Patent Publication Number: US-7905989-B2

Title: Process and apparatus for producing a tissue web

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a division of U.S. patent application Ser. No. 11/527,248, entitled “PROCESS AND APPARATUS FOR PRODUCING A TISSUE WEB”, filed Sep. 26, 2006 now U.S. Pat. No. 7,582,187, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a process and an apparatus for producing a fibrous web, in particular a tissue web. 
     2. Description of the Related Art 
     Tissue paper ideally has high absorbency or a high water absorption capacity in conjunction with a high tear resistance. The absorbency and the water absorption capacity are determined substantially by the volume and porosity of the tissue paper. 
     In order to increase the volume, it has already been proposed to press the tissue paper web only zonally during its production, in order, in addition to the pressed regions of higher tear resistance, and to obtain more lightly pressed or unpressed more voluminous regions. 
     During the production of tissue paper, in a last drying step, the tissue web is led over the circumferential surface of a heated Yankee drying cylinder, before the finished product is creped off the latter. While the tissue web is being led over the Yankee drying cylinder, it is held by a fabric. 
     In particular during the production of tissue paper with voluminous regions which have been compressed only slightly during the dewatering, there is, however, the problem that the tissue paper comes into contact with the hot circumferential surface of the Yankee drying cylinder with an excessively low dryness. This problem occurs to a greater extent at high machine running speeds, since here the dewatering times are reduced further and the voluminous regions accordingly carry still more moisture with them. 
     On account of the excessively low dryness, during contact of the tissue web with the heated circumferential surface of the Yankee drying cylinder, water vapor is produced between the hot circumferential surface and the tissue web, which can lead to the web lifting off the roll circumferential surface. 
     As a result of the tissue web lifting off the roll circumferential surface, it is possible for problems to occur with account to the runnability, up to breaking of the tissue web. 
     Furthermore, on account of the formation of water vapor between the tissue web and the heated circumferential surface of the Yankee drying cylinder, it is possible for the formation of bubbles and holes in the tissue web to occur. 
     It is already known to coat the drying or Yankee cylinder in order to counter the problems occurring during a transfer of the fibrous web from a TAD (through air drying) fabric to the surface of the Yankees cylinder. In addition, a doctor arrangement has already been proposed in which a doctor crepes the fibrous web and doctors it off the Yankee cylinder, and at least one further doctor is provided for the purpose of removing a layer of the roll coating containing dirt. 
     At present, there exist two different processes for producing tissue paper. Firstly, there is the conventional tissue production process, in which the fibrous web is formed, pressed and dried on the Yankee cylinder. Secondly, there is what is known as the TAD production process (TAD=through air drying), in which the fibrous web is dried between the sheet forming zone and the press section by way of an air stream. This method is associated with a high paper quality. 
     To address the aforementioned problems, various parameters, in particular those which relate to the region of the Yankee cylinder, must be chosen in a suitable way. 
     Typical values for some known parameters are listed in the following table: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Quantity of roll coating 
                 Blade or doctor 
               
               
                   
                 material 
                 loading 
               
               
                   
                 mg/m 2 ; mL/min 
                 kN/m 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Conventional tissue machine 
                 1-3; 
                 ~2.5 
               
               
                   
                 15-25  
                 2-3 
               
               
                 TAD machines 
                  5-15; 
                 ~7.0 
               
               
                   
                 80-100 
                 6-7 
               
               
                   
               
            
           
         
       
     
