Air separation unit (ASU) and process, ammonia synthesis plant comprising such an ASU, and method of revamping an asu

Abstract

An air separation unit or ASU (1) comprising: - a compression section (2) for compressing ambient air (3) to compressed air (4); - a heat exchanger (5), such as a water-cooled heat exchanger, to cool said compressed air (4) and thereby obtain cooled compressed air (6); - a processing section (7) of said cooled compressed air (6) to obtain processed air (13), said processing section (7) comprising at least one purification unit (8) and at least one cooling unit (9, 9'); and - a separation section (15) of said processed air (13) into an oxygen- or nitrogen-enriched stream (16) and a tail stream (17); wherein said ASU (1) is devoid of an absorption refrigeration unit, and wherein said at least one cooling unit (9, 9') comprises a multistage mechanical vapor-compression cycle (10) using ammonia as working fluid.

Description

[0001] “Air separation unit (ASU) and process, ammonia synthesis plant comprising such an asu, and method of revamping an asu”

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention is in the field of air separation, in particular pertaining to an air separation unit and process that can be implemented in ammonia synthesis.

[0005] Prior art

[0006] An air separation unit (ASU) is an industrial facility used to separate atmospheric air into its main constituent gases, principally nitrogen (N2), oxygen (O2), and eventually other more rare gases.

[0007] Fractional distillation is the most widespread technique for performing such separation: it involves that atmospheric air is compressed between 5 bar gauge (barg) and 10 barg, liquified by refrigeration, and subsequently distilled in a distillation column taking advantage of different boiling points of said constituent gases. More precisely, compressed nitrogen and oxygen have boiling points of minus 196 °C and minus 183 °C, respectively, so that this difference in temperature can be exploited for separating these elements by distillation.

[0008] However, the involved refrigeration requirements are significant due to the very low temperatures needed for liquefaction, and due to the fact that cooling is typically provided by lamination of compressed air at room temperature.

[0009] GB 1 460 993 A, GONG WENTAO ET AL: "Design and Analysis of Novel CO2 Conditioning Process in Ship-Based CCS", ENERGIES, vol. 15, no. 6, page 5928 (XP093117644), WO 2023 / 285001 A1 , US 2002 / 055545 A1 , US 2006 / 213224 A1 , and US 2003 / 209020 A1 belong to the prior art.

[0010] Summary of the invention

[0011] The invention aims to overcome the above drawbacks of the prior art.

[0012] The judicious insight of the present inventors is that a stream of air to be separated in an enriched stream and in a tail stream is fed to a separation section at lower temperatures with respect to the prior art. Hence, the problem addressed by the present invention is how to reduce power consumption of air separation unit (ASU) and process.

[0013] A reduction of power consumption involves that an ASU may have a higher capacity, or a same capacity may be obtained with an ASU of smaller dimensions.

[0014] A first aspect of the present invention is an air separation unit or ASU comprising:

[0015] - a compression section for compressing ambient air to compressed air;

[0016] - a heat exchanger to cool said compressed air and thereby obtain cooled compressed air;

[0017] - a processing section of said cooled compressed air to obtain processed air, said processing section comprising at least one purification unit and at least one cooling unit; and

[0018] - a separation section of said processed air into an oxygen- or nitrogen- enriched stream and a tail stream; wherein said ASU is devoid of an absorption refrigeration unit, and wherein said at least one cooling unit comprises a multistage mechanical vapor-compression cycle using ammonia as working fluid.

[0019] A second aspect of the present invention is an air separation process comprising the following steps:

[0020] - compression of ambient air to compressed air;

[0021] - cooling said compressed air and thereby obtaining cooled compressed air;

[0022] - processing of said cooled compressed air to obtain processed air, said processing comprising at least one step of purification and at least one step of cooling; and

[0023] - separation of said processed air into an oxygen- or nitrogen-enriched stream and a tail stream; wherein said air separation process is devoid of absorption refrigeration steps, and wherein said cooling in the processing step comprises a multistage mechanical vapor-compression cycle using ammonia as working fluid.

