WO2013085755A2

WO2013085755A2 - Feed spacers for spiral wound membrane element

Info

Publication number: WO2013085755A2
Application number: PCT/US2012/066695
Authority: WO
Inventors: Yatin Tayalia; Prasanna Rao Dontula; Upen Jayant Bharwada
Original assignee: General Electric Company
Priority date: 2011-12-09
Filing date: 2012-11-28
Publication date: 2013-06-13
Also published as: TW201341044A; WO2013085755A3

Abstract

A feed channel spacer for a spiral wound membrane element has Inlet and outlet edges that are thinner than the rest of the spacer material. The edges may be made thinner, for example, by passing the edges of a sheet of feed spacer material through a pair of hot rollers or by compressing the edges of the sheet in a heated press. The thinned edges of the feed spacer material are located in the element between glue lines applied to the permeate spacers of the element. The thin edges allow a greater membrane surface area to be provided in a given element diameter. A feed channel spacer may also have an area with obstructions to create micro-mixing effects. These obstructions may be provided on a feed spacer sheet of constant thickness, or one with thin edges.

Description

FEED SPACERS FOR SPIRAL WOUND MEMBRANE- ELEMENT

FIELD 00013 The present disclosure relates to spiral wound membrane elements and to feed channel spacers for spiral wound embraae elements.

BACKGROUND

[0002] The following discussion, is not an admission thai anything described below is common genera! knowledge or citable as prior art.

[0003] A spiral wound membrane element is anuiaeuwed by rolling one or more membrane envelopes around a perforated central robe. Each envelope comprises two

membrane sheets glued along their three outer edges to a permeate spacer. Adjacent

membrane sheets are separated on a feed side by feed channel spacer, which may also be called a brine channel spacer. In u e, the element is enclosed in a tubular pressure vessel.. Feed water eaters at an upstream end of tire tubular vessel and flows across the feed channel spacer. A portion of tire feed .flows throug the membrane sheets, through the permeate spacer, and out of the pressure vessel by way of the perforated central tube. The remainder of the feed water exits the feed channel as concentrate and is xvithdrawn from a downstream end. of the pressure vessel. US Patent Application Publication Number 2007/0068864 describes one example of a spiral wound membrane element.

[0004] A feed channel spacer is normally made of a sheet of plastic (for example polypropylene) mesh or netting. ^'The primary purposes of the feed, channel spacer is to create a space for the feed water to flow between adjacent membrane envelopes, and to create turbulence on the surfaces of the membranes. The turbulence reduces concentration polarization and so increases the tret driving pressure available to generate permeate.

However, the feed channel spacers also create a head loss to feed flow which reduces the net driving pressure. These effects must be balanced, along with the volume occupied by the .feed channel spacer and its ability to resist being fouled by contaminants in the feed water, £0005] Common thicknesses of feed channel spacers include 26 mils (0,66 ra), 2S mils

(0.71 mm), 31. mils. (0.70 turn) a»d 34 mils (0,86 .mm). In general, the thinner spacers consume less volume and so allow more membrane surface area to be pro vided in an element of a given outer diameter. However, the thinner spacers result in. greater head loss and are also more prone to fouling or plugging, which further increases head loss. Thicker feed spacers are better able to resist fouling and so are used with high folding feed water,

£0006J Numerous attempts have been made to provide feed channel spacers with special geometries that resist fouling or reduce concentration polarization at the membrane surface. For example, US200 01S2774 to llirofcawa et al discloses a feed side spacer having warps almost parallel to the direction of How, and wefts, thinner than the warps, at a prescribed pitch designed to reduce the pressure drop on the feed side as well as reduce clogging of the feed channel. For further example, Ahmad and Lau, in "impact of different spacer filaments geometries on 2D unsteady hydrodynamics and concentratio polarization in spiral wound membrane channel". Journal of Membrane Science 286 (2006) 77-92» use computational fluid dynamics to demonstrate that a mes spacer with strands of circular cross-section is more efficient at reducing the effect of concentration polarization than a mesh spacer with strands of rectangular cross-section. Lau et aL in "Feed spacer mesh angle: 3D modeling, simulation, and optimization based on unsteady hydrodynamic in spiral wound membrane channel". Journal of Membrane Science 343 (2009) ! 6~33_; use computational fluid dynamics to demonstrate an optimal included angle between the strands in a mesh-type spacer to reduce the effect of concentration polarization. However, a square or diamond shaped net, made with one set of parallel filaments below a second set of parallel filaments oriented obliquely to the first set, remains standard in the field.

