EPA-600/2-76-125
May 1976
Environmental Protection Technology Series
APPLICATION OF FOAM SCRUBBING TO
FINE PARTICLE CONTROL
Phase I
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the U. S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
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This document is available to the public through the National Technical Informa-
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EPA-600/2-76-125
May 1976
APPLICATION OF FOAM SCRUBBING
TO FINE PARTICLE CONTROL
PHASE I
by
T.E. Ctvrtnicek, T. F. Walburg,
C.M. Moscowitz, andH.H.S. Yu
Monsanto Research Corporation
1515 Nicholas Road
Dayton, Ohio 45407
Contract No. 68-02-1453
ROAPNo. 21ADL-029
Program Element No. 1AB012
EPA Project Officer: Geddes H. Ramsey
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
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ABSTRACT
This report summarizes the knowledge and the data obtained
during the first phase of investigations into foam scrubbing.
Detailed information is presented concerning foam scrubber
theory, experimentation, and economics. The theory pertains
to mechanisms influencing the behavior of fine particles and
the possibilities of their capture by foam. Collection
efficiencies obtained on a bench scale foam scrubber show
that foam scrubbing can be a viable fine particle control
device. Preliminary economic analysis indicates that foam
scrubbing can be competitive with other fine particle collec-
tion devices provided that surfactant solution is recycled
effectively. Verification of surfactant recycle is recom-
mended using a pilot scale foam scrubber.
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CONTENTS
Page
1. Summary 1
2. Conclusions and Recommendations 3
3. Introduction 7
4. Theoretical Aspects of Foam Scrubber Application 9
4.1 Foam Formation 9
4.2 Diffusion Phenomena in Foam 15
4.3 Particle Capturing by Other Mechanisms 23
4.3.1 Impaction and Sedimentation 27
4.3.2 Thermophoresis and Diffusiophoresis 43
4.3.3 Electrostatic Force 56
5. Experimental Work 59
5.1 Experimental Foam Scrubber 60
5.2 Foam Generation 60
5.3 Foam Destruction 63
5.4 Surfactant Selection 66
5.5 Particle Monitoring Equipment 66
5.6 Experimental Aerosol Generation 68
5.7 Foam Scrubber Fine Particle Collection Efficiency 68
6. Foam Scrubber Economics 79
6.1 Accuracy of Estimate 79
6.2 Base Collection Efficiencies 81
6.3 Results 88
7. References 93
Appendixes 97
111
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FIGURES
Number Page
1 Schematic of spray foam generator 10
2 Schematic of perforated plate foam generator 11
3 Cunningham slip correction for liquid and
solid particles 19
4 Displacement of particles in air by diffusion 21
5 Particle collection efficiency 24
6 Particle collection efficiency 25
7 Particle collection efficiency 26
8 Initial and boundary conditions 28
9 Particle collection efficiency 33
10 Particle collection efficiency 34
11 Particle collection efficiency 35
12 Particle collection efficiency 36
13 Particle collection efficiency 37
14 Particle collection efficiency 38
15 Particle collection efficiency 39
16 Particle collection efficiency 40
17 Particle collection efficiency 41
18 Schematic diagram of exhaust gas treatment 46
19 Schematic diagram of air washer using
directly recirculated spray water 52
20 The thermophoretic velocity in air as a
function of temperature gradient 55
21 The diffusiophoretic velocity in air as
a function of water-vapor pressure gradient 55
22 Schematic of electrostatic force between
particles and bubble wall 57
23 Experimental foam scrubber 61
24 Destruction chamber of the foam scrubber 62
IV
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FIGURES (continued)
Number
25 Foam destruction chamber 64
26 Monodispersed aerosol generating system 69
27 Foam scrubber collection efficiency of
polypropylene glycol 425 using 2% Tergitol foam
with rotating disk for foam destruction 72
28 Collection of DOP aerosol using foam scrubbing
with rotating disk for foam destruction 73
29 Foam scrubber collection efficiency of
polypropylene glycol 425 and DOP using
large-celled (3.9 mm) foam 76
30 Collection efficiency values for distilled water
sprayed on the screen versus water flow rate,
DOP particle size: 0.7 - 1.0 ym 78
31 Extrapolated fine particulate control efficiencies 82
32 Capital cost for fine particulate control 90
33 Operating cost for fine particulate control 91
A-l Bubble diameter versus air flow rate observed
for 1% Tergitol foam 100
A-2 Bubble diameter versus screen opening for 1%
Tergitol foam constant air and liquid flow 101
A-3 Bubble diameter versus surfactant concentration 102
A-4 Photos of foam for bubble size determinations 104
B-la Air flow 1.08 x 10~3 m3/s (2.28 cfm), 1% Tergitol 107
B-lb Air flow 2.32 x 10~3 m3/s (4.91 cfm), 1% Tergitol 107
B-2a Air flow 2.32 x 10~3 m3/s (4.91 cfm), 1% Alkanol 109
B-2b Air flow 1.08 x 10~3 m3/s (2.28 cfm), 1% Alkanol 109
B-3 Foam viscosity measured on Brookfield viscometer 110
C-l Foam drainage rates 115
C-2 Drainage rates for foams of varying stability 119
C-3 Portable sand blast device used for foam
stability measurement 121
C-4 Stability versus concentration for Tergitol foam 123
C-5 Stability of foam generated using Tergitol with
250 mesh screen 126
D-l Air jet whistle used by Branson for foam breakage 131
v
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FIGURES (continued)
Number Page
D-2 Deflectojet nozzle from Spraying Systems Company.
Nozzle has 180° spray angle 132
D-3 Modified and unmodified Teflon disk 135
E-l Expansion ratios for Tergitol foams 137
E-2 Expansion ratios for several surfactants 138
F-l Sample dilution system 142
F-2 Characterization of dilution system 144
G-l Particles generated by foam destruction
with rotating disk 146
VI
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TABLES
Number Paqe
1 Typical Sedimentation Velocity and Coefficient
of Sedimentation 32
2 Typical Coefficient of Impaction 43
3 Surfactant Types 67
4 Sample Data for DOP Particle Collection with
Foam Scrubber Using Rotating Disk - High
Quality Foam - 71
5 Cost Assumptions 84
6 Foam Scrubbing Assumptions 85
7 Electrostatic Precipitation Assumptions 86
8 Fabric Filter Assumptions 86
9 High Energy Wet Scrubber Assumptions 87
10 Foam Scrubbing Capital Costs for 23.6 m3/s
(50,000 acfm) Unit as a Function of
Residence Time 89
11 Foam Scrubbing Operating Costs for 23.6 m3/s
(50,000 acfm) Unit as a Function of
Surfactant Cycle 89
A-l Mean Bubble Diameter for Various Flow Conditions 98
A-2 Bubble Size of Foam Generated with Tergitol 105
B-l Threshold Air Flow Rates 112
C-l Results of Experimental Measurements 117
C-2 Bubble Volume vs. Diameter 124
D-l Destruction with Knockdown Spray 128
Vll
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SECTION 1
SUMMARY
This report summarizes the results of research work investi-
gating the application of foam to fine particle removal from
the gaseous phase. The foam is generated using aerosol gas;
hence the aerosol particles are enclosed inside the small
foam bubbles. Given enough time, the particles diffuse or
settle to the bubble wall and are removed from the gas phase.
Upon foam destruction the particles remain in the foam liquid
and the cleaned gas is discharged.
Section 2 summarizes the conclusions and recommendations re-
sulting from the first phase of this program involving the
foam scrubber theoretical evaluations, bench scale testing,
and preliminary economics. An introduction to the rationale
behind foam scrubbing is given in Section 3. Theories and
theoretical correlations describing the significance of
mechanisms that might be involved in the fine particle col-
lection process by foam are included in Section 4.
Application of foam to fine particle removal was experimen-
tally verified on a 1.2 x 10~3 m3/s (2.5 cfm) bench scale
scrubber. The results are presented in Section 5 and indicate
that 80% or higher collection efficiencies for particles be-
tween 0.2 and 1 ym are obtainable using this device and depend
mainly on foam bubble size, scrubber residence time, particle
and surfactant type, and particle size. Preliminary economics
are presented in Section 6.
1
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
Based on the results of this program's first phase involving
the foam scrubber theoretical evaluation, bench scale develop-
ment, and preliminary economic analysis the following is con-
cluded and recommended:
1. The foam scrubber is a viable method to remove fine
particles from gaseous streams. The method is simple
consisting of basic operations such as pumping the
liquid and gas and spinning the disk to destroy the
foam. The foam is generated by forcing aerosol gas
through a screen sprayed with a surfactant liquid.
Particle collection is believed to take place mainly
by difussion and sedimentation, the mechanisms that
are predictable and rather well understood.
2. Theoretical evaluations and later experimental verifi-
cation indicated that foam bubble size is an important
parameter for fine particle collection. This is due
to the diffusion arid sedimentation distance a particle
has to travel in order to reach the bubble collection
surface. Hence, the smaller the foam bubble the shorter
the residence time required for particle collection.
Uniform and sufficiently stable foams with bubbles
ranging in average size from 0.7 to 3 mm were success-
fully produced in this program.
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3. Foam bubble size is mainly dependent on screen size, gas
flow rate, and concentration of surfactant. The higher
the screen mesh, the gas flow rate, and the surfactant
concentration the smaller the size of foam bubbles. The
relative position of the screen and the spray nozzle are
also critical. With the screen properly wetted the liquid
flow rate was found to have little influence on foam
bubble size and stability. Generally, the smaller size
foam is more stable. Foam stability is further influenced
by the foam wetness (density) and surfactant type (the
wetter the foam, the higher its stability). Four sur-
factant types were used in this program, namely Tergitol®
TMN, Sterox® NM, Alkanol® DW, and Aerosol® OT. At 1%
surfactant concentration, the Alkanol DW foam was found
significantly more stable than foams made from the other
three surfactants.
4. The particle collection was experimentally verified for
fine aerosols of three size categories, 0.56 - 1.0 ym,
0.32 - 0.56 ym, and 0.18 - 0.32 ym, and two aerosol
types, dioctyl phthalate (OOP) and polypropylene glycol.
In order to obtain 90% polypropylene glycol aerosol
collection in Tergitol foam with average size bubbles
of 0.8 mm, 20 - 50 second scrubber residence times were
required (see Figure 27). The larger particles col-
lected better than the smaller ones. Comparable experi-
ments using foam with bubbles between 3 - 5 mm lowered
the collection efficiencies at 50 second residence time
to about 76 - 82%, the larger number representing col-
lection of larger particles.
Collection efficiencies in Tergitol foam with average
bubble size of 0.8 mm using the DOP aerosol,were deter-
mined at 55 - 64% for 20 second scrubber residence time.
The collection increased to 70 - 79% at scrubber
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residence time of 50 seconds. The smaller OOP particles
collected better than the large ones (see Figure 28).
Larger bubble foam (3-5 mm) reduced the DOP collection
at 50 seconds to about 35 - 40%.
5. A good agreement was found between theoretical collection
efficiencies and those determined experimentally. The
collection efficiencies are a function of particle size,
particle density, foam bubble size, and particle residence
time in the bubble. The effect of temperature on collec-
tion efficiency was not experimentally investigated. Due
to a proportional relationship of the particle diffusion
coefficient and the temperature an increase in collection
efficiency with an increased temperature may be expected.
Change in the gas relative humidity from 5% to 45% did
not influence the particle collection significantly.
6. Preliminary economics indicate that the foam scrubber can
be competitive in terms of capital investment costs with
other conventional particle collection devices including
fabric filter, high energy scrubber, and high efficiency
electrostatic precipitator. However, the operating costs
for the foam scrubber appear substantially higher than
those required for the aforementioned conventional de-
vices, mainly due to the cost of surfactants. While the
economic data produced useful comparisons, their appli-
cability must be considered with respect to the presently
limited availability of reliable information on fine
particle collection by the conventional devices and lack
of accurate economic information when these devices are
collecting significant amounts of fine particulates.
Also, the cost data for foam scrubber are based on exper-
imental data observed on a bench scale using non-
industrial aerosols.
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7. Based on the results of the experimental phase of this
program, we recommend that further evaluation of foam
scrubbing be made on a pilot scale preferably with
14 m3/hr (500 cfm) foam scrubber capacity. Additional
pilot scale information is needed in several areas.
These are listed below:
(a) Foam scrubber operation and collection capability
using an industrial fine aerosol.
(b) Surfactant recycle and recovery for reuse in foam
generation.
(c) Generation of uniform and sufficiently stable foam
on a large scale.
(d) Uniform foam transportation in large scale duct.
(e) Effective destruction of large volumes of foam.
(f) Scrubber collection ability of industrial aerosols
at elevated temperatures.
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SECTION 3
INTRODUCTION
A variety of air pollution equipment is available to industry
for containing solid particulate emissions (e.g., electro-
static precipitators, baghouses, inertial separators, wet
scrubbers, settling chambers, impingement separators, and
panel filters). In terms of mass removal efficiency, a
majority of the commercially available particulate control
devices are adequate, with mass removal efficiencies up to
+99%. However, submicrometer particles are not captured by
this equipment; they pass readily into the atmosphere, creat-
ing hazards to human health, as well as visibility and smog
problems.
New concepts and technology for the control of fine particles
are needed to help remedy this situation. In this category
is foam scrubbing. Prior investigations suggested that foam
capture of fine particles may be possible.1 However, in
order to effectively utilize this concept for fine particulate
control in industrial effluents, better understanding of the
mechanism of foam formation, behavior, and destruction and
mechanisms controlling the particle collection in foam are
needed.
Shannon, L. J. Control Technology for Fine Particulate
Emissions. Midwest Research Institute. Kansas City. U.S.
Environmental Protection Agency, EPA-650/2-74-027
(PB 236 646). May 1974. 225 p.
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This program in its first phase has attempted to (1) establish
the theoretical basis for fine particle collection mechanisms
using foam, (2) experimentally verify the established theories,
and (3) assess the preliminary economics of foam scrubbing.
The results are presented below.
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SECTION 4
THEORETICAL ASPECTS OF FOAM SCRUBBER APPLICATION
4.1 FOAM FORMATION
The most practical ways to generate foam for particulate
scrubbing are shown in Figures 1 and 2. Figure 1 shows de-
tails of a spray foam generator where foam is generated in
gas phase. Figure 2 shows a perforated plate foam generator
where foam is generated by forcing gas through a liquid phase.
A detail showing a bubble still attached to a wire screen is
also shown in Figure 1. The pressure of the gas stream pushes
the bubble forward with surface tension forces acting in the
opposite direction. Theoretically, when the pressure force
of the gas stream exceeds the surface tension force, the
detachment of a foam bubble occurs. The equation describing
the two acting forces is presented below.
where P = density of gas
V = velocity of gas
r = radius of screen opening
Y = surface tension of liquid
-------
FLOW OF AIR
FOAM
SOLUTION
FOAM
SPRAY SPRAY
NOZZLE
SCREEN
(a)
SCREEN WIRE
AEROSOL
> 4?rry
(b)
Figure 1. Schematic of spray foam generator.
10
-------
(a)
-p)>27rry
AEROSOL
(b)
Figure 2. Schematic of perforated plate foam generator,
11
-------
The factor of 2 on the right hand side of Equation 1 accounts
for two surfaces on both sides of foam film. Hence, the mini-
mum gas velocity, V . , required for the generation of foam
bubble by spray foam generator may be calculated as follows:
Vmin
In the case of multiple bubbles, due to sharing of film by
neighboring bubbles, the surface tension term in Equation 1
should be (2irry) instead of 2(2Trry), and Equation 2 becomes
Knowing the minimum gas velocity, the theoretical minimum
power requirement, N . , for the gas blower needed for bubble
formation may be determined according to the following
equation:
-------
the latter; at this moment, bubble detachment takes place.
Equation 4 describes the two forces for the perforated plate
foam generator.
vg(p£ - p) £ (2Trr)y (4)
>
where v = volume of bubble
PP = density of liquid
p = density of gas
r = radius of orifice
y = surface tensiion of liquid
Since p « p. and it is assumed that a spherical bubble with
X/
radius R is formed, further treatment of Equation 4 will
yield:
(5)
In this case, the energy requirement for the blower is simply
to overcome the static head of liquid layer and pressure drop
through perforated plate. Our discussion illustrated the
minimum energy requirement for the generation of foam. Let
us now consider how much surface energy is stored in foam per
unit volume of foam. Assuming uniform size bubbles, it is
reasonable to assume that bubbles are arranged in such a way as
to obtain maximum packing density. The maximum number of
spheres packed per unit volume can be determined:
2RA/3RA/V 4/2R:
VTR
13
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The total surface area of the bubble per unit volume can now
be calculated using Equation 6:
Total area = (4TrR2) —^— = -?— (7)
4/2R3 /2R
The total surface energy involved is, area x surface tension,
Total surface energy per unit volume = ——
/2~R
(8)
The conditions existing in real life situations are far more
complicated than our equations can describe. Consider now
the order of magnitude of Reynolds number of gas in an actual
scrubber. At typical scrubber flow velocity of 2.5 m/s
('vSOO fpm) , duct diameter, d, of 1 meter (^3 ft), gas viscos-
ity y = 0.02 x 10~3 Pa«s, and gas density p =1.2 kg/m3, the
Reynolds number (Re) is determined:
Re = = 2.5 x 1 x 1.2 ^ 150^000 (turbulent) (9)
0.02 x 10~3
Consequently, turbulent conditions are present in the main
gas stream. At the moment of the formation of bubbles, if
the bubble diameter is about 2 mm, the initial Reynolds
number of gas movement inside the bubble is:
OQ 2.5 x 2 x 10~3 x 1.2 ,nn ,, . , (10)
Re = =: 300 (laminar) v '
0.02 x 10~3
This suggests that at the formation of foam bubbles, the
transient phenomena of turbulent to laminar flow occurs. The
14
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description and analysis of the phenomena under such unsteady
conditions is extremely complicated.
4.2 DIFFUSION PHENOMENA IN FOAM
The summary of governing equations describing general flow
conditions, assuming constant gas properties, is presented
below.
