Topic 3 Mechanical Pulping.ppt - UBC Fibre Lab1 Mech 450 – Pulping and Papermaking Topic 3 –...
Transcript of Topic 3 Mechanical Pulping.ppt - UBC Fibre Lab1 Mech 450 – Pulping and Papermaking Topic 3 –...
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Mech 450 – Pulping and Papermaking Topic 3 – Mechanical Pulping
James A. Olson
Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia
Mechanical Pulping
Comparison of Mechanical and Chemical Pulps
Debarking
Stone Groundwood
Refiner Mechanical Pulp
Thermo mechanical pulping (TMP)
Chemi thermo mechanical pulping (CTMP)
Brightening
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Mechanical Pulping
Fibres mechanically removed from wood matrix
Chemical Pulping
Lignin holding fibres together is dissolved
Lignin
Fibres
In addition to fibre removal, fibres are broken and fines (fibres <
0.5mm) are created
About 1/3 of pulp mass is in form of fines
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In contrast, chemical pulping produces intact fibres
Chemical Mechanical
Yield Fibre/Wood - Low 40-70% - High 90-98%
Cellulose Purity - High - lignin - Low - lignindissolved remains
End Uses - Dissolving pulp - Low quality- High quality paper - High volume
paper(e.g. book) (e.g. newsprint)
- Reinforcement pkg. - Molded products
Raw Material Sensitivity - Low - High
General Parameters
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Chemical Mechanical
Strength - High - fibres intact - Low - fibres damaged
Bulk - Low - more and - High - few and lessflexible fibres flexible fibres
Optical - Dark but bleachable - Bright but hard- Poor light scattering to bleach high
- Good light scattering
Drainability - Good - long fibres, - Poor - short fibres,few fines many fines
Permanence - Good - Poor (optical)
Quality Parameters
Chemical Mechanical
Raw Material - High - low yield - Low - high yield
Capital - High - Low
Operating - High - Low - becoming(chemicals, energy etc) lower - high for
electrical energy
Auxiliary - High - Low - for slush pulp(pollution recovery etc)
Mechanical pulps are generally used forshort-life, inexpensiveproducts, e.g. newsprint
Cost Parameters
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History
Pre-mid 1800’s paper made of rags.
1841, Friedrick Keller “inventor” 1848 Johan Voith in Heidenheim
made first commercial grinder. 1859 Voith developed “Raffineur”
to break up any course material not properly ground. First success.
1867 Full plant powered by steam. Paper made with 70% wood (Worlds fair Paris)
1868 Tampella (finish company) started making grinders.
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Debarking Drum
Ring Debarking
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Debarking resistance
Factors:
Species
Moisture content
Felling season
Storage duration
Temperature
Jan Dec
4
12
May Sept0
Deb
arki
ng r
esis
tanc
e N
/cm
^2
Stone Groundwood (SGW)
Pulp produced by pressing
logs against rotating
grindstone
Unchanged for 150 years.
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Action of grinder
Circumferential speed 30 m/s Grinding pressure 250kPa Grits deform fibre-lignin matrix Repeated visco-elastic deformation
creates heat Increased heat in wood
Heat softens lignin that’s found in between fibres and helps to release the fibres
Action of grinder
Fibres are peeled back in layers
Grits pass over partially removed
fibres
Develops surface and flexibility of
fibres … paper strength.
Fibres are released
Next layer peeled off
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Operating Parameters
Species and property of wood
Amount of spray water
Temperature of spray water
Rate of wood feed
Pressure applied
Speed of grinder
Structure of stone
Pulp Constituents
Shives: fibre bundles (3%)
Long, intact fibres (20%)
Short, broken fibres (35%)
Fines (45%)
Flour 30x30
Fibrils 30x1
Dust 1x1
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Pulp Properties
Higher strength as
more energy applied
CSF drops 150-50 ml
as energy applied
Brightest of
unbleached pulps up
to 65 ISO
Stone Sharpening
Stones wear due to constant high-
speed abrasion
Ceramic stones
Sharpening every 6-14 days
Sharpness affects energy and
production
2.5mm
grits
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Continuous Grinding
Pressure Ground Wood (PGW)
Higher pressure leads to
higher temperatures
Softer lignin, easier to detach
whole fibres
Stronger pulp
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Example
The quality of the pulp produced during grinding is dependent on the
temperature in the grinding zone. Fibre can be liberated largely intact if
the lignin has been softened by temperature, however, if the temperature
is too low the fibres will be largely broken or if the temperature is too high
the wood will start to darken.
