Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to multicomponent membranes
for separating at least one gas from gaseous mixtures,to
processes for selectively separating at least one gas from
gaseous mixtures by permeation utilizing these multicomponent
membranes, and to appara~ utilizing these multicomponent
membranes.
Separating, including upgrading of the concentration
of,at least one selected gas from a gaseous mixture is an
especially important procedure in v;ew of the demands on the
supplies of chemical feedstocks. Frequently these demands
are met hy separating one or more desired gases from gaseous
mixtures and utilizing the gaseous products for processing.
Proposals have been made to employ separation membranes for
selectively separating one or more gases from gaseous mixtures.
To achieve selective separa.ion the membrane exhibits less
resistance to the tranaport of one or more gases than that
of at leas~t one other gas of the mixture. Thus, selective
separation can provide preferential depletion or concentration
of one or more desired gases in the mixture with respect to
at least one other gas and therefore provide a product having a
different proportion of the one or more desired gases to the
at least one other gas than that proportion in the mixture.
However, in order for selective separation of the one or
more desired gases by the use of separation membranes to be
commercially attractive, the membranes must not only be capa-
bIe of withstanding the conditions to which they may be
subjected during the separation operation, but also must
provide an adequately selective separation of the one or more
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desired gases at a sufficiently high flux, i.e., permeation
rate of the permeate per'unit surface area, so that the use of the
separation procedure is on an economically attractive basis.
Thus, separation membranes which exhibit adequately high
selective, separation, but undesirably low fluxes, may require
such large separating membrane surface area that the use of
these membranes is not economically feasible. Similarly
separation membranes which exhibit a high flux, but low
selective separation, are also commercially unattractive.
Accordingly, work has continued to develop separation
memhranes which can provide both an adequately seIective
separation of the one or more desired gases and a sufficiently
high flux such that the use of these separation membranes on a
commercial basis is economically feasible.
In general, the passage of a gas through a
membrane may proceed through pores, ;.e~, continuous channels
for fluid ~low in communication with both feed and exit
surfaces of the membrane (whi'ch pores may or may not be
suitable for separation by Knudsen flow or diffusion);
in another mechanism, in accordance with current views of
membrane theory the passage of a gas through the membrane
may be by interaction of the gas with the material of the
membrane. In this latter postulated mechanism, the permea-
bility of a gas through a membrane is believed to involve the
solubility of the gas in the membrane material and the
diffusion of the gas through the membrane. The permeability
constant for a single gas is presently viewed as being the
product of the soluhility and diffusivity of that gas in
the membrane. A given membrane material has a particular
permeability constant for passage of a given gas by the
interaction of the gas with the material of the membrane.
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The rate of permeation of the gas, l.e., flux, through the
membrane is related to ~he permeability constant, but i5 al80
influenced by variables such as the membrane thickness, the
physical nature of the membrane, the partial pressure
differential of the permeate gas across the membrane,~ the
temperature and the like.
DEVELOPMRNT OF MEMBRANES FOR LIQUID SEPARATION
HeretoforP, various modifications in membranes used
for liquid separations have been proposed in attempts to
solve particular problems associated with the separation
operation. The following discussion is illustrative of
specific modifications which have been made to membranes used
for liquid separations to solve particular problems and
provides a basis upon which the invention can be fully
appreciated. For example, cellulosic membranes were first
developed and utilized for the desalination of water, and
these membranes could generally be described as "dense",
or "compact", membranes. "Dense", or "compact", membranes are
membranes which are essentially free of pores, i.e., fluid flow
channels communicating between the surfaces of the membrane,
and are essentially free of voids, i.e., regions wlthin the
thickness of the membrane which do not contain the material
of the membrane. In the case of compact membranes, either
s~rface is suitable for the feed contact surface, because
properties of the compact mem~rane are the same from either
surface direction, i.e., the membrane is symmetric. Since
the membrane is essentially the same throughout the structure,
it falls within the definition of isotropic membranes.
Although ~ome of these compact membranes are fairly selective,
one of their main disadvantages is low permeate flux due to
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the relatively large thickness associated with the membranes.
It has therefore been uneconomical to build equipment
installations necessary for the desalination of appreciable
quantities of water using compact membranes. Attempts to
increase the flux of membranes for liquid separations have
included, for example, adding fillers to the membrane to
alter porosity and making the membranes as thin as possible
to increase rates of permeate flow. Although improved per-
meation rates have been achieved to a limited degree, generally
these improved rates have been obtained at the expense of the
selectivity of the particular membranes~
In another attempt to improve membrane performances
Loeb and his co-workers disclose in, for example~ U.S. Patent
No. 31133,132, a method for preparIng a modified cellulose
acetate membrane for desalination of water b.y first casting
a solution of cellulose acetate as a thin layer, and then
forming a dense membrane skin on the thin layer through
various techniques such as solvent evaporation followed by
quenching in cold water. The formation of these dense-skinned
membranes generally involved a final annealing treatment in
hot water. The membranes prepared by the Loeb method are
composed of two distinct regions made of the same cellulose
acetate material, a thin dense semi-permeable s.kin and a
less dens.e, void-containing, non-selective support region~
Since the membrane.s are not of ess.entially the same density
throughout their structure, they fall within the definition of
anisotropic membranes. Because of these distinct regions
and the difference in membrane properties whi.ch can be
observed depending on which.surface of the membrane faces a
brine feed solut;on, Loeb-type membranes can be described as
being asymmetric.
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7~
In, for instance, practical desalination tests,
asymmetric dense-skinned membranes have been shown to have
superior permeate flux when compared to the older style
compact membranes. The improvement in the permeation rate
of the Loeb-type membranes has been attributed to the decrease
in the thickness of the dense selective region~ The less
dense region in such a memhrane provides sufficient structur-
al support to prevent rupture of the membrane under operating
pressurea but offers. little resistance to permeate flow.
Hence, the separation is essentially accomplished by the
dense skin, and the primary functîon of the less dense
support region is to physically support the dense skin~
However, in such Loeb-type mem~ranes this less dense support
region îs frequently compacted by presaures such as those
desirable for desalination of water and under such conditions
the less dense support region loses some of its void volume.
Consequently, the free flow of permeate away from the effluent
side of the dense skîn is hindered,resulting in a reduced
permeation rate. Moreover, the cellulose acetate membranes
disclosed by Loeb et al, are also subject to fouling and
various chemical degradat;ons. Therefore, attention has been
directed to developïng Loeb-type membranes of materials other
than cellulose acetate which may provide stronger structural
properties and increased chemical resistance~ The "Loebing"
of polymer materials to obtain a single component membrane
exhîbiting good selectivity and a good permeation rate has
been found to be extremely difficult. Most attempts result
in producing mem~ranes which are either porous, i.e., have
fluid flow channel~ through the dense skin, and will not
Eeparate, or whîch have toq thick à dense skin to give useful
permeatïon rates. Thus, these aaymmetric membranes often
fail to meet with acceptance in liquid separation operations
such as reverse osmosis. As hereinafter further described, it
is even more difficult to provide Loeb-type membranes which
exhi~it good selectivity and permeation rates for gas separation
operations.
Further developments for providing advantageous separa-
t;on mem~ranes ~uitable for desalination of water and other
liquid-liquid separations such as separations of organic materials
from liquids have led to composi-te membranes comprising a porous
support whi.ch, due to the pre$ence of flow channels, can readily
pass liquid, yet is sufficiently strong to withstand operating
conditions and a thin semi-permeable membrane supported thereon.
Composite memhranes which have been proposed include the so-called
"dynamically-formed" membranes which are formed by continuously
depositing a polymeric film material from a feed solution on to
a porous support. This continuous deposition is required because
the polymeric film material is subject to being carried into the : :
pores and through the porous s-u~strate and hence needs to be -~
replenished. Moreover, the polymeric film material is frequently .
sufficiently soluble in the liquid m;xture being subjected to
aeparation that i.t is also usually subject to lateral erosion,
î.e., it washes off the support.
It has also heen proposed to prepare composite
desalinati.on membranes b.y provi.ding an essentially solid diffusion,
or separati.an, mem~rane on a porous support See, for instance
Sachs, et. al~ U.S. Patent No~ 3,676~2~3, who disclose a poly-
acrylic acid separation membrane on a porous support such as
ce.llulose acetate, polysulfone, etc~ The thickness of the
separating me~rane. i.s relatively large, e A g. up to 6~
microns, such that the separation membrane is sufficiently
strong that it does not tend to flow into or rupture at
the pores of the porous support~ Other proposals have
included the use of an anisotropic support having a denser
region at the surface, i.e., skin,as the immediate support surface
for the separation memhrane. See, for instance, Cabasso,
et. al., Research and Development of_NS-l and Related
Polysulfone Hollow Fibers for Reverse Osmosis Desalination of
Seawater, Gulf South Research Institute, July, 1975~ dis-
tributed by National Technical Information Service, U.S.Department of Commerce, publication PB 248,666. Cabasso,
et. al., disclose composite membranes for desalination of
water which consist of anisotropic polysulfone hollow fibers
which are coated with7 e.g., polyethyleneimine which is
cross-linked in s tu or furfuryl alcohol which is polymerized
in situ to provide a superimposed separation membrane. Another
approach to providing reverse osmosis membranes has been
disclosed by Shorr in U.S. Patent 3,556,305. Shorr discloses
tripartite separation membranes for reverse osmosis comprising
an anisotropic porous substrate, an ultrathin adhesive layer
over the porous substrate, and a thin semipermeable membrane
bound to the substrate by the adhesive layer. Often, these
ultrathin semipermeable membranes in composite form with
porous support materials are prepared by separately fabricating
the ultrathin membrane and a porous support followed by
placing the two in surface-to-surface contact.
Other types of memb.ranes which have been employed
for treating liquids are the so-called "ultrafiltration"
membranes in which pores of a desired diameter are provided.
Sufficiently small molecules can pass through the pores
whereas larger, more bulky molecules are retained on the feed
surface of the membrane. An example of types of ultrafiltration
membranes is provided by Massucco in U.S.Patent No. 3~556,992.
These memb,ranes have an anisotropic support and a gel
irreversibly compressed into the support to provide membranes
having suitable pore sizes for the separation of caustic
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hydroxides from hemicellulose, and the ultrafiltratlon takes
place through the gel.
DEVELOPMENT OF MEMBRANES FOR GAS SEPARATION
The above discussion of the background to this in-
vention has been directed to membranes for the separation of
a liquid from a liquid mixture such as in desalination of water.
More recently emphasis has been placed on developing separation
membranes which are suitable for separating a gas from a
gaseous mixture. The permeation of gases through separation
membranes has been the subject of various studies; however,
gas ~eparation membranes exhibiting both high flux and useful
selective separations have apparently not been provided, at
least commercially. The following discussion is illustrative
of specific modifications which have been made to membranes
used for gas separations and provides a basis upon which this
invention can be fully appreciated.
Attempts have been made to draw on knowledge developed
concerning liquid-liquid separation membranes. There are, however,
many different considerations in developing a suitable separation
membrane for gaseous systems as compared to developing a suitable
membrane for liquid systems. For instance, the presence of
small pores in the membrane may not unduly adversely affect
the performance of the membrane for liquid separations such
as desalination due to absorption on and swelling of the
membrane and the high viscosity and the high cohesive properties
of the liquids. Since gases have extremely low absorption,
viscosity and cohesive properties, no barrier is provided
to prevent the gases from readily passing through
the pores in such a membrane resulting in little,
if any, separation of gases. An extremely important differ-
ence between liquids and gases which might affect selective
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s.eparation hy permeat;on through mem~ranes is the generally
lower solubility of gases in memhranes as compared to the
solubility of liquids in such membranes, thus resulting
in lower permeability constants for gases as compared to those of
liquids. Other differences between liquids and gases
which might affect selective separation by permeation through
membranefi include density and internal pressure, the effect
of temperature on the viscosity, surface tension, and the
degree of order.
It has been realized that materials which exhibit
good separation of gases often have lower permeability
constants compared to those of materials exhibiting poor
separation of gases~ In general, efforts have been directed
to providing the material of a gas separation membrane in
as thin a form as possible,in view of the low permeabilities,
in order to provide adequate flux yet provide a membrane as
pore-free as possible,such that gases are passed through the
membrane b.y interaction with the material of the membrane.
One approach to developing separation membranes suitable
for gaseous systems has been to provlde composite membranes
having a superimposed membrane supported on an anisotropic
porous support wherein the superimposed membrane provides
the desired separation, i.e., the superimposed membrane is
semi-permeable. The superimposed membranes are advantageously
sufficiently th;n, i.e., ultrathin, to provide reasonable
fluxes. The essential function of the porous support is to
support and protect the superimposed membrane without
harming the delicate, thin superimposed membrane. Suitable
supports provide low resistance to permeate passage after
the superimposed membrane has performed its function of
selectively separating the permeate from the feed mixture. Thus,
these supports are desirably porous to provide low resistance
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110726~3
to permeate passage and yet su~fici.ently supportive, that
is, having pore sizes su~ficiently small to prevent
the rupture of the super;mposed membrane under separating
conditions. Klass, e.t. al , U.S. Patent No. 3,616,607,
Stancell, et; al., U.S. Patent No. 3,657,113 and Yasuda,
U.,S. Patent No. 3,775,3u.3 exemplify gas separation membranes
having superimposed membranes on a porous support.
Such composite membranes for gas separations have
.not heen without pr~blems~ For instance, Browall in U.S.
Patent 3,980,456 disclosesth.e fahrication of compos;te
membrane films for separati.on of oxygen from air comprising
a support of microporous polycarbonate sheet and a separately
formed, i.e., preformed,superimposed ultrathin separation
membrane. of 80 percent poly(:phenylene oxide~ and 20 percent
organopolysiloxane-polycar~onate copolymer. In the fabrica-
ti.on of the membrane.s the exclusion from the manufacturing
area of extremely small particulate impurities, i.e.,
particles beIow a~o`ut 3000 angs*roms in size, is stated by
Browall to be impractical or`i~possible.` These fine particles
may be deposited under or between preformed ultrathin
membrane layers and, because of their large sîze in comparison
to the ultrathin membranes, puncture the ultrathin membranes.
Such breaches reduce the effect;veness of the membrane.
The Browall patent discloses applying a preformed organo-
polysiloxane-polycarbonate copolymer sealing material over
the ultrathin membrane to cover the breaches caused by the
fine particles~. Br~wall also discloses employing a pre-
formed layer of *he organopolysiloxane-polycarbonate copoly-
mer be.tween the ultrathin memhrane and the porous polycarbonate
support as. an adhesive. Thus the composite membranes of
Bro~all are complex in materials and techniques of construc-
tion.
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In summary, apparently sui.table anisotropic
membranes have not been prov;ded for gas separations which,
in the absence of a superimposed membrane to provide the
selective separation, exhibit sufficient flux and selectiv-
S ity of separation for general commercial operations. It furtherappears that composite membranes for gas separation which
have a superimposed membrane to provide the selective
separation have achieved only slight or modest improvement
in membrane performance, and there appears to be no success-
ful, large scale commercial application of these gas separa-
tion membranes. Moreover, the superimposed membrane, although
possibly ultrathin ;n order to provide the desired selectiv-
ity of separation,may significantly reduce the flux of the
perm~ate gas through the composi.te membrane as compared
to that of the porous support not having the superimpos.ed
membrane thereon. . .
SUMMARY OF THE INVENTION
This: invention pertai.ns to part;cular mu.lticomponent,
or composite, membranes for gas separations comprising a
coating in contact with a porous separation membrane wherein
the separation properties of the multicomponent membranes are
principally determined by the porous separation membrane as
opposed to the material of the coating, processes for gas
separation employing the multicomponent membranes,and apparatus :
for gas separat;ons utilizing the multicomponent membranes.
These multicomponent membranes for the separation of at least
one gas from a gaseous mi.xture can exhibit a desirable
selectivity and st;ll exhib;t a usèful flux. This invention ::
provides multicomponent membranes for gas separation which
can be fabricated from a wide variety of gas separation
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membrane materials and thus enable greater latitude than has
heretofore existed in selecting such a membrane material which
is advantageous for a given gas separation. This invention
provides multicomponent membranes in which desired combinations
of flux and selectivity of separation can be provided by the
configuration and methods of preparation and combination of
the components. Thus, a material having high selectivity of
separation, but a relatively low permeability constant, can
be utilized to provide multicomponent membranes having desirable
permeation rates and desirable selectivity of separation.
Moreover, the membranes of this invention can be relatively
insensitive to the effects of contamination, i.e., fine
particles during their preparation which has previously caused
difficulty in preparing composite membranes of a preformed
; 15 ultrathin separating membrane superimposed on a support.
Advantageously, the use of adhesives in preparing the multi-
component membranes of this invention may not be necessary.
Hence, multicomponent membranes of this invention need not be
complex in techniques of construction. Multicomponent
membranes in accordance with this invention can be prepared
to provide high structural strength, toughness, and abrasion
and chemical resistances, yet exhibit commercially
ad*antageous flux and selective separation. These multi-
component membranes can also possess desirable handling
characteristics such as low susceptibility to static electrical
forces, low adhesion to adjacent multicomponent membranes and
the like.
