CHBE 401 Mechanical Pulping Screening Introduction
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1 CHBE 401 Mechanical Pulping Screening Introduction James A. Olson Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia Introduction Pulp screens are essential for contaminant removal and fractionation Cylinder and rotor are the key performance components Relevance
Transcript of CHBE 401 Mechanical Pulping Screening Introduction
1
CHBE 401 Mechanical PulpingScreening Introduction
James A. Olson
Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia
Introduction
Pulp screens are
essential for contaminant
removal and fractionation
Cylinder and rotor are
the key performance
components
Relevance
2
What we cover today
Develop a mathematical model of a single screen
Calculate the volumetric, mass and contaminant flow
through a screening system and a system of screens
Power consumption of a screen rotor
Development of a novel foil rotor
Objectives of Pulp Screening
Contaminant removal
Fibre fractionation
Deflocculation
Protection
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Contaminants
Shives are unpulped pieces of wood
Most common contaminant
Weakens paper, leads to machine
breaks
Source of visible dirt
Rougher surface
Returned to digester or sent to reject
refiner
Contaminants
Sclereids are inner bark cell clusters dense highly lignified show up as windows in
calendered sheets
Strings fibre bundles produced in the
process similar problems as shives
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Contaminants
Plastic cause streaks and cuts
during coating typical polyethylene,
SG < 1.0
Fibre fractionation remove coarse and long
fibres for further treatment
TMP Screening and Cleaning System
PrimaryRefiners
To Papermachine
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Pressure Screen Equipment
Parts of a Pressure Screen
Pulp and contaminants enter through feed port
Fibres pass through screen plate, shives retained by screen plate
Rotor produces pressure pulses to back flushes the apertures
Clean pulp exits through accept Contaminants exit via reject port Dilution counters reject
thickening Rock trap
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Cylinder Types
DR
ILLE
D
MA
CR
OF
LOW
WE
DG
EW
IRE
SLO
TT
ED
Holed cylinders 1.0-2.5 mm diameter holes 15-25 % open area probability screens durable, 20 year old technology
Slotted cylinders 0.1 mm to 0.5 mm wide slots 3-5 % open area barrier /probability screens always contoured Milled
• Cant make below 0.2 mm wide
Wedge wire (Welded wires) MacroFlow (constructed)
Slotted Screen Plate Manufacturing
Wire cylinders
Continuous slots increase open area
Higher capacity (shape and open
area)
Most expensive
Early plates were weaker
construction, susceptible to fatigue
failure and had questionable
tolerance … no longer true.
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Screen Plate Contours
Contours protrusion on feed side of
cylinder increase turbulence redirect flow into aperture increased barrier screening reduced probability
screening fractionation and reject thickening
Wire shapes
AFT has a range of ProfileTM
selections to suit a range of conditions.
Deeper contours tend to provide better capacities and reduced thickening - but at the expense of efficiency.
Broad Wire/Contour Selection
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Contoured Screen Surface
CFD simulation of flow
over surface
Contour height
Decreases Thickening
Decreases Efficiency
Increases Capacity
Slot tolerance is critical to contaminant removal and screen capacity.
Slot tolerance
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Slot tolerance specifications:
90% of slots are within +0.001” of average width
100% of slots are within +0.002” of average width
Average of slot widths are within +0.0008” of the nominal slot width.
Slot tolerance
Rotors
Produce pressure pulses to
backflush pulp accumulations
from the screen plate apertures
Induces high tangential fluid
velocity at screen surface
Can be foils or bumps … many
types available…
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Suction / cleansing action of rotor
Pressure pulse
What controls the passage of fibres through the screen plate?
0.2
0.04
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Barrier Screening
Probability Screening
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Fibre Trapping
Wall Effect
CONCENTRATIONGRADIENT
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Turning Effect
Mathematical Analysis of Screens and Screening Systems
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Pressure Screen Analysis
Develop a mathematical description of screens and screening systems
Allows us to engineer the process How?
Quantify probability of passage by a Passage ratio Derive performance equation for consistency, contaminant
efficiency and fractionation in terms of passage ratio and volumetric reject ratio
Derive equation for passage ratio in terms of fibre length, slot velocity and slot width
Calculate efficiency of screening system
Introduce fractionation simulation and optimization as a pulp processing design tool
Definitions for a single screen
Volumetric reject ratio
Rv = Qreject / Qfeed
Mass reject ratio
Rm = Creject Qreject / (Cfeed Qfeed)
Reject thickening factor
T = Creject / Cfeed
Shive removal efficiency
Es = Sreject Qreject / ( Sfeed Qfeed)
Long fibre removal efficiency
Q: Volumetric flow rate
C: Consistency
S: Concentration of shives
FEED ACCEPT
REJECT
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Passage Ratio
Quantifies the ability of
fibres to pass through a
single screen aperture
VS
VU
PC
CS
U
sC
uC
dQ C Pp
QC
(Q-dQ) (C-dC)
QC = CPp dQ + (Q-dQ)(C-dC)
Flow Model in Screening zone
Assume:
passage ratio constant
no axial mixing (plug flow)
perfect radial mixing
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T = Rv
Pp-1
CONSISTENCY CHANGES
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Efficiency of a single screen
Efficiency of a single screen
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Efficiency of a single screen
Efficiency: Barrier and Probability
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What factors affect passage ratio?
