CA1221617A - Production of pure nitrogen - Google Patents
Production of pure nitrogenInfo
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- CA1221617A CA1221617A CA000443004A CA443004A CA1221617A CA 1221617 A CA1221617 A CA 1221617A CA 000443004 A CA000443004 A CA 000443004A CA 443004 A CA443004 A CA 443004A CA 1221617 A CA1221617 A CA 1221617A
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Abstract
S P E C I F I C A T I O N
TO ALL WHOM IT MAY CONCERN:
BE IT KNOWN THAT I, RONALD DENIS OPENSHAW, a British citizen, of 8 Beech Crescent, Poynton, Stockport, Cheshire, United Kingdom, have invented certain new and useful improvements in IMPROVEMENTS IN THE PRODUCTION OF PURE NITROGEN of which the following is a description.
ABSTRACT OF THE DISCLOSURE
A process and apparatus for the production of nitrogen from air by the cryogenic fractionation of the air at superatmospheric pressure and which permits the production of variable quantities of nitrogen from a constant supply of air at superatmospheric pressure by periodically distilling less than all of the air which is supplied at superatmospheric pressure and work expanding a stream of air at superatmospheric pressure provided from the balance of said supply whereby to produce additional refrigeration for the process, and recovering condensed nitrogen vapour thereby formed in excess of that required for reflux.
TO ALL WHOM IT MAY CONCERN:
BE IT KNOWN THAT I, RONALD DENIS OPENSHAW, a British citizen, of 8 Beech Crescent, Poynton, Stockport, Cheshire, United Kingdom, have invented certain new and useful improvements in IMPROVEMENTS IN THE PRODUCTION OF PURE NITROGEN of which the following is a description.
ABSTRACT OF THE DISCLOSURE
A process and apparatus for the production of nitrogen from air by the cryogenic fractionation of the air at superatmospheric pressure and which permits the production of variable quantities of nitrogen from a constant supply of air at superatmospheric pressure by periodically distilling less than all of the air which is supplied at superatmospheric pressure and work expanding a stream of air at superatmospheric pressure provided from the balance of said supply whereby to produce additional refrigeration for the process, and recovering condensed nitrogen vapour thereby formed in excess of that required for reflux.
Description
lZ2161'~
IMP~OVEMENTS IN T~IE PRODUCTION OF PU~E NITROGEN
_ This invention !~elates to the production of nitrogen by its cryogenic separa-tion from air and in~particular to n process which permit~ a variable rate of supply of nitrogen from a relatively con~tant supply of compressed air.
A known proceas for the production of pure nitrogen by cryogenic air separation consists in compressing air to a superatmo~pheric pressure, usually about 9 bar abs, purifying the air of components which would ~olidify at the temperature~
employed, e.g. by removing most of the water content by cooling the air to about 5C with a refrigeration unit and eliminating the remainder of the water vapour and the carbon dioxide contained in the air by adsorption on molecular sieve~ or a similar material, cooling the compressed purified air to a temperature close to it~ dew-point, and feeding the cooled air to the base of a distillation column from which the nitrogen is recovered overhead.
Reflux for the distillation is supplied by expanding to an intermediate pre~sure the oxygen-rich liquid leaving the column base and evaporating said liquid in heat exchange with a portion of the nitrogen vapour leaving the top of the column whereby to condense said vapour. A second portion of the nitrogen vapour is recovered as product. The evaporated oxygen-rich liquid recovered from the reflux condenser is ;
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' ' :
, ~ , , :
,~ ' :, 12~1~;17 thereafter expanded in a turbine to ne~qr ntmospheric pre3sure, preferably after being pre-heated to avoid liquid form&tion in the -turbine, thu~ producing the refrigera-tion required to maintain the process. The cooling of the compressed purified air is effected by paa~ing it in countercurrent with the nitrogen product and expflnded oxygen-rich vapour in A main heat exchange.-wherein said nitrogen product and oxygen-rich vapour are warmed to near ambient temperature. The preheating of the oxygen-rich vapour prior to expanding may also be effected in this main heat exchanger. In general, the refrigeration available will permit the conden3ation of more of the nitrogen vapour than is required for reflux, thus providing a source of liquid nitro~en whi~h cAn be recovered and ~tored. ~lowever, in general only about 5 - 10%
of the total nitrogen production can be made available in this form, the actual amount depending on the ~ize of the plant.
Since on many industrial site~ nitrogen is required intermittently for purging equipment and similar purposes, there is a gro~ing demand for plants able to increase liquid produc-tion during periods in which little or no gaseous nitrogen is required, this liquid nitrogen to be stored in insulated tanks for use in subsequent periods of higher nitrogen demand. In conventional cryogenic nitrogen proces~es and plants of the kind described above the output of liquid nitrogen i~ independent of the amount of gaseous nitrogen delivered and cannot be increased by reducing the output of gaseous nitrogen.
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` ~LZ~l~;i7 T~le present invention provide~s a modification of a ni-trogen proce~q~ plant of the kind de~cribed sbove which enable~
the output of liquid nitrogen to be significantly increAsed during period~s in which the requirement for ga~eou~ nitrogen i~
low, thereby increa~ing the capability of the process and plant to satisfy intermittent periods of high nitrogen demand.
An increa~ed production of liquid nitrogen requires an increa~e in the amount of refrigeration supplied and, in conventional plants, this i9 limited by the amount of oxygen-rich gas available and the pres~ure at which it can be fed to the expansion turbine. In conventional operation the flmount of oxygen-rich ga3 flvailable is approximately 60~ of -the air treflted and the maximum preasure at which it can be fed to the expan~ion turbine is about 5.0 bar abs.
In periods during which the demand for ga3eouc nitrogen is low, it ir~ not nece~sary to feed the ~ame quantity of air to the distillation column a~ in periods of high demand, and in conventional plants it ir~ cuotomary to reduce the flow of air during such periods by ouction-throttling the air compre~or.
This leads to some reduction in power con~umption, but doe~ not permit more liquid nitrogen to be produced because there i~s no increar~e in the flow of oxygen-rich vapour through the turbine and hence the amount of refrigeration provided.
In the proce~s of this invention, only a part of the air is fed to the distillation column during periodr~ of low gas .
1~1617 demand and the remainder i~ work-expanded, either by joining the oxygen-rich waste gas ~tream after t;hrottling to intermediate pressure, or by passing at full pressure to a separate expander.
By means of this arrangement, additional refrigeration i~
produced thereby enabling more nitrogen to be withdrawn as liquid and this liquid can then be stored for use in a subsequent period of increa~ed nitrogen demand.
Thus according to the present invention, there i~
provided a process for the production of nitrogen from air by cooling and distilling a ~upply of air at ~uperatmospheric pressure to produce a nitrogen-rich vapour stream and nn oxygen-containing liquid re~iduc, rccovering a fir3t portion o~ ~ai(l vapour as product nitrogen gas, condensing a second portion of said vapour by indirect heat exchange with oxygen-containing liquid residue which has been expanded to an intermediate pressure, to provide reflux for the distillation, and providing refrigeration for the process by work-expanding evaporated oxygen-containing liquid residue recovered from said indirect heat exchange, said process further comprising periodically distilling less than all of the air which is supplied at superatmospheric pressure and work expanding a stream of air at superatmospheric pressure provided from the balance of said supply whereby to produce additional refrigeration for the process, and recovering conden~ed nitrogen vapour thereby formed in excess of that required for reflux.
- . . . . ~
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-', ' , ' ' '" . ' ' " ' ' :" : ' . ~ .' - ' The liqui~l nitrogen theroby produced mny be ~Itore(l and in a ~ulx~eqllerlt period of operntion, when demand for nitrogen gas exceeds the plnnt's capacity, the output of the plant mny be supplemented from this ~tore. By thi~ meane, therefore, it i3 possible to provide a variable supply of nitrogen from the plnnt while maintaining the supply o f oompressed air constant. Further plant flexibility is offered by the alternative of supplying at least a part of -the liquid nitrogen as such, as it is produced and/or after storaee.
