Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D46/00—Filters or filtering processes specially modified for separating dispersed particles from gases or vapours
  • B01D46/30—Particle separators, e.g. dust precipitators, using loose filtering material
  • B01D46/32—Particle separators, e.g. dust precipitators, using loose filtering material the material moving during filtering
  • B01D46/38—Particle separators, e.g. dust precipitators, using loose filtering material the material moving during filtering as fluidised bed
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D46/00—Filters or filtering processes specially modified for separating dispersed particles from gases or vapours
  • B01D46/42—Auxiliary equipment or operation thereof
  • B01D46/48—Removing dust other than cleaning filters, e.g. by using collecting trays
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
  • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
  • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
  • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
  • F01N3/24—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by constructional aspects of converting apparatus
  • F01N3/28—Construction of catalytic reactors
  • F01N3/2803—Construction of catalytic reactors characterised by structure, by material or by manufacturing of catalyst support
  • F01N3/2832—Construction of catalytic reactors characterised by structure, by material or by manufacturing of catalyst support granular, e.g. pellets
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27B—FURNACES, KILNS, OVENS, OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
  • F27B15/00—Fluidised-bed furnaces; Other furnaces using or treating finely-divided materials in dispersion
  • F27B15/02—Details, accessories, or equipment peculiar to furnaces of these types
  • F27B15/10—Arrangements of air or gas supply devices
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D17/00—Arrangements for using waste heat; Arrangements for using, or disposing of, waste gases
  • F27D17/008—Arrangements for using waste heat; Arrangements for using, or disposing of, waste gases cleaning gases
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2279/00—Filters adapted for separating dispersed particles from gases or vapours specially modified for specific uses
  • B01D2279/30—Filters adapted for separating dispersed particles from gases or vapours specially modified for specific uses for treatment of exhaust gases from IC Engines

Definitions

• the present invention relates to a suitably arranged fluidized bed device for the removal of fines, in particular fines of the classes PMi 0 and PM 2 .5, from gas exhausts downstream of combustion processes, both of an industrial (smokestacks) and of a residential (chimneys) nature.
• the invention also relates to the method of operation of the device.
• the fractions of particles having dimensions of less than 10 micrometers (pm) or of less than 2.5 pm, defined in the field with the abbreviations PM 10 and PM 2 .5, are especially harmful to human and animal health due to their ability to remain suspended in the air for long periods of time, to form nuclei for condensation of toxic substances suspended in the atmosphere and to be inhaled through breathing. In this way, these particles give rise to direct toxicity problems and to high-risk conditions that include, among other things, the risk of carcinogenic, teratogenic and mutagenic processes. It is therefore extremely important to successfully abate the maximum possible quantity of particulate from the gas exhausts before emission of the latter into the atmosphere.
• a first method devised was the use of filtering systems, both of the so-called “sleeve” type, wherein the filtering section comprises an enormous fabric sleeve that is kept inflated by the pressure of the carrier gas; and of the “ceramic filtration” type, wherein the filtering section is a rigid part made of porous ceramic material.
• these systems present several problems. Firstly, in order to retain PM10 (or worse PM2.5), these systems must have a porosity of extremely reduced dimensions; this causes considerable pressure drops in the flow of the exhaust gas.
• the problem gets worse and worse as filtration progresses, because the particulate that has already been blocked by the filters tends to obstruct the porosity thereof, thus further increasing the pressure drops and negatively affecting the processing upstream of the exhaust (for example, appreciably exacerbating the combustion characteristics of the boilers).
• the problem is particularly serious in ceramic filters.
• the "sleeve" filters are less critical in this regard, but because they are generally made from organic materials, their application is often impossible on account of the high temperatures of the gas exhaust to be treated or due to the presence of corrosive elements in the exhaust.
• Another method of removing the particulate is through the use of electrostatic precipitators, which when duly inserted along the gas flow, subject it to electrical fields having values typically of 25,000 Volts or higher; a "corona effect" is thus generated in the gas, which electrostatically charges the particulate, enabling its removal by means of an appropriate electrical field.
• electrostatic precipitators are very effective, being able to achieve particulate removal of the order of 90%, but their operating cost per gram of removed particulate is extremely high and is prohibitive for use in gas flows with high particulate loads (for example, loads in excess of 5 g of particulate per cubic metre of gas exhaust).
• wet abatement systems commonly known as "scrubbers". These are typically vertical towers wherein the flow of gas exhaust rises from low to high and meets, in counter-flow, a shower of a liquid, commonly water; the droplets of liquid entrap the particulate and transport it towards the bottom of the tower by gravity.
