Monolithic membrane filtration structure

Abstract

A monolithic membrane-type filtration structure for filtering liquids, includes a support formed of a porous inorganic material of permeability K.sub.s, the support having a tubular overall shape with a main axis, an upstream base, a downstream base, a peripheral wall delimiting an internal part and a plurality of passages parallel to the main axis of the support, formed in the internal part of the support, a membrane of permeability K.sub.m and of mean thickness t.sub.m covering the internal surface of the passages; the external hydraulic diameter of the structure satisfying the relationship Ø.sub.f=α×[A+B×log.sub.10 (K.sub.s×t.sub.m/K.sub.m)]; in which α is a coefficient between 0.85 and 1.15, and A=−21570×ent.sub.int.sup.2−18.6×D.sub.h+19.0×e.sub.int−2.5×e.sub.ext+0.1244 B=−11760×D.sub.h×e.sub.int+9.7×e.sub.int+3.1×e.sub.ext+0.04517. D.sub.h is the mean hydraulic diameter of the passages, e.sub.int is the minimum thickness of the internal walls between the passages, e.sub.ext is the minimum thickness of the peripheral wall of the filter.

Claims

1. A monolithic membrane-type filtration structure for filtering liquids, comprising: a support formed of a porous inorganic material of permeability K.sub.s, said support having a tubular overall shape with a main axis, an upstream base, a downstream base, a peripheral wall delimiting an internal part; a plurality of passages parallel to the main axis of the support, formed in the internal part of the support, said passages being separated from one another by internal walls formed of the porous inorganic material; all said passages being open at their upstream or downstream ends in a direction in which said liquid circulates, the filtered liquid being removed via said peripheral wall, a membrane of permeability K.sub.m and of mean thickness t.sub.m covering the internal surface of the passages; wherein an external hydraulic diameter of the monolithic membrane-type filtration structure Ø.sub.f satisfies the relationship (1):
Ø.sub.f=α×[A+B×log.sub.10(K.sub.s×t.sub.m/K.sub.m)]  (1) in which α is a coefficient comprised between 0.85 and 1.15, and
A=−21570×e.sub.int.sup.2−18.6×D.sub.h+19.0×e.sub.int−2.5×e.sub.ext+0.1244
B=−11760×D.sub.h×e.sub.int+9.7×e.sub.int+3.1×e.sub.ext+0.04517 in which: D.sub.h is the mean hydraulic diameter of the passages, e.sub.int is the minimum thickness of the internal walls between the passages and e.sub.int is from 0.3 mm to 3 mm, e.sub.ext is the minimum thickness of the peripheral wall of the filter and e.sub.ext is from 0.5 mm to 4 mm, t.sub.m, Ø.sub.f, e.sub.int, e.sub.ext and D.sub.h being expressed in meters and K.sub.s and Km being expressed in m.sup.2, and wherein the external hydraulic diameter Ø.sub.f of the support is from 30 mm to 100 mm.

2. The filtration structure as claimed in claim 1, wherein the ratio Ks×t.sub.m/K.sub.m is from 0.01 m to 10 m.

3. The filtration structure as claimed in claim 1, wherein the mean hydraulic diameter of the passages d.sub.h is from 0.5 to 7 mm.

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

5. The filtration structure as claimed in claim 1, wherein all the passages have the same hydraulic diameter.

6. The filtration structure as claimed in claim 1, wherein at least two passages have a different hydraulic diameter.

7. The filtration structure as claimed in claim 1, wherein the permeability Ks of the support is from 1.0×10.sup.−14 to 1.0×10.sup.−11.

8. The filtration structure as claimed in claim 1, wherein the permeability Km of the membrane is from 1.0×10.sup.−19 to 1.0×10.sup.−14.

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

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

11. The filtration structure as claimed in claim 1, wherein the median diameter of the pores of the support is from 20 to 50 micrometers.

