Polyethylene vs. stainless steel impellers for crystallization processes

Abstract

The importance of controlling the crystal size distribution of the product from an industrial crystallizer has long been recognized. To do this, some control over the nucleation and growth rates of crystals must be maintained. The crystal growth rate can be influenced by varying degrees of agitation, supersaturation, and temperature. However, these variables also have a major effect on the nucleation rate, so that it is seldom possible to change one of these quantities without simultaneously changing the other. It is the purpose of this note to discuss one means of substantially influencing the nucleation rate without affecting the growth rate.The role of crystal impacts in determining the nucleation rates was established by Clontz and McCabe (1971) and Johnson et al. (1972). Ottens and deJong (1972) have presented data to indicate that the major source of nucleation in an industrial crystallizer is at the impel'er , if there is internal mixing, or at the pump, if the mixing in the crystallizer is accomplished by external recycle Johnson et a]. (1972) also show that when a single crystal is impacted by a polyethylene rod, whose face is parallel to the crystal face being impacted, no new crystals are Kenerated. This can be contrasted to the generous number of crystals produced by impact with a metal rod. Their system was magnesium sulfate heptahydrate.On the basis of these reports an experimental plan to investigate the influence of the material of construction of the impeller in a stirred tank crystallizer was formulated. The crystallizer was a cylindrical Pyrex battery jar 15.25 cm in diameter and filled to a depth ol' 17.15 cm Three equally spaced plexiglass baffles extending to within 3 cm of the bottom were attached to the crystallizer. A fitted plexiglass cover was constructed to accommodate the impeller shaft, a thermometer which could be read to O.l"C, and seed crystal insertion. The entire apparatus was placed in a constant temperature bath. The impellers used were 5.1 cm in diameter, three bladed, with a square pitch, and located 9 cm from the bottom of the crystallizer.In all of the experiments of this series, the impeller speed was held at 700 rev./min., a value which kept the seed crystal adequately suspended. The size of the seed crystals was held essentially constant at about 5 mm x 2 mm x 2 mm; when deviations from this size occurred the results were erratic. Supersaturation was kept at 2°C and the temperature at about 33 to 35°C. A typical run went as follows: A solution of MgS04.7H20 was prepared, filtered, and placed in the crystallizer. The saturation temperature of the solution was determined by refractive index and the temperature of the solution was adjusted to 2°C above saturation. A seed crystal of the appropriate size and shape, and which had been recrystallized below 48.2"C to ensure the pure heptahydrate form, was inserted into the crystallizer and cured for 16 minutes with agitation. Shorter curing times gave excessively high crystal counts. Agitation was discontinued and the solut...

Mechanisms of initiation of incrustation

“…In our experiments the number of crystals found on the test surface is in general lower for the Teflon plate than for the steel plate. This corresponds to results on the rate of secondary nucleation (Shah et al, 1973) concerning the material of the agitator. For succinic acid and citric acid monohydrate, the number of new crystals formed on the plate increases with supersaturation, and for succinic acid on steel there is a linear relation (solubility data are given by Qiu and Rasmuson (1990)).…”

Contact nucleation of various crystal types

“…An examination of the experimental studies of Ottens and deJong ( 1972), Youngquist and Randolph (1972), Cise and Randolph ( 1972), Shah et al ( 1973), and Ness and White (1974) shows that the impact energy levels in their systems, as calculated using Equation (3) and the maximum seed size reported in their studies, were significantly lower than those investigated in this study. Nevertheless, Youngquist and Randolph (1972) observed that crystalimpeller collisions often resulted in macroscopic damage to the crystal, even at the relatively low energy levels of their study.…”

Factors affecting secondary nucleation rate of sodium chloride in an evaporative crystallizer

“…The notion that secondary'nucleation is tied up with a microattrition process is evidenced by the effects of hardness of the foreign contacting surface. Several authors (Shah et al, 1973;Randolph and Sikdar, 1974;Evans et al, 1974) studied nucleation rates in agitated systems fitted with stainless steel, aluminium and polyethylene impellers, and impellers covered with a soft rubber coating and teflon. It was shown that in all cases the softer polymeric and elastomeric materials dramatically reduce the formation of nuclei, probably by absorbing part of the impact energy.…”