JPS6324986B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an integrated process for producing urea and ammonia.

More particularly, the present invention relates to an integrated urea-ammonia synthesis process with improved carbon dioxide absorption in the ammonium carbamate synthesis zone.

According to Italian Patent No. 907469, absorption of CO ₂ contained in raw material gas for ammonia synthesis is achieved by supplying an ammonia aqueous solution from the bottom and top of the heat exchanger in a film-type heat exchanger. It is known to carry out this method.

However, about 20% of ammonia during absorption
evaporates, so this ammonia must be partially condensed and recycled to the CO ₂ absorber. The remaining ammonia still contained in the gas discharged from the CO ₂ absorption zone (which is also the reaction zone where carbamic acid is produced) is partially condensed as described above and then fed to the ammonia absorber;
Here, this ammonia is absorbed by water together with the ammonia from the ammonia synthesis zone.

The disadvantages of process complexity and high cost when operating with CO ₂ absorption according to conventional methods are overcome when carried out with the method according to the invention.

An object of the present invention is to provide a method for synthesizing urea in combination with ammonia synthesis, and according to this method, urea contained in the raw material gas for ammonia synthesis
CO ₂ is absorbed by introducing the CO ₂ -containing gas into an absorption device that is divided into two zones (i.e., a lower film type and an upper plate type) from or near the bottom. implement.

Other operations of this integrated process are known, namely producing ammonia in an ammonia synthesis reactor, absorbing ammonia from this synthesis reactor with water and incorporating it into the feed gas for ammonia synthesis. absorbing CO ₂ and producing ammonium carbamate by the method according to the invention; converting ammonium carbamate to urea in _a urea synthesis reactor; partial conversion, thermal decomposition of carbamates that have not been converted to urea, and the decomposition products are converted into ammonia, CO ₂
or removing with a gas selected from inert gases (preferably ammonia), removing the urine fluid still containing ammonium carbamate from this stripping zone, and recycling the ammonium carbamate decomposition products to the urea synthesis zone. to do,
and distilling the urea solution in one or more stages at a pressure of 3 to 30 atmospheres, on the one hand to obtain an ammoniacal solution of liquid ammonia and ammonium carbonate and on the other hand to obtain a urea solution free of ammonium carbamate.

The carbon dioxide is then absorbed in two different zones of the same device.

More specifically, most of the CO ₂ is absorbed in the area containing the tube. In this region, the absorption solution consisting of an ammoniacal solution of ammonium carbonate, obtained by distilling the urea solution at low pressure after removing most of the ammonium carbamate at a pressure substantially equal to the synthesis pressure, runs along the tube wall. It flows smoothly as a thin film, and the absorbed heat is removed by the coolant fluid flowing outside the tube wall. The remaining CO ₂ is removed in an adiabatic plate-shaped zone, where the absorption fluid is a concentrated aqueous ammonia solution.

The pressure during distillation of the urea solution to obtain the ammoniacal solution of ammonium carbonate is between 3 and 30 atmospheres.

In the plate absorption zone, liquid ammonia can also be advantageously used in addition to concentrated aqueous ammonia solutions. The preferred weight ratio of liquid ammonia to concentrated aqueous ammonia solution is from 1 to 5.

The absorption liquid is supplied to the film-type absorption zone by a distributor installed in the middle of the two zones.

Similarly, the absorption liquid (concentrated aqueous ammonia solution to which liquid ammonia has been added) is supplied at or near the top of the plate-type absorption zone. The solution from the plate area is fed directly to the urea synthesis reactor. Alternatively, it may be supplied to a film-type absorption zone through a suitable distributor and flowed in a thin film over the surface of the tube of the film-type absorption zone together with an ammoniacal solution of ammonium carbonate which is supplied directly to the film-type absorption zone. You can also do this.

In the film type absorption zone, CO ₂ can be absorbed up to a residual concentration of 2 to 3% (by volume).

During this process, evaporation of ammonia occurs, and the ammonia concentration in the carbon dioxide-free gas is approximately 10
or 12% (capacity). This gas is then passed to the upper plate region where it can completely absorb both the carbon dioxide and ammonia present by scrubbing as described above.

By operating according to the method of the invention, not only can the required heat exchange area be reduced, but also under very safe conditions due to the considerable excess of ammonia and its specific heat capacity.

According to the present invention, CO ₂ can be absorbed in two absorption areas installed in series (areas that do not overlap with each other). In the first zone, absorption takes place by a film method, while the second zone, comprising a set of plates, absorbs the remaining CO ₂ that could not be absorbed by the film-type absorption zone.

In this case, the crude gas for ammonia synthesis containing CO ₂ is fed at or near the bottom of the film-type absorption zone, and the absorption liquid (ammonium carbonate solution) is fed at or near the top, while this The gas taken out from the top of the film-type absorption zone is supplied to the bottom or near the bottom of the plate-type absorption zone. Also, at the top of the plate area, add a concentrated aqueous ammonia solution (liquid ammonia may be added).
Supply an absorption liquid consisting of: From the top of the plate-type absorption zone, a gas depleted of CO ₂ is obtained, while the solution obtained from the bottom of the plate-type absorption zone can advantageously be used as part of the absorption liquid of the film-type absorption zone, if required. .

Examples are illustrated to further explain the present invention, but the present invention is not limited thereto.