Continuity equation:
§£ + p divv = o (ll)
Momentum equation (Navier-Stokes equation; incompressible):
DV •> , o -»• /1 ^ \
pDt = pg ~ 9rac^ P + Vv v (12)
Energy equation (for normal engineering process):
PCV = kV2T (13)
Diffusion equation (2nd Pick's Law)
DV o±
fTjr = pDV2C (14)
Dt
where t = time
V = gas velocity
p = gas density
p = pressure
y = viscosity of gas
15
-------
C = specific heat of gas
k = conductivity of gas
D = diffusion coefficient
T = temperature
C = concentration
The above equations are very often difficult and tedious to
solve. In the following paragraphs, some simplifying
assumptions appropriate for particle collection by foam will
be made. Let us first consider the idealized case of a
bubble formation as shown in Figure 1. Aerosol with constant
concentration C rushes into a foam bubble and the detachment
of the bubble (assuming spherical shape) occurs at the time
t = 0. As long as the cross-sectional areas of foam genera-
tor and the foam scrubber remain constant, the mass of aerosol
moves with the same velocity before and after the formation
of a bubble. It is therefore reasonable to assume that the
aerosol inside the bubble and the foam bubble surface progress
with the same velocity through the scrubber. Consequently,
the impaction mechanism of aerosol on foam surface is negli-
gible.
As opposed to the case just mentioned (i.e., Figure 1), the
formation of a rising bubble through a liquid layer tends to
create a gas circulation pattern inside the bubble. This is
illustrated in Figure 2. The gas velocity at the surface of
the bubble is approximately given by classical inviscid flow
around a sphere. The conditions of perforated plate foam
generation will further be evaluated in the following section.
Assuming isothermal system (i.e., gas temperature = bubble
film temperature) and for a very small bubble, if the motion
of gas is negligible inside the bubble after the bubble has
16
-------
been formed, the set of Equations 11, 12, 13, and 14 reduces
to a single equation:
at
which after its transformation to spherical coordinates be
comes :
3C(r,t) _
at
To solve Equation 16 the initial and boundary conditions
must be summarized:
(a) C(r,0) = C initial concentration at
t = 0 is constant (16a)
(b) C(R,t) = 0 on the surface of the
sphere at t > 0 (16b)
(c) C(r,t) ? °°, t ^ 0 (16c)
The diffusion coefficient for small particles may be ex-
pressed by:
A( +Q'(
(17)
where k' = Boltzmann's constant (1.38 x 10~22 kg m/s2 K)
o.
T = temperature (23 C)
y = viscosity of gas (1.85 x 10~5 Pa-s)
r = radius of diffusing particle, ym
X = mean free path of the gas (0.653 x 10~5 cm
at 23°C, 760 mm Hg)
A,Q',b = empirical constants
17
-------
The quantity inside the brackets in Equation 17 is called
Cunningham slip correction factor, K.
The constants A, Q1, and b in Cunningham slip correction
depend on the value selected for the mean free path of the
gas molecules and the nature of the surface of the particles
(i.e., influenced by degree of specular reflection and dif-
fuse reflection of gas molecule at the surface of particles).
The calculated Cunningham correction factors are shown in
Figure 3 for particles of glass spheres and oil droplets.
The constants for glass spheres and oil droplets are pre-
sented below:
Glass spheres Oil droplets
A = 0.77 A = 0.86
Q1 = 0.40 Q1 = 0.29
b = 1.62 b = 1.25
Using the diffusion equations one can determine the distance
which a particle of a specific size can travel in a selected
period of time. This estimation is of interest to indicate
the size of foam bubbles that would be desirable for practical
foam scrubber applications. Basically, the diffusion of small
particles is caused by Brownian motion, the random movement of
particles as they are bombarded by surrounding gas molecules.
This motion can be expressed by the following equation:
X2 = 2Dt (18)
where X2 = mean square displacement
18
-------
0.00001
0.0001
0.001
GLASS SPHERES
OIL DROPLETS ,
1000
2r p (MICRON-ATM)*
ICRON(/U)=10 m
1 ATM = 1.013 x 10 5 Pa
Figure 3. Cunningham slip correction for liquid and solid particles.
-------
Figure 4 shows the calculated variations of X2 with r and t,
with and without the Cunningham slip correction. The calcula-
tions are based on oil droplets in air at 23°C and 760 mm Hg.
The data are plotted on a logarithmic paper to demonstrate
basic relationships between particle size, bubble size, and
probability of particle impaction with the bubble. It is
evident from Figure 4 that the Cunningham correction is
relatively unimportant for a 10 ym diameter particle but of
great importance for a 0.01 ym diameter particle.
Figure 4 can be used to get a qualitative idea of the behavior
of a foam scrubber. The following arbitrary example will
demonstrate this. Assuming that the foam is composed of
100 ym diameter bubbles, a 0.01 ym particle in the center of
the bubble will diffuse to the bubble surface in less than
0.1 second. A 0.1 ym particle will take 2.3 seconds, while
a 10 ym particle will take more than 100 seconds.
Equation 17 does not apply accurately to particles as small
as gas molecules. The uncertainty in the value due to
different particle substance does not usually exceed 2-3%.
With the above information available, the solution of Equa-
tion 16 will give particle concentration distribution inside
a bubble as a function of time for various particle sizes,
assuming that the particulate cloud is monodisperse and at
given concentration the coagulation of particles is negligible.
The partial differential equation, Equation 16 can be solved
by the separation-of-variables method. The general solution
will have a form as follows:
C(r,t) = (Cie~a2Dt + C2ea2Dt) [c3— J, (ar)
L /r % (19)
+ C,*— J , (ar)|
/F 2 J
20
-------
1000
100
10
1.0
0.1
0.1
ANDIOjjm (UNCORRECTED)
1.0
10.0 20.0
t, SECONDS
Figure 4. Displacement of particles in air by diffusion,
21
-------
where a, GI, C2/ C3, C^ = constants
J = Bessel's functions
n
By use of boundary conditions, Equations 16a, 16b, and 16c,
and lengthy derivation and transformation, Equation 19 becomes:
2 , R -
u ( F -f n - e sinmrs (19a)
0 n=l
This equation gives instantaneous particle concentration
inside the bubble.
The instantaneous rate of loss of particles to bubble surface
is now given by:
q(R,t) = -D(4TrR2)
= S^RDC
r=R n=l
The total loss of particles in time t from time t = 0 is:
(21)
Q(t) =
t oo 2
3r- t-^ i- -(—) Dt
o n=i
]
Hence, the particle collection efficiency, E (%) , in time t
may be determined as follows:
22
-------
(22)
x 100
00
^
note:
_
n2 - 6
Particle collection efficiency, Equation 22, was plotted for
various bubble sizes (0,. 5, 1.0, and 2.0 nun diameter) and
particle sizes (0.01, 0.1, and 1 ym diameter) as a function
of foam residence time in Figures 5, 6, and 7. Inspection
of Equation 22 shows that bubble diameter R is of primary
importance (appears as square term in denominator of expoten-
tial term). Also, Figure 3 and Equation 17 reveal that the
diffusion coefficient of 0.01 ym diameter particle is about
2,000 times larger than that of 1.0 ym particle.
Figures 5, 6, and 7 show that the collection efficiency of
0.01 ym particles within a reasonable practical residence
time (say 5 seconds) is very high. However, for 1.0 ym
particle, even a foam bubble diameter of 0.5 mm would not
appear to provide acceptable collection efficiency in most
practical engineering applications.
4.3 PARTICLE CAPTURING BY OTHER MECHANISMS
Other particle collection mechanisms (such as particle impac-
tion, sedimentation, etc.) were neglected in the above anal-
ysis. Consequently, the results presented represent the
lowest collection efficiency in the practical foam scrubbing,
except for adverse temperature or vapor gradient conditions.
(For non-isothermal system, the diffusion collection efficiency
23
-------
CJ>
o
o
o
o
PRESSURE -760 mm Hg *
TEMPERATURE - 23 ° C
BUBBLE DIAMETER-0.5 mm
0 10 20 30 40
RESIDENCE TIME, s
*lmmHg = 1.333xl02Pa
50
60
Figure 5. Particle collection efficiency,
24
-------
100
>
UJ
O
HZ
u_
o
i—
o
o
o
80
60
40
20
0
PRESSURE - 760 mm Hg *
TeV\PERATURE-23°C
BUBBLE DIAMETER-1.0 mm
0 10
* lmmHg=1.333xl02Pa
20 30 40
RESIDENCE TIME, s
50
60
Figure 6. Particle collection efficiency.
25
-------
>-
o
o
ul
u_
UJ
•z.
o
d
o
o
PRESSURE - 760 mm Hg *
TEMPERATURE - 23 C
BUBBLE DIAMETER-2.Omm
d = 0.01jLim
P
20
30 40
RESIDENCE TIME, s
50
lmmHg=1.333x!02Pa
60
Figure 7. Particle collection efficiency,
26
-------
may fall as the temperature of foam rises, on account of
repulsion of particles from the walls of the bubble owing
to Stefan flow and thermophoresis.) For very small particles
(e.g., 0.1 ym or larger) however, other mechanisms may in-
fluence the collection efficiency.
4.3.1 Impaction and Sedimentation
In order to evaluate the effect of impaction and sedimenta-
tion on the particle collection efficiency, the flow develop-
ment inside the bubbles during the period from foam generation
to final destruction must be known for the specific foam
generation technique. In the isothermal system, the flow
development can be obtained by solving Navier-Stokes equations
with appropriate initial and boundary conditions. Thus, the
momentum equation is as follows:
where 9 = angular 'direction
The initial and boundary conditions for two foam generating
techniques are shown in Figure 8. The solution of Equation
23 would give instantaneous velocity profile inside a bubble
as a function of time and coordinates. The solution can only
be obtained by numerical analysis, using a computer. It was
felt that it was outside the scope of this program to obtain
the solution of Equation 23.
However, as the first approximation, the order of magnitude
of relaxation time for gas flow inside the bubbles to reach
steady state can be estimated. This should provide an
27
-------
Vwall=0
WIRE SCREEN
(a) bubble formation over wire screen
O O
II
O LU
UJ —I
CX. CD
— CO
V,
CO
GAS BUBBLE
LIQUID
(AT t » 0)
Figure 8. Initial and boundary conditions,
28
-------
indication of significance of the unsteady state on the over-
all collection process.
Let us determine how fast the steady flow profile can be
attained in a bubble shown in Figure 8-b. The gas velocity
at the surface of the bubble is given by:
Vwall = 3/2 Vb sin9 (24)
where Vb = velocity of bubble rise
From the result of the analysis of Couette flow between the
annulus with one moving wall, the value required for the flow
to reach 95% of final velocity profile from the start was in-
vestigated by H. S. Yu and E. M. Sparrow:2
| = 0.32 Re
R (25)
where x = entrance length
R = channel width, or in this
case, radius of bubble
Re = Reynolds number
The time required for flow to reach 95% of final profile is
therefore:
= 2L- = 0.32R(Re)/V, (26)
2Yu, H. S., and E. M. Sparrow. Flow Development in a Channel
Having a Longitudinally Moving Wall. Journal of Applied
Mechanics. 37_(2) : 498-507, June 1970.
29
-------
For bubble size of 1 mm in diameter, rate of rise of bubble
at 0.30 m/s, and Reynolds number of 100, Equation 21 gives
t = 0.05 seconds. That means the gas flow in 1 mm bubble
would reach the steady state almost immediately, say in less
than 0.1 second. The motion of gas, however, would persist
as long as the bubbles are rising through the liquid layer.
By the same reasoning, as soon as foam bubble is out of
liquid, the internal gas motion would subside immediately in
the same relaxation time (<0.1 s) due to absence of surface
film motion. The impaction mechanism would, therefore, be
applicable only during the period of bubble rise through the
liquid. At a rising rate of 0.30 to 3.0 m/s, this amounts
to probably less than a second in most of the practical
engineering applications.
In the case of screen foam generator (Figure 8-a), initial
disturbance of gas movement from the foam generation process
would expect to subside in the same short period of time.
The pure diffusion process takes over during the rest of
foam residence time in the scrubber, which may be 10 seconds
or more.
Without computer analysis of initial flow disturbance, Equa-
tion 23, the extent of impaction or sedimation is difficult
to estimate. However, due to insignificant relative motion
between moving bubbles and gas, and short relaxation time
as we have just demonstrated, contribution of impaction would
appear insignificant for small particles which are of inter-
est in this project.
The number of particles deposited inside the bubble by sedi-
mentation in one second can be expressed as (irR2)CV . Using
S
this expression we can formulate the coefficient of absorp-
tion by sedimentation (ratio of the rate of particles
30
-------
deposited to the total number of particles in the bubble),
a , as follows:
s
7rR2CV 3V
a = - s - s
s
(4/37TR3)C 4R (27)
where V = particle settling velocity
The settling velocity can be expressed by:
(,, 0, -bd /2X\
1+A — + n' r P I
dp dp / (28a)
~bd /2X\
- P
where m = particle mass
p = particle density
Combination of Equations 27 and 28b gives:
as 2"4"yR
<28b)
(,, ,, -bd /2X\
L+A|A. + Q'±^e P J (29)
d d I
P P /
Typical sedimentation velocity of particles, Equation 27,
and coefficient of absorption by sedimentation, Equation 29,
are listed in Table 1 for bubble diameter of 1 mm and par-
ticle density of 1,000 kg/m3:
31
-------
Table 1. TYPICAL SEDIMENTATION VELOCITY AND
COEFFICIENT OF SEDIMENTATION
(for R = 0.0005 m, p = 1,000 kg/m3)
V , m/s
o
as, 1/S
d ( um)
P
0.01
6.5 x 10~8
0.0001
0.1
8.8 x 10~7
0.0013
1.0
3.5 x 10~5
0.052
Evidently, for particles larger than 1 ym, sedimentation
would significantly enhance the collection of particles during
the foam scrubbing process. For 1 ym particles, the table
indicates that about 5.2% of the remaining particles are re-
moved by sedimentation mechanism every second, assuming uni-
form particle concentration within the foam bubble. Because
of diffusion process, the particle concentration profile,
Equation 19, would not be uniform throughout the bubble (i.e.,
leaner near the wall) and, therefore, the actual sedimentation
coefficient would be somewhat less than indicated above.
The combined particle collection efficiency (including dif-
fusion and sedimentation) can now be determined in a step-by-
step calculation. Although the results obtained may not be
mathematically rigorous, they provide an additional tool for
estimating foam scrubber collection efficiency.
The computational results are presented in Figures 9 through
17 for various bubble diameters, particle sizes, and particle
densities. For convenience, the diffusion collection curves
are also shown in these figures. As evidenced by the collec-
tion curves, the enhancement of collection efficiency by the
sedimentation mechanism is far more important than the dif-
fusion mechanism for these larger particles. However, for
the particles smaller than 0.1 ym, the sedimentation mechanism
of particle collection is negligible.
32
-------
U)
U)
O
O
Lu
u_
LLJ
z:
o
O
o
o
BUBBLE DIAMETER -0.5mm
PARTICLE DENSITY- 1 g/cm3
PRESSURE-ATMOSPHERIC
TEMPERATURE23°C
d = l.Ojjm
r ,r- — J
DIFFUSION AND SEDIMENTATION
DIFFUSION ONLY
* CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
20
Figure 9
30 40 50 60
RESIDENCE TIME, s
Particle collection efficiency.
-------
00
100
s£
>.- 80
o
UJ
0
i±j 60
z
o
o
LjJ
S 40
~e
1
1
'1
1
i
1
-1
1
\-
1
J|
1
I/
HJ
d = 0.01pm*
20
BUBBLE DIAMETER-0.5 mm
PARTICLE DENSITY- 2 g/cm3
PRESSURE-ATMOSPHERIC
TEMPERATURE - 23 C
^
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
* CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
10
20
30 . 40 50
RESIDENCE TIME, s
60
Figure 10. Particle collection efficiency.
-------
U)
Ul
d = 0.01pm
*£
o
o
u_
u_
LU
O
G
O
O
BUBBLE DIAMETER-0.5 mm
PARTICLE DENSITY- 3g/cm:
PRESSURE-ATMOSPHERIC
TEMPERATURE-23 °C
d =lum --—
P J~~-
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
* CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
20
30 40
RESIDENCE TIME,
50
60
Figure 11. Particle collection efficiency.
-------
00
d = 0.01pm*
BUBBLE DIAMETER- 1.0mm
PARTICLE DENSITY- 1 g/cm3
PRESSURE-ATMOSPHERIC
TEMPERATURE - 23 ° C
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
p r
* CURVE IDENTICAL WITH CURVE
FOR DIFFUSION AND SEDIMENTATION
10
20
30 40 50
RESIDENCE TIME, s
60
Figure 12. Particle collection efficiency.
-------
U)
O
H 60
o
o
d = O.ljum
BUBBLE DIAMETER- 1.0mm
PARTICLE DENSITY- 2
PRESSURE- ATMOSPHERIC
TEMPERATURE- 23 °C
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
* CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
30 40 50
RESIDENCE TIME, s
60
Figure 13. Particle collection efficiency.
-------
u>
00
. 80
o
o
uT
O
o
O
o
BUBBLE DIAMETER- 1.0mm
PARTICLE DENSITY- 3g/cm3
PRESSURE-ATMOSPHERIC
TEMPERATURE -23°C
DIFFUSION AND SEDIMENTATION
DIFFUSION ONLY
* CURVE IDENTICAL WITH CURVE
FOR DIFFUSION AND SEDIMENTATION
30 40 50
RESIDENCE TIME, s
Figure 14. Particle collection efficiency,
-------
U)
BUBBLE DIAMETER -2.0mm
PARTICLE DENSITY- lg/cm3
PRESSURE -ATMOSPHERIC
TEMPERATURE-23 °C
d = 1pm
^B^V ^^"^ '
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
20
30 40 50
RESIDENCE TIME, s
60
Figure 15. Particle collection efficiency.
-------
BUBBLE DIAMETER -2.0mm
PARTICLE DENSITY- 2 g/cm3
PRESSURE-ATMOSPHERIC
TEMPERATURE-23 °C
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
10
20
30 40 50
RESIDENCE TIME, s
60
Figure 16. Particle collection efficiency.
-------
o
[Z
u_
Lu
O
I—
o
o
o
BUBBLE DIAMETER -2.0mm
PARTICLE DENSITY- 3 g/cm3
PRESSURE-ATMOSPHERIC
TEMPERATURE-23 °C
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
20
30 40 50
RESIDENCE TIME, s
60
Figure 17. Particle collection efficiency.
