Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OE THE INVENTION
1. Field of the Invention ':~
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This invention relates to a method and apparatus
for treating a gas and more particularly to a method and , ~ ,
apparatus for obtaining a unique state of high intensity ' -;
gas liquid sp~ay contact. : -
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2. Description of the Prior Axt .~ ' -
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Liquid sp~ay gas contacting devices, in,~hich the
gas as a continuous fluid phase is contacted with a dispersed ~ -
liquid phase or spray formed by a nozzle (pre-formed), have
long been used for gas washing for dust removal or absorption :;
in situations where low gas pressure drop or energy loss
is required. However, a concommitant feature of pre-formed ~- -
spray, low energy loss gas scrubbers'1s thcir relatively poor
contacting efficien~y relative to other:;liquid gas contacting ..
devioes such A3 packed towers. The deficienoies of spray
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contactors result from the short contact time of the liquid
droplets with the gas stream. The liquid spray residence
or dwell time in the flowing gas i9 limited to the short
"time of flight" between release from the spray nozzles and
interception by the vessel wall surfaces. Increases in
vessel size to compensate for this short contact time
deficiency of the spray contactor result in uneconomic
vessel cost~ without significant efficiency improvement.
For example, P. Kalika, "Chemical Engineering", 76, No. 16,
Pages 133-138, July 28, 1969, reports an evaporation efficaency
of only 60% for a large commercial spray contactor for cooling
hot incineration off-gases.
Various methods for improving spray/gas contacting
have been proposed or commercially used, with either no
; notable success or with the introduction of high energy losses.
Centrifugal or cyclone spray contactors wherein the gas is
introduced tangentially in a cylindrical vessel and in which
the spray is introduced axially, as in the Pease-Anthony
scrubber, ser~e to increase contact time by securing a curved
path for liquid droplet travel, thùs increasing residence time
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of the dispersed liquid in the gas. However, because of the
necessarily high centrifugal gas action in such cyclone
contactors, the centrifugal forces imposed on the drops
serves to shorten drop residence times theoretically obtain-
,1 able if the drops traveled with the gas at gas velocities.
Consequently, such centrifugal gas/liquid contacting devices
suffer from low a~erage spatial concentrations of liquid
droplets, with the only region of high drop concentration
being that near the spray injection nozzle, a characteristic
.of all conventional spray systems. The spatial concentration
of dropl.ets in a spray scrubber is a highly critical property
for dust removal applications, because efficient removal of
¦ the finer dust particles below 50-100 microns, requires
opportunity for dust/particle collisions, which, in turn, is
a function of both dust particle spatial volumetric concentra- -
tion and similarly sized droplet volumetric concentration.
Obviously, a high spatial concentration of smaller drops is
also desirable for heat and mass transfex applications because
of the increased active interphase contact area. This type
of desirable high concentration small droplet dispersion has
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hitherto been obtainable only in high energy 10s9 ga~ atomi-
zation devices such as Venturi Scrubber~, wherein gas
turbulence and impact is used to atomize liquid ~pray by
conducting the gas at very high velocities through a con-
verging nozzle or Venturi Throat and introducing liquid into
it at the throat region. Despite the expenditure of much
development effort, Ihe Venturi type of ~crubber remains an
example of the uncontrolled and wasteful use of turbulent
energy flow to secure efficient gas/liquid or du~t/liquid
contacting, with commercial unit gas energy losses varying
from 30 to 60 inches water column ~W.C.).
Prior art devices involving attempts at achieving
optimum spray/gas contact at low energy levels include
United States Patents 3,527,026 and 3,594,980.
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SUMMARY OF THE INVENTION
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In accordance with the present invention a method
is provided for treat~ng a gas that include~ introducing a
stream of untreated gas into a vessel in a first direction
and forming at least one toroidal vortex of the untreated
gas in the vessel. ~he toroidal vortex is substantially
perpendicular to the direction of flow of the stream of
untreated gas. A liquid in droplet form is introduced in~o
the vortex and the gas in the vortex is treated with the
liquid. The treated gas i8 separated from the liquid and
withdrawn from the vessel.
Further, in accordance with the present invention,
apparatus is provided for treating the gas which includes a
vessel having a sidewall portion, an open top portion and a
bottom;wa~li portion with a gas inlet opening therethrough. A
baffle plate is positioned above the inlet opening i~ spaced
relation thereto to form a clearance gap therebetween. The
baffle plate has a dimension less than the internal dimensions
of the ve~sel, and a peripheral gap is formed between the-
baffle plate and the sidewall portion. Spray nozzles are
positioned above the baffle plate and arranged to introduce
liquid in droplet form into a toroidal vortex of gas formed
above the baffle plate.
