MONOLITHIC MEMBRANE FILTRATION STRUCTURE

Abstract

A filtration structure with a membrane for filtering liquids, includes a monolith including a support formed from a porous inorganic material of permeability K.sub.s, the support having a tubular general shape with a main axis, an upstream face, a downstream face, a peripheral surface and an internal part; a plurality of channels parallel to the main axis of the support, formed in the internal part of the support, the channels being separated from each other by inner walls formed from the porous inorganic material; the channels being blocked at one or other of their upstream or downstream ends in the direction of circulation of the liquid, to define, respectively, inlet channels and outlet channels for the liquid, so as to force the liquid to pass through the porous walls separating the inlet and outlet channels, and a membrane covering the inner surface of at least the inlet channels.

Claims

1. A filtration structure with a membrane for filtering liquids, comprising at least one monolithic comprising: a support formed from a porous inorganic material of permeability K.sub.s, said support having a tubular general shape with a main axis, an upstream face, a downstream face, a peripheral surface and an inner part; a plurality of channels parallel to the main axis of the support, formed in the internal part of the support, said channels being separated from each other by inner walls formed from the porous inorganic material; said channels being blocked at one or other of their upstream or downstream end in the direction of circulation of said liquid, to define, respectively, inlet channels (4) and outlet channels for said liquid, so as to force said liquid to pass through the porous walls separating the inlet and outlet channels; a membrane of permeability K.sub.m and of mean thickness tm covering the inner surface of at least the inlet channels; wherein the mean path distance D of the liquid satisfies the relationship (1):
D=(Alog(K.sub.st.sub.m)+B) (1) in which: is a coefficient within a range between 0.0008 to 0.0013; A=272.sub.c+272p.sub.i+0.02; and B=601.sub.c+1757p.sub.i +0.28; .sub.c being the mean hydraulic diameter of all of the channels and p.sub.i being the mean thickness of the inner walls, D, t.sub.m, .sub.c, p.sub.i being expressed in m, and K.sub.s and K.sub.m being expressed in m.sup.2; D being defined, on a plane of cross section perpendicular to the main axis of said structure, by the arithmetic mean of the distances di between i portions of the membrane covering each inlet channel and the closest outlet channel of each portion i of membrane, a portion i being defined as a division of said membrane into at least i parts of equal length, i being greater than 10, each d.sub.i being measured from a central point of the inner surface of the membrane portion to a contact of the inner volume of said inlet channel up to a point of the inner wall of an outlet channel that is closest to said membrane portion.

2. The filtration structure as claimed in claim 1, wherein the ratio Kstm/Km is between 0.0005 and 5.

3. The filtration structure as claimed claim 1, wherein the hydraulic diameter of the support is between 50 and 300 mm.

4. The filtration structure as claimed in claim 1, wherein the mean hydraulic diameter of the channels .sub.c is between 0.5 and 5 mm.

5. The filtration structure as claimed in claim 1, wherein the mean inner wall thickness p.sub.i of the support is between 0.3 mm and 2 mm.

6. The filtration structure as claimed in claim 1, wherein the support has square, hexagonal or circular bases.

7. The filtration structure as claimed in claim 1, wherein the filter has a length of from 200 to 1500 mm.

8. The filtration structure as claimed in claim 1, wherein all the channels have an identical hydraulic diameter.

9. The filtration structure as claimed in claim 1, wherein the support has an open porosity of between 20% and 70%.

10. The filtration structure as claimed in claim 1, wherein the support has a median pore diameter of between 10 nm and 50 m.

11. The filtration structure as claimed in claim 1, wherein the mean thickness of the membrane t.sub.m is within a range from 0.1 to 300 m.

12. The filtration structure as claimed in claim 1, wherein the membrane has an open porosity of between 10% and 70%.

13. The filtration structure as claimed in claim 1, wherein the membrane has a median pore diameter of between 10 nm and 5 m.

14. The filtration structure as claimed in claim 1, wherein the channels have a circular or polygonal cross section.

15. A method comprising utilizing a filter as claimed in claim 1 for the purification and/or separation of liquids in the field of chemistry, pharmaceuticals, food, agrofood, bioreactors, or the extraction of oil or of shale gases.

16. The filtration structure as claimed in claim 2, wherein the ratio Kstm/Km is between between 0.001 and 1.

17. The filtration structure as claimed claim 3, wherein the hydraulic diameter of the support is between 80 and 230 mm.

18. The filtration structure as claimed in claim 4, wherein the mean hydraulic diameter of the channels c is between 0.5 and 3 mm.