• wet systems are the Venturi system, wherein the flow velocity of the exhaust is increased, causing it to pass through a narrowing and simultaneously nebulizing a liquid (the most common one is water) that retains the particulate in a manner analogous to that occurring in scrubbers; this system however requires a particulate settling chamber, which limits its applicability to exhausts of relatively low flow.
• the wet systems are also impacted by two other general problems, i.e. the need to use high volumes of water and the production of toxic waste sludge that is difficult to dispose of.
• US patent 5,198,002 describes a mechanical filtering system for a gas flow contaminated by fine solid material entrained in suspension.
• the system described is based on a filter that is formed "in-situ" within a special vessel containing a fluidizable bed of granular material of relatively large size compared to the material transported (fines) in suspension in the gas flow.
• the filter which is within the vessel, consists of a composite device formed by a rigid support (tube), which communicates, at one end, with the outside of the vessel.
• the tube is suitably perforated and covered by a porous membrane, on which a layer of granular material of the fluidized bed is formed.
• the filtering action on the fines is exerted by this layer of granular material, kept in place above the other filtering components of the device (the porous membrane and the perforated support tube) by the pressure drop that occurs across the filter itself.
• the material transported (fines), separated by filtration from the gas flow, remains in the filtering layer, from where it must be removed.
• the filter cleaning operation adopted comprises the temporary interruption of the gas flow, which causes the granular material to detach from the porous membrane and fall into the vessel of the fluidized bed.
• the layer readily reforms upon reactivation of the pressure of the gas flow.
• the active element of the US 5,198,002 system is thus a reversible filtering layer, composed in situ from granular material of an appropriate granule size.
• Japanese patent application JP 2001-259329 describes a method for the collection of fines transported by a gas flow based on the use of a specially designed fluidized bed.
• the document defines the mechanism for cleaning the gas flow generated by the gasification of coal used as the turbine fuel.
• the mechanism for removing particulate from the gas uses granular material (disposed within a relatively large vessel), which material has dimensions of preferably between 200 and 1000 micrometers and is preferably monodispersed, which is activated to an actual fluidized bed by the gas flow itself.
• the particulate present in the gas flow is retained on the surface of the granular material by adhesion; said granular material is gradually saturated by the particulate.
• the patent application teaches the periodic removal of a constant fraction of contaminated (used) granular material from the fluidized bed and its replacement with an equal quantity of fresh granular material, through dedicated channels.
• the method illustrated in this patent application represents a significant improvement over the prior art for the cleaning of fines from gas flows at high temperature, from 400 to 1000 °C for example, which thus justifies its application.
• US patent 2,548,875 describes. a method for the use of a fluidized bed in solid/gas reactions, wherein the solid component is typically a catalyst, and represents an improvement over analogous prior systems that necessitated systems for the final separation and recovery of the solid in suspension, consisting in general of a catalyst in its finest form.
• a relatively large filtering surface is employed, generally a ceramic or preferably metal filter membrane, a large part of which is immersed in the zone of the fluidized bed wherein the latter has the highest density.
• the membrane is kept relatively free from solid stratifications by the mechanical action of the fluidized bed itself through the collision of the particulate material having the largest dimensions; the material continuously removed from the membrane re-enters the fluidized bed; given the particular application, the removal of fine particulate by the system is not envisaged.
• the invention described in this patent therefore combines a filter of traditional type and a fluidized bed system, and allows the fluidized bed to remain in conditions of dynamic equilibrium, i.e. not to lose material.
• This system is, however, limited to this type of application, wherein the original gas does not transport a particulate in suspension: the only particulate in suspension consists of the finest component of the fluidized bed itself. In systems wherein the gas transports a particulate (as occurs in combustion fumes), this system is not applicable as it would become rapidly blocked by the accumulation of material.
• the aim of the present invention is to provide a device and a method for the abatement of particulate from gas exhausts capable of operating effectively, and at a cost compatible with the application, in the common conditions that arise in industrial or residential gas exhausts.
