Abstract:
A pulp pre-processor for the papermaking industry uses pulp fiber analyzers to determine the distribution of fiber parameters, such as length, curl or coarseness from a feed source, such as recycled pulp, and mixes metered portions of pulp from augmenting pulp sources on an as-needed basis to provide a uniform distribution of a particular fiber parameter while maximizing use of recycled pulp.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/844,439 filed on Sep. 14, 2006. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED R &amp; D 
     Not applicable 
     FIELD OF THE INVENTION 
     The present invention relates to papermaking and, in particular to a pulp pre-processor providing more uniform paper quality and having particular application for the manufacture of recycled paper. 
     BACKGROUND OF THE INVENTION 
     The process of making paper involves harvesting fibers from plants, typically trees, through a pulping process that separates the plant material into individual fibers. The resulting pulp fibers are diluted with 200 parts of water to one part fiber by weight. This fiber suspension, now referred to as stock, is supplied to a paper machine through a head box, which spreads the stock onto a moving wire to begin the process of de-watering the stock and consolidating the fibers into a formed paper sheet. 
     As water is drained from the forming sheet, individual fibers that were once relatively free to move independently become immobilized in a network of cellulose fibers held together by hydrogen bonding between the water and cellulose molecules. Water is then removed from the resulting sheet by mechanical pressing and then by drying until the finished sheet of paper is produced. 
     The production of paper has become more reliant on pulp that has been derived from waste paper recycled from many sources. Normally, recycled paper pulp is combined with un-recycled pulp to improve the quality of the resulting paper and to compensate for damage to the fibers of the recycled pulp that may occur during the recycling process. The large variation in the quality of recycled pulp effectively limits the amount of recycled pulp that may be added to the stock if a given paper quality is to be assured. 
     SUMMARY OF THE INVENTION 
     The present invention provides a pulp pre-processor that analyzes a pulp source, such as recycled pulp, and effects real-time correction of the pulp source by the addition of other pulp sources of known statistical characteristics. Real-time control can permit the addition of greater proportions of recycled pulp while still producing a uniform stock and, thus, uniform quality of paper. The present technique may also allow improved production on non-recycled paper by eliminating assumptions about pulp uniformity and allowing fine corrections of pulp consistency on a real-time basis. 
     Specifically then, the present invention provides a pulp pre-processor having a mixing cabinet and at least two pulp sources communicating with the mixing cabinet by means of controllable valves. Electronic pulp fiber analyzers associated with each of the two pulp sources provide corresponding signals measuring a statistical distribution of at least one physical parameter of the fibers of the pulp sources as delivered to the mixing cabinet. A controller receiving the signals from the electronic pulp fiber analyzers calculates a relative percentage of fiber from each of the two pulp sources necessary to produce a predetermined statistical distribution of fibers in the mixing cabinet. Based on this calculation, the controller provides real-time control signals to the valves to control them according to the calculated relative percentage. 
     Thus, it is one object of at least one embodiment of the invention to provide a more uniform stock by real-time adjustment of pulp percentages from multiple pulp sources. 
     One source may provide a distribution of fiber lengths relative to the other source that is skewed toward longer fiber lengths and the electronic pulp fiber analyzers may provide signals measuring statistical distributions of fiber length. 
     Thus, it is an object of at least one embodiment of the invention to control a fiber length distribution in the stock used to make paper by dynamically combining multiple sources having complex fiber length distributions. 
     Alternatively, or in addition, one source may provide a distribution of fiber curl relative to the other source that is skewed toward less fiber curl and the electronic pulp fiber analyzers may provide signals measuring statistical distribution of fiber curl. 
     Thus, it is another object of at least one embodiment of the invention to provide dynamic control of the distribution of fiber curl in papermaking stock. 
     The one source may alternatively or in addition provide for a distribution of fiber coarseness relative to the other source that is skewed toward greater fiber coarseness and the electronic pulp fiber analyzers may provide signals measuring statistical distributions of fiber coarseness. 
     Thus, it is another object of at least one embodiment of the invention to provide for more uniform pulp with respect to the parameter of fiber coarseness. 
     It is another object of at least one embodiment of the invention to allow use of variable pulp sources, such as from recycled paper, by allowing real-time “tuning” of the pulp fiber with respect to any one of: fiber length, curl, and coarseness. 
