U.S. patent application number 12/182230 was filed with the patent office on 2010-02-04 for membrane contactor systems for gas-liquid contact.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Vishal Bansal.
Application Number | 20100024651 12/182230 |
Document ID | / |
Family ID | 41058289 |
Filed Date | 2010-02-04 |
United States Patent
Application |
20100024651 |
Kind Code |
A1 |
Bansal; Vishal |
February 4, 2010 |
MEMBRANE CONTACTOR SYSTEMS FOR GAS-LIQUID CONTACT
Abstract
A membrane contactor system for removing a component from a gas,
comprising a housing defining a gas flow path; a microporous
membrane positioned in the housing to allow the gas to flow across
the membrane, wherein the membrane comprises a structure of nodes
connected by fibrils in which surfaces of the structure of nodes
and fibrils define a plurality of interconnecting pores extending
through the microporous membrane, wherein the plurality of
interconnecting pores are configured to allow the component to
diffuse therethrough; and an oleophobic coating disposed on the
microporous membrane to form a coated membrane and configured to
provide oleophobicity to the coated membrane without blocking the
plurality of interconnecting pores.
Inventors: |
Bansal; Vishal; (Overland
Park, KS) |
Correspondence
Address: |
CANTOR COLBURN, LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
41058289 |
Appl. No.: |
12/182230 |
Filed: |
July 30, 2008 |
Current U.S.
Class: |
96/13 ; 96/11;
96/12 |
Current CPC
Class: |
B01D 67/0088 20130101;
B01D 71/36 20130101; B01D 53/62 20130101; Y02A 50/20 20180101; Y02C
20/40 20200801; Y02C 10/04 20130101; B01D 53/229 20130101; B01D
2251/80 20130101; B01D 53/77 20130101; Y02A 50/2342 20180101; B01D
2325/38 20130101; B01D 53/228 20130101; Y02C 10/10 20130101; B01D
61/246 20130101; B01D 2257/504 20130101 |
Class at
Publication: |
96/13 ; 96/11;
96/12 |
International
Class: |
B01D 53/92 20060101
B01D053/92; B01D 71/06 20060101 B01D071/06; B01D 53/22 20060101
B01D053/22; B01D 71/36 20060101 B01D071/36; B01D 53/62 20060101
B01D053/62 |
Claims
1. A membrane contactor system for removing a component from a gas,
comprising: a housing defining a gas flow path; a microporous
membrane positioned in the housing to allow the gas to flow across
the membrane, wherein the membrane comprises a structure of nodes
connected by fibrils in which surfaces of the structure of nodes
and fibrils define a plurality of interconnecting pores extending
through the microporous membrane, wherein the plurality of
interconnecting pores are configured to allow the component to
diffuse therethrough; and an oleophobic coating disposed on the
microporous membrane to form a coated membrane and configured to
provide oleophobicity to the coated membrane without blocking the
plurality of interconnecting pores.
2. The membrane contactor system of claim 1, wherein the
microporous membrane is expanded polytetrafluoroethylene.
3. The membrane contactor system of claim 1, wherein the coated
membrane has an oleophobic rating of at least 2.
4. The membrane contactor system of claim 1, wherein the coated
membrane has an oleophobic rating of at least 4.
5. The membrane contactor system of claim 1, wherein the coated
membrane has an oleophobic rating of at least 6.
6. The membrane contactor system of claim 1, wherein the coated
membrane has an oleophobic rating of at least 8.
7. The membrane contactor system of claim 1, further comprising a
support layer upon which the microporous membrane is disposed.
8. The membrane contactor system of claim 7, wherein the support
layer comprises a textile material.
9. The membrane contactor system of claim 1, wherein the
microporous membrane is configured to be a hollow tube.
10. The membrane contactor system of claim 1, wherein the
oleophobic coating comprises a polymer comprising fluorinated
C.sub.1-32 hydrocarbon moieties.
11. The membrane contactor system of claim 10, wherein the
fluorinated polymer comprises units derived from polymerization of
fluoro(C.sub.1-16)alkyl acrylates, fluoro(C.sub.1-16)alkyl
methacrylates, perfluoro(C.sub.1-16)alkyl acrylates,
perfluoro(C.sub.1-16)alkyl methacrylates, fluorinated and
perfluorinated C.sub.1-12 olefins, fluoro(C.sub.1-12)alkyl maleic
acid esters, perfluoro(C.sub.1-12)alkyl maleic acid esters,
fluoro(C.sub.1-12)alkyl (C.sub.6-12)aryl urethane oligomers,
fluoro(C.sub.1-12)alkyl allyl urethane oligomers,
fluoro(C.sub.1-12)alkyl urethane acrylate oligomers,
fluoro(C.sub.1-12)alkyl urethane acrylate oligomers, or a
combination comprising at least one of the foregoing.
12. A membrane contactor system for use in separating carbon
dioxide from a gaseous stream in a continuous flow process, the
system comprising: a housing defining a gas flow path and
comprising a first outlet for the carbon dioxide and a second
outlet for the purified gas; an expanded polytetrafluoroethylene
microporous membrane positioned in the housing to allow the gaseous
stream to flow across a side of the expanded
polytetrafluoroethylene microporous membrane, wherein the expanded
polytetrafluoroethylene microporous membrane comprises a structure
of nodes connected by fibrils in which surfaces of the structure of
nodes and fibrils define a plurality of interconnecting pores
extending through the expanded polytetrafluoroethylene microporous
membrane, wherein the plurality of interconnecting pores are
configured to allow the carbon dioxide to diffuse therethrough; an
oleophobic enhancement coating disposed on the surfaces of the
structure of nodes and fibrils to form a coated membrane and
configured to provide oleophobicity to the coated membrane without
blocking the plurality of interconnecting pores; and an amine based
sorbent liquid disposed on a side of the expanded
polytetrafluoroethylene microporous membrane opposite the gas,
wherein the amine based sorbent liquid is configured to absorb the
carbon dioxide from the gaseous stream.
