Process and apparatus for separating air by cryogenic distillation

Abstract

In a process for separating air by cryogenic distillation, air is separated by distillation in a simple column (K1), nitrogen coming from the top of the column condenses at least partially in a top condenser (C) of the column when it meets bottom liquid (9) in the column that has been expanded (V1) and sent to the top condenser, a liquid nitrogen flow (15) is drawn off from the column, pressurised, and vaporised in a heat exchanger (E), and a gaseous oxygen flow (3, 7) from an external source (EL) cools and liquefies in the heat exchanger, by exchanging heat with the liquid nitrogen to be vaporised, is expanded and is sent to the top condenser of the column.

Description

[0001] Description

[0002] Title of the invention: Method and apparatus for air separation by cryogenic distillation

[0003] The present invention relates to a method and apparatus for air separation by cryogenic distillation. It also relates to an integrated method for electrolysis, air separation by cryogenic distillation, and ammonia production.

[0004] It is known to use a simple column to produce nitrogen, the simple column having a condenser at the top and sometimes a reboiler at the bottom. The book "Razdelenia Vozdukha" by Epifanova et al., published by Mashinostroenie, 1964, describes on page 241 an apparatus with a simple column where the cooling is provided by a Claude turbine, a pump pressurizes liquid nitrogen from the top of the column, and the pressurized liquid nitrogen vaporizes against air in a heat exchanger.

[0005] The invention aims to utilize an available oxygen flow at pressures between 5 and 30 bar, originating, for example, from a water electrolysis unit that produces hydrogen at pressures between 5 and 30 bar. The oxygen pressure is utilized by using it to vaporize a flow of liquid nitrogen drawn from the top of a single column and pressurized by a pump, without mixing the oxygen with the air intended for distillation. Alternatively, it can be utilized, according to a variant, by using it to heat the column's reboiler. The pressurized liquid nitrogen is vaporized in the process heat exchanger where the oxygen and air flow destined for the column are cooled for separation.

[0006] One aim of the invention is to produce nitrogen under pressure with particularly low energy consumption and / or reduced investment. It is necessary to treat the oxygen to remove any residual hydrogen it contains, as well as water and possibly other impurities that could freeze at cryogenic temperatures. The water and possibly other impurities can be removed by adsorption, for example, by a temperature or pressure switch. The vaporized liquid nitrogen can be heated in the heat exchanger, expanded in a turbine, and then heated again in the heat exchanger, thus providing the necessary cooling for separation. The expanded nitrogen can be used as a product at a pressure lower than that of the pump outlet. Alternatively, the cooling for separation can be obtained from other sources, for example, the expansion of air intended for the column and / or the addition of liquid nitrogen from an external source at the top of the column.In a particular variant, the process is optimized to be part of an integrated process of electrolysis, air separation by cryogenic distillation and ammonia production in which the oxygen from the electrolysis constitutes the pressurized gaseous oxygen which is used to vaporize liquid nitrogen produced by the air separation column, hydrogen from the electrolysis unit being combined with the nitrogen produced by the air separation to constitute an ammonia synthesis gas.

[0007] The aim is to determine the oxygen pressure at the electrolyzer outlet, given that the ratio is 1.5 O2 to 1 N2 when producing 2 NH3, and that electrolysis produces one oxygen atom for every two hydrogen atoms. This follows from the following simple chemical equilibria:

[0008] [Cheml] And

[0009] [Chem2]

[0010] 2NH3 <-> N2 + 3H2

[0011] The nitrogen yield of the apparatus to be described varies between 60 and 100%. US20080011015 describes a process according to the preamble of claim 1. According to one aspect of the invention, a process for separating air by cryogenic distillation is provided in which: i) Purified air cooled in a heat exchanger is separated by distillation in a single column operating at a pressure of at least 4 bar abs; nitrogen from the top of the column condenses at least partially in a column top condenser against column tank liquid that has been expanded and sent to the top condenser; ii) Condensed nitrogen is returned to the top of the column; iii) Vaporized liquid exits the column top condenser and is heated in the heat exchanger; iv) A flow of liquid nitrogen is withdrawn from the column, pressurized,heated and vaporized in the heat exchanger (v) A flow of gaseous oxygen from an external source produced by means other than air distillation in the column, is cooled and liquefied in the heat exchanger, without being mixed with air intended for distillation, by heat exchange with the liquid nitrogen to be vaporized, is expanded and is sent to the top condenser of the column to vaporize there, forming part of the vaporized liquid exiting the top condenser, characterized in that the column tank liquid which has been expanded is sent to the top condenser without having been enriched in oxygen,

[0012] Depending on other optional characteristics:

[0013] • the flow of gaseous oxygen comes from at least one water electrolysis unit.

