TUBULAR MEMBRANE COMPRISING LONGITUDINAL RIDGES, DEVICE PROVIDED THEREWITH AND METHOD FOR PRODUCING SUCH MEMBRANE

Abstract

A tubular membrane, a membrane module, a device including a number of such membranes, and a method for manufacturing such membranes. The tubular membrane includes a tubular base providing a support and having an inner and outer surface, where the tubular base defines a lumen for the feed flow, and a membrane layer provided on the inner surface of the tubular base, where the inner surface of the tubular membrane includes a number of inwardly projecting ridges that extend in a substantially longitudinal direction of the tubular membrane.

Claims

1. A tubular membrane comprising: a tubular base providing a support and having an inner and outer surface, wherein the tubular base defines a lumen for the feed flow; and a membrane layer that is provided on the inner surface of the tubular base, wherein the inner surface of the tubular membrane comprises a number of inwardly projecting ridges that extend in a substantially longitudinal direction of the tubular membrane.

2. The tubular membrane according to claim 1, wherein the ridges comprise membrane material.

3. The tubular membrane according to claim 1, wherein the inner surface of the tubular membrane comprises more than one inwardly projecting longitudinal extending ridge.

4. The tubular membrane according to claim 3, wherein the inner surface of the tubular membrane comprises more than 4 said ridges.

5. The tubular membrane according to claim 1, wherein the ridges comprise a cross section that is perpendicular to the average feed flow direction through the lumen, wherein the cross section has a non-circular shape, such as an ellipse.

6. The tubular membrane according to claim 1, wherein the ridge or ridges have an average height in the range of 50-2000 μm.

7. The tubular membrane according to claim 1, wherein the ridge or ridges have a width in the range of 0.1-10 mm.

8. The tubular membrane according to claim 1, wherein a ridge height of the ridge or ridges is related to the membrane inner diameter such that the ridge height is in the range of 0.4-40% of the membrane inner diameter.

9. The tubular membrane according to claim 1, wherein a ridge width of the ridge or ridges is related to the membrane inner circumference such that the ridge width is in the range of 0.2-63.7% of the membrane inner circumference.

10. A membrane module comprising a number of the tubular membranes according to claim 1.

11. A device for filtering a fluid, the device comprising a number of the membrane modules and/or tubular membranes according to claim 1.

12. A method for producing a the tubular membrane according to claim 1, comprising : providing a tubular base; and providing a membrane structuring tool configured for providing the membrane layer material to the inner surface of the tubular base, wherein the structuring tool is configured to provide a number of inwardly projecting ridges on the inner surface of the tubular base that extend in a substantially longitudinal direction of the tubular membrane.

13. The method according to claim 12, further comprising the step of moving the structuring tool such that the ridges are provided on the inner surface.

14. The method according to claim 12, wherein the structuring tool comprises a number of grooves configured for providing membrane material to the inner surface.

Description

[0035] Further advantages, features and details of the invention are elucidated on the bases of preferred embodiments thereof, wherein reference is made to the accompanying drawings, in which:

[0036] FIG. 1A schematically shows a tubular membrane of the embodiment of the invention;

[0037] FIG. 1B shows a cross section of the tubular membrane of FIG. 1a;

[0038] FIG. 1C schematically shows a device with a number of tubular membranes of FIG. 1A;

[0039] FIG. 2A shows a detailed cross section of the ridge of the tubular membrane of FIG. 1A;

[0040] FIG. 2B shows a detailed cross section of the surface area between two ridges in the tubular member of FIG. 1A; and

[0041] FIG. 3A-B shows experimental results with a tubular membrane according to the invention.

[0042] Tubular membrane 2 (FIG. 1A) has a length L, an inner diameter D.sub.in, and an outer diameter D.sub.out. Furthermore, tubular membrane 2 has outer wall 4 and inner wall 6. Outer wall 4 is defined by outer layer 8 that in the illustrated embodiment comprises a non-woven material. The non-woven material optionally comprises PET, PBT, PP, PE, PA, PAN or combinations thereof. The tubular member cross section is substantially circular shaped, although other shapes such as oval or ellipse shapes can be envisaged. It will be understood that other dimensions for tubular membrane 2 and/or its parts can also be envisaged in accordance with the present invention.

[0043] Inner wall 6 comprises polymer membrane material 10. The polymer membrane material preferably comprises one or more of polyethersulfone (PES), polysulfone (PSf), polyphenylsulfone (PPSU), polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyethylene (PE), polypropylene (PP) and combinations thereof. Part of the tubular base is intruded with polymer membrane material defining a transition region 12.

[0044] The inner wall 6 of tubular membrane 2 comprises a number of longitudinal ridges 14. In the illustrated embodiment tubular membrane 2 comprises eight ridges 14 that extend substantially parallel two central axes 16 of tubular membrane 2.

[0045] Ridge 14 (FIG. 1B) has height H and width W. In the illustrated embodiment height H is in the range of 200-350 μm and width W is in the range of 1.5-4 mm. Preferably all ridges have a height H and width W within this range.

