Experimental Investigation of Mixing Time in a Stirred, Torispherical-Bottomed Tank Equipped with a Retreat-Blade Impeller

Glass-lined reactors are typically used in the pharmaceutical and specialty chemicals industries because of their material compatibility with most reactants and their cleanability. These reactors are typically equipped with a retreat-blade impeller placed close to the bottom of the tank a single baffle mounted from the top. The mixing performance of such reactors has not received significant attention in the literature, despite their ubiquitous presence in the pharmaceutical industry. In particular, mixing time, i.e., the time required by the system to achieve a predefined level of homogeneity, has not been studied to any significant degree.

In this work, the mixing time of a 60 L, scale-down version of an actual industrial reactor having an industrial-type torispherical bottom and equipped with a retreat-blade impeller was experimentally determined for both the unbaffled case and the case in which the typical beavertail baffle was used (i.e., under partial baffling). A conductivity method using sodium chloride (NaCl) as a tracer was used as one of the approaches to determine mixing time. Experiments under unbaffled system were conducted by installing one two probes in the mixing vessel (at the wall and midway between the wall and the shaft, respectively). The presence of the conductivity probe(s) had a significant impact on mixing time. Results showed that different mixing times were obtained with the conductivity method depending on the location of the probe(s) and the number of probes.

A separate colorimetric method coupled with image processing was additionally used to determine the mixing time. Both methods produced similar results when two probes were present, probably because of the baffling effects introduced by the probes themselves. Results were also obtained with the partial baffling arrangement, but only using the conductivity method. Experiments were performed when the agitation speed was varied in order to establish correlation between mixing time and rotational speed under both unbaffled and partially baffled systems. The mixing time in partially baffled system appears to be inversely proportional to the impeller speed and the predicted mixing time from regression agrees well with the experimental results. The non-dimensional mixing number, ?95N, in partially baffled system was obtained within the speed range of 100–200 rpm.&'