Crystallization stages

In the second crystallizer stage, the crystals are usually reslurried with a higher purity PX stream from a later stage of purification. A second stage of centrifugation is sufficient in most cases to give PX purity >99%. 

At the crystallization stage, the rates of generation and growth of particles together with their residence times are all important for the formal accounting of particle numbers in each size range. Use of the mass and population balances facilitates calculation of the particle size distribution and its statistics i.e. mean particle size, etc. 

Before the crystallized material can be processed at higher SSP temperatures, the melting point of the crystals formed in the crystallization stage needs to... 

Batch crystallizers are often used in situations in which production quantities are small or special handling of the chemicals is required. In the manufacture of speciality chemicals, for example, it is economically beneficial to perform the crystallization stage in some optimal manner. In order to design an optimal control strategy to maximize crystallizer performance, a dynamic model that can accurately simulate crystallizer behavior is required. Unfortunately, the precise details of crystallization growth and nucleation rates are unknown. This lack of fundamental knowledge suggests that a reliable method of model identification is needed. 

One of the most commercially viable processes is the Nissan hemihydrate-dihydrate process. The conditions chosen for the digestion of the rock phosphate with sulphuric acid produces the hemihydrate phase. The slurry of fine hemihydrate crystals then passes to a crystallization stage where conditions are chosen (mainly temperature) so that gypsum is the stable species and it crystallizes out as substantially larger crystals, which aids subsequent filtration and washing. 

Figure 11.5. Three examples showing different Habitus of later-grown calcite crystals preferentially nucleated on the edges and corners of earlier-formed calcite crystal. Changes in Habitus depending on crystallization stages or growth temperatures are indicated [3]. (a) Earlier-formed hexagonal prism (A) and later-formed scalenohedral crystal (B). (b) Earlier-formed hexagonal prism (b) and later-formed thicker crystal (a), (c) The shaded area shows an earlier-formed rhombohedral crystal, and the remaining area represents later-formed scalenohedral crystals.

The first step in any chemical approach to crystalline structure is to determine the short-range order, i.e. which atoms are bonded. The most convenient way of doing this is by means of the bond graph described in Section 2.5. In many cases all or most of the bond graph can be determined from first principles, since, except for the weakest bonds created in the post-crystallization stage, the bond graph is determined by the rules of chemistry, particularly the hierarchical principle (Rule 11.5), the valence matching principle (Rule 4.2), and the principle of maximum symmetry (Rule 3.1). 

In the Sulzer-MWB process the naphthalene fractions produced by the crystallization process are stored in tanks and fed alternately into the crystallizer. The crystallizer contains around 1100 cooling tubes of 25-mm diameter, through which the naphthalene fraction passes downward in turbulent flow and pardy crystallizes out on the tube walls. The residual melt is recycled and pumped into a storage tank at the end of the crystallization process. The crystals that have been deposited on the tube walls are then partly melted for further purification. Following the removal of the drained liquid, the purified naphthalene is melted. Four to six crystallization stages are required to obtain refined naphthalene with a crystallization point of 80°C, depending on the quality of the feedstock The yield is typically between 88 and 94%, depending on the concentration of the feedstock fraction. 

The powder XRD-pattems of a typical ZSM-5 formation series are shown in figure 3. Again, the pattern labeled tk = 0 h is due to the starting porous glass. At a crystallization time of 46 h the typical MFI reflections are just detectable on top of the broad amorphous background. The crystallinity degree was at this crystallization stage QAi=0.03, which... 

Systems that exhibit behavior of the type illustrated in Fig. 4 cannot be purified in a single crystallization stage. They represent situations in which multiple stages or continuous-contacting devices may be useful. The principles of such operations are analogous to those of other countercurrent contacting operations—for example, distillation, absorption, and extraction. 

Since the formation of five-ring-rich zeolites like ZSM-5 can be easily envisaged starting from D5R silicates only, we have considered in this work the possibility of the D5R being a likely precursor species in either the nucleation and/or the crystallization stage. The results can be summarized as follows. 

FIGURE 10-24 Spherulidc growth. On the left is a snapshot during primary crystallization, while the snapshot on the right is during the secondary crystallization stage (Courtesy Prof. James Runt, Penn State). 

Most of the orogenic spinel Lherzolites also contain small amounts of amphibole ( 1 %) textu-rally equilibrated with the peridotite minerals. These amphiboles have been ascribed to the infiltration of melts/fluids in mantle conditions (Fabries et al., 1991), but their presence is not associated with noticeable chemical enrichments (Vannucci et al., 1995). A possible explanation is that the liquid/rock ratio was so low that the fluid composition was buffered by the LREE-depleted composition of the peridotites. Alternatively, these amphiboles might represent the products of a late crystallization stage during fertilization of the peridotites by LREE-depleted basaltic melts. 

Curve 4 exhibits very steep solubility. Yield is obtained by cooling the feed solution. To prevent fines formation, the cooling must exactly follow the solubility curve. This is done automatically in batch crystallizers. Continuous crystallizers in series must have the crystallizer stage temperatures selected so as not to cross the solubility curve. Benzoic acid and DMT exhibit this type of solubility. 

FIGURE 6.1 Conditions used for evaporation for impurity removal, and the crystallization stage(s) for solar salt (sodium chloride) production from seawater. Operating details from Korovessis and Lekkas [II]. 

The calculated weight percentages of the liquid phases observed at equilibrium provide further insight into the observed crystallization patterns. As shown in Fig. 11-18, the data suggest a direct correlation between the presence of a liquid phase during the freeze crystallization stage and the degree of... 

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