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In the paper machine, the paper is formed and most of the properties of the paper are determined. The paper machine is actually a large dewatering device consisting of a headbox, a wire section, press section and dryer section. The most common machine design to date is the Fourdrinier forming process in which the sheet is formed onto a continuous horizontal wire or fabric onto which the suspension of fibres is supplied from the headbox. Also, twin wire formers are used for web formation. In twin wire formers, the fibre suspension is fed between two wires operating at the same speed where the water is drained in two directions. There are different types of twin wire formers (e.g. gap formers). In gap formers, the diluted stock is injected directly into the gap between the two wires. A third alternative is to combine the Fourdrinier and a top wire. These are referred to as hybrid formers.
Figure 1 shows the key features of a twin wire paper machine.
Figure 1: Key features of a twin wire paper machine
Paper is made by feeding a dilute suspension of fibres, fillers (optional), dyes and other chemicals onto a fine wire mesh through which the water drains away, leaving a web of fibres, fines and fillers on the mesh. The fibre slurry, which at the wet end of the paper machine is typically between 0.2 % and 1.5 % consistency, is pumped to the wire section via the headbox. The objective of the headbox is to create a uniform dispersion of fibres across the total width of the papermaking wire, in order to achieve a uniform paper formation and grammage distribution. The water drains through the wire aided by 'dewatering elements'. Examples of these are rolls, foils and vacuum boxes below the wire. After typically 10 m, the web of paper is formed. In a twin wire former, additional dewatering pressure is formed by the fabric tension over a curved surface of blades or a roll. By the time the paper web has drained to around 10 – 20 % solids on the wire, the web is self-supporting and can be carried away from the wire and on to the subsequent pressing and drying stages. Machine speeds vary considerably, with the web on the fastest machines, typically newsprint, travelling up to 2 000 m/min with a web width of 11 m. Tissue machines, although generally of narrower width, now run at speeds in excess of 2 500 m/min. Some machines operate with several forming sections for making multi-ply papers or boards.
The paper web passes into a press section supported on felts between rollers and through vacuum sections to remove more water (final dryness of about 50 % moisture content) and then through the drying section. The drying is normally done using steam-heated cylinders enclosed in a hood. In the dryer section, the web is dried to the final dry content of 90 – 95 %. Practically all the heat used for drying ends up in the hood as exhaust air. The temperature of the exhaust air is normally 80 – 85 °C and the humidity is 140 – 160 g H2O/kg dry air. A part of the moisture (about 1 – 1.5 m3/t of paper) is driven off to the atmosphere. For economic reasons, all paper mills have installed heat recovery systems. Figure 7.2 shows a schematic picture of an example of the drying and heat recovery section of a paper machine.
In the first heat exchanger of the heat recovery system, heat is recovered in order to heat the incoming supplied air. The next heat exchanger is for the heating of incoming fresh water. In some cases, heat is also recovered to the wire pit water to compensate for the heat losses at the wet end. The last heat exchanger is for circulation water. The circulation water is used to heat the incoming ventilation air. The supply air and shower water are heated to their final temperatures (90 – 95 °C and 45 – 60 °C respectively) using steam.
Figure 2: Paper machine heat recovery system
Table 1 shows an example of heat flows in a typical large, modern paper machine. The production capacity of the machine is 240 120 t/yr (667 t/d). The dry content of the web entering the dryer section is 44.5 % and leaving is 91 % dryness. The temperature of the exhaust air is 82 °C and the humidity is 160 g H2O/kg dry air. The values are for Scandinavian winter conditions. In countries with a warmer climate, there is no need for the heating of circulation water that is used for machine room heating. Heat recovery to the circulation water decreases or disappears and the exhaust to atmosphere increases correspondingly.
Table 1: Example of heat recovery and heat losses of a paper machine with a production of 667 t/d
One alternative to drying the paper for the production of light-weight machine-glazed paper or conventional tissue is the use of a large diameter, heated 'Yankee' cylinder on the machine. The drying of the paper web is carried out during one rotation of the cylinder. In a simple papermaking set-up, the paper may then be reeled and sent for cutting and finishing. In more complex cases, a variety of different stages are incorporated within the machine, e.g. a film press where starch and other chemicals are applied on the surface of the paper by dipping or spraying, with residual water being removed in a short after-drying section.
In most applications, the edges of the web are continually trimmed with cutting water jets, into the couch pit, as it leaves the wire. Whenever the web breaks (it can happen a number of times per day), there is a considerable loss of paper. Similar losses occur on regular start-ups. All of this paper, termed 'broke', is repulped and returned to the stock chests in the stock preparation area. Losses of dry paper may be repulped immediately or stored and reintroduced later to the system. Coloured or coated broke is recycled if possible but sometimes needs to be bleached or chemically treated first.
