WO2010144057A1 - Double selective-layer membranes - Google Patents

Double selective-layer membranes Download PDF

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Publication number
WO2010144057A1
WO2010144057A1 PCT/SG2010/000219 SG2010000219W WO2010144057A1 WO 2010144057 A1 WO2010144057 A1 WO 2010144057A1 SG 2010000219 W SG2010000219 W SG 2010000219W WO 2010144057 A1 WO2010144057 A1 WO 2010144057A1
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WIPO (PCT)
Prior art keywords
layer
polymer
cellulose
membrane
polymeric membrane
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PCT/SG2010/000219
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French (fr)
Inventor
Kaiyu Wang
Tai-Shung Neal Chung
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National University Of Singapore
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Publication of WO2010144057A1 publication Critical patent/WO2010144057A1/en

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    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B01D69/1216—Three or more layers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/002—Forward osmosis or direct osmosis
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B01D69/1218—Layers having the same chemical composition, but different properties, e.g. pore size, molecular weight or porosity
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/02—Details relating to pores or porosity of the membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/02—Details relating to pores or porosity of the membranes
    • B01D2325/022—Asymmetric membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/02—Details relating to pores or porosity of the membranes
    • B01D2325/022—Asymmetric membranes
    • B01D2325/0233—Asymmetric membranes with clearly distinguishable layers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/02—Details relating to pores or porosity of the membranes
    • B01D2325/0283—Pore size
    • B01D2325/02831—Pore size less than 1 nm
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/04—Characteristic thickness
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
    • C02F1/445—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis by forward osmosis
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2103/00—Nature of the water, waste water, sewage or sludge to be treated
    • C02F2103/08—Seawater, e.g. for desalination
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A20/00—Water conservation; Efficient water supply; Efficient water use
    • Y02A20/124—Water desalination
    • Y02A20/131—Reverse-osmosis

Definitions

  • F Forward osmosis
  • this invention relates to a polymeric membrane.
  • the membrane includes a porous supporting layer having a first surface and a second surface opposed to the first surface, a first skin layer over the first surface of the supporting layer, and a second skin layer over the second surface of the supporting layer.
  • the supporting layer formed of a first polymer, has a first pore size and a first porosity.
  • the first skin layer, formed of a second polymer has a second pore size and a second porosity.
  • the second skin layer formed of a third polymer, has a third pore size and a third porosity. Both the second and third pore sizes are smaller than the first pore size and both the second and third porosities are smaller than the first porosity.
  • pore size refers to an average diameter of the pores or voids in different layers of the polymeric membrane and the term “porosity” refers to the volume ratio of the pores.
  • the pore size corresponds to the size of the largest molecule mat can permeate each individual layer of the membrane while the porosity corresponds to the density of each individual layer.
  • Embodiments of the membrane described above may include one or more of the following features.
  • the first skin layer is in contact with the first surface of the supporting layer.
  • the second skin layer is in contact with the second surface of the supporting layer.
  • the porous supporting layer has a thickness between 10 ⁇ m and 200 ⁇ m.
  • Each of the first skin layer and the second skin layer independently, has a thickness between 0.01 ⁇ m and 10 ⁇ m.
  • the first pore size is between 10 nm and 1000 nm.
  • Each of the second pore size and the third pore size independently, is between 0.1 nm and 0.5 nm (e.g., 0.2-0.5 nm).
  • the first porosity is between 30% and 90%. Both the second porosity and third porosity are less than 1%.
  • Two of the first, second, and third polymers are the same.
  • the first, second, and third polymers are the same.
  • Each of the first, second, and third polymers is a hydrophilic polymer such as cellulosic polymer (e.g., cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose dibutyrate, cellulose tributyrate, and a combination thereof), polyacrylonitrile, polyvinyl alcohol, polyamide, polyamide-imide, sulphonated polyethersulphone, sulphonated polysulphone, polybenzimidazole (PBI), and a combination thereof.
  • the membrane is in the form of a flat sheet or a hollow fiber.
  • the term "combination" refers to a polymer blend or a copolymer of the aforementioned polymers.
  • the invention in another aspect, relates to a method (e.g., a non-solvent induced phase inversion process) for producing a polymeric membrane.
  • the method includes mixing a polymer, a solvent, and a pore-forming additive to form a polymer solution; disposing the polymeric solution onto a surface of a substrate; immersing the substrate in a coagulation bath (e.g., a water coagulant bath), whereby the polymer solution coagulates to form on the substrate a polymeric film that includes a first surface layer in contact with the substrate, a second surface layer opposed to the first surface layer, and a porous middle layer between the first and second surface layers, the porous middle layer having a larger pore size and a higher porosity than both the first and the second surface layers; separating the polymeric film from the substrate; and heat-annealing the polymeric film in a water bath at 40 0 C- 120 0 C (e.g., 50-90 0 C).
  • non-solvent induced phase inversion refers to a process in which a polymer in a casting solution containing the polymer and a solvent (e.g., a ketone) is induced by a non-solvent (e.g., water) to precipitate out from the solution to form a film having a structure different from that of the polymer in the solution (i.e., polymer undergoing phase inversion).
  • a solvent e.g., a ketone
  • coagulation bath refers to a bath containing a suitable coagulant to cause a liquid or sol to coagulate.
