Mist recovering apparatus

Abstract: In a mist recovering apparatus for recovering a mist contained in a gas, provided with a filter (10) having pores of pore size increasing from a gas inlet side toward a gas outlet side, the gas containing the mist is passed from the gas inlet side of the filter, through the filter toward the gas outlet side of the filter. The mist is trapped by the filter and grows into droplets (16) as the gas containing the mist flows through the filter, and the droplets drip down along the gas outlet surface of the filter and are collected under the filter.

Description: The present invention relates to a mist recovering apparatus according to the pre-characterizing features of claim 1. Description of the Related Art Sodium contained in a gas in a sodium mist has been recovered by either a method which solidifies sodium from a sodium mist and collects solidified sodium by a filter for recovery or a method which changes a sodium mist into sodium droplets and collects the droplets for recovery. When recovering sodium by the former conventional method, the filter needs to be changed occasionally because solid sodium deposits on the filter and, eventually, clogs up the filter. When recovering sodium by the latter conventional method, the filter is not clogged up in a short time because sodium is collected in sodium droplets. However, the method needs another recovering apparatus below the filter because the sodium droplets are scattered again in a sodium mist from the filter. Thus, these conventional methods are unable to recover the sodium mist efficiently and require troublesome maintenance work. A metal mist trapping method which recovers sodium in sodium droplets from a sodium mist contained in a cover gas system is disclosed in JP-B No. 8-8968 published January 31, 1996. This metal mist trapping method uses a metal mist trapping apparatus having a vessel having a vertical gas passage packed with horizontal metal wire nets stacked in layers. The upstream metal wire nets with respect to the flowing direction of a gas containing a sodium mist are formed of finer metal wires and have smaller meshes than the downstream metal wire nets. The metal wire nets are heated at a temperature not lower than the melting point of sodium by a heating unit during operation. Since the gas passage of the vessel is fully packed with the horizontal metal wire nets stacked in layers, the sodium mist is trapped mainly by the metal wire nets near the gas inlet of the vessel and liquid sodium layers held on those metal wire nets enhances the sodium mist trapping performance of the metal mist trapping apparatus if the gas containing the sodium mist flows through the gas passage at a relatively low flow rate. However, if the gas containing the sodium mist flows through the gas passage at a high flow rate at which the gas flows when a dry cleaning apparatus is used, and the size of droplets produced by the condensation of the sodium mist is as small as several micrometers, liquid sodium layers held on the metal wire nets are blown off the metal wire nets and discharged through the gas outlet of the vessel, which reduces the sodium mist recovering efficiency of the metal mist trapping apparatus. In accordance with the pre-characterizing part of claim 1, EP-A-0 755 709 (claiming priority of 28/07/1995, but published on 29/01/1997, i.e. later than the priority date of this application) related to designated states CH, DE, ES, FR, GB, LI, and SE, discloses a mist recovering apparatus including plural filter walls constituted by metal mesh material in which the gas including mist is driven into the interior chamber of a cylindrical filter structure comprising said filter walls. The size of the pores defined by said metal meshes increases with the direction of flow of the gas, i.e. from the inner metal mesh in the direction toward the outer metal mesh. GB 960 640 discloses an oil separator comprising a cylindrical filter. The cylindrical filter consists of an outer cylindrical shroud having apertures, an inner tubular frame and fiber glass filter elements between said shroud and said tubular frame. Within the tubular frame there is provided a further filtering element comprising a plurality of stacked filter layers having the form of disks. There is provided an inner core member having apertures through which the gas flow can escape from the oil separator. US-A-4 759 782 discloses a cylindrical filter including a plurality of cylindrical filter elements. A gas flow