Secondary crystallisation

Secondary nucleation can, by definition, take place only if crystals of the species under consideration are already present. Since this is usually the case in industrial crystallisers, secondary nucleation has a profound influence on virtually all industrial crystallisation processes. 

The stage of rapid crystallisation ends when crystallinity reaches approximately 0.5—0.7, ie, when HDPE is 50—70% crystallised. After that, the crystallisation rate falls drastically. The slower process that follows is known as secondary crystallisation (20), which can be described by the empirical equation CR = A - - S-log(t). Secondary crystallisation can be accelerated if HDPE is heated. 

Codeinone, CjaHijOgN. This ketone (XLVII) corresponds to the secondary alcohol codeine and its stereoisomeride wocodeine. It may be prepared by oxidising codeine with potassium permanganate in acetone or with potassium dichromate in dilute sulphuric acid and in various other ways. Codeinone can be reduced to codeine electrolytically or by chemical methods. It crystallises from alcohol in prisms, m.p. 185-6, [a]J, ° — 205° (EtOH). The hydrochloride, B. HCl. HjO, has m.p. 179-80°, picrate, m.p. 205°, methiodide, B. CHjI. 2H2O, m.p. 180°. 

It is generally more convenient to employ the solid p-toluenesulphonyl chloride (m.p. 69°) rather than the liquid benzenesulphonyl chloride. More-ova-, the benzenesulphonamides of certain secondary amines are oils or low melthig point solids that may be difficult to crystallise the p-toluenesulphon-amides usually have higher melting points and are more satisfactory as derivatives. Technical p-toluenesulphonyl chloride may be purified by dissolving it in benzene and precipitating with light petroleum (b.p. 40-60°). 

Apart from deliberate or accidental introduction of tiny seed crystals to the system, and productive interactions between existing crystals and quasi-crystalline embryos or clusters in solution, the most influential mode of new crystal generation in an industrial crystalliser is contact secondary nucleation between the existing crystals themselves, between crystals and the walls or other internal parts of the crystalliser, or between crystals and the mechanical agitator. Secondary nucleation rates (in m-3 s 1) are most commonly correlated by empirical relationships such as ... 

A major problem, until recently, was the determination of the protein primary structure, but with the advent of modern analysis of DNA this has become comparatively easy. One of the first structures to be described was that of insulin which contains 60 amino-acids and has a molecular weight of 12,000. Once the primary structure is known, it is possible to predict the secondary and tertiary structures using additional information obtained through X-ray crystallography of the crystallised protein. 

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