Ceramic hollow fiber or ceramic capillary membranes

Abstract

A method of manufacturing a filter, the method comprising the steps of: forming a plurality of hollow fibre green bodies; arranging the plurality of hollow fibre green bodies to form an assembly of hollow fibre green bodies having a plurality of bore channels and a plurality of interstitial channels; heat treating the assembly of hollow fibre green bodies to form a filter substrate; and sealing the interstitial channels; wherein the method comprises manipulating the volume between the hollow fibre green bodies such that at least a portion of the interstitial channels in the filter are in fluid communication.

Description

[0001] CERAMIC HOLLOW FIBER OR CERAMIC CAPILLARY MEMBRANES

[0002] Field

[0003] The present invention relates to filters, methods of manufacture thereof, and methods of filtering. In particular, the invention relates to filters comprising a plurality of interstitial channels wherein at least a portion of the interstitial channels are in fluid communication.

[0004] Background

[0005] Ceramic membranes are widely used in microfiltration and ultrafiltration. This is due to a number of advantages that they have over polymer counterparts. The advantages include a greater mechanical strength and structural stiffness, greater corrosive and thermal resistance, stable operating characteristics during long service, and the possibility of multiple regenerations by calcination or by the backward stream of water or an appropriate solvent. This means that ceramic membranes can be operated over a wide pH range, at high temperatures and pressures, and in corrosive media. Furthermore, ceramics are typically made from inorganic minerals which may be better for the environment and more recyclable than polymers, which are typically derived from fossil fuels. Therefore, ceramics are often seen by end users as having a lower carbon footprint than polymers.

[0006] Ceramic membranes are of interest for filtration systems, such as forthe filtration of waterwhere the high strength material allows for the use of high pressure filtration. Examples of such filters are discussed in US2006 / 0175256.

[0007] Water filtration systems may use “dead-end” or “crossflow” configurations. In dead-end filtration, water is forced through the ceramic membrane such that all of the water is filtered. This may be achieved by blocking any open passages through the filter (or membrane element).

[0008] Crossflow filtration is a continuous process in which water is fed into a membrane module and flows along the surface of a membrane. A lower pressure on the other side of the membrane causes water to pass through the membrane as permeate. The remaining water and solids are referred to as retentate and may be recirculated to the feed reservoir. An advantage of crossflow filtration over dead-end filtration is that there is less tendency for solids to build up on the membrane surface because the high velocity of the fluid moving along the filter channels continuously strips material away from the membrane surface. This extends the useful lifetime of the filter before the layer of solids on the membrane surface must be removed, e.g. by backwashing. Conventional ceramic membrane filters typically comprise a small number of channels through which a feed composition is passed. Permeate must pass through the ceramic membrane and substrate to the outside of the filter to be collected. If the number of channels is increased (while maintaining the diameter of each channel) it becomes more difficult for the permeate to exit the filter because the permeate must as a whole pass through a greater volume of substrate. Therefore, drainage of the permeate from the filter can be a rate-limiting factor for the filtration process.

[0009] It is one aim of the invention, among others, to provide a filter that address at least one disadvantage of the prior art, whether identified here or elsewhere, or to provide an alternative to existing filters. For instance, it may be an aim of the present invention to provide a filter with improved drainage of permeate.

[0010] Summary of the Invention

[0011] According to a first aspect of the present invention, there is provided a method of manufacturing a filter, the method comprising the steps of: forming a plurality of hollow fibre green bodies; arranging the plurality of hollow fibre green bodies to form an assembly of hollow fibre green bodies having a plurality of bore channels and a plurality of interstitial channels; heat treating the assembly of hollow fibre green bodies to form a filter substrate; and sealing the interstitial channels; wherein the method comprises manipulating the volume between the hollow fibre green bodies such that at least a portion of the interstitial channels in the filter are in fluid communication.

[0012] According to a second aspect of the present invention, there is provided a filter comprising a substrate, wherein the substrate comprises a plurality of bore channels and a plurality of interstitial channels, wherein at least a portion of the interstitial channels are in fluid communication.

[0013] According to a third aspect of the invention, there is provided a method of filtering, the method comprising passing a composition through the filter according to the second aspect of the invention.

[0014] Detailed Description

[0015] Unless otherwise stated, the following terms used in the specification and claims have the meanings set out below. As used herein, the term “ceramic” is used in its ordinary sense, which is well-known to those skilled in the art. Specifically, a ceramic is an inorganic, nonmetallic solid, generally based on an oxide, nitride, boride, or carbide, that is fired at a high temperature.

[0016] As used herein, the term “substrate” is used in its ordinary sense, which is well-known to those skilled in the art. Specifically, a substrate is an underlying substance upon which another substance, such as a washcoat, may be applied. In some embodiments, a substrate may be used without another substance applied, such as without a washcoat applied. The substrate comprises a plurality of fused hollow fibres, preferably a plurality of parallel, fused hollow fibres. Such substrates may comprise open bore channels within the hollow fibres (e.g. the bore of the hollow fibres), interstitial channels between the hollow fibres, as well as micro-channels in the porous walls of the hollow fibres.

[0017] By “bore channels” we mean channels at the centre of the hollow fibres which are encompassed by the walls of the hollow fibres. The bore channels suitably correspond to the longitudinal axis of the hollow fibres. The bore channels suitably have a minimum diameter (i.e. at the narrowest part of the bore channel) of greater than 0.2 mm, such as greater than 0.5 mm, or even greater than 1 mm.

[0018] By “interstitial channels” we mean voids between the hollow fibres. When the hollow fibres are in a close-packed configuration the interstitial channels may generally run parallel to the bore channels.

[0019] By “micro-channels” we mean channels extending through the walls of the hollow fibres suitably having an entrance diameter of 5 pm to 200 pm.

[0020] The walls of the hollow fibres in the filter substrate are porous, i.e. comprise pores. Pore diameters are typically less than 13 pm, such as from 0.01 to 12 pm.

[0021] As used herein, the term “green body” is used in its ordinary sense, which is well-known to those skilled in the art. Specifically, by “green body” we mean a precursor to a substrate that has not yet been heat-treated or sintered. By “hollow fibre green body” is meant a green body in the form of a hollow fibre, e.g. a tube.

[0022] As used in the specification and the appended claims, the singular forms "a", "an," and "the" include both singular and plural referents unless the context clearly dictates otherwise. By way of example, "a filter" means one filter or more than one filter. Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The term “consisting essentially of’ or “consists essentially of’ means including the components specified but excluding other components except for components added for a purpose other than achieving the technical effect of the invention. The term “consisting of’ or “consists of’ means including the components specified but excluding other components.

[0023] Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising” may also be taken to include the meaning “consists essentially of’ or “consisting essentially of’, and also may also be taken to include the meaning “consists of’ or “consisting of’.

[0024] For the avoidance of doubt, where amounts of components in a composition are described in wt%, this means the weight percentage of the specified component in relation to the whole composition referred to.

[0025] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts of percentages may be read as if prefaced by the word “about”, even if the term does not expressly appear.

[0026] The term "about" as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, indicate that a value includes the standard deviation of error for the device or method being employed to determine the value. The term "about" is meant to encompass variations of + / -10% or less, + / -5% or less, or + / -0.1 % or less of and from the specified value, insofar such variations are appropriate to perform in the disclosure. It is to be understood that the value to which the modifier "about" refers is itself also specifically disclosed.

[0027] The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1 , 2, 3, 4 when referring to, for example, a number of elements, and can also include 1 .5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g. from 1 .0 to 5.0 includes both 1 .0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

[0028] The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each exemplary embodiment of the invention, as set out herein are also applicable to any other aspects or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each aspect or embodiment of the invention as interchangeable and combinable between different aspects of the invention.

[0029] As used herein, the term "and / or," when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a list is described as comprising group A, B, and / or C, the list can comprise A alone; B alone; C alone; A and B in combination; A and C in combination, B and C in combination; or A, B, and C in combination.

[0030] According to a first aspect of the present invention, there is provided a method of manufacturing a filter, the method comprising the steps of: forming a plurality of hollow fibre green bodies; arranging the plurality of hollow fibre green bodies to form an assembly of hollow fibre green bodies having a plurality of bore channels and a plurality of interstitial channels; heat treating the assembly of hollow fibre green bodies to form a filter substrate; and sealing the interstitial channels; wherein the method comprises manipulating the volume between the hollow fibre green bodies such that at least a portion of the interstitial channels in the filter are in fluid communication.

[0031] The inventors have found that by manipulating the volume occupied by the interstitial channels in the filter, it was possible to connect the interstitial channels such that in use, the filter shows increased drainage of permeate. This is highly advantageous as it increases the volume of a composition which can be filtered per unit volume of the filter.

[0032] Each of the hollow fibre green bodies suitably has an outer surface having a circular or oval cross section. Each ofthe hollow fibre green bodies suitably has a bore channel having a circular or oval cross section. Preferably, each of the hollow fibre green bodies has an outer surface having a circular cross section. Preferably, each of the hollow fibre green bodies has a bore channel having a circular cross section.

[0033] The step of forming a plurality of hollow fibre green bodies may comprise: providing:

[0034] -a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the polymer is soluble in the solvent; forming the fusing composition into a hollow fibre structure wherein the fusing composition forms a fusing layer; and contacting the fusing layer with a non-solvent in which the polymer is insoluble to precipitate said polymer, thereby forming a hollow fibre green body. The step of forming a plurality of hollow fibre green bodies may comprise: providing:

[0035] -a support composition comprising a substrate material, a polymer and a solvent, wherein the polymer is soluble in the solvent,

[0036] -a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the polymer of the fusing composition is soluble in the solvent of the fusing composition; forming the support composition and the fusing composition into a multilayer hollow fibre structure wherein the support composition forms a support layer and the fusing composition forms a fusing layer; and contacting the multilayer hollow fibre structure with a non-solvent in which the polymer of the support composition and the polymer of the fusing composition are insoluble to precipitate said polymers, thereby forming a hollow fibre green body.

[0037] The step of forming a plurality of hollow fibre green bodies may comprise: providing:

[0038] -a sacrificial composition comprising a polymer and a solvent, wherein the polymer is soluble in the solvent,

[0039] -a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the polymer of the fusing composition is soluble in the solvent of the fusing composition; forming the sacrificial composition and the fusing composition into a multilayer hollow fibre structure wherein the sacrificial composition forms a sacrificial layer and the fusing composition forms a fusing layer; and contacting the multilayer hollow fibre structure with a non-solvent in which the polymer of the sacrificial composition and the polymer of the fusing composition are insoluble to precipitate said polymers, thereby forming a hollow fibre green body.

[0040] The step of forming a plurality of hollow fibre green bodies may comprise: providing:

[0041] -a sacrificial composition comprising a polymer and a solvent, wherein the polymer is soluble in the solvent,

[0042] -a support composition comprising a substrate material, a polymer and a solvent, wherein the polymer of the support composition is soluble in the solvent of the support composition, and -a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the polymer of the fusing composition is soluble in the solvent of the fusing composition; forming the sacrificial composition, the support composition and the fusing composition into a multilayer hollow fibre structure wherein the sacrificial composition forms a sacrificial layer, the support composition forms a support layer, the fusing composition forms a fusing layer, and the support layer is adjacent to the fusing layer; and contacting the multilayer hollow fibre structure with a non-solvent in which the polymer of the sacrificial composition, the polymer of the support composition, and the polymer of the fusing composition are insoluble to precipitate said polymers, thereby forming a hollow fibre green body.

[0043] For the avoidance of doubt, by “(multilayer) hollow fibre structure” we mean a structure comprising a fusing layer, or a multilayer structure comprising a fusing layer and a sacrificial layer and / or support layer as defined herein.

[0044] The presence of the support composition and corresponding support layer advantageously provides structural support for the fusing layer and helps the hollow fibre green body to retain its shape when being fused to other hollow fibre green bodies.

[0045] References herein to a polymer or substrate material being (in)soluble or poorly soluble in a solvent or non-solvent mean that the polymer or substrate material is (in)soluble or poorly soluble in the solvent or non-solvent at a temperature of 25°C and at a pressure of 100 kPa.

[0046] Suitably, a polymer that is soluble in a solvent has a solubility of at least 10 g / L in the solvent, such as at least 100 g / L, in the solvent at a temperature of 25°C and at a pressure of 100 kPa. Suitably, a polymer that is poorly soluble in a solvent has a solubility of at least 1 g / L and less than 10 g / L, such as from 2 to 8 g / L in the solvent at a temperature of 25°C and at a pressure of 100 kPa. Suitably, a polymer or substrate material that is insoluble in a non-solvent has a solubility of less than 1 g / L in the non-solvent, such as less than 0.1 g / L, in the non-solvent at a temperature of 25°C and at a pressure of 100 kPa.

[0047] Alternatively, the solubility of a polymer or substrate material may be defined using Hansen solubility parameters. Hansen solubility parameters represent forces between two molecules (such as a polymer and a solvent). The use of Hansen solubility parameters is well known to those skilled in the art. The distance between the Hansen solubility parameters (Ra) of two molecules is calculated as follows:

[0048] Ra2= 4(6DI-6D2)2+ (6P1-6P2)2+ (6H1-6H2)2wherein 6D represents dispersion forces between the molecules, 6P represents polar forces between the molecules and 6H represents hydrogen bonding forces between the molecules.

[0049] The relative energy difference (RED) is calculated as follows:

[0050] RED = Ra / Ro wherein Ro is an experimentally determined interaction radius.

[0051] Suitably, a polymer and a solvent in which the polymer is soluble have a RED of less than 0.9. Suitably, a polymer and a solvent in which the polymer is poorly soluble have a RED of from 0.9 to 1 , preferably 1 . Suitably, a polymer or substrate material and a non-solvent in which the polymer or substrate material is insoluble have a RED of greater than 1.

[0052] The precipitation (i.e. phase inversion) of the polymers in the multilayer hollow fibre structure results in the formation of openings in a surface of the hollow fibre green body with microchannels extending away from the openings. This is believed to be due to the egress of the solvents present in the multilayer hollow fibre structure and ingress of the non-solvent. These micro-channels provide the hollow fibre green body, and the eventual hollow fibre substrate formed from the hollow fibre green body, with a large geometric surface area. However, the micro-channels typically do not extend all of the way from one surface of the hollow fibre green body to the other upon precipitation of the polymers.

[0053] Including a sacrificial composition in the multilayer hollow fibre structure in addition to the substrate material-containing support composition (when present) and fusing composition suitably causes the micro-channels to penetrate through at least the layers formed from the support composition (when present) and fusing composition when phase inversion is carried out. Although the micro-channels may terminate in the layer formed from the sacrificial composition, said layer can be removed from the hollow fibre green body, especially when said layer does not comprise a substrate material, so that the remaining hollow fibre green body has microchannels penetrating from one surface of the hollow fibre green body to another.