[0014] • the flow rate of gaseous oxygen is at a pressure of at least 7 bars abs., or even at least 10 bara.

[0015] • The flow rate of gaseous oxygen is not compressed upstream and / or downstream of the heat exchanger

[0016] • the flow of gaseous oxygen exits the heat exchanger at a pressure of at least 7 bar abs., or even at least 10 bara.

[0017] • The gaseous oxygen flow rate contains between 90 and 100 mol% oxygen, preferably more than 99 mol% oxygen

[0018] • a flow of gaseous oxygen from an external source upstream of the heat exchanger or the external source (e.g. a water electrolysis unit) is cooled in the heat exchanger and is sent to heat a tank reboiler of the single column in which it condenses, the condensed flow is expanded and sent to the head condenser to vaporize there.

[0019] • Nitrogen vaporized in the heat exchanger exits the heat exchanger at an intermediate temperature, is expanded in a turbine and is returned to the heat exchanger, possibly to a cold end of the heat exchanger, to be heated.

[0020] • the turbine provides at least 90%, or even substantially 100%, of the cooling required for the process.

[0021] • Vaporized nitrogen, possibly expanded, and heated in the heat exchanger is mixed with a flow of hydrogen gas to form a synthesis gas of ammonia.

[0022] • The turbine expands vaporized nitrogen to the pressure of the hydrogen gas with which it is mixed. • The flow of hydrogen gas, with which the vaporized and heated nitrogen is mixed, comes from the same electrolysis unit as the oxygen gas.

[0023] • The purified and cooled gaseous air from a heat exchanger is sent in gaseous form to the single column for separation. All the purified and cooled gaseous air from the heat exchanger is substantially at the pressure of the single column and is sent in gaseous form to the single column for separation without having been compressed.

[0024] • no part of the air sent to the column is used to reboil the single column

[0025] • The purified and cooled gaseous air, supplied by a heat exchanger, is sent in gaseous form to the single-column tank or to an intermediate level of the single column for separation.

[0026] • The purified and cooled gaseous air, in a heat exchanger, is sent in gaseous form to the single-column vessel or to an intermediate level of the single column to separate, without having been used for reboiling.

[0027] • all the liquid nitrogen withdrawn at the top of the column serves as product

[0028] • One of the gaseous oxygen flow rates sent to the column's head condenser was not used for reboiling the column

[0029] • Another of the gaseous oxygen flow rates sent to the column's head condenser was used for reboiling the column

[0030] • No liquid nitrogen withdrawn from the column is sent to another distillation column

[0031] • All the liquid nitrogen withdrawn from the column is then pressurized, heated, and vaporized in the heat exchanger

[0032] According to another aspect of the invention, an integrated process for the production of ammonia synthesis gas is provided, in which: i) Water is transformed by electrolysis into a flow of gaseous oxygen and a flow of gaseous hydrogen; ii) The flow of gaseous oxygen is sent to a cryogenic distillation air separation process operating as described above and is condensed, sent to the overhead condenser, and vaporized; iii) The vaporized and heated flow of liquid nitrogen is mixed with at least a portion of the flow of gaseous hydrogen to form the ammonia synthesis gas. According to another object of the invention, a cryogenic distillation air separation apparatus is provided, comprising a heat exchanger, a single column having an overhead condenser, and means for sending cooled purified air from the heat exchanger to the single column for separation by distillation.a conduit for sending nitrogen from the top of the column to condense at least partially in the column's top condenser, means for expanding a column tank liquid, means for sending the expanded tank liquid to the top condenser, a conduit for sending condensed nitrogen from the condenser to the top of the column, a conduit connected to the top condenser for sending vaporized liquid from the column's top condenser to be heated in the heat exchanger, a pump, a conduit connecting the column top to the pump for sending a flow of liquid nitrogen drawn from the column, a conduit for sending the liquid nitrogen pressurized by the pump to be heated in the heat exchanger, means for sending a flow of gaseous oxygen from an external source, supplied by means other than the column itself, to be cooled and liquefied in the heat exchanger.without being mixed with air intended for distillation, by heat exchange with liquid nitrogen, means for expanding gaseous oxygen downstream of the heat exchanger, means for sending the expanded oxygen, at least partially liquefied, to the top condenser of the column to vaporize there, forming part of the vaporized liquid exiting the top condenser, characterized in that the means for sending the expanded tank liquid to the top condenser are connected to send the liquid directly there.