  • Suitable coagulant for a cellulosic polymer solution include but are not limited to pure water, organic solvent miscible with water, an aqueous solution containing salts, or a combination thereof.
  • Different coagulants may be chosen for different polymer solutions.
  • the coagulants can be alcohol such as methanol, ethanol, and isopropanol.
  • water bath refers to a bath containing a heating medium having at least 50% of water by volume. The maximum heating temperature of the water bath can be controlled by selecting the composition of the heating medium. For example, when a mixture of water and a high boiling point organic solvent is used as the heating medium, the temperature of the water bath can reach beyond 100 0 C at room atmosphere.
  • Embodiments of the method described above may include one or more of the following features.
  • the polymer solution contains 10% to 35% (e.g., 18-35%) by weight of the polymer.
  • the polymer used is cellulosic polymer such as cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose dibutyrate, cellulose tributyrate, and a combination thereof.
  • the substrate can be formed of silica, alumina, mica, other metal oxides, or polymer.
  • the surface of the substrate upon which the polymer solution is disposed is a hydrophilic surface.
  • the substrate can either be a flat plate (e.g., a glass slide) or a rotating drum (e.g., that described in WO 2006/110497).
  • the solvent used is selected from the group consisting of N, N-dimethylacetamide(DMAc), dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), N-methyl-pyrrolidone (NMP), acetone, 1,4- dioxane, methyl ethyl ketone, and a combination thereof.
  • the pore-forming additive is selected from the group consisting of an alcohol (e.g., methanol, ethanol, propanol, and butanols), Methylene glycol , diethylene glycol, ethylene glycol, glycerol, ketone, an organic acid, polyethylene glycol, polyvinylpyrrolidone, formamide and a combination thereof.
  • the solvent and the pore-forming additive can be the same or different.
  • the coagulation bath may have a temperature between 0 0 C and 80 0 C (e.g., between 20 0 C and 60 0 C).
  • the porous middle layer of the polymeric film is in contact with both the first and second surface layers of the film.
  • the temperature of heat-annealing the polymeric film in a water bath can be controlled at 40 0 C- 120 0 C (e.g., 50-90 0 C).
  • the time of heat-annealing the polymeric film can be controlled between 5 min and 60 min.
  • the porous middle layer of the polymeric film still has a pore size of 10-1000 nm and a porosity of 30%-90% while each of the first and second surface layers, independently, has a pore size of 0.1-0.5 nm (e.g., 0.2-0.5 nm) and a porosity of less than 1%.
  • this invention relates to a method for extracting water from a saline solution through a forward osmosis process.
  • the method includes contacting a first saline solution with the first skin layer of the polymeric membrane described above and contacting a second saline solution with the second skin layer of the membrane to allow one of the first and second saline solutions to extract water from the other through a forward osmosis process.
  • the first and second saline solutions are separated by the membrane, the first saline solution has a first water content, and the second saline solution has a second water content different from the first water content or the two solutions have different osmotic pressures.
  • the membranes described herein include one or more of the following.
  • the membranes are suitable for use in the forward osmosis (FO) process, which requires less energy than the RO process for water treatment.
  • FO forward osmosis
  • FO offers the advantages of high rejection of a wide range of contaminants and lower membrane-fouling propensities than traditional pressure-driven membrane processes.
  • Membrane fouling can be significantly reduced, as the effect of internal concentration polarization has been eliminated by the two selective layers that prevent contact between the feed/draw solution and the middle porous supporting layer of the membrane.
  • Figure 1 illustrates the transport mechanism of an asymmetric membrane used in a pressure-retarded osmosis (PRO) process (left panel) and in a forward osmosis (FO) process (right panel), hi PRO mode, the draw solution is placed against the dense selective layer of the asymmetric membrane, while in FO mode, the feed is placed against the dense selective layer.
  • Figure 2 illustrates a cross section of a common composite reverse osmosis
  • Figures 3(a)-(e) are scanning electron microscopy (SEM) images that show the cross section and surface morphologies of a double selective-layer membrane made by the method described herein.
  • Figure 4 illustrates the transport mechanism of the double-selective layer membrane shown in Figure 2 in the top mode (left) or bottom mode (right), hi the top mode, the draw solution is placed against the thicker dense selective layer of the membrane, while in the bottom mode, the feed is placed against the thicker dense selective layer.
  • FIG 5 is a schematic of a forward osmosis testing set-up.
  • Figures 6 A and 6B demonstrate the effect of draw solution (MgCl 2 , 22 ⁇ 0.5 0 C) concentration on (6A) water permeation flux and (6B) salt leakage of the double selective-layer membrane shown in Figures 3(a)-(e). Pure water was used as the feed, hi the top mode, the draw solution was placed against the thicker selective layer of the membrane, while in the bottom mode, the feed was placed against the thicker selective layer. ⁇ o,b'- osmotic pressure of bulk draw solution.
  • Figure 7 demonstrate the effect of draw solution (MgCl 2 , 22 ⁇ 0.5 0 C) concentration on water permeation flux of the double selective-layer membrane shown in Figures 3(a)-(e).
  • draw solution MgCl 2 , 22 ⁇ 0.5 0 C
  • 3.5 wt% NaCl solution was used as the feed.
