MIXING AND DISPERSION MEASUREMENTS ON PACKED-BED FLOWS USING A FIBEROPTIC PROBE ARRAY

Scale-up of packed bed processes, particularly those involving chromat ographic separations, is made difficult by a seemingly inevitable incr ease in dispersion due to packing nonuniformity. To provide a suitable characterization, we measured the spatial distribution of dispersion and mixing in packed beds of uniform Impervious spherical glass partic les by a tracer impulse technique. The key feature in our work is the use of a fiberoptic array at the exit plane to obtain a time-resolved spatially-distributed response. All experiments wire in the creeping f low regime. We used a fluorescent dye with laser excitation through th e fiber terminations in the bed. The fluoresced radiation was collecte d through the same fibers. We analysed the data by the use of indices of the extent of micromixing based on Danckwerts's degree of segregati on. This requires the computation of moments of the responses of the i ndividual probes and their average. A simple model gives expressions f or the indices in terms of the Peclet number and is shown to provide a useful limiting case. The computed indices are also shown to be very sensitive to adsorption of dye on the surface of the glass spheres. Ho wever, for some of the experiments with the largest spheres made of Py rex glass, adsorption appears to have played an insignificant role. Th is technique successfully separates overall bed dispersion into two pa rts that due to large-scale transverse variation of the flow residence time, and that due to mixing on the microscale.