DRY 7: LAYER AND CONTACT DRYING
     PART 1: Introduction
         1. HOW TO USE THIS MANUAL
         2. MAIN CHARACTERISTICS OF CONTACT AND CONVECTIVE LAYER DRYERS
             2.1 Introduction
             2.2 Contact dryers
             2.3 Convective layer dryers
         3. REFERENCES
     PART 2: State of the Technology
         1. INTRODUCTION
             1.1 Purpose and layout
             1.2 Contact dryers
             1.3 Convective layer dryers
         2. DRYING UNDER VACUUM
             2.1 Introduction
             2.2 Selection of Operating Pressure
             2.3 Heating surface
             2.4 Condensers
             2.5 Vacuum Pumps
             2.6 Vapour Filtration
         3. CONTACT DRYERS
             3.1 Introduction
             3.2 Vacuum Tray Dryers
                 3.2.1 Introduction
                 3.2.2 Equipment
                 3.2.3 Applications
                 3.2.4 Process Design
                 3.2.5 Safety Hazards
             3.3 Vacuum Band Dryers
                 3.3.1 Introduction
                 3.3.2 Equipment
                 3.3.3 Applications
                 3.3.4 Safety Hazards
             3.4 Plate Dryers
                 3.4.1 Introduction
                 3.4.2 Equipment
                 3.4.3 Applications
                 3.4.4 Process Design
                 3.4.5 Safety Hazards
             3.5 Thin-Film Dryers
                 3.5.1 Introduction
                 3.5.2 Equipment
                 3.5.3 Applications
                 3.5.4 Process design
             3.6 Drum Dryers
                 3.6.1 Introduction
                 3.6.2 Equipment
                 3.6.3 Applications
                 3.6.4 Process Design
                 3.6.5 Safety Hazards
             3.7 Rotating Batch Vacuum Dryers
                 3.7.1 Introduction
                 3.7.2 Equipment
                 3.7.3 Applications
                 3.7.4 Process Design
                 3.7.5 Safety Hazards
             3.8 Horizontally Agitated Dryers (Low Speed)
                 3.8.1 Introduction
                 3.8.2 Equipment
                 3.8.3 Applications
                 3.8.4 Process Design
                 3.8.5 Safety Hazards
             3.9 Indirectly-Heated Rotary Dryers
                 3.9.1 Introduction
                 3.9.2 Equipment
                 3.9.3 Applications
                 3.9.4 Process Design
                 3.9.5 Safety Hazards
             3.10 Vertically Agitated Dryers
                 3.10.1 Introduction
                 3.10.2 Equipment
                 3.10.3 Safety
                 3.10.4 Process design
             3.11 Combination Dryers
                 3.11.1 Introduction
                 3.11.2 Filter dryers
                 3.11.3 Granulator/Bowl dryers
             3.12 Vibrated contact dryers
         4. FREEZE DRYERS
             4.1 Introduction
             4.2 The principles of freeze drying
             4.3 Limited Freeze Drying
             4.4 Commercial freeze dryers
         5. CONVECTIVE LAYER DRYERS
             5.1 Introduction
             5.2 Atmospheric tray dryers
                 5.2.1 Introduction
                 5.2.2 Equipment
             5.3 Rotating shelf dryers
                 5.3.1 Introduction
                 5.3.2 Equipment
                 5.3.3 Safety hazards
             5.4 Atmospheric band dryers
                 5.4.1 Introduction
                 5.4.2 Equipment
                 5.4.3 Safety hazards
             5.5 Tunnel dryers
                 5.5.1 Introduction
                 5.5.2 Equipment
                 5.5.3 Safety
             5.6 Batch through-circulation dryers
                 5.6.1 Equipment
                 5.6.2 Safety
             5.7 Moving bed dryers
                 5.7.1 Introduction
                 5.7.2 Equipment
                 5.7.3 Safety hazards
             5.8 Horizontal agitated dryers (high speed)
                 5.8.1 Introduction
