U.S. patent application number 14/074962 was filed with the patent office on 2015-05-14 for process for producing ammonia synthesis gas and a method for revamping a front-end of an ammonia plant.
This patent application is currently assigned to Ammonia Casale SA. The applicant listed for this patent is Ammonia Casale SA. Invention is credited to Ermanno Filippi, Raffaele Ostuni.
Application Number | 20150129806 14/074962 |
Document ID | / |
Family ID | 51691049 |
Filed Date | 2015-05-14 |
United States Patent
Application |
20150129806 |
Kind Code |
A1 |
Filippi; Ermanno ; et
al. |
May 14, 2015 |
Process for Producing Ammonia Synthesis Gas and a Method for
Revamping a Front-End of an Ammonia Plant
Abstract
A process for producing ammonia make-up synthesis gas and a
procedure for revamping a front-end of an ammonia plant for
producing ammonia make-up synthesis gas are disclosed, wherein the
make-up synthesis gas is produced by means of steam reforming of a
hydrocarbon gaseous feedstock; said front-end includes a primary
reformer, a secondary reformer, a shift conversion section, a CO2
removal section and optionally a methanation section; a
shell-and-tube gas-heated reformer is installed after said
secondary reformer, and a portion of the available feedstock is
reformed in the tubes of said gas-heated reformer, and heat is
provided to the shell side of said gas-heated reformer by at least
a portion of product gas leaving the secondary reformer, possibly
mixed with product gas leaving the tubes of said gas-heated
reformer.
Inventors: |
Filippi; Ermanno;
(Castagnola, CH) ; Ostuni; Raffaele; (Milano,
IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ammonia Casale SA |
Lugano-Besso |
|
CH |
|
|
Assignee: |
Ammonia Casale SA
Lugano-Besso
CH
|
Family ID: |
51691049 |
Appl. No.: |
14/074962 |
Filed: |
November 8, 2013 |
Current U.S.
Class: |
252/374 ;
29/401.1; 422/148 |
Current CPC
Class: |
C01B 3/26 20130101; C01B
2203/0205 20130101; B01J 7/00 20130101; C01B 2203/0475 20130101;
C01B 3/025 20130101; C01B 2203/0233 20130101; C01B 2203/0445
20130101; C01B 13/0229 20130101; C01B 3/382 20130101; C01B 2203/043
20130101; B01J 2219/00024 20130101; Y02P 20/52 20151101; C01B
2203/0244 20130101; C01B 2203/0283 20130101; C01B 2203/1235
20130101; C01B 2203/142 20130101; C01B 2210/0046 20130101; Y10T
29/49716 20150115; C01B 2203/068 20130101; C01B 3/50 20130101; C01C
1/0405 20130101; C01B 2203/0844 20130101; C01B 2203/141 20130101;
C01B 3/48 20130101 |
Class at
Publication: |
252/374 ;
422/148; 29/401.1 |
International
Class: |
C01B 3/26 20060101
C01B003/26 |
Claims
1. A procedure for revamping a front-end of an ammonia plant, said
front-end being arranged to produce ammonia synthesis gas
containing hydrogen and nitrogen by steam reforming of a
hydrocarbon gaseous feedstock, said front-end including a primary
reformer, a secondary reformer or an autothermal reformer, a shift
conversion section, a CO2 removal section and optionally a
methanation section, said procedure including at least:
installation of a gas-heated reformer after said secondary
reformer, said gas-heated reformer being a shell-and-tube heat
exchanger having a tube side and a shell side, and providing a
catalytic reforming of a first gas current passing in the tube side
and indirect heating of said first gas current by a second current
traversing the shell side, said first current including a portion
of the available hydrocarbon feedstock, the remaining portion of
said feedstock being directed to said primary reformer, and said
second current comprising at least a portion of a product gas
effluent from said secondary reformer or autothermal reformer.
2. The procedure of claim 1, wherein said first current is a
portion of a mixed flow comprising steam and the available
hydrocarbon feedstock, which is redirected to said gas-heated
reactor while the remaining portion is directed to said primary
reformer.
3. The procedure of claim 2, said mixed flow having a
steam-to-carbon ratio of between 2 and 3.5.
4. The procedure of claim 1, said second current comprising product
gas effluent from said secondary reformer or autothermal reformer,
and also comprising product gas leaving said tube side of said
gas-heated reformer.