• the present invention in a first aspect thereof relates to a fluidized bed device for the abatement of all types of particulate from all types of gas exhausts, even at high loads of particulate, which comprises:
• a chamber for containing the fluidized bed connected in its lower part to an arrival pipe of the exhaust gas to be treated and in its upper part to an opening or pipe for the outlet of the purified gas, said chamber having a cross-section of width higher than that of the arrival pipe of the exhaust gas to be treated, and being provided with lateral holes having dimensions lower than those of the material that forms the fluidized bed, for discharging outside the abated particulate;
• a porous or microperforated support for the material forming the fluidized bed upstream of said lateral holes in the direction of flow of the gas exhaust, a porous or microperforated support for the material forming the fluidized bed, the pores or microholes of which have dimensions higher than those of the particles of the particulate present in the exhaust gas and lower than those of the granular material of the fluidized bed;
• the material for forming the fluidized bed is made of granules of essentially spherical shape of an aerogel realized with silicon and/or germanium oxide or with mixed oxides based on silicon or germanium oxides.
• Fig. 1 represents, in a schematic cross-sectional view, the vessel in which the active elements of the device of invention are housed;
• Fig. 2 represents a top view of the support for the material of the fluidized bed
• Fig. 3 is a schematic representation of the material of the fluidized bed
• Fig. 4 represents, in a view analogous to that of Fig. 1 , a device of the invention assembled in a first embodiment thereof;
• Fig. 5 represents, in a view analogous to that of Fig. 1 , a device of the invention assembled in a second embodiment thereof;
• - Fig. 6 represents, in a top view and a side view, an element for retaining the material of the fluidized bed
• Fig. 7 represents, in a view analogous to that of Fig. 1 , a device of the invention in a third embodiment thereof;
• Fig. 8 represents a device of the invention in a further embodiment
• Fig. 9 represents a schematic view of a measurement system for evaluating the cost in terms of pressure drop of the particulate abatement results obtained by the device of the invention
• Fig. 10 represents schematically, in a view analogous to that of Fig. 1 , a particularly simple embodiment of the invention.
• the term “device” shall mean the innovative element of the invention, comprising the support of the fluidized bed, the widened cross- section part immediately upstream of this support and the parts immediately downstream the same, before any connection with other pipes for the discharge of the purified gas, naturally in addition to the material forming the fluidized bed;
• the term “system” shall mean an assembly of parts comprising the device and other elements, such as the pipes that carry the gas to the device and downstream thereof.
• Figure 1 is a schematic, vertical cross-sectional view of a possible chamber for containing the fluidized bed, in a basic version thereof.
• the vertical arrows indicate the direction of movement of the gas within the chamber.
• the chamber, 10, has a generally cylindrical form and comprises a pipe, 11 , for the inlet of the original carrier gas; this pipe will generally have a cylindrical cross-section and a diameter appropriate for connection with a discharge pipe in arrival from the apparatus that generates the gas exhaust.
• a part, 13 is connected to the pipe 11 , which part 13 has a generally conical shape that widens upwards from zone 12.
• Part 13 has a series of holes, 14, for discharging the fine particulate intercepted by the fluidized bed into the hopper 15; the holes 14 must have dimensions smaller than those of the granular material forming the fluidized bed.
• the hopper has, for its own hermetic sealing, an element, 16, that can be removed for the periodic discharge of the intercepted particulate.
• the chamber is connected to a pipe, 17, for outlet of the treated gas; the connection can occur for example through a conical connecting element, 18, between the upper edge of the cylindrical part of the chamber 10 and the pipe 17.
• Fig. 1 has been described with reference to cylindrical and conical parts, which represent the preferred shape, but could comprise parts of cross-sections of other shapes, for example the chamber may be prismatic with pyramidal connection parts 13 and 18.
• Figure 2 represents, in a top view, one possible embodiment of the support for the granular material, which, when the device is operational, forms the fluidized bed.
• the support, 20, has a rigid edge 21 , made for example of stainless steel, for the connection inside the part 13 of the chamber of Fig. 1 .
• the edge 21 supports the porous or perforated part 22, shown in the figure as a mesh; the openings of the part 22 (in the case shown in the figure, the perforations in the mesh) must have dimensions smaller than those of the granular material that, when activated, forms the fluidized bed, to avoid this material falling down towards zone 12 of the chamber 10; these dimensions must however be substantially greater than those of the particles of particulate to be abated, to avoid part 22 exerting a filtering effect and being capable of giving rise to cloggings and consequent severe pressure drops in the system.
• Figure 3 is a schematic representation of the granular material that, under operating conditions, forms the fluidized bed.
• the granular material, 30, is shown in the form of small spheres.
• the preferred shape of the granular material for the invention is spherical, monodispersed, which is recommended for all the most stringent applications; "monodispersed" material here means one such that all the granules have substantially the same dimensions.
