WO2013173872A1

WO2013173872A1 - Tubular membrane with helix

Info

Publication number: WO2013173872A1
Application number: PCT/AU2013/000535
Authority: WO
Inventors: Gilbert Erskine
Original assignee: Advanced Metallurgical Solutions Pty Ltd
Priority date: 2012-05-22
Filing date: 2013-05-21
Publication date: 2013-11-28

Abstract

A metallic tubular membrane with a helical groove or ridge.

Description

Tubular membrane with helix

The invention is described in the following statement: Field of the Invention

[0001] The present invention relates to metallic tubular membranes and, in particular, metallic tubular membranes with an internal or external helical groove or ridge.

Background

[0002] Tubular membranes are used in numerous industries to filter and separate particulates from fluid and gas. The membranes can be constructed from various materials depending on their application, including plastic mesh, fine plastic tubes, porcelain or stainless steel mesh.

[0003] Many currently available tubular membranes, such as the conventional fine plastic tubes, are prone to blockage. The configuration of the fine plastic tubes means that any blockage can result in putrefaction of the impurities which in turn leads to a reduction in the permeate quality due to the impartation of undesirable flavour characteristics.

[0004] Recently, stainless steel mesh has been put forward as a replacement to conventional filters. The advantage with this material is that it is easy to clean and more robust than porcelain which can have a tendency to shatter under high pressure.

However, present methods of producing stainless steel filters suffer from a number of drawbacks, including the fact that it is difficult to produce a pore size within the mesh to adequately filter small particles. Furthermore, it is difficult to produce a mesh with evenly spaced pores, which can limit the effective open area of the mesh.

[0005] Membranes, or indeed any other type of filtration media, are purely barriers to prevent the movement of particulates such as detritus and bacteria. In theory, a membrane with single channel pore would be an ideal filter. This is not, however, commercially viable. What actually occurs in filters, such as porcelain and metal filters, is that the fluid is forced along a torturous path from the retentate side of the membrane to the permeate side. In the process particulate material and bacteria is filtered out of the liquid. This has several disadvantages, for instance since there is a higher

transmembrane pressure drop, there is risk of permanent plugging from particulates being trapped within the membrane itself which makes it harder to clean. [0006] In order to minimize these disadvantages and reduce the effects of permanent plugging, manufactures have attempted to perfect the use of a thin layer on the inside or outside of the filter wall. These filters include an outer support tube produced with varying grades of metallic powder. This outer tube is fired and a thin coat is applied to either the internal or external surface using a much finer powder and the filter is then re-fired. One of the problems is that the layers can tend to laminate or separate due to the two step firing process.

[0007] Metallic tubular membranes, and in particular multilayered metallic tubular membranes, have overcome many of these aforementioned problems wherein the membrane includes a plurality of apertures extending there through, and at least some of said apertures increase in cross-sectional area from a first surface of the membrane to a second surface of the membrane.

[0008] Metallic tubular membranes are used in a variety of industries for the separation of particulates in liquid or gas and have a number of advantages over plastic tubular membranes. Metallic tubular membranes are robust and, depending on the metal used, can withstand temperatures up to 900^°C, have a high pressure tolerance and can tolerate highly corrosive environments.

[0009] WO 2008/064390 and WO 2008/064391 describes various multilayered metallic tubular membranes which are produced by building the membrane from the inside out, with powders having particles sizes that gradually increase thereby forming an aperture matrix wherein the cross-sectional area of the apertures increase as the apertures extend from the inside surface of the tube to the outside surface. This reduces the risk of plugging, which in turn reduces power input to operate the filtering machine where the metallic filter membrane is housed. WO 2008/064390 describes methods of production of metallic tubular membranes wherein the metal powder is loose gravity filled into a mould which has a solid mandrel and an elastomer outer. Once filled, the mould is then placed into an isostatic press and compressed under pressure up to 60,000 psi. The resultant green compact, as it is referred to, is then sintered in a furnace having an inert

atmosphere. This method produces a membrane with a substantially symmetric cross- sectional profile which suffers from similar permanent plugging issues as porcelain filters.

