U.S. patent application number 13/393740 was filed with the patent office on 2012-06-28 for production of ammonia make-up syngas with cryogenic purification.
This patent application is currently assigned to AMMONIA CASALE SA. Invention is credited to Ermanno Filippi, Geoffrey Frederick Skinner.
Application Number | 20120161079 13/393740 |
Document ID | / |
Family ID | 41560872 |
Filed Date | 2012-06-28 |
United States Patent
Application |
20120161079 |
Kind Code |
A1 |
Filippi; Ermanno ; et
al. |
June 28, 2012 |
Production of Ammonia Make-Up Syngas with Cryogenic
Purification
Abstract
A process and a related equipment for making ammonia make-up
synthesis gas are disclosed, where: a hydrocarbon feedstock is
reformed obtaining a raw ammonia make-up syngas stream; said raw
syngas is purified in a cryogenic purification section refrigerated
by a nitrogen-rich stream produced in an air separation unit; the
nitrogen-rich stream at output of said cryogenic section is further
used for adjusting the hydrogen/nitrogen ratio of the purified
make-up syngas; an oxygen-rich stream is also produced in said air
separation unit and is fed to the reforming section.
Inventors: |
Filippi; Ermanno;
(Castagnola, CH) ; Skinner; Geoffrey Frederick;
(Reading, GB) |
Assignee: |
AMMONIA CASALE SA
Lugano-Besso
CH
|
Family ID: |
41560872 |
Appl. No.: |
13/393740 |
Filed: |
August 25, 2010 |
PCT Filed: |
August 25, 2010 |
PCT NO: |
PCT/EP2010/062417 |
371 Date: |
March 1, 2012 |
Current U.S.
Class: |
252/374 ;
29/401.1; 422/187 |
Current CPC
Class: |
C01B 2203/0475 20130101;
C01B 2203/047 20130101; F25J 2210/20 20130101; F25J 2290/80
20130101; F25J 3/0219 20130101; C01B 3/506 20130101; C01B 3/48
20130101; C01B 3/586 20130101; C01B 2203/0233 20130101; F25J
2210/42 20130101; C01B 3/382 20130101; Y02C 20/40 20200801; C01B
3/34 20130101; F25J 2200/74 20130101; C01B 2203/068 20130101; F25J
3/04587 20130101; Y10T 29/49716 20150115; C01B 2203/0216 20130101;
C01B 2203/0445 20130101; Y02C 10/12 20130101; C01B 2203/1241
20130101; C01B 3/025 20130101; C01B 2203/0283 20130101; C01B
2203/046 20130101; F25J 3/0209 20130101; F25J 2270/904 20130101;
F25J 3/0276 20130101; F25J 3/04539 20130101; F25J 2200/02 20130101;
C01B 3/38 20130101 |
Class at
Publication: |
252/374 ;
422/187; 29/401.1 |
International
Class: |
C01B 3/02 20060101
C01B003/02; B23P 11/00 20060101 B23P011/00; B01J 7/00 20060101
B01J007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2009 |
EP |
09169289.7 |
Claims
1. A process for making ammonia make-up synthesis gas, comprising
the steps of: reforming a hydrocarbon feedstock, followed by steps
of shift, CO.sub.2 removal and methanation, to obtain a raw ammonia
make-up syngas stream comprising hydrogen and nitrogen; treating
said raw syngas in a cryogenic purification section obtaining a
purified syngas stream; feeding a liquid nitrogen-rich stream at a
cryogenic temperature to said cryogenic purification section; and
providing an indirect heat exchange between the syngas and said
liquid nitrogen-rich stream in the cryogenic section, said liquid
nitrogen-rich stream being at least partly evaporated to provide
refrigeration of said cryogenic section.
2. A process according to claim 1, where said liquid nitrogen-rich
stream, after at least a partial evaporation through the cryogenic
section, is recovered at an output of said cryogenic section, and
mixed with the purified syngas to provide at least a portion of the
nitrogen required to adjust the hydrogen/nitrogen ratio of the
ammonia make-up syngas.
3. A process according to claim 1, further comprising the step of
treating an air stream in an air separation unit, obtaining said
liquid nitrogen-rich stream and an oxygen-rich stream.
