SK281805B6 - Method for preparation nitric acid from solutions of nitrogen oxides in nitric acid - Google Patents

Abstract

The minimum nitric acid content of the solutions treated according to this solution is 60 wt. % HNO3, wherein the concentration of nitrogen oxides in these solutions is at least 10 wt. %. Nitrogen dioxide is predominantly present in the solutions treated in this way in the sense of this solution. Furthermore, for this process of reprocessing nitrogen oxides in nitric acid, it is further characterized that at least 0.15 and at most 0.65 parts by weight of hydrogen peroxide (such as 100% H 2 O 2) are present per weight part of nitrogen oxides in the reaction mixture, preferably 0.3%. up to 0.45 parts by weight of H2O2. The liberated reaction heat is removed from the process by cooling, and it is preferable to use cooling with the circulating reaction mixture. The process can be carried out in both batch and continuous processes

Description

Technical field

The invention relates to the production of nitric acid from solutions of nitrogen oxides in nitric acid and to an apparatus for carrying out this production method.

BACKGROUND OF THE INVENTION

In connection with the disposal of some special nitrogen products of chemical technology as well as materials belonging to areas of special types of human activity, there was an urgent social requirement to create an effective and environmentally harmless process of disposal of nitrogen oxide solutions in nitric acid.

The extremely strong oxidation properties of such concentrated solutions, which usually contain more than 70 wt. % nitric acid and at least 20 wt. % of nitrogen oxides, with the predominant presence of nitrogen dioxide (NO ₂ ), were also used as an effective oxidative component of rocket engines using liquid fuel for their propulsion a few years ago. Solutions of the subject type are commonly referred to in the chemical terminology as NITROLEUM. Generally, they are produced by absorbing nitrogen dioxide in concentrated nitric acid at a temperature in the region of -10 ° C, and this absorption is generally carried out at elevated pressure. Thus, NITROLEUM can be prepared whose physicochemical properties depend on the amount of nitrogen dioxide dissolved in the acid. When the concentration of nitrogen dioxide in the acid is close to 20 wt. %, NITROLEUM is characterized by high phase stability, which is important in terms of its use in most of its practical applications. The minimum equilibrium phase temperature of the binary system in question: HNO ₃ - N ₂ O ₄ , ie the highest "frost-resistance" of the solution - 73 ° C corresponds to an 18% nitrogen dioxide content in the acid. As the concentration of nitrogen dioxide in the binary HNO ₃ - N ₂ O _{4 system} increases, the phase stability of this system decreases relatively sharply, reaching a minimum level at 77 wt. % nitrogen dioxide in solution when the phase transition temperature is + 56 ° C.

Similarly, the boiling point of solutions in the binary system depends on the amount of nitrogen dioxide absorbed, while at normal atmospheric pressure the boiling point of NITROLEA drops sharply from about 88 ° C at zero nitrogen dissolved in acid to near 24 ° C, which corresponds to about 50 wt. % N ₂ O ₄ in acid, further increasing the concentration of the oxide up to a value slightly higher than 90 wt. %, the normal boiling point of the binary system is practically unchanged.

Already from the chemical composition of NITROLEA, it is a highly corrosive substance with a strong oxidizing effect on substances with which it comes into contact, which is extremely aggressive against all commonly used construction materials. Moreover, for solutions of nitrogen dioxide in nitric acid, even under normal conditions, a high partial pressure of strongly toxic nitrogen oxides, the so-called " nitrous gases. An essential component of these red-brown gases, which are released from the liquid phase without heating the solutions, is the monomeric form of nitrogen dioxide (NO ₂ ).

In considering the possibilities of effectively disposing of nitrogen dioxide solutions in nitric acid, the possibility of reprocessing to nitric acid of the usual commercial concentration (38-52% by weight of HNO ₃ ) seems to be the most appropriate.

However, the treatment of NITROLEA with normal dilution with water encounters in practice a number of difficulties which arise in particular from the fact that the reaction of nitrogen dioxide with water does not directly produce nitric acid, but only produces an equimolar mixture of nitric and nitric acids according to the reaction scheme:

NO ₂ + H ₂ O HNO _ä + HNOj Δ H = - 27,73 kcal, resp. N ₂ O ₄ + H ₂ O -> HNO ₃ + HNO ₂ Δ H = -14.13 kcal.

The resulting nitrous acid is unstable and decomposes almost immediately into nitric acid and nitrous oxide according to the reaction scheme:

HNO ₂ - HNO ₃ + 2 NO + H ₂ O Δ H = + 18.13 kcal.

