Air separation method and apparatus

Abstract: A method and apparatus for separating air in which a stream 16 of cooled air is separated in a single rectification column 12, a nitrogen stream 18 is withdrawn from the column 12, and an oxygen containing vapour stream 46 is removed from the column 12 and then divided into two subsidiary streams. The two subsidiary streams are condensed, one in condenser 22 associated with the heat of the column 12, and the other in a reboiler 52 associated with the bottom of the stripping column 14, and then combined for stripping within the stripping column 14 to produce ultra-high purity liquid oxygen as a bottom product 60.

Description: The present invention relates to an air separation method and apparatus in which air is separated and an ultra-high purity liquid oxygen product is produced. The invention may be employed in a single column nitrogen generator. It is well known in the art to separate air to produce an oxygen-rich fraction which is lean in the heavy components such as carbon dioxide, water and hydrocarbons and then to strip a liquid stream, composed of the oxygen-rich fraction, of light components such as nitrogen, argon, neon, krypton, and helium. For example, US-A-5,049,173 discloses a single column nitrogen generator in which a liquid stream is withdrawn from the nitrogen generator at a location thereof at which the liquid stream is composed of oxygen-rich liquid lean in the heavy components. The liquid stream is subsequently stripped within a stripping column by introducing the liquid into the top of the column to produce a descending liquid phase which becomes ever more concentrated in liquid oxygen and ever more dilute in the light components. US-A-5,049,173 also discloses a method of purifying an oxygen containing vapour stream removed from a high pressure column of a double column distillation unit. The oxygen containing vapour stream is subsequently liquefied in a reboiler of the stripping column before being stripped. In order to extract liquid from the stripping column, liquid nitrogen must be added to the stripping column. The problem in adding a liquid composed of nitrogen to a liquefied oxygen containing vapour stream is that the stripping column must be appropriately sized to strip a resultant combined stream having a lower purity than a liquid stream composed of oxygen-rich liquid. Furthermore, nitrogen production will suffer in direct portion to the liquid nitrogen removed. As will be discussed the present invention provides a method and apparatus for separating air in which an oxygen containing vapour stream lean in heavy components is liquefied and stripped within a stripping column without addition of a liquid nitrogen stream to reflux the stripping column. According to the present invention there is provided an air separation method comprising: cooling compressed and purified air stream to a temperature suitable for its rectification; producing an oxygen containing vapour fraction lean in heavy components by rectifying the cooled air stream; dividing the oxygen containing vapour fraction into first and second subsidiary streams; and separately condensing said subsidiary streams and stripping of light components said condensed subsidiary streams in a stripping column so that ultra-high purity liquid oxygen is produced as column bottoms therein; the first subsidiary stream being condensed through indirect heat exchange with said column bottoms, thereby to produce boil-up within said stripping column. The invention also provides an air separation apparatus comprising: a main heat exchanger for cooling a compressed and purified air stream to a temperature suitable for its rectification; a rectification column for rectifying said cooled air stream, a first outlet from the rectification column for an oxygen containing vapour fraction lean in heavy impurities; a stripping column having a reboiler associated with a bottom region thereof to provide boil-up within said stripping column; said reboiler communicating with the first outlet so that a first subsidiary stream of said oxygen containing vapour fraction is able in use to be condensed within said reboiler; a condenser for condensing a second stream of said oxygen-containing vapour fraction communicating with the first outlet; the condenser and the reboiler both communicating with an inlet to a top region of said stripping column and an outlet for an ultra-high purity liquid oxygen from a bottom region of the stripping column. As is evident from the foregoing description, the present invention has applicability to a single column nitrogen generator that is integrated with an ultra-high purity liquid oxygen stripping column having a reboiler. Since both liquid streams are separately condensed, the stripping column need only be designed to strip the oxygen-rich fraction and not an oxygen-rich fraction combined with nitrogen. Moreover, in case of a nitrogen generator, the other subsidiary stream can be condensed within a head condenser used in connection therewith. This of course will decrease the production of nitrogen product. However, such decrease will be less that would be the case had liquid nitrogen been removed because it is the coolant, usually oxygen rich liquid, that is condensing such subsidiary stream rather than liquid. Hence, nitrogen production does not suffer to the same extent as in prior art oxygen purification schemes where it is desired to remove an oxygen containing vapour fraction for further purification within a stripping column. High purity nitrogen has an impurity content of less than about 100 parts per billion by volume of oxygen. Ultra-high purity liquid oxygen has an impurity content of less than about 100 parts per billion (of impurities other than oxygen) by volume. When a stream is "fully warmed" it is warmed to a temperature of the warm end of the main heat exchanger or main heat exchanger complex. Similarly, when a stream is "fully cooled" it is cooled to a temperature of the cold end of the main heat exchanger or