U.S. patent application number 10/032315 was filed with the patent office on 2003-07-17 for crosslinked and crosslinkable hollow fiber mixed matrix membrane and method of making same.
Invention is credited to Koros, William J., Miller, Stephen J., Staudt-Bickel, Claudia, Vu, De Q., Wallace, David, Wind, John.
Application Number | 20030131731 10/032315 |
Document ID | / |
Family ID | 21864273 |
Filed Date | 2003-07-17 |
United States Patent
Application |
20030131731 |
Kind Code |
A1 |
Koros, William J. ; et
al. |
July 17, 2003 |
Crosslinked and crosslinkable hollow fiber mixed matrix membrane
and method of making same
Abstract
A composition of and a method of making high performance mixed
matrix hollow fiber membranes is described. The membranes have a
high resistance to plasticization by use of a predetermined amount
of crosslinking. The preferred polymer material for the membrane is
a polyimide polymer contineous phase comprising ester crosslinks
and a molecular sieve material dispersed within the polymer
contineous phase. The resultant mixed matrix hollow fiber membrane
exhibits a high permeability of CO.sub.2 in combination with a high
CO.sub.2/CH.sub.4 selectivity. Another embodiment provides a method
of making the mixed matrix hollow fiber membrane from a
monesterified polymer followed by final crosslinking after hollow
fiber formation.
Inventors: |
Koros, William J.; (Atlanta,
GA) ; Wallace, David; (Dunwoody, GA) ; Miller,
Stephen J.; (San Francisco, CA) ; Staudt-Bickel,
Claudia; (Heidelberg, DE) ; Wind, John;
(Austin, TX) ; Vu, De Q.; (San Pablo, CA) |
Correspondence
Address: |
David M. Tuck
Chevron Corporation
P. O. Box 6006
San Ramon
CA
94583-0806
US
|
Family ID: |
21864273 |
Appl. No.: |
10/032315 |
Filed: |
December 20, 2001 |
Current U.S.
Class: |
96/10 |
Current CPC
Class: |
Y10S 55/05 20130101;
B01D 71/64 20130101; Y02C 20/20 20130101; B01D 53/228 20130101;
B01D 2313/18 20130101; B01D 63/021 20130101; B01D 2323/30 20130101;
B01D 69/08 20130101; B01D 63/02 20130101; B01D 69/141 20130101;
B01D 53/22 20130101; B01D 67/0093 20130101; B01D 71/82
20130101 |
Class at
Publication: |
96/10 |
International
Class: |
B01D 053/22 |
Claims
What is claimed is:
1. A hollow fiber polymer membrane, comprising: a) a crosslinked
polymer continuous phase; and b) a molecular sieve material
dispersed within said continuous phase; wherein said membrane has a
CO.sub.2 permeability of at least 20 barrers and a
CO.sub.2/CH.sub.4 selectivity of greater than 30, at 35 degrees C.
and a pressure of 100 psia.
2. A hollow fiber mixed matrix polymer membrane, comprising: a) a
continuous phase polymer comprising a polyimide having
crosslinkable sites; and b) a molecular sieve material dispersed
within said continuous phase polymer; wherein the ratio of
crosslinkable sites to imide groups is between 3:8 and 1:16.
3. A hollow fiber mixed matrix polymer membrane material,
comprising: a molecular sieve dispersed within a continuous polymer
phase wherein said continuous polymer phase comprises a polyimide
polymer made from the monomers A+B+C; where A is a dianhydride of
the formula; 8where X.sub.1 and X.sub.2 are independently a
halogenated alkyl group, phenyl or halogen; where R.sub.1, R.sub.2,
R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are independently H, alkyl,
or halogen; where B is a diamino cyclic compound without a
carboxylic acid functionality; where C is a diamino cyclic compound
with a carboxylic acid functionality; and wherein the ratio of B to
C is between 1:4 and 8:1.
4. The hollow fiber polymer membrane of claim 1 wherein the
molecular sieve material dispersed within said continuous phase has
an average particle size of less than about 1 micron.
5. The hollow fiber polymer membrane material of claim 3 where
X.sub.1 and X.sub.2 are CF.sub.3.
6. The hollow fiber polymer membrane material of claim 3 where
R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are H.
7. The hollow fiber polymer membrane material of claim 3 wherein
the dianhydride is 6FDA.
8. The hollow fiber polymer membrane material of claim 3 wherein C
is DABA.
9. The hollow fiber polymer membrane material of claim 3 wherein B
is a diamino aromatic compound.
10. The hollow fiber polymer membrane material of claim 3 wherein B
is a diamino benzene compound having one or more methyl groups
attached to the benzene ring.
11. The hollow fiber polymer membrane material of claim 3 wherein
the ratio of B to C is between 17:3 and 3:2.
12. The hollow fiber polymer membrane material of claim 3 wherein
the ratio of B to C is between 17:3 and 3:1.
13. The hollow fiber polymer membrane material of claim 3 wherein
said membrane is subjected to esterification conditions in the
presence of a diol selected from the group consisting of ethylene
glycol, propylene glycol, 1,3 propanediol, 1,4 butanediol, 1,2
butanediol, benzenedimethanol, and 1,3 butanediol to form a hollow
fiber polymer membrane monoester.
14. The hollow fiber polymer membrane material of claim 13 wherein
at least 60% of the carboxylic acid functionality is converted to a
monoester.
15. The hollow fiber polymer membrane material of claim 13 wherein
the hollow fiber membrane monoester is subjected to
transesterification conditions to form a crosslinked hollow fiber
polymer membrane.
16. The hollow fiber polymer membrane of claim 1 wherein the
molecular sieve material dispersed within said continuous phase has
an average particle size of less than about 0.1 micron.
17. The hollow fiber polymer membrane material of claim 3 wherein B
is diamino durene.
18. The hollow fiber polymer membrane of claims 1, 2 and 3 wherein
the molecular sieve is a zeolite.
19. The hollow fiber polymer membrane of claims 1, 2 and 3 wherein
the molecular sieve is carbon molecular sieve.
20. The hollow fiber polymer membrane of claims 1, 2 and 3 wherein
the molecular sieve is a aluminophosphate zeolite.
21. The hollow fiber polymer membrane of claims 1, 2 and 3 wherein
the molecular sieve is a borosilicate.
22. The hollow fiber polymer membrane of claims 1, 2 and 3 wherein
the molecular sieve is a SAPO.
23. A method of making a crosslinked hollow fiber membrane,
comprising: preparing a continuous phase polyimide polymer
comprising a predetermined quantity of crosslinkable sites and a
molecular sieve material dispersed within said continuous phase
polymer; forming a hollow fiber from said continuous phase
polyimide polymer; treating the hollow fiber with a diol selected
from the group consisting of ethylene glycol, propylene glycol, 1,3
propanediol, 1,4 butanediol, 1,2 butanediol, and 1,3 butanediol, at
esterification conditions, to form a monoesterified hollow fiber;
and subjecting the monoesterified hollow fiber to
transeesterification conditions to form a crosslinked hollow fiber
membrane.
24. A method of making a crosslinked hollow fiber membrane,
comprising: preparing a polyimide polymer comprising a
predetermined quantity of crosslinkable sites and a molecular sieve
material dispersed within said continuous phase polymer; treating
the polyimide polymer with a diol selected from the group
consisting of ethylene glycol, propylene glycol, 1,3 propanediol,
1,4 butanediol, 1,2 butanediol, and 1,3 butanediol, at
esterification conditions, to form a monoesterified membrane
material; forming a monoesterified hollow fiber from the
monoesterified membrane material; and subjecting the monoesterified
hollow fiber to transesterification conditions to form a
crosslinked hollow fiber membrane.
25. The hollow fiber polymer membrane material of claim 13 wherein
at least 80% of the carboxylic acid functionality is converted to a
monoester.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the separation of mixtures
using polymer membranes.
BACKGROUND
[0002] Polymer membranes have been proposed for various
separations. It has been found that different molecules can be made
to diffuse through selected polymers differently. For example if
one component of a mixture is found to diffuse though a polymer
rapidly and a second component is found to diffuse through the
polymer very slowly or not at all, the polymer may be utilized to
separate the two components. Polymer membranes potentially can be
used for gas separations as well as liquid separations.
[0003] Polymeric membrane materials have been found to be of use in
gas separations. Numerous research articles and patents describe
polymeric membrane materials (e.g., polyimides, polysulfones,
polycarbonates, polyethers, polyamides, polyarylates,
polypyrrolones, etc.) with desirable gas separation properties,
particularly for use in oxygen/nitrogen separation (See, for
example, Koros et al., J. Membrane Sci., 83, 1-80 (1993), the
contents of which are hereby incorporated by reference, for
background and review).
[0004] The polymeric membrane materials are typically used in
processes in which a feed gas mixture contacts the upstream side of
the membrane, resulting in a permeate mixture on the downstream
side of the membrane with a greater concentration of one of the
components than the composition of the original feed gas mixture. A
pressure differential is maintained between the upstream and
downstream sides, providing the driving force for permeation. The
downstream side can be maintained as a vacuum, or at any pressure
below the upstream pressure.
[0005] The membrane performance is characterized by the flux of a
gas component across the membrane. This flux can be expressed as a
quantity called the permeability (P), which is a pressure- and
thickness-normalized flux of a given component. The separation of a
gas mixture is achieved by a membrane material that permits a
faster permeation rate for one component (i.e., higher
permeability) over that of another component. The efficiency of the
membrane in enriching a component over another component in the
permeate stream can be expressed as a quantity called selectivity.
