Pulping and Bleaching PSE 476/Chem E 471

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Pulping and BleachingPSE 476/Chem E 471

• Lecture 4
• Introduction to Chemical Pulping

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Introduction to Chemical PulpingAgenda

• Raw material issues
• Digesters
• Packing the Digester
• Presteaming
• Heating
• Liquor Impregnation
• Typical Cooking Schedule

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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

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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

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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

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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 in specific 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

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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

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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

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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

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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

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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

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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

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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

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Basic Process StepsBatch 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
  (175C)
• 4. Cooking temperature maintained for 20 - 45
  min
• a. Cooking proceeds till desired endpoint
  (kappa)
• 5. Contents of digester are discharged to blow
  tank

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Pulping Mechanics Loading the Digester

• What do breakfast cereal boxes and digesters have
  in common?
• Answer How do you get the most Capn 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

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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
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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.

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Improvement in pulp uniformity from pre-steaming
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Basic Process StepsBatch 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
  (175C)
• 4. Cooking temperature maintained for 20 - 45
  min
• a. Cooking proceeds till desired endpoint
  (kappa)
• 5. Contents of digester are discharged to blow
  tank

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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.

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Pulping Mechanics Internal Heating

• 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.

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Basic Process StepsBatch 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
  (175C)
• 4. Cooking temperature maintained for 20 - 45
  min
• a. Cooking proceeds till desired endpoint
  (kappa)
• 5. Contents of digester are discharged to blow
  tank

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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.

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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.

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Basic Process StepsBatch 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
  (175C)
• 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
26
Pulping Mechanics Typical Pulping Schedule
Batch
cooking
heating
Pressure relief
blowing
gas relief
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Basic Process StepsBatch 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
  (175C)
• 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
28
Basic Process StepsBatch 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
  (175C)
• 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
29
Pulping Mechanics Typical Pulping Schedule
Batch
cooking
heating
Pressure relief
blowing
gas relief
30
Basic Process StepsBatch 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
  (175C)
• 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