Note: Descriptions are shown in the official language in which they were submitted.
C~17-21-3203
~391~
. . .
,',':
. .
. .
..
. .
~1~` ...
.
ELECTROSTATICALL~ AUG~ENTED FIBER BED SEPARATOR
.. .
Background of the_Invention
This invention provides a method and apparatus for
removing particulates, including submicron particle size
insoluble solid particulates, from a gas stream using a
fiber bed collector from which collected insoluble solid
particulates can be removed during operation without the
10 need for removing the fiber bed from service for cleaning.
The invention permits high collection efficiency, extended
continuous operation without any unacceptable increase in
pressure drop and service life for the fiber bed separator
` not limited by the pluggage rate of the fiber bed.
Industxial waste gases and process gas stream fre~
quently contain undesirable solid particulates which must be
separated from the gas stream for environmental or process
requirements. Particulates having a particle size of 3 or
more microns are easily recovered with high efficiency in
20 many types of conventional separators. Smaller particulates,
and particularly those of submicron size, and especially
those down to about 0.2 micron in size are more difficult to
separate with a high degree of efficiency.
Fiber bed separators are commonly used for particu-
late separation. These fall into three classes: high
efficiency as regards submicron particulates, high velocity,
and spray catchers.
High ef~iciency riber bed separators typically use
5 to 20 micron fibers, such as glass fibers, packed to a bed
30 voidage of from 85 to 98% fox separation of submicron
,
.
.
:
:
~ C-17-21-3203
particulates with 95 to 98% efficiency~ but at low gas
flow rates (i.e., bed velocitiesl typically up to about
- 0.5 feet per second (or about Q,15 meters per second). At
bed veloclties about this level, significant increases in
pressure drop through the iber bed will result, with con-
sequent increase in power requirements. Liquid particulates
such as mists, etc. effectively drain from the fiber bed dur--
ing operation. Readily soluble solid particulates can be
removed from such fiber beds during operation by irrigating
10 the fiber bed with a liquid in which the solid particulates
are readily dissolved. Gas streams containing any appreciable
loading of insoluble solid particulates, and particularly
over l micron in size, cannot be treated in high efficiency
fiber bed separators, even with irrigation, without gradual
build-up of insoluble particulates wi-thin the fiber bed until
the fiber bed either becomes plugged or the pressure drop
through the fiber bed due to plugging has increased to an
unacceptable degree, at which point the fiber bed must be
taken out of service for cleaning or replacement.
High velocity fiber bed separators are used in appli-
cations where high gas flow rates (e.g., bed velocities of
from 5 to 10 feed per second; i.e., 1.5 to 3 meters per
second) are desired and less efficient separation of sub-
micron particulates can be tolerated. This type of fiber
bed separator typically used 25 to 50 micron diameter
fibers with a bed voidage of from 85 to 98~. For any
given bed voidage, the coarser fibers of a high velocity
fiber bed (as compared to a high efficiency fiber bed) has
considerably less fiber surface area per unit volume of
30 fiber bed and accordingly a more open network of fibers.
Therefore, this type of bed relies more heavily on an
inertial impaction mechanism of particulate separation and
thus has poorer efficiency for separation of submicron
particulates. Removal of insoluble solid particulates from
such fiber beds by irrigation or flushing is possible for
lower particulate loadings and particulates up to about
2 microns in size,buthere again gradual increase in
pressure drop and eventual pluggage will result due to
difficulty in removing larger sized particulates present in
~3~ C-17-21-3203
-3-
most applications, requiring removal from service for
cleaning or replacement after several days to weeks of
operation.
Finally, spray catcher fiber beds are used in
applications whereln high volumes of gases are to be
treated and separation of only large particulates of
3 microns or greater in size isofconcern. A spray catcher
typically uses fibers of about 100 to 300 microns average
diameter with a bed voidage of 90 to 98~. These fiber beds
10 have the lowest fiber surface area per unit volume of any of
the types of fiber bed separators discussed herein at any
given bed voidage. They rely almost entirely on the
inertial impaction separation mechanism. Accordingly, the
spray catcher has the poorest efficiency of all for separa-
tion of submicron particulates. However, even large (e.g.,
5 micron) insoluble solid particulates can be irrigated or
flushed out of spray catcher fiber beds.
This invention is directed to the use of the spray
catcher type of fiber bed for the separation of particulates,
20 and particularly insoluble solid particulates, from gas
streams at high bed velocities of about 5 feet per second
(i.e., 1.8 meters per second) or more, with a high effi-
ciency of at least 90~ on separationof submicron
particulates, and with liquid irrigation of the fiber bed to
remove the collected solid particulates therefrom for long
term continuous operation, without the need for taking the
fiber bed out of service for cleaning or replacement due to
pluggage.
As used herein, the term "insoluble solid particu-
30 lates" 'refers to solid particulates which will not dissolvein water or such other liquid system as may be selected as
the irrigation liquid, or which have such low solubility
rates in the liquid that their solubility cannot be effec-
tively used for their removal from the fiber bed.
Descript n of the_Prior Art
It is known that electrostatic augmentation will
improve the collection efficiency of most types of separa-
tors for removing particulates from gases. Nevertheless,
,.
,~ .
~3~ C~17-21-3203
we know of no electrostat~cally augmented fiber bed
separator which avoids the problem of gradual increase in
pressure drop through the fiber bed due to pluggage~ which
eventually requires that the ~iber bed be taken out of
service for cleanlng or replacement.
