Patent Application
20180304201
The invention relates to the field of filtering structures made of an inorganic material which are intended for the filtration of liquids, in particular structures coated with a membrane in order to separate particles or molecules from a liquid, more particularly water, for example produced water derived from oil or shale gas extraction.
Filters which use ceramic or nonceramic membranes to carry out the filtration of various fluids, in particular polluted water, have been known for a long time. These filters can operate according to the principle of frontal filtration, this technique involving the passage of the fluid to be treated through a filtering media perpendicularly to its surface. This technique is limited by the accumulation of particles and the formation of a cake at the surface of the filtering media, and therefore gives rise to a rapid drop in performance and also a drop in the level of filtration.
According to another technique to which the present invention also relates, use is made of tangential filtration which, in contrast, makes it possible to limit the accumulation of particles by virtue of the longitudinal circulation of the fluid at the surface of the membrane. The particles remain in the circulating stream, while the liquid can pass through the membrane under the effect of the pressure. This technique provides stability of the performance and of the level of filtration. It is more particularly recommended for the filtration of fluids highly laden with particles and/or with molecules.
The strong points of tangential filtration are thus its ease of use, its reliability, by virtue of the use of organic and/or inorganic membranes having a porosity suitable for carrying out said filtration, and its continuous operation.
Tangential filtration requires little or no adjuvant and provides two separate fluids which may both be of economic value: the concentrate (also known as retentate) and the filtrate (also known as permeate); it is a clean process which is environmentally friendly.
According to one alternative in particular useful for the purification of liquids laden with solid particles, the pollutants may remain trapped in the structure. In such a case, no concentrate is recovered at the outlet of the structure, only a filtrate.
Tangential filtration techniques are in particular used for microfiltration, ultrafiltration and nanofiltration.
Many structures of filters that operate according to the principles of tangential filtration are thus known from the current art. They comprise or are constituted from tubular supports made of a porous inorganic material that are formed of walls that delimit longitudinal ducts or channels parallel to the axis of said supports. The liquid to be filtered passes through the walls then the filtrate is discharged, usually at the peripheral outer surface of the porous support.
According to one configuration that is also used, the filters comprise at least one filtering element formed by a plurality of ducts separated by porous walls. Such a structure is commonly referred to in the field as a honeycomb.
The surface of said ducts is also customarily covered with a membrane, usually made of a porous inorganic material, referred to as a membrane or membrane separation layer in the present description, the nature and the morphology of which are suitable for stopping the molecules or the particles having a size that is close to or greater than the median pore diameter of said membrane when the fluid filtered spills into the porosity of the porous support.
In particular, in such filters, the median pore diameter of the material constituting the filtering membrane is normally much smaller than that of the material constituting the walls of the ducts, the ratio generally ranging from 1/1000 to 1/10. In addition, the thickness of the membranes is much thinner than that of the walls, the ratio ranging from 1/200 to ⅕.
The membrane is conventionally deposited on the inner surface of the channels by a process for coating a slurry of the porous inorganic material followed by a consolidating heat treatment, especially a drying and optionally a sintering of the ceramic membranes.
Patent application US 2013/0153485 describes a membrane filter comprising an assembly of filtration elements, the ends of which are connected by a material forming a mounting ring. However, no indication is provided in this application as regards the use of such an embodiment, in particular regarding the means to be used and the conditions that make it possible to obtain a filter that is reliable over time, i.e. having an integrity and leaktightness that are guaranteed and preserved over a prolonged service life of the filter.
In particular, no indication is provided in this publication as regards the mechanical strength conferred on the assembly by the mounting ring, in particular regarding its compression strength and resistance to deformation under the pressure exerted by the housing into which the filtering structure will be inserted during the use thereof, or regarding the filtration performance of a complex filter thus assembled.
It has been proven by the applicant company that the essential problem in such a structure lies in particular in the quality of the filtration of the incoming liquid. Specifically, the applicant company has been able to demonstrate that this quality is strongly dependent on the absence of so-called by-pass zones of the membrane, through which the liquid to be filtered may travel into the structure without having to pass through the active part where the filtration is carried out. In the presence of such zones, the liquid may pass through the filter, without coming into contact with the membrane separation layer of smaller porosity and present at the surface of the porous walls of larger porosity.