Definition of Terms
In accordance with this invention, the multicomponent
membranes for gas separation comprisea porous separation
membrane having feed and exit surfaces and a coating material
~ ` ~
in contact with.the porous separation membrane... The porous
separation memhrane has essentially the same composition,
or material, throughout its structure, i.e., th.e porous
separation membrane is su~stantially chemically homogenous.
The material of the porous. separation membrane exhibits
selective pPrme~t;~n f~r ~+ least one gas of a gaseous mixture
over that of at least one remaining ~as of the mixture, and
hence the porous separation membrane is defined as a
"separation" membrane. By describing the separation membrane
as ~Iporous~ it i~ meant that the membrane has continuous channels
for gaQ flow, i.e., pores, which communicate between the feed
surface and exit surface These continuous channels, if
sufficiently large in number and in cross-section, can
permit essentially all of a gaseous mixture to flow through
the porous separation membrane with little, if any, separation
due to inte.raction wîth the material of the porous separation
membrane. This invention advantageously provides multicompon-
ent membranes wherein the separation of at least one gas from
a gaseous mixture by interacti.on with the material of the
porous separation membrane is. enhanced, as compared to that
of the porous separat$on membrane alone~
The multi.component membranes of this invention
comprise porous separation membranes and-coatings which have
particular relationships. Some of these relationships can
con~eniently be stated i.n terms of relative separation factors
with respect to a pair of gases for the porous separation
membranes, coatings and the multicomponent membranes. A
separation factor ~C b) for a membrane for a given pair of
gases a and b is defined as the ratio of the permeability
3o constant (Pa~ ~f the. membArane for gas a to the permeability
constant (Ph) of the membrane for gas h. A separation factor
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is also equal to the ratio of the permeability (Pa/ ~)
of a membrane of thickness ~ for gas a of a gas mixture to the
permeability of the same mem~rane to gas b, (Pb/~ ) wherein the
permeability for a given gas is the volume of gas, standard
temperature and pressure (.STP)~ which passes through a membrane
per square centimeter of surface area, per second, for a partial
pressure drop of 1 centimeter of mercury across the membrane
per unit of thickness, and is expressed as P - cm3/cm2-sec-
cmHg/ ~
In practice, the separation factor with respect to a
given pair of gases for a given membrane can be determined
employing numerous. techniques which provide sufficient informa-
tion for calculation o~ permeability constants or permeabilities
for each of th~ pair of gases. Several of the many techniques
15 available for determ;ning permeability constants, permeabilities,
and separation factors is disclosed by Hwang, et. al.,
Techniques of Chemistry, Volume VII, Membranes in Separations,
John Wiley ~ Sons, 1975 at
Chapter 12, pages 296 to 322.
An intrinsic separati.on ~actor as referred to herein
is: the separation factor for a materi.al which has no channels
for gas flow across. the material~ and is the highest
achievab.le. separation factor for -the material Such a
material may be referred to as being continuous or non-porous.
The intrinsic ~eparation factor of a material can be approxi-
mated by measuring the separation factor of a compact membrane
of the material. However, several difficulties may exist in
the determination of an intrinsic separation factor including
imperfections ;ntroduced in the preparation of the compact
membrane such.as the presence of pores, the presence of fine
particles in the compact membrane, undefined molecular order
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~ 7~ 3
due to variations in membrane preparation, and the like.
Consequently, the determined intrinsic separation factor can
be lower than the intrinsic separation factor. Accordlngly,
a "determined intrinsic separation factor" as employed herein
refers to the separation factor of a dry compact membrane
~f ~h~ ~t~ri~l~
Brief Statements of the Invention
A multicomponent membrane for gas separations in
accordance with this invention exhibits, with respect to at
least one pair of gases, a separation factor which is signifi-
cantly greater than the determined intrinsic separation factor
of the coating material in occluding contact with a porous
separation membrane. By the term "significantly greater" in
describing the relationships of the separation factor of the
multicomponent membrane and the determined intrinsic separation
factor of the coating material, it is meant that the difference
in the separation factors is of import, e.g., is generally
at least about 10 percent greater. By the term "occluding
contact", it is meant that the coating contacts the porous
separation membrane such that in the multicomponent membrane the
proportion of gases passing through the material of the porous
separation membrane to gases passing through the pores is
increased co~pared to that proportion in the porous separation
membrane alone. Thus, the contact is such that in the multi-
component membrane an increased contribution of the material ofthe porous separation membrane to the separation factor exhibited
by the multicomponent membrane for at least one pair of gases
is obtained as compared to that contribution in the porous
separation membrane alone. Accordingly, with respect to said
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~1~37Z~33
at least one pair of gases, the separation factor which is
exhibited by the multicomponent membrane will be greater than
the separation factor exhibited by the porous separation
membrane. Additionally, with respect to at least one pair of
gases, the material of the porous separation membrane exhibits
a determined intrinsic separation factor greater than the
determined intrinsic separation factor of the material of the
coating. Also, with respect to said at least one pair of
gases, the separation factor exhibited by the multicomponent
membrane is often equal to or less than the determined intrinsic
separation factor of the material of the porous separation
membrane. Often, regardless of the intended gas separation
application of the multicomponent membrane, the relationships
of separation factors can be demonstrated for at least one
pair of gases which consists of one of hydrogen, helium,
ammonia, and carbon dioxide and one of carbon monoxide,
nitrogen, argon, sulfur hexafluoride, methane, and ethane.
Also, in some multicomponent membranes of this invention the
relationships of separation factors may be demonstrated for a
pair of gases which consists of carbon dioxide and one of
hydrogen, helium, and ammonia, or ammonia and one of carbon
dioxide, hydrogen, and helium.
Desirably a multicomponent membrane in accordance with
this invention exhibits a separation factor, with respect to
at least one pair of gases, which is at least about 35 percent
greater, and is preferably at least about 50 percent greater,
and sometimes is at least about 100 percent greater, than the
determined intrinsic separation factor of the material of
the coating. Frequently, with respect to said at least one
pair of gases, the separation factor of the multicomponent
membrane is at least about 5, often at least about 10 percent
greater, and sometimes at least about 50 or about 100 percent
greater, than that of the porous separation membrane.
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There are several features to this invention. One
feature pertains to multicomponent membranes for gas separations,
a second feature pertains to processes for gas separations
using the multicomponent membranes, and a third feature
pertains to apparatus for conducting gas separations,which
apparatus contain the multicomponent membranes.
In the feature of this invention pertaining to multi-
component membranes, one aspect involves multicomponent
membranes comprising a coating in occluding contact with a
porous separation membrane of a material,which material exhibits
selective permeation of at least one gas in a gaseous mixture
over that of one or more remaining gases in the gaseous
mixture, said porous separation membrane having a substantial
void volume, wherein the multicomponent membrane exhibits,
with respect to at least one pair of gases, a separation
factor significantly greater than the determined intrinsic
separation factor of the material of the coating. Voids are
regions within the porous separation membrane which are vacant
of the material of the porous separation membrane. Thus,
when voids are present, the density of the porous separation
membrane is less than the density of the bulk material of the
porous separation membrane. By describing the void volume
as "substantial" it is meant that sufficient voids, e.g., at
least about 5 percent by volume voids, are provided within
the thickness of the porous separation membrane to provide a
realizable increase in permeation rate through the membrane
as compared to the permeation rate observable through a
compact membrane of the same material and thickness.
Preferably the void volume is up to about 90, say, about 10
to 80, and sometimes about 20 or 30 to 70, percent based on
the superficial volume, i.e., the volume contained within the
gross dimensions, of the porous separation membrane. One method
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~ XJ~3
for determining the void volume of a porous separation membrane
is by a density comparison wi~h a volume of the bulk material
of the porous separation membrane which volume would correspond
to a membrane of the same gross physical dimensions and
configurations as the porous separation membrane. Hence,
the bore of a hollow fiber porous separation membrane would
not affect the density of the porous separation membrane.
The density of the porous separation membrane can be
essentially the same throughout its thickness, i.e., isotropic,
or the porous separation membrane can be characterized by
having at least one relatively dense region within its
thickness in barrier relationship to gas flow across the
porous separation membrane, i.e., the porous separation
membrane is anisotropic. The coating is preferably in
occluding contact with the relatively dense region of the
anisotropic porous separation membrane. Since the relatively
dense region can be porous, it can more easily be made quite
thin as compared to making a compact membrane of the same
thickness. The use of porous separation membranes having
relatively dense regions which are thin provides enhanced
flux through the ~ulticomponent membrane.
In a further aspect of the feature of the invention
relating to multicomponent membranes, the multicomponent
membranes comprise a coating in occluding contact with a
porous Qeparation membrane of a material,which material
exhibits selective permeation of at least one gas in a gaseous
mixture over that of one or more remaining gases in the gaseous
mixture, wherein the coating is applied using an essentially
liquid substance which is suitable for forming the coating,
and wherein, with respect to at least one pair of gases, the
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1~f2C~3
multicomponent membrane exhibits a separation factor
s~gnificantly greater than t4e determined intrinsic separation
factor of the material of the coating. The substance for
application to the porous separation membrane is essentially
liquid in tha~ it is incapable of maintaining a form in the
ahsen~e of ~t~rnal support. The material of the coating
may be liquid, or may be dissolved in or suspended as a finely
divided solid (e.g., colloidal size) in a liquid menstruum,
to provide the essentially liquid substance for application
to the porous separation membrane. Advantageously,the material
of the coating, or the material of the coating in the liquid
menstruum, wets, i.e., tends to adhere to, the material of
the porous separation membrane. Thus, the contact of the
coating with the porous separation membrane is frequently
facilitated. The use of an essentially liquid substance to
provide the coating on the porous separation membrane enables
simpler techniques to be employed than have been employed in
providing composite membranes of separately formed, solid
materials. Moreover, a wide range of materials for the coating
can be employed, and the application techniques can readily
be adapted to the use of porous separation membranes of various
configurations.
In an additional aspect of the feature of this invention
relating ~o the multicomponent membranes, the multicomponent
membranes comprise a coating in occluding contact with a
porous separation membrane comprising polysulfone wherein,
with respect to at least one pair of gases, the multicomponent
membrane exhibits a separation factor significantly greater
than the determined intrinsic separatlon~factor of the material
of the coating. In another aspect of this feature the
multicomponent membranes comprise a coating in occluding
-20-
contact with a hollow fiber porous separation membrane of
material, which material Pxhibits selective permeation of at
least one gas in a gaseous mixture over that of one or more
remaining gases in the gaseous mixture wherein, with respect
to at least one pair of gases, the multicomponent membrane
exhibits a separation factor significantly greater than the
determined intrinsic separation factor of the material of
the coating. In hollow filaments (i.e., hollow fibers) the
exterior surface can be the feed or exit surface of the
porous separation membrane and the interior surface will be
the exit or feed surface, respectively. Hollow filaments
advantageously facilitate providing gas separation apparatus
having high available surface areas for separation within
the given volumes of the apparatus. Hollow filaments are known
to be able to withstand greater pressure differentials than
unsupported films of essentially the same total thickness
and morphology.
A second feature of this invention pertains to processes
for gas separation employing the multicomponent membranes. In
this feature of the invention at least one gas in a gaseous
mixture is separated from at least one other gas by selective
permeation to provide a permeated product containing at least
one permeating gas. The process comprises: contacting the
gaseous mixture with one surface (feed surface) of a multi-
component membrane, which, with respect to at least one pair ofgases in the gaseous mixture, multicomponent membrane exhibits
selective permeation of one gas of the pair of gases over that
of the remaining gas of the pair of gases; maintaining the
opposite surface (exit surface) of the multicomponent membrane
at a lower chemical potential for the at least one permeating
gas than the chemical potential at the said one surface;
-21-
~ 2~ 3
permeating said at least one permeating gas into and through
the multicomponent membrane; and removing from the vicinity
of said opposite surface a permeated product having a different
proportion of said at least one gas of the gaseous mixture
to said at least one other gas of the gaseous mixture than
the proportion in the gaseous mixture of said at least one
gas to said at least one other gas. The separation processes
of this invention include concentrating the said at least one
gas on the feed side of the multicomponent membrane to provide
a concentrated product and include permeating the said at least
one gas through the multicomponent membrane to provide a
permeated product wherein said different proportion is a
higher proportion.
In one aspect of this feature, the multicomponent
membrane comprises a coating in occluding contact with a porous
separation membrane having a substantial void volume. In
another aspect of this feature of the invention, hydrogen is
selectively separated from a gaseous mixture also comprising
at least one of carbon monoxide, carbon dioxide, helium,
nitrogen, oxygen, argon, hydrogen sulfide, nitrous oxide,
ammonia, and hydrocarbon of 1 to about 5 carbon atoms. In a
further aspect of this feature of the invention, at least one
gas in a gaseous mixture is separated from at least one other
gas involving contacting the gaseous mixture with a multi-
`component membrane comprising a coating in occluding contactwith a porous separation membrane comprising polysulfone.
A further feature of this invention pertains to apparatus
for gas separations which utilize multicomponent membranes in
accordance with this invention. The apparatus comprise an
enclosure having at least one multicomponent membrane in
-22-
7Z~3
accordance with this invention therein, said milticomponent
membrane having a feed surface and an opposing exit surface,
and said enclosure having means to enable a gaseous mixture
to be supplied to and means to enable removal of gases from
the vicinity of the feed surface of the multicomponent membrane,
and means to enable a permeated product to be removed from the
vicinity of the exit surface of the multicomponent membrane.
It has surprisingly been found that a material for the
coating whi^h can have a low determined intrinsic separation
factor can be provided on a porous separation membrane,
which porous separation membrane can have a low separation
factor, to provide a multicomponent membrane having a
separation factor greater than either the coating or
porous separation membrane. This result is quite surprising
in contrast to previous proposals for gas separation
composite membranes having a superimposed membrane supported
on a porous support which have essentially required that
the superimposed membrane exhibit a high separation factor
in order to provide the selective separation for the
membraner The finding that coatings exhibiting low
separation factors can be employed in conjunction with porous
separation membranes to provide multicomponent membranes
having a greater separation factor than each of the coating
and porous separation membrane leads to highly advantageous
multicomponent membranes for separating gases. For instance,
materials having desirable intrinsic separation factors, but
which were difficult to utilize as superimposed membranes,
may be used as the material of the porous separation membrane
in accordance with this invention with the selectivity of
separation of the material of the porous separation membrane
contributing significantly to the separation factor of the
multicomponent membrane.
-23-
.
~Q7~
It can clearly be seen that the porous separation
membrane of the multicomponent membrane9 can be anisotropic
with a thin, but relatively dense, separating region. Thus,
the porous separation membrane can take advantage of the low
resistance to permeation offered by anisotropic membranes yet
provide multicomponent membranes which exhibit desirable
separation factors. Furthermore, the presence offlow channels
which can render unicomponent (non-composite) anisotropic
membranes unacceptable for gas separations, can be acceptable,
and even desirable, in porous separation membranes used in
the multicomponent membranes of this invention. The coating
can preferably provide a low resistance to permeation, and
the material of the coating exhibit a low determined intrinsic
separation factor. In some multicomponent membranes, the
coating may tend to selectively reject the desired permeate
gas, yet the resulting multicomponent membrane using that
coating can exhibit a separation factor greater than that of
the porous separation membrane.
This invention is concerned with the multicomponent
membranes formed through the combination of a preformed porous
separation membrane, i.e,, a porous separation membrane which
is prepared prior to application of the coating, and a coating.
The invention particularly relates to multicomponent gas
separation membranes wherein the selectivity of separation
of the material of the porous separation membrane contrîbutes
significantly to the selectivity of and relative permeation
ratesof the permeate gases through the multicomponent
membrane. The multicomponent membranes according to the
invention can, in general, exhibit higher rates of permeation
than do composite membranes describèd previous to this
invention which utilize superimposed membranes exhibiting
high separation factors. In addition, the multicomponent
-24-
.
Z~3
membranes of this invention provide a separation factor which
is superior to those of the coating and porous separation
membrane. The multicomponent membranes of the invention may
be in some ways analogous, but only superficially, to the
gas separation membranes described previous to this invention
which have superim?G3ed membranes exhibiting a high separation
factor on a porous support. These composite membranes
described previous to this invention do not utilize a support
or substrate which provides a substantial proportion of the
separation.
The multicomponent membranes of the invention allow
great flexibility for making specific separations because
both the coating and the porous separation membrane contribute
toward the overall separation performance. The result is an
increased ability to tailor these membranes for specific
separation requirements, e.g., for the separation of desired
gas or gases from various gas mixtures at commercially desirable
combinations of rate and selectivity of separation. The
multicomponent membranes can be fabricated from a wide variety
of gas separation materials and thus provide a greater latitude
than has heretofore existed in selecting an advantageous
membrane material for a given gas separation. In addition,
these multicomponent membranes are capable of providing good
physical properties such as toughness, abrasion resistance,
strength and durability, and good chemical resistance.
-25-
DET~ILED DESCRIPTION_OF THE INVENTION
The invention pertains to particular multicomponent
membranes for gas separations which comprise a coating in
contact with a porous separation membrane in which the
~.eparation properties of the multicomponent membrane are
principally determined by the porous separation membrane as
opposed to the coating, processes for gas separation employing
the multicomponent membranes and apparatusfor gas separations
utilizing the multicomponent membranes.
