阅读说明：本技术 氩气和氮气的生产方法 (Production method of argon and nitrogen ) 是由 贾科莫·科尔梅 于 2018-06-18 设计创作，主要内容包括：一种方法,包括在合适的吸收媒介(23)中使含有NOx的工艺气体(22)经受吸收NOx的步骤,获得硝酸(24)和含有氮气、氩气和残余NOx的尾气(25)；对所述尾气(25)进行包括至少一个NOx脱除步骤的处理,获得调节后的尾气(26)；对所述调节后的尾气的至少一部分(26b)进行分离处理,获得含有氩气的产物流(40)和含有氮气的产物流(37)。(A method comprising the steps of subjecting a process gas (22) containing NOx to a step of absorbing NOx in a suitable absorption medium (23), obtaining nitric acid (24) and a tail gas (25) containing nitrogen, argon and residual NOx; subjecting said tail gas (25) to a treatment comprising at least one NOx removal step, obtaining a conditioned tail gas (26); at least a portion (26b) of the conditioned off-gas is subjected to a separation treatment obtaining a product stream (40) comprising argon and a product stream (37) comprising nitrogen.)

1. A method comprising the steps of:

subjecting the process gas (22) containing NOx to a NOx absorption step in a suitable absorption medium (23) to obtain nitric acid (24) and a tail gas (25) containing nitrogen, argon and residual NOx;

subjecting said tail gas (25) to a treatment comprising at least one NOx removal step, obtaining a conditioned tail gas (26);

at least a portion (26b) of the conditioned off-gas is subjected to a separation treatment obtaining a first product stream (40) comprising argon and a second product stream (37) comprising nitrogen.

2. The process according to claim 1, characterized in that the argon content of the first product stream (40) is at least 99.5% (vol), more preferably at least 99.95% (vol), even more preferably at least 99.995% (vol).

3. The process according to claim 1 or 2, characterized in that the nitrogen content of the second product stream (37) is at least 99.5% (vol), more preferably at least 99.95% (vol), even more preferably at least 99.995% (vol).

4. The process according to any of the preceding claims, characterized in that the conditioned off-gas (26) comprises argon in an amount of at least 0.9% (vol), preferably at least 1.0% (vol), even more preferably at least 1.1% (vol).

5. A method according to any one of the preceding claims, characterized in that the conditioned tail gas (26) contains an amount of NOx of not more than 200ppm, preferably not more than 30ppm, more preferably not more than 5 ppm.

6. Method according to any of the preceding claims, characterized in that the conditioned tail gas (26) comprises N ₂The amount of O is not more than 1000ppm, preferably not more than 100ppm, more preferably not more than 30ppm, even more preferably not more than 10 ppm.

7. The method according to any of the preceding claims, characterized in that the conditioned off-gas (26) comprises oxygen in an amount of preferably not more than 5% (vol), more preferably 2-3% (vol).

8. The method according to any of the preceding claims, wherein the pressure of the conditioned off-gas (26) is more than 4bar, preferably 4-15 bar.

9. The method according to any of the preceding claims, characterized in that the process gas (22) comprising NOx is obtained by oxidizing an ammonia stream (20) in the presence of air or oxygen-enriched air (21).

10. The method according to any one of the preceding claims, wherein the conditioned tail gas comprises no more than 800ppm of CO ₂。

11. The method according to any one of the preceding claims, wherein the separation process is a cryogenic process.

12. The method of claim 11, wherein the separation process comprises: cooling and subsequently expanding the conditioned off-gas (26) obtaining a partial liquefaction and subjecting the liquefied fraction (30) to fractional distillation.

13. Method according to claim 11 or 12, characterized in that the method comprises removing CO prior to cryogenic treatment, preferably using molecular sieves ₂The step (2).

14. An apparatus, comprising:

an absorption column (4) fed with a process gas (22) containing NOx and adapted to absorb NOx in a suitable absorption medium (23) to provide nitric acid (24) and a tail gas (25) containing nitrogen, argon and residual NOx;

-a treatment unit (6) for said tail gas (25) adapted to remove NOx and to produce a conditioned tail gas (26);

a separation section (2) adapted to separate a first product stream (40) comprising argon and a second product stream (37) comprising nitrogen, said separation section (2) being fed with at least a portion (26b) of said conditioned off-gas (26).

