Impact of rheology on the digital optimisation of mixing in soap manufacture

Mixing profiles of formulated liquid and solid products such as soap is complicated by an evolving non-Newtonian rheology during saponification, which creates considerable difficulty for optimising process conditions and energy use. Poor mixing can result in an incomplete saponification process and a poor-quality end product. Whilst current work by Hart et al has led to the development of a digital optimisation workflow using simplified rheology, scope remains for improving the rheological model to more accurately represent processed material and final product. The main challenge in measuring soap rheology is the presence of slip on the rheometer geometry walls, making collection of data difficult with poor repeatability and reproducibility. Being unable to measure the rheological profile of soap limits the industry’s ability to optimise process, equipment design and process time which collectively limit the ability to control process energy input. This paper describes a scoping exercise to screen the capability of different instruments to measure soap rheology, to thus establish a validated rheological model for the digital workflow. This rheological model will then be compared with torque measurements obtained from a 5-blade ploughshare mixer, modelled on industrial mixers. This mixer contains regions of high shear, low shear, radial flow, and axial flow in close proximity, representing a more complex flow field. This study will thus determine the capability of the torque measurements to determine progress towards process endpoint, using the improved rheological model as a baseline to see whether in-situ determination of the product rheology matches expectations