With the increasing importance of renewable electricity as a primary source of energy on the planet, the importance of electricity–based technologies in process industries is expected to rise as well. Microwave heating is a well–known electricity–based industrial technology that is used in a variety of commercial applications.

This work addresses challenges and explores opportunities for industrial–scale utilization of microwave heating in heterogeneous catalytic gas-phase reactors. Through critical analysis of microwave applicators, the thesis highlights the limitations of traditional cavity–based reactors and underscores the potential of traveling–wave systems for achieving uniform heating profiles in heterogeneous catalytic flow reactors. The contribution of the thesis lies primarily in the design and optimization of traveling microwave reactors (TMRs). Challenges associated with catalyst heating profiles and process scale–up are addressed by introducing a coaxial waveguide structure and a tailored catalyst loading pattern. The TMR model demonstrates its effectiveness in accurately predicting temperature profiles and reaction dynamics along the reactor through simulation studies and experimental validation. Furthermore, the thesis introduces the Reverse Traveling Microwave Reactor (RTMR) as a novel reactor concept aiming to minimize temperature gradients along the catalyst bed by periodic reversal of microwave irradiation. Simulation–based studies showcase the RTMR’s potential in achieving temperature uniformity within the catalyst bed, offering new insights into reactor design and scale–up considerations for microwave–assisted catalytic flow processes.

The principal objective of the research focuses on the intensification of the batch and continuous crystallization processes through enhanced nucleation control, proper plug flow conditions in continuous tubular crystallizers and development of advanced image analysis based PAT tool for process monitoring.
Nucleation control is addressed through manipulation of the number of crystals in the crystallizer; by either controlling the rate of nuclei formation or through dissolution of the excess nuclei to limit the nucleation overshoot or through continuous seeding in case of flow crystallizer to suppress nucleation in the tubes. The following topics are addressed:

1. The efficiency of the Direct Nucleation Control(DNC) strategy using microwave heating.

2. Induction of high nucleation rates at low supersaturation by the application of laser or ultrasound energy.

3. Combination of the ultrasound assisted internal seed generation in the continuous tubular crystallizer, under plug flow conditions.

4. Characterization of nucleation and the crystal properties through development of in-situ imaging based PAT technology.

In this work, the doctoral thesis of G.S.J. Sturm on microwave reactors was continued. Microwave reactors are developed, because they have potential to increase safety and economically reduce waste. This work focused on microwaves and chemical processes, instead of just heating water, by designing a travelling microwave reactor for non-oxidative methane dehydroaromatization.

The travelling microwave reactor was designed using a structured approach. First, the objective was set to an economical conversion to aromatics of otherwise flared methane. Therefore, a successfully designed reactor is able to reduce the CO2 emissions and make many other reactions economical. Secondly, twenty-one challenges for microwave reactors were found using check-lists and a self-developed phenomena exploration method. This method was used to find unknown challenges on the intersection of established engineering fields. Then sub-solutions solving these challenges were extracted from existing reactors and reactor concepts. Finally, a few sub-solutions were selected and forced to work together to obtain the designed reactor.

The designed reactor consists of a high performance coated asymmetric annular monolith in an inert container with narrowing conductors in axial direction and an anisotropic porous media. The achieved production volume is three orders of magnitude larger than of a mono-mode microwave reactor at same operation frequency. The designed reactor is capable of obtaining another five orders of magnitude by increasing the temperature up to 1500K, pressure up to 50 atm, catalyst activity with at least two orders of magnitude and lowering the operating frequency. However, only four orders of magnitude were reached, because the designed reactor hit a flow limit.

The designed reactor with the three plus four order of magnitude improvement is not yet economical feasible. More advanced designs such as the spiralling narrowing rectangular microwave reactor might be. Furthermore, the designed reactor could become economically feasible in case of a more valuable product. Eventually, this work revealed some unsolved problems and opportunities of microwave reactors as well as information gaps. Moreover, it brought microwave reactors closer to industrial application.