The present invention relates to a filter element, a filter unit, and the manufacturing method thereof, for filtering solid particles from a fluid.

Particulate collection by the use of. a filter takes place by interception or impaction of the particles on the walls of the filter, as the fluid is forced through a porous filter wall of the filter.

Compact filtering systems able to operate in environments with high temperatures and pressures and potentially highly corrosive atmospheres are highly desired for a number of different industrial processes as the present filter systems have a number of disadvantages, such as a bulky design. Highly efficient particulate filters are, e.g., required for removing particles from hot gases having a temperature above. 400°C, in combustion plants for environmental protection. Refraining from removing particles in the combustion gases of systems where the combustion gases are reused across a secon¬ dary turbine will lead to the particles exposing the turbine parts to wear.

In modern PFCB (Pressurised Fluid Combustion Bed) plants, it is believed that the overall thermal efficiency can be increased by an extremely important few percent for a typical set-up, if a gas turbine/generator is connected directly to the combustion chamber outlet. Until now, this has proven very difficult, as the gas containing large amounts of solids will erode or wear out the gas turbine parts within an extre¬ mely short period of time. The gas in the combustion chamber preferably has a temperature of 850°C, and is kept at a pressure of 25 Bar, whereby the introduction of a highly efficient filter system in this environment has caused large problems.

PFCB test plants rarely exceed a total power output of more than 5 MW, yet power generation is preferable with plants delivering a total power output in the range of 50-100 MW. However, using a filter unit according to the present inven- tion will have a positive effect on the total economy of the PFCB power plant as replacement of gas turbine parts may be postponed. Furthermore, the emission levels will be reduced, improving the environment.

Other applications where compact and efficient filter units are desired are the removal of fine particles from hydro¬ carbon-processing gas streams having a temperature above the dew point, as well as the removal and/or recovery of cata¬ lysts. In addition, due to the increasing demands as to the reduction of emissions from incinerator applications, a high- quality cleaning of the combustion gases requires removal of particles from the gases.

Among the least expensive equipment for particulate removal from hot gas streams are cyclones. In fact, as much as 75-95% of the particulate matter (depending on the particle size thereof) may be removed by using two cyclones in series. For further reduction in the emission of particulate matter, electrostatic precipitators may be used. At present, however, these precipitators are very bulky, and as precipitators have decreasing efficiency at elevated temperatures a different type of particle filter is desired.

Only porous ceramic wall flow filters or barrier filter elements can achieve the desired filtering efficiency of 99,5 or even higher efficiency. Filters of this type are described in numerous patents and patent applications, such as EP 043 694 and WO 89/09648.

The conventional filter element technology for large size filter units is based on soft or rigid tubular filter elements, also called filter candles. Rigid filter candles may be manufactured with a maximum diameter of 80 mm and an

element length of 1000 mm. In order to form longer filter candles, two or more candles may be assembled to a total maximum length of 3 metres. Soft filter candles may be manu¬ factured in one piece with a maximum diameter of 200 mm and a maximum length of 2 metres.

All filter candles are porous members which are open at one end and closed at the opposite end. The fluid, containing solids to be separated therefrom, is supplied under pressure, with the greatest pressure on the collecting side of the filter, also called up-stream of the filter.

To form larger filter units, the candles are mounted side by side, either hanging from a support plate at the top of the filter vessel, or standing on a support plate at the bottom of the vessel. The candles are often only supported at one end thereof.

Generally, rigid candles have very thick walls as a result of the manufacturing method, which is a non-continuous, forming process carried out inside a steel cylinder. The result is a rather large wall thickness required by the often low strength of the filter material and requirements of the filter material as to the porosity thereof.

Ceramic candles typically have a high weight, which increases the vessel and support system weight considerably, and are prone to breakage, as they have a relatively low mechanical strength.

Filter candles are often put under stress by uncontrolled particle build-up, called dust-bridging, between two or more candles. This dust bridge may grow so thick that the thermal expansion of the filter candles, e.g. induced by load differ- ences of the boiler or differences in gas temperature, may be transferred from candle to candle resulting in filter candles breaking. Furthermore, vibrations caused by the back flushing pulses of pressurised fluid may cause the candles to break.

Furthermore, mechanical shock and vibrations caused by reson¬ ance frequencies created by the gas flow may break the brittle ceramic candles.

The rigid ceramic candles are generally produced using clay bonded SiC grains sintered in a low temperature oxide atmos¬ phere. The clay limits or reduces the mechanical strength of the candles. The corrosion resistance, however, is generally good.

Rigid metal based candles have a high mechanical strength, and candles made from advanced alloys also have a high cor¬ rosion resistance. However, as is the case in ceramic candles, the manufacturing method of metal candles has so far limited the size of metal candles to a maximum diameter of 100 mm and a maximum length of 3 metres. Metal candles are often manufactured by a compaction method where fibres or powder are put under pressure in the die of a hydraulic press.

The known soft filter candles are manufactured from vacuum formed ceramic fibres that are bonded together with inorganic binders to form thick walls of the candles. These candles have a low mechanical strength, and fibres from the down¬ stream areas of the filters are prone to loosening and to subsequently be carried away with the cleaned fluid to the outside of the filter system.

Today, the conventional method of cleaning filter elements, that is removing the solids from the up-stream surface of the filter candle, is preferably performed by "back flushing", i.e. introducing a reverse flow of compressed gas releasing or dislodging the particles that have formed a so-called "dust cake" on the inlet surface of the filter candle. The loosened dust subsequently falls down into, e.g., a dust hopper for final removal from the filter system.

In multiple filter units where as many as 1000 individual filter candles may be assembled inside one vessel, two or more filter elements may be cleaned at a time, or the com¬ plete filter system may be cleaned sequentially in groups of candles.

