Abstract:
A pressure diffuser is disclosed having a pressurized vessel having an inlet adapted to receive a slurry of cellulosic fibrous material and cooking liquor; an outer chamber having an assembly of screens, wherein the slurry flows through the outer chamber and a portion of the cooking liquor is extracted through the screens; a center chamber to receive cooking liquor extracted through the screens; an inner head capping the center chamber, wherein the inner head is impervious to gases escaping from the cooking liquor and collecting underneath the inner head, and a gas exhaust conduit extending from the inner head to an outlet external of the pressure diffuser, wherein the conduit has an inlet below the inner head.

Description:
RELATED APPLICATION  
       [0001]    Priority is claimed to U.S. Provisional Patent Application No. 60/382,604, filed May 24, 2002, the entirety of which is incorporated by reference. 
     
    
     
       FIELD OF INVENTION  
         [0002]    The present invention relates to pressure diffusers for pulp processing systems.  
         BACKGROUND OF THE INVENTION  
         [0003]    Conventional pressure diffusers for use in pulp processing systems generally include a center column chamber for liquor extracted from the pulp processed in the outer annular region of the diffuser vessel. The extracted liquor contains gases, usually in the form of air bubbles, which gradually drift upward in the center chamber where the gases separate from the liquor and form a gas pocket at the top of the chamber. The top of the chamber is capped with a conical inner head that separates the chamber from the outlet head above the chamber. Conventionally, the inner head is perforated, e.g., with screen slots, to allow gases to pass through the inner head and enter the pulp slurry flowing through the outlet head. Other types of inner heads are not perforated, but the center chamber is entirely filled with cooking liquor so that gases remained entrained in the liquor.  
           [0004]    A difficulty with the conventional methods of allowing gases from the center chamber to be reintroduced into the pulp slurry or entrained in the cooking liquor is that the gases are not removed from the diffuser separately of the pulp and liquor. It is preferable to remove gases from the pulp and cooking liquor. If the gases are not removed in the pressure diffuser, then they must be removed from the pulp and liquor streams downstream of the pressure vessel. Filtrate tanks and other devices are traditionally used to separate gases from the pulp and liquor downstream of the pressure diffuser.  
           [0005]    Traditionally, cooking liquor flows from the pressurized center chamber of the pressure diffuser into an unpressurized filtrate tank. The pressure diffuser is pressurized, for example, in a range of 75 psi (pounds per square inch) to 150 psi. Due this pressurization, gases in the cooking liquor and center chamber are compressed and have a reduced gas volume. As the cooking liquor flows from the pressure diffuser to the filtrate tank, which may be at atmospheric pressure, the gases in the cooking liquor increase in volume as they enter the filtrate tank. Typically, a large filtrate tank is provided to allow for the entrained gas bubbles in the cooking liquor to rise to the surface of the cooking liquor and fill the upper regions of the filtration tank. The liquor is held in the tank for a certain retention time to allow for the bubbles to rise out of the liquid and into the upper gas section of the tank.  
           [0006]    There is a desire to eliminate the filtrate tank or at least reduce the size of the filtrate tank. If there is to be no filtrate tank or only a small filtrate tank, then there is a need for the gas removal function previously performed by the filtrate tank to be performed upstream of the tank in the pulp processing system. Accordingly, there is a need for a means for removing at least some of the gas from the cooking liquor upstream of the filtrate tank so that the filtrate tank may be eliminated or at least reduced in size.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a means for extracting gases from the cooking liquor while the cooking liquor is in the pressure diffuser. Cooking liquor held in the center chamber. The inner head of the center chamber is impervious to gases. At least some of the gases that escape from the cooking liquor form a gas pocket at the top of the center chamber, if the liquid liquor does not entirely fill the chamber. By leaving a gas pocket at the upper region of the center chamber and by sealing the inner head at the top of the chamber such that gas may not flow from the center chamber into the pulp stream in the outlet head, gas will collect in a gas pocket underneath the inner head at the top of the conical center chamber. To remove the gas collected under the conical inner head of the center chamber, a gas relief pipe extends through the exterior wall of the pressure diffuser and into the center chamber so as to have an air inlet at the top of the center chamber. This air inlet allows gas to enter the gas pipe, flow through the pipe, and out of the pressure diffuser.  