The resulting nitric oxide must first be oxidized to nitrogen dioxide and only this can be converted to the desired nitric acid by subsequent absorption. In this regard, it should also be noted that the kinetics of the oxidation of nitric oxide (NO) to nitrogen dioxide (NO ₂ ) indicate that this reaction is only relatively slow. The rate of this oxidation decreases with increasing temperature. For this reason, it is appropriate to conduct the oxidation at the lowest possible temperature. Pressure has a favorable effect on the conversion of nitric oxide to nitrogen dioxide, since oxidation reduces the number of moles. However, this increases the demands on the machinery and equipment used, increases operating costs and generally complicates the technological process of disposal. They are also a source of unpleasant - toxic exhalation of nitrogen oxides.

Thus, when dissolving nitrogen dioxide in water, a maximum of two thirds of the oxide is converted into nitric acid, even under ideal reaction conditions, and the remaining one third which has been converted to nitric oxide must be subsequently oxidized again.

In principle, the present processing of NITROLEA into nitric acid could be utilized by existing production machinery and technological equipment used for the production of concentrated (usually 95-99%), or so-called. weak (47 - 70%) nitric acid direct production process.

When called. direct processes for the production of concentrated HNO ₃ are carried out by first condensing a portion of the nitrogen oxides and then processing them in autoclaves (50 atp, 70-85 ° C) to the acid of the desired concentration (usually 95 to 99 wt% HNO ₃ ) . However, the use of these processes to dispose of N ₂ O ₄ solutions usually prevents the presence of a variety of impurities often found in the solutions to be reprocessed (phosphates, fluorides and the like).

All these problems are solved in a simple and elegant manner by means of the process of processing the solutions of nitrogen oxides in nitric acid in the sense of this solution. It is typical of processes for the production of nitric acid from solutions of nitrogen oxides in nitric acid that the solution of nitrogen oxides in nitric acid (NITROLEUM) is subjected to a direct chemical reaction with an aqueous solution of hydrogen peroxide (H ₂ O ₂ ).

The basis of this method of nitric acid production, respectively. Thus, the process for the reprocessing of nitrogen dioxide in nitric acid is as follows:

NO ₂ + H ₂ O ₂ —► 2 HNO ₃

SK 281805-6

Under suitably selected and adhered reaction conditions using this process, no undesirable nitrous gases are produced, the only reaction product of such a process being the desired nitric acid.

Abroad, the related chemical process is used to capture nitrogen exhalations, mainly from the end offgases of nitric acid production. Methods of capturing nitrogen emissions of this type are described, for example, in earlier patents: Japanese Patent No. 5,916,549; 48-15766 (1973), U.S. Pat. No. 3,991,167 (1976) as well as EU patent no. 0 008 488 (Downey, G. Deborah, " Process of removing nitrogen oxides front gaseous mixtures "). All patented technologies operate in a two-phase system: liquid-gas, with a concentration of absorbed nitrous gases in the purified gas mixture of at most 500 ppm (500 mg / kg, corresponding to a concentration of 0.05 wt.% NO _x ). The absorbent is relatively dilute nitric acid containing 17-58 mass. % HNO ₃ (preferably 40-55 wt.% HNO ₃ ). According to the cited European patent, the peroxide content of the sorbent used is low (usually 0.4 to 2.0% by weight). The capture of nitrous gases in the sense of the cited solutions takes place in two absorption stages, with regard to the concentration ratios of the reactants used, the reaction temperature at contact of the gaseous with the liquid reactant is low (up to 60 ° C). 35 ° C).

SUMMARY OF THE INVENTION

The basis of the process for producing nitric acid from solutions of nitrogen oxides in nitric acid containing at least 60 mass. % HNO ₃ and at least 10 wt. % of the nitrogen oxides, with the predominance of nitrogen dioxide in the sense of this solution, is the chemical reaction of these solutions with an aqueous hydrogen peroxide solution. For this direct reprocessing process of nitrogen oxides solutions in nitric acid, which can also be used in the efficient reprocessing of NITROLEA, 0.15 to 0.65 parts by weight of hydrogen peroxide (H ₂₎ are added per part by weight of the nitrogen oxides fed into the process together with nitric acid. 0.1 to 0.45 parts by weight of peroxide (such as 100% peroxide). For technological and operational reasons, it is expedient if the concentration of hydrogen peroxide fed to the process is greater than 2 and less than 30 mass. %, and it is particularly preferred that the concentration of the aqueous solution of this reactant is maintained in the range of 8 to 14 percent by weight of H2O2;

