Patent Application
20120156485
This specification relates to hollow fiber membranes, to methods of making hollow fiber membranes, and to supporting structures for hollow fiber membranes.
The following discussion is not an admission that anything described below is common general knowledge or citable as prior art.
U.S. Pat. No. 5,472,607 describes a hollow fiber polymeric membrane supported by a braided textile tube. The braided tube is sufficiently dense and tightly braided that it is round stable before being coated with a membrane dope. The membrane polymer is located primarily on the outside of the braid. This type of structure has been used very successfully in the ZeeWeed™ 500 series membrane products currently sold by GE Water and Process Technologies. In particular, this type of supported membrane has proven to be extremely durable in use.
The following paragraphs are intended to introduce the reader to the detailed description to follow and not to limit or define any claimed invention.
The inventors have observed that the number of filaments required to make a round stable braided support tends to result in braids that have an over-abundance of tensile strength. Braiding a round stable support is also a slow process, typically done at a speed of about 20-30 meters per hour. In comparison, the dope coating process may be done at speeds up to 20-40 meters per minute and so the braid must be wound onto a spool as it is made, and then taken off again during the coating process. Reducing the number of filaments in a braided support could reduce the cost of the membrane while still providing adequate tensile strength. However, with fewer filaments the braided support might not be round stable and might not provide enough support for the membrane dope during coating. Further, even if fewer filaments were used in the braid, the braiding speed would still be much less than the coating speed and the braiding and coating operations still could not be combined in the same production line.
Various methods of making hollow fiber membranes are described herein in which a membrane dope is extruded onto a tubular supporting structure, alternatively called a support or tubular support. In a first set of membranes, the tubular support is porous and remains as a part of the finished membrane. The porous supporting structure may be made, for example, by a non-woven textile process, a sintering process using an extruder, or by extruding a polymer mixed with a second component. The second component may be a soluble solid or liquid, a super-critical gas, or a second polymer that does not react with the first polymer. The supporting structure supports the membrane dope until the dope is solidified into a membrane wall. Depending on the type and material of the tubular support, the resulting composite structure, comprising the supporting structure and the membrane wall, may have more tensile strength than a membrane wall of the same thickness made with the dope alone.
In a second set of membranes, the tubular support is surrounded by a textile reinforcement formed over a tubular support. For example, the textile reinforcement may be knitted around the tubular support. In the second set of membranes, the tubular support may be dissolved out of the finished membrane after the dope has formed a membrane wall, leaving the textile reinforcement embedded in the membrane wall. Alternatively, a porous support as in the first set of membranes may be surrounded by a textile reinforcement and remain in the finished membrane. The tubular support supports the membrane dope until the dope is solidified into a membrane wall. The textile reinforcement increases the tensile strength of the membrane, relative to a membrane wall of the same thickness made with the dope alone. The tubular support, if retained, may also contribute to an increase in textile strength. Optionally, the textile reinforcement may be knitted around the tubular support at the same speed as, and in line with, a dope coating process.
Referring to
The tubular support 12 may be porous and may remain inside of the finished hollow fiber membrane 10 in use. The tubular support 12 may have a higher tensile strength than a membrane wall 14 of the same thickness. Accordingly, the composite membrane 10 may also have a higher tensile strength than a membrane of the same thickness made with the dope 18 alone. A suitable permanent porous tubular support 12 can be made by extruding a mixture comprising two primary components though an annular die. A first primary component is a thermoplastic extrusion grade polymer that will remain as the porous support 12. A second primary component is a compatible substance that is soluble in a non-solvent of the first primary component. For example, the first primary component may be a water insoluble polymer and the second primary component may be a water-soluble substance such as sugar, salt, PVA or glycerol. The first and second primary components are chosen such that they do not react with each other. Instead, the first and second primary components are mixed with smooth agitation prior to extrusion so as to produce a homogenous mixture, for example an emulsion, of small particles of the second primary component in the first primary component. The mixture is extruded through an annular die and, optionally, a further outside diameter calibration die. The first primary component of the extruded mixture solidifies to form the support 12. The tubular support 12 is placed in a bath of a solvent of the second primary component, for example water, so that the second primary component can be leached out in the bath. This leaching step may occur before or after the membrane wall 14 is formed over the support 12. The first primary component could be an inexpensive (relative to the membrane dope 18) polymer, such as polyethylene (PE), if the tubular support 12 is required primarily to support the membrane dope 18 during coating. However, if a membrane 10 of greater strength is desired, then the first primary component may be a stronger polymer such as polyester (polyethylene terephthalate, PET).
Alternatively, a tubular support 12 may be made by extruding a mixture of two different thermoplastic materials. The two materials are chosen such that they are not reactive with each other. The two materials are also incompatible in the sense that the second material does not mix or dissolve well in the first material and instead the second material forms clusters with low adherence to a matrix provided by the first material. For example, one known second material is polymethylpentene (PMP). The first material may be, for example, PE or PET depending on whether cost or strength is more important for the finished membrane 10. After the extruded tube has solidified, it is stretched to set the final outside diameter of the tubular support 12, and to create small cracks between the two materials. The cracks provide the tubular support 12 with the desired porosity. A similar tubular support 12 can be made by mixing the first material with a non solute that is not a polymer such as CaCO3.
As further extrusion based alternatives, a commercially available filtration tube can be used for the support 12. A tubular support 12 with an open cell structure can also be extruded out of a polymer mixed with a super critical gas such as carbon dioxide. As the mixture comes out of the extrusion die, a reduction in pressure allows the super critical gas to escape leaving an open cell structure. An extruded solid tube can also be punched out mechanically, or with one or more pulsating laser beams, to form the support 12.