So-called ceramic cross flow filters have been investigated, but as these filters are impossible to manufacture in any¬ thing other than rather small substrate sizes, a large number is required in order to obtain large surface filter instal- lations.

In general, compared to filter units according to the inven¬ tion, all known filter candles and large filter systems offer a reduced exploitation of the vessel space. According to the invention, filter units offering a higher volume/surface ratio may be obtained which makes it possible to avoid build¬ ing large, bulky and very expensive filter vessels.

In a first aspect, the invention relates to a method for producing a filter unit, said method comprising forming a plurality of elongated tubular filter elements from a filtering material, arranging the elongated filter elements in a coextending relationship, bringing outer wall surfaces of the coextending filter elements in mutual contact along substantially the full length of said elements so as to define a plurality of flow passages for a fluid to be filtered and closing one end of each of the fluid passages, the closed ends being alternately at opposite ends of the filter unit, whereby fluid flowing into the open end of the fluid passages at one end of the filtering unit may flow transversely through filter element walls and out through open ends of fluid passages at the other end of the filter unit.

In the present context, mutual contact may mean both direct physical contact between filter elements and contact through,

e.g., a sealing material. The mutual contact is required in order to substantially prevent leakage of gas from filter element to filter element.

In a second aspect, the invention relates to a filter device for removing particles from a fluid or gas, in which said particles or solids have entrained or are suspended within, comprising in combination,

a filter tank or a vessel having a fluid inlet and a fluid outlet,

a filter unit comprising an assembled structure,

a filter structure cleaning device.

Thus, the present invention relates to a novel method of manufacturing a large filter unit. The filter elements of the filter unit may be manufactured from clay bonded SiC powder, SiC bonded SiC powder. Also other ceramic powders, such as Si ₃ N ₄ , SiONC, Alumina, Cordierite, Mullite, Spodume and members of the NaSiCON structural family may be used as the base structure. As described above, the sintering method for the clay bonded ceramic and most oxide ceramics is simpler than that of pure SiC. Cordierite and NaSiCON are known for their low thermal expansion coefficients making them suitable for filters subjected to large variations in the process gas temperature. Many combinations of grain structure and grain binder can be produced. Furthermore, as described above, also metal powders and metal fibre may be used in the present filter elements in certain applications.

Ceramics are often preferred, when the process environment contains hot gases with a low oxygen content, water and the possibility of a high sulphur content.

Compared to the known materials used in dust filters, such as metals, organic fibres and other ceramics, SiC has a number

of advantages, such as being stable in an oxidising atmos¬ phere to temperatures as high as 1500°C. Pure, solid SiC has a large physical strength, also at elevated temperatures: at 1000°C, the strength is 7 times the strength of solid Alumina. A pure SiC test piece made from grit P 150, 88-125 μm SiC grain having a pore size of 50 μm and a porosity of 50% has a bending strength of more than 40 MPa which is 3 times the strength of a commercially available clay bonded SiC filter element having a porosity of 36% and a pore size of 200 μm.

At high temperatures, SiC is more corrosion resistant than metals. In fact, the life of a filter element consisting of substantially pure SiC will probably not at all depend on corrosion at temperatures below 850°C. Oxidation tests have shown no change of the weight of a pure SiC filter element when exposed to 900°C for 2000 hours in an oxidising atmos¬ phere. At higher temperatures, however, gases with large amounts of water vapour/alkali/chlorine/sulfur will corrode the protecting Si0 ₂ layer on the filter grains, and the oxidation of the SiC grain may be accelerated. When the grain binder is pure SiC, an optimum lifetime of the filter element is obtained in contrast to the candles known per se, where the binders are a glass phase or clay based, as these binders do not have the optimum properties of SiC.

The base material is preferably a ceramic grain, such as SiC, and will normally have an average mean grain size in the range of 1-1.500 μm, such as in the range of 10-500 μm, preferably in the range of 30-250 μm. SiC grains having this grain size may be purchased in the following known Meshes: 24, 30, 40, 60, 80, 120, 150, 180, 220, 280, 320, 360, 400, 600 (according to FEPA abrasives standards) . If a different ceramic base material is to be used, similar sizes correspon¬ ding to FEPA standard are preferred.

The mean pore size of a porous ceramic filter element with no membrane thereon will normally be in the range of 0.1-750 μm, such as 1-150 μm in particular 2-50 μm.

As described above, the introduction of a membrane on the filtering surface of a filter element will increase the filtering efficiency of the filter element. Membranes in a two-layer set-up, where one layer is the base material of the filter element and the second layer is the membrane, may be manufactured from powder or fibres or from a combination thereof.

With a thin membrane coating, such as having a thickness of 0.05-2 mm, preferably 0.1-0.2 mm, the base structure may have a mean pore size in the range of 10-500 μm, preferably 20-250 μm, and the membrane may have a preferable mean pore size in the range of 0,1-50 μm preferably 1-15 μm. Tests have indi¬ cated that a separation efficiency as high as 99.9% for ashes may be reached using a membrane. This will ensure less than 0.2 mg/m ³ residual ash in the fluid stream up-stream of the filter from a typical coal fired power plant.

It may be preferred to manufacture multi-layered coatings on the filter elements, where the different layers are formed from coatings having different grain sizes, and where the coatings have increasing grain size from the outside toward the base structure of the filter element.

Final pore diameters of between 1 and 20 μm have been obtained, using fibres having a diameter of 2-4 μm and a length of 10-1000 μm, preferably 200-500 μm.

