CA2044727C - Hybrid prepurifier for cryogenic air separation plants - Google Patents
Hybrid prepurifier for cryogenic air separation plantsInfo
- Publication number
- CA2044727C CA2044727C CA002044727A CA2044727A CA2044727C CA 2044727 C CA2044727 C CA 2044727C CA 002044727 A CA002044727 A CA 002044727A CA 2044727 A CA2044727 A CA 2044727A CA 2044727 C CA2044727 C CA 2044727C
- Authority
- CA
- Canada
- Prior art keywords
- membrane
- gas
- air
- purge gas
- dry
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
- F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
- F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
- F25J3/04636—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air using a hybrid air separation unit, e.g. combined process by cryogenic separation and non-cryogenic separation techniques
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/22—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/22—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
- B01D53/225—Multiple stage diffusion
- B01D53/226—Multiple stage diffusion in serial connexion
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/22—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
- B01D53/229—Integrated processes (Diffusion and at least one other process, e.g. adsorption, absorption)
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/26—Drying gases or vapours
- B01D53/268—Drying gases or vapours by diffusion
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
- F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
- F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
- F25J3/04151—Purification and (pre-)cooling of the feed air; recuperative heat-exchange with product streams
- F25J3/04163—Hot end purification of the feed air
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
- F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
- F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
- F25J3/04151—Purification and (pre-)cooling of the feed air; recuperative heat-exchange with product streams
- F25J3/04163—Hot end purification of the feed air
- F25J3/04169—Hot end purification of the feed air by adsorption of the impurities
- F25J3/04181—Regenerating the adsorbents
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2205/00—Processes or apparatus using other separation and/or other processing means
- F25J2205/40—Processes or apparatus using other separation and/or other processing means using hybrid system, i.e. combining cryogenic and non-cryogenic separation techniques
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2205/00—Processes or apparatus using other separation and/or other processing means
- F25J2205/60—Processes or apparatus using other separation and/or other processing means using adsorption on solid adsorbents, e.g. by temperature-swing adsorption [TSA] at the hot or cold end
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2205/00—Processes or apparatus using other separation and/or other processing means
- F25J2205/60—Processes or apparatus using other separation and/or other processing means using adsorption on solid adsorbents, e.g. by temperature-swing adsorption [TSA] at the hot or cold end
- F25J2205/66—Regenerating the adsorption vessel, e.g. kind of reactivation gas
- F25J2205/70—Heating the adsorption vessel
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2205/00—Processes or apparatus using other separation and/or other processing means
- F25J2205/80—Processes or apparatus using other separation and/or other processing means using membrane, i.e. including a permeation step
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
- Y02C20/00—Capture or disposal of greenhouse gases
- Y02C20/40—Capture or disposal of greenhouse gases of CO2
Abstract
HYBRID PREPURIFIER FOR
CRYOGENIC AIR SEPARATION PLANTS
Abstract of the Disclosure Feed air to a prepurifier adsorption system/cryogenic air separation system for dry, high purity nitrogen and/or oxygen production is dried in a membrane dryer preferably characterized by a countercurrent flow path. Drying is enhanced by the use of purge gas on the permeate side of the membrane dryer, with adsorption system or cryogenic air separation system product or waste gas, dried feed air or ambient air being used as purge gas.
Two membrane materials are employed in the membrane dryer, in a single stage or in two stages, for enhanced removal of water and carbon dioxide from the feed air.
D-16,348
CRYOGENIC AIR SEPARATION PLANTS
Abstract of the Disclosure Feed air to a prepurifier adsorption system/cryogenic air separation system for dry, high purity nitrogen and/or oxygen production is dried in a membrane dryer preferably characterized by a countercurrent flow path. Drying is enhanced by the use of purge gas on the permeate side of the membrane dryer, with adsorption system or cryogenic air separation system product or waste gas, dried feed air or ambient air being used as purge gas.
Two membrane materials are employed in the membrane dryer, in a single stage or in two stages, for enhanced removal of water and carbon dioxide from the feed air.
D-16,348
Description
2~727 Y~BID PR~ IEIEB_E~
~YO~ AIR SE~RA~IQ~ PIIAN~
.. . ..... . .. ~, _ ,. .__ _ _" . . .... __ . .
The ;nvention r~lates to the cryogenic ~eparation ~f~air. ~More par~icularly, it relates to he pretrçatment of ~eed air to cryogenic air - separation systems.
Nitrogen and o~ygen are desired for many 20 chemi~al proces~ing, refinery, meta~ production and ~: ~ other i:ndu~trial applications. While var~ous ~echnigues are known for the production o~ nitrogen a~d~or o~ygen by air ~eparatio~,~cryogenic distillotion processes ~nd systems are widely used 25 ~or the production of ~itrogen and/or:o~y~en from air, or for the removal of nitrogen from we~l ~ases~ In each c~yog ni~c application, high ~r~ezing point~cont~minant;s, which ~ould otherwise ~olidify ~at the low temperatur~s at which the primary g~s 30 ~epa~ation tske~ pla~e,:must be removed rom the compressed ~eed ~as streàm. Such contaminants are commonly removed by r~frlgeration~adsorp~ion process combinations well ~own in the ~rt. In air s~paratio~:operations, this pre-cl~anup ca~ u~ilize ..
: D-16,34~ :
:
- , .
.:: ~: . ; :, , .: ,:;. . ,. :
. ; . :: .: ,. ~
- 2 ~ P~7 a reversing heat e~changer and cold end gel trap combination, or a mechanical air chiller/zeolite molec~lar sieve adsorber combination. In the former type of proces~ing unit, virtually all of the 5 contaminants are frozen out of the feed air when said air is thermally e~changed against the cryogenic waste and product gas streams.
Unfortunately, however, the self cleaning of the reversing heat e~changer unit requires a large purge 10 gas flow relative to the air feed. As a result, the a;r recovery of such cleanup cycles tends to be undesirably limited. Reversing heat e~changer units also require large valves, which must open and close on a ~yclie basis, switching the air feed and waste 15 purge flow passages. The valves are often located within the ~nsulated cold box portion of the eryogenic system, making maintenance difficult.
Furthermore, to act effectively, the heat e~change-gel trap combination must operate at low 20 temperature, and thus requires a eo~siderable cool down period during plant start-up.
In contrast to reversing heat e~changer and gel trap ~ombinations, mechanicaI chiller/
adsorptive unit combinations, ~s disclosed-in 25 Prentice, U.S. 4,375,367, can supply a clean, dry feed air stream within minutes of ~tart-up. The mechanical chiller reduces the air temperature to about 40F from the compressor aftercooler temperature of ~rom about 80F ~o about 115F. The ~0 air, which i~ satura~ed at the higher temperatures, loses the ~ulk of its water burden through condensation, thus reducing the inlet water concentration to ~he adsorptive unit. The D-16,~48 : _ 3 ~ 7 ~ 7 adsorption operation is typically carried out using a pair of pressure vessels, one bed being used for adsorbing purposes, while the other is underg~ing regeneration. The pressure vessels are filled with 5 an adsorbent material, such as alumina, zeolite molecular sieve or silica gel, which removes the - remaining water vapor, carbon dio~ide and~or other contaminants from the feed air stream. The adsorbent beds are usually regenerated at near 10 ambient pressure with a contaminant ree stream, either a portion of the cryogenic waste or dry air, which may be heated tv improve its desorbing capability. The operation o~ the mechanical chiller substantially improves the performance of the 15 adsorber beds by increasing their adsorption capacity, reducing the inlet water concentration, and, consequent~y, the purge f lvw and energy requirements of the operation. The mechanical chiller is limited to a minimum product dewpoint of 20 about 38F due to the necessity for avoiding the buildup of ice on the tubing walls. The chillers must also be followed by ~ moisture separator to remove the condensate formed fro~ the feed air and to protect the adsorbent beds ~rom egcessive 25 moisture. The mechanical chillRrs used in such operations tend to be e~pensive in terms of capital and power r~quirements, especially for small plants. In addition, such chillers are yenerally ~nown for requiring e~pensive maintenance.
In light of such factors, there has been a desire in the art for n~w prnces~es a~d ~ystzms that would either ~liminate or modify the furlction 2f the - ~omponent6 referred to above, particularly the .
D-16,398 - 4 - 2 0~ ~r~27 mechanical chiller and moisture separator so as to more economically provide clean, dry air to a cryogenic gas separation unit. One approach considered with interest i~ the use of membrane 5 systems to ~electively permeate water and carbon d;o~ide from feed air. Certain materials are well known as being capable of selectively permeating water and carbon dio~ide, while air or other gases, comprising less perme~ble components, are recovered 10 as non-permeate gas. ~ membrane system utilizing such a material would replace the function of the me~hanical chiller. Such membrane systems are well known to be relatîvely simple and easy to operate and maintain. As such membrane systems are normally 15 operated, however, the removal of moisture from the feed stream re~uires the co-permeation of signi~icant amounts ~f valuable product gas.
Operation of membrane systems at stagP cuts on the order of 10 to 20% might be re~uired to achieve the 20 aewpoint level achieved by the use of a mechanical chiller. Such cîrcumstance would, as a result, reduce the overall process recovery level achievable, încrPase the power requirements of the pro~ess, and be generally unattractiv~ from ~n 25 economic viewpoint. Despite such factors serving to deter the use of membrane dryer systems în place of mechanical chillers or said reversing heat e~changer and gel trap combinations, the use of membrane dryer ~ystems în new, împroved overall processes and 30 ~ystems, eliminating the need for the presently employed technî~ques, would represent a desîrable advance in the art.
It is ~n object of the inventîon, therefore, to provide an improved ~roc~s~ an~ system D-16,348 ..
2 ~ 7 for the production of dry nitrogen and/or o~ygen product.
It is another object of the invention to provide an impro~ed process and system utili2ing 5 cryogenic systems for gas separation and providing for desired for the use of a membrane system for the ~ removal of moisture and carbon dio~ide from the feed gas.
It is a further object of the invention to 10 provide ~ membrane dryer system capable of achieving enhanced drying efficiency and carbon dio~ide removal in an overall process and system for the recovery of dry nitrogen and/or oxygen using a eryogenic system or air separation.
With those and other obj~cts in mind, the invention is hereinafter described in ~etail, the novel f*atures thereo~ being particularly pointed o~t in the appended claims.
Summary of ~he I~v~n~ion : A membrane dryer system is employed in conjunction with an adsorption unit-cryogenic gas ~eparation u~it system to achiev~ a desired - produc~ion of dry nitrog~n ~nd/or o~ygen product.
25 The ~embrane dryer is preferably operated with a counter~urrent flow pattern and is re lu~ed on the low pressure permeate side ther~of. Waste gas from the adsorption-~ryogenic unit is used as purge gas.
The area requirements of the membra~e ars thereby 30 reduced, and the desired product recovery is appreciably increased. The membrane dryer removes water and carbon ~io~ide contaminants in the feed - air in si~gle or two ~tage units ~mploying ~eparate water ~nd carbon dioxide removal membr~n~s.
D-16,348 .