[0046] Device 18 (FIG. 1C) comprises a bundle 20 of tubular membranes 2 in a holder or housing 22. This enables feed flow to enter the lumen side of bundle 20. On the shell side the permeate is collected and is conducted from the system through permeate ports. It will be understood that the skilled person could envisage different embodiments of device 18 comprising a number of tubular membranes 2.

[0047] Ridge 14 (FIG. 2A) has asymmetric pore structure 24 with a small surface pores on the lumen side 26 that is positioned towards the center of tubular membrane 2 as seen in a cross section, and a large pores area 28 that is close to and/or attached to tubular base 8. In the illustrated embodiment the total (average) height H of ridge 14 is in the range of 250-350 μm. The membrane surface area between adjacent ridges 14 has a height that is much smaller (FIG. 2B). In fact, the thickness of this membrane material layer is about 40-60 μm.

[0048] Tubular membrane 2 is provided by providing tubular base 8. In the illustrated embodiment tubular base 8 is provided by helically winding porous material, preferably a non-woven material, and sealing or welding the overlap between adjacent strips together. In a next step, membrane material is cast and doctored onto the inner surface using a structuring tool that is provided with grooves such that ridges 14 are formed.

[0049] Tests have been performed with tubular membrane 2. In the tests an unfiltered apple juice fluid is used to determine the effect of fouling. Tubular membrane 2 is compared to conventional membranes without ridges. Tests have been performed at a TMP of 1 bar, with tubular membrane 2 having eight ridges, The flux is defined as the permeate volume collected in a defined time interval through a defined surface area of membrane. Flux/TMP is determined in L/m.sup.2/h/bar at cross flow velocity ranging from 1-4 m/s. Results with a first membrane are shown in FIG. 3A and results with a second membrane are shown in FIG. 3B. The tubular membrane with longitudinal ridges shows results (.square-solid.) that are substantially higher as compared to the reference membrane (.box-tangle-solidup.). This achieves a significant flux increase (∘, in %). This flux increase is achieved by enlargement of the effective membrane surface area and by the increased turbulence by the ridges acting as turbulence enhancers. These combined effects are synergetic and are surprisingly higher as would be expected from the membrane surface enlargement. Therefore, the performance of tubular membrane 2 according to the invention is even better as would be expected, thereby improving the possibilities for its industrial application. These possibilities are even further enhanced by the improved cleaning possibilities.

EXAMPLE 1

Experimental Results

[0050] The tubular membranes according to the invention have been tested in a leachate treatment MBR plant at a landfill. The purpose of the test was to compare the tubular membranes according to the invention with reference membranes, which in this are tubular flow membranes that do not have ridges (i.e. having a flat membrane wall). For the purpose the test, the membranes according to the invention have been labelled as Longitudinally-ridged membranes, whereas the reference membranes having a flat membrane wall have, been labelled as Reference membranes.

[0051] The design inflow to the test plant is 1.8 minimal liquid discharge (MLD), though it has actually been treating peak loads of 2.2 minimal liquid discharge (MLD). The process is a classical BIOMEMBRAT® with a pressurised bioreactor tank operating at a hydraulic retention time (HRT) of 15 hours, a solid retention time (SRT) of 53 days and an aeration rate of 4000 Nm.sup.3/h. Following flow balancing, the leachate is treated in two parallel lines, each consisting of a denitrification and two nitrification tanks. The sludge from each line is pumped into two ultrafiltration (UF) plants comprising three streams of 6 modules in series, the permeate being directly discharged.

[0052] Modules with the longitudinally-ridged and reference membranes were prepared. In order to provide similar test conditions, the longitudinally-ridged membranes and the reference membranes were placed in two different parallel loops, which are fed from the same bioreactor to provide similar test conditions.

[0053] Each module was a type 83G module having a 20.32 cm (8″) diameter and a module length of 3 meter, Each membrane in the module was 8 mm in diameter, which led to a total membrane area of 27.2 m.sup.2 in each module.

[0054] During testing, the following test parameters were applied: [0055] Filtration type; continuous, feed and bleed, bottom to top; [0056] Crossflow velocity (CFV): 4 m/s; [0057] Transmembrane pressure (TMP): 2.0-2.2 bar; [0058] Temperature 25-30° C.; [0059] Feed mixed liquor suspended solids (MLSS): 21-23 g/L; [0060] Inlet chemical oxygen demand (COD): 1500-2300 mg/L.

[0061] The test results shows that the COD-removal of the tubular membranes according to the invention achieved 85%-87%, whereas the total suspended solids (TSS) was below 75 mg/l. Moreover, the tubular membranes according to the invention, due to the straight longitudinally extending ridges, achieved a significantly higher flow rate than the reference membranes with no ridges. This is also shown in graph 1 provided below.

[0062] The present invention is by no means limited to the above described preferred embodiments thereof. The rights sought are described in the following claims, wherein the scope of which many modifications can be envisaged.