CA2048146C - Production of nitrogen free of light impurities - Google Patents

Abstract

ABSTRACT

This invention relates to a cryogenic process for the separation of air utilizing an integrated multi-column distillation system wherein an ultra high purity nitrogen rich, oxygen rich and optionally an argon rich product are generated. In the cryogenic distillation separation of air, air is initially compressed, pretreated and cooled for separation into its components. Ultra high purity, e.g., nitrogen having less than 0.1 ppm of volatile impurities is generated with enhanced nitrogen product recovery by withdrawing liquid nitrogen from the higher pressure column at an intermediate point and charging that fraction as reflux to the lower pressure column, withdrawing a nitrogen stream which is rich in volatile contaminants from the top of the high pressure column, partially condensing the nitrogen stream and removing the uncondensed portion as a purge stream, and, withdrawing an ultra high purity nitrogen product at a point below a nitrogen vapor withdrawal point at the top of the lower pressure column. Alternatively, no purge need be taken and the volatile impurities allowed to pass to the low pressure column. In that case, a nitrogen fraction rich in volatile impurities is removed from an upper portion of the low pressure column, condensed and at least a portion of the uncondensed fraction removed as a purge and the condensed portion returned to the low pressure column.

Description

PRODUCTION OF NITROGEN FREE OF LIGHT IMPURITIES

S TECHNICAL FIELD OF THE INVENTION
This invent~on relates to cryogenic process for the separatlon of alr and recovering ultra hlgh purity nitrogen with high nitrogen recovery.

Numerous processes are known for the separation of alr by cryogenic distillation into its const~tuent components. Typically the air separation process lnvolves removal of contaminant materials such as carbon dioxide and water from a compressed a~r stream prtor to cooling to near its dew point. -The cooled air then is cryogenically d~st~lled ~n an ~ntegrated mult~-column 15 distillation system having a high pressure column a low pressure column and a side arm column for the separat~on of argon. rhe s~de arm column for the -separation of argon typically commun~cates w~th the low pressure column ln that an argon/o~ygen stream contalnlng about 8-12% argon is removed and .
cryogenlcally distilled ln the slde arm column.
~o Processes to produce a high pur~ty n~trogen stream containing few light contaminants such as hydrogen helium and neon have been proposed.
Concentration of some of these contaminants in the feed air can be as high as 20 ppm. Almost all of these light components show up in final nitrogen product from an air separation unit (ASU). In some cases such as for the :
25 electronic industry~ this contaminat~on level is unacceptable in the end use of this nitrogen product.
The following patents d~sclose approaches to the problem.

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U.S. Patent 4 a24 453 discloses a process for producing ultra high purity oxygen as well as high purity nitrogen where the nitrogen purlty exceeds 99.998% and the amount of impurities is generally less than lO ppm.
More specifically air is compressed cooled and disl:illed in a rectif~cat~on system wherein in a first stage rectlflcation an oxygen enriched fractlon is removed from the bottom and a n~trogen rich liquld fraction ~s removed from an upper port~on of the first stage rectificat~on sub-cooled and returned as reflux to the top of the second stage rectificatlon. A nitrogen rich l~quid ~s re~oved from an upper portlon of the second stage at a point ~ust below an overhead removal point for nitrogen vapor from the second stage rect~cat~on. L~quid oxygen from the bottom of the first stage is sub-cooled expanded and used to drive a boiler/condenser in the top of the hlgh purity argon column. Nltrogen vapor ~rom the top of the flrst stage ~s used to drlve a rebo~lerl condenser ln the bottom of a hlgh purity oxygen column. To enhance product purlty a portion of the gaseous nitrogen stream from the top of the hlgh pressure column ls removed as purge.
U.S. 4 902 321 discloses a process for producing ultra hlgh purlty `~
nitrogen in a multi-column system. Alr ~s compressed cooled and charged to a high pressure column where it is separated ~nto its own components generatlng an oxygen llquld at the bottom and a n~trogen r~ch vapor at the top. The oxygen liqu~d ~s expanded and used to drive a boiler/condenser whlch is thermally llnked to the top of the hlgh pressure column for condenslng the nitrogen rlch vapor. A portion of the nltrogen rlch vapor ls 2g removed ~rom the top of the h~gh pressure column and condensed in the tube side of a heat exchanger. The result~ng l~qu~d n~trogen ls expanded and charged to a top of a stripptng column wherein nitrogen including impurities are flashed from the str~pping column. Any impurit~es not removed by :
flashing are stripped by passing a stream of substantially pure nitrogen upwardly through the column. The nitrogen liquid collected at the bottom of the ;tripping column ~s pumped to the shell s~de of the heat exchanger vaporized agalnst the nitrogen~rich vapor and removed as high purity product.
European Patent 0 0376 465 d~scloses an a~r separat~on process for ~
35 producing ultra high purity nitrogen product. In the process nitrogen - -product from a conventional air separation process is charged to the bottom of a column equ~pped with a reflux condenser. L~quid nitrogen ~s withdrawn from an upper portion of the column and flashed generatlng a liquid and a vapor. The liquid obtained after flashing is then flashed a second time and the resulting liquid recovered.
There are essentially two problems associated with the processes descr~bed for producing ultra-hlgh purity nitrogen and these problems relate to the fact that ~n the 453 dlsclosure purit~es are qulte often not sufficiently h~gh to meet ~ndustry specifications and tn the 321 process nitrogen recoveries are too low. The same can be said of the 465 European Patent.

