U.S. patent application number 14/496754 was filed with the patent office on 2016-03-31 for polyimide blend membranes for gas separations.
The applicant listed for this patent is UOP LLC. Invention is credited to Chunqing Liu, Howie Q. Tran.
Application Number | 20160089627 14/496754 |
Document ID | / |
Family ID | 55583449 |
Filed Date | 2016-03-31 |
United States Patent
Application |
20160089627 |
Kind Code |
A1 |
Liu; Chunqing ; et
al. |
March 31, 2016 |
POLYIMIDE BLEND MEMBRANES FOR GAS SEPARATIONS
Abstract
The polyimide blend membrane in the present invention was
prepared by blending a first aromatic polyimide with high
permeability and a second aromatic polyimide with high selectivity
for gas separation. The polyimide blend membrane in the present
invention showed improved permeability compared to membranes made
from the second aromatic polyimide and improved selectivity
compared to membranes made from the first aromatic polyimide.
Inventors: |
Liu; Chunqing; (Arlington
Heights, IL) ; Tran; Howie Q.; (Skokie, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UOP LLC |
Des Plaines |
IL |
US |
|
|
Family ID: |
55583449 |
Appl. No.: |
14/496754 |
Filed: |
September 25, 2014 |
Current U.S.
Class: |
95/45 |
Current CPC
Class: |
B01D 71/64 20130101;
Y02C 10/10 20130101; B01D 67/0006 20130101; Y02C 20/20 20130101;
B01D 53/228 20130101; Y02C 20/40 20200801; B01D 2325/20 20130101;
B01D 69/02 20130101 |
International
Class: |
B01D 53/22 20060101
B01D053/22; B01D 71/64 20060101 B01D071/64; B01D 69/12 20060101
B01D069/12 |
Claims
1. A process for separating at least one gas from a mixture of
gases, the process comprising (a) providing a polyimide blend
membrane comprising a miscible blend of a first aromatic polyimide
that comprises a plurality of repeating units of formula (I) and a
second aromatic polyimide that comprises a plurality of repeating
units of formula (II), wherein formula (I) is represented by:
##STR00005## wherein n, m, o, and p are independent integers from
20 to 500; and wherein formula (II) is represented by: ##STR00006##
wherein q and r are independent integers from 20 to 500; and
wherein the weight ratio of said first aromatic polyimide to said
second aromatic polyimide is in a range from about 10:1 to 1:10
wherein said polyimide blend membrane is permeable to said at least
one gas; (b) contacting a mixture of gases to one side of the
polyimide blend membrane to cause said at least one gas to permeate
the polyimide blend membrane; and (c) removing from an opposite
side of the polyimide blend membrane a permeate gas composition
comprising a portion of said at least one gas which permeated said
membrane.
2. The process of claim 1 wherein said mixture of gases comprises a
mixture of carbon dioxide and methane.
3. The process of claim 1 wherein said mixture of gases comprises a
mixture of hydrogen and methane.
4. The process of claim 1 wherein said mixture of gases comprises a
mixture of helium and methane.
5. The process of claim 1 wherein said mixture of gases comprises a
mixture of at least one volatile organic compound and at least one
atmospheric gas.
6. The process of claim 1 wherein said mixture of gases comprises
nitrogen and hydrogen.
7. The process of claim 1 wherein said mixture of gases comprises a
mixture of at least two gases selected from the group consisting of
carbon dioxide, oxygen, nitrogen, water vapor, hydrogen sulfide,
helium and methane.
8. The process of claim 1 wherein said mixture of gases comprises a
mixture of volatile organic compounds and at least one atmospheric
gas.
9. The process of claim 8 wherein said volatile organic compounds
are selected from the group consisting of toluene, xylene and
acetone.
10. The process of claim 1 wherein said mixture of gases comprises
a mixture of olefins and paraffins.
11. The process of claim 1 wherein said mixture of gases comprises
a mixture of hydrocarbons and hydrogen.
12. The process of claim 1 wherein said polyimide blend membrane is
cross-linked via UV radiation.
13. The process of claim 1 wherein said process is at a temperature
from about 20.degree. to about 100.degree. C.
14. The process of claim 1 wherein said mixture of gases comprises
a mixture of isoparaffin and normal paraffin.
15. The process of claim 1 wherein said polyimide blend membrane is
used in either a single stage membrane system or in either stages
of a two stage membrane system.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to polyimide blend membranes and
methods for making and using these membranes. In the past 30-35
years, the state of the art of polymer membrane-based gas
separation processes has evolved rapidly. Membrane-based
technologies have advantages of both low capital cost and
high-energy efficiency compared to conventional separation methods.
Membrane gas separation is of special interest to petroleum
producers and refiners, chemical companies, and industrial gas
suppliers. Several applications of membrane gas separation have
achieved commercial success, including N.sub.2 enrichment from air,
carbon dioxide removal from natural gas and from enhanced oil
recovery, and also in hydrogen removal from nitrogen, methane, and
argon in ammonia purge gas streams. For example, UOP's Separex.TM.
cellulose acetate spiral wound polymeric membrane is currently an
international market leader for carbon dioxide removal from natural
gas.
[0002] Polymers provide a range of properties including low cost,
permeability, mechanical stability, and ease of processability that
are important for gas separation. Glassy polymers (i.e., polymers
at temperatures below their T.sub.g) have stiffer polymer backbones
and therefore let smaller molecules such as hydrogen and helium
pass through more quickly, while larger molecules such as
hydrocarbons pass through more slowly as compared to polymers with
less stiff backbones. Cellulose acetate (CA) glassy polymer
membranes are used extensively in gas separation. Currently, such
CA membranes are used for natural gas upgrading, including the
removal of carbon dioxide. Although CA membranes have many
advantages, they are limited in a number of properties including
selectivity, permeability, and in chemical, thermal, and mechanical
stability.
[0003] The membranes most commonly used in commercial gas and
liquid separation applications are asymmetric polymeric membranes
and have a thin nonporous selective skin layer that performs the
separation. Separation is based on a solution-diffusion mechanism.
This mechanism involves molecular-scale interactions of the
permeating gas with the membrane polymer. The mechanism assumes
that in a membrane having two opposing surfaces, each component is
sorbed by the membrane at one surface, transported by a gas
concentration gradient, and desorbed at the opposing surface.
