The Bleaching of Pulp, 5th Edition
By:Peter W. Hart
Alan W. Rudie
Copyright© 2012 by
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ISBN: 978-1-59510-207-2
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Table of Contents
i. Authors List
ii. Table of Contents
1. Introduction
1.1
1.2.
1.3.
1.4.
Previous Pulp Bleaching Books
Current Pulp Bleaching Book
Organization of the Current Book
References Cited
2. Oxygen Delignification
2.1. Introduction
2.1.1. Comparison of Oxygen to Other Bleaching Agents
2.1.2. Advantages and Disadvantages of Oxygen Delignification
2.2. Chemistry of Oxygen Delignification
2.2.1. Lignin Reactions
2.2.2. Carbohydrate Reactions
2.3. Kinetics of Oxygen Delignification
2.3.1. Kinetic Rate Equations
2.3.1.1. Initial Kappa Number
2.3.1.2. Alkali Charge, Temperature and Oxygen Partial Pressure
2.3.1.3. Rate Equations
2.3.2. Carbohydrate Selectivity
2.4. Mass Transfer Effects
2.4.1. Oxygen Solubility
2.4.2. Liquid Phase Mass Transfer Coefficients for Mixers
2.4.3. Mass Transfer Coefficient in Retention Towers
2.5. Carry-over of Dissolved Solids
2.6. Control of Transition Metals
2.6.1. Addition of Magnesium Ion
2.6.2. Use of Chelating Agents
2.7. Commercial Medium Consistency Oxygen Delignification Systems
2.7.1. One-Stage Design
2.7.2. Two-Stage Designs
2.7.2.1. OxyTracTM Systems
2.7.2.2. GL&V System
2.7.3. Reductions in Kappa Number
2.8. Pulp Quality
2.9. Emission of Volatile Organic Compounds
2.10. Acknowledgments
2.11. References Cited
3. Chlorine Dioxide as a Delignifying Agent
3.1. Introduction
3.2. Delignification Chemistry
3.3. Standard D0 Stage Conditions
3.3.1.
3.3.2.
3.3.3.
3.3.4.
3.3.5.
3.3.6.
3.4.
3.5.
3.6.
3.7.
Furnish and Effect of Cooking Conditions
Chemical Charge
pH
Time and Temperature
Consistency
Carryover from Brownstock Washing
Pulp Quality - Viscosity and Strength
Summary
Acknowledgements
References Cited
4. Extraction and Oxidative Extraction
4.1. Introduction
4.2. General Overview of Alkaline Extraction
4.3. Extraction Delignification Process Variables for ECF Sequences
4.3.1.
4.3.2.
4.3.3.
4.3.4.
4.3.5.
4.3.6.
4.3.7.
4.4.
4.5.
4.6.
4.7.
4.8.
Chlorine Dioxide Delignification (D0) Chemical Charge
Caustic Charge and pH
Oxygen and Peroxide Reinforcement
Reaction Temperature
Retention Time
Consistency
Carryover
Extraction Delignification for TCF Sequences
Pulp Quality
Alternative Alkali Sources for Extraction
Second Extraction Stage
References Cited
5. The Hot Acid Stage for Hexenuronic Acid Removal
5.1. Hexenuronic Acids
5.1.1. Introduction
5.1.2. Formation and degradation during alkaline cooking
5.1.2.1. Effect of wood 4-O-methyl-α-D-glucuronic acid content
5.1.2.2. Effect of cooking technology
5.1.2.3. Effect of cooking conditions
5.1.2.4. Effect of delignification degree
5.1.3. Quantification in alkaline pulps
5.1.4. Impact on oxygen delignification
5.2. The Hot Acid Stage (A-stage)
5.2.1. Introduction
5.2.2. Chemistry
5.2.3. Kinetics
5.2.4. Effect of reaction time/temperature and pH
5.2.5. Effect on pulp constituents, yield and viscosity
5.2.7. Effect on pulp strength properties
5.2.8. Effect on pulp brightness stability
5.2.9. Effect on effluent load and treatability
5.2.10. Alternate methods for HexA removal
5.3. Hot acid stage technologies
5.3.1. Standalone A-stage
5.3.2. A/D technology
5.3.3. D/A technology
5.3.4. A/D versus D/A technologies
5.4. Industrial bleaching sequences with an A-stage
5.5. Acknowledgements
5.6. References Cited
6. The Use of Enzymes in Pulp Bleaching
6.1. The Use of Enzymes in Pulp Bleaching
6.1.1. Introduction
6.1.2. Enzyme types
6.1.3. Enzyme impacts upon mill operations and pulp yield
6.2. Application Criteria
6.2.1. Introduction
6.2.2. Temperature control
6.2.3. pH Control
6.2.3.1. General concerns
6.2.3.2. Use of mineral acids for pH control
6.2.3.3. Determining Acid demand
6.2.3.4. pH drift
6.2.3.5. Use of carbon dioxide for pH control
6.2.4. Mixing
6.2.4.1. Mixing Challenges
6.2.4.2. Determining mixing efficiency
6.2.4.3. Demonstration of mixing using showers on a washer
6.2.4.3. Mixing summary.
6.2.5. Retention time
6.2.5.1. Impact of time
6.2.5.2. Channeling
6.2.6. Application summary.
6.3. Converting Enzyme Performance to Benefits
6.3.1.
6.3.2.
6.3.3.
6.3.4.
Mill specific options
Change in fiber bleachability
Yield considerations
Economics
6.4. References Cited
7. Mineral Scale Management
7.1 Introduction
7.1.1 Trace Metals in Wood and Pulping
7.1.2 Generalized behavior of metals
7.1.3 Calcium
7.1.4 Oxalic Acid
7.1.5 Barium
7.2 Chemical Fundamentals
7.2.1 Acid and Base Equilibria
7.2.2 Precipitation Equilibria (Solubility Product)
7.2.3 Ion Activity
7.2.4 Supersaturation
7.3 Case Studies:
7.3.1 Digester (White Liquor) Chip Strainer
7.3.2 Lime Scale on Extraction stage washer
7.3.3 Barite scale in D0
7.3.4 Oxalate scale on D0 washer
7.3.5 Oxalate scale in an Extraction stage mixer
7.3.6 Barite scale in D0
7.4 Calculating the Trace Metals Partition in a Mill Environment
7.4.1 Ion Exchange using Solution State Equilibrium theory
7.4.2 Donnan Theory
7.4.3 Selectivity Coefficient Approach
7.4.4 Fundamental Approaches
7.4.5 Calculating soluble calcium
7.5 Summary
7.6 References Cited
8. Chlorine Dioxide as a Brightening Agent
8.1. Introduction
8.2. D Stage Conditions
8.2.1. Chemical Charge
8.2.2. Residual chlorine dioxide
8.2.3. pH
8.2.4. Time and Temperature
8.2.5. D Stage Consistency
8.2.6. Carryover to D Stages
8.3. Summary
8.4. Acknowledgements
8.5. References Cited
9. Ozone Delignification
9.1. Introduction
9.2. Fundamental Aspects of Ozone Bleaching
9.2.1 Ozone reactions with lignin and cellulose
9.2.2 Expression of ozone reactivity, effectiveness and selectivity
9.3. Process Conditions
9.3.1 Mass transfer of ozone to liquid phase
9.3.2 Pulp consistency
9.3.3 Ozone Charge
9.3.4 Effect of pH
9.3.5 Time
9.3.6 Temperature
9.3.7 Additives
9.3.8 Metal ions
9.3.9 Dissolved organic matter
9.3.10 Pulp processing before ozone stage.
9.3.11 Alkaline extraction after ozone stages
9.4. Role of Ozone in a Bleaching Sequence
9.4.1 Ozone delignification as a replacement for chlorination
9.4.2 Placement of ozone in a bleaching sequence
9.5. Process Equipment
9.5.1 High-Consistency Ozonation
9.5.2 Medium-Consistency Ozonation
9.5.3 Low-Consistency Ozonation
9.5.4 Materials of Construction
9.6. Environmental Considerations
9.7. References Cited
10. Peroxide Bleaching
10.1. Introduction
10.2. General Overview of Peroxide Bleaching
10.3. Factors Affecting Peroxide Bleaching
10.3.1.
10.3.2.
10.3.4.
10.3.5.
10.3.6.
Peroxide Charge
Caustic Charge and pH
Consistency
Mitigation of Peroxide Decomposition by Transitional Metal Ions
Washer Carryover
10.4. Pulp Viscosity, Strength and Particle Removal
10.5. Brightness Stability
10.6. Peroxide in Bleaching Sequences
10.6.1. ECF
10.6.2. ECF-Light
10.6.3. TCF
10.6.4 Post Bleaching in High Density Storage
10.7. Peroxide Catalysts and Activators
10.8. Peroxy Acids
10.9. Summary
10.10. References Cited
11. Dirt and Shive Management
11.1. Introduction
11.1.1. Types of Dirt
11.2. Dirt and Shive Reduction – Case Studies
11.2.1. Slotted Screens
11.2.2. Oxygen Delignification
11.2.3. Post Bleaching Screens
11.3. Dirt and Shive Measurement
11.3.1. Shives and Large Particles
11.3.2. Dirt measurement
11.4. Bleaching of Dirt and Shives
11.4.1.