  • the draw solution was placed against the thicker selective layer of the membrane, while in the bottom mode, the feed was placed against the thicker selective layer.
  • ⁇ ,b osmotic pressure of bulk draw solution
  • ⁇ F ,b osmotic pressure of bulk feed solution.
  • Figure 8 shows the fouling test results with the two membranes in the top mode.
  • This invention is based in part on the unexpected discovery that the membranes having double selective layers described herein have very high water flux and a salt rejection of more than 99.5% during forward osmosis separation.
  • membranes having double selective layers described herein have very high water flux and a salt rejection of more than 99.5% during forward osmosis separation.
  • almost all membranes used in the FO process are asymmetric, i.e., having only one selective layer and a porous supporting layer.
  • the cross section of an asymmetric membrane is illustrated in Figure 2, left panel.
  • the double selective- layer membrane (as shown in Figure 2, right panel) described herein does not suffer from the influence of internal concentration polarization (ICP) in the FO process.
  • the double selective layers ensure the rejections of ions, sugar and other dissolved solutes, but let water transport freely, which results in the elimination of ICP through preventing the contact between the feed or draw solution and the porous support layer as the concentration gradient in the middle porous layer is prevented to be established.
  • ECP exterior concentration polarization
  • ICP interior concentration polarization
  • the additional selective layer of the membrane described herein induces extra water transport resistance and subsequently decreases the water permeation flux.
  • the double selective-layer membranes have comparable or greater water flux than those asymmetric membranes in the FO process. More specifically, the membrane described herein can achieve a water flux as high as 48.0 LMH (i.e., L/(m 2 -h)) without elevated operation temperatures and the salt leakage less than 10.0 GMH (i.e., g/(m 2 -h)) in the FO process.
  • the two selective layers of the membrane described herein have a much smaller pore size and porosity that the porous supporting layer.
  • the supporting layer has a pore size of 10-1000 nm and a porosity of 30-90% while the selective layers have a pore size of 0.2-0.5 nm and a porosity less than 1%.
  • the two selective layers one may have a slightly greater or smaller pore size/porosity than the other.
  • the two selective layers of the membrane described herein are formed of the same polymer, e.g., cellulosic polymer.
  • the supporting porous middle layer can also be formed of the same polymer as the selective layer.
  • a double selective-layer membrane formed via the phase inversion process described herein has all three layers formed of the same polymer.
  • the two selective layers are formed of different polymers. This can be achieved by coating an asymmetric membrane on the side of its porous layer with a polymer different from that constitutes the selective layer. Alternatively, this can be achieved by subjecting a double selective-layer membrane formed via the method described herein to a surface treatment process that results in different chemical compositions for the two selective layers.
  • polymers in one of the two selective layers are branched or cross-linked while those in the other selective layer are not.
  • Membranes of other layer configurations such as that having two selective layers formed of the same polymer but a porous supporting layer formed of a different polymer, are also contemplated and can be produced by, e.g., coating, surface treatment, or combination thereof.
  • the polymers constituting the selective layers and the porous supporting layer are hydrophilic.
  • hydrophilic polymer include cellulosic polymer (e.g., cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose dibutyrate, cellulose tributyrate, and a combination thereof), polyacrylonitrile, polyvinyl alcohol, polyamide, polyaniide-imide, sulphonated polyethersulphone, sulphonated polysulphone, polybenzimidazole (PBI), and a combination thereof.
  • PBI polybenzimidazole
  • the process includes, among others, coagulating a polymer solution in a coagulation bath (e.g., a water bath) to form upon a substrate a polymer film that has two dense surface layers sandwiching a porous layer.
  • a coagulation bath e.g., a water bath
  • the formation of the two dense surface layers depends on many parameters such as temperature of the coagulation bath, coagulant, dope components of the polymer solution, and substrate chemistry.
  • the coagulant water is the generally used coagulant.
  • a substrate having a hydrophilic surface appears to promote forming two dense layers.
  • Example 1 Fabrication of a double selective-layer membrane using cellulose acetate A homogeneous polymer solution was prepared by dissolving cellulose acetate (CA, 22.5 wt%) in a mixture of N-methyl-pyrrolidone (72.5 wt%) and acetone (5 wt%). Next, the polymeric solution was coated onto a surface of the horizontal glass plate to form a uniform polymer layer by a casting knife with a gap of 50 mm. The coated plate was then immersed into the water coagulant bath (25°C) to allow CA to precipitate and a membrane was formed on top of the plate.
  • CA cellulose acetate
  • N-methyl-pyrrolidone 72.5 wt%)
  • acetone 5 wt%
  • the membrane was separated from the plate and rinsed with water to get rid of residual solvent before it was heat annealed in a hot water bath at 85 o C for l5 min.
  • the membrane as obtained was examined with SEM. As shown in Figure 3, the membrane produced has two dense surface layers and a porous middle layer.
  • Example 2 Water and salt transport characterization Water flux of the membrane made in Example 1 in the FO process was determined by measuring the volume change of the feed solution over a selected period, using a setup as shown in Figure 5.
  • the setup includes a crossflow cell of a plate and frame design with a rectangular channel on each side of the membrane.
  • the flow velocity during the testing was set at 6.4 cm/s for both feed and draw solutions, which concurrently flew through the cell channels.
  • the temperatures of the feed and draw solutions were maintained at 22 ⁇ 0.5 0 C.
