DE2813520C2 - - Google Patents

Description

The invention relates to a method for producing Ammonium nitrite by absorption of a nitrogen oxide, nitrogen dioxide, Containing oxygen and at least 65 vol .-% inert gas Gases with a molar ratio of nitrogen oxide to nitrogen dioxide of at least 2.0 in one on a pressure of 1.12 to 2.70 bar absorption system with a aqueous containing a basic governing ammonium compound Absorbent solution with a normality of at least 0.10 with respect to the basic ammonium compounds.

Such a method is described in US-PS 40 09 246. According to this patent, nitrogen dioxide is exclusively introduced into the absorption system before absorption. The process has the disadvantage that its exhaust gases have a relatively high nitrogen oxide concentration and pollute the environment.

This problem is also not addressed by the procedures US Pat. Nos. 10 61 630, 10 70 070, 27 97 144, 28 05 123, 28 05 122 and 39 32 593 solved.

The object underlying the invention was therefore in increasing the nitrogen oxide concentration in the exhaust gases reduce and improve the ammonium nitrite yield.

This task is carried out in a process with the aforementioned Features solved in that one in the absorption system gas to be absorbed when the absorption of the Nitrogen oxides, based on the nitrogen oxides content in the entering gas, 50 to 99% is done, additional Add nitrogen oxide in such an amount that one Molar ratio of nitrogen dioxide to nitrogen oxide in the resulting Mixture between 1 and 4 gets.

One of the uses of ammonium nitrite is for the production of hyroxylamine compounds. For this can an aqueous solution of ammonium nitrite can be used.  

The reactions of the method according to the invention include gas absorption including physical absorption gaseous components by the solution of the basic Ammonium compound and stripping of volatile components from this solution along with a variety of reactions in the liquid phase and gas phase which are regarded as side reactions can be. Despite the complexity of the reactions can be used to facilitate understanding of the implementation using, for example, ammonium bicarbonate than the basic ammonium compound, that of the absorbent Solution is added, set up the following reaction equations:

Main reactions

• 2 NO + O₂ 2 NO₂N₂O₄NO + NO₂ N₂O₃
  N₂O₃ + H₂O → 2 NHO₂
  HNO₂ + NH₄HCO₃ → NH₄NO₂ + CO₂ + H₂O

Undesirable side reactions

• 5 NO₂ +4 NH₄HCO₃ → 3 NH₄NO₃ + NH₄NO₂ + 2 H₂O + NO + 4 CO₂
  8 NH₃ + 6 NO₂ → 7 N₂ + 12 H₂O
  NH₄NO₂ → N₂ + 2 H₂O

The application of some super atmospheric pressure in the absorption system results in a controlled development of nitrogen dioxide in situ. This improves the ammonium nitrite yield strong, reduces the emission of nitrogen oxides, reduces the loss of ammonium as nitrogen and brings the formation of ammonium nitrate and containing ammonium nitrite Mist in the exhaust gas to a minimum.

The preferred embodiment of the invention can be briefly as follows to be discribed. In an absorption system with an aqueous absorbent solution which is basic contains reacting ammonium compound, an incoming  Gas with a molar ratio of nitrogen to nitrogen dioxide of at least 2.0 and an absorbent solution with a normality of at least 0.6 with respect to the basic Ammonium reactant is used, there is Improvement in that one

• a) the absorption system at a pressure of 1.12 to 2.70 keeps bar and
• b) additional nitrogen dioxide to that in the absorption system gas to be added, the additional Nitrogen dioxide is added to the gas when the absorption the nitrogen oxides 50 to 99%, based on the nitrogen oxides in the entering gas has, and the additional nitrogen dioxide this Sufficient gas is added to a molar ratio from nitrogen dioxide to nitrogen oxide in the resulting Mix between 1 and 4 to give.