13. The membrane contactor system of claim 11, wherein the coated
membrane has an oleophobic rating of at least 4.
14. The membrane contactor system of claim 11, wherein the coated
membrane has an oleophobic rating of at least 6.
15. The membrane contactor system of claim 11, wherein the coated
membrane has an oleophobic rating of at least 8.
16. The membrane contactor system of claim 11, wherein the
oleophobic coating comprises a polymer comprising fluorinated
C.sub.1-32 hydrocarbon moieties.
17. The membrane contactor system of claim 16, wherein the
fluorinated polymer is comprises units derived from polymerization
of fluoro(C.sub.1-16)alkyl acrylates, fluoro(C.sub.1-16)alkyl
methacrylates, perfluoro(C.sub.1-16)alkyl acrylates,
perfluoro(C.sub.1-16)alkyl methacrylates, fluorinated and
perfluorinated C.sub.1-12 olefins, fluoro(C.sub.1-12)alkyl maleic
acid esters, perfluoro(C.sub.1-12)alkyl maleic acid esters,
fluoro(C.sub.1-12)alkyl (C.sub.6-12)aryl urethane oligomers,
fluoro(C.sub.1-12)alkyl allyl urethane oligomers,
fluoro(C.sub.1-12)alkyl urethane acrylate oligomers,
fluoro(C.sub.1-12)alkyl urethane acrylate oligomers, or a
combination comprising at least one of the foregoing.
18. The membrane contactor system of claim 11, wherein the amine
based sorbent liquid comprises 2-amino-2-methyl-1, 3-propanediol,
2-hydroxyethyl piperazine, methyldiethanolamine, monoethanolamine,
tetraethylenepentamine, triethanolamine, polyethylene imine, or a
combination comprising at least one of the foregoing.
19. The membrane contactor system of claim 18, wherein the amine
based sorbent liquid further comprises a solvent, wherein the
solvent comprises alcohol, cyclic ketone, ester, ether, glycerol,
methoxy triethylene, glycol diacetate, polyethylene glycol,
propylene carbonate, 1,2-propylene glycol, or a combination
comprising at least one of the foregoing.
20. A gas turbine engine comprising: an exhaust treatment system
configured to remove carbon dioxide from a combustion exhaust
stream, wherein the system comprises: a housing defining a gas flow
path and comprising a first outlet for the carbon dioxide and a
second outlet for the purified exhaust; an expanded
polytetrafluoroethylene microporous membrane positioned in the
housing to allow the exhaust stream to flow across a side of the
membrane, wherein the membrane comprises a structure of nodes
connected by fibrils in which surfaces of the structure nodes and
fibrils define a plurality of interconnecting pores extending
through the membrane, wherein the pores are configured to allow the
carbon dioxide to diffuse therethrough; an oleophobic enhancement
coating disposed on the surfaces of structure of nodes and fibrils
and configured to provide oleophobicity to the membrane without
blocking the plurality of interconnecting pores; and an amine based
sorbent liquid disposed on a side of the membrane opposite the
exhaust stream, wherein the amine based sorbent liquid is
configured to absorb the carbon dioxide from the exhaust stream to
form the purified exhaust.
Description
BACKGROUND OF THE INVENTION
[0001] This disclosure relates generally to membrane contactor
systems for gas-liquid contacting in process industry and, more
specifically, to membrane contactors utilizing an
oleophobically-treated expanded polytetrafluoroethylene
membrane.
[0002] Systems for capturing and/or separating liquids and gases
are desirable in a variety of applications. Exemplary gas-liquid
contacting applications can include carbon dioxide separation,
natural gas sweetening, degasification of oil, and the like. For
example, the removal of carbon dioxide or other compounds from
gases may be desirable or necessary for a number of reasons. If a
gas is to be burned as fuel or emitted into the atmosphere as a
waste flow, the removal of carbon dioxide from the gas is necessary
in order to satisfy the carbon dioxide emission requirements which
are set by air pollution control authorities. In natural gas, for
instance, removing carbon dioxide (CO.sub.2) from the gas can
satisfy sales specifications or other process-dependent
requirements.
[0003] Several systems exist for removing components, such as
CO.sub.2, from gases. Packed bed scrubbers, distillation columns,
strippers, and the like, are all apparatuses used in such
gas-liquid contacting applications for separation/removal of
components. An example of a removal process using absorption
includes removing CO.sub.2 from flue gas by means of an aqueous
amine solution. The gas to be separated is led into an absorption
column where it comes into contact with amine solution, which
absorbs the CO.sub.2 molecules. The solvent can then be led to a
desorption process where the liquid is heated, and the CO.sub.2
molecules are removed from the amine solvent by means of a
desorption column. The solvent is cooled and passed back to the
absorption column, while the concentrated CO.sub.2 is removed.
[0004] In an absorption column, the amount of contact time with the
solvent can determine the degree of purification for the gas.
Therefore, a certain liquid surface area per volume must exist for
contact with the gas in order to purify the gas. Moreover, the
amount of gas which has to be treated factors into the size of the
apparatus. An absorption column, therefore, can require a large
diameter and height in order to treat a desired amount of gas to
the desired purification. Not only does this increase the cost of
the system, but it can also impact the system's utility in
applications where size and weight are a particularly expensive
commodity, such as in offshore installations.
[0005] Gas absorption membranes are used as contacting devices
between a gas and a liquid flow. These membrane contactors contain
a porous membrane, which promotes contact between the liquid and
the gas phase. The separation is caused by the presence of an
absorption liquid (e.g., an amine solvent) on one side the
membrane, which selectively removes certain components from the gas
flow (e.g., CO.sub.2) from the other side of the membrane. This
technology is currently being used as a substitute for the
apparatuses mentioned above in gas-liquid contacting
applications.
