Note: Descriptions are shown in the official language in which they were submitted.
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167
This invention relates to the method of ~eparating
gases b~ means of mem~ranes seIective for the permeation of
one or more gases of a mixture of gases. Mo~e particularly~
the invention relates to a method for the selective membrane
separation of gases by gaseous diffusion or permeation wherein
~he separating efficiency o the gaseous separating me~nbrane
stages are improved without increasing the work required to
accomplish the separation or altering the characteristics of
the membranes employed.
Gas separation membranes have been developed in recent
years for separating many gases from mixtures thereof with
many other gases and vapors. The separation of gases with
membranes, whether by diffusion or permeation through the
material of t~e mem~rane, depends to a large extent on the
differential pressures maintained on either side of the membrane.
The pressure considerations differ considerably from those
required when liquid materials are to be separated by means O
of semi-permeable membranes.
Generally the flux of one or more gases through a
semi-permeab~le membrane is enhanced as the pressure differen-
tial on the two sides o the membrane is incre~ased. However,
practical limits to such increased pressures are always imposed,
generally by the strength of the membranes employed, whether
in flat film or hollow fiber form. ID addition~ the cost of
compressing gases and gaseous mixtures through repeated stages
of separation such as cascaded separation steps or stages
rapidly becomes economicaily limiting.
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A method to separate such gases or gaseous mixtures
:
to a hîgh degree of purity which does not require large
increases in work necessary for compression would be advanta-
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geous and desira~le.
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There has ~een discovered a method for separating
gases wh;ch increases the purity or concentration of the
product gas or gases that does not require large increases @
E
in compressor work required. This novel method is applicable
:.
to any separation of gaseous mixtures which are separable
10 by permeation or diffusion through membranes, but in which a
single stage will not produce the desired purity of product
gas or gases. Thus the method is applicable to any multi-stage
cascade gas separation system as an additional stage or stages :
of such system.
:
~The novel method of the present invention involves
.
the steps of compressing an inîtial gaseous mixture and con-
tracting said mixture with a first stage membrane permeable to
at least one of the gases in said mixture to provide a first
stage concentrate, compressing said first stage concentrate
20 and contacting same with a second stage membrane permeable -
r:
to said at least one of the gases in the first concentrate -
providing a second concentrate and a depleted second permeant
mixture, passing said second permeant mixture to contact a
recycle stage membrane permeable to said at least one of the
gases in said permeant mixture to produce a recycle concentrate
mixture, blending said recycle concentrate mixture prior to
compression t~ereof, and recovering said second concentrate -
mixture.
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07-0365
In order to provide a clearer understanding of the L
present invention, reference is made to the preferred
embodiments exemplary of the ;nvention shown in the accompany-
ing drawing.
Figure l is a schematic diagram showi~g the prior
art method of separating one or more gases from mixtures there-
of with other gases ut;lizing two stages of membrane separation
and con~entration.
Figure 2 is a schematic drawing of the separation
10 of such gases from mixtures thereof by membrane permeation
using one embodiment of a two stage separation and recycle
method of this invention.
Figure 3 is a schematic diagram showing another
embodiment of a multi-stage method for separating and concen-
trating said gases by membrane permeation which employs a
recycle stage to realize a high degree of purity of the
deslred gas or gases.
Figure 1 illustrates a typical two stage gas separa-
tion system of the prior art in which the gas or mixture of
20 gases preferentially separated by the first stage membrane
separation is further concentrated by second stage membrane
separation. In operation the gaseous feed mixture 1 is
compressed to a desired pressure by compressor 2 and passed
to a first stage membrane apparatus 3. From the first stage
membrane apparatus 3 there is produced a permeate stream 4
enriched in the gas or gases desired to be concentrated and a
permeant 9 or unpermeated residue, stream 5 depleted in the
desired gas or gases which can ~e vented, burned for fuel,
or otherwise disposed cf depending upon its composition.
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The enrich.ed permeate mixture is re.compressed~ or
further compressed if des;red, by compressor 7 and passed to
a second stage membrane apparatus 8. From the second stage ~ .
membra.ne apparatus 8 there is produced a further enriched or
concentrated second permeate mixture 9 and a second stage
permeant mixture 10 depleted in the desired gas or gases.
This second permeant mixture 10 can be recycled to blend
: with the compressed feed mixture at a point 6 after th.e first
: compressor 2 or it can be disposed of as desired. The second
stage concentrate or permeate 9 generally constitutes the
desired product, a concentrated gas or mixture of gases.