11.4.2.
11.4.3.
11.4.4.
11.4.5.
11.4.6.
11.4.7.
Oxygen
Chlorine Dioxide
Mixing
Alkaline Extraction
Peroxide
Ozone
Enzymes
11.5. Summary
11.6. References Cited
12. Multi-Stage Bleach Plant Modeling and Optimization
12.1.
12.2.
12.3.
12.4.
Introduction
General Overview of Bleach Sequence
Modeling of Bleach Sequences
Kinetic Modeling of Bleach Sequences
12.4.1. Modeling Chlorine Dioxide Delignification (D0)
12.4.2. Modeling Caustic Extraction Delignification with Oxidant
Reinforcement
12.4.3. Modeling Chlorine Dioxide Brightening (D1 and D2)
12.4.4. Modeling Dispersion in Bleaching Towers
12.4.5. Modeling Bleach Effluent Characteristics
12.5. Steady-State Modeling of Bleach Sequences
12.5.1. Steady-State Models for D1 and D2
12.5.2. Steady-State Models for D0 and Extraction ((EO), (EP) or (EOP))
12.6. Optimization of Bleach Sequences Based on Models
12.6.1. Optimum Chlorine Dioxide Load Sharing between D1 and D2
12.6.2. Optimum Chlorine Dioxide Load Sharing between Delignification
and Brightening
12.6.2.1. Softwood Pulp
12.6.2.2. Hardwood Pulp
12.6.3. Optimization Involving Oxidative Extraction Variables and Chlorine
Dioxide Usage
12.6.3.1. Softwood Pulp
12.6.3.2. Hardwood Pulp
12.7. Impacts of Washer Carry-Over
12.8. Impact of Kraft Pulping Conditions on Bleaching
12.9. Other Factors Impacting Bleaching Optimization
12.10. Summary
12.11. References Cited
13. Metso Paper – Chemical and Mechanical Pulp Bleaching
Solutions
13.1. Introduction
13.2. Pulp Production
13.2.1. Wood/Chip Handling
13.2.2. Cooking
13.2.3. Deknotting, Screening and Washing
13.3. Bleach Plant Design
13.3.1.
13.3.2.
13.3.3.
13.3.4.
Pumping
Mixing
Reaction
Reactor/ Tower Tank Equipment
13.3.4.1. Reactor Inlet Distribution
13.3.4.2. Tower Dischargers
13.3.4.3. Reactor Blow Tank or Blow Tube
13.3.5. Reactor / Tower / Stock Tank Design
13.3.5.1. Pressurized Towers
13.3.5.2. Atmospheric Towers
13.3.6. Washing
13.3.6.1. TwinRoll Evolution Wash Press
13.3.6.2. The W-Press
13.4. Oxygen Delignification
13.5. Chlorine Dioxide Bleaching
13.6. Extraction
13.7. Peroxide Bleaching
13.7.1. (PO)-bleaching
13.7.2. P-Bleaching
13.8. Ozone Bleaching - The ZeTrac™
13.9. Other Bleaching Methods
13.9.1. Q-stage
13.9.2. Peracid Bleaching
13.9.2. Enzyme Treatment
13.10. Bleaching Sequences
13.10.1. ECF Light Bleaching
13.10.2. TCF Bleaching
13.11. Bleach Plant Closure
13.12. Mechanical Pulp Bleaching
13.12.1. Introduction
13.12.2. Bleach Plant Equipment
13.12.2.1. TwinRollTM Press
13.12.2.2. TwinWire Press
13.12.2.3. RotomixerTM
13.12.3. Overall System Design
14. Andritz Bleaching Technology for Chemical and
Mechanical Pulps
14.1. Chemical pulp bleaching
14.1.1. Background
14.1.2. Chemical Pulp Bleaching sequences
14.1.3. MC equipment in bleaching
14.1.3.1. Andritz MC pump
14.1.3.2. AC chemical mixer
14.1.3.3. AZ Chemical mixer
14.1.3.4. Other MC equipment with mixing
14.1.3.5. ARF and ARD Flow Discharger
14.1.3.6. ATS-MC and ATS-LC top scrapers
14.1.3.7. ADS Discharge Scraper
14.1.4. Reactor and tower technology
14.1.4.1. Pressurized towers
14.1.4.2. Atmospheric towers
14.1.5. Washing technology in bleaching
14.1.5.1. GasFree filter (GF filter)
14.1.5.2. Drum Displacer washer (DD washer)
14.1.5.3. Andritz Wash Press (AWP, AWP-D)
14.1.6. Bleaching stage technology
14.1.6.1. Alkaline bleaching stages
14.1.6.2. Two-stage oxygen
14.1.6.3. Oxygen stage and screenroom
14.1.6.4. Acidic bleaching stages
14.1.6.5. Hexenuronic acid hydrolysis
14.2. Mechanical Pulp Bleaching and Washing
14.2.1. Introduction
14.2.2. Main components
14.2.3. Dewatering of mechanical pulp
14.2.3.1. Dewatering and washing prior to the HC- bleach tower
14.2.3.2. Washing after bleaching
14.2.4. Dewatering equipment
14.2.4.1. Comparison of dewatering machines
14.2.4.2. Twin Wire Press
14.2.4.3. Screw press
14.2.5. High-Consistency Mixer
14.2.6. Bleach towers
14.2.6.1. HC-Tower discharge system
14.2.6.2. MC/LC-Tower discharge system
14.3. References Cited
15. GL&V Modern Bleach Plant Design and Operation
15.1. INTRODUCTION
15.2. PROCESS SELECTION
15.3. Modern ECF Bleach Plant Design and Equipment
15.3.1. Bleach Plant Process Design
15.3.2. DUALOX Oxygen Delignification
15.3.2.1. Introduction and Chemistry
15.3.2.2. Physical Arrangement
15.3.3. Pulp Washing
15.3.4. Pumping and Mixing
15.3.4.1. Medium Consistency pumping
15.3.4.2. Chemical Mixing
15.3.5. Bleaching
15.3.5.1. Introduction and Chemistry
15.3.5.2. First Stage Chlorine Dioxide Bleaching
15.3.5.3. Second Stage Oxidative Extraction
15.3.5.4. Wash Water and Filtrate Recirculation
15.3.6. Plant Design
15.4. Alternative Bleaching Sequences
15.5. References Cited
16. Instrumentation - Bleaching sensors and control
16.1. Introduction
16.2. Sensors
16.2.1. Flow meters
16.2.1.1. Magnetic Flow Meter
16.2.1.2. Vortex Flow Meter
16.2.1.3. Differential pressure type meters
16.2.1.4. Coriolis Mass Flow and Density Meters
16.2.1.5. Thermal Type Flow Meters
16.2.1.6. Doppler Flow Meter
16.2.1.7. Time of Flight Ultrasonic flow meter
16.2.1.8. Sonar Flow Meter
16.2.1.9. Turbine Flow Meters
16.2.1.10. Positive Displacement Flow Meter
16.2.2. Consistency meters
16.2.2.1. Blade Type
16.2.2.2. Rotary Type
16.2.2.3. Optical Consistency Meter
16.2.2.4. Microwave Consistency Meter
16.2.3. Brightness meters
16.2.3.1. Lab Brightness Meters
16.2.3.2.On-line Brightness Analyzers
16.2.3.3.In-line Brightness meters
16.2.4. Residual chemical meters
16.2.4.1. Polarographic meter
16.2.4.2. Oxidation-Reduction Potential Meter
16.2.5. Colorimeters
16.2.6. Spectrometers
16.2.7. pH meters
16.2.8. Kappa analyzers
16.2.9. Level sensors
16.2.9.1. Differential pressure meter
16.2.9.2. Radar and microwave level sensors
16.2.9.3. Ultrasonic and sonic level meters
16.2.9.4. Radio Frequency level meters
16.2.10. Temperature sensors
16.2.10.1. Thermocouples
16.2.10.2. Thermistors
16.2.10.3. Resistance Temperature Detectors
16.2.10.4. Temperature Probes
16.2.11. Conductivity sensors
16.3. Control strategies common to most bleaching stages
16.3.1.
16.3.2.
16.3.3.
16.3.4.