Using filter elements of SiC, the high strength of these filter elements will offer a lower total weight of the filter unit as the individual filter walls may be manufactured thinner than those of the filter elements known per se. Thus, a reduction in the wall thickness from 20-10 to 10-3 mm is obtainable. This will reduce the total system weight, reduce

manufacturing costs, reduce the pressure drop across the filter unit and, thus, give a higher total efficiency for the industrial plant.

The wall thickness of the filter elements may be chosen depending on the requirements for the maximum filter unit weight and on the forces which the filter elements are to withstand. Wall thickness may typically vary from 0.5-50 mm, preferably between 1-25 mm or 2-10 mm. The cell pitch or wall width is typically 1-250 mm, preferably between 10-100 mm.

However, if a low-strength ceramic base material is used, the wall thickness will typically be higher, such as 10-100 mm, preferably 15-40 mm, and the cell wall dimensions will be modified according to the specific wall thickness chosen.

Even though the presently preferred production method of the filter elements according to the invention employs continuous barrel/auger extrusion, it is contemplated that also tape- casting, iso-static casting, slip-casting and pressing methods may be used.

The assembled filter elements forming a complete filter unit may be cemented together at their contact points in order to obtain a large stability of the filter unit and in order to ensure gas tight sealing between the filter elements. The cement or compound used to interconnect the filter elements may be commonly used clay bonded SiC powder or other ceramic powders or cements, preferably materials having at least substantially the same expansion coefficient as the filter elements (in order to avoid or reduce thermal stress in the filter unit), such as Sauressen cement No. 8.

Alternatively, the filter elements may be separated at their contact points by mechanical seals giving longitudinal sup¬ port and/or axial centering. These seals may be manufactured from a ceramic fibrouous material or a composite of ceramics, organic materials and metals, preferably in the form of

fibres. The sealing material should be temperature and cor¬ rosion resistant and be able to tolerate the radial expansion of the filter elements. Alternatively, the fibres may be encapsulated in, e.g., a wire mesh rope made from a heat τ_ resistant metal, such as Inconel , or the like.

As the structure of the filter unit according to the inven¬ tion is formed by a number of filter elements, substantially any structure may be obtained. Even though a number of struc¬ tures are described in connection with the drawings, the preferred filter unit structure is the so-called honeycomb structure, which is one of the most compact filter structures presently known. A honeycomb structure is a structure where a number of channels extend in the filter from a gas inlet side thereof to a gas outlet side thereof, and where the channels are all closed in at least one end so as to prevent direct passage from the gas inlet side to the gas outlet side. Thus, in this structure, the gas is forced through at least one filter wall where the filtration of the gas is performed. In the preferred embodiment of a honeycomb filter, the channels are closed in a checquered pattern so that one inlet channel has four neighboring outlet channels. In this manner, all filtering walls take place in the filtering of the gas, making this filter type extremely compact.

By assembling a large number of filter elements into one or several large filter units according to the invention, rigid, low density filter units in the shape of giant honeycomb structures may be obtained. All single-piece honeycomb struc¬ tures, that is, honeycomb structures manufactured in a single piece, have the advantage of an extremely high mechanical strength; All mechanical forces are transferred correctly through the walls, to the canning container. By enclosing a number of filter elements according to the invention in a vessel, it is possible to obtain the same behaviour of the filter unit according to the invention as that of a single- piece honeycomb structure. At present, single-piece honeycomb

filters are rarely produced with diameters larger than 300 mm.

Compared to the known candle filter set-ups, the rigid filter unit of the invention will greatly reduce the risk of the extremely costly breakdown of one or more filter candles, whereby unfiltered PFCB gas is allowed to damage the gas turbine now connected directly to the boiler or where unfil¬ tered gas is led to the surroundings.

Even though the preferred filter elements for use in the filter unit according to the invention are manufactured by extrusion of a paste comprising SiC, acceptable filter elements may also be made of other materials and by other manufacturing methods. Thus, it is contemplated that vacuum formed fibre elements, made of either a ceramic or a metallic material may be used as a filter element for use in the filter unit according to the invention. Furthermore, the filter elements may be interconnected or sealed using a material comprising metal fibres.

In general, the filter unit according to the invention, preferably having a honeycomb structure, will have the fol¬ lowing advantages compared to standard filter systems based on filter candles:

thermal cracks over the cross-section of the filter unit structure are prevented as the the individual filter elements may be separated from each other by means of sealing, ■ the sealing may prevent mechanical stress from being transferred from a filter element to neighboring elements, - the manufacture of large honeycomb structures is pos¬ sible, such as a length of 2-5 metres and a diameter of 1-10 meter (prior to the present invention, honeycomb structures are not manufactured larger than 0 0.3 x L 0.3 meter) ,

virtually any size and shape of the filter unit is pos¬ sible due to the filter unit being built by separate modules (filter elements) , the filter unit is stable to vibration as the filter unit is rigid and as it may be provided by a relatively high weight, if this is preferred, the filter unit is tolerant to Boiler Load Transients, which will introduce temperature gradients in the filter unit, as each filter element due to the sealing may reach virtually any temperature regardless of the neighbor elements, the filter unit may be manufactured to be extremely compact and have a low weight.

In fact, a compactness (expressed as a volume/surface ratio) which is several factors higher than the present large filter systems may be obtained. The surface density of the filter unit according to the invention may be increased by a factor two-three compared to standard filter systems based on filter candles.

A simple calculation can illustrate the significance of the improvement in volume/surface ration obtained with this invention compared to the known hot gas cleaning systems for industrial use.

As described above, it is contemplated that the vol- ume/surface ratio obtainable using a filter unit having a honeycomb structure is the highest obtainable.

