Seed crystals surface area

Mirskii and Pirozhkov [66] reported on experiments in which seed crystals were added to normal batch zeoUte synthesis mixtures. In one set of experiments, different amounts of seed crystals of a desired phase (not specifically mentioned, but probably zeoUte NaA) were added to the synthesis mixture, and noted to ehminate the induction time and increase the rate of crystallization. Two additional factors were investigated and reported a) the rate of crystallization increased more with increased amounts of seed crystals added, and b) the rate of crystallization was enhanced more using the same mass of smaller seed crystals than with larger seed crystals. Both of these results were concluded to imply that the rate enhancement was due to the cumulative seed crystal surface area used to assimilate material from the solution. This point was illustrated further by adding seed crystals of one phase to a solution which nominally produced a different zeoHte phase. For example, zeoUte NaP seed crystals were added to a synthesis mixture, which was demonstrated to precipitate zeolite NaX, after about 30 % of the amorphous reagents had already crystalUzed. After two additional hours of crystallization, the absolute amoimt of zeoUte had... 

Equation (6) indicates that secondary nucleation is significantly affected by the crystal surface area A (m2/l). The nucleation process is usually viewed as a process where copious amounts of fines are produced, resulting in a corresponding increase in the specific crystal surface area (m2/g) and crystal surface areaA olume of the crystallizer content (m /l). Results obtained in this study concur with this nucleation view. However, Brown (9) reported that nucleation can be suppressed at high crystal surface areas ( 20 m2/l). Brown s observation was not evident in this study though the seed density of 200 g/litre used in this study had a surface area of 25 m2/l. The lower dependence of second nucleation on supersaturation or concentration driving force, AC, in comparison to crystal surface area. A, is obvious from equation (6). 

In zeolite systems chosen for study diffusion in the liquid phase and crystal growth on the crystal-liquid interface were the two major steps in converting gels to mordenite, zeolites A and X, the former being the rate-determining step for mordenite and the latter for zeolite X crystallization. In the mordenite system the effect of seed crystals, with surface areas per unit mass different by an order of magnitude, demonstrated the mechanism of nucleation on the seed crystal surfaces. The data support the hypothesis that crystal growth of the zeolite occurs from the solution phase rather than in the gel phase. 

Suppose that the batch crystallizer is seeded with a mass of crystals with a uniform size of Lseed. The number of seed crystals is Nseed, and, as the operation is to be free from nucleation, the number of crystals in the system remains the same as the number of seed crystals. The initial values of total crystal length, total crystal surface area, total... 

Although widely practiced for production of industrial chemicals, continuous evaporation for crystallization is rarely if ever used in pharmaceutical operations. Although continuous operation has the advantages of using massive seeding and increased control of supersaturation and the crystal surface area, the throughput necessary for its application is rarely, if ever, achieved for final bulk drug substances. In addition, continuous operation to achieve the conditions for crystallization (as discussed above for resolution of optical isomers) is often not... 

Goals. Consistent control of crystal surface area and crystal shape without charging dry seed from hatch to batch... 

Sonication is the means used in the fluidized bed crystallizers to maintain the number of seed particles in the magma to replace those removed in the product and at the same time to prevent the formation of overly large crystals. Excessive particle size starves the seed bed of crystal surface area for growth and, in the case of fluidized bed crystaUizers, causes sluggish solids movement, which can cause the particles to grow together. 

Seeding is commonly used as a control technique, especially in batch crystallization processes. The nucleation rate depends on the total surface area of the crystals. If the total crystal surface is relatively small in relation to the level of supersaturation, small crystals or crystals of undesired shapes may be formed in the early stage of crystallization. Otherwise, if the crystal surface area is sufficient, supersaturation will cause the existing crystals to grow. 

Nucleation is initiated by secondary mechanisms involving the seed crystals or low super-saturation and high surface area of seed crystals eliminate or minimize nucleation seed crystals grow... 

The experimental procedure was as follows. One litre of supersaturated aqueous solution was prepared, of which 0.8 was poured Into the crystallizer. After a constant temperature (of 298.2 K) was attained, the solution concentration was determined, and a seed crystal (0.59-0.71 mm) was put Into the solution and was allowed to grow for either 600 or 900 s. The crystal was then taken out of the crystallizer and put Into a 50 ml beaker In which a small quantity of alcohol was placed. The crystal was removed from the beaker and dried In the air for a few mlnltes, then Its weight and surface area were measured. The same crystal was again placed Into the crystallizer and allowed to resume growth. 

All synthesis experiments were carried out in rotating stainless steel autoclaves with an approximate volume of 250 cm3. After each experiment, the autoclaves were cleaned with 40 wt.-% HF to minimize seeding effects by residual crystallization products. The preparation of the synthesis mixture was as follows. Fumed silica with a surface area of ca. 200 m /g (CAB-O-SIL M-5) was mixed with about 90 % of the total amount of distilled water. To this mixture were added subsequently under stirring NaOH pellets (Merck, p.a.), pyrrolidine (Fluka, p.a.) as structure directing agent, AHNOgJj ... 

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