PROCESS FOR THE REMOVAL OF NITROGEN OXIDES (NX OY) FROM WASTE GASES OF AMMONIA OXIDATION PLANTS

Abstract: This invention relates to a process for the remotion of the nitrogen oxides contained in the waste gases coming from the catalytic oxidation plants of ammonia. The process includes the absorption of the gases in a mixture of sulphuric and nitric acid. The process allows to obtain the double advantage to eliminate the dangerous nitrogen oxides pollution and to recovery a very useful product, the nitrosyl hydrogen sulphate.

Description: The combustion between ammonia and air at 850°C in presence of catalysts gives rise to the formation of nitrogen oxide. The absorption in suitable liquid phases of such gases gives rise to known and important industrial products. So, i.e., the absorption of N₂O₅ in H₂O gives rise to HNO 3; the absorption of an equimolar mixture, NO +NO₂ =N₂O₃ in H₂SO₄ carries to the nitrosyl hydrogen sulphate (NOHSO₄). The plants for the production of such compounds, however, after the stage of absorption in the liquid phase, present a waste gas which still contains 0.1-0.5% by volume of nitrogen oxide which is discharged into the atmospheric environment. Such nitrogen oxides are toxic both for the acute and cronic intoxication; besides they cause acid rains with related environmental problems. The presence of the nitrogen oxides gives a coloring from yellow to ruddy to the waste gas according to their starting concentration and to the dilution in the air. Even if the simple elimination of the waste gas coloring, obtained through dilution with air, as happened in the past, reduces the danger of the gas becouse of the reduction of the nitrogen oxides concentration, it does not eliminate its presence in the atmosphere. In the literature numerous remotion methods of the nitrogen oxides are mentioned such as: - Oxides combustion with air on Pt catalysts -Pd (Anderson, H.C. et al. - Ind. & Eng. Chem. pp 199-204, March (1961)). -Catalytic reduction to N2 (Anderson, H.C. et al. - Ind. & Eng. Chem .pp 199-204, March (1961)). - Remotion by washing with caustic soda or lime milk (Sheigh, H.R.L.; Can. J. Chem. Eng. 3.36, (1958)). - Absorption in aqueous liquid phase after treatment with various chemical agents such as for example H202: (Belg. BE. 888902 (1981)), 03 (Gen. Off. 3.233.316 (1984)), NACIO (Ohr, Powietrza 1981, 15(3), 63-7 (Pol); Chem. Ab. 222518, 96, 1982)). The mentioned methods usually require complex and expensive plants and in some cases dangerous and without any recovery possibility of the nitrogen oxides (combustion, catalytic reduction) and generating on the contrary the continuous consumption of reagents. It is known from Ullmans Encyclopadie der techniscen Chemie 15, pag. 49 (1964) that the nitrogen oxides of NO + NO₂ in the molar ratio 1:1 (=N₂O₃) can be absorbed in H₂SO₄ to give NOHSO₄ according to the following relation: I) N₂O₃ + 2H₂SO₄-------- 2NOHSO₄ +H₂O. In the due kinds N₂O₃ and NOHSO₄ nitrogen is in its formal oxidation state N³⁺: the mentioned reaction is therefore formally a simple salifying reaction which happens after the absorption of the gas N₂O₃ in the liquid H₂SO₄. In case that oleum is used (SO₃ in H₂SO₄) the formation of H2O can be avoided and the said salifying reaction becomes: II) N₂O₃ + H₂SO₄ + SO₃ --------- 2NOHSO₄ In these reactions the compound NxOy is formed by the equimolar combination of the nitrogen oxides NO + NO₂ (NO + NO₂ ----- N₂O₃). However, in case of a generic mixture NO +NO₂ with molar ratio different from 1, as in the case of the N x O y present in the waste gas of the ammonia oxidation plants, the absorption in H₂SO₄ and the transformation of the oxides in NOHSO₄ remains limited to the formal share of N₂O₃ presents in the mixture. Actually it is found that the global reduction of the N x O y does not exceed the 25 -30% of the starting oxides. In fact in case of generic mixture of NxOy formed by: NO NO₂ N₂ /DL>N₂O₄ in indefinite and variable relations, such as in the waste gases of the ammonia oxidation plants, nitrogen is not in a constant formal oxidation state and such to allow a specific salifying reaction such as in I) and II). An aim of the present invention is therefore that to obtain a complete remotion of the nitrogen oxides resolving