AIP Con e ence P oceedings 1145, 97 (2009); h ps://doi.o g/10.1063/1.3180095 1145, 97
© 2009 Ame ican Ins i u e o Physics.
Al e na ing Field Elec onano luidiza ion
Ci e as: AIP Con e ence P oceedings 1145, 97 (2009); h ps://doi.o g/10.1063/1.3180095
Published Online: 01 July 2009
M. J. Espin, J. M. Val e de, M. A. S. Quin anilla, and A. Cas ellanos
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Al e na ing Field Elec onano luidiza ion
M.
J.
Espin ,
J.
M.
Val e de†,
M. A.
S.
Quin anilla†
and
A. Cas ellanos†
Depa men
o
Applied Physics
II.
Uni e si y
o
Se ille.
A enida Reina Me cedes
s/n,
41012
Se ille,
Spain.
†Facul y o
Physics.
Uni e si y
o
Se ille.
A enida Reina Me cedes
s/n,
41012
Se ille,
Spain.
Abs ac . The use o luidized beds o emo e submic on pa icles om gases has been in es iga ed since 1949. High
e iciency emo al was achie ed in he 1970’s by imposing an elec ic ield on a luidized bed o semi-insula ing g anules
ha we e able o collec he cha ged pollu an en ained in he luidizing gas. In spi e o hei ex ended use nowadays, he
collec ion e iciency o elec o luidized beds (EFB) is s ill hinde ed by gas bypassing associa ed o gas bubbling and he
consequen equi emen o oo high
gas
low
and
p essu e
d op.
In his pape we epo on he elec omechanical beha io o
an
EFB o insula ing nanopa icles. When
luidized
by
gas,
hese nanopa icles o m ex emely po ous ligh agglome a es o
size o he o de o hund eds o mic ons ha allow o a highly expanded nonbubbling luidized s a e a educed gas low. I
is ound ha luidiza ion uni o mi y and bed expansion a e addi ionally enhanced by an imposed AC elec ic ield o ield
oscilla ion equencies o se e al ens o he zs
and
ield s eng hs o he o de o 1
kV/cm.
Fo oscilla ion equencies o he
o de o he zs, o smalle , bed expansion is hinde ed due o elec opho e ic deposi ion o he agglome a es on o he essel
walls,
whe eas o oscilla ion equencies o he o de o kilohe zs, o
la ge ,
elec opho esis is nulli ied
and bed
expansion
is no
a ec ed.
Acco ding o
a
p oposed
model,
he size o nanopa icle agglome a es
s ems
om he balance be ween
shea ,
which depends
on
ield
s eng h,
and an
de Waals
o ces.
The
op imum ield s eng h o
enhancing bed expansion
p oduces
an
elec ic o ce
on he
agglome a es simila o hei weigh
o ce,
while
he
oscilla ion eloci y o
he
agglome a es is simila
o he
gas
eloci y.
Keywo ds: Fluidized
beds,
Nanopowde s,
Fil e s
PACS:
47.55.Lm,
81.07.Wx, 84.30.Vn
INTRODUCTION
E icien emo al o dus and mis om gases by pass-
ing hese h ough a bed o luidized solids was i s
claimed by Meisne and Mickley in 1949 [1]. Almos
wo decades
la e ,
p ac ical p oblems such as educing
ai pollu ion om diesel exhaus and il a ion o smoke
emissions om asphal ic pa emen ecycling p ocess,
mo i a ed Melche o de elop elec o luidized beds o
collec ion o submic on pa icles [2]. Melche and co-
wo ke s s essed a luidized bed by imposing an elec-
ic ield ha e ec i ely pola ized he millime e sized
pa icles.
These
semi-insula ing pa icles
hen se ed
he
unc ion o con en ional elec os a ic p ecipi a o s, ac -
ing as collec ion si es
o
he cha ged
pollu an s en ained
in he luidizing gas. In compa ison o elec os a ic p e-
cipi a o s, he collec ion su ace a ea pe uni olume o
elec o luidized
beds
is g ea ly
inc eased,
making i pos-
sible o educe he gas esidence ime and olume o
he il e [2]. A majo conce n ha bese s classical lu-
idized bed il e s lies in he bypassing o he con ami-
nan s h ough gas bubbles. Gas bubbling is he ypical
beha io ound in luidized
beds
o millime e
sized
pa -
icles [3], p o iding li le gas-solid con ac and hampe -
ing eac ion e iciency.
