ASSEMBLED FILTERS FOR THE FILTRATION OF LIQUIDS

Abstract

A membrane filter includes a plurality of honeycomb ceramic filtering elements, each element including a plurality of parallel ducts separated by walls and open on a face for introduction of the liquid to be filtered, an interstitial volume between the filtering elements, a filtration membrane positioned on the inner surface of the walls of the ducts, wherein the filtering elements are joined together by a curable material that forms after curing a sleeve in the form of a single part joining together, by sealing, all of the filtering elements separated the interstitial volume, the sleeve having a thickness e between 1% and 10% of the length of the filter, and the curable material being present in the open porosity and through the entire thickness of each porous wall forming the elements, over a minimum non-zero height h.

Claims

1. A membrane filter for liquid filtration comprising: a plurality of honeycomb ceramic filtering elements, each filtering element comprising a plurality of parallel ducts separated by walls made of a porous ceramic material, said ducts being open on an introduction face for introduction of the liquid to be filtered, an interstitial volume between said filtering elements, a filtration membrane formed from a ceramic material positioned on an inner surface of the walls of the ducts, a filtrate recovery system, positioned at an outlet of the ducts and/or at a periphery of the filter, wherein: said filtering elements are joined together, at least at the end of the filter that is open on said introduction face, by means of a curable material, that forms after curing a first sleeve in the form of a single part joining together, by sealing, all of said filtering elements, said first sleeve additionally being configured to maintain said interstitial volume between said filtering elements, said first sleeve has an average thickness e, measured along a longitudinal axis of the filter, between 1% and 10% of a length of the filter, and the curable material is present in the open porosity and through an entire thickness of each porous wall forming the filtering elements, over a minimum non-zero height h, said height being measured parallel to the longitudinal axis of the filtering element considered and starting from its end that is open on the introduction face.

2. The membrane filter as claimed in claim 1, wherein said minimum height h is less than 2.5?e.

3. The membrane filter as claimed in claim 1, wherein the maximum height according to which the curable material is present in an open porosity of the porous ceramic material and through the entire thickness of the porous walls forming the elements is less than 3?e.

4. The membrane filter as claimed in claim 1, further comprising at least one second sleeve.

5. The membrane filter as claimed in claim 4, wherein said second sleeve is positioned at the opposite end of the filter.

6. The membrane filter as claimed in claim 1, wherein the average thickness e of the first sleeve is between 2% and 5% of an average length of said elements.

7. The membrane filter as claimed in claim 1, wherein a median diameter of pores in the porous walls is between 5 and 50 micrometers.

8. The membrane filter as claimed in claim 1, wherein a median diameter of pores of the membrane is between 50 nm and 10 micrometers and is at least five times smaller than a median diameter of pores of the porous walls.

9. The membrane filter as claimed in claim 1, wherein the length of the filter is between 200 and 1500 mm.

10. The membrane filter as claimed in claim 1, wherein the thickness of the porous walls of the ducts is between 0.3 and 1.5 mm.

11. The membrane filter as claimed in claim 1, wherein the thickness of the membrane is between 20 nanometers and 50 micrometers.

12. The membrane filter as claimed in claim 1, wherein the ducts are of square, round or oblong cross section and have a hydraulic diameter between 1 and 5 mm.

13. (canceled)

14. (canceled)

15. (canceled)

16. The membrane filter as claimed in claim 1, wherein the ducts of the filtering elements are alternately blocked on the introduction face for introduction of the liquid to be filtered and on an opposite face to the introduction face.

17. The membrane filter as claimed in claim 1, wherein the ducts of the filtering elements are open on the liquid introduction face and closed on a recovery face.

18. (canceled)

19. The membrane filter as claimed in claim 1, wherein the filtering elements comprise particles of silicon nitride and/or of silicon carbide.

20. The membrane filter as claimed in claim 1, wherein the curable material is selected from epoxy resins and acrylate resins.

21. The membrane filter as claimed in claim 1, wherein the curable material comprises a filler consisting of mineral particles, a median diameter D.sub.50 of which is between 1 and 100 micrometers.

22. The membrane filter as claimed in claim 1, said filter being surrounded by a compartment wherein an opening is made that enables said recovery of the filtrate.

