LU82721A1 - PROCESS FOR SUPPRESSING THE EXPLOSIBILITY OF GASES OUT OF UREA MANUFACTURING FACILITIES - Google Patents

Description

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"Method for suppressing the explosiveness of gases leaving urea production plants."

The present invention relates to a process for suppressing the explosiveness of gases leaving urea production plants. As is known, urea is produced from ammonia and carbon dioxide by direct synthesis in a reactor working under high pressure (from approximately Λ 51 bars to 456 bars) and at a high temperature ( from 170 ° C 10 to about 220 ° C).

The reaction product, which is composed of urea, ammonium carbamate and water, leaves the reactor at the same time as the ammonia which has been used in excess for synthesis and is sent to a decomposition device. ; the latter operates at substantially the same pressure as that used for the synthesis and, in this high pressure decomposition apparatus, the carbamate contained in the urea solution is decomposed into ammonia and carbon dioxide.

The ammonia and carbon dioxide coming from the decomposition are sent to a condensing apparatus operating under high pressure which works at substantially the same pressure as that used for the synthesis; in this condenser, the ammonia and the carbon dioxide are condensed in order to again form ammonium carbamate, which will be recycled to the urea synthesis reactor.

The urea solution, freed from most of the ♦ carbamate, is removed from the high pressure decomposition apparatus and is sent to a decomposition apparatus operating at medium pressure, in which another part of the carbamate is decomposed into ammonia and carbon dioxide; the latter are condensed in a condensing apparatus under a pressure which is substantially the same as that existing in the decomposition apparatus operating under medium pressure (the average pressure indicated here is between approximately 25 bars 1/2 and 10 bars, approx. - iJ ron 18.3 bars being preferred), the major part of the products J [condensable is condensed in this condensing apparatus.

7 \ î ^ arf * ί n Λ / a 1 1 a nna vnî 1 Λω c? A4- n nr> f Arjnt T AMnani * \ ï! | i I * "| 2 under medium pressure, the flow of condensate and of non-condensed products, which will be better defined in this application subsequently, is sent to a rectification column.

From the top of this column we obtain pure ammonia and the 5 non-condensable products, while from the bottom we discharge a stream of ammonium carbonate which is recycled to the carbamate condensing apparatus operating under high pressure? the ammonia recovered by rectification is * also recycled to the synthesis.

1 ° The reagents to be sent to the reactor contain a certain amount of dissolved gas: these come from both the ammonia manufacturing plant and the CC ^ production unit and consist essentially of H2 " CO, CH ^, Ar and He.

1 'In addition, in the urea installation, the reactor and the decomposition apparatus are sent a determined volume of air or oxygen in order to protect the decomposition apparatus, the condensation apparatus and the reactor against the corrosive action of ammonium carbamate (this protection will be designated hereinafter by "passivation").

All the non-condensable gases mentioned above, both those originally contained in the fresh flow of C02 and ammonia, as well as those introduced to protect the devices subject to corrosion, are removed from the plant. -25 synthesis urea lation and after being rid of the saturation ammonia which accompany them, form an * explosive mixture because of the presence of the oxygen used for said passivation.

The gases in question are generally removed in part from the carbamate condensing apparatus operating at high pressure from where they are preferably directed to the rectification column operating at medium pressure to recover the ammonia and are then mixed in this column with the stream from the carbamate condensing apparatus operating at medium pressure.

I In the column intended for the “recovery of ammonia, J mentioned above, the ammonia is recovered at the top at the same time as the aaz in question and, after condensation of the ammonia, these gases removed from this product are discharged and "constitute the explosive flow mentioned above.

It is seen that the flow of non-condensable gases, which is explosive, can also be removed at other points in the installation without thereby modifying the problem in any way.

The means which has been adopted hitherto commonly to prevent the explosion of this mixture consists in mixing the flow of explosive gases with a volume of non-combustible gases such that it brings the composition of the resulting gaseous mixture beyond of its explosive limits.

The authors of the present invention have now discovered that the explosiveness of the mixture of gases leaving urea production plants can be suppressed but without diluting such a mixture with incompressible gases and therefore without reducing its combustibility; this can be achieved by using the streams from the ammonia synthesis facility which are commonly available in urea synthesis facilities.

In the synthesis of ammonia, the following streams are particularly available: 1. the stream of natural gas essentially composed of methane. This flow is sent to steam reforming in order to manufacture the synthesis gas for ammonia.

2. the gas flow from the steam reforming builds up. essentially killed by ^, N ^, CO and 5 3. The gas stream from steam reforming but freed from CO ^.

4. The flow of nitrogen and hydrogen, saturated with ammonia and possibly containing Ar, He and CH ^, leaving the ammonia manufacturing unit in order to avoid enrichment in argon , helium, and methane from the flow to be returned to the synthesis of ammonia.

The range of typical flow compositions mentioned above is as follows; ! Hg from 0.1% to 77% by volume *

Jp- N2 from 0.1% to 29% by volume.

