FR2736843A1 - Porous tube for tangential filtering, e.g. milk, (edible) oils etc. - comprises one surface of tube being irregular and the other smooth - Google Patents

Abstract

A porous tube for tangential filtering has one smooth surface, pref. the outside, and one surface, pref. the inside, which is irregular and designed to support a filtering membrane. Also claimed are: (i) forming the tube with a thickness which varies to create the irregularities; (ii) sintering the tube and forming a membrane on the irregular surface; (iii) and appts. for forming the tube by extrusion through an annular gap between a cylindrical die and a core, one of which is fixed while the other rotates and is non-circular in cross-section.

Description

 The present invention relates to new filter tubes, and more particularly new porous tubes with non-clogging wall intended for use in the tangential fluid filtration technique, as well as a method of manufacturing such tubes and an apparatus specially adapted to this manufacture.

 Various techniques for tangential filtration of liquids and suspensions are commonly used, for example, to separate one of the phases, miscible or not, in the fields of microfiltration, ultrafiltration and nanofiltration. In these techniques, tubular supports bearing a filtration membrane are generally used, and the mixture to be filtered is introduced under pressure into the tube while one of the phases, enriched in one of the components to be separated, is collected at the end of the reaction. outside the tube. The pore diameter of the membrane is chosen according to the characteristics of the products to be filtered. Alternatively, the pressure mixture is introduced into a container containing filtration tubes and the mixture enriched in one of the components is collected in the tubes.

 One of the difficulties limiting the use of these techniques results from the formation of a polarization layer on the surface of the filtration membrane. This layer is due to an accumulation of particles retained by the filtration membrane, and in some cases even the formation of a gel is observed. This polarization layer is unstable, and it causes a significant decrease in permeability and a variation in selectivity, the polarization layer superimposed on the filter membrane and modifying the sieve effect of the latter.

 Similar phenomena occur in the case of the filtration of gaseous mixtures, where a boundary layer is formed on the filtration membrane.

 To limit these adverse effects, it has been proposed to introduce into the tubes devices intended to create turbulence. These devices make it possible to improve the flow of permeate, and consequently the efficiency of the filtration, by limiting the clogging of the filter. However, the introduction and fixation of these devices in the filter tubes are difficult and complex. In addition, they cause annoying vibrations that affect the reliability of the equipment.

 The filtration tubes are conventionally constituted by a porous support with coarse texture, ensuring the mechanical strength of the assembly, coated by the filtration membrane.

 These tubes may be manufactured by coextruding separate powders for forming the support and the filter layer, or by applying the membrane on the previously formed porous support, and then performing a heat treatment by sintering, according to the technique described by example in FR-A-2,327,090.

 Patent FR-A-2 503 615 describes cylindrical tubes for filtering gaseous mixtures introduced under pressure inside the tubes, on the wall of which are formed imprints intended to create turbulences which avoid the accumulation of one of the gaseous phases on the tube wall and improve gas diffusion separation. These impressions are formed by passing the tubes out of the extrusion die, between rollers and tools that locally deform the tubes through their entire wall.

 Patent FR-A-2 503 616 describes a method of the same principle, consisting in deforming the wall of the tube at the outlet of the extrusion die by application of knobs disposed face to face on either side of the tube , or alternating positions.

 However, only small impressions can be obtained, and these deformations made in the wall of the tubes cause significant stresses in the material and the risk of cracking. As a result, the tubes produced by this process can only be used for gas filtration. In addition, these devices improve the gas separation coefficient, but do not modify the permeability of the filtration tube.

 The subject of the present invention is porous filtration tubes that can be used in the tangential filtration technique, which can be used as supports for the filtration of gases or liquids, or suspensions, avoiding any clogging of the filter.

 The invention also relates to porous cylindrical tubes with cavities avoiding or limiting the formation of a polarization layer or a boundary layer, increasing the coefficient of separation, and providing a higher permeability than the filtration tubes of the type known.

 The present invention also relates to a method of manufacturing such porous impression tubes, and a device specially adapted for their manufacture.

 The porous impression tubes according to the present invention are intended for tangential filtration and are distinguished in that they comprise imprints on one of their walls, and preferably on the inner wall, while the other wall is smooth.

 The imprints formed on one of the walls of the tube, and preferably on the inner wall, may have various forms, continuous or discontinuous. For example, the impression may have the form of a single or multiple helical rib, for example double or triple, variable pitch.

 The height of the impressions relative to the base surface of the tube may be between 0.1 mm and 2 mm, and preferably between 0.2 mm and 1.5 mm, for a wall thickness generally between 1, 5 mm and 5 mm, and an inner diameter of the tubes between 5 and 25 mm.