to be cleaned is fed from the inner core of the filter to the outside thereof. The pore size increases in the direction opposite to the flow direction. The filter elements are comprised of organic or inorganic fibers. DE 24 52 386 discloses an apparatus for treating a medium including mist. The apparatus comprises a porous body wherein the pore size increases in the direction of flow of the gas. The specific structure of the apparatus is not disclosed in this reference. As an example for the apparatus a ceramic body or a sintered metal body are mentioned. GB 1 082 810 discloses a process for separating droplets of a first liquid from a second liquid by using fibrous material, wherein the diameters of the fibers increase in the direction of flow. The apparatus used for carrying out this process is only roughly described as comprising a vertical vessel, preferably a cylindrical column, in which, preferably horizontally, a grid or sieve plate has been fitted on which there is provided at least one layer of fibrous material. US 5 415 676 discloses a mist collector cartridge for separating mist from a gas including a filter wrap, which is more porous on the outer region than on its inner region, wherein the flow of gas is from the inner region to the outer region. This filter wrap is used as a pre-filter and consists of a foamed urethane material. US 5 995 974 discloses a separator having a vertical cylindrical filter through which fluids flow radially inward. There are two filter layers. The pore size of these filter layers decreases in the direction of fluid flow. The internal space of the cylindrical filter is subjected to downwardly forced flow. US 3 880 626 discloses a separator to separate liquid from a gas stream. A laminate comprised of a plurality of wire nets is included in the separator. US 3 733 792 discloses a filter comprising a lamination of a wire net in the form of a cylindrical winding. Accordingly, it is an object of the present invention to provide a mist recovering apparatus to be applied to recovering a sodium mist capable of passing a gas satisfactorily through filters by avoiding the clogging of the filters, of preventing the rescattering of the sodium mist to omit any additional mist recovering apparatus and of efficiently recovering the mist at low costs. This object in achieved by the features of claim 1. Preferred embodiments are defined in the dependent claims. Preferably, the filter of the mist recovering apparatus for recovering a mist contained in a gas may be a filter structure constructed by laminating metal wire nets in a plurality of layers with the metal wire nets having finer meshes forming layers nearer to the gas inlet side and the metal wire nets having coarser meshes forming layers nearer to the gas outlet side and sintering the laminated metal wire nets, a filter structure constructed by forming layers of particles of a metal or a synthetic resin with the layers of the particles of smaller particle sizes formed nearer to the gas inlet side and the layers of the particles of greater particle sizes formed nearer to the gas outlet side or a ceramic filter structure having pores of pore size increasing from the gas inlet side toward the gas outlet side. The filter of the mist recovering apparatus may have the shape of a circular tube, a flat plate, a corrugated plate, a hollow truncated cone or an inclined plate. The mist recovering apparatus may be provided with a single or a plurality of filters. A louver or a baffle may be disposed on the gas outlet side of the filter of the mist recovering apparatus. The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagrammatic view of a dry cleaning system for recovering a sodium mist, incorporating a mist recovering apparatus according to the present invention; Fig. 2 is a schematic longitudinal sectional view of a mist recovering apparatus in a preferred embodiment of the present invention to be employed in the dry cleaning system of Fig. 1; Fig. 3 is an enlarged, fragmentary, sectional view of a tubular filter included in the mist recovering apparatus of Fig. 2; Fig. 4 is a graph comparatively showing the difference in mist recovering efficiency between a mist recovering method of the present invention and a conventional mist recovering method; Fig. 5 is an enlarged sectional view of a modification of the mist recovering apparatus of Fig. 2; Fig. 6 is a block diagram of an iodine laser including a mist recovering apparatus of the present invention for recovering a mist; Fig. 