Method of removing no_x and n₂o from nitric acid production residual gas

FIELD: inorganic compounds technologies.

SUBSTANCE: invention aims at reducing concentration of NO_x and N₂O in residual gas and provides a method wherein residual gas escaping absorption column, prior to enter turbine, is passed through two consecutive steps first reducing NO_x content by catalytic reduction and then reducing N₂O content by decomposing it into nitrogen and oxygen on catalyst containing one or more iron-loaded zeolites at working pressure 4-12 bar. Molar ratio NO_x/N₂O in residual gas before second step lies within a range of 0.001 to 0.5.

EFFECT: enhanced process efficiency.

10 cl, 2 dwg, 1 tbl, 2 ex

The present invention relates to a method for removal of NO_xand N₂O from the residual gas of the nitric acid production.

When industrial production of nitric acid, HNO₃catalytic combustion of ammonia is formed exhaust gas containing nitric monoxide NO, nitrogen dioxide NO₂(denoted together as NO_x), as well as laughing gas N₂O. whereas NO and NO₂has long been known as compounds with Ecotoxic effects (acid rain, smog formation) and are installed worldwide, limiting values for maximum permissible emissions, in recent years, laughing gas is also in an increasing volume moved into the center of attention of environmentalists because it is light weight contributes to stratospheric ozone depletion and the greenhouse effect.

After the reduction of emissions of laughing gas adipic acid production nitric acid represents the largest source of industrial emissions of laughing gas. Therefore, from the point of view of environmental protection, there is an urgent need for technical solutions to reduce emissions of laughing gas with emissions of NO_xduring the manufacture of nitric acid.

To remove the NO_xfrom the exhaust gas of the nitric acid production there are numerous vari the options method (denoted here as stage DeNO _x), such as chemical leaching, adsorption or catalytic reduction. An overview is given in Ullmann''s Encyclopedia of Industrial Chemistry, volume 17, VCH Weinheim (1991) (Dl). It has been noted that selective catalytic reduction (SCR) of NO_xammonia to N₂and H₂Oh, which proceeds with the catalyst at temperatures of from about 150°With up to 450°and provides a decomposition of NO_xmore than 90%. This is the most used option to reduce NO_xduring the manufacture of nitric acid, but he, like usual options, does not reduce the content of N₂O.

Thus, according to the prior art required a special second catalytic stage, which is appropriately combined with the stage DeNO_x.

This position is, for example, the basis described in the application US-A-5200162 method, which requires the inclusion of the decomposition of N₂O contains NO_xthe exhaust gas after stage DeNO_x. Moreover, at least part of the flow of exhaust gas that leaves the stage of decomposition of N₂O, cool and return at this stage, to reduce overheating this stage due to the exothermic decomposition of N₂O. the Invention relates to a flue gas with a content of N₂O up to 35%, for example, also to the flue gas in the production of adipic acid.

Represented Shell JV the property describes the simultaneous removal of NO _xand N₂O from the residual gas of the nitric acid production (Clark, D.M.; Maaskant, O. L.; Crocker, M., The Shell DeNO_xLuximoti offer you for sale: A novel and cost effective NO_xremoval technology as applied in nitric acid manufacture and associated processes, presented at Nitrogen '97, in Geneva, 9-11 February 1997, (D2)).

The system of the reactor Shell is based on the principle of the so-called reactor cross-currents, and phase DeNO_xworks already at relatively low temperatures (up to 120°). For the removal of N₂O apply amorphous catalyst based on a metal oxide.

When placing the respective catalysts in the residual gas, which leaves the absorption column at a temperature of 20-30°S, the interval of possible operating temperatures specified operating temperature of the turbine residual gas.

Turbine residual gas from a technical and economic point of view the whole process should work in the most economical temperature at the inlet <550°and possibly higher ΔT and ΔR.