5. The procedure of claim 1, said secondary reformer being an
air-fired secondary reformer, and the procedure including the step
of modifying said secondary reformer to operate with O2-enriched
air.
6. The procedure of claim 5, said O2-enriched air being obtained by
adding an oxygen flow to ambient air.
7. The procedure of claim 6, said oxygen flow being in an amount to
provide a molar concentration of oxygen in the enriched air between
25% and 70%.
8. The procedure of claim 6, said oxygen flow being delivered by an
air-separation unit.
9. The procedure of claim 8, further comprising the provision and
the installation of said air-separation unit.
10. The procedure of claim 1, further including the revamping of
said shift conversion section and/or the revamping of said CO2
removal section.
11. The procedure of claim 10, including the revamping of said
shift conversion section by means of one or more of the following:
the conversion of one or more existing axial-flow shift converters
into axial-radial shift converters; adding one or more shift
converters in parallel to the existing ones; replacing one or more
existing adiabatic high-temperature shift converters with one or
more isothermal medium-temperature shift converters.
12. The procedure of claim 11, including the provision of one or
more isothermal medium-temperature shift converters or the
modification of one or more existing shift converters to operate as
medium shift converters, wherein said medium-temperature shift
converters include a copper-based catalyst, and comprise a heat
exchanger immersed in the catalyst, to remove the heat produced by
the exothermic shift conversion.
13. The procedure of claim 12, said medium temperature being in the
range of 200-300.degree. C.
14. The procedure of claim 1, comprising the installation of a
purification section after said CO2 removal section, for removal of
inert gas from CO2-depleted product gas effluent form said CO2
removal section.
15. The procedure of claim 14, said purification section including
a methanation section.
16. The procedure of claim 14, said purification section including
a nitrogen wash section or a cryogenic condensation section for
condensation of nitrogen and inerts, or a PSA unit.
17. The procedure of claim 16, said purification section including
a nitrogen wash section or a cryogenic condensation section, said
procedure including the provision of a nitrogen line for addition
of nitrogen before or into said nitrogen wash section or said
cryogenic condensation section, and said nitrogen being in an
amount suitable to obtain a purified synthesis gas containing
hydrogen and nitrogen in a molar ratio around 3 to 1.
18. A process for producing ammonia synthesis gas containing
hydrogen and nitrogen by steam reforming of a hydrocarbon gaseous
feedstock, including: mixing said hydrocarbon gaseous feedstock
with steam, reforming a first portion of the so obtained mixed flow
of gaseous feedstock and steam in a primary reformer and then in a
secondary reformer or in an autothermal reformer, obtaining a first
product gas, reforming a second portion of said mixed flow in a
gas-heated reactor, obtaining a second product gas, said gas-heated
reactor being heated by a current of product gas comprising at
least a portion said first product gas.
19. The process of claim 18, said secondary reformer or autothermal
reformer operating with O2-enriched air having a concentration of
oxygen between 25% and 70% molar.
20. The process of claim 18, further comprising the treatment of
product gas comprising: shift conversion, removal of carbon
dioxide, and purification of CO2-depleted product gas after said
removal of carbon dioxide, and said purification including at least
one of the following: a methanation process; nitrogen wash;
cryogenic condensation; pressure-swing adsorption (PSA).
21. A plant for producing ammonia synthesis gas containing hydrogen
and nitrogen by reforming of a mixed flow of a gaseous hydrocarbon
gaseous feedstock and steam, said plant including: a train
including a primary reformer and a secondary reformer or an
autothermal reformer, a gas-heated reactor, said gas-heated reactor
being in parallel to said train of primary reformer and secondary
reformer or autothermal reformer, said gas-heated reactor having a
tube side for reforming and a shell side for a gaseous heat source,
said plant comprising feeding lines arranged to feed a first
portion of said mixed gaseous feedstock and steam to said primary
reformer and then to said secondary reformer or autothermal
reformer, and to feed a second portion of said mixed gaseous
feedstock and steam to said gas-heated reactor, and said plant
comprising a line feeding at least a portion of product gas from
said secondary reformer or autothermal reformer to the shell side
of said gas-heated reactor.
Description
FIELD OF THE INVENTION
[0001] The invention relates to reforming of hydrocarbons for the
preparation of a synthesis gas suitable for the production of
ammonia.
