(149e) Investigation of Fluidised Bed Dryer by Frequency-Depend Measurement and Different Normalisation Methods with Electrical Capacitance Tomography

During a drying process in a fluidised bed, the moisture content of particles changes from say 30% to 1%. Accordingly, the change in permittivity of the particle material can be estimated using Maxwell's equation, from 26.8 to 4.67, considering that the permittivity of water is 80 and the permittivity of dry particles is 4.0. In the meanwhile, the change in moisture content also results in the change in conductivity of material, from 0.028 to 40 S/m, which can be measured using an impedance analyser (HP4128) at 100 kHz measurement frequency. Because it is important to find the relationship between capacitance, moisture and frequency, an electrical capacitance tomography (ECT) system based on the HP4128 impedance analyser was used to measure the capacitances between different electrode pairs in fluidised bed dryer. During experiment, the measurement frequency was changed from 50 kHz to 13 MHz. It has been found that the capacitance values depend on not only the moisture content but also the measurement frequency. With a low moisture content, the relationship between capacitance and frequency is simple and linear. With a high moisture content, however, the relationship becomes more complex and nonlinear. This relationship with different moisture contents and different measurement frequency can be seen more clearly from the images generated by the ECT system. For image reconstruction, different models were used, series, parallel and Maxwell, to normalise the capacitance values. The results show that with low moisture content, the three models give nearly the same error. With the increase in moisture content, the differences between the three models become more and more obvious. With different gas-solids flow patterns, the three models also give slightly different images. It can be concluded that in a fluidised bed dryer, the measurements and hence the reconstructed images with different moisture contents depend on the measurement frequency and the adopted normalisation method. To improve the measurement accuracy and to control a fluidised bed dryer effectively, it is necessary to use an ECT system with programmable excitation frequency and with different normalisation models.