PROCESS AND APPARATUS FOR CRYOGENIC SEPARATION OF AIR WITH MIXED GAS TURBINE

Abstract

In this process and apparatus for cryogenic separation of air, the separation column system comprises a high-pressure column, a low-pressure column and a crude argon column. A mixed gas stream produced by mixing gaseous oxygen and a gas stream from the evaporation space of the argon top condenser, is work expanded in a mixed gas turbine.

Description

The invention concerns a process for cryogenic separation of air and a respective apparatus according to the first parts of the independent patent claims.

Cryogenic separation of air generating gaseous and liquid products is generally known, e.g., from H.-W. Haring, Industrial Gases Processing, Wiley-VCH, 2006, in particular section 2.2.5, “Cryogenic Rectification”.

Cryogenic air separation units classically comprise separation column systems in form of two-column systems, in particular Linde double columns. The may also have the form of three- or more-column systems. In addition to those columns for oxygen-nitrogen separation for generation nitrogen and/or oxygen in liquid and/or gaseous form, the separation column system may comprise additional columns for recovering further air components, in particular noble gases, or for producing particular high-purity oxygen and/or nitrogen products.

In the invention, a high-pressure column and a low-pressure column, which may be at least partially located above the high-pressure column and a main condenser, are used. The process of the invention is of the enhanced-pressure type, so that the high-pressure column is not operated at the classical pressure of about 5.3 bar (4 to 7 bar), but at a higher pressure of e.g. 8 to 14 bar, preferably 9 to 13 bar. The low-pressure column is not operated at the classical pressure of about 1.3 bar (1.2 to 1.5 bar), but at a higher pressure of e.g. 2 to 5 bar, preferably 2.5 to 4.5 bar. Those pressures are absolute and to be measured at the top of the respective column and also used in the invention.

If an air separator produces pressurized gaseous products, they may be compressed in a gas compressor (“external compression”). Alternatively, an “internal compression” process may be used by withdrawing cryogenic liquid from the column, pressurizing (e.g. pumping) is to the desired pressure and transferring it to the gaseous state by warming, e.g. in the main heat exchanger.

The invention has the object to find a further improved air separation process, in particular for co-production of pressurized nitrogen and argon and a relatively high liquid production, e.g. a liquid production (LIN equivalent [Nm3/h]: LIN [Nm3/h]+1.07×LOX [Nm3/h]+0.9×LAR [Nm3/h]) divided by pressurized GAN product amount is in a range from 0.00 to 0.06. (All those amounts are molar in this application, as long as nothing to the contrary is said.)

Such object is solved by the process and the apparatus according the independent patent claims.

The fraction withdrawn from the high-pressure column and introduced into the low-pressure column is frequently the bottom fraction of the high-pressure column. At least a part of it can be directly introduced into the low-pressure column, eventually through a subcooler, or that can be done indirectly by leading the high-pressure column fraction into the argon top condenser evaporation space and separately introducing gas and remaining liquid from the argon top condenser evaporation space to the low-pressure column.

The gaseous oxygen stream is normally withdrawn from a lower portion of the low-pressure column, e.g. from the very bottom of the low-pressure column.

The expansion machine may be of any type, e.g. a turbine; then it may be called a “mixed gas turbine”.

For enhancing of the rectification in the low-pressure column, a nitrogen recycle coming from the low-pressure column and directly or indirectly leading into the separation column system, in particular into the high-pressure column and/or into the low-pressure column may be used as described in claim 2. The “recycle gas” stream is the one coming from the low-pressure column, being compressed in the nitrogen compressor and afterwards being cooled, but not liquefied in the main heat exchanger. A product gas from the low-pressure column may or may not be lead jointly with the recycle gas through the warming in the main heat exchanger and the compression in the nitrogen compressor. The cooled recycle gas may at least partially be directly in gaseous form led into the high-pressure column, e.g. at the top or at 3 to 11 theoretical trays below. An alternative is an indirect introduction into the high-pressure column and/or the low-pressure column, e.g. by liquefying the recycle gas in a condenser, e.g. the main condenser and/or another column reboiler, and then introducing at least a portion of the liquefied recycle gas into a column, in particular the high-pressure column and/or the low-pressure column. In a first example, at least a portion of the cooled recycle gas is introduced into the high-pressure column via the liquefaction space of the main condenser. In another example, at least a portion of the cooled recycle gas is introduced into the low-pressure column via the liquefaction space of the bottom condenser of a pure oxygen column (preferably including subcooling this liquid in a separate channel in a subcooler and expansing the subcooled liquid in an expansion valve). For example, a first portion of the recycle gas is led via a first path (e.g. directly or via the main condenser) into the high-pressure column and a second portion of the recycle gas is led via a second path (e.g. through the bottom condenser of a pure oxygen column) into the low-pressure column.

