EPA-600/2-76-040
February 1976
Environmental Protection Technology Series
EVALUATION OF ELECTROFLUIDIZED BED
industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park. North Carolina 27711
-------
EPA-600/2-76-040
February 1976
EVALUATION OF ELECTROFLUIDIZED BED
by
K. P. Ananth and L. J. Shannon
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
Contract No. 68-02-1324. Task 13
ROAP No. 21ADL-029
Program Element No. 1AB012
EPA Project Officer: Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
-------
TABLE OF CONTENTS
Page
List of Figures iv
List of Tables iv
Acknowledgments '... v
Introduction 1
Background 2
Technical Evaluation . . 6
Model of Fluid Bed ........ 6
Predicted Performance of Fluid Bed 10
Effect of Changes in Bed Parameters on Performance 17
Collector-Particulate Interaction Mechanisms 17
Dynamics of Fluid Bed 18
Conclusions 21
References . 22
iii
-------
LIST OF FIGURES
No. Title Page
1 Possible Configuration for Electrofluidized Bed .... 4
2 Alternate Configurations for an Electrofluidized Bed
(A) Cros:-Flow; (B) Co-Flow 5
Overall Collection Efficiency as a Function of Particle
Size and Fluidization Velocity (D = 100 um) .... 15
Overall. Collection Efficiency as a Function of Particle
Size and Fluidization Velocity (D = 150 urn) .... 16
LIST OF TABLES
No. Title
1 Single Target Efficiency as a Function of Particle
Size and Fluidization Velocity (DC = 100 um) .... 11
2 Single Target Efficiency as a Function of Particle
Size and Fluidization Velocity (D,, = 150 um) .... 12
G
3 Overall Collection Efficiency as a Function of Particle
Diameter and Fluidization Velocity (D = 100 um) . . 13
4 Overall Collection Efficiency as a Function of Particle
Diameter and Fluidization Velocity (D = 150 um) . . 14
c
iv
-------
ACKNOWLEDGMENTS
This report was prepared for EPA/IERL-RTP under Contract No. 68-02-1324,
Task No. 13. The work was performed by Dr. K. P. Ananth, Senior Environ-
mental Engineer, and Dr. L. J. Shannon, Assistant Director, Physical Sci-
ences Division.
-------
INTRODUCTION
The work presented in this report was performed by Midwest Research In-
stitute for the Industrial Environmental Research Laboratory-RTF as Task
Order No. 13 on Contract No. 68-02-1324. The aim of this investigation
was to analyze and evaluate the concept of using an electrofluidized bed
for the collection of fine particulates.
A literature search has been conducted as part of this investigation and
it was found that the general concept is analogous to that of an electro-
static precipitator except for the electrode setup and actual mode of
collection. Instead of collecting particles on stationary electrodes as
in an electrostatic precipitator, the electrofluidized bed uses several,
largej mobile, charged particles which function as collectors in the
fluidized state.
The electrofluidized bed concept has been evaluated on a semiquantitative
basis, because even though it is possible to theoretically predict the
single target collection efficiency, it is not possible to model accurately
the dynamics of the bed. Our evaluation is contained in the following
sections and is preceded by a section on background information.
-------
BACKGROUND
Fluidized beds have been proposed as particulate filtration media from
time to time; but investigations of fluid beds as particulate control sys-
tems are quite limited. Meissner and Mickley conducted laboratory studies
of sulfuric acid mist removal via fluidized beds.i' Collection efficiencies
2 /
up to 93% were obtained in their tests. Scott and Guthrie- coiiducted
studies with fluidized beds using dioctyl phthalate droplets (0.5 to 1.1 um).
Collection efficiencies varied from 70% at high superficial velocities to
90% at low superficial velocities. Blackii-L' has conducted one of the more
detailed studies of fluid beds as particulate filters. In Black's experi-
mental program, the effectiveness of a fluidized bed in removing particulates
from an air stream was investigated at superficial gas velocities bf 8.75
to 25.0 ft/min.- Bed height-to-diameter ratios were varied from two to six.
The aerosols chosen were ammonium chloride and tobacco smoke. Concentra-
tion of aerosol ranged from 0.03 to 8.3 mg/cu m. Filtration efficiencies
of the fluidized bed in removing either ammonium chloride or tobacco parti-
cles of submicron size ranged from approximately 50 to 90% on a count basis.
