DE3307087A1 - Process for the elimination of nitrogen oxides from gas mixtures containing them by means of pressure swing adsorption - Google Patents

Abstract

In a process for the elimination of nitrogen oxides from gas mixtures containing them and also, possibly, steam, in particular from the waste gas from nitric acid plants, by means of pressure swing absorption, an improved operating life, reduced energy consumption and/or a reduction in size of the adsorber is achieved by the use of a carbon-containing adsorbent doped with sulphur, cobalt, manganese and/or elements of Group I, preferably potassium. In addition to the employment of a flushing gas, a thermal regeneration at preferably 25 to 120 DEG C is also possible with each pressure swing adsorption cycle or after a plurality of the same.

Description

The invention relates to a method for removing

Nitrogen oxides from gas mixtures containing them by means of pressure swing adsorption.

By developing high quality adsorbents such as activated carbon, It is possible to use carbon-containing molecular sieves or zeolitic molecular sieves become to separate a scene or several gases from gas mixtures by the gas mixture in a kind of plug flow through an elongated one with the adsorbent filled adsorber is passed. Due to the different inclination of different Gases for physical or chemical adsorption on the adsorbent well adsorbable gases mainly in the entrance area of the adsorber and heavier Adsorbable gases mainly in the subsequent adsorber areas on the adsorbent adsorbed, while the worst adsorbable component (s) the adsorber leaves at its exit. This adsorption step will be at the latest then ended when the adsorption capacity of the adsorbent for the or the retaining gas (s) is exhausted. Then the supply of the starting gas mixture stopped and the gas pressure in the adsorber on the inlet or outlet side by decompression and / or evacuate lowered. Here, at least one part of the gas components desorb. If the gas components are still adsorbed on the subsequent renewed The residual load of the adsorbent can have a disruptive effect on the adsorption step can be removed by further reducing the pressure and / or by using a suitable Flushing gas flushes the asorber through to desorption pressure and thereby the Gas components from the adsorbent repressed. After graduation During the desorption phase, the pressure in the adsorber returns to the adsorption pressure raised, which is at the level of the atmospheric pressure or higher.

Such a pressure swing adsorption process has also already been proposed been - as a working principle (without direct implementation) - to the exhaust gas of a plant for the production of nitric acid from nitrogen oxides (Ullmanns Enzyklopadie of technical chemistry, 4th edition, sand 20, pages 305 to 332, 331). There are also several adsorptive processes with exclusively thermal regeneration known so far (e.g. Chem.Eng.Progr. Vol. 72 (1976) No. 8, p. 98 ff). The nitrogen oxide removal by means of However, especially in the presence of water vapor in the starting gas mixture, bridge swing adsorption is problematic insofar as the nitrogen oxides adsorbed in the adsorption step not completely removed from the adsorbent by the above-mentioned measures in the desorption step remove so that the adsorbent experiences gradual contamination and exchanged after a relatively small number of pressure change cycles or one must be subjected to a special regeneration step. Because high quality adsorbent are relatively expensive, there is an interest, with corresponding time, energy and Capital expenditure the adsorbent, z.

@. by heating and rinsing be :. high temperatures or a reactivation, to regenerate again. However, such regeneration steps can also be gradual Do not prevent deterioration in the adsorption capacity of the adsorbent una also lead to a «slow consumption of the adsorbent, its service life behavior m @ thin still clearly to be desired laJit.

Because of the insufficient desorption of adsorbed nitrogen oxides in a normal pressure swing adsorption process has also been suggested to dispense with the tension desorption and instead use a purge gas desorption to be carried out at a relatively high temperature level (see Ullmann loc. cit., page 331 and Dutch patent application 66 07 036). These procedures are but, as already mentioned, very energy-intensive.

Finally, it is known for special applications, e.g. on the Field of wastewater treatment of mixtures containing phenol or for adsorption of SO2-2 and HCl-containing exhaust gases, the adsorption capacity of carbon-containing Adsorbents by doping the adsorbent with sulfur and / or To improve potassium compounds. Sulfur-containing dopings can be passed through Transfer of gas mixtures that contain components such as CS2, S02, H2S or others, Produce via activated carbon at temperatures between 400 and 700 ° C. The amount of doping let us know about the duration of the treatment and about the concentration of the sulphurous Control gases within wide limits. The maximum contents of a sulfur can be up to 30% by weight reach.

Doping with elements of the first main group and other metals such as copper, manganese, cobalt can be made by soaking Akttvkohlen with aqueous Prepare salt puffs, preferably sultate or nitrate solutions.

