Sublimation equipment

Very few standard forms of sublimation or de-sublimation equipment are in common use and most industrial units, particularly on the condensation side of the process, have been developed on an ad hoc basis for a specific substance and purpose. The most useful source of information on sublimation equipment is the patent literature, although as Holden and Bryant(95) and Kemp et al. 96> point out, it is not clear whether a process has been, or even can be, put into practice. 

A standard Pyrex sublimer equipped with a water-cooled cold finger is charged with 29.01 g (0.037 mol) crude pale-yellow solid. The sublimer is evacuated to 4 x 10-3 torr and then heated to 130°C with an oil bath. After 8 h, the white solid on the cold finger is scraped off and subjected to a second sublimation cycle. After the second sublimation cycle, the white solid on the cold finger is collected (scraped off) under ambient conditions, yielding 27.92 g (0.036 mol), 63% Ba(hfa)2 tetraglyme, based on 14.92 g (0.057 mol) Ba(NQ3)2. 

Besides the usual sublimation equipment (an example of the simplest type is a beaker covered with a water-filled flask) one should mention an apparatus for vacuum sublimation which can easily be assembled from a Pyrex evaporation vessel with a perforated cover. Figure 77 shows the construction. 

Many solids do not develop enough vapor pressure at atmospheric pressure (760 mm Hg) to be purified by sublimation, but they frequently can be sublimed at reduced pressure. Thus, most sublimation equipment has provision for connection to an aspirator or other vacuum source. Reduction of pressure also helps to prevent thermal decomposition of substances that would require high temperatures to sublime at ordinary pressures. 

AMMONIUM CARBAMATE. NH2 COO NH4. Ammonium carbamate has been produced and handled in aluminum alloy equipment, including tanks, piping and subliming equipment. See also Ref (Dp. 125, (3) p. 68. 

BENZOIC ACID. C HsCOOH. Alloys 3003 and 5154 were resistant to solid benzoic acid in laboratory tests under conditions of 100% relative humidity at ambient temperature. Aluminum alloy sublimating equipment, hoppers and piping have been used in the production of benzoic acid. See also Ref (I) p. 127, (2) p. 96, (3) p. 132, (7)p. 29. 

A simple apparatus for sublimation in a stream of air or of inert gas is shown in Fig. II, 45, 3.. d is a two-necked flask equipped with a narrow inlet tube B with stopcock and a wide tube C 12-15 inm. in diameter. The latter is fitted to a sintered glass crucible and the usual adapter and suction flask E. A well-fitting filter paper is placed on the sintered glass filter plate to collect any sublimate carried by the gas stream. 

The metal is very effective as a sound absorber, is used as a radiation shield around X-ray equipment and nuclear reactors, and is used to absorb vibration. White lead, the basic carbonate, sublimed white lead, chrome yellow, and other lead compounds are used extensively in paints, although in recent years the use of lead in paints has been drastically curtailed to eliminate or reduce health hazards. 

A high quahty version of a dye-sublimation printer has been developed specifically for color proofing. This device uses a laser writing head, rather than the typical thermal printhead, to produce higher resolution images. The device is capable of tme halftones, providing an accurate rendition of a printed page. It is, however, expensive both in equipment and materials cost. 

Metafile arsenic can be obtained by the direct smelting of the minerals arsenopyrite or loeUingite. The arsenic vapor is sublimed when these minerals are heated to about 650—700°C in the absence of air. The metal can also be prepared commercially by the reduction of arsenic trioxide with charcoal. The oxide and charcoal are mixed and placed into a horizontal steel retort jacketed with fire-brick which is then gas-fired. The reduced arsenic vapor is collected in a water-cooled condenser (5). In a process used by Bofiden Aktiebolag (6), the steel retort, heated to 700—800°C in an electric furnace, is equipped with a demountable air-cooled condenser. The off-gases are cleaned in a sembber system. The yield of metallic arsenic from the reduction of arsenic trioxide with carbon and carbon monoxide has been studied (7) and a process has been patented describing the gaseous reduction of arsenic trioxide to metal (8). 

The need to obtain color hard copies from electronic systems such as TV and video sets, or from personal computers, has been increasing. Several methods have been proposed to obtain hard copies of full color images. Among them, sublimation thermal-transfer printing has the following characteristics the quaHty of the printed color picture is extremely high, and the equipment is compact, quiet, and easy to manipulate as well as to maintain. 

Freeze-drying or dehydrating equipment for sublimation drying of ... 

Shelf Devices Equipment having heated and/or cooled shelves is available but is httle used for divided-sohds heat processing. Most extensive use of stationaiy shelves is freezing of packaged solids for food industries and for freeze drying by sublimation (see Sec. 22). 

Sublimation (diffusion) printing is a textile process in which color patterns in dry die crystals are transferred from a release film to the fabric under high heat and pressure. The process has been adapted to plastics. The equipment used is very similar to that used for hot stamping. Under heat and pressure, the dye crystals sublime (go directly to the vapor phase from the solid phase without melting) and the vapor penetrates the plastic product. As a result, the decoration is very durable and wear resistant. It is also cost competitive against other processes such as two-step injection molding or silk screening. 

Freshly sublimed 1,3-dithiane (1.0 g, 8.33 mmol) was placed in a flask equipped with a rubber septum. Tetrahydrofuran (20 ml) was added... 

Simple sublimation is a batch-wise process in which the solid material is vaporised and then diffuses towards a condenser under the action of a driving force attributable to difference in partial pressures at the vaporising and condensing surfaces. The vapour path between the vaporiser and the condenser should be as short as possible in order to reduce mass-transfer resistance. Simple sublimation has been used for centuries, often in very crude equipment, for the commercial production of ammonium chloride, iodine, and flowers of sulphur. 

Vacuum sublimation is a development of simple sublimation, which is particularly useful if the pressure at the triple-point is lower than atmospheric, where the transfer of vapour from the vaporiser to the condenser is enhanced by the increased driving force attributable to the lower pressure in the condenser. Iodine, pyrogallol, and many metals have been purified by vacuum sublimation processes in which the exit gases from the condenser are usually passed through a cyclone or scrubber to protect the vacuum equipment and to minimise product loss. 

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. 

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. 

Dimethyldibenzotellurophene. Powder tellurium (0.94 g 7.4 mmol) is thoroughly mixed with 2.61 g (6.1 mmol) of pure 4,4-dimethyl-2,2 -biphenyldiyl mercury, the mixture is placed into a sublimation apparatus equipped with a cold finger, and slowly heated under vacuum in a metal bath to 260-270°C. Colourless needles begin to sublime at 228°C. The temperature is held at 260°C for 12 h. The sublimate is recrystallized from a mixture of 300 mL of methanol and 10 mL of carbon tetrachloride. Yield 1.38 g (79%) m.p. 158°C. Dibenzotellurophene was similarly obtained in 82% yield. 

The oxidation of SO2 to SO3 is undesirable for several reasons. SO3 will result in a blue plume off the stack and increase opacity. SO3 will also continue to react to form sulfuric acid and ammonium bisulfate. Sulfuric acid will corrode downstream equipment like economizers and ductwork as well as leave the stack as an acid mist. SO3 will also reactant with NH3 to form ammonium bisulfate salt (ABS), which is corrosive and tacky. ABS will sublime from a gas to its solid form at and below its dew point. 

Ammonium bisulfate is a corrosive salt that sublimes from a vapor to a solid at temperatures below its dew point. The salt will foul downstream equipment resulting in higher pressure drops across economizers and preheaters. Ammonium bisulfate can be returned to its vapor state if the temperature is increased above its ABS dew point. 

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