CRYSTALLIZATION OF SUNFLOWER OIL ON A SCRAPED METAL SURFACE

摘要

The mechanism of sunflower oil crystallisation on a cold scraped surface was investigated. This involved observing the onset nucleation temperature of sunflower oil first by DSC, and then by running a flow cell which simulated the scraped region of a scraped surface heat exchanger. Video microscopy was used to observe the surface. For a scraped surface, results indicated that relatively high surface undercoolings were required to initiate nucleation on the surface and that the onset of nucleation was characterised by the generation of many tiny crystals or crystal aggregates floating in the vicinity of the metal surface. These very quickly settled onto the metal surface to form a layer of crystals. At surface undercoolings above 18°C, nucleation occurred within about 5 s. The induction time, and hence the nucleation kinetics, for this process was influenced predominantly by the surface undercooling and by whether the scraper touched the surface. Nucleation took place on the surface and the mechanism may involve a transition between homogeneous and heterogeneous nucleation. After nucleation, nuclei grew both parallel and perpendicular to the surface.