MANUFACTURE OF A FILTRATION MEMBRANE

Abstract

A filtration membrane made by a method that includes: a) selecting and preparing an organic polymer, such as a collodion; b) injecting a collodion into at least one channel of an extrusion die that also comprises an extrusion die core and at least one outlet; c) injecting an internal liquid into a hollow centering pin, the hollow centering pin comprising a channel positioned on the core of the extrusion die and also positioned at an axis of the outlet of the extrusion die; d) applying a holding film to the outlet of the extrusion die; e) unrolling the holding film onto a surface of at least one hollow fiber emerging from the outlet of the extrusion die; f) immersing the hollow fiber with the first holding film in a rinsing solution so as to obtain a flat hollow fiber filtration membrane; and ending the rinsing of the filtration membrane.

Claims

1. A flat filtration membrane, comprising: a first protective support film and a second protective support film, and a plurality of hollow fibers having an inner skin delimiting respectively a plurality of parallel filtration channels, the hollow fibers being formed by an organic polymer, wherein the first and second protective support films are applied on outer walls of the hollow fibers.

2. The filtration membrane according to claim 1, wherein at least one of the first and second protective support film is a nonwoven support.

3. The filtration membrane according to claim 1, wherein at least one of the first and second protective support film is of a porous nature.

4. The filtration membrane according to claim 1, wherein at least one of the first and second protective support film are polypropylene-based.

5. The filtration membrane according to claim 1, wherein the inner skin includes a small width area without channels, so as to ease a folding of the filtration membrane.

6. The filtration membrane according to claim 1, wherein each filtration channel has a diameter comprised between 0.2 and 3 mm.

7. The filtration membrane according to claim 1, having a ratio of the total thickness of the membrane and the channel diameter comprised between 1.6 and 1.8.

8. A flat filtration membrane, comprising: a first protective support film and a second protective support film, a plurality of hollow fibers having an inner skin delimiting respectively a plurality of parallel filtration channels, the hollow fibers being formed by an organic polymer, wherein the plurality of hollow fibers are positioned between the first and second protective support films.

9. A filtration membrane that includes a plurality of channels and is made a method comprising: a) selecting and preparing a collodion, b) injecting the collodion into at least one channel of an extrusion spinneret that further comprises a core and an outlet, c) injecting an internal liquid into a plurality of hollow centering needles, wherein each centering needle comprises a channel positioned at the core of the extrusion spinneret, d) applying a first protective support film at the outlet of the extrusion spinneret, unrolling the first protective support film onto a first face of a flat filtration membrane emerging from the outlet of the extrusion spinneret, the filtration membrane having a plurality of filtration channels, e) applying a second protective support film at the outlet of the extrusion spinneret, unrolling the second protective support film onto a second face of the filtration membrane emerging from the outlet of the extrusion spinneret, f) immersing the filtration membrane with the first and second support films in a rinsing solution, g) ending the rinsing of the filtration membrane.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0045] Other features and advantages of the invention will become apparent upon reading the following description. This is purely illustrative and should be read with reference to the accompanying drawings, in which:

[0046] FIG. 1 shows a schematic example of a spin assembly for implementing a conventional extrusion method,

[0047] FIG. 2 is a front sectional detail view of an extrusion spinneret for implementing a conventional extrusion method,

[0048] FIG. 3 is a front sectional view of an extrusion spinneret for implementing the proposed method,

[0049] FIG. 4 is a side sectional view of the extrusion spinneret shown in FIG. 3, for implementing the method,

[0050] FIG. 5 is a side sectional view of the extrusion spinneret for implementing the method, representing the application of the support films,

[0051] FIG. 6 is a cross-sectional view of a filtration membrane with a single row of channels obtained after implementing the method according to the invention,

[0052] FIG. 7 is a cross-sectional view of a filtration membrane with two rows of channels obtained after implementing the method,

[0053] FIG. 8 is a cross-sectional view of a filtration membrane with three rows of channels obtained after implementing the method,

[0054] FIG. 9 is a cross-sectional view of a filtration membrane with two rows of channels of large diameter, obtained after implementing the method,

[0055] FIG. 10 is a cross-sectional view of a filtration membrane with a single row of channels and a crushed buffer area, obtained after implementing the method.

