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 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.
 The present invention relates to pressure diffusers for pulp processing systems.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 Pressure diffusers
 In some conventional pressure diffusers, small vents in the top of the center chamber, e.g., the inner head
 Around the exterior of the center chamber is an annular volume
 In the embodiments of the invention disclosed here, a small gas exhaust vent pipe
 As shown in
 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
 Along with the rest of the vessel
 The air exhaust vent pipe
 As the pressure diffuser is pressurized, the pressure in the vessel
 Once the pipe
 The air vent pipe
 In operation, the air valve
 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
 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.