Abstract:
Methods and apparatus to condition the paper fiber stock of a wet paper web for enhanced dewatering and formation with an acoustic foil assembly used for removing water and redistributing fibers. A plane ultrasonic wave field is employed for interacting with water suspended pulp and paper fiber stock using acoustic techniques to subject particulate suspensions. The continuous process employees a piezoelectric ceramic transducer to selectively deflect flowing paper fiber stock as they penetrate the ultrasonic field. Depending upon the amount of dissolved gas in water, conditioning is obtained using a traveling wave field to improve paper properties for increased productivity and formation quality with headbox components having hydrodynamic optimization for paper and board forming. Acoustic transducers formed within the foil of a forming table control the fluidization of the paper fiber stock of a wet paper web to generate machine direction strain for imposing a shear in the cross-machine direction for increasing the orientation of paper fiber stock in the cross-machine direction. This generates shear layers which would result in cross-machine orientation of fibers and therefore would increase the strength and other physical properties in the CD.

Description:
CROSS-REFERENCE To RELATED APPLICATION  
       [0001]    This application claims benefit of U.S. Provisional Application No. 60/272,509, filed Mar. 1, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of Invention  
           [0003]    The present invention relates to a papermaking machine for making a continuous paper sheet from a wet paper web, and more particularly to conditioning for enhanced dewatering and formation with an acoustic foil assembly used for removing water and redistributing fibers within the suspension of the forming fabric to improve paper properties for increased productivity and formation quality with headbox components having hydrodynamic optimization for paper and board forming.  
           [0004]    2. Description of the Prior Art  
           [0005]    Currently in a papermaking machine, as the jet from a headbox impinges on moving wires, the layers of fibers are formed on the wire as the drainage occurs. This layer acts as an extra drainage-resisting mechanism on the forming wire. Paper fiber stock activity in a Fourdrinier forming table is critical to the production of a good sheet of paper. Generally, stock activity can be defined as turbulence in the fiber-water slurry on the forming fabric. This turbulence takes place in all three dimensions. Activity plays a major part in developing good formation by impeding stratification of the sheet as it is formed, by breaking up fiber flocs. As water is removed, activity becomes more difficult because the sheet becomes set, and because water, which is the primary media in which the activity takes place, becomes scarcer. Good paper machine operation is therefore a balance between activity and drainage. As the drainage proceeds, more fibers are formed and reduces the drainage amount of water.  
           [0006]    Conventional paper forming apparatus used primarily in the paper and board industry consists of a unit which is used to transform paper fiber stock, a dilute pulp slurry (i.e., fiber suspended in water at about 0.5 to 1 percent by weight) into a rectangular jet and to deliver this jet on top of a moving screen (referred to as wire in the paper industry). The liquid drains or is sucked under pressure through the screen as it moves forward leaving a mat of web fiber (e.g., about 5 to 7 percent concentration by weight). The wet mat of fiber is transferred onto a rotating roll, referred to as a couch roll, transporting the mat into the press section for additional dewatering and drying processes. The device which forms the rectangular jet is referred to as a headbox. These devices are anywhere from 1 to 9 meters wide depending on the width of the paper machine.  
           [0007]    In the forming section, foils and suction boxes are used for dewatering of the pulp suspension delivered from the headbox. As soon as the forming jet impinges on the forming board, a layer of concentrated fiber mat forms on the forming wire. It can be shown that by re-fluidizing the fiber mat at the early stages of the forming table, the dewatering rate increases resulting in additional drainage capacity along the same length of the forming section. Furthermore, this action enhances formation of the sheet by generating additional activity on the wire with tremendous benefits in quality and fiber savings. While such benefits can result, the prior art systems are limited to using undulating foils on slow speed machines. These undulation foils are ineffective in the forming section of the paper machine in that they yield an uncontrollable and inefficient system that is difficult to optimize.  
