IE46473B1

IE46473B1 - Method for the production of ammonium carbamate solution, and method for the production of urea and ammonia

Info

Publication number: IE46473B1
Application number: IE448/78A
Authority: IE
Original assignee: Snam Progetti
Priority date: 1977-03-03
Filing date: 1978-03-03
Publication date: 1983-06-29
Also published as: BE864554A; SE7802450L; DD134087A5; FR2382403A1; DK92978A; PL110971B1; JPS6324986B2; IT1115610B; CA1118185A; AU3338878A; LU79147A1; AU515786B2; DE2808831C3; AR221480A1; CS219889B2; TR19994A; NL7802158A; ES468020A1; DE2808831A1; BR7801343A

Abstract

In a process of the so-called integrated type for the production of ammonia and urea, the absorption of CO2 from the crude gases passing to the ammonia synthesis reactor is effected by means of an absorption apparatus which is divided into two sections, one of which is of the type with adiabatic plates (6) and the other is of the type with a thin-film tube bundle (9). Most of the CO2 is removed in the thin-film absorber, and the remainder in the adiabatic plate absorber.

Description

This invention relates to a method for the absorption of carbon dioxide in aqueous ammonia solution to produce an ammonium carbamate solution, and to a method for the concurrent production of urea and ammonia.

It is known from Patent Specification No. 35705 that the absorption of CO2 contained in raw gases for the synthesis of ammonia in an aqueous ammonia solution to form an ammonium carbamate solution can be carried out in a film-exchanger, the aqueous ammonia solution being fed to the exchanger both at the bottom and at the top thereof. However, during the absorption, evaporation of about 20% of the ammonia takes place, so that such ammonia must be partially condensed and fed back to the absorber. The remainder of the ammonia contained in the gases emerging from the absorber is sent, after the partial condensation described, to an ammonia-absorber wherein such ammonia, along with ammonia from the ammonia-synthesis apparatuses absorbed in water. This process is complicated and requires considerable expenditure on equipment.

According to the present invention, there is provided a -24 64 73 method for the absorption of carbon dioxide in aqueous ammonia solution to produce an ammonium carbamate solution, wherein the absorption is carried out by introducing the carbon dioxide at or near the bottom of an absorber which is divided into two sections the lower of which is a film-absorber, the absorbing solution in the upper section being the aqueous ammonia solution (with or without the presence therein of liquid ammonia) and the absorbing solution in the lower section being an aqueous ammoniacal solution of ammonium carbonate.

The invention also provides a modification of the above method, wherein the film-absorber and the plate-absorber are arranged serially and non-superposedly, and wherein the carbon dioxide is fed into the film-absorber at or near the bottom thereof, is then discharged from the top of the film-absorber and is then fed into the plate-absorber at or near the bottom thereof.

The present invention also provides a method for the concurrent production of ammonia and urea, comprising the steps of: (a) producing ammonia in an ammonia-synthesis reactor; (b) absorbing ammonia at the exit of the ammonia-synthesis reactor with water, thus obtaining a concentrated aqueous solution of ammonia; (c) utilizing the concentrated solution of ammonia for the absorption of CO2 contained in a raw gas for the synthesis of ammonia, thereby obtaining ammonium carbamate; (d, converting the ammonium carbamate partially into urea in a urea-synthesis reactor; (e) thermally decomposing the carbamate which has not been converted into urea and stripping the decomposition products with a gas selected from ammonia, CO2 and inert gas; (f) discharging from the stripping zone a solution of urea which still contains carbamate; (g)recycling the products of decomposition of the carbamate to the urea-synthesis 346473 reactor; and (h)distilling, under a pressure of from 3 to 30 atmospheres and in one or more stages, the solution of urea to obtain (i) liquid ammonia and one or more ammoniacal solutions of ammonium carbonate, and (ii) an aqueous solution of urea deprived pf ammonium carbamate; wherein the absorption of CO2 in step (c) is carried out by introducing the raw gas at or near the bottom of an absorber which is split into two sections, the lower section being a film-absorber and the upper section being a plate-absorber, the absorbing solution in the upper section being said concentrated aqueous solution of ammonia (with or without the addition of liquid ammonia thereto), and the absorbing solution in the lower section being one or more of said aqueous ammoniacal solutions of ammonium carbonate obtained by distillation of the solution of urea under a pressure of from 3 to 30 atmospheres.

Preferably, the main fraction of the CO., of the raw gas is absorbed in the film-absorber, which is equipped with tubes, wherein the absorbing solution smoothly runs along the tube walls in the form of a thin film, the absorption heat being removed by a coolant fluid which flows Outside the tube walls, and the remaining portion of the CO2 is removed in the plate-absorber, which is essentially adiabatic.

