Scale-up of Mixing Processes in Solid-Liquid Systems

The design and scale-up of processes in which solids are handled has historically been problematical (see for example Merrow, Chem. Innov., 2000 or Bell, Powder Tech., 2005). Plants in which solids are a raw material, intermediate and / or product often fail to reach design capacity within 12 months of plant start-up which plants which process only fluids often meet the design capacity and, in some cases, exceed it.

Two aspects of agitator design in solids processing will be reviewed in this presentation:

1. Determining the minimum agitator speed required to ensure that all particles are in motion. This is the optimum operating condition for mass transfer between the particles and the liquid phase.
The results of recent experiments examining the effect of solids concentration on this impeller speed will be briefly reviewed. Also, the minimum velocity for slurry transport in pipelines is a similar design criterion in that all particles must be in motion, suspended from the pipe floor, and design rules for mixing and pipe flow will be compared.

2. Control of particle size and distribution during precipitation reactions. Generation of large particles with a mono-modal distribution is desirable since this optimizes the performance of downstream washing and separation equipment. Poor mixing promotes primary nucleation and formation of fines.

Experiments that can be carried out at bench and pilot scale will be described along with their interpretation in terms of well-developed mixing models.