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Pulping and Bleaching PSE 476/Chem E 471

Pulping and Bleaching PSE 476/Chem E 471. Lecture #4 Introduction to Chemical Pulping. Introduction to Chemical Pulping Agenda. Raw material issues Digesters Packing the Digester Presteaming Heating Liquor Impregnation Typical Cooking Schedule.

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Pulping and Bleaching PSE 476/Chem E 471

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  1. Pulping and BleachingPSE 476/Chem E 471 Lecture #4 Introduction to Chemical Pulping PSE 476: Lecture 4

  2. Introduction to Chemical PulpingAgenda • Raw material issues • Digesters • Packing the Digester • Presteaming • Heating • Liquor Impregnation • Typical Cooking Schedule PSE 476: Lecture 4

  3. Chemical Pulping OverviewChip Quality Issues: Wood Species • Softwoods (under kraft conditions) • generally produces stronger pulp than hardwoods because of longer fibers • Lower yield than hardwoods because of higher lignin content and more soluble hemicelluloses • Hardwoods: • Forms paper with good printing quality • Smaller size produces smooth surface PSE 476: Lecture 4

  4. Chemical Pulping OverviewChip Quality: Chip Moisture • Very low moisture content makes liquor impregnation difficult • It is very important to cooking parameters to know the exact amount of wood mass in the digester as well as the amount of water to be able to add the correct amount of chemicals • Chip moisture can affect pulp yield, kappa number, and pulp quality PSE 476: Lecture 4

  5. Chemical Pulping OverviewChip Quality Issues • Chip Thickness • If chips are too thick, liquor cannot penetrate resulting in an incomplete cook • Formation of rejects • If chip is cooked longer to compensate, outside of chips will be overcooked lowering yield and pulp strength PSE 476: Lecture 4

  6. Chemical Pulping OverviewChip Quality Issues: Wood Density • Wood density (specific gravity) is important economically • More dense wood can be packed into a digester thus improving production • Large variations inspecific gravity between tree species and within same tree • Hardwood tend to have higher specific gravities than softwoods • Thicker cell walls responsible for higher densities • Pulp quality and washing efficiency affected by density • High density (thick walled fibers) wood have stiff fibers with high water drainage • Low density (thin walled fibers) wood produce flexible fibers which give high strength and high paper density PSE 476: Lecture 4

  7. Chemical Pulping OverviewChip Quality : Juvenile versus Mature Wood • Juvenile wood (5-20 years) characteristics • Shorter and narrower fibers • Thinner cell walls • High earlywood/latewod ratio • Lower specific gravity • Lower cellulose content (Reduced yield, high alkali consumption) • Higher hemicellulose content • Higher lignin content • Higher fraction of reaction wood PSE 476: Lecture 4

  8. Chemical Pulping OverviewChip Quality:Earlywood versus Latewood • Earlywood: Thin walled, large diameter fibers • Lower specific gravity • Fibers collapse easily and thus bond easier: stronger paper • Latewood: Thick walled, narrow diameter fibers • Higher specific gravity, less strength • Woods from the North contain higher percentage of earlywood than southern trees: • Higher tensile and burst strength, lower bulk, porosity, and tearing resistance PSE 476: Lecture 4

  9. Chemical Pulping OverviewChip Quality: Heartwood versus Sapwood • Heartwood Problems: • Higher level of extractives/lower moisture: • More difficult to chip • Extractives in voids slow liquor penetration • More rejects • Darker color a problem PSE 476: Lecture 4

  10. Chemical Pulping OverviewChip Quality: Reaction Wood • Compression Wood (conifers) • Darker color than normal wood • Shorter and thicker walled fibers • Higher specific gravity, higher lignin, lower cellulose • Lower pulp yields, weaker pulp, darker pulp • Tension Wood (hardwoods) • Lighter color than normal wood • Same length fibers but thicker walls • Higher specific gravity, higher cellulose , lower lignin • Lower strength, higher yield and brightness PSE 476: Lecture 4

  11. Chemical Pulping OverviewChip Quality: Decayed Wood • More fines are produced in chipping • Lower density material: lower production • Typically lower cellulose contents: lower yields • Higher percentage lignin and hemicellulose consume more cooking chemicals • Degradation of cellulose reduces pulp strength PSE 476: Lecture 4

  12. Chemical Pulping OverviewEquipment: Batch Digester • Large pressure reactors • 2500 to 12,500 ft3 • Typical mill will have banks of 6-8 digesters • Heating of system is either through direct heat (steam) or though indirect heating of the cooking liquor with forced circulation PSE 476: Lecture 4

