DE2645019A1 - PROCESS FOR THE PRODUCTION OF SODIUM NITRITE FROM NITROSE-CONTAINING GAS TROEMS - Google Patents

Description

Process for the production of sodium nitrite from gas streams containing nitrous

Various methods are known for removing nitrous, a _{gas mixture containing NO and NO 2.} According to one process, the nitrous oxide is catalytically reduced to nitrogen with the aid of gas burners (eg German Offenlegungsschrift 2 048 492) according to another process using reducing agents such as ammonia (German patent specification 1 545 445). These processes are too energy-intensive and the chemical compounds required for the reduction are relatively expensive. In another known method, the nitrous gases are washed out with water or aqueous alkali metal hydroxide solutions, such as sodium hydroxide solution, but the waste gas problem is shifted to a waste water problem.

The reuse of nitrous gases from exhaust gases for the production of nitric acid succeeds only in very complex processes (WMRamm: Absorption processes in chemical engineering, pages 344-354 VEB Verlag Technik Berlin, 2nd edition 1953). In another known method loc. cit., page 355/356) the nitrous-containing exhaust gas is used for the production of sodium nitrite according to the reaction equation NO + NO ₂ - » / N ₂ ⁰ J ~ ^ ⁺ 2NaOH - $> 2NaNO ₂ + H ₃ O. In this process, appropriate measures must be taken to ensure that the nitrous stream, which in an alkali wash turns into sodium

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nitrite is to be converted, contains exactly equivalent amounts of NO and NO ₂ , since only under this condition practically pure sodium nitrite is formed. The equivalent mixture of NO and NO ₉ is reacted with alkali hydroxide in the absence of oxygen to form sodium nitrite. If there is an excess of NO ₂ , nitrate is formed, while if there is an excess of NO, some of the NO does not react and enters the exhaust gas flow. This known method can therefore only be carried out with relatively uniform gas flows of a defined composition and then only with a high level of technical measurement and control effort. It is unsuitable for air currents containing nitrous.

The object of the present invention is to provide a method for the production of sodium nitrite solution from nitrous-containing gas streams of varying composition.

The present invention thus provides a process for the production of aqueous sodium nitrite solutions with simultaneous removal of NO and / or NO ₉ from gases by contacting with aqueous, basic sodium nitrite solutions with further formation of sodium nitrite, which is characterized in that an NO -containing gas or N0-N0 ₉ -containing gas mixture with oxygen-containing gases and an aqueous, alkali hydroxide-containing sodium nitrite solution mixed intensively, A-MoIe NO with B-moles NO ₉ and C-moles oxygen in the sodium nitrite-containing solution in such a stoichiometric ratio to the reaction

A-B · ■ are brought about that the conditions B <A and C> —j— are met

are.

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There are various embodiments of the method according to the invention. In a particularly simple embodiment, oxygen-free gas streams which already _{contain NO in a stoichiometric excess over NO 2} are mixed with air or with oxygen-enriched air or technical- _{grade oxygen, which, based on the N 2} O formation, is added in excess , mixed. The residence time of the gas streams mixed with oxygen must be so short until the subsequent contact with the aqueous alkali hydroxide-containing solution, preferably sodium or potassium hydroxide-containing solution, that when the gas mixture enters the alkali hydroxide-containing solution there is still an excess of NO over NO " is available. Sufficiently short residence times are generally less than 1 second, preferably less than 1/10 second.

To adjust the alkaline pH in the aqueous alkali hydroxide-containing sodium nitrite solution can also the alkali hydroxides, of course, also others in aqueous form Solution Alkali hydroxide-forming compounds, such as alkali carbonates, are used. The alkali hydroxide concentration of the aqueous solution, which is used to adjust the pH of the absorption solution, is usually at values between 1 to 50, preferably 10 to 25% by weight. The reaction temperatures of the process according to the invention are between about 20 to 100 ° C., preferably between 40 to 80 ° C. The pH of the alkali hydroxide solution should be kept between 7.5 and 13, preferably 8-9.5, during gas scrubbing will.

In another embodiment of the method according to the invention Oxygen-containing nitrous gas flows, e.g. nitrous-containing air flows, are both naturally occurring

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NO ₂ is present in excess of NO, treated. Here it is necessary to first subject the nitrous-containing gas stream to a process in which it is achieved by reduction or cleavage or acidic scrubbing that the gas formed has a molar excess of NO over NO ^ when entering the subsequent alkali scrubbing. If a reductive pretreatment takes place, the reducing agents which can reduce NO to NO can be used for this, for example Na ₃ SO ₃ , NaHSO-, FeSO ₄ .