7 is a block diagram of a MHD power generating system employing a mist recovering apparatus of the present invention for recovering a seed mist; Fig. 8 is a block diagram of a gas recovering system for recovering Freon gas and organochlorine solvents, employing a mist recovering apparatus of the present invention; Fig. 9 is a block diagram of a coal gasification hybrid power generating system employing a mist recovering apparatus of the present invention; Fig. 10 is a fragmentary sectional view of another filter to be employed in a mist recovering apparatus of the present invention; Fig. 11 is a fragmentary sectional view of third filter to be employed in a mist recovering apparatus of the present invention; Figs. 12(a), 12(b), 12(c) and 12(d) are perspective views of filters having the shapes of a flat plate, a corrugated plate, a hollow, truncated cone and an inclined plate, respectively; Figs. 13(a) and 13(b) are perspective views of an arrangement of a plurality of tubular filters and an arrangement of a planar filter, respectively; and Figs. 14(a) and 14(b) are schematic side views of a filter and a louver disposed on the gas outlet side of the filter, and a filter and a baffle disposed on the gas outlet side the filter, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A mist recovering apparatus C in a preferred embodiment the present invention will be described with reference to Fig. 1 as applied to recovering a sodium mist from an inert gas, such as argon gas, containing sodium vapor and flowing at a high flow rate, in a dry cleaning system A which blows the inert gas against spent nuclear fuel used by a sodium-cooled fast breeder in a cleaning tank B to remove sodium adhering to the spent nuclear fuel. Shown also in Fig. 1 are an economizing heater D , a blower E , a cooling unit F , and a heating unit G coated with a heat insulating material and surrounding the mist recovering apparatus C . As shown in Fig. 2, the mist recovering apparatus C has a vertical vessel 1 having a side wall provided with a gas inlet 2 in its lower portion and with a gas outlet 3 at its upper end, and a unidirectionally inclined bottom wall 4, and a liquid sodium drain pipe 6 connected to a lowermost portion of the bottom wall 4 and provided with a valve 5. An inside tube 8 is coaxially provided with a flange 7 at its upper end and is supported in a vertical attitude in a central region of a chamber defined by the vertical vessel 1 with its lower end positioned on a level lower than that of the gas inlet 2 of the vertical vessel 1. A gas discharge chamber 17 is formed between the upper wall of the vertical vessel 1 and the flange 7. An intermediate and upper portion of the tube 8 is formed of a tubular filter 10. As shown in Fig. 3, the tubular filter 10 is formed by laminating five metal wire nets 9a to 9e and sintering the laminated metal wire nets 9a to 9e. The metal wire nets 9a to 9e are arranged such that any outer net has a finer mesh than any inner net as clearly shown in Fig 3. A drain mechanism 11 is provided at the lower end of the tube 8. The drain mechanism 11 comprises a funnel 12 joined to the lower end of the tube 8, a drain pipe 13 connected to the outlet of the funnel 12 and extended through the bottom wall 4 of the vertical vessel 1, and a drain valve 14 connected to the drain pipe 13 outside the vertical vessel 1. A sodium mist recovering method in accordance with the present invention, to be carried out by the mist recovering apparatus C will be described below. A gas containing a sodium mist is introduced through the gas inlet 2 formed in a lower portion of the side wall of the vertical vessel 1 into the vertical vessel 1. Then, the gas flows through the tubular filter 10 into the tube 8. The sodium mist contained in the gas is trapped and separated from the gas by the metal wire nets 9a to 9e of the tubular filter 10 as the gas flows through the tubular filter 10. The sodium mist trapped by the tubular filter 10 grows gradually into sodium droplets 16 as the trapped sodium mist is forced to move through the tubular filter 10 by the gas. The sodium droplets 16 drip down along the inner circumference of the tubular filter 10 into the funnel 12 joined to the lower end of the tube 8 and liquid sodium is collected temporarily in the funnel 12. Then, the drain valve 14 is opened to discharge the liquid sodium collected in the funnel 12 through the drain pipe 13 for recovery. The gas which has released the sodium mist flows upward through the tube 