Patent Publication Number: US-2022234005-A1

Title: Entrance features in spiral wound elements

Description:
TECHNICAL FIELD 
     The subject invention relates to membrane systems utilized for the separation of fluid components, specifically spiral-wound membrane elements. 
     BACKGROUND ART 
     Spiral-wound membrane filtration elements are known in the art, and typically consist of a laminated structure comprised of a membrane sheet sealed to or around a porous permeate carrier which creates a path for removal, longitudinally to the axis of the center tube, of the fluid passing through the membrane to a central tube, while this laminated structure is wrapped spirally around the central tube and spaced from itself with a porous feed spacer to allow axial flow of the fluid through the element. Traditionally, a feed spacer is used to allow flow of the feed water, some portion of which will pass through the membrane, into the spiral wound element and allow reject water to exit the element in a direction parallel to the center tube and axial to the element construction. 
     Improvements to the design of spiral wound elements have been disclosed in U.S. Pat. No. 6,632,357 to Barger et al., U.S. Pat. No. 7,311,831 to Bradford et al., and patents in Australia (2014223490) and Japan (6499089) entitled “Improved Spiral Wound Element Construction” to Herrington et al., which replace the conventional feed spacer with islands or protrusions either deposited or embossed directly onto the inside or outside surface of the membrane. Typically, fluid feed flow is normal to the center tube of the spiral wound element. In fabrication, after winding the element in the spiral configuration, the membrane sheet envelope is cut off after gluing and the feed edge of the membrane envelope presents a flat surface to the flow of feed solution. US patent application PCT/US17/62425 entitled “Flow Directing Devices for Spiral Sound Elements” to Herrington, et al., describe anti-telescoping devices that incorporate turning vanes to cause fluid flow to sweep the feed end of the spiral wound element to help avoid blockage of particles in the feed stream from impinging on the end of the membrane envelope. None of these patents describe features that can be applied to the membrane sheet envelope on the inlet (also called the feed or entrance) and exit (also called the reject or outlet) end of the envelope of the spiral wound element that improve the flow path into the feed end of the element or from the reject end of the element. 
     DISCLOSURE OF INVENTION 
     Understanding of the present invention can be facilitated by the context of U.S. Pat. No. 6,632,357 to Barger et al., U.S. Pat. No. 7,311,831 to Bradford et al., and patents in Australia (2014223490) and Japan (6499089) entitled “Improved Spiral Wound Element Construction” to Herrington et al., each of which is incorporated herein by reference. 
     Embodiments of the present invention provide tapered ends on the membrane envelope to create a more aerodynamic or hydrodynamic entrance path into the feed spaces in a spiral wound membrane element, as well as a smoother transition from the element on the reject end of the element. The modified ends can be achieved by, as examples, combining (a) a narrow permeate carrier with bonding of the edges of the membrane envelope directly to one another with (b) a modified feed spacer in these regions to provide substantially uniform layer thickness. This configuration can be difficult to incorporate in conventional feed spacer mesh that has a uniform flat configuration. However, by employing feed spacers that are printed directly on the membrane surface tapered features can be integrated in the feed spacer print pattern on the feed and reject ends of the membrane sheet to facilitate more hydrodynamic entrance and exit flow paths. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustration of a conventional spiral wound membrane element prior to rolling. 
         FIG. 2  is an end view of a spiral wound membrane element. 
         FIG. 3  is a cross section view of a conventional mesh-type membrane element. 
         FIG. 4  is a section view of an example embodiment showing an inlet end of a spiral wound membrane element with feed spacers that force the end of the membrane envelope to taper to a close. 
         FIG. 5  is a section view of an example embodiment showing the inlet end of a spiral wound membrane element after the ends of the membrane envelope have been trimmed off. 
         FIG. 6  is a section view of an example embodiment showing an inlet end of a spiral wound membrane element after the ends of the membrane envelope have been trimmed off such that taller spacers at the inlet and exit end are maintained in place. 
         FIG. 7  is a section view of an example embodiment having an entrance end of a spiral wound membrane element with adhesive only applied to the membrane sheets. 
         FIG. 8  is a section view of an example embodiment having the entrance end of a spiral wound membrane element with the permeate carrier tapered on the edges. 
