CS203901B2 - Process for the treatment of gases at the preparation of ammonia - Google Patents

Description

CZECHOSLOVAK SOCIALIST REPUBLIC (19) DESCRIPTION OF THE INVENTION 203901 (11) (B2) • f. (22) Enlisted 13 11 70 (21) (PV 7673-70) 'f (51) Int. Cl.1 * 3 - C01C 1/04. , .J, · · (32) (31) (33) Priority from 15 11 69 (24469 A / 69) Italy · λ 'j.?; OFFICE FOR INVENTIONS AND DISCOVERIES (40) (45) Published 27 12 74 Published 15 11 83 • f- t (72) (73) Author of the invention and presenter of the patent GUADALUPI MARIO, MILAN (Italy) (54) Synthesis process ammonia 1

BACKGROUND OF THE INVENTION The present invention relates to a process for treating gases with ammonia synthesis.

Separation of unreacted gases leaving the synthesis reactor from ammonia is usually carried out by traction condensation, first in a water-cooled condenser, and subsequently in one or more of the condensers cooled by evaporation with ammonia.

The filling of the fresh synthesis mixture is carried out opposite to the gas flow direction in the final condensation stage cooled by the vaporized ammonia to remove the dewatering contained in the cartridge.

According to the prior art, gaseous ammonia is added to these gases to dehydrate the synthesis gases of the ammonium (N--i-H a) and the mixture is cooled to low temperatures by indirect heat exchange with the low temperature liquid cooled by the refrigerant. through the apparatus. The addition of ammonium gas should prevent the water from freezing, leaving water from the cooled gas mixture in the form of a strong ammonia solution.

The disadvantages of the prior art gas treatment process in ammonia synthesis are avoided by the present invention, which relates to the treatment of gases in the synthesis of ammonia, and which consists in absorbing the ammonia contained in the effluent from the synthesis reaction with water or dilute aqueous ammonia solution, thereby An ammonia solution containing more than 60% by weight ammonia is obtained, the water is then condensed from this gaseous stream by direct heat exchange with the terminal dehydrated gases to obtain a condensate in the form of ammonia solution containing up to 20% by weight ammonia and dried gases, whereupon the final dehydration of these gases is carried out by injection of liquid ammonia, whereby the dehydration is completed by adiabatic evaporation, whereupon the gases are fed to an ammonium nitrate separator where more than 60% by weight is obtained. % of ammonia and these terminal dehydrated gases are n they subsequently lead to said heat exchanger. In a preferred embodiment of the invention, the liquid ammonia is injected in excess with respect to the vaporized ammonia, wherein said excess is discharged in the form of a solution of ammonia containing more than 60% by weight of ammonia.

Compared to the prior art, the process of the invention allows for the elimination of refrigeration equipment and thus a significant reduction in production costs. The washing effect of the injected liquid ammonia also significantly reduces the residual water content to about 2 ppm. 203901 203901

The gas stream leaving the synthesis reactor is cooled to a film absorber after heat recovery if necessary. In this device, the gas stream flows upstream in countercurrent to the liquid film formed either by water or by a weak ammonia solution in water that absorbs the ammonia present in the gas stream. The apparatus is further equipped with a cooling device to remove the heat generated by the absorption of ammonium.

After wet cleaning, the gas leaving the absorber has a very low ammonia content (0.2 to 0.5 mol%) and is saturated with water.

In order to maintain a constant concentration of inert gases, a fresh charge is added to the gas stream to the dehydration stage after discharge. The dehydration is carried out by injecting liquid ammonia into the gas stream, whereby the water present is strongly cooled and condensed by the addition of ammonia. By using a suitable heat exchanger between wet and dry streams, very low saturation temperatures (2525 to 3030 ° C) are achieved with a small amount of evaporated ammonia.

The ammonia injected is in excess with respect to the necessary amount. This excess is removed from the reaction cycle in the form of a strong ammonia solution. The excess ammonia and the concentration of the discharged solution are regulated so that the water content of the dry stream does not exceed 2 to 3 ppm.

After the ammonia is injected, the stream is passed through a deseparator in which a strong ammonia solution is separated and the gas stream dried. The dried gases are passed through either the refrigerant heat exchanger in countercurrent due to the wet current to either the compressor or other suitable equipment and are recycled to the synthesis reactor. Most of the water in the heat exchanger is removed as a weak ammonia solution, the remaining water content is removed with a strong ammonia solution exiting the separator. The ammonia solution thus obtained is fed to a distillation zone where it is separated from the water by an ammoniacal method. The ammonia obtained had a purity of 99.9%.