Lowest efficiencies were encountered at highest gas flow rates and lowest
bed heights.
Jackson and coworkers have recently reported results of a study to com-
pare directly the- collection of fine particles by a bed of granules
operating in both the fixed and fluidized states.5,6/ Monodispersed aerosols
of dioctyl phthalate, in sizes of 0.67 and 1.2 microns were collected in.
a bed of granules of porous activated alumina having a mean size of 175
microns. Bed depths of 1 to 4 in. resulted in collection efficiencies up
to and exceeding 99% in the fixed state for either particle size; effi-
ciencies dropped markedly, upon bed fluidization and with increasing gas
velocity, to 70 to 80% at twice the initial fluidization velocity.
The mechanisms involved in electrostatic filtration of aerosols in fixed
and fluidized granular beds were studied at the Air Cleaning Laboratory
at Harvard University from 1955 to 1958.1' Polystyrene spheres were usec.
as the bed media in these studies. Polystyrene granules were charged in
situ by means of interspersed wires in the filter matrix or were remotely
-------
charged using a vibrating cylindrical Lucite trough. The test aerosol of
gentian violet microspheres was charged to 18 to 64 electron charges (posi-
tive) per particle by a spinning disc generator. A fixed bed of polystyrene
.granules (280 micron diameter) with a surface charge density of 0.09 esu/cm2
had a 64% collection efficiency for atmospheric dust as compared with a
96% efficiency for a fluidized bed expanded to 1207° of the original bed
depth.
Zahedi and Melcher at MIT have recently proposed the use of an electro-
fluidized bed (EFB) for the collection of fine particulates.l/ The EFB
concept is probably an outgrowth of studies on: (1) electrically induced
agglomeration between particles and (2) electrically augmented scrubbers
Q 11 /
which use charged water drops to collect oppositely charged particulates.
In the EFB, the collection sites are envisioned to be particles about
100 u in size.The charge on these particles is continuously renewed
by the application of an ambient electric field. The fine particles,
which are to be collected on the large bed particles through the agent
of the electric field, are charged prior to entering the collection
volume containing the large particles. The poles of the charged large
particles collect the oppositely charged fines. Melcher suggests that
gas velocities of the order of 3 to 8 ft/sec be used in the EFB.*
A schematic configuration, as presented by Melcher, is shown in Figure 1
to illustrate the general features of an EFB. Gas to be cleaned enters
from below through vertical ducts and is diverted through the parallel
sections of the EFB at relatively low velocity to be expelled at the top
through alternate vertical ducts. Particles are removed in the fluidized
bed by interaction with individual collector bodies comprising the bed
material. The charging section is used to charge the particles in the
gas prior to entering the EFB. Figure 2 illustrates alternate configu-
rations for an EFB.
Based on information presented above, as well as information contained in
the literature, the following evaluation of the EFB has been performed.
-------
outlet of cleaned gas f-e>-
electro-
fluidized
bed
1 Z
inlet of gas
to be cleaned
/ 7
corona charging of fines
Figure 1. Possible configuration for electrofluidized bed,
-------
11 iiiiiiiiiiiiiiiimm
H
BED
BED SUPPORT
GAS AND PARTICIPATE
(A)
(B)
Figure 2. Alternate configurations for an electrofluidized bed-
(A) cross-flow; (B) co-flow.
-------
TECHNICAL EVALUATION
The development of a model of a fluid bed as a particulate control device
is hampered by many of the difficulties involved in modelling a fabric
filter system (i.e., interference effects of neighboring particles, varia
tion of collector body surface characteristics with time, interaction of
collector mechanisms). A further complication arises because the precise
behavior of a mass of fluidized solids is difficult to define &nd is
strongly dependent u^on the particle size of the bed material.
Extensive effort to model a fluid bed was deemed to be outside the scope
of this evaluation, and a simple model of a fluid bed acting as a partic-
ulate collecting, device was developed in order to define some of the char
acteristics of such, a collection system.
MODEL OF FLUID/ BED"
The fluid bed was assumed to be composed of an array of collector bodies
with diameter Dc and interference effects of bed collector bodies were
assumed negligible. Utilizing the concept of single target efficiencies,
the following expression can be written for the number of particles re-
moved from an aerosol stream as it 'traverses an element of the bed of
length dL
- f5 - \ ".« -r «
where Neff = number of effective collector bodies per unit volume of bed,
Dc = diameter of collector body,
dL = incremental length of the bed,
n = number of particulates per unit volume at entrance to bed
element,
j\ = overall single particle target efficiency.
