Örebro universitet Örebro University
Institutionen för naturvetenskap och teknik School of Science and Technology 701 82 Örebro SE-701 82 Örebro, Sweden
Examensarbete 15 högskolepoäng C-nivå
BASIS AND A CONCEPT FOR A PAPER
MACHINE IN A LABORATORY SCALE
Mattias Andersson och Anders Berneke Maskiningenjörsprogrammet 180 högskolepoäng
Örebro vårterminen 2012
i
Abstract
This thesis studies paper and what it is made of and different ways to manufacture it in a laboratory scale. It also contains a comparison of different ways to dry tissue and which method that suits a small laboratory tissue machine best.
A small paper machine was found at Metso in Karlstad that was used as a basis for the concept machine. Since the machine at Karlstad used pressing and a cylinder drying method that involves pressing an alternative method to dry the paper had to be chosen.
For drying tissue there are several methods like, infrared and through air drying. A comparison was made between those methods to find out which suited the specification best.
The comparison shows that through air drying is the best drying method for the machine.
Sammanfattning
I detta examensarbete studeras hur man tillverkar papper och olika sätt att tillverka papper i
laboratorium skala. Den innehåller också en jämförelse av olika sätt att torka papper och vilken metod som passar bäst till en liten pappersmaskin.
En liten pappersmaskin hittades vid Metso i Karlstad. Den användes som grund för vårt förslag till maskin. Eftersom maskinen i Karlstad använde ett pressparti och flera torkcylindrar till att torka arket så behövdes de bytas ut. En jämförelse mellan olika torkmetoder gjordes för att finna en ny lämplig metod för ändamålet.
Litteraturstudien visar att det finns flera torkmetoder så som infraröd torkning och genomblåstorkning. En jämförelse gjordes mellan dessa metoder för att ta reda på vilken passade specifikationen bäst. Jämförelsen visar att genomblåstorkning är det bäst lämpade metoden för maskinen.
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Contents
1.1. Background ... 1 1.2. Requirements ... 1 1.3. Goal ... 1 2. Method ... 2 3. Product description ... 3 3.1. Tissue ... 3 3.2 Fiber pulp ... 3 3.2.1 Chemical pulp... 3 3.2.2 Mechanic pulp ... 3 4. Process description ... 4 4.1. Pulping ... 4 4.2. Beating ... 4 4.3. Wet end ... 5 4.3.1. Headbox ... 5 4.3.1.1. Cross-distributor ... 5 4.3.1.2. Pressure chamber ... 5 4.3.1.3. Slice ... 5 4.3.1.4. Multilayer slice ... 6 4.3.2. Open headbox ... 6 4.3.3. Forming table ... 7 4.3.4. Foil ... 8 4.3.5. Breast roll ... 9iii
4.3.6. Wire ... 9
4.3.7. Cleaning and drying the felt and wire ... 9
4.4. Press section ... 10 4.4.1. Felt ... 10 4.5. Dryer section ... 11 4.5.1. Cylinder dryer ... 11 4.5.2. Fan dryer ... 12 4.5.3. Infrared Dryer ... 13 4.5.3.1. Steel cylinder ... 13 4.5.3.2. Steel belt ... 13 4.6. Tissue machines ... 14
4.6.1. Dry crepe tissue ... 14
4.6.2. Through air drying ... 15
5. Results and discussion ... 16
5.1. Pulping ... 16 5.2. Beating ... 16 5.3. Headbox/open headbox ... 17 5.4. Forming table ... 18 5.5. Pressing ... 18 5.6. Drying ... 19 5.6.1. Infrared drying ... 19
5.6.2. Through air drying ... 21
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5.8. Concluding discussion ... 23
6. Conclusions ... 24
7. References ... 25
Appendix A: Besöksrapport – Korsnäs ... 1
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1.
Introduction
1.1. Background
Research in project Formulosa (Formulosa, 2012) have shown that, to use modified pulp fibers in a composite they can be in the form of tissue, and therefore obtain a continuous feed of material by mixing it into a compounder.
The process now works in the way that finished paper rolls from a variety of dealers are purchased. The paper roll is then set in a dryer to get maximum dryness before being used for the next process. The client has requested a technical specification on a machine that will continuously manufacture tissue so they do not have to buy tissue from other manufactures.
1.2. Requirements
Amount of pulp to be used in the process: 2 kg Concentration of water in the mix: 99.5 %
Width of produced tissue: 10 cm (minimum 6 cm & maximum 12 cm) Weight of produced tissue: 50-100 g/m2
Speed of production: 0.5-1.0 m/min (Only matters if it is decided to link the machine into the compounder)
Dryness of produced tissue: 90 %
The paper product should be as loosely bonded as possible but still able to carry its own weight The finished tissue rolls can have a diameter of max 30 cm
No crepe process should be included in the machine concept No limitations on energy usage of the paper machine
The paper machine has to be able to manufacture paper from different kinds of pulp
1.3. Goal
The goal of the thesis is to examine different methods to manufacture paper and specify a concept for a small laboratory paper machine.
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2.
Method
To get a good understanding of how the paper machine works and a foundation to stand on before deciding what methods and techniques that should be chosen as our basis, a thorough process description is needed. The process description is based on a litterateur review with several books, essays, information from manufacturers, producers, visits at manufacturers and their employees. With the help of the process description and discussions with manufacturers of paper and specified tissue machines a discussion is started on what is really possible to do with the given requirements. The contacted manufacturers and the person that the discussion was made with during the process were.
Metso - (Ivarsson) Elastolan - (Swartsköld) Teknik System AB - (Eriksson) Wefapress Papertech - (Kneifel) Ircon - (Björnberg)
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3.
Product description
3.1. Tissue
Tissue is a type of soft paper for hygiene and drying purposes.
