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WO2008137082A1 - Procédé de conception de membranes utiles dans des processus membranaires osmotiques - Google Patents

Procédé de conception de membranes utiles dans des processus membranaires osmotiques Download PDF

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WO2008137082A1
WO2008137082A1 PCT/US2008/005696 US2008005696W WO2008137082A1 WO 2008137082 A1 WO2008137082 A1 WO 2008137082A1 US 2008005696 W US2008005696 W US 2008005696W WO 2008137082 A1 WO2008137082 A1 WO 2008137082A1
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membrane
layer
semi
permeable membrane
water
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Menachem Elimelech
Jeffrey Mc Cutcheon
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Yale University
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Yale University
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    • 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
    • B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
    • 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
    • B01D61/0023—Accessories; Auxiliary operations
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0041—Inorganic membrane manufacture by agglomeration of particles in the dry state
    • B01D67/00412—Inorganic membrane manufacture by agglomeration of particles in the dry state by deposition of fibres, nanofibres or nanofibrils
    • 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
    • B01D69/10—Supported membranes; Membrane supports
    • B01D69/105—Support pretreatment
    • 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
    • B01D69/10—Supported membranes; Membrane supports
    • B01D69/107—Organic support material
    • B01D69/1071—Woven, non-woven or net mesh
    • 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
    • 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
    • B01D69/125—In situ manufacturing by polymerisation, polycondensation, cross-linking or chemical reaction
    • B01D69/1251—In situ manufacturing by polymerisation, polycondensation, cross-linking or chemical reaction by interfacial polymerisation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2311/00—Details relating to membrane separation process operations and control
    • B01D2311/26—Further operations combined with membrane separation processes
    • B01D2311/2653—Degassing
    • B01D2311/2657—Deaeration
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/30—Chemical resistance
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/36—Hydrophilic membranes

Definitions

  • the present invention relates generally to improved membranes for use in osmotically driven membrane processes including forward osmosis and pressure retarded osmosis processes.
  • Membrane technology has revolutionized the separations industry by providing a highly selective low cost alternative to standard separations processes.
  • Pressure driven membrane processes require the use of hydraulic pressure which uses electricity, a high quality and increasingly expensive form of energy. This has led to a growth in the field of osmotically driven membrane processes. These processes rely on transmembrane osmotic pressure which is created by a concentrated draw solution or osmotic agent.
  • Various configurations of osmotically driven membrane processes may be used for separation, concentration/dewatering, and power generation.
  • ammonia-carbon dioxide forward osmosis desalination process includes the ammonia-carbon dioxide forward osmosis desalination process, the ammonia- carbon dioxide osmotic heat engine, pressure retarded osmosis for generation of energy from salinity gradients (e.g., between river water and seawater), and treatment of various liquid wastes via forward osmosis utilizing brines as draw solutions.
  • salinity gradients e.g., between river water and seawater
  • Reverse osmosis is a pressure driven process in which the hydraulic resistance of the membrane is the primary resistance to water flux after the osmotic pressure has been overcome.
  • the hydraulic resistance is almost entirely limited to the thin dense surface layer (i.e., "rejection layer") as the microporous support layer and fabric backing layer have only minor or insignificant resistance.
  • forward osmosis is a diffusion (osmosis) driven process instead of a pressure driven process, so the factors affecting water flux are dramatically different than in reverse osmosis. Because of this, a high performance forward osmosis membrane requires a dramatically different structure than a reverse osmosis membrane.
  • an osmotic pressure gradient For water to transport across a membrane, an osmotic pressure gradient must be established across a selective portion of the membrane.
  • the osmotic pressure gradient is established between a relatively dilute feed solution and a far more concentrated draw solution.
  • draw solutions including for example ammonia-carbon dioxide, seawater, magnesium chloride, calcium chloride, sodium chloride, potassium chloride, sucrose, magnesium sulfate, potassium nitrate, ammonium carbonate, dextrose, and sucrose, by way of example and not limitation.
  • the selective portion of the membrane must not allow the passage of solutes from the feed to the draw solution or from the draw solution to the feed solution. High rejection by the membrane, high osmotic efficiency, low toxicity, high diffusivity, and low reactivity with the membrane and system components are all important criteria when selecting the draw solution.
  • a similar but concentrative CP phenomenon will occur when the support layer is facing the feed solution, as depicted in Figure 3.
  • This orientation of the membrane is typically used in PRO processes since the permeate stream is pressurized and the support layer is typically oriented away from hydraulic pressure.