The multicomponent membranes are widely applicable
in gas separation operations. Gaseous mixtures suitable for
feeds according to the invention are comprised of gaseous
substances, or substances that are normally liquid or solid
but are vapors at the temperature under which the separation
is conducted. The invention as described in detail hereinafter
pertains chiefly to the separation of, for example, oxygen
from nitrogen; hydrogen from at least one of carbon monoxide,
carbon dioxide, helium, nitrogen, oxygen, argon, hydrogen
sulfide, nitrous oxide, ammonia, and hydrocarbon of 1 to
about 5 carbon atoms, especially methane, ethane, and ethylene;
ammonia from at least one of hydrogen, nitrogen, argon,
and hydrocarbon of 1 to about 5 carbon atoms, e.g., methane;
carbon dioxide from at least one of carbon monoxide and
hydrocarbon of 1 to about 5 carbon atoms, e.g., methane;
helium from hydrocarbon of 1 to about 5 carbon atoms, e.g.,
methane; hydrogen sulfide from hydrocarbon of 1 to about 5
carbon atoms, for instance, methane, ethane, or ethylene;
and carbon monoxide from at least one of hydrogen, helium, .
nitrogen, and hydrocarbon of 1 to about 5 carbon atoms. It
-26-
a7z~3
is emphasized that ~he invention is not restricted to these
particular separation applications or gases nor the specific
multicomponent membranes in the examples.
The multicomponent membranes for gas separation,
r according to the invention, canbe films or hollow filaments,
or fibers, having a porous separation membrane, or substrate,
and a coating in occluding contact with the porous separation
membrane. Some factors which influence the behavior of the
multicomponent membranes are the permeability constants of
the materials of the coating and porous separation
membranes, the total cross-sectional area of the holes
(i.e., pores or flow channels) relative to the total surface
area of the porous separation membrane, the relative
thickness of each of the coating and the porous separation
membrane of the multicomponent membrane, the morphology
of the porous separation membrane, and most importantly
the relative resistance to permeate flow of each of the
coating and the porous separation membrane in a multi-
component membrane. In general, the degree of separation
of the multicomponent membrane is influenced by the
relative resistance to gas flow for each gas in the gas
mixture of the coating and the porous separation membrane,
which can be specifically chosen for their gas flow
resistance properties.
The material usèd for the porous separation membrane
may be a solid natural or synthetic substance having useful
gas separation properties. In the case of polymers, both
addition and condensation polymers which can be cast, extruded
or otherwise fabricated to provide porous separation membranes
are included. The porous separation me~branes can be prepared
in porous form, for example, by casting from a solution
comprised of a solvent for the polymeric material into
a poor or nonsolvent for the material. The spinning and/or
-27-
.
~1~)72~3
casting conditions and/or treatments subsequent to the initial
formation, and the like, can influence the porosity and
resistance to gas flow of the porous separation membrane.
Generally organic or organic polymers mixed with
inorganic filters are used to prepare the porous separation m~rane.
Typical polymers suitable for the porous separation membrane
according to the invention can be substituted or unsubstituted
polymers and may be selected from polysulfones; poly(styrenes),
including styrene-containing copolymers such as acrylonitrile-
styrene copolymers, styrene-butadiene copolymers and styrene-
vinylbenzylhalide copolymers; polycarbonates; cellulosic
polymers, such as cellulose acetate-butyrate, cellulose
propionate, ethyl cellulose, ~ethyl cellulose, nitrocellulose,
etc.; polyamides and polyimides, including aryl polyamides
and aryl polyimides; polyethe$s; poly(arylene oxides) such as
poly(phenylene oxide) and poly(xylylene oxide); poly(ester-
amide-diisocyanate); polyurethanes; polyesters
such as poly(ethylene terephthalate), poly(alkyl
methacrylates), poly(alkyl acrylates), poly(phenylene
terephthalate), etc.; polysulfides; polymer~ from monomers
having alpha-olefinic unsaturation other than mentioned above
such as poly(ethylene), poly(propylene), poly(butene-l),
poly(4-methyl pentene-l), polyvinyls, e.g., poly(vinyl
chloride), poly(vinyl fluoride), poly(vinylidene chloride), ~ .
poly(vinylidene fluoride), poly(yinyl alcohol), poly(vinyl
esters) such as poly(vinyl acetate) and poly(vinyl propionate),
poly(vinyl pyridines), poly(yinyl pyrrolidones), poly(vinyl
ethers), poly(vinyl ketones), poly(vinyl aldehydes) such as
poly(vinyl formal) and poly(vinyl butyral), polyCvinyl amides),
poly(vinyl amines), poly(vinyl urethanes), poly(vinyl ureas),
poly(vinyl phosphates), and poly(vinyl sulfates); polyallyls;
-28-
11~7Z~3
poly(benzobenzimidazole); polyhydrazides; polyoxadiazoles;
polytriazoles; poly(benzimidazole); polycarbodiimides;
polyphosphazines; etc., and interpolymers, including block
interpolymers containing repeating units from the above such
as terpolymers of acrylonitrile-vinyl bromide-sodium salt
of para-sulfophenylmethallylethers; and grafts and blends
containing any of the foregoing. Typical substituents
providing substituted polymers include halogens such as
fluorine, chlorine and bromine; hydroxyl groups; lower alkyl
groups; lower alkoxy groups; monocyclic aryl; lower acyl
groups and the like.
Selection of the porous separation membrane for the
present multicomponent membrane for gas separations may be
made on the basis of the heat resistance, solvent resistance,
and mechanical strength of the porous separation membrane,
as well as other factors dictated by the operating conditions
for selective permeation, as long as the coating and porous
separation membrane have the prerequisite relative separation
factors in accordance with the invention for at least one
pair of gases. The porous separation membrane is preferably
at least partially self-supporting, and in some instances may
be essentially self-supporting. The porous separation
membrane may provide essentially all of the structural support
for the membrane, or the multicomponent membrane may include
a structural support mèmber which can provide little, if any,
resistance to the passage of gases.
One of the preferred porous separation membranes
utilized in forming the multicomponent membranes comprises
polysulfone. Among the polysulfones which may be utilized
are those having a polymeric backbone comprised of the
-29-
~1~72~3
repeating structural unit:
~ ~0 1
_ -R- S -R - _
~ O _
where R and R' can be the same or different and are aliphatic
or aromatic hydrocarbyl-containing moieties, say, of 1 to about
40 carbon atoms, wherein the sulfur in the sulfonyl group is
bonded to aliphatic or aromatic carbon atoms, and the poly-
sulfone has an average molecular weight suitable for film or
fiber formation, often at least about 10,000. When
the polysulfone is not cross-linked, the molecular weight of
the polysulfone is generally less than about 500,000, and is
frequently less than about 100,000. The repeating units may . ~-
be bonded, i.e., R and R' may be bonded, by carbon to carbon
bonds or through various linking groups sueh as
O O O O
-0-, -S-, -C-, -C-N-I -N-~-N-, -0-C-, etc.
H H H
Particularly advantageous polysulfones are those in which
at least one of R and R' comprises an aromatic hydrocarbyl-
containing moiety and the sulfonyl moiety is bonded to at
least one aromatic carbon atom. Common aromatic hydrocarbyl-
containing moieties comprise phenylene and substituted
phenylene moieties; bisphenyl and substituted bisphenyl
moieties, bisphenyl methane and substituted bisphenyl methane
moieties having the nucleus
-30-
1~072~3
R4 R8 R R6
substituted and unsubstituted bisphenyl ethers of formula
R3 R7 Rg R5
~-X ~-
R4 R8 10 6
wherein X is oxygen or sulfur; and the like. In the depicted
bisphenyl methane and bisphenyl ether moieties Rl to Rlo
represent substituents which may be the same or different
and have the structure
Il
C ~ Z
X2
wherein Xl and X2 are the same or different and are hydrogen
or halogen (e.g., fluorine, chlorine, and bromine); p is
O or an integer, e.g., of 1 to about 6; and Z is hydrogen,
halogen (e.g., fluorine, chlorine and bromine) t Y-~ R
(in which q is O or 1, Y is -O-, -S-, -SS-, -0~ , or
-~-, and Rll is hydrogen, substituted or unsubstituted alkyl,
say, of 1 to about 8 carbon atoms, or substituted or
unsubstituted aryl, say, monocyclic or bicyclic of about
6 to 15 carbon atoms), heterocyclic with the heteroatom
being at least one of nitrogen, oxygen and sulfur and being
monocyclic or bicyclic with about 5 to 15 ring atoms, sulfato
-31-
1~07Z~3
and sulfono, especially lower alkyl-containing or monocyclic
or bicyclic aryl-containing sulfato or sulfono, phosphorous-
containing moieties such as phosphino and phosphato and
phosphono, especially lower alkyl-containing or monocyclic
or bicyclic aryl-containing phosphato or phosphono, amine
including primary, secGndary, tertiary and quaternary amines
in which the secondary, tertiary and quaternary amines often
contain lower alkyl or monocyclic or bicyclic aryl moieties,
isothioureyl, thioureyl, guanidyl, trialkylsilyl, trialkyl-
stannyl, trialkylplumbyl, dialkylstibinyl, etc. Frequently, ~-
the substituents on the phenylene groups of the bisphenyl
methane and bisphenyl ether moieties are often provided at
the ortho position, i.e., R7 to Rlo are hydrogen. The
polysulfones having aromatic hydrocarbyl-containing moieties
in general possess good thermal stability, are resistant
to chemical attack, and have an excellent combination of
toughnesR and flexibility. Useful polysulfones are sold
under trade names such as "P-1700", and "P-3500" by Union
Carbide, both commercial products having a linear chain of
the general formula
~}cc~O~ ~0~
3 n
where n, representing the degree of polymerization, is about
50 to 80.
Poly(arylene ether) sulfones are also advantageous. Polyether
sul~ones having the structure
-32-
1~72~3
~~0~11~
o
and available from ICI, Ltd., Great Britain, are also useful.
Still other useful polysulfones could be prepared through
polymer modifications, for example, by cross-linking,
grafting,quaternization, and the like.
In making hollow filament porous separation membranes,
a wide variety of spinning conditions may be employed. One
method for the preparation of polysulfone hollow filaments
is disclosed by Cabasso, et al, in Resear_h and Development
_ NS-l and Related Pol~ulfone Hollow Fibers for Reverse
Osmosis Desalination of Seawater, supra Particularly
. .
advantageous hollow fibers of polysulfones, e.g., of P-3500
polysulfone produ~ed by Union Carbide and polyether sulfones
of ICI, Ltd., can be prepared by spinning the polysulfone in
a solution comprised of solvent for the polysulfone. Typical
solvents are dimethylformamide, dimethylacetamide and N-methyl
pyrrolidone. The weight percent polymer in the solution may
vary widely but is sufficient to provide a hollow fiber under
the spinning conditions. Often, the weight percent of polymer
in the solution i8 about 15 to 50, e.g., about 20 to 35.
If the polysulfone and/or ~olvent contain contaminants, such
as water, particulates, etc., the amount of contaminants should
be sufficiently low to permit spinning. If necessary,
contaminants can be removed from the polysulfone andtor solvent.
The size of the spinning jet will vary with the desired inside
and outside diameters of the product hollow filament. One
class of spinning jets may have orifice diameters of about
-33-
lla7~3
15 to 35 mils (0 38 to 0.88 millimeters~ and pin diameters
of about 8 to 15 mils (0.2 to 0.38 millimeters) with
an injection capillary within the pin. The diameter of injection
capillary may vary within the limits established by the pin.
The spinning solution is frequently maintained under a
substantially inert atmosphere to prevent contamination and/or
coagulation of the polysulfone prior to spinning and to avoid
undue fire risks with volatile and flammable solvents. A
convenient atmosphere is dry nitrogen. The presence of
excessive amounts of gas in the spinning solution may result
in the formation of large voids.
The spinning may be conducted using a w~t jet or dry
jet technique, i.e., the jet may be in or removed from the
coagulating bath. The wet jet technique is often used for
the sake of convenience. The spinning conditions are preferably
not such that the filament is unduly stretched. Frequently,
spinning speeds are within the range of about 5 to 100 meters
per minute although higher spinning speeds can be employed
providing the filament is not unduly stretched and sufficient
residence time is provided in the coagulation bath. Any
essentially non-solvent for the polysulfone can be employed
for the coagulation bath. Conveniently, water is employed
as the primary material in the coagulation bath. A fluid is
commonly injected into the inside of the fiber. The fluid
may comprise, e.g., air, isopropanol, water, or the like.
The residence time for the spun fiber in the coagulation bath
is at least sufficient to ensure solidification of the
filament. The temperature of the coagulation bath may also
vary widely, e.g., from -15 to 90C or more, and is most
often about 1 to 35C,say, about 2 to 8 or 10C. The
coagulated hollow fiber is desirably washed with water to
remove solvent and may be stored in a water bath for periods
of time of at least about two hours. The fibers are generally
-34-
dried prior to applic:atioll of the coating and assembly in an
apparat~ls for gas separations. The drying may be conducted
at about 0 to 90C, conveniently about room temperature,
e.g., about 15 to 35C, and at about 5 to 95, preferably
about 40 to 60, percent relative humidity.
The foregoing description of methods for preparing
hollow ~liament porous sêparation membranes of polysulfone
is provided merely to illustrate techniques which are
available for producing porous separation membranes and is
not in limitation of the invention.
The coating may be in the form of an essentially
non-interrupted membrane, i.e., an essentially non-porous
membrane, in contact with the porous separation membrane,
or the coating may be discontinuous, or interrupted. When
the coating is interrupted, it is sometimes referred to as
an occluding material since it may occlude channels for gas
flow, i.e., pores. Preferably, the coating is not so thick
as to adversely affect the performance of the multicomponent
membrane, e.g., by causing an undue decrease in flux or by
causing such a resistance to gas flow that the separation
factor of the ml~lticomponent membrane i8 essentially that of
the coating. Often the coating may have an average thickness
of up to about 50 microns. When the coating is interrupted,
of course, there may be areas having no coating material.
The coating may often have an average thickness ranging
from about 0.0001 to 50 microns. In some instances, the
average thickness of the coating is less than about 1
micron, and may even be less than about 0.5 microns.
The coating may comprise one layer or at least two separate
layers which may or may not be of the same materials. When the
-35-
ilO72Q3
porous separation membrane is anisotropic, i.e., has a
relatively dense region within its thickness in gas flow
barrier relationship, the coating is desirably applied to be
in occluding contact in the relatively dense region. A
relatively dense region may be at either or both surfaces of
the porous separation membrane or may be at a mid-portion of
the thickness of the porou~ separation membrane. The coating
is conveniently applied to at least one of the feed and
exit surfaces of the porous separation membrane, and when the
multicomponent membrane is a hollow fiber, the coating may be
applied to the outside surEace to also provide protection to
and/or facilitate the handling of the multicomponent membrane.
While any suitable method can be employed, the method
by which the coating is applied can have some bearing on the
overall performance of the multicomponent membranes. The
multicomponent membranes according to the invention can be
prepared for instance, by coating a porous separation membrane
with a substance containing the material of the coating such
that in the multicomponent membrane the coating has a
re8istance to ga9 flow which is low in comparlson to the total
resistance of the multicomponent membrane. The coating may
be applied in any suitable manner, e.g., by a coating operation
such as spraying, brushing, immersion in an essentially liquid
substance comprising the material of the coating or the like.
As stated earlier, the material of the coating is preferably
contained in an essentially liquid substance when applied
and may be in a solution using a solvent for the material of
the coating which is substantially a non-solvent for the
material of the porous separation membrane. Advantageously,
-36-
1~07Z~3
the substance containing the material of the coating is
applied to one surface of the porous separation membrane, and
the other side of the porous separation membrane is subjected
to a lower absolute pressure. If the essentially liquid
substance comprises polymerizable material and the polymerizable
material is polymerized after application to the porous
separation membrane to provide the coating, the other surface
of the porous separation membrane is advantageously subjected
to a lower absolute pressure during or before the polymeriza-
tion. However, the invention itself is not limited by the
particular method by which the material of the coating is
applied.
Particularly advantageous materials for the coating
have relatively high permeability constants for gases such
that the presence of a coating does not unduly reduce the
permeation rate of the multicomponent membrane. The
resistance to gas flow of the coating is preferably relatively
small in comparison to the resistance of the multicomponent
membrane. As stated previously, the selection of materials
for the coating depends on the determined intrinsic separation
factor of the material of the coating relative to the determined
intrinsic separation factor of the material of the porous
separation membrane to provide a multicomponent membrane
exhibiting a desired separation factor. The material of the
coating should be capable of providing occluding contact with
the porous separation membrane. For instance, when applied
it should sufficiently wet and adhere to the porous separation
membrane to enable occluding contact to occur. The wetting
properties of the material of the coating can be easily
determined by contacting the material of the coating, either
alone or in a solvent, with the material of the porous
separation membrane. Moreover, based on estimates of the
~72Q3
average pore diameter of the porous separation membrane,
materials for the coating of appropriate molecular size can
be chosen. If the molecular size of the material of the coating
is too large to be accommodated by the pores of the porous
separation membrane, the material may not be useful to provide
occluding contact. If, on the other hand, the molecular size
of the material for the coating is too small, it may be drawn
through the pores of the porous separation membrane during
coating and/or separation operations. Thus with porous
separation membranes having larger pores, it may be desirable
to employ materials for coating having larger molecular sizes
than with smaller pores. When the pores are in a wide variety
of sizes, it may be desirable to employ a polymerizable
material for the coating material which is polymerized after
' application to the porous separation membrane,or to employ
two or more coating materials of different molecular sizes,
e.g., by applying the materials of the coating in order of
their increasing molecular sizes.