BRIEF DESCRIPTIO OF THE INVENTION

[0007] A feed channel spacer described herein has regions or borders, at the inlet and outlet edges of the feed channel spacer, thai are thinner than a central part, of the feed channel spacer. [00003 Ift a spiral wotrad membrane element, a feed channel spacer as described above is located between adjacent menihrane leaves of the element. In. each membrane leaf, a permeate carrier is located between upper and lower membrane sheets and an adhesive is applied at least in two lines along the side edges of the leaf, which are perpendicular to a central tube, The thinned edges of the feed channel spacers are located between .he lines of adhesive of adjacent membrane leaves. 00093 Ate the membrane leaves and feed channel, spacers are wonnd around the central tube, the side edges of the membrane leaves with their attached lines of adhesive extend in a spiral around the central tube, Because of the thickness of the adhesive, the ends of a typical me.mh.rane element with a feed channel spacer of uniform thickness have a larger diameter than the central part of the element. The outer diameter at the ends of the element limits the number or length of membrane leaves that may be placed i a pressure vessel of a aiven inside diameter. Providing .relatively this edaes on the feed channel spacers at least red uces any increase in diameter at the ends of an element that would otherwise be caused by the adhesive. Accordingly,, more or longer membrane leaves may be placed in a pressure vessel of a given inside diameter if feed channel spacers with thin edges are used, thus increasing the active membrane area of the element,

[0010] A process for making a feed channel spacer with thin edges comprises heating and compressing the edges o f a sheet of spacer material For example, the edges may be made thinner by passing the edges of a sheet of feed spacer material through a pair of hot rollers or by compressing the edges of the sheet in a heated press.

[00113 Another feed channel spacer described herein has an are with obstructions to fluid flow. The obstructions may be laid out in an array with subsequent rows staggered .from each other. The obstructions may be provided with a feed spacer sheet of constant thickness, or with one having relatively thin edges.

BRIEF DESCRIPTION OF THE DR WINGS

[0012] Figure ! is a cut-away perspective view of a spiral wound .membrane element [0013] Figure 2 s a schematic plan view feed channel spacer ha in thin edges.

[0014] Figure 3 is a schematic section along the l ne iU~!if of the feed channel s ac of Fi gure 2.

[00 S] Figure 4 is a schematic plan view of another feed channel spacer having thin edges and a region with obstructions.

[0016] Figure 5 A is a schematic section along the line V~V of the feed channel spacer of Figure 4.

[0017] Figure SB is a schematic section, along the line V~ of Figure 4 showing an. alternative construction of the feed channel spacer of Figure 4.

DETAiLED DESCRIPTION

[0018] Referring to Figure 1 , a spiral wound membrane element 1 is formed by wrapping one or more membrane leaves 1:2 and feed channel spacers 14 around a perforated centra! tube 16. The membrane leaves 12 may also be called envelopes. The feed channel spacers 14 may also be called brine channel spacers. The central tube 16 may also be called a core, a permeate tube or a product water collection tube. The leaves 1.2 comprise two generally rectangular membrane sheets 18 surmunding a permeate carrier 20. The edge of the membrane leaf 1 abutting the central tube 16 is open, bat the other three edges of a leaf 12 are sealed, for example by an adhesive. Less frequently, the two membrane sheets 18 of a membrane leaf 12 may be attached through a fokl line at the ti p of the leaf, in which case only the two side edges of a membrane leaf 12 are sealed with adhesive,

[0019] The membrane sheets 18 may have a separation layer cast onto a supporting or backing layer. The separation layer may e, for example, cellulose acetate, a polyamide, a thin Him composite or other materials that may he formed into a separation, membrane. The separation layer may have pores, for example, in the reverse osmosis, nanofiltration or ultrafiltration range. Filtered product water, also called permeate, passes through the membrane sheet while the passage of dissolved salts or suspended solids or other contaminants are rejected by the membrane sheet 18 tlepe titling on its pore ske. The permeate carrier 20 is i Ouid contact with rows of small holes 22 in th central tube 16 through, the open abutting edge of the -membrane leaf 32,

£0020] Each leaf 12 is separated by a feed channel spacer 14 that is also wound around the central tube 16, The feed channel spacer 14 is m fluid contact, with both ends of the element 1 and it acts as a conduit for feed solution across the surface of the membrane sheets 18. The dement 10 is placed inside of a pressure vessel (not shown) when in use, and feed water is introduced into one end of the pressure vessel. Feed water 70 flows through the element 10 from the entrance end 24 to the concentrate end 26 parallel to the axis A of the central tube 16, After passing through the element. 10, the feed water 70, less an water that has been permeated, leaves the elements as concentrate 90, alternatively called retentate or brine,

£0021] A membrane leaf 12 tends to haw side edges .132, whic are the edges perpendicular to the central tube 1 , that are 2 to 5 ra th or 10 to 22%, thicker than the remainder of the membrane leaf 12. The increase in thickness is caused by the adhesi ve, alternatively called glue lines, used to seal the edges of a membrane leaf 12. Since the outer diameter of an element 10 is typically maintained within a narrow range relative to the inside diameter of the pressure vessel that will surround the element 1 , the limiting diameter of the element 10 is typically formed in the area of the side edges 132 of the membrane leaves 12, The width of the glue lines may vary between, for example, about 25 mm with automatic glue application and about 30 mm to 45 mm with manual glue application.