  • solutes from the feed solution are dragged into the support layer by convection.
  • the solutes build up in concentration as the selective layer rejects them.
  • the solutes will attempt to diffuse back into the bulk feed solution but the support layer hinders this back diffusion and also negates the effects of turbulence and crossflow which occur on the external surface of the membrane.
  • Support layer hydrophobicity plays little role in pressure-driven membrane processes. Water permeates the active layer by a solution-diffusion mechanism and then simply percolates through the pores and cavities within the support layer. Thus, the support layer does not need to fully wet in order to ensure adequate permeate water flux.
  • the support layer when these asymmetric membranes are used in osmotically driven membrane processes, the support layer must fully wet to ensure effective water transport. If the support layer does not wet, vapor or air trapped in the pores not only blocks the passage of water, but it may also exacerbate internal concentration polarization by reducing the continuity of the water within the layer, and thereby reducing the effective porosity.
  • the inventors of the present invention have investigated the effects of membrane support layer wetting on water flux in osmotically driven membrane processes. Using various techniques, relative hydrophobicities of various support layer polymers were compared and correlated to water flux performance. The inventors of the present invention determined that support layer hydrophobic ity hindered osmotic flux through asymmetric membranes designed for pressure driven membrane processes. It was further demonstrated that improving the wetting of the membrane support layer results in a significant increase in water flux for osmotically driven membrane processes.
  • FO forward osmotic
  • PRO pressure retarded osmotic
  • Figure 1 depicts an osmotic pressure profile in a forward osmosis process across a dense, symmetric membrane.
  • Figure 2 depicts an osmotic pressure profile in a forward osmosis process across an asymmetric membrane oriented in the forward osmosis mode and illustrates dilutive internal concentration polarization.
  • Figure 3 depicts an osmotic pressure profile in a forward osmosis process across an asymmetric membrane oriented in the pressure retarded osmosis mode and illustrates concentrative internal concentration polarization.
  • Figure 4 depicts cross-sectional scanning electron microscope (SEM) images for three different membranes: (a) Osmonics CE; (b) Filmtec SW30 XE; and (c) Hydration Technologies CA membranes.
  • the bar in each SEM image represents 100 ⁇ m.
  • Figure 5 depicts cross-sectional SEM images for two different RO membranes with the fabric layer removed: (a) Filmtec SW30 XE; and (b) Osmonics CE.
  • the bar in each SEM image represents 100 ⁇ m.
  • Figure 6 depicts a top view of the polyethylene terephthalate (PET) fabric layer (right) and the porous cellulosic support layer (left) of the Osmonics CE membrane.
  • the porous material between the two layers is carbon tape.
  • Figure 7 depicts steady state forward osmosis flux measurements for the Filmtec SW30 XLE membrane averaged over at least 1 hour.
  • Figure 8 depicts s teady state forward osmosis flux measurements for the CE membrane averaged over at least 1 hour.
  • Figure 9 depicts forward osmosis flux measurements for the CE membrane with the fabric layer removed compared to flux measurements for the CA membrane over a range of NaCl draw solution concentrations.
  • Figure 10 shows a diagram depicting the osmotic pressure profile across a membrane oriented in the PRO mode (draw solution on the active layer).
  • Figure 11 depicts water flux data for the SW30 XLE membrane under 3 different conditions.
  • Figure 12 depicts water flux data for the SW30 XLE with the fabric layer removed with and without RO pretreatment.
  • Figure 13 depicts water flux data for the CE membrane under different conditions.
  • Figure 14 depicts forward osmosis flux measurements for the CE membrane with the fabric layer removed compared to flux measurements for the CA membrane over a range of NaCl draw solution concentrations.
  • Figure 15 depicts forward osmosis flux measurements for the CE and SW30 XLE membrane. Also, while not all elements are labeled in each figure, all elements with the same reference number indicate similar or identical parts.
  • the present invention relates generally to improved semi-permeable membranes for use in osmotically driven membrane processes including forward osmosis and pressure retarded osmosis processes.
  • the improvements of these new membranes over existing current generation reverse osmosis membranes leads to a reduced severity of internal concentration polarization and an improved wettability by using polymer materials of greater hydrophilicity.
