Press assembly and method for pressing a fibrous web

Abstract

The invention relates to a press assembly and to a method for pressing a fibrous web, comprising a main press having an extended press nip, wherein the press nip has a length of at least 150 mm, wherein the main press is designed in such a way that, when it is operated with a linear load LL of at least 1200 kN / m, a linear load ratio LLR of at least 0.69 and at most 1.52 results, wherein the linear load ratio LLR is the ratio of a weighted linear load WLL to the linear load LL, wherein the weighted linear load WLL is the result of an integration of the squared local pressure p(x)² weighted with a weighting factor A over the press nip length x, wherein the weighting factor A is 1 / 10 MPa, and wherein the linear load LL is the result of an integration of the local pressure p(x) over the press nip length x, so that the following formula results for the linear load ratio LLR: (I)

Description

[0001] Press arrangement and method for pressing a fibrous web

[0002] The present invention relates to a press arrangement for pressing a fibrous web, in particular a packaging paper web, comprising a main press with an extended press nip, wherein the press nip has a length of at least 150 mm, preferably at least 190 mm. Furthermore, the present invention relates to a method for pressing a fibrous web, in particular a packaging paper web, such as packaging testliner, wherein the fibrous web is guided through a main press with an extended press nip of at least 150 mm, preferably at least 190 mm.

[0003] Such a press arrangement and such a pressing method are described, for example, in the document WO2017207475A1, the disclosure of which is hereby incorporated by reference.

[0004] Machines for producing a fibrous web usually have a press arrangement in which the fibrous web is dewatered or dehumidified using mechanical pressure. The press arrangement is usually located between the forming and drying sections. A particularly efficient type of mechanical dewatering can be achieved using a so-called extended press nip. The advantages of an extended press nip are well known: The press nip is planar and not essentially linear like in conventional roller presses. As a result, the pressure exerted on the fibrous web to be dewatered in the press nip in the running direction does not set in suddenly, but can be continuously increased from a low value to a high value. This reduces the risk of the fibrous web to be dewatered being crushed in the press nip.In this way, the fibrous web can be dewatered very efficiently in an extended press nip, while at the same time preserving its volume.

[0005] For example, the fibrous web can 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 counter roll. Unlike in the production of tissue webs, the thickness or bulk is less important in the production of paper webs, especially packaging paper webs. For this reason, and because of the considerably higher basis weight of paper webs, especially packaging paper webs, significantly higher line loads are used in the press, namely line loads of at least 500 kN / m.

[0006] The topics of "energy costs" and "CO2 footprint" are playing an increasingly important role in the production of fibrous webs. Energy consumption in the dryer section, which is now almost exclusively gas-heated, is a particularly significant factor. To save energy and gas here, it would be highly advantageous if the fibrous web coming from the upstream press arrangement already had the highest possible dry content. An obvious idea might be to simply increase the line load in the press arrangement to achieve a higher dry content. However, this approach is only of limited practicality, as it carries the risk of the fibrous web to be dewatered being crushed in the press nip.In addition, the energy consumption in the press arrangement also increases with increasing line load at the same machine speed, and with increasing line load the investment costs increase because a considerably more massive frame is required.

[0007] It is an object of the present invention to reduce the energy costs and / or the CO2 footprint for the production of a fibrous web, in particular a packaging web.

[0008] This object is achieved by the independent claims. The dependent claims relate to advantageous developments of the present invention.

[0009] In concrete terms, the task is carried out by a generic method described at the beginning

[0010] Press arrangement, which is particularly characterized in that the main press is designed in such a way 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 results, wherein the line load ratio LLR is the quotient of a weighted line load WLL to the line load LL, wherein the weighted line load WLL is the result of an integration of the quadratic local pressure p(x) weighted with a weighting factor A 2 over the press gap length x, where the weighting factor A is one divided by ten megapascals, and where the line load LL is the result of an integration of the local pressure p(x) over the press gap length x, so that the line load ratio LLR is given by the following formula:

[0011] For a better understanding, reference is made to the diagram in Figure 3, which illustrates purely schematically how the line load ratio LLR can be easily determined and understood. First, the actual pressure profile p(x) is determined, i.e. the curve of the pressure acting over the length x of the extended press nip of the main press. In the present example, the main press has an extended press nip of 260 mm. Furthermore, for the sake of simplicity, it is assumed that the local pressure p(x) increases linearly along the extended press nip 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. The local pressures, which together make up the pressure profile, can be determined, for example, using a pressure film that is inserted into the extended press nip before pressure is applied to it by the main press.The printing film then measures, for example using the piezo effect, exactly where which local pressure is applied in the press nip. The pressure in the cross-machine direction is usually constant, so that only the pressure profile in the machine direction, i.e. along the length of the extended press nip, is important. At most, there may be a slight change in the pressure in the cross-machine direction in the page edge area, although these effects in the page edge area are to be ignored here. Such printing films with various resolutions are offered commercially, for example, by Fujifilm under the brand name "Prescale". The measurement should preferably be taken before the press starts up, after installing a fresh felt, in particular a fresh and still dry felt, which serves to guide the fiber web through the extended press nip. The measurement should preferably be taken with the maximum operating line load of the press.