                 5.8.2 Equipment
                 5.8.3 Safety
             5.9 Rotary-louvre dryers
                 5.9.1 Introduction
                 5.9.2 Equipment
                 5.9.3 Safety
             5.10 Layer dryers for sheets and films
                 5.10.1 Introduction
                 5.10.2 Cylinder dryers
                 5.10.3 Yankee dryers
                 5.10.4 Flotation dryers
                 5.10.5 Impingement drying
                 5.10.6 Through-drying
                 5.10.7 Infra-red drying
                 5.10.8 Stenters
                 5.10.9 Festoon drying
                 5.10.10 Drying of bobbins
                 5.10.11 Novel sheet dryers
                 5.10.12 Vibrated Through-Circulation Dryers
         6. SAFETY AND ENVIRONMENTAL CONSIDERATIONS
             6.1 Introduction
             6.2 Fires and explosions
                 6.2.1 Introduction
                 6.2.2 Preliminary screening of materials
                 6.2.3 Dust cloud explosions
                 6.2.4 Ignition of dust layers and material in bulk
                 6.2.5 Vapour explosions
                 6.2.6 Explosion protection
                 6.2.7 Good operating and maintenance practices
             6.3 Mechanical and noise hazards
             6.4 Toxic and hazardous materials
             6.5 Recycle systems
                 6.5.1 Introduction
                 6.5.2 Semi-closed with direct heating (self-inerting)
                 6.5.3 Semi-closed with indirect heating
                 6.5.4 Closed systems
             6.6 Environmental considerations
                 6.6.1 Introduction
                 6.6.2 The Environmental Protection Act (1990)
                 6.6.3 Exhaust emissions
         7. REFERENCES
         8. NOMENCLATURE
     PART 3: State of the Science
         1. INTRODUCTION
         2. HEAT TRANSFER IN LAYER DRYERS
             2.1 Overview of heat transfer in layer dryers
                 2.1.1 Convective layer dryers
                 2.1.2 Contact dryers
             2.2 Heat transfer to solids by convection
                 2.2.1 Introduction
                 2.2.2 Through-circulation dryers
                 2.2.3 Cross-circulation dryers
                 2.2.4 Impinging Jets
             2.3 Heat transfer to solids by radiation
                 2.3.1 Introduction
                 2.3.2 Radiation from black bodies
                 2.3.3 Radiation from grey bodies
                 2.3.4 Radiation from gases
             2.4 Heat transfer in solids by conduction
                 2.4.1 Introduction
                 2.4.2 The conduction equation
             2.5 Heat transfer by contact with a heating surface
                 2.5.1 Heat transfer to beds of particulates
                 2.5.2 Heat transfer to liquid, pastes and slurries
         3. DRYING PROCESSES IN CONVECTIVE LAYER DRYERS
             3.1 Overall description of the drying process
             3.2 Equilibrium moisture content
             3.3 Internal moisture transport
             3.4 External moisture transport by convection
                 3.4.1 Introduction
                 3.4.2 Mass transfer by diffusion through a boundary layer
                 3.4.3 Analogy between heat and mass transfer
                 3.4.4 Mass transfer coefficients
             3.5 Convective drying rate curve
             3.6 The characteristic drying curve
             3.7 Moisture transport within the layer
         4. DRYING PROCESSES IN CONTACT DRYERS
             4.1 Overall description of the drying process
             4.2 Equilibrium moisture content
             4.3 The contact drying curve
             4.4 The drying front
             4.5 The effect of agitation
             4.6 The effect of gas pressure and composition
                 4.6.1 Drying in the presence of pure evaporated vapour (boiling)
                 4.6.2 Drying of evaporated vapour with a noncondensible gas present (diffusion)