[0054] Preferably, the micro-channels do not penetrate all the way through the hollow fibre substrate. In some preferred embodiments, the sacrificial composition is not included in the multilayer hollow fibre structure, such the micro-channels may terminate in the layer formed from the support composition (when present) or the fusing composition.

[0055] Including a fusing agent in the fusing composition advantageously allows the hollow fibre green body to be fused with other hollow fibre green bodies prepared as described herein. Fusing of the hollow fibre green bodies may be conducted in a controlled manner, for example by exposing the hollow fibre green bodies to a stimulus, such as a high temperature (typically below sintering temperature). Using a combination of a polymer and a fusing agent in the fusing composition allows the fusing layer of the hollow fibre green body (precipitated from the fusing composition) to retain its shape in the absence of high temperatures or other stimuli, thereby preserving the geometry of the (multilayer) hollow fibre structure, while being able to fuse with fusing layers of other hollow fibre green bodies. A layer formed from the polymer of the fusing composition in the absence of the fusing agent would retain its shape, but would not be fusable with other hollow fibre green bodies.

[0056] The (multilayer) hollow fibre structure comprises a sacrificial layer (when present), a support layer (when present) and a fusing layer, formed from the sacrificial composition (when present), the support composition (when present) and the fusing composition, respectively. The hollow fibre green body formed by precipitation of the polymers in the (multilayer) hollow fibre structure comprises a sacrificial layer (when present), a support layer (when present) and a fusing layer corresponding to the sacrificial layer (when present), the support layer (when present) and the fusing layer, respectively, of the (multilayer) hollow fibre structure. The layer(s) of the hollow fibre green body suitably differ from the layer(s) of the (multilayer) hollow fibre structure in that the layer(s) of the hollow fibre green body are solid and are substantially free from the solvent of the sacrificial composition (when present), the solvent of the support composition (when present) and the solvent of the fusing composition. By “substantially free” from a solvent, we mean that the solvent is not present in an amount of more than 1 wt%, preferably 0.1 wt%, or even 0.01 wt% based on the total weight of the hollow fibre green body.

[0057] Suitably, the sacrificial composition comprises from 5 to 70 wt%, such as from 8 to 50 wt%, or even from 10 to 30 wt%, of a polymer and from 95 to 30 wt%, such as from 92 to 50 wt%, or even from 90 to 70 wt%, of a solvent.

[0058] The polymer of the support composition may be soluble in the solvent of the sacrificial composition (when present). The solubility of the polymer of the support composition in the solvent of the sacrificial composition may advantageously help the micro-channels penetrate into the sacrificial layer of the multilayer hollow fibre structure, especially when the support layer is adjacent to the sacrificial layer. Suitably, the support composition comprises from 5 to 50 wt%, such as from 6 to 25 wt%, or even from 8 to 15 wt%, of a polymer, from 5 to 85 wt%, such as from 20 to 70 wt%, or even from 30 to 50 wt%, of a solvent and from 10 to 90 wt%, such as from 20 to 70 wt%, or even from 30 to 50 wt%, of a substrate material.

[0059] In some preferred embodiments, the step of forming a plurality of hollow fibre green bodies does not comprise providing a sacrificial composition as defined herein. In such embodiments, the (multilayer) hollow fibre structure does not comprise a sacrificial layer. The hollow fibre structure may consist of the fusing layer, or the multilayer hollow fibre structure may consist of the support layer and the fusing layer. When a (multilayer) hollow fibre structure which does not comprise a sacrificial layer is contacted with the non-solvent, the micro-channels formed suitably terminate in the support layer (when present) or the fusing layer. The polymer of the fusing composition may be soluble in the solvent of the sacrificial composition (when present). The solubility of the polymer of the fusing composition in the solvent of the sacrificial composition may advantageously help the micro-channels penetrate into the sacrificial layer of the multilayer hollow fibre structure, especially when the fusing layer is adjacent to the sacrificial layer. Suitably, the fusing composition comprises from 2 to 50 wt%, such as from 5 to 25 wt%, or even from 8 to 15 wt%, of a polymer, from 2 to 50 wt%, such as from 5 to 25 wt%, or even from 8 to 15 wt%, of a fusing agent, from 5 to 85 wt%, such as from 20 to 70 wt%, or even from 30 to 50 wt%, of a solvent and from 10 to 90 wt%, such as from 20 to 70 wt%, or even from 30 to 50 wt% of a substrate material.

[0060] Suitably, substrate material of the fusing composition is present in an amount of from 60 to 95 wt%, such as from 63 to 85 wt%, or even from 66 to 75 wt%, based on the total solid weight of the fusing composition. By “solid weight” we mean the weight excluding the weight of solvents.

[0061] The polymer of the sacrificial composition is suitably immiscible with at least one of the polymer of the support composition (when present) and the polymer of the fusing composition. By “immiscible” we mean that the polymers do not form a substantially homogeneous mixture when stirred together in the first solvent at a temperature of 25°C and a pressure of 100 kPa. By “substantially homogeneous mixture”, we mean a mixture in which at least 80 vol%, such as at least 90 vol%, or even at least 95 vol%, of the mixture is homogeneous, e.g. does not contain phase separation.

[0062] Suitably, the polymer of the sacrificial composition is immiscible with any polymer in a layer adjacent to the sacrificial layer in the multilayer hollow fibre structure. For example, when the sacrificial layer is adjacent to the support layer in the multilayer hollow fibre structure, the polymer of the sacrificial composition is suitably immiscible with the polymer of the support composition. Preferably, the polymer of the sacrificial composition is immiscible with the polymer of the support composition and the polymer of the fusing composition. Suitably, the polymer of the sacrificial composition is immiscible with all other polymers in the multilayer hollow fibre structure.

[0063] The lack of miscibility between the polymer of the sacrificial composition and other polymers in the hollow fibre green body facilitates removal of the sacrificial layer from the hollow fibre green body to expose openings to micro-channels in the hollow fibre green body.

[0064] Suitably, the polymer of the sacrificial composition is insoluble in the solvent of the support composition (if present) and / or the solvent of the fusing composition. Suitably, the polymer of the sacrificial composition is insoluble in any solvent in a layer adjacent to the sacrificial layer in the multilayer hollow fibre structure. For example, when the sacrificial layer is adjacent to the support layer in the multilayer hollow fibre structure, the polymer of the sacrificial layer is suitably insoluble in the solvent of the support composition. Preferably, the polymer of the sacrificial composition is insoluble in the solvent of the support composition and the solvent of the fusing composition. Suitably, the polymer of the sacrificial composition is insoluble in all solvents in the multilayer hollow fibre structure other than the solvent of the sacrificial composition.

[0065] The insolubility of the polymer of the sacrificial composition in solvents of other layers in the hollow fibre green body facilitates removal of the sacrificial layer from the hollow fibre green body to expose openings to micro-channels in the hollow fibre green body.

[0066] The sacrificial composition may further comprise a substrate material. The substrate material of the sacrificial composition may comprise one or more of: a ceramic, cordierite, zirconia, yttrium- stabilised zirconia, titania, silicon carbide, clay, alumina, stainless steel, FeCr alloys, alloys of iron, alloys of aluminium, aluminium titanate, sintered metals, or a zeolite. Preferably, the substrate material comprises a ceramic.

[0067] Preferably, the sacrificial composition is substantially free from a substrate material. By “substantially free” from a substrate material, we mean that a substrate material is not present in an amount of more than 1 wt%, preferably 0.1 wt%, or even 0.01 wt%, based on the total weight of the sacrificial composition.

[0068] In some embodiments, a plurality of support compositions are provided and the multilayer hollow fibre structure is formed having a plurality of support layers, which are preferably adjacent to one another. For example, two support compositions may be provided and the multilayer follow fibre structure may be formed having two support layers, which are preferably adjacent to one another.

[0069] In some embodiments, the micro-channels do not penetrate into the support layer on precipitation of the polymers in the multilayer hollow fibre structure. In some embodiments where there is a plurality of support layers, the micro-channels do not penetrate into at least one of the support layers on precipitation of the polymers in the multilayer hollow fibre structure. For example, where there are two support layers, the micro-channels may only penetrate into one of the two support layers. Such embodiments may be advantageous, since the support layer not comprising micro-channels will be able to filter smaller particles than any layers comprising micro-channels. The micro-channels may be prevented from penetrating into the support layer by including an insufficient amount of solvent in the support composition, or ensuring poor solubility of the polymer of the support composition in the solvent of the support composition, for example by appropriate selection of the polymer and the solvent. By “insufficient amount” we mean that the support composition may comprise up to 20 wt%, such as up to 10 wt%, or even up to 5 wt% of a solvent. The polymer of the support composition may be poorly soluble in the solvent of the support composition. A solvent in which the polymer is poorly soluble may be prepared by mixing a solvent in which the polymer is soluble with a non-solvent in which the polymer is insoluble, wherein the solvent and non-solvent are miscible.

[0070] In embodiments where there is a plurality of support compositions, each support composition is preferably different from the other support compositions, and / or each support composition forms a support layer in the multilayer hollow fibre structure having a different geometry (e.g. thickness). For example, the plurality of support compositions may consist of two different support compositions. The substrate material of one of the support compositions is preferably different from the substrate material in another of the support compositions. Suitably, the substrate material of one of the support compositions has a different particle size from the substrate material of another of the support compositions. For example, the substrate material of one of the support compositions may have a larger particle size than the substrate material of another of the support compositions. A larger particle size typically corresponds to a larger pore size in the hollow fibre substrate prepared from the hollow fibre green body. Such an embodiment may be advantageous, as the larger particle size and corresponding larger pore size of the support layer enables fluid to more easily pass through the support layer to / from a micro-channel, especially when the hollow fibre green body comprises micro-channels terminating in the support layer.

[0071] The substrate material of the support composition may be different from the substrate material of the fusing composition. The substrate material of the support composition may have a different particle size from the substrate material of the fusing composition.

[0072] Preferably, the polymer of the support composition and the polymer of the fusing composition are the same. This improves the binding between the support layer and the fusing layer in the hollow fibre green body.

[0073] The polymer of the support composition and / or the polymer of the fusing composition suitably has a glass transition temperature above 25°C, such as above 50°C, such as above 100°C, or even above 200°C.

[0074] The fusing agent may comprise any suitable additive that enables the fusing layer of the hollow fibre green body to be fused to the fusing layer of other hollow fibre green bodies on exposure to a stimulus, such as heat or a solvent. Suitably, the fusing agent causes softening of the fusing layer of the hollow fibre green body (e.g. as measured by Shore Hardness, ASTM D2240) on exposure to a stimulus. For example, the fusing agent may comprise a polymer having a lower glass transition temperature than the polymer of the fusing composition (causing softening of the fusing layer on exposure to heat), or the fusing agent may comprise a temperature-activated catalyst (causing softening of the fusing layer on exposure to heat), or the fusing agent may be soluble in a solvent in which the polymer of the fusing composition is insoluble (causing softening of the fusing layer on exposure to the solvent). For the avoidance of doubt, by “polymer of the fusing composition” is meant the polymer which is not the fusing agent.

[0075] The fusing agent suitably comprises a polymer having a glass transition temperature below 25°C, such as below 20°C, such as below 15°C, or even below 10°C.

[0076] The polymer of the fusing agent is suitably soluble in solvent of the sacrificial composition (when present) and the solvent of the fusing composition. The polymer of the fusing agent is suitably insoluble in the non-solvent.

[0077] The fusing agent is suitably miscible with the polymer of the fusing composition. By “miscible” we mean that the fusing agent and the polymer of the fusing composition form a substantially homogeneous mixture when stirred together in the solvent of the fusing composition at a temperature of 25°C and a pressure of 100 kPa. This reduces the likelihood of the fusing agent and polymer of the fusing composition agglomerating in the fusing composition and improves the processability of the fusing composition.

[0078] The fusing agent may be present in the fusing composition in an amount of from 10 to 90 wt%, such as from 20 to 80 wt%, or even from 40 to 60 wt%, based on the total weight of the polymer of the fusing composition and the fusing agent in the fusing composition. For the avoidance of doubt, when the fusing agent is present in an amount of 10 wt% the polymer of the fusing composition is present in an amount of 90 wt% based on the total weight of the polymer of the fusing composition and the fusing agent in the fusing composition.

[0079] The support composition is preferably substantially free from the fusing agent. By “substantially free” from the fusing agent, we mean that the fusing agent is not present in an amount of more than 1 wt%, preferably 0.1 wt%, or even 0.01 wt%, based on the total weight of the support composition.

[0080] Suitably, the polymer of the sacrificial composition (when present), the polymer of the support composition (when present), the polymer of the fusing composition and the polymer of the fusing agent (when present) are each independently organic polymers. By “organic polymers” we mean polymers comprising carbon atoms in the backbone.

[0081] The polymer of the sacrificial composition may comprise a polyester. Suitably, the polymer of the sacrificial composition comprises an aliphatic polyester, such as polycaprolactone. The polymer of the sacrificial composition suitably has a number average molecular weight (Mn) or a weight average molecular weight (Mw) of from 5,000 to 200,000 g / mol, such as from 10,000 to 120,000 g / mol, or even from 20,000 to 80,000 g / mol.

[0082] The polymer of the support composition (when present) and / or the polymer of the fusing composition may each independently comprise one or more of polyethersulfone, polysulfone, cellulose and its derivatives, polyamide, polyetherimide, polyimide and its derivatives, polyvinylidene fluoride, and polydimethylsiloxane. Preferably, the polymer of the support composition and / or the polymer of the fusing composition comprises polyethersulfone.

[0083] The polymer of the support composition and / or the polymer of the fusing composition suitably independently have a number average molecular weight (Mn) or a weight average molecular weight (Mw) of from 5,000 to 200,000 g / mol, such as from 10,000 to 120,000 g / mol, or even from 20,000 to 80,000 g / mol.

[0084] The polymer of the fusing agent may comprise an epoxy resin. Suitable epoxy resins include bisphenol-based epoxy resins, such as bisphenol A-based epoxy resins. Preferably, the polymer of the fusing agent comprises bisphenol A diglycidyl ether.

[0085] The polymer of the fusing agent suitably has a number average molecular weight (Mn) or a weight average molecular weight (Mw) of from 200 to 5,000 g / mol, such as from 400 to 3,000 g / mol, or even from 600 to 1 ,000 g / mol.

[0086] The substrate material of the support composition is suitably insoluble in the solvent of the support composition such that the substrate material is present as a suspension in the solvent in the support composition. The substrate material of the fusing composition is suitably insoluble in the solvent of the fusing composition such that the substrate material is present as a suspension in the solvent in the fusing composition.

