Abstract:
A method of producing chemical cellulose pulp from sawdust utilizing a static down-flow retention vessel comprising the steps of continuously pressurizing a flow of sawdust using a progressive cavity pump to produce a pressurized sawdust feed for a treatment vessel, passing the pressurized sawdust feed at super-atmospheric pressure downwardly in the treatment vessel, adding treatment white black liquor to the treatment vessel, and treating the pressurized sawdust feed in the treatment vessel with treatment white black liquor to form a treated sawdust, and discharging treated sawdust from the treatment vessel.

Description:
[0001]    CROSS-RELATED APPLICATION 
         [0002]    This application is a non-provisional application claiming priority to U.S. Provisional Patent Application No. 62/270,494 filed Dec. 21, 2015, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Many forms of naturally occurring cellulose are used to produce chemical pulps for the production of paper. One source of cellulose is the waste from saw mills, namely sawdust. The pulping of sawdust has both advantages and disadvantages. One advantage for using sawdust as a source of cellulose is that smaller sawdust particles are relatively easy to impregnate with cooking liquor. For this reason the pretreatment systems for the chemical pulping of sawdust are less complex than those used to impregnate wood chips, which are generally more difficult to impregnate than sawdust. 
         [0004]    One disadvantage of the chemical pulping sawdust is that sawdust can be resistant to the flow of cooking liquors. The finely divided sawdust material tends to form a compact matrix when exposed to liquid flow. Exposure to this compact matrix can limit liquid&#39;s flow through the sawdust material, if not prevent liquid flow altogether. For example, because batch digesters are highly dependent upon the capability of providing a cooking liquor circulation through the medium being pulped sawdust), it is difficult, if not impossible, to pulp sawdust in a conventional batch digester. Also, conventional continuous digesters have difficulty handling sawdust without incorporating some form of special liquid distribution device. Conventional continuous sawdust pulping systems are known in the industry and are described in U.S. Pat. No. 3,684,651, EP 0157279, and U.S. Pat. No. 6,379,504 (all of which are included by reference in this document). 
         [0005]    EP &#39;279 introduces the use of a fluidizing, high-speed, degassing, centrifugal pump (such as an MC® brand pump manufactured by Kamyr AB as disclosed in U.S. Pat. Nos. 4,435,193 and 4,410,337) to pump a slurry of sawdust to a treatment step where the sawdust may be subjected to chemical treatment to produce pulp. A fluidizing, high-speed, degassing, centrifugal pump is required because the fine particles of the sawdust feed material tend to act as a solid when in a slurry and do not filter well. The fluidizing, high-speed, degassing, centrifugal pump has a fluidizer at the inlet of the pump. The fluidizer may have spokes through which the sawdust slurry is fed. As the sawdust slurry passes through the spokes, the spokes break apart the clumps of sawdust and thereby allow the solid-like slurry of sawdust feed material to be broken from a solid form into a fluidized suspension where there is no phase separation (e.g. no separation of the solids from the liquids). The fluidized suspension operates more like a Newtonian fluid than the non-fluidized suspension. Gases retained in the slurry of sawdust feed material must be removed from the sawdust feed material suspension. The removal of gases can also be accomplished by the fluidizing, high-speed, degassing, centrifugal pump. The disadvantage of the fluidizing, high-speed, degassing, centrifugal pump is the fluidizing requirement, because the fluidizer is prone to clogging. Replacing a clogged fluidizer requires the operators to deactivate the fluidizing, high-speed, degassing, centrifugal pump, which results in loss of production. Great care should generally be taken, however, to achieve sufficient fluidization in order to form a pumpable suspension of sawdust material. 
         [0006]    Another method for processing sawdust using a conventional continuous digester is given in U.S. Pat. No. 6,379,504. The &#39;504 patent utilizes a static retention vessel. A “static” retention vessel is a retention vessel without any significant internal circulation. Internal circulation typically includes (in conventional continuous digesters, for example) screens, conduits, pumps, heaters, and the like. While steam or heated liquid may be added to the pulp in the retention vessel, to ensure that the pulp is retained at cooking temperature (although that is not normally necessary), static vessels do not attempt to draw liquid uniformly through the vessel. In this manner, static retention vessels differ from conventional batch and continuous digesters. This conventional method involving a static retention vessel also includes a slurry pump. The slurry pump is located after a chute where the sawdust feed stock is diluted from an initial solids consistency of 20% to 35% at the inlet of the chute to a consistency of 10% to 15% (typically referred to as a “medium consistency slurry”) at the outlet of the chute where an inlet to the slurry pump is positioned. Transfer of medium consistency slurry by means of a pump prior to cooking is not energy efficient. Typically, such pumps are limited to medium consistency slurries of between 8% and 16% consistency. In heating such a slurry to cooking temperature, the excess liquid volume must also be heated to cooking temperature. For example, a 12% slurry contains 7.33 lbs. of liquid per pound of fiber. In contrast, a 30% slurry contains 2.33 lbs. of liquid per pound of fiber, or less than a third of the liquid per pound of fiber. The lower consistency slurry requires additional energy to heat excess liquid to cooking temperature. 
