Kinds of Crystallizers

Analym of Size Distribution Data Obtained in a CSTC Differential distribution data obtained from a continuous stirred tank crystallizer are tabulated. 

The last column is of the summation So at corresponding values of crystal length L. The volumetric shape factor is =0.866, the density is 1.5g/mL, and the mean residence time was 2.0 hr. The linear growth rate G and the nucleation rate B° will be found. 

The number of crystals per unit mass smaller than size L is 

The two unknowns G and n° may be found by nonlinear regression with the 12 available data for L,. However, two representative 

This distribution should be equivalent to the original one, but may not check closely because the two points selected may not have been entirely representative. Moreover, although the data were purportedly obtained in a CSTC, the mixing may not have been close to ideal. 

Analysis of Size Distribution Data Obtained in a CSTC 

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Occasionally an optically inactive sample of tartaric acid was obtained Pasteur noticed that the sodium ammonium salt of optically inactive tartaric acid was a mixture of two mirror image crystal forms With microscope and tweezers Pasteur carefully sep arated the two He found that one kind of crystal (m aqueous solution) was dextrorota tory whereas the mirror image crystals rotated the plane of polarized light an equal amount but were levorotatory... 

The optical activity of quartz and certain other materials was first discovered by Jean-Baptiste Biot in 1815 in France, and in 1848 a young chemist in Paris named Louis Pasteur made a related and remarkable discovery. Pasteur noticed that preparations of optically inactive sodium ammonium tartrate contained two visibly different kinds of crystals that were mirror images of each other. Pasteur carefully separated the two types of crystals, dissolved them each in water, and found that each solution was optically active. Even more intriguing, the specific rotations of these two solutions were equal in magnitude and of opposite sign. Because these differences in optical rotation were apparent properties of the dissolved molecules, Pasteur eventually proposed that the molecules themselves were mirror images of each other, just like their respective crystals. Based on this and other related evidence, in 1847 van t Hoff and LeBel proposed the tetrahedral arrangement of valence bonds to carbon. 

C, Pasteur made the surprising observation that two distinct kinds of ciys tals precipitated. Furthermore, the two kinds of crystals were mirror images an< were related in the same way that a right hand is related to a left hand. 

In this paper, the results obtained by applying this method to another 23 kinds of crystals have been shown, after a brief survey on the analytical procedures. 

Chemically speaking, SOAz (I) and SOAz (II) are strictly identical and pure their actions on animal tumors are also identical. Thus, we would expect that the difference in their melting points to be due to some structural peculiarities, either with regard to their space group (if the two kinds of crystals do not contain any insertion of solvent) or possibly by some inclusion of solvent in the unit cell (clathrate structure) as in the case of MYKO 63 when crystallized from C Hg or CCI4 (see above). 

If the ratio falls between these two ratios, a mixture of TiCl2 and TiCl3 will be formed, and microscopic examination or x-ray diffraction will show the product to consist of two kinds of crystals, one of TiCl2, the other of TiCl3. 

A different result will be observed when the oxide Ti02 is reduced with titanium. Ti and Ti02, in the ratios 1 1 and 1 3, will yield TiO and T Og, respectively, but intermediate ratios will not always yield a mixture of the two substances. In other words, one single kind of crystal, with an intermediate composition, will be formed, instead of two kinds of crystals with the compositions TiO and Ti203. 

The glaze or enamel, a kind of crystal, was composed as follows —... 

The setup for ESR spectroscopy is a cross between NMR and micro-wave techniques (Section 5.8). The source is a frequency-stabilized klystron, whose frequency is measured as in microwave spectroscopy. The microwave radiation is transmitted down a waveguide to a resonant cavity (a hollow metal enclosure), which contains the sample. The cavity is between the poles of an electromagnet, whose field is varied until resonance is achieved. Absorption of microwave power at resonance is observed using the same kind of crystal detector as in microwave spectroscopy. Sensitivity is enhanced, as in microwave spectroscopy, by the use of modulation The magnetic field applied to the sample is modulated at, say, 100 kHz, thus producing a 100-kHz signal at the crystal when an absorption is reached. The spectrum is recorded on chart paper. 