[0087] The substrate material of the support composition (when present) and / or the substrate material of the fusing composition may each independently comprise one or more of: a ceramic, cordierite, zirconia, yttrium-stabilised zirconia, titania, silicon carbide, clay, alumina, stainless steel, FeCr alloys, alloys of iron, alloys of aluminium, aluminium titanate, sintered metals, or a zeolite. Preferably, the substrate material of the support composition and / or the substrate material of the fusing composition comprises a ceramic. Preferably, the substrate material of the support composition and the substrate material of the fusing composition are the same. The substrate material of the support composition and / or the substrate material of the fusing composition may have a particle size of from 0.01 to 10 pm, such as from 0.05 to 5 pm, or even from 0.1 to 1 pm.

[0088] The solvent of the sacrificial composition (when present), the solvent of the support composition (when present) and / or the solvent of the fusing composition may each independently comprise an organic solvent, such as a dipolar aprotic solvent. Suitable dipolar aprotic solvents include acetonitrile, acetone, dimethylformamide, dimethylacetamide, hexamethylphosphoramide, N- methyl-2-pyrrolidone and dimethyl sulfoxide. Preferably, the solvent of the support composition and / or the solvent of the fusing composition each independently comprise N-methyl-2- pyrrolidone and / or dimethyl sulfoxide. Suitably, the solvent of the support composition and the solvent of the fusing composition are the same.

[0089] The no — solvent suitably comprises a polar protic solvent, such as water. Preferably the nonsolvent comprises water.

[0090] The support composition (when present) and / or the fusing composition may further comprise a dispersant. The dispersant is suitably effective to disperse the substrate material of the support composition in the solvent of the support composition and / or to disperse the substrate material of the fusing composition in the solvent of the fusing composition. Suitable dispersants are well known to persons skilled in the art and include polyalkylene glycols (such as polyethylene glycol), PEG-30 dipolyhydroxystearate (e.g. Cithrol® DPHS), Schwego® Wett 8082, alkylolammonium salt co-polymer (e.g. Disperbyk-180), and phosphoric ester salt (e.g. Disperbyk-145).

[0091] The step of forming a plurality of hollow fibre green bodies suitably further comprises degassing the sacrificial composition (when present), the support composition (when present) and the fusing composition (“the compositions”) prior to formation of the (multilayer) hollow fibre structure. Degassing the compositions prevents dissolved gas or gas bubbles from disrupting the formation of the micro-channels in the hollow fibre green body during the precipitation of the hollow fibre green body. Degassing the compositions suitably comprises subjecting the compositions to a vacuum, for example a pressure of less than 10,000 Pa, such as less than 6,600 Pa, or even less than 3,300 Pa. Degassing the compositions may comprise subjecting the compositions to a pressure of from 10,000 to 0.1 Pa, such as from 6,600 to 0.3 Pa, or even from 3,300 to 0.4 Pa.

[0092] In embodiments where the sacrificial layer, the support layer, and the fusing layer are present, the support layer is adjacent to the fusing layer in the multilayer hollow fibre structure. The sacrificial layer may be adjacent to the support layer and / or the fusing layer in the multilayer hollow fibre structure. For example, the support layer in the multilayer hollow fibre structure may be adjacent to the sacrificial layer and adjacent to the fusing layer (such that the support layer is in between the sacrificial layer and the fusing layer), or the fusing layer in the multilayer hollow fibre structure may be adjacent to the sacrificial layer and adjacent to the support layer (such that the fusing layer is in between the sacrificial layer and the support layer). Preferably, the support layer is adjacent to the sacrificial layer and is adjacent to the fusing layer in the multilayer hollow fibre structure.

[0093] Adjacent layers in the multilayer hollow fibre structure are suitably in contact with one another. For example, the support layer is suitably in contact with the fusing layer. This reduces the likelihood that the layers become detached from each other when the hollow fibre green body is formed. In embodiments where the structure is a multilayer hollow fibre structure comprising at least two layers (such as the fusing layer and the sacrificial layer or the support layer), the layers are preferably in contact with one another.

[0094] The sacrificial layer (when present) may be present in the hollow fibre green body in an amount of from 1 to 30% by bulk volume, such as from 5 to 25% by bulk volume, or even from 10 to 20% by bulk volume, based on the total bulk volume of the hollow fibre green body. The support layer (when present) may be present in the hollow fibre green body in an amount of from 20 to 90% by bulk volume, such as from 40 to 70% by bulk volume, or even from 50 to 60% by bulk volume, based on the total bulk volume of the hollow fibre green body. The fusing layer (when another layer is present) may be present in the hollow fibre green body in an amount of from 5 to 70% by bulk volume, such as from 10 to 50% by bulk volume, or even from 15 to 25% by bulk volume, based on the total bulk volume of the hollow fibre green body. By “bulk volume” we mean the volume including solid material and pores in the hollow fibre green body.

[0095] In some embodiments, the hollow fibre green body may consist of the fusing layer. The hollow fibre green body may consist of the sacrificial layer and the fusing layer. The hollow fibre green body may consist of the support layer and the fusing layer. The hollow fibre green body may consist of the sacrificial layer, the support layer and the fusing layer.

[0096] The (multilayer) hollow fibre structure may be formed by any suitable method. Suitably, the (multilayer) hollow fibre structure is formed by extrusion.

[0097] Preferably, the (multilayer) hollow fibre structure is extruded by forming the (multilayer) hollow fibre structure in an extrusion die and extruding the (multilayer) hollow fibre structure from the die. The (multilayer) hollow fibre structure is suitably extruded directly into the non-solvent (e.g. such that the (multilayer) hollow fibre structure does not come into contact with air). For example, the outlet of the extrusion die may be immersed in the non-solvent. Suitably, adjacent layers in the multilayer hollow fibre structure are contacted with one another before the multilayer hollow fibre structure is contacted with the non-solvent. For example, the extrusion die may contain a chamber in which the adjacent layers of the multilayer hollow fibre structure are contacted before the multilayer hollow fibre structure exits the die. This reduces the likelihood that the layers become detached from each other when the hollow fibre green body is formed. The extrusion may be carried out continuously to form an elongate (multilayer) hollow fibre structure.

[0098] The layers in the multilayer hollow fibre structure are suitably in the form of concentric hollow fibres. Suitably the sacrificial composition (when present) forms an inner layer and the fusing composition forms an outer layer in the multilayer hollow fibre structure. Preferably the fusing composition forms an outer layer in the multilayer hollow fibre structure.

[0099] When the hollow fibre structure consists of the fusing layer, the structure is preferably formed as a monolayer hollow fibre structure. The monolayer hollow fibre structure may be formed by extrusion of the fusing composition. Suitably the fusing composition is extruded as a hollow fibre around a bore fluid.

[0100] The multilayer hollow fibre structure may be formed by extrusion of the sacrificial composition (when present), the support composition (when present) and the fusing composition. Suitably the sacrificial composition (when present), the support composition (when present) and the fusing composition are extruded as concentric hollow fibres around a bore fluid.

[0101] The bore fluid preferably does not cause precipitation of dissolved polymers in the (multilayer) hollow fibre structure (e.g. the polymer of the sacrificial composition (when present), the polymer of the support composition (when present), the polymer of the fusing composition and the polymer of the fusing agent (when present)).

[0102] The bore fluid is suitably immiscible with the adjacent layer, such as the sacrificial layer, of the (multilayer) hollow fibre structure. Any polymer present in the layer of the (multilayer) hollow fibre structure adjacent to the bore fluid (e.g. the polymer of the sacrificial composition (when present), the polymer of the support composition (when present), the polymer of the fusing composition and the polymer of the fusing agent (when present)) is suitably insoluble in the bore fluid. The polymer of the sacrificial composition is suitably insoluble in the bore fluid.

[0103] The bore fluid may be non-aqueous. Suitably the bore fluid comprises a liquid hydrocarbon or a silicone oil. Suitable liquid hydrocarbons include C5-C19 hydrocarbons, such as hexane, heptane, and octane. Suitable silicone oils include polydimethylsiloxane. The step of forming a plurality of hollow fibre green bodies comprises contacting the (multilayer) hollow fibre structure with a non-solvent in which the polymer of the sacrificial composition (when present), the polymer of the support composition (when present), the polymer of the fusing composition and the polymer of the fusing agent (when present) are insoluble to precipitate said polymers, thereby forming a green body.

[0104] Suitably the multilayer hollow fibre structure is contacted with the non-solvent for less than 30 seconds, such as less than 10 seconds, or even less than 1 second, after formation of the multilayer hollow fibre structure. Minimising the time between formation and precipitation of the multilayer hollow fibre structure prevents excessive mixing of adjacent layers in the multilayer hollow fibre structure.

[0105] Suitably the volume of the non-solvent contacted with the (multilayer) hollow fibre structure exceeds the combined volume of the solvent of the sacrificial composition (when present), the solvent of the support composition (when present) and the solvent of the fusing composition in the (multilayer) hollow fibre structure. The volume of the non-solvent contacted with the (multilayer) hollow fibre structure may be at least two times, such as at least five times, or even at least ten times, the combined volume of the solvent of the sacrificial composition (when present), the solvent of the support composition (when present) and the solvent of the fusing composition in the (multilayer) hollow fibre structure. Suitably, the (multilayer) hollow fibre structure is contacted with a quantity of the non-solvent sufficient to remove at least 95 wt%, such as at least 98 wt%, or even at least 99 wt%, of the solvent of the sacrificial composition (when present), the solvent of the support composition (when present) and the solvent of the fusing composition from the (multilayer) hollow fibre structure.

[0106] The step of contacting the (multilayer) hollow fibre structure with the non-solvent may comprise immersing the (multilayer) hollow fibre structure in the non-solvent, spraying the (multilayer) hollow fibre structure with the non-solvent, or co-extruding the (multilayer) hollow fibre structure with the non-solvent. The step of contacting the (multilayer) hollow fibre structure with the nonsolvent preferably comprises immersing the (multilayer) hollow fibre structure in the non-solvent.

[0107] The step of forming a hollow fibre green body may be repeated to provide a plurality of hollow fibre green bodies. By “plurality of hollow fibre green bodies”, we mean 4 or more, such as 100 or more, or even 1000 or more of the hollow fibre green bodies. A single hollow fibre green body may be divided to provide a plurality of (shorter) hollow fibre green bodies. The hollow fibre green body may be divided by any suitably method, such as by cutting. Where the (multilayer) hollow fibre structure comprises an elongate (multilayer) hollow fibre structure formed by continuous extrusion, the hollow fibre green body may be cut into a plurality of (shorter) hollow fibre green bodies. The step of forming a plurality of hollow fibre green bodies suitably further comprises removing the non-solvent from the hollow fibre green body, for example by drying the hollow fibre green body at a temperature of from 10 to 70°C, such as from 20 to 60°C, or even from 30 to 50°C.

[0108] Suitably, the hollow fibre green body is precipitated having the same shape, size and arrangement of layers as the (multilayer) hollow fibre structure from which it is formed.

[0109] The step of forming a plurality of hollow fibre green bodies may comprise: providing:

[0110] -a sacrificial composition comprising a polymer and a solvent, wherein the polymer is soluble in the solvent,

[0111] -a support composition comprising a substrate material, a polymer and a solvent, wherein the polymer of the support composition is soluble in the solvent of the support composition and is immiscible with the polymer of the sacrificial composition, and

[0112] -a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the polymer of the fusing composition is soluble in the solvent of the fusing composition, and the fusing agent is miscible with the polymer of the fusing composition; forming the sacrificial composition, the support composition and the fusing composition into a multilayer hollow fibre structure wherein the sacrificial composition forms a sacrificial layer, the support composition forms a support layer, the fusing composition forms a fusing layer, the support layer is adjacent to the sacrificial layer and the support layer is adjacent to the fusing layer, wherein adjacent layers are in contact with one another; and contacting the multilayer hollow fibre structure with a non-solvent in which the polymer of the sacrificial composition, polymer of the support composition, and the polymer of the fusing composition are insoluble to precipitate said polymers, thereby forming a hollow fibre green body.

[0113] The step of forming a plurality of hollow fibre green bodies may comprise: providing:

[0114] -a sacrificial composition comprising a polymer and a solvent, wherein the polymer is soluble in the solvent, and the polymer comprises a polyester,

[0115] -a support composition comprising a substrate material, a polymer and a solvent, wherein the polymer of the support composition is soluble in solvent of the support composition, and

[0116] -a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the fusing agent comprises a polymer, wherein the polymer of the fusing composition and the polymer of the fusing agent are soluble in the solvent of the fusing composition, and the polymer of the fusing agent comprises an epoxy resin; wherein the polymer of the support composition and polymer of the fusing composition each independently comprise one or more of polyethersulfone, polysulfone, cellulose and its derivatives, polyamide, polyetherimide, polyimide and its derivatives, polyvinylidene fluoride, and polydimethylsiloxane; the substrate material of the support composition and the substrate material of the fusing composition each independently comprise one or more of a ceramic, cordierite, zirconia, yttrium-stabilised zirconia, titania, silicon carbide, clay, alumina, stainless steel, FeCr alloys, alloys of iron, alloys of aluminium, aluminium titanate, sintered metals, or a zeolite; and the solvent of the sacrificial composition, the solvent of the support composition and / or the solvent of the fusing composition each independently comprise a dipolar aprotic solvent; forming the sacrificial composition, the support composition and the fusing composition into a multilayer hollow fibre structure wherein the sacrificial composition forms a sacrificial layer, the support composition forms a support layer, and the fusing composition forms a fusing layer and the support layer is adjacent to the fusing layer; and contacting the multilayer hollow fibre structure with a non-solvent in which the polymer of the sacrificial composition, the polymer of the support composition, the polymer of the fusing composition and the polymer of the fusing agent are insoluble to precipitate said polymers, thereby forming a hollow fibre green body, wherein the non-solvent comprises a polar protic solvent.

[0117] The step of forming a plurality of hollow fibre green bodies may comprise: providing:

[0118] -a sacrificial composition comprising a polymer and a solvent, wherein the polymer is soluble in the solvent,

[0119] -a support composition comprising a substrate material, a polymer and a solvent, wherein the polymer of the support composition is soluble in the solvent of the support composition, and -a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the polymer of the fusing composition is soluble in the solvent of the fusing composition; wherein the substrate material of the support composition and the substrate material of the fusing composition are the same; the polymer of the support composition and the polymer of the fusing composition are the same; the solvent of the support composition and the solvent of the fusing composition are the same; and the support composition is substantially free from the fusing agent; forming the sacrificial composition, the support composition and the fusing composition into a multilayer hollow fibre structure wherein the sacrificial composition forms a sacrificial layer, the support composition forms a support layer, and the fusing composition forms a fusing layer and the support layer is adjacent to the fusing layer; and contacting the multilayer hollow fibre structure with a non-solvent in which the polymer of the sacrificial composition, the polymer of the support composition and the polymer of the fusing composition are insoluble to precipitate said polymers, thereby forming a hollow fibre green body.