  • the pressures at two channel inlets were kept at 1.0 psi.
  • the draw solutions used were MgCl 2 solutions with different concentrations.
  • the salt leakage was calculated by measuring the conductivity in the feed solution at the end of experiment.
  • the water permeation flux (Jv) is calculated from the feed volume change.
  • ⁇ F(L) is the permeation water collected over a predetermined time At (h) in FO process duration and A is the effective membrane surface area (m 2 ).
  • the salt concentration in the feed water was determined from the conductivity measurement using a calibration curve for the single salt solution.
  • the salt leakage, i.e., salt back-flowing from the draw solution to the feed, J s in g/(m 2 -h) (abbreviated as gMH), is thereafter determined from the increase of the feed conductivity: J s A(C t Vdl(AAi) (2) where Q and V t are the salt concentration and the volume of the feed at the end of FO tests, respectively.
  • Example 1 For sea water desalination, its water flux was assessed in a manner similar to that described in Example 2 except that 3.5 wt% NaCl instead of pure water was used. As shown in Figure 7, the water permeation flux was close to that shown in Figure 6A. Again, the water flux in the bottom mode was observed to be higher than that in the top mode.
  • Example 4 Fabrication and characterization of a double selective-layer membrane using cellulose triacetate
  • a homogeneous polymer solution was prepared by dissolving the cellulose triacetate (CTA, 20.0 wt%) in N-methyl-pyrrolidone (80.0 wt%). Then, a membrane was prepared in a manner similar to that described in Example 1 except that the membrane was thermal annealed in hot water at 9O 0 C for 15 min. The membrane formed had two dense surface layers and a porous middle layer.
  • the membrane was then characterized in a manner similar to that described in Example 2.
  • the water flux of the membrane reached 42.2 LMH in the bottom mode, which was much higher than that in the top mode, i.e., 30.2 LMH, when using 5.0 M MgCl 2 as the draw solution.
  • Example 5 Membrane fouling test Two CA membranes were produced using different coagulants: pure water and
  • NMP/water in a volume ratio of 90/10 The membrane formed with water coagulant had double selective layers while the other membrane formed with NMP/water coagulant had a single selective layer.
  • a colloidal solution of aluminum oxide nanoparticles whose actual size is around 200 nm was employed as the feed. Water flux was normalized to the initial value when the feed was DI water. As illustrated in Figure 8, the single- selective-layer membrane showed more serious fouling. Its water flux dropped to about 70 percent of its initial value after several hours.
  • the double selective-layer membrane showed only a slight decrease (less than 10 percent) in water flux, hi addition, after washing the two membranes with DI water for 1 h, the water flux of the double selective-layer membrane almost went back to the initial value, while the water flux of the single dense-layer membrane only reached 80 percent of the initial value.

Abstract

A polymeric membrane includes a porous supporting layer having a first surface and a second surface opposed to the first surface, a first skin layer over the first surface of the supporting layer, and a second skin layer over the second surface of the supporting layer. The supporting layer, formed of a first polymer, has a first pore size and a first porosity. The first skin layer, formed of a second polymer, has a second pore size and a second porosity. The second skin layer, formed of a third polymer, has a third pore size and a third porosity. Both the second and third pore sizes are smaller than the first pore size and both the second and third porosities are smaller than the first porosity. The polymeric membrane can be used as a forward osmosis membrane.

Description

DOUBLE SELECITVΕ-LAYER MEMBRANES
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of the priority of U.S. Provisional Application Serial No. 61/185,647, filed June 10, 2009, the content of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
Forward osmosis (FO) is a process that uses the osmotic pressure gradient generated by a draw solution (a highly concentrated solution) to induce water to pass through a selectively permeable membrane so as to effect separation of water from dissolved solutes.
It has been used in numerous applications, such as water reclamation, wastewater treatment, seawater desalination, concentration of liquid foods, controlled release of drugs, power generation, and water purification and reuse in space. See, e.g., T. Y. Cath, et al., J. Membr. Sci. 2006, 281 : 70; J. R. McCutcheon et al.,
Desalination 2005, 174: 1; J.O. Kessler et al., Desalination 1976, 18: 297; K. L. Lee et al., J. Membr. Sci. 1981, 8: 141; B. Jiao et al., J. Food Eng. 2004, 63: 303; K. B.
Petrotos et al., J. Food Eng. 2001, 49: 201; J. Herron, WO 2006/110497; J.R.
McCutcheon et al., J. Membr. Sci. 2006, 284: 237; A. SeppSla et al., J. Membr. Sci. 1999, 161: 115; S. Loeb, Desalination 20001, 141: 85; T. Y. Cath et al., J. Membr. Sci.
2005, 257: 85; and T. Y. Cath et al., J. Membr. Sci. 2005, 257: 111.
There is a need for developing new FO membranes with high water flux and high ion or solute rejection properties.
SUMMARY OF THE INVENTION
In one aspect, this invention relates to a polymeric membrane. The membrane includes a porous supporting layer having a first surface and a second surface opposed to the first surface, a first skin layer over the first surface of the supporting layer, and a second skin layer over the second surface of the supporting layer. The supporting layer, formed of a first polymer, has a first pore size and a first porosity. The first skin layer, formed of a second polymer, has a second pore size and a second porosity.