In one embodiment of the process, which is more suitable for use in relatively small plants, the stream of air thut if~ to b- work~expnndoA i~ e~pan(1ed to nn interm-~diqte pre~sure and combined with the oxygen-containing liquid residue which has been evaporated producing the condensed nitrogen vapour, and the combined stream is work expanded. By mean~ of thi3 embodiment, only one expansion engine is required.
In another alternative embodiment, which is more suitable for use in relatively larger plnnts, e.g. delivering at least about lOOO Nm3/hr of nitrogen, the stream of air and the evaporated oxygen-containing liquid residue are separately work expanded. Conveniently, however, the two work-expanded streams may then be combined into a single ~tream for subsequent indirect heat exchange with the feed air supply to cool the latter.
Preferably expansion turbine~ are used which may be radial inward-flow air-lubricated machines. In the second-~ ~1ti17 mentioned embodiment, the turbines may be equipped with variablenoz~les, in which case the two mflchineg may be interchangeable.
It i8 then no longer essential for the user to carry a spare turbine, since, if one machine should fail, operation can continue although without the production of the additional liquid nitrogen.
In general, the process may be controlled so that the proportion of the compressed air supply which is fed to the distillation i8 varied to match the demand for nitrogen gas.
Preferably the whole of the remainder of the supply of compressed air, when the nitrogen gas demand is low, is subjected to the work expan~ion since this will maximiso tho production of net available condensed nitrogen vapour e.g. for storage.
The invention also provides apparatus for the production of nitrogen by compressing a supply of air to supsratmospheric pressure, and after removing water vapour and carbon dioxide cooling it and subjecting the compressed air to separation by cryogenic distillation wherein the condensation of a portion of the nitrogen vapour recovered overhead to provide reflux for the distillation is effected by indirect heat exchange with evaporating oxygen-containing liquid residue of the distillation and refrigeration for the process is provided by work expanding evaporated oxygen-containing liquid residue recovered from the reflux condenser, said apparatus further including valve, conduit and work expansion means for "
61'~
intermittentl~y divQrting from the di~tillnt;ion a portion of the air ~,upplied at ~uperatmo~pheric pre~ure, work expanding it and pa~sing the work-expanded ~tream in indirect heat exchange with the feed to the di~tillation to provide additional refrigeration and means for recovering and optionally storing the excess conden~ed nitrogen vapour ~o obtained.
In accordance with one embodiment, said apparatus may include means for providing a supply of purified air at superatmo~pheric pre3sure main heat exchange means di~tillation mean~ having an inlet for air at superatmospheri.c pre~sure, an inlet for column reflux, a fir~t outlet for nitrogen-rich vapour and a ~econd outlet for oxygen-containing liquid residue reflux conden~ing means first and second expansion valve mean~
expansion engine means having an inlet and outlet for gas to be expanded optionally ~t least one in~ulated storage tank for liquid nitrogen, said tank being provided with an inlet for the liquid nitrogen and a valved outlet first conduit means for directing air at ~uperatmospheric pres~ure from said air supply means through ~aid main heat exchange means to said di~tillation means inlet .. . ' -:
.
~
~ econd conduit menn~ for directing a first portion of nitrogen vapour from snid distillation means first outlet through said main heat exchanger means in counter-current indirect heat exchange relation~hip with said air in said first conduit means and recovering said vapour a8 product nitrogen gas third conduit means for directing a second portion of nitrogen vapour fro~ said di~tillation means first outlet through said reflux condensing means whereby to condense said nitrogen vapour fourth conduit means for directing a first portion of condensed nitrogen vapour recovered from said reflux condenser means to said distillation means reflux inlet as reflux for t~e distillation fifth conduit means for recovering a second portion of condensed nitrogen vapour from said reflux condensing means and optionally directing it to said at least one insulated storage tank sixth conduit means for directing oxygen-containing liquid residue from said distillation means second outlet through said first expansion valve means and then through said reflux condensing means in indirect heat exchange relationship with condensing nitrogen vapour in said third conduit and thence to the inlet of said expansion engine means seventh conduit means for directing gas from the outlet of said expansion engine means through said main heat exchanger :' ' `
:.
, ' ' . ' . ` : ' ' '. '' '. ' ", :
' ' .
lZ~l~i7 ~, means in indirect cc)unter-current heat exchange relationship with ~aid nir at ~uperatmo~pheric pre~ure in ~aid fir~t conduit mean~
eighth conduit mean~ for directing air at superatmospheric pressure from said air supply mean3 through said second expansion valve mean~ to said ~ixth conduit mean~
downstream of said reflux condensing means, and valve means for controlling the flow of air in said eighth conduit means.
In accordance with another embodiment, the apparatus include~
means for providing a supply of purified air a-t superatmospheric pressure main heat exchange means distillation means having an inlet for air at superatmospheric pressure, an inlet for column reflux, a first outlet for nitrogen-rich vapour and a second outlet for oxygen-containing liquid residue reflux condensing means expansion valve means first and second expansion engine means each having an inlet and outlet for gas to be expanded optionally at least one insulated storage tank for liquid nitrogen, said tank being provided with an inlet for the liquid nitrogen and a valved outlet .. ,,,,.,,,, . .
- ' ~
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1;~2iti1~7 -- 1 o --fir~t; conduit mean~ for directing air at superatmospheric pressure from said nir supply means through sai.d main heat exchange means to said distillAtion means inlet second conduit means for directing a first portion of nitrogen vapour from said di~tillation means first outlet through said main heat exchanger means in co~mter-current indirect heat exchange relationship with said air in said first conduit means and recovering said vapour as nitrogen product gas third conduit means for directing a second portion of nitrogen vapour from said distillntion meAns first outlet through said reflux conden3ing means whereby to condense said nitrogen vapour fourth conduit means for directing a first portion of condensed nitrogen vapour recovered from said reflux condenser means to said distillation means reflux inlet fifth conduit means for recovering a second portion of condensed nitrogen vapour from said reflux condensing meane and optionally directing it to said at least one insulated ~torage tank sixth conduit means for directing oxyeen-containing liquid residue from said distillation means second outlet through said expansion valve means and then through said reflux condensing means in indirect heat exchange relation~hip with condensing nitrogen vapour in said third conduit and thence to the inlet of said first expansion engine means .
.. ' .
~z~1617 seventh conduit means for d:irecting gas from the outlet of ~aid fir~t expan~ion engine mean~ through ~aid main heat exchanger means in indirect counter-current heat exchange relation3hip with ~aid air at ~uperatmo~pheric pres~ure in said first conduit means eighth conduit means for directing air at superatmospheric pressure from said air supply means to the inlet of said second expansion engine mean~
ninth conduit means for directing air from the outlet of said second expansion engine means through the main heat exchanger mean~ in indirect counter-current heat exchange relationship with said air at ~uperatmospheric pressure in said first conduit means, and means for controlling the flow of air in said eighth conduit means.
In both embodiments, between the reflux condensing means and the first expansion engine mean3 said sixth conduit will normally pass through a part of the main heat exchange means at or towards the cold end thereof, superheat the oxygen-containing liquid re~idue which has been evaporated in said reflux condensing means to a temperature such that after expan~ion in the expansion engine it will be at or just above its dew point.
Likewise, in the second embodiment, it is preferred that the eighth conduit connects with said first conduit means to ', .
' . ~ . . .
. . - ~ ' ': ~ ' :' ' ' ' .
~Z~ii17 divert air therefrom at a point within ~aid main heat exchange means corresponding to a temperature such that after expansion through ~aid second expan~ion engine mean~ the air is at or ju~t above its dew point.