Thls ~nventlon relates to an a~r separat~on process for produclng ultra hlgh purity n~rogen as product with hlgh nltrogen recovery. In the baslc cryogenic process for the separat~on of a~r which comprises nitrogen oxygen and volatile and condenstble ~mpurities ~n an integrated multt-column d~stillat1On system having a hlgher pressure column and a lower pressure column an air stream is compressed freed of condensible ~mpuritles and cooled generating a feed for the integrated multi-column dlstlllation system. The improvement in th~s bas~c process for producing ultra hlgh purity nitrogen at h~gh nitrogen recovery comprises:
a~ generatlng a llqul~ nltrogen fractlon and a nltrogen rich vapor fraction contalnlng volatlle lmpur~t~es near the top of the hlgher pressure column;
b) removing a portton of the l~qu~d n~trogen fractlon from the hlgher pressure column;
c) expand~ng the l~quid nitrogen fractton and tntroduc~ng the expanded fraction to the top of the lower pressure column as feed;
d) generating a nitrogen rich vapor fraction containing residual volatile 1mpur~t~es at the top of the lower pressure column and removing that fraction as an overhead;
e) partially condensing at least one of sa~d nitrogen r~ch vapor fractlons generated ~n step (a) or (d) or both ~n a bo~ler/condenser;

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f) removing at least a portion of at least one of the uncondensed nitrogen rich vapor fractlons concentrated ln volatile impurities from the boiler/condenser as a purge stream;
g) returning at least a portton of at least one of the condensed nitrogen rich vapor fractions to a column as reflux; and h) generating and removing an ultra high purity nitrogen fractlon as product from the lower pressure column at a polnt blelo~ the removal polnt for the nitrogen rich vapor conta~n~ng volatlle ~mpuritles and below the point of return of the liquid n~trogen fraction as reflux to the lower pressure column.
In another embodiment wh~ch includes argon recovery an air stream is compressed freed of condensible ~mpur~t~es and cooled formlng a cooled air stream. The alr stream then ls cryogenically dlstilled ln an ~ntegrat~d multl-column dlstlllatlon system havlng a hlgher pre~sure colu~n a low~r pressure column and a slde arm column for e~ectlng separatlon of argon from oxygen. The improvement for produc~ng ultra hlgh purlty nltrogen product comprlses:
a) feed~ng substantlally all of sa~d cooled air stream to the h~gher pressure column;
b) generatlng a llquid n~trogen fractlon and a n~trogen rlch vapor fract~on conta~n~ng volat~le ~mpur~t~es near the top of the hlgher pressure column;
c) removlng a portion of the l~qu~d n~trogen fraction from the hlgher pressure column at a polnt below a removal point des~gnated for the removal of a nltrogen rlch vapor fractlon conta~nlng volatlle impur~t~es;
d) expanding the l~qu~d n~trogen fractlon and ~ntroducing the expanded fraction to the top of the lower pressure column as feed;
e) generat~ng a nitrogen r~ch vapor fract~on conta~ning residual volatlle ~mpuritles fraction at the top of the lower pressure column and removing that fraction as an overhead; and f) part~ally condensing at least one of said nitrogen rich vapor fract1Ons generated ~n step ~b) or step (e~ in a bo~ler/condenser and returning at least a portion of at least one the condensed nitrogen r~ch vapor fractlons to a column as reflux;

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2 ~ '~Ar 3 - s -g) removing at least a portion of at least one of the uncondensed nitrogen r~ch vapor fract~ons concentrated in volatile impuritles generated in step ~f) from the boiler/condenser as a purge stream;
h) removing an argon stream from the low pressure column and fraction~t~ng that argon stream in sald s7de arm column and recovertng an argon rich product as overhead; and j) generating and removlng an ultra high purity nitrogen fractlon as product from the lower pressure column at a point below the removal polnt for the nitrogen r~ch vapor containing residual volatile ~mpurities and below the po~nt of return of the l~quld nitrogen fraction as reflux to the lower pressure column.
The advantages for obta~ning ultra high purlty n~trogen at hlgh recovery are achleved by concentratlng volat~le lmpurltles ~n purge streams and mlnlmlzlng the volume of thesQ purge streams at strateglc locatlons ln the process , DRA~INGS
Flgure 1 ls a schematlc representat~on of an embodlment for generatlng ultra hlgh purlty nitrogen with enhanced nitrogen recovery.
F~gure 2 is a schemat~c representat~on of a varlat~on of the process ln Figure 1 wherein a portlon of the nltrogen from the n~trogen llquifler expanded to prov~de refr~gerat~on and charged to the h~gher pressure column and a nitrog2n vapor stream ~s removed ~n add~tlon to the llquld n~trogen stream near the top of the lower pressure column.
F~gure 3 ls a schemat~c representat~on of a varlatlon of the process of Fig 2 whereln a separate bo~ler/condenser ~s used to condense a portion of the nitrogen rich vapor conta~ning ~mpurttles from the h~gh pressure column against a portion of the l~quid nttrogen product from the lower pressure column. -Figure 4 ls a schematic representation of a variation of Figure 1 in that a portion of the liquid n~trogen from the lower pressure column ls expanded and used to condense a n~trogen fract~on ~n the overhead the lower pressure column. A turboexpander ~s provided for add~t~onal refrigeration.
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Figure 5 ~s a schematic representation of a variation of Figure 1 in that only a port~on of the uncondensed nitrogen fraction from the lower pressure column is removed as a purge.
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DETAILED DESCRIPTION OF T~E INVENTION
To facilitate an understanding of the invention and the concepts for generat~ng an ultra h~gh purity nitrogen product having a volatile l~purlty content of less than 5 ppm and preferably less than 0.1 ppm, reference ls made to Flgure 1. More part~cularly, a feed air stream 10 is initlally prepared from an air stream by compressing an alr stream comprlsing oxygen, nitrogen, argon, volattle ~mpur~t~es suc~l as hydrogen, neon, hellum, and the like, and condens~ble ~mpurttles, such as, carbon dlox~dQ and water ln a multl-stage compressor system to a pressure rang~ng ~rom abou~ ~0 to 300 psla and typlcally ~n the range of gO-180 ps~a. Volatlle ~mpurlttes hav~ a much lower bo11ing point than nitrogen. This compressed air stream ls cooled with cooling water and ch~lled agalnst a refr~gerant and then passed through a molecular s1eve bed to free ~t of condensable water and carbon dtoxide impurit1es.
The integrated multi-column dlst~llatlon system comprlses a high pressure column 202, a low pressure column 204 and, optionally, a side arm column 206 for effecting argon separatlon. High pressure column Z02 is operated at a pressure close to the pressure of feed a~r stream 10, e.g., 80 to 300 psia and air is separated ~nto ~ts components by ~nt~mate contact of the vapor and liquld in the column. H~gh pressure column 202 ls equipped w~th dist~llat~on trays or pack~ng, e~ther medium be~ng su~ted for effectlng llquid/vapor contact. A h~gh pressure n~trogen vapor stream conta~ning volatile impurities is generated at the top portion of high pressure column 202 and a crude liquid oxygen stream is generated at the bottom of high ~ -pressure column 202.
Low pressure column 204 typically ~s operated within a pressure range from about 15-80 psia and preferably ~n the range of abou~ 18 to 25 psia in order to produce an ultra h~gh purity nitrogen product. The obiective in the lower pressure column is to provide high purity nitrogen, e.g., greater than 99.998% preferably g9.999% by volume purity at the top of the column, :

