PRESS ARRANGEMENT AND METHOD FOR PRESSING A FIBROUS WEB

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of PCT application no. PCT/EP2024/054736, entitled “PRESS ARRANGEMENT AND METHOD FOR PRESSING A FIBROUS WEB”, filed Feb. 26,2024, which is incorporated herein by reference. PCT application no. PCT/EP2024/054736 claims priority to German patent application no. 10 2023 108 253.1, filed Mar. 31, 2023, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a press arrangement for a web of fibrous material.

2. Description of the Related Art

A press arrangement is described, for example, in publications WO8808051A1 and WO2017207475A1, the disclosure of which is hereby incorporated by reference.

Machines for the production of a fibrous web generally have a press arrangement in which the fibrous web is dewatered or demoistened by way of mechanical pressure. The press arrangement is generally arranged between the forming section and the dryer section. An especially efficient type of mechanical dewatering can be achieved via a so-called extended press nip. The advantages of an extended press nip are known. The press nip is planiform and not essentially linear as in conventional roll presses. As a result, the pressure to be exerted on the fibrous web to be dewatered does not start suddenly in the press nip in the direction of travel but can be continuously brought from a low value to a high value. This reduces the risk that the fibrous web to be dewatered is crushed in the press nip. Thus, the fibrous web can be dewatered very efficiently on the one hand and in a volume-preserving manner on the other hand in an extended press nip.

The fibrous web can, for example, be guided through the extended press nip together with a felt or between two felts. In a so-called shoe press, the extended press nip can be formed between a shoe press roll and a mating roll. In contrast to the production of tissue webs, the thickness or the so-called bulk is less important in the production of paper webs, in particular packaging paper webs. For this reason, and because of the considerably higher basis weight of paper webs, especially packaging paper webs, significantly higher linear loads are also applied in the second press nip, namely linear loads of at least 500 kN/m.

Topics like “energy costs” and “CO₂-foot print” play an ever increasingly important role in the production of fibrous webs. Energy consumption in the dryer section, which to date is still heated by gas for the most part, is a major key factor. To save energy, or rather gas, it would be very advantageous if the fibrous web incoming from the upstream press arrangement already had the highest possible dry content. An obvious solution might be to simply increase the linear load in the press arrangement to achieve a greater dry content. However, this approach is only of limited practicability, as it presents the risk that the fibrous web that is to be dewatered will be crushed in the press nip. In addition, energy consumption in the press arrangement also increases with increasing linear load at the same machine speed, and with increasing linear load investment costs rise, as a considerably more massive machine frame is required.

What is needed in the art is to reduce the energy costs and/or the CO2-foot print for the production of a fibrous web, in particular a packaging paper web.

SUMMARY OF THE INVENTION

The present invention relates to a press arrangement for pressing a fibrous web, in particular a packaging paper web, including at least two presses arranged one behind the other in the direction of travel of the fibrous web, specifically a first press with a first press nip and a following second press with a second press nip which is an extended press nip, wherein the second press is designed to be operated at a linear load of at least 500 kN/m. The present invention also relates to a method for pressing a fibrous web, optionally by using such a press arrangement.

The present invention provides this press arrangement, which is characterized in particular in that the second press is designed so that a quotient of average pressure and peak pressure in the second press nip is less than 0.5.

The average pressure can be easily determined by dividing the linear load by the press zone length of the press arrangement. The linear load herein is the total force applied by the press arrangement divided by the machine width or the width of the press zone over which the total force in the press nip of the press arrangement is distributed. The press zone length is the length measured in the press nip in machine direction, over which increased pressure is exerted on the fibrous web. A very low threshold value can be set for the increased pressure, for example 0.1 MPa above normal pressure. As a rule, the press zone length is a well-known parameter of the press arrangement. It can be easily measured, for example by way of a compression film that is inserted into the extended press nip prior to pressure being applied by the press arrangement. The compression film then measures where and how much pressure is applied in the press nip. In this way, the peak pressure in the press nip of the press arrangement can also be easily determined. Such compression films with different resolutions are offered commercially, for example, by Fujifilm under the brand name “Prescale”.