[0120] The method of the first aspect comprises arranging the plurality of hollow fibre green bodies to form an assembly of hollow fibre green bodies. By “assembly of hollow fibre green bodies”, we mean a predetermined arrangement of the hollow fibre green bodies. The assembly may comprise a regular repeating pattern of the hollow fibre green bodies.

[0121] The assembly may comprise hollow fibre green bodies having different shapes and / or sizes. Alternatively, the assembly may comprise hollow fibre green bodies having the same shape and / or size. Preferably, the assembly comprises hollow fibre green bodies having the same length. Preferably, all of the hollow fibre green bodies have the same length. Preferably, the hollow fibre green bodies are aligned such that the assembly has the same length as the hollow fibre green bodies.

[0122] The assembly suitably comprises the plurality of hollow fibre green bodies in a close packed configuration. The hollow fibre green bodies may be in a cubic close packed or in a hexagonal close packed configuration. A hexagonal close packed configuration is preferred, because it allows for a higher density of hollow fibres.

[0123] The assembly may have any suitable cross section. Suitably, the assembly has a circular, oval, square, rectangular or hexagonal cross section (i.e. out of plane with the longitudinal axis of the hollow fibres). Circular or hexagonal cross sections are preferred. In some embodiments, the assembly has a hexagonal cross section and the plurality of hollow fibre green bodies are in a hexagonal close packed configuration.

[0124] The assembly has a plurality of bore channels and a plurality of interstitial channels. In a cubic close packed configuration, each interstitial channel is suitably formed by the void between four hollow fibre green bodies. In a hexagonal close packed configuration, each interstitial channel is suitably formed by the void between three hollow fibre green bodies.

[0125] In the first aspect of the invention, the method comprises manipulating the volume between the hollow fibre green bodies such that at least a portion of the interstitial channels in the filter are in fluid communication (i.e. with each other). Suitably, the step of forming the plurality of hollow fibre green bodies and / or the step of forming the assembly of hollow fibre green bodies comprises manipulating the volume between the hollow fibre green bodies such that at least a portion of the interstitial channels in the filter are in fluid communication. By “fluid communication”, we mean that a fluid may flow from one interstitial channel to another without passing through a wall of a hollow fibre, i.e. the fluid may flow unfiltered between the interstitial channels.

[0126] The volume between the hollow fibre green bodies may be referred to as the interstitial volume. Manipulating the interstitial volume suitably comprises forming tunnels (which may also be referred to as gaps) between the interstitial channels. The interstitial channels and the tunnels form at least part of the interstitial volume. The interstitial volume may consist of the interstitial channels and the tunnels. The tunnels provide a means for fluid communication between the interstitial channels. The tunnels do not intersect with the bore channels. Therefore, the bore channels in the filter are not in fluid communication with the interstitial channels. The bore channels are also not in fluid communication with each other. Each bore channel is isolated (from the other bore channels, from the interstitial channels, and from the tunnels) by the wall of each hollow fibre. Therefore, fluid flowing through the bore channels must pass through the walls of the hollow fibres to enter the interstitial volume. In doing so, the fluid is filtered. Each bore channel suitably has only two openings, one at each end of the filter.

[0127] The presence of tunnels between the interstitial channels in the assembly of hollow fibre green bodies (and the filter) improves drainage of fluid from within the interstitial channels, as it avoids the need for the fluid to pass through (multiple) hollow fibre walls. Instead, the fluid may flow through the tunnels.

[0128] Suitably, the method comprises manipulating the volume between the hollow fibre green bodies such that each interstitial channel is in fluid communication. By this we mean that each interstitial channel in the assembly of hollow fibre green bodies (and in the filter) is suitably in fluid communication with at least one other interstitial channel. Each interstitial channel is preferably in fluid communication with at least one other interstitial channel via a tunnel. The assembly of hollow fibre green bodies (and the filter) suitably comprises at least one tunnel per interstitial channel. In some embodiments, the assembly comprises one tunnel per interstitial channel.

[0129] The assembly of the hollow fibre green bodies (and the filter) suitably comprises at least one tunnel putting the interstitial channels in fluid communication with the outside of the filter. This may further improve drainage of permeate from the filter.

[0130] The tunnels may be formed by using hollow fibre green bodies that are non-uniform along the lengths of the hollow fibre green bodies. The step of forming the plurality of hollow fibre green bodies may comprise forming a plurality of hollow fibre green bodies that are non-uniform along their lengths. When the hollow fibre green bodies that are non-uniform along their lengths are arranged in a close packed configuration, the hollow fibre green bodies are not in contact along their entire lengths. This creates gaps, corresponding to tunnels, between the hollow fibre green bodies. An advantage of using hollow fibre green bodies that are non-uniform along their lengths is that a large number of tunnels are formed, improving the drainage of the filter.

[0131] By “hollow fibre green bodies that are non-uniform along their lengths” we mean that the outer surface of the hollow fibre green bodies is non-linear along their lengths, such as to create gaps between the hollow fibre green bodies in a close packed configuration. Hollow fibre green bodies that are non-uniform along their lengths may be referred to herein as non-uniform hollow fibre green bodies. The outer surface of the non-uniform hollow fibre green bodies suitably undulates along their lengths. The non-uniform hollow fibre green bodies themselves may undulate along their lengths (e.g. such that the bore channels undulate). The non-uniform hollow fibre green bodies may have non-uniform diameters along their lengths. For example, the hollow fibre green bodies may have a plurality of sections along their lengths having a first diameter and a plurality of sections along their length having a second diameter, wherein the first diameter is different from the second diameter. Unless otherwise specified, by the “diameter” of a hollow fibre we mean the outer diameter thereof. The non-uniform hollow fibre green bodies may have outer surfaces having non-uniform cross sections along the lengths of the hollow fibre green bodies. For example, the hollow fibre green bodies may have a plurality of sections along their lengths having an outer surface with a first cross section (such as a circular cross section) and a plurality of sections along their length having an outer surface with a second cross section (such as an oval cross section), wherein the first cross section is different from the second cross section.

[0132] Non-uniform hollow fibre green bodies may be formed by extruding the (multilayer) hollow fibre structure in a non-uniform manner. For example, the (multilayer) hollow fibre structure may be subjected to oscillations (e.g. vibrations) during extrusion. Oscillations (e.g. vibrations) may be applied to the (multilayer) hollow fibre structure by oscillating (e.g. vibrating) the extrusion die. For example, the extrusion die may be vibrated directly by an external source of vibrations such as a vibrating device. Alternatively, the fibre diameter may be extruded in a non-uniform manner by modifying extrusion parameters (such as by independently or concurrently varying the extrusion rates of the compositions in the hollow fibre structure, for example by pulsing the extrusion rates), by adjusting the extrusion die during extrusion (e.g. when the extrusion die is adjustable so as to change its internal diameter), by intermittently modifying the cross section of the hollow fibre structure during extrusion (such as by crimping).

[0133] The hollow fibre green bodies having a non-uniform diameter suitably have a standard deviation of the fibre diameter along their lengths of at least 1 .2%, such as at least 1 .5%, for example at least 2.0% of the mean fibre diameter. The hollow fibre green bodies having a non-uniform diameter suitably have a standard deviation of the fibre diameter along their lengths of from 1 .2 to 10%, such as from 1 .5 to 7.0%, for example from 2.0 to 4.0% of the mean fibre diameter. The hollow fibre green bodies having a non-uniform diameter suitably have an interpercentile range of the fibre diameter of at least 1 .5%, such as at least 2.0%, for example at least 3.0% of the mean fibre diameter. The hollow fibre green bodies having a non-uniform diameter suitably have an interpercentile range of the fibre diameter of from 1 .5 to 12%, such as from 2.0 to 8.0%, for example from 3.0 to 5.0% of the mean fibre diameter.

[0134] The fibre diameter may be measured by a laser micrometer.

[0135] In some embodiments, the tunnels may be formed by placing a spacer in the assembly.

[0136] In embodiments where a spacer is inserted into the assembly, the hollow fibre green bodies are suitably uniform along their lengths. Such hollow fibre green bodies have a uniform diameter and cross section along their lengths. This suitably results in an assembly in which the hollow fibre green bodies are in contact along their entire lengths apart from the location of the spacer. This may advantageously allow for a greater degree of bonding between the hollow fibre green bodies and therefore a more robust filter substrate.

[0137] The hollow fibre green bodies having a uniform diameter suitably have a standard deviation of the fibre diameter along their lengths of less than 1.2%, such as less than 1.1 %, for example less than 1 .0% of the mean fibre diameter.

[0138] The hollow fibre green bodies having a uniform diameter suitably have an interpercentile range of the fibre diameter of less than 1 .5%, such as less than 1 .4%, for example less than 1 .3% of the mean fibre diameter.

[0139] The spacer is placed in between the hollow fibre green bodies (i.e. in the interstitial volume). The spacer may be placed in between the hollow fibre green bodies during the formation of the assembly or the spacer may be inserted in between the hollow fibre green bodies after the assembly has been formed. If the spacer protrudes beyond the outside of the assembly after being placed, the protruding portion may be removed.

[0140] The spacer is suitably formed from a combustible material. This allows the spacer to burn off during the heat treatment step and leave tunnels in the filter substrate.

[0141] In some embodiments, the spacer may be in the form of a thread or ribbon. The spacer is suitably at least as long as the width of the assembly. By “width of the assembly”, we mean a dimension perpendicular to the longitudinal axis (and the bore channels and the interstitial channels) of the assembly. The spacer may be placed such that it is perpendicular to the interstitial channels and extends from one side of the assembly to another. Preferably, the spacer is placed such that it intersects at least two interstitial channels. When the spacer is burned off, it will leave a tunnel between said interstitial channels. Each interstitial channel in the assembly is preferably intersected by a spacer.

[0142] Suitably, at least two spacers are placed in the assembly. The at least two spacers suitably comprise parallel spacers. The parallel spacers are suitably arranged in a staggered configuration. By “staggered configuration” we mean that no two parallel spacers are the same distance along the length of the assembly. For example, the assembly may comprise three parallel spacers, placed at 25%, 50% and 75% the length of the assembly, respectively. A staggered configuration of parallel spacers may be advantageous as a high concentration of tunnels at the same location along the filter substrate may create weak points in the substrate.

[0143] The at least two spacers may comprise non-parallel spacers. The non-parallel spacers may comprise perpendicular spacers. In some embodiments, at least three spacers may be placed in the assembly, wherein the spacers comprise a mixture of parallel and non-parallel spacers.

[0144] When the hollow fibre green bodies in the assembly are in a close packed configuration, the assembly comprises layers of the hollow fibre green bodies. Suitably, a spacer is placed in between each layer of hollow fibre green bodies.

[0145] The spacer is suitably thin or flat enough that it does not significantly disrupt the fusing of the hollow fibres at locations other than where the spacer is placed. Suitably, the spacer has a thickness that is less than 20%, such as less than 10%, for example less than 5% of the diameter of the hollow fibre green bodies. The spacer may have a thickness that is from 1 to 20%, such as from 2 to 10%, for example from 3 to 5% of the diameter of the hollow fibre green bodies. The spacer may have a thickness of from 0.1 to 1 mm, such as from 0.1 to 0.7 mm, for example from 0.1 to 0.5 mm.

[0146] Suitably, the spacer is in a linear conformation in the assembly. In some embodiments, the spacer is in a non-linear conformation in the assembly. For example, the spacer may be wrapped around one or more hollow fibre green bodies.

[0147] The spacer is suitably not adhesive. The spacer suitably does not adhere to the hollow fibre green bodies. The spacer is suitably flexible. The spacer suitably comprises a polymer. The polymer is suitably an organic polymer. The polymer may comprise a fluoropolymer. Examples of suitable fluoropolymers include polyvinylidene difluoride (PVDF) and polytetrafluoroethylene (PTFE). The fluoropolymer preferably comprises polytetrafluoroethylene (PTFE). In some embodiments, the spacer may be formed by coating a coating composition onto at least a portion of the plurality of hollow fibre green bodies. The coating composition may be coated by any suitable method, such as brushing or spraying. The coating composition is preferably coated on a portion of the outer surface of the hollow fibre green bodies. After coating, the coating composition is suitably dried to form a spacer on the outer surface of the hollow fibre green bodies. The hollow fibre green bodies may be arranged to form the assembly before the coating composition is dried. The coating composition may be coated on only a portion of the hollow fibre green bodies, or it may be coated on each hollow fibre green body. Each coated hollow fibre green body is preferably only partially coated with the coating composition. This allows the uncoated portion of the hollow fibre green bodies to fuse together.

[0148] The spacer formed from a coating composition suitably has a thickness that is less than 20%, such as less than 10%, for example less than 5% of the diameter of the hollow fibre green bodies. The spacer may have a thickness that is from 1 to 20%, such as from 2 to 10%, for example from 3 to 5% of the diameter of the hollow fibre green bodies. The spacer may have a thickness of from 0.05 to 0.7 mm, such as from 0.05 to 0.5 mm, for example from 0.05 to 0.3 mm.

[0149] The coating composition suitably comprises a polymer. The polymer is suitably an organic polymer. The polymer may comprise a fluoropolymer. Examples of suitable fluoropolymers include polyvinylidene difluoride (PVDF) and polytetrafluoroethylene (PTFE). The fluoropolymer preferably comprises polytetrafluoroethylene (PTFE). The coating composition suitably further comprises a liquid medium. The polymer may be dissolved or suspended in the liquid medium. The liquid medium may comprise an organic solvent and / or water.

[0150] The assembly may comprise an additional green body. For example, the assembly may comprise a single layer green body prepared by contacting a support composition or a fusing composition with a non-solvent. The preferred features of the support composition, the fusing composition and the non-solvent are as described in relation to the hollow fibre green bodies. Suitably, the assembly comprises a single layer green body prepared by contacting a fusing composition with a non-solvent. Preferably the fusing composition used to prepare the hollow fibre green bodies and the fusing composition used to prepare the additional green body are the same. The assembly may comprise a plurality of the additional green body.

[0151] The additional green body may have any suitable shape, such as a sheet or a hollow shape, such as a hollow cylinder or tube having a circular, oval, square, rectangular or hexagonal cross section. Suitably, the additional green body has a shape adapted to encompass the plurality of hollow fibre green bodies. The additional green body is suitably placed around the plurality of the hollow fibre green bodies, i.e. so as to encompass the plurality of hollow fibre green bodies. This improves the strength of the substrate ultimately obtained.

[0152] The additional green body may be formed by any suitable method. Suitably, the additional green body is formed by moulding or extrusion.