• the function of the material 30 is to provide the mass effect to block the fines by impact. This objective is ensured by the mass of the granule, which must be insensitive to collision of the individual particle to be abated: in practice, for the aim it is sufficient for the mass of the granule to be some orders of magnitude greater than the mass of the individual particles of the particulate to be abated.
• the granular material 30 is an aerogel in particulate form.
• the aerogels may be composed of silicon oxide, germanium oxide, a mix of the two in any molar ratio between them, or of a mixed oxide comprising silicon or germanium oxide as the main constituent and up to around 10% mole (relative to the sum of metals or metalloids) of th- or tetravalent elements, such as aluminium or titanium for example.
• the aerogels employed in the inventions preferably consist of a single oxide, and yet more preferably are of pure silica. These aerogels are obtained by the well known sol-gel method; for a description of the sol-gel method for the production of aerogels, reference should be made to the book "Sol-gel science: the physics and chemistry of sol-gel processing", by C. J. Brinker and G. W. Scherer, Academic Press, 1990.
• the inventors have ascertained that the best results in terms of the abatement of particulate in a gas exhaust are achieved with the use of aerogels produced using as precursors exclusively alkoxides, such as tetramethyl orthosilicate or tetraethyl orthosilicate (TMOS or TEOS), without addition of precursors in powder form (such as the micrometric silica powder known in the field as colloidal silica, or also as pyrogenic silica or "fumed silica”).
• precursors exclusively alkoxides, such as tetramethyl orthosilicate or tetraethyl orthosilicate (TMOS or TEOS)
• precursors in powder form such as the micrometric silica powder known in the field as colloidal silica, or also as pyrogenic silica or "fumed silica”
• Fluidized beds formed with these aerogels have a very low apparent density, for example below 1/15, relative to the corresponding dense material (in this case fused silica) and present a resistance to wind erosion that is adequate for the requirements of application on the fluidized bed according to the present invention.
• the granules of the material 30 can have dimensions of between 0.1 mm and 20 mm inclusive.
• Device 40 illustrated in Figure 4, is obtained from the union of the three parts shown in Figures 1 to 3, and constitutes a simple embodiment of the device.
• the support 20 can be rested on special tongues or on a continuous ring, but could also be simply supported by the cross-sectional narrowing of the part 13; in any case the connection between the support and the internal wall of the device must be secure, to ensure the stability of the fluidized bed.
• the contact zone between the outer perimeter of the edge 21 of the support and the part 13 must be at a sufficiently upstream point, along the direction of the flow, of the holes 14 to preclude to the carrier gas pathways other than passing though the fluidized bed; if this were not the case, and if the set of holes 14 also extended across the fluidized bed, there would be a possible flow of fluid between the holes 14 upstream and those downstream of the fluidized bed, through the hopper 15; at least part of the flow of gas entering the device could therefore bypass the fluidized bed by this path, and emerge at the outlet of the device itself without having been treated, therefore retaining part of the particulate that should have been abated.
• Figure 5 shows an alternative embodiment of the device of the invention.
• the device for abatement of the particulate, 50 comprises, in addition to the support, 20, a containment element, 60, in the shape of a cap, placed above the granular material of the fluidized bed with the function of modifying the distribution in its upper surface from planar to convex; the element 60 is shown in greater detail in Figure 6, in a top view (upper part of the figure) and a side view (lower part of the figure) respectively.
• the element 60 is formed by an edge 61 , analogous to and having the same function as the edge 21 of the support 20; a porous or micro- perforated part 62 that can be a metallic wire mesh is fixed to the edge 61 .
• a hole 63 suitable for top-loading of the granular material 30 At the centre of the cap is a hole 63 suitable for top-loading of the granular material 30; the hole 63 can be closed with a part (not shown in the figure) of diameter greater than that of the hole and made with the same material as part 62.
• the granular material 30 distributes itself in the space included between the support 20 and the element 60, which results in a path crossing the fluidized bed along its central axis, which is considerably longer than the embodiment illustrated in Figure 4.
• This configuration leads to a more rapid discharge outside of the particulate intercepted in the device under particular conditions, for example when the dimensions of the particulate, its density, or in any case its flowability within the fluidized bed should be such to cause its accumulation in the cross-sections of the fluidized bed that are less activated by the gas flow.
• Fig. 7 illustrates, in a view analogous to that of the preceding figures, a complete device in a third embodiment thereof.
• This device, 70 is analogous to that of Figure 4, but differs from the latter by the presence of an additional element 71 , made from a material impervious to the passage of gas (for example, steel), which has the dual function of defining, together with the hopper, the shape of the fluidized bed, limiting the volume thereof to that close to the hopper, and of channelling thereto all the carrier gas during the particulate abatement operations.