[0010] Metallic tubular membranes are often arranged into tubular modules, wherein a plurality of tubes, arranged in parallel, are contained within a sealed container or outer housing. The outer tube or housing often receives the permeate. The tubes are joined together via one or more tube face plates at one or more ends of the membrane module. This arrangement has the advantage of compacting the tubular membranes into a defined space resulting in significant capital and running cost savings. WO2012/009762 describes methods to produce membrane modules wherein the membranes have diameters between 1 mm and 20mm, which are welded onto face plates to form membrane modules.

[001 1] The performance of tubular membranes is limited by the efficiency of the liquid coming into contact with the surface of the membrane. The greater the fluid coming into contact with the surface of the membrane, the greater the membrane's efficiency.

Furthermore, the performance of tubular membranes is limited by the ability of the system to move the particles being filtered away from the membrane to prevent clogging of the membrane. Particulate matter has the potential to build up on the surface of the membrane and, overtime, reduces the flux by an order of up to 10. The performance of a tubular membrane is often significantly impaired by the generation of a gel layer on the surface of the membrane. A gel layer is a non moving layer on the surface of any tube carrying a fluid. In order to reduce this, manufacturers pump the liquid to be filtered at high velocities through the lumen which generates turbulence and shear which helps to disrupt the particles on the filter surface. The disadvantage with this method is that this consumes high amounts of power, given that large pumps are required and the friction heats the liquid which can damage the liquid product. In addition, shear forces can damage the liquid product directly.

[0012] There exists a need in the art for a tubular membrane which overcomes some or all of the deficiencies in the art. It is an object of the invention to overcome some or all of the deficiencies in the art.

Summary of the Invention

[0013] In a first aspect, the invention is a tubular membrane adapted to filter a liquid, comprising a plurality of apertures extending there through, wherein the membrane comprises a helical groove or ridge on its surface adapted to create a helical flow of the liquid passing through the tubular membrane.

[0014] Preferably, the membrane comprises a helical groove. Alternatively, the membrane comprises a helical ridge.

[0015] Preferably, the helical groove or ridge is on the internal surface of the tubular membrane. Alternatively, the helical groove or ridge is on the external surface of the tubular membrane.

[0016] Preferably, the helical groove or ridge is integral to the tubular membrane. More preferably, the membrane does not comprise a separate helical structure within the tubular membrane. For example, the membrane does not comprise a separate helical structure within the tubular membrane as described in US Patent Serial No. 5,628,909. [0017] Preferably, the helical groove or ridge runs continuous between both ends of the tubular membrane.

[0018] Preferably, the helical groove or ridge has a part-circular cross-sectional shape. Alternatively, the helical groove or ridge has a semi-circular cross-sectional shape.

Alternatively, the helical groove or ridge has a square cross-sectional shape.

Alternatively, the helical groove or ridge has a rectangular cross-sectional shape.

[0019] Preferably, the helical groove has a helix pitch P, a groove gap G, a channel width W, a channel depth D.

[0020] Preferably, the helical ridge has a helix pitch P, a ridge gap E, a ridge width F, a ridge height of H.

[0021 ] The helix pitch P for the groove, groove gap G, a channel width W and a channel depth D are dependent on the fluid and its viscosity, the internal diameter of the membrane and the speed at which the fluid needs to be filtered.

[0022] The helix pitch P for the groove, ridge gap E, ridge width F and a ridge height H are dependent on the fluid and its viscosity, the internal diameter of the membrane and the speed at which the fluid needs to be filtered.

