Plant for producing oxygen by cryogenic air separation

Abstract

The plant is used for producing oxygen by cryogenic air separation. The plant has a high-pressure column, a low-pressure column and a main condenser. An argon-elimination column is in fluid connection with an intermediate point of the low-pressure column and is connected to an argon-elimination column head condenser. An auxiliary column has a sump region, into which gas is introduced from the argon-elimination column head condenser. The head of the auxiliary column is connected to a return flow liquid line, in order to introduce a liquid stream from the high-pressure column or the head condenser. The liquid stream has an oxygen content which is at least equal to that of air. At least one part of the crude liquid oxygen from the sump of the high-pressure column is fed to the auxiliary column at a first intermediate point.

Claims

1. A method for producing oxygen by low-temperature separation of air in a distillation column system which comprises a high-pressure column and a low-pressure column, a main condenser which is a condenser evaporator having a liquefaction space and an evaporation space, wherein the liquefaction space of the main condenser is in fluid communication with the top of the high-pressure column and the evaporation space of the main condenser is in fluid communication with the low-pressure column, an argon discharge column which is in fluid communication with an intermediate point on the low-pressure column, an argon discharge column tops condenser which is a condenser-evaporator having a liquefaction space and an evaporation space, wherein the liquefaction space of the argon discharge column tops condenser is in fluid communication with the top of the argon discharge column, an auxiliary column whose bottom region is configured for introduction of gas from the evaporation space of the argon discharge column tops condenser, said method comprising: introducing liquid crude oxygen from the bottom of the high-pressure column into the auxiliary column, introducing a liquid stream from the high-pressure column or the main condenser as reflux into the top of the auxiliary column via a reflux liquid conduit, wherein the liquid stream has a nitrogen content at least equal to that of air, and supplying at least a first portion of the liquid crude oxygen to the auxiliary column at a first intermediate point, wherein the operating pressure at the top the auxiliary column is at least 50 mbar higher than the operating pressure at the top of the low-pressure column.

2. The method as claimed in claim 1, wherein a gaseous fraction is withdrawn from the top of the auxiliary column as a gaseous nitrogen product separately from a gaseous nitrogen product stream withdrawn from the top of the low-pressure column.

3. The method as claimed in claim 1, wherein an additional liquid fraction is introduced into the auxiliary column at a second intermediate point which is arranged above the first intermediate point.

4. The method as claimed in claim 3, wherein the additional liquid fraction is a liquid air fraction.

5. The method as claimed in claim 1, wherein at least a portion of liquid downflowing in the auxiliary column is collected immediately above the column bottom as collected fluid, and at least a portion of the collected liquid is introduced into the low-pressure column.

6. The method as claimed in claim 1, wherein no gas stream is passed from the low-pressure column into the auxiliary column.

7. The method as claimed in claim 1, wherein a second portion of the liquid crude oxygen is supplied to the auxiliary column at the bottom or to the evaporation space of the argon condenser and a third portion of the liquid crude oxygen is supplied to the low-pressure column at an intermediate point.

8. The method as claimed in claim 1, wherein the high-pressure column and the low-pressure column are arranged side by side and the argon discharge tops condenser and the auxiliary column are arranged over the high-pressure column.

9. The method as claimed in claim 1, wherein the argon discharge column and the argon discharge column tops condenser are arranged spatially separate from one another.

10. The method as claimed in claim 1, wherein the argon discharge column is arranged in a dividing wall column region of the low-pressure column.

11. The method as claimed in claim 1, wherein the mass transfer elements in the auxiliary column have an identical or higher specific surface area than those in the low-pressure column.

12. The method as claimed in claim 1, wherein the auxiliary column and the argon discharge column tops condenser are arranged in separate containers.

13. The method as claimed in claim 1, wherein no gas stream and no liquid stream are passed from the low-pressure column into the auxiliary column.

