Air separation method and apparatus

Abstract: A compressed and purified air stream is divided into first and second subsidiary streams 22 and 24 respectively. The first subsidiary stream is rectified within a double column air separation unit 30 comprising higher and lower pressure rectification columns 32 and 34 respectively. A liquid oxygen stream 58 is pumped from the lower pressure column 34 to a desired above-atmospheric delivery pressure. At the same time, the second subsidiary stream is expanded to substantially the delivery pressure by an expansion turbine 66 and the two streams are then counter-currently added to a mixing column 70 to vaporize the liquid stream and thereby produce a product oxygen stream at the delivery pressure. Liquid refrigerant streams 80 and 82 are withdrawn from the mixing column 70 and added to the lower pressure column 34 to refrigerate the process. The liquid refrigerant streams 80 and 82 increase the liquid to vapour ratio within the lower pressure column to enable enhanced production of liquid products and/or enhanced recovery of oxygen to be achieved.

Description: The present invention relates to an air separation method and apparatus for producing a gaseous oxygen product typically at an above-atmospheric delivery pressure. A variety of industrial processes require gaseous oxygen to be produced at an above-atmospheric delivery pressure. Such industrial processes include steel-making and glassmaking. Typically, air after having been filtered is compressed, purified and then cooled to a temperature suitable for its separation by rectification at cryogenic temperatures. The cooled air is introduced into an air separation unit that has higher and lower pressure columns connected to one another in a heat transfer relationship by means of a condenser/reboiler located within the lower pressure column. The air separates within the higher pressure column to produce a nitrogen-rich fraction and a liquid oxygen-enriched fraction, referred to herein as crude oxygen. The crude oxygen is separated within the lower pressure column to produce nitrogen at the top of the column and liquid oxygen at the bottom. A stream of the liquid oxygen is pumped to the delivery pressure and vaporized. The advantage of pumping is that a compressor does not have to be used to pressurize the oxygen product stream. Vaporization of the pumped liquid oxygen can be effected by direct heat exchange between the pumped liquid oxygen and a higher volatility stream within a mixing column. In operation of a mixing column, a less volatile stream is introduced in liquid state at the top thereof, and a more volatile vaporous stream is caused to ascend the mixing column from the bottom thereof. The descending liquid phase and ascending vapour phase are intimately contacted in the mixing column with the result that the vapour phase becomes progressively richer in a less volatile component, and the vapour phase progressively richer in a more volatile component. The pumped liquid oxygen stream may be introduced into a top region of the column as the less volatile stream and a compressed vaporous air stream introduced into the bottom of the mixing column as the more volatile stream. Gaseous oxygen is thus produced at the top of the mixing column and liquid air at the bottom. In any air separation plant, there will be heat leakage into the plant. In order to compensate for this, refrigeration is added by expanding a process stream with the performance of external work. In a common type of plant design, an air stream is cooled to an intermediate temperature (i.e. a temperature less than ambient by greater than those at which the air is rectified) and is expanded in an expansion machine with the performance of work to produce a refrigerant stream. The refrigerant stream may be introduced into the lower pressure column. This expanded gaseous stream, however, reduces the liquid to vapour ratio within the lower pressure column and can have the effect of reducing oxygen recovery. DE-A-4 219 160, FR-A-2 169 561 and EP-A-0 531 182 all relate to air separation processes in which a mixing column, as described above, is used to form a gaseous oxygen product. In the plant shown in Figure 2 of DE-A-4 219 160 air flows from the turbine 19 into the low pressure column 13; in that shown in Figure 8 of FR-A-2 169 561 air flows from the turbine 60 into the lower pressure column 59; and in that shown in Figures 1 to 3 of EP-A- 0 531 182 air flows from the turbine 9 into the low pressure column 4. The present invention provides a method and apparatus utilising higher and lower pressure rectification columns, an expansion turbine, and a mixing column, in which the mixing column is fed at its top with a liquid oxygen stream from the lower pressure rectification column and at its bottom with a vaporous, refrigerant, air stream from the expansion turbine, wherein a product gaseous oxygen stream is withdrawn from a top region of the mixing column and a liquid refrigerant stream is withdrawn from a bottom region of the mixing column and is introduced into the lower pressure rectification column. According to the present invention there is provided an air separation method for producing a gaseous oxygen product at a delivery pressure comprising: forming a compressed and purified air stream and dividing said compressed and purified air stream into first and second subsidiary streams; cooling said first