Understanding Rapid Dewatering of Cellulose Fibre Suspensions
1. Understanding Rapid Dewatering of
Cellulose Fibre Suspensions
Daniel Paterson
MASc. Thesis Defense
Mechanical Engineering
April 18, 2016
2. Outline
• Introduction to Industrial Problem
• Background: Past Dewatering Modeling Efforts
• Problem: Past Methods
• Project Objectives
• Part I: Extending the Modeling Approach
• Part II/III: Determining Material Parameters
• Part IV: Modeling Dewatering Trends
• Part V: Validating Dewatering Models
• Conclusion
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3. Introduction
• Dewatering: Important unit process in industry
• Pulp and paper, ceramics, mining, etc.
• Examples in pulp and paper:
• Paper machine: Forming and pressing sections
• Thickeners, screw presses, wash presses
• Twin roll presses
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4. • Understand dewatering in twin press rolls
• Used to optimize design
• Focused on “nip point”
• Modelled as 1D, constant dewatering rate consolidation
4(Hi – Hf) << L
5. Background
• Geometry
• Permeable piston at 𝑧 = ℎ 𝑡
• Closed base at 𝑧 = 0
• Compressive load: 𝜎 𝑡
• We want to model varying
dewatering rates
• Varying
𝑑ℎ 𝑡
𝑑 𝑡
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6. • Modeling Approach
• Follow work of Landman, Buscall, and White [1].
• Assumptions:
• Neglect gravity, inertial, and viscous terms
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(1)
(2)
(3)
(4)
(5)
Solid
Continuity
Fluid
Continuity
Darcian
Expression
Total Compressive
Stress Conservation
Constitutive
Equation
7. • Governing Equation (Base Model)
• What is needed?
• Permeability:
• Compressive Yield Stress:
7
`
8. • Material Parameters
• Compressive Yield Stress : The maximum
network stress (effective solid stress) that can be
withstood without solid network consolidation.
• Increasing function with (solidity)
• Permeability : A measurement of the resistance
fluid flow experiences flowing through a porous media
• Decreasing function with
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10. Problem: Preliminary Results
• Source?
• Suggested in the literature to be due to cellulose
fibres porous, hollow structure [2]
• Viscous component to compaction due to fluid
escaping fibres
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Nylon Fibres Cellulose Fibres
11. • Expand the modeling efforts of Landman, Buscall, and
White
• In conjunction with math postdoc
• Develop equipment and protocol for collecting material
parameters
• Compressive Yield Stress
• Permeability
• Validate base model and test suitability of extended model
for various suspensions of cellulose fibres
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Project Objectives
12. • Suggested in the literature the
source of discrepancy comes
from porous cellulose fibres
• Consolidation:
• Dynamic Compressibility:
12
= 0
Base Model
Instantaneous
particle compaction
Base Model
Remove this assumption
Part I: Extending the Modeling Approach
13. • Functional Form
• Proportional to fibre wall
permeability?
• Proposed Functional Form:
• Crude model of flow out of a porous
fibre to find fibre permeability:
13
L
2r
Suggested Form Only!
14. • Governing Equation (Extended Model)
• What is needed?
• Permeability:
• Compressive Yield Stress:
• Fitted Parameter: 14
15. • Compressive Yield Stress: The maximum network
stress (effective solid stress) that can be withstood without
consolidation
• Techniques:
Permeation Trials
• Approximation neglecting
flow induced compaction:
• Control error
Part II: Material Parameter
Slow Speed Compaction
• Simplified continuity:
• Uniform consolidation
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20. Account: Fit a Functional Form
• Model equations
• Fit functional form
• Permeability: A measurement of the resistance fluid flow
experiences flowing through a porous media
• Concern: Flow induced compaction
Part III: Material Parameter
Neglect: Manage Error
• Evaluate at
• Control error
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24. • Experimental dewatering trends collected for varying rates
• 0.001 – 10 mm/s
• Load versus solid volume fraction trends
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Part IV: Model Dewatering Trends
25. • Nylon Fibre Experimental Results:
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• Discussion
• Increased difficulty in dewatering with higher rates
• Initial load growth with high dewatering rates
26. • Nylon Fibre Model Results:
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Base Model
Experiment
• Discussion
• Predictive trends (no free parameters)
• Base model works well with “solid” nylon fibres
27. • Discussion
• Increased difficulty in dewatering with higher rates
• Dewatering curves do not trend back to
• NBSK (Series 1) Experimental Results:
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28. • NBSK (Series 1) Model Results:
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Base Model
Extended Model
Experiment
• Discussion
• Extended model trends fitted
• Improved with extended model
29. • Investigate models effectiveness in representing solid
phase movement during consolidation
• Nylon Fibres Base Model
• NBSK (Series 1) Extended Model
• Film dewatering events to develop velocity profiles
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Part V: Validating Models
30. 30
Experimental Base Model
3.0 mm/s
0.25 mm/s
• Nylon Fibre Velocity Profiles:
• Discussion
• 0.25 mm/s: Closer to linear, small solidity gradients
• 3.0 mm/s: Nonlinear velocity, large solidity gradients
• Base models provides good representation
31. • NBSK (Series 1) Velocity Profiles:
• Discussion
• Both velocities quite linear, small solidity gradients
• Base model provides poor representation
• Extended model provides improved representation
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Experimental Base Model Extended Model
10.0 mm/s
1.5 mm/s
32. • Equipment and experimental protocol developed for
collecting material parameters and
• Extended model provided improved representation of
cellulose fibre dewatering over the base model
• Acceptable form of
• Base model provided good representation of nylon fibre
suspension
• Constitutive function is well suited to hard particles
• Both models represented their corresponding suspensions
well in capturing the movement of the solid particles
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Conclusions
36. • Further cellulose trials
• Assess the continued suitability of dynamic
compressibility function:
• Continue cataloging dewatering behaviours of various
cellulose fibre suspensions
• Investigate a few experimental concerns
• Temperature impact on compressed cellulose fibres
• Retention challenges: TMP
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Future Work
