Membrane spacers for high salinity

Abstract

Printed spacer membrane elements offer the unique advantage of applying any pattern on the membrane surface to act as the feed spacer material. This is in contrast to conventional feed spacer mesh material that is uniform in thickness and density throughout the mesh. Concentration polarization is the accumulation of ions or other material at the membrane surface that reduces permeate quality, increases the opportunity for scale and biofilm, reduces recovery, and other deleterious effects. Random or mathematically generated feed space patterns can facilitate fluid mixing in the feed space in order to reduce concentration polarization that occurs at the membrane surface.

Description

Membrane Spacers for High Salinity

[0001] Technical Field

[0002] The subject invention relates to a membrane system utilized for the separation of fluid components, specifically membrane elements for spiral wound and other filtration systems.

[0003] Background Art

[0004] In cross-flow filtration, a feed fluid flows through a spiral wound 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, spiral wound systems, and radial flow 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 and the present invention is applicable to such systems.

[0005] Application of membrane filtration include water and wastewater, seawater, brine mining, electrodialysis, industrial water treatment for semiconductor manufacturing and recycling, water treatment for beverage applications, concentration of product streams such as paint thickening, water extraction from food processing such as syrup production, oil field water processing, and other membrane applications utilized in industry.

[0006] Spiral-wound membrane filtration elements are well known in the art which consist of 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. Traditionally, a feed spacer mesh has been used to allow flow of the feed water, some portion of which will pass through the membrane as permeate, and allow reject water to exit the element in a direction parallel to the center tube and axial to the element construction.

[0007] Membrane elements may be comprised of microfiltration elements, ultrafiltration elements, nano-filtration elements, or reverse osmosis elements. Classical reverse osmosis membranes are made of three layers. For example, the bottom or support layer may be a polyester fabric. Bonded to the polyester support may be aporous layer comprising polysulfone. The polysulfone layer with the polyester substrate may be sufficient for ultra-filtration. For reverse osmosis, a polymer layer is applied on the polysulfone to reduce the pore size sufficient to remove monovalent salts from the fluid stream. In seawater applications, the primary ions desiring rejection are sodium and chloride, but may include the class of di-valent ions, and may include boron and other constituents. Also being developed are graphene materials, carbon nano-tubes, chlorine tolerant polymers, and other materials that may offer lower energy reverse osmosis, or chlorine tolerance to facilitate cleaning.

[0008] 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 Herrington et al which replaces the feed spacer with islands or protrusions either printed, deposited or embossed directly onto the inside or outside 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 B2 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, Non-Deforming Patterns in Spiral Wound Elements” 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 at the adhesive lines during element fabrication.

[0009] One of the important features in spiral wound element design is consideration for concentration polarization. Concentration polarization occurs when ions in the bulk fluid stream accumulate at the membrane surface that creates locally high osmotic pressure which increases the required operating pressure for desalination. This tendency for concentration polarization occurs more readily at the reject end of the element, or last element in a series of elements, due to the fact that ions have accumulated at higher concentrations as the bulk fluid travels from the feed to reject end of the spiral wound element or element train. As ion concentrations increase at the reject end of the element, the tendency for precipitation to occur is greater. This can have a limiting effect on the recovery of the water that is produced as clean permeate. Concentration polarization can also damage the rejection characteristics of the element due to higher ion transport through the membrane. For this reason, any characteristics of the feed space that can reduce concentration polarization are desirable. This can include mixing patterns in the fluid space as well as increased shear in the fluid stream.

[0010] Brief Summary of the Invention

[0011] The present invention describes novel methods of fabricating cross-flow filtration devices that provide increased mixing and shear in the feed space to reduce concentration polarization, improve rejection, and has reduced energy consumption versus that available in the existing state-of-the-art that use conventional mesh or plastic feed spacers.