[0012] If the pressure profile p(x) is integrated over the length of the extended press nip x, the result is the line load LL. In the present example according to Figure 3, the line load LL corresponds to the triangular area under the straight line that represents the pressure profile p(x). Here it is 1,560 kN / m. In this way, the quality of the measurement taken can be checked very easily, namely by comparing the line load LL determined by integration with the value that was previously set as the line load LL of the main press. Basically, it should be noted at this point that the line load LL indicates the total force that the main press exerts per meter of width in the cross-machine direction or orthogonal to the direction of movement of the fibrous web.

[0013] Based on the measured pressure profile p(x), the curve of the weighted quadratic local pressure can now be easily determined using the following formula: ^ pa * p(x)2 This results in the curved curve shown in Figure 3, which intersects the straight line of the pressure profile p(x) at 10 MPa. The area below this curved curve corresponds to the weighted line load WLL. The graph in Figure 3 clearly shows that local pressures of less than 10 MPa lead to a smaller area and pressures of more than 10 MPa lead to an enlarged area compared to the triangular area, which represents the line load LL. In the present example, the weighted line load WLL is 1,250 kN / m. This results in a line load ratio LLR as the quotient of the weighted line load WLL to the line load LL of 0.8, which is within the scope of the claimed invention, namely in the range:

[0014] WLL

[0015] 0.69 1.52

[0016] ~ LL In practice, the pressure profile curve p(x) obviously doesn't have to be a straight line. Instead, there is a virtually inexhaustible variety of possible pressure profile curves p(x), even if the line load LL, i.e., the area under the curve, remains constant. The shape of the pressure profile curve p(x) depends significantly on the geometric design of the press elements that form the extended press nip between them, in particular on the geometric design of any press shoe when using a shoe press.

[0017] Until now, only the linear load (LL) was used to characterize the pressure distribution in an extended press nip. However, the inventors recognized that this parameter alone is not sufficient to accurately characterize the pressure distribution and thus also the dewatering behavior in the press nip.

[0018] Since integration involves a certain averaging, a constant pressure curve of p=10 MPa over the entire length of the press nip, for example, yields the same line load LL as a pressure curve of p=0 MPa in the first half and p=20 MPa in the second half of the shoe length. However, the effect of these two pressure curves on dewatering and on the sheet structure is very different.

[0019] This difference is made visible by the weighting in the weighted line load WLL. Thus, for the first case (p=10 MPa), WLL = LL, or a line load ratio LLR of 1, and for the second case, WLL = 1.5 LL, or a line load ratio LLR of 1.5.

[0020] The ratio therefore makes it very easy and efficient to differentiate between the effects of pressure profiles with the same line load, without having to analyze the course of the pressure curve in detail. Generally speaking, for the same line load, pressure profiles with strong pressure fluctuations and higher peak pressures tend to produce larger LLR values ​​than balanced profiles with medium pressure values. The inventors are to credit for having discovered that particularly efficient dewatering of the fibrous web can be achieved if the line load ratio LLR is at least 0.69 and at most 1.52, whereby the boundary conditions are that the line load is at least 1,200 kN / m and that the press nip has a length of at least 150 mm. If the extended press nip is provided by a shoe press, the counter roll to the shoe preferably has a diameter of less than 3,000 mm.

[0021] The line load ratio LLR thus indirectly describes the geometric design of the press elements, in particular of any press shoe. Since there is an incalculably large number of different geometric designs, all of which result in a line load ratio LLR between 0.69 and 1.52, the choice of the line load ratio LLR is appropriate here to describe the inventive solution. It is important that the line load ratio LLR can be determined simply and unambiguously for a press arrangement with a fixed design of the press elements. It is also within the skill of the person skilled in the art, with knowledge of the present invention, to design the press elements such that the desired line load ratio LLR is achieved.