[0153] The additional green body may be formed by introducing the support composition or the fusing composition into a mould and contacting said composition with the non-solvent, for example by immersing the mould containing the support composition or fusing composition in the nonsolvent.

[0154] The additional green body may be formed by extruding the support composition or the fusing composition from the die. The support composition or fusing composition is suitably extruded directly into the non-solvent (e.g. such that the support composition or fusing composition does not come into contact with air). For example, the outlet of the extrusion die may be immersed in the non-solvent. The extrusion may be carried out continuously to form an elongate additional green body.

[0155] The hollow fibre green bodies in the assembly are suitably in contact with one another. Preferably, the hollow fibre green bodies are in contact via the fusing layer of the hollow fibre green bodies.

[0156] The assembly may comprise a plurality of parallel, contacting hollow fibre green bodies. An outer surface of the hollow fibre green bodies is suitably the fusing layer thereof.

[0157] In some embodiments, the hollow fibre green bodies are linear. The plurality of parallel, contacting hollow fibre green bodies may be housed in an additional green body adapted to encompass said plurality, which is suitably prepared by contacting the fusing composition with the non-solvent.

[0158] In some embodiments, the hollow fibre green bodies are non-linear. The hollow fibre green bodies may be curved. The hollow fibre green bodies may be curved by any suitable means. The hollow fibre green bodies may be curved by weaving or braiding the hollow fibre green bodies together, or by twisting a plurality of linear, parallel hollow fibre green bodies (suitably about the longitudinal axis of the plurality of hollow fibres). The plurality of hollow fibre green bodies suitably comprises from 10 to 50, such as from 15 to 45, or even from 19 to 37 hollow fibre green bodies. A plurality comprising more than 50 non-linear hollow fibre green bodies may be more difficult to arrange. The presence of polymers in the hollow fibre green bodies makes the hollow fibre green bodies flexible, advantageously allowing the hollow fibre green bodies to be curved and arranged into a plurality of non-linear hollow fibre green bodies as described above. This is not possible with green bodies that do not comprise polymers, such as ceramic green bodies. Non-linear hollow fibre green bodies may advantageously create turbulent flow in a fluid flowing through the hollow fibre, such as by the formation of Dean vortices in the fluid. This may reduce the build-up of (solid) particles in the walls of the hollow fibre.

[0159] The plurality of non-linear, contacting hollow fibre green bodies may be housed in an additional green body adapted to encompass said plurality, which is suitably prepared by contacting the fusing composition with the non-solvent. When the plurality of hollow fibre green bodies is formed by twisting a plurality of linear, parallel hollow fibre green bodies, housing the plurality in the additional green body advantageously helps to hold the twisted plurality in place until fusion of the hollow fibre green bodies can take place.

[0160] The method of the first aspect of the invention comprises the step of heat treating the assembly of hollow fibre green bodies to form a filter substrate.

[0161] The step of heat treating the assembly suitably comprises fusing the hollow fibre green bodies in the assembly together, thereby forming a precursor substrate.

[0162] The hollow fibre green bodies may be fused in any suitable way, for example by heating the assembly or by contacting the assembly with a fusing solvent. Preferably the hollow fibre green bodies are fused by heating the assembly. Suitable ways of fusing the hollow fibre green bodies involve causing at least a portion of the fusing layers of the green bodies to soften and merge with one another. Suitably, at least a portion of the fusing layers is merged by at least 10%, such as at least 30%, for example at least 50% of the initial thickness of either of the fusing layers (i.e. the thickness of the fusing layers prior to merging).

[0163] After fusing, the degree of overlap between the walls of the fused hollow fibres is suitably from 1 to 75%, such as from 5 to 50%, for example from 10 to 30%. The degree of overlap may be calculated according to the equation:

[0164] 100 x (a+b-d) / (a+b) = degree of overlap wherein a is the unfused thickness of the wall a first hollow fibre (i.e. the thickness of the wall adjacent to an interstitial channel); b is the unfused thickness of the wall a second hollow fibre; and d is the combined thickness of the walls of the first hollow fibre and the second hollow fibre at the point of fusion, i.e. as measured between the longitudinal axis of each hollow fibre.

[0165] The hollow fibre green bodies may be fused by heating the assembly to a temperature at which the fusing layer of the hollow fibre green bodies softens (e.g. above the glass transition temperature of the fusing layer). The temperature is suitably at least 50°C, such as at least 100°C, for example at least 130°C. Once the fusing layer of a hollow fibre green body has become soft, it can merge with the fusing layer of another hollow fibre green body, thereby fusing the hollow fibre green bodies in the assembly together once the temperature is reduced. The hollow fibre green bodies are suitably fused at a temperature below the temperature at which the precursor substrate is sintered (e.g. below 1300°C, such as below 1200°C, for example below 1000°C). The hollow fibre green bodies are suitably fused at a temperature below the temperature at which organic polymers are removed from the precursor substrate (e.g. below 500°C, such as below 400°C, for example below 300°C). The hollow fibre green bodies may be fused by heating the assembly to a temperature of from 50 to 300°C, such as from 100 to 250°C, or even from 130 to 170°C.

[0166] The hollow fibre green bodies may be fused by contacting the assembly with a fusing solvent. Suitably the fusing agent is soluble in the fusing solvent and the polymer of the fusing composition is insoluble in the fusing solvent. The effect of this is that the fusing solvent causes the fusing layers of the hollow fibre green bodies in the assembly to soften and merge with one another. Suitably the polymer of the sacrificial composition (when present) and the polymer of the support composition (when present) are also insoluble in the fusing solvent.

[0167] Advantageously the hollow fibre green bodies can be fused under conditions that do not cause a softening of the support layer (when present). Therefore the hollow fibre green bodies may be fused while maintaining their shape.

[0168] Suitably, the hollow fibre green bodies are urged together whilst being fused. The hollow fibre green bodies may be urged together by compressing the assembly. For example, the assembly may be compressed by wrapping the assembly in a compression device. Suitable compression devices include a resilient band or sleeve (such as a silicone rubber band), a resilient tape (such as a spiral wound silicone tape), an inflatable bladder (such as a silicone bladder), or a mould. The mould may comprise at least two parts which may be urged together. Urging the hollow fibre green bodies together improves the fusing thereof.

[0169] The method of the first aspect of the invention may further comprise removing the polymer of the sacrificial composition (when present) from the hollow fibre green body, the assembly, or the precursor substrate. Suitably, the sacrificial layer consists of the polymer of the sacrificial composition. This step may be carried out before the polymer of the support composition (when present), the polymer of the fusing composition and the polymer of the fusing agent (when present) are removed from the precursor substrate during the heat treatment step. Preferably, the polymer of the sacrificial composition is removed from the hollow fibre green body before the hollow fibre green body is arranged into the assembly. The polymer of the sacrificial composition may be removed by any suitable means, such as manually, mechanically or by dissolution. The polymer of the sacrificial composition may be manually or mechanically pulled or peeled from the hollow fibre green body, the assembly, or the precursor substrate in order to remove the polymer of the sacrificial composition. Alternatively, the polymer of the sacrificial composition may be removed by dissolving the polymer in a removal solvent. Suitably, the polymer of the sacrificial composition is soluble in the removal solvent and the polymer of the support composition (when present), the polymer of the fusing composition and the polymer of the fusing agent (when present) are insoluble in the removal solvent.

[0170] Suitably, the hollow fibre green body comprises the sacrificial layer as an inner layer enveloping a bore fluid. The method may comprise removing the sacrificial layer from the hollow fibre green body, the assembly, or the precursor substrate together with the bore fluid. This prevents the bore fluid from being present during sintering where it may contaminate the substrate during sintering if the bore fluid is not volatile or combustible (for example when the bore fluid comprises a silicone oil).

[0171] The step of heat treating the assembly suitably comprises heating the precursor substrate to remove organic polymers from the precursor substrate prior to sintering. This step suitably also removes the spacer (when present) from the precursor substrate. This step is preferably carried out when the polymer of the support composition (when present), the polymer of the fusing composition, the polymer of the fusing agent (when present), and the polymer of the spacer (when present) are organic polymers. In order to remove organic polymers from the precursor substrate, the precursor substrate is suitably heated to a temperature of from 300 to 800°C, such as from 400 to 700°C, or even from 500 to 600°C. Preferably the heating is carried out under an oxygen-containing atmosphere, such as air. At these temperatures, the organic polymers typically ‘burn off’.

[0172] The step of heat treating the assembly suitably comprises sintering the precursor substrate, thereby forming a filter substrate.

[0173] The precursor substrate may be sintered at a temperature of at least 1000°C, such as at least 1200°C, or even at least 1300°C. The precursor substrate may be sintered at a temperature of from 1000 to 1600°C, such as from 1200 to 1500°C, or even from 1300 to 1400°C. The sintering is suitably carried out under an oxygen-containing atmosphere, such as air. Suitably, the step of heat treating the assembly comprises: fusing the hollow fibre green bodies in the assembly together by heating the assembly, wherein the hollow fibre green bodies are urged together whilst being fused, thereby forming a precursor substrate; heating the precursor substrate to remove organic polymers from the precursor substrate prior to sintering; and sintering the precursor substrate, thereby forming a filter substrate.

[0174] The filter substrate comprises a plurality of fused hollow fibres. Suitably, the filter substrate comprises a plurality of parallel, fused hollow fibres, preferably wherein the hollow fibres are linear. Alternatively, the filter substrate may comprise a plurality of fused hollow fibres wherein the fibres are non-linear.

[0175] The filter substrate is suitably free from polymers and solvents. The filter substrate may consist of the substrate material of the support composition (when present) and the substrate material of the fusing composition. Suitably, the filter substrate is porous and comprises micro-channels. Suitably, the substrate comprises pores having a pore size of from 0.01 to 12 pm, preferably from 0.01 to 5 pm, such as from 0.1 to 3 pm, or even from 0.5 to 1 pm. Pore size may be measured by mercury intrusion porosimetry or capillary flow porometry. Suitably, the microchannels extend from one surface of each hollow fibre to another surface of the hollow fibre (preferably from an outer surface to an inner surface of the hollow fibre) and have an entrance diameter of from 5 pm to 200 pm. In some embodiments, the micro-channels penetrate from one surface of each hollow fibre to another surface of the hollow fibre. In some preferred embodiments, the micro-channels do not penetrate from one surface of each hollow fibre to another surface of the hollow fibre. In such embodiments, the micro-channels suitably have openings in an outer surface of the hollow fibres. The micro-channels suitably only extend from 30 to 99%, such as from 50 to 95%, for example from 70 to 90% of the thickness of the walls of the hollow fibres.

[0176] The hollow fibres in the filter substrate may comprise two or more layers having different microchannel geometries. For example, the hollow fibres may comprise a first layer having microchannels having an entrance diameter of from 5 to 20 pm, such as from 5 to 15 pm, or even from 5 to 10 pm and a second layer having micro-channels having an entrance diameter of from 20 to 200 pm, such as from 30 to 200 pm, or even from 50 to 200 pm. It is believed that the micro-channel geometries of a hollow fibre layer can be varied by selecting the viscosities of the support composition (when present) and the fusing composition.

[0177] The method of the first aspect of the invention may comprise: forming a plurality of hollow fibre green bodies, which step comprises providing a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the polymer is soluble in the solvent; forming the fusing composition into a hollow fibre structure wherein the fusing composition forms a fusing layer; and contacting the fusing layer with a non-solvent in which the polymer is insoluble to precipitate said polymer, thereby forming a hollow fibre green body; arranging the plurality of hollow fibre green bodies to form an assembly of hollow fibre green bodies having a plurality of bore channels and a plurality of interstitial channels, wherein the hollow fibre green bodies in the assembly are in contact with one another via the fusing layer of the hollow fibre green bodies; fusing the hollow fibre green bodies in the assembly together by heating the assembly, wherein the hollow fibre green bodies are urged together whilst being fused, thereby forming a precursor substrate; heating the precursor substrate to remove organic polymers from the precursor substrate prior to sintering; and sintering the precursor substrate, thereby forming a filter substrate.

[0178] The method of the first aspect of the invention may comprise: forming a plurality of hollow fibre green bodies, which step comprises providing a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the polymer is soluble in the solvent; forming the fusing composition into a hollow fibre structure wherein the fusing composition forms a fusing layer; and contacting the fusing layer with a non-solvent in which the polymer is insoluble to precipitate said polymer, thereby forming a hollow fibre green body; arranging the plurality of hollow fibre green bodies to form an assembly of hollow fibre green bodies having a plurality of bore channels and a plurality of interstitial channels, wherein the hollow fibre green bodies in the assembly are in contact with one another via the fusing layer of the hollow fibre green bodies; fusing the hollow fibre green bodies in the assembly together by heating the assembly to a temperature of from 50 to 300°C, wherein the hollow fibre green bodies are urged together whilst being fused, thereby forming a precursor substrate; heating the precursor substrate to a temperature of from 300 to 800°C to remove organic polymers from the precursor substrate prior to sintering; and sintering the precursor substrate at a temperature of at least 1000°C, thereby forming a filter substrate. In some embodiments where the filter substrate comprises micro-channels penetrating from one surface of each hollow fibre to another, such as micro-channels penetrating all the way through the walls of the hollow fibres, the method of the first aspect may comprise blocking at least some of the micro-channels. Suitably, the method comprises blocking at least 50%, such as at least 70%, or even at least 90%, of the micro-channels penetrating from one surface of each hollow fibre to another. Preferably, the method comprises blocking all micro-channels penetrating from one surface of each hollow fibre to another.

[0179] The method of the first aspect of the invention may further comprise depositing a coating on the filter substrate. The coating may change the filtration properties of the filter substrate, such as reducing the size of particles that can pass through the filter substrate. The coating may comprise a refractory material. As used herein, the term “refractory material” is used in its ordinary sense, which is well-known to those skilled in the art. Specifically, a refractory material is a material that is resistant to high temperatures (such as above 538°C). The refractory material suitably comprises alumina, silica, titania, silicon carbide and / or zirconia. Such materials may be used for water filtration. The coating may comprise a dense ceramic material. By “dense ceramic material” we mean a ceramic material which does not comprise pores which form a path through the material. Movement of a substance through the dense ceramic material may only occur through atomic or ionic diffusion through the bulk dense ceramic material. The dense ceramic material may be used for the separation of gases. The dense ceramic material may comprise yttrium stabilised zirconia or zirconia doped with yttrium, which may be used for oxygen separation.

[0180] In some embodiments, the method of the first aspect does not comprise depositing a coating on the filter substrate.

[0181] The method of the first aspect of the invention comprises sealing the interstitial channels . The method may comprise sealing the interstitial channels of the precursor substrate and / or the filter substrate. Preferably the method comprises sealing the interstitial channels of the filter substrate.