BACKGROUND ART
[0002] Ammonia plants include a front-end for the generation of a
synthesis gas, which is then reacted to form ammonia in a synthesis
loop. The synthesis gas is generated in the front-end by steam
reforming of a hydrocarbon feedstock, for example natural gas or a
substitute natural gas (SNG).
[0003] A conventional and well known front-end includes: a primary
reformer, a secondary reformer, a shift reactor, a CO2 removal
section and optionally a methanation section. The purified
synthesis gas leaving the CO2 removal section or methanation
section has a molar ratio between hydrogen H.sub.2 and nitrogen
N.sub.2 of around 3:1 suitable for the synthesis of ammonia. Said
purified gas is then compressed to synthesis pressure and fed to a
synthesis loop.
[0004] The primary reformer converts methane from the hydrocarbon
source and steam into a gas containing carbon monoxide, carbon
dioxide and hydrogen. The secondary reformer provides a further
oxidation of said gas, using air as an oxidant. Shift conversion of
carbon monoxide to carbon dioxide takes place in an adiabatic
high-temperature shift (HTS) reactor operating around
350-500.degree. C. with an iron-based catalyst, and possibly in a
further adiabatic low-temperature shift (LTS) reactor. Carbon
dioxide is removed for example with a CO.sub.2 washing column.
Methanation, when provided, removes the residual carbon monoxide by
conversion to methane.
[0005] In recent years, the need of increase the capacity of
existing ammonia plants emerged. Various techniques have been
proposed to reach this goal. Most of said techniques rely on a
modification of the secondary reformer which is fired with
O2-enriched air or pure oxygen, instead of ambient air. However, a
drawback of this approach is the need of a large and expensive
air-separation unit to produce the required amount of oxygen.
SUMMARY OF THE INVENTION
[0006] The present invention discloses a novel way of revamping an
ammonia plant and increasing its capacity, according to the
attached claims. The invention also relates to a novel process and
plant according to the attached claims.
[0007] The invention provides that a gaseous feedstock is reformed
partly in a train of primary reformer and secondary reformer, or
autothermal reformer, and partly in a gas-heated reactor. The heat
source of said gas-heated reactor comprises at least part of a
product gas effluent from said secondary reformer. Preferably said
heat source comprises product gas from said secondary reformer and
product gas from the gas-heated reactor itself.
[0008] Said gas-heated reactor can be added to an existing
front-end in order to increase its capacity. Accordingly, a
front-end of an ammonia plant including a primary reformer, a
secondary reformer, a shift conversion section, a CO2 removal
section and optionally a methanation section can be revamped with a
procedure including the installation of a gas-heated reformer after
said secondary reformer.
[0009] Said gas-heated reformer is basically a shell-and-tube heat
exchanger having a tube side and a shell side, and providing a
catalytic reforming of a first gas current passing in the tube side
and indirect heating of said first gas current by a second current
traversing the shell side.
[0010] Said first current includes a portion of the available
hydrocarbon gaseous feedstock, the remaining portion of said
feedstock being directed to said primary reformer, and said second
current comprises at least part of the product gas effluent from
said secondary reformer.
[0011] Said first current is preferably a portion of a mixed flow
comprising steam and the hydrocarbon gaseous feedstock. Hence, a
portion of said mixed flow is directed to the gas-heated reactor
and the remaining portion is directed to the primary reformer. Said
mixed flow has preferably a steam-to-carbon ratio of between 2 and
3.5, and more preferably between 2.2 and 3. The steam-to-carbon
ratio of the first current can be different from that of the
remaining portion directed to the primary reformer.
[0012] A pre-reformer can also be provided before the primary
reformer, to use lower S/C ratios.
[0013] Said second current preferably comprises the effluent of
said secondary reformer mixed with the effluent gas leaving the
tube side of said gas-heated reformer. Accordingly, the product gas
collected from the tubes of said gas-heated reformer is joined with
the product gas from the secondary reformer, and the so obtained
hot product gas is introduced in the shell side of the gas-heated
reformer. In some preferred embodiments, the outlet temperature of
the tubes of said gas-heated reformer ranges from 750 to
850.degree. C. and the outlet temperature of said secondary
reformer ranges from 950 to 1050.degree. C.
[0014] Preferably, said procedure includes also the step of an
existing air-fired secondary reformer modified to operate with
O2-enriched air. According to some embodiments, said O2-enriched
air is obtained by adding an oxygen flow to ambient air, and said
oxygen flow is delivered by an air-separation unit. Preferred
oxygen concentration in the enriched air is between 25% and 70%
molar and more preferably between 30% and 50%.