A typical filter system for industrial applications may have a vessel with an inside diameter of 4.2 metres and a substrate volume of 21 m ³ . This vessel may be fitted with approx. 1385 filter candles having a diameter of 60 mm and a length of 1.5 metres, such as candles manufactured by Schuma¬ cher, Germany. All filter candles are typically hanging from a top plate inside the vessel with a distance of 100 mm from candle centre to candle centre. The calculated filtration

area is 18 m ² surface for each cubic metre of the vessel (18:1) .

In comparison, a honeycomb diesel filter trap as manufactured by Corning Inc. USA has a theoretical filtering area of 500 m ² for each cubic metre of the filter (500:1) . Even though this is obtained using extremely thin walls and a cell width of only 2.6 mm, this unit presently has the highest filtering surface/volume ratio in the world. However, it is presently not possible to manufacture single-piece CelCor filters, or other similar filters, with a diameter larger than 0.3 m and with a length of more than 0.3 m. The reason for this is that the gas/wall friction prohibits particle transfer to the bottom of a longer channel, whereby the remaining part of the channel will remain ineffective. The only solution to this problem is to increase the cell width, such as from 2.6 mm or 10 mm to 40 mm, so as to retain a cell size/cell length ratio of 1:125, which is preferred for diesel filters and 1:50 which is preferred for dust filters.

Thus, the filter unit of the invention, preferably having the structure as a giant honeycomb, is the best way of obtaining improvements in the important surface area/volume ratio. In fact, surface area/volume ratio of 45:1 may be obtained with a cell width of 40 mm and a wall thickness of 4 mm.

Thus, an increase in the area/volume ratio with a factor of two and a half may easily be obtained compared to filter systems using filter candles.

Even though the above-mentioned improvement is large, the above-mentioned example may not be the optimum ratio for all applications; it is merely an example of the advantages obtainable using a filter unit according to the invention.

The table of page 14 gives information, such as the specific filter area, for a comparison of filter elements according to the present invention, a NGK single honeycomb for e.g. die

Substrate manufacturer NGK Ltd. NoTox A/S NoTox A/S Schumacher Cerel

Type Honeycomb Honeycomb Honeycomb Candle Candle

Brand name Dust-Trap Dust-Trap DIA-Schumalith Cerafil

Filter substrate dimension 10x10xL500 20x20xL1500 40x40xL1500 060xL1500 0200xL1500

Vessel inside radius - m 2,1 2,1 2,1 2,1 2,1

Vessel inside diameter - m 4,2 4,2 4,2 4,2 4,2

Vessel inside length - m 0,5 1,5 1 ,5 1,5 1 ,5

Wall thickness - mm 1 2 4 15 20

Number of total cells across the center line 420 210 105 42 14

Candle diameter - mm 60 200

Cell spacing Center to Center - mm 10 20 40 100 300

Open side Width per Cell - mm 9 18 36

Effective filtration surface area per Cell - m2 0,02 0J 1 0,22 0,28 0,95

Total frontal face - m2 13,9 13,9 13,9 13,9 13,9

Number of substrate cells per m2 10000 2500 625 100 11

Number of inlet filter cells / candles 69.458 17.364 4.341 1.389 154

Effective total filtration area - m2 1250,24 1875,35 937,68 393,82 145,86

Filter substrate volume - m3 7 21 21 21 21

Specific Filter area - m2/m3 180,00 90,00 45,00 18,90 7,00

Filter area - m2/kg 0,3 0J 1 0,28 0,06 0,28

selgas filtration and two filter candles for industrial applications.

From the table on page 14, it is seen that the highest filter area/volume ratio is found in the NGK substrate. However, as described above, these substrates can not be manufactured in lengths larger than en the order of 0.5 metres. Thus, the filter area/volume is high for this type of substrate, but the total filter area is too small for industrial applica¬ tions.

However, it is clear that the Dust-trap filter elements according to the invention offer a large improvement compared to the filter candles, as the filter elements according to the invention obtain a filter area/volume ratio of 45-90, whereas the filter candles obtain a ratio of 7-19. Thus, the advantages of the filter elements and the filter unit accord¬ ing to the invention are obvious.

Thus, the present invention provides large, highly efficient filter units for industrial use and which are more compact and have a lower weight than the presently used standard filter systems. The weight reduction is important for boiler systems where load transients are common, such as emergency and peak-power electrical plants, as a lower thermal mass in the filter unit reduces the response time thereof. A rough calculation shows that the total weight of a filter unit according to the invention may be as low as only one third of that of a known candle based filter system.

A filter unit may be internally split into filter groups comprising a multiple of filter elements. This may be prefer¬ red in order to be able to clean one group at the time. In this manner, it will be possible directly to prevent the flow of gas from being filtered during cleaning. This may have certain advantages; the flow of gas to be filtered is oppo¬ site that of the cleaning gas pulse, whereby a stronger cleaning gas pulse is required in the presence of a reverse

gas flow. The filter groups in the filter unit may be mounted in parallel or in series, either side by side or above each other, depending on the manner in which the filter groups are to cooperate.

In order to prolong the period of time between two cleaning operations of a filter element or to facilitate the use of a smaller filter unit, the filter unit according to the inven¬ tion may be positioned down-stream of another particle remov¬ ing device. This other device may be a relatively low-effi- ciency particulate control device, such as a cyclone, which performs a first phase dust separation. A tangential flow inlet cyclone may remove the majority of particles, whereby the filter unit will be subjected to a reduced dust load prolonging the time between cleaning operations or allowing the use of a smaller filter unit. In fact, this combined set¬ up may offer a more compact installation taking the volume of the cyclone into account.

The vessel, chamber or container containing the filter unit may be manufactured from heavy gauge steel and may preferably be insulated by either a refractory or another suitable material, in order to reduce the thermal mass of the filter unit and in order to reduce the specific heat loss from the filter unit.