         FIG. 9  is a section view of the edge of an example embodiment comprising a membrane element which incorporates conventional feed spacer mesh. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION AND INDUSTRIAL APPLICABILITY 
       FIG. 1  is a schematic illustration of a conventional spiral wound membrane element prior to rolling, showing important elements of a conventional spiral wound membrane element  100 . Permeate collection tube  12  has holes  14  in collection tube  12  where permeate fluid is collected from permeate carrier  22 . In fabrication, membrane sheet  36  is a single continuous sheet that is folded at center line  30 , comprised of a non-active porous support layer on one face  28 , for example polysulfone, and an active polymer membrane layer on the other face  24  bonded or cast on to the support layer. In the assembled element, active polymer membrane surface  24  is adjacent to feed spacer mesh  26 , and non-active support layer  28  is adjacent to permeate carrier  22 . Feed solution  16  enters between active polymer membrane surfaces  24  and flows through the open spaces in feed spacer mesh  26 . As feed solution  16  flows through feed spacer mesh  26 , particles, ions, or chemical species, which are excluded by the membrane are rejected at active polymer membrane surfaces  24 , and molecules of permeate fluid, for instance water molecules, pass through active polymer membrane surfaces  24  and enter porous permeate carrier  22 . As feed solution  16  passes along active polymer membrane surface  24 , the concentration of materials excluded by the membrane increases due to the loss of permeate fluid in bulk feed solution  16 , and this concentrated fluid exits the reject end of active polymer membrane sheet  24  as reject solution  18 . Permeate fluid in permeate carrier  22  flows from distal end  34  of permeate carrier  22  in the direction of center tube  12  where the permeate fluid enters center tube  12  through center tube entrance holes  14  and exits center tube  12  as permeate solution  20 . To avoid contamination of the permeate fluid with feed solution  16 , non-active polymer membrane layers  28  are sealed with adhesive along adhesive line  32  through permeate carrier  22  thereby creating a sealed membrane envelope where the only exit path for permeate solution  20  is through center tube  12 . Typically, the width of the adhesive line  32  is 1-3″ after the adhesive has been compressed during the rolling process. 
     An assembled spiral wound membrane element  200  is shown in  FIG. 2 . A membrane envelope  102  comprises, as described in connection with  FIG. 1 , a membrane sheet  36  folded at one end with a permeate carrier  22  disposed therebetween the membrane sheet and sealed along the edges with a suitable adhesive. In the conventional design of membrane element, a feed spacer mesh  26  is placed adjacent to envelope  102  to allow the flow of feed fluid  16  to flow between membrane envelope  102  and expose all of the active polymer surfaces of the membrane sheet to feed fluid. Permeate, or product fluid is collected in the permeate carrier inside membrane envelope  102  and proceeds spirally down to center tube  12  where the product, or permeate fluid is collected. 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. 
     Referring to  FIG. 3 , a membrane envelope is created by sealing edges of a first  24  membrane sheet, a layer of permeate carrier  22 , and a second  28  membrane sheet together with an adhesive  104 . In the process of fabrication of a spiral wound element, the individual membrane leaves  24  and  28  are folded in half and permeate carrier  22  is placed between each folded sheet and the adhesive is applied on top of the permeate carrier and the element is rolled to produce the layered spiral configuration. During the rolling process, adhesive  104  must penetrate through permeate carrier  22  in order to properly seal membrane sheets  24  and  28  together to create membrane envelope  102  as in  FIG. 2 . To complete final construction of membrane element, the ends are trimmed through adhesive material  104  along cut line  44 . After trimming, the adhesive line  32  at the edges of the element typically extends 1-2″ inward into the permeate carrier from the face  134  of the membrane envelope  102 . In many fluid feed applications, fluid  16  may contain particles or impurities that may impinge on the flat end faces  134  of envelope  102  thereby allowing particles to collect on the end faces  134  thereby restricting fluid flow into the feed spaces between the leaves of envelope  102 . In addition, feed spacer mesh  23  can typically comprise a plastic webbing type mesh whereby the cut ends of the mesh will also act to accumulate particles in the entrance area of feed spaces between envelope  102 . Feed spacer mesh  23  comprises upper strands  136  and lower strands  138  that are bonded together at contact points  140 . Another undesirable characteristic of the existing mesh type spacer membrane elements is that feed fluid has to flow over and under strands  136  and  138  which creates pressure losses in the mesh spacer. These pressure losses increase the energy costs of operation of membrane systems. If pressure losses can be decreased, the overall energy requirements for the system can be reduced. In typical construction of a conventional membrane element  200 , membrane sheets  24  and  28  enclose permeate carrier  22  extending to the edge of the membrane sheets that allows the flow of permeate to the center collection tube  12  ( FIG. 2 ). 
