RU2296706C1 - Method of the non-concentrated nitric acid production - Google Patents

Abstract

FIELD: chemical industry; methods of production of the non-concentrated nitric acid.

SUBSTANCE: the invention is pertaining to the method of production of the nitric acid and may be used in chemical industry in the power-engineering operational layouts of production of the nitric acid containing the recuperation and high-temperature gas-turbine aggregate with application of the low-temperature catalytic selective purification of the exhaust gases from nitrogen oxides. The method of production of the non-concentrated nitric acid includes production of the nitrogen oxides by ammonia oxidizing, their absorption by the water, the catalytic purification of the exhaust gases after absorption by recovery of the residual nitrogen oxides by ammonia, their heating before feeding into the gaseous turbine by interaction of the methane with the oxygen on the catalytic agent ensuring decomposition of the nitrous oxide till the residual content of 5÷60 ppm. As the catalytic agent use the high-temperature manganese -alumina catalytic agent with the contents of the active component - Mn₂О₃ ≥ 10 mass % on the A1₂O₃ carrier at the volumetric speed of the flow of 15000-25000 hour^-1. The oxygen content in the exhaust gases after the absorption is maintained within the range of 2.9-3.5 volumetric %, the volumetric ratio _of CH₄/O₂ at the inlet of the reactor is within the range 0.37 - 0.48. The method ensures the high level of the ecological purity of the exhaust gases, reduces the consumption of the natural gas used for heating up of the purified exhaust gases up to the temperature of no less than 730°C.

EFFECT: the invention ensures the high level of the ecological purity of the exhaust gases, reduction of the consumption of the natural gas used for heating up of the purified exhaust gases up to the temperature of no less than 730°C.

3 ex, 5 tbl

Description

The present invention relates to a method for the production of non-concentrated nitric acid and can be used in the nitrogen industry, in particular, in energy technological schemes for the production of nitric acid containing a recovery high-temperature gas-turbine unit using low-temperature catalytic selective exhaust gas purification from nitrogen oxides.

In modern energy-technological installations, the gas mixture is heated after purification from nitrogen oxides for use as a working fluid of a gas turbine by burning natural gas. With a significant increase in natural gas prices, a decrease in its consumption in production processes is very important.

In addition, the current trends in the development of nitric acid technology, among others, include the reduction of nitrous oxide emissions, the third largest greenhouse gas emission. According to the Kyoto Protocol, nitrous oxide (N ₂ O) is recognized as one of six greenhouse gases, the content of which in the atmosphere should be reduced and controlled.

A known method of producing nitric acid using a method of purifying exhaust gases from nitrogen oxides while heating them to the temperature necessary to obtain energy in the turbine equal to the mechanical energy for compressing air and nitrous gas, including mixing the exhaust gas with natural gas, feeding the gas mixture into catalytic treatment reactor, where on a two-stage catalyst, with an excess of natural gas, high-temperature reduction of nitrogen oxides to nitrogen and water occurs (Nitrogen production acids in aggregates of large unit capacity, edited by V. M. Olevsky. M., Chemistry, 1985, pp. 51-52; 220-222).

As the first stage, a APK-2 grade palladium catalyst is used, containing 2 wt.% Palladium (Pd) supported on α-Al ₂ O ₃ aluminum oxide, or a mixture of APK-2 and APEK-0.5 catalysts (containing 0.5 wt.% applied Pd) in the ratio (1: 2). As the second stage, aluminum oxide α-Al ₂ O _{3 is used} .

The process of reducing nitrogen oxides to nitrogen and water proceeds in an environment that is practically free of oxygen, with an initial ratio of CH ₄ / O ₂ equal to 0.525: 0.6. The calculated temperature of the exhaust gas at the inlet to the reactor is 480 ° C, at the outlet 770 ° C. An increase in the temperature of the gaseous medium occurs mainly as a result of the “burning out” of oxygen on the first catalyst bed. Hot exhaust gases are sent as a working fluid to a recovery gas turbine.

The main disadvantages of this method:

- high cost of catalysts containing expensive Pd;

- strict restriction on the oxygen content in the exhaust gases entering the catalytic reactor for purification;

- tight connection between the operation of the catalytic reactor and the gas turbine unit;

- the presence in the purified gases of harmful impurities - carbon monoxide up to 0.15 vol.% and ammonia up to 0.035 vol.%;

- increased costs of reducing gas (natural gas) for cleaning due to the need for a reaction in a reducing environment.