     The pulp pre-processor may further provide a third controllable valve leading from the mixing cabinet and a third electronic pulp fiber analyzer providing a third signal measuring a statistical distribution of at least one physical parameter of fibers of the pulp as it is delivered from the mixing cabinet and the controller may further control the relative proportions of the pulp from the two sources based on the third signal and the predetermined statistical distribution. 
     Thus it is an object of at least one embodiment of the invention to measure both the quality of the pulp being mixed and the resultant product so as to provide for improved control accommodating effects of the mixing process, transport delay, or errors in the other pulp fiber analyzers. 
     The pulp pre-processor may further include a third source and a third controllable valve leading to the mixing cabinet and a third electronic pulp fiber analyzer providing a third signal measuring a statistical distribution of at least one physical parameter of fibers of the pulp as delivered to the mixing cabinet from the third source and the controller may further control the relative proportions of the pulp from the three sources based on the predetermined statistical distribution. 
     Thus it is an object of at least one embodiment of the invention to provide for the correction of a feedstock pulp, for example, recycled fibers, in either of two control directions through the use of multiple augmenting pulp sources, for example, having longer fiber distributions and shorter fiber distributions with respect to a principal pulp source to be corrected. 
     The controller may control the relative proportion of the pulp from the two sources based on the third signal to maximize use of the third source within the constraint of the predefined statistical distribution. 
     Thus it is an object of at least one embodiment of the invention to provide a control strategy that may maximize the percentage contribution of one pulp source, for example, recycled pulp, with improved stock uniformity. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
         FIG. 1  is a block diagram of a first embodiment of a pulp pre-processor of the present invention employing three pulp sources combined in the mixing cabinet to provide improved fiber distributions under the control of a controller; 
         FIG. 2  is a schematic representation of data flow from electronic pulp analyzers associated with each of the pulp sources of  FIG. 1  to the controller, and of control signal flow from the controller to control valves associated with each pulp source; 
         FIG. 3  is a block diagram of a program executed by the controller of  FIG. 2  in determining the control signals provided to the control valves; 
         FIG. 4  is a graphic representation of the control space for two control dimensions showing a strategy of maximizing use of one pulp source while in optimizing the pulp uniformity; 
         FIG. 5  is an alternative embodiment of the pulp pre-processor employing a single pulp fiber analyzer; and 
         FIG. 6  is an alternative embodiment of the pre-processor of  FIG. 1  providing for multiple dimensions of fiber augmentation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a pulp pre-processor  10  of the present invention may use three pulp sources  12 ,  14  and  16  being generally tanks containing cellulose fibers in water, for example, held in suspension by agitating impellers (not shown). 
     Each of the pulp sources  12 ,  14  and  16  connects by means of pipes  18  to respective metering devices  20 ,  22  and  24  which may, for example, be valve and pump combinations or metering pumps operating to control the volume of flow from the pulp sources  12 ,  14  and  16  into a mixing cabinet  26 , joined to the metering devices  20 ,  22  and  24  by pipes  28 . 
     Online pulp fiber analyzers  30 ,  31  and  32  sample the pulp flowing through each of pipes  28  from metering devices  20 ,  22  and  24  on a real-time basis to provide statistical distributions of fiber parameters including at least one or all of: fiber length, fiber curl and/or fiber coarseness. Suitable pulp fiber analyzers  30 ,  31 , and  32  may be the “Kajaani FSA Online Fiber Length Analyzer” commercially available from Metso Automation of Helsinki, Finland. The operation of pulp fiber analyzers of this type is described in U.S. Pat. No. 6,311,550, hereby incorporated by reference. 
     The mixing cabinet  26  may include a mixing impeller  34  so as to mix and suspend the pulp from each of the pulp sources  12 ,  14 , and  16  as metered through metering devices  20 ,  22 , and  24 . The combined pulp is then pumped from the cabinet  26  by metering device  36  to the head box of a papermaking machine  38  or a holding tank. 
     A fourth pulp fiber analyzer  40  may sample the pulp exiting the mixing cabinet  26 , for example between the cabinet  26  and metering device  36  to monitor the combined fiber distributions. 
