Method and apparatus for air separation by cryogenic distillation

Abstract

Title of the invention: Process and apparatus for air separation by cryogenic distillation. In a process for air separation by cryogenic distillation, air is separated by distillation in a simple column (K1). Nitrogen from the top of the column condenses at least partially in a column head condenser (C) against column tank liquid (9) that has been expanded (V1) and sent to the head condenser. A flow of liquid nitrogen (15) is withdrawn from the column, pressurized, and vaporized in a heat exchanger. A flow of gaseous oxygen (3,7) from an external source is cooled and liquefied in the heat exchanger by heat exchange with the liquid nitrogen to be vaporized, expanded, and sent to the column head condenser. FIG. 1

Description

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

[0001] 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.

[0002] It is known to use a simple column to produce nitrogen, the simple column having a top condenser and sometimes a tank reboiler. 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.

[0003] The invention aims to utilize an available oxygen flow at a pressure between 5 and 30 bar, originating, for example, from a water electrolysis unit that produces a hydrogen flow at a pressure between 5 and 30 bar. The oxygen pressure is utilized by using it to vaporize a liquid nitrogen flow drawn from the top of the single column and pressurized by a pump, without mixing the oxygen with the air intended for distillation. According to an alternative embodiment, it can also be utilized 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 intended to feed the column are cooled for separation.

[0004] One object of the invention is to produce nitrogen under pressure with particularly low energy consumption and / or reduced investment.

[0005] It is necessary to treat the oxygen to remove the 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 temperature or pressure switching.

[0006] 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 cooling necessary 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 come 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.

[0007] In a particular embodiment, 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 gaseous oxygen under pressure which is used to vaporize liquid nitrogen produced by the air separation column, with hydrogen from the electrolysis unit being combined with the nitrogen produced by the air separation to form a synthesis gas of ammonia.

[0008] The aim is to determine the oxygen pressure at the outlet of the electrolyzer, 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:

[0009] [Chem.l]

[0010] and

[0011] [Chem.2] hh * 3Ha

[0012] The nitrogen efficiency of the apparatus which will be described varies between 60 and 100%.

[0013] According to one object of the invention, a process for separating air by cryogenic distillation is provided in which: i. Purified and cooled air 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 head condenser against column tank liquid that has been depressurized and sent to the head condenser ii. Condensed nitrogen is returned to the top of the column iii. Vaporized liquid exits the column's 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 characterized in that v. A flow of gaseous oxygen from an external source 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 which exits the top condenser.

[0014] According to other optional features: • the flow of gaseous oxygen comes from at least one water electrolysis unit. • the flow rate of gaseous oxygen is at a pressure of at least 7 bars abs., or even at least 10 bara. • a flow of gaseous oxygen from an external source or the external source (e.g. a water electrolysis unit) is cooled in the heat exchanger and is sent to heat a single column tank reboiler in which it condenses, the condensed flow is expanded and sent to the head condenser to vaporize there. • 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 the cold end of the heat exchanger, to be heated. • the turbine provides at least 90%, or even substantially 100%, of the cooling required for the process. • Vaporized nitrogen, possibly expanded, and heated in the heat exchanger is mixed with hydrogen gas to form a synthesis gas of ammonia. • the turbine expands vaporized nitrogen to the pressure of the gaseous hydrogen with which it is mixed.

[0015] 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 oxygen gas and a flow of hydrogen gas ii. The gaseous oxygen flow is sent to a cryogenic distillation air separation process operating as described above and is condensed, sent to the head condenser and vaporized iii. The flow of vaporized and heated liquid nitrogen is mixed with at least a portion of the flow of hydrogen gas to form the synthesis gas of ammonia.

[0016] According to another object of the invention, a cryogenic distillation air separation apparatus is provided, comprising a heat exchanger, a single column having a top condenser, means for sending cooled purified air from the heat exchanger to the single column for separation by distillation, a line for sending nitrogen from the top of the column to condense at least partially in the top condenser of the column, means for expanding a column tank liquid, means for sending the expanded tank liquid to the top condenser, a line for sending condensed nitrogen from the condenser to the top of the column, a line connected to the top condenser for sending vaporized liquid from the top condenser of the column to be reheated in the heat exchanger, a pump,a pipe connecting the top of the column to the pump to send a flow of liquid nitrogen drawn from the column, a pipe, to send the pressurized liquid nitrogen from the pump to be heated in the heat exchanger

[0017] characterized in that it comprises means for sending a flow of gaseous oxygen from an external source to cool and liquefy in the heat exchanger, without being mixed with air intended for distillation, by heat exchange with liquid nitrogen, means for expanding the 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 which exits the top condenser.

[0018] The invention will be described in more detail with reference to the figures where:

[0019] [Fig-1] illustrates a process for separating air by cryogenic distillation according to the invention

[0020] [Fig.2] illustrates an integrated process of electrolysis, air separation and ammonia production according to the invention.

[0021] [Fig.3] illustrates a heat exchange diagram for the heat exchanger of [Fig.1] and

[0022] [Fig.4] shows a McCabe Thiele diagram for the column of [Fig.1].

[0023] [Fig. 5] illustrates another method of air separation by cryogenic distillation according to the invention

[0024] In [Fig. 1], a flow of air 1 purified of water and CO2 is at a pressure of at least 4 bar abs and is cooled in the heat exchanger E to a temperature close to its dew point, then sent to an intermediate section of a simple 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 structured packing or tray section above its entry point into the column and at least one structured packing or tray section below its entry point into the column KL. 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 the column K1 and is returned to the column KL.