at its roots the problem of pollution, with a process that does not require expensive plants in their installation and in their mainteining. Another aim is that to allow the recovery of the nitrogen oxides giving rise to a useful recyce product. The Applicant has surprisingly found a process based on the absorption in liquid phase of easy accomplishment, and of very high efficiency with about 90% recovery of the N x O y . Such process is based on the absorption of the N x O y in a liquid phase composed of a mixture including H₂SO₄ eventually partially substituted with oleum, and HNO₃. An object of the present invention is therefore a process for the remotion of the NxOy nitrogen oxides from the waste gases of catalytic oxidation plants of ammonia characterized by the fact that said nitrogen oxides are absorbed in a liquid phase formed by a mixture comprising at least (a) sulphuric acid, eventually partially substituted by oleum and (b) nitric acid. Such absorption is conveniently carried out at a temperature comprised between 10° and 90°C and preferably between 20° and 70°C. The component (b) of the mixture is preferably used in a concentration in the liquid phase outlet lower than 3% in weight and preferably comprised between 0.05% and 0.2%, the rest being formed by component (a). The absorption is carried out in countercurrent to make easy the contact between the gas containing the nitrogen oxides and the liquid phase, and it is preferably conducted in a filling-column in which the gas enters from the bottom, while the liquid is fed by an suitable distribution system according to the known methods used for such operations in the chemical industry. Further to the remotion carried out according to this invention, a sulphuric solution of nitrosulphuric acid (NOHSO₄) is obtained, which is the base reagent for the nitrosation of numerous organic compounds, i.e. nitrosation of the exahydrobenzoyc acid to give at the end caprolactam (US Patent No. 3356675). For concentration of the liquid phase in outlet it is understood the concentration of the nitric acid in the exaust liquid phase which is drawn from the nitrogen oxides remotion plant. It is, in fact, possible to analyse such concentration of the liquid phase in outlet and, according to the results of such analyses, to control the adding of HNO₃ or the drawing of a part of the exaust liquid phase, according to the cases. Therefore, from an applicative point of view, the data about the concentration of the nitric acid in the liquid phase in outlet are sufficient to consequently determinate, the other parameters of the process and, particularly, the amount of nitric acid in inlet. Surprisingly, the presence of HNO₃ in the sulphuric acid increases the absorption at values of at least 90%. Other components which can be present in the liquid phase without being indispensable are: H₂O 0-10% by weight SO₃ 0-30% by weight H₂O is partially formed by the chemical processes of absorption, partially it may be already present in the liquid phase depending if anhydrous acids or the common industrial products such as HNO₃ 65% and H₂SO₄ 96% are used. As already indicated instead of H₂SO₄ oleum can be used: the presence of SO₃ in such cases removes the water from the system. In this last case, the product recovered from the absorption column is composed of a anhydrous solution of NOHSO₄. Genera lly the end recovered product is formed in this way: NOHSO₄ 2.5-55% HNO₃ residue =0.05% H₂O 0.0-10% SO₃ 0.0-10% H₂SO₄ remaining up to 100 To better illustrate the invention, but without limiting it, the following examples are given, in which the titration of the N x O y is carried out with K MnO₄. Fig. 1 shows the scheme of a plant which can be used for carrying out the process according to the invention. With reference to Fig. 1, a waste gas flow 1 comes from an ammonia oxidation plant, such waste gas containes a nitrogen oxides concentration which is variable both regarding the type and the amount of oxides. Flow 1, controlled by valve 2, is feeded at the base of the absorption filling column 3. The gases pass upwards through column 3 and are discharged purifiedin the atmosphere, through ejector 4 fed by steam 5. Column 3 is fed on the top with the liquid phase 6 which goes out at the