Understanding Rapid Dewatering of

Cellulose Fibre Suspensions

Daniel Paterson

MASc. Thesis Defense

Mechanical Engineering

April 18, 2016

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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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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• 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

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 𝑑 ℎ 𝑡

𝑑 𝑡

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• 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

• Governing Equation (Base Model)

• What is needed?

• Permeability:

• Compressive Yield Stress:

7

`

• 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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• Model Solutions

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Slow Compression Fast Compression

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

• 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

• Suggested in the literature the source of discrepancy comes from porous cellulose fibres

• Consolidation:

• Dynamic Compressibility:

12

= 0Base Model

Instantaneous

particle compaction

Base Model

Remove this assumption

Part I: Extending the Modeling Approach

• 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!

• Governing Equation (Extended Model)

• What is needed?

• Permeability:

• Compressive Yield Stress:

• Fitted Parameter: 14

• 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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• Experimental Apparatus

• Movement: 100 mm

• Load: 1.3 MPa

• Rate 0.001 – 10 mm/s

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• Materials

• Varying species, pulping process, refinement, and flocculation state

• Ideal fibre suspension for base model validation

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• Equipment Validation

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: Softwood chemical pulp [2]

: Softwood chemical pulp [3]

: Coal-mining tailing [4]

: Zirconia suspension [5]

: Water treatment sludge [6]

: Alumina suspension [7]

: Series 1 (NBSK) Slow Speed Technique

: Series 1 (NBSK) Permeation Technique

• Representative Results

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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 error20

• Experimental Apparatus

• Movement: 160 mm

• Load: 1.0 MPa

• Pressure: 1.0 MPa

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• Equipment Validation

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: Series 1 (NBSK)

: Softwood chemical pulp [8]

: Softwood chemical pulp [9]

: Softwood chemical pulp [10]

: Nylon fibre suspension [11]

: Nylon fibre suspension [11]

: Glass fibre suspension [11]

: Acrylamide polymer gel [11]

: Lattice-Boltzmann simulation [12]

• Representative Results

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• 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

• Nylon Fibre Experimental Results:

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• Discussion• Increased difficulty in dewatering with higher rates• Initial load growth with high dewatering rates

• 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

• Discussion• Increased difficulty in dewatering with higher rates• Dewatering curves do not trend back to

• NBSK (Series 1) Experimental Results:

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• NBSK (Series 1) Model Results:

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Base Model

Extended Model

Experiment

• Discussion• Extended model trends fitted• Improved with extended model

• 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

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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

• 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

• 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

33

Thank You,Questions

Sponsors

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References

[1]

[2]

[3]

[4]

[5]

[6]

35

[7]

[8]

[9]

[10]

[11]

[12]

• 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 36

Future Work

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Functional form of Dynamic Compressibility

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