Powde s o mode a e densi y nanopa icles (such as
silica) can be uni o mly luidized and expe ience a
an-
si ion o elu ia ion a high gas eloci ies wi h ull sup-
p ession
o isible bubbles
[4].
In his pape
we
s udy he
beha io o
an
elec o luidized
bed
o d y silica nanopa -
icles,
which shows agglome a e pa icula e luidiza ion
in he absence o ex e nally applied elec ic
ield.
Sam-
ples we e subjec ed o DC and AC elec ic ields in o -
de o check he in luence o he equency o he ex-
e nal ield and o de e mine he size and cha ge o he
powde .
The supp ession
o
gas
bubbles in
luidized
beds
o nanopa icles has been causally ela ed o he o ma-
ion o po ous ligh agglome a es. A phenomenological
app oach o p edic he beha io o gas- luidized beds
o nanopa icles is o conside agglome a es
as
e ec i e
low-densi y
sphe es which
may exhibi nonbubbling
gas-
luidiza ion simila ly o coa se pa icles luidized by
liq-
uids.
In his way, he Richa dson-Zaki (RZ) equa ion,
o iginally in ended o desc ibe he expansion o
uni-
o m liquid- luidized beds [6], can be modi ied o
con-
side uni o m gas- luidiza ion o nanopa icle agglome -
a es
[4]
p (1)
whe e g is
he
supe icial
gas
eloci y, p is he e minal
se ling eloci y o a single pa icle, φ is he pa icle
olume
ac ion,
ka = d /dp is he a io o agglome a e
size o pa icle size, and D =
lnNa/ka,
whe e Na is he
numbe o pa icles in he agglome a e.
CPl
145,
Powde s and G ains 2009, P oceedings o he 6 In e na ional Con e ence on
Mic omechanics o G anula Media edi ed by M. Nakagawa and S. Luding
© 2009 Ame ican Ins i u e o Physics 978-0-7354-0682-7/09/S25.00
97
0.005
0.004
0.003
0.002
0.001
Solidlike - ... ,
T ansi ional
egime
egl„c
Uni o m luidlike egime
0 0.5 1 1.5 2 2.5 3
, (cm/s)
FIGURE 1. Pa icle olume ac ion as a unc ion o supe -
icial
gas
eloci y o luidiza ion wi hou ex e nal elec ic ield
applied.
The solid line ep esen s he modi ied RZ equa ion i
o he da a in he uni o m luidlike
egime.
The pho og aphs in
he inse illus a e bed expansion.
EXPERIMENTAL SETUP AND RESULTS
The luidized bed appa a us consis ed o a e ical 2.54
cm in e nal dia. polyca bona e essel, 16.2 cm in heigh ,
which is i ed a he bo om wi h
a
sin e ed s ainless s eel
pla e,
ha ing
a
po e size o
5
jim, ha ac s as gas dis ibu-
o . The luidiza ion cell is is placed be ween wo pa allel
squa e elec odes o 14 cm o side leng h. The dis ance
be ween he elec odes is ixed o I = 8 cm. One o he
elec odes is g ounded and high ol age V is applied o
he opposi e elec ode om an oscilla o /ampli ie ne -
wo k. The elec ic ield s eng h wi hin he bed can be
aken as app oxima ely cons an (E ~ V/l). The powe
supply sys em allowed us o p o ide peak ield s eng hs
up o E0 ~ 2 kV/cm o e
a
equency ange up o 10 kHz.
Di e en ypes o wa e o m, sinusoidal, squa e and
i-
angula shape, we e applied. The low o gas (d y ai )
o he column was con olled by means o a MKS low
con olle wi h a ull ange om 0 o 2000 cm3/min. The
ma e ial es ed in he expe imen s was p e-sie ed (using
a sie e opening o 500 jim) Ae osil©R974 , which is a
hyd ophobic SiO2 nanopowde wi h a pa icle densi y o
pp
=2250 kg/m3, and pa icle size dp = 12 nm.
