Note: Descriptions are shown in the official language in which they were submitted.
~0383Q6
This invention relates to a wet electrostatic precipitation pro-
cess for the removal of waste products from effluent gases and to a wet elec-
trostatic precipitator for performing the process of this invention.
It is well known that fine particles which can be electrically
charged can be collected in electrostatic precipitators. There are two types
of electrostatic precipitators that are commonly used
~ a) dry electrostatic precipitator - for which the mode of operation
is to electrically charge the particles which then collect on a grounded plate.
The particles collected on the grounded plate are removed by a rapping mech-
anism which dislodges the particles from the plate;
(b) wet electrostatic precipitator - this type of precipitator operates
on the same electrostatic principle but the dust particles collected on the
grounded plate are removed from the plate by a film of wash liquid which
passes over the grounded plate.
The advantage of using a wet electrostatic precipitator over a
dry electrostatic precipitator is that for materials with very low electrical
resistivity, the particles lose their electrical charge very quickly and if
collected in a dry electrostatic precipitator, would fall from the grounded
plate and would become re-entrained in the gas stream. On the other hand, in
the wet electrostatic precipitator the dust particles are entrapped in the
wash liquid on a continual basis.
Known wet electrostatic precipitator units operate with a very
high ratio of wash liquid to collected solids (e.g. 0.05% solids). When the
collected solids are separated out from the slurry of spent wash liquid and
collected solids, by settling and decanting, it has been found that the set-
tlement rate is slow.
The present invention enhances the settlement rate of collected
solids in the effluent. This is done by recycling some of the slurry back
into the precipitator, thus increasing the ratio of collected solids to wash
liquid in the slurry from about 0.05% up to 5% or 10%. This higher percen-
tage of solids in the wash liquid improves the settling characteristics of
the solids in the slurry.
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An additional advantage obtained by the process and apparatus
of this invention is that the collection efficiency of dust particles from the
flue gases is increased
More particularly, the present invention is a wet electrostatic
precipitator having
(i) an electrostatic precipitation zone having an electrode and a cor-
responding electrically chargeable plate;
tii) a gas inlet means for introducing a waste particle bearing gas into
the electrostatic precipitation zone and a gas outlet means for exhausting
the cleaned gas from the precipitator;
(iii) means for passing wash liquid over the electrically chargeable plate
to remove electrostatic precipitated particles from the plate,
~iv) inlet means for introducing fresh wash liquid into the means for
passing wash liquid over the electrostatically chargeable plate,
(v) a first reservoir for collecting waste particle bearing spent wash
liquid from the precipitation zone, and
(vi) circulation means to recycle a portion of the waste particle bear-
ing spent wash liquid to the means for passing wash liquid over the electri-
cally chargeable plate whereby the wash liquid contains in suspension a
predetermined amount of waste particles prior to passing over the electrically
chargeable plate.
The invention also includes a process for removing waste particles
from effluent gases by means of a wet electrostatic precipitator in which the
precipitated waste particles are washed out of the precipitator with a wash
liquid. A part of the wash liquid used to wash the precipitated particles
out of the precipitator is comprised by recirculating part of the waste par-
ticle-bearing, spent wash liquid through the precipitator until the wash
liquid contains in suspension a predetermined amount of waste particles prior
to passing through the precipitator.
An embodiment of the invention is illustrated in the accompanying
drawings in which:
Figure 1 is a schematic drawing and flow sheet of the wet electro-
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static precipitator according to the invention.
Figure 2 is a detailed drawing in plan of the wash liquid distri-
bution unit.
Figure 3 is an elevation of the embodiment illustrated in Figure 2.
Figure 4 is a view along section A-A of Figure 2.
Figure 1 illustrates three collection plates 1 in the form of
hollow cylindrical tubes. The collection plates are connected to an electri-
cal supply conduit 2 by which they are electrically charged. The effluent
gas from which the waste particles are to be removed is introduced at the
effluent gas inlet 3 and passes up through collection plates 1, which plates
electrostatically attract the waste particles from the effluent gas. The
effluent gas from which the waste particles have been removed are discharged
by means of exhaust gas fan 5 through the clean gas exhaust 4.
The fresh wash liquid, which in this case is water, enters the
precipitator through the fresh water inlet 6. This fresh wash liquid passes
down through collection plates 1 picking up the waste particles to form a
slurry of water and waste particles and passes into the slurry tank 8. The
slurry from the slurry tank 8 can either be passed through the slurry line 9
to a thickener 10 or can be recycled by means of the slurry recycle pump 13
through the slurry recycle line 14 back through the collection plates 1. By
recycling the slurry back through the collection plates the concentration of
waste particles in the wash liquid is increased.
A slurry tank agitator 7 can be provided in order to agitate the
slurry in slurry tank 8 and make the slurry more homogeneous. The waste par-
ticles are finally removed from the system when the slurry passes to the
thickener 10 which is used to decant the solids from the slurry. The slurry
is removed from the system through the thickener underflow discharge 11. Any
overflow from the thickener is recycled by the overflow recycle pump 12
through the thickener overflow line 15 back through the collection plates 1.
Figures 2, 3 and 4 illustrate slurry spray nozzles 20 located on
the slurry distributor header 19 which is connected to the slurry recycle
line 14. Fresh makeup water passing through the fresh water inlet 6 is
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introduced into the wash liquid through nozzles 21.