[0012] Embodiments of the present invention provide a spiral wound element comprising two or more membrane sheets, each membrane sheet being folded on itself at a fold line providing first and second membrane halves facing each other, wherein each folded membrane sheet is spirally wound around the center tube with the fold line proximal the center tube, and the first membrane half forming an inner wind relative to the second membrane half, each membrane sheet having a plurality of spacing features disposed on a surface of at least one of the first and second halves, where the spacing features utilize a pattern that reduces concentration polarization.

[0013] In some embodiments, the spacing elements are disposed on the first membrane half and not on the second membrane half. In some embodiments, the spacing elements are disposed on the second membrane half and not on the first membrane half. Some embodiments further comprise a permeable permeate carrier between each pair of adjacent folded membrane sheets.

[0014] In some embodiments, the areal packing density of the element is greater than 33in2 / in3. In some embodiments, the areal packing density of the element is greater than 35in2 / in3. In some embodiments, the areal packing density of the element is greater than 39in2 / in3. In some embodiments, the areal packing density of the element is greater than 41 in2 / in3. In some embodiments, the areal packing density of the element is greater than 43in2 / in3.

[0015] In some embodiments, the spacing features have a height extending above the corresponding membrane, where the height of spacing features near the fold line is less than the height of spacing features distant from the fold line. In some embodiments, the spacing features distant from the fold line by more than the radius of the center tube have a height that is constant, and spacing features that are closer to the fold line than the diameter of the center tube have a height that increases with distance from the fold line. In some embodiments, the spacing features distant from the fold line by more than the diameter of the center tube have a height that is constant, and spacing features that are closer to the fold line than the diameter of the center tube have a height that increases with distance from the fold line. In some embodiments, the spacing features distant from the fold line by more than the circumference of the center tube have a height that is constant, and spacing features that are closer to the fold line than the diameter of the center tube have a height that increases with distance from the fold line.

[0016] In some embodiments, the spacing features occupy less than 7% of the volume between the membrane halves. In some embodiments, the spacing features occupy less than 5% of the volume between the membrane halves. In some embodiments, the spacing features occupy less than 2% of the volume between the membrane halves. In some embodiments, the spacing features occupy less than 7% of the area of the membrane on which they are deposited. In some embodiments, the spacing features occupy less than 5% of the area of the membrane on which they are deposited. In some embodiments, the spacing features occupy less than 2% of the area of the membrane on which they are deposited.

[0017] Embodiments of the present invention provide an assembly, comprising (a) a plurality of folded membranes, each folded membrane comprising a membrane sheet having an active surface and an inactive surface opposite the active surface, the membrane sheet folded in half along a fold line with the active surfaces facing each other, wherein the membrane sheet has disposed thereon a plurality of spacingfeatures in a region less than 0.1 inch from the fold line having no spacing features; (b) a plurality of permeable permeate carrier sheets; (c) a center tube; (d) wherein the permeate carrier sheets are spirally wound about the center tube and welded together with a folded membrane in between each pair of permeate carrier sheets.

[0018] In some embodiments, the permeate carrier sheets are welded together with a leading edge of each successive permeate carrier sheet separated from the leading edge of the preceding permeate carrier sheet by no more than the diameter of the center tube divided by the number of permeate carrier sheets. In some embodiments, a second permeate carrier sheet is welded to a first permeate carrier sheet at a distance at least the circumference of the center tube and a third permeate carrier sheet is welded to the second permeate carrier sheets at a distance by no more than the diameter of the center tube divided by the number of permeate carrier sheets.

[0019] Embodiments of the present invention provide a spiral wound fluid treatment element, comprising: (a) a collection tube; (b) one or more membrane sheets, each having a first surface suitable for contacting fluid to be treated and a second surface, opposite the first surface, the membrane sheet spirally wound around the collection tube; (c) a plurality of spacing elements disposed on the first surface of each membrane sheets, wherein each spacing element has a cross-section viewed parallel to the axis of the collection tube that has a first end and a second end and extends from the first end to the second end, wherein the first end is wider than the second end.