[0006] The replacement of the conventional absorption columns with
membrane contactors can lead to significant reductions both with
regard to cost and weight for a separation system absorption unit.
Membrane contactors can provide up to multiple orders of magnitude
more surface area per volume than the conventional absorption
contactors mentioned above, due to the porous nature of the
membrane. Further, membrane contactors are free from problems like
channeling and flooding that can occur in packed and tray
columns.
[0007] Membrane contactors, however, can suffer from some
drawbacks. Often times the membrane material, and the porosity
thereof, is useful only for a limited range of liquids. Moreover,
the pores of the membrane can block over time, thereby reducing the
effectiveness of the contactor. Some membranes tend to absorb
certain liquids and/or contaminating agents. The materials can clog
up the pores of the membrane and prevent the desired gas from
diffusing therethrough, or the membrane can no longer effectively
resist penetration by the liquid phase.
SUMMARY OF THE INVENTION
[0008] Disclosed herein are membrane contactor systems,
particularly those used for carbon dioxide separation, and methods
for removing carbon dioxide from a flue gas stream in a gas turbine
system. According to an embodiment, a membrane contactor system for
removing a component from a gas comprises a housing defining a gas
flow path; a microporous membrane positioned in the housing to
allow the gas to flow across the membrane, wherein the membrane
comprises a structure of nodes connected by fibrils in which
surfaces of the structure of nodes and fibrils define a plurality
of interconnecting pores extending through the microporous
membrane, wherein the plurality of interconnecting pores are
configured to allow the component to diffuse therethrough; and an
oleophobic coating disposed on the microporous membrane to form a
coated membrane and configured to provide oleophobicity to the
coated membrane without blocking the plurality of interconnecting
pores.
[0009] In another embodiment, a membrane contactor system for use
in separating carbon dioxide from a gaseous stream in a continuous
flow process comprises a housing defining a gas flow path and
comprising a first outlet for the carbon dioxide and a second
outlet for the purified gas; an expanded polytetrafluoroethylene
microporous membrane positioned in the housing to allow the gaseous
stream to flow across a side of the expanded
polytetrafluoroethylene microporous membrane, wherein the expanded
polytetrafluoroethylene microporous membrane comprises a structure
of nodes connected by fibrils in which surfaces of the structure of
nodes and fibrils define a plurality of interconnecting pores
extending through the expanded polytetrafluoroethylene microporous
membrane, wherein the plurality of interconnecting pores are
configured to allow the carbon dioxide to diffuse therethrough; an
oleophobic enhancement coating disposed on the surfaces of the
structure of nodes and fibrils to form a coated membrane and
configured to provide oleophobicity to the coated membrane without
blocking the plurality of interconnecting pores; and an amine based
sorbent liquid disposed on a side of the expanded
polytetrafluoroethylene microporous membrane opposite the gas,
wherein the amine based sorbent liquid is configured to absorb the
carbon dioxide from the gaseous stream.
[0010] In still another embodiment, a gas turbine engine comprises
an exhaust treatment system configured to remove carbon dioxide
from a combustion exhaust stream, wherein the system comprises: a
housing defining a gas flow path and comprising a first outlet for
the carbon dioxide and a second outlet for the purified exhaust; an
expanded polytetrafluoroethylene microporous membrane positioned in
the housing to allow the exhaust stream to flow across a side of
the membrane, wherein the membrane comprises a structure of nodes
connected by fibrils in which surfaces of the structure nodes and
fibrils define a plurality of interconnecting pores extending
through the membrane, wherein the pores are configured to allow the
carbon dioxide to diffuse therethrough; an oleophobic enhancement
coating disposed on the surfaces of structure of nodes and fibrils
and configured to provide oleophobicity to the membrane without
blocking the plurality of interconnecting pores; and an amine based
sorbent liquid disposed on a side of the membrane opposite the
exhaust stream, wherein the amine based sorbent liquid is
configured to absorb the carbon dioxide from the exhaust stream to
form the purified exhaust.
[0011] The above-described and other features are exemplified by
the following Figures and detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Referring now to the Figures, which are exemplary
embodiments, and wherein the like elements are numbered alike:
[0013] FIG. 1 is a schematic illustration of an embodiment of
gas/liquid oleophobically-treated membrane contactor;
[0014] FIG. 2 is an enlarged schematic illustration of a portion of
the membrane illustrated in FIG. 1;
[0015] FIG. 3 is a greatly enlarged schematic sectional
illustration of a portion of the membrane in FIG. 2, illustrating a
coating disposed on the surfaces in the membrane;
[0016] FIG. 4 is schematic illustration of an exemplary embodiment
of a membrane contactor system for carbon dioxide removal; and
[0017] FIG. 5 is an enlarged schematic illustration of an
oleophobically-treated membrane used in the membrane contactor
system of FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The membrane contactor systems described herein include an
oleophobically-treated expanded polytetrafluoroethylene (PTFE)
membrane. The oleophobic treatment on the expanded PTFE membrane
can allow for the membrane contactor system to be effective in
applications covering a wider range of liquids than current
membrane contactors. The oleophobic treatment, moreover, can
increase the operating life of the membrane in the contactor by
reducing the amount of blockage of pores that can occur during use.
The membrane contactor described herein can be disposed in any
gas-liquid contacting application, such as, without limitation,
carbon dioxide separation/removal, natural gas sweetening, oil
degasification, and the like. In an exemplary embodiment, the
membrane contactor system can replace current absorption units in a
carbon dioxide (CO.sub.2) separation process. Employing the
membrane contactor system instead of current separation
technologies (e.g., packed bed scrubbers) for carbon dioxide
removal can reduce the installation and operation costs of the
system; reduce the space requirements and weight of the system; and
improve the environmental impact of the system by reducing the
amount of liquid stream carry over by the gas phase.