subsidiary stream to a temperature suitable for its rectification by cryogenic distillation; cooling said second subsidiary stream to an intermediate temperature above said temperature suitable for said rectification of said first subsidiary stream; introducing said first subsidiary stream into an air separation unit having higher and lower pressure rectification columns connected to one another in a heat transfer relationship so that liquid oxygen is produced at a column bottom region of the lower pressure column; pumping a liquid oxygen stream composed of said liquid oxygen to substantially said delivery pressure; expanding said second subsidiary stream with the performance of work to form a gaseous refrigerant stream at substantially said delivery pressure; introducing said liquid oxygen stream intoa top region of a mixing column; withdrawing a liquid refrigerant stream from said bottom region of said mixing column and introducing said liquid refrigerant stream into said lower pressure column; and forming said gaseous oxygen product by removing a product stream from the top of said mixing column, characterised in that: said gaseous refrigerant stream is introduced into a bottom region of said mixing column. The invention also provides an apparatus for separating air and for producing a gaseous oxygen product at a delivery pressure comprising: means for forming a compressed and purified air stream; heat exchange means for cooling a first subsidiary stream of the compressed and purified air to a temperature suitable for its rectification by cryogenic distillation and for cooling a second subsidiary stream of the compressed and purified air to an intermediate temperature above said suitable temperature; an air separation unit having higher and lower pressure rectification columns connected to one another in a heat transfer relationship an inlet to the higher pressure rectification column for the first subsidiary stream; an inlet to the lower pressure rectification column for oxygen-enriched liquid communicating with an outlet from the higher pressure rectification column; a pump communicating with said lower pressure rectification column for pumping a liquid oxygen stream from the lower pressure rectification column to substantially said delivery pressure; a turboexpander communicating with said heat exchange means for expanding the second subsidiary stream of the compressed and purified air with the performance of work to form a gaseous refrigerant stream at substantially said delivery pressure; a mixing column communicating at a top region thereof with said pump; an outlet for a liquid refrigerant stream from a bottom region of the mixing column communicating with said lower pressure column; and an outlet from the top region of the mixing column for the gaseous oxygen product, characterised in that: the mixing column has an inlet communicating at the bottom region thereof with the turboexpander. The introduction of a liquid refrigerant stream will increase the liquid to vapour ratio in the low pressure column to in turn increase liquid oxygen production or recovery. The increase in liquid oxygen production will increase production of the gaseous oxygen product over potential production of the gaseous oxygen product had the gaseous refrigerant stream been directly introduced into the low pressure column. It is to be noted that in the mixing column, as in any liquid-vapour contact column, there will be a pressure drop from bottom to top of the mixing column. Therefore, the pressure of the gaseous refrigerant stream is slightly greater than the liquid oxygen pressure. In a conventional air expansion plant, a stream of expanded air is introduced into the lower pressure column for refrigeration purposes. This added vapour reduces the liquid to vapour ratio within the lower pressure column and tends to reduce oxygen recovery within the lower pressure column. In the present invention the refrigerant air stream is introduced into the lower pressure rectification column in liquid state and thus enhances rather than reduces the liquid to vapour ratio. As a result, product oxygen recovery is greater and/or liquid products can be produced with less of an impact on recovery than in a prior art air expansion plant. The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an apparatus for carrying out a method in accordance with the present invention; and Figure 2 is a fragmentary view of an alternative embodiment of Figure 1. The same reference numerals are used in both Figures to indicate like elements performing the same or similar functions. With reference to Figure 1, an apparatus 10 in accordance with the present invention is illustrated. Apparatus 10 is an air expansion plant designed to produce an oxygen product at an above-atmospheric delivery pressure of approximately 2 bar (2 atm). An incoming air stream 12 in a manner well known in the art is filtered by a filter 14 and is compressed by a main compressor 16. After removal of the heat of compression by an aftercooler 18, air stream 12 is purified within a prepurification unit 20. The after-cooler 18 can be a conventional water-cooled indirect heat exchange unit, a direct contact cooler, a refrigeration unit, or, if desired, dispensed with entirely. Prepurification unit 20 utilizes adsorbent beds operating out of phase for regeneration purposes. The adsorbent is selected to remove water vapour and heavy components