REVAMPING OF AMMONIA-UREA PLANTS

Abstract

A method for revamping an ammonia-urea plant wherein: the ammonia section is modernized to produce an extra amount of low pressure steam; condensation stage of the high-pressure urea synthesis loop is modified to use part of the condensation heat of the urea stripper vapours to produce medium-pressure steam, said medium-pressure steam is fed to one or more steam users of the urea section, particularly for carbamate decomposition, the input of low-pressure steam to the urea section is balanced by importing the extra low-pressure steam produced in the ammonia section.

Claims

1-19. (canceled)

20. A method of revamping an ammonia-urea plant wherein: A) the ammonia-urea plant which is revamped has the following features: the ammonia urea plant includes an ammonia section and a urea section; said ammonia section includes a front-end for generation of a makeup syngas from reforming of a hydrocarbon and an ammonia synthesis section where the makeup syngas is reacted to produce ammonia; ammonia produced in the ammonia synthesis section, and at least part of the CO2 removed from the makeup syngas in the front-end, are fed to the urea section for use as starting material for the production of urea; the urea section includes a high-pressure urea synthesis section including at least a urea synthesis reactor, a stripper for removing unconverted ammonia and carbon dioxide from the reactor effluent, and a condensation stage arranged to condensate stripper vapours containing ammonia and carbon dioxide removed from the reactor effluent; the urea section further includes a recovery section for recovering unconverted ammonia and carbon dioxide from the solution effluent from the stripper, at a pressure lower than the urea synthesis pressure; the ammonia-urea plant also includes a steam network, including steam producers and steam users, and steam produced in the ammonia section is exported to the urea section and sent to at least one steam user in the urea section; the condensation stage of the high-pressure urea synthesis section is a steam producer in the steam network and produces low pressure steam at a first pressure using the heat of condensation of the stripper vapours, said low pressure steam being used internally in the urea section in one or more steam users of the urea section; and B) the revamping includes: the provision of an export of low pressure steam at a pressure not greater than 6 bar rel, from the ammonia section to the urea section, for use in at least one steam user in the urea section.

21. The method according to claim 20 wherein the urea section is modified to accommodate said low pressure steam exported from the ammonia section to the urea section.

22. The method according to claim 21 wherein: the urea section includes a steam user of steam at a second pressure greater than the pressure of said low pressure steam transferred from the ammonia section to the urea section; and the method further comprises modifying the urea section to produce steam at said second pressure for said user and balancing the input of low-pressure steam to the urea section with said low-pressure steam transferred from the ammonia section to the urea section.

23. The method according to claim 22 wherein the step of modifying the urea section to produce steam at said second pressure includes modifying the condensation stage of the high-pressure urea synthesis section to use part of the condensation heat of the stripper vapours to produce said steam at said second pressure.

24. The method according to claim 23, wherein the condensation stage of the urea synthesis section originally includes a first condenser arranged to produce low-pressure steam at said first pressure, and the revamping of the stage includes the installation of a second condenser arranged to produce said medium-pressure steam at said second pressure.

25. The method according to claim 24 wherein the second condenser is installed in series and upstream the first condenser, so that the stripper vapours coming from the stripper pass through the newly installed second condenser and then through the original first condenser.

26. The method according to claim 24, wherein the first condenser and/or the second condenser are shell-and-tube equipment, preferably horizontally arranged of the kettle type, where the stripper vapours flow in the tube side and steam is produced in the shell side.

27. The method according to claim 22 wherein the step of modifying the urea section to produce steam at said second pressure includes installation of a steam compressor which is fed with steam at low pressure and delivers steam at said second pressure.

28. The method according to claim 22 wherein: in the ammonia-urea plant, the steam at said second pressure for said user is originally produced by mixing medium-pressure steam with low-pressure steam produced in the condensation stage; and the method includes that said mixing is discontinued.