8 into the gas discharge chamber 17 and is discharged from the vertical vessel 1 through the gas outlet 3. The sodium mist which has deposited on the outer circumference of the tube 8 and the inner circumference of the vertical vessel 1 grows into sodium droplets 16, and the sodium droplets 16 drip onto and are collected on the bottom wall 4. The valve 5 is opened to discharge the sodium droplets 16 collected on the bottom wall 4 through the liquid sodium drain pipe 6. This sodium mist recovering method passes the gas containing the sodium mist through the tubular filter 10 formed by laminating the metal wire nets 9a to 9e, in which inner metal wire nets have coarser meshes than outer metal wire nets, whereby the sodium mist is trapped by the metal wire nets 9a to 9e and separated from the gas. The gas is able to flow smoothly through the tubular filter 10 without causing clogging because the mesh of the metal wire nets 9a to 9e becomes coarser as the gas flows inward, whereby the sodium mist trapped by the metal wire nets 9a to 9e is allowed to grow gradually into large sodium droplets 16 as the same is driven to move through the tubular filter 10 by the gas to the inner circumference of the tubular filter 10. Therefore, the sodium droplets 16 thus produced by the condensation of the sodium mist trapped by the tubular filter 10 are not scattered again into the gas outlet in a sodium mist. The large sodium droplets 16 drip by gravity along the inner circumference of the tubular filter 10 into the funnel 12 at the lower end of the tube 8. Since the sodium droplets 16 drip downward along the inner circumference of the tubular filter 10, while the gas flows upward through the tube 8, the rescattering of a sodium mist can effectively prevented and the sodium droplets 16 can efficiently be recovered. As is obvious from Fig. 4, the mist recovering efficiency of the sodium mist recovering method of the present invention is far higher than that of a conventional sodium mist recovering efficiency. While a maximum recovery achieved by the conventional sodium mist recovering method after a long period of operation is abut 68%, a recovery achieved by the sodium mist recovering method of the present invention in a short period of operation is about 99%. Fig. 5 shows a lower portion of a vertical vessel 1 in a modification of the vertical vessel 1 included in the mist recovering apparatus C of Fig. 2. The vertical vessel 1 shown in Fig. 5 has a conically tapered bottom wall 4. A sodium drain pipe 6' of a relatively great diameter is joined to the lower end of the tapered bottom wall 4. A drain pipe 13 has an upper end joined to a funnel 12 joined to the lower end of a tube 8 and is extended through the drain pipe 6'. Sodium droplets 16 are collected in and drained through the drain pipe 13 as indicated by the arrows 20 in Fig. 5. In the foregoing sodium mist recovering apparatus of the present invention, the sodium mist is trapped by the tubular filter 10 as the gas containing the sodium mist flows from the side of the outer circumference of the tubular filter 10 toward the side of the inner circumference of the tubular filter 10. However, the sodium mist recovering apparatus may be provided with a tubular filter formed by laminating metal wire nets 9a to 9e, in which the metal wire nets nearer to the inner circumference of the tubular filter among the metal wire nets 9a to 9e have finer meshes than those nearer to the outer circumference of the same, instead of the tubular filter 10 shown in Fig. 3, and the sodium mist may be trapped by the tubular filter as the gas containing the sodium mist flows from the side of the inner circumference of the tubular filter toward the side of the outer circumference of the same. The mist recovering apparatus C is applicable to the recovery of a mist in an iodine laser H as shown in Fig. 6 to remove continuously a mist contained in a gas and causing the reduction of laser output. The mist recovering apparatus C is applicable also to the recovery of a seed mist in a MHD power generating system J as shown in Fig. 7 to enhance the power generating efficiency of the MHD power generating system J . When applied to the recovery of a mist in a gas recovering system K as shown in Fig. 8 for recovering Freon gas and organochlorine solvents, the mist recovering apparatus C improves the recovery efficiency of the gas recovering system K . When applied to the recovery of a mist in a coal gasification hybrid power generating system L as shown