  13. Chemical Pulping OverviewEquipment: Continuous Digester • Tube shaped reactor in which the chips move through continuously • Various zones in the reactor: charging, impregnation, heating , cooking, etc. • Typical reactor: Kamyr digester PSE 476: Lecture 4

  14. Basic Process Steps:Batch Digester (Kraft) 1.Digester filled with chips and cooking liquor 2. Digester sealed, heating begins (cooking). a. Air and non condensable gasses vented as pressure builds. b. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank PSE 476: Lecture 4

  15. Pulping Mechanics: Loading the Digester • What do breakfast cereal boxes and digesters have in common? • Answer: How do you get the most Cap’n Crunch in the box and how do you get the most chips in a digester. • Chip Packing Methods • Loading with Liquor • Most common method of loading chips is simultaneously with liquor. • Liquor lubricates the chips and improves packing. • Reduces time to load digester • Mechanical Packing/Steam Packing PSE 476: Lecture 4

  16. Pulping Mechanics: Steam Packing the Digester • Steam packing results in a higher packing density. • Increased production. • Digester charge can be increased by up to 40% though packing. • Chips enter in tangential motion and drop in a spiral motion. • Flat bed of chips formed instead of conical profile. • Uniform packing eliminates “shortcuts” for liquor through chips. * The titles appear to be switched in this figure PSE 476: Lecture 4

  17. Pulping Mechanics: Presteaming • Chips will often contain a significant amount of air (when dry) which slows liquor penetration. • Application of steam to the chips forces air out which is replaced with water. • This results in: • Improved impregnation. • More even liquor distribution. • This process cannot be used when chips and liquor are charged together. PSE 476: Lecture 4

  18. Improvement in pulp uniformity from pre-steaming PSE 476: Lecture 4

  19. Basic Process Steps:Batch Digester (Kraft) 1. Digester filled with chips and cooking liquor. 2. Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank PSE 476: Lecture 4

  20. Pulping Mechanics: External Heating • External Heating: circulation of liquor through an external heat exchanger. • Liquor leaves digester though screens. • Pumped through heat exchanger and then back into the top of the digester. • Liquor turned over every 10 minutes. • Uniform heating and liquor distribution. PSE 476: Lecture 4

  21. Steam Pulping Mechanics: InternalHeating • Steam is applied through the bottom of the digester. • Heat transferred through convection: caused by the temperature difference between the top and bottom. • Very simple system. • Problems • Liquor diluted with steam. • Non-uniform temperature causes non-uniform cooking which reduces quality. PSE 476: Lecture 4

  22. Basic Process Steps:Batch Digester (Kraft) 1. Digester filled with chips and cooking liquor. 2.Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank PSE 476: Lecture 4

  23. Chip Impregnation: Penetration • Liquor penetrates the chip through pores • Hardwoods: through vessels • Softwoods: through lumina via pits. • Penetration is effective over long distances in grain direction. • Penetration is not effective across the grain. • Composition of liquor is relatively unimportant. • There are extreme differences in penetration rate between heartwood/sapwood, softwoods/hardwoods, and different hardwood species. PSE 476: Lecture 4

  24. Chip Impregnation: Diffusion • This process involves the diffusion of ions through liquid present in the chip internals (free water, cell walls). • Occurs wherever water is present. • Diffusion is optimum in water saturated wood. • Diffusion is effective only over short distances. • Diffusion is effective across the grain. • There are only small differences between different woods. • Diffusion is affected by wood density. • Diffusion is strongly affected by liquor composition. PSE 476: Lecture 4

  25. 1. Digester filled with chips and cooking liquor. 2.Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank Basic Process Steps:Batch Digester (Kraft) Text PSE 476: Lecture 4

  26. Pulping Mechanics: Typical Pulping Schedule: Batch cooking heating Pressure relief blowing gas relief PSE 476: Lecture 4

  27. Basic Process Steps:Batch Digester (Kraft) 1. Digester filled with chips and cooking liquor. 2.Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank Text PSE 476: Lecture 4

  28. Basic Process Steps:Batch Digester (Kraft) 1. Digester filled with chips and cooking liquor. 2.Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank Text PSE 476: Lecture 4

  29. Pulping Mechanics: Typical Pulping Schedule: Batch cooking heating Pressure relief blowing gas relief PSE 476: Lecture 4

  30. Basic Process Steps:Batch Digester (Kraft) 1. Digester filled with chips and cooking liquor. 2.Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank Text PSE 476: Lecture 4

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