4. A process according to claim 3, where said air separation unit
provides said liquid nitrogen-rich stream, and a second
nitrogen-rich stream at ambient temperature and in a gaseous state,
and where the amount of nitrogen required to adjust the HN ratio of
the ammonia make-up syngas is provided partly by the evaporated
liquid nitrogen-rich stream recovered at the output of the
cryogenic section and partly by said nitrogen-rich stream at
ambient temperature.
5. A process according to claim 3, where said oxygen-rich stream is
used as further oxidant in the reforming process, by injection of
said oxygen-rich stream into a secondary reformer of the reforming
section.
6. A process according to claim 1, wherein: said raw syngas is
cooled down to a cryogenic temperature in a main heat exchanger of
the cryogenic section, obtaining a cooled raw syngas; said cooled
raw syngas is fed to a contacting device where a liquid fraction
containing impurities is obtained by cryogenic liquefaction and
separated from the syngas; a purified syngas is recovered from said
contacting device and is further cooled and purified in a condenser
which is refrigerated by at least partial evaporation of said
liquid nitrogen-rich stream; a further purified syngas is taken at
the output of said condenser and re-heated in said main heat
exchanger, by heat exchange with the incoming raw syngas and with
evaporated nitrogen stream taken from said condenser.
7. A process according to claim 6, wherein said liquid fraction
containing impurities is further used as a refrigerating medium for
the main heat exchanger of the cryogenic section.
8. A process according to claim 1, wherein said liquid
nitrogen-rich stream and/or a second nitrogen-rich stream at
ambient temperature are substantially pure nitrogen.
9. An equipment for producing ammonia make-up synthesis gas
comprising: a front-end section comprising a reforming section
adapted to reform a hydrocarbon feedstock and to produce a raw
ammonia syngas stream; a cryogenic purification section treating
the raw syngas produced in the front-end; means feeding a liquid
nitrogen-rich stream at a cryogenic temperature to said cryogenic
purification section, for use as a heat exchange medium to
refrigerate said cryogenic purification section; and at least one
indirect heat exchanger between the syngas and said liquid
nitrogen-rich stream in the cryogenic section, said liquid
nitrogen-rich stream being at least partially evaporated in said
heat exchanger(s) to provide refrigeration of said cryogenic
section.
10. The equipment according to claim 9, further comprising means
for recovering the evaporated nitrogen-rich stream at an output of
the cryogenic purification section, and for mixing said
nitrogen-rich stream with purified syngas, to provide at least a
portion of nitrogen required for adjusting the hydrogen/nitrogen
ratio of the ammonia make-up syngas.
11. The equipment according to claim 9, comprising an air
separation unit delivering said liquid nitrogen-rich stream and a
second stream of nitrogen at ambient temperature for HN ratio
adjustment, and additionally delivering an oxygen-rich stream which
is fed as oxidizer to the reforming section.
12. An equipment according to claim 11, the front-end comprising a
primary reformer, a secondary reformer, and equipments for shift,
CO.sub.2 removal and methanation, said oxygen-rich stream being fed
to the secondary reformer of the reforming section.
13. The equipment according to claim 9, the cryogenic section
comprising: a contacting device such as a cryogenic condenser
column; a condenser receiving a partially-purified syngas obtained
in the contacting device, said condenser being refrigerated by the
liquid nitrogen-rich stream; a main heat exchanger where the
incoming raw syngas is cooled by heat exchange with one or more of
the following: the nitrogen stream evaporated in said condenser,
the purified syngas, a bottom effluent of said contacting
device.
14. A method for revamping the front-end of an ammonia plant, said
front-end section comprising a reforming section with at least a
primary reformer and a secondary reformer for converting a
hydrocarbon feedstock into ammonia raw make-up syngas, the method
comprising at least the steps of: installing an air separation unit
in parallel to said front-end; providing a cryogenic section for
treatment of the raw syngas, if not present in the original plant;
providing means feeding a liquid nitrogen-rich stream produced in
said air separation unit to said cryogenic section, for use as
refrigerating medium; providing a line feeding oxygen-rich stream
produced in said air separation unit to the secondary reformer, in
order to increase the capability of said reforming section.
Description
FIELD OF THE INVENTION
[0001] The invention relates to the production of ammonia make-up
syngas with cryogenic purification. More in detail, the invention
relates to production of a raw ammonia make-up syngas by steam
reforming of a hydrocarbon feedstock, such as natural gas, and
treatment of the raw syngas by cryogenic purification.
