EP3231936B1

EP3231936B1 - Method for producing fiber webs

Info

Publication number: EP3231936B1
Application number: EP16164657.5A
Authority: EP
Inventors: Mika Viljanmaa; Jari Ilomäki; Timo Nurmiainen
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2016-04-11
Filing date: 2016-04-11
Publication date: 2023-10-11
Anticipated expiration: 2036-04-11
Also published as: CN107287966A; EP3231936A1

Description

In general present invention relates to producing fiber webs in a fiber web production line, in particular to producing board webs. More especially the present invention relates to a method according to preamble part of claim 1.
As known from the prior art in fiber web producing processes typically comprise an assembly formed by a number of apparatuses arranged consecutively in the process line. A typical production and treatment line comprises a head box, a wire section and a press section as well as a subsequent drying section and a reel-up. The production and treatment line can further comprise other devices and/or sections for finishing the fiber web, for example, a pre-calender, a sizer, a final-calender, a coating section. The production and treatment line also typically comprises at least one slitter-winder for forming customer rolls as well as a roll packaging apparatus or a sheet cutter. In this description and the following claims by fiber webs are meant for example a paper and board webs.
In traditional drying methods of fiber webs thermal energy is provided to the fiber webs by contact, for example by drying cylinders; by convection, for example by impingement dryers; by condensation of steam on the surfaces of the fiber webs, for example by steam boxes; by radiation, for example by infrared dryers. During drying temperature of the fiber web increases and thus partial pressure of steam in the fiber web increases. Difference of partial steam pressures in the fiber web and the ambient air seeks to even out, whereby the fiber web evaporates moisture and dries. Especially towards the end of the drying process, when the fiber web is relatively dry and hot, efficiency of the traditional drying (in particularly when using drying cylinders) decreases, due to small temperature difference and small heat flow.
Calendering can be pre-calendering or intermediate calendering or final calendering depending on the type of the production line. Pre-calendering is typically used for creating required surface properties for further treatment for example for coating and final-calendering is generally carried out in order to improve the properties, like smoothness and gloss, of a web-like material such as a paper or board web. In calendering the web is passed into a nip, i.e. calendering nip, formed between rolls that are pressed against each other, in which nip the web becomes deformed as by the action of temperature, moisture and nip pressure. In the calender the nips are formed between a smooth-surfaced press roll such as a metal roll and a roll coated with resilient material such as a polymer roll or between two smooth-surfaced rolls. The resilient-surfaced roll adjusts itself to the forms of the web surface and presses the opposite side of the web evenly against the smooth-surfaced press roll. The nips can be formed also by using instead one of roll a belt or a shoe as known from prior art. Many different kinds of calenders to be used as a pre-calender and/or an intermediate calender and/or as an final-calender are known, for example hard nip calenders, soft nip calenders, supercalenders, metal belt calenders, shoe calenders, long nip calenders, multinip calenders etc.
Paper and board are available in a wide variety of types and can be divided according to basis weight in two grades: papers with a single ply and a basis weight of 25 - 300 g/m² and boards manufactured in multi-ply technology and having a basis weight of 150 - 600 g/m². It should be noted that the borderline between paper and board is flexible since board grades with lightest basis weights are lighter than the heaviest paper grades. Generally speaking, paper is used for printing and board for packaging.
The subsequent descriptions are examples of values presently applied for fibrous webs, and there may be considerable fluctuations from the disclosed values. The descriptions are mainly based on the source publication Papermaking Science and Technology, section Papermaking Part 3, edited by Rautiainen, P., and published by Paper Engineers' Association, Helsinki 2009; 404 pages.
Mechanical-pulp based, i.e. wood-containing printing papers include newsprint, uncoated magazine and coated magazine paper.
Today's newsprint furnishes mostly contain between 80 and 100 % deinked pulp (DIP). The rest of the furnish is mechanical pulp (typically TMP). However, there is also newsprint made of 100 % mechanical fiber furnishes. DIP based newsprint may contain up to 20 % filler. The filler content of a virgin-fiber based newsprint furnish is about 8 %.
General values for CSWO newsprint can be regarded as follows: basis weight 40 - 48.8 g/m², PPS s10 roughness (SCAN-P 76-95) 4.0 - 4.5 µm, Bendtsen roughness (SCAN-P21:67) 150 ml/min, density 600 - 750 kg/m³, brightness (ISO 2470:1999) 58 - 59 %, and opacity (ISO 2470:1998) 92 - 95 %.
