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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