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Received for review March 27,1978 Accepted April 20, 1978
ARTICLES
Crystal Regeneration and Growth of Small Crystals in Contact Nucleation Shakir Khambaty and Maurice A. Larson* Department of Chemical Engineering and Nuclear Engineering and Engineering Research Institute. lowa State University, Ames. lowa 5001 1
Magnesium sulfate heptahydrate crystals were contacted repetitively to determine crystal surface regeneration time and to determine the frequency at which edge contact produces nuclei as compared to face contacts. It was found that the regeneration time is nearly the same for edges and faces and that edge contacts produce more nuclei per contact. The presence of Cr3+ ions reduces both the growth rate and the number of nuclei produced. It is clear that contact nucleation rates are closely related to the rate at which a crystal is growing.
Introduction In designing crystallizers, the control of crystal size distribution is an important factor because of further marketing and processing specifications on crystal size. The principal physical phenomena affecting crystal size distribution in crystallization from solution are the nucleation rate and crystal growth rate. The most important of these is the nucleation rate. Many mechanisms exist by which nucleation can occur in operating mixed suspension crystallizers. It is believed, however, that contact nucleation is the principal source of nuclei. Contact nucleation occurs when a seed crystal touches or strikes another crystal, the agitator, or the crystallizer walls. In recent years considerable effort has been expended in studying the factors which influence contact nucleation. It has been found that supersaturation level, energy of contact, area of contact, frequency of contact, and the level of impurities all have considerable influence on nuclei production (Bauer, 1973; Bendig, 1974; Johnson, 1970). This evidence supports the assumption that the mechanism of 0019-7874/78/1017-0160$01.00/0
microattrition is the predominant mechanism of contact nucleation. Microattrition is defined here as attrition resulting in production of fragments of less than approximately 10 Pm. To study the contact nucleation phenomenon, it is essential to be able to measure nucleation rates effectively. A continuous mixed suspension crystallizer is a convenient and useful device for this purpose. In this experimental method, the crystal size distribution can be interpreted using the population balance to give a measure of the nucleation rate (Randolph and Larson, 1971). The size distribution can be determined either by screen analysis or by the use of a Coulter counter. The Coulter counter has made it possible to study the crystal size distribution a t very small sizes, and as a result of its use many investigators (Bauer, 1973; Bendig, 1974; Jancis and Garside, 1975; Sikdar, 1975) have noticed what appeared to be size-dependent crystal growth in the 1 to 30 pm size range. Recently, Sikdar and Randolph (1975) observed large numbers of crystals present at small sizes (