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BRIEF DESCRIPTION OF THE DRAWINGS
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Figure 1 i8 a view in ~ection and in elevation,
illustrating the improved gas cleaning apparatus.
Figure 2 is a schematic view in elevation and in
3ection, illustrating an array of separate compartmented
multiple baffle~ that may be utilized as single or multi-
staging contact units.
DESCRIPTIOt~ OF T~E PREFERRED EMBODIMENT -
Referring to the drawings, the ga~ treating or
cleaning apparatus is generally designated by the numeral 10 -:
and includes a cylindrical vessel 12 having a bottom wall 14
with an axial opening 16 therein. A cylindrical gas entry
duct 18 extends through the opening 16 upwardly into the
internal portion 20 of the vessel 12. The entry duct 18 has
a flow area that is le~s than half the flow area of the main
vessel 12. Alternatively, the gas entry duct could have a
configuration of a converging section tapering from the
cro~-sectional flow area of the main vessel 12 to a flow
area of less than approxi~ately one-half the area of the main
ve~sel 12. The annular area formed between the cylindrical
wall of vessel 12 and the entry duct 18 may be utilized as
a reservo~r 22 for the drainage liquor prior to removal of
the drainage liquar through-th~ outlet conduit 24.
A baffle plate 26, preferably of a conical config-
uration, is positioned above the outlet ena portion 28 of gas
entry conduit 18 within the vessel 12. The baffle plate 26
is spaced a preselected distance above the inlet duct
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outlet end 28 to form a clearance gap 30 therebetween to
deflect the gas flowing upwardly through the inlet duct 18
into the internal portion 20 of vessel 12. The baffle plate
26 is positioned axially within the vessel 12 and has a
transverse dimension less than the di~meter of the cylindrical
vessel 12 but greater than the gas outlet end portion 28 and
forms a peripheral and symmetrical gap 32 between the conical
baffle 26 and the wall of vessel 12. ~ :
A spray nozzle 34 is positioned slightly bëlow the -
upper end 28 of inlet duct 18 and is suitably connected by
means of a conduit 36 to a source of spray liquid such as
water or the like. The spray nozzle 34 is preferably a
nozzle that ejects a preformed spray in a configuration having
an angle of about 180. Second spray nozzles 38 are posi-
tioned within the vessel 12 at a location of at least one
vortex and preferably two or more vortices about the baffle . .,~.
plate 26 and are connected by means of conduit 40 to preferably :
the same source of liquid. The upper open end 42 of vessel 12
has a mist el minator pad 44 therein to capture wetted and
agglomerated particulate matter before the wetted particulate
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matter leaves the vessel 12. In lieu of the mist eliminator
pad 44 a mist eliminator bed comprised of packing of the
type disclosed in United States Patent 3,410,057 may be
positioned in the vessel 12 and provided with ~pray no~zles
to keep the mist eli~inator bed rinsed free of accumulated
matter. Alternatively, other types of mist eliminators may
also be employed.
The dirty or untreated gas enters the vessel 12
through the entry duct 18, as illustrated by the arrows in
the drawings and is w~tted by the preformed spray from nozzle- . .
34. The dirty gas impinges on the underside of baffle 26
and then flows laterally through the clearance gap 30 between
the upper end portion 28 of inlet duct 18 and the underside
of baffle 26. A portion of the dirty,untreated gas then
flows upwardly through the peripheral gap 32 between the
peripheral edge of the baffle 26 and the wall of vessel
12. When the gas velocities through the gas entry duct
exit outlet opening 28, clearance gap 30 and peripheral
gap 32 are in the range of 250 feet per minute to 2,000
feet per minute and preferably in the range of 500 to 1,500
feet per minute two ~etn of ~table gas vortice~ are formed.
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A primary vortex 46 is formed downstream and behind the
baffle plate 26 and a secondary vortex 48 is formed at a
location below the outlet end 28 of entry duct 18. The
secondary vortex surrounds the outer wall of entry duct
18. Both the primary vortex 46 and the secondary vortex
48 are toroidal vortices and in the range of the gas veloci-
ties specified are stable or standing vortices rotating in
opposite directions as illustrated in the drawings. The
gas continually enters and leaves the primary vortex 46 and
the secondary vortex 48 and flows upwardly through the vessel
12 and through the mist eliminator pad 44 and is exhausted
to the atmosphére.