The grammage of normal tissue is between 15 and 40 g/m2 (Gavelin, 1999). A lower grammage makes it possible to use higher production rate due to less water to dry off. A tissue with a higher grammage can be build up by several lighter plies. The most common amount of plies to use is two or three, sometimes four plies are used. Another way to build multiple plies is to use a special injection technique to create multiple layers and those layers be can made of different kind of pulp. (Gavelin, 1999)
3.2 Fiber pulp
3.2.1 Chemical pulp
In the manufacturing process of chemical pulp, woodchips are boiled with liquor that contains
chemicals. This process dissolves a substance called lignin which binds the fibers together in the wood. This process leads to a large proportion of wood substance is lost. The estimated yield is around 40 to 60 % depending on the process and the raw material. Hardwood (leaf tree) often have higher yield than softwood (Spruce and Pine). However, the chemical pulp is produced mainly from softwood because this gives a higher strength and produces less dust than hardwood pulp. But as the hardwood is more economical and gives a smoother paper it is common practice to mix in up to 40 % softwood pulp at single layer formation and up to 60 % at two-layer formation. (Kassberg, 1998)
3.2.2 Mechanic pulp
Tissue can be manufactured by using only pure mechanical processed pulp. This is most commonly used when producing cheaper tissue. There are three common types of mechanical pulp, groundwood, thermomechanical pulp and chemical thermo-mechanical pulp. Groundwood is the cheapest type of pulp because it consists of grounded wood logs that are mixed with water. This gives a rougher tissue with poor tear strength. To improve the characteristics of the paper it is common to use refined pulp where the most common types are thermomechanical pulp and chemical thermomechanical pulp. Both are produced by heating the woodchips with steam to about 120 ° C, and then refining the pulp under pressure. In order to make chemical thermomechanical pulp the pulp is mixed with sodium sulfate. (Kassberg, 1998)
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4.
Process description
4.1. Pulping
Pulping can be done in several ways, one of the method is with discontinues pulper. (Gavelin, 1999) As can been seen in Figure 1, a pulper is a circular trough with a rotor in the center of the bottom. Beneath the rotor a sieve is placed. A pulper can operate both continuous and discontinuous. (Fellers & Norman, 1996)
Figure 1: Configuration of a standard pulper.
In this type of pulping a certain amount of bales are added depending on how much pulp is
needed. Then the fibers are diluted with water to the desired percentage, and chemicals are added. This method is best suited to use when you want to know the exact concentration of chemicals, for example to dye the paper. This type of pulping takes about 15-20 minutes (Gavelin, 1999).
4.2. Beating
Beating the fibers can be compared to mill them and the purpose of beating the pulp is to increase the tensile strength and fiber softness (Gavelin, 1999). The fibers get more flexible during the beating and it will increase the density of the paper (Fellers & Norman, 1996).
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4.3. Wet end
4.3.1. Headbox
The main purpose of the headbox is to apply the stock (mix of pulp and water) on top of the wire (see chapter 4.3.6) to form an even layer of fibers which is aimed to be homogeneously. The headbox includes three main components, the cross-distributor, a pressure chamber and slice. The pressure chamber is not always included in all headboxes. Before the stock is fed into the headbox all the air has to be removed. (Persson, 1996)
4.3.1.1. Cross-distributor
The cross-distributor goes straight through the headbox and creates a steady flow of the stock so it can be applied onto the wire in an even layer. It also helps by dispersing the fiber flocks of pulp so the stock becomes more homogeneously. (Persson, 1996)
4.3.1.2. Pressure chamber
The purpose of the pressure chamber is to equalize and improve the speed of the mass flow between the cross distributor and the slice.
There are two different types of chambers with different technologies. One method is an air cushion headbox which is a further development of the old fully open inlet boxes. The second type is a box with a hydraulic technique, where instead of using the air bag it uses a liquid. The latter technique is primarily designed for twin-wire (the paper web is placed between two wires during several stages at the wet end) forming. The principle of both of them is to try to help the cross- distributor to create a steady flow by creating a positive pressure in the chamber. (Persson, 1996)
4.3.1.3. Slice
The slice is a taper out of the headbox that directs the flow of stock down onto the wire. A crucial moment for the initial dewatering process is that the stock hits the wire in a correct angle. Another important factor to consider when looking at the slice is the pressure in the box. The outlet rate
depends on the pressure in the box and at a higher production rates it may occur pressure deformation. (Persson, 1996)
The difference on the outlet flow and the speed of the wire must also be calculated as a ratio between each other. When the ratio is equal to one there is a risk that the fibers in the pulp will flock. This can be prevented by having a slightly higher or lower ratio on the speed. When the ratio differs from one it will result in shear deformations that will prevent the fibers to flock. (Persson, 1996)
6 4.3.1.4. Multilayer slice
In the manufacturing process of tissue paper a multilayer slice can be used. The slice is capable of spraying out layers with different types of stock out from the headbox. This can be used for example, if the tissue needs to have different characteristics on each side. (Persson, 1996)
4.3.2. Open headbox
Another method for applying the stock on to the wire is by having a regular flow of stock pour down onto the wire from an open box, see figure 2. The stock is continuously pumped into the box with a certain volume flow. The volume flow of the stock will be equally down on the wire as the rate of flow into the headbox, see Appendix B.
Figure 2: An open headbox. 1) The stock enters the box at a desired rate of flow. 2) Propeller to stir the stock and prevent it from sediment
7 4.3.3. Forming table
The forming table supports the wire, so when the stock first hits the wire it does not bend down, as can be seen in Figure 3. It also reduces the amount of air that the wire contains. It is therefore important to choose right distance between the breast roller and the forming table. If a too short distance is chosen the air will not have time to be removed from the wire. Or if an excessive distance is chosen it will result in vibration on the wire. The distance also determines the amount of initial dewatering. (Persson, 1996) The first dewatering starts at the forming table with the help of foils (see chapter 4.3.4) and vacuum foils. The foil drying process does not affect the tensile strength or the density.
