energy supply, e.g., vibration, may be divided into
two distinct regimes, a dense immobile phase and a smoothly
fluidized mobile phase above the dense immobile phase. The dense
immobile phase may be substantially eliminated according to the
present disclosure by adding heavy permanent magnetic particles to
the mix, preferably near the dense immobile phase, and then
exciting the magnetic particles via a magnetic field, e.g., an
oscillating magnetic field. The use of an external force, e.g.,
magnetic, acoustic, centrifugal/rotational and/or vibration
excitation forces, advantageously provides for better control of
the degree of particle movement. Combining such force(s) with
fluidizing gas flow advantageously achieves excellent mixing,
smooth fluidization, and high bed expansion with very little
particle loss in a safe and inexpensive manner.
[0027] The systems and methods of the present disclosure are
advantageously well suited for fluidization of nanoparticles
(extreme Geldart C powders), utilizing one or more external forces
and aeration (or a flow of another gas) to overcome fluidization
difficulties often associated with cohesive particles (e.g.,
channeling, spouting, plug formation) and to thereby advantageously
achieve vigorous fluidization in any of a variety of differently
shaped fluidization chambers or beds (e.g., tubular and/or
rectangular fluidization beds).
[0028] According to further aspects of the present disclosure,
fluidization characteristics of a variety of different
nanoparticles are provided and such fluidization characteristics
are advantageously correlated with macroscopic fluidization
behavior (APF or ABF) of the nanoagglomerates. To establish such
correlation, the properties of primary nanoparticles were
established in a conventional gravity-driven fluidized bed without
any additional external forces present. In addition, a simple and
effective method for estimating the average size of agglomerates
and bed voidage around the agglomerates is provided. The estimation
methodology can then be used in models to determine the minimum
fluidization velocity, pressure drop and other pertinent variables
of the fluidization process, and to determine the external force(s)
required to establish a desired particle size distribution to
achieve and support efficacious nanoparticle/nanoagglomerate
fluidization, as described herein.
[0029] These and other advantageous features and functionalities of
the disclosed fluidization system and method/process for fluidizing
nanoparticles will be apparent from the detailed description which
follows, particularly when read in conjunction with the figures
appended hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] To assist those having ordinary skill in the art to which
the subject matter of the present disclosure appertains in making
and using the disclosed fluidization systems and methods/processes,
reference is made to the appended figures, wherein:
[0031] FIG. 1 is a plot of particle size distribution for a
commercially available "as received" silica product;
[0032] FIG. 2 is a schematic drawing of an exemplary fluidization
apparatus in accordance with an illustrative aspect of the present
disclosure;
[0033] FIG. 3 is a plot showing bed height as a function of time
with variations in aeration and vibration conditions according to
an exemplary embodiment of the present disclosure;
[0034] FIGS. 4(a) and 4(b) are plots showing bed expansion ratios
as a function of time for different operating conditions according
to an exemplary embodiment of the present disclosure;
[0035] FIG. 5 is a plot showing pressure drop as a function of
superficial air velocity under specified operating conditions
according to an exemplary embodiment of the present disclosure;
[0036] FIGS. 6(a) and 6(b) are photographic images of fluidization
performance with and without the introduction of a magnetic
field;
[0037] FIG. 7 is a plot showing bed expansion ratio and pressure
drop for fluidization systems with and without magnetic
excitation;
[0038] FIG. 8 is a plot showing bed expansion ratio and pressure
drop for conventional fluidization of an 80/20 mixture before and
after magnetic processing;
[0039] FIG. 9 is a plot showing bed expansion ratio and pressure
drop for "soft" agglomerates with and without magnetic
excitation;
[0040] FIG. 10 is a plot showing bed expansion ratio and pressure
drop for "hard" agglomerates with and without magnetic
excitation;
[0041] FIG. 11 is a plot showing bed expansion ratio and pressure
drop for conventional fluidization of"hard" agglomerates before and
after magnetic processing;
[0042] FIG. 12 is a table showing minimum fluidization velocities
for "soft" agglomerates, "hard" agglomerates, and an 80/20 mixture
of hard/soft agglomerates;
[0043] FIGS. 13(a) and 13(b) are plots of particle size
distribution for "soft" agglomerates with and without magnetic
field application;
[0044] FIGS. 14(a) and 14(b) are plots showing bed expansion and
collapse for a soft agglomerate system with magnetic excitation
according to an exemplary embodiment of the present disclosure;
[0045] FIG. 15 is a table showing minimum fluidization velocities
and bed expansion ratios for "soft" agglomerates with different
mass ratios of magnets to nanoparticles;
[0046] FIG. 16 is a table showing minimum fluidization velocities
and bed expansion ratios for "soft" agglomerates with different
intensities of magnetic field;
[0047] FIG. 17 is a table showing minimum fluidization velocities
and bed expansion ratios for "soft" agglomerates with different
frequencies;
[0048] FIG. 18 provides a schematic diagram of an exemplary
sound-assisted fluidization system according to the present
disclosure;
[0049] FIGS. 19(a) and 19(b) provides images of bed behavior of
SiO.sub.2 nanoparticle agglomerates with and without sound
excitation, respectively;
[0050] FIG. 20 provides a plot of bed expansion relative to
superficial air velocity, with and without sound excitation,
according to an exemplary embodiment of the present disclosure;
[0051] FIG. 21 provides a plot of pressure drop relative to
superficial air velocity, with and without sound excitation,
according to an exemplary embodiment of the present disclosure;
[0052] FIG. 22 provides images of fluidization behavior at
different sound frequencies (300, 400, 500, 600 and 1000 Hz)
according to an exemplary embodiment of the present disclosure;
[0053] FIG. 23 provides a plot of dimensional bed height relative
to sound frequency according to an exemplary embodiment of the
present disclosure;
[0054] FIG. 24 provides a plot of dimensional bed height relative
to sound pressure level (dB) at two sound frequencies (100 and 400
Hz) according to an exemplary embodiment of the present
disclosure;
[0055] FIGS. 25(a) and 25(b) provide schematic diagrams of an
exemplary rotating fluidized bed system according to the present
disclosure;
[0056] FIG. 26 provides a plot of bed pressure drop relative to air
velocity for four (4) exemplary rotation speeds (indicated in terms
of equivalent gravity force, in G) according to an exemplary
embodiment of the present disclosure;
[0057] FIG. 27 provides a plot of bed height relative to air
velocity for four (4) exemplary rotation speeds according to an
exemplary embodiment of the present disclosure;
[0058] FIG. 28 provides a plot of pressure drop relative to air
velocity for four (4) exemplary rotation speeds according to an
exemplary embodiment of the present disclosure;
[0059] FIG. 29 provides a further plot of bed height relative to
air velocity for four (4) exemplary rotation speeds according to an
exemplary embodiment of the present disclosure;
[0060] FIG. 30 provides a further plot of pressure drop relative to
air velocity for four (4) exemplary rotation speeds according to an
exemplary embodiment of the present disclosure;
[0061] FIG. 31 provides an additional plot of bed height relative
to air velocity for four (4) exemplary rotation speeds according to
an exemplary embodiment of the present disclosure;
[0062] FIG. 32 provides a plot of fluidization velocity relative to
centrifugal force for three exemplary powder systems according to
the present disclosure;
[0063] FIG. 33 provides nanoparticle properties for a series of
powders in tabular form;
[0064] FIG. 34 provides fluidization characteristics of APF
nanoparticles in tabular form;
[0065] FIG. 35 provides fluidization characteristics of ABF
nanoparticles in tabular form;
[0066] FIG. 36 is an exemplary graphical representation of pressure
drop and bed expansion data as a function of velocity in accordance
with an illustrative aspect of the present disclosure;
[0067] FIGS. 37(a) and 37(b) are exemplary photographs showing a
fluidization bed with and without vibration, respectively, in
accordance with an illustrative aspect of the present
disclosure;
[0068] FIG. 38 provides a series of exemplary photographs showing a
progression of mixing during aerated vibrofluidization in
accordance with an illustrative aspect of the present disclosure;
and
[0069] FIG. 39 provides a series of exemplary photographs showing a
progression of mixing during magnetically assisted nanofluidization
in accordance with an illustrative aspect of the present
disclosure.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)
1. Fluidization of Nanoparticles Using Vibrational and/or Magnetic
Forces
[0070] According to an exemplary embodiment of the present
disclosure, homogeneous fluidization of nanoparticles is
advantageously achieved by coupling aeration with vibration.
Vibration (e.g., at frequencies in the range of 30 to 200 Hz, and
vibrational acceleration in the range of 0 to 5 g) has been found
to achieve smooth fluidization of nanoparticles. Through
introduction of vibrational energy, as described herein, the
nanoparticle/nanoagglomerate particle size distribution is
advantageously modified to and maintained in a distribution range
that supports and maintains efficacious fluidization. In exemplary
embodiments of the present disclosure, the minimum fluidization
velocity (defined as the lowest gas velocity at which the pressure
drop across the bed reaches a plateau) has been measured at
approximately 0.3-0.4 cm/s, and been essentially independent of
vibrational acceleration. Moreover, the bed expands almost
immediately after the air is introduced, reaching bed expansions of
three (3) times the initial bed height or higher. Hence, the bed
appeared to exhibit a fluid-like behavior at velocities much lower
than the minimum fluidization velocity. According to such exemplary
embodiments, fluidization of nanoparticles is achieved due to the
formation of stable, relatively large, and very porous agglomerates
and bubbles/elutriation of particles were essentially
non-existent.
[0071] Referring to the drawings and, in particular, FIG. 2, an
exemplary nanoparticle fluidization apparatus in accordance with an
illustrative aspect of the present invention is shown and generally
represented by reference numeral 1. Apparatus 1 essentially has a
gas supply source 2 suitable for supplying a fluidizing gas, a
vibration source 4 suitable for providing a mechanical force, and a
magnetic source 6 suitable for providing a magnetic force. Other
features that may preferably be associated with the apparatus 1
include a gas inlet 8, a gauge 10 for measuring gas flow, a
distributor 12, fluidization bed chamber 14, a vent 16 and/or an
accelerometer 18.
[0072] In a preferred aspect of the present invention, apparatus 1
is well suited for aerated vibrofluidization. In this aspect of the
invention, the fluidization bed chamber 14 may have a tube portion
15 of glass or any other suitable material, including, for example,
metal, plastic, or ceramic. The tube portion 15 has an inner
diameter that may preferably range from at least about one (1)
centimeter to several meters. If necessary, electrostatic charge
can be decreased, for example, via a DC-nozzle such as provided by
Tantec, Inc. A DC-nozzle can achieve static-neutralization by
ionizing the air flowing through the fluidization bed chamber 14 by
taking a voltage and transforming it into a high voltage. (Input
voltage: 120V AC+10-15%, 50/60 Hz, 3 ground plugs, Output Voltage:
12V DC, 300 mA). The fluidization bed chamber 14 may be mounted on
top of the vibration source 4. Optionally, acceleration may be
measured by the accelerometer 18 (e.g., a piezoelectric
accelerometer). Examples of fluidizing gases that may be utilized
in conjunction with this and other aspects of the present invention
include air, nitrogen, helium, oxygen, argon and/or other gases
suitable for fluidized bed chamber reaction.
