Scaling-up of reactive crystallizers

In precipitation reactors the formation of solids is a fast process. When stirred vessels are used nucleation occurs almost entirely near the feed stream inlet. In that case the mixing rate of the feed stream with the bulk contents, on a scale larger than the Kolmogorov scale, determines the product quality such as particle size or particle size distribution. It is recommended to keep the time needed for mixing at this larger scale, the mesomixing time, constant on scaling-up. Two models are known to calculate the mesomixing time, however both models need information on the turbulence characteristics near the feed stream inlet. In our laboratory laser-Doppler velocimetry (LDV) experiments have been performed to supply this information for a number of geometrical situations and stirrer speeds. The obtained data were used to calculate mesomixing times from the two models. Mesomixing times of the feed stream in a stirred vessel were also determined experimentally with a planar laser-induced fluorescence (PLIF) technique. The experimentally obtained mesomixing times were compared with the calculated mesomixing times. When the feed stream's velocity is lower than the local circulation velocity in the vessel the turbulent dispersion model gives the best results. For higher feed stream velocities the inertial-convective modelling should be used. The proposed method for calculating mesomixing times of feed streams in stirred vessels enables the designer to keep mixing times constant, and thus product quality, at scaling-up of stirred precipitators.