Uniform spacers for filtration

Abstract

Thermoplastic printed spacer membrane elements offer significant advantages in operating costs, capital costs, and compliance with toxicology issues in drinking water and other applications. It is desirable that thermoplastic spacers be uniform in height to facilitate high quality fabrication of membrane components. This patent application allows for uniform height printed spacers by adjusting the pressure on the dispensing nozzle or by changing the speed of the nozzle across the filter media or by combinations of speed and pressure

Description

UNIFORM SPACERS FOR FILTRATION

[0001] Technical Field

[0002] The subject invention relates to a membrane system utilized for the separation of fluid components, specifically spiral-wound membrane elements or flat sheet membrane systems.

[0003] Background Art

[0004] 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, radial, or spiral wound systems. Plate-and- frame, cassette, radial, and spiral-wound filtration modules often rely on stacked membrane layers which provide spacing between adjacent layers of filtration membrane. Spacing between the layers controls flow through the space, but may also facilitate lower pressure losses in the flow channel from the feed end of a crossflow system to the reject end of the crossflow system, thereby saving energy in the system. Alternatively, narrower feed spaces may facilitate more active area of filtration media in the membrane system, for example, more area in a spiral wound filtration element. More surface area in a filtration system can facilitate lower fluid flux (volume flow through a given surface area) due to lower applied pressure on the fluid media. This can save energy versus a filtration system with less surface area and higher applied pressure, but with equivalent finished product permeate flow. The present invention primarily relates to, but is not limited to, spiral wound membrane elements. The material being filtered can be liquids or gases.

[0005] Spiral-wound membrane filtration elements are well known in the art, and comprise a laminated structure comprised of 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. T rad itiona I ly , a feed spacer mesh 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 fluid to exit the element in a direction parallel to the center tube and axial to the element construction.

[0006] Improvements to the design of spiral wound elements have been disclosed in US Patent 6,632,357 to Barger et al, US Patent 7,311 ,831 to Bradford et al., and patents in Australia (2014223490), Japan (6499089), China (CN105163834B), Israel (240883), and South Korea (10-2196776) entitled “Improved Spiral Wound Element Construction” to Roderick et al which replaces the feed spacer with islands or protrusions either printed,deposited or embossed directly onto the active or inactive surface of the membrane, or on the permeate carrier. US patent 11 ,090,612 entitled “Graded spacers for filtration wound elements” to Roderick, et al, describe the use of height graded spacer features which are used to alter feed flow characteristics in a spiral wound element. US patent 11 ,040,311 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 11 ,745,144 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 envelop to provide support during gluing and rolling of the spiral wound element. US provisional application number PCT / US21 / 40353 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. US Patent 11 ,083,997 to Roderick, et al entitled “Non Nesting Patterns” describe denser patterns in the feed and reject ends of the membrane feed space, and a more open pattern in the middle, in order to avoid nesting of the printed patterns during element fabrication, particularly during the membrane envelop gluing process to support the glue lines. US Patent 11 ,633,700 entitled “Independent Spacers and Methods” to Herrington, et al, describes various methods for applying spacers to the membrane surface that does not expose the membrane surface to UV or visible light from inkjet, stencil, or screen printing processes that are photo cured. US patent application number 63294377 entitled “High Rejection Element” to Herrington, et al, describes membrane printing and assembly processes that provide support to the membrane sheet in high stress concentration areas to avoid damage to the membrane active layer. US patent application number 63294378 entitled “Spiral Element Enhanced Capacity” to Kurth, et al, describes membrane printing and assembly processes that provide improved membrane design features to increase the permeate flow capacity of spiral wound elements. US patent 8463418 B2 to Qing Liu, et al that describes a rapid prototype system utilizing a thermoplastic extrusion process that is not applied to a substrate, but rather makes a porous chamber for cell culture analysis or other biomedical processes.

[0007] Much of the printed spacer technology to date has used multi-pass UV or light cured inkjet processes that build up the height of the pattern in layers. Alternately, single layer stencil printing processes have been used that utilize epoxies or UV or light cured urethanes. Stencil printing offers the possibility of faster printing as compared to light or UV curedphoto-polymer inkjet processes because a stencil can provide a desired spacer height in a single pass, whereby ink-jet printing is usually applied in multiple passes.

[0008] Thermoplastic technology can have significant benefits over alternative spacer application technologies. Thermoplastic refers to any polymer-based glue that is applied in a molten state. The application of the adhesive can be by a glue gun, solenoid activated nozzles, high frequency tappet type nozzle heads, heated stencil, and other techniques. Hot melt is a common term for a thermoplastic, and is used in a variety of settings due to its versatility, including packaging, bookbinding, carton-making, graphic arts, tapes and labels, product assembly, as well as spacers in spiral wound, flat plate, pleated filtration, membrane systems, heat exchangers, and fabrics.

