Liquid-and gas-phase macromixing behavior was studied in gas-liquid high-aspect-ratio reactors stirred with multiple hydrofoil impellers pumping downward. Water, a sodium sulfate solution, and poly(vinylpyrrolidone) solutions of viscosity up to 110 mPa‚s were used as the liquid. For characterizing the liquid phase, mixing time experiments were conducted at various operating conditions, while detecting the response curves at several positions inside the tank. Comparison of the experimental curves with the theoretical ones provided by simple fluid dynamic models showed that the axial dispersion model is quite acceptable. The influence of impeller speed, gas flow rate, and viscosity on the model parameter was studied, and dimensionless relationships are given. The gas behavior was studied by means of the RTD and modeled with the axial dispersion model, which proved good for water and acceptable with coalescence-inhibiting electrolyte solutions. The model parameter dependence on the operating conditions was studied.Comparison between hydrofoil impellers and radial Rushton turbines is also attempted.

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 ªcageº, 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.