CN1617853A

CN1617853A - Process for the preparation of urea

Info

Publication number: CN1617853A
Application number: CN 02827637
Authority: CN
Inventors: J·H·米斯森
Original assignee: DSM IP Assets BV
Current assignee: Stamicarbon BV
Priority date: 2002-01-28
Filing date: 2002-12-30
Publication date: 2005-05-18
Anticipated expiration: 2022-12-30
Also published as: CA2473224C; MY140851A; CA2473224A1; AU2002360227B2; RU2301798C2; AR038220A1; WO2003064379A1; NL1019848C2; RU2004126243A; CN1260209C

Abstract

Process for the preparation of urea from carbon dioxide and ammonia by means of a urea stripping process, comprising a high-pressure stripper and a high-pressure condenser, whereby a urea synthesis solution is formed, and wherein the stream 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. The invention also relates to a process for improving a urea plant comprising a thermal stripping process.

Description

Process for the preparation of urea

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

Such a process is described, for example, in GB 1,542,371. According to the method, the urea synthesis solution obtained in the urea synthesis reactor is thermally stripped in a high-pressure stripper. In the high-pressure stripping column, heat is suppliedThe ammonium carbamate and the remaining ammonia are removed from the urea synthesis solution. Subsequently, the gaseous products obtained in the stripping process are condensed in a high-pressure condenser. The stripped urea synthesis solution is passed to a medium-pressure decomposer where the ammonium carbamate still present is decomposed to CO₂And NH₃. The heat required for the medium-pressure decomposition is provided by the steam condensate formed during the high-pressure stripping of the urea synthesis solution.

Here and in the following, high pressure means a pressure of 12.5-20MPa, and here and in the following, medium pressure means a pressure of 1.5-5 MPa.

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

The combined use of steam condensate and steam a has the advantage that at least a part of the steam condensate can be used for other purposes. This is because the steam condensate has a significantly higher temperature and pressure than is required for use in medium-pressure decomposition. The steam condensate may function better elsewhere in or outside the urea production process. For example, in a urea production process, the steam condensate may be expanded to a pressure of 1.2MPa and then used to drive a vacuum ejector or usedto drive, for example, CO₂Compressor, NH₃Or a carbamate pump.

The amount of steam condensate and steam a used depends on the temperature and pressure of the steam required for the medium-pressure decomposition. In medium pressure decomposition, higher desired temperatures and pressures require more steam condensate than lower desired temperatures and pressures. The amount of steam condensate and steam a also depends on the potential use of the steam condensate inside and outside the urea production plant. It is of course advantageous if the steam condensate can be used for various other purposes, the quantity of steam a being as large as possible.

Preferably, only steam a is used for the medium-pressure decomposition of ammonium carbamate in the urea synthesis solution. This is possible because where a submerged condenser is used, steam a in most cases has a temperature and pressure that can be used for medium pressure decomposition. In this way, all steam condensate can also be used elsewhere.

Urea can be prepared by feeding excess ammonia together with carbon dioxide into the synthesis zone at a suitable pressure (e.g. 12-40MPa) and a suitable temperature (e.g. 160-:

the ammonium carbamate formed is then dehydrated to form urea according to the following equilibrium reaction:

the extent to which these reactions are completed depends inter alia on the temperature and the excess of ammonia used. As reaction products, a solution is obtained which essentially consists of urea, water, free ammonia and ammonium carbamate. Ammonium carbamate and ammonia need to be removed from the solution and are preferably recycled to the synthesis zone. In addition to the above-mentioned solution, a gas mixture of unconverted ammonia and carbon dioxide and inert gases also occurs in the synthesis zone. Ammonia and carbon dioxide are removed from this gas mixture and preferably also recycled to the synthesis reactor.

Urea is prepared by, for example, a urea stripping process. A urea stripping process is understood to be a process for the manufacture of urea in which the ammonium carbamate that is not converted into urea is decomposed and the ammonia that is normally in excess is discharged at a pressure substantially equal to the pressure in the synthesis reactor. This decomposition/discharge takes place, for example, by adding heat, in one or more strippers located downstream of the synthesis reactor. The latter is called thermal stripping. Thermal stripping refers to the decomposition of ammonium carbamate and the removal of ammonia and carbon dioxide present from the urea solution by supplying heat. The gas stream containing ammonia and carbon dioxide leaving the stripper is condensed in a high-pressure carbamate condenser and recycled to the reactor as an ammonium carbamate-containing stream.

Urea stripping processes typically use two embodiments of high pressure condensers to condense the stripping gas.

In a first embodiment, the gas mixture to be condensed, optionally in combination with a suitable solvent (for example a recycled ammonium carbamate solution in water), is passed through a riser pipe, the condensed gas mixture forming a falling film on the pipe walls, whether or not in combination with the solvent.

