Tangential filter with a supporting element including a set of channels

Abstract

A tangential filter for filtration of a fluid includes a support element, wherein, along a transverse plane perpendicular to the central axis of the support element a) the support element includes in its central portion only inner channels that do not share a common wall with its outer surface, the inner channels having a substantially equivalent hydraulic diameter, b) the support element includes peripheral channels, including at least first and second adjacent peripheral channels, each of the two channels sharing a common wall with the outer surface, c) the ratio of the hydraulic diameter of the first channel to the hydraulic diameter of the second channel is greater than or equal to 1.1, d) the number of peripheral first channels is greater than or equal to the number of peripheral second channels, e) the second channel has a hydraulic diameter substantially identical to the hydraulic diameter of the inner channels.

Claims

1. A tangential filter for the filtration of a fluid, comprising a support element made from a porous inorganic material, said element having a tubular shape delimited by an outer surface and comprising in its inner portion a set of adjacent channels, having axes that are parallel to one another and separated from one another by walls of said porous inorganic material, wherein said channels are covered over their inner surface with a membrane separation layer intended to come into contact with said fluid to be filtered circulating in said channels, wherein, along a transverse plane perpendicular to a central axis of said tubular support element: a) said support element comprises in its central portion only inner channels that do not share a common wall with its outer surface, said inner channels having a substantially equivalent hydraulic diameter, b) said support element additionally comprises peripheral channels, including at least two adjacent peripheral first and second channels, each of said first and second channels sharing a common wall with said outer surface, c) a ratio D.sub.h of: a hydraulic diameter D.sub.hA of the first channel to a hydraulic diameter D.sub.hB of the second channel, is greater than or equal to 1.1, d) a number of peripheral second channels is greater than or equal to a number of peripheral first channels, e) the second channel has a hydraulic diameter Dh.sub.B substantially identical to a hydraulic diameter of the inner channels, wherein at least one inner channel does not share a common wall with a peripheral first channel.

2. The tangential filter as claimed in claim 1, wherein the second channel of smaller hydraulic diameter has, along said transverse plane, a shape substantially equivalent to that of said inner channels.

3. The tangential filter as claimed in claim 1, wherein, along said transverse plane, a ratio Rs=S.sub.A/S.sub.B of internal surface areas S.sub.A and S.sub.B, respectively of the peripheral first and second channels, is between 1.1 and 3.5.

4. The tangential filter as claimed in claim 1, wherein, along said transverse plane, the inner channels and the second channel have a substantially elliptical cross section, a ratio of a large axis to a small axis of the ellipse being between 2 and 1.

5. The tangential filter as claimed in claim 4, wherein the ratio of the large axis to the small axis of the ellipse is between 1.5 and 1.

6. The tangential filter as claimed in claim 1, wherein the inner channels have a substantially circular cross section, along said transverse plane.

7. The tangential filter as claimed in claim 1, wherein the second channel has a substantially circular cross section, along said transverse plane.

8. The tangential filter as claimed in claim 1, wherein the first channel is of flared shape, or else of elliptical shape, and wherein a surface of the first channel, along said transverse plane, extends principally along an axis from a periphery of the support element to its central axis.

9. The tangential filter as claimed in claim 1, wherein the surface of the common wall between the first channel and the outer surface is curved.

10. The tangential filter as claimed in claim 1, wherein the support element has a polygonal base or a circular base.

11. The tangential filter as claimed in claim 1, wherein the support element comprises in its peripheral ring only the first and second channels.

12. The tangential filter as claimed in claim 1, wherein the porous support element comprises a material selected from silicon carbide, SiC, silicon nitride, silicon oxynitride, silicon aluminum oxynitride, or a combination thereof.

13. The tangential filter as claimed in claim 12, wherein the silicon carbide SiC is liquid-phase or solid-phase sintered SiC or recrystallized SiC, the silicon nitride is Si.sub.3N.sub.4, and the silicon oxynitride is Si.sub.2ON.sub.2.

14. The tangential filter as claimed in claim 1, wherein the membrane separation layer comprises a ceramic material.

15. The tangential filter as claimed in claim 14, wherein the ceramic material is selected from the list consisting of silicon carbide SiC, silicon nitride, silicon oxynitride, silicon aluminum oxynitride, boron nitride BN, or a combination thereof.