To increase the effectiveness of drying the stock it can be preheated before being applied to the wire. Heating the stock also decrease the viscosity of the paper which gives a better dewatering process, see Appendix B.
Figure3: The wire section with the forming table (section 6, 3, and 4). 1) Slice, 2) Breast roller, 3) Foils, 4) Vacuum boxes, 5) Wire washer (pressurized water), 6) Water collecting tray, 7) Suction box
8 4.3.4. Foil
On the first part of the forming table it exist a number of foils. The point of using foils is to create a suction effect on the wire to help the dewatering process to start. By using this method correctly you will get a better formation on the bottom of the paper. (Persson, 1996)
A foil is built up first with a flat surface that touches the wire before leaning down from the wire in a small angle of 1-3° along the wire’s motion direction, see figure 4. This creates a pulse of under pressure that sucks down the water in the wedge, and before the water is sucked up again the wire has moved forward to a new foil. The new foil will scrape away the water drops that have not been sucked down from the wire and create a dewatering effect. However the suction pulse creates an opposite effect in the shape of a pressure pulse as it passes the next list. The pressure pulse will move the wire up from the foils instead. It is important that the pressure pulse is not too large or too small. At a small pressure pulse a small loosening on the pulp will happen which prevents dewatering. However an excessive pressure pulse damages the web and breaks the formation of the fibers. Important to remember is that the power of the pulses increases with the square of the velocity. (Persson, 1996)
Figure 4: High speed foils. The wire passes over the foils and pressure pulses sucks water down. It also scrapes of some water under the wire against the edges of the foils.
With a slow producing machine it is more difficult to create the pressure pulse as it depends on the speed out of the wire. A solution to this problem is to use foils that have a small curvature against the wire to cause a bigger pressure pulse, se Figure 5. (Persson, 1996)
It has been stated that a ribbed belt can be used instead of foils for a small paper machine (Covey, Helmer, & Raverty, 2003).
Figure 5: Slow speed foils. The curvature of the foils helps to create a higher pressure pulse during a slower manufacturing speed.
9 4.3.5. Breast roll
The breast roll is placed before area where the stock hits the wire and after the cleaning section. The task of the breast roll is to guide the wire during manufacturing since it tends to move sideways. For more information see Appendix B.
4.3.6. Wire
The wire is produced of plastic threads that are wowed into a desired pattern. The pattern determines the durability, drainage capacity and stability of the wire. There are wires with one, two and three layers with each different wire sizes and densities. (Persson, 1996)
4.3.7. Cleaning and drying the felt and wire
The wire need to be dried and cleaned after the stock leave its trajectory since some dirty water and other residues stays in there and reduce the dewatering process. The felt and wire can be washed with pressured water. After the cleaning process the water need to get out making room for the new water. Drying the wire after the cleaning section can be made with the help of suction boxes or heated air, see Appendix B.
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4.4. Press section
There are two different techniques that can be used when pressing the sheet, single or double felt. The number of presses varies depending on the characteristics and type of the paper that you want to get out of the process. When pressing, it is mainly the free water between the fibers that can be pressed out. The fibers can contain up to one third water and it can only be pressed out to a certain amount and then under the impact of a high pressure (Persson, 1996)
The water viscosity decreases with a rising temperature, which can be exploited to a certain degree depending on how much the felts and rollers can withstand. With a rising temperature the paper becomes more plastic deformable and can be more compressed at the press, which makes the paper to lose some of its stiffness. (Persson, 1996)
Pressing the paper will increase the density of the finished paper. A higher end density of the paper means a higher tensile strength of the paper. The pressing will decrease the distance between the fibers and the paper web will be more compact. (Fellers & Norman, 1996)
4.4.1. Felt
The felt has the purpose of even out the pressure between the sheet and the roll meanwhile to absorb and carry away the water that gets pressed out of the sheet.
To clean the felt from dirt that gets added in the press you can use a spray header. The spray headers are placed at a distance of about 250-400 mm away from the felt and the beam angle of the spray headers onto the felt is 90 °. An important thing for this method is that the water needs to be
droplets before they hit the felt in order to get a good effect. The water that gets added in this process is removed with the help of vacuum boxes, which also suck out the water from the felt that gets added in the press nips. (Persson, 1996)
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4.5. Dryer section
4.5.1. Cylinder dryer
Today the most common method of drying paper is with drying cylinders, see Figure 6. The drying is achieved by pressing the wet paper against hot cylinders. The heating of the cylinders is done by injecting steam through the cylinder shaft. The steam will condense against the cylinder walls and they are heated. Condensation also leads to a need to pump out the condensated water which is done with a siphon tube. The cylinders are placed in an above line and one below line that creates a drive group. Each line has its own wire, and so the paper can hang freely and dry evenly between cylinders where hot air is blow on the paper. This method where the paper hangs freely increases the drying capacity, but also increases the risk of folds and web breaks. To reduce these risks, this method has, since the 70's, been gradually switched (to something called) the slalom wire draw (Persson, 1996). This technique reduces the risk of web breaks because the wire supports paper at all times. This leads to a decrease of the drying capacity, because the wire is always attach to one side of web which will not dry as well as the other side. Because only one side of the paper is pressed against the cylinders the drying become inefficient compared to standard conventional drying. To enhance drying it is possible to replace the cylinders that the wire cannot press the paper against for a suction roll. But as the paper dries asymmetrically it will therefore be forced to feed the paper to a new drive group where the wire is on the opposite side. (Fellers & Norman, 1996)