A known method of purification of exhaust gas used in the production of nitric acid, with simultaneous thermochemical heating of the gas stream to prepare it as a working fluid, comprising mixing the exhaust gas with methane, supplying the mixture to two catalyst layers on which the process of reduction of nitrogen oxides takes place. In this case, methane or gas mixture is pre-mixed with ammonia or ammonia-containing gas; as a catalyst for the first layer, a zeolite catalyst is used, and as a second layer, a nickel-copper-cement catalyst or a zinc-chromium catalyst. Thermochemical heating is carried out to 620-790 ° C [RU 2234970, 2003, B01D 53/56, B01D 53/86].

The disadvantages of this invention:

- the use as a second layer of nickel-copper-cement catalysts, unstable when operating in the oxidizing mode - when the ratio of CH ₄ / O ₂ <0.5 decreases, it goes into the oxidizing form, while the degree of methane conversion decreases sharply and the temperature in the reaction zone decreases ;

- insufficient catalytic activity of the zinc-chromium catalyst during operation - verified in the process of pilot testing - the results in table 1; mechanical abrasion resistance is reduced to unacceptable values;

- CO, CH ₄ will be present in the purified exhaust gases after the catalytic system, since methane conversion on the proposed catalysts does not occur 100%;

- the issue of reducing nitrous oxide emissions is not being addressed;

- heating the gas mixture to 620 ° C is not optimal, since it does not increase the capacity of the gas turbine [Nitric acid production in units of large unit capacity. Ed. V.M. Olevsky. M., Chemistry, 1985, p. 141, 221, 224].

Table 1.
The activity of the zinc-chromium catalyst in the process of methane oxidation at a gas volumetric velocity of 20,000 h ^-1 . Volumetric ratio methane / oxygen Methane Conversion,% Restored form Oxidized form source catalyst the catalyst after run 2000 h source catalyst the catalyst after run 2000 h 0.52 84.3 79.9 42.9 7.2 0.48 82.0 41.2 94.4 7.2 0.41 97.3 83.9 99.7 4,5

A known method for the production of nitric acid using selective purification of exhaust gases from nitrogen oxides to nitrogen and water on a low-temperature alumino-zinc catalyst brand AMC-10 using ammonia and subsequent heating of the purified gases to the required temperatures [Production of nitric acid in units of large unit capacity. Ed. V. M. Olevsky, M., Chemistry, 1985, pp. 290-303].

The temperature in the catalytic reaction zone with this purification method is 280-300 ° C. The temperature increase during gas cleaning occurs at 10 ° C. The purified exhaust gas is heated from 290-310 ° C to a temperature of 760-780 ° C in the BNG-172 exhaust gas heating unit due to the heat of natural gas burning in the furnace of the radiation part of the heater.

After the reactor, the gas provides a NO _x content of not more than 0.005%, NH ₃ not more than 0.01 vol.%, Which is confirmed practically on industrial units.

The disadvantages of this method:

- significant consumption of natural gas as fuel for heating purified gases from 290-310 ° C to a temperature of 760-780 ° C;

- significant material costs for the construction of an exhaust gas heating unit and equipment installation;

- the issue of reducing emissions of nitrous oxide is not resolved.

A known method of producing nitric acid, including the oxidation of ammonia, the absorption of nitrogen oxides by water, the removal of production acid to the consumer, heating the exhaust gases, their selective purification in the presence of ammonia and heat recovery in a gas turbine [RU 2096316, 1995, C01B 21/40, B01D 53 / 34]. After selective purification, natural gas is introduced into the purified gas, the mixture is heated to a temperature above 450 ° C, and before being fed to the turbine, natural gas is burned on the catalyst.

The disadvantages of this method:

the issue of reducing nitrous oxide emissions from exhaust gases is not being addressed;

- the introduction of natural gas into the gas mixture before the stage of its heating from 300 ° C to a temperature of more than 450 ° C (ignition temperature of the catalyst) leads to an increase in energy consumption.

A known method of producing nitric acid, which proposes to reduce the content of nitrous oxide in the exhaust gas to increase the residence time of the gas mixture after oxidation of ammonia to 3.0 s [RU 2032612, 1989, C01B 21/26].

This method differs from that proposed by the decomposition of nitrous oxide in the "head" of the technological scheme - in the ammonia conversion department, due to the increase in the volume of conversion apparatuses, which is unacceptable for existing energy technological schemes for the production of nitric acid.

