Method for adsorptive waste gas cleaning

For cleaning waste air loaded with organic pollutants one uses an adsorptive cleaning plant with a rotating cylindrical body containing an adsorbent through which waste air to be cleaned is passed for adsorption of the pollutants, hot air for desorption of the adsorbed pollutants and cooling air for cooling the desorption zone. Waste air with desorbed pollutants is supplied to an afterburning plant with a firebox. The hot air is formed by mixture of a part of the firebox air heated to 700 to 900.degree. C. produced in the firebox, with the heated cooling air passing out of the cylindrical body. The afterburning plant has two chambers connected via the firebox and filled with a heat-retaining packing, waste air with desorbed pollutants being supplied alternatingly to one of the chambers and clean air removed from the other chamber.

This invention relates to a method for cleaning waste air loaded with 
organic pollutants according to the preamble of claim 1. It also relates 
to a plant for carrying out this method. 
Such a method and plant are known from WO95/01827. In one embodiment of the 
known plant, the waste air with the desorbed pollutants is supplied to the 
afterburning plant via a heat exchanger heated by the firebox air. The 
firebox air is thus cooled before being mixed with the heated cooling air 
passing out of the cylindrical body to form the hot air. In another 
embodiment of the known plant with a regenerative afterburning plant, the 
clean gas passing out of the afterburning plant is mixed with the heated 
cooling air passing out of the cylindrical body to form the hot air. 
In the known plant, the hot air is thus circulated between the cylindrical 
body and the particular afterburning plant. That is, the main part of the 
moisture expelled from the adsorbent of the cylindrical body with the hot 
desorption air is afterburned but then recycled to the cylindrical body as 
hot desorption air, causing the moisture content to accumulate 
increasingly and resulting in worse adsorption and corrosion due to 
failure to exceed the dew point. 
In the known plant, the waste air to be cleaned is therefore cooled with a 
cooling device before being supplied to the cylindrical body. For the 
cooled waste air to be heated back to a temperature suitable for 
adsorption, part of the air heated in the heat exchanger or the heated 
clean air passing out of the afterburning chamber is supplied to it before 
it enters the cylindrical body. In the embodiment with the heat exchanger, 
the total thermal energy demand of the known plant is about 150 kW per 
10,000 Nm.sup.3 /h waste gas to be cleaned. For the cooling effect to 
suffice, the cooling air stream must reach approximately the same 
throughput as the hot desorption stream. With a zeolite as an adsorbent, 
the hot desorption air must be heated to at least 160.degree. C. In order 
to heat the cooling air from 50.degree. C. to 160.degree. C. in the 
embodiment of the known plant with the afterburning chamber, however, one 
would have to admix 75% air component at 206.degree. C. from the 
afterburning chamber. Such a high hot air component is not available in 
this embodiment. But the embodiment of the known plant with the heat 
exchanger also requires a much greater component of hot air than cooling 
air and thus a disadvantageous mixture ratio for practical working. 
In such a cleaning plant, polluted waste gas flows through more than half 
the cross section of the cylindrical body containing adsorbent, while the 
hot air for desorbing absorbed pollutants flows into a smaller segment. 
The rotation of the cylindrical body and the sector-shaped connections for 
supplying and removing the individual gases to and from the body relative 
to each other permits continuous operation. That is, the cylindrical body 
of the cleaning plant can rotate while the sector-shaped connections do 
not move, or the connections can rotate while the cylindrical body is 
fixed, as described e.g. in DE 43 15 578 A1. 
Further, so-called regenerative thermal afterburning plants are known 
wherein the waste air with organic pollutants is supplied alternatingly to 
at least two chambers filled with heat-retaining packing and connected via 
a firebox. Due to the heat-retaining packing of the chamber which has been 
heated in the prior cycle, the waste air to be cleaned is preheated before 
entering the firebox (DE 29 51 525 A1, EP 0 472 605 B1). If they have a 
low concentration in the waste air to be cleaned, the organic pollutants 
can be concentrated with an absorbent and subsequent desorption before 
being supplied to the regenerative thermal afterburning plant (U.S. Pat. 
No. 5,297,954). 
The problem of the invention is to essentially reduce the total energy 
demand of such a plant without impairing the high cleaning efficiency and 
without increasing the equipment expense. 
This is obtained according to the invention with the method characterized 
in claim 1 and the plant characterized in claim 8. The subclaims state 
advantageous embodiments of the method and plant. 
According to the invention, waste air with desorbed pollutants is supplied 
to an afterburning plant having two chambers connected via the firebox and 
filled with a heat-retaining packing, the waste air with desorbed 
pollutants being supplied alternatingly to one chamber while clean air is 
removed from the other chamber. Part of the firebox air having a 
temperature of 700.degree. C. to 900.degree. C., in particular about 
800.degree. C., is removed from the firebox of the afterburning plant and 
mixed directly with the heated cooling air to form the hot air for 
desorbing pollutants adsorbed in the cylindrical body. 