Selectivity can be defined as the ratio of the permeabilities of
the gas components across the membrane (i.e., P.sub.A/P.sub.B,
where A and B are the two components). A membranes permeability and
selectivity are material properties of the membrane material
itself, and thus these properties are ideally constant with feed
pressure, flow rate and other process conditions. However,
permeability and selectivity are both temperature-dependent. It is
desired to develop membrane materials with a high selectivity
(efficiency) for the desired component, while maintaining a high
permeability (productivity) for the desired component.
[0006] The relative ability of a membrane to achieve the desired
separation is referred to as the separation factor or selectivity
for the given mixture. There are however several other obstacles to
use of a particular polymer to achieve a particular separation
under any sort of large scale or commercial conditions. One such
obstacle is permeation rate. One of the components to be separated
must have a sufficiently high permeation rate at the preferred
conditions or else extraordinarily large membrane surface areas are
required to allow separation of large amounts of material. Another
problem that can occur is that at conditions where the permeability
is sufficient, such as at elevated temperatures or pressures, the
selectivity for the desired separation can be lost or reduced.
Another problem that often occurs is that over time the permeation
rate and/or selectivity is reduced to unacceptable levels. This can
occur for several reasons. One reason is that impurities present in
the mixture can over time clog the pores, if present, or
interstitial spaces in the polymer. Another problem that can occur
is that one or more components of the mixture can alter the form or
structure of the polymer membrane over time thus changing its
permeability and/or selectivity. One specific way this can happen
is if one or more component of the mixture causes plasticization of
the polymer membrane. Plasticization occurs when one or more of the
components of the mixture acts as a solvent in the polymer often
causing it to swell and lose its membrane properties. It has been
found that polymers such as polyimides which have particularly good
separation factors for separation of mixtures comprising carbon
dioxide and methane are prone to platicization over time thus
resulting in decreasing performance of the membranes made from the
polyimides.
[0007] The present invention overcomes some of the problems of the
prior art membranes by providing a polymer membrane and a method of
making said polymer membrane that has the following properties/
advantages:
[0008] a) Excellent selectivity and permeability,
[0009] b) Sustained selectivity over time by resistance to
plasticization, and
[0010] c) Very large useable surface area by use of hollow
fibers.
SUMMARY
[0011] As discussed above the present invention seeks to provide a
membrane and method of making the membrane that achieves the result
of providing a commercially viable polymer membrane that overcomes
some of the drawbacks of the prior art membranes. The membranes of
the present invention can have very large available surface areas
using hollow fiber technology. The membranes of the present
invention also have a very high selectivity at a very high
permeability. The membranes of the present invention also are quite
resistant to plasticization and maintain their selectivity and
permeability properties over time as is required in commercial
applications of this technology. The membrane of the present
invention achieves this result by providing a predetermined number
crosslinkable sites in the polymer chain and by crosslinking the
polymer membrane using selected crosslinkers. The crosslinkable
polymer and crosslinked polymer is achieved without degradation of
the imide function (i.e. without altering the polyimide
structure.)
[0012] In one embodiment of the present invention a hollow fiber
mixed matrix polymer membrane is provided, comprising; a
crosslinked polymer continuous phase, and a molecular sieve
material dispersed within the polymer continuous phase. The
resulting membrane can have a CO.sub.2 permeability of at least 20
barrers and a CO.sub.2/CH.sub.4 selectivity of greater than 30,
when measured at 35 degrees C. and a pressure of 100 psia.
[0013] In an alternative embodiment of the present invention a
hollow fiber mixed matrix polymer membrane is provided, comprising:
a crosslinked polyimide polymer continuous phase and a molecular
sieve material dispersed within the polymer continuous phase having
a ratio of crosslinkable sites to imide groups of between 3:8 and
1:16. It has been found that too much crosslinking can cause the
hollow fiber polymer to be fragile and can also experience
performance problems. Too little crosslinking can lead to
plasticization of the polymer membrane over time resulting in
deteriorating performance and loss of selectivity.
[0014] In another alternative embodiment of the present invention a
hollow fiber mixed matrix polymer membrane is provided, comprising
a molecular sieve dispersed within a continuous polymer phase
wherein said continuous polymer phase comprises a polyimide polymer
made from the monomers A+B+C;
[0015] where A is the dianhydride of the formula; 1
[0016] where X.sub.1 and X.sub.2 are the same or different
halogenated alkyl group, phenyl or halogen;
[0017] where R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and
R.sub.6 are H, alkyl, or halogen;
[0018] where B is a diamino cyclic compound without a carboxylic
acid functionality;
[0019] where C is a diamino cyclic compound with a carboxylic acid
functionality; and
[0020] wherein the ratio of B to C is between 1:4 and 8:1.
[0021] A particularly preferred embodiment of the present invention
relates to using the crosslinked mixed matrix hollow fiber polymer
membrane of the present invention for the separation of carbon
dioxide (CO.sub.2) from methane (CH.sub.4). In particular this
embodiment of the invention relates to the removal of CO.sub.2 from
natural gas comprising CO.sub.2, CH.sub.4, and other light
gasses.
[0022] Among other factors the present invention provides the
composition of and the method of making a highly effective
polymeric membrane for the separation of mixtures. The invention
utilizes crosslinking of the mixed matrix polymer membrane to help
achieve the high selectivity required to make the separation
efficiently and to maintain the high selectivities and other
properties even after being exposed to extreme conditions such as
high temperatures and pressures. The invention also shows that
plasticization of the mixed matrix polymer membrane can be avoided
by appropriate degrees of crosslinking and appropriate selection of
the crosslinking units. It has also been determined that too much
crosslinking can lead to hollow fibers that are brittle and subject
to failure. The present invention has thus achieved a hollow fiber
polymer membrane that is both highly selective and highly permeable
for the preferred separations while also being stable and durable
for long term use in a commercial separation process at actual
working conditions. The present invention also provides a method of
making a mixed matrix hollow fiber polymer membrane precursor
material that is not excessively fragile allowing effective
spinning.
[0023] A preferred method for preparing hollow fibers is to
dissolve the polymer in a solvent or melt the polymer, and extrude
the polymer through a tubular capillary nozzle with a core fluid
used for the purpose of retaining the hollow fiber shape.
[0024] Any mixture of fluids that differ in size or other
properties, for example nitrogen and oxygen or ethylene and ethane,
may be separated using the membranes described herein. In one
embodiment, a gaseous mixture containing methane and carbon dioxide
can be enriched in methane by a gas-phase process through the
membrane. In other embodiments, the membranes can be used to purify
helium, hydrogen, hydrogen sulfide, oxygen and/or nitrogen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 shows the Monoesterification and Transesterification
Reactions.
[0026] FIG. 2 shows the synthesis of the monoester via the acid
chloride copolyimide route.
[0027] FIG. 3 is a proton NMR of an uncrosslinked polymer
(pre-esterification).
[0028] FIG. 4 is a proton NMR of the same polymer as FIG. 3 that
has been monoesterified with 1,4-butanediol.
[0029] FIG. 5 is a proton NMR of a polymer that has been
monoesterified with ethylene glycol.
[0030] FIG. 6 is a Single Fiber Test Module.
[0031] FIG. 7 is a permeation testing system for membrane fiber
modules.
[0032] FIG. 8 shows the CO.sub.2 permeability at increasing
pressures for a crosslinked material.
[0033] FIG. 9 shows the CO.sub.2 permeability at increasing
pressures for a uncrosslinked material.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The present invention relates to a highly durable mixed
matrix hollow fiber membrane exhibiting both high permeability of
CO.sub.2 and high CH.sub.4/CO.sub.2 selectivity and being resistant
to plasticization. Prior membranes have shown a significant decline
in selectivity over time. Not to be limited by theory it is
believed that the selectivity losses associated with exposure to
high levels of CO.sub.2 are the result of plasticization. Carbon
dioxide acts as a strong swelling agent, sorbing into the polymer
matrix and greatly increasing segmental motion. This increased
motion drastically reduces the difference in diffusion rates
between fast and slow gas species. If this swelling and segmental
motion could be limited, the selectivity of the membrane can be
maintained. In the present invention crosslinking has been shown to
reduce or eliminate CO.sub.2 plasticization in dense films. Proper
selection of the method of crosslinking, the chemical structure of
the polymer and crosslink, and proper degree of crosslinking are
important to achieve a hollow fiber membrane that achieves and
maintains superior permeability and selectivity need for a viable
commercial membrane.
[0035] U.S. Pat. No. 5,288,304 teaches generally a method for
preparation of carbon molecular sieve membranes. That disclosure is
incorporated herein by reference in its entirety. Patents that
relates to the separation of fluids by means of a mixed matrix or
composite membrane are U.S. Pat. Nos. 4,740,219 and 4,925,459 which
are incorperated herein by reference in their entirety. A copending
U.S. patent application that discloses a Process for
CO.sub.2/Natural Gas Separation is U.S. Pat. No. 6,299,668 is
incorporated herein by reference.
[0036] The polyimide is derived from a reaction of any suitable
reactants. Reactants can include monomers such as dianhydrides, as
well as tetra carboxylic acids, and furandiones. Other monomers
include diamino compounds, preferably diamino cyclic compounds,
still more preferably diamino aromatics. The diamino aromatics can
include aromatic compounds having more than one aromatic ring where
the amino groups are on the same or different aromatic ring. In the
present invention it is also important for the polyimide to have
incorporated in it a predetermined amount of crosslinkable sites.
These sites may include but are not limited to carboxylic acid
sites, ester functions, --OH groups, unreacted NH.sub.2 groups,
--SH groups, amide functions, and olefins. The preferred
crosslinkable sites in the process of the present invention are
carboxylic acid or ester groups, alcohols, and olefins.