         [0007]    The fluidizing, high-speed, degassing, centrifugal pump of EP &#39;279 and the slurry pump of the &#39;504 patent both have the disadvantages of being part of a multipart, complicated, energy inefficient feed system requiring dilution liquid be added to be within a pumpable consistency. Additionally, excessive mechanical action on sawdust slurries can be damaging to fiber properties, and is otherwise undesirable. 
         [0008]    U.S. Pat. No. 3,684,651 describes a pulping system for sawdust where washed and dewatered sawdust raw material is subjected to chemical treatment in a vapor phase. The method of the &#39;651 patent involves a step where washing and impregnation of the sawdust feed material is combined and the feed sawdust material is in a finely divided state. This method provides a simple feed system, but nevertheless a feed system requiring multiple screw conveyors and a rotary feeder prior to the chemical treatment vessel. 
         [0009]    Another common method used to continuously pulp sawdust is by using drag-chain type digesters, for example, an M&amp;D-type digester as shown in FIG. 138 of Volume 5 of TAPPI&#39;s  Pulp and Paper Manufacture  (1989), Grace, ed. These types of digesters have an inclined vessel through which sawdust is moved through the cooking liquor by means of a conveyor mechanism. The method of a drag-chain type digester is simplified compared to other sawdust pulping methods, especially the method of U.S. Pat. No. 3,684,651. The drag-chain type digester, however, does require a conveyor mechanism and a rotary valve. Other related hardware requires continuous maintenance that makes a drag-chain type digester system, especially the rotary valve, less than ideal in most pulp mills. The rotary valve is a typical star-type feeder that inherently experiences an unbalanced pressure load due to the large pressure difference between the inlet and outlet of the valve. This load imbalance typically causes significant wear and requires regular rebuilding of the valve. 
       SUMMARY OF THE INVENTION 
       [0010]    The present disclosure avoids the limitations presented by prior art continuous cooking systems for sawdust, and other finely divided comminuted fibrous material, by first eliminating the need for a rotary valve as a sawdust feeder to the treatment vessel. Second, the present disclosure eliminates the need for diluting the sawdust feed material to medium consistency prior to pumping the sawdust feed material to a treatment vessel. 
         [0011]    This disclosure addresses the problems inherent in treating sawdust, or other finely divided sources of cellulose material (which is within the scope of the term “sawdust” as used in the present specification and claims, e.g. initial cellulose particles that flow more like a powder than like conventional wood chips), and provides for more efficient pulping, and thereby allows for less maintenance. The invention is practiced using a progressive cavity pump attached to the feed screw at the discharge of the receiving vessel. The progressive cavity pump has an inlet and an outlet. The inlet is operatively connected to the receiving vessel and the outlet is operatively connected to the treatment vessel. 
         [0012]    It is known to use pumps, specifically a fluidizing, high pressure, degassing, centrifugal pump or a medium slurry pump in sawdust feeding sections of a sawdust pulping system. It has, however, previously been thought that a thick stock pump such as a progressive cavity pump could not be used in such a position. 
         [0013]    A progressive cavity pump is a rotary positive displacement pump utilizing a single helical rotor within a stator within a cylindrical casing or housing. The stator has a double helical shape and is mounted or otherwise connected to the casing or housing. As the rotor moves material (in this case sawdust) in a helical motion through the stator area within the casing or housing, the sawdust is pressed and the pressure of the sawdust is increased by the motion of the progressive cavity pump. Sawdust generally exits the progressive cavity pump at a pressure higher than the entering sawdust. The sawdust also exits the progressive cavity pump without the addition of steam as pressurizing medium or liquid to form a slurry. 