[0024] A further aspect of the present invention is an ammonia synthesis plant comprising said ASU and an ammonia catalytic converter arranged for reacting said nitrogen-enriched stream and a hydrogen stream in a stoichiometrically required ratio for ammonia synthesis.

[0025] A further aspect of the present invention is a method of revamping an air separation unit or ASU; wherein said ASU to be revamped comprises:

[0026] - a compression section for compressing ambient air to compressed air;

[0027] - a heat exchanger to cool said compressed air and thereby obtain cooled compressed air;

[0028] - optionally an absorption refrigeration unit;

[0029] - a separation section of said compressed air into an oxygen- or nitrogen- enriched stream and a tail stream; wherein said method comprises the following steps:

[0030] (I) installing a processing section of said cooled compressed air to obtain processed air, said processing section comprising at least one purification unit and at least one cooling unit; and

[0031] (II) optionally removing said absorption refrigeration unit;

[0032] (III) providing a line for feeding said processed air to said separation section; wherein said at least one cooling unit comprises a multistage mechanical vaporcompression cycle using ammonia as working fluid.

[0033] Advantages of the invention

[0034] Advantageously, the ASU and process of the present invention are capable of reducing power consumption of an ASU up to 15%.

[0035] Advantageously, the ASU and process of the present invention can be used to revamp or implement existing ammonia synthesis plants, in particular coal-based ammonia synthesis plants whose capacity is usually difficult to increase. More precisely, according to different embodiments, such revamping may include connection of an existing multistage mechanical vapour compression cycle to cool the cooled compressed air or revamping of an existing compressor.

[0036] Advantageously, the ASU and process of the present invention allow to reduce Total Investment Costs (TICs) for a target capacity. This is indeed true for layouts comprising at least a coldbox.

[0037] Advantageously, the ASU and process of the present invention interact synergistically with an ammonia synthesis plant or process due to additionally reduced requirements for ammonia refrigeration.

[0038] Advantageously, thanks to the use of ammonia as working fluid, the ASU and process of the present invention have a cooling capacity to below zero, e.g. minus 25 °C or lower.

[0039] Preferred embodiments

[0040] According to a preferred embodiment of the present invention, said at least one cooling unit 9, 9’ comprises one or more evaporator(s) 14 in heat-exchanging relationship with a purified stream fed by the purification unit.

[0041] Preferably, said evaporator(s) 14 is / are part of said multistage mechanical vaporcompression cycle 10.

[0042] According to another preferred embodiment, said multistage mechanical vaporcompression cycle 10 comprises at least two stages (e.g., only two stages), preferably at least three stages (e.g., only three stages), more preferably at least four stages (e.g., only four stages).

[0043] Said multistage mechanical vapor-compression cycle 10 preferably comprises compression stages I, II, III.

[0044] Each compression stage I, II, III preferably comprises - in sequence - a gaseous / liquid ammonia separator (optional) and a compressor. Said gaseous / liquid ammonia separator, when provided, is preferably a safety guard separator. Preferably, gaseous ammonia streams 20, 32 leaving said one or more, or each, evaporator(s) 14 is separated in a gaseous / liquid ammonia separator (optional) and then compressed in a compressor of a respective compression stage I, II, III thus obtaining gaseous ammonia phases 22, 24, 28.

[0045] More preferably, a gaseous ammonia phase 28 leaving the last compression stage III is cooled in a condenser 30 thus producing a condensate stream 29.

[0046] According to a preferred embodiment, said ASU 1 comprises at least a first lamination valve 25 for laminating said condensate stream 29 thus producing a first partially evaporated liquid ammonia stream 26 that is passed through an evaporator 14 of said cooling unit 9.