[0182] Suitably, the interstitial channels are sealed at one or both ends of the precursor substrate and / or filter substrate.

[0183] Suitably, the step of sealing the interstitial channels comprises sealing the interstitial channels at both ends of the precursor substate and / or filter substrate. By “ends of the precursor substrate and / or filter substrate” we mean the ends of the precursor substrate or filter substrate having openings to the bore channels. Suitably, the precursor substrate or filter substrate has two ends. Prior to the sealing step, the ends of the precursor substrate or filter substrate also have openings to the interstitial channels. Sealing the interstitial channels at the ends advantageously prevents leakage, i.e. it prevents transfer of fluid from the bore channels to the interstitial channels without passing through the walls of the hollow fibres. In some embodiments, the interstitial channels are sealed at both ends of the precursor substrate and / or filter substrate and the bore channels are kept open. Filters in which the interstitial channels are sealed at both ends and the bore channels are open may be particularly suitable for crossflow filtration. The seals in the interstitial channels may be referred to herein as interstitial seals.

[0184] The method of the first aspect of the invention may further comprise sealing the bore channels. Filters in which the interstitial channels and the bore channels are sealed may be particularly suitable for dead-end filtration. Each bore channel is preferably sealed at only one end of the precursor substrate and / or filter substrate. The bore channels may all be sealed at the same end of the precursor substrate and / or filter substrate. The bore channels may be sealed at one end of the precursor substrate and / or filter substrate and the interstitial channels may be sealed at the other end of the substrate, or the interstitial channels may be sealed at both ends of the substrate. Alternatively, the bore channels may be sealed at either end of the precursor substrate and / or filter substrate, preferably at alternating ends of the precursor substrate and / or filter substrate (e.g. such that each bore channel is adjacent to at least one other bore channel that is sealed at the other end of the substrate). The bore channels may be sealed at either end of the precursor substrate and / or filter substrate (preferably at alternating ends) and the interstitial channels may be sealed at one end of the substrate, or the interstitial channels may be sealed at both ends of the substrate. The seals in the bore channels may be referred to herein as bore seals.

[0185] The interstitial channels may be sealed by inserting a sealing composition therein. The bore channels may also be sealed by inserting a sealing composition therein. In some embodiments, the sealing composition is inserted into the interstitial channels and the bore channels (i.e. every opening) at the ends of the precursor substrate and / or filter substrate. The bore channels and / or interstitial channels may then be re-opened at one or both ends, as appropriate, for example by pushing through the sealing composition with a rod or pin. This may result in the sealing composition smearing against a portion of the inner surface of the hollow fibres. In some other embodiments, masking elements are inserted into the ends of the bore channels and / or interstitial channels to temporarily seal the bore channels and / or interstitial channels. The sealing composition is then inserted into the bore channels and / or interstitial channels that do not contain a masking element. The masking elements are then removed to re-open the bore channels and / or interstitial channels. The masking elements may be made from any suitable material, such as rubber. In embodiments where the interstitial channels are sealed at both ends of the filter, the interstitial channels are not re-opened or temporarily sealed with masking elements. The method of the first aspect of the invention may further comprise forming a seal around the outside of one or more of the ends of the precursor substrate and / or filter substrate. Such a seal may be referred to herein as an end cap. The end caps may advantageously allow the filter to have a tight fit in a filter membrane module without any leaks. Preferably, the method comprises forming a seal around each end of the precursor substrate and / or filter substrate.

[0186] An end cap is suitably formed by placing the ends of the precursor substrate and / or filter substrate in a mould and inserting a sealing composition between the outside of the ends of the substrate and the mould. The mould is then removed.

[0187] The end cap may have any suitable shape. Suitably, the shape of the end cap enables the filter to have a tight fit in a filter membrane module. Suitably, the end cap is circular, oval, square, rectangular or hexagonal. Circular or hexagonal end caps are preferred.

[0188] An end cap may not be required if the method of the first aspect of the invention comprises placing an additional green body around the plurality of the hollow fibre green bodies as described herein.

[0189] Suitably, the interstitial seals, the bore seals (when present) and the end caps (when present) are formed in the precursor substrate or the filter substrate. Preferably, they are formed in the filter substrate.

[0190] The interstitial seals, the bore seals (when present) and / or the end caps (when present) may be formed from any suitable material. In some embodiments, the interstitial seals, the bore seals (when present) and / or the end caps (when present) may be formed from a polymer, such as an epoxy resin. The polymer is suitably cured to form an impermeable (i.e. non-porous) seal.

[0191] In some embodiments, the interstitial seals, the bore seals (when present) and / or the end caps (when present) may be formed from a substrate material. In such embodiments, the substrate material may be applied to the precursor substrate prior to the sintering step such that the substrate material of the interstitial seals, bore seals and / or end caps sinters simultaneously with the substrate material of the hollow fibre green bodies. The substrate material of the interstitial seals, the bore seals (when present) and / or the end caps (when present) may be one or more of: a ceramic, cordierite, zirconia, yttrium-stabilised zirconia, titania, silicon carbide, clay, alumina, stainless steel, FeCr alloys, alloys of iron, alloys of aluminium, aluminium titanate, sintered metals, or a zeolite. Preferably, the substrate material comprises a ceramic. The substrate material may be porous. If the substrate material is porous, the interstitial seals, the bore seals and / or the end caps may be coated with an impermeable coating composition. Suitably, the interstitial seals, the bore seals and the end caps are formed from the same material.

[0192] The interstitial seals, the bore seals (when present) and / or the end caps (when present) suitably extend inwards from the ends of the filter along the length of the filter. The interstitial seals, the bore seals (when present) and / or the end caps (when present) may have thickness of up to 20%, such as up to 10%, for example up to 5% of the length of the filter. The interstitial seals, the bore seals and the end caps may have the same thickness.

[0193] According to a second aspect of the present invention, there is provided a filter comprising a substrate, wherein the substrate comprises a plurality of bore channels and a plurality of interstitial channels, wherein at least a portion of the interstitial channels are in fluid communication.

[0194] Suitably, the plurality of bore channels comprises 4 or more, such as 100 or more, or even 1000 or more bore channels. Suitably, the plurality of interstitial channels comprises 4 or more, such as 100 or more, or even 1000 or more interstitial channels.

[0195] Each of the bore channels suitably has a circular or oval cross section. Preferably, each of the bore channels has a circular cross section.

[0196] The bore channels and the interstitial channels are suitably parallel. The bore channels and the interstitial channels are suitably linear.

[0197] In some embodiments, the bore channels and the interstitial channels are non-linear. The bore channels and the interstitial channels may be curved. The plurality of bore channels suitably comprises from 10 to 50, such as from 15 to 45, or even from 19 to 37 bore channels.

[0198] Non-linear bore channels may advantageously create turbulent flow in a fluid flowing through the bore channels, such as by the formation of Dean vortices in the fluid. This may reduce the build-up of (solid) particles in the walls of the bore channels.

[0199] The interstitial channels are in fluid communication (i.e. with each other). This improves the drainage of permeate from the filter in use.

[0200] The interstitial channels are suitably in fluid communication by means of tunnels between the interstitial channels. The tunnels do not intersect with the bore channels. Therefore, the bore channels in the filter are not in fluid communication with the interstitial channels. The bore channels are also not in fluid communication with each other. Each bore channel is isolated (from the other bore channels, from the interstitial channels, and from the tunnels) by the substrate. Therefore, fluid flowing through the bore channels must pass through the walls of the hollow fibres to enter the interstitial volume. In doing so, the fluid is filtered. Each bore channel suitably has only two openings, one at each end of the filter.

[0201] Each interstitial channel is suitably in fluid communication with at least one other interstitial channel. Each interstitial channel is preferably in fluid communication with at least one other interstitial channel via a tunnel. The filter suitably comprises at least one tunnel per interstitial channel. In some embodiments, the filter comprises one tunnel per interstitial channel.

[0202] The filter suitably comprises at least one tunnel putting the interstitial channels in fluid communication with the outside of the filter. This may further improve drainage of permeate from the filter.

[0203] The substrate suitably comprises a plurality of fused hollow fibres. Each hollow fibre corresponds to one of the plurality of bore channels. The plurality of hollow fibres may comprise 4 or more, such as 100 or more, or even 1000 or more hollow fibres.

[0204] Each hollow fibre suitably comprises one or more layers. For example, each hollow fibre may consist of one layer. Alternatively, at least a portion of the plurality of hollow fibres may comprise two or more layers. The layers are suitably in the form of concentric hollow fibres. In some embodiments, each hollow fibre comprises two or more layers.

[0205] The substrate is suitably formed from a substrate material. The substrate material may comprise one or more of: a ceramic, cordierite, zirconia, yttrium-stabilised zirconia, titania, silicon carbide, clay, alumina, stainless steel, FeCr alloys, alloys of iron, alloys of aluminium, aluminium titanate, sintered metals, or a zeolite. Preferably, the substrate material of the support composition and / or the substrate material of the fusing composition comprises a ceramic. In embodiments where at least a portion of the plurality of hollow fibres comprises two or more layers, each layer may comprise a different substrate material. Preferably, the substrate material of each layer is the same. The substrate material is suitably sintered.

[0206] The substrate material may have a particle size of from 0.01 to 10 pm, such as from 0.05 to 5 pm, or even from 0.1 to 1 pm. In embodiments where at least a portion of the plurality of hollow fibres comprises two or more layers, each layer may comprise a substrate material having a different particle size. The substrate is suitably free from polymers and solvents. The substrate may consist of the substrate material. Suitably, the substrate is porous and comprises micro-channels. Suitably, the substrate comprises pores having a pore size of from 0.01 to 12 pm, preferably from 0.01 to 5 pm, such as from 0.1 to 3 pm, or even from 0.5 to 1 pm. Suitably, the micro-channels extend from one surface of each hollow fibre to another surface of the hollow fibre (preferably from an outer surface to an inner surface of the hollow fibre) and have an entrance diameter of from 5 pm to 200 pm. In some embodiments, the micro-channels penetrate from one surface of each hollow fibre to another surface of the hollow fibre. In some preferred embodiments, the microchannels do not penetrate from one surface of each hollow fibre to another surface of the hollow fibre. In such embodiments, the micro-channels suitably have openings in an outer surface of the hollow fibres. The micro-channels suitably only extend from 30 to 99%, such as from 50 to 95%, for example from 70 to 90% of the thickness of the walls of the hollow fibres. In some embodiments where at least a portion of the plurality of hollow fibres comprise two or more layers, the micro-channels do not penetrate into at least one of the layers.

[0207] In embodiments where at least a portion of the plurality of hollow fibres comprises two or more layers, each layer may have a different porosity and / or micro-channels having a different size.

[0208] The hollow fibres may comprise two or more layers having different micro-channel geometries. For example, the hollow fibres may comprise a first layer having micro-channels having an entrance diameter of from 5 to 20 pm, such as from 5 to 15 pm, or even from 5 to 10 pm and a second layer having micro-channels having an entrance diameter of from 20 to 200 pm, such as from 30 to 200 pm, or even from 50 to 200 pm.

[0209] The substrate may comprise a coating. The coating may be deposited on the substrate material. The coating may comprise a refractory material. The refractory material suitably comprises alumina, silica, titania, silicon carbide and / or zirconia. Such materials may be used for water filtration. The coating may comprise a dense ceramic material. The dense ceramic material is suitably gas-tight. The dense ceramic material may be used for the separation of gases, suitably by diffusion of certain gases through the ceramic material, for example via lattice diffusion or ionic diffusion. The dense ceramic material may comprise yttrium stabilised zirconia or zirconia doped with yttrium, which may be used for oxygen separation.

[0210] In some embodiments, the substrate does not comprise a coating.

[0211] The plurality of fused hollow fibres may comprise a regular repeating pattern of the hollow fibres. The plurality of fused hollow fibres may comprise hollow fibres having different shapes and / or sizes. Alternatively, the plurality of fused hollow fibres may comprise hollow fibres having the same shape and / or size. Preferably, the plurality of fused hollow fibres comprises hollow fibres having the same length. Preferably, all of the hollow fibres have the same length. Preferably, the hollow fibres are aligned such that the filter has the same length as the hollow fibres.

[0212] The plurality of fused hollow fibres is suitably in a close packed configuration. The hollow fibres may be in a cubic close packed or in a hexagonal close packed configuration. A hexagonal close packed configuration is preferred. In a cubic close packed configuration, each interstitial channel is suitably formed by the void between four hollow fibres. In a hexagonal close packed configuration, each interstitial channel is suitably formed by the void between three hollow fibres.

[0213] The plurality of fused hollow fibres may have any suitable cross section. Suitably, the plurality of fused hollow fibres has a circular, oval, square, rectangular or hexagonal cross section (i.e. out of plane with the longitudinal axis of the hollow fibres). Circular or hexagonal cross sections are preferred. In some embodiments, the plurality of fused hollow fibres has a hexagonal cross section and the hollow fibres are in a hexagonal close packed configuration.

[0214] The fused hollow fibres may be non-uniform along their lengths. When these hollow fibres are arranged in a close packed configuration, the hollow fibres are not fused along their entire lengths. This creates gaps, corresponding to tunnels, between the hollow fibres. An advantage of fused hollow fibres that are non-uniform along their lengths is that the filter may comprise a large number of tunnels, improving the drainage of the filter.

[0215] The fused hollow fibres that are non-uniform along their lengths may have a non-uniform diameter along their lengths. The hollow fibres having a non-uniform diameter suitably have a standard deviation of the fibre diameter along their lengths of at least 1.2%, such as at least 1.5%, for example at least 2.0% of the mean fibre diameter. The hollow fibres having a non- uniform diameter suitably have a standard deviation of the fibre diameter along their lengths of from 1.2 to 10%, such as from 1.5 to 7.0%, for example from 2.0 to 4.0% of the mean fibre diameter.

[0216] The hollow fibres having a non-uniform diameter suitably have an interpercentile range of the fibre diameter of at least 1.5%, such as at least 2.0%, for example at least 3.0% of the mean fibre diameter. The hollow fibres having a non-uniform diameter suitably have an interpercentile range of the fibre diameter of from 1 .5 to 12%, such as from 2.0 to 8.0%, for example from 3.0 to 5.0% of the mean fibre diameter.

[0217] Alternatively, the hollow fibres may be uniform along their lengths. Such hollow fibres have a uniform diameter and cross section along their lengths. This suitably results in filter in which the hollow fibres are fused along their entire lengths apart from the predetermined locations of the tunnels. This may advantageously result in a more robust filter. The hollow fibres having a uniform diameter suitably have a standard deviation of the fibre diameter along their lengths of less than 1 .2%, such as less than 1.1 %, for example less than 1.0% of the mean fibre diameter.