Consistency control
Fibre mass flow rate measurement and control
Chemical % applied control
Temperature control
16.3.5. pH control
16.3.6. Level control
16.3.7. Vacuum Washer controls
16.3.7.1. Shower Flow Controller(s)
16.3.7.2. Vat Level Controller
16.3.7.3. Drum Speed Controller-Dilution
16.3.7.4. Drum Speed Controller-Defoamer
16.3.7.5. Seal Tank Foam Probe
16.4. Control strategies by stage
16.4.1. D0 stage ClO2 control
16.4.2. E stage caustic control
16.4.3. D1 and D2 stage control
16.5. Bringing it all together
16.6. References Cited
17. Brightening of High-yield Pulps
17.1 Introduction to Brightening Technology
17.2. Principles of Hydrogen Peroxide Brightening
17.2.1. Wood Chemistry
17.2.2. Alkalinity
17.2.3. Sodium Silicate
17.2.4. Organic Stabilizers
17.2.5. Chelant Pretreatment
17.2.6. Magnesium Sulphate
17.2.7. Post-Brightening Neutralization
17.2.8. Temperature/Retention Time
17.2.9. Consistency Effects
17.2.10. Wood Species Effects
17.2.11. Pulp Type Effects
17.3. Mechanics of Peroxide Brightening
17.3.1. Single Tower
17.3.1.1. Single Tower Variations
17.3.1.2. Other Single Stage Tower Considerations
17.3.2. Two-Stage Sequences
17.3.3. Interstage Treatment
17.3.4. Post-Brightening Washing
17.3.5. Refiner Brightening
17.3.6. Steep or Flash Drier Brightening
17.3.7. Multi-Stage Sequences
17.4. Alkaline Peroxide for Strength Improvement and Energy
Reduction
17.4.1. Primary Line Treatment
17.4.2. Rejects Treatment
17.5. Principles of Sodium Hydrosulfite Brightening
17.5.1. Wood Chemistry
17.5.2. pH Effects
17.5.3. Temperature/Retention Time
17.5.4. Consistency Effects
17.5.5. Effects of Sequestrants
17.5.6. Effect of Wood Species
17.6. Mechanics of Sodium Hydrosulfite Brightening
17.7. Storage / Handling of Chemicals
17.7.1. Hydrogen Peroxide
17.7.2. Sodium Hydrosulfite
17.8. Additional Oxidizing/Reducing Agents
17.9. Conclusions
17.10. References Cited
18. Brightness: Basic Principles and Measurement
18.1. Introduction
18.2. History
18.2.1 Original Design
18.2.2 A newer Instrument design
18.3. Whiteness
18.4. Optical additives
18.4.1 Dyes and Fluorescent whitening agents
18.4.2 Fillers and Pigments
18.5. Brightness in the Industry
18.5.1 Brightness in Bleaching
18.5.2 Brightness in the Product
18.6 Measurement
18.6.1 Calibration
18.6.2 Measurement
18.7 Summary
18.8. References Cited
19. Pulp Bleaching and the Environment
19.1. Overview - Environmental Aspects of Pulp Bleaching
19.2. Effluent Characteristics and Composition
19.2.1. Effluent Characteristics
19.2.2. Monitoring and Testing of Bleach Plant and Mill Effluents
19.2.3. Environmental Performance at Today’s Bleached Mills
19.3. Assessing the Potential Effects of Pulping and Bleaching
Operations on the Aquatic Environment
19.3.1. Introduction
19.3.1.1 Lack of uniqueness of effluent biological responses from mills
practicing pulp bleaching
19.3.1.2 Lack of relevance of biomarker responses to measureable
environmental effects
19.3.1.3 Inconclusive results of limited field studies of pulp mill effluent
effects
19.3.2. Interpreting important findings from recent studies
19.3.2.1 Study structure
19.3.2.2 Judging the relevance of literature reports
19.3.2.3 Studies addressing fish reproduction
19.3.2.4 Mosquitofish studies
19.3.3. Long-term field assessments of pulp mill effluent effects
19.3.3.1 Background
19.3.3.2 Canadian environmental effects monitoring program
19.3.3.3 US long-term receiving water study
19.3.4. Conclusion
19.4. Bleach Plant Air Emissions
19.4.1. Introduction
19.4.2. Oxygen Delignification Emissions
19.4.3. Bleach Plant Emissions
19.4.3.1 Chlorine and Chlorine Dioxide Emissions
19.4.3.2 Ozone emissions
19.4.3.3. Organic Compound Emissions
19.4.3.4 Other organic compounds
19.4.4. Control of Chlorine and Chlorine Dioxide from Bleach Plants
19.4.4.1 Scrubbing media used in chlorine and chlorine dioxide scrubbers
19.4.4.2 Scrubber process control parameters and monitoring
19.5. Environmental Regulations
19.5.1. Introduction
19.5.2. Canadian Regulations
19.5.2.1. Fisheries act
19.5.2.2 Canadian Environmental Protection Act (CEPA)
19.5.3. United States Regulations – Water Discharges
19.5.3.1 Introduction
19.5.3.2 Water quality-based permit limits
19.5.3.3 Current technology-based limitations guidelines for wastewaters
19.5.3.4. Effects Monitoring
19.5.4. United States Regulations – Air Emissions
19.5.4.1 Introduction
19.5.4.2. Technology-based regulations for emissions to air from
bleached pulp mills
19.5.5. Other Regulations That Apply to Bleach Plants
19.5.5.1 Chlorine Dioxide Generating Systems
19.5.5.2 Annual chemical release reporting requirements
19.5.5.3 Risk Management Plans
19.6. List of Abbreviations and Acronyms
19.7. References Cited
20. Safe Storage and Handling of Bleaching Chemicals
20.1 Introduction
20.2 Chlorine Dioxide (ClO2)
20.2.1 Definition and Properties
20.2.2 Health and Safety Hazards
20.2.2.1 Major Hazards
20.2.2.2 Eye
20.2.2.3 Skin
20.2.2.4 Inhalation
20.2.2.5 Ingestion
20.2.2.6 Chronic Effects
20.2.2.7 Fire and Explosion
20.2.2.8 Other Hazards
20.2.3 Warning Properties
20.2.4 Personal Protective Equipment
20.2.5 Safety Equipment
20.2.6 Handling Spills and Leaks
20.2.7 Housekeeping and Routine Maintenance
20.2.8 Unloading Considerations
20.2.9 Design Considerations for Storage and Handling
20.2.9.1 General
20.2.9.2 Location
20.2.9.3 Storage Tanks
20.2.9.4 Containment
20.2.9.5 Lubricants
20.2.9.6 Chlorine Dioxide Generators
20.2.9.7 Materials of Construction
20.3 Hydrogen Peroxide (H2O2)
20.3.1 Definition and Properties
20.3.2 Health and Safety Hazards
20.3.2.1 Major Hazards
20.3.2.2 Eye
20.3.2.3 Skin
20.3.2.4 Inhalation
20.3.2.5 Ingestion
20.3.2.6 Chronic Effects
20.3.2.7 Fire and Explosion
20.3.2.8 Other Hazards
20.3.2.9 Hazard Classifications
20.3.3 Warning Properties
20.3.4 Personal Protective Equipment
20.3.5 Safety Equipment
20.3.6 Handling Spills and Leaks
20.3.7 Housekeeping and Routine Maintenance
20.3.8 Unloading Considerations
20.3.9 Design Considerations for Storage and Handling
20.3.9.1 General
20.3.9.2 Location
20.3.9.3 Storage Tanks
20.3.9.4 Insulation
20.3.9.5 Containment
20.3.9.6 Pumps
20.3.9.7 Lubricants
20.3.9.8 Piping
20.3.9.9 Pressure Relief Valves
20.3.9.10 Materials of Construction
20.4 Oxygen (O2)
20.4.1 Definition and Properties
20.4.2 Health and Safety Hazards
20.4.2.1 Major Hazards
20.4.2.2 Eye
20.4.2.3 Skin
20.4.2.4 Inhalation
20.4.2.5 Ingestion
20.4.2.6 Chronic Effects
20.4.2.7 Fire and Explosion
20.4.2.8 Other Hazards
20.4.3 Warning Properties
20.4.4 Personal Protective Equipment
20.4.5 Safety Equipment
20.4.6 Handling Spills and Leaks
20.4.7 Housekeeping and Routine Maintenance
20.4.8 Unloading Considerations
20.4.9 Design Considerations for Storage and Handling
20.4.9.1 General
20.4.9.2 Location
20.4.9.3 Storage Tanks
20.4.9.4 Containment
20.4.9.5 Piping
20.4.9.6 Lubricants
20.4.9.7 Materials of Construction
20.5 Sodium Chlorate (NaClO3)
20.5.1 Definition and Properties
20.5.2 Health and Safety Hazards of Sodium Chlorate
20.5.2.1 Major Hazards
20.5.2.2 Eye
20.5.2.3 Skin
20.5.2.4 Inhalation (of Dust or Mist)
20.5.2.5 Ingestion
20.5.2.6 Chronic Effects
20.5.2.7 Fire and Explosion
20.5.2.8 Other Hazards
20.5.3 Warning Properties
20.5.4 Personal Protective Equipment
20.5.5 Safety Equipment
20.5.6 Handling Spills and Leaks
20.5.7 Housekeeping and Routine Maintenance
20.5.8 Unloading Considerations
20.5.9 Design Considerations for Storage and Handling
20.5.9.1 General
20.5.9.2 Location
20.5.9.3 Storage Tanks
20.5.9.4 Containment and Sewers
20.5.9.5 Pumps
20.5.9.6 Valves
20.5.9.7 Lubricants
20.5.9.8 Water Heaters
20.5.9.9 Insulation
20.5.9.10 Materials of Construction
20.6 Methanol (CH3OH)
20.6.1 Definition and Properties
20.6.2 Health and Safety Hazards
20.6.2.1 Major Hazards
20.6.2.2 Eye
20.6.2.3 Skin
20.6.2.4 Inhalation
20.6.2.5 Ingestion
20.6.2.6 Chronic Effects
20.6.2.7 Fire and Explosion
20.6.2.8 Other Hazards
20.6.3 Warning Properties
20.6.4 Personal Protective Equipment
20.6.5 Safety Equipment
20.6.6 Handling Spills and Leaks
20.6.7 Housekeeping and Routine Maintenance
20.6.8 Unloading Considerations
20.6.9 Design Considerations for Storage and Handling