In the plate-absorber, liquid ammonia is also used with advantage, concurrently with the concentrated aqueous ammonia solution. The preferred weight ratio of liquid ammonia to concentrated aqueous ammonia solution is from 1:1 to J :5.

The absorbing solution may be fed to the film-absorber by means of a distributor which is located in a zone between the two sections. Likewise, the absorbing solution may be fed to a region -446473 at the top, or near the top, of the plate-absorber. The solution emerging from the plate-absorber may be sent directly to the ureasynthesis reactor, or, as an alternative, it can be fed into the film-absorber by means of an appropriate distributor and can be distributed in film form onto the surfaces of the tubes of the film-absorber, together with the ammoniacal solution of ammonium carbonate which is directly fed to the film-absorber.

In the film-absorber the CO2 is preferably absorbed to such an extent as to produce a residual gas containing 2%-3% by volume of CO2. During this stage, a certain evaporation of ammonia takes place, so that the gas deprived of the carbon dioxide usually has a content of ammonia of 10%-12% by volume. This gas subsequently enters the plate-absorber, wherein, by the scrubbing described above, it is possible completely to absorb both the carbon dioxide and the ammonia which are present.

By the method of this invention, it is possible not only to reduce the exchange surfaces which are necessary, but also to carry out the method under conditions of great safety on account of the considerable excess of ammonia and the relative thermal volume thereof.

It is possible, according to the present invention, to absorb the CO2 in two serially arranged absorption sections, that is, sections which are not superposed one upon another. Thus the invention provides a modified method wherein the film-absorber and the plate-absorber are arranged serially and non-superposedly; wherein the raw gas which contains CO2 is fed into the film-absorber at or near the bottom thereof, is then discharged from the top of the film-absorber and is then fed into the plate-absorber at or near the bottom thereof; wherein the absorbing solution for the film-54S47S absorber is fed into such section at or near the top thereof, and wherein the absorbing solution for the plate-absorber is fed into such section at or near the top thereof. The gas, deprived of its C02, is discharged from the top of the plate-absorber, whereas the solution obtained at the bottom of the plate-absorber can, if desired and with advantage, be used as an additional absorbing solution in the film-absorber.

This invention will now be illustrated by the following Example, in which reference is made to the single Figure of the accompanying drawing. , ·.

EXAMPLE; To produce 1,000 metric tons an hour of urea, there was fed to the bottom of a film-absorber 1 which operates under a pressure of 195 kg/cm , a raw gas 2 having the following composition: Ar ........215 3 normal m /hr ( 0.24% by volume)H2 ...... 55,425 do (61.52% do )N2 ...... 18,161 do (19.99% do ) ch4 ...... 251 do (0.28% do ) CO ......390 do ( 0.43% do ) CO, ..... 15,804 do (17.54% do ) The working temperature was 175°C. At 3, the following ammonia j solution, having a temperature of ’ 50°C, was introduced nh3 ..... 25,370 kg/hr (80% by weight ) h20 ...... 6,343 do (20% do ) Total .... 31,713 do (100% do ) At the top of the film-absorber 1, the following recycle carbonate , from 4, was fed in at a temperature of 103°C: -648473 NH3..... 5,558 kg/hr ( 26.75% by weight)C02..... 7,076 do ( 34.24% do )H2°..... 8,059 do ( 39.01% do ) Total .... 20,663 do (100.00% do ) The following concentrated solution of carbamate exited at the bottom and was sent to a urea reactor via line 5, at a tern- perature of 140°C. mh3 . CO2 . H2° . .... 24,834 .... 31,343 .... 13,920 ka/hr do do (35.43% (44.71% (19.86% by weight) do do ) ) Total . . . . 70,663 do (100.00% do ) From the film-absorber 1, a gas, partially stripped of its CO2 and having the following composition and a temperature of 125°C, was sent to plate-absorber 6: Ar . (0.25% by volume ) h2 . . . .55,425 do (63.44% do ) n2 . . . .18,161 do (20.78% do ) CH4 . . . . 251 do (0.29% do ) co . . . . 390 do (0.45% do ) CO2 . . . 3,451 do (3.95% do ) nh3 . . . 9,467 do (10.84% do ) Total • . . .87,360 do (100.00% do ) The heat of absorption was withdrawn by means of a jacket around the absorber, and was used to produce low-pressure steam.

To the top of the plate-absorber 6, there were fed: (1) Ammonia solution (temperature 50°) from 3: NH^..... 12,684 kg/hr (80% by weight) H2O.....3,172 do (20% do ) -746473 (2) recycled anhydrous ammonia, at a temperature of 38°C, from 7:35,874 kg/hr.