. Such a gas separation cascade is limited in the
degree of concentration or purification whlch can be obtained
by th.e permeabilities to the desired gas or gases of the
membranes employed in each of the stages, the selectivities
~or separation factors of these membranes for the desired gases
as compared~to;~the other gases:present in the gaseous feed
; mixtures and the ability of the membranes to withstand the
:: :
: pressure differentials imposed thereon. Consequently desirable
gas separations either do not reach the conce~trations or
purities desired or require additional stages of separation
which entail added steps of compression and membrane separation ~`
with resultlng~lncreases in equipment required and severely
increased costs. In many such instances the costs are not
economically feasible in comparison to other available means :
: of separating, concentrating or purifying the desired gas or
gases and consequently membrane separation means are not
employed.
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Referring now to Figure.2, which shows one. embodi- :
ment of the present invention, the improved operation and
result are illustra-ted.
In the operation of this embodiment the gaseo.us ;~
feed mixture 11 is compressed by compressor 12, separated in ~;
first stage membrane apparatus 13 into a first stage permea~t
stream 15 and enriched permeate 14 which is recompressed or
further compressed ~y compress.or 17 and passed to the second
stage membrane apparatus 18,as previously described. The
second stage. permeant 20, h.owever, is passed to a third or
recycle stage of membrane separation apparatus 21 to achieve
improved efficiency. The recycle stage 21 requires no addi- :
tional compression or work input since the second stage
permeant 20 is at essentially the same pressure as the feed
. mixture to the second stage membrane apparatus 18. The
recycle stage membrane apparatus 21 produces an enriched
permeate 22 suitable for blending w~ith -th~ permeate 14 from
the first separation stage -to afford additional enriched feed
mixture to the second separation stage. Such blending is
carried out by introducing the recycle stage permeate 22 into
the condui-t for first stage permeate 14 at a point 24 between
said first stage membrane apparatus 13 and the compressor 17
for the second separation stage. There is simultaneously
produced a recycle stage permeant 23 suitable for blending with
the compressed gaseous feed mixture 11 at a point 25 between
the first stage compressor 12 and the inlet to the first stage
separation membrane apparatus 13. Sizing the recycle stage
membrane area to produce a recycle stage permeant 23 of
approximateIy the same composition as the feed mixture 11 serves
to minimize the energy or w-ork required for compression by
compressor 17 of the permeate from the first stage membrane
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6707-o36s
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apparatus 13. Furth.ermore,' since the recycle stage' ,
permeant 23 ;s already at approximately the'pressure. of the
feed mixture to the'first stage separation apparatus 13,
total compression requirements for the process are kept to .'
a minimum. ''
T~e advantages of such a staged and recycle gas ~:
- ~separation are several. An increàse in the separation effi- i
ciency is realized by producing, a product gas mixture 19 of a .,
higher concentration of the desired gas or gases, i.e. a
10 higher degree of purity~ `
Such. increase is reali ~d without any additional :.
input of energy or work in the form of compression. It is
achieved without altering the gas separation characteristics
of the membranes used or requiring the use of a membrane
differing in such characteristics from those used previously
in multistage gas separa-tion cascades. It is applicable for
. the improved separation of all gases or mixtures of gases
separable by any given membranes and with the same membranes,
requiring no alteration of such membranes or theîr character
istics.
It has been found, for instance, that a two stage ~,
and recycle gas separation process for oxygen from air feed as ~:
shown in Fig. 2 operated at the same pressure differential in '~
both separation stages and employing fiber membranes of the
same separation factor and permeability for oxygen in relation
to nitrogen as those in the process of Fig. 1, can raise the
concentration of oxygen in the second stage permeated mixture
product, at the same weight of gas per hour, from 81.2% by
volume to 90.3% by volume with no increase in the horsepower
required for two stages of compression compared to the two-
stage cascade process ill'ustrated in Fig~ 1. Furthermcre, this
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improvement in efficiency is realized in this instance with
the use of a lower total separation membrane area o~ the
same membranes as in the two-stage process with which it is
compared, i.e. the total surface area o~ three items of
- separation mem~rane apparatus in the process of Fig. 2 is
57.7M ft,2 (5360 sq. meters) compared to a ~otal membrane Lur- ~
face area of two items of separation membrane apparatus of F
76.6M ft,2 (7116 sq. meters) employed in the two-stage process.
