Continuous Digester Models
Kinetic model
Hydraulic model
Continuous Digester Model
1
Continuous Digester Models
The problem
• As chips descend to the bottom
of the digester they get softer
with cooking. Forces on chips
cause them to conform
dramatically reducing void
fraction ε in the bed.
• The void volume in the chip
doesn’t appear to change much.
Pores in the chip don’t collapse.
Chip Flow
2
Continuous Digester Models
• As ε (void fraction in the bed) is reduced, the
residence time in that zone increases and the flow
of liquor is reduced.
» ε can be reduced to a level such that the digester is
plugged.
3
Continuous Digester Models
• Fundamental work on Kamyr hydraulics has been
done by Harkonen.
•Chips at various kappa
numbers can be placed in
apparatus
•Chips in digester can be
compressed
•Void fraction in bed is
measured
•Flow rate through the bed is
measured
4
Continuous Digester Models:
Compressibility
 v  0.644  ( p / 10 )
4 0.59
(void fraction in
chip bed)
Pressure
(0.831  0.139 ln k )
Kappa
5
Continuous Digester Models:
Flow Resistance
p
2
2
6
2
 4600 ( c /  v )V  3.9 *10 ( c /  v )V
l
Where:
Pressure drop
per unit length
εc = volume fraction of the chip
V = velocity of liquor
εc + εv = 1
6
Continuous Digester Models
• Combine experimental results with:
» Mass balance
» Force (momentum) balance
» Heat balance
• Predict chip pressure, Σ of all forces on chip
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Continuous Digester Models
Unfavorable
conditions if
chip pressure
is too high or
low
• Chip pressure too large then
the digester will plug (no
flow)
• Chip pressure < 0 then the
digester will hang
8
Models
Results
A: Chip pressure
in meters water
column
B: Chip volume
fraction in
percent
9
Models
Results
A: Liquor
Velocity in mm/s
B: Chip velocity
in mm/s
C: Relative
velocity
difference
between chip and
liquor in mm/s
10
Models
Results
A: Chip
temperature in °C
B: Cumulative
H-factor
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