Uncoated magazine paper (SC-supercalendered) grades usually contain 50 - 75 % mechanical pulp, 5 - 25 % chemical pulp, and 10 - 35 % filler. The paper may also contain DIP. Typical values for calendered SC paper (containing e.g. SC-C, SC-B, and SC-A/A+) include basis weight 40 - 60 g/m², ash content (SCAN-P 5:63) 0 - 35 %, Hunter gloss (ISO/DIS 8254/1) < 20 - 50%, PPS s10 roughness (SCAN-P 76:95) 1.0 - 2.5 µm, density 700 - 1250 kg/m³, brightness (ISO 2470:1999) 62 - 75 %, and opacity (ISO 2470:1998) 90 - 95 %.
Coated mechanical papers include for example MFC (machine finished coated), LWC (light weight coated), MWC (medium weight coated), and HWC (heavy weight coated) grades. Coated mechanical papers usually contain 45 -75 % mechanical or recycled fiber and 25 - 55 % chemical pulp. Semichemical pulps are typical in LWC paper grades made in the Far East. The filler content is about 5 -10 %. The grammage is typically in the range 40 - 80 g/m².
General values for LWC paper can be regarded as follows: basis weight 40 - 70 g/m², Hunter gloss 50 - 65 %, PPS S10 roughness 1.0 - 1.5 µm (offset) and 0.6 - 1.0 µm (roto), density 1100 - 1250 kg/m³, brightness 70 - 75 %, and opacity 89 - 94 %.
General values for MFC paper (machine finished coated) can be regarded as follows: basis weight 48 - 70 g/m², Hunter gloss 25 - 40 %, PPS S10 roughness 2.2 - 2.8 µm, density 900 - 950 kg/m³, brightness 70 - 75 %, and opacity 91 - 95 %.
General values for MWC paper (medium weight coated) can be regarded as follows: basis weight 70 - 90 g/m², Hunter gloss 65 - 70 %, PPS S10 roughness 0.6 - 1.0 µm, density 1150 - 1250 kg/m³, brightness 70 - 75 %, and opacity 89 - 94 %.
Woodfree paper is divided into two segments: uncoated and coated. Conventionally, the furnish of woodfree papers consists of bleached chemical pulp, with less than 10 % mechanical pulp.
Typical values are for uncoated WFU Copy paper: grammage 70 - 80 g/m², Bendtsen roughness 150 - 250 ml/min and bulk > 1.3 cm³/g; for uncoated offset paper: grammage 60 - 240 g/m², Bendtsen roughness 100 - 200 ml/min and bulk 1.2 - 1.3 cm³/g; and for color copy paper: grammage 100 g/m², Bendtsen roughness < 50 ml/min and bulk 1.1 cm³/g.
In coated pulp-based printing papers (WFC), the amounts of coating vary widely in accordance with requirements and intended application. The following are typical values for once- and twice-coated, pulp-based printing paper: once-coated basis weight 90 g/m², Hunter gloss 65 - 80 %, PPS s10 roughness 0.75 - 1.1 µm, brightness 80 - 88 %, and opacity 91 - 94 %, and twice-coated basis weight 130 g/m², Hunter gloss 70 - 80 %, PPS S10 roughness 0.65 - 0.95 µm, brightness 83 - 90 %, and opacity 95 - 97 %.
Containerboard includes both linerboard and corrugating medium. Liners are divided according to their furnish base into kraftliner, recycled liner and white top liner. Liners are typically 1- to 3-ply boards with grammages varying in the range 100 - 300 g/m².
Linerboards are generally uncoated, but the production of coated white-top liner is increasing to meet higher demands for printability.
The main cartonboard grades are folding boxboard (FBB), white-lined chipboard (WLC), solid bleached board (SBS) and liquid packaging board (LPB). In general, these grades are typically used for different kinds of packaging of consumer goods. Carton board grades vary from one- up to five-ply boards (150-400 g/m²). The top side is usually coated with from one to three layers (20-40 g/m²), the back side has less coating or no coating at all. There is a wide range of different quality data for the same board grade. FBB has the highest bulk thanks to the mechanical or chemimechanical pulp used in the middle layer of the base board. The middle layer of WLC consists mainly of recycled fiber, whereas SBS is made from chemical pulp, exclusively.
FBB's bulk typically is between 1.1 - 1.9 cm³/g whereas WLC is on range 1.1 - 1.6 cm³/g and SBS 0.95 - 1.3 cm³/g. The PPS-s10-smoothess is respectively for FBB between 0.8 - 2.1 µm, for WLC 1.3 - 4.5 µm and for SBS 0.7 - 2.1 µm.
Release paper is used in label base paper in various end-use applications, such as food packaging and office labels. The most common release paper in Europe is supercalendered glassine paper coated with silicone to provide good release properties.