29. The method according to claim 22, wherein the urea section includes a medium-pressure recovery section and a low-pressure recovery section and said steam user of said steam at the second pressure is a carbamate decomposer of the medium-pressure recovery section.

30. The method according to claim 29 wherein the urea section operates with ammonia-stripping or self-stripping process.

31. The method according to claim 22 wherein said second pressure is 5 bar to 7 bar.

32. The method according to claim 21 wherein: the urea section is a CO2-stripping section wherein the recovery section includes only a section at low pressure; the method includes reducing the amount of heat transferred to the stripper thus reducing also the amount of low pressure steam that can be produced in the high-pressure carbamate condenser; and the missing production of low pressure steam in the urea section is balanced with said low-pressure steam transferred from the ammonia section to the urea section.

33. The method according to claim 20, wherein the ammonia section is modified to increase the low pressure steam that can be exported form the ammonia section and therefore to provide said low pressure steam for transfer to the urea section, preferably as a result of increase efficiency of LP steam utilization or as a result of increased heat recovery.

34. The method according to claim 33, wherein at least part of the low pressure steam for balancing the input of the urea section is obtained from modernization of a CO2 recovery section in the front-end of the ammonia section, such modernization resulting in a reduced specific consumption of regeneration energy for the CO2 recovery section of less than 400 kcal/Nm3 of removed CO2.

35. The method according to claim 20, further including that the steam to carbon ratio in the reforming process is lowered, and including that the ammonia section is revamped to operate the reforming process with a S/C ratio of less than 2.7.

36. The method according to claim 35 wherein the shift section is modified to operate with said reduced S/C ratio and/or wherein at least one steam turbine driver is changed to a motor drive resulting in reduced steam consumption.

37. An integrated ammonia-urea process, comprising: ammonia is produced in an ammonia section by generation of a makeup syngas from reforming of a hydrocarbon and reaction of said makeup syngas to produce ammonia at least part of the synthesized ammonia and optionally also CO2 removed from the makeup syngas during its purification are used in a tied-in urea section for the production of urea, wherein urea is produced with a stripping process at high synthesis pressure and includes recovery of unconverted matter at one or more pressure levels lower than said synthesis pressure, wherein steam is produced internally in both the ammonia section and the urea section for use in steam users of the plant, wherein a low pressure steam at a pressure of pressure not greater than 6 bar rel, is transferred from the ammonia section to the urea section, for use in at least one steam user in the urea section.

38. The integrated ammonia-urea process according to claim 37, wherein: urea is produced with a self-stripping or with an ammonia-stripping process and recovery is performed at two pressure levels, namely at a medium pressure and at a low pressure, said steam transferred from the ammonia section to the urea section is used for carbamate decomposition in the medium-pressure recovery, or urea is produced with a CO2-stripping process and recovery is performed at a single pressure level, namely at a low pressure, said steam transferred from the ammonia section to the urea section is used together with steam produced in the carbamate condenser of the synthesis section, to provide steam for low-pressure steam users of the urea section.

Description

DESCRIPTION OF FIGURES

[0059] FIG. 1 is a block scheme of a ammonia-urea plant of the prior art.

[0060] FIG. 2 is a scheme of the plant of FIG. 1, modified in accordance with an embodiment of the invention.

[0061] FIG. 3 is a block scheme of a self-stripping urea section of the plant, according to a prior art.

[0062] FIG. 4 illustrates a scheme which is the result of a modification of the scheme of FIG. 3, after a revamping in accordance with an embodiment.

[0063] FIG. 5 illustrates a scheme a modification of the scheme of FIG. 3, after a revamping in accordance with another embodiment.

[0064] FIG. 6 illustrates an application of the invention to a plant where urea is produced with the CO.sub.2 stripping process.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0065] FIG. 1 illustrates a block scheme of a ammonia-urea integrated plant including an ammonia section A and a urea section U. The ammonia section A and urea section U are made according to known art and need not be described in detail.