[0009] Thermoplastic material is appealing as a spacer in filtration applications for many reasons. It can be applied to one surface of the filtration material where the opposing filtration surface does not have spacers. This can be advantageous by only requiring printing on one half of the total filtration surface, thereby speeding up the printing process. Since the application of spacers on a filtration surface typically blocks flow through the printed area, by only printing on one half of the filtration surface, the amount of blockage from the printing process is significantly reduced. However, if it is beneficial to the design of the process, printing can occur on both opposing surfaces of the filtration media to create interference in the printed patterns that cause more open flow channels, or to create distinct flow paths between the two patterns printed on opposing surfaces. The opposing surfaces between the spacers can be the same material on both surfaces or can be different materials on the two surfaces.

[0010] Thermoplastic printing is fast-acting and can be applied in one, two, or more layers depending on the desired height of the pattern. The time to cure, or set, can be adjusted based on the needs of the application. Thermoplastic material is safe to use and environmentally friendly. Thermoplastics can be used to bond difficult surfaces. It is inherently safe and is easy to ship and store with a long shelf life. Thermoplastics, which are polymer based, are faster, more cost-effective, can lead to improved bonding strength, and produces less volatile organic compounds than solvent-based adhesives. As such, thermoplastic material will produce less undesirable volatiles that can contaminate potable drinking water during the reverse osmosis (RO) process. This characteristic helps ensure compliance with toxicology protocols when tested against the requirements of NSF International. NSF International verifies that water contact materials do not extract materials that can contaminate fluids (such as water) and that can cause bad health effects for the consumer. Due to its chemical nature, thermoplastics can come in any number of forms as feed stock for thermal deposition. This includes granules, pellets, bags, cakes, drums, bricks, slats, and pillows. Thermoplastics can also be applied in several ways, includingthrough pressure feed manifolds with solenoid activated nozzles, high frequency print head nozzles, extrusion, melt blowing, spiral spraying, screen printing, stencil printing, and slot die coating. The dispensing equipment for thermoplastics can come in the form of melt reservoirs, pressure pumps, vacuum conveyance, drum or pail unloaders, and pre-melters.

[0011] Using conventional thermoplastic dispensing applications such as pressure fed solenoid-controlled feed through an orifice, the consistency in terms of volume dispensed, and consistent height of the dispensed thermoplastic bead are typically not a concern for material bonding applications. In printed spacer applications, the spacer can be a line, dot, or dashed pattern, as examples. Thermoplastics can be applied very quickly. The speed of application is a function of the speed of the substrate motion (or speed of the dispensing head), the size of the application orifice, the pressure applied to the thermoplastic material in the dispensing head, and the time the dispensing orifice is open. Of course, the volume dispensed will also be a function of the viscosity of the thermoplastic material. While uniform print height of a pattern is desirable, there may be applications where a variable height pattern may be desirable.

[0012] Brief Summary of the Invention

[0013] The present invention provides novel methods of rapidly applying a consistent height bead of thermoplastic material, whether the bead is a line, a dot, a dash, or other form. For the most rapid application of thermoplastics, it is desirable to allow continuous uninterrupted movement of the dispensing head or substrate being printed upon. However, conventional printing with the dispensing head turning on and off, while the head speed is constant, leads to features applied to the substate that are inconsistent in height. Typically, the start of the feature has more thermoplastic material applied at the beginning of the feature, than at the end of the feature. The present invention integrates a pressure regulator, e.g., an electronic pressure regulator, that can adjust the pressure on the thermoplastic material as the print head or substrate moves under the print head. By adjusting the pressure on the thermoplastic material in the dispenser, the volume of thermoplastic material at any one time along the length of the application can be increased or decreased to create a very consistent height bead. In another embodiment, the speed of the dispensing head across the substrate can be varied to produce a consistent thickness of thermoplastic material applied to the substrate. However, varying the print speed can lead to undesirable dynamic forces in the X- Y plotting system. Consistency in height is important for fabrication of many spiral wound membrane elements, flat sheet membrane assemblies, or other applications where one substrate is next to another in a fabricated package. Desirable height variations can be less than 50 percent height variation, or less than 10 percent height variation, or less than 5 percent height variation. Height variation can be quantified for example by use of height standard deviation, and compared with the median height.

[0014] Brief Description of the Drawings

[0015] FIG. 1 is an exploded view of a spiral wound membrane element.

[0016] FIG. 2 is an exploded view of a partially assembled spiral wound membrane element.

[0017] FIG.3 is a schematic illustration of a printed membrane surface showing a pattern with dots in the middle region with lines on the inlet and outlet regions.

[0018] FIG.4 is a schematic illustration of a printed membrane surface showing a pattern with dashes in the middle region with lines on the inlet and outlet regions.

[0019] FIG. 5 is a schematic illustration of a spacer printing system.

[0020] FIG. 6 is a schematic illustration of a membrane sheet with feed spacers printed outside a polyamide coated area.

[0021] FIG. 7 is a schematic illustration of a multi-manifold dispensing system

[0022] FIG. 8 shows plots of print head speed and applied pressure versus spacer feature height.

[0023] FIG. 9 is a schematic illustration of a side view of printed spacers on a membrane sheet with uneven height spacers

[0024] FIG. 10 is a schematic illustration of a side view of printed spacers on a membrane sheet with variable height spacer patterns.