- 2~4~7~7 ~lief Des~riDtion o ~hç Drawins The invention is hereinafter described in detail with reference to the ~ccompanying drawi~gs 5 in which:
Fig. 1 is a schematic flow diagram of an embodiment of the in~ention in which the waste gas from the cryogenic feed gas separation system is employed as purge gas for a membrane system for the 10 drying of the feed gas to the cryogenic system; and Fig. 2 is a schematic flow diagram of an embodiment in which purge gas xemoved from the absorhent bed prepurifier for the cryogenic ~ystem is employed as purge gas for a feed gas membrane 15 drying system. : -' e~ail~ De criP~i~n Qf ~he Invention The objects of the invention are accomplished by the integration of a membrane system 20 for ~eed air drying with a downstrPam adsorption-cryogenic air separation system under conditions enabling desired moisture~remo~al from the feed air to be accomplished without reduetion in the ~verall product recovery of the process and system to 25 unacceptable levels. Such conditions ad~antageously relate to the integration of the separate processing systems, he selectivity for moisture removal of the particular membran on~position employed, and membrane bundle design conditions under which 3~ ~ountercurrent flow is desirably achiev d in the membrane dryer system. This en3bles nitrogen and/or o~ygen to be recovered in dry form with minimum loss of æaid product during the drying operation.
DD 1~; ~ 3~! 8 _ 7 _ 2~7~7 In the practice of the invention, waste gas from the cryogenic air separation system is used to provide purge gas to a membrane dryer system and to the adsorption system upstream of said cryogenie 5 system. The invention enables a dry, high purity nitrogen and~or o$ygen product stream to ~e obtained ~ with minimum loss of desired product because of the requirements of the drying operation. The overall process and system of the invention is illustrated 10 with reference to the drawings. Further information relating to the overall cryogenic systems used in the pract ce of the invention, and the membrane systems integrated therewith to achieve enhanced drying o feed air are provided below.
lS In Fig. 1 of the drawings, feed air is passed in line l to ~ir compressor 2, from which wet compressed air is passed in line 3 to membrane dryer ~ystem 4. In said membrane system 4, water selectively permeates throu~h the membrane material 20 and is discharged from the system as waste ~as ~hrough lin~ 5. Feed air is recovere~ from membrane dryer system 4 as dry, non-permeate or retentate gas through line 6 for passage to adsorption system 7, which is used to remove contaminants from the dry 25 feed air prior to the passage of said feed air to the cryogenic air separation system. Adsorption system 7 is shown as including two beds of adsorbent material, i.e. b~ds 8 and 9, one b~d generally being used ~or its intended a~sorption purposes ~hile the 30 other bed is being regenerated. The dry, purified feed gas is passed from said adsorption system 7:in line 10 to cryogenic air separation system ll, from which the de ired dry, high purity product gas is D-16,348 ,,: : ::~ : , ~ , -: ~,.- - . .. : ;
, ~: .. . ~ .. . . .
::
- 8 ~ 7~7 recovered through line 12. A dry waste stream from said cryogenic system is withdrawn through line 13.
A portion of this dry waste stream, i.e o~ygen or nitrogen, is withdrawn through line 14 for passage 5 through adsorption system 7, that is through either bed 8 or bçd 9, as dry adsorbent purge gas for the - bed undergoing regeneration. An adsorbent waste stream is withdrawn from a~orption system 7 through line 15, said waste stream containing the adsorbent lO purge gas and contaminants desorbed rom the adsorbent beds during t~e regeneration thereof. The remaining portion Gf the ~ry waste gas from cryogenic air separation system 11 is passed through line 16 for introduction to membrane dryer system 9 15 as a dry purge gas on the lower pressure, permeate side of said membrane system. Said dry p~rge gas is used to facilitate th removal of permeate waste gas from the surface of the membrane, and is discharged, together with saia permeate gas, through line 5.
The embodiment of the invention illustrated in Fig. l~serves to eliminate the need for a chiller otherwise employed as part of a chiller/adsorbent bad combination ~or the removal of water and carbon dio~ide from the compressed air streams of ~5 conventional pre-purified cryogenic air separation plants. Such elimination of the ~hiller is desirable, as indicated above, because it is e~pensive in terms of both capital and power and becau~e it is well known ~or reguiring eg~ensive 30 maintenance. The membrane dryer sy tem used in the practi~e of the invention, on the other hand, is well known as being very ~imp}e and ine~pensive in - nature, and not reguirin~ e~tensive mainten~nce.
D-16,348 - g - 2~ 7 While this embodiment of the invention, integrating membrane systems with adsorption-cryogenic air separation systems, is an a~vantageous adva~ce over conventional pre-purified cryogenic air ~eparation 5 systems, further development in the art is also desirable. One limitation of the Fig. 1 embodiment of the invention is that the permeate purge gas re~uirements for the membrane dryer ~ystem, which typically are appro~imately 10-20% of the feed aix 10 to said membrane dryer system, are ;n addition to t~e 10-15% purge requirements for the pre-purifier adsorption system. Consequently, the relatively large overall purge requirements of the system, appro~imately 20-35%, make it difficult to achieve lS high recovery of nitrogen and o~ygen in cryogenic air separation systems when such large amounts of waste gas are not available.
The embodiment illustrat~d in Fig. 2 addresses the need for minimizing ~he overall purge 20 re~uirements of the system. In this embodiment, air in line 20 is compressed in air compressor 21, with the compressed air being passed in line 22 to coaleccer unit 23, from which water is removed through line 24. The thus-~rea~ed compres-sed air 25 stream is p3ssed in line 25 to fir~t ~taye membrane dryer 26, the first part of a two-stage membrane ~ryer system. Most of the water still present in the feed air is r~moved in this first stage dryer, which is refluxed in the permeate si~e by a dry 30 pur~e stream as hereinafter indicated. The partially dry, compressed feed air passes, as non-permeate gas, from first stage membrane ~Iryer 26 through line 27 to second stage mem}:~rane dryPr 28, D-16, 348 - :: ,' ' :. ~; '-.
- : -- ~ .",, - ., :
. . :
: . . ~
1~- 2~ 27 wherein residual water is removed so that dry feed air is passed therefrom as a non-permeate stream for passage ;n line 29 to prepurifier adsorption system 30 for purification before passage to the cryogenic 5 air separation system. Adsorption system 30 is shown as containing two adsorbent beds, namely bed 31 and bed 32, it being understood that one such bed will commonly be used for purification of dry feed gas while the other ~e~ is undergoing regeneration.
10 Dry, purified feed air leaving adsorption system 30 is passed in line 33 to cryogenic air separation system 34, from which the aesired dry, high purity nitrogen or oxygen product is recovered through line 35. Dry waste gas from cryogenic system 34 is 1~ withdrawn through line 36, heated in heat e~changer 37, and passed through line 38 to prepurifier adsorption system 30 as purge gas for use in the regeneration of whichever bed, i.e. bed 31 or bed 32, is being regenerated at any given time. Since 20 virtually ~11 of the water present in the feed air is removed in the membrane dryer system, the spent purge ga~ e~iting prepurifier adsorption system 30 will be re~atively dry, although.it will contain other contaminants such as carbon dio~ide ~nd 25 hydrocarbons. Such spent purye gas is passed in line 39 to first staQe membrane dryer 26 ~or use therei~ as purge gas on the permeate side of the membrane. Said purge ~as, together with water vapor that permeates through said membrane dryer 2S, is 30 withdrawn through linç 40 for dischsrge to waste.
The passage of such recycle purge ~as ~hrough membrane dryer 2fi ~acilitates the ~arrying o~ said permeate ~ater away from the surface of the membrane D-16,348 .
11- 2~f~7~7 on said permeate side so that a high driving force is maintained across membrane dryer 26 to sustain the desired moisture removal from the feed air stream being passed to said membrane dryer 26.
Second stage membrane dryer 28 is us0d, in the Fig. 2 embodiment, to dry the feed air to higher - levels than are achieved in first stage membrane ~ryer 26. For purging in this dryer, any dry, low pressure stream avail:able from the cryogenic 10 process, such as waste gas from cryogenic system 34, high purity nitrogen or o~ygen product gas, expanded feed air or the like, or waste gas from prepurifier adsorption system 30, can ~e used: a~ the dry purge gas. In Fig. 2, a portion of the cryogenic system 15 34 waste gas is shown as being passed through line ~1 to second stage membrane:dryer 28 for u~e as purge gas therein. Such purge gas facilitates the carrying away of the permeate water from the surface of the membrane on the permeate s~dP of the membrane 20 so that a high driving force is maintained across membrane 28 to sustain the desired additional drying of the ~eed air stream being passed to said membrane 28. Purge gas, together with additional permeat~
water, is withdrawn from membrane dryer throu~h line 25 42.
Those skilled in the art will appreciate that the use of second stage membra~e dryer 2B is optional, depending on the degree of feed air drying desired in any particular dry, high purity nitrogen 39 and/or o~ygen production operation. When employed, as in the Fig. 2 ~mbvdiment, second sta~e memhrane dryer 28 will typically be smaller and require much - less purge gas than first stage membrane dryer 26 D-16,3~8 .. ....
- 12 - 2~7~7 because most of the water removal from the feed air occurs in the first stage membrane dryer system.
The Fig. 2 embodiment will be seen to be of advantage in that it enables the overall purge 5 re~uirement of the process to be reduced in comparison to that of the Fig. l embodiment. Thus, if the total membrane dryer purge requireme~t ;s 20~
and the pre-purifier adsorption system 30 purge requirement is 15%, the~, in such embodiment, only lO S% of purge gas over and above that employed ~or pre-purification would be req~ired. Removal of ~irtually all of the water in the membrane dryer also greatly reduces the water load on the pre purifier adsorption system. This, in turn, greatly 15 reduces the thermal energy required for pre-purifier regeneration, making possible perhaps the use of compressor waste heat for prepurifier regeneration.
Since water is a very ~trongly adsorbed species in the prepurifier, the removal of most of 20 the water from the prepurifier feed gas can rPsult in improved aasorbent performance~with respect to other speci~s desired to be removed, such as carbon dio~;de, hydrocarbons and the like. It will be appreciated that this could lead to desir~bly 25 improved prepurifier operation. It should be noted that membrane dryers sui~able for the removal of water will slso generally be relati~ely selective for carbon dio~ide removal. Such carbon ~iv~ide removal will also reduce the load on the downstream 30 adsorpti:on unit.
While ~he removal of carbon dio~ide by the membrane dryers suita~le for water removal is thus desirable, other ~mbodiments further enhancing the D-16,348 : .:
. . ~. - :.... : -overall operation comprise the use of single or two stage membrane systems employing ~eparate water and carbon dio~ide removal membranes. In the single stage system, two membrane materials, one having a 5 selectivity optimized for water and the other being optimi~ed for carbon dio~ide, are zmployed. The separate membrane materials may be positioned in any des;rable form, as in a side-by-side or a layer-by-layer arrangement. The use of two different 10 permeable membranes capable of separating different compone~ts of a fluid mi~ture is described in the Perrin patent, V.S. 4,880,440. In such a single stage membrane system adapted for enhanced remcval of both water and carbon dio~ide, relatively dry 15 purge gas can be conveniently supplied from the prepurifier adsorption system and/or from the cryogenic air separation system as in the illustrated embodiments referred to above.