Crosslinking can also be induced by reaction of the imide function
itself to form a crosslinkable site and an amide. This method of
crosslinking will be discussed in more detail later. Another
preferred feature of the process of the present invention is that
the polyimide chains have limited rotational ability. One such
monomer that provides a polyimide chain with limited rotational
ability is: 2
[0037] This dianhydride is known as 6FDA or
4,4'-(hexafluoroisopropylidene- ) diphthalic anhydride, or
(2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride.
[0038] In the process of the present invention a carboxylic acid
functionality is intended to include the acid group itself as well
as acid derivatives such as esters and anhydrides as well as
activated carboxylic acid derivatives such as acid chlorides.
[0039] A preferred monomer for providing the carboxylic acid
functionality in the present invention is diamino benzoic acid:
3
[0040] A particularly preferred monomer is 3,5 diaminobenzoic acid.
4
[0041] The diamino cyclic compounds without a carboxylic acid
functionality can include aromatic compounds having more than one
aromatic ring where the amino groups are on the same or different
aromatic ring. Preferred examples include but are not limited to
4,4' isopropylidene dianiline, 3,3' hexafluoroisopropylidene
dianiline, 4,4' hexafluoroisopropylidene dianiline, 4,4'
oxydianiline, 3,3' oxydianiline and 4,4' diaminodiphenyl. Examples
of diamino aromatic compounds useful in the present invention
include diaminotoluene, diaminobenzotrifluoride, and di, tri, and
tetramethyidiaminobenzene.
[0042] The polymer membranes of the present invention can be used
for gas/gas separations, gas/liquid separations, liquid/liquid
separations, and liquid/solid separations.
[0043] As mentioned above one of the preferred crosslinkable sites
comprise carboxylic acid or esters or activated carboxylic acid
derivitives. Crosslinking groups or agents that have been found to
be useful in conjunction with the carboxylic acid functional sites
include: diols selected from the group consisting of ethylene
glycol, propylene glycol, 1,3 propanediol, 1,4 butanediol, 1,2
butanediol, benzenedimethanol, and 1,3 butanediol. Preferred
crosslinking agents include ethylene glycol, propylene glycol, 1,3
propanediol, and benzenedimethanol. Still more preferred
crosslinking agents are ethylene glycol, and 1,3 propanediol. It
has been found that having too long a crosslinking group can have
an undesirable impact on the permeability and/or selectivity of the
polymer however too short a crosslink can also have a negative on
the finished hollow fiber membrane. The most preferred crosslinking
agents for crosslinking carboxylic acid or ester sites is 1,3
propanediol.
[0044] Crosslinking can occur by the condensation reaction of
selected diol or diols with the crosslinkable acid functionality.
In the process of the present invention it has been found that
reaction of less reactive crosslinking agents can be facilitated by
activation of the carboxylic acid site on the polymer chain. One
way to do this is by converting the acid group to the corresponding
acid chloride. This can be effectively done by the use of thionyl
chloride. A method for this activation will be discussed in more
detail in the examples.
[0045] In a preferred embodiment of the process of the present
invention crosslinking can be achieved in a stepwise fashion by
first monoesterification of the acid function with the selected
diol or diols, followed by transesterification of the monoester to
the diester. (See FIG. 1)
[0046] In a particularly preferred embodiment of the present
invention the monoesterified polymer is spun into the hollow fiber
prior to transesterification to form the crosslinked hollow fiber
membrane. There are significant advantages to this process in
particular the monoester polymer can be more easily spun without
breaking or forming defects.
[0047] Alcohol or --OH groups can provide crosslinkable sites in
the present invention. Crosslinking groups useable with alcohol
crosslinkable sites include dicarboxylic acids, anhydrides, and
diesters. Examples of dicarboxylic acids useful as crosslink groups
include but are not limited to oxalic acid, malonic acid, succinic
acid, methylsuccinic acid, glutaric acid, and adipic acid. Non
limiting examples of anhydrides that may be used include maleic
anhydride, succinic anhydride, and methylsuccinic anhydride. Non
limiting examples of diesters are dimethylterephthalate,
dimethylisophthalate, dimethylphthalate, and diesters of the
dicarboxylic acids mentioned above. The dicarboxylic acids and
anhydrides can be reacted with the -OH containing polyimide at
esterification to form a crosslink. Likewise the diesters discussed
above can subjected to transesterification conditions in the
presence of the --OH containing polyimide to form the desired ester
crosslink.
[0048] In a preferred embodiment of the present invention the --OH
containing polyimide is subjected to monoesterification conditions
in the presence of one or more of the crosslinking groups to form a
monoesterified polyimide. It has been found that the monoesterified
polyimide can then be made into a hollow fiber as described in
detail later in this patent application. The hollow fiber can then
be subjected to transesterification conditions after hollow fiber
formation to form the crosslinked hollow fiber polymer
membrane.
[0049] Examples of reactants that can be used to provide a --OH
containing polyimide include diaminobenzyl alcohol,
diaminocyclohexanol, and other diaminoalcohols. 5
[0050] In some cases it may be preferable to protect the --OH
function prior to formation of the polyimide. This may be done by
conventional chemical means such as by masking the -OH group off as
an ether. The masked --OH group may then be hydrolyzed back to a
functional --OH group prior to crosslinking or prior to the
extrusion of the hollow fiber. 6
[0051] Also mentioned above are crosslinkable sites comprising
olefins. Crosslinking groups useable with olefins include but are
not limited to sulfur, divinylbenzene. Sulfur as a crosslinking
agent is thought to form a disulfide crosslink when reacted with an
olefin.
[0052] A particularly preferred diamino group that can be used to
make a crosslinkable polyimide polymer is diaminobenzoic acid
(DABA). The most preferred isomer of DABA is 3,5 diaminobenzoic
acid.
[0053] In an alternative embodiment of the present invention a
crosslinking-like effect can be achieved simply by the presence of
crosslinkable groups in the polymer chain. Crosslinkable groups
such as carboxylic acid functions can have an effect very similar
to actual covalent crosslinking. This effect can be referred to as
pseudocrosslinking. Not to be limited by theory pseudocrosslinking
is thought to occur because the crosslinkable groups can provide a
weak attractive interaction between polymer chains that behaves
similarly to actual crosslinking. The interaction can be a weak
ionic bond, hydrogen bond or Van der Waals forces. These weak
interactions cause the polymer to be weakly crosslinked.
[0054] In another alternative embodiment of the present invention
only a fraction of the available crosslinkable sites are actually
crosslinked. In this embodiment the resultant polymer membrane has
a combination of true crosslinks and pseudocrosslinks. Such a
combination can have processing and durability advantages.
[0055] In yet another embodiment of the present invention the
crosslinkable sites are selected such that some of said sites
interact with the molecular sieve material such that a weak bond is
formed between the polymer and the sieve via the crosslinkable
sites. In this way the crosslinkable site serves at least two
roles. It provides a site to facilitate crosslinking of the polymer
chains and it aids in making the polymer more compatible with the
molecular sieve. Thus the crosslinkable group acts as a self
primer.
[0056] Molecular Sieve Selection
[0057] One type of molecular sieving entity or particles useful in
the present invention is a carbon-based molecular sieve. The
molecular sieves used in the mixed matrix membranes described
herein are can be prepared by pyrolyzing polymeric particles.
Alternatively, the molecular sieve particles may first be prepared
by pyrolyzing a polymeric film or other continuous polymeric body.
For large scale production of carbon molecular sieve particles, it
is more difficult to form and subsequently pyrolyze polymeric films
than to use a powdered polymer as the material to be pyrolyzed.
U.S. Pat. No. 4,685,940, the contents of which are hereby
incorporated by reference in their entirety, discloses carbon
membranes for use in separation processes. The polymer used to
prepare the carbon membrane can be used to prepare the powder used
herein to prepare the molecular sieves particles that are
incorporated into the mixed matrix membranes. The resulting
particles have a predetermined pore size and function as molecular
sieves. Particles formed from the pyrolyzed polymer function well
even at elevated temperatures. The carbon particles are ideally
produced by the controlled pyrolysis of a suitable powder of
polymeric material under conditions that retain the basic integrity
of the original geometry. If the powder is not already in the right
size range, the pyrolyzed material is milled to a desired size
range.
[0058] Suitable materials include polyimides, polyamides, cellulose
and derivatives thereof, thermosetting polymers, acrylics,
pitch-tar mesophase, and the like. These materials are not
limiting, as other materials may be useful for fabricating carbon
membranes. Selection of the polymeric material for use in preparing
a powder to be pyrolyzed to form sieve particles 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. Especially preferred carbon molecular sieve particles
are those prepared from the pyrolysis of aromatic polyimides or
cellulosic polymers. Examples of aromatic polyimides are described,
for example, in U.S. Pat. Nos. 5,234,471 and 4,690,873. Other
patents describing useful polymers which can be subjected to
pyrolysis include European Patent Application 0 459 623. The
contents of each of these patents are hereby incorporated by
reference.
[0059] The pyrolysis can be generally effected in a wide range of
temperatures, between the decomposition temperature of the
carbonaceous material and the graphitization temperature (about
3000.degree. C.). Generally, pyrolysis will be effected in the
range of from 250 to 2500.degree. C., a preferred range being about
450 to about 800.degree. C.