The materials for the coating may be natural or
synthetic substances, and are often polymers, and
advantageously exhibit the appropriate properties to provide
occluding contact with the porous separation membrane.
Synthetic substances include both addition and condensation
polymers. Typical of the useful materials which can comprisè
the coating are polymers which can be substituted or
unsubstituted, and which are solid or liquid under gas
separation conditions, and include synthetic rubbers;
natural rubbers; relatively high molecular weight and/or
high boiling liquids; organic prepolymers; poly(siloxanes)
(silicone polymers); polysilazanes, polyurethanes;
poly(epichlorhydrin); polyamines; polyimines; polyamides;
acrylonitrile-containing copolymers such as
-38-
-`` li~72~3
poly(-chloroacrylonitrile) copolymers; polyesters
(including polylactams), e.g.,
poly(alkyl acrylates) and poly (alkyl methacrylates)
wherein the alkyl groups have, say, 1 to about 8 carbons,
polysebacates, polysu~cinates, and alkyd resins; terpinoid
resins such as linseed oil; cellulosic polymers; polysulfones,
especially aliphatic-containing polysulfones; poly(alkylene
glycols) such as poly(ethylene glycol), poly(propylene glycol),
etc.; poly(alkylene) polysulfates; polypyrrolidones; polymers
from monomers havingoC-olefinic unsaturation such as
poly(olefins), e.g., poly(ethylene), poly(propylene),
poly(butadiene), poly(2,3-dichlorobutadiene), poly(isoprene),
poly(chloroprene), poly(styrene) including poly(styrene)
copolymers, e.g., styrene-butadiene copolymer, polyvinyls
such as poly(vinyl alcohols), poly(vinyl aldehydes) (e.g.,
poly(vinyl formal) and poly(vinyl butyral)), poly(vinyl
ketones) (e.g., poly(methylvinylketone)), poly(vinyl esters) ~,
(e.g., poly(vinyl benzoate)), poly(vinyl halides) (e.g.,
poly(vinyl bromide)), poly(vinylidene halides), poly(vinylidene
carbonate), poly(N-vinylmaleimide), etc., poly(l,5-cyclo-
octadiene), poly(methylisopropenylketone), fluorinated
ethylene copolymer; poly(arylene oxides), e.g., poly(,xylylene
oxide); polycarbonates; polyphosphates, e.g., poly(ethylene-
methylphosphate); and the like, and any interpolymers
including block interpolymèrs containing repeating units from
the above, and grafts and blends containing any of the foregoing.
The polymers may or may not be polymerized after application
to the porous separation membrane.
-39-
1~7Z~3
Particularly useful materials for coatings comprise
poly~siloxanes). Typical poly(siloxanes) can comprise
aliphatic or aromatic moieties and often have repeating units
containing l to about 20 carbon atoms. The molecular weight of
the poly(siloxanes) may vary widely, but is generally at least about
1000. Often, the poly(siloxanes) have a molecular weight
; of about 1,000 to 300,000 when applied to the porous separation
membran~. Common aliphatic and aromatic poly(siloxanes)
include the poly(monosubstituted and disubstituted siloxanes),
e.g., wherein the substituents are lower aliphatic, for
instance, lower alkyl, including cycloalkyl, especially methyl,
ethyl, and propyl, lower alkoxy; aryl including mono or
bicyclic aryl including biphenylene, naphthalene, etc.;
lower mono and bicyclic aryloxy; acyl including lower aliphatic
and lower aromatic acyl; and the like. The aliphatic and
aromatic substituents may be substituted, e.g., with halogens,
e.g., fluorine, chlorine and bromine, hydroxyl groups, lower
alkyl groups, lower alkoxy groups, lower acyl groups and the
like. The poly(siloxane) may be cross-linked in the presence
of a cross-linking agent to provide a silicone rubber, and the
poly(siloxane) may be a copolymer with a cross-linkable
comonomer such as ~-methylstyrene to assist in the cross-linking. --
Typical catalysts to promote cross-linking include the organic
and inorganic peroxides. The cross-linking may be prior to
; 25 appli~ation of the poly(siloxane) to the porous separation
membrane, but preferably the poly(siloxane) is cross-linked
after being applied to the porous separation membrane.
Frequently, the poly(siloxane) has a molecular weight of about
1,000 to 100,000 prior to cross-linking. Particularly
advantageous poly(siloxanes) comprise poly(dimethylsiloxane),
poly(phenylmethylsiloxane), poly(trifluoropropylmethylsiloxane),
- -40-
7Z~3
copolymer ofo~-methylstyrene and dimethylsiloxane, and
post-cured poly(dimethylsiloxane)-containing silicone rubber
having a molecular weight of about 1,000 to 50,000 prior to
cross-linking. Some poly(siloxanes) do not sufficiently wet
a polysulfone porous separation membrane to provide as much
occluding contact as is desired. However, dissolving or
dispersing the poly(siloxane) in a solvent which does not
substantially affect the polysulfone can facilitate obtaining
occluding contact. Suitable solvents include normally liquid
alkanes, e.g., pentane, cyclohexane, etc.; aliphatic alcohols,
e.g., methanol; some halogenated alkanes; and dialkyl ethers;
and the like; and mixtures thereof.
The following materials for porous separation membranes
and coating are representative of useful materials and their
combinations to provide the multicomponent membranes of this
invention and gas separations for which they may be employed.
These materials, combinations, and applications, however, are
only representative of the wide range of materials useful in
the invention and, are not provided in limitation of the
invention, but only serve to illustrate the broad application
of the benefits thereof. Typical materials for porous
separation membranes for separation of oxygen from nitrogen
include cellulose acetate, e.g., cellulose acetate having a
degree of substitution of about 2.5; polysulfone; styrene-
acrylonitrile copolymer, e.g., having about 20 to 70 weightpercent styrene and about 30 to 80 weight percent acrylonitrile,
blends of styrene-acrylonitrile copolymers, and the li~e.
Suitable coating materials include poly(siloxanes)
(polysilicones), e.g., poly(dimethylsiloxane), poly(phenyl-
methylsiloxane), poly(trifluoropropylmethylsiloxane),
11~72G3
pre-vulcanized and post-vulcanized silicone rubbers, etc.;
poly(styrene~, e.g., poly~styrene) having a degree of
polymerization of about 2 to 20; poly(isoprene), e.g., isoprene
prepolymer and poly(cis-1,4-isoprene); aliphatic hydrocarbyl-
containing compounds having about 14 to 30 carbon atoms,
e.g., hexadecane, linseed oil, especially raw linseed oil,
etc.; and the like.
Typical materials for porous separation membranes for
separation of hydrogen from gaseous mixtures containing hydrogen
include cellulose acetate, e.g., cellulose acetate having a
degree of substitution of about 2.5; polysulfone; styrene-
acrylonitrile copolymer, e.g., having about 20 to 70 weight
percent styrene and about 30 to 80 weight percent
acrylonitrile, blends of styrene-acrylonitrile copoly~ers,
etc.; polycarbonates; poly(arylene oxides) such as poly(phenylene
oxide), poly(xylylene oxide), brominated poly(xylylene oxide),
brominated poly(xylylene oxide) post treated with trimethyl-
amine, thiourea, etc.; and the like. Suitable coating
materials include poly(siloxane) (polysilicones); e.g.,
poly(dimethylsiloxane), pre-vulcanized and post-vulcanized
silicone rubber, etc.; poly(isoprene);o~-methylstyrene-
dimethylsiloxane block copolymer; aliphatic hydrocarbyl-
containing compounds having about 14 to 30 carbon atoms;
and the like.
The porous separation membranes used in this invention
are advantageously not unduly porous and thus provide
sufficient area of the porous separation membrane material
for effecting separation on a commercially attractive basis.
The porous separation membranes significantly effect the
separation of the multicomponent membranes of this invention,
-42-
and accordingly, it is desirable to provide a large ratio
of total surface area to L:otal pore cross-sectional area in
the porous separation mem~rane. This result is clearly
contrary to the objective~ of the prior art in preparing
composite membranes wherein the superimposed membrane
substantially achieves the separation, and the supports are
desirably provided as porous as possible consistent with
their primary function, i.e., supporting the superimposed
membrane, and advantageously the support does not interfere
with thepermeategas either in slowing or inhibiting the
gas flow from the superimposed membrane.
Clearly, the amount of gas passing through the material
of the porous separation membrane and its influence on the
performance of the multicomponent membranes of the invention
is affected by the ratio of total surface area to total pore
cross-sectional area and/or the average pore diameter of the
porous separation membrane. Frequently, the porous separation
membranes have ratios of total surface area to total pore
cross-sectional area of at least about lO:l, preferably at
least about 103:1, to 103:1, and some porous separation m~ranes may
have ratios of about 103:1 to 103:1 or 1012:1. The average
pore cross-sectional diameter may vary widely and may often
be in the range of about 5 to 20,000 angstroms, and in some
porous separation membranes, particularly in some polysulfone
porous separation membranes, the average pore cross-sectional
diameter may be about 5 to 1000 or 5000, even about 5 to 200,
angstroms.
The coating is preferably in occluding contact with
the porous separation membrane such that, with respect to
the models which have been developed based on observation of
the performance of the multicomponent membranes of this
-43-
1~372~3
inventlon, increased resistance to the passage of gases
through the pores of the separation membrane is provided,
and the proportion of gases passing through the material
of the porous separation membrane to gases passing through
the pores is enhanced over that proportion uslng the porous
separation membrane not having the coating.
A useful characteristic with respect to gas separation
membranes is the effective separating thickness. The
effective separating thickness as employed herein is the
thickness of a continuous (non-porous) and compact membrane
of the material of the porous separation membrane which would
have the same permeation rate for a given gas as the multi-
component membrane, i.e., the effective separating thickness
i8 the quotient of the permeability constant of the material
of the porous separation membrane for a gas divided by the
permeability of the multicomponent membrane for the gas. By
providing lower effective separating thicknesses, the rate of
permeation for a particular gas is increased. Often the effective
separating thickness of the multicomponent membranes is
substantially less than the total membrane thickness,
especially when the multicomponent membranes are anisotropic.
Frequently, the effective separating thickness of the multi-
component membranes with respect to a gas, which can be
demonstrated by at least one of carbon monoxide, carbon
dioxide, nitrogen, argon, sulfur hexafluoride, methane
and ethane, is less than about 100,000, preferably
less than about 15,000, say, about 100 to 15,000 angstroms.
-44-
72~3
In multicomponent membrane~ comprising, e.g., polysulfone
porous ~eparation membranes, the effective separating thickness
of the multicomponent membrane for at least one of said gases i8
desirably less than about 5000 angstroms. In some multicomponent
membranes the effective separating thickness, especially with
respect to at least one of said gases is less than about 50,
preferably le~s than about 20, percent of the thi~kness of the
multicomponent membrane.
Prior to the lnvention, one method for preparing
membranes for gas separations from membranes containing pores
has been to treat at least one surface of the membrane containing
the pores to densify the surface and thereby decrease the
presence of pores,which pores decrease the selectivity of
separation of the membrane. This densification has been by,
for instance, chemical treatment with solvents or swelling
agents for the material of the membrane or by annealing
which can be conducted with or without the contact of a
liquid with the membrane. Such densification procedures
usually result in a 6ubstantial decrease in flux through
the membrane. Some particularly advantageous multicomponent
membranes of this invention exhibit a greater permeability
than that of a membrane substantially the same as the
porous separation membrane used in the multicomponent
membrane except that at least one surface of the membrane has
been treated to sufficiently densify the membrane or sufficiently
annealed, with or without the presence of a liquid to provide,
with respect to at least one pair of gases, a separation
factor equal to or greater than the sepa~ation factor exhibited
by the multlcomponent membrane. Another method for increasing
the selectivity of separation of a mèmbrane is to modify the
conditions of its manufacture such that it is less porous
than a membrane prod~ced under the unmodified condition~.
-45-
:: -
Generally, the increase in selectivity of separation due to
conditions of manufacture is accompanied by a substantially
lower flux through the membrane. Some particularly advantageous
multicomponent membranes of this invention, for instance, those
in which the porous separation membrane is an anisotropic hollow
fiber, exhibit a greater permeability than an anisotropic hollow
fiber membrane consisting of the material of the porous separation
membrane, which membrane is capable of maintaining the
configuration of the hollow fiber under gas separation
conditions, e.g., absolute pressure differentials of at least
about 10 kilograms per square centimeter, and which anisotropic
hollow fiber membrane exhibits, with respect to at least one
pair of gases, a separation factor equal to or greater than
the separation factor of the multicomponent membrane.
Advantageously, the porous separation membrane is
sufficiently thick that no special apparatus is required for
its handling. Frequently, the porous separation membrane has
a thickness of about 20 to 500, say, about 50 to 200, or
300 microns. When the multicomponent membrane i8 in the
configuration of a hollow fiber, the fiber may often have an
outside diameter of about 200 to 1000, say, about 200 to 800,
microns and wall thicknesses of about 50 to 200 or 300 microns.
In conducting gaseous separations, including concentra-
tions, employing the multicomponent membranes of this invention
the e~it side of the multicomponent membrane is maintained at
a lower chemical potential for at least one permeating gas
than the chemical potential at the feed side. The driving
force for the desired permeation through the multicomponent
membrane is a differential in chemical potential across the
multicomponent membrane such as described by Olaf A.
Hougen and K. M. Watson in Chemical Process Principles,
-46-
1~72C~3
Part II, John Wiley, New York (1947), for instance, as
provided by a differential in partial pressure. Permeating
gas passes into and through the multicomponent membrane and
can be removed from the vicinity of the exit side of the
multicomponent membrane to maintain the desired driving force
for the permeating gas. The functionality of the multicomponent
membrane does not depend upon the direction of gas ~low or
the surface of the membrane which is ~irst contacted by a
gaseous feed mixture.
In addition to providing a process for separating at
least one gas from gaseous mixtures which does not require
costly refrigeration and/or other costly energy inputs, the
present invention provides numerous benefits with a high
degree of flexibility in selective permeation operations. The
multicomponent gas separation membranes, whether in sheet or
hollow fiber form, are useful in the separation of industrial
gases, oxygen enrichment for medical applications, pollution
control devices, and any need where it is desired to separate
at least one gas from gaseous mixtures. Relatively infrequently
does a single component membrane possess both a reasonably
high degree of selectivity of separation and good permeation
rate characteristics, and even then these single component
membranes are suitable for separating only a few specific
gases. The multicomponent gas separation membranes according
to the invention may employ a wide variety of materials for
the porous separation membranes which previously have not
been desirable as single component membranes for gas separation
due to undesirable combinations of permeation rates and
separation factor. Since the selection of the material of
the porous separation membrane can be`based on its selectivity
and permeabilityconstants for given gases rather than its ability to
form thin and essentially pore-free membranes, the multi-
-47-
.~' ,- ~' .
~7ZC~3
component membranes of this învention can ad~ant~geously
be tailored for separations of a wide variety of gases from
gaseous mixtures.
-48-
~72G3
MATHEMATICAL MODEL
The cross-sectional diameter of pores in a porous
separation membrane may be on the order of angstroms, and
accordingly the pores of the porous separation membrane and
the interface between the coating and the porous separation
membrane are not directly observable employing presently available
optical microscopes. Presently available techniques which can
offer larger magnification of a specimen such as scanning electron
microscopy and transmission electron microscopy, involve special
specimen preparations which limit their viability in accurately
depicting features of the specimen, particularly an organic
specimen. For instance, in scanning electron microscopy, an
organic specimen is coated, e.g., with at least 40 or 50 angstrom
thick layer of gold, in order to produce the reflectance which
provides the perceived image. Even the manner in which the coat-
ing is applied can affect the perceived image. Furthermore, themere presence of the coating required for scanning electron
microscopy can obscure, or apparently change, features of the
specimen. Moreover, in both scanning electron microscopy and
transmission electron microscopy the methods used for obtaining
a sufficiently small portion of the specimen may substantially
alter features of the specimen. Consequently, the complete
structure of a multicomponent membrane can not be visually
perceived, even with the bèst microscopic techniques available.
The multicomponent membranes of this invention do perform
uniquely, and mathematical models can be developed which, as
demonstrated by various techniques,generally correlate with
the observed performance of a multicomponent membrane of this
invention. The mathematical models, however, are not in
limitation of the invention, but rather serve to further
illustrate the benefits and advantages provided by the invention.
