Crystallization desublimation

Crystals show enormous variation in external shape or habit, although all these shapes arise from ordered stacking of unit cells. This is illustrated in Figure 15.5, which speaks for itself. Flowever, for a given unit cell, the angles between faces that can exist are fixed. This thus allows determination of the crystal system and of the dimensions of the unit cell from a crystal (if it is perfect and not too small). Figure 15.5 shows that considerable variation in shape can be encountered. Even far more intricate shapes are observed in ice crystals present in snow (which are formed by desublimation of water from the air). The variation in shape is caused by variation in the growth rate of the various faces of a crystal, which rates often depend on the composition of the solution see Section 15.2. 

Also freeze drying can be applied to solutions. Water evaporates (desublimates) at low temperature and low pressure from the ice crystals present, leaving a highly porous structure of dry solutes, which often is in a glassy state. 

Increasing the sublimation and desublimation temperature (Td 200 °C) causes the transformation of initial primary product, )3-SeCl4 into Q -SeCl4. Thus crystallization from the gas phase obeys Ostwald s rule. Q -SeCl4 is also formed... 

The filtered gas is further cooled in the crystallizer to the desublimation temperature of the melamine product. Cooling is performed using the offgas from the urea scrubber. The melamine forms fine crystals, which are recovered from the process gas in the product-cyclone. Leaving the product-cyclone, the cooled melamine is stored and can be used without further treatment. It has a minimum purity of 99.8%. 

For industrial processing, it is not uncommon for liquefaction to be allowed in the vaporization stage, to facilitate better heat transfer, but this must never be allowed in the desublimation (crystallization) step. The condensation equipment, therefore, must operate well below the triple point. If liquefaction is employed before vaporization, the operation is often called pseudo-sublimation. 

A variant on the large-chamber desublimation condenser is a crystallization chamber fitted with gas-permeable walls. Vapour and carrier gas is cooled by... 

Crystallization is a thermal, phase-forming separation process from an amorphous phase, at least one solid crystalline phase is formed by phase transformation or by crystallization from a solution, which has one or more solids molecularly dispersed in a solvent, or by crystallization from a melt, or by desublimation out of a vapor phase. A crystalline product is produced from an initial amorphous phase, liquid or gas, with one or more components, for the purpose of forming, cleaning or mixture separation. 

Crystallization from a mixed vapor phase is also a thermal separation process. By removing heat from the system, desublimable components are converted directly to crystals. The remaining gas mixture then contains lower quantities of the desublimable components thus a partial separation is achieved by the phase separation. Desublimation may also be improved by countercurrent phase flow. 

Crystallization from a Vapor Phase, Sublimation and Desublimation... 

Crystallization from a Vapor Phase, Sublimation and Desublimation 529 Table 7-9. Selected sublimer designs. Representation according to Matz [7.2]. 

Fig. 7-41. Vacuum sublimator with fixed trays heated by steam or other heating agents and cooled desublimation areas. Sublimed vapor crystallizes at the desublimation area, where solid product is removed by means of rotating brushes.

Solid and vapor Preferential condensation (desublimation) preferential participation in crystal structure Purification of phthalic anhydride 18, 22, 23... 

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