RU2174946C1 - Method of preparation of nitric acid - Google Patents

Abstract

FIELD: preparation of nitrogen oxides and conversion thereof to nitric acid. SUBSTANCE: process of catalytic oxidation of molecular nitrogen is carried out at pressure similar to that at stage of absorption of the resulting nitrogen oxides with water, and energy for endothermal process of catalytic oxidation of molecular nitrogen is supplied with flow of streams directly into reaction zone. Process of catalytic oxidation of molecular nitrogen is carried out at temperatures of lower than 1000 C and at content of oxidizer in gas phase in front of catalyst of lower than 10 vol % of HNO₃+NO× vapors and at pressure of up to 25 atm in system. Catalysts for oxidation of molecular nitrogen include platinum alloys with platinum group metals or catalysts based on ferric, cobalt, chromium, aluminum oxides with promoting additives of high-melting metals. In order to form stream with oxidizer, not only atmospheric air, but gas mixture resulting from blowing off product acid is used. EFFECT: lower energy consumption, reduced capital outlays and simplified flowsheet of nitric acid production. 8 cl, 1 dwg, 1 ex, 2 tbl

Description

 The method relates to chemical technology and relates to methods for producing nitrogen oxides and processing them into nitric acid (AK).

A widely known method for the production of AA by water absorption of nitrogen oxides obtained by the conversion of ammonia by air. It consists of stages of the oxidation of ammonia by atmospheric oxygen on catalysts at temperatures of 800-900 ^o C; heat recovery of nitrous gases; allocation of reaction water; absorption of nitrogen oxides with water to produce AA and purification of exhaust gases (GH) from nitrogen oxides (Atroshchenko V.I., Kargin S.I. Nitric acid technology, M .: Chemistry, 1970).

 The disadvantage of this method is the high raw materials (ammonia, platinum) and material costs (specific capital investments in the production of ammonia and acid), as well as significant emissions of harmful substances into the atmosphere.

 In the method described in the patent of the Russian Federation N 2070865 dated 12/27/1996, it is proposed: "to introduce nitrous gases (NG) obtained during the oxidation of ammonia in the first stage of the catalyst, to introduce pure gases containing nitrogen oxygen compounds, and to carry out the oxidation of molecular nitrogen by NG second stage catalyst. "

The disadvantage of this method is that expensive ammonia is used as a raw material for the production of NG at the first stage of the catalyst, due to the oxidation energy of which the amount of nitrogen (N ₂ ) can be oxidized, which makes it possible to obtain only 15-35% of the production acid.

 Thus, this solution only partially reduces raw materials and capital costs against the first case.

 The closest in technological essence and the achieved effect is the "Method of producing nitric acid" according to patent N 2151736 from June 27, 2000 (priority from 10/15/1998).

In this case, nitrogen oxides are obtained only due to the catalytic oxidation of nitrogen by its oxygen compounds at 850-450 ^o C under atmospheric pressure, the absorption of nitrogen oxides is carried out under increased pressure, and the energy required for the nitrogen oxidation process is supplied with a stream of SH, containing nitrogen oxides and heated by a third-party energy carrier.

 The aim of the invention is to increase the efficiency of the process by reducing energy and capital costs, as well as simplifying the technological scheme of AK production.

 The novelty of the method lies in the fact that the process of catalytic oxidation of molecular nitrogen is carried out under pressure the same (uniform) with the stage of absorption by water of the obtained nitrogen oxides, and the energy for the oxidation process is supplied with the gas stream directly to the reaction zone from extraneous sources.

The process of catalytic oxidation of nitrogen is carried out at temperatures below 1000 ^o C and when the oxidant content in the gas phase in front of the catalyst is below 10 vol.% HNO ₃ + NO ₂ vapor, and the system pressure is up to 25 atm.

 Alloys of platinum with platinum group metals or catalysts based on oxides of iron, cobalt, chromium, aluminum with promoting additives of refractory metals are used as nitrogen oxidation catalysts. The catalysts can be located in the reactor in several layers, and the oxidizer is supplied to each layer in predetermined quantities, and the energy flow is necessary.

 To form a gas stream with an oxidizing agent, not only air, but also a gas mixture resulting from the blowing of production acid can be used.

