Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates generally to the field of gas
phase separation by the passage of gasses across a membrane and
more specifically to concentrating selected gasses in a mixture of
gasses.
The separation of gasses across a membrane is not new.
Patents to Frey (U.S. Patent No. 2,159,434) and Stahly (U.S.
Patent No. 2,388,095) disclose gas separation devices which apply
with pressure a mix~ure of gasses to one side of a semi-permeable
membrane. The membrane allows one gas to pass in preference to
another. It is also well known that the product gas from one such
device can be processed through subsequent serially-connected such
devices to further concentrate the gas.
Tapering of such separation devices is known. The
textbook Nuclear Chemical Enqineering, by Benedict and Pigford, p.
391, (McGraw-Hill, New York, 1957) describes tapering a plurality
of such gas separation devices which are connected in cascade
fashion to distribute a gas flow rate evenly through a gas
separation system. Tapering and cascade systems are described in
Introduction To Nuclear Enqineerinq, by Stephenson, pp. 362-368
(McGraw-Hill, New York, 1958). The article "Taperization of Step
Cascade for Uranium Enrichment by Gaseous Diffusion Procesq", by
Higashi and Myamoto (?ournal of Nuclear Science and Technology,
January 1976, pp. 30-34) also describes cascading and tapering.
Cascading to achieve tapering is also described in Introduction to
Nuclear Engineering, by Murray (George Allen, Unwin Ltd., London,
pl~. 68-79).
Countercurrent reflux is well known in distillation,
extraction and gas absorption processes. Countercurrent reflux
without backmixing in gas diffusion devices is described by
30 Pfefferle (~.S. Patent No. 3,144,313). Pfefferle's device
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separates a selected gas from a mixture of gasses across a
membrane which has a high diffusion rate for the selected gas.
Pfefferle describes a device which operates at relatively high
temperatures (300 to 400C) and pressures ~2.75 x 106 to 3.45 x
106 Pa) and which uses a metallic membrane.
Gas diffusion devices which employ hollow fibers or tubes
as membranes to separate feed and recovery chambers are known from
Pfefferle (U.S. Patent No. 3,144,313) and Skarstrom et al. (U.S.
Patent No. 3,735,558).
Practical applications of gas diffusion devices are many
and varied. They include oxygen enrichment for inhalation
therapy, sweetening of natural gas, stack gas cleaning, nitrogen
enrichment to reduce fire hazards, and the like. In most such
applications, economy and efficiency of operation are important to
the commercial advancement of such devices. Devices which operate
at relatively low temperatures and pressures and which require
relatively little energy input are desirable.
Gas separation devices which avoid numerous cascading
steps are desirable.
It is a principal object of this invention to concentrate
a selected gas or gasses in a mixture of gasses.
It is another object of this invention to concentrate a
gas or gasses at relatively low temperatures and pressures.
It i9 still another object of this invention to
concentrate a gas or gasses without cascading.
It is yet a further object of this invention to maintain
a substantially homogeneous flow rate throughout a countercurrent
reflux gas separation system.
It is also an object of this invention to remove moisture
from a gas.
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These and other objects are accomplished by a system
operable at room temperature for altering the relative
concentrations of the components of a mixture of fluids moving in
the system. The system includes at least one cell having chambers
separated by a semi-permeable membrane. The chambers communicate
in such a way as to produce countercurrent reflux flow of the
mixture and to result in one chamber having a higher pressure than
the other. The chambers have a geometry which avoids backmixing,
and the membrane has a different permeability constant for at
least two of the components of the mixture.
The system also includes a means for creating a pressure
differential across the membrane and at least one inlet means for
feeding the mixture into the system. At least one outlet means is
also included in the system for recovering the mixture after the
relative concentrations of its components have been altered. The
means for creating a pressure differential may create any such
suitable differential, although a pressure differential of less
than about 3.45 x 105 Pa is preferred.
The means for creating a pressure differential between
the chambers may be a pressure reducer positioned in the
communication between the chambers. It may also be a relatively
high pres~ure feed mixture or a compressor which communicates
; between the chambers or both.
The outlet means is positioned in the system to recover
an altered mixture having increased amounts of either most
permeable gas or the least permeable gas, or both. The inlet
means can be positioned at any suitable location in the system,
but it is preferably located at a point where the concentration of
gasses in the system matches the concentration of gasses in the
inlet mixture.
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In the relatively low pressure, room temperature system
of the present invention, any suitable semi-permeable membrane
material may be used. Representative useful materials include
cellulose acetate, polytetrafluoroethylene, cellulose triacetate,
cellulose acetate-styrene, cellulose acetate butyrate,
polyethylmethacrylate, cellulose propionate, polypropylene, epoxy,
ethyl cellulose, ethylene vinyl-acetate, methyl cellulose,
nitrocellulose, polyvinylchloride, polyvinyl acetate, nitroso
rubber, polyamide, polybutadiene, polyvinylchloride,
poly(butadiene methylmethacrylate), polytbutadiene styrene),
polycarbonate, polydialkylsiloxane resins, silicone rubbers,
polyethylene-acrylonitrile, polyethylenimine-polyvinylbutyral,
polyethylene, polyestermethane, polyethylene terephthalate and the
like.