DETAILED DESCRIPTION OF EMBODIMENTS

[0056] The description of embodiments of the method is given below with reference to some examples.

Example 1Membranes Based on Polyethersulfone (PES)

[0057] A spin assembly as shown in FIG. 1 can be used for the production of conventional filtration membranes by extrusion. Such an assembly comprises a tank of collodion 1 and a tank of internal liquid 2, both connected to an extrusion spinneret 3 that allows forming a nascent fiber. This nascent fiber falls into a precipitation bath 4 and, by means of take-up rollers 4b is, is guided toward a rinsing bay 5 and a bundling machine 6 which is used to roll it. For experimental spinning, the bundling machine 6 is not used and the nascent fiber falls directly into a basin of water extending to the exit from the large roller of the rinsing bay 5, which advances it forward.

[0058] To manufacture conventional membranes based on polyethersulphone (PES), collodion can be prepared from a polymer mixture containing 16% Veradel 3100P polyethersulfone in the presence of 6% polyvinylpyrrolidone K30 in N-methylpyrrolidone, while stirring and at a temperature maintained at 80 C.

[0059] The collodion is then filtered through metal mesh (5 micron filtration rating) and transferred to a storage tank where it is vacuum-degassed and then stored at a temperature of about 50 C.

[0060] The internal precipitation fluid in this example is water that is degassed by ultrasound and then stored prior to use in a tank maintained at, for example, 50 C.

[0061] The hollow fiber is produced with the collodion and internal fluid at a temperature of 50 C. A specific extrusion spinneret is used to produce a fiber having an inner diameter of 0.85 mm and an outer diameter of 1.45 mm. The spinning speed used is 16 m/min.

[0062] FIG. 2 illustrates the flow of the collodion 1 and of the internal liquid 2 within the extrusion spinneret 3 in more detail. The internal liquid 2 is injected into a centering needle 7 in which the channel is positioned at the core of the extrusion spinneret at its axis. The channel of the extrusion spinneret is fed collodion 1 upstream of the centering needle 7 so that the internal liquid 2 is fully covered by the collodion 1 when exiting the extrusion spinneret.

[0063] The precipitation bath 4 which begins the fiber rinse is filled with water maintained at 40 C. The fiber exiting the bath is advanced by a large motorized roller located at the rinsing bay 5 which releases it into a basin filled with water. The water temperature is also maintained at 40 C. in this basin, where the rinsing of the fiber and thus the removal of residual solvent is completed.

[0064] After 24 hours of soaking in water, a long piece of fiber is cut to produce a module containing 12 fibers. The total length of this micromodule is equal to 30 cm, which represents an effective filtration length of 26 cm and a filtration area of 83 cm.sup.2.

[0065] An initial test measurement of the water permeability of the membranes shows that the fibers thus obtained are only slightly permeable to water. In order to regain permeability in these fibers, two solutions are considered:

1) soaking the produced hollow fibers in water at 80 C. for 10 hours,
2) soaking the produced hollow fibers in water at 30 C. containing 500 ppm of NaClO (pH adjusted to 10) for 24 hours.

[0066] The permeability measurements in water brought to 20 C. (Lp20) in a micromodule of fibers treated in this manner are as follows:

TABLE-US-00001 Lp20 (L/h .Math. m.sup.2 .Math. bar @ 20 C.) Fibers rinsed with water at 80 C. for 10 hours 650 Fibers soaked in water at 30 C. with 500 ppm 720 of NaClO (pH 10)

[0067] The rise in the water permeability index observed here is mainly due to elimination of the free polyvinylpyrrolidone trapped between the polymer networks of polyethersulfone.

[0068] With the same collodion 1, it is possible to produce a filtration membrane according to the invention, formed of 33 channels covered with two nonwoven supports having the following characteristics: [0069] nonwoven polypropylene-based support of technical quality, specially processed for use as a membrane support. [0070] support thickness of 60 microns. [0071] a weight index of the support of 17 g/m.sup.2.

[0072] An extrusion spinneret as depicted in FIG. 3 can thus be used in the method of the invention. In this FIG. 3, the frontal section shows the upstream orifices 8 of the extrusion spinneret through which the collodion 1 flows and then traverses an upper portion 9 of the extrusion spinneret. This upper portion 9 is connected to a lower portion 10 by means of screws 11. The internal liquid 2 is injected via centering needles 7 inside the extrusion spinneret, through the side port 12. In this example, 33 centering needles are distributed in a row along the extrusion spinneret, to create a filtration membrane having 33 channels.