In particular, it is important for the removal of N₂O, since in the prior art explicitly required a higher temperature than the catalytic reduction of NO_x. The profitability of such a choice is associated with increased activity of the catalyst.

A review of numerous catalysts, the fundamental suitability for the decomposition and recovery laughing g is for confirmed, shown in Kapteijn F.; Rodriguez-Mirasol, J.; Moulijn, J.A., Appl. Cat. B: Environmental 9 (1996) 25-64, (D3).

As especially suitable for the decomposition of N₂O additional area represented zeolite catalysts subjected to the exchange of metal ions (patent US-A-5171533).

Used zeolites were obtained by ion exchange in an aqueous solution containing metal salts. For ion exchange were used metal salts from the group of copper, cobalt, rhodium, iridium, ruthenium or palladium. Copper zeolites are very sensitive to water vapor and rapidly lose activity (M.; Sandoval, VH; Schwieger, W.; Tissler, A.; Turek, T., Chemie Ingenieur Technik 70 (1998) 878-882, (D5)), while others listed here metals are relatively cheap.

With a loaded iron-type zeolite Fe-ZSM5 in appropriate conditions, which are described in table 1 in the application US-A-5171533, in the absence of NO_xH₂O and O₂at 450°achieved With only 20%decomposition of N₂O.

In the case of Fe-ZSM-5 activity in the decomposition of N₂O in the presence of appropriate amounts of NO, however, markedly increased, which leads to reactions with the formation of NO₂under NO+N₂O→N₂+NO₂catalyzed by Fe-ZSM-5 (Kapteijn F.; Marban, G.; Rodrigeuez-Mirasol, J.; Moulijn, J.A., Journal of Catalysis 167 (1997) 256-265, (D6); Kapteijn F.; Mul, G.; Marban, G.; Rodrigeuez-Mirasol, J.; Moulijn, J.A., Studies in Surface Science and Catalysis 101 (1996) 641-650, (D7)).

In the absence of NO_xfor zeolites, replaced the s Cu or Co, set a higher activity than the corresponding Fe-zeolites.

In the prior art methods (reference D6, D7) for the decomposition of N₂O in the presence of a catalyst Fe-ZSM-5 at 400°usually used With equimolar amounts of NO and N₂O. According to D6 and D7, the impact of NO_xon the decomposition of N₂O with decreasing ratio of NO/N₂O constantly decreasing, so that the ratio of NO/N₂O below 0.5 decomposition of N₂O more is not satisfactory.

The best results were observed when the molar ratio of NO/N₂O, is equal to 1 or greater than 1.

When using such a catalyst for recovery of N₂O in the exhaust gas of the nitric acid production according to the authors formed NO₂could be returned to the process for production of HNO₃. Concentrations of NO_xand N₂O in the exhaust gas was in a different way about 1000 nm

Iron-containing zeolites on the basis of ferrierite for recovery of N₂O-containing gases are the subject of the application WO 99/34901. Used catalysts contain 80-90% ferrierite, as well as additional binder components. The proportion of water in the recovered gas is in the range from 0.5 to 5%. Compared to zeolites of different types of zeolite FER (ferrierite) type decomposition of N₂O when the temperature of the 375 to 400° To give the best results (97% decomposition of N₂O at 375° and the ratio of NO/N₂O=1). Significantly less degradation was observed after application of zeolites of type pentasil (MFI) or mordenite (MOR). In the case of Fe-MFI zeolites of the above conditions could be achieved maximal decomposition of N₂O only in the amount of 62%.

In view of the prior art there is a problem, in particular, for the production of HNO₃, developing cost-effective way, which along with the high decomposition of NO_xalso provides sufficient decomposition of N₂O.

In particular, should be achieved good results in the decomposition of N₂O when the ratio of NO_x/N₂O below stoichiometric, in particular with respect to <0.5, and preferably <0,1, which takes place after reduction of NO_x.

