Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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APPARATUS FOR PRECIPITATING SOLID PARTICLES
AND AEROSOLS FROM GASES
The present invention relates to an apparatus for pre-
cipitating solid particles and aerosols from gases.
Various types of wet and dry electrostatic filters are
known for precipitating small solid particles and aerosols from
gaseous or aerosol-like substances, particularly waste gases.
The present invention relates to such an apparatus for wet pre-
cipitation.
German A-31 52 216 discloses an electric wet type
filter in which the gas is first of all conducted through an
ionizing device comprising a plurality of vertical, positively
charged metal wires arranged at a distance to one another,
between which negatively charged, similarly shaped wire counter-
electrodes are positioned.
The gas from the ionizing device reaches the electro-
static field of a filter which comprises a group of parallel
plate electrodes, one plate being thereby alternately connected
to the positive terminal of a voltage source while the other
intermediate plate is connected to the negative terminal.
The plate electrodes are made of glass and are washed
by a water film, whereby the actual electrode is formed by the
conductive water film. The plates are washed by means of pipes
which are disposed in the vicinity of the upper plate edge and
in which nozzle openings are formed so that the jets of water
from the pipes on the upper edge are directed obliquely downwards
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to the sides of the glass plates in the form of numerous fine
streams.
The known electrostatic filter exhibits various dis-
adwantages. Since the ionizing device lies in front of the
actual collecting plates, the danger exists that ionized solid
particles will accumulate predominantly on the leading edges of
the plates in the direction of the gas supply, whereby particu-
larly the edges are not washed entirely because the plates are
sprayed, as described, via nozzles, and thus it cannot always
be guaranteed that the solid particles will be carried away
immediately and completely with the washing liquid. In addition,
there is a danger that by spraying the waste gas to be treated
horizontally the water film on the plate edges will separate.
At the same time there is a danger of short circuits.
A so-called tube filter is known from French A-25 75 675,
the entire surface of its inner wall being washed by means of a
water reservoir arranged on the peripheral side but having, how-
ever, only very small precipitation surfaces, thus resulting in
a low specific gas throughput capacity and simultaneously a
high expense pertaining to the apparatus.
An object of the present invention is to provide a
wet ionization scrubber which allows a high gas throughput and
a high degree of precipitation of solid particles from the
gases or aerosols conducted through the scrubber while maintain-
inq the cost of the apparatus as low as possible.
The present invention provides an apparatus that com-
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prises a plurality of plates arranged parallel and at a distance
to one another and connected as electrodes with a specific
charge. These plates are washed continuously and over their
entire surfac~s with a washing liquid, charging wires which
have a different charge to that of the plates extend in the
spaces between the adjacent plates. The solid particles intro-
duced into the spaces between the plates by the flow of gas are
hereby provided in the area between the plates with a charge
corresponding to that of the charged wires and are subsequently
attracted directly by the plates connected as counterelectrodes.
Because the entire surface of the plates are washed with a wash-
ing liquid, the particles are quickly and completely carried
away to a collecting tank for discharge of the washing liquid
together with the solid particles.
The arrangement of the ionization wires between the
individual plates ensures that the charged solid particles
purposefully move along the shortest path to the plate electrodes
and cannot adhere to the edges or the unwashed surfaces.
With a higher gas throughput capacity, a higher degree
of precipitation which can be adjusted by the selected spacing
of the plates and the voltage applied is made possible through
this.
The apparatus according to the present invention is
characterized in detail by the features of claim 1.
According to a first examplary embodiment, it is pro-
posed to arrange the charging wires vertically between the
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plates, whereby this preferably occurs in such a way that the
wires lie in a vertical plane. The gas is preferably fed
hoxizontally and the wires are then arranged between the plates
at the end section facing the gas feed so that the solid parti-
cles are ionized uniformly and immediately following entry of
the gas flow between the plates.
There is also a danger in this case, particularly with
a high gas throughput, that the water film on the plate electrodes
at the gas inlet side could separate in places. Because of the
arrangement of wires between the plates, the apparatus according
to the present invention does not exhibit the disadvantage that
larger quantities of solid particles are precipitated on the un-
washed area of the plates. However, in a preferred exemplary
embodiment the present invention likewise proposes arranging the
ionization wires horizontally between the vertically oriented
plates and then guiding the gas in co-current or countercurrent
flow with the washing liquid through the spaces between the
plates.