U. S. Patents 3,874,858 and 3,~58,958 to Klugman et
al. teach an apparatus and method for removal of particu-
lates from a ~as stream using a packed wet scrubber after
first placing an electrical charge on the particulates in an
10 electrostatic field. The charged particulates are separated
in the packed wet scrubber, which is maintained electrically
neutral, by a "force of attraction" mechanism between the
charged particulate and the electrically neutral wet packing.
Such collection mechanism is taught by the patentees to
require that the gas stream be flowed at a velocity suf-
ficiently low for the attraction forces to overcome particle
velocity and the viscuous drag force of the gas on the
particulates. The patentees set an upper limit of 10 feet
per second (about 3 meters per second) bed velocity on their
20 invention, but their test results indicate that at 4 and
at 7 feet per second (i.e., about 1.2 and 2.1 meters per
second, respectively) their collection efficiency with
respect to submicron particulates was substantially below 90%.
Moreover, the apparatus itself is very large requiring a
great depth of packing in the direction of gas flow, i.e.,
24 and 48 inches in their examples.
More recently, U. S. Patent 4,029,482 to Postma et
al. teaches removal of particulates by a dry fiber bed,
after first placing an electrical charge on the particu-
30 lates in an electrostatic field, with at least initially ahigh efficiency for separation of submicron particulates.
The charged particles are separated in the dry fiber bed,
which is electrically-resistive (i.e., non-conducting), by
a "space charge" mechanism whereby the charged particulates
which are collected in he dry, non-conductive fiber bed
cause an appreciable electrical charge density to develop
within the fiber bed, causing gas-borne char~ed particulates
to deviate from the direction of gas 1OW to increase the
collection efficiency of the fiber bed.
,
~3 C-17-21-3203
-5-
Postma et al. distinyuish their "space charge"approach
from prior art "image forces" mechanisms ~Ihich appear to be
the same as the "force of attraction" mechanism used by
Klugman et al. The image forces approach of the prior art is
taught by Postma et al. to be limited to low gas flow rates
(i.e., bed velocities~ and to provide only modest improvement
in collection efficiency for submicron particulates. As men-
tioned above, these are the disadvantages noted above with
respect to Klugman et al.
However, while Postma et al. may have overcome these
disadvantages, it was at the expense of accepting another
major disadvantage; that is, the inability to clean the
fiber bed while it is in service to maintain a low pressure
drop. The Postma et al. separator operates by build-up of
collected particulates in the dry fiber bed to develop the
desired space charge. With constant bed velocity, the
pressure drop through the fiber bed will increase as the
fiber bed becomes increasingly plugged, until at some point
the fiber bed must be taken out of service and cleaned.
20 Moreover, it is apparent from Postma et al. that peak col-
lection efficiency of their separator is at bed velocities
of only 3 feet per second (about 0.9 meters per second),
with a sharp drop off in collection efficiency at higher bed
velocities.
Of great significance to the present invention is the
teaching in Postma et aL. of experiments with fiber beds wet
by a water spray and with conductive fiber beds, both of
which adversely affected collection efficiency. Postma et al.
attribute this loss of efficiency to the fact that such
30 wet fiber beds or conductive fiber beds would operate by the
"image forces" mechanism rather than by their "space charge"
mechanism.
Thus, the prior art teaches that use of the "image
forces" or "force of attraction!' collection mechanisms
require use of low bed velocities and pro~ide only moderate
efficiency with respect to separation of submicron particu-
lates. The prior art further teaches that these
disadvantages can be over come by using the "space charge"
mechanism, but requires acceptance of a new disadvantage,
C-17~21~3203
~ 6-
i.e" the inability to keep the fiber bed cleaned of
collected insoluble particulates while in service and
acceptance of gradual increase in pressure drop through the
fiber bed until the fiber bed finally must be taken out of
service and cleaned.
Summary of the Invention
It is an object o~ this invention to provide a
method and apparatus for removing particulates from a
gas using a fiber bed collector with low pressure drop
lO through the fiber bed.
It is another object of this invention to provide a
method and apparatus for removing particulates, and particu-
larly insoluble solid particulates, from a gas using a fiber
bed collector from which aollected insoluble solid particu-
lates can be removed during operation without the need for
removing the fiber bed from service for cleaning.
A further object is provision of such a method and
apparatus wherein even submicron particulates may be
separated from the gas stream with high collection efficiency.
A further object is provision of a method and appara-
tus wheerein a fiber bed may be used for high efficiency
separation of insoluble, submicron sized solid particulates,
with the insoluble solid particulates collected therein
being removed therefrom during operation, without inter-
ruption of service, to preclude any unacceptable increase
in pressure drop through the fiber bed during extended
continuous operation.
A further object is provision of a method and appara-
tus wherein the continuous service life of a fiber bed
30separator for insoluble solid particulates is no longer
limited by the pluggage rate of the fiber hed.
These, and further objects will be evident from the
disclosure set forth herein and from the following discussion
of the preferred embodiments, are attained as follows.
The method described herein comprises the steps of
passing a particulate containing gas stream through, irst,
an electrostatic or ionizing field to place a positive or
negative charge on said particulates~ and subsequently, a
C-17-21-3203
-7-
bed of fibers as hereinafter characterized, while
concurrently irrigating said flber bed with a liquld at a
liquid flow rate such that at least a sufficient portion
of the fiber bed contains sufficient liquid so as to
continuously dissipate the charge from the fiber bed and
the charged particulates collected therein through said
liquid to an electrical ground without developing any
significant space charge in the fibers and collected particu-
lates on the fiber bed. Any liquid may be used which is
10 either itself conductive or ~hich becomes conductive by virtue
of the particulates suspended and/or dissolved therein during
operation.