[0020] In some embodiments, the first ends of the spacing elements are in contact with the first surface of the corresponding membrane sheet, and wherein the first surface of the membrane sheet faces away from the collection tube. In some embodiments, the second ends of the spacing elements are in contact with the first surface of the corresponding membrane sheet, and wherein the first surface of the membrane sheet faces toward the collection tube.

[0021] In some embodiments, the ratio of the second end width to the first end width is less than 0.98. In some embodiments, the ratio of the second end width to the first end width is less than 0.95. In some embodiments, the ratio of the second end width to the first end width is less than 0.5.

[0022] In some embodiments printed spacers can comprise UV cured polymers, UV cured urethanes, thermoplastics including hot melt materials, thermoset materials, epoxies, fiberglass material, rubber material, or other materials suitable for applyingdirectly to the membrane surface, including polymers of similar composition as the active membrane surface material.

[0023] Embodiments of the present invention provide a spiral wound fluid treatment element, comprising: (a) a collection tube; (b) a plurality of membrane sheets, each having an active surface and an inactive surface opposite the active surface, wherein each membrane sheet is folded along a fold line such that the active surfaces face each other, and wherein each membrane sheet is disposed with the fold line proximal the collection tube; (c) wherein each membrane sheet has disposed on the active surface thereof a plurality of spacing elements, wherein each spacing element has a cross-section viewed parallel to the axis of the collection tube that has a first end and a second end and extends from the first end to the second end, wherein there is a feature space between the first end and the second end, wherein the feature space has a spacer pattern that reduces concentration polarization.

[0024] Brief Description of the Drawings

[0025] FIG. 1 is an exploded view of a spiral wound membrane element.

[0026] FIG. 2 is an exploded view of a partially assembled spiral wound membrane element.

[0027] FIG. 3 is a plan view of a membrane leaf with a printed spacer pattern with dots prior to folding and rolling.

[0028] FIG. 4 is a plan view of a membrane leaf with a printed spacer pattern with dashes prior to folding and rolling.

[0029] FIG. 5 is a plan view of a membrane leaf with a printed spacer pattern with random spacing prior to folding and rolling.

[0030] FIG. 6 is a plan view of a membrane leaf with a printed spacer pattern with random size spacing features prior to folding and rolling.

[0031] FIG. 7 is a plan view of a membrane leaf with a printed spacer pattern with a mathematical spacer arrangement prior to folding and rolling.

[0032] FIG. 8 is a plan view of a membrane leaf with a uniform distribution of features.

[0033] FIG. 9 is a plan view of a membrane leaf with a random pattern whereby a straight line cannot extend across a random pattern without contacting two or more features.

[0034] FIG. 10 is a plan view of a membrane leaf with a spacer pattern whereby a straight line cannot extend across a mathematically influenced pattern without contacting two or more features.

[0035] FIG. 11 is a plan view of a membrane leaf with a spacer pattern whereby multiple straight lines cannot extend across a mathematically influenced pattern without contacting two or more features.

[0036] FIG. 12 is a plan view of a membrane leaf with a random pattern with individual features located within individual regions with total volume of the individual regions equaling the total surface of the membrane space.

[0037] FIG. 13 is a plan view of a membrane leaf with a uniform pattern of individual features located within individual regions with total volume of the individual regions equaling the total surface of the membrane space.

