Note: Descriptions are shown in the official language in which they were submitted.
v
BACKGROUND OF T~IE INVE:NTION
1. ~'ield of the Invcntion
This invention relates to a dry method and apparatus
for treating an effluent gas stream in order to facilitate the
removal therefrom by conventional means of contaminating
particulates particularly those in the submicron range.
2. Des ription of the Prior Art
In many countries throughout the world, emission
standards have been or are being established to control the
articulate content of effluent gases being exhausted to the
atmosphere. Although these standards vary widely, most limit
the particulate content of effluent gases to levels below
0.1 gr./sdcf. In many industrial applications, such as for
example fiberglass furnaces, municipal incinerators, etc.,
such rigid standards can only be met by capturing and separating
a large ~ercentage of the submicron particulates suspended
in the effluent gas stream.
Conventional dry cleaning devices lack the ability
to effectively and reliably se~arate submicron contaminating
particulate suspended in effluent gas streams. For example,
in the case of baghouses where fabric filter bags are employed,
experience has indicated that the submicron particulates have
a tendency to rapidly plug or "mask" the fabric interstices,
thus requiring frequent disruptive bag shaking operations.
Where cyclone dust filters are employed, the submicron
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particulates have been found to lack the necessary mass required
for efficient centrifugal separation.
Wet venturi scrubbers have also been employed for the
purpose of separating contaminating particulates from effluent
gas streams. Basically, a wet venturi scrubber consists of a
constriction in the conduit carrying the contaminated effluent
gas stream. The effluent gas stream is accelerated through the
venturi constriction, and a liquid (usually water) is injected
into the gas stream at the venturi throat. The high gas
lO velocity atomizes the liquid and the relative velocities between
the contaminating particulates and the liquid droplets result
in a combination of one with the other through inertial
impaction. The liquid droplets and their captured contaminating
particulates are then separated from the effluent gas. While
15 this technique can lead to higher collection efflciencies, this
advantage is offset to a considerable extent by other associatéd
problems.
For example, it is known that the efficiency of the
inertial impaction technique can be improved by reducing the
20 size of the target liquid droplets. This however requires
igher gas velocities with accompanying pressure drops across
the venturi of 30"-60" w.g. A pressure drop of 30" w.g. results
¦in an excessively high energy usage of 240 kWh per million cu. ft.
¦of gas cleaned. Attempts at reducing the pressure drop across
25 the venturi have not been successful, primarily because a high
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gas velocity is essential at the venturi throat in order to
achieve optimum atomization of the injected liquid and still
have a remaining differential velocity between the liquid
droplets and the contaminating particulates which is sufficient
to produce the desired inertial impaction. Some thought has
been given to injecting a ~re-atomized liquid spray into the
gas stream in order to accommodate reduced gas velocities
through and reduced pressure dro~s across the venturi, but any
advantage gained in this regard has been found to be offset
by the power required ~o pre-atomize the liquid.
Another problem with wet venturi scrubbers is that
the atomized liquid droplets combine with acid components of
the effluent gas stream to produce a high corrosive medium.
This in turn makes it necessary to employ ducts and associated
downstream equipment constructed of expensive exotic corrosion
resistant materials. Even when this is done, however, corrosion
related maintenance problems are encountered. Moreover, the
resulting acid solutions must be neutralized, and even after
this is done, disposal problems are encountered.
Other known gas cleaning arrangements have involved
- the injection of solid material into the effluent gas stream.
An example of one such arrangement is shown in U.S. Patent No.
2,875,844. Such arranyements have resulted in little or no
capture of submicron particulates because the solids have been
dumped into the effluent gas stream at the outside diameter of
the conveying duct as a dense agglomerate. By the time
dispersion occurs, a condition which is essential for efficient
capture, the solids have attained approximately the same velocity
as that of the effluent gas and the contaminating particulates
suspended therein. Without an adequate relative velocity
between the contaminating particulates and the dispersed
target particulates, effective particulate capture through
inertion impaction is an impossibility.