6
-------
The number of effective collector bodies (Neff) is used in Eq. (1) rather
than the total number of collector bodies (N^p) in order to make allowance
for the actual behavior of a mass of solids fluidized by a gas. Under con-
ditions for fluidization some of the gas travels through the bed between
individual bed particles, but much of it travels through in "bubbles" or
pockets and experiences minimal contact with the bed particles. In the
bed itself the bed particles move in distinct aggregates which are lifted
by the bubbles or which move aside to let the bubbles pass.!/ The total
number of bed particles will not be involved in aerosol collection; hence,
the need to use the term Neff rather than
Integration of Eq. (1) within appropriate limits results in:
n_
rdn r~
J T= \^
n. v
eff 4
dL
(2)
or
- \ Nef f -T L
(3)
By defining the extent of aerosol penetration, R , as , Eq. (3) can
be written as 1
P = exp
Neff
The overall collection efficiency of the bed, E , is given by Eq. (5)
2
E = 1 - P = 1 - exp - 7)T Nef f
TTD,
(5)
In order to utilize Eqs. (4) or (5) to predict the performance of a fluid
bed, expressions for Neff and TL must be known. In any actual fluid bed
operating on an industrial source of particulate pollutants, Neff and 71T
will be dependent upon the characteristics of the particulate pollutant
and carrier gas stream. Furthermore, both parameters are likely to be
functions of bed age (i.e., vary in time). The manner in which the param-
eters vary with bed age is unknown and will not be specifically included
-------
in our model. However, both 7]^-- and Neff can be varied so that changes
in bed conditions with time can.be qualitatively assessed.
Neff is assumed to be a function of NTQT which is in turn a function of
the diameter of the bed material and bed porosity. Assuming spherical
bed particles, the total number of collector bodies, NTQT , is related to
the ideal bed porosity, e , by
_ _tptal volume of solids
NTOT
volume per collector body
_ (1-e) (total volume of bed)
(6)
TiD3/6
c
Vbed
As noted previously, Neff < NTQT but the functional relationship between
. F
we assumed that Eq. (7) is applicable
and NTor[, is not known. For the purpose of estimating bed performance,
6 or (l-e) Vbed
Neff = - - - , (7)
where a is the bed availability factor (a < 1). In an actual fluid bed,
the total bed porosity is the sum of the porosity between individual ag-
gregates, eb , and the porosity within individual aggregates, e^ (i.e.,
e = £b + e£ (1-6^))- Also, the bed availability factor is really related
to the microstructure of the bed and hence to e^ a^d e^ . An in-depth
analysis of this interrelationship was judged to be outside the scope of
this task and we assumed that &, e> et>» and e^ could be uncoupled as in-
dicated in Eq. (7). Assuming Vbe(j = 1 ft3 and porosity = 0.7 (typical
for fluid beds), Eq. (7) simplifies to
6 (1-0.7) a 1.8 a particles
Neff = ^ ^ - o- "^ -
nD ftJ of bed
-------
Substitution of Eq. (8) into Eq. (5), results in the following expression
for the collection efficiency of the bed
["
[
-0.45 a T}T L~|
^- (9)
The overall single target efficiency, TL » was calculated from Ref . 12
assuming that only electrostatic and inertial forces are important in an
electrof luidized bed composed of collector bodies of 100 urn and 150 urn
10 /
diameter particles. George and Poehleini=-' have determined single target
efficiencies for a two-body system expressing inertial and electrostatic
forces in terms of dimensionless parameters. The dimensionless constants
for inertial and coulombic force parameters given in Ref. 12 are
(10)
18 n Dc
and C Qi Qo
ES* -- 7 - -i-2= - x- , (11)
3T/ e0 )i V0 Dp (Dc+Dp)2
respectively. In the above equations
C = Cunningham correction factor (assumed to be 1),
p_ = density of particle f
= 2.8 *^r (assumed to be iron foundry particles),
cm
V = gas velocity in cm/sec,
= fluidization velocity in the EFB,
D = diameter of particle,
= 1 x 10~4 cm, 0.8 x 10'4 cm and 0.5 x 10"4 cm,
u = viscosity of gas,
= 1.8 x 10"4 poise,
D = collector diameter,
= 100 x 10'4 cm, 150 x 10'4 cm,
Q^ = particle charge in coulomb, and
0.2 = collector charge in coulomb.