[0047] According to another preferred embodiment, said ASU 1 further comprises a refrigeration user 27 cooled by said condensate stream 29 laminated in a second lamination valve 33. A further gaseous stream 21 leaving an evaporator 14 of said refrigeration user 27 is preferably fed to one of said compression stages I, II, III.

[0048] According to still another embodiment, at least one purification unit 8 and at least two cooling units 9, 9’ are arranged in series with respect to a flow direction of the cooled compressed air 6. Preferably, said ASU 1 comprises a plurality of cooling units 9, 9’. More preferably, said cooling units 9, 9’ comprise an upstream high-pressure (HP) cooling unit 9 and a downstream low-pressure (LP) cooling unit 9’.

[0049] More preferably, a liquid stream 34 generated by an evaporator 14 of said HP cooling unit 9 is fed to an evaporator 14 of the LP cooling unit 9’ through a third lamination valve 23.

[0050] Said at least one purification unit 8 is preferably arranged for separating an impurities-loaded stream 11 comprising water and carbon dioxide upstream of the first of said two cooling units 9, 9’.

[0051] Preferably, said separation section 15 comprises a feed-effluent heat exchanger 19 exchanging heat between said processed air 13 and said oxygen- or nitrogen- enriched stream 16. More preferably, said feed-effluent heat exchanger 19 is integrated in a coldbox 18.

[0052] According to a preferred embodiment, said purification step 8 and two cooling steps 9, 9’ are performed in sequence with respect to a flow direction of the cooled compressed air 6. Preferably, said at least one purification step is arranged for separating an impurities-loaded stream 11 comprising water and carbon dioxide upstream of said at least one cooling unit 9.

[0053] According to a preferred embodiment, said separation comprises exchanging heat between said processed air 13 and said oxygen- or nitrogen-enriched stream 16.

[0054] According to another preferred embodiment, at least 50% by volume, preferably at least 80% by volume, more preferably at least 90% by volume, even more preferably at least 97% by volume, still more preferably 100% by volume, of said ambient air 3 intended for separation is processed to obtain processed air 13.

[0055] Preferably, said ASU to be revamped is part of an ammonia synthesis plant, more preferably a coal-based ammonia synthesis plant.

[0056] The invention will now be described in view of the following non-limiting examples.

[0057] Description of the figures

[0058] Fig. 1 : schematic representation of the air separation unit or process according to an embodiment of the present invention;

[0059] Fig. 2: schematic representation of the air separation unit or process according to another embodiment of the present invention;

[0060] Fig. 3: schematic representation of the air separation unit or process according to still another embodiment of the present invention.

[0061] Detailed description of preferred embodiments

[0062] Fig. 1 shows a schematic representation of an air separation unit 1 according to a first embodiment of the invention. Ambient air 3 is compressed in a compression section 2 to obtain compressed air 4.

[0063] Said compressed air 4 is cooled in an indirect heat exchanger 5 thereby obtaining cooled compressed air 6. Said heat exchanger 5 may be a water-cooled heat exchanger.

[0064] The cooled compressed air 6 is fed to a processing section 7 comprising a purification unit 8 and a cooling unit 9. In the purification unit 8, the cooled compressed air 6 is separated in an impurities-loaded stream 11 (comprising mainly H2O and CO2) and a purified stream 12. The impurities-loaded stream 11 may be vented. The purified stream 12 is fed to the cooling unit 9, so that processed air 13 is obtained as a cooled-down effluent of the cooling unit 9.

[0065] In particular, the cooling unit 9 comprises an evaporator 14 in indirect heatexchanging relationship with the purified stream 12 fed by the purification unit 8, said evaporator 14 being part of a multistage mechanical vapor-compression cycle 10 - using ammonia as working fluid - explained in a greater detail herein below.

[0066] Said processed air 13 is fed to a separation section 15 wherein said processed air 13 is separated into an oxygen- or nitrogen-enriched stream 16 and a tail stream 17, this latter being exported from the ASU as an option. Said separation section 15 may comprise a coldbox or a cryogenic separator 18.

