DK141579B - Process of Ammonia Synthesis for Absorption of Ammonia. - Google Patents

Description

(11) PROCEDURE 141579 DENMARK (S,> ln, c'3 c 01 c 1/12 • (21) Note 2756/75 (22) Filed on 17 · Jun 1975

(23) Lebed · «15 · Nov. I97O

(44) The application and the proposal were made public on 20 April · 1 9 ° C

DIRECTORATE OF

PATENT AND TRADE MARKET (») Prtarilet begmt ft * dm

15 nov. 1969 * 24469/69, IT

29 May 1970 »25266/70, IT 27 Aug. 1970, 29055/70, IT

(71> SNAMPROGETTI S.P.A., Core o Venezia 16, Milan, IT.

(72) Inventor: Mario Guadalupi, Via Tiziano 18, Milan, IT.

(74) Proxy during the blessing · processing:

International Patent Office.

(54) Process of ammonia synthesis for the absorption of ammonia.

The present invention relates to a method of ammonia synthesis for the absorption of ammonia from the gases coming from the synthesis reactor.

It is known in the synthesis of ammonia to exclude the separation of ammonia from unreacted gases leaving the synthesis reactor by fractional condensation, first in a water-cooled condenser, then in one or more capacitors cooled by evaporation of ammonia. It is also known, e.g. from the specification to German Patent No. 1,244,746, to perform an absorption of ammonia from the synthesis agents by aqueous anmonia solutions, performing such absorption in two successive steps using ammonia solutions with increasing absorbency.

In the process of the invention, removal of ammonia from the synthesis gases is also effected by absorption, but this absorption takes place in a manner that is more efficient, technically simpler and cheaper.

The process according to the invention is characterized in that the ammonia is removed by absorption in a film absorber, consisting of a heat exchanger with a bundle in the substantially vertical, externally cooled pipe, in which the gases flow countercurrent with a downward flowing film of water or a weak, aqueous ammonia solution which absorbs the ammonia contained in the gases and leaves the bottom of the absorber as a strong ammonia solution, while removing the absorption heat of the ammonia by flowing water or other refrigerant on the outside of the tubes.

Thus, as in the prior art, several absorption apparatus or stages are not required. Due to the strong cooling of the liquid film, and due to the effective countercurrent mass transfer, very strong ammonia solutions (of a concentration of up to 80 wtZNH 2) are obtained, and it is possible to absorb ammonia to the extent that the gas stream leaving the absorber has a residual ammonia content of 0.2-0.5 molZ (and is saturated with water).

The gas flow from the absorption can be further processed in various ways for recycling, preferably in the following way:

After blowing inert gases to keep their concentration constant in the circuit, fresh feed mixture is added to the gas stream before treating the entire stream to remove water as far as possible.

This dewatering is carried out by injecting liquid ammonia into the gas stream, which causes both strong cooling of the gas stream and partly absorption of most of the water contained therein, so that the residual water content can thus reach as low as 1-3 ppm.

After said ammonia injection, which can be done by means of a venturi nozzle, the stream is sent to a separator in which the aforementioned strong ammonia solution and a dried gas stream are separated; the dried gases, after passing through a heat exchanger as a cooling medium in moist stream countercurrent, either to a compressor or to other suitable means and are recycled to the synthesis reactor. Most of the water contained in the moist gases is condensed inside the heat exchanger and removed as a weak ammonia solution, removing the excess water with the strong ammonia solution coming from the separator.

The ammonia solution obtained in this way is sent to a distillation zone where the ammonia is in a known manner separated from the water with a high degree of purity, ie. 99.9%

The operating pressure of the film absorber, and preferably throughout the synthesis plant, is preferably between 30 and 300 kg / cm, preferably between 80 and 150 kg / cm.

3 141579

If part or all of the produced anmonia is used to produce urea, part or all of the anmonia-water solution obtained in the film absorber may be used to absorb the carbon dioxide contained in the crude ammonia synthesis gases coming from carbon monoxide converters. which takes place under such conditions as to form anmonium carbamate, which is then sent for conversion to urea, while the carbon dioxide-free gases, possibly after further treatments of known kind, are sent to the dehydration and to the ammonia synthesis. In doing so, he can avoid two costly separation steps found in modern ammonia production plants, namely the carbon dioxide removal step with solvent regeneration and the ammonia separation stage.

The method according to the invention is further explained by means of the drawing, which shows a particular embodiment of the method according to the invention.