[0025] Oxygen under a pressure of at least 7 bar abs, or even at least 10 bar abs, preferably from an electrolysis unit, is purified to remove any impurities that could solidify in the heat exchanger in a purification 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. Flow 5, under a pressure of at least 7 bar abs, or even at least 10 bar abs, is purified in a separate stream. At -10 bar absolute pressure, the liquid cools almost to the cold end of the heat exchanger E and is sent as a gaseous liquid close to its dew point to the tank reboiler R of column Kl, where it condenses, providing heat for reboiling column Kl. The resulting liquid 11 is expanded through valve V2 and feeds the overhead condenser C. Oxygen-enriched liquid 9 is drawn from the tank of column Kl and expanded through valve VI. Both expanded liquids 9 and 11 are sent to the overhead condenser C of column Kl to cool the nitrogen at the top of the column.

[0026] The flow 7 under a pressure of 10 bar abs is cooled, then condenses to the cold end of the heat exchanger, is expanded in a valve V3 to the pressure of the head condenser C. The liquid formed is sent to the head condenser C of the column KL. The liquids are heated and partially vaporized by the condenser C forming a gas 13 richer in oxygen than the liquid 9 and slightly less rich in oxygen than the flow 11. The gas 13 is heated in the heat exchanger E from the cold end to the hot end.

[0027] A flow rate 15 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 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 19 is heated as it passes completely through the heat exchanger and exits the exchanger E to serve as a product of the air separation apparatus.

[0028] In [Fig. 2], an electrolysis unit EL is powered by water and preferably by 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.

[0029] 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 may 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 that at which the hydrogen 21 exits the electrolysis unit EL.

[0030] The gaseous oxygen 3 exiting under pressure from the electrolysis unit EL goes to the air separation device A, illustrated in [Fig. 1], after being dried and having residual hydrogen and other possible impurities removed. The air separation device processes dried and compressed air 1 and uses the pressurized oxygen 3 to provide nitrogen 19, preferably at the same pressure as the flow of hydrogen 21 to form the mixture 23.

[0031] Liquids 9 and 11 and 7 can be subcooled upstream of valves VI, V2, V3 in a subcooler against gas 13. The subcooler can be integrated into the main exchanger.

[0032] The presence of the reboiler R is not essential.

[0033] In this example, a single flow 3 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.

[0034] It will be understood that these flow rates 5, 7 may be at different pressures. Flow rate 5 may originate from one electrolysis unit and flow rate 7 from an external source 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.

[0035] [Fig-3] illustrates an exchange diagram for the heat exchanger E of the [Fig.1] where we see the nitrogen vaporization and oxygen condensation stages from the external source opposite each other. Since both fluids are pure, the condensation and vaporization stages are almost vertical, allowing for good alignment and efficient latent heat exchange.

[0036] [Fig.4] shows a McCabe-Thiele diagram for the column of [Fig.1] illustrating The x-axis represents the fraction of the most volatile compound in the liquid phase and the y-axis represents the fraction of the most volatile compound in the vapor phase along the height of the column.

[0037] [Fig. 5] illustrates a method of air separation in a simple column Kl with The unit has a top condenser but no tank reboiler. Water-purified air (1) is cooled in heat exchanger E. The cooled gaseous air then enters the tank of column K1 and separates, forming an oxygen-rich liquid. Oxygen gas, purified into water and hydrogen and at a pressure between 5 and 30 bar absolute, arrives from an electrolyzer or other source, liquefies in heat exchanger E, is expanded, and is sent to the top condenser C of column KL. The condenser is also supplied with expanded tank liquid (9) from column K1 to condense the top gas of column KL.

[0038] A flow of liquid nitrogen 15 is drawn from the top of the column Kl, 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 optionally expanded in a turbine T up to 10 bar abs and then returned to the heat exchanger to be heated.

[0039] This expansion of the turbine T provides at least 90% of the cooling capacity of the process, or even 100%. Otherwise, another way of providing the 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: i. Purified and cooled air in a heat exchanger (E) is separated by distillation in a single column (Kl) 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 (VI) 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. Characterized in that i.A flow of gaseous oxygen (3.7) from an external source 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 which exits the top condenser.

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

3. A method according to claim 1 or 2 wherein the flow rate of gaseous oxygen is at a pressure of at least 7 bar abs., or even at least 10 bara.

4. A method according to any one of the preceding claims wherein a flow of gaseous oxygen (5) from an external source is cooled in the heat exchanger (E) and is sent to heat a tank reboiler (R) of the single column (Kl) in which it condenses, the condensed flow (11) is expanded 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 exchanger heat (E) at an intermediate temperature of this, is expanded in a turbine (T) and is returned to the heat exchanger, possibly to the cold end of the heat exchanger, to be heated there.

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 wherein vaporized nitrogen, optionally expanded, and heated (19) in the heat exchanger is mixed with gaseous hydrogen (21) to form an ammonia synthesis gas (23, 25).

8. A method according to claims 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. An integrated process for producing 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).

10. A cryogenic distillation air separation apparatus comprising a heat exchanger (E), a single column (Kl) having a top condenser (C), means for sending cooled purified air from the heat exchanger to the single column for separation by distillation, a line 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 sending the expanded tank liquid to the top condenser, a line for sending condensed nitrogen from the condenser to the top of the column, and a line connected to the top condenser for to send vaporized liquid (13) from the column head condenser to be heated in the heat exchanger, a pump (P), a line connecting the column head to the pump to send a flow of liquid nitrogen (15) drawn from the column, a line to send the liquid nitrogen pressurized by the pump to be heated in the heat exchanger characterized in that it comprises means for sending a flow of gaseous oxygen (3,5) from an external source to cool and liquefy in the heat exchanger, without being mixed with air intended for distillation, by heat exchange with liquid nitrogen, means (V3) for expanding the gaseous oxygen downstream of the heat exchanger, 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.

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