In another embodiment of the hybrid 20 prepurifier of the invention, two separate membrane stages can be employe~. In the latter embodiment, each stage contains membrane modules containing membrane materials particularly suitable for the component primarily ~eing separated therein. The 25 two stage embodiment is preferably arranged so that th~ feed air passes to a first stage membrane adapted for water removall with the non-permeate, dried feed gas passing to a second stage membrane adapted primarily or the removal of carbon 30 diogide. In one such embodiment, a p~rtion of the waste gas from the ~ryogenic air ~eparation system may be passed, as in the ~ig. 1 embodiment, to the membrane ~ystem with separate portions of said purge D-16,348 .. : . ~ - .......... :
.... . .
:: -. , :~ , .. .. : -. . ~ . . : , - 14 - 2~ 7 gas b~ing passed to the first a~d seco~d membrane stages. It will be appreciated that the two stage membrane embodiment enables the purge gas r3tio to be optimized separately for each comp~nent, i.e.
5 water or carbon dio~ide, being removed ~rom the feed air.
~ t should be noted that with reference to the two stage process, the waste purge removed from the second stage, carbon dio~ide removal membrane 10 unit will be relatively dry and can be conveniently passed to the first stage, water removal membrane unit as purge gas therefor. As the adsorption waste will also be relatively dry, this stream can also be used for purge and can be passed to the first stage 15 membrane for such purge purposes, as in the Fig. 2 embodiment. Conveniently, the waste streams from the second membrsne ~tage and from the prepurifier adsorption system can be combined tv make up all or part of the purge gas for the first stage membrane 20 unit. This embodiment will typically result in a significant re~uction in the overall amount of gas required to be recycled for pur~e purposes. In this regard, it will be appreciated that, despite the use of separate water and carbon dio~ide removal 25 materials and stages, some carbon dio~ide will likely be removed in the first stage ~apted for water removal, ~nd some water will be removed in the ~econd stage carbon dioxide removal unit.
It should also be noted that adsorption of 30 water in the prepurifi~r is e~othermic in nature and ~enerates signi~icant amounts of heat. This tends ~o raise the t~mperature of the air leaving the prepurifier which, in turn, increases the D-l~, 3~a .
~; ~
_ 15 ~
refrigeration load on the cryogenic system. Removal of the water from the prepurifier feed by use of the membrane dryer system will tend to ~reatly reduce the heat generated in the prepurifier adsorption system during adsorption therein, thus benefiting the downstream cryogenie process.
In the practice of the invention, therefore, it will be seen that membrane dryer systems can be e~fectively inte~rated with lO prepurifier adsorption-cryogenic air separation systems so as to dry the feed air to said adsorption-cryogenic systems in a manner representing a highly desirable advance over the conventional approaches commonly employed in the 15 art. The membrane dryer system operation is enhanced by the use of purge gas on the perme~te side of the mem~rane, with dry waste gas from the adsoxption-cryogenic system, or a portion of the dry, high purity nitrogen product ~tream from the 20 cryogenic air separation system being passed to the membrane dryer system, including such system employing two membrane materials for separate water and carbon dio~ide removal, or to the two stage membrane systems referred to above, for use ther~in 25 as ~aid desired purge gas.
Certain membranes are known to selectively remove moisture rom compressed feed air, nitrogen streams or the like. Unfortunately it has been found, as di~closed in U.SO Patent No. 4,7B3,~01, 30 that, when operated in ~ crossflow permeation manner, such membranes may re~uire a stage c~t, i.e., the ratio of permeate gas to feed gas flow, of roughly 3~% at, for ~xamp1e, 150 psig operation to 16,348 - .,. . , ~:. ~:- ~ : ;
;~
_ 16 - ~ ~ 4 ~ 1? 2 7 achieve a relatively modest pressure dewpoint of -40F. Obviously, the product gas recovery of such a crossflow membrane unit would be quite low, and the power and dryer area requirements of ~uch an 5 overall system would be undesirably hi~h. In order to enhance the benefits of the integrated systems in the practice of the inventionr however, the membrane ~ dryer system is desirably operated in a countercurrent manner, with dry reflu~ purge gas 10 being passed on the permeate side of the membrane to facilitate the carrying away of moisture from said permeate side and the maintaining of a high driving force across the membrane for moisture removal.
This processing feature serves to minimi~e the 15 membrane area required and the product p~rmeation loss necessary to achieve a given product dewpoint, i.e. level of ~rying. It is desirable in preferred embodiments of .the invention, to maintain product loss due ~o co-permeation of said nitrogen and 20 o~ygen from the feed air to less than 1%, preferably less than 0.5%, of the total product flow.
It will be appreciated that the membrane composi~ion used in ~he dryer membrane sys~em should be on~ having a~high selectivity for water over 25 nitrogen and o~ygen. That is, moisture must be selectively permeated much more rapidly than air.
The water~air separation fa~tor should be at least 50, preferably greater than 1,000, for advantageous moisture removal from feed air. As indicated above, 30 such a dryer membrane system will also have a carbon dioside~air separation factor in the range of from about 10 to about 200. In addition, the membrane ~ompositio~ should have a ~elatively low D-16,348 - . .~ . . . ,- ., :
', . : !.
'.. ': ~ ' :' :
- permeability rate for ~oth nitrogen and oxygen Cellulose acetate is an e~ample of a preferred membrane separation material satisfying such criteria. It will be appreciated that a variety of 5 other materials can also be employed, such as ethyl cellulose, silicone rubber, polyurethane, polyamide, polystyrene and the like. In the single or t~o stage membrane systems employing separate materials for water removal and for carbon dioxiae removal, 10 cellulose acetate is a preferred material for water removal purposes, with ethyl cellulose also be desirable for such purposes. For the separate car~on ~io~ide removal membrane material, polybutadiene and natural rubber are e~amples of 15 suitable materials for this purpose.
The membrane dryer system having a me~brane material of desirable membrane composition, which is integrated with a pressure swi~g adsorption system and ~ryo~enic air separation system as disclosed and 20 claimed herein, is preferably operated in a countercurrent flow pattern ~s indicated above. In ~ hollow fiber membrane configuration or in other suit~ble membrane configurations, e.g. spiral wound membranes, bundle ~esigns providing for flow 2~ patterns of the cross-flow type have been commonly employea in commercial practice. In cross-flow operation, the flow direction of permeate gas on the permeate side of the membrane is at right angles to the flow of feed gas on the feed side of the 30 mem~rane. For example, in the use of hollow fiber bundles and the pass~ge of feed gas on the outside of the hollow fiber membranes, the flow direction of permeate in the bores of the fibers is ~enerally at D-16,348 .
- ~ : .: : ~ : .
. . ~
- ., .. ,~ : ~ ~: , - 18 - 20 ~ ~r~ 27 ~ right angles to the flow of feed over the e~ternal surface of the hollow fibers. Likewise, in th~
inside-out approach in which the feed gas is passed through the bores of the hollow fibers, the permeate 5 gas generally passes from the surface of the hollow fibers in a direction generally at right an~les to the direction of the flow of feed within the bores of the hollow fibers and then, within the outer shell~ in the ~irection of the outlet means for the 10 permeate gas. As ~hown in European Patent Application Publication No. 0 226 431, published 3une 24, 1987, countercurrent flow pattern-s can be crsa~ed by the encasing of the hollow fiber hundle within an impervious barrier over the entirety o 15 its longitudinal ou~er surface e~cept for a non-encased circumferential region near one end of said bundle. This enables the feed gas or permeate gas, depending on the desired manner of operation, i.e.
inside out or outside-in, to pass in countercurrent 20 flow outside the hollow f~ibers parallel to the 10w direction of permeate gas or :feed gas in the bores of the hollow fibers. The feed gas on the outside of the hollow fiber bundle, for ~ample, is caused to flow parallel to, r~ther than at right-angle to, 25 the ~ntral a~is of the iber ~undle. It will be understood that the membrane fibers may be organized either in.straight assemblies parallel to the central a~is of the bundle, or alternatively, can be wound in helical fashion around the central a~is.
39 In any event, the impermeable barrier material may be ~ wrap of impervious film, e.g., polyYinylidene or the like. Alternatively, the impermeable barrier may b an impervisus coating material, e.g. d .
D-16,34B
" , ~
-- l g ~ r~ 7 polysilo~ane, applied from an innocuous solvent, or a shrink sleeve installed over the membrane bundle and shrunk onto said bundle. The impermeable barrier thus encases the hollow fiber or other 5 membrane bundle and, as disclosed in said publication, has an opening therein permitting the flow of gas into or from the bundle SQ that the fluid flows in a direction substantially parallel to the a~is of the fiber bundle. For purposes of the 10 inv~ntion, the flow pattern should be one of countercurrent flow of the wet feed air stream and the permeate gas comprising purge gas supplied as indicated above, together with moisture that permeates through the membrane material in the 15 membrane dryer system.
It should b~ noted that membrane drying operations are co~monly carried out in the art using a dense fiber membrane. The membrane thickness for such a dense fiber is also the wall thickness, and 20 is very large in comparison to the ~kin portion of an asymmetric membrane or to the separation layer of a ~omposite membrane. For a dense fiber, it is necessary to have a large wall thickne~s to achieve a significant pressure capability. Thus, dense 25 fibers have a ~ery low permeability rate an~ require the use ~f a ~ery large surface area for ~dequate drying of the nitrogen product. By co~trast, asy~netric or composite membranes, preferred over dense membranes for purposes of the invention, have 30 very thin membrane separation layers, witb the xelatively more porous substrate portion of said membranes providing mechanical strength and support for the very thin portion that determines the D-16,348 , . . . ~ .
. - . .~ .
. . ; ., -. , : . .. : ~ .-. ~ .
_ 20 ~ 7~
separation characteristics of the membrane. Much less surface area is required, therefore, for asymmetric or composite membranes than ~or den~e, homogeneous membranes. Because of the inherently 5 improved permeability obtainable by the use ~f asymmetric or composite membranes rather than dense membranes, it is desirable to further enhance asymmetric and composite membrane performance in preferred embodiments of the invention, as related 10 to the drying of feed air, so as to achieve a significant reduction in the loss of valuable feed air by ~o-permeation that would occur in cross-flow operation of such membranes.