with min~mal argon loss and to generate a high purity oxygen stream. Low pressure column 204 is equipped with vapor liquid contact medium which comprises distillation trays or packing.
An argon side arm column 206 is in communication with the low pressure column 204 and generates an argon stream as an overhead and an oxygen stream as a bottoms. The column typically operates at a pressure close to the low pressure column pressure e.g. 15 to 80 psia and preferably in the range of about 18 to 25 psia.
In the process of Fig. 1 stream 10 which is free of condenslble ~mpurities is cooled to near its dew point in main heat exchanger system 200 which forms the feed v~a stream 12 to h~gh pressure column 202 associated wlth the lntegrated multl-column dlstlllation system. A hlgh pressure nltrogen rlch vapor contalntng volatlle llnpurltles ls sen~rated as an overhead and a llquld oxygen ~ractlon as a bottoms ~ractlon In one embodiment at least a portlon of the hlgh pressure nltrogen vapor generated in hlgh pressure column 202 ls wlthdrawn vla llne 14 and substantlally all of it is condensed in boiler/condenser 208 in the lower portion of low pressure column 204. Condensatlon of the n~trogen rich vapor contalnlng impurities prov~des bo~l-up and condensat~on reduces the level of volatile impurities ln the condensed l~qu~d phase and concentrates the lmpuritles ln the vapor phase. The condensed n~trogen ~s w~thdrawn from boiler/condenser 208 and a port~on ~s d~rec~ed to h~gh pressure column 202 as reflux vla llne 16. The condensed fractlon may be ~ntroduced as reflux as deslred to other columns. The uncondensed balance of the h~gh pressure nltrogen ls removed 2S via line 18 as a purge. However ~t ~s posslble to return the fractlon to first column 202 and remove volatile contam~nants at another polnt. In another embodiment no purge would be taken and substantially all of the nitrogen vapor fraction would be condensed in boiler/condenser Z08. The condensed fract~on then would be returned to the first or high pressure column 202.
A liquid nitrogen fract~on ~s collected at a po~nt typ~cally about 3-5 trays below the nitrogen removal point via l~ne 14 in h~gh pressure column 202. That liquid nitrogen fract~on is removed via line 20 typically isenthalpically expanded and the vola~ile ~mpurit~es flashed therefrom in separator 210. The liqu~d phase from separator 210 ~s ~ntroduced via line ' .

