GB1605214A

GB1605214A - Method of and apparatus for producing tubular filter element

Info

Publication number: GB1605214A
Application number: GB53533/73A
Authority: GB
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1973-01-08
Filing date: 1973-11-19
Publication date: 1984-08-30
Also published as: CA1173308A; DE2359505C1; IT1062795B; BE807638A; FR2527092A1; FR2527092B1

Description

(54) A METHOD OF AND APPARATUS FOR PRODUCING A TUBULAR FILTER ELEMENT (71) We, COMMISSARIAT A L'ENERGIE ATOMIQUE. an organisation created in France by ordinance No. 45-2563 of 18th October 1945, of 29 rue de la Federation, Paris 15e, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of and apparatus for producing tubular filter elements and more particularly filter elements of the kind formed from at least one thin coating of at least one metallic oxide and/or at least one metallic fluoride deposited on a ceramic or metallic support.

According to a known process, filter elements of this kind are generally tubular and are made by moulding pulverulent inorganic material and heat treatment to make the assembly mechanically strong, in accordance with techniques which are now conventional in the ceramics and powder metallurgy industries. These elements may be made from various metals and alloys such as steel, bronze and nickel or from oxides such as alumina, magnesia and silicates, or compounds which are substantially refractory, such as the fluorides, carbides, nitrides and borides.

To produce tubes, these pulverulent materials are mixed with an organic binder and extruded under pressure through an annular nozzle. The tubes are then dried and heat treated to eliminate the binders and ensure cohesion of the powder particles.

This technique is very suitable for the production of relatively thick tubes, for example of a thickness of several millimetres.

These porous supports or filters, however, must in most cases have a permeability which is as high as possible to permit maximum flow and very small pressure drop in the fluids passing through them.

In view of these requirements, therefore, an attempt has been made to make such tubes with very small thicknesses, but their industrial production is difficult and complex and these tubes have a very low mechanical strength.

Filter elements are therefore conventionally made with an appreciable flow, i.e. relatively high pore dimensions, i.e. using powders having coarse particles. It is, however, frequently necessary, particularly for the separation of gaseous isotopes or for checking very fine particles, that the said elements should have a very fine texture with very small pore radii. such a requirement is not compatible with the requirement of high permeability, which is a prime requirement for the filter elements in question.

To produce filter elements which satisfy these requirements, it is now conventional practice to make multi-layer materials, i.e.

materials which in respect of thickness are formed from a high-permeability layer with large pore radii, and low-permeability layers with very fine pore radii.

In actual fact, and in the most usual cases, these filter elements are formed from a high-permeability tubular support providing the mechanical strength of the assembly, on which a very thin coating is deposited, generally inside the tube and which imparts its own characteristics as to flow and given pore radii. The radius of the tubular support pores is usually between 1 and 20 microns. Of course the coating must be as thin as possible in order not to reduce the permeability of the assembly excessively.

Since the inside diameter of the highpermeability support is often small, it is not possible to deposit the fine coating by spraying a suspended powder, whether electro-statically or otherwise, as is conventional practice in painting. Various coating processes have therefore been proposed, for example: spraying dry powder inside a porous tube previously impregnated with a volatile liquid. Because of the difficulty of feeding a nozzle with a dry powder, this process results in heterogeneous deposits of non-uniform thickness, said deposits particularly being very fragile. In practice it is impossible to handle these elements after coating without damaging them and this very slow process is not an industrial process.

The coating processes already proposed do not however, provide the characteristics required for the filter element which are a tubular support having relatively large pores and a very thin coating deposited thereon having small pores, so that the resulting filter element comprises two layers, each layer having pores of uniform size but of different size from the pores of the other layer.

According to the invention there is provided a method of producing a tubular filter element having both high permeability and very small pores, from a rigid metal or ceramic support and at least one thin inorganic porous coating, the or each porous coating having pores smaller than those of the said support, said method comprising the steps of contacting the inner and/or outer wall of the support with a liquid containing in suspension an inorganic powder which is required to form the porous coating, said liquid ensuring transport of the inorganic powder and its adhesion to the support, draining the support, drying it, compressing it and heating it. The resulting coating may be compressed on its support by means of a resilient diaphragm to compact it and give it greater mechanical strength and, if required, reduce the pore size which is between 0.001 and 1 micron. It may also be heat-treated so as to be mechanically consolidated. The suspension of inorganic powder may contain a small proportion of an organic binder which is adapted to modify the rheological properties of the suspension so as to ensure good mechanical solidity of the thin coating after drying and good adhesion of the thin coating on the support.

In this method the nature and concentration of the organic binder used, and the pH of the suspension, determine the viscosity of said suspension and its minimum strength, which have a great influence on the regulation of the thickness of the deposit and on the homogeneity of the latter along the support.

Using suspensions having a Newtonian flow, during draining of the liquid the top part of the support is stripped while the base is loaded, and this results in appreciable heterogeneity of the deposit. By selecting the constituents of the suspension and, in particular, the organic binder so that it has a minimum strength point, the deposit can on the other hand be perfectly homogeneous. In this connection see "Céramique General Notions de Physico-Chimie", Vol. II, pages 137 to 144, by C.A. Jouenne.

