Continuous crystallizer, seeding effect

The most effective seeding is achieved in semicontinuous and continuous crystallizations by the nature of the operations themselves, in which the seed is always present and in large quantity. Although common in large industrial operations, these techniques have found more limited application in the pharmaceutical industry. Exceptions to this are detailed in Examples 7-6 and 11-6, on the continuous resolution of optical isomers in fluid bed crystal-fizers, and in Example 10-1, which presents a semicontinuous method of utilizing seed heel recycle in reactive crystallization to achieve primarily growth. 

Effects of Seeding on Start-up Operation of a Continuous Crystallizer... 

If the enhanced kinetic barrier, Ag, results in the observed antifreeze function, then according to Eq. (44), the continued growth of the nucleus is a rate phenomenon. In experiments conducted with seed crystals of ice nuclei held at — 0.2°C, there does not seem to be a time-dependent growth of the seed nucleus, even over extended periods (Feeney and Hofmann, 1973 Ahmed et al., 1975). The fact that no rate effects have been observed, however, is insufficient reason to eliminate a kinetic mechanism. 

As is well known, some compounds have never been crystallized, and phase separation results in a stable oil or an amorphous solid. The search for solvents and conditions, or the introduction of foreign particle seeds (e.g., by scratching a glass test tube) to induce crystal formation for a new compound, becomes a matter of trial and error. Combinatorial techniques continue to be developed that can aid in this evaluation. A critical factor for success may be removal of impurities to achieve a very high level of purity, because the effect of even very low levels of impurities on homogeneous nucleation will not be known at this stage. 

Figure 10-5 Crystals from Example 10-1 showing the effect of continuous crystaUization, with decreasing seed area for growth causing formation of line needles.

The use of seed crystals of CaCOs and Mg(OH)2 suspended in the evaporating salt solution was found to be an effective means of preventing the formation of carbonate and hydroxide scales, at least up to 250° F. The concentration of seeds in this slurry should be somewhere between 0.1 and 0.5% by weight. Although this technique has been reasonably well proved in the pilot plant, it has yet to be demonstrated that it will work continuously in a full-scale plant such as the one at Freeport, Tex. No tests on this scale have been made to date. 

The stability of a crystallizing system tends to increase with increasing crystal growth rate and magma density, and with decreasing nucleation order , minimum product size, and the relative amount of crystals withdrawn. Continuous seeding has the same effect as decreasing the effective nucleation order, and should lead to a stabilization of the system. 

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