12 4.5.2. Fan dryer
In the air-borne drying process the paper is transported overseveral boxes with nozzles that blow hot air on the paper. The boxes are often placed in a long line after each other. At the end of each line is a turning roll which turns the paper so it can be drawn back over the new line with the top side facing down. This way the machine can be build with a multi-storey box lines, see Figure 7. The low web tension also results in that the force needed to pull the web through the machine are relatively small. In long lines of fan dryers they are placed beneath the paper so an air cushion gets created and lifts the paper web up. This will reduce the force to drive the wire. (Fellers & Norman, 1996)
13 4.5.3. Infrared Dryer
At tissue manufacturing, it is common to replace the last press for an IR-element. By removing one of the presses the tissue changes its material properties. One example is the reduction of density, which means it increases the bulk of the paper resulting in a looser more porous paper. (Gavelin, 1999) An infrared drying is made up of several modules that are composed of six infrared lamps. The lamp consists of tungsten which is heated to above 2000° C. The lights will then emit radiation in the infrared spectrum. Behind the wires are different kinds of reflectors consisting of ceramic plate or a metal coated concave mirror. Approximately 30-60 % of rays will pass through the paper. The radiation is either reflected back by the cylinder or it will heat the cylinder surface. The amount of radiation which penetrates the paper is dependent on the type of paper produced, but also the temperature and water content. As the temperature increases and the water content decreases over time of exposure, it leads to reduced efficiency of the drying. To improve the efficiency, it is common to use infrared rays of different wavelengths. (Fellers & Norman, 1996)
The IR drying can be made in two different ways, either with a steel belt or a cylinder. 4.5.3.1. Steel cylinder
When using a cylinder the IR heaters heats up the cylinder from the inside and the paper dries on the cylinders surface. IR heaters can be added on the outside to decrease the drying time. There is a risk that the paper will adhere to the cylinder surface.
4.5.3.2. Steel belt
When using a steel belt the dryers heat the paper and the steel belt, see figure 8. The heat rays will dry the paper and the steel wire will help to reflect back some of the rays that pass through the paper and give a more effective drying. There is a risk of the tissue to adhere to the steel wire during the process.
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4.6. Tissue machines
There already exist several complete solutions for tissue machines (Metso.com, 2012). 4.6.1. Dry crepe tissue
Dry crepe tissue, see figure 9, is a classic machine that uses either a C-forming or Crescent forming with a Yankee cylinder. When using a Crescent former the stock is placed directly to a felt wire that
transports the paper web to the Yankee cylinder. At the Yankee cylinder it gets hot-pressed against it with a roll. A C-former uses a shaping wire first before transporting the paper web over to a felt wire before reaching it reaches the hot-press at the Yankee cylinder. (Lindquist, 2010)
Figure 9: Dry crepe tissue machine. 1) Headbox, 2) Forming roll, 3, 4) Hot presses 5) Yankee Cylinder, 6) Yankee hood, 7) Doctor’s blade for creping and cleaning, 9) Water cleaning and chemical adding
15 4.6.2. Through air drying
Through air drying differs from other manufacturing methods by having no press rolls. Instead it uses a through air drying wire that is placed between the forming section and the Yankee cylinder. The wire passes two large cylinders placed beneath the machine, see Figure 10. The cylinders works almost in the same way as the Yankee hood except the cylinders also suck the air through the paper into the cylinders with a under pressure. The cylinders are perforated so the air can pass through the surface. The paper gets a high bulk and absorption capacity from this process compared with the DCT method, due to the paper is forced onto the through air drying wire with the help of vacuum that shapes the paper web. (Lindquist, 2010) The air temperature can be between 100 – 150° C, see Appendix B.
Figure 10: Trough air drying machine. 1) Headbox, 2) Forming roll 3) Cylinders with suction boxes on the inside, 4) Fan system
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5.
Results and discussion
There exists one small paper machine at Metso in Karlstad that manufactures paper of 24cm width and at a speed of 2m/min (Appendix B). Even if the paper machine has some techniques that can be used for the intended paper machine there exists no paper machine that fulfill the requirements from the client. This means that the intended machine can be technically based of the paper machine in Karlstad with some changes to fulfill the requirements.
5.1. Pulping
The small paper machine in Metso Karlstad is directly connected to a small pulper that approximately can hold up to 200 liters of fluid (Appendix B). The pulper is built up like a version that is used in commercial manufacturing, see chapter 4.1.
According to the requirements, the machine need to be able to mix 2 kg pulp with 400 liter water if the concentration is 0.5 %, see equation 1. This means that a larger pulper or a separate storage tank needs to be built to suit the volume requirements.
⁄ (Equation 1) m1 = pulp mass [kg]
c = percentage of fiber concentration m2 = stock mass [kg]
5.2. Beating
The small paper machine at Karlstad uses a refiner in the process. Beating increases the tensile strength of the fiber bindings, see chapter 4.2.
With strong fiber bindings the paper is more difficult to break down by the compounder. It is therefore not desirable to use a refiner in the intended machine.
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5.3. Headbox/open headbox
The headbox used in the paper machine at Metso in Karlstad uses an open headbox, see chapter 4.3.2. The compounder machine used to mix the paper with the polymer can not use paper wider than 12 cm. This means that the intended machine can not produce a paper wider than 12 cm when it is used in line with the compounder. In order to make the machine in Karlstad fit the width requirement of
compounder, the intended machine needs to have an outlet with a width of 12 cm compared to 24 cm as the machine in Karlstad uses. The width of the headbox need to be a little wider than the finished paper since the web will shrink with about one percentage in the processes (Ivarsson). But since the shrinkage is about one percentage the total shrinking of the web will be minimal and there will be no need to include it in the calculations of the outlet width, see equation 2. The requirements of minimal width of the paper are 6cm.
