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Ultrasonic Technology for Desiccant Regeneration

Ultrasonic Technology for Desiccant Regeneration

By Ye Yao, Shiqing Liu

With global warming and the rapid improvement of people’s living standards, energy consumption by air conditioning (AC) systems in buildings is on the rise. It has been noted that the dehumidification process accounts for a large proportion of energy consumption by an AC system. In southern areas of China where the climate is very hot and humid, the percentage of energy to be consumed by the dehumidification process in an AC system will be more than 40%. By using adsorption/absorption dehumidifying technology, the heat and moisture load of air can be processed separately, and a higher energy efficiency will be achieved compared with the conventional cooling dehumidification method. In addition, no condensation of water happens during the air dehumidification process with the adsorption/absorption method, which effectively prevents virus and mold from breeding, and hence improves indoor air quality (IAQ). Therefore, people are paying more attention to the adsorption/absorption
dehumidifying method as the key technology for developing high-performance of AC systems. Regeneration of desiccant is a crucial process during the air dehumidification cycle with the adsorption/absorption method. It will produce great influence on the energy efficiency of desiccant AC systems. The conventional regeneration method by heating is found to be energy-wasting due to the relatively higher regeneration temperature of some desiccant materials. So, we have put forward the ultrasound-assisted regeneration method in this book.
The fundamental theory of the novel regeneration method is summarized as follows: 1 The mechanical effect of ultrasound causes a series of rapid and successive compressions. This can reduce the thickness of boundary layer near the surface of solid desiccants and bring about the enhancement of mass transfer during regeneration. Meanwhile, the ultrasonic heating effect causes a temperature rise in solid desiccants and enhances internal moisture diffusivity known as “rectified diffusion.” 2 For liquid desiccants, the cavitation effect induced by power ultrasound sprays the solution into numerous tiny droplets with a size range of 40–80 μm, which improves the regeneration rate of liquid desiccants through enlarging the contact area between the air and the desiccant solute instead of increasing the solution temperature. The study in this book demonstrates that ultrasound-assisted regeneration can significantly increase energy efficiency of regeneration, shorten regeneration time and hence improve
performance of the desiccant AC system. In addition, the temperature for regeneration can be reduced by introducing power ultrasound, which provides favorable conditions for the utilization of low-grade thermal energy (e.g., solar energy and waste heat) in the desiccant regeneration.

Ultrasonic Technology for Desiccant Regeneration is edited based on recent studies on ultrasound-assisted regeneration. It consists of six chapters as below:
Chapter 1 introduces the background of the topic to be illustrated in this book; it includes a literature review on up-to-date technologies related to desiccant materials, desiccant dryer systems and regeneration methods, and gives basic knowledge about ultrasound and methods for producing ultrasound.
Chapter 2 deals with models for ultrasound-assisted regeneration for silica gel, presenting experimental and theoretical results and including a parametric study of the new regeneration method.
Chapter 3 investigates the effect of ultrasound on the regeneration of a new honeycomb-type desiccant and includes a parametric study on ultrasound-assisted regeneration.
Chapter 4 introduces the mechanism of the ultrasound-assisted regeneration for the liquid desiccants, and studies the effects of the ultrasonic atomization on the liquid desiccant regeneration.
Chapter 5 deals with the working principle and design calculation method for longitudinal and radial vibration ultrasonic transducers that have potential applications in ultrasound-assisted regeneration.
Chapter 6 presents several desiccant air-conditioning systems in which ultrasound-assisted regeneration is employed.
The book is written by Dr Ye Yao (Associate Professor at the Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, China) and Dr Shiqing Liu (Professor at the Institute of Mathematics and Physics, Zhejiang Normal University, China). Chapters 1, 2, 3, 4 and 6 as well as the appendix have been written by Dr Ye Yao, and Chapter 5 has been written by
Dr Shiqing Liu and Dr Ye Yao.

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About the Authors ix
Preface xi
Acknowledgements xiii
Nomenclature xv
1 Introduction 1
1.1 Background 1
1.2 Literature Reviews 2
1.2.1 Desiccant Materials 2
1.2.2 Types of Desiccant Dryer 4
1.2.3 Regeneration Methods 10
1.3 The Proposed Method 19
1.3.1 Basic Knowledge about Ultrasound 19
1.3.2 Sound Generation 22
1.3.3 Fundamental Theory for Ultrasound-Assisted Regeneration 24
1.4 Summary 26
References 26
2 Ultrasound-Assisted Regeneration of Silica Gel 33
2.1 Theoretical Analysis 33
2.2 Experimental Study 38
2.2.1 Experimental Setup 38
2.2.2 Procedure for Experiments 39
2.2.3 Methods 40
2.2.4 Results and Discussions 42
2.3 Empirical Models for Ultrasound-Assisted Regeneration 51
2.3.1 Model Overviews 51
2.3.2 Model Analysis