  • the present invention relates generally to a semi-permeable membrane for use in a forward osmosis (FO) process, wherein said semi-permeable membrane is used to separate a draw solution and a feed solution, the semi-permeable membrane comprising:
  • draw solution i.e., basic, acidic or oxidative environments
  • the present invention relates generally to a semi-permeable membrane for use in a pressure retarded osmosis (PRO) process, the semi-permeable membrane comprising: a) an active layer having a high degree of selectivity for water and a very low resistance to water transport; b) a hydrophilic support layer exhibiting at least one of high porosity, low tortuosity and minimal thickness; and c) optionally, but preferably, a backing layer; wherein the semi-permeable membrane is capable of withstanding pressures generated during the PRO process.
  • a pressure retarded osmosis (PRO) process the semi-permeable membrane comprising: a) an active layer having a high degree of selectivity for water and a very low resistance to water transport; b) a hydrophilic support layer exhibiting at least one of high porosity, low tortuosity and minimal thickness; and c) optionally, but preferably, a backing layer; wherein the semi-permeable membrane is capable of withstanding pressure
  • the support layer does not wet out, then water cannot travel in any appreciable amount across the membrane in the absence of a hydraulic driving force. If the membrane only partially wets, then the internal concentration phenomena described above are enhanced in their severity as pathways for diffusion of solutes are limited by the presence of air or vapor in the support layer.
  • the inventors of the present invention have determined that in order to develop membranes tailored for osmotically driven membrane processes, the membrane support layers must have an increased degree of hydrophilicity throughout the entire layer so that they will wet spontaneously in the presence of aqueous solutions.
  • the inventors of the present invention have determined that there are several polymers that can be considered for the membrane support layer.
  • the type of polymer used for an actual membrane will greatly depend on the method of making the support layer.
  • the membrane support must be made with a polymer that has the following properties:
  • the film is highly porous and non-tortuous; 4) Once set, the polymer can withstand the chemical environment around the membrane during operation and cleaning; and
  • the polymer can withstand the hydraulic pressure of PRO processes, and the handling stress associated with module construction for either PRO or FO.
  • Polymers that may be suitable for use in membranes in accordance with the present invention include polyacrylonitrile; polyamide; poly (vinyl alcohol) after extensive cross linking; sulfonated polysulfone and sulfonated polyethersulfone (as described for example in U.S. publication No. 2007/0163951 to McGrath, the subject matter of which is herein incorporated by reference in its entirety); standard polysulfone/polyethersulfone polymers after treatment in oxidative environment (plasma or peroxide treatment); polysulfone/polyethersulfone polymers with hydrophilic additives (such as surfactants or hydrophilic nanoparticles); given by way of example and not limitation.
  • hydrophilic additives such as surfactants or hydrophilic nanoparticles
  • the material used will depend on the process.
  • the membrane support layer is subject to the ammonium salt solution and must be therefore very resistant to alkaline environments.
  • the ammonia-carbon dioxide osmotic heat engine i.e., PRO mode
  • the selective layer is exposed to concentrated alkaline environment, so its resistance to such an environment is a prime concern.
  • polymers are preferred for membranes used with the ammonia-carbon dioxide forward osmosis and osmotic heat engines: polyamides, sulfonated polysulfone and sulfonated polyethersulfone (as described for example in U.S. publication No. 2007/0163951 to McGrath); standard polysulfone/polyethersulfone polymers after treatment in oxidative environment (plasma or peroxide treatment); polysulfone/polyethersulfone polymers with hydrophilic additives (such as surfactants or hydrophilic nanoparticles); given by way of example and not limitation.
  • hydrophilic additives such as surfactants or hydrophilic nanoparticles
  • These support layers can be cast by phase inversion as is typically done in fabricating pressure driven membranes.
  • electrospun nanofiber supports may also be usable in the practice of the invention. Synthesis techniques are critical to the use of some of these specific polymers as osmotic membrane support layers as not all may be castable by phase inversion to yield highly porous and stable structures.
  • membranes are designed for use in either FO or PRO processes.
  • the support layers composed of hydrophilic polymers cast into porous films onto nonwoven hydrophilic polyester supports. These two-tiered support structures would then be coated with a highly selective active layer (typically polyamide based cast through interfacial polymerization).
  • a polymer solution containing a hydrophilic polymer is cast by phase inversion to a thickness of between about 30 and about 70 micrometers. In one embodiment, a thickness of about 50 micrometers is used for the cast polymer support layer.
  • An interfacially polymerized polyamide layer is then cast on top of the porous polymer film to a thickness of between about 50 and about 600 nanometers. In one embodiment the polyamide layer is cast to a thickness of about 200-400 nanometers.
