Contents. Handbook of Pulp. Edited by Herbert Sixta (Ed.)

Handbook of Pulp. Edited by Herbert Sixta (Ed.)

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-30999-3

VIII

2.1.3 The Microscopic Structure of Wood 48

2.1.3.1 Cell Types 48

2.1.3.2 Softwood and Hardwood Structure 50

2.1.3.3 Reaction Wood 54

2.1.3.4 Juvenile Wood 56

2.1.3.5 Secondary Changes 56

2.2 Outlook 59

2.2.1 Nano-structure of Fibers 59

2.2.2 Topochemical Distribution of Lignin and Phenolic Extractives 61

3 Wood Yard Operations 69

Jorg B. Ressel

3.1 Raw Material Storage 69

3.2 Debarking 71

3.2.1 Debarking Methods 72

3.2.1.1 Drum Debarker 72

3.2.1.2 Rotary Debarker 76

3.2.1.3 Ring Debarkers 77

3.3 Chipping and Screening 79

3.3.1 Disc Chipper 80

3.3.2 Drum Chipper 84

3.3.3 The Andritz HQ™-Sizer and Rechipper 86

3.3.4 Chip Conditioner 87

3.3.5 Chipper Canter Line: Profiling Line in Softwood Sawmills 88

3.4 Chip screening 89

3.4.1 Mechanical Screening 89

3.4.2 Wind Screening 93

3.4.3 Air Density Separator (ADS) 94

3.5 Process Control and Automation 95

3.6 Transport and Handling Systems 95

3.6.1 Log Handling 97

3.6.2 Stationary Conveyor Systems 97

3.6.3 Chip Storage 98

3.6.3.1 Chip Storage Systems 101

3.6.4 Wood Yard Losses and Waste Reduction 104

3.6.4.1 Specific Causes of Waste Generation 105

3.6.4.2 Pollution Prevention Options 105

4 Chemical Pulping Processes 109

Herbert Sixta, Antje Potthast, Andreas W. Krotschek

4.1 Introduction 109

4.2 Kraft Pulping Processes 111

4.2.1 General Description 111

4.2.2 Kraft Cooking Liquors 113

Contents

IX

4.2.3 Mass Transfer in Kraft Cooking 122

4.2.3.1 Purpose of Impregnation 122

4.2.3.2 Heterogeneity of Wood Structure 123

4.2.3.3 Steaming 130

4.2.3.4 Penetration 133

4.2.3.5 Diffusion 138

4.2.3.6 Diffusion Model 151

4.2.3.7 Effect of Impregnation on the Uniformity of Delignification 159

4.2.3.8 Numerical Solution of the Diffusion Model 163

4.2.4 Chemistry of Kraft Cooking 164

Antje Potthast

4.2.4.1 Lignin Reactions 164

4.2.4.2 Reactions of Carbohydrates 174

4.2.4.3 Reactions of Extractives 181

4.2.4.4 An Overview of Reactions During Kraft Pulping 183

4.2.4.5 Inorganic Reactions 184

4.2.5 Kraft Pulping Kinetics 185

Herbert Sixta

4.2.5.1 Introduction 185

4.2.5.2 Review of Kraft Cooking Models 188

4.2.5.3 Structure of a Selected Kinetic Model for Kraft Pulping 211

4.2.6 Process Chemistry of Kraft Cooking 229

4.2.6.1 Standard Batch Cooking Process 229

4.2.6.2 Modified Kraft Cooking 235

4.2.6.3 Polysulfide and Anthraquinone Pulping 306

4.2.7 Multistage Kraft Pulping 325

4.2.7.1 Prehydrolysis 325

4.2.7.2 Prehydrolysis: Kraft Pulping 345

4.2.8 Pulping Technology and Equipment 366

Andreas W. Krotschek

4.2.8.1 Batch Cooking versus Continuous Cooking 366

4.2.8.2 Batch Cooking Technology and Equipment 367

4.2.8.3 Continuous Cooking Technology and Equipment 377

4.3 Sulfite Chemical Pulping 392

Herbert Sixta

4.3.1 Introduction 392

4.3.2 Cooking Chemicals and Equilibria 395

4.3.3 Impregnation 403

4.3.4 Chemistry of (Acid) Sulfite Cooking 405

Antje Potthast

4.3.4.1 Reactions of Lignin 407

4.3.4.2 Reactions of Carbohydrates: Acid Hydrolysis 416

4.3.4.3 Reactions of Extractives 425

4.3.5 Process Chemistry of Acid Sulfite Pulping 427

Herbert Sixta

Contents

4.3.5.1 Basic Technology 427

4.3.5.2 Influence of Reaction Conditions 449

4.3.6 Alternative Sulfite Pulping Concepts 465

