RU2286943C2 - Method of intensification of the installation for production of nitric acid - Google Patents

Abstract

FIELD: chemical industry; method of intensification of the installations for production of the non-concentrated nitric acid.

SUBSTANCE: the invention is pertaining to the method of intensification of the installations for production of the non-concentrated nitric acid and may be used for raising productivity of the installations for production of the non-concentrated nitric acid under pressure. The invention provides for creation of the excess pressure on the inlet of the air compressor by preliminary compression of the atmospheric air in the high-pressure fan. At that the heat of the compression process in the warm season of the year is withdrawn by the direct contact with the water at the inlet of the fan, and in the cold season the heat is used for heating, at that in full or partially excluding heating of the air in the preheater mounted to prevent the icing up of the guiding apparatuses of the air compressor. At the enterprises with the high degree of the air dusting or chemical pollution for the contact cooling of the air by water it is possible to use scrubbers-washers, which combine the functions of the air cooler and the purification device. The method is effective for the operating installations, in which as a result of the wear-out of the flow-through section of the air compressors and the gas turbines decreases not only productivity, but also the pressure in the system, and as the result of it the concentration of the nitric acid. The method allows to realize the intensification of the installations using already existed equipment due to the increased pressure in the system. Concentration of the nitric acid is not lowered, the degree of purification of the tailing gases is preserved, production cost and the specific consumption of the steam and the natural gas are reduced.

EFFECT: the invention allows to realize the intensification of the installations using already existed equipment, to reduce production cost and the specific consumption of the steam and the natural gas.

4 cl, 2 ex, 2 tbl, 2 dwg

Description

The invention relates to the field of industrial production of non-concentrated nitric acid and can be used to intensify existing plants in which nitric acid with a concentration of 55-65% HNO _{3 is} obtained by oxidizing ammonia with atmospheric oxygen under pressure and absorbing nitrogen oxides under the same or higher pressure.

These units include a machine unit including an air compressor for compressing atmospheric air, a nitrous supercharger for compressing nitrous gases (in installations with a higher pressure at the absorption stage), and a drive for these compressors. The drive in modern installations includes a gas turbine for recovering the energy of compressed tail gases after absorption in combination with an electric motor or steam turbine.

In Russia and the CIS countries, the production of nitric acid is almost completely carried out at the plants according to the energy technological scheme, in which the machine unit is a gas turbine unit. In medium-pressure units under uniform pressure (UKL-7 units), the GTT-3 gas turbine unit includes an air compressor and a gas turbine with a calculated tail gas temperature of 700 ° C. In large-capacity units with two pressures of the AK-72 type, the GTT-12 gas-turbine unit includes an air compressor, a nitrous supercharger and a gas turbine with a calculated tail gas temperature of 760 ° С. Tail gas is heated by burning natural gas. The electric motor in the GTT-3 (10% of the total drive power), the steam turbine (8%) in the GTT-12 according to the design are designed to start the machine units, but in fact they are constantly operated as an additional drive.

The source of information

1. "Production of nitric acid in units of large unit capacity." / Edited by Olevsky V.M., Moscow: Chemistry, 1985, pp. 94-306.

2. The reference book of the nitrogen nitrogen, M .: Chemistry, 1987, pp. 66-92.

Well-known installations of Western firms are built on the basis of schemes without using the energy of burning natural gas. Accordingly, the drive of the machine units includes a gas turbine for recovering the energy of tail gases heated to 300-500 ° C by the heat of nitrous gases after the oxidation of ammonia, and the closing engine - a steam turbine or electric motor.

The performance of nitric acid production plants can decrease during operation, both due to deterioration of machine units and due to technological reasons (for example, due to a decrease in the degree of ammonia conversion and deterioration of heat exchange equipment).

But the main reason for reducing the productivity of plants is always a decrease in air supply by an air compressor due to wear, pollution of the flow part, and a decrease in the number of revolutions. This applies to machine units of any type, but to varying degrees. Machine units in which the speed of the compressors (air and nitrous) is constant due to the high-power electric motor are more stable. Less stable are machines with a steam turbine as the closing engine due to a reduction in RPM during wear. The most unstable are gas turbine driven machine units, such as GTT-3 and GTT-12, in which relatively quick wear of high-temperature gas turbines, with loss of efficiency, is added to the wear of the air compressor. At the same time, gas turbine engines of the GTT-12 type in the AK-72 unit with a low-power steam turbine are particularly sensitive to the loss of efficiency of gas turbines due to a drop in the speed of the air and nitrous compressor, which, in addition to reducing the air supply, i.e. plant performance, and leads to a significant decrease in pressure in the absorption system of nitrogen oxides and, as a consequence, to a decrease in the efficiency of the absorption column.