[0019] As mentioned, the membrane contactor system comprises a
porous membrane and is configured to promote contact between a
liquid and a gas phase. When the membrane is placed between the gas
and the liquid (i.e., the solvent), the liquid will not be in
direct contact with the gas, which is in motion. This division
between the gas and liquid phases makes it possible to employ a
high gas rate in the system without the liquid being carried along
by the gas. FIG. 1 illustrates an exemplary embodiment of a
membrane 10. The size of the pores 12 in the membrane 10 can be
selected according to the following reasoning: the pores 12 are so
large that the "X" gas molecules (e.g. CO.sub.2) move (diffuse)
rapidly through the pores 12 and into the solvent, and the pores 12
are so small that solvent does not penetrate into the pores 12 and
through the membrane 10. The membrane of the system described
herein is advantageously treated with an oleophobic enhancement
material.
[0020] "Oleophobicity" of the membrane can be rated on a scale of 1
to 8 according to AATCC test 118-1992. This test evaluates the
membrane's resistance to wetting. Eight standard oils, labeled #1
to #8, are used in the test. The #1 oil is mineral oil (surface
tension: 31.5 dynes/cm @ 25 degrees Celsius (.degree. C.)) and the
#8 oil is heptane (surface tension: 14.8 dynes/cm @ 25.degree. C.).
Five drops of each rated oil are placed on the membrane. Failure
occurs when wetting of the membrane by a selected oil occurs within
30 seconds. The oleophobic rating of the membrane corresponds to
the last oil successfully tested. The higher the oleophobic rating,
the better the oleophobicity. After treatment, the membrane 10 can
have an increased oleophobicity. In an exemplary embodiment, the
oleophobicity of the membrane 10 is at least 1, specifically at
least 2, more specifically at least 4, even more specifically at
least 6, and most specifically at least 8.
[0021] The membrane 10 is comprised of an expanded PTFE (ePTFE). As
illustrated in FIG. 2, the ePTFE is porous, specifically
microporous, with a three-dimensional matrix or lattice-type
structure of nodes 22 interconnected by numerous fibrils 24.
Surfaces of the nodes 22 and fibrils 24 define numerous
interconnecting pores 26 that extend through the membrane 10
between opposite sides of the membrane. Expansion of the PTFE
material to form the microporous structure is well known to those
having skill in the art, and essentially involves stretching the
PTFE at a certain rate and temperature as described, for example,
in U.S. Pat. No. 3,953,566. Exemplary ePTFE materials are available
commercially from, for example, Tetratec #1305 (Tetratec,
Philadelphia, Pa.) or Poreflon.RTM. WP-100, Sumitomo Electric
Industries, Osaka, Japan.
[0022] ePTFE typically provides air permeability, while being
hydrophobic. This property of the ePTFE material is useful in
applications where restriction of water is desirable. The ePTFE
material can be coated with an oleophobic material in such a way
that provides oleophobic properties to the ePTFE without
compromising the hydrophobicity or the permeability of the
material. Turning now to FIG. 3, the oleophobic treatment can
adhere a coating 28 to the nodes 22 and fibrils 24 that define the
pores 26 in the membrane 10. The oleophobic coating 28 can also
conform to the surfaces of at least most, specifically all, the
nodes 22 and fibrils 24 that define the pore 26 in the membrane 10.
Again, the coating 28 improves the oleophobicity of the membrane 10
by resisting contamination from absorption of contaminating
materials. This permits the treated ePTFE membrane to be useful
with a wider range of liquids and gases, because the oleophobic
coated pores prevent the contaminating materials from absorbing in
the membrane. As used herein, "contaminating
materials/contaminants" is generally used to mean any material that
is not the desired molecule to be absorbed in the membrane for the
particular application, or any material that can cause blockage of
the pores in the membrane.
[0023] The size of the pores of the oleophobically-treated ePTFE
can contribute to determining the effective range of molecules that
can be prevented or restricted from flow through the membrane 10.
Some factors considered in choosing pore size are viscosity and
pressure on the liquid side of the membrane. In addition, pore size
can have an effect on the rate at which gas can migrate across the
membrane. Exemplary average pore size can range from about 0.05
micrometers (.mu.m) to about 1.5 .mu.m, specifically from about 0.2
.mu.m to about 1.0 .mu.m, and more specifically from about 0.5
.mu.m to about 0.8 .mu.m. However, larger or smaller average pore
sizes may be used. The average pore size of the particular membrane
10 will depend upon the intended application for the membrane
contactor system, and the range of fluids involved therewith.
[0024] Another factor in the flow of the fluid through the
oleophobically-treated ePTFE membrane 10 is the porosity of the
membrane, (i.e., the percentage of open space in the volume of the
membrane, as determined by comparison of the density of the ePTFE
with respect to the density of nonporous, non-expanded PTFE).
Exemplary porosity of the membrane 10 can be in the range from
about 20% or to about 95%. Again, depending upon the application of
the membrane contactor system, the porosity of the membrane can
range from about 70% to about 95%, specifically from about 80% to
about 95%, and more specifically from about 85% to about 95%, for
most gas-liquid contacting applications.
[0025] The dimensions (e.g., length, width, or diameter) of the
membrane 10 can vary and will again depend on the use of the
membrane contactor system. For example, a membrane contactor system
used as an absorption unit for removal of CO.sub.2 from a turbine
exhaust gas can have dimensions based on, among other things, the
size of the port through which the exhaust gas flows, the flow rate
through the port, the average amount of CO.sub.2 in the exhaust,
the amount of CO.sub.2 to be absorbed during the lifetime of the
system, and the like. Depending upon the size of the intended
application, the membrane contact system can comprise a single
layer of oleophobically-treated ePTFE membrane, or multiple layers.
In one embodiment, the membrane can be configured as a hollow tube
or tubes. In such a configuration, liquid can be disposed on the
outside of the tube(s) and the gas can be disposed inside.
