Spiral wound element construction for improved capacity

Abstract

Printed spacer membrane elements have the unique advantage of being able to impart any pattern to the membrane surface to act as a feed spacer material, in contrast to conventional feed spacer mesh materials, which are uniform in thickness and density throughout the mesh. They allow for closer spacing between membrane sheets in the feed space while utilizing more open feed space, which allows more membrane sheets to be wrapped around the membrane element, resulting in more permeate flow while simultaneously reducing pressure drop from the feed end to the reject end of the element. The unique design features of the printed spacer dramatically improve the performance of the membrane element, in contrast to prior art feed spacer mesh elements.

Description

[Technical field]

[0001] Technical Field The present invention relates to membrane systems utilized in the separation of fluid components, and in particular to spiral wound membrane elements. [Background technology]

[0002] Background technology In cross-flow filtration, the feed fluid flows through a spiral wound filter and is discharged at the other end, while a portion of the fluid is removed by filtration through the membrane surface parallel to the direction of fluid flow. Cross-flow filtration can take a variety of forms, including plate-and-frame, cassette, hollow fiber, or spiral wound systems. Filtration modules in plate-and-frame, cassette, and spiral wound systems often rely on stacked membrane layers to provide spacing between adjacent filtration membrane layers, and the present invention is applicable to such systems.

[0003] Spiral wound membrane filtration elements are well known in the art, consisting of a laminate structure constructed of a membrane sheet sealed to or around a porous permeate carrier that forms a path for removal of fluid through the membrane to a central tube, longitudinally relative to the axis of the central tube, which laminate structure is spirally wrapped around the central tube and spaced from itself by a porous feed spacer to allow axial flow of fluid through the element from the feed end to the reject end of the element. Conventionally, a feed spacer mesh is used to allow the flow of feed water (a portion of which passes through the membrane) into the spiral wound element and to allow reject water to exit the element in a direction parallel to the central tube and axially along the element structure.

[0004] Improvements to spiral wound element designs are disclosed in U.S. Patent No. 6,632,357 to Barger et al., U.S. Patent No. 7,311,831 to Bradford et al., and Australian (Patent No. 2014223490), Japanese (Patent No. 6499089), Chinese (CN No. 105163834B), Israeli (Patent No. 240883), and Korean (Patent No. 10-2196776) patents entitled "Improved Spiral Wound Element Construction" to Herrington et al., which replace feed spacers with islands or protrusions printed, deposited, or embossed directly onto the inner or outer surface of the membrane or onto the permeate carrier. U.S. Patent No. 11,090,612 to Roderick et al. entitled "Graded spacers for filtration wound elements" describes the use of height graded spacer features used to modify the feed flow characteristics in spiral wound elements. U.S. patent application PCT / US17 / 62424 to Roderick et al., entitled "Interference Patterns for Spiral Wound Elements," describes patterns of spiral wound elements that keep the membrane feed space open but provide support for the membrane envelope bond area during winding. U.S. patent application PCT / US18 / 55671 to Roderick et al., entitled "Bridge Support and Reduced Feed Spacers for Spiral-Wound Elements," describes support features applied to the distal end (end furthest from the central tube) of the membrane envelope to provide support during bonding and winding of the spiral wound elements.U.S. Provisional Patent Application No. 63,051,738 to Herrington et al., entitled "Variable Velocity Patterns in Cross Flow Filtration," describes support patterns that vary in size from the feed end to the reject end of the membrane feed space in feed flow channels parallel to a central tube to control the velocity of the feed solution as its concentration increases from the feed end to the reject end of the spiral wound element. U.S. Patent No. 11,083,997 to Roderick et al., entitled "Non-Nesting, Non-Deforming Patterns in Spiral Wound Elements," describes providing denser patterns at the feed and reject ends of the membrane feed space and more open patterns in the middle to avoid nesting of printed patterns at the bond line during element manufacture. Summary of the Invention [Means for solving the problem]

[0005] Summary of the Invention The present invention describes a novel method for manufacturing spiral wound membrane elements that provides increased permeate flow with reduced energy consumption versus that available in the existing state of the art using conventional mesh feed spacers.

[0006] An embodiment of the present invention provides a spiral wound element including one or more membrane sheets, each membrane sheet folded upon itself at a fold line providing first and second membrane halves opposing each other, each folded membrane sheet spirally wound around a central tube with the fold line proximal to the central tube, the first membrane half forming an inner wrap against 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 membrane halves, the distance from the fold line to the first spacing feature being equal to or greater than the diameter of the central tube divided by the number of membrane sheets having a fold line proximal to the central tube.