[0025] FIG. 11 is a schematic illustration of a side view of printed spacers on a membrane sheet with spacer heights that are uniform.

[0026] Modes for Carrying Out the Invention and Industrial Applicability

[0027] A feed spacer in a spiral wound filtration element is required to maintain a channel for fluid to flow from the feed to reject end of the feed channel, 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 spiral wound membrane manufacturing due to the ease of integration in the production process, but by the nature of their design many of their hydrodynamic characteristics are dependent on the thickness of the spacer. Conventional mesh spacers also provide uniform support characteristics in the feed space all the way from the distal end from the center tube to the proximal end of the membrane sheet near the center tube. 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. However, in any particular application, it is desirable that the spacers be uniform in height, for example having a height variation of less than .001 to .002 inch (25 to 50 microns) height in a spacer that is in the range of .010 to .020 inches (250 to 500 microns) tall. This represents a height variation of less than 10 percent. Feed spacers can be applied to any number of filtration applications includingliquids or gasses, and in various configurations including spiral wound elements, flat plate elements, radial elements, pleated elements, ion exchange processes, heat exchange applications, and others where liquid or gas fluids require spacing features.

[0028] Cross-flow filtration, by its nature, relies on some portion of the feed fluid to pass through the membrane and become part of the permeate (product fluid), thus creating a situation where the quantity of the feed fluid is constantly being reduced as it passes through the membrane. The higher the portion of permeate produced, the lower the portion of feed / concentrate fluid that remains flowing through the membrane element. 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 from the feed to the reject end of the feed space in the element. In reality, the amount of fluid passing through any location along the feed flow path 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 in the feed space as well as from the permeate side of the element locally. In some processes such as forward osmosis, a fluid such as water is removed from the feed stream via osmosis. The resulting concentrated reject stream may be the desired product, for instance when concentrating a food product. Another example where the concentrated solution is the product is maple syrup concentration using reverse osmosis. To obtain syrup from raw sap, the fluid is reduced 10 to 1 , historically via boiling. Extracting water from the raw sap via reverse osmosis is a much more energy efficient method of concentrating the raw sap. The resulting concentrated reject stream is the desired product.

[0029] During fabrication of a spiral wound element, permeate carrier material is attached to the center tube by tape or bonding, the membrane envelope is placed adjacent to the permeate carrier, and the flat sheet assembly is glued - to seal the permeate carrier envelope - and the envelope is rotated around the center tube with a rotating mechanism similar to a lathe. The center tube can be captured or keyed to the lathe so that the lathe can rotate the center tube and wind the membrane envelope and permeate carrier around the center tube. Torque on the center tube should be adequate to roll the envelope until the entire envelope is wrapped around the center tube. Sufficient tension should be maintained in the membrane envelope to ensure the glue penetrates completely through the permeate carrier and contacts both membrane leaves to ensure the membrane envelope is completely sealed. As the membrane envelope is wrapped around the center tube, the diameter of the element increases. However, the torque and forces on the membrane envelope are greatest at the center tube where the diameter is smallest. Greater force proximal to the center tube creates greater force on the membrane envelope, particularly the feed space, during rolling. An important advantage of printed spacer technology is that more open feed spacerchannels can be created, and closer spacing of printed spacers can be achieved at the center tube where stress concentrations are higher. During the rolling process, the feed spacer components are slipping relative to the opposing unprinted membrane surface. Consistency in height of the feed spacer elements is important to provide uniform load distribution and avoid damage to the opposing membrane surface from a feed spacer feature that is much taller than the surrounding membrane features.

[0030] Important economics in manufacturing a printed feed spacer element are the costs associated with printing the feed spacer features on the membrane surface.

[0031] The present invention concerns features and techniques for rapid application of feed spacer features using thermoplastic material in a consistent and uniform matter. Thermoplastic material is applied as a liquid at elevated temperature which solidifies as temperature decreases and does not require UV or light to drive chemical reactions leading to solidification of a liquid resin in the form of a printed pattern. Accordingly, with thermoplastic deposition there is no UV or other wavelength of energy damage imparted to the membrane active surface that can damage the flux or rejection of the membrane surface. UV cured material is also typically applied in many layers to build up the height of the features. The UV cured print head makes a number of passes across the membrane surface, all of which take time. Most applications of thermoplastic material is for bonding two surfaces together. In those applications, the height of the thermoplastic material in the molten state is not important. And when the thermoplastic is dried, it is usually spread out between the bonded surfaces of the opposing materials. For filter spacer applications, an important feature of thermoplastic material is that it can be applied in one pass and quickly dried to achieve the desired feature height. Thermoplastic material also has a significant cost advantage versus light cured adhesives. In addition, in contrast to UV or light cured photo polymers, thermoplastic material can be recovered and recycled at the end of life of the membrane element.