The apparatus described herein is particularly adapted
to carrying out the above process and comprises a housing
ha~ing inlet and outlet ends, a particle charging section
within said housing comprising electrostatic or ionizing
field means, a bed of fibers as hereinafter characterized
disposed between said particle charging section and the
outlet, irrigating means for providing a flow of liquid
20 through said fiber bed at a liquid flow rate such that at
least a sufficient portion of the fiber bed contains
sufficient liquid so as to continuously dissipate the charge
from the fiber bed and the charged particulates collected
therein through said liquid to an electrical ground without
developing any appreciable space charge in the fiber bed.
While this method and apparatus are suited for
separatior. of any liquid or solid particulates from gas
streams at high bed velocity with high collection efficiency
for even submicron particulates, this method and apparatus
30 will find particular application to the separation of insol-
uble solid particulates from gases with high efficiency and at
high bed velocities of from about 6 to 15 feet per second
(i.e., about 1.8 to 4.6 meters per second~ or more.
The fiber bed used in this apparatus and method is a
bed of fibers of from about 50 to 1000, and particularly
from about 100 to 500, microns a~erage diameter, packed to a
bed voidage of at least 90%. Preferable fibers of rom
about 100 to 250 microns in avera~e diameter are used, with
a bed voidage of from about 95 to 98%. For example, fibers
C-L7-21~3203
~ 8-
of 2Q0 microns avera~e diameter packed to a bed voidage of
95~ will provide a fiber surface area of about 305 square
feet per cubic foot of fiber bed (i.e., about 1013 s~uare
meters per cubic meter2.
Although the charge is dissipated from the fiber
bed so that no appreciable charge is permitted to develop
in the fibers of the fiber bed or to remain in the collected
particulates, space charge effects are present within the
gas phase in the fiber bed particularly at larger fiber
10 diameters and with larger particulates. In the preferred
embodiment using 100 to 250 micron average diameter fibers,
however, the image forces collection mechanism is maxi-
mized and the space charge collection mechanism is minimized.
In the less preferred embodiment of 500 to 1000 micron
average diameter fibers, conversely, the space charge collec-
tion mechanism (in terms of the gaseous phase) is maximized
and the image forces mechanism is minimized.
Preferably, the fibers used will have a relatively
uniform distribution of fiber diameters within + 30% of the
20 average fiber diameter selected. However, as long as the
average fiber diameter is within the broad range set forth
in the previous paragraph, a wide range of fiber diameter
distribution can be tolerated provided not more than about
10% of the fibers have diameters below 40 microns.
Bed voidage, expressed as volume percent void fraction,
is defined by the following equation:
V~ D Df _ Dp x lO0
Df
where V~ = percent void fraction, Df = density of the fiber
30 material, and Dp = packing density of the fiber bed. For
example, using glass fibers having a specific gravity of,
for example, 2.55, the density of the fiber material (Df)
will be 159 pounds per cubic foot (2.55 grams per cubic
centimeter2. A bed of 200 micron average diameter glass
fibers packed to a packing density (Df2 of 7.0 pounds per
cubic foot ~0.11 grams per cubic centimeter~ will have a
bed voidage ~V~2 of ~5.6~.
As is apparent from the above, the fiber beds used in
this invention are a rather loosely packed, open network of
.,
~ 3~2~
C-L7-21-3203
_g_
relatively coarse fibers, At ~ void fraction o~ about 95~,
the inter-fiber distance is typically about 5 times the
fiber diameter. While insoluble solid particulates can
readily be flushed out of guch a fiber bed~ heretofore
such fiber beds have not been suitable for use in removing
submicron particles from gas streams with any acceptable
degree of efficiencyO Rather~ their use has been limi~ed
to applications where either there are not enough submicron
particulates present to be of concern, or where a sub-
stantial quantity of submicron particulates are present butneednotbe removed, or are removed by a prior or subsequent
device designed to remove submicron particulates.
The present invention is not limited bv the fiber
material used. In ~he preferred embodiment, glass fibers,
as well as fibers of other dielectric materials are used; the
irrigating liquid and/or the particulates contained therein
providing sufficient conductivity for dissipating the
electrical charge from the collected charged particles so
that no interfering space charge can develop on the fibers
and collected particulates in the fiber bed. Such
dielectric fiber materials include, f~r example, such
plastic fibers as polyesters, polyvinylchloride, polyethylene
terephthalate,fluorocarbon polymers; nylon;
polyalkylenes such as polyethylene and polypropylene;
mineral wools; asbestos.
If desired, however, conductive fiber materials can
also be used since it is suitable to practice of the present
invention that the fiber bed itself be conductive without
relying solely on the irrigating liquid/particulates. In
this embodiment, irrigation of the fiber bed is still
necessary in order to continually wet and flush the
collected particulates to prevent or minimize the tendency
of dry particulates to become re-entrained in the gas stream.
Suitable conduc~ive fiber materials include, for example,
metals such as stainless steel, titanium, copper, brass,
or any wire mesh of proper fiber diameter, as well
as carbon or graphite fibers.
i ~
.. _ . . ... ... . .
. .
:
:; .
.: , ; ': . ', - :
: ...
