Continuous Digester Models Kinetic model Hydraulic model Continuous Digester Model

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

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.

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

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

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

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

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

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

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

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