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Mechanical pulping

  1. Lecture 4: Mechanical Pulping, Chemi- mechanical Pulping, Paper Making Staff Scientist, Dr. Sc. (Tech.) Timo Laukkanen
  2. Benefits and drawbacks of mechanical pulping • + The material yield (pulp/wood) of chemical pulping is 40-50% when in mechanical pulping it can be 85-95%. • + Low cost • + High ink absorbency, compressibility, opacity, and bulk. • + Heat recovered can be used in paper machines • - Low strength, low permanence, tendency to yellow with time (primarily caused by high levels of lignin). • - Paper made with mechanical pulps also contain shives, or incompletely ground fiber bundles. • - Needs a lot of electricity 21.1.2020 2
  3. Mechanical pulping methods • Mechanical pulp is the oldest pulping process. New mechanical pulping methods were developed to reduce disadvantages without compromising the advantages of mechanical pulping. • Thermomechanical pulping (TMP) operates under higher temperature and pressure that further soften the lignin to make fiber separation easier. Thermomechanical pulp is stronger than mechanical pulp, and still retains the high-yield of mechanical pulps. • Chemi-thermomechanical pulps use mild chemicals to increase pulp brightness and reduce shive content. • Nowadays pulps are custom made for specific end uses by affecting the degree of refining, chemical treatments of chips, and peroxide bleaching. 21.1.2020 3
  4. Mechanical pulping methods • Some terminology • TMP Thermomechanical pulp • CTMP Chemithermomechanical pulp • GW Groundwood pulp • Stone groundwood (SGW) pulp: small logs are ground against artificial bonded stones, high yield, short fibers-> weak • Refiner mechanical pulping (RMP): similar to SGW, but can use chips longer fibers and stronger than SGW • PGW Pressurized groundwood pulp • NSSC Neutral sulfite semichemical pulp • CMP Chemimechanical pulp 21.1.2020 4
  5. TMP 21.1.2020 5
  6. CTMP 21.1.2020 6
  7. Chipping and chip storage • After barking chips are made • Important for pulp quality • Chips should be homogenous • Screened chips are then wetted and heated rapidly in the preheating stage • Short storing time • Chips dry fast (high temperature in storage) • accelerates discoloration and decay of the wood 21.1.2020 7
  8. Chip washing • Sand, pieces of metal, etc. carried along with the wood can accelerate the wear on the refiner discs. • The chips are washed with hot water. 21.1.2020 8
  9. Steam treatment of chips • Chips are moistened and heated (+ chemical treated in CTMP) in preheater, which causes the lignin bond between the fibers to soften. • In CTMP chips are given a 20-30 minute steaming, followed by impregnation with sodium sulfite (Na2SO3). • The quantity of sodium sulfite, reaction time and temperature can be used to adjust pulp properties. A typical sulfite dosage is 2-3% of the wood. 21.1.2020 9
  10. Refining • The chips are defiberized and refined to finished pulp in a refiner. This takes usually place in two stages. • The wood chips are fed between the refiner discs of refiner. At least one of the discs rotates at a speed of 1500 to 2300 rpm. The wood fibers are separated as a result of the action of mechanical forces. • A majority of the electricity used by the refiners is converted to steam by the shear forces the pulp is exposed to. • Steam is sent to heat recovery to generate fresh steam and used. e.g. in pulp drying or in a paper machine. 21.1.2020 10
  11. Refining • Todays refiners are almost always pressurized. • Advantages include reduced volumetric steam flow and improved stability of the refiner load  easier production of uniform pulp. • Largest single refiners operate at a rating of about 40 MW. • Refining to finished pulp usually takes two stages arranged in series. A reject refiner is used for refine fiber bundles that pass main line refining. 21.1.2020 11
  12. Refining • Lot of steam is produced in the making of mechanical pulp. • Recovering the heat formed in the process is essential for plant economy. • The most of the power used is spent on vaporizing of water in the refiner and about ton of steam is generated per MWh of the energy used. • The steam is first separated from the fibers and then condensed in the reboiler to raise clean steam. 21.1.2020 12
  13. Refining 21.1.2020 13
  14. Refining 21.1.2020 14
  15. Latency removal • Removal of fiber curling, which occurs at high-consistency refining, by means of mixing the pulp a certain period of time at a lower consistency (2 to 4 %) and a temperature of 70 to 80 0C. 21.1.2020 15
  16. Grinding For groundwood pulp, chipping and refining is replaced by grinding  OK for newsprint paper 21.1.2020 16
  17. Screening and rejects treatment • After refining, the pulp always contains shives and coarse fibers, which must be treated according to the requirements of each paper grade. 21.1.2020 17
  18. Bleaching • Brightness (57 to 64 %) of mechanical pulp is very near the brightness of the wood material used. OK for newsprint • Bleaching needed if higher demands on brightness is required. • Peroxide is the most common chemical used for bleaching. • Pulp is bleached usually in two stages, first in medium consistency and latter in high consistency. • Depending on its intended use, CTMP can be bleached to a brightness of as much as 80%. The maximum brightness for aspen CTMP is 85%. • As most of the lignin still stays in the pulp (to maintain high yield), the whiteness achieved in bleaching is temporary as exposure to air and light can turn paper to yellow. 21.1.2020 18
  19. MATERIALS 21.1.2020 19
  20. PRODUCTS 21.1.2020 20
  21. ENERGY CONSUMPTION 21.1.2020 21
  22. CASE TMP: EOK (specific energy consumption) vs production Why large differences? 0 0.5 1 1.5 2 2.5 3 0 2 4 6 8 10 12 EOK, MWh/t Production t/h
  23. 0 1 2 3 4 5 6 7 8 9 10 0 2 4 6 8 10 12 TMP-LTO:n kokonaisteho, MW Tuotanto, t/h CASE TMP: LTO (heat recovery) vs production Why huge deviations?