[0007] In some embodiments, the spacing feature is disposed on the first membrane half and not on the second membrane half. In some embodiments, the spacing feature is disposed on the second membrane half and not on the first membrane half. Some embodiments further include a permeable permeate carrier between each pair of adjacent folded membrane sheets, the permeate carrier attached adjacent the inactive side of the folded membrane sheets. There can be one or more folded membrane sheets in a spiral wound element.

[0008] In some embodiments, the areal packing density of the membranes within an element is greater than 33 in2 / in3. In some embodiments, the areal packing density of the membranes within an element is greater than 35 in2 / in3. In some embodiments, the areal packing density of the membranes within an element is greater than 39 in2 / in3. In some embodiments, the areal packing density of the membranes within an element is greater than 41 in2 / in3. In some embodiments, the areal packing density of the membranes within an element is greater than 43 in2 / in3.

[0009] In some embodiments, the spacing features have a height that extends above the corresponding membrane, where the height of the spacing features closer to the fold line is less than the height of the spacing features further from the fold line. In some embodiments, spacing features more than a radius of the central tube from the fold line have a constant height, and spacing features closer to the fold line than a diameter of the central tube have a height that increases with distance from the fold line. In some embodiments, spacing features more than a diameter of the central tube from the fold line have a constant height, and spacing features closer to the fold line than a diameter of the central tube have a height that increases with distance from the fold line. In some embodiments, spacing features more than a circumference of the central tube from the fold line have a constant height, and spacing features closer to the fold line than a diameter of the central tube have a height that increases with distance from the fold line.

[0010] In some embodiments, the spacing features occupy less than 7% of the volume between the film halves. In some embodiments, the spacing features occupy less than 5% of the volume between the film halves. In some embodiments, the spacing features occupy less than 2% of the volume between the film halves. In some embodiments, the spacing features occupy less than 7% of the area of ​​the film on which they are deposited. In some embodiments, the spacing features occupy less than 5% of the area of ​​the film on which they are deposited. In some embodiments, the spacing features occupy less than 2% of the area of ​​the film on which they are deposited.

[0011] An embodiment of the invention provides an assembly including: (a) a plurality of folded membranes, each folded membrane including a membrane sheet having an active surface and a non-active surface opposite the active surface, the membrane sheets folded in half along a fold line with the active surfaces facing each other, the membrane sheets including a plurality of spacing features disposed thereon, but without the spacing features in an area less than 0.1 inches from the fold line; (b) a plurality of permeable permeate carrier sheets; and (c) a central tube, (d) the permeate carrier sheets spirally wrapped around the central tube and welded together with a folded membrane sandwiched between each pair of permeate carrier sheets.

[0012] In some embodiments, the permeate carrier sheets are welded together such that the leading edge of each successive permeate carrier sheet is not spaced from the leading edge of the preceding permeate carrier sheet by more than the circumference of the central tube divided by the number of permeate carrier sheets, hi some embodiments, the second permeate carrier sheet is welded to the first permeate carrier sheet a distance at least the circumference of the central tube, and the third permeate carrier sheet is welded to the second permeate carrier sheet a distance equal to or less than the circumference of the central tube divided by the number of permeate carrier sheets.

[0013] An embodiment of the invention provides 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 a fluid to be treated and a second surface opposite the first surface, the membrane sheet being spirally wound around the collection tube; and (c) a plurality of spacing features disposed on a first half of the first surface of each membrane sheet, each spacing feature having a first end and a second end, each spacing feature having a cross-section perpendicular to the axis of the collection tube extending from the first end to the second end, the first end being wider than the second end.

[0014] In some embodiments, the first ends of the spacing elements contact the first halves of the corresponding membrane sheets, and a first surface of the first halves of the membrane sheets faces away from the collection tube. In some embodiments, the first ends of the spacing elements contact the first halves of the corresponding membrane sheets, and a first surface of the first halves of the membrane sheets faces towards the collection tube.

[0015] In some embodiments, the ratio of the width of the second end to the width of the first end is less than 0.98. In some embodiments, the ratio of the width of the second end to the width of the first end is less than 0.95. In some embodiments, the ratio of the width of the second end to the width of the first end is less than 0.85. In some embodiments, the spacing element comprises one or more of a thermoplastic, a reactive polymer, a wax, or a resin.