[0032] Another issue with previously used printing methods is that before solidification of the printing materials - due to low surface tension and / or viscosity of the printing material - the photo-polymer material is able to spread, or overspray, and cover more of the membrane surface than was intended. This additional coverage can block membrane flow leading to reduced efficiency. Due to the higher viscosity of thermoplastic materials, and the fact that it cools and stiffens as it hits the surface, the ability to spread over the membrane is reduced.

[0033] Another benefit of the present invention is that the thermoplastic material can be applied quickly by printing the pattern in one direction with one or more print head print heads. The print head(s) first lay down a continuous line pattern at the inlet edge of the membrane, typically 3 inches, more or less, that provides support for the glue line when the membrane envelop is glued to seal the permeate carrier between the two membrane sheets.The print head continues to print across the length of the membrane sheet laying down a series of short dashed or curved segments, forming spacers, that create the support patterns that are more widely spaced in the middle of the membrane sheet. At the opposite side of the sheet, the print head then lays down the support lines on the reject end of the membrane sheet in the same fashion as the inlet feed support lines. The print head is then indexed down the longitudinal length of the membrane sheet by a distance, e.g., .100 inches, and the print head then proceeds back in the opposite direction of the first printed line. Alternative line spacings can range from .040 inches to 1.00 inches. In this second pass, the dense pattern on the edges of the membrane is printed, but the short patterns in the open area of the membrane may or may not be printed, based on the desired longitudinal spacing of the short patterns in the middle of the membrane sheet. For example, the print pattern may skip printing the short dashes, or segments, on two passes of the print head. Thereby, the dense line pattern on the feed and reject ends of the membrane sheet will be printed every .100 inches down the longitudinal length of the membrane, but the short dashed pattern will only be printed every .300 inches down the longitudinal length of the membrane sheet. Of course, many variations can be made in the longitudinal length of the spacing as well as the length of the edge print pattern, and the spacing of shorter straight or curved patterns in the middle of the membrane.

[0034] A system for applying the thermoplastic material to the membrane surface can comprise a two-axis gantry system as well as an adjustment mechanism for changing the height of the print head above the membrane sheet. This system can be controlled by a programmable logic controller (PLC) and the print pattern can be loaded into the program with conventional software designed to control plotters. In an example application, high pressure solenoid-controlled dispensing nozzles are utilized to apply the thermoplastic material to the membrane sheet. The membrane sheet can be held in place with a vacuum table to ensure the membrane sheet does not move, and can be held at a fixed height relative to the print head(s). In an example embodiment, the print head nozzle is .2 inches (5 mm) above the membrane sheet surface, but can be as little as .05 inches (1.27 mm) or as high as .5 inches (12.7 mm) or greater. The thermoplastic feed system can be configured from known industry standard bulk thermoplastic feed systems with heated feed lines to the print heads. A pressure control system can be integrated in the thermoplastic feed system to ensure a constant and steady supply of thermoplastic material is delivered to the print heads. While one print head can print a membrane sheet, multiple print heads can reduce the time required to print a full sheet.

[0035] Printing with thermoplastic material requires the applied drop to have a low enough thermal energy to avoid damaging the membrane, which in many cases has a thin 30nm to 10 micron separating layer on the membrane being coated. This corresponds to a processusing resins having a solidification temperature from 45C to 200C. Thermoplastic materials having solidification temperatures lower than that are often unable to withstand cleanings. Materials with higher temperature than that can lead to damage of the membrane. Suitable features are typically .020 to .03 inches (500 to 750 microns) in width, .05 to .5 inches in length (1270 to 12,700 microns), and .002 to .05 inches (50 to 1270 microns) in height, but can be shorter or taller in height.

[0036] Membrane materials can have an oleophobic surface for printing that helps reduce spreading. Surfaces can be chemically modified, or have microfeatures that minimize spreading.

[0037] Example print features can have the same width, but alternatively can also have a section in the middle of the feature (halfway between the membrane base and the top of a feature) where the diameter of the feature is wider or narrower than the base. This geometry can give improved mixing performance in some applications.

[0038] Thermoplastic features usually have a dome shaped characteristic. Sharp edges at the top of a feature can lead to damage to the facing unprinted (or printed) opposing membrane surface in which the feature is in contact. Example features can have a profile that resembles a dome or flat surface that is free of sharp features.

[0039] The typical durometer range for thermoplastic materials applied to the membrane surface are about 70 durometer. Alternative durometers can range from 80 to as low as 40.

[0040] Example patterns have a repeating pattern in the center region (i.e. not the denser edge pattern) that are divisible into integer ratios. More specifically the center section can have features that are in a row from edge to edge, where the feature position is the same for the left half and right half, or the left third, center third, and right third, etc. down to the case where every feature in a row from edge to edge have the same spacing. The position of one row to the next can align, or can be offset, but the row position symmetry can be the same. This can lead to more effective mixing and an improved application process.

