Composite Membranes for Osmotically Driven Membrane Processes, Mass and/or Heat Transfer

CROSS REFERENCE TO RELATED U.S. PATENT APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Patent Application Serial No. 61/876,184, entitled "Composite Membranes for Osmotically Driven

Membrane Processes, Mass and/or Heat Transfer" filed September 10, 2013. The entire contents of the provisional application are incorporated herein by reference in their entirety.

FIELD

[0002] One or more aspects relate to membranes used for selective mass transport, heat and/or mass transport, their means of manufacture, and their means of use. Examples of fields of use for such membranes include: osmotically driven membrane processes (ODMPs) such as forward osmosis (FO), pressure retarded osmosis (PRO), direct osmotic concentration (DOC), and osmotic dilution (OD); membrane contactors for use in membrane distillation (MD) for desalination, chemical separations, or direct heat and mass transfer, which collectively may be referred to as mass exchange (MX); and heat exchange (HX) processes, in which the primary transport is of thermal energy.

BACKGROUND

[0003] A number of currently practiced and emerging water and gas separations and treatment processes require effective means for bringing one or more fluid streams into contact with a device to enable the transport of mass, such as water, hydrocarbons, and gases; energy, such as heat; and selective transport, such as separation of water from solutes, particles from solutions, or gases from liquids or mixed gas streams. A common set of requirements for such mass and energy transport devices is that they ideally offer low resistance to the transport of the desired energy or mass, and that they be robust, inexpensive, and compact. Ideally these devices will additionally allow for reasonably high flow rates of both fluid streams in a system without excessive pressure drop, such as may be achieved with relatively large diameter hollow fiber membranes. It is also important that these devices achieve these goals without compromising their structural integrity, such as their resistance to forces such as pressure and tension that might cause bursting, collapse, or tearing of the membrane device.

SUMMARY

[0004] The invention herein describes methods of fabrication, characteristics of composition, and processes for use of composite supported thin polymer membranes for separations, mass transfer, and heat exchange.

[0005] Methods include, but are not limited to: the use of frames, guides, or other mechanical supports for protection of thin membrane substrates during the casting of thin polymer films on their surface to form flat sheet membranes; the use of tubular or tube/fiber composite substrate supports, to provide protection from mechanical damage during the formation and use of thin film polymer flat sheet, hollow fiber or tubular membranes; the formation of multiple layers of polymer on flat, hollow fiber, or tubular supports.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention.

[0007] FIG. 1 is a depiction of the use of a perforated belt with spikes on its edges to convey a thin paper substrate through a membrane fabrication process.

[0008] FIG. 2 is a depiction of the use of a frame with cross braces, clamping on the edges of a flat sheet of paper to convey a thin paper substrate through a membrane fabrication process.

[0009] FIG. 3 is a depiction of the formation of a tubular paper support from substrate material and casting of a polymer solution to form a supported hollow fiber membrane.

[0010] FIG 4 is a depiction of the casting of a polymer solution on a tubular support, made in a separate wet-laid process (not shown), to form a supported hollow fiber membrane. [0011] FIG. 5 is a diagram of a forward osmosis membrane module with supported hollow fiber membranes suspended in a solution.

DETAILED DESCRIPTION

[0012] Several characteristics of polymers make their use in interfaces for heat and mass transfer devices desirable. A wide variety of polymers are available, enabling selection from a wide variety of attributes for the optimization of such devices. For example, appropriate selections of polymers enable appropriate separation

characteristics for solution-diffusion based selective separations of gases and liquids, such as water desalination through polyamide films, or methane separation from carbon dioxide through films of polysulfone. Additionally, polymers may be selected to offer exceptional chemical resistance to aggressive chemical environments, including chemical reactions such as corrosion, a common deleterious process encountered in the use of metallic interfaces in applications such as heat exchange. Polymers may be formed into very thin films, offering excellent mass and heat transport properties, and may be combined with other polymers to obtain benefits offered by each, in dense, asymmetric, layered, and other configurations.