It will be understood that the cryogenic 15 air separation system employed for purposes of the invention can be any conv~ntional, c~mmercially available system capable of producing high purity hitrogen and/or o~ygen in desirable quantities by the cryogenic rectificatîon of air. The details of 20 the cryogenic air separation system are not a part of the essence of the invention, relatihg to ~he integration of the cryogenic system with a membrane dryer system and with a conventi~nal prepurifier adsorption system. ~epresen~ative e~amples of such 25 cryogenic air separation technoloyy are di~losed in the Gheung patent, V.S. 4,44B,545, the Pahade et.
al. patent, U.S. 4,453,957, and the Cheung paterlt, U.S. 4,594,0B5. Similarly the prepurifier adsorption system employed in the practice of the 30 invention comprises any desirable adsorption system well known in the art and capable of r~moving undesired ~ontaminants from the dry ~eed air stream ~efore it~ passage to the cryogenic air ~eparation D-16,348 ~ . :
- 21 - 2Q~727 - system. The prepurifier adsorption system employed can ~e any convenient, commercially available system capable of removing carbon dio~ide and/or other.
contaminants, including re~idual water, from the dry 5 feed air stream. The adsorption system is commonly a pressure swing adsorption system operated so ~s to selectively adsorb said contaminants from the feed air at an elevated pressure and to desorb said contaminants at lower pressure, e.g. near ambient 10 pressure, for removal from the system. Such pressure systems typically employ a pair of adsorbent beds, with one bed being used f OE
adsorption purposes while the other bed is being regenerated. Typical adsorbent materials employed 15 in said beds include alumina, zeolite molecular sieves or silica gel. Alterna~ely, such systems can be operated on a thermal swing adsorption cycle, wherein the desired adsorption is carried but at a lower temperature, with desorption being 20 accomplished at an elevated temperature.
For purposes of the invention, a purge ratio, i.e. reflu~ pur~e gas/feed air flow on th~
non-permeable side, of at least about I0%, but preferably about 20% or above, is desired to keep 25 area requirements, product loss and back diffusion to a minimum. The purge ratio requirements also tend to ~e greater at relatively lower feed air pressures than at higher pressures.
In an illustrative e~ample of the practice 30 of tAe invention, the ~ryogenic air separation sy~tem is adapted to produce ~0 tons of dry, hi~h purity nitrog~n. Since nitrogen recovery based on air in the conventi~n~l pre-purified cryogenic D-16,34~
. , .
~ : :
- 2~ - 2 ~
system is typically on the order of 52%, appro~imately 48% of the feed air flow is available as low pressure waste. The cryogenic system can conveniently be operated with a feed air pressure of 5 91 psia, at an air temperature of 115F, with a waste gas pressure of 18 psia. In a conventional ~ystem, an aftercooler dewpoint of 115F, ~hiller product air dewpoint of 40F, an~ an absorbent product air dewpoint of -100F san conveniently be 10 empl~yed. A conventional mechanical chiller for use in such a system would cost appro~imately $30,000 and eonsume about 10 KW of electrical power. The air pressure drop in such a chiller and moisture separator would be on the order of about 2 psi. The 15 chiller is ~esirably replaced in the practice of the invention, as in the Fig. 1 embodiment, with a membrane dryer system having an o~ygen/n~troyen separation factor of 5.9, and a water/air separation ~3ctor of 1,000 or more. The membrane dryer system 2D is desirably comprised of hollow fiber membranes wound in a helical co~figuration, and operated using an impervious barrier of polyvinylidene to e~case the membF3ne and create a countercurrent flow pattern. In order to minimize the amo~nt of 25 compressed air lost due to permeation durin~ the dryi~g operation, the stage cut, i.e. permeate~feed ~low, of the membrane is kept very low. However, it ~hould be recognized, as i~dicated above, that a portion of the actual operating stage cut is due to 3Q th~ desired rejection of water and is unavoidable ;f th~ desired:drying is to be achieved. For enhanced drying, therefore, it is the dry stage cut resulting from the ~o-permeation of o~ygen and nitro~en that , ,348 .; ,1, ~ ~' " , - 23 - 2 ~ ~ri27 is minimized, i.e. to not more than about 5~, preferably to less than 0.5% of the inlet feed air.
A dry reflu~ purge ratio on the order of 18-20~ is used under the particular operating conditions and 5 membrane characteristics referre~ to above. The membrane ~ryer system is found to achieve a significant reduction in capital and power costs, and other benefits, provided that said dry f lux purge ratio of at least 18% is available.
An added advantage of the membrane dryer system is that it is not limited to providing a ~O~F
air dewpoint feed to the adsorption-cryogenic system. A given membrane area can ~e used to provide air of varying quality depending on the 15 purge ratio employed and the membrane characteristics. The residual water concentrat;on of the dried air can be reduced by the use of more purge gas, or membranes with higher water separ~tion characteristics, apart from the use of increased 20 membrane area. Any such reduction in residual water content will serve to reduce the amount of water vapor that must be removed by the adsorbent beds in the prepurifier adsorption system, thereby increasing the sapacity of said system and reducing 25 the purge gas and energy requirements thereofO The optimum membrane dryer dewpoint will thus be seen to depend on the relative cost of removing water in the membrane dryer system and in the prepurifier adsorption system.
It will be appreciated that various changes and modifications can be made in the details of the process and 6ystem as herein described without departing ~rom th~ scope of the invention a~ ~et D-16,348 :
, . ::
; , . . . :
- 24 ~ 4 rj 2 7 forth in the appended claims. Thus, asymmetric or composite membrane ~tructures can be employed in the dryer membrane system of the invention. While dense membrane~ are commonly used for product drying 5 applications, such dense membranes are not preferred ~ecause of the inherent limitatîons thereof noted above, although they can be used in the practice of the invention.
The permeable membranes employed in the 10 practice of the invention, in either a single stage or the two stage embodiments employing a single material or separate material~ for water and carbon dio~ide removal, will co~monly be employed in assemblies of membrane bundles, typically positioned 15 within enclosures to form membrane modules that comprise the principal element of a membrane system. A membrane system may comprise a single module or a number of such modules, arranged for either parallel or series operation. The membrane 20 modules can be constructed using bundles of membranes in convenient hollow fi~er form, or in spiral wound, pleated flat sheet, or other desired membrane configurations. Membrane modules are constructed to have a feed air side, and an 25 opposite, permeate gas e~it side. For hollow fiber membranes, the feed side can be either the bore side for inside-out operation, or the outside of the hollow fibers for outside-in operation. Means are provided for introducing feed air to the system and 30 for withdrawing ~oth permeate and non-permeate streams.
As indi~ated above, the purge ~as employed in the invention should be a dry or a rela~ively dry D-16,348 - . ., -;
.
- 25 - 20~ ~27 gas, as from the sources reerred to herein. As used herein, a relatively dry purge gas is one having a moisture partial pressure not e~ceeding the partial pressure of moisture in the dried feed air 5 stream. Preferably, said purge gas moisture partial pressure will be less than half the moisture partial pressure in said stream, as will be the case with respect to the sources of purge gas disclosed above.
Membranes will be seen to provide a highly 10 desirable system and process for drying feed ~ir before its passage to air adsorption-cryogenic air separat~on system for the production of dry, high purity nitrogen. By accomplishing the drying in convenient membrane systems, the use of the more 15 cos~ly chillers for moisture removal can be avoided. By integratiny the processing streams of the memhrane dryer system, utilizing single or two stage units of a single material or of separate materials for enhanced water and carbon dioxide 20 removal, with the cryogenic air separation system and the prepurifier adsorption system, a purge of the low pressure, permeate side of the membrane dryer æystem with relatively dry ~urge gas is ~onveniently accomplished. By utilizing a bundle 25 arrangement so as to establish a countercurrent flow pattern, preferred embodiments of the drying operation can be carried out with an enhanced recovery of dry feed air, avoiding the co-permeation of ~ignificant amounts of compressed ~ir as occurs 30 in cross-flow permeation operations.
D-16,348
~YO~ AIR SE~RA~IQ~ PIIAN~
.. . ..... . .. ~, _ ,. .__ _ _" . . .... __ . .
The ;nvention r~lates to the cryogenic ~eparation ~f~air. ~More par~icularly, it relates to he pretrçatment of ~eed air to cryogenic air - separation systems.
Nitrogen and o~ygen are desired for many 20 chemi~al proces~ing, refinery, meta~ production and ~: ~ other i:ndu~trial applications. While var~ous ~echnigues are known for the production o~ nitrogen a~d~or o~ygen by air ~eparatio~,~cryogenic distillotion processes ~nd systems are widely used 25 ~or the production of ~itrogen and/or:o~y~en from air, or for the removal of nitrogen from we~l ~ases~ In each c~yog ni~c application, high ~r~ezing point~cont~minant;s, which ~ould otherwise ~olidify ~at the low temperatur~s at which the primary g~s 30 ~epa~ation tske~ pla~e,:must be removed rom the compressed ~eed ~as streàm. Such contaminants are commonly removed by r~frlgeration~adsorp~ion process combinations well ~own in the ~rt. In air s~paratio~:operations, this pre-cl~anup ca~ u~ilize ..
: D-16,34~ :
:
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- 2 ~ P~7 a reversing heat e~changer and cold end gel trap combination, or a mechanical air chiller/zeolite molec~lar sieve adsorber combination. In the former type of proces~ing unit, virtually all of the 5 contaminants are frozen out of the feed air when said air is thermally e~changed against the cryogenic waste and product gas streams.
Unfortunately, however, the self cleaning of the reversing heat e~changer unit requires a large purge 10 gas flow relative to the air feed. As a result, the a;r recovery of such cleanup cycles tends to be undesirably limited. Reversing heat e~changer units also require large valves, which must open and close on a ~yclie basis, switching the air feed and waste 15 purge flow passages. The valves are often located within the ~nsulated cold box portion of the eryogenic system, making maintenance difficult.
Furthermore, to act effectively, the heat e~change-gel trap combination must operate at low 20 temperature, and thus requires a eo~siderable cool down period during plant start-up.
In contrast to reversing heat e~changer and gel trap ~ombinations, mechanicaI chiller/
adsorptive unit combinations, ~s disclosed-in 25 Prentice, U.S. 4,375,367, can supply a clean, dry feed air stream within minutes of ~tart-up. The mechanical chiller reduces the air temperature to about 40F from the compressor aftercooler temperature of ~rom about 80F ~o about 115F. The ~0 air, which i~ satura~ed at the higher temperatures, loses the ~ulk of its water burden through condensation, thus reducing the inlet water concentration to ~he adsorptive unit. The D-16,~48 : _ 3 ~ 7 ~ 7 adsorption operation is typically carried out using a pair of pressure vessels, one bed being used for adsorbing purposes, while the other is underg~ing regeneration. The pressure vessels are filled with 5 an adsorbent material, such as alumina, zeolite molecular sieve or silica gel, which removes the - remaining water vapor, carbon dio~ide and~or other contaminants from the feed air stream. The adsorbent beds are usually regenerated at near 10 ambient pressure with a contaminant ree stream, either a portion of the cryogenic waste or dry air, which may be heated tv improve its desorbing capability. The operation o~ the mechanical chiller substantially improves the performance of the 15 adsorber beds by increasing their adsorption capacity, reducing the inlet water concentration, and, consequent~y, the purge f lvw and energy requirements of the operation. The mechanical chiller is limited to a minimum product dewpoint of 20 about 38F due to the necessity for avoiding the buildup of ice on the tubing walls. The chillers must also be followed by ~ moisture separator to remove the condensate formed fro~ the feed air and to protect the adsorbent beds ~rom egcessive 25 moisture. The mechanical chillRrs used in such operations tend to be e~pensive in terms of capital and power r~quirements, especially for small plants. In addition, such chillers are yenerally ~nown for requiring e~pensive maintenance.