According to one simple process, the filter elements according to the invention may be constructed by filling the support or supports with the suspension, by leaving them in contact therewith for a given time, emptying out the suspension and allowing the element coated in this way to drain and dry. This filter is then subjected to a pressure of between 500 and 3000 bars, and finally subjected to a heat treatment at between 500 and 1800"C for about one hour to eliminate the organic binder or binders or volatile or chemical products and consolidate the deposited inorganic film.

The invention also relates to apparatus for performing the method according to the invention, said apparatus comprising a closed container formed with an orifice for the supply of the inorganic powder suspension, a supporting tube immersed to near the bottom of the container and adapted to support one end of the tubular filter element above said container and in communication with said tube, a second tube surmounting the other end of said tubular filter element, and means for raising the level of said suspension in said supporting tube and through said filter element into said second tube so as to contact the inner wall of said element with said suspension.

According to a specific embodiment of this apparatus, the ends of the tubular element are provided with sealing-tight gaskets, the top end communicating with a glass tube which, as a result of its transparency, allows the suspension level to be monitored, for example by a photo-electric cell which ensures that coating stops when the level of the suspension reaches said glass tube. The bottom end of the tubular element communicates with the supporting tube immersed in the suspension in a sealed tank to which excess air pressure is applied. This excess pressure causes the suspension to rise in the support and results in its coating.

One embodiment of the apparatus of the invention will now be described by way of example with reference to the accompanying drawing, in which the single figure is a sectional view of said apparatus.

A tubular filter element 1, the ends of which are provided with two sealing-tight gaskets 2 and 3, is kept in communication with a glass tube 4 at the top and with a metal supporting tube 5 at the bottom, the tube 5 being connected to a tank 6. The latter is provided with a T-tube 7, the limbs of which are provided with a valve 9 and a valve 8 which respectively allow the admission of a compressed gas and connection to atmospheric pressure. The supporting tube 5 is immersed almost to the bottom of the tank which is filled with liquid suspension 10, filling being via the sealed orifice 11. The suspension level is monitored by the photoelectric cell 12, its receiver 13 and a control unit 14. Coating is effected by placing the filter element 1 between the tubes 4 and 5, closing the atmospheric pressure venting valve 8 and opening the pressure gas admission valve 9.

This pressure, which is applied to the liquid suspension 10, causes the latter to rise in the supporting tube. When the suspension level reaches the detection zone of the photoelectric cell 12 and 13, the latter closes the valve 9 and opens the valve 8 via the control unit 14. The suspension falls back into the tank 6. After draining, the support is disconnected from the machine and the operation can be re-commenced with a new support.

Of course, instead of using a pressure on the liquid suspension, a negative pressure could be produced via the tube 4 so as to cause the suspension to rise in the same way in the support, and this adaptation is within the scope of the invention.

The invention is illustrated with the aid of the following examples, which are given by way of example only.

Example 1: A tubular refractory ceramic support having an outside diameter of 20 mm, a thickness of 2 mm, a length of 500 mm and pores having radii of 5 microns, is filled with the suspension prepared as below: 300g of electrically melted alumina powder having a grain size very close to 5 microns, are suspended in a well-homogenised solution of: 15g of carboxymethyl cellulose, 30g of glycerine, 500cc of ethyl alcohol, 500cc of water.

This suspension is introduced into the support by means of the apparatus as described. After draining and drying at ambient temperature and then in an oven the element obtained in this way is provided with two inner and outer polymer diaphragms and subjected to an isostatic compression of 1500 bars in a water-filled pressure vessel. The object of this compression is to make the deposited layer very compact and anchor said layer on the support.

After this operation the layer is subjected to heat treatment at 18000C for one hour. The resulting filter has a very high permeability and the radius of the pores of the deposited layer or coating is 1 micron.

Example 2: A tubular support identical to that in Example 1 is coated as described in Example 1 by means of the following suspension: 300g of a calcined alumina powder of a grain size betwen 1 and 15 microns and a specific surface of 8m2 per gram are added to 1 litre of an aqueous gel containing 0.2% of ethyl methyl cellulose. The suspension is agitated and then left for two days and finally screened on a 40 micron cloth. The coating obtained has a thickness of 20 microns and is compressed, as in Example 1, between two diaphragms at a pressure of 800 bars. After heat treatment in air at 15000C, the resulting element has pores of a radius of 0.4 micron on average.

Example 3: A sintered nickel tube having an outside diameter of 15 mm, a thickness of 0.5 mm and a length of 500 mm is coated as described in Example 1 with a suspension formed from the following: 250g of calcium fluoride of a grain size of the order of 3 microns, 2g of polyvinyl alcohol, 1 litre of water.

After deposition, drying, compression at 500 bars and heat treatment at 5500C, the resulting element has a very high permeability and a pore radius of approximately 0.3 microns.