~'~3~
C-17-21-3203
10~
Distribution of the fibers within the fiber bed can
be random, but for best results the fibers will be at
least partially oriented parallel to the vertical plane
transverse to the direction of gas flow and less oriented
or substantially randomly distributed across the face of
such plane. This is graphically portrayed hereinafter in
tne disclosure.
This oriented fiber bed can be quantified by satura-
ting a cubic volume of a test fiber bed (as shown nerein-
after) with water while the cube is disposed such thatthe more oriented plane is vertical and measuring the
residual saturation of water held up in the fiber bed
after gravity drainage stops, then rotating the test bed
such that the surface thereof which normally ~lould be the
downstream surface is now down in the base position, and
repeating the residual saturation test. The residual
saturation of the cubic fiber bed with the more oriented
plane disposed vertically should be at least 15~ less
than the residual saturation with the other plane
oriented vertically. Any capillary and surface tension
hold-up of water in the bottom of the fiber bed should be
corrected for in such residual saturation tests, or
alternately, a large enough fiber bed, e.g., 10 inches
(or about 25 centimeters) in each dimension, can be used
so that such hold-up becomes negligible with respect to
the residual saturation.
Generally, a fiber bed depth in the direction of gas
flow of from about 2 to 6 inches (or about 5 to 15 centi-
meters) will be su~ficient for efficient separation of
even submicron particulates from the gas stream. Deeper
fiber beds will provide only marginally improved collection
efficiency, but at the expense of proportionately greater
pressure drop. If desired, the fiber bed may comprise a
series of fiber beds of shallower depth, e.g., 1 to 2
inches (or about 2.5 to 5 centimeters), in fiber-to-fiber
contact with each and~or spaced, e.g., by 0.25 to
several inches (or about 0.5 to several centimeters) from
each other.
. ...
.
~ ~ ~f~ C-17-21-3203
This invention is not lim~ted in the nature of the
means or method by~which the D.C, electrostatic or ionizing
fleld is created whi~h places the electrical charge on the
particles. Such means and methods are well known in the
art and typically comprise one or more discharge electrodes
of one polarity in conjunction with one or more grounded
electrodes, the discharge electrodes being connected to a
D.C. power source of up to 35,000 volts or more.
It is necessary, however, that the strength of the
lO electrostatic or ionizing field be sufficient to place the
desired electrical charge upon the particu;ates, and for
best results such field should extend across the entire
cross-sectional area of the housing in the particle charging
zone. For example, at a D.C. current of about 6 to 20
millamperes at 25,000 volts will provide a corona power
of 150 to 500 watts suitable for treating about 700 to 1000
actual cubic feet per minute (i.e., 19.8 to 28.3 cubic
meters per minute) of gas.
Suitable means for irrigating or flushing the fiber
20 bed to remove particulates therefrom are well known in the
art and are not limiting of this invention. It is only
required that the irrigation means be suited to irrigat-
ing at least the upstream portion of the fiber bed to
the bed depth where the substantial majority of the
particulates are collected. Preferably, however, the
entire fiber bed is irrigated.
Thus, the fiber bed may be irrigated with liquid
from a liquid supply header disposed above, or within
the upper part of, the fiber bed. As is well known in the
30 art, various liquid distribution means can be employed to
distribute the liquid over the top surface of a fiber bed,
~ such as, for example, perforated distributor plates, etc.
Baffles may also be advantageously positioned at the top of
the fiber bed to prevent gas flow from by-passing the
fiber bed around such liquid supply header or liquid
distribution means,
Alternatively to, or in conjunction with, such an
overhead fiber bed irrigating means, ~he irrigation liquid
... , , ;
C-17-21-3203
~ 12-
can be sprayed onto the upstream surface of the fiber bed
where viscous drag from the flowing gas will carr~ the
liyu~d at least partially through the depth of the fiber
bed before gravity drainage will carry the liquid out of
the bottom of the fiber bed~
Such viscous gas phase drag on the liquid increases
with increasing bed velocity of the gas. At higher bed
velocities, e.g., above about 11 feet per second (i.e.,
about 3.3 mete`rs per second), entrainment of the liquid as
10 droplets in the gas leaving the fiber bed can become a
problem, but one which can be eliminated or at least
substantially reduced by any of a variety of means for
either precluding entrainment at high velocities or
separating entrained droplets in subsequent entrainment
separators.
The rate of liquid flow required in the fiber bed will
depend upon the specific application. The minimum flow rate
will be that necessary to prevent a space charge from
developing in the fiber bed which will be a function of the
20 fiber bed dimensions, the nature of the fibers with
respect to conductivity and the conductivity of the liquid
particulates combination within the fiber bed. Beyond this,
however, the liquid flow rate must be sufficient to flush
the solid particulates out of the voidage of the fiber
bed, and will be dependant upon the fiber diameter, the
bed velocity of the gas being treated, and the loading
of solid particulates in the yas stream per unit volume of
fiber bed. For example, water flow rates of from about 1 to
10 gallons per minute/1000 CFM of gas flow (i. e.,
30 about 3.8 to 38 liters per minute/cubic meter) have been
found satisfactory for glass fiber beds separating bark fed
boiler dust at loadings of from about 200 to 1000 milligrams
per cubic meter of gas at bed velocities of from about 6 to
12 feet per second (i.e., about 1.~ to 3.7 meters per
second).
In a preferred embodiment of this invention, the
grounded electrodes of the electrostatic field means are
also irrigated with liquid to flush collected particulates
'~ off the groùnded electrodes. Suitable irrlgating means,
,
C-17-21-3203
~ 13~
methods a~d liquid flow rates are well known to those
skilled in the art of electrostatic precipitators.