[0013] The thinness of the material used for transport, selectivity, and exchange processes, however, is often of great importance. For example, in solution diffusion based separations, the rate of transport of a material is inversely proportional to the thickness of the selective layer. For direct contact between streams through a porous film, the length of the pore is directly proportional to the resistance to mass transfer. For heat transfer, the conductivity of a material is similarly inversely proportional to the thickness of that material - a thin polymer with a relatively low intrinsic thermal conductivity can be a more effective conductor than a relatively thick plate of metal.

[0014] One example of relationship between material thickness, properties, and performance may be found in the conductivity of a film of polyimide compared to that of a plate stainless steel. Polyimide has an intrinsic thermal conductivity of 0.52 Watts, per meter of thickness, per degree Kelvin difference in temperature, or 0.52 WmK. In contrast, stainless steel has a thermal conductivity over 30 times higher, at 16 WmK. However, metal heat exchangers typically have tube thicknesses of not substantially less than 1 mm to allow for sufficient strength and corrosion resistance. Polymer films, however, may be made to be much thinner, by means described herein - as thin as 1 micron in supported asymmetric configurations, or 10-100 microns or greater in supported dense configurations. Assuming a stainless steel tube thickness of 1 mm, one may calculate a heat transfer coefficient of 16,000 Wm K. Assuming a polyimide film thickness of 50 microns, or 0.05 mm, the heat transfer coefficient is 10,400 Wm K. This would seem to still leave the polymer at a disadvantage, but one must also consider the overall heat transfer coefficient of a heat exchange surface, which in many cases is dominated by the resistance to heat transfer of the boundary layers at the interface of the heat transfer material and fluids. For example, a stainless steel heat exchanger transferring heat between steam and water is expected in practical use to have an overall heat transfer coefficient of approximately 680 Wm K, substantially below the ideal coefficient for stainless steel without boundary layer resistances (16,000 Wm K for 1 mm thickness). In such an environment, the practical difference between a 1 mm thick stainless steel heat exchanger and one made of 0.05 mm thick polyimide may be expected to be minimal. It is also possible, by the methods described herein, in such applications as might warrant it, to reduce the thickness of the polymer further, and indeed a 32.5 micron film of polyimide offers the same ideal thermal conductivity as a 1 mm stainless steel plate. Generally, the thinner the polymer interface may be made, the better its separation, mass transfer, or heat transfer characteristics.

[0015] Thin polymer interfaces, however, due to the inverse relationship between material thickness and mechanical strength, require fabrication and composition characteristics that cause them to be well-supported and resistant to mechanical damage, both during their manufacture and thereafter during use. Currently, polymer interfaces for such purposes are made in ways that compromise one or more of these goals, such as with making flat sheet membranes excessively thick to allow for fabrication on roll- to-roll manufacturing equipment, or such as with fabricating hollow fiber membranes in such a way that the polymer is self supporting, and therefore necessarily thick and/or dense, reducing performance and increasing cost.

[0016] Flat sheet embodiments

[0017] In accordance with one or more non-limiting embodiments, a thin polymer membrane, consisting of one or more layers, may be cast onto a thin substrate material in a flat sheet roll-to-roll fabrication process, such that the thin substrate material is supported in or on a protective device to prevent mechanical damage from occurring to the substrate or polymer during fabrication. Mechanical forces acting on flat sheet membranes during fabrication include longitudinal and lateral forces that may cause tears, perforations, deformation, or creasing, which may degrade the quality of the finished membrane, or in many cases prevent its successful fabrication. The protective device may be a frame, which attaches to the edges of the thin substrate, by means of, by way of non-limiting example: clamps; adhesives; friction-inducing surfaces over the entire surface, portions of the surface, and/or at the edges; portions of the guide that perforate the edges of the substrate. The device may be a plate or semi-flexible or flexible mechanically robust carrier surface that may attach, by way of non-limiting example: by adhesive; electrostatic forces; vacuum seal; and the like. Materials may include, by way of non-limiting example: metals; polymers; inorganic materials;

inorganic and polymer composites; woven or non-woven polymer substances; silicone or similar materials, with or without fiber reinforcements. The protective device may have cross braces or tensioning devices; may interact with, attach to, or otherwise be directed or moderated in its function by the roll-to-roll process equipment, to maintain or enhance alignment, processing speed, or other characteristics; may be perforated or impermeable; but in many cases allows for the use of membrane substrates in the fabrication process that would not be readily usable if employed without the protective device.