In plants of the UKL-7 type with a GTT-3 machine, in which constant revolutions are maintained due to the electric motor, wear of the air compressor and gas turbine leads to a decrease in HNO ₃ production not only due to a direct decrease in air supply, but also because of the need to increase fractions of compressed air directed by the technology to a gas turbine.

The technical problem to which the present invention is directed is to achieve a significant increase in the productivity of nitric acid plants - at least 8-15% (depending on the type of units) without costly upgrading or replacing machine units on existing process equipment by simultaneously increasing the air supply and pressure. The invention can be used in the construction of new plants, but its main purpose is the intensification of existing plants such as UKL-7 and AK-72 with gas turbine engine units.

The essence of the invention is to provide excess air pressure at the inlet to the air compressor of the machine unit (instead of vacuum) by pre-compressing the air in a high-pressure fan and cooling it by direct contact with water.

The novelty of the method according to the invention consists in creating excess pressure at the inlet of the air compressor and lowering the air temperature after preliminary compression in the high-pressure fan by direct contact of air with water.

According to the present invention, the amount of excess pressure at the inlet of the air compressor is limited by the permissible limit pressure in the apparatus of the nitric acid plant, which is intensified. All other things being equal (compressor shaft revolutions, inlet air temperature, polytropic efficiency, etc.), the air pressure after compression is approximately directly proportional to the inlet air density. Technological equipment is usually calculated on the maximum air pressure that can be achieved in the air compressor in the cold season, when the density of air at the suction is maximum.

Standard conditions for calculating GTT-3 and GTT-12 engine assemblies for suction conditions of an air compressor:

- nominal air temperature + 20 ° С,

- in winter - 5 ° С (taking into account heating at a lower temperature, in order to prevent icing of the guide apparatus),

- barometric pressure 745 mm Hg (99.3 kPa),

- vacuum at the inlet 500 mm century (0.49 kPa), i.e. Absolute pressure at the inlet 98.81 kPa.

This means that in winter the maximum air density, and therefore the maximum design air pressure, is (273 + 20) / (273-5) = 1.0932 times higher than the nominal. Accordingly, the permissible absolute pressure at the inlet at a nominal temperature of + 20 ° C will be 98.81 × 1.0932 = 108.02 kPa, which is achieved by preliminary air compression of 11-11.5 kPa. Accordingly, the performance of the air compressor due to the air density factor can be maximally increased by (108.02-98.81) × 100 / 98.81 = 9.3%. For machine units with a gas turbine drive of the GTT-12 type, the speed of which depends on the wear of the drive, the performance of the air compressor grows to a greater extent. This is due to the fact that the degree of air compression and specific energy consumption in the air compressor does not depend on the density of air, while the energy output in a gas turbine increases due to an increase in the degree of expansion, which leads to an increase in the number of revolutions. Table 1 shows the results of a calculation study of the existing GTT-12 machine in one of the AK-72 units after many years of operation using actual indicators of air consumption, shaft speed of the air compressor and nitrous supercharger, gas pressure and temperature along the system path.

When installing a high-pressure fan with a pressure of 11-11.5 kPa, an excess pressure is created at the inlet of the air compressor 0.09 kgf / cm ² higher than the original.

Based on such excess pressure, the calculations (Table 1) of various modes of increasing the air supply of the existing GTT-12 were performed. The initial mode, option 0, reflects the actual indicators before intensification. They served as the initial data for a design study program of the effect of increasing the pressure at the inlet of the air compressor on its operation. Options 1-5 relate to the operation of GTT-12 before the repair of a nitrous supercharger, 6-7 after repair.

As can be seen from the calculation results, after increasing the pressure at the inlet, the intensification (air supply) is 12% without changing the energy consumption from the steam drive (option No. 3), and after repair No. 4 - with a decrease in steam consumption in the backpressure steam turbine from 41 to 24 t / h (option No. 6), while the pressure at the stage of ammonia oxidation rises from 3.9 to 4.51-4.47 kgf / cm ² , at the absorption stage - from 9.2 to 10.42 and 10.38 kgf / cm ² , i.e. at - 1.2 atm; OK shaft revolutions increase from 4860 to 4960 rpm.