[0218] The hollow fibres having a uniform diameter suitably have an interpercentile range of the fibre diameter of less than 1.5%, such as less than 1.4%, for example less than 1.3% of the mean fibre diameter.

[0219] The filter may comprise a group of tunnels extending from one side of the filter to another. By “group of tunnels” we mean tunnels at the same distance along the length of the filter, wherein each of said tunnels branches from an interstitial channel that branches into another of said tunnels. The group of tunnels is suitably perpendicular to the interstitial channels. The filter may comprise at least two groups of tunnels extending from one side of the filter to another, which are suitably perpendicular to the interstitial channels. The at least two groups of tunnels suitably comprise parallel groups of tunnels. The parallel groups of tunnels are suitably arranged in a staggered configuration. By “staggered configuration” we mean that no two parallel groups of tunnels are the same distance along the length of the filter. For example, the filter may comprise three parallel groups of tunnels, placed at 25%, 50% and 75% the length of the filter, respectively. A staggered configuration of parallel groups of tunnels may be advantageous as a high concentration of tunnels at the same location along the filter may create weak points in the filter.

[0220] The at least two groups of tunnels may comprise non-parallel groups of tunnels. The non-parallel groups of tunnels may comprise perpendicular groups of tunnels. In some embodiments, the filter may comprise at least three groups of tunnels, wherein the groups of tunnels comprise a mixture of parallel and non-parallel groups of tunnels.

[0221] When the fused hollow fibres in the filter are in a close packed configuration, the filter comprises layers of the hollow fibres. Suitably, the filter comprises a group of tunnels in between each layer of hollow fibres.

[0222] Suitably, the tunnels have a width that is less than 20%, such as less than 10%, for example less than 5% of the diameter of the hollow fibres. The tunnels may have a width that is from 1 to 20%, such as from 2 to 10%, for example from 3 to 5% of the diameter of the hollow fibres. The tunnels may have a width of from 0.1 to 1 mm, such as from 0.1 to 0.7 mm, for example from 0.1 to 0.5 mm. Suitably, the group of tunnels is in a linear conformation in the filter. In some embodiments, the group of tunnels is in a non-linear conformation in the filter. For example, the group of tunnels may form a pathway through each interstitial channel around a hollow fibre.

[0223] The substrate may comprise an additional structure. The additional structure suitably comprises the same substrate material as the fused hollow fibres. The substrate may comprise a plurality of the additional structure. The additional structure may have any suitable shape, such as a sheet or a hollow shape, such as a hollow cylinder or tube having a circular, oval, square, rectangular or hexagonal cross section. Suitably, the additional structure has a shape adapted to encompass the plurality of fused hollow fibres. The additional structure is suitably placed around the plurality of fused hollow fibres, i.e. so as to encompass the plurality of fused hollow fibres. The additional structure is suitably fused to the plurality of fused hollow fibres.

[0224] The substrate may comprise a plurality of parallel, fused hollow fibres, preferably wherein the hollow fibres are linear. The plurality of parallel, fused hollow fibres may be housed in an additional structure adapted to encompass said plurality, which is suitably fused to the hollow fibres and comprises the same substrate material as the hollow fibres.

[0225] In some embodiments, the hollow fibres are non-linear. The hollow fibres may be curved. The hollow fibres may be woven or braided together, or twisted (suitably about the longitudinal axis of the plurality of hollow fibres). The plurality of hollow fibres suitably comprises from 10 to 50, such as from 15 to 45, or even from 19 to 37 hollow fibres.

[0226] Non-linear hollow fibres may advantageously create turbulent flow in a fluid flowing through the hollow fibre, such as by the formation of Dean vortices in the fluid. This may reduce the build-up of (solid) particles in the walls of the hollow fibre.

[0227] The plurality of non-linear, fused hollow fibres may be housed in an additional structure adapted to encompass said plurality, which is suitably fused to the hollow fibres and comprises the same substrate material as the hollow fibres.

[0228] The plurality of hollow fibres is suitably fused via an outer layer thereof. Suitably, at least a portion of the outer layers of the hollow fibres are merged with one another. Suitably, at least a portion of the fusing layers is merged by at least 10%, such as at least 30%, for example at least 50% of the unfused thickness of either of the outer layers (i.e. the thickness of said layer adjacent to an interstitial channel). Suitably, the degree of overlap between the walls of the fused hollow fibres is from 1 to 75%, such as from 5 to 50%, for example from 10 to 30%. The degree of overlap may be calculated as described herein.

[0229] The interstitial channels are suitably sealed. Suitably, the interstitial channels are sealed at one or both ends of the filter. The interstitial channels are suitably sealed at both ends of the filter. By “ends of the filter” we mean the ends of the filter having openings to the bore channels. Suitably, the filter has two ends. Sealing the interstitial channels at the ends advantageously prevents leakage, i.e. it prevents transfer of fluid from the bore channels to the interstitial channels without passing through the substrate. In some embodiments, the interstitial channels are sealed at both ends of the filter and the bore channels are open. Such filters may be particularly suitable for crossflow filtration. The seals in the interstitial channels may be referred to herein as interstitial seals.

[0230] The bore channels may be sealed. Filters in which the interstitial channels and the bore channels are sealed may be particularly suitable for dead-end filtration. Each bore channel is preferably sealed at only one end of the filter. The bore channels may all be sealed at the same end of the filter. The bore channels may be sealed at one end of the filter and the interstitial channels may be sealed at the other end of the filter, or the interstitial channels may be sealed at both ends of the filter. Alternatively, the bore channels may be sealed at either end of the filter, preferably at alternating ends of the filter (e.g. such that each bore channel is adjacent to at least one other bore channel that is sealed at the other end of the filter). The bore channels may be sealed at either end of the filter, preferably at alternating ends of the filter, and the interstitial channels may be sealed at one end of the filter, or the interstitial channels may be sealed at both ends of the filter. The seals in the bore channels may be referred to herein as bore seals.

[0231] The filter may further comprise seals around the outside of the ends of the filter. Such seals may be referred to herein as end caps. The end caps may advantageously allow the filter to have a tight fit in a filter membrane module without any leaks.

[0232] The end cap may have any suitable shape. Suitably, the shape of the end cap enables the filter to have a tight fit in a filter membrane module. Suitably, the end cap is circular, oval, square, rectangular or hexagonal. Circular or hexagonal end caps are preferred.

[0233] An end cap may not be required if the filter comprises an additional structure around the plurality of fused hollow fibres as described herein.

[0234] The interstitial seals, the bore seals and / or the end caps may be formed from any suitable material. In some embodiments, the interstitial seals, the bore seals and / or the end caps may be formed from a polymer, such as an epoxy resin. The polymer is suitably cured to form an impermeable (i.e. non-porous) seal.

[0235] In some embodiments, the interstitial seals, the bore seals and / or the end caps may be formed from a substrate material. The substrate material may be one or more of: a ceramic, cordierite, zirconia, yttrium-stabilised zirconia, titania, silicon carbide, clay, alumina, stainless steel, FeCr alloys, alloys of iron, alloys of aluminium, aluminium titanate, sintered metals, or a zeolite. Preferably, the substrate material comprises a ceramic. The substrate material may be porous. If the substrate material is porous, the interstitial seals, the bore seals and / or the end caps may be coated with an impermeable coating composition.

[0236] Suitably, the interstitial seals, the bore seals and the end caps are formed from the same material.

[0237] The interstitial seals, the bore seals and / or the end caps suitably extend inwards from the ends of the filter along the length of the filter. The interstitial seals, the bore seals and / or the end caps may have thickness of up to 20%, such as up to 10%, for example up to 5% of the length of the filter. The interstitial seals, the bore seals and the end caps may have the same thickness.

[0238] Preferably, the filter according to the second aspect is manufactured according to the method of the first aspect of the invention. A filter obtainable according to the method of the first aspect of the invention may be provided.

[0239] According to a third aspect of the invention, there is provided a method of filtering, the method comprising passing a composition through the filter according to the second aspect of the invention.

[0240] The composition passed through the filter suitably comprises a mixture of substances, such as a solid and a liquid, a solid and a gas, a liquid and a gas, a mixture of solids, a mixture of liquids, and / or a mixture of gases. The filter suitably retains at least one of the substances in the mixture, while allowing other substances in the mixture to pass through. Suitably, the composition comprises a liquid or a gas. Preferably, the composition comprises a mixture of a solid and a gas or a mixture of a solid and a liquid. Alternatively, the composition may comprise a liquid and a gas, a mixture of liquids, and / or a mixture of gases.

[0241] Suitably, the composition passed through the filter comprises solid particles to be filtered, i.e. solid particles larger than the pores in the substrate. For example, the composition may comprise an aqueous composition that comprises particulates. The method of the third aspect of the invention may comprise crossflow filtration. In such embodiments, the interstitial channels are suitably sealed at both ends of the filter and the bore channels are open. The crossflow filtration suitably comprises feeding the composition being filtered (which may be referred to as the feed composition) into the openings of the bore channels at one end of the filter and collecting retentate from the openings of the bore channels at the other end of the filter. A pressure is applied to the bore channels that is higher than the pressure in the interstitial channels. This causes a portion of the feed composition (which may be referred to as the permeate) to pass through the walls of the hollow fibres from the bore channels to the interstitial channels. The permeate then drains from the interstitial channels to the side of the filter (via the tunnels) and is collected. The retentate corresponds to the feed composition after removal of the permeate. The retentate may be re-used. For example, the retentate may be used as a feed composition.

[0242] The method of the third aspect of the invention may comprise dead-end filtration. In such embodiments, the interstitial channels and the bore channels are suitably sealed. Each bore channel is preferably sealed at only one end of the filter. The dead-end filtration suitably comprises feeding the feed composition into the openings of the bore channels at one end of the filter and collecting permeate from the openings of the bore channels and / or interstitial channels at the other end of the filter, and / or from the side of the filter (where the permeate may flow to via the tunnels). The interstitial channels are preferably sealed at the end of the filter where the feed composition is introduced. When the interstitial channels are sealed at a first end of the filter and the bore channels are sealed at a second end of the filter, the permeate is suitably collected from the openings of the interstitial channels at the second end of the filter and from the side of the filter. When the bore channels are sealed at one end of the filter and the interstitial channels are sealed at both ends of the filter, permeate is suitably collected from the side of the filter. When the bore channels are sealed at alternating ends of the filter (e.g. such that each bore channel is adjacent to at least one other bore channel that is sealed at the other end of the filter), feed composition is suitably fed into the openings of the bore channels at one end of the filter and permeate is suitably collected from the openings of the (other) bore channels at the other end of the filter. Permeate is suitably also collected from the side of the filter. Permeate may also be collected from openings of the interstitial channels at the other end of the filter, when present.

[0243] The method of the third aspect of the invention may further comprise regenerating the filter by removing solid particles that have accumulated in the filter. Regeneration may be carried out by backwashing the filter. Backwashing typically comprises reversing the flow of fluid through the filter. This may comprise feeding a composition into the interstitial channels and collecting the composition from the openings of the bore channels. The collected composition will comprise the solid particles that were accumulated in the filter. Suitably, the composition passed through the filter comprises a liquid. The composition may comprise a liquid and solid particles, such as solid particles larger than the pores in the substrate. The liquid may comprise water and / or an organic solvent, such as an alcohol. Preferably, the liquid comprises water. The method of the third aspect of the invention is preferably a method of filtering water. Suitably, the filter through which the composition is passed does not comprise a catalytic species or a precursor thereof.

[0244] In embodiments where the composition passed through the filter comprises a liquid or a gas, in particular when the composition comprises a liquid and a gas, a mixture of liquids, and / or a mixture of gases, the filter suitably comprises a coating as described above. For example, when the composition passed through the filter comprises water, the coating suitably comprises a refractory material such as alumina, silica, titania, silicon carbide and / or zirconia. In another example, when the composition passed through the filter comprises a mixture of gases, the coating suitably comprises a dense ceramic material, such as yttrium stabilised zirconia. The method of the third aspect of the invention may be a method of separating gases.

[0245] Brief Description of the Drawings

[0246] For a better understanding of the invention, and to show how example embodiments may be carried into effect, reference will now be made to the accompanying drawings in which:

[0247] Figure 1 shows a cross-sectional view of a filter according to the present invention.

[0248] Figure 2 shows a different cross-sectional view of the filter of figure 1 .

[0249] Figure 3 shows a perspective view of an assembly of hollow fibre green bodies and spacers.

[0250] Figure 4 shows a side view of hollow fibres having a non-uniform diameter along their lengths.

[0251] Figure 5 shows a perspective view of an end of a filter having an end cap.

[0252] Figure 6 shows an experimental setup for crossflow filtration.

[0253] Figure 7 shows a graph with the results of a membrane permeation test.

[0254] Detailed Description of the Example Embodiments Figure 1 shows a cross-sectional view of a filter 10 comprising a plurality of hollow fibres 11 in accordance with the invention, out of plane with the longitudinal axis of the fibres. Each fibre 11 has a wall 12 and a bore channel 13. The fibres 11 are arranged in a hexagonal close packed configuration, such that the space in between the fibres 11 takes the form of interstitial channels 14 running parallel to the bore channels 13. If each hollow fibre 11 has the same outer diameter and is uniform (e.g. has a uniform outer diameter) along its length, then each interstitial channel 14 will be isolated from the other interstitial channels 14 by the walls 12 of the hollow fibres 11. However, according to the present invention, tunnels 15 are introduced between the interstitial channels 14 so as to allow the flow of fluid between the interstitial channels 14. However, the tunnels 15 do not extend into the bore channels 13, to ensure that any fluid in the bore channels 13 must pass through a wall 12 in order to move to an interstitial channel 14. The tunnels 15 may be introduced by inserting a spacer between layers of hollow fibre green bodies prior to sintering of the green bodies.

[0255] Figure 2 shows a cross-sectional view of the filter 10 of Figure 1 which is in plane with the axis of the fibres 11. The bore channels are not shown, but form the centre of the fibres 11. The interstitial channels 14 run parallel to the fibres 11 . The tunnels 15 run perpendicular to the fibres 11 and interstitial channels 14, intersecting with the interstitial channels 14 but not extending into the bore channels of the hollow fibres 11. Fluid flowing through the bore channels of the fibres 11 must pass through the walls of the fibres 11 to move to the interstitial channels 14. From the interstitial channels 14, the fluid may pass through the tunnels 15 to rapidly exit the filter 10 as permeate 16, as indicated by the arrow. In the absence of the tunnels 15, fluid flowing through the centre of the filter 10 would need to pass through the walls of multiple hollow fibres 11 to exit the assembly 10 as permeate 16. The presence of the tunnels 15 therefore increases the rate at which fluid can be filtered by the filter 10.