[0041] Thermoplastic material application is a non-contact process whereby the print head is positioned preferably 0.2 inches (5 mm) above the membrane surface, but the range can alternatively be .02 to .5 inches (.5 to 12.7 mm) above the membrane surface. Non-contact printing avoids damage to the membrane surface that can be a problem for other application techniques such as screen or stencil printing. Thermoplastic dispensing also has the advantage of using fewer print heads than photo polymer inkjet type printing. Fewer print heads means that maintenance to clean heads is faster with less operational down time.

[0042] There are several characteristics of thermoplastic materials and dispensing equipment that affect the deposition rate of material to a substrate. These include viscosity, temperature, print head travel speed, nozzle size, pressure of the thermoplastic material on the nozzle, and other factors. A wide variety of characteristics will determine the build heightand print speed. Various manufacturers can offer thermoplastic materials that work well for creating printed spacers on membrane substrates. With these various characteristics and precision control of the X and Y directions of the gantry system, various configurations of printed spacers can be fabricated at practical heights for feed spacer applications.

[0043] One aspect of more open feed spaces is that the concentration of ferees applied to the membrane envelope, and consequently, the feed spacers, is that higher forces are applied to the feed spacer elements, particularly near the center tube. While this is discussed in the prior art, thermoplastic printing techniques can also address these concerns.

[0044] The feed shaping features employed can be of 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 the fluid must traverse around the outside of the features, the fluid flow velocity will change locally in the areas between the feed spacing features from the feed to reject end of the membrane element. Efficient printed spacer patterns will allow maximum flow from the feed to the reject end of the membrane, will provide very little resistance to flow from the feed to reject end, the features will minimize formation of stagnation points on the leading and trailing edge of the spacer, and will help promote mixing of the feed solution to reduce concentration polarization in the feed space.

[0045] In spiral wound elements, the membrane leaf is often folded at the center line where the center line comes in contact with the permeate carrier at the center tube prior to rolling. Fold protection is described in the prior art. Fold protection usually consists of tape applied along the width of the membrane sheet where it is folded. Prior art also discusses fold protection that is applied by printing or otherwise applying a polymer or other resin as the fold protection material. Fold protection is used to protect the membrane leaf where it is creased when folded to avoid damage from the crease. Damage in the crease without fold protection can result in loss of rejection and flux in the finished membrane element. Fold protection can be utilized uniquely in printed spacer technology by extending the fold protection over the top of the printed spacer features near the center tube to help avoid stress concentration of the printed spacer features from damaging the active surface of the membrane on the unprinted side of the membrane leaf.

[0046] FIG. 1 is a schematic illustration of a conventional spiral wound membrane element prior to rolling, showing important elements of a conventional spiral wound membrane element 100. Permeate collection tube 12 has holes 14 in collection tube 12 where permeate fluid is collected from permeate carrier 22. In fabrication, membrane sheet 36 is a single continuous sheet that is folded at center line 30, comprised of a non- active porous support layer on one face 28, for example polyester or polysulfone, and an active polymermembrane layer on the other face 24 bonded or cast on to the support layer. In the assembled element, active polymer membrane surface 24 is adjacent to feed spacer mesh 26, and non-active support layer 28 is adjacent to permeate carrier 22. Feed solution 16 enters between active polymer membrane surfaces 24 and flows through the open spaces in feed spacer mesh 26. As feed solution 16 flows through feed spacer mesh 26, particles, ions, or chemical species, which are excluded by the membrane are rejected at active polymer membrane surfaces 24, and molecules of permeate fluid, for instance water molecules, pass through active polymer membrane surfaces 24 and enter porous permeate carrier 22. As feed solution 16 passes along active polymer membrane surface 24, the concentration of materials excluded by the membrane increases due to the loss of permeate fluid in bulk feed solution 16, and this concentrated fluid exits the reject end of active polymer membrane sheet 24 as reject solution 18. Permeate fluid in permeate carrier 22 flows from distal end 34 of permeate carrier 22 in the direction of center tube 12 where the permeate fluid enters center tube 12 through center tube entrance holes 14 and exits center tube 12 as permeate solution 20. To avoid contamination of the permeate fluid with feed solution 16, non-active polymer membrane layers 28 are sealed with adhesive along adhesive line 32 through permeate carrier 22 thereby creating a sealed membrane envelope where the only exit path for permeate solution 20 is through center tube 12. Typically, the width of the adhesive line 32 is 1”-3” after the adhesive has been compressed during the rolling process.

[0047] A partially assembled spiral wound membrane element 200 is shown in FIG. 2. A membrane envelope 40 comprises, as described in connection with FIG. 1 , a membrane sheet 36 folded at one end with a permeate carrier 22 disposed therebetween the membrane sheet and sealed along the edges with a suitable adhesive line 32 (FIG. 1). In the conventional design of a membrane element once rolled, a feed spacer mesh 26 is placed adjacent to envelope 40 to allow the flow of feed fluid 16 to flow between layers of membrane envelope 40 and expose all of the active polymer surfaces 24 of the membrane sheet to feed fluid. Permeate 20, or product fluid is collected in the permeate carrier 22 inside membrane envelope 40 and proceeds spirally down to center tube 12 where the product, or permeate fluid 20 is collected while the reject stream 18 exits the element. A single spiral wound element may comprise a single membrane envelope and feed spacer layer, or may comprise multiple membrane envelopes and feed spacer layers stacked and rolled together to form the element.