           [0008]    Because the gas is pressurized in the diffuser, gas flows from the high-pressure diffuser to atmospheric pressure on the outside of the diffuser or in an environmental-friendly gas recovery system. A valve on the gas pipe may be manually or automatically activated to periodically vent the gas from the center chamber, or may be automatically operated when gas is sensed in the center chamber or in the pipe. The valve is closed when there is no or little gas in the center chamber such as when the cooking liquor fills the center chamber. By removing pressurized gas from the center chamber, the amount of gas entrained in the cooking liquor and pulp slurry is reduced and the need for devices, e.g., a large filtrate tank, to remove entrained gases is eliminated or minimized.  
           [0009]    In a first embodiment, the invention is a pressure diffuser comprising: a pressurized vessel having an inlet adapted to receive a slurry of cellulosic fibrous material and cooking liquor; an outer chamber having an assembly of screens, wherein the slurry flows through the outer chamber and a portion of the cooking liquor is extracted through the screens; a center chamber to receive the cooking liquor extracted through the screens; an inner head capping the center chamber, wherein the inner head is impervious to gases escaping from the cooking liquor and collecting underneath the inner head, and a gas exhaust conduit extending from the inner head to an outlet external of the pressure diffuser, wherein the conduit has an inlet below the inner head.  
           [0010]    With respect to the first embodiment, the inlet to the conduit may also be adjacent a lower surface of an apex to the inner head; the inner head may be conical and the inlet to the conduit be adjacent an apex of the inner head; a valve may be in the conduit, and the valve may be automatically actuated when gas is at the inlet of the conduit. Further, in the first embodiment, the conduit may include a downwardly extending section between the inlet and an aperture in the inner head for the conduit, wherein the aperture is below a gas pocket level in the inner head.  
           [0011]    In a second embodiment, the invention is a method for venting gases from a pressure diffuser vessel having an outer chamber with an assembly of screens and a center chamber with an upper gas containing region, the method comprising: pressurizing the vessel and feeding into the outer chamber of the vessel a slurry of cellulosic fibrous material and cooking liquor; from the slurry in the outer chamber, extracting at least a portion of the cooking liquor through the assembly of screens and introducing the extracted liquor into the center chamber; at least partially filling the center chamber with the extracted cooking liquor; collecting gases from the cooking liquor in a gas pocket at an upper region of the center chamber, and venting gases from the gas pocket through a gas exhaust conduit having an inlet positioned in the gas pocket and an outlet external of the pressure diffuser vessel.  
           [0012]    In the second embodiment, the outlet of the a gas exhaust conduit may extend downward from the gas pocket and into a liquid filled portion of the center chamber before the conduit extends through a wall of the center chamber; the conduit may include a valve and the method further comprises automatically actuating the valve when gas detected at the inlet of the conduit, when gas detected in the gas pocket, on a predetermined periodic basis, or automatically closing the valve when liquid is detected at the inlet of the conduit. The method of the second embodiment may further comprise outputting the cooking liquor from the central chamber to a filtration tank, wherein the filtration tank has a small interior volume insufficient to contain a large gas volume. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 comprises a side schematic cross-sectional view of an upper section of an exemplary variable pressure diffuser.  
         [0014]    [0014]FIG. 2 is a detailed side schematic cross-sectional view of an exemplary upper portion of a pressure diffuser, such as shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIGS. 1 and 2 show a variable pressure diffuser  10  having a gas vent for releasing gases entrained in the cooking liquid in the diffuser vessel  11 . The pressure diffuser vessel  11  comprises a first interior (annular) volume  12  for containing communited cellulosic fibrous material (cellulosic/paper pulp) to be treated under pressure. Except for the components relating to gas removal, the pressure diffuser  10  may be a conventional pressure diffuser having a vessel  11 ; a pulp inlet  13  to the vessel, typically at the bottom thereof; a pulp outlet  14  from the vessel, typically at the top thereof, and a screen assembly  15  upstanding within the vessel, and cooperating with the interior wall of vessel  11  to define the annular first interior volume. An exemplary pressure diffuser (without the gas removal means of the present invention) is shown in the U.S. Pat. No. 5,567,279, which is incorporated by reference.  
