Fluidizedbed agitation
As described above, the Oslo-Krystal unit is a fluidized-bed agitated crystal-lizer in which the gentle action minimizes secondary nucleation and allows large crystals to grow. Oslo-Krystal vacuum crystallizers can be of the 'open' Figure 8.51 or 'closed' Figure 8.45 types. In the former the crystallization zone is at atmospheric pressure. In the latter all parts of the equipment are under reduced pressure. Figure 8.50. Escher-Wyss Tsukishima double propeller DP crystallizer Figure 8.50....
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Figure 9.26. Cold-finger test for assessing encrustation behaviour A rough assessment of the propensity for encrustation can be gained by using the simple cold-finger technique shown in Figure 9.26. Coolant is supplied at constant temperature and flow rate to the finger e.g. 2.5 cm diameter, 15 cm long which dips into solution flowing through the larger vessel. The maximum temperature difference allowed between the coolant and solution without encrustation occurring can be determined by visual...
Brodie purifier
The Brodie purifier Brodie, 1971 is a countercurrent melt purification system with the added feature of an imposed temperature gradient between the residue and product outlets. The unit thereby acts as a countercurrent multistage fractionator with partial melting and recrystallization occurring along its length. The essential features are shown in Figure 8.20. The heat exchangers, provided with slow-moving scraper-conveyors, are arranged into refining and recovery sections separated by the feed...
81 Precipitation
Precipitation is a widely used industrial process. It is also a very popular laboratory technique, especially in analytical chemistry, and the literature on this aspect of the subject is voluminous see Kolthoff et al., 1969 . Precipitation plays an important role not only in chemistry but also in metallurgy, geology, physiology, and other sciences. In the industrial field, the manufacture of photographic chemicals, pharmaceuticals, paints and pigments, polymers and plastics utilizes the...
762High temperature solution growth
Many substances normally considered insoluble in water have an appreciable solubility at elevated temperatures and pressures. This property is utilized in the technique called 'hydrothermal crystallization', which is basically crystallization from aqueous solution at high temperature 350-550 C and pressure 1-3 kbar . The operation is carried out in a steel autoclave Figure 7.18b , which can be provided with a silver or platinum liner for protection. The technique has proved satisfactory for the...
Trough crystallizers
The first truly continuous crystallizer to be introduced to the chemical industry, between 1905 and 1910, was the Wulff-Bock unit Figure 8.36 , frequently referred to as the crystallizing cradle or rocking crystallizer. It consists of a long shallow trough, about 1.2 m wide, rocked on supporting rollers. The solution to be crystallized is fed in at one end and the crystals are discharged at the other end, continuously. Transverse baffles may be fitted inside the trough to prevent longitudinal...
OsloKrystal evaporating crystalllzer
The principles of the Oslo-Krystal process, already referred to above in connection with cooling crystallizers, can also be applied to evaporative crystallization. Three forms of the Oslo-Krystal evaporating crystallizer are shown in Figure 8.45. The construction of these crystallizers, commonly used in multiple-effect systems, is of the 'closed' form, i.e. the vaporizer is directly connected with the crystallizer body to form a sealed unit. There are two basic types, the waisted a and the...
Adiabatic evaporative cooling
An early report of adiabatic evaporative cooling Takegami, 1993 considered the purification of caprolactam melting point 690 C in the presence of water Figure 8.26. Apparatus for melt DCC crystallization, using air as a coolant. A Reservoir B Data acquisition system C Thermostatic baths and circulators D Particle analysis sensor E Dilution tank F Temperature recorder G Crystallizer H Flowmeter I Refrigerator J Temperature programming controller K Heat exchanger. After Kim and Mersmann, 1997...
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Figure 9.27. The mixing criterion see text Figure 9.27. The mixing criterion see text phenomenon of acoustic microstreaming at the solid-liquid interface. Although successful in small-scale pilot plant equipment, scale-up to large-scale industrial crystallizers has not yet been achieved. The effect of an externally applied magnetic field on crystalline scale deposition, typically of CaCO3 from hard water flowing in metal pipes, still remains a controversial subject despite the considerable...