 A significant novelty of the proposed patent compared to the prototype is that the processes of nitrogen oxidation and absorption of nitrogen oxides are carried out under uniform pressure, there is no exhaust gas circulation, and the energy carrier is air (gas with a high oxygen content).

Schematic diagram is presented in the drawing, where
1 - turbocharger air
2 - saturator steam acid mixture (PCVS),
3 - heating pipe-heat exchanger,
4 - reactor for the oxidation of nitrogen,
5, 14, 15 - waste heat boilers,
6 - heat exchanger, heat carrier heater,
7 - exhaust gas heater (VG),
8, 8 ^* - refrigerator-condenser of nitrous gases (NG),
9 - absorption column,
10 - bleaching column
11 - combustion chamber,
12 - reactor catalytic purification of VG,
13 - gas turbine,
16 - air heater
17 - capacity for the coolant,
18 - capacity for acid oxidizing agent.

The proposed method for the production of AK is as follows: the air in a predetermined amount is compressed to the required pressure by the compressor 1 and distributed:
- to saturator 2 and then in the form of PCVS - to reactor 4 to provide an oxygen oxidation reaction;
- for the oxidation of nitric oxide II in NG at the exit from the reactor 4;
- to improve the process of acid formation in the column 9 through the bleaching column 10;
- for heating the air carrier energy in the heater 16.

 The shelf type saturator 2 is irrigated with acid from the tank 18. The acid is transferred to the tank from the absorption column 9.

 Coils are located on the shelves of the saturator, through which energy is supplied to evaporate the acid - to obtain a PCVS stream.

The temperature of the acid is determined by the pressure and a given concentration of AK vapors in PCVS and will be in the range 130-190 ^o C.

When leaving the saturator 2 PCVS in the heating pipe-heat exchanger 3 is heated to 150-250 ^o C to avoid condensation of acid and water vapor and is fed into the reactor 4 in front of each catalyst bed. Here, the PCVS is mixed with air (energy carrier), which is supplied from the heater 16. The resulting mixture at a temperature below 800 ^o C enters the catalyst. The amount of gases introduced in front of the shelf depends on a number of factors - mainly on the specified oxidizer content in the mixture and the temperature in front of the catalyst bed.

The catalyst is located in several layers, which is also determined by the set parameters, activity and selectivity of the catalyst. However, there is no need to have the NG temperature at the outlet of the reactor below 450 ^o C, because the reaction of nitrogen oxidation
2HNO ₃ + N ₂ = 3NO + NO ₂ + 2H ₂ O - 355 kJ (1)
practically stops, and at this temperature the decomposition reaction of HNO ₃ vapor prevails:
4HNO ₃ = 4NO ₂ + 2H ₂ O + O ₂ - 191.0 kJ. (2)
At temperatures below 600 ^o C (depending on the selected pressure) reaction (3)
2NO + O ₂ = 2NO ₂ + 113.9 kJ, (3)
which is favorable in terms of energy for the main process.

When leaving reactor 4, air is introduced into the NG stream in the amounts necessary for the oxidation of nitric oxide II by oxygen and the conversion of NO _x into acid. Then the NG stream is separated. A smaller part is directed to heating the PCVS in the heating pipe 2, exiting from which it is mixed with the general NG stream in front of the condenser-refrigerator 8. The main NG stream passes through the recovery boiler 5, heat carrier 6 and exhaust gas heaters 7 and also enters the condenser-coolers 8.

In condensers 8, 8 ^* NG is cooled to a temperature of about 40 ^o C and together with the resulting solution of nitric acid enters the absorption column 9.

 In the absorption column 9, a normal acidification process proceeds. The VH leaving the column (9) are heated in a heat exchanger 7, a combustion chamber 11, subjected to high-temperature or selective catalytic purification from nitrogen oxides 12, go through a turbine 13, a steam boiler 14 and are released into the atmosphere.

 The nitric acid formed in column 9 through the bleaching column 10 is discharged as a production and oxidizing agent. Air with nitrogen oxides, acid vapor and water is transferred to the lower part of the absorption column 9.

If desired, an acid of 2 concentrations can be given out on the unit. With a high content of nitrogen oxides in NG, an acid concentration above azeotropic can easily be obtained by condensation of reaction water in condensers (8, 8 ^* ), and production acid, up to 62.0% HNO ₃ , in column 9. At the same time, at high PNO ₂ acid up to 70% can be obtained in column 9. In this case, it should be diverted from 3-5 plates of the column.