Improved efficiencies are also realized when
10 employing the novel features of the present invention in other
multi-stage gas separation processes such as those employing
3, 4 or more stages of membrane separation in series. When
employing the improved process of the present inven-tion with
such multiple stage cascade systems~ the novel recycle stage
membrane separation can ~e associated with any two successive
stages of separation to a~ford improved efficiency. However,
to minimi2e equipment addition, it is generally preferred to
employ the novel recycle stage for the permeant from the final
separation stage in series to provide a recycled permeate to
20 be blended with the enriched permeate from the penultimate
stage, i.e. the stage prior to the final stage of compression,
and a recycle stage permeant to be blended with the enriched
feed stream to the penultimate stage of membrane separation.
Such an imprDved multi-stage gas separation process is illus-
trated in Fig. 3 and further described hereinafter.
-In the operation of the embodiment as shown in Fig. 3
the gaseous feed mixture 31 i5 compressed by compressor 32
and separated in the first stage membrane apparatus 33 to
produce an enriched permeate 34 and a depleted first stage
permeant 35. As shown in Fig. 3 the first stage permeant 35
- depleted in the gas or gases to be concentrated, can be
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directed, if desired, prior to disposal to a pressure reducer
or expander 36 for power recovery from such compressed
permeant and applîcation of such recovered energy by conven-
tional means to perform useful work in the present or any
desired process. Likewise, such power recovery, if desired,
~an be carried out on the compressed permeant 15 from stage 1
of the two-stage process as illustrated in Fig. 2 by the
pressure reducer or expander 16. The enriched Rermeate 34
from stage`l is recompressed, or further compressed, by
compressor 37 and passed to the second stage membrane apparatus
38 which produces a stage 2 permeant 40 suitable for recycle ;.
to the compressed feed mixture to stage } membrane apparatus
33 at a point 50 prior to the inlet of said stage 1 apparatus
and also produces a stage 2 permeate 39 further enriched in
the desired gas or gases. The stage 2 permeate 39 is re~
compressed or:further compressed by compressor 41 and passed
to stage 3 mem~rane separation apparatus 42. From the third
stage membrane separation apparatus there are recovered a still
further enriched permeate mixture 43 and a third stage
permeant 44, still at approximately the inlet pressure of
stage 3, which is passed to the stage 4 or recycle stage
membrane separation apparatus 45. The recycle stage membrane
separation apparatus is sized so as to produce a recycle stage
permeant 47 of approximately the same gaseous composition as
the compressed feed to stage 2 membrane separation apparatus
38 and suitable for blending with such feed at a point 49
just prior to the inlet of the stage 2 membrane separation
apparatus 38. There is simultaneously produced a recycle
stage permeate 45 enriched in the desired gas or gases and
approximating the composition of the permeate 39 from the stage
- 2 membrane separation apparatus 38 which is blended with said
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permeate 3~ at a point 48 just prior to the compressor 41
associated ~ith the third stage of membrane separation~
Thls multi-stage gas separation process illustrated
in Fig. 3 likewise `results in improved efficiencies in gas
separation and generally th~ same advantages set forth here-
ina~ove with respect to the two stage gas separation process.
For example it has been found that the three stage and recycle
gas separation process as shown in Fig. 3 when applied to
the concentration of oxygen from an air feed can raise the
concentration of oxygen in the third stage permeated mixture
product, at the same weight of gas product per hour~ from
94.5% by volume oxygen to 9:6.6% by volume with no increase in
the horsepower requîred for the three stages of compression
when compared to a three stage membrane separation process
employing three stages in series of membrane separation
apparatus with the same hollow fiber membranes and pressure .
:dlfferent~a1s but with no recycle stage. In the instance of
the specific oxygen separation membrane studied the three
stage plus recycle stage cascade required an increased surface
area for the four items of separation membrane apparatus
in the process of Fig. 3 i~ comparison to that of the three
ltems of separatlon membrane apparatus utilized in a three
stage series cascade due to sizing of the membrane surface
areas required to produce a recycle stage permeant 47
approximating the composition of the first stage permeate 34
and the second stage permeant 40 approximating the composition
of the feed mixture 31.
