Method and apparatus for the cryogenic separation of air

Abstract

A method and apparatus serve for the cryogenic separation of air in an air separation plant which has a main air compressor, a main heat exchanger and a distillation column system with a high-pressure column and a low-pressure column. All of the feed air is compressed in the main air compressor to a first air pressure which is at least 3 bar higher than the operating pressure of the high-pressure column. A first part of the compressed total air flow, as first air flow at the first air pressure, is cooled and liquefied or pseudo-liquefied in the main heat exchanger, then expanded and introduced into the distillation column system. A second part of the compressed total air flow, as second air flow, is post-compressed in a turbine-driven post-compressor to a second air pressure.

Claims

1. A method for cryogenic separation of air in an air separation plant which has a main air compressor, a main heat exchanger and a distillation column system with a high-pressure column and a low-pressure column, said method comprising: compressing feed air in the main air compressor to a first air pressure, which is at least 3 bar higher than an operating pressure of the high-pressure column, to form a compressed total air flow, splitting, at the first pressure, the compressed total air flow into a first air flow and a second air flow, cooling and liquefying or pseudo-liquefying said first air flow, at the first pressure, in the main heat exchanger, then expanding the first air flow to form an expanded first air flow, and introducing the expanded first air flow into the distillation column system, compressing said second air flow in a first turbine-driven compressor from the first pressure to a second air pressure which is higher than the first air pressure, to form a compressed second air flow, branching off, at the second pressure, a first partial flow of said compressed second air flow to form a third air flow, and introducing said third air flow at the second pressure into a first turbine, where said third air flow is expanded, while performing work, to form an expanded third air flow, and introducing said expanded third air flow into the distillation column system, wherein the first turbine drives the first turbine-driven compressor, producing one or more liquid products in the distillation column system and withdrawing said one or more liquid products from the air separation plant, withdrawing a first product flow in liquid form, from the distillation column system, elevating the pressure of said first product flow in liquid form to a first elevated product pressure, and evaporating or pseudo-evaporating and further heating said first product flow at said first elevated pressure in the main heat exchanger to form a first compressed gas product, withdrawing said first compressed gas product from the air separation plant, forming, at the second pressure, a second partial flow of said compressed second air flow to form a fourth air flow, cooling said fourth air flow in the main heat exchanger at the second pressure to a first intermediate temperature, and further compressing said fourth air flow in a cold compressor to a third air pressure which is higher than the second air pressure, to form a further compressed fourth air flow, cooling and liquefying or pseudo-liquefying said further compressed fourth air flow at the third air pressure in the main heat exchanger, to form a liquefied or pseudo-liquefied fourth air flow, and then expanding said liquefied or pseudo-liquefied fourth air flow to form an expanded fourth air flow, and introducing said expanded fourth air flow into the distillation column system, wherein in a first mode of operation a first quantity of said one or more liquid products is withdrawn from the air separation plant, wherein in the first mode of operation a first ratio of the first quantity of said one or more liquid products to a first quantity of said first compressed gas product is between 20% and 30%, in a second mode of operation a second quantity of said one or more liquid products, which is at least 50% of the first quantity and is less than the first quantity, is withdrawn from the air separation plant, and in both the first mode of operation and the second mode of operation at least a part of one of the following air flows is introduced into the high-pressure column: said expanded first air flow, said expanded third air flow, and said expanded fourth air flow, the fourth air flow, which flows through the cold compressor, has at least one of the following properties: a quantity thereof is greater in the second mode of operation than in the first mode of operation, and a pressure thereof at an outlet of the cold compressor is higher in the second mode of operation than in the first mode of operation.

2. The method according to claim 1, wherein a fifth air flow is branched off from said first air flow, and at the first air pressure and at a second temperature is introduced into a second turbine where said fifth air flow is expanded, performing work, to form an expanded fifth air flow, the second turbine drives a second turbine-driven compressor which is said cold compressor, said expanded fifth air flow is introduced into the distillation column system, wherein in the first mode of operation a first quantity of said fifth air flow is introduced into the second turbine and in the second mode of operation a second quantity of said fifth air flow is introduced into the second turbine, and the first quantity of said fifth air flow is less than the second quantity of said fifth air flow.

3. The method according to claim 2, wherein in the first mode of operation a first quantity of air forms said first air flow and a second quantity of air forms said second air flow, and in the second mode of operation a third quantity of air, which is equal to or less than the first quantity of air, forms the first air flow, and a fourth quantity of air, which is less than the second quantity of air, forms the second airflow.

4. The method according to claim 1, wherein said third airflow is expanded in the first turbine to an outlet pressure which is equal to the operating pressure of the high-pressure column.

5. The method according to claim 2, wherein said fifth air flow is expanded in the second turbine to an outlet pressure which is equal to the operating pressure of the high-pressure column.

6. The method according to claim 2, wherein, in the second mode of operation, the fifth air flow is expanded in the second turbine to an outlet pressure which is equal to an operating pressure of the low-pressure column.