14. A plant for producing oxygen by low-temperature separation of air comprising: a high-pressure column and a low-pressure column, a main condenser which is a condenser evaporator having a liquefaction space and an evaporation space, wherein the liquefaction space of the main condenser is in fluid communication with the top of the high-pressure column and the evaporation space of the main condenser is in fluid communication with the low-pressure column, an argon discharge column which is in fluid communication with an intermediate point on the low-pressure column, an argon discharge column tops condenser which is a condenser-evaporator having a liquefaction space and an evaporation space, wherein the liquefaction space of the argon discharge column tops condenser is in fluid communication with the top of the argon discharge column, an auxiliary column whose bottom region includes an inlet for introduction of gas from the evaporation space of the argon discharge column tops condenser, and via a crude oxygen conduit for introduction of liquid crude oxygen from the bottom of the high-pressure column into the auxiliary column, a reflux liquid conduit for introducing a liquid stream from the high-pressure column or the main condenser as reflux into the top of the auxiliary column, wherein the liquid stream has a nitrogen content which is at least equal to that of air, and the crude oxygen conduit is configured for introducing crude oxygen into the auxiliary column at a first intermediate point, wherein the auxiliary column is configured to operate at a pressure at the top of the auxiliary column that is at least 50 mbar higher than the pressure at the top of the low-pressure column.

15. The plant as claimed in claim 14, further comprising means for obtaining a gaseous tops fraction from the auxiliary column as a gaseous nitrogen product separately from a gaseous tops nitrogen from the low-pressure column.

16. The plant as claimed in claim 14, further comprising a conduit for introduction of an additional liquid fraction into the auxiliary column at a second intermediate point which is arranged above the first intermediate point.

17. The plant as claimed in claim 14, further comprising means for collecting at least a portion of the liquid downflowing in the auxiliary column immediately above the column bottom and means for introducing the collected liquid into the low-pressure column.

18. The plant as claimed in claim 17, wherein the high-pressure column and the low-pressure column are arranged side by side, the argon discharge column is arranged above the low-pressure column and the auxiliary column is arranged next to the combination of the low-pressure column and the argon discharge column and above the high-pressure column above the main condenser.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The inventionand further details of the inventionare more particularly elucidated hereinbelow with reference to two exemplary embodiments depicted in schematic form in the drawings. The drawings depict only the most important elements, in particular those which distinguish the system of the invention from customary air separation systems.

(2) FIG. 1 shows a first exemplary embodiment for a plant according to the first variant of the invention having a double column composed of an auxiliary column and an argon discharge column above the high-pressure column,

(3) FIG. 2 shows a second exemplary embodiment according to the second variant of the invention where the argon discharge column is arranged in a dividing wall column region of the low-pressure column,

(4) FIG. 3 shows a third exemplary embodiment similar to FIG. 1 but with one-above-the-other arrangement of the high-pressure column and the low-pressure column,

(5) FIG. 4 shows a modification of FIG. 3 having a shorter auxiliary column,

(6) FIG. 5 shows the exemplary embodiment of FIG. 3 supplemented with an oxygen column,

(7) FIG. 6 shows a further exemplary embodiment having the auxiliary column over the low-pressure column,

(8) FIG. 7 shows a variant having the auxiliary column over the high-pressure column and the main condenser and

(9) FIG. 8 shows a system similar to FIG. 2 but with the argon condenser arranged in the low-pressure column.

(10) Air compression means, air purification means and main heat exchangers are not shown in the drawings. The representation is also simplified in other respects; some streams which are not relevant to the understanding of the invention are not marked.

DETAILED DESCRIPTION OF THE INVENTION

(11) The plant of the exemplary embodiment in FIG. 1 comprises a high-pressure column 1, a low-pressure column 2 and a main condenser 3. The main condenser 3 is here configured as a multi-level bath evaporator, more particularly as a cascade evaporator. The high-pressure column 1 and the low-pressure column 2 are arranged side by side; in particular their lower ends are situated at the same geodetic level.

(12) A first substream 4 of the feed air flows in gaseous form into the high-pressure column 1 immediately above the column bottom. A second portion 5 of the feed air is at least partly liquid and is supplied to the high-pressure column 1 at an intermediate point. At least a portion of the liquid air is immediately withdrawn again via conduit 6, cooled in a countercurrent subcooler 7 and via the conduits 108 and 108b at least partly supplied to the low-pressure column 2 at a first intermediate point.