Typical values for supercalendered release papers are basis weight 60 - 95 g/m², caliper 55 - 79 µm, IGT 12 - 15 cm, Cobb Unger for dense side 0.9 - 1.6 g/m² and for open side 1.2 - 2.5 g/m².
Coated label paper is used as face paper for release, but also for coated backing paper and flexible packings. Coated label paper has a grammage of 60 - 120 g/m² and is typically sized or precoated with a sizer and single-blade coated on one side. Some typical paper properties for coated and calendered label paper are basis weight 50 - 100 g/m², Hunter gloss 70 - 85 %, PPS s10 roughness 0.6 - 1.0 µm, Bekk smoothness 1500 - 2000 s and caliper 45 - 90 µm.
One problem with calendering of fiber webs is to achieve required surface properties and simultaneously achieve required bulkiness i.e. relation of thickness of the web to its grammage (basis weight). When the fiber web has high bulkiness the basis weight can be reduced which results as considerable savings in raw material. Thus in recent times it has been one of the main focus points in developing calenders, mostly due to environmental and cost saving reasons.
Typically the fiber web is guided from the drying section to a precalender, when the temperature of the fiber web is about 80 - 90 °C. In the thickness direction of the web the middle layers of the web are hot and near plastic state, whereby during calendering the fiber web will compact also in the middle layers, which leads to bulk loss.
It is known from prior art that bulkiness can be saved in calendering by cooling the fiber web before calendaring.
Publication EP 2682520 A1 discloses a production line for producing fiber webs, which comprises at least one calender with at least one calendering nip and a reel-up after the calender and at least one cooling means before the at least one calender for cooling the fiber web to temperatures of not higher than 40 °C, preferably to temperature in the range of 10 - 30 °C
Publication WO 2005/042837 A1 discloses a treatment line for paper in which on the travel path of a paper web that is arranged to convey forward the paper web moving at web speed, there is a moistening device, a calender for precalendering the surface of the paper web and a coating unit for coating the paper web precalendered in the calender and the travel path of the paper web between the moistening device and the calender is arranged to have such a length with respect to the web speed that the delay time of the web between the moistening device and a hot precalendering nip surface of the calender located on the same side with respect to said moistening device is 0.6 to 6.0 s.
Publication WO 00/70144 A1 discloses a paper machine line in particular for the manufacture of fine paper, which line comprises a short circulation, a headbox, a wire section, a press section, a dryer section, a precalender, a precoater and a drying section after that, a coating station/stations and after-drying section/sections, a calender and a reel-up, which may comprise a moistening device based on steam or water mist, placed before the calender, for profile control of curl.
Publication EP2876206 A1 discloses a method, in which fiber web is cooled at least partially by moisturizing and evaporative cooling process, advantageously by combination of applying moisture, for example water, and of blowing dry cool gas, for example air, onto the surface of the fiber web. The moisture evaporates and cools the fiber web.
An object of the invention is to create a production line which is simple, cost effective and raw material saving production line and a method of producing fiber webs with high production capacity.
A further object of the present invention is to approach the above problems from a new point of view and to suggest novel solutions contrary to conventional modes of thinking.
One further object of the present invention is to provide an energy efficient drying to the production line of a fiber web and to the method of producing a fiber web. To achieve the objects mentioned above and later the method according to the invention is defined by the features of claim 1.
Advantageous embodiments and features of the are defined in dependent claims.
According to the invention advantageously in drying of a fiber web thermal energy of the fiber web is utilized such that difference of partial steam pressures of the fiber web and the ambient air is created by providing dry gas, for example air or air-mixture or gas-mixture, blows on at least one surface of the fiber web. The thermal energy of the fiber web will thus be utilized for evaporation as dry gas is blown onto at least surface of the fiber web, whereby moisture of the fiber web evaporates, where by latent thermal energy is combined with the evaporation and thus the fiber web cools according to equation: $dT = m_{water} \times H / (m_{paper} \times c_{p}),$
in which heat of evaporation of water H = 2350 kJ/kg and specific heat capacity of the fiber web c_p = 1,65 kJ/(kg°C).
According to an advantageous feature of the invention after drying with dry gas blows the fiber web is further dried by traditional drying methods, for example by drying cylinders. This is advantageous as the fiber web is again cool and thus the temperature difference and the heat flow have increased.
According to the invention relative humidity of the dry gas blows is at the most 70 %.
According to an advantageous feature of the invention cycle of drying with dry gas blows and drying with traditional drying methods is repeated at least once.