According to U.S. patents 3,969,482 and 3,995,005,
solid particulate material for sorbing and/or reacting with acid
gases may be dispersed in a secondary air stream and radially
injected through one or more points along an effluent gas
carrying conduit. However, in the absence of adequate relative
velocity between the gas streams together with means for
inltially distributing the secondary stream into the effluent
stream, little or no capture of contaminating particulates
occurs, es~ecially at conduit ~ositions radially removed from
the injection yoints. sy the time the added material is
dispersed throughout the effluent gas, the relative velocity of
the streams approaches zero.
While electrostatic preciyitators have met with
B some success, their operation has been plagued by~ ~ }~,
uildups of oils and fats where combustion practices are less
han optimum, and variations in the conductivity of the
ontaminating particulates.
SUMMARY OF THE INVENTION
It is a general objective of the presen-t invention -to
obviate or at least considerably reduce -the aforementioned
problems and disadvantages.
According to one aspect of the present invention,
there is provided in a dry method for -trea-ting a primary gas
stream flowing in a conduit -to facili-tate the removal -therefrom
of submicron contaminating particulates, wherein a secondary
gas stream with targe-t particulates dispersed therein is intro-
duced into the primary gas s-tream to promote inertial impaction
between said contaminating and target particulates, the improve-
men-t comprising: introducing said secondary gas stream counter-
currently to said primary gas stream through an orifice circum-
scribing an area within said conduit; the average particle
size of the target particulates being between about 3-50
microns; and deflecting said primary gas stream away from
said area and across said orifice while simul-taneously accel-
erating said primary gas stream to achieve a relative velocity
between said primary and secondary gas streams at said orifice
of about 20-200 feet per second.
The target particulates are large enough so that
after they combine with contaminating particulates, the
resulting particle size and mass will ~e such as to permit
efficient capture and separation by conventional gas cleaning
equipment.
According to another aspect of the present invention,
there is provided an apparatus for treating a primary gas
stream flowing in a conduit to facilitate the removal there-
from of submicron contaminating particulates, wherein a
secondary gas stream with target particulates dispersed
therein is introduced into the primary gas stream to promote
inertial impaction between said contaminating and target
particulates, the improvement comprising: distributor means
for introducing said secondary gas stream countercurrently
to said primary gas stream through an orifice circumscribiny
an area within said conduit; and deflecting means for deflect-
ing said primary gas stream away from said area and across
said orifice while simultaneously accelerating said primary
gas stream.
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The invention will be more clearly understood from
the following description and the accompanying drawing~, both
of which refer to preferred but nonlimiting embodiments of
the inventi.on.
S BRIEF DESCRIPTION OF 'rHE DRAWINGS
Figure l is a schematic illustration of an industrial
installation embodying the method and apparatus of the present
invention;
Figure 2 is an enlarged side view, partially broken
- 10 away, of the preferred embodiment of the injection apparatus
of the present invention;
Figure 3 is an end view of the apparatus sl1own in
Figure 2;
Figure 4 is a partially exploded view of the distributor .
means of the embodiment shown in :Figures 2 and 3;
Figure 5 is a side view, with portions broken away,
of an alternate e~bodiment of the invention;
Figure 6 is a sectional view taken along the line 6-6
f Figure 5 and rotated 90;
Figure 7 is a side view, again with portions broken
way, of a third embodiment of the invention;
Figure 8 is an end view of the embodiment of Figure 7;
nd,
E~igure 9 is a sectional view of still ano-ther embodiment
f the present invention.
f~
Referring lnit:ially to Figure 1, -there is shown an
optional quench chamher 10 receiving an indus-trial effluent
gas Sl from a conven-tional source (not shown) such as a municipal
incincerator, glass furnace, or the like. The effluent gas S2
from chamber 10 is directed through first conduit 12 to conven-
tional gas cleaning apparatus depicted at 14, which may comprise
a fabric filter such as a baghouse, a cyclone separator, or -the
like. At section 22 of conduit 12, a secondary air stream A2 is
introduced through second conduit 18 and the resulting gas
mixture S3 fed to cleaning apparatus 14 and exhausted to the
atmosphere through stack 16 which may include a conventional
exhaust blower (not shown).