4.3.6.1 Magnefite Process 466

4.3.6.2 Two-Stage Neutral Magnefite (Bisulfite-MgO) 467

4.3.6.3 Sivola Processes 468

4.3.6.4 Stora Processes (Hydrogen Sulfite or Monosulfite-Acid Sulfite) 472

4.3.6.5 Alkaline Sulfite Pulping 475

5 Pulp Washing 511

Andreas W. Krotscheck

5.1 Introduction 511

5.2 Pulp Washing Theory 512

5.2.1 Overview 512

5.2.2 Drainage 513

5.2.3 Compressive Dewatering 517

5.2.4 Diffusion 517

5.2.5 Sorption 519

5.3 Principles of Washing 523

5.3.1 Dilution/Extraction Washing 524

5.3.2 Displacement Washing 524

5.3.3 Compressive Dewatering 525

5.3.4 Multi-Stage Washing 526

5.3.5 Fractional Washing 528

5.4 Washing Parameters 528

5.4.1 Overview 528

5.4.2 Dilution Factor 529

5.4.3 Feed and Discharge Consistencies 532

5.4.4 pH 533

5.4.5 Entrainment of Air 534

5.4.6 Temperature 535

5.4.7 Equipment-Specific Parameters 535

5.5 Washing Efficiency 537

5.5.1 Overview 537

5.5.2 Wash Yield 537

5.5.3 Displacement Ratio 538

5.5.4 Norden Efficiency Factor 539

5.5.5 Standardized Norden Efficiency Factor 545

5.6 Washing Equipment 547

5.6.1 General Remarks 547

5.6.2 Rotary Drum Washers 547

5.6.2.1 Conventional Drum Washers 547

5.6.2.2 Drum Displacer 549

5.6.3 Belt Washers 551

X Contents

5.6.4 Diffusion Washers 552

5.6.4.1 Atmospheric Diffuser 552

5.6.4.2 Pressure Diffuser 554

5.6.5 Roll Presses 556

5.6.6 In-Digester Washing 557

6 Pulp Screening, Cleaning, and Fractionation 561

Andreas W. Krotscheck

6.1 Introduction 561

6.2 Screening Theory 563

6.2.1 Introduction 563

6.2.2 Flow Regime 564

6.2.3 Fiber Passage and Reject Thickening 566

6.2.4 Selective Fiber Passage 570

6.3 Screening Parameters 572

6.3.1 Equipment Parameters 572

6.3.1.1 Screen Basket 572

6.3.1.2 Rotor 573

6.3.2 Operating Parameters 575

6.3.2.1 Reject Rate 575

6.3.2.2 Accept Flow Rate 575

6.3.2.3 Feed Consistency 577

6.3.2.4 Temperature 577

6.3.2.5 Rotor Tip Velocity 577

6.3.3 Furnish Parameters 578

6.3.3.1 Pulp Fibers 578

6.3.3.2 Contaminants 579

6.3.3.3 Entrained Air 579

6.4 Centrifugal Cleaning Theory 579

6.4.1 Introduction 579

6.4.2 Flow Regime 580

6.4.3 Sedimentation 581

6.4.4 Underflow Thickening 584

6.4.5 Selective Separation 585

6.5 Centrifugal Cleaning Parameters 586

6.5.1 Cyclone Parameters 586

6.5.2 Operating Parameters 587

6.5.2.1 Flow Rate and Pressure Drop 587

6.5.2.2 Feed Consistency 587

6.5.2.3 Temperature 587

6.5.3 Furnish Parameters 587

6.5.3.1 Pulp Fibers 587

6.5.3.2 Contaminants 588

6.6 Separation Efficiency 588

Contents XI

6.6.1 Screening and Cleaning Efficiency 588

6.6.2 Fractionation Efficiency 590

6.6.2.1 Removal Efficiency 590

6.6.2.2 Fractionation Index 591

6.7 Screening and Cleaning Applications 592

6.7.1 Selective Contaminant Removal 592

6.7.1.1 Knots 593

6.7.1.2 Shives 593

6.7.1.3 Bark 593

6.7.1.4 Sand and Stones 593

6.7.1.5 Metals and Plastics 594

6.7.2 Fractionation 594

6.8 Systems for Contaminant Removal and Fractionation 594

6.8.1 Basic System Design Principles 594

6.8.2 Systems for Contaminant Removal 596

6.8.2.1 Arrangement 596

6.8.2.2 Fiber Loss versus Efficiency 598

6.8.3 Systems for Fractionation 599

6.9 Screening and Cleaning Equipment 601

6.9.1 Pressure Screens 601

6.9.2 Atmospheric Screens 604

6.9.2.1 Secondary Knot Screens 604

6.9.2.2 Vibratory Screens 605

6.9.3 Hydrocyclones 605

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