A Lecture on Low Consistency Refining of Mechanical Pulp
James A. Olson Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia
Outline: what we will cover today
. LC refining equipment and overview
. Fibre morphology changes
. Characterization of the refining effect
. Total energy transfer, no load, refiner efficiency
. Intensity of energy transfer . Heterogeneity of treatment
. Fibre cutting during LC refining
1 LC Refining
. LC refiners operate at 3-5% consistency
. Differs from HC refining in that:
. Pump through operation
. Decouples the flow in the refiner from refiner operation and design (speed / diameter / plate geometry)
. Smaller, more controlled plate gap than HC refiners
. No steam production
LC Refining
. Conventional LC refining done in stock prep area of papermachine
. Originally chemical pulp
. Increase pulp strength, sheet smoothness . Increasingly important in the manufacture of mechanical pulp
2 LC Refining
. Flow in a LC refiner
. Conical Refiner
. Double Disc
LC refining
. Bars
. Fibre capture and transport
. Cyclic compression and shear
. Permanently deforms fibre wall . Grooves provide capacity
. Angles provide uniform bar contact area
Page - 1985
3 LC Refining - paper properties
. Fibre flexibility and higher bonded area increases sheet strength
. More flexible fibres increase sheet smoothness
LC refining – fibre morphology
. Imposes cyclic compression on fibres . Internal delamination – break down of cell wall – increases flexibility . External fibrillation – increases relative bonded area . Reduces wall thickness - increases fibre flexibility, fines production . Fibre cutting
4 LC refining – fibre morphology
. Dislocations
. Fibre curl
. Fibre wall delamination
. Presence of tension during refining
Page 1985
Wall thickness reduction in LCR
5 LC refining in mechanical pulping
. Mechanical pulping has traditionally only used high consistency (HC) refining
. Low consistency refiners are used in 3 main areas
. Reject refining
. Post refiners
. (Low consistency) Third stage refining
LC reject refiners
. Effective at removing shives
. Significant energy savings
. Minimum capital investment
. Limit to the amount of energy that can be transferred to the pulp before fibre cutting
. Good for lower grades of mechanical pulp …
6 LC reject refiners Port Hawkesbury – Newsprint (blended into SCA) Lowest capital cost per ton of any modern TMP mill (900 admt/d:C$90M)
Mokvist et al, IMPC – Norway, 2005
LC mechanical pulp post-refiners
. Located in papermachine stock prep area
. Small freeness change, low tear loss, relatively small tensile improvement
. Coupled to papermachine to facilitate immediate response to changing pulp quality
. Enables better pulp quality control
. Freeness at TMP disc thickener is higher
. Not typically used for energy reduction or capacity increase
7 LC refining – third stage refiners Low consistency (LC) 3-4%
Chip feeder Latency removal Tertiary Refiner (LC)
Primary Refiner Secondary Refiner (HC) (HC)
High Consistency (HC) 20-40%
LC refining – third stage refiners
. LCR add 5% of total energy applied to pulp
. typically 100-150 kWh/t. . Increased production for minimum capital investment, typically, 10% increase in production for northern softwood
. Decrease shive content
. Allow for low latency residence time . Improve pulp quality, improve tensile at slightly lower Freeness
. Energy reduction, typically 5-7% savings
. Note: Energy, capacity and quality are trade-offs
8 LC refiners – energy savings potential
HCR HCR+LCR
Sabourin et al, 2007 IMPC
LC refiners – energy savings potential
HCR HCR+LCR
Sabourin et al, 2007 IMPC
9 LC refining – third stage refiners
. Why is LC refining more energy efficient than HC refining ?
. Control flow and power independently
. More uniform, controllable plate gap
. Higher, controllable intensity, more efficient energy transfer
. More homogeneous treatment
LC Refining – homogeneity
HC Andritz 36-1CP chip refiner
. Vertical lines indicate bar crossings
(Olender et al, in press)
LC Metso JC-00 Conflo
- CTMP 3.1%
(Prairie et al, in press)
10 LC refiners – energy savings potential
. Strategy to increase energy to LC refiners and reduce energy to HC refiners
. What limits the energy saving potential of LC refiners?