An increase in pressure at the stage of ammonia oxidation by 15%, at the absorption stage - by 13% allows intensification to be performed on the same technological equipment.

Under mode No. 8, without decreasing the load of the steam turbine, an increase in the production of nitric acid of ~ 19% can be obtained.

Accepted abbreviations in table 1:

OK - axial air compressor;

NN - nitrous supercharger;

TVD - high pressure turbine.

For UKL-7 units with a GTT-3 gas-turbine engine unit, the ratio between the increase in air pressure at the inlet of the air compressor and the increase in nitric acid production remains disproportionate, but to a lesser extent due to the constant speed provided by the electric motor.

Table 2 shows the results of computational studies of the GTT-3 machine in a UKL-7 nitric acid installation under uniform pressure at the stages of ammonia oxidation and absorption.

The initial mode, version 0.1, reflects the actual indicators before the intensification of the GTT-3M machine operating as part of the UKL-7 installation in a region with a temperate climate and barometric pressure of 745 mm Hg.

Option 1.1 - the mode after intensification with the installation of a fan for preliminary air compression up to 11-11.5 kPa and the load of the electric motor (motor generator FAZ-800) with reduced power consumption.

Options 0.2, 1.2 relate to the operation of the GTT-3M in a region with a hot climate (up to 35 ° C in summer) and low barometric pressure (Central Asia, the Caucasus).

Computational studies were carried out on the basis of indicators of operating machines as part of installations.

The baseline for installation 1 in a temperate climate corresponds to a production of 14 t / h mng. HNO ₃ , for installation 2 in Central Asia, production of 12 t / h mng. HNO ₃ .

After intensification, ceteris paribus, at facility 1, output increases to 15.3 t / h (~ 9.0%), at plants 2 - up to 13.8 t / h (~ 15.0%).

Accepted abbreviations in table 2:

OS - axial compressor;

TsN - centrifugal blower of an air compressor;

GT - gas turbine;

FAZ-800 - a reversible electric motor as part of the GTT-3M (M - modernized).

Pre-compression of air to 11-11.5 kPa in the fan leads to an increase in its temperature by 10-12 ° C, which somewhat eliminates the increase in its density. The air must be cooled to a design temperature of + 20 ° C. In the known method of cooling through a wall, a refrigerant with a temperature of + 5 ° C is required.

To remove the heating of air during compression, it is necessary to divert 12500-18700 kcal per 1 t of HNO ₃ , which is equivalent to the evaporation of 42-60 kg of liquid ammonia.

The energy consumption for producing cold with a potential of + 5 ° C in modern ammonia refrigeration machines will be at least 5-6 kWh per 1 ton of HNO ₃ .

Capital expenditures for the construction of an ammonia-refrigeration station and an air cooler are also significant, while air cooling takes 5-8 months a year.

Therefore, according to the present invention, the air is cooled by direct contact with water, i.e. evaporative cooling.

To drain 12500-18700 kcal, 21.7-26 kg of water per 1 ton of HNO ₃ must be evaporated (for AK-72 installations with intensification by 12% - ~ 1215 kg / h, for UKL-7 with intensification by 9% - ~ 420 kg / h).

With evaporative air cooling, the degree of cooling is affected by the relative humidity of the air.

As calculations show, for air with an initial temperature of + 20 ° С and with an initial relative humidity of air <65% by evaporative cooling, it is possible to cool the air to an initial temperature of + 20 ° С, at a relative humidity of 70% - up to 21 ° С, at a relative humidity of 90 % - up to 23 ° С.

But even on such days, in comparison with cooling through the wall, contact cooling does not lose. Undercooling of air by 1-3 ° C during contact cooling is compensated by lower hydraulic losses due to the exclusion of the heat exchanger.

Contact cooling of air with water is especially effective from the point of view of energy consumption if it is injected with water through a nozzle with fine atomization at the fan inlet when the compression process in the fan approaches isothermal.