-------
Next, let us consider the effect of inertial deposition for
the situation shown in Figure 8-b, i.e., during the rising
of bubble through the liquid layer. Even though it has been
shown that unsteady conditions exist only for a limited period
we will attempt to determine whether in this short time any
significant particle collection could occur. It is reported
that small amounts of surface active agents cause a marked
decrease in surface film movement because of the formation
of a "skin" around the bubble that effectively prevents
circulation.3 The circulation has only been substantiated
for gas bubbles of about 0.003 to 0.005 m in diameter rising
through carefully purified water. Nevertheless, further
discussion is warranted. In calculation of inertia deposi-
tion, the radial velocity of the particles due to the cen-
trifugal force is:
VwallVs 9 VVssin29
rad Rg 4' Rg (30a)
d2 V. Zsinz6/ ,, _, -bd /2A\
= P P P I-I_I_A^A j. ,M^A~ P I (30b)
(0,
L«|i
P
3UR i—dp • « a^ y
Hence, the number of particles deposited on the bubble sur-
face in one second will be:
9VK2V^n / 67TV, 2V nR
sin2927rR2sin9de = —-—
g
OT -, 9 '
fi C\ (~*\7 4
r> D h "" / ")} Ti ~i-"J-_/^A\
P ?.. b fl + AiA + Q'i^ p » (31b)
/ ,, ,, -bd /2X\
1 + A2A+Q,2^ p \
\ P P I
3Bird, R. B., W. E. Stewart, and E. N. Lightfoot. Transport
Phenomena. New York, John Wiley and Sons, Inc., 1960. 780 p,
42
-------
The ratio of the rate of particles deposited per second to
the total number in the bubble, or coefficient of inertia
deposition a., is:
ai=
:R
2A ?, -bd /2X'
±A -u r>i±A^ P
Ad~+ Q
)
(4/3TTR3)C
ap<
V
-bd
4R
/2X\
(32)
Typical coefficient of inertial absorption for case of
Figure 8-b is shown below for bubble diameter of 1 mm and
particle density of 1,000 kg/m3, assuming bubble rising
velocity of 0.30 m/s.
Table 2. TYPICAL COEFFICIENT OF IMPACTION
(for R = 0.0005 m, p = 1,000 kg/m3 , Vb = 0.30 m/s)
ou, 1/s
d (ym)
0.01
0.011
0.1
0.146
1.0
5.84
Surprisingly, the above simplified analysis shows that the
inertial absorption is quite efficient in removing micrometer
(1 ym) size particles in relatively short time. The table
shows that all the particles above 1 ym could be removed in
less than 0.2 second, only if the circulation of the gas
inside the bubble could be maintained.
4.3.2 Thermophoresis and Diffusiophoresis
Other diffusion mechanisms can contribute to the foam scrub-
bing operation. A small particle in a temperature gradient
is subject to thermophoresis: gas molecules striking the
43
-------
side of the particle facing the high temperature region
have more kinetic energy on the average than those hitting
on the low temperature side and the particle moves toward
the low temperature region. As a result, if the particulate-
laden air passing through the foam scrubber is warmer than
the bubble surface, thermophoresis will aid in particle col-
lection.
Another important phenomenon to consider is Stefan flow
caused by the evaporation or condensation of water vapor
from or onto a bubble surface. This is a hydrodynamical
flow of a vapor-gas mixture, normal to a liquid surface,
which compensates for gas diffusion. Because of Stefan flow
it is important that air going into a foam scrubber be satur-
ated with water vapor. If warm, saturated air cools and con-
denses as it passes through a foam scrubber, Stefan flow will
sweep particles toward the liquid surface where water vapor
is condensing. If unsaturated air is used, evaporation will
hinder particle deposition.
However, the water vapor can also condense on fine particles,
which act as nucleation sites for water droplets. If this
occurs, the now larger particles will diffuse more slowly
to the bubble surface, and the particle collection efficiency
will be reduced. Of course, if submicron particles grow
large enough (>^1 ym), they can be collected by gravitation
or inertial impaction.
One final diffusion mechanism, called diffusiophoresis, oc-
curs in binary gas-vapor mixtures. In a process analogous
to thermophoresis, a small particle suspended in a concen-
tration gradient is struck from one side by a greater number
of heavier molecules. As a result, the particle migrates
toward the low concentration region of the higher molecular
44
-------
weight species. In contrast to Stefan flow, diffusiophoresis
ceases when the molecular weights of gas and vapor are equal,
even though a concentration gradient exists. The following
discussion will attempt to quantify the diffusional phenomena
mentioned above.
The following examples will illustrate some practical aspects
of the application of a. foam scrubber to industrial gases.
A typical foundry furnace exhaust gas temperature is 721°C
(1,330°F). In some installations heat is reclaimed from the
furnace exhaust gases, and the gas temperature is reduced to
within the design limits of applicable pollution control
equipment. Further cooling may be accomplished by water
spray nozzles, dilution air, or convective radiation cooling.
According to the Air Pollution Engineering Manual (1967) , the
test results of a typical outlet gas temperature from the
waste heat boiler are 238°C (460°F) with a moisture content
of 12.4% (or humidity ratio of 0.08).
Figure 18 shows schematically a possible process for a total
particulate removal system using a foam scrubber. The tem-
perature and humidity of the dust-laden gas entering the foam
scrubber will influence the particle collection efficiency.
In previous discussions we have only dealt with the isothermal
bubble system and neglected the effect of vapor condensation
(or evaporation), temperature gradient, and electrostatic
effect inside the foam bubbles. In engineering practice,
water spray is used to cool and humidify the gas stream
entering wet scrubbers. However, the simple water spray is
not always sufficient to saturate the gas with moisture.
The psychrometric process of gas inside the foam bubble
therefore can be that of vapor evaporation from the bubble
film surface or condensation of moisture from the gas phase
45
-------
TO STACK -
©
1
FOAM SCRUBBERS
FOAM LI QUID
©
WATER
SPRAY
©
238 °C
(460 ° F)
WASTE
HEAT
BOILER
CD
CTl
0.08
70 140 460
21 60 238
DRY - BULB TEMPERATURE
1330 °F
721 °C
FROM FURNACE
721 ° C (1330 ° F)
Figure 18. Schematic diagram of exhaust gas treatment.
-------
to the film surface depending on the operating conditions
of the foam scrubber.
In the typical foam scrubber psychrometric process shown in
Figure 18 (i.e., process 3->4) f the entering gas is not
saturated. Due to the gas high humidity and colder foam
liquid film, the process (3->4) is that of condensation and
cooling which would enhance the particle capturing capability
of a foam scrubber. If the entering gas is relatively dry
(not saturated and low in humidity ratio), evaporation of
water vapor can take place inside the bubble even though
the foam liquid is colder than the gas as shown by the dashed
line 3'-4' in Figure 18. Evaporation of foam liquid is detri-
mental to the diffusion of particles to the foam bubble sur-
face. Apart from the pure diffusion process discussed pre-
viously, the particles in general move in the direction of
the diffusion flux of the heavier gas component. This is
called diffusiophoresis. It is possible that inside the
evaporating bubbles, a dust free zone can exist in the
boundary layer, thus, blocking the accumulation of particles
on the foam surface by the pure diffusion process.
In the practical application of foam scrubbing, it is unlikely
that the foaming liquid and particle laden gas will be at
isothermal conditions. Under the influence of a temperature
gradient in the gas, particles inside.the bubble would move
towards the lower temperature. This phenomenon is called
thermophoresis.
It should be noted that both diffusiophoresis and thermo-
phoresis processes stop soon after the bubble film and gas
inside the bubbles reach equilibrium state. If the time
required to reach the equilibrium state is comparable to
the foam residence time in the scrubber, the diffusiophoresis
47
-------
and thermophoresis could become important factors for
particle capturing.
Let us now consider a small particle with radius r and
velocity V suspended in a gas which itself move with velo-
city V and in which a temperature gradient AT exists. Then
the force exerted by the gas on such a particle is given
by the following equations:1*
2 /k+C. - k \
( t r p) ,
F = -6irur _ - _ £ _ — VT
P /1+3C ^ 2k+k +2C. - k \ 5p (33)
^ } (
r M
.
m r t r
p
where k = conductivity of particle
a = coefficient depending on both gas and
particle surface properties and temperature,
* 1/5
15 2-g
Ct ~ 8 at t
C = 2~am
m am
a. = thermal accomodation coefficient
a = momentum accomodation coefficient
m
In the steady state, the force given by Equation 33 is equal
to the friction of the particle, i.e.,
^Aerosol Science. Davies, C. N. (ed). New York, Academic
Press, 1966. 468 p.
48
-------
6-n-yr^ Vr
A_ + Q. A_ e~^p| (34)
r r 2 A I
P P I
The thermophoretic velocity of the particle is therefore:
v = - \ P/" J L *\"p/~PJ k_
p r A \ i r /i \ i SP <35)
2k+k +2C. —k
It was first suggested by Stefan that there must exist a
hydrodynamic flow directed away from the evaporating and
towards the condensing surface near a surface of an evap-
orating or condensing body. The velocity of this hydrodyna-
mic flow (Stefan flow) , U, is given by the following equa-
tion applicable for a binary mixture:
D dpi
u = -
06a)
where pj, P2 = partial pressures of the vapor and air, and
- -
dx ~ dx
Particle matter near a condensing or evaporating surface
might be expected to move with a velocity of about that of
Stefan flow. However, the diffusion current of the vapor
and gas have a modifying effect on the velocity of the par
ticles. For small particles in a binary gas mixture,
49
-------
Waldmann and Schmitt4 derived the particle velocity as
follows:
• /ma j,,
n Vl
i—-* dx
+ y2 /ma
(37)
where mi, ma = mass of vapor and gas
yi/ y2 = molar fraction of vapor and gas
This means that the particle moves in the direction of the
diffusive flux of the heavier gas molecules.
Adding V1 to the Stefan velocity, U, the diffusiophoretic
velocity of particle becomes:
V = V1 + U = - _ —
p p P2 dx (38)
Before we estimate the effect ot thermophoresis and diffu-
siophoresis we must estimate the magnitude of AT and Ay in
the practical engineering application. It must be emphasized
that because of the limited relative amount of gas and liquid
mass involved in the process, AT and Ay in Equations 35 and 36
are not constant values; rather, they diminish to zero in a
relatively short period and vary within the bubble from point
to point.
It is always desirable to saturate the gas stream entering
a wet scrubber with water vapors. A common method is water
quenching. The efficiency of this type of spray varies with
50
-------
the design, the quantity and pressure of the spray water and
other operation factors. The following are, approximately,
the efficiencies obtainable with commercial air washers of
average construction in which the same water is continuously
recirculated without being either heated or cooled (see
Figure 19).
Single bank of sprays: n = 65%
Two banks of sprays: n =80%
Three banks of sprays: n =90%
W
tz-ts W3-W2 D %
where n = = = !_e —^
W t2-t* W *-W2 a
M = mass rate of flow of dry air
3.
W = humidity ratio
h = convection mass transfer coefficient
A. = surface area of water droplets
W * = humidity ratio of saturated moist air
s
t* = thermodynamic wet bulb temperature
V = volume of air washer
It is obvious that it is practically impossible to achieve
100% saturation with this kind of spray device. It is esti-
mated that mass transfer coefficient h A has a value of
1.33 kg/s-m3 [300 lb/(hr) (cu ft) (lbw/lba)] for commercial
air washers with a pressure drop of 498 Pa (2 in. of water)
or less.
51
-------
TO FOAM SCRUBBER
W t h
¥Vo> lo> "o
dV'
FROM WASTE
HEAT BOILER
i/2» ^, h.
i
T
1
ttt * ft
HI HI
MI iii
1
(a)
w
t*61uC
(142°F)
W
o
h-
a:
0.08
460 °F
238 °C
Figure 19.
100 140
38 60
DRY - BULB TEMPERATURE
(b)
Schematic diagram of air washer using
directly recirculated spray water.
52
-------
Instead of using recirculating water sprays as just shown
above, one might use cold water sprays, possibly from a
cooling tower, well water, or city water. For such a device,
the process line, 2-3, would not follow a straight line
(wet-bulb temperature line) as shown in Figure 19. The con-
dition line would be like the one shown in Figure 18.
The equation describing the condition line on the psychro-
metric chart for the changes in the state for moist air passing
the cold water spray can be expressed as follows:
§|
where h = enthalpy of moist air
L = Lewis number usually between 0.8 and
1.0 and depending on air velocity and
ratio of thermal diffusivit.y and water
vapor diffusivity
h = specific enthalpy of saturated moist
g' air evaluated at t
w
W = humidity ratio of saturated moist air
' evaluated at t
w
h = enthalpy of saturated moist air at t
To obtain an accurate solution for Equation 39 on the psychro-
metric chart, a step-by-step construction of the condition
line is necessary. If the humidity ratio of the gas enter-
ing the foam scrubber is sufficiently high, there is no need
for 100% saturation to achieve effective thermophoresis and
diffusiophoresis for the removal of small particles inside
the bubbles. Process 3+4 shown in Figure 18 is that of
53
-------
cooling and condensation of unsaturated hot humid gas in cool
foam bubbles. In this case the thermophoretic and diffusio-
phoretic forces are additive. On the other hand, super-
saturation with respect to water vapor may result in the
modification of the particle size distribution (formation of
larger size particles), thus, increasing the importance of
the gravitational settling or decreasing the importance of
the pure diffusional process.
Previous experimental results of the thermophoretic velocity
as a function of temperature gradient are shown in Figure 20. *
For estimation of initial thermophoretic velocities of parti-
cles we may assume:
Gas temperature = 60°C
Foam liquid temperature = 20°C
Hence, for 1 mm diameter foam bubbles, the initial tempera-
ture gradient (300/T) dT/dx is about 800, which is extremely
high (see Figure 20) indicating significant initial thermo-
phoretic particle velocities. The exact temperature history
of bubble liquid film and the gas inside the bubble are
extremely difficult to quantify. However, we can estimate
the time required to reach the final equilibrium temperature
of the solution for the sudden emersion of a spherical gas
bubble in a constant temperature liquid bath. With a thermal
diffusivity, a, of 2.2 x 10~5 m2/s (0.8587 ft2/hr) and a
Nusselt number of unity, calculation shows that it requires
only 0.02 second for the central temperature of the gas
bubble to reach 99% of the final temperature. This means
that thermophoretic velocities go to zero in 0.02 second
from the start of bubble formation.
Experimental results in diffusiophoretic velocity agree very
well (±5%) with theory described by Equation 38 as shown in
54
-------
40
30
§ 20
10
ROSENBLATT AND LaMER
20 40 60 80 100 120 140 160
TEMPERATURE GRADIENT ^ ?r
IK/cm) T dx
Figure 20. The thermophoretic velocity in air as a
function of temperature gradient.
VAPOR GRADIENT x 10 ( Pa / cm )
dp
•ax-
Figure 21.
The diffusiophoretic velocity in air as a
function of water-vapor pressure gradient.
55
-------
Figure 21. It is seen that the maximum diffusiophoretic
velocity is in the order of 0.1 mm/s. Similarly, for the
thermophoresis process we can estimate the time required for
the dry gas inside the bubble to reach an equilibrium satura-
tion vapor pressure. With the diffusion coefficient of water
vapor in air at D = 0.24 cm2/s and a Schmidt number of unity,
a quick calculation shows that it requires about 0.08 second
for the central vapor pressure of the gas bubble to reach 99%
of saturation. This is about 4 times slower than the gas
reaches equilibrium temperature. With a diminishing vapor
gradient the total displacement of a particle within the
bubble by diffusiophoretic velocity would be far less than
0.1 x 0.08 = 0.008 mm.
4.3.3 Electrostatic Force
In addition to the previously mentioned thermophoresis and
diffusiophoresis, electrostatic effects are also important
in .the particle collection by foam. Since particulate emis-
sions from industrial effluents are highly charged, let us
consider the effect of electrostatic charge on the collection
of small particles by a foam scrubber.
As shown in Figure 22, the foam bubble can be considered
conductive and grounded at all times. Imaginary charges,
therefore, are always created opposite to charged particles
regardless of the charge sign. By Coulomb's law of electro-
static forces, the attracting or repelling force between the
charged particle and opposing surface is:
F ' = JL £i
Coulomb 47TE 62 (40)
56
-------
FOAM BUBBLE
Figure 22.
Schematic of electrostatic force
between particles and bubble wall,
57
-------
where 6 = distance between particles and surface
a = number of charges on a particle
e = permittivity of gas
In cgs system, Equation 40 becomes:
F = (ae)2 _ (4.8 x 10~10)2 n2 . 2
Coulomb k52 k62 ynes cm
where e = 1 electron charge = 4.8 x 10~10 cgs esu
k = dielectric constant of gas
Equation 41 indicates the electrostatic force is inversely
proportional to the square of distance between the bubble
surface and the particle. While this force may be negligible
when the particle is away from the bubble film, the attract-
ive force between the particle and the bubble wall could
become significant in the final step of the diffusion process
as 6+0, F_ , i.-*00-
Coulomb
58
-------
SECTION 5
EXPERIMENTAL WORK
Many experiments were performed in order to develop additional
knowledge of foam scrubber performance. These experiments in-
cluded particle collection efficiency determinations, investi-
gations of optimum conditions for foam generation, investiga-
tions of various techniques of foam destruction, and foam
characteristics measurements, e.g., foam stability, bubble
size, foam density, and foam viscosity. In several cases we
had to develop and devise a new technique to perform some of
the experiments in order to obtain data representative of the
foam scrubbing process. Descriptions of these techniques as
well as some of the initial foam characterization results ob-
tained before we were able to optimize the foam scrubber
operation with respect to particle collection do not have a
direct bearing on the particle collection efficiency data
presented later in this section. We feel, however, that
these descriptions may be useful in further development of
fine particle foam scrubbing process since they provide some
quantitative or qualitative indications for the relationships
of variables influencing the foam characteristics. In order
not to interfere with the main text of this report the des-
criptions will be presented in several appendixes and referred
to throughout the main text.
59
-------
5.1 EXPERIMENTAL FOAM SCRUBBER
The experimental apparatus to investigate the performance of
a foam scrubber was constructed and is shown in Figure 23.
