Abstract:
Method and apparatus for introducing liquors into a batch digester to produce enhanced or plug-flow of the liquors through the bed of cellulosic material. Wash out liquor can be introduced into the digester during the discharging in a manner to add wash out liquor into the digester and to prevent formation of a vortex of material during discharge.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This present invention relates to improved methods and apparatus for delignifying and discharging the delignified cellulosic materials from a Batch Digester during and at the end of a pulping or cooking step.  
         [0002]     In conventional batch pulping operations, the wood material (softwoods, eucalyptus, or hardwoods) or the lignocellulosic crop material (e.g., bagasse, bamboo, kenaf, reeds, and so forth) is reacted with cooking liquor for a given time at a specified temperature. The cooking liquor may be any of kraft, soda, alkaline, sulfite, polysulfide, or modifications thereof, such as with anthraquinone. At the desired terminating point of the delignification reaction, the cooked material still resides at high pressure and high temperature inside the digester.  
         [0003]     Conventional Batch Reactors (digesters) used for producing fibrous cellulose from fiber bearing sources, such as wood, have traditionally been filled with the wood chips and cooking chemicals required to liberate the cellulosic fiber. Recently, new processes have emerged that reuse stored liquors (spent liquor) from the previous batch cooks to save energy and to take advantage of the residual chemicals in the spent liquor. Some of these new types of digesters require that 1) the reuse of spent liquor is pumped into the bottom of the digester with the excess liquor exiting out the top of the digester (hence they are referred to as Displacement Batch Digesters) and 2) the contents of the digester after delignification must be pumped from the bottom of the digester with gravity as the only pressure available inside the digester during most of the discharging operation feeding the discharge pump.  
         [0004]     In order for all the contents of the digester to have the same chemical activity and to produce consistent quality pulp, the liquor that is pumped into the bottom of the digester must flow upward in a plug flow (i.e. no channeling) fashion. The fluid dynamics of this type of arrangement favors the rising liquors to follow the path of least resistance which is to flow against the smooth outer digester walls instead of flowing uniformly through the chip bed. Therefore, the rising fluid tends not to form a plug flow profile and the contents of the digester are not exposed to the same chemicals and temperatures.  
         [0005]     Conventional Batch reactors (digesters) used for producing fibrous cellulose from fiber bearing sources, such as wood, have traditionally been emptied by opening the hot pressurized vessel to an atmospheric tank. This process is referred to as a “blow” and the atmospheric tank is referred to as a “blow tank”. Recently, new processes using batch digesters have been developed to remove the hot liquor from the pressurized digester before the “blow” thereby cooling the contents which precludes the violent flashing that occurs from high temperature liquor flashing into a blow tank from a pressurized vessel. The processes have been commercialized by various companies and are marketed under trade names such as RDH, Superbatch, CBC and DDS. Since the cooled digester does not have pressure and thermal energy to flash (blow), the contents (the product pulp fiber) must be pumped out of the digester in what is sometimes referred to as a discharge operation.  
         [0006]     The discharging of the cooled digester contents is very difficult because the material in the digester, the product cellulose fiber, does not flow smoothly due to the pulp&#39;s inherent thixotropic properties. This also occurs because the fibers in the pulp may clump together and the black liquor in the pulp can easily separate from the mixture leaving behind fibers stuck inside the digester.  
         [0007]     The commercial systems use nozzle jets at the bottom of the digester to spray fluid into the pulp mass in order to direct and maintain the flow of pulp with liquor out of the digester. The spray nozzles may be placed so as to create a circular flow of liquor in the digester. The flow out of the digester must exit through a “drain” opening at the bottom of the digester by gravity alone in an operation very similar to draining of a sink or tub of water. Unfortunately, a vortex forms due to the earth&#39;s rotation which interferes with the flow into the drain opening (outlet flange). The vortex in a large vessel such as a digester becomes substantial and the air funnel in the middle of the vortex can block 50% or more of the drain opening which greatly interferes with the draining process.  
         [0008]     U.S. Pat. Nos. 4,814,042; 6,719,878; 5,800,674; H1,681; 4,764,251; 4,849,052; 6,306,252; 6,346,166; 6,346,167; 6,350,348; 6,391,628; 6,451,172 and 6,514,380 disclose various aspects of the delignification (cooking) process for providing wood pulp including the removal of the delignified pulp from the digester.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The present invention pertains to methods and modifications to a Displacement Batch Digester to facilitate in-flow of pulping liquors through the material being pulped or cooked and the removal of cooked (delignified) pulp from any type of batch digester. The methods involve introducing liquid or liquor adjacent the bottom of the digester to accomplish a plug-flow regime in the digester during the cooking or pulping phase and also to introduce liquor during the discharge phase.  
