1 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
41
Embed
Topic 3 Mechanical Pulping.ppt - UBC Fibre Lab1 Mech 450 – Pulping and Papermaking Topic 3 – Mechanical Pulping James A. Olson Pulp and Paper Centre, Department of Mechanical Engineering,
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
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
15
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
16
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
17
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)
18
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
19
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
20
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
21
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
22
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)
23
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
24
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
25
Effect of refining on long fibres
Effect of refining on fibre width
Refining reduces fibre
width by removing outer
wall material.
26
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
27
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
28
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
29
Latency removal
Latency removal result in:
Chemi Thermo Mechanical Pulping (CTMP)
30
Chemi-Mechanical Pulps
• To decrease energy cost or to improve pulp quality, chemical treatments are often added to mechanical pulping