19. The filtration structure as claimed in claim 5, wherein the mean inner wall thickness p.sub.i of the support is between 0.4 mm and 1.4 mm.

20. The filtration structure as claimed in claim 11, wherein the mean thickness of the membrane t.sub.m is within a range from 10 to 70 m.

21. The filtration structure as claimed in claim 15, wherein the membrane has a median pore diameter of between 50 nm and 1000 nm.

22. The filtration structure as claimed in claim 14, wherein the channels have a square, hexagonal or octagonal and square cross section.

Description

[0069] The figures attached hereto illustrate in greater detail certain aspects of the present invention. The information given herein below should not, however, be considered as restricting the scope of the invention, in any of the aspects of the invention described in the figures.

[0070] FIG. 1 illustrates an overall view of a common filtering structure (or filter).

[0071] FIG. 2 is a front view of a part of the upstream face of the filter which illustrates in greater detail the subject matter of the present invention.

[0072] FIGS. 3 to 10 are also front views of a part of the upstream face of the filter in which the configuration of the channels is different.

[0073] FIGS. 11 and 12 illustrate two embodiments of a filter according to the present invention.

[0074] FIG. 1 illustrates a frontal filtration filter comprising a cylindrical support 1 having a main axis (X), an upstream face 2 and a downstream face 3, in the direction of circulation of the liquid to be filtered. A plurality of channels parallel to the main axis (X) are formed in the internal part of the support and separated from each other by porous inner walls, comprising inlet channels 4 which are open on the upstream face and outlet channels 5 which are open on the downstream face, in the direction of circulation of the liquid. The inlet channels 4, emerging on each of the upstream bases 2 and downstream bases 3, are covered on their inner surface with a membrane (not shown in FIG. 1) and are blocked on their downstream face 3. The outlet channels 5 are blocked on their upstream face 2.

[0075] FIG. 2 is a view of the upstream face of a filter for illustrating the subject of the present invention in greater detail. A central inlet channel 4 and several outlet channels 5 of the filter, and also the filtering membrane 6 which lines the interior of each inlet channel, are shown in FIG. 2. This membrane is divided into i portions of equal length, as shown in FIG. 2. For each portion i, a distance di from the central point 7 of the inner surface of the membrane portion in contact with the inner volume of said inlet channel up to point 8 of the inner wall of an outlet channel that is closest to said membrane portion is determined. As may be seen in FIG. 2, various outlet channels 5 can and must be considered as a function of the position of the membrane portion i and of the blocking configuration and of the geometry of the channels.

[0076] The distance D that is relevant according to the present invention is the arithmetic mean of the di thus determined for all the portions i of all the inlet channels of each monolith.

[0077] The number of portions chosen in the section plane is advantageously chosen as a function of the configuration of the channels and of the number of outlet channels with regard to each inlet channel, but must be sufficient to be representative of the mean path of the liquid derived from an inlet channel to an outlet channel, across the porous wall of the support. Typically, the number of measurements of di per channel is greater than 10, or even greater than 20, preferably greater than 50, or even greater than 100. According to the invention, at least 20, preferably at least 50 or even 100 distances d.sub.i are thus determined per inlet channel, for the calculation of D.

[0078] FIG. 3 is a front view of the upstream face of a filtration filter whose inlet and outlet channels are of square cross section, according to a first configuration of the blocking of the channels.

[0079] FIGS. 4 to 7 are front views of the upstream face of a filtration filter whose inlet and outlet channels are of square cross section, according to other configurations of the blocking of the channels.

[0080] FIGS. 8 to 10 are front views of the upstream face of a filtration filter whose inlet and outlet channels are of hexagonal cross section, according to several configurations of the blocking of the channels.

[0081] FIGS. 11 and 12 illustrate two modes of functioning of such filters:

more particularly, FIG. 11 illustrates a longitudinal section (along a plane passing through the main axis) of a filtering structure (or filter) inserted in a compartment (housing).

[0082] FIG. 12 schematically represents a longitudinal section of a filter immersed in a reservoir of the liquid to be filtered.

[0083] FIG. 11 describes a frontal filtration filter inserted in a compartment 10 comprising a cylindrical support 1 having a main axis (X), an upstream face 2 and a downstream face 3. A plurality of channels parallel to the main axis (X) are formed in the inner part of the support and separated from each other by porous inner walls, comprising inlet channels 4 which are open on the upstream face and outlet channels 5 which are open on the downstream face, in the direction of circulation of the liquid. The inlet channels 4, emerging on each of the upstream bases 2 and downstream bases 3, are covered on their inner face with a membrane (6) and are blocked on their downstream face 3. The outlet channels 5 are blocked on their upstream face 2. The leaktightness of the system is ensured by a seal 9.