20.6.9.1 General
20.6.9.2 Location
20.6.9.3 Storage Tanks
20.6.9.4 Containment
20.6.9.5 Electrical Equipment
20.6.9.6 Static Electricity
20.6.9.7 Piping
20.6.9.8 Materials of Construction
20.7 Sodium Hydroxide (NaOH)
20.7.1 Definition and Properties
20.7.2 Health and Safety Hazards
20.7.2.1 Major Hazards
20.7.2.2 Eye
20.7.2.3 Skin
20.7.2.4 Inhalation
20.7.2.5 Ingestion
20.7.2.6 Chronic Effects
20.7.2.7 Fire and Explosion
20.7.2.8 Other Hazards
20.7.3 Warning Properties
20.7.4 Personal Protective Equipment
20.7.5 Safety Equipment
20.7.6 Handling Spills and Leaks
20.7.7 Housekeeping and Routine Maintenance
20.7.8 Unloading Considerations
20.7.9 Design Considerations for Storage and Handling
20.7.9.1 General
20.7.9.2 Location
20.7.9.3 Storage Tanks
20.7.9.4 Containment
20.7.9.5 Pumps
20.7.9.6 Piping
20.7.9.7 Materials of Construction
20.8 Sulfuric Acid (H2SO4)
20.8.1 Definition and Properties
20.8.2 Health and Safety Hazards
20.8.2.1 Major Hazards
20.8.2.2 Eye
20.8.2.3 Skin
20.8.2.4 Inhalation
20.8.2.5 Ingestion
20.8.2.6 Chronic Effects
20.8.2.7 Fire and Explosion
20.8.2.8 Other Hazards
20.8.3 Warning Properties
20.8.4 Personal Protective Equipment
20.8.5 Safety Equipment
20.8.6 Handling Spills and Leaks
20.8.7 Housekeeping and Routine Maintenance
20.8.8 Unloading Considerations
20.8.8.1. Railcars
20.8.8.2. Tank Trucks
20.8.9 Design Considerations for Storage and Handling
20.8.9.1 General
20.8.9.2 Location
20.8.9.3 Storage Tanks
20.8.9.4 Containment
20.8.9.5 Piping
20.8.9.6 Insulation
20.8.9.7 Flexible Hoses
20.8.9.8 Air Unloading Lines
20.8.9.9 Materials of Construction
20.9 Ozone (O3)
20.9.1 Definition and Properties
20.9.2 Health and Safety Hazards
20.9.2.1 Major Hazards
20.9.2.2 Eye
20.9.2.3 Skin
20.9.2.4 Inhalation
20.9.2.5 Ingestion
20.9.2.6 Chronic Effects
20.9.2.7 Fire and Explosion
20.9.2.8 Other Hazards
20.9.3 Warning Properties
20.9.4 Personal Protective Equipment
20.9.5 Safety Equipment
20.9.6 Handling Spills and Leaks
20.9.7 Housekeeping and Routine Maintenance
20.9.8 Unloading Considerations
20.9.9 Design Considerations for Storage and Handling
20.9.9.1 General
20.9.9.2 Location
20.9.9.3 Storage Tanks
20.9.9.4 Containment
20.9.9.5 Piping systems
20.9.9.6 Lubricants
20.9.9.7 Materials of Construction
20.10 Respirators
20.10.1 Overview
20.10.2 Selecting a Respirator
20.10.2.1 Selecting a Respirator for Emergency Escape
20.10.2.2 Selecting a Respirator for Firefighting in a Hazardous
Atmosphere
20.10.2.3 Selecting a Respirator for Emergencies, Unknown
Concentrations, or Concentrations Above IDLH
20.10.2.4 Selecting a Respirator for a Hazardous Atmosphere With
Known Concentrations Below IDLH
20.10.3 Respirator Descriptions
20.10.3.1 Cartridge Respirator
20.10.3.2 Full Facepiece Cartridge Respirator
20.10.3.3 Full Facepiece Canister Gas Mask
20.10.3.4 Powered Air Purifying Respirator
20.10.3.5 Full Facepiece Type C Supplied Air Respirator (SAR)
20.10.3.6 Full Facepiece Type C Supplied Air Respirator with Auxiliary
SCBA
20.10.3.7 Self Contained Breathing Apparatus (SCBA)
20.11 Labels and Classifications
20.11.1 Overview
20.11.2 Transportation-Related Categories and Labels
20.11.2.1 UN Numbers
20.11.2.2 DOT and TDG Hazard Classes
20.11.3 Storage and Use-Related Categories and Labels
20.11.3.1 NFPA Ratings
20.11.3.2 WHMIS Classification
20.11.3.3 HCS Labeling
20.11.3.4 HMIS® Labeling
20.11.3.5 American National Standards Institute (ANSI) Labeling
20.11.4 GHS Hazard Class
20.12 Definitions
20.13 Acknowledgements
20.14 References Cited
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CHAPTER 1
Introduction
ALAN W. RUDIE and PETER W. HART
1.1. PREVIOUS PULP BLEACHING BOOKS
The current book, The Bleaching of Pulp, is the fifth in a series of
books that has spanned more than 65 years. The first book in this ongoing series was published in 1953. The book was a project of the TAPPI Pulp Purification Committee with Ward D. Harrison (Riegel Paper
Corp) assigned to recruit authors and Raymond S. Hatch (Hudson Bay
Pulp and Paper) as editor [1]. Hatch wrote four chapters, while Harrison
wrote the chapter on chlorine dioxide. Other authors were Alexander
Meller (Australian Pulp Manufactures) J.N. Swartz (Howard Smith Paper Mills), K.G. Booth (Crown Zellerbach), Simmonds and Kingsbury
(FPL), Lamar Moss (Whiting Plover Paper Co), M.W. Phelps (Peter J.
Schweitzer Inc.), Beeman and Reichert (Finch Pruyn and DuPont respectively), T.A. Pasco (Nekoosa-Edwards Paper Company), and M.G.
Lyon (Champion Paper and Fiber). The project was started in 1947 and
took six years to complete. It was published as The Bleaching of Pulp:
TAPPI Monograph No. 10 [1]. Since that time, new versions of the
book have been published as bleaching technology and understanding
have changed.
The second edition of Bleaching of Pulp was published in 1963 with
Alan W. Rudie, Supervisory Research Chemist, USDA Forest Products Laboratory,
Madison, WI 53726
Peter W. Hart, Manager, New Products, MeadWestvaco Corporation, Atlanta, GA
30309
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2
INTRODUCTION
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E. Howard Rapson as the editor [2]. Well-known chapter authors included J. A. van den Akker, Carlton Dence, and Howard Rapson. The
number of chapter authors from industry was notable: eight from paper
companies, five from engineering consultants or suppliers. There were
a total of 20 chapters in this book.
The third book in the series, The Bleaching of Pulp, was published in
1973 with Rudra P. Singh as the editor [3]. Of 28 contributors, 14 were
from paper companies, and nine were unattached consultants or academics. The remaining five contributors worked for Hewlett-Packard
and other supplier companies.
The fourth book in this series changed the name to Pulp Bleaching:
Principles and Practice. The editors were Carlton W. Dence and Douglas W. Reeve [4]. The book was published in 1996. The fourth book
diverged from the format and style of the first three books in the series.
The 36 chapters were organized under eight major subject headings in
an effort to give subjects relating to bleach plant engineering, bleaching chemistry, and environmental issues a more distinct identity than in
the past. Seven of the chapters were written by the editors. The fourth
book stressed the principles undergirding the selection, design, and implementation of a successful bleaching operation. In a major departure
from past books, 38 of the 58 authors were academicians, research institution employees, or consultants. Only six of the chapters had input
from industrial workers. The remaining 14 authors were from supplier
companies, with the majority of the supplier contributions coming from
DuPont.
1.2. CURRENT PULP BLEACHING BOOK
The current book has reverted to the style of the first three in the series. It aims to provide highly practical information on the current state
of industrially applicable pulp bleaching. This book includes direct industry input into 15 of the 20 chapters. In general, the chemistry associated with pulp bleaching has not changed much over the last 20 years
or so and therefore has been adequately covered in the fourth book in
this series. Notable exceptions to this statement are the impact and understanding of hexenuronic acid (covered in Chapter 5), the application
of enzymes, specifically xylanase (Chapter 6), and the impact of nonprocess elements and scale formation on bleaching equipment, covered
in Chapter 7. Both hexenuronic acid and nonprocess elements are now
significantly better understood than in earlier years and are therefore
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Current Pulp Bleaching Book
3
covered in detail. Improvements in the understanding of the fundamental chemistry and practical application of various bleaching chemicals
are also discussed in the current book. Other bleaching chemicals for
which no significant new advances in fundamental understanding have
been achieved are reviewed from a practical application standpoint.
Some bleaching chemicals covered in the previous books, such as chlorine and hypochlorite, have been eliminated from the current book.