[0067] More precisely, said processed air 13 enters a feed-effluent heat exchanger 19 of the separation section 15 wherein it indirectly exchanges heat with the oxygen- or nitrogen-enriched stream 16. After such indirect heat exchange:

[0068] - the oxygen-enriched stream 16 may optionally be fed to a reforming section for reforming a hydrocarbon feedstock into a raw syngas comprising H2, CO2, CO. E.g., the reforming section comprises an autothermal reformer (ATR);

[0069] - the nitrogen-enriched stream 16 may optionally be fed to an ammonia catalytic converter arranged for reacting said nitrogen-enriched stream 16 and a hydrogen stream in a stoichiometrically required ratio for ammonia synthesis. Said ammonia catalytic converter is typically part of an ammonia synthesis loop.

[0070] Said multistage mechanical vapor-compression cycle 10 comprises three stages I, II, III. Each stage is a compression stage comprising - in series and in this sequence - a gaseous / liquid ammonia separator (optional) and a compressor.

[0071] The first stage I receives a first gaseous ammonia stream 20 leaving said evaporator 14. This first gaseous ammonia stream 20 is subjected to optional separation and then to compression to obtain a first gaseous ammonia phase 22.

[0072] The first gaseous ammonia phase 22 is fed to separation (optional) and compression of the second stage II, thus producing a second gaseous ammonia phase 24. Optionally, a further gaseous stream 21 originated from an evaporator 14 of a refrigeration user 27 may be fed to the first stage I together with the first gaseous ammonia stream 20 or, in any case, may be mixed with the first gaseous ammonia phase 22 upstream of the second stage II.

[0073] The second gaseous ammonia phase 24 is fed to separation (optional) and compression of the third stage III to obtain a third gaseous ammonia phase 28. The third gaseous ammonia phase 28 is cooled in a condenser 30, and then fed as a condensate stream 29 to a first lamination valve 25 and to an optional second lamination valve 33.

[0074] At least part of the condensate stream 29 is expanded in the first lamination valve 25 thus producing a first partially evaporated liquid ammonia stream 26 that is passed through the evaporator 14 of the cooling unit 9 thus producing the first gaseous ammonia stream 20 feeding the first stage I as explained herein before.

[0075] Optionally, any remaining part of the condensate stream 29 may be expanded in the second lamination valve 33 and passed through the evaporator 14 of the refrigeration user 27 thus producing the further gaseous stream 21 .

[0076] Fig. 2 shows a schematic representation of an air separation unit according to a second embodiment of the invention.

[0077] As a difference with respect to the embodiment of Fig. 1 , the processing section 7 comprises the purification unit 8 and two cooling units 9, 9’, in particular a high- pressure (HP) cooling unit 9 and a low-pressure (LP) cooling unit 9’ - arranged in series with respect to a flow direction of the cooled compressed air 6.

[0078] In particular, the purified stream 12 is passed through said upstream HP cooling unit 9 to give a partially cooled stream 31. The partially cooled stream 31 is passed through said downstream LP cooling unit 9’ thereby providing the processed air 13.

[0079] Each of the two cooling units 9, 9’ is provided with an evaporator 14.

[0080] The evaporator 14 of the LP cooling unit 9’ is connected to the three compression stages I, II, III in the same way as explained herein before with reference to Fig. 1.

[0081] A second gaseous ammonia stream 32 leaving the evaporator 14 of the HP cooling unit 9 is fed to the second stage II for being separated (optionally) and compressed, and leaving the second stage II as second gaseous ammonia phase 24.

[0082] As a further difference with respect to Fig. 1 , a liquid stream 34 generated by the evaporator 14 of the HP cooling unit 9 is fed to the evaporator 14 of the LP cooling unit 9’ through a third lamination valve 23. The first partially evaporated liquid ammonia stream 26 that is passed through the evaporator 14 of the LP cooling unit 9’ is thus produced.

