Abstract:
A separation module utilizing a feed spacer and a method for forming such a separation module are provided. A gasket comprising a flexible waterproof material is disposed on at least part of one or more edges of the feed spacer. A membrane layer is disposed on a first surface of the feed spacer. A permeate carrier is disposed on a surface of the membrane element opposite the feed spacer.

Description:
BACKGROUND 
       [0001]    Embodiments presented herein relate to a separation module and more particularly to reverse osmosis, forward osmosis, and physical filtration modules. Physical filtration can include micro, ultra and nano filtration processes. 
         [0002]    Membrane modules are widely used for separating fluids with dissolved and suspended organic and inorganic solids. Processes used for this purpose can include reverse osmosis, forward osmosis, and physical filtration. In reverse osmosis, a feed solution such as, but not limited to brackish or impure water, sea water, and so forth, is passed through a semi-permeable membrane at a pressure higher than the osmotic pressure of the feed water. A permeate, for example, purified water is obtained on the other side of the semi-permeable membrane. 
         [0003]    In forward osmosis, water from a feed solution such as, but not limited to brackish or impure water, seawater, and so forth, is drawn through a semi-permeable membrane due to the osmotic pressure difference between the feed solution and a draw solution. The draw solution therefore exits the separation module with a reduced concentration of draw chemical due to the increased percentage of water. 
         [0004]    Lastly, for the physical filtration processes such as micro-, ultra- and nano-filtration, a feed solution containing suspended solids is introduced to the separation module at higher pressure than exists in the permeate channels of the module. Water flows through the pores of the separation membrane and exits the separation module through a permeate channel. 
         [0005]    In the above processes for reverse osmosis, forward osmosis and physical filtration, the feed channels are typically defined by geometry of the module and more typically, by adhesives that are disposed on the edges of the feed spacer materials. Though channels defined by this method have been shown, a robust implementation has yet to be achieved. For example, the adhesive adheres to the membrane face which in the case of RO or FO is typically very thin on the order of 100 nm. When the feed channel is pressurized above the pressure of the permeate channel, a stress concentration at the membrane-adhesive joint develops and generally results in a tear in the membrane. The tear in the membrane then results in decreased purification. In other cases, where the feed pressures are not high enough to immediately tear the membrane at the stress concentration, handling, pressure fluctuations or periodic cycling can have similar effect on the membrane tearing. 
         [0006]    In other cases, end caps, or end potting can be disposed on the ends of the modules to define the feed solution flow path. Lastly, chemical joining of the layers can also be used to define the feed solution flow path within the feed channel. However, such joints may be susceptible to leaks at high pressures of the feed. Further, processes involved in producing chemical joints may be expensive and time consuming. Variations in the thickness of each feed channel along the flow direction may also not be possible in such cases. The chemically joined edges may also cause damage to the layer of membrane element. Moreover, chemical joints may not provide additional rigidity to the layer of permeate carrier. In spiral wound modules, it may also be difficult to roll chemically joined leaves around the core. 
         [0007]    Therefore there is a need for a feed spacer gasket technology that overcomes these and other shortcomings of the prior art. 
       BRIEF DESCRIPTION 
       [0008]    A separation module utilizing a feed spacer and a method for forming such a separation module are provided. A gasket comprising a flexible waterproof material is disposed on at least part of one or more edges of the feed spacer. A membrane layer is disposed on a first surface of the feed spacer. A permeate carrier is disposed on a surface of the membrane element opposite the feed spacer. 
         [0009]    Several embodiments of a separation module are provided. The membrane module includes at least one layer of a permeate carrier, at least one layer of a membrane element, and at least one layer of a feed spacer. The membrane module further includes at least one layer of a feed spacer wherein edges of the feed spacer are at least partly covered by one or more strips of a waterproof flexible material. A seal is formed between the membrane element and the feed spacer, wherein the one or more strips of the waterproof flexible material is compressed against the membrane element to form the seal. The flexible waterproof material may be compressed against the membrane element either by winding the membrane stack around a central core, or by compressing the flexible waterproof material against the membrane element using a suitable frame and plate assembly. 