[0084] FIG. 12 schematically represents a frontal filtration filter immersed in a reservoir 11 comprising the liquid to be filtered. The components of the filter are similar to those of FIG. 11, except for the fact that the filter moreover comprises a coating 6 on its outer periphery so that the liquid to be filtered does not circumvent the membrane by passing via the periphery directly into a peripheral outlet channel. This coating may be leaktight. If this coating is permeable, it comprises at least the membrane. The leaktightness on contact with the reservoir is ensured by a seal 9.

[0085] The present invention is illustrated with the aid of the nonlimiting examples that follow, in connection with the attached FIGS. 1 to 10.

EXAMPLES

[0086] Examples of frontal filters according to the invention (examples 1-3, 2-1, 3-4, 3-5 and 4-2) and comparative examples (1-1; 1-2; 2-2; 2-3; 2-4; 3-1; 3-2; 3-3; 3-5; 4-1 and 4-3) were prepared according to the processes described below.

Example 1-1 (Comparative)

[0087] A support was made according to the techniques well known to those skilled in the art by forming a silicon carbide honeycomb. To do this, the following are mixed in a blender: [0088] 3000 g of a mixture of the two powders of silicon carbide particles with a purity of greater than 98% comprising 70% by mass of a first powder of grains with a median diameter of about 11 m and 30% by mass of a second powder of grains with a median diameter of about 0.9 m; and [0089] 300 g of an organic binder of the cellulose derivative type.

[0090] About 25% by mass of water relative to the mass of SiC and of organic binder are added and blending is performed until a homogeneous paste is obtained, the plasticity of which allows extrusion to obtain a support with a porosity of 35%.

[0091] The support is extruded using this paste by means of a die to obtain a crude cylindrical monolithic block with a diameter of 150 mm and a length of 300 mm, the inner part of which has a plurality of channels of square cross section. The shape of the die is adapted to obtain channels of square cross section with a hydraulic diameter of 1.8 mm and inner walls with a mean thickness of 400 micrometers.

[0092] The crude monolith obtained is then dried to bring the content of water not chemically bound to less than 1% by mass, and then baked under argon up to a temperature of 2100 C. which is maintained for 5 hours. The support obtained has an open porosity of 35% and a median pore diameter of about 10 m, as measured by mercury porosimetry.

[0093] The channels of the monolith are alternately blocked according to well-known techniques, for example described in patent application WO 2004/065088. So as to obtain a blocking geometry as shown in FIG. 3. The outer peripheral wall of the support is rendered nonfiltering.

[0094] A filtration membrane is then deposited on the inner surface of the channels. The deposition of the membrane is performed by coating with slips. To do this, a membrane attachment primer is made in a first stage, using a slip whose mineral formulation includes 48% by mass of a powder of black SiC grains (SIKA DPF-C), the median diameter D.sub.50 of which is about 10 micrometers, 32% by mass of a powder of black SiC grains (SIKA FCP-07), the median diameter D.sub.50 of which is about 2 micrometers, 13% by mass of a powder of silicon metal grains, the median diameter D.sub.50 of which is about 4 micrometers, 7% of an amorphous carbon powder, the median diameter D.sub.50 of which is about 1 micrometer. The whole is mixed in a solution of deionized water, the amount of water representing about 50% of the total mass of the mixture.

[0095] The membrane separating layer (the membrane) is obtained using a slip whose mineral composition is as follows: 67% by mass of powder of silicon metal grains, the median diameter D.sub.50 of which is about 4 micrometers, 33% of amorphous carbon powder, the median diameter D.sub.50 which is about 1 micrometer. The whole is mixed in a solution of deionized water, the amount of water representing about 50% of the total mass of the mixture.

[0096] The supports are then dried at room temperature for 10 minutes and then at 60 C. for 12 hours. The supports thus dried are then baked under argon at a temperature of 1470 C. for 4 hours at ambient pressure under argon.

[0097] The primer and the membrane are deposited according to the same process. The slip is introduced into a reservoir with stirring at 20 rpm. After a phase of de-aeration under a mild vacuum, typically 25 mbar, with continued stirring, the reservoir is placed under a mild excess pressure of about 0.8 bar so as to be able to coat the interior of the support from the bottom to the top. This operation only takes a few seconds for a support 300 mm long. The slip coats the inner wall of the channels of the support and the excess is then evacuated by gravity immediately after deposition. In practice, irrespective of its thickness, this layer of primer has no influence on the filtration performance qualities of the filter, given its porosity characteristics (median pore diameter and overall porosity) which are greater than that of the membrane itself, which thus acts alone as the separating layer.

