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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
An intuitively organized and incisive exploration of UV radiation and its modern applications In Photochemical Reactors: Theory, Methods, and Applications of Ultraviolet Radiation, distinguished civil engineer and researcher Dr. Ernest R. Blatchley III delivers a comprehensive exploration of the theory, methods, and contemporary and emerging applications of ultraviolet (UV) radiation. The author describes the fundamentals of the history of photochemistry and photochemical reactions before moving on to consider the dynamic behavior of UV-based reactor systems and the physical concepts that govern natural and man-made sources of UV radiation. The book also covers the numerical and empirical methods used to evaluate photochemical kinetics, photobiological kinetics, and the dynamics of UV photoreactors. Common and emerging applications of UV radiation--like the disinfection of water, wastewater, air, and surfaces--are discussed, and UV-induced transformation processes are also explored. Readers will also find: * Thorough introductions to methods and principles that are universal to UV processes, as well as comparisons between those processes * Critical explorations of the physics of natural and artificial sources of ultraviolet radiation * Practical discussions of modern applications of UV radiation, including the disinfection of water, air, wastewater, and surfaces, as well as the use of UV photoreactors to promote photolysis and photo-initiated, radical-mediated reactions Perfect for UV professionals, academics, and scientists, Photochemical Reactors: Theory, Methods, and Applications of Ultraviolet Radiation will also earn a place in the libraries of professionals working in companies that manufacture UV reactors, as well as engineering consultants with a professional interest in ultraviolet radiation.
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Photochemical Reactors
Theory, Methods, and Applications of Ultraviolet Radiation
Ernest R. Blatchley III
Purdue University
Indiana, USA
This edition first published 2023© 2023 John Wiley & Sons, Inc.
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Preface
As compared to other common reactor types, development and application of photochemical reactors have been relatively recent. The fundamental principles that guide our current understanding of photochemistry and photochemical reactors started to emerge only within roughly the last century. Despite this relatively short period of development, interest in photochemical reactors has expanded rapidly, especially over the last 20–30 years.
Applications of photochemical reactors, which are dominated by ultraviolet (UV) radiation sources, are expanding rapidly. However, common misunderstandings or simplifications of the behavior of these systems have negatively affected the implementation of these technologies. Implementation of UV‐based reactor systems has also been hampered by a lack of any comprehensive reference or textbook to describe these systems.
A central goal of this work is to provide a comprehensive summary of the theory, methods, and common applications of UV radiation. Heavy emphasis has been placed on the refereed literature as the sources of information for this book; however, for parts of the material presented herein, it was necessary to cite documents from the gray literature.
The book is intended for use as a reference for practicing engineers and scientists, and as a textbook for a graduate‐level class in engineering. Readers are assumed to have a formal educational background in calculus and differential equations, general chemistry, calculus‐based physics (Newtonian mechanics), and fluid mechanics; previous educational or practical experience related to process engineering or physico/chemical processes of environmental engineering will be beneficial, though not required.
This book was prepared as a series of three logically ordered modules. When taught as a 1‐semester class, each module can be taught effectively in roughly five weeks. Module 1 (Chapters 1–4) presents the background and history of the development of principles of photochemistry and photochemical reactors. The principles presented in Module 1 are foundational to the remainder of the book. Module 2 (Chapters 4–8) presents detailed descriptions of the numerical and experimental (empirical) methods that are used to characterize and quantitatively describe photochemical reactions and photochemical reactor dynamics. Module 3 (Chapters 9–11) presents detailed summaries of three common groups of applications, including disinfection of water and wastewater, photochemical reactors designed to impart chemical changes on water and wastewater, and the use of UV to disinfect air and surfaces. Below are brief summaries of the material included in each of the chapters of this book.
Chapter 1. Background and History – This chapter includes descriptions of early, foundational discoveries in photochemistry, including the “Laws of Photochemistry,” the people and personalities involved in their development, and some intriguing philosophical points that were raised by some of the pioneers in this field. The chapter also includes descriptions of important milestones in the development of our current understanding and application of photochemistry and photochemical reactors.
Chapter 2. Photochemical Reactions – Most students will be familiar with the logic and methods that are used to describe the mechanisms and kinetics of (thermal) chemical reactions. These principles are summarized as a refresher, followed by parallel presentation of analogous principles that are used to define our understanding of the mechanisms and kinetics of photochemical reactions. Important similarities, differences, and connections between chemical and photochemical reactions are also presented.
