Mixing, reaction and precipitation:: Semibatch barium sulphate precipitation

This paper reports both experimental and modelling studies on a double feed semibatch process, aimed at developing a better understanding of the role of mixing in precipitation processes. Experimental work, using precipitation of barium sulphate as a model system, showed the great sensitivity of the process to the time over which the reactants were fed into the precipitator. The roles of stirrer speed, feed position and the presence or absence of seed crystals were also explored. Simulations were based on the concept of a partially segregated inhomogeneous precipitator and used a simple three-environment model. The influence of macro-and micromixing were characterised by the corresponding time constants and calculations showed the influence of these two parameters on the size distribution characteristics of the crystal product. Macromixing was shown to be particularly important for this precipitation arrangement, micromixing being of lesser significance. Reasonable agreement was obtained between experimental results and the corresponding simulations.