The use of controlled superatmospheric pressure in the Absorption system combined with the late introduction of additional nitrogen dioxide reduced in the absorption system air pollution and exhaust gas losses from nitrogen oxides up to 20 times or more compared to conventional ones Methods. It also improves the late addition of nitrogen dioxide to the system also the absorption efficiency, because the optimal molar ratio of nitrogen dioxide to nitric oxide for maximum absorption with decreasing Concentration of nitrogen oxides increases. Such an improvement leads to a significant cost advantage in terms of to the large tonnages of ammonium nitrite that are produced annually used by industry, such as in the manufacture of hydroxylamine compounds. Hydroxylamine compounds are used in large quantities for manufacture of caprolactam via the intermediate cyclohexanone oxime used.

Gaseous streams for the method according to the invention  can be used by catalytic combustion of ammonia-air mixtures by burning Get air with an electric arc. Containing nitrogen oxides Gas flows caused by the catalytic combustion of ammonium-air mixtures are preferred because they are common most economical for effective implementation of the procedure. Especially gaseous streams that come from the combustion of ammonia-air mixtures with about 6 to 10 vol .-% ammonia are preferred for reasons of their Economy and effectiveness. Such gases contain about 6 to 10% by volume at the outlet of the oxidation device Nitrogen oxide combined on an anhydrous and ammonia-free basis with about 8 to 12 vol.% oxygen and 65 to 80 vol.% Nitrogen.

If a gaseous stream, but not nitrogen Contains oxygen, can be used, oxygen from a oxygen-containing gas, conveniently air, in the gaseous Electricity will be introduced.

Although various means can be used to Introducing nitrogen dioxide into the absorption system involves the process in its preferred form that one the nitrogen dioxide continuously directly into the absorption system injected.

The term "basic ammonium compound" as it is used here is said to be one or more of the compounds Ammonium carbonate, ammonium bicarbonate and / or ammonium hydroxide lock in. Those in the process of the invention The alkaline ammonium compound used can conveniently be used in the aqueous solution before its introduction into incorporated the absorption system for ammonium nitrite production will. Preferably the absorbent Solution always a normality of at least 0.6 regarding of the basic ammonium reactants.

Although the selected basic ammonium compound  in the aqueous solution in various concentrations the concentration of the compound can be used in the aqueous solution generally at 1 to 21% by weight and preferably 4 to 20% by weight when ammonium bicarbonate used as the basic ammonium compound becomes. When ammonium carbonate as the basic ammonium compound the concentration is used the compound in the aqueous solution generally at 1 to 30% by weight and preferably 4 to 20% by weight. More than about 21% by weight ammonium bicarbonate or more than about 30% by weight Ammonium carbonate are under normal working conditions due to the solubility limit of these compounds in Water not possible. The highest yields of ammonium nitrite is obtained when the aqueous solution, which is the basic contains reacting ammonium compound at a temperature is kept between about -5 ° C and 35 ° C.

The aqueous absorption solution that enters the absorption system should be fed according to the present invention at a pH of at least 7.5, preferably at 8.0 to 9.0, so as to cause substantial decomposition to avoid the ammonium nitrite product, which is acidic Conditions would occur.

The additional nitrogen dioxide for addition to the absorption system according to the invention can according to any suitable and known method. For example it is conveniently based on the known ammonia oxidation Nitric oxide oxidation method. ammonia becomes catalytic with the formation of nitrogen oxide and water catalytically oxidized. Under normally used conditions are about 94 to 96% of the ammonia in nitrogen oxide converted when the reaction at a very high speed runs. Nitrogen dioxide then becomes oxidized formed by nitrogen oxide (NO). The oxidation takes place spontaneously at ambient temperature, but contrary to the general one Usually in such cases with a relatively slow speed. The reaction differs from general ones  Also regulate by having lower temperatures runs faster. For these reasons, the oxidation of nitrogen oxide usually at a temperature like this is low, how economically viable, and at a relatively long residence time. The implementation is exothermic, and therefore must be cooled to heat the reaction to remove and turn the gas on for nitrogen oxide oxidation to cool the desired temperature. Usually the temperature for the final reaction is 15 to 75 ° C with response times of up to 5 minutes or longer.

