Patent Application
20230211312
The present invention relates to the field of ammonia-urea integrated plants. Particularly, the invention relates to revamping of ammonia-urea plants and efficient integration of the steam networks of the ammonia section and urea section.
Urea is produced industrially by reacting ammonia and carbon dioxide. The production of ammonia typically involves the generation of a makeup synthesis gas (syngas) from reforming, purification of the syngas and catalytic conversion of the syngas to produce ammonia. The purification of the syngas normally includes shift conversion of carbon monoxide CO to carbon dioxide CO2 and removal of CO2.
The production of ammonia and urea, therefore, can be advantageously integrated. A urea plant coupled to an ammonia plant may use as source material all or part of the ammonia produced in the ammonia plant, as well as the CO2 removed from the syngas during purification.
The term ammonia-urea plant denotes an installation including an ammonia section where ammonia is produced and a urea section where at least part of the ammonia produced in the ammonia section is used for the synthesis of urea. Preferably, also part or all of the carbon dioxide removed from the syngas in the ammonia section is used for the synthesis of urea. An input of fresh carbon dioxide may also be provided if necessary.
The ammonia section generally includes the front-end for generation of syngas by steam reforming and its purification, and a synthesis section where the syngas is catalytically reacted to produce ammonia. The front-end in turn includes a reforming section and a purification section. The reforming section may have different arrangements, e.g. including a primary reformer and a secondary reformer, an autothermal reformer, and others.
The purification section generally includes one or more shift converters to convert CO into CO2 and a CO2 removal section. The CO2 removal section may use various CO2 separation techniques such as, for example, amine-based absorption or other like temperature-swing adsorption (TSA) or a pressure-swing adsorption (PSA).
The purified syngas shall have a suitable nitrogen to hydrogen ratio for the synthesis of ammonia. If necessary, nitrogen may be added to the syngas in order to reach the target ratio.
The synthesis of ammonia is performed at a high pressure which means that the syngas needs be compressed. The compressor is typically a multi-stage compressor driven by a steam turbine.
The starting material for the steam reforming may be a suitable hydrocarbon, such as natural gas.
The urea section may use any of the known techniques for the synthesis of urea but most preferably, urea is produced with a stripping process. The term stripping process identifies a process wherein the effluent of a urea synthesis reactor is processed in a stripper to remove unconverted ammonia and carbon dioxide.
In accordance with the stripping process, the urea section may include a high-pressure urea synthesis section and a recovery section. The high-pressure synthesis section operates at a pressure over 100 bar, typically 140 to 200 bar. The reagents need be compressed at such high pressure. Particularly the carbon dioxide, be it a fresh stream and/or the CO2 removed from the syngas, needs be compressed to the urea synthesis pressure. Like the ammonia syngas compressor, also the CO2 compressor is a large, multi-stage compressor typically driven by a steam turbine.
The recovery section has basically the function of remove and recover unconverted reagents contained in the urea-containing solution produced in the synthesis section and may operate at one or more lower pressure(s). For example a urea plant may include a low-pressure recovery section or a medium-pressure recovery section followed by a low-pressure recovery section.
The high pressure synthesis section includes at least a urea synthesis reactor, a high-pressure stripper and a high-pressure condenser. Optionally it may include a scrubber for vapours extracted from the reactor.
The urea synthesis rector produces a reactor effluent (urea-containing aqueous solution) which contains a significant amount of unconverted ammonia and carbon dioxide, mostly in the form of ammonium carbamate. In the stripper, the effluent of the reactor is heated and optionally contacted with a stripping gas (e.g. carbon dioxide) to decompose the ammonium carbamate into ammonia and carbon dioxide and then remove the ammonia and carbon dioxide in a gaseous phase. Typically, the stripper is a steam-heated shell-and-tube apparatus.
The stripper vapours, containing the ammonia and CO2 removed from the reactor effluent, are condensed in the high-pressure condenser to obtain a condensate which is returned to the reactor.
The urea-containing solution obtained from the stripper is further processed in the recovery section. The processing in a recovery section includes heating the solution in a carbamate decomposer and condensation of the so obtained vapours, obtaining a carbamate solution which can be returned to the high-pressure section, for example introduced in the high-pressure condenser.
