Investigation of bubble behaviour in a gas-solid fluidized bed by means of a twin-plane ECT sensor

Fluidized beds are used extensively in chemical engineering applications, such as food processing, combustion, gasification processes and pharmaceutical industry. However, due to their complex fluid mechanical characteristics (for example bubble size and bubble velocity), conventional measuring techniques do not permit a sufficient understanding. Electrical Capacitance Tomography (ECT) has been developed over the last few decades as a non-invasive measurement technique and applied to study gas-solids fluidized beds. Although many researchers investigated relationships governing bubble sizes, the influence of cut-off value (a grey level value distinguishing gas and solid phases) of bubble boundary on the bubble size estimation is still not fully understood.

A twin-plane ECT sensor with 10 mm-long measuring electrodes has been designed and fabricated to study the single bubbling regime in a gas-solids model fluidized bed. To investigate the influence of the cut-off value on bubble size estimation, different diameter hollow plastic balls have been imaged while the bed is at packed bed state. Appropriate MATLAB code has been developed to extract the cut-off values of the boundary for these plastic balls. Based on the cut-off values, bubble size within single bubbling regime has been estimated by LBP and iterative LBP image reconstruction algorithms.

Finally, estimated bubble size has been compared and analysed with existing bubble size estimation correlations. Future work will focus on the effect of different reconstruction algorithms and permittivity models on bubble size estimation to get more comprehensive understanding of bubble behaviour to examine the capability of ECT at estimating single bubble size within fluidized bed.