The second skin layer, formed of a third polymer, has a third pore size and a third porosity. Both the second and third pore sizes are smaller than the first pore size and both the second and third porosities are smaller than the first porosity.
As used herein, the term "pore size" refers to an average diameter of the pores or voids in different layers of the polymeric membrane and the term "porosity" refers to the volume ratio of the pores. The pore size corresponds to the size of the largest molecule mat can permeate each individual layer of the membrane while the porosity corresponds to the density of each individual layer.
Embodiments of the membrane described above may include one or more of the following features. The first skin layer is in contact with the first surface of the supporting layer.
The second skin layer is in contact with the second surface of the supporting layer. The porous supporting layer has a thickness between 10 μm and 200 μm. Each of the first skin layer and the second skin layer, independently, has a thickness between 0.01 μm and 10 μm. The first pore size is between 10 nm and 1000 nm. Each of the second pore size and the third pore size, independently, is between 0.1 nm and 0.5 nm (e.g., 0.2-0.5 nm). The first porosity is between 30% and 90%. Both the second porosity and third porosity are less than 1%. Two of the first, second, and third polymers (e.g., the second and third polymers) are the same. The first, second, and third polymers are the same. Each of the first, second, and third polymers is a hydrophilic polymer such as cellulosic polymer (e.g., cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose dibutyrate, cellulose tributyrate, and a combination thereof), polyacrylonitrile, polyvinyl alcohol, polyamide, polyamide-imide, sulphonated polyethersulphone, sulphonated polysulphone, polybenzimidazole (PBI), and a combination thereof. The membrane is in the form of a flat sheet or a hollow fiber. The term "combination" refers to a polymer blend or a copolymer of the aforementioned polymers.
In another aspect, the invention relates to a method (e.g., a non-solvent induced phase inversion process) for producing a polymeric membrane. The method includes mixing a polymer, a solvent, and a pore-forming additive to form a polymer solution; disposing the polymeric solution onto a surface of a substrate; immersing the substrate in a coagulation bath (e.g., a water coagulant bath), whereby the polymer solution coagulates to form on the substrate a polymeric film that includes a first surface layer in contact with the substrate, a second surface layer opposed to the first surface layer, and a porous middle layer between the first and second surface layers, the porous middle layer having a larger pore size and a higher porosity than both the first and the second surface layers; separating the polymeric film from the substrate; and heat-annealing the polymeric film in a water bath at 400C- 1200C (e.g., 50-900C). The term "non-solvent induced phase inversion" refers to a process in which a polymer in a casting solution containing the polymer and a solvent (e.g., a ketone) is induced by a non-solvent (e.g., water) to precipitate out from the solution to form a film having a structure different from that of the polymer in the solution (i.e., polymer undergoing phase inversion). The term "coagulation bath" refers to a bath containing a suitable coagulant to cause a liquid or sol to coagulate. Examples of suitable coagulant for a cellulosic polymer solution include but are not limited to pure water, organic solvent miscible with water, an aqueous solution containing salts, or a combination thereof. Different coagulants may be chosen for different polymer solutions. For example, when PBI solution is used for producing a double selective- membrane via a phase inversion process, the coagulants can be alcohol such as methanol, ethanol, and isopropanol. The term "water bath" refers to a bath containing a heating medium having at least 50% of water by volume. The maximum heating temperature of the water bath can be controlled by selecting the composition of the heating medium. For example, when a mixture of water and a high boiling point organic solvent is used as the heating medium, the temperature of the water bath can reach beyond 1000C at room atmosphere.
Embodiments of the method described above may include one or more of the following features.
The polymer solution contains 10% to 35% (e.g., 18-35%) by weight of the polymer. The polymer used is cellulosic polymer such as cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose dibutyrate, cellulose tributyrate, and a combination thereof. The substrate can be formed of silica, alumina, mica, other metal oxides, or polymer. The surface of the substrate upon which the polymer solution is disposed is a hydrophilic surface. The substrate can either be a flat plate (e.g., a glass slide) or a rotating drum (e.g., that described in WO 2006/110497). The solvent used is selected from the group consisting of N, N-dimethylacetamide(DMAc), dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), N-methyl-pyrrolidone (NMP), acetone, 1,4- dioxane, methyl ethyl ketone, and a combination thereof. The pore-forming additive is selected from the group consisting of an alcohol (e.g., methanol, ethanol, propanol, and butanols), Methylene glycol , diethylene glycol, ethylene glycol, glycerol, ketone, an organic acid, polyethylene glycol, polyvinylpyrrolidone, formamide and a combination thereof. The solvent and the pore-forming additive can be the same or different. The coagulation bath may have a temperature between 0 0C and 80 0C (e.g., between 20 0C and 60 0C). The porous middle layer of the polymeric film is in contact with both the first and second surface layers of the film. The temperature of heat-annealing the polymeric film in a water bath can be controlled at 400C- 1200C (e.g., 50-900C). The time of heat-annealing the polymeric film can be controlled between 5 min and 60 min. Upon heat annealing, the porous middle layer of the polymeric film still has a pore size of 10-1000 nm and a porosity of 30%-90% while each of the first and second surface layers, independently, has a pore size of 0.1-0.5 nm (e.g., 0.2-0.5 nm) and a porosity of less than 1%.