A reservoir 30 is provided to retain the slurry before it passes
through the collection plates 1. Also provided but not shown in Figures 2, 3
and 4 are frusto-conical weirs which are positioned within the reservoir 30
and communicate with the top of the collection plates 1.
A shroud 17 is provided so that slurry passing from the slurry
spray nozzles 20 is directed in a manner conducive to keeping the circulating
velocity of the slurry in the reservoir 30 sufficiently high to eliminate the
solids in the slurry from settling out in certain "dead" areas of the reser-
voir, particularly around the weirs. The fresh makeup water nozzles 21 are
also positioned so that the fresh makeup water entering the reservoir 30
also enhances the circulating velocity of the slurry in the reservoir 30.
The amount of solids in the slurry passing through collectionplates 1 can be regulated by the regulation of the fresh water entering
through fresh water inlet 6 and the regulation of the slurry recycle line 14.
The following example compares removal of waste production case A,
where there was no recirculation of slurry, and case B, where there was re-
circulation of the slurry.
Case A Case B
Fresh Water Usage*
(U.S. gallons per dry ton of
dust recovered) 960** 270**
Solids in Precipitation Discharge (%) ~ 0.5 5.0
Final Solid Density ~%) 24 59
Thi~kener Minimum Area
~ft /ton.24 hours) 4
Efficiency ~%) 97.5 99-5
* Based on thickener underflow % solids being 80% of final solid density.
** Calculated based on no thickener.
From the above it can be seen that recycling the slurry gives the
advantages of the use of less fresh water, a smaller minimum thickener area
and a greater final density of solids. It is this greater density of final
solids which promotes faster settling of the solids in the slurry and conse-
s ~038306
quently better separation capabilities.
The test results also show that efficiency of the precipitator
is increased from 97.5% to 99.5% when the per cent solids in the discharge
from the wet electrostatic precipitator is 5% rather than less than 0.5%.
The probable mechanism for this efficiency improvement is that with higher
contained solids the water film flowing over the collection plates is more
turbulent thus creating better collection characteristics.
The recirculation slurry is not limited to 5% solids. The higher
limit is the % solids in the slurry that is pumpable with each material; or
the density that increases the settling characterisitics of the material to
a point where the weir in the top of the collection plate cyclindrical tube
becomes filled with settled material.
The following Table I sets out wet electrostatic precipitator
(WESP) efficiency data for six tests in which the sluTTy was recycled in
tests 3, 4 and 5 but not l, 2 and 6 generally, when the carbon content of
the dust is high the efficiency is lower. Test No. 3 in Table I shows,
however, that even when the carbon content of the dust is as high as 26.6%,
by recycle to 5.0%, the efficiency has been increased to 99.91%. Table II
sets out test data setting out thickener area requirements and settling
rates for varying concentrations of solids in the recycle solution. From
Table II it can be seen, generally, that a better settling rate and a lesser
thickener area requirement is obtained withaa high per cent solids in the
wash liquid. The additional settling rate of the recycled solution seems
to be the result of flocculation of the particles in the distribution shroud
and in the collector pipes.
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TABLE I
Test 1 2 3 4 5 6
Kiln R.K. R.K. R.K. D.K. R.K. R.K.
Volume of gases acfm 799 644 629 1428 630 515
Temp: Inlet WESP F 730 628 660 298 712 720
Outlet wesP F 256 232 204 174 170 240
Dust Load Concentration
Inlet wesP gr/scf 2.29 2.28 3.12 8.51 7.50 7.53
Outlet wesP gr/scf .031 .060 .003 .010 .028 .217
Clean water flow USgpm 14 11 11
Water slurry recycle " 11 9.4 11
Clean water addition " .25 1.0 1.0
Water to swamp " 14 11.0 0.25 1.0 1.0 11
% Solids Slurry 0.02 0.11 5.0 5.5 2.6 0.27
wesP Efficiency 98.56 96.95 99.91 99.88 99.3 96.5
Inlet wesP % C Dust 10.7 26.6 1.31 4.25 4.07
Slurry wesP % C Solids 9.6 10.7 16.2 1.91 6.12 5.03
Inlet Gases Flow %
CO/CO21.3/2 6/1 6/1 0/9.7 6.2/0
Outlet Gases Flow %
CO/CO21/5.52.5/3.72/3.7
Time of Tests hrs 8 13 25 20 25 11
D.K. - Drying Kiln
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TABLE II
% Solids Ultimate Settling Thickene~ Overflow TemperOature
in Wash % SolidsRatej Area, t / Clarity, Flow, F
Test Liquid hr/ft ton/24 hrs. ppm
A3.87 37 4.6* 6.70 20 138
B0.82 20 43.3 3.6 5 160
C1.09 23 51.2 2.28 10 166
D0.46 24 59.1 4.81 10 172
E0.40 16 43.3 7.50 10 114
F0.40 16 39.4 8.24 10 115
G0.40 14 43.3 5.28 5 106
H0.75 21 49.2 3.48 5 156
I5.64 59 19.7 1.08 25 138
J 0.4 25.0 44.0 2.95 46 144
K 1.0 37.0 32.5 3.99 46 144
L 2.0 40.8 21.2 3.00 25 144
M 3.0 41.9 20.0 2.05 15 144
N 4.0 43.1 15.3 1.97 10 144
0 5.0 43.5 11.8 2.01 10 144
P15.0 50.0 5.2 1.20 46 144
* The settling rates were difficult to determine due to high turbidity of
the effluent.