According to this solution-diffusion model, the membrane
performance in separating a given pair of gases (e.g.,
CO.sub.2/CH.sub.4, O.sub.2/N.sub.2, H.sub.2/CH.sub.4) is determined
by two parameters: the permeability coefficient (abbreviated
hereinafter as permeability or P.sub.A) and the selectivity
(.alpha..sub.A/B). The P.sub.A is the product of the gas flux and
the selective skin layer thickness of the membrane, divided by the
pressure difference across the membrane. The .alpha..sub.A/B is the
ratio of the permeability coefficients of the two gases
(.alpha..sub.A/B=P.sub.A/P.sub.B) where P.sub.A is the permeability
of the more permeable gas and P.sub.B is the permeability of the
less permeable gas. Gases can have high permeability coefficients
because of a high solubility coefficient, a high diffusion
coefficient, or because both coefficients are high. In general, the
diffusion coefficient decreases while the solubility coefficient
increases with an increase in the molecular size of the gas. In
high performance polymer membranes, both high permeability and
selectivity are desirable because higher permeability decreases the
size of the membrane area required to treat a given volume of gas,
thereby decreasing capital cost of membrane units, and because
higher selectivity results in a higher purity product gas.
[0004] One of the components to be separated by a membrane must
have a sufficiently high permeance at the preferred conditions or
an extraordinarily large membrane surface area is required to allow
separation of large amounts of gases or liquids. Permeance,
measured in Gas Permeation Units (GPU, 1 GPU=10-.sup.6 cm.sup.3
(STP)/cm.sup.2 s (cm Hg)), is the pressure normalized flux and is
equal to permeability divided by the skin layer thickness of the
membrane. Commercially available gas separation polymer membranes,
such as CA, polyimide, and polysulfone membranes formed by phase
inversion and solvent exchange methods have an asymmetric
integrally skinned membrane structure. Such membranes are
characterized by a thin, dense, selectively semipermeable surface
"skin" and a less dense void-containing (or porous), non-selective
support region, with pore sizes ranging from large in the support
region to very small proximate to the "skin". However, fabrication
of defect-free high selectivity asymmetric integrally skinned
polyimide membranes is difficult. The presence of nanopores or
defects in the skin layer reduces the membrane selectivity. The
high shrinkage of the polyimide membrane on cloth substrate during
membrane casting and drying process results in unsuccessful
fabrication of asymmetric integrally skinned polyimide flat sheet
membranes using phase inversion technique.
[0005] US 2005/0268783 A1 disclosed chemically cross-linked
polyimide hollow fiber membranes prepared from a monoesterified
polymer followed by final cross-linking after hollow fiber
formation.
[0006] U.S. Pat. No. 7,485,173 disclosed UV cross-linked mixed
matrix membranes via UV radiation. The cross-linked mixed matrix
membranes comprise microporous materials dispersed in the
continuous UV cross-linked polymer matrix.
[0007] U.S. Pat. No. 8,016,124 disclosed a thin film composite
membrane (TFC) comprising a blend of polyethersulfone and aromatic
polyimide polymers. The TFC membrane has a layer of a blend of
polyethersulfone and aromatic polyimide with a thickness from about
0.1 to about 3 microns.
[0008] U.S. Pat. No. 8,337,598 disclosed a TFC hollow fiber
membrane with a core player and a sheath UV-crosslinked polymer
layer.
[0009] The selective thin layer on the non-selective porous layer
of a thin film composite (TFC) membrane can be delaminated easily
from the non-selective porous layer, which will result in
significantly decreased selectivity for gas separations. On the
other hand, the integrally-skinned asymmetric membranes have a
selective thin layer and a porous layer from the same membrane
material and formed from the same membrane solution at about the
same time. Therefore, the selective thin layer of an
integrally-skinned asymmetric membrane cannot be delaminated easily
from the non-selective porous layer.
[0010] The present invention discloses polyimide blend membranes
and methods for making and using these membranes.
SUMMARY OF THE INVENTION
[0011] This invention pertains to polyimide blend membranes and
methods for making and using these membranes. This invention
pertains to polyimide blend thin film composite or asymmetric
membrane with either flat sheet or hollow fiber geometry.
[0012] The term "polyimide blend membrane" in the present invention
refers to a membrane prepared from a blend of two or more polyimide
polymers.
[0013] The present invention provides a polyimide blend membrane
comprising a miscible blend of an aromatic polyimide I that
comprises a plurality of repeating units of formula (I) and an
aromatic polyimide II that comprises a plurality of repeating units
of formula (II), wherein formula (I) is
##STR00001##
wherein n, m, o, and p are independent integers from 20 to 500; and
wherein formula (II) is:
##STR00002##
wherein q and r are independent integers from 20 to 500; and
wherein the weight ratio of said aromatic polyimide I to said
aromatic polyimide II is in a range of 10:1 to 1:10.
[0014] The polyimide blend membrane in the present invention was
prepared by blending an aromatic polyimide I with high permeability
and an aromatic polyimide II with high selectivity for gas
separation. The polyimide blend membrane in the present invention
showed improved permeability compared to the aromatic polyimide II
and improved selectivity compared to the aromatic polyimide I.
[0015] The polyimide polymers used for making the polyimide blend
membranes with high selectivities described in the current
invention have a weight average molecular weight in the range of
50,000 to 1,000,000 Daltons, preferably between 70,000 to 500,000
Daltons.
[0016] In another embodiment of the invention, the polyimide blend
membranes in the present invention have undergone an additional UV
cross-linking process. The polyimide blend membranes in the present
invention have UV cross-linkable benzophenone functional
groups.
[0017] The cross-linked polyimide blend membranes comprise polymer
chain segments where at least part of these polymer chain segments
are cross-linked to each other through possible direct covalent
bonds by exposure to UV radiation. The cross-linking of the
polyimide blend membranes provides membranes with superior
selectivity and improved chemical and thermal stabilities compared
to the corresponding uncross-linked polyimide blend membranes.
[0018] The polyimide blend membrane in the present invention can be
either asymmetric integrally skinned membrane or thin film
composite (TFC) membrane.
[0019] The asymmetric integrally-skinned flat sheet or hollow fiber
polyimide blend membranes in the present invention were prepared by
a phase inversion process. In some cases, UV radiation was applied
to the surface of the membrane to further improve the membrane
selectivity.