[0060] The carbon membranes contain pores larger than the
ultramicropores required for the molecular sieving process. It is
believed that these larger pores connect the ultramicropores that
perform the molecular sieving process and allow for high
productivities in the dry membrane state. Generally, the higher the
final temperature used for the pyrolysis of the polymer, the
smaller the pores of the product, and thus the smaller the
molecules that can permeate through such membranes.
[0061] The pyrolysis of suitable precursors, generally under
conditions conventionally used for the production of carbon fibers,
results in a product that has a certain microporosity of molecular
dimensions which is responsible for the molecular sieve properties
of the carbons.
[0062] During the pyrolysis process, the heating is preferably
effected under a vacuum or inert gas (e.g., nitrogen, argon)
atmosphere. Controlled thermal degradation of the polymer precursor
results in a pore opening, and thus pre-determined pore-size ranges
can be obtained, suitable for the intended separation process.
[0063] For the intended use, it is advantageous to obtain fluid
separation membranes having pore size and a pore size distribution
that effectively separate specific mixtures of fluids. Generally, a
pore size distribution of 3-10 Angstroms is suitable and 3-5
Angstroms is preferable for gas separations.
[0064] The carbon molecular sieves (CMS) or particles of the
present invention are believed to be nano- and microporous
materials that have distributions of pore sizes and interconnected
channels that enable fast transport of gas molecules. Within the
distribution of pore sizes are constricted, ultramicroporous pore
openings with dimensions that are of the same order of magnitude as
molecular sizes of gas molecules. It is generally believed that
"ultramicropores" (less than 5 Angstroms) perform the molecular
sieving (size-selective) process in carbon molecular sieve
materials, while larger "micropores" (6 to 10 Angstroms) connecting
ultramicropores provide sorption cavities and allow for high fluxes
of gas penetrants by promoting larger average diffusional jumps.
Thus, the porous nature of carbon molecular sieves of the present
invention provides their capability for high gas permeabilities,
yet their molecular sieving morphology permits precise
discrimination of gas penetrants to yield highly selective
membranes.
[0065] Zeolites are another class of molecular sieve that can be
used in making the mixed matrix membranes of the present invention.
Small, medium, and large pore zeolites can be used in mixed matrix
membranes depending on the intended separation. In the lists that
follow zeolite structure types will be identified by their
structure type code as assigned by the IZA Structure Commisission
following the rules set up by the IUPAC Commission on Zeolite
Nomenclature. Each unique framework topology is designated by a
structure type code consisting of 3 capital letters.
[0066] Examples of useful small pore size zeolites include but are
not limited to the following types:
[0067] ABW, AEI, AFT, AFX, APC, APD, ATN, ATT, ATV, AWW, BIK, BRE,
CAS, CHA, DDR, EAB, EDI, ERI, GIS, GOO, ITE, JBW, KFI, LEV, LTA,
MER, MON, NAT, PAU, PHI, RHO, RTE, RTH, THO, VNI, YUG, ZON.
Examples of specific small pore zeolites that can be used are Linde
Type A, Chabazite, Erionite, and SAPO-56.
[0068] Medium pore zeolite types that may be used in the process of
the present invention include but are not limited to:
[0069] AEL, AFO, AHT, CGF, DAC, EPI, EUO, FER, HEU, LAU, MEL, MFI,
MFS, MTT, NES, PAR, SFF, STF, STI, TER, TON, WEI, and WEN.
[0070] Examples of specific useful intermediate pore size zeolites
include but are not limited to ZSM-5, ZSM-1 1, ZSM-22, ZSM-23,
ZSM-35, ZSM-48, ZSM-57, SUZ-4, SSZ-23; SSZ-25, SSZ-28, SSZ-32,
SSZ-36, NU-87, and silicalite. ZSM-5 is described in U.S. Pat. No.
Re. 29,948 (of original U.S. Pat. No. 3,702,886). ZSM-11 is
described in U.S. Pat. No. 3,709,979. ZSM-22 is described in U.S.
Pat. No. 4,556,477. ZSM-23 is described in U.S. Pat. No. 4,076,842.
ZSM-35 is described in U.S. Pat. No. 4,016,245. ZSM48 is described
in U.S. Pat. No. 4,585,747. SUZ-4 is described in EP Application
No. 353,915. SSZ-23 is described in U.S. Pat. No. 4,859,422. SSZ-25
is described in U.S. Pat. Nos. 4,827,667 and 5,202,014. SSZ-28 is
described in U.S. Pat. No. 5,200,377. SSZ-32 is described in U.S.
Pat. No. 5,053,373. SSZ-36 is described in U.S. Ser. No.
60/034,252. Silicalite is a hydrophobic crystalline silica-based
molecular sieve which has been developed and patented (see U.S.
Pat. No. 4,061,724) to Gross et al.). A detailed discussion of
silicalite may be found in the article "Silicalite, A New
Hydrophobic Crystalline Silica Molecular Sieve"; Nature, Vol. 271,
Feb. 9, 1978, incorporated herein by reference. The entire contents
of all these patents and patent applications are incorporated
herein by reference.
[0071] Large pore zeolite types that may be used in the process of
the present invention include but are not limited to:
[0072] AFI, AFR, AFS, AFY, ATO, ATS, BEA, BOG, BPH, CAN, CON, CZP,
DFO, EMT, FAU, GME, LTL, MAZ, MEI, MOR, MTW, OFF, OSI, RON, SAO,
and VET.
[0073] Examples of specific useful intermediate pore size zeolites
include but are not limited to Linde Type L, Beta zeolite, CIT-1,
Faujasite, Mazzite, Mordenite, ZSM-12, and Offretite.
[0074] Very large pore zeolite types that may be used in the
process of the present invention include but are not limited
to:
[0075] CLO, VFI, AET, and CFI.
[0076] Specific examples of very large pore zeolites include
Cloverite and CTI-5.
[0077] Cation modification of zeolites can be used to affect the
separation characteristics of the zeolite. Such cation modification
includes ion exchange where sodium or potassium ions in the zeolite
are replaced with other ions such as barium, calcium, cesium, or
any other selected exchangeable ion. This can be done to adjust the
adsorption characteristics of the zeolite thus increasing the
selectivity.
[0078] The average particle size of the molecular sieve useful in
the present invention is less than 5 microns, more preferably less
than 1 micron, still more preferably less than 0.1 microns. Smaller
particle size facilitates bonding between the molecular sieve and
the polymer. The zeolite particle size can be reduced after
synthesis such as by high shear wet milling. Prior to membrane
formation, the zeolite may be silanated, either during wet milling
or separately.
[0079] It is believed that silanation permits improved bonding
between the zeolite outer surface and the polymer. Suitable silane
compounds include 3-aminopropyldimethylethoxysilane and
3-isocyanopropyldimethylchlorosilan- e. Silanation can be carried
out, for example, by mixing the zeolite in an ethanol/water mixture
containing the silane compound for a period of time (a few minutes
up to a few hours), then recovering the treated zeolite and washing
with ethanol to remove excess silane.
[0080] Other types of molecular sieves that are useable in the
present invention include borosilicate, silico-aluminophosphate
(SAPO), aluminophosphate (ALPO), and other zeolite-like molecular
sieves. These zeolite-like molecular sieves can have structures
similar to the aluminosilicate zeolites discussed above.
[0081] Polymer Selection
[0082] An appropriately selected polymer can be used which permits
passage of the desired gases to be separated, for example carbon
dioxide and methane. Preferably, the polymer permits one or more of
the desired gases to permeate through the polymer at different
diffusion rates than other components, such that one of the
individual gases, for example carbon dioxide, diffuses at a faster
rate through the polymer. In a preferred embodiment, the rate at
which carbon dioxide passes through the polymer is at least 10
times faster than the rate at which methane passes through the
polymer.
[0083] It is preferred that the membranes exhibit a carbon
dioxide/methane selectivity of at least about 5, more preferably at
least about 10, and most preferably at least about 30. Preferably,
the polymer is a rigid, glassy polymer as opposed to a rubbery
polymer or a flexible glassy polymer. Glassy polymers are
differentiated from rubbery polymers by the rate of segmental
movement of polymer chains. Polymers in the glassy state do not
have the rapid molecular motion that permit rubbery polymers their
liquid-like nature and their ability to adjust segmental
configurations rapidly over large distances (>0.5 nm). Glassy
polymers exist in a non-equilibrium state with entangled molecular
chains with immobile molecular backbones in frozen conformations.
The glass transition temperature (Tg) is the dividing point between
the rubbery or glassy state. Above the Tg, the polymer exists in
the rubbery state; below the Tg, the polymer exists in the glassy
state. Generally, glassy polymers provide a selective environment
for gas diffusion and are favored for gas separation applications.
Rigid, glassy polymers describe polymers with rigid polymer chain
backbones that have limited intramolecular rotational mobility and
are often characterized by having high glass transition
temperatures (Tg>150.degree. C.).
[0084] In rigid, glassy polymers, the diffusive selectivity tends
to dominate, and glassy membranes tend to be selective in favor of
small, low-boiling molecules. The preferred membranes are made from
rigid, glassy polymer materials that will pass carbon dioxide
preferentially over methane and other light hydrocarbons. Such
polymers are well known in the art and are described, for example,
in U.S. Pat. Nos. 4,230,463 to Monsanto and 3,567,632 to DuPont.
Suitable membrane materials include polyimides, polysulfones and
cellulosic polymers among others.
