Transcript No Slide Title

Pulping
and
Bleaching
Pulping and Bleaching
PSE 476/Chem E 471
Lecture #4
Introduction to Chemical Pulping
PSE 476: Lecture 4
1
Pulping
and
Bleaching
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Introduction to Chemical Pulping
Agenda
Raw material issues
Digesters
Packing the Digester
Presteaming
Heating
Liquor Impregnation
Typical Cooking Schedule
PSE 476: Lecture 4
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Pulping
and
Bleaching
Chemical Pulping Overview
Chip 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
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Pulping
and
Bleaching
Chemical Pulping Overview
Chip 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
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Chemical Pulping Overview
Pulping
and
Bleaching
Chip Quality Issues
• Chip Thickness
Screenings (%)
20
3mm
5mm
7mm
15
» If chips are too thick, liquor
cannot penetrate resulting in
an incomplete cook
- Formation of rejects
10
» If chip is cooked longer to
compensate, outside of
chips will be overcooked
lowering yield and pulp
strength
5
0
33
34
36
40
42
43
46
48
Kappa Number
PSE 476: Lecture 4
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Pulping
and
Bleaching
Chemical Pulping Overview
Chip 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
PSE 476: Lecture 4
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Pulping
and
Bleaching
Chemical Pulping Overview
Chip Quality : Juvenile versus Mature Wood
• Juvenile wood (5-20 years) characteristics
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»
»
»
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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
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Pulping
and
Bleaching
Chemical Pulping Overview
Chip 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
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Pulping
and
Bleaching
Chemical Pulping Overview
Chip 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
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Pulping
and
Bleaching
Chemical Pulping Overview
Chip Quality: Reaction Wood
• Compression Wood (conifers)
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»
»
»
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)
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»
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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
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Pulping
and
Bleaching
Chemical Pulping Overview
Chip 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
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Pulping
and
Bleaching
Chemical Pulping Overview
Equipment: 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
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Chemical Pulping Overview
Pulping
and
Bleaching
Equipment: Continuous Digester
W L and
C hips In
Impr e gnation Zone
C ounte r - c ur r e nt
he at ing/w ashing
( st ar t of bulk
de lignific at ion)
Extr ac t ion
Liquor O ut
W hite
Liquor
B ulk
D e lignif ic ation
W ash
W at e r
C ounte r - c ur r e nt
he at ing/w ashing
( r e sidual
de lignific at ion)
W hite
Liquor
• Tube shaped reactor in
which the chips move
through continuously
• Various zones in the
reactor: charging,
impregnation, heating ,
cooking, etc.
• Typical reactor: Kamyr
digester
W ash W ate r
P ulp O ut
PSE 476: Lecture 4
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Pulping
and
Bleaching
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
14
Pulping
and
Bleaching
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
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Pulping
and
Bleaching
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.
* The titles appear to be switched in this figure
» Flat bed of chips formed instead of
conical profile.
» Uniform packing eliminates
“shortcuts” for liquor through chips.
PSE 476: Lecture 4
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Pulping
and
Bleaching
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
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Improvement in pulp uniformity
from pre-steaming
30
No Pre-steaming
Pre-steaming
25
% of Fibers
Pulping
and
Bleaching
20
15
10
5
0
0
20
40
Kappa
PSE 476: Lecture 4
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80
100
18
Pulping
and
Bleaching
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
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Pulping
and
Bleaching
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
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Pulping
and
Bleaching
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 nonuniform cooking which reduces quality.
Steam
PSE 476: Lecture 4
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Pulping
and
Bleaching
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
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Pulping
and
Bleaching
Chip Impregnation:
Penetration
• Liquor penetrates the chip through pores
» Hardwoods: through vessels
» Softwoods: through lumina via pits.
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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
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Pulping
and
Bleaching
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
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Pulping
and
Bleaching
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
25
Pulping Mechanics:
Pulping
and
Bleaching
Typical Pulping Schedule: Batch
170
cooking
heating
150
Pressure
relief
130
110
gas relief
90
blowing
0
22
0
20
0
18
0
16
0
14
0
12
0
10
80
60
40
20
70
0
Temperature (°C)
190
Time (minutes)
PSE 476: Lecture 4
26
Pulping
and
Bleaching
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
27
Pulping
and
Bleaching
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
Pulping Mechanics:
Pulping
and
Bleaching
Typical Pulping Schedule: Batch
170
cooking
heating
150
Pressure
relief
130
110
blowing
gas relief
90
0
22
0
20
0
18
0
16
0
14
0
12
0
10
80
60
40
20
70
0
Temperature (°C)
190
Time (minutes)
PSE 476: Lecture 4
29
Pulping
and
Bleaching
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
30