16. The tangential filter as claimed in claim 1, wherein a porosity of the material constituting the support element is between 20% and 60%, a median pore diameter of the material constituting the porous support being between 5 and 50 micrometers.

17. The tangential filter as claimed in claim 1, wherein an equivalent median pore diameter of the material forming the membrane separation layer is between 1 nm and 5 micrometers.

18. The tangential filter as claimed in claim 1, wherein the fluid is a liquid.

19. The tangential filter as claimed in claim 1, wherein the support element is made from a non-oxide inorganic material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the appended figures:

(2) FIG. 1 illustrates a conventional configuration of a tubular filter according to the current art, along a transverse sectional plane P.

(3) FIG. 2 schematically shows a first most advantageous configuration according to the invention.

(4) FIG. 3 schematically shows another configuration of a tubular filter according to the invention.

(5) FIG. 4 illustrates an embodiment comparative to that shown schematically by FIG. 3, and not in accordance with the present invention.

(6) FIG. 5 illustrates the transverse sectional plane P of a filtering structure and its central axis A, within the meaning of the present invention.

(7) FIG. 6 is a microscopy image of a filter according to the invention, showing the membrane separation layer.

(8) FIG. 7 schematically shows another configuration of a tubular filter according to the invention.

(9) FIG. 1 illustrates a tangential filter 1 according to the current art, as used for filtering a fluid such as a liquid. FIG. 1 represents a schematic view of the transverse sectional plane P illustrated by FIG. 5. The filter comprises, or most often consists of, a support element 1 made of a porous, preferably non-oxide, inorganic material. The element conventionally has a tubular shape, delimited by an outer surface 2. It comprises in its inner portion 3 a set of adjacent channels 4, having axes that are parallel to one another and separated from one another by walls 8. The walls are made of a porous inorganic material that lets the filtrate pass from the inner portion 3 to the outer surface 2. The channels 4 are covered over their inner surface with a membrane separation layer 5, as illustrated by the electron microscopy image shown in FIG. 6. This membrane separation layer 5 (or membrane) comes into contact with said fluid to be filtered circulating in said channels. The channels 4 of the structure 1 may be split into various groups: all the inner channels Ci generally have a substantially equivalent hydraulic diameter and are of circular shape (in cross section) and form the central portion of the structure, within the meaning of the present invention. The filtering structure additionally comprises, around said central portion, peripheral channels Cp that occupy the outermost (or peripheral) ring of channels of the filter, said channels Cp sharing a common wall with the outer surface 2. According to a conventional configuration in which the channels have a circular shape, a minority portion of the channels (denoted by A) of the peripheral ring necessarily has a truncated shape, in order to retain a sufficient thickness of the outer wall. Even if the majority of the other peripheral channels have a circular shape (channels B in FIG. 1) identical to that of the channels Ci, the research carried out by the applicant company has shown that the presence of these peripheral channels A of restricted hydraulic diameter, even in a small number, had a negative and substantial influence on the performance and efficiency of the filtration of the filter, as will be described hereinafter.

(10) FIGS. 2 and 3 illustrate various embodiments according to the invention of a tangential filter according to the invention, in which the structure also comprises a group G of two adjacent peripheral channels Cp A and B that share a common wall (respectively 6 and 7 in FIG. 2), with the outer surface 2, which make it possible to solve such a problem. The filter according to FIG. 2 additionally comprises inner channels Ci of substantially equivalent hydraulic diameter and of circular shape (in cross section), which together constitute the central portion of the structure, within the meaning of the present invention.

(11) Unlike the prior art illustrated by FIG. 1, according to FIG. 2 the channels A are configured this time in such a way that their hydraulic diameter is greater than the hydraulic diameter of the channels B of circular cross section, the latter however being present in a larger number on said peripheral ring.
The filter according to FIG. 3, in accordance with the subject of the present invention, shows a configuration comprising three types of peripheral channels of different hydraulic diameter Dh. In accordance with the present invention, the number of circular peripheral channels B, the Dh of which is identical to that of the inner channels, is greater than the number of adjacent channels having a larger Dh (channels A).
On the contrary, according to the comparative configuration not in accordance described in FIG. 4, the number of peripheral channels B of smaller Dh is less than that of the adjacent channels A of larger size.
FIG. 4 therefore illustrates a comparative embodiment in which the channels A of larger hydraulic diameter this time outnumber the channels B, contrary to the requirements of the present invention.