To secure the novel dispersed liquid vortex
entrapment and retention, it is necessary to inject spray
liquid into the vortices 46 and 48 through the spray nozzles
34 and 38. The spray is preferably injected into the vortices
either transverse to or co-current with the`direction of gas
flow at that point of injection so as not to disrupt the
stability of the vortex. The droplets of liquid that are
small enough to lack sufficient momentum (or escape veloeities)
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to leave the vortex are trapped in the vortex and circulate ;~
along with the gas in the closed loop toroidal circulation
pattern. However, because the gas flow within the vortex is
turbulent and because there are a range of droplet sizes and
hence slip velocities, droplet collision and coalescence
mechanisms are continuously operative to cause droplet growth
to sizes having sufficient momentum to escape the gas vortex.
Because of this continuous droplet escape from the
vortex, the high spatial liquid holdup and concentration of this
invention is not achieved unless the rate of droplet injection
into the vortex equals or exceeds the escape rate. The point at
which the rate of droplet spray injectionequals the escape rate
is termed the critical minimum liquid rate or nozzle pressure
and this critical spray rate will be a function of the gas
velocity in the vortex, the gas density, the diameter of the
vortex,--and the size distri~ution of the droplets injected by
the preformed spray from spray nozzles 34 and 38 which, in
turn, is a function of the liquid pressure at the nozzle used.
Higher spray nozzle pressures are generally advantageous
because they result Ln smaller droplet size~ whioh are more
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susceptible to entrapment in the gas vortices and, for a
given spray nozzle, higher nozzle pressures yield higher
liquid spray rates so that the critical minimum rate also
corresponds to a critical nozzle pressure. Because of the
variation in spray nozzle types and sizes and the fact that
more than one spray nozzle may be advantageously used for
spray injection, it is not possible to state a general range
of critical minimum liquid spray rate or nozzle pressure and
it is necessary to determine the spray rate and nozzle
pressures required by preliminary testing in a laboratory or
pilot unit. It is advantageous to conduct such tests in trans-
parent equipment or equipment having a sight port for visual
inspections because the visual appearance of the primary
vortex undergoes an abrupt transition when the critical
minimum iiquid spray-rate and/or pressure is reached, becoming
dense and opaque because of the accumulation of the trapped -
liquid droplets. Observation of the primary vortex in the
high liquid holdup zone of operation shows that the standing
vortex slowly precesses in a clockwise direction, when viewed
from above, releasing streams of coalesced large drops at
the region where the standing vortex l~fts slightly off the
baffle.
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It ha~ been determined that an annular vena contrac~a
is formed downstream of the baffle 26 and it is preferable ~ -
not to inject the spray liquid into thic annular vena contracta
for optimal liquid retention. A spray directed toward the
general area of this low pressure vena contracta is directed
; into this high velocity wall region and then directed upwardly
along or onto the walls. If the wall extends hpward beyond
the baffle at least one or preferably two vortex diameters
a portion of this aspired liguid recycles in the vortex trail.
However, this recycle consists primarily of relatively large
droplets formed by agglomeration in the vena contracta upon
initial aspiration of the fine spray in this zone. As
previously stated, liquid in the form of fine droplets i8
preferred in the vortex holdup pattern and cannot occur when
the spray injection is into the vena contracta zone. It ha~
been found when the spray is injected into a region other
than the vena contracta preferably from one to two vortex
diameters above the baffle a high density liquid holdup in
the vortex zone is obtained. Where the spray is injected into
the vortex at a location far enough away from the baffle to
avoid aspiration of most of the spray into the vena contracta
of the annular Venturi throat formed upstream of the baffle,
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it is believed the only way that liquid can l~ave the circu-
; lating loop is to run down the baffle wall. Liquid running
down the baffle wall tends to get picked up and re-entrained
as spray in the vena contracta section adjacent the wall.
Liquid holdup as recirculating loop spray increases until
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it reaches equilibrium between spray input and liquid escaping
the vena contracta down the wall. Where the spray nozzles 38
J are spaced two or more diameters of the primary vortex above
the baffle plate 26, two or more vortices are formed in
overlying relation and increase the efficiency of the gas
cleaning appara~ s.