Over the last 15 to 20 years since the last bleaching book was published, the pulp bleaching industry as a whole has tended to be far more
application-oriented than research-oriented. Therefore, the current
book tends to focus more on the applied aspects of pulp bleaching that
have been developed in the last 20 years. An example of such efforts
include multistage bleach plant modeling (Chapter 12), which includes
efforts to determine optimal operating targets while minimizing bleaching costs.
At approximately the time that the last book was published, in 1996,
the industry was making the process changes needed to comply with
the cluster rules. As part of the cluster rules, the use of elemental chlorine and hypochlorite was eliminated. Therefore, the current book no
longer contains chapters devoted to these bleaching chemicals. Several
bleaching chemicals were evaluated for the production of both elemental-chlorine-free (ECF) pulps and totally-chlorine-free (TCF) pulps. After a short period of “alphabet soup” bleach sequence exploration, the
majority of the industry settled on ECF bleaching, with a small percentage of the industry opting for a TCF process. Since 1996, equipment
and technology suppliers have taken a more holistic approach towards
supplying bleaching solutions. Expanded emphasis has been placed on
the use of chlorine dioxide, oxygen, and peroxide. Several different
bleaching sequences have been used since the advent of ECF processes.
Figure 1.1 shows the evolution of several types of bleaching sequences
over the last 30 years [5]. Equipment suppliers have also made significant advances in washing devices (Figure 1.2), which have resulted in
improved bleaching processes and designs [5]. Developments in fiber
processing, production, and bleaching technology which have resulted
in significant bleach plant changes are shown in Figure 1.3 [5].
Initially, the use of 100% chlorine dioxide in the first bleaching stage
resulted in severe calcium oxalate scale formation in many mills. Since
that time, the fundamentals of scale formation have become well understood, and the industry has developed the ability to model and predict
the operating conditions which will lead to scale formation. The use of
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INTRODUCTION
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FIGURE 1.1. Evolution of bleaching sequences over the last 40 years. Figure adapted
from Ref. [5].
chlorine dioxide at lower than natural pH in the first stage has been extensively studied, and typical application and troubleshooting methods
are discussed here.
Since the initiation of the cluster rule, the use of oxygen delignification and oxygen bleaching has expanded significantly as well. Better
FIGURE 1.2. Bleach plant washer developments from the early 1970s to date. Figure
adapted from Ref. [5].
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Current Pulp Bleaching Book
5
FIGURE 1.3. Developments in fiber processing, production, bleaching technology, and
emerging trends in the production of bleached pulp. Figure adapted from Ref. [5].
understanding of the associated chemistry and reactor design (plumbing) has been developed [6]. These developments are discussed in detail.
Another area that has experienced a significant level of development
and application is the use of multistage models to optimize bleach plant
operation by predicting cost-optimal operating conditions [7]. When
bleach plants used chlorine as the primary first-stage bleaching chemical, the low-cost bleaching strategy was invariably to maximize delignification in the chlorination stage. When instead the bleach sequence
is started with chlorine dioxide in the first stage, the optimum low-cost
strategy requires balancing the amount of chlorine dioxide used across
two or three chlorine dioxide stages. Chapter 12 provides a detailed review of both the fundamental models and the global models developed
over the last 10–20 years.
The last chapter in the book, Safe Storage and Handling of Bleaching
Chemicals, focuses on safety aspects associated with pulp bleaching.
Detailed chemical properties of importance to bleach plant engineers
such as heat of mixing, specific gravity, melting and freezing points of
various process solutions, and materials of construction are covered for
process chemicals. In addition, Chapter 20 also reviews the respirator
options available for use by bleach plant personnel and the labeling and
placard systems used for various bleaching chemicals.
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INTRODUCTION
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In general, the current book has been laid out to be user-friendly and
practical. Each chapter has been written in several well-documented
subsections to provide immediate access to subject content. Chapters 2,
3, and 4 deal with delignification of chemical pulps. Chapters 5, 6, and
7 cover modifications to traditional bleaching stages and include the
use of hot acid treatment, enzymes, and metals management. The next
four chapters deal with brightening stages and the removal of dirt and
shives. Chapters 12 through 18 cover bleach plant design and control.
Chapter 19 reviews the environmental impact of bleaching effluents,
and Chapter 20 covers bleach plant safety.
1.3. ORGANIZATION OF THE CURRENT BOOK
One of the hardest tasks in organizing a book of this sort is recruiting authors and then getting the willing recruits to follow through with
the promised chapters. Dr. Joseph Genco of the University of Maine
graciously agreed to spearhead the chapter on Oxygen Delignification.
The chapters on chlorine dioxide delignification and brightening were
written by Daniel Connell and Scott Carmichael of EKA Chemicals.
Brian Brogdon of Future Bridge Consulting led the efforts on Extraction, Peroxide, and Multistage Modeling. The important chapter dealing with Hexenuronic acid and A stages was directed by Jorge Colodette of Universidade Federal de Viçosa, Brazil. Enzyme bleaching
was led by Harold Petke of Iogen Corporation, Ontario, Canada. Alan
Rudie of the USDA Forest Products Lab directed the chapter on Nonprocess Element Management. Douglas Freeman of New Page Corporation brought the chapter on Ozone up to date. Charles Couchene
of Georgia-Pacific Corporation led the effort to update the section on
Dirt and Shive Management. Peter Bräuer of Andritz, Lew Shackford
of GL&V, and Kevin McCanty of Metso spearheaded the three vendorrelated chapters on holistic vendor-specific solutions to modern bleaching problems. The chapter on modern instrumentation and controls was
written by Gerry Pageau of Howe Sound Pulp & Paper Corporation.
Stan Heimberger of Arkema Inc. completed the chapter on Mechanical
Pulp Bleaching, and Patrick Robinson of XR Laboratory LLC reviewed
pulp brightness. The chapter on Environmental Impact of Bleaching
was led by Alan E. Stinchfield, retired. Finally, the new section on Safe
Storage and Handling of Bleaching Chemicals was led by Doug Reed
of EKA Chemicals. Several of these chapters had multiple coauthors.
The authors listed here were the primary points of contact for each of
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References Cited
7
the chapters. The contributions of each and every chapter author to this
book are greatly appreciated.
The first book in the series required six years from initiation to publication. The fourth in the series required four years. This reality is responsible for one other change in the format of the book. The major
equipment suppliers have each been given one chapter to present the
equipment line that they supply for new bleach plants. Where this borders on advertising, it is honest advertising. For the editors, it gave us
the leverage of publishing without a supplier chapter. Thankfully, it did
not come to that. However, five authors complied with the initial target
deadline, and another five chapters were received before we had completed the editing on the first five. Not to force the remaining authors
to complete their chapters in a timely manner would have devalued the
efforts of those who had. Having watched Doug Reeve struggle with
this issue for years to complete the 1996 version, and knowing some of
the chapter authors who had to revise substantial portions of their text to
catch up with changes that had occurred in the two-year-plus delay, we
were determined to complete this version in a single year. With the help
of all our chapter authors and considerable work on the part of TAPPI,
we have achieved this goal. We hope that you, too, appreciate the effort
and most importantly, find the book useful for operating and optimizing
your bleach plants.
1.4. REFERENCES CITED
1. Beeman, L.A., MacDonald, R.G. (eds.), The Bleaching of Pulp—Tappi Monograph
10, Tappi Press, New York, 1953.
2. Rapson, W.H. (ed.), The Bleaching of Pulp—Tappi Monograph 27, Tappi Press,
New York, 1963.
3. Singh, R.P. (ed.), The Bleaching of Pulp, 3rd ed., revised, Tappi Press, Atlanta,
GA, 1979.
4. Dence, C.W., Reeves, D.W. (eds.), Pulp Bleaching: Principles and Practice, Tappi
Press, Atlanta GA, 1996.
5. Andrede, M., The fiber line of the future for eucalyptus kraft pulp, Keynote Presentation, 5th Intl. Colloquium on Eucalyptus Pulp, Porto Seguro, Brazil, May 9–11,
2011.
6. Agarwal, S., Genco, J.M., Miller, W., et al., Medium-consistency oxygen delignification kinetics and tower design. In Brogdon, B. (ed.), Innovative Advances in
the Forest Products Industries, AIChE Symposium Series No. 319, 94, pp. 32–46,
1998.
7. Hart, P.W., The chemical versus energy tug of war: a pulp mill perspective. Tappi
J. 10(7): 37–42, 2011.
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CHAPTER 2
Oxygen Delignification
JOSEPH M. GENCO, ADRIAAN R. P. van HEININGEN and
WILLIAM MILLER
2.1. INTRODUCTION
Oxygen was recognized as a potential bleaching agent as early as
1867, at which time a process was patented to improve pulp bleaching
by running “heated air through an agitated pulp suspension.” The early
history of oxygen bleaching extended over one hundred years and has
been reviewed by McDonough [1] and Rodriguez [2]. Successful commercialization required technological advances in pressurized operations, the separation and purification of oxygen from air, and the discovery of chemicals that could serve as carbohydrate protectors. With these
advances in place, the first commercial oxygen delignification plant
was started up at the Sappi kraft mill at Enstra, South Africa [3]. The
pulp produced in these first operations had strength equivalent to that of
unbleached pulp. By 1996, worldwide installed capacity had increased
dramatically to 145,000 A.D. metric tons per day of bleached kraft pulp
[4], and by 2010, the installed capacity had approximately doubled to
about 300,000 A.D. metric tons per day. Therefore, most of the world’s
bleachable-grade pulp is presently treated using oxygen delignification.