22 o the top of low pressure column Z04 as reflux. An optional impure liquid fraction is removed from the mlddle to lower section of hlgh pressure column 202 via line 24 expanded and charged to a middle section of low pressure column 204 as reflux. Another vapor fraction is removed via line 26 and split into two fractlons. One fractlon is removed vla line 28 and warmed in main heat exchanger system 200 and recovered as high pressure gaseous ni~rogen (HPGAN) and the other is passed via line 30 to nitrogen recycle liquefier 212 for liquefact~on.
It is in low pressure column 204 where ultra high purity nitrogen and high purity oxygen is produced. A nitrogen rich stream conta~ning residual volatile impurities is generated if not removed in high pressure column 202, and removed from the top or upper-most portion of low pressure column 204 via line 32 whereln lt ls warmed aga~nst other process flulds ~n heat exchanger system 200 and recovered AS low pressure gaseous nltrogen ~LPGAN). The warmcd nltrogen vapor stream ls removcd ~rom heak exchang~r system 200. The concentratlon of resldual volatlle lmpurit~es In nttrogen vapor stream 32 is primarily controlled by any nltrogen purge stream removed from an upper portion of hlgh pressure column 204 v~a llne 18. Recovery of nitrogen in the process is controlled by the volume of the purge stream 18.
An ultra high purity n~trogen product is generated as a llqùid fract10n (LIN) near the upper port~on of the low pressure column 204 and removed via line 34. Typlcally thls removal polnt is a few trays e.g. 2-5 trays below the removal polnt of the n~trogen vapor v~a l~ne 32. It ~s also below the introduction point for the hlgh pur~ty nltrogen reflux from separator 210 and above the waste nltrogen vapor removal v~a l~ne 36 near the mlddle to upper portion of low pressure column 204. A high purity oxygen stream ls removed as liquid ~rom low pressure column 204 via llne 37 and a h~gh purlty oxygen stream is removed as vapor v~a l~ne 39 warmed in ma~n heat exchanger system 200 and recovered as gaseous oxygen (GOX).
An argon containing vapor stream having a concentration of from about 8 to 12X argon is removed from an intermed~ate point in low pressure column 204 via llne 38 and charged to s~de arm column 206 ~or separation. Argon is separated from oxygen in side arm column 206 and a bottoms fract~on rich ~n oxygen is withdrawn from the bottom of column 206 and returned via line 40 ~$ ~
g to low pressure column 204. Side arm column 206 like hlgh pressure column 202 and low pressure column 20~ is equ~pped with vapor-liquid contact medium such as trays or pack~ng. An argon rich vapor stream is removed from the side arm column 206 via line 42 wherein ~t is condensed in boilerl condenser 214 at the top of the column 206. A portlon of the condensed argon stream is returned to argon column 204 as reflux and the balance of the stream is removed via llne 44 and recovered as a crude liquid argon stream (crude LAR). The botler/condenser in argon stde arm column 206 ls cooled by remov~ng liqu~d oxygen from the bottom of hlgh pressure column 202 via line 46 expanding that liquid and partlally vaporlzing a fract~on of that liqu~d in bo~ler/condenser 214. A port~on of the unvapor~zed fractlon is charged as reflux v~a llne 48 to low pressure colu~n 204 and the vapor~zed por~on ~s removed from bo~ler/condenser 214 and charged vla llne 50 as f~ed to the low pressure column.
Hlgh purlty nltrogen reflux for the low pressure column ls obtaln~d by expand~ng a l~quid nltrogen fractlon obta~ned from the hlgh pressure column as stated and obtain~ng supplemental n~trogen from nitrogen recycle liquefier 212 as requ~red. The n~trogen from n~trogen recycle l~quefier Z12 is conveyed vla llne 52 expanded and flashed in separator 210. The overhead ~rom separator 210 contalns some volatlle contamlnants and is removed ~rom the system v~a l~ne S4 as waste. Opt~onally ~t ~s comblned with the waste stream 36 from the low pressure column.
Flgure 2 represents a schematic representat~on of another embod~ment of a variatjon of the process of F~gure 1 for generat~ng ultra hlgh purlty gaseous n~trogen product ln the low pressure column. A numbering system similar to that of Figure 1 has been used for common equlpment and streams and comments regarding column separat~ons w~ll be l~mlted to the significant differences between this process and that described in Figure 1.
One difference between the process of Figure 1 to that in Figure 2 is that in Figure 2 the liquid nitrogen from nitrogen liquefier 21Z which has a signiflcant concentration of lights is fed via llne 52 to the high pressure column rather than to separator 210. Preferably the feed location of this liquid is chosen to match the concentrat~on of the lights on the tray at its feed po~nt. Typically ~t wi~l be fed at least one tray above - -, '::
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the withdrawal tray for liquid nitrogen stream 20. This allows a higher rejection of the lights through purge stream 18 from boiler/condenser 208 and the liquid nitrogen feed to separator 212 has a much lower concentration of the lights. This reduces the concentration of light contaminants in liquid stream 22 exiting separator 210 and fed to low pressure column 204.
Another distinction between the flow sheets of Figure 2 and that of Figure 1 is that a vapor stream rich in residual lights or impurities is removed vla llne 32 as a purge allo~ing a h~gher pur~ty low pressure ultra hlgh pur~ty gaseous nltrogen fraction to be produced. It contains contaminants and aids in controlling the contaminant level in l~quid stream 34. Almost all of th~
low pressure ultra high purlty nitrogen product is recovered through line 35. Although not shown vapor stream 54 from the separator 210 may not need to be vented. It can be mlxed wlth the gaseous nitrogen stream 32 from the top of the low pressure column 204 and sent to nltrogen recycle llquQ~lQr 212.
In the processes of F~gure 1 and F~gure 2 a large fractlon of the contaminants in the gaseous n~trogen from the top section of the low pressure column (streams 32 ~n F~g. 1 and 35 ln Flg. 2) comes from crude liquid oxygen stream 46. Th~s crude l~qu~d oxygen stream in the bottom of the high pressure column is in contact with the incoming feed air and accordingly picks up a s~gn~cant concen~ration of light contaminants. In order to reduce the level of ~mpurtt~es ~n the crude l~quid oxygen prior to lntroductlon as reflux to low pressure column Z04 an alternatlve ls to expand the crude llqu~d oxygen stream from the bottom of the hlgh pressure column prlor to feeding it to the low pressure column or the boiler/con-denser located at the top of the crude argon column. The expanded fractlon then is fed to separator 211 and the flashed vapor from the top of this separator is taken as a purge stream 51. A fraction of this liquid is returned to the vaporizer side of boiler/condenser 214 and the other ls fed to the low pressure column.
Figure 3 shows a flowsheet for the product~on of all gaseous products and a part of the nitrogen product is produced at a h~gh pressure and at extremely high-purity. This is achieved by taKing a portion of liquid nitrogen stream 34 from the low pressure column and pump~ng ~t to a higher pressure. The higher pressure l~quid n~trogen stream 55 is then fed to an :, . .
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auxiliary boiler/condenser 216 where it is botled against a high pressure nitrogen stream containing volatile impurittes wh~ch is with~rawn from the top of the high pressure column 202 via line 57. The condensed ltqutd nitrogen stream 59 is returned to the high pressure column as reflux while boiled stream 61 is warmed tn main heat exchanger system 200 to provide high purity gaseous nitrogen stream at htgh pressure. A portton of the uncondensed stream rich in volatile impurtt~es is removed v~a l~ne 63 and discharged as waste. A turboexpander 45 ~s used to prov7de refrigeratton.
Figure 4 represents a process scheme for the product~on of ultra htgh purity gaseous nttrogen. In this process a portton of the llqutd nttrogen from low pressure column 204 removed via line 34 ts tsenthalptcally expanded. That stream ts conveyed v~a l~ne 65 to botler/condenser 218 at the top of the low pressure column 204 and used to condense the nltrogen rtch factlon. A purge stream 67 ls taken from th~s sectlon. Purge str~ams 18 and 67, whlch contaln a hlgh cQncent,ra~lon of llght contamlnants, can both be combtned wlth waste stream 39 prtor to expanslon ln turboexpander 45.
Referrlng to Ftgure 5, a feed alr stream ls ~nlttally prepared from an atr stream by compress1ng an atr stream comprtstng oxygen, nttrogen, argon, volattle tmpurtttes such as hydrogen, neon, heltum, and the ltke, and condenstble t~purtttes, such as, carbon dtox~de and water tn a multt-stage compressor system to a pressure rangtng from about 80 to 300 psta. Th~s compressed atr stream is cooled w1th coollng water and chtlled against a refrigerant and then passed through a molecular steve bed to free tt of condensable water and carbon dtox~de ~mpur~ttes to provtde stream 510.
In the process an alr stream 510 free of condenslble tmpurtttes is cooled to near its dew polnt ~n maln heat exchanger system 500. The atr stream then forms the feed via stream 512 to high pressure column 502 assoctated wtth the integrated multi-column dtsttllation system. A nitrogen rich vapor contatning volatile impurities is generated as an overhead and a crude ltquid oxygen fractton as a bottoms fraction. At least a portion of the nitrogen vapor generated ~n h~gh pressure column 502 is wtthdrawn vta line 514 and parttally condensed tn botler/condenser 508 tn the bottom portion of low pressure column 504. Condensat~on of the n~trogen rtch vapor containing impurities provides boil-up in the low pressure column. The . . .
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condensed nitrogen which has a fractional amount of impurities is withdrawn from boiler/condenser 508 and at least a port~on d~rected to the top of h~gh pressure column 502 as reflux via line 516. The uncondensed portion which in concentrated ln volatile impurities is taken as purge via line 518. A
nitrogen-r~ch liquid is withdrawn from the upper portion of the hlgh pressure column 502 via llne 530 and is expanded and fed to an upper portton of low pressure column 504.
It is in low pressure column 504 where the ultra high pur~ty n~trogen product is produced. In the embodiment of Flgure 5 a nitrogen stream rkh in volatile impurltles is generated ln the the top or upper most port~on of the low pressure column 504. Depend1ng on the amount of impur~tles removed in first column 502 volat~le ~mpur~t~es will be present ~n the upper most portion of low pressure column 504. The nltrogen rlch fractlon containlng volatlle impurltles ls removcd as an overhead v~a llnQ 5ZO and partlally condensed ln boller/condenser 51~ Uncondensed gases whlch ara rl~h In volatlle lmpur~tles are removed as a purge stream vla llne 522 wlth the condensed fraction being returned to low pressure column 504 vla llne 5Z4.
An ultra hlgh purlty nltrogen product e.g. product contalnlng less than 5 ppm and preferably less than 0.1 ppm resldual contaminants ls re~oved v~a line 540 at a polnt below the removal po1nt for volatile lmpur~tles ln column 504.
To obtain the necessary refrigerat~on for produc~ng ultra high purity nitrogen product ln this process crude llquld oxygen is removed from hlgh pressure column 502 as a bottoms fract~on v~a l~ne 54Z expanded and then ;~
charged to the vapor~zer sectlon of bo~ler/condensed 514 located at the top of the low pressure column 504. The vaporlzed oxygen ls removed vla llne 54~ as an overhead. Some of the overhead is dlverted to a turboexpander 546 via llne 548 with the balance being warmed in main heat exchanger 500 and then dlverted to turboexpander 546. The exhaus~ from turboexpander 546 is 30 warmed against process fluids in lleat exchanger 500 and then discharged as : :
waste. Opt~onally a small fraction o~ the ~eed to turboexpander 546 may be diverted through an expansion valve and then discharged as waste as shown.
Further embod~ments of Figures ~-5 are env~s~oned and generally lnvolve the reduction of volatile impurities in feed streams prior to introduction ,, :.