[0085] Examples of suitable polymers useable as either the membrane
material or the porous support include substituted or unsubstituted
polymers and may be selected from polysulfones; poly(styrenes),
including styrene-containing copolymers such as
acrylonitrilestyrene copolymers, styrene-butadiene copolymers and
styrene-vinylbenzylhalide copolymers; polycarbonates; cellulosic
polymers, such as cellulose acetate-butyrate, cellulose propionate,
ethyl cellulose, methyl cellulose, nitrocellulose, etc.; polyamides
and polyimides, including aryl polyamides and aryl polyimides;
polyethers; polyetherimides; polyetherketones; polyethersulfones;
poly(arylene oxides) such as poly(phenylene oxide) and poly(xylene
oxide); poly(esteramide-diisocyanate); polyurethanes; polyesters
(including polyarylates), such as polyethylene terephthalate,
poly(alkyl methacrylates), poly(acrylates), poly(phenylene
terephthalate), etc.; polypyrrolones; polysulfides; polymers from
monomers having alpha-olefinic unsaturation other than mentioned
above such as poly (ethylene), poly(propylene), poly(butene-1),
poly(4-methyl pentene-1), polyvinyls, e.g., poly(vinyl chloride),
poly(vinyl fluoride), poly(vinylidene chloride), poly(vinylidene
fluoride), poly(vinyl alcohol), poly(vinyl esters) such as
poly(vinyl acetate) and poly(vinyl propionate), poly(vinyl
pyridines), poly(vinyl pyrrolidones), poly(vinyl ethers),
poly(vinyl ketones), poly(vinyl aldehydes) such as poly(vinyl
formal) and poly(vinyl butyral), poly(vinyl amides), poly(vinyl
amines), poly(vinyl urethanes), poly(vinyl ureas), poly(vinyl
phosphates), and poly(vinyl sulfates); polyallyls;
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-sulfophenylmethallyl ethers; 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.
[0086] Preferred polymers useable in the hollow fiber membrane of
the present invention include polyimides. poyletherimides,
polyethersulfones and polysulfones. More preferred polymers useable
in the membrane material of present invention include polyimides.
poyletherimides, and polysulfones made using analogs of 6FDA.
Particularly preferred polyimides useable in the present invention
comprise polyimides or polyetherimides made using 6FDA.
[0087] In a particularly preferred embodiment of the present
invention the hollow fiber polymer membrane is a composite material
comprising a membrane layer comprising an effective skin layer as
well as a porous support. The porous support material can be the
same or different polymer as the membrane. Ideally the porous
support is an inexpensive porous polymer. In a composite hollow
fiber polymer membrane the porous support layer can be either the
inside layer or the outside layer. Most preferably the porous
support layer is the inside layer in this embodiment and the "skin"
layer is on the outside of the hollow fiber. A composite membrane
material is discussed in copending U.S. patent applications Ser.
Nos. 09/834,857 and 09/834,808 which are incorporated herein in
their entirety. A Patent that discusses composite membranes is U.S.
Pat. No. 4,925,459 which is also incorporated herein by reference
in its entirety.
[0088] Molecular Weight of the Polymer
[0089] Another parameter that needs to be controlled in order to
achieve the high permeability, high selectivity hollow fiber
membrane of the present invention is the molecular weight of the
polymer material. Molecular weight of the polymer material can be
critical to forming a hollow fiber membrane that is not too brittle
and has an effective skin layer. Molecular weight of the polymer
material can also be critical in achieving a spinnable dope
solution. A feature of the present invention is that the selection
of polymer having a proper molecular weight (MW) can be important
in the formation of a hollow fiber membrane. It is preferable to
have a MW above the entanglement molecular weight of the polymer.
It has been found that if the molecular weight of the polymer is
too low the polymer is too brittle and a effective skin layer may
not form. If the molecular weight is too high processability can
become difficult. In the present invention it is preferable to have
an average polymer molecular weight of between 40,000 and 200,000,
more preferably between 60,000 and 160,000, still more preferably
between 70,000 and 140,000, and most preferably between 80,000 and
120,000. Not to limited by theory is thought that the MW of the
polymer should be above, ideally well above, the entanglement MW of
the polymer in order to achieve a material that has high strength
and is not brittle. A paper that discusses the effect of molecular
weight on polymer properties such as entanglement is in Fundamental
Principles of Polymeric Materials, SPE Monograph Series 2.sup.nd
ed., John Wiley & Sons, New York: (1982), page 259 written by
Stephen L. Rosen; the contents of which are hereby incorporated by
reference, for background and review.
[0090] It is also believed that the molecular weight of the
polyimide chain can be degraded during the monoesterification
process. A sufficiently high molecular weight polymer should be
used to allow for some loss of MW during the esterification process
yet still be within the desired range after completion. The
molecular weights used in the present application are Number
Average Molecular Weights and can be determined by GPC (Gel
Permeation Chromatography).
[0091] Dispersing and "Sizing" Carbon Molecular Sieve Particles
[0092] In a preferred embodiment, the Carbon Molecular Sieve (CMS)
particles are preconditioned at high temperature in a vacuum oven,
for example at a temperature of about 300.degree. C. under vacuum
for at least 12 hours. After the preconditioning treatment, a
desired quantity of CMS particles can be dispersed into a suitable
solvent, preferably one which can be used to dissolve the desired
organic polymer material that will be the continuous polymer phase
in the eventual mixed matrix membrane. It is particularly preferred
that the desired organic polymer is miscible in this solvent. The
slurry is well agitated and mixed using any suitable method (e.g.,
shaker or two parallel rollers, etc.) for preferably between about
30 minutes to an hour. To enhance dispersal and homogeneity, a
suitable ultrasonic sonicator can be applied to the slurry, for
example, for approximately one minute. In a preferred embodiment, a
long stainless steel rod from an ultrasonic high-frequency source
generator is used to agitate the slurry. The sonication step breaks
up conglomerations of CMS particles.
[0093] The CMS particles can optionally, but preferably, be
"primed" (or "sized") by adding a small amount of the desired
matrix polymer or any suitable "sizing agent" that will be miscible
with the organic polymer to be used for the matrix phase.
Generally, this small amount of polymer or "sizing agent" is added
after the CMS particles have been dispersed in a suitable solvent
and sonicated by the ultrasonic agitator source. Optionally, a
non-polar non-solvent, in which the polymer or "sizing agent" is
insoluble, may be added to the dilute suspension to initiate
precipitation of the polymer onto the carbon particles. The
"primed" carbon particles may be removed through filtration and
dried by any conventional means, for example in a vacuum oven,
prior to re-dispersion in the suitable solvent for casting. The
small amount of polymer or "sizing agent" provides an initial thin
coating (i.e., boundary layer) on the CMS particle surface that
will aid in making the particles compatible with the polymer
matrix.
[0094] In a preferred embodiment, approximately 10% of total
polymer material amount to be added for the final mixed matrix
membrane is used to "prime" the CMS particles. The slurry is
agitated and mixed for preferably between about 6 and 7 hours.
After mixing, the remaining amount of polymer to be added is
deposited into the slurry. The quantity of CMS particles and the
amount of polymer added will determine the "loading" (or solid
particle concentration) in the final mixed matrix membrane. Without
limiting the invention, the loading of CMS particles is preferably
from about 10 vol. % to about 60 vol. %, and more preferably, from
about 20 vol. % to about 50 vol. %. To achieve the desired
viscosity, the polymer solution concentration in the solvent is
preferably from about 5 wt. % to about 25 wt. %. Finally, the
slurry is again well agitated and mixed by any suitable means for
about 12 hours.
[0095] This technique of "priming" the particles with a small
amount of the polymer before incorporating the particles into a
polymer film is believed to make the particles more compatible with
the polymer film. It is also believed to promote greater
affinity/adhesion between the particles and the polymers and may
eliminate defects in the mixed matrix membranes. This "priming"
technique is believed to be generally applicable to particles
placed in polymer membranes, for example those membranes using
zeolites rather than CMS particles. Accordingly, the invention
described herein also includes a general method for "priming"
particles for inclusion in polymer membranes.
[0096] Methods of Forming the Mixed Matrix Membrane
[0097] The mixed matrix membranes are typically formed by casting
the homogeneous slurry containing CMS particles and the desired
polymer, as described above. The slurry can be mixed, for example,
using homogenizers and/or ultrasound to maximize the dispersion of
the particles in the polymer or polymer solution. The casting
process is preferably performed by three steps:
[0098] (1) pouring the solution onto a flat, horizontal surface
(preferably glass surface),
[0099] (2) slowly and virtually completely evaporating the solvent
from the solution to form a solid membrane film, and
[0100] (3) drying the membrane film.
[0101] To control the membrane thickness and area, the solution is
preferably poured into a metal ring mold. Slow evaporation of the
solvent is preferably effected by covering the area and restricting
the flux of the evaporating solvent. Generally, evaporation takes
about 12 hours to complete, but can take longer depending on the
solvent used. The solid membrane film is preferably removed from
the flat surface and placed in a vacuum oven to dry. The
temperature of the vacuum oven is preferably set from about
50.degree. C. to about 110.degree. C. (or about 50.degree. C. above
the normal boiling point of the solvent) to remove remaining
solvent and to anneal the final mixed matrix membrane.
[0102] The final, dried mixed matrix membrane can be further
annealed above its glass transition temperature (T.sub.g). The
T.sub.g of the mixed matrix membrane can be determined by any
suitable method (e.g., differential scanning calorimetry). The
mixed matrix film can be secured on a flat surface and placed in a
high temperature vacuum oven. The pressure in the vacuum oven
(e.g., Thermcraft.RTM. furnace tube) is preferably between about
0.01 mm Hg to about 0.10 mm Hg. Preferably, the system is evacuated
until the pressure is 0.05 mm Hg or lower. A heating protocol is
programmed so that the temperature reaches the T.sub.g of the mixed
matrix membrane preferably in about 2 to about 3 hours. The
temperature is then raised to preferably about 10.degree. C. to
about 30.degree. C., but most preferably about 20.degree. C., above
the T.sub.g and maintained at that temperature for about 30 minutes
to about two hours. After the heating cycle is complete, the mixed
matrix membrane is allowed to cool to ambient temperature under
vacuum.