In particular, the tests carried out and reported hereinbelow by the applicant company have demonstrated that a large portion of the liquid in such a structure was able to pass directly into the channels of the structure via the porosity of the support, starting from the end of the filter open on the face for introduction of the liquid to be filtered, without therefore having to pass through the membrane covering the inner walls of the honeycomb ceramic elements. Such a by-pass zone of the membrane is for example illustrated by the arrows 100 in the appended
As indicated above, without this however being a certainty, such a phenomena could be explained on account of the large differences in porosity, in pore size and in thickness between the materials respectively constituting the membrane and the walls.
The object of the present invention is to resolve the problems disclosed above, and it proposes in particular to provide a mechanically strong filter, assembled from a set of honeycomb ceramic filtering elements, each comprising a plurality of ducts, and the filtration efficiency of which is optimal, in particular by preventing the presence of by-pass zones in the assembled filter, through which a portion of the liquid is not filtered, while preserving as far as possible the filtration surface area available to the liquid within said filter.
In its most general form, the present invention thus relates to a membrane filter for liquid filtration comprising:
According to certain preferred embodiments of a filter according to the invention which may of course be combined together where appropriate:
In particular, in operation, the filter according to the invention is normally positioned in a compartment (also referred to as housing in the present description). According to this conventional configuration, said compartment therefore surrounds the filtering elements and the sleeve(s). Such a compartment enables in particular the containment of the liquids (filtrate and/or retentate) within a filtration unit.
According to one preferred embodiment according to the invention, in particular according to which the filtrate is recovered at the periphery or at the outlet of the filter, said filtrate recovery means may therefore include the compartment (housing) into which said filter is inserted. According to such an embodiment, said means comprise in particular an opening in said compartment, as is described for example in publication US 2013/0153485) or in the appended figures of
Thus, according to one preferred embodiment of the invention, said recovery means may comprise or else consist of an opening made in the housing surrounding the filter.
The invention also relates to a filtration unit comprising a filter as described above inserted into its compartment, including the filtrate recovery means.
The invention also relates to a process for manufacturing a membrane filter as claimed in one of the preceding claims, comprising the following successive steps:
According to one particular embodiment of the manufacturing process described above, before step c, the ends of each of the filtering elements are preimpregnated with a resin, for example a curable resin that plugs the porosity of the porous ceramic material on the face for introduction of the liquid to be filtered.
Within the meaning of the present description, the following definitions are given:
A channel or duct is understood to mean the space delimited by the porous walls of the structure in which the liquid to be filtered is introduced and flows. Inner channels or ducts are understood for the purposes of the invention to mean the ducts that do not share a common wall with the outer or peripheral surface of the filtering element. In a complementary manner, a duct that has at least one common wall with the outer surface of the filtering element is referred to as peripheral. This wall is referred to as an outer wall. The other walls are referred to as inner walls.
The open porosity and the median diameter of the pores of the porous walls described in the present description are determined in a known manner by mercury porosimetry.
The pore volume is measured by mercury intrusion at 2000 bar using a Micromeritics AutoPore IV 9500 Series mercury porosimeter, on a 1 cm3 sample taken from a block of the product, the sampling region excluding the skin typically extending up to 500 microns from the surface of the block. The applicable standard is ISO 15901-1.2005 part 1. The increase in pressure up to high pressure results in “pushing” the mercury into pores of increasingly small size. The intrusion of mercury is conventionally carried out in two steps. In a first step, mercury intrusion is carried out at low pressure up to 44 psia (around 3 bar), using air pressure to introduce the mercury into the largest pores (>4 μm). In a second step, high-pressure intrusion is carried out with oil up to the maximum pressure of 30 000 psia (around 2000 bar).