After the impregnation, at temperatures between ambient temperature and 900 C can be carried out, the activated carbon is dried and then to temperatures which are above the decomposition temperature of the respective salts milssen, brought. It is with a stream of inert gas, preferably nitrogen flows through. The concentration of the doping can be between a few mg / kg Adjust activated charcoal up to approx. 100. g / kg activated charcoal. The same goes for mixtures made of different perforated elements. Such treated adsorbents are so far only for the combined nitrogen oxide and sulfur oxide separation from furnace flue gases been investigated. A moving layer reactor is constantly supplied with fresh or regenerated adsorbent fed in at the top and withdrawn at the bottom, while the gas mixture to be cleaned continuously through the reactor transversely to the direction of the wall. Nox can only be bound adsorptively to a very small extent. In a demonstration facility degrees of NOx removal were below 10% between entry and exit of the moving bed adsorber measured.

Only by adding ammonia can the conditions be the same Reduce WO catalytically on activated carbon. This possibility of catalytic reduction is implemented for some nitric acid plants with noble metal catalysts but presupposes that the exhaust gas has temperatures above 2100 C. Dles is at Nitric acid plants with a medium pressure absorption but not given; lay here in front of the expansion turbine, maximum temperatures of 1800 C.

Activated coke from a plant for flue gas desulphurisation, which is on top is largely fully charged in the manner described, is regenerated thermally at temperatures above 3500 C.

This process has the disadvantage of large abrasion of the adsorbent due to the circulation of the adsorbent. In such processes' will therefore the aim is to achieve the highest possible adsorption capacity of the adsorption material, to decrease the circulatory speed.

It must be accepted that the improved adsorption properties at the same time a deterioration in the desorption properties due to the result in higher affinity between the adsorbent and the adsorbate.

On the other hand, there is one special feature in pressure swing adsorption processes easy desorbability of the adsorbed components must be ensured, otherwise the desorption step takes disproportionately more time compared to the adsorption step, so that of a multitude of adsorbers only one on adsorption and all the rest would have to be switched to desorption, which is an unexpected increase in investment costs would result. For this reason, adsorbents whose properties are the special requirements in moving layer reactors with thermal regeneration are adapted, particularly unfavorable for a pressure swing adsorption process - and vice versa - . Because of the comparatively small system size of pressure swing adsorption layers and the low abrasion of the adsorbent in such systems, these procedures apply but as particularly elegant (see Ullmann, loc. cit.).

The invention is therefore based on the object of a method of to provide the type mentioned at the beginning, the service life behavior of the adsorbent improved.

This is according to the invention by the use of a sulfur, Cobalt, manganese and / or elements of the 1st group. especially potassium-doped carbonaceous Adsorbent reached. This procedure has compared to the known method the advantage that the Adsorptlonsniittel before a thorough regeneration or longer in use before being replaced, and / or the size of the pressure swing adsorption system can be reduced in size compared to conventional systems. In addition, surrender a number of simplifications in the procedure, which are described below will.

The possibility of using the adsorbents known per se according to the invention was extraordinarily surprising for the reasons given above - It is attributed to the fact that the doping of the carbon-containing adsorbent with sulfur, cobalt, manganese and / or an element of the 1st group (main or minor croup of the periodic table), especially potassium in the adsorption of nitrogen oxides not only - as is known - to an improved adsorption capacity but - contrary the general experience - also to the improved desorbability of nitrogen oxides leads to de-aparentation desorption. A sure explanation for this surprising Behavior of carbon adsorbents doped according to the invention has not yet been found.

The method according to the invention can be particularly effective use with gas mixtures that also contain water vapor, especially with the Exhaust gases from plants for nitric acid production * from which they may still be Existing nitric acid mist as a precaution by means of upstream mist separators should be removed so that the adsorbent is not overly adsorbed with Contaminate nitric acid.

In general, it applies to the process according to the invention that with higher Pressure is adsorbed and desorbed at lower pressure. In principle, they can Differences in pressure may be very small, but then there is the enrichment of the deposited Limits are set for the gas in the desorption step.

The adsorption pressure is used in particular for exhaust gas purification on the level of the exhaust gas pressure and adjust the desorption pressure accordingly. For nitric acid plants this means that the adsorption pressure corresponds to the pressure in the Corresponds to absorption towers. Desorption pressure can be used for absorption pressures above 2 bar 1 bar can be selected; this is the pressure with which secondary air is fed into the process will. If the absorption pressure is only 1 bar, then it must be in negative pressure, preferably between 0.05 and 0.5 bar, no additional vacuum pump is desorbed needed).

