AU2002360227A1

AU2002360227A1 - Process for the preparation of urea

Info

Publication number: AU2002360227A1
Application number: AU2002360227A
Authority: AU
Inventors: Jozef Hubert Meessen
Original assignee: Stamicarbon BV
Current assignee: Stamicarbon BV
Priority date: 2002-01-28
Filing date: 2002-12-30
Publication date: 2003-09-18
Anticipated expiration: 2022-12-30

Description

PROCESS FOR THE PREPARATION OF UREA

The invention relates to a process for the preparation of urea from carbon dioxide and ammonia in a urea synthesis reactor by means of a urea stripping process.

Such a process is described in for example GB-1.542.371. According to that process a urea synthesis solution obtained in the urea synthesis reactor is thermally stripped in a high-pressure stripper. In the high-pressure stripper the ammonium carbamate and the remaining ammonia are removed from the urea synthesis solution by supplying heat. The gaseous products obtained during stripping are subsequently condensed in a high-pressure condenser. The stripped urea synthesis solution is passed on to a medium-pressure decomposer wherein the ammonium carbamate still present is decomposed into CO₂ and NH₃. The heat required for medium-pressure decomposition is supplied by steam condensate that is formed during high-pressure stripping of the urea synthesis solution.

High pressure here and hereinafter means a pressure of 12.5-20 MPa and medium pressure here and hereinafter means a pressure of 1.5-5 MPa.

Surprisingly, it has now been found that when in the aforesaid process a submerged condenser is used as high-pressure condenser, the steam A which forms in the high-pressure condenser has such a temperature and pressure that it can be utilized in combination with the steam condensate for medium-pressure decomposition of the ammonium carbamate in the urea synthesis solution.

Utilization of a combination of the steam condensate and steam A presents the advantage that at least a portion of the steam condensate becomes available for other purposes. This is because the steam condensate has a significantly higher temperature and pressure than needed for application in the medium-pressure decomposition. The steam condensate can be used to better effect elsewhere within or outside the urea process. Within the urea process the steam condensate could for example be expanded to a pressure of 1.2 MPa and then utilized for driving a vacuum ejector or for driving for example the CO₂ compressor, the NH₃ - or the carbamate pumps. The utilized quantities of steam condensate and of steam A depend on the temperature and pressure of the steam needed for medium-pressure decomposition. More steam condensate is needed for a higher desired temperature and pressure in the medium-pressure decomposition than for a lower desired temperature and pressure. The quantities of the steam condensate and of steam A are also dependent on the potential applications of the steam condensate within or outside the urea plant. If the steam condensate can be utilized for various other applications, it is of course advantageous for the quantity of steam A to be as large as possible.

Preferably, only steam A is used for the medium-pressure decomposition of the ammonium carbamate in the urea synthesis solution. This is possible since, where a submerged condenser is used, steam A in most cases has such a temperature and pressure as to be able to be utilized for the medium-pressure decomposition. In this manner it is also possible to utilize all steam condensate elsewhere.

Urea can be prepared by introducing excess ammonia together with carbon dioxide into a synthesis zone at a suitable pressure (for example 12-40 MPa) and suitable temperature (for example 160-250°C), which first results in the formation of ammonium carbamate according to the reaction:

2NH₃ + CO₂ → H₂N-CO-ONH₄

Dehydration of the ammonium carbamate formed then results in the formation of urea according to the equilibrium reaction:

H₂N-CO-ONH₄ *-» H₂N-CO-NH₂ + H₂O

The extent to which these reactions go to completion depends on inter alia the temperature and the excess amount of ammonia used. As a reaction product there is obtained a solution consisting essentially of urea, water, unbound ammonia and ammonium carbamate. The ammonium carbamate and the ammonia need to be removed from the solution and are preferably returned to the synthesis zone. Besides the aforementioned solution there arises in the synthesis zone also a gas mixture of unconverted ammonia and carbon dioxide as well as inert gases. Ammonia and carbon dioxide are removed from this gas mixture and are preferably also returned to the synthesis reactor.

Urea is prepared by means of for example a urea stripping process. A urea stripping process is understood to be a process for the production of urea in which the decomposition of the ammonium carbamate thatis not converted into urea and the expulsion of the customary excess ammonia largely take place at a pressure that is essentially virtually equal to the pressure in the synthesis reactor. This decomposition/expulsion takes place in one or more strippers located downstream of the synthesis reactor, for instance with addition of heat. The latter is called thermal stripping. Thermal stripping means that ammonium carbamate is decomposed and the ammonia and carbon dioxide present are removed from the urea solution exclusively by supplying heat. The ammonia and carbon dioxide-containing gas stream exiting from the stripper are condensed in a high-pressure carbamate condenser and returned to the reactor as an ammonium carbamate-containing stream.

Urea stripping processes usually employ two embodiments of a high- pressure condenser for condensation of the stripping gases.

In a first embodiment, the gas mixture to be condensed, optionally in combination with a suitable solvent (for example a recirculated ammonium carbamate solution in water) is passed through vertical tubes, with the condensed gas mixture, whether or not in combination with the solvent, forming a falling film on the tube wall. In a second embodiment, described in for example GB-1.542.371 , the gases to be condensed, together with the solvent, flow through horizontally positioned tubes wherein the condensation process takes place. In both of the aforementioned embodiments the required cooling is effected by passing a suitable coolant along the outside surface of the tubes. Water is usually used as a coolant.

