Measurement of Bubble Property Distributions for Better Description of Mass Transfer Limitations in Chemical Reactors

Tomographic X-ray measurements of fluidized beds were evaluated by pseudo three-dimensional reconstruction of individual bubbles in a freely bubbling fluidized bed. It was found that several aspects of the applied methodology significantly influenced the results: i) the consideration of the individual rise velocities of the bubbles for the calculation of the vertical dimension of each bubble, ii) the determination of the surface area and shape of each bubble which turned out to be correlated with the rise velocity, as well as iii) a new adaptive threshold method for the binarization of the reconstructed void fractions, ensuring that the measured void fraction is maintained throughout the reconstruction procedure.

The X-ray tomography measurements showed that the bubble properties are broadly distributed for a given location within the investigated BFBs. This indicates that the description of the hydrodynamics in BFB merely by mean values of the bubble properties might not be sufficient. This is especially relevant for the prediction of critical reactor characteristics such as mass transfer limitations which could be caused by bubbles that are faster and larger than the corresponding mean values suggest.

Another important findings are the strong correlations between the size, the rise velocity and the shape of the bubbles. These correlations are particularly important in the context of measurements with optical probes or similar methods.

Correlations from literature that predict one average bubble size as a function of the fluidization number and measurement height did not describe the data well, especially for higher fluidization numbers.

The individual bubble property distributions can be described with generalized extreme value distribution (GEV) functions and their correlations can be accounted for with t-copula functions. Data sampled by these distribution functions can be used for the verification of CFD models or as a foundation for the development of semi-empirical correlations.