23. A process for manufacturing a membrane filter as claimed in claim 1, comprising the following successive steps: a. manufacturing a set of honeycomb filtering elements comprising a plurality of parallel ducts separated by walls made of a porous ceramic material, an open porosity of which is between 15% and 60%, b. depositing, on the inner surface of the porous walls, a filtration membrane formed from a ceramic material, c. aligning the ends of the filtering elements, according to an arrangement that is substantially parallel along their length, said elements arranged in parallel additionally being held spaced apart so that an interstitial volume is present between each filtering element, d. preparing a curable material, and adjusting its viscosity in such a way that said curable material penetrates the entire thickness of each porous wall of all the elements over a non-zero height h, said height being measured along the longitudinal axis of the filter, e. applying said curable material, starting from at least one end of the filtering elements, to said interstitial volume, over a thickness between 1% and 10% of the length of the elements, f. curing said curable material to give a sleeve in the form of a single part joining together, by sealing, all of said tubular elements, separated from one another by said interstitial volume.

24. (canceled)

25. The membrane filter as claimed in claim 1, wherein the plurality of honeycomb ceramic filtering elements are positioned substantially parallel in said membrane filter.

26. The membrane filter as claimed in claim 1, wherein an open porosity of the porous ceramic material is between 15% and 60%.

27. The membrane filter as claimed in claim 1, wherein the curable material is a curable resin optionally incorporating a mineral filler.

Description

[0103] The figures associated with the examples that follow are provided in order to illustrate the invention and its advantages, without of course the embodiments thus described being able to be considered to limit the present invention. In the appended figures:

[0104] The figures of FIG. 1 (FIGS. 1A and 1B) illustrate a conventional configuration of a filter comprising 19 unitary filtering elements according to the invention, along a frontal view corresponding to the inlet face for the liquid to be filtered (figure LA) and along a longitudinal cross section (FIG. 1B). FIG. 1C shows the filter from FIG. 1B inserted in its housing, during operation.

[0105] FIG. 2 depicts another configuration of a filtering element according to the invention, before the assembly thereof in a filter according to the invention.

[0106] FIG. 3 depicts an alternative configuration of a filtering element according to the invention, before the assembly thereof in a filter according to the invention.

[0107] The figures of FIG. 4 (4A and 4B) illustrate the phenomenon of impregnation of the resin in the porosity of the walls of a filtering element during the formation of the sleeve according to the invention. FIG. 4B is a schematic representation, provided for greater clarity, illustrating the characteristics visible in FIG. 4A.

[0108] FIG. 5 is a photo of a filter in accordance with the present invention.

[0109] The figures of FIG. 6 (6A and 6B) schematically represent a filtering element extracted from the assembly with its portion of the sleeve, along an elevation view (FIG. 6A) and along a three-dimensional cross section (FIG. 6B). FIGS. 6A and 6B are provided in order to illustrate the pathways for possible by-passing of the membrane by the fluid to be filtered in an assembled filter according to the invention.

[0110] FIG. 7 represents the plane of a filter according to the invention comprising 7 filtering elements. In FIG. 7, the dimensions are indicated are in mm.

[0111] The figures of FIG. 8 (8A to 8C) schematically represent, according to other configurations, a filtration unit in accordance with the present invention.

[0112] In FIGS. 1 to 8, the same numbers are used to denote the same objects.

[0113] FIGS. 1A and 1B illustrate a tangential filter 1 according to the invention, as can be used for the filtration of a liquid. FIG. 1A depicts a frontal view of a filter according to the invention, from the inlet face 3 of the liquid to be filtered. FIG. 1B represents a schematic view of the filter along the longitudinal cross-sectional plane AA indicated in FIG. 1A. The filter 1 has a longitudinal central axis 13, perpendicular to the frontal face 3, and passing through its center. The filter comprises a set of filtering elements 2 made of a porous, preferably non-oxide, inorganic material, such as recrystallized SiC. Each element has for example a tubular shape, which may be of hexagonal cross section as illustrated by FIG. 1A or preferably of circular cross section as illustrated by FIG. 2. Each element has a longitudinal central axis 12. The filter is inserted into a housing or compartment, a portion of the walls 5 of which is represented in FIG. 1B. Each element 2 comprises in its internal portion a set of adjacent ducts (or channels) 7, with axes parallel to one another and that are separated from one another by walls 8 formed from the porous material. The walls 8 are therefore formed from a porous inorganic material that lets the filtrate pass from the internal portion of the elements to the outer surface thereof. The ducts 7 are covered over their inner surface with a membrane separation layer (also referred to as filtration membrane or else membrane), coating the inside of the ducts (not represented in the figures of FIG. 1). This filtration membrane comes in contact with said fluid to be filtered flowing in said channels after the introduction thereof into the assembled structure at the inlet face 3. The filtering structure comprises inner ducts and peripheral ducts occupying the ring of outermost channels of the filter. According to one most conventional configuration illustrated by FIG. 1, all the ducts have a cross section of circular shape.