In this connection it is proposed that the device for
continuously and completely washing the plates with a washing
liquid, which preferably has its own electric conductivity, be
designed (from top to bottom) with distribution channels that
are open at the top and are arranged directly on the upper edqe
of each plate. The washing liquid then runs over the edge of
the channels, over the outer surface of the channels and then
over the entire length of the plates to thereby uniformly and
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completely cover their surfaces. The channels can be wider
than the plates.
Even when feeding the gas countercurrently, a separa-
tion of the liquid film is avoided since the lower edges of the
plates are always washed over their entire length/width and new
water is continuously supplied from the top.
In an advantageous exemplary embodiment the wires are
situated between the adjacent plates at an angle less than 90
to the horizontal. The advantage of this is that any water
droplets that should be deposited on the charging wires (also
called electrical discharge wires) are immediately carried off
to the lower-lying end so that the danger of a short circuit
between adjacent plates with the same charge is prevented. The
electrical discharge wires are preferably inclined between 10
and 30 to the horizontal.
The level of high voltage which can be applied to the
electrical charging wires depends on the spacing between the
plates. The plate spacing and the applied voltage also depend
on the quality and quantity of gas/aerosol to be cleaned. At
a corresponding high voltage, a single "discharge plane" of
electrical charging wires suffices, that is only one charging
wire runs between adjacent plates. If a lower power supply must
be selected or to even further improve the degree of precipita-
tion, it is proposed in a further alternative to arrange a plural-
ity of "charging planes" between the plates. A plurality of
electrical charging wires then positioned at a distance to one
another between adjacent plates, whereby adjacent electrical
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charging wires between adjacent plates are again preferably
each arranged in one plane. The individual ~Icharging planes
can thereby be connected jointly or separately to a voltage
source.
The advantage of the distribution channels on the
upper edges of the plates, described above, having the washing
liquid flowing over their edge onto the plates is that disper-
sions can also be used for washing. The apparatus according to
the present invention thus has a further advantage over the
electric filter known from German A-31 52 216 in which the wash-
ing liquid is sprayed onto the plates via nozzles, since these
nozzles can easily become clogged when using dispersions.
The distribution channel can consist of a pipe whose
upper side is cut open. The distribution channel can be designed
with a weir to uniformly distribute the rinsing liquid. To
achieve large precipitation surfaces it is in particular pro-
posed to arrange precipitation aids between the washed plates
and at an adequate distance to the electrical discharge wires,
these precipitation aids being wetted by the washing liquid.
Aqueous solutions of acidic or basic electrolytes,
solutions containing surfactants or polyelectrolytes can be used,
for example, as the washing liquid.
Furthermore, it is proposed that an evaporative cooler
through which the gas/aerosol is conducted precede the apparatus.
The evaporative cooler represents a type of high-capacity ab-
sorber and can consist of a spray tower or a fixed bed which is
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sprayed with a washing liquid. The gas/aerosol is in this case
cooled to the dew point. Coarser solid particles are thereby
already precipitated. Aerosol-like solid particles are con-
ducted through the fixed bed or the spray tower and are then
preferably precipitated in countercurrent flow through the fol-
lowing wet ionization scrubber of the type described above.
This alternative represents a two-stage process which is advanta-
geous particularly with heavily loaded waste gases as well as
tarry, acidic, basic and saline aerosols from waste gases. The
entire device preferably comprises a vertical tower with a first
charging plane through which the waste gas is first of all con-
ducted before it reaches the wet ionization scrubber. The wash-
ing liquid from the wet ionization scrubber can thereby be used
again in the preceding charging stage; however, it is also
equally possible to design the spray tower with a separate
feeding mechanism for a washing liquid and to drain the washing
liquid from the wet ionization scrubber separately.
If the evaporative cooler is formed from a fixed bed,
then it can consist, for example, of a packed bed or an ordered
packing, for example a fabric packing.
The present invention will be explained in greater
detail herebelow with reference to two exemplary embodiments.