The apparatu~ may suitably include a valve adapted to control flow in said eighth conduit means and means respon~ive to pressure and/or flow changes in the compre~sed air supply and/or product nitrogen gas for controlling said valve whereby a fall in the rate of flow of said product nitrogen ga~ below a predetermined level opens said valve and a rise in said rate of flow closes said valve and at rates of flow of said product nitrogen ga3 below said predetermined va]ue the rate of flow of air through said eighth conduit is proportional to the difference between the actual product nitrogen ga~ flow rate and said predetermined value.
The invention will now be described in greater detail with reference to preferred embodiment3 thereof and with the aid of the accompanying drawings in which Figure 1 is a flow diagram of one embodiment of the invention wherein there i8 provision for a part of the compressed air supply to be combined with the evaporated oxygen-containing liquid recovered from the reflux condenser for subsequent work expansion, and Figure 2 is a flow diagram of an alternative embodiment wherein there i8 provision for a part of the compressed air `
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~ ` - ' , -~Z~1~17 ~upply to be expanded ~eparately from the evaporated oxygen-containing liquid.
In the figure~ the numeral~ 1 and 2 represent heat exchangers, 3 is a distillation column, 4, 5 and 6 are expansion turbines, 7, 8 and 9 are automatically controlled valves, 10 is a manually adju~table valve, which can be set as required, PIC
represents a pressure-responsive controller and FIC represents a flow-responsive controller.
In the e~bodiment shown in Figure 1 air, which ha3 been compressed e.g. to about 9 bar abs., and from which moisture and carbon dioxide have been removed by conventional means;
enters heat exchanger 1 through line 11 at near ambient temperature and is cooled to a temperature slightly below its dew-point in counter-current heat exchange with gaseous nitrogen product in line 24, and a waste gas, the nature of which is identified below, in line 22. Leaving the exchanger the air feed can divide into two streams passing through lines 12 and 26, re~pectively. Control is provided by valve 8.
In one mode of operation, suitable for use in periods when demand for nitrogen gas is high, the whole of the air feed passes through line 12 to the base of the distillation column 3 in which it is separated into a pure nitrogen overhead stream leaving through line 14 and an oxygen-containing liquid, which leaves through line 15. This liquid is first employed as the coolant for reflux condenser heat exchanger 2. To this end it .
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-mu3t rir~-t be oxpanded to an intermc(iiate ~re~ re, e.g.
approximntely 5.0 bar nb~. in the vnL~e 10 and i~ then pa~ed to the reflux condon3er 2 wherein it i~ evnporated thereby conden~ing a fir~t part of the nitrogen overhend stream from the column which i~ withdrawn through line 16. The conden3ed nitrogen leave~ the condenser through line 17, at leAst a part being returned through line 18 to the column a~ reflux and any remainder, in line 19, being drawn off through level-controlled valve 9 a~ liquid nitrogen product. The remainder of the overhead nitrogen ~tream i3 recovered in line 24, pa~3ed through heat exchanger 1 where it i9 warmed to near ambient temperature and recovered through line 25 and pres~ure-controlled valve 7 a~
nitrogen product gas.
The evaporated oxygen-containing liquid leaving the reflux conden3er in line 20 i~ pas~ed via line 21 to the expan~ion turbine 4, in which it i3 work-expanded to near atmospheric pre3~ure. Leaving the turbine through line 22, it i~
warmed to near ambient temperature in exchanger 1 in countercurrent heat exchange with the compressed air, finally leaving through line 2~. Before entering the turbine, the stream in line 21 is ~uperheated in heat exchanger 1, a~
illustrated, to the degree neces~ary to avoid liquid formation occurring in the expansion turbine. To ensure the proper heat exchange in exchanger 1, this stream is 90 superheated that when work expanded in the expansion turbine the out]et temperature i~
,' ' ' just above or at the dew point of the expanded ~tream.
In accordance with the invention, provision is made via conduit 26 to divide a portion of the compressed air from line 12 when demand for nitrogen product ~a~ is low and, after expanding it through valve 8 to about the same pressure as the evaporated oxygen-rich liquid in line 20, to inject it in line 20 whereby it i~ expanded with the evaporated-oxygen-rich liquid in expansion turbine 4. Thi~ increaees the amount of refrigeration produced and thus also the amount of nitrogen liquid that is produced.
The excess over that required for reflux i9 recovered through line 19.
One arraneement for automatic control of the plant is now described. In a first mode of operation, the demand for nitrogen gas from line 25 equals the capacity of the plant and valve 8 is clo~ed. When demand for nitrogen eas increases, the Pre~sure Indicator Controller (PIC) will throttle to maintain the system pres~ure. A reduction in demand, on the other hand, will initially cause a rise in pre~sure and the PIC will open fully.
If demand continue~ to fall, the further increase in pressure can be employed by any suitable means, e.~. a suction control valve on the air compressor (not ~hown), to decrease the air flow in -::
~ Iine 11. This reduction in flow ic then detected by the Flow :
Indlcator Controller (FIC) which i9 arranged to open valve 8 whereupon the pressure fall~ thereby causing the suction control valve to reopen and re~tore the floN in line 11.
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~L2~i~i'7 The openine of v~-lve 8 permit~ the process to move to a ~econd mode wherein the air flow in line 26, which i~ the exce~
of the compre~sed air ~upply over and above that requlred to ~ati~fy the reduced demand for nitrogell ga~, ic expanded through ~aid valve into the evaporated oxygen-rich liquid in line 20 and thence pa3~ed to expan~ion turbine 4 thereby increasin~ the refrigeration delivered to heat exchanger 1. Thi~ re~ults in an increace in the proportion of liquid in the air to the di~tillation column which in turn allows for the withdrawal of a greater proportion of the reflux a~ liquid nitrogen product.
Thi~ can be achieved without affectine the purity of the nitrogen product oince the nitrogen product rate i~ low and the reflux available ic much greater than nece~3ary to achieve the de~ired separation of nitrogen from air.
A portion of the compres~ed air in line 12 will continue to flow through line 26 while the demand for nitrogen i~ below that obtainable froM the full air supply.
When demand for nitrogen gas i9 ~ubsequently increa~ed, the 3y~tem pre~sure will decreace and flow in line 11 will tend to increase. Thi~ flow increase i~ detected by the FIC which will act to throttle valve 8 to restore the correct flow in line 11. When valve 8 i~ fully clo~ed, whereby all the compres~ed air in line 11 i~ fed to the distillation column, the ~y~tem reverts to it~ fir~t mode of' operation and any further increa~e in demand for nitrogen gao will cau~e the PIC to throttle to maintain . - ' , : ' - ~ . ' :' ' ':
- : -' lZ'~16~7 system pressure, a9 before. Means may be provided for then automatically opening a valve in the outlet from a liquid nitrogen storage tank in which liquid nitrogen produced during the second mode of operation hfls been stored, whereby to supply additional nitrogen from that source to satisfy the increased demand for nitrogen gns.
An alternative embodiment of the invention i9 illustrated in ~igure 2 in which all parts and conduits common with the arrangement of Figure 1 are identified by the same reference numerals. While otherwise arranged and operating in substantially the same manner as that of Figure 1, in this embodiment turbine 4 is replaced by two turbines 5 and 6 through which respectively the evaporated oxygen-contnining liquid and any compressed air diverted from line 11 (in this case through line 13) are separately expanded before being combined in line 28 and passed through main heat exchanger 1 as the refrigerant stream.
In order to provide the compressed air stream to the turbine 6 at the desired temperature such that the expanded air leaving the turbine at about atmospheric pressure is at about its dew point and substantially no liquid formation occurs in the turbine, this air i3 withdrawn from the main feed line 11 at an appropriate point in main heat exchanger 1, e.g. corresponding to a temperature of about 130K. For the same reason, the evaporated oxygen-containing liquid recovered from the reflux condenser heat . -, , ` ~
~z~
exchanger ~ in line 20 is fir~t guperheated to about 120K in the main heat exchanger 1 before being passed to the turbine 5.