[0020] The membrane dope formulation for the preparation of
asymmetric integrally-skinned flat sheet or hollow fiber polyimide
blend membrane with high selectivities for gas separations in the
present invention comprises good solvents for the polyimide
polymers that can completely dissolve the polymers. Representative
good solvents for use in this invention include N-methylpyrrolidone
(NMP), N,N-dimethyl acetamide (DMAC), methylene chloride,
N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dioxanes,
1,3-dioxolane, mixtures thereof, others known to those skilled in
the art and mixtures thereof. In some cases, the membrane dope
formulation for the preparation of asymmetric integrally-skinned
flat sheet or hollow fiber polyimide blend membrane with high
selectivities for gas separations in the present invention also
comprises poor solvents for the polyimide polymers that cannot
dissolve the polymers such as acetone, methanol, ethanol,
tetrahydrofuran (THF), toluene, n-octane, n-decane, lactic acid,
citric acid, isopropanol, and mixtures thereof. It is believed that
the proper weight ratio of the solvents used in the present
invention provides asymmetric integrally-skinned flat sheet or
hollow fiber polyimide blend membrane with <200 nm super thin
nonporous selective skin layer which results in high
permeances.
[0021] The thin film composite polyimide blend membrane described
in the current invention comprises a thin nonporous selective
separation layer comprising the polyimide blend described in the
present invention and a porous nonselective mechanical support
layer made from a material different from the polyimide blend
described in the present invention or a polymer blend comprising a
first polymer different from any of the polyimides in the polyimide
blend described in the present invention and a second polymer that
is the same as one of the polyimides in the polyimide blend
described in the present invention. The porous nonselective
mechanical support layer described in the present invention with a
low selectivity and high flux can be made from materials including
cellulosic polymers, polysulfone, polyethersulfone, polyamide,
polyimide, polyetherimide, cellulose nitrate, polyurethane,
polycarbonate, polystyrene, polybenzoxazole, or mixtures
thereof.
[0022] One asymmetric integrally-skinned hollow fiber polyimide
blend membrane PI-1/PI-2 described in the present invention is
fabricated from a blend of a poly(3,3',4,4'-benzophenone
tetracarboxylic dianhydride-pyromellitic
dianhydride-2,4,6-trimethyl-m-phenylenediamine-2,4-toluenediamine)
(PI-1) derived from the condensation reaction of
3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) and
pyromellitic dianhydride (PMDA) with a mixture of
2,4,6-trimethyl-m-phenylenediamine (TMPDA) and 2,4-toluenediamine
(2,4-TDA) (BTDA:PMDA:TMPDA:2,4-TDA=3:2:3:2 (molar ratio) and a
poly(3,3',4,4'-benzophenone tetracarboxylic
dianhydride-2,4-toluenediamine-4,4'-methylenedianiline) (PI-2)
derived from the condensation reaction of BTDA with a mixture of
2,4-TDA and 4,4'-methylenedianiline (MDA) (BTDA:2,4-TDA:MDA=5:4:1
(molar ratio). The weight ratio of PI-1 to PI-2 is 1:1.2. The blend
PI-1/PI-2-O-5Si2.5U asymmetric integrally-skinned hollow fiber
membrane with 2.5 minutes of UV treatment showed high
H.sub.2/CH.sub.4 separation performance with H.sub.2 permeance of
198 GPU and H.sub.2/CH.sub.4 selectivity of 184 for
H.sub.2/CH.sub.4 separation. The blend PI-1/PI-2-O-5Si2.5U
asymmetric integrally-skinned hollow fiber membrane with 2.5
minutes of UV treatment also showed high CO.sub.2/CH.sub.4
separation performance with CO.sub.2 permeance of 74 GPU and
CO.sub.2/CH.sub.4 selectivity of 30.1 for CO.sub.2/CH.sub.4
separation.
[0023] The invention provides a process for separating at least one
gas from a mixture of gases using the polyimide blend membrane
described herein, the process comprising: (a) providing a polyimide
blend membrane described in the present invention which is
permeable to said at least one gas; (b) contacting the mixture on
one side of the polyimide blend membrane to cause said at least one
gas to permeate the membrane; and (c) removing from the opposite
side of the membrane a permeate gas composition comprising a
portion of said at least one gas which permeated said membrane.
[0024] The polyimide blend membranes described in the current
invention are not only suitable for H.sub.2 purification
application, but also suitable for a variety of other gas
separations such as CO.sub.2/CH.sub.4, O.sub.2/N.sub.2, and
H.sub.2S/CH.sub.4 separations.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The use of membranes for separation of both gases and
liquids is a growing technological area with potentially high
economic reward due to the low energy requirements and the
potential for scaling up of modular membrane designs. Advances in
membrane technology, with the continuing development of new
membrane materials and new methods for the production of high
performance membranes will make this technology even more
competitive with traditional, high-energy intensive and costly
processes such as distillation. Among the applications for large
scale gas separation membrane systems are nitrogen enrichment,
oxygen enrichment, hydrogen recovery, removal of hydrogen sulfide
and carbon dioxide from natural gas and dehydration of air and
natural gas. Also, various hydrocarbon separations are potential
applications for the appropriate membrane system. The membranes
that are used in these applications must have high selectivity,
durability, and productivity in processing large volumes of gas or
liquid in order to be economically successful. Membranes for gas
separations have evolved rapidly in the past 25 years due to their
easy processability for scale-up and low energy requirements. More
than 90% of the membrane gas separation applications involve the
separation of noncondensable gases: such as nitrogen from air, and
hydrogen from nitrogen, argon or methane. Membrane gas separation
is of special interest to petroleum producers and refiners,
chemical companies, and industrial gas suppliers. Several
applications of membrane gas separation have achieved commercial
success, including nitrogen enrichment from air, hydrogen from
nitrogen, argon or methane, carbon dioxide removal from natural gas
and biogas and in enhanced oil recovery.
[0026] The present invention provides polyimide blend membranes.
This invention also pertains to the application of the polyimide
blend membranes for H.sub.2 purifications such as H.sub.2/CH.sub.4
separation, and also for a variety of other gas separations such as
separations of CO.sub.2/CH.sub.4, H.sub.2S/CH.sub.4,
CO.sub.2/N.sub.2, olefin/paraffin separations (e.g.
propylene/propane separation), and O.sub.2/N.sub.2 separations.
[0027] The present invention provides a polyimide blend membrane
comprising a miscible blend of an aromatic polyimide I that
comprises a plurality of repeating units of formula (I) and an
aromatic polyimide II that comprises a plurality of repeating units
of formula (II), wherein formula (I) is
##STR00003##
wherein n, m, o, and p are independent integers from 20 to 500; and
wherein formula (II) is:
##STR00004##
wherein q and r are independent integers from 20 to 500; and
wherein the weight ratio of said aromatic polyimide I to said
aromatic polyimide II is in a range of 10:1 to 1:10.