In light of such factors, there has been a desire in the art for n~w prnces~es a~d ~ystzms that would either ~liminate or modify the furlction 2f the - ~omponent6 referred to above, particularly the .
D-16,398 - 4 - 2 0~ ~r~27 mechanical chiller and moisture separator so as to more economically provide clean, dry air to a cryogenic gas separation unit. One approach considered with interest i~ the use of membrane 5 systems to ~electively permeate water and carbon d;o~ide from feed air. Certain materials are well known as being capable of selectively permeating water and carbon dio~ide, while air or other gases, comprising less perme~ble components, are recovered 10 as non-permeate gas. ~ membrane system utilizing such a material would replace the function of the me~hanical chiller. Such membrane systems are well known to be relatîvely simple and easy to operate and maintain. As such membrane systems are normally 15 operated, however, the removal of moisture from the feed stream re~uires the co-permeation of signi~icant amounts ~f valuable product gas.
Operation of membrane systems at stagP cuts on the order of 10 to 20% might be re~uired to achieve the 20 aewpoint level achieved by the use of a mechanical chiller. Such cîrcumstance would, as a result, reduce the overall process recovery level achievable, încrPase the power requirements of the pro~ess, and be generally unattractiv~ from ~n 25 economic viewpoint. Despite such factors serving to deter the use of membrane dryer systems în place of mechanical chillers or said reversing heat e~changer and gel trap combinations, the use of membrane dryer ~ystems în new, împroved overall processes and 30 ~ystems, eliminating the need for the presently employed technî~ques, would represent a desîrable advance in the art.
It is ~n object of the inventîon, therefore, to provide an improved ~roc~s~ an~ system D-16,348 ..
2 ~ 7 for the production of dry nitrogen and/or o~ygen product.
It is another object of the invention to provide an impro~ed process and system utili2ing 5 cryogenic systems for gas separation and providing for desired for the use of a membrane system for the ~ removal of moisture and carbon dio~ide from the feed gas.
It is a further object of the invention to 10 provide ~ membrane dryer system capable of achieving enhanced drying efficiency and carbon dio~ide removal in an overall process and system for the recovery of dry nitrogen and/or oxygen using a eryogenic system or air separation.
With those and other obj~cts in mind, the invention is hereinafter described in ~etail, the novel f*atures thereo~ being particularly pointed o~t in the appended claims.
Summary of ~he I~v~n~ion : A membrane dryer system is employed in conjunction with an adsorption unit-cryogenic gas ~eparation u~it system to achiev~ a desired - produc~ion of dry nitrog~n ~nd/or o~ygen product.
25 The ~embrane dryer is preferably operated with a counter~urrent flow pattern and is re lu~ed on the low pressure permeate side ther~of. Waste gas from the adsorption-~ryogenic unit is used as purge gas.
The area requirements of the membra~e ars thereby 30 reduced, and the desired product recovery is appreciably increased. The membrane dryer removes water and carbon ~io~ide contaminants in the feed - air in si~gle or two ~tage units ~mploying ~eparate water ~nd carbon dioxide removal membr~n~s.
D-16,348 .
- 2~4~7~7 ~lief Des~riDtion o ~hç Drawins The invention is hereinafter described in detail with reference to the ~ccompanying drawi~gs 5 in which:
Fig. 1 is a schematic flow diagram of an embodiment of the in~ention in which the waste gas from the cryogenic feed gas separation system is employed as purge gas for a membrane system for the 10 drying of the feed gas to the cryogenic system; and Fig. 2 is a schematic flow diagram of an embodiment in which purge gas xemoved from the absorhent bed prepurifier for the cryogenic ~ystem is employed as purge gas for a feed gas membrane 15 drying system. : -' e~ail~ De criP~i~n Qf ~he Invention The objects of the invention are accomplished by the integration of a membrane system 20 for ~eed air drying with a downstrPam adsorption-cryogenic air separation system under conditions enabling desired moisture~remo~al from the feed air to be accomplished without reduetion in the ~verall product recovery of the process and system to 25 unacceptable levels. Such conditions ad~antageously relate to the integration of the separate processing systems, he selectivity for moisture removal of the particular membran on~position employed, and membrane bundle design conditions under which 3~ ~ountercurrent flow is desirably achiev d in the membrane dryer system. This en3bles nitrogen and/or o~ygen to be recovered in dry form with minimum loss of æaid product during the drying operation.
DD 1~; ~ 3~! 8 _ 7 _ 2~7~7 In the practice of the invention, waste gas from the cryogenic air separation system is used to provide purge gas to a membrane dryer system and to the adsorption system upstream of said cryogenie 5 system. The invention enables a dry, high purity nitrogen and~or o$ygen product stream to ~e obtained ~ with minimum loss of desired product because of the requirements of the drying operation. The overall process and system of the invention is illustrated 10 with reference to the drawings. Further information relating to the overall cryogenic systems used in the pract ce of the invention, and the membrane systems integrated therewith to achieve enhanced drying o feed air are provided below.
lS In Fig. 1 of the drawings, feed air is passed in line l to ~ir compressor 2, from which wet compressed air is passed in line 3 to membrane dryer ~ystem 4. In said membrane system 4, water selectively permeates throu~h the membrane material 20 and is discharged from the system as waste ~as ~hrough lin~ 5. Feed air is recovere~ from membrane dryer system 4 as dry, non-permeate or retentate gas through line 6 for passage to adsorption system 7, which is used to remove contaminants from the dry 25 feed air prior to the passage of said feed air to the cryogenic air separation system. Adsorption system 7 is shown as including two beds of adsorbent material, i.e. b~ds 8 and 9, one b~d generally being used ~or its intended a~sorption purposes ~hile the 30 other bed is being regenerated. The dry, purified feed gas is passed from said adsorption system 7:in line 10 to cryogenic air separation system ll, from which the de ired dry, high purity product gas is D-16,348 ,,: : ::~ : , ~ , -: ~,.- - . .. : ;
, ~: .. . ~ .. . . .
::
- 8 ~ 7~7 recovered through line 12. A dry waste stream from said cryogenic system is withdrawn through line 13.
A portion of this dry waste stream, i.e o~ygen or nitrogen, is withdrawn through line 14 for passage 5 through adsorption system 7, that is through either bed 8 or bçd 9, as dry adsorbent purge gas for the - bed undergoing regeneration. An adsorbent waste stream is withdrawn from a~orption system 7 through line 15, said waste stream containing the adsorbent lO purge gas and contaminants desorbed rom the adsorbent beds during t~e regeneration thereof. The remaining portion Gf the ~ry waste gas from cryogenic air separation system 11 is passed through line 16 for introduction to membrane dryer system 9 15 as a dry purge gas on the lower pressure, permeate side of said membrane system. Said dry p~rge gas is used to facilitate th removal of permeate waste gas from the surface of the membrane, and is discharged, together with saia permeate gas, through line 5.
The embodiment of the invention illustrated in Fig. l~serves to eliminate the need for a chiller otherwise employed as part of a chiller/adsorbent bad combination ~or the removal of water and carbon dio~ide from the compressed air streams of ~5 conventional pre-purified cryogenic air separation plants. Such elimination of the ~hiller is desirable, as indicated above, because it is e~pensive in terms of both capital and power and becau~e it is well known ~or reguiring eg~ensive 30 maintenance. The membrane dryer sy tem used in the practi~e of the invention, on the other hand, is well known as being very ~imp}e and ine~pensive in - nature, and not reguirin~ e~tensive mainten~nce.
D-16,348 - g - 2~ 7 While this embodiment of the invention, integrating membrane systems with adsorption-cryogenic air separation systems, is an a~vantageous adva~ce over conventional pre-purified cryogenic air ~eparation 5 systems, further development in the art is also desirable. One limitation of the Fig. 1 embodiment of the invention is that the permeate purge gas re~uirements for the membrane dryer ~ystem, which typically are appro~imately 10-20% of the feed aix 10 to said membrane dryer system, are ;n addition to t~e 10-15% purge requirements for the pre-purifier adsorption system. Consequently, the relatively large overall purge requirements of the system, appro~imately 20-35%, make it difficult to achieve lS high recovery of nitrogen and o~ygen in cryogenic air separation systems when such large amounts of waste gas are not available.
The embodiment illustrat~d in Fig. 2 addresses the need for minimizing ~he overall purge 20 re~uirements of the system. In this embodiment, air in line 20 is compressed in air compressor 21, with the compressed air being passed in line 22 to coaleccer unit 23, from which water is removed through line 24. The thus-~rea~ed compres-sed air 25 stream is p3ssed in line 25 to fir~t ~taye membrane dryer 26, the first part of a two-stage membrane ~ryer system. Most of the water still present in the feed air is r~moved in this first stage dryer, which is refluxed in the permeate si~e by a dry 30 pur~e stream as hereinafter indicated. The partially dry, compressed feed air passes, as non-permeate gas, from first stage membrane ~Iryer 26 through line 27 to second stage mem}:~rane dryPr 28, D-16, 348 - :: ,' ' :. ~; '-.
- : -- ~ .",, - ., :
. . :
: . . ~
1~- 2~ 27 wherein residual water is removed so that dry feed air is passed therefrom as a non-permeate stream for passage ;n line 29 to prepurifier adsorption system 30 for purification before passage to the cryogenic 5 air separation system. Adsorption system 30 is shown as containing two adsorbent beds, namely bed 31 and bed 32, it being understood that one such bed will commonly be used for purification of dry feed gas while the other ~e~ is undergoing regeneration.
10 Dry, purified feed air leaving adsorption system 30 is passed in line 33 to cryogenic air separation system 34, from which the aesired dry, high purity nitrogen or oxygen product is recovered through line 35. Dry waste gas from cryogenic system 34 is 1~ withdrawn through line 36, heated in heat e~changer 37, and passed through line 38 to prepurifier adsorption system 30 as purge gas for use in the regeneration of whichever bed, i.e. bed 31 or bed 32, is being regenerated at any given time. Since 20 virtually ~11 of the water present in the feed air is removed in the membrane dryer system, the spent purge ga~ e~iting prepurifier adsorption system 30 will be re~atively dry, although.it will contain other contaminants such as carbon dio~ide ~nd 25 hydrocarbons. Such spent purye gas is passed in line 39 to first staQe membrane dryer 26 ~or use therei~ as purge gas on the permeate side of the membrane. Said purge ~as, together with water vapor that permeates through said membrane dryer 2S, is 30 withdrawn through linç 40 for dischsrge to waste.
The passage of such recycle purge ~as ~hrough membrane dryer 2fi ~acilitates the ~arrying o~ said permeate ~ater away from the surface of the membrane D-16,348 .
11- 2~f~7~7 on said permeate side so that a high driving force is maintained across membrane dryer 26 to sustain the desired moisture removal from the feed air stream being passed to said membrane dryer 26.