Conversely, the hollow tube membrane can be as effective when the
liquid is disposed within the tube(s) and the gas is disposed
outside.
[0026] The thickness of the membrane 10 can depend, for example, on
the amount of absorption desired, the average pore size of the
membrane 10, the expected or desired lifetime of the membrane
contactor system, the average pore size of other possible membrane
layers in the system, and the durability of the membrane 10.
Generally, the thicker the membrane 10, the more restricted the
flow of fluid and/or molecules through the membrane 10. Thus, for
example, the thickness of a membrane in a CO.sub.2
separation/removal process may be determined by a balance of a
desired rate of mass transport of fluid (e.g., exhaust gas) across
the membrane and a desired amount of filtration of CO.sub.2. In
some embodiments, useful, for example as absorption units,
thicknesses of the membrane in the membrane contactor system can be
in a range of about 0.5 .mu.m to about 500 .mu.m. In exemplary
embodiments, the thickness of the membrane can range from about 4
.mu.m to about 200 .mu.m, specifically from about 10 .mu.m to about
150 .mu.m, and more specifically from about 25 .mu.m to about 100
.mu.m. However, larger and smaller thicknesses can be used.
[0027] In another embodiment, the membrane can be disposed on a
substrate layer. The substrate layer can impart strength and
durability to the membrane. In an exemplary embodiment, the
substrate layer can be a textile material. A textile material can
be a fabric, netting, and the like. In one embodiment, the membrane
can be laminated to the substrate layer. The lamination of membrane
to the substrate layer or layers can be by thermal means, adhesive
means or the like. In this specific embodiment, the overall
thickness of the system can be bigger due to the addition of the
substrate.
[0028] The membrane 10 is treated using an oleophobic coating
material, in one embodiment to increase the oleophobicity of the
membrane. Exemplary oleophobic coating materials include
fluorinated polymers, which as used herein includes homopolymers
and copolymers having fluorohydrocarbon and/or perfluorohydrocarbon
moieties. The fluoro- or perfluorohydrocarbon moieties can be
incorporated into the polymer backbone, pendant from the polymer
backbone, or a combination thereof. Accordingly, a variety of
different types of polymers can be used, including, for example,
polyolefins, polyacrylates, polymethacrylates, polyesters,
polysulfones, polyethersulfones, polycarbonates, polyethers,
polyamides, polyacrylamides, polysulfonamides, polysiloxanes, and
polyurethanes.
[0029] The fluorinated polymers can be derived from polymerization
of a variety of monomers or oligomers known to produce the desired
backbone ands that include fluorinated or perfluorinated C.sub.1-32
hydrocarbon moieties, in particular fluoro(C.sub.1-32)alkyl and/or
perfluoro(C.sub.1-32)alkyl moieties. In one embodiment,
perfluoro(C.sub.1-16)alkyl moieties are present, in particular,
--CF.sub.3, --CF.sub.2CF.sub.3, and --CF.sub.2CF.sub.2CF.sub.3. In
another embodiment, perfluoro(C.sub.1-4)alkylene moieties are
present, in particular, --CF.sub.2--, --CF.sub.2CF.sub.2--, and
--CF.sub.2CF.sub.2CF.sub.2--. Exemplary monomer or oligomer units
can include, for example, fluoro(C.sub.1-16)alkyl acrylates,
fluoro(C.sub.1-16)alkyl methacrylates, perfluoro(C.sub.1-16)alkyl
acrylates, perfluoro(C.sub.1-16)alkyl methacrylates, fluorinated
and perfluorinated C.sub.1-12 olefins such as tetrafluoroethylene,
fluoro(C.sub.1-12)alkyl maleic acid esters,
perfluoro(C.sub.1-12)alkyl maleic acid esters,
fluoro(C.sub.1-12)alkyl (C.sub.6-12)aryl urethane oligomers,
fluoro(C.sub.1-12)alkyl allyl urethane oligomers,
fluoro(C.sub.1-12)alkyl urethane acrylate oligomers,
fluoro(C.sub.1-12)alkyl urethane acrylate oligomers, and the like.
The fluorinated monomers or oligomers can optionally be
copolymerized with additional non-fluorinated monomers or oligomers
including, for example, unsaturated hydrocarbons (e.g., olefins),
(C.sub.1-12)alkyl acrylates, and (C.sub.1-12)alkyl
methacrylates.
[0030] Specific exemplary classes of these oleophobic polymers
include, without limitation, apolar perfluoroalkylpolyethers having
--CF.sub.3, --CF.sub.2CF.sub.3, and
-CF.sub.2CF.sub.2CF.sub.3moieties (PFPE), mixtures of apolar (PFPE)
with polar monofunctional PFPE, polar water-insoluble PFPE with
phosphate, silane, or amide end groups, mixtures of apolar PFPE
with fluorinated or perfluorinated (C.sub.1-12)alkyl methacrylate
polymers or fluorinated or perfluorinated (C.sub.1-12)alkyl
acrylate polymers, and copolymers comprising
perfluoro(C.sub.1-3)alkylether units and fluorinated or
perfluorinated (C.sub.1-12)alkyl methacrylate units or fluorinated
or perfluorinated (C.sub.1-12)alkyl acrylate units. The
above-mentioned polymers can be crosslinked by, for example, UV
radiation in aqueous form solution or emulsion. Mixtures of the
fluorinated polymers can be used as well.
[0031] One specific form that the oleophobic polymers are
commercially available is emulsions. Exemplary emulsions can
include, without limitation, those based on copolymers of siloxanes
and perfluoro(C.sub.1-12)alkyl-substituted acrylates or
methacrylates, emulsions based on fluorinated or perfluorinated co-
or terpolymers, one type of unit containing at least
hexafluoropropene or perfluoroalkyl vinyl ether, emulsions based on
perfluoro(C.sub.1-12)alkyl-substituted polyacrylates and
methacrylates, and the like. These polymers and their preparation
are well known to those with skill in the art. A specific
oleophobic fluorinated polymer is a perfluoroalkyl acrylic
copolymer and/or perfluoroalkyl methacrylic copolymer water-based
dispersion of Zonyl.RTM. 8195, 7040, 8412, and/or 8300, available
from Dupont of Wilmington, Del.