(12) Four filtering supports corresponding respectively to the representations according to FIGS. 1 to 4 were produced according to the techniques of the art by shaping and firing structures consisting of porous recrystallized silicon carbide, which correspond respectively to the following examples 1 to 4.

(13) The filters according to all the examples are obtained according to the same experimental protocol below:

(14) Mixed in a mixer are: 3000 g of a mixture of two powders of particles of silicon carbide having a purity of greater than 98% in the following proportions: 75% by mass of a first powder of particles having a median diameter of the order of 60 micrometers and 25% by mass 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 mass of the population of said particles is found). 300 g of an organic binder of the type derived from cellulose.
Water is added in an amount of around 20% by mass relative to the total mass of SiC and of organic additive and mixing is continued 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 desired configuration and as represented in appended FIGS. 1 to 4. Thus, for each configuration, 5 to 10 green supports having a diameter of 25 mm and a length of 30 cm are synthesized.

(15) The green monoliths thus obtained are dried using a microwave system for a sufficient time to bring the content of water that is not chemically bound to less than 1% by mass.

(16) The 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 mean pore distribution diameter of the order of 25 micrometers, as measured by mercury porosimetry.

(17) A membrane separation layer is then deposited on the inner wall of the channels of the support structure according to the process described below:

(18) A primer for adhesion of the separation layer is formed, in a first step, from a slurry, the formulation of which comprises 50% by mass of SiC grains (d.sub.50 of around 10 micrometers) and 50% deionized water.

(19) A slurry of the material constituting the membrane filtration layer is also prepared, the formulation of which comprises 50% by mass of SiC grains (d.sub.50 of around 0.6 micrometer) and 50% deionized water.

(20) The rheology of the slurries was controlled at 0.5-0.7 Pa.Math.s at 1 s.sup.?1. In order to control the rheology of these slurries and to comply with a viscosity typically between Pa.Math.s approximately under a shear gradient of 1 s.sup.?1 measured at 22? C. according to the DINC33-53019 standard.

(21) These two layers are deposited successively according to the same process described below: the slurry is introduced into a stirred tank (20 rpm). After a deaeration phase under a slight vacuum (typically 25 millibar) while maintaining the stirring, the tank is placed under a positive pressure of around 0.7 bar in order to be able to coat the inside of the support from its bottom portion up to its upper end. This operation takes only a few seconds for a 30 cm long support. Immediately after coating the slurry onto the inner wall of the channels of the support, the excess is discharged by gravity.

(22) The supports are then dried at ambient temperature for 30 minutes then at 60? C. for 30 h. The supports thus dried are then fired at a temperature of greater than 1600? C. The firing temperature depends on the characteristics required for the final porosity of the membrane, namely a median pore diameter of around 1 micrometer and an open porosity of 40%, by volume.

(23) A transverse cut is made through the filters thus obtained. The structure of the membrane is observed with a scanning microscope. One of the electron microscopy images obtained is shown in FIG. 6. Observed in this figure are the porous support 100 of high porosity, the primer layer 102 that enables the adhesion of the membrane separation layer 103 of finer porosity.

(24) On the basis of the electron microscopy images, the mean thickness of the membrane 103 on the channels A and on the channels B on the various structures according to examples 1 to 4 was measured by image analysis. The results measured are reported in table 1 below.

(25) More particularly, reported in table 1 is the ratio of the mean thickness of the membrane (103) thus measured on all the channels A to the mean membrane thickness thus measured on all the channels B. A ratio close to 1 thus indicates an ideal distribution of the filtering inorganic material in all the channels of the filter. On the contrary, the further the ratio is from 1, the more heterogeneous the deposition of the membrane separation layer is. In table 1, the filtration surface area is calculated from the combined sum of the perimeters of all the channels of the structure.

(26) A flow measurement is carried out on the filters according to the following method.