           [0008]    Often, Fourdrinier tables are mechanically shaken or vibration/oscillation elements are provided to promote stock activity, especially on slower, narrower machines. While the shaking might be a good way to enhance formation, it is undesirable because it is difficult and expensive to control and maintain, and generally punishing on the equipment on and around the Fourdrinier table. High quality typically means good formation, uniform basis weight profiles, uniform sheet structure and high sheet strength properties. These parameters are affected to various degrees by paper fiber distributions, fiber orientations, fiber density and the distributions of fines and fillers. Optimum fiber orientations in the XY plane of the paper and board webs which influences MD/CD elastic stiffness ratios across the width is of significant importance in converting operations and end uses for certain paper grades.  
           [0009]    A table roll causes a large positive pressure pulse to be applied to the sheet resulting from water under the forming fabric being forced into the incoming nip formed by the roll and forming fabric. This positive pulse has a positive effect on stock activity by causing flow perpendicular to the sheet surface. Similarly, on the exiting side of the roll, large negative pressures are generated which greatly enhance drainage. Foils are used to promote and control activity and drainage. A vacuum pulse is generated by the nip formed by the forming fabric and conventional foil as the fabric passes over the foil. Activity is generated by using a number of consecutively placed foils, encouraging a positively reinforced activity in the stock. The foils may require replacement periodically, particularly in a high-speed operation. It is important to have a mounting system to enable rapid replacement of the foils. Another type of foil incorporates a positive incoming nip to generate a positive and negative pressure pulse. The amplitude of the pressure pulse is determined in a large part by the angle formed by the fabric and the incoming edge of the foil. This type of foil simulates a table roll, but with much lower amplitude positive and negative pressure pulses.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention shows that the fiber layer initially forming on the wire as the forming jet impinges on the wire can be re-fluidized removing the resistance to drainage as well as redistributing the fibers in an isotropic formation. The net effect is to increase the rate of drainage as well as reduce the MD/CD ratio delivered from the headbox. Considering that the degree of re-fluidization can be accurately and easily controlled by adjusting the power delivered to the acoustic foils, the re-fluidization and consequently the rate of drainage and the redistribution of the fiber orientation can be easily optimized on-line as during the machine operation, and as the machine parameters change.  
           [0011]    One objective of the present invention is to distribute the fibers on the forming table and create a flow with large uniform shear in the cross-machine direction. The wire can only move in MD. It can be shown that the maximum shear occurs near the surface of the forming wire. By imposing a uniform shear in the cross-machine direction on the forming table/wire, the average fiber orientation or polar angle cans shift from MD to CD.  
           [0012]    The other objective is to increase the rate of drainage by re-fluidizing and, consequently, removing the resistance due to the initial layer of fiber forming on the forming wire. Furthermore, the easily adjustable control on the rate of drainage will allow profiling the water removal in the CD by placing narrow section of adjustable acoustic foils in the CD from the left to the right side of the forming wire. The acoustic foils will now be in the form narrow blocks placed next to each other under the wire, much like the control system on a slice or the dilution control devices in the headbox in operation today.  
           [0013]    To align more fibers in CD on average can be performed by imposing shear in CD on the forming table. To increase the CD elastic stiffness or CD strength of paper or board can be accomplished by generating a cross-stream component or velocity component in CD which is constant in CD or varies periodically in CD. The variation can be spatially periodic in the CD. The velocity component in the cross-machine direction results in large CD component of shear. This will increase the tendency of fibers to be aligned in the CD. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:  
         [0015]    [0015]FIG. 1 is a cross-sectional view of a prior art blade arrangement;  
         [0016]    [0016]FIG. 2 is a cross-sectional view showing the paper forming machine headbox and wire components;  
         [0017]    [0017]FIG. 3 illustrates use of acoustic transducers on a forming board for increasing water drainage and formation in accordance with the present invention;  
         [0018]    [0018]FIG. 4A is a polar diagram illustrating the anisotropic fiber orientation with a preference to the machine direction; and  
         [0019]    [0019]FIG. 4B illustrates the reorientation of paper fibers providing isotropic fiber orientation for discharge as a web product resulting in higher cross-machine direction (CD) strength providing fiber orientation substantially equally in all in-plane directions as shown in the polar diagram. 
