(389f) Further Studies of Micromixing: Scale-up, Baffling and Feed Pipe Backmixing

The work reported was undertaken at the Institute of Chemical and Engineering Sciences, Singapore. It consisted of an experimental study of micromixing in a semi-batch reactor agitated by a Rushton turbine using two different reaction schemes. With one, the iodide-iodate scheme, at the same mean specific energy dissipation rate and feed location, the selectivity was similar at the 3.6 L scale to that found earlier at the 25 L scale (Assirelli et al., 2002). Also, at the 3.6 L scale, a study of back mixing at different feed locations showed that it was greatest when feeding into the trailing vortex, which is also the best feed location to improve micromixing. At that location, all pipes gave a similar selectivity at the same exit velocity. However, on closer inspection, it was found that even at the same velocity, in general, the bigger the pipe, the greater the level of backmixing. This finding raises certain questions about micromixing based on local specific energy dissipation rates. However, it is in agreement with some previous work (Bourne et al., 1981). Comparison with the limited literature also showed that one of the two criteria for predicting backmixing fitted the present results quite well (Vicum et al., 2004) but the other overpredicted it by two orders of magnitude (Jo et al., 1994). The other reaction scheme involved two parallel reactions, consisting of an acid-base neutralization and the alkaline hydrolysis of ethyl monochloroacetate (Bourne and Yu, 1994). It was used to compare micromixing in a 3L vessel under baffled conditions with that without them. Such a comparison has only been made once before at a larger 25 L scale and with the iodide-iodate scheme (Assirelli et al., 2008). The earlier surprising result was confirmed, namely that the mean specific energy dissipation rate required for equivalent selectivity without baffles was less than that required with them.

REFERENCES

Assirelli, M., W. Bujalski, A. Eaglesham and A. W. Nienow, “Study of Micromixing in a Stirred Tank using a Rushton Turbine: Comparison of Feed Positions and Other Mixing Devices", Chem. Eng. Res. Des., 80, 855-863 (2002).

Assirelli, M., W. Bujalski, A. Eaglesham and A. W. Nienow, “Macro- and Micro-Mixing Studies in an Unbaffled Vessel Agitated by a Rushton Turbine”, Chem. Eng. Sci., 63, 35– 46 (2008).

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Bourne, J. R. and S. Yu, Investigation of Micromixing in Stirred Tank Reactors using Parallel Reactions, Ind. Eng. Chem. Res., 33, 41-55 (1994).

Jo, M. C., W. R. Penney and J. B. Fasano, Backmixing into Reactor Feed Pipes Caused by Turbulence in an Agitated Vessel. AIChE Symp. Ser., 90, 41-49 (1994)

Vicum, L., S. Ottiger, M. Mazzotti, L. Makowski and J. Baldyga, Multi-Scale Modelling of a Reactive Mixing Process in a Semi-Batch Stirred Tank, Chem. Eng. Sci., 59, 1767-1781 (2004).