Due to the high concentration of nitrous gases in such treated solutions, the reaction with peroxide has a distinctly exothermic character, resulting in a significant heating of the reaction mixture. If the reaction is to be maintained in desirable proportions, it is necessary to remove the released heat of reaction from the process by cooling. The exothermic nature of the reaction should be borne in mind when selecting the feed rate of the reactants. Especially in the continuous guided process for the processing of nitrogen oxides solutions in nitric acid (NITROLEA), it is advisable to remove the heat of reaction by cooling the circulating reaction mixture and also by adding water to the process in order to adjust the concentration of nitric acid to be prepared. In this way, it is possible to ensure the required level of operational reliability, even at higher levels of feed into the process of dosing reactants, and also to prevent unwanted formation of nitrous gases.

Simultaneously with the reaction in progress, or anytime after its partial or complete termination, the nitric acid concentration prepared by the process according to the invention is adjusted by the addition of water, respectively. dilute nitric acid to the desired concentration.

The process for producing nitric acid from solutions of nitrogen oxides in nitric acid, according to this solution, can be used in operational manufacturing practice in both batch (batch) and continuous mode of operation. Due to the extremely corrosive aggressiveness of the treated solutions of nitrogen oxides in nitric acid (NITROLEA), it is advantageous to carry out the reaction in a continuous or at least semi-continuous operating mode.

In the context of the chemical technology research of the method of the present invention, it has been shown experimentally that for the operational use of the process for producing nitric acid from solutions of nitrogen oxides in nitric acid in the sense of this solution, it is advantageous to use a device consisting of film reactor 1. it can be advantageously designed in the form of a vertically situated column with a trickle packing.

It is expedient to select the feeds of the reactants in such a way that liquid contact to the dosing system is effected in the lower part of the film reactor 1.1, i.e., in the form of a liquid. j. a solution of nitrogen oxides in nitric acid B with a mixture of nitric acid and unreacted peroxide, which flows from the top of the film reactor 1.2. In the upper part of the film reactor 1.2, in particular, the liberated nitrogen oxides react with the metered fresh hydrogen peroxide solution C.

The bottom of the film reactor 1 flows into the collecting part of the device 2, which serves to homogenize, react and possibly adjust the concentration of nitric acid produced.

Optionally, at least one heat exchanger-cooler 3 is also part of the apparatus assembly. This may form part of the film reactor 1 and / or may be incorporated into an optionally used circulation path 4 between the collection part of the device 2 and the used film reactor 1 (Figures 1 and 2) ).

The packing of the used film reactor 1 may be organized and / or unorganized, formed, for example, by classical Raschig rings, by means of mesh or cap trays, or also in the form of turbogrid trays with internal cooling.

For the removal of the heat of reaction, it has proved to be particularly suitable to use the system of so-called. by direct cooling through the circulating reaction mixture, respectively. acid, or a combination of such direct cooling with a so-called. by internal cooling incorporated into the reaction zone of the film reactor being used, for example by incorporating a tubular, plate or cross-drilled heat exchanger-cooler (Figures 3 and 4).

The nitric acid produced by the process can still contain some small amounts of dissolved nitrogen oxides. These can be removed from the product, for example. by acid denitration, i. j. by blowing the acid through the air in the packed column, at a slightly elevated temperature (50-60 ° C).

DETAILED DESCRIPTION OF THE INVENTION

The following examples illustrate, supplement and illustrate but do not limit the claims of the present invention.

Example 1

A sulphonation flask equipped with a two-blade stirrer, a thermometer and a 1500 ml dropping funnel was charged with 258.6 g of waste for reprocessing - a solution of nitrogen oxides in concentrated nitric acid containing an average of 74.6 wt. % HNO ₃ and 21.65 wt. % of nitrogen oxides, expressed as nitrogen dioxide (NO ₂ ). The treated liquid waste (NITROLEUM) was clear, reddish-brown in color, while a layer of dark brown-red vapors of nitrogen oxides (mainly nitrogen dioxide) was maintained above the liquid phase.

While stirring, 182.3 g of an approximately 11.4% aqueous hydrogen peroxide solution were fed over a 58 minute dropping funnel. Due to the exothermic nature of the reaction, the temperature of the reaction mixture increased from the initial temperature of 21 ° C to 56.2 ° C during dosing of the peroxide solution.