In a first variant, cooled recycle gas may be directly introduced into the high-pressure column, e.g. at its top. Claim 3 describes a second variant, where recycle gas is introduced into the main condenser, liquefied therein and then introduced as liquid onto the top of the high-pressure column. Both variants can be combined by introducing one portion of the cold recycle gas into the main condenser, another portion directly into the column. Another portion of the recycle gas may be used at a different place in the plant.

A pressurized pure argon product may be generated by internal compression as lined out in claim 4. A part of the total argon product can be produced in a liquid form and stored in the tank.

The crude argon column may have the form of a split column as described in claim 5. There are at least two parts. In principle, there can be three or more parts.

The separation may further comprise a pure oxygen column as described in claim 6. The feed liquid for the pure oxygen column comes from the bottom of the crude argon column or from an intermediate point of the crude argon column, e.g. a few theoretical trays above the bottom.

Such pure oxygen column is preferably pure oxygen column is arranged below the first part of the crude argon column and within a common vessel with the first part of the crude argon column.

The pure oxygen column preferably has a bottom reboiler as described in claim 8, which may be heated high-pressure column gaseous nitrogen and/or by a portion of the cooled recycle gas, which is not directly going into the high-pressure column—see claim 9. The recycle gas is preferably at least partially liquefied in the bottom reboiler of the pure oxygen column and then sent to the high-pressure column or to the low-pressure column as reflux liquid.

In an operation mode, in which not the entire argon product is needed, an argon-oxygen mixture may be withdrawn from the crude argon column via an intermediate gas outlet according to claim 10. That feature reduces the load of the crude argon column. The argon-oxygen mixture is warmed in the main heat exchanger in order to recover its energy.

This particular embodiment is applicable to a one-part crude argon column as well as to a split crude argon column. In the last case, the intermediate gas outlet may be in either part of the crude argon column. Preferably it is arranged at an intermediate height of the second part.

In the invention, it may be advantageous to use a split low-pressure column as described in claim 11.

In a variant of a process, there is preferably not recycle gas and top gas of the high-pressure column (12) is withdrawn (302) as a pressurized gaseous nitrogen product as lined out in claim 13. Alternatively or additionally, top gas (64, 65) from the low-pressure column (13, 113/213) is compressed in a nitrogen compressor and withdrawn as a pressurized gaseous nitrogen product, in particular by admixing it the warmed top gas from the high-pressure column (12). The nitrogen compressor preferably does not compress further streams, in particular no recycle gas.

The invention and further details of the invention will be illustrated in the following by exemplary embodiments, which are shown in the drawings.

FIG. 1 a first embodiment of the invention with single-part low-pressure column,

FIG. 2 a second embodiment with split low-pressure column and

FIG. 3 a third embodiment with partial withdrawal of GAN product from top of the high-pressure column

In the embodiment of FIG. 1, atmospheric air (AIR) 1 flows through a filter 2 to a main air compressor 3 and is compressed therein to a pressure of about 11 to 12 bar. The compressed air stream is cooled in coolers 4 and 5 and the sent to separator 6, from where liquid water (H2O) is discharged. The air from separator 6 is sent to purification unit 7 removing water vapour, carbon dioxide and further impurities by adsorption. The purified air 8 is introduced into a main heat exchanger 9. The total feed air is fully cooled until the cold end of the main heat exchanger 9 and then introduced into the high-pressure column 12 of a double column further comprising a low-pressure column 13 and a main condenser 14.