[0016] Embodiments of the present invention provide a scroll-type fluid treatment element that includes the following: (a) a collection tube; (b) a plurality of membrane sheets, each having an active surface and an inert surface opposite the active surface, each membrane sheet being folded along a fold line such that the active surfaces face each other, and each membrane sheet being disposed proximal to the collection tube with the fold line present; (c) here, each membrane sheet has a plurality of spacing retention features disposed on its active surface, each spacing retention feature having a first end and a second end and having a cross-section perpendicular to the axis of the collection tube extending from the first end to the second end, the first end being wider than the second end.

[0017] Embodiments of the present invention provide a scroll-type element that includes printed features disposed on a membrane, where the width R of the feature on the membrane surface is different from the width S of the feature at the top of the feature, S < R, and the membrane sheet is oriented such that for a given feature, the position of width R is further from the central tube than the position of width S.

[0018] In some embodiments, the spacing retention features extend a distance of from 0.003 inches to 0.050 inches from the corresponding membrane.

Brief Description of the Drawings

[0019] Brief Description of the Drawings [Figure 1] It is an exploded view of a scroll-type membrane element. [Diagram 2] It is an exploded view of a partially assembled scroll-type membrane element. [Diagram 3] It is an end view of a scroll-type element before winding a plurality of membrane sheets around a central tube. [Figure 4] Two figures showing the unit cross-sectional areas of a prior art mesh element and a printed spacer element of a feed channel of the same height. [Diagram 5] Two figures showing the effective flow cross-sections of a prior art mesh element and a thin printed spacer element having a shorter height feed channel. [Figure 6] FIG. 2 is a diagram of a printed membrane sheet having high density glue support lines in the inlet and outlet areas of the membrane sheet and a low density area in the center. [Figure 7A] FIG. 13 is a diagram of a low density hole pattern for the center tube. [Figure 7B] FIG. 13 is a diagram of a dense hole pattern for a central tube. [Figure 8] FIG. 1 is an end view of two membrane leaves wrapped around a central tube with trapezoidal feed spacer elements. [Figure 9] FIG. 1 is an end view of two membrane leaves wrapped around a central tube with trapezoidal feed spacer elements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] MODE FOR CARRYING OUT THEINVENTION AND INDUSTRIAL APPLICABILITY Feed spacers in spiral wound filtration elements are necessary to maintain a channel through which fluid can flow from the feed end of the feed channel to the reject end, but the design of the spacer also affects the differential pressure loss, local flow velocity, turbulence, stagnation zones, and other fluid flow conditions from the feed end to the reject end of the element. Extruded mesh feed spacers have been traditionally used in membrane manufacturing, but by the nature of their design, many of the hydrodynamic properties depend on the thickness of the spacer. Traditional mesh spacers also provide uniform support properties in the feed space from the distal end of the central tube to the proximal end of the membrane sheet near the central tube. Printed feed spacers allow for unique design properties not available with traditional extruded or woven mesh spacers. The thickness and shape of the printed spacer can be independently varied to obtain a wide range of configurations that can be tailored to the specific challenges found in a particular application or spiral wound membrane element application. One of the key features of the printed spacer technology is the open structure of the feed channel. Traditional mesh spacers consist of two layers of stringers crisscrossed together and welded or glued at the intersection of the two stringers to provide structure to the mesh. By its very nature, all fluid must pass through one stringer, go around the stringer, and return over the stringer on the other side. If a mesh stringer crosses the feed space, even if it is not perpendicular to the flow path, there is an inherent limitation in the height of the flow channel. A printed feed spacer does not have an inherent limitation. The channel height of a printed spacer can be half that of a traditional mesh spacer, providing the same mass flow characteristics and the same pressure drop feed to rejection as a mesh spacer. Traditional mesh spacers in 40 inch long elements typically range from 26 to 32 mils (0.026 to 0.032 in), but can be higher or lower for unique applications. Printed spacer technology allows the spacer height to be much lower, typically 15 to 20 mils (0.015 to 0.020 in), providing the same mass flow characteristics as a mesh spacer in a 40 inch long element, with an equivalent pressure drop feed to rejection.For elements 12 inches long or less, the spacer height can be as low as 0.003 inches, but is typically 0.006 inches to 0.008 inches tall. One of the main features of mesh spacers that is generally 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 pile up at the membrane surface as fluid molecules pass through the membrane, increasing the concentration of total dissolved solids (TDS) at the membrane surface, thereby increasing the osmotic pressure required to move the fluid molecules across the membrane surface. Another disadvantage of concentration polarization is that the increased TDS migrating through the membrane surface reduces the quality of the product water, also known as rejection loss. Concentration polarization can have a more pronounced effect in waters with high TDS, such as seawater, where the increase in osmotic pressure is more dramatic due to the higher TDS at the reject end of the element. For freshwater, high rejection is usually less critical. However, in some industrial applications, such as semiconductor processing, high rejection is desired even for low TDS applications. For waters containing rarely dissolved species, the combined effects of system recovery and polarization can also result in precipitation at the reject end of the element, or in the last element of a multi-element pressure vessel. These characteristics can limit recovery in reverse osmosis systems. In any case, printed spacer technology can have a wide variety of printed features to create the mixing necessary to minimize concentration polarization.