• the element 71 is removable to enable the routine maintenance of the device.
• the device 70 may also comprise in its uppermost part a retention element (not shown in the figure) analogous to element 60, and of a shape such that it may adapt to the presence of the element 71 .
• Figure 8 illustrates a complete device for the abatement of particulate, having a non-axial shape and suitable in particular in industrial applications of considerable dimensions.
• the device, 80 adopts as a wall 81 on which are present the holes 82 for the discharge of the intercepted particulate, an inclined plane connected to the hopper (not shown in the figure) for the collection of the abated fines.
• the wall 81 is fixed to another wall, 83, that can have the geometry of a cross-section of cone or of pyramid, so as to form, together with the wall 81 , a chamber 84 having a cross-section increasing upwards.
• the carrier gas enters the device 80 through the pipe 85 and is forced to pass through the support 86, which supports the material of the fluidized bed (not shown) from below;
• the support 86 is of analogous construction to the support 20, analogously thereto, its external edge must provide a secure seal against the internal wall of the device, and the fixing line between said external edge and the internal wall must be entirely upstream, along the direction of the gas flow, of the holes 82.
• a retention element, 87 having a construction and function analogous to those of the element 60 previously described; in particular, the curved form of the element 87 determines a distribution of the material of the fluidized bed such as to appropriately lengthen the possible paths of the gas from the wall 81 to the parts of the wall 83 more distant therefrom, thus adjusting the length of said paths to the level of activity required in the fluidized bed.
• the purpose of this modulation is that, in operation, the accumulation of fines within the fluidized bed will be greater the closer it comes to the wall 81 ; the resistance of the fluidized bed to the passage of the carrier gas will thus be greater in the areas of accumulation of the fines, i.e.
• the carrier gas After having passed through the fluidized bed in the chamber 84 and the element 87, the carrier gas reaches a chamber 88, connected at height by means of a generally conical or pyramidal part, to the pipe for the outlet of the gas from the device, 89.
• a first important dimension is the cross-section of the chamber in which the fluidized bed is formed.
• the insertion of a fluidized bed into a gas exhaust always constitutes an obstacle to the flow for which the device was originally designed; in the context of the invention, the drawback is offset with a widening of the area of the cross-section of the device, at the level of the fluidized bed and proportional to the reduction in the flow (measured for example in m 3 /hour); for example, if it is observed that the insertion of a certain fluidized bed entails a loss of flow equal to 50%, to maintain the flow of the carrier gas unaltered it is necessary to increase by a factor of two the cross-sectional area of the device in the zone of the fluidized bed: with this arrangement, the linear velocity of the carrier gas will be reduced by 50% of the original value, but the value of the flow remains constant; more generally, a widening of the cross-sectional area by a factor x results in a value for the linear velocity of the carrier gas of 1 /x
• an anti-particulate device designed for a given activation velocity typically causes in the gas exhaust of a heater for residential heating a reduction in the flow of around 1/3 of its value; the flow can be restored to its original value by means of a widening of the cross-section of the device, in the zone of the fluidized bed, by a factor of 3, which in the case of a circular cross- section will result in an increase in radius equal to a factor of 1.732.
• the insertion of the device will have caused no pressure drop to the system of the exhaust gas, but will have reduced the velocity of the carrier gas through the widened cross-section to a third of its value.
• the cross-sectional area can return, without pressure drop, to the original value of the gas exhaust before the abatement, with consequent restoration of the velocity to the original value, as illustrated for example in Fig. 4.
• the second important dimension is that of the granules of aerogel that forms the fluidized bed; this is determined once known the density of the specific material used, and in particular the type of particulate that is to be abated. Indeed, the dimensions of the particulate to be abated determine the settling velocity thereof (i.e. the maximum velocity that the carrier gas can have to allow the fall by gravity of the particles of particulate); this maximum velocity must be at least equal to the activation velocity of the fluidized bed, which depends on the dimensions of the granules of aerogel, as explained in greater detail below with reference to the method of the invention.
• the dimensions of the granules of aerogel determine the dimensions of the openings of the supports 20 and 86, as well as the dimensions of the holes 14 and 82; these dimensions must obviously be smaller than those of the granules of aerogel, to allow the containment thereof within the zone of the device destined for formation of the fluidized bed; on the other hand, these dimensions must be the maximum possible dimensions, to minimise the pressure drops in the case of the openings of the supports 20 and 86, and to promote to the maximum the discharge of the particulate in the case of holes 14 and 82.