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to the low pressure column. For example dîst~llation columns can be used in place of separators 210 and 211 where the incoming streams to the separator are cooled expanded and introduced to the top of the column.
Volatile impurities are flashed from the descending liquld and strtpped by ascending vapor. Usually the incoming feed is cooled in a boiler/condenser against the liquid at the bottom of this column. Acc~rdingly dist~llatton columns may be ut~l~zed in place of separators 210 and 211.
An embodiment which may be utilized to reduce the volatile contamlnants in crude l~quid nitrogen stream 20 from the high pressure column ls to I0 charge the liqu~d n~trogen stream from the high pressure column to the top of a third distillation colllmn. Descendlng l~quld then is strlpped of volatile ~mpur~t~es by ascend~ng vapor. A portion of the incoming feed alr stream may be used to vapor~ze the nltrogen l~quid in a bo~ler/condenser ln the bottom o~ the column. The overhead from th~ flxed dlstlllatlon column lS may be returned to an upper portlon o~ hlgh pressure column 202 and th~
llquid fractlon would be trans~erred to separator 210.
I~ oxygen is not a destred product ~n the overall process the flow scheme ~n Flgure 4 ~ay be mod~f~ed by associatlng boiler/condenser 208 wlth the hlgh pressure column and add~ng an add~tional boiler/condenser and associatlng lt wlth the bottom of the low pressure column.
Figure 5 shows a volat~le ~mpurl~y r~ch purge stream 518 taken ~rom the boilerJcondenser 508. Alternat~vely thls purge may not be taken at all and the nitrogen rich stream 514 can be totally condensed ln the boiler/condenser 508 located at the top of the h~gh pressure column 508. In this optlon the tray sectlon between the top of the h~gh pressure column and llquld n~trogen stream 530 withdrawl po~nt w~ll not be needed and l~qu~d nitrogen stream 530 can be withdrawn as a portlon of the condensed nitrogen stream 516 and fed to the low pressure column 504.
The following examples are provided to illustrate the embodiments of the invention and are not intended to restrict the scope thereof Example 1 ;
Ultra Hlah PuritY Llauld Nltroaen An a~r separation process using the apparatus descrlbed in Figure 1 was carrled out. Figure 1 shows a plant where primar~ly ultra h~gh purity liquid :
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nitrogen llquid oxygen and liquid argon are produced. In this flgure feed air stream 12 containing light contaminants is fed at the bottom of the h1yh pressure column. A gaseous nitrogen stream 26 is withdrawn a couple of trays below the top tray and is sent to nitrogen recycle liquefier 212 whereln the nitrogen is condensed. A liquid nitrogen stream 20 is also withdrawn fro~
roughly the same location which eventually supplles the reflux to the low pressure column. No major product streams are withclrawn from the top of the high pressure column and the top 3 5 trays ~ncrease the concentrat1On of the lights in the vapor phase. A non-condens~ble purge Sstream 18) is taken from the bollerlcondenser located at the bottom o~ the low pressure column. Thls purge contalns a fairly hlgh concentratton of the lights and ls respons~ble for remov~ng the major~ty of the l~ght contaminants from the system prlor to introduct~on of any feed to the lower pressure column.
Feed streams to the n~trogen llque~ler parttcularly strealn 26 Pro~ th~
hlgh pressure column have a slgnlflcantly h~gher concentratlons oF llght contamlnants than other streams. Therefore the return~ng llquld nltrogen stream 52 from the l~qulf~er also has an undeslrably hlgh concentratlon of these co~ponents. Thls stream along with reflux stream 20 from the high pressure column ls expanded and fed to separator 210. In thts separator about 2-15X of the total feed stream ts flashed and comes out as vapor stream 54. The l~quld nltrogen stream from the separator is fed as reflux to the top of the low pressure column. In some ~nstances th~s l~qu~d stream ltself may meet product l~quid nltrogen spec~flcat~ons. Generally a lower concentrat~on ls more deslrable and lt ~s favorable to feed th~s stream to the top of the low pressure column as reflux. The result~ng descendlng llqu~d nltrogen stream at the top of the low pressure column ts str~pped further of the l~ght components and a f~nal llquid nitrogen product (stream 34) of hlgh purlty ls withdrawn 1 to 5 trays below the top of the low pressure column. The vapor -stream 54 from separator 210 ls rich ~n light contaminants and is preferably discarded as a waste stream. In that way a gaseous n~trogen product is obtained from the low pressure column whtch is relatively pure.
Sample calculatlons for the flowsheet ~n F~gure 1 were done for a preselected process des~gn. The concentrat~on of hydrogen ~n the feed air was 6 ppm and the objective was to produce h~gh purlty l~quid nitrogen. It ls 3S observed that the concentratlon of hydrogen ~n purge stream 18 ls 0.8X. Its _ 15 -flow rate is falrly small at 0.06% of total flow rate of air stream 12 to the high pressure column and th~s stream ~s responsible for the removal of about 75% hydrogen contained in the feed air stream 12. The reflux liquid nltrogen stream 20 withdrawn from the high pressure column has 0.21 ppm hydrogen. ~hen this stream along with liquid nitrogen stream 52 from the liquifier is flashed ln separator 210 the concentration of hydroqen ln llquid nltrogen from separator (stream 22) ls reduced to 0.09 ppm. Even though thls concentratlon ls low lt may not meet the more strlngent requlrement of today s lndustry. If not th~s result~ng llquid from the separator may be fed to the top of the low pressure column and a llquid nltrogen product w~thdrawn from the low pressure column (stream 34) has about 0.2 ppb concentratlon of hydrogen.
One reason that the llquld nltrogen product ln Flgure I ls so pure ls because the contentratlon of hydrogen ln the vapor stream ascendlng ln the low pressure column below the wlthdrawal polnt of llquld nltrogen product ls fairly low. For example thls concentratlon ls 0.06 ppm. Thls vapor as lt ascends the top few trays of the low pressure column strips the descend~ng llquid nltrogen of the l~ght ~mpur~t~es and the l~quid nltrogen product ls purif~ed. W1th the ascens~on of the vapor stream the concentratlon of llghts in the vapor phase lncreases and the gaseous nltrogen leav~ng the top of the low pressure column wlll have about 0.19 ppm hydrogen.
The process of F~gure 1 as w~th others to be descrlbed takes advantage of the fact that the equ~llbrlum constant of the llght contamlnants ls falrly high. For example the equ~l1br~um constant ~or hydrogen at the typ~cal hlgh pressure column pressures of 90-105 ps~a ~s 35-SO. Thls lmplles that the concentrat10n of hydrogen ~n the llquid phase ~n the h~gh pressure column ls about 113S to 1/50 of that in the vapor phase. Another fact which is exploited ls that the value of thls equil~br~um constant increases rapidly as the pressure ~s decreased. Thus the value of the equillbrium constant for hydrogen at typ1cal low pressure column pressures of 18-25 psia is in the neighborhood of 300. As a result when a l~qu~d from the h~gh pressure column is let down ln pressure most of the l~ght contam~nants are contalned ln the flaihed vapor and ~solat~on of th~s flashed vapor leads to a gaseous n~trogen stream from the top of the low pressure column w~th much lower concentrat~on ~ :
of light contam~nants. - :
.

