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.

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; 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; and treating an air stream in an air separation unit, obtaining said liquid nitrogen-rich stream and an oxygen-rich stream.

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, 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.

4. A process according to claim 1, 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.

5. 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.

6. A process according to claim 5, wherein said liquid fraction containing impurities is further used as a refrigerating medium for the main heat exchanger of the cryogenic section.

7. 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.

8. 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; an air separation unit 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; wherein said air separation unit further delivers a liquid nitrogen-rich stream and a second stream of nitrogen at ambient temperature for HN ratio adjustment, and additionally delivers an oxygen-rich stream which is fed as oxidizer to the reforming section; and wherein the front-end section further comprises equipments for shift, CO.sub.2 removal and methanation.

9. The equipment according to claim 8, further comprising a line 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.

10. An equipment according to claim 9, 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.

11. The equipment according to claim 8, 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.

12. 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 a line for feeding a liquid nitrogen-rich stream produced in said air separation unit to said cryogenic section, for use as refrigerating medium; providing 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; and 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

DESCRIPTION OF THE FIGURES

[0047] FIG. 1 is a simplified block scheme of the front-end of an ammonia plant operating according to the invention.

[0048] FIG. 2 is a more detailed scheme of a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] A preferred embodiment of the cryogenic section 2 and of use of the nitrogen stream 32 is disclosed in FIG. 2.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.