The separation column system of the embodiment of FIG. 1 consists of the double column 12/13, a pure oxygen column 16, a methane rejection column 17, a single-part crude argon column 18 and a pure argon column 19. The pure oxygen column has a bottom reboiler 20, the crude argon column a top condenser 21 and the pure argon column a top condenser 22 and a bottom reboiler 23. All these condensers and reboilers as well as the main condenser 14 are condenser-evaporators, each having a liquefaction space and an evaporation space. An exception is the bottom reboiler 23 of the pure argon column 19, which is warmed by sensible heat.

The crude liquid oxygen 24 from the bottom of the high-pressure column 12 is cooled in a subcooler 25. A first portion 27 of the cooled crude liquid oxygen 26 is partially fed through the bottom reboiler 23 of the pure argon column and then introduced into the evaporation space of the top condenser 21 of the crude argon column 18. The remaining liquid 28 is sent to the low-pressure column 13. A first part 30 of the evaporated portion 29 is sent to the low-pressure column as well. A second part 31 is taken as “the stream having a higher nitrogen content” 31 according to the invention and is described in detail later.

A second portion 32 of the cooled crude liquid oxygen 26 is introduced into the evaporation space of the top condenser of the pure argon column 19. Remaining liquid 33 is sent to the low-pressure column 13. The evaporated portion 34 is mixed to the evaporated portion 29 from the evaporation space of the top condenser 21 of the crude argon column 18. Thereby is goes to the low-pressure column 13 or into the “the stream having a higher nitrogen content” 31.

Most 36 of the gaseous nitrogen 35 from the top of the high-pressure column 12 is at least partially liquefied in the main condenser 14. The remainder 37 in at least partially liquefied in the bottom reboiler of the pure oxygen column. The liquid nitrogen from the pure oxygen column bottom reboiler is cooled in the subcooler 25. The cooled liquid nitrogen 39 is sent to the top of the low-pressure column 13.

The liquid nitrogen 40 from the main condenser 14 is partially fed back to the top of the high-pressure column 12. Another portion 42 is cooled in the subcooler 25. A first part of the cooled liquid nitrogen 43 is sent to the top of the low-pressure column 13, whilst a second part 45 is withdrawn as pure liquid nitrogen product (PLIN).

A gaseous argon-containing fraction, the argon transition fraction 46, from the low-pressure column 13 is introduced into the bottom of the methane rejection column 17. In the other direction, the bottom liquid 47 of the methane rejection column 17 is reintroduced into the low-pressure column 13. Such bottom liquid contain practically all methane from fraction 46, so that the top of the methane rejection column 17 is methane-free. The top gas 48 of such column is sent to the bottom of the crude argon column 18, together with the top gas 80 from the pure oxygen column 16.

The bottom liquid 78 of the crude argon column 18 is lifted via pump 79. A first portion 49 enters the pure oxygen column 16 as methane-free reflux. From the bottom of the pure oxygen column 16, ultra-high purity liquid oxygen 50 is withdrawn and led into a storage tank 51. The tank liquid may be pressurized in the tank or downstream the tank by a pump (not shown). The high-pressure liquid oxygen can be warmed in the main heat exchanger 9 and be recovered as an internally compressed ultra-high purity gaseous oxygen product (GOXIC).

A second portion 52 of bottom liquid 78 of the crude argon column 18 is fed into the top of the methane rejection column 17.

The liquefaction space of the top condenser 21 of the crude argon column 18 is a bath type condenser. At its top, a crude argon stream 58 is withdrawn from the crude argon column 18 and introduced into the pure argon column 19. From the top of the pure argon column, a waste gas 60 is withdrawn and released to the atmosphere (ATM). At the bottom, a pure argon product 59 is recovered and the sent to an internal compression with pump 61 and (line 62) warming in main heat exchanger 9. At the warm end of the main heat exchanger 9 (line 63), an internally compressed gaseous argon product is (GARIC) withdrawn in pressurized form.