[0028] The polyimide polymers used for making the polyimide blend
membranes with high selectivities described in the current
invention have a weight average molecular weight in the range of
50,000 to 1,000,000 Daltons, preferably between 70,000 to 500,000
Daltons.
[0029] The polyimide blend membrane in the present invention was
prepared by blending an aromatic polyimide I with high permeability
and an aromatic polyimide II with high selectivity for gas
separation. The polyimide blend membrane in the present invention
showed improved permeability compared to the aromatic polyimide II
and improved selectivity compared to the aromatic polyimide I.
[0030] In another embodiment of the invention, the polyimide blend
membranes in the present invention have undergone an additional UV
cross-linking process. The polyimide blend membranes in the present
invention have UV cross-linkable benzophenone functional groups.
The cross-linked polyimide blend membranes comprise polymer chain
segments where at least part of these polymer chain segments are
cross-linked to each other through possible direct covalent bonds
by exposure to UV radiation. The thickness of the UV cross-linked
surface layer of the polyimide blend membrane described in the
present invention is in a range of 20 nm to 5 microns. The
cross-linking of the polyimide blend membranes provides membranes
with superior selectivity and improved chemical and thermal
stabilities compared to the corresponding uncross-linked polyimide
blend membranes.
[0031] One preferred aromatic polyimide polymer I that is used for
the formation of the polyimide blend membrane in the present
invention is poly(3,3',4,4'-benzophenone tetracarboxylic
dianhydride-pyromellitic
dianhydride-2,4,6-trimethyl-m-phenylenediamine-2,4-toluenediamine)
(PI-1) derived from the condensation reaction of
3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) and
pyromellitic dianhydride (PMDA) with a mixture of
2,4,6-trimethyl-m-phenylenediamine (TMPDA) and 2,4-toluenediamine
(2,4-TDA). The molar ratio of BTDA to PMDA is in a range of 5:1 to
1:5. The molar ratio of TMPDA to 2,4-TDA is in a range of 5:1 to
1:5. The molar ratio of the total dianhydrides of BTDA and PMDA to
the total diamines of TMPDA and 2,4-TDA is 1:1.
[0032] One preferred aromatic polyimide polymer II that is used for
the formation of the polyimide blend membrane in the present
invention is poly(3,3',4,4'-benzophenone tetracarboxylic
dianhydride-2,4-toluenediamine-4,4'-methylenedianiline) (PI-2)
derived from the condensation reaction of BTDA with a mixture of
2,4-TDA and 4,4'-methylenedianiline (MDA). The molar ratio of
2,4-TDA to MDA is in a range of 10:1 to 1:10. The molar ratio of
BTDA dianhydride to the total diamines of 2,4-TDA and MDA is
1:1.
[0033] The polyimide blend membrane in the present invention can be
either asymmetric integrally skinned membrane or thin film
composite (TFC) membrane.
[0034] The asymmetric integrally-skinned flat sheet or hollow fiber
polyimide blend membranes in the present invention were prepared by
a phase inversion process. In some cases, UV radiation was applied
to the surface of the membrane to further improve the membrane
selectivity.
[0035] The membrane dope formulation for the preparation of
asymmetric integrally-skinned flat sheet or hollow fiber polyimide
blend membrane with high selectivities for gas separations in the
present invention comprises good solvents for the polyimide
polymers that can completely dissolve the polymers. Representative
good solvents for use in this invention include N-methylpyrrolidone
(NMP), N,N-dimethyl acetamide (DMAC), methylene chloride,
N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dioxanes,
1,3-dioxolane, mixtures thereof, others known to those skilled in
the art and mixtures thereof. In some cases, the membrane dope
formulation for the preparation of asymmetric integrally-skinned
flat sheet or hollow fiber polyimide blend membrane with high
selectivities for gas separations in the present invention also
comprises poor solvents for the polyimide polymers that cannot
dissolve the polymers such as acetone, methanol, ethanol,
tetrahydrofuran (THF), toluene, n-octane, n-decane, lactic acid,
citric acid, isopropanol, and mixtures thereof. It is believed that
the proper weight ratio of the solvents used in the present
invention provides asymmetric integrally-skinned flat sheet or
hollow fiber polyimide blend membrane with <200 nm super thin
nonporous selective skin layer which results in high
permeances.
[0036] The thin film composite polyimide blend membrane described
in the current invention comprises a thin nonporous selective
separation layer comprising the polyimide blend described in the
present invention and a porous nonselective mechanical support
layer made from a material different from the polyimide blend
described in the present invention or a polymer blend comprising a
first polymer different from any of the polyimides in the polyimide
blend described in the present invention and a second polymer that
is the same as one of the polyimides in the polyimide blend
described in the present invention. The thin film composite
polyimide blend membrane described in the current invention has
either hollow fiber or flat sheet geometry.
[0037] The porous nonselective mechanical support layer has low
selectivity and high flux. Selection of the porous nonselective
mechanical support layer for the preparation of TFC polyimide blend
membrane in the present invention may be made on the basis of the
heat resistance, solvent resistance, and mechanical strength of the
porous nonselective mechanical support layer, as well as other
factors dictated by the operating conditions for selective
permeation. The porous nonselective mechanical support layer is
preferably at least partially self-supporting, and in some
instances may be essentially self-supporting. The porous
nonselective mechanical support layer may provide essentially all
of the structural support for the membrane. Some preferred polymers
that are suitable for the preparation of the porous nonselective
mechanical support layer for the TFC polyimide blend membrane
according to the present invention include, but are not limited to,
polysulfones, sulfonated polysulfones, polyethersulfones (PESs),
sulfonated PESs, polyethers, polyetherimides such as Ultem,
cellulosic polymers such as cellulose acetate and cellulose
triacetate, polyamides, polyimides, polyether ketones, and blends
thereof.
[0038] Some preferred solvents that can be used for dissolving the
polyimide blends for the preparation of TFC polyimide blend
membrane described in the current invention include NMP,
N,N-dimethyl DMAC, methylene chloride, DMF, DMSO, dioxanes,
1,3-dioxolane, acetone, isopropanol, and mixtures thereof. For the
preparation of TFC polyimide blend flat sheet membrane, it is
preferred that the polyimide blend solution has a concentration of
from about 1 to about 20 wt % to provide an effective coating. The
dilute polyimide blend solution is applied to the surface of the
porous nonselective mechanical support layer by dip-coating, spin
coating, casting, spraying, painting, and other known conventional
solution coating technologies. For the preparation of TFC polyimide
blend hollow fiber membrane, it is preferred that the polyimide
blend solution has a concentration of from about 20 to about 40 wt
%. The polyimide blend solution and the polymer solution for the
formation of the porous nonselective mechanical support layer were
co-extruded from a spinneret.