(Equation 2)
The stock flow in to the headbox is the same as the out flow of stock and can be calculated by using equation 2. For an example, with a grammage of 50 g/m2, width of paper web 0.12 m, and a speed of the wire of 2 m/min the stock flow rate out from the open headbox to the wire need to be 2.4 kg/min, see equation 3.
(Equation 3)
m = grammage [g/m2]
b = width of paper web [m]
c = concentration of stock [kg/H2O]
v = speed of wire [m/min]
In comparison with for an example a headbox described in chapter 4.3.1 uses complex parts like cross-distributor, pressure chamber and slice that directs the flow of stock down onto the wire, the open headbox does not need any of this cross-distributor, pressure chamber and slice parts and is much simpler to build sense it is mostly constructed out of acrylic sheets glued together to form a box. And this headbox is already proven to work in this productions speed, this means that there is no need to design a new headbox similar to the one described in chapter 4.3.1. The main advantage with the headbox described in chapter 4.3.1 is that it can handle much higher productions speeds then the open headbox. But since the production speed requirements for the intended machine is 2 m/min this advantage is not important.
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5.4. Forming table
The requested machine needs a wire with a forming table
The small paper machine at Metso in Karlstad uses a forming table with both normal foils and vacuum foils. By having foils first, it gives the water that naturally flows away a place to go, instead of staying on the wire. The foils will also help the dewatering process by scraping of the water from the underside of the wire. Instead of getting foils from a manufacturer ribbed belts can be used (Covey, Helmer, & Raverty, 2003). It is a cheaper and easier method to create foils of ribbed belts since there is no need to order customized foils from a manufacturer to fit the intended machine. The customized foils are manufactured individually for each paper machine depending on the manufacturing speed (Persson, 1996). Instead standardized ribbed belts can be used. The suction boxes after the foils will help to suck out a little more water from the web by having a lower air pressure in the boxes. The dryness of the paper should be at around 20 % after the forming table if the same concept on the forming table is used as in the small paper machine at Metso (Appendix B).
In order to make the wire to go around the forming table the wire must have a certain tension to be driven by the drive roll. Otherwise the driving roll will not be able to make the wire to go around (Appendix B). To create the tension one of the roles can be made moveable in the machine direction (Ivarsson). This is how the wire tension in the small paper machine at Metso was controlled. Bearing housings had oval hole that allows one to move the bearing housings, as far as necessary to tension the wire.
The forming table should be as similar as possible to the forming table at Metso in Karlstad since it is a proven machine that works.
5.5. Pressing
The small paper machine at Karlstad uses a press to dry the web. Pressing will increase the tensile strength and density of the paper, see chapter 4.4. A higher density means that the distance between the fibers is closer to each other and they are more bonded. With a compact paper with strong fiber bindings, the paper is more difficult to break down by the compounder.
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5.6. Drying
The paper machine in Karlstad uses a cylinder dryer, see chapter 4.5.1. This type of drying technique presses the paper against hot cylinders. Since pressing increases the tensile strength and the density of the paper this method is not desirable to use in the intended machine. Instead a alternative method has to be chosen to dry the paper.
After a discussion with companies (Björnberg) (Eriksson) (Ivarsson), comparing methods (infrared drying and through air drying) from the literature review and visit to a manufacturer (Metso.com, 2012), two different solutions for drying the paper was found that could be implemented to the requested paper machine.
5.6.1. Infrared drying
Infrared drying can be used in two different methods, either with a horizontal surface or on a cylindrical surface. The cylinder will save more space than the flat steel wire. The Infrared dryers require less equipment than through air drying systems (Björnberg).
The cylinder width needs to be 400 mm and have a diameter of 500 mm to make room for the required equipment, and the effective use of the cylinder length is 240 degrees (Eriksson). With a diameter of 500 mm the cylinder will get a 1047 mm length that the paper will dry on, see Equation 4 and Figure 11.
(Equation 4)
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The effect needed to heat up the drying cylinder with infrared heat to the working temperature is 2 kW and additional 2 kW to evaporation of the water in the web (Eriksson) with the given requirements of starting dryness of 20 %, finished dryness of 95 % at a manufacturing speed of 2 m/min.
By putting the paper web directly on a steel surface there is a risk of the paper to adhere to the surface. This can be prevented if the steel is coated with chrome or Teflon (Björnberg).
To test if the paper will adhere to the surface a small experiment can be made by putting a paper with a dryness of 20 % onto a heated steel plate and put into an oven until it is dry. The heated steel plate should have same working temperature as the cylinder will have. If it does not adhere to the steel plate it will not probably adhere to the steel cylinder. If it adheres to the surface, test can be made with coating materials like chrome or Teflon (Björnberg).
If the experiments show that the paper adheres to the steel surface a crepe process (a metallic blade is put against the surface of the drying cylinder to scrape the paper away from the cylinder) will be needed at the end of the drying process (Ivarsson). Since the request is not to include any crepe process the infrared drying would not be available method for drying.
21 5.6.2. Through air drying
The drying process can be made on a horizontal surface or on a cylinder depending on how much space is available. The surface of the drying cylinder has to be perforated with many small holes so the air and water can be sucked out by suction boxes on the inside of the cylinders. The paper web should be placed on the through air drying wire before the drying cylinder to avoid web breaks (Ivarsson). Drying with heated air compared to infrared drying requires more energy due too that the air has to be heated in a through air drying method (Lindquist, 2010) (Persson, 1996). To increase the effectiveness of fan drying, hoods can be added to the sides of the drying process to maintain the heated air around the paper web (Ivarsson). Since the request gives no limitations on energy efficiency, through air drying is an available method.
A through air drying process also produces a lot of noise (Ivarsson) making it an unhealthy work environment. By adding protective hoods to the dryer, the noise can be reduced.