[0048] FIG. 3 depicts printed membrane sheet 300 and includes printed membrane sheet 24, with printed dot spacers 50 in the middle, and printed lines 46 of width W on the feed and reject sides of printed membrane sheet 24. Prior to rolling, membrane sheet 36 is folded at fold line 30. This pattern typically has printed dots 50 that are .030 to .050 inches (750 to1250 microns) in diameter and printed lines 46 that are typically .020 to .040 inches (500 to 1000 microns) wide. This pattern is well suited to UV cured inkjet or stencil printing because the print resolution is precise enough to print characters that have dimensionally different widths and diameters.

[0049] FIG. 4 depicts an example efficient print pattern 400 for a thermoplastic printer. With a thermoplastic printer, a .050 inch (1250 micron) diameter dot requires that the print head print a small circle and fill in the dot. This slows down the speed of the print head and extends the time necessary to print a full sheet of patterns on printed membrane sheet 24 beginning at inlet edge 66 and continuing to exit edge 64. However, the print head can travel in one direction and print the longer support line 76 on the inlet end 16 of the membrane sheet, print spacers 68 and continue to the other end of printed membrane sheet 24 to print exit lines 72 on the discharge end of the membrane. In the example embodiment in the figure, note that every second edge support line, 72 or 76, does not have spacers 68 printed in-line with entrance and exit edge support lines 72 and 76. When spacers 68 are printed in the same line as edge support lines 72 and 76, the print head can run at the same speed from the inlet to the exit end of printed membrane sheet 24, for instance, less dense space 74. When spacers 68 are not printed in-line with support lines 72 and 76, then the print travel head can be accelerated at high speed to the other end of printed membrane sheet 24 in less dense space 74 before slowing down to print support line 76, or support line 72, depending on the direction of travel of the print head. This process can reduce the time to print a full pattern on printed membrane sheet 24. The time necessary to print the full membrane spacer pattern on half of membrane sheet 36 will determine the ultimate efficiency and cost of printing a membrane sheet. Generally, faster print times are preferred. Spacing X of print lines 72, 74, 76, can be 0.10 inches (2500 microns), but can range in spacing from 0.20 to 1 .0 inches (5000 to 25000 microns). Also note that only half of membrane sheet 36 is printed and folded at fold line 30. This also speeds up the printing process by only printing one half of membrane sheet 36, for instance printed membrane sheet 24. Spacers 68 can be printed tall enough to create the necessary space between membrane sheets to create the fluid feed space.

[0050] FIG. 5 represents a system 500 for printing pattern 92 on membrane sheet 36. Membrane sheet 36 can be individually cut and placed in printer system 500, or membrane sheet 36 can be roll fed on and off of printer system 500. Membrane sheet 36 is placed on printer system 500 so that the left end of membrane sheet 36 is positioned over vacuum table 90 to secure membrane sheet 36 in place, and to ensure membrane sheet 36 is flat and maintains a consistent vertical space between membrane sheet 36 and print heads 94. The right end of membrane sheet 36 can be placed on flat support table 86. Print heads 94 are mounted on print head support 84 which is mounted to X-axis stage or stages 82. Printhead support 84 may also be mounted on Y-axis stages 80 depending on the design of the gantry system. Assuming print heads 94 are less than or equal to 2.0 inches wide, they can be offset and face each other to maximize the number of print heads 94 in the system. The more print heads 94 there are in the system, the faster a membrane sheet 88 can be printed.

[0051] FIG. 6 represents a membrane sheet 36 where there are uncoated regions 65 on both sides of membrane sheet 36. A membrane sheet comprises typically three layers. The first layer is a support layer made of, for instance, polyester. The second layer is a porous support layer, for instance, made of polysulfone. A thin polymer membrane material is then cast on top of the polysulfone layer. Uncoated regions 65 consist only of the polyester support layer. There is no polysulfone or polymer layer in uncoated regions 65. Hence, uncoated regions 65 cannot be used for fluid separation purposes. Printed support lines 46 are necessary to support the element during the gluing and rolling operation. When printing using UV cured inks, support lines 46 do not need to extend into uncoated regions 65. Typically, printed support lines 46 are trimmed short, about half of the way into the length of printed support lines 46 on both sides of the membrane element. Uncoated regions 65 can be useful when printing spacers utilizing thermoplastics. Without modification of the thermoplastic print head 96 relative to travel speed or pressure variations applied to the thermoplastic material, the printed support lines 64 or 66 (FIG. 4) can have taller thickness regions, or upsets 70 (FIG. 9) at the beginning of the print process. By starting the printing process in uncoated regions 65 the upsets 70 may exist in those regions, but can then be trimmed off at the inner edges of uncoated regions 65 prior to gluing and rolling the element. In an alternative embodiment, print head 96 mounted on either X-axis gantry stage 82 (FIG. 5) or Y-axis gantry stage 80 (FIG. 5) can begin moving in uncoated regions 65 without activating print head nozzle 94 (FIG. 9) to allow time to overcome startup inertia, and allow X-axis gantry stage 82 and / or Y-axis gantry stage 80 to accelerate to full speed before beginning to dispense thermoplastic material 78 (FIG. 9) on support lines 72 or 76 in the active regions of printed membrane sheet 24 as shown in FIG. 4.