[0022] Figures 2 and 3 show a feed channel spacer 14 having edges 1 10, 1 0 that are thinner than, a central, part 120 of die feed channel spacer 14, Referring to Figure .1 , a feed edge portion 1 10 of the feed channel, spacer 1 is located between the glue lines 132 on the s ide edges of t he mem brane leaves 12 near the entrance end 24 of the el ement 1.0. A concentrate edge portion 130 of the feed channel spacer 1.4 is located betwee glue lines 132 on the side edges of the membrane leaves 1.2 near the concentrate end 26 of the element 10. |0023] The edges 1 10, 130 may be made of a differ nt material ihm the central part

120 of the feed channel spacer 14, or the edges 1 10, 130 may b treated to reduce their thickness. For xample, the edges my be made by taking a. sheet of theroiopiastie mesh or netting, as is typically used to create feed channel spacers, and compressing the edges of the sheet between a pair of rollers or a press. The rollers or press are preferably heated to above the heat deflection temperature of the sheet while pressing the edges 1 1.0, 130 of the sheet such that the edges 1 10, 130 are permanently deformed to a reduced thickness. Alternatively, a feed channel spacer 1.4 may be made in three pieces, in this case, the central portion 120 is made of feed channel spacer material having one thickness and the edges 1 10, 130 are made of feed charmel spacer material having a lesser thickness,

[0024] Referring to Figure 3, the central portion 120 has a first thickness 170. The first thickness 1.70 may be any thickness appropriate- for a spiral wound membrane, for example between 0.6 mm and 1 ,2 mm. The feed edge portion 1 1 , and the concentrate edge portion 130, have a second thickness .160. The second thickness 160 is less than the first thickness 170. For example, the second thickness 160 may he between 2 and 12 mil less than the first thickness 1.70. Using a feed channel spacer 14 having relativel thinner edges 1 10, BO allows a larger membrane area to be packed into the spiral wound element 1 of a given outside diameter resulting in a higher permeate flow per element 1 , 002S] While the thinned edges might at first appear to increase head loss, the flow through the edge 110. 130 is generally laminar, and the edges are typically less than 5 cm wide. Accordingly, any increase in head loss is small However, the membrane surface area may be increased, for example by 5% or more or 10% or more. The increase in membrane area more than compensates for any decrease m net driving pressure allowing permeate flax to be increased for a given outside diameter of an .-element 10. Alternatively, if an existing element 10 has difficulty filtering feed liquids with a propensity to foul the feed channels, ft may be advantageous to use a feed channel spacer 14 with the same thickness as the existing feed channel spacer at the edges 1 10, 130, but with a greater thickness in the centra! portion 120, The centra! portion 120 may be made thicker, for example, between 2 and 12 mils thicker, without materially decreasing the membrane surface area. [0026] Referring to figure , a second eed channel spacer 1 0 shown is similar to the feed channel spacer 1 in avina thinner eckes 1 10. 130, However, in the second feed channel spacer 140, the thinner edges H , 130 are optional and the second, feed channel spacer may have a uniform thickness.

[0027] The second teed channel, spacer 140 has a region 200 that, includes a plurality of obstructions 1 0. The region 200 shown in Figure 4 occurs only at the tip of the second feed channel spacer 140, which is the region of the second, feed channel spacer 40 that will be located near the outside of the element 10, furthest from the central tube 16, Alternatively, the region 200 may occur in other parts of the second feed channel, spacer 140 including throughout the entire second feed channel spacer 140.

[0028] In plan view; as shown in Figure 4, the obstructions are provided in a staggered pattern or array across a portion of the width (for example between edges 1 10, 130) of the second feed channel spacer 140. Subsequent rows are staggered in a nominal direction 80 of feed flow through the second feed channel spacer 140. Figure 4, and the other Figures, are not to scale, and the obstructions 190 are much more numerous and closer together than illustrated. The centre to centre distance between adjacent obstructions 1.90 may he 20 mm or less. The gap between obstructions 1 0, measured perpendicular to the nominal direction feed flow direction SO, may be between 0.5 and 5 mm. The diameter of the obstructions 1.90, or their width perpendicular to the nominal direction 80 of feed flow if they are not round, may be between 1. and 5 ram.