A drawback of condensation in one of the two aforesaid embodiments of a high- pressure condenser is that the liquid residence time in the tubes is short. Because of this short residence time, hardly any urea is formed in condensers according to the two aforesaid embodiments.

A third type of condenser is a so-called submerged condenser. A submerged condenser is described in for example EP-155735-A1. In a submerged condenser the gas mixture to be condensed is passed through the shell side of a shell- and-tube heat exchanger, through which shell side may also be passed a dilute carbamate solution originating from for example the high-pressure scrubber. The heat of solution and condensation released here is removed with the aid of a medium, for example water, flowing through the tubes, which is converted into steam A.

In a urea plant, the high-pressure scrubber scrubs out of the inert stream the raw materials that are not converted in the reaction, which materials leave the reactor through the top along with inert gases. Next the inert stream is vented. Scrubbing is effected with the aid of a dilute carbamate stream which forms in the urea recovery section.

The submerged condenser may be placed in horizontal or vertical position. It is however particularly advantageous for condensation to be effected in a horizontal submerged condenser (also known as a pool condenser; see for example Nitrogen No. 222, July-August 1996, pages 29-31) in that, in comparison with other condenser designs, the liquid as a rule has a significantly longer residence time in the pool condenser. As a result, besides a carbamate solution, extra urea is formed in the pool condenser. This urea is returned to the urea reactor along with the carbamate solution.

A particular embodiment of a submerged condenser is a pool reactor. Such a reactor comprises a horizontal condensation zone and heat exchanger that are designed as a submerged condenser. A proportion of the gas mixture to be condensed is passed through the shell side of a shell-and-tube heat exchanger, through which shell side the ammonia and a dilute carbamate solution are also passed, with the heat of solution and condensation being removed with the aid of a medium, usually water, which is converted into steam A.

A pool reactor has the advantage that the heat exchanger/condenser is integrated in the reactor, allowing a urea plant to be built at lower capital investment. A pool reactor is described in further detail in US-5,767,313. The condensation zone in the pool reactor has essentially the same advantages as a submerged condenser. In this condensation zone, too, urea formation takes place to a significant extent already in the condensation section, resulting in improved heat transfer as a result of which it is possible to produce in the condensation zone a steam A having such a pressure and temperature that it can be utilized in the medium- pressure decomposition.

Steam A has a temperature of 150-175°C, preferably of 160-170°C. The steam A has a pressure of 0.3-1 MPa, preferably 0.4-0.8 MPa and most preferably 0.6-0.8 MPa.

The application of a submerged condenser, a pool condenser or a pool reactor is possible in a new urea plant that employs thermal stripping. It is however also well possible in an existing urea plant employing thermal stripping to replace the existing condenser with a submerged condenser. This can take place during a regular maintenance shut-down of the plant. The existing condenser can also be replaced with a pool condenser. It is also possible in an existing urea plant to replace both the reactor and the condenser with a pool reactor.

An example of a urea stripping process is the process as described in GB-1.542.371. In the urea stripping process as described in GB-1.542.371 high- pressure stripping of the urea synthesis solution is effected with the aid of steam having a pressure of 26 atm (2,6 MPa) and a temperature of 225°C. This steam is used in the medium-pressure decomposition. In this stripping process, the steam produced by the high-pressure condenser is not suitable for use in the medium-pressure decomposition, because this steam has a pressure of only 0.45 MPa and a temperature of 147°C. Medium-pressure decomposition is effected at a pressure of approx. 1.8 MPa and a temperature of approx. 155°C.

If in the aforesaid process a submerged condenser is applied, steam A can be produced with a temperature of 160-170°C and a pressure of 0.6-0.8 MPa. This steam A is suitable for use in the medium-pressure decomposition at a pressure of 1.8 MPa and a temperature of 155°C.

Claims

1. Process for the preparation of urea from carbon dioxide and ammonia by means of a urea stripping process comprising a high-pressure condenser and a high-pressure stripper, whereby a urea synthesis solution is formed and, wherein the steam condensate that forms during high-pressure stripping of the urea synthesis solution is utilized for medium-pressure decomposition of the ammonium carbamate in the urea synthesis solution, characterized in that a combination of the steam condensate and steam A, which forms in the high- pressure condenser, is used for medium-pressure decomposition, with the high-pressure condenser being a submerged condenser.

2. Process according to Claim 1 , characterized in that a pool condenser is used as submerged condenser.

3. Process according to either of Claims 1-2, characterized in that only steam A is used for medium-pressure decomposition.

4. Process according to Claim 1 , characterized in that high-pressure condensation is effected in a pool reactor.

5. Process according to any one of Claims 1-4, characterized in that steam A has a temperature of 160-170°C. 6. Process according to any one of Claims 1-5, characterized in that steam A has a pressure of 0.

6-0.8 MPa.

7. Process for improving a urea plant comprising a thermal stripping process and a condenser, characterized in that the existing condenser is replaced with a submerged condenser.

8. Process according to Claim 7, characterized in that the submerged condenser is a pool condenser.

9. Process for improving a urea plant comprising a thermal stripping process, a condenser and a urea reactor, characterized in that the existing condenser and the existing urea reactor are replaced with a pool reactor.

10. Urea plant comprising a thermal stripping process, characterized that the urea plant contains a submerged condenser.

11. Urea plant according to Claim 10, characterized in that the submerged condenser is a pool condenser.

12. Urea plant comprising a thermal stripping process, characterized in that the urea plant contains a pool reactor.