[0021] Cross-flow filtration, by its very nature, relies on a portion of the feed fluid passing through the filter to become part of the filtrate, thus creating a situation where the amount of feed fluid is constantly decreasing as it passes through the filter. The greater the portion of filtrate that is produced, the smaller the portion of the feed / concentrate fluid that continues to flow through the filter. As the fluid passes through the element, a portion of the fluid passes through the membrane. A simple modelling would be that if the flux through the membrane is constant, the flow of the feed solution will gradually decrease as it flows through the element from the feed end of the feed space to the reject end. In reality, the amount of fluid passing at any point along the feed flow path depends on the local flow conditions, the local concentration of solutes or suspended matter, and the local pressure, which in turn depends on the back pressure in the feed space as well as locally from the permeate side of the element.

[0022] A significant advantage of the printed spacer technology is the ability to create more open feed spacer channels. The present invention provides a means to maximize permeate flow, increase recovery or reduce differential pressure loss from the feed to reject end of the element, reduce the likelihood of scale formation, simplify cleaning protocols, or a combination of all these advantages.

[0023] The feed shaping features employed can be any of a number of shapes including rounded dots, ellipses, rounded rods, lenticular shapes, elongated polygons, arcs, lines or other geometric shapes. Due to the shape of the features and the fact that the fluid must move around the outside of the features, the fluid flow rate varies locally in the areas between the feed spacing features from the feed end to the reject end of the membrane element.

[0024] FIG. 1 is a schematic diagram of a conventional spiral wound membrane element prior to wrapping, showing the key elements of a conventional spiral wound membrane element 100. The permeate collection tube 12 has holes 14 therein through which permeate is collected from the permeate carrier 22. In manufacture, the membrane sheet 36 is a single continuous sheet that is folded about a centerline 30 and is comprised of an inactive porous support layer, such as polyester, polysulfone, or a combination thereof, on one side 28 and an active polymer membrane surface 24 that is adhered or cast onto the support layer. In the assembled element, when wrapped around the permeate collection tube 12, the active polymer membrane surface 24 is adjacent to the feed spacer mesh 26 and the inactive support layer 28 is adjacent to the permeate carrier 22. In a preferred embodiment, the feed spacer mesh 26 is replaced with printed features on the active polymer membrane surface 24, or alternatively on the inactive support layer 28, to form embossed features on the active polymer membrane surface 28. The feed solution 16 enters between the activated polymer membrane surfaces 24 and flows through the open spaces of the feed spacer mesh 26. As the feed solution 16 flows through the feed spacer mesh 26, particles, ions, or species rejected by the membrane are rejected at the activated polymer membrane surfaces 24, and permeate molecules, e.g., water molecules, pass through the activated polymer membrane surfaces 24 and enter the porous permeate carrier 22. As the feed solution 16 moves along the activated polymer membrane surfaces 24, the concentration of the substances rejected by the membrane increases due to the loss of permeate in the bulk feed solution 16, and this concentrated liquid exits the reject end of the activated polymer membrane sheet 24 as rejected liquid 18. The permeate in the permeate carrier 22 flows from the distal end 34 of the permeate carrier 22 in the direction of the central tube 12, where the permeate enters the central tube 12 through the central tube inlet holes 14 and exits the central tube 12 as permeate 20. To avoid contamination of the permeate with the feed solution 16, the non-activated polymeric membrane layer 28 is adhesively sealed along a bond line 32 through the permeate carrier 22, thereby forming a sealed membrane envelope in which the only exit path for the permeate 20 is through the central tube 12. Typically, the width of the bond line 32 is 1-3 inches after the adhesive is compressed during the wrapping process.