Second stage membrane dryer 28 is us0d, in the Fig. 2 embodiment, to dry the feed air to higher - levels than are achieved in first stage membrane ~ryer 26. For purging in this dryer, any dry, low pressure stream avail:able from the cryogenic 10 process, such as waste gas from cryogenic system 34, high purity nitrogen or o~ygen product gas, expanded feed air or the like, or waste gas from prepurifier adsorption system 30, can ~e used: a~ the dry purge gas. In Fig. 2, a portion of the cryogenic system 15 34 waste gas is shown as being passed through line ~1 to second stage membrane:dryer 28 for u~e as purge gas therein. Such purge gas facilitates the carrying away of the permeate water from the surface of the membrane on the permeate s~dP of the membrane 20 so that a high driving force is maintained across membrane 28 to sustain the desired additional drying of the ~eed air stream being passed to said membrane 28. Purge gas, together with additional permeat~
water, is withdrawn from membrane dryer throu~h line 25 42.
Those skilled in the art will appreciate that the use of second stage membra~e dryer 2B is optional, depending on the degree of feed air drying desired in any particular dry, high purity nitrogen 39 and/or o~ygen production operation. When employed, as in the Fig. 2 ~mbvdiment, second sta~e memhrane dryer 28 will typically be smaller and require much - less purge gas than first stage membrane dryer 26 D-16,3~8 .. ....
- 12 - 2~7~7 because most of the water removal from the feed air occurs in the first stage membrane dryer system.
The Fig. 2 embodiment will be seen to be of advantage in that it enables the overall purge 5 re~uirement of the process to be reduced in comparison to that of the Fig. l embodiment. Thus, if the total membrane dryer purge requireme~t ;s 20~
and the pre-purifier adsorption system 30 purge requirement is 15%, the~, in such embodiment, only lO S% of purge gas over and above that employed ~or pre-purification would be req~ired. Removal of ~irtually all of the water in the membrane dryer also greatly reduces the water load on the pre purifier adsorption system. This, in turn, greatly 15 reduces the thermal energy required for pre-purifier regeneration, making possible perhaps the use of compressor waste heat for prepurifier regeneration.
Since water is a very ~trongly adsorbed species in the prepurifier, the removal of most of 20 the water from the prepurifier feed gas can rPsult in improved aasorbent performance~with respect to other speci~s desired to be removed, such as carbon dio~;de, hydrocarbons and the like. It will be appreciated that this could lead to desir~bly 25 improved prepurifier operation. It should be noted that membrane dryers sui~able for the removal of water will slso generally be relati~ely selective for carbon dio~ide removal. Such carbon ~iv~ide removal will also reduce the load on the downstream 30 adsorpti:on unit.
While ~he removal of carbon dio~ide by the membrane dryers suita~le for water removal is thus desirable, other ~mbodiments further enhancing the D-16,348 : .:
. . ~. - :.... : -overall operation comprise the use of single or two stage membrane systems employing ~eparate water and carbon dio~ide removal membranes. In the single stage system, two membrane materials, one having a 5 selectivity optimized for water and the other being optimi~ed for carbon dio~ide, are zmployed. The separate membrane materials may be positioned in any des;rable form, as in a side-by-side or a layer-by-layer arrangement. The use of two different 10 permeable membranes capable of separating different compone~ts of a fluid mi~ture is described in the Perrin patent, V.S. 4,880,440. In such a single stage membrane system adapted for enhanced remcval of both water and carbon dio~ide, relatively dry 15 purge gas can be conveniently supplied from the prepurifier adsorption system and/or from the cryogenic air separation system as in the illustrated embodiments referred to above.
In another embodiment of the hybrid 20 prepurifier of the invention, two separate membrane stages can be employe~. In the latter embodiment, each stage contains membrane modules containing membrane materials particularly suitable for the component primarily ~eing separated therein. The 25 two stage embodiment is preferably arranged so that th~ feed air passes to a first stage membrane adapted for water removall with the non-permeate, dried feed gas passing to a second stage membrane adapted primarily or the removal of carbon 30 diogide. In one such embodiment, a p~rtion of the waste gas from the ~ryogenic air ~eparation system may be passed, as in the ~ig. 1 embodiment, to the membrane ~ystem with separate portions of said purge D-16,348 .. : . ~ - .......... :
.... . .
:: -. , :~ , .. .. : -. . ~ . . : , - 14 - 2~ 7 gas b~ing passed to the first a~d seco~d membrane stages. It will be appreciated that the two stage membrane embodiment enables the purge gas r3tio to be optimized separately for each comp~nent, i.e.
5 water or carbon dio~ide, being removed ~rom the feed air.
~ t should be noted that with reference to the two stage process, the waste purge removed from the second stage, carbon dio~ide removal membrane 10 unit will be relatively dry and can be conveniently passed to the first stage, water removal membrane unit as purge gas therefor. As the adsorption waste will also be relatively dry, this stream can also be used for purge and can be passed to the first stage 15 membrane for such purge purposes, as in the Fig. 2 embodiment. Conveniently, the waste streams from the second membrsne ~tage and from the prepurifier adsorption system can be combined tv make up all or part of the purge gas for the first stage membrane 20 unit. This embodiment will typically result in a significant re~uction in the overall amount of gas required to be recycled for pur~e purposes. In this regard, it will be appreciated that, despite the use of separate water and carbon dio~ide removal 25 materials and stages, some carbon dio~ide will likely be removed in the first stage ~apted for water removal, ~nd some water will be removed in the ~econd stage carbon dioxide removal unit.
It should also be noted that adsorption of 30 water in the prepurifi~r is e~othermic in nature and ~enerates signi~icant amounts of heat. This tends ~o raise the t~mperature of the air leaving the prepurifier which, in turn, increases the D-l~, 3~a .
~; ~
_ 15 ~
refrigeration load on the cryogenic system. Removal of the water from the prepurifier feed by use of the membrane dryer system will tend to ~reatly reduce the heat generated in the prepurifier adsorption system during adsorption therein, thus benefiting the downstream cryogenie process.
In the practice of the invention, therefore, it will be seen that membrane dryer systems can be e~fectively inte~rated with lO prepurifier adsorption-cryogenic air separation systems so as to dry the feed air to said adsorption-cryogenic systems in a manner representing a highly desirable advance over the conventional approaches commonly employed in the 15 art. The membrane dryer system operation is enhanced by the use of purge gas on the perme~te side of the mem~rane, with dry waste gas from the adsoxption-cryogenic system, or a portion of the dry, high purity nitrogen product ~tream from the 20 cryogenic air separation system being passed to the membrane dryer system, including such system employing two membrane materials for separate water and carbon dio~ide removal, or to the two stage membrane systems referred to above, for use ther~in 25 as ~aid desired purge gas.
Certain membranes are known to selectively remove moisture rom compressed feed air, nitrogen streams or the like. Unfortunately it has been found, as di~closed in U.SO Patent No. 4,7B3,~01, 30 that, when operated in ~ crossflow permeation manner, such membranes may re~uire a stage c~t, i.e., the ratio of permeate gas to feed gas flow, of roughly 3~% at, for ~xamp1e, 150 psig operation to 16,348 - .,. . , ~:. ~:- ~ : ;
;~
_ 16 - ~ ~ 4 ~ 1? 2 7 achieve a relatively modest pressure dewpoint of -40F. Obviously, the product gas recovery of such a crossflow membrane unit would be quite low, and the power and dryer area requirements of ~uch an 5 overall system would be undesirably hi~h. In order to enhance the benefits of the integrated systems in the practice of the inventionr however, the membrane ~ dryer system is desirably operated in a countercurrent manner, with dry reflu~ purge gas 10 being passed on the permeate side of the membrane to facilitate the carrying away of moisture from said permeate side and the maintaining of a high driving force across the membrane for moisture removal.
This processing feature serves to minimi~e the 15 membrane area required and the product p~rmeation loss necessary to achieve a given product dewpoint, i.e. level of ~rying. It is desirable in preferred embodiments of .the invention, to maintain product loss due ~o co-permeation of said nitrogen and 20 o~ygen from the feed air to less than 1%, preferably less than 0.5%, of the total product flow.
It will be appreciated that the membrane composi~ion used in ~he dryer membrane sys~em should be on~ having a~high selectivity for water over 25 nitrogen and o~ygen. That is, moisture must be selectively permeated much more rapidly than air.
The water~air separation fa~tor should be at least 50, preferably greater than 1,000, for advantageous moisture removal from feed air. As indicated above, 30 such a dryer membrane system will also have a carbon dioside~air separation factor in the range of from about 10 to about 200. In addition, the membrane ~ompositio~ should have a ~elatively low D-16,348 - . .~ . . . ,- ., :
', . : !.
'.. ': ~ ' :' :
- permeability rate for ~oth nitrogen and oxygen Cellulose acetate is an e~ample of a preferred membrane separation material satisfying such criteria. It will be appreciated that a variety of 5 other materials can also be employed, such as ethyl cellulose, silicone rubber, polyurethane, polyamide, polystyrene and the like. In the single or t~o stage membrane systems employing separate materials for water removal and for carbon dioxiae removal, 10 cellulose acetate is a preferred material for water removal purposes, with ethyl cellulose also be desirable for such purposes. For the separate car~on ~io~ide removal membrane material, polybutadiene and natural rubber are e~amples of 15 suitable materials for this purpose.
The membrane dryer system having a me~brane material of desirable membrane composition, which is integrated with a pressure swi~g adsorption system and ~ryo~enic air separation system as disclosed and 20 claimed herein, is preferably operated in a countercurrent flow pattern ~s indicated above. In ~ hollow fiber membrane configuration or in other suit~ble membrane configurations, e.g. spiral wound membranes, bundle ~esigns providing for flow 2~ patterns of the cross-flow type have been commonly employea in commercial practice. In cross-flow operation, the flow direction of permeate gas on the permeate side of the membrane is at right angles to the flow of feed gas on the feed side of the 30 mem~rane. For example, in the use of hollow fiber bundles and the pass~ge of feed gas on the outside of the hollow fiber membranes, the flow direction of permeate in the bores of the fibers is ~enerally at D-16,348 .
- ~ : .: : ~ : .
. . ~
- ., .. ,~ : ~ ~: , - 18 - 20 ~ ~r~ 27 ~ right angles to the flow of feed over the e~ternal surface of the hollow fibers. Likewise, in th~
inside-out approach in which the feed gas is passed through the bores of the hollow fibers, the permeate 5 gas generally passes from the surface of the hollow fibers in a direction generally at right an~les to the direction of the flow of feed within the bores of the hollow fibers and then, within the outer shell~ in the ~irection of the outlet means for the 10 permeate gas. As ~hown in European Patent Application Publication No. 0 226 431, published 3une 24, 1987, countercurrent flow pattern-s can be crsa~ed by the encasing of the hollow fiber hundle within an impervious barrier over the entirety o 15 its longitudinal ou~er surface e~cept for a non-encased circumferential region near one end of said bundle. This enables the feed gas or permeate gas, depending on the desired manner of operation, i.e.
inside out or outside-in, to pass in countercurrent 20 flow outside the hollow f~ibers parallel to the 10w direction of permeate gas or :feed gas in the bores of the hollow fibers. The feed gas on the outside of the hollow fiber bundle, for ~ample, is caused to flow parallel to, r~ther than at right-angle to, 25 the ~ntral a~is of the iber ~undle. It will be understood that the membrane fibers may be organized either in.straight assemblies parallel to the central a~is of the bundle, or alternatively, can be wound in helical fashion around the central a~is.