             4.7 Heat transfer considerations
         5. MATERIAL TRANSPORT AND AGITATION
             5.1 Introduction
             5.2 Solids residence times in continuous layer dryers
                 5.2.1 Band dryers
                 5.2.2 Plate dryers
                 5.2.3 Drum dryers
                 5.2.4 Rotating shelf dryers
                 5.2.5 Moving bed dryers
                 5.2.6 Indirect rotary dryers
                 5.2.7 Rotary louvre dryers
                 5.2.8 Horizontally agitated dryers
             5.3 Agitation and mixing
                 5.3.1 Introduction
                 5.3.2 Flow properties
                 5.3.3 Mixture quality
                 5.3.4 Models of mixing in agitated contact dryers
         6. CONVECTIVE LAYER DRYER MODELS
             6.1 Introduction
             6.2 Simplified analysis of convective layer drying (no agitation)
             6.3 Detailed analysis of thick layer convective drying (no agitation)
                 6.3.1 Static bed drying equations
                 6.3.2 Co-current and counter-current drying equations
                 6.3.3 Cross-flow drying equations
             6.4 Agitation in convective layer dryers
             6.5 Conclusions: practical application of the convective layer dryer models
         7. CONTACT DRYING MODELS
             7.1 Introduction
             7.2 Heat transfer in packed beds
                 7.2.1 Principles of contact heat transfer to packed beds
                 7.2.2 The Wall-to-Surface Heat Transfer Coefficient hws
                 7.2.3 The Surface-to-Bed Heat Transfer Coefficient hsb for unagitated beds
                 7.2.4 The Wall-to-Bed Heat Transfer Coefficient hwb for unagitated beds
                 7.2.5 The Wall-to-Bed Transfer Coefficient hwb for Agitated Beds
                 7.2.6 Thermal Conductivity of Packed Beds of Dry Particles
                 7.2.7 The Thermal Conductivity of Packed Beds of Wet Particles
             7.3 Drying Models
                 7.3.1 Introduction
                 7.3.2 Agitation
                 7.3.3 Drying rates of non-hygroscopic solids in pure vapour
                 7.3.4 Drying rate of hygroscopic solids in pure vapour
                 7.3.5 Drying rates of solids in vapour + gas
                 7.3.6 Drying rates of polydisperse particles
                 7.3.7 Schlunder's contact drying model: Discussion and conclusions
         8. SCALE-UP METHODS AND DESIGN PROCEDURES
             8.1 Introduction
             8.2 Simple scale up: specific drying rate (SDR) method drying rate
                 8.2.1 Basis of SDR method
                 8.2.2 Example 1: Continuous rotating shelf (tray) dryer
                 8.2.3 Example 2: Continuous vacuum plate dryer
                 8.2.4 Example 3: Batch horizontal agitated dryer
                 8.2.5 Assessment of the SDR scale-up method
             8.3 Scaling batch drying curves (convective layer dryers)
                 8.3.1 Theoretical basis of scaling rules
                 8.3.2 Application of scaling rules
         9. DISCUSSION AND CONCLUSIONS
             9.1 Basis for design procedure
             9.2 Contact Drying
             9.3 Convective Layer Drying
         10. NOMENCLATURE
         11. REFERENCES
         12. APPENDIX A1. GURNEY-LURIE CHARTS
         13 APPENDIX A2. CALCULATION OF CONTACT HEAT TRANSFER COEFFICIENT
         14 APPENDIX A3. CALCULATION OF THE EFFECTIVE DRY BED THERMAL CONDUCTIVITY
     PART 4: Design guide
         1. INTRODUCTION
         2. HEAT AND MASS BALANCES
             2.1 Dryer system flowsheet
             2.2 Overall mass balance
                 2.2.1 General points
                 2.2.2 Batch dryers
                 2.2.3 Continuous dryers
             2.3 Heat balance
                 2.3.1 Generic form
                 2.3.2 Continuous dryers
                 2.3.3 Batch dryers
                 2.3.4 Dominant terms in the heat balance