[0015] The procedure may also include the revamping of the existing
shift conversion section and/or the revamping of the existing CO2
removal section.
[0016] Revamping of the shift conversion section may include one or
more of: modification of existing axial-flow shift converters into
axial-radial shift converters; adding one or more shift converters
in parallel to existing ones; replacing one or more existing
adiabatic HTS converters with one or more isothermal MTS converters
or revamping to MTS.
[0017] An isothermal MTS converter is understood as a shift
converter with a copper-based catalyst, for example a Cu--Zn
catalyst, working at a medium temperature and comprising a heat
exchanger immersed in the catalyst, to remove the heat produced by
the exothermic shift conversion. Said medium temperature is for
example in the range of 200-300.degree. C.
[0018] The technique used for the revamping of the existing CO2
removal section is known in itself and may vary depending on the
kind and size of said section, e.g. number and size of columns.
[0019] According to a further preferred aspect of the invention,
the procedure comprises the installation of a purification section
for removal of inert gas. The term inert gas denotes gaseous
components which are considered inert to the synthesis of ammonia,
for example methane and Argon. Said purification section may
include for example a pressure swing adsorption (PSA) unit or a
cryogenic unit.
[0020] Examples of suitable cryogenic units for said purification
and removal of inert gas include nitrogen wash and cryogenic
condensation.
[0021] Nitrogen wash is carried out with a stream of nitrogen
having a suitable high purity, preferably containing not more than
10 ppmv (parts per million in volume) of oxygen. Said nitrogen can
be generated by an air separation unit. For example the ASU
providing the above mentioned oxygen for air enrichment can also
provide this nitrogen stream.
[0022] An example of applicable cryogenic condensation is disclosed
in EP2292554.
[0023] In all the above cases, the amount of nitrogen added to the
synthesis gas is regulated in such a way that the final ratio
between hydrogen and nitrogen is around 3:1 as desired.
[0024] The procedure may include the revamping of other equipment,
e.g. of the main synthesis gas compressor, according to the needs.
In some embodiments, the ammonia synthesis loop is also revamped to
cope with the increased amount of synthesis gas delivered by the
modified front-end.
[0025] Thanks to the addition of the gas-heated reformer, a
considerable increase of capacity is obtained without an expensive
revamping of the primary reformer, and with a relatively small
amount of oxygen for the secondary reformer. Hence, the size and
cost of the air separation unit are less than in prior-art
solutions. The use of O2-enriched air has the advantage that less
nitrogen is introduced with the oxidant and then the ratio of
volumetric flow rate over capacity (amount of synthesis gas
produced) is more favorable.
[0026] This means that not all the nitrogen necessary for the
production of ammonia is introduced in the secondary reformer, as
in the background art. A relevant portion of said nitrogen is
introduced in the final purification step or after said
purification step, depending on the purification technique (e.g.
nitrogen wash or PSA). The advantage of this practice is to reduce
the flow of gas in the front end of the plant, allowing more space
for the capacity increase.
[0027] In some embodiments of the invention, the capacity, in terms
of the amount of synthesis gas that can be produced by the
front-end, is increased by 50% and more. In some cases the capacity
increases by 100% i.e. the capacity of the revamped plant is twice
the original one.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a scheme of a front-end of an ammonia plant
according to the invention.
[0029] FIGS. 2 to 5 illustrate some embodiments of the invention
concerning purification of the raw synthesis gas.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0030] FIG. 1 illustrates a front-end of an ammonia plant including
a tube primary reformer 1, a secondary reformer 2, a shift
conversion section 3, a CO2 removal section 4, and a gas-heated
reactor (GHR) 5 after the secondary reformer 2, and before the
shift conversion section 3.
[0031] Said gas-heated reactor 5 is basically a shell-and-tube
equipment for indirect heat exchange between a first current in the
tube side and a second current in the shell side. Said first
current contains a gaseous hydrocarbon and steam. Said tubes of
reactor 5 are filled or coated with a suitable catalyst for steam
reforming.
[0032] A gaseous hydrocarbon feedstock, for example desulphurized
natural gas 10, is added with steam 11 forming a mixed flow 12. A
first part 13 of said mixed flow 12 is directed to the tubes of the
primary reformer 1, and the remaining part 14 of said mixed flow 12
is fed to the tube side of the gas-heated reactor 5. Further steam
can be added to stream 14, according to some embodiments.