of the air such as carbon dioxide and potentially dangerous hydrocarbons. After air stream 12 has been compressed and purified as described above, it is divided into first and second subsidiary streams 22 and 24. As illustrated, the air stream 12 is also preferably divided into a third subsidiary air stream 26. The first subsidiary air stream 22 is cooled within a main heat exchanger 28 to a temperature suitable for its rectification by cryogenic distillation. For purposes of illustration, the main heat exchanger 28 is shown as being a single unit, but could consist of a series of units. Each heat exchanger unit 28 may be of the plate-fin kind. The first subsidiary stream 22, which consists of the major part of the undivided air stream, is introduced into an air separation unit (i.e. double rectification column) 30 having a higher pressure column 32 and a lower pressure column 34 connected to one another in a heat transfer relationship by means of a condenser/reboiler 36. The air contained within first subsidiary stream 22 is distilled within the higher pressure column 32 into a nitrogen-rich fraction that collects at the top and an oxygen-rich fraction which collects at the bottom of the column 32. A stream 38 composed of the oxygen-rich liquid is withdrawn from the column 32, is subcooled within a subcooler unit 40, is reduced in pressure to lower pressure column 34 pressure by means of a pressure reduction valve 42, and is introduced into the lower pressure column 34 for further separation. The further separation produces liquid oxygen at the bottom and nitrogen vapour at the top of the column 34. A nitrogen-rich vapour stream 44 is withdrawn from the top of higher pressure column 32. Part of the nitrogen-rich vapour stream 44 is introduced into the condenser/reboiler 36 to boil liquid oxygen separated in the lower pressure column 34. A stream 46 of the condensate is introduced into the top of higher pressure column 32 as reflux. Another stream 48 of the condensate can also be withdrawn as liquid nitrogen product. The other part of the nitrogen-rich vapour stream 44 forms a medium pressure nitrogen product stream 50 which downstream of being warmed to ambient temperature within main heat exchanger 28 can be sent to another plant. In order to reflux lower pressure column 34, a further stream 52 of the liquid nitrogen condensate is removed from the top of higher pressure column 32, is reduced in pressure by passage through a valve 54, and is introduced into the top of the lower pressure column 34. A waste nitrogen stream 56, composed of the nitrogen vapour fraction produced in lower pressure column 34, can be extracted from the column 34 and warmed within the subcooler 40 to subcool both the oxygen-rich stream 38 and the nitrogen reflux stream 52. The waste nitrogen stream 56 is warmed to about ambient temperature within the main heat exchanger 28 and may be vented therefrom. A liquid oxygen stream 58 is withdrawn from the column 34 by a pump 60 and is raised to substantially the required delivery pressure of apparatus 10. At the same time, the second subsidiary stream 24 is further compressed by a booster compressor 62. After removal of heat of compression from the further compressed stream 24 by an aftercooler 64, it is partially cooled within main heat exchanger 28 and is expanded in a turboexpander 66 to a pressure that is substantially though a little in excess of the delivery pressure. The turboexpander 66 is preferably coupled to the booster compressor 62 to apply at least a portion of the work done by the expanding air to the operation of booster compressor 62. A gaseous refrigerant stream 68 flows out of the turboexpander 66, and would in conventional processes be introduced directly into lower pressure column 34. In an alternative embodiment (not shown) the feed to the turboexpander 66 may comprise an air stream which is fully cooled and then rewarmed to a temperature intermediate the cold and warm ends of the main heat exchanger 28. The term "fully warmed" as used herein means warmed to the temperature of the warm end of main heat exchanger 28, and the term "fully cooled" means cooled to the temperature of the cold end of main heat exchanger 28. In the present invention, the gaseous refrigerant stream 68 is introduced into a mixing column 70, specifically into a bottom region 72 thereof. At the same time, the liquid oxygen stream 58 is pumped by pump 60 into a top region 74 of the mixing column 70. The mixing column 70, through direct heat exchange between the two streams, produces at its top region a gaseous oxygen product at a pressure a little in excess of the delivery pressure. The gaseous oxygen product is removed from top region 74 of mixing column 70 as a product stream 76, which downstream of its having been fully warmed within main heat exchanger 28, is delivered as a product. As can be appreciated, a liquid product (as a stream 77) could also be taken from pump 60. The previously mentioned third subsidiary stream 26 is reduced in pressure by passage through a valve 78 to approximately the same pressure as that of the gaseous refrigerant stream 68. Downstream of its being fully cooled within main heat exchanger 28, the third subsidiary stream 26 is introduced into the bottom region 72 of the mixing column 70 so as to augment the gaseous refrigerant stream 68. A liquid refrigerant stream 80 is withdrawn from the bottom region 72 of the mixing