In Figu e 1 we ha e plo ed expe imen al da a o
</>
as
a unc ion o g in he absence o ex e nal ield applied.
The bed expands mono onously as g is inc eased. By
i ing he da a o g s.
</>
o he modi ied Richa dson-
Zaki equa ion (Eq. 1), i is ob ained d** = 226jUm and
D = 2.588, which is in good ag eemen wi h p e ious
measu emen s [4].
Figu e 2 shows expe imen al da a on
</>
as a unc ion
o g and o di e en alues o he s eng h o he c oss-
low elec os a ic ield applied. The main e ec o he
elec ic ield is an inc ease o F om local obse a-
ions [7], i is seen ha he dynamics is uled by elec-
0.012
4 0.01
0.008
0.006
0.004
0.002
0
A 4 kV/cm
o 2.6kV/cm
• 1.3 kV/cm
- 0 kV/cm
2 3
(cm/s)
FIGURE 2. . A e age pa icle olume ac ion o he lu-
idized bed φ as a unc ion o he supe icial gas eloci y g
o di e en s eng hs o he elec os a ic ield applied.
(Hz)
FIGURE 3. Rela i e a ia ion o he pa icle olume ac-
ion
as a
unc ion o
he
oscilla ion equency o he al e na ing
elec ic
ield.
Peak ield s eng h is ixed o 1.25 kV/cm. Da a
is shown o h ee di e en alues o he supe icial
gas
eloc-
i y g. The inse shows elec opho e ic deposi ion (1Hz) and
enhanced
bed
expansion (20 Hz).
opho e ic deposi ion a he
wall.
In he s a iona y s a e,
luidiza ion quali y is hampe ed and bed expansion is de-
c eased.
Insula ing d y pa icles in a luidized bed accumu-
la e a signi ican amoun o cha ge. Mos in es iga ions
sugges ha he main cause o he cha ge build-up on
g anula ma e ials du ing indus ial handling and p o-
cessing is con ac cha ging [5]. Cha ges a e exchanged
whene e any wo su aces come in o con ac wi h each
o he e en o cases whe e he bulk ma e ials a e he
same due o su ace impu i ies and impe ec ions. The
elec opho e ic o ce on ou agglome a es is Fe ~
Q**E,
whe e Q** is hei cha ge. Using he S okes’ law o
an isola ed agglome a e, i can be es ima ed Q** =
3nnd** *h*/E,
whe e n is he gas iscosi y and
*h*
is
he e minal eloci y o he agglome a e. We ha e ana-
lyzed he ajec o ies o some agglome a es a he ini ial
s age o applica ion o he elec ic ield by means o a
98
E=1.25kV/cm, =1Hz £=125
kV/cm,
= 20 Hz E=1.25kV/cm, =1 kHz
FIGURE 4. Pho og aphs o he elec onano luidized bed il-
lus a ing bed expansion o h ee di e en equencies o he
al e na ing elec ic ield (indica ed)
as
compa ed o
bed
expan-
sion in he
absence
o ex e nally applied elec ic
ield.
Supe i-
cial
gas
eloci y is ixed o g = 2.7 cm/s.
-0.15
E0
(kV/cm)
•Squa e
—•— Sinusoidal
—±—
T iangula
0.2 0.4
Ems (kV/cm)
0.1
0.01
0.
A+%
• /
1
-0.15
-0.2
FIGURE 5. Rela i e a ia ion o he pa icle olume ac-
ion o he elec onano luidized bed as a unc ion o he ield
s eng h
o
h ee
di e en wa e o m
ypes.
Top:
Da a
is shown
as a unc ion o he peak ield s eng h E0. Bo om: Da a is
shown as a unc ion o he oo -mean-squa ed ield s eng h
E ms.
elec ic ield oscilla ion equency
and
supe icial
gas
e-
loci y a e
ixed
o 20 Hz
and
g = 2.7
cm/s,
espec i ely.
CMOS high-speed came a. The agglome a e cha ge o
he agglome a es manually acked was ound o be o
he o de o 10-14C, gi ing a cha ge o mass a io
Qm
in he ange 10-5 -10-4C/kg.