[0025] In the existing art, a partially assembled spiral wound membrane element 200 is shown in FIG. 2. The membrane envelope 40 comprises a membrane sheet 36 folded at one end as described in connection with FIG. 1, and a permeate carrier 22 is placed between the membrane sheets and sealed along the edges with a suitable bond line 32 (FIG. 1). In the conventional design of the membrane element once wound, a feed spacer mesh 26 is placed adjacent to the envelope 40 so that the flow of the feed solution 16 flows between the layers of the membrane envelope 40, exposing all of the active polymer surfaces 24 of the membrane sheet to the feed solution. The permeate, or product, fluid is collected on the permeate carrier 22 within the membrane envelope 40 and spirals up to the central 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 wound together to form the element. In the case of multiple membrane sheet elements, the permeate carrier assembly (tricopack) consists of multiple permeate carrier sheets ultrasonically (or otherwise) welded together with an offset distance that is typically the circumference of the central tube divided by the number of leaves in the element. The first permeate carrier sheet is typically longer so that it can wrap around the central tube once or twice to allow volume for permeate from all the leaves to enter the central tube bore without creating excessive back pressure on the individual permeate leaves.

[0026] FIG. 3 illustrates an important advantage of the printed spacers 50 applied to the active polymer surface 24 of the membrane sheet 36. The printed spacers 50 can be printed at a height of 50 percent or less than that of the conventional mesh 26 (FIG. 2). Because the thickness of the membrane envelope 40 (FIG. 2) using the printed spacers 50 is significantly thinner than that of the conventional mesh 26, more layers of the membrane envelope 40 (FIG. 2) can be wrapped around the central tube 12 for a comparable outer diameter of the membrane element 200 constructed with the conventional mesh 26. In practice, each of the membrane envelopes 40 should start at the fold line 30 (FIG. 1) where the permeate carrier 22 is attached to the central tube 12 at the attachment point 23. The thickness of the membrane envelope 40, including the thickness of the feed spacers, determines the total thickness per layer around the central tube 12. This total thickness determines the minimum spacing X at which the permeate carrier 22 can be attached to the central tube 12, assuming that all membrane leaves start from the central tube and are not stacked on top of other leaves to wrap the central tube with the first layer. For an element 40 inches (1016 mm) long and 8 inches (203.2 mm) in diameter, the central tube 12 typically has an outside diameter of 1.5 inches (39.1 mm), but can be smaller or larger. And, of course, the larger the diameter of the central tube 12, the less amount of membrane that can be wrapped around it before the outside diameter exceeds 8 inches (203.2 mm). The circumference of the central tube 12 is given by πD, where D is the outside diameter of the central tube 12. For a 1.5 inch (39.2 mm) central tube, the circumference is 4.71 inches (119.6 mm). With thin mesh feed spacers, about 25 membrane envelopes 40 can be wrapped around the central tube 12. Feed spacer mesh 26 using conventional manufacturing techniques typically has a thickness as low as 0.028 inches (0.71 mm), and is limited to this minimum thickness by the feed-to-reject feed pressure loss within the membrane element. This means that the spacing X has a distance of 4.71 inches divided by 25 leaves, or 0.188 inches (4.79 mm).With a printed spacer 50 of 0.017 inches (0.43 mm) height, as many as 35 membrane sheets can be used with a leaf length equivalent to a conventional mesh spacer element before an 8 inch (203.2 mm) outer diameter is achieved. This results in a 40% increase in membrane surface area (40% increase in permeate flow) but with feed to reject pressure drop equivalent to a mesh element. If 35 leaves are used with a 1.5 inch (39.2 mm) central tube, the spacing X can be as small as 0.135 inches (3.43 mm). The illustrations and descriptions herein assume that the permeate carrier is attached directly to the central tube. In some embodiments, a layer of permeate carrier material is wrapped around the central tube, either as a separate element or as an extension of the permeate carrier. Such wrapping is considered to be part of the central tube for purposes of this discussion.

[0027] For 4 inch (101.6 mm) elements, the outside diameter of the central tube is typically 0.84 inch (21.34 mm). 4 inch diameter elements with mesh spacers typically have five membrane leaves. A typical 4 inch diameter element has a central tube with an outside diameter of 0.84 inch and a circumference of 2.64 inch (67 mm). The spacing between the permeate carrier leaves around the central tube is 0.528 inch (13.41 mm). With a printed spacer element, seven leaves can be wrapped around the central tube for an equivalent length of membrane sheet. This results in a permeate carrier spacing of 0.377 inch (9.58 mm) between sheets at the central tube.