4

Inert gases (CO + CO ~ + Ar + He) of 0.1% to 50% by volume.

CH ^ the rest to make 100% by volume.

! Furthermore, in the streams mentioned above under Nos 2, 3 and 4, the molar ratio between H2 and N2 is between 2.5 and 3.3.

An object of the present invention is to provide a method for suppressing the explosiveness of gas leaving urea manufacturing plants, said method comprising the stage of mixing said gases leaving with one or more. fluxes available in installations for the manufacture of ammonia, and which have the following composition: H2 from 0.1% to 77% by volume; from 0.1 to 29% by volume; CO and / or C02 and / or Ar and / or He from 0.1 to 50% by volume and methane the rest to complete to 100% by volume.

In fact, it is quite a surprising fact that, operating according to the present invention, the explosiveness is suppressed not only as a result of the change in the composition of the mixture, but also in a completely unforeseen manner, 20 because the explosive area is made narrow.

It has therefore become possible, when operating according to the present invention, not only to make the flow of non-condensable gases leaving the urea installation non-explosive, but also to allow the use of the mixture as a gaseous fuel in the installation.

The present invention is illustrated by the example of the | * descriptive but not limiting below with reference to the drawings i! attached.

EXAMPLE

Reference is made to the diagram in FIG. 1 which shows in a simplified manner a plant for the production of urea.

It is assumed that the installation in question has an average production of 1,500 tonnes of urea per day, that the volume of gases, 3 dissolved in the reactants is 410 normal meters per hour, and that the composition is as follows: I H2 51.47% by volume N2 40.73%% O. 7r56% 5 CO Ο, 12% by volume CH3 0.12% The decomposition apparatus operating under high pressure receives 333 m normal air passivation per hour.

All these gases are released from the reaction solution 5 and are removed from the installation by the top of the ammonia rectification column: the latter operates at approximately 18.3 bars. The overhead gases from this column 12, after an initial condensation of most of the ammonia in the exchanger 8, are separated from this ammonia in the separator 1, and the ammonia thus separated is removed by line 9 and used in other units of the installation. Saturated ammonia gases at the temperature of separator 1 (35 ° C) have the following composition: 3 H2 211.0 m normal per hour 15 CH4 0.5.

CO O, 5 N2 + Ar 430.0. "o2 101.0 NH3 2977.0 '2o and must be sent via line 2 to the absorber 6 in which the ammonia is recovered after having been absorbed with the water arriving via line 7.

By removing this gas from its ammonia, a gaseous mixture having the following composition is obtained; 25 flammable gas 28.54% by volume of which: = 28.40 8 CH4 = 0.07% CO = 0.07% N2 + Ar 57.87% 02 13.59% 30 As can be seen in the diagram FIG. 2 - (which shows the explosion limits of ga2) the representative point for gas.D is inside the explosion zone; on the triangular diagram A = 100% of 02 <B = IOO% of inert gases and C = 100% of flammable gases.

Conversely, when operating according to% the present invention, I the gases of line 2 are mixed at point 11 with L the aaz leaving the ammonia synthesis cycle (production 6! Of ammonia 865 tonnes per day) which arrive via line 3: these gases have the following flow rate and composition:! 3 I Flow 7633 normal m per hour 1 Composition = 54.86% by volume I 5 CH4 = 10.92% "N2 = 21.94% NH3 = 12.28% 6r ___ 100.00% by volume ··

Gaseous mixture which is sent via line 4 10 to the absorber 6 has the following composition: 3 H2 = 4398.0 normal meters per hour CH3 = 833.5 CO = 0.5 "N_ + Ar = 2105.0 15 2 o2 = 110.0.

NH3 · = 3914.0 "3 11 352.0 normal m per hour.

After removing the ammonia, the gas has the following composition: H2 59.14% by volume CH4 11.21% "28.30 total total flammable gas · - ,.

02 1.35 * '70.35 * by volume | 25 | The diagram in FIG. 3, which is quite similar to that in FIG. 2, indicates the explosive limits of this latter gas mixture and its representative point D is largely beyond this limit. A, B and C have

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30 the same meanings as in FIG. 2.

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Claims (1)

1.- Method for suppressing the explosiveness of gases leaving urea production plants, characterized in that said released gases are mixed with one or more of the gas streams which are available in the manufacturing plants of the ammonia, and which consist of H2 from 0.1% to 77% by volume; from 0.1 to 29% by volume; CO and / or CO2 and / or Ar: and / or He of O, 1 to 50% by volume, and methane to make up to IOO% by volume. 2. Method according to claim 1, characterized in that the flow available in the ammonia synthesis installations is an element chosen from: a) the flow of natural gas, essentially consisting of methane, b) the stream from steam reforming of methane essentially consisting of H2, N2, CO and C02 c) the stream of gases from steam reforming stripped of CO ^; and d) the flow of nitrogen and hydrogen saturated with ammonia and 2o optionally saturated with water, containing Ar, He and CH ^; and from the ammonia manufacturing facility. ^ _ 'ψ \