             2.4 Practical use of the equations
             2.5 Ancillary calculations
                 2.5.1 Heater duty
                 2.5.2 Gas recycle
                 2.5.3 Gas leaks
                 2.5.4 Solids recycle (backmixing)
         3. SCOPING DESIGN METHODS
             3.1 Introduction
             3.2 Contact dryers
             3.3 Convective through-circulation dryers
             3.4 Convective cross-circulation dryers
             3.5 Batch dryers with fixed batch size
             3.6 Other heating methods and combinational dryers
             3.7 Costing
             3.8 Performance mode calculations
         4. SMALL-SCALE TESTS AND MATERIAL PROPERTIES
             4.1 Drying behaviour
                 4.1.1 Drying kinetics
                 4.1.2 Equilibrium moisture content/desorption isotherms
             4.2 Handling and product quality properties
                 4.2.1 Cohesion and Adhesion
                 4.2.2 Attrition, dust and fines
                 4.2.3 Thermal Sensitivity
             4.3 Safety and environmental factors
             4.4 Laboratory simulation of selected layer dryers
                 4.4.1 Continuous rotating shelf dryer
                 4.4.2 Continuous vacuum plate dryer
                 4.4.3 Horizontally and vertically agitated dryers
                 4.4.4 Continuous rotary-louvre dryer
         5. SCALE-UP METHODS FOR LAYER DRYERS
             5.1 Introduction
             5.2 Simple scale up: specific drying rate (SDR) method drying rate
                 5.2.1 Basis of SDR method
                 5.2.2 Example 1: Continuous rotating shelf (tray) dryer
                 5.2.3 Example 2: Continuous vacuum plate dryer
                 5.2.4 Example 3: Batch horizontal agitated dryer
                 5.2.5 Assessment of the SDR scale-up method
             5.3 Detailed Scale-up of Convective Layer Dryers
                 5.3.1 Scaling batch drying curves
                 5.3.2 Example 3 Scaling of a through-circulation drying curve
             5.4 Overall Scale-up Procedure for Convective Layer Dryers
             5.5 Detailed Scale-up of Contact Dryers
                 5.5.1 Drying heat transfer requirements
                 5.5.2 Heat and Mass Transfer Resistances
                 5.5.3 Scaling Rules
                 5.5.4 Application of Detailed scaling rules
             5.6 Overall Scale-up procedure for Contact Dryers
         6. DETAILED SIMULATION OF CONVECTIVE LAYER DRYERS
             6.1 Introduction
             6.2 Elements of Detailed model
             6.3 Detailed Calculation Procedure
             6.4 Example case study
         7. DETAILED SIMULATION OF CONTACT LAYER DRYERS
             7.1 Introduction
             7.2 Rigorous model of contact drying
         8. RECOMMENDED DESIGN METHODS FOR INDIVIDUAL DRYER TYPES
             8.1 Classification of layer dryers
             8.2 Methods for individual dryer types
                 8.2.1 Cross-circulated tray dryers
                 8.2.2 Perforated tray (through-circulated) dryers
                 8.2.3 Contact or vacuum tray dryers
                 8.2.4 Deep bed through-circulation dryers
                 8.2.5 Rotating shelf (turbo-tray) dryers
                 8.2.6 Plate dryers
                 8.2.7 Cross-circulated belt and tunnel dryers
                 8.2.8 Moving Bed Dryers
                 8.2.9 Perforated band through-circulation dryers
                 8.2.10 Vacuum band dryers
                 8.2.11 Vertical pan and conical agitated dryers
                 8.2.12 Double-cone or tumbler dryers
                 8.2.13 Horizontal pan dryers
                 8.2.14 Horizontal agitated dryers with jacket heating
                 8.2.15 Indirect rotary steam-tube dryers
                 8.2.16 Rotary kilns (indirect rotary furnace)
                 8.2.17 Rotary Louvre Dryers
         9. NOMENCLATURE
         10. REFERENCES