The quotient of average pressure and peak pressure depends interestingly only on the geometric design of the press elements that together form the extended press nip, in particular on the geometric design of a possible press shoe, not however on the peak pressure. In other words, for a defined geometric design, the quotient of average pressure and peak pressure remains essentially constant even if the peak pressure is increased, since an increase in peak pressure leads to an essentially proportional increase in average pressure. The quotient of average pressure and peak pressure thus describes indirectly the geometric design of the press elements, in particular of a possible press shoe. Since there is an incalculable number of different geometric designs, that all result in a quotient of average pressure and peak pressure that is less than 0.5, the selection of this quotient appears appropriate in order to describe the inventive solution. It is important that for a press arrangement with an established design of press elements, the value of the quotient of average pressure and peak pressure can be determined easily and clearly.

The inventors have found that in regard to efficient dewatering of the fibrous web, it is particularly advantageous if the geometric design of the press elements of the second press, which is operated with a linear load of at least 500 kN/m, is selected so that the quotient of average pressure and peak pressure is less than 0.5. Thus, the geometric design of the press elements of the second press of the press arrangement according to the present invention differs from known press arrangements, where this quotient is always above 0.5. To achieve a quotient of less than 0.5 with a linear load of at least 500 kN/m, the product of the press zone length and the peak pressure must be relatively large. Thus, the present invention describes optional geometric designs that lead to a relatively large press zone length and at the same time a relatively great peak pressure.

The first press, located upstream of the second press, serves to pre-solidify and demoisten the fibrous web so that it is not crushed in spite of a high peak pressure in the second press nip. The press arrangement according to the present invention is not limited to the first press and the second press but can also include one or a number of additional presses if required. In particular, another press can be located upstream of the first press. Optionally, however, no additional press is provided between the first press and the second press.

In particular, the quotient of average pressure and peak pressure can be less than 0.46 and/or greater than 0.3. A geometric design of the press elements, especially the press shoe of the second press, which results in a quotient of between 0.3 and 0.46, has proven to be particularly advantageous in tests for efficient dewatering.

Regarding the linear load with which the second press can be operated, this can also be significantly higher than 500 kN/m, namely at least 1,000 kN/m, or even at least 1,300 kN/m. As remarked above, there are practical limits to an increase in the linear load due to an increasing risk of crushing the fibrous web, increasing drive energy, and rising investment costs, particularly for the design of a more solid frame. The greater the line load selected, the larger the product of press zone length and peak pressure must be selected according to the present invention in order to achieve a value of less than 0.5 for the quotient of average pressure and peak pressure.

Good results regarding the dry content increase in the fibrous web in the press arrangement according to the invention can be achieved if the second press is designed to apply a peak pressure of at least 9 MPa, optionally at least 10 MPa, optionally at least 12 MPa to the fibrous web. These peak pressures are noticeably higher than those in known press arrangements of this type.

The first press, which has the specific function of pre-solidifying or demoistening the fibrous web so that it can pass through the second press undamaged, should basically be designed differently than the second press, even if the first press nip can also be an extended press nip. In particular, the first press should be designed so that at least one of the following statements applies:

• • a) a quotient of average pressure and peak pressure in the first press nip is greater than 0.5;
  • b) the maximum linear load with which the first press can be operated is less than the maximum linear load with which the second press can be operated;
  • c) the peak pressure that can be applied to the fibrous web in the first press nip is less than the peak pressure that can be applied to the fibrous web in the second press nip.

One optional embodiment of the invention provides that the second press is a shoe press, including a press shoe with a concavely curved surface and a shoe press shell rotatably mounted around the press shoe.