[0039] The invention provides a process for separating at least one
gas from a mixture of gases using the polyimide blend membrane
described in the present invention, the process comprising: (a)
providing a polyimide blend membrane described in the present
invention which is permeable to said at least one gas; (b)
contacting the mixture on one side of the polyimide blend membrane
described in the present invention to cause said at least one gas
to permeate the membrane; and (c) removing from the opposite side
of the membrane a permeate gas composition comprising a portion of
said at least one gas which permeated said membrane.
[0040] The polyimide blend membrane described in the present
invention is especially useful in the purification, separation or
adsorption of a particular species in the liquid or gas phase.
[0041] The polyimide blend membrane described in the present
invention is especially useful in gas separation processes in air
purification, petrochemical, refinery, and natural gas industries.
Examples of such separations include separation of volatile organic
compounds (such as toluene, xylene, and acetone) from an
atmospheric gas, such as nitrogen or oxygen and nitrogen recovery
from air. Further examples of such separations are for the
separation of CO.sub.2 or H.sub.2S from natural gas, H.sub.2 from
N.sub.2, CH.sub.4, and Ar in ammonia purge gas streams, H.sub.2
recovery in refineries, olefin/paraffin separations such as
propylene/propane separation, and iso/normal paraffin separations.
Any given pair or group of gases that differ in molecular size, for
example nitrogen and oxygen, carbon dioxide and methane, hydrogen
and methane or carbon monoxide, helium and methane, can be
separated using the polyimide blend membrane described in the
present invention. More than two gases can be removed from a third
gas. For example, some of the gas components which can be
selectively removed from a raw natural gas using the membrane
described herein include carbon dioxide, oxygen, nitrogen, water
vapor, hydrogen sulfide, helium, and other trace gases. Some of the
gas components that can be selectively retained include hydrocarbon
gases. When permeable components are acid components selected from
the group consisting of carbon dioxide, hydrogen sulfide, and
mixtures thereof and are removed from a hydrocarbon mixture such as
natural gas, one module, or at least two in parallel service, or a
series of modules may be utilized to remove the acid components.
For example, when one module is utilized, the pressure of the feed
gas may vary from 275 kPa to about 2.6 MPa (25 to 4000 psi). The
differential pressure across the membrane can be as low as about 70
kPa or as high as 14.5 MPa (about 10 psi or as high as about 2100
psi) depending on many factors such as the particular membrane
used, the flow rate of the inlet stream and the availability of a
compressor to compress the permeate stream if such compression is
desired. Differential pressure greater than about 14.5 MPa (2100
psi) may rupture the membrane. A differential pressure of at least
0.7 MPa (100 psi) is preferred since lower differential pressures
may require more modules, more time and compression of intermediate
product streams. The operating temperature of the process may vary
depending upon the temperature of the feed stream and upon ambient
temperature conditions. Preferably, the effective operating
temperature of the membranes of the present invention will range
from about -50.degree. to about 150.degree. C. More preferably, the
effective operating temperature of the polyimide blend membrane of
the present invention will range from about -20.degree. to about
100.degree. C., and most preferably, the effective operating
temperature of the membranes of the present invention will range
from about 25.degree. to about 100.degree. C.
[0042] The polyimide blend membrane described in the present
invention are also especially useful in gas/vapor separation
processes in chemical, petrochemical, pharmaceutical and allied
industries for removing organic vapors from gas streams, e.g. in
off-gas treatment for recovery of volatile organic compounds to
meet clean air regulations, or within process streams in production
plants so that valuable compounds (e.g., vinylchloride monomer,
propylene) may be recovered. Further examples of gas/vapor
separation processes in which the polyimide blend membrane
described in the present invention may be used are hydrocarbon
vapor separation from hydrogen in oil and gas refineries, for
hydrocarbon dew pointing of natural gas (i.e. to decrease the
hydrocarbon dew point to below the lowest possible export pipeline
temperature so that liquid hydrocarbons do not separate in the
pipeline), for control of methane number in fuel gas for gas
engines and gas turbines, and for gasoline recovery.
[0043] The polyimide blend membrane described in the present
invention also has immediate application to concentrate olefin in a
paraffin/olefin stream for olefin cracking application. For
example, the polyimide blend membrane described in the present
invention can be used for propylene/propane separation to increase
the concentration of the effluent in a catalytic dehydrogenation
reaction for the production of propylene from propane and
isobutylene from isobutane. Therefore, the number of stages of a
propylene/propane splitter that is required to get polymer grade
propylene can be reduced. Another application for the polyimide
blend membrane described in the present invention is for separating
isoparaffin and normal paraffin in light paraffin isomerization and
MaxEne.TM., a process for enhancing the concentration of normal
paraffin (n-paraffin) in the naphtha cracker feedstock, which can
be then converted to ethylene.
[0044] The polyimide blend membrane described in the present
invention can also be operated at high temperature to provide the
sufficient dew point margin for natural gas upgrading (e.g,
CO.sub.2 removal from natural gas). The polyimide blend membrane
described in the present invention can be used in either a single
stage membrane or as the first or/and second stage membrane in a
two stage membrane system for natural gas upgrading.
EXAMPLES
[0045] The following examples are provided to illustrate one or
more preferred embodiments of the invention, but are not limited
embodiments thereof. Numerous variations can be made to the
following examples that lie within the scope of the invention.