The problem is solved by the invention, which relates to a method of reducing the concentration of NO_xand N₂O in the tail gas of nitric acid production, while leaving the absorber column residual gas prior to its entry into the turbine residual gas is passed through the combination of two stages; the first stage reduces the content of NO_x(DeNO_x-stage and the second stage reduces the content of N₂O in Gaza (DeN₂O-phase); the ratio of the NO /N₂O before entering the gas in the second stage is in the range from 0.001 to 0.5, preferably in the range from 0.001 to 0.2, especially in the range from 0.01 to 0.1, and the gas in the second stage are in contact with the catalyst, which contains mainly one or more loaded iron zeolites.

Used according to the invention, the catalysts contain basically, preferably >50 wt.%, in particular >70 wt.% one or more loaded iron zeolites. So, for example, along with the zeolite Fe-ZSM-5 is used according to the invention the catalyst may contain other iron-containing zeolite, such as, for example, iron-containing zeolite of the MFI type or MOR. Moreover used according to the invention the catalyst may contain other well-known specialist additives, such as binders.

Catalysts introduced on stage DeN₂O, preferably based on zeolites, in which the iron entered by solid state ion exchange. Usually come from commercially available ammonium zeolites (e.g., NH₄-ZSM-5) and the corresponding salts of iron (for example, FeSO₄·7H₂O) and intensively stirred them with each other mechanically in a ball mill at room temperature (Tutek and others; Appl. Catal. 184 (1999) 249-256; application EP-A-0955080). These literary sources are listed here as references. The floor is built powder catalyst then calicivirus in a chamber furnace in air at temperatures in the range from 400 to 600° C. After calcination Fe-zeolite intensively washed with distilled water and after otfilrovanna zeolite is dried. Thereafter, thus obtained Fe-containing zeolite is mixed with a suitable binder, mix and ekstragiruyut, for example, to obtain cylindrical pellets of catalyst. As a binder applicable to all commonly used binders, the most commonly used are aluminium silicates, such as kaolin.

According to the present invention are used zeolites loaded with iron. Moreover, the iron content per mass of zeolite up to 25%, but preferably from 0.1 to 10%. In particular, suitable zeolites of type MFI, BETA, FER, MOR and/or MEL. Precise instructions for the composition and structure of these zeolites are listed in the Atlas of Zeolithe Structure Types, Elsevier, 4th edition, 1996, which is here referred to. The preferred zeolites are zeolites of the MFI type (pentacel) or MOR (marginal). In particular, the preferred zeolites of the type Fe-ZSM-5.

According to the present invention, DeN₂O-catalysts in combination with the proposed DeNO_xthe stage is thus located between the absorption column and the turbine residual gas to leaving the absorber column residual gas was directed first at a temperature of <400°With, in particular <350°With, in the reactor (the first stage), in which sod is neigh NO _xin Gaza was reduced to, for example, <100 nm (cf. figure 2). Working pressure at this first stage is preferably from 1 to 15 bar, in particular from 4 to 12 bar.

Previously used stage DeNO_xmatches usually used in the prior art method of reducing the emissions of NO_xin devices for nitric acid. The content of NO_xin the residual gas must be, however, is still high enough could be effective socialisticheski NO effect or NO₂in subsequent DeN₂O-stage.

When implementing DeN₂O-stage without prior DeNO_x, that is, when one input stream with approximately equimolar amounts of NO and N₂O return NO₂formed by the reaction NO+N₂O→N₂+NO₂in HNO₃the process is inefficient because of the relatively low concentrations of NO₂<2000 CNM

The content of N₂O in the gases leaving the stage DeNO_xthat does not change substantially. So the gas after the first stage is usually characterized by a content of NO_xfrom 1 to 200 nm, preferably from 1 to 100 nm, in particular from 1 to 50 nm, and the proportion of N₂O equal to from 200 to 2000 nm, preferably from 500 to 1500 nm After stage DeNO_xthe ratio of NO_x/N₂O is from 0.001 to 0.5, preferably from 0.001 to 0.2, in particular from 0.01 to 0.1. The water content of the gases to the to the absorption column at the stage DeNO _xand after DeN₂O-stage is typically in the range from 0.05 to 1%, preferably in the range from 0.1 to 0.8%, in particular in the range from 0.1 to 0.5%.