[0033] The single column is not thermally connected to another column. The invention will be described in more detail with reference to the figures where: [FIG.1] illustrates a process for separating air by cryogenic distillation according to the invention.

[0034] [FIG.2] illustrates an integrated process of electrolysis, air separation and ammonia production according to the invention.

[0035] [FIG. 3] illustrates a heat exchange diagram for the heat exchanger of [FIG. 1] and

[0036] [FIG. 4] shows a McCabe-Thiele diagram for the column of [FIG. 1]. [FIG. 5] illustrates another method of air separation by cryogenic distillation according to the invention. In Figure 1, a flow of air 1 purified of water and CO2 is at a pressure of at least 4 bar abs and preferably substantially at the pressure of the single column K1 and is cooled in the heat exchanger E to a temperature close to its dew point and then sent to an intermediate section of a single column K1 operating at a pressure of at least 4 bar abs having a top condenser C and optionally a tank reboiler. The air enters the column in essentially gaseous form with at least one body of structured packing or trays above its entry point into the column and at least one body of structured packing or trays below its entry point into the column K1 and preferably without having been compressed.The air separates in the column, forming an oxygen-enriched liquid in the tank and a nitrogen-enriched gas at the top. The gas condenses at least partially in the top condenser C of column K1 and is returned to column K1.

[0037] Oxygen at a pressure of at least 7 bar abs, or even at least 10 bar abs, from an external source upstream of the heat exchanger E, preferably from an electrolysis unit EL, containing between 90 and 100 mol% oxygen (considered dry), for example, more than 99 mol% oxygen, is purified to remove any impurities that could solidify in the heat exchanger E. This purification takes place in a unit such as a catalyst and in an adsorption unit, for example, of the TSA or PSA type, to remove water and possibly other impurities. The catalyst for removing residual hydrogen from the oxygen can be palladium, platinum, cerium, or one of their oxides. The purified oxygen 3 is cooled in the heat exchanger and is split in two, preferably upstream of the heat exchanger E, forming a flow 5 and a flow 7.The flow 5, under a pressure of at least 7 bar abs or even at least 10 bar abs, cools almost to the cold end of the heat exchanger E and is sent in a gaseous state close to its dew point to the tank reboiler R of column K1, where it condenses, providing heat for reboiling column K1. The resulting liquid 11 is expanded through a valve V2 and feeds the overhead condenser C. Oxygen-enriched liquid 9 is drawn from the tank of column K1 and expanded through a valve V1. Both expanded liquids 9 and 11 are sent to the overhead condenser C of column K1, without oxygen enrichment of liquid 9, to cool the nitrogen at the top of the column. Preferably, at least one of the flow 5 and 7 is not compressed upstream and / or downstream of the heat exchanger.

[0038] Preferably, neither flow 5 nor flow 7 is compressed upstream of the column or downstream of the heat exchanger E and / or upstream of the heat exchanger.

[0039] Flow 7, under a pressure of 10 bar abs, is cooled, then condenses to the cold end of the heat exchanger. It is then expanded through valve V3 to the pressure of the head condenser C. The resulting liquid is sent to the head condenser C of column K1. The liquids are heated and partially vaporized by condenser C, forming a gas 13 that is richer in oxygen than liquid 9 and slightly less rich in oxygen than flow 11. Gas 13 is heated in the heat exchanger E from the cold end to the hot end.

[0040] Air 1 is not used to reboil column K1.

[0041] A flow rate of liquid nitrogen is drawn from the top of column K1, pressurized by a pump to a pressure of 18 bar abs, and sent to vaporize in the heat exchanger (against the flow rate of 7). At least some of the pressurized vaporized nitrogen exits the heat exchanger at an intermediate temperature, is expanded in a turbine T to approximately 10 bar abs to provide cooling for separation, and is then returned to the cold end of the heat exchanger at a pressure of 10 bar abs. The expanded nitrogen is heated as it passes completely through the heat exchanger and exits exchanger E to serve as a product of the air separation apparatus.

[0042] This implies that the pressure of the liquid nitrogen is chosen according to the pressure at which oxygen 7 is available, in order to take advantage of the free pressure at which oxygen comes out of the external source.

