Patent Application
20240017217
The subject invention relates to a membrane system utilized for the separation of fluid components, and especially to cross-flow and spiral-wound membrane elements.
In cross-flow filtration, a feed fluid flows through a filter and is released at the other end, while some portion of the fluid is removed by filtration through a membrane surface, which is parallel to the direction of fluid flow. Various forms of cross-flow filtration exist including plate-and-frame, cassette, hollow-fiber, and spiral wound systems. Plate-and-frame, cassette, and spiral-wound filtration modules often rely on stacked membrane layers which provide spacing between adjacent layers of filtration membrane. The present invention is applicable to such systems. Several references are listed herein to facilitate understanding of the invention; each of those references is incorporated herein by reference.
Spiral-wound membrane filtration elements known in the art consist of a laminated structure having a membrane sheet sealed to or around a porous permeate carrier, which creates a path for removal, longitudinally to the axis of the center tube, of the fluid passing through the membrane to a central tube, while this laminated structure is wrapped spirally around the central tube and spaced from itself with a porous feed spacer to allow axial flow of the fluid through the element from the feed end of the element to the reject end. Traditionally, a feed spacer is used to allow flow of the feed water, some portion of which will pass through the membrane, into the spiral wound element and allow reject water to exit the element in a direction parallel to the center tube and axial to the element construction.
Improvements to the design of spiral wound elements have been disclosed in U.S. Pat. No. 6,632,357 to Barger et al., U.S. Pat. No. 7,311,831 to Bradford et al., and patents in Australia (2014223490) and Japan (6499089) entitled “Improved Spiral Wound Element Construction” to Herrington et al. which replaces a conventional feed spacer with islands or protrusions printed, deposited or embossed directly onto the inside or outside surface of the membrane. US patent application PCT/WO2018190937A1 entitled “Graded spacers for filtration wound elements” to Roderick, et al., describes the use of height graded spacer features which are used to alter feed flow characteristics in a spiral wound element. US patent application PCT/US17/62424 entitled “Interference Patterns for Spiral Wound Elements” to Roderick, et al., describes patterns in spiral wound elements that keep membrane feed spaces open but also provide support for the membrane envelope glue areas during rolling. US patent application PCT/US18/55671 entitled “Bridge Support and Reduced Feed Spacers for Spiral-Wound Elements” to Roderick et al. describes support features that are applied to the distal end (farthest end from the center tube) of the membrane envelope to provide support during gluing and rolling of the spiral wound element. US provisional application number 63,051,738 entitled “Variable Velocity Patterns in Cross Flow Filtration” to Herrington et al. describes support patterns that vary in size from the feed to the reject end of the membrane feed space in the feed flow path parallel to the center tube in order to control the velocity of the feed solution as the concentration of the feed solution increases from the feed to the reject end of the spiral wound element. Each of the foregoing is incorporated herein by reference.
Patterns on membrane surfaces utilizing interfacial polymerization are described by Sajjad H. Maruf, et al., entitled “Fabrication and characterization of a surface-patterned thin film composite membrane” and published in the Journal of Membrane Science, 452 (2014) pages 11-19. These patterns have been fabricated for controlling cellular responses for the purpose of biofilm control. Typical groove depths of 200 nano meters are described. These groove depths are much, much smaller than 1 thousandth of an inch.
Printing of a polyamide coating on the polysulfone substrate has been described by Chris Arnush with the Zukerberg Institute of Water Technology of Ben Gurion University in a paper entitled “2-D and 3-D Printing Assisted Fabrication and Modification of UF/NF/RO Membranes for Water Treatment”. Polyamide coatings applied by electrospray have also been described by Jeffery McCutcheon with the University of Connecticut.
The top selective layer of a membrane is referred to as the active layer. In some embodiments the active layer can comprise polyamide. A typical thin-film composite (TFC) reverse osmosis (RO) membrane is made through interfacial polymerization of polyamide on the surface of a microporous substrate. As a general description, interfacial polymerization of polyamide occurs when an amine solution contacts a chloride solution. There are many possible formulations for specific amine and chloride solutions that can be utilized. In some embodiments the amine solution comprises an aqueous amine solution and the chloride solution comprises an organic chloride solution. In conventional membrane manufacturing a substrate is placed in contact with an amine solution, typically comprising m-phenyldiamine for RO membranes and piperazine for nanofiltration membranes, and subsequently placed in contact with a chloride solution, typically comprising trimesoyl chloride (TMC).
An embodiment of the present invention provides a method of using interfacial polymerization to fabricate feed spacers comprising polyamide on the active layer of a membrane. For example, interfacial polymerization of the feed spacers can be facilitated by printing an aqueous amine solution and printing a chloride solution on to the membrane surface or active layer.
In a specific example embodiment of the current invention, an aqueous amine solution comprising 1,6-hexanediamine can be printed on to the membrane active layer, followed by printing another solution comprising sebacoyl chloride. Other specific example embodiments can utilize different chloride solutions, including solutions comprising TMC, sebacoyl chloride, or any mixture thereof. The chloride solution can also comprise one or more organic solvents. For example, the chloride solution can comprise solvents such as hexane, toluene, or any mixture thereof. Other materials and solutions, now known or later discovered, that create interfacial polymerization reactions can also be utilized.
These spacers are of sufficient height to create the fluid flow space between two membrane sheets in a spiral wound element or flat sheet membrane system. Fluid flow spacer heights can be in the range of 0.001 inch to 0.050 inches, or greater. For thin spacers to maximize the surface area of membrane sheet in a spiral wound element in some applications, spacer heights can be in the range of 0.003 inches to 0.017 inches. For spacer heights to minimize energy losses in some applications from pressure losses from the feed to reject end of the element, spacer heights can be in the range of 0.015 inches to 0.035 inches in height, or greater.
