Crystallization results

The Phillips process is a two-stage crystallisation process that uses a pulsed column in the second stage to purify the crystals (79,80). In the pulsed column, countercurrent contact of the high purity PX Hquid with cold crystals results in displacement of impurities. In the first stage, a rotary filter is used. In both stages, scraped surface chillers are used. This process was commercialized in 1957, but no plants in operation as of 1996 use this technology. 

Liquid crystals may be divided into two broad categories, thermotropic and lyotropic, according to the principal means of breaking down the complete order of the soHd state. Thermotropic Hquid crystals result from the melting of mesogenic soHds due to an increase in temperature. Both pure substances and mixtures form thermotropic Hquid crystals. In order for a mixture to be a thermotropic Hquid crystal, the different components must be completely miscible. Table 1 contains a few examples of the many Hquid crystal forming compounds (2). Much more is known about calamitic (rod-Hke) Hquid crystals then discotic (disk-like) Hquid crystals, since the latter were discovered only recendy. Therefore, most of this section deals exclusively with calamities, with brief coverage of discotics at the end. 

Gumylphenol. -Cumylphenol (PGP) or 4-(1-methyl-l-phenylethyl)phenol is produced by the alkylation of phenol with a-methylstyrene under acid catalysis. a-Methylstyrene is a by-product from the production of phenol via the cumene oxidation process. The principal by-products from the production of 4-cumylphenol result from the dimerization and intramolecular alkylation of a-methylstyrene to yield substituted indanes. 4-Cumylphenol [599-64-4] is purified by either fractional distillation or crystallization from a suitable solvent. Purification by crystallization results in the easy separation of the substituted indanes from the product and yields a soHd material which is packaged in plastic or paper bags (20 kg net weight). Purification of 4-cumylphenol by fractional distillation yields a product which is almost totally free of any dicumylphenol. The molten product resulting from purification by distillation can be flaked to yield a soHd form however, the soHd form of 4-cumylphenol sinters severely over time. PGP is best stored and transported as a molten material. 

FIG. 18-60 Fop Illation density of crystals resulting from Bujacian behavior. 

A further confirmation that mirrorlcss lasing is restricted to single domains comes from an experiment in which an Oocl-OPV5 film has been crystallized from the isotropic melt phase (above 204 "C). Melt crystallization resulted in the formation of large domains with dimensions up to several millimeters (see Fig. 16-29 C). Tlie normalized emission spectra for different excitation energies are shown in Figure 16-47. The excitation spot diameter was 1 mm in these ex-... 

To avoid misunderstanding, it should be emphasized that if the transition from one type of crystallization to the other one is considered, this does not imply a transformation of crystals of one type into the other one during stretching. In contrast, if the molecule enters a folded-chain crystal, it is virtually impossible to extend it. In this case, we raise the question, which of the two crystallization mechanisms controls the process at each given value of molecular orientation in the melt (this value being kept constant in the crystallization process during subsequent cooling of the system). At /J < /3cr, only folded-chain crystals are formed whereas at / > only fibrillar crystals result at /8 /3cr, crystals of both types can be formed. 

Figure 2. An octahedron resulting from rapid growth perpendicular to faces of cube and a skeleton crystal resulting from rapid growth along diagonals of cube...

When growth in the direction perpendicular to the face of the cube is rapid, the cube face disappears and an octahedron results. Salt octahedron shape can be obtained under certain conditions—for example, when grown in the presence of urea. When growth in the direction of the diagonal of the cube is rapid, the edges and comers of the cube grow in preference to the center of the cube face and a skeleton crystal results. One common table salt is composed largely of crystals of this type. 

Following this, attempts were made to remove a particle from the extruded plastic sheeting. When a particle was finally isolated, applying pressure with the ATR crystal resulted in cracking of the particle, so a representative spectrum could not be obtained. This brittle behavior usually indicates that the sample is inorganic in nature. 

Before discussing the crystallization results for these elastomers, it is useful to summarize two of the important distinguishing features of the Ba-Mg-Al polymerization system ... 

If electron-pair, or covalent, bonding is periodic in two or three dimensions, crystals result. The most important case is the carbon-carbon bond. If it is extended periodically in two-dimensions the result is graphite in three-dimensions it is diamond. Other elements that form electron-pair bonds are Si, Ge, and a-Sn. Some binary compounds are A1P (isoelectronic with Si),... 

On the other hand, if the enantiomeric purity of the original solid is less than that of the eutectic (as in the case of M2 in Fig. 25b), crystallization results in a decrease in enantiomeric purity. For example, when sufficient solvent has been added to correspond to point P2, the tie line shows that the solid N2 contains less of the predominant enantiomer D than M2 and is in equilibrium with E, which corresponds to a saturated solution of the eutectic solid, e. When the system reaches the composition represented by point Q2, the solid that crystallizes out is the racemic compound, R, which is in equilibrium with the saturated solution, U2, containing the racemic compound and enantiomer D. 

One approach to the production of high-performance dielectrics relies on the use of mixed-metal, multiple-component oxides. These oxides provide convenient means for controlling the dielectric-constant breakdown-field product through incorporation of components that specifically contribute to performance via dielectric constant or breakdown. At the same time, the mixed materials can inhibit crystallization, resulting in deposition of amorphous films with extremely flat surfaces. Common candidates, base oxides for tuning these properties, are listed in Table 4.1. 

Modern pump designs also include a means for flushing the piston with solvent behind the pump seal (not shown in Figure 13.4). The solvent for this is drawn in from a separate reservoir and pumped back into this same reservoir. The purpose is to continuously rinse the piston free of mobile phase residue such that abrasive solute crystals resulting from a mobile phase that has dried out on the piston will not deposit there. These solutes, such as the salts dissolved in the buffered mobile phases used in ion exchange chromatography, may otherwise crystallize on the piston and then damage the piston or the pump seal when the piston moves back and forth. Mobile phases that contain such solutes must be flushed from the system after use so that there is also no crystallization on the front side of the seal. 

As determined from X-ray diffraction measurements, the unit cell of crystalline PET is triclinic with a repeat distance of 1.075 nm along the major axis [5, 6], This corresponds to >98 % of the theoretical extended length of the monomer repeat unit [6], There is very little molecular extensibility remaining in a PET crystal, resulting not only in a high modulus but also a relatively short extension range over which the crystal can be extended and still recover elastically. The density of the crystalline structure is 1.45 g/ml, or about 9% higher than the amorphous structure [3],... 

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

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