Freeze-crystallization techniques

Chemical precipitation has traditionally been a popular technique for the removal of heavy metals and other inorganics from wastewater streams. However, a wide variety of other techniques also exist. For example, ion-exchange, reverse osmosis, evaporation, freeze crystallization, electrodialysis, cementation, catalysis, distillation, and activated carbon have all been used for removal of inorganics. 

The Atomizing Freeze Crystallization (AFC)-Snowfluent technology is specifically designed for cold climates. The technology combines freeze crystallization and snow-making techniques... 

Separation of the aromatics from each other and from other hydrocarbons by distillation is not economical because of the limited boiling-point differences and the formation of azeotropic mixtures. Instead, extractive or azeotropic distillation and liquid-liquid extraction are applied.234,235 The latter process is by far the most often used technique. The three processes are applied according to the aromatic content of the gasoline source. p-Xylene, the most valuable of the isomeric xylenes, is isolated by freezing (crystallization) or solid adsorption. 

Other crystallization techniques that are less frequently applied in the pharmaceutical industry, such as melt and freeze crystallization, may be applicable for some processes. In Example 11-4, purification of dimethyl sulfoxide (DMSO) is presented. In this case, low-level impurities, primarily dimethyl sulfide, are removed by controlled fractional crystallization from the melt (DMSO is a liquid above 18.45°C), in combination with adsorption of impurities from the unfrozen liquid. In the feed DMSO prior to the crystallization step, the impurities, while unacceptable, are at too low a level to be removable by adsorption alone. 

The presence of liquid-crystalline material at the emulsion interface has been shown by electron microscopy using the freeze-etching technique 18). Typical liquid-crystalline structures are shown in Figure 16. These liquid-crystalline compositions are viscous, and the lamellar phase displays pseudoplastic rheology. The lamellar phase is the most important of all liquid-crystalline phases for emulsion stability. The presence of a liquid-crystalline phase causes a reduction of the available London-van der Waals forces for coalescence 16). As a consequence of the reduction of the influence of these dispersion forces and the high viscosity of the liquid-crystal layer, the time for coalescence is increased dramatically. 

Process development work at Ranbaxy began with the aim to develop a simple, practical, and efficient method for the preparation of crystalline imipenem monohydrate, which is thermally stable, has a uniform degree of crystallinity, and is of high purity. After extensive experimentation, a process was developed for the isolation of pure crystalline imipenem monohydrate from a solution containing imipenem. The process does not use capital-intensive techniques of lyophilization or freeze crystallization nor the time-consuming purification process of column chromatography using expensive hydro-phobic resins. 

The early sixties were golden times for the progress of investigation techniques in this field of science the first NMR studies in the anisotropic media of lyotropic liquid crystals [48] were done, the introduction of the freeze fraction technique for the preparation of electron microscopy probes (e.g., of cell membranes [49]) and the separation of different lyomesophases by centrifugation [47] were introduced all between 1960 and 1962. 

It is no news that, compared with the solid state reaction, the glycine-nitrate route, sol-gel and freeze-drying techniques require lower calcination temperatures to yield pnire crystals of perovskite phases, with the resulting energy saving (very important nowadays). The disadvantage is, however, the fact that they are more time consuming and require more controlled synthesis conditions. In the same way, it is clearly observed that the ceramic... 

An alternative method is the freeze-etching technique [24, 25], which consists of producing platinum shadowed replicas of fractures created in rapidly frozen biological systems or lyotropic liquid crystals. The fracture orientation is somewhat haphazard, but occurs preferentially within the paraffinic level of the bilayers. These methods offer the possibility of viewing bilayers directly and preparing stereoviews. Beautiful pictures of liquid crystalline DNA, both cholesteric and hexagonal, have been obtained, but individual molecules are not easily resolved. However, the director distribution can be deduced from the images [26],... 

Microtomy and freeze-etching techniques have revealed many similarities between liquid crystals and a large series of biological materials, which show the same symmetries as nematic, smectic and cholesteric phases, but without being fluid [27-34]. The biological materials are stabilized... 

In the case of other types of materials better control has been achieved by incorporating freeze-drying techniques in which the entire matrix sample mixture was dissolved and then the solvent removed at a low temperature and pressure, ensuring the deposition of extremely fine crystals. The coprecipitation of matrix and sample also gives a uniform dispersion. 

An important breakthrough was achieved in the late 1980s when the vertical gradient freeze (VGF) technique was developed [3, 4]. In the following period it was possible to improve the crystal-growth conditions continuously with respect to decreased thermal stress, especially by a rigorous application of modeling and process-simulation tools [5, 6]. 

An alternative to the above preparation methods, albeit a rather involved one, is the freeze-fracture technique, in which the dispersion is shock frozen by being poured into liquid nitrogen. The freezing process has to be fast enough to avoid crystallization of the water phase. The sample is then cryo-transferred to the electron microscope where it is fractured. The fracture surfaces can then be imaged using, for example, replica techniques. 

Ciyst lliz tion. Low temperature fractional crystallization was the first and for many years the only commercial technique for separating PX from mixed xylenes. As shown in Table 2, PX has a much higher freezing point than the other xylene isomers. Thus, upon cooling, a pure soHd phase of PX crystallizes first. Eventually, upon further cooling, a temperature is reached where soHd crystals of another isomer also form. This is called the eutectic point. PX crystals usually form at about —4° C and the PX-MX eutectic is reached at about —68° C. In commercial practice, PX crystallization is carried out at a temperature just above the eutectic point. At all temperatures above the eutectic point, PX is stiU soluble in the remaining Cg aromatics Hquid solution,... 

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