Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a method and apparatus for purifying
a gas of particles suspended therein.
Dust-laden gases or those which are loaded with other impurities can
be cleaned in known manner by the crude gas stream being caused to flow through
a layer of granular filter material, consisting for example of silica sand.
During the course of operation, the granular filter bed becomes progressively
more laden with dust particles until its increase in flow resistance necessi-
tates regeneration, i.e. freeing the granular filter bed from dust.
Several methods for regenerative cleaning of granular filter material
are known. It has for instance been suggested to draw off the filter material
from time to time, to lead it outside of the filter system through a cleaning
apparatus (cyclone) and then to return it to the filter after the dust has been
successfully extracted. These systems require a large amount of conveying means
since the filter material must firstly be drawn off at the lower part of the
filter system, then conveyed vertically upwards and finally led into the filter
again from above.
In order to overcome this disadvantage, the suggestion has been made
to arrange the granular filter bed between two gas-permeable, coaxial tubes and
to circulate the filter material in a closed cycle during regenerative cleaning.
For this purpose, the material is carried upwards by a conveyor gas stream from
a central conveyor tube and falls again through the annular filter bed into the
range of the conveying gas stream. Arranged in such systems between the outer
tube of the filter bed and the casing shell is a crude gas chamber, through
which crude gas flows during filtration, and dust-laden scavenging gas flows
during the regeneration phase. It is unavoidable that dust swirling in the
crude gas space also settles on the outer tube and thereby impairs the effec-
tiveness of the filter system in the long run. Also, the dust-laden conveying
gas must be led out of the filter system and cleaned by a separator before
--1--
~,
~
lP~ 6~
discharge.
The present invention provides apparatus for purifying a gas of
particles suspended therein which comprises:
(a) a shell;
(b) two walls in said shell
(1) said walls being annular about a common, vertically
extending axis,
(2) said shell and one of said walls defining therebetween a
first space annular about said axis,
(3) said walls defining therebetween a second space annular
about said axis,
(4) the other one of said walls bounds a third space therein;
(c) a filter bed of granular material in said second space,
(1) said walls being formed with perforations permeable to the
gas to be purified and to at least a portion of said particles;
(d) a conveying conduit having a bottom orifice communicating with
the lowermost portion of said second space and upwardly extending therefrom;
(e) feeding means for feeding a gas under pressure to said bottom
orifice and the communicating lowermost portion of said second space, said con-
duit having a top orifice, whereby a mixture of said gas under pressure and of
said granular material is conveyed upwardly through said conduit and discharged
from said top orifice when said gas under pressure is fed to said bottom orificeand said lowermost portion;
(f) separating means for separating the granular material from the
gas in said mixture, said separating means communicating with said top orifice
in such a manner so as to receive the mixture discharged from said top orifice
and with the topmost portion of said second space in such a manner so as to
permit downward movement of the separated granular material from said separating
75~'~
means to said second space;
(g) a connecting conduit connecting said separating means with said
first space for flow of the separated gas from said separating means to said
first space;
(h) inlet means for admitting the gas to be purified to said first
space,
(i) outlet means for withdrawing purified gas from said third
space; and
(j) supply means for supplying a scavenging gas to said third space.
From another aspect, the invention provides in a method of purifying
a gas of suspended particles in which the gas to be purified is passed from a
first space to a third space through a filter bed of granular material confined
in a second space between perforated, vertically extending walls, whereby at
least a portion of said particles is retained by said granular material, said
second space having topmost and lowermost portions, and purified gas is with-
drawn from said third space in each of a plurality of filtering phases, granular
material carrying retained particles is withdrawn from said lowermost portion
during each of a plurality of filter bed cleaning phases alternating with said
filtering phases by suspending the granular material in a stream of conveying
gaseous material flowing toward a zone above said topmost portion, the suspended
granular material is separated in said zone from said stream and said particles,
the particles being suspended in said stream, the granular material being
returned to said second space by gravity, the improvement which comprises:
(a) withdrawing the separated gaseous material and the particles
suspended therein from said zone,
(b) introducing the withdrawn material and the particles into said
first space, and
(c) separately withdrawing said gaseous material and the particles
, ~
75t~
from said first space during each of said cleaning phases.