The most viable option is the through air drying system, see Figure 12. A through air drying process gives a loosely bonded paper (Ivarsson). Since the paper web is always placed on a wire during the drying section, there will be no risk for the paper to adhere to the drying cylinder. Therefore no restrictions exist of which type of pulp can be used regarding the drying section.
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5.7. Reel-up
A reel-up is only needed if the intended machine will not be connected to the compounder; otherwise the reel-up section will be bypassed.
To get the paper into a roll there are several techniques used in commercial manufacturing (Persson, 1996)(Appendix B). Those machines are designed to work with a continuous high feed of paper at a high speed that automatically switch paper rolls when they are full. Since the intended machine will not produce same amount of tissue at same speed an automatically technique for changing paper rolls are not needed.
An alternative solution to make paper rolls is to make us of the gravity and the wire speed, see Figure 13. By letting an empty roll coated with glue touch the tissue after the drying process, the empty roll will start to rotate with the same speed of the wire. Due to the glued roll the paper will attach to the empty roll. With the tissue rolling up the diameter of it will increase. By letting an arm holding the roll it will start to move upwards with the increasing diameter. It results in the peripheral speed of the paper roll will be constant regardless of the diameter of the paper roll. When the tissue roll has achieved desired diameter it can be removed by cutting of the tissue manually and a new empty roll can be placed on the arm.
Figure 13: Reel-up
To make a simple reel-up system for the paper the given method above should be used. The method does not require any extra motors which gives a cheaper machine since there is no need to buy more equipment.
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5.8. Concluding discussion
The machine is based on the machine (Appendix B) at Metso in Karlstad with the following changes: The pulper needs to be larger to accommodate 400 liters of stock.
The refiner needs to be removed since it makes the finished paper stronger and therefore more difficult to mix with the material in the compounder.
The outlet of the head box needs to be 12cm instead of 24cm in order to make the finished paper fit the width requirement of compounder.
The forming table can stay the same as the Metso machine but the foils can be changed to ribbed belt instead. This should be cheaper to purchase then special made foils.
The pressing section of the Metso machine needs to be removed since it makes the paper stronger and therefore more difficult to mix with the material in the compounder.
The dryer section needs to be replaced by a through air drying process with a cylinder configuration. Since the current process presses the paper on hot cylinders which leads to a stronger paper. The machine at Metso does not have any reel-up system. A reel-up is only needed if the intended machine will not be connected to the compounder; otherwise the reel-up section will be bypassed. When the machine is not connected to the compounder a reel-up system using glue and the weight of the paper roll will be used.
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6.
Conclusions
The conclusion is to use the same forming table and headbox as used in Karlstad since it is a method that works and the machine already exists; there is no need to build a complete new machine. For the drying part the through air drying is the most viable option since it is already used in commercial manufacturing and no risk of adhesion of the paper on to the cylinder.
Figure 14: The whole concept as a picture
Figure 15: The open headbox used in the small paper machine at Karlstad that can be used for this machine as well
Figure 16: The through air drying cylinder. 1) Fan dryers, 2) Perforated cylinder, 3) Suction box, 4) Reel-up
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7.
References
Björnberg, T. (n.d.). Ircon - Personal communications.
Covey, G., Helmer, R., & Raverty, W. (2003). A low cost pilot paper machine. Appita journal , 55 (6), pp. 453-456.
Ek, M., Gellerstedt, G., & Henriksson, G. (2006). Ljungberg textbook, Book 4. Stockholm: Fibre and Polymer Technology, KTH.
Eriksson, K. (n.d.). Teknik system AB - Personal communications.
Fellers, C., & Norman, B. (1996). Pappersteknik. Stockholm: Avdelningen för pappersteknik, Kungl tekniska högskolan.
Formulosa. (2012, 04 27). Formulosa. Retrieved 04 27, 2012, from http://formulosa.com/ Gavelin, G. (1999). Mjukpapper. Markaryd: Skogsindustrins utbildning Markaryd AB. Ivarsson, H. (n.d.). Metso - Personal communications.
Kassberg, M. (1998). Massa och papper. Markaryd: Skogsindustrins utbildning i Markaryd AB. Kneifel, A. (n.d.). wefapress papertec - Personal communications.
Lindquist, G. (2010). Energiförbrukning vid mjukpapperstillverkning. Umeå: Umeå universitets tekniska högskola.
Metso.com. (2012, 04 11). Retrieved 04 11, 2012, from
http://metso.com/pulpandpaper/MPwTissue.nsf/WebWID/WTB-041125-2256F-39C4D?OpenDocument Persson, K.-E. (1996). Papperstillverkning. Markaryd: Skogsindustrins utbildning i Markaryd AB.
Ramasubramanian, M. (2000). An experimental investigation. Journal of manufacturing science and engineering , 122 (3), pp. 576-581.
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Appendix A: Besöksrapport – Korsnäs
2012-03-21Syfte
Syftet med besöket var att få en grundlig beskrivning på hur en pappersmaskin fungerar för att kunna göra en beskrivning på en pappersmaskin till en specifikation, samt att få information om olika producenter utav pappersmaskiner och andra tillverkare utav papper. Dessutom vill vi studera den småskaliga pappersmaskin som Korsnäs har i laborationssyfte.