. As power increases so does “Intensity” of treatment
. As Intensity increases fibres become increasingly broken / cut.
. At high power you get an unacceptable loss in pulp tensile strength.
LC Refiner characterization
. E - Specific energy: Total amount of energy transferred to the pulp per unit mass. P P . P [kW] power applied to the refiner E noload M . Pno-load [kW] . Power applied before fibre quality begins to change.
. Many people use a rule of thumb 0.050 inch (1.25mm) gap
. Pnet = P – Pno-load Net power is the power that goes to the pulp . Measured with pulp moving through refiner . M = [o.d. tonnes / day] Mass flow rate of pulp.
11 No-load power
. HC refining no load is small / negligible
. Steam provides small viscous drag on plate . LC refining no load power can be significant
. Can be up to 40% of total power
. High viscous friction in 4% pulp suspension
Power – plate gap
2500
2000 Current operation 1500 (1200kw) No-load ? (500kw) 1000
Total power, kW 500
0 01234567891011 Gap, mm
58 inch LC twin flow refiner, 4% consistency TMP
12 Refiner Efficiency
. If the refiner is not fully loaded than the efficiency of the process can be very low.
. Refiner Efficiency Net Power E Total Power . The previous power curve showed that current operating power is 1200kW and the no load power is approximately then the efficiency would be 1200 750 E 37.5% 1200 . Only 37% of the energy is transferred to the pup.
No – load power correlation
. From fluid dynamics a dimensional analysis
suggests that Pno-load should be
Pnoload 3 5 C p 3 5 Pnoload C p N D N D Example, Herbert and Marsh 1968 . Cp = power coefficient, approximately constant . N = RPM (Angular velocity) Other published correlation: . D = Diameter of plate No load power = k * D4.3 *N3
. = fluid density . Approximately independent of flow rate
13 Energy saving opportunity Reduced periphery plate
. 58 inch no-load is 477kW
. 55 inch no-load is 307 kW
48 . No-load reduced by 47 approx 170 kW or 46 45
35% reduction in no- 44
43 load TSR3 58 inch plate 42 Tensile Index (Nm/g) Index Tensile TSR4 55 inch plate . No loss is tensile 41 40
strength 39 0 20 40 60 80 100 120 140 160 Specific Energy Consumption (Kwh/ton)
Refiner efficiency
. LC refiners should be operated at full power to increase efficiency
. Optimize plate design and geometry to enable fully loaded operation
. Utilize reduced periphery plates
. Shutdown refiners in parallel . Need to optimize plate and HC operation to enable full power to LC refiners
14 Specific energy ranges
. E Specific energy: Total amount of energy P P transferred to the pulp per unit mass. E noload M Grade Pulp kWh/t Fine Papers Hardwood 80-120 Softwood 80-140 News/directory SW Kraft 40-100 Post-refineTMP 20-60 TSR 60-120 Linerboard OCC 30-60
Refining intensity
. Specific Energy is not enough
. Need to describe intensity of treatment
. Range of 2-parameter characterizations (energy and intensity)
. Energy can be expended in different ways
. Large number of low energy impacts
. Small number of large energy impacts . Break energy down into these components (Number and intensity).