The gaseous nitrogen fraction 64 from the top of the low-pressure column 13 is partially used as a recycle gas and first pre-warmed in subcooler 25. The pre-warmed gaseous nitrogen fraction 65 is sent to the cold end of the main heat exchanger 9 and fully warmed therein. The warmed gaseous nitrogen fraction 66 is compressed in a nitrogen compressor 67 to a product pressure of preferably 8 to 15 bar, more preferably 9.5 to 12.5 bar. The compressor 67 has an aftercooler. The compressed nitrogen fraction 68 is split into a product fraction 69, which is withdrawn as pressurized gaseous nitrogen product (PGAN) and the recycle gas 70. The pressurized recycle gas is fully cooled again in the main heat exchanger 9. The cooled recycle gas (89) is mixed with the gaseous nitrogen 35 from the top of the high-pressure column 12, i.e. liquefied either in the main condenser 14 or in the pure oxygen column bottom reboiler 20. Thereby, a portion of the recycle gas (now as liquid) enters the high-pressure column via line 41.

Pressurized gaseous oxygen is produced by internal compression. Liquid oxygen 84 from the bottom of the low-pressure column 13 (or from the evaporation space of the main condenser 14) is pumped in pump 85 to the desired product pressure, fully warmed in the main heat exchanger 9 and finally recovered via line 86 as internally compressed product (GOXIC).

The previously mentioned “the stream having a higher nitrogen content” 31, which comes at least partially from the evaporation space of the top condenser 21 of the crude argon column 18 is warmed in the subcooler 25. The warmed stream 71 is mixed with a gaseous oxygen stream 72 from the bottom of the low-pressure column 13. The mixed gas 73 is partially warmed in the main heat exchanger 9 to an intermediate temperature of 150 to 230 K and work-expanded in a mixed-gas turbine 75, which is operated as a generator turbine. The expanded mixed gas 76 is reintroduced into the main heat exchanger 9 and fully warmed. The warmed low-pressure mixed gas 77/78 can be released to the atmosphere (ATM) or sent to the purification unit 7 as regeneration gas.

In the embodiment of FIG. 1, some of the gas rising in the crude argon column 18 may be withdrawn via an intermediate gas outlet 81, in order to reduce the amount of argon product 59/62/63 and thereby reducing energy consumption. The gas withdrawn gas 82 is fully warmed in a separate passage of the main heat exchanger 9. The warmed gas 83 may be admixed to the expanded mixed gas 77 and either released to the atmosphere or used as regeneration gas in the purification unit 7.

The process of FIG. 2 mainly differs from FIG. 1 by a split argon column and a split low-pressure column. The explanations on FIG. 1 above are also valid for the respective steps and units of FIG. 2. Reference numbers in FIG. 2 are partially taken from FIG. 1 in order to identify the same or similar features and functions.

The crude argon column is split into a first part 118 and a second part 218, the argon top condenser 21 being arranged on the top of the second part 218. A gas fraction 190 from the top of the first part 118 is introduced into the bottom of the second part 218. At least a first portion 193 of the bottom liquid 191 of the second part 218 is introduced into the top of the first part 118.

The low-pressure column is split into a bottom part 113 and a top part 213. Different from a one-part low-pressure column, those two parts are arranged side by side. A gaseous connection stream 195 is taken from the top gas 194 of the bottom section and introduced into the bottom of the top section 213. A liquid connection stream 196 is withdrawn from the bottom of the top section 213 and sent to the top of the bottom section 213 via the bottom of the first part 118 of the crude argon column, line 197, pump 198 and line 199. Another portion of the top gas 194 of the bottom section 113 of the low-pressure column is taken as argon transition fraction 46 and introduced into the bottom of the first part 118 of the crude argon column. The bottom liquid of the first part 118 (mixed with the bottom liquid 196 from the top part 213 of the low-pressure column) is sent via line 197, pump 198 and line 199 to the top of the bottom part 113 of the low-pressure column.

