Pulping and Bleaching PSE 476

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Pulping and BleachingPSE 476

• Lecture 5
• Continuous Reactors

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Continuous ReactorsAgenda

• Last lecture
• Kamyr Digester
• Impregnation and Heating
• Cooking
• Washing
• Batch versus Continuous

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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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Diffusion
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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
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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
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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
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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
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Continuous ReactorsChip and Liquor Charging

• It is very important that the chips are added to
  continuous digesters in a consistent and accurate
  fashion. A chip meter is used to accomplish
  this.
• This is a rotating wheel which meters chips.
• A presteaming vessel is used to heat the chips
  and remove air and volatiles.
• The chips are steamed for 2-3 minutes at 103-124
  kPa.

Chip Feeding System (blow the slide up to see
better)
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Continuous Reactors
Notes
Kamyr Digester an example of a continuous
digester
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Kamyr Digester
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Continuous Reactors
Chips
Chips and
Steam
Liquor
Impregnation
Chips
Zone
Silo
Steam
Liquor
s
n
Cooking
e
e
Zone
r
c
S
White Liquor
High
Liquor
Pressure
Washing
Feeder
Zone
Blow Line
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High Pressure Feeder

• Because the digester is under pressure, a high
  pressure feeder is used to get the chips into the
  digester.
• Chips are mixed with white liquor and fed through
  the top of the digester.
• In kraft cooking, white liquor is the cooking
  liquor. It is mostly a combination of NaOH and
  NaHS.
• Impregnation of the chips begins at this point.
• Excess liquor must be used to convey the chips.

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Chip and Liquor Charging

• At the top of the digester, the chips and the
  liquor are added via a screw surrounded by
  strainer.
• The excess liquor needed to convey the chips is
  removed at this point.
• Chips form a plug which moves through down the
  digester.
• Additional new white liquor is added to the chips
  at the top of the plug.

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Impregnation Zone

• The top zone in the digester is the impregnation
  zone.
• It takes approximately 45 minutes to pass through
  this zone.
• The temperature of this section is between 105C
  to 130C. Like in batch cooking, it is important
  that good impregnation is obtained before rapid
  delignification begins.
• Most modern digesters have separate vessels for
  impregnation,

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Heating Zone

• Heating Zone
• After liquor impregnation, the chips are rapidly
  heated to cooking temperature (160C-170 C)
• This is accomplished by removing the liquor in
  this section through screens, pumping the liquor
  through a heat exchanger, and returning the
  liquor to the digester. This is done twice in
  this section.
• Heating is accomplished in a small section of the
  digester.

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Cooking Zone

• The chips are cooked for 1-2 hours _at_ 160C -
  170C.
• Unlike a batch digester, the liquor composition
  in a continuous digester is controllable. Liquor
  is removed through screens and new liquor pumped
  back into the system.

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Washing Zone

• The chips are cooled rapidly in the washing zone
  with fresh washing liquor to (130C) using a
  countercurrent process to quench degrading
  reactions.
• The liquor use to wash is the black liquor from
  cooking.
• Chemicals are removed through a diffusion
  controlled process.
• Washing takes about 1.5 hours.
• Pulp is washed at bottom with fresh water prior
  to removal.

Notes
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Process simulationhttp//www.psl.bc.ca/equipment/
digester/
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Batch versus Continuous

• Batch
• More Flexible
• Grade Changes
• Fiber Source
• Maintenance Issues
• Less down time, production loss
• Production Flexibility
• Add another digester

• Continuous
• Lower Energy Costs
• Easier to control non-condensable gases odor
• Lower environmental impact
• Compact (space efficient)
• Steady-state flow rate