Process for the separation of air components, such as difficultly absorbable air impurities, out of air-gas mixtures.

0001The invention relates to a process for the separation of air components, such as difficultly absorbable air impurities, out of air-gas mixtures by ringing the mixture together with absorbents and to apparatuses for carrying out this process.

0002Air-gas mixtures with air components such for example as nitrous gases, hydrogen chloride, sulphur dioxide, hydrogen sulphide, mercaptans, amines were hitherto usually brought together with the absorbents, for example caustic soda, caustic soda - sodium hypochlorite, water or hydrogen peroxide, in absorption installations with plate columns, packed columns, spray washers or Venturi washers. The degree of separation achieved with such installations, in dependence upon the nature of the separated medium and the selected form of installation, was poor, adequate, but sometimes even good. With the spray or Venturi washers for nitrogen dioxide and nitrogen triooxide it was possible to achieve degrees of separation in the order of magnitude of 30 to 70 %; however higher degrees of separation up to 80 % were achievable only when using very expensive installations, for example 10-storey plate columns or packed columns with packings of 4 to 6 m. Very low degrees of separation resulted in the case of nitrogen oxide; when using caustic soda or water as absorbent they lay below 10 % and could not be increased beyond 30 % even in the case of caustic soda - sodium hypochlorite and hydrogen peroxide.

0003The invention is based upon the problem, dispensing with the use of expensive plate columns or packed columns, of achieving significantly higher degrees of separation with relatively low expense and at the same time also separating other difficultly absorbable air constituents, for example soluble sublimates such as ammonium chloride, ammonium bifluoride, indium chloride and zinc chloride, with good degrees of separation. Even difficultly absorbable aerosols of oils, fats and plasticisers, which are emulsifiable in the aqueous phase, can be separated with high degrees of separation. Sulphuric acid aerosols and sulphur trioxide, which ordinarily are absorbed in high-percentage sulphuric acid, are to be absorbable with water.

0004The problem is solved in that air with one or more of the above-listed constituents is brought together, in passage through one or more packs of stratified filter mats of chemical-proof unwoven fabrics with the absorbents, as for example caustic soda, sodium hypochlorite, hydrogen peroxide or water. The packs of =filter mats are here compressed to.' ₁0 to 50 % or more of their initial volume in dependence upon the concentration of the substances to be separated in the air-gas mixture. The metering of the quantities of absorption medium to be fed to the air-gas mixture takes place, as proposed by the invention, by regulation of pH or oxidation-reduction or by colorimetric measurement. It is possible in accordance with the invention, in the case of feed of hydrogen peroxide to a mixture of nitric acid separated with nitrile gases, by the use of hydrogen peroxide solutions of high concentration, to separate out nitric acid in a concentration suitable for re-use in the initial process.

0005The apparatus for carrying out the process in a reaction tower with feed of the air-gas mixture from above or from beneath consists in accordance with the invention of spray nozzles for the absorbent arranged above a pack of filter mats arranged in the tower cross-section, a collecting vessel arranged beneath the pack of filter mats with overflow and discharge to a circulating pump, and an inwardly or outwardly blowing fan arranged above or below the pack of filter mats in an opening of the tower wall and, in the case of an inwardly blowing fan, a discharge pipe leading through the pack of filtermats out of the tower. The apparatus can also consist of two packs of filter mats arranged with spacing one above the other and a second collecting vessel arranged between the two packs and connected with a further circulating pump to the collecting vessel.

0006The invention will be explained in greater detail by reference to the examples of embodiment illustrated in the drawing and the experimental results achievable therewith. In the drawing: - <ul id="ul0001" list-style="none"><li>Fig. 1 shows an absorption installation with reaction tower, in diagrammatic representation,</li><li>Fig. 2 shows another form of embodiment and</li><li>Fig. 3 shows a further form of embodiment of the installation.</li></ul>

0007As may be seen from Figure 1, the filter mat pack 2 formed from layers of unwoven fabrics is arranged in the reaction tower 1 beneath the inlet pipe 4 for the air-gas mixture which enters the tower in the direction of the arrow marked 3. Above the filter mat pack 2 in the tower a spray nozzle 5 is arranged from which the absorbent issues and charges the upper side of the filter mat pack 2. The spray nozzle 5 is connected through a conduit 6 with the collecting vessel 7 situated at the bottom of the tower 1 at a distance below the filter mat pack 2. In the conduit 6 a circulating pump 8 is arranged. The collecting vessel 7 has a collecting hopper 9. Between this hopper 9 and the under side of the filter mat pack 2 and aperture 1b is provided in the wall 1a of the tower 1, which aperture is joined by a droplet separator 10 which leads by way of a conduit indicated by 11 to the fan 12. Between the droplet separator 10 and the collecting vessel 7 a discharge conduit 13 is arranged. The overflow of the collecting vessel 7 is marked by 14. The absorbent medium is fed from a reservoir 15 through conduits 16 and 17 by means of the pump 18 to the conduit 6 and the spray nozzle 5. Regulation of feed is controlled by the measuring and regulating unit 19.