     The high quantity of coating material previously required to coat the Yankee cylinder is obviously associated with economic disadvantages. The same applies to the relatively high blade or doctor loading previously required. 
     What is needed in the art is an improved process and an improved apparatus of the type mentioned at the beginning. What is needed in the art is an improved process and an improved apparatus which ensures the highest possible quality of the tissue paper and, at the same time, ensures that this high product quality can also be achieved with a lower quantity of coating material for the Yankee cylinder and a lower blade or doctor loading. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for producing a fibrous web, in particular a tissue web, having the following steps: 
     a) in a pressing zone, the fibrous web is pressed lying between the structured belt and a circulating, unstructured permeable supporting belt, 
     b) the fibrous web and the structured belt are fed to a press nip provided on a Yankee cylinder, 
     c) the fibrous web is transferred from the structured belt to the surface of the Yankee cylinder in the region of the press nip, 
     d) the surface of the Yankee cylinder is doctored off continuously and then recoated again, so that a renewed coating is always present in the press nip. 
     With this process according to the invention, a high quality of the tissue paper is achieved with, at the same time, a reduced required quantity of coating material for the Yankee drying cylinder and reduced blade or doctor loading. The fact that optimal results can be achieved in particular even with a reduced quantity of coating material and reduced doctor loading can be attributed to various factors. 
     After the fibrous web has been pressed in the pressing zone, lying between the structured belt and a circulating unstructured, that is to say relatively smooth, permeable supporting belt, the tissue web can be brought into contact with the Yankee drying cylinder with a relatively smooth side, while on the other hand, on account of the structured belt lying on the other side of the web, only part of the fibrous or tissue web is pressed. In a departure from the conventional TAD machines, the tissue web can therefore come into contact with the Yankee cylinder with approximately 100% of the surface of the relevant side, while only part thereof is pressed. 
     During the pressing of the fibrous web in the pressing zone between the structured belt and the circulating unstructured, that is to say relatively smooth, permeable supporting belt, the fibrous web is dewatered in the direction of the permeable supporting belt. In this case, the pressing pressure is preferably 1.5 bar or less. 
     Because the fibrous web is dewatered further toward the side facing away from the structured belt through the supporting and/or dewatering belt, the fibers are forced in the direction of the relatively flat or smooth surface of the supporting belt, formed in particular by a dewatering belt. The dewatering can be carried out for example by way of an appropriately high vacuum and/or mechanically, for example by way of a tensioning belt, by way of which the structured belt, the fibrous web and the supporting belt are pressed against a preferably smooth surface. According to the invention, the relevant gas stream therefore flows firstly through the permeable structured belt, then the fibrous web and finally the permeable supporting belt. By contrast, in the case of a conventional TAD process, the relevant gas stream flows firstly through the fibrous web and then the structured belt. In such a conventional TAD process the fibrous web is therefore not given a smooth surface. Apart from this, the differential pressure produced in the conventional TAD process is relatively low. 
     According to the present invention, the coating of the Yankee cylinder additionally ensures improved transfer of the fibrous web from the structured belt to the surface of the Yankee cylinder. By way of the coating, the heat transfer from the Yankee cylinder to the fibrous web is improved considerably. In addition, bubble formation, lifting of the web and so on are counteracted. Since at least part of the coating has been removed continuously again by way of the relevant doctor, no dirt can accumulate. 
     For optimal support of the aforementioned transfer of the fibrous web from the supporting belt to the surface of the tissue cylinder, an adhesive coating material is applied to the surface of the Yankee cylinder. 
     According to another embodiment of the present invention, the quantity of material applied continuously to the surface of the Yankee cylinder preferably lies in a range from about 3 to about 9 mg/m 2 . 
     In order to doctor off the surface of the Yankee cylinder continuously, a doctor or the like is loaded in such a way that the result is a line force in a range from about 3 to about 7 kN/m, preferably in a range from about 5 to about 6 kN/m. The relevant doctor loading can thus be reduced considerably without any kind of costs in terms of quality. 
     The fibrous web is formed on a circulating permeable structure belt, as a result of which the depressions of the structured belt are filled up with fibers, instead of the fibers of a web already formed being sucked into the depressions of the structured belt in a conventional TAD process. In this way, a voluminous fibrous web is produced as compared with the process in which a smoothly formed fibrous web is pressed into the depressions of a structured belt. Furthermore, as a result of forming the fibrous web between the permeable structured belt and a smooth unstructured forming fabric, the effect brought about by the dewatering of the fibrous web in the pressing zone in the direction of the supporting belt is further intensified such that approximately 100% of the surface of the relevant side of the fibrous web can come into contact with the Yankee cylinder. 
     As already mentioned, the pressing zone is expediently delimited on the side adjacent to the supporting belt by an at least substantially smooth surface. In this case, this smooth surface can in particular be formed by a rotating roll. 
     In the region of the pressing zone, a gas stream can be produced which flows through the structured belt, the fibrous web and the supporting belt, the gas stream flowing first through the structured belt, then the fibrous web and finally the supporting belt. 
     The gas stream that is produced can in particular be an air stream and/or steam stream. 
     According to another embodiment of the present invention, the gas stream is at least partly produced by way of a suction zone of a rotating suction roll, which delimits the pressing zone on the side adjacent to the supporting belt. 
     However, the gas stream can also at least partly be produced by way of a flat or curved suction box or the like, which delimits the pressing zone on the side adjacent to the supporting belt. 