0 Iron (II) sulfate solution is particularly preferred in the sense of the present invention for a pre-reduction of NO ₃ to NO, because this reducing agent is very inexpensive and can easily be electrolytically regenerated in a known manner (e.g. Gmelin's Handbuch der Anorg. Chem., 8 Ed., Eisen Teil B, System No. 59, page 271 (1932)). - _y

If the NO excess in the output stream is to be broken down by cleavage, this requires, for example, thermal cleavage at temperatures above 400 ° C., which leads to a significant excess of NO over NO ₃ . (Lewis, von Elbe: Combustion, Flames and Explosions of Gases, p. 681, Ac. Press Inc.-New York and London 1961).

Also through the known acidic scrubbing of nitrous gases according to the reaction equation 3NO ₂ + 2H ₂ O 2HNO ₃ + NO, for example described in WM Ramm, loc. cit., pp. 344-354, the stoichiometric excess of NO over NO ₃ necessary for the process according to the invention can be generated. According to the invention, the resulting NO can be converted into nitrite.

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When the nitrous-containing exhaust gases are brought into contact with alkali metal hydroxide solution according to the invention, a sodium nitrite solution is formed which is kept at a pH of 7.5 to 13, preferably 8 to 9.5, with the aid of sodium hydroxide solution. Generally above 9, the nitrate content of the nitrite solution produced falls to values of about 1 to 3 g per liter the nitrite solution, but allows the removal of nitrous from the nitrous-containing exhaust gases without a significant amount of CO _{2 being} bound.

The residence time required for the conversion of nitrous to nitrite in the gas scrubbing process depends on the initial concentration of the reactants NO, NO ₂ and oxygen (see Gmelin's Handbook of Inorganic Chemistry, 8th Edition., Volume 4, page 764- 771). If technical oxygen or oxygen-enriched air is used instead of air, shorter residence times can then be achieved; surprisingly, no increased formation of nitrate is observed in the process according to the invention despite large oxygen surpluses (based on nitrite formation) of the order of magnitude of over 100%.

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The inventive method can be one-step or Carry out in multi-stage washing systems. Bubble columns, water ring pumps, Rotary scrubbers and / or packed columns.

If the gas scrubbing is carried out in several stages, the nitrous scrubbing should preferably be started with a water ring pump, continues it with a bubble column or a rotary washer and ends it with one or more packed columns.

A continuous process for the production of sodium nitrite from nitrous-containing sodium nitrite is shown below with reference to a figure Exhaust gas flows described, which represents a preferred embodiment of the method according to the invention. In the figure, the numbers have the following meaning:

1 supply line for gases containing nitrous;

2+

2 supply line for reducing agent (e.g. Fe -

solution) or water;

3 reduction column or acid scrubbing

4 line for pretreated nitrous gases (containing molar excess of NO over NO ₂ );

5 supply line for gases containing oxygen; '6 gas compressor;

7 line for nitrous gas;

8 bubble column;

9 addition of sodium hydroxide solution; '

10 Drain line for the NaN0 ₂ solution formed

11 discharge line for treated gas;

12 Drainage line for Fe ⁺ or ENT solution.

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The exhaust gases containing nitrous are treated as follows:

Nitrous exhaust gas, the NO2 in excess compared to NO contains, is subjected to a reduction or acid scrubbing in column 3 via line 1, a gas stream being formed, which has a molar excess of NO compared to N0 ". This excess is preferably over 5% held. The gas compressor 6 transports this gas stream via lines 4 and 7 into the bubble column 8, where the absorption of NO, N0 "is carried out using sodium hydroxide solution to form sodium nitrite. About 9 is fresh caustic soda supplied, via 10 sodium nitrite solution, discharged. Treated gas leaves the bubble column via line 11 and can be treated further via line

2+

2 becomes reducing solution (eg Fe) or H _o 0 in a column

3 + -

3 supplied, over 12 resulting Fe - or

• HNO ₃ solution discharged.

Nitrous-containing waste gas, which contains NO compared to NO ₂ in a molar excess, does not need to be pretreated in the pre-reduction zone and can therefore be passed directly over

4 fed to the gas compressor and in it with oxygen-containing Gases added over 5 are mixed. This The mixture is then passed via line 7 to the absorption as in the case described.

In the context of the method according to the invention, preference is given to First of all, an intensive wash with a high exchange effect was carried out, with over 90% of the nitrous

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is absorbed. This is preferably followed by a less intensive wash with the longest possible residence times at. Retention vessels can be used between these washout stages, which allow partial regression of NO-, enable and thus the washing effect of the following Increase the washing system. If the composition of the gas mixture fluctuates greatly, it is necessary to dispense with such retention vessels (so as not to generate an excess of NO ~ compared to NO) and the lingering of the gas mixture can only be transferred to the alkali scrubbing, where permanent Interaction of gas and liquid in each case formed NO, together with excess NO is converted to nitrite.

