PRESS ARRANGEMENT AND METHOD FOR PRESSING A FIBROUS WEB

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation, under 35 U.S.C. § 120, of copending International Patent Application PCT/EP2024/057156, filed Mar. 18, 2024, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German Patent Application DE 10 2023 108 255.8, filed Mar. 31, 2023; the prior applications are herewith incorporated by reference in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a press arrangement for pressing a fibrous web, in particular a packaging paper web, including a main press with an extended press nip, where the press nip has a length of at least 150 mm, preferably at least 190 mm. The invention further relates to a method for pressing a fibrous web, particularly a packaging paper web such as packaging testliner, in which the fibrous web is guided through a main press having an extended press nip of at least 150 mm in length, preferably at least 190 mm in length.

Such a press arrangement and method are described, for example, in International Publication WO 2017/207475 A1, corresponding to U.S. Pat. No. 11,286,617 B2, the disclosure of which is incorporated herein by reference.

Machines for producing a fibrous web regularly include a press arrangement in which the fibrous web is dewatered by mechanical pressure. The press arrangement is typically located between the forming section and the dryer section. A particularly efficient type of mechanical dewatering can be achieved via a so-called extended press nip. The advantages of an extended press nip are well known: the press nip is an area contact rather than substantially linear as in conventional roll presses. As a result, the pressure exerted on the fibrous web in the press nip does not rise abruptly at nip entry in the machine direction, but increases continuously from a low value to a high value. That reduces the risk of crushing the fibrous web in the press nip. Accordingly, in an extended press nip the fibrous web can be dewatered both efficiently and with minimal loss of bulk.

For example, the fibrous web may be guided together with one felt or between two felts through the extended press nip, wherein the extended press nip may be formed in a shoe press between a shoe press roll and a backing roll. Unlike the production of tissue webs, in the production of paper webs, and particularly packaging paper webs, the thickness or “bulk” is of less importance. For that reason, and because of the considerably higher basis weights of paper webs, especially packaging paper webs, significantly higher line loads are employed in the press section, namely line loads of at least 500 kN/m.

The issues of energy costs and CO₂ footprint are becoming increasingly important in the production of fibrous webs. Energy consumption in the dryer section, which today is almost exclusively gas-heated, is a major contributing factor. In order to reduce energy or gas consumption, it would be highly advantageous if the fibrous web leaving the upstream press arrangement already had the highest possible dry content. A straightforward idea might be simply to increase the line load in the press arrangement to achieve a higher dry content. However, that approach is only practicable to a limited extent, since it increases the risk of crushing the fibrous web in the nip. In addition, at a given machine speed, energy consumption in the press arrangement also rises with increasing line load. Moreover, investment costs increase with increasing line load, because a substantially more massive framing structure is required.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a press arrangement and a method for pressing a fibrous web, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and which reduce the energy costs and/or the CO₂ footprint in the production of a fibrous web, in particular a packaging web.

With the foregoing and other objects in view there is provided, in accordance with the invention, a press arrangement, which comprises a main press configured such that, when operated with a line load LL of at least 1,200 kN/m, a line-load ratio LLR of at least 0.69 and at most 1.52 is obtained. The line-load ratio LLR is defined as the quotient of a weighted line load WLL to the line load LL, the weighted line load WLL being the result of an integration of the squared local pressure p(x)², weighted with a weighting factor A, over the nip length x, the weighting factor A being one divided by ten megapascal, and the line load LL being the result of integrating the local pressure p(x) over the nip length x, such that the following formula for the line-load ratio LLR applies:

LLR = WLL LL = ∫ A * p ⁡ ( x ) 2 ⁢ dx ∫ p ⁡ ( x ) ⁢ dx = ∫ 1 10 ⁢ MPa * p ⁡ ( x ) 2 ⁢ d ⁢ x ∫ p ⁡ ( x ) ⁢ d ⁢ x

Advantageous embodiments set forth in the dependent claims.