     Furthermore, the gas stream can be produced at least partly by way of a positive pressure hood arranged on the side of the permeable structured belt. The positive pressure hood can be, for example, a steam blower box. 
     Alternatively or additionally, the structured belt, the fibrous web and the supporting belt can also be pressed against an at least substantially smooth surface by way of a press belt under tension. In this case, the smooth surface can in particular again be formed by a rotating roll. 
     The structured belt used is expediently a structured fabric. 
     The structured belt used can also be a (TAD) fabric, for example. 
     The fibrous web is formed in a way as has been described previously. However, in specific cases it may also be expedient to form the fibrous web by the fibrous stock suspension being sucked into the structure of the permeable structured belt by way of a vacuum device. 
     The supporting belt, as already mentioned, can in particular be a dewatering belt. 
     The relatively depressed and the relatively elevated regions of the structured belt are formed and arranged relative to one another in such a way that at most 35% and preferably at most 25% of the structured belt is pressed in the press nip. The gentlest possible pressing is achieved by the press nip provided on the Yankee cylinder being a shoe press nip. If the fibrous web to be produced does not require any bulk but does have a high dryness with a high production output, as an alternative to this the press nip formed on the Yankee cylinder can be formed with a suction press roll or a press roll. 
     According to another embodiment of the present invention, the Yankee cylinder is assigned only one doctor, by way of which, firstly, at least part of the coating is removed and, secondly, the fibrous web is creped and lifted off the cylinder surface. 
     According to another embodiment of the present invention, the Yankee cylinder is assigned a first doctor for removing at least part of the coating and a second doctor, by means of which the fibrous web is creped and lifted off the cylinder surface. 
     The outlet pocket between fibrous web and structured belt, occurring at the outlet from the press nip, is acted on by way of an air knife. 
     According to another embodiment of the present invention, in the region in which the structured belt runs off a mating roll forming the press nip with the Yankee cylinder, the outlet pocket between the structured belt and the mating roll is acted on by way of an air knife. As a result, fibers situated in the depressions of the structured belt are released, which assists the transfer of the fibrous web on the Yankee cylinder. In addition, the formation of bubbles on the fibrous web is counteracted. 
     The fibrous web removed from the Yankee cylinder again is subsequently wound up. 
     According to the present invention, an apparatus for producing a fibrous web, in particular a tissue web, includes a sheet forming zone, in which the fibrous web is formed on a circulating permeable structured belt, a pressing zone, through which the fibrous web is led lying between the structured belt and a circulating unstructured permeable supporting belt, and a press nip provided on a Yankee cylinder, through which the fibrous web is led together with the structured belt, the fibrous web in the region of the press nip being transferred from the structured belt to the surface of the Yankee cylinder, at least one doctor, which doctors off the surface of the Yankee cylinder continuously, and a coating device, by way of which the Yankee cylinder is subsequently recoated again, so that a renewed roll coating is always present in the press nip. 
     The coating device is driven or regulated in such a way that the quantity of coating material applied continuously to the surface of the Yankee cylinder lies in the range from about 3 to about 9 mg/m 2 . 
     A doctor is provided to scrape off the surface of the Yankee cylinder continuously and to crepe the fibrous web, this doctor being loaded in such a way that the result is a line force in a range from about 3 to about 7 kN/m, preferably in a range from about 5 to about 6 kN/m. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of an embodiment of an apparatus for producing a fibrous web, in particular a tissue web; 
         FIG. 2  is a fragmentary, side view of the Yankee cylinder with an associated coating device and a doctor, by way of which at least part of the coating applied is removed again continuously; 
         FIG. 3  is a fragmentary, side view, comparable with  FIG. 2 , of the Yankee cylinder with a single associated doctor, by way of which, firstly, the surface of the Yankee cylinder is doctored off and at least part of the coating applied is removed again, and by way of which, secondly, the tissue web is creped and lifted off the Yankee cylinder; and 
         FIG. 4  is a fragmentary, side view, comparable with  FIG. 3 , of the Yankee cylinder, but the Yankee cylinder being assigned two doctors. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to  FIG. 1 , there is shown a schematic illustration of an embodiment of an apparatus  10  for producing a fibrous web, which is a tissue web in the present case. 
     A headbox  12  delivers a stock suspension jet into an inlet gap  14 , which is formed in the region of a forming roll  16  between an inner circulating permeable structured belt  18  and an outer circulating forming fabric  20 , which run together in the region of the forming roll  16  and are subsequently led jointly around this forming roll  16 . 
     The structured belt  18  can be in particular a three-dimensionally structured fabric. 
     Facing the tissue web  22 , the forming fabric  20  has a side that is relatively smooth as compared with the relevant side of the structured belt  18 . 
     In contrast, the side of the structured belt  18  facing the tissue web  22  has depressed regions and regions elevated with respect thereto, the tissue web  22  being formed in these depressed and elevated regions of the structured belt  18 . 
     The structured belt  18  can be formed by a TAD fabric, for example. 
     In the region of the forming roll  16 , the tissue web  22  is dewatered substantially through the outer forming fabric  20 . Then, in the region of a deflection roll  24 , the forming fabric  20  is separated from the tissue web  22  again which, together with the structured belt  18 , is led further to a pressing zone  26 , in which the tissue web  22  is pressed lying between the structured belt  18  and a circulating unstructured permeable supporting belt  28 . 
     The permeable supporting belt  28  can in particular be a felt. In the region of the pressing zone  26 , pressure is exerted on the structured belt  18 , the tissue web  22  and the supporting belt  28 , the tissue web  22  being dewatered in the direction of the supporting belt  28  formed, for example, by a felt. 
