Cryogenic Rectification System with Liquid Oxygen Boiler

Europaisches Patentamt J European Patent Office © Publication number: 0 624 766 A1 Office europeen des brevets

EUROPEAN PATENT APPLICATION

@ Date of filing: 12.05.94

© Priority: 13.05.93 US 60136 © Applicant: PRAXAIR TECHNOLOGY, INC. 39 Old Ridgebury Road © Date of publication of application: Danbury, CT 06810-5113 (US) 17.11.94 Bulletin 94/46 @ Inventor: Roberts, Mark Julian © Designated Contracting States: 1966 Harvey Road BE DE ES FR GB IT Grand Island, 14072 New York (US) Inventor: Beddome, Robert Arthur 94 Elmwood Park East Tonawanda, 14150 New York (US) Inventor: Bonaquist, Dante Patrick 1036 Ransom Road Grand Island, 14072 New York (US)

© A cryogenic rectification system wherein com- compressor, and a portion (102) of the feed which pressed feed (101) is further compressed in a com- does not undergo turboexpansion is condensed pressor (5) coupled to a turboexpander (7) and tur- against liquid oxygen in a product boiler (4). boexpanded through the turboexpander to drive the

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Rank Xerox (UK) Business Services (3. 10/3.09/3.3.4) 1 EP 0 624 766 A1 2

Technical Field feed air from undergoing said turboexpansion; (D) introducing turboexpanded feed air into the This invention relates generally to cryogenic higher pressure column of a double column rectification and more particularly to cryogenic rec- cryogenic rectification plant; tification wherein liquid oxygen is vaporized to con- 5 (E) condensing at least some of the feed air dense feed. which is not turboexpanded; (F) introducing condensed feed air into the high- Background Art er pressure column; and (G) withdrawing liquid oxygen from the cryo- Oxygen is produced commercially in large io genie rectification plant, vaporizing liquid oxygen quantities by the cryogenic rectification of feed air, by indirect heat exchange with said condensing generally employing the well known double column feed air, and recovering resulting oxygen as system, wherein product oxygen is taken from the product. lower pressure column. At times it may be desir- Another aspect of the invention is: able to produce oxygen at a pressure which ex- 75 Apparatus for producing oxygen by cryogenic ceeds its pressure when taken from the lower pres- rectification comprising: sure column. In such instances, gaseous oxygen (A) a first compressor, a second compressor may be compressed to the desired pressure. How- and a turboexpander coupled to the second ever, it is generally preferable, both for safety and compressor; for operating cost purposes, to remove oxygen as 20 (B) means for passing feed to the first compres- liquid from the lower pressure column, pump it to a sor and from the first compressor to the second higher pressure, and then vaporize the pressurized compressor; liquid oxygen to produce the desired elevated (C) a double column cryogenic rectification plant pressure product oxygen gas. comprising a higher pressure column; Cryogenic rectification requires refrigeration in 25 (D) means for passing feed from the second order to operate. The requisite refrigeration is in- compressor to the turboexpander and from the creased when oxygen is withdrawn from the col- turboexpander into the higher pressure column; umn as liquid and pumped prior to vaporization (E) a product boiler, means for passing feed to because the pump work is added to the system. the product boiler and from the product boiler Refrigeration may be provided to the cryogenic 30 into the higher pressure column; process by the turboexpansion of a stream fed into (F) means for passing fluid from the cryogenic the rectification column system. However, the com- rectification plant to the product boiler; and pression of a stream for the turboexpansion con- (G) means for recovering product fluid from the sumes a significant amount of energy. product boiler. Accordingly, it is an object of this invention to 35 As used herein, the term "feed air" means a provide a cryogenic rectification system wherein mixture comprising primarily nitrogen, oxygen and liquid oxygen is removed from the column system argon, such as air. for boiling and wherein process refrigeration is pro- As used herein, the terms "turboexpansion" vided by turboexpansion of a feed stream, which and "turboexpander" mean respectively method has improved operating efficiency over conven- 40 and apparatus for the flow of high pressure gas tional oxygen product boiler cycles. through a turbine to reduce the pressure and the temperature of the gas thereby generating refrig- Summary Of The Invention eration. As used herein, the term "column" means a The above and other objects which will be- 45 distillation or fractionation column or zone, i.e. a come apparent to those skilled in the art upon a contacting column or zone wherein liquid and vapor reading of this disclosure are attained by the phases are countercurrently contacted to effect present invention, one aspect of which is: separation of a fluid mixture, as for example, by A method for producing oxygen by cryogenic contacting or the vapor and liquid phases on a rectification comprising: 50 series of vertically spaced trays or plates mounted (A) compressing feed air to a first pressure; within the column and/or on packing elements (B) further compressing at least 55 percent of which may be structured packing and/or random the compressed feed air to a second pressure packing elements. For a further discussion of dis- by passing it through a compressor coupled to a tillation columns, see the Chemical Engineers' turboexpander; 55 Handbook fifth edition, edited by R. H. Perry and (C) turboexpanding at least some of the further C. H. Chilton, McGraw-Hill Book Company, New compressed feed air by passing it through said York, Section 13, The Continuous Distillation Pro- turboexpander but maintaining at least some cess. The term, double column is used to mean a