The liquid used ma~ be ~ater or any other liquid
dictated by or useful in the particular application. For
example, if the partlculates being collected a~e a mixture
of insoluble solids ~ith resinous, greasy or solid particu-
lates which are soluble in a given solvent or solution,
then that solvent or solution is advantageously used so as
to dissolve the solubles as well as flush away the insolubles.
10 Similarly, if the gas stream contains objectionable odor-
iforous substances or gaseous components such as sulfur
dioxide, nitrogen oxides, etc. as well as particulates to
be removed, the liquid used will advantageously be one
which will either react with or absorb the odoriferous
substance or gaseous component. Thus, in various embodi-
ments the liquid may be, for example, aqueous solutions
of detergents, ammonium hydroxide or other alkalis, sulfuric
acid, acidic or basic salts; non-aqueous liquids
such as diethanolamine or aqueous solutions thereof
20 Other useful liquids and liquid systems will be obvious to
those skilled in the art of gas treatment. I conductive
fibers are used, non-conductive liquid/particulate systems
may be used.
When treating the particulates laden gas, an electro-
static charge, preferably a negative charge, is placed upon
the particulates as they pass through the electrostatic
field. Negatively charged particulates permit use o~
electrostatic fields of higher voltage per centimeter of
electrode spacing before spark-over will occur. In a
30 preferred embodiment, particularly when treating ~ery
dirty gases having a high dust loading, the electrostatic
field means is operated with irrigated grounded electrodes
to collect a substantial portion of the charged particles
on the grounded electrodes. The grounded electrodes will
primarily attract and capture the larger particul~tes
which because of their greater mass acquire a higher
charge. While some submicron particulates may be captured
on the grounded electrode, they are primaril~ captured
?
' ' " ,'' ~.' '' ~ ~ ' '
,
': ', ' ;.~' ' :~ '
~3~ C-17-21-3203
~ 14~
in the fiber bed. Collected particulates may be removed
from the ~rounded electrodes by any conventional means such
as rapping or air blast, but more preferably by irrigating
the grounded electrodes with liqu~d continuously or at
least intermittantly to flush of~ the particulates collected
thereon be$ore they or agglomerates thereof can be blown off
by the gas stream and carried to the fiber bed. In this
way as much as 50 to 95~ of the dust contained in the gas
stream can be removed prior to the fiber bed and reduce
10 the dust loading which must be removed by the fiber bed.
The remaining charged particles, or substantially all
if the grounded electrodes are not being used as a collector,
are carried in the gas stream to the fiber bed and separated
from the gas stream therein while irrigating the fiber bed
with a liquid to flush away collected solid particulates and
dissipate the electrical charge from the charged particles
to prevent any significant space charge from developing in
the fiber bed.
The bulk of the particulates are collected in the
upstream one-third of the depth (in the dlrection of gas
flow) of the fiber bed, with progressively lesser amounts,
particularly of larger particles, collected through the
remaining depth. The fiber bed irrigation system is
preferably designed to distribute the bulk of the liquid
in this upstream portion of the fiber bed, with allowance
for viscous gas phase drag on the liquid (which is a func-
tion of the bed velocity of the gas) which will carry the
liquid deeper into the bed before gravity drainage carries
the liquid out of the bottom of the fiber bed.
~In a preferred embodiment the irrigating system is
designed such that after allowing for gas phase drag on the
liquid at least 50% of the irrigating liquid used in the
fiber bed flows through the upstream one-third of the depth
of the fiber bed. This can be accomplished by irrigating
from the top of the fiber bed, or by applying a liquid spray
uniformly across the upstream face of the fiber bed (partic-
ularly at high bed velocities), or by a combination of both.
With this expedient, irrigating liquid requirements can
be minimized. Another expedient to further minimize
C-17~21-3203
~15-
irrigating liquid requirements is to use fiber beds which
are high and narrow insofar as the aerodynamics of fiber
bed design will perm~t.
Depending upon the appllcation, it may or may not be
desirable to recirculate used liquid back to the fiber bed
irrigattng means, the optional grounded electrode irrigation
means, or koth. Conservation and cost, particularly of
expensive liquids, make recirculation desirable when
feasible. In the simplest situation, it will merely be
necessary to separate undissolved solid particulates, by
any conventional means, from the liquid prior to recircula-
tion. However, in many applications, the used liquid may
contain soluble particulates or absorbed gases or reaction
products which will preclude recirculation without at least
some prior treatment of the used liquid to regenerate it
and remove objectionable constituents.
For example, if the apparatus and method of this
invention is used to scrub objectionable gases together
with particulates from a waste gas stream, at least partial
regeneration of the liquid will be necessary before
recirculating the liquid back to the irrigation system. An
example would be any of the scrubbing processes for removal
of sulfur dioxide from gas streams, such as sodium or
ammonia scrubbing, or an alkanolamine scrubbing as taught in
U. S. Patent 3,873,673.
It will therefore be apparent that in addition to the
above uses the apparatus and method of this invention can
be used in a wide variety of industrial and environmental
applications for removal of liquid or solid (soluble or
insoluble) particulates, or mixtures thereof. In a pre-
ferred embodiment, however, the present invention is more
particularly directed to applications requirin~ sspara~ion of
insoluble solid particulates from gases, and especially
where separation of submicron size, insoluble particulates
with high efficiency is required. Examples of such insol-
uble particulates to which the present in~ention is suited
include fly ash, bark fed boiler dust, incineration dusts
and fumes, carbon, s;lica dust, pi~ment dusts, metalurgical
fumes, and the like.