Oxygen delignification is a process which uses oxygen and alkali to
Joseph M. Genco, Calder Professor of Pulp and Paper Science and Engineering,
University of Maine, Orono, ME 04469
Adriaan R. P. van Heiningen, J. Larcom Ober Professor of Chemical Engineering,
University of Maine, Orono, ME 04469
William Miller, Operations and Maintenance Coordinator, Evergreen Packaging,
Canton, NC 28716
9
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OXYGEN DELIGNIFICATIONCopyrighted Material
remove a substantial fraction of the lignin that remains after pulping
(Figure 2.1). In most cases, the term is used synonymously with oxygen bleaching. In the recent past, because of the trend in the industry
towards ECF (elemental-chlorine-free) bleaching combined with minimal emission of chlorinated organic compounds, oxygen delignification
has emerged as a very important process.
The advantages of an oxygen delignification stage are both environmental and economic. The effluent from the oxygen stage is free from
chloride ions and can be recycled back to the recovery furnace (Figure
2.1). Installing an oxygen stage before a traditional bleach plant considerably reduces emissions of potentially hazardous chlorinated lignins,
COD (chemical oxygen demand), BOD (biochemical oxygen demand),
and color in bleach plant effluent [5]. There are also savings in operating costs through the use of lower amounts of chlorine dioxide, ozone,
hydrogen peroxide, and other oxidizing agents, because oxygen has a
lower cost than all other oxidizing agents. The main disadvantage of an
oxygen stage is that compared to a chlorine dioxide stage, it has both
lower reactivity and lower selectivity [6]. The other disadvantage is the
high capital cost of installing an oxygen delignification system.
The oxygen delignification process is operated at relatively high temperature and pressure at either medium or high consistency in a singleor two-stage system. The degree of delignification achieved is normally
in the range of 40% to 60%. Medium consistency is the most commonly
used in industrial application. Typical conditions are shown in Table
1 for both medium- and high-consistency processes. High-consistency
oxygen delignification processes have been reviewed by McDonough
[1]. However, few high-consistency systems are being operated commercially.
FIGURE 2.1. Kraft Mill with Oxygen Delignification.
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Introduction
11
TABLE 2.1. Process Conditions for the Oxygen Delignification Process.
Variable
Medium Consistency
High Consistency
Consistency (%)
Alkali Consumption (Kg/Tonne )
Temperature (°C)
Pressure (Bars)
10 to 14
15 to 35 (1.5% to 3.5%)
85 to 105
In (7 to 8)
Out (4.5 to 6.0)
50 to 60 (1 Stage)
20/60 (2 Stage)
20 to 24 (2.0% to 2.4%)
0–0.02
25 to 30
15 to 25 (1.5% to 2.5%)
100 to 115
In (4 to 6)
Out (4 to 6)
25 to 35
Retention Time (Minutes)
Oxygen Consumption (Kg/Tonne)
MgSO4 (%) (When Required)
15 to 24 (1.5% to 2.4%)
0–0.02
The oxygen delignification process is flexible and is best viewed as a
“bridging strategy” between pulping and final bleaching [7]. This is illustrated in Figure 2.2 for fully bleached softwood pulp. In the past, the
kappa number leaving the softwood digester was approximately 30, and
the pulp was bleached using the (C+D)EDED bleach sequence. In modern fiber lines, delignification in the softwood digester is extended to a
kappa number of 25 to 26, followed by oxygen delignification which
may be considered a further extension of pulping. The oxygen delignification process typically lowers the kappa number by approximately
50% to 60%, say to 13, and the process is then finished by bleaching
the pulp with a D(EO+P)DED sequence. An alternative way to look at
oxygen delignification is to consider an oxygen stage as the first stage
of an OD(EOP)DED bleach sequence. In either case, the oxygen stage
is a bridging strategy between conventional pulping and bleaching.
2.1.1. Comparison of Oxygen to Other Bleaching Agents
With the exception of caustic, all chemicals used in kraft pulp bleaching are oxidizing agents that break down lignin macromolecules into
fragments small and hydrophilic enough to dissolve in water and alkali.
McDonough [6] compared various bleaching agents and distinguished
them on reactivity, selectivity, efficiency, the ability to bleach particles
or shives, and environmental effects (Table 2.2). Chemical reactivity
pertains to the rate and extent of reaction of the bleaching agent with
lignin. Selectivity refers to the rate of reaction of the oxidizing agent
with lignin relative to the rate of reaction with carbohydrates. Selectivity is often measured as the ratio of the reduction in kappa number to the
drop in pulp viscosity. Efficiency relates to the ability of the bleaching
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OXYGEN DELIGNIFICATIONCopyrighted Material
FIGURE 2.2. Evolution of Bleaching Illustrating Oxygen as a Bridging Technology between Pulping and Bleaching for Softwood.
agent to remove lignin and to increase brightness at the lowest chemical
charge. Other important considerations include the ability of the bleaching agent to break down impurities in the pulp such as shives, knots,
and dirt particles, and the potential for the bleaching chemical to harm
the environment.
Oxygen may be classified as an oxidizing agent with low reactivity, moderate selectivity, and low efficiency. It has moderate ability to
bleach shives and other particulate matter and most significantly has
low impact on the environment. The low reactivity necessitates elevated temperatures (85°C to 115°C) and pressures (4 to 8 bars) for oxygen
TABLE 2.2. Qualitative Properties of Selected Bleaching Agents [6]
(McDonough, 1992) (H= High, M = Medium, and L = Low).
Chemical
Chlorine Dioxide
(ClO2)
Oxygen (O2)
Hydrogen Peroxide
(H2O2)
Ozone (O3)
Particle Environmental
Reactivity Selectivity Efficiency Bleaching
Concerns
M
H
H
H
M
L
L
M
H
L
L
M
L
L
L
H
L
M
L
L
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Introduction
13
delignification reactions to proceed. Oxygen’s moderate selectivity can
sometimes lead to appreciable loss in pulp viscosity and may necessitate adding a viscosity protector such as MgSO4 to the pulp in the case
of softwood. Low efficiency means that appreciable quantities of the
reagent must be added to the oxygen delignification system (15 to 25
kg O2 per tonne of pulp).
2.1.2. Advantages and Disadvantages of Oxygen Delignification
Oxygen delignification significantly improves bleach process efficiency and can shorten a bleaching sequence provided that effective
washing is used after the oxygen stage. Because less lignin enters the
bleach plant, there is a significant decrease in consumption of bleaching
chemicals and a reduction in cost because oxygen is less expensive than
ClO2 and H2O2.
A major advantage of oxygen delignification is its impact on discharge to the wastewater treatment system, because liquid effluent
from an oxygen delignification system is recycled back to the recovery
boiler (Figure 2.1). Without the oxygen system, all the effluent from
the bleach plant would go to wastewater treatment because of the presence of chlorides and the possibility of corrosion in the recovery cycle,
especially in the recovery boiler. With oxygen delignification, effluent from the oxygen stage is recycled back to the recovery boiler, and
thus BOD, COD, and color going to the waste treatment system are
reduced. The reduction of chlorinated organic material makes the use
of oxygen delignification technology a valuable component of TCF and
ECF bleaching. There is, of course, some discharge of trace amounts
of carbon monoxide and volatile organic compounds such as methanol,
acetaldehyde, terpenes, and other compounds from the blow tank and
the post-oxygen washer filtrate tanks [8].
Oxygen delignification involves free radical reactions, and therefore
its selectivity is limited. This can lead to significant reduction in pulp
viscosity and intrinsic fiber strength, depending upon the final kappa
number of the pulp. Physical properties of paper depend both on fiberto-fiber bonding and on the intrinsic fiber strength of the pulp. However,
reductions in pulp viscosity are usually significant only at low freeness
values when the paper is well bonded and fiber failure determines paper strength. Moreover, because high pressures are involved in oxygen
delignification, the cost of an oxygen system can require a significant
capital investment. Finally, because the dissolved solids extracted from
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OXYGEN DELIGNIFICATIONCopyrighted Material
the pulp are recycled back to the recovery system, there is an increase
in the solids loading of the recovery boiler, but not particularly in the
total heat input or in the resulting steam generation rate of the boiler [9].
2.2. CHEMISTRY OF OXYGEN DELIGNIFICATION
The mechanism of oxygen delignification has been studied by many
investigators [10–13]. The normal (lowest-energy) configuration of the
oxygen molecule is the triplet state. This molecule contains two electrons that are unpaired. Therefore, each of these electrons has an affinity for other electrons of opposite spin. For this reason, oxygen is a
di-radical in the triplet state, which makes it unreactive unless heated
to elevated temperatures. At higher temperatures, oxygen has a strong
tendency to react with organic substances, and radical chain reactions
are initiated which liberate superoxide anion radicals (O2·–) and hydroperoxy radicals (HOO·) [6]:
RO– + O2 → RO· + O2·–
(1)
RH + O2 → R· + HO2·
(2)
The one-electron reduction reactions transform oxygen into a hydroperoxy radical, then to hydrogen peroxide, subsequently to a hydroxyl
radical, and finally to water. These reactions are illustrated in Figure
2.3 [6].