In accordance with Washburn's law mentioned in the ISO 15901-1.2005 part 1 standard, a mercury porosimeter thus makes it possible to establish a size distribution of the pores by volume. The median pore diameter of the porous walls corresponds to a threshold of 50% of the population by volume.
The porosity of the membrane, corresponding to the total volume of the pores in the membrane, and the median pore diameter of the membrane are advantageously determined according to the invention with the aid of a scanning electron microscope. Typically, sections of a wall of the support in transverse cross section are produced, so as to visualize the entire thickness of the coating over a cumulative length of at least 1.5 cm. The images are acquired from a sample of at least 50 grains, preferably of at least 100 grains. The area and the equivalent diameter of each of the pores are obtained from photographs by conventional image analysis techniques, optionally after a binarization of the image that aims to increase the contrast thereof. A distribution of equivalent diameters is thus deduced, from which the median pore diameter is extracted.
The porosity of the membrane is obtained by integration of the curve of distribution of equivalent pore diameters.
Likewise, a median size of the particles constituting the membrane layer can be determined by this method.
The median size of the particles constituting the membrane layer is in general between 20 nanometers and 10 micrometers, preferably between 100 nanometers and 2 micrometers.
An example of determination of the median pore diameter or of the median size of the particles constituting the membrane layer, by way of illustration, comprises the following sequence of steps, which is conventional in the field:
A series of SEM photographs is taken of the support with its membrane layer observed along a transverse cross section (that is to say, over the whole thickness of a wall). For greater clarity, the photographs are taken on a polished section of the material. The image is acquired over a cumulative length of the membrane layer at least equal to 1.5 cm, in order to obtain values representative of the whole of the sample.
The photographs are preferably subjected to binarization techniques, well known in image processing techniques, in order to increase the contrast of the outline of the particles or pores.
A measurement of the area is carried out for each particle or each pore constituting the membrane layer. An equivalent pore or grain diameter is determined, corresponding to the diameter of a perfect disk of the same area as that measured for said particle or for said pore (it being possible for this operation to be optionally carried out using dedicated software, in particular Visilog® software sold by Noesis). A distribution of particle or grain size or of pore diameter is thus obtained according to a conventional distribution curve and a median size of the particles and/or a median diameter of pores constituting the membrane layer are thus determined, this median size or this median diameter respectively corresponding to the equivalent diameter dividing said distribution into a first population comprising only particles or pores with an equivalent diameter greater than or equal to this median size and a second population comprising only particles with an equivalent diameter less than this median size or this median diameter.
The median diameter D50 of the powders of the particles, in particular of the silicon carbide SiC powders, used to produce the support or the membrane is evaluated conventionally by a particle size distribution characterization carried out with a laser particle size analyzer in accordance with the ISO 13320-1 standard. The laser particle size analyzer may be, for example, a Partica LA-950 from the company HORIBA. Within the meaning of the present description and unless otherwise mentioned, the median diameter of the particles respectively denotes the diameter of the particles below which 50% by weight of the population is found.
Within the meaning of the present invention, within a filtering element, all the ducts have a cross section and a distribution that are substantially constant and identical over the entire length of the filter, irrespective of the transverse sectional plane considered.
The thickness e of the sleeve is understood to mean the average thickness of said sleeve measured parallel to the longitudinal central axis of the filter. Without departing from the scope of the invention, the thickness of the sleeve may however vary locally substantially in the longitudinal direction of the filter, in particular as a function of the technique for producing the sleeve, for example as a function of the profile of the mold used for casting the curable material binding the filtering elements together.
An example is given below for producing a filtering element that is incorporated in the structure of a filter according to the invention, which of course does not limit the processes for obtaining such an element:
According to a first step, the filtering element is obtained by extruding a paste through a die configured according to the geometry of the structure to be produced according to the invention. The extrusion is followed by a drying and a firing in order to sinter the inorganic material constituting the support and to obtain the porosity and mechanical strength characteristics necessary for the application.