: ''
C-17-21-3203
-16~
The present apparatus and method are particularly
suited to treatment of gas streams at hi~h bed v~locities
through the fiber bed. Overall collection efficiencies of
98 to 99.9~, with at least 85%, and preferably 90 to 95~,
efficiency for submicron particulates in the 0.2 to 0.9
micron range are attainable at bed velocities of 6 to 11
feet per second (i.e., about 1,8 to 3.3 meters per second),
or even 15 feet per second (i.e., about 4,6 meters per
second) or more with provision for re-entrainmen~ prevention
10 or removal. Despite such high bed velocities, the present
invention provides high collection efficiency with minimal
pressure drop, in a more compact apparatus than, for example,
the packed bed scrubber of Klugman e~ al. Balancing
reduced blower horse power requirements against the power
required for the electrostatic field means offers energy
savings versus other high efficiency separators such as
fiber bed separators, making use of the present invention
advantageous in applications where insoluble solid
particulates are not present, e.g,, ammonium nitrate and0 urea prill towers, char-broiler fumes, etc.
Description of the Drawin~s
Figure 1 is a side cross-sectional view of one
embodiment of this invention.
Figure lA is a top cross-sectional view of the
electrostatic field means of Figure 1.
Figures 2 and 2A are top and end views, respectively,
of just one segment of a preferred electrostatic field
means useful in the practice of this invention.
Figure 3 is a side cross-sectional view of the
30 upper portion of a fiber bed with a preferred embodiment
of overhead irrigating liquid distribution.
Figure 4 is a side cross-sectional view of an
inclined fiber bed, which comprises one embodiment of this
invention,
Figure 5 is graphic portrayal of the plane of orien-
tation of the fibers in a preferred fiber bed embodiment of
this invention.
,
. .~"
C-17-21-3203
~17~
Des~ ption of the Preferred Embodiments
In the embodiment represented by Figures l andl~ , the
apparatus of this invention comprises a housing 2 with gas
inlet end 4 and outlet end 6. Disposed within said houslng
proximate said inlet end is an electrostatic field means
comprising a plurality of high voltage discharge electrodes 8
connected to a high voltage D.C. source ~not shown) and a
plurality of grounded electrodes lO. The discharge
electrodes 8 may be of either positive or negative polarity,
but preferably negative as shown in Figures 2 and 2A . The
grounded electrodes lO will have the opposite polarity with
respect to the discharge electrodes, e.g., positive as
shown in Figures 2 and 2A..
Disposed within the housing downstream (in the
direction of gas flow) of the electrostatic field means is
fiber bed 12 with overhead irrigating means 14 for dis-
tributing irrigating liquid within the fiber bed. Except
for provision for flow of the irrigating liquid, the periphery
(edges) of the fiber bed should be appropriately sealed
in the housing by a frame (not shown) with gasketing or
other conventional edge sealing means to prevent leakage of
the gas around the edges of the fiber bed. Spray means 16
is also shown for spraying irrigating liquid uniformly
across at least the upper portion of the upstream face of
the fiber bed. Under most circumstances overhead irrigation
means 14 will provide sufficient irrigation of the fiber
bed, making the additional use of spray means 16 unnecessary.
However, in appropriate circumstances use of both overhead
irrigating means 14 and spray means 16 may be advantageous.
In other applications, particularly with light dust ].oading
in the gas or with fiber beds only 2 to 3 inches (5 to 7.6
centimeters) deep in the direction of gas flow or with
high enough bed velocity gas flow, spray means 16 may pro-
vide sufficient liquid penetration of the fiber bed to make
unnecessary the use of overhead irrigating means 14.
In operation, dirty gas containing par~iculates 18
enters inlet end 4 and passes through the electrostatic field
between discharge electrodes 8 and grounded electrodes lO
wherein the particulates become charged, e.g., negatively as
.. - ,
, ~
, , .:
:
C~17-21-3203
-18~
shown in the drawings. Charged particles 20 flow downstream
to be collected in fiber bed 12 which is connected to an
electrical groundO Particlates collected in the fiber
bed are flushed therefrom by irrigatlng with a liquid from
either or both of overhead irrigating means 14 and spray
means 16. The liquid and partlculates contained therein are
drained from the bottom of fiber bed 12 and removed from
the apparatus.
In an optional embodiment of this invention the
lO used liquid may be treated to remove solid particulates and
any other contaminants present and then recirculated back
to the irrigation system. In the simplified flow scheme
shown in Figure l, the used liquid drains from the fiber
bed into a series of conduits 22, or into a trough (not
shown) and then into one or more conduits 22, into a
manifold 24 and thence to a liquid treatment means 26.
This liquid treatment means may be simply a single or
multiple stage clarifier or settling tank or other system
for separating solid particulates from the liquid.
20 Alternatively, liquid treatment means 26 may be any liquid
treatment or regenerating system and process for at least
partially restoring the used liquid to its original con-
dition for recirculation to the irrigation system. From
liquid treatment means 26, all or part of the liquid may be
recirculated back to overhead irrigating means 14, or to
spray means 16, or both, together with any fresh make-up
liquid which may be necessary.