When flushing the adsorbent to desorption pressure with a Purge gas can also be preheated to improve the purge effect come, e.g.

with air preheated to 25 to 1200 C, in particular at the Removal of nitrogen oxides from the exhaust gases of nitric acid plants. Here is a special one Preference in countercurrent flushing (with regard to the adsorption direction) because so the adsorbent is gradually heated from the adsorber outlet and is in the subsequent adsorption step also gradually cools down again. It will purging ends when this heat front has reached the adsorber inlet. This approach is particularly useful for the treatment of exhaust gases from nitric acid plants advantageous, as this exhaust gas contains NO2, O2 and H2O vapor, which leads to the formation of nitric acid on the adsorbent and a corresponding contamination of the same for Consequence. The inventive method namely a sufficient Desorption reached in the pressure change cycle, although the desorption temperature of e.g. 800 C well below the nitric acid decomposition temperature, namely below of at least 120 ° C.

A combination of the pressure adsorption process according to the invention is particularly elegant with a thermal regeneration after the expiry of several pressure swing adsorption cycles, because, surprisingly, there is also no major energy or plant related Effort must be made, since the regeneration temperature is also in this case is below the decomposition temperature of nitric acid, so that expensive fittings, that can withstand the high regeneration temperatures that have been customary up to now, are omitted and can be replaced by simpler ones.

So far it has been assumed that the regeneration temperatures are significant must be above the temperature of the nitric acid decomposition, as stated in Ullmann a.a.0. and Dutch patent application 66 07 036. - It has in contrast, even shown that the inventive combination of the pressure swing adsorption process with a thermal regeneration, at each or after a plurality of pressure swing adsorption cycles, even then a quite satisfactory reduction in contamination, i.e. Improvement of the service life of the adsorbent if this is not according to the invention is doped, which is especially true for regeneration temperatures below the saltpetre acid decomposition temperature is surprising. Based on this approach becomes, of course, a further significant improvement by doping according to the invention achieved.

The invention is illustrated below with reference to one in the drawing Embodiment explained below.

For the production of nitric acid in absorption towers 1 and 2 are the nitrous gases after a catalytic ammonia oxidation in a compressor 5 compressed with secondary air 3 from about atmospheric pressure to e.g. 3 to 9 bar and after cooling in two heat exchangers 6,7 into the absorption towers 1, 2, from which silver a line 8, the nitric acid is deducted, which via a line 9 abandoned water and from which via a line 10 an exhaust gas at about 2.5 to 9 bar and 20 to 300 C and with a NOx content of 200 to 2000 ppm is withdrawn, The exhaust gas flows through according to the state of the Technique: immediately afterwards the secondary side dea heat exchanger 6 and leaves it at a temperature from 150 to 300 ° C. The exhaust gas then relaxes in one with the compressor 5 in the drive direction coupled expansion turbine 11 and is to the AtmosphEre Delivered via a line 12. - According to the invention, the exhaust gas happens from the Line 1d: first a mist separator 13 and then one of the three adsorbent units Adsorber 14 to 16 to subsequently the secondary side of the heat exchanger 6 happen. The information for that which is composed of about 50:50 parts of NO and NO2 Exhaust gas refers to so-called low pressure adsorption systems for the production of nitric acid; the exhaust gases are basically saturated with water vapor.

Of the adsorbers 14 to 16 there are always two adsorbers in a pressure swing adsorption cycle, while the third adsorber is thermally regenerated is or is in reserve. The adsorption step (in adsorber 14 and accordingly in the other adsorbers) so that the exhaust gas after passing the Nebelabscfleiders 13 under the above-mentioned conditions, the inlet valve is open 17, then the adsorber 14 and finally the opened outlet valve 18, before it is fed to the secondary set of the heat exchanger 6 via a Sawn line 19 will. All other valves relating to the adsorber 14 are closed. Before a breakthrough of nitrogen oxides can be observed at the outlet of the adsorber 14, this is switched to desorption.

The desorption consists of two steps together, The first step is an expansion desorption, in which only the expansion valve 20 is open at the Adsorberauslaßseite and at which the pressure in the adsorber in the secondary air line 3 lowers to about atmospheric pressure; the desorption gases are fed back into the process. After reaching about the atmospheric Pressure begins as a second desorption step, a purge gas desorption, in which Uber a scavenging air line 27 with the scavenging air inlet valves 21 and 22 open and at open expansion valve 20 air is passed through the adsorber 14 and consequently the desorption gases produced in this case are also fed back into the process.

Because of the inevitably low desorption temperature only know Substantial parts of the adsorbed nitrogen oxides but not traces of the Adsorbents forming nitric acid are desorbed in this way.

The rebuilding of the adsorptive pressure is otherwise closed Valves the adsorber 14 by opening the inlet valve 17 and flowing in Exhaust gas reached.