2 3 EP 0 624 766 A1 4 higher pressure column having its upper end in Figure 2 is a schematic representation of an- heat exchange relation with the lower end of a other preferred embodiment of the invention lower pressure column. A further discussion of dou- wherein all of the compressed feed is further com- ble columns appears in Ruheman "The Separation pressed by the compressor coupled to the turboex- of Gases", Oxford University Press, 1949, Chapter 5 pander. VII, Commercial Air Separation. Vapor and liquid contacting separation pro- Detailed Description cesses depend on the difference in vapor pres- sures for the components. The high vapor pressure The invention employs a compressor coupled (or more volatile or low boiling) component will io to a turboexpander and passes fluid compressed tend to concentrate in the vapor phase whereas the by the compressor to the turboexpander for tur- low vapor pressure (or less volatile or high boiling) boexpansion to generate refrigeration for introduc- component will tend to concentrate in the liquid tion into the cryogenic rectification plant. The fluid phase. Partial condensation is the separation pro- passing through the turboexpander drives the com- cess whereby cooling of a vapor mixture can be 15 pressor by virtue of the coupling of these two used to concentrate the volatile component(s) in pieces of equipment thus eliminating the need for a the vapor phase and thereby the less volatile generator to capture energy produced by the tur- component(s) in the liquid phase. Rectification, or boexpansion and improving the operating efficiency continuous distillation, is the separation process of the compressor. A portion of the feed which has that combines successive partial vaporizations and 20 not been turboexpanded is condensed to boil liquid condensations as obtained by a countercurrent oxygen. Thus, the condensing feed is uncoupled treatment of the vapor and liquid phases. The from the remaining feed enabling this fluid to be countercurrent contacting of the vapor and liquid compressed independently to the pressure level phases is adiabatic and can include integral or required for the product boiler thus allowing oxygen differential contact between the phases. Separation 25 product pressure requirements to be independent process arrangements that utilize the principles of of plant refrigeration requirements. rectification to separate mixtures are often inter- The invention will be described in detail with changeably termed rectification columns, distillation reference to the Drawings. Referring now to Figure columns, or fractionation columns. Cryogenic rec- 1, feed such as feed air 100, is compressed by tification is a rectification process carried out at 30 passage through first or base load compressor 1 to least in part at temperatures at or below 150 de- a first pressure generally within the range of from grees Kelvin. 70 to 150 pounds per square inch absolute (psia). As used herein, the term "indirect heat ex- The compressed feed air is cooled through cooler change" means the bringing of two fluid streams 29 to remove heat of compression and cleaned of into heat exchange relation without any physical 35 high boiling impurities such as carbon dioxide and contact or intermixing of the fluids with each other. water vapor by passage through adsorber 2. The As used herein, the term "argon column" feed air is then divided into a first portion 101 and means a column which processes a feed compris- a second portion 102. Portion 101 comprises at ing argon and produces a product having an argon least 55 percent and preferably comprises from 65 concentration which exceeds that of the feed and 40 to 80 percent of the compressed feed air. Portion which may include a heat exchanger or a top 101 is further compressed to a second pressure, condenser in its upper portion. As used herein, the which exceeds the first pressure and is generally term "liquid oxygen" means a liquid having an within the range of from 80 to 170 psia, by passage oxygen concentration of at least 95 mole percent. through second compressor 5 which is coupled to As used herein, the term "coupled" means 45 turboexpander 7. The further compressed feed air mechanically connected so as to transfer work di- 103 is passed through cooler 30 to remove heat of rectly from one device to another without any inter- compression and then cooled by indirect heat ex- mediate gears. The coupling may be by means of change with return streams by passage through a single rotating shaft connecting the two devices. main heat exchanger 8. Resulting cooled stream 50 104 is passed to turboexpander 7. A small portion Brief Description Of The Drawings 105 of cooled stream 104 may be liquefied by passage through heat exchanger 9 by indirect heat Figure 1 is a schematic representation of one exchange with return streams. Resulting liquefied preferred embodiment of the invention wherein stream 106 is then passed into column 11 which is only a portion of the compressed feed is further 55 the higher pressure column of a double column compressed by the compressor coupled to the cryogenic rectification plant which also comprises turboexpander. lower pressure column 14.

