RU2253614C1 - Method of modernization of an installation for production of nitric acid with its increased productivity - Google Patents

Abstract

FIELD: chemical industry; modernization of installations of nitric acid production according to a combined scheme.

SUBSTANCE: the invention is pertaining to the field of chemical industry, in particular, to the method of modernization of installations of nitric acid production. The method of modernizing of installation of production of the nitric acid providing for oxidation of ammonia with the help of air under a rarefaction, compression of the cooled nitrous gases in a nitrose supercharger, an absorption of nitric oxides in a tower absorber under pressure of 3.5-4.0 kg / cm², expansion of the waste tail gases in the turbo-expander, consists in the fact that they increase the pressure of the nitrous gases at the inlet of the supercharger in the range from rarefaction up to the pressure of 1.02-1.05 kg / cm², by installing a new or an additional gas blowers on the lines of the ammonia - air mixture or the nitrous gases or by decrease of hydraulic resistance of apparatuses and devices of pipelines at absorption inlet of the nitrose supercharger. At a pressure increase at the inlet of absorption of the nitrose gasses supercharger by decrease of the hydraulic resistance of the apparatuses and pipelines before absorption of the nitrose supercharger the increase of productivity of the installation will be much lower (~ up to 8 %). The method ensures an increase of productivity of the installation using the existing equipment at addition of small investments, which pay off less than for 1 year (from 0.6 up to 0.8 of a year) due to decrease of specific consumptions of raw material and power and a significant decrease of the conditionally-permanent expenses. Simultaneously the method allows to increase concentration of nitric acid and a degree of absorption, that may achieve its maximum at addition of pressure systems.

EFFECT: the invention ensures increased productivity of the installation at addition of small investments, which pay off less than for 1 year.

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Description

The method relates to the modernization of the installation of nitric acid production according to combined schemes with the oxidation of ammonia by air under vacuum, compression of nitrous gases in a supercharger with a turboexpander and absorption of nitrogen oxides under increased pressure, mainly 3.5 kg / cm ² .

Such plants with relatively low power and pressure at the absorption stage of 3.5 kg / cm ² continue to be operated at a number of enterprises in Russia and the CIS countries.

The technological scheme of such plants includes cleaning the air drawn in from the atmosphere, compressing the mixture of air and ammonia coming from the gas tank in a gas blower to a pressure of ~ 1.08 kg / cm ² , heating the ammonia-air mixture (ABC) with nitrous gases, oxidizing the ABC in the contact apparatus operating under a small vacuum (200-400 Pa), on a two-stage catalyst at a temperature of 800-820 ° C, cooling of hot nitrous gases (NG) in a waste heat boiler, then in an ABC heater, in a refrigerator-washer with water cooling to 45- 55 ° C, cooling mix associated NG with additional atmospheric air, which is blown off from production of nitric acid from NO _x , compression of NG in a centrifugal supercharger (NIS) driven by an electric motor and turboexpander to 3.5-4.1 kg / cm ² , cooling of compressed NG, absorption of nitrogen oxides in an absorption column, catalytic purification of tail gases from NO _x residues in a selective low-temperature purification reactor at a temperature of ~ 260-300 ° С, expansion of tail gases in a recovery gas turbine before discharge into the atmosphere [1, p. 69-78].

As the nitrous blower wears out, the unit's performance decreases; at the same time, the degree of compression of NG and the concentration of nitric acid decrease, which in the aggregate negatively affects its cost.

Therefore, at present, the urgent task is to modernize these plants with an increase in over-design productivity over the design range within the pressure increase allowed by the strength characteristics of the absorption stage apparatuses.

During the long-term operation of these plants, partial modernizations were carried out in order to increase the concentration of nitric acid (installing an NG cooler at the entrance to the absorption column), to reduce the content of nitrogen oxides in the tail gases at the exit of the absorption column (cooling a part of the plates with water cooled to + 5 ° C) ; installation of a reactor for selective catalytic treatment of tail gases to a residual content of NO _{x of} 0.005 vol.% [see 2, pp. 63-66].

The indicated modernizations solved fragmented private problems without increasing the productivity of the units, which led to an increase in the cost of nitric acid.

The aim of the present invention is a method of modernizing the installation of 3.5 ati with an increase in productivity by a maximum of 20% while increasing the concentration of nitric acid from 47-49% to 50-53% HNO ₃ , reducing the content of nitrogen oxides in the tail gases after the column to 0.1 -0.11 vol.%. As a result, the costs of ammonia, electricity, demineralized water are reduced by 1 ton of HNO ₃ , and the cost of production.

