Modern Drying Technology, Energy Savings Book

Series Preface XI

Preface of Volume 4 XV

List of Contributors XIX

Recommended Notation XXIII

EFCE Working Party on Drying; Address List XXIX

1 Fundamentals of Energy Analysis of Dryers 1
Ian C. Kemp

1.1 Introduction 1

1.2 Energy in Industrial Drying 2

1.3 Fundamentals of Dryer Energy Usage 3

1.3.1 Evaporation Load 3

1.3.2 Dryer Energy Supply 4

1.3.3 Evaluation of Energy Inefficiencies and Losses: Example 5

1.3.3.1 Dryer Thermal Inefficiencies 6

1.3.3.2 Inefficiencies in the Utility (Heat Supply) System 8

1.3.3.3 Other Energy Demands 13

1.3.4 Energy Cost and Environmental Impact 14

1.3.4.1 Primary Energy Use 14

1.3.4.2 Energy Costs 14

1.3.4.3 Carbon Dioxide Emissions and Carbon Footprint 15

1.4 Setting Targets for Energy Reduction 16

1.4.1 Energy Targets 16

1.4.2 Pinch Analysis 17

1.4.2.1 Basic Principles 17

1.4.2.2 Application of Pinch Analysis to Dryers 19

1.4.2.3 The Appropriate Placement Principle Applied to Dryers 21

1.4.2.4 Pinch Analysis and Utility Systems 24

1.4.3 Drying in the Context of the Overall Process 25

1.5 Classification of Energy Reduction Methods 26

1.5.1 Reducing the Heater Duty of a Convective Dryer 28

1.5.2 Direct Reduction of Dryer Heat Duty 29

1.5.2.1 Reducing the Inherent Heat Requirement for Drying 29

1.5.2.2 Altering Operating Conditions to Improve Dryer Efficiency 30

1.5.3 Heat Recovery and Heat Exchange 31

1.5.3.1 Heat Exchange Within the Dryer 31

1.5.3.2 Heat Exchange with Other Processes 32

1.5.4 Alternative Utility Supply Systems 32

1.5.4.1 Low Cost utilities 33

1.5.4.2 Improving Energy Supply System Efficiency 33

1.5.4.3 Combined Heat and Power 34

1.5.4.4 Heat Pumps 36

1.6 Case Study 37

1.6.1 Process Description and Dryer Options 37

1.6.2 Analysis of Dryer Energy Consumption 38

1.6.3 Utility Systems and CHP 42

1.7 Conclusions 43

References 45

2 Mechanical Solid–Liquid Separation Processes and Techniques 47
Harald Anlauf

2.1 Introduction and Overview 47

2.2 Density Separation Processes 51

2.2.1 Froth Flotation 51

2.2.2 Sedimentation 54

2.3 Filtration 61

2.3.1 Cake Filtration 61

2.3.2 Sieving and Blocking Filtration 72

2.3.3 Crossflow Micro- and Ultra-Filtration 73

2.3.4 Depth and Precoat Filtration 75

2.4 Enhancement of Separation Processes by Additional Electric or Magnetic Forces 80

2.5 Mechanical/Thermal Hybrid Processes 83

2.6 Important Aspects of Efficient Solid–Liquid Separation Processes 85

2.6.1 Mode of Apparatus Operation 85

2.6.2 Combination of Separation Apparatuses 87

2.6.3 Suspension Pre-Treatment Methods to Improve Separation Conditions 91

2.7 Conclusions 94

References 95

3 Energy Considerations in Osmotic Dehydration 99
Hosahalli S. Ramaswamy and Yetenayet Bekele Tola

3.1 Scope 99

3.2 Introduction 100

3.3 Mass Transfer Kinetics 101

3.3.1 Pretreatments 101

3.3.2 Product 102

3.3.3 Osmotic Solution 103

3.3.4 Treatment Conditions 103

3.4 Modeling of Osmotic Dehydration 104

3.5 Osmotic Dehydration – Two Major Issues 105

3.5.1 Quality Issues 105

3.5.2 Energy Issues 106

3.5.2.1 Osmo-Convective Drying 107

3.5.2.2 Osmo-Freeze Drying 109

3.5.2.3 Osmo-Microwave Drying 111

3.5.2.4 Osmotic-Vacuum Drying 113

3.6 Conclusions 114

References 116

4 Heat Pump Assisted Drying Technology – Overview with Focus on Energy, Environment and Product Quality 121
Sachin V. Jangam and Arun S. Mujumdar