         [0010]    A method for fabricating a separation module is provided. The method includes providing a feed spacer and impregnating at least part of one or more edges of the feed spacer with a flexible waterproof material. The method further includes providing a membrane element on one side of the feed spacer, and providing a permeate carrier on the opposite side of the membrane element. In several embodiments, the method further includes winding the feed spacer, the membrane element, and the permeate carrier around a core. The flexible waterproof material compresses against the membrane element to form a seal in the feed channel. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  illustrates the sequence of layers of materials, according to an embodiment; 
           [0012]      FIG. 2  illustrates the waterproof gasket, according to several embodiments; 
           [0013]      FIG. 3  is a cross section view of a membrane stack of a separation module, according to one embodiment; 
           [0014]      FIG. 4  is a cross section view of a membrane stack of a separation module, according to another embodiment; 
           [0015]      FIG. 5  illustrates a membrane stack of a separation module, according to one embodiment; 
           [0016]      FIG. 6  illustrates a membrane stack of a separation module, according to another embodiment; 
           [0017]      FIG. 7  illustrates a membrane stack of a separation module, according to yet another embodiment; and 
           [0018]      FIG. 8  is a profile plot of gasket thickness against a length of the feed spacer, according to various embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Various embodiments presented herein will be described in detail below with reference to accompanying drawings. It will be apparent, however, that these embodiments may be practiced without some or all of these specific details. In other instances, well known process steps or elements have not been described in detail in order not to unnecessarily obscure the description of the embodiments. The following example embodiments and their aspects are described and illustrated in conjunction with apparatuses, methods, and systems which are meant to be illustrative examples, not limiting in scope. 
         [0020]    Embodiments presented herein describe a feed spacer and a separation module employing the feed spacer. Depending on the particular embodiment, the separation module can be used for reverse osmosis, forward osmosis or physical filtration applications. Exemplary embodiments for the applications will become evident through the descriptions provided with the accompanying figures. 
         [0021]      FIG. 1  illustrates an example sequence of materials in a typical spiral wound separation module  100  applicable for reverse osmosis, forward osmosis and physical filtration, according to various embodiments. The separation module includes one or more layers of membrane element  102  disposed between one or more layers of feed spacer  104  and one or more layers of permeate carrier  106 . The layers of the membrane elements  102 , the feed spacer  104 , and the permeate carrier  106  are wound around a central core  108 . The central core  108  may include separate channels for the feed solution, the permeate and the retentate. The sequence of layers may be repeated any number of times depending on the desired geometry of the separation module. 
         [0022]    The basic function of the spiral wound separation module  100  for reverse osmosis, forward osmosis, and physical filtration is described in the following paragraphs. 
         [0023]    Reverse Osmosis 
         [0024]    The feed solution may be pumped through the feed spacer  104  at high pressure, usually 2-17 bar (30-250 PSI) for brackish water, and 40-70 bar (600-1000 PSI) for seawater. Due to the pressure of the feed solution, the feed solution flowing through the feed spacer  104  is forced into the membrane element  102 . The permeate, for example, purified water, may pass through the membrane element  102  and collect in the permeate carrier  106 . The permeate carrier  106  carries the permeate to a permeate discharge port. The retentate, for example, brine, does not pass through the membrane element  102 , and remains in the feed spacer  104 . The feed spacer  104  carries the retentate to a retentate discharge port. 
         [0025]    Physical Filtration 
         [0026]    The feed solution may be pumped through the feed spacer  104  at high pressure. Due to the pressure of the feed solution, the feed solution flowing through the feed spacer  104  is forced into the membrane element  102 . The filtrate may pass through the membrane element  102  and collect in the permeate carrier  106 . The permeate carrier  106  carries the filtrate to a filtrate discharge port. The impure feed solution does not pass through the membrane element  102 , and remains in the feed spacer  104 . The feed spacer  104  carries the impure feed solution to a impure feed discharge port. 