0008The air-gas mixture passes in the direction of the arrow 3 into the reaction tower 1 and is brought together with the absorbent in the filter mat pack 2, which is impregnated with the absorbent sprayed on through the nozzle 5. The combined material produced from bringing together, after it has left the filter mat pack, travels by way of the collecting hopper 9 into the collecting vessel 7 while the air and slight liquid constituents still contained therein depart from the reaction tower 1 through the aperture 1b. The liquid residues are precipitated in the droplet separator and pass likewise through the conduit 13 into the collecting vessel, while the finally purified air is blown away through the conduit 11 by the fan 12.

0009In the embodiment according to Figure 2, two filter mat packs 2a and 2b are provided arranged one above the other with spacing. Between these a further collecting vessel 7a is arranged which is connected through a conduit 21 with the collecting vessel 7 arranged at the bottom of the reaction tower 1-r From the collecting vessel 7 a second conduit 6a leads to the spray nozzle 5a arranged above the upper filter mat pack 2a.

0010Otherwise the installation has the same features as that illustrated in Figure 1.

0011In the embodiment according to Figure 3 the filter mat pack21 is likewise arranged in the reaction tower 1 beneath the inlet nozzle, here marked by 23, for the air-gas mixture which is blown into the tower in the direction of the arrow 24 by the fan 20. Above the filter mat pack 21 here several spray nozzles 5a and 5b are arranged which are likewise in communication through a conduit 6 and a circulating pump 8 with the collecting vessel 7 arranged at the bottom of the tower 1 at a distance below the filter mat pack 21. The collecting vessel 7 is provided with a level regulator 26; it is fed through a conduit 16 with fresh absorbent media and further has an outlet conduit 27. A withdrawal pipe 22, which leads out of the tower 1 in the direction of the arrow 25, is conducted centrally through the filter mat pack 21.

0012The air-gas mixture introduced into the tower 1 by the fan 20 in the direction of the arrow 24 is impregnated with the sprayed-on absorbent medium in the filter mat pack 21 and the occurring combination, after it has left the filter mat pack 21, passes as in the already described apparatuses into the collecting vessel 7, while the air and slight liquid constituents still contained therein depart from the reaction tower 1 through the withdrawal pipe 22 in the direction of the arrow 25.

0013In the operation of the apparatus for the process as described, as shown by the following comparisons in Table form, with the various kinds of absorbent media significantly higher degrees of separation were achieved in each case. In the case of caustic soda, caustic soda - hypochlorite and water, hydrogen peroxide these usually lie at the highest achievable limit of 90 to 99.9 %. In the separation of nitrogen oxide using hydrogen peroxide as absorbent, the increase of the degree of separation in comparison with the results achievable with other installations is especially distinct, since here it was possible to achieve 90 to 95 %, compared with 30 % and less. These results in the use of hydrogen peroxide are of especially great economic significance because the recovery of nitric acid, saving neutralisation and decontamination costs for oxidation of the formed nitrite, produced in the case of absorption of nitrogen dioxide a 44 % nitric acid, in the absorption of nitrogen trioxide a 26 % nitric acid and in the absorption of nitrogen oxide even a 17 % nitric acid. When using still more highly concentrated hydrogen peroxide solutions it was also possible to produce nitric acids of correspondingly higher percentage.

Comparison Table I

0014Degrees of separation of nitrogen dioxide N0₂

0015<tables id="tabl0001" num="0001"><img file="EP0065584A1_D0001.tif" /></tables>

Comparison Table II

0016Degrees of separation of nitrogen trioxide N₂0₃ or NO₂/NO in molar ratio 1:1 <tables id="tabl0002" num="0002"><img file="EP0065584A1_D0002.tif" /></tables>

Comparision Table III

0017Degrees of separation of nitrogen oxide NO

0018<tables id="tabl0003" num="0003"><img file="EP0065584A1_D0003.tif" /></tables>For the absorption of various air constituents with hydrogen peroxide or caustic soda, with a sprinkling density of 20 cu.m/sq.m. x h the packing height of the filter mats in the case of SO₂ was compressed from 0.3 to 0.15 m., for SO₃ from 1.80 to 0.60 m. and for ammonium chloride and other sublimates form 0.60 to 0.30 m.