[0114] In the configuration according to FIG. 1, the filtering elements have a hexagonal transverse cross section. Of course, the filtering elements may have shapes other than that represented in FIG. 1. In particular, according to one preferred embodiment according to the invention, the filtering elements have a circular cross section, along a cross sectional plane perpendicular to the length thereof. FIG. 2 illustrates such a configuration of the filtering elements. As can be seen in FIG. 2, half of the channels of the peripheral ring have however a truncated shape, in order to retain a sufficient thickness of the outer wall. FIG. 3 illustrates another filter configuration comprising elements having ducts that are arranged in a similar manner to those of FIG. 2, the cross section of the ducts this time being square. According to this configuration, all the ducts have the same cross section.

[0115] According to one possible embodiment illustrated by FIGS. 2 and 3, the filter according to the invention comprises a plurality of ducts distributed in several rings around a central axis. A ring of ducts is understood to mean a set of ducts, the barycenter of which is located on one and the same concentric circle of the central axis of the filtering element.

[0116] According to the invention, the filtering elements 2 according to FIGS. 1A and 2 and 3 are grouped together in the form of an assembled filter, as illustrated by FIGS. 1B, 5 and 7. Each of the elements 2 is separated from the next by an interstitial volume 6 through which the filtrate resulting from the elements 2 flows, after crossing the membrane. As illustrated by FIGS. 1B, 5 and 7, the filtering elements 2 are held at a distance from one another in order to make an interstitial volume 6 between them, in a single and mechanically strong structure by means of two sleeves 9 and 10 positioned preferably on either side of the elements 2 and at each end thereof. The two sleeves are in contact and are held in compression by the walls 5 of the filtration compartment (often referred to as housing or casing in the field).

[0117] In operation, the liquid to be filtered is introduced from the introduction face 3 of the filter thus obtained, passes through the membrane coating the inside of the ducts 7, a filtrate being collected in the interstitial volumes 6 in order to be ultimately collected at the filter outlet, generally through an opening made in the housing surrounding the filter (see for example US 2013/0153485).

[0118] By way of example, the embodiment illustrated by FIG. 1C describes the operation of a filter 1 according to the invention as described in FIG. 1B. In FIG. 1C, the arrows 14 indicate the path of the liquid to be filtered in the filtration unit 20. The filter 1 is positioned in its compartment, the walls 5 of which comprise an opening 15 for discharging the filtrate 16. At the outlet of the filter, a retentate 17 is also recovered through the openings of the channels positioned at the opposite end from the introduction face, for optional recycling.

[0119] According to other embodiments of the invention illustrated by the figures of FIG. 8 (8A to 8C), only a filtrate 15 is collected. The configurations described in the appended FIGS. 8A to 8C should not however be considered to limit the scope of the present invention, under any of the aspects described.

[0120] According to one configuration of a filtration unit incorporating a filter according to the invention, for example illustrated by FIG. 8A, the liquid to be filtered 14 is introduced at the inlet face 3 of the filter. This filter has a structure in which the ducts of the filtering elements 2 are alternately blocked on the face for introduction of the liquid to be filtered and on the opposite face by plugs 18 that are preferably impermeable to the liquids, so as to force the liquid to pass through the porous walls of said filtering elements and the membrane covering them. A distinction is thus made in this embodiment, as illustrated in FIG. 8A, between inlet ducts 7 for the liquid to be filtered and outlet ducts 7 for the filtrate after having passed through the porous walls provided with a filtration membrane. The filter 1 is contained in the compartment that surrounds it in the filtration unit and one portion of the walls 5 of which is represented in FIG. 8A. The filtrate 16 is recovered at the outlet of the unit 20 through an opening 15 made in the compartment containing the liquids in the filtration unit. Another portion of the filtrate 16, recovered from the interstitial volumes 6, is collected through an opening 15 on the peripheral portion of the compartment surrounding the filter.

