System and method for providing refrigeration to a cryogenic separation unit

Abstract

A system and method for providing refrigeration to a cryogenic separation unit is provided. The disclosed system and associated methods employ both a warm recycle turbine arrangement and cold turbine arrangement to provide the refrigeration required to produce a large amount of liquid products, such as liquid oxygen, liquid nitrogen and liquid argon when used in a cryogenic air separation unit.

Claims

1. A method for providing refrigeration to a cryogenic separation unit, the cryogenic separation unit having a distillation column system with a lower pressure column and a higher pressure column, the method comprising the steps of: compressing at least a portion of a feed stream in a multi-stage main feed compression system to a first pressure; purifying the compressed feed stream to remove high boiling contaminants and other impurities; further compressing at least a portion of the purified, compressed feed stream in a booster compression system to a second pressure; still further compressing at least a portion of the further compressed feed stream at the second pressure in the booster compression system to a third pressure, wherein the third pressure is greater than the second pressure; cooling a first portion of the still further compressed feed stream at the third pressure in a primary heat exchanger and expanding the cooled first portion of the feed stream at the third pressure in a first turbine to a pressure suitable for introduction into the higher pressure column; cooling and condensing a second portion of the still further compressed feed stream at the third pressure in the primary heat exchanger and feeding the condensed second portion from the primary heat exchanger to the lower pressure column of the distillation column system; directing a first portion of the exhaust stream from the first turbine to the higher pressure column of the distillation column system, wherein the first portion of the exhaust stream from the first turbine comprises between 30% and 50% of the exhaust stream from the first turbine; warming a second portion of the exhaust stream from the first turbine which comprises between 50% and 70% of the exhaust stream from the first turbine and compressing the warmed second portion of the exhaust stream from the first turbine in a booster loaded recycle compressor to produce a compressed recycle stream at a recycle pressure between the first pressure and the second pressure, wherein the recycle compressor is disposed apart from the main feed compression system and the booster compression system and the compressed recycle stream is separate from the purified, compressed feed stream; recycling the compressed recycle stream to the purified, compressed feed stream; diverting a portion of the purified, compressed feed stream at the second pressure to a second turbine and expanding the diverted portion of the purified, compressed feed stream to a pressure at or above the first pressure and below the second pressure; and warming all of the exhaust stream from the second turbine in the primary heat exchanger and recycling all of the warmed exhaust stream from the second turbine to the purified, compressed feed stream; wherein the shaft work of expansion from the second turbine drives one or more stages of compression in the booster loaded recycle compressor; and wherein the shaft work of expansion from the first turbine drives one or more stages of compression in the booster compression system.

2. The method of claim 1, wherein the cryogenic separation unit is a cryogenic air separation unit and the feed stream further comprises air or a stream comprised of one or more constituents of air.

3. The method of claim 1, wherein the pressure of the warmed exhaust stream from the second turbine is equal to the recycle pressure.

4. The method of claim 1, wherein the step of directing the first portion of the exhaust stream from the first turbine to the distillation column system of the cryogenic separation unit further comprises directing the first portion of the exhaust stream to a higher pressure column of the distillation column system.

5. The method of claim 1, further comprising the step of even further compressing the second portion of the still further compressed feed stream prior to the steps of cooling and condensing the second portion such that the second portion is liquefied at a pressure not less than the third pressure.

6. The method of claim 1, further comprising the steps of: splitting the second portion of the still further compressed feed stream at the third pressure into a third high pressure portion and a fourth high pressure portion; directing the third high pressure portion to the lower pressure column in the distillation column system; and directing the fourth high pressure portion to the higher pressure column in the distillation column system.

