WO2000078691A1

WO2000078691A1 - Plasma assisted gas processing

Info

Publication number: WO2000078691A1
Application number: PCT/GB2000/002269
Authority: WO
Inventors: David Leslie Segal; Andrew Holt
Original assignee: Accentus Plc
Priority date: 1999-06-22
Filing date: 2000-06-09
Publication date: 2000-12-28
Also published as: AU5544200A; GB9914472D0

Abstract

A reactor for the plasma assisted processing of gaseous media, in which there is included a bed of pellets made from sintered barium titanate or other ferroelectric powder formed into spherical pellets using a fugitive binder sintering technique.

Description

Plasma Assisted Gas Processing

The present invention relates to the processing of gaseous media by means of plasmas established in the gaseous media, and more specifically to the removal of carbonaceous particles and other pollutants from the exhaust emissions of internal combustion engines.

Our earlier patent number GB 2 274 412 discloses a method and apparatus for removing carbonaceous particulate and other pollutants from internal combustion engine exhaust gases in which the exhaust gases are passed through a bed of pellets of a ferroelectric material having a high dielectric constant to which a voltage of some tens of.kilovolts is applied. This voltage is sufficient to excite plasma discharges in the exhaust gases which remove carbonaceous particles from the exhaust gases by oxidative processes especially by electric discharge assisted oxidation. Among the materials disclosed for use in the bed is barium titanate, which can be adapted to catalyse the removal of

N0_X from the exhaust gases by reduction to N₂. The use of barium titanate for the plasma assisted processing of gaseous media also is disclosed in A. Mizumo IEEE Transactions on Industry Applications, Vol 28 pp 535-540 May/June 1992 and R. Zhang et al IEEE Transactions on Industry Applications 1996, Vol 32 pp 113-117.

In each of the above cases, the barium titanate is in the form of spheres or pellets, or other shapes which may be extrudates. Barium titanate is a material which is difficult to fabricate into shapes particularly spherical pellets as relatively high temperatures, of the order of 1200°C to 1500°C are required to sinter it effectively. This is also true for other ferroelectric materials. These high temperatures mean also that the resultant material is dense with a low porosity: the theoretical density of essentially non-porous tetragonal barium titanate is 6.017 g cm and the density of hexagonal barium titanate is 5.806 g cm . Also, in the case of reactors for the treatment of vehicle engine exhaust emissions, the weight is a disadvantage as it is desirable for the reactor to be as light as possible, both to reduce its contribution to the weight of the vehicle and to facilitate its mounting on the vehicle.

US patent 5,258,338 discloses a method of manufacturing monolithic ceramic capacitors in which barium titanate powder is mixed with a cadmium silicate or borate sintering flux and a grain growth inhibitor such as niobium oxide in an organic carrier. The mixture is deposited on a substrate which is dried, coated with a metal layer to form the electrodes of the capacitor and sintered to consolidate the ceramic dielectric. The carrier is present purely to facilitate the deposition of the ceramic layer upon the substrate electrode and appears to have no part to play in the formation of the actual ceramic dielectric material.

US patent 4,558,020 discloses a method of manufacturing disc capacitors in which barium titanate powder is mixed with additives namely calcium titanate and niobium oxide to shift the Curie point and a binder, an aqueous solution of polyvinylalcohol . The mixture is blended, filtered through a sieve and granulated after which the granulated powder is pressed by application of a pressure to a die to discs. The discs are typically 13 mm in diameter and 0.5 mm thickness and are then sintered at up to 1400°C. The binder is removed in the sintering process. This method uses prereacted ceramic powders for example barium titanate that are then blended with a binder for manufacture exclusively of discs and shapes derived from a rectangular die typically 4.7 mm x 12.5 mm x 1.5 mm. However use of applying pressure to a die is not an economic process for the production of dense ceramic spheres.

US patent 5,637,542 discloses a method of manufacturing discs of a piezoelectric material based on a binary system solid solution derived from Bi_0-5 Na_0-5 iO₃ containing sodium niobate , NaNb0₃. Powders of bismuth oxide, sodium carbonate, niobium oxide and titanium dioxide were mixed in a ball mill, the resulting powder was press -molded at 800°C, pulverised and added with polyvinylalcohol as a binder. The powder mixture containing binder was uniaxially-molded under a pressure to discs typically 20 mm diameter and 1.5 mm thickness, heat-treated at temperature to volatilize the binder and then heated at elevated temperature, 1300°C. The method discloses that powders are prereacted at 800°C before mixing with the binder and the method is restricted to production of ceramic discs only. It is not disclosed whether the polyvinyl alcohol is used as an aqueous or non-aqueous solution or as a dry powder when mixed with reactants. In addition the method is restricted to the bismuth sodium titanium oxide system. As is well understood in the art, the sintering behaviour of ceramic powders is dependent on ceramic composition, and a binder suitable for one composition may not be applicable to a different composition.

It is an object of the invention in one aspect to provide substantially dust -free spherical bodies of ceramic ferroelectric material and in another aspect to provide a reactor incorporating such a material for the plasma-assisted processing of a gaseous medium.