An embodiment of the inventive subject matter is illustrated, by way
of example only, in the accompanying drawings, wherein:-
Figure 1 is a simplified vertical section of a filter unit showingit in its operational phase (cleaning crude gas);
Figure 2 is a cross-section on the line II - II in Figure 1 and
simultaneously illustrates the parallel connection of several filter units;
Figure 3 is a cross-section on line III - III in Figure l;
Figure 4 is a cross-section on line IV - IV in Figure 5;
Figure 5 shows the filter unit shown in Figures 1 to 4 during the
regenerative cleaning phase (cleaning filter bed); and
Figure 6 is a vertical cross-section of a variant.
The schematic filter unit (illustrated without showing unimportant
details) has a casing 1 which has on its upper part an inlet opening 2 for
scavenging gas which enters via a scavenging gas pipe 3. Cn the opposite side
of the casing 1 there is an outlet opening 4 for the purified gas which flows
off through a pipe 5. By means of two gates 7 and 8 the openings 2 and 4 can
be closed alternately, under command impulses that can, for example, originate
from a schematically indicated servomotor M.
Inside the practically circular cylindrical casing 1 are two gas-
permeable, coaxial cylindrical pipes 9, 10. The space between these two pipes
is filled with a filter bed 11 consisting of a granular material, for example
silica sand.
The two gas-permeable perforated pipes 9,10 taper towards the bottom.
Joining the lowest part of the outer pipe 9 is a conveying gas pipe 12, in
which a throttle valve 13 and a porous, i.e. gas-permeable, plate 29 are mounted.
A central conveyor tube 14 extends upwards from the lowest, central point of the
inner pipe 10. Above the upper mouth of the conveyor tube 14 is a deflecting
S6~
member 15 which is shaped like an inverted cone and therefore has a downward-
tapering deflection surface. The deflecting member 15 is attached to the
housing 18 of a separating device by means of struts 16. The housing 18 is
constructed as a double cone, the widest section of which lies approximately
in its middle area. This housing 18 is connected by a pipe 19 to the space
between the outer pipe 9 and the casing 1, which will now be called the crude
gas chamber 20.
The two gates 7 and 8 are joined together by two rods 6 and traverses
(not shown) arranged in the region of the gates. The rods 6 lie therefore on
both sides of the separating housing 18 and rigidly couple the gates 7,8.
; A crude gas channel 21 opens into the casing 1 and arranged in the
conically tapering lower section of the casing 1 in a conduit 22 a discharge
conveyor, preferably a discharge screw 23. The crude gas channel 21 is
limited laterally by two parallel, vertical walls 39 and 40.
As shown in Figure 1 and Figure 2, several filter units are arranged
consecutively in a filter system and are fed by a common crude gas channel 21.
Neighbouring filter units are separated from one another in their lower sectionsbelow a floor 34 by vertical walls 33 and are connected to the crude gas
channel 21 by openings 35.
Four radially-directed drop pipes 24 open into the bottom of the
housing 18, connecting the space enclosed within the housing 18 to the filter
bed 11. The drop pipes 24 are inclined downwards towards the filter bed at an
angle which is larger than the angle of repose of the granular material used.
The connecting pipe 19 from the separating housing 18 to the crude
gas chamber 20 as shown in Figure 3 opens preferably tangentially into the uppersection of the crude gas chamber 20.
As mentioned before, the filter unit shown in Figure 1 is to be
thought of as a part of a system which comprises several such units, so that one
of the elements can be regenerated as desired while the other elements maintain
the cleaning operation of the crude gas.