Since virtually all of the grinding power is dissipated as heat in the
grinding zone, it follows that temperature in that zone is controlled by the
addition of shower water.
For a given grinding operation, wood, F, and dilution, D, (kg/s) enter the
grinder at Tin degrees C. The suspension leaving the grinder at Tout and
at a consistency, C. Assume that the steam is not formed. Determine the
electrical energy applied, E (J/kg), to maintain these outlet conditions.
C
F
D
E
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Mech 450 – Pulping and Papermaking Topic 3b – Refiner Mechanical Pulping
James A. Olson
Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia
Refiner Mechanical Pulp (RMP)
Wood chips are comminuted into fibres by bars on rotating
and stationary discs
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History
1957 Stora (Sweden) installed a Defibrator “raffinator”. Bauer shortly after
1963 Both companies modified to operate under pressure to make Thermo-mechanical pulp
1970’s First 100% TMP newsprint 1980’s 2-stage refining and heat recovery 1985 Large refiners 15MW. Chemicals added to further soften lignin
(CTMP). Mechanical pulps are replacing chemical pulps
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Chip Handling
Wood is typically chipped in a disc chipper
Goal is to have a high proportion of acceptable chips
3-16 knives on a disc 4 m diameter 450 m^3 / hr of solid wood Low cutting speed (20 m/s) as pin
chips increase with speed
Effect of chip size
Over size chips
Uneven feed in refiner
Reduces quality
Over thick fraction
Contains most of the knots
Decreases fibre length and long fibre portion
Decreases strength and brightness
Fines Fraction
Lowers energy consumption
Decreases strength, sheet density, brightness and light
scattering
Creates linting problems and increases shive content
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Chip washing
Immersed in a tank fed by a paddle wheel (Sunds). Removes: Rocks, metal, sawdust, bark Adds moisture Raises temperature
Chip Screening
Chips are passed through a series of screens Oversize: left on screen with 45 mm holes Overthick: left on screen with 7 mm slots Accept: left on screen with 7 mm holes Pin chips: left on screen with 3 mm holes Fines: pass through last screen
Overthick chips don’t react well to pre-treatments, lower yield
Fines and pin chips produce too many shives (not refined)
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Chip Steaming/Preheating
Atmospheric type Steam to 80 - 95 C
Most are pressurized (50kPa to 110kPa over pressure) Objective is to warm chip and equalize the moisture
content Can optimize a bit:
Higher temperature gives longer fibres, higher tensile Lower temperatures give better optical properties
Chip impregnation systems Used in CTMP Processes Compresses chips
• Water is removed and is high in extractives… fed to effluent• 4:1 compression ratio or higher
Passes chips into a pool liquor containing chemicals Increase moisture content by 6-7%
Disc Refiner
Refining Equipment
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Self Pressurization
Refining imposes cyclic
compression of visco-elastic
material
Generates tremendous amount
of heat and steam
Dilution required to maintain
approx 30% consistency
Steam pressure reaches max
and flows both ways
Can cause blow-back
Types of Refiners
Single disc, Moving rotor staionary stator 1.7m Dia. 15 MW
Double Disc Two counter-rotating discs More power delivered Less energy required per ton
• Higher shives, less long fibres, (similar to SGW)
Twin refiner One rotor, two stators… more
refining surface• Low intensity refining possible
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Refiner size over time
Conical Disc Refiners
Flat disc section and
conical section
Increases grinding surface
without increasing
diameter
Power: CD70, 76, 82 uses
15, 24 32 MW
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Refining Action
Chips are preheated to soften lignin Chips hit breaker bars and undergo a
series of normal and shear forces Rapid Breakdown in screw feeder,
entrance zone and breaker bars section. Fractures along grains, mostly along
fracture planes initiated in chipping Match stick size fragments accumulate
in refining zone with major axis along tangential direction
Match sticks defibred by longitudinal grinding and brooming
Fibres form flocs and flow out by steam drag and inertial forces
Flocs caught on bar edges and repeatedly compresssed by passing bars.