In oxygen bleaching, the substrate is activated by providing alkaline
conditions to ionize the free phenolic hydroxyl groups in the residual
lignin, which then form superoxide anion and a phenoxy radical. The
superoxide anion can then react further with the different resonance
structures of the phenoxy radical, which eventually leads to ring opening and muconic acid or to quinone structures, among others. Alterna-
FIGURE 2.3. Oxygen Chemistry in Aqueous Solution [6].
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tively, the superoxide anion may be converted into other oxygen-based
radicals such as the hydroxyl radical (see Figure 2.3) which then may
react with cellulose to lower the pulp viscosity [14] or may hydroxylate
lignin to form new phenols. Superoxide anions do not degrade carbohydrates [15].
2.2.1. Lignin Reactions
Two general approaches have been used to determine the mechanisms of lignin removal during oxygen bleaching. In the first method,
model compounds have been used to study the reaction of lignin with
oxygen under alkaline conditions [10,16–18]. In the second approach,
experimental studies have been conducted in batch-scale reactors to investigate the effects of process variables on the structural features of
both residual and dissolved lignins and have led to an increased understanding of the chemical and physical aspects of the mechanism [19,20].
Important reactions of lignin are initiated when a phenolic hydroxyl
group in lignin dissociates in alkali to form a phenolate ion (Figure
2.4). The ion then reacts with oxygen to form a resonance-stabilized
phenoxy radical and a superoxide anion [21].
The resonance-stabilized intermediates then undergo reaction with
themselves (lignin condensation) or with oxygen species such as hydroxyl (HO·), hydroperoxy (HOO·) and superoxide anion (O2·–) radicals to form organic acids, carbon dioxide, and other low-molecularweight organic products through side-chain elimination, ring opening,
and demethoxylation reactions [10]. Figure 2.5 illustrates these types of
reaction pathways.
Johansson and Ljunggren found that phenolic structures with a conjugated side chain, like stilbene and enol ethers, react very rapidly,
whereas diphenylmethane-type condensed structures are particularly
resistant to oxygen bleaching [22]. The p-hydroxylphenyl and 5,5'-biphenolic units in the residual lignin are quite stable and tend to accumulate during oxygen delignification [23,24]. During oxygen delignification, it has been confirmed that the number of phenolic hydroxyl
groups and carboxyl groups in the lignin decreases and the number of
carboxylic acid groups increases. The final degradation products from
oxygen delignification are predominately organic acids and carbon dioxide [25].
Lawoko and coworkers showed that ~90% of the residual lignin in
softwood kraft pulp is linked to various carbohydrates, mainly xylan
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FIGURE 2.4. Initial Attack of Oxygen on Phenolic Nuclei.
and glucomannan, as so-called lignin-carbohydrate complexes (LCCs)
[26]. The delignification rates of various LCC types were found to be
different, with the xylan- and cellulose-rich LCCs being delignified
faster than the glucomannan-rich LCCs toward the end of a kraft cook
and during oxygen delignification [27]. Consequently, after oxygen delignification, nearly all the residual lignin was found in the glucomannan
LCC fraction.
2.2.2. Carbohydrate Reactions
Carbohydrate reactions also occur during oxygen delignification and
have been studied by numerous researchers. Carbohydrate-degradation
reactions are commonly monitored by measuring the decrease in intrinsic viscosity [η] of the pulp. Initially, delignification was limited to re-
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moval of approximately half the lignin in the pulp entering the oxygen
stage. With some of the newer processes for softwood delignification,
the reduction in kappa number can be greater than 60% while still preserving the viscosity of the pulp [28,29].
The reactions involved can be divided into two categories: one is
random chain cleavage by radical species, and the other is carbohydrate
peeling reactions. Random chain cleavage occurs at any glycosidic
linkage along the chainlike molecule, while in carbohydrate peeling reactions, individual sugar units on the end of the chain are attacked and
successively removed one unit at a time [30]. Both types of reactions
may occur during oxygen delignification, but random chain cleavage is
thought to be the most significant. The radical species responsible for
random chain cleavage originate mostly from the peroxyl radicals and
hydroxyl radicals generated by lignin oxidation reactions (see Figures
2.3 and 2.4). However, metals such as iron and copper are also capable
of creating hydroxyl radicals from generated hydrogen peroxide. The
random attacks on the cellulose chain decrease the average length of the
cellulose, as indicated by a decrease in pulp viscosity, which if excessive, leads to the loss of pulp strength. Guay and coworkers used computational methods on the cellulose model compound methyl cellobiose
to determine that the step involving the elimination of the superoxide is
FIGURE 2.5. Possible Reactions of Lignin via the Phenoxy Radical.
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FIGURE 2.6. Mechanism for Cellobiose Formation.
energetically unfavorable [14]. The first pathway is a hydroxyl-radical
substitution reaction at the anomeric carbon, forming cellobiose and
methanol (Figure 2.6). The second degradation pathway is a hydroxylradical substitution reaction at the glycosidic linkage between the two
pyranose rings, forming methyl β-D-glucoside and D-glucose (Figure
2.7).
The peeling reaction is responsible for decreasing the carbohydrate
yield of the process. Both primary and secondary peeling reactions are
possible. Primary peeling occurs when the carbohydrates in the kraft
pulp contain reducing ends. It has been proposed that so-called “peeling
delignification” may occur when lignin fragments covalently bound to
hemicelluloses undergo peeling reactions [31]. Lignin attached to these
“peeled” hemicelluloses will then also be removed from the pulp. However, the impact of “peeling delignification” was found to be relatively
small, and this phenomenon occurred only during the initial phase of
oxygen delignification [32]. Secondary peeling is initiated on the freshly generated reducing sugar unit immediately following random hydro-
FIGURE 2.7. Mechanism for Formation of Methyl β-D-glucoside and D-glucose.
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lysis. This will then lead to a significant yield loss, as quantified by Ji,
who also showed that the carbohydrate yield loss is linearly correlated
with the cellulose chain cleavages [33].
2.3. KINETICS OF OXYGEN DELIGNIFICATION
Oxygen delignification is a heterogeneous reaction involving three
phases: solid (fiber), liquid (aqueous alkali solution), and gas (oxygen)
(Figure 2.8). Oxygen gas must dissolve in the liquid, diffuse through
the liquid film surrounding the fiber, and move into the fiber wall before
an oxygen delignification reaction can occur. The products of the reaction must then diffuse out of the cell wall and back into the bulk of the
liquid surrounding the pulp. It has been shown by van Heiningen and
coworkers that mass transfer of oxygen to the fibers as well as the actual
delignification reactions may be rate-limiting, while diffusion inside the
fiber wall is not [34].
2.3.1. Kinetic Rate Equations
Kinetic data that represent the decrease in kappa number with time
during oxygen delignification exhibit two distinct stages or periods at
a fixed initial alkali concentration and temperature. This is illustrated
in Figure 2.9 for Scandinavian softwood, for which the oxygen delignification experiments were carried out at 110°C temperature, 0.02
FIGURE 2.8. Phenomenological View of Oxygen Delignification.
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FIGURE 2.9. Kappa No. After Oxygen Delignification for Scandinavian Softwood.
moles NaOH/liter concentration, and 0.98 MPa (142 psi) oxygen pressure [35]. Olm and Teder described the kinetics for the lignin remaining
in the pulp using two pseudo-first-order reactions: a rapid initial delignification followed by a slow final delignification. K02 in Figure 2.9 is
the amount of “slowly eliminated lignin,” and the value K1 = K0 – K02
is the amount of “easily eliminated lignin,” where both are expressed in
terms of kappa number.
Lignin removal in the initial rapid stage is thought to involve lignin
moieties that react readily with the oxygen free-radical species present
in the cell wall. In the second, slower delignification stage, it is thought
that the moieties in the lignin react very slowly with the free radicals
being generated by the caustic and oxygen. The moieties in the lignin
that are difficult to remove may originate in the pulp from the digestion
process or may result from the condensation reactions shown in Figure
5. The two delignification stages are directly paralleled by two corresponding cellulose reaction phases [35].
2.3.1.1. Initial Kappa Number
It is easier to delignify pulps with high rather than low initial kappa
numbers. It has been reported that southern hardwood kraft pulps with
high initial kappa number have lower resistance to oxygen delignifi-
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cation and a higher reaction rate compared to pulps with low initial
kappa number [36]. The reason for this observation is that the kappa
number represents a combination of different oxidizable components:
lignin, nonlignin oxidizable structures, and hexenuronic acids (HexA)
[37], and the relative contribution of HexA, which are unreactive under
standard oxygen delignification conditions, decreases with increasing
kappa number. This same effect of kappa number has been observed for
southern softwood by Tao [38]. However, when the kappa numbers are
corrected for HexA content, the delignification rate is shown to be proportional to the amount of residual lignin in loblolly pine kraft pulps,
irrespective of their initial kappa numbers of 23, 26, and 34 [32]. Other
studies have shown that stopping the cooking process at a high kappa
number, at 40 for example, instead of the conventional kappa number of
30, and then using an oxygen delignification stage leads to an increase
in pulp yield of approximately 2% [39].