[0066] The ammonia section A includes the following blocks: a reforming section REF, a shift conversion section SH, a syngas purification section PUR, an ammonia synthesis SYN. The blocks REF, SH and PUR are part of a front-end for the generation of ammonia makeup gas.

[0067] The urea section U includes the blocks of urea synthesis US and urea finishing UF.

[0068] The reforming section REF receives a first input of natural gas 1 as process gas (i.e. natural gas to be reformed) and a second input of natural gas 2 for use as fuel.

[0069] The syngas produced in the block REF is processed through blocks SH and PUR and finally reacted in the block SYN to produce ammonia 3.

[0070] Said ammonia 3 is reacted in the urea synthesis US together with carbon dioxide 4 removed from the syngas in the block PUR. An input of fresh CO2 may also be provided. The block US produces a urea solution or a urea melt which is processed in the block UF to produce solid urea 5. The solid urea may be obtained by prilling or granulation.

[0071] The block 10 denotes the steam network of the ammonia section A. Similarly the block 20 denotes the steam network of the urea section U. The blocks 11, 12 denote the power users and thermal users of the ammonia section A; the blocks 21, 22 denote the power users and thermal users of the urea section U.

[0072] The line 6 denotes high grade heat (heat at elevated temperature) which is transferred from the ammonia process to the steam network (e.g. heat removed from hot effluent of steam reforming, etc.).

[0073] The ammonia steam network 10 provides steam to the power users 11 (e.g. the syngas compressor) and thermal users 12 (e.g. preheaters, CO2 regeneration). Similarly, the urea steam network 20 provides steam to respective power and thermal users 21, 22.

[0074] A medium-pressure steam is typically exported via line 30 from the ammonia steam network 10 to the urea steam network 20, due to excess production of steam in the ammonia section. This steam may be integrated with steam 31 produced in auxiliary boilers AUX to form the steam in line 32 which is actually imported by the urea steam network 20. The steam imported with line 32 satisfies the need of the urea section U for high grade steam, primarily to operate the CO2 compressor and for the high-pressure stripper.

[0075] The boilers AUX require an additional amount of fuel 7. Then the fuel input of the process includes the fuel streams 2 and 7.

[0076] FIG. 2 illustrates, conceptually, the modification of the invention. A low-pressure steam 40 (low-grade heat) is exported from the ammonia steam network 10 to the urea steam network 20. This additional steam 40 is made available thanks to a modernization of the ammonia section A. The urea section U is modified to use some of the heat internally recovered for producing an amount of medium-pressure steam instead of low-pressure. The corresponding deficiency in low-pressure steam in the urea steam network 20 is compensated by the imported steam 40. As a result, the overall consumption (fuels 2 and 7) can be reduced.

[0077] FIG. 3 illustrates a scheme of a urea section U according to the self-stripping concept showing the following items: urea reactor R, high-pressure stripper S, carbamate condenser HPC, ammonia pump AP, CO2 compressor COM, medium-pressure recovery section MP-R and low-pressure recovery section LP-R. The block TU of thermal users is illustrated as a separate block; however it should be understood that some of the thermal users of TU may belong to the recovery section MP-R or LP-R.

[0078] The reactor R, stripper S and condenser HPC form a high-pressure synthesis loop. The loop may also include a scrubber for the gas vented from the reactor (not shown). The solution effluent from the stripper S is processed in the recovery section MP-R at medium pressure and then in the section LP-R at low pressure. The effluent of the section LP-R is an aqueous solution of urea consisting mainly of urea and water which may be fed to a finishing section. The unconverted ammonia and carbon dioxide removed in the sections MP-R are returned to the high-pressure synthesis loop normally as a carbamate-containing solution (not shown).

[0079] Only some of the connections and process lines are shown in FIG. 3 for simplicity. The main process lines illustrated are the following. [0080] 100 reactor input [0081] 101 reactor effluent [0082] 102 stripper vapours [0083] 103 solution effluent from the stripper [0084] 104 recycle solution from the HPC [0085] 105 input of CO2

[0086] FIG. 3 also shows some lines of the steam network. The high grade steam 32 imported from the ammonia section and from the auxiliary boilers is used to operate the CO2 compressor COM particularly to power its steam turbine.