The closest in technical essence and the achieved technical result is a method for producing non-concentrated nitric acid, including the production of nitrogen oxides, their absorption with aqueous solutions, mixing of exhaust gas with ammonia, its purification on an aluminum-zinc-zinc catalyst of the AMC brand, followed by mixing of the gas stream with methane and heterogeneous oxidation of methane on aluminum-nickel catalyst brand GIAP-2. Before mixing with methane, the purified exhaust gas is heated in a tube furnace to 500 ° C [SU 1835156, 1990, C01B 21/40].

This invention is selected by us for the prototype.

The content of nitrogen oxides in the proposed method does not exceed 0.005 vol.% However, as shown by the results of our tests of aluminum-nickel catalysts of various grades, including GIAP-3-6N, which is close in composition and characteristics to GIAP-2, it is not possible to use these catalysts to heat a purified gas mixture by heterogeneous oxidation of methane:

- the maximum degree of methane conversion does not exceed 91.6% at a space velocity of 10,000 h ^-1 ;

- the required gas heating temperature is not achieved, which is necessary for the normal operation of the gas turbine unit (730-780 ° C) with an oxygen content in the mixture of 2.5-3.8 vol.%

The test results are shown in table 2.

Table 2.
Methane conversion on nickel-aluminum catalysts. Catalyst Name Volumetric velocity of gases, h - ¹ The gas content at the inlet to the reactor: The temperature in the catalyst layer, ° C The degree of methane conversion,% oxygen, vol.% methane / oxygen volume ratio GIAP-3-6N 25000 2,5 0.51 560 19,4 3.0 0.44 520 5.3 20000 2.7 0.58 670 71.0 3,5 0.47 700 72.7 15,000 3,1 0.51 660 56.1 3.6 0.46 660 58.3 10,000 2,3 0.57 720 91.6 3.2 0.46 640 65.6 ICI 20000 2,3 0.53 570 17.5 2.3 0.38 560 4.7

Due to the failure to achieve the claimed technical result, this invention cannot find practical application.

In addition, as in previous cases, the issue of reducing nitrous oxide emissions is not being addressed.

An object of the present invention is to develop a method for heating purified exhaust gases to a temperature of 730-780 ° C by catalytic combustion of combustible gases using an optimal catalyst, while solving the problem of reducing nitrous oxide formed in the stage of ammonia oxidation.

The problem is solved in a method for producing non-concentrated nitric acid, which includes the production of nitrogen oxides by oxidation of ammonia, their absorption with water, mixing of exhaust gases with ammonia after absorption, their purification by reduction of residual nitrogen oxides on a selective catalyst, heating the purified exhaust gases before the recovery gas turbine in the reactor by the interaction of methane with oxygen on the catalyst, while the exhaust gases are heated on a high-temperature alumina-manganese a catalyst with an active component content of Mn ₂ O ₃ ≥10 wt.% on an Al ₂ O ₃ support, with a volumetric rate of exhaust gases of 15000-25000 h ^-1 . After absorption, the oxygen content in the exhaust gases is maintained in the range of 2.9-3.5 vol.%, And the volumetric ratio of CH ₄ / O ₂ at the outlet to the reactor is in the range of 0.37-0.48.

The use of the proposed catalyst:

- allows to reduce the consumption of natural gas for heating purified exhaust gases;

- allows to reduce material costs when transferring the unit to selective cleaning;

- eliminates the use of expensive platinum group metals in the exhaust gas heating system;

- provides heating of the purified exhaust gases to a temperature of at least 730 ° C;

- provides a degree of oxidation of methane of 94-99% in the oxidative mode (with a ratio of CH ₄ / O ₂ equal to 0.37-0.48) at a temperature of the onset of reaction not higher than 510 ° C, a space velocity of 15000-25000 h ^-1 , concentration O ₂ in the gas mixture of 2.9-3.5 vol.%;

- provides a residual content in the exhaust gases of CH ₄ - 0.04-0.1 vol.%, CO <0.0005 vol.%, H ₂ - not more than 0.03 vol.%;

- provides the destruction of nitrous oxide formed at the stage of ammonia oxidation, the results are shown in table 3;

- provides decomposition of residual ammonia contained in the gas after selective purification.