The design of the gas inlet and gas outlet of the vessel may benefit from flow optimalisation, such as by using cones, baffles, manifolds or other suitable apparatus to direct the gas flow and ensure a more even pressure load of the individ¬ ual filter elements.

An interesting topic in the field of gas filtration is the use of catalytically active coatings. Catalytically active coatings have for many years been used to obtain reductions in oxides of nitrogen, etc. However, catalytically active coatings may be used for many other purposes. Thus, it is contemplated that the surface structure of the filter

elements according to the invention may function as a support for a catalytically active coating and, thus have a filter body performing not only a filtration but which may simulta¬ neously perform a multitude of other gas filtration or gas cleaning functions.

A problem encountered in the field of catalytically active coatings is the problem of the filtered particles forming a layer on the catalytically active coating and, thus, reducing the efficiency thereof. This problem may be solved as dis- closed in Danish patent application DK 1099/93, where the catalytically active coating is coated on at least the up¬ stream surface of the filter element and preferably on the total surface of the filter element. As the particles fil¬ tered by the filter element will only travel a small distance into the porous material of the filter element, the catalyti¬ cally active coating on the rest of the surface of the material will remain active during the filtering operation.

As the coating is preferably applied internally in the struc¬ ture (on the surface of each individual grain in the struc- ture) the optimum contact between the gas flowing through the porous material and the catalyst is obtained.

Alternatively, the filter elements may be manufactured with ribs for enlarging the potential surface of the coating. Catalytic coatings may also be coated on separate structures, which are introduced in down-stream sections of the present filter unit. In this case, the base material or substrate for the catalytic coating may be chosen to be different from that of the filter material and, thus, be chosen to be more appro¬ priate for the actual coating process than the filter material itself.

Further alternatively, beds or pellets comprising a catalyti¬ cally active material may be filled into the open volume between walls of the filter elements or between the individ¬ ual elements themselves.

An alternative use of a catalytically active coating is the use thereof for the reduction of the combustion temperature of, e.g., carbon particles filtered from the combustion gases of a diesel engine.

Large marine diesel engine electrical power plants of between 1 MW and 50 MW (installed power) are currently not fitted with particulate filters at all, as the current diesel particulate filter technology is not able to produce filters large enough to cope with the large exhaust gas flow from large engines. However, using a filter unit according to the invention, a filter of a suitable size may be manufactured also for such applications.

Particles from diesel engines consists mainly of carbon, which may be oxidised at elevated temperatures (in the order of 550°C) . However, using a catalytically active coating, the regeneration temperature may be reduced to on the order of 400°C. Due to the use of low cost diesel fuel having a high sulphur content, non-organic matter from fuel and lubrication oil is also known to be present in considerable amounts in combustion gases from diesel engines. In order to remove the accumulated metal oxide ashes, the filter substrate will need periodical cleaning or alternatively, it may be cleaned using a built-in catalytic process.

Embodiments of the present invention will now be described with reference to the drawing, wherein

Fig. 1 is a partly cut away view of a first embodiment of a filter unit according to the invention with a simple can¬ ning,

Fig. 2 is a side elevational cross-section of an embodi- ment of a filter system comprising a filter unit according to the invention,

Fig. 3 is a cross-section of the embodiment of Fig. 2 in which the filter cleaning system is removed,

Fig. 4 is a cross-sectional view of first embodiment of a sealing construction of a filter element according to the invention,

Fig. 5 is a cross-sectional view of second embodiment of a sealing construction of a filter element according to the invention,

Fig. 6 is a cross-sectional view of third embodiment of a sealing construction of a filter element according to the invention, - Fig. 7 is a cross-sectional view of third embodiment of a sealing construction of a filter element according to the invention,

Fig. 8 is a cross-sectional view of a filter unit accord¬ ing to the invention, - Fig. 9 illustrates a cross-section of four embodiments of sealing elements for use in a filter unit according to the invention,

Fig. 10 illustrates a cross-section of four assembled filter elements according to the invention having sealing elements therebetween,

Fig. 11 is a cross-sectional view of an alternative filter element for use in the assembly of filter elements of Fig. 10,

Fig. 12 illustrates a cross-section of three assembled filter elements according to the invention having sealing elements therebetween,

Fig. 13 illustrates a cross-section of a number of filter elements according to the invention and of a suitable sealing element therefor, - Fig. 14 illustrates a cross-section of a number of filter elements according to the invention and of a suitable sealing element therefor,

Fig. 15 is a cross-sectional view of an alternative filter element for use in the assembly of filter elements of Fig. 10 and of suitable sealing elements therefore,

Fig. 16 is a partly cut away view of a first embodiment of a single-piece honeycomb filter.

Fig. 17 is a partly cut away view of a second embodiment of a single-piece honeycomb filter.

Fig. 18 is a partly cut away side view of a filter unit according to the invention comprising four honeycomb-shaped filter blocks,

Fig. 19 is a first embodiment of a carrier element for a catalytically active coating, and

Fig. 20 is a second embodiment of a carrier element for a catalytically active coating.

The filter unit of the honeycomb structure of Fig. 1 is illustrated with a part 12 of the canning 11 cut away and seen from the down-stream side 13. A total of 43 individual filter elements 16 having a triangular cross-section may be seen at the down-stream side 13. However, any number of filter elements may be incorporated in a filter element according to the invention, such as from a few to more than 1000. Each of the filter elements are provided with an element plug or end cover 14 to ensure that the fluid is forced through the filter wall and to give the filter unit the honeycomb structure.

Seals 15 are located between contact points between the filter elements and between the filter elements and the canning 11.