[0073] FIG. 4 shows a side sectional view of the arrangement of one of the centering needles 7 at the core of the extrusion spinneret, in the center of the lumen traversing said extrusion spinneret and through which the collodion 1 flows.

[0074] The above precipitation conditions are used. The conditions of the collodion and internal liquid flow rates are fixed so as to produce a membrane with channels 0.85 mm in diameter. Two nonwoven supports (effective width of 35.2 mm and total width of 46 mm) are applied to the nascent multi-bore membrane in order to obtain a total thickness of 1.38 mm as shown in FIG. 6.

[0075] FIG. 5 illustrates the application of the support film onto the nascent fiber, resulting in formation of the filtration membrane according to the method of the invention. The support films 13, in this case the nonwoven materials of this example, unwind and pass through various rollers 14 positioned at the perimeter of the extrusion spinneret and pressing said nonwoven supports 13 onto the outer walls of the nascent fiber exiting the extrusion spinneret. The support film immediately adheres to the nascent fiber flowing from the extrusion spinneret, due to capillary action and the wetting by the solvent used to prepare the collodion.

[0076] The two nonwoven supports serve to protect the fibers forming the channels of the membrane of the invention. The membrane so formed is produced at a spinning speed of roughly 16.5 m/min. This membrane is then advanced by a set of two rollers placed on both sides of the spinneret outlet. The membrane then slides directly into the U of two flattened half-tubes prepared and placed on both sides of the spinneret and which are oriented in the direction in which the membrane exits. This imposes a specific path for a distance that can be varied according to the rate of precipitation of the membrane. In the present case, the path is 3 m, which is sufficient for solidification of the nascent membrane and continuing with the other manufacturing steps. The membrane of the invention is then conveyed to a cutting member, which cuts it into lengths of 1.4 m that fall into a basin containing water where the rinsing continues.

[0077] Samples of these membranes are subjected to the same tests as above and are used to fabricate micromodules containing a single membrane that has an effective length of 26 cm (total length of 30 cm and filtration area of 229 cm.sup.2).

[0078] Permeability measurements at 20 C. are as follows:

TABLE-US-00002 Lp20 (L/h .Math. m.sup.2 .Math. bar @ 20 C.) Untreated NovaMem membrane +/0 NovaMem membrane rinsed with water at 80 C. 1500 for 10 hours Fibers soaked in water at 30 C. with 500 ppm 1640 NaClO (pH 10)

[0079] One can see that the water permeability of the membranes of the invention is higher than that of conventional hollow fibers. One will also note that the conventional hollow fiber, although it has satisfactory mechanical properties (breaking load: 7 N-elongation at break: 45%), requires more care during manipulation than the filtration membrane of the invention which is protected by the nonwoven backing. In addition, the membrane of the invention is considerably more robust yet flexible and is manipulated via the support that surrounds it and protects its outer surfaces. Moreover, the covering of the contact surface of the support film with collodion is controlled so as to form a membrane where its performance is linked to the composition of the collodion and to the process conditions used during spinning, and is no longer dependent on the advancement conditions used, further contributing to maintaining said performance.

[0080] For reference purposes, a 300 DN module of effective filtration length of 1.2 m and a fill factor of 55% provides a filtration area equal to: [0081] 70.5 m.sup.2 with conventional hollow fibers as described above. [0082] 73 m.sup.2 with filtration membranes according to the invention produced as described above.

[0083] In both cases, a ring 5 mm thick is installed on the inner circumference of the housing in order to distance the fibers and membranes from the inner surface of the housing which requires bonding. In the case of the membrane of the invention, the bonding area per membrane is 1.3738 mm. Therefore, only 693 membranes, each individually protected and each containing 33 filter channels, are used to produce this module instead of 22,000 hollow fibers. This allows concluding that a high level of security is achieved with this novel manufacturing method.

[0084] The method thus enables the production of qualitatively more reliable membranes and also the creation of filtration modules that provide a more advantageous filtration area, the created filtration channels being produced in a smaller space.