  • the method is similar to the making of a reverse osmosis membrane, but yields a selective membrane with a relatively thick, albeit hydrophilic support layer, which would be useful in certain configurations of the PRO process, most notably the ammonia-carbon dioxide osmotic heat engine with deionized water as a working fluid.
  • the present invention relates to a membrane that is suitable for use in an osmotically driven process that incorporates a porous layer formed by electrospun nanofibers instead of the porous polymer support layers used previously. While nanofiber electrospinning is generally well known, it has not been used previously to provide a support layer for a semi-permeable membrane usable in an osmotically driven process.
  • Typical water filtration membranes are based on a composite structure which includes a nonwoven polyester-like support (100-125 micrometers thick), a thinner porous polymer support structure which is cast upon this nonwoven film (about 50 micrometers thick), and a very thin selective barrier layer which conducts the actual separation (perhaps 200-400 nanometers thick).
  • the relatively thick support layers support the thin and relatively fragile selective barrier while operating under pressure.
  • a semi-permeable membrane that is usable in osmotically driven processes such as forward osmosis and pressure retarded osmosis
  • ultra thin, highly porous, hydrophilic support layers beneath the thin selective barrier. This would include optimization of both the thick polyester (PET) nonwoven and the polymer layer.
  • PET thick polyester
  • the inventors have discovered that the use of nanofibers in place of the polymer layer would help achieve this with use of a thinner polyester support layer and three general procedures were considered:
  • An ultra-thin, porous polyester nonwoven support film is used as a substrate, and polymer nanofibers that are electrospun into a nonwoven mat are deposited onto the polyester nonwoven film.
  • the polyester support film is custom designed to have a highly porous structure, be more hydrophilic, and be as thin as 80 micrometers for membranes intended for PRO applications (though thicker is reasonable for higher pressure applications).
  • the electrospun nanofiber mat will typically have a thickness of about 20 micrometers but can be thicker for higher pressure applications.
  • a thin barrier layer is cast upon this nanofiber mat which must be compatible with the draw solution chemistry. This three tiered structure (polyester nonwoven, nanofibers, selective layer) is capable of providing a good result in PRO applications.
  • This active layer is typically a polyamide based polymer layer cast by interfacial polymerization as is typical with salt rejecting RO membranes.
  • Both the nanofiber and polyester backing layer must have chemical resistance to the various draw solutions that are of significant alkalinity, acidity, or oxidative potential.
  • Electrospun nanofiber mats are themselves quite strong for a given thickness. These mats may serve to provide all of the necessary support to the selective layer during operation since there is no hydraulic pressure in FO.
  • the nanofiber mats are spun onto a substrate to which there is little adhesion (for example, a woven netting or a Teflon sheet).
  • a thin active layer is cast upon this nanofiber mat which must be compatible with the feed solution chemistry (perhaps 200-400 nm).
  • This active layer is typically a polyamide based polymer layer cast by interfacial polymerization as is typical with salt rejecting RO membranes. Once the active layer is set, the nanofibers are removed from the substrates, resulting in a nanofiber supported membranes.
  • the thickness of the nanofiber mat may be at least 30 micrometers.
  • the use of additives to create nanocomposite nanofibers with greater mechanical strength than the polymer alone may reduce the required thickness.
  • the nanofiber backing layer must have chemical resistance to the various draw solutions that are of significant alkalinity, acidity, or oxidative potential.
  • these nanofiber webs can create supporting structures in membranes with incredibly high porosity (in excess of 80%) and pore interconnectivity (tortuosity approaching 1). Because of the high degree of fiber interconnectivity, the mechanical strength of these webs exceeds that of typical porous polymer films, allowing them to be made thinner, typically in the range of about 20 micrometers. As discussed previously, porosity, pore interconnectivity, and thinness are all critical aspects of support structures for FO and PRO membranes.
  • Hydrophilicity of the nanofiber web can be tailored by selecting polymers or miscible polymer blends of polymers that are hydrophilic or by chemically treating hydrophobic polymers.
  • Hydrophilic polymers by way of example and not limitation, that can be used for phase inversion casting or electrospinning nanofiber webs include polyacrylonitrile, crosslinked polyvinyl alcohol, sulfonated polysulfone, sulfonated polyethersulfone, block copolymers that incorporate hydrophilic functional groups such as hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, and phosphate groups, and combinations of one or more of the foregoing.