In a preferred embodiment, also shown in Figure 1,
the grounded electrodes lO may be used as collectors for a
30 portion of the charged particulates in which event the
grounded electrodes are also irrigated with liquid to
flush collected particulates there~rom. Any conventional
means and method may be used for such irrigation, for
example spray means 28 disposed upstreamo~grounded
electrodes 10. Such spray means 28 may, for example
comprise a plurality of tubes off of a manifold (not
shown~, each tube having a plurality of nozzles and being
disposed substantially upstream of a discharge electrode 8,
with the nozzles oriented such as to spray liquid on the
.' ~
.
' ' .. ,:
~34~8~ C-17-21-3203
~ 14~
surfaces of the grounded electrodes facing that discharge
electrode.
Liquid and particulates draining down the ~rounded
electrodes is carried away from the bottom thereof by
conventional means, not shown but graphically represented
by line 30. If recirculation of this used liquid is
desired,it may be treated separa~ely, particularly if this
liquid is not the same as the liquid used in the fiber bed.
If both liquids are the same, however, the used liquid
rom the grounded electrodes may also be treated in
liquid treatment means 26. In various embodiments, only
fresh liquid may be used for irrigating the grounded
electrodes with recirculation only to the fiber bed
irrigation system, or treated liquid can be used for
irrigating the grounded electrodes.
In the preferred embodiment shown in Figures 2 and
2A~, the electrostatic field means uses as discharge
electrodes a plurality of rods 8A, each rod having a plural-
ity of needles 8B projecting therefrom parallel -to the
direction of gas flow in both the upstream and the down-
stream directions. ~As best can be seen in end view
Figure 2 , the spacing between needles 8B projecting
upstream from rod 8A (solid circles with dot in center) is
substantially equidistant. Similarly the spacing between
needles 8B projecting downstream from rod 8A (dotted
circles) is also substantially equidistant, but the down-
stream series of needles is staggered from the upstream
needl~s about half way therebetween. In this way the
corona between each needle tip and the grounded electrode
as represented by the light parabolic lines in each of
Figures 2 and 2A overlap to insure that all particulates
will be subjected to the electrostatic field.
In another preferred embodiment where grounded
electrodes 10 are used as particulate collectors they are
flat or slightly conve~ plates at least 12 centimeters (or
about 5 inchesl wide in the direction of ~as flow, e.g.,
from 12 to 25 centimeters (or about 5 to 10 inches) wide.
These wide grounded electrodes provide increased residence
time of particulates in the electrostatic field which aids in
B
, ` . .
.- ~ .. ; . .. .
. ~
.. ~.. . . .
.
C-17-21-3203
-20~
their capture on the grounded electrodes.
Though these drawings show onl~ one bank o~
discharge electrodes and ~rounded electrodes in ~he
electrostatic field section of this apparatus, it will be
obvious to those skilled in the art that two or more such
banks can be provided in series in the direction of gas
flow depending upon the needs of a given application.
Figure 3 represents in one drawing both one
embodiment of overhead irrigating means 14 and one way to
lO distribute a high proporation of the irrigating liquid into
the upstream portion of the fiber bed. Liquid manifold 15
is provided with a plurality of liquid discharge tubes or
outlets 15A. By appropriate positioning of -tubes or
outlets 15A along the length of manifold 15 the liquid can be
distributed in varying quantities, as desired, along the depth
of the fiber bed. The liquid discharges from tubes or out-
lets 15A over a perforated plate 32 and then flows through
the perforations therein into fiber bed 12. A series
of baffles 34 are also advantageously provided to
20 confine the liquid into a series of compartments,
here shown as 3-compartments a, b and c, as well as to
prevent gas from by-passing the fiber bed. As shown,
three tubes or outlets 15A project into compartment a, two
i~to compartment b, and one into compartment c. Allowing
for pressure drop along manifold 15, this arrange~ent will
discharge at least 50% of the liquid into compartment a
which serves about the upstream 1/3 of the depth of the
fiber bed. After allowing for viscous gas phase drag on
the liquid in the fiber bed, compartment c may be operated
30 dry, i.e., no liquid flow therein, particularly at high gas
bed velocities.
Figure4 represents another embodiment in which
fiber bed 12 is inclined at an angle such that irrigating
liquid draining down through the fiber bed under the forces
of both gravit~ and viscous gas phase drag will flow
downward through the fiber bed as shown by the arrows sub-
stantially along the planes parallel to each face of the
fiber bed. This counteracts the effect of viscous gas
phase drag which otherwise would carry individual portions
L~
~3~ c 17-21-3203
~ 21~
of the liquid deeper into the fiber bed and at very high
bed velocities off the downstream surface of the fiber bed
as re-entrainment. The proper an~le of inclination of the
fiber bed can readily be calculated by one skilled in the
art using vector analysis of the ~as phase drag force and
gravity force on the liquid at design gas bed velocity
and liquid flow rate. This embodiment allows use of
shallower fiber beds in terms of depth in the direction of
gas flow, and/or higher bed velocities, e.g. 12 to 15 feet
lO per second (3.6 to 4.6 meters per second) or more, with
less re-entrainment of liquid from the downstream surface of
the fiber bed.
WORKING EXAMPLES
The following table is illustrative of this invention
using apparatus as described in Figure l/lA and 2/2~. In
each test run the fiber bed is a vertical 2 or 4 inch
(i.e., 5 or lO.l centimeters) deep bed of jackstraw
type chemically resistant glass fibers of about 200
microns average fiber diameter with a packing density
20 of about 7 pounds per cubic foot (0.11 grams per cubic
centimeter) which calculates to provide a bed voidage
of about 95.6~. In each instance, the height and width
of the fiber bed are each a nominal 12 inches (i.e.,
30.5 centimeters). The fibers are partially oriented
within the fiber bed such that residual saturation in
such vertical position is about 0.4 grams of water per
gram of fiber and its residual saturation, when rotated
90 such that its downstream surface (in the direction
of gas flow) is in the bottom position is about 1.55
30 grams of water per gram of fiber.