It may be seen that part of the filter elements 16 have an end plug not similar to the end plug 14 of the most common filter element in the filter unit. These special end plugs are designed in order to allow the filter unit to have a circular cross-section. Naturally, the shape or cross-section of the filter unit may be chosen freely and, thus, be square or any other suitable, e.g., even non symmetrical, shape.

In Fig. 2, a filter unit 23 is positioned inside a steel chamber 21 designed for use at high pressure. A gas inlet or duct 22 is preferably located at the bottom of the vessel, in order to keep the dust and the dust loaded gases below the

filter unit 23. An inlet diffuser or cone 24 is introduced for optimisation of the gas flow, before the gas reaches the filter unit 23, so as to obtain a more laminar flow. A sup¬ port arrangement 25 transfers the weight of the many filter elements of the filter unit to the steel construction of the filter system.

Cleaning of the filter elements of the filter unit may be performed by back flushing with compressed fluid delivered by a pressure fluid tank 26 and a pipe system 27 that directs a short pulse (which is controlled by a (valve not shown) ) of pressurised fluid through the open outlet channel so as to loosen the dust collected on the inside up-stream side of the inlet channels. Subsequent to loosening of the duct, gravity will lead the loosened dust cakes down to a dust collecting hopper 28 positioned beneath the filter unit 23.

The gas outlet 29 of the filter system may be located in the side or at the top of the chamber depending on space require¬ ments of the system.

In Fig. 3, the system of Fig. 2 is seen from the top, where the back-flushing system is removed. It is seen that the giant honeycomb filter unit 31 (reference numeral 23 in Fig. 2) takes full advantage of all the available space in the vessel. In the circumference between the filter unit 31 and the refractory wall 32, an expansion tolerant material 33, which may additionally be thermally insulating, is canning the honeycomb filter unit 31. Especially designed filter elements may be required at certain locations in the filter unit in order to ensure that the honeycomb structure takes full advantage of the vessel, such as where there is not enough space for a complete filter element. Furthermore, it may be preferred to add an additional layer of thermal insu¬ lation outside the vessel walls.

Fig. 4 illustrates a sealing structure in which a free stand¬ ing filter element 41 is supported on a support plate 43. The

filter element 41 is positioned on a conically shaped sealing element 42 which is subsequently compressed by the filter element 41 for complete sealing between the filter element 41 and the support structure.

As shown in Fig. 4, the filter element 41 may be manufactured in such a way that a separate top support plate is not required.

Figs. 5-7 show alternative embodiments of sealing structures 51, 61 and 72 of the type seen in Fig. 4.

Compared to the filter elements illustrated in Figs. 4 and 6, the filter elements illustrated in Figs. 5 and 7, are of a type in which a separate top cover 52 is required. This top cover may be made of the same material as the filter elements, or, alternatively may be formed directly from the wall material of the filter element.

Expansion of the filter elements in the axial, longitudinal direction will naturally cause no problems, as the elements can expand freely in this direction.

Tests have indicated that carefully designed bottom support plates 43, 53 with suitably shaped gas channel inlets may reduce turbulence at this position and, thus, lower the total system pressure drop by 2-3%.

The filter elements 82 of Fig. 8 are supported by a support¬ ing grid 83 through support blocks 81. This embodiment offers reduced flow resistance, as the filter elements are supported only at positions where the gas velocity is low. The support blocks 81 may be reinforced so as to be able to transfer the weight of the filter elements 82 standing thereon to the lower support grid 83. The support blocks 81 may optionally be made of a porous material so as to also contribute to the total filter surface area of the filter unit. Thus, the structure is in principle self-supporting.

The support blocks 81 may be formed as an integral part of the filter unit or they may be made separately and then cemented, clamped or otherwise joined to the filter unit. The support blocks 81 may additionally be used at both ends of the filter unit for sealing alternate channels at alternate ends.

The support blocks 81 may have a conical, hemi-spherical or pyramidal form with a flat area on the inside of the apex on which a catalytically active insert may be supported. This insert may be of the type shown in Fig.s 19 and 20. Further¬ more, gas seals 84 are preferably mounted between the blocks 81 and the filter elements 82. Support blocks or channel plugs placed at the down-stream side of the filter channels may additionally incorporate Venturis for the cleaning sys- tern.

In Fig. 9, different embodiments of sealing elements for longitudinal support or centering are illustrated. These sealing elements may, e.g., be fabricated from a ceramic material or a composition of ceramic, organic materials and metals, preferably in the form of fibres. The material should be temperature and corrosion resistant and able to tolerate the radial expansion of the filter elements. Alternatively, sealing elements may be formed by encapsulating fibres in a wire mesh made from a heat resistant metal, such as Inconel .

In Fig. 10, four filter elements 101 having a triangular cross-section are juxtaposed around sealing elements 102. A filter element having a triangular cross-section, has a larger surface area per unit volume than that of cylindrical- ly shaped elements. The triangular section may, at each corner, incorporate part of a circle or radius so that circu¬ lar sealing elements 102 may be used, when the corners from three individual elements 101 are positioned around the sealing element 102. The sealing element 102 may, additional¬ ly to sealing the elements 101, guide and support the elements 101 and absorb any possible thermal expansion or

prevent mechanical forces from travelling from one element 101 to the neighboring element 101.

As seen in Fig. 11, several internal support walls 112 may be incorporated in the filter elements 101 of Fig. 10, so as to form the filter element 111 of Fig. 11 in which the strength required during high pressure back-flushing may be obtained.

In Fig. 15 an alternative embodiment of a triangularly shaped element with internal stiffening structure is shown together with suitable sealing elements 151.

In addition to increasing the strength of the filter element 111, the fins 112 may be used as carriers for, e.g., a cata¬ lytically active coating. Depending on the direction of flow of the gas to be filtered, the fins 112 should be incorpor¬ ated on the inside or the outside of the element 111.