         [0014]    Until recent developments to progressive cavity pumps, progressive cavity pumps were small in size, which limited a progressive cavity pump&#39;s throughput capacity and thus encouraged the use of multiple pumps operating in series to handle the flow of material in a conventional sawdust pulping system. Progressive cavity pumps also have pressure limitations. Additionally, the costs associated with purchasing and maintaining multiple progressive cavity pumps were considered to be unreasonable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The foregoing will be apparent from the following more particular description of exemplary embodiments of the disclosure, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, with emphasis instead being placed upon illustrating the disclosed embodiments. 
           [0016]      FIG. 1  is a schematic of a conventional sawdust pulping system using a rotary valve. 
           [0017]      FIG. 2  is a side schematic view of a conventional sawdust pulping system using dilution and a slurry pump. 
           [0018]      FIG. 3  is a schematic view of an exemplary embodiment of a system according to the present disclosure using a progressive cavity pump. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The following detailed description of the preferred embodiment is presented only for illustrative and descriptive purposes and is not intended to be exhaustive or to limit the scope and spirit of the invention. The embodiments were selected and described to best explain the principles of the invention and its practical application. One of ordinary skill in the art will recognize that many variations can be made to the invention disclosed in this specification without departing from the scope and spirit of the invention. 
         [0020]    The present disclosure describes a feed system for a sawdust pulping process where a progressive cavity pump is used in place of a rotary feeder to pressurize and transport sawdust feed material from a receiving vessel to a treatment vessel. 
         [0021]      FIG. 1  is a schematic view of a conventional sawdust pulping system  100  with a rotary valve  105  configured to feed sawdust  101  into the sawdust chute  122 ,  127 . Sawdust  101  is fed to a feed conveyor  102 . Recycled digester rejects  103  from other digesters in other pulping systems may also be fed to feed conveyor  102 . The teed conveyor  102  transports sawdust  101  and recycled digester rejects  103  to a receiving vessel  104 . The receiving vessel  104  may receive exhaust steam  126 . Exhaust steam  126  is purge steam  112  which has been distributed to the rotary valve  105  to aide in purging solid material caught in the pockets of a rotary valve  105 . Sawdust  101  from the receiving vessel  104  is discharged to a feed screw  124  located at the discharge end of the receiving vessel  104 . Sawdust  101  in the feed screw  124  moves from the receiving vessel  104  into an upper section of sawdust chute  127 , connecting the discharge end of the feed screw  124  and the inlet pockets of the rotary valve  105 . 
         [0022]    The rotary valve  105  is comprised of multiple pockets to receive sawdust  101  feed material from the upper section of the sawdust chute  127  at atmospheric or near atmospheric pressure and pressurize the sawdust  101  while within the rotary valve  105  to the operating pressure of the treatment vessel  106 . The operating pressure of the treatment vessel  106  may be between 2 bar and 15 bar absolute. Purge steam  112  may be distributed through purge steam distributor  114  to rotary valve  105  as chute purge steam  112 . From rotary valve  105 , pressurized sawdust  101  is transported via sawdust chute lower section  122  where treatment white black liquor  119  is added as the pressurized sawdust  101  enters the treatment vessel  106 . 
         [0023]    Black liquor  110  (source of black liquor  110  may be elsewhere in the mill) may be pumped via black liquor pump  111  to a stream of white liquor  108  associated with white liquor pump  109 . White liquor  108  and black liquor  110  may be combined to give a combined white black liquor  113 . Combined white black liquor  113  is heated in a heat exchanger  107  using purge steam  112  from purge steam distributor  114 . It may be desirable to bypass heat exchanger  107  with at least a portion of the combined white black liquor  113  as unheated white black liquor  117 . Heated white black liquor  118  may be mixed with at least a portion of unheated combined white black liquor  117  to form treatment white black liquor  119 . Treatment white black liquor  119  is used in the treatment vessel  106  to chemically treat the pressurized sawdust  101  entering treatment vessel  106 . 
         [0024]    Purge steam  112  from purge steam distributor  114  is typically provided to treatment vessel  106  as digester purge steam upper  115  and digester purge steam lower  116  to aide in the movement of sawdust  101  through the treatment vessel  106 . It is possible for only one of the digester purge steam upper  115  or digester purge steam lower  116  to be used. 