Table 3.
The content of N ₂ O in the gases before and after the alumina-manganese catalyst. Experience number The temperature in the catalyst layer, ° C At the entrance to the reactor: Nitrous oxide content: oxygen, vol.% methane / oxygen volume ratio to the reactor
ppm after the reactor, ppm one 780 3,5 0.37 28.5 0.9 2 760 2.9 0.4 580 12 3 765 3.2 0.4 1360 35

This catalyst has high heat resistance - up to 900 ° C. Resistant to nitrogen oxides, which can enter the catalyst in case of leakage of untreated exhaust gases.

In the presence of NO _{x, the} efficiency of the methane oxidation reaction on the catalyst does not decrease and the process initiation temperature does not increase (480-510 ° С).

The lower limit of oxygen content of 2.9% is determined from the conditions of stable heating of exhaust gases from 490-510 ° C to 760 ° C while maintaining the oxidative state.

The upper limit of 3.5 vol.% Is determined from the condition of heating the exhaust gases from 480 ° C to 780 ° C in the oxidative mode, so that the CO content in the exhaust gases emitted into the atmosphere is not more than 0.01 vol.%.

The range of volume ratios is determined by the activity of the catalyst: CH ₄ / O ₂ - 0.37 refers to the initial period of operation, 0.48 - to the end of catalyst operation.

The performance of the proposed catalyst in an oxidizing environment without the presence of NO _x and NH ₃ in gases, which corresponds to the mode when starting the nitric acid aggregate, are shown in table 4.

Table No. 4.
Methane conversion on an alumina-manganese catalyst in an oxidizing medium without the presence of NO _x and NH ₃ in the gas stream at a space velocity of 20,000 h ^-1 . Reactor inlet gas stream: The temperature in the catalyst layer, ° C The degree of methane conversion,% oxygen, vol.% methane / oxygen volume ratio temperature, ° С 3.9 0.44 490 760 85.6 3.9 0.39 490 790 95.3 3,5 0.43 500 780 98.8 3,7 0.40 505 780 98.9 3.35 0.46 510 830 99.6

The performance of the catalyst in an oxidizing environment in the presence of NO _x and NH ₃ in the exhaust gas, corresponding to the nominal mode during the production of nitric acid and selective purification, are shown in table 5.

Table No. 5.
Methane conversion on an alumina-manganese catalyst in an oxidizing medium in the presence of NO _x 0.12 vol% and NH ₃ 0.02 vol% in a gas stream at a space velocity of 20,000 h ^-1 . Reactor inlet gas stream: The temperature in the catalyst layer, ° C The degree of methane conversion,% oxygen, vol.% methane / oxygen volume ratio temperature, ° С 3,1 0.45 495 780 94.7 3,5 0.41 490 770 94.0 3,5 0.38 500 800 97.2 3.2 0.39 500 800 96.5 3.2 0.44 495 805 95.7

Examples of carrying out the invention.

Example 1. Ammonia is oxidized with atmospheric oxygen in contact devices under a pressure of 0.35 MPa. Nitrous gases, after cooling, condensation of water vapor and separation of the resulting 40-47% nitric acid, are compressed in a supercharger to a pressure of 1.0 MPa. The absorption of nitrogen oxides is carried out by their absorption with water with the formation of 58-60% nitric acid.

After absorption, the exhaust gases containing 0.1% nitrogen oxides, 2.9% oxygen, and 0.2% nitrous oxide are heated to 290 ° C.

Heated exhaust gases are mixed with gaseous ammonia in a volume ratio of NH ₃ : NO _x - 1.05: 1 from stoichiometry and sent to a catalytic purification reactor.

Purified exhaust gases after selective purification contain not more than 0.005 vol.% NO _x and NH ₃ . Then they are heated to 510 ° C in a preheater, mixed with methane in a volumetric ratio of methane and oxygen in the exhaust gas CH ₄ / O ₂ = 0.48 and sent to a catalytic reactor, where the exhaust gas is heated by the interaction of methane and oxygen on the catalyst up to 730 ° C.

As a catalyst, a high-temperature alumina-manganese catalyst is used with an active component content of Mn ₂ O _{3 of} 11.8 wt.% On an Al ₂ O ₃ support, which is loaded into the reactor in an amount corresponding to an exhaust gas velocity of 15,000 h ⁻¹ .

In the process of heating exhaust gases to 730 ° C in an oxidizing environment, excess ammonia is oxidized to nitrogen, nitrous oxide decomposes to a residual content of 0.006 vol.% Or 60 ppm also with the formation of nitrogen, and CO is formed in insignificant amounts - <0,0005 vol.% This ensures a low total content of harmful substances in the gases emitted into the atmosphere.