For better understanding, reference is made to the diagram in FIG. 3, which schematically illustrates how the line-load ratio LLR can be determined. To this end, the actual pressure profile p(x)—that is, the course of the pressure acting along the nip length x of the extended press nip of the main press—is first determined. In the present example, the main press has an extended press nip of 260 mm. For simplicity, it is assumed that the local pressure p(x) increases linearly from 0 MPa at the beginning of the extended press nip at x=0 mm to 12 MPa at the end of the extended press nip at x=260 mm, so that the pressure profile p(x) is a straight line. Determination of the local pressures that together form the pressure profile can be carried out, for example, using a pressure-sensitive film inserted into the extended press nip before the main press applies pressure. The film then records, for example by using the piezoelectric effect, the specific local pressures present in the nip. In practice, the pressure in the cross-machine direction is generally constant, so only the pressure profile in the machine direction, i.e. along the length of the extended press nip, is relevant. At most, minor variations in pressure may occur in the cross-machine direction at the edge regions, but such edge effects should be disregarded here. Pressure-sensitive films with different resolutions are commercially available, for example from Fujifilm under the trade name “Prescale.” Preferably, the measurement is performed prior to start-up of the press, after installation of a fresh felt, in particular a fresh and still dry felt used for guiding the fibrous web through the extended press nip. The measurement is preferably carried out at the maximum operating line load of the press.

If the pressure profile p(x) is integrated over the nip length x of the extended press nip, the line load LL is obtained. In the present example according to FIG. 3, the line load LL corresponds to the triangular area under the straight line representing the pressure profile p(x). In this case it amounts to 1,560 kN/m. In this way the accuracy of the measurement can also be readily verified by comparing the line load LL obtained by integration with the value that was set as the line load LL of the main press. It should be noted that the line load LL specifies the total force exerted by the main press per meter width in the cross-machine direction, i.e. orthogonal to the running direction (machine direction, MD) of the fibrous web.

Based on the measured pressure profile p(x), the curve of the weighted squared local pressure can then be determined using the following formula:

1 10 ⁢ MPa * p ⁡ ( x ) 2 .

This results in the curved line shown in FIG. 3, which intersects the straight line of the pressure profile p(x) at 10 MPa. The area beneath this curved line corresponds to the weighted line load WLL. FIG. 3 clearly shows that local pressures below 10 MPa lead to a smaller area, while pressures above 10 MPa lead to a larger area compared with the triangular area representing the line load LL. In the present example, the weighted line load WLL is 1,250 kN/m. This yields a line-load ratio LLR, defined as WLL/LL, of 0.8, which falls within the claimed invention, namely in the range:

0.69 ≤ WLL LL ≤ 1 . 5 ⁢ 2

In practice, the pressure profile curve p(x) need not be a straight line. Rather, there is an almost limitless variety of possible pressure profiles p(x), even if the line load LL, i.e. the area under the curve, remains the same. The shape of the pressure profile curve p(x) depends primarily on the geometric configuration of the press elements forming the extended press nip, in particular on the configuration of any press shoe in a shoe press.

Until now, only the line load LL has been used to characterize the pressure profile in an extended press nip. The inventors, however, have recognized that this parameter alone is insufficient to accurately characterize the pressure profile, and hence the dewatering behavior in the press nip.

Because integration inherently averages values, a constant pressure profile of p=10 MPa over the entire nip length yields the same line load LL as a profile with p=0 MPa in the first half and p=20 MPa in the second half of the nip. Yet the effects of these two pressure profiles on dewatering and sheet structure are, however, markedly different.

Through the weighting in the weighted line load WLL, this difference becomes apparent. In the first case (p=10 MPa), WLL=LL, in other words a line-load ratio LLR of 1. In the second case, WLL=1.5×LL, i.e. a line-load ratio LLR of 1.5.