15. The plant according to claim 14, wherein the separation section (2) comprises:

-a heat exchanger (8) adapted to refrigerate said at least a portion (26b) of said conditioned off-gas, thereby obtaining a refrigerant gas (28);

an expander (9) for said refrigerant gas (28) to obtain a partially liquefied gas (29);

a separator (10), in which separator (10) a liquefied fraction (30) of the partially liquefied gas (29) is separated from a non-liquefied fraction (31).

A distillation apparatus (11), said distillation apparatus (11) receiving said liquefied fraction (30) and separating said stream (40) comprising argon and said stream (37) comprising nitrogen.

16. The plant according to claim 14 or 15, comprising a reactor (3), said reactor (3) being adapted to oxidize an ammonia stream (20) in the presence of air or oxygen-enriched air (21) to obtain said NOx-containing process gas (22).

Technical Field

The invention relates to the technical field of argon and nitrogen production.

Background

Argon (Ar) is a rare gas and therefore chemically inert. By virtue of this property, it can be used in a variety of industrial applications, for example, in the formation of inert atmospheres.

Molecular nitrogen (N) due to its low reactivity ₂) And is also particularly suitable for use in the formation of inert atmospheres in various industrial and process environments. Large amounts of nitrogen are also used in cryogenic applications, but the main use is still the synthesis of ammonia from which fertilizers, polymers, explosives and colorants can be obtained.

Argon and nitrogen are typically obtained from air separation processes along with oxygen.

Most air separation plants perform a fractionation process of liquid air. The process is called cryogenic process and it is substantially separated by the difference in boiling points of argon, nitrogen and oxygen (-186 c, -196 c and-183 c, respectively). Air liquefaction processes known in the art are for example the linde process and the claude process. Fractionation is typically carried out in a system comprising several distillation columns (usually three columns).

One problem with this technique is that the boiling points of argon and oxygen are very close, which makes it difficult to separate argon from oxygen to obtain high purity argon. The nitrogen obtained with this technique typically also contains ppm levels of Ar and oxygen, which is undesirable.

To facilitate the separation of argon and nitrogen as much as possible, a large distillation column with a large number of trays may be used, the number of columns may be increased, or an adsorption bed may be installed downstream of the column, thereby achieving further purification of the argon and nitrogen streams obtained by distillation. However, all these solutions are expensive in terms of device design and energy consumption.

Other devices for producing argon and nitrogen use a selective adsorption process through a membrane. However, these devices are still not very popular and are costly.

Disclosure of Invention

The object of the present invention is to provide a method which makes it possible to obtain substantially pure argon and nitrogen, while at the same time being simple and inexpensive.

Applicants have found that it is a convenient source for the production of argon and nitrogen based on the composition of the tail gas of the nitric acid synthesis process.

As is well known, according toOstwald process synthesis of nitric acid comprises a step of absorption of nitrogen oxides NOx in water, which produces a nitric acid stream and a nitric acid stream containing nitrogen, argon, residual NOx and optionally also N ₂Tail gas of O. In the prior art, the exhaust gases are generally treated to remove NOx and possibly N according to the prescribed limits for emission into the atmosphere ₂O, and then discharging. The basic idea of the invention is to treat at least a portion of said off-gas to separate the argon and nitrogen contained therein, so as to obtain a product of high commercial value.

The aforementioned object is achieved by a method according to claim 1, comprising the steps of:

subjecting the process gas containing NOx to a NOx absorption step in a suitable absorption medium (absorptionmeans) to obtain nitric acid and a tail gas containing nitrogen, argon and residual NOx;

subjecting said tail gas to a treatment comprising at least one NOx removal step to obtain a conditioned tail gas;

subjecting at least a portion of said conditioned off-gas to a separation treatment to obtain a first product stream comprising argon and a second product stream comprising nitrogen.

The absorption medium used during the NOx absorption step is preferably water.

Preferably, said conditioned off-gas is divided into at least two portions, the first portion being subjected to the above separation treatment and the second portion being advantageously treated in a suitable expander.

The argon concentration of the first product stream is greater than the argon concentration in the conditioned tail gas as a result of the separation treatment. Similarly, the nitrogen concentration of the second product stream is greater than the nitrogen concentration in the conditioned tail gas.

Preferably, the first product stream comprises at least 99.5% (vol) argon, more preferably at least 99.95% (vol), even more preferably at least 99.995% (vol).

Preferably, the second product stream comprises at least 99.5% (vol) nitrogen, more preferably at least 99.95% (vol), even more preferably at least 99.995% (vol).