2.4 Theoretic Model for Ultrasound-Assisted Regeneration 59
2.4.1 Physical Model 62
2.4.2 Mathematical Model for Ultrasonic Wave Propagation 62
2.4.3 Mathematical Model for Heat and Mass Transfer in Silica Gel Bed 67
2.4.4 Model Validation 75
2.4.5 Error Analysis for Experimental Data 85
2.5 Parametric Study on Silica Gel Regeneration Assisted by Ultrasound 89
2.5.1 Acoustic Pressure and Oscillation Velocity in the Packed Bed 89
2.5.2 Thermal Characteristics of the Bed during Ultrasound-Assisted
Regeneration 91
2.5.3 Enhancement of Regeneration Assisted by Ultrasound 106
2.5.4 Comparisons between the Transverse- and Radial-Flow Beds 110
2.6 Quantitative Contribution of Ultrasonic Effects to Silica Gel Regeneration 110
2.6.1 Theoretical Analysis 110
2.6.2 Method 113
2.6.3 Results and Discussions 114
2.7 Energy-Saving Features of Silica Gel Regeneration Assisted by Ultrasound 119
2.7.1 Specific Energy Consumption 119
2.7.2 Results and Discussions 120
2.7.3 Brief Summary 125
2.8 Effects of Ultrasound-Assisted Regeneration on Desiccant System Performance 126
2.8.1 Study Objective and Method 126
2.8.2 Results and Discussions 127
2.8.3 Brief Summary 139
References 139
3 Ultrasound-Assisted Regeneration for a New Honeycomb Desiccant
Material 141
3.1 Brief Introduction 141
3.2 Experimental Study 142
3.2.1 Experimental System 142
3.2.2 Raw Material and Experimental Conditions 142
3.2.3 Analysis Parameters 144
3.2.4 Experimental Results 145
3.2.5 Energy Attenuation and Absorptivity of Ultrasound in the Material 154
3.3 Theoretical Model for Honeycomb-Type Desiccant Regeneration 159
3.3.1 Basic Assumptions 159
3.3.2 Governing Equations 159
3.3.3 Determination of Key Parameters 160
3.3.4 Model Validation 161
3.4 Model Simulations and Analysis 163
3.4.1 Parametric Study 163
3.4.2 Quantitative Contributions of Ultrasonic Effects to the Regeneration
of Honeycomb-Type Desiccant 172
3.5 Summary 176

4 Ultrasound-Atomizing Regeneration for Liquid Desiccants 177
4.1 Overview 177
4.1.1 Principles and Features of the Liquid-Desiccant Dehumidification 177
4.1.2 Thermo-Physical Properties of Liquid Desiccant Materials 178
4.1.3 Research Status of Solution Regenerators 182
4.2 Theoretical Analysis 183
4.2.1 Mass Transfer Coefficients for the Droplets 183
4.2.2 Atomized Size of Droplet by Ultrasonic Atomizing 192
4.2.3 Droplet Distribution Characteristics and Measurement Techniques 194
4.2.4 Vapor Pressure of Liquid Desiccant Mixture 196
4.3 Theoretical Modeling for the Ultrasound-Atomizing Regenerator 201
4.3.1 Assumptions 201
4.3.2 Basic Equations 201
4.3.3 Determination of Key Parameters 202
4.3.4 Model Validation 203
4.3.5 Parametric Study 208
4.4 Performance Analysis of Liquid-Desiccant Dehumidification System with
Ultrasound-Atomizing Regeneration 221
4.4.1 The Ultrasound-Atomizing Regenerator versus the Packed One 221
4.4.2 Performance of Liquid Desiccant System with Different Regenerators 226
References 233
5 Ultrasonic Transducers 235
5.1 Longitudinal Vibration of Sandwich Piezoelectric Ultrasonic Transducer 235
5.1.1 Overview 235
5.1.2 Theoretical Analysis 240
5.1.3 State Equations of Sandwich Piezoelectric Electromechanical
Transducer 248
5.1.4 Design Case 256
5.2 Radial Vibration Ultrasonic Transducer 258
5.2.1 Overview 258
5.2.2 Theoretical Analysis and Design of a Binary Radial Transducer 259
5.2.3 Radial Vibration Sandwich Piezoelectric Transducer 267
5.2.4 Summary 275
5.3 Ultrasonic Atomization Transducer 275
5.3.1 Basic Principle of Ultrasonic Atomization 275
5.3.2 Basic Structure of Ultrasonic Atomizers 275
5.3.3 Research Status and Applications 277
References 281
6 Desiccant System with Ultrasonic-Assisted Regeneration 283
6.1 For Solid-Desiccant System 283
6.1.1 Based on the Longitudinal Vibration Ultrasonic Transducer 283
6.1.2 Based on the Radial Vibration Ultrasonic Transducer 284
6.2 For Liquid-Desiccant System 287
6.3 Future Work

6.3.1 Development of Ultrasonic Transducer 289
6.3.2 Development of Desiccant Materials Adaptive to Ultrasound-Assisted
Regeneration 290
6.3.3 Development of Demister 290
6.3.4 Environmental Impact 290
References 292
A Basic Equations for Properties of Common Liquid Desiccants 293
A.1 Lithium Chloride (LiCl) 293
A.2 Calcium Chloride (CaCl2) 297
A.3 Lithium Bromide (LiBr) 299
A.4 Vapor Pressure (Pa) 302
A.5 Specific Thermal Capacity (J/(kg⋅
∘C)) 303
A.6 Density (kg/m3) 303
A.7 Dynamic Viscosity (Pa s) 303


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