15 Refining intensity
“High Intensity” N: Number of impacts on fibres
EA I: Intensity of each impact I E: Net specific energy “Low Intensity”
EB E = N • I N EA = EB
Refining intensity
. Most 2 parameter break energy into N and I in the following ways:
P C P E N I M M C C N Number of impacts – related to flow rate M Intensity of impacts – related to power P I C C is a machine parameter
16 Refining intensity
. Specific Edge Load theory
. C is the “Cutting edge length” CEL
. CEL is the total length of bar edges a fibre will see in a revolution [km/rev]
R2 n n n n CEL r s dr ri si r R1 cos i cos
CEL= Number of bars on rotor X no of bars on stator X bar length divided by cosine of bar angle to radius TAPPI – technical information sheet
Refining intensity Cutting edge length
. Example of using the discrete form of the equation
. Divide disc up and count …
R2 n n n n CEL r s dr ri si r R1 cos i cos
17 Refining intensity
. Integral example …
. Can re-write integral in terms of bar width and groove width, i.e, 2r 2r n (r) n (r) r w g s w g
2 R22 2 r 2 2 R3 R3 CEL dr 2 1 2 R1 cos g w cos 3g w
Refining intensity
. Specific Edge Load [J/s]
P P I SEL RPMCEL . Although derived empirically, in rigorous terms SEL is the energy expended per bar crossing per unit bar
length ( Kerekes and Senger, 2006)
. CEL x RPM=SEL is the “machine parameter” – Specific-edge-Load
. SEL is not directly the energy expended on pulp
18 Refining intensity
. Example plate pattern
. ICPM
Refining intensity
. Typical specific edge loads for various paper grades
19 Other “Machine Intensities”
. Specific Surface Load LUMIAINEN, 1990 SEL bar. width
. Modified Edge Load MELTZER F.P., RAUTENBACH R., 1994,
(.Bar width groove . width ) XSEL bar.2tan widthX
Specific Surface Load
. Accounts for bar width Specific Edge Load SSL Bar Width . Considered to Jm/ 2 Ws 1 Ws be energy per mmm2 unit area of bar surface
20 “Fibre Intensity”
. “Fibre Intensity” is the energy expended on pulp by bar crossings
. It requires assumptions about how fibres are captured from grooves and impacted during bar crossings
. It is a “Specific Energy per Impact”, S
Probability of Fibre Impact
. SEL is the energy expended per bar crossing per unit bar length … don’t know how much pulp that energy is expended on …
. Not all fibres are impacted in every bar crossing
. Can show this readily from mass balance of fibres in groove vs. fibres in gap
. C-Factor used to estimate fibre captured
21 From mass balance, only about 5-10% of pulp in groove can fit in gap.
Fibre capture zone Kerekes & Olson (2003)
C Factor for Disc Refiner
P I C
82GDC n31 2tan R3 R3 C F 2 1 3w D
22 Specific Edge Load
5 Softwoods 4
3
2 Change in Hardwoods 1 Breaking Length (km) Breaking Length E=120 kWh/t 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Specific Edge Load (J/m)
Specific Intensity
6 Irreversible 5 Deformation
4 E=120 kWh/t
3
Change in 2 Softwoods
Breaking Length (km) 1 Hardwoods 0 01020 30 40 50 60 Specific Intensity, (kJ/kg)
23 Comparison of Intensities
. J.C. Roux (FRS Oxford 2009)
. Recently a comparison of intensity estimates
. Specific Edge Load (SEL), Specific Surface Load (SSL), Modified edge load (MEL) and his own Net Normal Force per Bar Crossing
. Gives correlations to fibre cutting for each intensity
Comparison of Intensities
J.C. Roux (FRS Oxford 2009)
24 Comparison of Intensities
. Demonstrates that:
. Roux / Kerekes: SEL is not a great predictor of refining effect
. Roux: MEL is a better predictor
. Kerekes: C-factor is better than SEL
. Roux: Net normal force per bar crossing gives best correlation . Problem is that it is difficult to measure effect of intensity … need to measure a pulp property interpolated to a Specific Energy
Our recent work …
. We hypothesize that Intensity is directly proportional to the refiner gap
. Gap is easily measured
. Power is proportional to 1/Gap
. Not the first to say this
. Ulla-britt Mohlin (2005)
. Miles, May, et al (1987) on reject refiner
25 Pilot Trials
. Vary Total Energy Applied
. Specific energy
• 60 kWh/t increments from 0 to 420 kWh/t
. Vary Intensity
. Specific Edge Load (SEL) [J/m]
• 0.14 (low), 0.28 (medium), 0.56 (high) Andritz R&D laboratory Springfield OH . Vary plate geometry (BEL) [km/rev.], flow rate 22 inch TwinFlow refiner [kg/s], power [kW] and RPM [1/s]
. Achieve intensities with several combinations
. Measure pulp quality response
Trial plan
26 Results, Freeness-SE
140
120
100
80 [ml]
60 CSF
40
20
0 0 50 100 150 200 250 300 350 400 450 Specific Energy [kWh/t]
Results, Tear-Tensile 11
10 Low Intensity
/g] 9 2
8 [mNm 7
6 Index High Intensity 5 Tear 4
3 40 45 50 55 60 65 Tensile Index [Nm/g ]
27 Results, Tensile-Intensity 12
10
8 [Nm/g]
6 kWh/t
200 4 increase
@ 2 Tensile 0 0 0.1 0.2 0.3 0.4 0.5 0.6
‐2 Intensity [J/m]
Results, Tensile-Gap
9
8 Fibre cutting Elastic deformation 7
6 [Nm/g]
5 kWh/t 4 Fibrillation 200 increase
3 @ 2
Tensile 1
0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 ‐1 Gap [mm]