The lowermost section 117 of the first part 118 of the crude argon column simultaneously acts as methane rejection column. At an intermediate height immediately above the lowermost section 117, the first part 118 is connected to the top of a pure oxygen column 16 by a liquid line 149 and a gas line 180.

Ultra-high purity liquid oxygen 50 from the bottom of the pure oxygen column 16 is pressurized in this particular embodiment in a multiple tank system 200 according to U.S. Ser. No. 10/209,004 B2 and then (via line 201) fully warmed in the main heat exchanger 9. The warm ultra-high purity oxygen gas 202 is recovered as a final product (UHPGOX).

The liquid oxygen 84 from the bottom of the low-pressure column 113 (or from the evaporation space of the main condenser 14) is subcooled in subcooler 25 (not shown) and then withdrawn as a liquid oxygen product (LOX).

The cooled recycle gas 89 is fed to the liquefaction space of the main condenser 14 (together with some of the top nitrogen 35 from the high-pressure column 12). There it is liquefied. A first portion 41 of the liquefied recycle gas is fed into the top of the high-pressure column 12; a second portion 42, 44 of the liquefied recycle gas is fed into the top of the low-pressure column 213.

Alternatively, the cooled recycle gas 89 could be split into a first portion to the main condenser and a second portion, which is introduced into the liquefaction space of the bottom reboiler of the pure oxygen column 16. In another alternative, the recycle gas is completely fed to the liquefaction space of the bottom reboiler of the pure oxygen column 16, if necessary supplemented by some gaseous nitrogen 35 from the top of the high-pressure column 12.

FIG. 3 is in many parts similar or identical to FIG. 2, but deviates in two main aspects:

• • Gaseous nitrogen is from the top of the high-pressure column and withdrawn as a pressurized gaseous nitrogen product (UHPGAN) via lines 300, 301 and 302.
  • There is no recycle gas. All of the top gaseous nitrogen 64/65/66/369 from the low-pressure column 213 is withdrawn as a pressurized gaseous nitrogen product (UHPGAN) downstream the nitrogen compressor 67 by admixing it to the nitrogen from the high-pressure column 12.

The invention in general can be applied as well to systems without a methane rejection column and/or without a pure oxygen column.

Claims

1. Process for cryogenic separation of air in a separation column system comprising a high-pressure column, a low-pressure column, a main condenser, which is a condenser-evaporator having a liquefaction space and an evaporation space and brings high-pressure column top and low-pressure column bottom in heat-exchange relationship, and a crude argon column having an argon top condenser, which is a condenser-evaporator having a liquefaction space and an evaporation space, comprising compressing a total feed air stream,cooling the compressed feed air in a main heat exchanger,introducing at least a portion of the feed air into the high-pressure column,introducing at least one fraction from the high-pressure column directly or indirectly to the low-pressure column,introducing an argon transition fraction from the low-pressure column to the crude argon column,introducing a liquid cooling fraction from the high-pressure column into the evaporation space of the argon top condenser,withdrawing a gaseous oxygen stream from the low-pressure column,mixing the gaseous oxygen stream with another gas stream having a higher nitrogen content than the gaseous oxygen stream to form a mixed gas stream,warming the mixed gas stream in the main heat exchanger,work expanding the warmed mixed gas stream in an expansion machine andcompletely warming the expanded mixed gas stream in the main heat exchanger, whereinthe above stream having a higher nitrogen content is withdrawn from the evaporation space of the argon top condenser.

2. Process according to claim 1, wherein a gaseous nitrogen fraction from the low-pressure column is used as a recycle gas,the recycle gas is warmed in the main heat exchanger,the warmed recycle gas is compressed in a nitrogen compressor,the compressed recycle gas is cooled in the main heat exchanger, and withdrawn from the main heat exchanger in gaseous form andat least a first portion of the cooled recycle gas is introduced either in gaseous form or in liquefied form into the separation column system, in particular into the high-pressure column and/or the low-pressure column.

3. Process according to claim 2, wherein at least a portion of the cooled recycle gas is introduced into the high-pressure column via the liquefaction space of the main condenser.