Hot exhaust gases from the reactor are sent to a recovery gas turbine, where when expanded to atmospheric pressure, they are cooled to 380 ° C.

Then they are cooled to 200 ° C in the exhaust gas heater after absorption, heating the gases to 290 ° C in a mixture with flue gases from the tubular heater.

After cooling, the cleaned exhaust gases are emitted into the atmosphere.

Example 2. It differs from example 1 as follows. After absorption with the content of 0.08% nitrogen oxides, 3.2% oxygen, up to 0.2% nitrous oxide, the exhaust gases are heated to 280 ° C. Further, as in example 1, the heated exhaust gases are mixed with ammonia and sent for purification. Purified exhaust gases after selective purification with a content of not more than 0.005 vol.% NO _x and NH ₃ are heated to 495 ° C in a heater, mixed with methane in a volume ratio of CH ₄ / O ₂ = 0.42 and sent to a catalytic reactor, where As a result of the interaction of methane and oxygen on the catalyst, the purified exhaust gases are heated to 760 ° C.

The high-temperature alumina-manganese catalyst is loaded into the reactor in an amount corresponding to a space velocity of 20,000 h ^-1 .

When the exhaust gases are heated to 760 ° C in an oxidizing environment, excess ammonia is oxidized to nitrogen, nitrous oxide decomposes to a residual content of 0.0005 vol.% Or 5 ppm, CO is formed <0.0005 vol.%, Which ensures a low total content of harmful substances in gases released into the atmosphere.

Hot exhaust gases from the reactor, expanding to atmospheric pressure in a recovery gas turbine, are cooled to 400 ° C and then cooled to 220 ° C in a heater, heating to 280 ° C in a mixture with flue gases from a tube heater after absorption. Cooled cleaned exhaust gases are emitted into the atmosphere.

Example 3. It differs from example 2 in the composition of the exhaust gases after the absorption of nitrogen oxides, heating temperature, the ratio of CH ₄ / O ₂ . After absorption with the content of 0.06% nitrogen oxides, 3.5% oxygen, up to 0.2% nitrous oxide, the exhaust gases are heated to 280 ° C, mixed with ammonia, as in example 2, and sent for purification. Purified exhaust gases after selective purification with a content of not more than 0.005 vol.% NO _x and NH ₃ are heated to 480 ° C in a heater, mixed with methane in a volume ratio of CH ₄ / O ₂ = 0.37 and sent to a catalytic reactor, where as a result of the interaction of methane and oxygen on the catalyst, the purified exhaust gases are heated to 780 ° C.

The high-temperature alumina-manganese catalyst is loaded into the reactor in an amount corresponding to a space velocity of gases of 25,000 h ^-1 .

When the exhaust gases are heated to 780 ° C in an oxidizing environment, excess ammonia is oxidized to nitrogen, nitrous oxide decomposes to a residual content of 0.0005 vol.% Or 5 ppm, CO is formed in insignificant amounts - <0.0005 vol.%, Which ensures low the total content of harmful substances in the gases emitted into the atmosphere.

Hot exhaust gases are cooled in a recovery gas turbine to 370 ° C and then cooled to 200 ° C in a heater, heating to 280 ° C in a mixture with flue gases from a tubular heater the exhaust gases after absorption. Cooled cleaned exhaust gases are emitted into the atmosphere.

Thus, the method of producing non-concentrated nitric acid of the present invention provides a higher level of environmental cleanliness of the exhaust gases compared to existing methods involving high-temperature heating of the exhaust gases.

Claims (1)

A method for producing non-concentrated nitric acid, including the production of nitrogen oxides by oxidation of ammonia, their absorption with water, mixing with exhaust ammonia after absorption, their purification by reduction of residual nitrogen oxides on a selective catalyst, heating the purified exhaust gases before the recovery gas turbine in the reactor by reacting methane with oxygen on the catalyst, characterized in that the heating of the exhaust gases is carried out on a high temperature alumina-manganese catalyst with a content su- component Mn ₂ O ₃ ≥10 wt.% of the supported Al ₂ O ₃ at a space velocity of exhaust gas 15,000-25,000 h ^-1, the content of oxygen in the exhaust gas after the absorption is maintained in the range of about 2.9-3.5. %, and the volumetric ratio of CH ₄ / O ₂ at the inlet to the reactor is in the range of 0.37-0.48.