         [0025]    Sawdust  101  from treatment vessel  106  is discharged into surge tube  121 . From surge tube  121 , treated sawdust  123  may be sent for further processing, including depressurization, cooling, washing, bleaching, etc. Volatile gases are removed from treatment vessel  106  via DNCG release  120 . 
         [0026]      FIG. 2  shows a schematic diagram of a conventional sawdust pulping system  200  with dilution and a slurry pump  10  for pulping finely divided comminuted cellulose material referred to as “sawdust”  101  herein. The sawdust  101  is fed continuously by feed conveyor  11  into a receiving vessel  12  where pretreatment may take place. Pretreatment may consist of steaming or treatment with black liquor  110  or some other strength or yield enhancing chemical, for example polysulfide or anthraquinone and their derivatives. Treatment and retention in receiving vessel  12  may be from 5 minutes to 60 minutes, but is preferably between 5 minutes and 20 minutes. The receiving vessel  12  may operate at atmospheric or super-atmospheric pressures. 
         [0027]    The receiving vessel  12  may exhibit single-convergence and side relief as disclosed in U.S. Pat. No. 5,500,083 and U.S. Pat. No. 5,628,873. Receiving vessel  12  discharges into a conveyor  13  which includes a conventional conveying screw as shown in  FIG. 1 , or any other conventional means of conveying the pretreated sawdust may be provided. The conveyor  13  typically comprises a screw  13 ′ driven by a drive device such as an electric motor  13 ″, for example a variable speed electric motor. If the conveyor  13  is pressurized, some form of pressure-isolation device can be used between the receiving vessel  12  and the conveyor  13 . For example, a star-type feeder, such as rotary valve  14 , may be used. The conveyor  13  is a first mixer for mixing steam and cooking liquor with the sawdust  101 . 
         [0028]    Cooking liquor, for example Kraft white liquor, is added to the conveyor  13  in white liquor line  43  to begin the impregnation of the material with cooking chemicals. Steam may be, but is not necessarily, added to the conveyor  13  via steam line  15  to begin the heating or continue the heating of the material begun in the vessel  12  and to remove unwanted air from the material. The conveyor  13  may also include a vent  16  for releasing non-condensable gases (NCG) to a conventional NCG collection system. A slurry having a consistency of about 25% or more and a temperature of between about 125° F. to 175° F. may be discharged from conveyor  13 . 
         [0029]    The conveyor  13  discharges to a teed chute  17  in which the sawdust  101  slurry is diluted to a consistency of between about 5% to about 15%. The temperature of the sawdust  101  slurry in the feed chute  17  may be between about 150° F. to about 250° F. The feed chute  17  feeds a conventional slurry pump  18 . The slurry pump  18  pressurizes and transfers the sawdust  101  slurry to a conventional dewatering conveyor  19  via slurry conduit  20 . The slurry may be diluted to lower the consistency of about 5% to about 10% in the slurry conduit  20 , e.g. by dilution liquid (e.g. recirculated liquor, filtrate, or hot water), added via dilution liquor conduit  21 . The dewatering conveyor  19  may be a conventional separator such as a “top separator” or an “inverted top separator” or another suitable conveyor. 
         [0030]    The liquor removed from this dewatering conveyor  19 , via hot liquor line  22  is typically at about 250° F. to about 300° F., may be used as the source of dilution in the dilution liquor conduit  21 , after being pressurized in pump  23  and heated in heat exchanger  26 . All or part of hot liquor in hot liquor line  22  may be flashed to produce a source of steam using conventional flash tank  24 . For example, the pressure of the hot liquor in hot liquor line  22  may be decreased under controlled conditions, i.e. flashed, in flash tank  24  to produce a source of contaminated steam  25  and hot flashed liquor  25 ′. The contaminated steam  25  may be used as the source of steam introduced to the conveyor  13  or receiving vessel  12 . This contaminated steam  25  may be supplemented by clean steam as needed. The hot flashed liquor  25 ′ from flash tank  24  may be used as the source of dilution liquid in teed chute  17 , or elsewhere. 
         [0031]    The dewatering conveyor  19  increases the consistency of the sawdust  101  slurry to between about 20% to about 40% and discharges the sawdust  101  slurry to a conventional steam mixer  27 . The steam mixer  27  may be any conventional device (e.g. having an internal conveying screw) for introducing steam to the slurry and heating the slurry to cooking temperature, typically about 250° F. to about 350° F. (from 2 bar to 10 bar), while the slurry&#39;s consistency is being diluted by the steam addition to between about 15% to about 35%. 