The most common urea stripping processes are: the CO2 stripping process, the self-stripping process and the ammonia stripping process. In the CO2 stripping process (originally developed by Stamicarbon), the stripping of the urea-containing solution is performed with the aid of gaseous CO2 as stripping agent and the subsequent recovery is typically performed in a single recovery section at low pressure. In a self-stripping process the stripping is performed without a stripping agent and recovery is performed in a medium-pressure section followed by a low-pressure section. The ammonia-stripping process is a variant of the self-stripping process where gaseous ammonia is introduced in the stripper to act as stripping agent and recovery is still performed at medium pressure and low pressure.
Both the ammonia section and the urea section include a steam network. The steam network denotes the assembly of various steam producers and steam users and the headers that connect them.
A steam producer is typically a piece of equipment where heat is removed from a process stream. Examples are a gas cooler, e.g. to cool the hot gas form reforming before the shift conversion, or a condenser (e.g. the high-pressure condenser of the urea section) where condensation heat is transferred to cooling water and steam is generated.
A steam user may be a steam expander, such as the steam turbines of the syngas compressor and CO2 compressor, or a piece of equipment which requires heat. For example the CO2 removal section of the ammonia front-end may require heat for regeneration of a CO2 absorbing solution. Other examples of steam users include e.g. the high-pressure stripper and the carbamate decomposers in the recovery section. Steam users can be classified as power users (PUs) when the steam is used to generate power (e.g. turbines) and thermal users (TUs) where steam is used to provide a desired heat input to the process (e.g. a stripper or decomposer).
A steam producer may generate steam at a high pressure or low pressure depending on the temperature of the heat source, e.g. of the process stream from which heat is removed. Generally, the amount of HP steam that can be produced by heat recovery is limited, and HP steam may be produced by auxiliary boilers when needed.
In the prior art of ammonia-urea plants, each of the ammonia section and urea section has its own steam network. Some high-pressure steam normally produced in an auxiliary boiler, is typically exported from the steam network of the ammonia section to the steam network of the urea section, to power the steam turbine of the CO2 compressor. A medium-pressure (about 23 bar) steam extracted from said turbine is typically used to heat the high-pressure stripper (i.e. to provide heat for decomposition of carbamate). Low pressure steam for other thermal users of the urea section is produced by the carbamate condenser of the synthesis loop, typically at a pressure of around 3.5 bar.
The medium-pressure recovery section, when provided, includes typically a carbamate decomposer which requires steam at a pressure higher than the pressure of the steam produced in the carbamate condenser, for example around 5.5 bar. There is no steam producer in the urea section which is able to produce steam directly at such pressure, therefore this steam is normally produce by mixing a portion of the medium-pressure steam taken from the above mentioned turbine of the CO2 compressor with a portion of the low-pressure steam produced in the carbamate condenser in an ejector. However ejectors are very inefficient as known in the art.
The ammonia section may also produce an excess of low-pressure steam, possibly as a consequence of a revamping. For example, an excess of LP steam may be the result of installation of a backpressure turbine to drive auxiliary equipment or of modernization of existing thermal users, for example modernization of a CO2 recovery section to reduce its thermal consumption.
In such a case, when an additional LP steam is available, the prior art suggests its exploitation in the ammonia section by installing suitable users like vapour absorption cooling systems or chemical heat pumps, which are however expensive and inefficient.
Also, the above mentioned mixing of medium-pressure steam with LP steam is inefficient from the energetic point of view. An inefficient recovery of thermal energy increases the fuel consumption of the plant. The fuel may include for example fuel of a fired furnace used for primary reforming and fuel of auxiliary boilers for the production of steam.
There is therefore the need to provide a more efficient use of steam and integration between the steam networks in the ammonia-urea plants, particularly when a plant is revamped and more LP steam is available in the ammonia section as a result of the revamping.
An aspect of the invention is a method of revamping an ammonia-urea plant according to claim 1.
The revamping includes the provision of an export of low pressure steam from the ammonia section to the urea section, for use in at least one steam user in the urea section. Said low pressure stream which is transferred (exported) from the ammonia section to the urea section has a pressure not greater than 6 bar preferably 2 to 6 bar. Even more preferably said steam is at a pressure of 3 bar to 4 bar. This is the pressure at the thermal user in the urea plant.
This extra amount of low pressure steam for the urea section can be exploited in different ways in accordance with the features of the urea section and kind of urea synthesis process. The urea section may be modified in view of the extra amount of LP steam coming from the ammonia section.