  • hydrophilicity of hydrophobic polymers such as polysulfone and polyethersulfone
  • oxidative treatment with plasma or chemicals e.g., peroxide, acid
  • plasma or chemicals e.g., peroxide, acid
  • surfactant such as sodium dodecyl sulfate
  • hydrophilic nanomaterials such as hydrophilic inorganic nanoparticles
  • membranes for membranes to perform well over long periods of time, they must be compatible with their environment and must be designed with consideration to process specifications. Thermal and chemical conditions need to be optimized for enhanced performance of various osmotically driven membrane processes and the membranes must be able to withstand these conditions.
  • the membranes must be compatible with both the feed and the draw solution and not degrade or be physically or chemically altered in the presence of either the feed or the draw solution.
  • Current generation reverse osmosis membranes consisting of polyamide active layers, for example, are highly resistant to acidic and basic conditions and high temperatures yet will degrade in the presence of chlorine. Cellulosic membranes are chlorine resistant, yet degrade outside of ambient pH ranges due to hydrolysis reactions and will also break down at higher temperatures. This particular aspect of compatibility is critical to draw solutions such as ammonia-carbon dioxide draw solutions.
  • the present invention relates generally to a method of increasing the degree of saturation of a semi-permeable membrane for use in an osmotically driven process, wherein the semi-permeable membrane comprises a backing layer, a porous support layer, and an active layer having a high degree of selectivity for water and a very low resistance to water transport, the method comprising the step treating the semi-permeable membrane to increase its degree of water saturation by at least one of:
  • the CE and SW30 XLE membranes are both fabricated using a polyester (PET) non-woven fabric support. This fabric is used as a substrate to cast the cellulosic CE membrane and the SW30 XLE polysulfone (PS) support layer by phase inversion. In order to study the hydrophobicity of the support layer, the fabric layers were carefully removed by peeling them away from the other layer (or layers) of each membrane. SEM micrographs in Figure 5(a) and (b) show the cross sections of the CE and SW30 XLE membrane, respectively, with their fabric layers removed. The membrane selective layers are facing up with the support layers facing down.
  • PET polyester
  • PS polysulfone
  • the first membrane listed in Table 1 is the CA membrane, which has no fabric support, although it does have a self-supporting porous structure.
  • This membrane which has been studied in previous investigations on forward osmosis, shows a moderate degree of hydrophilicity (62.0° for active layer, 63.6° for backing layer). This is expected due to its cellulosic composition in which the cellulose polymer backbone contains hydrophilic hydroxyl groups.
  • the similarity between the active and backing layer is not surprising, since the membrane is made through the phase inversion process and the polymer material is the same throughout the membrane.
  • the standard deviation of the measurement is reasonably high given the macroscale roughness caused by the embedded polyester mesh. Roughness increases the variability in the contact angle measurement due to pinning and other surface effects which create metastability within the sessile droplet. This also explains the increased standard deviation of the measurements on the backing layer, which has an increased roughness due to surface porosity.
  • the SW30 XLE membrane is an RO thin film composite membrane with a polyamide active layer supported by a polysulfone (PS) support layer.
  • the SW30 XLE porous support is a polysulfone UF membrane, which is cast upon a PET non-woven fabric layer.
  • the active layer is hydrophilic, which is expected since the water must diffuse through this part of the membrane and increased hydrophilicity decreases the active layer fouling propensity.
  • the support layers are more hydrophobic.
  • the PET fabric is moderately hydrophilic (67.5°) and the PS support is hydrophobic (95.2°).
  • the CE membrane is comprised of an asymmetric cellulosic layer which has an active layer and a supporting porous structure. This self-supported cellulosic layer is cast upon a PET fabric support.
  • the active layer, or the dense portion of the cellulosic layer, has nearly the same hydrophilicity as the CA membrane (59.1°). This is not surprising considering both membranes are cellulosic.
  • the lower standard deviation is due to the smoothness of the CE active layer.
  • the non-woven PET fabric layer has a moderate degree of hydrophilicity (61.8°), similar to that of the SW30 XLE PET fabric layer.
  • the cellulosic support of the CE membrane was measured as being very hydrophilic (19.4°), though this is unexpected due to the fact that the material is the same as the active layer. This low contact angle measurement is likely due to the rough surface morphology, which is formed by contact with the fabric layer during membrane casting.
