Achim Schmidt

Solid to Gas Temperature Gradient in a Microwave Fluidized Bed

Convective heat transfer from solid to gas was investigated in a microwave heated fluidized bed and compared to an identical conventionally heated bed. Fluidizing gas was fed at room temperature and heated in contact with the bed solid phase. Solid phase temperature was varied in range of 300-600°C. Temperature difference of the two phases and heat loss by gas stream were measured in both heating types and discussed regarding chemical reaction engineering and energy efficiency. Advantages of microwave heating in such systems were elaborated with an outlook to perspective applications of the process in field of materials science and engineering.

Analysis of microwave heating in a fluidized bed reactor

Analysis of microwave heating in a fluidized bed reactor, FBR, is presented. Simulation yields a detailed picture of selective heating by energy conversion from electromagnetic to thermal. However, particle movement and gas convection which distribute thermal radiation generated locally by microwave absorption into the volume of the FBR are not included. In order to analyze the superposition of microwave heating with convection and conduction in the FBR a unique 3D-temperature recording device has been applied. The simulated and the dynamic heating profiles are compared for an FBR filled with a microwave-absorbing material, like e.g., SiC.

Microwave antenna array for high temperature materials processing

Industrial scale high temperature microwave processing of materials, like e.g., dielectric heating for sintering or heat treatment of powder metals and ceramics has not been successful yet, mainly because of lack of suitable equipment. The paper presents results of antenna development aiming at 2.45 GHz microwave transmission into hot processing chambers, with minimum heat loss because only minor cooling of windows or transmission lines is required. High power microwave coupling is enabled by an array of high temperature resistant rod antennas. The design is performed in two steps: first simulation (FDTD) and experimental validation is done for a single antenna, than the results are transferred into the design of a 6 antenna array. This antenna array is capable of transmitting up to 36 kW 2.45 GHz radiation into a 1m3 volume process chamber at temperatures up to 1500°C. The homogeneity of E-H-field distribution in the chamber and the limitations of material selection for the antenna array are discussed.

Microwave assisted Platinum recycling

Purification of recycled platinum is a multi-step process of repeated dissolving of the scrap metal, precipitation of Platinum salts and thermal treatment of the salts to obtain a pure metal. As a new process, microwave heating is applied for dissolving platinum as well as for decomposition of the Pt-salts into sponge Platinum metal. The goal of microwave heating is reduction of process time, reduced utilization of chemicals like chlorine and savings on energy consumption. By use of microwave radiation at 2.45 GHz frequency all goals could be achieved. In addition, application of pulsed microwave heating enables tailoring of the Pt-metal morphology.

Production Scale m3 Batch Furnace for Hybrid-Heating and Microwave Sintering

Production scale inert atmosphere microwave sintering has not been successful yet, mainly because of lack of suitable equipment. For sintering in air microwave furnaces are available at different industrial scale, e.g., gas-microwave hybrid heated batch kilns or microwave “adiabatic casket” tunnel kiln [1]. Inert atmosphere furnaces require vacuum and gastight coupling of microwave energy. In the State of the Art technology, microwave-coupling windows represent a heat sink inside the sintering furnace. Although, radiation heat loss at temperatures > 1600 °C can be reduced by using a thermally insulating casket inside the microwave furnace, heat loss and temperature gradients caused by the microwave transparent windows are still not acceptable. Therefore, a new concept was developed assisted heating in inert atmosphere sintering furnaces, enabling high power microwave coupling by means of high temperature resistant antennas [2]. This paper presents microwave sintering results of commercial powder metallurgical (PM)-parts, details of the antenna system and the processing technology.