A tubular support 12 can also be made without extrusion. For example, a non-woven support can be made by directing electro-spun, melt blown or spray blown fiber segments onto a rotating mandrel. The mandrel is porous with its inner bore connected to a source of suction such that the semi-solidified fiber segments collect on the mandrel. The fiber segments fully solidify and bond to each other on the mandrel to form a non-woven tube. One end of the tube is continually pulled off of the mandrel to create the tubular support 12. Optionally, a non-woven structure may have sufficient density to increase the strength of the membrane 10. A support 12 may also be made by a continuous sintering process with an extrusion machine. In this process, semi molten polymer granules are pressed together in an extruder and sintered under pressure pushing them through the extrusion die in a manner similar to metallic sintering, but into a tubular shape. This tube is than used as a support 12. The sintering process does not produce a support 12 with as much strength as a non-woven support, but sintering and non-woven techniques may be performed in line with the coating process.
Referring to
The knitted reinforcement 22 is applied to the tubular support 12 by passing the support 12 through a knitting machine while knitting one or more mono or multi-filaments 24 in a tube around the support 12. The knitting may be done at the same speed as the dope coating process described in relation to
Alternatively, the textile reinforcement 22 may be provided in the form of one or more spiral wrappings made by passing a support 12 through a wrapping machine.
Further alternatively, a textile reinforcement 22 may be provided in the form of many short segments of filaments 24. For example, the textile reinforcement may be made by mixing micro fibers into the membrane dope 18. Micro fibers can also be applied to the outside surface of a semi-solidified membrane dope 18 by spraying chopped, electro-spun or melt blown fibers on the surface of the dope 18. The fibers are sprayed onto the membrane dope 18 as it leaves the coating head 16 or soon after such that the dope 18 may have started to solidify, but it is not yet solid.
Optionally, the one or more filaments 24 in the reinforcement 22 may be coated or bi-component filaments 24, or a yarn or other multi-filament form of filament 24 may have two or more types of component mono-filaments. Crossing or adjacent filaments 24 may be fixed to each other at points of contact. For example, filaments 24 may be fixed to each other by heat setting, ultraviolet light, welding, plasma, resins or other adhesives. The material of the filaments 24, the coating of coated filaments 24, one or more exposed material in bi-component filaments 24, or one or more mono-filaments in multi-filaments may be chosen to be suitable for a chosen fixation method if required. The flexibility of the textile reinforcement 22 may be adjusted by altering the density of the filaments 24, by deciding whether or not to fix filaments 24 together at points of contact or intersection, and by adjusting the density of points of contact or intersection between filaments 24.
The support 12 may remain inside the second membrane 20. In that case, the support 12 may be made by any of the methods described for the first membrane 10. Regardless of the method of making the permanent support 12, the reinforcement 22 is formed around the support 12, for example by being knit or cable wrapped around the support 12. Since the support 12 keeps the reinforcement 22 in a round shape and at a desired diameter, the density of the reinforcement 22 can be chosen based on the intended use of the membrane 20. For example, a membrane 20 intended for use in drinking water filtration may have a reinforcement 22 that is less dense than in a membrane 20 intended for use in waste water. After the reinforcement 22 is placed around the support 12, the combined structure is passed through the coating head 16 as described above in relation to
If the tubular support 12 will not remain inside the finished membrane 20, then the support 12 is made from a material that is soluble in a non-solvent of the separating layer 14. For example, the support 12 may be water-soluble. The reinforcement 22 is applied around the support 12 as described above and the combined structure is passed through the coating head 16 as described in relation to
After a solid membrane wall 14 has formed, the support 12 is washed out in the coagulation bath or a separate solvent. The reinforcement 22 is preferably loose enough that the dope 18 will have penetrated through the reinforcement 22 to the outer surface of the support 12. At least outer filaments 24, or segment of filaments 24, are encapsulated in the membrane wall 14. The reinforcement 22 can also be very thin such that the outside diameter of the membrane 20 can be 1 mm or less. The total amount of material used in the support 12 and the reinforcement 22 may be similar to or less than the amount of bore fluid required to make an unsupported membrane. Accordingly, a light duty supported membrane 22 can be made for a similar price as an unsupported membrane but with increased strength. The dissolved material from the support 12 can be recovered from the coagulation bath or other solvent.
With a knit reinforcement 22 over a tubular support 12, for example in the form of a water soluble tube, the applied tension during knitting and during the coating process helps ensure that the circular knit reinforcement 22 lies smoothly on the surface of the support 12. The wall thickness of the support 12 is chosen such that the support 12 is not fully dissolved until the membrane material in the dope 18 coagulates. Accordingly, although the filaments 24 may be fixed to each other before the dope 18 is applied to the support 12, this is not mandatory since the coagulated dope 18 is sufficient for maintaining the position of the filaments in the knit in the finished second membrane 20.
A knit reinforcement 22 is preferably made by circular warp knitting. Warp knits consist of multiple yarn systems (using mono or multi-filament yarns), which run longitudinally along the surface of the cylindrical knit. Each yarn system has a designated needle and the yarn guiding elements alternate between the loop forming needles. In comparison, a weft knit is formed from one or multiple yarn systems, which are interconnected by loops, formed by needles. In weft knitting the adjacent loops (stitches) run in a spiral shaped line around the length axis of the circular knit. A plurality of the loop (stitch) forming needles and the yarn guiding elements rotate with reference to each other. A weft knit provides a more nearly closed and cylindrical surface than a warp knit, but uses more filament 24 material and is less stable in length. While the weft material might be better if its primary purpose was to support the dope 18, in the present membrane 10 the dope is cast on to a tubular support 12 and the knit is intended to increase the strength of the membrane 10. A warp knit is more rigid or stable in length than a weft knit, and so the warp knit is preferred for use as the reinforcement 12.