A series of test runs are reported using fly
ash at various loadings in air. In each test runs
1 through 16 only the fiber bed is used as a collec-
tor, with water as the irrigating liquid from a
distributor above the fiber bed~ and no irrigation
of the grounded electrode. ~or comparative purposes,
some of these test runs are reported with the electro-
,~
' ~
C-17-21-3203
~ 2~
static field turned 4ff as indicated by "None" in the
caro~a po~er column.
ln test run 17 both the fiber bed and the ~rounded
electrodes are used as collectors, with water irriga-
tion of both.
23
__ _ ~ ~ v v .r 1-- ~ v CD ___ J~ ~ _ . _ .
a ~n(n ~n .n ~n n ___ ~n ID ~n ~n ~n <n Q ~n _ _ .
a o cn a~ ~n ~ ~D ~n 0 <n i~ ~n ~ ~ n ~D I _
O CD ~ ~ ~ .~ _1 ~ ~ 0 .~ ~ '~ ~r . r~
O _ C~ ~D ~n O <D 0 `~D O <n o 0 c~ c~ CD lo ~n____,_
. ~ ~ Q_ __ __ __ _
~: O ~D ~D CD CD O ~ I` V _~ ~ ~ n , , ~
U '~ O ~ ~D CD Cl~ Dl ID ~n c~ <n ~D ~D ~ ~D ~n ~n ~n ~n
7~ 0 _ _ _ _~
;l ~ i~ ~ O~r~ ~ CD N V~ _ CD D O ~ CD ~ CD
-Uu'O - ~ -- _ I _
_~ v. c~ Oc~ n ~ c-- .~_~ c~ ~0 _ ~n c;~ 0 m ,~ r~
~ a ¦ CC~ ~CDCD ~` ~ ~ 0 ~_ 0 V 1~ cn o cn CD
- ~~I ll l l ~CD ~ _~ .
g.
r~ ~D ~a CD L_ _ ~0, '~ ~ ~ r ~ CD cac CD O
u c ~ r~ c r-- v u~ o o o o o o o o o o
rr~ ~
u~,
a _ ~ o n o o r~ r~ r r~ v r N Or~ o
_
a ~ ~ z N W ~ V~ 3 v z ' ~ ~D ~ ~ ~ CO
r~ r~.-
. ~ .
~ q _~ ~ CD r~ r~ r~ N O O r~ r~ _ ~ c ~ u O __~.
r~ I c~ ~ ~ ~ <~ # ~DCD r~ O r
a ~ ~ r~ o ~ ~ c~r~ r~ ~ ~a cD ,,, ~_ ro r ~
~Q : ' _ ~ __ . ' .
r ~ v ," ~ ~ ~ ~n ~ ~ v~ c~ ~ ~ ~ ~ o
c.. ~ __ ~ ,
~ O O O O ~ r1u~ ~ r~ rc o~ ~o c ~ ~ co;o o
~- a <7 r r~ ~ ~ r~#1 #1 ~ -~ .~ C~ C'~
~ _ __ .__
C D ~ N r~ V Q 1~ O ¦ ~ O _~ N _ ~ ____
.
.
. ' ' ' . .
C-17-21-3203
-24~
In reviewing the data presented in the table, it
should be noted that the fiber bed used in test runs 1
through 10 and 13 through 16 is only 5 centimeters (i.e.,
2 inchesl deep in the direction of gas flow and yet even
such a shallow fiber bed provides signiicant improvement
in collection efficiency, particularly of submicron
particulates with the practice of this invention.
Test runs 1 through 12 use two stages of wire to
plate electrostatic field means (as shown in Figures 1 and
10 1~ in series. Runs 13 through 16 use three stages of such
wire to plate electrostatic field means in series, providing
~onger residence time of particulates in the electrostatic
field, to give over 95~ average collection efficiency on
submicron particulates, even at the high dust loadings
shown.
The 10.1 centimeter ~i.e., 4 inch) deep fiber bed
of runs 11 and 12 (which is a more commercially useful
depth to use) gives over 96% average collection efficiency
on submicron particulates, even though only 2 stages of
20 wire to plate electrostatic field means are used in series.
Test run 17 illustrates the use of a single stage
of the needle to plate electrostatic field means of Figures
2 and 2A with water irrigation of the grounded electrodes
(i.e., plates). The results shown are lower than desired
but are consistent with the objects of this invention in
view of the fact that the desired corona power for the
needle to plate electrostatic means used is about 500 to
600 watts while in this test run only 118 watts of corona
power was obtained. Collection efficiencies of at least
30 95% on submicron particulates and higher for larger particu-
lates are extrapolatable from this data at 500 to 600
watts corona power.
The foregoing description of the several embodiments
of this invention is not intended as limiting of the
invention. As will be apparent to those skilled in the art,
the inventive concept set fo~th herein can find many appli-
cations in the art of separation of particulates from gases
and many variations on and modifications to the embodiments
. .
~3~ C-17-21-3203
~ 25~
described aboYe ~ay ~e made without departln~ fro~ the
spirit and scope o~ this invention.
: . . ~ .
: ,
,
.
":
.