In certain cases, for example, when operating plants in hot and dry climates (a significant part of Russia, Central Asia), air pre-cooling in a scrubber can be used, since in these areas during the hot season the air has low relative humidity, but is often contaminated with dust from dust storms. When installing a scrubber to the fan, the air temperature at the suction decreases, the introduction of water splashes and their evaporation in the fan during air compression contributes both to lowering the air temperature at the inlet of the air compressor and reduces the energy consumption for air compression.

Contact cooling of the air slightly increases the content of water vapor in the air, but not more than 0.4-0.5% by volume.

As a result of this, in comparison with cooling of compressed air through the wall, the productivity of the air compressor decreases by 0.45%, and the thermal load of the refrigerator-condenser on nitrous gases increases by 2%.

These negative aspects of contact cooling of the air are insignificant in comparison with the advantages with respect to cooling through the wall, especially since air cooling is required only in the warm season (at an air temperature of + 10 ° С it may not be performed).

The implementation of the invention can be demonstrated in examples 1 and 2.

Example 1, figure 1. Intensification of installations like AK-72 and UKL-7.

Fan 1 is re-installed with a capacity of 230-240 thousand nm ³ / h, which is 15-20% higher than the nominal capacity of the GTT-12 air compressor; fan head 11-11.5 kPa.

To the fan inlet through nozzles 2, 1-1.25 m ³ / h of steam condensate or demineralized water is supplied.

At a temperature of atmospheric air + 20 ° С, relative humidity 60-65% and removal of heat of compression by evaporation of water, the air temperature at the fan discharge is maintained at + 20 ° С, and relative humidity rises to 90-95%.

At a relative humidity of atmospheric air of 90%, the water injection remains at about the same level, the temperature of the compressed air after evaporation of the water until it is completely saturated is stabilized at ~ 23 ° C.

At a temperature of atmospheric air below 10 ° C, the injection of water into the fan can be stopped.

In the cold season, heating the air during compression in the fan allows a long period of time not to spend water vapor on heating the air in the heater installed at the inlet to the filter 3 in front of the air compressor of the installation.

Periodically, the injection of demineralized water for a short time for washing increases to prevent the deposition of salts in the flow part of the fan with the drainage of water from the fan into the drain.

With the intensification of the UKL-7 installations, there are no fundamental differences from the invention used in comparison with the described one. Accordingly, a high-pressure fan is installed with a capacity of 88-90 nm ³ / h of air and a pressure of 11-12 kPa.

Example 2, figure 2.

In regions with a hot climate and / or with high dustiness of atmospheric air, a rough air filter 3 and a washing scrubber 2 with desalted water circulation using pump 4 are installed on the fan inlet 1.

Upon contact of air with water on the mass transfer nozzle of the scrubber 2, air is saturated with water vapor. The saturation process occurs with varying degrees of air cooling depending on relative humidity.

At a relative humidity of 40-50% and an ambient temperature of + 30 ° C, the air is cooled to a temperature close to the temperature of the "wet thermometer", in this case, up to 22 ° C.

When this air is compressed, taking into account cooling during evaporation of droplets carried away with air and additionally injected water at the outlet of fan 1, the air temperature is ~ 25 ° С and at the same time it will have a humidity close to full saturation. Through the scrubber 2 to install AK-72 runs ~ 220 thousand Nm ³ / h of air, to install UKL-7 -.. 90 thousand Nm ³ / hr into the scrubber injected fresh water in an amount equal to the amount of evaporated water and outputted as a drainage with trapped dust.

There should not be a splash trap in the scrubber; if the entrainment of the water spray is insufficient to remove the heat of compression of the air by their evaporation, additional water is supplied to the fan through the nozzles.

The examples and drawings to them illustrate the application of the method for installing AK-72, and for UKL-7, which does not exhaust the scope of the invention. It can be used for any nitric acid production plant.

Among the most important advantages that provide a significant economic effect in the implementation of the method include:

- low capital investment on capacity gains;

- short terms for implementation, and without stopping the current installation;

- during intensification, the pressure in the system increases, which allows it to be implemented in existing technological equipment without lowering the concentration of nitric acid and impairing the discharge of harmful substances into the atmosphere with tail gases;

- decreases the cost of production.

Literature

1. The production of nitric acid in units of large unit capacity, edited by V.M. Olevsky. M .: Chemistry, 1985, pp. 94-306.