Then, by opening the outlet valve 18, a new Adsorptionsschri start tt.

While the adsorber 14 is switched to adsorption, passes through the adsorber 15 includes the steps of relaxation desorption, purge air desorption and pressure build-up. The adsorbers 14 and 15 run through this Druckweeflselrnythmus until a residual charge (sontamination) has built up in one of the two, which is a thermal Regeneratoon required night. uis at this point was the Adsorber 16 to the thermal regeneration described below for adsorber 14 switched or was in reserve. During this thermal regeneration of the adsorber 14 the above-described Druckwecnseirhythm runs now with the adsorbers 15 and 16 next. A partial gas stream of the exhaust gas at around 1 bar and 800 ° C the line 12 is branched off from this via a line 23 and the adsorber 14 via the opened regeneration valve 24 in countercurrent to the Adsorptionsrichtun, supplied and leaves it together with the ire essentials of nitrogen oxides and Water vapor existing regeneration gases via purge air outlet valves 25/26 to to be fed back to the nitric acid production process via the air line 3. Since this regeneration step takes a long time, the adsorbent becomes zenit warmed up to the temperature of the regeneration gas, so that the desorption the residual charge can run off practically completely. After finishing the thermal in The adsorber 14 can still regenerate for a certain time with the valves open 21, 22 and 20 are cooled before being taken back into the pressure swing cycle and the regeneration step for the adsorber 15 begins while it is running the pressure change rhythm is now carried out via the adsorbers 14 and 16. By the return of the desorption and regeneration gases to the process increases the efficiency the nitric acid plant increased. By returning a partial flow from the exhaust pipe 12 e3n part of the otherwise uselessly dissipated process heat is used.

The use of the Adsorptoonsri, itte1 according to the invention enables an increase in the number of pressure swing cycles before thermal regeneration must be done. what sicyi favorable to the adsorbent consumption or else the Size of the pressure swing adsorption system.

Experimental examples: On an adulteration plant of the one shown in the figure Kind were in the previously described catfish under different conditions and with different adsorbent experiments among those in the following Test conditions shown in the table.

The information in brackets relates to the left column exclusively to example 9; for this example the valves were in part designed as throttle valves instead of pure shut-off valves.

Table: Experimental example No. 1 2 3 4 5 6 7 8 9 adsorption pressure of the nitric acid plant 3 3 6 4 4 3 5 7 1.05 in bar exhaust gas volume flow in 1000 1000 1000 1000 1000 1000 1000 1000 1000 m³ / h in the normal state NOx volume fraction of the 800 1100 900 700 1000 2000 850 800 1100 exhaust gas in ppm exhaust gas temperature in ° C 22 22 22 22 22 22 22 22 22 Adsorber volume in m³ 0.16 0.17 0.095 0.13 0.14 0.26 0.11 0.085 0.48 adsorber diameter at 0.5 0.5 0.4 0.5 0.5 0.6 0.4 0.4 0.80 m bed height of the Adsorbent layer 0.81 0.87 0.76 0.66 0.71 0.92 0.88 0.68 0.97 in m inner Surface of the adsorbent in 800 800 800 760 760 740 700 720 740 m² / g Brunauer, Emmett and Teller pressure build-up time from about 1 (0.1) bar to 5 5 5 4 4 4 4 4 2 Adsorption pressure in min.

Experimental example No. 1 2 3 4 5 6 7 8 9 adsorption time to 40 40 40 30 30 25 30 30 25 Adsorption pressure in min.

Duration of relaxation desorption (evacuation) 2 2 3 2 2 2 2.5 3 5 from adsorption pressure to approx. 1 (0.1) bar in min.

Duration of desorption rinsing at approx. 1 (0.1) bar 33 33 32 24 24 19 23.5 23 18 in min.

NO content in the exhaust gas 130 165 140 120 160 330 115 110 210 ter dem Adsorber in ppm NO2 content in the exhaust gas - <10 <10 <10 <10 <10 <10 <10 <10 <10 ter the adsorber in ppm Number of pressure changes 18 18 20 22 22 16 27 25 16 cycles of a thermal regeneration Amount of regeneration 40 40 38 45 45 80 37 38 150 gas in m³ / h temperature of the regenerator 90 90 90 90 90 90 90 90 90 tion gas in ° C Duration of the regeneration 19 19 20 17 17 10 21 20 10 gas application in h Experimental Example No. 1 2 3 4 5 6 7 8 9 amount of cooling air in 100 100 100 100 100 150 100 100 250 m³ / h Duration of the cooling air 5 5 6.67 5 5 3.33 6 5 3.33 turn in h Impregnation of the adsorbent S / 0.25 S / 0.35 K / 0.083 K / 0.42 K / 0.42 Cu / 0.32 S / 1.7 Na / 0.26 K / 0.42 with S, K, Cu or Na in Wt% The adsorbents were made in the following manner and impregnated: S-doping: CS2-N2 mixture flows through activated carbon reactor for 2 h at 550 ° C.