In still another aspect, this invention relates to a method for extracting water from a saline solution through a forward osmosis process. The method includes contacting a first saline solution with the first skin layer of the polymeric membrane described above and contacting a second saline solution with the second skin layer of the membrane to allow one of the first and second saline solutions to extract water from the other through a forward osmosis process. The first and second saline solutions are separated by the membrane, the first saline solution has a first water content, and the second saline solution has a second water content different from the first water content or the two solutions have different osmotic pressures.
Advantages of the membranes described herein include one or more of the following. The membranes are suitable for use in the forward osmosis (FO) process, which requires less energy than the RO process for water treatment. In addition, FO offers the advantages of high rejection of a wide range of contaminants and lower membrane-fouling propensities than traditional pressure-driven membrane processes. Membrane fouling can be significantly reduced, as the effect of internal concentration polarization has been eliminated by the two selective layers that prevent contact between the feed/draw solution and the middle porous supporting layer of the membrane.
The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the following drawings, detailed description of several embodiments, and also from the appending claims.
BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the transport mechanism of an asymmetric membrane used in a pressure-retarded osmosis (PRO) process (left panel) and in a forward osmosis (FO) process (right panel), hi PRO mode, the draw solution is placed against the dense selective layer of the asymmetric membrane, while in FO mode, the feed is placed against the dense selective layer. Figure 2 illustrates a cross section of a common composite reverse osmosis
(RO) membrane with a single selective layer (left) and that of a membrane described herein with double selective layers (right).
Figures 3(a)-(e) are scanning electron microscopy (SEM) images that show the cross section and surface morphologies of a double selective-layer membrane made by the method described herein.
Figure 4 illustrates the transport mechanism of the double-selective layer membrane shown in Figure 2 in the top mode (left) or bottom mode (right), hi the top mode, the draw solution is placed against the thicker dense selective layer of the membrane, while in the bottom mode, the feed is placed against the thicker dense selective layer.
Figure 5 is a schematic of a forward osmosis testing set-up. Figures 6 A and 6B demonstrate the effect of draw solution (MgCl2, 22 ± 0.50C) concentration on (6A) water permeation flux and (6B) salt leakage of the double selective-layer membrane shown in Figures 3(a)-(e). Pure water was used as the feed, hi the top mode, the draw solution was placed against the thicker selective layer of the membrane, while in the bottom mode, the feed was placed against the thicker selective layer. πo,b'- osmotic pressure of bulk draw solution. Figure 7 demonstrate the effect of draw solution (MgCl2, 22 ± 0.50C) concentration on water permeation flux of the double selective-layer membrane shown in Figures 3(a)-(e). 3.5 wt% NaCl solution was used as the feed. In the top mode, the draw solution was placed against the thicker selective layer of the membrane, while in the bottom mode, the feed was placed against the thicker selective layer. ππ,b: osmotic pressure of bulk draw solution; πF,b: osmotic pressure of bulk feed solution.
Figure 8 shows the fouling test results with the two membranes in the top mode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
This invention is based in part on the unexpected discovery that the membranes having double selective layers described herein have very high water flux and a salt rejection of more than 99.5% during forward osmosis separation. Currently, almost all membranes used in the FO process are asymmetric, i.e., having only one selective layer and a porous supporting layer. The cross section of an asymmetric membrane is illustrated in Figure 2, left panel.
On the one hand, unlike these asymmetric membranes, the double selective- layer membrane (as shown in Figure 2, right panel) described herein does not suffer from the influence of internal concentration polarization (ICP) in the FO process. The double selective layers ensure the rejections of ions, sugar and other dissolved solutes, but let water transport freely, which results in the elimination of ICP through preventing the contact between the feed or draw solution and the porous support layer as the concentration gradient in the middle porous layer is prevented to be established. Only exterior concentration polarization (ECP) exists during the FO process, which has less effect on the water permeation flux and can be reduced by enhancing the flowing turbulence by crossflow. The water transport mechanism for the double selective-layer membrane is demonstrated in the Figure 4.
ICP seriously counteracts the driving force (forming a sharp concentration gradient within the porous support layer) and subsequently decreases the water permeation flux. Moreover, ICP within the membrane porous supporting layer cannot be eliminated by enhancing the crossflow along the membrane surface whereas the enhanced turbulence from the crossflow reduces the effect of exterior concentration polarization (ECP) near the dense selective layer, as shown in Figure 1.
On the other hand, as a selective layer of a membrane typically only allows water to permeate while rejecting ions, sugar, or other dissolved solutes, the additional selective layer of the membrane described herein induces extra water transport resistance and subsequently decreases the water permeation flux. Unexpectedly, the double selective-layer membranes have comparable or greater water flux than those asymmetric membranes in the FO process. More specifically, the membrane described herein can achieve a water flux as high as 48.0 LMH (i.e., L/(m2-h)) without elevated operation temperatures and the salt leakage less than 10.0 GMH (i.e., g/(m2-h)) in the FO process.
The two selective layers of the membrane described herein have a much smaller pore size and porosity that the porous supporting layer. Typically, the supporting layer has a pore size of 10-1000 nm and a porosity of 30-90% while the selective layers have a pore size of 0.2-0.5 nm and a porosity less than 1%. Between the two selective layers, one may have a slightly greater or smaller pore size/porosity than the other.