2. The reference book of the nitrogen nitrogen, volume 2. M.: Chemistry, 1987, pp. 66-92.

Table 1
Table of the results of a calculation study of the operating modes of the current GTT-12 Options 0 one 2 3 four 5 6 7 8 Atmosphere pressure kg / cm ² 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Fan pressure kg / cm ² 1.00 1.09 1.09 1.09 1.09 1.09 1.09 1.09 1.09 OK pressure kg / cm ² 3.90 4.23 4.32 4.51 4.55 4.60 4.47 4.52 4.57 Pressure before NN kg / cm ² 3.40 3.69 3.73 3.95 3.97 3.98 3.92 3.94 3.95 Pressure beyond LV kg / cm ² 9.20 9.89 10.38 10.42 10.70 11.07 10.38 10.66 11.04 Gas pressure in front of the theater kg / cm ² 8.40 8.95 9.38 9.43 9.68 10.01 9.40 9.65 9.99 Rotor speed OK rpm 4860 4930 5050 4960 5035 5,130 4960 5040 5,130 LV rotor speed rpm 4880 4840 5000 4830 4900 5000 4800 4870 4970 Resilience margin OK % 18.8 18.4 23.9 17.8 20.9 25.2 18.6 21.8 26.2 NN stability margin % 44.7 42.5 46.2 42.3 43.7 46.0 43.3 44.8 47.2 Atm temperature air ° C twenty twenty twenty twenty twenty twenty twenty twenty twenty Air temperature before OK ° C twenty thirty thirty twenty twenty twenty twenty twenty twenty Outlet air temperature OK ° C 187 201 205 196 198 200 195 197 200 Air temperature before LV ° C 65 65 70 70 70 70 70 70 70 Air outlet temperature ° C 215 213 226 217 221 227 207 210 216 Temperature, gas in front of the turbine ° C 745 745 745 745 745 745 745 745 745 Gas temperature behind the turbine ° C 391 385 380 380 377 374 380 377 373 Steam flow to turbine t / h 1.0 1.0 1.0 1.5 1.5 1.5 1.0 1.0 1.0 Steam turbine power kw 1700 1700 2400 1700 2050 2550 950 1250 1700 Total steam consumption t / h 41 41 57 42 49 61 24 thirty 41 Air flow before OK t / h 254.0 270.7 283.7 284.5 292.1 302.1 284.5 292.2 302.5 Performance gain % - 6.6 11.7 12.0 15.0 18.9 12.0 15.1 19.1

table 2
The table of the results of a calculation study of the operating modes of the current GTT-3 in UKL-7 units No. p / p Indicators Unit. Options 0.1 1.1 0.2 1.2 one Barometric air pressure kPa 99.3 99.3 96.6 96.6 2 Air pressure kPa 2.1 - behind the inflatable fan - 110.0 - 106.0 2.2 - on suction OK 95 105 92 102 2.3 - on forcing the central nervous system 716 780 680 745 2.4 - before GT 560 609 540 580   3 Temperature ° C 3.1 - air at suction OK twenty twenty thirty 25 3.2 - tail gases before GT 680 680 690 690 four Rotor speed rpm 5100 5100 5100 5100 5 Air consumption t / h 5.1 - before OK one hundred 108,4 85.3 97.2 5.2 - on technology 72 78.5 61.57 70.8 6 Natural gas consumption st.m ³ / h 1680 1800 1500 1725 7 Power consumption kWh 7.1 - on the fan - 400 - 370 7.2 - on the FAZ-800 350 150 400 200 8 The increase in the production of HNO ₃ % - 9 - fifteen

Claims (4)

1. The method of intensification of installations for the production of nitric acid, including the compression of air in an air compressor, the stage of ammonia oxidation by atmospheric oxygen and absorption of nitrogen oxides under pressure, the recovery of heated tail gas energy in a gas turbine, characterized in that excess pressure is created at the air compressor inlet by preliminary compression of atmospheric air in a high-pressure fan, and in the warm season, heat of compression is removed by direct contact of air with water. 2. The method according to claim 1, characterized in that the air is cooled during compression by injection of water through nozzles at the fan inlet. 3. The method according to claim 1, characterized in that the air is cooled in a scrubber upon contact with water, combining air cooling with preliminary cleaning of dust and chemical impurities. 4. The method according to claim 1, characterized in that the absorption of nitrogen oxides is carried out under the same pressure as the oxidation of ammonia, or a higher pressure.

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