         [0032]    The sawdust  101  slurry discharged from the steam mixer  27  proceeds to a retention vessel/digester  28  in which the cooking reaction is allowed to proceed. The retention time in the retention vessel/digester  28  may range from about 30 minutes to about 6 hours. It should be noted that retention vessel/digester  28  is static, that is, retention vessel/digester  28  does not include any real cooking circulations or associated screens, because cooking circulations would be difficult to operate for such a finely comminuted material as sawdust  101 . The retention vessel/digester  28  need not include an agitator at a retention vessel/digester discharge  29  but preferably includes as the discharge  29  a non-mechanical means, such as a single-convergence outlet with side relief as illustrated schematically in  FIG. 2  or liquid discharge jets or nozzles. 
         [0033]    The material is discharged through discharge  29  from retention vessel/digester  28 , typically at between about 5% and about 20% consistency, and is transferred, while still at cooking temperatures and pressures (and without destructive reduction of pressure), via transfer conduit  30  to a second treatment vessel  31 . In the second treatment vessel  31  the cooked, hot, pressurized material is cooled by means of filtrate from filtrate line  32 . The heat of the treated material entering second treatment vessel  31  is removed via liquid extraction line  33  and used, for example, as a source of heat for heat exchanger  26 . The hot liquor in liquid extraction line  33  is cooled somewhat in heat exchanger  26  and may then be sent to a conventional chemical recovery system, for example, to one or more flash tanks, to evaporators, to a recovery boiler, etc. The liquor in liquid extraction line  33  may also be used to treat material in receiving vessel  12 , conveyor  13  or feed chute  17 . 
         [0034]    The second treatment vessel  31  may be a pressure diffuser where the cooked sawdust  101  is typically cooled by diffusing the cooler liquid from filtrate line  32 , typically brownstock washer filtrate, through a pulp bed of cooked sawdust  101  (pulp) formed in the second treatment vessel  31 . The pulp is cooled to below cooking temperature (e.g. below about 250° F.) in the second treatment vessel  31 . The hot cooking liquor is displaced by the cooler liquid in this process and the hot displaced liquor is extracted as is conventional from the bottom of the pressure diffuser (in liquid extraction line  33 ). The cooled pulp is discharged from the top  34  of the second treatment vessel  31  and passed by cooled material discharge conduit  35  to a high density brown stock storage vessel  36  or the like. The pulp stored in the high density brown stock storage vessel  36  may be further treated by, for example, washing or bleaching, and sent to a paper, board, or pulp machine. 
         [0035]      FIG. 3  is a schematic view of an embodiment of a system according to the present disclosure using a progressive cavity pump. A sawdust pulping system with a progressive cavity pump  300  has been conceived. Sawdust  101  is fed to a receiving vessel  104 . Sawdust  101  is discharged from receiving vessel  104  into feed screw  124 . Connected to feed screw  124  is a thick slurry pump, specifically a progressive cavity pump  225 . The progressive cavity pump  225  has a stator piece  226  attached to the housing  228  and a rotor  227  within the housing  228 . An inlet  229  is operatively connected to feed screw  124  and an outlet  230  is operatively connected to the treatment vessel  106 . In some instances, the outlet  230  may be connected to a sawdust chute  122 A or it may be connected directly to treatment vessel  106 . If sawdust chute  122 A exists, it may be a pipe and may be short in length, such as 1 foot (“ft.”) to 2 ft. in length. If the sawdust chute  122 A is used, treatment white black liquor  119  may be added to sawdust chute  122 A. It is also possible to add treatment white black liquor  119 ′ to the outlet  230  of the progressive cavity pump  225 . If desired, it is also possible to add treatment white black liquor  119 ″ directly to treatment vessel  106 . If desired, any combination of locations for treatment white black liquor  119 ,  119 ′,  119 ″ may be used. 
         [0036]    When using progressive cavity pump  225 , it is not necessary to provide purge steam  112  to the progressive cavity pump, but it may be desirable to add purge steam  112  to treatment vessel  106  through one or both digester purge steam upper  115  and digester purge steam lower  116 . Any volatile gases produced in treatment vessel  106  are removed via DNCG release  120 . As part of the system purge, steam  112  may first flow to a steam distributor  114  and then to treatment vessel  106 . 
         [0037]    While the invention has been particularly shown and described with references to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.