         [0010]     The apparatus of the invention involves modification to the digester to facilitate introduction of the wash out liquor (liquid) into the digester to assist in the discharging of the pulp. An overhead shower can be used to aid in emptying the cooked material. Mechanical devices can also be placed in the lower digester area to aid in removing the cooked pulp.  
         [0011]     Therefore, in one aspect the present invention is a method of discharging delignified cellulosic material from a cooled Batch Digester at the end of a cook cycle, after removal or cooling of the hot liquor in the digester, comprising the steps of; introducing a shower of clean out liquor into the digester at a location above the material, the shower distributing the liquor substantially evenly across a cross-sectional area of the digester, and withdrawing the pulp from the digester while maintaining a flow of liquor sufficient to maintain consistency of the pulp until said digester is empty.  
         [0012]     In another aspect the present invention is a batch digester of the type having a generally cylindrical section and a bottom generally conical section the conical section having an outlet located proximate the apex of the conical section, the improvement comprising; installing a distribution channel for liquid inside the digester at a location proximate a transition from the cylindrical section to the conical section, the distribution channel installed adjacent an inner surface of the digester and having means to direct a plurality of streams toward a vertical center line of the digester.  
         [0013]     In yet another aspect the present invention is a batch digester of the type having a generally cylindrical section and a bottom generally conical section the conical section having an outlet located proximate the apex of the conical section, the improvement comprising; a plurality of distribution channels on the inner surface of the conical section the distribution channels extending from proximate where the conical section meets the cylindrical section to proximate the outlet, the distribution channels containing means to direct a plurality streams of liquor toward a vertical center-line of the digester.  
         [0014]     In still another aspect the present invention is a method for enhancing plug-flow of pulping liquors introduced into a batch digester for delignifying cellulosic materials comprising the steps of: introducing a plurality of streams of the pulping liquor proximate a bottom location of the digester, the streams oriented to flow in a direction generally parallel to a vertical axis (up or downward flow) of the digester; and continuing flow of the liquor until the cellulosic material is delignified to a desired delignification.  
         [0015]     In still another aspect the present invention is a method for enhancing plug-flow of pulping liquors introduced into a batch digester for delignifying cellulosic materials comprising the steps of: introducing a plurality of streams of the pulping liquor proximate a bottom location of the digester, the streams oriented to flow in a direction generally parallel to a vertical axis (up or downward flow) of the digester; and continuing flow of the liquor until the cellulosic material is removed from the digester.  
         [0016]     In still a further aspect the present invention is a method of enhancing plug flow of pulping liquor introduced into a bottom of a batch digester for counter-current flow through a charge of cellulosic material to be delignified comprising the steps of: placing a flow directing device in the digester at a location proximate a bottom portion of a vertical portion of the digester, the flow directing device causing the pulping liquor to flow in a direction generally vertical to a center line of the digester; and containing flow of the liquor until the cellulosic material is delignified to a desired delignification. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]      FIG. 1  is a schematic representation of a conventional system used to introduce cooking liquor into a batch digester.  
         [0018]      FIG. 2  is a schematic representation of two different methods and apparatus to facilitate introduction of cooking liquors into cellulosic material during the cook in a Batch Digester.  
         [0019]      FIG. 3   a  is a schematic representation of a method and apparatus to facilitate removal of cooked pulp from a Batch Digester.  
         [0020]      FIG. 3   b  is a schematic representation of a device to aid the method and the apparatus of  FIG. 3   a.   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Referring to  FIG. 1 a  batch digester  10  is used to pulp or cook lignocellulosic material. As part of the cooking process cooking liquor represented by arrow  12  is introduced into the bottom of Digester  10  and forced under pressure to flow upwardly through the lignocellulosic material shown as  14 . Although the intent is to have the cooking liquor move in a plug flow regime, in practice this does not happen. In fact arrows  16 ,  18  illustrate the flow path of liquor inside the digester and line  20  illustrates the resulting flow pattern of liquor inside digester  10 . As stated above this condition leads to inconsistencies in the pulp/chemical interface and problems in removing cooked pulp from the digester.  
         [0022]     According to the present invention three modifications can be made to a Displacement Batch Digester to mitigate the problems discussed above. The first is a liquor distribution channel placed at or above the bottom tangent (knuckle) line of the digester. The second is the use of liquor distribution channels placed on the bottom of the digester cone to distribute liquor and to help mitigate the formation of a large vortex during discharge or to inject liquors to entice plug flow. The third is to add a top shower head to keep the pulp on the top of the discharging mass diluted and to break up the flocs (which has been tested).  