         [0016]    Pressure diffusers  10  generally have a large center chamber  20 , which contains cooking liquor extracted from pulp. The center chamber  20  may be a cylindrical volume extending substantially the entire vertical length of the vessel  11 . The center chamber  20  contains a substantially-still pool of cooking liquor extracted from the pulp slurry via the screen assembly  15 . The pool allows air bubbles in the extracted liquor to rise to the top of the center chamber  20  n the pressure diffuser.  
         [0017]    In some conventional pressure diffusers, small vents in the top of the center chamber, e.g., the inner head  22 , allow the air to pass through the top of the center chamber, into the pulp stream at the top of the diffuser, and onto the next stage or blow tank. In other conventional pressure diffusers, air remains entrained in the cooking liquor which fills the entirety of the center chamber. In these conventional diffusers, the entrained air is extracted along with the liquor into the filtrate tank. The entrained air necessitates larger filtrate tanks to remove the air from the liquor. In addition, air in the pulp reduces the capacity of pulp washing equipment, and removal of air from the liquor is desirable.  
         [0018]    Around the exterior of the center chamber is an annular volume  12  between the center chamber and the vessel wall. Within this annular chamber  12  is arranged screen assembly  15  which may be a series of screens that extract cooking liquor from the pulp slurry flowing through the annular chamber  12  in the pressure diffuser vessel  11 . As the pulp flows through the annular section, cooking liquor is extracted from the pulp via the screens  15 . The cooking liquor flows from the screens into the center chamber  20 . The cooking liquor collects in the center chamber and may fill the entirety of the center chamber  20 .  
         [0019]    In the embodiments of the invention disclosed here, a small gas exhaust vent pipe  24  is extends from the inner head  22  to the outer head  26  and then out of the vessel  11 . Inside the inner head, the vent pipe  24  extends to a high point  28  of a cone formed by the lower, inside surface of the inner head  22 . An inlet end  36  of the vent pipe is positioned at the high point  28  underneath the inner head. An opposite end of the vent pipe  24  is positioned externally of the diffuser vessel and may vent to a gas recovery system  42 . The vent pipe is connected to valve  32  (either manually activated or automatically controlled) so that air in the inner head can be released through the pipe  24 .  
         [0020]    As shown in FIG. 2, the center chamber  20  is a generally cylindrical vertical volume aligned with the centerline of the pressure diffuser vessel. At the top of the center chamber is an inner head  22 , which may be a conical housing formed over the center chamber. The inner head and center chamber define a pressurized vessel for the cooking liquor extracted from the pulp in the first volume  12  of the diffuser vessel.  
         [0021]    Some gases, such as air, become entrained in the cooking liquor. These gases may tend to rise in the relatively-stagnant pool of cooking liquor that forms in the center chamber  20 . The air, which is typically in the form of various sized air bubbles, may separate from the cooking liquor and form a gas pocket  34  under the conical umbrella formed by the inner head  22  of the center chamber. The conical inner head  22  is impervious to air. Air separating from the cooking liquor in the center chamber collects in the air pocket  34 , underneath the inner head.  
         [0022]    Along with the rest of the vessel  11 , the air pocket  34  is pressurized, such as to 75 psi-150 psi, which is substantially the same pressure as in the interior of the pressure vessel. Accordingly, the air pocket  34  may be a relatively-high pressure volume of compressed air.  