(13) In the main condenser 3 a portion 10 of the gaseous tops nitrogen 9 from the high-pressure column 1 is at least partly condensed. A first portion 12 of the thus obtained liquid nitrogen 11 is applied to the top of the high-pressure column 1 as reflux. A second portion 13 is supplied to an internal compression means (not shown) and finally obtained as gaseous compressed nitrogen product. Another portion 14 of the gaseous tops nitrogen 9 is heated in the main heat exchanger (not shown) and obtained directly as gaseous compressed product.

(14) Liquid crude oxygen 15 from the high-pressure column 1 is cooled in the countercurrent subcooler 7 and is supplied via the conduits 16 and 18 and also through an argon discharge column tops condenser 17 to the low-pressure column 2 at a second intermediate point which is situated below the first intermediate point.

(15) Liquid impure nitrogen 35 is withdrawn from an intermediate point on the high-pressure column 1, cooled in the countercurrent subcooler and via conduit 36/136a applied to the top of the low-pressure column 2. A portion thereof may be obtained via conduit 37 as liquid nitrogen product (LIN). Gaseous impure nitrogen 138a is withdrawn from the top of low-pressure column 2 and after heating in the countercurrent subcooler 7 sent on via conduit 39 to the main heat exchanger (not shown).

(16) A first portion 22 of the liquid oxygen 20 from the bottom of the low-pressure column 2 is conveyed using a pump 21 into the evaporation space of the main condenser 3 and at least partially evaporated therein. Gas thus formed 23 is recycled into the bottom of the low-pressure column 2 and serves therein as ascending gas. A second portion 24 of the liquid oxygen 20 is cooled in the countercurrent subcooler 7 and withdrawn via conduit 25 as liquid oxygen product (LOX). A third portion 26 of the liquid oxygen 20 is supplied to an internal compression means (not shown) and finally obtained as gaseous compressed oxygen product which is the primary product of the plant.

(17) An argon discharge column 31 is as usual connected via a gas feed 32 and a liquid return conduit 33 to an intermediate point on the low-pressure column 2. Liquid reflux for the argon discharge column is produced in the liquefaction space of the argon discharge column tops condenser 17. The gaseous residual product 34 is withdrawn from the liquefaction space and heated in the main heat exchanger.

(18) An auxiliary column 140 is situated in the same container as the argon discharge column tops condenser 17 which functions as a bottoms heating means for the auxiliary column and produces ascending vapor therefor. A portion 136b of the subcooled liquid impure nitrogen 36 from the high-pressure column 1 is employed as reflux liquid at the top of the auxiliary column 140.

(19) A portion 108a of the subcooled liquid air 108 may be supplied to the auxiliary column 140 at a second intermediate point. Another portion 108b, along with a stream 141 of turbine-decompressed air 141, is supplied to the low pressure column 2 at the same intermediate point or higher (not shown).

(20) Gaseous impure nitrogen 138b is withdrawn from the top of the auxiliary column 140 and mixed with the gaseous impure nitrogen 138a from the top of the low-pressure column 2. The overall stream 38 after heating in the countercurrent subcooler 7 is sent on via conduit 39 to the main heat exchanger (not shown). Alternatively, the two nitrogen streams 138a, 138b may also be passed to, and through, the main heat exchanger separately.

(21) With the aid of auxiliary column 140 the top section of the low-pressure column is disburdened. Said section can therefore be configured with a lower capacity; conversely for the same dimensions of the low-pressure column the capacity of the plant is a whole can be increased.

(22) In this exemplary embodiment the pressure difference at the column top between the auxiliary column and the low-pressure column is 50 to 150 mbar. Departing from the pictorial representation in FIG. 1, the tops fractions 138a, 138b from the low-pressure column 2 and the auxiliary column 140 may be withdrawn at slightly different pressures, passed through the countercurrent subcooler 7 and supplied to the main heat exchanger (not shown). This also applies to the following exemplary embodiments.