Example 2Membranes Based on Polyacrylonitrile (PAN)

[0085] To manufacture conventional membranes based on polyacrylonitrile (PAN), a polymer (collodion) mixture is prepared containing 18% polyacrylonitrile in the presence of 2% lithium chloride in N-methylpyrrolidone, while stirring at a temperature maintained at 70 C.

[0086] The collodion is then filtered through a wire mesh (5 micron filtration rating) and transferred to a storage tank where it is vacuum-degassed and then stored at a temperature of 40 C.

[0087] The precipitation liquid is water, degassed by ultrasound and then stored prior to use in a tank maintained at 40 C.

[0088] The hollow fiber is produced with the collodion and internal fluid at a temperature of 40 C. A specific extrusion spinneret is used to produce a fiber having an inner diameter of 0.90 mm and an outer diameter of 1.62 mm.

[0089] The spinning speed used is 18 m/min.

[0090] The external rinsing bath is filled with water maintained at 40 C. The fiber exiting the bath is advanced by a motorized roller which releases it into a basin filled with water. The water temperature is also maintained at 40 C. in this basin, where the rinsing of the fiber and thus the removal of residual solvent is completed.

[0091] After 24 hours of soaking in water, a long piece of fiber is cut to produce a module containing 12 fibers. The total length of this micromodule is equal to 30 cm, which represents an effective filtration length of 26 cm and a filtration area of 91 cm.sup.2.

[0092] Measurement of water permeability in the micromodule yields a constant water permeability index equal to 290 l/h.Math.m.sup.2.Math.bar at 20 C. The fibers obtained in this manner have a breaking load equal to 7.6 N and an elongation at break equal to 45%. Although these mechanical properties of elongation appear satisfactory, the crushing strength of these fibers appears low (feel flexible when touched, and crush quickly).

[0093] With the same collodion, a filtration membrane according to the invention is produced that is formed of 33 channels covered with two nonwoven supports having the following characteristics: [0094] nonwoven polypropylene-based support of technical quality, specially processed for use as a membrane support. [0095] support thickness of 95 microns. [0096] weight index of the support is 34 g/m.sup.2.

[0097] The above precipitation conditions are used. The conditions of the collodion and internal liquid flow rates are fixed so as to produce a membrane with channels 0.90 mm in diameter. Two nonwoven supports (effective width of 37 mm, total of 46 mm) are applied to the nascent multi-bore membrane in order to obtain a total thickness of 1.49 mm. As the mechanical strength of the nascent polyacrylonitrile membrane provides insufficient crushing strength, a thicker nonwoven support of 95 microns is used although it is quite possible to obtain a membrane of the invention having satisfactory mechanical performance with a support only 60 microns thick.

[0098] The two nonwoven supports serve to protect the fibers forming the channels of the filtration membrane of the invention. The membrane so formed is produced at a spinning speed of roughly 18 m/min. This membrane is then advanced as explained in Example 1, to be rinsed and cut into lengths of 1.4 m which fall into a basin containing water where the rinsing continues.

[0099] Samples of these membranes are then collected in order to fabricate a filtration module containing a single membrane that has an effective length of 26 cm (total length of 30 cm) and a filtration area of 243 cm.sup.2.

[0100] As above, the water permeability of the filtration module just produced according to the invention is measured, to find that the water permeability index of the membrane is equal to 850 l/h.Math.m.sup.2.Math.bar at 20 C. One can see here that the difference between the water permeability of the membrane of the invention and of a conventional hollow fiber membrane is greater than with the fibers produced in Example 1. In Example 1 the ratio Lp.sub.Invention/Lp.sub.Fiber was close to 2.3, while in this example the same ratio is close to 2.9.

[0101] This is explained by the fact that the rate of precipitation of the collodion used to produce PES membranes is faster than that of the collodion used to produce PAN membranes. In technical terms, the PAN nascent fiber falls into the rinsing bath when its outer surface has not completely finished gelling, so that the rinsing bath has more influence. In the case of the PES fiber, the outer surface of the nascent fiber is in a more advanced state of gelification. Here, the rinsing bath has less impact on the fiber performance.