Cu or K doping: aqueous solution of potassium sulfate is used for impregnation of activated charcoal at 300 C for 4 hours. Then drying at 170 ° C in inert gas and thermal treatment at 6000 ° C. for 2 hours in a stream of nitrogen.

The following can be seen from the figures given in the experimental examples: i trose gases from nitric acid absorption systems can be under different 3 conditions are cleaned, whereby the absorption pressures in ranges of about 1 up to 7 bar and the NOx concentration between 700 and 2000 ppm. These design values do not give the limits of the NOx removal process rather, they indicate the range of conditions as they are after nitric acid absorption systems are present, which are operated partially normal or medium pressure.

Examples 1, 2 and 7 demonstrate the suitability of sulfur-doped activated carbons. Example 1 and 2 show how the design works with a variable NOx inlet concentration and low doping changes. Example 7 shows that at higher adsorption pressure and similar NOx entry concentrations, the adsorber volume is greatly reduced can be.

Examples 3, 4 and 5 demonstrate the suitability of activated carbons doped with potassium at different adsorption pressures and NOx loads.

Example 6 and 8 show that doping with copper and sodium are suitable.

Compared to processes with exclusively thermal regeneration essentially the following advantages; The adsorber volumes are smaller, since the regeneration times be: the pressure swing adsorption considerably smaller than are in thermal regeneration.

The energy expenditure is lower for several reasons: In comparison to zeolites, which are also used, which are very hydrophilic, is regeneration easier, since less water vapors have to be desorbed. Compared to less hydrophilic Adsorbents have the advantage of less frequent thermal regeneration and in the comparatively low regeneration temperature.

Claims (16)

"Process for removing nitrogen oxides from them containing Gas mixtures by means of pressure swing adsorption "Claims: 0 Removal method of nitrogen oxides from gas mixtures containing them by means of pressure swing adsorption, characterized by the use of one with sulfur, cobalt, manganese and / or Group I elements, preferably potassium, doped carbonaceous adsorbent. 2. The method according to claim 1, characterized in that the gas mixture also contains water vapor. 3. The method according to claim 1 or 2, characterized by the use one at elevated temperature with sulfur-containing compounds (gases) such as CS2, SO, or H23 reacted activated carbon. 4. The method according to one or more of claims 1 to 3, marked by using one with a salt of an element of the first group of the Periodic table, especially activated carbon soaked with potassium salt. 5. The method according to one or more of claims 1 to 4, characterized characterized in that gas mixtures containing nitric acid mist prior to the adsorption step Be passed through a mist separator. 6. The method according to one or more of claims 1 to 5, characterized characterized that the pressure in the adsorber is reduced during the desorption and that Adsorbent is purged with a purge gas at desorption pressure. 7. The method according to claim 6, d a d u r c h g e k e n n z e i c h n e t that the purge gas is air. 8. The method according to claim; «. or 7, characterized in that the purge gas is preheated. 9. The method according to claim 8, d a d u r c h g e k e n n z e i c h n e t that the preheating temperature is between 25 and 1200C. 10. The method according to claim 8 or 9, characterized in that the purge gas in the opposite direction to the direction of adsorption over the adsorbent is directed. 11. The method according to one or more of claims 1 to 10, characterized characterized in that the adsorbent after several pressure swing adsorption cycles is thermally regenerated. 12. The method according to one or more of claims 1 to 11, characterized through the use of at least three adsorbers, one of which is alternately loaded, one is desorbed under pressure reduction and one is thermally regenerated. 12. The method according to one or more of claims 1 to 12, characterized characterized in that the gas mixture is the exhaust gas of a plant for nitric acid production is. 14. The method according to claim lo, characterized in that the desorption gases from pressure swing adsorption and thermal regeneration (secondary) for dLe nitric acid production are supplied. 15. The method according to claim 13 or 14, d a d u r c h g e k e n n z e i c hnet that air and / or exhaust gas in the bypass for the nitric acid production as flushing gas is passed through the adsorber. 16. The method according to one or more of claims 13 to 15, characterized characterized in that a partial flow during a thermal regeneration step the exhaust gas of the plant for the production of nitric urine through the adsorber and then is fed into the (secondary) air flow for nitric acid production.