In he case o an al e na ing
ield,
i is expec ed ha
he cha ged agglome a es will be o ced o oscilla e a
he ield equency. Figu e 3 shows he ela i e a ia ion
A*/*,
Uni o m luidlike
egime
FIGURE 6. Rela i e a ia ion o he pa icle olume ac ion
o he elec onano luidized bed as a unc ion o he supe icial
gas eloci y. Elec ic ield oscilla ion equency and s eng h
a e ixed o 500 Hz and E0 = 1.25 kV/m (squa e wa e), espec-
i ely. The lines a e p edic ed cu es by he model. Solid line:
complex-agglome a e cha ge
Q^*
= 1.9 x 10~14 C. Do ed line:
Q** = 1 x 10-14 C. Dashed line: Q** = 3 x 10"14 C
o he pa icle olume ac ion ( φ/φ0, whe e φo is he
pa icle olume ac ion in he absence o ex e nal elec-
ic ield) o
he
elec onano luidized
bed as a
unc ion o
he oscilla ion equency o a ixed peak ield s eng h
(E0 =
1.25
kV/cm).
Fo equencies o
he
o de o he zs
he main mechanism is s ill elec opho e ic deposi ion o
he agglome a es on he walls, which gi es ise o bed
channeling and
collapse.
In
he ange
o in e media e
e-
quencies, be ween ens and hund eds o he zs, he op-
posi e beha io is
obse ed.
Bed
expansion is g ea ly
en-
hanced by he al e na ing
ield.
Finally, o equencies
o he o de o kilohe zs
and
la ge ,
he al e na ing ield
has no app eciable e ec on bed expansion (see
Fig.
4).
The e ec o he s eng h o he ield in he ange o in-
e media e oscilla ion equencies and wa e o m ype is
shown in Fig. 5. Bed expansion is u he enhanced as
he s eng h o he ield is inc eased. The da a ma ches
o
a
single
end when
i is
plo ed
agains
he
oo -mean-
squa ed
ield
E ms,
which indica es ha
enhanced bed
ex-
pansion is he esul o a ime a e aged
p ocess.
F om a
log-log plo (see inse ) i is obse ed ha φ/φ0 scales
as he
ield s eng h
squa ed,
which is p opo ional o he
kine ic ene gy gained by he agglome a es du ing one
semipe iod. This sugges s a possible ole o collisions
be ween agglome a es o di e en cha ge on deagglom-
e a ion.
DISCUSSION
A change o he pa icle olume ac ion when he
ield is u ned on could be a ibu able o a a ia ion
o he complex-agglome a e size. In o de o es ima e
99
heo e ically
he
complex-agglome a e size, simple-
agglome a es, which exis be o e luidiza ion
[4],
will
be conside ed as e ec i e pa icles unde going agglom-
e a ion
due
o
a ac i e o ces be ween each o he
[8].
This a ac i e o ce
is
coun e balanced
by
he gas
low
shea
o
suppo he weigh
o
he complex-agglome a e
in
he
g a i y ield plus
he
shea
due o
hei o ced
oscilla ions
in he
elec ic
ield.
The
balance be ween
he o e all ime-a e aged shea
and he
a ac i e o ce
de e mines
he
size
o
he
complex-agglome a es.
The
complex-agglome a e weigh o ce
W** is
balanced
by
he
hyd odynamic ic ion om
he
su ounding
gas,
which ac s mainly
a i s
su ace
due o he
low
sc eening e ec . Using
a
sp ing model
[8],
he
ypical
s ain
on he
su ace
o he
complex-agglome a e
can
be es ima ed
as
jg ~
w**/K**R**.
He e
W** =
N*W*,
whe e
N* is
he
numbe
o
simple-agglome a es
in he
complex-agglome a e and
W*
is
he
simple-agglome a e
weigh o ce.
K** is he
complex-agglome a e sp ing
cons an , which, using
a
heo y
on he
elas ic p ope ies
o andom pe cola ing sys ems, can be ob ained om
he
simple-agglome a e sp ing cons an
K*
and he elas ici y
exponen J3
(K** ~
K*/{k* ,
whe e
k* is
he
a io
o
complex-agglome a e adius
R**
o
simple-agglome a e
adius R* and
J3
=
3
in
3D). When he al e na ing elec ic
ield is u ned
on,
he o ced complex-agglome a e oscil-
la ions gi e ise
o
an
addi ional shea o ce
o
balance
he elec ic o ce
Q**E.