The test scrubber can be supplied with compressed and filtered
air. Before entering the foam generator, the air can pass
through a humidification chamber and become saturated with
moisture. The air can also be humidified using a dry steam
generator (not shown in Figure 23). Humidified air is then
passed through a rotameter and into the foam production and
scrubbing section made of about O.lm (4 inches) I.D. plexi-
glass pipe. The aerosol is introduced at an orifice to insure
thorough mixing. Downstream of the orifice, the surfactant
solution supplied by a metering pump is sprayed onto the
stainless steel screen. Foam is generated by the interaction
of the air and liquid passing through the screen. Theoretical
aspects of foam generation using a screen were presented in
Section 4.1.
The foam then proceeds down the pipe of variable length (up
to 6 m) and into the destruction chamber. A portion of the
scrubber and the destruction chamber are shown in Figure 24.
5.2 FOAM GENERATION
It was initially felt that a 40 to 60 mesh screen would be of
sufficient fineness to generate foam with the desired bubble
diameter (approximately 1 mm). However, visual observation
of foam made with the 60 mesh screen indicated that this foam
consisted of bubbles about 6 mm in diameter. Since the bubble
diameter is a critical factor for particle collection, some
fine screens, 250 mesh and 325 mesh, have been investigated.
The size of foams produced with different screens and sur-
factants and with different operating conditions will be
60
-------
ROTAMETER
HUMID I Fl CATION
CHAMBER
COMPRESSED
AIR REGULATOR
:•*.•
0
•Hi?
EXHAUST
AEROSOL
ANALYZERS
PLEXIGLASS PIPE
SCREEN
r- MIXING SECTION
k
FILTER
ORIFICE PLATE
(AEROSOL
GENERATOR
I i
FOAMING
SOLUTION
FOAM
DESTRUCTION
CHAMBER
DRAIN
Figure 23. Experimental foam scrubber.
-------
Figure 24. Destruction chamber of the foam scrubber
62
-------
further discussed in Appendixes A and B. The majority of the
particle collection measurements were performed with the 250
mesh screen.
5.3 FOAM DESTRUCTION
The design of the foam destruction chamber permitted the in-
vestigation and determination of the minimum destruction
energy requirements for several destruction approaches. The
chamber is schematically shown in Figure 25; its physical
appearance was shown in Figure 24. According to the theory,
the foam generation process is associated with the formation
of new surface and consumption of energy. On the contrary,
the reversed process of foam destruction is associated with
energy release. This means that foam should have sufficient
energy to self-destruct.
The difficulty in destroying foam is directly related to the
foam stability which appears as a function of surfactant con-
centration and foam generation conditions. At a given sur-
factant concentration and foam generation conditions which
influence the foam density (wetness) and the foam bubble
size, one should be able to produce foams with the same
stability. The wet foams, however, undergo continuous drain-
age and change in their density. Generally, the drier foams
are less stable. In the foam scrubber process therefore the
foam stability will be a function of residence time. Since
the foam drainage is caused by gravitational forces, the
physical arrangement of the scrubber, that is, whether the
major portion of the scrubber is horizontal, vertical, or
inclined, will also have some influence on the stability of
foam exiting the scrubber.
Ideally, the foam should possess marginal stability and col-
lapse by itself immediately after leaving the foam scrubber.
63
-------
VARIABLE
DISTANCE
FOAM INLET
ENERGY
SOURCE
•SAMPLING TUBE
TO EXHAUST
OBSERVATION WINDOW
EXCHANGEABLE ELEMENTS SUCH AS:
1. HIGHSPEED ROTATING DISK
2. RADIANT HEATING
3. ULTRASONIC ENERGY
TO DRAIN
Figure 25. Foam destruction chamber.
64
-------
This would eliminate the foam destruction step and any energy
requirements associated with it. In practical foam scrubber
application, however, the self-destruction of foam would be
difficult to control due to the presence of gaseous and par-
ticle impurities that can highly influence the foam stability
in a not exactly predictable manner. We have observed the
self-destruction of foam to be a relatively slow process of
collapsing first the small bubbles into larger bubbles which
is followed by the larger bubbles collapsing to still larger
bubbles. Because the bubble size is a very important para-
meter for fine particle collection, once the larger size
bubbles are formed, the particle collection efficiency is
markedly reduced. Early collapse of foam would simply mean
that either insufficient collection of particles would result
or longer scrubber residence times would need to be provided.
Since the major collection mechanisms utilized in the foam
scrubber are slow and r€>quire residence times as high as
30 - 60 seconds any additional increase of the residence time
appears undesirable. This means that at least at present
some form of foam destruction is needed.
Five foam destruction techniques were investigated during
this program and included liquid spray, thermal destruction,
ultrasonic destruction, compressed air spray, and high speed
rotating disk. The high speed rotating disk was found to be
the most effective device for foam destruction as to the
energy requirements and possibility of foam liquid recycle.
Consequently, the high speed disk was selected and used in
particle collection determinations. Additional discussions
on foam stability and its measurement are presented in
Appendix C. Related information on foam destruction techni-
ques and foam density determination is presented in Appendixes
D and E, respectively.
65
-------
5.4 SURFACTANT SELECTION
Four surfactants have been selected for foam scrubber evalua-
tions. Tergitol TMN-6, made by Union Carbide, was chosen
because it was used successfully in our previous coal dust
suppression work utilizing foam. As suggested by our consul-
tant, Aerosol OT produced by American Cyanamid possesses
exceptional foaming characteristics and was therefore included
in our study. Alkanol DW, made by DuPont, a sodium alkylaryl
sulfonate, was selected because it is a good general purpose
surfactant and is characterized as a medium quality foaming
agent. Sterox NM, made by Monsanto, was also chosen because
it is a high foaming general purpose surfactant. We feel
that these four surfactants well represented the surface
active agent market in the United States, and were good candi-
dates for initiation of foam scrubbing work. Table 3 contains
a list of the four surfactants with some of their character-
istics.
5.5 PARTICLE MONITORING EQUIPMENT
Three particle counters were utilized on this program for
aerosol characterization and the determination of fine parti-
cle collection efficiency by foam scrubbing. The counters
were a nuclei condensation counter, an optical counter, and
an electrical aerosol analyzer. The instruments have a com-
bined capability to measure aerosol particles in the size
range from 0.003 ym to about 30 ym. Because the counters
have limited counting capacity a sample dilution system was
devised and used in connection with the counters. Additional
description of the aerosol sampling instruments and their
capabilities is provided in Appendix F.
66
-------
Table 3. SURFACTANT TYPES
Surfactant
Tergitol TMN
Sterox NM
Alkanol DW
Aerosol OT
(75%)
Manufacturer
Union Carbide
Corp. ,
Plastics Div.
Monsanto Co. ,
Inorganic
Chemicals Div.
E.I. du Pont
de Nemours &
Co. , Inc.
American
Cyanamid Co. ,
Industrial
Chemicals, Div.
Composition
Trimethyl nonyl
polyethylene
glycol ether
Alkyl phenol
ethylene
oxide adduct
Sodium alkyl-
aryl sulfonate
Dioctyl ester
of sodium sul-
fosuccinic acid
Chemical
type
Nonionic
Nonionic
Anionic
Anionic
Concen-
tration, %
90
99.5
28
75
Comments
Wetting and re-
wetting agent
General purpose
detergent
(melting point
30-35°C)
General use
detergent; du
Pont says it is
in tight supply
at the present
time
Fruity pungent
odor
CTi
-------
5.6 EXPERIMENTAL AEROSOL GENERATION
Two types of aerosol particles were used during the bench
scale tests of foam scrubber for collection efficiency.
They were dioctyl phthalate and polypropylene glycol. Rela-
tive to water the two particle types possess different wet-
tability properties, the dioctyl phthalate being essentially
nonwettable and the polypropylene glycol being wettable and
miscible with water in any proportions. The experimental
aerosols were generated by means of a condensation aerosol
generator developed at the University of Minnesota by Liu,
Whitby, and Yu,5 and illustrated in Figure 26.
5.7 FOAM SCRUBBER FINE PARTICLE COLLECTION EFFICIENCY
Particle collection efficiencies were obtained for the two
particle types by measuring the aerosol concentration just
prior to the surfactant spray nozzle and after the foam was
destroyed in the destruction chamber. (Refer to Figure 23.)
The difference in particle counts was used as a measure of
particle collection in the foam scrubber. The measurements
were made for different particle sizes as permitted by the
particle counters used.
The destruction of foam resulted in formation of additional
fine aerosol particles. This is associated with bubble burst-
ing and liquid droplet dispersion at the time of foam de-
struction. Upon droplet evaporation any non-volatile material
5Liu, B. Y. H., K. T. Whitby, and H. H. S. Yu. A Conden-
sation Aerosol Generator for Producing Monodispersal
Aerosols in the Size Range of 0.036 to 1.3 Microns. (Pre-
sented at the 6th Inst. Conf. on Condensation Nuclei. Paris.
May 1966). J. de Recherches Atmos. (Paris.) No. 3:397-406
(1966) .
68
-------
ORIFICE
DRY COMPRESSED
AIR
PRESSURE
GAUGE
ABSOLUTE FILTER
TIL
fl
FLOW METER
-52.3<:nr/s
ONE ORIFICE
COLLISON
FLOW DIVIDER
MADE UP FROM
STANDARD
COPPER FITTING
a
NO. 65
DRILL HOLE
H->
NO. 75
DRILLHOLE ,
!-»~^
-MADEUP FROM STANDARD
COPPER FITTING
-40 MESH SCREEN
.HEATING TAPE
[ 192 WATTS WITH LINE VOLTAGE)
VARIABLE TRANSFORMER
• 2cm I. D. QUARTZ
GLASS TUBE
5kV
NO. 47
DRILL HOLE
55.3 cm3/s
S-PXJCIT
3XE
. 74 DRILL
L \ HOLE
[U
^.OcrrT/s
DILUTING AIR
llf
VALVES CRITICAL
ORIFICE
1)
r
aECTROSTATIC
PRECIPITATOR
FOR EXCESS
AEROSOL
AEROSOL OUT
Figure 26. Monodispersed aerosol generating system.
69
-------
present in the liquid drops remains in the gas stream as a
particle. Some additional data of number of particles pro-
duced from foam destruction are presented in Appendix G.
During the tests evaluating the scrubber collection efficiency
the destruction disk was operated so as to minimize the
secondary particle formation.
In order to evaluate the fine particle collection occurring
in the foam scrubber the concentration of the secondary par-
ticles at foam destruction was determined by operating the
scrubber with no input aerosol and counting the particles
after the foam was destroyed. This established the baseline
of particulates which was then subtracted from the counts
obtained under identical scrubber operating conditions and
with a characterized aerosol input. Typically, the input
aerosol concentration was maintained at a level which would
hold the number of particles generated at foam destruction
to about 2 - 10% of the input aerosol in the investigated
size range.
Collection determinations were observed with different scrub-
ber residence times. The scrubber residence times were
varied by changing the scrubber length. At the same time
the air flow velocity through the scrubber was maintained
constant. This arrangement permitted performance of collection
measurements with identical foam characteristics. Typical
collection data using the high speed disk for the foam de-
struction are presented in Table 4. The results are sum-
marized in a graphical form in Figures 27 and 28. Because
the collection of fine particles in foam takes place mainly
by diffusion and sedimentation the collection efficiency is
a strong function of scrubber residence time. Both Figures
27 and 28 demonstrate curves asymptotically approaching 100%
collection with an increase of scrubber residence time which
70
-------
Table 4. SAMPLE DATA FOR OOP PARTICLE COLLECTION WITH FOAM
SCRUBBER USING ROTATING DISK - HIGH QUALITY FOAM
Exper-
iment
number
ia
2b
3C
4d
5
6
7
8
Operating conditions
Concen-
tration
of Ter-
gitol, %
1
1
1
4
2
2
2
2
Air flow
rate ,
IO"3 raVs
(cfm)
1.137
(2.41)
1.370
!2.91)
1.370
(2.91)
1.137
(2.41)
1.137
(2.41)
0.541
(1.15)
1.137
(2.41)
1.137
(2.41)
Resi-
dence
time,
s
40
30
30
40
40
84
13
13
Dilu-
tion
ratio
30/1
30/1
30/1
30/1
30/1
30/1
30/1
None
Surfactant
solution
flow rate,
cm3/s
5.83
5.83
5.83
5.83
5.83
5.83
5.83
5.83
Input OOP
concen-
tration,
106 parti-
cles/cm3
2.25
0.55
2.59
3.35
3.28
3.13
2.72
1.06
Results
Parameter
Input aerosol
Aerosol & foam
Baseline -
foam only
% Collected
Input aerosol
Foam & aerosol
Baseline -
foam only
% Collected
Input aerosol
Foam & aerosol
Baseline -
foam only
% Collected
Input aerosol
Aerosol & foam
Baseline -
foam only
% Collected
Input aerosol
Aerosol & foam
Baseline -
foam only
% Collected
Input aerosol
Aerosol & foam
Baseline -
foam only
% Collected
Input aerosol
Aerosol & foam
Baseline -
foam only
% Collected
Input aerosol
Aerosol & foari
Baseline -
foam only
% Collected
Particle concentration, jjm
0.18 - 0.32
1.0 x IO6
5.19 x 10s
7.92 x iO1*
56.4
1.61 x 10s
1.35 x IO5
7.53 x 10*
63.0
1.20 x IO6
6.44 X IO5
8.99 x 10"
53.3
1.64 X IO6
7.32 x IO5
3.48 x 10s
76.6
1.70 x IO6
6.60 X 10!
2.73 x IO5
77.2
1.40 x IO6
3.96 x IO5
1.44 x IO5
82.1
1.41 X IO6
7.44 x IO5
1.11 x IO5
54.9
4.71 x IO5
3.06 x IO5
7.38 x 10*
50.6
0.32 - 0.56
6.30 X IO5
3.06 x 10s
2.30 X 10"
55.1
1.81 X 10s
9.30 X 10*
2.93 X 10*
64.8
2.94 X IO5
1.75 x 10s
3.00 x 10*
50.7
6.99 X 10s
3.15 x 10s
1.22 x IO6
72.6
7.23 X IO5
2.86 X 10s
1.00 x IO5
74.3
7.50 x IO5
2.10 x IO5
5.10 x 101*
78.7
6.15 x 10^
3.24 x 10
4.20 x 10*
54.2
1.41 X IO5
8.27 x ID*
1.93 x 1011
54.2
0.56 - 1.0
1.92 x 105
1.00 x 10s
1.02 x 10"
54.6
1.14 x IO5
5.05 X 10"
1.09 X 10"
65.3
5.89 X 10"
4.00 X 10"
1.04 X 10*
49.9
1.63 x IO5
8.73 X 10"
3.36 x 10"
67.1
1.58 X IO5
7.47 X 10"
2.74 X 10"
66.0
2.00 X IO5
6.33 x 10"
1.49 X 10*
75.8
1.49 X lof
9.27 X 10
1.05 y 10*
45.7
3.20 x 10*
1.86 X 10*
4.91 x IO3
57.1
aCoarse foam, somewhat unstable. This experiment as a comparison with high quality foam data below.
bCoarse foam, somewhat unstable. Aerosol generated from 10% OOP solution.
GCoarse foam, somewhat unstable. Aerosol generated from 2% OOP solution.
dBaseline high because of high disk speed with 4% Tergitol. Uniform, small-celled, stable foam.
-------
CO
O
o
O
100
80
60
o
rr 40
o
i—
o
o
o
20
FOAM CHARACTERISTICS
250 MESH SCREEN
AIRFLOW 1. 137 x 10
WATER FLOW 5.83cnT/s
BUBBLE SIZE 0.83 mm
-
m5/ s
RELATIVE HUMIDITY
45% -5% PARTICLE SIZE
o
D
A
0.18-0.32pm
0.32-0.56pm
0.56 -1.00pm
10
20
30 40 50
RESIDENCE TIME, s
60
70
80
Figure 27. Foam scrubber collection efficiency of polypropylene glycol 425 using
2% Tergitol foam with rotating disk for foam destruction.
-------
en
t/i
o
100 -
80
60
TERGITOL
40
o
o
o
o
20
FOAM CHARACTERISTICS
2% SOLUTION , ,
AIRFLOW 1.137x10" m/s
WATER FLOW 5.83cm Is
PARTICLE SIZE
0.18-0.32 jum
0.32-0.56 pm
0.56 -1.00 jum
10
20
30 40 50
RESIDENCE TIME, s
60
70
80
Figure 28.
Collection of DOP aerosol using foam scrubbing with
rotating disk for foam destruction.
-------
agrees with the theoretically predicted curves reasonably-
well (see Figures 6, 12, 13, and 14). Bubble diameter in
all cases was 0.8 mm nominal. Somewhat higher collection
efficiencies than those predicted have been observed using
polypropylene glycol (wettable) aerosol especially at the
lower scrubber residence times (Figure 27). This indicates
that the compatibility between particles and the foam as for
example suggested by the aerosol wettability is an important
factor to consider in optimization of foam scrubber design
and operation. Figure 28 shows several additional data
points obtained with the OOP aerosol and different surfactants
including Tergitol, Sterox, and Aerosol OT. These points
further support the factor of aerosol/foam compatability since
some changes in collection efficiency were noted due to the
change of surfactant. Even though the collection efficiencies
were found generally lower than those observed using Tergitol
foam, proper surfactant selection might improve OOP collection
and approach the efficiency that was observed for polypropy-
lene glycol aerosol.
Also, a comparison of the collection data persented in Figures
27 and 28 suggests that there might be somewhat different
collection rate control factors in the cases of DOP and poly-
propylene glycol aerosols. While the small size particles
showed better collection efficiency than large size particles
using DOP aerosols, this trend was found to be reversed for
particles of polypropylene glycol.
Figure 27 also includes two sets of data measured with dif-
ferent humidity in the scrubber. Two relative humidity
levels, 5% and 45%, were investigated using glycol aerosol
and Tergitol foam. More humid input air increased the parti-
cle collection by about 1-2% for residence times of 13 and
26 seconds. Since the accuracy of the particle concentration
74
-------
measurement is believed to be about 10 - 20% it is not certain
whether the effect of humidity is or is not significant.
The effect of bubble size on collection efficiency was also
evaluated. The 60 mesh screen was used to generate coarser
foam with the other operating parameters kept constant. The
size of cells for the 60 mesh screen was photographically
determined to be about 3.9 mm. The observed collection rates
are presented in Figure 29. The collection rates are signi-
ficantly less than the values determined for the small-celled
foam (0.83 mm). The DOP aerosol showed a collection rate of
about 40% with a particle size between 0.56 and 1.0 ym and 80
second residence time (75% collection was observed with the
small-celled foam). The glycol achieved about 84% collection
under similar conditions (98% with small-celled foam). Two
data sets are also displayed using a large-celled 1% Alkanol
foam. While this foam improved collection of DOP aerosol,
the collection of polypropylene glycol aerosol was essentially
unchanged.
