Optimization of experiment span and data acquisition rate for reliable electrical capacitance tomography measurement in fluidization studies - A case study

In a conventional fluidized bed, the particle behaviour is highly unpredictable and therefore the data collected during a specific experimental scenario is often highly representative of that isolated result, rather than being reproducible. To quantify the degree of representation/reproducibility, different scenarios of sampling span and measurement frequency were tested for studying the bed hydrodynamics. The experiments were carried out at ambient conditions in a non-reacting fluidized bed. A twin plane electrical capacitance tomography system was used to measure the solid fraction distribution in a 150 mm diameter acrylic column and at a selected gas velocity of 0.9 m s^-1. The images produced from capacitance measurements are relatively low-resolution images. Thus an iterative method based on a LPB algorithm has been used and the recommended number of iterative required for enhanced images is presented. In measuring the hydrodynamic parameters, it is demonstrated that increasing the recording span considerably increases the measurement accuracy. It is also observed that the visualization of the axi-symmetrical nature of the bed hydrodynamics is not easy to achieve with a short recording span of 20 s, due to the high bed non-uniformity. For solid fraction measurements, the minimum recommended experimental sample is 4000 data points. When measuring dynamic parameters such as frequencies or standard deviation of solid fraction fluctuations, high data capture rates are of vital importance to properly characterize the hydrodynamic fluctuations. Local measurements were found to only represent the specific measuring location. In order to have a global estimate of a dynamic parameter such as the bubble frequency the analyses should be based on the average bed signal. When measuring the bubble rise velocity from the average bed signal it has been found that, within the operating conditions considered in this study, a combination of 60 Hz and 80 s (4800 points) or 100 Hz and 60 s (6000 points) produces an acceptable level of accuracy. Low data capture rates <40 Hz may dramatically underestimate or completely fail to provide an estimate of bubble rise velocity.