     Since the tissue web  22  is dewatered in the direction of the permeable supporting belt  28  in the pressing zone  26 , and the structured belt  18  led through this pressing zone  26  is identical with the structured belt on which the tissue web  22  was formed, the more voluminous sections of the tissue web  22  are compressed less highly than the less voluminous sections, so that, as a result, the voluminous structure of the relevant more voluminous sections is maintained. 
     The dewatering pressure for the tissue web  22  in the pressing zone  26  is produced simultaneously, at least in some sections, by a gas stream and by a mechanical pressing force. 
     The gas stream flows firstly through the structured belt  18 , then the tissue web  22  and finally the permeable supporting belt  28 . 
     As can be seen from  FIG. 1 , the gas stream is produced by a suction zone  30  of a suction roll  32 . 
     The mechanical force applied alternatively or additionally is produced by the structured belt  18 , the tissue web  22  and the supporting belt  28  in the pressing zone  26  being led between a press belt  34  under tension and a smooth surface, which is formed here by the roll  32 , for example. 
     The pressing zone  26  is at least substantially defined by the wrap region of the press belt  34  around the circumferential surface of the suction roll  32 , this wrap region being defined by the distance between the two deflection rolls  36 ,  38 . 
     In the region  40  through which the tissue web  22  is led together with the structured belt  18 , the tissue web  22  can be subjected to at least one further drying step. 
     Following that, the tissue web  22  is led together with the structured belt  18  through a press nip  44  formed on a drying cylinder, specifically a Yankee cylinder  42 . In this press nip  44  the tissue web  22  lies between the structured belt  18  and the smooth surface of the Yankee cylinder  42 . The press nip  44  is formed by a shoe press nip. The Yankee cylinder  42  is therefore assigned a shoe press unit, here a shoe press roll  46 , in order to form the press nip  44 . As can be seen from  FIG. 1 , a hood  62  can be assigned to the Yankee cylinder  42 . 
     The fact that the tissue web has been formed between the structured belt  18  and a forming fabric  20  which is relatively smooth in relation thereto, means only the side of the tissue web  22  formed on the structured belt  18  has an undulating surface. By contrast, the surface of the tissue web  22  formed on the smooth forming fabric  20  is relatively smooth. The tissue web  22  now comes with this smooth side into contact with the surface of the Yankee cylinder  42  in the press nip  44 . The tissue web  22  therefore touches the Yankee cylinder with a relatively large area. Since the structured belt  18  in the press nip  44  is identical with the structured belt on which the tissue web  22  was formed, it is moreover ensured that the more voluminous regions of the tissue web  22  are also virtually not pressed in this press nip  44 . On the other hand, the less voluminous regions of the tissue web  22  are pressed, which means that the strength of the tissue web  22  is increased further. 
     Following the press nip  44 , the structured belt  18  is separated from the tissue web  22 , which is led on the Yankee cylinder  42  as far as a doctor  48 , by which the tissue web  42  is creped and lifted off the Yankee cylinder  42 . The tissue web  22  is subsequently fed to a reeler  50 , in which it is wound up with the aid of a pressure roll  52  to form a roll  54 . 
     Therefore, in the region of the press nip  44 , the tissue web  22  is transferred from the structured belt  18  to the surface of the Yankee cylinder  42 . The surface of the Yankee cylinder  42  is doctored off continuously, for example by way of the doctor  48 , and is subsequently recoated again by way of a coating device  56 , so that a renewed coating is always present in the press nip  44 . An adhesive coating material can be applied to the surface of the Yankee cylinder  42 . 
     The quantity of coating material applied continuously to the surface of the Yankee cylinder  42  expediently lies in a range from about 3 to about 9 mg/m 2 . In order to doctor off the surface of the Yankee cylinder  42  continuously, a doctor or the like, here for example the doctor  48  simultaneously creping the tissue web, can be loaded in such a way that the result is a line force in a range from about 3 to about 7 kN/m, preferably in a range from about 5 to about 6 kN/m. 
     The gas stream that is produced which, in the pressing zone  26 , flows firstly through the structured belt  18 , then the tissue web  22  and finally the supporting belt  28 , can in particular be an air stream and/or steam stream. 
     Alternatively or additionally to the suction roll  32 , the gas stream can in particular be produced at least partly by way of a positive pressure hood  58  arranged on the side of the permeable structured belt  18 . 
     As already mentioned, the structured belt  18  used can in particular be a structured fabric, for example a TAD fabric. 
     The supporting belt  28  provided can in particular be a dewatering belt, as already mentioned. 
     The relatively depressed and the relatively elevated regions of the structured belt  18  are preferably formed and arranged relative to one another in such a way that at most 35% and preferably at most 25% of the structured belt  18  is pressed in the press nip  44  formed with the Yankee cylinder  42 . 
       FIG. 2  shows an enlarged illustration of a section of the Yankee cylinder  42 , to which the coating device  56  and a doctor  48  are assigned, by way of which at least part of the coating  60  applied is removed again continuously. 
       FIG. 3  shows a schematic partial illustration, comparable with  FIG. 2 , of the Yankee cylinder  42  having a single associated doctor  48 , by way of which, firstly, the surface of the Yankee cylinder  42  is doctored off and at least part of the coating applied is removed again, and by way of which, secondly, the tissue web  22  is creped and lifted off the Yankee cylinder  42 . 
     In  FIG. 3 , the coating unit  56  assigned to the Yankee cylinder  42  can be seen. 
       FIG. 4  shows a schematic partial illustration, comparable with  FIG. 3 , of the Yankee cylinder  42  with associated coating unit  56 . However, the Yankee cylinder  42  is assigned two doctors  48 ′,  48 ″. The first doctor  48 ′ serves to crepe the fibrous web  22  and lift it off the cylinder surface. By way of the second doctor  48 ″, at least part of the coating  60  is removed, including dirt which has accumulated on the cylinder surface. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claim. 
     
