Abstract:
A positive pressure disc-type filtration apparatus ( 10 ) including a collection trough ( 40 ) internal to the cylindrical pressure vessel housing the filter discs ( 24 ). The collection trough ( 40 ) is entirely disposed within the cylinder of the pressure vessel ( 12 ) and configured and located to receive and direct filter cake removed from the discs ( 24 ) and then reslurried and sluiced toward the center of the trough using jets of liquid ( 32 ), to be discharged longitudinally centrally through a single outlet extending downwardly from the trough and through the wall of the pressure vessel ( 12 ). A method of filtration is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/255,213, filed Dec. 12, 2000 for DISC-TYPE FILTRATION APPARATUS INCLUDING DISCHARGE COLLECTOR INTERNAL TO PRESSURE VESSEL AND METHOD OF FILTRATION. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to disc-type filtration apparatus employed for separation of liquid from a suspension or slurry, such as a lime mud. More specifically, the invention relates to a disc-type filtration apparatus housed within a pressure vessel and employing a positive pressure in conjunction with filter discs to effect the liquid separation, as well as a method of filtration.  
           [0004]    2. State of the Art  
           [0005]    Disc-type filtration apparatus are well known, and have been employed for decades. In its simplest form, such an apparatus employs one or more hollow filter discs, the sidewalls of which discs are either formed of a porous material or define a frame covered with a porous cover, commonly termed a filter cloth. The degree of porosity is selected for the suspension to be filtered, considering the viscosity of the liquid component as well as the particle size or sizes of the solid component. The discs are usually circumferentially sectored into compartments, are oriented vertically and mutually laterally spaced along a tube extending along a horizontal axis. The interiors of the disc sectors are in communication with the interior of the tube. During operation, the discs are rotated about the longitudinal axis so as to become coated with the suspension A positive pressure differential is created between the exteriors of the discs and the interiors thereof, to draw much of the liquid component of the suspension coating the disc exteriors into the interiors and significantly reducing the liquid content of the suspension on the disc exteriors to form a “cake”. The cake is then removed from the disc exteriors.  
           [0006]    Two principal approaches have been used to create the pressure differential between the disc interiors and exteriors. In one approach, exemplified by the teachings of U.S. Pat. Nos. 2,699,872 and 2,899,066, a vacuum is drawn in the tube and the exteriors of the discs are at ambient atmospheric pressure. A currently more favored approach, due to its ability to provide a greater pressure differential, particularly in the case of large, multi-disc filters, is to deploy the filter discs within a pressure vessel which is pressurized above ambient, as by an air compressor. This latter approach is exemplified by the teachings of U.S. Pat. Nos. 2,079,755; 3,252,577; 5,849,202; 6063,294; Canadian Patent No. 1,192,142 and International Patent Application No. WO 90/10490. A commercially available filtration apparatus of the latter type is offered by EIMCO Process Equipment Co. (formerly Baker Process), of Salt Lake City, Utah, assignee of the present invention, as the ClariDisc®) Filter. Another commercially available apparatus of the latter type is offered by Kvaerner Pulping AB of Karlstad, Sweden, as the CAUSTEC® PDW™ filter.  
           [0007]    The design of the aforementioned ClariDisc® Filter has been well accepted by customers, as the solids from this filter (comprising filter cake which has been re-slurried), are collected in a sloping outlet trough portion integral with and protruding beyond the cylindrical envelope of a cylindrical pressure vessel portion housing the discs, and discharged from the pressure vessel through a single outlet or multiple outlets. This is in contrast to the CAUSTEC® PDW™ filter, which uses a plurality of discharge outlets extending from the exterior of the pressure vessel, one outlet for each disc. The CAUSTEC® PDW™ filter is unduly complex and expensive to both build and deploy, due to the use of multiple outlets extending from the cylindrical pressure vessel portion. However, it is also somewhat costly to design and manufacture the trough and outlet assembly for the ClariDisc® Filter, since it functions as part of the outer wall of the pressure vessel and, therefore, must comply with certain governmental or professional engineering standards, the latter exemplified by the American Society of Mechanical Engineers (ASME) Section 8 Pressure Vessel Code.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a positive pressure disc-type filtration apparatus including a collection trough internal to a cylindrical pressure vessel housing the filter discs. The collection trough for the inventive apparatus is entirely disposed within the cylinder or cylindrical envelope of the pressure vessel and thus may be configured and located without concern for pressure differentials to receive and direct filter cake removed from the discs and then reslurried and sluiced toward the center of the trough using flowing liquid, to be discharged longitudinally centrally through a single outlet extending downwardly from the trough and through the side wall of the pressure vessel cylinder.  