• Part 1 Introduction
• Part 2 State of the technology
• Part 3 State of the science
• Part 4 Design guide

Volume DRY VII   Part 1 Introduction.

This describes the basic features of layer dryers, where the solids form a continuous layer with gas/vapour confined to the interstices, unlike the dispersion-type dryers described in Volumes DRY III-VI. Part 1 distinguishes between contact dryers (heated by conduction) and convective-heated dryers. It briefly reviews the advantages and disadvantages of the various types relative to other types of dryer, especially dispersion dryers.

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Volume DRY VII   Part 2 State of the technology.

In contact dryers, heat is supplied indirectly by conduction from a heated surface rather than by convection from a hot gas as in most other types of dryer. Early development of these devices was based on their ability to handle toxic and explosive materials safely. However, in recent years, their comparatively low energy consumption and compact design has stimulated wider interest. Eleven types (indirectly-heated rotary dryers, horizontal agitated dryers, rotating batch vacuum dryers, vacuum band dryers, vacuum tray dryers, plate dryers, drum dryers, thin film dryers, vertically agitated dryers, combination dryers and vibrated contact dryers) are described. In each case the topics addressed include equipment options, applications, process design, fire and explosion hazards, maintenance, environmental problems and costs. Contact dryers are frequently operated under vacuum and there is a separate chapter on vacuum systems.

Convective layer dryers can be divided into two sub-types: through-circulation, in which the drying gas passes through a layer of wet material; and cross-circulation, in which the drying gas passes over the wet material. Through-circulation convective dryers include band and belt dryers, perforated trays, moving bed and rotary-louvre dryers. Quite high drying rates can be obtained with less dust entrainment than for dispersion dryers. Pellets and preforms are often dried by these units, especially perforated band dryers. Through-circulation dryers are unsuitable for small particles as the solids can fall through the holes. Cross-circulation units include tunnel dryers, solid trays and rotating shelf (turbo-tray) dryers. The low degree of agitation makes these a popular choice for fragile materials, but conversely gives very slow drying rates.

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Volume DRY VII   Part 3 State of the science. 

This Part covers the underlying Science behind layer drying (both contact and convective). Layer dryers can be divided into two main types: contact and convective which operate under quite different principles. To avoid confusion the two types are dealt with separately where appropriate. The following areas are covered:

• Heat transfer: For contact dryers - direct contact heat transfer between heated walls and wet materials; heat transfer by conduction through layers of wet material. For convective layer dryers - heat transfer by convection.
• Drying kinetics and mass transfer: physical fundamentals, convective mass transfer coefficients; boiling and diffusion mechanisms in contact dryers; the influence of pressure and atmosphere in contact dryers; the characteristic drying curve concept.
• Material transport and agitation: the methods of material transport in layer dryers (moving belts, rotating plates, impellers etc.) and how this affects throughput and residence time are described. The related topic of agitation and mixing is also discussed.
• Dryer modelling: theoretical models of layer dryers are described. This includes a complete description of Schlünder’s contact dryer model and differential equation models of deep bed convective layer dryers.
• Dryer design and scale-up: Methods of dryer design based on experimental tests and scale-up are considered as a basis for the design procedure described in Part 4

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Volume DRY VII   Part 4 Design guide.

The subject matter for this guide, will include design procedures for both convective layer and contact dryers, is currently being researched. At present it is anticipated that, depending on the type of equipment and the test data available, a combination of theoretical models and experimental data will be the most appropriate basis for design. In many cases non-agitated dryers may be designed by a set of experimental tests and a simple scale-up procedure. Agitated dryers present more of a problem because it is very difficult to replicate the mixing patterns of a full-scale dryer in pilot scale equipment. At this stage it seems that Schlünder’s N_mix mixing model will be the most likely approach.