[0121] According to another configuration of a filtration unit incorporating a filter according to the invention, illustrated by FIG. 8B, the liquid to be filtered is introduced at the inlet face 3 of the filter. This filter is contained in the compartment that surrounds it in the filtration unit and one portion of the walls 5 of which is represented in FIG. 8B.

[0122] This filter has a structure in which the ducts of the filtering elements 2 are open on the liquid introduction face and closed on the recovery face, by plugs 18, so as to force the liquid to pass through the porous walls of said filtering elements 2 and the membrane covering them.

[0123] As illustrated by FIG. 8B, in operation, the liquid to be filtered passes through the porous walls of said filtering elements and the membrane covering them. The filtrate 16 is recovered in the interstitial volumes 6 present around the filtering elements then collected at the outlet of the unit 20 via an opening 15 made in the compartment containing the liquids in the filtration unit. According to one possible embodiment, the ducts 7 are plugged by liquid-tight plugs in order to force all of the filtrate to pass through the interstitial volumes 6. According to another possible embodiment, the sleeve itself ensures the leaktightness of the opposite face of the filter with respect to liquids.

According to an alternative configuration of a filtration unit 20 incorporating a filter according to the invention, illustrated by FIG. 8C, the liquid to be filtered 14 is firstly introduced at the inlet face 3 of the filter. This filter is contained in the compartment that surrounds it in the filtration unit and one portion of the walls 5 of which is represented in FIG. 8C. As illustrated by FIG. 8C, in operation, the liquid to be filtered passes through the porous walls of said filtering elements and the membrane covering them. The filtrate 16 is firstly recovered in the interstitial volume 6 present around the filtering elements. Openings 19 are made in the rear sleeve 10 of the filter to allow the filtrate 16 which is finally collected at the outlet of the unit 20 to be discharged via an opening 15 made in the compartment containing the liquids in the filtration unit. According to one alternative embodiment, as represented in the bottom part of FIG. 8C, a space may be made between compartment 5 and sleeve 10 in order to allow the filtrate 16 to be discharged.

[0124] Illustrated in FIGS. 6A and 6B are the difficulties encountered in the implementation of such an assembled filter: when the liquid to be filtered is introduced from the introduction face 3 of the filter, a portion of this liquid passes directly into the largest porosity of the porous walls 8 of the filtering element, without entering into the ducts 7 and passing through the membrane 11. Such a by-pass circuit is illustrated by the arrows 100 in FIG. 6B. The implementation of the present invention makes it possible to solve such a problem.

[0125] The following examples make it possible to illustrate the invention and its advantages but in no way limit the scope thereof.

Examples 1 to 8

[0126] Filtering elements, the transverse cross section of which is depicted in FIG. 2, were produced according to the techniques of the art by shaping and firing structures consisting of porous recrystallized silicon carbide, according to the production process described above.

[0127] The structural features of the filtering element are listed in table 1 below:

TABLE-US-00001 TABLE 1 Examples 1-8 FIGURE illustrating the filtering element 2 Total number of ducts 19 S.sub.A = Total surface area of channel A (mm.sup.2) 16.98 (non-truncated channels) S.sub.B = Total surface area of channel B (mm.sup.2) 9.22 (truncated channels) Surface area ratio Rs = S.sub.A/S.sub.B 1.84 Hydraulic diameter D.sub.hA of channel A (mm) 4.65 Hydraulic diameter D.sub.hB of channel B (mm) 3.26 Ratio D.sub.h = D.sub.hA/D.sub.hB 1.42 Average thickness of the outer wall (mm) 0.7 Filtration surface area m.sup.2/m of filter length 0.26 OFA % 56

[0128] The hydraulic diameter D.sub.h of a channel is calculated, in any transverse sectional plane P of the tubular structure, from the surface area of the cross section of the channel S of said channel and from its perimeter P, along said sectional plane and by applying the following conventional expression:

D.sub.h=4?S/P

[0129] The OFA (open front area) is obtained by calculating the ratio, as a percentage, of the area covered by the sum of the transverse cross sections of the channels to the total area of the corresponding transverse cross section of the porous support.

[0130] The elements according to examples 1 to 6 are obtained according to the same experimental protocol below:

[0131] Mixed in a mixer are: [0132] 6000 g of a mixture of the two powders of silicon carbide particles with a purity of greater than 98% in the following proportions: 75% by weight of a first powder of particles having a median diameter of the order of 60 micrometers and 25% by weight of a second powder of particles having a median diameter of the order of 2 micrometers. (Within the meaning of the present description, the median diameter d.sub.50 denotes the diameter of the particles below which 50% by weight of the population of said particles is found). [0133] 600 g of an organic binder of the cellulose derivative type.