In drawings illustrating the invention:
Figure 1: a partially cut away side view of a first
exemplary embodiment of a wet type electro-
static filter,
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Figure 2: a per~p~ctive view showing the position-
ing of precipitation plates
and electrical discharge wires of the
apparatus according to Figure 1,
Figure 3: a side view of a second exemplary embodi-
ment of a wet ionization scrubber preceded
by a high-capacity absorber,
Figure 4: illustrates an alternative positioning of
the electrical discharge wires relative to
the precipitation plates in the apparatus
according to Figure 3.
The apparatus according to Figure 1 comprises a c~lin-
drical housing 10 with a rectangular cross-section which merges
at both endsvia a cones 12 into a feed connector 14 or an out-
let connector 16 for the waste gas to be cleaned. A fan 18 for
conveying the waste gas G through the housing 10 is disposed in
the inlet connector 14.
A plurality of vertically oriented plates 20 are arrang-
ed parallel and spaced from one another in the centre portion of
the housing 10. The plates 20 are connected to one another by
means of spacers. The plates are spaced equidistantly and an
electrical charging wire 22 is connected between adjacent plates
20 such that the electrical charging wires 22 are positioned one
below the other in a plane. As can be seen from Figure 1, the
vertically extending electrical charging wires 22 are positioned
at the gas inlet end of the plates 20 at a distance behind the
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front faces 20a. The electrical charging wires 22 run equi-
distantly between adjacent plates 20 and extend above and
below the plates 20 where they are fastened to the frame by
horizontally extending clamps 24 (Figure 2) isulated on the
outside by insulators 26.
The plates 20 are smooth and free of notches. A
distribution channel 28 is arranged on the upper edge 20b of
each plate 20. This channel can be configured as a weir taper-
ing conically to the plate edge 20b. Figure 2 illustrates one
embodiment in which the distribution channel 28 is formed from
a pipe that is firmly connected to the associated plate 20. The
diameter of the pipe 28 is greater than the thickness of the
associated plate 20 so that the distribution channel 28 extends
somewhat beyond the plate 20 on both sides. The channel 28 is
open at the top and a washing liquid is supplied by means of a
feeding mechanism (not illustrated). This washing liquid flows
over both of the edges of the channel 28, along its outer wall
and then reaches the outer surfaces of the plates 20 which are
thus washed completely and uniformly. These sections can be
covered with an increased film of rinsing water by changing the
incline over the length as well as by forming a notch trans-
versely to the channel or by mounting a displaceable ring on the
end facing the ionizing device. This structural feature is
particularly adapted to deal with the increased precipitation
of pollutants at the front end section of the plates 20 in the
direction of gas supply.
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To obtain precipitation, the plates 20, on the one
hand, and the electrical discharge wires 22, on the other hand,
must be provided with different charges. In the exemplary
embodiment illustrated in Figures 1, 2, the electrical charging
wires 22 are positively charged and the plates 20 are negatively
charged, or rather the plates 20 form the earth potential for
which a voltage source (not illustrated) is used. The fine
particles, which are contained in the gas blown into the housing
10 via the gas inlet connection piece 14, are charged (in this
case, positively) while passing the electrical charging wires 22
and are subsequently attracted by the plates 20 acting as counter
electrodes where they contact the continuous water film and are
carried downwards by the water film. To this end, the base 30
of the housing has openings, not illustrated in detail, through
which the washing liquid reaches a collecting tank 32. The flow
of rinsing water containing the solid particles preferably then
passes through a settling tank.
Figure 3 illustrates a further exemplary embodiment
of the apparatus which is in this case arranged in the upper
portion of a tower 34. Analogous to the exemplary embodiment
according to Figures 1, 2, the apparatus comprises a plurality
of water-washed plates 20 arranged parallel and at a distance to
one another. These plates are suspended at the upper end from
a supporting structure 36. Analogous to Figures 1, 2, a rinsing
channel 28 runs above each plate. The water supply is schema-
tically illustrated at 38.
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In this case also an electrical charging wire 22 runs
between adjacent plates 20; however, in contrast to the exemp-
lary embodiment according to Figures l, 2, it is arranged hori-
zontally, namely in the area of the lower end section of the
plates 20. The ends of the electrical discharge wires 22 extend-
ing beyond the plates 20 are stretched over clamps 40 that are
insulated from the frame by means of insulators.