[0032] The microporous ePTFE membrane is rendered oleophobic by
treating it with an oleophobic coating composition. The process of
treating the membrane can comprise any suitable method for
oleophobically coating an article, and are well known to those
skilled in the art. Exemplary techniques can include applying the
oleophobic coating composition in a liquid form, e.g., a melt, or
solution, or latex dispersion of the material. Exemplary methods
for applying the liquid oleophobic enhancement material can
include, without limitation, dipping, painting, spraying,
roller-coating, brushing, and the like, over the surface of the
membrane. Regardless of the technique, the application can be
carried out until internal surfaces of the microporous membrane
structure are coated with the oleophobic coating composition, but
not until the pores are filled as that could lessen the gas-liquid
absorption property of the membrane. Thus, the presence of the
oleophobic coating composition has little effect on the porosity;
that is, the walls defining the voids in the microporous membrane
have only a very thin coating of the oleophobic material (as
illustrated in FIG. 3). Application of the oleophobic coating
composition can be achieved by varying the concentration, solids
content of the solution or dispersion, and/or by varying the
application temperature, or pressure.
[0033] The use of an organic solvent can help to facilitate the
distribution of the oleophobic fluorinated polymer throughout the
microporous membrane. Typically, the microporous membrane is not
initially oleophobic and may be oleophilic. Thus, use of an organic
solvent can sometimes reduce difficulties in wetting and/or
saturating the membrane structure with the oleophobic coating
composition. A variety of organic solvents can be used. The term
"organic solvent" is intended to generally refer to non-aqueous
solvents and combinations of non-aqueous solvents, and, in
particular, to solvents comprising organic compounds. As used
herein, the oleophobic material is "dissolved in an organic
solvent" if at least about 50 wt. % of the material is dissolved in
the organic solvent. Exemplary organic solvents can include,
without limitation, alkanes, ketones, esters, ethers, alcohols, and
the like, as well as combinations of these solvents. For example,
exemplary organic solvents can include heptane, ethyl acetate,
butyl acetate, isoamyl acetate, dioctyl adipate, acetone, methyl
ethyl ketone, methyl isobutyl ketone, isopropanol, diethyl ether,
mineral spirits, petroleum distillate, and combinations thereof The
choice of organic solvent or solvents for use with the oleophobic
material can be affected by a variety of factors including, without
limitation, solubility of the oleophobic fluoropolymer, boiling
point of the solvent, molecular weight of the solvent, polarity of
the solvent or solvent combination, and the like.
[0034] During application to the membrane, the oleophobic coating
composition can wet and saturate the membrane 10. The solvent can
then be removed, for example, by air drying or heating. The
oleophobic is disposed on the membrane 10 and can impart
oleophobicity to the membrane contactor system. It is possible in
some embodiments to achieve covalent coupling between the
oleophobic coating and the membrane. In an optional embodiment, the
oleophobically-treated ePTFE membrane can be "cured" by heating.
This "curing" process can possibly increase the oleophobicity by
allowing rearrangement of the fluoropolymer into a specific
oleophobic orientation. The application of heat can permit the
oleophobic fluoropolymer to flow around the nodes 22 and fibrils 24
to form the coating 28. The curing temperature can vary among the
oleophobic fluoropolymers. Exemplary ranges can include from about
40.degree. C. to about 140.degree. C., specifically about
50.degree. C. to about 130.degree. C., and more specifically about
70.degree. C. and about 125.degree. C.
[0035] In one embodiment, the fluorinated polymer is in the form of
a stabilized water-miscible dispersion of the polymer solids. In
this embodiment, the oleophobic fluoropolymer solids can also
contain relatively small amounts of acetone and ethylene glycol or
other water-miscible solvents and surfactants that were used in the
polymerization reaction when the fluorinated polymer solids were
made. Optionally, the dispersion of oleophobic fluorinated polymer
solids is stabilized with a stabilizing agent, such as, but not
limited to, deionized and/or demineralized water. The stabilizing
agent reduces the propensity of the oleophobic fluorinated polymer
solids from settling out and agglomerating to a size which cannot
enter a pore in the membrane to be coated. Although the coating
composition may include other amounts of stabilizing agent, in some
embodiments the coating composition forming coating layer includes
an amount of stabilizing agent in the range of about 5 wt % to 50
wt %. For example, in some embodiments the coating composition
includes an amount of stabilizing agent in the range of about 15 wt
% to about 25 wt %.
[0036] The stabilized dispersion of oleophobic fluorinated polymer
solids can be diluted in one or more suitable solvents to form the
coating composition that will form coating layer. Although other
solvents may be used, suitable solvents can include, but are not
limited to, water, ethanol, isopropyl alcohol, acetone, methanol,
n-propanol, n-butanol, N,N-dimethylformamide, methyl ethyl ketone
and water soluble e-and p-series glycol ethers. Moreover, although
the solvents can have other surface tensions, in some embodiments,
the coating composition includes a solvent having a surface tension
of less than about 31 dynes per centimeter. After coating, as
described above, the coating composition is then consolidated, for
example by heating the coated membrane such that the oleophobic
fluorinated polymer solids flow and coalesce, and such that the
stabilizing agents and solvents are removed. During the application
of heat, the thermal mobility of the oleophobic fluoropolymer
solids allows the solids to be mobile and flow around, engage, and
adhere to surfaces of the membrane, and therefore coalesce to form
the coating layer.