[0038] Modes for Carrying Out the Invention and Industrial Applicability

[0039] The 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 the differential pressure loss from the feed to reject end of the element, local flow velocities, turbulence, stagnation zones and other fluid flow conditions. Extruded mesh feed spacers have been used traditionally in membrane manufacture, 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. Printed spacers 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 application. One of the key features of printed spacer technology is the open architecture of the feed channel. Conventional mesh spacers consist of two layers of stringers criss-crossed over each other and welded or bonded at the intersection of the two stringers to provide structure to the mesh. By its very nature, all of the fluid must pass over one of the stringers, around the stringer, and back over the opposing stringer. A mesh stringer running all the way across the feed space, even if it is not normal to the flow path, creates an inherent restriction in the height of the flow channel. Printed feed spacers have no inherent restriction. The channel height on printed spacers can be half the height of the conventional mesh spacer, providing the same mass flow characteristics and same pressure drop feed to reject as the mesh spacer. Conventional mesh spacers in 40-inch long elements are typically in the range of 26 to 32 mils (.026 to .032 inches) but can be higher or lower for particular applications. Printed spacer technology allows spacer heights much lower, typically 15 to 20 mils (.015 to .020 inches) and provide the same mass flow characteristics with equivalent pressure drop feed to reject as mesh spacers in 40” long elements. In 12” long or shorter elements, the spacing height can be as low as .003”, but typically .006” to .008” tall.

[0040] One of the primary characteristics of mesh spacers commonly expressed as an advantage is that the mesh creates substantial mixing in the flow stream that helps reduce concentration polarization. Concentration polarization is the tendency of ions in the fluid stream to stack up at the membrane surface as fluid molecules pass through the membrane and increase the concentration of the total dissolved solids (TDS) at the membrane surface, and thereby increases the osmotic pressure required to drive the fluid molecules through the membrane surface. Other disadvantages of concentration polarization are an increase in the TDS that transfers through the membrane surface reducing the quality of the product water, otherwise known as a loss of rejection. Concentration polarization can have more obvious effects in high TDS water such as seawater where increases in osmotic pressure are more dramatic due to higher TDS at the reject end of the element. For fresh water, high rejection is typically not as important. However, in some industrial applications such as semiconductor processing, high rejection is desirable even in low TDS applications. For water containing sparingly soluble species, the combined effects of system recovery and polarization can also lead to precipitation at the reject end of the element, or at the last element in a multi-element pressure vessel. These characteristics may restrict the recovery of reverse osmosis systems. In any event, printed spacer technology can have a wide variety of printed features to create mixing that is needed for concentration polarization to be minimized. Theoretically, laminar flow characteristics of thinner channel printed spacers achieve good levels of concentration polarization as compared to mesh type spacers.

[0041] Cross-flow filtration, by its nature, 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 fluxthrough 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.

[0042] An important advantage of printed spacer technology is that more open feed spacer channels can be created. The present invention provides a means to maximize permeate flow, increase recovery, reduce concentration polarization, decrease differential pressure losses from the feed to reject end of the element, reduce scale formation potential, simplify cleaning protocols, or combinations of all of these benefits.

[0043] The feed shaping features employed may be of any of a number of shapes, including round dots, ovals, bars with rounded ends, lenticular forms, stretched polygons, arcs, 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.

[0044] The arrangement and location of features in a print pattern serve multiple roles in membrane filtration and as such need to be designed properly to maximize the performance benefit of printed spacers.

[0045] One role the specific location features in a pattern play is to create a uniform gap between two adjacent membrane layers. To work well at this, the features need to have a relatively consistent distance to adjacent features. This decreases the likelihood of some regions with too few features either coming into contact with each other, or being subjected to high pressures during the assembly process. The distances to nearby features doesn't need to be perfectly uniform however, and as such a number of statistical approaches could be used to determine that there is a relatively uniform distance between features. This may include an average number of features in a given area, statistics on the distance distribution to neighboring features, or looking at the closest or furthest feature.

[0046] Balancing the interest in having a uniform coverage distribution, is the desire to minimize the amount of membrane surface covered with features. Each feature potentially blocks the membrane underneath it, reducing the effectiveness of resultingproducts such as membrane elements, and particularly spiral wound membrane elements. The combination of the area of each feature, along with the number of features and the area of membrane being covered can be converted into a number that represents the area of membrane in the print that is obscured or covered by features. Note that in spiral wound elements many print patterns may only cover one half of the used area, with the other half being used without any features. In this case the coverage amount is calculated based on the area where features are being deposited.