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Example 2 Ultra Hlqh Puritv Gas~ous Nltroqen The flowsheet descr~bed so far in Figure 1 may have one major short-coming. If a gaseous nitrogen product of h~gh purity ls required the purest gaseous nltrogen stream ava~lable fro~ th~s process ls represented by stream 32 from the top of the low pressure column. The concentrat~on of hydrogen ln this stream may be about .021 ppm. In some applications it may be des!rable to produce a gaseous nitrogen stream of even hlgher purlty. Flgure 2 offers a variat~on for produclng gaseous nltrogen of ultra hlgh pur~ty. The gaseous nitrogen product stream ~s w~thdrawn a couple of trays below the top tray in the low pressure column ~stream 35). The concentratlon of the llght contamlnants ln the vapor ascend~ng the low pressure column from the bottom ls extremely low l.e. essent~ally the sam~ as the concentratlon o~ stream 36.
Therefore the concentratlon of llghts ln the nltrogQn vapor str~am 35 ls also extremely low. A small amount of vapor ls allowed to trav~l upwards ln the column and ls collected as stream 3Z from the top of the column. Th1s vapor stream ls requlred to strip the descend~ng reflux llquld nitrogen stream 22 of the light contam~nants. The flow of vapor s~ream 3Z ~s such as to ach1eve the desired strlpplng ln the top few trays of the low pressure column. Stream 32 can e~ther be dlscarded as waste or ~t can be recycled to the nltrogen recycle liquefler. The maln po~nt ~s that the gaseous nltrogen product stream 35 should not be mlxed wlth contam1nated nltrogen stream 3Z. Thus for the feed and product condltlons of th~s flow sheet lt ~s poss~ble to produce an addltlonal gaseous nltrogen product stream w~th hydrogen concentrations of equal to or less than Z.5 ppb.

