A capacitance probe to characterize gas bubbles in stirred tank reactors

An invasive plate and frame capacitance probe was designed for dielectric constant measurements over a wide frequency range (10–5000 kHz) in a 5 l stirred tank reactor. Preliminary measurements with polyethylene beads showed a linear variation of the effective permittivity with the 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; the technique is thought to allow precise local holdup measurements. Existing limitations and possible further improvements of the technique are discussed, especially in view of the information that can be gained from the standard deviation of the measurements.