Example 1
Preparation of PI-1/PI-2(5:6) and UV Cross-Linked PI-1/PI-2(5:6)
Polyimide Blend Dense Film Membranes
[0046] 2.5 g of aromatic polyimide poly(3,3',4,4'-benzophenone
tetracarboxylic dianhydride-pyromellitic
dianhydride-2,4,6-trimethyl-m-phenylenediamine-2,4-toluenediamine)
(PI-1) containing UV cross-linkable carbonyl groups and 3.0 g of
poly(3,3',4,4'-benzophenone tetracarboxylic
dianhydride-2,4-toluenediamine-4,4'-methylenedianiline) (PI-2)
containing UV cross-linkable carbonyl groups were dissolved in a
solvent mixture of 12.0 g of NMP and 15.0 g of 1,3-dioxolane. The
mixture was mechanically stirred for 2 hours to form a homogeneous
casting dope. The resulting homogeneous casting dope was filtered
and allowed to degas overnight. The PI-1/PI-2(5:6) polyimide blend
dense film membrane was prepared from the bubble free casting dope
on a clean glass plate using a doctor knife with a 20-mil gap. The
membrane together with the glass plate was then put into a vacuum
oven. The solvents were removed by slowly increasing the vacuum and
the temperature of the vacuum oven. Finally, the membrane was dried
at 200.degree. C. under vacuum for at least 48 hours to completely
remove the residual solvents to form a polymer membrane in dense
film.
[0047] The PI-1/PI-2(5:6) polyimide blend dense film membrane was
further UV cross-linked by exposure to UV radiation using 254 nm
wavelength UV light with a radiation time of 10 minutes at
50.degree. C.
Example 2
Evaluation of the CO.sub.2/CH.sub.4 and H.sub.2/CH.sub.4 Separation
Performance of PI-1/PI-2(5:6) and UV Cross-Linked PI-1/PI-2(5:6)
Polyimide Blend Dense Film Membranes Prepared in Example 1
[0048] The PI-1/PI-2(5:6) and UV cross-linked PI-1/PI-2(5:6)
polyimide blend dense film membranes were tested for
CO.sub.2/CH.sub.4 and H.sub.2/CH.sub.4 separations at 50.degree. C.
under 791 kPa (100 psig) pure gas feed pressure. The results in
Tables 1 and 2 show that the new PI-1/PI-2(5:6) polyimide blend
dense film membrane has intrinsic CO.sub.2 permeability of 6.94
Barrers (1 Barrer=10.sup.-10 cm.sup.3 (STP) cm/cm.sup.2 s (cm Hg))
and single-gas CO.sub.2/CH.sub.4 selectivity of 34.4 at 50.degree.
C. under 791 kPa for CO.sub.2/CH.sub.4 separation. This membrane
also has intrinsic H.sub.2 permeability of 28.1 Barrers and
single-gas H.sub.2/CH.sub.4 selectivity of 139.1 at 50.degree. C.
under 791 kPa for H.sub.2/CH.sub.4 separation. It can be seen from
Tables 1 and 2 that the PI-1/PI-2(5:6) polyimide blend dense film
membrane showed significantly improved CO.sub.2/CH.sub.4 and
H.sub.2/CH.sub.4 selectivities after UV cross-linking
TABLE-US-00001 TABLE 1 Pure gas permeation test results of
PI-1/PI-2(5:6) and UV cross-linked PI-1/PI-2(5:6) polyimide blend
dense film membranes for CO.sub.2/CH.sub.4 separation* Dense Film
Membrane P.sub.CO2 (Barrer) .alpha..sub.CO2/CH4 PI-1/PI-2(5:6) 6.94
34.4 UV cross-linked PI-1/PI-2(5:6) 6.33 48.0 *P.sub.CO2 and
P.sub.CH4 were tested at 50.degree. C. and 791 kPa (100 psig); 1
Barrer = 10.sup.-10 cm.sup.3(STP) cm/cm.sup.2 sec cmHg.
TABLE-US-00002 TABLE 2 Pure gas permeation test results of
PI-1/PI-2(5:6) and UV cross-linked PI-1/PI-2(5:6) polyimide blend
dense film membranes for H.sub.2/CH.sub.4 separation* Dense Film
Membrane P.sub.H2 (Barrer) .alpha..sub.CO2/CH4 PI-1/PI-2(5:6) 28.1
139.1 UV cross-linked PI-1/PI-2(5:6) 27.6 209.0 *P.sub.H2 and
P.sub.CH4 were tested at 50.degree. C. and 791 kPa (100 psig); 1
Barrer = 10.sup.-10 cm.sup.3(STP) cm/cm.sup.2 sec cmHg.
Example 3
Preparation of PI-1/PI-2(5:6) Polyimide Blend Hollow Fiber
Membranes
[0049] A hollow fiber spinning dope containing 11.9 g of PI-1
polyimide, 14.3 g of PI-2 polyimide, 65.0 g of NMP, and 6.0 g of
1,3-dioxolane was prepared. The spinning dope was extruded at a
flow rate of 3.0-4.0 mL/min through a spinneret at 50.degree. C.
spinning temperature. A bore fluid containing 10% by weight of
water in NMP was injected to the bore of the fiber at a flow rate
of 0.6 mL/min simultaneously with the extruding of the spinning
dope. The nascent fiber traveled through an air gap length of 5-7
cm at room temperature, and then was immersed into a water
coagulant bath at 19.degree. C. and wound up at a rate of 23-30
m/min. The water-wet fiber was annealed in a hot water bath at
85.degree. C. for 30 minutes. The annealed water-wet fiber was then
sequentially exchanged with methanol and hexane for three times and
for 30 minutes each time, followed by drying at 100.degree. C. in
an oven for 1 hour to form PI-1/PI-2(5:6) polyimide blend hollow
fiber membranes with the spinning conditions listed in Table 3.
TABLE-US-00003 TABLE 3 Spinning conditions for PI-1/PI-2(5:6)
polyimide blend hollow fiber membranes Air gap Dope rate Bore rate
Take-up rate Hollow Fiber Membrane (cm) (mL/min) (mL/min) (m/min)
PI-1/PI-2-O 5 3.0 0.6 23.5 PI-1/PI-2-P 5 3.0 0.6 30.2 PI-1/PI-2-W
10 3.6 0.6 23.5 PI-1/PI-2-Y 7 3.0 0.6 23.5 PI-1/PI-2-G 3 4.0 0.6
30.2
Example 4
Preparation of UV Cross-Linked PI-1/PI-2(5:6) Polyimide Blend
Hollow Fiber Membranes
[0050] The PI-1/PI-2(5:6) polyimide blend hollow fiber membranes
prepared in Example 3 were coated with 5 wt % of thermally curable
RTV silicone solution in hexane and then cured at 85.degree. C. for
1 hour. The RTV silicone-coated polyimide blend hollow fiber
membranes were cross-linked via UV radiation for 2.5 minutes using
a UV lamp with intensity of 1.45 mW/cm.sup.2.