ExamDle 3 Ultra Hiah Purltv Gaseous Nltroaen Flgure 4 ~llustrates other var~at~ons to the processes of Figures 1 and 2 and produces gaseous nltrogen at extremely h~gh-purlty. In th~s flowsheet the pressure of the low pressure column is increased to a pressure whlch ls hlgher than the conventional low pressure column pressure of 17-22 psia.
- -.

_ ~ ~e ~

Thus the low pressure column is run such that pressure at the top of the low pressure column ~s higher than 22 psia. The l~quid nitrogen stream 20 from the high pressure column is fed to a suitable location in the low pressure column wh~ch preferably is a couple of trays (l to 5) below the top tray in the low pressure column. A liquid nltrogen stream 34 is w~thdrawn from a suitable locat~on in the low pressure column. Thls suitable locat~on ~s at least one tray below the feed point of liquid nitrogen reflux stream 20. A
portton or all of th~s stream ls then let down ln pressure (stream 65) and vaporized in boiler/condenser 218 located at the top of the low pressure }o column. The vaporlzed stream 35 prov1des the desired hlgh-purity gaseous nitrogen product. The top few trays in the low pressure column concentrate the llghts ~n the vapor phase and when th~s stream ls condensed ~n boiler/condenser 218 a stream 67 rlch in llght components is w~thdrawn as a ;
purge stream. By assoclatlng a botler/eondenser wlth the low pressurQ column and by removlng volatlle ~mpur~tles vla l~ne 67 lt ls poss~ble to ellm)nate the taklng of a purge vla llne 18 from the hlgh pressure column. In that case substantlally all of the vapor removed vla llne 14 would be condensed and returned v~a line 16 to the h19h pressure column.
The advantage of the scheme ~n Flgure 4 ls that the purlty and recovery of the gaseous n~trogen product ls hlgh. Th~s ~s because the flowrate of the purge stream 67 is much smaller than the purge streams from the top of the low pressure column and separator 210 as for example ~n F~gure 2.