• the fluidized bed There may be present, downstream or upstream of the fluidized bed (both inside a device of the invention and external thereto), along the direction of the flow of the gas exhaust, other elements for removal of the particulate, even of a different type, or even gas conversion elements, for example porous baffles the porous walls of which are covered by a catalyst for the complete oxidation of species present in the carrier gas, such as the conversion of CO to CO 2 , or active agents for the desulphurisation and/or the denitrification of the carrier gas.
• a catalyst for the complete oxidation of species present in the carrier gas such as the conversion of CO to CO 2
• active agents for the desulphurisation and/or the denitrification of the carrier gas for this purpose it is also possible to envisage chemical functionalization (for example with appropriate catalysts and chemical agents) of the material of the fluidized bed itself.
• All the materials constituting the device of the invention must clearly have the characteristic of being inert to the exhaust gases at all possible operating temperatures; metal, ceramic or oxide materials are therefore preferably used for the construction of the various components of a device of the invention.
• the invention relates to a method for the abatement of particulate that uses a device previously described, and which comprises, in the case of fines, the operations of:
• the settling velocity of the particulate to be abated i.e. the velocity below which the particulate is no longer entrained by the carrier gas, indicated as V s in the remainder of the text;
• the mass of the granules of aerogel be at least a few orders of magnitude greater than that of the individual particles of the particulate.
• Vs > VA the most common operating condition of the method of the invention is however the one wherein V A > V s .
• the first operation of the method of the invention consists in slowing down the linear velocity of the gas exhaust from Vi to Vc", this operation of slowing down of the velocity of the carrier gas will be defined in the remainder of the text as "conditioning".
• the conditioning must be such that is 1 / 0 Vi > Vc, and preferably such that V c is comprised between 1/ 00 and 1 /500 of Vi; if Vc were equal to too high a fraction of Vi, for example equal to 1/5 of Vi, the slowing down of the flow would be too meagre and would not be useful for the invention.
• the aim of the conditioning is to minimise the difference Vc - Vs; with particulate of the PM-io type, the difference V c - V s remains typically considerable even after the conditioning of V c ; it is however important to acknowledge that the abatement system can operate correctly even in conditions where V c is equal to 50 times Vs; the lesser the difference V c - V s is, the lesser will be the extension in thickness of the fluidized bed necessary for discharging outside the particulate to be abated.
• the velocity Vc must however be greater than the activation velocity of the fluidized bed, which would otherwise act as a normal static filter, with all the pressure drops inherent to filters and with the problem of settling of the fines inside the filter itself; in addition, this velocity must not be so high as to cause the transport of the granules of aerogel (a situation known in the field as "pneumatic transport") and therefore the destruction of the fluidized bed; normally, however, in the operating conditions of the method to which the present invention relates, the flow velocities of the carrier gas within the zone of the fluidized bed are significantly smaller than those that cause the pneumatic transport.
• V s For particulates of relatively large dimensions (or for the larger-dimension fractions of a particulate having a large granulometric distribution), that have values of V s greater than around 5 cm/s, it is normally relatively easy to produce an aerogel in the form of granules that gives rise to a fluidized bed with a value of V A ⁇ V s . In this case, it is sufficient to slow down the flow of the gas exhaust within or immediately before the zone of the fluidized bed to a value Vc slightly greater than Vs; the fact that V c is only slightly greater than V s facilitates blocking of the particulate, and avoids occurrence of pneumatic transport conditions of the fluidized bed.
• V A > V s The most characteristic operating condition of the devices and the method of the invention is however the one wherein V A > V s . This means that, in order to come to activate the fiuidized bed, the velocity of the gas carrier must be conditioned to a value that stays above V A .
• the settling velocity is typically greatly below 5 cm/s and is also substantially below the activation velocity of a fluidizable bed realisable on an industrial scale. Under these conditions, it would therefore seem impossible, a priori, to attain a condition of velocity of the gas exhaust (to be always achieved by means of widening of the pipe cross-section, as mentioned above) such as to allow it to abate the particulate.
• the inventors have observed that, surprisingly, with the systems of the present invention, it is possible to abate particulate operating with values of Vc substantially greater than V s , for example with values of V c equal to 50 times Vs.
• the most innovative characteristic of the method of the invention is that the abatement of the particulate is also operational at velocities of the carrier gas within the zone of the fiuidized bed, V c , significantly greater than the settling velocity of the particulate, Vs. This apparently paradoxical result is made possible by the distinctive characteristics of the device of the invention.
