A helicopter view of microwave application to chemical processes: reactions, separations, and equipment concepts

Abstract

We present a helicopter view of microwave technology application to various reaction and separation processes, including liquid-phase organic syntheses, gas-solid catalytic reactions, polymerizations, extraction, distillation, crystallization, membrane separation, and adsorbent regeneration/dehydration. The overarching aim is to demonstrate the breadth of potential applications of microwave technology to chemical industry, with particular attention to separations, as this is a less explored microwave application area. In this context, some key findings, opinions, and developments in the relevant literature are summarized. In addition, the present microwave equipment concepts for chemical processes are critically reviewed and new ones are put forward, as we believe that an important milestone in the road from laboratory-scale microwave experimentation to industrial-scale microwave-assisted chemical processing is the design and development of innovative microwave equipment concepts tailored for specific chemical processes.

In situ temperature measurements in microwave-heated gas-solid catalytic systems. Detection of hot spots and solid-fluid temperature gradients in the ethylene epoxidation reaction

“…In this case, heat can be supplied directly to the catalyst, with a huge potential regarding energy savings. Microwave heating, often in combination with microreaction technology has also been heralded as a keystone of intensification in chemical processing [8][9] .…”

Microwaves and Heterogeneous Catalysis: A Review on Selected Catalytic Processes

“…Microwave electromagnetic radiation is situated between 300 MHz and 300 GHz, the radio and infrared frequencies, with this corresponding to wavelengths in a vacuum of about 1 m to 1 mm. Current legislation makes the frequencies of 915 MHz, 2.45 GHz, and 5.85 GHz the most commonly available for chemical processes and Industrial Scientific and Medical (ISM) bands, in order to avoid any interference with broadcast and communications bands [1]. The frequency of 2.45 GHz is used as a source of heating commonly found in domestic microwave ovens.…”

“…The displacement rate of the charges, which is called a "displacement current" is proportional to the change rate of the electric field in respect to time. In the case of microwave irradiation, the electric field changes rapidly with time, and the complex permittivity, ε*, and the complex permeability, μ*, are expressed as follows: * = − (1) in which: ε′ (the real part), represents the dielectric constant, and is a representation of the ability of the material to store electrical energy, ε″ (the imaginary part) represents the loss factor and reflects the ability of the material to dissipate electrical energy * = − j (2) in which: μ′ (the real part) is the amount of magnetic energy stored within the material, μ″ (the imaginary part) represents the amount of magnetic energy which can be converted into thermal energy.…”

“…For an ideal solid in which each dipole has several equilibrium positions, the Boltzmann statistics can be used for the calculation of the relationship between τ and a dielectric constant [1]:…”

“…Even if for more than twenty years Process Intensification (PI) has been only identified as a kind of technological "toolbox" containing some spectacular examples of process improvement, it is one of the most important areas of progress for modern chemical engineering [1]. In 2009, Van Gerven and Stankiewicz [2] proposed a fundamental view on PI, with the definition of four basic principles and four domains, which must be considered if the aim is the intensification of a chemical process: spatial, thermodynamic, functional, and temporal.…”