IE43281B1 - Formation of ammonium carbamate solution - Google Patents

Description

This invention relates to a method for the formation of ammonium carbamate solution, e.g. in urea-synthesis plants.

In the various methods for the synthesis of urea, there are usually provided exchangers for condensing the vapours of NHg+COg+HgO produced by decomposition in various stages of the synthesis, and means for recycling the ammonium carbamate solution formed to the urea reactor.

There are several procedures for recycling the carbamate solution to the reactor, varying in the number of stages used and in the pressures and temperatures employed.

In stripping procedures, almost all of the carbamate solution is recycled in a single stage at a pressure which is substantially equal to that of the urea reactor, and the vapours of NHg+COg+F^O in the stripper (which also operates at a pressure substantially equal to that of the urea reactor) are condensed in a carbamate condenser at a high temperature. The heat evolved in the condensation is used for the production of steam.

In other conventional processes, the carbamate solution is recycled in a number of stages at decreasing pressures, these pressures being substantially different from that of the urea reactor. Only a fraction of the heat of condensation of the carbamate solution is recovered at a temperature which is high -243281 enough to produce steam in the carbamate condenser, the latter being operated at a high pressure.

In all cases, the condensation of vapours of NHg+COg+HgO is a critical operation, especially in the case of mixtures of NH3+C02+H20 having a low content of HgO, such as those encountered in the carbamate recycle stages which ODerate at a higher pressure.

To make this condensation possible, especially when the vapours of NH3+C02+ H.,0 to be condensed emanate from a high-pressure distillation stage (and thus are lean in H20), there is fed to the carbamate condenser, along with the vapours, a carbonate solution which is rich in H.,0, namely that normally produced in the carbamate condesnation and recycle stages operated at a lower pressure. High pressures are pressures from 100 to 300 atmospheres, and low pressures are pressure of from 2 to 50 atmospheres. This solution is absorbent and hence the condenser functions as an absorber. It is very important that uniform distribution is achieved between the vapours of NH3+C02+H20 and the carbonate solution. This is attained in conventional techniques by the use of appropriate systems for mixing and distributing the vapours and the carbonate solution.

In the known art, in order to achieve an even distribution of the vapours and the absorbent liquor, the absorbers operated at the higher pressures are generally of the vertical type with the absorbent liquor in the interior of the tubes. The absorption is carried out as shown in Figure 1 of the accompanying drawing. The vapours of NH3+C02+H20 are fed in via line 3 and, in the mixer 4, are mixed with ammonium carbonate solution emanating from the low-pressure stage and fed through line 5 to a vertical absorber 1 in the jacket of which water, fed in through line 6, is circulated. The water vapour produced emerges from the jacket of absorber 1 through line 9 and pass to a steam separator 7 from which water and steam are drawn along lines 6 and 8 respectively. The solution of ammonium carbamate produced is discharged through line 10.

As can be seen, when heat is to be removed by the production of steam, it is necessary to provide apparatus for separating the steam, which apparatus is connected to the absorber. -3fit w * In as much as even distribution of the liquid and the vapours is extremely critical for correct operation of the absorber and for obtaining as much as possible of the heat evolved by the absorption of the NHg-i-COgtHgO vapours, expensive distribution systems are usually provided at the absorber inlet. It is a cumbersome task to secure correct operation of such distributors and to bar preferential paths, since it is likely that clogging may occur as a result of the crystalization of the carbamate. -44 3 2 81 In addition, it is extremely expensive to provide a vertical absorber on account of the supporting structures it requires, especially in installations having a high potential out-put and also when the steam separator is provided with means for the removal of heat from the steam produced. It is very difficult, moreover, to achieve even distribution when the apparatus has a considerable bulk (i.e. when the apparatus is a high-output installation).

According to the present invention, there is provided a method of forming ammonium carbamate solution, which comprises feeding a mixture of (1) ammonium carbamate solution and (2) a gas comprising ammonia, carbon dioxide and water vapour, at a pressure of from 50 to 300 atmospheres, into a horizontal tubular absorption zone maintained under a static pressure.

The invention is particularly suitable for making use of the ammonium carbonate solution emanating from a low-pressure stage of a urea-synthesis plant and of the gas comprising NHg, C02 and H20 emanating from a high pressure, carbamatedecomposition stage of a urea-synthesis plant, and the description below relates to the use of such solution and gas.

It has been surprisingly found in accordance with the present invention that if the mixture of the solution of ammonium carbonate emanating from the low-pressure stage (which solution may be compressed) and of the NHg, C02 and H20 vapour emanating from the decor position of the carbamate at a high pressure (the pressure of such a mixture being from, for example, 50 to 300 atmospheres) is introduced into a horizontal tubular absorption zone at an inlet pressure of from 50 to 300 atmospheres maintained under a static pressure P, all the above enumerated drawbacks are overcome or mitigated.