12. The filtration structure as claimed in claim 1, wherein the passages are of circular or polygonal cross section.

13. A filtration device comprising: said filter structure as claimed in claim 1, and an enclosure sealing around said filter structure, said enclosure comprising: means for introducing the liquid that is to be filtered, in fluidic communication with the passages on the upstream face of said filter structure, means for removing the permeate at the periphery of the filter structure and in fluidic communication with the peripheral wall of said filter structure, means for removing the retentate or concentrate on the downstream face of said filter structure.

14. A method comprising purifying and/or separating liquids in the chemical, pharmaceutical, food or agrifoodstuffs industry, in bioreactors, or in the extraction of oil or gases from shale with a filter structure as claimed in claim 1.

15. The filtration structure as claimed in claim 3, wherein the external hydraulic diameter Ø.sub.f of the support is from 50 to 80 mm.

16. The filtration structure as claimed in claim 3, wherein the mean hydraulic diameter of the passages d.sub.h is from 1.5 to 3.5 mm.

17. The filtration structure as claimed in claim 1, wherein the minimum thickness e.sub.ext of the peripheral wall of the filter is from 1 to 2 mm.

Description

(1) The attached figures illustrate certain aspects of the present invention in greater detail. The information given hereinafter must not, however, be considered as restricting the scope of the invention, in any of the aspects of the invention described in the figures.

(2) FIG. 1 illustrates an overall view of a common filter structure.

(3) FIG. 2 is a view of the upstream face of a filter structure explaining how the parameters measured according to the invention are measured.

(4) FIG. 3 is a frontal view of the upstream face of a filtration filter, the inlet and outlet passages of which are of round cross section.

(5) FIGS. 4 and 5 depict two complete configurations of the front face of a filter according to the present invention. In these figures, the dimensions are quoted in millimeters.

(6) FIG. 1 illustrates a tangential filtration structure comprising a support 1 of cylindrical shape having a main axis (X), an upstream face 2 and a downstream face 3, according to the direction in which the liquid to be filtered circulates. A plurality of passages 4, parallel to the main axis (X) are formed in the internal part 8 of the support and separated from one another by porous internal walls 5. Peripheral walls 7 separate the passages situated in the internal part 8 of the support from the outside. The passages 4 are open on the upstream face and on the downstream face, in the direction in which the liquid to be filtered circulates. The passages 4 are covered on their internal surface by a membrane 6 (depicted in FIG. 1 on just one passage for greater clarity). In operation, the liquid is brought onto the downstream face and passes through the structure via the passages 4. According to the principles of tangential filtration, some of the liquid passes through the porous walls 5 of the structure 1 and is filtered via the membrane 6 applied to the walls. The filtered liquid (filtrate) is recovered at the periphery of the filter after having passed through the peripheral wall 7, via recuperating means known per se (and not depicted in FIG. 1), for example of the type of those described in publication WO2017/085551, and the liquid (retentate) remaining in the passages is removed on the upstream face of the structure, for example toward other filtering units of the same type.

(7) As a general rule, as depicted in FIG. 2, the filtering structure described hereinabove is inserted in a filtration device 10 comprising an enclosure 11 sealing around said filtering structure. The enclosure in particular comprises means 12 for introducing the liquid 15 that is to be filtered, in fluidic communication with the passages on the upstream face of said filter structure, means 13 for removing the permeate 9 at the periphery of the filter structure and in fluidic communication with the peripheral wall of said filter structure and means 16 for removing the retentate or concentrate 14 on the downstream face of said filter structure and seals 17 as illustrated in FIG. 2.

(8) FIG. 3 is a frontal view of the upstream face of a filtration filter the inlet and outlet passages of which are of round cross section. FIG. 3 depicts the parameters e.sub.int (the minimum thickness of the internal walls between the passages), e.sub.ext (the minimum thickness of the peripheral wall of the filter), t.sub.m and D.sub.h described hereinabove.