;~ Once the desired high density liquid holdup zones
have been created in the standing gas vortices, the dirty gas
~ entering through the inlet duct 18 is deflected by theb ffle
', 26 and a portion of the dirty gas enters the reflected or
,~ ~econdary vortex 48 while the major por~ion flows through
the peripheral gap 32 and enters the primary vortex 46. If
the dirty gas contains particulates, the smaller particulate
;~ sizes having insufficient momentum to escape from the vortex
flow in either the primary or reflected vortices 46 and 48,
respectively, and are trapped by the closed circulation vortex
flow. Thus, the dirty gas preferentially deposits its smaller
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size particulate contaminants in the vortices while the
larger sized particulates are thrown out to the wall of
vessel 12 or the surface of the baffle 26 by the tortuous
path of the gas flow around the baffle and the action of the
spinning vortices. In the vortices the smaller particulates
which lack the momentum to escape and which also constitute
the normally difficultly removable fraction for conventional
spray scrubbers are trapped in a long residence time zone and
exposed to a high concentration density of similarly trapped
liquid droplets causing them to collide with the liquid
droplets. The liquid droplets containing the wetted particu-
lates collide with other liquid droplets under going agglomera-
tion and growth in size until sufficient momentum is achieved
for the larger drops to leave the circulation loop of the
vortex and spin out to the vessel walls where the liquid and
wetted particulate matter drains down into the reservoir 22
and is withdrawn from the vessel 12 through drain pipe 24. ;~
In cases of gas absorption or heat transfer,
particularly where long liquid residence times are required
to achieve close approach to either mass transfer or heat
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transfer equilibria, the retention of the trapped liquid spray
in the vortices yields high-efficiency contact. A significant
contributing factor to high transfer efficiencies, particularly
in cases where the major resistance to transfer lies in the
liquid phase, is the dynamic droplet collisibn and growth
processes occurring in the high holdup vortex zones once the
critical liquid minimum rate or pressure is exceeded. Because
external stagnant liquid films develop on small drops and
because such films tend to go to equilibrium saturation with
a soluble component of a gas stream or with external temper-
ature, it is well known that most of the heat or ma~s transfer
in a conventional spray scrubber takes place in the region of
the spray nozzle where fresh external surface is generated,
as shown by Pigford and Pyle, "Industrial and ~ngineering
Chemistry", Volume 43, Pages 1649-1662, ~1951). To improve
mass or heat transfer, it is necessary that this saturated `
external liquid film on the liquid drop be mixed with the
internal unsaturated core of the liquid drop. In the present
invention, this is accomplished by the high intensity collision
mechanism occuring in the vortex zones of flow, thus continually
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generating fresh, mixed, unsaturated or non-equilibrium
droplet surface, thus yielding unusually high heat and mass
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transfer rates.
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Due to the collision process, the trapped spray
zone of the vortices is self-equilibrating. If too high a
!~ droplet injection rate is imposed for the existing equili-
brium rate of removal, then the concentration of droplets
per unit vortex volume increases and the rate of droplet
~ collision, which is dependent on the volumetric concentration
F;'~ of droplets, al80 increases, yielding a greater number of
~¦ 10 drops larger than the escape size, and a new drop concentra-
:~ tion equilibrium level is achieved. Such higher dynamic
equilibrium droplet input and escape levels may be advantageous
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~i for certain heat or mass transfer applic~tions, depending to
the extent that such processes are rate controlled in con-
ventional spray contactors by the transfer resistance of the
~¦ stagnant external film on the liquid droplets. The present
`l invention allows operation under conditions where the external
' drop film resistance can be minimized purely by variation
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`~ in the liquid spray input rate in combination with the appro-
priate design for achieving stable gas flow vortices.
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The versatility of the above described gas treating
apparatus is illustrated in Figure 2 where a plurality of the
previously described gas cleaning devices are po~itioned in
what can be termed a tray desiqn with a plurality of units
formed in a suitable manner to enclose the entire tray. Where
desired, the units and/or trays may be positioned in overlying
re-lation with each other to provide multi-staging. The
previously described gas cleaning apparatus is of cylindrical
configuration. ~t should be understood that the gas cleaning
units may have any other suitable configuration as, for
example, a rectangular configuration.
Referring to Figure 2, the vessel generally
designated hy the numeral 50 has a plurality of gas cleaning
units 52, 54, 56, 58, 60,and 62. The vessel 50 has an
outer wall 64 that forms one of the walls for the units 52
and 62. A common wall 66 separates the units 52 and 54.
Further, if there are a series of units behind the unit 52 a
common back wall would serve for adjacent units. The gas to
be cleaned is supplied through conduits 68 to all of the
units 52-62 and the gas to be cleaned is introduced into the
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respective units 52-62 and subjected to a cleaning action as
previously described. Vortices 70 are formed in the units
52-62 and spray nozzles 72 supply liquid in fine droplet form
as previously described to the vortices 70. A common header
74 supplies liquid for the respective spray nozzles 72. ~ -
Positioned above the units 52-62 is a mist eliminator 76
similar to the mist eliminator 44 previously described. Spray
nozzles 78 and 80 are provided to continuously wash and clean
the mist eliminator 76. It will be apparent from the above -
that the previously described gas cleaning apparatus may be
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utilized in a number of gas cleaning operations.