In this second embodiment of the invention, two alternative method~ of operation are applicable in periods in ~hich the demand for gaseous nitrogen is low and a high rate of production of liquid nitrogen i~ desired.
In the first method of operation, which is preferable when the low demand for gaseous nitrogerl i9 rectricted to a r61atively short poriod, such as a night shift or week-end, the flow of oxygen-containing liquid residue i~ left unchanged; i.e., the setting of valve 10 is not altered. As in the embodiment of Fi~ure 1, the valve ~ i~ operated to divert through line 13, expansion turbine 6 and line 28, the excess of the compressed air feed in line 11, over and above that required to satisfy the reduced demand for nitrogen gas. A significant increase is thus obtained in the output of liquid nitrogen through valve 9.
If the low demand for gaseous nitrogen is to continue for an extended period, a further increase in liquid nitrogen output can be obtained in accordance with n second method of operation by adjusting the eetting of valve 10 to reduce the flow of oxygen-rich liquid from the bottom of the column and hence the flow of vapour to turbine 5. To maintain the total flow of air to the plant, the flow indicator controller (FIC) will then automatically divert a larger flow of air through valve 8 to the auxiliary turbine 6, thereby re~ulting in a further increaced .
; ' ' ~ ' - : :
-lZ'Zl~
, 9 output of liquid nitrogen.
The reduction of flow through valve 10 with theconsequent increase of flow through valve ~ can be continued as long as the resultlng reduced flow of air to the column i~
adequate to satisf~ the hydraulic requirement~ of the trays and downcomers. In a typical arrangement, the m mimum flow of air to the column to sati~fy the~e requirement~ is approximately 45~ of normal loading. However, greater degrees of turn-down are po~sible by suitable choice of column design, as is known in the art.
The invention i9 now illustrated but in no way limited by the following Example~.
Example 1 In this Example, nitrogen gas was produced from a compressed air supply delivered in line 11 at about 9 bar ab~.
and 12C using the arrangement illustrated in Figure 1 of the accompanying drawings. In a first mode of operation, valve ~ was closed and in conventional manner all the compressed air supply in line 11 was passed to the distillation column ~. The resultant flows are recorded in Column A of Table 1 below. The arrangement was then switched to a second mode in which only about two-thirds of the compressed air supply was fed to the coluDLn, the remaining one-third being diverted through valve ~
and pipeline 26 to be expanded in turbine 4. The re~ultant flows are recorded in Column B of Table 1 from which it can be seen : - .
: .~ .
iZ~ 1'7 that the net production of liquid nitrogen for Ytoraee i~
doubled.
A B
Flow~ Nm3/hrNm3/hr Compre~3ed Air Supply (Line 11) 1570 1570 Air to Di3tillation Column (Line 12) 1570 1050 Total Wa~te gas ~upplied to turbine 4 (Line 21) 960 1480 Compre3sed Air diverted through valve 8 to turbine 4 (I,ine 26) - 520 Product Nitrogen Ga~ (I.ine 25) 570 10 I,iquid Nitro~en mflko (Line 19) ~0 80 .
.
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Example 2 In this Example, nitro~en ~a3 was again produced from a compressed air supply delivered in line 11 at about 9 bar abs.
and 12C but using ~he arran~ement illustrated in Figure 2 of the accompanying drawings.
In a first mode of operation, valve 8 was closed and in a conventional manner all the compressed air supply in line 11 was passed to distillation column 3. The resultant flows are recorded in Column A of Table 2 below. The arrangement was then switched to a second mode in which only about 70~0 of the compres~ed air supply in line 11 was fed to the column, thc remaining 30% being diverted through line 13 and valve ~3 to be expanded in turbine 6. The resultant flow3 are recorded in Column B Or Table 2. In a third mode, valve 10 was throttled until only 45% of the air in line 11 was passed to the column.
The resultant flows are recorded in Column C of Table 2.
A B C
Nm3thr Nm3/hrNm /hr Compressed Air Suuply (Line 11 )4540 4540 4540 Air to Distallation Column (Line 12) 4540 3280 2040 Evaporated oxy~en-containing liquid to Turbine 5( Line 20/21 ) 2770 2770 1490 Compressed Air diverted to Turbine 6 (Line 13) - 1260 2500 Product Nitrogen Gas (Line 25) 1616 222 225 Liquid Nitrogen make (Line l9) 154 28~ 325 .- ~j, ": '~ . ' ' , , :.
12~1~17' From the Table, it can be ~een tl1at in the second mode of operation (Column B) the liquid nitrogen output is increased over that of the conven-tional mode (Column A) by a factor of 1.87. In the third mode of operation (Column C) the factor is increased to
IMP~OVEMENTS IN T~IE PRODUCTION OF PU~E NITROGEN
_ This invention !~elates to the production of nitrogen by its cryogenic separa-tion from air and in~particular to n process which permit~ a variable rate of supply of nitrogen from a relatively con~tant supply of compressed air.
A known proceas for the production of pure nitrogen by cryogenic air separation consists in compressing air to a superatmo~pheric pressure, usually about 9 bar abs, purifying the air of components which would ~olidify at the temperature~
employed, e.g. by removing most of the water content by cooling the air to about 5C with a refrigeration unit and eliminating the remainder of the water vapour and the carbon dioxide contained in the air by adsorption on molecular sieve~ or a similar material, cooling the compressed purified air to a temperature close to it~ dew-point, and feeding the cooled air to the base of a distillation column from which the nitrogen is recovered overhead.
Reflux for the distillation is supplied by expanding to an intermediate pre~sure the oxygen-rich liquid leaving the column base and evaporating said liquid in heat exchange with a portion of the nitrogen vapour leaving the top of the column whereby to condense said vapour. A second portion of the nitrogen vapour is recovered as product. The evaporated oxygen-rich liquid recovered from the reflux condenser is ;
.
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, ~ , , :
,~ ' :, 12~1~;17 thereafter expanded in a turbine to ne~qr ntmospheric pre3sure, preferably after being pre-heated to avoid liquid form&tion in the -turbine, thu~ producing the refrigera-tion required to maintain the process. The cooling of the compressed purified air is effected by paa~ing it in countercurrent with the nitrogen product and expflnded oxygen-rich vapour in A main heat exchange.-wherein said nitrogen product and oxygen-rich vapour are warmed to near ambient temperature. The preheating of the oxygen-rich vapour prior to expanding may also be effected in this main heat exchanger. In general, the refrigeration available will permit the conden3ation of more of the nitrogen vapour than is required for reflux, thus providing a source of liquid nitro~en whi~h cAn be recovered and ~tored. ~lowever, in general only about 5 - 10%
of the total nitrogen production can be made available in this form, the actual amount depending on the ~ize of the plant.
Since on many industrial site~ nitrogen is required intermittently for purging equipment and similar purposes, there is a gro~ing demand for plants able to increase liquid produc-tion during periods in which little or no gaseous nitrogen is required, this liquid nitrogen to be stored in insulated tanks for use in subsequent periods of higher nitrogen demand. In conventional cryogenic nitrogen proces~es and plants of the kind described above the output of liquid nitrogen i~ independent of the amount of gaseous nitrogen delivered and cannot be increased by reducing the output of gaseous nitrogen.
'' , ~ - :
` ~LZ~l~;i7 T~le present invention provide~s a modification of a ni-trogen proce~q~ plant of the kind de~cribed sbove which enable~
the output of liquid nitrogen to be significantly increAsed during period~s in which the requirement for ga~eou~ nitrogen i~
low, thereby increa~ing the capability of the process and plant to satisfy intermittent periods of high nitrogen demand.
