MONOLITHIC FILTER

Abstract

A monolithic membrane filter for the filtration of liquids includes a support formed from a porous inorganic material of permeability K.sub.s, the support exhibiting a tubular shape having a main axis, an upstream base, a downstream base, a peripheral surface and an internal portion; channels parallel to the main axis of the support, formed in the internal portion of the support, the channels separated from one another by internal walls formed from the porous inorganic material; a formed in the internal portion of the support and opening onto the peripheral surface so that the filter has an external surface formed by the peripheral surface of the support and the surface of said at least one slot; and a membrane of permeability K.sub.m and of mean thickness t.sub.m covering the internal surface of the channels.

Claims

1. A monolithic membrane filter for the filtration of liquids comprising: a support formed from a porous inorganic material of permeability K.sub.s, said support exhibiting a tubular shape having a main axis, an upstream base, a downstream base, a peripheral surface and an internal portion; a plurality of channels parallel to the main axis of the support, formed in the internal portion of the support, said channels separated from one another by internal walls formed from the porous inorganic material; at least one slot formed in the internal portion of the support and opening onto the peripheral surface so that the filter has an external surface formed by the peripheral surface of the support and the surface of said at least one slot; and a membrane of permeability K.sub.m and of mean thickness t.sub.m covering the internal surface of the channels; wherein a mean path distance D satisfies the equation:
D=?*exp(B)*(K.sub.st.sub.m/K.sub.m).sup.A wherein ? is a coefficient within a range of from 0.0008 to 0.0012; A=?21.5*?.sub.c+15.4*p.sub.i+0.16*?.sub.f+0.31; and B=561*p.sub.i+101?.sub.c+1.16; where ?.sub.c is the mean hydraulic diameter of the channels, ?.sub.f is the hydraulic diameter of the filter and p.sub.i is the mean thickness of the internal walls; and D, t.sub.m, ?.sub.c, p.sub.i and ?.sub.f are expressed in m, and K.sub.s and K.sub.m are expressed in m.sup.2; D being defined by the arithmetic mean of all of the minimum distances d.sub.i between the center of each channel c.sub.i and the external surface of the filter, the distances d.sub.i being measured for each filtering channel c.sub.i by considering a sectional plane perpendicular to the main axis onto which all of the slots are transferred.

2. The filter as claimed in claim 1, wherein a ratio ?/D is less than 0.65 wherein ? is the standard deviation relative to the distance D of the minimum distances d.sub.i between the center of each channel c.sub.i and the external surface of the filter.

3. The filter as claimed in claim 1, wherein said at least one slot is/are each in a plane parallel to the main axis.

4. The filter as claimed in claim 1, wherein each of said at least one slot is formed by a cavity opening onto the peripheral surface of the support, with the channels directly connected to said cavity.

5. The filter as claimed in claim 4, wherein said cavity has a length of from 1% to 20% of a length of the filter.

6. The filter as claimed in claim 1, wherein said at least one slot is/are blind.

7. The filter as claimed in claim 1, wherein the support has square, hexagonal or circular bases.

8. The filter as claimed in claim 1, wherein the hydraulic diameter of the filter ?.sub.f is within a range extending from 50 to 300 mm.

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

10. The filter as claimed in claim 1, wherein the mean hydraulic diameter of the channels ?.sub.c is within a range extending from 1 to 5 mm.

11. The filter as claimed in claim 1, wherein all the channels have an identical hydraulic diameter.

12. The filter as claimed in claim 1, wherein the mean thickness of the internal walls p.sub.i is within a range extending from 0.3 to 2 mm.

13. The filter as claimed in claim 1, wherein the support has an open porosity of from 20% to 70%.

14. The filter as claimed in claim 1, wherein the mean thickness of the membrane t.sub.m is within a range extending from 0.1 to 300 ?m.

15. The filter as claimed in claim 1, wherein the membrane has an open porosity of from 10% to 70%.

16. A method comprising purifying and/or separating liquids in the chemistry, pharmaceutical, food, agri-food, bioreactor, or oil or shale gas extraction fields with a filter as claimed in claim 1.