In the crude gas cleaning operation, the dust-laden crude gas flows
according to Figure 1 through the channel 21 into the crude gas chamber 20,
flows subsequently through the filter bed 11 and leaves the inner, clean gas
chamber surrounded by the filter bed through the spaces 25 between the pipes 24
(Figure 3). The clean gas then reaches the outlet 5 through the opening 4, the
conveying gas valve 13 being closed.
If the filter bed material is to be regenerated, the conveying gas
throttle valve 13 is opened (Figure 5). The gate 7 uncovers the flow-in opening
for scavenging gas which then flows through the spaces 25 (Figure 3) into the
clean gas chamber of the filter and from there through the filter bed 11 (see
Figure 4) radially outwards (arrows 27) into the crude gas chamber 20.
The conveying gas stream flows, as mentioned, through the pipe 19
tangentially into the crude gas chamber 20. Since the crude gas chamber 20 is
one of two annular spaces bounded by the cylindrical walls 1,9, the conveying
gas $tream flows downwards in a helical path S (Figure 5). In this way, it
mixes with the scavenging gas stream (arrows 27) which picks up dust from the
filter bed 11, so that dust granules flushed out of the filter bed 11 by the
scavenging gas follow the downward helical path.
If this dust-laden gas stream were to enter the lower collecting room
26 at all parts of its periphery, a large portion of the dust would then enter
the crude gas channel 21 and therefore pass to the other filter elements. To
avoid this, the entrance point E of the conveying gas pipe 19 is arranged in
relation to the dimensions of the casing 1 and the gas flow speed in such a
position on the circumference of the casing 1 that the end point Z of the
helical path S lies at least substantially diametrically opposite the opening
35 (which serves as a flow-in opening during the crude gas cleaning operation).
i;6~
Thanks to this arrangement, the major part D of the dust is separated from the
gas, not in the flow out region of the gas, but opposite the outlet opening
35, and slides downwards along the inclined wall 36 and is removed by the screw
conveyor.
It will be clear to the person skilled in the art that the point Z
not only lies diametrically opposite the opening 35, but in fact covers a
: relatively wide zone, as indicated in Figure 2.
The entrance point E (Figure 5) is determined experimentally by
firstly determining the pitch h of the helical path S and then, beginning from
point Z, plotting the helical path along the inner wall of the casing 1.
The vertical distance of the points E and Z is a whole multiple of
the pitch h.
Through the co-operation of the radially directed scavenging gas
stream 27 and the tangentially inflowing conveying gas stream 28, a cyclone
effect results which provides intensive dust separation and prevents con-
tamination of the outer surface of the pipe 9,
Due to the abrupt interruption of the helical line S at point Z
caused by the sudden widening of the lower chamber 26, dust circulating near the
walls quickly falls.
Since the crude gas is sucked through the crude gas channel 21 by
means of a suction fan (not shown) arranged at the end of the channel, there
is a sub-pressure in this channel so that the mixed scavenging and conveying
gas stream is drawn without difficulty through the opening 35 (Figures 2 and 5)
into the crude gas stream.
As shown in Figure 5, the granular filter material is caught by the
conveying gas stream at the lowest point of the filter bed 11 and is conveyed
upwards in the conveyor tube 14. After leaving the conveyor tube 14, the
granular material strikes the deflecting member 15. The filter material
l~Z75~j~
granules rebound from the deflecting member 15 and are guided downwards by the
inner surface of the casing 18 and are returned to the filter bed 11 through the
drop pipes 24. The conveying gas leaves the separating device 17 through the
pipe 19 and enters, as already mentioned, the crude gas chamber 20 tangentially
at point E (Figure 5).
Figure 6 shows a variant where like parts are marked with the same
reference numerals. Contrary to the embodiment as per Figures 1-5, the convey-
ing gas pipe 37 is led concentrically downwards in the conveyor tube 14 and opens
into a deflection chamber 38 arranged below the conveyor tube 14. The conveying
gas entering deflection chamber 38 is deflected upwards and sweeps the granules
of the filter bed 11 in the annular space 12 upwards. Apart from this differ-
ence, the filter unit works as described on the basis of Figures 1 to 4.
-8-