7. A cryogenic separation unit comprising: a distillation column system with a lower pressure column and a higher pressure column, a multi-stage main feed compression system configured for compressing at least a portion of a feed stream to a first pressure; a pre-purifier unit disposed downstream of the main feed compression system and configured for purifying the compressed feed stream to remove impurities; a booster compression system disposed downstream of the pre-purifier unit and configured for further compressing the purified, compressed feed stream to a second pressure and then still further compressing a portion of the purified, compressed feed stream at the second pressure to a third pressure, wherein the third pressure is greater than the second pressure; a primary heat exchanger configured to receive a first portion and a second portion of the still further compressed stream at the third pressure, partially cooling the first portion of the still further compressed stream at the third pressure, and condensing the second portion of the still further compressed stream at the third pressure to temperatures suitable for rectification in the lower pressure column of the distillation column system; a first turbine arrangement configured to receive the partially cooled first portion of the still further compressed stream at the third pressure, expand such first portion to provide refrigeration, wherein a first portion of the expanded stream comprising between 30% and 50% of the expanded stream is directed to the higher pressure column and wherein another portion of the expanded stream comprising between 50% and 70% of the expanded stream is directed to the primary heat exchanger where it is warmed; a recycle compression circuit configured to receive another portion of the expanded stream from the first turbine arrangement, warm said portion of the expanded stream in the primary heat exchanger, further compress the warmed expanded stream in a booster-loaded recycle compressor to produce a recycle stream at a recycle pressure between the first pressure and the second pressure, wherein the booster-loaded recycle compressor is disposed apart from the main feed compression system and the booster compression system and the compressed recycle stream is separate from the purified, compressed feed stream; wherein the compressed recycle stream is recycled to a location upstream of the boosted compression system; a second turbine arrangement configured to receive a portion of the further compressed feed stream at the second pressure and expand such portion to provide refrigeration, wherein the expanded stream from the second turbine arrangement is warmed in the primary heat exchanger and recycled to a location upstream of the boosted compression system; and a warm turbine recycle circuit configured to receive all of the expanded stream from the second turbine arrangement, warm the expanded stream in the primary heat exchanger, and recycle all of the warmed expanded stream from the second turbine arrangement to a location upstream of the boosted compression system; wherein the shaft work of expansion from the second turbine arrangement drives one or more stages of compression in the booster loaded recycle compressor; and wherein the shaft work of expansion from the first turbine drives one or more stages of compression in the booster compression system.

8. A method for providing refrigeration to a cryogenic separation unit, the cryogenic separation unit having a distillation column system with a lower pressure column and a higher pressure column, the method comprising the steps of: compressing at least a portion of a feed stream in a multi-stage main feed compression system to a first pressure; purifying the compressed feed stream to remove high boiling contaminants and other impurities; further compressing at least a portion of the purified, compressed feed stream in a booster compression system to a second pressure; still further compressing at least a portion of the further compressed feed stream at the second pressure in the booster compression system to a third pressure, wherein the third pressure is greater than the second pressure; cooling a first portion of the still further compressed feed stream at the third pressure in a primary heat exchanger and expanding the cooled first portion of the feed stream at the third pressure in a first turbine to a pressure suitable for introduction into the higher pressure column; wherein the first turbine is coupled to the booster compression system and the first turbine is configured such that the shaft work of expansion from the first turbine drives one or more stages of compression in the booster compression system; cooling and condensing a second portion of the still further compressed feed stream at the third pressure and feeding the condensed second portion to the lower pressure column of the distillation column system; directing a first portion of the exhaust stream from the first turbine to the higher pressure column of the distillation column system, wherein the first portion of the exhaust stream from the first turbine comprises between 30% and 50% of the exhaust stream from the first turbine; warming a second portion of the exhaust stream from the first turbine which comprises between 50% and 70% of the exhaust stream from the first turbine and compressing the warmed second portion of the exhaust stream from the first turbine in a booster loaded recycle compressor to produce a compressed recycle stream at a recycle pressure between the first pressure and the second pressure, wherein the recycle compressor is disposed apart from the main feed compression system and the booster compression system and the compressed recycle stream is separate from the purified, compressed feed stream; recycling the compressed recycle stream to the purified, compressed feed stream; diverting a portion of the purified, compressed feed stream at the second pressure to a second turbine and expanding the diverted portion of the purified, compressed feed stream to a pressure at or above the first pressure and below the second pressure; and warming all of the exhaust stream from the second turbine in the primary heat exchanger and recycling all of the warmed exhaust stream from the second turbine to the purified, compressed feed stream; wherein the shaft work of expansion from the second turbine drives one or more stages of compression in the booster loaded recycle compressor; and wherein the shaft work of expansion from the first turbine drives one or more stages of compression in the booster compression system.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) While the specification concludes with claims specifically pointing out the subject matter that Applicant regards as the invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawing in which

(2) FIG. 1 is a schematic illustration of an embodiment of an integrated cryogenic separation and liquefaction system outlining a process or method for cryogenic separation and liquefaction in accordance with the present invention.