The effluent gas Sl has solid particulates suspended
therein, for example soot, salts, oxides or the like, a portion
of which are typically of submicron size. Such gas also frequent-
ly contains~acid gases such as CO2, SOx, HCl, HF, and the like.
Quench chamber 10 may be used to cool the gas by aqueous spray,
or to remove a portion of the acid gases as salts by alkaline
aqueous sprays as described, for e~ample, in U. S. Patents
3,969,482 and 3,995,005. The spray water is evaporated and
raises the dew point of the gas which may be of assistance
in the particulate capture hereinafter described. It is
not necessary in all cases, however, and may be omitted. The
spray additions, if used, should be limited to avoid raising the de~ point
sufficiently hic3h to provide saturation in any part of the
i~rocess, pa~ticularly in appara~us 14 where excess rnoisture
can interfere with separation. Preferably a difference of at
least about 40F or more betwcen dry and wet bulb temperature
is maintained.
The secondary gas stream A2 is ~roduced in conduit
1~ by means of blower 20 and introduced into the primary
effluent gas stream S2 at portion 22 of first conduit 12,
between flanges 24 and 26. Target particulates P are fed
through vibratory hopper 28 and feed screw 30 and injected into
stream A2 at a point within conduit 18 remote from conduit 12 to
assure dispersal prior to introduc:tion into the effluent
stream S2. Target particulates P can comprise any suitable
solid and should have an average particle size of at least
bout 3 microns, preferable 3-50 microns, more preferably 3-20
icrons, and most preferably 10-20 microns. Particulate nepheline
syenite or phonolite is preferred.
Figures 2-9 and the following description illustrate
ethods and apparatus for introducing secondary gas stream A2
into ~rimary stream S2 with a relative velocity and distribution
sufficient to capture smaller contaminating solid aerosol
particulates by inertial impact with target particulates P.
The mechanis~ of the inelastic impact captur~ is not thoroughly
understood but is believed to include mass attraction and
mechanical effects between the roughened surfaces of the
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. , impacting solids. In some cases, electrostatic forces and
h;;midity effects may be involved. Capture has been demonstrated
by air elutriation studies in which the finer captured parti-
culates were not separated.
The method and apparatus currently pre~erred are shown
, in Fiyures 2-4. A curved upper section 34 of second conduit 18
extends through the wall of section 22 of first conduit 12
and has a flange ~2 for joining to the vertical portion of
conduit 18 shown in Figure 1. The inner end of section 34 is
fixed by supports 42a, 42b and 42c and is joined to a first
right truncated cone 38 of sheet material with its larger base
directed upstream with respect to the flow path of gas Sl.
. Located concentrically within and syaced from cone 38 is a second
right cone 50 having its apex directed downstream with respect
to the flow of S2 and upstream w.ith respect to the flow of gas
A2. Cones 38 and 50 define therebetween an annular passageway
for the secondary gas stream A2 which terminates in an annular
orifice 40 through which the stream A2 is introduced and injected
countercurrently into effluent gas S2. Together they constitute .
distributor means for introducing and injecting stream A2 into
. stream S2 at a plurali.ty of positions about the axis of duct 12.
A third right cone 52 is provided with its base
joined to the base of cone 50 and with its apex directed upstream
l with respect to the flow of gas S2. Cone 52 serves both as
accelerating means for the acceleration of gas stream S2 during
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introduction of stream A2, and as deflector means for deflecting
; the flow of 52 toward the walls of conduit 12 transversely
across the orifice 40.
Cone 50 is secured to cone 38 by means of bolts 54
and is spaced therefrom by means of four spacer members 58.
As shown in Figure 3, a hinged door 46 is provided upstream of
flange 24 for providing access to the interior of conduit 12.
Also shown is a pipe stub 44 for connection to a fume hood
(not shown) over hopper 28 to prevent escape of ~arget particulate
to the atmosphere.