Silicone materials are often preferred because of their
permeability and relative chemical inertness. An especially
preferred material is a poly(alphamethylstyrene-co-dimethyl-
siloxane) copolymer (described in U.S. Patent No. 4,107,227) which
has many of the permeability characteristics of silicone rubber
but which has shaping and handling characteristics typical of
plastics. In the preferred embodiment, the membrane is a hollow
fiber.
A cell may be formed ~rom a plurality of interconnected
` modular units. Each such unit includes chambers separated by a
membrane. In the preferred embodiment, the modular units are
interconnected in a tapered configuration so as to form a system
in which the rate of movement of the mixture is substantially
constant throughout the system.
FIG. 1 shows schematically and in cross-section a system
according to the present invention.
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FIG. 2 shows schematically the relative membrane area
required to obtain substantially even flow in the system of
FIG. l.
FIG. 3 shows schematically and in cross-section an
arrangement of modular units which together form the system of
claim l having a membrane area approximating that shown in
FIG. 2.
~ IGS. 4 and 5 show alternative embodiments of the system
of the present invention.
Referring more specifically to FIG. l, there is shown a
system according to this invention for altering the relative
concentrations of the components of a mixture of fluids moving in
the system. The system includes cell l which has a high pressure
chamber 2 and a low pressure chamber 3 separated by a membrane 4.
Chambers 2 and 3 communicate through pressure reducer 25 at one
end and compressor 5 at the other.
Each chamber 2 and 3 has a geometry which avoids
backmixing of a gas flowing through the system. Such a geometric
limitation usually requires that the chambers have a small
ao cross-section. In a representative embodiment, eor example, cell
l is a bundle of hollow microfibers packed in a tube, each hollow
microfiber having an I.D. of 0.239 mm. and an O.D. of 0.610 mm..
The inside of the microfibers forms chamber 2 and the area outside
the fibers but inside the tube forms chamber 3.
An inlet means 6 allows entry of a mixture of gasses into
the system. Inlet means 6 may be a pressurized feed. The
pressurized feed creates a pressure differential across the
membrane with the aid of pressure reducer 25. Inlet means 6 can
be located anywhere along either side of cell l, but it is most
preferahl~ located at about the p int where the concentration of
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gasses in the mixture moving through the system is about the same
as the concentration of gasses entering the system.
In the embodiment of FIG. 1, the pressure differential
across the membrane can be provided by compressor S which
increases the gas pressure as it moves from chamber 3 to chamber
2.
Outlet 7 for altered mixture 11 enriched in the least
permeable gas is located at one end of the system. Outlet 8 is
provided at the other end of the system to collect altered mixture
12 enriched in the most permeable gas.
A plurality of outlets such as outlets 7 and 8, or one
such outlet, could be placed anywhere along either side of the
cell to collect any desired mixture of gasses.
In operation of the embodiment of FIG. 1, a pressurized
feed mixture 9 of gasses enters the system at inlet 6 and moves
around the system in the direction shown by the arrows.
Amounts of the most permeable component 10 pass through
membrane 4 as mixture 9 moves along chamber 2. As mixture 9
approaches the end of chamber 2, its relative concentrations of
least permeable and most permeable components has changed because
of the loss of most permeable component 10.
Altered mixture 11 which has a relatively low
concentration of component 10 may be recovered at outlet 7.
As mixture 9 moves along chamber 3, countercurrent to its
flow in chamber 2, it gathers additional amounts of most permeable
component 10 so that an altered mixture 12 containing a relatively
high ratio of most permeable component 10 may be collected at
outlet 8.
In most operations, the volume of the gas stream
recirculated through cell 1 may exceed considerably that of feed
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stream 9, however, at steady state operation, the volume of
altered mixtures 11 and 12 will total the volume of feed mixture
9.
It can readily be seen that such a flow pattern would
normally result in a greater volume of mixture 9 being present
near the compressor end of cell 1 where most permeable gas 10
tends to accumulate. For example, in a system for concentrating
oxygen in air, condensed air twhich is 21.1% 2) is introduced at
inlet 6 at a rate of about 0.137 cc/sec under a pressure of 172.1
cm Hg (the system operates at a temperature of about 23.1C.). At
inlet 6, mixture 9 already in cell 1 is flowing in the direction
shown by the arrows at a rate of about 0.093 cc/sec.
By the time mixture 9 gets to the end of chamber 2, a
distance of 2.11 m in a 35-member bundle of silicone rubber hollow
microfibers, it has a pressure of only 171.5 cm Hg because of
passage of 2 through membrane 4. The gas stream has an oxygen
concentration of only 15.1% at the end of chamber 2. About 0.0972
cc/sec of the oxygen reduced (nitrogen-enriched) altered mixture
11 is drawn off at outlet 7 in this example.