The operating conditions which have been used in our experi-
ments give an air (or foam) velocity in the scrubber of about
0.15 m/s (0.5 ft/s). It is postulated that a higher average
velocity might influence the collection rate, especially at
the point of foam generation where higher turbulence could
enhance the effect of collection by impaction. Consequently,
the velocity was increased by a factor of three to about
0.45 m/s (1.5 ft/s). At the same time the scrubber was
lengthened to 5.5 m (18 ft) so that a 13 second residence
time remained unchanged. The collection results for both
DOP and the polyglycol were essentially the same as for the
13 second residence time at the lower air velocity.
75
-------
ON
CO
O
o
O
100
80
60
>-
o
o
[I 40
O
i^
o
20
2% TERGITOL FOAM EXCEPT WHERE SHOWN OTHERWISE
POLYPROPYLENE GLYCOL AEROSOL -
10
OOP AEROSOL {Si
1 % ALKANOL FOAM
I
PARTICLE SIZE
0.32- 0.56 jum
0.56 -l.OOjum
20
30 40 50
RESIDENCE TIME, s
60
70
80
Figure 29. Foam scrubber collection efficiency of polypropylene glycol 425
and DOP using large-celled (3.9 mm) foam.
-------
This supports the theoretical evaluations presented in Section
4 which indicated that the collection rates during the foam
formation will be negligible and that the majority of parti-
cles will have to be collected by diffusion and sedimentation
mechanisms.
There does, however, appear to be a point where collection
efficiency is enhanced by capture of particles in the foam
generation step. This occurs at high liquid to gas ratios
causing the foam scrubber to behave similar to a wet scrubber.
Figure 30 shows the effect of distilled water sprayed on the
250 mesh screen in relation to collection efficiency of DOP
particles in the size range between 0.7 and 1.0 ym. No col-
lection of particles is seen for liquid to gas ratios below
5,130 cm3/™3 (38.4 gal/1,000 ft3). Thus, the collection
efficiencies for foam scrubbing reported above primarily
represent the mechanisms of diffusion and sedimentation.
77
-------
00
(S)
00
o
O
100
P 80
C£
* 60
o
s
LU
3 40
o
o
? 20
o
o
0
0
20
2.0
BENCH SCALE
OPERATION
60
GALLONS / 1000 ft
80 100
4.0 6.0 8.0
WATER FLOW RATE, 103 cm3/m3
10.0
12.0
14.0
Figure 30. Collection efficiency values for distilled water
sprayed on the screen versus water flow rate,
OOP particle size: 0.7 - 1.0 ym.
-------
SECTION 6
FOAM SCRUBBER ECONOMICS
Based upon the data collected during the bench scale evalu-
ation of foam scrubbing, a cost model was generated to evaluate
the economics of this fine particulate removal technique. The
costs generated were compared with costs for particulate con-
trol using conventional techniques: high efficiency elec-
trostatic precipitation, fabric filtration, and high energy
wet scrubbing. Before presenting the cost estimates and com-
parisons, their limitations must be comprehended.
6.1 ACCURACY OF ESTIMATE
The accuracy of the foam scrubbing cost estimates is limited
by the amount of information gained during the bench scale
testing. This information was used to estimate the costs of
a foam scrubber process with a handling capacity several
magnitudes larger. Hence, the accuracy of the estimates for
the foam scrubber is judged to be about ± 50%. Additional
information to determine the foam scrubber costs was taken
from Modern Cost-Engineering Techniques edited by H. Popper6
and Plant Design and Economics for Chemical Engineers by
6Modern Cost-Engineering Techniques. Popper, H. (ed).
New York, McGraw-Hill Book Company, 1968. 850 p.
79
-------
M. S. Peters and K. D. Timmerhaus.7 The costs for conven-
tional control equipment were obtained from three journal
references: Dust Collection Equipment by G. D. Sargent,8
A Systematic Procedure for Determining the Cost of Controlling
Particulate Emissions from Industrial Sources by N. G. Edmisten
and F. L. Bunyard,9 and Estimating the Costs of Gas-Cleaning
Plants by J. R. F. Alonso.10
Even though the economic data for the conventional devices
should be representative of fabric filter, high energy scrubber,
and high efficiency electrostatic precipitation, the corres-
ponding collection efficiencies of these devices in the fine
particle range are questionable. They all are defined as high
efficiency collectors based on total particle mass collection.
Since fine particles in most industrial gases represent only
a small fraction of the total particle mass (e.g., an esti-
mated 68% of fine particles in the range between 0.01 and
3 ym from pulverized coal fired power plants are in the
1 - 3 pm range on a mass basis, while the same size range
represents less than 1% of particles on a number basis),11
7Peters, M. S., and K. D. Timmerhaus. Plant Design and
Economics for Chemical Engineers, 2nd Edition. New York,
McGraw-Hill Book Company, 1968. 850 p.
8Sargent, G. D. Dust Collection Equipment. Chemical
Engineering. 76^130-150, January 27, 1969.
9Edmisten, N. G., and F. L. Bunyard. A Systematic Procedure
for Determining the Cost of Controlling Particulate Emis-
sions from Industrial Sources. Journal of the Air Pollution
Control Association. 2_0_:446-452, July 1970.
10Alonso, J. R. F. Estimating the Costs of Gas-Cleaning Plants,
Chemical Engineering. 7j8:86-96, December 13, 1971.
1!Shannon, L. J., P. G. Gorman, and M. Reichel. Particulate
Pollutant System Study. Vol. II - Fine Particle Emissions.
Midwest Research Institute. Kansas City. Environmental
Protection Agency (Durham), Contract CPA 22-69-104
(PB 302 521). August 1, 1971. 348 p.
80
-------
these devices may not remove fine particles even if a very
high overall mass collection efficiency (99%+) is obtainable
as claimed. As a result, we can be comparing the economics
of a foam scrubber device collecting fine particles with a
conventional collector whose collection efficiency in the
fine particle range is not very significant. Nevertheless,
the economic data for the three conventional control tech-
niques are based on extensive past experience in design, con-
struction, and operation and are believed to be 25 - 30%
accurate.
6.2 BASE COLLECTION EFFICIENCIES
To substantiate the questionability of collection efficiencies
for the three conventional devices in the fine particle range
the following discussion is presented. In a study of fine
particulate emissions performed by Midwest Research Insti-
tute,11 collection efficiency data for the conventional
devices to the submicrometer particle size range were extrap-
olated. These are presented in Figure 31. For convenience
the collection data from bench scale experiments for the
foam scrubber are also included.
The collection efficiencies for wet scrubbers and fabric
filters in the particle size between 0.1 and 1 ym can be
sufficiently supported by experimental data. Data from at
least 10 scrubbers and six fabric filters of different designs
were compiled to produce the extrapolated curves. The curve
for fabric filters was produced using data which ranged be-
tween about 0.5% and 65% penetration of 0.1 ym particles.
Thus, this curve represents a questionable average with a
relatively large spread rather than a typical fabric filter
operation. Wet scrubbers show a fairly steep curve with the
collection efficiencies in fine particle size range quickly
81
-------
00
to
99.99
99.9
99.8
99.0
95.0
90.0
50.0
20.0
10.0
5.0
1.0
0.5
0.2
0.05
A -
B -
"• -
POLYPROPYLENE GLYCOL 425-60 SECOND RESIDENCE TIME (BEST COLLECTION)
OOP AEROSOL - 60 SECOND RESIDENCE TIME (BEST COLLECTION)
COLLECTION EFFICIENCIES FOR FOAM SCRUBBER AREON A NUMBER BASIS -
I
0.01
0.1 1.0
PARTICLE DIAMETER, pm
0.01
0.05
0.2
0.5
1.0
5.0
10.0 *
20.0 o
UJ
O
LJ-
50.0 Us
90.0
95.0
99.0
99.8
99.9
99.99
Figure 31. Extrapolated fine particulate control efficiencies
-------
decreasing with the decreasing particle size. The collection
curves for the foam scrubber do not extend below 0.1 ym
particle size, but as indicated by the theoretically cal-
culated collection efficiencies (see Figures 9 through 17)
reasonably high collection of particles in this range may be
expected. The extrapolated collection curve of electrostatic
precipitation to particle size range between 0.1 and 1 ym
can be supported by only one experimental datum measured for
particles of about 0.7 ym. The rest of the experimental
data were taken in the range above 1 ym.
In conclusion, the collection of fine particles by the three
conventional devices is questionable and will need further
experimental verification. This may lead to changes in the
cost estimates if these devices should collect significant
amounts of particles in fine particle size range. The changes
are more likely to increase than to decrease the costs pre-
sented here. The costs for foam scrubber are based on bench
scale tests and can increase or decrease in the future. To
obtain meaningful comparison of capital and operating cost
estimates for particulate control alternatives, the esti-
mates should be based upon consistent parameters and have
similar accuracy. However, a preliminary economic analysis
of foam scrubbing can be useful by providind order-of-magnitude
comparisons with conventional control techniques and identi-
fying the critical cost sensitive areas to provide direction
for further experimental work. But the limitations of these
estimates and comparisons should not be overlooked. The
economic analysis of foam scrubbing and comparison with the
three conventional control devices follow. General assump-
tions applicable to all cost estimates are listed in Table 5
followed by the assumptions for each specific type of control
equipment in Tables 6 through 9.
83
-------
Table 5. COST ASSUMPTIONS
General
Capital and operating costs do not include waste treatment
and disposal
Gas temperature, gas composition, grain loading, and control
efficiency were not considered as variables due to insuffi-
cient information concerning their influence on cost
Time reference 1975 Marshall and Stevens index 440
Chemical Engineering fabricated
equipment index 203
Capital
Cost scaling exponent 0.6
Installation charge includes field installation, startup
cost, working capital, and interest on construction loan
Operating
Stream time 8,000 hours/yr
Pump and air mover efficiency 50%
Utility costs
electricity $0.015/kWh
water $0.079/m3 ($0.30/103 gal)
Depreciation - 7% installed cost
Capital charges - 10% installed cost (includes interest,
taxes, insurance, overhead, general and administration, etc.)
84
-------
Table 6. FOAM SCRUBBING ASSUMPTIONS
Capital
Minimize surface area of scrubber (length = diameter) with
maximum diameter of about 9 m (30 ft) - if larger add another
scrubber train
Scrubber - 6.35 mm (1/4") carbon steel with cost as function
of weight (spray nozzle 45 kg, 100 Ib)
Screen - $215/m2 ($20/ft2)
Foam destruction system (10% of scrubber cost)
Residence time 10 - 50 seconds
Surfactant makeup vessel or surge vessel with recycle 1/2
hour capacity
Pump requirement - 500 cm3/m3 (38 gal/1,000 acf) (either feed
or recycle)
Installation charge - 150% purchase cost
Operating
Pressure drop 2.2 kPa (9" H20)
Solution nozzle pressure 138 kPa (20 psig)
Labor 1.5 man @ $6/hour
Control laboratory labor and operating materials 30% operat-
ing labor cost
Surfactant solution recycle - none, 90%, 99%
Energy required for foam destruction - 847 watts/m3
(24 watts/ft3)
Surfactant - 2% solution utilized for Tergitol
Cost on 100% basis - Tergitol $1.43/liter ($5.40/gal)
Aerosol $1.27/liter ($4.80/gal)
Sterox $0.53/liter ($2.0/gal)
Alkanol $3.17/liter ($12.00/gal)
use 1% solution
85
-------
Table 7. ELECTROSTATIC PRECIPITATION ASSUMPTIONS
Capital
References listed in text
Installation charge - 70% purchase cost
Operating
Maintenance, labor and materials $64/m3 s"1 ($0.03/acfm)
Pressure drop 249 Pa (1.0" H20)
Contact power 9 watts/m3 (0.00034 hp/acfm)
For costs based on installed cost use high end of range
Table 8. FABRIC FILTER ASSUMPTIONS
Capital
References listed in text
Installation charge - 75% purchase cost
Operating
Maintenance, labor and materials $170/m3 s"1 ($0.08/acfm)
Pressure drop 1.99 kPa (8.0" H20)
For costs based on installed cost use high end of range
86
-------
Table 9. HIGH ENERGY WET SCRUBBER ASSUMPTIONS
Capital
References listed in text
Installation Charge - 200% purchase cost
Operating
Maintenance, labor and materials $127/m3 s"1 ($0.06/acfm)
Pressure drop 14.9 kPa (60" H20)
Liquid head 1.79 x 105 Pa (26 psig)
Liquid circulation 2.67 m3/l/000 m3 (20 gal/103 acf)
Makeup water 6.68 x 10~5 m3/m3 (0.03 gal/acfm hr)
For costs based on installed cost use high end of range
87
-------
6.3 RESULTS
The results of the capital cost estimates are presented in
Table 10 and Figure 32. Table 10 presents foam scrubbing
capital cost as percentage of total costs and as a function
of residence time for a 23.6 m3/s (50,000 aefm) unit, and
Figure 32 depicts the capital costs for foam scrubbing as
well as conventional controls as a function of unit capacity.
Table 10 shows that the largest capital cost is incurred for
the surfactant mixing vessel (from 35% to 61% of total cost).
This is due to the large quantity of solution required for
treatment [5,100 cm3/m3(38 gal/103 ft3)]. A 57,000 gal
vessel is required for 1/2 hour storage capacity. Figure 32
indicates that capital costs for foam scrubbing are similar
to the costs for electrostatic precipitation, the most
expensive conventional control device.
Table 11 and Figure 33 present the results of the operating
cost analysis. Foam scrubbing operating costs as a percent-
age of total operating costs are presented in Table 11 as a
function of surfactant recycle for a 23.6 m3/s (50,000
acfm) unit. The operating cost is independent of residence
time since the operating costs based on installed equipment
cost are minimal compared to the cost of the surfactant
(76 - 99+%). Costs for this estimate are based on using
Tergitol. Other surfactant costs were listed in Table 6.
Figure 33 presents a comparison of operating costs for foam
scrubbing with the conventional collection methods. Even
with 99% recycle, the operating cost of foam scrubbing is
an order of magnitude greater than the most expensive con-
ventional method (high energy wet scrubbing).
For foam scrubbing to be competitive with other particulate
control equipment, operating costs must be reduced. A
88
-------
Table 10. FOAM SCRUBBING CAPITAL COSTS FOR 23.6 m3/s
(50,000 ACFM) UNIT AS A FUNCTION OF RESIDENCE TIME
Component
Scrubber
Screen
Destruction system
Mixing (or storage) vessel
Feed (or recycle) pump
TOTAL purchase cost
Residence time, s
10
20
30
40
50
Capital cost as % of total
21
12
2
61
4
100
26
16
3
51
4
100
31
22
3
41
3
100
33
24
3
37
3
100
33
26
3
35
3
100
Table 11. FOAM SCRUBBING OPERATING COSTS FOR 23.6 m3/s
(50,000 ACFM) UNIT AS A FUNCTION OF SURFACTANT CYCLE
Component
Labor
Materials
Surfactant
Other
Utilities
Depreciation
Capital charges
TOTAL operating cost
0
90
99
Operating cost as % of total
<1
99+
<1
<1
<1
<1
100
1
97
<1
2
<1
<1
100
7
76
1
13
1
2
100
Percent recycle
89
-------
VD
O
io6r
g i«r
10
- FOAM SCRUBBER
FABRIC FILTER
ELECTROSTATIC
PRECIPITATOR
RESIDENCE
TIME, s
l i i i t I
10J
10
10
1 acfm = 4.719 x 10 "4 m3 / s
CAPACITY, acfm*
HIGH ENERGY
WET SCRUBBER
10°
Figure 32. Capital cost for fine participate control.
-------
10,000r
o
o
Q-
o
1000
100
10
1.0
FOAM SCRUBBER
NO RECYCLE —
FOAM SCRUBBER
90% RECYCLE "
FOAM SCRUBBER
99'/.RECYCLE
HIGH ENERGY
WET SCRUBBER
ELECTROSTATIC
PRECIPITATOR
FABRIC FILTER
0.11 . i i I i i 11 I
•lacfm -4.719xlO"4m3/s
104 105
CAPACITY, acfm-
Figure 33. Operating cost for fine particulate control,
91
-------
critical area in which operating cost reduction can be sought
is the expenditure for surfactant. Reducing this expenditure
may be accomplished by any one or combination of the following
ways: using less expensive surfactant, increasing recycle of
surfactant, using more dilute surfactant solution, or reducing
the liquid to gas ratio. If savings in surfactant cost can
be accomplished by these methods, foam scrubbing will become
economically more attractive and may become cost competitive
with conventional particulate control methods.
92
-------
SECTION 7
REFERENCES
1. Shannon, L. J. Control Technology for Fine Particulate
Emissions. Midwest Research Institute. Kansas City.
U.S. Environmental Protection Agency, EPA-650/2-74-027
(PB 236 646). May 1974. 225 p.
2. Yu, H. S., and E. M. Sparrow. Flow Development in a
Channel Having a Longitudinally Moving Wall. Journal of
Applied Mechanics. 37_(2) :498-507, June 1970.
3. Bird, R. B., W. E. Stewart, and E. N. Lightfoot. Trans-
port Phenomena. New York, John Wiley and Sons, Inc.,
1960. 780 p.
4. Aerosol Science. Davies, C. N. (ed). New York, Academic
Press, 1966. 468 p.
5. Liu, B. Y. H., K. T. Whitby, and H. H. S. Yu. A Conden-
sation Aerosol Generator for Producing Monodispersal
Aerosols in the Size Range of 0.036 to 1.3 Microns.
(Presented at the 6th Inst. Conf. on Condensation Nuclei.
Paris. May 1966). J. de Recherches Atmos. (Paris).
No. 3:397-406 (1966).
6. Modern Cost-Engineering Techniques. Popper, H. (ed).
New York, McGraw-Hill Book Company, 1976. 538 p.
7. Peters, M. S., and K. D. Timmerhaus. Plant Design and
Economics for Chemical Engineers, 2nd Edition. New York,
McGraw-Hill Book Company, 1968. 850 p.