Air-conditioned so the exhaust gas is fed to the subsequent stage DeN₂O, where through the use of socialisticheskogo effect NO_xin the presence of the corresponding zeolite catalyst spend decomposition of N₂O N₂and O₂.

Unexpectedly, it was found that in the presence of applied according to the invention of iron-containing zeolite catalysts for the decomposition of N₂O greatly increased even in the presence of minor amounts of NO_x, that is, when the molar ratio of NO_x/N₂O <0,5 (cf. figure 1). The effect that significantly increases with increased temperature. Thus, according to the present invention, for example, at 450°With a molar ratio of NO_x/N₂O 0,01 enough yet in the presence of Fe-ZSM-5 catalyst to reduce the concentration of N₂O from 72% to 33%. This is all the more surprising that in the prior art rapid decomposition of N₂O leads to the already mentioned stoichiometric interaction of N₂O with NO. NO_xat sufficient temperature and low ratio of NO_x/N₂O plays the role of homogeneous socializaton, which accelerates the decomposition of N₂O according to N₂O→N_{2 +1/2 O2. With a ratio of NOx/N2O previously named the boundaries of the possible maximum decomposition of N2O on subsequent DeN2O-stage. As soon as this ratio falls below 0,001, reduces the decomposition of N2O to unsatisfactory values (see example 5). After stage-DeN2O the content of N2O the proposed method is in the range from 0 to 200 nm, preferably in the range from 0 to 100 nm, in particular in the range from 0 to 50 nm}

Moreover, the operating temperature stage DeN₂O define, in particular, the desired degree of decomposition of N₂O and the number of NO_xcontained in the residual gas, and, as is well known to specialists in this field, and as almost all of the catalytic purification of gases, it largely depends on the loading of the catalyst, i.e. the number of catalyst on the exhaust gas flow. Preferably the operating temperature of the second stage is in the range from 300 to 550°With, in particular in the range from 350 to 500°at a pressure in the range from 1 to 15 bar, in particular from 4 to 12 bar. With increasing pressure increases Socialisticheskaya action NO_xon the decomposition of N₂O, so that by increasing pressure it is possible to further reduce the operating temperature.

Moreover, when calculating or setting the working temperature of the factors you should take into account the presence of oxygen and H ₂O that in the production method and variants of the method of production of nitric acid may vary within known boundaries and exert an inhibitory effect on the conversion of N₂O. the Contents Of₂is in the range from 1 to 5 vol.%, in particular in the range from 1.5 to 4%vol.

With iron-containing zeolite catalysts, ispolzuemye according to the invention at temperatures in the range from 300 to 550°C, preferably from 350 to 500°able to achieve decomposition of N₂O>90%, in particular >95%. At higher temperatures it is possible to achieve sufficient decomposition of N₂O when the ratio of NO_x/N₂O 0,01.

Predlagaemy method allows combinations of DeNO_x-stage and DeN₂O-stage lower the levels of NO_xand N₂O residual gases during the manufacture of nitric acid to a minimum. Due to the location DeNO_x-the stage before DeN₂O-stage and between the absorption column and the turbine residual gas method according to the invention is very economical due to the increasing temperature profile.

Additionally, the processing at the location of both stages before decompression turbine particularly preferably, both stages can be carried out under pressure (each, depending on the options the production of HNO₃between the 4 and 11 bars), hence effectively reducing the required volumes of the reactor or catalyst.

By conducting DeNO_x-phase at relatively low temperatures in excess of that guaranteed a significant reduction in the content of NO_xalso when you start the device, which requires only a small heat process.