         [0027]    Forward Osmosis 
         [0028]    The feed solution may be pumped through the feed spacer  104 , and a suitable draw solution may be pumped through the permeate carrier  106 . Due to the osmotic pressure gradient across the membrane element  102 , a net flow of permeate from the feed solution in the feed spacer  104  to the draw solution in the permeate carrier  106  occurs. The permeate may pass through the membrane element  102  and collect in the permeate carrier  106 . The permeate carrier  106  carries the permeate to a permeate discharge port. The permeate may then optionally be subject to a second separation process such as reverse osmosis, or draw solute separation techniques. The retentate does not pass through the membrane element  102 , and remains in the feed spacer  104 . The feed spacer  104  carries the retentate to a retentate discharge port. 
         [0029]      FIG. 2  illustrates flexible waterproof gaskets impregnated on a feed spacer in the separation module, according to several embodiments. Membrane stack  200  may be employed in several different separation module configurations for the reverse osmosis, forward osmosis and physical filtration processes. The membrane stack  200  includes one or more layers of membrane element  202  disposed between one or more layers of feed spacer  204  and one or more layers of permeate carrier  206 . A flexible waterproof gasket  208  is disposed on the lateral edges of the feed spacer that are perpendicular to the axis of a cylindrical separation module. The flexible waterproof gasket  208  may preferably be disposed on the feed spacer  204  prior to assembly of the membrane stack  200 . The layers of the membrane elements  202 , the feed spacer  204 , and the permeate carrier  206  are wound around a central core  210 . Due to winding of the membrane elements  202 , the feed spacer  204 , and the permeate carrier  206  around the central core  210 , a seal is formed between the membrane elements  202  and the flexible waterproof gasket  208  due to compression. The seal thus defines a feed solution channel between the membrane elements  202  adjacent to the feed spacer  204 . 
         [0030]      FIG. 3  illustrates a cross section view  300  of an exemplary membrane stack, according to one embodiment. The membrane stack includes a feed spacer  302 . The feed spacer  302  includes an open mesh structure  304 . The lateral edges of the open mesh structure  304  may be covered, at least partly, with one or more flexible waterproof gaskets  306 . The flexible waterproof gasket may be made of a rubbery material having a glass transition temperature below typical operating temperatures (5-6 degree Centigrade) of the separation module. The flexible waterproof gasket may be made of materials such as including thermoplastics and thermosets. Example materials include, without limitation, hot melt adhesives such as ethylene-vinyl acetate (EVA) copolymers, ethylene-acrylate copolymers such as ethylene-vinylacetate-maleic anhydride, ethylene-acrylate-maleic anhydride, terpolymers, ethylene n-butyl acrylate, ethylene-acrylic acid, and ethylene-ethyl acetate; polyolefins such as low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene, polybutene-1, polyamides and polyesters, polyurethanes such as thermoplastic polyurethanes and reactive urethanes; styrene block copolymers including styrene-butadiene-styrene, styrene-isoprene-styrene, styrene-ethylene/butylene-styrene, styrene-ethylene/propylene block copolymers, polycaprolactone, polycarbonates, fluoropolymers, silicone rubbers, and thermoplastic elastomers. In particular, ethylene-vinyl acetate (EVA) may be used to form the flexible waterproof gaskets  306 . In the embodiment illustrated in  FIG. 3 , the lateral edges of the open mesh structure  304  may be covered completely with one or more flexible waterproof gaskets  306 . 
         [0031]    The flexible waterproof gasket  306  may be disposed on the open mesh structure  304  using any suitable technique. In one embodiment, the open mesh structure  304  is impregnated with a hot thermoplastic material, such as EVA. The feed spacer  302  is then stacked with one or more membrane elements  308 , and one or more permeate carriers  310  to form the membrane stack for a separation module. Compression of the flexible waterproof gasket  306  against the membrane elements  308  effectively forms a seal for the feed channel. In several embodiments (such as the embodiments described in conjunction with  FIG. 5 ) for spiral wound and flat module configurations, the pressure difference between the feed channel and the applied feed solution pressure is small. In such embodiments the flexible waterproof gaskets  306  easily seal the feed channel. 
