Table of Contents

Food & Feed Drying Technology:
A Practical Approach

Preface
Section 1: Basic Principles
1. Introduction and Basic Principles
1.1 – General Considerations
1.2 – Defining the Product Moisture Content
1.2.1 – Wet basis
1.2.2 – Dry basis
1.3 – How does the Water Evaporate?
1.4 – A Definition for Drying Efficiency
1.5 – What Controls the Drying Rate?
1.6 – The Product Drying Curve

2. Drying Systems used in the Food Industry
2.1 – Examples of “Uncontrolled” vs “Controlled” Drying Processes
2.2 – Alternative Dryer Configurations and their Classification
2.2.1 – Batch vs Continuous Operation
2.2.2 – Dryer Classification and Air Condition
2.2.3 – Dryer Classification and the Energy Sources in Drying
2.2.4 – Dryer Classification and the Product Bed Condition
2.2.5 – Dryer Classification and the Product Transport Mechanism
2.3 – Design and Operational Attributes of the Various Alternative Drying Systems
2.3.1 – Bin Dryers and Coolers
2.3.2 – Tray Dryers
2.3.3 – Drum Dryer
2.3.4 – Agitated Batch Dryer
2.3.5 – Cascade Dryers and Coolers
2.3.6 – Continuous Belt Dryers
2.3.7 – Dynamic Convective Drying – Fluidised Bed Dryer and the Spouted Bed Dryer
2.3.8 – The Pneumatic Conveyor Dryer or Flash Dryer
2.3.9 – The Spray Dryer
2.3.10 – Rotary Dryers
2.3.11 – Freeze-Drying Technology
2.4 – Dryer Technology Selection Criteria

Section 2: Product Water Activity, Psychrometrics and Product Quality
3. Product Water Activity & Product Stability
3.1– Background and Some History
3.2 – A Definition for Water Activity
3.3 – Measurement of Water Activity
3.4 – Water Activity and Sorption Isotherms
3.5 – Determining a Product Isotherm
3.6 – Calculation of the Water Activity
3.6.1 – Raoult’s Law
3.6.2 – Norrish Model
3.6.3 – Grover Model
3.6.4 – The Ross Equation
3.6.5 – Isotherm Models
3.7 – Prediction of the Equilibrated Water Activity and Moisture Content for Multi-Component Mixtures
3.8 – Water Activity and Product Microbial Stability
3.9 – Water Activity and Product Chemical Stability
3.10 – The Role of the Product Packaging

4. Psychrometry
4.1 – Background
4.2 – Definitions for the Psychrometric Parameters
4.3 – The Psychrometric Chart
4.4 – Use of Psychrometrics in Dryer Performance Evaluation

5. Drying and Product Quality Attributes
5.1 – The Drying Process and Product Quality
5.2 – Change in Nutrient Content and Flavour
5.3 – Shrinking and Cracking (Checking)
5.3.1 – Modelling Shrinkage During Drying
5.4 – Effect of Drying on Texture and Colour
5.5 – Effect of the Drying Process on Product Rehydration
5.5.1 – Basics of Rehydration Technology
5.5.2 – Modelling the Rehydration Process
5.5.3 – Influence of Product Pretreatment on Rehydration
5.6 – Microbiological Quality
5.6.1 – Reducing Microbial Contamination After Drying
5.6.2 – Mould Growth in Dried Products During Storage and Transport
5.7 – Product Losses Due to Thermal Damage
5.7.1 – Sources of Thermal Damage and its Mitigation
5.8 – Migration of Soluble Constituents
5.9 – Case Hardening
5.10 – Effect of Product Pretreatment on the Drying Process and the Product Quality
5.11 – Two Case Studies Relating to Drying and Product Quality
5.11.1 – Case Hardening and the Aquafeed Sink/Float Phenomenon
5.11.2 – Influence of Drying on Pasta Quality

Section 3: Drying Process Development and Drying Equipment Design
6. Mathematical Modelling of the Drying Process
6.1 – Development of Simplified Drying Process Models
6.2 – Effect of Porosity and Temperature Upon Diffusivity
6.3 – Effect of Product Pretreatment on the Drying Time
6.4 – Heat and Mass Transfer During Drying
6.5 – Models for the Diffusivity Using the Average Concentrations
6.6 – Modelling the Heat and Mass Transfer
6.7 – Non-Equilibrium Effects During the Drying Process
6.8 – Effect of Product Shrinkage on the Mass Transfer Rate
6.9 – Thin-Layer Drying Equations
6.10 – Product Drying Kinetics
6.11 – Calculation of Product Drying Times
6.11.1 – Use of Data from Drying Tests
6.11.2 – Calculations Based on the Drying Flux
6.12 – Estimation of Product Moisture Profiles (for Standard Geometries)