  • the fabric fibers will imprint on the surface, leaving a mold of the fibers in the polymer, as shown in Figure 6. This roughness, which can also be seen in Figure 5(b), allows for a drop of fixed volume to spread by capillary action into the crevices on the surface.
  • Reverse osmosis pretreatment In some of the experiments, the membrane was "pretreated" in a reverse osmosis mode prior to running a flux test in the forward osmosis cell. This pretreatment under hydraulic pressure was used to purge air and/or vapor out of the support layers prior to forward osmosis. It was thought that moderately high hydraulic pressures might induce liquid water to displace air or vapor within the porous support layers, partially or perhaps fully wetting the membrane. An RO crossflow unit with identical channel dimensions to the FO crossflow unit was used for this pretreatment. Deionized feedwater was used at a
  • feed solution degassing was done to eliminate the possibility of air bubbles moving into the support layer from the feed during FO tests. Degassing can also improve the wetting of the support layer, as air or vapor trapped in the support layer might be induced to diffuse out into the degassed bulk solution. This treatment was only done with the feed solution when the membrane was oriented with the feed against the support layer (i.e., in the PRO mode).
  • the hydrophobic polypropylene hollow fibers are typically used in membrane vacuum distillation processes for degassing and this particular module is used for degassing small scale deionized water systems.
  • the feed solution (deionized water) flowed through the lumen of the fibers while a vacuum was drawn on the shell side.
  • a vacuum of at least 29 inches Hg was drawn by a direct drive vacuum pump (Edwards, Murray Hill, NJ).
  • the feed solution was run in closed loop and dissolved oxygen levels were measured with a dissolved oxygen probe (Fisher Scientific, Waltham, MA).
  • SDS sodium dodecyl sulfate
  • the first membrane orientation that was considered is the FO mode.
  • the membrane active layer is facing the feed solution and the support layer is facing the draw solution.
  • This is the typical orientation for most membrane separation processes, such as reverse osmosis.
  • flux is osmotically driven and no hydraulic pressure is used.
  • the first tests were conducted with the SW30 XLE membranes.
  • This membrane is typically used for seawater desalination and therefore has a very high salt rejection and thick porous support layers intended to withstand the high hydraulic pressures of seawater RO (up to 1200 psi or 82.7 bars).
  • Osmotically driven water flux was measured using a deionized water feed and a 1.5 M NaCl draw solution, corresponding to a transmembrane osmotic pressure of about 70 atm (1029 psi, 70.9 bar), after various pretreatments to the membrane.
  • Conductivity of the deionized feedwater was monitored to measure salt transport from the draw solution to the feed. For all tests, salt transport from the draw solution was found to be small and did not appreciably impact the transmembrane osmotic pressure difference.
  • Figure 7 presents the averaged steady state water flux data.
  • the water flux through this membrane is very low when operated in the FO mode: less than 0.5 gallons/ft 2 membrane area/day (gfd).
  • the RO pretreatment conditions were as follows: deionized feedwater against the active layer, 450 psi (31.0 bar) pressure, 25 ⁇ 2 0 C, run for at least 1 hour.
  • ⁇ W159767 2 ⁇ osmosis experimental conditions were as follows: membranes oriented in the FO mode, with the feed (DI water) against the active layer and the draw solution (1.5 M NaCl) against the support layer. Crossflow rate and temperature of both feed and draw solutions are 21.3 cm/s and 20 0 C, respectively. Note that a water flux of 10 ⁇ m/s corresponds to 21.2 gal ft "2 d "1 (gfd) or 36.0 1 m "2 h “1 . Note also that typical seawater RO systems operate between 9-11 gfd, and those are operated at lower transmembrane driving forces.
  • the PET fabric layer of the membrane could be removed and flux slightly increased. This is likely due to a decreased internal concentration polarization effect due to the decreased overall thickness of the support layer.
  • the membrane undergoes reverse osmosis pretreatment the flux increases for both the full membrane and the membrane with no fabric, though the effect is far more noticeable for the membrane with the fabric removed.
  • Pretreating the membrane with hydraulic pressure i.e., reverse osmosis mode
  • By purging the air from the layer a greater degree of water continuity can exist within the support layer, reducing the prevalence of internal CP by increasing the number of solute diffusion pathways while simultaneously increasing the available pathways for water transport.
  • the slight decrease in flux after RO pretreatment may be due to compaction of the PET fabric or the cellulosic porous support, causing a decreased porosity which may increase the hydraulic resistance within these layers or increase the severity of internal CP.