           [0009]    The use of a cylindrical pressure vessel having a single outlet provided by the design of the filtration apparatus of the present invention results in a much lower design and fabrication cost for the pressure vessel, even if a slightly larger cylinder diameter for a given disc diameter is required. In addition, relocation of the trough within the pressure vessel allows for the height of the filtration apparatus to be significantly reduced for easier placement and reduced installation costs.  
           [0010]    The present invention also encompasses a method of filtration wherein a suspension may be separated within a cylindrical envelope of a pressure vessel into filter cake and filtrate at a plurality of laterally spaced locations and the filter cake collected from the plurality of spaced locations to a single location within the cylindrical envelope for discharge from the pressure vessel. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0011]    In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention:  
         [0012]    [0012]FIG. 1 is a transverse sectional view of the filtration apparatus of the present invention;  
         [0013]    [0013]FIG. 2 is a top view of a portion of the filtration apparatus of the present invention showing the collection trough disposed within the cylinder of the pressure vessel, with the pressure vessel shown in broken lines for clarity;  
         [0014]    [0014]FIG. 3 is a side view of a lower portion of the filtration apparatus of the present invention showing the collection trough disposed within the cylinder of the pressure vessel, with the pressure vessel shown in broken lines for clarity;  
         [0015]    [0015]FIG. 4 is a side view of a discharge chute and the collection trough. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring now to the drawing figures, filtration apparatus  10  of the present invention includes a pressure vessel  12  having a substantially cylindrical envelope and defined in part by side wall  14 , which substantially defines a cylinder concentric with a longitudinal vessel axis PV. Access hatches  16 , slurry inlet conduit  18  and filter cake (also termed “mud cake”) outlet conduit  20  extend through the side wall  14 . A plurality of filter discs  24  (for example, and in this instance, six) are disposed within pressure vessel  12 . The filter discs  24  each include multiple sectors  26  as known in the art, which communicate with the interior of one or more central filtrate discharge tubes  28  (a single tube being employed in this embodiment), an end of which (not shown) also extends through an end of pressure vessel  12  to effectuate removal of the liquid component of the suspension under filtration drawn therefrom under differential pressure through filter cloths disposed on the exteriors of the sectors  26  which collectively form the filter discs  24 . It should be noted that, in the preferred embodiment the axis DT of the filtrate discharge tube  28  is laterally offset from the longitudinal axis PV of the pressure vessel  12 . A disc wash assembly  30  is associated with each filter disc  24 , as is a high pressure shower assembly  32  for disc cleaning, the latter movable by drive assembly  34  in an oscillating fashion through arc  36  as shown in FIG. 1. A discharge scraper  38  in the form of doctor blades flanks each filter disc  24  for removal of mud cake from the exterior thereof. Such an apparatus as described to this point is entirely conventional, as is its operation. Therefore, no further description of these features is required.  
         [0017]    Disposed within pressure vessel  12  and partially laterally offset and extending below filter discs  24  (see FIG. 1) is a mud cake collection trough  40 . Trough  40  is elongate and extends beyond the longitudinally outermost filter disc  24  of the row of filter discs  24 . Trough  40  includes two inclined bottom walls  42  extending to the side wall  14  of pressure vessel  12  and sloping downwardly away from the horizontal midplane of pressure vessel  12  as well as toward the longitudinal center of the trough  40 , where opening  44  communicates trough  40  with mud cake outlet conduit  20  (FIGS. 1 and 3). Trough  40  is further configured on its inner side with a plurality of discharge chutes  46 , one substantially parallel to and flanking each side of each filter disc  24 , to receive mud cake removed from the sides of the filter discs  24  by discharge scraper  38 . Referring to FIGS. 2 and 4, discharge chutes  46  each include an elongated open mouth  48  at an upper extent thereof. Each open mouth  48  is of a length sufficient to receive mud cake falling from the discharge scrapers  38  substantially spanning the radius of the filter discs  24  between central filtrate discharge tube  28  and the filter discs&#39; outer edges. More particularly, the open mouth  48  is desirably of a length which corresponds to a radial length of the filtering area of the sectors  26  (e.g., the distance between the outer radius of the filter discs  24  and the radius of the central filtrate discharge tube  28 ).  