The
oo -mean-squa ed shea
s ain
is
hus inc eased
up o
j ms
=
V yg)2
+
(YE)2,
whe e
JE
~
Q**E ms/K**R**.
Thus,
he
shea o ce,
which ac s
on he
simple-agglome a es a ached
o
he
ou e laye
o he
complex-agglome a e,
is
Fs ~
K*Y msR*
~
(k*)D+W(W*)2
+
(Q*E ms)2. He e
i
is
assumed
N* = (k*)D and
ha
he
complex-
agglome a e cha ge
is
equally dis ibu ed among
he
simple-agglome a es
(Q** =
N*Q*,
whe e
Q* is he
simple-agglome a e cha ge). Simple-agglome a es
a ach
o he
complex-agglome a e
as
long
as he
shea o ce
is
smalle han
he
a ac i e o ce. Thus,
he limi condi ion
Fs = F dW
leads
o he
equa ion
k* ~
5o1/(D+2)
o
es ima ing
he
complex-agglome a e
size
d**,
whe e
he
agglome a e Bond numbe
Bo is
de ined
as he
a io
o he
a ac i e o ce
F dW o
V(w*)2
+
(Q*E ms)2.
The
simple-agglome a e weigh
is
W* =
NPWP,
whe e Wp
is
he nanopa icle weigh
and
Np
is
he numbe
o
p ima y nanopa icles
in
he simple-
agglome a e, which
is
assumed
o be Np =
(d*/dp)D.
The simple-agglome a e cha ge
Q* is
needed
o
es ima e
he complex-agglome a e size. P o ided ha simple-
agglome a es
a e
unal e ed
by he
elec ic
ield,
i is
Q*
=
Q*0*/N%,
whe e
Q*0*
and
N%
a e he
complex-
agglome a e cha ge and numbe
o
simple-agglome a es
in
he
complex-agglome a e
o E = 0,
espec i ely.
Thus
k*=k*01+ Q 0* ™
(2)
whe e
k*0
=
d^*/d*
is
he
ela i e complex-agglome a e
size
in
he absence
o
elec ic
ield.
Using a ypical alue
o
F dW
=
10 nN,
d* =
30,11m, and
D =
2.5
-
2.6 [4],
i
is p edic ed
d*0*
~
150^m
in
he absence
o
applied elec-
ic
ield.
Eq.
2
can
be
used
in
he
modi ied
RZ
equa-
ion
(Eq.
1).
Figu e
6
shows
he
ela i e a ia ion
o
he pa icle olume ac ion
as
a
unc ion
o
he
supe -
icial gas eloci y
o
an
imposed ex e nal ield
o
peak
s eng h
E0 =
1.25
kV/cm.
The
lines ep esen
he
p e-
dic ions
by
he model
o
di e en alues
o
he
complex-
agglome a e cha ge. Complex-agglome a e size
in he
absence
o
elec ic ield and ac al dimension
a e
aken
om ou p e ious de i a ion based on bed expansion
ex-
pe imen al da a
(d** =
226 /im,
D =
2.588). Rema k-
ably, he p edic ion i s
o
he da a
in
he luidlike egime
o
a
alue
o
he
complex-agglome a e cha ge
(Q*0*
=
1.9
x
10-14
C)
simila
o he
o de
o
magni ude
es i-
ma ed
by
acking agglome a e ajec o ies.
The
model
p edic ion
is
howe e qui e sensi i e
o
he
agglome a e
cha ge. Conside able de ia ion om he da a
is
ob ained
o o he cha ge alues
o
same o de
o
magni ude.
This
p e en s
us
om
a
conclusi e s a emen
on
he
model
alidi y. Mo eo e , he ole
o
collisions
on
deagglome -
a ion
is no
conside ed
by
he model.
ACKNOWLEDGMENTS
We acknowledge Spanish Go e nmen Agency Minis e-
io
de
Ciencia
y
Tecnologia (con ac FIS2006-03645)
and
Jun a
de
Andalucia (con ac FQM 421).
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H.
P.
Meissne , H.
S.
Mickley,
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J R.
Melche , P oceedings
o
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IEEE
65
(1977)
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&
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