In a second embodiment, as described in GB 1,542,371, the gas to be condensed flows together with the solvent through a horizontally placed tube, where the condensation process takes place.

In both of the above embodiments, the required cooling is achieved by passing a suitablecoolant over the outer surface of the tube. Water is typically used as the coolant.

A disadvantage of the condensation in one of the two embodiments of the high-pressure condenser described above is the short residence time of the liquid in the tubes. Due to the short residence time, hardly any urea is formed in the condenser according to the two embodiments described above.

A third type of condenser is the so-called submerged condenser. Submerged condensers are described, for example, in EP 155735 a 1. 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 also the dilute carbamate solution produced, for example, by a high-pressure scrubber, can be passed. The heat released here by dissolution and condensation is removed by a medium, for example water, flowing through the pipe, which medium is converted into steam a.

In a urea production plant, a high-pressure scrubber scrubs the raw materials unconverted in the reaction from an inert stream, which leaves the reactor through the top thereof together with inert gas. Next, an inert stream is discharged. The washing is effected by means of a dilute carbamate stream formed in the urea recovery section.

The submerged condenser may be in a horizontal or vertical position. However, it is particularly advantageous to effect condensation in a horizontal submerged condenser (also known as a pool condenser; see, for example, Nitrogen No.222, 7-8, 1996, pages 29-31) because, in general, liquids have a significantly longer residence time in the pool condenser compared to other condenser designs. Thus, in addition to the carbamate solution, additional urea is formed in the pool condenser. This urea is recycled to the urea reactor together with the carbamate solution.

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

The pool reactor has the advantage that a heat exchanger/condenser is integrated into the reactor, thereby enabling the construction of a urea production plant with a low capital investment. A pool reactor is described in more detail in US5,767,313. The condensation zone in a pool reactor has essentially the same advantages as a submerged condenser. In this condensation zone, too, urea is already formed in large quantities in the condensation section, so that an increased heat transfer results, so that in the condensation zone it is possible to produce steam a, the pressure and temperature of which make it possible to use it in medium-pressure decomposition.

The temperature of the steam A is 150-175 ℃, preferably 160-170 ℃. The pressure of the steam A is 0.3 to 1MPa, preferably 0.4 to 0.8MPa, most preferably 0.6 to 0.8 MPa.

In the new urea production plant using thermal stripping, a submerged condenser, a pool condenser or a pool reactor may be used. However, in existing urea plants using thermal stripping, the existing condenser can also be replaced by a submerged condenser. This can be done during regular maintenance shutdowns of the plant. It is also possible to replace the existing condenser with a pool condenser. In existing urea plants, the reactor and the condenser can also be replaced by a pool reactor.

An example of a urea stripping process is the process as described in GB 1,542,371.

In a urea stripping process as described in GB 1,542,371, high pressure stripping of the urea synthesis solution is achieved by steam at a pressure of 26atm (2.6MPa) and a temperature of 225 ℃. This steam is used for medium pressure decomposition. In this stripping process, the steam generated by the high-pressure condenser is not suitable for use in medium-pressure decomposition, since the pressure of this steam is only 0.45MPa and the temperature is only 147 ℃. Whereas medium pressure decomposition is achieved at a pressure of about 1.8MPa and a temperature of about 155 ℃.

If a submerged condenser is used in the above process, it is possible to generate the vapor A at a temperature of 160 ℃ and 170 ℃ and a pressure of 0.6 to 0.8 MPa. This steam A is suitable for use in medium-pressure decomposition at a pressure of 1.8MPa and a temperature of 155 ℃.

Claims

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

2. The process according to claim 1, characterized in that a pool condenser is used as the submerged condenser.

3. Method according to claim 1 or 2, characterized in that only steam a is used for medium-pressure decomposition.

4. The process according to claim 1, characterized in that the high-pressure condensation is effected in a pool reactor.

5. Process according to any one of claims 1 to 4, characterized in that the temperature of the steam A is 160-170 ℃.

6. Process according to any one of claims 1 to 5, characterized in that the pressure of the steam A is between 0.6 and 0.8 MPa.

7. A method for improving a urea production plant, said plant comprising a thermal stripping process and a condenser, characterized in that the existing condenser is replaced by a submerged condenser.

8. The process according to claim 7, characterized in that the submerged condenser is a pool condenser.

9. A process for a urea production 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 by a pool reactor.

10. A plant for the preparation of urea comprising a thermal stripping process, characterized in that it comprises a submerged condenser.

11. Plant for the preparation of urea according to claim 10, characterized in that said submerged condenser is a pool condenser.

12. A plant for the preparation of urea comprising a thermal stripping process, characterized in that it comprises a pool reactor.