These electrical charging wires 22 are arranged in
such a way that they lie in a horizontal plane.
As shown in Figure 3, in addition to the electrical
charging wires 22, additional electrical charging wires 22a are
arranged between the upper end sections of the plates 20 in a
manner corresponding to that of the electrical charging wires
22, and thus results in a second charge/ionization plane.
An evaporation cooling zone 42, which extends over
the entire cross-section of the tower 34 and is configured in
this case as a countercurrent spray tower, is arranged in the
tower at a specific distance below the apparatus described above.
A spray head 44, which is supplied with a separate flow 48 of
washing liquid via a line 26, is arranged in the centre of the
tower 34 above the evaporation cooling zone 42. The washing
liquid is sprayed in a star shape by means of the spray head
44 over the entire cross-sectional area of the evaporative
cooler 42 which consists of a gas permeable plastic network.
This installation Operates as follows; The waste
gas/aerosol is conveyed via an inlet connector 14 located at
:
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the lower end of the tower 34 through the tower 34 by means of
a fan and thereafter flows through the tower 34 from bottom to
top. In this way the waste gas first flows through the evapora-
tive cooler 42 where a first precipitation of coarse particles
occurs. The coarser particles are absorbed by the sprayed
washing liquid and on account of gravity are returned with the
flow of washing water against the gas direction G to the base
area of the tower 34 and discharged laterally via an outlet 50
below the gas inlet connection piece 14.
The already preconditioned waste gas continues to
flow upwards by the action of the fan and then reaches the actual
wet ionization scrubber in which the particle-like gas constitu-
ents are first of all ionized in the area of the first discharge
plane (electrical charging wires 22) and are subsequently pre-
cipitated in the manner described earlier by means of precipita-
tion on the film of rinsing water flowing off the plates.
The thus further cleaned gas continues to flow upwards
through the tower 34 until it reaches the area of the second
discharge plane (electrical charging wires 22a) where any solid
particles still remaining in the gas are removed in the same
manner.
The cleaned waste gas subsequently leaves the apparatus
at the top and is drawn off via the gas outlet connection piece
16 at the upper end of the tower 34 and the very fine particles
precipitated on the plates 20 or the film of rinsing water are
discharged downwardly via the flow of rinsing water which is
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conducted through the evaporative cooler 42 to the outlet 50 to
wh:ich a clarifying stage is connected.
The effectiveness of precipitation of the installation
described is particularly high since the distribution channels
described guarantee that the entire surface of the plates 20 is
washed and the very fine particles precipitated in the area of
the discharge planes thus always reach the counter electrodes
(plates 20) on the shortest path and are carried away with the
flow of rinsing water. Thus, the gas is guided countercurrently
to the rinsing liquid. It would also be equally possible, how-
ever, to conduct the gas and the rinsing liquid co-currently.
In contrast, the apparatus according to Figures 1, 2 operates
in a crosscurrent manner, i.e. the gas is guided through the
ionizing devices (plates 20, electrical charging wires 22) per-
pendicularly to the direction of flow of the washing liquid.
Instead of arranging the electrical charging wires 22,
22a horizontally, they can also be arranged at an angle ~ to
the horizontal as illustrated in Figure 4. The advantage of
this exemplary embodiment is that water droplets which may be
deposited on the electrical discharge wires 22, 22a, are carried
to the lower-lying end of the electrical discharge wire and thus
away from the actual ionizing device. In this manner the danger
of a short circuit between the electrical discharge wires 22
and the adjacent plates 20 is avoided.
Furthermore, the electrical charging wires are fastened
and charged as described above.
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The plate spacing depends on the voltage applied to
the electrical discharge wires 22, 22a. In view of the limited
dielectric strength of the unheated insulators inserted in the
ionizing device, working voltages of up to + 20 kV or - 20 kV
preferably are used in handling of moist gases. Accordingly,
as a function of the gas to be treated, plate spacings of 15
to 60 mm are preferred. The plates themselves can consist of
almost any type of material and are as a rule grounded. In any
event, the plate material should be easily wettable and resist-
ant for the respective field of use. Furthermore, the plates
should have as smooth a surface as possible in order to prevent
the rinsing liquid from separating.