(23) The exemplary embodiment in FIG. 2 differs from that of FIG. 1 in that the argon discharge column 17 is not arranged below the argon discharge column tops condenser 17 but rather in a dividing wall section A2 of the low-pressure column 2. Equivalent elements bear the same reference numerals in both drawings.

(24) FIG. 2 describes three sections of the low-pressure column 2: a lower section A1, a middle section A2 and a top section A3.

(25) The middle section A2 of the low-pressure column 2 is configured as a dividing wall section. A vertical dividing wall 27 separates a first subspace 28 and a second subspace 29 from one another. The dividing wall is formed in the example by a flat piece of sheet metal which is welded to the column wall on both sides. Both subspaces contain mass transfer elements, for example structured packing. The mass transfer layers in the subspaces may, but need not, be of identical height. The two subspaces may be of identical or different sizes.

(26) The first subspace 28 forms the argon section of the low-pressure column 1. It is in fluid communication with the lower section at the bottom and with the upper section at the top. Thus a first portion of the gas can flow from the lower section through the first subspace 28 to the upper section A3. Conversely, liquid flows from the upper section A3 through the first subspace 28 into the lower section A1.

(27) The second subspace 29 forms the argon discharge column 31. Said subspace is likewise in fluid communication with the lower section A1 and a second portion of the gas ascending from the first section A1 can therefore flow in from there. However, said subspace is gas tightly sealed with respect to the upper section A3 with a horizontal wall 30. The horizontal wall has an approximately semicircular configuration and is welded to the column wall and the dividing wall 27. Neither can gas flow from the top of the argon discharge column 31 into the top section A3 nor can liquid from there penetrate into the argon discharge column 31.

(28) At the top of the argon discharge column 31 argon-enriched gas 32 is withdrawn and partly liquefied in the liquefaction space of the argon discharge column tops condenser 17. The thus produced liquid 33 is recycled as reflux into the argon discharge column 31. The proportion remaining in gaseous form is withdrawn from the argon discharge column tops condenser 17 in gaseous form as argon-enriched product or residual gas 34 and passed through the main heat exchanger (not shown) through a separate passage group.

(29) Due to the integration of the argon discharge column 31 into the low-pressure column 2 and due to the arrangement of the argon discharge column tops condenser over the high-pressure column 1, the argon discharge requires no additional setup area compared to the pure nitrogen-oxygen separation. The increase in the oxygen yield can accordingly be achieved without any appreciable enlargement of the plant.

(30) In addition, the exemplary embodiment in FIG. 2 comprises a cup 150 in the auxiliary column 140 and a conduit 151. The liquid downflowing in the auxiliary column 140 is collected in the cup 150 above the argon discharge column tops condenser completely, partly or not at all. The collected liquid is partly or completely introduced into the low-pressure column 2 via the conduit 151, preferably above the conduit 18. This avoids mixing of this liquid with the liquid crude oxygen 16 from the high-pressure column 1/the unevaporated liquid from the evaporation space of the argon discharge column tops condenser 17. Advantageous control of the argon discharge column tops condenser is also possible.

(31) The cup 150 and the conduit 151 may also be employed in all other exemplary embodiments. Instead of the cup, any other collecting device for liquid may be used. For example, the liquid may be collected in a chimney tray or withdrawn from a rectifying tray or its downcomer.

(32) In FIG. 3 the high-pressure column 1, main condenser 3 and low-pressure column 2 are arranged one above the other in the form of a conventional double column. The auxiliary column 140, argon discharge column tops condenser 17 and argon discharge column 31 likewise form a double column similarly to FIG. 1. However, said column is not arranged above the high-pressure column 1 but rather next to the double column composed of the high-pressure column 1 and the low-pressure column 2, for example on a scaffold.

(33) In addition, not the entirety of the crude oxygen 16 is passed from the bottom of the high-pressure column 1 into the evaporation space of the argon discharge column tops condenser, but rather, via conduit 16b, only a portion. Another portion passes directly via conduit 16a directly into the low-pressure column 2, the remainder via conduit 16c to a first intermediate point on auxiliary column 140.