(27) At a temperature of 25? C. a fluid consisting of demineralized water loaded with 300 ppm of synthetic oil feeds the filters to be evaluated under a transmembrane pressure of 0.5 bar and a circulation rate in the channels of 2 m/s. The permeate (water) is recovered at the periphery of the filter. During the test, the filter gradually clogs up due to the deposition of the oil in the channels at the surface of the separation membrane, resulting in a decrease in the amount of permeate recovered at the periphery of the filter. The flow rate is measured after 20 h of filtration. The characteristic flow rate measurement of the filter is expressed in L/min per meter of filter length after 20 h of filtration. In the table, the flow rate results have been expressed with reference to the data recorded for comparative example 1. More specifically, a value of greater than 100% indicates an increased flow rate with respect to the reference and therefore an improvement in the filtration capacity.

(28) The comparison of the homogeneity values characterizing the structure according to example 1 (obtained according to current techniques) and the structure according to example 2 (modified according to the criteria of the present invention) shows that it is possible to substantially improve the performance of the filter by applying the principles of the present invention.

(29) Specifically, the filter according to example 2 shows a substantially higher ratio of mean thickness of the membrane than for example 1 and also a very significantly higher filtrate flow rate after 20 hours of activity.

(30) The filter according to the embodiment of the invention illustrated by FIG. 3 (corresponding to example 3) also makes it possible to improve the homogeneity of deposition of the membrane while maintaining a permeate flow rate after 20 h that is substantially improved relative to reference example 1.

(31) The filter according to example 4 is comparable with that of example 3 (according to the invention), the channel sizes and geometries being similar. Contrary to the requirements of the present invention, the number of peripheral channels B (of smaller hydraulic diameter) according to example/FIG. 4 is however less than the number of peripheral channels of larger hydraulic diameter (channels A). The comparison of the filtration performance of the filters according to examples 3 and 4, on the basis of the data reported in table 1, shows that it is necessary for the number of peripheral channels B of smaller size to be greater than or equal to the number of peripheral channels A of larger size, in order to obtain a homogeneous distribution of the deposition while maintaining an improved filtration capacity. Such a result appears completely unexpected in view of the current knowledge in the field of tangential filters.

(32) Represented in FIG. 7 is a transverse cross-sectional view of another filter support according to the present invention, comprising a central portion that comprises only channels Ci of circular shape along said sectional plane and peripheral channels A and B, the channel B having a cross section identical to that of the channels Ci and channels A of larger hydraulic diameter, having a flared (ovoid) shape, in the shape of a drop and the end of larger dimension of which is oriented toward the center of the filter. Such a configuration has proved particularly advantageous for maximizing the filtration surface area while preserving a sufficient wall thickness between the channels A and B for maximizing the permeate flow rate and obtaining an acceptable ratio of the mean thickness of the membranes, as measured respectively on the channels A and the channels B.

(33) TABLE-US-00001 TABLE 1 Filter from Example 2 Example 3 Example 4 the art (according to (according to (comparative Example 1 the invention) the invention) example 4) Associated FIG. FIG. 1 FIG. 2 FIG. 3 FIG. 4 S.sub.A = Channel A surface area 1.52 5.89 5.65/7.08 5.65/7.08/7.64 (mm.sup.2) S.sub.B = Channel B surface area 3.14 3.14 3.14 3.14 (mm.sup.2) Surface area ratio 0.48 1.87 1.80/2.25 1.80/2.25/2.43 Rs = S.sub.A/S.sub.B Hydraulic diameter Dh.sub.A of 1.25 2.41 2.57/2.77 2.57/2.77/2.83 channel A (mm) Hydraulic diameter Dh.sub.B of 2.00 2.00 2.00 2.00 channel B (mm) Ratio Dh = Dh.sub.A/Dh.sub.B 0.62 1.20 1.28/1.38 1.28/1.38/1.41 Mean thickness of the outer 1.1 1.1 0.5 0.6 wall (mm) Number of channels A/ 1/3 1/3 4/5 4/3 number of channels B Filtration surface area m.sup.2/ 0.38 0.37 0.44 0.42 m of filter length OFA % 37 39 48 50 Ratio of mean thickness of 0.85 0.91 0.89 0.85 the membrane Measurement of relative flow 100 116 107 101 rate (in %)