17. The filter as claimed in claim 14, wherein the mean thickness of the membrane t.sub.m is within a range extending from 10 to 70 ?m.

Description

EXAMPLES

[0047] Examples of tangential filters according to the invention (examples 1A, 1B and 2 to 7) and comparative examples (C1 to C7) were prepared according to the processes described below.

Examples 1A and 1B

[0048] A support was produced according to techniques well known to a person skilled in the art by forming a silicon carbide honeycomb. In order to do this, the following are mixed in a kneader: [0049] 3000 g of a mixture of the two powders of silicon carbide particles with a purity of greater than 98% comprising 75% by weight of a first powder of grains having a median diameter of around 60 ?m and 25% by weight of a second powder of grains having a median diameter of around 2 ?m; and [0050] 300 g of an organic binder of the cellulose derivative type. Water, around 25% by weight relative to the weight of SiC and of organic binder, is added and kneading is carried out until a homogeneous paste is obtained, the plasticity of which allows extrusion.

[0051] The support is extruded from this paste using a die to obtain a cylindrical green monolith block with a diameter of 150 mm and a length of 300 mm, the internal portion of which has a plurality of channels of square section. The shape of the die is suitable for obtaining channels having a hydraulic diameter of 4 mm and internal walls with a mean thickness of 1.2 mm.

[0052] The green monolith obtained is subsequently 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, then fired up to a temperature of at least 2050? C., which is maintained for 5 hours. The support obtained has an open porosity of 50% and a median pore diameter of around 10 ?m.

[0053] A membrane is then deposited on the internal surface of the channels. The deposition of the membrane is carried out by slip coating. For this, a first primer layer is deposited from a slip comprising 50% by weight of SiC grains having a median diameter of around 20 ?m and 50% of deionized water. A separating layer is then deposited on the primer layer from a slip comprising 50% by weight of SiC grains having a median diameter of around 1 ?m and 50% of deionized water. The viscosity of the slips, measured at 22? C. under a shear gradient of 1 s.sup.?1 according to the DIN-53019-1:2008 standard, is adjusted to 0.1 Pa.Math.s with the aid of additives well known to a person skilled in the art.

[0054] The primer and the membrane are deposited according to the same process. The slip is introduced into a tank stirred at 20 rpm After a phase of deaerating under slight vacuum (typically 25 mbar), while maintaining the stirring, the tank is placed under a slight positive pressure of around +1 bar in order to be able to coat the inside of the support from the bottom up to the top. This operation only takes a few seconds for a 300 mm long support. The slip coats the internal wall of the channels of the support and the excess is then discharged by gravity immediately after deposition.

[0055] Next, the coated support is dried at ambient temperature for 30 minutes then at 60? C. for 30 h. The coated support thus dried is then sintered at a temperature of 1350? C. under an argon atmosphere for 4 hours in order to obtain a porosity of the membrane of 40% with a median pore diameter of 200 nm.

[0056] Cavities were machined in the dried support and the discharge channels connected to the cavities were plugged according to well-known techniques as described in application WO 2004/065088 in order to create slots, before sintering the support. The slots were positioned so as to obtain a mean path distance D that satisfies the equation (1) according to the invention, that is to say a distance D of between 5.7 and 8.5 mm in the case of examples 1A and 1B. In the case of example 1A, 4 through cavities with a length of 50 mm and a thickness equal to a channel (4 mm), which are parallel to one another and to the main axis, are machined according to the diagram illustrated in FIG. 3. The slots are positioned in a regular manner with respect to one another, that is to say that the number of channels between two neighboring slots is constant to within plus or minus one channel. In the case of example 1B, 10 blind slots with a length of 50 mm, which are parallel to one another and to the main axis, are machined according to the diagram illustrated in FIG. 4. In FIGS. 3 and 4 representing sectional planes of the filters according to examples 1A and 1B respectively, the hollowed out portions represent the slots with no distinction between the cavities and the discharge channels.