[0073] In an aspect of the invention, the flow rate of the
fluidizing gas, which is preferably a dry compressed air, may be
measured with the gauge 10, such as, for example, by a rotameter or
alternatively, any other suitable flow measuring device. Depending
on the application and size of the fluidization bed chamber 14 and
the powder utilized, a typical flow rate may fall anywhere in the
range of about a fraction of a centimeter/second to several
centimeters/second. For instance, if the fluidization bed chamber
14 having a cylindrical bed of about 3 inches is used with 12 nm
silica particles (Aerosil.RTM. R972 silica), a velocity of 1
cm/sec. may preferably be employed to achieve vigorous fluidization
with high bed expansion.
[0074] In another aspect of the invention, vibrational parameters
(e.g., frequency, amplitude, and vibrational acceleration), which
may be controlled by an inverter, for example, can be varied to
achieve a desired effect on the degree of mixing and behavior of
fluidization. For instance, suitable values for frequency might
preferably range from about 20 to about 500 Hz; suitable values for
amplitude preferably range from about 0.001 to about 13.81 mm; and
suitable values for acceleration may preferably be as high as about
20 g's.
[0075] It follows from the foregoing that vibrational acceleration
or intensity may be defined as a ratio of vibration acceleration to
gravity acceleration: r=(A.omega..sup.2) /g where A=amplitude and
.omega.=2 nf. Still further, as appropriate and/or needed, pressure
drop may easily be measured by a manometer or a pressure
transducer, for example, and recorded either manually or
electronically via a computer. Operating efficacy may be monitored
and/or observed, as desired, by photographing the apparatus 1 with
a suitable camera, e.g., a digital camera to capture the behavior
of fluidization, such as smooth or bubbling.
[0076] In illustrative embodiments of the present disclosure
wherein fluidized nanoparticles were generated by aeration and
vibration, a fluidized bed consisting of a glass tube with an inner
diameter of 6.25 cm and height of 35 cm was employed. The fluidized
bed was equipped with a series of ports for sampling and pressure
measurements. The distributor consisted of a porous sintered metal
material. The bed was mounted on top of a Ling Dynamic System
vibrator, which can produce AC vertical sinusoidal waves with
accelerations up to 5.5 g (where g is the acceleration due to
gravity) measured by a piezoelectric accelerometer. The frequency
(f) of vibration could be varied from 30 to 200 Hz.
[0077] The powder used was Aerosil.RTM. R974 (Degussa) hydrophobic
silica having a primary particle size, particle density, bulk
density, and external surface area of 12 nm, 2200 kg/m.sup.3, 30
kg/m.sup.3, and 200 m.sup.2/g, respectively. These silica
nanoparticles were at the extreme end of Geldart's group C powder
classification. Humidity is an important issue when dealing with
powders (especially hydrophilic powders) because of liquid bridges
and electrostatic effects. However, in the experiments described
herein, hydrophobic silica was employed such that humidity did not
play as large a role, and bone dry compressed air was used as the
fluidizing gas. The airflow rate was measured by a rotameter.
[0078] When the bed was in its typical mode of aeration (i.e.,
evidencing undesirable fluidization behavior, such as channeling,
lifting as a plug, etc.), the vibration was turned on. Flow rate,
pressure drop, vibrational acceleration, frequency and bed height
measurements, as well as visual observation of the fluidization
behavior for each experiment, were all recorded. The vibration
intensity is defined as the ratio of vibrational acceleration to
gravitational acceleration, .GAMMA.=(A.omega..sup.2)/g, where A is
the amplitude of vibration and .omega.=27 .pi.f. The pressure drop
was measured by a pressure transducer and recorded on a computer.
Photos were taken with a digital camera.
[0079] Two methods of sampling were employed, both of which yielded
similar results when the powder samples were viewed with a scanning
electron microscope (SEM). The first involved aspirating out
samples at different heights of the bed through small openings
along the side of the tube. These samples were then gently placed
on SEM sample disks. The second method consisted of gently dipping
a SEM sample disk adhered with a double-side carbon tape into the
fluidized bed. The sample disk was then directly used for SEM
analysis. In addition, to aid in viewing agglomerates in situ, an
argon laser generator (Reliant 1000M, Laserphysics) with 3-watt
power and a high-speed CCD camera with an extremely short exposure
time were used. As described herein, it was observed experimentally
that mechanical vibration helped break up the channeling and
spouting in a bed of nano-sized powders. Considerably smaller gas
velocities (but still much larger than that based on the primary
nanoparticle size) were adequate in these experiments, because
vibration provided sufficient energy to the system to overcome
interparticle forces and form stable agglomerates.
[0080] Visual observation of a highly expanded bed revealed the
presence of two distinct layers: a small bottom layer consisting of
very large agglomerates and a larger top layer consisting of very
smoothly fluidized smaller agglomerates. SEM micrographs indicated
that the fluidized agglomerates in the top layer ranged in size
from approximately 5 to 50 .mu.m. The bottom layer consisted of
agglomerates that were measured to be as large as 2 mm. When the
two layers were separated by aspirating out the smoothly fluidized
agglomerates, and this top portion was reused as the next bed, the
bottom dense layer did not reappear. This suggests that the dense
layer simply contained the hard agglomerates, which were present in
the as-received nanoparticles; such hard agglomerates could have
formed during handling and storage. Under vibration, these large
agglomerates would sink to the bottom of the bed since the
vibration energy was not sufficient to break them up and the
airflow was not large enough to fluidize them. In order to avoid a
large agglomerate size distribution, only the top portion of the
bed (smooth layer) was used in all of our experiments described
below.
[0081] A Beckman Coulter counter (dry module) was used to determine
the agglomerate size distribution of the as-received silica powder.
Representative Coulter counter results for pre-experiment powder
indicated a mean agglomerate size of about 30-40 .mu.m. This is
highly suspect since large agglomerates of size on the order of
millimeters (perhaps formed during storage) could be observed
visually. These contradictory results suggest that the agglomerates
are in general so fragile that any measurement method involving
direct contact with the sample is not effective and reliable. It is
believed that the agglomerates were broken up during the course of
Coulter counter size distribution measurements, leading to
agglomerate sizes of about 30-40 .mu.m.
[0082] As mentioned above, agglomerate samples were aspirated out
of the bed at different heights of the expanded fluidized bed and
examined under SEM. The agglomerate sizes averaged approximately 30
.mu.m. However, the agglomerates appeared very porous and fragile,
and might have broken during their removal from the bed and/or
during sample preparation for the SEM. The agglomerate size
estimated from pressure drop and bed height data in fluidization
experiments was considerably larger (.about.160 .mu.m). Given the
fragile nature of the agglomerates, it is reasonable to expect that
an equilibrium between agglomerate breakage and agglomerate
formation is reached during the process of fluidization. Therefore,
the true agglomerate size can only be found from measuring the
agglomerates dynamically as fluidization is occurring. The use of a
high-speed digital camera with an extremely short exposure time and
a laser beam may be effective to estimate the dynamic agglomerate
size in situ.
[0083] Although the bed was not initially (before application of
vibration) fluidizable with aeration alone, the bed appeared to
have a short-term memory after vibration was applied. This memory
effect was apparent in an experiment where the bed was first fully
fluidized with vibration and aeration, and then was allowed to
settle down by turning off the vibration and aeration. This settled
bed could then be fluidized by aeration alone as long as it was
done within a few minutes, which is contrary to expectations given
the Geldart Group C character of the primary particles. Thus, once
the bed was fluidized with introduction of vibrational force, the
interparticle networks in the original sample were disrupted and
the resulting agglomerates did not form strong cohesive networks
for several minutes, even after the bed was allowed to settle.
However, if the bed was left longer than a few minutes in its rest
state, it became difficult to fluidize the bed.
[0084] Additionally, once the bed was fluidized with the aid of
vibration and aeration, the vibration could be turned off and the
bed would remain expanded and fluidized for a considerable amount
of time (approximately 30 hours). FIG. 3 shows a comparison between
the settling of a fully expanded bed after (a) aeration was left on
and vibration was turned off, and (b) both aeration and vibration
were turned off. Without both vibration and aeration, the bed
collapsed to its initial height within two (2) minutes. Based on
these experimental observations, it appears that once the
interparticle forces are disrupted, it takes a finite time to
return to the original undisturbed conditions.
[0085] In these experimental studies, the bed of nanoparticles,
when aerated without vibration, exhibited plug flow, channeling,
and spouting. When airflow was coupled with sufficient vibration
(so that .GAMMA.=(A.omega..sup.2)/g>1), the immobile bed would
almost immediately begin to expand. More particularly, the channels
would close, the spouting would stop, and/or the solid plug would
break up. Increasing vibrational intensity, .GAMMA., weakly
affected bed height. The same phenomenon was also seen when only
the top portion of the bed was used. At high vibration frequencies
(f>100 Hz) and airflow rate, relatively large bubbles could be
seen. At low frequencies (<50 Hz), many of the bubbles appeared
to break and dissipate throughout the bed forming microbubbles
(estimated to be about 200 .mu.m). Bubbles were not seen to
coalesce, grow or break the upper surface of the bed.
[0086] At modest fluidization gas velocities, the surface of the
bed was very smooth, there was no apparent disturbance from bubbles
and practically no elutriation of particles was observed. At higher
gas velocities (>2 cm/s), the surface became unstable and
elutriation of particles out of the tube could be observed. FIG.
4(a) shows bed expansion rate at different .GAMMA. at a constant
frequency of 50 Hz and constant superficial air velocity of 0.28
cm/s. In each experiment, the vibrational parameters were first set
at the desired conditions, and then the aeration was turned on (at
time t=0) at the desired superficial velocity. The steady state bed
expansion increased with increasing .GAMMA., but appeared to become
independent of .GAMMA. at sufficiently large values of .GAMMA.. In
this series of experiments, the vibrational intensity was varied by
changing the amplitude (A), while holding the frequency of
vibration constant. This bed expansion behavior may be rationalized
as follows: as the vibrational intensity was increased, the size of
the agglomerate decreased at first and then became roughly
independent of .GAMMA., i.e., reached a state of dynamic
equilibrium.
[0087] The scaled acceleration .GAMMA. was not the only vibrational
parameter affecting steady state bed expansion. FIG. 4(b)
illustrates that the steady state bed expansion, at a constant
superficial air velocity of 0.28 cm/s, depended on the frequency of
vibration, even when .GAMMA. was maintained constant; however, no
systematic trend was manifest. It was found that at higher values
of .GAMMA., the effect of vibration frequency on the steady state
bed expansion decreased. It is clear from FIGS. 4(a) and 4(b) that
at least two dimensionless groups involving A and .omega. would be
needed to capture the effect of vibration on fluidization
behavior.
[0088] It is also clear from FIGS. 4(a) and 4(b) that the rate at
which the bed expanded depended on the vibrational parameters. The
higher the frequency or the lower the .GAMMA., the slower the bed
expanded. The rate of bed expansion was roughly the same for
.GAMMA.=4-6, but appreciably smaller at .GAMMA.=3 (see FIG. 4(a)).
As seen in FIG. 4(b), the rates of bed expansion at frequencies of
50, 70 and 100 Hz were comparable, while those at 30 and 150 Hz
suggest an inverse dependence on the frequency.