Preferably the urea section is modified to accommodate said low pressure steam exported from the ammonia section to the urea section. The consumption of medium-pressure steam may be consequently reduced.
In some embodiments the urea section may be modified to produce internally steam at a pressure greater than the pressure of the steam imported from the ammonia section, for a particular steam user like for example a carbamate decomposer of a medium-pressure recovery section. The imported steam is then used to balance the request of low-pressure steam. Said internal production of steam may replace an inefficient mixing of MP steam and LP steam, to the advantage of the overall fuel consumption.
In an embodiment, the method includes that the condensation stage of the urea section is modified for direct production of steam for use in a MP carbamate decomposition in a medium-pressure recovery section. Some of the heat that can be recovered from the stripper vapours is used, in the modified condensation stage, to produce said steam for the MP carbamate decomposition. As a consequence of this, the urea section will produce less LP steam and the need of LP steam in the urea section is then balanced by the LP steam imported from the ammonia section.
In a variant of this embodiment, some steam produced in the carbamate condenser of the synthesis section may be compressed to a higher pressure required by a steam user.
In another embodiment, a CO2 stripping urea section can be modified by reducing the heat transferred to the high-pressure stripper. In terms of the urea process, this means that part of the duty of the stripper is shifted to the low-pressure recovery section and the stripper consumes less steam for its heating. As the stripper requires a valuable steam at medium pressure, this is a considerable advantage from the energetic point of view. The reduced duty of the stripper causes that less LP steam can be produced in the carbamate condenser of the synthesis section, however the missing LP steam is provided by the LP steam import from the ammonia section.
Therefore the main advantage of the invention is a better recovery and use of thermal energy within the ammonia-urea process, which ultimately results in reducing the fuel consumption of the overall process. For example the invention may reduce the need of MP steam extraction from the turbine of the CO2 compressor. A reduced MP turbine extraction results in a reduced input of HP steam of the turbine which is produced at the expense of fuel, and therefore is a fuel saving.
Some preferred embodiments of the invention are given in the dependent claims. Another aspect of the invention is a process according to the claims.
The term of low pressure (LP), when referring to steam, denotes a pressure of less than 6 bar and preferably 2 to 6 bar. The term medium-low pressure (MLP) may be used to denote a pressure in the upper range or slightly above the low-pressure, typically of 4 to 10 bar. The term medium pressure (MP) typically denotes a pressure of 15 to 40 bar. The term high pressure (HP) typically denotes a pressure well above 40 bar, for example above 90 bar.
It should be noted that the terms of low-pressure, medium-pressure and high-pressure may also refer to the pressure in the process side. For example the high-pressure carbamate condenser of the urea synthesis section has a high pressure (generally greater than 100 bar) in the process side but the condensation heat removed from the stripper vapours has a low temperature and therefore the condenser produces steam at low pressure, typically of 3.5 bar. Also the terms of low-pressure recovery section and medium-pressure recovery section refer to the pressure in the process side.
In absence of a different indication all pressures are relative to the atmospheric pressure (bar rel). The atmospheric pressure is understood as 101325 Pa.
The urea section may include 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. This is typically the case of a urea section including a medium pressure recovery section, where decomposition of carbamate at medium pressure requires steam at around 6 bar while the imported steam is at about 3.5 bar.
In such a case the imported steam cannot be used as such in this steam user. The method may include 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 the low-pressure steam transferred from the ammonia section to the urea section.
The steam at said second pressure may be produced by 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. This can be made, in a preferred embodiment, with installation of a second condenser. Said second condenser may be a heat exchanger fitted in the same pressure vessel of an existing condenser or can have a separate pressure vessel.
Said second condenser may be 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.
Preferably, the first condenser and/or the second condenser are configured as shell-and-tube equipment. More preferably they are horizontally arranged vessels of the kettle type, where the stripper vapours flow in the tube side and cooling water is evaporated in the shell side.
A step of modifying the urea section to produce steam at said second pressure may also include installation of a steam compressor which is fed with steam at low pressure and delivers steam at said second pressure.
In the ammonia-urea plant, the steam at said second pressure for said user may be originally obtained by mixing medium-pressure steam with low-pressure steam produced in the condensation stage. In such a case the method of the invention preferably includes that said mixing is discontinued.
The above embodiments including internal production in the urea section of steam at said second pressure (e.g. MLP steam around 6 bar) are typically applicable to a plant including a self-stripping urea section (e.g. Snamprogetti urea section) because this kind of urea process has a peculiar steam user requiring steam at about 6 bar, namely the carbamate decomposer of the medium-pressure recovery section.
