Microwave Field Applicator Design in Small-Scale Chemical Processing

Abstract

Ever since the first experiments nearly three decades ago, microwave enhanced chemistry has received incessant scientific attention. Many studies report improved process performance in terms of speed and conversion under microwave exposure and therefore it is recognized as a promising alternative method of process activation. It has also raised skepticism though, since the mechanisms behind the process enhancement remain unclear. Nevertheless, in the context of process intensification, the combination of microwave fields and microreactor systems has a promising quality; the enhanced reaction rates of the former and the superior heat and mass transfer rates and tightly controlled processing conditions of the latter together may provide a well-controlled and highly intensified processing environment.The objective of this thesis is to explore the possibilities to apply a microwave field in microstructured reactor systems. The familiar (domestic) multimode cavity systems are discounted as a viable means to apply a microwave field to a microreactor; the electromagnetic conditions in such systems simply are too poorly defined and controlled. In order to give each molecule the same processing experience, the field applicator needs to apply a spatially uniform microwave field. Therefore it is investigated what the theoretical limitations are on the uniformity of the electromagnetic field and heating rate distribution under parametric variation in a hypothetical resonant system. Design charts are presented that illustrate how important operating, geometric and medium parameters relate with each other. It is demonstrated how these simple configurations can provide design guidelines and first approximations for more realistic process equipment geometries. In a next step, the practical limitations encountered in commonly applied cavity systems are investigated. To this end, a simple exemplary process was analyzed both by experiment and simulation. The process under consideration is heating of water contained in a vial inside a popular, off-the-shelf, single-mode microwave cavity device. Both the heating rate distribution and the overall heating rate are investigated as well as the sensitivity of these measures to parametric variation. It is found that the resonant microwave field in generic, non-tailored systems is highly sensitive to parametric variation, that the heating process is hard to predict, and that that such systems do not lend themselves for control or optimization. Currently, the types of microwave equipment that are used in microwave chemistry research are principally limited to the aforementioned generic microwave systems. To widen this scope, the potential of standard sized, rectangular waveguides to form a basis for microwave applicator systems is explored. It is demonstrated that such systems support microwave fields that are relatively simple and predictable, which enables a higher degree of adaption and optimization to fit specific process requirements. The feasibility of long residence time continuous flow processing under microwave activation is experimentally demonstrated in a novel reactor type that the rectangular waveguide uniquely supports. Up to this point only cavity systems that support resonant fields have been considered. Resonant conditions are associated with hard-to-predict electromagnetic field patterns, difficulty in controlling and optimizing heat generation, and intrinsic spatial non-uniformity. The novel Coaxial Traveling Microwave Reactor concept is proposed as a means to address these issues by avoiding resonance altogether. Thus the highly optimized processing conditions characteristic of microreactors may be retained. Two concept variants are presented, one for liquid phase processing and one for heterogeneous gas phase catalytic reactions, respectively. A method to optimize the applicator geometry is demonstrated. The thesis is concluded by a discussion on the design principles that were identified in the course of the research and a on a framework for further development of equipment for electromagnetically enhanced chemical processing systems.