     Each of the pulp fiber analyzers  30 ,  31 ,  32 , and  40  provide signals to a central controller  42  which in turn provides control signals controlling each of metering devices  20 ,  22 ,  24 , and  36 . The controller  42  may further receive commands from a terminal  44 , for example, defining a desired pulp distribution and variance. A central controller  42  suitable for use in the present invention may be a Logix series controller commercially available from Rockwell Automation of Milwaukee, Wis., or other suitable device. 
     Referring to  FIGS. 1 and 2 , in the first embodiment, pulp source  14  may supply recycled paper pulp having varying fiber parameter distribution  56  reflecting a varying source of recycled fiber and affects on the fiber caused by recycling. In this example, pulp sources  12  and  16  provide augmenting pulp sources having distributions  54  and  58  intended to correct the fiber distribution  56  of the pulp of pulp source  14 . For example, pulp sources  12  may be softwood fiber having a relatively higher concentration of long fibers greater than 3 millimeters, whereas pulp source  16  may supply hardwood fiber having a relatively higher concentration of short fibers and longest fibers of around 1 millimeter, meaning that the distribution of fibers in pulp sources  12  is skewed toward longer fibers with respect to the distribution of fibers of pulp source  16 . 
     Examples of soft woods include Jack pine, Ponderosa pine, and redwood. Examples of hardwoods include sugar maple, silver birch, and aspen. 
     Referring now to  FIG. 2 , a controller  42  executes a stored control program  46  that receives a command data set  48  from a user, for example, entered through terminal  44 . The command data set  48  provides, for example, a target distribution  50  showing a desired percent of fibers in each of a number of fiber length bins, for example, at every millimeter from one to seven millimeters. Typically, the target distribution  50  will provide for error bands  52  indicating a desired tolerance in the distribution obtained. 
     The controller  42  also receives corresponding distributions  54 ,  56  and  58  from each of the pulp fiber analyzers  30 ,  31  and  32 , providing histograms counting numbers of fibers in each bin or providing weighted fiber counts (weighting the fibers according to their representative mass). In either case, the distributions  54 ,  56 , and  58  provide the same domain and range as the desired distribution. 
     The controller will also receive a distribution  60  representing a sampling of the output of the tank from pulp fiber analyzers  40 . As will be understood, this distribution  60  will normally be very close to the target distribution  50  based on the control action of the controller  42  executing the stored control program  46  to provide control signals  62  to each of the metering devices  20 ,  22 , and  24 . 
     Referring now also to  FIG. 3 , the control program  46  starts, as indicated by process block  64 , by accepting the command data set  48 . This command data set  48  may be entered manually based on empirically discovered formulas for paper or may be selected from a pre-existing library of formulas. 
     At process block  64 , the distribution  56  from the online pulp fiber analyzer  31  monitoring the recycled pulp source  14  is read and at succeeding process block  68 , amounts of augmenting pulp from pulp sources  12  and  16  are calculated based on current measurements of those pulps, taken at process block  70 , using online pulp fiber analyzers  30  and  32 . The positioning of the pulp fiber analyzer  31  may be upstream of the metering devices  20 ,  22 , and  24  and of the introduction of the pulp into the mixing cabinet  26  to provide for sufficient calculation time to control metering devices  20 ,  22 , and  24  for the right augmenting fiber addition. 
     The calculation of the necessary amounts of augmenting pulp from pulp sources  12  and  16  may be done by characterizing each of the distributions by one or more moments and using an algebraic decomposition, for example, as described in Ring, Gerard, J. F.; Bacon, Aric J., “Multiple-Component Analysis of Fiber Length Distributions”,  TAPPI Journal , Vol. 78, No. 7, pp. 224-231 (1997). Particularly when multiple-augmenting fiber sources are used, other calculation techniques may also be used, for example, those employing hill climbing techniques, or Monte Carlo or simulated annealing techniques. The predicted distribution of the mixed fibers will be a bin-by-bin summing of the distributions of each of the pulp sources  12 ,  14 , and  16  weighted by their percentage representation in the mix established by the control of metering devices  20 ,  22 , and  24 . 
