Measurement of holdup and propagation velocities in a liquid-solid circulating fluidized bed using Electrical Resistance Tomography

The application of liquid-solid circulating fluidized beds (LSCFB) is steadily increasing in the chemical, biochemical and other industries. Electrical Resistance Tomography (ERT) as an imaging technique was employed for the study of flow parameters such as phase holdup, velocity distribution of individual phases in LSCFB. ERT is a non-invasive technique based on conductivity measurement of the continuous phase, which also provides color-coded cross sectional view of phases with a frequency of up to 250 images per second. The local conductivity measured by a number of electrodes located at the periphery of the plane was then further converted into a local phase concentration distribution based on Maxwell's relation. Local solids holdup also measured by using optical fibre probe to verify and compare result produced by ERT. Both give good experimental agreement each other. By crosscorrelation between the data obtained from a pair of planes of ERT, one upstream and one downstream, the phase propagation velocity was determined. Water was used as the continuous and conductive phase and glass beads of 500 micron size used as the solid and non-conductive phase in this investigation. Radial non-uniformity of phase holdups was observed at different superficial liquid velocities. Solids holdup was higher in regions close to the wall and low in the central area. Such non-uniformity in phase holdup decreased with increasing liquid velocity. Both solids holdup and radial non-uniformity increased with solids circulation rate. The propagation velocity did not change radially at low liquid velocities, but increased in the central region at higher superficial liquid velocities. The propagation velocities were lower than superficial liquid velocity due to local slip velocity and drag force. A good correlation between the ERT and Optical Fibre probe methods was verified.