An increa~ed production of liquid nitrogen requires an increa~e in the amount of refrigeration supplied and, in conventional plants, this i9 limited by the amount of oxygen-rich gas available and the pres~ure at which it can be fed to the expansion turbine. In conventional operation the flmount of oxygen-rich ga3 flvailable is approximately 60~ of -the air treflted and the maximum preasure at which it can be fed to the expan~ion turbine is about 5.0 bar abs.
In periods during which the demand for ga3eouc nitrogen is low, it ir~ not nece~sary to feed the ~ame quantity of air to the distillation column a~ in periods of high demand, and in conventional plants it ir~ cuotomary to reduce the flow of air during such periods by ouction-throttling the air compre~or.
This leads to some reduction in power con~umption, but doe~ not permit more liquid nitrogen to be produced because there i~s no increar~e in the flow of oxygen-rich vapour through the turbine and hence the amount of refrigeration provided.
In the proce~s of this invention, only a part of the air is fed to the distillation column during periodr~ of low gas .
1~1617 demand and the remainder i~ work-expanded, either by joining the oxygen-rich waste gas ~tream after t;hrottling to intermediate pressure, or by passing at full pressure to a separate expander.
By means of this arrangement, additional refrigeration i~
produced thereby enabling more nitrogen to be withdrawn as liquid and this liquid can then be stored for use in a subsequent period of increa~ed nitrogen demand.
Thus according to the present invention, there i~
provided a process for the production of nitrogen from air by cooling and distilling a ~upply of air at ~uperatmospheric pressure to produce a nitrogen-rich vapour stream and nn oxygen-containing liquid re~iduc, rccovering a fir3t portion o~ ~ai(l vapour as product nitrogen gas, condensing a second portion of said vapour by indirect heat exchange with oxygen-containing liquid residue which has been expanded to an intermediate pressure, to provide reflux for the distillation, and providing refrigeration for the process by work-expanding evaporated oxygen-containing liquid residue recovered from said indirect heat exchange, said process further comprising periodically distilling less than all of the air which is supplied at superatmospheric pressure and work expanding a stream of air at superatmospheric pressure provided from the balance of said supply whereby to produce additional refrigeration for the process, and recovering conden~ed nitrogen vapour thereby formed in excess of that required for reflux.
- . . . . ~
' ~
-', ' , ' ' '" . ' ' " ' ' :" : ' . ~ .' - ' The liqui~l nitrogen theroby produced mny be ~Itore(l and in a ~ulx~eqllerlt period of operntion, when demand for nitrogen gas exceeds the plnnt's capacity, the output of the plant mny be supplemented from this ~tore. By thi~ meane, therefore, it i3 possible to provide a variable supply of nitrogen from the plnnt while maintaining the supply o f oompressed air constant. Further plant flexibility is offered by the alternative of supplying at least a part of -the liquid nitrogen as such, as it is produced and/or after storaee.
In one embodiment of the process, which is more suitable for use in relatively small plants, the stream of air thut if~ to b- work~expnndoA i~ e~pan(1ed to nn interm-~diqte pre~sure and combined with the oxygen-containing liquid residue which has been evaporated producing the condensed nitrogen vapour, and the combined stream is work expanded. By mean~ of thi3 embodiment, only one expansion engine is required.
In another alternative embodiment, which is more suitable for use in relatively larger plnnts, e.g. delivering at least about lOOO Nm3/hr of nitrogen, the stream of air and the evaporated oxygen-containing liquid residue are separately work expanded. Conveniently, however, the two work-expanded streams may then be combined into a single ~tream for subsequent indirect heat exchange with the feed air supply to cool the latter.
Preferably expansion turbine~ are used which may be radial inward-flow air-lubricated machines. In the second-~ ~1ti17 mentioned embodiment, the turbines may be equipped with variablenoz~les, in which case the two mflchineg may be interchangeable.
It i8 then no longer essential for the user to carry a spare turbine, since, if one machine should fail, operation can continue although without the production of the additional liquid nitrogen.
In general, the process may be controlled so that the proportion of the compressed air supply which is fed to the distillation i8 varied to match the demand for nitrogen gas.
Preferably the whole of the remainder of the supply of compressed air, when the nitrogen gas demand is low, is subjected to the work expan~ion since this will maximiso tho production of net available condensed nitrogen vapour e.g. for storage.
The invention also provides apparatus for the production of nitrogen by compressing a supply of air to supsratmospheric pressure, and after removing water vapour and carbon dioxide cooling it and subjecting the compressed air to separation by cryogenic distillation wherein the condensation of a portion of the nitrogen vapour recovered overhead to provide reflux for the distillation is effected by indirect heat exchange with evaporating oxygen-containing liquid residue of the distillation and refrigeration for the process is provided by work expanding evaporated oxygen-containing liquid residue recovered from the reflux condenser, said apparatus further including valve, conduit and work expansion means for "
61'~
intermittentl~y divQrting from the di~tillnt;ion a portion of the air ~,upplied at ~uperatmo~pheric pre~ure, work expanding it and pa~sing the work-expanded ~tream in indirect heat exchange with the feed to the di~tillation to provide additional refrigeration and means for recovering and optionally storing the excess conden~ed nitrogen vapour ~o obtained.
In accordance with one embodiment, said apparatus may include means for providing a supply of purified air at superatmo~pheric pre3sure main heat exchange means di~tillation mean~ having an inlet for air at superatmospheri.c pre~sure, an inlet for column reflux, a fir~t outlet for nitrogen-rich vapour and a ~econd outlet for oxygen-containing liquid residue reflux conden~ing means first and second expansion valve mean~
expansion engine means having an inlet and outlet for gas to be expanded optionally ~t least one in~ulated storage tank for liquid nitrogen, said tank being provided with an inlet for the liquid nitrogen and a valved outlet first conduit means for directing air at ~uperatmospheric pres~ure from said air supply means through ~aid main heat exchange means to said di~tillation means inlet .. . ' -:
.
~
~ econd conduit menn~ for directing a first portion of nitrogen vapour from snid distillation means first outlet through said main heat exchanger means in counter-current indirect heat exchange relation~hip with said air in said first conduit means and recovering said vapour a8 product nitrogen gas third conduit means for directing a second portion of nitrogen vapour fro~ said di~tillation means first outlet through said reflux condensing means whereby to condense said nitrogen vapour fourth conduit means for directing a first portion of condensed nitrogen vapour recovered from said reflux condenser means to said distillation means reflux inlet as reflux for t~e distillation fifth conduit means for recovering a second portion of condensed nitrogen vapour from said reflux condensing means and optionally directing it to said at least one insulated storage tank sixth conduit means for directing oxygen-containing liquid residue from said distillation means second outlet through said first expansion valve means and then through said reflux condensing means in indirect heat exchange relationship with condensing nitrogen vapour in said third conduit and thence to the inlet of said expansion engine means seventh conduit means for directing gas from the outlet of said expansion engine means through said main heat exchanger :' ' `
:.
, ' ' . ' . ` : ' ' '. '' '. ' ", :
' ' .
lZ~l~i7 ~, means in indirect cc)unter-current heat exchange relationship with ~aid nir at ~uperatmo~pheric pre~ure in ~aid fir~t conduit mean~
eighth conduit mean~ for directing air at superatmospheric pressure from said air supply mean3 through said second expansion valve mean~ to said ~ixth conduit mean~
downstream of said reflux condensing means, and valve means for controlling the flow of air in said eighth conduit means.
In accordance with another embodiment, the apparatus include~
means for providing a supply of purified air a-t superatmospheric pressure main heat exchange means distillation means having an inlet for air at superatmospheric pressure, an inlet for column reflux, a first outlet for nitrogen-rich vapour and a second outlet for oxygen-containing liquid residue reflux condensing means expansion valve means first and second expansion engine means each having an inlet and outlet for gas to be expanded optionally at least one insulated storage tank for liquid nitrogen, said tank being provided with an inlet for the liquid nitrogen and a valved outlet .. ,,,,.,,,, . .
- ' ~
' .