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Cooled further compressed stream 104 is tur- waste stream 24 is withdrawn from column 14 at a boexpanded by passage through turboexpander 7 point below the point where stream 22 is with- thus generating refrigeration and also driving com- drawn, passed through heat exchangers 20, 16, 9 pressor 5. Turboexpanded stream 107 is intro- and 8 and removed from the system as stream duced into higher pressure column 11 which is 5 116. operating at a pressure generally within the range An argon containing fluid is passed from col- of from 65 to 100 psia. umn 14 to argon column 18 in line 25, and is Feed stream 102, which does not pass through separated by cryogenic rectification in argon col- turboexpander 7, preferably is compressed to a umn 18 into argon-richer vapor and oxygen-richer third pressure by passage through compressor 3. io liquid. The oxygen-richer liquid is returned to col- Generally, the third pressure will differ from the umn 14 by line 27. Argon-richer vapor is passed in second pressure and will be within the range of line 117 into top condenser 17 wherein it is par- from 100 to 1400 psia. Resulting stream 108 is tially condensed by indirect heat exchange with cooled by passage through main heat exchanger 8 oxygen-enriched fluid. Resulting argon-richer fluid and is passed to product boiler 4 wherein it is is is passed into phase separator 118 and liquid 119 condensed by indirect heat exchange with boiling from phase separator 118 is passed into column 18 liquid oxygen as will be more fully described later. as reflux. Vapor 26 from phase separator 118 is Condensed feed air 109 is subcooled by passage recovered as product crude argon having an argon through heat exchanger 10 by indirect heat ex- concentration of at least 90 mole percent. change with liquid oxygen and is passed into col- 20 Liquid oxygen is withdrawn from column 14 in umn 1 1 preferably at a point above the point where line 23 and preferably is pumped to a higher pres- stream 107 is passed into column 11. Generally, sure by passage through liquid pump 28. The this point will be at least 2 equilibrium stages liquid oxygen is then warmed by passage through above the stream 107 introduction point. If desired, heat exchanger 10 and then passed into product stream 109 may be combined with stream 106 and 25 boiler 4 wherein it is vaporized by indirect heat this combined stream 110, as illustrated in Figure exchange with condensing feed air. Resulting va- 1 , may be introduced as described into column 1 1 . porized oxygen 120 is warmed by passage through Within column 11, the feeds are separated by main heat exchanger 8 and recovered as product cryogenic rectification into nitrogen-enriched top oxygen gas 121 having an oxygen concentration of vapor and oxygen-enriched bottom liquid. The em- 30 at least 95 mole percent. The pressure of the bodiment illustrated in Figure 1 also includes a product oxygen gas will vary, depending upon third column which in this case is an argon column whether and how liquid pump 28 is employed, from for the production of crude argon. Nitrogen-en- the pressure prevailing at the column 14 withdrawal riched top vapor 111 is passed into main con- point to a pressure of about 1200 psia. If desired, denser 15 wherein it is condensed against reboiling 35 some liquid oxygen may be recovered from prod- column 14 bottoms. Resulting condensed fluid 112 uct boiler 4 in line 122. is passed in stream 113 as reflux into column 11, Figure 2 illustrates one preferred embodiment and in stream 13 through heat exchanger 20 and of the invention wherein all of the feed air which is valve 21 into column 14 as reflux. Oxygen-enriched compressed to the first pressure is further com- liquid is passed in stream 12 from column 11 40 pressed to the second pressure. The numerals in through heat exchanger 16, wherein it is subcooled Figure 2 correspond to those of Figure 1 for the by indirect heat exchange with return streams, and common elements and these common elements through valve 19 into top condenser 17 of argon will not be described again in detail. column 18. In top condenser 17, the oxygen-en- Referring now to Figure 2, the entire feed 150 riched liquid is partially vaporized and the resulting 45 which has been compressed through first compres- vapor and liquid (shown for convenience in Figure sor 1 to the first pressure is further compressed to 1 as one stream 114) is passed into column 14. the second pressure by passage through second Column 14 is operating at a pressure less than compressor 5. Resulting further compressed feed that of column 1 1 and generally within the range of stream 151 is cleaned of high boiling impurities by from 16 to 30 psia. Within column 14 the fluids fed 50 passage through adsorber 2 and resulting stream into column 14 are separated by cryogenic rec- 152 is divided into two portions 153 and 154. tification into nitrogen-rich vapor and oxygen-rich Portion 153 comprises from about 65 to 80 percent liquid, i.e. liquid oxygen. Nitrogen-rich vapor is of the feed and is cooled by passage through main withdrawn from column 14 in line 22, warmed by heat exchanger 8 prior to being passed as stream passage through heat exchangers 20, 16, 9 and 8 55 155 through turboexpander 7. A portion 105 of and, if desired, recovered as product nitrogen 115 stream 155 may be liquefied as was the case with having a nitrogen concentration of a least 98 mole the embodiment illustrated in Figure 1. Feed percent. For product purity control purposes, a stream 154, which does not pass through turboex-