Numeraus modifications within the scope of the
~ present invention will occur to those skilled in the art.
i For example, it may be preferable in some cases to achieve
~, longer gas contact times or more efficiency by multi-staging
contact units of the present invention and it is obvious
~ that the contact unit of the present invention lends itself
iJ readily to multi-staging in series, whether vertically within
one vessel, or within several separate vessels. Additionally,
~0 rectangular vessels may be employed, utilizing one baffle, or
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,( an array of separate compartmented multiple baffles, to handle
a range of gas flows. Other variations and modifications will
also readily occur to those skilled in the art. ~he ranges
of the gas velocities previously mentioned constitute preferred
embodiments of present invention when applied to gases such
;, as air or steam and the preisent invention may be practiced
outside of these ranges in oita~le instances.
, An example of industrial app}ica~on of the present !'~
. invention will now ~e described.
~, , ~ 10 EXAMP~hE
'~ A commercial food proce~sing plan~ was intermittently
~; ~ ~ emitting vinegar fume- and spice odors from four 750 gallon
kettles in whlch condiments were prepared. The part of the`
process responsible for the emis~ion and air pollution problem
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, ~ wa-~a~ rapid boil of the kettle contents for a period of from ;~ ~,
S~to 15 m~inutes during which time approximately~300 pounds
per minute of steam at 220F. were evolved from the kettle
content8 and vented directly to the air. This steam contained
igntficant amounts of vlnegar and~spice oils and the area
~around the proce-- building and plant not only had an offensive
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acrid odor, but under certain wind-free climatic conditions,
the concentration of vinegar was high enough to make the air
unbreathable. A test unit comprising a 42 inch diameter
main vessel was initially installed on one of the kettle
stacks, said vessel containing a gas inlet duct tapering to
a 20 inch diameter outlet, a 26 inch diameter conical baffle
(15 flate cone) spaced 10 inches from the end of the gas
entry duct, and spray injection nozzles located so that four
of the spray nozzles were located directly on the baffle,
directed upward, Spray nozzles used were Bete Fog Nozzle
Company nozzles, Type TF14FCN nozzles, described in United
States Patent 2,612,407, having a 90 full-cone spray
angle and a 7/32 inch orifice diameter. Tests with this
unit showed that with this nozzle size, injection spray
rates of 8.1 gallons of water at 60F.per minute per
nozzle at a nozzle pressure of 40 pounds per square inch
gage gave the high liquid holdup vortex zone by visual
inspection, and an accompanying efficiency of 100%
removal of the acetic acid and spice odors. Below this set
of operating conditions, at 30 psig at the spray nozzles, -
corresponding to an input rate of 7.1 gallons of water per
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minute per nozzle, the acid odor reappeared, and the high ; ;~
intensity liquid holdup zone appeared to visually empty its ~-
liquid content to a low holdup level. Additionally, when
the higher nozzle pressure of 40 psig was used wlth the
TF14FCN nozzles, the liquid drainage from the scrubber was
found to be at its boiling point, 212F., indicating
extremely high thermal contact efficiency. Further, the
fact that the scrubber was able to quantitatively remove acetic
acid from 220F. steam with an effluent liquor temperature of
212F. indicated a highly unusual and unique mass transfer
capability of the contactor.
Replacement of the TF14FCN nozzles with size
TF20FCN nozzles, having a 5/16 inch orifice, conferred .
d~fferent characteristics on the scrubber. With the large ;
spray nozzles, which require a higher nozzle pressure to
achieve the same degree of spray atomization than do the
~; smaller nozzles, operation of the scrubber to nozzle pressures
J~; ~ of 50 psig failed to produce the high liquid holdup observed ~ -
in the vortex zones for the smaller nozzles, and the odor and
acid content of the effluent steam from the scrubber could
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not be removed. When the smaller nozzles, TF14FCN were
substituted, high holdup zones were again visually observed,
and analyses of the draina~e liquor showed ~hat the scrubber
had returned to 100% efficiency, as was also evident by the
acid odor free steam emission.
Measurement of pressure drop across the scrubber
showed a gas pressure loss of less than 2.0 inch of water
under ~ull flow conditions, and removal of the demister pad
ànd secondary measurement showed pressure losses of less than
1.0 inch of water across the baffle/spray section alone.
According to the provisions of the patent statutes,
I have explained the principle, preferred construction and
mode of operation of my invention and have illustrated and
described what I now consider to represent its best embodi-
~, ment. However, it should be understood that, within the
scope of the appended claims, the invention may be practiced
otherwise than as specifically illustrated and described.
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