 The porous tube of the invention may be made of any material customary for the filtration tubes, for example alumina, alumino-silicate, titanium oxide, silicon carbide, zirconia or any other pure ceramic material, these materials being used singly or in combination.

It may comprise at least one filtration membrane of conventional type, for example of the zirconia type on an alumina support.

 The process according to the present invention, for the production of porous single-walled impression tubes of the type described above, consists in shaping the tube, for example by extrusion, from a composition adapted according to the invention. the desired porosity, by varying its thickness longitudinally, then to sinter and then apply, if necessary the filter membrane on the wall bearing the imprint.

 The composition of the dough is adapted according to the desired properties. It comprises a liquid part, generally consisting of water, and a solid part comprising a powder of suitable particle size and usual additives such as binders, plasticizers and water retenters, such as cellulose derivatives (for example methylcellulose or methylcellulose). ethylcellulose), polysaccharides, polyvinyl alcohols, polyethylene glycol. The weight ratio of the liquid part to the solid part may be preferably between 1/20 and 2/3, but values outside this range may be considered in some cases.

 The powder used in the composition may be chosen from alumina, an alumino-silicate, especially cordierite, a zirconia, titanium oxide, or a natural clay, whose grain size may be between 5 and 200 μm. about.

 Sintering is carried out according to conventional techniques, by heat treatment at a temperature of between about 900 ° C. and about 1800 ° C., depending on the nature of the material used.

 The filtration membrane, consisting for example of zirconia (zirconium oxide), alumina, hafnium, spinel, silico-aluminate, or titanium dioxide, may be applied to the wall by impregnation, coating, casting or soaking. Ceramic membranes are also suitable for high temperature filtration (above 100c).

 The device of the present invention for the implementation of the above method is of the type comprising an extrusion die, and is distinguished in that the die has a cylindrical pin disposed along its axis, the pin or the output die the die being rotatably mounted and non-circular section, to form at its output a wall of irregular thickness.

 In a first embodiment, the spindle is rotatably mounted in the fixed die, and has a non-circular section. This produces tubes with indentations on the inner wall.

 The spindle preferably has a frusto-conical shape, the large base of which is disposed on the exit side of the die, and has one or more recesses for the formation of the cavities.

 According to a variant according to the invention, the spindle has a circular cylindrical section and is fixedly mounted in the axis of the die, while the die is non-circular section and is rotatably mounted on the die head, to thereby form imprints on the outer wall of the tube.

 The spinning technique used is perfectly suited to any ceramic powder that can be processed by extrusion (piston extruder or stretch extruder). The tubes can be extruded at different diameters and speeds.

 The inner diameter of the tubes obtained by this spinning technique can range from 5 to 25 mm and the extrusion rate can vary from 2 to 10 cm / s. The length of footprints can vary from 10 to 100 mm and more. The pitch can be reversed during the extrusion of the tubes. The rotational speed of the spindle (or rotating die) is generally between 4 and 100 rpm, and preferably between 20 and 60 rpm.

 The tangential filtration tubes of the invention can be used in microfiltration, ultrafiltration and nanofiltration processes, depending on the nature of the filtering membrane used if appropriate and the media to be filtered or enriched in certain components. such as liquid or colloidal media. They can be used directly without a filtration membrane in filtration processes for purification or depollution.

They can also be used in liquid pervaporation processes, for example for the filtration of a water / alcohol azeotropic mixture, or else in gas permeation processes, for example for the separation of a mixture of carbon dioxide and carbon dioxide. methane.

 The tubes of the invention provide many advantages over the tubes known in the art and, by substantially eliminating or attenuating the boundary layer or the polarization layer, they significantly improve the separation coefficient and permeability. It is thus possible to considerably reduce the membrane area necessary for filtration, of the order of 3 to 5 times compared to the usual surfaces of the conventional tubes.

 These advantages of the filtration tubes of the invention can be exploited in various applications, for example for the filtration of liquids in the food industry such as milk or oils, for the regeneration of used motor oils. or waste oils, or for the purification of highly polluted effluents in the chemical industry, as well as for the treatment of water.

The features and advantages of the invention will appear in more detail in the following description with reference to the accompanying drawings, which represent
Figure 1 is a schematic view of an extrusion die according to the invention, comprising a rotating pin.

 Figure 2 is an enlarged view of a detail of Figure 1 showing the position of the spindle in the die.

 Figure 3a: a bottom view of a rotary spindle according to the invention.

 Figure 3b: a bottom view of an alternative embodiment of the spindle of the invention.

 As shown in Figure 1, the die according to the invention is in the form of a conventional die, modified by the incorporation of a rotary spindle mounted along its axis.

 The paste, after mixing, is placed in the tank (1) and is pushed by the piston (2) out of the die by the gap separating the pin (3) and the inner wall of the die head forming the die (5). The spindle (3), of substantially frustoconical shape, is rotated on its axis by the motor (4) which can be provided with an inverter to ensure rotation in one or the other direction.