28 Results, Gap-S.E.
0.6
0.5
0.4
0.3 [mm]
0.2 Gap
0.1
0 0 50 100 150 200 250 300 350 400 450 Specific Energy [kWh/t]
Results, LFF-SE
60
55
50
45
40
35 [%]
30
LFF 25
20
15
10 0 50 100 150 200 250 300 350 400 450 Specific Energy [kWh/t]
29 Results, LFF-Gap
1.00
0.90 in out 0.80 LFF LFF
0.70 Fibre cuttingFibrillation Elastic deformation
0.60 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 Gap [mm]
Refiner gap as intensity
. Currently Gap is not a predictable parameter, unlike the refiner characterization
. If you cant predict its not useful from an engineering perspective
. To predict gap we start from a simple dimensional analysis and assume that:
Pnet D C p f N 3D5 G
30 Results, Power Number-Gap
0.003 Pnet ρω3D5
0.002
0.001
D G 0 0 1000 2000 3000 4000 5000 6000 7000
Refining intensity - Gap
. Predict Refiner gap from power, speed and size
. Refiner gap controls:
. Fibre cutting – long fibre content – critical gap
. Freeness change . Forgacs (1963) pulp properties can be predicted from a measure of fibre length and surface area
. Use Specific energy and Gap to predict CSF and Length changes
. Use CSF and fibre length to predict tear and tensile changes
31 Results, Tear-LFF
11
10
/g] 9 2
8 [mNm 7
6 Index
5 Tear 4
3 10 15 20 25 30 35 40 45 50 55 60 LFF [%]
Results, Tensile-LFF2/CSF
70
65
60
55 [Nm/g]
50
45 Index
40
35 Tensile
30 0 0.003 0.006 0.009 LFF2[%] CSF [ml]
32 Intensity Summary
. Increasing refiner power increases intensity
. Several methods to estimate intensity
. Plate gap controls intensity of treatment
. High normal forces on fibres at smaller gap (Roux)
. Force based analysis shows gap controls forces on fibres (kerekes)
. Increasingly active area of research . Implies:
. Operate LC refiner at highest possible power (smallest gap) before onset of cutting
. Gap measurement and control is increasingly important
HETEROGENEITY OF REFINING
66
33 Heterogeneity
. Specific energy and Intensity are mean values
. Refining is tremendously heterogeneous;
. Circulating flow patterns in refiners
. Plug flow from low velocity/high consistency
. Non uniform distribution of loading on fibres . Degree of heterogeneity is important
Heterogeneity
34 Heterogeneity
. Model refining experiment – compression refiner
. Only a fraction of the fibres
are compressed / refined MTS compression tester and test cell. . On repeated compression cycles the same fibres are refined .. No change in tensile
. Fibres redistributed … new fibres are refined and continuous change in tensile
Heterogeneity
5.0
Without Redistribution 4.0 Redistribution After Every Cycle
3.0
2.0
Breaking LengthBreaking (km) 1.0
0.0 0 50 100 150 200 No. of Cycles
35 Heterogeneity
. Results of single fibre compression studies showed that (P. Wild et al 2001):
. Fibre modulus changed during first compression and no subsequent change after that. . We postulate that small fraction of fibres, P, are refined during any one cycle in our refiner
. Statistical Analysis: fraction of refined fibres after n cycles:
Rn11 P n
. Tensile increase is proportional to fraction of refined fibres
Heterogeneity
4.8 1.0 4.3
0.8 3.8
0.6 3.3
0.4 2.8 Probability
Breaking Length (km) Compression Refining Trials 2.3 0.2 Cumulative Probability of a Fibre Being Refined
1.8 0.0 0 1020304050 Cycles
36 Heterogeneity
. From this analysis we can predict the probability of a fibre being refined during each compression: P = 0.06
. Similar analysis can be performed for a disc refiner assuming a bar crossing is a compression cycle.