[0087] The steam 33 discharged by the turbine is still at medium pressure, for example around 20 bar or higher, and is used partially to heat the stripper S (line 34).

[0088] The medium-pressure recovery section MP-R includes a carbamate decomposer which requires steam at around 6 bar, which is a comparatively low pressure but still higher than the pressure of steam that can be produced in the condenser HPC, which is typically around 3.5 bar.

[0089] The steam for this carbamate decomposer is therefore produced by mixing part of the MP steam from the turbine, at line 35, with LP steam 36 from the condenser HPC. The resulting steam 37 at the desired pressure of around 6 bar is sent to the decomposer CD. The mixing is performed in a steam ejector SE.

[0090] It can be seen that this process involves a let-down of the valuable steam 35 and therefore

[0091] A remaining portion of the LP steam produced in the condenser HPC (line 38) is used to heat other thermal users of the urea section.

[0092] For example in an embodiment, the MP steam in line 35 has a pressure of about 25 bar; the steam in line 36 has a pressure of about 3.5 bar and the steam in line 37 has a pressure of about 5.5 bar.

[0093] FIG. 4 is an example of implementation of the invention. The urea section U is modified by installing a second condenser HPC-2 in the high-pressure synthesis section. The vapours 102 withdrawn from the stripper S are passed first in the new condenser HPC-2 where a first condensation is performed, and then the effluent of the condenser HPC-2 is further condensed in the original condenser HPC.

[0094] The new condenser HPC-2 is also arranged to produce steam at a higher pressure than the original condenser HPC. Particularly, said condenser produces a medium pressure steam 50 which is used to heat the medium-pressure decomposition step in the MP-R section, instead of the stream 37.

[0095] As some of the heat contained in the stripper vapours is used to generate said steam 50, the amount of LP steam in the line 38 is reduced. The input of LP steam for the thermal users is therefore balanced with the imported steam 40.

[0096] The mixing of medium pressure steam and low pressure steam with the ejector SE is removed. Therefore the process is made more efficient from the point of view of the use of energy. The steam 33 from the turbine of the compression section COM can be entirely destined to the stripper S via line 34.

[0097] FIG. 5 illustrates another embodiment where steam 40 imported from the ammonia section is raised to the pressure required by the section MP-R in a steam compressor SC. For example the steam 40 is raised from 3.5 bar to 5.5 bar. Also in this embodiment the inefficient mixing in the ejector SE can be removed. Part of the steam 40 may also be used in the users TU.

[0098] The embodiments of FIG. 4 with double-pressure condensation and of FIG. 5 with compression of steam are usually alternative embodiments, however they may be combined if appropriate.

[0099] FIG. 6 illustrates a scheme of a CO2 stripping urea section. The carbon dioxide delivered by the compression section COM is introduced in the synthesis loop via the stripper S, where it acts as a stripping aid (line 105a). The synthesis loop typically comprises also a high-pressure scrubber SCR where gas 106 vented from the reactor are scrubbed with a recycle solution 107 coming from the low-pressure recovery section LP-R. The fresh ammonia enters the loop via the condenser HPC, together with recycle solution 108 from the scrubber.

[0100] The recovery is performed only at a low pressure in the LP-R section. Therefore the urea section normally has no user of MLP steam like the MP carbamate decomposer previously described.

[0101] In such a case a preferred embodiment of the invention provides that the duty of the stripper S is reduced and shifted to the LP-R section. To some extent, this can be made without affecting the performance of the reactor R. Accordingly the steam 34 required by the stripper S is reduced, which is a saving of energy. The condenser HPC as a consequence has also a reduced duty and can produce less steam 38; the missing steam is compensated by the import 40 from the ammonia section.