[0018] In a preferred embodiment, shown in FIG. 1, a perforated, flexible silicone sheet with spikes along its edges is attached to a thin wet-laid membrane substrate by means of electrostatic attraction and perforation of the edge of the substrate by the spikes at the edge of the sheet, providing mechanical support and protection to the substrate in a roll-to-roll polymer application process. The combined silicone guide and substrate are directed through a slot die application of polymer solution onto the substrate, and subsequent immersion precipitation of the polymer in an aqueous bath. The polymer, substrate, and guide are subsequently directed through a polymer film application to the surface of the first polymer to form a thin dense film of the second polymer. The combined first polymer, second polymer, substrate, and guide are directed into a series of baths for post treatments, including heat annealing and solvent exchange. The combined membrane and guide are then dried through a heating process, to form the finished membrane product, substantially protected from mechanical disruption during the fabrication process. The guide is then cleaned and prepared for reuse in a continuous process. In the illustration of FIG. 1, 100 is the membrane substrate, 101 is the silicone guide sheet, 102 is the perforations in the sheet, 103 is the spikes at the edges of the sheet used to hold the substrate in place.

[0019] In an alternate embodiment, shown in FIG. 2, the guide device is a semi-rigid frame with cross-frame supports that attaches to the substrate by pressure clamping at the edges of the substrate sheet, such that the guide protects the membrane during fabrication and maintains a smooth, evenly tensioned surface for polymer application to the substrate. In the illustration of FIG 2, 105 is the membrane substrate, 106 is the flexible guide frame, 107 is the cross braces of the frame, and 108 are the clamps at the edges of the frame to hold the substrate in place.

[0020] Hollow fiber embodiments

[0021] In accordance with one or more non-limiting embodiments, a thin polymer membrane, consisting of one or more layers, may be cast onto or around a thin substrate material in a hollow fiber or tubular fabrication process, such that the thin substrate material allows the polymer layer to be thinner than it would be if the polymer was self- supporting, while simultaneously allowing the combined support and polymer to be more mechanically robust. The supporting material may consist of, by way of non- limiting example: a woven or non-woven mesh hollow fiber or tube; a wet or dry laid paper formed in a hollow fiber or tubular configuration; an electrospun material; a perforated hollow fiber or tube. The materials that may be employed in the supporting material include, by way of non-limiting example: carbon fiber; cellulosic or non- cellulosic polymers; inorganic substrates, such as ceramics. Means of introducing the polymer to the surface of the substrate include, by way of non-limiting example: casting a film of the polymer onto the substrate from a spinneret other device through which the substrate is directed, such that the polymer is applied uniformly around the

circumference of the substrate; dip-coating the polymer onto the substrate; vapor deposition or spraying of the polymer onto the substrate. The polymer may be applied in the form, by way of non-limiting example: of a polymer solution, followed by bath immersion causing precipitation of the polymer; a polymer solution, followed by solvent evaporation from the polymer; melt extrusion of the polymer; spray coating of a polymer solution or melt. The polymer may in some cases react with the materials of the substrate to cause bonding between them forming a composite material.

