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MODELLING OF KRAFT TWO-PHASE DIGESTER PULP PROCESSES. P. He M. Salcudean I. Gartshore E.L. Bibeau*. *. Engineering / Process & Product Quality Conference & Trade Fair September 12-16, 1999 Hilton Anaheim. OUTLINE. Introduction Continuos Kamyr Digester Previous numerical models

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slide1

MODELLING OF KRAFT TWO-PHASE DIGESTER PULP PROCESSES

P. He

M. Salcudean

I. Gartshore

E.L. Bibeau*

*

Engineering / Process & Product Quality Conference & Trade FairSeptember 12-16, 1999Hilton Anaheim

outline
OUTLINE
  • Introduction
    • Continuos Kamyr Digester
  • Previous numerical models
  • New process model
  • Simulation results
  • Conclusions
slide3

CONTINUOUS DIGESTER

Chips and

Chips

Steam

Liquor

Impregnation

Chips

Zone

Silo

Steam

Liquor

s

n

Cooking

e

e

Zone

r

c

S

White Liquor

High

Liquor

Pressure

Washing

Feeder

Zone

Blow Line

digester
DIGESTER
  • Wood
    • Lignin (glue that binds fibres)
    • Carbohydrates
      • cellulose and hemi-cellulose
  • White liquor
    • NaOH and Na2S
  • Weak black liquor
    • Contains the degradation products
  • Wash flow
digester5
DIGESTER
  • Wood
    • Lignin (glue that binds fibres)
    • Carbohydrates
      • cellulose and hemi-cellulose
  • White liquor
    • NaOH and Na2S
  • Weak black liquor
    • Contains the degradation products
  • Wash flow
digesters modelling
DIGESTERS MODELLING

OBJECTIVES

  • Model the delignification processes occurring within a digester
  • Calculation of solid and liquid conservation equations for simulating the dynamic industrial pulping process in a digester
why digester model
WHY DIGESTER MODEL
  • Huge capital investment for mill
    • Heart of Kraft operation
  • Improve ROR
    • Fluid mechanics
      • Fibre property variations
    • High corrosion rates
    • Improve process control and knowledge
    • Enhance equipment performance and reliability
    • Reduce risk in capital expenditures
previous models
PREVIOUS MODELS
  • 1-D Block Flow “0 order models”
    • Control of Kappa number (lignin concentration)
  • 1-D Models
    • Model free liquor + impregnated chips
    • Simple assumptions for flow
  • One 2-D model (Harkonen)
    • No convection terms; partial diffusion terms
    • Irrotational flow
    • Interaction between phases: Ergun’s equation
previous models9
PREVIOUS MODELS
  • Delignification models (NaOH + Na2S)
    • 3 stages: initial, bulk, residual
    • Model different wood components
      • Lignin and carbohydrates
      • Low/high reactive lignin, cellulose, 2 components of hemi cellulose
  • Diffusion process
difficulties
DIFFICULTIES
  • Complex 3-D two-phase flow with high solid fraction
  • Some constitutive relationships difficult to obtain
  • Variation of solid pressure with solid fraction and kappa number
  • Tangential stress in both phases
  • Reaction rates and diffusion characteristics
  • Poor characterisation and variability of chip
ubc digester model
UBC DIGESTER MODEL

MODEL CHARACTERISTICS

  • 3-D and two-phase solid-liquid flow
    • two-phase treatment implemented
  • Curvilinear grids with segmentation
  • Quasi-steady mass, chemical reaction, and heat transfer calculation
    • option to expand to transient
  • Liquid-solid flow model coupled with the energy and conservation of species equations
ubc digesters model
UBC DIGESTERS MODEL

MODEL CHARACTERISTICS

  • Both free and entrapped liquor
    • mass exchange accounted
  • Energy equation
    • includes heat generation through chemical reactions
  • Transport equations for
    • lignin and carbohydrates
model assumptions
MODEL ASSUMPTIONS
  • Flow is two-phase, solid-liquid, 3D quasi-steady flow

Volume fractions of solid

Volume fraction of liquid

Solid pressure

  • The liquor occurs in two forms
    • free liquor
    • liquor entrapped into the chips

Volume fraction of liquid in a chip

Volume fraction of wood material in a chip

digester model
DIGESTER MODEL
  • MASS CONSERVATION EQUATIONS
  • MOMENTUM EQUATIONS

Liquid

Solid

Liquid

Phase momentum

exchange by

diffusion

Stress

tensor

Convection phase

momentum exchange

Body

force

Convection

Solid

digester model15
DIGESTER MODEL
  • Sample Equations

Liquid Momentum Equation

Solid Pressure Equation

digester model16
DIGESTER MODEL
  • ENERGY BALANCE

Liquid

Solid

 = thermal conductivity  hr heat of reaction

r = reaction rate k = heat conduction

c = heat capacity Subscript w indicates the side walls

digester pulping model
DIGESTER PULPING MODEL
  • COMPONENT BALANCES

where

Ds = diffusion coefficient [OH] = concentration of effective alkali

Initial:

Bulk:

Residual:

digester pulping model18
DIGESTER PULPING MODEL
  • DELIGNIFICATION REACTION RATES

Stage

Rlig

Rcarb

Initial

Bulk

Residual

= lignin content = carbohydrate content

= sulfide sulfur content [S] = concentration of sulfide sulfur

numerical scheme
NUMERICAL SCHEME
  • Finite volume
  • Curvilinear grids
  • Block structured
  • Second order accurate for cross derivative terms
  • Steady and unsteady
digester model history
DIGESTER MODEL HISTORY
  • Year 1
    • Define fundamental 3-D two-phase equations
  • Year 2
    • Implement model
      • Liquid flow
      • Solid flow
      • Energy
      • Kraft pulping process
    • Starting point
      • UBC 3-D curvilinear flow solver
digester model history21
DIGESTER MODEL HISTORY
  • Current Year
    • Validation
      • experimental digesters
      • in-situe measurements
      • compare to other numerical predictions
    • Investigate first commercial digester
    • Add sub-models
      • discharge pipe, rake, chip level
    • Constitutive relationships
      • solid shear
      • compaction
conclusions
CONCLUSIONS
  • 3-D digester model
    • fairly advanced state of development
    • can predict flow channelling
  • Computations are feasible with ability to capture essential features of the process
  • Effective tool to improve operations, obtain better performance, and address mill issues
  • Need for extensive collaboration with industrial partners and research players to address issues of measurements and validation
slide33

ACKNOWLEDGEMENT

NSERC

Canfor Forest Products

copy of presentation
COPY OF PRESENTATION
  • Copy of results with solid shear and movies
    • Go to www.psl.bc.ca
    • Press on “Public Download” button
    • Go to directory “Anaheim”
    • Download file “Digester.ppt”