Chapter 3. Photochemical Reactor Theory – Students with experience in reactor engineering or physico/chemical processes of engineering will be familiar with the logic and mathematical basis of basic reactor models that used to describe the dynamic behavior of thermal and biochemical reactors. These principles are presented briefly, followed by parallel presentation of the principles that are used to define our understanding of the dynamics of photochemical reactors. Important similarities, differences, and connections between chemical and photochemical reactors are also presented.
Chapter 4. Ultraviolet Radiation Sources – The fundamental principles of physics and chemistry that govern the generation and spatial distribution of radiant energy from natural and “artificial” sources of UV radiation are presented. Most artificial sources involve processes that convert input electrical power into output UV photon power; the basic principles that govern the behavior of these sources are presented.
Chapter 5. Actinometry and Radiometry – This chapter provides a detailed summary of the methods and instruments that are used to measure radiant energy from UV sources. Chemical actinometry represents a family of methods that are used to measure UV energy based on known photochemical behavior. Radiometric methods rely on detectors that convert input photon energy into an output electronic signal.
Chapter 6. Numerical Models for Simulation of Photochemical Reactor Behavior – The dynamic behavior of photochemical reactors is governed by the dose distribution that is delivered by that system. In turn, the dose distribution is governed by the spatial distribution of radiant energy and fluid mechanical behavior within the irradiated zone of the reactor. These attributes of photochemical reactors can be accurately simulated using contemporary computational tools, including fluence rate field models and computational fluid dynamics, both of which are summarized in this chapter. The chapter also includes a summary of integrated applications of these numerical methods for simulation of photochemical reactor dynamics.
Chapter 7. Validation of Photochemical Reactors – Applications of UV photoreactors often require that they meet standards of performance. Common methods that have been developed to define the performance of photochemical reactors are presented in this chapter, along with discussion of some of the benefits and limitations of these methods. Modern and emerging methods for characterization of photochemical reactor performance are also presented.
Chapter 8. Methods for Quantification of Microbial Responses to UVC Irradiation – Methods that are used to quantify and characterize the effects of UVC radiation on microbes need to account for the mechanisms that govern these effects. This chapter includes descriptions of the methods that have been developed to account for this mechanistic behavior for several groups of microbes, including bacteria, viruses, protozoa, and algae. The implications of the use of inappropriate methods are also described.
Chapter 9. UV Disinfection of Drinking Water and Municipal Wastewater – These represent the most common applications of UV‐based photoreactors. The fundamental principles that govern the design and application of these systems are presented, including summaries of important behavioral characteristics of these systems. Differences and similarities among systems that are used for disinfection of drinking water, municipal wastewater, and intermediate applications are also presented.
Chapter 10. Photolysis and Advanced Reaction Processes for Control of Trace Contaminants – UV‐based reactors are often used to alter the chemical characteristics of water and wastewater. The fundamental principles that are used to simulate and design reactors that are used for direct photolysis, advanced oxidation processes, and advanced reduction processes are presented.
Chapter 11. UV Disinfection of Air and Surfaces – The use of UVC‐based systems for disinfection of air and surfaces was first demonstrated in the 1930s and has since been shown to be effective for reducing the risks of exposure to airborne and surface‐associated pathogens. The emergence of hospital‐acquired infections, antibiotic‐resistant microbes, and the COVID‐19 pandemic have heightened interest in these systems. This chapter provides a summary of the fundamental principles that are used to design and implement UVC‐based systems for disinfection of air and surfaces.
It is likely that applications of photochemistry and photochemical reactors will continue to expand into other areas of engineering and the physical sciences. The principles and methods described in this book will provide a foundation for understanding their dynamic behavior, but adaptations may be required to account for differences between these new applications and those that are described herein.
Ernest R. Blatchley IIIPurdue University – West Lafayette, IN
Acknowledgements
I started writing this book during a sabbatical leave from Purdue University. Prior to leaving for this sabbatical, a graduate student in one of my classes asked a simple question about my plans for this book: “fiction or nonfiction?” As you might guess, this student was a bit of a wise‐guy, but I appreciated his wry smile as he asked this question. Among my goals in writing this book was to be comprehensive and rigorously correct in presenting the current state‐of‐knowledge as related to photochemical reactors. So, to answer this question posed by my former student (who will remain un‐named), nonfiction!