[0089] In all of the experiments performed, the measured pressure
drop across the bed at high gas velocities approximately equaled
the weight of the bed per unit cross sectional area. FIG. 5 shows a
typical set of results obtained in a vibrated fluidized bed of
silica nanopowder, where both the pressure drop across the bed and
the bed expansion at increasing gas velocities are presented. The
pressure drop has been scaled with the actual measured weight of
the bed per unit cross sectional area of the bed, while the bed
height has been scaled with the height of the settled bed. It is
clear from FIG. 5 that the pressure drop increased initially with
gas velocity and then leveled off at high gas velocities. In the
plateau region, the scaled pressure drop is very close to the
expected value of unity. A lower measured pressure drop than the
weight of the bed could be due either to a loss of powder sticking
to the wall, powder elutriation, or possibly to some
non-uniformities in the gas flow due to the relatively porous
distributor that was used in the experiments. On the other hand,
wall friction (Loezos et al., 2002) and cohesion between the bed of
particles with a layer of particles adhering tightly to the
distributor (Castellanos et al., 1999; Sundaresan, 2003) would
result in a higher measured pressure drop than the weight of the
bed. Our studies have revealed only a weak effect of vibrational
parameters on the constant (plateau) pressure drop obtained at high
gas velocities. Thus, there is no clear consensus on the effect of
vibration on pressure drop across the bed.
[0090] FIG. 5 also shows that bed expansion behavior in an
exemplary system according to the present disclosure was different
than that observed with Geldart group A particles where bed
expansion begins only after the minimum fluidization velocity is
exceeded. As soon as a vibrofluidized bed (with .GAMMA.>1) was
aerated, it began to expand even though the actual gas phase
pressure drop was only a fraction of the bed weight per unit cross
sectional area. As gas flow rate was increased, the bed continued
to expand and this was accompanied by a systematic increase in the
gas phase pressure drop. The bed expansion continued into the
constant pressure drop regime. The overall bed expansion could be
in excess of five times the original height, and even at such
dramatic bed expansion levels the quality of fluidization appeared
to be smooth.
[0091] Based on an agglomerate size of about 50 microns, the
Reynolds number is less than 1. A number of studies (Mawatari et
al., 2002; Noda et al., 1998; Tasirin et al., 2001; Erdesz et al.,
1986) have found that, as the vibration intensity, .GAMMA., is
increased, the minimum fluidization velocity is decreased. Here,
minimum fluidization velocity refers to the lowest gas velocity for
which the pressure drop across the bed becomes constant. However,
in experiments according to the present disclosure (with
.GAMMA.>1), frequency and other vibrational parameters had only
a small effect on the minimum fluidization velocity, and this
effect became unobservable as .GAMMA. was increased.
[0092] In the exemplary experimental studies of the present
disclosure, the minimum fluidization velocity (based on the
definition above) was determined to be around 0.3-0.4 cm/s (see
FIG. 5). However, it is noted that the bed exhibited fluid-like
properties as soon as it started to expand at velocities as low as
0.1 cm/s. Such a minimum fluidization velocity cannot be obtained
empirically based on the primary particle size, which demonstrates
unequivocally that the disclosed system is only fluidizing
agglomerates.
[0093] Since agglomerates are being fluidized according to
exemplary embodiments of the present disclosure, it is valuable to
identify the manner in which voidage is defined. According to the
present disclosure, voidage, .epsilon., is defined as the fraction
of the total bed volume occupied by the fluid. Using 0.03
g/cm.sup.3 and 2.2 g/cm.sup.3 for the bulk density of a settled bed
and primary nanoparticles density, respectively, it is possible to
calculate that .epsilon..about.0.9864. Thus, the bed of
nanoparticles is already highly fluffy even before fluidization. As
the bed expands, .epsilon. increases to above 0.99. The agglomerate
themselves were very porous.
[0094] In a further experimental study according to the present
disclosure, a small amount of silica was dyed blue with methylene
blue for mixing/tracer experiments. In such experimental study, the
progression of mixing of a small layer of blue particles placed at
the top of the vibrated fluidized bed was observed. Within a few
minutes, the entire bed turned blue and appeared well mixed, even
though aeration was applied at a level well below the minimum
fluidization conditions. This result clearly showed that even in
the region where the gas phase pressure drop was considerably
smaller than the bed weight per unit cross sectional area, active
mixing of the agglomerates occurred.
[0095] Preliminary SEM and EDX analyses showed that the
agglomerates were well mixed, at least on the agglomerate level,
indicating that vibrofluidization could be used to mix agglomerates
of different species of nanoparticles. For example, nano-silica has
been effectively mixed with nano-titania and nano-molybdenum oxide.
It is not known if the agglomerates retained their integrity during
fluidization or if they broke and formed again rapidly; in the
former case, one would achieve little mixing at the nano-scale.
However, if mixing were indeed observed on a scale smaller than the
agglomerate size, it would be indicative of fragile agglomerates,
which broke and formed repeatedly in a vibrofluidized bed.
[0096] Thus, according to the preceding experimental studies, it
has been demonstrated that nanosized silica could be easily and
smoothly fluidized in the form of stable, very porous agglomerates
with negligible elutriation with the aid of vibration and aeration.
Since the bed remained fluidized for a considerable amount of time
with only air flow after vibration was turned off, vibration
appeared to be necessary only initially to disrupt interparticle
network and establish a desired particle size distribution, after
which aeration was sufficient to sustain the bed in a fluidized and
expanded state for an extended period of time, i.e., a dynamic
equilibrium was established. The mixing studies described above
show that the application of vibrofluidization of nanoparticles to
mix different nanoparticles together to form nanocomposites also
yields promising and advantageous results.
[0097] In another preferred aspect of the present invention, the
apparatus 1 may be well suited for aerated-magnetically assisted
fluidization. In this aspect of the invention, the apparatus 1 may
preferably be substantially similar to that previously identified
and/or described. However, the vibration source 4 may preferably be
either replaced by or supplemented with the magnetic source 6
preferably having one or more magnetic elements or particles, such
as, for example, barium-ferrite polyurethane coated magnets. Other
magnetic particles may also be used whose sizes range from about
0.5 to about 5.0 mm. The magnetic source 6 also preferably has one
or more magnetic field generators preferably surrounding a base
portion 17 of the fluidization bed chamber 14.
[0098] In this aspect of the invention, the energy dissipated from
the collisions and/or spins of the magnetic particles due to
interaction with a magnetic field induced by the magnetic field
generators may be utilized to facilitate effective fluidization of
nanoparticles. Further, utilizing different loads of the magnetic
particles may be an effective way to affect the energy inputted
into the fluidization bed chamber 14. That is, the more magnetic
particles used, the greater the energy provided. The magnetic field
may also be varied in order to change the energy input. The
magnetic field may, in one aspect of the invention, be induced via
a copper coil, for example, to induce an oscillating magnetic field
strength of approximately 40 mT.
[0099] Typical magnetic particles may comprise hard barium ferrite
(BaO.6 Fe.sub.20.sub.3), AlNiCo, rare earth metals, ceramics or
various mixtures thereof. Such magnetic particles preferably have
coercivities ranging from about 200 to about 3000 oersteds. In
order to minimize the attrition of the magnetic particles and the
attrition by them on the container walls and screen, it may be
preferable to provide a soft coating over the magnetic particles.
For example, the magnetic particles may be coated with polymeric
materials such as, for example, cured epoxy or
polytetrafluoethylene, to smooth the surface and make the magnetic
particles more durable and resistant to wear. The magnetic
particles may also be comprised of magnetic powder embedded in a
polymeric matrix, such as barium ferrite embedded in sulfur cured
nitrile rubber such as ground pieces of PLASTIFORM.TM. Bonded
Magnets, available from Arnold Engineering Co., Norfolk, Nebr. The
size of the magnetic particles may vary from about ten times to
about thousand times the size of the powder material to be
fluidized. The appropriate size of the magnetic particles may
depend on and/or be based on the type of application, the density
of the powder material, and/or the cohesive strength of the powder
material. The appropriate size of the magnetic particles may be
readily determined by one skilled in the pertinent art. The shape
of the magnetic particles may also vary, and may be spherical,
elongated, irregular or other suitable shape.
[0100] The quantity of the magnetic particles required may be
dependent on the quantity of the powder material to be moved, the
bulk density of the particular powder material, the cohesiveness of
the particular powder material, and/or environmental factors such
as moisture, temperature, or time of consolidation. Preferably,
only that quantity of magnetic particles needed to cause the powder
material near the container outlet zone to move and/or flow may be
used. In general, the weight of the magnetic particles should be
approximately equal to the weight of the powder material near the
outlet zone, for example, if a conical bottomed hopper is used, the
weight of the magnetic particles should be approximately equal to
the weight of the powder material in the lower half of the conical
section. However, the amount or weight of magnetic material may be
less or more depending upon the nature of application.
[0101] The magnetic field generator(s) may preferably be supplied
with power by means of oscillators, oscillator/amplifier
combinations, solid-state pulsating devices and/or motor
generators. A magnetic field may preferably be generated by means
of a solenoid coil, an air core or laminated metal cores, and/or
stator devices or the like. Further, the in a preferred aspect of
the present invention, the magnetic field generator(s) may have one
or more AC motor stators, i.e., motors preferably with armatures
removed, which may be powered by an alternating current supply
through variable output transformers. In addition, metal strips may
be placed outside the magnetic field generator(s) to preferably
confine the magnetic field to a specific volume of space. The
magnetic field preferably oscillates either by changing the value
in a sinusoidal fashion but keeping the direction the same, or by
changing the direction of the field itself, so that the field
rotates. The oscillating magnetic field can be caused, for example,
by using multiphase stators to create a rotating magnetic field, as
disclosed in U.S. Pat. No. 3,848,363 to Loveness, or by using a
single phase magnetic field generator with an AC power supply at a
specified frequency to create a bipolar oscillating magnetic field.
In highly cohesive powder materials, a rotating field is preferred
because the magnetic particles do not have a possibility of not
being moved due to having an alignment with the direction of the
field as in a bipolar field.
[0102] A useful magnetic field is preferably one with an intensity
sufficient to cause desirable motion and excitation of the magnetic
particles, but not large enough to demagnetize the magnetic
character of magnetic particles that are moved by the oscillating
magnetic fields. The magnetic field intensity may range between
about 1 oersted and about 3000 oersteds, preferably between about
200 and about 2500 oersteds.
[0103] An important characteristic of the magnetic field may be
defined by the frequency of oscillations. The frequency of
oscillations in the oscillating magnetic field affects the movement
and subsequently the number of collisions that take place between a
magnetic element preferably moved in the magnetic field and the
surrounding powder material/particles preferably caused to move
and/or to be fluidized. If the oscillating frequency is too low,
the magnetic particles may move too slowly and may not have
sufficient motion to cause the other powder material to flow. If
the oscillating frequency is too high, the magnetic particles may
not be able to spin in the fast changing field due to their
inertia. The frequency may be from about 5 hertz to about 100,000
hertz, preferably from about 50 hertz to about 1000 hertz, and even
more conveniently at the hertz which is commonly used in AC power
supplies (i.e., 50 hertz, 60 hertz, and/or 400 hertz).
[0104] Having identified and/or described some of the various
aspects associated with this exemplary aspect of the present
invention, different methods of preparation and processing
applications are now discussed.