[0022] In a preferred embodiment, shown in FIG. 3, a tube of high porosity, thin substrate is formed by feeding a strip of substrate paper, with a thickness of between 10-75 microns and a width of between 0.1 and 1.5 inches, around a guide rod and into the inlet of a spinneret, in a diagonal configuration similar to that used in the formation of a paper straw, producing a tubular substrate with a diameter of approximately 1-2 mm. A polymer solution is introduced to the exterior of the tubular substrate, coating and partially surrounding the support material with a thickness of between 10-75 microns. The supporting material and polymer move through an air gap before entering a water bath, which causes a porous, asymmetric polymer membrane with a dense film to be formed, producing the desired supported hollow fiber membrane. The composite membrane is subsequently suitable for forward osmosis desalination, heat exchange, or mass exchange, depending on the substrate paper and polymer used. In the illustration of FIG. 3, 109 is the spinneret, 110 is the guide rod, 111 is membrane substrate, 112 is the inlet for the formed tubular substrate, 113 is the inlet for the polymer solution which coats the substrate, 114 is the polymer coated substrate passing through an air gap towards a precipitation water bath (not shown).

[0023] In a preferred embodiment, shown in FIG 4, a high porosity wet-laid tube of PET substrate with a thickness of between 10-75 microns and a diameter of between 0.8-2 mm is directed to an inlet of a spinneret. A solution of cellulose acetate polymer is directed to form a coating on the surface of the substrate that is approximately 10-75 microns in thickness. The combined polymer and substrate are immersed in a nonaqueous bath, inducing the formation of a dense film on the polymer surface, and subsequently immersed in an aqueous bath, inducing the formation of a porous asymmetric support beneath the film, thereby producing the desired supported hollow fiber membrane. The resulting membrane is suitable for use in forward osmosis desalination. In the illustration of FIG. 4, 115 is the spinneret, 116 is the tubular membrane substrate, 117 is the inlet for the tubular substrate, 118 is the inlet for the polymer solution which coats the substrate, and 119 is the polymer coated substrate moving towards the non-aqueous bath (not shown). [0024] In a preferred embodiment, a high porosity, semi-rigid carbon fiber mesh, approximately 0.5-3 mm in diameter, is directed to the annulus of a spinneret. A volatile solution of polyimide is directed to form a coating of between 10-150 microns on the surface of the mesh. The combined mesh and polymer are exposed to convective air, causing evaporation of the volatile solvent, forming a dense, symmetric film of polyimide, partially enclosing the mesh support, resulting in a composite hollow fiber membrane which may be used for heat exchange.

[0025] In an alternate embodiment, a high porosity, semi-rigid nylon mesh, approximately 0.5-2 mm in diameter, is directed to the annulus of a spinneret. A solution of PVDF is directed to form a 10-75 micron coating on the surface of the mesh. The combined mesh and polymer are immersed in an aqueous bath, causing the polymer to precipitate in a porous, asymmetric layer, partially enclosing the mesh at its base. The composite membrane is subsequently immersed in a bath of ethanol, and thereafter dried, resulting in a composite hollow fiber membrane that may be used to contact a liquid with a gas without permitting liquid permeation of the small, hydrophobic membrane pores.

[0026] In an alternate embodiment, a strip of wet laid nylon paper substrate of approximately 10-75 microns in thickness and between approximately 0.25-1.5 inches wide is directed to encircle a guide rod placed in the annulus of a spinneret, in a spiral pattern as might be found in the formation of a paper straw, forming a tubular substrate of between 0.5 - 3 mm in diameter. An adhesive and/or stitching are applied to the edge of the paper strip to provide enhanced mechanical strength to the support. A solution of polyimide polymer is directed to form a coating on the surface of the substrate of approximately 10-100 microns in thickness. The combined polymer and substrate are allowed to pass through an air gap prior to entering an aqueous bath, inducing the formation of a dense film on the polymer surface, and subsequently immersed in an aqueous bath, inducing the formation of a porous asymmetric support beneath the film. The composite membrane is subsequently suitable for gas separations or heat exchange.