     The mix of the pulp from the pulp sources  12 ,  14  and  16  may be further adjusted according to the monitored outflow distribution from fiber analyzer  40  per process block  73  to accommodate errors between the target distribution  50  and the output of the fiber analyzer  40  caused by the action of the mixing chamber or other systematic offsets. 
     Referring now to  FIG. 4 , the calculation of process block  68  produces multiple solutions  74  on a solution surface  72 . These multiple solutions may, for example, lie within the error bands  52  about a given target distribution  50 , or may be the result of different combinations of pulp from different tanks, providing competing solutions. 
     Under these circumstances, a particular solution  74 ′ may be selected so as to maximize the amount of recycled fiber used in the stock per process block  76  or alternately to maximize use of the most cost-effective fiber source and to minimize more costly fiber sources. 
     Finally, at process block  78 , metering devices  20 ,  22 , and  24  are set. 
     Upon completion of the setting of the metering devices  20 ,  22 , and  24 , the control program  46  cycles again to process block  66  to repeat these steps. 
     Metering device  36  may be controlled according to the desired delivery rate of pulp but may also be adjusted to control the dwell time of pulp within the mixing cabinet  26  to improve the mixing as may be determined by monitoring variations in the pulp distribution  60 . 
     In alternative embodiments, other physical fiber parameters such as curl or coarseness maybe be monitored by the pulp fiber analyzers  30 ,  31  and  32  instead of fiber length, and pulp sources  12  and  16  may hold pulp sources selected to provide appropriately skewed fiber distributions to allow for correction of curl or coarseness. 
     Referring now to  FIG. 5 , a simplified embodiment of the invention may, for example, include two pulp sources  14  and  12  and a single pulp fiber analyzer  31 . In this case, the pulp in pulp source  12  is pre-characterized, for example, by pulp fiber analyzer  31  before start of the pre-processing, after which the pulp fiber analyzer  31  is switched over to pulp source  12  for real-time monitoring of the pulp source  14 . The initial distribution of the fibers in pulp source  12  is provided to the controller  42  and it is assumed the pulp from pulp source  12  is essentially homogenous and invariant. 
     Otherwise, a similar control strategy as that described above may be adopted, however, with a lesser ability to correct for distribution deficiencies in the recycled pulp source  14 . Even so, the simplified pulp pre-processor  10  of  FIG. 5 , by providing precise metering of the augmenting pulp from pulp sources  12 , can potentially provide a high-quality and uniform-quality paper pulp with a large percentage of recycled fibers and efficient conservation of un-recycled fibers from pulp source  12 . 
     Referring now to  FIG. 6 , a multi-dimensional pulp pre-processor  10  may make use of a pulp source  14  and six augmenting pulp sources  12   a  through  12   f , each with corresponding metering devices  20  and pulp fiber analyzers  30 , all under the control of controller  42 . In this case, each of the augmenting pulp sources  12   a  through  12   f  may be selected to have polar distributions of a given physical pulp parameter, for example, long fiber length, short fiber length, high fiber curl, low fiber curl, fine fibers and coarse fibers. The controller  42  may thus affect multiple control loops to correct the distribution of pulp source  14  for any of these parameters. 
     This pulp pre-processor  10  provides bi-directional parameter control and thus potentially can handle a wide range recycled pulp from pulp source  14  providing uniform output pulp while incorporating a large percentage of recycled pulp into the resultant mixed pulp. 
     It will be understood that alternatively, each of the pulp sources  12   a  through  12   f  may represent different polar combinations of pairs of the parameters so that appropriate combinations of the pulp from these tanks can still effect arbitrary bi-directional correction of any distribution of fiber parameters. 
     The pulp sources  12   a  through  12   f  may be selected from pulps of particular wood species or may, in fact, be pulp sources that have been pre-processed to accentuate the desired characteristics. For example, a pulp source with a fiber length distribution weighted toward long fibers can be prepared through centrifugal separation or other known techniques. 
     It will be understood that the present process is not limited strictly to use with recycled fibers but may also be used to provide for extremely uniform pulp for exacting papermaking processes or to handle variations in un-recycled pulp sources. Further it will be recognized that multiple fiber analyzers can in fact be implemented with a single computational or optical unit shared among pulp streams so that separate real time measurements are nevertheless obtained. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.