Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention starts from a process for biological waste
air purification and the use of a percolation system,
wherein the wa6te air stream is passed through a percol-
ation system fed with a washing fluid and serving at the
same time as a carrier body for a microorganism culture
which degrades the pollutant. Processes of this type have
been described, for example, in the article by H. Brauer,
Umschau 1984, Issue No. 20, page 598, and in EP-A 100,024
and EP-A 133,222. In this case, the washing water is
passed through the percolation system in co- or counter-
current with the waste air which is to be purified. As
carrier bodies, for example, activated carbon or hollow
bodies of plastics such as polypropylene, polyamide or
polyethylene are used. A biofilm of adapted microorganism
cultures gradually grows on the surface of the carrier
bodies. During the passage of the waste air through the
percolation system, waste air constituents are, on the
one hand, washed out by the wa~hing fluid and, on the
other hand, also absorbed directly by the microorganisms.
Biological waste air purification has proved particularly
suitable when the pollutant concentrations remain more or
less con~tant, that i8 to say when no load fluctuations
occur.
Thi~ is the starting po$nt of the invention. It i8 based
on the ob~ect of developing a biological waste air
purif$cat$on proce~s of enhanced flexibility, 80 that
reliable adherence to preset limits can be ensured even
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in the case of varying loading of the waste air with
pollutants. In particular, a surge load with twice the
pollutant concentration should still be intercepted.
Starting from the process described at the outset, this
ob~ect is achieved according to the invention when the
waste air stream and the washing fluid are pa~sed through
the percolation system in co-current and in counter-
current in periodic alternation. This means that there is
a periodic change-over between the co-current and
counter-current procedures.
The frequency for the alternating co-and counter-current
procedures is advantageously in the range from 0.2 to 2
changes per hour.
The carrier body used is preferably a reticulated poly-
urethane foam which can be partially coated with carbon
powder fixed to the surfaces of the foam skeleton.
Advantageously, the carrier body is inserted into the
percolation tower in the form of a polyurethane foam mat
rolled up to give a cylindrical body.
According to a further development of the invention, the
washing fluid is recirculated through the percolation
system and passed through a purification filter which
contains the ~ame carrier body with immobilised micro-
organisms as the percolation system. In this case it is
advantageous when the purification filter i8 likewi6e
designed as a percolation sy~tem and i~ operated in such
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a way that the prepurified gas leaving the first percola-
tion system is passed in co-current with the washing
fluid through the second percolation system.
The process according to the invention has proved es-
pecially suitable in the separation of dichloromethane
from a waste air stream.
By means of the invention, the following advantages are
achieved:
- Because of the alternating co-current and counter-
current procedures, a better purification perfor-
mance can be achieved. Surge loads can be reliably
intercepted. The biomass necessary for the purific-
ation forms in this procedure in the upper and lower
parts of the percolation tower.
- In the process variant with a downstream second
percolation tower as a purification filter, the
biomass is formed mainly by pollutants which, in the
counter-current procedure, pass with the washing
fluid (water) into the second percolation system and
are degraded therein. In this way, the pollutant
stream is distributed over both percolation systems
and can be influenced via the circulating water
rate.
- The use of reticulated polyurethane ether foam has
the advantage that this material requires only about
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3% of the reaction space and does not contain any
"dead~ spaces. Moreover, the flow resistance is
markedly le~s than in other percolation systems. The
free space is therefore available for both the gas
S purification and the growth of biomass. In this way,
a longer service life with a significant Laving of
purification cycles for the carrier material is
achieved. The frequency of required purification
cycles depends on the nature and quantity of the
pollutants fed, in particular on the carbon content
thereof.
- The use of wound-up polyurethane mats as the carrier
material allows easily effected mechanical cleaning
without 1088 of carrier material and of its
biological purification capacity. Because of its
skeleton structure, the carrier material is almost
dimensionally ~table even in the moist state and has
a very low density.