Beskrivning
Vi mötte upp Tobias Söderholm1 på Korsnäs för att få en kort överskådlig beskrivning utav
pappersindustrin och en utav deras kartongmaskiner. Därefter så förklarade han mer ingående hur de själva momenten i tillverkningsprocessen gick till. Först tillverkar Korsnäs sin egen fibermassa på
fabriksområdet innan den transporteras till kartongmaskinen där den först mals och blandas med vatten och kemikalier till en mäld och sedan späds ut med vatten till lagom koncentration. Mälden pumpas sedan ut till inloppslåda som med ett givet tryck placerar mälden med ett jämnt tunt lager mellan två valsar för att forma ett pappersark. På valsarna sitter det en vira som för vidare arket över foilslister där en stor del utav vattenmängden försvinner. När arket har passerat foilslisterna så guskas de olika lagren på kartongen ihop igenom att de olika virorna med ark av olika massatyper möts i fyra steg. När arken guskats ihop förs den vidare över till presspartiet i maskinen på en filt. Filten för vidare arket genom ett visst antal valsar som pressar ut vatten ur arket och för bort vattnet. När denna process är klar så är kartongen starkt nog att hålla ihop utav sin egen vikt och förs in torkpartiet som är det längsta partiet i processen. Eftersom kartongen klarar av att dras i detta stadium så behövs det ingen vira eller filt att transportera fram kartongen med utan det dras fram självmant via cylindrar. Cylindrarna är placerade i sicksack mönster i en lång bana så att ena sidan utav kartongen torkas mot en cylinder och andra sidan torkas mot nästa cylinder. Cylindrarna är uppvärmda under hela denna process. Efter torkprocessen så beläggs kartongen med olika ämnen beroende på order innan det sluttorkas och rullas upp i stora rullar för avkylning innan det förs vidare till ett lager.
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Efter att vi fått se vart Korsnäs packar och lagerför sina produkter gick vi vidare mot deras
utvecklingslabb för att få se på en liten kartongmaskin som de använder. Maskinen är uppbyggd på det sättet att operatören blandar vatten med fibrer bredvid för att få till önskad mäld. Mälden öses sedan ned i ett kar på maskinen som håller blandningen i kontinuerlig rörelse för att hindra den från att sedimentera. Därefter pumpas mälden till en spruta som förs fram och tillbaka i vertikal riktning för att lägga på ett tunt lager med massa på en roterande vira. När all massa har placerats på viran så förs den upp och läggs mellan 2 filtar innan den pressas mellan två valsar. Maskinen tillverkar två stycken lika stora ark samtidigt. Vardera pappersarken pressas två gånger med bestämt tryck innan det läggs in i en elektrisk torkugn. Efter pressningen mellan valsarna så är pappret starkt nog att hålla för sin egen vikt.
Slutsats
Mälden som blandas ihop utav vatten och massa består till ca 99.98% vatten. Trycket på mälden vid inloppslådan fick vi inte reda på utan att de varierades beroende olika faktorer t.ex. på vilken sorts kartong som tillverkas och hastighet på viran. Hastigheten på maskinen var ca 650m/min. Vid torkning utav arket så minskar bredden, dock så visste Tobias inte hur mycket. Tack vare att pappret innehåller så mycket vatten så kan man använda sig utav temperaturer vid torkning som överstiger 100°C.
Volymen massa som användes på laboratoriemaskinen var 50 l. Den roterande viran snurrade med en hastighet på ca 800 rpm. Trycket på valsarna vid pressning låg på 1bar respektive 2bars övertryck. Pappersarken låg inne i torkugnen i 15 min.
I besöket fick vi reda på två stycken olika maskintillverkare som är stora inom branschen och de är Metso och Voith. Samt att det finns en tillverkare utav mjukpapper i Mariestad som heter Mestä tissue.
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Appendix B: Besöksrapport – Metso Paper Karlstad AB
2012-04-12Syfte
Syftet med detta besök är att se hur en fungerande pilotmaskin för tillverkning utav tissue fungerar samt att inledda diskussioner med Metso som är en tillverkare utav pappersmaskiner om vad som är möjligt och inte när man tillverkar en maskin och vad man ska tänka på.
Beskrivning
Dagen börjar med ett möte i ett konferensrum med Hans Ivarsson (Project Manager R & D) där han presenterade sig och vad Metso gör i Karlstad. Därefter gick vi på en rundvandring vid deras pilotmaskiner.
Mälderi
Vi gick först till massuppslagningsdelen av anläggningen. De torkade pappersmassabalarna tillverkad av Södra, förs ner i en uppslagare där massa blandas med vatten. En bal med pappersmassa väger
ca:250kg. Hans Ivarsson berätta att beroende på massan sammansättning får pappret olika egenskaper som påverkar slutprodukten och tillverknings processen. Barrmassa (Gran, Tallfibrer) ger ett starkare papper medan lövmassa (eukalyptus) ger ett mjukare papper. Hans sa även att man kan använda flera skikt i tissue tillverkning. Man använder då lövmassa på baksidan av pappret och barrmassa på
framsidan. Efter uppslagningsmaskinen så visade Hans oss några färdiga rullar. Dessa var 50cm breda och en diameter på ca 50cm. Efter det gick vi vidare till lagerkaren där Hans berättade att man använder ett kar för varje lager i pappret.