A very important aspect of the described process is that the ammonia dissolves very quickly in water, and the greater the contact surface between the gases to be treated and the intermediate liquid ammonia, the higher the rate. This fact is utilized in the process according to the invention, since excellent dehydration of the gases is achieved by shortening the dough without saturating them with ammonia. Therefore, injection of liquid ammonia into the gas stream leaving the synthesis reactor in the production of ammonia is carried out by a suitable device , allowing a large surface area, thereby reducing the yield.

By way of example, a venturi-tube may be used to guide said gases where there is considerable turbulence that forms a large contact surface. The liquid ammonia is injected into said apparatus. The reduction of the contact surface in a further cycle is achieved by a separator or other suitable apparatus. The larger the contact surface, the closer the separator should be. In this way, excellent dehydration is achieved without saturating the gases with ammonia, since the reduced contact time does not allow vaporization of large amounts of ammonia, while the high surface finish provides good dehydration.

Another important aspect of the present invention where part or all of the ammonia obtained is used in the production of urea is the use of part or all of the ammonia / water solution obtained in the film absorber to absorb carbon dioxide. contained in the crude ammonia synthesis gases, resulting in the descending conversion of carbon monoxide. This absorption is carried out under the conditions of ammonium carbonate production, which is subsequently converted to urea, while the carbon dioxide-containing gases are generally referred to after further known dehydration and ammonia synthesis.

Nitrogen and hydrogen-synthesized ammonia gases are produced by weak oxidation of hydrocarbons (i.e., decomposition by steam, partial oxidation by oxygen, and the like). The gases obtained are led to further operations, such as the conversion of carbon monoxide, carbon dioxide and removal of the remaining carbon compounds. Another advantage of the invention is that it is possible to omit two expensive separation steps used in modern ammonia production plants, i.e. the carbon monoxide removal step and the ammonia removal step.

Thus, in a suitable device that allows a large contact surface between the gases and the liquid to be reached, some or all of the ammonia solution in the water obtained by the process of the invention can be utilized to remove the carbon monoxide present in the crude ammonia synthesis gases, to obtain a solution of ammonium carbonate which is subsequently converted to the wetting agent.

The absorption of carbon dioxide is preferably carried out within the range of pressures conventionally used in urea synthesis. As mentioned, the carbon monoxide-free gases are conducted into the synthesis reactor for the production of ammonia after the other known process steps.

The invention will be further described with reference to the accompanying drawing, which does not indicate the degree of absorption of carbon dioxide.

Liquids flowing out of the synthesis reactor 1 at a pressure in the range

Claims (2)