[0256] Figure 3 shows a perspective view of an assembly 30 of hollow fibre green bodies 31 . Each green body 31 has the same outer diameter and is uniform (e.g. has a uniform outer diameter) along its length. Spacers 32 are inserted between layers of the green bodies 31. When heat is applied to the green bodies 31 while urging the green bodies 31 together, the outside of the green bodies 31 will fuse together along their lengths except for where the spacers 32 are located. Subsequent removal ofthe spacers 32 will result in tunnels extending through assembly between layers of hollow fibres.

[0257] Figure 4 shows a side view of a filter 40 comprising two hollow fibres 41 having a non-uniform diameter along their lengths. The fibres 41 are bonded at a number of points 42, leaving a number of gaps 43 between the fibres 41. In a filter 40 comprising a plurality of such hollow fibres 41 in a hexagonal close packed configuration, the gaps 43 form tunnels between the interstitial channels (not shown). Figure 5 shows a perspective view of an end of a filter 50 comprising a plurality of fused hollow fibres 51 . The end of the filter 50 is sealed around the hollow fibres by an end cap 52. The interstitial channels between the hollow fibres 51 (not shown) are also sealed. At the end of the filter 50, only the bore channels 53 of the hollow fibres 51 are exposed. The seal at the end of the filter 50 allows a tight fit of the filter in a filter membrane module and prevents mixing of permeate in the interstitial channels with the feed composition fed into the bore channels 53.

[0258] The invention will now be described with reference to the following non-limiting examples.

[0259] Examples

[0260] Sample 1 (comparative)

[0261] Sample 1 was a filter membrane obtained from Tami Industries with the following properties.

[0262] Sample 2 (comparative)

[0263] Sample 2 was made in accordance with the teaching of WO 2023 / 285827, as described below.

[0264] 1. Preparation of extrusion feeds

[0265] 1.1 Bore fluid

[0266] Dimethicone (polydimethylsiloxane) with a viscosity between 5 and 500 cSt was transferred to an air-tight reservoir and degassed under vacuum for about 1 hour.

[0267] 1.2 Support composition 44.85 wt% aluminium oxide (alumina) with a particle size between 0.1 and 1 |j.m was added to 43.60 wt% dimethylsulfoxide (DMSO) and 0.34 wt% Cithrol DPHS dispersant (PEG-30 dipolyhydroxystearate). The materials were then mixed at high shear (4000 rpm) at about 40°C for about 4 hours. After mixing at high shear 11 .21 wt% polyethersulfone (PES, Mw = 58,000 g / mol) was added and stirring was continued at about 2000 rpm. The mixed suspension was then transferred to a gas tight reservoir and vacuum was applied until no bubbles could be seen at the surface.

[0268] 1.3 Fusing composition

[0269] 39.88 wt% aluminium oxide (alumina) with a particle size between 0.1 and 1 |j.m was added to 39.88 wt% dimethylsulfoxide (DMSO) and 0.30 wt% Cithrol DPHS dispersant. The materials were then mixed at high shear (4000 rpm) at about 40°C for about 4 hours. After mixing at high shear 9.97 wt% polyethersulfone (PES, Mw = 58,000 g / mol) and 9.97 wt% bisphenol-A-d ig lycidy I ether (Epon 828, Mn = 700 g / mol) were added and stirring was continued at about 2000 rpm. The mixed suspension was then transferred to a gas tight reservoir and vacuum was applied until no bubbles could be seen at the surface.

[0270] 2. Extrusion

[0271] The extrusion feed reservoirs were pressurized to about 50 kPa (0.5 bar) to supply material to progressive cavity pumps for the support composition and the fusing composition and to syringe pumps for the bore fluid.

[0272] The three extrusion feeds were pumped to an extrusion die. The flow rates were 13 mL / min, 10 mL / min, 4 mL / min for the bore fluid, the support composition, and the fusing composition, respectively. The fluids were pumped through three concentric internal nozzles in the die and exited the die through a single outlet immersed in a water tank. The fluids exited the die in the form of a cylinder with concentric layers, the order of the layers being the bore fluid, the support composition, and the fusing composition from innermost to outermost.

[0273] Contact with the water caused the three fluids exiting the die to solidify, forming a solid, polymer / ceramic, flexible mixed matrix tube (hollow fibre) with a mean outer diameter of about 3.1 mm and a mean inner diameter of about 1 .8 mm. The hollow fibre was drawn from the water and 145 mm lengths of fibre were cut therefrom and deposited into another container of water.

[0274] The collected fibres were soaked in water and surfactant (15-30% anionic surfactants and 5- 15% non-ionic surfactants) with agitation to remove the remaining solvent and dimethicone (bore fluid) from the hollow fibre green body. The fibres were then dried at a temperature between 30°C and 80°C.

[0275] 3. Assembly

[0276] Dry fibres were placed vertically on a template consisting of 5 mm long pins arranged so that the outer surfaces of the fibres were in contact and the fibres were arranged in a hexagonal close packed configuration. For a final substrate diameter of 12 mm (post-sintering), 19 fibres were arranged on the template with a hexagonal profile and two halves of a mould having a hexagonal cross section were placed around the outside of the assembly of fibres and clamped together. After the mould was in place the assembly was removed from the template. The assembly was placed in a preheated oven at 150°C for 2 hours. After heating, the assembly was allowed to cool to room temperature and the mould was removed. The result was a fused precursor substrate.

[0277] 4. Heat treatment

[0278] The precursor substrate was heated in a furnace according to the following steps.

[0279] Step 1 : The precursor substrate was heated from 20°C to 250°C at a rate of 0.5°C / min.

[0280] Step 2: The substrate was held at 250°C for 1 hour to control exothermic reactions in the substrate.

[0281] Step 3: The substrate was heated from 250°C to 440°C at a rate of 0.5°C / min.

[0282] Step 4: The substrate was held as 440°C for 15 hours to remove the Epon 828 and to partially remove the polyethersulfone.

[0283] Step 5: The substrate was heated from 440°C to 540°C at a rate of 0.5°C / min, during which time the remaining poly ethersulfone was removed.

[0284] Step 6: The substrate was heated from 540°C to 1350°C at a rate of 5°C / min.

[0285] Step 7: The substrate was held at 1350°C for 4 hours to sinter the substrate.

[0286] Step 8: The substrate was cooled from 1350°C to 20°C at a rate of 5°C / min to obtain a cooled, sintered substrate. The sintered substrate consisted of alpha-alumina having a nominal pore size of 0.17 pm as measured by mercury intrusion porosimetry and capillary flow porometry.

[0287] 5. End capping and sealing of interstitial channels

[0288] A circular mould was provided which was open at both ends and which had an inner diameter of 20 mm. A piece of adhesive tape was used to cover one end of the mould. The mould was placed on a hard surface with the adhesive tape between the mould and the hard surface. The sintered substrate was then placed in the mould with the flat end of the sintered substrate on the adhesive tape to create a seal. Araldite (a two-part epoxy resin) was then injected into the gap between the mould and the sintered substrate. The resin was allowed to cure for 24 hours. After the epoxy resin had cured the mould was removed, leaving a circular end cap having an outer diameter of 20 mm.

[0289] To seal the interstitial channels, fresh un-cured epoxy resin was pushed into the end of the sintered substrate to fill both the bore channels and the interstitial channels. Then a pin was used to push the epoxy resin down the bore channels, smearing it against the channel walls and reopening the bore channels. The epoxy resin was then allowed to cure, resulting in a substrate having open bore channels and closed interstitial channels.

[0290] The epoxy resin coated on the walls of the bore channels at the end of the substrate is not considered to impair the performance of the substrate, because the end cap alone prevents permeation of fluid from the bore channels to the outside of the substrate at that location.

[0291] The process was repeated for the other end of the sintered substrate.

[0292] Sample 3 (inventive)

[0293] Sample 3 was made in the same way as sample 2, except that in step 2 (extrusion) the extrusion parameters were adjusted so as to create a non-uniform diameter along the length of the hollow fibre. The mean inner diameter of the fibre was about 1 .8 mm.

[0294] Sample 4 (inventive)

[0295] Sample 4 was made in the same way as sample 2, except that step 3 (assembly) was carried out as follows.

[0296] 61 dry fibres were gathered in a bundle. One end of the bundle was placed in a holder to create a hexagonal close packed configuration of fibres. The other end of the bundle was placed in a polytetrafluoroethylene (PTFE) mould having a hexagonal cross-section. Pieces of PTFE tape having a thickness of 0.1 -0.5 mm were fed through the bundle widthways in between each layer of fibres in a staggered configuration. The pieces of tape were then trimmed so that they did not protrude substantially beyond the outside of the bundle of fibres, and the bundle was fully pushed into the PTFE mould.

[0297] The bundle was placed in a preheated oven at 150°C for 2 hours. This caused the fibres to fuse except at the locations of the PTFE tape. After heating, the bundle was allowed to cool to room temperature and the mould was removed. The result was a fused precursor substrate.

[0298] Step 4 (heat treatment) was carried out as for sample 2. At around 600°C the PTFE tape burned off, leaving gaps between the layers of fibres.

[0299] Analysis of extruded fibre diameter of samples 2 and 3

[0300] A random sample of 10 dry fibres from batches used to fabricate samples 2 and 3 were passed through a laser micrometer one at a time, whilst being continuously rotated to measure diameter around the fibre circumference and along the entire length. The data for each batch of 10 fibres was pooled before being analysed statistically.

[0301] Table 1. Summary of fibre diameter data for samples 2 and 3

[0302] The standard deviation of mean fibre diameter along the length of the fibre for sample 3 is 2.3x that for sample 2, showing greater variation of fibre diameter for sample 3 compared to sample 2. Furthermore, the interpercentile range is 2.6x that for sample 2, again showing greater variation of fibre diameter for sample 3 compared to sample 2.

[0303] Experimental setup

[0304] Figure 6 shows a schematic of the experimental setup for crossflow titration. 1" internal diameter stainless steel pipe with standard tri-clamp fittings was used for both the feed and permeate lines. The membrane elements were housed in a 4" tri-clamp tube (length 180 mm) with a 1" tri-clamp side outlet for permeate and 4" to 1" tri-clamp conical reducers for both the feed and retentate streams (module inlet and outlet respectively).

[0305] A 180 L polyethylene drum was used as a feed reservoir. Permeate was collected in a 12 L polyethylene container. A 12V diaphragm pump was used to pump feed fluid through the membrane element and back into the feed reservoir, i.e. in crossflow configuration.

[0306] Feed, retentate and permeate pressures were measured in-line and the data was collected using a 20 bit datalogger. Crossflow volumetric flow rate and permeate volumetric flow rate were measured in-line and data was collected using a 20 bit datalogger.

[0307] Permeation flux

[0308] Deionized water was used for all tests.

[0309] Each sample was inserted into the membrane module individually. Needle valves 1 and 2 were set to fully open and feed pump voltage was increased to a low value to flood the entire system with water.

[0310] Needle valve 2 was then set to fully closed. Feed pump voltage and Needle valve 1 were adjusted to circulate feed at a membrane crossflow velocity of 0.44 m / s. Crossflow velocity was calculated from Flow sensor 1 and the total membrane channel cross-sectional area. Needle valve 2 was then opened to create a transmembrane pressure differential (TMP). After a stable TMP was achieved, permeation flow rate was recorded for 20 minutes (Flow sensor 2). Membrane permeate flux was taken as the mean flow rate over the 20 minute time period. Needle valve 2 was then opened further to increase TMP and the permeate flux data collection process was repeated for a series of increasing TMP values.

[0311] Figure 7 shows membrane permeation vs. TMP for Samples 1 - 3. For all samples permeate flux increased with increasing TMP, as expected. Sample 1 showed the lowest permeate flux at all values of TMP. Sample 2 showed similar but slightly higher permeate flux compared to Sample 1 . Sample 3 showed the highest permeate flux of all the samples across the entire range of TMP values, ~4.6x the permeate flux of Sample 1 across the entire TMP range studied.

[0312] Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0313] All of the features disclosed in this specification (including any accompanying claims), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[0314] Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0315] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

Claims1 . A method of manufacturing a filter, the method comprising the steps of: forming a plurality of hollow fibre green bodies; arranging the plurality of hollow fibre green bodies to form an assembly of hollow fibre green bodies having a plurality of bore channels and a plurality of interstitial channels; heat treating the assembly of hollow fibre green bodies to form a filter substrate; and sealing the interstitial channels; wherein the method comprises manipulating the volume between the hollow fibre green bodies such that at least a portion of the interstitial channels in the filter are in fluid communication.

2. The method of claim 1 , wherein the step of forming a plurality of hollow fibre green bodies comprises: providing:-a fusing composition comprising a substrate material, a polymer, a fusing agent, and a solvent, wherein the polymer is soluble in the solvent; forming the fusing composition into a hollow fibre structure wherein the fusing composition forms a fusing layer; and contacting the fusing layer with a non-solvent in which the polymer is insoluble to precipitate said polymer, thereby forming a hollow fibre green body.

3. The method of claim 1 or claim 2, wherein the assembly comprises the plurality of hollow fibre green bodies in a close packed configuration.

4. The method of any preceding claim, wherein the method comprises manipulating the volume between the hollow fibre green bodies such that each interstitial channel is in fluid communication.

5. The method of any preceding claim, wherein manipulating the volume between the hollow fibre green bodies comprises forming tunnels between the interstitial channels.

6. The method of claim 5, wherein the tunnels are formed by using hollow fibre green bodies that are non-uniform along their lengths.

7. The method of claim 5, wherein the tunnels are formed by placing a spacer in the assembly.

8. The method of claim 7, wherein the spacer is in the form of a thread or ribbon.

9. The method of claim 7, wherein the spacer is formed by coating a coating composition onto at least a portion of the plurality of hollow fibre green bodies.

10. The method of any preceding claim, wherein the step of heat treating the assembly comprises: fusing the hollow fibre green bodies in the assembly together by heating the assembly, wherein the hollow fibre green bodies are urged together whilst being fused, thereby forming a precursor substrate; heating the precursor substrate to remove organic polymers from the precursor substrate prior to sintering; and sintering the precursor substrate, thereby forming a filter substrate.11 . The method of any preceding claim, further comprising forming a seal around the outside of one or more of the ends of the filter substrate.

12. A filter comprising a substrate, wherein the substrate comprises a plurality of bore channels and a plurality of interstitial channels, wherein at least a portion of the interstitial channels are in fluid communication.

13. The filter of claim 12, wherein the filter is manufactured according to the method of any of claims 1 to 11 .

14. A method of filtering, the method comprising passing a composition through the filter according to claim 12 or claim 13.

15. The method of claim 14, wherein the composition comprises a liquid, preferably wherein the liquid comprises water.

Images