[0105] Depending on the bed chamber size, a measured amount of
nano-sized powders may be carefully placed in the bed chamber 14
preferably above the distributor 12. The bed chamber 14 may of any
suitable shape or configuration (e.g., tubular (3D) or rectangular
(2D)) and may preferably be placed vertically in operative
association with the equipment (i.e., the vibration source 4 and/or
the magnetic source 6). The distributor 12 may be made of several
materials and take a variety of different forms. For example, the
distributor 12 may be a sintered metal disk, a ceramic porous
plate, or simple wire meshes or clothes, all with apertures
preferably small enough (usually less than about 40 microns) to
distribute the fluidizing medium as evenly as possible. Highly
cohesive nanoparticles may not appreciably fall through the
distributor 12. The top of the bed chamber 14 may be sealed with a
cap and hose or tube, for example, leading to the vent 16 in case
of any powder elutriation, which may occur at relatively high
velocities. Once the bed chamber 14 is operatively connected to the
vibration source 4 and/or the magnetic source 6, the vibration
and/or magnetic field may be set at the desired settings (e.g.,
acceleration, frequency, etc.). Preferably, when the bed chamber 14
operatively cooperates with the vibration source 4 and/or with the
magnetic source 6 at the desired parameters, an air flow may be
slowly turned on. One may verify effective fluidization using bed
chamber expansion and pressure differentials. That is, not only
does the bed chamber expand to provide a good indicator of
fluidization, but the pressure drop may also be a good indicator
when it equals the weight of the bed chamber per unit area.
Pressure taps may be drilled into the bed chamber at various
desired heights thereof so that pressure drops across different
places of the bed chamber may be obtained or quantified. One should
remember that if the measured pressure drop includes the
distributor 12, the pressure difference of the distributor 12 must
be subtracted from the total pressure drop recorded to obtain the
drop across the powder bed. If a dense layer forms at the bottom of
the bed chamber 14 near the distributor 12, the top portion of the
powder bed may be elutriated or physically taken out with an
aspirator, for example, for later use in other applications (e.g.,
experiments without another dense layer forming). During successful
fluidization in accordance with one or more aspects of the present
invention, samples may be taken for testing and analysis with SEM,
EDX, TEM, EELS, etc. Average size, an overall mapping of the
composition, and/or the degree of mixing may be obtained using such
techniques. It is noted that if the powder material used is
energetic, different and appropriate means of analysis may be used
and extra caution should be taken when using energetic materials
for fluidization. For example, an electrostatic charge may be
significantly decreased using a DC nozzle that can ionize the
fluidizing medium (e.g., air). Nonetheless, energetic samples such
as, for example, nano-aluminum and nano-sized molybdenum oxide
(MIC) may also be fluidized and well mixed in accordance with one
or more aspects of the present invention.
[0106] This system is applicable at temperatures that range from
about -100 degree C. to about 2000 degree C. and pressures that
range from about 0.2 bars to about 2000 bars. The temperature
and/or pressure may be limited mainly by the particular material
being fluidized and/or the materials used in constructing the
apparatus 1. Preferably, ambient temperatures and/or pressures
(e.g., room temperature and/or atmospheric pressure) may be
utilized. Humidity should preferably be regulated so that moisture
may be kept to a minimum. The presence of moisture may affect
agglomeration of the powder material, although some humidity may be
helpful to minimize electrostatic charges. After the fluidization
process is completed, the powder material may preferably be
collected in a clean container.
[0107] Additional experimental studies directed to the introduction
of magnetic forces using an oscillating AC magnetic field to excite
relatively large (mm size) magnetized particles mixed in with
nanoparticles agglomerates to effect fluidization are further
described herein. As demonstrated by such experimental studies,
with the aid of an oscillating at low frequencies, the bed of
nanoparticle agglomerates can be smoothly fluidized, and the
minimum fluidization velocity may be significantly reduced. In
addition, channeling or slugging of the bed disappears and the bed
expands uniformly without bubbles and with negligible elutriation.
The bed expansion and the minimum fluidization velocity depend on
the mass ratio of magnetic particles to nanoparticles, and the
intensity and frequency of the oscillating magnetic field. The
effects of the intensity and frequency of the oscillating magnetic
field and the weight ratio of magnets to non-magnetic nanoparticles
are described herein, as well as important fluidization parameters
(such as the minimum fluidization velocity, pressure drop across
the bed, and bed expansion) in such systems are demonstrated.
Unlike traditional magneto-fluidized beds, the magnetic particles
used according to the present disclosure are permanent magnets,
which furiously spin and create intense shear and agitations under
an oscillating magnetic field.
[0108] The experimental system utilized herein consisted of a
fluidized bed of nanoparticle agglomerates, an oscillating
electromagnetic field and a visualization apparatus. The fluidized
bed was a vertical transparent column with a distributor at the
bottom. The column was a section of acrylic pipe with an inner
diameter of 57 mm and a height of 910 mm. The distributor was a
sintered metal plate of stainless steel with a thickness of 2 mm
and pore size of 20 .mu.m. To generate a uniform gas field before
the distributor, glass beads of diameter between 2.5 and 3.5 mm
were charged into a chamber placed below the distributor and above
the gas inlet to form a packed bed about 100 mm high. An ultra-fine
mesh filter was located at the gas outlet to filter out any
elutriated nanoparticle agglomerates.
[0109] The fluidization behavior was visualized with the aid of a
lighting device (Illumination Technologies, Model 150SX) and
recorded by a digital camcorder (Sony, Digital 8). The magnetic
particles were barium ferrite (BaO-6Fez03) coated with polyurethane
(supplied by Aveka, USA), about 1.0-3.0 mm in size. These were
permanent magnetic particles, which were recharged by contacting
them with a strong permanent magnet before each experiment and were
then added to the bed of nanoparticles at a prescribed mass ratio.
The shafts of two 1/20 HP electric motors (Dayton 5M064B) were
removed and the electromagnetic coils were placed opposite one
another around the lower part of the vertical transparent column by
mounting them on an acrylic plate which holds the distributor. The
coils were driven by an alternating current generated by a power
supply and were capable of generating an oscillating magnetic field
with an intensity up to 140 Gauss at the center of the coil. The
power supply (Triathlon Precision AC Source) was rated to supply AC
current with adjustable frequency and voltage. A strong cooling fan
(Comair Rotron TNE2A) was used to prevent the coils from
overheating.
[0110] Fumed SiO.sub.2 nanoparticles (Degussa Aerosil@ R974) with a
primary particle size of 12 nm and a bulk density of about 30
kg/m.sup.3 were used in these experimental studies. Due to surface
treatment by the manufacturer, the nanoparticles were hydrophobic.
Before the experiments, the particles were sieved using a shaker
(Octagon 2000) and a 35-mesh sieve opening (about 500 .mu.m). The
sieving process functioned as a "pre-treatment" step with respect
to the nanoparticle feedstock and served to separate very large
agglomerates, which may have been generated during packing,
storage, and transportation. The selection of a mesh opening of 500
.mu.m was based on previous experimental findings that the typical
size of fluidized nanoparticle agglomerates is between 100 to 400
.mu.m. The size range of the fluidized nanoparticle agglomerates
was measured by analyzing digital images of the fluidized
agglomerates with the help of a laser source (Laser Physics Reliant
1000m), a CCD camera (LaVision FlowMaster 3S), and an image
processing system (Dual Xeon CPU).
[0111] For purposes of the present disclosure, the smaller
nanoagglomerates that pass through the openings of the 500 .mu.m
sieve are designated as "soft" and the larger agglomerates, from
about 500 .mu.m to more than 10 mm are designated as "hard". These
two different sized agglomerates and a "mixture" consisting of 80%
soft agglomerates and 20% hard agglomerates by weight (80/20) were
selected to conduct the fluidization experiments described
herein.
[0112] To minimize any effect of humidity on the fluidization
experiments, pure nitrogen from a compressed N.sub.2 tank was used
as the fluidizing gas. The gas flow rate was measured and adjusted
by two calibrated rotameters (Gilmont) with a combined flow rate
range of up to 51.0 liters per minute. The pressure drop across the
bed was measured with a differential pressure transmitter
(Cole-Parmer) with a measurement range of up to 1.0 inch of water;
the lower pressure tap was placed slightly above the distributor
(approximately 3 mm), so that it was not necessary to measure the
pressure drop across the distributor. A Gaussmeter (Walker
Scientific Inc. MG-3A) with a range of from 1 to 10.sup.4 G was
used to measure the intensity of the oscillating magnetic field,
which was measured at the center point between the coils in the
empty column (before charging the nanoparticles into the bed).
[0113] We have found that, even when using the same nanoparticles,
if the experiments are run with agglomerates of different sizes,
the bed shows very different fluidization behavior. For example,
the soft R974 agglomerates fluidize smoothly with large bed
expansion (APF) at a low minimum fluidization velocity of 0.23
cm/s. Here, we define the minimum fluidization velocity as the gas
superficial velocity beyond which the bed pressure drop is no
longer dependent upon the gas velocity and becomes constant, and a
relatively large bed expansion (typically 2 or more times the
initial bed height) occurs. A mixture consisting of 80% soft
agglomerates and 20% hard agglomerates (80/20) also behaves as APF,
but the minimum fluidization velocity is much higher (5.67 cm/s)
than that of the soft agglomerates. However, the hard R974
agglomerates do not fluidize at all, even at a gas velocity as high
as 13.2 cm/s. At this high gas velocity, significant particle
elutriation was observed, and the fluidization experiment had to be
interrupted to avoid large losses of nanoparticles.
[0114] Typical fluidization behavior of the 80/20 mixture of
SiO.sub.2 nanoparticle agglomerates with and without the external
oscillating magnetic excitation are shown in the photographic
images of FIGS. 6(a) and 6(b), respectively. Without the external
oscillating magnetic excitation, at a superficial gas velocity of
0.65 cm/s (FIG. 6(a)), the nanoparticle agglomerates are first
lifted as a plug and then the plug disintegrates to form
undesirable, stable channels through which the gas passes; the bed
expands slightly with an uneven surface and the pressure drop is
much less than the bed weight, indicating that the nanoagglomerate
bed is not fluidized.
[0115] However, if a sufficiently strong oscillating magnetic field
is applied, the magnetic particles are set in motion (translation
and rotation) and the nanoparticle agglomerates are fragmented into
smaller agglomerates because of collisions with the magnets, the
vessel wall, and the distributor. After a few minutes, the particle
size distribution of the nanoparticle agglomerates are brought into
a desirable range, the channels disappear, and the bed begins to
expand slowly and uniformly until it reaches its full expansion, of
up to five (5) times the initial bed height. At the same time, the
pressure drop reading is very close to the weight of the bed,
indicating fluidization of the entire bed. A homogenous
fluidization state is established, as shown in FIG. 6(b), and the
surface is very smooth and even. After the experiment, the powder
was poured out and, from visual observation, most of the original
large hard agglomerates are gone and the average agglomerate size
appears very much smaller.
[0116] The pressure drop normalized with the bed weight per unit
area and the bed expansion ratio as a function of superficial gas
velocity through the bed are shown in FIG. 7 (with and without
magnetic excitation). As shown therein, solid lines reflect bed
expansion ratios and dashed lines reflect pressure drops. The
magnetic field intensity was 140G at the center of the field, and
the mass ratio of magnets to nanoparticles was 2:1 (with AC
frequency of 60 Hz). With further reference to FIG. 7, U.sub.mf1
represents the minimum fluidization velocity without magnetic
excitation, whereas U.sub.mf2 represents the minimum fluidization
velocity with magnetic excitation. It is clear from FIG. 7 that the
magnetic excitation causes the bed to expand almost immediately as
the velocity is increased and the bed fluidizes at a velocity more
than one order of magnitude lower than that without magnetic
assistance.