203901 was almost completely evaporated, amounting to 10,665 kg / h. Injected ammonia cooled the gas stream at -18.5 ° C to -28 ° C. Separator 4 provided at equilibrium: a) 350 kg / h 90% by weight ammonia solution. (b) 846 000 m3 / h of gases having the following composition: H2 66,0 vol% N2 22,0 vol% CH4 7.0 vol% Ar 3,0 vol% NH3 2,0 vol% H2O less than 3 ppm Advantages of the process according to the invention are clearly evident from these values. These economic and operational advantages result from the excellent absorption of the α-moniac in the film absorber. The absorption is favorably influenced by the refrigerant used in the devices, which dissipates the heat generated during absorption. The advantages of the novel process for drying recycled gases and fresh fillers are also obvious. The new process allows very good drying and very good thermal efficiency, while using a very simple device. It should also be pointed out that the process of the invention may be used in part in conventional processes for the production of ammonia, since either only the absorber stage with a film absorber or the dehydration step for recycled gases and freshly supplied gaseous mixture by rapid ammonia cooling and exchange The heat exchanger can be used in conjunction with other parts of a known process and apparatus for producing ammonia. OBJECT A process for the treatment of gases in the synthesis of an α-moniacal, characterized in that the ammonia contained in the gaseous stream emerging from the synthesis reactor is absorbed with water or dilute aqueous ammoniacal solution to obtain an ammonia solution containing more than 60% by weight of ammonia, water from this gas stream is then condensed by indirect heat exchange with the dehydrated tail gas, thereby obtaining a condensate in the form of an ammonia solution containing up to 20% by weight of ammonia and dried gases, followed by dehydration. These gases are injected by means of liquid ammonia to complete dehydration on the basis of adiabatic evaporation, whereupon the ammonia solution separators are passed through the gases, where more than 60% by weight of ammonia is obtained, and the final dehydrated gases are subsequently fed to of said heat exchanger. 2. A process as claimed in claim 1, wherein the liquid ammonia is injected in excess with respect to the evaporated ammonia, said excess being discharged in the form of an ammonia solution containing more than 60% by weight ammonia. 1 of the drawings of the drawings, n, p., Plant 7, bridge 203901 from 8 to 18 MPa are conducted after a possible thermal regeneration and cooling (not plotted) through line 6 to a film absorber 2, in which the gas flow is conducted internally by tubes upstream of the fluid flowing downwardly through the inner surface of the tubes and absorbing the ammonia present in the gases. The liquid may be either a water or a weak solution of ammonia and the dofilm absorber 2 is fed through line 10. The heat generated by the absorption of ammonia is dissipated by the water flowing outside the tubes, that is to say on the jacket side, whereby the water is fed via line 11 and discharged via line 12 The strong ammonia solution is discharged via line 7 from the bottom of the absorber 2, and a stream of gases with a very low ammonia content, i.e. 0.2 to 0.5 moles, is discharged through the line 8. %, practically saturated with water. After the inert gases have been removed by means of pipes 9, a fresh charge is supplied via line 12. The resulting gas stream is fed via line 14 to heat exchanger 3 in which most of the heat is released into the cooled dried gases fed through tube 18 from separator 4. This cooling effect causes condensation of most of the water contained in the reaction gases while The exchanger 3 is discharged via line 15 with a weak ammonia solution. Drying is accomplished by injecting liquid ammonia into the gases supplied from the exchanger 3 via line 17, whereby the ammonium solution causes the gas to cool down by adiabatic evaporation and subsequently condensate the water contained therein. Liquid ammonia is fed through line 17 via line 16. The resulting mixture enters separator 4, a strong ammonia stream is withdrawn from the bottom of the separator via line 19, the dried gas stream leaving the separator via line 18, wherein its water content does not exceed 2-3 ppm and is conducted in a coolant in the heat exchanger 3; the gases are then fed through the conduit 20 and 21 to a compressor 5 which recycles to the synthesis reactor 1. The following example serves to illustrate the invention without limiting it. After cooling the stream to 40 ° C, the stream was fed into the film absorber 2 to the bottom of the film absorber. A 10 wt.% Solution of ammonia at 48969 kg / h was added to the top of the absorber via line 10 in day. The absorber 2 exited via line 7 a 60 wt% a-moniacal solution and 108,865 kg / h and 8701190 m 3 / h of washed gases. These gases have the following composition, at 45 ° C, on a dry basis: H 2 65.59 vol. N 2 21.90 vol. CH 4 8.40 vol. Ar 3.61 vol. NH 3 .50 vol. Saturation water 525 kg / h. 10,900 m 3 / h of these gases were discharged via line 9. The fresh mixture fed through line 13 exhibited the following properties: Flow rate 142 170 m 3 / h. Composition: H2 74.17 volumetric N2 24.92 volumetric CH4 0.82 vol. Ar 0.29 vol. Saturation water from conduit 11 60 kg / h. The two streams were mixed before being fed to exchanger 3, with the combined stream showing the following Characteristics: Flow rate 832 460 m3 / h. Pressure 12.2 MPa Temperature 44 ° C Composition (on dry basis) H2 68.08 Volume N2 22.35 Volume CH4 7.12 Volume Ar 3.04 Volume NH3 0.41 Volume%%%%% Saturation Water 607 kg / h. The current exiting the exchanger showed the following characteristics: Example The example relates to an ammonium nitrate synthesis plant with a daily production of 1,200 tonnes of ammonia. a) Gas temperature -18.5 ° C The synthesis reactor leaves the water content of 35 kg / h. 780 930 m3 / h stream having the following composition at a pressure of 12.5 MPa: b) ammonia solution h2 58.99 volume% flow rate 818 kg / h N2 19.68 volume% ammonia content 30 wt% CH4 7.58 vol% water content 70% by weight Ar 3.25% by volume NH3 10.50% by volume Ammonia injected