There is a continuous need to prevent the build-up of solids on the fast-moving wires, felts or rollers as these would quickly lead to web breaks. The showers or sprays for this purpose are the primary consumers of fresh water and/or cleaned water in the system. Vacuum systems can also consume substantial amounts of fresh water. However, water-free vacuum systems are also available.
The retention of solids (fibres, fines and fillers) and solubles (added chemicals, organic material from the pulp, etc.) in the paper web, rather than passing them through the mesh and allowing them to remain in the water circuit, is important. It affects the likely destination of any substance – either to the product or to the effluent. On-line consistency monitoring is often used to stabilise retention. The retention of solids on the wire can be increased by the addition of retention aids (chemicals improving retention), and this is normal practice for most paper grades. However, this is constrained for some grades by product quality.
Energy consumption efficiency of paper machine
In order to reduce the consumption of thermal and electrical energy, BAT is to use a combination of the techniques given below.
Usually sizing means wet-end sizing where starch or synthetic sizing agents are added directly to the furnish to reduce the natural absorption capacity of the paper. In sizing, starch or other sizing agents are applied to the fibre matrix to increase the strength of the base paper web and to modify the surface properties with respect to liquid uptake during writing, printing or coating. Wet-end sizing is applied for instance to fine papers and some special paper grades.
The potential environmental impact of size application is mainly the releases to water. If sizes are added to the paper stock, a significantly higher concentration of COD in the water circuits can be measured. Also, the repulping of sized broke somewhat increases the COD in the water circuits. For instance, the repulping of starch-coated broke is one of the main sources of COD effluents in the writing and fine paper sector.
Sizeing may also be applied to the surface of the paper sheet (surface sizing) to avoid dusting (linting) of the paper in offset printing processes. Surface sizing also increases the surface strength of the paper. In surface sizing, the web is passed through the sizing liquor pond, which is located above a roll nip that presses on the web (size press). As a result, the paper web absorbs the sizing liquor. The amount of sizing agent taken up depends on the dry content of the web which can reach 98 % before the size press. Size press technology has advanced, with the film size press becoming the norm in preference to the older flooded pond two-roll technology. Film size presses involve the application of a controlled amount of water-based size mixture evenly to the paper sheet by first creating a uniform film thickness on an adjacent roll and then transferring the film onto the paper sheet as if printing the size film onto the paper. The water applied in the size press is evaporated in the after-dryer section. Film size presses are mainly used for printing and writing papers and packaging grades made from recycled fibre. If size is applied by a film size press, only relatively small amounts of concentrated size have to be discharged when operational conditions are changed.
Although size press treatment is a form of paper coating to improve its surface properties, the term 'coating' is usually reserved for the application of a pigmented slurry to the surface of the paper in order to improve printability or for other specialist applications.
Dyeing of paper
Coloured papers are obtained by dyeing the paper stock or the paper surface (size press, paper coating). Optically brightened papers can be produced in the same manner. Stock dyeing is the most widely used type of paper dyeing. Dyes, pigments, and optical brighteners are added either batch-wise in the pulper or mixing chest or introduced continuously into the stock flow. Continuous addition has the advantage of a shorter zone in the stock line that must be cleaned when the colour is changed. However, because of the lower contact time compared to batch addition, a lower colour yield is obtained for intensely coloured papers and more complex equipment is required for this dyeing process. When the surface of the paper is coloured in the size press, the dyes are added to the size press liquor. Surface dyeing has gained acceptance only in individual cases because uniform dyeing of the paper is difficult to achieve. However, this process has the advantage of the absence of dyes in the water circuits.
Surfaces of papers can also be coloured by coating. In normal coating the surface of the paper or board is covered with a pigment coating. In the case of coloured coatings, the starting material is the white coating mixture, and the desired shade is attained by adding a dispersion of an organic or inorganic pigment. Depending on the fibrous material to be dyed and the intended purpose of the paper, different types of pigments and dyes are used as basic dyes (cationic dyes), direct dyes, and acid dyes. Additionally, fixing agents and other additives are used to improve dye fixation and to obtain better dyeing results. Inorganic pigments or organic pigments (e.g. azo and phthalocyanine types) and carbon black are pigments used for paper dyeing.
The potential environmental impact of dyeing is mainly the releases to water. Especially in mills with several changes of tints or shades per day, the water circuits have to be cleaned after a certain time. Usually, the paper mills work in campaigns producing first the paler tints, changing step by step to the deeper tints. The colouration of the water then just has to be readjusted. However, when for instance deep green is reached, the water system has to be washed. The coloured waste water is sent to the external recipient via the waste water treatment plant. Several times per month, the piping is subjected to a chemical treatment to remove deposits and colour in the piping. In some mills, elemental chlorine and hypochlorite are used as cleaning agents.
Source: Best Available Techniques (BAT), Reference Document for the Production of Pulp, Paper and Board, 2015
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