The separation process is adapted to selectively separate at least argon from nitrogen. Preferably, the separation treatment is a low-temperature treatment performed at a temperature not exceeding 133K (-140 ℃).

In a preferred embodiment, the process gas is obtained by oxidizing an ammonia stream in the presence of air or oxygen-enriched air (enrichedair). Therefore, the sources of nitrogen and argon contained in the off-gas are essentially air or oxygen-enriched air introduced during the oxidation step.

The oxidation step essentially comprises a first stage of catalytic oxidation of ammonia, providing nitric oxide, NO, and a small amount of nitrous oxide, N ₂O; and a second stage of NO oxidation to provide nitrogen dioxide NO ₂Or dinitrogen tetroxide N ₂O ₄. According to extensive practice in the art, the compounds NO, NO ₂And N ₂O ₄Represented by the general formula NOx.

Advantageously, the absorption step of the process gas is carried out in an absorption tower in which the NOx contained in the process gas is at least partially absorbed in the absorption medium, preferably water, to provide nitric acid and the above-mentioned off-gas.

Optionally, the process gas may be subjected to N prior to the absorbing step ₂O removal (so-called "N ₂O secondary removal "). In some embodiments, N will be ₂O is removed from the off-gas (so-called "three removal"); some embodiments include a second removal and a further three removals.

The conditioned tail gas comprises mainly nitrogen. Preferably, the gas comprises nitrogen in an amount equal to or greater than 80% (vol), preferably greater than 90% (vol), even more preferably between 95% and 98% (vol).

The conditioned tail gas also comprises a non-negligible amount of argon, typically at least 0.9% (vol), preferably at least 1.0% (vol), even more preferably at least 1.1% (vol).

The conditioned tail gas may also contain a small amount of water, preferably in an amount of not more than 0.5% (vol), more preferably 0.2 to 0.3% (vol).

The adjusted tailThe gas preferably contains negligible amounts of NOx and N ₂O。

Preferably, the conditioned tail gas contains no more than 200ppm NOx, more preferably no more than 30ppm, even more preferably no more than 5 ppm.

Preferably, the conditioned tail gas contains N ₂The amount of O is not more than 1000ppm, preferably not more than 100 ppm; more preferably not more than 30ppm, even more preferably not more than 10 ppm.

Taking into account NOx and N ₂Tendency of O to freeze during the separation process, NOx and N if present ₂O in an amount greater than the above-identified amount causes a series of operational problems during the operation of the relevant device, and during the stoppage of the device due to NOx and N released into the atmosphere ₂The cumulative amount of O can cause safety problems.

In some embodiments, the conditioned tail gas has an oxygen content, preferably no greater than 5% (vol), more preferably 2% to 3% (vol).

The treatment of the tail gas preferably comprises a denitration step (DeNOxstage) by catalytic reduction, more preferably Selective Catalytic Reduction (SCR), in the presence of a reductant, preferably ammonia.

In other embodiments of the invention, the treatment of the tail gas comprises a non-selective catalytic reduction (NSCR) denitration step. In this case, the conditioned tail gas is substantially free of oxygen and may contain trace amounts of hydrocarbons or hydrogen, CO ₂And ammonia.

Preferably, the pressure of the conditioned off-gas is greater than 4bar, preferably in the range of 4 to 15 bar. Said pressure corresponds to the preferred pressure for treating said off-gas.

The conditioned tail gas contains no or very little carbon dioxide. For example, the conditioned tail gas contains no more than 800ppm CO ₂Preferably not more than 700ppm, more preferably not more than 600 ppm. The ppm symbol represents a volume of parts per million.

The separation treatment preferably comprises fractional distillation using different boiling points, which are-186 ℃ for argon, -196 ℃ for nitrogen and-183 ℃ for oxygen (at standard conditions STP).

Preferably, the separation process comprises: cooling and subsequently expanding the conditioned off-gas to obtain a partial liquefaction; and subjecting the liquefied fraction to fractional distillation. Thus, the process preferably comprises fractional distillation of at least one of argon, nitrogen and oxygen at the respective boiling points.

The method may include removing CO prior to cryogenic treatment ₂To avoid CO ₂Freezes and accumulates in the refrigeration cabinet. The CO is ₂The removing of (b) preferably comprises passing the gas through a molecular sieve.