-49-
~3
For a better understanding of the following mathematical
model of multicomponent membranes o~ this invention, reference
can be made to the depicted model~ provided as Figures 1, 2,
3, 4, 6 and 7 of the drawings. The depicted models are only
intended to facilitate the understanding of the concepts
developed in the mathematical model and do not, and are not
intended to depict actual s~ructures of the multicomponent
membranes of the invention. Furthermore, consistent with the
purpose of assisting in understanding the concepts of the
mathematical model, the depicted models illustrate the presence
of features involved in the mathematical model; however, the
depicted models are greatly exaggerated with respect to the
relative relationships among these features in order to facilitate
observing the features. Figure 5 is provided to assist in
demonstrating the analogy between the roncept of resistance
to permeate flow of the mathematical model and resistance to
electrical flow.
Figures 1, 2 and 4 are depicted models for purposes of
understanding the mathematical model and illustrate an
interface of coating and a porous separation membrane, i.e.,
an enlarged region, indicated in Figure 6 as that region
between lines A-A and B-B, but not necessarily on the same
scale. Figure 3 is an enlarged depicted model of the region
indicated in Figure 7 as that region between lines C-C and
D-D. In the depicted models~ like designations refer to the
same features.
Figure 1 is an enlarged cross-sectional view, for
illustrative purposes, of one depicted model of an essentially
continuous and non-interrupted overlay 1 of the material X
of the coating in contact with material Y of a porous
separation membrane with solid portions 2 having pores 3
-50-
~ ~ 7 Z ~3
filled or partially filled by the material of X;
Figure 2 is an enlarged illustration of another depicted
model, wherein material Y of the porous separation membrane
is in the form of curved surface in~erface areas either being
void or partially filled with material X of the coating in a
uniform contact, i.e., uninterrupted, fashion;
Figure 3 iq an enlarged illustration of a depicted
model having material X within pores 3, bu~ no uninterrupted
overlay 1 is present.
Figure 4 is still another depicted model to assist in
describing concepts in accordance with the mathematical model
of the invention. Figure 4 in cooperation with Figure 5
demonstrates an analogy with the well-known electrical
current resistance circuit illustrated in Figure 5;
Figure 6 is yet another cross-sectional view of a
depicted model in which material X of the coating is provided
as a pore-blocking film cast on a denser surface of the
porous separation membrane, which is characterized by a
reciprocally graduated density and porous structure through
the membrane thickness; and
Figure 7 i8 a cross-sectional view of a depicted
model of an occluded anisotropic separation membrane which
does not of necessity require a continuous or uninterrupted
overlay 1.
-51-
~1~72~3
The following equations illustrate a mathematical model
which has been developed to explain the observed performances
of the multicomponent membranes of this invention. By
appropriate use of this mathematical model, porous separation
membranes and materials for coatings can be chosen which will
provide advantageous multicomponent membranes of this
invention.
As will be demonstrated below, the flux, Q , for gas
T,a
a through a multicomponent membrane can be represented as a
function of the resistance to flow of gas a through each
portion (see, for instance the depicted model of Figure 4)
of the multicomponent membrane by analogy to the mathematically
equivalent electrical circuit of Figure 5.
¦: R2 a R3 a
where ~p is the pressure differential for gas a across
the multicomponent membrane and Rl a' R2 ~ and R3
represent the resistance to flow of gas a of the overlay 1,
the solid portions 2 of the porous separation membrane, and
the pores 3 of the porous separation membrane, respectively.
The flux, QT b~ of a second gas b through the same multi-
component membrane can be expressed in the same manner, but
with the appropriate terms for the pressure differential of
gas b and the resistances to flow of gas b through the
overlay 1, the solid portions 2 of the porous separation
-52-
1~7Z~3
membrane and the pores 3. Each of these resistances for gas b
can be different than each of those for gas a. Thus selective
permeation can be achieved by the multicomponent membrane.
Advantageous multicomponent membranes can be modeled by
varying Rl, R2, and R3 relative to each other for each of
gases a and b to produce desirable calculated fluxes for each
o~ gases a an~ b, ~lld by v~ryiLlg the resistances for gas a
relative to those for gas b to provide a calculated selective
permeation of gas a over gas b.
Other equations which are useful in understanding the
mathematical model are listed below.
For any given separating material, the separation factor
for two gases a and b, ~b~ is defined by Equation 2 for a
membrane of material n of a given thickness Q and surface
area A:
a Pn,a Qa a Pb
o~. = -- =
2) Pn,b Qb a Pa
where Pn a and P b are the respective permeability constants
of material n for gases a and b, and Qa and Qb are the
respective fluxes of gases a and b through the membrane
when ~Pa and apb are the driving forces, i.e., partial
pressure drops, for gases a and b across the membrane.
The flux Qa through a membrane of material n for gas a can
be expressed as
3) Q = aPaPn,a An ~Pa
-53-
~ 3
where An is the surface area of ~he membrane of material n,
Qn isthe thickness of the membrane of material n, and Rn a
is defined for the purposes of the model as the resistance
of a membrane of material n to the flow of gas a.
From Equation 3 it can be seen that the resistance Rn a
is represented mathematically by Equation 4.
n
~ ~ n
~, ~n,a Pn~a An
This resistance is analogous in a mathematical sense to the
electrical resistance of a material to current flow.
For the purposes of illustrating this mathematical
model, reference can be made to the depicted model of, for
instance, Figure 4. The porous separation membrane is
represented as comprising solid portions 2 of material Y
and pores, or holes 3. Material X is present in the
depicted model of Figure 4 as overlay 1 and as the
material which enters into the pores 3 of the porous
separation membrane. Each of these regions, the overlay 1,
the solid portions 2 of the porous separation membrane, and
the pores 3 containing material X, has a resistance to gas
flow such that the total multicomponent membrane can be
compared to the analogous electrical circuit represented in
Figure 5, in wh$ch a resistance, Rl, is in series with two
resistances, R2 and R3, which are in parallel. ::
If material X is provided in the form of a continuous,
compact overlay 1, then its resistance Rl to flow for a
given gas can be represented by Equation 4 and will be a
function of the thickness, Ql~ of the overlay, the surface
-54-
7Z~3
area, Al,of the overlay, and the per~eability constant, Px,
of the material X.
The porous separation membrane of a multicomponent
membrane of this invention is represented by the model as
S two resistances in parallel. In accordance with Equation 4,
the resistance R2 f the solid portions 2 of the porous
separation membrane, comprising material Y, is a function of
the thickness Q2 of these solid portions, the total surface
area A2 of the solid portions 2, and the permeability constant
Py of material Y. The resistance R3 of the pores 3 in the
porous separation membrane is in parallel to R2. The
resistance R3 of the pores is represented, as in Equation 4,
by a thickness Q3 divided by a permeability constant P3 and
a total pore cross-sectional surface area A3. For the
purposes of the mathematical model, the assumption is made
that Q3 is represented by the average depth o~ the penetration
of material X into the pores 3, as illustrated in the depicted
model of Figure 4, and the permeability constant P3 is
represented by the permeability constant PX of material X
present in the pores.
The permeability constants, PX and Py~ are measurable
properties of materials. The surface area Al can be
established by the configuration and size of the multicomponent
membrane, and surface areas A2 and A3 can be determined, or
limits to them can be estimated, using conventional scanning
electron microscopy in combination with procedures based on
ga~ flow measurements of the porous separation membrane. The
thickness Ql~ Q2 and Q3 can be determined in the same
manner. Thus, QT a for a multicomponent membrane can be calcu-
lated from Equations 1 and 4 using values for ~PT a~ Ql~ Q2~ Q3
-55-
1~7Zg~3
Pxl Py, Al~ A2 and A3 which may be established. The separation
factor (~ba) can also be determined in a like manner from
Equations l and 2.
The mathematical model may be of assistance in developing
advantageous multicomponent membranes of this invention. For
instance, since the separation of at least one gas in a gaseous
mixture from at least one remaining gas is substantially
effected by the porous separation membrane in particularly
advantageous multicomponent membranes, a material for the
porous separation membrane can be selected on the basis of
its determined intrinsic separation factor for said gases as
well as its physical and chemical properties such as strength,
toughness, durability, chemical resistance, and the like.
The material can then be made into porous membrane form using
any suitable technique. The porous separation membrane can
be characterized, as stated above, by scanning electron
microscopy, preferably in combination with gas flow
measurement techniques such as described by H. Yasuda
et al, Journal of Applied Science, Vol. 18, p. 8~5-819 (1974).
The porous separation membrane can be represented,
for the purposes of the model, as two resistances to gas flow
in parallel, the solid portions 2 and the pores 3. The
resistance of the pores, R3, is dependent upon the average
size of the pores, which determines whether the gas flow
through the pores will be iaminar flow or Knudsen diffusive
flow (as discussed, for example in Hwang, et al, supra at
p. 50 ff), and on the number of pores, Since the diffusion
rates for gases through open pores is much greater than through
solid materials, the calculated resistance to gas flow of the
-56-
-
11~)7Z~3
pores, R3, is usually substantially less than the calculated
resistance of the solid portions, R2, of the porous separation
membrane, even when the total pore cross-sectional area
is much less than the total surface area of the
solid portions. In order to cause an increase in the proportion
of permeate gas flow through the solid portions 2 with respect
to flow through the pores 3, the resistance of the pores, R3,
must be increased relative to the resistance of the solid
portions, R2. This can be accomplished, in accordance with
this model, by placing a material X in the pores to decrease
the diffusion rate of gases through the pores.
Upon obtaining an estimation of the resistance to gas
flow through the pores and having knowledge of the resistance
to gas flow of the material of the porous separation membrane,
the desired increase in resistance to gas flow through the
pores necessary toprovide a multicomponent membrane having
a desired separation factor can be estimated. Conveniently,
but not necessarily, it can be assumed that the depth of
the material of the coating in the pores (Q3) and the
distance (Q2) of minimum permeation of the gas through the
material of the porous separation membrane are the same. Then,
based on knowledge of the permeability constants of materials
for coatings, a material for the coating can be selected
to provide the desired resistance. The material for the
coating can also be selected for other properties, in addition
to increasing R3, as will be described below. If the material
for the coating also forms an overlay on the porous separation
membrane, as illustrated in Figure 4, it can decrease the
flux. Such a situation is described according to the
mathematical model by Equation 1. In such a case the properties
57-
11~7Z(3 3
of the material of the coating should also be such that
the flux is not unduly reduced.
The selection of a material for the coating depends on
its determined intrinsic separation factor relative to the
determined lntrinsic separation factor of the material of the
porous separation membrane and its ability to provide the
- desired resistance in the multicomponent membrane. The
material of the coating should be capable of occluding contact
with the porous separation membrane. Based on the average
pore size of the porous separation membrane, materials for the
coating of appropriate molecular size can be chosen. If
the molecular size of the coating is too large, or if the
coating material bridges the pores at the surface, the model
provides that the resistance of the pores, R3, would not be
increased relative to the resistance, R2, of the solid portions
of the porous separation membrane, and in such a case, the
proportion of gases permeating through the solid portions 2
relative to gases diffusing through the pores would not be
increased with respect to that proportion in the porous
separation membrane alone. If, on the other hand, the
molecular size of the material of the coating is too small,
it may be drawn through the pores during coating and/or
separation operations.
Frequently, the coating is provided in the form of an
overlay 1 (see the depictea model of Figure 4) in addition
to the material of the coating which enters the pores. In
-58-
li~7Z~3
these instances, the overlay 1 represents a resistance to
gas flow, Rl, which is in series with the combined resistances
of the porous separation membrane. When this situation
occurs, the material of the coating should be advantageously
selected such that the overlay in the multicomponent membrane
will not provide too great a resistance to gas flow (while
the coating still provides sufficient resistance in the pores),
in order that the porous separation membrane substantially
effects the separation of at least one pair of gases in the
gaseous mixture. This might be accomplished, for example,
by providing a material for the coating which exhibits high
permeability constants for gases and low selectivity.
The thickness Ql of the overlay, as represented by
the model, can also have some affect on the flux and
selectivity exhibited by the multicomponent membrane, since
the resistance (Rl) of the overlay 1 is a function of its
thickness Rl
If a suitable material X and a material Y have been
chosen, various configurations of multicomponent membranes
comprising these materials can be modeled, using Equations 1,
2, and 4. Information concerning, for example, more desirable
ratios of total pore cross-sectional area (A3) to total
surface area (A2 + A3) for the porous separation membrane
and more desirable thicknesses for the separating layer Q2
of the porous separation membrane can result from this
mathematical modeling. This information may be useful, for
example, in determining procedures for the production of the
porous separation membranes having desirable area ratios,
A3/(A2 + A3), and desirable separating thicknesses Q2 as
well as desirable overlay thicknesses Ql In the case
of anisotropic hollow fiber porous separation membranes,
this might be accomplished by appropriate choice of
-59-
i~7Z6~3
spinning conditions and/or post-treatment conditions.
The above dlscussion is illustrative of the way in
which various configurations of multicomponent membranes
can be mathematically modeled.~ Several methods have been
discussed for varying, with respect to at least one pair of
gases, the relative resistances of the overlay 1, the solid
portions 2 and the pores 3 of the porous separation membrane
to provide advantageous multicomponent membranes exhibiting
high flux and high selectivity for at least one pair of gases.
The following is the mathematical derivation which, in
combination with Equations 3 and 4, will yield Equation 1.
From the well-known Ohm's law of electrical
resistances a mathematical expression for the total resistance,
RT, of the electrical circuit illustrated in Figure 5 can be
obtained.
R2R3
5) ~ = Rl + R23 = Rl + -
where R23 is the combined resistance of R2 and R3 in parallel
and is equal to the last term in Equation 5.
By analogy, the mathematical model described above
utilizes this same mathematical equation to express the total
resistance to flow of a given gas for a multicomponent
membrane, as illustrated in an exaggerated way by the depicted
model of Figure 4. The resistance, R23, represents the
combined resistance of both portions of the porous separation
membrane, the solid portions 2 and the pores 3 filled with
material X. If the coating is not provided as an essentially
-60-
`` ~11~7283
continuous overlay 1, but only as material X whlch enters
the pores 3, a situation illustrated in the depicted model of
Figure 3, then the resistance of the overlay Rl is zero, and
the term is dropped from Equation 5 and all subsequent
S equations derived from Equation 5.
The total flux of a given gas a through the multicomponent
membrane i~ ~quiv~ L~ che current in electrical flow and,
at steady state, is given by Equation 6.
6) QT'a Ql,a Q23,a
where Ql a is the flux of gas a through the overlay 1, and
Q23 a is the combined flux of gas a through both the solid
portions 2 and pore~ 3 (filled with material X) of the porous
separation membrane.
7) Q23,2 Q2,a + Q3,a
The total partial pressure drop for gas a across the
multicomponent membrane is the sum of the partial pressure
drop across the overlay 1, 4Pl a,and the par~ial pressure
drop across the solid portions 2 and the filled pores 3 of
the porous separation membrane, ~P23 a
8) ~PT,a = aPl~a + ~P23,a
The flux of gas a through each portion of the multicomponent
membrane can be expressed by Equation 3, using the resistances
and partial pressure drops specific to each por~ion.
~1~7Z~3
9) Ql,a Rl ~
10~ Q23 a = 23,a = P23~a(R2~a + R3,a)
R23,a R2,a R3,a
From Equations 6, 8, 9, and 10, an expression for ~P23 a
in terms of resistances and total partial pressure drop can
be derived.
_1
11) ap23,a = ~PT,a 1 + l,a ( 2, + R3,a)
Equation 11, in combination with Equations 6 and 10, yield
Equation 1.
In accordance with this invention a coating is in
occluding contact with the porous separation membrane to
provide a multicomponent membrane. This mathematical model,
which has been developed to explain the phenomena exhibited
by the multicomponent membranes of this invention, provides
that pores 3 in the porous separation membrane contain
material X. The resistance to gas flow, R3, of the pores
containing material X is much greater than the resistance
to gas flow of pores not filled with material X, since the
permeability for gases of any material is much less than the
permeability of an open flow channel. Accordingly, R3 is
increased in the multicomponent membrane, and with reference
to Equation 10, R2 becomes more significant in affecting R23.
Since R3 is increased relative to R2 in the multicomponent
membrane, an increased proportion o gas passes through the
solid portions oftheporous separation membrane as compared
11~)7Z~3
to that through pores 3 filled with material X than that
proportion in the porous separation membrane alone.
Consequently, the separation factor of at least one gas pair is
enhanced in the multicomponent membrane by interaction with
the material Y, as compared to that separation factor in
the porous separation membrane alone.
The following examples are illustrative of the invention
but are not in limitation thereof. All parts and percentages
of gases are by volume, and all parts and percentages of
liquids and solids are by weight, unless otherwise indicated.
-63-
2~3
EXAMPLES 1-3
Examples 1 thrDugh 3 in Table I represent multicomponent
membranes comprised of cellulose acetate porous separation mem-
branes and a coating. Examples 2 and 3 show the same composite
hollow fiber mernbranes separating two different gas mixtures. It
is seen in these two examples that the porous substrate membrane
separates both gas mixtures to some degree even in the absence of a
coating, but in both cases the separation factor is much less than the
determined intrinsic separation factor of the cellulose acetate. In
such porous separation membranes most of the gas passes through
the pores and relatively little pçrmeate flow is through the cellulose
acetate.