[0022] In known press arrangements, the line load ratio LLR for the above-mentioned boundary conditions is always below 0.69. In the press arrangement according to the invention, however, the press elements are designed such that a line load ratio LLR between 0.69 and 1.52 results, preferably between 0.71 and 1.35, particularly preferably between 0.73 and 1.14. It has surprisingly been found that such a line load ratio LLR leads to more efficient dewatering of the fibrous web compared to known presses whose line load ratio LLR lies outside this value range, in particular below 0.69.

[0023] An interesting sub-range is the range for LLR values ​​less than one, i.e. the range between 0.69 and 0.99, or 0.71 and 0.97, or 0.73 and 0.95. The inventors have also discovered that, with regard to efficient dewatering of the fibrous web, it is advantageous if the press arrangement further comprises a pre-press arranged upstream of the main press in the running direction of the fibrous web, preferably immediately. The pre-press serves to pre-consolidate and dehumidify the fibrous web to such an extent that it is not crushed despite a high peak pressure in the main press. The press arrangement according to the invention can, if required, also comprise one or more further presses. In particular, another press can be arranged upstream of the pre-press.

[0024] The press arrangement using a pre-press can be carried out in many different ways.

[0025] In particular, the pre-press may preferably also have an extended press nip, wherein the pre-press is or may be designed such that the line load ratio LLR of the pre-press is less than 0.69.

[0026] Alternatively or additionally, the pre-press can also have a classic roller gap.

[0027] A very advantageous embodiment is a press type marketed by the applicant under the name "DuoCentri NipCo Flex" press. This press features two press nips on a central roll. The first press nip is a conventional roll nip that functions as a pre-press.

[0028] The main press is then represented by a shoe press nip, which is formed by a shoe press roll and the central roll as a counter roll.

[0029] Another alternative design consists of three shoe presses arranged one behind the other. The main press can be the second or third of these shoe presses.

[0030] Regarding the line load at which the main press can be operated, this can be higher than 1,200 kN / m, namely at least 1,300 kN / m, or even significantly higher. As mentioned above, however, there are practical limits to increasing the line load due to the increasing risk of crushing the fiber web, increasing drive energy, and rising investment costs, especially for the design of a more solid frame.

[0031] Even if the peak pressure cannot be arbitrarily high for the reasons described above, it is preferably above 10 MPa, which, when using the line load ratio LLR according to the invention, leads to a significant increase in dry content without, however, unduly compressing the fibrous web.

[0032] A preferred embodiment of the invention provides that the pre-press is a shoe press which comprises a press shoe with a substantially concavely curved surface and a shoe press jacket rotatably mounted around the press shoe.

[0033] It has proven advantageous if the shoe press cover consists at least partially of polyurethane, which is formed by reacting 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 C2-u-diol. It is further preferred if the polyol component of the prepolymer comprises polytetramethylene ether glycol (PTMEG) and at least one polycarbonate polyol. Alternatively or additionally, the crosslinker component can comprise polytetramethylene ether glycol (PTMEG) and / or at least one polycarbonate polyol.

[0034] A press sleeve with such a polyurethane layer has proven surprisingly resistant even under high peak pressures. Peak pressures of significantly more than 10 MPa pose no problem. At the same time, this polyurethane mixture is capable of maintaining good adhesion to the reinforcing threads embedded therein, even after many alternating load cycles under high peak pressures. According to a further aspect, the present invention relates to a machine for producing a fibrous web, preferably packaging testliner, comprising a press arrangement according to the invention as described above.

[0035] Furthermore, the invention relates to a method for pressing a fibrous web, in particular packaging paper web, such as packaging testliner, preferably using a previously described press arrangement according to the invention, wherein the fibrous web is guided through a main press with an extended press nip of at least 150 mm in length, preferably at least 190 mm in length, wherein the main press is operated with a line load of at least 1,200 kN / m, preferably at least 1,300 kN / m, wherein the main press is designed such that a line load ratio LLR of at least 0.69 and at most 1.52 results, wherein the line load ratio LLR is the quotient of a weighted line load WLL to the line load LL, wherein the weighted line load WLL is the result of an integration of a quadratic local pressure p(x) weighted with a weighting factor A 2over the press gap length x, where the weighting factor A is one divided by ten megapascals, and where the line load LL is the result of an integration of the local pressure p(x) over the press gap length x, so that the line load ratio LLR is given by the following formula:

[0036] The effects and advantages previously stated with regard to the press arrangement according to the invention also apply mutatis mutandis to the method according to the invention and vice versa.

[0037] Thus, in the method according to the invention, the line load ratio LLR is preferably at least 0.71 and at most 1.35, more preferably at least 0.73 and at most 1.14.

