A Lecture on Low Consistency Refining of Mechanical Pulp

1

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

2

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

3

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

4

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

5

LC refining – fibre morphology

Dislocations

Fibre curl

Fibre wall

delamination

Presence of

tension during

refining

Page 1985

Wall thickness reduction in LCR

6

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 …

7

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

8

LC refining – third stage refiners

High Consistency (HC) 20-40%

Primary Refiner(HC)

Chip feeder

Tertiary Refiner(LC)

Secondary Refiner(HC)

Low consistency (LC) 3-4%

Latency removal


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

9

LC refiners – energy savings potential

Sabourin et al, 2007 IMPC

HCR HCR+LCR


Sabourin et al, 2007 IMPC

HCR HCR+LCR

10


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)

11


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 [kW] power applied to the refiner

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

= [o.d. tonnes / day] Mass flow rate of pulp.

M

PPE loadno

M

12

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

0

500

1000

1500

2000

2500

0 1 2 3 4 5 6 7 8 9 10 11

Tota

l pow

er, k

W

Gap, mm

No-load ?(500kw)

Current operation(1200kw)

58 inch LC twin flow refiner, 4% consistency TMP

13

Refiner Efficiency

If the refiner is not fully loaded than the efficiency of

the process can be very low.

Refiner Efficiency

The previous power curve showed that current operating power is

1200kW and the no load power is approximately then the efficiency

would be

Only 37% of the energy is transferred to the pup.

%5.371200

7501200

E

PowerTotal

PowerNet E

No – load power correlation

From fluid dynamics a dimensional analysis

suggests that Pno-load should be

Cp = power coefficient, approximately constant

N = RPM (Angular velocity)

D = Diameter of plate

= fluid density

Approximately independent of flow rate

Other published correlation:No load power = k * D4.3 * N3

53DNCP ploadno 53DN

PC loadno

p

Example, Herbert and Marsh 1968

14

Energy saving opportunity Reduced periphery plate

58 inch no-load is

477kW

55 inch no-load is

307 kW

No-load reduced by

approx 170 kW or

35% reduction in no-

load

No loss is tensile

strength 39

40

41

42

43

44

45

46

47

48

0 20 40 60 80 100 120 140 160

Ten

sile

Ind

ex (N

m/g

)

Specific Energy Consumption (Kwh/ton)

TSR3

TSR4

58 inch plate

55 inch plate

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

15

Specific energy ranges

E Specific energy: Total amount of energy

transferred to the pulp per unit mass. M

PPE loadno

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).

16

Refining intensity

I

“High Intensity”

BE

AE

“Low Intensity”

N

N: Number of impacts on fibres

I: Intensity of each impact

E: Net specific energy

E = N • IEA = EB

Refining intensity

Most 2 parameter break energy into N and I

in the following ways:

INC

P

M

C

M

PE

M

CN

C

PI

Number of impacts – related to flow rate

Intensity of impacts – related to power

C is a machine parameter

17

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]

i

srR

R

sr rnn

drnn

CEL ii

coscos

2

1

TAPPI – technical information sheet

CEL= Number of bars on rotor X no of bars on stator X bar length divided by cosine of bar angle to radius

Refining intensity Cutting edge length

Example of using the

discrete form of the

equation

Divide disc up and count …

i

srR

R

sr rnn

drnn

CEL ii

coscos

2

1

18

Refining intensity

Integral example …

Can re-write integral in terms of bar width

and groove width, i.e,

2

31

32

22

1

22

3cos

2

cos

2

wg

RRdr

wg

rCEL

R

R

gw

rrnr

2)(

gw

rrns

2)(

Refining intensity

Specific Edge Load [J/s]

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

CELRPM

P

SEL

PI

19

Refining intensity

Example plate pattern

ICPM

Refining intensity

Typical specific edge loads for various paper

grades

20

Other “Machine Intensities”

Specific Surface Load LUMIAINEN, 1990

Modified Edge Load MELTZER F.P., RAUTENBACH R., 1994,

( . . )

. 2 tan

Bar width groove widthXSEL

bar widthX

.

SEL

bar width

Specific Surface Load

Accounts for bar

width

Considered to

be energy per

unit area of bar

surface

2

Specific Edge LoadSSL

Bar WidthW s 1 W s

m m m

2/J m

21

“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

22

Fibre capture zoneKerekes & Olson (2003)

From mass balance, only about 5-10% of pulp in groove can fit in gap.