[0023] The helix pitch is the angle that the groove or ridge makes against the longitudinal axis of the membrane (the longitudinal direction of the flow of the fluid). The closer the helix pitch is to the longitudinal axis (the lower the pitch is to 0°; with parallel to the longitudinal axis of the membrane being 0°), the lower the centrifugal motion and force on the liquid (the liquid is twisting at a slower rate; although the laminar flow of the fluid will be at a higher rate). The higher the pitch and the closer the pitch is to the perpendicular line of the longitudinal axis of the membrane (that is, 90°) the higher the centrifugal motion and force on the liquid (the liquid is twisting at a high rate). The higher the helix pitch angle, the higher the rate of twisting of the fluid and the lower the laminar flow rate of the fluid.

[0024] The groove gap G, is the distance between the crests of the helix lines.

[0025] The channel width W, is the width of the particular groove line; the width of the impression in the surface of the membrane which makes the groove.

[0026] The channel depth D, is the depth of the particular groove line; the depth of the impression in the surface of the membrane which makes the groove.

[0027] The ridge gap E, is the distance between the crests of the helix lines of the ridge. [0028] The ridge width F, is the width of the particular ridge line; the width of ridge in the surface of the membrane.

[0029] The ridge height H, is the height of the particular ridge line; the depth of the ridge in the surface of the membrane which makes the ridge.

[0030] Preferably, the helix pitch is selected from the group consisting of: between 0° and 90°; between 10° and 80°; between 20° and 70°; between 30° and 60°; between 40° and 50°; and 45°

[0031] Preferably, the groove gap G or ridge gap E is selected from the group selected from: at least 10 times smaller than the internal diameter of the tubular membrane;

between 10 and 60 times smaller than the internal diameter of the tubular membrane; between 30 and 50 times smaller than the internal diameter of the tubular membrane.

[0032] Preferably, the groove gap G or ridge gap E is selected from the group selected from: between 0.01 mm and 500mm; between 0.01 mm and 250mm; between 1 mm and 100mm; between 1 mm and 10mm.

[0033] Preferably, the channel width W or ridge width F is no greater than three times the depth of the groove or height of the ridge.

[0034] Preferably, the channel width W or ridge width F is selected from the group selected from: between 0.001 m and 10mm; between 0.01 mm and 5mm; between 0.1 mm and 1 mm.

[0035] Preferably, the channel depth D or ridge height H is selected from the group selected from: between 0.001 m and 10mm; between 0.01 mm and 5mm; between 0.1 mm and 1 mm.

[0036] In another preferred embodiment;

• the internal diameter of the tubular membrane is 20mm;

• the helix pitch P is 25°;

• the groove gap G is 20mm;

• channel width W is 1 mm; and

• channel depth D is 1 mm.

[0037] In another preferred embodiment; the internal diameter of the tubular membrane is 20mm;

the helix pitch P is 25°;

the ridge gap E is 20mm; • ridge width F is 1 mm; and

• ridge height H is 1 mm,

[0038] Preferably, the tubular membrane comprises a second helical groove or ridge running in parallel with the first helical groove or ridge. Preferably, the tubular membranes comprises a plurality of helical grooves or ridges running in parallel with the first helical groove or ridge. In one embodiment, the tubular membrane comprises both grooves and ridges.

[0039] Preferably, the liquid to be filtered is passed from one axial end along the passage defined by the tubular membrane in a helical flow motion.

[0040] Preferably, the tubular membrane utilises helical flow to induce centrifugal forces to improve flushing of the membrane surface which at the same time maintains substantially laminar flow to prevent damage to the particular components in the fluid being filtered.

[0041 ] Preferably, the tubular membrane is adapted to break down gel layers that form on the surface of the membrane when in operation.

[0042] Preferably, the tubular membrane is metallic.

[0043] Preferably, the tubular membrane is in the form of a cylinder.

[0044] Preferably, the tubular membrane has a wall thickness selected from the group consisting of: between 1 and 10mm; between 2 and 8mm; 5mm.

[0045] Preferably, the internal diameter of the tubular membrane is selected from the group consisting of: between 1 and 500mm; between 2 and 400mm; between 4 and 300mm; between 10 and 20mm.