In one embodiment, the two selective layers of the membrane described herein are formed of the same polymer, e.g., cellulosic polymer. Further, the supporting porous middle layer can also be formed of the same polymer as the selective layer. For example, a double selective-layer membrane formed via the phase inversion process described herein has all three layers formed of the same polymer. hi another embodiment, the two selective layers are formed of different polymers. This can be achieved by coating an asymmetric membrane on the side of its porous layer with a polymer different from that constitutes the selective layer. Alternatively, this can be achieved by subjecting a double selective-layer membrane formed via the method described herein to a surface treatment process that results in different chemical compositions for the two selective layers. For instance, polymers in one of the two selective layers are branched or cross-linked while those in the other selective layer are not. Membranes of other layer configurations, such as that having two selective layers formed of the same polymer but a porous supporting layer formed of a different polymer, are also contemplated and can be produced by, e.g., coating, surface treatment, or combination thereof.
Preferably, the polymers constituting the selective layers and the porous supporting layer are hydrophilic. Examples of a hydrophilic polymer include cellulosic polymer (e.g., cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose dibutyrate, cellulose tributyrate, and a combination thereof), polyacrylonitrile, polyvinyl alcohol, polyamide, polyaniide-imide, sulphonated polyethersulphone, sulphonated polysulphone, polybenzimidazole (PBI), and a combination thereof. One way to synthesize the membrane of this invention is a phase inversion process. The process includes, among others, coagulating a polymer solution in a coagulation bath (e.g., a water bath) to form upon a substrate a polymer film that has two dense surface layers sandwiching a porous layer. The formation of the two dense surface layers depends on many parameters such as temperature of the coagulation bath, coagulant, dope components of the polymer solution, and substrate chemistry. As for the coagulant, water is the generally used coagulant. For example, a substrate having a hydrophilic surface appears to promote forming two dense layers.
Without further elaboration, it is believed that the above description has adequately enabled the present invention. The following examples are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. All of the publications cited herein are hereby incorporated by reference in their entirety.
Example 1 : Fabrication of a double selective-layer membrane using cellulose acetate A homogeneous polymer solution was prepared by dissolving cellulose acetate (CA, 22.5 wt%) in a mixture of N-methyl-pyrrolidone (72.5 wt%) and acetone (5 wt%). Next, the polymeric solution was coated onto a surface of the horizontal glass plate to form a uniform polymer layer by a casting knife with a gap of 50 mm. The coated plate was then immersed into the water coagulant bath (25°C) to allow CA to precipitate and a membrane was formed on top of the plate. After the plate was taken out of the coagulation bath, the membrane was separated from the plate and rinsed with water to get rid of residual solvent before it was heat annealed in a hot water bath at 85oC for l5 min. The membrane as obtained was examined with SEM. As shown in Figure 3, the membrane produced has two dense surface layers and a porous middle layer.
Example 2: Water and salt transport characterization Water flux of the membrane made in Example 1 in the FO process was determined by measuring the volume change of the feed solution over a selected period, using a setup as shown in Figure 5. The setup includes a crossflow cell of a plate and frame design with a rectangular channel on each side of the membrane. The flow velocity during the testing was set at 6.4 cm/s for both feed and draw solutions, which concurrently flew through the cell channels. The temperatures of the feed and draw solutions were maintained at 22 ± 0.50C. The pressures at two channel inlets were kept at 1.0 psi.
The draw solutions used were MgCl2 solutions with different concentrations. When using pure water as the feed, the salt leakage was calculated by measuring the conductivity in the feed solution at the end of experiment. The water permeation flux (Jv) is calculated from the feed volume change.
Jv = AV/(AAt) (1)
where ΔF(L) is the permeation water collected over a predetermined time At (h) in FO process duration and A is the effective membrane surface area (m2). The salt concentration in the feed water was determined from the conductivity measurement using a calibration curve for the single salt solution. The salt leakage, i.e., salt back-flowing from the draw solution to the feed, Js in g/(m2-h) (abbreviated as gMH), is thereafter determined from the increase of the feed conductivity: Js = A(CtVdl(AAi) (2) where Q and Vt are the salt concentration and the volume of the feed at the end of FO tests, respectively.
As shown in Figures 6 A and 6B, the water flux of the membrane went up with an increase in draw MgCl2 concentration while the salt leakages were satisfactorily low in any circumstances. Unexpectedly, the water flux reached 48.2 LMH in the bottom mode, which was much higher than that in the top mode. This result is opposite to the result using commercial RO and FO membranes during the FO process in which the water flux in the PRO mode is higher than that in the FO mode.
Example 3: Sea water desalination
To test the performance of membrane of Example 1 for sea water desalination, its water flux was assessed in a manner similar to that described in Example 2 except that 3.5 wt% NaCl instead of pure water was used. As shown in Figure 7, the water permeation flux was close to that shown in Figure 6A. Again, the water flux in the bottom mode was observed to be higher than that in the top mode.
Example 4: Fabrication and characterization of a double selective-layer membrane using cellulose triacetate
A homogeneous polymer solution was prepared by dissolving the cellulose triacetate (CTA, 20.0 wt%) in N-methyl-pyrrolidone (80.0 wt%). Then, a membrane was prepared in a manner similar to that described in Example 1 except that the membrane was thermal annealed in hot water at 9O0C for 15 min. The membrane formed had two dense surface layers and a porous middle layer.
