Time-series analysis of pressure fluctuation in gas-solid fluidized beds.
Fluidized beds have been used extensively in chemical process industries for several years. The success of fluidized operations is largely dependent on a well-defined and stable contact regime. Hence, the ability to understand the flow regime behaviour plays a vital role in the design and operation of fluidized bed units in order to achieve a particular stable fluid dynamic state. Fluidization regime is dependent on factors such as particle properties (size, density and geometry), column properties (size and geometry) as well as the fluidizing medium properties (density, viscosity and velocity) (Fan, et al., 1981). In order to expand the understanding of the hydrodynamics of a fluidized bed system, a substantial amount of research has been committed to the measurement and analysis of pressure fluctuations in a fluidized bed. This is due to the strong relationship between pressure fluctuation and the hydrodynamic factors which include bubble size, bubble rising velocity and the motion of the bed surface with time (Hartman, et al., 2009). Identification of each regime could be accomplished through the time-series analysis of the pressure fluctuation in the time domain, frequency domain and state space domain (van Ommen, et al., 2011). The main focus of this dissertation was to apply time-series analysis in the frequency domain, for the characterisation of the different fluidization regimes (particulate, bubbling, slugging and turbulent) in a gas-solid fluidized bed. This was achieved through the use of spectral analysis and the mathematical tool known as the Fast Fourier Transform (FFT) was used to analyse and interpret the pressure fluctuation in the fluidized bed. Analysis was also performed in the time domain by analysing the time-pressure behaviour as well as the change in the standard deviation of pressure fluctuations. Experimental measurements were conducted in three different columns with varying column height and diameter. Three different solid particles were used namely, sand particles (Geldart Group B), plastic beads (Geldart Group D) and spent Fluid Cracking Catalyst (Geldart Group A). The sampling frequency used for pressure measurements in this work was fixed at 500 Hz with a sampling time of 30 minutes. Results indicated that the pressure fluctuation signal is useful in providing information about fluidized bed behaviour. Analysis in the time domain revealed that this technique could be used primarily to identify whether fluidization has occurred or not. Analysis in the frequency domain indicated a better representation of the fluidization behaviour and the different regimes could clearly be identified and distinguished based on a dominant frequency. In the 5 cm diameter column, the Geldart Group B particles displayed a distinct dominant frequency for the bubbling regime while a dominant frequency could not be obtained for the Group A and D particles respectively. In the 11 cm diameter column, the Geldart Group B materials were observed to fluidize very easily, with three dominant frequencies corresponding to the bubble, slugging and turbulent regimes, being identified. The Geldart Group D particles were found to fluidize at high velocities in the 11 cm diameter column with the bubbling and slugging regimes being identified. Geldart Group A particles were found to behave very differently from the other two materials. A noticeable bed expansion was seen before fluidization actually occurred. The only regime achieved with the Group A particles was the bubbling regime. Results for the 29 cm diameter column indicated a dominant frequency for the bubbling regime for the Group B particles while measurements could not be performed using other materials due to limitations on the pressure transmitter. It was further observed that the dominant frequencies were much more pronounced at higher bed heights. In addition, a change in the aspect ratio (bed height: column diameter) had a significant influence on the dominant frequency as a visible shift was apparent. An increase in the aspect ratio indicated a noticeable decrease in the dominant frequency component. This was valid for all fluidization regimes investigated.