         [0032]      FIG. 4  illustrates a cross section view  400  of an exemplary membrane stack, according to one embodiment. The membrane stack includes a feed spacer  402 . The feed spacer  402  includes an open mesh structure  404 . The lateral edges of the open mesh structure  404  may be covered, at least partly, with one or more flexible waterproof gaskets  406 . Example materials and techniques suitable for forming the flexible waterproof gasket  406  are described in conjunction with  FIG. 3 . The membrane stack of  FIG. 4  further includes an adhesive  408  applied between the flexible waterproof gasket  406  and the adjacent membrane elements  410 . The adhesive  408  may be applied on the flexible waterproof gasket  406  and around the outer edges of the flexible waterproof gasket  406  to improve the sealing. 
         [0033]    In embodiments for spiral wound and flat module configurations, where the pressure difference between the feed channel and the applied feed solution pressure is large, the adhesive  408  may further bond the feed spacer  402  to a membrane element  410 . Suitable materials for the adhesive  408  form a bond with the flexible waterproof gasket  406  as well as with the membrane element  410 . One example of a suitable adhesive is a thermosetting urethane. 
         [0034]    Although  FIGS. 2 ,  3 , and  4  illustrate flexible waterproof gaskets disposed on lateral edges of the open mesh structure, in various other embodiments, flexible waterproof gaskets may also be is disposed on the axial edges of the open mesh structure, particularly the axial edge distal from the central core. Such embodiments are described in conjunction with  FIGS. 6 and 7 . 
         [0035]      FIG. 5  illustrates a membrane stack  500  for use in a separation module, according to one embodiment. Membrane stack  500  may be suitable for use in a spiral flow separation module. The membrane stack  500  includes one or more layers of membrane element  502  disposed between one or more layers of feed spacer  504  and one or more layers of permeate carrier  506 . A flexible waterproof gasket  508  is disposed on the lateral edges of the feed spacer  504  at the axial ends of a cylindrical separation module. Membrane stack  500  may be employed in several different separation module configurations for the reverse osmosis, and physical filtration processes. 
         [0036]    The feed solution may flow spirally inwards from an inlet disposed on a circumferential edge of the spiral flow separation module, into the central core  510 . Alternatively, the feed solution may flow spirally outwards from the central core  510  into an outlet disposed on a circumferential edge of the spiral flow separation module. As the feed solution flows through the feed spacer  504 , the membrane element  502  recovers a permeate. The permeate flows across the membrane element  502  into the permeate carrier  506 . The permeate then flows spirally inwards from a circumferential edge of the permeate carrier  506 , into the central core  510 . 
         [0037]    Similar to the flexible waterproof gasket  508  disposed on the feed spacer  504 , the permeate carrier  506  may also include a flexible waterproof gasket  512  disposed thereon. The flexible waterproof gasket  512  may form a seal, the seal defining a permeate channel between the membrane elements  502  adjacent to the permeate carrier  506 . 
         [0038]      FIG. 6  illustrates a membrane stack  600  for use in a separation module, according to one embodiment. Membrane stack  600  may be suitable for use in a cross permeate flow separation module. The membrane stack  600  includes one or more layers of membrane element  602  disposed between one or more layers of feed spacer  604  and one or more layers of permeate carrier  606 . The feed spacer  604  further includes a flexible waterproof gasket  608  disposed on the lateral edges of the feed spacer  604  at the axial ends of a cylindrical separation module. Membrane stack  600  may be employed in several different separation module configurations for the reverse osmosis, forward osmosis and physical filtration processes. 