       
         
           
               
             
               
                   
               
               
                 List of designations 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 10 
                 Apparatus 
               
               
                   
                 12 
                 Headbox 
               
               
                   
                 14 
                 Inlet gap 
               
               
                   
                 16 
                 Forming roll 
               
               
                   
                 18 
                 Structured belt 
               
               
                   
                 20 
                 Forming fabric 
               
               
                   
                 22 
                 Tissue web 
               
               
                   
                 24 
                 Deflection roll 
               
               
                   
                 26 
                 Pressing zone 
               
               
                   
                 28 
                 Supporting belt 
               
               
                   
                 30 
                 Suction zone 
               
               
                   
                 32 
                 Suction roll 
               
               
                   
                 34 
                 Press belt 
               
               
                   
                 36 
                 Deflection roll 
               
               
                   
                 38 
                 Deflection roll 
               
               
                   
                 40 
                 Region 
               
               
                   
                 42 
                 Yankee cylinder 
               
               
                   
                 44 
                 Press nip 
               
               
                   
                 46 
                 Shoe press roll 
               
               
                   
                 48 
                 Doctor 
               
               
                   
                 48′ 
                 Doctor 
               
               
                   
                 48″ 
                 Doctor 
               
               
                   
                 50 
                 Reeler 
               
               
                   
                 52 
                 Pressure roll 
               
               
                   
                 54 
                 Roll 
               
               
                   
                 56 
                 Coating device 
               
               
                   
                 58 
                 Positive pressure hood 
               
               
                   
                 60 
                 Coating 
               
               
                   
                 62 
                 Hood