[0134] Water is added in an amount of around 20% by weight relative to the total weight of SiC and of organic additive and mixing is carried out until a homogeneous paste is obtained, the plasticity of which allows the extrusion of a structure of tubular shape, the die being configured for obtaining monolith blocks, the channels and the outer walls of which have a structure according to the configuration represented in the appended FIG. 2. Thus, green supports having a diameter of 25 mm and a length of 120 cm are synthesized.

[0135] The green monoliths thus obtained are dried by microwave radiation for a time sufficient to bring the content of water that is not chemically bound to less than 1% by weight.

[0136] The honeycomb monoliths are then fired up to a temperature of at least 2100? C. which is maintained for 5 hours. The material obtained has an open porosity of 43% and a median pore distribution diameter of the order of 25 micrometers, as measured by mercury porosimetry.

[0137] A membrane separation layer is then deposited on the inner wall of the channels of the support structure according to the process described below:

A primer for adhesion of the separation layer is formed, in a first step, from a slip, the mineral formulation of which comprises 30% by weight of a powder of grains of black SiC (Sika DPF-C), the median diameter d.sub.50 of which is approximately 11 micrometers, 20% by weight of a powder of grains of black SiC (Sika FCP-07), the median diameter d.sub.50 of which is approximately 2.5 micrometers, and 50% of deionized water.

[0138] A slip of the material constituting the filtration membrane layer is also prepared, the formulation of which comprises 40% by weight of SiC grains (d.sub.50 of approximately 0.6 micrometer) and 60% of demineralized water.

[0139] The rheology of the slips was adjusted, by addition of the organic additives, to 0.5-0.7 Pa.Math.s under a shear gradient of 1 s.sup.?1, measured at 22? C. according to the standard DIN C 33-53019.

[0140] These two layers are successively deposited according to the same process described below: the slip is introduced into a tank with stirring (20 rpm). After a phase of deaerating under slight vacuum (typically 25 millibar) while continuing to stir, the tank is overpressurized to approximately 0.7 bar in order to be able to coat the inside of the support from its bottom part up to its upper end.

[0141] This operation only takes a few seconds for an element with a length of 120 cm. Immediately after coating the slip over the inner wall of the channels of the support, the excess is discharged by gravity.

[0142] Next, the elements are dried at ambient temperature for 30 minutes and then at 60? C. for 30 h. The supports thus dried are then fired at a temperature of 1800? C. under argon for 2 h and at ambient pressure.

[0143] The thicknesses of the primer layers and of the membrane filtration layer after sintering are substantially equal and are of the order of 45 micrometers. The firing temperature depends on the characteristics required for the final porosity of the membrane, namely a median pore diameter d.sub.50 of around 1 micrometer and a total porosity of 40%, by volume.

[0144] Unlike the other examples, the coated supports of examples 7 to 8 were fired at a firing temperature of 1600? C. under nitrogen for 2 h and at ambient pressure. The median pore diameter d.sub.50 of the membrane is measured as being equal to around 250 nanometers.

[0145] The lower portion of the elements, comprising an accumulation of the materials of the various layers applied, is cut over a length of 10 mm.

[0146] A transverse cut is made through the filters thus obtained. The structure of the membrane is observed with a scanning microscope. Observed on an electron microscopy image are the porous wall of the element, of high porosity, the primer layer that enables the adhesion of the membrane separation layer of finer porosity, that ultimately covers the inside of the ducts.

[0147] The filtering elements thus synthesized are then dropped into a silicone container so as to rest on one of their ends.

[0148] The same initial volume of resin is added for all the examples. Epoxy-based thermosetting resins are introduced into the container so as to make a sleeve between the elements. The viscosity of the resin used is different and is adjusted according to examples 1 to 6 through the chemical nature of the epoxide used, or else through the addition to the initial epoxy resin, before curing, of a mineral filler in the form of a larger or smaller amount of SiC particles of various sizes.