After coating, the separation factor for the gases exhibited by
the multicomponent membranes of Examples 2 and 3 is greater than
either the determined intrinsic separation factor of the coating
material or the separation factor of the porous separation membrane.
Thus, in the multicomponent membrane a greater proportion of the gas
flow is through the cellulose acetate as compared to that through the
pores; hence, the separation factor of the multicomponent membrane
is much closer to the determined intrinsic separation factor of the
cellulose acetate.
Example 1 shows a different sample of cellulose acetate hollow
fiber with somewhat different coated and uncoated properties and can
be compared to Example 2. Although the porous separation membrane
has a higher permeability for 2 and a lower separation factor, the
multicomponent membrane exhibits a higher separation factor than each
of the material of the coating and the porous separation membrane
separately.
-64-
7z~3
8 ~ u~
-I o
~8 ~
~ ~ b ~ ~ ~ X ~ =. x D~ :
~ ~ :~ ~ 8 ~ ~ ~9
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,~o ,z _ .
N X N r X )
~ ~ .
0~ ,
R ~¦ l a v R e ~ 4;~ e ,, e ~ . ~ R p
~ ~ ,~j u ~ o ~ o ~
~ z " a ~ ~ z~ ~ R Z~ ~ R Z~ ~ Sl '¢ R z~
c ~ 3 ~ o 1 ~ ~ -;~ ~ ~ s o
~ u U ,~ fi U ~ ~ ~ ~ ~ s o
11~7Z~3
QJ
~1 u ~ u,
O ~ ~ 'I i~
ul ~ .c . 0
o ~ ~ ~
O ,~ p O
h -~1 0 0 0
U7 ~ ~ O
0
h E~ h h
O h
~ ~ D7 U ~(o
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O ~
d 111 ,0~ U
0fi ~ m uO'
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o
N :~
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~ X 0
P ~ ~o
æ ~ ~ ~
O
S U ~ ~ o)~ H al -
~g ~
o~
. . ,q ~ ~ o
~ P~ O ~j ~ 3 ~ ~ 0 0
tn ~ oQ, ~ 0 41 ~
$ ~ o 0 0h 0 ~
P U 4~
~q ~ B ~
~ ~: h ~ I 13 0
O P~
uj u~
o
u 0~ .~ ~ tj .~
h oj ~ ~ O ~.
o p,
o --,~
O h # ~3~ ~1 0 U
8 ~j
u ~q~ u
uj ~j 0 K 0 t~
~ h ~ h
fi ~ C
O ~ .4 0 ~, Q. t~
0 h ~ I rl O 0 O~ ,l
0 ` M ~ 1 u .~ Uj ~a
0 ~ 00 ~ ~ ~ 0 ~l 0
5 o ~ 0j
0 5~ u o ~
-66-
11~7Z~3
EXAMPLES 4-10
Examples 4 through 10 illustrate various liquid coatings on
porous polysulfone hollow fiber separation membranes for selectively
separating oxygen from air and are presented in Table II. The porous
~p~ratio~ niblan~ du nol sepai7ate oxygen from nitrogen. The
coating materials used are representative of high molecular weight
organic and silicone liquids which have sufficiently low vapor pressures
that they will not readily evaporate from the coated surface and have
separation factors for oxygen over nitrogen which are generally less
than about 2. 5. The molecular sizes of the coatings are sufficiently
small to provide occluding contact with the porous separation membrane
but are not unduly small such that the coating material can pass through
the pores under coating and/or separating conditions. The observed
separation factors for the multicomponent membranes are greater
than each of the separation factor of the porous separation membrane
(1. 0 in all Examples) and the coating material (2. 5 or less for the
coating materlals of the Examples).
-67 -
11~7;~3 c0
U~
C hC) h "
D ~) ~1 0 C~ U~
I
CJ o N I O ~ ~ ~ id O C~
O ~, 2J h ~( X 3 ~ 'h
~1 u~ X . O ~ ~ C O
'J' ~ ' ~1 ':r u-) ~ c ,-( H 1~ 1
~o ~ ~ ~~0 ~ ~ ~I ~lo
~ .o I ~, OC o ~ C V
, a) ,~ ~a) o O.C 0
.~1 ~a C u~ ~: 0 3 ~ C~.
N 3 ~ ~ X X O h -,1 h
.,~ . o~ ~ 1 ~ ~ O O ` O
1~ ~1 0 ~ O ~ O
~ ~ r u) C~
d o ~ ~ ~D ,~ ~ ~ h o o
o ~ 1 1 ~ C) ~ ~ ~ ~,
u c ~ a ~
r~1 0 r ~ r-l ~ X , r~lr l ,C ~ ~ tJ~
~ ~ 0 a~ r-l ~ ~ O
E~ ~ ~ rl ~ 1r l ~ ~ r-l C ~
rl u7 ~ ~ ~ ~ .C O -rl 0 3
r-l R~ D ~ ~ ~ 0
C C tJ~ ~ ~ I 1~l ~ 0 ~ 0
O ~3 C rl IU rl U O 0 ~ O e
0 C ~ ~ I t r~ ~) -rl ~)
O ~J h ~r) C h O O rl CJ ~ 0 r l 0 rd
U C O h o ~I C) r-l r-l ~ ~ ~-1 .C4
r( 11~ ~ .) /D e o o
~J h I_ tJ~ rl ,~ ~ t) X 0 ~ C rl i~ ~
r~ ~ 3 r-l r-l ,~ r-l r-l ~ O ~ ~ h C~
e ~ ~ v ~
C r l r-l~ O h ~ h V
,J O ~1 ~ r-l O ~ O
O rl -r~ ~ -rl ~ ~Da) ~ C O C) ,n
h ~) 0 I I 0 ~h U~ r 0 oJ
h ~ O D l r-l ~~ O ~ r-~ ~ O 11l ~ ,1 0
~ ~ ~1 ~ C X ~ X rcC U 0 ~
uJ rl) 3 r l ~ rd r-l ~ ~ ~ ~ C C . -
~ ~ .~ ~ ~ e X ~ ~ C H -rl
t~ h ~,~f ~ . r3~ h ~ (~
E3 Q( ~Clo O ~
O r.) r-l rl U~ ~D ~ n ~ 0
~ Ul ~ ~X r; ~ ~' Pl ,~ _ U ~ O
~o x u~
O ~1 a) ..
X ~ ~ ~ ~ H ~ h
O ~ ~ o h ~ ~ ~
.,~ _ ~ O ~ ~1 ~ O
~ C X ~ X ~i 0 rl U 3
h ~ O~ ~1 ~i ~ C ~ rl ~ h
P~ P~ P~ 0 ~J O o ~ ~ h
H ¦ Q) O ,~ U -- C q-l -
C ~ _IUI,~ C c
O ~ o ~ ~ ~ ô H O ~
.IJ ~ U ~ U ~ ~
C~ C ~ C~I ~ ~l ~ O
Z ù p, ~ c o C c ~~J c ~ 0
~D ~; C0 '4 0 ~ ~ U J~ u ~ d 0 ~ a C ,.
o ~ h ~
x o oo cl o ~ o :~ ~ o ~ a)* ~ I x o a~ ' C
.
~, - 68
11~7Z03
EXAMPLES 11-15
Examples 11 through 15 illustrate various coatings which are
either applied to the porous separation membranes as liquids and
reacted in place to become solid polymer coatings (post-vulcanized) or
S are applied as normally solid polymers dissolved in a solvent. The
results are reported in Table III. In the Examples oxygen is enriched
from an air feed by the multicomponent membrane, and a variety of
treated polysulfons hollow fiber porous separation membranesis utilized.
.
-69-
~i~7Z~3
~, ~
'C ~ D
c 0 ~ h I I
~ O ,_1 o
h ~ ~ ~I
u~ 8 ~, 8 ~ . o . x
r-~1~ ~) .,1 ~ N ,n 11) 1`
3.~
c ~ a ~ ~ ~
N C $ a)
o ~ ~ C ~ ~ O
Q) $ . .~ O ~
_ ~ ~ ~
~1 3 ~ ~q 'I ,_1
~ ~ a ~
o
~ ~ ~_
C ~ ~ C ~ O O
c ~ ~ ~ a U. x ~ ~ x :
'~ ~ ~0 1 ~ .
.
c) ~d
e Q) c
~ * N ,~
~ o c In C '~ ~ In
~ ~C ~
S~ O ~ ~ O ~ ~
o ~ ~ o r~ . x x
` ~ ~
O
~ C~ .
o ~ ~ h ~ ~ c ~ o co ,i
O :~ ~ t) b, I ~ ~ X -i ~r x
3 ;'10 rl ~_
. H H 1-
,1
~ C C
Cl C ~I C ~ H H ~I C)I C
~Id p ~ $ ~ o~ tn~
H . Ual H ~4 ~I h h ~ o C `1 C ~ 'Y ~ ~ c C~
z ~ Z ~'~ ~c ~ ~o o
E~ c cn ~ ~ W C~ W h o ~ 0 ,1 ~ ~d w w
~ ~ ~ o ~ o ~ ,1 o ~ ~ ~ ~ ~ 6 E~
x v v v a u~ - o G) ~ -- P~ * _----~ _
'4' " - ~ O -
,.,~,
: ' ' . ' ' '' ~' ' ' :
~1~372~3
EXAMPLES 16-18_ '
Examples 16, 17 and 18 show that the multicomponent
membranes utilizing polysulfone hollow fiber porous separation
membranes can also effectively separate H2 from C0/H2 mixtures
according to the invention. The porous separation membrane
separa~iGn factor was not ~easure~ pri,or to coating for Examples
16 and 18, but numerous experiments with similar porous separation
membranes indicate that the separation factors can be expected
to be between about 1~3 and about 2~5. This expectation is
verified in Example 17 f wbe.re the porous separation membrane
separation factor for H2 over C0 was measured as 1,3. These
porous separation membranes thus exhibit some separation between
H2 and CO due to Knudsen diffusion., These examples illustrate
the use of different coatings, coating procedures, permeabilities
and separation factors of the multicomponent and porous separa-
tion membranes in providing the multicomponent membranes of the
invention. Examples 14 and 17 and Examples 15 and 18 are
carried out with the same multicomponent membrane and comparison
of these examples shows that use of a multicomponent membrane
for one separation or with one mixture of gases will not
prevent it from late.r functioning with another set of gases,
The Examples are reported iD Table IV.
7Z~3
~ N
~ O
o~ ~
.~ ~D
~q ~
.~ ~ ~ 4~
~q U~ O O ~ O
~ ~o ,,~ i o ~
8 ~
~ ~ .,~ In E~
r~ ~ ~ ~ HH
E E~
¦ ~_ ~c~ H 11 0 ,~
~1 g ~C a) ~
~ ~ ,~ ~ ~ 0~ : ~
,~ C ~ ~ q
. Q)~ ~ O
aJ +' c ~ li C 1~1
~; O ~ O rl
a~ ~ C~ ~ ,. ,~ ~_
~1 ~ l ~) ~D J ) .~ ,n
~ .~
t~
-72-
:
,
11~7Z~3
EXAMPLES 19-21
_
Examples 19, 20, arld 21 (Table V) show permeation properties
of multicomponent membranes composed of various coating materials
on porous hollow fiber copoly(styrene-acrylonitrile) separation mem-
branes for air and CO/H2 separation. In each example, the multi-
component membrane has a higher separation factor than each of the
coating and the porous separation membrane alone. Example 21
demonstrates a porous separation membrane which exhibits a
separation factor for H2 over CC~ of 15 before applying the coating, i. e.,
there are relatively few pores in the porous separation membrane and
the average pore diameter may be small. A comparison of Examples
20 and 21 shows that the multicomponent membrane of Example 20 has
a higher permeation rate and a higher separation factor than the
porous separation membrane of Example 21, even though that membrane
has a greater separation factor than that of the porous separation
membrane of Example 20. Thus multicomponent membranes in
accordance with the invention can have a superior rate of permeation
than a membrane of equal or greater separation factor consisting
essentially of the material of the porous separation membrane.
~1~7Z~3
¦ x~
a ~ D9
e ~
~ h . . ~ ~ o ~ o
h P ~ O O ~
c ~ ~0 0 0 0 ~ ~ ~a v
o o ~? a O ~ C ,," ~ X O U a ~ o :
o ,~ v u~
~ 0 ~ .~
u~ r-l O O ~ ,0 1 ~
X I o~ a) ~ a ~~r~ al N ,LI -r .~
Ei ~ .* ,~ ~ ~j o ~
c`~h ~: 0 c~
:~ ,1~ u h O ~ - O ~ h U v
E o r-l~ a ~ ~ 0~ ~ X ~ X . O ~
4-J O~ ~O~r ~d
U . O g
.0~ ~rl r~
'` E ~ ~ h ô q P Fq H ~i
t~ ~ ~ ~ ~ h ~ O ~ ~ o ~1 ~ , .
q E E o , ~5 S U Eq ~, Eq a r ~3 , a ~ ¦
. '~ h O ~ `-- r ~ h O ~ H
zl bao aO ~ .u ~ q~ ~", E--' E`'F'~ a al~ O O E
~ ~ ~ o ~ 0 ~ ~ ~
X ~ c~ ~ O O a) O P~ ~ U ~: C~J u ~ ^ c~ *
~S - ;~
~ -74-
EXAMPLE 22
A five component gas stream is utilized as the feed to a
multicomponent membrane of Examples 15 and 18. The feed
stream is comprised of hydrogen, carbon dioxide, carbon monoxide,
nitrogen, and methane with amounts of water and methanol up
to satu~at.ion v~lues The feed stream,at 4~$ ~g/cm pressure and
at a temperature of 40C, is introduced to the shell side
of the multicomponent membrane. The bore pressure is one
atmosphere. The following gas permeabilities and separation
factors relative to hydrogen are observed:
Permeability* For: Separation Factor H2 over
H2~8.5xlO-5) ---
C02~3.7xlO-5) ~2 2-3
C0 (.27x10~5) C0 31 0
N2 (.68~10-5) N2 12.4
CH4(.23x10-5) CH4 36.9
- *in cm3(STP)/cm2-sec-cm Hg.
It is clear in this example that the separation of
h~drogen from gaseous mixtures containing at least one of
2~ C0, N2 and CH4 can be ~asily effected. The presence of one
or more additional gases in the gaseous mixture, such as satu-
rated water and methanol vapor,apparently does not hi~der the
multicomponent membrane from effecting the separation. Also
it is clear that various of the other gases in the mixture
can be separ~ted from each other, e.g., the separation factor
for C02 over C0 woùld be the ratios of permeabilities, i.e.,
about 14. Example 22 also illustrates the effect of the
-75-
372(~3
the porous separation me2nbrane in p.roviding the relative permeation
rates through t~e multicompo2lent membrane. Thus, the coating
material (Sylgard 184*)exhibits a determined intrinsic separation
factor of about 0. 3 to 0. 4 for H2 over CO2 (i. e., CO2 is faster than
H2~, yet the multicomponent membrane exhibits a separation factor
.2f 2. 3 fsr ~2 over C02. This value is essentially equal, within
experimental error, to the determined intrinsic separation factor of
the polysulfone for H2 over CO2.
* Trademark
-76-
i~972~3
EXAMPLE 23
Example 23 (Table VI) shows the permeabilities (P/R) for a
number of gases through a multicomponent membrane utilizing a hollow
fiber polysulfone porous separation membrane. Example 23 shows the
same values for the same gases through a continuous compact film of
the polysulfone material. The ratio of any two P or P/,Q values defines an
approximate separation factor for those gases through the compact film
or the multicomponent membrane, respectlvely. The Example
illustrates a clear trend in that the permeabilities for the multicomponent
membrane generally vary from gas to gas in the same order as those for
the polysulfone compact film. This trend indicates that the material
of the porous separation membrane is providing a significant portion
of the separation exhibited by the multicomponent membrane. This
example also shows that a multicomponent membrane may be used to
separate any of a number of gases from each other. For example, from
the table it is seen that NH3 could be readily separated from H2 or N2,
He from CH4~ N20 from N2- 2 from N2, or H2S from CH4, using this
multicomponent membrane. The advantage of high permeation rates
of the multicomponent membranes is indicated by the data represented
in Table VI.
S ~
Y ~ I b~ ~
~ sD ~ ~
C~i o cn ~ . .
~ ~ ~ ~ ~ 0
~ 3 ~ ~
~ ~ . ~ ~ ~
~i ~ X o ~
~ ~ U p~ . ~ ~ o U~ ~ .-~
8 ~ ~ 8 ~ ,
1 ~ o ~ ~ ~ z~
-7B-
CO~\1PARA'rIVE F,X~MPLES 24 through 26
~not in accordance with the invention)
Examples 24 through 26 are reported in Table VII and
illustrate that nol all composite membranes will fall within the scope
of the invention, i. e., pro-vide a multicomponent membrane exhibiting
a separation factor significantly greater than the determined intrinsic
separation factor of the coating material, even though they are
comprised of porous separation membranes and coating materials each
of which may be employed with other coating materials or porous
separation membranes to provide multicomponent membranes in
accordance with this invention.