         [0023]    The air exhaust vent pipe  24  has an inlet  36  at the apex  28  of the inner head. The inlet end  36  extends into the air pocket  34  below the inner head. The vent pipe extends from its inlet  36  down along an inner surface of the inner head until it reaches an aperture  38  in the inner head through which the pipe extends radially outward to the wall of the pressure diffuser. The vent pipe may be supported by a brace arm for the inner head. The vent pipe  24  extends through the wall of the inner head at a level below the gas pocket  34 , in order to reduce the potential of gas seeping through the aperture  38  for the inlet pipe in the wall of the inner head. To the extent that cooking liquor flows through that aperture in the inner head, the cooking liquor re-enters the pulp. While it is not particularly troublesome if cooking liquor seeps through the aperture in the inner head for the pipe, it is preferable to avoid such seepage.  
         [0024]    As the pressure diffuser is pressurized, the pressure in the vessel  11  forces gases, e.g., air, air out from the gas pocket  34  underneath inner head, into the inlet  36  and through the vent pipe  24 . The vent pipe may extend generally radially out a side wall of the vessel  11 . Alternatively, the gas vent pipe  48  may extend vertically down through the top  26  of the diffuser and to the inner head where the inlet includes a funnel inlet  50  to capture gases accumulated under the inner head. The supports for the inner and outer heads in the diffuser may provide supports for the air pipe  24 .  
         [0025]    Once the pipe  24  extends through the inner head  22 , it may extend radially outward through a sealed aperture  40  in the outer wall of the pressure vessel. The aperture in the outlet head  26  of the pressure vessel for the air pipe is necessarily sealed to preserve the pressurization within the pressure vessel. Once outside the pressure vessel, the air vent pipe  24  is coupled to a gas valve  32  which regulates the flow of air from the pocket  34  and through the pipe. The gas valve  32  is used to prevent the flow of excessive amounts of cooking liquor from the center chamber through the vent pipe and out of the pressure diffuser. The valve  32  is normally closed when the inlet to the valve is submerged in cooking liquor, such as occurs when the cooking liquor totally fills the center chamber including the interior of the inner head. When the inlet  36  to the gas pipe  24  is exposed to the gas pocket, air can enter the air pipe by opening the air valve  32 . When the air valve  32  is open, air passes through the air pipe and out of the pressure vessel. Air passes through the gas vent pipe  24  and the open air valve to an atmospheric exhaust or to a gas recovery system  42 . To the extent cooking liquor enters the air pipe, such as when the cooking liquor totally fills the inner head, the amount of cooking liquor in the air pipe is relatively small and can be purged from the air pipe by the pressurized air in the air pocket.  
         [0026]    The air vent pipe  24  may be a relatively small diameter, e.g. quarter-inch pipe, suitable for handling pressurized air. An air pipe cover shield  44 , such as angle iron, may be used to shield the pipe as it extends through the slurry flow in the outlet head. The cover shield may be a ⅛ inch thick steel shield that extends from the vessel wall to the inner head just above the air pipe.  
         [0027]    In operation, the air valve  32  is operated to purge gas (air) pockets  34  forming in the inner head of the pressure diffuser. By purging air pockets, air is exhausted from the pressure diffuser without being entrained in the cooking liquor. When the cooking liquor is extracted from the pressure diffuser, it has a relatively-low air content. An advantage of the present invention is that it removes air from the pressure diffuser separately from the cooking liquor, and allows relatively-low air content cooking liquor to be extracted from the pressure diffuser.  
         [0028]    The air valve may be manually operated and on a periodic schedule, e.g., once a day, to purge any air that may have formed in the inner head from time to time. Alternatively, the air valve may have sensors  45  to detect when air is in the gas pipe. When air in the gas pipe is detected, the air valve automatically opens to purge any air from the interior head. When cooking liquor enters the gas pipe, the air valve detects the liquid present and closes the air valve. Further, a gas detector  46  may be incorporated in the pressure monitor device that extends to the inner head. The gas detector may be arranged near the apex  28  of the inner head  32  to detect air pockets  34  forming in the inner head. When an air pocket is sensed, the air valve may be opened to purge the air pocket.  
         [0029]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.