bottom and is stored in tank 7. The liquid phase 6 is therefore heated on heater 8 and recycled on top through pump 8. Valve 10 allows the regulation of the flow of the liquid phase 6. Through valve 12 the exaust liquid phase 6 is drawn, when the concentration of nitric acid, measured i.e. in tank 7, is in the preferred range. When, further to the analysis of the nitric acid concentration in the liquid phase in outlet of the column 3, it happens that such concentration falls to too low levels, it is possible to feed a solution containing fresh nitric acid by valve 11. The process can work discontinuously, in this case the starting liquid phase 6 containes a high concentration of nitric acid, for example 5%. Such liquid phase, at high concentration, is continuously recycled by pump 9 carrying out the nitrogen oxides remotion, without making any drawing through valve 12. During the prosecution of the nitrogen oxide absorption, the nitric acid concentration gradually falls till reaching the wanted concentration for example 0.5%. At that point the exaust liquid phase 6 is drawn and replaced by fresh liquid. The exaust phase contains a high NOHSO₄ concentration. The process can work continously, in this case the exaust phase 6 is drawn through valve 12, such exaust phase 6 is analysed to determinate the nitric acid concentration, while, through valve 11, a fresh nitric acid solution is continuously fed, in such amount to mantain the nitric acid concentration in the liquid phase 6 at the wanted concentration. Example 1 shows a comparison example in which the absorption phase is conducted in absence of nitric acid. Example 2 to 7 show a first embodiment of the invention in which the process is carried out discontinuously. Example 8 show a second embodiment of the invention in which the process is carried out continuously. COMPARISON EXAMPLE 1 (Absorption liquid: H₂SO₄) In a 1,850 mm high absorption column, with a 200 mm diameter, filled of rashing rings up to 1,000 mm height, 45 H₂SO₄ Kg, contained in a tank linked with the bottom of said column, are mantained in recycle at the speed of 1,500 1/h through pump. From the bottom of the column 31.4 mc/h waste gas coming from an ammonia oxidation plant are fed continously, such gas in inlet in the column contains 7,334 ppm of NxOy nitric oxides. The outlet gas on top of the column containing 5,476 ppm of nitric oxides. The calculated remotion is equal to 25.3%. EXAMPLES 2 - 6 (Absorption liquid:HNO₃ in H₂SO₄ in various concentration) In a 1,850 mm high absorption column, with a 200 mm diameter filled of rasching rings up to 1,000 mm height, 45 Kg of a mixture containing HNO₃ in the per cents indicated in Table 1 are maintained in recycle at the speed of 1,500 1/h through pump, the rest being sulphuric acid. From the bottom of the column 31.4 mc/h waste gas coming from an ammonia oxidation plant are fed, such gas in inlet in the column contains an amount of oxides as indicated in Table I. The gas in outlet on top of the column contains an amount of nitric oxides as indicated in Table 1. The remotion, calculated as (N x O y inlet -N x O y outlet)/N x O y inlet, is indicated in Table 1. The absorption is continued till the concentration of the circulating nitric acid does not fall to the level indicated in the last column of Table 1. Then the exaust liquid phase is drawn. Considering in particular the Example 6, NOHSO₄ crystalline precipitation due to the starting high concentration of HNO₃ and consequently to the final high concentration of NOHSO₄ were noted. EXAMPLE 7 (Absorption liquid: HNO₃ in oleum) In a 1,850 mm high absorption column, with a 200 mm diameter filled of rasching rings up to 1,000 mm height, 45 Kg of a mixture containing 5% of HNO₃, 25% of free SO₃ the rest being sulphuric acid are maintained in recycle at the speed of 1,500 1/h through pump. From the bottom of the column 31.4 mc/h waste gas coming from an ammonia oxidation plant are fed, such gas in inlet in the column containes an amount of oxides indicated in Table 1. The remotion, calculated as (N x O y inlet -N x O y outlet)/N x O y inlet, is indicated in Table 1. EXAMPLE 8 (Absorption in liquid phase composed of HNO₃ in H₂SO₄ with continuous feeding of waste gas and absorption liquid) In a 1,850 mm