column 70 and is introduced into lower pressure column 34. Additionally, a liquid refrigerant stream 82 is removed from an intermediate region of the mixing column 70 and introduced into an intermediate region of the lower pressure column 34. Withdrawal of the stream 82 ensures the liquid to vapour ratio within top region 74 of mixing column 70 is greater than that than in the bottom region 72. Operation of the mixing column 70 at a reduced liquid to vapour ratio below the level from which the stream 82 is withdrawn helps to provide relatively efficient operation of the column 70. The liquid oxygen stream 58 is warmed to essentially its saturation temperature upstream of its introduction into top region 74 of mixing column 70. This is done through an auxiliary heat exchanger 84 which further cools gaseous refrigerant stream 68 and an auxiliary crude liquid oxygen stream 86 which is withdrawn from and returned to the higher pressure column 32. Optionally, although not shown in Figure 1, the third subsidiary stream 26 can be cooled in heat exchanger 84 modified with a pass designed to accommodate it. As illustrated, appropriate pressure reduction valves 87, 88 and 90 are provided to adjust the pressure of the streams 80, 82 and 86 flowing into high and low pressure columns 32 and 34. With reference to Figure 2, an alternative embodiment of apparatus 10 is illustrated. In this embodiment, the medium pressure nitrogen stream 50 is compressed in a compressor 92 downstream of the warm end of the main heat exchanger 28. The compressor 92 is driven by a turboexpander 94 that expands a second subsidiary stream of air 24a. An auxiliary crude liquid oxygen stream 86 is not utilized in this embodiment. Gaseous oxygen stream 68 is further cooled by being used to heat the liquid oxygen stream 58 in a heat exchanger (not shown) that would serve the same purpose as auxiliary heat exchanger 84 but would not have a passageway for auxiliary crude liquid oxygen stream 86. The following is a calculated example in tabular form illustrating the operation of apparatus 10. It is to be noted that mixing column 70 has stages formed by sieve or bubble cap trays, structured packing or random packing. It is also to be noted that the oxygen product taken from the mixing column 70 is less pure than the liquid oxygen stream introduced into the mixing column 70. This is generally true for all examples of the method according to the invention. Stream Vapour Fraction Temperature (C) Pressure (atm) (bar) Mass Flow (kg/h) O 2 Composition (mole fraction %) Air stream 12 after prepurification unit 20 1.00 11.11 5.17 55407.65 0.21 First subsidiary stream 22 after main heat exchanger 28 0.98 -174.65 5.02 42688.69 0.21 Second subsidiary stream 24 before main heat exchanger 28 1.00 7.78 7.02 6114.89 0.21 Gaseous refrigerant stream 68 at inlet to auxiliary heat exchanger 28 1.00 -157.03 2.14 6114.89 0.21 Gaseous refrigerant stream 68 after auxiliary heat exchanger 84 1.00 -176.11 2.03 6114.89 0.21 Auxiliary crude liquid oxygen stream 86 1.00 -174.70 5.01 712.85 0.20 Third subsidiary stream 26 1.00 -174.62 2.03 6604.08 0.21 Product stream 76 after main heat exchanger 28 1.00 8.67 1.90 12974.35 0.95 Liquid oxygen stream 58 prior to mixing column 70 0.00 -176.79 2.02 36711.00 0.99 Liquid refrigeration stream 80 0.00 -183.97 2.03 15039.99 0.55 Liquid refrigeration stream 82 0.00 -179.54 2.03 21415.62 0.83 Medium pressure nitrogen product stream 50 1.00 -179.17 4.95 3253.31 0.00

Claims: An air separation method for producing a gaseous oxygen product at a delivery pressure comprising: forming a compressed and purified air stream and dividing said compressed and purified air stream into first and second subsidiary streams; cooling said first subsidiary stream to a temperature suitable for its rectification by cryogenic distillation; cooling said second subsidiary stream to an intermediate temperature above said temperature suitable for said rectification of said first subsidiary stream; introducing said first subsidiary stream into an air separation unit having higher and lower pressure rectification columns connected to one another in a heat transfer relationship so that liquid oxygen is produced at a bottom region of the lower pressure column; pumping a liquid oxygen stream composed of said liquid oxygen to essentially said delivery pressure; expanding said second subsidiary stream with the performance of work to form a gaseous refrigerant stream at essentially said delivery pressure; introducing said liquid oxygen stream into a top region of a mixing column; withdrawing a liquid refrigerant stream from said bottom region of said mixing column and introducing said liquid refrigerant stream into said lower pressure column; and forming said gaseous oxygen product by removing a product stream from the top of said mixing column, characterised in that: said gaseous refrigerant stream is introduced into a bottom region of said mixing column. A method according to claim 1, further comprising: further compressing said second subsidiary stream upstream of its expansion; removing heat of compression from said further compressed second subsidiary stream upstream of its expansion; recovering at least part of the performance of work of expansion and applying said work to the compression of said second subsidiary stream. A method according to claim 2, wherein: nitrogen-rich vapour is produced in said higher pressure column; a