  • the data set was expanded to include a range of draw solution concentrations up to 1.5 M NaCl.
  • the data are shown in Figure 9 and are compared to the previously studied CA membrane in the same orientation.
  • the membranes are oriented in the FO mode, with the feed (DI water) against the active layer and the draw solution (NaCl) against the support layer.
  • Experimental conditions crossflow rate and temperature of both feed and draw solutions are 21.3 cm/s and 20 0 C, respectively.
  • a water flux of 10 ⁇ m/s corresponds to 21.2 gal ft "2 d "1 (gfd) or 36.0 1 m "2 h “1 .
  • the similarity of the flux data is notable, being that the modified CE membrane is a commercial RO membrane and the CA is a commercial FO membrane. While the membrane support layers differ in thickness (slightly), porosity, and tortuosity, support layer characteristics which play a role in determining the severity of internal CP, as well as other structural characteristics (the embedded mesh support in the CA membrane), their materials are similar and both are cast through the phase inversion process. Thus, a commercial RO membrane was shown to compare favorably with commercial FO membranes after simple modification.
  • the membrane When the draw solution is placed against the active layer and the feed solution is against the support layer, the membrane is oriented in the PRO mode.
  • This mode is typically used for PRO applications where the draw solution is pressurized, and hence, requires the mechanical support of the porous layer on the feed side of the membrane.
  • salinity in the feedwater will create concentrative internal concentration polarization effects.
  • the flux tests described below were designed to solely test the impact of membrane hydrophobicity on water flux and therefore were conducted with a deionized water feed solution. In this case, no internal CP effects will occur. Note, however, that salt may leak across the membrane active layer from the draw solution and enter in the support layer. If this occurs, diffusion out of this layer is hindered by the porous structure and internal CP would form.
  • FIG. 10 This form of internal CP is illustrated in Figure 10.
  • the feed (the right side) is deionized water.
  • a dilutive external concentration polarization layer exists on the permeate side of the membrane.
  • No internal concentration polarization exists in the support layer since the feed is deionized water.
  • Small amounts of internal concentration polarization may occur due to salt leakage from the draw solution, but the amount of leakage is expected to be small due to the high salt rejection characteristics of the membrane.
  • the first tests in this mode were conducted with the SW30 XLE membrane. As above, a 1.5 M NaCl draw solution was used along with a deionized water feed solution, but the membrane was oriented in the PRO mode. The first test was done with the intact membrane.
  • RO pretreatment conditions deionized feedwater against the active layer, 450 psi (31.0 bar) pressure, 25 ⁇ 2 0 C, run for at least 1 hour.
  • Forward osmosis experimental conditions membranes oriented in the PRO mode, with the feed (DI water) against the support layer and the draw solution (1.5 M NaCl) against the active layer. Crossflow rate and temperature of both feed and draw solutions are 21.3 cm/s and 20 0 C, respectively.
  • Membrane is oriented in the PRO mode. This was surprising considering the feed is deionized water and therefore no internal CP should be occurring.
  • Previous investigations on water flux through commercial membranes (including the PA-300, NS-101, NS-200, BM-I-C and the Permasep B-IO hollow fiber) oriented in the PRO mode concluded that salt transport from the draw solution was the primary cause of low flux due to internal concentration polarization effects.
  • high rejection desalination membranes such as the SW30 XLE, salt leakage from the draw solution should be minimal, even for higher concentration salt solutions. Salt leakage should be further mitigated by the water flux itself, which is in the opposite direction of the salt flux. Therefore, we would expect to see far greater fluxes given the high transmembrane osmotic pressure (70 atm)
  • the membrane performed similarly with and without the fabric support layer. However, after RO pretreatment, the flux was steady at nearly 5 gfd (about 6 times higher than before the pretreatment). What is interesting to note, though, is that the flux does not decrease with time as it did with both support layers present. This suggests that the wetting and drainage mechanisms are different for the PET and the PS support layers. It would appear that after partial wetting by RO, the PS support does not drain which would cause a reduction in flux. However, when the PET fabric is attached, the overall wetting and drainage mechanisms of the entire support layer change.
  • the reasoning behind the transient water flux is likely due to differing drainage and imbibition mechanisms of the PET fabric layer.
  • the PET fabric is initially wet to some degree which fully wets the cellulosic membrane.
  • Upon osmosis water is drawn out of the cellulosic portion of the membrane which consequently drains the PET fabric. If water continuity is broken (i.e., drying) before water can enter the fabric, then the fabric may have difficulty rehydrating. If this is the case, the drainage mechanism by osmosis is faster than the wetting mechanism by water cohesiveness and capillarity.