(9) The present invention is illustrated using the following nonlimiting examples, in conjunction with the appended FIGS. 1 to 5.

EXAMPLES

A—First Series of Examples

(10) A first series of examples of structures according to the invention (example 1) and comparative structures (examples C11 to C16) were prepared according to the methods described hereinbelow.

Example 1 (Invention)

(11) A support was produced according to techniques well known to those skilled in the art by forming a honeycomb of silicon carbide. In order to do that the following were blended in a mixer: 3000 g of a mixture of the two powders of particles of silicon carbide with a purity higher than 98% containing 75 wt % of a first powder of grains having a median diameter of around 60 μm and 25 wt % of a second powder of grains having a median diameter of around 2.0 μm; and 300 g of an organic binder of the cellulose derivative type.

(12) Approximately 25 wt % of water with respect to the mass of SiC and of organic binder was added and the mixture mixed to form a homogeneous paste the plasticity of which allows extrusion in order to obtain a support exhibiting a porosity of 40%.

(13) The support was extruded from this paste using a die in order to obtain a cylindrical raw monolithic block with a diameter of 51 mm and a length of 300 mm, the internal part of which had a plurality of passages of circular cross section. The shape of the die was suited to obtaining passages of circular cross section having a hydraulic diameter of 3 mm and internal walls with a minimum thickness of 1200 micrometers, according to FIG. 4.

(14) The raw support obtained was then dried in order to bring the content of non-chemically bound water down to less than 1 wt %, and then fired under argon to a temperature of 2100° C. which was maintained for 5 hours. The support obtained had an open porosity of 40% and a median pore diameter of around 25 micrometers, as measured by mercury porosimetry.

(15) A filtration membrane was then applied to the internal surface of the passages. The membrane was applied by slurry coating. To do that, a membrane-keying primer was formed first of all, from a slurry the mineral formulation of which contained 50 wt % of a powder of grains of black SiC with a median diameter D50 of around 20 microns and 50% deionized water.

(16) A separating layer was then applied to the primer layer from a slurry containing 50 wt % of grains of SiC having a median diameter of around 1 μm and 50% deionized water. The viscosity of the slurries, measured at 22° C. under a shear gradient of 1 s.sup.−1 in accordance with standard DINC33-53019-1:2008 was adjusted to 0.1 Pa.Math.s using additives well known to those skilled in the art.

(17) The primer and the membrane were applied using the same method. The slurry was introduced into a tank with stirring at 20 revolutions/min. After a phase of air removal under a slight vacuum, typically 25 mbar, with the stirring maintained, the reservoir was placed under a slight overpressure of around +1 bar so as to be able to coat the inside of the support from the bottom up to the top. This operation takes just a few seconds for a support of 300 mm in length. The slurry coats the internal wall of the passages of the filter element and the excess is then removed under gravity immediately after deposition.

(18) The coated support was then dried at ambient temperature for 30 minutes and then at 60° C. for 30 h. The coated support thus dried was then sintered at at temperature of 1400° C. under an atmosphere of argon for 4 hours in order to obtain a membrane porosity of 40% with a median pore diameter of 250 nm.

(19) The filter structure thus obtained had the characteristics reported in the attached Table 1. Its diameter Ø.sub.f satisfies relationship (1) according to the present invention.

Comparative Examples C11 and C12

(20) Unlike the example according to the invention, the die was modified to obtain supports of smaller diameter to make up a multi-element filter with the same diameter Ø.sub.f as the filter according to example 1, all the other structural parameters remaining otherwise unchanged. Three units were assembled in accordance with the principles described in application WO2017/085551 to constitute the filter according to example C11, and four units were assembled in accordance with the principles described in application WO2017/085551 to constitute the filter according to example C12. The spacing between each unit in the assembled filter was equal to 3 mm.