None of the gas stream passes through the pressure
reducer 25, but at a point in chamber 3 opposite inlet valve 6 the
gas flow i9 0.133 cc/sec and the 2 concentration is about 25.9%.
The pressure in chamber 3 i9 7~. 92 cm Hg.
By the time the gas stream reaches the end of chamber 3
(a distance of about 4.24 m), it has a flow rate of 0.284 cc/sec
` and an altered oxygen concentration of about 36.8~. About 0.0401
cc/sec of the oxygen enriched altered mixture 12 is taken at
outlet 8 and about 0.244 cc/sec is reintroduced into chamber 2 at
a pressure of about 172.6 cm Hg. Because of passage of most
permeable gas 10 (oxygen) through membrane 4, mixture 9 has a
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reduced pressure of about 172.1 cm Hg after it travels the 2.13 m
back to inlet 6.
FIG. 2 shows schematically a sheet membrane 14 which is
~haped to correspond with the changing volume of flow of mixture 9
as it circulates around cell 1 of FIG. 1 in countercurrent fashion
to that there is a homogeneous rate of flow. As mixture 9 enters
chamber 2 at inlet 6, much of most permeable component 10 passes
through membrane 4 and adds to the volume of the portion of
mixture 9 which is already in chamber 3. The volume of mixture 9
is continually reduced as it passes along chamber 2 because of the
continual passage of portions of most permeable component 10
through membrane 4.
Sheet membrane 14 of FIG. 2 is constructed to correspond
with the changes in volume of mixture 9 so as to result in a
substantially uniform flow of mixture 9.
A ~ystem accord ing to the present lnvention can be
constructed to correspond to the ~hap~ of sheet membrane 14, with
an outlet for altered mixture 11 enr~ched with the least permeable
product and outlet 8 for altered mixture 12 enriched with the most
permeable product.
However, the construction of such a system presents
practical construction difficulties which do not make it a
preferred embodiment. Such difficulties are largely overcome by
the representative embodiment shown schematically in FIG. 3.
Cell 15 is constructed of modular units 16. Each unit 16
comprises a high pressure chamber 17 and a low pressure chamber 18
separated by a membrane 19. Units 16 are interconnected in such a
way as to result in a membrane surface area and configuration
approximating that shown in FIG. 2 so that the rate of flow is
substantially constant at all parts of cell 15.
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In cell 15, a mixture of gasses is introduced at inlet
20. It moves through chambers 17 and 18 and through membrane 19
as in FIG. 1. In the embodiment of FIG. 3 the gas moves at a
substantially uniform rate throughout the system because of the
tapering effect, provided by the arrangement of modular units 16.
A gas mixture having an altered concentration of components can be
removed at outlet 21 (for mixtures having a reduced concentration
of the most permeable component) or at outlet 22 (for mixtures
having an increased concentration of the most permeable
component).
Pressure reducer 23 and compressor 24 function in the
same manner as pressure reducer 25 and compressor 5 of FIG. 1.
FIG. 4 shows schematically and in cross-section an
embodiment of the present invention wherein the pressure
differential is accomplished by a pressurized feed of mixture 27
through inlet 26, as mixture 27 moves through high pressure
chamber 28 to pressure reducer 29, most permeable component 30
moves through semi-permeable membrane 31. In low pressure chamber
32 most permeable component 30 joins mixture 27 in countercurrent
reflux flow.
An altered mixture 32 having a high concentration of the
most permeable gas can be collected or discharged at outlet 33.
Similarly, in this embodiment altered mixture 34 is collected at
outlet 35. Altered mixture 34 has been stripped of some of its
concentration of most permeable gas 30, leaving it with an
enriched concentration of the least permeable gas.
Referring more specifically to FIG. 5, there is shown yet
another embodiment of the present invention. The pressure
differential across membrane 36 is accomplished by compressor 37
which draws feed mixture 38 (such as air) into low pressure
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chamber 3~ at inlet 41, increases its pressure and moves it into
high pressure chamber 40 in countercurrent flow. Most permeable
component 42 of mixture 38 moves through membrane 36 in the
direction shown by the arrows and countercurrent flow of mixture
38 results in an altered mixture 43 which is enriched in the most
permeable component. Altered mixture 43 may be collected at
outlet 44. An altered mixture 45, which is usually considered as
residue in this embodiment may be collected at outlet 46.
The present invention has been disclosed in the above
teachings and drawings with sufficient clarity and conciseness
to enable one skilled in the art to make and use the invention, to
know the best mode for carrying out the invention and to
distinguish it from other inventions and from what is old. Many
variations and obvious adaptations of the inventions will readily
come to mind, and these are intended to be contained within the
scope of the invention as claimed below.
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