The essence of the present invention lies in the fact that to achieve this goal increase the pressure of nitrous gases at the inlet of the nitrous supercharger in one of the following ways:

- installation of a new gas blower of ammonia-air mixture with a higher pressure at the discharge or additional to the existing one;

- installation of a gas blower on a line of nitrous gases after cooling to 40-50 ° C in a refrigerator-washer;

- reducing the hydraulic resistance of apparatuses and devices on pipelines from the air intake from the atmosphere to the absorption of nitrous gases by a supercharger (NIS).

With increasing NG pressure at the inlet of the supercharger, its productivity and compression ratio increase according to the aerodynamic characteristics of the supercharger 541-41-1 (see FIGS. 3-6), which is installed in modernized installations. The greatest increase in pressure at the inlet of the NIS is achieved by installing an additional (or new with a higher pressure) gas blower on the ammonia-air mixture line or a new gas blower on the line of nitrous gases.

The authors of the present invention are not aware of modernization methods with increasing the productivity of plants according to a combined scheme with ammonia oxidation by air under vacuum, and with absorption of nitrogen oxides under an average pressure of 3-5 kg / cm ² , in which, to intensify, the pressure of nitrous gases at the inlet of the nitrous supercharger was increased.

Units with ammonia oxidation under vacuum, which initially had excess pressure at the suction of the NIS, are also unknown.

In modern large-capacity plants, according to the combined scheme of ammonia oxidation, they are produced under a pressure of 3-5 kg / cm ² , and nitrogen oxides are absorbed under a pressure of 9-15 kg / cm ² [2]. Therefore, the authors cannot indicate a close analogue of the proposed method for the modernization of plants according to the combined scheme with oxidation under vacuum and in general any plants for the production of nitric acid.

One of the essential features of the present invention is the limitation of the upper level of pressure increase at the NIS intake with a pressure of 1.02-1.05 kg / cm ² , which is associated with the strength characteristics of the devices of the absorption stage and the aerodynamic characteristics of the nitrous supercharger 540-41-1. An absorption column is adopted as the determining apparatus, the strength of which, according to the passport data, allows a pressure of not more than 4 kg / cm ² in the upper part of the apparatus. According to the calculations, at a maximum productivity of the nitric acid aggregate of 6.9 t / h (20% higher than the design maximum permissible for the modernized installation), that is, at a flow rate of 29,500 m ³ / h of NG through the column, the pressure loss in the NIS - oxidizer path - tail gas heater - refrigerator - lower part of the absorption column "is ~ 0.8 kg / cm ² . Therefore, the permissible pressure at the injection of the NIS after intensification is 4.8 kg / cm ² .

According to the aerodynamic characteristics of the supercharger 540-41-1, such pressure at the gas flow rate indicated above is reached at a temperature of 40 ° C and an inlet pressure of 1.02 kg / cm ² , and at 60 ° C - 1.05 kg / cm ² .

An increase in the pressure of nitrous gases at the inlet of the NIS can be made according to two schemes:

- The first scheme is to increase the pressure of the ammonia-air mixture (ABC) in the installation head. The ABC pressure increase level to obtain the nitrous gas pressure at the NIS intake within 1.02-1.05 kg / cm ² taking into account the resistance losses of the ABC path “ABC gas blower - gas pipelines - ABC heater - control dampers - ABC filter” at the end of the run and nitrous gas path “bottom of the contact apparatus — boiler — utilizer — heater ABC — refrigerators-washers — mixing unit with air — gas pipelines”, according to calculations, is 0.12 kg / cm ² . To do this, you can install a new gas blower, but it is preferable to install the same gas blower in series with the existing TG-500-1,08 gas blower, which, in addition to reducing the amount of investment, allows you to economically solve the problem of starting the contact apparatus under vacuum (turning on only one gas blower). According to this scheme, in the nominal mode, the contact apparatus for the oxidation of ammonia will operate at a pressure of 1.125 kg / cm ² ;

- the second scheme is the compression of nitrous gases cooled to 45-55 ° C after refrigeration washers with gas blowers with a total pressure of 0.1-0.12 kg / cm ² . Blowers must be made of stainless steel. According to this scheme, contact devices operate under vacuum.