main heat exchange complex. Further, when a stream is "partly warmed" or "partly cooled" it is respectively warmed or cooled to a temperature between the warm end and cold end temperatures of the main heat exchanger or main heat exchange complex. Light components include nitrogen, argon, neon, helium, and hydrogen and heavy components include carbon dioxide, water, krypton and hydrocarbons. The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawing which is a schematic flow diagram of an air separation plant. With reference to the drawing, an air separation plant 1 is illustrated that is designed to separate air into a high purity nitrogen fraction and an ultra-high purity liquid oxygen fraction. Air after having been compressed and purified in a manner well known in the art is cooled in a main heat exchanger (complex) 10 to a temperature suitable for its rectification which would normally be at or near the dewpoint of air. The air is then rectified within a single rectification column 12 into a high-purity nitrogen-rich top fraction ("tower overhead") and an oxygen enriched bottom liquid fraction ("column bottoms"). An oxygen containing vapour fraction is removed from the single column 12 at a location thereof at which such vapour fraction will be lean in heavy components. After condensation, such vapour fractions stripped within a stripping column 14 to produce the ultra-high purity liquid oxygen product. The present invention is not limited to single column nitrogen generators and in fact, has wider applicability to plural column arrangements. A compressed and purified air stream 16 which, as has been previously mentioned, is cooled within a main heat exchanger 10, is formed by compressing the air, removing the heat of compression, and then purifying the air of heavier components such as carbon dioxide, moisture and hydrocarbons. It is to be noted that even after such purification, however, such heavy components still be present within compressed and purified air stream 16 and will concentrate within liquid fractions produced from the rectification thereof. Compressed and purified air stream 16 is then introduced into the single rectification column which contains liquid-vapour contacting elements such as trays, random packing or structured packing to rectify the air into a top high-purity, nitrogen fraction and a bottom oxygen enriched liquid fraction. A nitrogen product stream 18 is taken from the high purity, nitrogen fraction. A part 20 of nitrogen product stream 18 is condensed within a head condenser 22 and then is recycled to the column 12 as reflux. The head condenser 22 is a single pass unit of plate-fin construction. The other part 24 of nitrogen product stream 18 is fully warmed within main heat exchanger complex 10 where it is expelled at ambient temperatures as product nitrogen (PGN). Coolant is supplied to head condenser 22 by way of removal from the column 12 of a liquid air stream 26 and a liquid oxygen enriched stream 28. Liquid air stream 26 and oxygen enriched stream 28 are expanded within valves 30 and 32, respectively vaporised within head condenser 22. The vaporised liquid air stream 26 is recompressed within a recycle compressor 34 to the operating pressure of the column 12 to produce a recycle stream 36, which after having been partly cooled within the main heat exchanger complex 10, is introduced into a bottom region of the column 12. In the illustrated embodiment, recycle stream 36 is not fully cooled so as to prevent its liquefaction. The oxygen rich liquid stream 28 after having been vaporised is introduced into a turboexpander 38 to produce a refrigerant stream 40. Refrigerant stream 40 can be combined with other waste streams and then fully warmed within the main heat exchanger complex 10 as a waste nitrogen stream 42. Such warming decreases the enthalpy of the incoming air in order to compensate for irreversibilities such as heat leakage into air separation plant 1. The recycle compressor 34 and the turboexpander 38 can be coupled, for example, by an energy disapative oil brake or a generator, so that some of the energy of the work of expansion can be recovered to power recycle compressor 34. It is possible to use a liquid stream having the same composition as oxygen-rich liquid stream 28 as the sole coolant for head condenser 22 and which thereafter is recirculated back to the column. However, the use of the streams 26 and 28 is particularly advantageous because the liquid air stream 26 has a higher nitrogen content than the oxygen-rich liquid stream 28. As such, the stream 26 has a higher dewpoint pressure for the same temperature of oxygen-rich liquid. Therefore, the supply pressure of vaporised liquid air stream 26 to the compressor is higher and, thus, more flow can be compressed for the same amount of work. This increase in flow allows for an increase in heat pumping action which boosts recovery over that which would have been obtained had oxygen-rich liquid stream 28 been used as the sole coolant. Moreover, the composition of vaporised liquid air stream 26 is close to the equilibrium vapour composition in the sump of the column. This allows the bottom of the column to operate more reversibly than in known processes. The oxygen containing vapour fraction lean in the heavy components is withdrawn from column 12 as an oxygen containing vapour stream 46 which is divided into two subsidiary streams 48 and 50. Subsidiary stream 48 is condensed by passage through a reboiler 52 located within a bottom region 54 of stripping column 14. This provides boil up for stripping column 14. The resultant condensate is then reduced in pressure by pressure reduction valve 56. The other of the two subsidiary streams 50 is condensed within head condenser 22 and then is reduced in pressure by a pressure reduction valve 58. The two subsidiary streams 48 and 50 are combined and then introduced into stripping column 14 to be stripped and thereby to produce the ultra-high purity liquid oxygen as an ultra-high purity liquid oxygen product stream 60.