2.3.1.2. Alkali Charge, Temperature, and Oxygen Partial Pressure
Increasing the alkali concentration (Figure 2.10), temperature (Fig-
FIGURE 2.10. Effect of Caustic Addition on Reduction in Kappa No. for Southern Softwood (90°C and 100 psig pressure).
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FIGURE 2.11. Effect of Temperature on Reduction in Kappa No. For Southern Softwood
(2.5% NaOH and 100 psig pressure).
FIGURE 2.12. Effect of Oxygen Pressure on Reduction in Kappa No. for Southern Hardwood (100°C and 2.5% NaOH).
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ure 2.11), and oxygen pressure (Figure 2.12) accelerates both delignification and cellulose degradation reactions [36]. In laboratory studies
where the pressure is kept constant, beyond a minimum value of approximately 4 atmospheres, the effect of oxygen pressure is generally
small in comparison to the effects of alkali charge (Figure 2.10) and
temperature (Figure 2.11). Increases in oxygen pressure in laboratory
studies have relatively little effect in the absence of an excess of alkali
or an increase in temperature. In commercial installations, because the
pressure decreases as the pulp proceeds upward through the delignification tower, the pressure in the second stage is sometimes increased, as
for example in the OxytracTM process [28].
2.3.1.3. Rate Equations
The kinetics of oxygen delignification are complex and vary for
different wood species and pulping processes. The kinetics of oxygen
delignification can be described using either one-region or two-region
kinetic models [13,35,36,40–43]. These studies express the kinetics
of oxygen delignification in terms of reaction temperature (T), oxygen pressure (PO2), and alkali concentration [OH–]. Most of the kinetic
models are empirical and neglect mass transfer effects in the mathematical representation of the data. Empirical models have been used to size
oxygen delignification towers in one- and two-stage systems [44,45].
The most widely used one-region model for the rate of lignin removal (rL) is described by a power-law equation:
−rL = −
dK
= k[OH− ]m [ PO2 ]n K q
dt
(3)
where (K) is the kappa number; [OH–] is the sodium hydroxide concentration, and (PO2) represents the oxygen pressure [43]. The empirical
constants m, n, and q in Equation (3) are determined from experimental data. The reaction rate coefficient k depends on temperature and is
given by the Arrhenius equation:
 E 
k = A exp − A 
 RT 
(4)
where (EA) is the activation energy, (R) is the gas constant, and (T) is
the absolute temperature. Two-region models are also widely used. Olm
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and Teder summarized the drop in lignin content (L) expressed as kappa
number for Scandinavian softwood by a two-region model assuming
first-order kinetics for the kappa number (q = 1) [35]. The empirical
constants for the exponents on the hydroxyl ion and the oxygen pressure were determined from experimental data:
−rL =
dL
= k1[OH− ]0.1 Pox0.1K 01 + k2 [OH− ]0.3 Pox0.2 K 02
dt
(5)
Some studies have found that the exponent on the kappa number for
one-region models is greater than one [36,38,40–42]. One explanation
given for these high reaction orders is that the delignification reaction is
the sum of a great number of parallel first-order reactions taking place
simultaneously and corresponding to the different lignin moieties in the
pulp [46].
Ji and coworkers were able to express the delignification rate as a
first-order reaction with respect to lignin content calculated from kappa
number corrected for the presence of nonreactive HexA [32]. The effect
of alkali concentration was accounted for by fitting the pKa value (11.5
at 90°C) of the rate-limiting active lignin site. This site was identified
as the cyclohexadienone hydroperoxide anion formed by superoxide
reacting with the phenolate radical located at the carbon-3 position. The
effect of oxygen pressure was modeled by Langmuir-type adsorption
of oxygen on the active lignin site. The final rate equation at 90°C is:
−
PO2
dLC
[OH− ]
= 0.175
⋅
⋅ LC
−
dt
0.111 + [OH ] 1 + 3.39 PO2
(6)
with –dLc/dt expressed in mg lignin/g pulp/min, [OH–] in mol/l, and
PO2 in MPa.
2.3.2. Carbohydrate Selectivity
Competing delignification and carbohydrate degradation reactions
occur simultaneously during oxygen delignification. The degree of delignification is normally measured by determining the kappa number of
the pulp. Similarly, carbohydrate degradation is monitored by measuring the decrease in intrinsic viscosity [η]. Selectivity can be defined as
the ratio of the oxygen reaction rate with lignin to the reaction rate with
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the carbohydrate polymers present in the pulp. Selectivity is usually
expressed as the change in kappa number relative to the reduction in
pulp viscosity. These concepts are illustrated in Figures 2.13 and 2.14
for southern hardwood [45]. Figure 2.15 compares the selectivity for
southern softwood kraft pulps having initial kappa numbers of 90.2,
65.0, 40.0, and 25.0 [38]. The oxygen delignification experiments were
conducted in a single-stage oxygen delignification system using constant conditions of 100°C, 3% alkali application rate, and 75 psig (517
kPa). The oxygen delignification selectivity was higher for the pulps
with higher initial kappa numbers than for lower-kappa-number pulps.
Perfect selectivity would be obtained if the slope of the intrinsic viscosity versus kappa number curve were zero. It can also be seen that the
selectivity line of kraft delignification is similar to that of oxygen delignification for lower-kappa-number pulps (27.3 and 39.2).
The correlation between intrinsic viscosity [η] and the degree of polymerization (DP) is expressed by the Mark-Houwink-Sakurada equation [47].
[η ] = 0.6061× DP 0.90
FIGURE 2.13. Intrinsic Viscosity versus Time (minutes) for Southern Hardwood.
(7)
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FIGURE 2.14. Selectivity for Southern Hardwood (100°C and 100 psig pressure).
FIGURE 2.15. Intrinsic Viscosity [η] versus Kappa Number for Southern Softwood Pulps
in Single-Stage Oxygen Delignification (100°C, 75 psig and 3% NaOH).
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The intrinsic viscosity can be used to estimate DP, which can be
converted into the number average molecular weight and the moles of
cellulose per ton of pulp [35]. Similarly to kappa number, empirical
power-law models have also been proposed for the rate of cellulose
degradation (rC):
−rC = −
dM
= k[OH]m [ PO2 ]n M q
dt
(8)
where (M) is the number average molecular weight of cellulose and m,
n, and q are empirical constants. Note that Equation (7) does not account for the presence of hemicellulose polymers in the pulp.
The effect of initial kappa number on cellulose degradation rate expressed as moles of cellulose per ton of pulp is illustrated in Figure 2.16.
The initial kappa number has a large effect on the number of moles of
cellulose formed by reaction of oxygen with the pulp. The reduction in
the molecular weight of cellulose is less at the higher kappa-number
values. The change in the number of moles of cellulose at the zero (0)
time point in Figure 2.16 represents the degradation of cellulose taking
place in the digester during the pulping process [38].
FIGURE 2.16. Effect of Initial Kappa No. on the Number of Moles of Cellulose per Tonne
of Pulp Formed During Oxygen Delignification at 100°C, 75 psig (517 kPa), and 3%
NaOH.
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2.4. MASS TRANSFER EFFECTS
The rate of oxygen transport can limit the rate of the overall process [34,48–52]. Transport effects on the overall reaction rate can be
divided into interphase and intraphase factors (Figure 2.8). The mass
transfer resistance of oxygen in the gas phase is insignificant compared
to the resistance in the liquid phase [53]. There was no improvement in
the oxygen delignification rate after southern hardwood kraft pulp was
heavily refined. This suggests that intrafiber mass transfer effects also
do not greatly influence the delignification rate [36].
2.4.1. Oxygen Solubility
Mass transfer limitations are aggravated by the low solubility of oxygen in water and in aqueous sodium hydroxide solutions [54,55]. Medium-consistency oxygen delignification systems use pressure to improve
mass transfer by reducing gas volume and bubble size. The elevated
pressure also improves the solubility of oxygen gas. The important factors affecting oxygen solubility are temperature (T), oxygen pressure
(PO2), and the presence of inorganic solutes such as NaOH. Equations
for the solubility of oxygen in water have been given by Tromans [55].
The elevated temperature that is beneficial to oxidation reaction kinetics is detrimental to gas volume and solubility. Table 2.3 presents data
on the range of oxygen-gas solubility in water which are applicable to
commercial oxygen delignification process conditions and which come
from several sources. The solubility has been converted to an equivalent charge on pulp at 12% consistency [56].
The oxygen-gas charge used in commercial softwood oxygen delignification systems often ranges from 15 to 25 kg/tonne pulp. The
purpose of the gas mixer is pulp “fluidization,” a term which refers to
TABLE 2.3. Published Oxygen Gas Solubility in Water.
Pressure, Atm
0.7
7
9
10
15
Temp., °C
Solubility,
Grams/liter
Equivalent O2 at 12% Consistency
(Kg O2/Tonne Pulp)
100
100
100
100
100
0.004
0.105
0.224
0.38
0.373
0.029
0.77
1.64
2.79
2.73
(a) based on 12% o.d. pulp consistency.