The present invention includes the upgrade of the installation without additional pumping devices; in this embodiment, an increase in the pressure of nitrous gases at the inlet of the NIS is carried out by reducing the hydraulic resistance of the apparatus and devices on the tract to the NIS.

On the path of the ammonia-air mixture, a reduction in gas pressure losses can be achieved:

- changing the design of the air filter and replacing the filter material with a material with lower resistance;

- exception throttle on the ABC line;

- installation of a second heater ABC, which also gives a decrease in resistance along the path of nitrous gas;

- design changes and replacement of filter material in the ABC filter.

According to rough calculations, this can give a reduction in pressure loss in the ABC and NG lines to 400-500 mm c. Art. In this embodiment, a vacuum is maintained at the suction of the NIS, and the increase in productivity will reach no more than 8% of the design.

The following are examples of the implementation of the present invention according to schemes with the installation of additional discharge devices.

Example 1 (figure 1).

Air taken from the atmosphere is filtered in filter 1. An ammonia-air mixture compressed in an existing gas blower (item 2) under a pressure of 1.06-1.08 kg / cm ² is fed to the inlet of a newly installed gas blower (item 3) with with a productivity up to 30,000 m ³ / h and a pressure up to 0.12 kg / cm ² under suction conditions (temperature - 20-35 ° С, pressure 1,06-1,08 kg / cm ² ). In the newly installed gas blower, the ABC pressure rises to 1.18-1.2 kg / cm ² . ABC heating and cooling of nitrous gases is carried out in existing technological equipment. Heated ABC is fed into the contact apparatus (item 4), which is transferred to the operating mode under pressure up to 1.125 kg / cm ² . For this, the contact apparatus is being reconstructed in order to increase its tightness. Nitrous gas is cooled in existing devices and enters the inlet of the nitrous supercharger (item 5) under a pressure of 1.02-1.05 kg / cm ² . Compressed to 4.7-4.8 kg / cm ² nitrous gases are then sent for processing to the absorption department, where, if necessary, work is carried out to ensure the operation of the apparatus under this pressure. In connection with the transition to the operating mode from rarefaction to excess pressure at the inlet of the nitrous supercharger to supply additional air and purge gases from the purge column (pos. 6) of nitric acid from dissolved nitrogen oxides, a fan (pos. 7) is installed, which supplies air under a pressure greater than the excess pressure at the inlet of the NIS by the resistance value of the purge column and piping system.

Example 2 (figure 2).

In this embodiment, the modernization of the installation after cooling of nitrous gases in refrigerator-washers to 50-55 ° C increases the pressure of nitrous gases from 0.92-0.93 kg / cm ² to 1.02-1.05 kg / cm ² by gas blowers ( item 1). At the inlet of these gas blowers, additional and purge air enters, which before modernization was supplied to the inlet of the nitrous supercharger. This option does not require sealing of the contact apparatus.

Next, nitrous gases are compressed in a nitrous supercharger 2 to a pressure of 4.7-4.8 kg / cm ² and enter the absorption system.

The modernization of the installation according to the options described in examples 1 and 2, provides an increase in productivity by 15-20% from the initial to modernization, while a 20% increase in productivity relates to the upper limit of the increase in pressure at the inlet of the nitrous supercharger to 1.05 kg / cm ² .

The modernization of the installation without the use of additional gas blowers on the lines of ammonia-air mixture or nitrous gases, but with a decrease in pressure losses along the ABC path and nitrous gases to the suction of the nitrous supercharger due to the technical solutions described above, allows to increase the plant productivity by 8%.

The above examples illustrate the essence of the invention, but do not limit it.

Literature:

1. The Nitrogen Handbook, Volume 2. M.: Chemistry, 1969, pp. 69-78

2. The Nitrogen Handbook, edition 2, book 2. M.: Chemistry, 1987, pp. 63-66.

Claims (1)

A method of modernizing a nitric acid production plant, including the oxidation of ammonia by air under vacuum, the compression of chilled nitrous gases in a nitrous supercharger, the absorption of nitrogen oxides in an absorption column under a pressure of 3.5-4.0 kg / cm ² , the expansion of exhaust tail gases in a turboexpander, characterized in that the pressurized nitrous gases suction blower ranging from vacuum to pressure 1,02-1,05 kg / ^cm2 installing new or additional blowers on lines ammonia-air mixture or nitrous gas or reduction in the hydraulic resistance devices and devices on pipelines to nitrous suction blower.

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