The process of writing this book was a tremendous learning experience for me and one that would not have been possible without the support of many other people. First and foremost are the contributions of many engineers and scientists at Trojan Technologies. Special thanks to Linda Gowman for championing this project and for her steadfast interest. I would also like to thank Ted Mao and Mark Kustermans who continued to support this project after Linda retired. I am indebted to the many current and former employees of Trojan Technologies who provided support for this project in the form of formal reviews of individual chapters and informal conversations. These include: Jim Robinson, Mike Sasges, Gord Knight, Gang Fang, and Domenico Santoro; for anyone who provided comments or other input that I have not named here, please accept my sincere apologies. I am also grateful to Professor Ronnie Gehr of McGill University whose thoughtful, detailed comments and suggestions led to notable improvements of several chapters.
For the spring 2022 semester, I developed an online graduate‐level class based on the material presented in this book. Students in this class were gracious and helpful in identifying errors within the text, which were completely my fault. I would like to thank them for their patience and willingness to help: Sudharshan Anandan, Kevin Brown, Xiaosu Ding, Rachel Gehr, Chunxu Huang, Kartikeya Pandey, Satya Patra, and Jamaie Scott.
Barbara Blatchley, Professor of Psychology at Agnes Scott College and one of my sisters, has also been an important source of inspiration for me. She is a gifted author and has written both a textbook (Statistics in Context) and a great book on the subject of luck (What are the Chances: Why We Believe in Luck). I regularly bent her ear to get advice on this project. Her guidance was important to me, not only in moving me toward completion of this project, but also in doing it the right way. Thank you Barbie!
Finally, I want to thank my wife, Marcia Blatchley, for her interest and support of this project. The process of writing this book has been a huge consumer of my time, but she has always been patient, supportive, and understanding, even (or especially) in the times when I was pulling my hair out.
About the Companion Website
This book is accompanied by a companion website.
www.wiley.com/go/blatchley/photochemicalreactors
This website includes the entire textbook.
1Background and History
1.1 Introduction
Photochemistry is defined as “a branch of chemistry that deals with the effect of radiant energy in producing chemical changes.” [1] In a more formal sense, photochemistry involves any overall process:
where R represents a molecule that absorbs the energy of a photon (hν) to yield an electronically excited molecule (R*), which in turn reacts to yield one or more products (P) [2]. As we will see in Chapter 2 and other parts of this book, there are many other possible fates for electronically excited intermediates, R*. In some cases, the excess energy within R* is dissipated by physical processes that may return the reactant molecule to its ground state. These may be viewed as examples of photophysical processes: [2]
Photochemical reactors include natural and engineered systems in which one or more photochemical processes take place to bring about changes to system composition. Common changes within these systems may involve chemical or microbial components of the system, but in principle any component or phase of the system may be susceptible to photochemical change. In most applications, we are concerned with changes to fluid (i.e. liquid or gas) composition; however, photochemical and photophysical processes may also be induced within solids or on solid surfaces.
As compared with other fields of science, our understanding of the fundamental principles of photochemistry and photochemical reactor theory is relatively young. Much of the foundational material that informs our current understanding of photochemistry and photochemical reactors has been formulated over the last century. As a point of reference, calculus was developed independently by Isaac Newton and Gottfried Leibniz in the mid‐17th century. Newton also played critical roles in establishment of many of the basic principles of modern physics and mechanics; he defined the “laws of motion” in 1687 in the Principia Mathematica Philosophiae Naturalis (Latin for “Mathematical Principles of Natural Philosophy”).
This first chapter is intended to provide brief introductions to photochemistry and photochemical reactors, as well as the people and personalities involved in the development of the fundamental principles that inform our understanding of these processes. In addition to providing historical context, we will see that the people who developed these principles represented a remarkable cross section of humanity. However, among many (not all) of these individuals, several admirable personal attributes were evident. In general, the people who informed our current understanding of photochemical processes had remarkably clear, profound perceptions of the world that surrounds us and were able to distill their understanding of nature and natural phenomena into clear, concise statements. Some were able to describe these phenomena in mathematical terms, and many had a passion to communicate their understanding to others. These pioneers of photochemistry and photochemical processes were often progressive, courageous individuals who were willing to challenge scientific dogma, sometimes at the cost of significant professional risk.
