Crystallization content

The data for a plot like Fig. 18-60 are easily obtained from a screen analysis of the total crystal content of a known volume (e.g., a liter) of magma. The analysis is made with a closely spaced set of testing sieves, as discussed in Sec. 19, Table 19-6, the cumulative number of particles smaller than each sieve in the nest being plotted against the aperture dimension of that sieve. The fraction retained on each sieve is weighed, and the mass is converted to the equivalent number of particles by dividing by the calculated mass of a particle whose dimension is the arithmetic mean of the mesh sizes of the sieve on which it is retained and the sieve immediately above it. 

The Investigation was carried out using a seeded, batch crystallization In the absence of nucleatlon. Supersaturated solutions were prepared, seeded and maintained at a constant temperature while crystallization proceeded. Samples were taken periodically to give a solution for analysis and crystals for size analysis and crystal content determination. 

The main processing options open to the crystallizer designer are the solubility gap (transition temperature, acid content), the operating temperature and the values of the rate coefficients (affected by Impurities) and crystal surface areas (eg. altering crystal content). The computer model generated In this study allows these effects to be evaluated. 

Figure 6 shows the size distributions for the samples taken from one of the runs, presented as the cumulative number oversize per ml of slurry. From the lateral shift of the size distributions, the growth rate can be determined. Figure 7 shows values of growth rate, G, supersaturation, s, and crystal content determined during the run. As a material balance check, the crystal contents were evaluated from direct measurements, from solution analyses and from the moments of the size distribution. The agreement was satisfactory. No evidence of size dependent growth or size dispersion was observed. 

Figure 7. Crystal content, supersaturation and growth rate vs time for gypsum growth run at 50 C.

To determine the rate of dissolution of hemlhydrate crystals, the same vessel was used as for the crystallization study. The vessel was filled with the sulphate-rich solution (zero Initial calcium concentration). An amount of sieved hemlhydrate seed crystals, about 10% In excess of that required to saturate the solution, was added. At very short time Intervals, samples were taken using a similar procedure to that for the gypsum growth Investigation. Samples were separated Into crystals for size analysis (with a 190pm orifice) and crystal content and solutions for analysis. Further details are given by Mukhopadhyay (17). 

Figure 12 shows the size analyses from one run. Crystal content and solution analysis measurements were In good agreement. The size shown In Figure 12 Is the volume equivalent size, which Is acceptable for crystal growth where the crystals retain the same elongated shape as they grow. However when needle-1Ike crystals dissolve, there Is no longer preferable transfer on particular faces, so the shape changes. If allowance Is not made for this. Incorrect dissolution rates will result. Thus for purposes of...

A = crystal surface area per litre of the crystallizer content (m2/l)... 

Equation (6) indicates that secondary nucleation is significantly affected by the crystal surface area A (m2/l). The nucleation process is usually viewed as a process where copious amounts of fines are produced, resulting in a corresponding increase in the specific crystal surface area (m2/g) and crystal surface areaA olume of the crystallizer content (m /l). Results obtained in this study concur with this nucleation view. However, Brown (9) reported that nucleation can be suppressed at high crystal surface areas ( 20 m2/l). Brown s observation was not evident in this study though the seed density of 200 g/litre used in this study had a surface area of 25 m2/l. The lower dependence of second nucleation on supersaturation or concentration driving force, AC, in comparison to crystal surface area. A, is obvious from equation (6). 

The conversion rates of n-hexane are shown as a function of the crystallinity parameter Qai for different temperatures. We found that the catalytical activity increases simultaneously with the increased crystallinity of the composites, the crystallization products. According this linear correlation it can be concluded that the catalytical active sites, the acidic centers in the zeolitic framework, are always, independent of the crystal content of the composite material, accessible for educt of the test reaction, the n-hexane molecules. This leads to the assumption that the crystallization must start on the interface (at the phase border) between the solution (contains the alkalinity and the template) and the solid (porous glass) surface and has to carry on to the volume phase of the glass resulting finally in complete transformed granules. 

By means of x-ray studies it is possible to demonstrate chain orientation into the fiber structure without much crystallization, if any, occurring. Calorimetric measurements, Smith and Dole (1956), are entirely in accord with this structural interpretation i.e. the existence of fiber structure without crystallinity. Dole (1959) reviewed specific heat differences between drawn and undrawn filaments and could find none that could not be interpreted on the basis of crystal content. 