For example, when it is a question of an SiC support, it may in particular be obtained according to the following manufacturing steps:
For example, the average thickness of the outer walls of an element according to the invention is between 0.5 and 2.0 mm. Such a thickness makes it possible in particular to ensure a cohesion and a suitable adhesion between the porous ceramic material constituting the outer wall and the curable resin that is incorporated into its porosity. If necessary, the outer surface of the porous walls may also be made rough to further facilitate the adhesion and the penetration of the resin into the porosity of the walls. The average thickness of the inner walls of the elements is generally thinner than that of the outer walls and is preferably between 0.3 and 1.5 mm.
The length of the filtering elements is in principle between 200 and 1500 mm.
The hydraulic diameter of the ducts (also sometimes referred to as channels in the present description) is preferably between 1 and 5 mm, preferably between 1.5 and 4 mm.
Depending on the type of filtration envisaged (tangential or frontal) and/or the configuration envisaged for the recovery of the filtrate (at the outlet of the channels and/or at the periphery of the filter), certain ducts may or may not be blocked at the ends, in particular at the opposite end of the ducts with reference to the introduction of the liquid into the filter. Preferably, in the case of tangential filtration, no duct is blocked.
The filtering element is then coated according to the invention with a membrane (or membrane separation layer). One or more layers, referred to as primer layers, may be deposited before forming the filtering membrane according to various techniques known to a person skilled in the art: techniques for deposition using suspensions or slips, chemical vapor deposition (CVD) techniques or thermal spraying, for example plasma spraying, techniques.
Preferably, the primer layers and the membrane are deposited by coating using slips or suspensions comprising ceramic particles. A first layer is preferably deposited in contact with the substrate (primer layer), acting as a tie layer. The formulation of the primer comprises 50% by weight of SiC grains (median diameter between 2 and 20 micrometers) and 50% deionized water. A second layer of finer porosity is deposited on the primer layer, and constitutes the actual membrane. The porosity of this latter layer is suitable for giving the filtering element its final properties. The formulation of the membrane preferably comprises 50% by weight of SiC grains (in particular having a median diameter of between 0.1 and 2 micrometers) and 50% deionized water.
In order to control the rheology of these slips and to comply with a suitable viscosity (typically from 0.01 to 1.5 Pa·s, preferably 0.1 to 0.8 Pa·s under a shear gradient of 1 s−1 measured at 22° C. according to the DINC33-53019 standard), thickeners (in proportions typically of between 0.02% and 2% of the weight of water), binders (typically between 0.5% and 20% of the weight of SiC powder), and dispersants (between 0.01% and 1% of the weight of SiC powder) may be added. The thickeners are preferably cellulose derivatives, the binders are preferably PVAs or acrylic derivatives and the dispersants are preferably of ammonium polymethacrylate type.
These coating operations typically make it possible to obtain a primer layer having a thickness of around 30 to 50 micrometers after drying and sintering. During the second coating step, a membrane layer having a thickness of around to 50 micrometers is obtained after drying and sintering.
The element thus coated is then dried at ambient temperature typically for at least 30 minutes then at 60° C. for at least 24 hours. The supports thus dried are sintered at a firing temperature typically between 1700° C. and 2200° C. under a non-oxidizing atmosphere, preferably under argon, so as to obtain a membrane porosity (measured by image analysis as described above) preferably of between 10% and 40% by volume and an equivalent median pore diameter (measured by image analysis) preferably of between 50 nm and 10 micrometers, or even between 100 nm and 5 micrometers.
The lower end of the elements is then leveled off in order to eliminate the excess of the coating materials, which are much more concentrated in this portion of the ceramic part, over a length of around 5 to 20 mm.
The filtration membranes according to the invention preferably have the following features:
A plurality of the filtering elements thus obtained are then assembled in order to form the filter according to the invention, so as to leave an interstitial void between them through which the filtrate introduced on an introduction face of the filter thus obtained will be able to flow.
