Process for the synthesis of urea

Abstract

A process for synthesis of urea from CO.sub.2 and NH.sub.3 wherein a steam flow (13) produced in the condenser (3) of a high-pressure synthesis loop is compressed to raise its pressure and temperature before using the steam as a heat source for a downstream step of the process.

Claims

1. A process for synthesis of urea from CO.sub.2 and NH.sub.3 comprising: a) reacting CO.sub.2 and NH.sub.3 at a reaction pressure to form an aqueous solution of urea; b) stripping said aqueous solution of urea at a stripping pressure obtaining a purified solution and a gaseous phase containing ammonia and carbon dioxide, wherein the stripping step involves passing said aqueous solution as a falling film in an externally heated tube bundle; c) condensing said gaseous phase in at least one condenser at a condensation pressure to form a recycle solution which is sent back to the reaction step, thus forming a synthesis loop; d) producing at least a first steam flow at a first recovery pressure and first recovery temperature using heat removed from said gaseous phase during the condensation step c); e) using said first steam flow as a heat source in at least one downstream step of the process, wherein: f) a compression of said first steam flow to reach a pressure greater than said recovery pressure and a temperature greater than said recovery temperature, before said use as a heat source in the at least one downstream step, wherein no fresh CO.sub.2 is added during the condensation step and wherein said step of compression of said first steam flow is performed with a compressor driven by an electric motor.

2. The process according to claim 1, wherein the stripping pressure and the condensation pressure are lower than the reaction pressure, the synthesis loop being then non-isobaric.

3. The process according to claim 2, wherein the stripping pressure and the condensation pressure are at least 20 bar lower than the reaction pressure.

4. The process according to claim 1, wherein said condensation pressure is 70 to 90 bar.

5. The process according to claim 1, wherein said stripping pressure is the same as the condensation pressure.

6. The process according to claim 1, wherein the compression of said step f) is performed with a compression ratio not greater than 2.

7. The process according to claim 1, wherein said first recovery pressure is 1.8 to 4.0 barg and the steam after compression of step f) has a pressure of 3 barg to 6 barg.

8. The process according to claim 1, wherein said first recovery temperature is not greater than 145° C. and the steam after compression of step f) has a temperature of at least 150° C.

9. The process according to claim 1, wherein the step d) includes the generation of at least two steam flows at different pressures and at least one of said steam flows is compressed according to step f).

10. The process according to claim 1, wherein the reaction pressure is greater than 140 bar.

11. The process according to claim 1, including the use of the compressed steam as a heat source in any of: a step of recovery of non-converted carbamate; a step of evaporation to remove water from a urea solution; and a step of crystallization to remove water from a urea solution.

12. The process according to claim 1, wherein the stripping of the solution is performed with the aid of a gaseous stripping medium and said stripping medium is CO.sub.2 or ammonia.

13. The process according to claim 1, wherein the step d) further includes the generation of at least a second steam flow, which is not compressed after generation, and is used to provide heat to a crystallization section for the concentration of a urea solution.

14. The process according to claim 2, wherein said recycle solution, which is obtained from condensation, is pumped to the reaction pressure by means of a centrifugal pump.

15. A plant for synthesis of urea from CO.sub.2 and NH.sub.3 comprising at least: a reactor where CO.sub.2 and NH.sub.3 react at a reaction pressure to form an aqueous solution of urea; a stripper fed with said aqueous solution of urea, wherein the solution is treated at a stripping pressure obtaining a purified solution and a gaseous phase containing ammonia and carbon dioxide, wherein the stripper includes a falling-film vertical tube bundle; a condenser for condensing said gaseous phase from the stripper at a condensation pressure to form a recycle solution which is sent back to the reactor, thus forming a synthesis loop; a steam system comprising at least a first steam line arranged to produce a first steam flow using heat removed from the condenser, wherein said first steam flow is used as a heat source in at least one downstream section of the plant; wherein: said steam system comprises a steam compressor arranged to raise the pressure of said first steam flow and deliver the so obtained compressed steam flow to said at least one downstream section, wherein said steam compressor is driven by an electric motor.

16. The plant according to claim 15, wherein the synthesis loop is non-isobaric and said stripper and condenser operate at a pressure lower than the pressure of the reactor.