[0037] Irrespective of the solvent or carrier used, the coating
compositions can include an amount of oleophobic fluoropolymer
solids in the range of about 0.1 wt % to about 10 wt % based on a
total weight of the coating composition. For example, in some
embodiments, the coating composition includes oleophobic
fluoropolymer solids in the range of about 0.5 wt % to about 1.5 wt
%. When the coating composition includes other amounts of solvent,
other than water, the coating composition that forms coating layer
includes an amount of solvent, other than water, in the range of
about 40 wt % to about 80 wt %. For example, in some embodiments
the coating composition includes an amount of solvent, other than
water, in the range of about 50 wt % to about 75 wt %.
[0038] The coating composition has a surface tension and a relative
contact angle that enable the coating composition to wet pores in
the membrane such that pores are coated with the oleophobic
fluorinated polymer solids in the coating composition. However, in
some embodiments where an organic solvent is used as described
above, the membrane is wet with a solution containing a solvent
before the coating composition is applied to membrane such that the
coating composition will pass through membrane pores and "wet-out"
surfaces of membrane.
[0039] The thickness of coating layer formed and the amount and
type of fluorinated polymer solids in the coating layer can depend
on several factors, including the affinity of the solids to adhere
and conform to the surfaces of the membrane that define membrane
pores, the final solids content within the coating composition, the
coating process, and the intended use and desired durability during
use.
[0040] It is not necessary that the coating composition completely
encapsulate the entire surface of the membrane network, or be
continuous to increase oleophobicity of the membrane. However, in
one embodiment, at least 50%, specifically at least 75%, and more
specifically at least 90% of the membrane surfaces are coated.
[0041] The oleophobically-treated ePTFE membrane can be
advantageously employed as a membrane contactor in gas/liquid
contacting applications. The membrane contactor system can be
particularly useful as a as the absorption unit of a CO.sub.2
removal system for a gas turbine. While the discussion below
focuses on the use of the membrane contactor in a downstream
application for gas turbines, it is to be understood that the
membrane contactor described herein can be employed in any system
employing liquid-gas contacting technology. The membrane contactor
system can be particularly effective in processes where weight,
size, cost, energy consumption, and environmental aspects are key
concerns.
[0042] FIG. 4 illustrates an exemplary embodiment of a membrane
contactor system 100 that functions as a CO.sub.2 absorption unit
in a gas turbine engine system. The membrane contactor system 100
is configured to remove CO.sub.2 or other compounds from combustion
gases (e.g., natural gas). With recent trends in environmental
regulation, it is becoming increasingly important to cheaply and
effectively remove CO.sub.2 emissions in hydrocarbon combustion
processes. Exhaust gas is created after natural gas, or some other
combustive fuel, are ignited in a gas turbine for energy. After
being expanded through the turbine, the exhaust gas stream 102 is
fed into the absorption unit 100. The exhaust gas stream 102 can be
cooled first in order to help reduce the volume of the gas, and
therefore, to reduce the size and weight of the membrane contactor
system 100. Again, the membrane contactor system is configured to
remove the CO.sub.2 from the exhaust gas stream 102. The purified
gas 104 (i.e., the CO.sub.2-free exhaust) exits the top of the
membrane contactor system 100, and can be vented to atmosphere or
further treated or recycled for energy recovery. The CO.sub.2-rich
stream 106 can exit a lower portion of the membrane contactor
system 100 and continue on for further treatment, such as
desorption, compression, sequestration, and the like.
[0043] The membrane contactor system 100 is shown cutaway to reveal
the individual membrane contactors 110 disposed in the system
housing 108. The membrane contactor system 100 can contain a single
membrane contactor or multiple membrane contactors, and will depend
on the volume of the exhaust gas, the concentration of CO.sub.2 in
the gas, and the like. The oleophobically-treated ePTFE membranes
in the CO.sub.2 removal system can have a variety of shapes
including, without limitation, sheets, hollow fibers, and the like.
As shown in FIG. 4, the membranes contactors 110 comprise sheets of
membranes 112.
[0044] The carbon dioxide removal system of the present disclosure
can comprise the venting membrane contactor system 100 with an
amine based absorption liquid. This system can utilize an
amine-based sorbent and a membrane contactor at the appropriate
partial pressure to absorb the CO.sub.2 from the exhaust stream.
The amine based sorbent is configured to absorb CO.sub.2 from the
gaseous stream while the membrane contactor system, comprised of a
plurality of membranes, is configured to support the amine based
sorbent and separate the amine based sorbent, the gaseous stream,
and the CO.sub.2.
[0045] The amine based sorbent is a liquid sorbent which is capable
of absorbing CO.sub.2. Exemplary characteristics of the amine based
sorbent can include low volatility, nontoxicity, low viscosity, the
ability to absorb CO.sub.2 from low partial pressures (e.g., less
than about 1 kilopascal (kPa)), and the like. Exemplary amine based
sorbents can include amines, such as 2-amino-2-methyl-1,
3-propanediol, 2-hydroxyethyl piperazine, methyldiethanolamine,
monoethanolamine, tetraethylenepentamine, triethanolamine,
polyethylene imine, and other like amine based sorbents. In an
exemplary embodiment, the amine base sorbent can be a
monoethanolamine (MEA).
[0046] In order to further enhance the amine based sorbent's
CO.sub.2 sorption rates, reduce its viscosity, and facilitate
transport of the absorbed carbon dioxide, a solvent can be added to
the amine based sorbent. Exemplary solvents can possess the same
low volatility, low viscosity, and nontoxic properties of the amine
based sorbent. Exemplary solvents can include, without limitation,
alcohols, cyclic ketone, esters, ethers, and mixtures thereof,
including dimethyl ether of polyethylene glycol, glycerol, methoxy
triethylene, glycol diacetate, polyethylene glycol, propylene
carbonate, 1,2-propylene glycol, and the like. The solvent used
with a particular amine base sorbent for a given application can
readily be determined by one of skill in the art. Exemplary factors
for the selection of the solvent and the amine based sorbent can
include chemical compatibility, solubility of the amine based
sorbent in the solvent, absorption/desorption kinetics,
nontoxicity, low viscosity, low volatility, and the like.
