INTERACTIONS BETWEEN MIXING ON VARIOUS SCALES IN STIRRED TANK REACTORS

Fast, competitive-consecutive reactions exhibit product distributions which are influenced by micro- and macroscale concentration gradients in a reactor. If a reaction is run many times under identical conditions in a semi-batch reactor, except that different feed rates are used, the product distribution is constant at low feed rates, but indicates increasing non-uniformity of composition at higher feed rates. Competition between micromixing and reaction determines product distribution at low feed rates. The additional inhomogeneity appearing at higher feed rates signals a mixing constraint at scales larger than microscopic. The relevant scale is not the macroscopic one of the whole vessel, but rather a mesoscale reflecting the interaction of the plume of fresh feed with its surroundings. Dispersion of this plume has been calculated for homogeneous turbulence, as well as when the mean plume velocity and the turbulence properties in its vicinity were spatially varying. Micromixing and chemical reaction could then be calculated in the concentration field resulting from turbulent dispersion. The relative importance of meso- and micromixing could be expressed by the ratio of their time constants. The description of simultaneous meso- and micromixing developed here successfully predicted product distributions from fast diazo-coupling reactions which had been measured at two scales, two feed points, three concentration levels and various stirrer speeds. The model requires knowledge of the reaction kinetics and the flow field, but does not, however, contain any arbitrary parameters.