An important feature of the disclosed embodiments of the invention is that the up-stream and down-stream regions of the filter unit are only separated by real filter walls and not by any seals or the like in the longitudinal direction. Thus, optimum exploitation of the filtering walls of the filter is obtained.

Similar to Fig. 10, filter elements 121 having square cross- sections are juxtaposed in Fig. 12. The elements 121 have connection points at each corner adapted to circular sealing elements 122. Again, the sealing element 122 will ensure that no thermal expansion loads are transferred to neighboring elements 121.

Also in this embodiment internal support walls may be incor¬ porated for increasing the structural strength or to facili¬ tate the extrusion process of the element 121.

In Fig. 13 a number of round filter elements 131 with inter¬ nal supporting walls 132 are shown. Separated by 120° three

outside connection points 133 for connection to the neighbor¬ ing elements 134 through sealing elements 135 are shown. The internal 132 walls transfer and distribute the mechanical forces acting on the elements 131. The spaces between the elements are preferable for use as inlet channels, carrying the dust-loaded gas. The filter elements 131 are preferably used so that the particles are filtered on the outside there¬ of, i.e. so that the gas outlets and the down-stream side of the filter elements are on the inside of the elements 131.

In Fig. 14 an alternative design of filter elements 141 are illustrated together with a suitable sealing element 142.

In Fig. 16 a single-piece honeycomb filter building-block 161 is illustrated which has a channel closing 172 according to that disclosed in Danish patent application DK 0402/93. This way of closing the filter element channels gives funnel- shaped gas entrances and gas exits which may be preferred due to the improved gas flow characteristics obtained thereby.

Fig. 17 shows a single-piece honeycomb filter building-block 171 in which the channels have been closed by plugs 172. This way of closing the channels, however, has the disadvantage that part of the filtering walls 171 will also be blocked, leading to a reduction in the total filtering area of the filter element 171.

Fig. 18 illustrates a large filter unit assembled from four honeycomb building blocks 181. In between the blocks 181 an insulation material 182 protects the individual blocks from mutual mechanical contact and isolates the blocks 181 from thermal expansion in other blocks 181. The insulation material 182 is preferably based on ceramic fibres, including a Vermiculite expanding compound or a composite of different ceramic fibres. From 4 to 600 blocks 181 may be assembled into a single filter unit. The shape of the filter blocks 181 may vary between triangular, 4, 5 or 6 sided elements.

Figs. 19 and 20 illustrate embodiments of suitable structures for supporting a catalytically active coating which is intro¬ duced in the filter unit. These structures may either be an integral part of the filter elements of the filter unit or they may be separate individual elements, such as elements especially suited for a specific coating. By shaping the substrates according to the structure of the filter elements, the substrates may slide into the down-stream or outlet channels or be placed in a space between the inlet channel elements of the filter unit.

Example 1

SiC substrates were manufactured according to the method disclosed in patent application EP 0 336 883, where a con¬ tinuous extrusion process was used. The compound comprised 69-72 wt% very inexpensive, commercially available Mesh 180 SIKA I SiC grinding grains having a particle size of 75-105 μm from Arendal in Norway and 4-13 wt.% ultra fine SiC having a particle size of less than 2 μm, mixed into a plastic paste further comprising 4-6 wt% Methyl Cellulose from Hoechst, 8- 25 wt% water and 0-12 wt% ethanol.

The compound was extruded in a water cooled single screw auger extruder with a vacuum chamber through a die head. The extrusion speed was from 1.5-2 meter pr. minute.

After a very high temperature sintering process taking place at a temperature between 2200°C and 2600°C in a protective atmosphere, such as Argon, the structure became a low den¬ sity, rigid and highly porous filter element.

The properties of the SiC (Silicon Carbide) based filter material are characterised by an extremely high thermal conductivity (10-30 W/m ² K) , giving a thermal shock resistance three times greater than that of, e.g., Cordierite filters. The decomposition point of SiC was found to be around 700°C

higher than the melting point of Cordierite (1300°C) . The thermal expansion coefficient was measured to be 4 μm/10 ^6o C

The high strength of the filter element is obtained due to the use of ultra fine SiC powder as the ceramic binder. This ultra fine SiC powder evaporates at the high sintering tem¬ perature and condenses at the grain. contacts between the larger Mesh 180 grains. Thus, the structure obtained by this method becomes a pure SiC structure.

A noticeable feature, when working with large grains is the very low shrinkage during sintering; below 1%. The high tolerances thereby obtainable are extremely important when the filter units are to be assembled from many similar but individual filter elements.

Several different filter elements were produced from a powder having a very controlled grain size with different cross- sections and having diameters of up to 100 mm. The filter elements were characterised by an extremely homogeneous and controlled pore size and distribution, measured to be 40 μm, which gives a filtering efficiency that has been measured to be on the order of 80% and better than 98% using a.membrane made of 5 μm particles.

In this example, the pores were formed, and their size was determined by, the grain size of the base material. As an alternative production method, the pore size may be con- trolled by incorporating a pore forming agent into the ceramic compound before the extrusion process. The pore- forming agent typically consists of an organic material, such as in the form of a powder, granulate or fibres, that can be removed by oxidation, either before,- during or after the sintering process.

The sintering temperature for SiC may be lowered by adding a few percent of additives to the raw compound. The additives should be included in the green body and be sintered together

therewith. Additives such as Carbon Black, amorphous Boron, Aluminum, Beryllium, Silicon and Aluminum Oxide are known as sintering additives that lower the sintering temperature. Using Aluminum, it is possible to sinter SiC powder at a temperature of 1900-2200°C. Furthermore, the mechanical strength of the sintered element may be increased noticeably by adding Aluminum as an additive. A protective sintering atmosphere such as Nitrogen may also influence the sintering temperature and the specifications of the final product.