DETAILED DESCRIPTION

(3) Turning now to FIG. 1, there is shown a simplified illustration of the present cryogenic separation system 10 and process. In a broad sense, the present system and method comprises: a multi-stage main feed compression train 20; one or more booster compression circuits 30, a main or primary heat exchange section 40; two or more turbine based refrigeration circuits 70A and 70B, and a distillation column system 50.

(4) In the main feed compression train 20 shown in FIG. 1, the incoming feed air 11 is compressed in a multi-stage main air compressor arrangement 12 to a pressure P1 generally in the range of about 130 psia to about 190 psia. The compressed air feed 13 is then purified in a pre-purification unit 14 to remove high boiling contaminants from the incoming feed air. Such a pre-purification unit 14 typically has beds of adsorbents to adsorb such contaminants as water vapor, carbon dioxide, and hydrocarbons.

(5) As described in more detail below, the compressed, purified feed air stream 15 is separated into oxygen-rich, nitrogen-rich, and argon-rich fractions in a plurality of distillation columns including a higher pressure column 52, a lower pressure column 54, and optionally, argon column (not shown). Prior to such distillation however, the compressed, pre-purified feed air stream 15 is split into a plurality of feed air streams that are cooled to temperatures suitable for rectification. Cooling the compressed, purified feed air streams is accomplished by way of indirect heat exchanger with the warming column system 50 streams which include the oxygen, nitrogen and/or argon waste. Refrigeration is generated by the cold and warm turbine arrangements disposed within the turbine based refrigeration circuits.

(6) In the present embodiment, the compressed, pre-purified air stream 15 is further compressed in a recycle air compressor (RAC) 22 to a pressure P2 in range of about 450 psia to about 550 psia. A first portion of this warm, further compressed, pre-purified air 23A is still further compressed by way of a boosted air compressor 24 preferably powered by way of the shaft work of expansion from a first turbo-expander 32 to a third pressure P3. As illustrated, the first turbo-expander 32 providing the shaft work is preferably one of the turbo-expanders associated with the cold-turbine arrangement 72, and preferably a lower column turbine (LCT). The resulting pressure, P3, of this first portion of compressed, pre-purified feed air 23A is preferably in the range of about 650 psia to about 850 psia. A second portion of the warm, further compressed, pre-purified air 23B is diverted to the refrigeration circuits 70B, and more particularly to the warm recycle turbine (WRT) arrangement 74 as a warm recycle air stream 23B, described below.

(7) The first portion of compressed, pre-purified feed air 23A is high pressure feed air stream that is further split into a first subportion high pressure feed air stream 37 and a second subportion high pressure feed air stream 39. The first subportion high pressure feed air stream 37 is partially cooled in the primary heat exchanger 42 and expanded in the first turbo-expander 32 associated with the LCT cold turbine arrangement 72, while the second subportion high pressure feed air stream 39 is liquefied in the primary heat exchanger 42 and fed to the distillation column system 50. As illustrated, part of the second subportion high pressure feed air stream 39 is liquefied in the primary heat exchanger 42 and the resulting liquid air stream 41 is expanded in valve 46 and introduced at an intermediate location of the higher pressure column 52 while another part of the second subportion high pressure feed air stream 39 is liquefied in the primary heat exchanger 42 and the resulting liquid air stream 43 is expanded in valve 44 and introduced as liquid air to the lower pressure column 54. The splitting of the high pressure feed air stream 23A may be accomplished either upstream of the primary heat exchanger 42 or within the primary heat exchanger at selected locations to achieve the desired cooling profiles of the different portions and subportions of the high pressure feed air stream.