[0038] In addition, the fibrous web is preferably further guided through a pre-press arranged upstream of the main press in the running direction of the fibrous web, preferably immediately upstream, wherein the pre-press may also have an extended press nip, wherein the pre-press may be designed such that the line load ratio LLR of the pre-press is less than 0.69.

[0039] The method and apparatus according to the invention are particularly suitable when the fibrous web comprises graphic paper grades or grades belonging to the board and packaging sector. It is particularly preferred when the fibrous web is a packaging paper web, such as Packaging Testliner. Tissue grades, on the other hand, are of lesser or no importance.

[0040] The method according to the invention can be used particularly efficiently if the fibrous web consists of at least 20 wt.%, preferably at least 50 wt.% OCC fibers. OCC is an abbreviation known in the art and stands for "old corrugated containers." In other words, the press arrangement according to the invention and the method according to the invention are particularly well suited for efficiently dewatering fibrous webs that have a significant or even decisive proportion of used fibers, i.e., no fresh fibers. This is related 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 pulp or wood pulp, such as TMP, CTMP, and / or PGW.

[0041] The 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 1000 m / min, in particular more than 1200 m / min or even more than 1400 m / min, are possible. In this case, the provision of a pre-press often proves advantageous, as this usually allows for an increase in production speed.

[0042] The invention will be explained in more detail below with reference to an exemplary embodiment described with the aid of schematic figures. Figure 1 shows a press arrangement according to the invention, comprising a

[0043] Main press and a pre-press;

[0044] Figure 2: an enlarged and detailed view of the main press of the

[0045] Press arrangement shown in Figure 1,

[0046] Figure 3: an example pressure curve in the extended press nip of the

[0047] Main press.

[0048] Figure 1 shows a very schematic illustration of a press arrangement according to the invention, comprising a main press 1 and a pre-press 11 arranged directly upstream in the direction of movement BR of a fibrous web 8. In this exemplary embodiment, both the main press 1 and the pre-press 11 are designed as shoe presses and thus each have an extended press nip. Alternatively, however, the pre-press 11 could also not have an extended press nip and / or the pre-press 11 and the main press 1 could share a common central roller. The central roller would then be a press element by means of which both the extended press nip of the pre-press 11 and the extended press nip of the main press 1 would be formed.

[0049] Figure 2 shows an enlarged and detailed illustration of the main press 1, which is of particular importance according to the invention. This is designed 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 provided by two press elements, namely a shoe press roll 2 and a counter roll 3. The shoe press roll 2 comprises a press shoe 5, which is supported on a stationary yoke 4, and a shoe press sleeve 6, which is arranged so as to be rotatable around the press shoe 5. The fibrous web 8 is preferably guided through the extended press nip 7 in a sandwich-like manner between two press felts 9. The press shoe 5 has a substantially concavely shaped surface, over which the shoe press sleeve 6 runs, while the press shoe 5 presses it with a high compressive force F in the direction of the counter roll 3.The geometric design of the pressing elements, in particular of the press shoe 5, is selected such that a line load ratio LLR of at least 0.69 and at most 1.52 results.

[0050] The compressive force F is preferably selected to be large enough 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.

[0051] At such peak pressures, it has proven particularly advantageous if the shoe press cover 6 consists at least partially of polyurethane, which is formed by reacting a prepolymer and a crosslinking 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 crosslinking component contains a C2-μ-diol. For example, the shoe press cover 6 can have a reinforcing structure made 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 C5-β-polycarbonatediol and 50 wt.-% of a mixture of 1,4-phenylene diisocyanate (PPDI) and polytetramethylene ether glycol (PTMEG), and wherein the crosslinker comprises polytetramethylene ether glycol (PTMEG) and 1,6-hexanediol or is preferably formed substantially therefrom.

[0052] The pre-press 11, shown schematically in Figure 1, can and preferably is configured differently than the main press 1. In particular, unlike the main press 1, it can 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 sufficiently pre-consolidate the fibrous web 8 for passage through the main press 1 so that it is not subjected to excessive compression despite a relatively high peak pressure in the second press 1.

[0053] The fibrous web 8 preferably serves for the production of a packaging paper web or is such a packaging paper web. Furthermore, the fibrous web preferably consists of at least 20 wt.%, more preferably at least 50 wt.%, of OCC fibers, which are characterized by particularly high resistance even to high peak pressures.

[0054] The press arrangement 1 according to the invention could theoretically also comprise more presses than just the pre-press 11 and the main press 1. However, the main press 1 is preferably the last press of the press arrangement, ie, the last press before the fibrous web 8 is transferred to a drying section downstream of the press arrangement.