Alternatively, SiC powder based filter substrates may also be manufactured using the well known reaction sintering method, where bonding takes place at temperatures in the region of 1600-2000°C.

Example 2

Oxide-based ceramic substrates were manufactured from Cordie¬ rite, Spodumene and Mullite compositions. The ceramic precur¬ sors are listed in Table 1.

Table 1. Ceramic precursors. wt%

Mix A Mix B Mix C

Cordierite Spodumene Mullite

China Clay grade E (APS 2-3) 4 400..44 6 655..88 51.5 Talc (APS 1-4) 43 . 6 - A1 ₂ 0 ₃ CT 3000 SG (APS 0.4-0.6) 1 166..00 - - 48.5

Si0 ₂ Fyleverken (APS 3-6) - 15 .3 i ₂ C0 ₃ anal. quality - 18 .9

As binder/plasticiser, a methyl-hydroxy-ethyl-cellulose was used (Tylose MH 300 P from Hoechst) . In the case of Cordieri¬ te and Spodumene ceramics, the precursors were cal¬ cined/sintered to a grog and crushed into a coarse-grained partly porous powder with a particle size of less than 156 μm

and an average particle size (APS) on the order of 10 μm. The green body compounds were mixed according to Table 2, where the dry elements were mixed for 30 min. Ethanol was subsequently added, and after another 10 min. of mixing, the water was introduced, and the final paste was mixed for another 30 min. As a pore-forming agent, a filler of poly¬ styrene spheres (Shell N 2000) was added along with the dry raw materials. Size fractions from 800 μm to <200 μm were tested. The spheres pack to a dry tap porosity of 40 vol% which results in a max. filler/compound ratio of 0.68.

The compounds were extruded in a single screw auger extruder with a vacuum chamber, through a die head. The extruded bodies were dried at ambient temperature and humidity, and sintered in an electrical furnace with a normal atmosphere according to Table 2.

Table 2. Green body compounds.

Cordierite Spodumene Mullite

Mix A 46.5 - -

Mix B - 48.5 -

Mix C - - 73.7

Tylose 8.6 9.0 0.7

Water 15.7 11.1 9.0

Ethanol 29.2 31.4 16.7

Filler/Compound 56-64 58-62 58-64

(vol/vol)

Sintering temp. 'C 1340 1270 1400

Linear shrinkage % 5.4 6.7 7.1

Aluminum Oxide fibre coatings were applied as a thin membrane having a thickness of 0.1-0.2 mm and a pore size of 2-10 μm have been tested. The membrane was applied to the structure as a fluid comprising 80% water, 10% MH 300 P and 10% fibres which was flushed through the porous filter element. Tests indicated that separation efficiencies as high as 99.99% for ashes may be obtained using filter elements of this type.

Example 3

SiC substrates were manufactured according to Example 1, and comprising 69-72 wt% very inexpensive commercially available Mesh 240 SIKA I SiC grinding grains from Arendal in Norway and 4-13 wt% ultra fine SiC, mixed into a plastic paste further comprising of 4-6 wt% Methyl Cellulose from Hoechst, 8-25 wt% water and 0-12 wt% ethanol.

The paste was extruded in a water cooled single screw auger extruder with vacuum chamber through a die head and at an extrusion speed of 1.5-1.8 meter pr. minute.

Several different designs were produced from a powder having a very controlled grain size and with different cross-sec- tions and diameters and with maximum cross-section of 100 mm.

The elements were characterised by an extremely homogeneous and controlled pore size and distribution, measured to be 20 μm, which gave a filtering efficiency measured to be around 98% prior to the application of a membrane. Furthermore, even the addition of a catalytically active coating on the surface of the filter elements will reduce the pore size of the element and, thus, increase the filtering efficiency thereof.

Membranes have indicated that filtration efficiencies in the region of 99.8% or better may be obtained. The membranes were produced on the basis of oxide fibres of a controlled diam¬ eter from 1-100 μm and a length of 0.1-10 mm, preferably a

diameter of 5-30 μm and a length of 0.2-5 mm. The coating thickness was 0.05-0.5 mm, preferably 0.1-0.2 mm.

Example 4

High alloy metal powders such as AISI 321 or 310 have a good corrosion resistance and may, thus, be used in particle filters in certain applications. A compound comprising 69-72 wt% commercially available Mesh 240 powder from BSA, Ametek, or the like, mixed into a plastic paste further comprising 4- 6 wt% Methyl Cellulose from Hoechst, 8-25 wt% water and 0-12 wt% ethanol.

The compound was extruded in a water cooled single screw auger extruder with a vacuum chamber through a die head and at an extrusion speed of 1.5-1.8 meter pr. minute.

In order to remove the binder completely, the binder removal was performed in a oxidising atmosphere at 350-500°C for 30 minutes.

The sintering was performed in a electrical resistance heated furnace with a dry Hydrogen atmosphere at 1100-1400°C for 20- 60 minutes.

Experiments have shown that individual filter elements for the invented giant honeycomb filter may also be fabricated from metal powders or metal fibres. This material may be shaped directly to giant honeycomb filter elements by extru¬ sion or isostatic, uni-axial compacting. Alternatively, the powder or fibre may be pre-shaped to sheets or tubes, receiv¬ ing after-treatment by forming, bending, welding or pressing, into filter elements according to the invention.

In addition, woven ceramic or fibre-ceramic composite materials or metal fibre fabrics, clothing, felts, plates or boards or tape-casted ceramic boards can also be mechanically shaped into giant honeycomb filter elements and made rigid by

coating or soaking with binders, coatings of various kinds followed by a sintering or calcination process.