[0102] To explain this phenomenon, we can say that the nascent fiber must be produced at a sufficiently slow spinning speed for it to be immersed in the rinsing bath only when its outer structure has solidified. The manufacture of membranes according to the method of the invention allows us to reach this state because it allows us to reduce the thickness of the porous support formed around the filter channels. In the case of hollow fibers, the thickness of the fiber must be fairly high in order to give it sufficient crushing strength. The nonwoven backing we use offers two advantages. Firstly, it protects the membrane surfaces from the handling devices as mentioned above. In addition, it also appears that decreasing the thickness of the membrane obtained according to the method of the invention has a positive impact on the filtration performance of the membrane.

[0103] We can show that a 300 DN module of effective filtration length of 1.2 m and a fill factor of 55% produces a filtration area equal to: 60 m.sup.2 with conventional hollow fibers as described above.

[0104] 68 m.sup.2 with membranes according to the invention produced as described above.

[0105] In both cases, a ring 5 mm thick is installed on the inner circumference of the housing in order to distance the fibers and membranes from the inner surface of the housing which requires bonding. The bonding area of the membrane according to the invention is 1.4940 mm. Therefore 610 membranes containing 33 filter channels are used instead of close to 17,600 conventional fibers.

[0106] On the other hand, in order to produce PAN hollow fibers, the diameter ratio is increased to 1.8 instead of the 1.7 for PES fibers. This gives the PAN fibers satisfactory crushing strength. With the method according to the invention, it is possible to maintain an equivalent ratio (1.62 for the PES membrane and 1.65 for the PAN membrane). For the membrane of the invention, this ratio is the ratio of the total thickness of the membrane and the channel diameter.

Example 3Filtration Modules

[0107] The table below gives the filtration areas that can be provided by a filtration module manufactured according to the invention, having an inner diameter of 300 mm and an effective filtration length equal to 1,200 mm. Three configurations of membranes of the invention are considered:

[0108] 1A membrane 15 as illustrated in FIG. 6, having a single series of thirty-three channels 16 (Series 1) placed between two layers of support film 17 measuring 1.5 mm in total thickness, which is a bonding area of 1.540 mm. This membrane is suitable for all potential cut-offs, from microfiltration to nanofiltration.

[0109] 2A membrane 15 as illustrated in FIG. 7, having two series of thirty channels 16 (Series 2) placed between two layers of support film 17 measuring 2.7 mm in total thickness, which is a bonding area equal to 2.740 mm. This membrane is more suitable for high ultrafiltration, ultrafiltration, or nanofiltration applications.

[0110] 3-. A membrane 15 as illustrated in FIG. 8, having three series of thirty channels 16 (Series 3) placed between two layers of support film 17 measuring 3.7 mm in total thickness, which is a bonding area of 3.742 mm. This membrane is more suitable for ultrafiltration and nanofiltration applications.

[0111] The filtration areas were calculated for two potential fill factors of the module (55 and 60%), while reducing the inner radius of the housing by 5 mm in order to place a centering ring allowing better adhesion of the head plate.

TABLE-US-00003 TABLE Filtration areas provided according to fill factor and number of channels Number of Number of Fill factor membranes per Filtration area * channels (%) module (m.sup.2) Series 1 33 55 605 68 60 660 74 Series 2 60 55 336 68 60 367 75 Series 3 90 55 234 71 60 255 78

[0112] We can immediately see that the filtration area which can be provided with membranes of the invention far exceeds that achievable with conventional hollow fibers of the same inner diameter. The advantage of this geometry is not limited to this aspect. Indeed, the number of membranes used is clearly reduced, and they have fibers that are particularly well-protected. In addition, the time taken to produce the membranes needed to equip the S3 module is less than 20 min, a very short time compared to standard values, despite a linear spinning speed equal to 20 m/min. This is also achieved while fully protecting the integrity of the membrane, since the filtration surface of the membrane and its outer surface do not come into contact with any element that could affect integrity. Similarly, the tools for producing this novel generation of membranes are limited to the members for preparing and feeding the collodion and internal precipitation liquid. Finally, the membrane can be advanced during manufacture by members mounted at its ends and which can securely adhere the two support layers. However, this is not always necessary because the compression of the two supports by the two drive rollers and the fiber covering which constitutes the support film may be sufficient to ensure complete adhesion of the membrane to the supports so applied. Finally, the path of the nascent membrane within the two slides (flattened U-shaped tube) positioned one on each side, forms a rinsing path that maintains membrane integrity better than any known method.