(8) Part of the exhaust stream 36A from the first turbo-expander 32 of the LCT based cold turbine arrangement 72 is fed directly to the distillation column system 50, and more preferably to the higher pressure column 52 while another part of the exhaust stream 36B from the first turbo-expander 32 of the LCT based cold turbine arrangement 72 is diverted to the primary heat exchanger 42 where it is warmed to near ambient temperatures and the resulting LCT recycle stream 45 is compressed in the WRT booster compressor 79. Stream 36A may be optionally subcooled against a waste nitrogen stream and/or phase separated prior to column entry. The compressed LCT recycle stream 76 is then combined with the warmed WRT exhaust stream 78 and recycled back to the compressed and purified feed air stream 15, preferably at a location upstream of the RAC 22. One of the key aspects or features of the present system and method is this recompression of the LCT recycle stream 45 to a pressure, P4, that is not less than the pressure P1 of the compressed air feed exiting the multi-stage main feed air compressor 12 or pre-purification unit 14.

(9) In the illustrated embodiment, between about 50% and 70%, and more preferably about 60% of the exhaust stream 36 from the first turbo-expander 32 of the LCT based cold-turbine arrangement 72 is recycled back through the primary heat exchanger 42 while the remaining 30% to 50% of the exhaust stream 36 from the first turbo-expander 32 of the LCT based cold turbine arrangement 72 is fed to the distillation column system 50. In a preferred mode of operation, the remaining exhaust stream 36A is fed directly to the higher pressure column 52. In cases where the exhaust stream is a two phase stream, the exhaust stream may also be directed to a phase separator either upstream or downstream of the LCT exhaust split to further condition the stream prior to introduction into the distillation column system.

(10) Within the illustrated distillation column system 50, the various feed air streams in both gaseous and liquid forms are separated in manners well known to those persons skilled in the art into various product streams, kettle streams, and waste streams, including a liquid nitrogen product stream 62 and a liquid oxygen product stream 64, which are preferably directed to suitable storage vessels (not shown). A portion of the liquid nitrogen stream 67 may be used to reflux the lower pressure column 54. Likewise, a portion of the kettle stream 65 may be re-introduced to the lower pressure column 54.

(11) The waste streams comprised of excess gaseous oxygen 66 and lower pressure column overhead gaseous nitrogen 68 are preferably returned to the primary heat exchanger 42 where they are warmed to temperatures at or near ambient temperature and indirectly cooling the high pressure incoming air feed streams. Optionally, the gaseous nitrogen overhead stream 68 may be used as a source of subcooling streams entering the distillation column system 50. Optionally, the gaseous oxygen stream 66 and gaseous nitrogen overhead stream 68 may be combined into a single waste stream 69 prior to warming in the primary heat exchanger 42.

(12) Key features of the present system and method are derived from the management of the various warming recycle streams obtained from both the cold and warm turbines. In the illustrated embodiment, a warm recycle air stream 23B is extracted from the discharge of the RAC 23 and directed via a warm recycle circuit to the primary heat exchanger 42, partially cooled in the primary heat exchanger 42 and expanded in a second turbo-expander 75 of the warm recycle turbine (WRT) arrangement 74 to a pressure not less than the pressure of the compressed air feed exiting the multi-stage main feed air compressor 12 or pre-purification unit 14. While the stream 23B is shown as being partially cooled in the primary heat exchanger, the stream 23B could alternatively be cooled by other cooling means such as a refrigeration system. The exhaust 77 from the second turbo-expander 75 is then warmed in the primary heat exchanger 42 thereby producing WRT recycle stream 78 which is returned or recycled back to the compressed, purified feed air stream 15, preferably at a location upstream of the RAC 22.

(13) While the present invention has been described with reference to a preferred embodiment and operating method associated therewith, it should be understood that numerous additions, changes and omissions to the disclosed system and method can be made without departing from the spirit and scope of the present invention as set forth in the appended claims.

(14) For example, the warm recycle air stream 23B may be extracted or diverted from the discharge of the LCT booster compressor 24, partially cooled in the primary heat exchanger 42 and subsequently expanded in the second turbo-expander 75 of the warm recycle turbine (WRT) arrangement 74 to generate refrigeration.

(15) Also, the warm booster compressor discharge pressure and the WRT exhaust pressure are preferably equivalent so that the streams 76 and 78 may be combined prior to recycling the combined stream 78 to the purified, compressed feed air stream 15. However, in arrangements where the warm booster compressor discharge pressure and the WRT exhaust pressure differ, the LCT recycle stream 76 and the warmed WRT exhaust stream 78 may be returned or recycled separately to selected locations in the purified, compressed feed streams 15 or 23.