13/ ,
* A more recent report, in response to reference 12, actually shows that
the denominator of Eq. (11) should contain D_ instead of (D,. + D_) .
c c p
However, DC » D , therefore the two expressions are almost identical.
This fact is also acknowledged in reference 13.
-------
The range of superficial gas- velocities for stable bed f luidization (Vo
in Eqs. (10) and (11)) is about 3 to 10 times the minimum velocity for
f luidization. The minimum velocity for f luidization is given b
Pc em
_
vmin 150 u (l-em)
where g = acceleration due to gravity,
Pg - density of bed solids,
Pf = density of fluid (i.e., air in this case)
D = diameter of collector bodies in bed,
]i - viscosity of fluid, and
^ = minimum porosity.
The minimum velocity for fluidization is typically about 0.1 ft/sec
depending upon the physical characteristics of the bed and the fluidizing
medium. For purposes of illustrating the effect of fluidizing velocity
on the inertial and electrostatic constants and hence on the single particle
target efficiency, we used a range of velocities from 0.1 to 3.0 ft/sec.
Equations (10) and (11) were used to calculate ij- and ES for different
particle and collector diameters under varying velocity conditions.
Aerosol particles were assumed to be charged to saturation in a field of
2.5 x 10 v/m. Bed particles were assumed to be charged to saturation
in a field of 5 x 105 v/m.' The single target efficiency was then ob-
tained from the graph of Ref. 13 which shows the single particle target
efficiency as a function of i|r and ES, the inertial and coulombic force
parameters. The results of the calculations are shown in Tables 1 and 2.
PREDICTED PERFORMANCE OF FLUID BED
The collection efficiency for a bed length of 1 ft was calculated using
Eq. (9) and the single particle target efficiencies given in Tables 1 and
2. The constant, ot , was assumed to vary from 10"^ to 10"* which is
equivalent to assuming that Neff varies from about 10-* to 10 particles
per cubic foot for collector bodies of 100 urn and 150 urn in diameter.
The computed overall collection efficiencies are shown in Tables 3 and
4 and graphically presented in Figures 3 and 4, respectively.
10
-------
Table 1. SINGLE TARGET EFFICIENCY AS A FUNCTION OF
PARTICLE SIZE AND FLUIDIZATION VELOCITY^/
Particle
Diameter
(cm)
1 x 10~4
1 x 10'4
1 x 10"4
1 x 10'4
1 x 10'4
0.8 x ID'4
0.8 x 10"4
0.8 x KT4
0.8 x 10"4
0.8 x 10'4
0.5 x 10'4
0.5 x 10"4
0.5 x 10'4
0.5 x 10"4
0.5 x 10'4
Fluidization Particle
Velocity Charge
(cm/sec)
3.04
15.20
30.48
60.96
91.44
3.04
15.20
30.48
60.96
9-1 .44
3.04
15.20
30.48
60.96
91.44
(coulombs)
4.97 x 10"17
4.97 x 10"17
4.97 x 10"17
4.97 x 10 ~17
4.97 x 10'17
3.20 x 10-17
3.20 x 10"17
3.20 x 10-17
3.20 x 10-17
3.20 x 10"17
1.25 x 10~*7
1.25 x 10"
1.25 x 10-17
1.25 x 10-17
1.25 x ID"17
ES
3.53
0.70
0.35
0.18
0.12
2.86
0.57
0.28
0 . 14
0.095
1.79
0.36
0.18
0.09
0.02
1
0.0026
0.0132
0.0263
0.0527
0.0790
0.0017
0.0084
0.0169
0.0337
0.0506
0.0007
0.0033
0.0066
0.0132
__
Single
Target
Efficiency
14.0
2.50
0.90
0.25
0.16
10.0
2.0
0.80
0.25
1.40
0.64
__
a/ Collector diameter = 100 x 10"4 cm; collector charge = 1.04 x 10"
coulomb.