About 440 g of clear, light green-green nitric acid having a specific gravity of 1.373 g were prepared according to the present invention. cm ^-3 (about 61 wt% HNO ₃ ).

Example 2

The solution of nitrogen oxides in nitric acid used during this laboratory experiment had a chemical composition similar to that described in Example 1.

The procedure was also carried out using a 1500 ml sulfonation flask equipped with a two-blade all-glass stirrer and a thermometer (0-100 ° C).

The flask was charged with 301 g of a treated solution of nitrogen oxides in nitric acid of the NITROLEUM type, which, in addition to the aforementioned content of nitrogen and concentrated nitric acid, still contained approximately 1.2 wt. % trihydrogenphosphoric acid (H ₃ PO ₄ ) and about 0.5 wt. % hydrofluoric acid (HF).

During this laboratory experiment, the feed of the diluted hydrogen peroxide solution (ca. 11 wt% H ₂ O ₂ ) to the reaction system was carried out through an all-glass packing column (column height ca. 150 mm), in a dropping funnel.

A total of 212.1 g of an approximately 11% aqueous hydrogen peroxide solution was fed over 54 minutes. The peroxide dosing rate was controlled so that the level of red-brown nitrogen dioxide vapor was maintained at about half the height of the glass filler column used. During peroxide dosing in this way, the highest temperature in the reaction medium stabilized at the interface level of the red-brown vapors in the packing column (60-80 ° C).

In this way, about 513 g of pale yellow, weak greenish nitric acid having a specific gravity density at room temperature of 1.363 g were obtained. cm ' ³ .

Example 3

Assembly of apparatus used for laboratory verification of nitric acid production containing about 52 wt. % Of HNO ₃ , from solutions of nitrogen oxides in nitric acid, so-called. NITROLEA, in the sense of this solution, consisted of: a 1500 ml sulfonation flask equipped with a glass stirrer and a thermometer (0-100 ° C), into which a glass-filled split absorption column filled with loose glass filling (cut glass tubes) ).

The inlet of the reprocessed solution of nitrogen oxides in nitric acid (NITROLEA) resulted in approximately half the height of the absorption column, above the lower part of the height of its fill. The aqueous peroxide solution was dosed above the top of the column packed from where the degassing of the apparatus used was also discharged.

A solution of nitrogen oxides in nitric acid (NITROLEUM), a processing for the laboratory experiment was characterized as follows: the content of nitric nitrogen oxides content of the orthophosphoric acid content of hydrofluoric acid density - the density, d ₂ of the _'C boiling point at atmospheric pressure

74.6 wt. % HNO ₃

21.65 wt% (as NO ₂ )

1.2 wt. % H ₃ PO ₄

0.5 wt. % HF

1.593 g. cm- ³ to 55 ° C phase transition temperature: -60 ° C

The aqueous hydrogen peroxide solution used contained approximately 10 wt. % H ₂ O ₂ and was prepared by diluting about 30% technical peroxide in a weight ratio of about 1; Second

During this laboratory experiment, a total of 440.8 g of about 10% hydrogen peroxide was fed to 559.2 g of the treated solution of nitrogen oxides in nitric acid, and the dosing was controlled so that nitrous gasses from the packed-bed reactor did not appear nitrous. gases. On average, 0.79 parts by weight of about 10% hydrogen peroxide was dosed per part by weight of the treated solution of oxides in nitric acid (NITROLEA).

Under these conditions, about 1000 g of pale yellow nitric acid was obtained, practically free of free nitrous gases, whose specific gravity at 20 ° C was 1.32 g. cm ' ³ .

Claims (1)

PATENT CLAIMS Process for producing nitric acid from solutions of nitrogen oxides in nitric acid, characterized in that nitric acid containing at least 60% by weight of HNO ₃ and at least 10% by weight of nitrogen oxides, predominantly containing nitrogen dioxide, is subjected to chemical reaction with an aqueous hydrogen peroxide solution, wherein 0.15 to 0.65 parts by weight of hydrogen peroxide, preferably 0.3 to 0.45 parts by weight of hydrogen peroxide, are fed into the process per part by weight of nitrogen oxides, the heat released from the process being removed by cooling, preferably by so-called. by direct circulating acid cooling, and the concentration of the nitric acid reaction product at the same time as the chemical reaction in progress, or at any time after its partial or complete termination, is optionally adjusted by dilution.