A hydrodynamic study on the soil removal mechanisms of liquid jets and sprays

This thesis presents a hydrodynamic study on the cleaning mechanisms of liquid jets and sprays used in batch cleaning processes in the pharmaceutical industry. By analysing an exemplar wash rack employed for cleaning-out-of-place (COP) on a pharmaceutical manufacturing facility, the flow rate distribution through the rack was predicted using the open source software EPANET. The flow rates obtained using this method were then applied to the design of experiments (DOE).
Two jet cases were considered; horizontal impingement on to a vertical wall and vertical impingement through a pipe. For the horizontal case, flow rates used were in the range 1 – 4 l/min. Water temperatures were in the range 20 – 60 degrees centigrade and soil layer thicknesses 0.19 – 1.9 mm. The soil used for horizontal experiments was white soft paraffin (WSP), an excipient commonly used in the manufacture of liquids, creams and ointments. Results from these experiments showed that cleaning occurred via a roll-up mechanism of WSP. Using an energy framework it was observed that cleaning efficiency decreased with time as the distance of the cleaning frontier from the impingement point of the jet increased. Cleaning beyond the drop point of WSP showed a significant increase in cleaning performance as phase transition occurred to mobile. For the vertical jet, the material cleaned was a gel. The primary failure mode was misalignment of the jet relative to the pipe, causing flow to miss the entrance to the pipe. Also residues were observed when the gel was preheated at 85 degrees centigrade prior to cleaning. For the spray experiments, WSP was again used and the same flow conditions. Large residual films were observed at room temperature but again beyond the WSP drop point all material in contact with the flow was removed.
Computational simulations of single droplet impingement on to a wetted wall with contact angles varying from 0 – 83 degrees and a wall with a liquid film on the surface were also conducted using the computational fluid dynamics software COMSOL. This was intended to represent a constituent droplet of a spray and gauge its cleaning mechanism on a wall. Shear exerted on the wall was observed to significantly reduce with the presence of a liquid film on the surface.