Example 24 provides a multicomponent membrane having a
pre-vulcanized silicone rubber coating on a porous polysulfone separa-
tion membrane. Since the pre -vulcanized silicone rubber may have
too large molecular dimensions to be expected to occlude the pores
in accordance with the model, e g., the molecules can only bridge
the pores, the coating will not alter the resistance of the pores to gas
flow. In Example 24, the coating compound is a pre~ lcanized polyrner
with the sarne essential polyrneric backbone as the Sylgard 184*
illustrated, for instance, in Examples 11, 14 and 15 in Table III.
However, the pre-vulcanized silicone rubber has a much higher
molecular weight and size due to pre-vulcanization than does Sylgard
184, and thus apparently cannot fill the pores, and consequently the
composite membrane exhibits a separation factor equal (within
experimental error) to that of the coating material.
* Trademark
_79_
-.
11~7ZC33
F~xample 25 illustrates a multicomponent membrane wherein
Sylgard 184*is used as the coating material and a porous polyacrylonitrile
separation membrane is utilized. Polyacrylonitrile exhibits a very low
permeability for gases when in continuous, non-porous form. With
reference to the model, such a porous separation membrane will have
a very high resistaulce to flow through the solid portions thereof, such
that ~hen a coating material of high permeability such as Sylgard 184 *
is in occluding contact therewith, the gas flow occurs predominantly
through the coating and the plugged pores, and thus the multicomponent
membrane exhibits a separation factor which is equal to or less than
that of the coating mernbrane,
A multicomponent membrane which is illustrated in
Example 26 exhibits a separation factor that is lower than the deter-
mined intrinsic separation factor of the coating material. This situation
is similar to that of Example 24 in that the poly(vinylbutyral) used as
the coating material has a high molecular weigm.; In addition, it does
not wet polysulfone as well as many silicones and other preferred
coatings. More~ver, the poly(vinylbutyral) has a relatively low
permeability. The observation that the separation factor exhibited by
the rnulticomponent membrane is less than that expected of the coating
material suggests imperfections in the coating itself.
2 5 * Trademark
~ --80--
~i :37Z~3
TABLE VII (not in accordance with the invention)
EXAMYLE 24
General Electric 4164 Pre-
Coat~ vulcanized Silicone Rubber
Porous Hollow Fiber Membrane Polysulfone P-3500 Union Carbide
~ _ _ _ _ _ _ _ _ _ _ _ _
Gas Feed Air
Enriched Gas 2
(Permeate)
Coating Procedurea E
Coating Material
Determined Intrinsic
Separation Factorb 2 over N2 l.7
Porous Separation
Membrane PermeabilityC For Air l.8 x lO 4
Porous Separation b
Membrane Separation Factor 2 over N2 l~0
Multicomponent Me~brane
Separation Factor 2 over N2 l.61
Multicomponent Membrane
Permeabilityc For 2 4~l x lO 5
EXAMPLE 25
- Dow Cor.ning Syl.gard 184*post-
Coatin~ vulcanized Silicone Rubber
_ __ ___ _.
Porous Hollow Fiber Membrane Polyacrylonitrile
.
Gas Feed Air
Enriched Gas
(Permeate) . 2
Coating Procedurea F
Coating Material
Determined Intrinsic
Separation Factorb 2 over N2 2,3
Porous Separation
Membrane PermeabilityC For Air 2 x 10-3
Porous Separation ~Iembrane
Separation Factorb -- 2 over Nz l.0
Multicolnponent Membrane
Separation Factorb 2 over N~ l.9
Multicomponent Membrane
Permeabilityc For 2 l.7 x lO 5
* Trademark
1~7ZGi3
EXAMPLE 26
Coatino Poly-(vinylbutyral)
Porous Hollow Fiber Membrane Polysulfone P-3500 Union Carbide
.
.
Gas Feed Air
Enriched Gas o
(Permeate) 2
Coating Procedurea C
Coating Material
Determined Intrin~ic
Separation Factor 2 over N2 4.7
Porous Separation
Membrane PermeabilityC For Air 1.8 x 10 4
Porous Separation Membrane
Separation Factorb 2 over N2 1.0
Multicomponent Me~brane
Separation Factor 2 over N2 4 0
Multicomponent Membrane
PermeabilityC For 2 1.4 x 10 6
.. . . . . . . . . .. .
(a) As footnote d in Table I.
~b) As footnote e in Table I.
(c) As footnote a in Table I.
-82-
~1~)72~)3
EXAMPLES 27-34
. _ _
Examples 27 through 34 are reported in Table VIII and illustrate
a series of porous separation membrane post-spinning treatments and
the way in which these treatments affect the separation properties of
multicomponent membranes made from such post-treated porous
separation membranes. In Table VIII the coating material and the method
of application are essentially the same in order to emphasize that
changes in the permeation rate and separation factor of the multicomponent
membranes (for both air and CO/H2 gas mixture feed) are apparently
due to changes in the relatively dense regions of the porous separation
membrane. The treatments are believed to affect the available pore
cross-sectional area (~3) relative to total surface area (A2 + A3) of
porous separation membrane A decrease in A3 relative to the total
surface area will cause an increase in the relative resistance to flow
through the pores in the porous separation and multicomponent membranes.
This, in turn, will force more of the permeate gas to flow through the
porous separation membrane material, and the separation factor
exhibited by the multicomponent membrane will move closer to the
intrinsic separation factor of the material of the porous separation
membrane.
In all of the examples in Table VIII, the porous separation
membrane utilized is a porous hollow fiber membrane of polysulfone
(Union Carbide, P-3500) from the same bobbin, which was wet-spun from
a dope of 25% solids in dimethylformamide solvent into a coagulant of
water at approximately 3C through a tube-in-orifice jet through which
water was injected to the bore and the fiber taken up at a speed of 21. 4
mpm. The porous separation membrane used in each example is stored in
deionized water at room temperature after spinning until the post-
treatment process is applied.
-83 -
1~.0~11~3
TABLE VIII
Examples 27 through 34
Hol1Ow Fiber_Membrane Post_Treatment
The multicomponent membranes of Examples 27 through 34
utilize a coating of Dow Corning Sylgard 184*post-vulcanized
silicone rub~er followin~ ~oating procedure F as in Table XVI.
Post treatment was performed on the hollow fiber membrane -
after spinning but before the coating was applied~
EXAMPLE 27
Post Treatment Air evaporation of the water
Gas Feed Air
Enriched Gas O
(Per~eate) 2
Multicomponent Membrane
Permeabilitya For 2 1.5 x 10 5
Multicomponent Membrane
Separati~on Factorb 2 over N2 4~7
Post-treated Porous Separa-
tion Membrane Permeabilitya For air 3.7 x 10 4
Post-treated Porous Separa-
tion Membrane Separation
Factorb 2 over N2 1~0
EXAMPLE 28
Post Treatment Air evaporat;on of the water
Gas Feed CO,
H2
Enriched Gas H
(Permeate) 2
Multicomponent Membrane
Permeabilitya For H2 7.6 x 10 5
Multicomponent Membrane
Separation Factorb H2 over CO 23.1
Post-treated Porous Separa-
tion Membrane Permeabilitya For H2 ~2.0 x 10 4
Post-treated Porous Separa- ..
tion Membrane Separation
Factorb H2 over CO ~2.6
* Trademark
-84-
~ ` ~
03
EXAMPLE 29
Post Treatment Air evaporation of the water;
followed by exposure to acetone
vapor at 25C with bore vacuum;
followed by alternating water
immersion and methanol immer-
sion with bore vacuum (3 cycles);
followed by alternating water
immersion and isopropyl alcohol
immersion (2 cycles),
Gas Feed Air
Enriched Gas O
(Permeate) 2
Multicomponent Membrane
Permeabilitya For 2 7.7 x 10 6
Multicomponent Membrane
Separation Factorb 2 over N2 5.3
Post-treated Porous Separa-
tion Membrane Permeabilltya For 2 3.5 x 10 5
Post-treated Porous Separa-
tion Membrane Separation
Factorb 2 over N2 1.0
EXAMPLE 30
Post Treatment Air evaporation of the water
followed by exposure to acetone
vapor at 25C with bore vacuum;
followed by alternating water
immersion and methanol immer-
sion with bore vacuum (3 cycles);
followed by alternating water
immersion and isopropyl alcohol
immersion (2 cycles).
Gas Feed CO 5
H2
Enriched Gas
(Permeate) H2
Multicomponent Membrane
Permeabilitya For H2 4.5 x 10 5
Multicomponent Membraneb
Separation Factor H2 over CO 30.4
Post-treated Porous Separa-
tion Membrane Permeabilitya For H2 1,5 x 10 4
Post-treated Porous Separa-
tion Membrane Separation
Factorb H2 over CO 5.1
7Z~;~
EXAMPLE 31
Post Treatment Air evaporation of the water;
followed by heating in hot
air oven to 80-95 for
approximately 3 hours.
Gas Feed Air
Enriched Gas
(Permeate) 2
Multicomponent Membrane
Permeabilitya For 2 1.6 x 10 5
Multicomponent Me~brane
Separation Factor 2 over N2 5 0
Post-treated Porous Separa-
tion Membrane Permeabilitya . For air 3.7 x lQ-4
Post-treated Porous Separa-
tion Membrane Separation
Factorb 2 over N2 1.0
. EXAMPLE 32
Post Treatment Air evaporation of the water;
followed by heating in hot `
air oven to 80-95 for
~ approximately 3 hours.
Gas Feed CO ,
H2
Enriched Gas
(Permeate) H2
Multicomponent Membrane
Permeabilitya For H2 9.8 x 10 5
Multicomponent Membrane
.Separàtion Factorb H2 over C0 23
Po~t-treated Porous Separa-
tion Membrane Permeabilitya For H2 ~2.5 x 10 4
Post-treated Porous Separa-
tion Mbembrane Separation
Factor H2 over C0 ~1.3
86-
EXAMPLE 33
Post Treatment Dry by exchanging water
with isopropyl alco~ol;
followed by exchanging
the isopropyl alcohol with
pen~ane; followed by air
evaporation of the pentane
Gas Feed Air
Enriched Gas
(Permeate) 2
Multicomponent Membrane
Permeabilitya For 2 2.0 x 10 5
Multicomponent Membrane
Separation Factorb 2 over N2 ' 4.2
Post-treated Porous Separa-
tion Membrane Permeabilitya For air 1.5 x 10-3
Post-treated Porous Separa-
tion M,embrane Separation
FactorD 2 over~,N2 1.0 ~:
EXAMPLE 34
Post Treatment Dry by exchanging water with
isopropyl alcohol; followed
by exchanging the isopropyl
alcohol with pentane; followed
by air evaporation of the
pentane
Gas Feed CO ,.
H2
Enriched Gas
(Permeate) H2
Multicomponent Membrane
Permeabilitya For H2 1.2 x lO 4
Multicomponent Membrane ,
Separation Factorb H2 over C0 15.9
Post-treated Porous Separa-
tion Membrane Permeabilitya For H2 ~2.5 x 10 4
Post-treated Porous Separa-
tion M,embrane Separation
FactorD H2 over C0 ~1.3
. ~
(a) as footnote a in Table I
(b) as footnote e in Table I
. 11~7ZC~3
EXAMPLE S 35-3 ~
Exdm~le~ 35 Lhr~ugh 3~ reported in Table Ix illustrate the effect
which additi~es in tne c~l~ting material have on the separation factor
of a r~ lticomponent m~l~rane for tw~ gas mLxture feedstreams (air and
Co/~2~ Th~se addltives aL~ inc3rporated into the coating material in
small amounts before the coating is applied to the porous separation
membrane. Such additives can change the separating properties of the
multicom~onent membranes, for example, by changing the w.etting properties
of the coating material and thereby affecting its ability to be in
occluding contact with the porous separation me~ rane. If the additive
enhances occluding contact the separation factor of a multicomponent
~embrane with such an additive is expected to be cloæer to the intrinsic
separation factor of the porous separation membrane material. than the
; separation factor of a similar multicomponent membrane without such an.
additivè.
The porous separation membrane hollo~ fibers utilized in Examples
35 to 39 were all from the same bobbin and were made of polysulfone
(P-3500) in a highly porous form (see footnote a), by spLnning according
to the same procedure as the hollow fibers of Examples 27-34~ The
.20 determined intrinsic separation factor of polysulfone for 2 over
N2 from an alr feed is about 6.0 and for H2 over CO from a CO~2
mdxture is about 40.
-88-
2~3
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-` ~.13~ZC~3
~JES 40-43
Examples 40 through 43 in Table X illustrate multicomponent
membranes in which the porous separation memoranes are prepared under
different spinning conditions. The multLcomponent membranes of
Examples 40 to 43 employ a Dow Corning Sylgard 184*post-vulcanizea
silicone rL~ber coating (Coating Procedure F, Table XVI) on porous
polysulfone ~tJnion Carbide, P-3500) separation membranes. ~Le porous
polysulfone hollow fiber sL~strate membranes are wet jet spun from the
indicated dopes into a water coagulant at the indicated temperature
and spinning rate by means of a hollow fiber spinnerette having an orifice~
to provide for injection of coagulant into the fiber bore as formed.
The range of permeabilities (2 and H2) and separation factors of the
multicomponent membranes (2 over N2 and H2 over CO) e~hibited in
Examples 40 to 43 for either air feed or COjH2 mixture feed can be
related to the variability in relative resistances of the pores and
material of the porous separation mel~branes to gas flow. The conditions
under which the porous substrate material is spun determine to a great
extent the porosity characteristics and the effective separating thickness.
which that substrate will possess. In addition, these characteristics
can be altered with post-spinning treatments of the porous s~lbstrate
(see Examples 27 to 34).
.
* Trademark
~..,
- -90-
. . . . . ....
11~'72/~;~
TABLE X
Spinning Conditions for P-3500 Polysulfone
Hollow Fiber Porous Separation Membranes
~XAMPLE 40
Solvent Dimethylformamide
Coagulation
Temp C
Spin Rate, mpm 21.4 mpm
Dope Concentration-wt. % polymer 25~
Multicomponent Membrane b 4.5
Separation Factor, 0~ over ~2
Permeability for 2 a 7.7 x 10 6
Multicomponent Membrane b 16.7
Separation Factor, H2 over C0
Permeability for H2 5.0 x 10 5
Porous Separation Memorane 6 x 10-4
Permeability for Air
~5
EXA~LS 41
Solvent Dimeth~lformamide
Coagulation 5
Temp. C
Spin Rate, mpm 2104 mpm
Dope Concentration-wt. % polymer 254 ;~
Multicomponent Membrane b 5.09
Separation Factor, 2 over N2
Permeability for 2 6.2 x 10-6
Multicomponent Membrane b 25
Separation Factor, H2 over C0
Permeability for H2 4.9 x 10
Porous Separation Membrane 9 x 10
Permeability for Air
-91-
~7Z~3
EXAMPLE 42
Solvent Dimethylformamide
Coagulation 4
Temp. C
Spin Rate, ~pm 33 ~pm
Dope Concentration-wt. ~ polymer 28~
Multicomponent Membrane b 5.9
Separation Factor, 2 over N2
Permeability for 2 8.0 x 10
Multicomponent Membrane b 30
Separation Factor, H2 over CO
Permeability for H2 5.9 x 10
Porous Separation Membrane 2 x 10
Permeability for Air
EX~MPLE 43
Solvent Dimeth~lacetamide
CoaguOation 5
Temp. C
Spin Rate, mpm 33 mpm
Dope Concentration-wt. % polymer 27%
Multicomponent Membrane b 5.6
Separation Factor, 2 over N2
Permeability for 2 6.0 x 10-6
Multicomponent Membrane b 27
Separation Factor, H2 over CO
Permeability for H2 3.8 x ~0 5
Porous Separation Membrane 4.5 x 10
Permeability for Air
a) As in Table I
b) As footnote e in Table I
-92-
EXAMPLES 44-51
.
Examples 44 through 47 in Table XI illustrate multicomponent membranes
wherein the porous separation membrane is in the form of an anisotropic
film comprising an acrylonitrile/styrene copolymer having a determined
intrinsic separation factor for ~2 over CO of 76. The films had been
cast fxom solvents compri~ing dimethylformamide and non-solvents as
indicated in the Table onto a plate~ desolvated in air for 5-45 seconds,
coagulated as indicated belo~ then suhmerged in water at 25C to wash,
removed and dried. Examples 48 to 51 illustrate multicomponent membranes
in the form of films which are dense. These exaLples illustrate multi-
component membranes according to the invention which are in the form of
films, and may include porous separation membranes that have coatings on
both surfaces.
-93-
7Z~3
TABLE XI
Multico~ponent Mem~ranes in Film Form
F.XAMPLE 44
Coating Dow Corning Sylgard 184*
post-vulcani7ed silicone
rubber
Porous Separation Membrane Copoly(acrylonitrile/styrene)
32%/68% by weight
.
Gas Feed H2,CO
Coating Procedure B
Coating Material Determinedb l.9
Intrinsic Separation Factor
H2 over CO
Porous SeparationbMembrane 13
Separation Factor
H2 over CO
Multicomponent Me ~ rane 34.8
Separation Factor , H2
EXAMPLE 45
Coating Dow Corning 200 poly~di-
methyl siloxane)
Porous Separation Membrane Copoly(acrylonitridle/styrene)
32%/68~ by weight
_ _ .
