Technical Papers and Presentations

An electrically heated fixed bed (19 mm diameter, 200 mm length) is investigated regarding its electrical conductivity. Various electrically conductive activated carbons are used. The fixed bed is constructed from activated carbon pellets and/or activated carbon fabric. Electrical heating is provided by axially arranged electrodes to which a voltage of about 20 V is applied. In addition, nitrogen flows through the vessel. Experimentally, the resulting current, the resulting resistance, the temperatures at four points and a thermal image are recorded. The applied voltage and measured current are used to calculate the electrical power introduced into the fixed bed. Although the integral power is easy to calculate, it must be considered that the power does not drop homogeneously across the fixed bed but as a function of the local electrical conductivity. The electrical conductivity of semiconductors such as activated carbon is temperature dependent. Thus, one needs the temperature distribution in the fixed bed for the calculation of the local introduced power. In addition, it is expected that the electrical conductivity of bulk materials depends on their compression. At the same time, an increase in temperature can lead to thermal expansions of the fixed bed and the container, which also influences the compression of the packed bed. The COMSOL Multiphysics® software is used to describe a multi-physics model of the experiment. The following interfaces are used: Heat Transfer in Porous Media, Electric Currents, Electromagnetic Heating. A simple heat balance shows that the nitrogen flow has only a small influence on the energy balance. Most of the electrical power introduced is dissipated through the vessel or flanges in steady state. The electrical conductivity of the bed as well as the thermal conductivity of the bed are crucial. The model is used to compare the simulated temperature distributions with the experimental temperature distributions and thus allows assumptions about, for example, the electrical conductivity of the packed bed. The results show the significant influence of the compression of the packed bed on the electrical conductivity and thus on the temperature distribution. Further investigation results are the simulated behavior for homogeneous material properties and a parameter estimation for the description of the temperature dependence of the electrical conductivity under defined assumptions.