7. Principles of Mass and Energy Balances in Drying Systems
7.1 – Mass (Material) Balances – Steady-State Scenario
7.2 – Energy (or Heat) Balances
7.3 – Combined Mass and Energy Balances
7.4 – Utilising the Data from Mass and Energy Balances
7.5 – Industry Example

8. Dryer Selection
8.1 – General Considerations
8.2 – Preliminary Selection Considerations
8.3 – Secondary Selection Criteria
8.4 – Final Selection

9. Batch Tray Dryer Technology
9.1 – General Considerations
9.2 – Pros & Cons of Tray Drying Systems
9.3 – Alternative Configurations of Tray Drying Systems
9.4 – Design of the Tray Dryer System
9.5 – Operation of Tray Drying Systems
9.6 – Estimation of the Batch Drying Time 200

10. Belt Dryer Technology
10.1 – General Considerations
10.2 – Operational Considerations
10. 3 – Evaluation of alternative Conveyor-Belt Dryer Configurations
10.4 – Preliminary Sizing of a Belt Dryer

11. Spray Drying Technology
11.1 – Introduction
11.2 – Operational Characteristics of a Spray Dryer
11.3 – Sizing the Drying Chamber
11.4 – The Atomisation Process
11.5 – Controlling the Atomisation Process
11.5.1 – Control of Fluid Atomisation – Nozzle Systems
11.5.2 – Control of Atomisation – Centrifugal Systems
11.6 – Estimation of the Droplet Diameter
11.7 – Estimation of the Mass Transfer Coefficient
11.8 – Extrusion Porosification Technology

12. Pneumatic, Fluidised Bed and Spouted Bed Dryers
12.1 – Pneumatic Conveyor Dryers
12.2 – Fluidised Bed Dryers
12.3 – Basic Theory of Fluidised Bed Technology
12.4 – Minimum Fluidisation Velocity and Minimum Bubbling Velocity
12.5 – Understanding the Bed Hydrodynamics
12.6 – Understanding Drying in a Fluidised Bed System
12.6.1 – Handling Sticky or Cohesive Materials
12.6.2 – Heat and Mass Transfer in a Fluidised Bed
12.6.3 – Estimation of the Fan Power for a Fluidised Bed
12.7 – Static Fluid Bed Dryers versus Vibrating Fluid Bed Dryers
12.8 – Spouted Bed Dryers
12.8.1 – Benefits of Spouted Bed Technology
12.8.2 – The Performance of Spouted Bed Systems

13. Heat Pump Dryers
13.1 – General Background
13.2 – Basic Operating Principle of a Heat Pump
13.3 – The Application of Heat Pumps in Drying Applications
13.4 – The Advantages and Disadvantages of Heat Pump Dryers
13.5 – Performance Comparison for Heat Pump Drying Systems
13.6 – A Brief Comment on Refrigerants
13.7 – High Temperature Heat Pumps

14. Freeze-Drying Technology
14.1 – General Considerations
14.2 – The Freeze-Drying Process
14.3 – Equipment Types and Performance Characterisation
14.4 – Considerations for Freeze-Drying Process Design, Development and Scale-Up
14.5 – Putting it all together
14.6 – Analysis of Freeze-Drying Processes
14.7 – Design Considerations in Freeze-Drying Plant Design

15. Osmotic Dehydration
15.1 – General Considerations
15. 2 – The Mechanism of Osmotic Dehydration
15.3 – Factors affecting the Osmotic Dehydration Process
15.4 – Selection of the most appropriate Osmotic Agent
15.5 – The Pros and Cons of Osmotic Dehydration
15.6 – Mathematical Modelling of the Osmotic Dehydration Process
15.6.1 – A Methodology for Osmotic Dehydration Tests
15.6.2 – The Solution of Fick’s 2nd Law
15.6.3 – The Solution of the Weibull Distribution Model
15.6.4 – Worked Example – Dried Apricots