4 7 EP 0 624 766 A1 8 pander 7, is preferably compressed to the third product boiler 4. pressure by passage through third compressor 3, The example described above represents a cooled by passage through main heat exchanger 8 very advantageous turboexpander-compressor and passed into product boiler 4 wherein it is combination because high efficiencies can be condensed by indirect heat exchange with boiling 5 achieved for both the turboexpander and the boost- liquid oxygen. The other elements of the embodi- er compressor while operating them at the same ment illustrated in Figure 2 are substantially the speed, without gearing. same as those illustrated in Figure 1 . For these process conditions, radial turboex- The following example is provided for illustra- panders and radial compressors are most often tive purposes and is not intended to be limiting. io employed. For these machines, the design proce- The example reports a computer simulation of the dure is to choose the operating speed such that invention employing the embodiment of the inven- optimum efficiency is obtained. The optimum tion illustrated in Figure 1 and employing air as the speed is dependent on the particular pressure ratio feed. The numerals correspond to those of Figure and flowrate required of the application. Use is I. Air at atmospheric temperature and pressure is is made of a dimensionless parameter ns, or specific compressed by passage through first compressor 1 speed. This parameter is proportional to the rota- to a first pressure of 115 psia. The air is then tional speed in RPM. Optimal efficiency for a radial cooled to 299 Kelvin in cooler 29. After purification turbine occurs at a value of ns of about 0.5. Simi- and water removal in adsorber 2, the air tempera- larly for a radial compressor, the optimal efficiency ture is 289 Kelvin. A portion 102, equalling 24 20 occurs at about 0.95. In the example described percent of the air is compressed in third compres- above, a speed of 5800 RPM for both the turbine sor 3 to a third pressure of 346 psia. The remaining and the booster results in values on ns of 0.48 and 76 percent of the air in line 101 is compressed in 0.92 for the turbine and the compressor, respec- second compressor 5 to a second pressure of 127 tively. These values will give maximum efficiencies psia. Stream 103 is then cooled in cooler 30 to a 25 for both wheels. It is thus possible to obtain very temperature of 290 Kelvin and then further cooled high efficiencies for both the turbine and the com- in heat exchanger 8 to 111 Kelvin by heat ex- pressor without gearing by properly choosing the change with cold returning streams. A small portion stream to be compressed. 105 of this stream equalling 3.5 percent of the air The choice of the stream to be compressed is flow is liquefied in heat exchanger 9. The remain- 30 important. It is not effective to simply combine the ing portion is turboexpanded to a pressure of 78 turboexpander on a shaft with any booster com- psia by passage through turboexpander 7 and in- pressor. Rather, this combination produces an ef- troduced into the bottom of higher pressure column fective match only when the major fraction of the II. Compressor 5 is mechanically coupled to tur- feed air or the total feed air is employed for further boexpander 7, thus providing the work of compres- 35 compression. It has the advantage of uncoupling sion. the minor feed air stream associated with the prod- After exiting compressor 3, the stream 108 is uct boiler which allows that stream to be com- cooled in main heat exchanger 8 to 158 Kelvin and pressed independently to the pressure level re- is then condensed in product boiler 4, by heat quired for the product boiler. This allows the oxy- exchange with liquid oxygen boiling at a pressure 40 gen product pressure level to be independent of of 150 psia. 3.0 percent of the liquid oxygen is any plant refrigeration requirements. withdrawn as liquid product 122. Stream 109 is Further, the arrangement has design flexibility then subcooled in heat exchanger 10 by heat ex- relative to the amount of turbine refrigeration pro- change with warming liquid oxygen, before being duced by the arrangement. If the required plant combined with stream 106 and introduced into 45 refrigeration is increased, for example to produce higher pressure column at an intermediate point. liquid products, the turbine work is increased, Higher pressure column 11 is operated at a which increases the booster compressor energy pressure of 78 psia. The pressure at the top of available and the resultant air feed pressure level column 14 is 15.2 psia. to the turbine. The shaft work balances and cor- The purity of liquid oxygen stream 23 is 99.6 50 responding air feed pressure levels are self-com- percent oxygen on a molar basis. Nitrogen product pensating as plant refrigeration requirements stream 22 contains 99.98 mole percent nitrogen change. and has a flowrate equal to 20 percent of the air Although the invention has been described in flow. The composition of the argon product stream detail with reference to certain preferred embodi- 26 is 98.5 mole percent argon. 55 ments, those skilled in the art will recognize that Liquid oxygen stream 23 is pumped in liquid there are other embodiments of the invention within pump 25 to a pressure of 150 psia before being the spirit and the scope of the claims. introduced into heat exchanger 10 and then into