The drawing shows a schematic representation the process according to the invention in a continuous manner is explained.

A larger proportion of a gaseous stream, which was produced, for example, by ammonia oxidation and contains nitrogen oxide, nitrogen dioxide and oxygen and has a molar ratio of nitrogen oxide to nitrogen dioxide of at least 2.0, is carried via line 10 into the lower part of the absorption chamber 11 with one or more Packs or packing sections 12, 13 and 14 , which is kept at a pressure of 1.12 to 2.70 bar, introduced. The gaseous stream goes up through the absorption chamber 11 in countercurrent contact with an aqueous solution containing a basic ammonium compound, such as a mixture of ammonium carbonate and ammonium bicarbonate. The aqueous solution is introduced into the upper part of the chamber 11 via line 15 and flows down over the packs or packing elements of the sections 12, 13 and 14 . A portion of the aqueous solution enriched with ammonium nitrite is withdrawn as a recycle stream from the absorption chamber 11 via line 16 and passed through the pump 17 through the cooling apparatus 18 , in which the recycle stream is brought to the desired temperature, for example 5 to 25 ° C. is cooled. The current coming from the cooling apparatus 18 recycle stream is recycled via line 19 and line 15 to the absorption chamber. 11 A smaller proportion of the gaseous stream in line 10 goes via line 20 to the nitrogen oxide oxidation device 21 , which is dimensioned such that it gives an average reaction time of 2 to 5 minutes. Optionally, oxygen or an oxygen-containing gas can be added to the gaseous stream which is fed into the oxidizing device 21 via line 20 . The gaseous stream from line 20 goes up through oxidizer 21 at the desired reaction temperature, such as 45 ° C to 55 ° C, with a substantial portion, such as 80 to 90%, of the nitrogen oxide being oxidized to nitrogen dioxide. The nitrogen oxide oxidizer 21 includes baffles 28 to prevent mixing of the gas at one end of the oxidizer with the other end. The oxidized gas leaves the nitrogen oxide oxidizing device 21 via line 23 and goes to the absorption chamber 11 , where the gas is fed in under the section 13 provided with packings or packing elements. Exhaust gas from chamber 11 is discharged via line 26 . A product stream, which is an aqueous solution enriched with ammonium nitrite and contains remaining basic ammonium compounds, is withdrawn from the process via line 27 .

In the examples below are all parts and percentages Parts by weight and percentages by weight if nothing else is expressly stated.

Example 1 (comparative example) Part 1 (conventional method)

An absorption tower with three sections with packing, packed with porcelain saddles of 2.5 and 5 cm, as in the Drawing shown was at about 11 ° C and at atmospheric pressure charged with an aqueous solution containing 5.5% Contained ammonium bicarbonate and 16.4% ammonium carbonate. The feed was at a flow rate of 516 parts per hour. The solution in the tower became continuous  and one in counterflow with 518 parts per hour gaseous stream with a temperature of 35 ° C and the brought into contact the following volume percent composition: 5.6% water vapor, 79.2% nitrogen, 8.25% nitrogen oxide, 0.92% nitrogen dioxide and 5.96% oxygen. A Part of the aqueous solution enriched with ammonium nitrite was cooled and continuously added to the aqueous feed solution returned. A product stream became continuous of the recycle stream at a flow rate deducted from 598 parts per hour, taking this stream Contained 13.4% ammonium nitrite. Exhaust gas from the absorption tower contained 1.2% by volume of nitrogen oxides.

Part 2 (method according to the invention)

In a second experiment, the above absorption tower kept at a pressure of 1.65 bar and continuously with an aqueous solution of the above composition fed at a flow rate of 516 parts per hour. The solution in the tower was continuous and countercurrent with 466 parts per hour of a gaseous Current of the above composition contacted. How shown in the drawing was approximately 52 parts per hour a gaseous stream of the above composition continuously fed into the nitrogen oxide oxidation device. The gas temperature in the nitrogen oxide oxidizer was kept at 55 ° C, and about 90% of the nitrogen oxide converted into nitrogen dioxide.