medium pressure nitrogen stream composed of said nitrogen-rich vapour is removed from said higher pressure column and warmed by indirect heat exchange with said first and second subsidiary streams; said medium pressure nitrogen stream is compressed to a nitrogen delivery pressure; and at least part of the work of expansion is recovered and applied to the compression of said medium pressure nitrogen stream. A method as claimed in any one of the preceding claims wherein: said compressed and purified air stream has a pressure above said delivery pressure; said compressed and purified air stream is further divided into a third subsidiary air stream; said third subsidiary air stream is reduced in pressure to essentially said delivery pressure; and said third subsidiary air stream is cooled and introduced into said bottom region of said mixing column. A method as claimed in any one of the preceding claims, wherein an intermediate liquid refrigeration stream is removed from an intermediate region of the mixing column and introduced into said lower pressure column. A method as claimed in claim 5, in which: said liquid oxygen stream is in a subcooled state after having been pumped; and said gaseous refrigerant stream is heat exchanged with said liquid oxygen stream so that said liquid oxygen stream is in a saturated state and said gaseous refrigerant stream further cools. A method as claimed in any one of the preceding claims, wherein: nitrogen vapour is separated at the top of said lower pressure column; a waste nitrogen stream composed of said nitrogen vapour is removed from said lower pressure column; and said waste nitrogen stream is warmed by counter-current, indirect heat exchange with said first, second and third subsidiary streams. An apparatus for separating air and for producing a gaseous oxygen product at a delivery pressure comprising: means (16, 20) for forming a compressed and purified air stream (12); heat exchange means (28) for cooling a first subsidiary stream (22) of the compressed and purified air to a temperature suitable for its rectification by cryogenic distillation and for cooling a second subsidiary stream (24) of the compressed and purified air to an intermediate temperature above said suitable temperature; an air separation unit (30) having higher and lower pressure rectification columns (32, 34) connected to one another in a heat transfer relationship; an inlet to the higher pressure rectification column (32) for the first subsidiary stream (22); an inlet to the lower pressure rectification column (34) for oxygen-enriched liquid communicating with an outlet from the higher pressure rectification column (32); a pump (60) communicating with said lower pressure rectification column (34) for pumping a liquid oxygen stream (58) from the lower pressure rectification column (34) to essentially said delivery pressure; a turboexpander (66, 94) communicating with said heat exchange means (28) for expanding the second subsidiary stream (24) of the compressed and purified air with the performance of work to form a gaseous refrigerant stream (68) at essentially said delivery pressure; a mixing column (70) communicating at a top region (74) thereof with said pump (60); an outlet for a liquid refrigerant stream (80) from a bottom region (72) of the mixing column (70) communicating with said lower pressure column (34); and an outlet from the top region (74) of the mixing column (70) for the gaseous oxygen product, characterised in that: the mixing column (70) has an inlet communicating at the bottom region (72) thereof with the turboexpander (66, 94) Apparatus as claimed in claim 8, further comprising: a booster-compressor (62) for further compressing said second subsidiary stream (24); an aftercooler (64) downstream of said booster compressor (62) for removing heat of compression from said second subsidiary stream (24); in which said heat exchange means (28) has a configuration to enable said second subsidiary stream to be cooled to said intermediate temperature; and said booster compressor (62) is coupled to said turboexapander (66) for recovering the performance of work of expansion and applying said work to the compression of said second subsidiary stream (24). An apparatus as claimed in claim 8, further comprising: an outlet for a medium pressure nitrogen stream (50) from the higher pressure rectification column (32); a compressor (92) for compressing the medium pressure nitrogen stream (50) to a nitrogen delivery pressure; and said compressor (92) coupled to said turboexpander (94) so that the work of expansion is recovered in the compression of said medium pressure nitrogen stream (50). An apparatus as claimed in any one of claims 8 to 10 wherein: the bottom region (72) of said mixing column (70) has an additional inlet for a third subsidiary air stream (26). An apparatus as claimed in any one of claims 8 to 11, wherein said mixing column (70) has an outlet for an intermediate liquid refrigerant stream (82) at an intermediate level thereof communicating with the lower pressure rectification column (34). An apparatus as claimed in any one of claims 8 to 12, further comprising means (84) for exchanging heat between said gaseous refrigerant stream (68) and said liquid oxygen stream (58).

Inventors: HOGG NEIL, STRAUB JOSEPH, HOGG, NEIL, STRAUB, JOSEPH
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1995
Main classification: F25J3
IPC codes: F25J3