[0046] Preferably, the tubular membrane has an aperture size selected from the group consisting of between 1 nm and 1 mm; between 700nm to 900nm.

[0047] Preferably, the tubular membrane is comprised of 316 stainless steel.

[0048] Preferably, the tubular membrane comprises a multilayered filter membrane.

[0049] Preferably, at least some of the said apertures increase in cross-sectional area from a first surface of the membrane to a second surface of the membrane.

[0050] In a second aspect, the invention comprises a tubular membrane module comprising a plurality of metallic tubular membranes, at least one metallic face plate adapted to receive and support said membranes, and housing for the tubular membrane module adapted to receive the permeate from the tubular membranes. [0051 ] In a third aspect, the invention comprises an extrusion head used for the production of a tubular membrane using powder metallurgy techniques, wherein said extrusion head has at least one internal helix grove, wherein the extrusion head comprises a helix on the inner or outer component of the dye.

[0052] In a fourth aspect, the invention comprises a method for producing a tubular membrane as described herein using powder metallurgy techniques.

[0053] In one example, the membrane is produced according to the methods widely available in the art. In one embodiment, the membrane is produced according to some or all of the methods described in WO 2008/064390 and WO 2008/064391.

[0054] For example, the membranes are manufactured using the following method: a) Metal powders of various sieve sizes (depending on micron finish required) are mixed with various binders and are either heated or cooled depending on the binder selected; b) The mix is then extruded using specifically designed die heads (single or multi head dies) of various diameters from 3mm to 20mm or greater; c) As the membrane is extruded from the die head, the material is cured using a heating or cooling source, again depending on the binder selected (typically either hot air, induction heating or a cooling media); d) The membranes are cut to length and by the known art of sintering, are sintered; e) An inner coating is applied to the membranes either in a high vacuum furnace or a low hydrogen continuous furnace; and f) The membranes are re-sintered.

[0055] Preferably, the membrane is produced wherein at least one layer of porous metallic material is built upon a cylindrical forming member (which is subsequently removed to leave the membrane) which comprises an external helix ridge on the surface of the forming member (to form an internal groove or ridge on the tubular membrane) which is integral to the forming member (and will be integral to the tubular membrane). The resultant membrane includes a plurality of apertures extending there through, and at least some of said apertures increase in cross-sectional area from a first surface of the membrane to a second surface of the membrane.

[0056] For example, the cylindrical forming member is produced by extrusion equipment using a feedstock made from metal powder waxes and polyethylene or any other feed stock suitable for extrusion or injection moulding. For example, the feedstock is a metallic particulate and the metallic particulate comprises a base material such as N-metal, priodyne, ethylene, glycol or similar and further including a metal base power, such as but not limited to, stainless steel, tungsten, silica, boron, cobalt, chromium, nickel and/or silver nitride. More preferably, the mixture is blended into a homogenous consistency at a constant temperature. Preferably, the mixture is heated to a temperature ranging from 38°C to 1 10°C and constantly stirred for a period of between 2 and 24 hours.

[0057] The extrusion head for the forming member comprises a helix groove or ridge on the inner male part of the dye. As the forming member is produced, a puller grips the forming member and pulls it at a constant speed, during which time it rotates through 360°. The speed of the rotation influences the helix pitch.

[0058] The forming member is then subject to a chemical de-binding step to remove the wax. Preferably, the de binding of the binding agents is performed using controlled heat and ramp rates in an oxygen atmosphere. Then, the forming member is placed in a furnace to remove the polyethylene. Then the item is loaded into a hydrogen furnace and sintered.

