RU2372568C1 - Method of extracting ammonia from purge gases - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of extracting NH₃ from purge gases with the following composition, vol %: ammonia - 0.1 to 15, hydrogen - 20 to 55, other gases - the rest, involves washing purge gas in a scrubber and an ammonia extraction stage. Purge gases are washed in a multi-stage scrubber, while cooling ammonia solution between absorption stages, and ammonia is extracted after the first stage of the scrubber by evaporation in volume- or film-type apparatus at synthesis pressure or storage of ammonia in containers with subsequent drying and liquefaction.

EFFECT: improved economical and ecological characteristics of production.

Description

The invention relates to the field of chemical-technological energy-saving processes that use gas mixtures containing such valuable products as ammonia and hydrogen. It can find application in the reconstruction of existing and the creation of new highly efficient ammonia plants.

Known methods for the allocation of hydrogen, nitrogen, argon and a krypton-xenon mixture from purge gases for the synthesis of ammonia provide for their preliminary purification from ammonia. This is due to the fact that the ingress of ammonia on the membranes causes their irreversible compaction; in cryogenic systems, ammonia can become solid; in adsorption plants, the presence of ammonia in the processed gas reduces the activity of adsorbents.

A known method of extracting ammonia from purge and tank gases [RU 2217669, cl. F25J 3/06], which provides for their cooling by refrigerant streams in order to condense liquid NH ₃ .

The disadvantage of this method is the high cost of cold, due to the fact that for the partial condensation of ammonia with a volume concentration of 0.5-10%, it is necessary to cool the entire gas stream.

The closest in technical essence is the method of separation of ammonia, described in the book: Reference book of nitrogen. 2nd ed. revised., M.: Chemistry. 1986. P.385. It includes the absorption of gases by water in a column type apparatus and the burning of a purified gas mixture in a tube furnace. The produced ammonia water contains about 25 wt.% Ammonia and is used mainly as nitrogen fertilizer for sowing. Demand for it is limited, and transporting this type of fertilizer over long distances is unprofitable due to the low content of nutrients in it. Thus, most of the produced ammonia-containing water from the purge gas absorption system is discharged into the sewer, which leads to an increase in the cost of processing industrial effluents.

An object of the invention is to further improve the process for purging purge gases from ammonia to produce commercial ammonia in liquid form.

The problem is achieved in that the absorption of NH _{3 is} carried out by washing the purge gas in a scrubber with cooling the ammonia solution between the absorption steps and the ammonia evolution after the first step of the scrubber by evaporation in a volumetric or film type apparatus with its subsequent drying and liquefaction.

The essence of the invention is a method of extracting NH ₃ from the purge gases of the composition, vol.%:

ammonia 0.1-15 hydrogen 20-55 other gases rest

including washing the purge gas in the scrubber and the stage of ammonia separation, moreover, the washing is carried out in a multi-stage scrubber, with cooling of the ammonia solution between the absorption steps, and the ammonia is extracted after the first stage of the scrubber by evaporation in a volumetric or film-type apparatus under pressure, synthesis or storage of ammonia in food containers with its subsequent drying and liquefaction.

The claimed invention can be implemented as follows.

Ammonia synthesis purge gases containing hydrogen, methane, nitrogen, ammonia and inert gas impurities are fed into a nozzle scrubber for washing ammonia with water. The specified scrubber consists of at least two steps. At the same time, a weak aqueous ammonia solution is sent to irrigate the nozzle of the first stage after the second stage, for irrigation of which water from the treatment system or fresh water is supplied. In order to increase the rate of chemisorption and absorption capacity of the target component (ammonia), an interstage water cooling is provided in the scrubber. After the last stage of the scrubber, the purge gases purified from dust and ammonia enter the hydrogen evolution system based on one of the principles described above.

The use of a multi-stage scrubber with an increased mass transfer area makes it possible to obtain a strong aqueous ammonia solution at the outlet of the first stage with an ammonia concentration close to 25 wt.%, Which minimizes the cost of thermal energy for its evaporation.

After the first stage of the scrubber, washing water containing ammonia is sent to the ammonia recovery system, which works on the principle of evaporation. The proposed method provides two options for its submission to the evaporator:

1) with the pressure of ammonia synthesis;

2) with the storage pressure of liquid ammonia in food containers by expanding the ammonia solution in a hydraulic turbine.