PRIOR ART
[0002] It is known to produce ammonia by reaction of a so called
ammonia make-up synthesis gas (syngas) comprising hydrogen and
nitrogen in a ratio around 3:1, in a suitable high-pressure
synthesis loop.
[0003] The make-up syngas is usually produced by catalytic steam
reforming of a hydrocarbon feedstock, in a front-end section of the
ammonia plant. Conventional equipments of the front-end are a
primary reformer, a secondary reformer, a cooling/shift converter,
a CO.sub.2 separation section, and a methanation section. The
front-end operates at a pressure not greater than 60-80 bar, and
usually in the range of 15 to 35 bar, while the ammonia synthesis
loop operates at a higher pressure, e.g. over 100 bar. Hence,
another component of the front-end is a main syngas compressor,
generally with a multi-stage arrangement, to feed the synthesis
loop.
[0004] Cryogenic treatment of the syngas is also known in the prior
art. U.S. Pat. No. 3,572,046 discloses an apparatus for
purification of the raw syngas where excess nitrogen is removed in
a cryogenic section, and net refrigeration of said cryogenic
section is provided by expansion of the syngas.
[0005] U.S. Pat. No. 5,736,116 discloses a retrofitting method by
installation of an air separation furnishing an oxygen-rich and a
nitrogen-rich stream. The oxygen-rich stream is used to enrich the
air feed of the secondary reformer, and increase the hydrogen
content of the make-up gas substantially above the design
stoichiometry and capacity; the nitrogen-rich stream is supplied to
the synthesis loop to obtain a desired hydrogen to nitrogen ratio
in the syngas feed to the ammonia converters and compensate for the
excess hydrogen in the make-up gas.
[0006] The capability of the front-end section is decisive for the
capability of the overall ammonia plant. There is a continuous
effort to increase the production rate of the ammonia plants, and
hence of their front-end section, in relation to size and cost of
the equipments. These problems are encountered in the realization
of new hydrocarbon steam-reforming based ammonia plants, as well as
in the retrofitting of existing ones.
[0007] Boosting the capacity of the primary reformer in a
substantial manner may be quite expensive. Old tube reformers can
be retrofitted by installing replacement tubes made of a more
resistant material and, hence, having a greater diameter and
smaller thickness (thus providing more passage section) than tubes
of the original design. This is possible, however, only for few
outdated units. Installing additional tubes is possible but subject
to the size of the original reformer; increasing the size of the
reformer is also possible but, of course, is expensive and
time-consuming. Other solutions are to lower the steam/carbon
ratio, which may be effective only in older plants and, in any
case, involves a corresponding revamping of the downstream treating
section, or to install an additional pre-reformer, which however a
relatively low benefit of 10-15% of production rate.
[0008] The volumetric rate through the reformers and the following
equipments, such as shift converters and CO.sub.2-removal units, is
often the limit for the maximum achievable output. Many drawbacks
are connected with a larger flow rate through the front-end, and
can be summarized as: the need to increase the capacity of both the
compressor of the air flow and the compressor of the syngas flow,
and their driving turbines; the more pressure losses in the
front-end; the need to increase the capability of the
CO.sub.2-removal unit. Increasing the volumetric flow rate through
the front-end also involves higher pressure drops and higher duty
of the CO.sub.2-removal section. Generally, the pressure drop can
be reduced only with expensive modifications such as substitution
of some valves, transformation of axial reactors into axial-radial
units, and so on. Also the CO.sub.2-removal section, in general,
requires a substantial revamping (e.g. substitution of one or more
columns, provision of new columns) to obtain a significant increase
in capacity.
[0009] A second problem is to increase the air stream from the air
compressor, to provide more oxygen to the secondary reformer.
Installing new internals of the compressor and possibly of the
driving turbine of the compressor itself is effective, but costly,
as well as the provision of a further compressor in parallel to the
existing one. Installation of a booster, i.e. a pre-compressor
disposed to raise the pressure at the intake of the main air
compressor, is less expensive but also less effective.
[0010] The capacity of the main syngas compressor is also a
critical point. Said compressor is a special and expensive item,
especially designed to operate with the syngas. It is generally
preferred not to install any booster or additional compressor in
parallel to the main compressor, because failure of any additional
equipment may compromise the reliability of the whole plant and may
cause severe damage to the main compressor. A compressor can be
revamped by replacing the internals of the compressor and turbine,
but this modification is quite expensive.
