Surface-Cooled Crystallizers

Although surface-cooled types of MSMPR crystalhzers are available, most users prefer crystallizers employing vaporization of solvents or of refrigerants. The primary reason for this preference is that heat transferred through the critical supersaturating step is through a boil-ing-hquid-gas surface, avoiding the troublesome solid deposits that can form on a metal heat-transfer surface. 

FIG. 18 66 Forced-circulation baffle surface-cooled crystallizer. (Swenson Process Equipment, Inc.)... 

An Oslo surface-cooled crystallizer is illustrated in Fig. 18-71. Supersaturation is developed in the circulated liquor by chilling in the cooler H. This supersaturated liquor is contacted with the suspension of ciystals in the suspension chamber at E. At the top of the suspension chamber a stream of mother hquor D can be removed to be used for fines removal and destruction. This feature can be added on either type of equipment. Fine ciystals withdrawn from the top of the suspension are destroyed, thereby reducing the overall number of ciys-tals in the system and increasing the particle size of the remaining product ciystals. 

The following cheap procedure serves for the preparation of large amounts of methylamine hydrochloride (Brochet and Gambier, Bull. Soc. chim., 1895 [iii.], 13, 533). Heat together in a distilling flask attached to a downward condenser 250 g. of ammonium chloride and 570 c.c. of 35 per cent formaldehyde solution. With the thermometer in the liquid, slowly raise the temperature to 104° and maintain at this point until distillation ceases (about 4-5 hours from the start). By then 100-120 g. of water and methyl alcohol will have collected in the receiver. Cool the flask, remove the ammonium chloride which separates by filtration at the pump, and evaporate the filtrate to half its volume on the water bath. Again remove ammonium chloride by filtration and concentrate the filtrate until a film of crystals forms on the surface. Cool and separate the methylamine hydrochloride by Ultra-... 

Clavilier s currently recommended procedure is as follows. Two cooling processes of the crystal after flame-annealing treatment are carried out between steps (a) and (b) in Fig. 1 i.e., air cooling until the temperature of the crystal becomes 200-300°C and cooling in pure water under an H2 -I- Ar stream. The transfer of the crystal into the cell in step (c) of Fig. 1 with a droplet of H2 + Ar-saturated pure water on the crystal face is important for protecting the crystal surface from impurities in air. Then the crystal surface is exposed to the solution by the dipping method of Fig. 1. 

The type of vessel employed depends on volatility of the solvent obviously the conical flask already recommended for "crystallization by cooling" is not suitable for spontaneous evaporation, while a beaker or shallow dish is. When the latter type of vessel is used, "crusts" often form on the sides above the surface of the liquid. Such cmsts seldom consist of pure substance so they should be removed carefully with a spatula or spoon before attempting to filter off the crystals. 

Fig. 7.20 AFM image of polyethylene grown at 160 °C and subsequently crystallized during cooling on the surface of a planar CrOx/Si02 catalyst. The left inset indicates schematically how polyethylene molecules fold into lamellar structures. The AFM image...

Figure 13 shows a schematic diagram illustrating the configuration of a surface cooling (indirect heat transfer) crystallizer. Heat can be transferred to a coolant in an external heat exchanger, as shown, or in coils or a jacket... 

By October 1949, I started experimenting with crystallization at 100 C, reasoning that the higher water content zeolites with larger pore volumes and, presumably, larger pore sizes, would be more likely to crystallize at temperatures lower than 200 -300 C. In nature the anhydrous aluminosilicates were formed at relatively high temperatures, and the hydrous ones were believed to have been formed later as the earth s surface cooled. Not surprisingly, when we first tried low temperature synthesis with relatively insoluble silica and alumina in mildly alkaline solutions, there was no reaction in reasonable time periods. 

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