Comparative Examples C13 and C14

(21) The supports and membranes were produced in the way as for the example according to the invention. Unlike in example 1 according to the invention, the die was modified to obtain monolithic supports for examples C13 and C14 with diameters Ø.sub.f of 40 mm and 62 mm respectively, all the other structural parameters remaining unchanged. The diameters of these two structures are therefore outside, respectively below and above, the interval defined by application of formula (1) according to the invention.

Comparative Examples C15 and C16

(22) The filters according to examples C15 and C16 had the same diameter as comparative example C14, but were made up by assembling, in accordance with the principles of application WO2017/085551, units in order to make up a multi-element filter with the same diameter Ø.sub.f as the monolithic filter structure according to C14, all the other structural parameters remaining otherwise unchanged with respect to that example. The spacing between each unit in the assembled filter was equal to 3 mm.

(23) All of the data and results obtained for examples 1 and C11 to C16 are reported in table 1 below.

B—Second Series of Examples

(24) According to this second series of examples, structures according to the invention (examples 2 and comparative structures (C21 to C26) were prepared using the same methods and the same principles as those described hereinbelow. In this second series of examples, the permeability of the support Ks of the support was varied.

Example 2 According to the Invention

(25) In order to obtain another value for the permeability Ks, unlike in example 1, for this second series of examples the support was produced from a blend of two powders of particles of silicon carbide of a purity higher than 98% containing 70 wt % of a first powder of grains having a median diameter of around 11 μm and 30 wt % of a second powder of grains having a median diameter of around 0.5 μm.

(26) The raw support obtained was then dried to bring the content of non-chemically bound water down to under 1 wt %, then fired under argon to a temperature of 2150° C., which was maintained for 5 hours. The support obtained had an open porosity of 40% and a median pore diameter of around 15 microns, as measured by mercury porosimetry.

(27) Supports were thus obtained the permeability of which was lower than that measured in the first series of examples as reported in table 2. The same procedure as used for the example was then used to apply the same membrane associated with the same primer as with the structure according to example 1.

Comparative Examples C21 to C26

(28) Examples C21 to C26 respectively correspond to examples C11 to C16, with the difference that supports of lower permeability, according to example 2, were used to form the filters in these examples.

(29) All of the data and results obtained are reported in table 2 below.

(30) Test and Results Table:

(31) For each of these monolithic or multi-element filter structures, the ratio Φ/Φ.sub.max was determined, where Φ is the characteristic flow of the filter structure in question and Φ.sub.max is the flow measured for the filter structure according to example 1 according to the invention for the first series of examples or according to example 2 according to the invention for the second series of examples, to which an effectiveness of 100% is assigned. The characteristic flow of a filter was evaluated using the following method: at a temperature of 25° C., a fluid made of demineralized water is fed to the filters that are to be evaluated at a trans-membrane pressure of 0.5 bar and a rate of flow along the passages 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 20 h of filtration. The results obtained, together with all the relevant dimensional characteristics of the filters thus obtained are summarized in table 1 below.

(32) Interpretation of the Results:

(33) Regarding the first series of examples (example 1 according to the invention and C11 to C16):

(34) The multielement filters according to examples C11 and C12 have a filtration capability inferior to that of the monolithic structure according to example 1 according to the invention, as indicated by the ratio of the flows mentioned in table 1.

(35) The monolithic filter structures according to example C13, the diameter of which is not in accordance with the invention, exhibit flows which are very much degraded by comparison with the reference structure.

(36) Comparing examples C14 to C16 makes it possible to show that the monolithic filter structure selected according to the invention exhibits a flow and filtration capabilities that are optimal, taking account of the combined structural characteristics of the support and of the filter membrane. In particular, the flows measured according to the filters obtained by assembling several filter units (examples C15 and C16) appear to perform better than those for the monolithic structure of the same diameter (example C14). In such a circumstance, the multi-element filter appears to perform better, even though it is more complex to implement.