EXAMPLE In order to produce 1000 tons/hour of urea, a raw material gas 2 having the following composition was fed to the bottom of a film-type absorption zone 1 (operating at a pressure of 195 Kg/cm ² ) shown in the accompanying drawings. Composition Ar215 (m ³ \hour) 0.24 (% by volume) H ₂ 55425 61.52 N ₂ 18161 19.99 CH ₄ 251 0.28 CO390 0.43 CO ₂ 15804 17.54 The operating temperature was 175°C.

On the other hand, an ammonia solution having the following composition at a temperature of 50° C. was supplied from No. 3. Composition NH ₃ 25370 (Kg/hour) 80 (wt%) H ₂ O 6343 20 total 31713 100 Ammonium carbonate circulating fluid at a temperature of 103°C with the following composition is connected to the top of the film type absorption zone 1 via 4. supplied. Composition NH ₃ 5558 (Kg/hour) 26.75 (% by weight) CO ₂ 7076 34.24 H ₂ O 8059 39.01 Total 20663 100.00 A concentrated solution of carbamate with the following composition is thereby obtained from its bottom, which is was fed into the urea reactor at a temperature of 140°C. Composition NH ₃ 24834 (Kg/hour) 35.43 (wt%) CO ₂ 31343 44.71 H ₂ O 13920 19.86 Total 70097 100.00 From the film area, a portion with the following composition
A gas from which CO ₂ has been removed is obtained, which is heated to a temperature of 125°C.
and fed into plate area 6. Composition Ar215 (m ³ / hour) 0.25 (volume %) H ₂ 55425 63.44 N ₂ 18161 20.78 CH ₄ 251 0.29 CO390 0.45 CO ₂ 3451 3.95 NH ₃ 9467 10.84 Total 87360 00.00 Absorbed heat in absorption region Take it out from the jacket side, Used to generate low pressure steam.

An ammonia solution (temperature 50°C) having the following composition was fed to the top of the plate area. Composition NH ₃ 12684 (Kg/hour) 80 (% by weight) H ₂ O3 172 20 35874 Kg/hour of anhydrous ammonia was also circulated through 7 at a temperature of 38°C.

From the bottom of the plate area through 8 a solution with the following composition was obtained at a temperature of 117°C. Composition NH ₃ 45939 (Kg/hour) 82.20 (wt%) CO ₂ 6778 12.13 H ₂ O 3172 5.67 Total 55889 100.00 This solution was supplied to the urea synthesis reaction zone.

From the top of the plate area a CO ₂ -free gas with the following composition was obtained via 9 at a temperature of 43°C. Composition Ar215 (m ³ /hour) 0.25 (volume %) H ₂ 55425 63.44 N ₂ 18161 20.79 CH ₄ 251 0.29 CO390 0.45 NH ₃ 12918 14.78 Total 87360 100.00 This gas was supplied to the methanation process.

[Brief explanation of the drawing]

The drawing is a schematic illustration of an absorber suitable for carrying out the method of the invention. 1...Film type absorption area, 6...Plate type absorption area.

Claims (1)

[Claims] 1. A step of producing ammonia in an ammonia synthesis reactor, in which ammonia is absorbed by water at the outlet of the ammonia synthesis reactor, and CO ₂ contained in the raw material gas for ammonia synthesis is absorbed. A process to obtain a concentrated ammonia aqueous solution used in the production process, a process to obtain ammonium carbamate, a process to partially convert ammonium carbamate to urea in a urea synthesis reactor, a process to thermally decompose ammonium carbamate that has not been converted to urea. removing the decomposition products with a gas selected from ammonia, CO ₂ and an inert gas; removing a urea solution still containing ammonium carbamate from this stripping zone; removing the decomposition products of ammonium carbamate with urea; recycling to the synthesis reactor and distillation of the urea solution in one or more stages at a pressure of 3 to 30 atmospheres to obtain an ammoniacal solution of liquid ammonia and ammonium carbonate on the one hand and to remove ammonium carbamate on the other hand. In a method for simultaneously producing ammonia and urea, which includes the step of obtaining an aqueous urea solution, an absorption apparatus is used which is divided into two zones, the lower zone being a film type absorber and the upper zone being a plate type absorber. The raw material gas containing CO ₂ is introduced from the bottom or near the bottom, and the raw material gas is
In the lower zone, countercurrent contact is made with an absorption liquid consisting of an ammoniacal solution of ammonium carbonate obtained by distilling a urea solution under a pressure of 3 to 30 atmospheres, and then in the upper zone, an absorption liquid consisting of the concentrated aqueous ammonia solution is brought into contact. A method for simultaneously synthesizing ammonia and urea, characterized by absorbing CO ₂ by bringing them into countercurrent contact with a liquid. 2 Weight ratio 1 to 5 relative to ammonium carbonate
2. The method of claim 1, wherein liquid ammonia is added to the absorption liquid in the lower zone in an amount of . 3 The film type absorption area and the plate type absorption area are arranged in series without overlapping, and the CO ₂
A raw material gas containing gas is supplied at or near the bottom of the film-type absorption area, and then taken out from the top of the film-type absorption area and supplied at or near the bottom of the plate-type absorption area, and at or near the top of the plate-type absorption area. The absorption liquid for the film-type absorption zone that is supplied at or near the top is an ammoniacal solution of ammonium carbonate to which liquid ammonia is added if necessary, and the absorption liquid for the plate-type absorption zone that is supplied at or near the top is a concentrated aqueous ammonia solution. The method according to claim 1 or 2, wherein