[0117] After separation from the magnetic particles, the
nanoparticle agglomerates were recharged back into the column, and
a second fluidization experiment without magnetic assistance is
conducted using these agglomerates. FIG. 8 is a comparison of the
fluidization characteristics of the 80/20 mixture, before and after
magnetic processing. Solid lines represent bed expansion ratios and
dashed lines represent pressure drops. The magnetic field intensity
was 140 G at the center of the field and the mass ratio of magnets
to nanoparticles was 2:1 (AC frequency of 60 Hz). A significant
reduction in the minimum fluidization velocity from 5.67 cm/s
(before magnetic "fragmentation" processing) to 1.25 cm/s (after
magnetic "fragmentation" processing) is observed, indicating that
previous fluidization with magnetic assistance causes the
agglomerates to be fragmented into smaller ones and the average
agglomerates size is reduced. However, the minimum fluidization
velocity of these smaller agglomerates is still about an order of
magnitude larger than the minimum fluidization velocity observed
when the magnetic assistance is turned on.
[0118] The fluidization behavior of exemplary soft agglomerates is
shown in FIG. 9. Solid lines represent bed expansion ratios and
dashed lines represent pressure drops. The magnetic field intensity
was 140 G at the center of the field and the mass ratio of magnets
to nanoparticles was 2:1 (AC frequency of 60 Hz). U.sub.mf1
represents the minimum fluidization velocity without magnetic
excitation, whereas U.sub.mf2 represents the minimum fluidization
velocity with magnetic excitation. The much smaller agglomerates
fluidize well with and without magnetic excitation. In both cases,
the minimum fluidization velocities appear to be quite close to
each other, but at higher gas velocities (above minimum
fluidization velocity), the bed expansion with magnetic assistance
is higher than that without magnetic assistance. It is also noted
that the ratio of the measured pressure drop to the weight of the
bed per unit area is below unity for magnetic assisted
fluidization. This may mean that some of the nanoagglomerates are
not participating in the fluidization and may be sticking to the
magnets.
[0119] FIG. 10 shows the typical fluidization behavior (pressure
drop and bed expansion) of hard SiO.sub.2 nanoparticle agglomerates
(R974) with and without magnetic excitation. Solid lines represent
bed expansion ratios and dashed lines represent pressure drops. The
magnetic field intensity was 140 G at the center of the field and
the mass ratio of magnets to nanoparticles was 2:1 (AC frequency of
60 Hz). U.sub.mf1 represents the minimum fluidization velocity
without magnetic excitation, whereas U.sub.mf2 represents the
minimum fluidization velocity with magnetic excitation. The size of
the hard agglomerates was in a wide range, from 0.5 mm to about 10
mm. Without the magnetic excitation, even at superficial gas
velocity as high as 13.2 cm/s, the hard agglomerates could not be
fully fluidized. Visual observation reveals that the smaller hard
agglomerates are in motion at the top of the bed, but the larger
agglomerates remain at the bottom of the bed, causing the gas to
flow in large channels between them. The bed showed almost no
expansion and the pressure drop was much less than the bed weight,
indicating that the bed was not fluidized.
[0120] After turning on the external magnetic field, however, the
large agglomerates become smaller and smaller due to fragmentation
(disruption of interparticle forces) caused by collisions with the
magnetic particles, and these smaller agglomerates participate in
the circulation of the bed. After a few minutes, the nanoparticle
size distribution reaches a desired range and assumes a dynamic
equilibrium. From that point, even at the relatively low gas
velocity of 0.94 cm/s, all of the large agglomerates disappear, and
the bed expands slowly and uniformly until it reaches full
expansion, while the pressure drop reading is very close to the
weight of the bed, indicating that the entire bed is fluidized.
[0121] The fragmentation caused by the magnetic processing is so
obvious that the reduction in size of the hard agglomerates could
be seen by inspection after the magnetic field and air flow were
shut down. Upon removing the magnetic particles, the nanoparticle
agglomerates are recharged into the chamber and a conventional
fluidization experiment (no magnetic assistance) is performed. FIG.
11 is a comparison of the fluidization characteristics between the
powder before and after undergoing a magnetic assisted fluidization
(fragmentation) process according to the present disclosure. Solid
lines represent bed expansion ratios and dashed lines represent
pressure drops. The magnetic field intensity was 140 G at the
center of the field and the mass ratio of magnets to nanoparticles
was 2:1 (AC frequency of 60 Hz). U.sub.mf1 represents the minimum
fluidization velocity before magnetic fragmentation processing,
whereas U.sub.mf2 represents the minimum fluidization velocity
after magnetic fragmentation processing. A very large reduction in
the minimum fluidization velocity (U.sub.mf) from greater than 13.2
cm/s to 2.29 cm/s indicates that the average agglomerates size has
been significantly reduced through the magnetic fragmentation
processing.
[0122] The U.sub.mf for the hard agglomerates after magnetic
processing is 2.29 cm/s, which is larger than the U.sub.mf of 1.25
cm/s for the 80/20 mixture, and also much larger than the U.sub.mf
of 0.23 cm/s for the soft agglomerates. This indicates that the
average size of hard agglomerates and of the mixture after the
fragmentation process is still larger than that of the soft
agglomerates. Hence, in order to only investigate the effect of
magnetic excitation (e.g., magnet to nanoparticle mass ratios, AC
frequencies, and different magnetic field intensities), and to
minimize the influence of non-uniformity of the initial agglomerate
size distribution, the soft agglomerates represent a good choice to
conduct the comparison experiments.
[0123] At low gas velocities, conventional fluidization (no
magnetic assistance) of soft agglomerates or of the 80/20
agglomerate mixture, produced only slugging and channeling, while
at sufficiently high gas velocities, the bed can be fluidized
smoothly. If the gas velocity is increased above a certain level,
bubbles can be observed in the fluidized bed. Fluidization of
nanoparticle agglomerates occurs due to the disruption of
interparticle forces by the large hydrodynamic forces generated at
high gas velocities. However, for conventional fluidization of hard
agglomerates, even at a very high gas velocity, the bed could not
be fully fluidized.
[0124] The mechanism of fluidization with the assistance of an
oscillating magnetic field is two-fold: (1) fragmentation of large
agglomerates into smaller ones, and (2) transferring kinetic energy
generated by the oscillating magnetic excitation to the
nanoparticle agglomerates due to collisions to disrupt the large
interparticle forces between them. The table of FIG. 12 presents a
summary of the minimum fluidization velocities for the soft, hard
and 80/20 agglomerate mixture. For the soft agglomerates, magnetic
excitation has little effect, but it produces a definite
improvement in fluidization behavior for the 80/20 mixture. Even
for the hard agglomerates, magnetic excitation changes the
fluidization characteristics significantly, from no fluidization to
smooth, bubble-less, agglomerate particulate fluidization (APF)
with very large bed expansion up to five (5) times the initial bed
height.
[0125] The minimum fluidization velocity is also significantly
reduced from higher than 13.2 cm/s to 0.38 cm/s. Without magnetic
excitation, at a gas velocity of 13.2 cm/s or higher, extremely
strong elutriation could be observed, while with magnetic
excitation, at the low gas velocity of 0.38 cm/s, elutriation was
negligible. The substantial reduction in the minimum fluidization
velocity resulting in smooth and bubble-less fluidization with
little elutriation offers significant benefits for industrial
applications where good mixing and high rates of heat and mass
transfer with little gas by-passing are required.
[0126] Moreover, optical measurements demonstrate that the mean
agglomerate size of the decreases by roughly 100 .mu.m during
magnetic processing (from mean measurement of 315 .mu.m to mean
measurement of 196 .mu.m). As shown in the plots of FIGS. 13(a) and
13(b), the agglomerate size distribution is advantageously shifted
downwards through magnetic processing according to the present
disclosure, establishing a dynamic equilibrium that facilitates
effective bed fluidization. FIG. 13(a) reflects the particle size
distribution for a "soft" agglomerate system without magnetic field
application (i.e., control) and FIG. 13(b) reflects particle size
distribution with magnetic field application (140 G, 60 Hz, mass
ratio of magnets to nanoparticles of 2:1). The data reflected in
FIGS. 13(a) and 13(b) was generated through in situ optical
measurements on the fluidized bed surface.
[0127] According to experimental observation, when the magnetic
excitation is turned on (140 G, 60 Hz; 2:1 ratio of magnets to
nanoparticles), the fluidization behavior of the nanoparticle bed
does not change immediately, and it will take several minutes for
the bed to begin expanding. The fluidized bed does not reach full
expansion for a period of about 5 to 15 minutes, i.e., a state of
dynamic equilibrium. The bed expansion as a function of time for
R974 silica at different gas velocities is shown in FIG. 14(a); the
higher the velocity, the quicker the bed expansion. Similarly, when
turning off the magnetic excitation, it also takes a short period
of time, typically 10-30 seconds, for the bed to begin to collapse,
and the collapse will last from 1 to 3 minutes before reverting
back to a fixed bed with uneven surface. The bed collapse as a
function of time is shown in FIG. 14(b); the higher the gas
velocity, the longer it will take for the bed to collapse.
[0128] Additional fluidization experiments with magnetic assistance
(140 G, 60 Hz) were conducted using soft agglomerates for four
different mass ratios of magnets to nanoparticles, varying from 1:4
to 2:1. The table of FIG. 15 presents the values of U.sub.mf and
the bed expansion ratios at two different gas superficial
velocities that were observed for these four cases. This table
shows that the minimum fluidization velocity and bed expansion
depends on the magnet to nanoparticles mass ratio, with U.sub.mf
decreasing from 1.61 cm/s to 0.26 cm/s as the mass ratio increases
from 1:4 to 2:1. This reduction indicates that adding more magnetic
particles to the bed results in more kinetic energy transported
from the magnets to the nanoagglomerates, causing more
fragmentation and easier fluidization. The results set forth in the
table also show that there is little benefit in increasing the
ratio of magnets to nanoparticles above 1:1. It is also noted that
the minimum fluidization velocities for low mass ratios of magnets
to nanoagglomerates are actually higher than were observed for the
nanoagglomerates without any magnetic assistance. This behavior is
probably due to the additional drag of the gas on the magnetic
particles.
[0129] The table of FIG. 16 presents the values of U.sub.mf and bed
expansion ratio at a fixed superficial gas velocity for three
different magnetic field intensities when fluidizing soft
nanoagglomerates, keeping the ratio of magnets to nanoparticles at
2:1. The center point of the column around which the 2 coils are
placed was selected as the reference point for measuring the
intensity of the magnetic field and it was observed that, when
using a magnetic field intensity of less than 80 G, the bed could
not be fluidized. Hence, three (3) different intensities (100, 120,
and 140 G) were selected to conduct the fluidization experiments.
As shown in FIG. 16, the minimum fluidization velocity is a strong
function of the magnetic field intensity and U.sub.mf and decreases
rapidly as the intensity of the magnetic field increases,
indicating better fluidization. The values of the bed expansion are
quite close to one another, but they are nonetheless consistent
with the trend that the bed will expand more in a stronger magnetic
field.
[0130] The table of FIG. 17 presents the values of U.sub.mf and bed
expansion ratio at a fixed superficial gas velocity for three (3)
different frequencies of AC power, keeping the mass ratio of
magnets to nanoparticles at 2:1 and the magnetic field intensity at
120 G at the center of the field. The table shows that the
frequency of the magnetic field can significantly affect the
minimum fluidization velocity. At the lower frequencies, i.e., 45
Hz and 60 Hz, the beds show similar fluidization behavior, and can
be fluidized easily at a U.sub.mf of 0.65 cm/s and 0.51 cm/s,
respectively. But at higher frequency, i.e., 80 Hz, the bed is
difficult to fluidize, U.sub.mf is as high as 2.64 cm/s, and the
bed expansion is much smaller than at the lower frequencies. At a
frequency higher than 90 Hz, the bed could not be fluidized at
all.