16. Process Design of Alternative Drying Systems
16.1 – General Considerations
16.2 – Rules of Thumb for the Process Design of Various Types of Drying Technology
16.3 – Batch Tray Dryers
16.4 – Batch Agitated Dryers
16.5 – Belt Dryers
16.5.1 – Worked Example 1
16.5.2 – Worked Example 2
16.6 – Drum Dryers
16.6.1 – Worked Example 1
16.6.2 – Worked Example 2
16.7 – Pneumatic Dryers
16.8 – Fluidised Bed Dryers
16.9 – Spouted Bed Dryers
16.10 – Spray Dryers
16.11 – Estimation of the Drying Time in a Freeze-Dryer
16.11.1 – Worked Example 1
16.11.2 – Worked Example 2

17. Mechanical Design Considerations for the Design of Drying Systems
17.1 – General Considerations
17.2 – The Basic Principles of Clean Design
17.3 – Additional Considerations for the Incorporation of Clean Design Principles
17.3.1 – Edge Reinforcement of Sheet Metal
17.3.2 – Inclusion of Bolts and Bolt Finishing
17.3.3 – Inspection Windows / Access Hatches
17.3.4 – Product Transfer
17.3.5 – Structural Members and Supports
17.3.6 – Product Conveyor Guides and Supports
17.3.7 – Drive Pulleys
17.3.8 – Dryer Installation
17.3.9 – Guarding
17.4 – The Selection of the Materials of Construction
17.4.1 – Material Properties
17.4.2 – Process Considerations
17.4.3 – Common Materials of Construction for Dryers
17.5 – The Selection of Bed Support or Conveyor Configuration
17.6 – Dryer Maintenance
17.6.1 – Feed and Distribution System Maintenance
17.6.2 – The Product Transport System Maintenance
17.6.3 – Heater System Maintenance
17.6.4 – Fan System and Ducting Maintenance
17.6.5 – Filter System Maintenance
17.7 – Preventive Maintenance

Section 4: Drying Process Control and Drying Process Optimisation
18. Process Control for Drying Systems
18.1 – Process Control of Drying Systems
18.2 – Benefits of an Optimised Process Control for Drying Systems
18.3 – Basic Principles for the Process Control of Drying Systems
18.3.1 – The Control Loop
18.3.2 – Process Control Terms
18.4 – Process Control for Tray Dryers
18.5 – Process Control for Vacuum Dryers
18.6 – Process Control of Fluidised Bed Dryers
18.7 – Process Control for Drum Dryers
18.8 – Process Control of Rotary Dryers
18.9 – Process Control of Spray Dryers

19. Process Optimisation & Energy Efficiency for Drying Systems
19.1 – Background
19.2 – Impact of Variable Input Moisture Content
19.3 – Understanding Drying Process Energy Efficiency
19.4 – Process Optimisation Studies
19.4.1 – Optimisation of Convective Hot Air Drying – Response Surface Methodology examples
19.4.2 – Optimisation of the Spray Drying Process
19.4.3 – Optimisation of the Drying via the use of Online Process Mass Spectrometer Gas Analysis
19.5 – A Checklist of Drying Process Improvement Opportunities
19.6 – Quick Determination of Modified Process Parameters for a Dryer
19.6.1 – Worked Example: Productivity Improvements
19.6.2 – Worked Example: New Operating Temperature
19.7 – Assessing Dryer Efficiency
19.8 – A Strategy for Improving Dryer Efficiency

20. Economic Evaluation of Drying Systems
20.1 – Background
20.2 – Determining the Total Drying Cost
20.3 – Preliminary Estimation of the Capital Cost of a Drying System
20.4 – Preliminary Estimation of the Operating Cost of a Drying System
20.5 – Design of an Economically Optimal Drying System

21. Troubleshooting the Drying Process
21.1 – Background
21.2 – An Introduction to the Principles of Process Troubleshooting
21.3 – A Simplified Dryer Performance Assessment
21.4 – A Step-by-Step Guide to Dryer Troubleshooting
21.5 – Preventing Thermal Damage during Drying
21.6 – Preventing Self-Heating and Ignition in Dryers

Appendix 1 – Estimation of the Transport Properties of Air, Water and Saturated Steam
Appendix 2 – Estimation of Material Thermal Properties
Appendix 3 – Effect of Pressure on the Boiling Point of Water
Appendix 4 – Correlations for the Estimation of Heat Transfer Coefficients
Appendix 5 – Calculation of Psychrometric Data
Appendix 6 – Estimation of Moisture Diffusivity in Foods
Appendix 7 – Technical Analysis of Particulate Materials
Appendix 8 – Solids Density Determination

Nomenclature and Symbols
Technical References
Index