[0098] The coated support is then dried at room temperature for 30 minutes and then at 60 C. for 30 hours.

[0099] The coated support thus dried is then sintered at a temperature of 1300 C. under an argon atmosphere for 4 hours to obtain a membrane porosity of 40% with a median pore diameter of 100 nm.

Example 1-2 (Comparative)

[0100] A filter was prepared in an identical manner to that of example 1-1, the only difference being that blocking is performed according to the configuration described in FIG. 4.

Example 1-3 (According to the Invention)

[0101] A filter was prepared in an identical manner to that of example 1-1, except that the blocking is performed according to the configuration described in FIG. 5.

Examples 2-1 (According to the Invention) and 2-2 to 2-4 (Comparative)

[0102] A filter was prepared in an identical manner to that of example 1-1, except that the die was modified so as to obtain channels with a hydraulic diameter of 2.6 mm and a mean inner wall thickness of 800 micrometers. The mixture intended for the extrusion of the support comprises 65% by mass of a first powder of silicon carbide particles with a median diameter of about 11 m and 35% by mass of a second powder of silicon carbide particles with a median diameter of about 0.9 m.

[0103] In this series of examples, a membrane separating layer made of silicon carbide is then deposited on the inner wall of the channels according to the process described below:

[0104] a primer for attaching the separating layer is formed, in a first stage, using a slip whose mineral formulation includes 30% by mass of a powder of black SiC grains (SIKA DPF-C), the median diameter D.sub.50 of which is about 11 micrometers, 20% by mass of a powder of black SiC grains (SIKA FCP-07), the median diameter D.sub.50 of which is about 2.5 micrometers, and 50% of deionized water. A slip of the material constituting the separating layer is also prepared, the formulation of which includes 40% by mass of SiC grains (d50 in the region of 0.6 micrometer) and 60% of demineralized water. The rheology of the slips was adjusted by adding organic additives at 0.7 Pa.Math.s under a shear gradient of 1 s.sup.1, measured at 22 C. according to standard DIN C33-53019.

[0105] These two layers are successively deposited according to the same process described below: the slip is introduced into a reservoir with stirring (20 rpm). After a phase of de-aeration under a mild vacuum (typically 25 millibar), with continued stirring, the reservoir is placed under a positive pressure of about 0.7 bar in order to be able to coat the interior of the support from its bottom part to its top extremity. This operation only takes a few seconds for a support 30 cm long. Immediately after coating the slip on the inner wall of the channels of the support, the excess is evacuated by gravity.

[0106] The supports are then dried at room temperature for 10 minutes and then at 60 C. for 12 hours and the channels are blocked according to the same procedure as for the series of examples 1-1 to 1-3.

[0107] The supports thus dried are then baked under argon at a temperature of 1540 C. for 2 hours at ambient pressure.

Examples 3-1 to 3-3 (Comparative Examples) 3-4 and 3-5 (According to the Invention)

[0108] A filter was prepared in an identical manner to that of example 2-1, except that the die was modified so as to obtain channels with a hydraulic diameter equal to 1.9 mm and a wall thickness of 635 micrometers. Furthermore, the crude monolith obtained is baked up to a temperature of 2200 C. The support obtained has an open porosity of 50% and a median pore diameter of about 35 m.

[0109] Blocking of the structures according to examples 3-1 to 3-4 was performed, respectively, in the same manner as for examples 1-1 to 1-3, respectively, according to FIGS. 3 to 6. A design according to FIG. 7 was produced according to example 3-5.

[0110] In this series of examples, the step of deposition and of drying of the separating membrane is performed twice successively (once only) so as to obtain a layer with a mean thickness of 50 micrometers. Furthermore, the rheology of the slip was adjusted by adding organic additives at 0.5 Pa.Math.s under a shear gradient of 1 s.sup.1, measured at 22 C. according to standard DIN C33-53019.

Examples 4-1 and 4-3 (Comparative Examples) and 4-2 (According to the Invention)

[0111] A filter was prepared in an identical manner to that of example 2-1, except that the die was modified so as to obtain a hexagonal structure as shown in FIG. 8, the channels of which have a hydraulic diameter of 2.0 mm and a mean inner wall thickness of 600 micrometers. Furthermore, the crude monolith obtained is baked up to a temperature of 2130 C. The support obtained has an open porosity of 40% and a median pore diameter of about 9 m.