The method for separatlng gaseous mixtures of the :~
present invention is applicable to the separation, concentra-
tion or purifica.ion of any gas or gases from any other gases
or vapors which can be separated by semi-permeable membranes
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a7 -0365
by virture of selective permeation or diffusion therethrough
of one or more gas or gases from gaseous mixtures. Such
separat;ons can include oxygen or nitrogen from air, water
vapor from any mixture of gases~ hydrogen from any of carbon
monoxide, carbon dioxide, helium~ nitrogen~ argon, ammonia,
alkanes suc~ as methane, ethane and the li]ce, ~nd alkenes
such as ethylene, propylene and the li]ce, ammonia from hydrogen,
nitrogen, argon, water vapor, alkanes and the like, carbon
monoxide rom carbon dioxide, hydrogen, nitrogen, argon,
alkanes and the like, carbon dioxide from hydrogen, nitrogen,
carbon monoxide, alkanes and the like, hydrogen sulfide from
hydrogen, nitrogen, carbon dioxide, alkanes and the like,
alkanes such as methane from hydrogen, nitrogen, ammonia,
carbon monoxide, carbon dioxide, methanol, water vapor and -the
like, alkanols such as methanol or ethanol from hydrogen,
carbon monoxide, carbon dioxide, nitrogen, and alkanes such
as methane or ethane and the like and any other gases or
vapors separable by contacting a semi-permeable membrane such
as isotopes of vaporizable metals or salts thereof such as
uranium or uranium hexafluoride and the like.
The seml-permeable membranes which can be employed
in the novel process of the present invention can be of any
type useful for separating gases. Such membranes can be in the
form of films, asymmetric or symmetric, supported or self-
supporting, rolled into tubular form or employed flat. The
gas separation membranes are preferably in the form of hollow
fibers in order to realize the greatest surface areas in the
most economically sized apparatus. Such hollow fiber membranes
can likewise be either symmetric or asymmetric, isotropic or
anisotropic, and can be homogenous insofar as their composition.
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07-0365
They can be hollow fibers of one material coated on either
or both the exterior or interior thereof with one or more
other materials, i.e. composite or multicompo~ent hollow
fiber membranes. In the case of many gas separations such
composite or multicomponent membranes have been found to
exhibît ver~ superior selectivities for the desired gases and
high permeabilities for such gases and hence are often
preferred to realî~e efficient separations processes. In fact,
any hollow fiber gas separation membranes are preferably
l~ emplo~ed in -the novel process of the present invention so
long as such hollow fiber membranes are effective to separate
one or more gases from a mixture of gases such as any of those
described above.
The gas separation membrane apparatus which can be
employed in the present invention is likewise not limiting.
Any apparatus suitable for holding and providing contact on one
surface of a membrane in film form together with provision
for inlet of a feed gas mixture and outlet of an unpermeated
permeant mixture and a selectively permea-ted mixture can be
used. In the instance of the preferred hollow fiber membranes
the apparatus can be of any form which provides for an assembly r
of hollow fiber membranes manifolded to provide separate
contact to the exterior of the fibers for a gaseous feed stream
and removal of an unpermeated stream as well as removal of a
permeated stream from the interior or bores thereof, or
alternatively for the feed stream to the interior and removal
of the unpermeated mixture therefrom as well as removal of
the permeated mixture from the exterior of the fibers. The
apparatus need provide an outer housing, inlet means manifolded
to either the exterior or interior of said fibers, and exit
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means manifolded from hoth the:'exterior and interior of said
fibers. Such.'apparatus can ~e of the' straight thr'ou.gh type ,,
wherein the'fi~ers are manifolded at both ends of the housing ' ,,.,
of the apparatus,with U-shaped fi~er ~illing where the
manifolding ls at one'end only o~ the housing or ~hèIl of E
the apparatus, or of any other configuration providing for ,
- .effective'separation of one or more gase.s or vapors from a ~;
mixture of gases or vapors. . .~
The'other items of equipment or apparatus illustrated ::
10 diagramatically in the drawing such as compressors~ expanders ',, ,'
and conduits between the various stages are of conventional
design and any convenient type of such apparatus can be r
employed and forms no part of the present invention. i
While the invention has been described with reference 1,
to part;cular em~odiments thereof:, it will be appreciated that
modifications and variations are possible without departing ,.:~
from the scope of the invention.
For example, a multistage cascade is not limited , .
to a single recycle stage as shown in Figure 3. In an n ' '
stage cascade, 'n-1 recycle stages could be employed if desired.
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