8. Sargent, G. D. Dust Collection Equipment. Chemical
Engineering. 76_: 130-150, January 27, 1969.
9. Edmisten, N. G. , and F. L. Bunyard. A Systematic Proce-
dure for Determining the Cost of Controlling Particulate
Emissions from Industrial Sources. Journal of the Air
Pollution Control Association. 20:446-452, July 1970.
93
-------
10. Alonso, J. R. F. Estimating the Costs of Gas-Cleaning
Plants. Chemical Engineering. _7£:86-96, December 13,
1971.
11. Shannon, L. J., P. G. Gorman, and M. Reichel. Particu-
late Pollutant System Study. Vol. II - Fine Particle
Emissions. Midwest Research Institute, Kansas City.
Environmental Protection Agency (Durham), Contract
CPA 22-69-104 (PB 203 521). August 1, 1971, 348 p.
12. Mahalingam, R., H. S. Surate, and J. A. Brink, Jr., High
Expansion Foam Flow Analyses. Washington State Univer-
sity. (Presented at the AIChE 78th National Meeting.
Salt Lake City. August 1976.)
13. Bikerman, J. J. Foams. New York, Spring-Verlag, 1973.
337 p.
14. Bransky, D. W., and F. F. Diwoky. Removal of Sulfuric-
Acid Fog by Bubble-Phase Absorption. Refiner and Natural
Gasoline Manufacturer. 19^191-195, June 1940.
15. Boucher, R. M., and A. L. Weiner. Foam Control by
Acoustic and Aerodynamic Means. British Chemical
Engineering. £: 808-812, December 1963.
16. Personal communeiation. Branson Ultrasonics Co. October
1975.
17. McCabe, W. L., and J. C. Smith. Unit Operations of
Chemical Engineering. New York, McGraw-Hill, 1967,
p. 217.
18. Goldberg, M., and E. Rubin. Mechanical Foam Breaking.
Ind. and Eng. Chemistry Process Design and Development.
6_(2) :195-200, April 1967.
19. Rubin, E., and M. Golt. Foam Breaking with a High Speed
Rotating Disk. Ind. and Eng. Chemistry Process and
and Development. £(2):341-344, April 1970.
20. Liu, B. Y. H., R. N. Berglund, and J. K. Agarwal. Experi-
mental Studies of Optical Particle Counters. Atmospheric
Environment. £: 717-732, July 1974.
21. Liu, B. Y. H., K. T. Whitby, and Y. H. Pui. A Portable
Electrical Aerosol Size Analyzer for Size Measurement
of Submicron Particles. (Presented at the 66th Annual
Meeting of the Air Pollution Control Association. Chicago.
June 23, 1973), 15 p.
94
-------
22. Quon, J. E., and R. A. Phillips. A Laboratory Investi-
gation of a Foam Phase Air Cleaning Device. (Presented
at the 148th National Meeting of the American Chemical
Society. Chicago. September 1964), 15 p.
23. Willeke, K., K. T.. Whitby, W. E. Clark, and V. A. Marple.
Size Distributions of Denver Aerosols - A Comparison of
Two Sites. Atmospheric Environment, 8:609-633, June 1974
95
-------
APPENDIX A
DETERMINATIONS OF FOAM BUBBLE SIZE
The theoretical deviations for particle collection indicated
that the bubble size is a very influencing parameter for par-
ticle collection efficiency. Therefore, there was a need to
define the foam bubble size in order to investigate the con-
ditions under which fine bubble foams can be generated.
The foam bubble size determinations were made using a photo-
graphic technique. In order to eliminate optical distortion
of curved pipe wall we have designed a plexiglass pipe ele-
ment which has a flat viewing section. All our bubble size
measurements were made using this section and through flat
plexiglass. Photographs were taken with a 35 mm camera fitted
with an extension tube attachment for close-up photography to
measure approximately 1 mm bubbles. A series of shots were
required at various lighting conditions to determine optimum
light conditions for bubble resolution. Each photo was eval-
uated by the random selection of 10 bubbles. These 10 bubbles
were measured and their mean volumetric diameter calculated.
This was accomplished by converting the 10 measured diameters
to volume figures (assuming spherical bubbles) and then
averaging these for a mean volume. The mean volume was then
converted back to a mean volumetric diameter.
Early work on bubble size involved experiments which should
establish the influencing parameters on bubble size. Param-
eters investigated were air flow rate, liquid flow rate, sur-
factant type and concentration, and screen mesh size. The
results of our experiments are presented in Table A-l.
Several general trends can be observed from data in Table A-l.
The dependence of air flow rate on bubble size can be
97
-------
Table A-l. MEAN BUBBLE DIAMETER FOR VARIOUS FLOW CONDITIONS
Surfactant Flow rates
Concen-
tration, %
4
4
2
1
1
1
1
1
1
1
1
1
1
1
1
1
0.5
1
1
1
2
1
1
1
2
1
1
1
1
1
1
1
1
Name
Tergitol
it
H
it
n
H
ii
n
n •
n
n
ii
n
ii
n
n
ii
Aerosol
Aerosol
Aerosol
Sterox
Sterox
Sterox
Sterox
Alkanol
Alkanol
Alkanol
Tergitol
Tergitol
Tergitol
Tergitol
Tergitol
Tergitol
Water
cm3/s
18.08
11.42
18.08
11.42
11.42
11.42
18.08
18.08
18.08
18.08
18.08
18.08
20.70
15.30
10.05
4.80
18.08
18.08
18.08
18.08
18.08
18.08
18.08
18.08
18.08
18.08
18.08
11.42
11.42
11.42
18.08
18.08
2.320
Air
m3/s x 10=3
1.076
1.076
1.076
1.076
1.680
2.320
0 .077
1.076
1.350
1.680
2.001
2.320
1.076
1.076
1.076
1.076
1.076
1.076
1.680
2.320
1.076
1.076
1.680
2.320
1.076
1.076
1.076
1.076
1.680
2.320
1.076
1.680
2.320
Liquid gas
ratio,
cm3/m3 x 10~3
16.8
12.6
16.8
10.6
6.8
4.9
23.5
16.8
13.4
10.8
9.0
7.8
19.2
14.2
9.3
4.5
16.8
16.8
10.8
7.8
16.8
16.8
10.8
7.8
16.8
16.8
16.8
10.6
6.8
4.9
16.8
10.8
7.8
Screen
mesh size
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
60
60
60
350
350
350
Mean
volumetric
bubble diameter
(mm)
0.84
0.87
0.94
1.10
0.91
0.82
1.09
0.99
0.92
0.79
0.77
0.74
0.92
0.93
0.93
0.94
1.01
0.85
0.81
0.77
0.90
0.99
0.94
0.89
0.70
0.83
0.76
2.94
2.58
2.54
0.76
0.62
0.66
vo
oo
-------
demonstrated by Figure A-l. The curve indicates that the
mean bubble diameter decreases with an increasing air flow
rate. As lone as the foam generation screen is maintained
properly wet, the increased water flow rate seemed to de-
crease the mean air flow rate bubble size. This effect,
however, appeared rather insignificant and very much within
the experimental error of foam bubble size determinations.
The screen has been observed to be the most critical param-
eter affecting bubble size. A 60 x 60 mesh screen under
stable condition produced foam with bubble diameter of about
2.5-3 mm. A 250 x 250 mesh screen produced foam of about
1 mm diameter, and 350 x 350 mesh screen produced bubbles
about 0.7 mm in diameter. The effect of screen size on the
foam bubble size is represented by Figure A-2.
Figure A-3 shows the effect of surfactant concentration on
bubble size. An increaising concentration of Tergitol tends
to reduce the mean bubble size. An additional effect of
surfactant concentration was observed at low surfactant con-
centrations (about 0.1%). At these low concentrations the
foam is unstable or marginally stable and the bubbles tend
to be much larger than when higher surfactant levels are
used and stable foams are produced. No further investigation
of these marginal conditions was made since large bubble
foams were of no interest to this program.
In conclusion, the most directly influencing parameters on
bubble size are air flow rate, surfactant concentration, and
the screen size. However, only the screen size changes the
bubble size significantly (factor of 5) while the flow rate
and surfactant concentration exhibit less significant (about
a 10 - 20%) change in bubble size.
99
-------
t
fc
o
LU
—I
OQ
CQ
ZD
00
CO
<
LU
Qi
O
INCREASE IN AIR FLOW RATE
Figure A-l. Bubble diameter versus air flow rate
observed for 1% Tergitol foam.
100
-------
LU
LU
00
CD
CD
LU
S
o
z
CO
<:
LU
C£
O
INCREASING; SCREEN OPENING
Figure A-2.
Bubble diameter versus screen opening for
1% Tergitol foam constant air and liquid flow.
101
-------
LU
fc
DO
CO
00
-z.
<
CO
<:
LU
C£
o
INCREASE IN %TERGITOL
Figure A-3.
Bubble diameter versus
surfactant concentration.
102
-------
In the later part of the program we were able to improve the
technique of foam generation to an extent that much lower
liquid flow rates were needed to produce foams of satisfactory
stability. This was accomplished mainly through the physical
position of the liquid spray nozzle in relation to the foam
generation screen. The position of the nozzle was adjusted
manually on the "try and see" basis. With the nozzle in
certain positions a uniform and stable foam was produced.
We feel that this position was in the exact center of the
duct with the nozzle spray pattern covering the whole screen.
Exact measurements of the nozzle positions were not made be-
cause of the limited access to the nozzle location.
The measurements of foam bubble size were repeated with the
reduced liquid flow rates and the data obtained suggest that
the position of the spray nozzle can significantly influence
the foam bubble size as well as its stability. Table A-2 sum-
marizes the results determined under specific operating con-
ditions. Comparison of the data in Tables A-l and A-2 will
indicate the significance of nozzle position on the foam
characteristics. Even though correlating the data from both
tables may lead to some discrepancies, general trends between
air flow rate, surfactant concentration, screen size, and
foam bubble size discussed earlier in this appendix are be-
lieved real since they were gotten with the spray nozzle in
a specific position. Examples of typical photos used to
determine foam bubble size are shown in Figure A-4.
One weakness of the correlations and data presented here is
that the foam was photographed through the plexiglass wall
and only the bubbles against the wall could be analyzed.
The validity of the results therefore rests mainly in indi-
cating the foam bubble size range as well as the influencing
bubble size. Assuming that the bubbles on the duct walls
103
-------
(a)
(b)
(c)
(a) Tergitol foam, 1% solution; air flow 1.076 x 10" m3/s;
liquid flow 11.4 cm3/s; 250 mesh screen; volumetric mean
bubble diameter 1.10 mm; scale graduated
in fractions of an inch
(b) Tergitol foam, 2% solution; air flow 1.137 x 10~3 m3/s;
liquid flow 5.83 cm3/s; 60 mesh screen;
volumetric mean bubble diameter 3.90 mm
(c) Tergitol foam, 1% solution; air flow 1.137 x 10~3 m3/s;
liquid flow 5.83 cm3/s; 250 mesh screen;
volumetric mean bubble diameter 0.84 mm
Figure A-4. Photos of foam for bubble size determinations
104
-------
Table A-2. BUBBLE SIZE OF FOAM GENERATED WITH TERGITOLC
Percent
Tergitol
1
2
2
2
Screen size
250 mesh
250 mesh
325 mesh
60 mesh
Arithmetic mean
bubble size,
mm
0.84
0.82
0.72
3.90
Bubble size range,
mm
0.60 - 1.10
0.65 - 1.05
0.40 - 1.05
3.00 - 4.60
aWater flow 5.83 cm3/s; air flow 1.137 x 10~3 m3/s
(38.4 gal/1,000 acfm).
are same or proportional in size to the bubbles in the main
foam flow the correlations would have broader application.
Th evaluations of qualitative and quantitative differences
between the bubbles on the wall and in the main stream were
not the objective of this program and would probably require
very sophisticated experiments and a separate research effort,
It has been observed that during high flow rates the bubbles
in the vicinity of the wall exhibit laminar flow while those
in the center of the flow regime may exhibit turbulent flow.
The turbulence then results in coarser bubbles which migrate
to the center of the flow regime. The correlations pre-
sented, however, were based on no significant turbulence
observed in the foam flow. Additional discussion of foam
turbulence is presented in Appendix B.
105
-------
APPENDIX B
TURBULENT FLOW OF FOAM
The minimum air velocity required for foam formation was
empirically determined using the bench scale scrubber with
the 250 mesh screen. The air flow was first turned off and
then the water sprayed onto the screen. The air flow was
very gradually increased until foam formation was just ob-
served across the entire screen face. At this air flow
rate the foam was clearly exhibiting plug flow. A slight
variation was observed with the various surfactants. How-
ever, no variation was seen by varying the water delivery
rate which is in agreement with our observations relative
to the negligible influence of liquid flow on bubble size.
This is probably because the excess water simply drains off
the screen. The measurements of minimum air velocities are
presented below.
Surfactant Water flow rate v minimum
1% Tergitol 11.42 cm3/s ' 2.03 x 10~2 m/s (4 ft/min)
1% Tergitol 18.08 cm3/s 2.03 x 10~2 m/s (4 ft/min)
1% Alkanol 11.42 cm3/s 2.54 x 10~2 m/s (5 ft/min)
1% Aerosol 11.42 cm3/s 2.34 x 10~2 m/s (4.6 ft/min)
It has been observed that the air flow has an effect on the
bubble size uniformity. An air flow of 1.076 x 10~3 m3/s
(2.28 cfm) with a 1% Tergitol solution spray produces a
reasonably uniform bubble size across the flow patterns
(see Figure B-la). A 2.317 x 10~3 m3/s (4.91 cfm) flow rate
with the same solution gives a large number of coarse bubbles
in the center of the flow with the coarser bubbles being
primarily in the more turbulent section of the flow and the
smaller bubbles accumulating in the laminar flow near the
wall (Figure B-lb). The Reynolds numbers for the air flow
106
-------
Figure B-la. Air flow,
1.076 x 10- m3/s (2.28 cfm)
1% Tergitol
Figure B-lb. Air flow,
2.317 x 10~ m3/s (4.91 cfm)
1% Tergitol
107
-------
at 1.076 x lO"3, 1.685 x 10"3, and 2.317 x 10~3 m3/s (2.28,
3.57, and 4.91 cfm) are 8,840, 13,430, and 19,100, respect-
ively. All of these values are in the transition region
considering gas flow. Our observations of foam flow at
2.317 x 10~3 m3/s (4.91 cfm), however, revealed significantly
more turbulence in the center of the flow than that observed
with 1.076 x 10~3 m3/s (2.28 cfm) flow.
Similar phenomena were observed for the other surfactants
except the Alkanol DW. At the 2.317 x 10~3 m3/s (4.91 cfm)
flow rate as well as the 1.076 x 10~3 m3/s (2.28 cfm) flow
rate, Alkanol exhibits uniform, consistent bubbles across
the flow field (see Figures B-2a and B-2b). This foam tends
to be more stiff and rigid. In effect it seems to exhibit a
higher bulk foam viscosity. Readings were taken to several
foams with a Brookfield FVT viscometer and the values are
presented graphically in Figure B-3. The results of viscosity
measurements indicate that the Alkanol foams are more viscous
than the other foams at a 1% concentration. The values of
viscosity are relative only and are not meant to be a deter-
mination of absolute foam viscosity. The apparent foam
viscosity in Figure B-3 appears as a strong function of rate
of shear indicated by the rotational speed of the viscometer
plate.
This phenomenon is in agreement with the findings of others,
e.g., Mahalingam who observed that foam viscosity is charac-
teristically non-Newtonian and increases with decreasing
shear rate.12 Since these viscosity measurements were not
considered absolute and are a function of shear they were
12Mahalingam, R., H. S. Surate, and J. A. Brink, Jr., High
Expansion Foam Flow Analyses. Washington State University
(Presented at the AIChE 78th National Meeting. Salt Lake
City. August 1974.) 34 p.
108
-------
Figure B-2a. Air flow,
1.076 x 10~3 m3/s (2.28 cfm)
1% Alkanol
Figure B-2b. Air flow,
2.317 x 10-[ m3/s (4.91 cfm)
1% Alkanol
109
-------
4.0
3.0
CO
Q_
£2.0
CO
O
O
oo
1.0
1
I
1%ALKANOLDW
(READINGS AT HIGHER RPM OFF SCALE)
1%TERGITOL
1 MINUTE = 60 SECONDS
j i i i i
4
8 10 12 14 16
SPINDLE SPEED, RPM
18 20
Figure E-3.
Foam viscosity measured on
Brookfield viscometer.
110
-------
not included in the Reynolds number calculations mentioned
above.
Measurements were taken to determine the minimum air velocity
at which the turbulence effect (the presence of large coarse
bubbles in the foam) is observed. The data are given in
Table B-l. As the air flow rate is increased the turbulence
effect becomes more pronounced, i.e., the coarse bubbles be-
come larger and more frequent. The method for the threshold
velocity determination is described below.
The water flow rate was held constant at 18.08 cm3/s (1,085
ml/min), and the surfactant concentrations varied between
0.5% and 4%. The air flow rate was gradually incrementally
increased until turbulence developed. It should be pointed
out that this is not a sharply defined point but rather the
beginning of a transition region. It should also be noted
that even at low flow rates some surfactants exhibit a cer-
tain amount of large bubbles while others do not. Alkanol
appears also to be superior to the other surfactants in its
resistance to large bubble formation.
The validity of the data presented in this appendix is in
the observation of two gas velocities which determine the
range of optimum foam generation. The minimum velocity is
required to produce uniform foam of satisfactory quality and
has been theoretically defined as the velocity needed to
detach a bubble from the generation screen. As the velocity
increases, more and more turbulence is observed in the nucleus
of the flow which results in coarse bubble formation. The
latter phenomenon is further complicated by higher shear
forces which reduce the apparent foam viscosity and produce
flow with a higher Reynolds number.
Ill
-------
Table B-l. THRESHOLD AIR FLOW RATES
Surfactant
Tergitol TMN
Aerosol OT
Sterox NM
Alkanol DW
Concentration, %
0.5
1
2
4
0.5
1
2
4
0.5
1
2
4
1
Threshold air flow rate
mVs x 10~d
Very low
1.298
1.519
1.822
1.345
1.68
1.519
1.217
1.076
1.345
1.68
1.68
2.66
cfm
2.75
3.22
3.86
2.85
3.57
3.22
2.58
2.28
2.85
3.57
3.57
5.65
112
-------
APPENDIX C
FOAM STABILITY
The foam stability is an important variable affecting the
overall performance of the scrubber. The foam must resist
any destructive interaction of particulates and any tendency
for the individual cells to destruct and coagulate into
larger cells during scrubber operation. At the same time,
it is desirable to operate the scrubber at a minimum sur-
factant concentration to minimize destruction requirements
and surfactant costs.
