Dry-cleaning with high-pressure carbon dioxide - The influence of mechanical action on washing-results

Previous studies indicate that the removal of non-polar soils in CO ₂ is comparable to the level of dry-cleaning in perchloroethylene (PER). However, these studies show that the removal of particulate soil is insufficient in CO₂ compared to PER. Particle removal can be increased by the use of more mechanical action and/or the use of surfactants. From experiments, it is concluded that the removal of relatively large particles like sand increases with increasing mechanical action. However, the level of mechanical action has no influence on the removal of relatively small particulate and non-particulate soils (with the exception of clay on wool). Increasing the amount of mechanical action is therefore not the solution that will lead to the increase of the washing-results for relatively small particles (like carbon black and clay) up to the level of the washing-results using PER. The removal of relatively small particulate soils in CO₂ can be improved by the use of suitable surfactants that reduce adhesion forces. A model for quantifying the amount of mechanical action has been developed. This model can be used to predict the optimal process conditions for relatively large particle removal. It is concluded (from the model and experiments) that the level of highest mechanical action in CO₂ is obtained in a two-phase environment at low pressure and temperature and that 75 RPM is the optimal number of revolutions in our system. At these conditions, however, the washing-results for small particles are not as good as the washing-results for these particles in PER.