The drawing shows a synthesis reactor 1, from which the effluents at a pressure .2 in the range 80-180 kg / cm after possible heat recovery and cooling, which is not shown in the drawing, are sent through 6 to a film absorber 2 in which the gas flow flows upwards in the vertical. countercurrent with a liquid film that flows down the tube surface and absorbs the ammonia present in the gases.

This liquid can be either water or weak anmonolac solution, which is passed to 2 through 10.

The heat absorption of the ammonia is removed with water flowing outside the pipes, ie. on the casing side, which water is fed through 11 and comes out through 12.

A strong ammonia solution comes out through 7 from the bottom of the fila absorber, and from its top through 8 comes a gas stream with a very low residual ammonia content, ie. 0.2-0.5 mol%, practically saturated with water and, after blowing off the inert gases through 9, the fresh food mixture is added through 13.

The resulting gas stream is passed through 14 to a heat exchanger 3, where it dispenses most of its heat content to the cooled dried gases entering through 18 from a separator 4.

This cooling causes condensation of most of the water contained in the gases, resulting in a weak ammonia solution from the bottom of the heat exchanger 3 through 15. The drying is completed by spraying liquid anmonia into the gases coming from 3 through 17, which, through adiabatic evaporation in the gas stream, causes cooling thereof and consequently condensation of the contained water. This liquid ammonia is sent into 17 through 16.

The resulting mixture enters into a separator 4, a strong ammonia solution is withdrawn from the bottom of this separator through 19, a dried gas stream leaves this separator through 18 and contains no more than 2-3 ppm water and is sent as a cooling medium to the heat exchanger 3. then through 20 to 4 a compressor 5 which through 21 recycles them to the synthesis reactor 1.

The following example explains the reaction conditions and advantages of treating gases from the ammonia synthesis according to the process of the invention.

Example

This example relates to a synthesis plant for producing 1200 tons of ammonia per liter. day.

3

A current of 780,930 Nm / h leaves the synthesis reactor; this stream had the following composition at a pressure of 125 absolute atmospheres: Η £ 58.99 volume ?.

N2 19.68 »CH4 7.58"

Ar 3.25 "NH3 10.50”

After cooling this stream to 40 ° C, it was sent to the bottom of the film absorber 2.

48,969 kg / h of 10 weight ?, ammonia solution was sent to the top of the absorber.

108,865 kg / b of an ammonia solution of 60 wt% ammonia left absorber 2 through 7 and 701,190 Nm / h of washed gases through 8.

These gases had, at a temperature of 45 ° G, the following composition on a dry basis: H2 65.59 volume ?.

N2 21.90 "CH4 8.40"

Ar 3.61 "NH3 0.50» l water 525 kg / h 3 10,900 Nm / h of these gases was blown through 9.

The fresh food mix fed through 13 had the following data:

Flow rate 142,170 Nm / h

Composition: H2 74.17 volume ?.

N2 24.92 "CH4 0.82»

Ar 0.29 "Saturation water 60 kg / h

The two gas streams were mixed before going into heat exchanger 3, the whole stream having the following data: 5 U1579

Flow rate 832,460 Nm3 / h

Pressure 122 absolute atmospheres

Temperature 44 ° C

Composition (on a dry basis): H2 68.08 volume% N2 22.35 »CH4 7.12"

Ar 3.04 »NH3 0.41« »Saturation water 607 kg / h

At the output of the heat exchanger, the flow had the following data: a) gas

temperature -18.5 ° G

water content 35 kg / h b) ammonia solution flow rate 818 kg / h aranonia content 30 wt% water content 70 w * gt%

The liquid ammonia injected was 10.665 kg / h and evaporated almost completely while cooling the gas stream from -18.5 ° C to -28 ° C. Separator 4 gave at equilibrium: a) 350 kg / h 90 wt. Ammonia solution 3 b ) 846,000 Nm / h gases having the following composition: H2 66.0% by volume N2 22.0 "CH4 7.0"

Ar 3.0 "NH 3 2.0" H 2 O <3 ppm

The economic benefits and operating benefits of the method according to the invention are evident from the foregoing.

These advantages are due to the excellent absorption of the ammonia in the film absorber; the absorption is promoted by the refrigerants past in this apparatus, which remove the heat generated during absorption.

It is worth mentioning that the process according to the invention can also be used in conventional processes in the preparation of ammonia, and whereby the dewatering of recycle gases and fresh food gases is made use of known methods and apparatus.