A CAPACITANCE PROBE TO CHARACTERIZE GAS-BUBBLES IN STIRRED-TANK REACTORS

An invasive plate and frame capacitance probe was designed for dielect ric constant measurements over a wide frequency range (10-5000 kHz) in a 5 1 stirred tank reactor. Preliminary measurements with polyethylen e beads showed a linear variation of the effective permittivity with t he volume fraction of the beads, in accordance with the Maxwell-Wagner theory. Results obtained by bubbling air under various agitation and aeration rates at 500 kHz in deionized water, and the effect of a non- ionic antifoam on permittivity variations, are shown and discussed; th e technique is thought to allow precise local holdup measurements. Exi sting limitations and possible further improvements of the technique a re discussed, especially in view of the information that can be gained from the standard deviation of the measurements.