Paper making and fiber liberation – Processes and products – Non-uniform – irregular or configured web or sheet
Reexamination Certificate
2002-08-07
2004-09-14
Fortuna, José A (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-uniform, irregular or configured web or sheet
C162S109000, C162S207000, C162S359100, C162S358300, C162S361000, C162S358500, C162S296000, C034S400000, C034S111000, C034S132000, C034S618000, C034S236000
Reexamination Certificate
active
06790315
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apparatus and methods for drying a continuous tissue paper web in a papermaking machine. The invention relates more particularly to a drying section and method in which the web is passed through a pre-drying section and a final drying section, wherein the pre-drying section has at least one press that includes a first press device and a counter roll arranged in contact with each other so as to form a press nip therebetween, and an endless fabric is arranged to pass through the press nip together with the web.
The term “tissue” as used herein is intended to include all kinds of soft hygiene paper, whether creped or not, including but not limited to handkerchief paper, bathroom tissue, and towel.
BACKGROUND OF THE INVENTION
In the production of soft paper or tissue at lower basis weights, which is used for the production of household paper such as paper towels and other sanitary products, it is a general requirement that the bulk, i.e., the ratio between the volume and the weight of the paper, should be substantially higher than for other types of paper. Paper with a high bulk exhibits a desired combination of softness and high water absorption capacity.
The present commercially used technology to dry a continuous tissue web in a drying section employs large-diameter cylinders, so-called Yankee dryers, which are steam heated internally, or employs porous cylinders (so-called TAD cylinders) on which thermal through-air drying of the web is performed by blowing hot air through the web, either outwardly from the inside of the cylinder or inwardly from the outside of the cylinder.
Use of Yankee dryers for drying tissue has a major disadvantage in that the smooth surface of the cylinder causes the web to be compressed as it is pressed onto the Yankee dryer by means of a press roll, which is wrapped by a felt that carries the web to the Yankee dryer. This is especially the case since the web generally is still relatively wet when it is transferred to the Yankee dryer, leading to its structure and bulkiness being impaired by the treatment on the Yankee dryer. Hence, in a conventional drying section for tissue, only limited bulk levels can be reached. Typically, the web is creped from the Yankee dryer in order to somewhat improve its softness and bulkiness, but generally that improvement is lost when the tissue is used and thereby becomes wet.
In the development of drying sections for tissue, it has also been suggested to partially dewater the web in a press section comprising one or more press nips, possibly of shoe press type, before finally drying the web on a conventional drying cylinder such as a Yankee dryer. A papermaking machine of this type, including a pre-drying section and a final drying section, is disclosed in U.S. Pat. No. 5,393,384, the web being carried by an impermeable belt through the drying sections. Although a certain beneficial effect results from this type of system, the press section cannot be made adequately efficient, since only limited linear loads can be used in the press nips in order to avoid compressing the tissue web and thereby reducing the bulk of the web. Consequently, one press is usually not sufficient to reach the desired dry content levels in the press section, and hence at least two presses have to be used, which requires more space, investment costs, and energy. Despite using at least two presses, the resulting tissue becomes relatively flat and compact, with a bulk that is lower than the desired high bulk. Another problem with the use of such presses is that rewetting of the web at the outlet from the press nip usually occurs, which reduces the effectiveness of the press in dewatering the web.
Drastically improved bulk levels have been reached by the use of thermal through-air drying cylinders, i.e., TAD cylinders. Typically, the bulk is increased by about 60 to 200%, compared to the bulk levels that are reached with a conventional press nip followed by a Yankee dryer. Usually, the TAD cylinder is preceded by a suction apparatus, by which an imprinting pattern is achieved in the web in its wet state. The web is thereafter dried in the TAD cylinder, by inward or outward air flow, while retaining the structure of the imprinting pattern and thereby retaining the high bulk of the web. In the TAD drying, the imprinted web structure is, in a matter of speaking, frozen in its structure.
The TAD cylinder, however, has the disadvantages that it requires a lot of space, has a limited capacity, and yet requires a lot of energy. Moreover, the TAD cylinder requires the use of very large air volumes that have to be handled. Also it entails high investment costs.
Another known method to dry a fibrous web is to use so-called impulse drying. The wet web is pressed at a high temperature and with a high linear load in a press section comprising one or more press nips. The technique is described in SE 7803672-0, corresponding to U.S. Pat. No. 4,324,613, and is used in web types other than tissue webs, or for tissue webs with limited bulk. In these patents, there are described maximum specific pressures of 3-8 MPa and surface temperatures on the counter roll of a press of conventional type of between about 150° C. and 350° C. By “conventional type” is meant a press nip in which two rolls with cylindrical cross sections are counter-acting against each other under pressure. The time that a given region of the moving web resides in this type of press nip, however, is only a few milliseconds because of the short length of the press nip in the machine direction, which is too short a time for the beneficial effects of the high pressure acting at a high temperature to be fully developed. Therefore, it has also been suggested to use impulse drying in a heated shoe press in which the press nip is extended to about 20-30 cm, giving much longer residence times for the web in the nip.
U.S. Pat. No. 5,556,551 discloses such an impulse drying process for drying paper webs such as toilet paper. The web and a water-absorbing felt are fed into a press nip of a shoe press. The web may be heated by steam prior to its entry into the nip, or the smooth surface of the counter roll may be heated prior to the nip. Delamination, caused by the sudden expansion of flash steam when the web leaves the high-pressure nip, is said to improve the volume and softness of the web. The dried web is creped from the counter roll by a doctor. High bulk levels cannot be reached by the process of U.S. Pat. No. 5,556,551, since the web will be compressed in the nip between the absorbing felt which has a flat surface and the smooth surface of the counter roll.
Yet another known method to dry tissue web is to use a drying cylinder, such as a Yankee dryer, as the counter roll in a press nip. The roll that co-acts with the counter roll may also be equipped with a press shoe so that an extended nip is formed. Systems of this type are shown in DE 196 54 345 (FIG. 5) and DE 43 21403 (FIG. 11). The press shoe may include a heating device. In U.S. Pat. No. 3,806,406, there is shown a system for the formation of a tissue web, which system also is of the type with a press nip between a first roll and a Yankee dryer. This system seeks to avoid compressing of the high-bulk tissue web by providing the surface of the Yankee dryer with a relief pattern with depressions and elevated parts therebetween. Essentially, only the parts of the web that abut the elevated parts of the cylinder surface are pressed together in the press nip, the intermediate parts being relatively unaffected. The web is thereby provided with an imprinting pattern consisting of parts that are pressed together and other parts that are not pressed together, corresponding to the pattern of the cylinder surface.
Despite several methods, devices and systems being known for drying tissue webs, there is no commercially available drying section that gives a high bulk, high water absorption levels, a good softness, and a distinct imprinting pattern in the web, while also requiring only a relatively small
Alston & Bird LLP
Fortuna José A
Metso Paper Karlstad AB
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