[0149] More specifically, two types of resins are used: [0150] an epoxy resin sold by Ebalta under the reference AH110/TG?, with a viscosity of 1950 mPa.Math.s at 25? C., [0151] an epoxy resin sold by Struers under the reference Epofix?, with a viscosity of 390 mPa.Math.s at 25? C. Two mixtures of different particles are also used to modify the viscosity of the resins: [0152] SiC particles, having a mean diameter d.sub.50 of 2 micrometers (sold under the reference FCP 07), [0153] SiC particles, having a mean diameter d.sub.50 equal to 45 micrometers (sold under the reference F240).

[0154] The smaller the mean diameter of the SiC powder added, the greater the viscosity of the mixture with the resin.

[0155] The details of the conditions for preparing the resins for each example are given in table 2 below.

[0156] In each case, the curable material is cured at ambient temperature, according to the recommendations and the conditions recommended by the supplier, until a rigid sleeve is obtained that is in the form of a single part surrounding the filtering element, as illustrated schematically in FIG. 1.

[0157] After curing the resins, the filtering elements are cut in their center and along the longitudinal direction, that is to say along a longitudinal sectional plane passing through the central axis 12 of the element, and a visual observation of the penetration depth and profile of the resin in each duct is carried out. As indicated in FIG. 4B that schematically illustrates the photograph shown in FIG. 4A, the heights of penetration of the cured material 4 in the porosity of the walls 8 are measured for each of the ducts 7 forming the filtering element.

[0158] A minimum height and a maximum height of penetration of the curable material within the walls of the filtering element are thus measured, from the end of the element, as illustrated by FIGS. 4A and 4B. The values are compared to the thickness e finally obtained for the sleeve. The results are reported in table 2 below.

TABLE-US-00002 TABLE 2 Example Example 2 Example 3 Example 7 Example 1 (comparative) (comparative) Example 4 Example 5 Example 6 (comparative) Example 8 Element According to According to According to According to According to According to According to According to configuration FIG. 2 FIG. 2 FIG. 2 FIG. 2 FIG. 2 FIG. 2 FIG. 2 FIG. 2 Median diameter D50 = D50 = D50 = D50 = D50 = D50 = D50 = D50 = of the pores in 1000 nm 1000 nm 1000 nm 1000 nm 1000 nm 1000 nm 250 nm 250 nm the membrane Characteristics of the curable material Resin alone Resin Resin Resin Resin Resin Resin Resin Resin AH110TG AH110TG Epofix AH110TG AH110TG AH110TG AH110TG AH110TG Viscosity of resin 1950 1950 390 1950 1950 1950 1950 1950 alone (mPa .Math. s at 25? C.) Mineral filler in no Yes: FCP 07 no Yes: F240 Yes: F240 Yes: FCP 07 Yes: FCP 07 Yes: F240 the resin (D50 = 2 ?m) (D50 = 45 ?m) (D50 = 45 ?m) (D50 = 2 ?m) (D50 = 2 ?m) (D50 = 45 ?m) % mineral filler NA 50% NA 5% 50% 5% 5% 5% (weight) Characteristics after curing of the sleeve Sleeve thickness 13 21 7 16 15 17 10.5 12 (mm) h.sub.max 25 27 24 28 27 27 19 21 h.sub.min 13 0 20 13 13 9 0 4 h/e ratio h.sub.max/e 1.9 1.3 3.4 1.7 1.8 1.45 1.8 1.7 h.sub.min/e 1 2.8 0.8 0.9 0.3 0.3

[0159] The results reported in table 2 above indicate that the adjustment of the viscosity of the initial curable material, before the curing thereof, is critical during this step of forming the sleeve, in order to enable the correct operation of the assembled filter finally obtained, in particular to ensure the filtration quality of the device and the leaktightness thereof.

[0160] Firstly, it is observed that the sleeve thicknesses obtained after impregnation and curing are highly variable depending on the nature of the curable material and that the resin always impregnates the peripheral walls over a maximum height greater than the thickness of the final sleeve, due to capillary action.

[0161] Moreover, the results show that the use of a resin having an excessively high viscosity (comparative examples 2 and 7) prevents the impregnation of all the walls of the ducts of the element, throughout the thickness thereof and in particular the impregnation of the most central channels of the elements forming the filter.