 In any case, a higher concentration acid should be used as an oxidizing agent. The concentration of production acid can always be adjusted according to need.

To provide energy for the process of nitrogen oxidation (reaction 1), a special energy carrier heater 16 is installed. In it, due to external energy (for example, burning natural gas), the air is heated to a temperature of 800 ^o C and fed to the reactor 4 in front of the catalyst beds.

 The energy of the flue gases after the heater 16 is used to produce steam 15.

 The mode and parameters of the heater are determined by the specified mode of operation of the catalysis zone of the reactor.

 The proposed scheme allows you to widely vary the parameters of its work.

 This primarily refers to the stage of processing NG. Here, various options for utilizing NG energy, withdrawing reaction water, changing the concentration of the resulting nitric acid, etc. can be used.

 There are various options for processing nitrous gases, but they will not affect the basic technological solutions that determine the process of oxidation of molecular nitrogen.

Research and development of the process was carried out on a pilot plant made of stainless steel and titanium. The installation had: an air compressor with a pressure of up to 30 atm (had gas circulation); saturator to obtain a steam-acid mixture (PCVS); a reactor having two catalyst shelves and a stage for the processing of nitrogen oxides — heat exchangers, condensers, an absorption column and catalytic purification of high-temperature gases from NO _x .

 The set temperatures in the reactor and saturator were maintained by external heating. Depending on the stage of research, the gas supply to the reactor could vary.

 The amount of gas supplied, temperature and pressure were continuously measured by instrumentation; the content of nitrogen oxides was determined by chemical analyzes, and in some cases spectrophotometric. The oxygen content in the SH, if necessary, was determined by chemical analyzes.

In studies, the air flow rate was 350-1500 l / h NU; pressure from 1 to 20 atm, the temperature in front of the catalyst varied from 600 to 800 ^o C. At the exit from the catalyst layer, due to the endothermic reaction of nitrogen oxidation, it decreased to 450-550 ^o C (depending on the given mode).

Based on the air consumption and the results of chemical analyzes of PCVS and exiting NG on a computer, the material balance of experience was calculated using a special program. The change in the volumes of the components entering the catalyst and the exhaust gases calculated the degree of growth of nitrogen compounds, β ₀ (degree of useful use of HNO ₃ vapor, reaction 1) and the degree of thermal decomposition of vapor of HNO ₃ (according to reaction 2).

 Based on the balance data, other process indicators could be calculated, for example, the degree of use of nitrogen, oxygen, etc.

Example 1. (Table 1, paragraph 4). Checked the tightness of the installation. Electric heating raised the temperature in the reactor to 750 ^o C in a saturator filled with 70% HNO ₃ , to 80 ^o C and the supply line of PCVS to 240 ^o C, the compressor starts, the air is compressed to 7.4 atm and the pressure in the apparatus is gradually increased. The temperature in the reactor is adjusted and rises in the saturator to 140 ^o C. After reaching the set temperature in the reactor and saturator, the air purified from the oil is taken from the main compressor discharge stream and the installation is launched into the duct. From the compressor, air is sent to the saturator, where it is saturated with nitric acid and water vapors. The resulting steam-acid-air mixture (PCVS) through the heated line enters the reactor, passes the catalyst bed and then all the nitrogen oxide processing apparatus.

At the set air flow rate (1000 l / h), the temperature of the nitric acid solution is again adjusted to 140 ^o C and the gas mixture in front of the catalyst bed is 750 ^o C.

 The established regime was maintained for 2 hours, after which at the same time (before and after the catalyst layer) gas samples were taken for the content of nitrogen compounds. The absorbing liquid was hydrogen peroxide. The absence of nitrous oxide in the gases is confirmed by spectral analysis.

 4 parallel gas samples were taken.

 The content of nitrogen oxides was: in front of the catalyst layer (PCVS) - 16.03 + 0.23%; at the exit from the layer - 25.50 + 0.2%; the absolute increase in nitrogen compounds is 9.5 vol.%.

Having the air flow rate, the concentration and temperature of the acid in the saturator, the equilibrium vapor pressure of HNO ₃ and H ₂ O for these conditions, and also knowing the content of AA vapors in PCVS according to a special program on a computer, we calculated the material balance of the reactor.