7. The method according to claim 2, wherein at least one part of the expanded fifth air flow is introduced into the high-pressure column.

8. The method according to claim 1, wherein a second product flow is withdrawn in liquid form from the distillation column system, is raised in the liquid state to a second elevated product pressure, is evaporated or pseudo-evaporated and heated in the main heat exchanger, and the heated second product flow is withdrawn from the air separation plant, wherein the first product flow consists of oxygen from a lower region of the low-pressure column and/or the second product flow consists of nitrogen from an upper region of the high-pressure column or from a top condenser of the high-pressure column.

9. The method according to claim 1, wherein said expanded third airflow is introduced into the high-pressure column.

10. The method according to claim 1, wherein said third air pressure is higher in the second mode of operation than in the first mode of operation.

11. The method according to claim 1, wherein, in the first mode of operation, the cold compressor operates with a lower load than in the second mode of operation.

12. The method according to claim 1, wherein in the second mode of operation a second ratio of the second quantity of said one or more liquid products to a second quantity of said first compressed gas product is reduced compared to said first ratio.

13. The method according to claim 2, wherein in the first mode of operation a power of the first turbine is less than 20% of a power of the second turbine and in the second mode of operation a ratio of the power of the first turbine to the power of the second turbine is less than 30%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention, and further details of the invention, are explained in more detail below with reference to an exemplary embodiment of an air separation plant represented schematically in the drawing.

DETAILED DESCRIPTION OF THE INVENTION

(2) The exemplary embodiment of the invention is first explained below with reference to the first mode of operation, which in this case is configured for maximum liquid production. Atmospheric air 1 (AIR) is drawn in, via a filter 2, by a main air compressor 3 and is compressed to a first air pressure of for example 22 bar. Downstream of the main air compressor 3, the compressed total air 4 at the first air pressure is treated in a pre-cooling device 5 and then in a purification device 6. The purified total air 7 is split into a first air flow 100 and a second air flow 200.

(3) The first air flow 100 is cooled in a main heat exchanger 8, from the hot to the cold end, and in that context is (pseudo-)liquefied and then expanded in a throttle valve 101 to approximately the operating pressure of the high-pressure column explained below, which is preferably 5 bar to 7 bar, for example 6 bar. The expanded first air flow 102 is fed, via the line 9, to the distillation column system which has a high-pressure column 10, a main condenser 11, which is designed as a condenser-evaporator, and a low-pressure column 12.

(4) The second air flow 200 is post-compressed in a first turbine-driven post-compressor 202c with aftercooler 203 to a second air pressure of for example 28 bar. The post-compressed second air flow 204 is split into a third air flow 210 and a fourth air flow 230.

(5) The third air flow 210 is fed to the hot end of the main heat exchanger 8 and is removed again at a first intermediate temperature T1. The third air flow is fed, at this intermediate temperature and the second air pressure, to a first turbine 202t where it is expanded, performing work, to the operating pressure of the high-pressure column 10, which is 5 bar to 7 bar, for example 6 bar. The first turbine 202t is mechanically coupled to the first post-compressor 202c. The third air flow 211 which has been expanded so as to perform work is introduced into a separator (phase separator) 212 where a small liquid fraction is removed therefrom. It then flows, in purely gaseous form, via the lines 213 and 13 to the sump of the high-pressure column 10. The turbine inlet pressure is in this case equal to the second air pressure.

(6) In the distillation column system, the sump liquid 15 of the high-pressure column is cooled in a countercurrent subcooler 16 and is fed via line 17 to an argon part 500 which will be explained later. Thence, it flows in part in liquid form (line 18) and in part in gaseous form (line 19) at the low-pressure column pressure back out and is introduced at a suitable point into the low-pressure column 12. (If no argon part is present, the subcooled sump liquid is immediately expanded to low-pressure column pressure and introduced into the low-pressure column.)

(7) At least part of the liquid air guided via line 9 into the high-pressure column 10 is removed again via line 18, also cooled in the countercurrent subcooler 16 and is fed to the low-pressure column 12 via valve 21 and line 22.

(8) A first part 24 of the gaseous overhead nitrogen 23 of the high-pressure column 10 is introduced into the liquefaction space of the main condenser 11 where it is essentially entirely liquefied. A first part 26 of the liquid nitrogen 25 so obtained is given up to the high-pressure column 10 as recirculation. A second part 27 is cooled in the countercurrent subcooler 16 and is fed via valve 28 and line 29 to the top of the low-pressure column 12. In the first mode of operation, part of this is removed again via line 30 and is obtained as liquid nitrogen product (LIN) and is drawn off from the air separation plant.

(9) From the top of the low-pressure column, in which there prevails a pressure of 1.2 bar to 1.6 bar, for example 1.3 bar, gaseous low-pressure nitrogen 31 is removed, is heated in the countercurrent subcooler 16 and in the main heat exchanger 8 and is drawn off via line 32 as gaseous low-pressure product (GAN). Gaseous impure nitrogen 33 from the low-pressure column is also heated in the countercurrent subcooler 16 and the main heat exchanger 8. The hot impure nitrogen 34 can either be vented into the atmosphere (ATM) via line 35 or can be used, via line 36, as regeneration gas in the purification device 6.