 As shown in Figure 3a which is a bottom view of the spindle (3), it has a non-circular section at its large base. This irregularity is obtained by providing withdrawals (6) on the periphery. In the case of the example shown in Figure 3a, the pin has three withdrawals and then allows to form a tube bearing a triple helical shaped footprint.

 The pin (7) shown in Figure 3b has four recesses (8) arranged symmetrically with respect to the axis on its periphery. This pin allows to form on the inner wall of the tube a quadruple helix which can, if desired, have a discontinuity by reversing the rotation of the pin during extrusion.

 As an example of applications of the filtration tubes of the present invention, the results obtained in the filtration of several liquid media are given below.

EXAMPLE 1
Manufacture of a porous alumina-based tube
A paste having the following composition (composition given for 3 kg of dough) is prepared.

solid part
powder A1203 (grain diameter 23 ssm): 82%
- organic products (binders, plasticizers and retainers
of water): 18%.

Liquid part
- H20: 18% relative to the solid mass.

 After dry mixing of the inorganic powder and organics for about 15 minutes, the H 2 O solvent is added in its entirety. Mixing after adding water lasts an average of 15 minutes.

 An extrusion is then carried out in a die equipped with a spindle as described above, having a section identical to that shown in FIG. 3a, and rotating at a speed of 40 rpm. for a step of 10 mm.

 Tubes 20 mm in outer diameter and 16 mm in internal diameter are extruded. The extrusion is done under vacuum to avoid the formation of defects on the walls. The spinning takes place at a speed of 2 cm / s.

 The extruded tubes thus obtained are dried and sintered according to a conventional technique. The drying is carried out on rolls at 25 ° C. Sintering of this material takes place from 1600 C.

 This produces tubes having an imprint in the form of surface roughness. The depth of the asperities in this case is 1 mm and the pitch of 10 mm. A second type of tube was spun with a speed of rotation different from the spindle (20 rpm) to obtain a step length of 20 mm.

EXAMPLE 2
Manufacture of a porous cordierite-based tube
A paste having the following solid part composition is prepared
- cordierite powder (grain diameter <50 m): 80%
- organic products (binders, plasticizers and reten
water): 20% liquid part
- H20: 30% relative to the solid mass
The mixing of the components, the kneading of the dough and the extrusion are carried out as indicated in Example 1.

 Drying is carried out on rolls at 25 ° C as in Example 1, while sintering of this material is carried out by applying a temperature gradient of 2 "/ min. up to the temperature of 300 C, hold at this temperature for one hour, temperature rise with a gradient of 4 "/ min.

up to 1250 ° C. then sintering for 10 h at this temperature.

 The values recorded for the extrusion, the asperities and the pitch are identical to those of the alumina support of Example 1.

EXAMPLE 3
Manufacture of a porous tube based on natural clay
A natural clay paste having the following composition is prepared
- sio2: 51.7%
- CaO: 15.5%
- A1203: 18.2%
- MgO: 4%
- Fe203: 7.2%
Na2O: 1%.

solid part
- clay powder: 82%
- organic products (binders, plasticizers and retainers
of water): 18%.

liquid part
- H20: 15% relative to the solid mass
The mixing of the components and the kneading of the dough are as described in Example 1.

 The extrusion is carried out using a die identical to that of Example 1 with the exception of the dimensions of the head.

 Tubes 10 mm in outer diameter and 6 mm in inner diameter are extruded. The extrusion is done under vacuum to avoid the formation of defects in the ceramic. The spinning takes place at a speed of 3 cm / s. The rotational speed of the spindle is 60 rpm.

 The drying is carried out on rolls at 25 ° C. The sintering of this material is between 900 and 1100 C.

 The depth of the asperities in this case is 0.5 mm. The length of the step is 10 mm.

EXAMPLE 4
Application to ultrafiltration of milk.

 It is reconstituted skimmed milk. The filtration is carried out on a membrane with a pore diameter of 50 nm. The speed of circulation is 4m / s. The inside diameter of the tube is 16 mm.

Aspérités de la pin: pitch of the helix = 12 mm, height = 1.5 mm.

<Tb>

<SEP> Pressure <SEP> (Pa) <SEP> 4.105 <SEP> 6.105
<tb> Flow <SEP> M. <SEP> T. <SEP> (l / m2.h) * <SEP> 95 <SEP> 95
<tb> Flow <SEP> M. <SEP> L. <SEP> (l / m2.h) ** <SEP> 40 <SEP> 45
<tb><SEP> Gain <SEP>&num;<SEP> 2 <SEP>&num;<SEP> 2
<Tb>
* MT = twisted membrane
** ML = smooth membrane
This example shows that the twisted membrane filter according to the invention makes it possible to double the permeate flow rate.