[0112] In this series of examples, the preparation of the separating membrane is performed as for example 2-1, but the coated supports are then baked under argon at a temperature of 1480 C. instead of 1540 C.

[0113] Blocking of the structures according to examples 4-1 to 4-3 was performed in the same manner as previously so as to obtain a blocked structure, respectively, according to FIGS. 8 to 10.

[0114] Table of Results and Test:

[0115] For each of these filters, the ratio /.sub.max is determined, in which is the characteristic flow of the filter under consideration and .sub.max is the flow measured for the most efficient filter of the same series of examples, for which an efficacy of 100% is attributed. The characteristic flow of a filter was evaluated according to the following method: at a temperature of 25 C., a fluid formed from demineralized water feeds the filters to be evaluated at a transmembrane pressure of 0.5 bar and a circulation speed in the channels of 2 m/s. The permeate is recovered at the filter outlet. Measurement of the characteristic flow of the filter is expressed in L/h/m/bar after 20 hours of filtration. The results obtained and also all the pertinent size characteristics of the filters thus obtained are collated in table 1 below.

[0116] Examples 1-3, 2-1 and 3-4 according to the invention correspond to optimum structures whose configuration moreover depends on the physical characteristics of the membrane and of the support. These examples demonstrate the importance of adapting the pattern and the number of inlet and outlet channels of the filter as a function of the physical parameters of the filter, such as the shape of the channels, the mean thickness of the inner walls, the mean thickness of the membrane, the median pore diameter of the membrane and the porosity of the membrane or of the support, so as to obtain a distance D according to the invention to maximize the flow of filtrate. The filters according to the invention thus dimensioned are characterized by an optimized and maximal flow of filtrate, as may be observed in the results reported in table 1.

[0117] The advantages of the present invention are also demonstrated on other types of filters different from the preceding examples by the totally different configuration of the inlet and outlet channels, which in this case have a hexagonal cross section, as shown in the attached FIGS. 8 to 10. The configurations according to FIGS. 8 to 10 differ in their number of inlet and outlet channels. The results obtained are reported in table 2 below. As for the filters whose channels have a square cross section, it is observed that maximum filtration efficiency, in the sense described previously, is obtained for the filter according to example 4-3 in accordance with the present invention, but the channels of which are in this case of hexagonal cross section.

TABLE-US-00001 TABLE 1 ex 1-1 ex 1-2 ex 1-3 ex 2-1 ex 2-2 ex 2-3 ex 2-4 ex 3-1 ex 3-2 ex 3-3 ex 3-4 ex 3-5 Hydraulic diameter of the channels 1.8 2.6 1.9 c (mm) Thickness of the inner walls 0.400 0.800 0.635 pi (mm) Open porosity of the substrate (%) 35 40 50 Median pore diameter of the 10 5 35 substrate (m) Membrane porosity (%) 40 50 45 Median pore diameter of the 100 350 350 membrane (nm) Mean thickness of the membrane 30 30 50 (separating layer) (m) Ks.tm/Km (m) 0.171 0.002 0.830 Theoretical distance D according to 1.3 < D < 2.1 0.6 < D < 1.0 2.0 < D < 3.2 the invention for = 0.0008 and 0.0013 (m) Blocking pattern according to FIG. 3 FIG. 4 FIG. 5 FIG. 3 FIG. 4 FIG. 5 FIG. 6 FIG. 3 FIG. 4 FIG. 5 FIG. 6 FIG. 7 D measured (mm) 0.41 0.73 1.51 0.81 1.31 2.45 3.15 0.64 1.01 1.85 2.37 3.15 Relative flow 40% 87% 100% 100% 96% 55% 37% 56% 84% 96% 100% 98%

TABLE-US-00002 TABLE 2 ex 4-1 ex 4-2 ex 4-3 Hydraulic diameter of the channels c (mm) 2.0 Thickness of the inner walls pi (mm) 0.600 Open porosity of the substrate (%) 40 Median pore diameter of the substrate (m) 9 Membrane porosity (%) 40 Median pore diameter of the membrane (nm) 250 Mean thickness of the membrane 50 (separating layer) (m) Ks .Math. tm/Km (m) 0.065 Theoretical distance D according to the 1.3 < D < 2.2 invention for = 0.0008 and 0.0013 (mm) Blocking pattern according to FIG. 8 FIG. 9 FIG. 10 D measured (mm) 0.75 1.80 2.3 Relative flow 29% 100% 88%