• the aerogel which is an extremely low-density material that therefore allows the production of fiuidized beds with extremely low activation velocities (compared to the fiuidized beds produced with conventional materials); the lightness of the granules of aerogel can be controlled during the production thereof, as described above.
• V c values lower than 1/10 of V
• a progressive reduction in the velocity of the particles of particulate within the fiuidized bed is obtained, on account of the collisions between these and the granules of aerogel, until net instantaneous velocities (in the direction of the flow) equal to zero are achieved.
• V c Since the value of V c must also satisfy the condition Vc > V A , in the case of fines the granular material 30 must be selected to allow the lowest possible value of V A . V c must not however be excessively greater than V A , to keep the V c - V s difference virtually at a minimum, because, as described below, the cost of the discharge of the abated particulate to outside the fluidized bed is proportional to the difference between these two velocities.
• the inventors have experimentally observed how in abating fines from gas exhausts the outside discharge of the abated fines is a function of the thickness of the fluidized bed: the greater the Vc - V s difference, then the greater the thickness of the fluidized bed must be, with the consequent implications on costs and on dimensional availability.
• a fluidized-bed thickness equal to 4 mm (when idle), of the type described in the present invention, is sufficient to abate PM 10 type particulate in suspension.
• a fluidized bed thickness of around 100 mm (when idle) is required.
• V c at which to operate has been established on the basis of the particulate to be abated (which value for reasons of dimensions and of costs is unlikely to exceed that of V s by more than a couple of orders of magnitude), it is possible to determine, by means of known calculation formulas and/or with few pilot tests, the characteristics of a granular aerogel capable of forming a fluidized bed under these conditions.
• the inventors have observed experimentally that it is possible to effectively abate a particulate of the PM2.5 type (Aerosil OX 50 Degussa, the granulomere distribution of which presents the maximum value for a diameter of 0.28 micrometers), which has a theoretical settling velocity of under 0.1 cm/s, with a linear velocity of carrier gas equal to 2.5 cm/s, which is capable of activating a bed of granules of aerogel of only silica having a density of 0.2 g/cm 3 and a diameter of 0.1 mm, obtained starting with silicon alkoxides alone; this fluidized bed can be supported by a support of type 20 wherein the part 22 is a wire net in which the "perforations" have dimensions of between 0.07 and 0.08 mm, ensuring that the support 20 does not significantly contribute to pressure drops in the device.
• the part 22 is a wire net in which the "perforations" have dimensions of between 0.07 and 0.08 mm
• the kinetics of the particulate in suspension is dependant on the balance of the effect of transport of the carrier gas and on the vertical fall due to gravity; the transport would have a vertical component and a lateral component, while gravity always has just a vertical component. If the conditions inside the fluidized bed are not those of vertical flow, but of inclined flow, the vertical component of the transport would reduce its intensity to the extent that gravity would not be effectively balanced; with a suitable design of the vessel of the fluidized bed, the result would bring the particles in suspension closer to the holes in the hopper positioned on the downwardly-facing inclined wall of the device. A net movement of the particles in a substantially transversal direction relative to the direction of flow of the carrier gas would thus occur, which would in conclusion cause the particles to fall from the holes communicating the fluidized bed with the hoppers for collecting the particulate.
• the abated particulate can settle through the support of the fluidized bed. It comprises: a funnel part, 1 10, to which an arrival pipe 120 from the gas exhaust is attached; the part 1 0 fulfils the condition of slowing down the velocity of the exhaust flow due to the increase in cross-section.
• the wider cross-section end of the part 1 10 continues with a cylindrical part 130, having a constant cross- section, with an opening 140 at the end opposite to the part 1 10, allowing the carrier gas to be vented to the air once it has been purified of the particulate.
• a highly porous support 150 is present, and on this the fluidized bed 160 is formed with granular material 170.
• the support 150 is rested on a continuous ring, in such a way that the connection between the support and the internal wall of the device ensures stable support for the fluidized bed. Furthermore, the support 150 is held firmly against said ring (by screws not shown in the figure), to ensure that, to be discharged outside, the gas is forced to pass through the fluidized bed.
• the device in Fig. 10 is created to scale and with laboratory materials and does not comprise the openings for discharge of the particulate towards an external hopper at the level of the fluidized bed; this device is however sufficient to demonstrate, limited to the described conditions, the effect of abatement of the particulate, which in tests is observed to form deposits in the funnel zone immediately upstream of the support of the fluidized bed.
• This test is aimed at evaluating the pressure drop produced on the system by the fluidized bed device of the invention.