- In the removal of dichloromethane (DCM) from a waste
air stream, it was possible to achieve particularly
high degradation rates. The purification performance
amounted to 90~ and more, depending on the raw gas
concentration. In this way, it ha~ been possible,
for example, to achieve values significantly below
the limits now demanded by the German Clean Air
Regulations (20 mg/m3) at starting values of 200
mg/m3 in a gas stream of 6 m3/hour.
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The invention is explained in more detail below by
reference to an illustrative example. The drswing show~
a flow diagrsm of the process according to the invention.
The waste air to be purified is passed via the line 1
through its three-way valve 2 into the percolation tower
3. The prepurified air leaves the percolation tower 3
through the line 4 and i~ fed to a second percolation
tower 5. The line 4 is connected via a three-way valve 6
to either the upper end or the lower end of the first
percolation tower 3.
The washing water for operating the percolation tower 3
is delivered by means of the pump 8 from a mixing tank 7
through the line 9 to the spray nozzle 10 in the percola-
tion tower 3. At the lower end of the percolation tower
3, the washing water runs via the line 11 to a pH value-
measuring point 12 and is pumped from there by the pump
13 to the spray nozzle 14 of the second percolation tower
5. From there, the washing water flows back into the
mixing tank 7 through the line 15. About 1% strength
aqueous soda ~olution, which optionally further nutrient
components such as urea or other nitrogen-containing
substances as well as phosphates, is pumped by means of
a pump 17 from a stock tank 16 into the mixing tank 7.
Fresh water is fed via the line 18 to a further stock
tank 19. Excess water flows out of the stock tank 19
through the line 20. Excess water from the mixing tank 7
i~ discharged via the line 21. A stirrer 22 i8 installed
ln the tank 7 for mixing.
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The gas feed and discharge in the first percolation tower
3 is effected, as already described, by means of electri-
cally switchable three-way valves 2 and 6. The valves 2
and 6 are switched over at a preset frequency of between
0.2 and 2 cycles per hour in such a way that the gas
flows through the percolation system alternatively in co-
current and in counter-current to the washing water. The
connections here are such that, when the feed line 1 is
connected to the upper end of the percolation tower 3,
the lower end is connected to the line 4 leading to the
second percolation tower 5 and, if the line 1 is
connected to the lower end of the tower, the upper end is
connected to the line 4. In the second percolation tower,
the prepurified gas flows in co-current with the washing
fluid. The purified gas flows out through the line 23
downstream of the second stage. In the alternating co-
and counter-current procedure in the first percolation
tower 3, a sufficiently large quantity of an adapted
mixed microorganism culture is maintained in an active
state in both the upper and the lower region of the
percolation system, so that the microorganisms are always
alternately exposed to a feeding phase and a starvation
phase. The starving microorganisms can intercspt the
concentration peaks in the event of surge loads.
As already described, the washing fluid is circulated
through the two percolation towers 3 and 5, a purific-
ation taking place in the second tower. The percolation
tower S in the ~econd stage thus has a dual function; it
~erve~, on the one hand, for final purification of the
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gas and, on the other hand, for purification of the
washing fluid. This purification effect i8 important, in
as much as the washing water, coming from the first tower
in the counter-current procedure, contains a pollutant
concentration corresponding to the phase equilibrium,
which pollutants are degraded while passing through the
second percolation tower 5. On the other hand, the
gaseous pollutant contents, which is briefly increased
in the change-over phases of the three-way valves 2 and
6, is reduced in the second stage, in addition to the
final purification of the gas.
The percolation system 24 in the first tower 3 consists
of a wound-up mat of reticulated polyurethane foam. The
PU foam here forms a skeleton of thin small rods of
approximately triangular cross-section, taking up about
3% of the total volume. ~ecause of the reticulation,
there are no membranes between the cells and hence also
no closed cavities or niches. As a result of the
skeleton-like structure, the material is largely dimen-
~ionally stable even in the operating state.