In a more detailed manner, the set of the elements obtained previously are deposited in a container so as to rest on one of their ends. They are also held spaced apart from one another by calibrated spacers. A curable resin, the viscosity of which is suitable according to the invention, is introduced into the container so as to fill the interstices between the elements then the resin is cured, at ambient temperature or under the effect of heating for the case of a thermosetting resin, until a rigid sleeve is obtained in the form of a single part surrounding and joining all of the filtering elements, as illustrated in
Preferably, the same operation is carried out according to a second step at the other end of the filtering elements, in order to obtain the final filter thus comprising a plurality of honeycomb ceramic filtering elements positioned substantially parallel, with an interstitial volume present between said filtering elements.
According to the invention, the adjustment of the resin and in particular of its viscosity during this step of forming the sleeve has turned out to be critical for enabling the correct operation of the assembled filter thus obtained, in particular for ensuring the filtration quality of the device. Very particularly, it has been observed, according to the present invention, that the viscosity of the curable resin injected into the container should be low enough for the curable material to penetrate to the core of the open porosity of the walls of the filtering elements, that is to say through the entire thickness of all the porous walls constituting the plurality of elements, in particular through the entire thickness of the inner walls of all the filtering elements used to form the assembled filter. As will be demonstrated by the examples provided in the present description, the presence of the resin throughout the porosity of the walls, over a non-zero height h (cf.
In the end, the most suitable viscosity was determined to be between 1000 and 3000 mPa·s at 25° C. (or more generally at the temperature at which the curing thereof is carried out), even though its optimal value is capable of varying substantially, in particular as a function of the porosity of the porous walls and/or of the geometry of the ducts. In principle, the assembled filter thus obtained is then inserted into a housing (also referred to as a compartment or casing), comprising inlet openings for the liquid to be filtered and outlet openings for the filtrate and optionally the retentate, according to conventional configurations as for example described in application US 2013/0153485.
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:
The figures of
The figures of
The figures of
The figures of
In
In the configuration according to
According to one possible embodiment illustrated by
According to the invention, the filtering elements 2 according to
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).
By way of example, the embodiment illustrated by
According to other embodiments of the invention illustrated by the figures of
According to one configuration of a filtration unit incorporating a filter according to the invention, for example illustrated by
According to another configuration of a filtration unit incorporating a filter according to the invention, illustrated by
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.
As illustrated by
According to an alternative configuration of a filtration unit 20 incorporating a filter according to the invention, illustrated by
Illustrated in
The following examples make it possible to illustrate the invention and its advantages but in no way limit the scope thereof.
Filtering elements, the transverse cross section of which is depicted in
The structural features of the filtering element are listed in table 1 below:
The hydraulic diameter Dh 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
h=4×S/P
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.
The elements according to examples 1 to 6 are obtained according to the same experimental protocol below:
Mixed in a mixer are:
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
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.
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.
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 d50 of which is approximately 11 micrometers, 20% by weight of a powder of grains of black SiC (Sika FCP-07), the median diameter d50 of which is approximately 2.5 micrometers, and 50% of deionized water.
A slip of the material constituting the filtration membrane layer is also prepared, the formulation of which comprises 40% by weight of SiC grains (d50 of approximately 0.6 micrometer) and 60% of demineralized water.
The rheology of the slips was adjusted, by addition of the organic additives, to 0.5-0.7 Pa·s under a shear gradient of 1 s−1, measured at 22° C. according to the standard DIN C 33-53019.
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.
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.
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.
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 d50 of around 1 micrometer and a total porosity of 40%, by volume.
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 d50 of the membrane is measured as being equal to around 250 nanometers.
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.
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.
The filtering elements thus synthesized are then dropped into a silicone container so as to rest on one of their ends.
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.
More specifically, two types of resins are used:
The smaller the mean diameter of the SiC powder added, the greater the viscosity of the mixture with the resin.
The details of the conditions for preparing the resins for each example are given in table 2 below.
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
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
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
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.
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.
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.
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 hmin/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.
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.
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.
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.
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.
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.
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
More specifically, a turbidity measurement is carried out on the filters corresponding to assemblies of 7 filtering elements in accordance with the appended
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
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.
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.
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.
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
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1553899 | Apr 2015 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2016/051021 | 4/29/2016 | WO | 00 |