11
-------
Table 2. SINGLE TARGET EFFICIENCY AS A FUNCTION OF
PARTICLE SIZE AND FLUIDIZATION VELOCITY!/
Particle
Diameter
(cm)
1 x 10'4
1 x 10"4
1 x 10'4
1 x ID'4
1 x ID'4
0.8 x 10 ~4
0.8 x IO"4
0.8 x 10 ~4
0.8 x 10'4
0.8 x IO"4
0.5 x 10 "4
0.5 x 10 "4
0.5 x IO"4
0.5 x' 10'4
0.5 x 10"4
Fluidization
Velocity
(cm/sec)
3
15
30
60
91
3
15
30
60
91
3
15
30
60
91
.04
.20
.48
.96
.44
.04
.20
.48
.96
.44
.04
.20
.48
.96
.44
4
4
4
4
4
3
3
3
3
3
1
1
1
1
1
Particle
Charge
(coulombs)
.97
.97
.97
.97
.97
.20
.20
.20
.20
.20
.25
.25
.25
.25
.25
x
x
X
X
X
X
X
X
X
X
X
X
X
X
X
io-17
10"17
io-17
io-17
io-17
10"17
10 -17
io-17
io-17
io-17
10-17
io-17
io-17
10-17
io-17
ES
3
0
0
0
0
2
0
0
0
' 0
1
0
0
0
0
.57
.71
.36
.18
.12
.88
.58
.29
.14
.10
.81
.36
.18
.09
.06
0
0
0
0
0
0
0
0
0
0
0
0
. 0
0
0
Single
Target
ty Efficiency
.0018
.0090
.0180
.0370
.0550
.0011
.0056
.0110
.0230
.0340
.0004
.0020
.0043
.0085
.0130
14
2.5
1.2
0.40
0.13
11.0
2.0
1.0
0.34
0.14
1.30
0.70
0.28
__
a/ Collector diameter = 150 x 10 cm; collector charge = 2.35 x 10"13
coulomb.
12
-------
Table 3. OVERALL COLLECTION EFFICIENCY AS A FUNCTION
OF PARTICLE DIAMETER AND FLUIDIZATION VELOCITY-/
Particle
diameter
(cm)
-4
1 x 10 7
-4
i x 10 ;
4
i x 10 ;
~£L
1 x 10 7
-4
1 x 10
-4
0.8 x 10 7
-4
0.8 x 10 ;
A
0.8 x 10 ;
.A
0.8 x 10 ;
A
0.8 x 10
0.5 x 10"4
0.5 x 10~7
4
0.5 x 10
0.5 x 10~4
0.5 x 10
Fluidization
velocity
(cm/sec)
3.04
15.20
30.48
60.96
91.44
3o04
15.20
30.48
60.96
91.44
3.04
15.20
30.48
60.96
91.44
Single
target
efficiency Overall collection efficiency (%)
(7.)
14.0
2.50
0.90
0.25
0.16
10.0
2.0
0.80
0.25
--
1.4
0.64
--
--
a = lO-^
1.9
0.34
0.12
0.03
0.02
1.36
0.27
0.11
0.03
--
0.19
0.09
__
a = 10-3
17.45
3.37
1.23
0.31
0.22
12.8
2.7
1.09
0.31
--
1.9
0.87
__
a = 10'2
85.31
29.0
11.6
3ol
2.17
74.59
23.97
10.38
3.1
17.45
8.39
__
a = 10"1
100.0
96.75
70.86
27.03
19.68
lOOoO
93.55
66.58
27.0
85.31
58.39
__
a/ Collector diameter = 100 um.
13
-------
Table 4. OVERALL COLLECTION EFFICIENCY AS A FUNCTION
OF PARTICLE DIAMETER AMD FLUIDIZATION VELOCITY-/
Particle
diameter
(cm)
-4
1 x 10 7
-4
i x 10 ;
-4
1 x 10 .
-4
i x 10 7
-a
1 x 10
-4
0.8 x 10 7
4
0.8 x 10 7
A.
0.8 x 10 7
-4
0.8 x 10 7
-4
0.8 x 10
-4
0.5 x 10 7
-4
0.5 x 10 7
-4
0.5 x 10 ;
-a.
0.5 x 10 7
4
0.5 x 10
Fluidization
velocity
(cm/sec)
3.04
15.20
30.48
60.96
91.44
3.04
15.20
30.48
60.96
91.44
3.04
15.20
30.48
60.96
91.44
Single
target
efficiency
a)
14.0
2.50
1.20
0.40
0.13
11.0
2.0
1.0
0.34
0.14
«
1.30
0.70
0.28
< B
Overall
cy = lO'4.