The method invention is also applicable to plants having a urea section where urea is produced according to the CO2 stripping process. In such a case, to make use of the extra amount of LP steam from the ammonia section, the method may include 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. The missing production of low pressure steam in the urea section is then balanced with the low-pressure steam imported from the ammonia section.
The method of the invention may include that the ammonia section is modified to provide the low pressure steam which is required for balancing the low pressure steam input of the urea section. It can be seen that the modification of the ammonia section and of the urea section are synergetic. The modernization of the ammonia section may provide an extra amount of low-pressure steam, which may be caused by increased thermal efficiency of existing components and/or the provision of additional means for heat recovery. The modification of the urea section, at the same time provides that the urea section can import this extra amount of low-pressure steam, because the internal production of LP steam in the urea section is lowered to the advantage of a more valuable MP steam. From energetic and thermodynamic perspective, the plant is made more efficient.
In a preferred embodiment, 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. As stated above, the CO2 recovery section is a large user of low pressure steam, typically for regeneration of a CO2 solvent in an amine-based process or hot potassium carbamate process.
The ammonia-urea plant to be modified may originally include mixing of medium-pressure steam with low-pressure steam produced in the condensation stage and using the so obtained steam at a medium-low pressure to provide heat to a carbamate decomposer. In such a case, the method of the invention may include that said mixing is discontinued and said decomposer is heated with the MP stream produced in the modified condensation stage, for example in newly installed second condenser of the stripper vapours in the high-pressure synthesis section.
According to another preferred embodiment, the method includes that the steam to carbon ratio in the reforming process is lowered. This reduction of the steam to carbon ratio may be advantageous for the reforming process but reduces the amount of high pressure steam that can be produced by heat recovery. In this case, the overall reduction of high pressure steam consumption in the ammonia urea plant given by the invention compensates for this drawback of a lower heat recovery downstream the syngas generation units (e.g. primary and secondary reformers) due to a reduction of the steam to carbon ratio in the ammonia plant therefore enabling to maximise the benefits of lowered S/C in the plant.
More preferably, the steam to carbon ratio in the reforming process is lowered to a value in the range 2.6 to 2.0. If necessary the shift catalyst may be replace with a shift catalyst which is selected accordingly to sustain the modified low steam to carbon ratio. The reduction of the S/C in the ammonia plant leads to a trade-off between fuel consumption reduction, inert formation and the possibility to recover heat downstream the syngas generation units to operate the CO2 recovery section.
In a preferred embodiment, a modernization of the CO2 recovery section is performed, which reduces the regeneration heat required by the CO2 solvent through the installation of a new flash regeneration column. Said modernization, in synergy with the lower S/C, may reduce the specific regeneration heat required by CO2 recovery. In a preferred embodiment said specific regeneration heat is reduced down to less than 400 kcal/Nm3 of removed CO2.
A low S/C ratio may reduce the heat that can be recovered from the process gas in the ammonia section downstream the syngas generation units. This can be compensated, according to an embodiment of the invention, by a retrofit of large and inefficient consumers of steam, particularly of consumers of high-pressure steam. Said retrofit may include replacing a steam turbine drive with an electric motor drive. For example at least one of the boiler feed water pumps of the ammonia plant steam network is changed from a turbine driver to motor drive.
In a highly preferred embodiment, the ammonia section is revamped to operate the reforming process with a S/C ratio in the range of 2.6 to 2.0; the shift section is modified to operate with said reduced S/C ratio; at least one steam turbine driver is changed to a motor drive resulting in reduced steam consumption.
The above described embodiments can effectively increase the energy efficiency of the ammonia-urea plant, particularly with the adoption of a reduced S/C ratio in conjunction with the overall reduction of high-pressure steam consumption.
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.
The urea section U includes the blocks of urea synthesis US and urea finishing UF.
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.
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.
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.
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.
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.).
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.
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.
The boilers AUX require an additional amount of fuel 7. Then the fuel input of the process includes the fuel streams 2 and 7.
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).
Only some of the connections and process lines are shown in
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).
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.
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.
It can be seen that this process involves a let-down of the valuable steam 35 and therefore
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.
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.
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.
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.
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.
The embodiments of
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20170057.2 | Apr 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/059098 | 4/7/2021 | WO |
