1. Continuous Digester Models
Kinetic model
Hydraulic model
Continuous Digester Model

2. 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

3. 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.

4. 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

5. Continuous Digester Models: Compressibility
(void fraction in chip bed)
Pressure
Kappa

6. Continuous Digester Models: Flow Resistance
Where:
εc = volume fraction of the chip
V = velocity of liquor
εc + εv = 1
Pressure drop per unit length

7. Continuous Digester Models
Combine experimental results with:
Mass balance
Force (momentum) balance
Heat balance
Predict chip pressure, Σ of all forces on chip

8. Continuous Digester Models
Chip pressure too large then the digester will plug (no flow)
Chip pressure < 0 then the digester will hang
Unfavorable conditions if chip pressure is too high or low

9. Models Results A: Chip pressure in meters water column
B: Chip volume fraction in percent

10. 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

11. Models Results
A: Chip temperature in °C
B: Cumulative H-factor