Embodiments of the present invention provide elements for use in fluid filtration comprising a permeable support layer, a selectively permeable active layer applied thereon, and one or more spacing features applied on the active layer. As an example, the spacing features can comprise polyamide feed spacers printed on the membrane active layer. As examples, the spacing features can have the thicknesses and other characteristics described elsewhere herein.
The feed spacer in a spiral wound filtration element is required to maintain a channel for fluid to flow through, but the spacer design also impacts local flow velocities, turbulence, stagnation zones and other fluid flow conditions. Extruded mesh feed spacers have been used traditionally in membrane manufacture due to their ease of integration in the production process, but by their nature many of their hydrodynamic characteristics are dependent on the thickness of the spacer. Printed feed spacers allow for unique design characteristics unobtainable with conventional extruded or woven mesh spacers, since their thickness and geometry can be changed independently to yield a wide range of configurations which can be tailored to specific applications or specific challenges found in spiral wound membrane element construction.
Cross-flow filtration relies on some portion of the feed fluid to pass through the filter and become part of the filtrate, thus creating a situation where the quantity of the feed fluid is constantly being reduced as it passes through the filter. The higher the portion of filtrate produced, the lower the portion of feed/concentrate fluid that remains flowing through the filter. As a fluid flows through the element, a portion of the fluid passes through the membrane. Modeled simply, a constant flux through the membrane produces a gradually decreasing flow of the feed solution as it flows through the element. In reality the amount of fluid passing through depends on local flow conditions and local concentrations of solutes or suspended materials, as well as the local pressure which also depends on any back-pressure from the permeate side of the element locally.
Many cross-flow filtration systems, such as spiral wound elements and stack filters, rely on parallel flat sheets of membrane material through which the feed fluid flows. In such systems where the feed channel occupies a fixed volume, the loss of feed fluid to the filtrate stream creates a situation where the fluid stream flowing from the feed inlet to the concentrate outlet decreases in cross-flow velocity along the length of the filter, but increases in ion concentration toward the reject end of the fluid stream. Hydrodynamic conditions in the filter, including the cross-flow velocity, as well as the filter geometry and the feed spacer, affect several important characteristics of the fluid flow such as fluid shear, boundary layer thickness, and concentration polarization which in turn affect filter performance characteristics including membrane flux, frictional pressure losses, biological fouling, and scaling. Thus, for a system with a fixed filter geometry and feed spacer, the changing cross-flow velocity induces changes in these characteristics throughout the system, which can lead to less desirable performance.
Embodiments of the present invention provide processes to produce feed spacers wherein the spacer material comprises polyamide and is located on the active layer of membrane sheet. In an example embodiment of the present invention a printing process is used to create polyamide feed spacers on a membrane surface. In one example embodiment the polyamide spacer material is the same or similar to the active layer of a thin film composite membrane sheet. Other embodiments include spacer material comprising of polyamide and other additives, such additives can be added for various purposes including: reducing fouling; improving membrane permeability or rejection performance; modifying physical and chemical spacer properties including surface chemistry, height, stiffness, permeability, porosity, and roughness. Other example embodiments include various layers of spacer material comprising polyamide and one or more materials. By way of a non-limiting example, a spacer can be mostly comprised of polyamide with a top “capping” layer of a different material. Such embodiment can be desirable in order to obtain suitable surface characteristics that allow the various layers in a spiral wound membrane to be rolled while avoiding undesirable interactions between the top of the spacer and any adjacent material.
Spacers constructed of polyamide material provide several benefits relative to those previously known. First, the spacer material does not require application by inkjet type printing, screen printing, or other techniques that utilize ultraviolet (UV) or light in the visible spectrum to cure the spacer material. The use of photopolymer curing can add heat or other forms of energy to the membrane sheet, which can damage the structure of the thin film composite (TFC) and adversely affect the flux or ion rejection characteristics of the membrane sheet. Photopolymer ink jet applied materials can also add organics to the membrane surface thereby reacting with the charged membrane surface which can negatively affect flux or rejection characteristics. Interfacial polymerization as used with example embodiments of the present invention can be accomplished at room temperature by a rapid chemical reaction thereby significantly reducing temperature rise on the TFC. Further, polyamide feed spacers can allow feed solution flow through the physical spacer. This is not possible with materials used in conventional ink jet printers using photopolymer processes. As such, the loss of active membrane surface area will be greater with conventional inkjet spacer materials relative to spacers applied with polyamide material. This feature can enhance permeation rates in membrane systems and improve overall permeate production and efficiency for any given size membrane element or flat sheet membrane system. Furthermore, in embodiments where both the feed spacers and the membrane active layer comprise polyamide, there is less risk of adverse interactions or material incompatibility between the feed spacers and the membrane surface.
The feed spacing features employed can comprise any of a number of shapes, including round dots, ovals, bars with rounded ends, lenticular forms, stretched polygons, lines or other geometric shapes. Due to the shape of the features and the fact that fluid in many cases must traverse around the outside of the features, the fluid flow velocity will change locally in the areas between the feed spacing features, but if the features are uniform in size and pattern, the bulk fluid velocity is only affected by the reduction in fluid volume caused by filtrate flowing through the membrane. The result is a net reduction in fluid volume and therefore fluid velocity from the inlet to the reject stream of the element.
A partially assembled spiral wound membrane element 200 is shown in
In a representative embodiment of an existing reverse osmosis spiral wound membrane elements shown in
In an example embodiment of the present invention shown in
The present invention has been described in connection with various example embodiments. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those skilled in the art.
| Number | Date | Country | |
|---|---|---|---|
| 63148002 | Feb 2021 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US22/15688 | Feb 2022 | US |
| Child | 18359304 | US |