Example 5
Evaluation of CO.sub.2/CH.sub.4 Separation Performance of UV
Cross-Linked PI-1/PI-2(5:6) Polyimide Blend Hollow Fiber
Membranes
[0051] The PI-1/PI-2-O-5Si2.5U, PI-1/PI-2-P-5Si2.5U, and
PI-1/PI-2-Y-5Si2.5U UV cross-linked polyimide blend hollow fiber
membranes were tested for CO.sub.2/CH.sub.4 separation at
50.degree. C. under 5617 kPa (800 psig) feed gas pressure with 10%
of CO.sub.2 and 90% of CH.sub.4 in the feed. The results are shown
in Table 4. It can be seen from Table 4 that all membranes
described in the current invention showed high CO.sub.2 permeances
of 68-74 GPU and high CO.sub.2/CH.sub.4 selectivities of 28-30.
TABLE-US-00004 TABLE 4 CO.sub.2/CH.sub.4 separation performance of
PI-1/PI-2(5:6) and UV cross-linked PI-1/PI-2(5:6) polyimide blend
hollow fiber membranes Membrane CO.sub.2 permeance (GPU)
CO.sub.2/CH.sub.4 selectivity PI-1/PI-2-O--5Si2.5U 73.7 30.1
PI-1/PI-2-P--5Si2.5U 70.2 30.3 PI-1/PI-2-Y--5Si2.5U 68.1 28.2 1 GPU
= 10.sup.-6 cm.sup.3 (STP)/cm.sup.2 s (cm Hg)Testing conditions:
50.degree. C., 5617 kPa (800 psig) feed gas pressure, 10% CO.sub.2
and 90% of CH.sub.4 in the feed.
Example 6
Evaluation of H.sub.2/CH.sub.4 Separation Performance of UV
Cross-Linked PI-1/PI-2(5:6) Polyimide Blend Hollow Fiber
Membranes
[0052] The PI-1/PI-2-O-5Si2.5U, PI-1/PI-2-P-5Si2.5U, and
PI-1/PI-2-Y-5Si2.5U UV cross-linked polyimide blend hollow fiber
membranes were tested for H.sub.2/CH.sub.4 separation at 50.degree.
C. under 5617 kPa (800 psig) feed gas pressure with 10% of H.sub.2
and 90% of CH.sub.4 in the feed. The results are shown in Table 5.
It can be seen from Table 5 that all membranes described in the
current invention showed high H.sub.2 permeances of 187-232 GPU and
high H.sub.2/CH.sub.4 selectivities of 145-224.
TABLE-US-00005 TABLE 5 H.sub.2/CH.sub.4 separation performance of
PI-1/PI-2(5:6) and UV cross-linked PI-1/PI-2(5:6) polyimide blend
hollow fiber membranes Membrane H.sub.2 permeance (GPU)
H.sub.2/CH.sub.4 selectivity PI-1/PI-2-O--5Si2.5U 198.1 184.2
PI-1/PI-2-P--5Si2.5U 231.6 224.0 PI-1/PI-2-Y--5Si2.5U 187.3 145.1 1
GPU = 10.sup.-6 cm.sup.3 (STP)/cm.sup.2 s (cm Hg)Testing
conditions: 50.degree. C., 5617 kPa (800 psig) feed gas pressure,
10% H.sub.2 and 90% of CH.sub.4 in the feed.
Example 7
Preparation of PI-1/PI-2(6:5) and UV Cross-Linked PI-1/PI-2(6:5)
Polyimide Blend Dense Film Membranes
[0053] 3.0 G of aromatic polyimide poly(3,3',4,4'-benzophenone
tetracarboxylic dianhydride-pyromellitic
dianhydride-2,4,6-trimethyl-m-phenylenediamine-2,4-toluenediamine)
(PI-1) containing UV cross-linkable carbonyl groups and 2.5 g of
poly(3,3',4,4'-benzophenone tetracarboxylic
dianhydride-2,4-toluenediamine-4,4'-methylenedianiline) (PI-2)
containing UV cross-linkable carbonyl groups were dissolved in a
solvent mixture of 12.0 g of NMP and 15.0 g of 1,3-dioxolane. The
mixture was mechanically stirred for 2 hours to form a homogeneous
casting dope. The resulting homogeneous casting dope was filtered
and allowed to degas overnight. The PI-1/PI-2(6:5) polyimide blend
dense film membrane was prepared from the bubble free casting dope
on a clean glass plate using a doctor knife with a 20-mil gap. The
membrane together with the glass plate was then put into a vacuum
oven. The solvents were removed by slowly increasing the vacuum and
the temperature of the vacuum oven. Finally, the membrane was dried
at 200.degree. C. under vacuum for at least 48 hours to completely
remove the residual solvents to form a polymer membrane in dense
film.
[0054] The PI-1/PI-2(6:5) polyimide blend dense film membrane was
further UV cross-linked by exposure to UV radiation using 254 nm
wavelength UV light with a radiation time of 10 minutes at
50.degree. C.
Example 8
Evaluation of the CO.sub.2/CH.sub.4 and H.sub.2/CH.sub.4 Separation
Performance of PI-1/PI-2(6:5) and UV Cross-Linked PI-1/PI-2(6:5)
Polyimide Blend Dense Film Membranes Prepared in Example 7
[0055] The PI-1/PI-2(6:5) and UV cross-linked PI-1/PI-2(6:5)
polyimide blend dense film membranes were tested for
CO.sub.2/CH.sub.4 and H.sub.2/CH.sub.4 separations at 50.degree. C.
under 791 kPa (100 psig) pure gas feed pressure. The results in
Tables 6 and 7 show that the new PI-1/PI-2(6:5) polyimide blend
dense film membrane has intrinsic CO.sub.2 permeability of 10.8
Barrers (1 Barrer=10.sup.-10 cm.sup.3 (STP) cm/cm.sup.2 s (cm Hg))
and single-gas CO.sub.2/CH.sub.4 selectivity of 30.0 at 50.degree.
