Crystallization practice scale

A practical scaled-up study was performed on samples of 3 kg of fat in square tin cans one with 1% DK F-10 in vegetable ghee and the other as control. The sample containing DK F-10 reached complete solidification after 13 h, whereas the control did not solidify until 5 h later. In the sample containing Dk F-10, solidification took place in all parts of the oil simultaneously, whereas in the control, solidification occurred from bottom to top. This fact indicates that the emulsifier was absorbed in the fat during crystallization in sufficient amounts to bring about a faster solidification process. 

Example 7-6 illustrates the applicability of good crystallization practice to achieve continuous production of large-volume pharmaceutical compounds. It also illustrates a crystallization process that is inherently unfeasible by any method other than continuous operation. When carried out using fluidized bed crystallizers, ultrasonic crystal disraption is used, even at factory scale, to maintain a steady-state population of seed particles in this all-growth system. 

When the salt derived from the hydrogenation products is crystallized, a part of the racemic modification crystallizes as a racemate but not conglomerate, while a part of the excess enantiomer gives crystals of a single enantiomer. In all cases, the former crystals are much soluble than the latter. Hence crystals of a single enantiomer are effectively isolated from racemic modification. This optical enrichment procedure is effectively applied for 3-hydroxyalkanoic acids of more than 80% e.e. on a practical scale. 

Nucleation can often be induced by agitation, mechanical shock, friction and extreme pressures within solutions and melts, as shown by the early experiments of Young (1911) and Berkeley (1912). The erratic effects of external influences such as electric and magnetic fields, spark discharges, ultra-violet light. X-rays, 7-rays, sonic and ultrasonic irradiation have also been studied over many years (Khamskii, 1969) but so far none of these methods has found any significant application in large-scale crystallization practice. 

Aluminum sulfate is a starting material in the manufacture of many other aluminum compounds. Aluminum sulfate from clay could potentially provide local sourcing of raw materials for aluminum production. Processes have been studied (24) and the relative economics of using clay versus bauxite have been reviewed (25). It is, however, difficult to remove impurities economically by precipitation, and purification of aluminum sulfate by crystallization is not practiced commercially because the resulting crystals are soft, microscopic, and difficult to wash effectively on a production scale (26—28). 

A highly detailed picture of a reaction mechanism evolves in-situ studies. It is now known that the adsorption of molecules from the gas phase can seriously influence the reactivity of adsorbed species at oxide surfaces[24]. In-situ observation of adsorbed molecules on metal-oxide surfaces is a crucial issue in molecular-scale understanding of catalysis. The transport of adsorbed species often controls the rate of surface reactions. In practice the inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their catalytic performance extremely difficult. In order to reduce the level of complexity, a common approach is to study model catalysts such as single crystal oxide surfaces and epitaxial oxide flat surfaces. 

It is often found that the ratio R (measured, for instance, by gas adsorption methods) of actual metal surface area accessible to the gas phase, to the geometric film area, exceeds unity. This arises from nonplanarity of the outermost film surface both on an atomic and a more macroscopic scale, and from porosity of the film due to gaps between the crystals. These gags are typically up to about 20 A wide. However, for film thicknesses >500 A, this gap structure is never such as completely to isolate metal crystals one from the other, and almost all of the substrate is, in fact, covered by metal. In practice, catalytic work mostly uses thick films in the thickness range 500-2000 A, and it is easily shown (7) that intercrystal gaps in these films will not influence catalytic reaction kinetics provided the half-life of the reaction exceeds about 10-20 sec, which will usually be the case. 

The addition of one or more heavy atoms to a macromolecule introduces differences in the diffraction pattern of the derivative relative to that of the native. If this addition is truly isomorphous, these differences will represent the contribution from the heavy atoms only thus the problem of determining atomic positions is initially reduced to locating the position of a few heavy atoms. Once the positions of these atoms are accurately determined, they are used to calculate a set of phases for data measured from the native crystal. Although, theoretically, one needs only two isomorphous derivatives to determine the three-dimensional structure of a biological macromolecule, in practice more than two are needed. This is due to errors in data measurement and scaling and in heavy-atom positions, as well as lack of isomorphism. 

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