Scale-Down and Scale-up of a Slurry Mixing Process

Scale-up is a common activity in process development. The goal is to develop the understanding of how a particular unit operation affects the outcome of the process in small scale equipment and then design equipment for the full-scale plant based on this understanding.

If a piece of equipment in a plant is not operating as designed, the operation can be scaled down, recreating the problem at small scale to develop an understanding of its causes. Then further tests can be carried out to develop a solution to the problem at small scale followed by the design of equipment to implement at plant scale.

A case study will be presented which will discuss how mixing issues were created, due to changes in the operating procedures, in the production of a viscous, non-Newtonian, high solids concentration slurry. The changes resulted in the slurry being more viscous and shear-thinning. The agitator was not capable of moving the slurry in regions far from the impeller blades and severe stagnation and fouling was observed. The process would have to be shut down on a regular basis so that the vessel could be cleaned.

Scale-down studies were carried out in a combined stirred tank / viscometer developed for the project enabling a mixing test to be performed identifying the degree of agitator required to eliminate stagnant regions and, at the same time, to measure the slurries rheology. The results were scaled-up and the new agitator has been working as specified for nearly five years.

Finally, some of the considerations that must be taken into account when making rheology measurements, especially for high concentration slurries, will be addressed.