[0011] Summarizing, the boosting of the front-end section of a
steam-reforming ammonia plant is faced with a number of limitations
and constraints from a technical-economical point of view.
[0012] A further technical problem to be considered is the amount
of impurities, such as unconverted methane and carbon oxides, and
inerts such as Argon, which is contained in the syngas fed to the
synthesis loop. The synthesis loop is very sensitive to said
impurities, and so there is the need to achieve the best possible
purification of the syngas.
[0013] The above cited retrofitting method disclosed in U.S. Pat.
No. 5,736,116 gives a partial solution to the above problems,
disclosing enriched air reforming coupled with injection of
nitrogen into the synthesis loop. However, it does not provide a
satisfactory solution to all the above problems, and does not take
into account the impact on the downstream ammonia loop and the
problem of impurities contained in the syngas.
SUMMARY OF THE INVENTION
[0014] The problem underlying the invention is to solve the above
listed limitations in a cost-effective way. This problem is solved
by a process, a plant and a method of revamping according to the
following disclosure.
[0015] A process for making ammonia make-up synthesis gas,
according to the invention, comprises the steps of:
[0016] reforming a hydrocarbon feedstock, followed by steps of
shift, CO.sub.2 removal and methanation, obtaining a raw ammonia
make-up syngas stream comprising hydrogen and nitrogen;
[0017] treating said raw syngas in a cryogenic purification section
obtaining a purified syngas stream;
[0018] feeding a liquid nitrogen-rich stream at a cryogenic
temperature to said cryogenic purification section;
[0019] providing an indirect heat exchange between the syngas and
said liquid nitrogen-rich stream in the cryogenic section, said
liquid nitrogen-rich stream being at least partly evaporated to
provide refrigeration of said cryogenic section.
[0020] The liquid nitrogen-rich stream is preferably a
substantially pure nitrogen in a liquid state, having a temperature
preferably between 185.degree. C. and 190.degree. C. below zero
(around 88-93 K). Preferably said liquid nitrogen-rich stream is at
least partly evaporated to refrigerate said cryogenic section.
[0021] Said nitrogen-rich stream is preferably recovered at output
of said cryogenic purification section, after evaporation and
heating through the cryogenic section itself, and is mixed with the
purified syngas to provide at least a portion of the nitrogen
required to adjust the hydrogen/nitrogen ratio of the ammonia
make-up syngas.
[0022] The liquid nitrogen-rich stream is preferably obtained from
an air separation unit. In a preferred embodiment of the process,
the nitrogen-rich stream and additionally an oxygen-rich stream are
produced in an air separation unit, and said oxygen-rich stream is
used as oxidant in the reforming section, preferably by injecting
said oxygen-rich stream in a secondary reformer of said reforming
section, to increase the production of the make-up syngas.
[0023] More preferably, said air separation unit delivers the
liquid nitrogen at cryogenic temperature, and additionally a second
stream of nitrogen at ambient temperature. The amount of nitrogen
required to adjust the HN ratio of the ammonia make-up syngas is
provided partly by the evaporated liquid nitrogen-rich stream
recovered at the output of the cryogenic section, and partly by
said nitrogen-rich stream at ambient temperature.
[0024] The above embodiment is preferred for the following reasons.
The amount of nitrogen that is necessary to adjust the HN ratio is
usually greater than the amount of liquid nitrogen that needs to be
evaporated to refrigerate the cryogenic section. The higher is the
fraction of liquid nitrogen, the higher is the energy consumption
of the air separation unit. Then, in order to save energy, it is
preferred that only the minimum amount of nitrogen necessary for
the cryogenic process is supplied in liquid form, the remaining
nitrogen being delivered at ambient temperature.
[0025] Further preferred aspects of the process are as follows. The
raw syngas is cooled down to a cryogenic temperature in a main heat
exchanger of the cryogenic section, recovering frigories form the
cold, purified syngas and from the at least partly evaporated
nitrogen-rich stream. A cooled raw syngas is obtained, which is fed
to a contacting device for separation of impurities by cryogenic
liquefaction. A partially purified syngas is recovered from said
contacting device and is further cooled and purified in a
condenser, which is refrigerated by said nitrogen-rich stream; a
further purified syngas and a condensed fraction are taken at the
output of said condenser; the syngas is then re-heated in said main
heat exchanger, by heat exchange with the incoming raw syngas and
with the nitrogen stream from said condenser.