(34) In FIG. 4 the auxiliary column 140 is slightly shorter than in FIG. 3, the tops reflux is here formed by liquid air 108. This is applied via the reflux liquid conduit 408b to the top of the auxiliary column 140.

(35) In FIG. 5 the argon discharge column is effectively extended downward compared to FIG. 3. Situated in the same container as the argon discharge column 31 is an oxygen column 336 in the form of an additional distillation section. The lower end of the oxygen column 336 communicates via the gas conduit 332 and the liquid conduit 333 with the low-pressure column 2 immediately above the bottom thereof.

(36) The top of the oxygen column 336 receives reflux liquid from the conduit 33 and/or via at least a portion of the liquid effluxing from the argon discharge column 31. The capacity of the oxygen column 36 may be adjusted with the two conduits 32, 33. If the liquid conduit 33 is closed (or is omitted), the capacity is precisely distributed between the two columns such that the conversion of the oxygen column 336 is equal to the conversion of the argon discharge column 31. If more capacity is to be shifted into the oxygen column 336, liquid is transportedcounter to the flow direction marked in FIG. 1from the low-pressure column 2 into the oxygen column 36 via the liquid conduit 33. This additional capacity is withdrawn from the oxygen column 336 below the argon discharge column 31 and supplied to the low-pressure column 2 as the corresponding gas amount.

(37) FIG. 5 also depicts with dashed lines two bypass conduits 501, 502 which make it possible to shut down the argon discharge column tops condenser 17 and continue to operate the rest of the plant. Conduit 501 then passes the liquid from the bath of the argon discharge column tops condenser 17 to the top of the argon discharge column 31. In countercurrent, via conduit 502, the tops steam from the argon discharge column 31 is passed into the auxiliary column 140. This feature may be combined with all other exemplary embodiments.

(38) The plant depicted in FIG. 6 comprises an entry filter 302 for atmospheric air (AIR), a main air compressor 303, an air pre-cooling unit 304, and air purification unit 305 (typically formed by a pair of molecular sieve adsorbers), a three-stage, intermediately cooled and post-cooled booster air compressor 306 (BAC) and a main heat exchanger 308. A first substream 4 of the feed air flows in gaseous form into the high-pressure column 1 immediately above the column bottom. A second portion 5 of the feed air is at least partly liquid and is supplied to the high-pressure column 1 at an intermediate point. At least a portion of the liquid air is immediately withdrawn again via conduit 6, cooled in a countercurrent subcooler 7 and via the conduits 108 and 108b at least partly supplied to the low-pressure column 2 at a first intermediate point.

(39) In the main condenser 3 a portion 10 of the gaseous tops nitrogen 9 from the high-pressure column 1 is at least partly condensed. A first portion 12 of the thus obtained liquid nitrogen 11 is applied to the top of the high-pressure column 1 as reflux. A second portion 13 is supplied to an internal compression means (pump 313) and finally obtained as gaseous compressed nitrogen product. Another portion 14 of the gaseous tops nitrogen 9 is internally compressed (pump 621), heated in the main heat exchanger 308 and obtained directly as gaseous compressed product (GANIC).

(40) Liquid crude oxygen 15 from the high-pressure column 1 is cooled in the countercurrent subcooler 7, sent on via conduit 16 and then via the conduits 18a, 18b, 18c divided among the argon discharge column tops condenser 17, the low-pressure column 2 and the auxiliary column 140, supplied at a second intermediate point which is situated below the first intermediate point.

(41) Liquid impure nitrogen 35 is withdrawn from an intermediate point on the high-pressure column 1, cooled in the countercurrent subcooler and via the conduits 36 and 136a/136b applied to the top of the low-pressure column 2 to the top of auxiliary column 140. A first stream of gaseous impure nitrogen 138a is withdrawn from the top of the low-pressure column 2 and after heating in the countercurrent subcooler 7 via conduit 39. After heating main heat exchanger (308), this stream is blown off to the atmosphere (ATM).