     From a fluid dynamic standpoint, feed fluid  16  impinging on flat end faces  134  of membrane envelop  102  is not optimal, and creates additional resistance to fluid flow as the fluid transitions from bulk flow into the feed channels. 
     In an example embodiment of the present invention shown in  FIG. 4 , a cross section of a portion of the end of the membrane module  400  is shown. Feed spacer mesh  23  ( FIG. 3 ) is replaced with spacing features  70  having a first thickness applied directly to one active polymer surface of membrane leaf  24 . Spacing features  70  can be any pattern or height compatible with the desired performance of the system. The opposing side of membrane leaf  28  can optionally have spacing features applied to the surface. In the example embodiment shown, permeate carrier  22  is not extended all the way to the end of membrane sheets  24  and  28 , and is terminated at edge  74  of permeate carrier  22 . In this embodiment the edge of the permeate carrier  74  still extends within the width of the adhesive line  72 . To facilitate sealing of membrane sheets  24  and  28  together during element manufacture without the presence of the permeate carrier between the membrane sheets, spacing features  76  of a second thickness, greater than the first thickness, are applied at the end edges of membrane sheet  24 , extending approximately to the edge of the permeate carrier  74 . As membrane element is rolled together during fabrication, thicker spacing features  76  cause membrane sheets  24  and  28  to squeeze together at the ends thereby bringing membrane sheets  24  and  28  in contact at the end edges. Adhesive  72  seals the ends of membrane sheets  24  and  28  and also the permeate carrier  22  to seal the membrane envelope and separate it from the feed and reject fluid flow. After the adhesive has cured, the end of membrane element can be trimmed off, e.g., at one of cut lines  78  producing the configurations shown in  FIG. 5  and  FIG. 6 . 
     The thicker spacing features  76  can be uniform in thickness, or can vary in thickness from thicker toward the edge away from the permeate carrier to thinner towards the permeate carrier so as to create a thickness transition from the outer edge to the area of the permeate carrier. The thickness of thinner spacer  70  and thicker spacers  76  are selected such that the thicker spacer is, at its maximum thickness, equal to or nearly equal to the thickness of permeate carrier  22  that is present between the inner portions of the sheets but not present near the edge. This allows the overall thickness of each complete layer of the element, including membrane sheets  24  and  28 , permeate carrier  22 , thinner and thicker spacers  70  and  76 , and adhesive  72 , to be effectively constant so that element rolls to a substantially uniform diameter throughout. 
     A cross-sectional portion of the element end  500  as in  FIG. 5  in the example embodiment provides a smooth and tapered inlet channel for feed fluid  16  to enter feed space  67  between membrane sheets  24  and  28  after the excess edge and feed spacer have been trimmed. For example, permeate spacer  22  can be 0.010 inches in height. Membrane sheets  24  and  28  can be, for example, 0.005 inches in height. In the example embodiment shown in  FIG. 5 , end face  82  of the membrane envelope can be, for example, 0.010 inches tall in contrast to the thicker end face  134  shown in  FIG. 3 . Thinner end face  82  can be advantageous because it provides less frontal surface area to collect particles which can restrict flow of feed fluid  16 . Thinner end face  82  also allows a tapered inlet channel to fluid feed space  67  which can be advantageous in reducing pressure losses in the inlet of the feed channel, thereby reducing the energy required to pump fluid through the element. 
     Trimming the element such that thicker end feed spacers  76  ( FIG. 4 ) are removed can provide an open configuration for flow. As described in  FIG. 5 , however, it is difficult to roll and trim a spiral wound element with precision to ensure that design. The configuration shown in  FIG. 6  shows another embodiment of the cross section of the edge of membrane element  600  that retains the thicker end feed spacers  76  by trimming the end further from permeate carrier  22 . In this example, some of thicker end feed spacers are trimmed off while some  76  remain in the feed channel. These features provide more of an obstruction to the feed channel than the configuration shown in  FIG. 5 , but still provide more open area for flow than can be achieved if feed spacer  70  were uniform in height from one end of the element to another. 