1;~2iti1~7 -- 1 o --fir~t; conduit mean~ for directing air at superatmospheric pressure from said nir supply means through sai.d main heat exchange means to said distillAtion means inlet second conduit means for directing a first portion of nitrogen vapour from said di~tillation means first outlet through said main heat exchanger means in co~mter-current indirect heat exchange relationship with said air in said first conduit means and recovering said vapour as nitrogen product gas third conduit means for directing a second portion of nitrogen vapour from said distillntion meAns first outlet through said reflux conden3ing means whereby to condense said nitrogen vapour fourth conduit means for directing a first portion of condensed nitrogen vapour recovered from said reflux condenser means to said distillation means reflux inlet fifth conduit means for recovering a second portion of condensed nitrogen vapour from said reflux condensing meane and optionally directing it to said at least one insulated ~torage tank sixth conduit means for directing oxyeen-containing liquid residue from said distillation means second outlet through said expansion valve means and then through said reflux condensing means in indirect heat exchange relation~hip with condensing nitrogen vapour in said third conduit and thence to the inlet of said first expansion engine means .
.. ' .
~z~1617 seventh conduit means for d:irecting gas from the outlet of ~aid fir~t expan~ion engine mean~ through ~aid main heat exchanger means in indirect counter-current heat exchange relation3hip with ~aid air at ~uperatmo~pheric pres~ure in said first conduit means eighth conduit means for directing air at superatmospheric pressure from said air supply means to the inlet of said second expansion engine mean~
ninth conduit means for directing air from the outlet of said second expansion engine means through the main heat exchanger mean~ in indirect counter-current heat exchange relationship with said air at ~uperatmospheric pressure in said first conduit means, and means for controlling the flow of air in said eighth conduit means.
In both embodiments, between the reflux condensing means and the first expansion engine mean3 said sixth conduit will normally pass through a part of the main heat exchange means at or towards the cold end thereof, superheat the oxygen-containing liquid re~idue which has been evaporated in said reflux condensing means to a temperature such that after expan~ion in the expansion engine it will be at or just above its dew point.
Likewise, in the second embodiment, it is preferred that the eighth conduit connects with said first conduit means to ', .
' . ~ . . .
. . - ~ ' ': ~ ' :' ' ' ' .
~Z~ii17 divert air therefrom at a point within ~aid main heat exchange means corresponding to a temperature such that after expansion through ~aid second expan~ion engine mean~ the air is at or ju~t above its dew point.
The apparatu~ may suitably include a valve adapted to control flow in said eighth conduit means and means respon~ive to pressure and/or flow changes in the compre~sed air supply and/or product nitrogen gas for controlling said valve whereby a fall in the rate of flow of said product nitrogen ga~ below a predetermined level opens said valve and a rise in said rate of flow closes said valve and at rates of flow of said product nitrogen ga3 below said predetermined va]ue the rate of flow of air through said eighth conduit is proportional to the difference between the actual product nitrogen ga~ flow rate and said predetermined value.
The invention will now be described in greater detail with reference to preferred embodiment3 thereof and with the aid of the accompanying drawings in which Figure 1 is a flow diagram of one embodiment of the invention wherein there i8 provision for a part of the compressed air supply to be combined with the evaporated oxygen-containing liquid recovered from the reflux condenser for subsequent work expansion, and Figure 2 is a flow diagram of an alternative embodiment wherein there i8 provision for a part of the compressed air `
`'.': `` ' .' ' ' ' ~ .
."' '., `. ' ' ':, '' ' .' ' : ~ -,"' ~,.",,', ~" . ', ' - . ' ' ' ' ` ~ .' :'. '; ' : ' ' ' ` ' .
~ ` - ' , -~Z~1~17 ~upply to be expanded ~eparately from the evaporated oxygen-containing liquid.
In the figure~ the numeral~ 1 and 2 represent heat exchangers, 3 is a distillation column, 4, 5 and 6 are expansion turbines, 7, 8 and 9 are automatically controlled valves, 10 is a manually adju~table valve, which can be set as required, PIC
represents a pressure-responsive controller and FIC represents a flow-responsive controller.
In the e~bodiment shown in Figure 1 air, which ha3 been compressed e.g. to about 9 bar abs., and from which moisture and carbon dioxide have been removed by conventional means;
enters heat exchanger 1 through line 11 at near ambient temperature and is cooled to a temperature slightly below its dew-point in counter-current heat exchange with gaseous nitrogen product in line 24, and a waste gas, the nature of which is identified below, in line 22. Leaving the exchanger the air feed can divide into two streams passing through lines 12 and 26, re~pectively. Control is provided by valve 8.
In one mode of operation, suitable for use in periods when demand for nitrogen gas is high, the whole of the air feed passes through line 12 to the base of the distillation column 3 in which it is separated into a pure nitrogen overhead stream leaving through line 14 and an oxygen-containing liquid, which leaves through line 15. This liquid is first employed as the coolant for reflux condenser heat exchanger 2. To this end it .
.
-mu3t rir~-t be oxpanded to an intermc(iiate ~re~ re, e.g.
approximntely 5.0 bar nb~. in the vnL~e 10 and i~ then pa~ed to the reflux condon3er 2 wherein it i~ evnporated thereby conden~ing a fir~t part of the nitrogen overhend stream from the column which i~ withdrawn through line 16. The conden3ed nitrogen leave~ the condenser through line 17, at leAst a part being returned through line 18 to the column a~ reflux and any remainder, in line 19, being drawn off through level-controlled valve 9 a~ liquid nitrogen product. The remainder of the overhead nitrogen ~tream i3 recovered in line 24, pa~3ed through heat exchanger 1 where it i9 warmed to near ambient temperature and recovered through line 25 and pres~ure-controlled valve 7 a~
nitrogen product gas.
The evaporated oxygen-containing liquid leaving the reflux conden3er in line 20 i~ pas~ed via line 21 to the expan~ion turbine 4, in which it i3 work-expanded to near atmospheric pre3~ure. Leaving the turbine through line 22, it i~
warmed to near ambient temperature in exchanger 1 in countercurrent heat exchange with the compressed air, finally leaving through line 2~. Before entering the turbine, the stream in line 21 is ~uperheated in heat exchanger 1, a~
illustrated, to the degree neces~ary to avoid liquid formation occurring in the expansion turbine. To ensure the proper heat exchange in exchanger 1, this stream is 90 superheated that when work expanded in the expansion turbine the out]et temperature i~
,' ' ' just above or at the dew point of the expanded ~tream.
In accordance with the invention, provision is made via conduit 26 to divide a portion of the compressed air from line 12 when demand for nitrogen product ~a~ is low and, after expanding it through valve 8 to about the same pressure as the evaporated oxygen-rich liquid in line 20, to inject it in line 20 whereby it i~ expanded with the evaporated-oxygen-rich liquid in expansion turbine 4. Thi~ increaees the amount of refrigeration produced and thus also the amount of nitrogen liquid that is produced.
The excess over that required for reflux i9 recovered through line 19.
One arraneement for automatic control of the plant is now described. In a first mode of operation, the demand for nitrogen gas from line 25 equals the capacity of the plant and valve 8 is clo~ed. When demand for nitrogen eas increases, the Pre~sure Indicator Controller (PIC) will throttle to maintain the system pres~ure. A reduction in demand, on the other hand, will initially cause a rise in pre~sure and the PIC will open fully.
If demand continue~ to fall, the further increase in pressure can be employed by any suitable means, e.~. a suction control valve on the air compressor (not ~hown), to decrease the air flow in -::
~ Iine 11. This reduction in flow ic then detected by the Flow :
Indlcator Controller (FIC) which i9 arranged to open valve 8 whereupon the pressure fall~ thereby causing the suction control valve to reopen and re~tore the floN in line 11.
~-:'~, ' , ., .... , . , ~
. . . . . .
. . . ' .. . .