The external dimen~ions of the mat unit are, for example,
5 x 100 x 200 cm. These units can be sewn up along the
100 cm edges to give mats of any de~ired length. The
first and last mat units are flattened along the 200 cm
edge 80 that, when the long mat is wound up upon a
winding core, formation of gaps is avoided. The winding-
up produce~ an almost cylindrical block of the polyur-
ethane carrier material, having a height of 100 cm and a
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diameter of any desired magnitude. These blocks can then
be stacked on top of one another in the percolation tower
3 up to the desired height. The interspace between the
percolation block 24 and the inside wall of the tower 3
S is filled with an inflatable sealing mat 25. It consists
of an annular double-walled sleeve, whose edges are
welded up. The sealing sleeve 25 laid around the
percolation system 24 is then filled with air through a
small connecting tube and thus seals the interspace
between the percolation system 24 and the inside wall of
the percolation tower 3. As a result, the waste air to be
purified is forced to flow through the percolation ~ystem
24, who~e surface is colonised by microorganisms.
The percolation system 26 arranged in the second percola-
lS tion tower 5 is in its structure identical to the first
percolation system 24. Nowever, it has in general a
smaller height and is operated exclusively in co-current;
that is to say the gas and water streams coming from the
first percolation tower 3 are passed downwards through
the percolation system 26.
ExamDle
U~ing the unit described, dichloromethane was removed
from a waste air stream. Dichloromethane (DCN) was
uniformly fed via a gas-washing bottle which was filled
with a defined quantity of DCM, held at a constant
t-~perature (~bout lO-C) by external cooling water and
c~rried a flow of a defined air r~te (~bout 1 l/h) in
, .
:
~,
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fine bubbles. This air laden with DCM in accordance with
saturation was pas~ed into the feed line 1, and a DCM
concentration of about 300 mg/l was thus set. The 3-way
valves 2 and 6 controlled by a timer effected the change-
over of co-current and counter-current procedures in the
percolation tower 3 in a rhythm of about one hour, that
is to say the DCM-laden air and the washing fluid with
added soda solution of about 170 l/h passed the percola-
tion system 24 alternatively in co-current and counter-
current.
The pH value of the washing fluid running out at thebottom was measured at the measuring point 12, and it was
then fed for final biological purification to the second
percolation tower 5. This final purification is of great
importance when the apparatus operates in the counter-
current phase, because the washing water absorbs about
5 mg/l dichloromethane from the laden air. When sprayed
onto the first percolation tower 3, this amount would
partially be released again to the purified air and thus
reduce the efficiency. The final biological purification
in the second percolation tower avoids this disadvantage.
While the washing water passes through the first tower 3,
the pH value falls from about 8.1 to about 7.2, and by
about 0.1 in the final purification. The original pH
value i~ restored by pH-controlled addition of 1%
~trength soda solution.
Por working, the microorgani~ms additionally require
nitrogen and phosphorus and diverse trace elements.
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Nitroqen and phosphorus were added in the form of urea or
nitrate (0.2 g/l) and R2HP0~ (0.1 g/l) simultaneou~ly with
the 1~ strength soda solution. The addition of 60 ml of
nutrient salt solution wa~ carried out discontinuously
from a 60 1 ~tock tank which was charged with about 5 1
of potable water per hour. The pressure drop of the air
during the passage through the apparatus was 30 mm or 35
mm water gauge at 10 m3/h air throughput. No measures were
taken for cooling or heating the large parts of the
apparatus. The reaction temperature was thus identical to
the room temperature (20- to 25-C).
The DCM concentration before and after the purification
was measured by means of a flame ionisation detector. At
a superficial loading of 140 m3/m2 h on an inlet
concentration of about 300 mg/m3 of DCM, the DCM con-
centration in the pure gas, that is to say in the outlet
line 23, was about 10 mg/m3. It was possible to maintain
the requirements of the German Clean Air ~egulations for
class 1 (maximum 20 mg/m3) even at surge loads of twice
the concentration.
The PU carrier material used in this test for the per-
colation systems 24 and 26 was immersed for pretreatment
into a suspension of specific DC~-degrading micro-
organisms and then inserted into the gas column. ~icro-
organism cultures suitable for this purpose are describedin the literature (see, for example, Rohler, Staub et
al., J. Gen; Mlcrobiol. 132, 283~ to 2843 (1986)).
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