[0162] On the contrary, the use of an excessively fluid resin (comparative example 3) results in the diffusion of the curable material throughout the porosity of the structure and ultimately in a thickness of the sleeve that is significantly reduced relative to the expected thickness. A thin thickness of the sleeve appears to be highly prejudicial to the structural rigidity and to the final integrity of the filter finally assembled from a plurality of elements using the resin of comparative example 3. Moreover, the very high value of the impregnation for all of the ducts of the element (as indicated by the value of 2.8 for the h.sub.min/e parameter according to example 3), also results in a substantial reduction of the filtration surface area accessible to the liquid to be filtered and therefore of the overall filtration capacities of the filter.

[0163] Within the meaning of the present invention, the filtration surface area of a filtering element (or of a filter) corresponds to the combined internal surface area of all of the inner walls, covered by the membrane and accessible to the fluid to be filtered in said element (or said filter). In particular, the portion of the walls for which the internal porosity is plugged by the cured material during the manufacture of the sleeve is not considered to be the filtration surface area.

[0164] According to examples 4 to 6 and 8 according to the invention, it appears possible to insert a mineral filler into the organic resin in order to increase the mechanical properties thereof, without impairing the quality of the filtration of the incoming liquid and without reducing the filtration surface area. Such a configuration additionally makes it possible to ensure a much better compression strength of the sleeve when the filter thus assembled is inserted into its housing, as explained above.

[0165] Example 7 shows that the mixture of curable resin that was suitable for example 6 is no longer suitable in the case of a membrane with significantly smaller pore diameter, the most central ducts of the elements not being impregnated by the curable material, which implies the presence of membrane by-pass zones in the filter.

[0166] Example 8 shows that a device with a sleeve thickness and an impregnation of all of the inner walls is possible again, on condition that the filler (and therefore the viscosity) of the mixture in the resin is modified, using particles of significantly larger size.

[0167] In summary, the results reported in the preceding table show that the viscosity of the curable material injected into the porosity of the walls of the elements should be adjusted: it should be low enough for the curable material to penetrate into the open porosity and through the entire thickness of all the porous walls forming the plurality of elements, in particular through the entire thickness of the innermost walls of all the filtering elements used to form the assembled filter. As demonstrated by the preceding examples, the presence of the resin throughout the porosity of the walls, over a non-zero height h (said height being measured along the longitudinal axis of the filter and from said end) ensures the best operation of the complex structure by effectively preventing the abovementioned by-pass zones. According to another essential aspect of the invention, the viscosity should not however be too low, in order to avoid the obstruction of an excessively large part of the filtration surface area remaining within the ducts and the general weakening of the assembled filter due to a lack of thickness of the sleeves that join the constituent elements of the structure.

[0168] In order to compare the filtration performances for the filters according to the invention, a filtration is carried out using assembled filters having the configuration represented in FIG. 7.

More specifically, a turbidity measurement is carried out on the filters corresponding to assemblies of 7 filtering elements in accordance with the appended FIG. 7, using the filtering elements and the sleeve compositions respectively described in examples 1 and 2 above.

[0169] More specifically, two filters are synthesized and assembled, each from 7 filtering elements as described in the preceding examples.

The first filter according to the invention is obtained by joining the 7 filtering elements together, over the two ends by sleeves 9 and 10, by means of the curable material as described in example 1. According to the invention and as illustrated in FIG. 7, the filter is obtained after curing the sleeves in the form of a single part that joins, by sealing, all of the 7 filtering elements.

[0170] The second comparative filter is obtained in the same way as the first, but using this time, as curable material, the mixture of the resin and of the filler described in example 2.

[0171] The following method is used:

Use is made of synthetic dirty water comprising clay, salt, oil and surfactants at contents respectively equal to 100 ppm, 4000 ppm, 300 ppm and 2 ppm.

[0172] The dirty water supplies, at a constant temperature of 25? C., the two filters to be evaluated under a trans-membrane pressure of 0.5 bar and a flow rate in the channels of 3 m/s. The filtrate (purified water) is recovered at the periphery of the filter, via the interstices 6.

[0173] In order to estimate the filtration performance of the filter, the turbidity of the filtrate is measured continuously using a LAT N1 series beam turbidity meter supplied by Kobold Instrumentation, by the end of 10 filtration cycles. A lower value after the turbidity test therefore corresponds to a better quality of filtration of the incoming liquid, which may itself be directly linked to the absence of by-pass zones 100 of the filtering membrane, as described in FIG. 6B.

[0174] This expressed turbidity is 0.8 NTU for the first filter (according to the invention) and 3.5 for the comparative filter. Such a difference proves the increased filtration efficiency of the filter obtained according to the principles of the present invention.