         [0039]    The feed solution may flow spirally inwards from an inlet disposed on a circumferential edge of the cross permeate flow separation module, into the central core  610 . Alternatively, the feed solution may flow spirally outwards from the central core  610  into an outlet disposed on a circumferential edge of the cross permeate flow separation module. As the feed solution flows through the feed spacer  604 , the membrane elements  602  recover a permeate. The permeate flows across the membrane element  602  into the permeate carrier  606 . The permeate then flows through the permeate carrier  606  axially out towards the axial ends of the cross permeate flow separation module. The permeate may flow out axially through one end, or both ends. In one embodiment, the cross permeate flow separation module may be used for forward osmosis process. The draw solution flows axially through the permeate carrier  606 . 
         [0040]    Flexible waterproof gaskets  612  disposed on the permeate carrier  606  may form a seal, similar to the seal formed by the flexible waterproof gasket  608 . The flexible waterproof gaskets  612  defining a permeate/draw channel between the membrane elements  502  adjacent to the permeate carrier  506 , and direct the flow of the permeate/draw solution axially through the cross permeate flow separation module. 
         [0041]      FIG. 6  illustrates a cross permeate flow separation module having a spiral feed flow, and an axial permeate/draw solution flow. However, it should be appreciated that flow paths of the permeate/draw solution and the feed solution may be reversed. In other words, the cross permeate flow separation module may have a spiral permeate/draw solution flow and an axial feed flow. In such embodiments, the feed spacer  604  may have disposed thereon a flexible waterproof gasket along the axial edges parallel of the cross permeate flow separation module, similar to flexible waterproof gasket  612 . On the other hand, the permeate carrier  606  may have disposed thereon a flexible waterproof gasket along the lateral edges of the cross permeate flow separation module, similar to flexible waterproof gasket  608 . 
         [0042]      FIG. 7  illustrates a membrane stack  700  for use in a separation module, according to one embodiment. Membrane stack  700  may be employed in several different separation module configurations for the reverse osmosis, forward osmosis and physical filtration processes. The membrane stack  700  includes one or more layers of membrane element  702  disposed between one or more layers of feed spacer  704  and one or more layers of permeate carrier  706 . The feed spacer  704  further includes a flexible waterproof gasket  708  disposed on the lateral edges and the distal axial edge of the feed spacer  704 . The feed spacer  704  also includes a flexible waterproof gasket  710  disposed perpendicular to the axis of the cylindrical separation module. The flexible waterproof gasket  710  may be disposed substantially mid way between the lateral edges of the feed spacer  704 . The flexible waterproof gasket  710  may not extend up to the distal axial edge of the feed spacer  704 . The flexible waterproof gasket  708  and the flexible waterproof gasket  710  define a U-shaped feed channel for feed solution flow. 
         [0043]    The feed solution may flow into the central core  712  from an inlet at one axial end of the central core  712 . The feed solution flows into the feed spacer  704  and spirally outwards to the end of the feed spacer  704 . The feed solution turns the corner at the distal end of the flexible waterproof gasket  710  and flows spirally inwards to the central core  712 . The feed solution then drains out of an outlet at the opposite axial end of the central core  712 . 
         [0044]    As the feed solution flows through the feed spacer  704 , the membrane elements  702  recover a permeate. The permeate flows across the membrane element  702  into the permeate carrier  706 . The permeate then flows through the permeate carrier  706  axially out towards the axial ends of the cross permeate flow separation module. The permeate may flow out through one axial end, or both axial ends. In one embodiment, the separation module may be used for forward osmosis process. The draw solution flows axially through the permeate carrier  706 . 
         [0045]    Flexible waterproof gaskets  712  disposed on the permeate carrier  706  may form a seal, similar to the seal formed by the flexible waterproof gasket  708 . The flexible waterproof gaskets  712  defining a permeate/draw channel between the membrane elements  702  adjacent to the permeate carrier  706 , and direct the flow of the permeate/draw solution axially through the separation module. 
         [0046]    Similar to the embodiment illustrated in  FIG. 6 , the flow paths of the permeate/draw solution and the feed solution may be reversed. In other words, the separation module may have a spiral permeate/draw solution flow and an axial feed solution flow. In such embodiments, the feed spacer  704  may have disposed thereon a flexible waterproof gasket along the proximal and distal axial edges similar to flexible waterproof gasket  712 . On the other hand, the permeate carrier  606  may have disposed thereon a flexible waterproof gaskets configuration similar to flexible waterproof gaskets  708  and  710 . 
