best milling practices in feeding, extraction and power ...roll surface drainage • if juice cannot...

Queensland University of Technology CRICOS No. 00213J

a university for the worldrealR

Best milling practices in

feeding, extraction and

power consumption

Geoff Kent

1

CRICOS No. 00213J


Overview of presentation

• Three topics

– Feeding

– Extraction

– Power consumption

• Focus on individual milling units

– Extraction in particular is also considerably

affected by cane preparation and added water

rate

2



Feeding

3

CRICOS No. 00213J


Feeding

• Definition

– The cane or cane fibre rate that can be

achieved at a particular mill speed

• Associated with the capacity of the mill

4

CRICOS No. 00213J


Basis for feeding theory

• Based on two-roll mill

• Compaction (γ) at

Donnelly chute exit:

γ =𝑄𝑓

𝐿ℎ𝑆 cos 𝛼• Qf Cane fibre rate

• L Roll length

• h Chute setting

• S Roll surface speed

• α Contact angle

h

α

CRICOS No. 00213J


Compaction at Donnelly chute

exit• Compaction at chute

exit not affected by

mill settings

• Function only of

conditions in chute

• Essentially constant

year to year

CRICOS No. 00213J


Predicting mill speed

• Assumption of constant compaction at Donnelly

chute exit can be used to predict mill speed

• Rearranging the compaction equation:

γ =𝑄𝑓

𝐿ℎ𝑆 cos 𝛼 𝑆 =

𝑄𝑓γ𝐿ℎ cos 𝛼

– Compaction (γ) from previous performance

– Roll length (L), chute setting (h) and contact angle

(α) from mill geometry

– Fibre rate (Qf) selected

CRICOS No. 00213J


Application of the feeding

theory concept

• As originally conceived, the

theory was applied to the

pressure feeder

• Theory indicates that mill

speed is affected by:

– Donnelly chute exit setting

– Underfeed setting

– Pressure feeder work opening

CRICOS No. 00213J


Application to

four-roll mill

• Pressure feeder geometry

very similar to first three

rolls in four-roll mill

• Theory indicates that mill

speed is affected by:

– Donnelly chute exit setting

– Underfeed setting

– Feed work opening

CRICOS No. 00213J


Implications

• Choice of settings will affect mill speed

• If the mill is operating near the middle of its

speed range, there are no consequences

• If the mill is operating near the top or bottom

of its speed range, chute height cannot be

properly maintained

• Feeding theory can be used to adjust settings

so mill operates in a better speed range

CRICOS No. 00213J


Best practices in feeding

• Setting selection

• Roll surface grip and drainage

11

CRICOS No. 00213J


Setting selection

• Choice of settings

affects speed:

– Donnelly chute exit

– Underfeed nip

– Feed nip

• Case study for a

dewatering mill at

Sezela (South Africa)

reported SASTA 2014

12

CRICOS No. 00213J


Effect of mill ratio on feed

work opening

• Mill ratio is ratio of work

openings between feed

and delivery nips

• Typically used to

calculate feed setting

• As set:

– Work opening ratio 1.8

– Feed work opening 93

mm Feed to delivery work opening ratio

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Feed w

ork

open

ing (

mm

)

20

40

60

80

100

120

140

160

180

200

CRICOS No. 00213J


Effect of mill ratio on roll

speed

• As set:

– Work opening

ratio 1.8

– Speed 1.0

r/min

Feed to delivery work opening ratio

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Roll

sp

eed

(r/

min

)

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

CRICOS No. 00213J


• Feed work

opening 93 mm

• As set:

– UF 216 mm

– Donnelly 369 mm

Donnelly chute opening (mm)

300 400 500 600 700 800

Ro

ll sp

ee

d (

r/m

in)