high absorption column, with a 200 mm diameter filled of rasching rings up to 1,000 mm height, 45 Kg of a solution containing 10% of NOHSO₄ dissolved in H₂SO₄: are maintained in recycle at the speedy of 1,500 1/h through pump: in such starting solution nitric acid is completely absent. From the bottom of the column 31.4 mc/h waste gas coming from an ammonia oxidation plant containing 7,200 ppm of N x O y are fed. On top of the column, mixed together with the recycle liquid, see scheme 1, 15.5 Kg/h of a solution at 2% of HNO₃ in H₂SO₄ (tot. fed 100% HNO₃ = 310 g/h) were fed; comtemporaneously, from the tank containing the hold-up at bottom column, 15.5 kg/h were drawn in order to maintain constant and equal to 45 total Kg the recycled liquid in the column. The analysis of the drawn liquid gave: - NOHSO₄ : 11.1% therefore about 1,720 g /h of nitrosylsulphuric acid were totally produced - HNO₃ : 0.07% therefore 10.8 g/h out of the 310 g/h eliminated, remain not reacted. The gas on the top of the column contains an amount of nitrogen oxides equal to 590 ppm with a remotion efficiency of 91.8%. As far as the NHOSO₄ production in the exaust liquid phase is concerned, it has been generally founded the following relation between the initial concentration of the nitric acid in the mixture and the concentration of the nitrosylsulphuric acid in solution after the complete HNO₃ exhaustion: For handling facility of the NOHSO₄ solutions even at low temperatures (10 - 30°C) without involving phenomenons of crystalline separations it is convenient (but not obligatory) to maintain the NOHSO₄ concentration below 30% during the discontinuous operation; consequently it is advisable that the nitric acid is lower than 5% in the starting charge phase of the discontinuos process. It has been therefore verified that in practice even very low concentrations of nitric a cid allow to obtain nitrogen oxides remotion in order of 90%, see in particular the example 8 carried out with a concentration of 0.07%. The practicability of the process with so low nitric acid concentration clearly allows to obtain a useful product NOHSO₄ having a low concentration of HNO₃ both with a continuos process which a continuously maintains a low concentration of HNO₃ and with a discontinuos process which, even using high starting concentration of gives rise to the consumption of such high starting concentration and allows equally at the end of the process to obtain low concentration of HNO₃. The fact that the NOHSO₄ contains low HNO₃ concentrations is of basic importance for the further uses of NOHSO₄ as nitrosation agent, which must be sufficiently pure. On the other hand a direct NOHSO₄ purification to eliminate the HNO₃ is particularly complex and expensive.

Claims: 1. Process for the remotion of NxOy nitrogen oxides contained in the waste gases of the ammonia catalytic oxidation plants characterized in that such nitrogen oxides are absorbed in a liquid phase formed by a mixture including at least: a) sulphuric acid b) nitric acid 2. Process according to Claim 1, characterized by the fact that said mixture includes also oleum. 3. Process according to Claims 1 and 2, characterized by the fact that said nitric acid is present in said mixture in the outlet liquid phase in a concentration by weight lower than 3% 4. Process according to Claim 3, characterized by the fact that said nitric acid is preferably comprised in such mixture in the outlet liquid phase in a concentration by weight comprised between 0.05% and 0.2%. 5. Process according to any one of the previous claims characterized by the fact that it is carried out at a temperature comprised between 10C and 90°C. 6. Process according to any one of the previous claims characterized by the fact that it is carried out at a temperature comprised between 20° and 70°C. 7. Process according to Claim 1 characterized in that it is carried out discontinuosly, the starting HNO₃ concentration on the total of the liquid phase being lower than 5% by weight. 8. Process for the nitrogen oxides remotion according to what has been described and illustrated.

Inventors: DAL MORO, ANACLETO, SCALTRITTI, GIOVANNI
Applicants: CHIMICA DEL FRIULI S.P.A.
Application year: 1987
Main classification: B01D53
IPC codes: B01D53, C01B21