[0052] FIG. 7 represents a set of four thermoplastic manifolds 100, each of which has 25 solenoid controlled thermoplastic print heads 96. Manifolds 100 are fed under pressure via flexible thermoplastic feed hoses 98 from a central thermoplastic feed pot. This configuration enables 100 print heads to print 100 lines at the same time. A typical 1 meter by 1 meter print pattern will have, for example, 400 lines, each 2.5 mm apart. 400 lines in one meter of inlet glue support lines 66 and reject support lines 64 (FIG. 4) are sufficient to keep the membrane sheets apart during rolling. Support lines with a height of .020 inches (500 microns) in height have been shown to allow the print head to travel at a speed of 500 mm / second. For a 1 meter long pattern, this is a print speed of 2 seconds from the beginning to the end of the pattern. Since four passes are needed to obtain 400 lines with 100 printheads, the total length of time to print each one meter by one meter pattern is 8 seconds. If we allow 2 seconds for the gantry system to shift 2.5 mm at the end of each print line, then it would take approximately 16 seconds to print a full membrane sheet pattern that is .02 inches (500 microns) tall. Experience has shown that the fastest pattern that can be printed with a high speed UV cured inkjet printer is about 3 minutes. This demonstrates that a 100- nozzle thermoplastic print system can print a membrane sheet 12 times faster than a UV cured inkjet system. While prices vary, it is worth noting that UV cured ink is approximately 10 times more expensive per 1 -liter volume than thermoplastic material. Accordingly, thermoplastic printing offers a significant price reduction versus UV cured inkjet printing.

[0053] FIGs. 8A and 8B represent two plot charts. FIG. 8A shows the relationship between print head speed and the printed feature height when the applied pressure on print head 96 (FIG. 9) held constant. FIG. 8B shows the relationship between the applied pressure of the thermoplastic material in print head 96 with printed feature height when the speed of print head 96 is held constant. The speed of print head 96 as well as the applied thermoplastic pressure in print head 96 can be simultaneously adjusted to achieve the desired feature height or pattern along the length of the feature.

[0054] FIG. 9 represents a side view of typical thermoplastic printed segments 68 on printed membrane sheet 24 using conventional thermoplastic dispensing systems. Under a constant applied thermoplastic pressure represented by pressure line 110, and constant speed of print head 96, when thermoplastic dispensing nozzle 94 is activated, more thermoplastic material 78 is discharged at the beginning of printed line segment 68, or at the end of line segment 68 resulting in excess thermoplastic material being discharged creating upsets 70 in thermoplastic printed segments 68. Upsets 70 can occur at the beginning of line segments 68, at the end of line segments 68, or on both ends of line segments 68. While the desired height of the thermoplastic pattern might be Y height, upset feature 70 create an undesirable stress concentration having a height of X. These upset features 70 can cause stress concentrations in spiral wound or flat membrane assemblies during fabrication, which can damage the rejection properties of the treated solution.

[0055] FIG. 10 represents a side view printed membrane 24 with printed pattern features 68 with variable heights that are controlled by variable pressure in print head 96 or by variable speed in print head 96 or by combinations of both speed and pressure. Standard thermoplastic pressure is defined by region 110. Reduced thermoplastic pressure is defined by region 112. In the embodiment shown in FIG. 10, deposition of thermoplastic material 78 from print head nozzle 94 can be terminated in regions 113 thereby eliminating the deposition of line segments 68. Likewise, line segments 68 can have increased height segments 70 or 71 by virtue of increasing thermoplastic pressure 115 or by reducing the speed of print head 96.

[0056] FIG. 11 represents a solution to the stress concentration patterns 70 shown in FIGs. 9 and 10. This solution is provided by one or both options of changing the pressure in print head 96, or increasing the speed of print head 96 as it traverses over printed membrane 24. Alternately, printed membrane sheet 24 can be moving under a fixed row of print heads 96. The normal pressure on print head 96 is represented by pressure line 110. At the beginning of printing on line segment 68, the pressure on the thermoplastic material in print head 96 is reduced to pressure value 112. This reduces the volume of thermoplastic material that is dispensed at the beginning or end of printed segment 68, thereby facilitating a uniform height Y of line segment 68. Alternately to reducing the pressure on print head 96, the speed of print head 96 can be increased to allow less thermoplastic material 78 to be dispensed over a specific region, and thereby providing a uniform height Y of printed features 68.

