Crystallizers cascade

The triple-point crystallization of carbon dioxide is illustrated in Figure 7, which shows a schematic carbon dioxide phase diagram expanded about the triple-point and a closed-cycle triple-point crystallizer operating with pure carbon dioxide. The operation of this closed-cycle unit is identical to that of a unit in the stripping section of a continous crystallizer cascade, except that in the cascade vapor would pass to the unit above, and liquid would pass to the unit below. 

If maximum product size is desired, the process stream flow is parallel, with a fresh feed and slurry discharge to product recovery equipment from each crystallizer. Cascaded product flow is avoided because flow through a series of tanks narrows the residence time distribution and the CSD and reduces the mass mean size by reducing the number of larger crystals in the distribution tail. This tail represents the major mass-weighted fraction. Population models have been solved to verify this effect, assuming an equal nucleation rate in each stage (Randolph and Larson 1988). The actual mean in industrial practice is lower because transfer of... 

Radiation Damage. It has been known for many years that bombardment of a crystal with energetic (keV to MeV) heavy ions produces regions of lattice disorder. An implanted ion entering a soHd with an initial kinetic energy of 100 keV comes to rest in the time scale of about 10 due to both electronic and nuclear coUisions. As an ion slows down and comes to rest in a crystal, it makes a number of coUisions with the lattice atoms. In these coUisions, sufficient energy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by an incident ion are caUed primary knock-on atoms (PKA). A PKA can in turn displace other atoms, secondary knock-ons, etc. This process creates a cascade of atomic coUisions and is coUectively referred to as the coUision, or displacement, cascade. The disorder can be directiy observed by techniques sensitive to lattice stmcture, such as electron-transmission microscopy, MeV-particle channeling, and electron diffraction. 

By 1969, when a major survey (Thompson 1969) was published, the behaviour of point defeets and also of dislocations in crystals subject to collisions with neutrons and to the eonsequential collision cascades had become a major field of researeh. Another decade later, the subjeet had developed a good deal further and a highly quantitative body of theory, as well as of phenomenological knowledge, had been assembled. Gittus (1978) published an all-embracing text that eovered a number of new topics chapter headings include Bubbles , Voids and Irradi-ation(-enhanced) Creep . 

The optimal network increases total residence time by 48 per cent when compared with an equivalent MSMPR of the same volume and throughput. This increase would translate into a similar increase in mean crystal size and a 78 per cent increase in yield. Exactly the same residence time as for the single crystallizer have been reported from simple cascade configurations previously designed for stage-wise crystallization processes for slight improvements in... 

In another approach, two cascaded microstructure-collapses on a photonic crystal fiber are intentionally introduced by C02 laser or electric arc heating31. In this way, the mismatch of core size along the fiber causes light coupling between the core and cladding modes. 

For a given element, the sputter yield depends on the surface morphology surfaces that are rough on the scale comparable to the dimensions of the sputter cascade give higher yields of secondary particles than flat surfaces [4], For single crystals, sputter yields have been observed to vary with the plane exposed 17]. 

Some chiral compounds exhibit a different crystal structure for each pure enantiomer and their mixture the difference can be used to distinguish them. Again, the use of cascading libraries allows pure compounds to be distinguished from compounds contaminated with relatively small amounts of the other chiral forms. 

Fig. 8.2 Principle of the MALDI process. Initially, analyte and matrix are co-crystal I ized. After evaporation of the solvent, a nanosecond laser pulse is directed onto the crystalline surface, and both matrix and analyte molecules are desorbed. A complex reaction cascade leads to the formation of charged analyte molecules that reach the mass spectrometer without significant fragmentation.

Most of the now synthetically used quantitative cascade reactions involve an initial substitution step. That is quite clear for the reactions of acyl hahdes with thioureas to give 2-aminothiazoliiun salts. The 3-cascade consists of substitution to form the thiuronium salt, specific cychzation with the more nucleophihc of the amino groups, and elimination of water. In all reported cases, the water of reaction is taken up by the product crystal and it can be removed by heating to about 80 °C in a vacuiun. For example, if the thioureas 162 and phenacyl bromide 217 are stoichiometrically ball-milled at room temperature for 30 min, quantitative yields of the pure products 428 are obtained in all cases after drying at 0.01 bar at 80 °C [ 10] (Scheme 67). The free bases 429 can be obtained by trituration of 428 with NaHCOj solution. Furthermore, the thioureido-acet-amides 275 react correspondingly with 217 to give quantitative yields of the salts 430 from which the free bases can be obtained by NaHCOj trituration [96]. 

The easy access of a hydroxy group of ninhydrin (254) to its substitution and the presence of the carbonyl groups makes this highly reactive compound an interesting starting point for cascade reactions with amino compounds. These have been mechanistically investigated with AFM on six different faces of 254 and strictly relate to the crystal packing [94]. 

The solid-state interaction of enamines (428, 333a) with trans-l,2-diben-zoylethene (87) provides quantitative yields of the pyrrole derivatives 445 or 446 [140]. These remarkable 5-cascades consist of initial vinylogous Michael addition, enol/keto tautomerism, imine/enamine tautomerism, cyclization, and elimination, all within the crystal without melting. A waste-free extraordinary atom economy is achieved that cannot nearly be obtained in solution. The milling times are unusually long here (3 h) but it s certainly worth the effort... 

It should be remembered that F was derived on the basis of assuming an amorphous solid. The same is true for Sigmund s sputtering theory. However, it is the suspicion of the present author that the sequence of collisions, which are present in every cascade, tends to make the random assumption more realistic. Therefore, it is possible that this theory can be applied with some degree of confidence to polycrystalline materials. It cannot, of course, be applied to single crystal materials where chaneling and related phenomena occur. 

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