According to an advantageous feature of the invention the fiber web is dried by cool, dry gas after the drying section before a calender. By this waste heat of the fiber web will be utilized and thus energy savings are achieved and additionally the calendering is improved, especially the bulkiness / roughness relation is improved 2 - 6 % as the fiber web entering the calender is cool and as the cooler fiber web endures more pressure.
According to an advantageous feature of the invention temperature of the cool, dry gas blows is lower than temperature of the fiber web.
According to an advantageous feature of the invention the fiber web is dried with cool, dry gas at least before calendering and/or at least before reeling.
According to the invention a 7 - 20 m, advantageously 10 - 15 meters long run of the fiber web is before the calendering or reeling, advantageously without processing or treating the fiber web or provided with drying with cool, dry gas. According to an advantageous feature of the invention a 7 - 20 m, advantageously 10 - 15 meters long run of the fiber web is before the calendering and reeling, advantageously without processing or treating the fiber web or provided with drying with cool, dry gas. The distance is calculated from the point where the fiber web leaves the last means processing or treating the fiber web i.e. the previous drying or cooling cylinder before the calender, to the point where the fiber web contacts the first roll of the calender or contacts the reeling cylinder. Temperature of the dry, cool gas blows is advantageously 10 - 60 °C. Thus drying without using extra energy supply is achieved. During this run the free surfaces of the fiber web are cooled, whereby the partial pressure of steam on the surfaces decreases and in middle of the fiber web the partial pressure of steam remains high. Thereby the difference in partial steam pressures conveys the moisture by diffusion from middle of the fiber web to the surfaces of the fiber web and drying is enhanced.
According to an advantageous feature of the invention the fiber web is cooled by the long run or by the long run provided with cool, dry gas blows is located before a pre-calender. By this calendering is improved and as thermal effect of a roll calender nip is low the fiber web is still cool when entering a coating station and thus also the coating is improved as coating cools fast, whereby viscosity of water of the coating increases fast and the coating remains on the surface of the fiber web.
According to an advantageous feature of the invention the fiber web that comprises starch is cooled before calendering by the long run or by the long run provided with cool, dry gas blows and the fiber web is calendered in a calendering nip comprising a thermo roll, temperature of which is at the most 150 °C, advantageously at the most 100 °C. By this adherence of the starch onto the surface of the thermo roll is prevented.
The production line advantageously comprises at least one head box, which can be a two or three layer head box, forming means for each layer or layer combination, a press section with at least one press nip, a drying section, at least one calender.
According to an advantageous embodiment the production line further comprises a Yankee cylinder and/or belt arrangement, a size press and an after drying section located after the Yankee cylinder and/or belt arrangement and/or the size press and/or the calender.
According to an advantageous embodiment the production line further comprises a coating section for coating the fiber web by 1 - 6 layers of coating and drying means for drying the coating.
The production speed of the production line is advantageously 100 - 2000 m/min.
The basis weight of the fiber web produced by the production line is 50 - 1000 g/m².
The end product of the production line is a fiber web with 1 - 10 fiber layers.
The end product of the production line is a fiber web with 1-10 coating layers.
According to an advantageous feature the head box is a two or a three layer head box.
According to an advantageous feature the press section comprises at least one roll press nip and/or at least one shoe press nip.
According to an advantageous feature the drying section comprises at least one drying cylinder group with one wire draw and/or at least one drying cylinder group with twin wire draw.
According to an advantageous feature the calender is a pre- or an intermediate or an end calender.
According to an advantageous feature the size press is a bond sizer or a spray sizer or a film sizer.
According to an advantageous feature the after coating section comprises at least one of the following: a bond coater, an air brush coater, a sizer, a blade coater, a rod coater, a curtain coater, a spray coater, a cast coater.
In the following the invention is further explained in detail with reference to the accompanying drawing in which:
In figures 1 - 4 is schematically shown an advantageous example of a production line according to the invention.
In the following disclosure and the accompanying drawings corresponding parts, part components, sections etc. are marked by same reference signs unless otherwise mentioned.