Comparative Example C1

[0057] A filter was prepared in a manner identical to that of example 1A except that only 2 slots were produced by machining 2 through cavities, parallel to one another and to the main axis, in the support according to the diagram illustrated in FIG. 5.

Example 2

[0058] A filter was prepared in a manner identical to that of example 1A except that the shape of the die is suitable for obtaining channels having a hydraulic diameter of 2 mm and internal walls with a mean thickness of 1.2 mm; and 5 slots were produced by machining 5 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (2 mm).

Comparative Example C2

[0059] A filter was prepared in a manner identical to that of example 2 except that 3 slots were produced by machining 3 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (2 mm).

Example 3

[0060] A filter was prepared in a manner identical to that of example 1A except that the shape of the die is suitable for obtaining channels having a hydraulic diameter of 4 mm and internal walls with a mean thickness of 0.4 mm; and 7 slots were produced by machining 7 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (4 mm).

Comparative Example C3

[0061] A filter was prepared in a manner identical to that of example 3 except that 10 slots were produced by machining 10 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (4 mm).

Example 4

[0062] A filter was prepared in a manner identical to that of example 1A except that the dried coated support is sintered at a temperature of 1300? C. under an argon atmosphere for 4 hours in order to obtain a porosity of the membrane of 40% with a median pore diameter of 125 nm and 3 slots were produced by machining 3 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (4 mm).

Comparative Example C4

[0063] A filter was prepared in a manner identical to that of example 4 except that 7 slots were produced by machining 7 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (4 mm).

Example 5

[0064] A filter was prepared in a manner identical to that of example 1A except that the slip used for the deposition of the membrane comprises 12.3% by weight of SiC grains having a median diameter of around 0.5 ?m, 64.4% of deionized water, 23.1% of PVA and 0.2% of deflocculant with reference to example 2 of EP 0 219 383. The coated and dried support is sintered at a temperature of 1050? C. under a nitrogen atmosphere for a hold of 4 hours in order to obtain a porosity of the membrane of 25% with a median pore diameter of 200 nm; and 2 slots were produced by machining 2 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (4 mm).

Comparative Example C5

[0065] A filter was prepared in a manner identical to that of example 5 except that 1 slot was produced by machining 1 through cavity, parallel to the main axis, in the support. The cavity has a length of 50 mm and a thickness equal to a channel (4 mm).

Example 6

[0066] A filter was prepared in a manner identical to that of example 1A except that a longer contact time between the suspension and the support is selected in order to obtain a membrane having a mean thickness of 200 ?m, this thickness being obtained by combining 4 layers of 50 ?m with the deposition and drying process described in example 1A. The support thus coated and dried is sintered under the same conditions as example 1A; and 3 slots were produced by machining 3 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (4 mm).

Comparative Example C6

[0067] A filter was prepared in a manner identical to that of example 6 except that 6 slots were produced by machining 6 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (4 mm).

Example 7

[0068] A filter was prepared in a manner identical to that of example 1A except that the mixture used for manufacturing the support comprises: [0069] 3000 g of a mixture of the two powders of silicon carbide particles with a purity of greater than 98% comprising 70% by weight of a first powder of grains having a median diameter of around 11 ?m and 30% by weight of a second powder of grains having a median diameter of around 0.5 ?m; and [0070] 300 g of an organic binder of the cellulose derivative type;
in order to obtain a support having a porosity of 35%; and 7 slots were produced by machining 7 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (4 mm).

Comparative Example C7

[0071] A filter was prepared in a manner identical to that of example 7 except that 4 slots were produced by machining 4 through cavities, parallel to one another and to the main axis, in the support. The cavities have a length of 50 mm and a thickness equal to a channel (4 mm).

[0072] For each of these filters, the ratio ?/?.sub.0 is determined in which ? is the characteristic flow of the filter and ?.sub.0 is the characteristic flow of an identical filter without any slots. The characteristic flow of a filter was evaluated according to the following method: at a temperature of 25? C. a fluid consisting of demineralized water supplies the filters to be evaluated under a transmembrane pressure of 0.5 bar and a rate of circulation in the channels of 2 m/s. The permeate is recovered at the periphery of the filter. The measurement of the characteristic flow of the filter is expressed in L/h/m/bar after filtering for 20 h. The results obtained and also the dimensional characteristics of the filters thus obtained are summarized in table 1 below.