By the method and a~paratus shown in Figures 2-4,
the gas stream A2 and dis~ersed target partic~lates are intro-
duced into the interior of stream Slj with good distribution
and high relative velocity. Relative velocity as used herein
refers to the algebraic sum of the vectors of the flow of gas
stream S2 and A2 parallel to the axis of duct 12 during intro-
duction of A2 into S2. ~ relative velocity of about 20-200
feet per second is ~referred and from about 50-150 is more
referred. By the means shown, an efficiency of cap~ure of
ubmicron particulates equivalent to a liquid venturi having
n inlet to throat pressure drop of from 50 to 150 inches water
auge can be obtained with substantially reduced power, without
ontaminated wash li~uid, and with reduced corrosion.
While circular sections for conduits and cones are
referred as shown, other polygonal or oval sections can be
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usedO Preferably the same sections are used on all concentric
parts to maintain uniformity of flow about the longitudinal
axis of conduit section 22.
A second embodiment of the present invention is shown
in Figures 5 and 6. A venturi is provided within conduit
section 22 by means of truncated, conical member 60 fixed to
its walls and having a narrowed throat opening 62. Concentric
conical sheet metal members 64 and 66 are mounted between the
outer wall of member 60 and the inner walls of section 22 and
define between them a tapering circumferential passageway 67
which terminates in an annular orifice 68 surrounding throat
. pening 62. Secondary gas stream A2 is fed through duct 70
tangentially into passageway 67, around member 60, and outwardly
through orifice 68 where it is introduced cocurrently into the
ccelerated effluent gas stream S2. Relative velocity is
provided by feeding stream A2 at a different, preferably lower,
velocity than stream S2. Both streams are turbulent with good
istributed mixing and efficient impact capture of contaminating
articulates is obtained Duct 70 is provi~ed with a flange
71 by which it is joined to the vertical portion of conduit
18 of Figure 1 and comprises the outlet portion of that conduit
ithin the conduit 12.
Fro~ Figures 2-4 and Figures 5-6, it will be noted
that while the flow is countercurrent and cocurrent, respectively,
there are also radial components of flow as the streams S2 and
A2 merge. As used herein, -the terms countercurrent and cocurrent
include such flows where there is a substantial vector component
of motion along or against the direction of flow of the primary
effluent stream S2 parallel to the axis of conduit 12. Co-
current and countercurrent injection of the dispersed target
particles distributed interiorly of the walls of the conduit
are preferred, with countercurrent flow being most preferred
since it provides the greatest effective relative velocity.
A third embodiment is shown in Figures 7 and 8 in
which duct 82 terminates within conduit section 22 with four
nozzles 86 spaced even about the longitudinal axis of section 22.
Nozzles 86 are angled away from that axis toward the conduit
walls and divides the secondary gas stream A2 into four sub-
streams. Duct 82 has flange 84 for joining to vertical conduit
18 and constitutes the upper outlet end thereof within conduit
12. Secondary stream A2 is fed into effluent stream S2 either
cocurrently or countercurrently, but is preferred countercurrent
s shown. Conduit section 22 is provided with access door
80 and the end of duct 82 is supported within section 22 by
eans of T-shaped support 88. Since the nozzles 86 and
their associated structure significantly reduce the flow area
ithin conduit section 22, gas stream S2 will be accelerated
as the stream A2 is lntroduced therein.
While countercurrent or cocurrent flows are preferred,
impact capture can also be obtained by radial injection from
a plurality of circumferential positions as shown in the fourth
embodiment of Figure 9, although with re~uced efficiency. As
shown in Figure 9, the secondary gas stream ~2 with dispersed
target particulates P is fed through conduit 18 into a circum-
S ferential manifold 92 surrounding the wall of conduit section
22. Manifold 92 communicates with a plurality of openings 94
in the wall of section 22 which are preferably uniformly spaced
therearound. Relatively high velocity for stream A2 and a
relatively even distribution of flow through openings 94 is
required.