  24. CASE TMP: Rejects vs production Why deviations? 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 2 4 6 8 10 12 Reject flow, % Tuotanto, t/h
  25. Case TMP energy efficiency analysis Data ja efficiency metric • TMP-data from mill: • Year 2017, 8760 h • ~ 200 parameters • 15 parameters were chosen for more in detail analysis • The energy efficiency metric was theoretic heat recovery potential • The temperature of the incoming district heating flow was not studied (untypically in this mill the steam from refiners didn’t go to the paper mill, but into heating of district heat)
  26. Case TMP: Initial results • From the data a clear reason for the variation of energy efficiency was found: • One of the refiners produced substantially less steam than the other refiners  energy efficiency decreased • Following studies: • Why did the temperature of district heating flow coming into the TMP process varied so much • Why was the steam production of one of the refiners so much less than the others? • What improvements and/or investments are needed in order for the faulty refiner to achieve the same steam production as the other refiners? • If there is no chance to affect the district heating temperature, where else could the steam from the refiners be used?
  27. PAPER MAKING • In cases where pulp mill is integrated with a paper mill, pulp is pumped through a pipe from the pulp mill • For a non-integrated mill, pulp is delivered in the form of pulp bales, which must be disintegrated (slushed) in a bale pulper. • Fibers are the most important component of paper and board. • Properties of wood fibers vary to great extent in different pulp types, and the use of mechanical, chemi-mechanical, chemical and recycled pulps is very much subject to paper and paperboard grades. • Other raw materials are fillers, adhesives, chemical additives and coating agents. 21.1.2020 27
  28. Energy use of wood-free coated paper making 21.1.2020 28
  29. Water removal of a paper machine 1 ADt of paper 21.1.2020 29
  30. PRODUCTS • The main grades of magazine paper are wood-containing, light-weight coated paper (LWC) and supercalendered or calendered, but uncoated paper (SC). • Special papers include papers processed as household and sanitary paper, wallpaper base paper, label paper, sack paper, wrapping paper, packaging paper, envelope paper, flexible packaging paper or industrial papers. Insulation paper, such as cable paper, is used to insulate electric cables. • Fine paper is made of pulp which generally contains no more than one tenth of mechanical pulp. Good-quality printing and writing papers and copier papers are examples of fine papers. • Hygiene papers are soft grades called tissues – such as toilet and kitchen paper and hand or facial towels – which have a good absorption ability, disintegrate rapidly in water and are recyclable as biomass. • Wood-free pulp or paper is a product whose raw material does not include mechanical pulp. 21.1.2020 30
  31. PRODUCTS: Printing and fine papers • Printing paper grades are manufactured in the largest possible production units using the most economic possible raw materials. • The main raw material is mechanical pulp; for newsprint also recycled fibre pulp is used. • A typical basis weight range of paper is from 35 to 55 g/m2, the production speed from 1200 to 1600 m/min, and the web width exceeds eight meters. • Fine paper machines also belong to the same size class as the machines for wood containing grades. • A typical production speed range is from 1000 to 1300 m/min and the basis weight range from 50 to 90 g/m2 . 21.1.2020 31
  32. PRODUCTS: Special papers • In the production of fine and special paper grades chemical pulps are mainly used, which are more expensive than mechanical pulps. • Production output of special papers are normally small; a lot of flexibility is required from the production line and the machines are generally slower and narrower. • The basis weight range of special papers is extremely wide varying between 40 and 250 g/m2 . • The running speeds range from 400 to 1100 m/min and the web width from three to eight meters. 21.1.2020 32
  33. PRODUCTS: Boards • Packaging boards include: folding boxboard, white lined chipboard, liquid packaging board, solid bleached sulfate board, carrier board; and container boards: kraftliner, testliner, fluting • The number of special board grades is very high, but production volumes (compared to the previously mentioned grades) rather small. • Production of board machines are usually very high because of the high basis weight. For packaging box board the basis weight range is between 170 and 450 g/m2, and for liner board between 100 to 330 g/m2 • Packaging boards are normally formed in three or four plies. • Due to high surface quality requirements the surface ply is produced from chemical pulps and in most cases is double coated. • For the centre ply, less expensive mechanical pulp is used 21.1.2020 33