4.1 Introduction 121

4.2 Heat Pump Drying System – Fundamentals 122

4.2.1 Heat Pump 122

4.2.2 Refrigerants 125

4.2.3 Heat Pump Dryer 127

4.2.4 Advantages and Limitations of the Heat Pump Dryer 130

4.3 Various Configurations/Layout of a HPD 131

4.4 Heat Pumps – Diverse Options and Advances 132

4.4.1 Multi-Stage Heat Pump 132

4.4.2 Cascade Heat Pump System 133

4.4.3 Use of Heat Pipe 134

4.4.4 Chemical Heat Pump (CHP) 135

4.4.5 Absorption Refrigeration Cycle 138

4.5 Miscellaneous Heat Pump Drying Systems 140

4.5.1 Solar-Assisted Heat Pump Drying 140

4.5.2 Infrared-Assisted Heat Pump Dryer 143

4.5.3 Microwave-Assisted Heat Pump Drying 143

4.5.4 Time-Varying Drying Conditions and Multi-Mode Heat Pump Drying 145

4.5.5 Heat Pump Assisted Spray Drying 147

4.5.6 Modified Atmosphere Heat Pump Drying 148

4.5.7 Atmospheric Freeze Drying Using Heat Pump 149

4.6 Applications of Heat Pump Drying 150

4.6.1 Food and Agricultural Products 150

4.6.2 Drying of Wood/Timber 150

4.6.3 Drying of Pharmaceutical/Biological Products 152

4.7 Sizing of Heat Pump Dryer Components 153

4.8 Future Research and Development Needs in Heat Pump Drying 156

References 158

5 Zeolites for Reducing Drying Energy Usage 163
Antonius J. B. van Boxtel, Moniek A. Boon, Henk C. van Deventer, and Paul J. Th. Bussmann

