Tomography-controlled Microwave Drying of Porous Materials
State of the art: Microwave technology faces growing interest in industry due to the possibility of volumetric and selective heating of dielectric materials. With that significant energy and time saving as compared to conventional, convective and radiative heating can be achieved. In the latter case heating of process materials is limited to the surface only and heat transfer into the volume is defined by the material’s heat conductivity. Microwave heating has already proven its efficacy in e. g. debindering or sintering of ceramics, reducing processing time by a factor of 10 and more. For microwave assisted curing of carbon fibre reinforced composites in the automotive and avionic industries, a reduction of energy consumption up to 70% and processing time by about 50% has been demonstrated. Due to the strong microwave absorption of water microwave assisted drying is a wide spread application of microwave in e. g. ceramics, food, chemical and pharmaceutical industries. Selective heating of residual moisture, within e. g. agricultural products or impregnated foams for heat insulations, is easily achieved by microwave heating. Non-uniform moisture distribution could be more efficiently addressed by intelligent control of distributed microwave sources. However, applying such a precise microwave control requires in-situ and non-invasive measurement of the unknown distribution of moisture inside the bulk.
TOMOCON objectives: For tomography-controlled microwave drying of porous materials, such as impregnated foams, both Electrical Capacitance Tomography (ECT) and Microwave Tomography (MWT) sensors will be qualified for moisture distribution measurement and will be used to control microwave antennae power drivers. Technological challenges are the coupling of ECT electrodes either via contact using robotic actuators or contactless. For MWT a grand challenge is the derivation and solution of the inverse problem for limited data of a non-regular arrangement of only few microwave emitters. Here extensive field modelling and development of appropriate iterative reconstruction schemes is necessary. Targeted are 10 Hz acquisition speed for bimodal measurement and < 1 s latency. The controller concept shall incorporate knowledge-based control and self-learning by making use of a data repository for different goods, geometries and process parameters. Multi-physics process modelling is used to predict microwave propagation, heat transfer and moisture removal by combining electromagnetic field simulations with CFD simulations for convective moisture removal using a porous body approach. The developed ECT and MWT sensor concepts will be implemented and validated on an industrial conveyor belt microwave system at Karlsruhe Institute of Technology (KIT) and tested for drying of polymer foams with real relevance to industry. Demonstration will be performed at KIT’s microwave laboratory HEPHAISTOS with a number of microwave processing and heating systems. The microwave lines there have an excellent technological level and highest flexibility for microwave system development, characterization and application. Different microwave systems up to walk-in ovens are available as infrastructure.