The membrane was then characterized in a manner similar to that described in Example 2. The water flux of the membrane reached 42.2 LMH in the bottom mode, which was much higher than that in the top mode, i.e., 30.2 LMH, when using 5.0 M MgCl2 as the draw solution.
Example 5: Membrane fouling test Two CA membranes were produced using different coagulants: pure water and
NMP/water in a volume ratio of 90/10. The membrane formed with water coagulant had double selective layers while the other membrane formed with NMP/water coagulant had a single selective layer. hi the fouling test, a colloidal solution of aluminum oxide nanoparticles whose actual size is around 200 nm was employed as the feed. Water flux was normalized to the initial value when the feed was DI water. As illustrated in Figure 8, the single- selective-layer membrane showed more serious fouling. Its water flux dropped to about 70 percent of its initial value after several hours. In contrast, the double selective-layer membrane showed only a slight decrease (less than 10 percent) in water flux, hi addition, after washing the two membranes with DI water for 1 h, the water flux of the double selective-layer membrane almost went back to the initial value, while the water flux of the single dense-layer membrane only reached 80 percent of the initial value.
OTHER EMBODIMENTS
All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. For example, the membranes of this invention can be applied for gas separation. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features. From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:
1. A polymeric membrane comprising a porous supporting layer having a first surface and a second surface opposed to the first surface, the supporting layer being formed of a first polymer; a first skin layer over the first surface of the supporting layer, the first skin layer being formed of a second polymer; and a second skin layer over the second surface of the supporting layer, the second skin layer being formed of a third polymer; wherein the supporting layer has a first pore size and a first porosity, the first skin layer has a second pore size and a second porosity, and the second skin layer has a third pore size and a third porosity, both the second and third pore sizes being smaller than the first pore size and both the second and third porosities being smaller than the first porosity.
2. The polymeric membrane of claim 1 , wherein the first skin layer is in contact with the first surface of the supporting layer and the second skin layer is in contact with the second surface of the supporting layer.
3. The polymeric membrane of any preceding claim, wherein the second and third polymers are the same.
4. The polymeric membrane of any preceding claim, the first, second, and third polymers are the same.
5. The polymeric membrane of any preceding claim, wherein each of the first, second, and third polymers is a hydrophilic polymer.
6. The polymeric membrane of claim 5, wherein the hydrophilic polymer is selected from the group consisting of cellulosic polymer, polyacrylonitrile, polyvinyl alcohol, polyamide, polyamide-imide, sulphonated polyethersulphone, sulphonated polysulphone, polybenzimidazole, and a combination thereof.
7. The polymeric membrane of claim 6, wherein the cellulosic polymer is selected from the group consisting of cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose dibutyrate, cellulose tributyrate, and a combination thereof.
8. The polymeric membrane of any preceding claim, wherein the porous supporting layer has a thickness between 10 μm and 200 μm.
9. The polymeric membrane of any preceding claim, wherein each of the first skin layer and the second skin layer, independently, has a thickness between 0.01 μm and 10 μm.
10. The polymeric membrane of any preceding claim, wherein the first pore size is between 10 nm and 1000 nm and the first porosity is between 30% and 90%.
11. The polymeric membrane of any preceding claim, wherein each of the second pore size and the third pore size, independently, is between 0.1 nm and 0.5 nm.
12. The polymeric membrane of any preceding claim, wherein both the second porosity and third porosity are less than 1%.
13. The polymeric membrane of any preceding claim, wherein the membrane is in the form of a flat sheet or a hollow fiber.
14. A method of producing a polymeric membrane, the method comprising: mixing a polymer, a solvent, and a pore-foπning additive to form a polymer solution; disposing the polymeric solution onto a surface of a substrate; immersing the substrate in a coagulation bath, whereby the polymer solution coagulates to form on the substrate a polymeric film that includes a first surface layer in contact with the substrate, a second surface layer opposed to the first surface layer, and a porous middle layer between the first and second surface layers, the porous middle layer having a larger pore size and a higher porosity than both the first and the second surface layers; separating the polymeric film from the substrate; and heat-annealing the polymeric film in a water bath at 400C- 1200C.
15. The method of any of claims 14, wherein the polymer is cellulosic polymer.
16. The method of any of claims 15, wherein the cellulosic polymer is selected from the group consisting of cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose dibutyrate, cellulose tributyrate, and a combination thereof.
17. The method of any of claims 14-16, wherein the surface of the substrate upon which the polymer solution is disposed is a hydrophilic surface.
18. The method of any of claims 14-17, wherein the solvent is selected from the group consisting of N, N-dimethylacetamide, dimethyl sulfoxide, dimethyl formamide, N-methyl-pyrrolidone, acetone, 1,4-dioxane, methyl ethyl ketone, and a combination thereof.
19. The method of any of claims 14-18, wherein the pore-forming additive is selected from the group consisting of an alcohol, triethylene glycol, diethylene glycol, ethylene glycol, glycerol, ketone, an organic acid, polyethylene glycol, polyvinylpyrrolidone, formamide and a combination thereof.
20. The method of any of claims 14-19, wherein the polymer solution contains 10%-35% of the polymer by weight.
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