[0083] Fig. 3 shows a schematic representation of an air separation unit according to a third embodiment of the invention.

[0084] As a difference with respect to the embodiment of Fig. 2, the air separation unit 1 comprises a separation section 15 with a feed-effluent heat exchanger 19 that is fed with the processed air 13 leaving the LP cooling unit 9’, as already explained with reference to Fig. 1 . LIST OF THE REFERENCE SIGNS

[0085] 1 air separation unit (ASU)

[0086] 2 compression section or step

[0087] 3 ambient air

[0088] 4 compressed air

[0089] 5 heat exchanger, preferably water-cooled heat exchanger

[0090] 6 cooled compressed air

[0091] 7 processing section or step

[0092] 8 purification unit or step

[0093] 9 cooling unit or step, preferably high-pressure (HP) cooling unit or step

[0094] 9’ cooling unit or step, preferably low-pressure (LP) cooling unit or step

[0095] 10 multistage mechanical vapor-compression cycle

[0096] 11 impurities-loaded stream

[0097] 12 purified stream

[0098] 13 processed air

[0099] 14 evaporator or evaporation step

[0100] 15 separation section or step

[0101] 16 oxygen- or nitrogen-enriched stream

[0102] 17 tail stream

[0103] 18 coldbox or cryogenic separator or separation

[0104] 19 feed-effluent heat exchanger

[0105] 20 first gaseous ammonia stream

[0106] 21 further gaseous stream

[0107] 22 first gaseous ammonia phase

[0108] 23 third lamination valve 24 second gaseous ammonia phase

[0109] 25 first lamination valve

[0110] 26 first partially evaporated liquid ammonia stream

[0111] 27 refrigeration user 28 third gaseous ammonia phase

[0112] 29 condensate stream

[0113] 30 condenser

[0114] 31 partially cooled stream

[0115] 32 second gaseous ammonia stream 33 second lamination valve

[0116] 34 liquid stream

[0117] I first compression stage

[0118] II second compression stage

[0119] III third compression stage

Claims

CLAIMS1 ) An air separation unit or ASU (1 ) comprising:- a compression section (2) for compressing ambient air (3) to compressed air (4);- a heat exchanger (5), such as a water-cooled heat exchanger, to cool said compressed air (4) and thereby obtain cooled compressed air (6);- a processing section (7) of said cooled compressed air (6) to obtain processed air (13), said processing section (7) comprising at least one purification unit (8) and at least one cooling unit (9, 9’); and- a separation section (15) of said processed air (13) into an oxygen- or nitrogen-enriched stream (16) and a tail stream (17); wherein said ASU (1 ) is devoid of an absorption refrigeration unit, and wherein said at least one cooling unit (9, 9’) comprises a multistage mechanical vapor-compression cycle (10) using ammonia as working fluid.2) The ASU (1 ) of claim 1 , wherein said at least one cooling unit (9, 9’) comprises one or more evaporator(s) (14) in heat-exchanging relationship with a purified stream (12) fed by the purification unit (8), said evaporator(s) (14) being part of said multistage mechanical vapor-compression cycle (10).3) The ASU (1 ) according to any of the previous claims, wherein said multistage mechanical vapor-compression cycle (10) comprises compression stages (I, II, III), each compression stage (I, II, III) comprising- in sequence - a gaseous / liquid ammonia separator and a compressor.4) The ASU (1 ) according to claims 2 and 3, wherein gaseous ammonia streams (20, 32) leaving said one or more, or each, evaporator(s) (14) is / are separated in a gaseous / liquid ammonia separator and then compressed in a compressor of a respective compression stage (I, II, III) thus obtaining gaseous ammonia phases (22, 24, 28), and wherein a gaseous ammonia phase (28) leaving the last compression stage (III) is cooled in a condenser (30) thus producing a condensate stream (29).5) The ASU (1 ) according to the previous claim, comprising at least a first lamination valve (25) for laminating said condensate stream (29) thus producing a first partially evaporated liquid ammonia stream (26) that is passed through an evaporator (14) of said cooling unit (9).6) The ASU (1 ) according to any of the previous claims, further comprising a refrigeration user (27) cooled by said condensate stream (29) laminated in a second