Most of the firebox air thus leaves the afterburning plant as 
low-temperature clean gas. The mixture ratio of heated cooling air to 
firebox air heated to 700.degree. C. to 900.degree. C. is preferably 
between 3 and 10 parts by weight cooling air, preferably 4 to 6 parts by 
weight cooling air, per 1 part by weight firebox air heated to 700.degree. 
C. to 900.degree. C. That is, the partial stream of firebox air removed 
from the firebox to form the hot gas is 1/3 to 1/10, preferably 1/4 to 
1/6, of the total firebox air of the afterburning plant. Thus, 2/3 to 9/10 
or preferably 3/4 to 5/6 of the firebox air in the afterburning plant is 
used for heating the heat-retaining packing before leaving the 
afterburning plant as clean air cooled to less than 100.degree. C. The 
energy consumption of the total plant is thus reduced according to the 
invention by about 60% over the conventional plant. A partial stream of 
preferably 5% to 20% is diverted from the waste air loaded with organic 
pollutants to be cleaned as cooling air for cooling the just desorbed 
sector of the cylindrical body. That is, the throughput of waste air with 
desorbed pollutants in the afterburning plant is 5 to 20 wt % of the 
throughput of waste air to be cleaned passing through the cylindrical 
body. 
The adsorptive cleaning plant used according to the invention can be a 
conventional adsorptive cleaning plant with such a cylindrical body filled 
with adsorbent, whereby the connections for admitting and discharging the 
gases, on the one hand, and the body, on the other hand, rotate relative 
to each other. 
The cylindrical body preferably has air-permeable end walls on each side 
and a plurality of radially extending partitions between the two end 
walls, and the connections for admitting waste air to be cleaned and for 
discharging clean air, for admitting hot air and for discharging waste air 
with desorbed pollutants and for passing cooling air through the 
cylindrical body are formed as segments of a circle which are disposed on 
the end walls concentric to the rotational axis. The connections for 
admitting waste air to be cleaned and for discharging clean air, for 
admitting hot air and for discharging waste air with desorbed pollutants 
and for passing cooling air are located exactly opposite each other on 
each side of the cylindrical body. The connections for admitting waste air 
to be cleaned and for admitting cooling air and for discharging waste air 
with desorbed pollutants are located on one side of the cylindrical body 
and the connections for discharging clean air, for admitting hot air and 
for discharging heated cooling air are disposed on the other side of the 
cylindrical body. 
The afterburning plant used according to the invention is preferably formed 
in accordance with German patent application 196 43 821.7, to which 
reference is made in all points. 
When waste air with desorbed pollutants is supplied to one chamber of the 
afterburning plant while clean air is removed from the other chamber, and 
the chambers are changed in order to supply waste air with desorbed 
pollutants and remove clean gas to and from the other chamber in each 
case, the chamber to which waste air with desorbed pollutants was supplied 
is filled with this waste air which then flows into the clean gas pipe 
when the latter is connected to this chamber. Thus, at least one chamber 
filling of waste air with desorbed pollutants would flow into the clean 
gas pipe. 
In order to prevent this, heated cooling air is supplied according to the 
invention to the chamber to which waste air with desorbed pollutants was 
supplied before clean air is removed therefrom. Heated cooling air thus 
displaces pollutants from this chamber into the firebox, where they are 
burned. When the chamber is connected to the clean gas pipe and the 
direction of flow in the afterburning plant reversed, only some cooling 
air, i.e. some of the waste air to be cleaned which is much less loaded 
with pollutants, passes into the clean gas pipe at each change of flow. 
The chambers are preferably rinsed with cooling air for no longer than 5 
seconds. 
For rinsing the chambers with cooling air, the pollutant supply pipe in 
which waste air with desorbed pollutants flows to the afterburning plant 
is connected via a bypass pipe with the hot gas supply pipe supplying hot 
gas to the cylindrical body, suitable shut-off devices being provided for 
blocking the supply of waste air with desorbed pollutants from the 
cylindrical body and for supplying hot gas from the hot gas supply pipe. 
In order to keep pressure relations constant on the cylindrical body, a 
separate fan is preferably provided for sucking cooling air out of the 
cylindrical body. 
The adsorbent filling the cylindrical body is preferably a hydrophobic 
zeolite. A mixture of zeolite, in particular a hydrophobic zeolite, with 
active carbon has also proven suitable. The hydrophobic zeolite typically 
consists of 90 to 97 wt % SiO.sub.2, 2 to 10 wt % Al.sub.2 O.sub.3 and &lt;1% 
Na. If a mixture of zeolite and active carbon is used, the mixture ratio 
is preferably 50 to 90 wt % zeolite and 10 to 50 wt % active carbon.