[0027] In an alternate embodiment, a 1 - 5 mm diameter carbon fiber mesh is directed to the annulus of an extrusion coater. A 10 - 150 micron thick film of PTFE is thermally extruded onto the surface of the mesh and allowed to cool, forming a PTFE hollow fiber, which may be used for high temperature, corrosive, and/or high salinity heat exchange.

[0028] Membrane module and system embodiments

[0029] In accordance with one or more non-limiting embodiments, a membrane module intended for separation, heat exchange, or mass exchange operations, is composed of a multitude of hollow fiber or tubular composite polymer membranes, consisting of thin polymers cast on strong substrate materials, resulting in a high performance, robust and durable device.

[0030] In accordance with one or more non-limiting embodiments, a separation, heat exchange, or mass exchange process is carried out by use of a hollow fiber membrane module consisting of a multitude of hollow fiber or tubular composite polymer membranes, consisting of thin polymers cast on strong substrate materials, enabling separation, heat exchange, and/or mass exchange operations.

[0031] In a preferred embodiment, shown in FIG. 5, a membrane module intended for forward osmosis desalination consists of supported hollow fiber membranes of between 1 - 2 m in length, with a diameter of 1-3 mm, potted by use of epoxy to headers at either end of the fibers. The headers and fibers are immersed in a tank such that the water to be desalinated is on the exterior of the fibers, and the draw solution is directed through the headers through the fiber lumen. In the illustration of FIG 5, 120 is the membrane module, 121 is the hollow fiber membranes epoxied into the module at either end of their length, 122 are the inlets for a solution to flow through the lumen of the fibers, 123 are the outlets for the lumen solution, 124 is the tank in which the membrane module is immersed, and 125 is the solution surrounding the outer surface of the hollow fiber membranes.

[0032] In a preferred embodiment, a membrane module intended for heat exchange consists of substrate / polymer composite hollow fiber heat exchange membranes, the ends of which are potted within high temperature epoxy, such that a first fluid may be directed through the lumen of the fibers, and a second fluid may be directed around the outer surface of the fibers, effecting an exchange of heat between the fluids. [0033] In an alternate embodiment, a membrane module intended for mass exchange consists of a substrate/polymer composite hydrophobic porous hollow fiber membranes, the ends of which are potted within an epoxy, such that a liquid may be directed through the lumen of the fibers and a gas directed around the outer surface of the fibers, such that mass transfer of the gas may be achieved through the membrane pores.

[0034] In an alternate embodiment, a membrane module intended for forward osmosis or pressure retarded osmosis consists of substrate / polymer composite solution diffusion or molecular sieve separation hollow fiber membranes, the ends of which are potted within an epoxy, such that a first fluid with a first osmotic pressure and hydraulic pressure may be directed through the lumen of the hollow fibers, and a second fluid with a second osmotic pressure and hydraulic pressure may be directed around the outer surface of the fibers, such that transport of solvent between the two fluids is driven by osmotic pressure differences, facilitating separation or power production.

[0035] In a preferred embodiment, a heat exchange process is carried out by means of heat exchange through substrate / polymer composite hollow fiber membranes.

[0036] In an alternate embodiment, a mass exchange process is carried out by means of membrane contactor substrate / polymer composite hollow fiber membranes.

[0037] In an alternate embodiment, a separations process is carried out by means of solution diffusion or molecular sieve selective polymer layer as part of substrate / polymer composite hollow fiber membranes.

[0038] In an alternate embodiment, a separations process is carried out by means of a membrane module composed of thin, supported flat sheet membrane manufactured using a protective guide device.

[0039] It will also be recognized by those skilled in the art that, while the invention has been described above in terms of preferred embodiments, it is not limited thereto.

Various features and aspects of the above described invention may be used individually or jointly. Further, although the invention has been described in the context of its implementation in a particular environment, and for particular applications (e.g. forward osmosis), those skilled in the art will recognize that its usefulness is not limited thereto and that the present invention can be beneficially utilized in any number of environments and implementations where it is desirable to transfer components or attributes between two or more streams. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the invention as disclosed herein.