• a controlled-flow gas feed system is prepared as shown in Figure 9.
• the system, 90 comprises an inlet line 91 for a pressurized gas (e.g. nitrogen); a pressure reducer 92 calibrated from 0 to 10 5 Pascal (Pa) placed on the line 91 ; downstream the pressure reducer 92, an asameter (i.e.
• a pressurized gas e.g. nitrogen
• a pressure reducer 92 calibrated from 0 to 10 5 Pascal (Pa) placed on the line 91 ; downstream the pressure reducer 92, an asameter (i.e.
• a measurer of the rate of a gas flow 93, calibrated to measure flow rates from 0 to 30 litres/minute (l/m); at the asameter outlet, a flow diverter 94, that can discharge the incoming gas from the asameter towards the outside environment (outlet line 95) or towards a chamber 96 in which it is possible to introduce fines, in its turn connected to an outlet line 97 that can be connected to an anti-particulate device of the invention (in the figure, in deviator 94, the continuous lines illustrate the case in which the flow is channelled towards the chamber 96; the broken lines instead indicate the condition of the deviator 94 in the case in which the incoming gas is discharged towards the outside).
• the line 97 of the feed system of figure 9 is connected to the inlet pipe 120 of a device 100 designed according to figure 10.
• This device has an internal pipe 120 diameter of 4 mm, a maximum diameter of part 1 10 (corresponding to the diameter of the outer wall of the device) of 80 mm; a support 150, consisting of a Schott porous baffle of porosity grade 2 (corresponding to pore dimensions of between around 40 and 100 pm); on the support 150, a fluidized bed of granules of aerogel 170 obtained by sol-gel synthesis starting from TEOS alone, having a density of around 0.2 g/cm 3 , for a fluidized-bed height of around 5 mm and a mean density thereof of around 0.1 g/cm 3 .
• the device of the invention operates with no or insignificant pressure drops; in any case, it is possible to dimension the device itself to reduce any pressure drops; for example, the inventors have ascertained, by extrapolation on an industrial scale model, that by operating at a flow of 15m 3 /min, it is possible to have virtually no pressure drops if the diameter of the inlet duct 120 to the device 100 is equal to 126.5 mm and the diameter of the support 150 is 2,530 mm.
• This example relates to a test for removing particulate with the device of the invention.
• example 1 The experimental set-up of example 1 is used, this time with the introduction of a synthetic particulate (Carbon Black CABOT, XC 72 R, No. 136) into chamber 96.
• a synthetic particulate Carbon Black CABOT, XC 72 R, No. 1366
• the behaviour of the device in retaining these particles is observed visually, by illuminating the parts of the device immediately beneath the porous baffle 150 and immediately above the fluidized bed; illumination takes place in accordance with the principle of trans-illumination, that is by irradiating the zones of interest with high-intensity light along an axis perpendicular to the axis of the device 100, and observing the system along a direction perpendicular to both the axis of the device 100 and to the direction of illumination.
• a white card is placed at the opposite part of the device 100 relative to the direction of observation, which helps to identify the carbon black particles (the device 100 is made of glass and is therefore transparent).
• the flow in the system is adjusted, with the deviator 94 in a position such as to discharge the incoming gas from the asameter towards the outside along the line 95; the flow is adjusted to 15 l/min.
• the deviator 94 is then brought into the position wherein the incoming flow from the asameter is channelled into the chamber 96, containing the carbon black, and from there to the device 100.
• test of example 2 is repeated, while preserving all the conditions but eliminating the fluidized bed and retaining only the porous baffle 150.
• gas from the asameter is channelled into chamber 96 and then to the modified device, a net flow of carbon black is observed above the baffle, confirming the essential role of the fluidized bed.
• example 2 The test of example 2 is repeated, in this case however employing as particulate a silica powder (Aerosil Degussa OX-50, of mean granule size around 280 nanometres and without particles in excess of 1 pm); in this case to demonstrate the course of this powder, which is white in colour, a black card is placed behind the device 100.
• a silica powder Alignment Degussa OX-50, of mean granule size around 280 nanometres and without particles in excess of 1 pm
• a black card is placed behind the device 100.
• the results are identical to those of example 2, no traces of particulate are observed above the fluidized bed.
• test of example 3 is repeated with the particulate of example 4 and employing a black card; after having channelled the flow from the asameter into the chamber 96 and to the modified device, a large quantity of particulate is observed, which passes beyond the support 150 (in an amount greater than in example 3), once again confirming the need for a fluidized bed.

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