EXAMPLE 5
Application to ultrafiltration of olive oil
Pore diameter: 50 mm
Filtration speed: 5 m / s
Temperature, approx. 100 ° C Viscosity: 6.10 3 Pa.s
The imprints on the tube are identical to those of Example 4.

<Tb>

<SEP> Pressure <SEP> (Pa) <SEP> 5.105 <SEP> 6.105
<tb> Flow <SEP> M. <SEP> T. <SEP> (l / m2.h) * <SEP> 65 <SEP> 50
<tb> Flow <SEP> M. <SEP> L. <SEP> (l / m2.h) ** <SEP> 15 <SEP> 12
<tb><SEP> Gain <SEP>&num;<SEP> 4 <SEP>&num;<SEP> 4
<Tb>
* MT = twisted membrane
** ML = smooth membrane
Thus, in the case of olive oil, the permeate flow rate is multiplied by 4 with the twisted membrane filter of the invention.

EXAMPLE 6
Application to the regeneration of used oils
The used motor oils are in the form of a fine, black colloidal suspension consisting of a base oil and degraded additives during operation.

The used oil used in this example contains about 12-15% of total impurities (carbon colloids, sludge, traces of water and gasoline, etc.).

 Ultrafiltration, which constitutes the first step of a regeneration process, is carried out by means of a ceramic membrane on a porous alumina tube according to the invention (pore diameter of the membrane = 25 nm, height of the imprints = 1 mm, pitch: 12 mm).

 The same experiment is made for comparison on a smooth filter according to the usual technique, comprising a membrane of the same structure.

 The used oil is pretreated at 190 ° C for 1 hour to remove water and volatile components, and is maintained at 150 ° C during testing; the viscosity is about 4.5.10 3 Pa.s.

The permeate flow measurement is made on the membrane of each of the two filters successively using the following operating conditions for 1 hour.
Pressure 2 bars; 4 bars; 6 bars.

Tangential velocity 2 m / s; 3.5 m / s; 5 m / s.

 In the case of the smooth filter according to the known technique, it is found that the permeate flow rate is substantially constant as soon as the pressure reaches 2 bars, irrespective of the tangential velocity. This is due to the formation of a polarization layer which causes clogging of the filter. The maximum flow rate at 6 bar and 5 m / s is 15.4 l / h.m2.

 In the case of the impression filter according to the invention, it is found that the permeate flow increases continuously with the pressure, and all the more clearly that the speed is high. Downstream of the cavities is formed a vortex which, by detaching itself from the wall, brews the clogging layer and drives it in the axis of the tube, thus causing a permanent declogging action.

 The flow rate then reaches 49 l / h.m2 for a speed of 5 m / s and a pressure of 6 bar.

 The quality of the ultrafiltered oil is equivalent to the output of the two filters.

Claims (13)

 1. Porous impression tube for use as a filter medium in the tangential filtration technique, characterized in that it comprises impressions on one of its walls, while the other wall is smooth.  2. Porous tube according to claim 1, characterized in that it has indentations on its inner wall, while its outer wall is smooth.  3. Porous tube according to any one of claims 1 and 2, characterized in that the imprint has a simple or multiple helical rib shape.  4. Porous tube according to any one of the preceding claims, characterized in that the height of the impression is between 0.1 and 2 mm, the thickness of the wall between 1.5 and 5 mm, and the inner diameter. of the tube between 5 and 25 mm.  5. A method of manufacturing a porous impression tube having impressions on one of its walls while the other wall is smooth, characterized in that a tube is shaped from a composition of matter adapted, by varying its thickness longitudinally, then sintering is performed and then applied a filter membrane on the wall bearing the imprint.  6 Process according to claim 5, characterized in that the tube is shaped by extrusion.  7. Method according to any one of claims 5 and 6, characterized in that the composition of material contains at least one powder and a liquid previously kneaded to form a paste.  8. Process according to claim 7, characterized in that the powder is chosen from alumina, a silicon aluminosilicate, titanium oxide, zirconia or silicon carbide.  9. The method of claim 5, characterized in that the membrane is applied by impregnation or coating on the wall bearing the imprint.  10. Device for the manufacture of a porous tube, comprising an extrusion die, characterized in that the die has a cylindrical pin disposed along its axis, the pin or the output matrix of the die being rotatably mounted and sectional not circular.  11. Device according to claim 10, characterized in that the spindle is rotatably mounted and the matrix is fixed.  12. Device according to claim 10, characterized in that the matrix is rotatably mounted and the pin is fixed.  13. Device according to any one of claims 10 to 12, characterized in that the pin is of frustoconical shape, its large base being directed towards the exit of the die.