[0055] List of reference symbols

[0056] 1 main press

[0057] 2 shoe press rollers

[0058] 3 Counter roller

[0059] 4 standing yoke

[0060] 5 press shoe

[0061] 6 Shoe press cover

[0062] 7 extended press nip

[0063] 8 Fibrous web

[0064] 9 Press felt

[0065] 10 Press arrangement

[0066] 11 Pre-press

[0067] BR direction of movement

[0068] F compressive force

Claims

Patent claims 1. Press arrangement (10) for pressing a fibrous web (8), in particular a packaging paper web, comprising a main press (1) with an extended press nip (7), wherein the press nip (7) has a length of at least 150 mm, preferably of at least 190 mm, characterized in that the main press (1) is designed 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 results, wherein the line load ratio LLR is the quotient of a weighted line load WLL to the line load LL, wherein the weighted line load WLL is the result of an integration of the quadratic local pressure p(x) weighted with a weighting factor A. 2over the press gap length x, where the weighting factor A is one divided by ten megapascals, and where the line load LL is the result of an integration of the local pressure p(x) over the press gap length x, so that the line load ratio LLR is given by the following formula:

2. Press arrangement (10) according to claim 1, characterized in that the line load ratio LLR is at least 0.71 and at most 1.35, preferably at least 0.73 and at most 1.

14.

3. Press arrangement (10) according to claim 1 or 2, characterized in that the press arrangement (10) further comprises a pre-press (11) arranged upstream of the main press (1) in the running direction of the fibrous web (8), preferably immediately upstream.

4. Press arrangement (10) according to claim 3, characterized in that the pre-press (11) also has an extended press nip, wherein the pre-press (11) is designed such that the line load ratio LLR of the pre-press is less than 0.

69.

5. Press arrangement (10) according to one of the preceding claims, characterized in that the main press (1) is designed to be operated with a line load LL of at least 1,300 kN / m.

6. Press arrangement (10) according to one of the preceding claims, characterized in that the main press (1) is a shoe press which comprises a press shoe (5) with a substantially concavely curved surface and a shoe press jacket (6) rotatably mounted around the press shoe (5).

7. Press arrangement (10) according to claim 6, characterized in that the shoe press jacket (10) consists at least partially of polyurethane which is formed by reacting a prepolymer and a crosslinking 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 crosslinking component contains a C2-u-diol.

8. Press assembly (10) according to claim 7, characterized in that the polyol component of the prepolymer comprises polytetramethylene ether glycol (PTMEG) and at least one polycarbonate polyol.

9. Press arrangement (10) according to claim 7 or 8, characterized in that the crosslinking component is polytetramethylene ether glycol (PTMEG) and / or at least one Polycarbonate polyol.

10. Machine for producing a fibrous web (8), preferably packaging testliner, comprising a press arrangement (10) according to one of the preceding claims.

11. A method for pressing a fibrous web (8), in particular a packaging paper web, such as packaging testliner, preferably using a press arrangement (10) according to one of claims 1 to 9, wherein the fibrous web (8) is guided through a main press (1) with an extended press nip (7) of at least 150 mm in length, preferably at least 190 mm in length, characterized in that the main press (1) is operated with a line load LL of at least 1,200 kN / m, preferably at least 1,300 kN / m, wherein the main press (1) is designed such that a line load ratio LLR of at least 0.69 and at most 1.52 results, wherein the line load ratio LLR is the quotient of a weighted line load WLL to the line load LL, wherein the weighted line load WLL is the result of an integration of a quadratic local pressure p(x) weighted with a weighting factor A. 2over the press gap length x, where the weighting factor A is one divided by ten megapascals, and where the line load LL is the result of an integration of the local pressure p(x) over the press gap length x, so that the line load ratio LLR is given by the following formula:

12. Method according to claim 11, characterized in that the line load ratio LLR is at least 0.71 and at most 1.35, preferably at least 0.73 and at most 1.

14.

13. Method according to claim 11 or 12, characterized in that the fibrous web (8) is further guided through a pre-press (11) arranged upstream of the main press (1) in the running direction of the fibrous web (8), preferably directly upstream.

14. The method according to any one of claims 11 to 13, characterized in that the method is carried out at a speed of at least 1,000 m / min, preferably at least 1,200 m / min, in particular more than 1,400 m / min.

15. Method according to one of claims 11 to 14, characterized in that the fibrous web (8) is at least 20 % by weight, preferably at least 50 % by weight, consists of OCC fibres.

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