Aperture type (holes or slots)
Size of aperture
Fibre length / contaminant size
Fluid velocity through aperture
Rotor tip velocity
Contour type
Rotor type
?
Which ones are the most important?
Fibre length, L
Slot width, W
Aperture Velocity, Vs
Rotor speed, Vt
Dimensional analysis might suggest that
LV
WVPe
t
s
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Penetration number analysis:
Penetration number analysis:
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System efficiency - Definitions
System efficiency - Analysis
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System efficiency - Analysis
System efficiency - Analysis
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System efficiency - Analysis
System efficiency - Analysis
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System efficiency - Analysis
Power requirements
How much power is
required?
What are the key
variables?
D
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Dimensional analysis
Power Coefficient,
Reynolds Number,
Capacity coefficient,
Reject Ratio, Rv= Qr/Qf
2353 DV
P
D
PC
t
p
a
t
a
DVD
2
Re
23 DV
Q
D
QC
t
ffq
3 2Re, ,q v
T
Pfn C R
v D
Experimental results
3 different screen sizes
Dimensional form
0
10
20
30
40
50
60
70
15 17 19 21 23 25 27 29 31 33
Tip Speed (m/s)
Po
wer
(kW
)
M200 GHC
M400 GlHC
M800 GHC
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Experimental results
Non Dimensional
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1.5E+07 1.7E+07 1.9E+07 2.1E+07 2.3E+07 2.5E+07 2.7E+07 2.9E+07 3.1E+07 3.3E+07 3.5E+07
Reynolds No., Re
Po
wer
Co
effi
cien
t, C
p
M200 GHC
M400 GHC
M800 GHC
Effect of element geometry
0
10
20
30
40
50
60
70
80
15 17 19 21 23 25 27 29 31 33
Tip Speed (m/s)
Po
wer
(kW
)
LR
GHC
XR
GHC XRLR
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Effect of element geometry (non Dim)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1.4E+07 1.6E+07 1.8E+07 2.0E+07 2.2E+07 2.4E+07 2.6E+07
Reynolds No, Re
Po
wer
Co
effi
cien
t, C
p
LR
GHC
XR
Effect of Feed Flow Rate
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Capacity Coefficient, Cq
Po
wer
Co
effi
cien
t, C
p
LR
GHC
XR
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Effect of Rv
0.0
0.5
1.0
1.5
2.0
2.5
0.00 0.05 0.10 0.15 0.20 0.25
Capacity Coefficient, Cq
Po
wer
Co
effi
cien
t, C
p
GHC Const Rv
GHC Const Qa
Power conclusions
1.
2.
Depends on element shape
3. Little dependence on Rv
fP Q
3 2tP V D
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Computational Fluid Dynamics
The foil shape was designed using
Computational Fluid Dynamics (CFD)
Divide domain into fine grid
Solve fundamental equations for pressure,
velocity and turbulence
Obtain theoretical calculations for flow and
pressure on the cylinder
Examine large number of design variables
(shape, angle, etc.)
( d)
High Performance Rotor Design
Rotors generate pulsations to clear screen
cylinder apertures
Optimal pulsations ensure high efficiency,
capacity and runnability
There are more than 200 screens in the
province
200 HP motors
Consumes ~ 180 GW h / yr
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CFD Results
CFD Results
Optimal pulse for 5 Deg angle of attack:
Strong negative pulse No positive pulse
212
p
t
PC
V
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Prototype Pilot Trials
Experimental conditions:
Ahlstrom F1 Screen
Cylinder: 0.1mm MF1232
Rotors:
Prototype EP (60 mm Foil)
Prototype EP (130 mm Foil)
Gladiator HC (GHC – Solid core)
Andritz VF
De-ink market pulp
1.4% consistency
Pilot Trial Results
0
10
20
30
40
50
60
0 5 10 15 20 25 30Tip Speed (m/s)
Po
wer
(H
P)
VF
GHC
EPL
EPS
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Pilot Trial Results
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60
Power (HP)
Max
imu
m C
apac
ity
(t/d
)Pump Maxmum
Commerical foil rotor
EP - Foil rotor
GHC - Solid core rotor
Current BC Mill Trials
BC Hydro / Canfor / AFT demonstration project to
conduct 2 mill trials
Northwood: softwood kraft mill
GHC solid core rotor
Replace conventional “Stingray” rotor
TMP mill:
EP foil rotor
Jan. 2006
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Canfor-Northwood SW Kraft Trial
0
20
40
60
80
100
120
140
18 20 22 24 26 28 30
Tip Speed (m/s)
Po
we
r (k
W)
GHC
Stingray
52% Energy Savings
Canfor-Northwood SW Kraft Trial
50
55
60
65
70
75
80
85
90
95
100
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
Slot Velocity (m/s)
De
bri
s R
em
ov
al E
ffic
ien
cy
(%
)
GHC (24 m/s)
Stingray (29 m/s)
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Conclusions Two new low energy rotors have been developed
Performance has been demonstrated in computational, laboratory,
pilot and mill trials
GHC - Solid core
52% energy savings
Maintained high efficiency and runnability
EP – Foil rotor
Potential for 80% energy savings
Increased efficiency and runnability
The end