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Fines destruction
Even though significant improvements in product CSD can be made by employing controlled operation, the benefits are often diminished because fine crystals are created by secondary nucleation Mullin and Nyvlt, 1971 Jones and Mullin, 1974 . In an attempt to combat this problem, the effects of fines destruction FD , commonly employed to improve product CSD in continuous crystallizers section 9.1.2 was investigated by Jones, Chianese and Mullin 1984 . Runs were made with potassium sulphate in a 30...
462 Eutectic formation
Equilibrium relationships in three-component systems can be represented on a temperature-concentration space model as shown in Figure 4.20. The ternary system ortho-, meta- and para-nitrophenol, in which no compound formation occurs, is chosen for illustration purposes. The three components will be referred to as O, M and P, respectively. Points O', M' and P' on the vertical Figure 4.20. Eutectic formation in the three-component system o-, m- and p-nitrophenol a temperature-concentration space...
473 Janecke diagrams
In order to simplify the interpretation of the phase equilibria in reciprocal salt pair systems, the water content may be excluded. The curves of the space model can then be projected on to the square base to give a two-dimensional graph, called a Janecke diagram as described in section 4.7.1. A typical projection is shown in Figure 4.31a the lettering is that used in Figure 4.30. The enclosed areas, which represent saturation surfaces, indicate solutions in equilibrium with one salt, the...
15
always remains lower than the partial pressure of the water vapour in the atmosphere. Calcium chloride, with a critical humidity of 32 per cent at 150C, is a well-known example of a deliquescent salt. The term 'efflorescence' refers to the loss of water of crystallization from a salt hydrate this occurs when the vapour pressure exerted by the hydrate exceeds the partial pressure of water vapour in the atmosphere. Sodium sulphate decahydrate, with a critical humidity of 93 per cent, is an...
14 Crystal systems
There are only 32 possible combinations of the above-mentioned elements of symmetry, including the asymmetric state no elements of symmetry , and these are called the 32 point groups or classes. All but one or two of these classes have been observed in crystalline bodies. For convenience these 32 classes are grouped into seven systems, which are known by the following names regular 5 possible classes , tetragonal 1 , orthorhombic 3 , monoclinic 3 , triclinic 2 , trigonal 5 and hexagonal 1 . The...
15 Miller indices
All the faces of a crystal can be described and numbered in terms of their axial intercepts. The axes referred to here are the crystallographic axes usually three which are chosen to fit the symmetry one or more of these axes may be axes of symmetry or parallel to them, but three convenient crystal edges can be used if desired. It is best if the three axes are mutually perpendicular, but this cannot always be arranged. On the other hand, crystals of the hexagonal system are often allotted four...
13 Crystal symmetry
Many of the geometric shapes that appear in the crystalline state are readily recognized as being to some degree symmetrical, and this fact can be used as a means of crystal classification. The three simple elements of symmetry which can be considered are 1. Symmetry about a point a centre of symmetry 2. Symmetry about a line an axis of symmetry 3. Symmetry about a plane a plane of symmetry It must be remembered, however, that while some crystals may possess a centre and several different axes...
113 Imperfections in crystals
Very few crystals are perfect. Indeed, in many cases they are not required to be, since lattice imperfections and other defects can confer some important chemical and mechanical properties on crystalline materials. Surface defects can also greatly influence the process of crystal growth. There are three main types of lattice imperfection point zero-dimensional, line one-dimensional and surface two-dimensional . The common point defects are indicated in Figure 1.25. Vacancies are lattice sites...
12 Crystalline solids
The true solid crystal comprises a rigid lattice of molecules, atoms or ions, the locations of which are characteristic of the substance. The regularity of the internal structure of this solid body results in the crystal having a characteristic shape smooth surfaces or faces develop as a crystal grows, and the planes of these faces are parallel to atomic planes in the lattice. Very rarely, however, do any two crystals of a given substance look identical in fact, any two given crystals often...