Breakerbars
Refining action
Fibre development
step
Fibres undergo cyclic
compressions
between bars
Internally and
externally delaminates
the fibres
Increases flexibility
and surface area
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Refiner Segment Design Parameters
Width of Grooves and Bars Traditionally the main parameter Wide grooves - narrow bars
• reduce specific energy consumption in refiner• Open volume allows gap to be narrower and can result in lower pulp
quality Wide bars / narrower grooves
• Increase specific energy consumption and improve quality• When Volume in groove is reduced steam flow is impeded and axial
load is higher and infeed of fibres is more difficult. This can lead to unstable feed
Height of the bars Higher the more open the groove volume, the better steam
removal Low bar height forces fibres to the plate gap an pulp quality
improves.
Dam number, height, and placement Forces pulp from the grooves to the plate gap Residence time increases. Hinders steam removal
Bar taper and angle When bars form a pumping angle fibre are forced through, lower
residence time which reduces energy consumption
Thermo-mechanical Pulp (TMP)
Pulping carried out in two refiners in
tandem
First refiner - pressurized with steam
(along with pre-steamer)
Second refiner is atmospheric
Produces longer fibre (stronger paper) and
fewer shives (small bundles of fibres)
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Theory
Specific Energy
Intensity:
Number of impacts
Intensity of each impact:
Specific energy per
impact
No LoadP PE
QC
I
“High Intensity”
BE
AE
“Low Intensity”
N
Ee
How do we calculate residence time?
Force balance on element of pulp
1 2r rF C F F bS
224 ( ) ( ) ( )( ) ( )
2r m
f p
rP r c rdv r b c rU r C A r
dr v m v
2
1
r
r
dr
v
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Operating parameters
Refiner speed (increase)
Increase intensity at same power
Lower energy at same freeness, lower length, and tear
Inlet Consistency (increase)
Increase moisture content and fibre length
Production rate (increase)
Reduce energy and lower length and strength
Preheating and steaming temperature
Not too critical
Plate Gap
Increases intensity
Lead to pad collapse
Effect of refining on coarseness
Coarseness:
Decreasing coarseness support
delamination theory
Lower coarseness of small fraction
indicate they are created from
fragments of cell wall
Not always evident if we measure
coarseness of whole pulp
Difficult to measure coarseness of pulp
with fines
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Effect of refining on long fibres
Effect of refining on fibre width
Refining reduces fibre
width by removing outer
wall material.