[0029] The obstructions 1 0 cause the feed water to flow in a curving local feed .flow path 82. The curvature of the local feed flow path. 82 may vary, but at least some portions, for example 50% or more, of the local feed flow path 82 preferably have a radius of curvature of between ! mm and 10 mm. The curvature causes the feed water flowing through the local feed flow path 82 to experience micro-mixing effects such as Dean vortices. This micro- mixing inhibits concentration polarkation on the surfaces of the membranes 18.

[0030] The obstructions 1 0 are oriented generally vertically when the second feed channel spacer 140 is oriented horizontally. The obstructions 190 may be cylindrical, or they ma have a conical or dome shape to reduce their area of contact wi th th membranes 3S. Referring to Fignre 5 A,, in a second feed channel spacer HO A the obstructions 1 0 are supported on. a central sheet 192. Each obstruction 190 extends from the centra! sheet 192 to the top or bottom of the second feed channel spacer. The obstructions 1.90 may be made, for example, of deposits of an adhesive or lastic placed on the central sheet 1 2. Alternatively, the obstructions 190 may he made by forming dimples in a the m pl stic central sheet ! 92, hi thai case, the pattern of obstructions 190 on one side of the central sheet 192 is offset from the pattern of obstructions 19 on other side of the centra! sheet 192, Referring to figure 58, in another second feed channel spacer 14 A._; the obstructions 190 are supported by a mesh or et feed spacer materia! 1 4 of the first width. 170, These obstructions 190 pass throtigh the width of the second feed channel spacer 140.A, In thi s case, the obstructions 1.90 are .formed, by deposits of an adhesive or plastic attached to the feed spacer material 1 4,

[0031] in Figure 4, the region 200 occurs onl at the ti of the second feed channel spacer 1 0. The tip of a feed channel spacer tends to carry less feed flow than the base of a feed channel spacer. In some eases, the tips of the membrane leaves 12 do no fully participate in filtration because the feed flow velocity is insufficient to inhibit concentration polartzatfon. The obstructions 190 reduce the area available for flow through the region 200 which increases the local velocity of the feed water in region 200, In addition to an mixing that might be caused by the curving flow path, the increase in local velocity also helps reduce concentration polarization in the region 200. Optionally, the location and dimension of the obstructions 190 may be chosen to increase the local velocity of feed flow without also attempting to create micro-mixing effects.

[0032] This written description uses examples to disclose embodiments of the invention and also to enable any person sk lled in the art to practice embodiments of the invention, including making and using any devices or systems and perforating any incorporated methods. The patentable scope of embodiments of the inventio is defined by the claims, and may include other examples that occur to those skilled in the art.

Claims

CLAIMS:

1. A feed channel spacer for a spiral wound, raetnkane element having a central area with a first thickness and opposed edges having a second thickness that is less tha the first thickness.

2. The feed channel spacer of claim 1 wherein the edges me located on the reed channel spacer such that, in a completed spiral wound membrane element, the edges are placed between lines of adhesive in membrane leaves of the element,

3. The feed c nn l spacer of claim .! wherein, the second thickness is between 2 and 12 mils less than the first thickness.

4. The feed channel spacer of c laim 1 wherein the edges are 50 mm or less in. width,.

5. The feed channel spacer of claim 1 wherein the edges and the central portion are parts of a common piece of a net or mesh material

6.. The feed channel spacer of claim t w herein the edges and die centra! portion are distinct pieces of material.

7. The Iced channel spacer of claim I iwther comprising a region comprising m array of obstructions.

8. A method of making a feed channel spacer comprising a step of compressing two opposed edges of a sheet of a feed channel spacer .material to reduce the thickness of the edges,

9. The method of claim 8 further comprising heating the feed channel spacer material while compressing It.

10. The method of claim 9 wherein the feed channel spacer material is m d of a thermoplastic ma tenal and the step of heating comprises heating the thermoplastic material t at least its heat deflection temperature.

1 1. The .method of claim 8 comprising compressing the edges in a pre ss or between two rollers.

! 2. A feed channel spacer comprising 8» array of obstructions in at least a regio of the feed channel spacer.

13, The feed channel spacer of claim 12 wherein the bst uctions are supported oft a net or mesh material or on a central sheet.

14. The teed channel, spacer of claim. 13 wherein the obstructions are formed m a sheet of thermoplastic material,

! 5. The feed channel spacer of claim 13 wherein t he obstructions are formed by depositing w adhesive or plastic onto a central sheet or onto a mesh or net.

1.6, The feed channel spacer of claim 12 wherein the region is located in a dp portion of the feed channel spacer.

17. The feed channel spacer of claim 12 wherein the obstructions are positi oned in an array wherein, subsequent rows of obstructions are offset from each other,

18. The feed channel spacer of claim 17 wherein the obstructions- create a curving feed flow path having sections with a radius of curvature of 10 mm or less.