[0047] A third aspect of feature layout in a pattern is related to improving the way flow profiles develop membrane devices. When print patterns are used that have features laid out on a regular grid, there are often regions where flow can occur over long distances without interacting with any features. For example, in regular rectangular grids, flow can preferentially occur in the space between features and flow from inlet to outlet undisturbed. We have found that by intentionally modifying the layout of features so that there is a reduced distance flow can occur in a straight line before being disrupted, fluid flow occurs in multiple directions leading to reduced concentration polarization and improved ability to direct solids away from the membrane and out of the membrane device.

[0048] One method to produce feature spacing with highly uniform distribution while preventing long straight-line distances between features employs placing the spacing features as points along a Vogel spiral. A Vogel spiral is an aperiodic arrangement of points generated using a mathematical formula, where each point's angle (0) is a multiple of an index number (i) divided by an irrational number (n), and its distance from the center (r) is proportional to the square root of the point's index (i), and C is a scaling constant.

[0049] r= C *(i), / 2

[0050] 0 = i*3607n

[0051] This equation requires an irrational angle to prevent points from lining up and to ensure they are distributed evenly without rotational symmetry. For uniform distribution, using the golden ratio, defined as (1 +51 / 2) / 2, as the irrational number (n) provides an optimal distribution.

[0052] Example embodiments include patterns that include:

[0053] Example 1 . A print pattern made up of an array of features on a specific membrane surface wherein at least 90% of the pattern meets the following criteria: i. acoverage of less than 10% (or 8%, 6%, 4%, 2%) of that specific membrane’s surface area; ii. has a feature-to-feature distance having a standard deviation of nearest neighbor distance less than 1 / 3 of the average nearest neighbor distance; iii. has a longest straight line that can be fit within the pattern while not intersecting a feature that is less than 100 times the average nearest neighbor distance.

[0054] Defined in an algorithmically defined manner meeting the criteria described in example 1 .

[0055] A pattern as in example 1 where the features are disposed along a Fibonacci or Vogel spiral.

[0056] A pattern as in example 1 where the features are disposed in a random distribution.

[0057] A pattern as in example 1 , in a spiral wound membrane element, flat sheet membrane pattern, flat circular membrane pattern, or other membrane sheet configuration.

[0058] A print pattern made up of an array of features on a specific membrane surface having at least 90% of the pattern meets the following criteria: i. a coverage of less than 10% (or 8%, 6%, 4%, 2%) of that specific membrane’s surface area; ii. straight line distance a from a feature to other features in the pattern without intersecting a third feature and are less than 100 times (or 50 times, 25 times) the nearest neighbor distance, iii. has a longest straight line that can fit within the pattern which doesn’t intersect a feature that is less than 100 times (or 50 times, 25 times) the average nearest neighbor distance.

[0059] 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 20 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 polysulfone, and active polymer membrane surface 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 when rolled around permeate collection tube 12. In the preferred embodiment, feed spacer mesh 26 is replaced with printed features on active polymer membrane surface 24 or alternatively on non-active support layer 28 to createembossed features on active polymer membrane surface 28. 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 to 3 inches after the adhesive has been compressed during the rolling process.

[0060] In the existing art, 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 permeate carrier 22 disposed therebetween membrane sheets 36a and 36b and sealed along the edges with a suitable adhesive line 32 (FIG. 1). In the conventional design of 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 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 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. For multiple membrane sheet elements, the permeate carrier assembly (Trico pack) consists of multiple permeate carrier sheetsthat are ultrasonically (or otherwise) welded together at an offset distance which is typically the distance of the center tube diameter divided by the number of leaves in the element. The first permeate carrier sheet is typically longer so that it can wrap around the center tube once or twice in order to create volume for the permeate from all of the leaves to enter the holes 14 (FIG. 1) in center tube 12 without causing undue back pressure in the individual permeate leaves.