It has proven advantageous if the shoe press shell consists at least partially of polyurethane, which is formed by the reaction of a prepolymer and a crosslinker component, wherein the prepolymer is a reaction product of 1,4-phenylene diisocyanate (PPDI) and a polyol-component containing at least one polyether polyol and/or at least one polycarbonate polyol, and wherein the crosslinker component contains a C_2-14 diol. It is further optional if the polyol component of the prepolymer includes poly-tetramethylene ether glycol (PTMEG) and at least one polycarbonate polyol.

Alternatively, or in addition, the crosslinking component can include poly-tetramethylene ether glycol (PTMEG) and/or at least one polycarbonate polyol. A press shell with such a polyurethane layer has proven surprisingly resilient even at high peak pressures. At the same time, this polyurethane mixture is suited for maintaining good adhesion of the reinforcement threads embedded therein, even after many alternating load cycles under high peak pressures.

According to a further aspect, the current invention relates to a machine for the production of a fibrous web, optionally of packaging testliner, including the previously described inventive press arrangement.

The invention moreover relates to a method for pressing a fibrous web, in particular a packaging paper web, optionally by using a previously described inventive press arrangement, wherein the fibrous web is guided through at least two presses consecutively, namely a first press with a first press nip and a subsequent second press with a second press nip, wherein the second press nip is an extended press nip, and wherein the fibrous web is pressed in the second press with a linear load of at least 500 kN/m, wherein the fibrous web is pressed in the second press in such a way that a quotient of average pressure and peak pressure in the second press nip is less than 0.5.

The efficacy and benefit statements previously given with regard to the inventive press arrangement apply analogously to the pressing process according to the invention and vice versa.

Thus, in the method according to the invention, the quotient of average pressure and peak pressure can be less than 0.46 and/or greater than 0.3.

Moreover, the fibrous web can be pressed in the second press with a linear load of at least 1,000 kN/m, optionally at least 1,300 kN/m.

The fibrous web is pressed in the second press with a peak pressure of at least 9 MPa, optionally with at least 10 MPa, optionally with at least 12 MPa.

Also, in the method according to the invention, the first press can also be operated in such a way that at least one of the following statements applies:

• • a) a quotient of average pressure and peak pressure in the first press nip is greater than 0.5;
  • b) the maximum linear load with which the first press is operated is less than the maximum linear load with which the second press is operated;
  • c) the peak pressure that is applied to the fibrous web in the first press nip is less than the peak pressure that is applied to the fibrous web in the second press nip.

The inventive method can be utilized especially efficiently if the fibrous web consists of at least 20 wt. %, optionally at least 50 wt. %, of OCC fibers. OCC is an abbreviation known in expert circles for “old corrugated containers”. In other words, the press arrangement according to the invention and the pressing method according to the invention are particularly well suited for efficient pressing of fibrous webs that have a significant or even a decisive proportion of recycled fibers, hence no fresh fibers. This is due to the high resistance of the OCC fibers to high pressures. The remaining fibers of the fibrous web to be pressed can be selected, for example, from mechanical wood pulp or chemical pulp, such as TMP, CTMP and/or PGW.

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 press arrangement according to the present invention, including a first press and a second press; and

FIG. 2 is an enlarged and detailed presentation of the second press of the press arrangement shown in FIG. 1.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one embodiment 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

In an entirely schematic representation, FIG. 1 shows a press arrangement according to the invention, including a first press 11 and a second press 1 arranged directly behind it in the direction of travel BR of a fibrous web 8. In this exemplary design example, first press 11 as well as second press 1 are designed as shoe presses and thus each have an extended press nip. Alternatively, however, it would be possible for first press 11 to not have an extended press nip, and/or first press 11 and second press 1 could share a common central roll. The central roll would then be a press element by way of which both the first press nip of first press 11 and the second press nip of second press 1 would be formed.