According to FIG. 1 the plant consists of adsorptive cleaning plant 1 and 
afterburning plant 2. 
Adsorptive cleaning plant 1 has cylindrical body 3 which is filled with an 
adsorbent and set rotating about axle 4 of a motor not shown. 
Cylindrical body 3 is supplied waste air to be cleaned via polluted gas 
pipe 5. For this purpose a connection (not shown) is provided which is 
formed like a segment of a circle corresponding to adsorption zone 6, the 
edge area of the connection being sealed with a seal from side wall 7 of 
body 3 which is of gas-permeable design. 
A corresponding segment-shaped connection in the area of adsorption zone 6 
is disposed opposite, i.e. on other side wall 8 of cylindrical body 3 
which is likewise of gas-permeable design. Connected to this connection 
likewise sealed from side wall 8 is clean gas pipe 9 which contains 
exhaust fan 10 and leads to chimney 11. 
Preliminary filter 12 is provided in polluted gas pipe 5 in order to 
separate particles harmful for cylindrical body 3. Connected to polluted 
gas pipe 5 after preliminary filter 12 is cooling air supply pipe 13 which 
leads to a connection not shown having a of segment form corresponding to 
cooling zone 14 and sealed from side wall 7 of body 3. A corresponding 
segment-shaped connection is disposed in the area of cooling zone 14 on 
other side wall 8 of cylindrical body 3 and sealed from the same. 
Connected to this connection is cooling air removal pipe 15 which leads to 
mixing chamber 16. Exhaust fan 17 is provided in cooling air removal pipe 
15. 
Hot gas supply pipe 18 leads from mixing chamber 16 to a connection (not 
shown) on side wall 8 of cylindrical body 3 which is formed like the 
segment of a circle corresponding to desorption zone 19 and sealed from 
side wall 8. A corresponding segment-shaped connection (not shown) is 
disposed in the area of desorption zone 19 on other side wall 7 of 
cylindrical body 3 and sealed from side wall 8. Connected to the 
connection on side wall 7 in the area of desorption zone 19 is pollutant 
pipe 20. While cylindrical body 3 rotates, the connections for admitting 
air to be cleaned and for discharging clean air, for admitting hot air for 
desorption and for discharging waste air with desorbed pollutants and for 
passing cooling air are fixed 
Afterburning plant 2 has two chambers 25, 26 connected via firebox 21 with 
burner 22 and provided with heat-retaining packing 24 which can be 
designed in accordance with EP 0 472 605 B1, to which reference is made in 
all points. 
Each chamber 25, 26 is connected via valve 27, 28 with pollutant pipe 20 
and via valve 29, 30 with clean gas pipe 31 of afterburning plant 2 which 
opens after fan 10 into clean gas pipe 9 of adsorptive cleaning plant 1. 
A partial stream of firebox air heated to 700 to 900.degree. C. is removed 
from firebox 21 via firebox air removal pipe 33 and supplied to mixing 
chamber 16. The ratio of firebox air supplied to mixing chamber 16 via 
pipe 33 to heated cooling air supplied via cooling air removal pipe 15 is 
adjusted via control valve 34 in firebox air removal pipe 33. 
Between hot air supply pipe 18 and pollutant pipe 20 there is bypass pipe 
35 which is connected to pollutant pipe 20 with three-way valve 36. 
Exhaust fan 37 is disposed in pollutant pipe 20 after three-way valve 36. 
The plant works as follows. Waste air loaded with pollutants is supplied 
via polluted gas pipe 5 to adsorption zone 6 of cylindrical body 3 in 
which the pollutants are adsorbed. The cleaned waste gas flows via clean 
gas pipe 9 into chimney 11. 
Cylindrical body 3 rotates in accordance with arrow 38 around axle 4 so 
that the absorbent loaded with pollutants in cylindrical body 3 is rotated 
from adsorption zone 6 into desorption zone 19, while desorption zone 19 
passes into cooling zone 14 and the cooling zone into adsorption zone 6 in 
order to adsorb pollutants from waste air to be cleaned again. 