[0026] Preferably the contacting device is a cryogenic column. The
condenser can be a part of the column or a separate item,
preferably over the column. Refrigeration of said condenser is
given by total or partial evaporation of the liquid nitrogen-rich
stream.
[0027] More in detail, and in a preferred embodiment, the syngas is
treated in a column for cryogenic liquefaction, which is part of
the cryogenic section, and purified syngas recovered at top of said
column is further cooled in a condenser which is refrigerated by
partial or total evaporation of the liquid nitrogen-rich stream. A
fraction containing methane and others impurities is liquefied in
said condenser, and sent back to the column; the further purified
syngas is taken at output of the condenser and re-heated in the
main heat exchanger, cooling the incoming raw syngas. The nitrogen
stream at the output of the condenser and/or a liquid stream
containing methane, nitrogen and impurities, recovered at the
bottom of the column, may also be used as further heat-exchange
media, e.g. fed to the same main heat exchanger to refrigerate the
incoming raw syngas stream.
[0028] The nitrogen required for adjusting the H/N ratio of the
ammonia make-up syngas, i.e. the liquid nitrogen evaporated in the
cryogenic section and/or the second nitrogen stream delivered by
the ASU at ambient temperature, can be mixed with the purified
syngas upstream the main syngas compressor feeding the downstream
ammonia synthesis loop, or downstream said main syngas compressor,
providing separate compression of the nitrogen. Both embodiments
are possible, the separate compression of N2 being however
preferred. In this way, a pure make-up syngas substantially
consisting of nitrogen and hydrogen in the suitable 3:1 ratio, with
very low impurities, is obtained.
[0029] The hydrocarbon feedstock is preferably natural gas or
substitute natural gas (SNG), but any suitable reformable
hydrocarbon may be used.
[0030] An aspect of the invention is also a process for producing
ammonia, where a make-up syngas is obtained with the above process
and reacted in a per se known ammonia synthesis loop. Hence, in
accordance with the invention, a plant for the synthesis of ammonia
make-up synthesis gas comprises at least:
[0031] a front-end section comprising a reforming section adapted
to reform a hydrocarbon feedstock and to produce a raw ammonia
syngas stream;
[0032] a cryogenic purification section treating the raw syngas
produced in the front-end;
[0033] means feeding a liquid nitrogen-rich stream at a cryogenic
temperature to said cryogenic purification section, for use as a
heat exchange medium to refrigerate said cryogenic purification
section.
[0034] At least one indirect heat exchanger between the syngas and
said liquid nitrogen-rich stream in the cryogenic section, said
liquid nitrogen-rich stream being at least partially evaporated in
said heat exchanger(s) to provide refrigeration of said cryogenic
section.
[0035] According to a preferred aspect of the invention, said means
for feeding the nitrogen-rich stream to the cryogenic section
comprise at least an air separation unit, also referred to as ASU.
The air separation unit delivers the nitrogen-rich stream and
additionally delivers an oxygen-rich stream which is preferably
used as oxidizer in the reforming section. The ASU may further
deliver a nitrogen-rich stream at ambient temperature, for HN ratio
adjustment, with the above discussed advantages in terms of energy
savings. The ASU can use a conventional process such as cryogenic
distillation.
[0036] In a preferred embodiment, the front-end comprises a primary
reformer, a secondary reformer, and equipments for shift, CO.sub.2
removal and methanation. The oxygen-rich stream delivered by the
air separation unit is preferably fed to the secondary reformer of
the reforming section.
[0037] According to a preferred arrangement of the cryogenic
section, said cryogenic section comprises at least a contacting
device such as a cryogenic condenser; a condenser receiving a
partially-purified syngas obtained in the contacting device, and
refrigerated by the nitrogen-rich stream; a main heat exchanger
where the incoming raw syngas is cooled by heat exchange with one
or more of the following available streams: the nitrogen stream,
the purified syngas and possibly a liquid fraction separated in the
contacting device.
[0038] The invention is also applicable to retro-fitting of an
existing ammonia plant or of the front-end thereof.
