Abstract:
A method and apparatus for reducing the amount of rewet of in a press section of an industrial press including a press fabric for carrying a sheet, a press for applying pressure to the press fabric and for forcing water out of the sheet and into the press fabric, and means for applying a reusable displacing agent to at least one side of press fabric, the displacing agent absorbing interfacial water upon exiting the press and preventing rewet of the sheet.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of U.S. Provisional Patent Application Ser. No. 60/620,746 filed Oct. 21, 2004 entitled “SACRIFICIAL RECOVERABLE DISPLACING AGENT FOR IMPROVED PAPER DEWATERING”, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to the papermaking arts. More specifically, the present invention relates to a reusable and recoverable sacrificial displacing agent for use with a press fabric in a press section of a paper machine.  
         [0004]     2. Description of the Prior Art  
         [0005]     During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of the cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.  
         [0006]     The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric or belt, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.  
         [0007]     The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.  
         [0008]     It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.  
         [0009]     The present invention relates specifically to the press fabrics used in the press section. Press fabrics play a critical role during the paper manufacturing process. One of their functions, as implied above, is to support and to carry the paper product being manufactured through the press nips.  
         [0010]     Press fabrics also participate in the finishing of the surface of the paper sheet. That is, press fabrics are designed to have smooth surfaces and uniformly resilient structures, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper.  
         [0011]     Perhaps most importantly, the press fabrics accept the large quantities of water extracted from the wet paper in the press nip. In order to perform this function, there literally must be space, commonly referred to as void volume, within the press fabric for the water to go, and the fabric must have adequate permeability to water for its entire useful life. Finally, press fabrics must be able to prevent the water accepted from the wet paper from returning to and rewetting the paper upon exit from the press nip.  
         [0012]     Contemporary press fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven base fabric into which has been needle-punched a batt of fine, non-woven fibrous material. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered or laminated. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.  
         [0013]     The woven base fabrics themselves take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a woven seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the machine-direction (MD) yarns thereof. In this process, the MD yarns weave continuously back and forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a paper machine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are brought together, the seaming loops at the two edges are interdigitated with one another, and a seaming pin or pintle is directed through the passage formed by the interdigitated seaming loops.  
         [0014]     Further, the press fabric may be formed of several layers. For example, the fabric may include a woven base and an intermediate layer that are laminated together.  
         [0015]     The present invention relates primarily to an improvement in the dewatering characteristics of press fabrics through the use of a sacrificial reusable water displacing agent applied to the surface of the press fabric.  
       SUMMARY OF THE INVENTION  
       [0016]     The present invention is directed to a method of reducing the amount of rewet in a press section of paper machine. The method includes steps of providing a press fabric, depositing a water containing fibrous paper web on the press fabric to form a sheet, and depositing a displacing agent on one side of the press fabric. After pressing the combination of the press fabric, fibrous paper web and displacement agent in a press nip, upon exiting the press nip, the displacement agent absorbs interfacial water and prevents, or at least minimizes rewet of the paper web.  
         [0017]     Another aspect of the present invention is directed to an apparatus for reducing the amount of rewet of a paper web in a press nip. The apparatus includes at least one press fabric for carrying a sheet, and a press for applying pressure to the press fabric and for forcing water out of the sheet and into the press fabric. The apparatus also includes means for applying a reusable displacement agent to at least one side of press fabric, and recovering the displacement agent for reuse.  
         [0018]     The present invention will now be described in more complete detail with frequent reference being made to the drawings identified below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The following detailed description, given by way of example and not intended to limit the present invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, wherein like reference numerals denote like elements and parts, in which:  
         [0020]      FIG. 1  is a profile view of a press section of a papermaking machine according to one embodiment of the present invention; and  
         [0021]      FIG. 2  is a profile view of a press section of a papermaking machine according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     A preferred embodiment of the present invention will be described in the context of papermaking press fabrics. However, it should be noted that the invention may find other applications with fabrics used in other sections of a paper machine, as well as to those used in other industrial settings.  
         [0023]     Some examples of other fabric types to which the invention is applicable include papermakers&#39; forming fabrics, papermakers&#39; dryer fabrics, through-air-drying fabrics and pulp forming fabrics.  
         [0024]     One aspect of the present invention is directed to the use of a particulate dispersion placed at the sheet/press fabric interface that is active in the press nip of a papermaking machine to successfully compete with the paper sheet in absorbing interfacial water that is between the sheet and press fabric. Typically, in a pressing process interfacial water migrates into both the press fabric and sheet exiting a nip. However, the migration of water back into the sheet results in “rewet”, which is a very undesirable occurrence in papermaking.  
         [0025]     The mechanism thought to be responsible for this undesirable rewet is the pore size of the sheet as compared to the press fabric as they both exit the nip and expand. The paper sheet typically has a smaller pore size distribution than the press fabric. It is believed that the pore size of the paper sheet is up to 10× smaller than the pore size in press fabrics containing the finest surface batt fibers. Estimates are 0.5-1 micron mean flow pore diameters in the paper sheet as compared to 10-20 microns in the press fabric. Very fine pores preferentially allow more water to re-enter the paper web structure.  