Claims: An air separation method comprising: cooling compressed and purified air stream to a temperature suitable for its rectification; producing an oxygen containing vapour fraction lean in heavy components by rectifying the cooled air stream; dividing the oxygen containing vapour fraction into first and second subsidiary streams; and separately condensing said subsidiary streams and stripping of light components said condensed subsidiary streams in a stripping column so that ultra-high purity liquid oxygen is produced as column bottoms therein; the first subsidiary stream being condensed through indirect heat exchange with said column bottoms, thereby to produce boil-up within said stripping column. An air separation method as claimed in claim 1, wherein: the cooled air stream is rectified within a single rectification column and a nitrogen fraction is also produced thereby; a head condenser communicating with said single column condenses part of said nitrogen fraction, thereby to produce reflux for said single rectification column; a remaining part of said nitrogen fraction is fully warmed in a main heat exchanger in countercurrent heat exchange relationship with the air stream being cooled; and the second subsidiary stream is condensed within said head condenser. An air separation method as claimed in claim 2, wherein: coolant for said head condenser is produced by extracting a liquid stream from said single rectification column and expanding said liquid stream through a valve; the extracted liquid stream vaporises within said head condenser; and the vaporised liquid stream is recompressed to the operating pressure of said single rectification column is cooled to said temperature suitable for rectification and is recycled to said single rectification column. An air separation method as claimed in claim 3, further comprising: supplying additional coolant to said head condenser by withdrawing an oxygen-rich liquid stream from a bottom region of said single rectification column and expanding said oxygen-rich liquid stream through a valve; vaporising said oxygen-rich liquid stream within said head condenser and partially warming said vaporised oxygen-rich liquid stream; turboexpanding said warmed, vaporised oxygen-rich liquid stream to produce a refrigerant stream; and fully warming said refrigerant stream in indirect heat exchange within said compressed and purified air stream being cooled. An method as claimed in claim 4, wherein a stream of a top fraction separated in said stripping column is fully warmed in the main heat exchanger. An air separation apparatus comprising: a main heat exchanger (10) for cooling a compressed and purified air stream to a temperature suitable for its rectification; a rectification column (12) for rectifying said cooled air stream, a first outlet (46) from the rectification column (12) for an oxygen containing vapour fraction lean in heavy impurities; a stripping column (14) having a reboiler (52) associated with a bottom region thereof to provide boil-up within said stripping column (14); said reboiler (52) communicating with the first outlet (46) so that a first subsidiary stream of said oxygen containing vapour fraction is able in use to be condensed within said reboiler (52); a condenser (22) for condensing a second stream of said oxygen-containing vapour fraction communicating with the first outlet (46); the condenser (22) and the reboiler (52) both communicating with an inlet to a top region of said stripping column (14) and an outlet (60) for an ultra-high purity liquid oxygen from a bottom region of the stripping column (14). An air separation apparatus as claimed in claim 6, wherein: said rectification column (12) is a single rectification column (12) which has a second outlet (18) for a nitrogen product stream; the condenser (22) is arranged so as to be able to condense nitrogen separated in the rectification column (12). Air separation apparatus as claimed in claim 7, additionally comprising a third outlet (26) for a liquid stream containing oxygen from the rectification column (12) communicating with an inlet to the condenser (22) via an expansion valve (30); and a recycle compressor (34) having an inlet communicating with an outlet for vaporised liquid stream containing oxygen from the condenser (22) and an outlet communicating with an inlet to a bottom region of the rectification column (12). An air separation apparatus as claimed in claim 8, further comprising additionally comprising a fourth outlet (28) for a further liquid stream containing oxygen from the rectification column (12) communicating with another inlet to the condenser (22) via another expansion valve (32); and a turboexpander (38) communicating via the main heat exchanger (10) with an outlet from the condenser (22) for vaporised further liquid stream containing oxygen.

Inventors: BROOKS CHARLES M, NAUMOVITZ JOSEPH P, BROOKS, CHARLES M., NAUMOVITZ, JOSEPH P.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1997
Main classification: F25J3
IPC codes: F25J3