(42) A first portion 22 of the liquid oxygen 20 from the bottom of the low-pressure column 2 is conveyed using a pump 21 into the evaporation space of the main condenser 3 and at least partially evaporated therein. Gas thus formed 23 is recycled into the bottom of the low-pressure column 2 and serves therein as ascending gas. A second portion 24 of the liquid oxygen 20 is cooled in the countercurrent subcooler 7 and withdrawn via conduit 25 as liquid oxygen product (LOX). A third portion 26 of the liquid oxygen 20 is internally compressed, i.e. brought to the desired product pressure by means of a pump 321, heated in the main heat exchanger 308 and finally obtained as gaseous pressurized oxygen product (EOXIC) which is the primary product of the plant.

(43) The argon discharge column 31 is as usual connected via a gas feed 32 and a liquid return conduit 33 to an intermediate point on the low-pressure column 2. Liquid reflux for the argon discharge column is produced in the liquefaction space of the argon discharge column tops condenser 17. The gaseous residual product 34, 334 is withdrawn from the liquefaction space, heated in the main heat exchanger 308 and finally released to the atmosphere (ATM); it could alternatively be obtained as an argon-enriched product.

(44) The auxiliary column 140 and the argon discharge column tops condenser 17 are situated in separate containers. However, the gas conduit 61 ensuresas in the preceding exemplary embodimentsthat gas produced in the evaporation space of the argon discharge column tops condenser 17 continues to be introduced into the bottom of the auxiliary column 140 and is available there as ascending vapor. Liquid generated in the bottom of the auxiliary column 140 is supplied to the low-pressure column 2 at a suitable intermediate point via a liquid conduit 62. A portion 136b of the subcooled liquid impure nitrogen 36 from the high-pressure column 1 is employed as reflux liquid at the top of the auxiliary column 140.

(45) A portion 108a of the subcooled liquid air 108 may be supplied to the auxiliary column 140 at an intermediate point. From the top of the auxiliary column 140 a second stream of gaseous impure nitrogen 138b is withdrawn at a slightly higher pressure than the stream 138a, heated separately from the first stream 138a in countercurrent subcooler 7 and main heat exchanger 308 and via conduit 638 at least partly/at least intermittently employed as regeneration gas in the air purification unit 305.

(46) In all exemplary embodiments the gas conduit 32 and the liquid conduit 33 between the low-pressure column and the argon discharge column may also be combined in a single conduit having a particularly large cross section. Furthermore, the low pressure column may be supplemented by an additional nitrogen section which receives a dedicated reflux, preferably liquid nitrogen from the high-pressure column or from the main condenser. Alternatively, the auxiliary column may also produce purer nitrogen than the low-pressure column when the auxiliary column receives reflux from a purer part of the high-pressure column. Furthermore, individual elements, a plurality of elements or all elements such as the air compression, the air pre-cooling, the air purification, the interconnection of the main heat exchanger and the turbines and the management of the impure nitrogen products from FIG. 6 may each be combined with other exemplary embodiments.

(47) In terms of process engineering, FIG. 7 corresponds largely to FIG. 6, though the argon discharge column 31 and the auxiliary column 140 are interchanged here. The auxiliary column stands above the high-pressure column 1 and the main condenser 3, the argon discharge column 31 is arranged above the low-pressure column 2. In addition, a nitrogen compressor 777 is also provided here in order to further increase product pressure of the gaseous nitrogen 14, 714 with respect to the high-pressure column pressure.

(48) FIG. 8 depicts a system similar to that of FIG. 3. In particular, the low-pressure column 2 contains a dividing wall section 253. In contrast to FIG. 2 the argon condenser 17 is incorporated in the low-pressure column and is not configured as a simple bath evaporator but rather as a bilevel pocket evaporator (also known as a cascade evaporator). The bottom of the auxiliary column 140 is in fluid communication with the evaporation space of the argon condenser 17 via a gas conduit 237 and a liquid conduit 238. Departing from the pictorial representation in FIG. 8, the tops fractions 138a, 138b from the low-pressure column 2 and the auxiliary column 140 are withdrawn at slightly different pressures, passed through the countercurrent subcooler 7 separately and supplied to the main heat exchanger (not shown) separately.