- . ' ' , :- , , : ' . -- . ' .
.
, . , ~ .
, . :
.
~L2~i~i'7 The openine of v~-lve 8 permit~ the process to move to a ~econd mode wherein the air flow in line 26, which i~ the exce~
of the compre~sed air ~upply over and above that requlred to ~ati~fy the reduced demand for nitrogell ga~, ic expanded through ~aid valve into the evaporated oxygen-rich liquid in line 20 and thence pa3~ed to expan~ion turbine 4 thereby increasin~ the refrigeration delivered to heat exchanger 1. Thi~ re~ults in an increace in the proportion of liquid in the air to the di~tillation column which in turn allows for the withdrawal of a greater proportion of the reflux a~ liquid nitrogen product.
Thi~ can be achieved without affectine the purity of the nitrogen product oince the nitrogen product rate i~ low and the reflux available ic much greater than nece~3ary to achieve the de~ired separation of nitrogen from air.
A portion of the compres~ed air in line 12 will continue to flow through line 26 while the demand for nitrogen i~ below that obtainable froM the full air supply.
When demand for nitrogen gas i9 ~ubsequently increa~ed, the 3y~tem pre~sure will decreace and flow in line 11 will tend to increase. Thi~ flow increase i~ detected by the FIC which will act to throttle valve 8 to restore the correct flow in line 11. When valve 8 i~ fully clo~ed, whereby all the compres~ed air in line 11 i~ fed to the distillation column, the ~y~tem reverts to it~ fir~t mode of' operation and any further increa~e in demand for nitrogen gao will cau~e the PIC to throttle to maintain . - ' , : ' - ~ . ' :' ' ':
- : -' lZ'~16~7 system pressure, a9 before. Means may be provided for then automatically opening a valve in the outlet from a liquid nitrogen storage tank in which liquid nitrogen produced during the second mode of operation hfls been stored, whereby to supply additional nitrogen from that source to satisfy the increased demand for nitrogen gns.
An alternative embodiment of the invention i9 illustrated in ~igure 2 in which all parts and conduits common with the arrangement of Figure 1 are identified by the same reference numerals. While otherwise arranged and operating in substantially the same manner as that of Figure 1, in this embodiment turbine 4 is replaced by two turbines 5 and 6 through which respectively the evaporated oxygen-contnining liquid and any compressed air diverted from line 11 (in this case through line 13) are separately expanded before being combined in line 28 and passed through main heat exchanger 1 as the refrigerant stream.
In order to provide the compressed air stream to the turbine 6 at the desired temperature such that the expanded air leaving the turbine at about atmospheric pressure is at about its dew point and substantially no liquid formation occurs in the turbine, this air i3 withdrawn from the main feed line 11 at an appropriate point in main heat exchanger 1, e.g. corresponding to a temperature of about 130K. For the same reason, the evaporated oxygen-containing liquid recovered from the reflux condenser heat . -, , ` ~
~z~
exchanger ~ in line 20 is fir~t guperheated to about 120K in the main heat exchanger 1 before being passed to the turbine 5.
In this second embodiment of the invention, two alternative method~ of operation are applicable in periods in ~hich the demand for gaseous nitrogen is low and a high rate of production of liquid nitrogen i~ desired.
In the first method of operation, which is preferable when the low demand for gaseous nitrogerl i9 rectricted to a r61atively short poriod, such as a night shift or week-end, the flow of oxygen-containing liquid residue i~ left unchanged; i.e., the setting of valve 10 is not altered. As in the embodiment of Fi~ure 1, the valve ~ i~ operated to divert through line 13, expansion turbine 6 and line 28, the excess of the compressed air feed in line 11, over and above that required to satisfy the reduced demand for nitrogen gas. A significant increase is thus obtained in the output of liquid nitrogen through valve 9.
If the low demand for gaseous nitrogen is to continue for an extended period, a further increase in liquid nitrogen output can be obtained in accordance with n second method of operation by adjusting the eetting of valve 10 to reduce the flow of oxygen-rich liquid from the bottom of the column and hence the flow of vapour to turbine 5. To maintain the total flow of air to the plant, the flow indicator controller (FIC) will then automatically divert a larger flow of air through valve 8 to the auxiliary turbine 6, thereby re~ulting in a further increaced .
; ' ' ~ ' - : :
-lZ'Zl~
, 9 output of liquid nitrogen.
The reduction of flow through valve 10 with theconsequent increase of flow through valve ~ can be continued as long as the resultlng reduced flow of air to the column i~
adequate to satisf~ the hydraulic requirement~ of the trays and downcomers. In a typical arrangement, the m mimum flow of air to the column to sati~fy the~e requirement~ is approximately 45~ of normal loading. However, greater degrees of turn-down are po~sible by suitable choice of column design, as is known in the art.
The invention i9 now illustrated but in no way limited by the following Example~.
Example 1 In this Example, nitrogen gas was produced from a compressed air supply delivered in line 11 at about 9 bar ab~.
and 12C using the arrangement illustrated in Figure 1 of the accompanying drawings. In a first mode of operation, valve ~ was closed and in conventional manner all the compressed air supply in line 11 was passed to the distillation column ~. The resultant flows are recorded in Column A of Table 1 below. The arrangement was then switched to a second mode in which only about two-thirds of the compressed air supply was fed to the coluDLn, the remaining one-third being diverted through valve ~
and pipeline 26 to be expanded in turbine 4. The re~ultant flows are recorded in Column B of Table 1 from which it can be seen : - .
: .~ .
iZ~ 1'7 that the net production of liquid nitrogen for Ytoraee i~
doubled.
A B
Flow~ Nm3/hrNm3/hr Compre~3ed Air Supply (Line 11) 1570 1570 Air to Di3tillation Column (Line 12) 1570 1050 Total Wa~te gas ~upplied to turbine 4 (Line 21) 960 1480 Compre3sed Air diverted through valve 8 to turbine 4 (I,ine 26) - 520 Product Nitrogen Ga~ (I.ine 25) 570 10 I,iquid Nitro~en mflko (Line 19) ~0 80 .
.
.
Example 2 In this Example, nitro~en ~a3 was again produced from a compressed air supply delivered in line 11 at about 9 bar abs.
and 12C but using ~he arran~ement illustrated in Figure 2 of the accompanying drawings.
In a first mode of operation, valve 8 was closed and in a conventional manner all the compressed air supply in line 11 was passed to distillation column 3. The resultant flows are recorded in Column A of Table 2 below. The arrangement was then switched to a second mode in which only about 70~0 of the compres~ed air supply in line 11 was fed to the column, thc remaining 30% being diverted through line 13 and valve ~3 to be expanded in turbine 6. The resultant flow3 are recorded in Column B Or Table 2. In a third mode, valve 10 was throttled until only 45% of the air in line 11 was passed to the column.
The resultant flows are recorded in Column C of Table 2.
A B C
Nm3thr Nm3/hrNm /hr Compressed Air Suuply (Line 11 )4540 4540 4540 Air to Distallation Column (Line 12) 4540 3280 2040 Evaporated oxy~en-containing liquid to Turbine 5( Line 20/21 ) 2770 2770 1490 Compressed Air diverted to Turbine 6 (Line 13) - 1260 2500 Product Nitrogen Gas (Line 25) 1616 222 225 Liquid Nitrogen make (Line l9) 154 28~ 325 .- ~j, ": '~ . ' ' , , :.
12~1~17' From the Table, it can be ~een tl1at in the second mode of operation (Column B) the liquid nitrogen output is increased over that of the conven-tional mode (Column A) by a factor of 1.87. In the third mode of operation (Column C) the factor is increased to
2.11.