~eferring again to Figure 1 t the gas streams S2 and A2
flow past flange 26 as combined stream S3 and into the cleaning
apparatus 14. The effluent gas Sl has been cooled by admixture
~ith the ambient air stream A2 and by any quench liquid applied
in chamber 10. If further cooling is desired, an additional
air stream A3, controlled by a conventional damper or the like
(not shownJ, may be admitted through the duct 100 prior to
ntry into the apparatus 14.
While good results have been obtained without applying
lectrostatic charge to the particles P,for some applications
he efficiency of removal can be increased by providing a charge
f either polarity. This can be accomplished, for example by
assing the air stream A2 with entrained target particulates
etween charged electrodes 102 and 104 within conduit 18.
lternatively, the target particulates can be triboelectrically
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charged by passing them in contact with a suitable triboelectric
material such as ylass or the like located as a lining or as
one or more collars within conduit 18. ~ venturi collar of
such material can be used for increasing friction. The contam-
inatiny particulates in stream Sl need not be charged, but
can be similarly charged with the opposite polarity if desired.
EX~iPLE 1
The present invention has been tested in a pilot
installation using a slip stream of effluent gas from a glass
meltiny furnace used for the manufacture of glass fibers,
employing the apparatus of Figures 1 and 5. Ninety percent
of the contaminating ~articulates were less than 1 micron in
size and the particulates were estimated to have an average
;~ particle size of about 1/2 micron. These particulates were
omposed typically of the following salts and oxides: sodium
fluoride, calcium fluoride, calcium oxides, silica, sodium
ulfate and boron oxid~s. The gas also contained acid gas
omponents, particularly oxides of sulfur and hydrogen fluoride
s indicated in Table 1 below. ~etween about 10 and 35 pounds
er hour of ne~heline syenite having an average particle size
etween about 10 and 20 microns was metered into the secondary
ir stream ~2 The incoming gas stream Sl was quenched in
hamber 10 with a spray at the rate oE 2.5 gallons per minute
ith a 2.5% by weight slurry of calcium hydroxide in water.
he results of this test are given below in Table 1 wherein
CFI~ means actual cubic feet per minute, PPr~l means parts per
nillion, and gr./SCF means grains per standard cubic Eoot.
TABLE I
Sl S2 ~2 ~A3 S~
GAS VOL.
ACFM: 7000 5600 1000 1400 7000
TEI~IPE~TURE, F:
Dry Bulb: 700 235 Ambient Ambient 170
Wet ~ulb: 95 134 Ambient Ambient 117
Velocity, fps: ~ 50 50-80*
SO~, ppm:100~200 20-30
F , ppm: 90
B , gr./SCF0.25 0.003
ther
Particulates
gr./SCF: 0.2 0.01
*at injection into S2
Capture of the fine contaminating ~articulates in
the effluent gas stream by the target particulates injected
ith the stream A2 was verified by gas elutriation tests.
sample of the separated mixture of particulates shaken from
the bags in the baghouse was placed in a column. A stream of
air at various velocities was passed upwardly through the
particulate sample. Particulates entrained with the air were
seyarated, tested, and compared with like tests from the
2S riginal material. The tests were substantially the same
indicati.ng that the fine particulates were bound to the
heavier target particulatcs.
EX~PLE 2
A typical flue gas from a municipal incinerator
containing a mixture of solid particulates of oxides and salts
and treated according to the method and apparatus of Figures
2-4 will give results substantially as follows, when quenched
with 60 gallons per minute of water in chamber 10 and supplied
with 100 to 150 pounds per hour of target particulates of 3-15
micron nepheline syenite in stream A2:
TABLE II
Sl ' s2 A2 S~l
as Vol., ACFM (OOO's) 250 150 5 156
emperature, ~F
Dry Bulb: 1600 400 80 380
Wet Bulb: 100 164 60 162
elocity, f~s, about: 50-80 50-80*
articulates, gr./SCF: 2.0 0.03
HC1, PPMo 200 5
HF, PP~: 10 3
. *at injection
It should bc understood that the foregoing description
nd e~amples are given for the purpose of illustration and that
he invention includes all modifications and equivalents within
the scope of the appended claims.