The ratio therefore provides a simple and efficient way of distinguishing between pressure profiles with the same line load, without the need to analyze the detailed course of the pressure curve. In general, for the same line load, pressure profiles with large fluctuations and higher peak pressures tend to yield larger LLR values than balanced profiles with moderate pressure values.

It is the contribution of the inventors to have discovered that particularly efficient dewatering of the fibrous web can be achieved when the line-load ratio LLR is at least 0.69 and at most 1.52, under the boundary conditions that the line load is at least 1,200 kN/m and the press nip length is at least 150 mm. Where the extended press nip is provided by a shoe press, the backing roll facing the shoe preferably has a diameter of less than 3,000 mm.

The line-load ratio LLR thus indirectly characterizes the geometric configuration of the press elements, in particular of any press shoe. Since there is an almost unlimited number of geometric configurations that may result in a line-load ratio LLR between 0.69 and 1.52, the use of the line-load ratio LLR is an appropriate way to describe the inventive solution. Importantly, for a press arrangement with a fixed configuration of press elements, the line-load ratio LLR can be determined simply and unambiguously. Moreover, it is within the ability of the skilled person, with knowledge of the present invention, to configure the press elements such that the desired line-load ratio LLR is obtained.

In known press arrangements, the line-load ratio LLR under the above boundary conditions is always below 0.69. By contrast, in the press arrangement according to the invention, the press elements are configured such that a line-load ratio LLR between 0.69 and 1.52 is obtained, preferably between 0.71 and 1.35, and more preferably between 0.73 and 1.14. It has surprisingly been found that such a line-load ratio LLR results in more efficient dewatering of the fibrous web compared with known presses whose line-load ratio LLR lies outside this range, and in particular below 0.69.

An interesting sub-range is that for LLR values less than one, i.e. the range between 0.69 and 0.99, or between 0.71 and 0.97, or between 0.73 and 0.95.

The inventors have further found that, for efficient dewatering of the fibrous web, it is advantageous if the press arrangement also includes a pre-press located upstream of the main press in the running direction of the fibrous web, preferably directly upstream. The purpose of the pre-press is to preconsolidate and dewater the fibrous web sufficiently such that it is not crushed despite the high peak pressure in the main press. The inventive press arrangement may also include one or more additional presses as required. In particular, another press may be located upstream of the pre-press.

The press arrangement with a pre-press can be realized in many different ways.

In particular, the pre-press may likewise preferably have an extended press nip, and may be configured such that its line-load ratio LLR is less than 0.69.

Alternatively or additionally, the pre-press may include a conventional roll nip.

A particularly advantageous embodiment is a press type marketed by the Applicant Voith under the name “DuoCentri NipcoFlex” press. In this configuration, two press nips are provided on a central roll. The first press nip is a conventional roll nip functioning as the pre-press.

The main press is then formed by a shoe press nip defined between a shoe press roll and the central roll acting as the backing roll.

Another alternative embodiment includes three shoe presses disposed in series, the main press being formed by either the second or the third of these shoe presses.

With regard to the line load at which the main press can be operated, this may also be higher than 1,200 kN/m, namely at least 1,300 kN/m, or even significantly higher. As noted above, however, practical limits are placed on increasing the line load due to the increased risk of crushing the fibrous web, the rising drive energy, and the higher investment costs, particularly the need for a more massive framing structure.

Although the peak pressure cannot be increased arbitrarily for the reasons described above, it is preferably above 10 MPa, which, when used together with the inventive line-load ratio LLR, leads to a significant increase in dry content without impermissibly crushing the fibrous web.

In a preferred embodiment, the pre-press is a shoe press including a press shoe with a substantially concave surface and a press jacket (press belt) rotatably supported about the press shoe.