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Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for the cryogenic separation of air which comprises nitrogen, oxygen and volatile impurities in an integrated multi-column distillation system, having a higher pressure column and a lower pressure column wherein the air stream is compressed, freed of condensible impurities, and cooled generating a feed for the integrated multi-column distillation system, the improvement for producing ultra high purity nitrogen at high nitrogen recovery which comprises:
a) generating a liquid nitrogen fraction and a nitrogen rich vapor fraction containing volatile impurities near the top of the higher pressure column;
b) removing a portion of the liquid nitrogen fraction from the higher pressure column;
c) expanding the liquid nitrogen fraction and introducing the expanded fraction to the top of the lower pressure column as feed;
d) generating a nitrogen rich vapor fraction containing residual volatile impurities at the top of the lower pressure column and removing that fraction as an overhead;
e) partially condensing at least one of said nitrogen rich vapor fractions generated in step (a) or (d) or both in a boiler/condenser;
f) removing at least a portion of at least one of the uncondensed nitrogen rich vapor fractions concentrated in volatile impurities from the boiler/condenser as a purge stream;
g) returning at least a portion of at least one of the condensed nitrogen rich vapor fractions to a column as reflux; and, h) generating and removing an ultra high purity nitrogen fraction as product from the lower pressure column at a point below the removal point for the nitrogen rich vapor containing volatile impurities and below the point of return of the liquid nitrogen fraction as reflux to the lower pressure column.

2. The process of Claim 1 wherein the liquid nitrogen from the higher pressure column is expanded and the volatile impurities flashed therefrom in a separator.

3. The process of Claim 2 wherein at least a portion of the nitrogen liquid obtained from the separator is returned to an upper portion of the lower pressure column as nitrogen reflux.

4. The process of Claim 3 wherein a liquid nitrogen product is withdrawn from the lower pressure column at a point about 2-5 trays below the removal point for the nitrogen vapor containing residual volatile impurities.

5. The process of Claim 3 wherein a nitrogen vapor product stream is withdrawn from an upper portion of the lower pressure column at a point below the removal point for nitrogen-rich vapor containing residual impurities.

6. The process of Claim 2 wherein liquid nitrogen from the high pressure column is charged at the top of a distillation column and volatile components stripped therefrom with the resulting liquid fraction being charged to the separator used for introducing liquid nitrogen to the lower pressure column as reflux.

7. The process of Claim 6 wherein the separator for returning liquid nitrogen to the lower pressure column is a distillation column.

8. The process of Claim 7 wherein a nitrogen fraction rich in volatile impurities is generated in the lower pressure column and a portion of the nitrogen rich vapor fraction containing volatile impurities from the lower pressure column is charged to a separate boiler/condenser and con-densed with the condensed fraction returned as reflux to the lower pressure column and the uncondensed fraction removed as a purge.

9. In a process for the cryogenic separation of an air stream in an integrated multi-column distillation system having a higher pressure column, a lower pressure column, and a side arm column for separation of argon, the improvement for producing ultra high purity nitrogen product, while enhancing nitrogen recovery which comprises:

a) feeding substantially all of said cooled air stream to the higher pressure column;
b) generating a liquid nitrogen fraction and a nitrogen rich vapor fraction containing volatile impurities near the top of the higher pressure column;
c) removing a portion of the liquid nitrogen fraction from the higher pressure column at a point below a removal point designated for the removal of a nitrogen rich vapor fraction containing volatile impurities;
d) expanding the liquid nitrogen fraction and introducing the expanded fraction to the top of the lower pressure column as feed;
e) generating a nitrogen rich vapor fraction containing residual volatile impurities fraction at the top of the lower pressure column and removing that fraction as an overhead; and, f) partially condensing at least one of said nitrogen rich vapor fractions generated in step (b) or step (e) in a boiler/condenser and returning at least a portion of at least one the condensed nitrogen rich vapor fractions to a column as reflux;
g) removing at least a portion of at least one of the uncondensed nitrogen rich vapor fractions concentrated in volatile impurities generated in step (f) from the boiler/condenser as a purge stream;
h) removing an argon stream from the low pressure column and fractionating that argon stream in said side arm column and recovering an argon rich product as overhead; and, j) generating and removing an ultra high purity nitrogen fraction as product from the lower pressure column at a point below the removal point for the nitrogen rich vapor containing residual volatile impurities and below the point of return of the liquid nitrogen fraction as reflux to the lower pressure column.

10. The process of Claim 9 wherein a nitrogen fraction rich in volatile impurities is generated in the high pressure column and a portion of the nitrogen rich vapor fraction containing volatile impurities from the high pressure column is charged to a separate boiler/condenser and condensed with the condensed fraction returned as reflux to the high pressure column and the uncondensed fraction removed as a purge.

11. The process of Claim 10 wherein the liquid nitrogen from the higher pressure column is expanded and the volatile impurities flashed therefrom in a separator.

12. The process of Claim 11 wherein at least a portion of the nitrogen liquid obtained from the separator is returned to an upper portion of the lower pressure column as nitrogen reflux.

13. The process of Claim 12 wherein liquid nitrogen product is withdrawn from the lower pressure column at a point about 2-5 trays below the removal point for the nitrogen vapor containing residual volatile impurities.

14. The process of Claim 13 wherein a nitrogen vapor product stream is withdrawn from an upper portion of the lower pressure column as a point below the removal point for nitrogen-rich vapor containing residual impurities.

15. The process of Claim 14 wherein liquid nitrogen from the high pressure column is charged at the top of a distillation column and volatile components stripped therefrom with the resulting liquid fraction being charged to the separator used for introducing liquid nitrogen to the lower pressure column as reflux.

16. The process of Claim 14 wherein the separator for returning liquid nitrogen to the lower pressure column is a distillation column.

17. The process of Claim 16 wherein liquid oxygen is withdrawn from the bottom of the lower pressure column, expanded, separated into liquid and vapor components in a separator and the liquid component vaporized in a boiler/condenser in the argon column.

18. The process of Claim 17 wherein the separator for separating the expanded liquid oxygen is a distillation column and the liquid oxygen is cooled in a boiler/condenser with said distillation prior to expansion and the resulting expanded oxygen charged to the top of said distillation column.