Search

Many modern food, pharmaceutical and fast moving consumer goods manufacturing facilities clean their process equipment by automated circulation of cleaning solutions through the units. The soiling layers are often non-Newtonian, soft solids while the fluid mechanics are inherently dynamic and involve coupling between the cleaning solution and the soil. This talk will present results from recent studies of the removal of viscoplastic layers from smooth rigid surfaces by coherent, turbulent, impinging water jets, where there is a rich range of fluid phenomena on display. We pose the engineering question: what level of detailed modelling do we require to obtain acceptable predictive capability, and along the way introduce some of the challenges that colleagues might want to look into.

A new technique, fluid dynamic gauging (FDG), is introduced for studying the swelling and removal of polymer films from various surfaces. Films of two different polystyrene copolymers, labeled PX (30 kDA) and PA (125 kDa), were deposited in the laboratory on, inter alia, SS 316 plates. FDG was used to track, in situ and in real time, the films' evolving thickness during swelling and removal in three agents:  NaOH, MEK, and a commercial aqueous cleaning solution, TPU. The effects of initial film thickness, solvent concentration, solvent temperature, nature of the substrate, and applied stress were investigated. Swelling profiles suggest that Type II diffusion is the governing transport process. Different modes of behavior were found. In NaOH, swelling occurred after a lengthy induction period, but no surface cleaning was detected. MEK and TPU swelled the films with a negligible induction period and then cleaned the surfaces, with the former working faster. For MEK applied to PX and PA, and for TPU with PX, the film was removed by dissolution, i.e., the cohesion of the film was attacked. By contrast, TPU applied to PA removed the film by destroying the adhesion. Application of TPU to films composed of mixed layers of PX and PA showed that the deposition sequence determines removal behavior. A simple model allows prediction of the removal kinetics of composite films from those of single-component films. The potential for substantially accelerated reactor cleaning by appropriate reactor scheduling is thereby identified.