The value of P is preferably from 0.5 to 5 atmospheres and can be obtained by use of any system known from the prior art, for example by means of a static liquid head ( H) at the outlet. A pressure-reduction valve may be used for reducing the height of the static liquid head. For examole, a static pressure of 5 atmospheres would require a static liquid head of 40-50 metres in height, and the use of a pressure-reduction valve enables this height to be reduced to a more practical value. -543281 The method according to the present invention can b.e advantageously carried out in a horizontal or substantially horizontal tube-bundle absorber, preferably one in which the ratio of the static pressure, measured as a water column height H, to the diameter D of the circumference which encompasses the outermost tubes of the bundle of horizontal tubes Is from 5 to 30, more preferably about 10.

The mixture of the aqueous ammonium carbonate solution and the gaseous mixture of C02, NH3 and Η,,Ο, preferably having a carbonate solution content above 20%, enters the tubes and the gaseous mixture is absorbed in the carbonate solution.

The resulting solution is discharged from the absorber. The heat of ahsorption may be used for producing steam in the absorber jacket.

The invention will now be described by way of example with reference to Figure 2 of the accompanying drawings.

The inlet end of a horizontal tube absorber 1 is fed via lines 2 and 3 with a mixture of a carbonate solution emanating from the low-pressure decomposition stages of a urea-synthesis plant and a gaseous mixture of C02+NH3JrH20 emanating from the high-pressure decomposition of the carbonate. The mixture passes through a plurality of tubes 7 wherein the gaseous Mixture of C02, NR3 and H.,0 is ahsorhed in the carbonate solution to form a product which leaves the absorher 1 through a tube 5, this tube 5 maintaining the desired H (6) to be overcome Ky the product. The tubes 7 are immersed in water, the surface of which is shown by dotted line 8. The heat produced by absorption of the gaseous mixture in the carbonate solution heats the water, and the steam produced leaves absorber 1 through line 4.

The advantages of the method described with referenc to Figure 2 are: 1. A reduction of the invested capital (the distribution system is not necessary, the supporting structures are not as extensive, and, in the case in which heat is removed by producing steam, the steam separator is not necessary). 2. Reliability of operation under optimum conditions, without crystallization or preferential paths. 3. Removal of large quantities of beat (i.e. the production of better quality steam) when the condensation takes place at a high pressure.

An example will now be given, illustrating the invention. -543281 EXAMPLE A Kettle-type horizontal absorber 1 containing a bundle of tubes, is used (see Figure 2).

The vapour in line 3 has a pressure of 150 atmospheres, a temperature of 190°C and a rate of flow of 60 tons per hour. The vapour emanates from a carbamate decomposition section (not shown) and consists of NHg+COg+HgO. The vapour is absorbed in the interior of tubes 7 of the tube-bundle after having been mixed with an aqueous solution of ammonium carbonate recycled through line 2 from a low-pressure section of a urea-synthesis plant. The solution in line 2, which is compressed to a pressure of 150 atmospheres, had a temperature of 70°C and a rate of flow of 25 tons per hour.

The heat of absorption boils the water in the jacket to produce steam which leaves through line 4.

The H:D ratio is maintained at 10, and under these conditions, the tubes of the horizontal tube absorber completely fill with liquid and absorption takes place unhindered.

The steam produced has a temperature of 147°C, while a conventional apparatus is incapable of producing steam at a temperature higher than 138°C.

Claims (13)

1. A method of forming ammonium carbamate solution, which comprises feeding a mixture of (1) ammonium carbonate solution and (2) a gas comprising ammonia, carbon dioxide and water vapour, at a pressure of from 50 to 300 atmospheres, into a horizontal or substantial horizontal tubular absorption zone maintained under a static pressure.

2. A method according to Claim 1, wherein the tubular absorption zone is maintained under a static pressure by means of a static liquid head.

3. A method according to claim 1 or 2, wherein the ammonium carbonate solution emanates from a low-pressure stage of a urea-synthesis plant.

4. A method according to claim.!, 2 or 3, wfterein the gas comprising ammonia, carbon dioxide and water vapour emanates from a high-pressure, carbamate-decomposition stage of a urea-synthesis plant.

5. A method according to any of the preceding claims, wherein the static pressure is from 0.5 to 5 atmospheres.

6. A method according to any of the preceding claims, wherein the absorption zone is a horizontal or substantially horizontal tube-bundle absorber.

7. A method according to claim 6, wherein the ratio of the static pressure, measured as a water column height, to the diameter of the circumference which encompasses the outer-most tubes of the tube-bundle is from 5 to 30.

8. A method according to claim 7, wherein the said ratio is about 10.

9. A method according to any of the preceding claims, wherein the content of ammonium carbonate solution of the mixture is greater than 20$.

10. A method according to any of the preceding claims, wherein generation of steam takes place concurrently with the formation of the ammonium carbamate solution, the steam being generated by use of the heat evolved by the absorption.

11. A method of forming ammonium carbamate solution, substantially as hereinbefore described with reference to Figure 2 of the accompanying drawing.

12. A method of forming ammonium carbamate solution, substantially as described in the foregoing Example. -843281

13. Ammonium carbamate solution, whenever formed by the method claimed in any of the preceding claims. Dated this the 26th day Of July, 1976.