GB2060614A - Improved Process for the Production of Urea by Reducing the Tendency of a Discharge Gas to Explode - Google Patents

Abstract

To reduce the tendency of a discharge gas from a urea-producing plant to explode, the discharge gas is mixed with one or more gas streams which are obtained from an ammonia- producing plant and whose composition is from 0.1 to 77% by volume of H2, from 0.1 to 29% by volume of N2, from 0.1 to 50% by volume of CO and/or CO2 and/or Ar and/or He, and a balance to 100% by volume of CH4.

Description

SPECiFICATION Process for Reducing the Tendency of a Discharge Gas From a Urea-Producing Plant to Explode This invention relates to a process for reducing the tendency of a discharge gas from a urea producing plant to explode.

As is known, urea may be produced from ammonia and carbon dioxide by direct synthesis in a reactor at a high pressure (from 50 to 450 atmospheres) and at a high temperature (from 1 700C to 2200C). The reaction product (which consists of urea, ammonium carbamate and water), together with excess ammonia, is fed from the reactor to a decomposer which operates under substantially the same pressure as that used for the urea synthesis, and, in this high-pressure decomposer, the carbamate contained in the urea solution is decomposed to form ammonia and carbon dioxide.

The ammonia and the carbon dioxide derived from the decomposition are fed to a high-pressure condenser which operates under substantially the same pressure as used for the urea synthesis. In this condenser, ammonia and carbon dioxide are condensed to form ammonium carbamate again, -which is recycled to the urea-synthesis reactor.

The solution of urea, stripped of a predominant amount of the carbamate, is fed from the high pressure decomposer to a medium-pressure decomposer (the term "medium pressure" used herein means a pressure of from 10 to 25 atmospheres, preferably 18 atmospheres), wherein a further amount of the carbamate is decomposed to form ammonia and carbon dioxide. The latter are condensed in a condenser under a pressure which is substantially the same as that used in the medium-pressure decomposer.

The major amount of condensable products are condensed in this decomposer.

From the medium-pressure condenser, the stream of condensates and uncondensed products, to be better described hereinbelow, is fed to a rectification column. From the top of the column, pure ammonia and uncondensable products are obtained, whereas from the bottom of the column, a stream of ammonium carbonate is obtained. The latter is recycled to the high pressure carbamate condenser. The ammonia recovered from the rectification column is likewise recycled for synthesis.

The reactants fed to the reactor contain a certain portion of dissolved gases. These gases emanate from the ammonia-producing plant and from a CO2-producing unit as well, and essentially consist of H2, N2, CO, CH4, Ar and He.

To the urea installation, to the reactor and to the decomposer, there are fed certain quantities of air or of oxygen in order to protect the decomposer, the condenser and the reactor against the corrosive action of ammonium carbamate.

All of the uncondensable gases mentioned above, both those which were originally contained in the fresh stream of carbon dioxide and ammonia, and those introduced for providing protection against corrosion, are discharged from the urea-producing plant. After having been stripped of ammonia, these gases form an explosive mixture, on account of the presence of the oxygen which has been employed for the protection against corrosion.

The gases in question are usually discharged, partly from the high-pressure carbamate condenser from which they are preferably forwarded to the medium-pressure rectification column for recovering ammonia. In this column, they are mixed with the stream from the mediumpressure carbamate condenser.

In the ammonia-recovery column, ammonia is recovered at the top, together with the gases in question, and, upon condensation and stripping of the ammonia, the aforementioned explosive mixture is obtained. It is clear that explosive mixtures of uncondensable gases may also be drawn from other points of the plant.

The expedient which has heretofore been commonly adopted to prevent explosion of such mixtures is to mix the explosive mixtures with an amount of incombustible gas such as to bring the composition of the resultant gaseous mixture beyond the explodability limits.

It has now been found, in accordance with the present invention, that the tendency of the mixtures of discharge gases from urea-producing plants to explode can be reduced, without diluting the mixture with incombustible gas and thus without reducing its combustibility, by using gas streams from an ammonia-producing plant, which streams are currently available in urea-producing plants.

Thus, according to the present invention there is provided a process for reducing the tendency of a discharge gas from a urea-producing plant to explode, which process comprising mixing the discharge gas with one or more gas streams obtained from an ammonia-producing plant and comprising (i) from 0.1 to 77% by volume of hydrogen, (ii) from 0.1 to 29% by volume of nitrogen, (ii) from 0.1 to 50% by volume of one or more of carbon monoxide, carbon dioxide, argon and helium, and (iv) methane.

In ammonia-producing plants, the following gas streams in particular are obtained: (1) A natural-gas stream, essentially composed of methane, which is subjected to steam reforming for the production of ammonia synthesis gas.

(2) A gas stream obtained by the steam reforming and essentially composed of H2, N2, CO and CO2.

(3) A gas stream obtained by this steam reforming but stripped of CO2.