39 In any event, the impermeable barrier material may be ~ wrap of impervious film, e.g., polyYinylidene or the like. Alternatively, the impermeable barrier may b an impervisus coating material, e.g. d .
D-16,34B
" , ~
-- l g ~ r~ 7 polysilo~ane, applied from an innocuous solvent, or a shrink sleeve installed over the membrane bundle and shrunk onto said bundle. The impermeable barrier thus encases the hollow fiber or other 5 membrane bundle and, as disclosed in said publication, has an opening therein permitting the flow of gas into or from the bundle SQ that the fluid flows in a direction substantially parallel to the a~is of the fiber bundle. For purposes of the 10 inv~ntion, the flow pattern should be one of countercurrent flow of the wet feed air stream and the permeate gas comprising purge gas supplied as indicated above, together with moisture that permeates through the membrane material in the 15 membrane dryer system.
It should b~ noted that membrane drying operations are co~monly carried out in the art using a dense fiber membrane. The membrane thickness for such a dense fiber is also the wall thickness, and 20 is very large in comparison to the ~kin portion of an asymmetric membrane or to the separation layer of a ~omposite membrane. For a dense fiber, it is necessary to have a large wall thickne~s to achieve a significant pressure capability. Thus, dense 25 fibers have a ~ery low permeability rate an~ require the use ~f a ~ery large surface area for ~dequate drying of the nitrogen product. By co~trast, asy~netric or composite membranes, preferred over dense membranes for purposes of the invention, have 30 very thin membrane separation layers, witb the xelatively more porous substrate portion of said membranes providing mechanical strength and support for the very thin portion that determines the D-16,348 , . . . ~ .
. - . .~ .
. . ; ., -. , : . .. : ~ .-. ~ .
_ 20 ~ 7~
separation characteristics of the membrane. Much less surface area is required, therefore, for asymmetric or composite membranes than ~or den~e, homogeneous membranes. Because of the inherently 5 improved permeability obtainable by the use ~f asymmetric or composite membranes rather than dense membranes, it is desirable to further enhance asymmetric and composite membrane performance in preferred embodiments of the invention, as related 10 to the drying of feed air, so as to achieve a significant reduction in the loss of valuable feed air by ~o-permeation that would occur in cross-flow operation of such membranes.
It will be understood that the cryogenic 15 air separation system employed for purposes of the invention can be any conv~ntional, c~mmercially available system capable of producing high purity hitrogen and/or o~ygen in desirable quantities by the cryogenic rectificatîon of air. The details of 20 the cryogenic air separation system are not a part of the essence of the invention, relatihg to ~he integration of the cryogenic system with a membrane dryer system and with a conventi~nal prepurifier adsorption system. ~epresen~ative e~amples of such 25 cryogenic air separation technoloyy are di~losed in the Gheung patent, V.S. 4,44B,545, the Pahade et.
al. patent, U.S. 4,453,957, and the Cheung paterlt, U.S. 4,594,0B5. Similarly the prepurifier adsorption system employed in the practice of the 30 invention comprises any desirable adsorption system well known in the art and capable of r~moving undesired ~ontaminants from the dry ~eed air stream ~efore it~ passage to the cryogenic air ~eparation D-16,348 ~ . :
- 21 - 2Q~727 - system. The prepurifier adsorption system employed can ~e any convenient, commercially available system capable of removing carbon dio~ide and/or other.
contaminants, including re~idual water, from the dry 5 feed air stream. The adsorption system is commonly a pressure swing adsorption system operated so ~s to selectively adsorb said contaminants from the feed air at an elevated pressure and to desorb said contaminants at lower pressure, e.g. near ambient 10 pressure, for removal from the system. Such pressure systems typically employ a pair of adsorbent beds, with one bed being used f OE
adsorption purposes while the other bed is being regenerated. Typical adsorbent materials employed 15 in said beds include alumina, zeolite molecular sieves or silica gel. Alterna~ely, such systems can be operated on a thermal swing adsorption cycle, wherein the desired adsorption is carried but at a lower temperature, with desorption being 20 accomplished at an elevated temperature.
For purposes of the invention, a purge ratio, i.e. reflu~ pur~e gas/feed air flow on th~
non-permeable side, of at least about I0%, but preferably about 20% or above, is desired to keep 25 area requirements, product loss and back diffusion to a minimum. The purge ratio requirements also tend to ~e greater at relatively lower feed air pressures than at higher pressures.
In an illustrative e~ample of the practice 30 of tAe invention, the ~ryogenic air separation sy~tem is adapted to produce ~0 tons of dry, hi~h purity nitrog~n. Since nitrogen recovery based on air in the conventi~n~l pre-purified cryogenic D-16,34~
. , .
~ : :
- 2~ - 2 ~
system is typically on the order of 52%, appro~imately 48% of the feed air flow is available as low pressure waste. The cryogenic system can conveniently be operated with a feed air pressure of 5 91 psia, at an air temperature of 115F, with a waste gas pressure of 18 psia. In a conventional ~ystem, an aftercooler dewpoint of 115F, ~hiller product air dewpoint of 40F, an~ an absorbent product air dewpoint of -100F san conveniently be 10 empl~yed. A conventional mechanical chiller for use in such a system would cost appro~imately $30,000 and eonsume about 10 KW of electrical power. The air pressure drop in such a chiller and moisture separator would be on the order of about 2 psi. The 15 chiller is ~esirably replaced in the practice of the invention, as in the Fig. 1 embodiment, with a membrane dryer system having an o~ygen/n~troyen separation factor of 5.9, and a water/air separation ~3ctor of 1,000 or more. The membrane dryer system 2D is desirably comprised of hollow fiber membranes wound in a helical co~figuration, and operated using an impervious barrier of polyvinylidene to e~case the membF3ne and create a countercurrent flow pattern. In order to minimize the amo~nt of 25 compressed air lost due to permeation durin~ the dryi~g operation, the stage cut, i.e. permeate~feed ~low, of the membrane is kept very low. However, it ~hould be recognized, as i~dicated above, that a portion of the actual operating stage cut is due to 3Q th~ desired rejection of water and is unavoidable ;f th~ desired:drying is to be achieved. For enhanced drying, therefore, it is the dry stage cut resulting from the ~o-permeation of o~ygen and nitro~en that , ,348 .; ,1, ~ ~' " , - 23 - 2 ~ ~ri27 is minimized, i.e. to not more than about 5~, preferably to less than 0.5% of the inlet feed air.
A dry reflu~ purge ratio on the order of 18-20~ is used under the particular operating conditions and 5 membrane characteristics referre~ to above. The membrane ~ryer system is found to achieve a significant reduction in capital and power costs, and other benefits, provided that said dry f lux purge ratio of at least 18% is available.
An added advantage of the membrane dryer system is that it is not limited to providing a ~O~F
air dewpoint feed to the adsorption-cryogenic system. A given membrane area can ~e used to provide air of varying quality depending on the 15 purge ratio employed and the membrane characteristics. The residual water concentrat;on of the dried air can be reduced by the use of more purge gas, or membranes with higher water separ~tion characteristics, apart from the use of increased 20 membrane area. Any such reduction in residual water content will serve to reduce the amount of water vapor that must be removed by the adsorbent beds in the prepurifier adsorption system, thereby increasing the sapacity of said system and reducing 25 the purge gas and energy requirements thereofO The optimum membrane dryer dewpoint will thus be seen to depend on the relative cost of removing water in the membrane dryer system and in the prepurifier adsorption system.
It will be appreciated that various changes and modifications can be made in the details of the process and 6ystem as herein described without departing ~rom th~ scope of the invention a~ ~et D-16,348 :
, . ::
; , . . . :
- 24 ~ 4 rj 2 7 forth in the appended claims. Thus, asymmetric or composite membrane ~tructures can be employed in the dryer membrane system of the invention. While dense membrane~ are commonly used for product drying 5 applications, such dense membranes are not preferred ~ecause of the inherent limitatîons thereof noted above, although they can be used in the practice of the invention.
The permeable membranes employed in the 10 practice of the invention, in either a single stage or the two stage embodiments employing a single material or separate material~ for water and carbon dio~ide removal, will co~monly be employed in assemblies of membrane bundles, typically positioned 15 within enclosures to form membrane modules that comprise the principal element of a membrane system. A membrane system may comprise a single module or a number of such modules, arranged for either parallel or series operation. The membrane 20 modules can be constructed using bundles of membranes in convenient hollow fi~er form, or in spiral wound, pleated flat sheet, or other desired membrane configurations. Membrane modules are constructed to have a feed air side, and an 25 opposite, permeate gas e~it side. For hollow fiber membranes, the feed side can be either the bore side for inside-out operation, or the outside of the hollow fibers for outside-in operation. Means are provided for introducing feed air to the system and 30 for withdrawing ~oth permeate and non-permeate streams.
As indi~ated above, the purge ~as employed in the invention should be a dry or a rela~ively dry D-16,348 - . ., -;
.
- 25 - 20~ ~27 gas, as from the sources reerred to herein. As used herein, a relatively dry purge gas is one having a moisture partial pressure not e~ceeding the partial pressure of moisture in the dried feed air 5 stream. Preferably, said purge gas moisture partial pressure will be less than half the moisture partial pressure in said stream, as will be the case with respect to the sources of purge gas disclosed above.
Membranes will be seen to provide a highly 10 desirable system and process for drying feed ~ir before its passage to air adsorption-cryogenic air separat~on system for the production of dry, high purity nitrogen. By accomplishing the drying in convenient membrane systems, the use of the more 15 cos~ly chillers for moisture removal can be avoided. By integratiny the processing streams of the memhrane dryer system, utilizing single or two stage units of a single material or of separate materials for enhanced water and carbon dioxide 20 removal, with the cryogenic air separation system and the prepurifier adsorption system, a purge of the low pressure, permeate side of the membrane dryer æystem with relatively dry ~urge gas is ~onveniently accomplished. By utilizing a bundle 25 arrangement so as to establish a countercurrent flow pattern, preferred embodiments of the drying operation can be carried out with an enhanced recovery of dry feed air, avoiding the co-permeation of ~ignificant amounts of compressed ~ir as occurs 30 in cross-flow permeation operations.
D-16,348
Claims (27)
1. An improved system for the production of dry, high purity nitrogen and/or oxygen from air comprising:
(a) a membrane dryer system capable of selectively permeating water and carbon dioxide present in wet feed air, said system comprising separate membrane materials, one for the selective permeation of water and the other for the selective permeation of carbon dioxide;
(b) a prepurification adsorption system capable of selectively adsorbing residual water, carbon dioxide, and other contaminants Prom dry feed air removed as non-permeate gas from said membrane dryer system;
(c) a cryogenic air separation system for the cryogenic rectification of air, and the production of dry, high purity nitrogen and/or oxygen product gas, together with a dry waste gas;
(d) conduit means for passing relatively dry purge gas to the low pressure permeate side of the membrane dryer system to facilitate the carrying away of water vapor and carbon dioxide from the surface of the membrane and maintaining the driving force for removal of water vapor and carbon dioxide through the membrane from the feed air stream for enhanced moisture separation therefrom, said relatively dry purge gas comprising waste or product gas from said cryogenic air separation system and/or the prepurifier adsorption system or ambient air, whereby the provision of purge gas on the permeate side of the membrane dryer system facilitates the D-16,348 desired moisture and carbon dioxide removal with minimum loss of feed air.