1.27
0.23
0.11
0.04
0.01
0.99
0.18
0.09
0.03
0.01
M
0.12
0.06
0.03
~~
collection
a = 10"3 a
11.96
2.25
lolO
0.36
0.12
9.53
1.80
0.91
0.31
0.13
w M
1.18
Oo63
0.25
efficiency
= 10~2 a
72.03
20.35
10.34
3.57
1.13
63.25
16.64
8.70
3.05
1.27
__
11.16
6.17
2.52
"
(%)
= 10'1
100
89.72
66.45
30.51
11.16
100
83.8
59.75
26.61
11.96
«_
69.36
47.11
22.49
"""
£/ Collector diameter = 150 pm.
14
-------
o
c
LU
c
u
J
6
100.0
10.0
1.0
0.1
COLLECTOR DIA
= 100 MICRONS
~ Legend
A A ex = 1Q-3
e 9 c* = 10"2
B ex = 1Q-1
V0 = 0.5 ft/sec
(15.2 cm/sec)
VQ = 1 ft/sec
(30.48 cm/sec)
0
0.2
0.4 0.6 0.8
Particle Size (Microns)
1.0
1.2
Figure 3. Overall collection efficiency as a function of particle size
and fluidizatio.n velocity (Dc = 100 urn).
15
-------
100.0
x
o
.2J
'o
O
JU
"o
U
S
(U
10.0
Legend
1.0 -
0.1
0
COLLECTOR DIA
= 150 MICRONS
ex = io
VQ = 0.5 ft/sec
(15.2 cm/sec)
V0 = 1 ft/sec
(30.48 cm/sec)
V0 = 2 ft/sec
(60. 96 cm/ sec)
J I I I I I I I
I I
0.2
0.4
0.6
0.8
1.0
1.2
Particle Size (Microns)
Figure 4. Overall collection efficiency as a function of particle size
and fluidization velocity (D = 150 urn)
c
16
-------
Inspection of Tables 3 and 4 or Figures 3 and 4 indicates that: (1) pre-
dicted collection efficiencies decrease with decreasing aerosol particle
size for a given superficial., velocity and bed availability factor (Q?);
(2) predicted collection efficiencies decrease with increasing superficial
velocity at a given bed availability factor; (3) collection efficiencies
decrease with decreasing bed availability at a given superficial velocity;
and (4) collection efficiency decreases with increasing size of the fluid
bed particles. The predicted behavior of the fluid bed is in general agree-
ment with the experimental results of work on fluidized beds discussed in
the background section of this report.
A major weakness of the simple model proposed is that it makes no allowance
for changes in bed parameters with time. Bed age is expected to exert
some influence on performance in an actual industrial gas cleaning applica-
tion. However, the agreement between the predicted and experimental per-
formance suggests that the very simple model can be used to assess qualita-
tively the changes in bed performance with changes in major bed parameters.
The probable influence of time dependent factors (i.e., bed age) on fluid
bed performance is discussed in more detail in the next section.
EFFECT OF CHANGES IN BED PARAMETERS ON PERFORMANCE
As mentioned in the preceding section, changes in bed parameters are
anticipated with time in any actual industrial application of electro-
fluidized beds. Among the changes anticipated are:
1. Changes in bed particle size
2. Changes in surface characteristics of bed particles
3. Changes in bed availability
These changes are expected to influence both the collector-particulate
interaction mechanisms and the dynamics of the fluid bed itself. Some
of the possible effects that may occur are discussed next.
Collector-Particulate Interaction Mechanisms
Electrostatic phenomena are a function of particle properties as well as
particle charge. The influence of an increase in collector body size can
be qualitatively assessed, in terms of our simple model, by writing the
electrostatic constant in the alternate form suggested by George and
Poehleiul2./
17
-------
ES =
If Dc » Dp as is our case
ES »
D
(Dp+Dc)
2
(13)
C e
3 u
(14)
where epsi = surface gradient of charge on particulates,
eps -surface gradient of charge on collector bodies,
6 = dielectric constant.
Equation (14) indicates that the parameter, ES , is a direct function of
the charge surface gradient of the collector. An increase or decrease
in eos , which is a function of collector size and the charge-to-mass
ratio of particles, will be reflected in a corresponding increase or
decrease in ES , and depending upon the superficial velocity, a change
in the single target collection efficiency and overall bed collection ef-
ficiency.
Changes in bed particle size and shape can also alter the particle charging
characteristics of the bed. The net effect may be detrimental or favorable
depending upon the change in the charge per bed particle.