5.1 Introduction 163

5.2 Zeolite as an Adsorption Material 164

5.2.1 Zeolite 164

5.2.2 Comparing the Main Sorption Properties of Zeolite with other Adsorbents 166

5.3 Using Zeolites in Drying Systems 168

5.3.1 Drying Systems 168

5.3.2 Direct Contact Drying 169

5.3.3 Air Dehumidification 170

5.4 Energy Efficiency and Heat Recovery 173

5.4.1 Defining Energy Efficiency 173

5.4.2 Energy Recovery for a Single-Stage System 174

5.4.3 Energy Recovery in a Multi-Stage System 176

5.4.4 Energy Recovery with Superheated Steam 178

5.5 Realization of Adsorption Dryer Systems 180

5.5.1 Adsorption Dryer Systems for Zeolite 180

5.5.2 Adsorption Wheel Versus Packed Bed 181

5.5.3 Zeolite Mechanical Strength 182

5.5.4 Long Term Capacity of Zeolite 183

5.5.5 Zeolite Adsorption Wheel 183

5.6 Cases 185

5.6.1 Zeolite-Assisted Drying in the Dairy Industry 185

5.6.2 Zeolite-Assisted Manure and Sludge Drying 189

5.6.3 Direct Contact Drying of Seeds with Zeolites 191

5.7 Economic Considerations 193

5.8 Perspectives 195

References 196

6 Solar Drying 199
Joachim Müller and Werner Mühlbauer

6.1 Introduction 199

6.2 Solar Radiation 200

6.3 Solar Air Heaters 204

6.4 Design and Function of Solar Dryers 210

6.4.1 Classification of Solar Dryers 210

6.4.2 Solar Dryers with Natural Convection for Direct Solar Drying 212

6.4.3 Solar Dryers with Natural Convection for Indirect Drying 213

6.4.4 Solar Dryers with Forced Convection for Direct Drying 214

6.4.5 Solar Dryers with Forced Convection for Indirect Drying 218

6.4.6 Dryers with Roof-Integrated Solar Air Heaters 223

6.5 Solar Drying Kinetics 226

6.5.1 Empirical Drying Curves in Solar Drying 226

6.5.2 Equilibrium Model for Solar Drying Kinetics 227

6.6 Control Strategies for Solar Dryers 231

6.6.1 Airflow Management During the Night 231

6.6.2 Recirculation of Drying Air 232

6.6.3 Back-Up Heating Systems 232

6.7 Economic Feasibility of Solar Drying 234

6.7.1 Drying of Timber in Brazil 235

6.7.2 Drying of Tobacco in Brazil 237

6.8 Conclusions and Outlook 239

References 242

7 Energy Issues of Drying and Heat Treatment for Solid Wood and Other Biomass Sources 245
Patrick Perré, Giana Almeida, and Julien Colin

7.1 Introduction 245

7.2 Wood and Biomass as a Source of Renewable Material and Energy 245

7.3 Energy Consumption and Energy Savings in the Drying of Solid Wood 254

7.3.1 Kiln-Drying of Solid Wood: A Real Challenge 254

7.3.2 The Conventional Drying of Wood 258

7.3.2.1 The Design of Conventional Kilns 258

7.3.2.2 Drying Time and Energy Efficiency 259

7.3.3 Theoretical Evaluation of the Kiln Efficiency 263

7.3.4 Two Case Studies of Kiln Efficiency 266

7.3.5 Rules for Saving Energy 269

7.3.5.1 Energy Savings in Conventional Kilns 269

7.3.5.2 Energy Saving by Alternative Technologies 270

7.4 Preconditioning of Biomass as a Source of Energy: Drying and Heat Treatment 271

7.4.1 Importance of Biomass Drying as a Preconditioning Step 271

7.4.1.1 Dryers for Biomass 273

7.4.1.2 Numerical Approach to the Continuous Drying ofWoody Biomass 276

7.4.2 Interest of Heat Treatment as a Preconditioning Step 281

7.5 Conclusions 287

References 289

8 Efficient Sludge Thermal Processing: From Drying to Thermal Valorization 295
Patricia Arlabosse, Jean-Henry Ferrasse, Didier Lecomte, Michel Crine, Yohann Dumont, and Angélique Léonard

8.1 Introduction to the Sludge Context 295

8.1.1 Origin, Production and Valorization Issues 295

8.1.2 Sludge: A Complex Material 297

8.1.3 Useful Properties for Energy Valorization 299

8.2 Sludge Drying Technologies 300

8.2.1 General Remarks 300

8.2.2 Convective Drying Methods and Dryer Types 301

8.2.3 Indirect Contact Drying Methods and Dryer Types 305

8.2.3.1 Rotor Design and Operation of the Drying Process 306

8.2.3.2 Drying Performances 308

8.2.4 Solar Drying and Dryer Types 310

8.2.5 Combined and Hybrid Drying 311

8.2.6 Sludge Frying, an Alternative to Conventional Drying 311

8.2.6.1 Heat and Mass Transfer During Fry-Drying 312

8.2.6.2 Energy and Environmental Aspects 313

8.2.7 Pathogen Reduction 314

8.3 Energy Efficiency of Sludge Drying Processes 315

8.3.1 Specific Heat Consumption of Sludge Dryers 315

8.3.2 Towards the Reduction of Energy Consumption Associated with Sludge Drying 316

8.3.3 Case Studies 316

8.4 Thermal Valorization of Sewage Sludge 318

8.4.1 General Description of the Thermal Processes Available for Sewage Sludge 318

8.4.2 Desired Water Content for Thermal Processes 319

8.4.3 Including a Drying Step Before Thermal Valorization 320

8.5 Energy Efficiency of Thermal Valorization Routes 321

8.5.1 Importance of Dryer Efficiency 321

8.5.2 Combining Sludge Drying and Thermal Valorization by Integrating on Site 322

8.6 Conclusions 324

References 325

Index 331