lamination valve (33), a further gaseous stream (21 ) leaving an evaporator (14) of said refrigeration user (27) being fed to one of said compression stages (I, II, III).7) The ASU (1 ) according to any of the previous claims, wherein said at least one purification unit (8) and at least two cooling units (9, 9’) are arranged in series with respect to a flow direction of the cooled compressed air (6); said at least one purification unit (8) being arranged for separating an impurities- loaded stream (11 ) comprising water and carbon dioxide upstream of the first (9) of said two cooling units (9, 9’).8) The ASU (1 ) according to any of the previous claims, comprising a plurality of cooling units (9, 9’), wherein said cooling units (9, 9’) comprise an upstream high-pressure (HP) cooling unit (9) and a downstream low- pressure (LP) cooling unit (9’), wherein a liquid stream (34) generated by an evaporator (14) of said HP cooling unit (9) is fed to an evaporator (14) of the LP cooling unit (9’) through a third lamination valve (23).9) The ASU (1 ) according to any of the previous claims, wherein said separation section (15) comprises a feed-effluent heat exchanger (19) exchanging heat between said processed air (13) and said oxygen- or nitrogen-enriched stream (16); preferably said feed-effluent heat exchanger (19) being integrated in a coldbox (18).10) The ASU (1 ) according to any of the previous claims, wherein said separation section (15) comprises a coldbox or a cryogenic separator (18).11 ) An ammonia synthesis plant comprising an ASU (1 ) according to any of theprevious claims and an ammonia catalytic converter arranged for reacting said nitrogen-enriched stream (16) and a hydrogen stream in a stoichiometrically required ratio for ammonia synthesis.12) An air separation process comprising the following steps:- compression of ambient air (3) to compressed air (4);- cooling said compressed air (4) and thereby obtaining cooled compressed air (6);- processing of said cooled compressed air (6) to obtain processed air (13), said processing comprising at least one step of purification (8) and at least one step of cooling (9); and- separation of said processed air (13) into an oxygen- or nitrogen- enriched stream (16) and a tail stream (17); wherein said air separation process is devoid of absorption refrigeration steps, and wherein said cooling in the processing step comprises a multistage mechanical vapor-compression cycle (10) using ammonia as working fluid.13) The air separation process according to the previous claim, wherein said purification step (8) and two cooling steps (9) are performed in sequence with respect to a flow direction of the cooled compressed air (6); said at least one purification step being arranged for separating an impurities-loaded stream (11 ) comprising water and carbon dioxide upstream of said at least one cooling unit (9).14) The air separation process according to claim 12 or 13, wherein said separation comprises exchanging heat between said processed air (13) and said oxygen- or nitrogen-enriched stream (16).15) A method of revamping an air separation unit or ASU, wherein said ASU to be revamped comprises: a compression section for compressing ambient air to compressed air;- a heat exchanger to cool said compressed air and thereby obtain cooled compressed air;- optionally an absorption refrigeration unit;- a separation section of said compressed air into an oxygen- or nitrogen- enriched stream and a tail stream; wherein said method comprises the following steps:(IV) installing a processing section (7) of said cooled compressed air (6) to obtain processed air (13), said processing section (7) comprising at least one purification unit (8) and at least one cooling unit (9); and (V) optionally removing said absorption refrigeration unit;(VI) providing a line for feeding said processed air (13) to said separation section (15); wherein said at least one cooling unit (9) comprises a multistage mechanical vapor-compression cycle (10) using ammonia as working fluid. 16) The method of revamping according to the previous claim, wherein saidASU to be revamped is part of a coal-based ammonia synthesis plant.

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