An additional advantage of placing both stages between the absorption column and the turbine residual gas while increasing the temperature profile is that the residual gas originating from the proposed combination, may be sent directly to the turbine residual gas without pre-cooling and without additional measures for cleaning gas for optimum return compression and heat.

Examples:

Stage-DeNO_x:

Pre-DeN₂O-catalyst as DeNO_xthe catalyst was introduced classical SCR catalyst based on the V₂O₅-WO₃-/TiO₂(cf. G. Erti, H. Knozinger, J. Weitkamp: Handbook of Heterogeneous Catalysis, volume 4, pages 1633-1668), as described, with the use of NH₃as the reductant. The catalyst was operated at a temperature of 350°C. depending on the entered number of NH₃at the exit from the stage DeNO_xreceived a different content of NO_xand, therefore, the ratio of NO_x/N₂O

Stage-DeN₂O:

Receiving iron-containing MFI catalyst was carried out by solid-phase ion exchange on the basis of commercially available zeolite in the ammonium form (ALSI-PENTA, SM27). Details of the preparation can be taken from: M. Rauscher, K. Kesore, R. Monnig, W. Schwieger, A. Tissler, I. Turek, Appl. Catal. 184 (1999) 249-256.

The powder catalyst was caliciviral on the air for 6 hours at 823 K, washed and dried overnight at 383 K. After addition of the appropriate binder was followed by extrusion to obtain cylindrical pellets of catalyst (2×2 mm).

The experiments were carried out in stationary operating a flow device with a systematic analysis in flow rate every time 10000 h^-1.

Part of the boot was:

By varying the added quantities of NH₃can be obtained the following final concentrations of NO_xand N₂O:

As follows from the above examples, it is possible a higher degree of decomposition of N₂O up to a ratio of NO_x/N₂O 0.001, in particular of 0.01. If the ratio is reduced to such boundary values, sufficient decomposition is no longer guaranteed because of the lack of sufficient socialisticheskogo of NO_x.

1. A method of reducing the concentration of NO_xand N₂O in the tail gas of nitric acid production, while leaving the absorber column residual gas before the turbine inlet residual gas is passed through a combination of the two stages and the first with the adiya's lower content of NO _xby catalytic reduction, and in the second stage lower content of N₂O in the gas by decomposition into nitrogen and oxygen, the molar ratio of NO_x/N₂O before entering the gas in the second stage is in the range from 0.001 to 0.5, and the gas in the second stage in contact with a catalyst which contains one or more loaded iron zeolites, and the working pressure in the second stage is from 4 to 12 bar.

2. The method according to claim 1, characterized in that the catalyst or catalysts contain loaded with iron zeolites types MFI, BETA, FER, MOR and/or MEL.

3. The method according to claim 2, characterized in that the zeolite or zeolites loaded with iron, are zeolites of the MFI type.

4. The method according to claim 3, characterized in that the zeolite is Fe-ZSM-5.

5. The method, at least one of the preceding paragraphs, characterized in that the temperature in the first stage is <400°C, preferably <350°C.

6. The method, at least one of the preceding paragraphs, characterized in that the temperature of the second stage is in the range from 300 to 550°C, preferably in the range from 350 to 500°C.

7. The method, at least one of the preceding paragraphs, characterized in that the two stage carried out under pressure in the range from 4 to 12 bar.

8. The method, at least one of the pre is striding points characterized in that the first stage is performed by the method of selective catalytic reduction.

9. The method, at least one of the preceding paragraphs, characterized in that enter into the process of the residual gas after leaving the absorption column and the entrance to the first or second stage, the water content in the gas is in the range from 0.05 to 1%, in particular in the range from 0.1 to 0.8 vol.%.

10. The method, at least one of the preceding items, wherein introducing the residual process gas before entering the second stage, and the content of NO_xin the gas is in the range from 1 to 200 nm and the content of N₂O in the range from 200 to 2000 nm