  • the same "hysteresis" apparently occurs in the SW30 XLE membrane as well and can now be attributed to the PET fabric layer.
  • the porous portion of the cellulosic membrane is very hydrophilic and absorbent and therefore will spontaneously wet even when in contact with a partially hydrated PET layer regardless of RO pretreatment.
  • a slightly higher initial water flux after RO pretreatment is likely due to a marginal increase in saturation of the PET layer, which improves water continuity for a short time at the beginning of a test (initial fluxes of over 10 gfd vs. 8 gfd without pretreatment).
  • the RO pretreatment will improve wetting of both layers.
  • the PET layer drains while the PS layer does not (as indicated in Figure 12). Therefore, in the SW30 XLE membrane, the overall wettability of the intact support layer is limited by the PS support at first which is very likely less saturated than the more hydrophilic PET. As the PET fabric drains, flux is limited by that layer. Therefore, in the CE membrane, the limiting factor to water flux is the wettability of the fabric layer while in the SW30 XLE membrane, the limiting factor is the wettability of the PS support at first and then the PET fabric after it dries. In other words, the drier layer will control the overall wettability of the composite support layer, not necessarily the more hydrophobic layer.
  • Sodium dodecyl sulfate an anionic surfactant
  • SDS Sodium dodecyl sulfate
  • the flux tests involved a brief equilibration time (50 minutes) to obtain a baseline flux.
  • SDS was added to the feed solution, raising its concentration to 1 mM. This concentration was chosen because it is high enough to significantly reduce the surface tension of water (from 70.9 to 53.9 mN/m as determined by using a Wilhelmy plate tensiometer), yet low enough not to create foam or bubbles.
  • the data shows that after the increase in flux caused by SDS for the SW30 XLE membrane, there is a subsequent flux decline. This decrease is due to the accumulation of SDS molecules within the porous support layer. SDS is rejected by the active layer of the membrane and thus will be retained within the support layer.
  • ⁇ WI597672 ⁇ amasses near the active layer and effectively fouls its pores.
  • Membrane support layer hydrophobicity significantly hinders water flux in osmotically driven membrane processes. Lack of sufficient support layer wetting not only exacerbates internal concentration polarization phenomena, but also disrupts water continuity within the membrane, thereby reducing the pathways for water transport. Improved wetting of the support layer has been shown to increase water flux, especially for pressure retarded osmosis applications with dilute feed solutions. This improved wetting was achieved by purging the air and vapor out of the support layer with RO pretreatment prior to testing, or with the use of a surfactant to improve wetting within the layer. It was shown, though, that these treatments impacted the various layers of each membrane differently depending on the structure and hydrophobicity of the layers.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Water Supply & Treatment (AREA)
  • Nanotechnology (AREA)
  • Inorganic Chemistry (AREA)
  • Manufacturing & Machinery (AREA)
  • Separation Using Semi-Permeable Membranes (AREA)

Abstract

L'invention concerne des processus membranaires osmotiques tels que l'osmose directe (FO) et l'osmose retardée par pression (PRO), qui sont fondés sur l'utilisation de grands différentiels de pression osmotique à travers des membranes semi-perméables pour produire un flux d'eau. L'invention concerne des membranes améliorées s'utilisant dans de tels processus membranaires osmotiques. Les membranes polymères actuelles utilisées dans des séparations de liquides sont généralement constituées d'une barrière sélective maintenue par une structure poreuse. Cette structure n'est pas idéale pour les processus osmotiques, à moins que certaines caractéristiques de membrane soient personnalisées de manière appropriée. La porosité, l'épaisseur, la tortuosité et les propriétés hydrophiles des couches support jouent toutes un rôle crucial dans l'efficacité du flux d'eau à travers des membranes semi-perméables asymétriques. Les couches support de membrane doivent être minces, fortement poreuses, non tortueuses et/ou hydrophiles si elles sont utilisées dans des processus FO et PRO. Ces objectifs doivent être atteints sans sacrifier les propriétés de perméabilité à l'eau et de rejet de sel. Divers procédés de fabrication de ces nouvelles membranes sont décrits.
PCT/US2008/005696 2007-05-02 2008-05-02 Procédé de conception de membranes utiles dans des processus membranaires osmotiques Ceased WO2008137082A1 (fr)

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