         [0023]     As shown in  FIG. 2 a  generally circular or toroidal distribution channel or pipe  22  is put inside the digester  10  at or above the digester tangent or knuckle line  26 . The tangent or knuckle line  26  is the location where an upper generally cylindrical portion  11  of digester  10  is joined to a lower or generally conical section  13 . A pipe, man-way or outlet  15  is placed at the apex of the conical section  13  of digester  10 . The channel  22  is constructed so that cooking liquor is pumped into the digester in generally vertical streams across the lignocellulosic material as shown by arrows  24 . A minor portion of the liquor flowing into channel  22 , as shown by line  23 , is withdrawn and introduced into the digester through outlet  15  as shown by line  30 . The channel  22  could also be put at a lower position in the digester  10 . The liquors flowing into the digester inside channel  22  will be distributed through perforations in channel  22  selectively located towards the center of the digester  10  as shown as  22 a. That is, the channel will have perforations towards the center line of the digester to allow the inflow liquor to exit the channel towards the center of the digester. A small percentage of the liquors put into channel  22  are permitted to exit the distribution channel  22  via line  30  and be directed into the bottom of the digester  10  in the traditional method used today for displacement type cooking. The liquor entering channel  22  will perform two functions: one, it will keep the channel  22  clear of deposits and unwanted debris and two, it will allow treatment of the material located below the distribution channel  22  and above outlet  15 . The distribution channel can be one or more pieces that could form several spokes. However, due to obstruction inside the digester, more than two channels (i.e. a simple cross pattern) is not suggested unless the digester diameter warrants it.  
         [0024]      FIG. 2  also illustrates an alternate embodiment of the invention where two or more liquor channels  34 ,  36  can be placed on the inner surface of the conical portion  13  of digester  10 . Channels  34 ,  36  will extend from a location proximate the tangent  26  to a location proximate the discharge outlet  15 . Perforations or holes will enable liquor introduced into channels  30 ,  32  to exit in a generally vertical direction as shown by arrows  38 ,  40 . As opposed to the embodiment discussed above, all of the liquor will be introduced into channels  34 ,  36  individually or via a common inlet. More than two channels  34 ,  36  can be used, depending upon the size, etc. of the digester.  
         [0025]     As shown in  FIG. 2  the arrangement of the channels according to the invention will result in plug flow pattern through the lignocellulosic material or batch  14  inside the digester as illustrated by line  42 .  
         [0026]     Referring to  FIG. 2  the liquor channels  22 ,  34 ,  36  placed at the bottom of the digester  10  can be pipes, conduits or hollow channels to allow liquor flow into the digester through perforations or holes. The liquor channels could be used to dilute the pulp while the pulp passes the metal channels. The channels would be connected to a liquor supply pipe(s). Radial liquor channels or solid bars would also minimize the creation of a large vortex that interferes with the draining of the digester by dissipating the kinetic energy of the swirling liquor in the bottom of the cone. Creating obstructions on the smooth interior surface of the cone by placing channels therein would also help to improve flow of pulped material out of the digester.  
         [0027]     Referring to  FIG. 3   a  batch digester  50  would be fitted with a pipe shown as  52  to inject liquor into the top of the digester. The liquor would exit the pipe  52  through a distribution shower head  54  to evenly distribute the liquor over the digester&#39;s cross sectional area as shown by arrow  56 . This flow of liquor would maintain the proper consistency in the pulp that is the last to exit the digester (the pulp floating and coagulating on the top of the digester contents) which is also the most probable to be “dewatered” as the discharging operation progresses. Further, as the level of the pulp in the digester decreases, the liquor from the shower head will have a greater distance to fall which increases it&#39;s kinetic energy. This momentum in the liquor must be dissipated into the pulp (as a force due to the change in momentum) which helps to break apart the pulp flocs and increase the flow out of the digester. This method and apparatus has been successfully tried at a commercial installation. In place of channels  22 ,  30 ,  32  solid members which would not distribute liquor but would act only as an energy dissipation device to minimize the creation of a large vortex during the draining of the digester can be used.  
         [0028]     Obstructions attached to the bottom conical section of the digester will dissipate the circular fluid energy and mitigate the creation of a large interfering vortex. Such devices can be in the form of anti-vortex vanes  60  shown schematically in  FIG. 3   b.  The vanes would be placed on the inside surface of the conical section  51  of digester  50  extending from a location proximate the bottom tangent  53  to a location proximate the outlet  58  of digester  50  as sown by lines  60  in  FIG. 3   a.  The digester will drain in a shorter time due to the absence of a vortex by making available the full area of the drain opening as well as creating less foam from air being entrained into the liquor. The anti-vortex vanes could increase in height normal to the digester shell as the distance to the center drain opening decreases or any other configuration (e.g. constant height). Anti-vortex vanes are constructed to end at the drain opening as illustrated schematically in  FIG. 3B . Additionally, the anti-vortex vanes could be made in the form of a hollow channel with openings or passages on the surface of the channel facing upwardly in the digester to allow liquor to be injected into the digester at any phase of the cooking operation (e.g. discharging or liquor charging).  
         [0029]     Having thus described my invention what is desired to be secured by Letters Patent of the United States is set forth in the appended claims which should be read without limitation.