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Claims (B) means for passing feed to the first compressor and from the first compressor to the 1. A method for producing oxygen by cryogenic second compressor; rectification comprising: (C) a double column cryogenic rectification (A) compressing feed air to a first pressure, 5 plant comprising a higher pressure column; (B) further compressing at least 55 percent (D) means for passing feed from the second of the compressed feed air to a second compressor to the turboexpander and from pressure by passing it through a compres- the turboexpander into the higher pressure sor coupled to a turboexpander; column; (C) turboexpanding at least some of the io (E) a product boiler, means for passing feed further compressed feed air by passing it to the product boiler and from the product through said turboexpander but maintaining boiler into the higher pressure column; at least some feed air from undergoing said (F) means for passing fluid from the cryo- turboexpansion; genic rectification plant to the product boil- (D) introducing turboexpanded feed air into 15 er; and the higher pressure column of a double (G) means for recovering product fluid from column cryogenic rectification plant; the product boiler. (E) condensing at least some of the feed air which is not turboexpanded; 8. The apparatus of claim 7 further comprising a (F) introducing condensed feed air into the 20 third compressor for compressing feed prior to higher pressure column; and passing the feed to the product boiler. (G) withdrawing liquid oxygen from the cryogenic rectification plant, vaporizing liq- 9. The apparatus of claim 7 wherein the means uid oxygen by indirect heat exchange with for passing fluid from the cryogenic rectifica- said condensing feed air, and recovering 25 tion plant to the product boiler comprises a resulting oxygen as product. liquid pump.

2. The method of claim 1 wherein all of the 10. The apparatus of claim 7 further comprising a compressed feed air is further compressed to third column, means for passing fluid from the the second pressure by passing it through the 30 cryogenic rectification plant to the third column compressor coupled to the turboexpander. and means for recovering fluid from the third column. 3. The method of claim 1 further comprising compressing feed air to a third pressure prior to the condensation by indirect heat exchange with vaporizing liquid oxygen.

4. The method of claim 1 wherein the liquid oxygen is pumped to a higher pressure after withdrawal from the cryogenic rectification plant 40 and prior to vaporization.

5. The method of claim 1 further comprising recovering a nitrogen-rich fluid from the double column cryogenic rectification plant. 45

6. The method of claim 1 further comprising passing an argon-containing fluid from the double column cryogenic rectification plant into an argon column and recovering an argon-richer 50 fluid from the argon column.

7. Apparatus for producing oxygen by cryogenic rectification comprising: (A) a first compressor, a second compres- 55 sor and a turboexpander coupled to the second compressor;

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8 European Paten, EUROPEAN SEARCH REPORT " Office Ep 94 1Q 74Q7

DOCUMENTS CONSIDERED TO BE RELEVANT Category Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE of relevant passages to claim APPLICATION (Int.CI.5) US-A-5 069 699 (AIR PRODUCTS AND 1,7 F25J3/04 CHEMICALS) * abstract * * figure 3 * * column 8, line 19 - column 10, line 16

Y,P US-A-5 251 451 (AIR PRODUCTS AND 1,7 CHEMICALS) 12 October 1993 * abstract * * figure 1 * * column 5, line 30 - column 6, line 37 * A 4,5,9

A EP-A-0 465 929 (UNION CARBIDE INDUSTRIAL 1,5-7,10 GASES TECHNOLOGY CORPORATION) * abstract * * column 4, line 31 - column 7, line 30 * * figure 1 *

EP-A-0 063 318 (AIR PRODUCTS AND 1,2,7,8 CHEMICALS) TF.CHNICAI. FIELDS * abstract * SEARCHED (lnt.CI.5) * page 5, line 26 - page 9, line 11 * F25J * figure 1 *

The present search report has been drawn up for all claims Place of search Date of completion of the search Examiner THE HAGUE 12 August 1994 Siem, T CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D : document cited in the application document of the same category I, : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document