A systematic study of the conventional and microwave (MW) kinetics of an industrially relevant demethylation reaction is presented. In using industrially relevant reaction conditions the dominant influence of the solvent on the MW energy dissipation is avoided. Below the boiling point, the effect of MWs on the activation energy E_a and k₀ is found nonexistent. Interestingly, under reflux conditions, the microwave-heated (MWH) reaction displays very pronounced zero-order kinetics, displaying a much higher reaction rate than observed for the conventionally thermal-heated (CTH) reaction. This is related to a different gas product (methyl bromide, MeBr) removal mechanism, changing from classic nucleation into gaseous bubbles to a facilitated removal through escaping gases/vapors. Additionally, the use of MWs compensates better for the strong heat losses in this reaction, associated with the boiling of HBr/water and the loss of MeBr, than under CTH. Through modeling, MWH was shown to occur inhomogeneously around gas/liquid interfaces, resulting in localized overheating in the very near vicinity of the bubbles, overall increasing the average heating rate in the bubble vicinity vis-à-vis the bulk of the liquid. Based on these observations and findings, a novel continuous reactor concept is proposed in which the escaping MeBr and the generated HBr/water vapors are the main driving forces for circulation. This reactor concept is generic in that it offers a viable and low cost option for the use of very strong acids and the managed removal/quenching of gaseous byproducts.

Microwave plasma (MWP) technology is currently being used in application fields such as semiconductor and material processing, diamond film deposition and waste remediation. Specific advantages of the technology include the enablement of a high energy density source and a highly reactive medium, operational flexibility, fast response time to inlet variations and low maintenance costs. These aspects make MWP a promising alternative technology to conventional thermal chemical reactors provided that certain technical and operational challenges related to scalability are overcome. Herein, an overview of state-of-the-art applications of MWP in chemical processing is presented (e.g. stripping of photo resist, UV-disinfection, waste gas treatment, plasma reforming, methane coupling to olefins, coal/biomass/waste pyrolysis/gasification and CO₂ conversion). In addition, two potential approaches to tackle scalability limitations are described, namely the development of a single unit microwave generator with high output power (>100 kW), and the coupling of multiple microwave generators with a single reactor chamber. Finally, the fundamental and engineering challenges to enable profitable implementation of the MWP technology at large scale are discussed.

The control of nucleation in crystallization processes is a challenging task due to the often lacking knowledge on the process kinetics. Inflexible (predetermined) control strategies fail to grow the nucleated crystals to the desired quality because of the variability in the process conditions, disturbances, and the stochastic nature of crystal nucleation. Previously, the concept of microwave assisted direct nucleation control (DNC) was demonstrated in a laboratory setup to control the crystal size distribution in a batch crystallization process by manipulating the number of particles in the system. Rapid temperature cycling was used to manipulate the super(under)saturation and hence the number of crystals. The rapid heating response achieved with the microwave heating improved the DNC control efficiency, resulting in halving of the batch time. As an extension, this work presents a novel design in which the microwave applicator is integrated in the crystallizer, hence avoiding the external loop though the microwaves oven. DNC implemented in the 4 L unseeded crystallizer, at various count set points, resulted in strong efficiency enhancement of DNC, when compared to the performance with a slow responding system. The demonstrated crystallizer design is a basis for extending the enhanced process control opportunity to other applications.