(a) a membrane dryer system capable of selectively permeating water and carbon dioxide present in wet feed air, said system comprising separate membrane materials, one for the selective permeation of water and the other for the selective permeation of carbon dioxide;
(b) a prepurification adsorption system capable of selectively adsorbing residual water, carbon dioxide, and other contaminants Prom dry feed air removed as non-permeate gas from said membrane dryer system;
(c) a cryogenic air separation system for the cryogenic rectification of air, and the production of dry, high purity nitrogen and/or oxygen product gas, together with a dry waste gas;
(d) conduit means for passing relatively dry purge gas to the low pressure permeate side of the membrane dryer system to facilitate the carrying away of water vapor and carbon dioxide from the surface of the membrane and maintaining the driving force for removal of water vapor and carbon dioxide through the membrane from the feed air stream for enhanced moisture separation therefrom, said relatively dry purge gas comprising waste or product gas from said cryogenic air separation system and/or the prepurifier adsorption system or ambient air, whereby the provision of purge gas on the permeate side of the membrane dryer system facilitates the D-16,348 desired moisture and carbon dioxide removal with minimum loss of feed air.
2. The system of Claim 1 in which said membrane dryer system contains membrane bundles adapted for a countercurrent flow pattern with the permeate gas flowing generally parallel to the flow of wet feed air.
3. The system of Claim 1 in which said dry purge gas for the membrane dryer system comprises waste gas from said cryogenic air separation system.
4. The system of Claim 3 and including conduit means for passing a portion of said waste gas from the cryogenic air separation system to said prepurifier adsorption system as purge gas therefor.
5. The system of Claim 1 and including conduit means for passing waste gas from said cryogenic air separation system to said prepurifier adsorption system as purge gas therefor, the waste gas from said prepurifier adsorption system comprising said purge gas for the membrane dryer system.
6. The system of Claim 1 in which said membrane dryer system comprises a two-stage membrane system.
7. The system of Claim 6 in which the first membrane stage is adapted for the removal of water from the feed air and comprising one membrane material, and the second stage is adapted for the D-16,348 removal of carbon dioxide from feed air and comprising a separate membrane material, and including additional conduit means to pass waste or product gas from said cryogenic air separation system or expanded air to the second stage of said membrane system as purge gas.
8. The system of Claim 5 in which said membrane dryer system contains membrane bundles adapted for a countercurrent flow pattern with the permeate gas flowing generally parallel to the flow of wet feed air.
9. The system of Claim 7 and including additional conduit means to pass waste or product gas from the cryogenic air separation system or expanded air separately to the first and second stages of the membrane system as purge gas.
10. The system of Claim 7 and including means to pass purge and permeate gas exhausted from the second stage of the membrane system to the first stage thereof as purge gas.
11. The system of Claim 10 and including conduit means for passing waste gas from said cryogenic air separation system to said prepurifier adsorption system as purge gas therefor.
12. The system of Claim 10 and including means to pass the waste gas from the prepurifier adsorption system to the membrane dryer system as purge gas therefor.
D-16,348
D-16,348
13. The system of Claim 12 in which said waste gas from the prepurifier adsorption system is passed to the first stage of the membrane system as purge gas, together with said gas exhausted from the second stage of said membrane system.
14. An improved process for the production of dry, high purity nitrogen and/or oxygen from air comprising:
(a) passing wet feed air to a membrane dryer system capable of selectively permeating water and carbon dioxide therefrom, said system comprising separate membrane materials, one for the selective permeation of water and the other for the selective permeation of carbon dioxide;
(b) passing the thus-dried feed air to a pre-purification adsorption system capable of selectively adsorbing carbon dioxide, residual water and other contaminants from dry feed air removed as non-permeate gas from said membrane dryer system;
(c) passing the dry, pre-purified feed air from said pre-purification adsorption system to a cryogenic air separation system for the cryogenic rectification of air, and the production of dry, high purity nitrogen product gas, together with a dry, oxygen-containing waste gas;
(d) recovering dry, high purity nitrogen product gas f from said cryogenic air separation system; and (e) passing relatively dry purge gas to the low pressure permeate side of the membrane dryer system to facilitate the carrying away of water vapor and carbon dioxide from the surface of D 16,348 the membrane and maintaining the driving force for removal of water vapor and carbon dioxide through the membrane from the feed air stream for enhanced moisture separation therefrom, said relatively dry purge gas comprising waste or product gas from said cryogenic air separation system and/or the repurifier adsorption system or ambient air, whereby the provision of purge gas on the permeate side of the membrane dryer system facilitates the desired moisture and carbon dioxide removal with minimum loss of feed air.
(a) passing wet feed air to a membrane dryer system capable of selectively permeating water and carbon dioxide therefrom, said system comprising separate membrane materials, one for the selective permeation of water and the other for the selective permeation of carbon dioxide;
(b) passing the thus-dried feed air to a pre-purification adsorption system capable of selectively adsorbing carbon dioxide, residual water and other contaminants from dry feed air removed as non-permeate gas from said membrane dryer system;
(c) passing the dry, pre-purified feed air from said pre-purification adsorption system to a cryogenic air separation system for the cryogenic rectification of air, and the production of dry, high purity nitrogen product gas, together with a dry, oxygen-containing waste gas;
(d) recovering dry, high purity nitrogen product gas f from said cryogenic air separation system; and (e) passing relatively dry purge gas to the low pressure permeate side of the membrane dryer system to facilitate the carrying away of water vapor and carbon dioxide from the surface of D 16,348 the membrane and maintaining the driving force for removal of water vapor and carbon dioxide through the membrane from the feed air stream for enhanced moisture separation therefrom, said relatively dry purge gas comprising waste or product gas from said cryogenic air separation system and/or the repurifier adsorption system or ambient air, whereby the provision of purge gas on the permeate side of the membrane dryer system facilitates the desired moisture and carbon dioxide removal with minimum loss of feed air.
15. The process of Claim 14 in which said membrane dryer system contains membrane bundles adapted for a countercurrent flow pattern with the permeate gas flowing generally parallel to the flow of wet feed air.
16. The process of Claim 14 in which said dry purge gas for the membrane dryer system comprises waste gas from said cryogenic air separation system.
17. The process of Claim 16 and including passing a portion of said waste gas from the cryogenic air separation system to said prepurifier adsorption system as purge gas therefor.
18. The process of Claim 14 and including passing waste gas from said cryogenic air separation system to said prepurifier adsorption system as purge gas therefor, the waste gas from said pre-purifier adsorption system comprising said purge gas for the membrane dryer system.
D-16,348
D-16,348
19. The process of Claim 13 in which said membrane dryer system comprises a two-stage membrane system.
20. The process of Claim 19 in which the first membrane stage is adapted for the removal of water from the feed air and comprising one membrane material, and the second stage is adapted for the removal of carbon dioxide from feed air and comprising a second membrane material, and including passing waste or product gas from said cryogenic air separation system or expanded air to the second stage of said membrane system as purge gas.
21. The process of Claim 20 in which waste gas from the cryogenic air separation system is passed to the second stage of said membrane system as purge gas.
22. The process of Claim 18 in which said membrane dryer system contains membrane bundles adapted for a countercurrent flow pattern with the permeate gas flowing generally parallel to the flow of wet feed air.
23. The process of Claim 19 and including passing waste or product gas from the cryogenic air separation or expanded air separately to the first and second stages of the membrane system.
24. The process of Claim 19 and including passing purge and permeate gas exhausted from the second state of the membrane system to he first stage thereof as purge gas.
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25. The process of Claim 24 and including passing waste gas from the cryogenic air separation system to said prepurifier adsorption system as purge gas therefor.
26. The process of Claim 24 and including passing waste gas from the prepurifier adsorption system to the membrane dryer system as purge gas therefor.
27. The process of Claim 26 in which the waste gas from the prepurifier adsorption system is passed to the first stage of the membrane system as purge gas, together with said exhausted from the second state of said membrane system.
D-16,348
D-16,348
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/539,808 US5116396A (en) | 1989-05-12 | 1990-06-18 | Hybrid prepurifier for cryogenic air separation plants |
| US7-539,808 | 1990-06-18 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2044727A1 CA2044727A1 (en) | 1991-12-19 |
| CA2044727C true CA2044727C (en) | 1995-01-10 |
Family
ID=24152739
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002044727A Expired - Fee Related CA2044727C (en) | 1990-06-18 | 1991-06-17 | Hybrid prepurifier for cryogenic air separation plants |
Country Status (9)
| Country | Link |
|---|---|
| US (1) | US5116396A (en) |
| EP (1) | EP0463535B1 (en) |
| JP (1) | JPH04227021A (en) |
| KR (1) | KR960004608B1 (en) |
| CN (1) | CN1031924C (en) |
| BR (1) | BR9102492A (en) |
| CA (1) | CA2044727C (en) |
| DE (1) | DE69101591T2 (en) |
| ES (1) | ES2051052T3 (en) |
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-
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- 1990-06-18 US US07/539,808 patent/US5116396A/en not_active Expired - Fee Related
-
1991
- 1991-06-17 DE DE69101591T patent/DE69101591T2/en not_active Expired - Fee Related
- 1991-06-17 ES ES91109910T patent/ES2051052T3/en not_active Expired - Lifetime
- 1991-06-17 EP EP91109910A patent/EP0463535B1/en not_active Expired - Lifetime
- 1991-06-17 JP JP3170386A patent/JPH04227021A/en active Pending
- 1991-06-17 CA CA002044727A patent/CA2044727C/en not_active Expired - Fee Related
- 1991-06-17 BR BR919102492A patent/BR9102492A/en not_active IP Right Cessation
- 1991-06-17 CN CN91105023A patent/CN1031924C/en not_active Expired - Fee Related
- 1991-06-18 KR KR1019910010054A patent/KR960004608B1/en active IP Right Grant
Also Published As
| Publication number | Publication date |
|---|---|
| ES2051052T3 (en) | 1994-06-01 |
| CN1058545A (en) | 1992-02-12 |
| JPH04227021A (en) | 1992-08-17 |
| CN1031924C (en) | 1996-06-05 |
| DE69101591D1 (en) | 1994-05-11 |
| US5116396A (en) | 1992-05-26 |
| EP0463535B1 (en) | 1994-04-06 |
| CA2044727A1 (en) | 1991-12-19 |
| BR9102492A (en) | 1992-01-21 |
| DE69101591T2 (en) | 1994-10-27 |
| KR960004608B1 (en) | 1996-04-09 |
| KR920000360A (en) | 1992-01-29 |
| EP0463535A1 (en) | 1992-01-02 |
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