[0043] In Figure 2, an electrolysis unit EL is powered by water, preferably green electricity, and produces a flow of hydrogen 21 which exits under pressure, for example 10 bar abs, and mixes with nitrogen 19 to form a synthesis gas 23 containing approximately one mole of nitrogen for every three moles of hydrogen. The gas 23 is compressed by a compressor V and then sent to an ammonia synthesis unit N which produces a flow of ammonia 27.

[0044] Preferably, the pressures of hydrogen 21 and the expanded nitrogen 19 are chosen to be the same. However, they may differ, and one of the gases can be compressed to the pressure of the other, preferably nitrogen, which is easier to compress. Preferably, the pressure to which the nitrogen 19 has been expanded is chosen to be the pressure at which the hydrogen 21 exits the electrolysis unit EL.

[0045] The pressurized oxygen gas 3 exiting the electrolysis unit EL is sent to the air separation unit A, illustrated in Figure 1, after being dried and having residual hydrogen and other possible impurities removed. The air separation unit processes the dried and compressed air 1 and uses the pressurized oxygen 3 to supply nitrogen 19, preferably at the same pressure as the hydrogen flow rate 21, to form the mixture 23.

[0046] Liquids 9 and 11 and 7 can be subcooled upstream of valves V1, V2, V3 in a subcooler against gas 13. The subcooler can be integrated into the main exchanger.

[0047] The presence of the reboiler R is not essential.

[0048] In this example, a single flow 3 containing between 90 and 100% mol of oxygen considered dry, for example more than 99% mol of oxygen, comes from the electrolysis unit and is divided into two flows at the same pressure 5, 7, of which one 7 is used to vaporize the pressurized liquid nitrogen and the other 5 is sent to the tank reboiler R. Preferably, at least one of the flows 5, 7 is not compressed downstream of the heat exchanger.

[0049] It will be understood that these flow rates 5 and 7 may be at different pressures. Flow rate 5 may originate from one electrolysis unit and flow rate 7 from an external source upstream of the heat exchanger, other than an electrolysis unit, and vice versa. Flow rate 5 may originate from one electrolysis unit and flow rate 7 from another electrolysis unit, and the vaporized nitrogen may be mixed with hydrogen from either electrolysis unit.

[0050] [FIG. 3] illustrates a heat exchange diagram for heat exchanger E in [FIG. 1], showing the nitrogen vaporization and oxygen condensation stages from the external source upstream of the heat exchanger opposite each other. Since both fluids are pure, the condensation and vaporization stages are nearly vertical, allowing for good alignment and efficient latent heat exchange.

[0051] [FIG. 4] shows a McCabe-Thiele diagram for the column in [FIG. 1] illustrating on the x-axis the fraction of the most volatile compound in the liquid phase and on the y-axis the fraction of the most volatile compound in the vapor phase along the column height. [FIG. 5] illustrates an air separation process in a simple column K1 with a top condenser but without a tank reboiler. Water-purified air 1 is cooled in the heat exchanger E.Thus, the cooled gaseous air arrives in the tank of column K1 and separates there, forming an oxygen-rich liquid in the tank. Purified gaseous oxygen, containing between 90 and 100% mol of oxygen considered dry, for example more than 99% mol of oxygen, at a pressure between 5 and 30 bar abs, arrives from an electrolyzer or other source, liquefies in the heat exchanger E, is expanded and is sent to the head condenser C of column K1, without having been compressed downstream of the heat exchanger E. The condenser C is also supplied by the expanded tank liquid 9 from column K1, sent directly to the condenser C without having been enriched in oxygen, in order to condense the head gas of column K1.

[0052] A flow of liquid nitrogen 15 is drawn from the top of column K1, pressurized by a pump up to a pressure of 18 bar abs, vaporized in the heat exchanger, at least part of the vaporized liquid is possibly expanded in a turbine T up to 10 bar abs and then returned to the heat exchanger to be heated.

[0053] Air 1 is not used to reboil column K1.

[0054] This expansion of the turbine T provides at least 90% of the cooling capacity of the process, or even 100%. Otherwise, another method of supplying cooling capacity, such as a nitrogen cycle or feed nitrogen, can be used.