         [0026]     Previously, efforts have been made to produce a press fabric surface agent having pore sizes closer to those of the paper sheet. One of these efforts has been through the use of membranes which are integral with the press fabric and help to expedite water removal. However, these previous attempts have generally failed because these membranes suffer from poor durability when compared to a traditional press fabric having a paper contacting surface formed of fine batt fibers.  
         [0027]     One aspect of the present invention eliminates the durability limitation of the prior art membranes by eliminating the requirement that the competitive pore size material be durably affixed to the press fabric. Instead, the present invention, which is used to reduce operating costs while decreasing the amount of paper rewet, is directed to the application, recovery and reformulation of a “displacing agent” to assist dewatering in a press nip. A further aspect of the present invention is directed to a particulate collection process that recovers the water-competitive particulate either from the sheet surface, the press fabric surface, or the “water spray” exiting the nip.  
         [0028]     In  FIG. 1 , one embodiment of the pressing process using a displacement agent is shown. The agent  10  can be applied to the surface of press fabric  12  or bottom side of the paper web or the sheet as a barrier to rewet and may be either recovered immediately by the recovery means  14  after the press section  16  action, or recovered after several pressing or drying steps then recovered for reuse if the particle temporarily attaches itself to the paper web. The determination of when to recover the agent may depend on the characteristics of the agent, as well as the individual press section configurations. The application of the agent can be performed on any press.  
         [0029]     This concept is similar to what a catalyst does in a chemical reaction. In essence, the agent is not consumed, but likely needs refreshing or reformulation in for example a reformulator  18 . The agent may be applied by full width showers when the agent is dispersed in a fluid. Alternatively, the agent can be a solid and be applied from a spool in the form of a film. Also, the agent can be applied using conventional coating applications from a roll for example, as would be used to apply a coating or sizing agent on a paper web.  
         [0030]     The two primary requirements for the displacing agent are that it, compete for boundary layer water as well or better than the paper web to be dewatered, and that it is not intrinsically attached to the paper web that is being dewatered in order for subsequent collection and reformulation to occur.  
         [0031]     A further aspect of the present invention is shown in  FIG. 2  where displacing agent is recovered from the surface of the paper sheet by contact with another PMC fabric or belt specially designed to attract the displacing agent. In this regard a displacing agent may be applied to an underside of a press fabric as a particulate and carried to the nip by the press fabric to interface with the paper sheet in the nip.  
         [0032]     A substantial percentage of the displacing agent may temporarily adhere to the paper sheet surface exiting the nip; however, this can be easily removed. In  FIG. 2 , a second papermaker&#39;s belt or fabric  20  may be used to attract and remove the agent from the paper sheet  22 .  
         [0033]     The belt  20  may, for example, have a smoother surface than the paper sheet, or perhaps a surface that capitalizes on a characteristic of the displacing agent to separate it from the paper sheet  22  without adversely effecting the paper sheet  22 .  
         [0034]     Modifications to the above would be obvious to those of ordinary skill in the art, but would not bring the invention so modified beyond the scope of the appended claims.  
       EXPERIMENTS  
       [0035]     Testing has been undertaken to demonstrate the effectiveness of the displacing agent described above when compared to either a press fabric alone or a press fabric in combination with a membrane. In addition, the experiments were undertaken to show that the displacing agent could effectively be reused and recycled. Finally, an experiment was undertaken to determine if there were significant advantages to the use of the displacing agents and a surface membrane. These experiments were undertaken on a device known as a press nip simulator (“SPNS”) which can simulated the pressure, residence time, and pressure pulse that exist in commercially available press nips. The results of these experiments are shown in  FIG. 3 . Note, our  FIG. 3  reference to “SRDA” is when the sacrificial recoverable displacing agent is used.  
         [0036]     As can be seen with reference to  FIG. 3 , the press fabric alone had a final dryness of only 34.6%, whereas when the press fabric was used with the displacing agent described herein, the final dryness increased to over 45%. This is exceeds the dryness achieved by a pressfabric and a surface membrane which resulted in a dryness of 44.6%. It was found that the drying capabilities of the displacing agent were not significantly reduced when the agent was reused and recycled. Final dryness for the recycled particles was 44.7% which, though less than on the first pass remained favorably comparable to the press fabric and the surface membrane. Finally, an experiment was undertaken to determine whether combined use of a displacing agent and a surface membrane resulted in even greater effect. The result showed that though when used in combination there was a somewhat increased dryness achieved, 46.1% final dryness.  
         [0037]     Accordingly, the testing demonstrates that the use of a displacing agent as described by herein achieves improved drying characteristics which are comparable or exceed that achieved by the use of surface membranes as previously known. Further, even when reused these displacing agents retain the ability to achieve an acceptable final dryness.