[0103] The resulting mixed matrix membrane is an effective membrane
material for separation of one or more gaseous components from
gaseous mixtures including the desired component(s) and other
components. In a non-limiting example of use, the resulting
membrane has the ability to separate carbon dioxide from methane,
is permeable to these substances, and has adequate strength, heat
resistance, durability and solvent resistance to be used in
commercial purifications. While not wishing to be bound to a
particular theory, the molecular sieves are believed to improve the
performance of the mixed matrix membrane by including selective
holes/pores with a size that permits carbon dioxide to pass
through, but either not permitting methane to pass through, or
permitting it to pass through at a significantly slower rate. The
molecular sieves should have higher selectivity for the desired gas
separation than the original polymer to enhance the performance of
the mixed matrix membrane. For the desired gas separation in the
mixed matrix membrane, it is preferred that the steady-state
permeability of the faster permeating gas component in the
molecular sieves be at least equal to that of the faster permeating
gas in the original polymer matrix phase. An advantage of the mixed
matrix membranes described herein over membranes including
primarily the continuous carbon-based molecular sieve membrane
films and fibers is that they are significantly less brittle.
[0104] The membranes can be used in any convenient form such as
sheets, tubes or hollow fibers. Hollow fibers can be preferred,
since they provide a relatively large membrane area per unit
volume. Sheets can be used to fabricate spiral wound modules
familiar to those skilled in the art.
[0105] For flat-sheet membranes, the thickness of the mixed matrix
selective layer is between about 0.001 and 0.005 inches, preferably
about 0.002 inches. In asymmetric hollow fiber form, the thickness
of the mixed matrix selective skin layer is preferably about 1000
Angstroms to about 5000 Angstroms. The loading of CMS particles in
the continuous polymer phase is between about 10% and 60%,
preferably about 20% to 50% by volume.
[0106] Mixed Matrix Membrane Enhancement Test For CMS Membranes
[0107] A test can be prepared to verify that the carbon molecular
sieves formed via pyrolysis have been properly and successfully
made to produce mixed matrix membranes with enhanced permeation
properties. This test involves preparation of a sample mixed matrix
membrane film using a test polymer and a specified loading of
carbon molecular sieve particles, and comparing the
CO.sub.2/CH.sub.4 permeation selectivity versus a membrane film of
the same test polymer without added sieve. The CO.sub.2/CH.sub.4
permeation selectivity is determined by taking the ratio of the
permeability of CO.sub.2 over that of CH.sub.4. The permeability of
a gas penetrant i is a pressure- and thickness-normalized flux of
the component through the membrane and is defined by the
expression: 1 P i = N i l p i
[0108] where P.sub.i is permeability of component i, is thickness
the membrane layer, N.sub.l is component i's flux (volumetric flow
rate per unit membrane area) through the to membrane, and
.DELTA.p.sub.l is the partial pressure driving force of component i
(partial pressure difference between the upstream to the
downstream). Permeability is often expressed in the customary unit
of Barrer (1 Barrer=10.sup.-10 cm.sup.3 (STP) .cm/cm.sup.2.s.cm
Hg). Permeability measurements can be made using a manometric, or
constant volume, method. The apparatus for performing permeation
measurements in films are described in O'Brien et al., J. Membrane
Sci., 29, 229 (1986) and Costello et al., Ind. Eng. Chem. Res., 31,
2708 (1992), the contents of which are hereby incorporated by
reference.
[0109] In the Mixed Matrix Enhancement Test, permeation tests of
pure gases of CO.sub.2 and CH.sub.4 are performed on the mixed
matrix membrane. The mixed matrix membrane film is separately
tested with each gas using an upstream pressure of about 50 psia
and a vacuum downstream. A temperature of about 35.degree. C. is
maintained inside the permeation system. Similar permeation tests
of pure gases of CO.sub.2 and CH.sub.4 are performed on a prepared
membrane film of the same test polymer without added sieve
particles. To confirm that the carbon molecular sieves particles
have been properly produced and prepared by the methods described
herein, the mixed matrix membrane film should exhibit a
CO.sub.2/CH.sub.4 selectivity enhancement in the Mixed Matrix
Enhancement Test, of 10% or more over the CO.sub.2/CH.sub.4
selectivity of the pure test polymer membrane alone.
[0110] The method for forming the sample mixed matrix membrane for
use in the Enhancement Test is as follows:
[0111] (1) The CMS fine particles are preconditioned at high
temperature in a vacuum oven at a temperature of about 300.degree.
C. under vacuum for at least 12 hours. After the preconditioning
treatment, these CMS particles can be used to prepare a sample
mixed matrix membrane film. For the purpose of the Enhancement
Test, the CMS particles are dispersed in the solvent
dichioromethane (CH.sub.2Cl.sub.2).
[0112] (2) After dispersal in CH.sub.2Cl.sub.2, the CMS particles
are sonicated in solution for about 1 minute with an ultrasonic rod
in the vial and are well-mixed, as described previously. Large CMS
particles in the slurry are separated from the fine particles by
any conventional means, for example, decantation or centrifugation.
After sonication and isolation of finer CMS particles, the CMS
particles are ready for "priming" (or "sizing") with the matrix
polymer. For the purpose of the Enhancement Test, the polymer to be
used for the matrix phase is Ultem.RTM. 1000 (GE Plastics). Its
chemical structure is shown below. 7
[0113] Prior to use, the Ultem.RTM. 1000 polymer is dried at a
temperature of about 100.degree. C. under vacuum for at least 12
hours in a vacuum oven. For "priming" the CMS particles, typically
10 wt. % of the total amount of matrix polymer (Ultem.RTM. 1000) to
be added to the slurry is used. For the Enhancement Test, it is
desired to prepare the final slurry of CMS particles and polymer
with the following properties: a weight ratio of Ultem.RTM. 1000 to
CMS particles of about 4 to 1 (i.e., a "loading" of about 20 wt. %
of CMS particles in the final mixed matrix membrane) and a slurry
concentration of about 15 to about 20 wt. % solids (CMS particles
and polymer) in CH.sub.2Cl.sub.2 solvent. After "priming" the CMS
particles with Ultem.RTM. 1000, the slurry is well-mixed by any
conventional means for about 12 hours. The remaining amount of
Ultem.RTM. 1000 polymer is added to the slurry, and the final
slurry is again well-mixed by any conventional means for about 12
hours.
[0114] (3) The polymer/CMS slurry is poured onto a flat, leveled,
clean horizontal glass surface placed inside a controlled
environment (e.g., plastic glove bag). To decrease the evaporation
rate, the controlled environment is near-saturated with
CH.sub.2Cl.sub.2 solvent. A stainless steel film applicator (Paul
N. Gardner Co.) is used to draw/spread the CMS/polymer slurry to a
uniform thickness. An inverted glass funnel was used to cover the
solution. The tip of the funnel is covered with lint-free tissue
paper to further control the evaporation rate. The solvent from the
polymer film slowly evaporates over about a 12-hour time period.
The dried film approximately has a thickness of about 30 to about
60 microns. After drying, the membrane film is annealed at a
temperature of about 100.degree. C. for about 12 hours in
vacuum.
[0115] (4) To perform the Enhancement Test, permeability
measurements of the flat mixed matrix membrane films are required.
The measurements can be made using a manometric, or constant
volume, method. The apparatus is described in references previously
cited in this section. A sample film area from final mixed matrix
film is masked with adhesive aluminum masks having a circular,
pre-cut, exposed area for permeation through the membrane. The
masked membrane can be placed in a permeation cell and the
permeation system. Both the upstream and downstream sections of the
permeation system are evacuated for about 24 hours to 48 hours to
remove ("degas") any gases or vapors sorbed into the membrane.
Permeation tests of the membrane can be performed by pressurizing
the upstream side with the desired gas at the desired pressure. The
permeation rate can be measured from the pressure rise of a
pressure transducer and using the known downstream (permeate)
volume. Following the permeation testing of a given gas, both the
upstream and downstream sections are evacuated for at least 12
hours before permeation testing of the next gas.
[0116] With the above procedure, the CO.sub.2 and CH.sub.4
permeabilities are measured for the test mixed matrix membrane and
the pure test polymer (Ultem.RTM. 1000). The CO.sub.2/CH.sub.4
selectivity of the mixed matrix membrane is compared to the
CO.sub.2/CH.sub.4 selectivity of the pure test polymer (Ultem.RTM.
1000) alone. A CO.sub.2/CH.sub.4 selectivity enhancement of 10% or
more should be observed in the mixed matrix membrane film.
[0117] Separation Systems Including the Membranes
[0118] The preferred form membrane used in the present invention is
hollow fibers. However, the membranes may take any form known in
the art. Some other membrane shapes include spiral wound, pleated,
flat sheet, or polygonal tubes. Multiple hollow fiber membrane
tubes are preferred for their relatively large fluid contact area.
The contact area may be further increased by adding additional
tubes or tube contours. Contact may also be increased by altering
the gaseous flow by increasing fluid turbulence or swirling.
[0119] The preferred glassy materials that provide good gas
selectivity, for example carbon dioxide/methane selectivity, tend
to have relatively low permeabilities. A preferred form for the
membranes is, therefore, integrally skinned or composite asymmetric
hollow fibers, which can provide both a very thin selective skin
layer and a high packing density, to facilitate use of large
membrane areas. Hollow tubes can also be used.