C. under 791 kPa for CO.sub.2/CH.sub.4 separation. This membrane
also has intrinsic H.sub.2 permeability of 36.1 Barrers and
single-gas H.sub.2/CH.sub.4 selectivity of 100.7 at 50.degree. C.
under 791 kPa for H.sub.2/CH.sub.4 separation. It can be seen from
Tables 6 and 7 that the PI-1/PI-2(6:5) polyimide blend dense film
membrane showed significantly improved CO.sub.2/CH.sub.4 and
H.sub.2/CH.sub.4 selectivities after UV cross-linking
TABLE-US-00006 TABLE 6 Pure gas permeation test results of
PI-1/PI-2(6:5) and UV cross-linked PI-1/PI-2(6:5) polyimide blend
dense film membranes for CO.sub.2/CH.sub.4 separation* Dense Film
Membrane P.sub.CO2 (Barrer) .alpha..sub.CO2/CH4 PI-1/PI-2(6:5) 10.8
30.0 UV cross-linked PI-1/PI-2(6:5) 9.40 36.3 *P.sub.CO2 and
P.sub.CH4 were tested at 50.degree. C. and 791 kPa (100 psig); 1
Barrer = 10.sup.-10 cm.sup.3(STP) cm/cm.sup.2 sec.cmHg.
TABLE-US-00007 TABLE 7 Pure gas permeation test results of
PI-1/PI-2(6:5) and UV cross-linked PI-1/PI-2(6:5) polyimide blend
dense film membranes for H.sub.2/CH.sub.4 separation* Dense Film
Membrane P.sub.H2 (Barrer) .alpha..sub.CO2/CH4 PI-1/PI-2(6:5) 36.1
100.7 UV cross-linked PI-1/PI-2(6:5) 35.2 135.9 *P.sub.H2 and
P.sub.CH4 were tested at 50.degree. C. and 791 kPa (100 psig); 1
Barrer = 10.sup.-10 cm.sup.3(STP) cm/cm.sup.2 sec cmHg.
Example 9
Preparation of PI-1/PI-2(6:5) Polyimide Blend Hollow Fiber
Membranes
[0056] A hollow fiber spinning dope containing 14.4 g of PI-1
polyimide, 12.0 g of PI-2 polyimide, 65.0 g of NMP, and 6.0 g of
1,3-dioxolane was prepared. The spinning dope was extruded at a
flow rate of 3.0-3.8 mL/min through a spinneret at 50.degree. C.
spinning temperature. A bore fluid containing 10% by weight of
water in NMP was injected to the bore of the fiber at a flow rate
of 0.6 mL/min simultaneously with the extruding of the spinning
dope. The nascent fiber traveled through an air gap length of 7-10
cm at room temperature, and then was immersed into a water
coagulant bath at 19.degree. C. and wound up at a rate of 23-30
m/min. The water-wet fiber was annealed in a hot water bath at
85.degree. C. for 30 minutes. The annealed water-wet fiber was then
sequentially exchanged with methanol and hexane for three times and
for 30 minutes each time, followed by drying at 100.degree. C. in
an oven for 1 hour to form PI-1/PI-2(6:5) polyimide blend hollow
fiber membranes with the spinning conditions listed in Table 8. The
PI-1/PI-2(6:5) polyimide blend hollow fiber membranes were then
coated with 2 wt % of thermally curable RTV silicone solution in
hexane and then cured at 100.degree. C. for 1 hour.
TABLE-US-00008 TABLE 8 Spinning conditions for PI-1/PI-2(6:5)
polyimide blend hollow fiber membranes Air gap Dope rate Bore rate
Take-up rate Hollow Fiber Membrane (cm) (mL/min) (mL/min) (m/min)
PI-1/PI-2-6-5-O-2RTV 7 3.8 0.6 30.2 PI-1/PI-2-6-5-G-2RTV 7 3.8 0.6
23.5 PI-1/PI-2-6-5-B-2RTV 7 3.0 0.6 23.5
Example 10
Evaluation of CO.sub.2/CH.sub.4 Separation Performance of
PI-1/PI-2(6:5) Polyimide Blend Hollow Fiber Membranes
[0057] The PI-1/PI-2-6-5-O-2RTV and PI-1/PI-2-6-5-B-2RTV polyimide
blend hollow fiber membranes were tested for CO.sub.2/CH.sub.4
separation at 50.degree. C. under 5617 kPa (800 psig) feed gas
pressure with 10% of CO.sub.2 and 90% of CH.sub.4 in the feed. The
results are shown in Table 9. It can be seen from Table 9 that
PI-1/PI-2-6-5-O-2RTV membrane described in the current invention
showed CO.sub.2 permeances of 57 GPU and high CO.sub.2/CH.sub.4
selectivities of 28. PI-1/PI-2-6-5-B-2RTV membrane described in the
current invention showed high CO.sub.2 permeances of 78 GPU and
high CO.sub.2/CH.sub.4 selectivities of 25.5.
TABLE-US-00009 TABLE 9 CO.sub.2/CH.sub.4 separation performance of
PI-1/PI-2(6:5) polyimide blend hollow fiber membranes Membrane
CO.sub.2 permeance (GPU) CO.sub.2/CH.sub.4 selectivity
PI-1/PI-2-6-5-O-2RTV 57.0 28.0 PI-1/PI-2-6-5-B-2RTV 77.8 25.5 1 GPU
= 10.sup.-6 cm.sup.3 (STP)/cm.sup.2 s (cm Hg)Testing conditions:
50.degree. C., 5617 kPa (800 psig) feed gas pressure, 10% CO.sub.2
and 90% of CH.sub.4 in the feed.
Example 11
Evaluation of H.sub.2/CH.sub.4 Separation Performance of
PI-1/PI-2(6:5) Polyimide Blend Hollow Fiber Membranes
[0058] The PI-1/PI-2-6-5-G-2RTV and PI-1/PI-2-6-5-B-2RTV polyimide
blend hollow fiber membranes were tested for H.sub.2/CH.sub.4
separation at 50.degree. C. under 5617 kPa (800 psig) feed gas
pressure with 10% of H.sub.2 and 90% of CH.sub.4 in the feed. The
results are shown in Table 10. It can be seen from Table 10 that
both membranes described in the current invention showed high
H.sub.2 permeances of 285-298 GPU and high H.sub.2/CH.sub.4
selectivities of about 150-160.
TABLE-US-00010 TABLE 10 H.sub.2/CH.sub.4 separation performance of
PI-1/PI-2(6:5) polyimide blend hollow fiber membranes Membrane
H.sub.2 permeance (GPU) H.sub.2/CH.sub.4 selectivity
PI-1/PI-2-6-5-G-2RTV 285 149.7 PI-1/PI-2-6-5-B-2RTV 298 161.9 1 GPU
= 10.sup.-6 cm.sup.3 (STP)/cm.sup.2 s (cm Hg)Testing conditions:
50.degree. C., 5617 kPa (800 psig) feed gas pressure, 10% H.sub.2
and 90% of CH.sub.4 in the feed.
* * * * *