So far, only normal diffraction (with sharp maxima at the points of reciprocal lattice) has been mentioned. Investigation of background scattering can provide extended information on various kinds of crystal defects (well beyond stacking faults and the like) as has been demonstrated e.g. on metal and feldspar structures. 

In discussions to this point, no significant interaction between a guest and its medium has been considered. This is probably the case in the reaction cavity model of Cohen [13] as well, since product selectivity was attributed mainly to the presence or absence of free volume within the cavity. The analogy of guests in hosts to balls in boxes is very deficient, but is really not different from the situation in the kinds of crystal systems which first inspired the Cohen nomenclature. Interatomic attraction and repulsion was important in analyzing those systems and was even critical to the crystal engineering used to assemble some of the systems used in the studies by Schmidt and his co-workers [1,48,89]. In addition to being stiff or flexible, cavity walls must... 

Nickel.—Vanadium and nickel are miscible in all proportions in the liquid state up to 36 per cent, vanadium. The solid alloys, which contain up to 20 per cent, vanadium, appear to be homogeneous, but those richer in vanadium consist of two kinds of crystals.10 These alloys are made by reducing a mixture of vanadium pentoxide and nickel oxide.u... 

Biiltemann 2 observed that vanadium ammonium alum separates out in blue crystals from a solution containing sulphuric acid, but from solutions containing a weak acid, or from neutral solutions, red crystals are obtained. (The chromium alums can also be prepared in differently coloured modifications.) The analytical data, melting-point, electrical conductivity, rate of efflorescence, and general behaviour of both kinds of crystals are identical, so that it is difficult to ascribe different constitutions to them. Meyer and Markowitz3 have shown that both forms separate out when the molecular proportion of sulphuric acid in the solution is less than that theoretically required, and attribute the red colour to the presence of traces of vanadous oxide, V203, or its hydroxide, V(OH)3. Vanadium rubidium and vanadium ciesium alums behave in the same way. A vanadium guanidine alum has also been prepared.4... 

Figure 16.6. Crystal size distributions of several materials in several kinds of crystallizers (Bamforth, 1965).

The main kinds of crystallizers are represented in Figure 16.10. They will be commented on in order. Purification of products of melt crystallization is treated separately. 

The crystallization procedure employed by Pasteur for his classical resolution of ( )-tartaric acid (Section 5-1C) has been successful only in a very few cases. This procedure depends on the formation of individual crystals of each enantiomer. Thus if the crystallization of sodium ammonium tartrate is carried out below 27°, the usual racemate salt does not form a mixture of crystals of the (+) and (—) salts forms instead. The two different kinds of crystals, which are related as an object to its mirror image, can be separated manually with the aid of a microscope and subsequently may be converted to the tartaric acid enantiomers by strong acid. A variation on this method of resolution is the seeding of a saturated solution of a racemic mixture with crystals of one pure enantiomer in the hope of causing crystallization of just that one enantiomer, thereby leaving the other in solution. Unfortunately, very few practical resolutions have been achieved in this way. 

Critical micelle concentrations depend almost entirely on the nature of the lyophobic part of the surfactant. If micelle structure involved some kind of crystal lattice arrangement, the nature of the lyophilic head group would also be expected to be important. 

This kind of crystal design is more often a failure than a success, and crystal structure prediction, particularly hard crystal structure prediction is still extremely difficult (Section 8.8). Generally, a large majority of the currently known host structures discussed in Chapter 7, for example, were discovered by accident rather than design. However, as our knowledge of the factors involved grows, in tandem with more powerful computational and modelling tools, more successes may be expected. 

In 1848, the French scientist Louis Pasteur prepared the sodium ammonium salt of racemic tartaric acid and allowed it to crystallize in large crystals which are visually distinctive from hemihedral forms.4 By discriminating the asymmetric faces of the crystals, he picked out the two kinds of crystals mechanically with a pair of tweezers and a loupe. Finally he obtained two piles of crystals, one of (+) and one of (-)-sodium ammonium tartrate. This was the first separation of optically active compounds from their racemate. 

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