1.2 Early Applications, Discoveries
In the 5th century BC, Hippocrates prescribed heliotherapy (i.e. sunbathing) for medical and psychological purposes [3]. This practice has continued until today, although a formal understanding of the specific medical conditions that were improved by this practice did not emerge until the 1700s. It was not until the 1800s that adverse human health effects were linked to exposure to sunlight.
Isaac Newton's contributions to science were not limited to the discovery of calculus or the formulation of the laws of motion. In 1666, he showed that white sunlight could be divided into colors; he coined the term “spectrum” to describe this range of colors and developed an instrument that involved optical elements, including a prism, which may have been the first spectrometer [4].
Ancient civilizations understood that sunlight provides visibility, warmth, health, and vitality; however, their understanding of these phenomena was almost entirely empirical. Physical explanations evolved through mythology. The first report of an organic photochemical reaction was attributed to Trommsdorff (1834), who reported that crystals of α‐Santonin turned yellow and “burst” when exposed to sunlight [5]. Between the 17th century and early 20th century, scientific principles related to electromagnetic radiation and photochemistry evolved considerably [3, 6].
In 1614, Sala noted that sunlight turned silver nitrate (AgNO3) crystals black. More than a century and a half later (1777), Scheele observed that paper soaked in silver chloride (AgCl) darkened when exposed to sunlight. Though it was not evident at the time, this discovery was related to chemical (film) photography that would emerge in the late 19th century. Subsequent experiments by Ritter (1801) indicated that radiation just beyond the violet portion of the electromagnetic spectrum was more effective than other colors (wavelengths) at promoting this reaction. Radiation in this region of the spectrum was referred to as “deoxidizing” rays; today, we identify this as ultraviolet (UV) radiation.
A seminal contribution was provided by Kirchhoff and Bunsen in 1859 in the form of the spectroscope, an early form of a spectrometer (see Figure 1.1) [7]. This device was used to demonstrate that atoms absorb and emit electromagnetic radiation at characteristic wavelengths. Based on observations made with this device, they speculated that gaps in the spectrum of solar radiation were attributable to selective absorption by constituents of earth's atmosphere. This device and the observations made by Kirchhoff and Bunsen provided the first clear links between chemical composition and spectroscopic behavior. As such, this device represents the foundation for contemporary analytical spectroscopy.
Figure 1.1 Schematic illustration of spectroscope developed by Kirchhoff and Bunsen [7]. (a) was an internally blackened, trapezoidal box; two small telescopes (b and c) were directed at a prism (f). The eyepiece of (b) was removed and replaced by a plate in which a slit formed by two brass edges was adjusted at the focus of the objective lens. Light was provided by a device later identified as a Bunsen burner (d ). A sample of the material of interest was held in the flame by a platinum wire mounted on a stand (e). The prism rested on a brass plate; the prism and a mirror (g) were rotated using handle (h).
Source: Kirchhoff and Bunsen [7], figure 2 (p.91)/with permission of Taylor & Francis.
In 1865, Maxwell proposed that light and sound both belong to a larger energy spectrum that has wave‐like properties. The term “electromagnetic” wave was assigned because Maxwell believed that both were generated by the interactions of electric and magnetic fields. Maxwell's theory was later supported by Hertz who demonstrated the existence of microwaves, which fell beyond (i.e. at wavelengths longer than) the UV, visible, and infrared (IR) portions of the spectrum. These findings established the basis for the wave theory of light, which still informs some aspects of our current understanding of the behavior of electromagnetic radiation.
1.3 Development of Modern Principles of Photochemistry
The observations and discoveries leading up to the work of Maxwell and Hertz formed the foundation of modern photochemistry. However, formalization of many of the fundamental principles of photochemistry required a number of other profound insights and discoveries. In many cases, the people who made these discoveries emerged as icons of the scientific world and are recognized among the giants of scientific history.
Max Planck was a German physicist who studied under Kirchhoff and Helmholz [8]. He succeeded Kirchhoff as Professor at the University of Berlin. Like many of the other scientists who participated in the development of principles that define photochemistry, he made numerous, wide‐ranging scientific contributions, largely in the areas of thermodynamics and the behavior of electromagnetic radiation. Perhaps his greatest contribution came from his examination of radiation emitted from heated bodies, in which he pointed out that the principles that had been used to describe the behavior of radiation to that point were incapable of describing the emission spectrum from a “black body.” This led to the development of quantum theory, and ultimately to Planck being awarded the Nobel Prize in Physics in 1919.
