Fast reactions in rotor-stator mixers of different size

Abstract

Mixing problems can become severe when carrying out fast reactions in industrial stirred tank reactors. It is of interest for such purposes to investigate in-line mixers having short residence times and high energy dissipation rates. Chemical reactions in aqueous solution have been carried out in semi-batch as well as in in-line operating modes and using two geometrically similar rotor-stator mixers having a linear scale-up factor of two. A fast and complex diazo coupling reaction was employed as a test system to investigate the relevant flow characteristics. During semi-batch operation aqueous 1-naphthol solution circulated from a stirred tank through the rotor-stator machine and then back to the tank, whilst diazotised sulphanilic acid solution was fed directly into the rotor. The primary reaction products (monoazo dyes) reacted to bisazo dye to an extent which depended upon the rate of turbulent energy dissipation ε within the reaction zone. A good coarse scale distribution of reagents was obtained using a feed nozzle having fine radial outlets to introduce diazotised sulphanilic acid into the rotor, so ensuring that reaction could only start just inside the rotor's teeth. The test reaction was complete before the solution had reached the highly sheared annulus between rotor and stator. Experiments indicated that the energy dissipation rate was uniform within the reaction zone. By comparing the measured product distribution with the predictions of a micromixing model it was possible to deduce ε and its dependence on the rotor speed (ε ∼ N 2.5 approx.). For in-line operation both reagent solutions fed to the rotor-stator mixer pass through it only once. Both sizes of mixer showed good performance. The scale-up criterion for constant product distribution in the micromixing controlled regime was constant rotor tip speed. Modifications of rotor-stator mixer are proposed with which it should be possible to get further improvements.