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Effect of refining on wall thickness
High intensity refining
reduces wall thickness more
at same energy
Outer part of fibre wall is
being peeled away
Effect of refining on fibre collapse
X-section measured by CLSM
Collapse index is an indication
of fibres ability to form ribbons
High intensity process creates
more collapsed fibres at same
energy
Wall stiffness about the same
Therefore wall thickness is
less for high intensity
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Effect of refining on fibre flexibility
Effect of increasing
energy plateaus at
moderate energies
Fibre development is
mostly through removal
of outer wall
Not through internal
delamination
Comparison of Pulp Properties
SGW RMP TMP
Energy required (GJ/ton) 5.0 6.4 7.0
Freeness 100 130 100-150
Burst index 1.2 1.6 1.8-2.4
Tear index 3.5 6.8 7.5-9.0
Breaking length (km) 3.2 3.5 3.9-4.3
Shive content (%) 3 2 0.5
Long fibre content (R48) 28 50 55
Fines content (P100) 50 38 35
Brightness (unbleached) 61.5 59 58.5
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Miscellaneous Other Data
Typical Production Rate 300 Bdt/d
(of one refiner) 800 Bdt/d - modern
Typical gap between plates 0.5-1 mm
Typical Specific Energy 7 GJ/t
Typical Power to Refiners 20-30 MW
(27,000 – 42,000 horsepower
10-15 train diesel locomotive)
Latency Removal
After refining fibres are kinked and
curled and not suitable for
papermaking
Lignin cools and holds kinked shape
Latency removal straightens fibres
Low consistency
30 minutes
90 degrees C
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Latency removal
Latency removal result in:
Chemi Thermo Mechanical Pulping (CTMP)
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Chemi-Mechanical Pulps
• To decrease energy cost or to improve pulp quality, chemical treatments are often added to mechanical pulping
• Pretreatment of chips• to lower energy
• Interstage treatment• lower energy, fibre flexibilization
• Post-treatment• fibre flexibilization
Sulphonation reactions
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1. increase in tear index2. increase in freeness
Increasedlong fibrecontent
Decreasedshive content
Improved fibreseparation
Softening ofmiddle lamella
lignin
Low sulphonatecontent (0-1%)
Usual means is sulphonation using sodium sulphite or sodium bisulphite
Decrease in freenessIncrease in breaking length
Decrease in specific scattering
Increase infibre flexibility
and conformability
Softening offibre wall
lignin
High sulphonatecontent (1-2%)
Pulp Properties
RMP fibres broken
TMP separated at primary wall,
some fibre broken
CTMP Middle lamella very soft,
almost all fibres separated at
M.L.
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Pulp Properties
Light scattering reflects
fines content
Tensile reflects surface
area and flexibility of
long fibres.
Pulp Properties Changes during Refining
Strength increase
Corresponds to energy
increase without cutting
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“Alphabet” Pulps
Many combinations of treatment and pulping processes are
possible
PUREMECHANICAL
SGWPGWRMP
TRMPPRMP
TMP
CHEMICALLYMODIFIED
HEAVYFRACTIONAL
LIGHT
HEAVY
LFCMPCTLF
TCMPCRMPCTMP
OPCOSCMPBCMP
UHYBSUHYS
MO
NO
PU
LP
S
PR
INT
ING
PU
LP
S
RE
INF
OR
CE
ME
NT
PU
LP
S
Effect of sulphonation on Lignin softening temperature
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Effect of yield on fibre stiffness
Effect of sulphonation on fibre length
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Effect of sulphonation on tensile
Effect of Sulphonation Energy required
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Effect of sulphonation on light scattering
Scattering vs Energy
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Mechanical Pulp Brightening
Often desirable to make pulp brighter (whiter)
Do not want to remove lignin to keep yield high
Use “brightening” chemicals, e.g. hydrogen peroxide
Problem: If lignin not removed, brightness not permanent
(reversion, yellowing)
Example: BCTMP (Bleached Chemi-Thermo-Mechanical
Pulp)
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Screening and Cleaning
Pulping process imperfect
Small bundles of fibres (shives) remain
These must be removed and further refined
Mechanical pulping is therefore follows by an elaborate
screening system
Subject of next lecture (after LC-refining)
TMP System
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Process may also include “cleaners” (hydrocyclones)
Energy Recovery
Enormous volume of steam produced from heat created
in mechanical pulping
This steam can be recovered and used for mill process
steam, e.g. for paper drying
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Energy balance
About 65% of electrical energy can be
recovered in this manner
RTS results
Retention: Short retention in pre-
heater (10-20s).
The short time at elevated temperature
reduces the brightness losses
Temperature: increase pressure
to 5.5-6.0 bar
Speed: Increase speed to 2000-
2500 RPM. Decreases specific
energy to get same ‘quality of
pulp’. 15% energy reduction.
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Conclusions
Refining characterized by specific energy and intensity
Refining removes outer wall material
Thin wall, collapsible fibres
Smoother, stronger paper
Heat softens lignin
More long fibres and less fines
CTMP softens lignin in fibre wall
Even more long fibres, less fines
Makes fibres more collapsible at same wall thickness
Less fines
The end