[0039] In particular, the invention provides a method for revamping
the front-end of an ammonia plant, said front-end section
comprising at least a primary reformer and a secondary reformer for
converting a hydrocarbon feedstock into ammonia raw make-up syngas,
and a cryogenic section for treatment of the raw syngas, the method
comprising at least the steps of: installing an air separation unit
in parallel to said front-end; providing means for feeding a
nitrogen-rich stream produced in said air separation unit to said
cryogenic section, for use as refrigerating medium; providing a new
line feeding oxygen-rich stream produced in said air separation
unit to the secondary reformer, in order to increase the capability
of said reforming section. If not present in the original plant, a
new cryogenic section may also be provided in the revamping.
[0040] The use of nitrogen-rich stream as a cooling medium for the
cryogenic section has been found an effective measure to increase
the capability of the plant and improve the overall efficiency of
the process. A first advantage is that the invention makes use of
nitrogen-rich stream as a cooling medium to provide the net
refrigeration to the cryogenic section, instead of energy-consuming
expansion of the raw syngas, as suggested in the prior art.
Expanding at least a portion of the raw syngas, however, is not
excluded by the invention and can be adopted--if appropriate--as a
further means to refrigerate the cryo section. In such a case, the
refrigerated syngas, or at least a part thereof, is expanded in a
suitable expander or turbine.
[0041] A further advantage is that the nitrogen-rich stream is used
in a highly efficient way, i.e. first as a refrigerating medium for
the cryo section, and then for H/N ratio adjustment of the purified
syngas, avoiding the feed of a substantial amount of inert nitrogen
through the purification equipments downstream the reformers.
Hence, a significant advantage is obtained without the drawback of
a substantial increase of the volumetric flow rate processed in the
reformers, shift converter(s) and CO.sub.2-removal equipment.
[0042] The feeding of the reheated nitrogen stream downstream the
front-end, preferably at the intake of the main syngas compressor,
reduces the increase in volumetric flow rate through the whole
front-end and related problems, including pressure drops and duty
of the CO.sub.2-removal and methanation section. In fact, the
front-end receives only the pure oxygen stream, necessary for
boosting the reforming capacity, while the nitrogen stream, which
would pass through the front end substantially as inert gas, is
appropriately fed only to the synthesis loop, where it is required
as one of the reagents to produce ammonia, and in order to
establish the correct HN ratio of the make-up syngas.
[0043] The invention, moreover, is particularly efficient in the
removal of methane, and other impurities from the syngas, thanks to
the treatment in the nitrogen-refrigerated cryo section. Less
inerts means a more efficient conversion of the reagents nitrogen
and hydrogen into ammonia, with consequent reduction in the
recirculation of unreacted syngas and lower energy consumption.
[0044] Integration with an air separation unit is particularly
efficient, making also available an oxygen-rich stream which is
advantageously injected into the secondary reformer, thus boosting
the capability of the front-end section in terms of production of
raw syngas.
[0045] The advantages will be more evident with the following
detailed description of a preferred embodiment.
DESCRIPTION OF THE FIGURES
[0046] FIG. 1 is a simplified block scheme of the front-end of an
ammonia plant operating according to the invention.
[0047] FIG. 2 is a more detailed scheme of a preferred embodiment
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0048] Referring to FIG. 1, the front-end of an ammonia plant
comprises a reforming section 1 where a hydrocarbon feedstock 11
and steam 12 react to a raw syngas stream 13, comprising hydrogen,
nitrogen, plus amounts of CO, CO.sub.2, H.sub.2O, residual methane,
argon and other impurities. The reforming section 1 for example
comprises a primary reformer, a secondary reformer and known
equipments for treating the reformed syngas with the process steps
of shift conversion, CO.sub.2 removal and methanation.
[0049] The raw syngas stream 13 is fed to a cryogenic section 2
where it is subject to cryogenic liquefaction and removal of
impurities, said section 2 delivering a purified syngas 17. This
purified syngas 17 is compressed in a syngas compressor and fed to
an ammonia synthesis loop.
[0050] According to the invention, a liquid nitrogen-rich stream,
such as a substantially pure liquid nitrogen 32, is used as a
cooling medium to provide net refrigeration to said cryogenic
section 2. The liquid nitrogen 32 is at least partly evaporated to
furnish the required frigories to the cryo section 2, and recovered
from the cryogenic section as flow 34 which is used to adjust, at
least partially, the hydrogen/nitrogen ratio of the make-up syngas,
i.e. is mixed with the purified syngas 17 or fed to the ammonia
synthesis loop.