[0028] For economy and ease of manufacture, a 1.8 inch (45.7 mm) diameter by 12 inch (304.8 mm) long domestic element using a mesh spacer usually has only one leaf. Because the element is only 12 inches long, the mesh thickness can be much thinner than a 40 inch long element, since the differential pressure loss feed to reject is a much shorter distance. In any case, however, the feed space height of the print spacer can be half that of the mesh spacer, and more than one leaf can be used for each leaf of the mesh element. A typical diameter of the central tube of a 12 inch element is 0.67 inches (17 mm), or a circumference of 2.1 inches (53.46 mm). Sometimes shorter leaves are used on the mesh element, with two leaves wrapped around the central tube. Rarely, three leaves are used. For two leaves and a 0.67 inch diameter central tube, the permeate carrier spacing is 1.05 inches. Printed spacer elements can have more leaves for the same pressure drop feed to rejection, so the permeate carrier spacing to the central tube can be less than 1.05 inches. 3 inch diameter by 12 inch long elements are also becoming popular because they allow much more permeate flow and may eliminate the need for a pressure tank in home systems. In either case, the permeate spacing X on the central tube will be half the distance for an equivalent printed spacer element than for a mesh spacer element.

[0029] A narrow overall width is preferred to prevent the membrane sheet from pulling away from the attachment point 23. In particular, the distance V from the fold to the first printed feature is preferably greater than the spacing X of the permeate carrier. This has the effect of holding the leaf in place during wrapping and prevents leaks from forming at the insertion point. It is desirable for the distance from the fold to the first printed feature to be less than 4 inches, more preferably less than 3 inches, more preferably less than 2 inches, and even more preferably less than 1 inch, due to the altered mixing characteristics caused by the reduced channel height.

[0030] In summary, a 40 inch long element can use 40 percent more membrane leaves than a mesh type element, i.e., the spacing can be about 30% less (5 leaves vs. 7 leaves = 0.71), and a 12 inch long element can use twice as many leaves, i.e., the spacing can be 50% less (1 leaf vs. 2 leaves = 0.5). Of course, there are many combinations of spacer width, number of leaves, element length, and acceptable pressure drop feed to reject. In all cases, however, the spacing height of the printed spacer can be shorter and the number of leaves can be greater for a mesh type element for an equivalent pressure drop feed to reject.

[0031] 4A and 4B highlight one of the key advantages of the printed spacer technology. FIG. 4A shows a view of a feed channel of width Y with a mesh spacer 26 in the feed space. The mesh spacer 26 includes stringers 26a and 26b aligned perpendicular to each other. In practice, the stringers 26a and 26b may be at a non-normal angle of less than 90 degrees to each other. The mesh spacer 26 also has the stringers oriented at 45 degrees to the flow path. However, FIG. 4A shows the effective blockage of the string 26a, allowing the flow 52a to be blocked for half the flow path width - the feed channel width Y. The printed spacer 50 allows the fluid 52b to flow the entire height of the feed channel height Y. Clearly, this is a much larger cross-sectional area flow path providing less resistance and lower pressure drop to flow from the feed end to the reject end of the element 100 (FIG. 1). 5A and 5B show the true effective flow 52a in the mesh spacer and the flow 52c in the reduced height printed feed spacer element. The cross-sectional flow area of ​​both elements is substantially the same. Testing was performed with industry standard 40 inch long by 4 inch diameter membrane elements, i.e., 4040 elements with a conventional 28 mil feed spacer mesh. As a comparison, flow testing was also performed with a 13 mil printed spacer element to measure the pressure drop from the feed end to the reject end of the element. No performance data on flux and rejection was collected in these tests. The purpose of this testing was to demonstrate the significant advantage of the printed spacer for reducing differential pressure. Flow rates were tested from 4 gallons per minute (GPM) to 18 GPM in 2 GPM increments and the differential pressure from the feed end to the reject end of the element was measured. The printed spacer element had a lower differential pressure from the feed end to the reject end of the element, even though the height of the feed channel of the printed spacer (13 mil) is less than half the height of the mesh spacer element (28 mil).

[0032] [Table 1]

[0033] Furthermore, the differential pressure as a function of feed flow is best described as the following power law: Differential pressure = a * feed flow ^ b where b is about 1.4 for mesh elements and 1 for print elements. This results in increased savings as the feed flow rate increases, and significantly reduced energy losses. Spiral wound elements of the present invention preferably have an exponent b of less than 1.35, more preferably less than 1.2, even more preferably less than 1.1, and even more preferably about 1.