         [0047]    In some embodiments, the flexible waterproof flexible waterproof gaskets may allow variable height feed channels. Variable height feed channels may facilitate optimal interaction of the feed water with the semi-permeable membrane, while minimizing pressure drop through the feed channel. 
         [0048]      FIG. 8  illustrates a profile plot  800  of thickness of the flexible waterproof gasket against the length of the feed spacer, according to various embodiments. For spiral wound configurations, the length of the feed spacer is the spiral length measured from the central core. As would be apparent to one skilled in the art, the direction of the feed flow would determine the direction of the thickness gradient. Accordingly, the variation of the feed channel can be tailor for any of the feed flow configuration embodiments shown in  FIGS. 2 ,  3 ,  4 ,  5  and  6 . 
         [0049]    Profile  802  is a straight line indicating that the thickness of the flexible waterproof gasket is constant throughout the length of the feed spacer. Thus the height of the feed channel remains unchanged as the feed water flows from the inlet to the core. 
         [0050]    Profile  804  is a straight line indicating a linearly increasing thickness of the flexible waterproof gasket. The thickness is lowest at the end near the retentate outlet from the module, and highest at the end near the feed solution inlet. In other words, the height of the feed channel linearly decreases as the feed water traverses from the feed solution inlet to the retentate outlet. 
         [0051]    Profile  806  is a step type profile indicating that the thickness of the flexible waterproof gasket increases in steps with the length of the feed spacer. In one example implementation, the profile  806  may provide a feed channel having a different height for every turn of the membrane stack. For implementations where the feed solution enters through an axial inlet port, the height of the feed channel for the outermost turn of the membrane stack would be highest, and the height of the feed channel for the innermost turn of the membrane stack would be lowest. Whereas, for implementations where the feed solution enters through central core, the height of the feed channel for the innermost turn of the membrane stack would be highest, and the height of the feed channel for the outermost turn of the membrane stack would be lowest. 
         [0052]    Profile  808  is a curve indicating that the thickness of the flexible waterproof gasket increases non-linearly and gradually with the length of the feed spacer. In one example implementation, the profile  808  may become substantially flat after a predefined length of the feed spacer. 
         [0053]    The thickness profile of the flexible waterproof gasket may be determined using factors such as, but not limited to, the decrease in feed volume due to purification of the feed water as it flows through the feed channel. Such a decrease in feed volume reduces the feed solution velocity in a fixed height feed channel. Thus, the thickness profile may be selected based on the required velocity gradient from the feed solution inlet to the retentate discharge port, without changing the operating parameters of the pump used to pressurize the feed water. Maintaining the feed solution velocities may also decrease concentration polarization and maintain mass transport across the membrane, thus improving efficiency of the spiral feed flow RO element. 
         [0054]    The foregoing description includes various embodiments of the separation module in a spiral wound configuration. However, the teachings of these embodiments may equally be applied to flat-type separation modules. In particular, the embodiments described in conjunction with  FIG. 6  may readily be practiced in a separation module in the flat-type configuration. Flat-type separation modules include a membrane stack similar to that described in conjunction with  FIG. 1 . However, the membrane stack is laid flat on a frame or plate assembly, rather than being wound around a central core. Various arrangements of plates and frames may be used to compress the flexible waterproof gaskets against the membrane elements to effectively seal the feed channels. Further, as described in conjunction with  FIG. 8 , the flexible waterproof gaskets may have a thickness varying along the longitudinal length of the feed spacer. Flat-type configurations of separation modules typically include feed inlet ports and retentate discharge ports connected to the feed carriers, and permeate discharge ports connected to the permeate carriers. 
         [0055]    Although specific implementations and application areas are described in conjunction with the embodiments presented herein, such description is solely for the purpose of illustration. Persons skilled in the art will recognize from this description that such embodiments may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims.