CA2058500A1 - Cryogenic process for producing ultra high purity nitrogen - 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 product is 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 light impurities is generated with enhanced nitrogen product recovery by withdrawing liquid nitrogen from a first column at an intermediate point and charging that fraction as feed to the second column withdrawing a nitrogen stream which is rich in volatile contaminants from the top of the first column partially condensing that nitrogen stream against crude liquid oxygen and removing the uncondensed portion which has been concentrated in volatile contaminants as a purge stream. An ultra high purity nitrogen product is obtained from a second column.

Description

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PATENT - 2ll~US04429 CRYOGENIC PROCESS FOR PRODUCING
ULTRA HIGH PURITY NITROGEN

_CHNICAL FIELD OF THE lNVENTION
This invention relates to a cryogenic process for the separation of air and recovering ultra hlgh purity nitrogen with high nitrogen recovery.

BACKGROUND OF THE INVENTION
Numerous processes are known for the separation of air by cryogenic distillation into lts constituent components. Typicallyt the air separatlon process involves removal of contaminant materlals such as carbon dloxlde and water from a compressed air stream prior to cooling to near its dew point.
20 The cooled air then is cryogenically dlstilled in an ~ntegrated multl-column dlstillation system.
Processes to produce a high purity nitrogen stream containing few light contaminants such as hydrogen helium and neon have been proposed.
Concentration of some of these contamlnants in the feed alr can be as high 25 as 20 ppm. Almost all of these light components sho~ up in final nltrogen product from an air separat~on unit (ASU). In some cases such as for the electronic industry thls contam~nation 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 4 824 453 discloses a process for producing ultra h~gh purity oxygen as well as h~gh pur~ty n~trogen where the nitrogen purity exceeds 99.998% and the amount of impuritles is generally less than 10 ppm.
More specif~cally alr ~s compressed cooled and d~stilled in a rect~f~cat~on system wherein ~n a f~rst stage rect~f~catlon an oxygen enr1ched ~raction is removed from the bottom and a n~trogen r~ch liqu~d fract~on is removed from an upper port~on of the f~rst stage rect~f~cat~on sub-cooled and returned as reflux to the top o~ the second stage rectif~cat~on. A nitrogen rich l~quld ls remoYed from an upper port~on of the second stage at a polnt just below an overhead removal point for nitrogen vapor ~rom the second stage rect~flcat~on. L~quid oxygen from the bottom of the f~rst stage is sub-cooled expanded and used to dr~ve a bo~ler/condenser in the top of the h~gh pur~ty argon column. Nltrogen vapor from the top of the first stage ~s used to drive a reboiler/ condenser ~n the bottom of a h~gh purity oxygen column. To enhance product purity a port~on of the gaseous nitrogen stream from the top of the f~rst column ls removed as purge.
U.S. 4 902 321 dlscloses a process for produc~ng ultra h~gh purity n~trogen in a mult~-column system. A~r ~s compressed cooled and charged to a f~rst column where lt ls separated lnto ~ts own components generat~ng an oxygen llqu~d at the bottom and a n~trogen r~ch vapor at the top. ThP
oxygen l~quld ls expanded and used to dr~ve a bo~ler/condenser wh~ch ~s thermally l~nked to the top of the f~rst column for condensing the n~trogen r~ch vapor. A portion of the nitrogen rich vapor is remcved from the top of the f~rst column and condensed in the tube s~de o~ a heat exchanger. The result~ng l~quid nitrogen is expanded and charged to a top of a str~pp~ng column wherein nitrogen includ~ng impurit~es are flashed from the stripping column. Any impurit~es not removed by flash~ng are stripped by passing a stream of substant~ally pure nitrogen upwardly through the column. The n~trogen li~uid collected at the bottom o~ the stripping column is pumped to the shell slde of the heat exchanger vaporized against the n~trogen-r~ch vapor and removed as h~gh pur~ty product.
European Patent 0 0376 465 d~scloses an a~r separation process for producing ultra h~gh purity nitrogqn product. In the process n~trogen ~ f -J

product from a conventional air separat~on process ~s charged to the bottom of a column equ~pped w~th a reflux condenser. L~qu~d nitrogen is withdrawn from an upper portion of the column and flashed generating a liquid and a vapor. The l~qu~d obta~ned after flashlng is then flashed a second time and the resulting llquid recovered.
There are essentially two problems associated with the processes descr~bed for produc~ng ultra-high purity nitrogen and these problem; relate to the fact that ~n the 453 d~sclosure purit~es are quite often not sufficiently hlgh to meet industry specifications and in the 321 process nitrogen recover~es are too low. The same can be said of the 465 European patent.

SUMMARY OF THE INVENTION
This invention relates to an air separat~on process for producing ultra high purity n~trogen as product with h~gh n~trogen recovery. In the baslc cryogen~c process for the separatlon of air which comprises nitrogen oxygen and volatile and condensible impur~t~es ~n an integrated multi-column d~stillation system an a~r stream 1s compressed freed of condensible impurities and cryogen~cally d~stilled. Nltrogen ~s recovered as a product. The ~mprovement for produc~ng an ultra hlgh pur~ty nitrogen product in a multi-column distillation system compris~ng a first column and an ultra high purity nitrogen column which comprises:
a) generating a n~trogen r~ch vapor containing volatile impuriti2s near the top of the first column and a crude liquid oxygen fraction in the bottom of sa~d first column;
b) removlng a fraction of said nitrogen-rich vapor containing volat~le impurities and at least part~ally condens~ng at least a port~on of said stream thereby forming a first conden~ed fractlon and an uncondensed fraction;
c) returning at least a portion of said f~rst condensed fraction as reflux to a column in the distillation system;
d) removing at least a portion of the uncondensed nitrogen rlch vapor fraction rich in volatile impurit~es generated in step b) as a purge stream;

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e) removing a liquid nitrogen fraction from the first column at a point below the removal point for the n~trogen rich vapor contalning volatile impuritles from the f rst column;
f) introduclng the liquid nitrogen fraction to an upper part of the ultra hlgh purity nitrogen column as feed;
g) generating a nitrogen rich vapor fraction containlng residual volatile impurities at the top of the ultra high pur~ty n~trogen colu~.n and removing that fraction as an overhead; and h) removing an ultra high purity nitrogen fraction from the ultra high purlty nitrogen column.

There are several advantages associated with thls process those being the abillty to produce nitrogen via a standard nitrogen generator plant with the resultant nitrogen being of ultra hlgh purity and w~th high recovery of n~trogen based on feed a~r ~ntroduced to the process.

DRAWING

The figure is a schemat~c representat~on of an embodiment for generatlng ultra high purity nltrogen with enhanced nltrogen recovery.

DETAILED DESCRIPTION OF THE INVENTION

To faci1itate an understanding of the inventlon and the concepts for 2s generating an ultra high purlty nltrogen product having a volatlle impuritycontent of less than 5 ppm and preferably less than 0.1 ppm reference ~s made to the figure. More particularly a feed alr stream lO is initially prepared from an air stream by compressing an air stream comprlsing oxygen nitrogen argon volat~le impur~tles such as hydrogen neon helium and the llke and condensible impur~t~es such as carbon d~oxide and water in a multl-stage compressor system (MAC) to a pressure ranging from about 70 to 300 psia. Volat~le impurities have a much lower bo~ling point than nitrogen. This compressed a~r stream is cooled with cooling water and chilled against a refrigerant and then passed through a molecular sieve bed to free it of condensable water and carbon dioxide impurities.
The integrated multi-column distillation system comprises a first column 102 and an ultra high purity nitrogen column 104. Both columns 102 and 104 are operated at similiar pressures and pressures which are close in pressure to that of the fe~d air stream 10, e.g., 70 to 300 psia, and typically from 90L150 psia. Air is separated into its components by intimate contact of the vapor and liquid in the f~rst column 102. First column 102 is equlpped with distillation trays or packing, either medium being suited for ef~ecting liquid/vapor contact. A nitrogen vapor stream contain~ng a high concentrat~on of volatile impurities is generated at the top portion of first column 102 and a crude liquid oxygen stream is generated at the bottom of first column 102.
In the process an air stream 10 free of condensible impurities is cooled to near ;ts dew point ln main heat exchanger system tO0. The air stream then forms the feed via stream 12 to first column 102 associated with the integrated multt-column distillation system. A nitrogen r1ch vapor conta~nlng volatile impur~ties ~s generated as an overhead and a crude llquid oxygen fractlon as a bottoms fraction. At least a portion of the nitrogen vapor generated in first column ls withdrawn via line 14 and partially condensed in boiler/condenser 108 located at the top of ~irst column 102. Condensation of the nitrogen rich vapor containing light impurities concentrates these impurities in the uncondensed vapor phase.
The condensed nitrogen which has a fractional amount of impurities is withdrawn from boiler/condenser 108 and at least a portion directed to the top of first column 102 as reflux v~a line 16. The uncondensed n1trogen vapor conta~n~ng a large portion of the ~mpurities ~s remsved via l~ne 18 as a purge.
A liquid nitrogen fraction is collected at a point typically about 2-5 trays below the nitrogen removal polnt via line ~4 in flrst column 102.
That liquid nitrogen fractlon is removed via line Z0 and introduced to the top of ultra high purity nitrogen column 104 as ~eed and reflux. ultra high purity nitrogen column 104 is operated within a pressure range from about 70-300, typically 90-150 psia, in order to produce an ultra high purity nitrogen product. The objective in the ultra hish purity nitrogen column is to provide ultra high purity n~trogen e.g. greater than 99.99870 preferably 99.999% by volume purity at the bottom of the column. Ultra high purlty nitrogen column 104 is equipped w~th vapor liqu~d contact medium wh~ch comprises dist~llation trays or packing.
It ls in ultra high purity nltrogen column 104 where ultra h~gh pur~ty nltrogen ts generated. The key to ~ts success is the ultimate concentratlon and removal of a large part of the volatile impurities from a n~trogen vapor. More partlcularly a nltrogen-rich stream contalning residual 1~ volatlle impur~t~es is generated and removed from the top or upper most portion of ultra high purity nitrogen column 104 as an overhead via llne 32 wherein it ~s returned to the upper to middle portlon of flrst column 102.
The concentration of residual volat~le impuritles in nitrogen vapor stream 32 is primarily controlled by the purge nitrogen stream removed from an upper port~on of first co1umn 102 as this governs the amount of volat11es submitted to the ultra h~gh pur~ty n~trogen column. An ultra high purlty nitrogen product is generated as a l~qu1d fractlon (LIN) in the bottom port~on of the ultra high purlty nitrogen column 104 and removed vla l~ne 34.
The ultra high purity liqu~d nitrogen (stream 34) is vaporized by feedlng it to a boiler/condenser 114 th2re~n. The l~quld stream is cxpanded through a valve and charged to the vaporizer s~de of the boiler/condenser 114. This vaporization of the llquid nitrogen at least part~ally condenses the nitrogen rich stream containing volatiles taken as an overhead from 2~ flrst column 102 v~a line 35. An ultra hlgh purlty nitrogen product ~s obtained as a liquid fract~on from the bollerlcondenser v~a l~ne 38 and as a vapor fract~on via llne 40. The condensed fraction is returned to the flrst column 10Z as reflux via line 37. If the nitrogen feed conta~n~ng volatiles in T1ne 35 is partially condensed ~n boiler/condenser 114 then the uncondensed portion is removed as a purge stream via line 41. Th~s purge stream may be comblned with purge stream 18 and d~scarded. Alternat~vely the purge streams may be collected for the recovery of light contaminants helium hydrogen and neon.
Oxygen is not a desired product in thls nitrogen generating process.
Crude l~qu~d oxygen is removed from first column 102 as a bottoms fraction ;