17. The plant according to claim 15, wherein the synthesis loop comprises a centrifugal pump arranged to feed the recycle solution from the condenser to the reactor, raising the pressure of the solution to the reaction pressure.

18. A method for revamping a urea plant, wherein: the plant comprises: a reactor where CO.sub.2 and NH.sub.3 react at a reaction pressure to form an aqueous solution of urea; a stripper fed with said aqueous solution of urea, wherein the solution is treated at a stripping pressure obtaining a purified solution and a gaseous phase containing ammonia and carbon dioxide and wherein the stripper includes a falling-film vertical tube bundle; a condenser for condensing said gaseous phase from the stripper at a condensation pressure to form a recycle solution which is sent back to the reactor, thus forming a synthesis loop; a steam system comprising at least a first steam line arranged to produce a first steam flow using heat removed from the condenser, wherein said first steam flow is used as a heat source in at least one downstream section of the plant; and the method includes: adding a steam compressor to said steam system, the added compressor being arranged to raise the pressure of said first steam flow, wherein the added compressor is driven by an electric motor.

19. The process according to claim 4, wherein said condensation pressure is 80 bar or about 80 bar.

20. The process according to claim 10, wherein the reaction pressure is greater than 200 bar.

21. The plant according to claim 15, wherein the synthesis loop is non-isobaric and said stripper and condenser operate at a pressure at least 20 bar lower than the pressure of the reactor.

Description

DESCRIPTION OF FIGURES

(1) FIG. 1 is a scheme of a CO.sub.2 stripping urea plant revamped according to a first embodiment of the invention.

(2) FIG. 2 is a scheme of a CO.sub.2 stripping urea plant according to a second embodiment of the invention.

(3) FIG. 3 is a scheme of a self-stripping urea plant according to a third embodiment of the invention.

DETAILED DESCRIPTION

(4) FIG. 1 illustrates in a simplified manner a conventional CO.sub.2 stripping urea synthesis loop comprising a reactor 1, a stripper 2 and a condenser 3 which operate substantially at the same pressure, for example at 120 to 210 bar. Accordingly, said reactor 1, stripper 2 and condenser 3 form an isobaric HP loop. The loop may comprise additional items (e.g. a scrubber) which are not illustrated.

(5) A fresh CO.sub.2 stream 7 is fed to the stripper 2 and a feed of NH.sub.3 8 is fed to the condenser 3, for example with an ejector (not shown). In a further embodiment of the invention the feed of fresh CO.sub.2 can be separated in two streams feeding the reactor 1 and the stripper 2 (not shown).

(6) The CO.sub.2 stream 7 acts as a stripping medium in the stripper 2.

(7) An aqueous solution 4 containing urea and carbamate formed in the reactor 1 is sent to the stripper 2, where a purified urea solution 9 and a gaseous phase 5 are obtained.

(8) The purified urea solution 9 is sent to one or more recovery stage(s), for example at a recovery section 10 at low pressure (for example 2-6 bar) passing through an expansion valve 11.

(9) In the recovery section 10, the urea solution 9 after decompression is subjected to further treatments including decomposition of carbamate and condensation of vapours of ammonia and carbon dioxide. A so obtained solution of carbamate 16 is pumped back to high-pressure condenser 3. A purified urea solution 19 is sent to a downstream finishing section.

(10) The gaseous phase 5 from the stripper 2 is at least partially condensed in the condenser 3 and recycled via line 6 to the reactor 1.

(11) In the condenser 3, the heat of condensation of the gaseous phase 5 is transferred to water 12 and used to produce steam 13 for use in a downstream section of the plant, for example in the recovery section 10, as illustrated in the FIG. 1, or in a finishing section after the recovery section 10.

(12) Particularly, the steam 13 produced in the condenser 3 is compressed in a steam compressor 14 to raise its pressure and temperature. The so obtained compressed/heated steam 15 delivered by said compressor 14 may be directed to the recovery section 10 where it provides heat to one or more related equipment, for example to one or more decomposer(s) operating at a medium pressure and/or at a low pressure.

(13) The steam compressor 14 may be a multi-stage compressor. In some embodiments, the steam compressor 14 is an electric compressor.

(14) In another embodiment, at least part of the steam 15 may be used in a finishing section after the recovery section 10. Said finishing section may include an evaporation section or a crystallization section to remove water from the solution 19. The hot steam 15 may be used for example to furnish heat to the evaporation section or to the crystallization section.