0.7

0.8

0.9

1.0

1.1

1.2

UF 200 mm

UF 300 mm

UF 400 mm

UF 500 mm

Effect of underfeed and

Donnelly chute exit opening

CRICOS No. 00213J


Roll surface grip

• Feeding dependent

on rolls gripping the

bagasse

• Picots seem a good

way of doing that

16

CRICOS No. 00213J


Roll surface drainage

• If juice cannot easily

drain away from the

roll surface, it has to

force its way back in

the direction of

feeding

• Bad for feeding

• Solutions

– Messchaert grooves

– Lotus rolls

17



Extraction

18

CRICOS No. 00213J


Volumetric theory of juice

extraction

Feed to mill

Escribed volume

Delivery bagasse

Expressed juice

CRICOS No. 00213J


Increasing extraction

approach 1• Volumetric theory shows that reducing escribed

volume will reduce volume of delivery bagasse

– Hence increase extraction

20

Feed to mill

Escribed volume

Delivery bagasse

Expressed juice

CRICOS No. 00213J


Escribed volume (𝑽𝑬)

• Escribed volume

of final nip

influences

extraction

• 𝑉𝐸 = 𝐿𝑊𝑆

• L = roll length

• 𝑊 = work opening

• 𝑆 = surface speed

21

L

W

S

CRICOS No. 00213J


Influencing escribed volume

• Option 1: change top

roll hydraulic pressure

• Increases delivery nip

compaction

• If increasing pressure,

need to ensure top

roll shaft has

sufficient strength

22

CRICOS No. 00213J


Influencing escribed volume

• Option 2: enable

higher delivery nip

compaction at the

same hydraulic

pressure

– Better preparation

– Softer bagasse

23

CRICOS No. 00213J


How to change bagasse

preparation?• Change cane

preparation

– Change power

consumption

• Change preparation

in mills

– Increase hydraulic

pressure in earlier

mills

24

Energy absorbed (kW.h/t)

0 2 4 6 8 10 12 14 16

Po

l in

op

en

ce

lls (

%)

65

70

75

80

85

90

95

CRICOS No. 00213J


How to change bagasse

softness?• Change added water

temperature

• Higher temperature

softens the fibres

25

Juice temperature difference (°C)

0 5 10 15 20 25Com

paction d

iffe

rence (

kg/m

3)

10

20

30

40

50

60

CRICOS No. 00213J


The real extraction process

Feed to mill

Escribed volume

Delivery bagasse

Expressed juice

Reabsorption

CRICOS No. 00213J


Increasing extraction

approach 2

• Based on volumetric

theory

• Reduce reabsorption

factor (k)

• Volume of bagasse

(based on fibre rate

and delivery bagasse

constituents) larger

than escribed volume E

B

V

Vk

Feed to mill

Escribed volume

Delivery bagasse

Expressed juice

Reabsorption

CRICOS No. 00213J


Understanding reabsorption

• Strongly dependent on definition of

escribed volume

– Definition of work opening W

– Definition of surface speed S

CRICOS No. 00213J


Definition of work opening

• Defined half way into

grooves

• Assumes:

– Tip area inside work

opening space where

bagasse cannot fill is same

as groove area outside work

opening space where

bagasse can fill

– Groove is filled with bagasse

Wo

rk o

pe

nin

g

29

CRICOS No. 00213J


Definition of surface speed

• Also defined half way

into grooves

• Defined for top roll

only

30

Wo

rk o

pe

nin

g

Surface speed definition

CRICOS No. 00213J


For reabsorption to occur…

• Need to have more bagasse passing through nip

than indicated by escribed volume definition

– Consider fibre and juice

31

CRICOS No. 00213J


Possible mechanisms of

reabsorption• Juice flowing forward through or around

grooves

• Bagasse slipping forward between rolls

– Insufficient roll roughness to grip bagasse

• Bagasse extruding forward due to internal

shear

CRICOS No. 00213J


Juice flow over rolls

• Particular problem for

four-roll mills

33

CRICOS No. 00213J


Juice suction system

• Option to extract juice

from behind the top roll

before it flows over the

top

• Four-roll mill experiment

found juice suction

reduced reabsorption

factor by 5%

34

CRICOS No. 00213J


Lotus rolls

• Significant increase in

popularity in last few

years

• Ability to draw in juice

and prevent it from

flowing over top roll

• Good option for four-

roll mills

35

CRICOS No. 00213J


Bagasse slipping

forward• Tests have

shown that

a rough

sandpaper

(P24) is

sufficient

for

bagasse

to grip

36

CRICOS No. 00213J


Bagasse

shearing• Stress/strain

behaviour of bagasse

is complex

• Finite element

analysis modelling

has provided some

insights of stress

states of bagasse in

mills

37

0

50

100

150

200

250

300

350

400

450

0.00 0.05 0.10 0.15 0.20 0.25

She

ar s

tre

ss (

kPa)