(4) A stream of nitrogen and hydrogen, saturated with ammonia and possibly with water, which contains Ar, He-and CH4, as discharged from the ammonia-producing plant in order to offset the enrichment, with Ar, He and Cm4, of the stream which is recycled for ammonia synthesis.

In streams (2), (3) and (4) above, the molar ratio of H2:N2 is usually from 2.5:1 to 3.3:1.

Typically, the gas stream comprises from 0.1% to 77% by volume of H2, from 0.1% to 29% volume of N2, from 0.1% to 50% by volume of inert gas (i.e. one or more of CO, CO2, Ar and He), and a balance, to 100% by volume, of CH4.

A surprising fact is that, when operating in accordance with this invention, the tendency of the discharge gas to explode is reduced not only due to the change of the composition of the discharge gas but also, quite unpredictably, on account of a narrowing of the explodability zone.

Thus, it is possible, by operating in accordance with the Invention, not only to render nonexplosive the gas stream of uncondensable gases from the urea plant, but also to exploit the mixture as a gaseous fuel in the plant.

The invention will now be illustrated by the following Example, wherein reference is made to Figures 1 to 3 of the accompanying drawings.

Example Reference is made to Figure 1, which is a simplified representation of a urea-producing plant. Assuming that the plant has an average output of 1500 metric tons of urea perday, the volume of gases dissolved in the reactants is 410 normal m3 per hour, and the composition of the gas is as follows: by volume H2 51.47% N2 40.63% 02 7.56% CO 0.12% CH4 0.12% The high-pressure decomposer of the plant receives 333 normal m3 per hour of passivatlon air.

All of these gases are evolved from the reaction solution and are drawn out of the plant through the top of an ammonia-rectification column operating at about 18 atmospheres. The head gas 12 from this column, after initial condensation, in an exchanger 8, of a predominant amount of the ammonia therein, is separated from such ammonia in a separator 1, and the separated ammonia is removed along line 9 and used in other units of the plant. The gas, saturated with ammonia and at the temperature of the separator t (350C), has the following composition: Normal m3 per hour H2 211.0 CH4 0.5 CO 0.5 N2+Ar 430.0 02 101.0 NH3 2977.0 This gas is passed along line 2 to an absorber 6, wherein ammonia is absorbed with water which enters the absorber along line 7.The resulting aqueous ammonia solution is passed along line 10.

By stripping the gas of Its ammonia in the absorber 6, there is obtained a gas which has the following composition: by volume H2 28.40% CH4 0.07% CO 0.07% N2+Ar 57.87% O2 13.59% Thus the content of flammable gases (H2, CH4 and CO) of this gas Is 28.54% by volume.

As can be seen from the triangular diagram In Figure 2, which shows the explodablilty limits of gaseous mixtures of 02, inert gas and flammable gas, and wherein A is 100% of 02, B Is 100% of inert gas and C is 100% of flammable gas, the above gas, which has the composition D, is In the interior of the explodability region.

However, when operating in accordance with this invention, the gas passed along line 2 Is admixed at point 11 with a discharge gas of an ammonia synthesis plant having an output of ammonia of 865 metric tons per day, which gas Is passed along the line 3 at a rate of flow of 7633 normal m3 per hour and has the following composition: by volume H2 54.86% CH4 10.92% N2 21.94% NH3 12.28% Total 100.00% The gas mixture which is sent along line 4 to the absorber 6 thus has the following composition: normal m3 per hour H2 4398.0 CH4 833.5 CO 0.5 N2+Ar 2105.0 O2 110.0 NH3 3914.0 Total 11352.0 Upon ammonia stripping, the gas has the following composition: : by volume H2 59.14% CH4 11.21% N2+Ar 28.30% O2 1.35% Thus, the total content of inflammable gases Is 70.35% by volume.

The triangular diagram of Figure 3, which Is similar to that of Figure 2, indlcates the limits of explodability of this latter gas mixture, and the particular gas above, which has the composition D, is widely beyond these limits. A, B, and C have the same meanings as In Figure 2.

Claims (3)

Claims

1. A process for reducing the tendency of a discharge gas from a urea-producing plant to explode, which process comprising mixing the discharge gas with one or more gas streams obtained from an ammonia-producing plant and comprising (i) from 0.1 to 77% by volume of hydrogen, (ii) from 0.1 to 29% by volume of nitrogen, (iii) from 0.1 to 50% by volume of one or more of carbon monoxide, carbon dioxide, argon and helium, and (iv) methane.

2. A process according to claim 1, wherein the gas stream is one or more of (a) a natural gas stream comprising methane, (b) a gas stream obtained by the steam reforming of methane and comprising hydrogen, nitrogen, carbon monoxide and carbon dioxide, (c) a gas stream obtained by the steam reforming of methane and by the stripping of carbon dioxide from the resulting product, and (d) a gas stream of nitrogen and hydrogen, saturated with ammonia and possibly saturated with water, and containing argon, helium and methane.

3. A process according to claim 1, substantially as described in the foregoing Example.