[0057] Typical performance characteristics at constant operating temperature are presented for Nordson thermoplastic dispensing heads using Fuller thermoplastic material. At 500 psi pressure and nozzle speed of 200 mm / sec, the feature height is .019 inches. At a pressure of 600 psi and nozzle speed of 400 mm / sec, the feature height is between .014 to .016 inches. At 625 psi pressure and nozzle speed of 600 mm / sec, the initial feature height is .028 inches followed by continuous printing of feature heights of .021 to .024 inches. At 650 psi pressure and nozzle speed of 400 mm / sec, the initial feature height is .026 inches followed by continuous printing of feature heights of .021 to .024 inches. At 675 psi pressure and nozzle speed of 500 mm / sec, the initial feature height is .028 inches followed by continuous printing of feature heights of .020 inches. At 700 psi pressure and nozzle speed of 500 mm / sec, the initial feature height is .030 inches followed by continuous printing of feature heights of .021 to .023 inches. In an example embodiment of the present invention, the nozzle pressures can be in the range of 100 to 1000 psi, e.g., in the range of 500 to 700 psi. In an example embodiment of the present invention, the nozzle speed can be in the range of 100 to 1000 mm / second, e.g., in the range of 400 to 600 mm / sec. In an example embodiment of the present invention, the change in pressure applied to the thermoplastic can be less than 100 milliseconds, e.g., less than 5 milliseconds.

[0058] 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.

Claims

CLAIMSWhat is claimed is:

1. A method of applying a pattern, comprising a continuous or discontinuous thermoplastic material, to a substrate, comprising providing a substrate, supplying the thermoplastic material under pressure to a dispensing head having a dispensing nozzle and mounted proximal to but not in contact with the substrate, traversing the dispensing head along a path across a surface of the substrate at a travel speed, and regulating the pressure, controlling the travel speed, and opening the dispensing nozzle such that thermoplastic material dispensed from the nozzle forms a pattern along the path on the substrate, wherein the pattern has an actual height profile that is within 10 percent of a predetermined height profile.

2. The method of claim 1 , comprising regulating the pressure to a constant value and controlling the travel speed and opening of the nozzle.

3. The method of claim 1 , comprising controlling the travel speed to a constant value and regulating the pressure and opening of the nozzle.

4. The method of claim 1 , wherein the predetermined height profile is not a constant height.

5. The method of claim 4, comprising regulating the pressure to a first pressure when the predetermined height profile comprises a first height, and regulating the pressure to a second pressure when the predetermined height profile comprises a second height, wherein the first pressure is greater than the second pressure and the first height is greater than the second height.

6. The method of claim 1 , comprising controlling the dispensing nozzle so that no thermoplastic material is dispensed when the local value of the predetermined height is zero.

7. The method of claim 1 , wherein the substrate comprises a membrane.

8. The method of claim 9 wherein the membrane comprises a substrate, a porous support layer, and a separating layer.

9. The method of claim 8, wherein the substrate comprises polyester, and the porous support layer comprises polysulfone, and where the separating layer comprises polyamide10. The method of claim 8, wherein the thermoplastic material has a solidification temperature between 45C and 200C.11 . The method of claim 1 , wherein maximum value of the predetermined height profile is between 50 and 1270 microns and wherein the height varies less than 10 percent.

12. The method of claim 1 , wherein the pattern has a width between250 and 1000 microns.

13. The method of claim 1 , wherein the pattern comprises a single segment, or a plurality of segments, and where each segment has a length from 250 microns to 1 meter.

14. The method of claim 1 , wherein the dispensing nozzle travels at a speed of greater than 25 centimeters per second.

15. An apparatus for printing a pattern on a membrane sheet, comprising: a table configured to securely hold the membrane sheet; a dispensing head support; an x-axis stage mounted with the dispensing head support, and configured to translate the dispensing head support in an x dimension; one or more dispensing heads mounted with the dispensing head support; a y-axis stage configured to translate the membrane sheet relative to the dispensing head support in a y dimension, or to translate the dispensing head support relative to the membrane sheet in the y dimension; a source of thermoplastic material, configured to supply thermoplastic material to the one or more dispensing heads; a pressure regulator, configured to regulate the pressure of thermoplastic material in the one or more dispensing heads; a control system, configured to operate the apparatus to print a pattern on the membrane sheet according to the method of any of claims 1-16.

16. An apparatus for printing a pattern on a membrane sheet, comprising: a dispensing head support, and an apparatus for translating the dispensing head relative to the membrane sheet in the X, Y, and / or Z dimensions at a controlled velocity according to the method of any of claims 1-14.

17. The apparatus of claim 16 comprising: a table configured to securely hold the membrane sheet; a dispensing head support; an x-axis stage mounted with the dispensing head support, and configured to translate the dispensing head support in an x dimension; one or more dispensing heads mounted with the dispensing head support; a y-axis stage configured to translate the membrane sheet relative to the dispensing head support in a y dimension, or to translate the dispensing head support relative to the membrane sheet in the y dimension; a source of thermoplastic material, configured to supply thermoplastic material to the one or more dispensing heads; a pressure regulator, configured to regulate the pressure of thermoplastic material in the one or more dispensing heads; a control system, configured to operate the apparatus to print a pattern on the membrane sheet.

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