Another aspect of the invention is a process for producing an argon-containing stream and a nitrogen-containing stream by separating a conditioned off-gas of a plant for the treatment of nitric acid, said conditioned off-gas being obtainable by:

subjecting the process gas containing NOx to a NOx absorption step in a suitable absorption medium to obtain nitric acid and a tail gas containing nitrogen, argon and residual NOx;

subjecting said tail gas to a treatment comprising at least one NOx removal step to obtain said conditioned tail gas.

Another aspect of the invention relates to an apparatus for producing argon and nitrogen according to the claims.

The conditioned tail gas has a greater argon content and a smaller oxygen content than air. For these reasons, obtaining argon from the conditioned off-gas is substantially easier and more advantageous than separating argon from air. In particular, a smaller oxygen content (or absence of oxygen in the case of NSCR) promotes the production of a product stream comprising argon, since oxygen with close boiling points is the most difficult component to separate from argon.

Another advantage of the tail gas is its low pollutant content (in particular NOx and N) ₂O) which allows to obtain a high purity argon and nitrogen flow and to allow the correct operation of the plant avoiding the contact with N ₂O and the management of toxic gases (e.g., NOx).

Another advantage is that said conditioned off-gas is available at high pressure (typically more than 4bar, for example 4-15 bar), which allows the gas to be partially liquefied by expansion. In this way, it is no longer necessary to use a dedicated compressor for the offgas, which is advantageous from an economic point of view compared to conventional air fractionation plants, since the compressor constitutes its most expensive component.

The investment costs of the fractionation plant according to the present invention will be much less than conventional air fractionation plants, considering that there is no compressor due to the fact that the tail gas is under pressure and the structure of the fractionation column is simplified due to the low oxygen content in the tail gas. Thus, argon and nitrogen are available at competitive prices.

It is also true to say that argon and nitrogen are available at competitive prices, considering that nitric acid plants must purchase the required electricity from the market to compensate for the power generated, which is lost due to the fact that the tail gas is not fully expanded in the expander but fractionated.

For these reasons, the conditioned off-gas represents a particularly advantageous source for the production of argon and nitrogen.

In addition, the invention increases the value of the tail gas discharged from nitric acid production plants which are discharged to the atmosphere in the prior art. Thus, the present invention adds an important source of revenue for the nitric acid plant. Thus, one aspect of the present invention is represented by the combined production of nitric acid, argon and nitrogen. The nitrogen thus obtained can be for example marketed or used to increase the production capacity of a possible ammonia plant in combination with a nitric acid plant.

Another advantage of the present invention is the saving of natural and energy resources compared to prior art methods of argon and nitrogen production (i.e. distillation or selective adsorption) which use air as the feedstock, which prior art methods require large amounts of energy.

The present invention is particularly attractive in situations where the local argon and nitrogen markets (where the nitric acid plant is present) are not balanced with respect to the composition of air.

The advantages of the present invention will become more apparent by reference to the following detailed description of the preferred embodiments of the invention.

Drawings

Fig. 1 shows a simplified diagram of a device according to the invention.

Fig. 2 shows a diagram of an apparatus for co-producing nitric acid, argon and nitrogen according to a preferred embodiment of the present invention.

Detailed Description

The plant according to figure 1 essentially comprises an absorption column 4, a treatment unit 6 for the off-gas leaving the column, an expander 7 and a section 2 for separating the argon flow and the nitrogen flow.

The device operates as follows.

Will contain NOx and a small amount of N ₂The process gas 22 of O and the aqueous stream 23 are fed to the absorption column 4. In said column 4, the NOx is partially absorbed in water, forming a stream 24 comprising nitric acid and a stream comprising mainly nitrogen and small amounts of oxygen, argon, water, N ₂O and residual NOx tail gas 25.

Said tail gas 25 is sent to said treatment unit 6, in which treatment unit 6 NOx and optionally also N are at least partially removed ₂O to provide a conditioned tail gas 26. The pressure of the gas 26 leaving the treatment unit 6 is preferably in the range of 4 to 15 bar.

The conditioned gas 26 is advantageously divided into two portions: the first portion 26a is expanded in the expander 7 and discharged to the atmosphere as stream 27, and the second portion 26b is sent to the section 2 and subjected to separation treatment, obtaining a stream 40 containing argon and a stream 37 containing nitrogen.