According to the present invention in one aspect there is provided a method of manufacturing substantially dust free bodies of a ferroelectric material, comprising the operations of combining particles of the ferroelectric material with a material adapted to act as a fugitive binder, characterised by forming the combined ferroelectric and binder materials into spherical green bodies and firing the green bodies to produce substantially dust and binder free spherical bodies of ferroelectric material.

Preferably the spherical green bodies are formed by tumbling particles of ferroelectric material in a rotating chamber and simultaneously spraying the particles of ferroelectric material with a solution of the binder material until about 5% by weight of the binder material has been added to the particles of ferroelectric material. The tumbling operation is continued until spherical bodies of a desired size have been formed. The green bodies of ferroelectric material are then dried and fired at temperatures sufficient to drive out the binder and sinter the ferroelectric material into cohesive, substantially dust and binder free spheres which are particularly useful to form a gas- permeable bed in a plasma-assisted reactor for the treatment of gaseous media to remove noxious components therefrom.

Examples of ferroelectric materials which can be produced by the process of the invention are barium or calcium titanate. Suitable binder materials are solutions of guar gum and carbohydrates such as starch and also polyvinyl alcohols. According to the invention in a second aspect there is provided a reactor for the plasma assisted processing of gaseous media, comprising a chamber including an inlet and an outlet for a gaseous medium to be processed, a gas permeable bed of a particulate ferroelectric material, means for directing the gaseous medium through the bed of ferroelectric material and means for applying across the bed of ferroelectric material a potential sufficient to excite an electrical discharge in the said gaseous medium in the interstices of the bed of ferroelectric material, characterised in that the ferroelectric material is in the form of spherical pellets made by a process according to the first aspect of the invention.

Embodiments of the invention will now be described by way of the following examples and with reference to the accompanying drawing which is a longitudinal section of a plasma assisted reactor for the treatment of the exhaust gases from an internal combustion engine to remove noxious components therefrom.

Example 1

5 kg of barium titanate powder, of mesh size 100% less than 50 microns, were added to a rotating inclined pan. The pan was rotated at 80 revolutions per minute and a solution of 5 weight % of an organic gum dissolved in demineralised water was sprayed onto the rotating powder until spherical agglomerates of the required size had formed. When the required size of spheres had formed the aqueous spray was discontinued and the pan was rotated for a further period of time.

The rotation of the pan was stopped and the spheres removed from the pan and dried in an electrically heated oven at 60°C for 12 hours. The dried spheres were removed from the oven and loaded into pure alumina crucibles and heated to a temperature of 1420°C in an electrically-heated, air- atmosphere furnace and maintained at this temperature for 1 hour. When the furnace had cooled to 20°C the crucibles were removed from the furnace and the barium titanate spheres were collected as a hard non-dusting and dense material with an Average Bulk Density (ABD) of 2.95 g cm . A representative sphere diameter for the barium titanate was 3 mm.

Example 2

10 kg of barium titanate powder, of mesh size 100% less than 50 microns, were added to a rotating inclined pan. The pan was rotated at 90 revolutions per minute and a solution of 2.5% weight of an organic gum dissolved in demineralised water was sprayed onto the rotating powder until spherical agglomerates of the required size had formed. When the spheres had formed to a diameter of between 3-5 mm the aqueous spray was discontinued and the pan was rotated for a further period of time, as before.

The rotation of the pan was stopped and the spheres removed from the pan and dried in an electrically-heated oven at 60°C for 15 hours.

The dried spheres were removed from the oven and loaded into pure alumina crucibles and heated to a temperature of 1350°C in an electrically-heated furnace with a purged atmosphere containing 3% oxygen by volume and maintained at this temperature for 3 hours . When the furnace had cooled to 20°C the crucibles were removed from the furnace and the barium titanate spheres were collected as a hard non-dusting and dense material with an Average Bulk Density (ABD) of 2.91 g cm .

It is envisaged that an alternative approach would be to prepare first a dry blended mix of dry barium titanate powder with dry powdered binder which is then added to the rotating inclined pan for forming spheres of barium titanate held together by the binder. If necessary a spray of solvent for the binder onto the powder mix on the rotating pan would be used to assist the agglomeration into spheres.

Example 3

1 kg of barium titanate powder, of mesh size 100% less than 150 microns was dry blended with 4% weight of graphite powder and 0.3% weight of an organic polymeric gum. The dry blended powder was then pressed into pellets in an hydraulic press at an applied pressure of 10 tons and the 'green' pellets dried in an electrically- heated oven at 60°C for 12 hours.

The dried pellets were removed from the oven, loaded into pure alumina crucibles and heated to a temperature of 1350°C in an electrically-heated furnace for 4 hours. When the furnace had cooled the crucible was removed from the furnace and the barium titanate pellets were collected as a hard, non-dusting and dense material with an Average Bulk Density (ABD) of 2.9 g cm ^■3

This pressing approach of this Example 3 offers a possible route to forming pellets from a micronised powder without any significant agglomeration of the powder prior to loading in the die press. Spherical pellets can be formed in this way. The method comprising dry blending of the barium titanate powder with the binder also provides a novel process for forming pellets of other shapes. However, pressing is not at present a commercially attractive route to manufacturing pellets in bulk quantities.