4. Process according to claim 1, wherein the separation column system further comprises a pure argon column,a crude argon stream is withdrawn from the crude argon column or the argon top condenser,the crude argon stream is introduced into the pure argon column,a liquid pure argon stream is withdrawn from the pure argon column,the liquid pure argon stream is pressurized in liquid state,the pressurized pure argon stream is warmed in the main heat exchanger andfinally recovered as a pressurized argon product.

5. Process according to claim 1, wherein the crude argon column is split into a first part and a second part, the argon top condenser being arranged on the top of the second part, whereby a gas fraction from the top of the first part is introduced into the bottom of the second part and at least a first portion of the bottom liquid of the second part is introduced into the top of the first part.

6. Process according to claim 5, wherein the separation column system further comprises a pure oxygen column,a liquid fraction from the crude argon column is introduced into the top of the pure oxygen column anda liquid pure oxygen fraction is withdrawn from the bottom of the pure oxygen column.

7. Process according to claim 6, wherein the pure oxygen column is arranged immediately below a methane rejection column having just a single bottom/top plate between each other.

8. Process according to claim 6, wherein the pure oxygen column has a bottom reboiler, which is a condenser-evaporator having a liquefaction space and an evaporation space.

9. Process according to claim 2, wherein a second portion of the cooled recycle gas is introduced into the liquefaction space of the pure oxygen column bottom reboiler.

10. Process according to claim 1, wherein, at least temporarily, an argon-oxygen mixture is withdrawn from the crude argon column via an intermediate gas outlet and the argon-oxygen mixture is warmed in the main heat exchanger.

11. Process according to claim 1, wherein the low-pressure column is split into a bottom part and a top part,a gaseous connection stream is withdrawn from the top of the bottom sectionthe gaseous connection stream is introduced into the bottom of the top section,a liquid connection stream is withdrawn from the bottom of the top section andthe liquid connection stream is introduced into the top of the bottom section.

12. Process according to claim 1, wherein a portion of the top gas of the high-pressure column is warmed in the main heat exchanger andthe warmed gas is withdrawn as a pressurized gaseous nitrogen product.

13. Process according to claim 1, characterized in that a top gas from the low-pressure column is warmed in the main heat exchanger,the warmed gas is compressed in a nitrogen compressor andthe compressed gas is withdrawn as a pressurized gaseous nitrogen product, in particular by admixing it the warmed top gas from the high-pressure column.

14. Process according to claim 1, characterized in that the cooled recycle gas is introduced into the high-pressure column in gaseous form.

15. Apparatus for cryogenic separation of air comprising a separation column system comprising a high-pressure column, a low-pressure column, a main condenser, which is a condenser-evaporator having a liquefaction space and an evaporation space and is configured to bring high-pressure column top and low-pressure column bottom in heat-exchange relationship, and a crude argon column having an argon top condenser, which is a condenser-evaporator having a liquefaction space and an evaporation space, and further comprising a main air compressor for compressing a total feed air stream,a main heat exchanger for cooling the compressed feed air,a main air compressor for compressing a total feed air stream,a main heat exchanger for cooling the compressed feed air,means for introducing at least a portion of the feed air into the high-pressure column,means introducing at least one fraction from the high-pressure column directly or indirectly to the low-pressure column,an argon transition line for introducing an argon transition fraction from the low-pressure column to the crude argon column,means for introducing a liquid cooling fraction from the high-pressure column into the evaporation space of the argon top condenser,means for withdrawing a gaseous oxygen stream from the low-pressure column,means for mixing the gaseous oxygen stream with another gas stream having a higher nitrogen content than the gaseous oxygen stream to form a mixed gas stream,means for introducing the mixed gas stream into the main heat exchanger for warming,an expansion machine for work expanding the warmed mixed gas stream andmeans for completely warming the expanded mixed gas stream in the main heat exchanger,whereinmeans for mixing the gaseous oxygen stream with another gas stream having a higher nitrogen content are connected to the evaporation space of the argon top condenser.