It has proven advantageous for the press jacket to be made at least in part of polyurethane formed by reaction of a prepolymer and a crosslinker component, the prepolymer being 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 the crosslinker component containing a C2-14 diol. More preferably, the polyol component of the prepolymer includes polytetramethylene ether glycol (PTMEG) and at least one polycarbonate polyol. Alternatively or additionally, the crosslinker component may include polytetramethylene ether glycol (PTMEG) and/or at least one polycarbonate polyol.

A press jacket with such a polyurethane layer has proven surprisingly resistant even at high peak pressures. Peak pressures well above 10 MPa do not present a problem. At the same time, this polyurethane composition has been found to maintain good adhesion to reinforcement threads embedded therein, even after many load cycles at high peak pressures.

With the objects of the invention in view, there is also provided a machine for producing a fibrous web, preferably a packaging testliner, comprising a press arrangement according to the invention as described above.

With the objects of the invention in view, there is concomitantly provided a method for pressing a fibrous web, in particular a packaging paper web such as a packaging testliner, preferably using a press arrangement according to the invention as described above, wherein the fibrous web is guided through a main press having an extended press nip of at least 150 mm in length, preferably at least 190 mm. The main press is operated with a line load of at least 1,200 kN/m, preferably at least 1,300 kN/m, and is configured such that a line-load ratio LLR of at least 0.69 and at most 1.52 results. The line-load ratio LLR is defined as the quotient of a weighted line load WLL to the line load LL, the weighted line load WLL being the result of integration of the squared local pressure p(x)², weighted with a weighting factor A, over the nip length x, the weighting factor A being one divided by ten megapascal, and the line load LL being the result of integrating the local pressure p(x) over the nip length x, such that the following formula for the line-load ratio LLR applies:

LLR = WLL LL = ∫ A * p ⁡ ( x ) 2 ⁢ dx ∫ p ⁡ ( x ) ⁢ dx = ∫ 1 10 ⁢ MPa * p ⁡ ( x ) 2 ⁢ d ⁢ x ∫ p ⁡ ( x ) ⁢ d ⁢ x

The statements regarding the effects and advantages set forth above for the inventive press arrangement apply correspondingly to the inventive method and vice versa.

Accordingly, in the inventive method, the line-load ratio LLR is preferably at least 0.71 and at most 1.35, and more preferably at least 0.73 and at most 1.14.

In addition, the fibrous web is preferably also guided through a pre-press disposed upstream of the main press in the running direction of the fibrous web, preferably directly upstream. The pre-press may likewise have an extended press nip and may be configured such that its line-load ratio LLR is less than 0.69.

The inventive method and press arrangement are particularly suitable when the fibrous web is a graphic paper grade or a grade belonging to the Board & Packaging sector. It is particularly preferred for the fibrous web to be a packaging paper web, such as a packaging testliner. By contrast, tissue grades are of minor or no significance.

The inventive method can be used particularly efficiently when the fibrous web is formed of at least 20 percent by weight (wt. %), preferably at least 50 wt. %, of OCC fibers. OCC is an abbreviation well known in the art and stands for “old corrugated containers.” In other words, the inventive press arrangement and method are especially well suited for efficiently dewatering fibrous webs having a significant or even predominant proportion of recycled fibers, i.e. without virgin fibers. This is due to the high resistance of OCC fibers to high pressures. The remaining fibers of the fibrous web to be pressed may, for example, be mechanical pulp or chemical pulp such as TMP, CTMP, and/or PGW.