(15) FIG. 2 illustrates a CO.sub.2 stripping plant in accordance with a second embodiment where items corresponding to FIG. 1 are denoted by the same numerals.

(16) In FIG. 2 the high-pressure loop formed by the reactor 1, stripper 2 and condenser 3 is non-isobaric. In particular, the stripper 2 and condenser 3 operate at a pressure lower than the pressure of reactor 1. For example, the reactor 1 operates at 210 bar while the stripper 2 and condenser 3 operate at around 80 bar.

(17) The reaction effluent 4 is de-compressed to the stripping pressure through a valve 17 and the recycle solution withdrawn from the condenser 3 is brought to reaction pressure through a pump 18.

(18) An advantage of the embodiment of FIG. 2 is that the stripper and condenser can be operated at relatively low pressure, compared to the reaction pressure; the heat recovery from the condenser 3 to the recovery section 10 is however not penalized by the relatively low condensation pressure, thanks to the intermediate compression through the compressor 14. This compression raises the temperature of the steam, thus making the steam 15 useful for heating the recovery section 10, e.g. a decomposer of the same.

(19) Also in the embodiment of FIG. 2, the fresh CO.sub.2 feed 7 may be split in two streams 7a and 7b directed respectively to the reactor 1 and to the stripper 2 as illustrated.

(20) The fresh ammonia 8 is split into stream 8a directed to the reactor 1 and stream 8b directed to the condenser 3.

(21) The compressor 14 implements a heat pump transferring heat from the condenser 3 to the recovery section 10 while raising the temperature of said heat.

(22) In a preferred embodiment, the steam 13 produced by the condenser 3 has a temperature of around 135° C. and a pressure of around 2.1 barg; the steam 15 delivered by the compressor 14 has a temperature at least of 150° C. and a pressure of above 3.5 barg.

(23) In further advantageous embodiments of the invention, the high-pressure loop may comprise more than one condenser. For example, in a urea synthesis process including a non-isobaric high-pressure loop, a biphasic solution produced in the condenser 3 is subjected to a second condensation step, wherein a saturated water steam is produced. Said saturated steam can be used to pre-heat the ammonia and/or in a pre-concentration step of the solution 19, before the solution is fed to an evaporation section to remove water and produce a urea melt. Alternatively it can be also used to remove water in a concentration section based on the crystallization technology.

(24) Said pre-concentration step may be carried out for example in a shell-and-tube pre-concentrator by sending the biphasic solution to the shell side.

(25) FIG. 3 is a scheme of a self-stripping urea plant comprising a reactor 1, a stripper 2 and a condenser 3.

(26) A fresh CO.sub.2 stream 7 and a feed of NH.sub.3 8 are fed to the reactor 1. The reactor effluent 4 is depressurized through a valve 17 and sent to the stripper 2. After the stripping process, a purified urea effluent 9 is sent to one or more recovery section(s) 10 downstream by passing through an expansion valve 11. Typically the recovery sections include a medium-pressure recovery section and a low-pressure recovery section. After treatment in the recovery sections, a purified solution 19 is then sent to a finishing section.

(27) The gaseous phase 5 formed during the self-stripping step is combined with the carbamate recycle solution 16 from the recovery sections. The so obtained stream 26 is at least partially condensed in the condenser 3 and the condensed carbamate solution 20 is sent to a carbamate separator 21, wherein a liquid solution 23 and a gaseous phase 22 are produced.

(28) The liquid solution 23 is recycled to the reactor 1 by means of a pump 24 which brings the solution 23 back to the reaction pressure. Before reaching the reactor 1, the liquid solution 25 delivered by the pump 24 is combined with the ammonia fresh feed 8 from outside to form a flow 26. The flow 26 is then sent to the reactor 1.

(29) The gaseous phase 22 produced in the carbamate separator 21 is sent to the recovery section, for example to a medium-pressure decomposer.

(30) In a variant of FIG. 3, the stripper 2 and condenser 3 may operate at a pressure which only slightly less than the reaction pressure. In this case the solution 23 can be fed to the reactor 1 with an ejector (instead of pump 24) wherein the motive stream of the ejector is the ammonia feed 8.

(31) The schemes of FIGS. 1 to 3 may result from the revamping of an existing plant, wherein the revamping includes the addition of the compressor 14 and if necessary the provision of the related steam line.