Sublimation entrainer sublimators

The cometary coma The coma and the nucleus form the head of the comet the streams of dust and gas released by the comet form a very large, extremely tenuous atmosphere called the coma, which can have a spread up to around 104—105 km. The coma is not developed when the comet is a long way from the sun, but when it comes closer (at around 5 AU), the ice mixture begins to sublime and is ejected as a gas stream. Dust particles are entrained at a velocity of around one kilometre per second. 

If you re going to pull a vacuum in the sublimator, do it now. If the vacuum source is a water aspirator, put a water trap between the aspirator and the sublimator. Otherwise you may get depressed if, during a sudden pressure drop, water backs up and fills your sublimator. Also, start the vacuum slowly. If not, air, entrained in your solid, comes rushing out and blows the crude product all over the sublimator, like popcorn. 

In entrainer sublimation, an entrainer gas is blown into the vaporisation chamber of a sublimer in order to increase the vapour flowrate to the condensing equipment, thereby increasing the yield. Air is the most commonly used entrainer, though superheated steam can be employed for substances such as anthracene that are relatively insoluble in water. If steam is used, the vapour may be cooled and condensed by direct contact with a spray of cold water. Although the recovery of the sublimate is efficient, the product is wet. The use of an entrainer gas in a sublimation process also provides the heat needed for sublimation and an efficient means of temperature control. If necessary, it may also provide dilution for the fractional condensation at the desublimation stage. Entrainer sublimation, whether by gas flow over a static bed of solid particles or through a fluidised bed, is ideally suited to continuous operation. 

A general-purpose, continuous entrainer-sublimation plant is shown in Figure 15.34. The impure feedstock is pulverised in a mill and, if necessary, a suitable entrainer gas,... 

The product yield from an entrainer-sublimation process may be estimated as follows. The mass flowrate G of the inert gas and the mass sublimation rate S are related by ... 

The theoretical maximum yield from an entrainer sublimation process is the difference between the calculated sublimation rates corresponding to the conditions in the vaporisation and condensation stages. 

As discussed in Section 15.5.2, the separation of two or more sublimable substances by fractional sublimation is theoretically possible if the substances form true solid solutions. Gillot and Goldberger(10°) have reported the development of a laboratory-scale process known as thin-hlm fractional sublimation which has been applied successfully to the separation of volatile solid mixtures such as hafnium and zirconium tetrachlorides, 1,4-dibromobenzene and l-bromo-4-chlorobenzene, and anthracene and carbazole. A stream of inert, non-volatile solids fed to the top of a vertical fractionation column falls counter-currently to the rising supersaturated vapour which is mixed with an entrainer gas. The temperature of the incoming solids is maintained well below the snow-point temperature of the vapour, and thus the solids become coated with a thin film (10. im) of sublimate which acts as a reflux for the enriching section of the column above the feed entry point. 

Quenching the vapour with cold air in the chamber may increase the rate of heat removal although excessive nucleation is likely and the product crystals will be very small. Condenser walls may be kept free of solid by using internal scrapers, brushes, and other devices, and all vapour lines in sublimation units should be of large diameter, be adequately insulated, and if necessary, be provided with supplementary heating to minimise blockage due to the buildup of sublimate. One of the main hazards of air-entrainment sublimation is the risk of explosion since many solids that are considered safe in their normal state can form explosive mixtures with air. All electrical equipment should therefore be flame-proof, and all parts of the plant should be efficiently earthed to avoid build-up of static electricity. 

As was previously mentioned, trace elements that sublime at temperatures below those attained during coal combustion (e.g., As, Se, Hg, Zn), and are associated with thermally unstable solid phases (in particular organic matter and sulphide minerals), are subject to vaporization into furnace gases. Once these gases, and fly ash particles entrained in the gases, are vented from the combustion furnace they quickly cool, leading to the condensation of volatilized elements onto the... 

Only a few solids have vapor pressures near atmospheric at safe temperatures, among them COz, UF , ZrCL(, and about 30 organics. Ammonium chloride sublimes at 1 atm and 350°C with decomposition into NH3 and HC1, but these recombine into pure NH4C1 upon cooling. Iodine has a triple point 113.5°C and 90.5 Torr it can be sublimed out of aqueous salt solutions at atmospheric pressure because of the entraining effect of vaporized water. 

Sublimation pressures down to 0.001 bar are considered feasible. At lower pressures and in some instances at higher ones, entrainer gas is used, usually air or nitrogen or steam. By such means, for instance, salicyclic add is purified by sublimation at 150°C with an entrainer of air with sufficient C02 to prevent decarboxylation of the acid. At the operating temperature, the vapor pressure is only 0.0144 bar. Operating conditions corresponding to equilibrium in the sublimer appear in Figure 19.11. Equilibrium may be approached in equipment where contact between phases is intimate, as in fluidized beds, but in tray types percent saturation may be as low as 10%. 

Condensers usually are large air-cooled chambers whose walls are kept clear with brushes or scrapers or even swinging weights. Scraped or brushed surface crystallizers such as Figure 16.10(a) should have some application as condensers. When a large rate of entrainer gas is employed, a subsequent collecting chamber will be needed. One of the hazards of entrainer sublimation with air is the possibility of explosions even of substances that are considered safe in their normal states. 

The enthalpy of formation Is calculated from that of the crystal by use of Ag jjH"(298.15 K) = 71.965 0.5 kcal/mol. Sublimation studies by mass-spectrometrlc and entrainment techniques suggest the presence of a small amount of dimer In the saturated vapor. These studies are reviewed on the table for Al2Fg(g). The heats of sublimation for the monomer and dimer are selected by adopting the JANAF entropies for crystal, monomer and dimer, the mass spectrometrlc value of 2 (monomer) -... 

From the desublimation chamber , the gaseous phase is vented while the product is retained by a filter. Furfuryl alcohol polymers, not being sufficiently volatile, remain in the sublimer and are discarded as residue. The entrained particles separated in the cyclone are returned into the feed stream. 

To minimize entrainment of sublimed selenium (IV) chloride, a flow rate not in excess of 1 or 2 bubbles of chlorine every 2 seconds through a 6-mm. orifice in the exhaust line is maintained. 

In addition to being able to fit in the evaporator, the sample must tolerate the conditions required for the vacuum evaporation of the metal film and carbon backing. The specimen must be virtually dry, i.e., it must not give off appreciable or uncontrolled amounts of water vapor in the evaporator. This can be accomplished by either drying the specimen or keeping the entrained water frozen at a temperature that precludes significant sublimation [discussed under freeze-etch techniques (see Section 2.4.5.4)]. 

If the phase change is solid to vapor, then this is a sublimation but the compound is lost. The preferred technique is solid to vapor back to solid, so the product can be collected. If the vapor is carried away from the sample surface as soon as it forms, so as to reduce the vapor pressure immediately above the sample, then this is called an entrainer sublimation, also in Chapter 7. If a liquid is solidified, changed to a vapor, and then to a solid, this is called freeze drying. This is discussed in Chapter 8. 

The types of distillation processes to be examined will be simple, fractional, steam, immiscible solvent, azeotropic, extractive, vacuum, molecular, entrainer sublimation, and freeze drying. 

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