Example 4Filtration Membrane of the Invention Having Channels of Large Diameter

[0113] In the case of hollow fibers of large diameter, the crushing and bursting strength of the fibers requires a substantial thickness. For a fiber having an inner diameter of 2.7 mm, the outer diameter must be at least equal to 5 mm. This creates many difficulties related to the following: [0114] First, the amount of collodion used is quite large: for one m.sup.2 of effective area produced, close to 2.3 liters of collodion will be used, excluding waste. [0115] The nascent fiber is manipulated very carefully, as the fiber quickly bends and may flatten whenever it touches a handling device. This forms a fiber which has an oval shape and unequal thickness, which is very fragile when compressed or crushed. [0116] The outer skin of such a fiber requires using a very low spinning speed in order to be as independent as possible of the rinsing bath composition.
Other difficulties relate to the mechanical strength of the fibers during use. A minimal loss of fiber integrity can easily result in fracture propagation from a starting point of fracture, which affects retention of the products to be stopped by the membrane. In contrast, it is possible to produce a membrane according to the invention equipped with numerous filtering channels having a diameter equal to 2.7 mm and to use a thinner wall. This is possible due to the presence of the support film acting as a protective layer advantageously lined with a separating mesh (here acting as a reinforcement), which gives the membrane the following advantages: [0117] The support layer and its irrigation mesh become a layer of mechanical reinforcement, which reduces the thickness of the membrane wall without any risk of reducing the crushing strength of the membrane. In the present case, a protective layer having a total thickness equal to 0.250 mm is used, of which 0.050 mm is provided by the mesh. [0118] FIG. 9 shows a membrane 15 according to the invention equipped with twenty-two channels 16 that are 2.7 mm in diameter. This membrane is made with a support film 17 having a total thickness of 7.7 mm and a width of 42 mm (bonding area 7.746 mm). To produce one m.sup.2 of this membrane, only 1.1 liters of collodion are used, excluding waste: this is a 52% reduction in the amount of collodion, replaced in part by the support which is formed of less technical material but better reinforces the mechanical strength of the membrane and its stability over time.

[0119] Note that the time required to produce 1 m.sup.2 of membrane according to the invention is at least halved. Although some membrane manufacturers are able to produce up to 16 hollow fibers in parallel, the method of the invention can increase the linear production speed by 50 or even 100%. In addition, the method of the invention ensures that the membrane, and more particularly the hollow fibers thereof, are not damaged by the handling devices, which is not the case with other known manufacturing methods. Finally, the membrane channels formed by the hollow fibers so created are perfectly cylindrical, while individually produced hollow fibers of large diameter are often flattened or given an oval form by their contact with the advancement means. This last detail is very important because such deformed fibers age very badly and fairly quickly end up generating mechanical fractures that propagate, similarly to a welded tube split along its length.

Example 5Filtration Membrane of the Invention with Buffer Zone

[0120] FIG. 10 illustrates a membrane according to the invention, created with an intermediate buffer zone. In this membrane 15, formed for example with one row of channels, fifty channels 16 of 0.9 mm are placed along a total effective width of the support film 17 that is equal to 70.7 mm (which is a bonding area of 1.575 mm). As before, the thickness of this membrane is equal to only 1.5 mm, which allows providing a large filtration area per unit volume.

[0121] This configuration provides an important advantage:

[0122] 1For an effective filtration length equal to 1.2 m, the filtration area for each membrane element is 0.17 m.sup.2.

[0123] 2In a housing which has an inner diameter of 300 mm, it is easy to obtain a total filtration area of between 55 and 60 m.sup.2 depending on the fill factor applied (respectively 55 and 60%).

[0124] 3Although some of the available filtration area is lost per module, the produced membranes are fitted into the module more quickly (+50%), and production is managed with a more compact and more productive tool.

[0125] 4To produce a module providing 60 m.sup.2 of filtration area, only 352 properly protected membranes are manipulated rather than 19,200 self-supporting, fragile hollow fibers that require more collodion.

[0126] Many opportunities therefore exist for producing diverse forms, each providing specific advantages. The main and key advantage of the method of the invention lies in the fact that it ensures production of filtration membranes in the manner that best provides the appropriate performance. This concept also allows producing membranes which have unmatched hydraulic and mechanical properties.