[0061] FIG. 3 depicts printed spacer membrane sheet 300 whereby membrane sheet 36 with center line 30 has spacing features 68 applied to one-half of membrane sheet 36 with spacing features 68 arrayed in a staggered linear pattern in central printed region 74 where printed features 68 are not uniformly spaced in a triangular arrangement. A straight line may pass horizontally or vertically through the pattern but the spacings are not uniform. Spacing between features 68 may be 0.05 inches (1270 microns) to 1 .0 inches (25,400 microns) apart, or 0.25 inches (6350 microns) to 0.5 inches (12700 microns) apart. Feed fluid 16 enters at entrance side 66 of membrane sheet 36 and exits at discharge side 64 as reject fluid 18. Higher density feed patterns 76 and reject end patterns 72 provide support for membrane sheet 36 during sealing of the membrane envelope. Entrance and exit support features 72 and 76 may have spacings X of .05 inches (1270 microns) to 0.30 inches (7620 microns) or 0.08 inches (2032 microns) to 0.12 inches (3048 microns).

[0062] FIG. 4 depicts printed spacer membrane sheet 400 whereby membrane sheet 36 with center line 30 has spacing features 70 applied to one-half of membrane sheet 36 with spacing features 70 arrayed in a staggered linear pattern in central printed region 74 where printed features 70 are not uniformly spaced in a triangular arrangement. Spacing between features in the linear array may be 0.05 inches (1270 microns) to 1 .0 inches (25,400 microns) apart, or 0.25 inches (6350 microns) to 0.5 inches (12700 microns) apart. Feed fluid 16 enters at entrance side 66 of membrane sheet 30 and exits at discharge side 64 as reject fluid 18. Higher density feed patterns 76 and reject end patterns 72 provide support for membrane sheet 36 during sealing of the membrane envelope.

[0063] FIG. 5 depicts printed spacer membrane sheet 500 whereby membrane sheet 36 with center line 30 has spacing features 80 applied to one-half of membrane sheet 36 with spacing features 80 arrayed in a random pattern in central printed region 74 where printed features 80 are not uniformly spaced relative to other printed features. Spacing of features can be determined by a random number generator in the printingsystem software or by a randomly generated pattern in software loaded into a printing system. Spacing Y between features in the array may be 0.05 inches (1270 microns) to 1.0 inches (25,400 microns) apart, or 0.25 inches (6350 microns) to 0.5 inches (12700 microns) apart. Feed fluid 16 enters at entrance side 66 of membrane sheet 36 and exits at discharge side 64 as reject fluid 18. Higher density feed patterns 76 and reject end patterns 72 provide support for membrane sheet 36 during sealing of the membrane envelope.

[0064] FIG. 6 depicts printed spacer membrane sheet 600 whereby membrane sheet 36 with center line 30 has spacing features 82 and 84 applied to one-half of membrane sheet 36 with spacing features 82 and 84 arrayed in a random pattern in central printed region 74 where printed features 82 and 84 are not uniformly spaced relative to other printed features. Spacing features may comprise more than two different sizes. Spacing features may also comprise various shapes including circular dots, line segments, triangles, squares, diamonds, or other randomly shaped features. Spacing of features can be determined by a random number generator in the printing system software or by a randomly generated pattern in software loaded into a printing system. Spacing Z between features 82 and 84 in the array may be 0.05 inches (1270 microns) to 1 .0 inches (25,400 microns) apart, or 0.25 inches (6350 microns) to 0.5 inches (12700 microns) apart. Feed fluid 16 enters at entrance side 66 of membrane sheet 30 and exits at discharge side 64 as reject fluid 18. Higher density feed patterns 76 and reject end patterns 72 provide support for membrane sheet 36 during sealing of the membrane envelope.