(37) Example 2 shows that a lower support permeability Ks requires a lower diameter Ø.sub.f of the monolithic filter structure, by application of the present invention.

(38) In the same way as for the first series of examples, it is found that it is possible, by applying the same formula (1) according to the invention, to optimize the diameter of the filter according to structural parameters both of the support and of the membrane.

(39) The various filters described in applications WO2015/177476A1 (D1) and WO2016/097661 (D2), likewise filed by the present applicant company, were also studied. The results of the tests and calculations are reported in table 3 below. It may be seen that the value of the diameter of the structures described in these examples is not optimal and does not meet the criteria of the present invention as described in particular in the claims which follow.

(40) TABLE-US-00001 TABLE 1 Example Ex. 1 C11 C12 C13 C14 C15 C16 Membrane D.sub.50 nm 250 250 250 250 250 250 250 Membrane AP % 40 40 40 40 40 40 40 Km (×10.sup.17) m.sup.2 6.2 6.2 6.2 6.2 6.2 6.2 6.2 tm μm 50 50 50 50 50 50 50 Support D.sub.50 μm 25 25 25 25 25 25 25 Support AP % 40 40 40 40 40 40 40 Ks (×10.sup.13) m.sup.2 6.2 6.2 6.2 6.2 6.2 6.2 6.2 (Ks × tm)/Km m 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Dh mm 3 2 2 3 3 2 2 e.sub.int mm 1.2 0.7 0.7 1.2 1.2 0.7 0.7 e.sub.ext mm 1.5 1 1 1.5 1.5 1 1 Filter type — monolithic multi-element multi-element monolith monolith multi-element multi-element Number of filters — 1 3 4 1 1 3 4 Ø individual mm 51 22 19 40 62 27 24 structure Ø filter mm 51 51 51 40 62 62 62 Øf calculated mm 43-59 — — 43-59 43-59 — — according to relationship (1) Φ/Φmax — 1.00 0.92 0.92 0.60 1.37 1.41 1.47

(41) TABLE-US-00002 TABLE 2 Example Ex. 2 C21 C22 C23 C24 C25 C26 Membrane D.sub.50 nm 250 250 250 250 250 250 250 Membrane AP % 40 40 40 40 40 40 40 Km (×10.sup.17) m.sup.2 6.2 6.2 6.2 6.2 6.2 6.2 6.2 tm μm 50 50 50 50 50 50 50 Support D.sub.50 μm 15 15 15 15 15 15 15 Suppor AP % 40 40 40 40 40 40 40 Ks (×10.sup.13) m.sup.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 (Ks × tm)/Km m 0.18 0.18 0.18 0.18 0.18 0.18 0.18 Dh mm 3 2 2 3 3 2 2 e.sub.int mm 1.2 0.7 0.7 1.2 1.2 0.7 0.7 e.sub.ext mm 1.5 1 1 1.5 1.5 1 1 Filter type — monolith multi-element multi-element monolith monolith multi-element multi-element Number of filters — 1 3 4 1 1 3 4 Ø individual mm 43 18 16 33 51 22 19 structure Ø filter mm 43 43 43 33 51 51 51 Øf calculated mm 36-48 — — 36-48 36-48 — — according to relationship (1) Φ/Φmax — 1.00 1.00 1.00 0.57 1.34 1.45 1.53

(42) TABLE-US-00003 TABLE 3 Example ex 1 (D1) ex 2 (D1) ex 3 (D1) ex 4 (D1) ex 1 (D2) ex 2 (D2) ex 3 (D2) ex 4 (D2) ex 5 (D2) ex 6 (D2) disclosed mm 25 25 25 25 25 25 25 25 25 25 monolith Ø optimal Ø mm 35 35 35 33 48 30 46 75 48 81 according to the invention Ø max according mm 40 40 40 38 56 35 53 86 56 94 to the invention Ø min according mm 30 29 30 28 41 26 39 64 41 69 to the invention