In the schematical example of a production line for producing coated fiber webs, in particular coated board webs shown in figures 1 - 4 the production line for producing fiber webs comprises three head boxes 7, 8, 9 each for providing furnish for one fiber layer of the fiber web W and each followed by a forming unit 101, 102, 103 in a forming section 10 of the production line, in which forming section the fiber web W is formed and moisture is removed from the fiber web. In a press section 11 the fiber web W is pressed in press nips 111, 112. A drying section 12 of the production line comprises traditional drying in drying cylinder group/-s 121 of one-wire draw and/or in drying cylinder group/- s 122, 12N of twin-wire draw. The drying section 12 is followed by a size press 131 of a sizing section 13, which comprises a drying section 14 for the size, which drying section comprises a turning device 141, non-contact drying means 142, drying cylinder group 143 with twin-wire draw. After the drying section 14 for the size is provided a cooler 144. After the cooler 144 the fiber web is calendered in a calendering nip formed between two calender rolls in a calender 15 followed by drying by non-contact drying means 152. There after the fiber web W is coated in coating section 16, 17, which provides coating for two coating layers by coaters 161, 171. Each coater 161, 171 is followed by a drying section comprising non-contact drying means 162, 172 and/or a drying cylinder group 163, 173. After the coating section an end calender 18 is located, in which the fiber web W is calendered in two calendering nips 181, 182 formed between calender rolls. At the end of the production line the fiber web W is reeled to a parent roll 192 having full width fiber web in a reel-up 19 by a reeling cylinder 191. The parent rolls 192 are transferred to an unwinder 201 of the slitter-winder 20. The unwound full width fiber web W is cut in longitudinal direction of the fiber web W i.e. slitted in a slitter 202 to partial fiber webs WN by slitter blades and the partial fiber webs WN are wound to partial fiber web rolls i.e. customer rolls in a winder 203.
These devices and sections can be constructed in various different designs and constructions known as such to one skilled in the art. Advantageously the head box is a two or a three layer head box 7, 8, 9, the press section comprises at least one roll press nip 111 and/or at least one shoe press nip 112, the drying section comprises at least one drying cylinder group 121 with one wire draw and/or at least one drying cylinder group 122, 12N with twin wire draw and the size press 131 is a bond sizer or a spray sizer or a film sizer.
The production line comprises at least one cooler 144 providing gas blows after the press section, at least one moisturizing device located before at least one cooler 144, at least one calender 15, a reel-up 19, a slitter-winder 20 and/or a sheet cutter. The cooler 144 comprises means to blow dry, cool gas towards at least one surface of the fiber web W. Many different kinds of calenders 15 can be used as a pre-calender and/or as an intermediate and/or as an final-calender, for example hard nip calenders, soft nip calenders, supercalenders, metal belt calenders, shoe calenders, long nip calenders, multinip calenders.
The production line can further comprise a Yankee cylinder and/or belt arrangement, a size press 131 and an after drying section 14, 152 located after the Yankee cylinder and/or belt arrangement and/or the size press and/or the calender 15 and a coating section 16, 17 for coating the fiber web by 1 - 4 layers of coating and drying means for drying the coating. The coating section 16, 17 comprises at least one coater 161, 171 of the following: a bond coater, an air brush coater, a sizer, a blade coater, a rod coater, a curtain coater, a spray coater, a cast coater.
Between the calender 15 of the production line and the last drying or cooling cylinder before it a non-contacting means for cooling the fiber web i.e. the cooler 144 providing cool gas is located and length of the fiber web run between last contact point of the fiber web on the last drying or cooling cylinder before the calender 15 and the first contact point of the fiber web on the first calender roll forming calendering nip of the calender 15 it is 7 - 20 m, advantageously 10 - 15 m.

Claims

Method for producing a fiber web (W), in which method furnish is fed from at least one head box (7, 8, 9) to at least one forming section (10) for forming the furnish to the fiber web, the fiber web is pressed in a press section (11), the fiber web is dried in at least one drying section (12), the fiber web is calendered in at least one calender (15, 18) and reeled in a reel-up (19), wherein in drying of a fiber web (W) thermal energy of the fiber web (W) is utilized such that difference of partial steam pressures of the fiber web (W) and the ambient air is created by providing dry gas blows having relative humidity at the most 70 % on at least one surface of the fiber web whereby thermal energy of the fiber web is utilized for evaporation of moisture from the fiber web and that the fiber web is run 7 - 20 m, advantageously 10 - 15 meters before the calendering or the reeling.
Method according to claim 1, wherein the fiber web is run 7 - 20 m, advantageously 10 - 15 meters before the calendering and the reeling.
Method according to claim 2, wherein during said run before the calendering and/or the reeling the fiber web (W) is dried with the cool, dry gas blows and temperature of the dry, cool gas blows is 10 - 60 °C.
Method according to claim 3, wherein after drying with the cool, dry gas blows the fiber web (W) is further dried by drying cylinders.
Method according to claim 4, wherein the drying with the dry gas blows and the drying with drying cylinders is repeated at least once.
Method according to any of previous claims 2 - 5, wherein the fiber web (W) that comprises starch is cooled before the calendering by the run or by the run provided with the cool, dry gas blows and the fiber web is calendered in a calendering nip comprising a thermo roll, temperature of which is at the most 150 °C, advantageously at the most 100 °C.