[0065] FIG. 7 depicts printed spacer membrane sheet 700 whereby membrane sheet 36 with center line 30 has spacing features 86 applied to one-half of membrane sheet 36 with spacing features 86 arrayed in a mathematically generated pattern in central printed region 74 where printed features 86 are not spaced relative to each other in a linear array. Spacing of features 86 can be determined by a mathematical sequence such as a Fibonacci or Vogel Spiral pattern generated in software loaded into a printing system. Examples of other patterns include a binomial expansions, Pascal’s triangle, Sierpinski triangle, a hexagonal grid, fractal pattern, or other mathematical configurations. Spacing between features 86 in the array may be 0.05 inches (1270 microns) to 1 .0 inches (25,400 microns) apart, or 0.25 inches (6350 microns) to 0.5 inches (12700 microns) apart. Feed fluid 16 enters at entrance side 66 of membrane sheet 36 and exits at discharge side 64 as reject fluid 18. Higher density feed patterns76 and reject end patterns 72 provide support for membrane sheet 36 during sealing of the membrane envelope. FIG. 10 and FIG. 11 are embodiments of FIG. 7.

[0066] FIG. 8, membrane sheet 800 is an alternative embodiment that allows a straight line 90 and 92 across the printed pattern as compared to FIG. 3 or FIG. 4. but the pattern is generally uniform in both X and Y axis in all respects. The pattern of FIG. 8 violates the non-uniformity characteristics which are the intent of this patent.

[0067] FIG. 9 is an embodiment of the present invention that shows membranae 900 with membrane sheet 36 having center line 30 with a straight line through the spacing pattern that cannot extend across the entire pattern from one feature to the next without hitting a third spacing feature. A straight line distance A from a feature to other features without intersecting a third feature are less than 100 times (or 50 times, or 25 times) the nearest neighbor distance - for instance a random spacer pattern. This supports the spacing characteristics of the pattern in FIG. 5 and FIG. 6.

[0068] FIG. 10 is an embodiment of the present invention that shows membrane sheet 1000 a straight line through the spacing pattern that cannot extend across the entire pattern from one feature to the next without hitting a third spacing feature. A straight line distance A from a feature to other features without intersecting a third feature are less than 100 times (or 50 times, or 25 times) the nearest neighbor distance - for instance a random spacer pattern. This supports the spacing characteristics of the pattern in FIG. 7.

[0069] FIG. 11 , printed space membrane sheet 1100 with membrane sheet 36 and center line 30 is an embodiment of the present invention that shows an example of three straight lines, D, E, and F, that go through a printed spacer pattern. This case defines the case where no single line that can be fit with the pattern does not intersect a feature that is less than 100 times (or 50, times, or 25 times) the average nearest neighbor distance. A straight line cannot pass through this pattern without intersecting another feature.

[0070] FIG. 12, printed spacer membrane sheet 1200 is an embodiment of the present invention that defines a random pattern on a printed spacer membrane sheet where the printed spacer area is defined by the dimensions a and b, and where N is the number of printed spacer features 80 within area a X b. The average spatial area, Asavg, that each spacer 80 resides in is defined as (a X b) / N = As avg. Spacer feature 80, is located near the geometric center of an average spatial area, As avg. Only one spacer feature 80 is located within each average spatial area, As avg. Each individualspatial area, (A individual) 94 can vary in size by less than 99% of the average spatial area, As avg, or less than 50% of the average spatial area, As avg, or less than 20% of the average spatial area, As avg. FIG. 5, FIG. 6, and FIG. 9 are example embodiments of FIG. 12.

[0071] FIG. 13 shows a regular repeating array of spacing features and a single geometric shape that is able to tile the plane of the array. Arrays that have the ability to be tiled with a single geometric shape have regions in which flow through the feed channel may not be disturbed or disrupted leading to the potential for reduced mixing which may lead to increased concentration polarization or solid build up. In contrast FIG. 12 with its random distribution or FIG. 7 with a Vogel spiral design, there isn’t a way to create connected subregions using a single geometric shape, instead multiple different geometric subregions are required to tile the plane of the array. As a result water flow through such channels occur through a more meandering path leading to improved mixing.

