Comparison of experimental techniques for the measurement of mixing time in gas-liquid systems

Measurements of the homogenisation characteristics during the agitation of a liquid and the mixing time by simple in situ conductivity probes are very well established. However, unless special precautions are taken, in the presence of the second phase such as gas, the conductivity trace becomes distorted to a greater or lesser extent, so that it is not possible to follow the transient change of concentration in the liquid phase or estimate the mixing time. In this paper it is confirmed that, without special precautions, simple in situ probes are unsatisfactory. However, by shielding the probe with a cage, the ingress of bubbles into the probe region is essentially prevented and satisfactory results can be obtained in situ with responses having as little noise as in the case without gas. A second technique involves elimination of the gas from a small sample stream and measurement of the stream's conductivity transient. By suitable and rather simple treatment of the response, results equivalent to that from the in situ shielded probes can be obtained. The latter technique is especially useful where the placement of in situ probes is difficult. It is also suggested that recent results, which disagree with much of the literature on liquid phase mixing times in gassed systems, arose due to the use of in situ unshielded conductivity probes.