Several techniques to measure foam stability were examined
in order to evaluate this important property of foam. These
techniques included the static measurements in a graduated
cylinder and foam destruction using a water spray. Both of
these techniques were found to have some limitations as to
their applicability to foam conditions existing in the foam
scrubber. A method involving destruction of foam using sand
was devised and felt to be sufficiently independent of other
variables influencing foam stability measurements. A de-
tailed discussion of the results interpretation is presented
here.
Most classical studies on foam stability have investigated
broad classes of foaming agents and involved filling a con-
tainer with foam and measuring the volume change with time
as the foam collapsed.13 Two methods can be employed here:
(1) covering the top of the container to eliminate the
evaporative effects, and (2) allowing the container to
remain open. The second method is based on the convective
13Bikerman, J. J. Foams. New York, Springer-Verlag, 1973.
337 p.
113
-------
and evaporative destruction on the foam surface. These are
stability measurements under essentially static conditions
as opposed to flow in a pipe where the foam is subject to a
more dynamically changing environment.
The ideal method of measuring the stability for our purpose
would be to have an infinitely long pipe and simply record
the time required for a significant amount of foam destruc-
tion. Since this is not a practical method much effort was
expended attempting to statistically measure the stability
of the foams produced and correlate the measured values
with observations of foam flow in our bench scale scrubber.
The foam was first put into a 1,000 cm3 graduate (this is
only slightly more difficult than putting the toothpaste
back in the tube). The graduate was then covered with a
dampened cloth to eliminate any external effect. Gravita-
tional effects gradually caused the water to drain from the
bubble walls and collect in the bottom of the graduate.
Some typical drainage curves for Tergitol are presented in
Figure C-l. The foam soon resembles a shadowy sheath of
entangled surfactant molecules. The stability in this case
is probably dependent on intermolecular forces, and the
degree of molecular entanglement. The destruction observed
did not proceed at a consistent rate and large sections of
the foam mass would suddenly destruct as if by an explosion.
Using this technique a reasonable degree of reproducibility
was achieved with Tergitol and the values indicated that
higher surfactant concentrations tend to increase the stabi-
lity; this is what was observed in the flowing foam. For
Aerosol OT, however, the reproducibility was much less, and
a 0.1% foam which was very marginally stable for foam pro-
duction gave a static stability reading of greater than
114
-------
E
o
1 12 K
1%TERGITOL
AIR FLOW RATE 1.08x10
LI QUID LEVEL
WATER FLOW 18.08 cm" / s
LI QUID LEVEL
WATER FLOW 11.42 cm3/
FOAM LEVEL
18.08 cm' / s
i I I i i
4h
2h
-\ 200
0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900
TIME, s
Figure C-l. Foam drainage rates.
115
-------
5,400 seconds (90 minutes) with negligible destruction. A
1% solution of Aerosol on the other hand which is adequately
stable for foam production required 600 - 1,200 seconds
(10 - 20 minutes) for 500 cm3 of foam loss. From the experi-
mental results determined for destruction of 500 cm3 of foam
and summarized in Table C-l, it was felt that this method of
measuring the stability did not correlate well with what is
observed in foam production. More than one value presented
in Table C-l indicates duplicate runs.
It was attempted to measure the stability by leaving the top
of the graduate open to the air and in some cases blowing
air into the graduate. The results achieved were similar
to those described above. Foams which were not adequately
stable for tube flow exhibited the same or higher readings
as foams which were stable enough.
It was decided to abandon measuring the stability under
essentially static conditions and to look for a method which
measured the foam's resistance to destruction by external
forces. A new method was devised. A 0.21 m3 (55 gallon)
container with a bottom drain was filled with foam which was
destroyed to 0.10 m3 (25 gallon) level with a hand operated
water spray. The time required to accomplish this was
recorded as a measure of foam stability. The results also
presented in Table C-l (dynamic stability) were reproducible
within several seconds and seemed to correlate well with
foam behavior observed under flowing conditions.
The experimental data also indicate that increased surfactant
concentration, increased foam wetness, and a decreased bubble
size all tend to increase foam stability. Measurements with
the 250 mesh screen indicate that a stability reading of
approximately 40 - 60 seconds is required for stable foam
116
-------
Table C-l. RESULTS OF EXPERIMENTAL MEASUREMENTS
Surfactant
0.1% Tergitol
0.5% Tergitol
1% Tergitol
2% Tergitol
5% Tergitol
1% Tergitol
(60 mesh screen)
0.1 Aerosol OT
1% Aerosol CT
0.1% Alkanol DW
It .Alkanol DW
0.1% Sterox NM
1% Sterox NH
Air flov
rate
«3/B X 10" 3
1.076
1.68
2.32
1.076
1.68
2.32
1.076
1.68
2.32
1.076
1.68
2.32
1.076
1.68
2.32
1.076
1.68
2.32
1.076
1.076
1.68
2.32
1.076
1.68
1.076
1.68
2.32
1.076
1.68
2.32
1.076
1.076
1.68
2.32
1.076
1.68
2.32
1.076
1.68
2.32
1.076
1.68
2.32
1.076
1.68
2.32
Air flow
rate
cfm
2.28
3.57
4.91
2.20
3.57
4.91
2.28
3.57
4.91
2.28
3.57
4.91
2.28
3.57
4.91
2.28
3.57
4.91
2.28
2.28
3.57
4.91
2.28
3.57
2.28
3.57
4.91
2.28
3.57
4.91
2.28
2.28
3.57
4.91
2.28
3.57
4.91
2.28
3.57
4.91
2.28
3.57
4.91
2.28
3.57
4.91
Water flow
rate
cmVs
18.08
18.08
18.08
11.42
11.42
11.42
18.08
18.08
18.08
11.42
11.42
11.42
18.08
18.08
18. OS
11.42
11.42
11.42
18.08
11.42
18.08
18.08
18.08
18.08
18.08
11.42
11.42
11.42
18.08
18.08
18.08
18.08
11.42
11.42
11.42
18.08
18.08
18.08
18.08
18.08
18.08
11.42
11.42
11.42
Expansion
ratio
cmVg
89
99.5
130
107
121
149
65
90
123
84
138
192
66
83
120
83.5
' 132
182.5
"
132
157.5
194
117
132.5
76
119.5
153
98
145.5
205
—
54.2
98.5
155
— •
Static
stability
seconds
312,315
420,432
330,345
258,270
312,333
375,480
450,576
510,516
630,555
396,375
420,390
456,492
576
636
600
492
510
495
-
—
>5400
>5400
660,738
1110
1020
1197
—
>5400
>5400
--
Dynamic
stability
seconds
21
18.5
58,53
44,45
45,53
42,41
38,37
37,39
85,84
60,55
135
21
20
19
—
65
45
—
240,250
105,106
15
55
60
47
Coaments
Stable foam not formed
Foam narginally stable
bubbles Sam - 10 no
" ^" ~
Approx 1 mm bubbles
stability adequate
Bubbles approximately
5mm - 10 mm in diameter
Marginally stable bubbles
5 mm - 10 mm
Approx 1 mm bubbles
stability adequate
Stability adequate bubbles
5 mm - 10 mm
Approx 1 nm bubbles
Marginal stability very
large bubbles
Approx 1 mm bubbles
stability adequate
Note: All data taken with 250 mesh screen unless otherwise stated.
117
-------
production. The 40 mesh screen, which produced about 5 mm
bubbles, exhibited stable foam with only a 20 second spray-
down time. One surfactant, Alkanol DW at 1% concentration,
gave a foam which was about four times as stable as the other
foams at 1%. This foam was so stable we had much difficulty
destroying it. This may indicate that the 72% "inactive
component" present in Alkanol may not be inactive as far as
foam production is concerned. This is particularly interest-
ing since Alkanol, a sodium alkylaryl sulfonate, was chosen
because alkylaryl sulfonates are generally thought to be
poor-to-medium foaming agents.
Speculating that perhaps foam which is more stable tends to
hold the water longer and would exhibit a water drainage
rate which is different from a less stable foam, drainage
rate curves were prepared for foams of varying surfactant
concentrations (see Figure C-2). It is seen that all of
the foams exhibit about the same drainage rates in the time
periods used in foam scrubbers. Therefore, the drainage
rates do not appear to be the influencing factor for the
observed differences in foam stability.
The water spray destruction method to determine foam stabi-
lity became questionable when we had established correlations
representing foam stability as a function of surfactant con-
centration . The foam stability seemed to proportionally in-
crease with the surfactant concentration. This is contrary
to the theory since the foam stability should be increasing
while the foam surface is being saturated with the surfactant
to form a monolayer. Once this phase is reached the foam
stability will start to decrease. As a result, the foam
stability correlation with concentration should contain a
maximum. The proportional dependence of foam stability on
the surfactant concentration using water spray for foam
118
-------
E
o
UJ
o
o
o
26
24
22
20
18
16
14
12
5 10
8
6
2
0
FOAMS PRODUCED AT 18.08 cnT / s - WATER FLOW
1.076 cm3/ s- AIR FLOW
LIQUID LEVEL COLLECTED IN IMHOFF CONE
(1240 cm5 VOLUME)
1% ALKANOL
2%TERGITOL
1%TERGITOL
24 6 8 10 12 14
TIME, MINUTES*
*1 MINUTE = 60 SECONDS
16 18 20
Figure C-2. Drainage rates for foams
of varying stability.
119
-------
destruction is perhaps due to the dilution of surfactant.
Considering that there is a certain minimal concentration
of surfactant at which the foam is reasonably destroyed with
a water spray, the more concentrated foam will require more
water to dilute this foam to reach this minimal concentration
and a proportional relationship will result.
In order to determine foam stability, a method was needed
which would destroy the foam with an inert material, one
that does not interact with the foam. Sand was felt to be
the most readily available material to satisfy this require-
ment. Consequently, a method was investigated whereby the
sand was sprayed into a 3.8 x 10~3 m3 (1 gallon) container
of foam with a portable hand operated sand blast gun. The
sand blast gun used (similar to a paint spray gun) is a
Porta-Blast unit made by Lindberg Products Co., Placerville,
California. The arrangement of the sand foam destruction
method is shown in Figure C-3.
The sand was screened into a uniform fraction with 18 - 20
mesh U.S. Standard sieve screens. The screened sand was
held in a separate container and fed into the sand spray by
the suction force developed inside the gun. The gun nozzle
did not provide a wide enough spray pattern for our purposes.
Consequently, a brass pipe fitting was inserted in the gun
to make a suitable nozzle and provide a suitable spray pat-
tern. The fitting was 1/4" pipe to 3/8" Swagelok with the
swage end outside the gun. The Porta-Blast gun was operated
at 6.89 x lO4 pascals (10 psig), and 1.12 x 10~3 m3/s (2.40
cfm). The distance between the nozzle and the container
was held at 40 cm and the spray time was fixed at 10 seconds.
After spraying, the foam exhibited a level surface and the
final height could be measured. The resultant height was
reproducible to +0.25 inch. The sand flow rate was measured
and determined to be 4.83 g/s (290 g/min) +10%.
120
-------
PORTA-BLASTGUN
COMPRESSED AIR
69k Pa (10 PS IG)
CARDBOARD BOX FOR
DUST ELIMINATION
Figure C-3.
Portable sand blast device used for
foam stability measurement.
-------
Reproducible results were achieved with this method. Visually
observed quality of foam in the scrubber correlated well with
the results obtained using sand for foam destruction. The
resulting stability curve for Tergitol foams is presented in
Figure C-4. The stability curve indicates that the Tergitol
foam steadily increases in stability up to about 3-4% con-
centration and beyond this the stability levels off. Higher
concentrations of Tergitol foams were not investigated since
they were considered impractical for foam scrubber applica-
tion.
We first attempted to uniformly impact the foam with sand by
shaking the sand from a screen. This method appeared promis-
ing but it would not give consistent reproducible results.
Sand was then screened into various uniform fractions using
standard sieve screens. The uniform fraction of sand was
then placed on the smallest sieve screen which would pass
all of the particles and held over the container of foam.
The sieve was vibrated with a mechanical vibrator to help
insure a uniform delivery rate of sand. Various size frac-
tions of sand were tried and various types of vibrators and
shakers were also evaluated with different vibrational fre-
quencies and amplitudes.
The height above the container and the time increment employed
were also varied during evaluation of this method. However,
the irregularly shaped sand particles tended to clog the
sieve and consequently a uniform consistent sand flow rate
could not be achieved with this technique. Circular glass
beads were also investigated, but they passed easily through
the screen without vibration.
It was noticed that foam stability tends to increase signif-
icantly with a reduction in bubble size. From purely
122
-------
3200
3000
2800
2600
2400 -
•o
§ 2200-
2 2000 -
o
ft 1800
UJ
:> 1600
o
£ 1400
i 1200
1000
800-
600 -
400
200-
O
>
0,
AIRFLOW 1.076x 10"3 m3 / s
WATER FLOW 18.08 cm3 Is
SCREEN SIZE 250MESH
0 1
4567
%TERGITOL
10
Figure C-4. Stability versus concentration
for Tergitol foam.
123
-------
geometric considerations, a reduced bubble diameter increases
the number of cell units which must be destroyed. Table C-2
indicates the bubble volume as a function of bubble diameter.
It can be seen that about two thousand 1 mm bubbles are re-
quired to occupy the volume of a 1/2 inch bubble or about
fifteen thousand 0.5 mm bubbles are needed for the same
volume. Smaller bubbles exhibit an increased stability simply
because the reduced film area is more resistant to hole ini-
tiation.
Table C-2. BUBBLE VOLUME VS. DIAMETER
Bubble diameter
inches mm
; 1/2
1/4
1/8
12.7
6.35
5
3.17
2.0
1.0
0.5
Volume,
mm3
1,072.53
134.07
65.45
16.68
4.19
0.52
0.065
Number
of units
per cm3
0.93
7.46
15.28
59.95
238.6
1,910
15,384
In the latter part of the program we were able to produce
foam of sufficient quality for aerosol collection with liquid
flow rates as low as 5.83 cm3/s. Since our previous foam
stability measurements did not include this condition, we
have obtained additional measurements of the foam stability.
As indicated above, in the earlier measurements the foam was
destroyed using a 10 second spray of 18 - 20 mesh sand with
a portable sandblast gun at 6.89 x 101* Pa (10 psig) with the
nozzle placed 40 cm above the foam container. For the new
conditions the foam was much less stable and a 10 second
spray resulted in a complete destruction of the foam.
124
-------
Consequently, the spray time was reduced to 5 seconds. The
stability curves of the Tergitol foam for three scrubber
residence times and three surfactant levels are presented in
Figure C-5.
125
-------
4000
3000
LU
5
to
UJ
Q
<
O
2000
o
>
1000
0
AIR
FOAM RESIDENCE TIME
* 13s
• 40 s
• 80 s
FLOW 1.137 x 10"3 m3 / s
3,
WATER FLOW 5.83 cnT/ s
FOAM DESTROYED USING A 5 SECOND SPRAY OF 18 - 20
h MESH SAND WITH A SANDBLAST GUN AT A PRESSURE OF
10PSIG * AND A NOZZLE TO FOAM DISTANCE OF 40 cm
3
* lPSI=6.895x 10 Pa
I I I
_L
1
3 4
%TERGITOL
6
Figure C-5.
Stability of foam generated using
Tergitol with 250 mesh screen.
126
-------
APPENDIX D
TECHNIQUES OF FOAM DESTRUCTION
A recycled surfactant solution that is sprayed into the foam
can destroy foam.11* For our purposes it was desired not to
dilute the surfactant solution so it could be recycled for
reuse. Consequently, a spray method was evaluated using the
non-diluted surfactant solution.
The 1% Tergitol solution was fed through a positive displace-
ment pump to a Fulljet spray nozzle (Spraying Systems Co.).
The spray nozzle pressure was adjusted to give 5.15 x 105
Pa (60 psig). Data for 1% Tergitol foam are presented in
Table D-l and compared with the data obtained with pure water
spray.
The data indicate that the recycled solution is not nearly
as effective as pure water. It was also observed that the
spray method is more effective for less stable foam and
would be quite efficient for foam comprised of large, 6 mm
or larger, bubbles. An increase in destruction rate is
noted for an increased spray rate. The spray appears to
destroy the foam easily at first, but as time proceeds the
wet solution probably cannot drain off the foam as fast as
it is delivered by the spray. Consequently, the spray begins
to interact with the very "wet" foam and gives the appearance
of generating more foam. In conclusion this method did not
appear to be an effective method for foam destruction but
it might have some potential for less stable foams or could
possibly be used in conjunction with another method.
luBransky, D. W.,.and F. F. Diwoky. Removal of Sulfuric-
Acid Fog by Bubble-Phase Absorption. Refiner and Natural
Gasoline Manufacturer. 19:191-195, June 1940.
127
-------
Table D-l. DESTRUCTION WITH KNOCKDOWN SPRAY
to
00
Water flow,
cm3/s
18.08
18.08
11.41
11.41
18.08
18.08
Air- flow,
m3/s x 10 ~3
1.076
1.076
2.32
2.32
1.076
1.076
(cfm)
2.28
2.28
4.91
4.91
2.78
2.78
Knockdown
spray
Pure water
1% Tergitol
Pure water
1% Tergitol
Pure water
1% Tergitol
Knockdown
spray
flow rate
cm3/s
26.66
26.66
26.66
26.66
52.92
52.92
Spraydown
time
seconds
53,55
101,97
33,36
54.57
31,30
60,63
Destruction rate
m3/s x 10~2 (cfm)
2.10 44.5
1.15 24.3
3.27 69.7
2.02 43.3
3.72 78.9
1.84 39.1
-------
THERMAL DESTRUCTION
It was felt than an abrupt delivery of thermal energy to the
foam might be effective for foam destruction. In our eval-
uation a ring burner, 0.152 m (6 inches) in diameter, was
placed in the foam destruction chamber. A propane bottle
was attached to the burner and the regulator adjusted for
maximum propane gas flow. The propane gas flow was deter-
mined to be 0.166 g/s (10 g/min) by weighing the container
before and after the experiment. Assuming complete combus-
tion, the energy input for 10 Ib/min of propane is 8,281 W
(11.1 hp). The air intake on the burner was adjusted for a
completely blue flame. Foam produced from 18.08 g/s of
liquid and 1.076 x 10~3 m3/s of air was introduced into the
destruction chamber. The foam passed through the center of
the flame and enclosed the burner and extinguished the flame.
