Transcript No Slide Title

Pulping and Bleaching
PSE 476
Lecture #1
Introduction
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Introduction to Pulping:
History of Papermaking
• 3000 BC: Papyrus
» Exterior of reed is laminated and pressed to form
sheet
» Developed by Egyptians
» Word paper derived from papyrus
• Making of papyrus
http://www.lib.umich.edu/pap/exhibits/papyrus_making/slides.html
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Papyrus Making-Harvesting
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Papyrus Making-Peeling
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Papyrus Making-Cutting
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Papyrus Making-Soaking
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Papyrus Making-Rolling
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Papyrus Making-Pressing, Drying
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Papyrus Making-Final Product
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Introduction to Pulping:
History of Papermaking
• 200 BC: Parchment
» Tanned skin of animals
» Today parchment refers to high quality paper from
vegetable fibers
• 105 AD: Paper from plant (mulberry) fibers
» Developed by Chinese Emperor by Ts'ai Lun
» National secret stolen by Arabs in ~700 AD
• 1796: First continuous paper machine
developed near Paris. Fiber source - rags
(cotton)
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Introduction to Pulping:
History of Papermaking
• 1854: Soda pulping process developed in
England
• 1840: Groundwood pulping developed in
Germany
• 1867: Sulfite (acid) pulping process developed
in US
• 1884: Kraft pulping process developed in
Germany
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Introduction to Pulping:
Two Ways to Generate Fibers
• Mechanically: “Grind up” raw material (most of the
lignin retained).
» Newsprint
• Chemically: Dissolve away the lignin
» Kraft pulping: NaOH/NaSH (dominant process)
» Sulfite: (Sulfur Dioxide/bisulfite/sulfite) (limited number of
mills)
» Other: Organosolv, steam explosion, etc (very minor)
• In this class, we are going to discuss the
methods and chemistry used in the processes
which convert fibrous materials to bleached
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fibers.
Raw Material
• Fibrous material can come in the form of:
» Wood (main form in the United States)
» Other plant material (straw, reeds, etc)
• Wood
» Softwoods & hardwoods
» Logs (chipped directly from logs for pulp
production)
» Chips (residuals from saw mill operations)
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Regional Affects on Raw
Material
For this lecture,
we will be looking
at raw material use
by these regions.
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Raw Material
Log/Chip Makeup
Roundwood
(%)
38
Residual
(%)
62
Softwood
(%)
92.5
Hardwood
(%)
7.5
Total
cords*103
10,420
N.C.
86.5
13.5
28
72
6,715
S. C.
71
29
63
37
39,137
N. E.
78
22
40
60
6,902
S. E.
74
26
73
27
29,387
Region
W.
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Raw Material
Log/Chip Makeup
Roundwood
(%)
materials * 103M3
Residual
(%)
materials * 103M3
1953
60,661
92
4,937
8
1996
145,170
71
60,341
29
(%)
Hardwood
Softwood
3
materials * 10 M
3
3
materials * 10 M
(%)
3
1953
54,006
82
11,592
18
1996
136,072
65
72,439
35
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Raw Material Handling
Physical Measurements
• Physical measurements important for:
» Determining how much wood is coming into the
mill.
» How much wood is being charged into the digesters.
» How much actual wood mass (dry) is being charged
so to get the correct liquor to wood ratio.
• Mills use scales and other devices
» Moisture content meters
» Laboratory chip screening
» Wood species determination
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Raw Material Handling
Physical Measurements
•
Moisture Content - Green (paper industry)
»
»
Wet basis, amount of water in wood as a fraction of wet weight
of wood
Typical MCgr of freshly cut wood = 50% (30-60%)
MC gr = Mass of water in wood x 100%
Wet wood mass
•
Moisture Content - Oven Dry (wood
scientists/foresters)
»
»
Oven dry basis
Typical MCOD of freshly cut wood = 100% (45-150%)
MC OD =
Mass of water in wood
Oven dry wood mass x 100%
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Raw Material Handling
Physical Measurements
•
Solid Wood Density
Dry weight of wood
Density =
Unit volume of green wood
»
»
Wood contracts 8-15% on volume basis when it dries
below 30% moisture. This needs to be taken into
account when determining density.
Typical units lb/ft3 or kg/m3
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Raw Material Handling
Debarking and Chipping
• There are many different mechanical systems
used to debark and chip wood. We will not
cover these in this class. There are plenty of
references available for you to read.
Important points:
» Get all of the bark of the log.
» Bark typically used as fuel source.
» Chip to a very consistent size.
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Raw Material Handling
Chip Dimensions
• Uniform Chip Size is very
important!
»
»
»
Large chips undercook
leaving shives (rejects).
Small chips clog liquor
circulation, use large
amount of chemicals, and
give a low yield of weak
pulp.
Chip thickness the primary
concern.
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1/2 to 1” long
TOP
Side
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Raw Material Handling
Wood Deterioration
• Wood decay requires moisture and oxygen.
»
»
»
»
Moisture content > 20% MCOD.
Remove either and slow/stop degradation.
Sprinkle solid wood (logs) with water to keep the
wood saturated and therefore limit oxygen
content.
Drying wood to <20% MCOD would slow
degradation.
-
Economically unsound.
Would slow liquor penetration and therefore
pulping.
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Raw Material Handling
Chip Pile Degradation
• Conditions that accelerate degradation.
»
»
»
»
Tall chip piles
Chip pile compaction
Whole tree chips
Storage of hardwoods (high starch contents).
• Method to reduce degradation.
» FIFO (first in first out)
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Raw Material Handling
Deterioration in Wood Chip Piles
•
General rule of thumb:
»
•
•
•
•
1% loss to decay/month.
Respiration of parechyma cells
responsible for heat generation.
Above 45-55°C, fungal and
bacterial degradation stop.
Chemical autoxidation takes
over above 55°C. This results in
severe losses during pulping.
Loss of extractives high during
storage.
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Raw Material Handling
Debris
• Debris is not a good thing!
• Sources
»
Bark, foliage, plastic, metals, dirt, decayed
wood
• Problems
»
Dirt specks, loss of strength, structural
imperfections, wear on equipment.
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Raw Material Handling
Screening
• Most mills use a screening system to
achieve relative consistency in chip
size.
» Overs: Chips which are oversized or
over thick.
» Accepts: Chips that are in the correct
size distribution.
» Fines: Chips that are too small (includes
sawdust).
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