An increase in the collector body size or a change in collector body shape
can also influence particle to particulate cohesion or adhesion. Here we
refer to the interaction between collector body and aerosol particle fol-
lowing the electrically induced collision of the two.
Dynamics of Fluid Bed
Because of the complexity of the flow and the inherent mechanical in-
stablity of gas-solids fluidized systems, operational problems may occur
if the characteristics of the bed particles change during the course of
particulate collection.
18
-------
It may very well be that the gas-solids mixing patterns determine the ef-
fect of electrostatic forces, and hence the performance of an EFB.
The dynamics of fluid beds are influenced by the design and internal
configuration of the bed, the design of the gas-inlet system, the size
and size distribution of bed particles, and the shape and density of
the bed particles. Bed design and internal configuration can change
gas-solids mixing patterns and slugging (i.e., formation of large gas
bubbles) can occur in improperly designed beds. Proper placement of
electrodes may be quite important in this regard. The gas distribu-
tion system will in-fluence fluidization characteristics--especially
channeling tendencies. Channeling will decrease the bed availability
factor, o/ , and. as shown in Figures 3 and 4, overall collection efficiency
is a strong function of bed availability.
The characteristics of the solid phase are related to various abnormalities
of fluid beds. Bed particle size and size distribution, bed particle shape
and bed particle density all influence channeling. Quantitative correla-
tions of bed particle properties with channelization tendencies are not
available. However, irregular particles exhibit a greater tendency to
channeling than do smooth spherical particles. Increasing the size of
bed particles generally results in a decrease in channeling tendencies.
Even under normal or good conditions for fluidization, much of the gas
travels through the bed in bubbles. This phenomenon is called aggrega-
tion. The causes of aggregative fluidization are not well defined. How-
ever, particle size of the bed material is a factor with a. trend toward
less aggregation with increasing particle size.
Size segregation will also occur in gas-fluidized systems if the solids
are not of uniform size or density. The finer or less dense bed material
will tend to move toward the upper part of the bed.
The preceding brief discussion of bed dynamics suggests several problems
which might occur in an EFB during aging or between cleaning or regenera-
tion cycles. First, the size of the bed particles will increase and the
shape will change as a result of particulate collection. The net in-
fluence of these two changes on collection efficiency is difficult to pre-
dict because increasing bed particle size generally improves bed dynamics
while a change in shape to a more irregular particle can result in in-
creased channeling. Also, as shown in Figures 3 and 4, an increase in bed
particle size would be expected to result in a decrease in overall collec-
tion efficiency.
19
-------
The suggestion that the most attractive application of the EFB is where
the large particles comprising the bed are made up of the same material
as that being collected is open to question. The start-up of such a
device might involve seeding the bed with foreign particles. The par-
ticulate will then have to be captured on these particles starting the
agglomeration process. At some point in the operation, presumably under
steady state conditions, the bed particles will have to be removed to
retain the required population density of the collector bodies. In light
of the preceding comments on bed dynamics, attainment of a stable fluidized
self-agglomerating system may be very difficult. It is also questionable
whether stable agglomerates of 100 to 200 urn can actually be produced in a
fluid bed system.
20
-------
CONCLUSIONS
Our analysis of the EFB concept indicates that theoretically at low super-
ficial velocities and high values of electrostatic forces fluidized beds
augmented by electrostatic forces will be more effective for the removal
of particulates than are conventional fluid beds. It is not clear that
the expected performance can actually be achieved because of the inherent
problems involved in operating fluid beds. Previous experience would indi-
cate that single pass fluidized beds are not likely to attain collection
efficiencies much in excess of 90%. Attainment of high efficiencies by
staging may be possible.
The performance of an EFB will be dependent upon both the electrostatic
phenomena occurring, in the bed and the bed dynamics. Electrostatic phe-
nomena have been considered in detail, in the literature, but the im-
portance of bed dynamics needs further investigation. Experience gained
from the use of fluid beds in the chemical industry indicates that bed
dynamics may actually be the more important factor influencing collec-
tion efficiency.
21
-------
REFERENCES
1. Meissner, H. P., and H. S. Mickley, "Removal of Mists and Dusts from
Air by Beds of Fluidized Solids," Industrial and Engineering Chem-
istry. 4L,:;i238 (1949).
2. Scott, D. S., and D. A. Guthrie, "Removal of a Mis; in *» Fluidized
Bed," Can. J. Chem. Engr.. 37_, 200 (1959).