     
    
       [0020]    While the invention will be described in connection with preferred embodiments described herein, it will be understood that it is not intended to limit the invention to these embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Referring now to the drawings in detail in known paper forming machines, the Fourdrinier forming table  10  depicted in FIG. 1 shows a long blade or foil  12  that has undulations which generally decline in the machine direction (MD). The forming wire  14  traverses a path immediately above and is supported by the foil  12  and then immediately above and supported by trail blade  16 . Drainage is enhanced if the area between the foil  12 , the forming wire  14 , and trail blade  16  remains flooded with surface tension maintained between the water above and below the forming wire  14 . A counterflow zone  18  is formed above foil  12 . Water present both above and below forming wire  14  is drained through a passageway  20  immediately between foil  12  and trail blade  16 . In the area of the undulations of the foil  12 , one or more generally downwardly extending vents  22  may be formed at the foil  12 . The vents  18  may be used to allow liquid flow therethrough to equalize the pressure with the atmosphere. This venting of any counterflow to atmosphere equalizes the pressure above and below the forming fabric  14  and controls the downward force on the forming wire  14 , controlling deflection with respect to the trail blade  16  and eliminating the vacuum or deflection of the forming wire  14 . The control of the venting across the surface of the foil  12  provides cross-machine profile control or variable drainage in the machine direction (MD). The surface of the foil  12  may be undulating either locally or across the cross-machine direction (CD).  
         [0022]    [0022]FIG. 2 illustrates an embodiment of a paper forming machine headbox component  24  for receiving a paper fiber stock and generating a rectangular jet  30  therefrom for discharge upon a wire component  32  moving in a machine direction (MD). One method to increase shear in CD is by generating a uniform secondary flow should inside the headbox. The uniform secondary flow should have a uniform velocity component in CD. The velocity component in CD is a function of the height or can be constant with respect to height. Applicant&#39;s assignee has also patented componentry and a design for a distributor  26  provided, e.g., for distributing the paper fiber stock flowing into the headbox component  24  in a cross-machine direction (CD) which would be generally perpendicular to the machine direction of the wire component in a conventional hydraulic headbox. See, e.g., U.S. Pat. No. 6,153,057 to Aidun issued Nov. 28, 2000 for “Methods And Apparatus To Enhance Paper And Board Forming Qualities”.  
         [0023]    The distributor  26  supplies a flow of paper fiber stock across the width of the headbox  24  in the machine direction. A nozzle chamber  28  is shown having an upper surface and a lower surface converging to form a rectangular output lip defining a slice opening for the rectangular jet at opening. As shown, the paper fiber stock flows as indicated by the arrows in the nozzle chamber  14  to output the rectangular jet  30  upon the forming wire  14 .  
         [0024]    In accordance with a preferred embodiment of the present invention, acoustic foils can be used for enhanced dewatering and formation by creating the motion of redistributed fibers locally within the suspension and excitation of forming fabric. This may be performed by placing ultrasonic transducers, e.g., acoustic transducers  34 ,  36 , and  38  discussed below, at the early stages of the drainage process below the wire, preferably on the forming board of a Fourdrinier machine. In a twin-wire former, the transducers can be installed around the forming roll and outer-wire side alternatively. When ultrasonic waves are applied to pulp fibers in a liquid suspension, the fibers move away from the source due to the acoustic force acting on the fibers. The acoustic force consists of three major forcing mechanisms acting on a fiber. These are acoustic radiation, acoustic streaming, and cavitation bubble-induced force. These mechanisms are inter-dependent and non-linear as well as frequency-dependent. It is the combination of these forces that contributes to the manipulation of pulp fibers suspended in liquid. This generates shear layers which would result in cross-machine orientation of fibers and therefore would increase the strength and other physical properties in the CD.  
         [0025]    For a Fourdrinier machine, this fiber-manipulation effect can be used at the early stages of the forming table to generate controlled normal force on the fiber mat in the forming section to: (1) re-fluidize the fibers in the fiber mat to increase the drainage rate, and (2) generate controlled activity on the forming wire to enhance formation.  