It was observed that as the foam encountered the flame the
water was driven off and the dehydrated "foam" was able to
encircle the burner. Modifications were made to the burner,
such as a metal plate above the flame to deflect the heat
down in the foam. Also, holes were drilled in the bottom
of the burner to shoot flame down in the foam. These modi-
fications did not significantly alter the results observed
before.
A more effective method of thermal foam destruction was
found using a steam coil. A 15 m (50 ft) length of 0.63 cm
(1/4 inch) copper tubing was coiled into a 0.65 m (25 inch)
long pipe section with diameter of 0.15 m (4 inches) and
placed inside the destruction chamber. Steam at 1.03 x 101*
Pa (15 psig) passed through the coil and resulted in a com-
plete destruction of 1% Tergitol foam at an air flow of
1.076 x 10~3 m3/s (2.28 cfm) and a water flow of 18.08 cm3/s
(1,085 ml/s). The condensed steam was collected at a rate
of 85 cm3/min. This corresponds to an energy load of 4.98 hp.
129
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ULTRASONIC DESTRUCTION
The majority of the ultrasonic devices on the market employ
the transmission of ultrasonic energy to incompressible
materials. Foam by its nature is a compressible substance
and in fact foam materials are used as acoustic insulators.
Some studies have been done investigating frequencies from
0.7 to 29 kHz/s and intensities up to 150 db by passing air
through high frequency horns and whistles.15 An apparatus
of this sort is shown in Figure D-l. The method destroyed
foam but the maximum destruction rate achieved was
1.7 x I0~k m3/s (0.36 cfm) per 4.7 x I0~k m3/s (1 cfm) of
air supplied to the sound source. As a result a 1,470 watt
(2 hp) compressor would be required to destroy 4.7 x lO"4 m3/s
(1 cfm) of foam. Further evaluation of this approach re-
vealed that the large air flow from the sound source tended
to dehydrate the foam in some cases and produce a sticky
material creating some operation problem.16 Also, a sound-
proof chamber would probably be required for industrial ap-
plications. Perhaps a more promising technique to accomplish
foam breakage is the use of an air jet as illustrated in
Figure D-2.
We have investigated the application of an air jet to foam
destruction using Deflectojet 8686-37 nozzle procured from
Spraying Systems Company, Figure D-2. This nozzle simulated
the flow pattern and the air jet action suggested in refer-
ence 15. Using the air jet in our destruction chamber re-
sulted in a complete destruction of 1% Tergitol foam which
was produced with a 1.076 x 10~3 m3/s (2.28 cfm) air flow
15Boucher, R. M., and A. L. Weiner. Foam Control by Acoustic
and Aerodynamic Means. British Chemical Engineering.
£:808-812, December 1963.
16Personal communication. Branson Ultrasonics Co. Oct. 1975.
130
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COMPRESSED GAS
WHISTLE
H
;i|HIGH SPEED AIR
IjSf JET WITH FOAM
DROPLETS
LIQUID DROPS
SUCTION AREA
Figure D-l.
Air jet whistle used by
Branson for foam breakage,
131
-------
Hollow Cone Spray Pattern
Figure D-2. Deflectojet nozzle from Spraying
Systems Company.
spray angle.
Nozzle has 180
132
-------
and 18.08 cm3/s water flow. The foam rate was gradually in-
creased and at 1.345 x 10~3 m3/s (2.85 cfm) the rate of de-
struction became slower than the foam flow rate. The air
flow through the nozzle at this condition was measured to
be 1.16 x 10~3 m3/s (2.45 cfm) at 3.7 kPa (46 psi). This
air flow corresponds to a compressor requirement of 261 W
(0.35 hp) assuming a compressor efficiency of 80%.17
DESTRUCTION USING HIGH SPEED ROTATING DISK
A high speed rotating disk was investigated by Goldberg and
Rubin18 and by Rubin and Golt.19 Their results indicated
that disk speeds in excess of 2,500 rpm provided an effective
technique for foam breakage. Their work also indicated the
disk material was not critical and a Teflon disk exhibited
quite effective foam destruction. We mounted a 0.24 cm
(0.062 in.) thick Teflon disk on a 50 watt (1/15 hp) Bodine
electric motor and placed the disk in the 0.1 m (4 in.)
vertical plexiglass pipe which was located inside the de-
struction chamber.
The motor was operated at its rated output of 10,000 rpm.
The disk was evaluated with 1% Tergitol foam, 1.076 x 10~3 m3/s
(4.91 cfm). The destruction was inadequate. It was felt
that the disk-to-pipe clearance would be a critical parameter
(as any undestroyed foam had to pass through this opening).
17McCabe, W. L., and J. C. Smith. Unit Operations of Chemical
Engineering. New York, McGraw-Hill, 1967. p. 217.
18Goldberg, M., and E. Rubin. Mechanical Foam Breaking.
Ind. and Eng. Chemistry Process Design and Development.
6^(2) :195-200, April 1967.
19Rubin, E., and M. Golt. Foam Breaking with a High Speed
Rotating Disk. Ind. and Eng. Chemistry Process Design
and Development. 9(2):241-244, April 1970.
133
-------
Clearances from 0.05 - 0.15 cm (0.125 - 0.375 in.) were
investigated and the effect of the clearance distance on the
destruction rate was observed to be minimal. The disk was
then modified by cutting some sections out of it as shown
in Figure D-3. This modification greatly increased the ef-
ficiency of foam destruction. The modified disk (with a
disk-to-wall clearance of 0.08 cm) efficiently destroyed 1%
and 4% Tergitol foam at flow rates up to 3.6 x 10~3 m3/s
(7.5 cfm). The input power to the motor under no torque was
measured at 50 watts (1/15 hp), while in use the power re-
quirement was 60 - 65 watts.
A small pneumatic hand grinder (Buckeye 2.6, Model 510D) was
used to spin the disk instead of the electric motor. The
grinder listed a maximum speed of 24,000 rpm. Once again
no attempt was made to optimize the rotational speed. The
grinder was operated with air at 317 kPa (46 psi) and
1.84 x 10~3 m3/s (3.9 cfm) and was as efficient as the disk
driven by the electric motor. The compressor requirement
for this air would be 410 watts (0.55 hp), which is signifi-
cantly greater than the power needed for the electric motor.
The success of the high speed disk indicated than an efficient
implementation of shear forces is an effective technique for
foam destruction. Since the high speed disk destroyed foam
efficiently with the least energy consumption this technique
was mainly utilized in our foam scrubber collection efficiency
evaluations.
134
-------
Figure D-3. Modified and unmodified Teflon disk
135
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APPENDIX E
FOAM DENSITY
As indicated by our results above, the stability of foam with
the same bubble size may vary with time. This is due to foam
drainage. Fresh foam prepared with an adequate liquid spray
will contain more surfactant liquid between the bubbles (wet
foam) than the foam which was allowed to drain some of this
solution (dry foam). The thickness of the bubbles may be
considered as a measure of foam wetness. However, the diffi-
culty in measuring the thickness of the foam films restricts
the use of this variable. Similar results that could be
considered a measure of foam wetness or wall thickness might
be achieved by measuring the density of the foam.
A technique was developed to consistently and reproducibly
measure the foam density. A 0.1 m (4 in.) flexible hose was
attached to the plexiglass pipe about 1.52 m (5 ft) down-
stream from the foam production screen. A plastic container
(^0.08 m3) was then put on a platform so that it was above
the level of the plexiglass pipe. This prevents water runoff
from entering the container. The container is then filled
with foam and weighed. From these measurements the expansion
ratio defined as the volume of the container divided by the
weight of the foam, or foam density, can be calculated. Ex-
pansion ratios for foams produced varied between about 50
and 200 cm3/g. The data are presented graphically in Figures
E-l and E-2. The expansion ratio is simply the inverse of
the density, and an expansion ratio of 100 has a bulk foam
density of 0.01 g/cm3.
The data indicate that the water flow rate is increased the
expansion ratio is reduced. Increasing the surfactant con-
centration also tends to increase the density or reduce the
136
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220
200
180
160
140
o
£ 120
z
-------
220
200
180
160
140
eg
z
o
n_
a
120
100
80
60-
1 % STEROX
11.42cm3/s
1% AERO SOL
11.42cm3/s
1%TERGITOL
11.42cm3/3
1 % AEROSOL
18.08 cm^/s
1 % ALKANOL
18.08 cm3/s
1%TERGITOL
18.08 cm3/s
1 % STEROX
18.08 cm3/s
NOTE:
ALL DATA TAKEN WITH 250 MESH SCREEN
0.5 1.0 1.5 2.0
AIR FLOW RATE, I0"3xm3/s
2.5
Figure E-2. Expansion ratios for several surfactants.
138
-------
expansion ratio. Reducing the bubble size also tends to
reduce the expansion ratio as is seen from the foam pro-
duced with the 60 mesh screen. An interesting observation
is that increasing the bubble size (data with the 60 mesh
screen and the 0.5% Tergitol) seems to reduce the slope of
the curve. Data with very fine bubbles throughout the
operating range (the Alkanol) increases the slope of the
curve.
The possible explanation is that the larger bubbles which
are definitely less stable than fine bubbles might undergo
some destruction during the sampling.
139
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APPENDIX F
INSTRUMENTATION FOR AEROSOL SAMPLING
Three types of particle count instruments were used for aero-
sol characterization and the determination of particle col-
lection efficiency by the foam scrubber. The three instru-
ments included a model Rich 100 Condensation Nuclear Monitor
manufactured by the Environment/One Corporation, a Royco
Model 220 Optical Counter from Royco Instruments, Inc., and
a Model 3030 Electrical Aerosol Size Analyzer manufactured
by Thermo Systems, Inc.
These three instruments characterize particles in the sub-
micrometer size range desired for evaluation of foam scrubber
operation. The condensation nuclei counter operates on the
principle of utilizing the aerosol particles to nucleate con-
densation of water vapor and form larger particles which are
analyzed by light attenuation phenomena. The condensation
counter provides the total of all particles over 0.0025 ym
in diameter, for concentrations above 300 particles/cm3
(8.5 x 106/ft3). Environment/One lists the accuracy of their
instrument at ±20% with a repeatability of ±3%. The instru-
ment computes a new reading about once per second and requires
less than a minute to reach equilibrium in a new environment.
The Royco optical counter operates by projecting a light beam
through the gas stream and detecting the particles by moni-
toring the light scatter with a photomultiplier tube. The
particles are individually monitored by the photo tube and
the detected light pulse is a function of particle size.
Consequently, the output pulses from the tube can be counted
and classified according to particle size at the same time.
The instrument counts particles in five particle size classes:
0.3 - 0.5 ym, 0.5 - 0.7 ym, 0.7 - 1.0 ym, 1.0 - 2.0 ym, and
•140
-------
>2.0 ym. As reported by the manufacturer, the optical sensor
has a maximum concentration limit, of 35 - 70 particles/cm3
1 - 2 x 106 particles/ft3). The accuracy of this instrument
has been reported to approach 100%.20
The electrical analyzer generates a positively charged aero-
sol by passing the input gas through an electrically induced
corona. The charged aerosol is then passed along a charged
wire with known voltage. Particles less than a certain size
are attracted to the wire and the remaining particles are col-
lected on a charged plate. The voltage across the wire is
incrementally varied and the charge collected upon the plate
can be transformed into a size distribution of particles from
0.003 ym to 1.0 ym in 10 incremental steps. The accuracy of
the instrument has been reported as "good" with a variability
of 5% to 10%.21
SAMPLE DILUTION
The high aerosol concentrations which were employed in the
investigations required the use of a dilution system so that
the sample stream would not exceed the capacity of the optical
and electrical aerosol monitors. The dilution system had a
dilution capability from 25/1 and 500/1 and is illustrated
in Figure F-l. The Cyclonair blower was used to pull the
aerosol sample and the dilution air through the mixing
section of the dilution apparatus.
20Liu, B. Y. H., R. N. Berglund, and J. K. Agarwal. Experi-
mental Studies of Optical Particle Counters. Atmospheric
Environment. 8^:717-732, July 1974.
21Liu, B. Y. H., K. T. Whitby, and Y. H. Pui. A Portable
Electrical Aerosol Size Analyzer for Size Measurement of
Submicron Particles. (Presented at the 66th Annual Meeting
of the Air Pollution Control Association. Chicago.
June 23, 1973). 15 p.
141
-------
t"O
TEST DUCT
UNDILUTED SAMPLE
FLOW MEASUREMENT —
V
FLO\
SAM RLE STREAM
FLOW
ADJUSTMENT
COMPRESSED AIR
ABSOLUTE
FILTER
ADJUSTMENT
•STATIC MIXER
<
DILUTION AIR
-FLOW MEASUREMENT
DILUTED SAMPLE
FOR ANALYSIS
CYCLONAIR
BLOWER
Figure F-l. Sample dilution system.
-------
The overall operation of the dilution system was checked by
comparing the actual dilution ratio (as determined by the
particle concentration measured with the condensation nuclei
counter) with the theoretical dilution ratio (as determined
from the rotameter settings). Figure F-2 indicates accurate
dilutions up to the ratio of 400 to 1. The dilution system
was also verified and its accuracy demonstrated by the use
of three aerosol counters. Good agreement of all instruments
was observed.
143
-------
2 500
£
O UJ
< g
°= 8
OS
Si
O
Q_
ID
400
300
200
OQ 100
0
0
100 200 300 400
THEORETICAL DILUTION RATIO
500
Figure F-2. Characterization of dilution system.
144
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APPENDIX G
SECONDARY FORMATION OF PARTICLES FROM FOAM DESTRUCTION
Upon droplet evaporation any non-volatile material which is
present in the liquid drop then remains in the gas stream as
a particle. The destruction of foam is associated with the
formation of fine aerosol. This has been observed by other
investigators and confirmed in our experiments (e.g., Quon
and Phillips22 have indicated that a significant number of
particles in the 0.3 to 2.0 ym range are produced when foam
is destroyed by bubbling through isoamylalcohol). Destruction
of foam results in bubble bursting and liquid droplet disper-
sion. While using the high speed rotating disk for foam
destruction, it was observed that the number of particles
generated varies with the disk speed and the foam density
(see Figure G-l). The data indicate that for 2% Tergitol
foam (o.37 x 10~3 m3/s air flow, 5.83 cm3/s water flow) at
minimum destruction speed of 5,000 rpm, the baseline number
of fine particles (ranging from 0.01 to 1.0 ym) was about
0.79 x 106 particles/cin3. The number of these fine particles
increased to about 1.65 x 106/cm3 at a disk speed of 10,000
rpm. The foam used in this experiment consisted of about
1 mm cells and had a bulk density of 4.95 kg/m3.
A significantly less dense foam was prepared using 1% Tergi-
tol. Under the same flow conditions this foam appeared
somewhat unstable and consisted of coarser, non-uniform cells
(approximately 3-10 mm). The density could not be measured
accurately because of the low foam stability. The baseline
number of fine particles generated by disk destruction of
22Quon, J. E., and R. A. Phillips. A Laboratory Investigation
of a Foam Phase Air Cleaning Device. (Presented at the 148th
National Meeting of the American Chemican Society. Chicago.
September 1964.). 15 p.
145
-------
0.01
10
CO
LU
O
cc
UJ
DO
0.1
DISK SPEED =10,000 RPM
^^^*^r
1.0
DISKSPEEDX
= 5000 RPM
EFFLUENT AIR
FROM FOAM SCRUBBER
WITH NO" INPUT AEROSOL
2%TERGITOLFOAM
\.\3U \Q'3m3ls AIRFLOW
5.83 cm3/s LIQUID FLOW
10J
0.01
0.1
1.0
PARTICLE SIZE,
Figure G-l.
Particles generated by foam
destruction with rotating disk
146
-------
this foam varied from 0.34 x 106/cm3 at 5,000 rpm to 0.57 x
106/cm3 at 10,000 rpm. During actual collection efficiency
determinations the disk was operated at the minimum destruction
speed to reduce the particle baseline to a minimum.
Even though some emission of fine particles from foam de-
struction can be expected, our results should be regarded as
very preliminary. We do not know whether or not the number
of fine particles emitted from the foam destruction in the
commercial operation would be proportionally higher relative
to process capacity. Also, we feel that the design optimi-
zation of the disk foam destruction device in respect to fine
particle emissions could substantially reduce this problem.
147
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA- 600/2 -76-12 5
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Application of Foam Scrubbing to Fine Particle
Control--Phase I
5. REPORT DATE
May 1976
6. PERFORMING ORGANIZATION CODE
7 AUTHOR(S)T.E. Ctvrtnicek, T. F. Walburg,
C.M. Moscowitz, andH.H.S. Yu
8. PERFORMING ORGANIZATION REPORT NO
MRC-DA-556
I. PERFORMING ORGANIZATION NAME AND ADDRESS
Monsanto Research Corporation
1515 Nicholas Road
Dayton, Ohio 45407
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-029
11. CONTRACT/GRANT NO.
68-02-1453
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
Phase I Final; 6/74-12/75
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES JERL-RTP project officer for this report is Geddes H. Ramsey,
Mail Drop 61, Ext 2298.
repOrt summarizes knowledge and data obtained during the first phase
of investigations into foam scrubbing. It gives detailed information concerning foam
scrubber theory, experimentation, and economics, The theory pertains to mecha-
nisms influencing the behavior of fine particles and the possibilities of their capture
by foam. Collection efficiencies obtained on a bench scale foam scrubber show that
foam scrubbing can be a viable fine particle control device. Preliminary economic
analysis indicates that foam scrubbing can be competitive with other fine particle
collection devices if the surfactant solution is recycled effectively. The report
recommends that surfactant recycle be verified, using a bench scale foam scrubber.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFJERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Dust
Scrubbers
Foam
Air Pollution Control
Stationary Sources
Fine Particulate
Foam Scrubbing
13B
11G
07A
. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReportj
Unclassified
21. NO. OF PAGES
152
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
148
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