[0051] The nitrogen content of said substantially pure nitrogen
stream 32 is more then 99% molar, preferably produced in an air
separation unit (ASU) 3. The ASU 3 receives an air feed 31 and
provides the liquid nitrogen stream 32 and an oxygen-rich stream
35, which is fed as oxidizer to the secondary reformer of the
section 1. The ASU 3 also delivers a nitrogen stream 32a at ambient
temperature. The nitrogen required to adjust the HN ratio of the
syngas is furnished partly by the stream 34 and partly by said
ambient temperature nitrogen 32a.
[0052] A preferred embodiment of the cryogenic section 2 and of use
of the nitrogen stream 32 is disclosed in FIG. 2.
[0053] The cryo section 2 basically comprises a main indirect heat
exchanger 201, a gas-washing column 202 and a condenser 203. The
raw syngas 13 is cooled to a cryogenic temperature in the main heat
exchanger 201, and cooled raw syngas 14 is fed to the column 202,
where cryogenic separation of methane, nitrogen and other
impurities takes place. The heat exchanger 201 recover frigories
from a purified syngas 16 obtained in the column 202 and previously
cooled in a condenser 203, from a gaseous nitrogen stream 33 and
from a liquid stream 20 separated at bottom of said column 202.
[0054] More in detail, the product gas 15 obtained at top of said
column 202 is further cooled in the condenser 203, which is
refrigerated by the evaporation of the cold, at least partly liquid
nitrogen stream 32, obtaining the purified syngas 16 and removing
further amounts of methane, nitrogen, and other impurities that are
recycled to the column 202 via the liquid recycle stream 18.
[0055] The nitrogen stream 32 at least partly evaporates through
the condenser 203 and exits as stream 33, which is heated through
the main exchanger 201, so cooling the incoming raw syngas 13.
[0056] A liquid stream 19, mainly consisting of methane and
nitrogen, is recovered at bottom of the column 202, expanded and
possibly evaporated in a device 22 such as an expansion valve or a
turbine, obtaining a stream 20. Said stream 20 is also re-heated in
the main exchanger 201, exiting as a stream 21 that can be used as
a fuel. Expansion of stream 19 in a turbine allows to recover some
useful work.
[0057] Hence, the main exchanger 201 is refrigerated by the
nitrogen stream 33, the cold purified syngas 16 and the methane
stream 20, all of which contribute to refrigeration of the incoming
raw syngas 13.
[0058] The reheated and purified syngas 17, exiting the cryo
section 2 around ambient temperature, is sent to a main syngas
compressor 40 and then to the ammonia synthesis loop. The stream 34
of gaseous, re-heated nitrogen is fed to an appropriate nitrogen
compressor 41, and mixed with the compressed purified syngas
together with the ambient-temperature nitrogen 32a delivered by the
unit 3, to adjust the H/N ratio in the ammonia synthesis loop. The
compressed nitrogen 35 is mixed with the output of the syngas
compressor 40 forming a syngas stream 23 with the correct HN ratio
of around 3:1.
[0059] FIG. 2 shows a separate-compression embodiment, where syngas
and nitrogen are compressed separately in the compressors 40 and
41, respectively. In other embodiments of the invention, the
nitrogen can also be mixed with the purified syngas upstream (e.g.
at the intake) the main syngas compressor 40. In this last case,
when revamping an existing plant, an existing syngas compressor may
need to be revamped in order to accommodate the additional
nitrogen.
[0060] One of the aspects of the invention is a method for
revamping the front-end of an existing ammonia plant. A front-end
section comprising at least a primary reformer and a secondary
reformer, and the cryogenic section 2 for treatment of the raw
syngas, is revamped for example by at least the following
operations: installing the air separation unit 3 in parallel to the
front-end; providing means feeding the liquid nitrogen-rich stream
32 produced in said air separation unit 3 to said cryogenic section
2, providing a line feeding the oxygen-rich stream 35 produced in
the same unit 3 to the secondary reformer of the front-end, in
order to increase the capability of the reforming section 1. As
clear to a skilled person, the above are the basic steps and
further equipments such as valves, piping, auxiliaries etc. will be
provided according to the specific needs.
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