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via line 42, cooled in boilerlcondenser 110, expanded and then charged via line 43 to the vaporizer section of boiler/condensed 108 located at the top of first column 102. The Yaporized portion of the oxygen is removed via line 44 as an overhead and the balance as a liquid purge via line 45. Some of the overhead is diverted to a turboexpander 116 via line 46 with the balance being warmed in main heat exchanger 100 and then diverted to turboexpander 116. The exhaust from turboexpander 116 is warmed aga~nst process fluids in heat exchanger lGO and the dlscharged as waste.
Optionally, a small fractlon of the feed to turboexpander 116 may be diverted through an expansion valve and then discharged as waste.
In the figure, the boilup at the bottom of the ultra high purity nitrogen column 104 is provided by cooling crude liquid oxygen 42 in the boiler/condenser 110. Alternatively, this boilup can be achieved by heat exchange with any suitable fluid. An example can be condensation of a 15 portion of the feed a~r stream 12 in the boiler/condenser 110 to proivde the boilup at the bottom of the ultra high purity nitrogen column 104. In thls case, the condensed air strea~ will be returned to a suitable location ~n the flrst d~stillat~on column 102. Also, ~t ~s possible to use more than one fluid for heat exchange in the bottom boiler/condenser 110.
In the f~gure, two purge strea~s 18 and 41 rich in light volatile impurities are shown, one from boiler/condenser 108 and one from boilerlcondenser 114. HoweYer, it is not totally necessary to take purge from both of these bo~ler/condensers and nitrogen r~ch stream containing volatiles may be totally condensed 1n any one of them. A purge stream from 25 at least one of the boilerlcondensers 108 or 114 is necessary but purge from both as shown ~n the figure will decrease the conçentratlon of volatiles ~n the feed to the ultra h~gh purity nitrogen column 104.
Even though not shown in the figure, it is also possible to withdraw an ultra high purity gaseous nitrogen stream as-product from the bottom of the ultra high purity nitrogen column 104. This will particularly b~ more attractive when only a fraction of the total nitrogen product is needed as an ultra high purity gaseous nitrogen. In such a case, most of the nitrogen product will be producQd of standard purity from the top section of the first distillation colu~n 102 and a gaseous ultr~ high purity nitrogen product from the bottom of the ultra hlgh purity nitrogen column 104. The pressure of both the nitrogen products w~ll be nearly ~dent~cal. In th~s case no ultra hlgh purity l~quid nitrogen stream 34 may be withdrawn from the bottom of the ultra h~gh purity n~trogen column 104 to be vaporized ~n the bo~ler/condenser 114. Thus for th~s case where only a fract~on of the total nltrogen product ~s produced as ultra h~gh purlty n~trogen boller/condenser 114 may not be used.
The followtng examples are prov~ded to ~llustrate the embod~ments of the ~nvent~on and are not ~ntended to restr~ct the scope thereof.
Exam~le 1 Ultra H~gh Pur~tv L~quid N~trogen An air separat~on process using the apparatus descrlbed in the f~gure was s~mulated. In th~s f~gure feed a~r stream 12 conta~n~ng l~ght contam~nants ~s fed at the bottom of the first column. A gaseous n~trogen stream 14 ~s w~thdrawn from the top of first column 102 and ~s r~ch ~n volatile contamlnants. A l~qu~d nltrogen stream 20 ls also w~thdrawn from about 2-5 trays below the nitrogen w~thdra~al po~nt as feed and reflux to the ultra h~gh purity nitrogen column 104. No major product streams are w~thdrawn from the top of the f~rst column and the top 2-5 trays ~ncrease the concentrat~on of the l~ghts in the vapor phase. A non-condens~ble purge (stream 18) ~s taken from the bo~ler/condenser located at the top of the f~rst column. Th~s purge contalns a fa~rly h~gh concentrat~on cf the lights and ~s responsible for removing the majority of the light contam~nants from the system.
Alternatlvely no purge need be taken and substantially all of the stream may be condensed and the volat~les allowed to concentrate for rcmoval via llne 41. These two streams are responsible for recovery ~n the process ~n the sense that the higher the flow rate the lower the recovery. However because each stream is concentrated in l~ghts the~r volume may be ma~ntained at a low 3~ level thereby enhancing recovery.
Sample calculations for the flowsheet in Figure 1 were done for a preselected process design. The table sets forth the conditlons:

TABLE
AIR SEPARATION FOR PRODUCING ULTRA HI~H PURITY NITROGEN
~BÇ~iS CONDITIONS FOR THE FIGURE

Com- T P F Impurity Concentratlon Stream ~Qnent F ps~a lb moles He H2 Ne 12 alr -269.9 126 100 5.2 ppm 10 ppm 18.2 ppm N2 -277.6 122 41.1 0.05 ppm 0.35 ppm 0.58 ppm 28 purge -279.9 122 0.05 1.04% 1.97% 3.58X
32 N2 -277.6 122 2.9 0.65 ppm 4.96 ppm 8.32 ppm 34 N2 -277.5 122 38.2 <0.01 ppb O.C5 ppb 0.05 ppb N2 277.7 122 37.7 89 ppm 0.06% o.llX
N2 -280 110 38.2 <0.01 ppb 0.05 ppb 0.05 ppb The process descr~bed ~n the f~gure results ~n h~gh nltrogen recovery of ultra h~gh purlty product vla l~ne 38 and l~ne 40 w~th an extremely low 2~ ~mpurlty level. Note the level of total contam~nants is 0.11 ppb ~mpurlt~es.