[0072] 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 printed membrane sheet having disposed thereon a plurality of spacing features, wherein spacing features are disposed in a pattern comprising an array on a specific membrane surface and where the spacing features cover less than 10% of the membrane surface and wherein at least 70% of the spacing features have an average nearest neighbor distance, defined as the average of the 6 next closest spacing features, and wherein the longest straight line that can be fit within the array which doesn’t intersect a feature is less than 100 times the average nearest neighbor distance.

2. A printed membrane sheet having disposed thereon a plurality of spacing features, wherein the spacing features are disposed in a pattern comprising an array on a specific membrane surface and where the spacing features cover less than 10% of the membrane surface and wherein at least 70% of the array are regions wherein the longest straight line that can be fit within the array without intersecting a feature is less than 20 inches.

3. A printed membrane sheet having disposed thereon a plurality of spacing features, wherein the spacing features are disposed in a pattern comprising an array on a specific membrane surface where the spacing features cover less than 10% of the membrane surface and wherein at least 70% of the array can be divided into subregions with each subregion containing one spacing feature, wherein the subregions completely tile the plane of the printed membrane sheet and wherein the placement of features in the array makes it impossible for all subregions to possess a common geometry.

4. A printed membrane sheet of claims 1-3 wherein the spacing elements are disposed in a mathematical pattern.

5. The printed membrane sheet of claim 4 wherein spacing features are positioned in polar coordinates in locations given by (r, 0) relative to a center point wherein:(a) there is a scaling factor C that is 0.1 to 20mm;(b) there is an index number i, consisting of integers with a number of values which corresponds to the number of features located about the center point;(d) the distance r from the center to the feature is given by the r= C *(i)1 / '2;(e) wherein n is a number within 0.5% of an irrational number;(f) the angle 0 = i*3607n .

6. The printed membrane sheet of claim 5 where n is within 0.5% of the golden ratio, defined as (1+51 / 2) / 2, being approximately 1.618.

7. The printed membrane sheet of claim 5 where n is within 0.5% of Euler’s number, approximately 2.718.

8. The printed membrane sheet of claim 5 where n is within 0.5% of 2.618.

9. A filtration element comprising one or more printed membrane sheets according to any of claims 1-8, wherein each membrane sheet is folded on itself at a fold line providing first and second membrane sheet halves facing each other, wherein the plurality of spacing features is disposed on a surface of at least one of the first or second membrane sheet halves, or on both membrane sheet halves.10.. The filtration element of claim 9, wherein membrane sheets are spirally wound to form a spiral wound membrane element.11 . A method of treating a fluid by reverse osmosis, comprising supplying the filtration element of claim 10 and flowing the fluid therethrough.

12. A reverse osmosis system comprising one or more membrane elements according to claim 10.

13. The printed membrane sheet of claim 1 , wherein the spacing features cover less than 8% of the membrane surface.

14. The printed membrane sheet of claim 1 , wherein the spacing features cover less than 6% of the membrane surface.

15. The printed membrane sheet of claim 1 , wherein the spacing features cover less than 4% of the membrane surface.

16. The printed membrane sheet of claim 1 , wherein the spacing features cover less than 2% of the membrane surface.

17. The printed membrane sheet of claim 1 , wherein the longest straight line that can be fit within the array which doesn’t intersect a feature is less than 50 times the average nearest neighbor distance.

18. The printed membrane sheet of claim 1 , wherein the longest straight line that can be fit within the array which doesn’t intersect a feature is less than 10 times the average nearest neighbor distance.

19. The printed membrane sheet of claim 2, wherein the longest straight line that can be fit within the array without intersecting a feature is less than 10 inches.

20. The printed membrane sheet of claim 2, wherein the longest straight line that can be fit within the array without intersecting a feature is less than 5 inches.21 . The printed membrane sheet of claim 2, wherein the longest straight line that can be fit within the array without intersecting a feature is less than 2 inches.

22. The printed membrane sheet of claim 5, wherein the scaling factor C is 0.5 to 10mm.

23. The printed membrane sheet of claim 5, wherein the scaling factor C is 1 to 5mm.

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