Heterogeneity
. Similar analysis for LC 8.7 1.0 C=2%; SEL=2J/m 0.9 Commulative probability of fibre being refined refiner 7.7 0.8
0.7 P=0.0021 6.7 0.6 [-] ) . Large number of bar 5.7 0.5 B 0.4 R(N 4.7 crossings [km] Breaking Length 0.3
RN 1 1 PN B 0.2 3.7 B P 0.0021 . Small number of fibres 0.1
2.7 0.0 affected during each rbar 0 100 200 300 400 500 600 700 800 900 Number of bar crossings, N B [-] crossing
37 Heterogeneity - Summary
. Small number of fibres are impacted during each bar crossing
. Large amount of heterogeneity. Some fibres remain unrefined. . Large number of bar crossing are required to ensure sufficient fraction of pulp is refined.
. Opposed to the fatigue hypothesis of fibre deformation used by many
FIBRE CUTTING
38 Fibre cutting model
. Want to understand fibre cutting because it limits the application of LC refining.
. Want to know
. What fibres are cut?
. How does specific energy determine cutting?
. How does intensity affect cutting?
Fibre Comminution
. Comminution model adapted from crushing and grinding industries to describe fibre length reduction using the following equation:
1000 900 dNi 800 Si Ni 2 Bij S j N j 700 dE ji 600
N 500 400 300 200 Ni = Fibre length distribution 100 E = Specific Energy (kWhr/t) 0 Si = Selection function (Cutting rate) 0.00 1.00 2.00 3.00 4.00 5.00 Fibre Length (mm) Bij = Breakage function Olson, et al 2001 Heymer, 2009
39 Initial Validation Experiments
. Test the experimental and computational methodology
. Handsheets made from the same chemical pulp
. Handsheets cut into strips of varying width:
. 2mm, 5mm, 7mm . Fibre length measured before and after
. And S calculated from fibre length distributions
. Exclude fibres less than 0.5mm
Validation Results
. Fibre cutting is function is linear with fibre length
. Measured cutting is smaller than theory
40 Experimental
. Pilot Refining trials . 5 different plate patterns . 2 different refiners (22” Beloit DD, EW) . Range of Consistencies, Energy, Flowrates . Fibre length distribution measured using an optical fibre analyzer
. Calculate Si numerically knowing the fibre length distribution and the applied energy
Pilot Refining Experiments
. Determine selection function for:
. Increasing specific energy at a constant intensity
. Increasing SEL at a constant specific energy
. Varying SEL and refiner plates and refiners . Examine relationship between cutting and tensile strength
41 Varying Specific Energy
. How:
. Vary mass flow rate
. Constant cconsistency, power
. High intensity plate . Probability of cutting per kWh/t is
. Pproportional to fibre length
. Independent of specific energy . Fibre cutting is not a fatigue process
Varying SEL
. How: . Vary applied power . Constant Specific energy (120 kWhr/t) . Cutting is dependent on SEL . Cutting independent of consistency . Cutting proportional to length . Propose to characterize Cutting by constant of proportionality
Si Li
42 All Trials
. Vary SEL by
. Power
. Plate type
. Refiner type . Constant specific energy (120 kWhr/t)
. Cutting a function of SEL
. Independent of consistency
Tensile Strength
. Constant specific energy, 120kWhr/t
. Small cutting rate strength increases
. Large cutting rate strength decreases
. Not a great correlation
. Same cutting, significant changes in tensile
43 Conclusions
. Developed experimental and computational methods to measure fibre cutting distribution . Fibre cutting is . Proportional to fibre length • Implies random cutting process . Proportional to specific energy • More energy more cutting • Not a fatigue process . Function of refining intensity (SEL) . Independent of consistency . Developed a predictive model of fibre length distribution changes during refining . Optimal tensile strength development may be at the onset of fibre cutting
44