Shear strain

Measurements

Predictions

2.745

2.750

2.755

2.760

2.765

2.770

2.775

2.780

2.785

2.790

2.795

0.00 0.05 0.10 0.15 0.20 0.25

Spe

cifi

c vo

lum

e

Shear strain

Measurements

Predictions

Shear strain

Shear

str

ess

Specific

volu

me

CRICOS No. 00213J


Other methods of controlling

reabsorption• There are few conclusive methods for

reducing reabsorption

• Some options that may help

– Groove profile

– Roll roughness

– Mill ratio

– Trash plate design and setting

– Mill control

CRICOS No. 00213J


Groove profile

• Conducted compression

tests with a range of

groove profiles

– Pitch

– Angle

• Better extraction with

– Higher pressure

– Larger sample (work

opening)

– Wider angle

39

CRICOS No. 00213J


Why may wider grooves be

better?

Initial state Possible final states

CRICOS No. 00213J


Roll roughness

• One Australian sugar

factory used to use

reabsorption factor

measurements to

determine when to

arc rolls

41

CRICOS No. 00213J


Mill ratio

• Ratio of escribed volumes

of feed to delivery

• Low mill ratio expresses

more juice in feed nip so

cannot be reabsorbed in

delivery nip

• Low mill ratio reduces load

that can be applied at

delivery nip and so reduces

delivery escribed volume

42

Feed

Delivery

CRICOS No. 00213J


Mill ratio

• Some Australian mills use reabsorption

factor as a guide for adjusting settings

• High reabsorption factor

– Reduce feed nip setting

• Low reabsorption factor

– Reduce delivery nip setting

43

CRICOS No. 00213J


Trash plate design and

setting• Expect that trash plate

design and setting will

affect reabsorption factor

• No real evidence to

suggest how

• Wide range of designs and

settings that seem to

function

44

Trash plate

CRICOS No. 00213J


Mill control

• Good mill

control can

lead to lower

reabsorption

factors

45

Time

04:00:00 08:00:00 12:00:00

Ch

ute

le

ve

l (%

)

0

10

20

30

40

50

60

70



Power consumption

46

CRICOS No. 00213J


Reducing power consumption

• From roll load theory

𝑅 ∝ 𝐿𝐷 𝐶𝐹 − 0.1

• R Roll load

• L Roll length

• D Roll diameter

• 𝐶𝐹 Filling ratio (compaction)

• Power

consumed

between pairs

of rolls

• Power

consumed

when pushing

bagasse

across plates

CRICOS No. 00213J


Power consumption in a four-

roll mill• Not much can be done to reduce power

consumption in a four-roll mill without

affecting performance

• Two-roll mill designs have the advantage

of no trash plate

– Generally lower capacity

51

CRICOS No. 00213J


Trash plate effect

• From studies of mills with

independent drives, torque

on feed roll is of similar

magnitude to torque on

delivery roll, even though

compaction is a lot lower

• Hypothesis that feed roll

pushes bagasse across trash

plate

CRICOS No. 00213J


Power consumption

innovations• More efficient drives

– Low efficiency steam

turbines replaced by

higher efficiency

electric motors

• Hydraulic transmissions

• Variable speed drives

and mechanical

transmissions

• Independent drives

– Pinionless mills

CRICOS No. 00213J


Power consumption

innovations• More efficient mill

couplings

• Rope couplings

reported to reduce

power consumption

by 8% compared to

tailbar couplings

Image and results from Sundaram et al (2016)

best milling practices in feeding, extraction and power ...roll surface drainage • if juice cannot...

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