[0034] Using a 13 mil printed spacer height, 40 percent more membrane area can be incorporated into a 40 inch long element. A 28 mil mesh spacer element with a 4 inch diameter contains five leaves wrapped around a central tube for a final outside diameter of just under 4 inches. A 13 mil tall printed spacer element allows seven leaves of the same length to be wrapped around the central tube while still maintaining a diameter of just under 4 inches. The added membrane material surface area of ​​the printed spacer element allows for up to 40 percent more permeate volume under the same operating conditions when the same membrane sheet material is used for both the mesh and printed spacer elements. In practice, the 40 percent increased surface area is capped by the fact that the printed spacer 50 is printed directly onto the membrane sheet 36, occluding as much as 3 percent of the active surface area. Still, 37 percent more square feet of active membrane area can be utilized in a 40 inch long printed spacer element compared to a mesh spacer element for the same pressure drop feed to rejection. This can be described as the areal packing density of the membrane element [active area (in2) per volume (in3) of the element]. Conventional elements using mesh spacers have an areal packing density of up to 31 in2 / in3. Exemplary spiral wound elements of the present invention can achieve areal packing densities of greater than 33 in2 / in3, greater than 35 in2 / in3, and more preferably greater than 39 in2 / in3, greater than 41 in2 / in3, and greater than 43 in2 / in3.

[0035] FIG. 6 illustrates a high density edge pattern 46 on the inlet and outlet of the printed active membrane surface 24. This high density edge pattern 46 is preferred to support the edges of the membrane element during and after the glue lines 32 (FIG. 1) are pressed against the backside of the membrane sheet 28 (FIG. 1) so that the adhesive is forced to flow through the permeate carrier 22 (FIG. 1) and into adhesive contact with the opposite non-active surface of the membrane sheet 28 (FIG. 1) during wrapping. The width W of the inlet and outlet edge patterns 46 has a width W that is determined by the adhesive manufacturing technique, adhesive viscosity, and other factors. For a 40 inch long element using manual gluing and wrapping techniques, the width W of the edge pattern 46 is typically 3 to 3.5 inches (76 to 89 mm). The length of W needs to be wide enough to supply pressure to the glue lines and allow trimming of the element to occur through the pattern area, but W should be minimized because this area will lead to greater active area loss and increased pressure drop from the feed end to the reject end of the element. Using automated wrapping and gluing techniques, the width W of the edge pattern 46 as printed (before cutting) can be minimized to a width of 2.0 inches (50.4 mm), but more preferably is 1.5 inches (38.1 mm).

[0036] 7A and 7B are diagrams of a central tube 12 with holes 14 that provide fluid flow from the permeate carrier 22 (FIG. 1). Due to the shorter height of the printed spacer features 50 (FIG. 3), more membrane envelopes 40 (FIG. 2) can be utilized in any comparable outer diameter element constructed with a conventional mesh spacer 26 (FIG. 1). As more permeate flows into the central tube 12, the holes 14 must be either larger in diameter or more closely spaced to accommodate the additional permeate flow without increasing the back pressure of the permeate carrier 22. In a 12 inch long element, up to twice as many membrane envelopes 40 can be wrapped around the central tube. Thus, the spacing density for a 12 inch long element must be increased by 2 to 1 to avoid back pressure issues in the permeate carrier 22. In a 40 inch long element, 40 percent more leaves can be wrapped around the central tube 12. In that case, a 40 percent increase in the number of holes 14 or a 40 percent increase in the cross-sectional area of ​​the holes 14 would be required relative to the number and / or size (diameter) of the central tube holes of elements made with conventional mesh. This is particularly important for high flow rate ultrafilter (UF) and microfilter (MF) spiral wound elements.

[0037] FIG. 8 illustrates an embodiment of the invention where the flow path from the feed end 16 (FIG. 1) to the reject end 18 (FIG. 1) of the spiral wound element 100 (FIG. 1) maximizes the flow path due to the shape of the spacer features 50. As an example, as the printed membrane 36 is wrapped around the central tube 12 with the spacer features 50 applied to the membrane 36, the spacer features 50 can be formed, for example, in a trapezoidal shape or other shape with different widths of the features 50, S at the top and then narrower at the bottom R where it contacts the membrane. This is especially important for the edge features 46 (FIG. 6), which make the flow path narrower and longer as the fluid passes through, thereby contributing to a larger pressure differential. As the membrane 36 is wrapped around the central tube 12, the tops of the trapezoids are closer together such that the sides of the spacer features 50 create parallel walls P, providing a more open feed flow path for fluid to enter and exit the spiral wound element 100 (FIG. 1). As an alternative to parallel walls after wrapping, the side walls and edge features 46 (FIG. 6) of the spacer features 50 may not be parallel, but may form an arc greater than 0 degrees such that the feed spacer path is more open than paths with parallel walls alone. The net result of a more open flow path is less pressure loss from the feed end to the reject end of the element, thereby saving energy in the membrane filtration process. It has been found that to improve flow channel differential pressure characteristics, the folded membrane sheet 28 may be oriented in such a way that for a given feature 50, its position, described by its width S, is closer to the center tube than the side of the feature, described by its R, in the assembled spiral wound element.