A partial stream of 5 to 20% of waste air to be cleaned is diverted from 
polluted gas pipe 5 as cooling air via cooling air removal pipe 15 in 
order to cool desorption zone 19 just desorbed with hot air from hot air 
supply pipe 18. After flowing through cooling zone 14 then heated cooling 
air is urged via cooling air removal pipe 15 with fan 17 into mixing 
chamber 16, where it is mixed with a small quantity of firebox air heated 
to 700 to 900.degree. C. via firebox air removal pipe 33, controlled via 
control valve 34, in a mixture ratio of for example 5 parts by weight 
cooling air to 1 part by weight hot firebox air, into hot air which is 
supplied via hot air supply pipe 18 to desorption zone 19 in order to 
expel pollutants therefrom. That is, according to the invention the 
portion of cooling air is many times, for example five times, greater than 
the portion of hot air, thereby obtaining perfect cooling. This ensures 
that the ratio of hot air to cooling air does not exceed 1.0 to 1.5, in 
particular 1.1 to 1.33. At the same time, only a small portion of firebox 
air of e.g. 1/6 of the supplied cooling air is supplied to mixing chamber 
16, thereby preventing overloading with water vapor and thus a danger of 
corrosion and impairment of the adsorptive power of adsorbent in 
cylindrical body 3. 
Waste gas loaded with a high pollutant concentration due to the desorption 
passes in pollutant pipe 20 with fan 37 via valves 27, 28 into one of the 
two chambers 25, 26 of afterburning plant 2. Heat-retaining packing 24 in 
particular chamber 25, 26, which was heated in the previous cycle, heats 
the waste gas containing desorbed pollutants, whereupon the desorbed 
pollutants are burned in firebox 21. While e.g. 1 part of the firebox air 
is diverted via firebox air removal pipe 33, the remaining e.g. four parts 
of hot firebox air heat heat-retaining packing 25 in other chamber 26, 25, 
so that largely cooled clean gas with a temperature of less than 
100.degree. C. passes into clean gas pipe 31 
Chambers 25, 26 are charged with waste air with desorbed pollutants 
alternately e.g. every 1 to 3 minutes by a switchover of valves 27, 28 in 
pollutant pipe 20 and valves 29, 30 in clean gas pipe 31. The high mixture 
ratio of cooling air to firebox air avoids substantial accumulation of 
moisture in the circuit between afterburning plant 2 and adsorptive 
cleaning plant 1. 
In order to prevent waste gas with desorbed pollutants contained in chamber 
25, 26 last charged with waste gas with desorbed pollutants via pipe 20 
from passing from said chamber 25, 26 into clean gas pipe 31 after 
switchover, hot air little loaded with pollutants is supplied shortly 
before the beginning of switchover of valves 27 and 28 in pollutant pipe 
20 and valves 29, 30 in clean gas pipe 31 by means of three-way valve 36 
for a few seconds to particular chamber 25, 26 via bypass pipe 35. 
Directly thereafter bypass pipe 35 is closed again with three-way valve 36 
so that hot air in hot air supply pipe 18 is supplied back to desorption 
zone 19. 
Thermal afterburning thus reaches a mean degree of cleaning of over 99.5%. 
In the afterburning plant of FIGS. 2 and 3, which is designed according to 
German patent application 196 43 821.7, valves 27, 28 and 29, 30 are 
provided in box 41 disposed in space 39 between two chambers 25, 26. Box 
41 has two side walls 42, 43, upper wall 44, lower wall 45, upper 
partition 46 and lower partition 47. Upper wall 44 and upper partition 46 
enclose with side walls 42, 43 gas removal channel 48 to which clean gas 
pipe 31 is connected, while lower wall 45 and lower partition 47 enclose 
together with side walls 42, 43 gas supply channel 49 to which pollutant 
pipe 20 is connected. Channels 48 and 49 are sealed at the ends at the 
back in FIGS. 2 and 3. Upper partition 46 and lower partition 47 form 
together with side walls 42, 43 the housing of shut-off device 51 of the 
plant. In this housing vertical partition 52 is provided between upper 
wall 46 and lower wall 47, separating the housing into two single chambers 
54, 55. 
Single chambers 44, 45 are provided on upper housing wall 47 and lower 
housing wall 48 with openings 56, 58; 57, 59 in each case. For opening and 
closing the two openings 56, 57; 58, 59 disposed one on the other, each 
single chamber 54, 55 has plate-shaped closing body 60, 61 which is 
fastened to the lower end of piston rod of 62, 63. 
Single chambers 54, 55 of shut-off device 51 are connected with one firebox 
25 or with other firebox 26. For this purpose shut-off device 51 is 
designed to be open in the area of particular single chambers 54, 55 on 
the side facing particular chamber 25, 26, i.e. side wall 42 or 43 is 
lacking in this area. This results in large opening 65, 66 through which 
waste air with desorbed pollutants flows into particular chamber 25, 26 or 
clean gas from particular chamber 25, 26 into main shut-off device 51. 
Piston rods 62, 63 are operated in each case by piston/cylinder unit 74, 75 
fastened to holding device 76 above box 41 in space 39. In the end 
positions of piston rods 62, 63 and thus of plate-shaped closing bodies 
60, 61, one of two openings 56, 57 or 58, 59 disposed one on the other in 
each single chamber 54, 55 is thus closed in each case while other opening 
57, 56 or 59, 58 is open.