Claims (8)

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 wherein an air stream is compressed freed of condensible impurities and cooled generating a feed for the integrated multi-column distillation system the improvement for producing an ultra high purity nitrogen product in a multi-column distillation system comprising a first column and an ultra high purity nitrogen column which comprises:
a) generating a nitrogen rich vapor containing volatile impurities near the top of the first column and a crude liquid oxygen fraction in the bottom of said first column;
b) removing and partially condensing at least a portion of said nitrogen rich vapor fraction containing volatile impurities thereby forming a first condensed fraction and an uncondensed fraction;
c) returning at least a portion of said first condensed fraction as reflux to a column in the distillation system;
d) removing at least a portion of the uncondensed nitrogen rich vapor fraction rich in volatile impurities generated in step b) as a purge stream e) removing a liquid nitrogen fraction from the first column at a point below the removal point for the nitrogen rich vapor containing volatile impurities from the first column;
f) introducing the liquid nitrogen fraction to an upper part of the ultra high purity nitrogen column as feed;
g) generating a nitrogen rich vapor fraction containing residual volatile impurities at the top of the ultra high purity nitrogen column and removing that fraction as an overhead; and h) removing an ultra high purity nitrogen fraction from the ultra high purity nitrogen column.

2. The process of Claim 1 wherein a portion of said nitrogen rich vapor fraction containing volatile impurities from the first column is at least partially condensed against crude liquid oxygen in a boiler/condenser located at the top of the first distillation column to provide a condensed fraction which is returned to the first distillation column as reflux.

3. The process of Claim 2 wherein the liquid nitrogen from the ultra high purity nitrogen column is expanded and warmed against a fraction of the nitrogen rich vapor containing volatile impurities from the first column in a boiler/condenser thereby partially condensing a fraction of the nitrogen rich vapor, separating the condensed fraction from the uncondensed vapor fraction and removing the uncondensed vapor fraction as a purge stream.

4. The process of Claim 2 wherein the condensed nitrogen rich vapor fraction reduced in volatile impurities is returned to the first column at an upper portion as reflux.

5. The process of Claim 3 wherein a liquid and vapor fraction are generated on the vapor side of the boiler/condenser and at least a portion of the nitrogen liquid is recovered as product.

6. The process of Claim 5 wherein at least a portion of the nitrogen vapor is recovered from the vapor side of the boiler/condenser as product.

7. The process of Claim 4 wherein crude liquid oxygen from the bottom of the first column is charged to a boiler/condenser in the bottom portion of the ultra high purity nitrogen column, cooled by indirect heat exchange, expanded and charged to the vaporizer side of the boiler/condenser located at the top of the first column.

8. In a process for the cryogenic separation of air which comprises nitrogen, oxygen and volatile impurities in an integrated multi-column distillation system wherein an air stream is compressed, freed of condensible impurities, and cooled generating a feed for the integrated multi-column distillation system, the improvement for producing an ultra high purity nitrogen product in a multi-column distillation system comprising a first column and an ultra high purity nitrogen column which comprises:
a) generating a nitrogen rich vapor containing volatile impurities near the top of the first column and a crude liquid oxygen fraction in the bottom of said first column;
b) removing and partially condensing at least a portion of said nitrogen rich vapor fraction containing volatile impurities thereby forming a first condensed fraction and an uncondensed fraction;
c) returning at least a portion of said first condensed fraction as reflux to a column in the distillation system;
d) removing at least a portion of the uncondensed nitrogen rich vapor rich fraction rich in volatile impurities generated in step b) as a purge stream;
e) removing a liquid nitrogen fraction from the first column at a point below the removal point for the nitrogen rich vapor containing volatile impurities from the first column;
f) introducing the liquid nitrogen fraction to an upper part of the ultra high purity nitrogen column as feed;
g) generating a nitrogen rich vapor fraction containing residual volatile impurities at the top of the ultra high purity nitrogen column and removing that fraction as an overhead; and h) removing an ultra high purity nitrogen fraction from the ultra high purity nitrogen column.