[0033] The first part 13 of mixed steam and methane flow 12 is
reformed in the primary reformer 1, obtaining a partial conversion
of methane contained therein, and the effluent 15 is further
converted in the secondary reformer 2 with oxygen-enriched air 18.
Said oxygen-enriched air 18 is obtained by adding a suitable amount
of oxygen 17 to ambient air 16. The oxygen 17 may be provided for
example by an air-separation unit 25. Preferably said
oxygen-enriched air 18 contains 25% to 50% of oxygen. Said
air-separation unit 25 may also deliver a current of nitrogen of a
high purity for a further use in the process, as illustrated for
example in the FIGS. 2 to 5.
[0034] Referring again to FIG. 1, the second part 14 of said flow
12 is reformed in the tubes of said gas-heated reactor 5. Here, the
heat input to the reforming process is provided by the current 21
of hot product gas which traverses the shell side of the gas-heated
reactor 5. Said current 21 comprises the product gas 19 from the
secondary reformer 2 and also the product gas 20 leaving the
tube-side of gas-heated reactor 5, which is joined with said
product gas 19 as illustrated.
[0035] Hence it can be said that the gas-heated reactor 5 operates
in parallel to the train of primary reformer 1 and secondary
reformer 2. Part of the available mixed flow 12 is converted
through the reformers 1 and 2 to the first product gas 19, while
another part is converted through the gas-heated reactor 5 to the
second product gas 20.
[0036] Preferably the first part 13 is more than 50% of the flow
12. In a preferred embodiment, the first part 13 is around 70% and
the second part 14 is around 30% of the total amount of mixed flow
12. This ratio however may vary.
[0037] After a passage in the shell side of the gas-heated reactor
5, said hot current 21, now cooled to 22, is fed to the shift
conversion section 3.
[0038] The effluent 23 of said shift conversion section 3 is
treated in the CO2-removal section 4.
[0039] The CO2-depleted stream 24 is preferably purified for
example by removing residual methane and other inert gaseous
components (e.g. Argon) before it is fed to an ammonia synthesis
loop.
[0040] FIGS. 2 to 5 illustrate some of the possible embodiments for
the purification of said stream 24.
[0041] According to FIG. 2, the CO2-depleted gas 24 is purified in
a PSA section 26 and in a methanation section 27. Then the purified
gas 28 is compressed in a compression section 29 and sent to a
synthesis loop 30. The PSA may also be installed downstream the
methanation section in a variant embodiment.
[0042] A suitable amount of nitrogen is added via line 31 to the
stream effluent from the PSA section 26. Said nitrogen 31 may come
from the same ASU 25 which generates the oxygen 17 (FIG. 1).
[0043] FIG. 3 illustrates a variant where said CO2-depleted gas 24
is purified in a nitrogen wash section 33, optionally after a
methanation section 27. The necessary nitrogen 34 may be provided
by the ASU 25 as above.
[0044] FIG. 4 illustrates an embodiment where purification of said
CO2-depleted gas 24 includes a cryogenic condensation in a suitable
unit 35, after a first step of purification in a methanation
section 27. A suitable amount of nitrogen 36 is added to the gas
before it enters said cryogenic condensation unit 35.
[0045] FIG. 5 illustrates a variant of FIG. 4 which includes a
first compression section 29A before the cryogenic condensation
unit 35, and a second compression section 29B after said unit 35.
The first compression section 29A provides an initial compression
and the second compression section 29B provides final compression
after the purification in the cryogenic unit 35. The nitrogen 36 is
preferably added to the gas stream after the initial compression
and before it enters the cryogenic condensation unit 35.
[0046] In the above embodiments, the amount of nitrogen via lines
31 or 34 or 36 is regulated in such a way that the purified product
gas 30 contains the desired concentration of nitrogen for ammonia
synthesis.
[0047] Thanks to the reforming in parallel through the reformer 1
and gas-heated reactor 5, a front-end as illustrated in FIG. 1 is
able to convert a greater amount of natural gas 10, i.e. it has a
greater capacity, compared to a conventional front-end.
[0048] According to some embodiments, the gas-heated reactor 5 is
added during a revamping procedure of the front-end originally
comprising the reformers 1, 2 and sections 3, 4. The other
equipment, in particular the shift conversion section 3 and CO2
removal section 4, can also be revamped.
* * * * *