[0059] Next at least one layer of membrane is built upon the forming member such that, when the forming member is removed, the resultant membrane includes a plurality of apertures extending there through and at least some of said apertures increase in cross- sectional area from a first surface of the membrane to a second surface of the

membrane. Preferably, at least one of said layers is formed by partially submersing the forming member into a mixture containing, in part, metallic particulate. Preferably, the mixture includes a base material such as N-metal, priodyne, ethylene, glycol or similar and further including a metal base powder, such as but not limited to, stainless steel, tungsten, silica, boron, cobalt, chromium, nickel, and/or silver nitride. Preferably, the mixture is blended into homogenous consistency at a constant temperature. Preferably, the mixture includes a methanol based solution. Preferably, the methanol based solution includes 1000ml of denatured alcohol, 10 grams to 20 grams of Teflon, 7 grams of wax, 2ml to 9ml of glycerine and 2m to 7ml of polyethylene glycol, which is mixed with a metallic powder to produce a mixture having a paint-like constancy.

[0060] Preferably, the forming member is immersed in a sequence of different mixtures to form a plurality of layers wherein the different mixtures contain particles of different grain size and wherein the different mixtures contain materials having different melting points.

[0061 ] Preferably, the method further comprises the steps of: a. providing a rod with an external helix protruding on the surface of the rod, which is adapted to act as a forming member onto which the membrane is built; b. immersing at least a portion of the rod in a first mixture containing a first particulate and a methanol based solution, whereby a first layer is formed; c. removing the rod from the first mixture and allowing it to drip in a controlled atmosphere for a first pre-determined period of time; d. immersing the rod into a liquid and allowing the rod to stand for a second pre-determined period of time; e. allowing the rod to stand for a third predetermined period of time; f. drying the rod; g. placing the rod into a sheath such that a the rod is separated from the sheath by a cavity; h. filling the cavity with a second mixture containing a second particulate; i. sealing the ends of the sheath; j. placing the sheathed rod into a press and applying a known pressure to the rod and coating thereby forming a cast; k. separating the cast from the rod; and I. treatment of the cast to thereby form a metallic filter membrane.

[0062] Preferably, the treatment involves sintering the cast in a furnace.

[0063] Preferably, steps b. to f. are repeated at least twice such that the rod is sequentially immersed into a plurality of mixtures containing particulates of different diameters.

[0064] Following sintering, solid ends are fitted to the ends of the membrane.

[0065] In a fifth aspect, the invention comprises a metallic tubular membrane

substantially as herein described with reference to examples.

[0066] In a sixth aspect, the invention comprises a metallic tubular membrane module substantially as herein described with reference to examples.

[0067] In a seventh aspect, the invention comprises a method for producing a metallic tubular membrane substantially as herein described with reference to examples.

[0068] The invention described herein has application for the filtration of fluids. The invention can have application in one or more of the following industries; mining, oil and gas, refining, light and heavy manufacturing, food and wine processing and

manufacturing, water purification and management and agricultural industries.

[0069] Other aspects and advantages of the invention will become apparent to those skilled in the art from a review of the ensuing description.

Brief Description of the Drawings

[0070] Figure 1 : Provides a graphical representation of the extruder barrel and tubing die used to produce a tubular membrane with an internal helical ridge.

[0071 ] Figure 2: Provides a graphical representation of a tubular membrane with an internal helical ridge, which is the sintered product of extruded from the tubing die of Figure 1 . ₉ 35

Detailed Description of the Invention

General

[0072] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[0073] The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein.

[0074] The invention described herein may include one or more ranges of values (e.g. size, concentration etc). A range of values will be understood to include all values within the range, including the values defining the range, and values adjacent to the range that lead to the same or substantially the same outcome as the values immediately adjacent to that value which defines the boundary to the range.

[0075] The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference. Inclusion does not constitute an admission is made that any of the references constitute prior art or are part of the common general knowledge of those working in the field to which this invention relates.

[0076] The disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein does not constitute an admission that any of the references constitute prior art or are part of the common general knowledge of those working in the field to which this invention relates.