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'' . -.' ' '.' . .' ' , - .: - ' ': ' ~' . - - . -. . -"
Claims (9)
1. A process for the production of nitrogen from air by cooling and distilling a supply of air at superatmospheric pressure to produce a nitrogen-rich vapour stream and an oxygen-containing liquid residue, recovering a first portion of said vapour as product nitrogen gas, condensing a second portion of said vapour by indirect heat exchange with oxygen-containing liquid residue which has been expanded to an intermediate pressure, to provide reflux for the distillation, and providing refrigeration for the process by work-expanding evaporated oxygen-containing liquid residue recovered from said indirect heat exchange, said process further comprising periodically distilling less than all of the air which is supplied at superatmospheric pressure and work expanding a stream of air at superatmospheric pressure provided from the balance of said supply whereby to produce additional refrigeration for the process, and recovering condensed nitrogen vapour thereby formed in excess of that required for reflux.
2. A process as claimed in claim 1 wherein said stream of air is expanded to an intermediate pressure and combined with the oxygen-containing liquid residue which has been evaporated by said indirect heat exchange, and the combined stream is work expanded.
3. A process as claimed in claim 1 wherein said stream of air and said oxygen-containing liquid residue which has been evaporated by said indirect heat exchange are separately work-expanded.
4. Apparatus for the production of nitrogen by compressing a supply of air to superatmospheric pressure and after removing water vapour and carbon dioxide cooling it and subjecting the compressed air to separation by cryogenic distillation wherein the condensation of a portion of the nitrogen vapour recovered overhead to provide reflux for the distillation is effected by indirect heat exchange with evaporating oxygen-containing liquid residue of the distillation and refrigeration for the process is provided by work expanding evaporated oxygen-containing liquid residue recovered from the reflux condenser, said apparatus further including valve, conduit and work expansion means for periodically diverting from the distillation a portion of the air supplied at superatmospheric pressure, work expanding it and passing the work-expanded stream in indirect heat exchange with the feed to the distillation to provide additional refrigeration and means for recovering and optionally storing the excess condensed nitrogen vapour so obtained.
5. Apparatus as claimed in claim 4, including means for providing a supply of purified air at superatmospheric pressure main heat exchange means distillation means having an inlet for air at superatmospheric pressure, an Inlet for column reflux, a first outlet for nitrogen-rich vapour and a second outlet for oxygen-containing liquid residue reflux condensing means first and second expansion valve means expansion engine means having an inlet and outlet for gas to be expanded optionally, at least one insulated storage tank for liquid nitrogen, said tank being provided with an inlet for the liquid nitrogen and a valved outlet first conduit means for directing air at superatmospheric pressure from said air supply means through said main heat exchange means to said distillation means inlet second conduit means for directing a first portion of nitrogen vapour from said distillation means first outlet through said main heat exchanger means in counter-current indirect heat exchange relationship with said air in said first conduit means and recovering said vapour as product nitrogen gas third conduit means for directing a second portion of nitrogen vapour from said distillation means first outlet through said reflux condensing means whereby to condense said nitrogen vapour fourth conduit means for directing a first portion of condensed nitrogen vapour recovered from said reflux condenser means to said distillation means reflux inlet as reflux for the distillation fifth conduit means for recovering a second portion of condensed nitrogen vapour from said reflux condensing means and optionally directing it to said at least one insulated storage tank sixth conduit means for directing oxygen-containing liquid residue from said distillation means second outlet through said first expansion valve means and then through said reflux condensing means in indirect heat exchange relationship with condensing nitrogen vapour in said third conduit and thence to the inlet of said expansion engine means seventh conduit means for directing gas from the outlet of said expansion engine means through said main heat exchanger means in indirect counter-current heat exchange relationship with said air at superatmospheric pressure in said first conduit means eighth conduit means for directing air at superatmospheric pressure from said air supply means through said second expansion valve means to said sixth conduit means downstream of said reflux condensing means, and valve means for controlling the flow of air through said eighth conduit means.
6. Apparatus as claimed in claim 4 including means for providing a supply of purified air at superatmospheric pressure main heat exchange means distillation means having an inlet for air at superatmospheric pressure, an inlet for column reflux, a first outlet for nitrogen-rich vapour and a second outlet for oxygen-containing liquid residue reflux condensing means expansion valve means first and second expansion engine means each having an inlet and outlet for gas to be expanded optionally at least one insulated storage tank for liquid nitrogen, said tank being provided with an inlet for the liquid nitrogen and a valved outlet first conduit means for directing air at superatmospheric pressure from said air supply means through said main heat exchange means to said distillation means inlet second conduit means for directing a first portion of nitrogen vapour from said distillation means first outlet through said main heat exchanger means in counter-current indirect heat exchange relationship with said air in said first conduit means and recovering said vapour as nitrogen product gas third conduit means for directing a second portion of nitrogen vapour from said second conduit after the said distillation means first outlet through said reflux condensing means whereby to condense said nitrogen vapour fourth conduit means for directing a first portion of condensed nitrogen vapour recovered from said reflux condenser means to said distillation means reflux inlet fifth conduit means for recovering a second portion of condensed nitrogen vapour from said reflux condensing means and optionally directing it to said at least one insulated storage tank sixth conduit means for directing oxygen-containing liquid residue from said distillation means second outlet through said expansion valve means and then through said reflux condensing means in indirect heat exchange relationship with condensing nitrogen vapour in said third conduit and thence to the inlet of said first expansion engine means seventh conduit means for directing gas from the outlet of said first expansion engine means through said main heat exchanger means in indirect counter-current heat exchange relationship with said air at superatmospheric pressure in said first conduit means eighth conduit means for directing air at superatmospheric pressure from said air supply means to the inlet of said second expansion engine means ninth conduit means for directing air from the outlet of said second expansion engine means through the main heat exchanger means in indirect counter-current heat exchange relationship with said air at superatmospheric pressure in said first conduit means, and means for controlling the flow of air in said eighth conduit means.
7. Apparatus as claimed in claim 5 including a valve associated with said eighth conduit means and means responsive to pressure and/or flow changes in the compressed air supply and/or product nitrogen gas for controlling said valve whereby a fall in the rate of flow of said product nitrogen gas below a predetermined level opens said valve and a rise in said rate of flow closes said valve and at rates of flow of said product nitrogen gas below said predetermined value the rate of flow of air through said eighth conduit is proportional to the difference between the actual product nitrogen gas flow rate and said predetermined value.
8. Apparatus as claimed in claim 6 including a valve associated with said eighth conduit means and means responsive to pressure and/or flow changes in the compressed air supply and/or product nitrogen gas for controlling said valve whereby a fall in the rate of flow of said product nitrogen gas below a predetermined level opens said valve and a rise in said rate of flow closes said valve and at rates of flow of said product nitrogen gas below said predetermined value the rate of flow of air through said eighth conduit is proportional to the difference between the actual product nitrogen gas flow rate and said predetermined value.
9. Apparatus as claimed in claim 6 including a variable area nozzle control means on the second expansion engine and means responsive to pressure and/or flow changes in the compressed air supply and/or product nitrogen gas for controlling said variable area nozzle control means whereby a fall in the rate of flow of said product nitrogen gas below a predetermined level opens said variable area nozzle control means and a rise in said rate of flow closes said variable area nozzle control means and at rates of flow of said product nitrogen gas below said predetermined value the rats of flow of air through said eighth conduit is proportional to the difference between the actual product nitrogen gas flow rate and said predetermined value.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000443004A CA1221617A (en) | 1983-12-09 | 1983-12-09 | Production of pure nitrogen |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000443004A CA1221617A (en) | 1983-12-09 | 1983-12-09 | Production of pure nitrogen |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1221617A true CA1221617A (en) | 1987-05-12 |
Family
ID=4126717
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000443004A Expired CA1221617A (en) | 1983-12-09 | 1983-12-09 | Production of pure nitrogen |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1221617A (en) |
-
1983
- 1983-12-09 CA CA000443004A patent/CA1221617A/en not_active Expired
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| Date | Code | Title | Description |
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| MKEX | Expiry |