in Fig. 9, the mist recovering apparatus C enhances the power generating efficiency of the coal gasification hybrid power generating system L . The mist recovering apparatus C of the present invention is applicable to recovering a molten salt mist or emulsion. The mist recovering apparatus C may be provided with a filter structure 20 shown in Fig. 10 and formed by laminating layers of particles 18a, 18b, 18c and 18d of a metal or a resin with the particles of smaller particle sizes forming the layers nearer to the gas inlet side and the particles of greater particle sizes forming the layers nearer to the gas outlet side. As shown in Fig. 11, ceramic filter structure 30 may also be used that has pores 19 of pore size increasing from the gas inlet side toward the gas outlet side, instead of the tubular filter 10 of the sectional structure shown in Fig. 3. The mist recovering apparatus C may be provided with a flat filter 10a as shown in Fig. 12(a), a corrugated filter 10b as shown in Fig. 12(b), a filter 10c of the shape of a hollow truncated cone as shown in Fig. 12(c) or an inclined flat filter 10d as shown in Fig. 12(d). The mist recovering apparatus C may be provided, if necessary, with a plurality of filters each of the type of any one of the foregoing filters as shown in Fig. 13(a) or 13(b). A louver 21 as shown in Fig. 14(a) or a baffle 22 as shown in Fig. 14(b) may be disposed on the gas outlet side of the filter of the mist recovering apparatus C for the further enhancement of the mist recovering efficiency of the mist recovering apparatus. Although the invention has been described in its preferred forms with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope and spirit thereof.

Claims: A mist recovering apparatus for recovering a mist contained in a gas, comprising a cylindrical filter (10) made of a lamination of wire nets (9a-9e) and having a gas inlet surface and an opposite gas outlet surface, said wire nets having pores formed by metal wires, respectively, and assembled in a face-to-face lamination with the pores disposed in mutually adjoining and communicating relation, said pores on one wire net having a greater pore size than an adjoining wire net with respect to a direction from said gas inlet surface to said gas outlet surface, said pores being enlarged in pore size in said direction so that the pores have largest sizes in said outlet surface, and gas flow producing means (1, 2 and 3) provided to cause a gas containing a mist to flow through the gas inlet surface, the pores and the gas outlet surface in this order, characterised in: that said cylindrical filter (10) is a vertical filter with said gas inlet surface and said gas outlet surface formed as a vertical outer cylindrical surface and a vertical inner cylindrical surface, respectively, said cylindrical inner surface defining therein a vertical internal space, that said lamination of wire nets (9a-9e) is formed of at least five metal wire nets (9a-9e) so that the pores of the adjoining wire nets are gradually enlarged in size without any interruption to cause the mist passing through the pores from the outer cylindrical surface to the inner cylindrical surface to gradually grow in size, and that said gas flow producing means (1, 2 and 3) produces an upward forced flow within said vertical internal space of the cylindrical filter (10). The mist recovering apparatus according to claim 1, characterised in that the metal wire nets (9a-9e) are made of a sintered metal. The mist recovering apparatus according to claim 1 or 2, characterised in that said cylindrical filter (10) is disposed within a vessel (1) which has a gas inlet (2) at a lower portion of the vessel (1) and a gas outlet (3) at an upper portion of the vessel (1), and the gas outlet (3) is in communication with said vertical internal space of the cylindrical filter (10). The mist recovering apparatus according to any one of claims 1 to 3, characterised in that said vertical internal space of the cylindrical filter (10) is connected at the bottom thereof to a funnel (12) which is connected to a draining pipe (13).

Inventors: FUJII SADAO, ITO HIDEKI, KIYOTA MASAHIKO, MURAI MASAHIRO, TAKATA TAKASHI, UMEOKA TOSHIHIRO, FUJII, SADAO, ITO, HIDEKI, KIYOTA, MASAHIKO, MURAI, MASAHIRO, TAKATA, TAKASHI, UMEOKA, TOSHIHIRO
Applicants: KANSAI ELECTRIC POWER CO, KAWASAKI HEAVY IND LTD, KAWASAKI JUKOGYO KABUSHIKI KAISHA, THE KANSAI ELECTRIC POWER CO., INC.
Application year: 1997
Main classification: B01D46
IPC codes: B01D46, B01D45