In the first embodiment (Fig. 1), the washing water after the first stage of the scrubber 1 enters the evaporator 13 through a recuperative heat exchanger 14. As an evaporator, a volume or film type apparatus can be used in which ammonia is evaporated by supplying heat from an external source. As a source of heat, any process stream of steam or condensate can be used. Ammonia released from the water passes sequentially through a recuperative heat exchanger 14 and a moisture separator 15, after which it enters one of two alternately working adsorbers of a deep-drying unit 10. Drained to dew point temperature (minus 40 ° С), ammonia is throttled through valve 11 to pressure in food containers liquid ammonia (1.8 MPa) and liquefies. To regenerate the adsorbents of the ammonia deep drying unit 10, preheated technical air or any other process stream of a heated moisture-free gas is used. Water from the lower part of the evaporator 13 is pumped to a recuperative heat exchanger 14 by a pump 12, then through the cooler 4 for irrigation to the second stage of the scrubber 1. Pump 12 is used only to compensate for pressure losses in the respective apparatuses of the installation. After the second stage of the scrubber 1, a weak water-ammonia solution enters the buffer tank 2, from which it is pumped through the cooler 3 to irrigate the first stage of the scrubber 1 with a pump 5. A strong water-ammonia solution after the first stage of the scrubber 1 is fed to the ammonia extraction system by evaporation. The cycle closes.

In the second embodiment (figure 2), a strong aqueous ammonia solution from the first stage of the scrubber 1 enters a hydraulic turbine 6, in which it expands to a pressure equal to the storage pressure of liquid ammonia in food containers. After that, a strong ammonia-water solution is fed through a recuperative heat exchanger 15 to the evaporator 14, where ammonia is evaporated by supplying heat from an external source. Ammonia released from the water passes sequentially through the recuperative heat exchanger 15 and the moisture separator 9, after which it enters one of two alternately working adsorbers of the deep adsorption drying unit 11. Ammonia dried to the dew point temperature of minus 40 ° С condenses in the air condenser 12 when it is blown by the fan 13. The washing water circulation in the second case is carried out according to a similar scheme, except that before the water is supplied to the scrubber 1, its pressure must be increased to the initial value (giving eniya purge gases). For this, a pump 7 is provided, located on the same shaft with a hydraulic turbine 6. As in the first case, an additional electric motor 8 is used to compensate for the losses for the drive of the pump 7.

The productivity of the installation that implements the claimed method, in terms of ammonia in terms of 1000 nm ³ / h of processed purge gases containing 1.5 vol.% Ammonia, will be approximately 10 kg / h. Accordingly, for the installation of ammonia synthesis AM-76, the purge gas flow in which is 4000 ... 6000 nm ³ / h, the amount of ammonia will be 40 ... 60 kg / h or 350 ... 480 tons per year.

The specific water consumption for flushing the purge gases, as a rule, does not exceed 0.15 kg / nm ³ , which for ammonia aggregates of the type AM-76 corresponds to a water consumption for a scrubber of 600 ... 900 kg / h. In cases where ammonia water is not dispensed to consumers, the total consumption of fresh water for recharge does not exceed 2% of its indicated consumption in the apparatus.

The main power-consuming equipment for the purge gas purification system from ammonia are water pumps. The consumption of electric energy for their drive when used to expand a strong aqueous ammonia solution of a hydraulic turbine is insignificant.

An additional effect of the introduction of the proposed method for the extraction of ammonia from purge gases is the elimination of discharges of ammonia-containing water into the sewers, which helps to improve the environmental situation in the region and reduce the cost of waste disposal of the enterprise.

Claims (1)

The method of extraction of NH ₃ from the purge gas composition, vol.%:
ammonia 0.1-15 hydrogen 20-55 other gases rest,
comprising flushing the purge gas in a scrubber and an ammonia separation step, characterized in that the purging gas is flushed in a multi-stage scrubber with cooling the ammonia solution between the absorption steps, and the ammonia is extracted after the first scrubber step by evaporation in a volumetric or film type apparatus at a synthesis pressure or storage of ammonia in food containers with its subsequent drying and liquefaction.

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