[0073] These examples demonstrate the importance of adapting the geometry of the filter on the basis of the physical parameters of the filter, such as the shape of the channels, the mean thickness of the internal 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 in order to maximize in order to maximize the filtrate flow. The filters according to the invention an increase in the characteristic flow that is at least 5% higher.

TABLE-US-00001 TABLE 1 Example 1A 1B C1 2 C2 3 C3 4 C4 ?f 150 150 150 150 150 150 150 150 150 ?c 4 4 4 2 2 4 4 4 4 p.sub.i 1.2 1.2 1.2 1.2 1.2 0.4 0.4 1.2 1.2 D50.sub.m 200 200 200 200 200 200 200 125 125 PO.sub.m 40 40 40 40 40 40 40 40 40 t.sub.m 50 50 50 50 50 50 50 50 50 PO.sub.s 50 50 50 50 50 50 50 50 50 D50.sub.s 10 10 10 10 10 10 10 10 10 K.sub.s * t.sub.m/K.sub.m 0.352 0.352 0.352 0.352 0.352 0.352 0.352 0.90 0.90 D.sub.inv 5.7-8.5 4.4-6.7 3.6-5.5 7.3-10.9 N.sub.f .sup.4.sup.(a) 10.sup.(D) .sup.2.sup.(a) .sup.5.sup.(a) .sup.3.sup.(a) .sup.7.sup.(a) .sup.10.sup.(a) .sup.3.sup.(a) .sup.7.sup.(a) D.sub.actual 6.7 6.3 8.8 6.0 9.1 3.8 3 8.5 4.1 ?/D 0.53 0.57 0.53 0.51 0.52 0.64 0.81 0.52 0.59 ?/?.sub.0 2.77 2.74 2.19 2.75 2.14 4.33 3.99 1.91 1.79 Example 5 C5 6 C6 7 C7 ?f 150 150 150 150 150 150 ?c 4 4 4 4 4 4 p.sub.i 1.2 1.2 1.2 1.2 1.2 1.2 D50.sub.m 200 200 200 200 200 200 PO.sub.m 25 25 40 40 40 40 t.sub.m 50 50 200 200 50 50 PO.sub.s 50 50 50 50 35 35 D50.sub.s 10 10 10 10 10 10 K.sub.s * t.sub.m/K.sub.m 2.25 2.25 1.406 1.406 0.0701 0.0701 D.sub.inv 9.3-13.9 8.2-12.3 3.7-5.5 N.sub.f .sup.2.sup.(a) .sup.1.sup.(a) .sup.3.sup.(a) .sup.6.sup.(a) .sup.7.sup.(a) .sup.4.sup.(a) D.sub.actual 11.4 16.1 8.4 4.7 4.1 6.7 ?/D 0.53 0.59 0.56 0.57 0.58 0.58 ?/?.sub.0 1.40 1.19 1.77 1.68 6.17 4.77 ?.sub.f hydraulic diameter of the filter (mm) ?.sub.c: mean hydraulic diameter of the channels (mm) P.sub.i: mean thickness of the internal walls (mm) D50.sub.m: median pore diameter of the membrane (nm) PO.sub.m: open porosity of the membrane (%) t.sub.m: mean thickness of the membrane (?m) PO.sub.s: open porosity of the support (%) N.sub.f number of through .sup.(a) or blind .sup.(b) slots D50.sub.s: median pore diameter of the support (?m) K.sub.s: permeability of the support (m.sup.2) K.sub.m: permeability of the membrane (m.sup.2) D.sub.inv: range of D calculated according to the invention (mm) D.sub.actual: value of D measured according to the actual geometry of the filter (mm) ?.sub.: characteristic flow of the filter (L/h/m/bar) ?.sub.0: characteristic flow of an identical filter with 0 slots (L/h/m/bar)