From the bottom of the plate-absorber 6, the following solution emerged via 8 at a temperature of 117°C.

NH3 . . . . . 45,939 kg/hr (82.20% by weight) C°2 . , . . 6,778 do (12.13% do ) h2o . . . . 3,172 do ( 5.67% do ) Total . . . . 55,889 do (100.00% do ) This solution was sent to the urea-synthesis reactor 10 From the , top of the plate-absorber 6, the following CO2 - stripped gas emerged via 9 at a temperature of 43°C. Ϊ 4 Ar . . . . . . . 215 3 normal m /hr (0.25% by volume) h2 . . . . . .55,425 do (63.44% do )N2 · * · . . .18,161 do (20.79% do ) 15 ch4 . . . . . . 251 do (0.29 % do ) CO . . . . . . , 390 do (0.45% do ) NH3 . . . . .12,918 do (14.78% do ) Total . . . . 87,360 do (100.00% do ) This gas was subjected to methanization.

Claims

CLAIMS:

1. A method for the concurrent production of ammonia and urea, comprising the steps of: (a) producing ammonia in an ammoniasynthesis reactor; (b) absorbing ammonia at the exit of the ammonia-synthesis reactor with water, thus obtaining a concentrated aqueous solution of ammonia; (c) utilizing the concentrated solution of ammonia for the absorption of CO 2 contained in a raw gas -846473 for the synthesis of ammonia, thereby obtaining ammonium carbamate; (d) converting the ammonium carbamate partially into urea in a urea-synthesis reactor; (e) thermally decomposing the carbamate which has not been converted into urea and stripping 5 the decomposition products with a gas selected from ammonia, CO 2 and Inert gas; (f) discharging from the stripping zone a solution of urea which still contains carbamate; (g) recycling the products of decomposition of the carbamate to the urea-synthesis reactor; and (h) distilling, under a pressure of from 3 to 30 atmospheres θ and in one or more stages, the solution of urea to obtain (i) liquid ammonia and one or more ammoniacal solutions of ammonium carbonate, and (ii) an aqueous solution of urea deprived of ammonium carbamate; wherein the absorption of CO 2 in step (c) is carried out by introducing the raw gas at or near the bottom of an absorber 5 which is split into two sections, the lower section being a film absorber and the upper section being a plate-absorber, the absorbing solution in the upper section being said concentrated aqueous solution of ammonia (with or without the addition of liquid ammonia thereto), and the absorbing solution in the lower section j being one or more of said aqueous ammoniacal solutions of ammonium carbonate obtained by distillation of the solution of urea under a pressure of from 3 to 30 atmospheres.

2. A method according to claim 1, wherein in the absorbing solution in the upper section, the weight ratio of the liquid ammonia i to said concentrated aqueous solution of ammonia is from 1:1 to 1:5.

3. A modification of a method according to claim 1 or 2, wherein the film-absorber and the plate-absorber are arranged serially and non-superposedly, wherein the raw gas which contains CO 2 is fed -946473 into the film-absorber at or near the bottom thereof, is then discharged from the top of the film-absorber and is then fed into the plate-absorber at or near the bottom thereof; wherein the absorbing solution for the film-absorber is fed into such section 5 at or near the top thereof; and wherein the absorbing solution for the plate-absorber is fed into such section at or near the top thereof.

4. A method for the concurrent production of ammonia and urea, substantially as hereinbefore described with, reference to -,0 the accompanying drawing.

5. Ammonia whenever produced by the method according: to any of claims 1 to 4, ’

6. Urea whenever produced by the method according to any of claims 1 to 4. 15

7. A method for the absorption of carbon dioxide in aqueous ammonia solution to produoe an ammonium carbamate solution, wherein the absorption is carried out by introducing the carbon dioxide at or near the bottom of an absorber which is divided into two sections the lower of which is a film-absorber and the upper of which is a 20 plate-absorber, the absorbing solution in the upper section being the aqueous ammonia solution (with or without the presence therein of liquid ammonia) and the absorbing solution in the lower section being an aqueous ammoniacal solution Of ammonium carbonate.

8. A modification of a method according to claim 7, wherein 25 the film-absorber and r the plate-absorber are arranged serially and non-superposedly; and wherein the carbon dioxide is fed into the film-absorber at or near the bottom thereof, is then discharged from the top of the film-absorber and is then fed into the plate-absorber -1046473 at or near the bottom thereof.

9. A method for the absorption of carbon dioxide in aqueous ammonia solution to produce an ammonium carbamate solution, sub stantially as hereinbefore described with reference to the accompanying drawings.

10. An ammonium carbamate solution whenever produced by the method according to claims 7, 8 or 9.