         [0018]    Each discharge chute  46  further includes two side walls  50  oriented substantially parallel to filter discs  24 , and an inclined lower wall  52  extending between side walls  50  and sloping downwardly from mouth  48  and away from the center of pressure vessel  12  toward the side wall  14  thereof. The outer extents of discharge chutes  46  comprise openings  54  into the outer collection portion  56  of trough  40 , wherein inclined bottom walls  42  lead to opening  44 . Spray heads  58  fed by manifold  60  direct jets or other flows of fluid, such as, for example, water into discharge chutes  46  to prevent accumulation of mud cake on the side walls  50  thereof and to dilute the mud cake scraped off filter discs  24  to enhance sluicing thereof into opening  44  for discharge from pressure vessel  12  through outlet conduit  20 .  
         [0019]    No auger or other mechanical device is required to move the mud cake scraped from filter discs  24  to outlet conduit  20 , the diluted mud cake flowing under gravity down inclined lower walls  52  to openings  54  and then along one of the bottom walls  42  of trough  40  to opening  44 . The side walls  50  at each longitudinal end of trough  40  extend to the side wall  14  of pressure vessel  12  to complete trough  40 .  
         [0020]    The configuration of the trough  40  and discharge chutes  46  is such that the open mouths  48  are positioned above an intended slurry liquid level during operation of the filter  10 . The discharge chutes  46  and trough  40  thus form a barrier separating discharged filter cake from the slurry contained in the pressure vessel  12 . Such an arrangement allows the discharge and initial collection of the filter cake to take place entirely within the substantially cylindrical envelope of the pressure vessel  12  in a continuous filtration process.  
         [0021]    It is noted that in larger filters having a large quantity of filter discs  24 , it may be desirable to incorporate multiple troughs  40  for removal of the filter cake. For example, if the filter  10  includes twelve filter discs  24  it may be desirable to include two troughs  40 , each having an associated opening  40  and each being configured to collect filter cake from six filter discs  24 . It should be noted that the invention is not limited to a single trough  40  for each six filter discs  24  and that the above described embodiments are simply exemplary. Rather, numerous configurations are contemplated wherein one or more troughs  40  may be used with each trough  40  being associated with multiple discharge chutes  46  such that the number of outlet conduits  20  formed in the sidewall  14  of the pressure vessel  12  is less than the number of discharge chutes  46  contained therein.  
         [0022]    It is noted that the number of troughs  40  formed in a given filter  10  may depend on such factors as the number of filter discs  24  present in the filter  10 , the viscosity or flowability of the filter cake, and the minimum allowable angle of the bottom walls  42  which still provides sufficient gravity for collection and removal of the filter cake.  
         [0023]    It is also noted that the outlet conduits  20  and associated openings  44  need not be in the longitudinal center of the trough  40 . Rather, the opening  44  may be formed at any longitudinal location along the trough  40 . Thus, for example, a single bottom wall  42  may extend from one longitudinal end of the trough  40  to an opening  44  at the opposing longitudinal end of the trough. Thus, the present invention may be easily adapted to various plant layouts wherein locating associated piping may be a concern.  
         [0024]    Additionally, while the invention has been described with respect to a pressure vessel  12 , the above trough configuration may be implemented into a tank or vat of a nonpressurized vessel. Thus, for example, a filter which draws a vacuum through the discharge tube  28  while maintaining an ambient pressure at the exteriors of the filter discs  24  may likewise incorporate a trough  40  such as described above.  
         [0025]    While the present invention has been described in terms of a preferred embodiment, those of ordinary skill in the art will understand and appreciate that it is not so limited. Accordingly, many additions, deletions and modifications to the invention as disclosed may be effected without departing from the scope of the invention as set forth in the claims which follow.