[0120] Sheets can be used to fabricate a flat stack permeator that
includes a multitude of membrane layers alternately separated by
feed-retentate spacers and permeate spacers. The layers can be
glued along their edges to define separate feed-retentate zones and
permeate zones. Devices of this type are described in U.S. Pat. No.
5,104,532, the contents of which is hereby incorporated by
reference.
[0121] The membranes can be included in a separation system that
includes an outer perforated shell surrounding one or more inner
tubes that contain the mixed matrix membranes. The shell and the
inner tubes can be surrounded with packing to isolate a contaminant
collection zone.
[0122] In one mode of operation, a gaseous mixture enters the
separation system via a containment collection zone through the
perforations in the outer perforated shell. The gaseous mixture
passes upward through the inner tubes. As the gaseous mixture
passes through the inner tubes, one or more components of the
mixture permeate out of the inner tubes through the selective
membrane and enter the containment collection zone.
[0123] The membranes can be included in a cartridge and used for
permeating contaminants from a gaseous mixture. The contaminants
can permeate out through the membrane, while the desired components
continue out the top of the membrane. The membranes may be stacked
within a perforated tube to form the inner tubes or may be
interconnected to form a self-supporting tube.
[0124] Each one of the stacked membrane elements may be designed to
permeate one or more components of the gaseous mixture. For
example, one membrane may be designed for removing carbon dioxide,
a second for removing hydrogen sulfide, and a third for removing
nitrogen. The membranes may be stacked in different arrangements to
remove various components from the gaseous mixture in different
orders.
[0125] Different components may be removed into a single
contaminant collection zone and disposed of together, or they may
be removed into different zones. The membranes may be arranged in
series or parallel configurations or in combinations thereof
depending on the particular application.
[0126] The membranes may be removable and replaceable by
conventional retrieval technology such as wire line, coil tubing,
or pumping. In addition to replacement, the membrane elements may
be cleaned in place by pumping gas, liquid, detergent, or other
material past the membrane to remove materials accumulated on the
membrane surface.
[0127] A gas separation system including the membranes described
herein may be of a variable length depending on the particular
application.
[0128] The gaseous mixture can flow through the membrane(s)
following an inside-out flow path where the mixture flows into the
inside of the tube(s) of the membranes and the components which are
removed permeate out through the tube. Alternatively, the gaseous
mixture can flow through the membrane following an outside-in flow
path.
[0129] In order to prevent or reduce possibly damaging contact
between liquid or particulate contaminates and the membranes, the
flowing gaseous mixture may be caused to rotate or swirl within an
outer tube. This rotation may be achieved in any known manner, for
example using one or more spiral deflectors. A vent may also be
provided for removing and/or sampling components removed from the
gaseous mixture.
[0130] The membranes are preferably durable, resistant to high
temperatures, and resistant to exposure to liquids. The materials
may be coated, ideally with a polymer, to help prevent fouling and
improve durability. Examples of suitable polymers include those
described in U.S. Pat. Nos. 5,288,304 and 4,728,345, the contents
of which are hereby incorporated by reference. Barrier materials
may also be used as a pre-filter for removing particulates and
other contaminants which may damage the membranes.
[0131] Methods of Forming Hollow Fibers
[0132] Hollow fibers can be formed, for example, by extruding a
polymer/molecular sieve mixture through a tubular capillary nozzle
with a core fluid used for the purpose of retaining the hollow
fiber shape. These fibers typically have the diameter of a human
hair and offer the advantage of maximizing the surface area per
unit volume. Industrial hollow fiber membrane modules typically
contain hundreds of thousands of individual hollow fibers.
Specifically, to maximize productivity, the hollow fibers typically
include an ultrathin (<2000 Angstroms) "skin" layer on a porous
support. Gas separation is accomplished through this selective
"skin." This outer "skin" layer may be supported on the same
polymer to form an integrally skinned asymmetric hollow fiber
membrane. The most advanced membranes have an asymmetric sheath
with the selective skin supported on an inexpensive porous core
layer (different polymer) to form a composite hollow fiber
membrane. This type of device is described in U.S. Pat. No.
5,085,676, the contents of which are hereby incorporated by
reference.
[0133] Hollow fibers can be employed in bundled arrays potted at
either end to form tube sheets and fitted into a pressure vessel
thereby isolating the insides of the tubes from the outsides of the
tubes. Devices of this type are known in the art. Preferably, the
direction of flow in a hollow fiber element will be counter-current
rather than co-current or even transverse. Such counter-current
flow can be achieved by wrapping the hollow fiber bundle in a
spiral wrap of flow-impeding material. This spiral wrap extends
from a central mandrel at the center of the bundle and spirals
outward to the outer periphery of the bundle. The spiral wrap
contains holes along the top and bottom ends whereby gas entering
the bundle for tube side flow at one end is partitioned by passage
through the holes and forced to flow parallel to the hollow fiber
down the channel created by the spiral wrap. This flow direction is
counter-current to the direction of flow inside the hollow fiber.
At the bottom of the channels the gas re-emerges from the hollow
fiber bundle through the holes at the opposite end of the spiral
wrap and is directed out of the module.
[0134] Purification Process
[0135] A mixture containing gases to be separated, for example
carbon dioxide and methane, can be enriched by a gas-phase process
through the mixed matrix membrane, for example, in any of the
above-configurations. The preferred conditions for enriching the
mixture involve using a temperature between about 25.degree. C. and
200.degree. C. and a pressure of between about 50 psia and 5000
psia. These conditions can be varied using routine experimentation
depending on the feed streams. Other gas mixtures can be purified
with the mixed matrix membrane in any of the above configurations.
For example, applications include enrichment of air by nitrogen or
oxygen, nitrogen or hydrogen removal from methane streams, or
carbon monoxide from syngas streams. The mixed matrix membrane can
also be used in hydrogen separation from refinery streams and other
process streams, for example from the dehydrogenation reaction
effluent in the catalytic dehydrogenation of paraffins. Generally,
the mixed matrix membrane may be used in any separation process
with gas mixtures involving, for example, hydrogen, nitrogen,
methane, carbon dioxide, carbon monoxide, helium, and oxygen. The
gases that can be separated are those with kinetic diameters that
allow passage through the molecular sieves. The kinetic diameter
(also referred to herein as "molecular size") of gas molecules are
well known, and the kinetic diameters of voids in molecular sieves
are also well known, and are described, for example, in D. W.
Breck, Zeolite Molecular Sieves, Wiley (1974), the contents of
which are hereby incorporated by reference.
[0136] Additional Purification
[0137] If additional purification is required, the product in the
permeate stream can be passed through additional membranes, and/or
the product can be purified via distillation using techniques well
known to those of skill in the art. Typically, membrane systems may
consist of many modules connected in various configurations (See,
for example, Prasad et al., J. Membrane Sci., 94, 225-248 (1994),
the contents of which are hereby incorporated by reference for
background and review). Modules connected in series offer many
design possibilities to purify the feed, permeate, and residue
streams to increase the separation purity of the streams and to
optimize the membrane system performance.
[0138] The effectiveness of the polymer membrane for gas
separations can be determined. As discussed above the membrane to
be commercially viable must have high permebility of at least one
component in combination with excellent selectivity. Preferably the
crosslinked polyimide polymer; having a CO.sub.2 permeability of at
least 20 barrers and a CO.sub.2/CH.sub.4 selectivity of greater
than 30. The permeability and selectivity of the membrane is
measured at 35 degrees C. and a pressure of 100 psia.
[0139] Methodology of Fiber Module Construction
[0140] For laboratory or commercial use, a suitable plurality of
the fibers is bundled together to form a separation unit. The
number of fibers bundled together will depend on fiber diameters,
lengths, and porosities and on desired throughput, equipment costs,
and other engineering considerations understood by those in the
chemical engineering arts.
[0141] The fibers are held together by any conventional means. This
assembly is then typically disposed in a pressure shell such that
one end of the fiber assembly extends to one end of the pressure
shell and the opposite end of the fiber assembly extends to the
opposite end of the pressure shell. The fiber assembly is then
fixably or removably affixed to the pressure shell by any
conventional method to form a pressure tight seal.
[0142] The unit is then operated, e.g., as a shell-tube heat
exchanger, where the feed is passed to either the shell or tube
side at one end of the assembly and the product is removed from the
other end. For maximizing high-pressure performance, the
high-pressure feed is typically fed to the shell side of the
assembly. At least a portion of the CO.sub.2 in the feed passes
through the membrane to the tube side, i.e., inside the membranes.
CO.sub.2 depleted feed is then removed from the opposite end of the
shell side of the assembly. Any conventional recycle scheme may be
optionally used to optimize a desired purity level.
[0143] In order to perform permeation tests, for example, a test
module consisting of a single fiber is constructed, as shown in
FIG. 2. Details of fabricating the module are given in the
Illustrated Embodiments section below.
[0144] Operating Conditions
[0145] The process is operated with a feed pressure of from about
20 psia to about 4000 psia, preferably at least about 50 psia, and
more preferably from about 200 psia to about 1000 psia. The feed
temperature can be ambient or can be selected to optimize the
selectivity.