In a preferred variant of the process according to the invention, oxygen-free nitrous-containing and C0 ₂ -containing gas streams from an acidic scrubbing process with removal of NO ₂ according to the reaction equation 3NO ₂ + 2H ₂ O ^ =? 2HN0., + N0, then the resulting gases are contacted with aqueous alkali hydroxide solution to obtain alkali carbonate or alkali hydrogen carbonate, practically all of the CO2 being removed, followed by the mixing of the residual gas with oxygen-containing gases and the aqueous alkali hydroxide-containing sodium nitrite solution. *

In a further variant of the method according to the invention According to the invention, 90 to 98% of the nitrous can be extracted from the exhaust gas removed and the remaining nitrous gases using another known method for removing nitrous, for example by known reduction with NH.

The method according to the invention is exemplified below Haft explained:

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example 1

_{500 liters per hour of air containing 7000 ppm NO 2} were introduced through a laboratory bubble column with a diameter of 50 mm and a height of 700 mm, which was filled with about 20% strength by weight aqueous iron sulfate solution. 5000 ppm NO and 2000 ppm NO _{2 were} found at the exit of the bubble column (Beckmann Analyzer).

Example 2

200 liters per hour of gas containing about 30% NO ₂ (remainder 40% N ₂ , 10% CO ₂ , 10% O ₂ , 10% N ₂ O) were washed with acid (40% aqueous HNO _n ) and then treated in a stirred bubble column with a diameter of 120 mm and a height of 400 mm with a NaNO ₂ solution at 25 ° C. The pH of the solution was maintained at about 9 with about 5 wt% NaOH solution. About 2.5% NO was found at the gas outlet. In addition to 40 g per liter of NaNO _{2, the} solution contained about 1.3 g per liter of NaNO-.
Example 3

Before entering a stirred bubble column (diameter 120 mm, height 400 mm), 100 liters per hour of nitrous gas containing 60% by volume NO, 5% by volume NO ₃ , 20% by volume CO ₂ , 15% by volume were added .-% N ₃ O, brought together with 25 _{liters of O 3} per hour and then absorbed in the bubble column in an NaNO ₂ solution at 50 ° C. The pH was kept at 8.9 with about 20% strength by weight aqueous NaOH solution. Between 3000 and 4000 ppm NO were measured at the gas outlet. In addition to 200 g per 1 NaNO ₃ , 7 g per 1 NaNO ₃ , the solution also contained about 50 g per 1 sodium carbonate.

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Claims (8)

hl Process for the preparation of aqueous sodium nitrite solutions with simultaneous removal of NO and / or NO ₂ from gases by contacting with aqueous basic sodium nitrite solutions with further formation of sodium nitrite, which is characterized in that a NO-containing gas or NO-NO ₂ -containing gas mixture with oxygen-containing gases and an aqueous, alkali hydroxide-containing sodium nitrite solution intensively mixed, with A-mols of NO being reacted with B-mols of NO ₂ and C-mols of oxygen in the sodium nitrite-containing solution in such a stoichiometric ratio, AB that the conditions B <A and C> - j— are fulfilled. 2. The method according to claim 1, characterized in that when B = 1, A = 1.01, preferably 1.1 to 10, is. 3. The method according to any one of claims 1 to 2, characterized in that AB indicates that C is 1.05, preferably 1.1 to 1.3 - j-, ' . 4. The method according to any one of claims 1 to 3, characterized in, that the aqueous, alkali hydroxide-containing solution is kept at a pH of 7.5-13, preferably 8-9.5 will. 5. The method according to any one of claims 1 to 4, characterized in that that a NO-containing gas immediately before the subsequent contact with the aqueous alkali hydroxide containing solution mixed with oxygen-containing gases. Le A 17 413 - 10 - 809815 / 0Ö55 6. The method according to any one of claims 1 to 5, characterized in that NO-NO ₂ -containing gas mixtures containing a stoichiometric excess of NO ~, reductively, preferably with iron (II) sulfate solution, pretreated until NO is stoichiometric is present in excess over NO ₂ and the gas mixture obtained is contacted with the alkali hydroxide-containing solution in the presence of oxygen-containing gases. 7. The method according to any one of claims 1 to 5, characterized in that oxygen-free nitrous and C0 ₂ ~ -containing gas streams are first subjected to acid scrubbing with removal of NO ₂ , then the resulting gases with aqueous alkali hydroxide solution to obtain alkali carbonate or alkali hydrogen carbonate contacted and then the mixing of the residual gas with oxygen-containing gases and the aqueous, alkali hydroxide-containing sodium nitrite solution carried out. 8. The method according to any one of claims 1 to 7, characterized in that in a first stage over 90% of the NO or N0 ₂ proportion of the alkali hydroxide-containing aqueous sodium nitrite solution are absorbed and at least in a further stage the remaining nitrogen oxides by alkali hydroxide containing sodium nitrite solution are largely removed. Le A 17 413 - 11 - 809815/0055