Figure 2 shows the device of figure 1 in more detail. The plant comprises in particular a section 1 for the synthesis of nitric acid and a section 2 for the production of argon and nitrogen.

Said section 1 essentially comprises a reactor 3 for the catalytic oxidation of ammonia, an absorption column 4, a heat exchanger 5, for removing NOx and optionally N ₂O unit 6 and expander 7. The section 1 also comprises a compressor located between the reactor 3 and the absorption column 4, in particular in the case of high-capacity plants.

The station 1 operates as follows.

An ammonia stream 20 and an air stream 21 are fed to the reactor 3. In reactor 3, the ammonia is catalytically oxidized to produce nitric oxide, NO, and smaller amounts of nitrous oxide, N ₂O and at least a part of the NO is further oxidized to produce nitrogen dioxide NO ₂Or dinitrogen tetroxide N ₂O ₄Generating a gas stream 22.

The gaseous stream 22 and the aqueous stream 23 are introduced into an absorption column 4, where at least part of the NOx is absorbed to produce nitric acid 24 in the absorption column 4.

The absorber column 4 also provides a tail gas 25 as an overhead product, the tail gas 25 comprising mainly nitrogen and small amounts of oxygen, water, argon, N ₂O and residual NOx.

The off-gas 25 is preheated in heat exchanger 5 and then fed to the unit 6. According to the example shown in fig. 2, the unit 6 comprises a denitration section in which NOx is at least partially removed by Selective Catalytic Reduction (SCR).

The unit 6 works under a pressure of 4-15 bar and provides a gas containing mainly nitrogen, 2-3% oxygen, 0.2-0.3% water,<30ppm NOx and<30ppmN ₂gas 26 of O.

The gas 26 is divided into two portions: a first portion 26a is expanded inside said expander 7 and a second portion 26b is discharged from section 1 for the synthesis of nitric acid and is sent to section 2 for the production of argon and nitrogen.

The expander 7 produces at least a part of the power required by the compressor (not shown) in the nitric acid section 1. The expanded gas 27 is vented to atmosphere.

Section 2 for the production of argon and nitrogen mainly comprises a heat exchanger 8, an expander 9, a separator 10 and a distillation unit 11.

According to the example of fig. 2, the device 11 comprises: a first distillation column 12 operating at a pressure of about 4 to 5bar, a second distillation column 13 operating at atmospheric pressure and a third distillation column 14 for separating argon.

The station 2 operates as follows.

The gaseous fraction 26b from section 1 is mixed with a recycle stream 32 and sent to said heat exchanger 8, cooled in said heat exchanger 8, releasing heat into stream 31 from separator 10, obtaining a refrigeration gas 28.

The refrigerant gas 28 is then sent to the expander 9 to be partially liquefied. According to an embodiment, the expander 9 is a valve or a turbine.

The partially liquefied gas 29 is fed to the separator 10. The separator 10 separates a liquid phase 30 and a gas phase 31. The liquid phase 30 is sent to the distillation unit 11 and the gaseous phase 31 is sent to the heat exchanger 8 to refrigerate the incoming gas 26b before it is reintroduced into the cycle as stream 32.

In more detail, the liquid phase 30 is fed to a first column 12, the first column 12 separating gaseous nitrogen 33 from the top and a liquid fraction 34 comprising nitrogen, oxygen and argon from the bottom.

The liquid fraction 34 is sent to the second column 13, while the nitrogen 33 is fed to the condenser 15 where it is condensed in heat exchange with the tails fraction 35 of the column 13 in the condenser 15.

According to the example of fig. 1, the condensed nitrogen stream 36 leaving the condenser 15 is divided into two portions: the first portion 36a is sent to the second column 13 and the second portion 36b is sent to the first column 12 as a reflux stream.

The second column 13 separates nitrogen 37 and oxygen 38.

A fraction 39 comprising argon and oxygen is collected at an intermediate point of said second column 13 and sent to the third column 14, the third column 14 separating substantially pure argon 40 and oxygen 41.

Examples of the invention

In a plant for the production of 500MTD (metric ton per day) nitric acid, a process gas containing 5-6% NOx is obtained at the inlet of the absorption column. At the outlet of the aforesaid column, the tail gas comprises about 300 to 500ppm NOx and at the outlet of the treatment Section (SCR) the gas comprises about 0 to 22ppm NOx. This gas is passed to a separation section, which makes it possible to obtain about 77'000kg/h of nitrogen and about 1'300kg/h of argon.