     Referring to  FIG. 7 , cross section of the edge of a membrane element  700  shows an example embodiment in which less adhesive is used to seal the membrane envelope. The amount of adhesive required to seal the membrane leaf can be reduced significantly by applying adhesive  72  such that only tapered cavity  84  ( FIG. 5 ) at the edge of the membrane envelope contains adhesive after trimming; adhesive does not need to extend inward to the outer edge  74  of permeate carrier  22 . This can be accomplished by employing a narrower permeate carrier  22  which does not extend as far towards the edges of the element, by reducing the amount of adhesive  72  used to create the adhesive line  32 , or a combination of the two. In conventional membrane element fabrication, glue must penetrate permeate carrier  22  and come in contact with the back sides of membrane sheets  24  and  28 . This fabrication approach is often challenging for the characteristics of adhesive  72 . Adhesive  72  in the conventional fabrication approach must have very specific viscosity, thixotropy, and wetting properties in order to penetrate permeate carrier  22  and seal the back side of the membrane sheet. When adhesive  72  is only required to seal the two back sides of membrane leaves  24  and  28 , as in the example embodiment of  FIG. 7 , the characteristics of adhesive  72  can be much less specific, which can result in lower cost materials as well lower volume of adhesive. Utilizing only enough adhesive to seal membrane sheets  24  and  28  minimizes the amount of adhesive required. The present invention will function both with and without the adhesive layer extending into the outer edge of permeate carrier  74 . 
     In an example embodiment shown in  FIG. 8 , the cross section of the edge of membrane element  800  contains a modified permeate carrier edge. The outside edges of porous permeate carrier  92  are thinner relative to portions distal from such edges to better facilitate the transition area between thinner  70  and thicker  76  feed spacers. In embodiments where permeate carrier  22  comprises a woven or extruded thermopolymer material, the thinning can be done by heating the edge under pressure, using a heat sealer, for example. Having the edge of permeate carrier  22  taper or step down reduces the severity of flexing or deformation required by membrane sheets  24  and  28  as it is compressed during rolling. Other example methods of creating a thinner outer edge of permeate carrier  92  include custom pressing, extrusion, custom weaving, co-molding of a tapered edge onto an existing sheet of permeate carrier, and combinations thereof. 
     While the previous examples have all incorporated feed spacers printed directly on the membrane surface, which can provide the benefit in the areas of reduced pressure loss through the element and reduced end fouling, similar stepped- or tapered-end configurations can also be produced using conventional feed spacer mesh and a permeate carrier layer that does not extend across the entire length of the membrane element. An example embodiment depicted in  FIG. 9  shows the cross section of the edge of a membrane element  900  which incorporates conventional feed spacer mesh. A single continuous feed spacer of one thickness  23  provides the feed spacing between membrane sheets  24  and  28  while additional mesh strips  94 , the same thickness as the permeate carrier, are disposed at each edge of the feed spacer to create a uniform thickness during element rolling. While such an example embodiment might not provide all of the advantages of the printed embodiments, a stepped mesh spacer can still provide improvements in pressure loss as compared to conventional elements rolled with a single thickness feed spacer. 
     Similarly, a combination of mesh and printed elements can be employed to provide stepped or graded entrance/exit features provided that they are configured such that the combined spacers at the edges create an opening for the feed to reject flow while adding thickness to make up for the area at the edge where the permeate carrier is not present. 
     Filtration membrane, particularly thin-film composite reverse osmosis membrane, is typically very fragile, and can be damaged by contact with feed spacer mesh or by the process of printing or depositing features onto the film surface. Accordingly, it can be advantageous to enable assembly of an element where there is no printed or mesh feed spacer contacting the active surface of the membrane sheets. Australian patent 2014223490 describes printing features on the permeate carrier, which in turn deform or emboss the membrane sheet in order to provide feed spacing, but it is difficult to provide feed spacing separation at the edges adjacent to the adhesive in such a configuration. By combining thinner feed spacers disposed on the permeate carrier with thicker spacers at the edges disposed on the membrane surface, an element can be created without any printed or mesh spacing features in contact with the active surface of the membrane, while providing additional spacing at the ends of the element. The areas of the membrane sheet that are in contact with the thicker spacer are sealed to one another with adhesive, preventing permeation through the membrane in these areas. Therefore, any damage to the surface of the membrane sheet caused by the thicker edge spacers will not affect the permeation or salt rejection characteristics of the membrane sheet. 
     Although the primary hydrodynamic and fouling improvements occur when this technique is applied to the edge seals of the membrane envelope, it can be seen that the techniques described can also be applied to the adhesive seal at the distal edge  34  of the membrane envelope which will still benefit from reduced adhesive and permeate carrier usage. In the case of the end seal, the addition of a thicker feed spacer can be used, but it is not as necessary as keeping the thickness constant across the end seal which can also be achieved by having no spacer features across the end of the leaves. 
     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.