FIG. 2 shows an enlarged and detailed representation of second press 1, which is of particular importance according to the invention. It is designed to be operated with a linear load of at least 500 kN/m. Second press nip 7 features two press elements, namely a shoe press roll 2 and a mating roll 3. Shoe press roll 2 includes a press shoe 5, which is supported by a standing yoke 4, and a shoe press shell 6, which is arranged rotatably around press shoe 5. Fibrous web 8 is optionally guided through second extended press nip 7 in a sandwich-like manner between two press felts 9. Press shoe 5 has an essentially concave surface covered by shoe press shell 6 while press shoe 5 presses the latter with a high compressive force F in the direction of mating roll 3. The geometric design of the press elements, in particular of press shoe 5, is selected in such a way that a quotient of average pressure and peak pressure in second press nip 7 is less than 0.5 according to the invention. Said quotient can optionally be between 0.3 and 0.46.

The compressive force F is optionally selected to be strong enough that the peak pressure acting on fibrous web 8 in the second, extended press nip 7 is at least 9 MPa, optionally at least 10 MPa, optionally at least 12 MPa. In order to achieve a quotient of average pressure and peak pressure in second press nip 7 that-according to the invention-is less than 0.5, with such a high peak pressure and a line load of at least 500 kN/m, press shoe 5 must be relatively long in order to provide a correspondingly large press zone length for fibrous web 8 in second press nip 7.

It turned out to be especially advantageous at such high peak pressures if shoe press shell 6 consists at least partially of polyurethane which is formed by the reaction of a prepolymer and a crosslinker component, wherein the prepolymer is a reaction product of 1,4-phenylene diisocyanate (PPDI) and a polyol-component containing at least one polyether polyol and/or at least one polycarbonate polyol, and wherein the crosslinker component contains a C_2-14 diol. For example, shoe press shell 6 can have a reinforcement structure consisting of threads embedded in the polyurethane layer, wherein the prepolymer of the polyurethane layer consists of 50 wt. % of a mixture of 1,4-phenylene diisocyanate (PPDI) and C_5-6 polycarbonate diol and 50 wt. % of a mixture of 1,4-phenylene diisocyanate (PPDI) and poly-tetramethylene ether glycol (PTMEG), and wherein the crosslinker includes poly-tetramethylene ether glycol (PTMEG) and 1,6-hexanediol or is optionally formed substantially therefrom.

First press 11, shown schematically in FIG. 1, can be and optionally is designed differently than second press 1. In particular, unlike second press 1, it can be designed such that a quotient of average pressure and peak pressure in the first press nip is greater than 0.5, and/or that the maximum linear load with which the first press can be operated is less than the maximum linear load with which the second press can be operated, and/or that the peak pressure that can be applied to the fibrous web in the first press nip is less than the peak pressure that can be applied to the fibrous web in the second press nip. Optionally, all three statements apply. In the press arrangement according to the invention, first press 11 serves in particular to sufficiently pre-solidify fibrous web 8 for traveling through second press 1 so that it is not crushed to an unacceptable state in spite of a high peak pressure in second press 1.

Fibrous web 8 optionally serves for the production of a packaging paper web or is such a packaging paper web. Moreover, the fibrous web consists of at least 20 wt. %, optionally at least 50 wt. %, of OCC fibers, which are characterized by an especially high resistance even to high peak pressures.

Press arrangement 1 according to the invention could theoretically also have more presses than only first press 11 and second press 1. However, second press 1 is optionally the last press in the press arrangement, that is, the last press before fibrous web 8 is transferred to a dryer section located downstream of the press arrangement. Only in exceptional cases may there be a smoothing press arranged after first press 1 and a downstream dryer section, however, said smoothing press does not have an extended press nip.

Component Identification Listing

• • 1 second press
  • 2 shoe press roll
  • 3 mating roll
  • 4 standing yoke
  • 5 press shoe
  • 6 shoe press shell
  • 7 second (extended) press nip
  • 8 fibrous web
  • 9 press felt
  • 10 press arrangement
  • 11 first press
  • BR direction of travel
  • F compressive force

While this invention has been described with respect to at least one embodiment, 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 claims.