(80a) Effect of Temperature and Shear On the Hydrogenated Castor Oil Crystal Morphology in the Crystallization of Oil-in-Water Emulsions
AIChE Annual Meeting
Monday, November 8, 2010 - 12:30pm to 12:53pm
Crystallization of Hydrogenated castor oil-in-water emulsions has been studied by polarized light microscopy, scanning electron microscopy, differential scanning calorimetry and small and wide angle X-ray powder diffraction. Three types of crystal morphologies have been observed: rosette, fiber and irregular crystal. The effect of each type of crystal morphology on the rheology of crystal suspension is the main goal of this research. First, the effect of temperature and shear on making different types of crystal morphologies has been studied. Isothermal crystallization has been studied with the crystallization temperature from 70°C to 55°C. Rosettes and fibers are main morphologies but the volume fraction of rosettes decreases from 99.9% at 70°C to 90.0 wt% at 55°C. X-ray diffraction analysis shows no polymorphism changes within the above temperature range. These results combined with microscopy indicate that the nucleation mechanism of rosettes and fibers are different. There are mainly two types of nucleation: volume heterogeneous nucleation and homogeneous nucleation. Through calculation of activation free energy by Fisher-Turnbull equation at 70°C, a value of 7.152kJ/mol has been obtained. If the same nucleation mechanism happens at 55°C, the activation energy would be 1.580kJ/mol but it is much lower than the actual value of 3.675kJ/mol at 55°C. It means that the nucleation of fibers needs much higher activation energy than that of rosettes, and the difference can only be caused by heterogeneous and homogeneous nucleation. Therefore it is suggested that rosettes are heterogeneously nucleated while fibers are nucleated homogenously. Crystals other than rosettes and fibers are categorized as irregular crystals that form due to high supercooling or breakup of rosettes or fibers under shear. By further increasing the supercooling to 45°C, irregular crystals formed first because growth unit (i.e., molecules) tends to pack disorderedly to form thermal unstable structure. With the increase of hold time at 45°C, metastable irregular crystals transform to rosettes and thermal stable irregular crystals. Constant cooling rate study showed that at lower cooling rate (1°C/min) more rosettes and fibers form and at higher constant cooling rate (5°C/min) more irregular crystals form. Steady shear has been applied by a Linkam Shear stage on the HCO-in-water emulsions while cooling at constant rate. Onset temperature for crystallization has been obtained by microscopy method. Induction time has been calculated based on the onset temperature and a linear decrease of induction time versus logarithm of shear rate has been obtained. Morphology analysis indicated that fibers favored gentle shear, i.e. 1 s-1, which is suggested that shear lowers the activation energy for nucleation so that homogeneous nucleation has more chance to happen. But fibers are easily broken down with an increase of time at shear at 1 s-1. Rosettes tended to grow elongated perpendicular to the shear direction due to the mass transfer distribution caused by laminar flow field around rosettes. At the cooling rate of 1°C/min, mean crystal size decreased with shear rate. However, at the cooling rate of 5°C/min, irregular crystals were the only morphology but the crystal size became larger with the increase of shear rate.