[0131] This foregoing experimental studies have shown that silica
nanoparticle agglomerates can be easily and smoothly fluidized with
the assistance of magnetic particles in an oscillating magnetic
field. Due to a significant reduction in the minimum fluidization
velocity with magnetic assistance, both elutriation of nanoparticle
agglomerates and gas bypass in the form of bubbles is greatly
reduced. With magnetic excitation, hard (larger than 500 .mu.m)
agglomerates change their fluidization pattern from no fluidization
to agglomerate particulate fluidization (APF) with large bed
expansion. The minimum fluidization velocity of an 80% soft
(smaller than 500 .mu.m) and 20% hard agglomerate (80/20) mixture
can also be significantly reduced. Magnetic-assisted nanoparticles
fluidization is easier to achieve and yields more uniform
fluidization, and such approach can be used for "as-received
powders", i.e., straight out of the bag, without any
pre-processing, and hence is very useful for practical
applications. Overall, the introduction of the magnetic energy
according to the present disclosure significantly alters
agglomerate size, reducing it to achieve a desired size
distribution, and allowing for advantageous fluidization
performance results.
[0132] The fluidization of nanoparticles and/or nanoagglomerates in
accordance with one or more aspects of the present invention may
have a great impact on the processing and manufacturing of
nanostructured products. It is known that mechanical, electronic,
catalytic, optical, and/or other properties of a material are
significantly enhanced when made of nanoparticle components. For
example, copper preferably composed of nanocrystalline copper may
be 5 times harder than copper that is composed of micron-sized
copper particles. Further, the mixing of nanosized aluminum and
molybdenum oxide to produce MIC, an energetic material that may
have a variety of important military applications. It has been
ascertained that good mixing of the two components on the
nanoscale, as provided by the present invention, is essential for
obtaining a viable, highly energetic product.
[0133] There are also several coating applications for
nanoparticles/nanoagglomerates which may be imminent with the
present invention. For example, in an alternative aspect of the
present invention, the apparatus of the present invention may be
provided with a spray nozzle preferably located above the bed
surface. The spray nozzle is preferably suitable to spray the
surface, where the particles are continuously circulating
throughout the bed. The spray nozzle may preferably be sized to
deliver an appropriate amount of material for a desired amount of
coating. Due to the loose structure of the agglomerates, individual
coating of primary particles may only be forthcoming.
2. Sound-Assisted Fluidization of Nanoparticle Agglomerates
[0134] According to the present disclosure, it has been found that,
with the aid of sound wave excitation at low frequencies, a bed of
nanoparticle agglomerates can be readily fluidized and the minimum
fluidization velocity is significantly reduced. For example, in the
case of an exemplary nanoparticle material, namely hydrophobic
fumed silica nanoparticles (Degussa Aerosil.RTM. R974 having a
primary particle size of 12 .mu.m) in the form of large 100 to 400
.mu.m agglomerates, the minimum fluidization velocity was decreased
from 0.14 cm/s in the absence of sound excitation to 0.054 cm/s
with the assistance of sound wave excitation. In addition, under
the influence of sound, channeling or slugging of the bed quickly
disappeared and the bed expanded uniformly. Within a certain range
of the sound frequency, typically from 200 to 600 Hz, bubbling
fluidization occurred. Both the bed expansion and bubble
characteristics have been determined to be strongly dependent on
the sound frequency and sound pressure level. However sound has
almost no impact on fluidization when sound frequency is extremely
high, e.g., above 2000 Hz. A relatively high sound pressure level
(such as 115 dB) is needed to initiate the fluidization at such
high frequencies.
[0135] Thus, according to the present disclosure, sound waves are
advantageously employed for fluidization purposes, either alone or
in combination with other external energy sources, to provide
excitation to nanoparticles that is relatively inexpensive, affects
the entire particle bed, and does not require any physical contact
between the sound generator and the nanoparticles. The
advantageously disclosed sound-assisted fluidization of
nanoparticle agglomerates and their fluidization characteristics
are not only different from those observed using other fluidization
methods for nanoparticle agglomerates, but are also different from
sound-assisted fluidization of micron or sub-micron sized
particles. The effects of sound frequency and sound pressure level
on the fluidization behavior, such as the minimum fluidization
velocity, bubbling regime, pressure drop across the bed, and bed
expansion, are also disclosed herein.
[0136] A schematic diagram of an exemplary sound-assisted
fluidization system 100 is shown in FIG. 18. The exemplary system
100 includes a fluidized bed 102 containing nanoparticle
agglomerates 104, a sound excitation device 106, and a
visualization apparatus 108. The visualization apparatus 108 is
provided for the sole purpose of monitoring the activities and/or
behavior of the nanoparticles within fluidized bed 102, and is not
required for implementations wherein such monitoring is not
necessary or desirable. The exemplary fluidized bed 102 is a
vertical transparent column with a distributor 110 at the bottom.
In an exemplary embodiment of the present disclosure, the column is
fabricated from a section of acrylic pipe with an inner diameter of
57 mm and a height of 910 mm. The exemplary distributor 110 is a
sintered metal plate of stainless steel with a thickness of 2 mm
and pore size of 20 .mu.m. Ultra-fine mesh filters 112 are located
at the gas outlet to filter out any elutriated nanoparticle
agglomerates.
[0137] The disclosed sound excitation device 106 includes a 63 mm
loudspeaker 114 that is powered by a sound amplifier 116 that
communicates with a signal generator 118. The loudspeaker 114 is
installed on the top of fluidized bed 102. A precision sound
pressure level meter (not pictured) may be used to measure the
sound pressure level. According to an exemplary embodiment of the
present disclosure, sound excitation system 106 is capable of
generating a sound wave in fluidized bed 102 with a sound pressure
level up to 125 dB and the sound frequency from signal generator
118 is typically adjustable, e.g., within a range extending from 10
to 2 MHz. The fluidization behavior of the nanoparticles is
visualized with the aid of a lighting device (not pictured) and is
recorded by a digital camcorder 120. The visual images may be
advantageously analyzed directly by a computer 122.
[0138] According to an experimental use of the system 100,
synthetic silicon dioxide nanoparticles (Degussa, R974) with a
primary particle diameter of 12 nm and a primary density of 2560
kg/m.sup.3 were employed. The disclosed sound-assisted fluidization
system is not limited to use with silicon dioxide nanoparticles,
however, but may be employed with a variety of nanoparticle
materials finding application in a variety of commercial fields.
Before use in exemplary system 100, the nanoparticles were sieved
using a shaker (Octagon 2000) and a sieve of Mesh No. 35 (mesh
opening, about 500 .mu.m). The sieving process served to remove
very large agglomerates which may have been generated during
packing, storage, and transportation. The selection of a mesh
opening of 500 .mu.m reflects the fact that the typical size of
fluidized nanoparticle agglomerates is between 100 to 400 .mu.m
(although the present disclosure is not limited to such particle
size distributions). The bulk density of the sieved nanoparticle
agglomerates was 33.8 kg/m.sup.3.
[0139] Due to surface treatment by the manufacturer, the silicon
dioxide nanoparticles are hydrophobic. To minimize any potential
effect of humidity on the nanoparticle fluidization, pure nitrogen
from a compressed N.sub.2 tank 122 was used as the fluidizing gas.
The gas flow rate was measured and adjusted by a calibrated
rotameter 124. With the aid of an inclined tube monometer 126, the
pressure drop across the bed was measured. By measuring the
pressure in the manner schematically depicted in FIG. 18, the
pressure drop across distributor 110 was excluded.
[0140] Typical bed behavior of SiO.sub.2 nanoparticle agglomerates
with and without sound excitation are shown in FIGS. 19(a) and
19(b), respectively. The nanoparticle agglomerates were first
lifted in a slugging mode and then the bed disintegrated to form
stable channels. The bed only expands slightly with an uneven
surface, as shown in FIG. 19(a). Once a sufficiently strong sound
is applied, the instabilities in the bed collapse in a couple of
seconds, the channels disappear, and the bed expands rapidly and
uniformly until it reaches the full expansion. A homogenous
fluidization state is easily established, as shown in FIG.
19(b).
[0141] Typical fluidization characteristics, including the minimum
fluidization velocities, bed expansions and bed pressure drops with
and without sound excitation, are illustrated in FIGS. 20 and 21,
respectively. An advantageous substantial reduction in the minimum
fluidization velocity with the introduction of the disclosed sound
energy is apparent. For the test material, i.e., the DeGussa
Aerosil.RTM. R974 nanoparticles, the minimum fluidization velocity
was reduced from 0.14 cm/s in the absence of sound energy to 0.054
cm/s with sound excitation. As used herein, the minimum
fluidization velocity is defined as the gas superficial velocity
beyond which the bed pressure drop is no longer dependent upon the
gas velocity and becomes nearly constant.
[0142] As noted above, at low gas velocities, only the slugging and
channeling occur in a fluidized bed of nanoparticle agglomerates
while, at sufficiently high gas velocities, the bed can be
fluidized smoothly. Fluidization of nanoparticle agglomerates
occurs due to the effective breakup of large agglomerate clusters
by the large hydrodynamic forces at high gas velocities. With the
aid of sound excitation, however, the breakup of large agglomerate
clusters takes place due to a combined effect of hydrodynamic
forces and acoustic excitations. Through the introduction of such
energy, the particle size distribution is advantageously shifted
downward, thereby facilitating efficient and efficacious
fluidization according to the present disclosure.
[0143] FIG. 22 shows a series of representative snapshots of the
fluidizing bed at different sound frequencies. At a fixed sound
level output (e.g., 125 dB in FIG. 22), the bed of nanoparticle
agglomerates can only be fluidized in a relatively narrow band of
low sound frequency from 20 to 1000 Hz. Furthermore, bubbles appear
in an even narrower range, 200-600 Hz, and as seen in FIG. 22, both
the occurrence of bubbling and bubble size are strongly dependent
on the sound frequency. Due to the relatively high bed voidage
observed when fluidizing nanoparticle agglomerates in the bubbling
fluidization regime, the bubble size and the bubble rising velocity
can be easily identified using visualization technology. The bed
expansion is also strongly dependent on the sound frequency, as
seen in FIG. 23.
[0144] The effect of sound pressure level on the bed expansion is
shown in FIG. 24. It is noted that below a critical value of sound
pressure level (e.g., 112 dB at 1000 Hz and 105 dB at 400 Hz in
FIG. 24), there is no fluidization. The critical sound pressure
level appears to be a function of sound frequency. Within the range
of the test conditions reflected in FIG. 24, the bed expansion
increases monotonically as the sound pressure level increases.
[0145] Bed expansion and overall fluidization performance according
to this exemplary embodiment of the present disclosure are related,
at least in part, to the balance between the sound-assisted
agglomerate breakup and the sound-assisted agglomeration of the
nanoparticles. The introduction of sound energy reduces agglomerate
size and, once a desired agglomerate size distribution,
advantageous fluidization performance results. At low frequencies,
the introduction of sound energy to a nanoparticle system
contributes to a reduction in particle size distribution, thereby
enhancing bed expansion and fluidization (e.g., up to frequencies
of about 1000 Hz), as well as reductions in minimum fluidization
velocities (e.g., R974 reduced from 0.2 cm/s to 0.05 cm/s;
TiO.sub.2 reduced from 5.17 cm/s to 2.29 cm/s). In addition, at
sound pressure levels of greater than about 90dB, fluidization
behavior of nanoparticle systems is enhanced. The enhanced
fluidization behavior achieved through sound energy introduction
supports or facilitates more uniform mixing, faster surface
reaction and/or better surface coating.