[0038] The present invention has been described in connection with various exemplary embodiments. It is to be understood that the above description is merely illustrative of the application of the principles of the invention, the scope of which is determined by the claims viewed in light of the specification. Other variations and modifications of the present invention will be apparent to those skilled in the art.

Claims

1. 1. A spiral wound element comprising two or more membrane sheets, each membrane sheet folded upon itself at a fold line providing first and second membrane halves opposing each other, each folded membrane sheet spirally wound around a central tube with the fold line proximal to the central tube, the first membrane half forming an inner wrap 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 membrane halves, a distance from the fold line to a first spacing feature being equal to or greater than a diameter of the central tube divided by the number of the membrane sheets having a fold line proximal to the central tube.

2. The element of claim 1 , wherein the spacing features are disposed on the first membrane half and not on the second membrane half.

3. The element of claim 1 , wherein the spacing features are disposed on the second membrane half and not on the first membrane half.

4. 10. The element of claim 1 further comprising a permeable permeate carrier between each pair of adjacent folded membrane sheets.

5. 2. The element of claim 1, wherein the membrane sheet has an areal packing density of greater than 33 in2 / in3.

6. 6. The element of claim 5, wherein the membrane sheet has an areal packing density of greater than 35 in2 / in3.

7. 7. The element of claim 6, wherein the membrane sheet has an areal packing density of greater than 39 in2 / in3.

8. 8. The element of claim 7, wherein the membrane sheet has an areal packing density greater than 41 in2 / in3.

9. 9. The element of claim 8, wherein the membrane sheet has an areal packing density of greater than 43 in2 / in3.

10. 2. The element of claim 1, wherein the spacing features have a height that extends above a corresponding membrane, the height of the spacing features closer to the fold line being less than the height of the spacing features away from the fold line.

11. 2. The element of claim 1, wherein the spacing features further away from the fold line than a diameter of the central tube have a constant height and spacing features closer to the fold line than a diameter of the central tube have a height that increases with distance from the fold line.

12. 12. The element of claim 11, wherein the spacing features further from the fold line than the circumference of the central tube have a constant height and the spacing features closer to the fold line than the diameter of the central tube have a height that increases with distance from the fold line.

13. 12. The element of claim 11, wherein the spacing features further away from the fold line than a diameter of the element have a constant height and spacing features closer to the fold line than a diameter of the element have a height that increases with distance from the fold line.

14. The element of claim 1 , wherein the spacing features occupy less than 7% of the volume between the membrane halves.

15. The element of claim 14 , wherein the spacing features occupy less than 5% of the volume between the membrane halves.

16. The element of claim 14 , wherein the spacing features occupy less than 2% of the volume between the membrane halves.

17. The element of claim 1 , wherein the spacing features occupy less than 7% of the area of ​​the film on which they are deposited.

18. 20. The element of claim 17, wherein the spacing features occupy less than 5% of the area of ​​the film on which they are deposited.

19. 20. The element of claim 18, wherein the spacing features occupy less than 2% of the area of ​​the film on which they are deposited.

20. (a) a plurality of folded membranes, each folded membrane comprising a membrane sheet having an active surface and a non-active surface opposite the active surface, the membrane sheet folded in half along a fold line with the active surfaces facing each other, the membrane sheet having a plurality of spacing features disposed thereon, the spacing features being absent in an area less than 0.1 inches from the fold line; (b) a plurality of permeable liquid carrier sheets; (c) a central tube; (d) the permeate carrier sheets are spirally wound around the central tube and welded together with a folded membrane between each pair of the permeate carrier sheets; assembly.

21. 21. The assembly of claim 20, wherein the permeate carrier sheets are welded together such that the leading edge of each successive permeate carrier sheet is not spaced apart from the leading edge of the preceding permeate carrier sheet by more than the circumference of the central tube divided by the number of permeate carrier sheets.

22. 21. The assembly of claim 20, wherein a second permeate carrier sheet is welded to the first permeate carrier sheet at a distance at least the circumference of the central tube, and a third permeate carrier sheet is welded to the second permeate carrier sheet at a distance not greater than the circumference of the central tube divided by the number of permeate carrier sheets.

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