         [0026]    With reference to FIG. 3, one example is to simulate the effect of acoustic force on the forming table  32 , a 5×10 cm acoustic transducer at 150 kHz as described in the present preferred embodiment. The transducers  34 ,  36 , and  38  were sealed off at the bottom of an enclosure of the forming table  32 . A forming fabric was put on the transducer. A thin layer of fiber mat (approximately 3 mm) was positioned right above the forming fabric. Next, an acoustic pulse of 4 W/cm was applied to the fiber mat and is forming fabric. The transducers  34 ,  36 , and  38  may be used to control liquid flow and equalize the counterflow, re-fluidization, and pressure therein. The use of the acoustic foil transducers  34 ,  36 , and  38  in the foil  12  may be used to equalize the pressure above and below the forming wire  14  and therefore controls the downward force on the paper fiber mat formed therein while controlling fluidization. The control can be uniform or non-uniform across the surface of the foil  12  for cross-machine profile control or variable drainage in the machine direction. The transducers  34 ,  36 , and  38  can be throttled independently or in gangs of any combination. The surface of the foil  12  can be oscillated locally or across the cross-machine (CD) direction. The acoustic force caused excitation of forming fabric and fibers within the mat.  
         [0027]    In the forming table section  32 , foils and suction boxes are also often used for dewatering of the pulp suspension delivered from the headbox. As soon as the forming jet impinges on the forming board, a layer of concentrated fiber mat forms on the forming wire  14 . It can be shown that by re-fluidizing the fiber mat at the early stages of the forming table, the dewatering rate increases resulting in additional drainage capacity along the same length of the forming section. Furthermore, this action enhances formation of the sheet by generating additional activity on the wire  14  with tremendous benefits in quality and fiber savings. While such benefits can result, the prior art systems are limited to using undulating foils  12  on slow speed machines. These undulation foils  12  are ineffective in the forming section of the paper machine in that they yield an uncontrollable and inefficient system that is difficult to optimize.  
         [0028]    Fiber mat forming very early on the forming board can be disturbed by re-fluidization creating less resistance to drainage, therefore increasing the rate of drainage. Furthermore, the fluidization of the fiber mat re-orients the fibers to some degree and consequently reduces the machine direction/cross direction (MD/CD) ratio. In prior known systems, undulating foils  12  (with wavy surface) are used to push back some of the water into the wire from below, thereby re-fluidizing the fiber by injecting liquid back into the layer. In the present invention, the surface of a foil  12  or a drainage element or part of the forming board is equipped with one or more acoustic transducers as shown in FIG. 3, to provide for a more efficient process of re-fluidization, whereby injection of water back into the layer is eliminated or greatly reduced.  
         [0029]    By replacing the undulating foils  12  or the like with the acoustically excited foils using transducers  34 ,  36 , and  38 , a normal force due to acoustic pressure on the fibers can be exerted on the fiber mat re-fluidizing the fibers in the fiber mat  18 . The present invention has the advantage of superior control and on-line optimization capability for any given grade or process condition. The paper machine forming table thus receives the paper fiber stock discharged upon the wire component moving in a machine direction (MD) and an information processor in communication with a transducer coupled with the foil component controls the fluidization of the paper fiber stock of a wet paper web discharged upon the wire component. The information processor and acoustic transducers align paper fibers in the cross-machine direction (CD) as formation occurs in the wet paper web with dewatering being controlled by energizing the transducer coupled with the foil component under the control of the information processor.  
         [0030]    One or more of the acoustic transducers formed within the foil of a forming table controls the fluidization of the paper fiber stock of the wet paper web to generate machine direction strain for imposing a shear in the cross-machine direction for increasing the orientation of paper fiber stock in the cross-machine direction. In addition, the acoustic transducers control the fluidization of the paper fiber stock as dewatering occurs of the wet paper web progressing along the transducers for paper fiber orientation increasing the elastic content and distribution of strength in the web as between the MD to the CD direction.  
