A FLUID MECHANICAL APPROACH TO TURBULENT MIXING AND CHEMICAL REACTION PART III COMPUTATIONAL AND EXPERIMENTAL RESULTS FOR THE NEW MICROMIXING MODEL

The basic ideas for modelling micromixing, given in Part II, are applied to a pair of consecutive, competitive reactions conducted in semibatch as well as in continuous stirred tank reactors. The objections to earlier work, given in Part I, no longer apply, The principles of calculating the product distribution X for each reactor type are outlined and numerical results are given, using the parameters for the diazo coupling of 1-naphthol with diazotised sulphanilic acid. These showed the dependence of X on mode of reactor operation (SBR and CSTR), volume ratio of reagent solutions (V_A/V_B), Schmidt number (S_c) and mixing modulus (M = k₂c_B0δ_v²/DA). Under typical experimental conditions, the first reaction was instantaneous, whilst the second was fast (see Appendix), permitting a transformation of composition variables and an enormous saving of computer time.

Measured product distributions for the diazo couplings were available covering two tank sizes, three impeller types, three volume ratios, several initial concentrations of reagents, semibatch and continuous operating modes and several feed points. These points were grouped together as (a) suction side of impeller, (b) just below liquid surface, and (c) midway between impeller and wall. (The only major independent variable not covered here is fluid viscosity). In order to make a prediction with the new model, only the rate of energy dissipation (ε) in the reaction zone must still be known. This was expressed as a multiple (φ) of the average rate(ε = ε) and the values consistent with the product distributions measured for the three feed points were (a) 8, (b) 0.23, and (c) 1. These are in good agreement with flow visualisations as well as local ε-measurements using hot film and laser doppler anemometry. These and other results suggest that the new model represents a significant advance on previous methods