[0146] Based on the test results set forth herein, it is apparent
that nanoparticle agglomerates can be easily and smoothly fluidized
with the assistance of sound energy at an appropriate sound
pressure level and sound frequency. Since there is a significant
reduction in the minimum fluidization velocity in the presence of
sound, elutriation of nanoparticle agglomerates is much reduced.
The ability to fluidize the exemplary fumed silica nanoparticle
agglomerates could only be achieved within a given range of sound
frequency with a sound pressure level above a critical value.
Bubbling fluidization occurs within an even smaller range of sound
frequency.
3. Fluidization of Nanoparticles and/or Nanoagglomerates in a
Rotating Fluidized Bed
[0147] According to a further aspect of the present disclosure, a
rotating fluidizing bed (RFB) system and associated method/process
are provided for use in advantageously fluidizing
nanoparticles/nanopowders/nanoagglomerates. Use of the disclosed
rotating fluidized bed system demonstrates a linear dependence
between the minimum fluidization velocity and the centrifugal force
delivered thereby. The centrifugal force is generally dependent on
such factors as the dimensions of the rotating system and the
rotational speed thereof. For example, conditions may be selected
whereby the rotating system generates various force levels, e.g.,
forces that are 10, 20, 30 and 40 times normal gravity force. Of
note, it has been determined that one disadvantage associated with
fluidization of nanoparticles at normal gravity force is that high
powder elutriation takes place at high gas velocities; however,
using a centrifugal force field, higher gas flow rates may be
advantageously employed without having such high levels of
elutriation, while generating even smaller agglomerates sizes than
in conventional fluidization.
[0148] Several factors can influence pressure drop across a
rotating bed containing powders, such as elutriation, radial
velocity, and overall unit design. It is further believed according
to the present disclosure that Coriolis forces and their effects
should be considered as an additional cause of pressure drop
variations for rotating fluidized bed systems. Indeed, it has
previously been mentioned that rotation is an additional factor for
destabilization [Brouwers, Phase separation in centrifugal fields
with emphasis on the rotational particle separator, Experimental
Thermal and Fluid Science 26 (2002) 325-334], and that rotation
becomes important when the Reynolds number based on rotation is
higher than a certain value; secondary flows may also occur as a
consequence of the Coriolis force.
[0149] With reference to FIGS. 25(a) and 25(b), an exemplary
rotating unit 200 according to the present disclosure is
schematically depicted. Rotating unit 200 includes a chamber 202
that encloses a cylindrical porous stainless steel sintered mesh
204 with an aperture size of 20 .mu.m, 2 mm of thickness, 400 mm of
diameter and 100 mm of depth. Mesh 204 functions to distribute the
gas that passes through the bed, i.e., as a gas distributor. This
gas distributor turns along its axis of symmetry, moved by a motor
206 which is controlled by a speed variator.
[0150] Rotating unit 200 also includes a stationary cylindrical
filter 208 of 100 .mu.m mesh with 2 mm of thickness, 100 mm of
diameter and 90 mm of depth; the function of stationary filter 208
is to retain elutriated fine powder. The covers of chamber 202 and
mesh/gas distributor 204 are typically fabricated of an
appropriately rigid material, e.g., acrylic plastic. In an
exemplary material, the covers are fabricated from a transparent or
translucent material which allows the behavior of the bed inside
the unit to be viewed.
[0151] Pressure taps 210 are placed between gas distributor 204 and
the inner filter mesh 208, as shown in FIGS. 25(a) and 25(b). The
pressure drop across the air distributor may be measured using a
differential pressure transmitter. The gas, e.g., air, delivered to
the distributor may be measured by an area variable type flowmeter
212. Since it is generally not possible to measure the bed pressure
drop directly in a rotating fluidized bed, the pressure drop across
the air distributor mesh 204 may be determined as a function of air
velocity or flow rate before loading rotating unit 200 with powder.
Then the bed pressure drop can be quantified by subtracting the
pressure drop measured when the unit is loaded with powder, and
when the unit is empty.
[0152] Among other accessories, a digital camera may be associated
with rotating unit 200 for use in recording the behavior of
nanoparticle agglomerates during fluidization. In addition, a laser
light may be used to determine the expansion of the bed as well as
the homogeneity of the bed's surface. Further; a vacuum system may
be employed to remove exhaust from rotating unit 200 and the
pressure transmitter may be advantageously connected to a computer
system for processing of data received therefrom.
[0153] Experiments have been conducted using a system that
corresponds to the system schematically depicted in FIGS. 25(a) and
25(b). The powders employed in such experimental runs belonged to
Geldart C classification since they are very fine and cohesive
particles; however, some of them behave like group "A" powders,
specifically the APF behavior [Wang et al., Fluidization and
agglomerate structure of SiO.sub.2 nanoparticles, Powder
Technology, 124 (2002) 152-159], while others have bubbling
fluidization, specifically, the ABF behavior as found in recent
experiments by our group, described later in this document. The
tested powders showed a strong cohesive behavior, but they were
quite different than C powders due to their fluidization behavior
and bulk density. For purposes of the noted experiments, the
powders were sieved using a shaker and a sieve of Mesh No. 60 (mesh
opening about 250 .mu.m). This sieving procedure was followed
because it is believed that the large agglomerates break the
homogeneity of the flow field and make the fluidization more
difficult. Fumed Silica Aerosil was employed having an approximate
tapped density of 50 g/l. The R974 material had an average particle
size of 12 nm, while the R972 material had an average particle size
of 16 nm. In both cases, 70 grams were used; the bulk density of
these powders was approximately 30 g/l. The tested titanium dioxide
P25 material had an average particle size of 21 nm, a tapped
density of 130 g/l, and a bulk density of about 90 g/l. A total of
250 grams were used in the experiments and the initial bed height
was close to 0.02 m.
[0154] The experimental steps can be summarized as follows. The
unit was cleaned very carefully so as to ensure a uniform air field
would be generated by the air distributor. All of the component
parts of the rotating unit were assembled and all joints sealed in
order to prevent leaks. The presence of leaks would undesirably
distort collected pressure drop data. The pressure drop across the
air distributor was then measured. For this purpose, the unit was
run empty, and the air flow and the rotating speed were changed
successively in order to find the relationship between the
distributor's pressure drop and the air flow.
[0155] Next, the test material was loaded into the unit and the
rotating speed was set at the desired value in order to increase
the centrifugal force. Immediately thereafter, the air flow was
increased slowly and relevant data was recorded, i.e., air flow,
pressure drop and bed height. Subsequently, the rotating speed was
increased to higher values and the same procedures were followed
with respect to data collection.
[0156] FIG. 26 shows the measured air pressure drops at different
values of air velocity and at different rotating speeds translated
in "G"s (i.e., translated into gravity forces). The pressure drop
increases until the minimum fluidization velocity is reached, then
a constant pressure drop is observed. It is noted that the pressure
drop does not uniformly maintain a linear trend before reaching the
minimum fluidization velocity (U.sub.mf); it is believed that due
to the centrifugal force imparted by the rotating unit, the powder
was compacted and therefore the changing pressure is due to the
irregularities of the bed before reaching the fluidized state.
[0157] FIG. 27 shows the relative bed height as a function of air
velocity for the tested R974 material. It is noted that the
compaction effect over the bed that is effected by the centrifugal
field changes the bulk density of the powder. Bed pressure drop
data related to the fluidization behavior of R972 material is shown
in FIG. 28. FIG. 29 shows the relative bed height during
fluidization of the R972 material. In the case of R972 material (as
with the R974 material), there is a compaction effect over the
powder as the centrifugal field increases. It is believed that the
centrifugal force is transmitted to all particles in the bed by the
particles that are in closer proximity to the air distributor.
[0158] FIG. 30 shows the fluidization behavior for titanium dioxide
P 25 material. Of note, the amount of titanium dioxide loaded into
the unit for the experimental runs described herein was higher than
the amount of fumed silica because the bulk density of the titanium
dioxide is approximately three (3) times that of the silica;
therefore, a larger pressure drop was expected due to the increase
of the weight of the bed within the system. With reference to bed
height response for the titanium dioxide material and as shown in
FIG. 31, there is not a large bed expansion as was experienced with
the R974 and R972 silica powder materials. In addition, measurement
of the increase in bed height was very difficult to achieve. This
difficulty can be explained due to the higher density (bulk and
particle) of the titanium dioxide powder. No significant
elutriation was observed during the titanium dioxide experimental
runs.
[0159] Therefore, it can be concluded that fluidization behavior in
the rotating bed systems of the present disclosure differs based,
at least in part, on the characteristics of the particles processed
in such systems. For the R974 and R972 fumed silica materials, the
bed expansion behavior can facilitate determination of the fully
fluidized system state. By contrast, for the TiO.sub.2 P25
material, the bed expansion was poor and unstable, thereby giving
no useful insight with respect to the fluidization state of the
system.
[0160] FIG. 32 shows the relationship between minimum fluidization
velocity and centrifugal force for the three tested material
systems. A linear dependence between the minimum fluidization
velocity and centrifugal force, as observed in prior studies and
consistent with a model proposed by Kao et al. [Kao et al., On
Partial Fluidization in Rotating Fluidized Beds, AIChE J. 33 (1987)
858].
[0161] The experimental pressure drop measurements can be affected
by several problems during the experimental runs described herein,
such as clogging of the distributor, leaks across the distributor
assembly, inaccuracies in the readings of the flow rate, problems
in the pressure reading system, etc. However, these systematic
errors generally exhibit a level of repeatability that can be
determined and, therefore, actions can be taken to address the
underlying problem(s). Preliminary analysis shows that the
theoretical predictions for the pressure drop in a rotating
fluidized bed only consider the effect due to the centrifugal
forces, and do not account for the effects of the relative
magnitude between the radial and tangential velocities and the
gradient of the tangential velocity in the radial direction.
Nanoparticles differ in this respect from micron and larger
particles, because the radial air velocities for nanoparticles are
much lower than those for the micron and larger sized particles.
When such factors are all taken into account, one may find that the
current theoretical predictions such as Kao et al. [Kao et al., On
Partial Fluidization in Rotating Fluidized Beds, AIChE J. 33 (1987)
858], may not be fully valid and may need to be corrected for other
effects, including but not limited to the Coriolis effects.
[0162] Based on the foregoing experimental data, the advantageous
ability to fluidize nanoparticles, nanopowders and/or
nanoagglomerates in a rotating fluidized bed according to the
present disclosure is clearly demonstrated. The foregoing
experimental data also shows that different nanopowders exhibit
different behaviors during fluidization in a rotating bed. The
advantages of the disclosed rotating fluidized bed relative to
conventional, non-fluidized systems include: less elutriation of
powder, higher air flow rate, higher powder load by unit area of
distributor, reduction of the size of agglomerates due to the
higher shear rate, and shorter processing time. The foregoing test
results further demonstrate a linear dependency of the minimum
fluidization velocity and the artificial gravity force generated by
the centrifugal effect. In addition, it is believed that when
fluidizing agglomerates of nanoparticles, effects such as the
Coriolis forces and others affect the pressure drop.