Press arrangements according to aspects of the present invention are also advantageous because they can be operated at high production speeds. Speeds of more than 1,000 m/min, in particular more than 1,200 m/min, or even more than 1,400 m/min are possible. In this case, providing a pre-press often proves advantageous, as it typically enables an increase in production speed.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in a press arrangement and a method for pressing a fibrous web, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic, cross-sectional view of a press arrangement according to the invention, which includes a main press and a pre-press;

FIG. 2 is an enlarged and detailed cross-sectional view of the main press of the press arrangement shown in FIG. 1; and

FIG. 3 is a diagram showing an exemplary pressure curve in the extended press nip of the main press.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a diagrammatic illustration of a press arrangement 10 according to the invention including a main press 1 and a pre-press 11 disposed immediately upstream in the running direction BR of a fibrous web 8. In this embodiment, both the main press 1 and the pre-press 11 are configured as shoe presses and therefore each have an extended press nip. Alternatively, however, the pre-press 11 may have no extended press nip and/or the pre-press 11 and the main press 1 may share a common central roll. In that case, the central roll would be a press element forming both the extended press nip of the pre-press 11 and the extended press nip of the main press 1.

FIG. 2 shows an enlarged and detailed view of the main press 1, which is of particular importance in the invention. The main press 1 is configured to be operated with a line load LL of at least 1,200 kN/m, preferably at least 1,300 kN/m. The extended press nip 7 of the main press 1 is formed by two press elements, namely a shoe press roll 2 and a backing roll 3. The shoe press roll 2 includes a press shoe 5, which is supported on a stationary yoke 4, and a press jacket 6 (press belt), which is disposed to rotate about the press shoe 5. The fibrous web 8 is preferably guided sandwich-like between two press felts 9 through the extended press nip 7. The press shoe 5 has a substantially concave surface over which the press jacket 6 runs, while the press shoe 5 presses it with a high pressing force F toward the backing roll 3. The geometric configuration of the press elements, in particular the press shoe 5, is selected such that a line-load ratio LLR of at least 0.69 and at most 1.52 is obtained.

The pressing force F is preferably selected such that the peak pressure acting on the fibrous web 8 in the extended press nip 7 is at least 10 MPa. The length of the extended press nip of the main press 1 is at least 150 mm, preferably at least 190 mm.

At such peak pressures, it has proven particularly advantageous if the press jacket 6 is made at least in part of polyurethane formed by reaction of a prepolymer and a crosslinker component. 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 the crosslinker component contains a C2-14 diol. For example, the press jacket 6 may include a reinforcement structure of threads embedded in the polyurethane layer, the prepolymer of the polyurethane layer being formed of 50 wt. % of a mixture of 1,4-phenylene diisocyanate (PPDI) and C5-6 polycarbonate diol and 50 wt. % of a mixture of 1,4-phenylene diisocyanate (PPDI) and polytetramethylene ether glycol (PTMEG), and the crosslinker including, or preferably predominantly being formed of, polytetramethylene ether glycol (PTMEG) and 1,6-hexanediol.

The pre-press 11 shown diagrammatically in FIG. 1 may, and preferably does, differ in configuration from the main press 1. In particular, unlike the main press 1, it may be configured such that the line-load ratio LLR of the pre-press 11 is less than 0.69. In the press arrangement, the pre-press 11 serves in particular to preconsolidate the fibrous web 8 sufficiently for passage through the main press 1, so that despite the relatively high peak pressure in the second press 1 the web is not excessively crushed.

The fibrous web 8 is preferably intended for the production of a packaging paper web or is such a packaging paper web. Furthermore, the fibrous web preferably is formed of at least 20 wt. %, more preferably at least 50 wt. %, of OCC fibers, which are characterized by particularly high resistance even to large peak pressures.

The inventive press arrangement 1 could theoretically include more presses than just the pre-press 11 and the main press 1. Preferably, however, the main press 1 is the last press of the press arrangement, i.e. the last press before the fibrous web 8 is transferred to a dryer section downstream of the press arrangement.

The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention:

• • 1 Main press
  • 2 Shoe press roll
  • 3 Backing roll
  • 4 Stationary yoke
  • 5 Press shoe
  • 6 Press jacket
  • 7 Extended press nip
  • 8 Fibrous web
  • 9 Press felt
  • 10 Press arrangement
  • 11 Pre-press
  • BR Running direction (machine direction, MD)
  • F Pressing force