Preface

The use of alternative energy sources, such as alternating electromagnetic fields at different operating frequencies, acoustic and hydrodynamic cavitation, magnetic fields, plasma and high gravity fields in chemical processing are some of the key approaches of process intensification to enable greener chemical processes and sustainable chemical manufacturing. Some of these technologies have already been commercialized for certain niches. However, the breadth of industrial implementation will depend on the production and operating costs, robustness, flexibility and safety. The progress in the development of alternative energy source-based processes in various disciplines of chemicals and materials manufacturing reported in the open and patent literature gives confidence that the above criteria will be met. In this book, world leading researchers demonstrate the potential of several alternative energy transfer technologies to enable greener chemical processing in different industries through attainment of resource- and energy-efficient reaction and separation processes. Rather than being comprehensive in a specific application area or technology, the book aims at highlighting the broad impact that the aforementioned technologies may have in various application areas.

In Chapter 1, the impressive impact of microwave irradiation in the field of organic chemistry is discussed. The ability of microwaves to deliver energy rapidly and selectively to those components of the reaction mixture that are strongly microwave-dissipative, whether a reagent, a catalyst or a solvent, can enable greener chemistry in terms of decreased process times, higher energy efficiency and processing under solvent-free or green solvent conditions.

Chapter 2 presents different strategies for the application of microwaves to extract high value chemicals from plant matter. The volumetric heating of microwaves allows for their direct interaction with the plant matrix, intracellular water heating and vaporization, overpressure inside the plant matrix and, eventually, more effective cell wall rupture. This effect combined with rapid heating of a polar solvent may result in significantly faster extraction kinetics and improved materials efficiency, in terms of using less solvent and producing higher yields, compared to conventional heating.

Chapter 3 places the focus on the potential use of microwave technology for low temperature (and thus energy efficient) decomposition of biomass and biomass constituents (cellulose, lignin, hemicellulose) to high value chemicals. Although most of the relevant work in this area has been carried out with lab-scale microwave equipment, microwave process upscaling possibilities are also discussed.

Chapter 4 concludes the first part of the book devoted to microwave technology. The chapter discusses design aspects of different microwave applicator concepts suitable for chemical processing. The discussion extends beyond standard off-the-shelf monomode and multimode cavities to advanced non-cavity applicator types that can be used for efficient and tailored microwave activation of chemical reactors. In this context, an important suggestion put forward is that well-controlled and optimized microwave-assisted chemical processing requires transition from the current processing paradigm of chemical reactors activated by standing wave fields, as in conventional resonant cavity-based equipment, to chemical reactors activated by travelling electromagnetic fields.

Chapter 5 gives an overview of applications of cavitational (ultrasonic and hydrodynamic) reactors to different reactive and separation processes and the associated benefits in terms of greener and intensified processing. Faster chemical syntheses, improved yields and selectivities and safer operation at ambient conditions, mostly due to radical formation and mass transfer intensification, are some of the benefits expected in reactive processes. Cavitation, in synergy with oxidants, can also enable effective decontamination of wastewater. Regarding separation processes, application of ultrasound to crystallization can affect the crystal size distribution and product polymorphism. Further, ultrasound can enable shorter extraction processes with improved recovery at milder temperatures and lower amounts of solvents, compared to conventional extraction. Ultrasound can also improve adsorbents' activity and enhance adsorptives' desorption. Finally, it has been reported that ultrasound can improve vapor–liquid mass transfer and possibly break azeotropes in distillation processes.

Chapter 6 and 7 are concerned with magnetic fields. Chapter 6 presents applications of magnetic fields to separation processes in the chemical and biotechnology industries. In particular, an overview of mechanical magnetic separations, magnetic separations involving magnetic solids with non-tailored surfaces and magnetic separations involving tailored and functionalized magnetic solids is presented.

Chapter 7 introduces magnetic field-assisted mixing concepts. In most chemical reactive processes, the mixing rate determines the spatiotemporal distribution of the temperature and concentration fields, which in turn determine the reaction rates and product yield and distribution. Chapter 7 highlights intensification of mixing of fluids using magnetic fields in the context of ferrohydrodynamics and magnetohydrodynamics.

Chapter 8 discusses past achievements and future trends in the field of heterogeneous photocatalysis for solar fuel synthesis and pollutant degradation. The chapter is organized in two parts. First, novel developments in catalyst design are presented with a special focus on the application of MOFs. Second, the current state-of-the-art and challenges in the design of photocatalytic reactors are discussed including alternative options for the light source to enhance efficiency.

Chapter 9 reviews the most important reactor design concepts, which form building blocks for photocatalytic reactor designs aimed at wastewater treatment. The two popular performance indicators used in the literature to assess photocatalytic reactors are the photonic efficiency and the pseudo-first order rate constant. The former does not account for the total electricity consumption; the latter is process volume dependent. In this work, a new benchmark is introduced, the photocatalytic space-time yield, to address these limitations. The new benchmark has been demonstrated by comparing three different photocatalytic reactor designs, namely a microreactor, a membrane reactor and a parallel plate reactor. This comparative study points at a new direction in the research field of photocatalytic wastewater treatment. This is the efficient illumination of existing reactor geometries, instead of seeking new reactor geometries.

Plasma reactors are seen as an enabling technology for decentralized chemicals and fuels production and efficient utilization of renewable electricity generation from solar energy or wind. In this vein, Chapter 10 summarizes and evaluates plasma-assisted nitrogen fixation reactions (NO, NH₃ and HCN synthesis) in different types of plasma reactors. Despite the current limitation in scalability of plasma reactors, non-thermal plasma processing in certain operating windows in combination with solid catalysts has the potential to enable energy efficient chemistries.

The last two chapters of the book give an overview of applications of high gravity fields to green intensified chemical processing through intensification of mixing, heat and mass transfer and the enablement of ideal flow patterns and short contact times. In this context, Chapter 11 reviews the application of spinning disc reactors and rotating packed beds, including some novel recent versions of the latter, on polymerization, reactive-precipitation, catalytic and enzymatic transformation and adsorption processes. Chapter 12 introduces the concept of rotating fluidized beds in static vortex chambers. The hydrodynamic aspects and design characteristics of the vortex chambers are discussed in detail based on experiments and CFD simulations. The technology can intensify various processes, including low temperature pyrolysis and gasification of biomass, and particle drying and coating, when compared to conventional fluidized beds.

Georgios D. Stefanidis

Andrzej I. Stankiewicz