Claims

Demands 1. A process for separating air by cryogenic distillation in which:

1. Purified and cooled air in a heat exchanger (E) is separated by distillation in a single column (K1) operating at a pressure of at least 4 bar abs. Nitrogen from the top of the column condenses at least partially in a top condenser (C) of the column against tank liquid (9) of the column that has been depressurized (V1) and sent to the top condenser. ii. Condensed nitrogen is returned to the top of the column. iii. Vaporized liquid (13) exits the top condenser of the column and is heated in the heat exchanger. iv. A flow of liquid nitrogen (15) is withdrawn from the column, pressurized, heated and vaporized in the heat exchanger. v.A flow of gaseous oxygen (3,7) from an external source (EL), produced by means other than the distillation of air in the column, is cooled and liquefied in the heat exchanger, without being mixed with air intended for distillation, by heat exchange with the liquid nitrogen to be vaporized, is expanded and is sent to the top condenser of the column to vaporize there forming part of the vaporized liquid exiting the top condenser characterized in that the column tank liquid (9) which has been expanded (V1) is sent to the top condenser without having been enriched in oxygen.

2. Method according to claim 1 wherein the flow of gaseous oxygen (3,5,7) comes from a water electrolysis unit (EL).

3. Method according to claim 1 or 2 wherein the flow of gaseous oxygen is at a pressure of at least 7 bar abs., or even at least 10 bara and preferably is not compressed downstream of the heat exchanger.

4. A method according to any one of the preceding claims, wherein a flow of gaseous oxygen (5) from an external source (EL) upstream of the heat exchanger is cooled in the heat exchanger (E) and is sent to heat a tank reboiler (R) of the single column (K1) in which it condenses, the condensed flow (11) is relaxed and sent to the head condenser to vaporize there.

5. A method according to any one of the preceding claims, wherein nitrogen vaporized in the heat exchanger exits the heat exchanger (E) at an intermediate temperature therefrom, is expanded in a turbine (T), and is returned to the heat exchanger, possibly to a cold end of the heat exchanger, to warm up.

6. A process according to claim 5 in which the turbine (T) provides at least 90%, or even substantially 100%, of the cooling required for the process.

7. A process according to any one of the preceding claims in which vaporized nitrogen, optionally expanded, and heated (19) in the heat exchanger is mixed with a flow of gaseous hydrogen (21) to form an ammonia synthesis gas (23, 25).

8. Method according to claim 5 and 7 in which the turbine (T) expands vaporized nitrogen to the pressure of the gaseous hydrogen (21) with which it is mixed.

9. A method according to any one of the preceding claims wherein all the air is purified and cooled in the heat exchanger substantially to the pressure of the single column.

10. A method according to claim 9 wherein all the purified and cooled air in the heat exchanger is sent to the single column without having been compressed downstream of the purification step.

11. Integrated process for the production of ammonia synthesis gas (27) in which: i. Water is transformed by electrolysis (EL) into a flow of gaseous oxygen (3) and a flow of gaseous hydrogen (21) ii. The flow of gaseous oxygen is sent to a cryogenic distillation air separation process operating according to one of the preceding claims and is condensed, sent to the head condenser (C) and vaporized iii. The vaporized and heated flow of liquid nitrogen (19) is mixed with at least a portion of the flow of gaseous hydrogen (21) to form the ammonia synthesis gas (23, 25).

12. Cryogenic distillation air separation apparatus comprising a heat exchanger (E), a single column (K1) having a top condenser (C), means for sending cooled purified air from the heat exchanger to the single column for separation by distillation, a conduit for sending nitrogen from the top of the column to condense at least partially in the top condenser of the column, means (V1) for expanding a tank liquid (9) from the column, means for to send the expanded tank liquid to the overhead condenser, a line for sending condensed nitrogen from the condenser to the top of the column, a line connected to the overhead condenser for sending vaporized liquid (13) from the overhead condenser of the column to be heated in the heat exchanger, a pump (P), a line connecting the top of the column with the pump for sending a flow of liquid nitrogen (15) drawn from the column, a line for sending the liquid nitrogen pressurized by the pump to be heated in the heat exchanger, means for sending a flow of gaseous oxygen (3,5) from an external source (EL), supplied by means other than the simple column, to be cooled and liquefied in the heat exchanger, without being mixed with air intended for distillation, by heat exchange with the liquid nitrogen,means (V3) for expanding gaseous oxygen downstream of the heat exchanger and means for sending the expanded oxygen, at least partially liquefied, to the head condenser (C) of the column to vaporize there, forming part of the vaporized liquid exiting the head condenser, characterized in that the means for sending the expanded tank liquid to the head condenser are connected to send the liquid directly there.

13. Apparatus according to claim 12 in which the single column is not thermally connected to another column.

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