C Factor for Disc Refiner

Dw

RRtannCGDC F

3218 3

132

32

C

PI

23

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

1

2

3

4

5

Specific Edge Load (J/m)

Cha

nge

in

Bre

akin

g L

engt

h (k

m) Softwoods

E=120 kWh/t

Hardwoods

Specific Edge Load

6

0 10 20 30 40 50 600

1

2

3

4

5

Specific Intensity, (kJ/kg)

Cha

nge

in

Bre

akin

g Le

ngth

(km

)

E=120 kWh/t

IrreversibleDeformation

Softwoods

Hardwoods

Specific Intensity

24

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


J.C. Roux (FRS Oxford 2009)

25


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

26

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)

Vary plate geometry (BEL) [km/rev.], flow rate

[kg/s], power [kW] and RPM [1/s]

Achieve intensities with several combinations

Measure pulp quality response

Pilot Trials

Andritz R&D laboratory Springfield OH

22 inch TwinFlow refiner

Trial plan

27

Results, Freeness-SE

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300 350 400 450

CSF [ml]

Specific Energy [kWh/t]

Results, Tear-Tensile

3

4

5

6

7

8

9

10

11

40 45 50 55 60 65

Tear In

dex [mNm

2/g]

Tensile Index [Nm/g ]

High Intensity

Low Intensity

28

Results, Tensile-Intensity

‐2

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4 0.5 0.6

Tensile increase [Nm/g]

@ 200 kWh/t

Intensity [J/m]

Results, Tensile-Gap

‐1

0

1

2

3

4

5

6

7

8

9

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Tensile increase [Nm/g]

@ 200 kWh/t

Gap [mm]

Fibre cutting Elastic deformation

Fibrillation

29

Results, Gap-S.E.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300 350 400 450

Gap

[mm]


Results, LFF-SE

10

15

20

25

30

35

40

45

50

55

60

0 50 100 150 200 250 300 350 400 450

LFF [%

]


30

Results, LFF-Gap

0.60

0.70

0.80

0.90

1.00

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55

LFFout

LFFin

Gap [mm]

Fibre cutting Elastic deformation

Fibrillation

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:

G

Df

DN

PC net

p 53

31

Results, Power Number-Gap

0

0.001

0.002

0.003

0 1000 2000 3000 4000 5000 6000 7000

Pnetρω3D5

DG

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

32

Results, Tear-LFF

3

4

5

6

7

8

9

10

11

10 15 20 25 30 35 40 45 50 55 60

Tear In

dex [mNm

2/g]

LFF [%]

Results, Tensile-LFF2/CSF

30

35

40

45

50

55

60

65

70

0 0.003 0.006 0.009

Tensile In

dex [Nm/g]

LFF2[%]CSF [ml]

33

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

34

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

35

Heterogeneity

Model refining experiment –

compression refiner

Only a fraction of the fibres

are compressed / refined

On repeated compression cycles the same

fibres are refined .. No change in tensile

Fibres redistributed … new fibres are

refined and continuous change in tensile

MTS compression tester and test cell.

Heterogeneity

0.0

1.0

2.0

3.0

4.0

5.0

0 50 100 150 200

No. of Cycles

Bre

ak

ing

Le

ng

th (

km

)

Without Redistribution

Redistribution After Every Cycle

36

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:

Tensile increase is proportional to fraction of refined fibres

1 1n

R n P

Heterogeneity

1.8

2.3

2.8

3.3

3.8

4.3

4.8

0 10 20 30 40 50

Cycles

Bre

akin

g L

eng

th (

km)

0.0

0.2

0.4

0.6

0.8

1.0

Pro

bab

ilit

y

Compression Refining Trials

Cumulative Probability of a Fibre Being Refined

37

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

refiner

P=0.0021

Large number of bar

crossings

Small number of fibres

affected during each rbar

crossing

2.7

3.7

4.7

5.7

6.7

7.7

8.7

0 100 200 300 400 500 600 700 800 900

Number of bar crossings, N B [-]

Bre

akin

g L

eng

th [

km]

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

R(N

B)

[-]

C=2%; SEL=2J/m

Commulative probability of fibre being refined

0021.0P

BNB PNR 11

38

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

39

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:

Ni = Fibre length distribution E = Specific Energy (kWhr/t) Si = Selection function (Cutting rate)Bij = Breakage function

jjij

ijiii NSBNS

dE

dN

2

0100200300400500600700800900

1000

0.00 1.00 2.00 3.00 4.00 5.00

N

Fibre Length (mm)

Olson, et al 2001Heymer, 2009

40

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

41

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

42

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

ii LS

43

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

44

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

A Lecture on Low Consistency Refining of Mechanical Pulp

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