_ _ . . . _ . .. . _
Gas Feed H2,CO
Coating Procedure B
25 Coating Material Determinedb 1.9
Intrinsic Separation Factor
H2 over CO
Porous SeparationbMembrane 12.2
Separation Factor
H2 over CO
Multicomponent Me ~ rane 23.8
Separation Factor
H2 over CO
* Trademark -94~
..`f"
37Z~3
EXAMPLE 46
Coating DoW Cbrning Sylgard 184*
vulcanized Silicone Rub~er
Poro~s Separation Membrane Copoly(acrylonitrile/styrene)
32%/68% hy weight
.
Gas Feed H2,CO
Coating Procedure B
Coating Material Detellnine~ 1.9
Intrinsic Separator Factor
H2 over CO
Porous SeparationbMembrane 4.0
Separation Factor
H2 over CO
Multicomponent Me ~ rane 23.5
Separation Factor
H2 over CO
- EXAMPLX 47
Coating Dow Corning 200 poly~
. methyl siloxane)
Porous Separation Mel~rane Copoly(acrylonitr~ e/styrene)
32%/68~ by weight
Coating Dow Corning 200 poly(di-
. ~ethy~ silo~ane)
. . ~
_ _
Gas Peed H2,CO
Coating Procedure . B
Coating Material Determine ~ 1.9 ~ ,
Intrinsic Separation Factor
over CO
Porous SeparationbMembrane 3.4
Separation Factor
H2 over CO
Multicomponent Membrane,
Coating on One Sibe
Separation Factor , H2 over CO 7.6
Multico~ponent Membrane, 34
Coating on Both S~des
Separation Factor , H2 over CO
* Trademark
9 5--
..
.
a) ~.s e~pla~ed in Table XVI
b) As footnote e in Table I
c) Coagulated in 50/50 by voluine ethylene glycol/~ater for 30 min.
at 25 C
5d) Coagulated in 90/10 by ~olu~e isopropyl a]oohol/water for 30 min.
at 25 C
e) Coag~llated in 10/90 by volume isopropyl alcohol/water for 30 min.
at 25 C
f) Coagulated in water at 25 C
EXAMPLE 48
Coating Dow Corning Sylgard 184 *
post-~lcanized silicone
ru~ber
Porous Separation Mei~brane Copoly(acrylonitrile/styrenè
. 25~/75~ by weight
Gas Feed Air
Enriched Gas
(Permeate)
Coating Procedure E
Coating Material Determinedb 2.3
Intrinsic Separation Factor
Porous SeparationbMembrane 0 over N 3.6
Separation Factor 2 2
~lulticomponent Me ~ rane 0 over N 5.4
Separation Factor 2 2
* Trademark
-96-
7203
EXAMPLE 49
Coating Do~ Corning 200 poly(di-
methyl siloxane)
Porous Separation Membrane Polyblend of two acrylo-
nitrile/styrene copolymers
Gas Feed Air
Enriched Gas 2
(Permeate)
Coating Procedure A
Coating Material Determane ~ 2.3
Intrinsic Separation Factor
Porous SeparationbMembrane O2 over N2 4.9
Separation Factor
Multiconponent Meb~brane P2 over N2 6.1
Separation Factor
EXAMPLE 50
Coating Do~ Corning 200 poly(di-
methyl siloxane?
Porous Separation Membrane Copoly(acrylonitrile/styrene)
32%/68~ by ~eight, suspen-
sion polymerized
...._
.
Ga9 Feed Air
~nriched Gas 0
~Permeate)
Coating Procedure A
Coating Material Determinedb 2.3
Intrinsic Separation Factor
Porous SeparationbMembrane 02 over N2 1.0
Separation Factor
Multicomponent Me ~ rane 0 over N 6.3
Separation Factor 2 2
,
-97-
2Q;~
EXAMPLE 51
Coatinq Dow Corning 200 poly(di-
methyl siloxane)
Porous Separation Membrane Copoly(acrylonitrile/styrene)
32%/68% by weight,mass
polymerized
. _
. . _
Gas Feed Air
Enriched Gas 0
~Permeate)
Coating Procedure A
Coating Material Determine ~ 2.3
Intrinsia Separation Factor
Porous SeparationbMembrane O over N 3.6
Separation Factor 2 2
Multicomponent Me ~ rane 0 over N 4.9
Separation Factor 2 2
:' ~
~ a)~ As explained in ~able XVI
::
b) As footnote e in Table I
EXAMPLES 52-57
_
Examples 52 through 57 illustrate several multicomponent membranes
in hollow fiber form. The porous hollow fibers can be produced by wet-
spinning as generally described above. The polycarbonate fiber of Examples
52 and 53 was wet-jet spun from a dope of 27.5 wt. percent polycarbonate
in N-methylpyrrolidone into a water coagulant at 25C at a rate of 21.4
mpm. The polysulfone hollow fiber of Example 54 was spun from a dope of
27.5 wt. percent polysulfone ~P-3500) in a mixed 80/20 dimethylacetamide~
acetone solvent into a water ooagulant at 2C at a rate of 21.4 mpm. m e
acrylonitrile-styrene copolymer fi~er of Example 55 was spun from a dope
of 27.5 wt. percent copolymer in a mixed solvent of 80/20 dimethylformamide/
~ormamide into a water coagulant at 3 & at~a rate of 21.4 mpm. The
acrylonitrile-styrene copolymer fiber of Examples 56 and 57 was spun from
-98-
- ~ :
11`~'72~3
a dope of 25 wt. percent copoly~r in the same mlxed solvent as in
Example 55 into a water coagulant at about 20 C at a rate of 21.4 mpm.
The results of testing the multicomponent hollow fibers in separating
a hydrogen/carbon monoxide gas mixture are set out in Table XII below.
'
_99_
`" 13L~72~3
,
... ~ o,~ ~o
~` ~ L~ h O ~ C C ~ 1` X
8 ~ B, ~ .. 5 ~
~*~rco UV ~
~D ~ O ~ Z c ~ o
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:~ O N ~ ~
o 8
8 o~
U~ O ,5 " W ~ ~ O ~ N ~
H ~ V ~ ~ ~ ~ 1~, ~ V X X
X ,~3 ~ O N ~ t`l C U ~ Ei 1'1 N
C -,~0~ O
" D ~ ~o " 4 ~ O ~ 8 ~ o
o I I ~ a 3 o r~ I ~N N t'') ~ rX X O
.~ ~ X ~ w ~ X
.;~ ~3 ~ ~ U) ~ 10
-~ o ~ ~J 8 _, x
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3 ~X ~ X ;~
u~ u ~ ~ X x
i~ Z ~ 8 a
J~O ~o ;~
-100-
~.
li~72(~3
58
~his exam~le illustrates a multicomponent membrane ha~ing multiple
coatings to achieve a desired separation factor. A porous hollow fiber
` separation m~brane composed of a copolymer of 63% acrylonitrile and 37%
styrene was wet-spun from a so1ution of 27.5 wt percent copolymer in
93/7 dimethylformamide/rormamide mi~ed solvent into 2 C water at a rate
of 21.4 m~n. This fiber ~as first treated by dipping in methanol ~hile
a vacuum was pulled on the bore, dried and the methanol treatment and
drying repeated. The dried substrate fiber was then coated by procedure
D with poly(cis-isoprene) in pentane solvent, cured for 30 minutes at
85 C and then coated with 10% solution of Sylgard 184*in pentane by
procedure F. The coated substrate was then recoated with the poly(cis-
isoprene) solution, dried and recoated with the Sylgard 184*solution
and thereafter cured 30 minutes at 90 C, 30 minutes at 100 C and finally
~ 15 30 minutes at 105C. The results of testing the uncoated porous and
.. the multi.ply coated multicomponent membrane are set out in Table XIII.
,~ `
TABLE ~III
Coating MaterialDetermined Cis-Isoprene 3.5
Intrinsic Separation Factor,H2 over C0 Sylgard 184* 1.9
Porous Separation Membrane 5.09
Separation Factor, H2 over C0
Multicomponent Membrane 82
Separation Factor, H2 over C0
Multicomponent Membrane 6.5 x 10
Permeability to H2
Porous Separation Membrane 2.65 x 10
Permeability to H
Determined Intrinsic 320
Se~aration Factor, H~ over CO
~! of Material of Porous Separation
Menbrane
* Trademark
101--
, ,, . ~
. ~
,
~03
EXAMPI,ES 59 and 60
_ _ _____ :
Examples 59 and 60 illustrate multicomponent membranes
utilizing a brominated poly(xylylene oxide) porous separation membrane
in hollow fiber form with a coating. The hollow fiber was wet-spun from
a dope of 30 wt. percent polymer in N-methylpyrrolidone into a water
coagulant at 85C at a rate of 14. 8 mpm. In Example 59 the brominated
poly(xylylene oxide) in which the bromination is essentially upon methyl
groups is coaled without a post-spinning treatment. In Example 60 the
brominated poly(xylylene oxide) is post-treated by soaking for 20 hours
in a solution of 10% trimethylamine in watet. The coating in each instance
was Dow Corning Sylgard 184*silicone rubber applied by procedure B
(see Table XVI). The results are set out in Table XIV below.
TABLE XIV
_ 59 60
Dow Corning Dow Corning
Sylgard 184* Sylgard 184*
post-vulcanized post-vulcanized
silicone rubber silicone rubber
Brominated Poly- Brominated Poly-
(xylylene oxide) (xylylene oxide)
Post-treated with
3 ) 3
Coating Material
Determined Intrinsic
Separation Factor,
H2 over CO 1. 9 1. 9
Porous Separation Membrane
Separation Factor,
H2 over CO i. 48 2, 85
Multicomponent Membrane
Separation Factor
H2 over CO 11.1 9. 59
Multicomponent Membrane
Permeability to E~2 9 58xlO 5 1. 27xlO
Porous Separation Membrane
Permeability to E~2 1. 25xlO 3 3. 83xlO
Determined Intrinsic
Separation Factor, 15 34
H2 over CO, for
Material of Porous
Separation Membrane
* Trademark
-102-
EXAMPLE 61
_ _ _
This example illustrates a multicomponent membrane
utilizing a different modified brominated polytxylylene oxide)
porous separation membrhne in hollow fiber form. The hollow
fiber of brominated poly(xylylene oxide~ o~ Example 59
was post-treated by soaking for about ~0 hours at 50C. in
a solution of 5% by weight th;ourea dissolved in 95/5 by
volume water/methanol. After drying,the hollow fiber membrane
was coated with a 5% solution of Dow Corning Sylgard 184*in
pentane by procedure F (see Table XVI~. Testing of the
post-treated hollow fiber porous separat;on membrane and of
the coated multicomponent membrane gave the following results:
Coat;ng Material Determined
Intri`nsic Separation Factor t H2 over C0 1.9
Porous Separati`on Memhrane
Separation Factor, H2 over C0 5.6
Multicomponent Me~hrane
Separation Factor, H2 over CO 46.1
Multicomponent Mem~rane
Permeability to H2 7.2xlO 6
Porous Separation Membrane
Permeability to H2 3.9xlO 5
Determined Intrinsic
Separation Factor, H2 over CO,
for Material of Porous
Separation Membrane ~ 150
* Trademark
-103-
"` il~172~3
~X .~ I' I L S _6 ?_~ nd_6 3
These ex2mples serve to illustrate the flexibility of
the invenlion wherein the coating may be present on the
interior and both interior and exterior surfaces of a hollow
fiber porous separation membrane. ~hey also illustrate the
invention in a process wherein the gaseous feed stream is
contacted at the surface of the multicomponent n,embrane
opposite the coating. In Example 62 a porous polysulfone
hollow fiber separation membrane was coated on the interior
with a 3% solution of Sylgard 134*post-vulcanized silicone
rubber in pentane by pumping such solution slowly through
the bore of the hollow fiber substrate and allowing the fiber
to air dry~ The pc-rmeability was determined by permeation
of an H2-C0 mixture from the exterior into the bore of the
resulting composite membrane. In Example 63 the bore coated
fiber of Example 62 was additionally coated wi-th the same
Sylgard 184*solution by procedure F. The results of testing
these multicomponent membranes are set out in Table XV below~ ~-
* Trademark
-104-
~07Z~3
~ABLE XV
_ _ 6~ ~3
Dow Con~ing D~w CornIng
Sylgard 184 * Sylgard 184 *
post-vulcanized post-vulcanized
silicone rubber silicone rubber
Polysulfone(a) Polysulfone(a)
(bore only coated) (bore and outside coated)
Coating M~terial
Deter~ined IntrLnsic
Separation Factor,
H2 over CO 2.3 2.3
~orous Separation M~mbrane
Separation Factor,
H2 over CO 3.23 3.23
Multicomp~nent ~brane
Separation Factor
H2 over CO 22.0 21.2
M~lticcmponent M~mb~ane
PermYability to H2 3-6x.Lo-5 2.31x10-5
PorDus Separation ~brane
Permeability to H2 2.06x10-4 2.06x10-4
.
(a) Polysulfone, Ukion Carbide, P-3500, wet-syun f~om a 30 wt. percent
dope in a 50/50 dimethylformamide/~etllyl pyrrolidone solvent
m~xture into water at 2C at a rate of ~1.4 ~ym and taken up, after
w~shing and stretching,at 33 mpm.
* Trademark
-iVJ--
1~1)7Z03
EXAMPLE 64
This example illustrates a procedure for making a multicomponent
membrane in hollow fiber form which utilizes a polysulfone porous
separation membrane and a coating of Sylgard 184*. Polysulfone polymer
(P-3500 available from Union Carbide) is dried at 100C at 125 rnm. of
mercury pressure for about 25 hours. The dried polysulfone is adrnixed
at a tenperature of about 65 to 70C with dimethylacetamide (moisture
content less than about 0.1 weight percent) to provide a solution containing
- 27. 5 weight percent polysulfone. The solution is transported to a holding
tank having a nitrogen atmosphere at about 1. 4 kilograms per square
centimeter. The solution is not heated while in the holding tank and
thus can cool to ambient temperature.
The polyrner solution is pumped from the holding tank to a hollow
fiber spinnerette which is immersed in an aqueous bath at a temperature
of about 4C. The spinnerette has an outer orifice diameter of . 0559 cm,
an inner pin of . 0229 cm and an injection orificé in the pin of . 0127 cm.
The polyrner solution is pumped and metered to the spinnerette at a
rate of about 7. 2 milliliters per minute and is drawn from the spinnerette
at a rate of about 33 meters per minute. The polyrner solution
coagulates in the form of a hollow fiber upon contacting lhe aqueous bath.
Through the injection orifice of the spinnerette is provided distilled water
to coagulate the inside of the hollow fiber. The fiber p~sses through the
aqueous bath for a distance of about one meter. A quantity of the
aqueous bath is continuously purged to maintain a dimethyl acetamide
concentration of less than about 4 weight percent in the bath.
' ' ' '
* Trademark
-106-
.~. ,
. .. .
,
. -
2~3
The fiber is then immersed in a second aqueous bath which is
maintained at a temperature of about 4C for a distance of abDut five
meters. Upon lea-ring the second aqueous bath the fiber contains some
dim ethylacetami de .
The fiber from the second aqueous bath is immersed in two
additional aqueous baths at room temperature, each for a distance
of about five meters, and the fiber is wound on a bobbin under only
sufficient tension to effect the winding. The fiber is maintained wet
with water during winding, and after winding the bobbin is immersed in
an aqueous vat and stored at room ternperature. Thereafter, the fiber
is dried under ambient conditions, preferably at about 20~C and 50
percent relative humidity. The dried fiber is then coated with a solution
of about 5 percent dimethylsiloxane-containing silicone rubber prepolymer
(Sylgard 184*available from Dow Corning) and a curing agent in
n-pentane. The application of the coating is conducted by immersing
the fiber in the prepolymer solution while maintaining the solution under
a positive pressure. The fiber is allowed to air dry and cross-link to
provide the silicone rubber coating.
* Trademark
`~ -107-
7Z03
Table XVI - Coating Procedure&
A. The porous hollow fi~er membrane was dipped in undiluted
liquid coating material, The excess liquid was allow-ed to
dr;p off.
B. The por~us hollow fiber mem~rane was dipped in undiluted
liquid coating material while a vacuu~l was applied to the bore
of the porous hollow fi~er~ After the fiber was removed the
vacuum was broken and excess liquid was allowed to drip off.
C. The porous hollow fiber memhrane was dipped in liquid
coating material diluted with a hydrocarbon solvent. The
solvent was allowed to evaporate~
D. The porous hollow fïber membrane was dipped in liquid
coating material with a hydrocarbon solvent~ while a vacuum
was applied to the bore of the hollow fiher. After the fiber
was removed the vacuum wa~ broken and the solvent was allowed
to evaporate.
E. The porous hollow fiber membrane was dipped in a solution
containing coating material in the form of a polymerizable
prepolymer, appropriate curing a~ent~ and hydrocarbon solvent.
The solvent was allowed -to evaporate and the membrane prepolymer
was cured in place.
F. The coating procedure was used as described in E~ except
that a vacuum was applied to the bore of the hollow fiher
while it was dipped in the coating solution.
-108-
., .