[0077] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations, such as "comprises" or "comprising" will be understood to imply the inclusion of a stated integer, or group of integers, but not the exclusion of any other integers or group of integers. It is also noted that in this disclosure, and particularly in the claims and/or paragraphs, terms such as "comprises", "comprised", "comprising" and the like can have the meaning attributed to it in US Patent law; e.g., they can mean

"includes", "included", "including", and the like. [0078] The present invention will now be described with reference to the following non- limiting Examples. The description of the Examples is in no way limiting on the preceding paragraphs of this specification, but is provided for exemplification of the methods and compositions of the invention.

Examples

[0079] It will be apparent to persons skilled in the materials and arts that numerous enhancements and modifications can be made to the above described processes without departing from the basic inventive concepts. All such modifications and enhancements are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims. Furthermore, the following Examples are provided for illustrative purposes only, and are not intended to limit the scope of the processes or compositions of the invention.

EXAMPLE 1

[0080] Figure 1 provides a graphical representation of a tubing die used to produce a tubular membrane with an internal helical ridge. The spiral spigot (C) comprises a die (A), a tip (B) and a die land (E).

[0081] The tip B comprises a helical groove, which will produce an internal helical ridge on the internal surface of the tubular membrane.

[0082] The tubing die further comprises a die land E.

[0083] Figure 2 provides a detailed graphical representation of a cross section of a sintered tubular membrane produced from extruded product from the die presented in Figure 1.

[0084] The tubular membrane has an external diameter of D.

[0085] The tubular membrane has an internal diameter including the height of the ridge (ID) of D/1.5.

[0086] The width (WT) of the tubular membrane is D/10. [0087] The width of the helical ridge (A) is D/10. [0088] The diagonal of the ridge (B) is D/50. [0089] The number of ridges (No. of RIBS) is D/2.5. [0090] The pitch is D x 25. [0091] The tubular membrane (Figure 2) is produced by extrusion equipment using a feedstock made from metal powder waxes and polyethylene or any other feed stock suitable for extrusion or injection moulding. The feedstock is a metallic particulate and the metallic particulate comprises a base material such as N-metal, priodyne, ethylene, glycol or similar an further including a metal base power, such as but not limited to, stainless steel, tungsten, silica, boron, cobalt, chromium, nickel and/or silver nitride. The mixture is blended into a homogenous consistency at a constant temperature. The mixture is heated to a temperature ranging from 38°C to 1 10°C and constantly stirred for a period of between 2 and 24 hours.

[0092] The extrusion head (Figure 1 ) for the tubular membrane comprises a helix ridge on the inner male part of the dye. As the membrane is produced, a puller grips the membrane and pulls it at a constant speed, during which time it rotates through 360°. The speed of the rotation influences the helix pitch.

[0093] The membrane produced with the internal helical ridge is sintered or combined with a plurality of membranes and a supporting end and sintered as a single module.

[0094] The resultant membrane includes a plurality of apertures extending there through and at least some of said apertures increase in cross-sectional area from a first surface of the membrane to a second surface of the membrane.

[0095] A metallic mixture is added to the internal surface of the membrane to form a metallic layer. For example, the mixture includes a base material such as N-metal, priodyne, ethylene, glycol or similar and further including a metal base powder, such as but not limited to, stainless steel, tungsten, silica, boron, cobalt, chromium, nickel, and/or silver nitride. The mixture is blended into homogenous consistency at a constant temperature and re-sintered.

Claims

CLAIMS:

1. A tubular membrane adapted to filter a liquid, comprising a plurality of apertures extending there through, wherein the membrane comprises a helical groove or ridge on its surface adapted to create a helical flow of the liquid passing through the tubular membrane.

2. A tubular membrane of claim 1 , wherein the helical groove or ridge is on the

internal surface of the tubular membrane.

3. A tubular membrane of claims 1 or 2, wherein the helical groove or ridge runs continuous between both ends of the tubular membrane.

4. A tubular membrane of any one of the preceding claims, wherein the helical groove or ridge is integral to the tubular membrane.