[0047] The chemical reaction for amine absorption is:
2(R--NH.sub.2)+H.sub.2O+CO.sub.2--(R--NH.sub.3).sub.2CO.sub.3,
where R.dbd.C.sub.2H.sub.4OH
[0048] The reaction is reversible and the equilibrium can be
altered by altering the temperature. The CO.sub.2 is absorbed by an
absorption medium in a temperature range of about 20.degree. C. to
about 70.degree. C. The oleophobically-treated ePTFE membranes are
employed as contact devices between a gas and a liquid flow in the
membrane contactor system 100. FIG. 5 illustrates an exemplary
embodiment of an oleophobically-treated ePTFE membrane 150 in one
of the membrane contactors. The separation is caused by the
presence of the amine based sorbent liquid 158 on one side of the
membrane 150, which selectively removes CO.sub.2 156 from the
exhaust gas flow 157 on the other side of the membrane 150. The
membrane 150 is intended to provide a contacting area which
prevents mixing of the exhaust gas 157 and the amine based sorbent
158. The membrane 150, however, is advantageously permeable to the
CO.sub.2 156, which is required to be removed. The selectivity in
the separation process is derived from the amine based sorbent 158.
A highly selective separation can be achieved through an
appropriate choice of the amine based sorbent 158.
[0049] The removal of the exhaust gas CO.sub.2 component is
achieved by use of porous, oleophobic ePTFE membrane 150 in
combination with a suitable amine based sorbent liquid 158. As a
result of the membrane oleophobicity and small pore size, the
exhaust gas 157 and amine based sorbent liquid 158 can be kept
separate, and exhaust contaminant are prevented from blocking the
pores by the oleophobic coating of the membrane. The amine based
sorbent solution contacts the CO.sub.2 in the gaseous exhaust
stream within the membrane contactor. The membrane contactor system
100 promotes CO.sub.2 removal via partial pressure gradients by
utilizing the plurality of oleophobically-treated ePTFE membranes
that contain or support the amine based sorbent. As a result of the
porosity and oleophobic characteristics of the membranes, the
contactor system allows direct liquid-gas contact, while preventing
sorbent leakage, or blockage of the pores by contaminants in the
gaseous exhaust stream. The membrane pore size is chosen dependent
upon the operational characteristics of the amine based sorbent and
the oleophobically-treated ePTFE membrane utilized. In an exemplary
CO.sub.2 removal system application, the membrane pore size can be
less than about 0.1 .mu.m, specifically less than about 0.05
.mu.m.
[0050] The use of gas absorption membrane contactor systems
comprising oleophobically-treated ePTFE membranes have several
advantages over conventional contacting devices such as packed
columns. The compactness of the equipment through the use of hollow
fiber or sheet shaped membranes is typically smaller than packed
column filter media, because the membranes have a much greater
surface area per volume (cm.sup.2/cm.sup.3). The height of the
absorption unit in the CO.sub.2 removal system of a gas turbine
will likewise be significantly reduced compared to current columns
for the same reason. In some cases, the reduction can be as great
as about 4/5 of the packed column height. Moreover, the
oleophobically-treated ePTFE membranes do not have the same
entrainment, flooding, channeling or foaming issues typically found
in current packed bed columns. Still further, the oleophobic
coating on the membranes provide a membrane contactor system that
can be employed in a wider variety of liquid compositions, because
of the non-wetting nature of the pores. The coating also helps to
increase the use life of the membrane contactors, because the
oleophobic characteristics reduce the blockage of pores. All of
these benefits lead to size, weight, cost, and environmental
savings over current gas-liquid separation systems. The benefits of
which can be particularly useful in off-shore applications, where
space and weight are at a premium.
[0051] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. Ranges disclosed herein are inclusive and combinable
(e.g., ranges of "up to about 25 wt %, or, more specifically, about
5 wt % to about 20 wt %", is inclusive of the endpoints and all
intermediate values of the ranges of "about 5 wt % to about 25 wt
%," etc.). "Combination" is inclusive of blends, mixtures, alloys,
reaction products, and the like. Furthermore, the terms "first,"
"second," and the like, herein do not denote any order, quantity,
or importance, but rather are used to distinguish one element from
another, and the terms "a" and "an" herein do not denote a
limitation of quantity, but rather denote the presence of at least
one of the referenced item. The modifier "about" used in connection
with a quantity is inclusive of the stated value and has the
meaning dictated by context, (e.g., includes the degree of error
associated with measurement of the particular quantity). The suffix
"(s)" as used herein is intended to include both the singular and
the plural of the term that it modifies, thereby including one or
more of that term (e.g., the colorant(s) includes one or more
colorants). Reference throughout the specification to "one
embodiment", "another embodiment", "an embodiment", and so forth,
means that a particular element (e.g., feature, structure, and/or
characteristic) described in connection with the embodiment is
included in at least one embodiment described herein, and may or
may not be present in other embodiments. In addition, it is to be
understood that the described elements may be combined in any
suitable manner in the various embodiments.
[0052] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which the
embodiments of the invention belong. It will be further understood
that terms, such as those defined in commonly used dictionaries,
should be interpreted as having a meaning that is consistent with
their meaning in the context of the relevant art and the present
disclosure, and will not be interpreted in an idealized or overly
formal sense unless expressly so defined herein.
[0053] While the disclosure has been described with reference to an
exemplary embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the disclosure. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
disclosure without departing from the essential scope thereof.
Therefore, it is intended that the disclosure not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this disclosure, but that the disclosure will include
all embodiments falling within the scope of the appended
claims.
* * * * *