         [0031]    The shear on the forming table  32  is the dominant factor in orienting the fibers in the average sense. By manipulating the hydrodynamic, the principal direction of shear can be oriented such that the average angle of the fiber distribution is forced to be large with respect to the machine direction, and therefore, add to the cross direction (CD) strength properties. Oriented shear and activity are both shear-producing processes that differ only in their degree of orientation on a fairly large scale, that is, a scale that is large compared to the size of individual fibers. Oriented shear is shear flow having a distinct and recognizable pattern in the undrained fiber suspension. Cross Direction (“CD”) oriented shear improves both sheet formation and test. The primary mechanism for CD shear (on paper machines that do not shake) is the creation, collapse, and subsequent recreation of a well defined Machine Direction (“MD”) orientation in the paper fiber stock. This can be done either by directing the flow in the headbox such that the jet/wire interaction results in a large CD orientation, or achieve the same result by facilitating predetermined drainage which directs the flow sideways.  
         [0032]    The acoustic section is composed of piezoelectric ceramic transducers mounted to generate a propagating ultrasonic wave field upon excitation. Transducers  34 ,  36 , and  38  and optimally absorbers are removable and easily replaceable. Both the flow development and the acoustic sections have similar rectangular cross-sections. One dimension is, e.g., several centimeters (corresponding to the width of transducers) and the normal dimension is adjustable. Mechanical separation is provided with an adjustable mechanical foil assembly.  
         [0033]    The narrow-band, single-element transducers  34 ,  36 , and  38  were designed to resonate at, e.g., 60 kHz, 150 kHz or 1.5 MHZ (different transducers). Their rectangular cross-section insures paper fiber penetration with the ultrasonic field submitted to the same acoustic dwell length. Although different transducers  34 ,  36 , and  38  may be used for different frequencies, they are typically made of a piezoelectric ceramic material, but any device for generating an ultrasonic signal responsive to a time varying input will suffice the transducer  34 ,  36 , and  38 . In the embodiment, slicing of the piezoelectric material, in the transducers  34 ,  36 , and  38 , was used to optimize thickness mode vibrations and field uniformity.  
         [0034]    With reference to FIGS. 4A and 4B, whereas the ratio of the major axis to the minor axis in FIG. 4A illustrates anisotropic fiber orientation with a preference to the machine direction, FIG. 4B shows an isotropic fiber orientation which is achieved when the paper fibers are oriented approximately equally in all in-plane directions resulting in the circular polar diagram of FIG. 4B. It will be appreciated that the isotropic fiber orientation results in higher cross-machine (CD) strength as indicated from the dashed line polar plot outline in FIG. 4B extending in the solid circular polar diagram in the CD direction. The additional CD strength accordingly results in the ability to manufacture a paperboard product having lighter weight sheets with the same CD performance, additional use of recycled fiber product, improved printing surface and superior dimensional stability. Additionally, the process results in a system requiring less energy consumption and thus fiber conception while providing increased productivity advantages. Additionally, common problems such as twist, warp, and sheet curl, and the dependence upon jet wire speed, are substantially alleviated.  
         [0035]    With reference to FIG. 4B, the CD fiber alignment of the resulting web product has a fiber orientation polar plot achieving a strength distribution orientation that illustrates the ability to turn the axis of the fiber content being produced in the web product more towards the CD direction with less fibers being oriented in the MD direction. To increase the CD elastic stiffness or CD strength of paper or board can be accomplished by generating a cross-stream component or velocity component in CD which is constant in CD or varies periodically in CD. The variation can be spatially periodic in the CD. The velocity component in the cross-machine direction results in large CD component of shear. This will increase the tendency of fibers to be aligned in the CD. This provides for the strength in the CD direction as being as large as the strength in the MD direction. Accordingly, the specification of paper manufacture for packaging and other paper products with respect to the CD machine direction strength may be achieved with less fiber content.  
         [0036]    Once the strength in the CD direction is increased, the production process may be optimized for predetermined weight per square meter of sheet of paper or a certain amount of fiber content to meet minimum CD strength specifications. Of course, under such circumstances the strength in the MD direction will be lower because of the reorientation of fibers, providing less fibers oriented in the MD direction. However, the MD direction usually has substantially more strength than the CD direction in conventional paperboard manufacture.  
         [0037]    It will be appreciated by those skilled in the art that modifications to the foregoing preferred embodiments may be made in various aspects. The present invention is set forth with particularity in the appended claims. It is deemed that the spirit and scope of that invention encompasses such modifications and alterations to the preferred embodiment as would be apparent to one of ordinary skill in the art and familiar with the teachings of the present application.