Paper making and fiber liberation – Processes and products – Non-uniform – irregular or configured web or sheet
Reexamination Certificate
2000-06-16
2002-06-04
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-uniform, irregular or configured web or sheet
C162S117000, C162S204000, C162S206000, C162S118000, C162S305000, C162S358300, C162S359100, C162S360200, C162S361000, C034S400000, C034S242000, C034S168000, C034S370000
Reexamination Certificate
active
06398909
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for processing a wet web of fiber material with a cellulose content, preferably a soft paper web, so as to imprint the web to impart a desired surface structure to the web and to dry the web, and for reeling the imprinted and dried web into a roll.
BACKGROUND OF THE INVENTION
In conventional production of paper, a continuous web is formed from a suspension of pulp delivered onto a moving wire, and the web with originally a very high water content is partially dewatered on a wire to form a wet web with at least some degree of coherence, although the fibers are still capable of undergoing rearrangement at this stage of the paper making process. The wet web is then fed to a press section, comprising one or several wet presses each having at least one roll nip through which the web runs, and in which roll nip still more water is mechanically pressed out of the web. The web is then made to pass through a drying section in which remaining water is evaporated to a predetermined dry solids content for the web through a supply of heat. Finally, the web is reeled up into a finished material reel or roll in a reel-up. At all web transfers between two consecutive machine sections, the web must transferred from one drive section to the next over an open or closed draw. If the web in the draw lacks support from a belt or the like then the draw is termed an open draw. At the open draw the web must bear its own weight and also be able to tolerate increased tensions caused by speed differences between the drive sections in question. The risk for web breakage in an open draw is high and therefore open draws should be avoided.
In recent years press sections have been developed that have made it possible to transfer the web from the forming section of the paper machine through all roll nips without any considerable tension on the wet paper web. Through the gradual increase of the dry solids content of the web, the strength of the web grows continually during its passage through the press section. Nevertheless, the web still is sensitive to the increased tensions, which occur at transfers between different sections of production. Furthermore, in a conventional wet press, rewetting of the web tends to occur at the outlet of every roll nip, which negates some of the gained strength of the web.
Accordingly, it would be desirable to provide a higher dry solids content of the wet web at an earlier stage of the production process. It would also be desirable to eliminate transfers and open draws throughout the pressing, drying, and reeling operations of a papermaking process, which would facilitate a higher web velocity and thereby a substantial increase in production with subsequent benefits for the paper industry. It would also be desirable to provide such benefits while also providing a higher bulk for soft paper and better quality in the final product.
Furthermore, in production of paper, board, and cardboard, currently it is common to have a very elongated drying section with a great number of drying cylinders and guide rolls that alternate with each other, around which the wet web runs during heating and evaporation of water. In production of soft paper, which has lower grammages and which for instance is used for production of household paper, paper towels and other hygiene products, it is more typical to employ a drying section that comprises either a single drying cylinder with a relatively large diameter (e.g., up to 6 m in diameter) and with a polished mantle surface that is steam heated from its inner side. i.e. a so called Yankee dryer, or a combined machine which consists of both a Yankee dryer and a number of common drying cylinders in succession. Consequently, these machines employing a large number of drying cylinders tend to be quite long.
However, attempts have been made to reduce one or several of the problems mentioned above. For instance, from U.S. Pat. No. 4,324,613 it is known that pressing of a wet web in a hot press simultaneously at increased temperature and high pressure leads to several favorable effects, e.g., a considerably increased dewatering speed and thereby a shortened drying section. The term “hot press” which is used in the present context and which constitutes an integration between a press and a drying arrangement is improperly named by many an “impulse dryer.” The patent describes a roll nip that is formed between two cylindrical rolls movable towards each other, one of the rolls being heated from the outside to a high surface temperature. The simultaneously applied high temperature and high-pressure give a high thermal flow to the web, which results in a high dewatering speed. The good dewatering most likely results in part from the considerable steam generation at the roll nip, which helps to blow or force remaining liquid out of the web, and from the increased temperature that reduces the water viscosity so that it is more easily pressed out of the web. Additionally, the water, which during the exertion of the roll nip pressure attains a temperature in excess of 100° C., rapidly evaporates when the pressure rapidly drops as the web exits the roll nip.
However, in a conventional roll nip the dwell time for the paper web is too short in order to get any real benefits from the favorable effects of the hot pressing. However, from U.S. Pat. No. 5,393,384 it is known to employ a shoe press with a concave press shoe that acts against a heated drying cylinder and forms an elongated nip through which the web passes. Since the nip of the shoe press is extended to approximately 20 to 30 cm, the dwell time of the web in the nip is substantially extended relative to a roll press, so that the web is subjected to high pressure and temperature for a much longer period of time.
Furthermore, when producing soft paper such as tissue, there is generally a desire to attain a web with a high bulk (i.e., ratio of volume to weight), since high bulk tends to provide both softness and high power of absorption.
There have been many attempts to manufacture soft paper with substantially increased bulk. For instance. U.S. Pat. No. 3,806,406 describes a method and a device for forming of a soft paper web with high bulk. A wet web is fed on a felt through a roll nip comprising a press roll and a heated Yankee dryer. Because the pressing of the web tends to reduce the bulk, the mantle surface of the Yankee cylinder comprises alternately raised and lowered surface portions, which constitutes a relief pattern for placing against the web. Thus, only those parts of the web in contact with the raised surface portions are compressed, while the parts situated in between arc left relatively unaffected. The entire drying is done thermally through the supply of heat from the Yankee dryer, during which the soft paper web is held in a fixed position in relation to the raised and lowered surface portions. Accordingly, the web acquires a certain texture, i.e., a predetermined pattern of compressed and uncompressed surface portions, which surface portions match the relief pattern of the mantle surface. The web thereby also obtains an increased bulk.
However, the Yankee dryer has some disadvantages. One problem, as mentioned above, is that the Yankee dryer has a large diameter and therefore is very voluminous. Another problem is that when using a Yankee dryer it is difficult to achieve as high a temperature as can be achieved in hot pressing. Despite the fact that the steam within the drum may have a very high temperature, the drum mantle surface typically only reaches approximately 95° C. to 100° C. The mantle of the Yankee dryer must have a considerable thickness, typically approximately 50 to 100 mm, in order to be able to resist both the internal steam pressure and the external pressure against the mantle surface. The dryer thus represents a very large mass of material that must be heated up, and the thick mantle effectively blocks the thermal flow from the drum interior to the exterior mantle surface, leading to high energy co
Alston & Bird LLP
Fortuna Jose
Valmet-Karlstad Aktiebolag
LandOfFree
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