(10) Liquid oxygen is drawn off, via line 37, from the sump of the low-pressure column 12 (specifically from the evaporation space of the main condenser 11). As the case may be, a first part 38 is subcooled in the countercurrent subcooler 16 and is obtained via line 39 as liquid oxygen product (LOX) and is drawn off from the air separation plant. A second part 40 forms the “first product flow”, is raised in a pump 41 to a first product pressure of for example 37 bar, is evaporated at this high pressure in the main heat exchanger 16 and is heated to near ambient temperature. The hot high-pressure oxygen 42 is given off as oxygen-rich first compressed gas product (GOX IC).

(11) A further internal compression product can be obtained from a third part 43 of the liquid nitrogen 25 from the main condenser 11. This is raised as “second product flow” in a pump 44 in liquid form to a second product pressure of for example also 37 bar. At this second product pressure, it is evaporated in the main heat exchanger 8 and heated to near ambient temperature. The hot high-pressure nitrogen 45 is then given off at the second product pressure as nitrogen-rich compressed gas product (GAN IC).

(12) A third part 230 of the second air flow 204 forms a “fourth air flow”; this is cooled in the main heat exchanger (8) to a first intermediate temperature (T3), further compressed in a cold compressor (14c) to a third air pressure of for example 40 bar and flows through the main heat exchanger as far as the cold end at this very high pressure. The cold pseudo-liquefied third part 232 is expanded in a throttle valve 233 to the high-pressure column pressure and is fed via the lines 234 and 9 to the high-pressure column 10.

(13) The cold compressor 14c is driven by a second expansion turbine 14t, in which a third partial flow 301 of the compressed total air flow 7, as “fifth air flow”, is expanded so as to perform work from the first air pressure to the operating pressure of the high-pressure column 10. The second turbine has an inlet temperature T2. The fifth air flow 302 expanded so as to perform work is introduced via line 13 into the high-pressure column 10.

(14) If an argon product is required, the air separation plant also has an argon part 500 which functions as described in EP 2447563 A1 and produces a further liquid product in the form of pure liquid argon (LAR) which is drawn off via line 501.

(15) The “first total quantity of liquid products”, which is drawn off from the air separation plant in a first mode of operation, consists in this exemplary embodiment of the flows 30 (LIN), 39 (LOX) and 501 (LAR). In the first mode of operation, the ratio of the total quantity of liquid products (LOX, LIN, LAR) to the quantity of oxygen-rich compressed gas product 42 (GOX IC, “first compressed gas product”) is between 20% and 30%. The power of the turbine 14t is less than 20% of the power of the turbine 202t.

(16) In a second mode of operation, the plant is operated with a reduced “second total quantity of liquid products” and reduced ratio of the total quantity of liquid products (LOX, LIN, LAR) to the quantity of oxygen-rich compressed gas product 42 (GOX IC, “first compressed gas product”). In general, the flow quantity is reduced in at least one of the lines 30 and 39, preferably in both. The argon production is generally not restricted in a targeted manner since in most cases the maximum argon yield is desired. The quantities and pressures of the internal compression products 42, 45 also remain constant.

(17) In a variant of the exemplary embodiment represented here, the two turbine inlet temperatures T1 and T2 can also, within the scope of the invention, be identical.

(18) In the second mode of operation, the turbine powers are shifted, the turbine 14t is run up, in particular to full load and the power of the turbine 202t is reduced. The ratio of the powers of the turbines 14t/202t is for example below 30%.

(19) Moreover, the total quantity of air and the final pressure of the compressor are reduced, such that the main air compressor 3 uses less energy. The internal compression process is, however, improved by the fact that the fourth and fifth partial flows 230, 301 are increased and thus more high-pressure air 232 is available. The quantity of air through the line 100 is less than or equal to that in the first mode of operation. With the reduction in liquid production when transitioning from the first to the second operation case, the load on the second turbine 14t is increased and the load on the first turbine 202t is reduced.

(20) Fundamentally, the described process can occasionally also be operated in stationary fashion, that is to say with constant liquid production. In another application case, it can be expedient in the first mode of operation to entirely shut down the combination of second turbine 14t and cold compressor 14c.

(21) The second turbine 14t can also be formed such that it injects not into the high-pressure column 10 but into the low-pressure column 12; by virtue of the correspondingly raised pressure ratio, more energy can be made available for the cold compressor.

(22) The effect of the invention can be further increased by connecting, downstream of the cold compressor 14c, a second cold compressor which can be switched off. In the second mode of operation, the flow from the first cold compressor 14c is fed through a second cold compressor before it is fed back into the main heat exchanger. The second cold compressor is driven with an electric motor. In the first mode of operation, the second cold compressor is switched off and the flow from the first cold compressor 14c flows via a bypass line past the second cold compressor.