3. Black, C. H., and R. W. Boubel, "Effectiveness of a Fluidized Bed
in Removing Submicron Particulate from an Air Stream," I/EC
Process Design and Development. 8, 573 (1969). .
4. Black, C. H., "Effectiveness of a Fluidized Bed in Filtration of
Airborne Particulates of Submicron Size," Ph.D. Thesis No. 67-5646,
University Microfilms, Ann Arbor, Michigan (1967).
5. Yankel, A. J., R. G. Patterson, and M. L. Jackson, "Fine Particle
Collection with a Fixed-Fluidized Bed," under review by Industrial
and Engineering Chemistry, Process Design and Development, 1973.
Presented at the Annual Meeting, Air Pollution Control Associa-
tion, Pacific Northwest International Section, Eugene, Oregon,
15 November 1972.
6. Jackson, M., "Fluidized Beds for Submicron Particle Collection,"
presented at the 75th National Meeting, AIChE, Detroit, Michigan,
3-6 June 1973.
7. Juvinall, R. A., et al., "Sand-Bed Filtration of Aerosols: A Re-
view of Published Information on Their Use in Industrial and
Atomic Energy Facilities," Argonne National Laboratory Report
ANL-7683, June 1970.
8. Zahedi, K., and J. R. Melcher, "Electrofluidized Beds in the Filtration
of Submicron Particulate," Paper No. 75-57.8, Presented at the APCA
Meeting, June 1975.
22
-------
9. Melcher, J. R., and K. S. Sachar, "Electrical Induction of Particulate
Agglomeration," Final Report, EPA Contract No0 68-002-0018, Task
No. 7, 10 August 1971.
10. Melcher, J. R., K. S. Sachar, J. F. Hoberg, and E. B. Devitt, "Re-
search on Systems of Charged Droplets and Electric Fields for the
Removal of Sub-Micron Particulates from Industrial Gases," Progress
Report, Phase 1, EPA Contract No. 68-002-0018, 1 April 1973.
11. Melcher, J. R.', and K. S. Sachar, "Charged Droplet Technology for
Removal of Particulates from Industrial Gases," Final Report, EPA
Contract No. 68-002-0018, Task No. 8, 1 August 1971.
12. George, H. F., and G. Wo Poehlein, Environ. Sci. and Tech., <8_ (1),
46 (1974).
13. Nielsen, K. A., and J. C. Hill, Environ. Sci. and Tech., £ (8), 767
(1974).
14. McCabe, W. L., and J. C. Smith, Unit Operations of Chemical Engineering,
Second Edition, McGraw-Hill, New York (1967).
23
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing}
1. REPORT NO.
EPA-600/2 -76-040
4. TITLE AND SUBTITLE
Evaluation of Electrofluidized Bed
3. RECIPIENT'S ACCESSION-NO.
5. REPORT DATE
February 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
K. P. Ananth and L. J. Shannon
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING CRSANIZATION NAME AND ADDRESS
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-029
11. CONTRACT/GRANT NO.
68-02-1324, Task 13
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 6/74-1/75
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
EPA project officer for this report is D. C. Drehmel, 919/549-8411, Ext 2925.
6. ABSTRACT
The report gives results of an evaluation of the concept of using electro-
fluidized beds for fine particle collection. A simple model was developed to des-
cribe the interaction between an aerosol stream and the bed material. Overall
collection efficiency of the device was theoretically predicted from single target
efficiency values based on electrostatic and inertial forces. Maximum predicted
overall collection efficiencies range from 96% for a 1 micron aerosol particle to
85% for a 0. 5 micron aerosol particle, with collector bodies of size 100 micron,
an availability factor of 0.1, and fluidization velocities of 0. 5 ft/sec. An increase
in fluidization velocity or a decrease in collector number density (i. e. , larger
collector bodies) diminishes the overall collection efficiency for the conditions used
in this investigation. The performance of the electroflmdized bed depends upon the
interaction of the electrostatic forces and bed dynamics. The inherent instability
of solids flow patterns in gas-fluidized beds may limit the collection efficiency that
can be achieved.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
Air Pollution
Fluidizing
Electrostatics
Aerosols
Air Pollution Control
Particulate
Fine Particles
Electrofluidized Beds
Collection Efficiency
c. COSATI Field/Group
13B
07A,13H
20C
07D
S. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
29
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 O-73)
24
------- |