Comparison of Experimental Techniques for the Measurement of Mixing Time in Gas-Liquid Systems

Abstract

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 situprobes 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.

Design of and initial results from a Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC)

“…Mixing time was defined as τ 95 , the time required for the concentration of a stable species to reach 95% of its maximum value (Pinelli et al, 2001). NO in N 2 was measured with a commercial NO/NO 2 analyser (discussed below), and several combinations of injection and sampling points across the chamber were investigated with respect to the number of fans running as well as the speed at which the fans were run.…”

Towards a CFD model of bubble columns containing significant surfactant levels

“…To reduce the noise in the conductivity signal caused by the passage of bubbles in the vicinity of a probe two measures were applied together; firstly each probe was wrapped in a layer of fine, nonconductive mesh (a similar method being used by Pinelli et al (2001)); secondly a symmetrical moving-average filter (using a period of 1 s) was applied to the data using an approach outlined elsewhere (Rampure et al, 2007). In this work the mixing time has been defined as the time necessary for the tracer concentration to stabilise within 7 5% of the final equilibrium value.…”

Gas hold-up distribution and mixing time in gas–liquid stirred tanks

“…Although the curves shape depends on the position, the comparison of the calculated t 95 on the two planes confirms that generally the time to achieve the fixed 95% homogeneity level is not significantly different at the various elevations [28]. The mixing time estimated at different gas flow rates and impeller speeds with the RT is shown in Figure 9.…”

“…On the experimental side, the techniques based on conductivity probes for single phase systems require appropriate modifications in aerated conditions [28], the optical methods are limited to very low gas hold-up, while in principle the application of ERT does not suffer of any limitation. Indeed, it has been successful adopted for the dynamic characterization of single-phase stirred tanks [29][30][31], as well as for two-phase systems [22,32,33], but, at the best of our knowledge, comprehensive investigations on the mixing time of gas-liquid stirred tanks based on ERT are not available to date.…”