[0146] Methodology of Single Fiber Module Construction
[0147] Reference is made to FIG. 6. In order to perform permeation
tests, a module 200 consisting of a single fiber 205 was
constructed. The module 200 is fabricated from two stainless steel
(316) Swagelok.RTM. 1/4-inch tees 210, stainless steel 1/4-inch
tubing and nuts, two brass NPT 1/4-inch female-tube adapters 215,
two brass NPT 1/4-inch male-tube adapters 220, and two brass
Swagelok.RTM. 1/4-inch nuts. The hollow fiber membrane 205 is
threaded through the module housing, so that a length of carbon
fiber extends on each end. The ends of the module are then plugged
with Stycast.RTM. 2651 epoxy 225 (from Emerson-Cuming Company)
cured for overnight. The ends of the membrane 205 are snapped off
after the epoxy hardens.
[0148] Methodology of Membrane Testing System
[0149] Reference is made to FIGS. 6 and 7. The permeation testing
for the fibers 205 was performed with single-fiber test modules
200. Gas transport through the membranes was examined with a
pressure-rise permeation testing system 300. The system permitted
high-pressure testing of mixed feed gas and sampling of gas streams
with a gas chromatograph. The module 200 was attached in a shell
feed method of operation. Mixed feed gas 305 from a compressed gas
cylinder 310 was supplied on the shell-side of a single-fiber test
module 200. The module 200 and ballast volumes were placed in a
circulating water bath 315 to control and maintain a constant
temperature.
[0150] Vacuum was pulled on both the shell- and bore-side of the
hollow fiber membrane 205 first for overnight before testing.
Permeate at the two ends from the bore-side of the fiber was pulled
by vacuum through a downstream sample volume. The permeation rate
was measured from the pressure rise of a Baratron.RTM. pressure
transducer 320 over time after closing the valve to vacuum. The
pressure rise was plotted on chart recorder. The compositions of
all the streams can be determined by a gas chromatograph.
Individual gas fluxes were then calculated. The plumbing of the
system consisted of stainless steel (316) Swagelok.RTM. 1/4-inch
and 1/8-inch fittings and tubing, Whitey.RTM. and Nupro.RTM. valves
with welded elements. The system is rated for over 1500 psia
pressure.
[0151] The present invention will be better understood with
reference to the following non-limiting examples.
EXAMPLES
[0152] The present examples are intended to help illustrate the
process of the present invention and are not meant to limit the
scope of the application.
Example 1
Synthesis of Monoester via Activated Carboxylic Acid
[0153] The reactivity of the diols strongly depends on their
structure. Due to the electron releasing effect of the methylene
groups the reactivity of diols is increasing with increasing chain
length. For example, 1,4 butanediol>1,3-propane diol>ethylene
glycol. Synthesis of the monoester over activated carboxylic acid
derivatives as shown in FIG. 2 might be especially interesting for
less reactive crosslinking agents.
[0154] The monoesterification reaction was carried out as follows:
the DABA-copolyimide is dissolved in THF (10 wt %) under nitrogen
atmosphere and 2 times of the stoichiometric amount of thionyl
chloride is added. The reaction is heated to reflux and the excess
of thionyl chloride and THF is distilled out of the reaction
solution. The residual copolyimide acid chloride (brown crystals)
was stored under vacuum at low temperature (50.degree. C)
overnight. The acid chloride was dissolved in THF and dropped
slowly to an excess of glycol (70 times excess) dissolved in THF.
Elevated temperature was necessary because phase separation tends
to occur during this reaction step. It was found that the acid
chloride showed a much lower solubility in THF compared to the DABA
copolyimide, so using NMP as a solvent for the second step of the
reaction might be useful.
[0155] With the procedure described above it is assumed that a
certain amount of diester is already obtained during the monoester
formation. This is indicated by the fact that a 10 wt % solution of
the monoester (over the acid chloride route) in NMP showed a
significant higher viscosity than monoester solutions obtained over
the self catalyzed reaction. It was also obvious that the
mechanical properties drastically improved. Starting with a
material forming very brittle films, probably due to low molecular
weight, over the acid chloride route films with high flexibility
were obtained. Therefore, the synthesis of monoester over the acid
chloride route can be a valuable method for building up molecular
weight, which itself leads high viscosity and can help in the
process of hollow fiber spinning.
Example 2
Self Catalyzed Monoesterification Reaction
[0156] Some of the diols such as 1,4 butanediol have been found to
form the monoester without the use of a catalyst. For the
self-catalyzed reaction, DABA-copolyimides are dissolved in dry NMP
(15-17 wt %) and 70 times excess of diol is added. The reaction
mixture is stirred for 12 hours at 130.degree. C. under nitrogen
purge. Precipitation, blending and filtration lead to fluffy
particles of monoester which are dried at 70.degree. C. under
vacuum.
Example 3
Acid Catalyzed Monoesterification Reaction
[0157] For the acid catalysed reaction, per 2 g of polymer 1 mg of
p-Toluene sulfonic acid was added. The procedure for the reaction
was the same as for the self-catalyzed reaction.
Example 4
Conversion of the Monoesterification Reaction
[0158] We have found that .sup.1H-NMR is a useful method to show
the conversion which can be reached in the monoesterification
reaction. This should be explained on two examples. FIG. 4 shows
the .sup.1H NMR of 6FDA-DAM/DABA 3:2 non-crosslinked in DMSO-D6.
The presence of the DABA units can be proven by comparing the ratio
of all aromatic protons and aliphatic protons (3 methyl groups of
DAM). For this case we found:
[0159] After the self-catalyzed monoesterification reaction with 1
,4-butanediol and low temperature drying of the monoester
(70.degree. C. under vacuum), again .sup.1H NMR was performed. The
spectrum is shown in FIG. 4. For the monoester-NMR we can calculate
the conversions of the reaction by the ratio of aromatic protons
and aliphatic protons of the methylene group next to the ester
group. We can check the calculations also by the ratio of
DAM-methyl protons and the methylene group next to the ester
group.
[0160] From the spectrum obtained for the monoesterification
reaction it can be concluded that nearly complete conversions can
be obtained with 1,4-butanediol using the self-catalyzed reaction
conditions.
[0161] As already mentioned we assume that the ethylene glycol is
less nucleophilic than the butylene glycol. It has been found that
using ethylene glycol in a self-catalyzed monoesterification
reaction, the conversions seems to be much lower. The .sup.1H-NMR
of the 6FDA-DAM/DABA 2:1 ethylene glycol monoester is shown in FIG.
5
[0162] Table 1 summarizes the results for the conversions obtained
with copolyimides having different DAM/DABA compositions. Thereby
different methods for synthesizing the monoester copolyimide were
investigated. The conversion of the reactions was indepently
calculated from the ratio of aromatic protons (without the aromatic
DAM proton) and the methylene protons next to the ester group as
well as from the ratio of the aliphatic DAM methyl protons and the
methylene protons next to the ester. The follwing conclusions can
be drawn:
[0163] The monoesterification reaction can be catalyzed by protons.
Therefore it is obvious that with increasing DABA content of the
copolyimide structure higher conversion rates are obtained. The
DAM/DABA 4:1 with butanediol shows a conversion of less than 50%
whereas for the DAM/DABA 3:2 a conversion of over 90% was obtained
(self catalyzed). Ethylene glycol generally has a lower
nucleophilic character than butanediol, the conversion for a
DAM/DABA 2:1 composition is rather low (less than 40%) although a
high number of protons are present due to the high DABA
content.
[0164] For DAM/DABA 4:1 monoesterification with ethylene glycol
very low conversion was expected (at least less than 40%) in the
self-catalyzed reaction due to the low DABA content. By adding
p-Toluene sufonic acid to the monoesterification reaction
conversions of more than 60% preferably more than 80% can be
reached.
[0165] The acid chloride groups are highly reactive groups,
therefore conversions are over 95% for the monoester synthesis over
the acid chloride route. This method might be important for the
monoesterification reactions using multifunctional crosslinking
agents and DABA structures with low DABA contents.
1TABLE 1 Calculated Conver- sion based Calculated on DAM-
Conversion based CH.sub.3 on Aromatics protons/ Ratio: (without
DAM/ methylene Copolyimide Aromatics (total)/ methylene protons
protons Monoester DAM-CH3 protons (on ester) (on ester) DAM/DABA
4:1 theoretical: 22 23 Butanediol 1.03 Self-catalyzed experimental:
1.08 DAM/DABA 2:1 theoretical: 38 40 Ethylene glycol 1.28
Self-catalyzed experimental: 1.23 DAM/DABA 3:2 theoretical: 94 98
Butanediol 1.44 Self-catalyzed experimental: 1.50 DAM/DABA 3:2
theoretical: 97 98 Butanediol 1.44 Over acid experimental: chloride
1.49
Example 5
Plasticization Resistance
[0166] In order to show CO.sub.2 plasticization resistance, pure
CO.sub.2 permeation experiments have been performed with the
6FDA-DAM/DABA 3:2 film crosslinked with 1,4 butanediol at
140.degree. C. To determine the CO.sub.2 plasticization the
CO.sub.2 permeability is measured at increasing CO.sub.2 pressure.
The CO.sub.2 pressure was held at a given pressure for 24 hours
then the CO.sub.2 permeability was measured. The CO.sub.2 pressure
was then held for an additional 24 hours and again measured. A
substantial increase in the CO.sub.2 permeability indicates
plasticization. Results of plasticization test are shown in FIG. 8.
FIG. 8 shows that the crosslinked material has surprising
resistance to plasticization.
Comparative Example 6
Plasticization Resistance of Uncrosslinked Film
[0167] To contrast the plasticization resistance of a crosslinked
membrane (of Example 5), an uncrosslinked film was tested under
similar conditions to Example 5. The results of this test are shown
in FIG. 9. FIG. 9 shows a substantial increase in the CO.sub.2
permeability indicating plasticization.
* * * * *