4. Gas Fluidization Characteristics of Nanoparticle
Agglomerates
[0163] According to a further aspect of the present disclosure, the
effects of different types of nanoparticles on gas fluidization
characteristics of nanoparticle agglomerates was determined. Taking
advantage of the extremely high porosity of the bed, optical
techniques were used to visualize the flow behavior, as well as to
measure the sizes of the fluidized nanoparticle agglomerates at the
bed surface. Upon fluidizing a series of different nanoparticle
materials, two types of nanoparticle fluidization behavior were
observed, namely agglomerate particulate fluidization (APF) and
agglomerate bubbling fluidization (ABF).
[0164] Highly porous nanoparticle agglomerates exhibit two distinct
fluidization behaviors, APF (smooth fluidization without bubbles at
minimum fluidization) and ABF (bubbles at minimum fluidization).
APF agglomerates show very large bed expansions, up to five times
the initial bed height as the superficial gas velocity is raised,
and the Reynolds numbers for these nanoagglomerates at minimum
fluidization are very low (0.05 to 0.35), which indicate that the
agglomerates are in creeping flow. ABF nanoagglomerates fluidize
with large bubbles and show very little bed expansion as the
superficial gas velocity is raised and the Reynolds numbers at
minimum fluidization are close to or higher than 2.0, which
indicate that hydrodynamic inertial effects cannot be
neglected.
[0165] The difference in fluidization behavior between smooth,
liquid like, bubble-less, particulate fluidization with high bed
expansion (APF), and non-homogeneous, bubbling, aggregative
fluidization with low bed expansion (ABF) has been found according
to the present disclosure to largely depend on the bulk density and
the primary particle size of such nanoparticles. Indeed, the
fluidization of relatively small (less than 20 nm) nanoparticles
with a bulk density less than 100 kg/m.sup.3 appear to behave as
APF, whereas larger and heavier nanoparticles are more likely to
behave as ABF (see the Table included as FIG. 33 hereto).
[0166] On the basis of experimental data using classical fluidized
particles such as FCC catalyst, UOP catalyst, and hollow resin,
Romero and Johanson [Romero et al., Factors affecting fluidized bed
quality, Chem. Eng. Progr. Symp. Series., 58 (38) (1958) 28-37]
present a criterion to characterize the quality of fluidization as
either smooth or bubbling, depending on the value of a combination
of dimensionless groups. These dimensionless groups consist of the
product (.PI.) of the particle to fluid density ratio, the Reynolds
and Froude number (these are based on calculated agglomerated
properties, and not on primary particle properties) at minimum
fluidization, and the bed height to bed diameter ratio: = Fr mf
.times. Re mf .times. .rho. a - .rho. g .rho. g .times. H mf d t
< 100 , .times. smooth .times. .times. fluidization = Fr mf
.times. Re mf .times. .rho. a - .rho. g .rho. g .times. H mf d t
> 100 , .times. bubbling .times. .times. fluidization ( 1 )
##EQU1## wherein:
[0167] d.sub.t diameter of vessel (chamber), cm
[0168] Fr.sub.mf Froude number at minimum fluidization velocity, Fr
mf = u mf 2 d a .times. g , ##EQU2## dimensionless
[0169] H.sub.mf bed height at minimum fluidization velocity, cm
[0170] Re.sub.mf Reynolds number at minimum fluidization velocity,
dimensionless
[0171] .rho..sub.a density of agglomerate in fluidized bed,
kg/m.sup.3
[0172] .rho..sub.g density of gas, kg/m.sup.3
[0173] The porous nanoparticle agglomerates of the present
disclosure behave differently than the classical solid particles
used to obtain equation (1). Nonetheless, the values of the
dimensionless groups (which are designated as ".PI.") were
calculated for a series of tested nanoparticle materials.
Unexpectedly and as shown in the Tables included as FIGS. 34 and 35
herein, the calculated results agree remarkably well with this
criterion of formula (1). For the eight APF nanoparticle materials
set forth in the Tables of FIGS. 34 and 35, the values of .PI. are
within the range of 0.008.about.1.55 (which is much less than 100),
whereas for the three ABF nanoparticle materials, the values of
.PI. are within the range of 398.about.1441 (which is much larger
than 100). Hence, the criteria set forth in formula (1) appear to
be valid for nanoparticle agglomerates and therefore provide a
valuable tool or methodology for determining whether a nanoparticle
of interest will behave as APF or ABF.
[0174] Thus, according to the present disclosure, a classification
criterion based on the value of a combination of dimensionless
groups to differentiate between particulate and bubbling
fluidization for classical solid fluidized particles may be
advantageously employed to predict whether nanoparticles will
behave as APF or ABF. Indeed, utilization of this criterion may be
superior to using the size and bulk density of the nanoparticles to
predict their fluidization behavior.
[0175] Moreover, it is demonstrated herein that fluidization of
well-sieved nanopowders may be effectively achieved in the absence
of external excitation, e.g., external excitations based on
vibration, magnets, etc. Thus, according to the present disclosure
and without external excitation, nanopowders may be fluidized
provided the nanoparticles are well-sieved so that large, hard
agglomerates are removed. This advantageous result further enhances
the flexibility and effectiveness of nanoparticle fluidization
systems according to the present disclosure.
5. Combined Systems
[0176] According to the present disclosure, it is specifically
contemplated that one or more of the energy modalities disclosed
herein may be advantageously employed either alone or in
combination. Thus, for example, the following energy source
combinations may be employed to achieve advantageous fluidization
of nanoparticles according to the present disclosure: [0177]
Vibratory force in combination with magnetic force; [0178]
Vibratory force in combination with sound energy; [0179] Vibratory
force in combination with a rotating fluidized bed; [0180]
Vibratory force in combination with at least two of magnetic force,
sound energy and a rotating fluidized bed; [0181] Vibratory force
in combination with magnetic force, sound energy and a rotating
fluidized bed; [0182] Magnetic force in combination with sound
energy; [0183] Magnetic force in combination with a rotating
fluidized bed; [0184] Magnetic force in combination with at least
two of vibratory force, sound energy and a rotating fluidized bed;
[0185] Sound energy in combination with a rotating fluidized bed;
[0186] Sound energy in combination with at least two of vibratory
force, magnetic force and a rotating fluidized bed; and [0187] A
rotating fluidized bed in combination with at least two of
vibratory force, magnetic force and sound energy.
[0188] Thus, according to the present disclosure, systems and
methods/processes for fluidization of nanoparticles are provided
that exhibit numerous advantageous properties and results,
including: less elutriation of powder, lower minimum fluidization
velocities, in certain cases, higher air flow rate, higher powder
load by unit area of distributor, reduction of the size of
agglomerates due to the higher shear rate, improved mass-transfer
and shorter processing time. Moreover, in exemplary implementations
of the present disclosure wherein multiple energy sources are
combined with a fluidizing gas source, e.g., combinations of at
least two ancillary energy sources selected from among vibratory
forces, magnetic forces, sound/acoustic forces, and
rotational/centrifugal forces, the application of such external
energy sources may be supplied at levels such that, in combination,
the ancillary energy supplied to the fluidization system affects
the desired nanoparticle fluidization results. The disclosed
systems and methods/processes may also be employed with a variety
of fluidizing gases, e.g., air, N.sub.2, He, Ar, O.sub.2 and/or
combinations thereof. Thus, the ability to supply multiple types
and levels of energy provides significant control and flexibility
to the fluidization of nanoparticle systems. The advantageous
fluidization systems and methods/processes disclosed herein may be
used in processing a wide variety of nanoparticle materials for use
in various applications, including applications that involve the
manufacture of drugs, cosmetics, foods, plastics, catalysts,
energetic and bio materials, high-strength or corrosion resistant
materials, and in mechatronics and micro-electro-mechanical
systems. Effective dispersion of nanoparticles is achieved
according to the present disclosure, thereby facilitating a host of
nanoparticle-related processing regimens, e.g., mixing,
transporting, surface property modifications (e.g., coating),
and/or downstream processing to form nano-composites.
[0189] The present disclosure having been thus described with
particular reference to exemplary forms thereof, it will be readily
apparent that various changes and modifications may be made therein
without departing from the spirit of the present disclosure as
defined herein. The following additional examples are intended for
illustrative purposes only and should not be construed so as to
limit or narrow the scope of the present invention in any way.
EXAMPLE 1
[0190] An apparatus as shown in FIG. 2 was used to fluidize
nanopowders using any gas such as air or nitrogen and
vibration.
[0191] FIG. 36 shows an exemplary plot of observed pressure drop
and bed expansion vs. superficial air velocity. At gas velocities
greater than 0.1 cm/sec and a vertical sinusoidal vibration of 5.5
g's, the bed begins to expand and continues to expand both before
and after the minimum fluidization velocity, defined as the
velocity at which the pressure drop across the bed is equal to the
weight of the bed divided by its cross sectional area. The bed
expanded to four times its initial height and appeared to be
uniformly fluidized with negligible elutriation.
EXAMPLE 2
[0192] Using the apparatus of FIG. 2, and 12 nm silica powders with
a constant flow rate and vibrational parameters of 50 Hz and 2 g's,
the silica powders were fluidized.
[0193] FIGS. 37(a) and 37(b) illustratively show what may typically
occur during a fluidization process. With air or vibration alone,
nothing useful occurs to a conventional nanoparticle powder bed.
When the two are coupled together, however, the nanoparticle size
distribution is reduced/lowered and the powder bed expands with
vigorous particle movement.
EXAMPLE 3
[0194] Using the apparatus of FIG. 2, and 12 nm silica, tracer
silica dyed with methylene blue and constant flow rate of dry air
and vibrational parameters of 50 Hz and 4 g's, was fluidized.
[0195] FIG. 38 shows the progression of mixing 12 nm silica with a
small amount of the same nano-sized silica dyed with methylene
blue. The bed was operated at a constant air velocity of 0.45
cm/see with a vertical sinusoidal vibration of 4 g's at a frequency
of 50 Hz. As can be seen in the figure, as soon as the vibration
was turned on the bed started to expand and uniform bubble less
fluidization was observed. Within 2 minutes, the entire bed turned
blue, indicating not only good fluidization, but also very good
mixing.
EXAMPLE 4
[0196] Using the apparatus of FIG. 2, and 12 nm silica, carbon
black nanoparticles and constant flow rate of dry air and
vibrational parameters of 50 Hz and 4 g's, the silica was
fluidized.
[0197] Similar results, depicted in FIG. 39, were obtained with
magnetic assistance instead of vibration. In this particular
example, the weight of the magnets, whose size range from 1.4 to
1.6 mm, was double that of the silica bed. A small amount of carbon
black, another nanosized powder, was used as the tracer and placed
on top of the silica bed at the start of the experiment. Again,
within minutes, the entire bed showed very good and complete
mixing.
PROPHETIC EXAMPLE 1
[0198] Using the apparatus of FIG. 2 along with vibration and
magnetic excitations, a coated nano-powder mixture of pigment and
polymeric material may be fluidized for powder coating application.
Metallic objects to be coated may be heated to temperatures above
the melting temperature of the polymer and dipped in the fluidized
bed for an amount of time dependent upon the coating thickness
desired. Very uniform, thin coatings may be achieved after
processing.
[0199] Although the present disclosure has been described with
reference to exemplary embodiments thereof, the present disclosure
is not to be limited to such exemplary embodiments. Rather, it is
contemplated that modifications, enhancements and/or variations to
the disclosed fluidization systems and methods/processes may be
made without departing from the spirit or scope of the present
invention.
* * * * *