For illustrative purposes, consider that the protocol for a cooling crystallizer can involve either natural cooling— cooling resulting from exposure of the crystallizer contents to a heat sink without intervention of a control system—or manipulation of cooling to reduce the system temperature in a specific manner. In both cases, the instantaneous heat-transfer rate is given by ... 

Aromatic pitches were produced by treating CCB-distillate at 400°C, 410°C, 420°C, 430°C, and 440°C. We found that process temperature is a very important parameter in determining the rate of toluene and pyridine insoluble formation. It was also found that a relatively high temperature (around 430°C) is required to produce a pitch with a high liquid crystal content. Table II gives the composition of pitches produced at 400-440°C. Figure 1 illustrates graphically, the effect of process temperature on the rate of toluene, pyridine, and quinoline insolubles formation. 

X-ray results on the tubes showed that in both cases the crystal content was virtually all form I, indicating that possibly form II to form I conversion occurred on poling the lower draw ratio tube 1, where incomplete conversion to form I had been obtained on drawing alone. The densities of tubes 1 and 2 were 1.7887 and 1.7744 g cm 3 respectively, corresponding to form I contents of 38% and 35%. Slip had occurred between the caterpillar haul-off rubber tracks and the tubing, and consequently the true draw ratios measured by tube dimensions were less than the machine draw ratios obtained from the haul-off speeds. It is encouraging to note that the coefficients in... 

The regularity of stacking of unit cells can be represented by a crystal lattice, which expresses only the periodicity of the crystal contents, not the actual molecule that is repeated. As shown in Fig. 2, the crystal structure is a combination (convolution is the mathematical term) of a motif (the macromolecule or a group of macromolecules within a unit cell) and the crystal lattice. It is only necessary to find the atomic arrangement in one unit cell, and then this atomic arrangement is repeated in a manner defined by the crystal lattice to give the entire crystal structure. 

This structural information can also help explain changes observed in the mechanical properties of the nanocomposites. As the amorphous content of the samples decreases from UM to dPC and the material becomes more crystalline, the nanocomposites become stronger. Also in the core of the injection moulded test bars where slow cooling is prevalent, the more stable a structure appears to form readily. As the y crystal structure is said to be more ductile than the a, it would be expected that the tensile strength of materials containing mostly a crystals, like DdPC-OdPC, to be much stronger than those with high levels of y crystal in the core. So not only is the increase in modulus due to the reinforcement provided by the clay layers and increase in crystallinity, but also the reduction in y crystal content. 

Mwhar liquor is ciroalated, with or without crystal content, through a heat exchanger where heal is added or removed from the system. (Adiabatic vacuum cooling ciyslaliizers do not require this step.)... 

Hardness, softening range, crystallization, content uniformity, dissolution (37° C)... 

The amorphous phases that cannot be detected by XRD can be determined from the difference between the calculated paste composition and the crystal content as determined by XRD. The hydrated and carbonated phases in the paste can be detected through differential scanning calourimetry (DSC) [18]. 

The thick juice is fed into evaporator crystallizers in which a further gentle concentrating under vacuum (0.2 bar abs.) is carried out to supersaturate the solution. After adding seed crystals, crystal growth proceeds via parallel further evaporation and reduction of progressing supersaturation through crystallization. For mechanical reasons, the crystallization is finished at about 50% crystal content. 

The dehydration of moist inorganic salts with water of crystallization content is, in general, a complex task. The loss of crystalline water usually consists of several components with different temperature or residence time requirements for each case. In the course of dehydration, the material to be processed (e.g., ZnCl2 or MnCy may form undesirable compounds. In numerous cases, as with magnesium chloride and iron chloride, oxygen impedes the full loss of crystal water as a result of heat effects. On the basis of these problems, publications dealing with successful dehydration experiments conducted in spouted bed equipment are of particular interest. 

Nematic ordering can be observed and quantified based on a change in a characteristic band of the liquid crystal. Moreover, the phase separation and the onset of nematic ordering can be temporarily resolved. The conversion at phase separation decreased strongly with an increase in liquid crystal content, while the conversion required for phase separation increased with increasing temperature. The fraction of liquid crystal present as nematic droplets and total fraction of nematic domains in NOA65 were quantified from changes in the vibrational spectrum of the liquid crystal (E7, which refers to the commercial label for the liquid crystal) (37). 

Fig. 15. Plot of bending modulus (along and across the flow direction) versus liquid crystal content. Specimen thickness = 1/16 and molding temperature = 275°C. 

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