по реакции (2) - 16%,

объем NO_x на выходе;

объем HNO₃ на входе.From the obtained volumes of gas components before and after the catalyst layer, the degree of HNO ₃ vapor conversion was calculated by reaction (1)

according to reaction (2) - 16%,

output NO _x

volume of HNO ₃ at the inlet.

The degree of oxidation of NO (α) - 31.5%
Gas speeds under operating conditions: V _p = 0.115 m / s, W _p = 16080 hours, where V _p is the linear velocity under operating conditions; W _p - space velocity in operating conditions.

 The increase in nitrogen compounds in the experiment amounted to: 1082 l / h.

on the installation described in example 1 (table 1, paragraph 4) a fairly broad study of the process was carried out in the parameters:
pressure - from 1 to 20 atm, temperature in the catalyst bed 850 - 500 ^o C, space velocity from 7 to 12.5 thousand hours under excess pressure and up to 80 thousand hours under atmospheric pressure. The amount of catalyst loaded also varied.

 Part of the experiments are shown in table 1.

The degree of useful use of HNO ₃ vapor according to reaction (1) ranges from 60 to 85% with a gas content of up to 17% oxygen and up to 25% water vapor.

 The difference between this scheme and the prototype is to carry out all stages of the process of producing AK under uniform pressure in the absence of SH circulation, some other technological parameters and solutions.

The feasibility study of several options for the production of non-concentrated nitric acid from nitrogen oxides obtained by the oxidation of molecular nitrogen has been completed. Aggregates of AK production were tied to one construction site and were calculated on a single capacity of an analogue - the reconstructed UKL-7 unit - 120 thousand tons of HNO ₃ per year.

 As follows from Table 2, all production options based on the use of NG obtained by nitrogen oxidation have significantly better performance than AA production from ammonia. In particular, the prime cost of AK for all options (Schemes 1, 2, 3) is 24-35% lower than modern schemes for the production of AK from ammonia (Scheme 4).

 The AK production unit according to the claimed method (sche. 2, direct-flow) has against the prototype (sche. 1, 3) 11.5–13.0% lower energy consumption and 5–8% lower cost of acid.

 This is mainly due to the lack of circulation of exhaust gases in the proposed scheme.

Claims (8)

 1. A method of producing nitric acid from nitrogen oxides, comprising oxidizing molecular nitrogen on a catalyst with oxygen compounds at elevated temperatures to produce nitrogen oxides, oxidizing nitric oxide II with oxygen, utilizing heat, absorbing nitrogen oxides with water, characterized in that the process of catalytic oxidation of molecular nitrogen carried out under a pressure identical with the stage of water absorption of the obtained nitrogen oxides, and the energy for the oxidation process is supplied with a gas stream directly to the reaction zone ie outside sources. 2. The method according to claim 1, characterized in that the process of catalytic oxidation of nitrogen is carried out at a temperature below 1000 ^o C, preferably 800 to 450 ^o C and the content of the oxidizing agent in the gas phase before the catalyst is below 10 vol.%, Preferably 1-5.0 % vapor HNO ₃ + NO ₂ .  3. The method according to claim 1 or 2, characterized in that the process of nitrogen oxidation and absorption of nitrogen oxides is carried out under pressure up to 25 atm.  4. The method according to any one of claims 1 to 3, characterized in that as the carrier gas of energy, use air, including oxygen enriched.  5. The method according to any one of claims 1 to 4, characterized in that the energy of nitrous gases after oxidation of nitrogen, oxidation of nitric oxide II by oxygen and absorption is used to obtain a gas stream with an oxidizing agent.  6. The method according to any one of paragraphs. 1 to 5, characterized in that the nitrogen oxidation catalyst is arranged in several layers in the reactor, the oxidizing agent being supplied to each layer in predetermined quantities, and the energy flow as necessary.  7. The method according to any one of claims 1 to 6, characterized in that for the formation of a gas stream with an oxidizing agent, a gas mixture obtained by blowing production acid is used.  8. The method according to any one of claims 1 to 7, characterized in that platinum alloys with platinum group metals or catalysts based on iron, cobalt, chromium, aluminum oxides with promoting additives of refractory metals are used as nitrogen oxidation catalysts.

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