WHAT WE CLAIM IS: 1. A method of producing a tubular filter element having both high permeability and very small pores, from a rigid metal or ceramic support and at least one thin inorganic porous coating, the or each porous coating having pores smaller than those of the said support, said method comprising the steps of contacting the inner and/or outer wall of the support with a liquid containing in suspension an inorganic powder which is required to form the porous coating, said liquid ensuring transport of the inorganic powder and its adhesion to the support, draining the support, dyring it, compressing it and heating it.

2. A method of producing a filter element according to Claim 1, characterised in that the inorganic powder suspension contains a small proportion of an organic binder adapted to modify the rheological properties of the suspension so as to ensure good mechanical solidity of the thin coating after drying and good adhesion of the thin coating on the said support.

3. A methiod of producing a filter element according to Claim 2, characterised in that the organic binder is a cellulose ester such as carboxymethyl cellulose or ethyl methyl cellulose.

4. A method of producing a filter element according to Claim 2, characterised in that the organic binder is polyvinyl alcohol.

5. A method of producing a filter element according to Claim 1, characterised in that an inorganic porous coating is formed from at least one metallic oxide.

6. A method of producing a filter element according to Claim 1, characterised in that an inorganic porous coating is formed from at least one metallic fluoride.

7. A method of producing a filter element according to any of the preceding claims, characterised in that the radius of the pores of the porous coating is between 0.001 and 1 micron.

8. A method of producing a filter element according to any of the preceding claims, characterised in that the radius of the pores of the support is between 1 and 20 microns.

9. A method of producing a filter element according to any of the preceding claims, characterised in that the deposited and dried coating is compressed isostatically by means of two diaphragms disposed on the inner and outer walls of the element, the assembly being disposed in a liquid-filled pressure vessel and

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. suspension falls back into the tank 6. After draining, the support is disconnected from the machine and the operation can be re-commenced with a new support. Of course, instead of using a pressure on the liquid suspension, a negative pressure could be produced via the tube 4 so as to cause the suspension to rise in the same way in the support, and this adaptation is within the scope of the invention. The invention is illustrated with the aid of the following examples, which are given by way of example only. Example 1: A tubular refractory ceramic support having an outside diameter of 20 mm, a thickness of 2 mm, a length of 500 mm and pores having radii of 5 microns, is filled with the suspension prepared as below: 300g of electrically melted alumina powder having a grain size very close to 5 microns, are suspended in a well-homogenised solution of: 15g of carboxymethyl cellulose, 30g of glycerine, 500cc of ethyl alcohol, 500cc of water. This suspension is introduced into the support by means of the apparatus as described. After draining and drying at ambient temperature and then in an oven the element obtained in this way is provided with two inner and outer polymer diaphragms and subjected to an isostatic compression of 1500 bars in a water-filled pressure vessel. The object of this compression is to make the deposited layer very compact and anchor said layer on the support. After this operation the layer is subjected to heat treatment at 18000C for one hour. The resulting filter has a very high permeability and the radius of the pores of the deposited layer or coating is 1 micron. Example 2: A tubular support identical to that in Example 1 is coated as described in Example 1 by means of the following suspension: 300g of a calcined alumina powder of a grain size betwen 1 and 15 microns and a specific surface of 8m2 per gram are added to 1 litre of an aqueous gel containing 0.2% of ethyl methyl cellulose. The suspension is agitated and then left for two days and finally screened on a 40 micron cloth. The coating obtained has a thickness of 20 microns and is compressed, as in Example 1, between two diaphragms at a pressure of 800 bars. After heat treatment in air at 15000C, the resulting element has pores of a radius of 0.4 micron on average. Example 3: A sintered nickel tube having an outside diameter of 15 mm, a thickness of 0.5 mm and a length of 500 mm is coated as described in Example 1 with a suspension formed from the following: 250g of calcium fluoride of a grain size of the order of 3 microns, 2g of polyvinyl alcohol,

1 litre of water.

subjected to a pressure of between 500 and 3,000 bars.

10. A method of producing a filter element according to Claim 9, characterised in that heating is carried out between 500 and 1800"C for about one hour.

11. Apparatus for performing the method according to Claim 1, said apparatus comprising a closed container formed with an orifice for the supply of the inorganic powder suspension, a supporting tube immersed to near the bottom of the container and adapted to support one end of the tubular filter element above said container and in communication with said tube, a second tube surmounting the other end of said tubular filter element, and means for raising the level of said suspension in said supporting tube and through said filter element into said second tube so as to contact the inner wall of said element with said suspension.

12. Apparatus according to Claim 11, characterised by a control unit for monitoring the level of the suspension in said second tube.

13. Apparatus as claimed in Claim 12, wherein the container has a venting conduit and conduit for connecting the interior of the container to pressure, and the control unit is operable to control the venting and pressurising of the interior of the container.

14. Apparatus according to Claim 12, characterised in that the suspension level control system comprises a photoelectric cell, a receiver and a control unit which controls the venting and pressurising of the interior of the container.

15. A method of producing a tubular filter element substantially as hereinbefore described with reference to the accompanying drawing.

16. Apparatus for performing the method of Claim 15, said apparatus being substantially as described and as shown in the accompanying drawing.