In the above Examples, the gum used was guar gum, but other materials such as carbohydrates, for example starches, polymers such as polyvinyl alcohols, or polysaccharides , or mixtures of these can be used. The criteria for the binder are that the material should be capable of providing sufficient cohesion to the green bodies to enable them to be handled and survive the early stages of the sintering process, but to be completely eliminated by the end of the sintering process so that they do not affect the ferroelectric properties of the final material.

Using a fugitive binder technique for producing the spherical particulate ferroelectric material has the advantage that the particles can have an appreciable porosity that can be reduced by heating at a controlled temperature .

Referring to the drawing, a reactor 1 for removing pollutants, such as NO_x or carbonaceous combustion products, from the exhaust from an internal combustion engine consists of a cylindrical stainless steel chamber 2 which has an inlet stub 3 and an outlet stub 4 by means of which it can be connected into the exhaust system of an internal combustion engine. The chamber 2 is arranged, in use, to be connected to an earthing point 5. Perforated cylindrical stainless steel inner and outer electrodes 6 and 14 are positioned co-axially within the chamber 2 by means of two electrically insulating supports 7 and 8. The upstream end of the inner electrode 6 is arranged to be connected via an insulating feed through 10 to a source 9 of an electrical potential of the order of kilovolts to tens of kilovolts which may be a regularly pulsed direct or continuously varying alternating potential, or an interrupted continuous potential. The space 11 bounded by the electrodes 6 and 14 and the supports 7 and 8 is filled, in this example, with a bed illustrated highly diagrammatically at 12 of particulate barium titanate, which, in accordance with the present invention, are in the form of spheres.

The support 7 nearer the inlet stub 3 has a number of axial holes 13 disposed regularly around its periphery so that incoming exhaust gases are constrained to pass into the space 15 between the outer electrode 14 and the chamber 2 of the reactor 1 and thence radially through the bed 12 of barium titanate spheres before passing through the inner electrode 6 and leaving the chamber 2 via the exhaust stub 4.

The barium titanate spheres in the bed 12 are produced by a fugitive binder sintering technique such as those described above and have a bulk packing density of around 3 kg per litre. The permittivity of the barium titanate spheres can be adjusted by calcination temperature which affects the porosity and crystallite size both of which affect the magnitude of the permittivity.

Claims

1. A method of manufacturing substantially dust -free bodies of a ferroelectric material, comprising the operations of combining particles of the ferroelectric material with a material adapted to act as a fugitive binder, characterised by forming the combined ferroelectric and binder materials into spherical green bodies and firing the green bodies to produce substantially dust and binder free spherical bodies of ferroelectric material .

2. A method according to claim 1, further characterised in that the said green bodies include less than five weight per cent of binder.

3. A method according to claim 1 or claim 2 , further characterised in that the said green bodies are fired at a temperature exceeding 1000°C.

4. A method according to any preceding claim, further characterised in that there is included the operation of agitating with a rotary motion a powder of ferroelectric material together with a binder material to form spherical particles of ferroelectric material powder bound together by the binder material .

5. A method according to any preceding claim, further characterised in that there is included the operations of agitating particles of ferroelectric material with a rotary motion while being sprayed with a solution of the binder material, the operation being continued until spherical green bodies of a desired size have been formed, drying the said green bodies and firing the green bodies at a temperature sufficient to drive out the binder material so as to form spherical particles of substantially dust and binder free ferroelectric material .

6. A method according to any of claims 1 to 3 , further characterised in that there is included the operations of dry blending particles of the ferroelectric material with graphite powder and an organic gum, pressing the mixture to form spherical pellets of green material, drying the green pellets and firing the green pellets at a temperature sufficient to remove both the graphite powder and the organic gum so as to form spherical pellets of substantially dust and binder free ferroelectric material .

7. A method according to claim 1, further characterised in that the binder is an organic material.

8. A method according to claim 7, further characterised in that the binder is selected from the group comprising guar gum, starches and polymeric materials including polyvinyl alcohols.

9. A method according to claim 8, further characterised in that the polymeric materials are polysaccharides.

10. A method according to any preceding claim, further characterised in that the ferroelectric material is barium titanate or calcium titanate.

11. A reactor for the plasma assisted processing of gaseous media, comprising a chamber (2) including an inlet (3) and an outlet (4) for a gaseous medium to be processed, a gas permeable bed (12) of a ferroelectric material , means for directing the gaseous medium through the bed (12) of ferroelectric material and means for applying across the bed (12) of ferroelectric material a potential sufficient to excite an electrical discharge in the gaseous medium in the interstices of the bed (12) of ferroelectric material , characterised in that the ferroelectric material is made by a method according to any preceding claim.