  34. PROCESSES: Stock preparation Stock preparation is located between the pulp mill & the paper mill. • Integrated pulp mills do not require treatment & pulping system for bales, but pipe stock is refined in accordance with the paper grade. • For a non-integrated mill, bales are disintegrated in a bale pulper. 21.1.2020 34
  35. PROCESSES: Web forming • The web forming section on a paper machine comprises • the headbox approach piping • the headbox and • the forming section. • The tasks of the headbox and the approach piping are: • Uniform distribution of stock suspension • Stabilize pressure variations and pulsation in the infeed flow • Produce suitable turbulence level in the stock suspension for fibre floc dispersal • Produce a stock suspension jet in the forming section with a desired consistency (typically from about 0.5 to 1.0 %), speed and direction 21.1.2020 35
  36. PROCESSES: Press section • The function of the press section is to • remove maximum amount of water from the web and to compress it, and • to achieve sufficiently high wet strength to ensure that the web is transferred to the dryer section without any breaks. • The dry content of the web is 17 - 20% in the beginning, and 35 - 50 % after the press, depending on the paper grade and press design. • 1 % increase in dry content at the press will diminish the dryer-section steam consumption by 3-4 %. 21.1.2020 36
  37. PROCESSES: Drying • Raising the web dry content to 50 % is economically and technically possible by using mechanical dewatering methods (centrifugal force & under-pressure at the wire section, pressing at the press section). • Water is then removed by evaporation, in order to reach the desired final dry content. 21.1.2020 37 Factors contributing to drying section energy economy
  38. PROCESSES: Drying EVAPORATION • There are three different methods normally applied to paper and board drying: • contact or cylinder-drying • air-drying • radiation drying • Common to all these methods is that the web is provided with external energy that evaporates water away from the web. • Then evaporated water exits the dryer with air. • Main difference in drying methods is the way energy is supplied, which leads to different type of equipment being used. 21.1.2020 38
  39. PROCESSES: Drying Cylinder drying 21.1.2020 39 Condebelt drying
  40. PROCESSES: Drying AIR DRYING • The heat transfer of air dryers is performed by blowing air through nozzles against the web. • Drying is performed by means of convection in the air dryer, allowing hot air jets to be discharged against the paper web surface . • The web is supported and stabilized by blowing air so as not to let the dryer parts touch the web. The air dryer's air circulation system circulates the impingement air. • The exhaust air system ensures low moisture & heat loads for the machine hall. 21.1.2020 40
  41. PROCESSES: Drying INFRARED DRYING • Thermal radiation serves as the heat source. • When radiation encounters another material layer, part of it penetrates it, part is reflected back and part is absorbed into the layer itself. • This absorbed part of radiation is utilized for infrared drying. The radiating surface can be heated either electrically or with the combustion flame of gas. 21.1.2020 41
  42. PROCESSES: Finishing • Finishing after paper or board machine depends on grade requirements. For example, the product can be: • Surface sized. Surface sizing is common with fine papers, coated raw papers, boards. • Coated. For example, printing papers (LWC, MWC) and some boards. • Calendered. Uncoated SC grades are finished by calendering. • Coated with plastic and aluminum extrusion. For example some liquid packaging boards. 21.1.2020 42
  43. ENERGY CONSUMPTION • The paper machine needs around 2500 – 5500 MJ thermal energy and around 1800 – 2200 MJ electrical energy for producing one ton of paper, depending on the grade, machine type and process energy efficiency. • Nearly all the energy removed from the dryer section is transferred along with the exhaust air  heat recovery 21.1.2020 43
  44. HEAT RECOVERY • Main purpose of heat recovery system is to replace primary energy with recovered heat from the process in an economically profitable way. • Total heat recovery in a modern paper or board machine can be over 50 MW. • This is equivalent to 86 t/h of steam, therefore the energy flows are significant. • It is important to utilize this energy efficiently. Heat recovery solutions provide excellent possibilities for optimizing the energy and air flows in paper and board making. 21.1.2020 44
  45. ENERGY CONSUMPTION 21.1.2020 45
  46. ENERGY CONSUMPTION 21.1.2020 46