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Tissuemaskinen
Sedan fortsatte vi till den första tissue maskinen (tm1) som är den maskin som oftast används. När vi var där så stod maskinen stilla och viran var bortplockad. Vi började med att titta på formningspartiet där mälden först kom till en blandningspump där den späds ut med mer vatten så att koncentrationen blir ca 0,1 % enligt Hans. För att öka effektiviteten så värms mälden upp till ca 40° C, det ger en högre starttemperatur vid torkningen med lägre viskositet på vattnet. Efter det så sprutas mälden ut mellan två viror som förs ihop över en formeringsvals. Formeringsvalsen är den drivande valsen i systemet. När mälden träffar virorna så sker en första avvattning genom den höga hastighet som mälden sprutas ut med, viran har små hål i sig som vattnet passerar. Dock så åker inte allt vatten igenom och en del fibrer försvinner i denna process. När mälden passerat formeringsvalsen så delas virorna och mälden följer med den med hjälp utav suglådor bort mot yankeecylindern. Formeringsviran lämnar mälden efter formeringsvalsen för att passera ett tvättsystem. Tvättsystemet består utav ett visst antal spritsrör som med högt tryck spolar rent viran med vatten. Strax efter avvattningen passerar viran en vals som kan förspännas för att man ska få viran att åka runt. Viran har också en tendens att röra sig i sidled längs med valsarna, för att förhindra att viran åker av har man gjort en vals vinklbar. Där sitter det en sensor som känner av när viran rör på sig och signaler sänds till valsen som då vinklar sig för att motverka förflyttningen. Mälden som har transporterats bort till yankeecylindern pressas först med en varmpress mot yankeecylindern för att klistra fast på den och en del vatten ska pressas bort i en DCT-process. För att öka på klistringen utav pappret mot cylindern sprutas det på kemikalier med hjälp utav spritsrör dessa kan placeras på olika sätt så att varje punkt på yankeecylindern träffas av en, två eller tre strålar. Arket följer med yankeecylindern runt när den roterar och varmluft (ca 180° C) blåses emot papprets yta. På andra sidan utav yankeecylindern placerar man en stålskena som kallas schaber som kräppar bort pappret från cylinderns yta. Detta för att man ska få veck på pappret som ökar töjbarheten. Längden på pappret minskar med 15-20 % under kräppningen, detta leder till att efterföljande viror och processer måste minska sina hastigheter med motsvarande längdförminskning för att inte papprets ska dras ut igen. Vinkeln på kräppschabern avgör kvalitén och antalet veck som skapas på pappret men normalt så körs det med en 90° vinkel, det är mest vid specialfall som det körs andra vinklar. Efter schaber sitter det en till som är tillför att göra rent ytan på cylindern innan kemikalier appliceras. När papper kräppats av förs det bort med en vira och suglådor mot en process som kallas kalandrering som ska göra papper ännu mjukare. Kalandrering görs med genom att pappret passerar mellan två stycken uppvärma valsar. Efter denna process är pappret klart för upprullning och förvaring. Upprullningen sker genom att pappret passerar ett kartongrör som har klister på ytan. Röret fångar upp pappret från viran och rullar upp det.
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Maskineriet
Efter att Hans visat maskinen gick vi vidare mot deras andra tissuepilotmaskin som de inte använder lika ofta, den har samma principer som den första förutom att man kan göra några annorlunda inställningar och prover. Vi fick även se deras vakuumsystem, pumpsystem och kemikalieförråd.
Through air drying cylindrar
I källaren där maskineriet fanns stod även cylindrarna för TAD systemet. De är uppbyggda på det sättet att ytan på dem påminner om mönstret till en bikupa. Runt cylindrarna sitter det fläktar som blåser ut varmluft mot pappret och de småhålen på cylindrarna suger luften igenom pappret. temperaturen på luften ligger på 100-150° C. Nackdelen med detta är att även vattnet och viran värms upp och
effektiviteten försämras. När pappret passerat through air drying cylindrarna åker de runt
yankeecylindern och sedan kalandreras. Vid användning utav through air drying system kräppas normalt inte pappret bort ifrån yankeecylindern.
Liten pappersmaskin
I deras lokaler så höll en gymnasieskola praktiska övningar på en liten pappersmaskin. Pappersmaskinen var 3 – 4 m lång och tillverkade papper med ytvikten 60 – 100 g/m2 i en hastighet på 1-2 m/min.
Bakgrunden till vem som hade byggt maskinen och vart den kom ifrån visste inte Hans direkt utan skulle kolla upp information om det.
Själva maskinen bestod först utav ett koniskt kar med en rotor centralt placerat i mitten på botten ovanför en sil. Själva uppslagningskaret rymde minst 200 liter. Därefter gick det slangar över till ett annat kar där man förvarade den färdiga massan innan den skulle pumpas till maskinen. Deras
pappersmaskin använde sig inte utav en traditionell inloppslåda utan en plastbalja men ett plant utlopp över viran. Mälden pumpades med ett kontrollerat flöde in i baljan och på så sätt bestämdes också hur mycket massa som rann ned på viran. Baljans bredd kunde ställas in och max var 24cm. Efter att mälden hade applicerats på viran så passerade mälden ett antal foils och sedan vacuumfoils. Därefter lämnades mälden över till en enkelsidig filtvira där pappret pressades mellan ett par valsar. När pappret hade pressats åkte det in i en cylindertork med fem stycken cylindrar innan det var färdigtorkat. Till maskinen finns det ett fungerande mälderi med en konkvarn.
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Slutsats
Pappret får sämre kvalité vid sidorna så någon gång under processen tas ca 10cm bort ifrån sidorna utav pappret för att få jämn kvalité. Pappret krymper med någon % totalt i bredd från början till slut enligt Hans.
Man kan värma mälden innan den läggs på viran för att sänka viskositeten på vattnet vilket ger en bättre avvattning samt öka effektiviteten på torkningen (ca 40° C på mälden i Metso).
Hans trodde var praktiskt svårt att ta fram och göra en säker liten Yankeecylinder. Det gick att använda sig utav TAD principen för torka papper på en rak bana istället för cylindrar (Temperaturen på
varmluften är ca 100 - 150° C). Att man använde cylindrar där var för att spara plats. Men stora delar av den lilla skolmaskinen skulle kunna var intressanta att använda i vår maskin. Hela formningskonceptet kommer att passa oss men en del förbättringar kommer att införas. Press och torkdelarna är inte optimala för tissuetillverkning och kommer därför att behöva bytas ut till en annan sorts torkning.
Den lilla pappersmaskinen har dessa egenskaper: Virans hastighet: 2 m/min
Bredden på pappret: 240 mm Ytvikt på pappret: 60 – 100 g/m2 Maskinens längd: ca 3-4 m
Volym på uppslagaren: ca 200 liter Torrhalt på färdig produkt: ca 95 % Torrhalt efter formningsbordet: ca 20 %