The gaseous stream from the nitrogen oxide oxidizer at about 55 ° C was continuously in the absorption tower just below the middle section with packing fed in at which point in the tower the absorption about 95% of the nitrogen oxide, based on the nitrogen oxides, which were fed to the tower below. By this addition of nitrogen dioxide to the absorption tower became the molar ratio of the gas mixture of nitrogen dioxide to nitric oxide increased to 2 to 3. Part of the watery  Solution at the bottom of the tower enriched with ammonium nitrite was cooled to 11 ° C and continuously returned to the aqueous feed solution. A product stream was continuously fed from the recycle stream with a Flow rate of about 600 parts per hour and one Subtracted content of about 14% ammonium nitrite. Exhaust gas off the absorption tower contained about 0.06% by volume of nitrogen oxides. This means a surprising reduction in air pollution compared to the conventional method. Exploit of ammonium nitrite up to 95% of theory and there is little ammonium nitrate and ammonium nitrite containing mist in the exhaust gas.

Surprisingly, the amount of ammonium nitrate ammonium nitrite mist increases in the exhaust gas with increasing pressure, and Such fog is no longer detected at 2.65 bar. Desirably, the pressure in the absorption tower is 1.65 up to 231 bar.

Desirably, the exhaust from the tower was through a conventional venturi scrubber performed before the gas the atmosphere was released. When this occurred, contained the exhaust gas from the scrubber is about 100 to 170 ppm Nitrogen oxides. That used in the venturi scrubber Water (or an aqueous solution that is a basic reagent Contains ammonium compound), can be recycled, a mixture of ammonium nitrite and ammonium nitrate with a molar ratio of nitrite to nitrate of about 0.6 to win.

Example 2 (comparative example)

The procedure of Example 1, Part 1 was repeated, however with the exception that the absorption tower on a Maintained pressure of 1.65 bar instead of atmospheric pressure has been. Surprisingly, the nitrogen oxide content in the exhaust gas from the absorption tower when in use this pressure reduced from 1.65 bar to 0.38 vol .-%. In  little ammonium nitrate-ammonium nitrite mist appeared in the exhaust gas detected.

Example 3 (comparative example)

The procedure of Example 1, Part 2 was repeated, however with the exception that the absorption tower is at atmospheric pressure instead of being kept at 1.65 bar. The exhaust off the absorption tower contained approximately 0.87% by volume of nitrogen oxides plus a substantial amount of ammonium nitrate ammonium nitrite mist.

Regarding pollution with nitrogen oxides in the Exhaust gas shows this example by compiling the values of Examples 1 to 3 that a synergistic improvement from a combination of low positive pressure in the absorption system with later addition of additional quantities Nitric oxide to the absorption system according to the invention results.

table

Absorption system

Claims (3)

1.A process for the production of ammonium nitrite by absorption of a gas containing nitrogen oxide, nitrogen dioxide, oxygen and at least 65% by volume of inert gas with a molar ratio of nitrogen oxide to nitrogen dioxide of at least 2.0 in one at a pressure of 1.12 to 2.70 bar-held absorption system with an aqueous absorption solution containing a basic ammonium compound with a normality of at least 0.10 with respect to the basic ammonium compounds, characterized in that the gas to be absorbed in the absorption system when the absorption of nitrogen oxides, based on the content 50 to 99% of the nitrogen oxides in the incoming gas is added, additional nitrogen oxide is added in such an amount that a molar ratio of nitrogen dioxide to nitrogen oxide in the resulting mixture of between 1 and 4 is obtained. 2. The method according to claim 1, characterized in that ammonium carbonate is used as the basic ammonium compound, Ammonium bicarbonate and / or ammonium hydroxide used. 3. The method according to claim 1 or 2, characterized in that the exhaust gas emerging from the absorption system additionally treated with water in a Venturi gas scrubber.