5. A tubular membrane of any one of the preceding claims, wherein the helical groove has a helix pitch P, a groove gap G, a channel width W, a channel depth D.

6. A tubular membrane of any one of claims 1 to 4, wherein the helical ridge has a helix pitch P, a ridge gap E, a ridge width F and a ridge height of H.

7. A tubular membrane of claims 5 or 6, wherein the helix pitch P is selected from the group consisting of: between 0° and 90°; between 10° and 80°; between 20° and 70°; between 30° and 60°; between 40° and 50°; and 45°

8. A tubular membrane of claim 5, wherein the groove gap G selected from the

group selected from: at least 10 times smaller than the internal diameter of the tubular membrane; between 10 and 60 times smaller than the internal diameter of the tubular membrane; between 30 and 50 times smaller than the internal diameter of the tubular membrane.

9. A tubular membrane of claim 5, wherein the groove gap G is selected from the group selected from: between 0.01 mm and 500mm; between 0.01 mm and 250mm; between 1 mm and 100mm; between 1 mm and 10mm.

10. A tubular membrane of claim 5, wherein the channel width W is no greater than three times the depth of the groove.

11 . A tubular membrane of claim 5, wherein the channel width W is selected from the group selected from: between 0.001 m and 10mm; between 0.01 mm and 5mm; between 0.1 mm and 1 mm.

12. A tubular membrane of claim 5, wherein the channel depth D is selected from the group selected from: between 0.001 m and 10mm; between 0.01 mm and 5mm; between 0.1 mm and 1 mm.

13. A tubular membrane of claim 6, wherein ridge gap E selected from the group selected from: at least 10 times smaller than the internal diameter of the tubular membrane; between 10 and 60 times smaller than the internal diameter of the tubular membrane; between 30 and 50 times smaller than the internal diameter of the tubular membrane.

14. A tubular membrane of claim 6, wherein the ridge gap E is selected from the

group selected from: between 0.01 mm and 500mm; between 0.01 mm and 250mm; between 1 mm and 100mm; between 1 mm and 10mm.

15. A tubular membrane of claim 6, wherein the ridge width F is no greater than three times the height of the ridge

16. A tubular membrane of claim 6, wherein the ridge width F is selected from the group selected from: between 0.001 m and 10mm; between 0.01 mm and 5mm; between 0.1 mm and 1 mm.

17. A tubular membrane of claim 6, wherein the ridge height H is selected from the group selected from: between 0.001 m and 10mm; between 0.01 mm and 5mm; between 0.1 mm and 1 mm.

18. A tubular membrane of any one of the preceding claims, wherein the liquid to be filtered is passed from one axial end along the passage defined by the tubular membrane in a helical flow motion.

19. A tubular membrane of any one of the preceding claims, which utilises helical flow to induce centrifugal forces to improve flushing of the membrane surface while at the same time maintaining substantially laminar flow to prevent damage to the particular components in the fluid being filtered.

20. A tubular membrane of any one of the preceding claims, adapted to break down gel layers that form on the surface of the membrane when in operation.

21 . A tubular membrane of any one of the preceding claims, wherein the membrane is metallic.

22. A tubular membrane module comprising at least one metallic tubular membrane according to any one of the preceding claims, at least one metallic face plate adapted to receive and support said membranes, and housing for the tubular membrane module adapted to receive the permeate from the tubular membranes.

23. An extrusion head used for the production of a tubular membrane according to any one of the preceding claims using powder metallurgy techniques, wherein said extrusion head has at least one internal helix grove or ridge, wherein the extrusion head comprises a helix ridge or groove on the inner or outer component of the dye.

24. A method for producing a tubular membrane according to any one of the

preceding claims using powder metallurgy techniques.

25. A method for producing a tubular membrane module according to claim 22 using powder metallurgy techniques.

26. A metallic tubular membrane substantially as herein described with reference to examples.