Desublimator

The term sublimation strictly refers to the phase change solid -> vapour, with no intervention of a liquid phase. In industrial applications, however, the term usually includes the reverse process of condensation or desublimation solid -> vapour -> solid. In practice, it is sometimes desirable to vaporise a substance from the liquid state and hence the... 

Although there are few data available on sublimation-desublimation, a considerable amount of information can be calculated using the Clausius-Clapeyron equation provided that information on vapour pressure is available at two or more temperatures. In this way ... 

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. 

Although the application of fluidisation techniques to sublimation-desublimation processes was first proposed by Matz" 11, the technique has not yet been widely adopted for large-scale commercial use, despite its obvious advantage of improving both heat and mass transfer rates. G aiko 112 1 has, however, reported on a fluidised-bed de-sublimation unit operating in the United States for the production of aluminum chloride at the rate of 3 kg/s (11 tonne/h). 

Sublimation is the transformation of a solid directly into vapor and desublimation is the reverse process of condensing the vapor as a... 

Desublimation Vapour Solid ESA Recovery of phthalic anhydride from non-condensible gas... 

Sublimation is the transfer of a substance from the solid to the gaseous state without formation of an intermediate liquid phase, usually at a relatively high vacuum. The reverse process, desublimation, is also practised, for example, in the recovery of phthalic anhydride from gaseous reactor effluent. 

The adsorption enthalpy equals the sum of desublimation enthalpy and net adsorption enthalpy. The net adsorption enthalpy is the enthalpy difference between a pure solid compound and its adsorbed state on a surface at zero surface coverage. Hence, the net adsorption enthalpy characterizes the interaction, which depends on the nature of both metals. On the other hand, the desublimation enthalpy is an exclusive property of the adsorbate. 

Increasing the sublimation and desublimation temperature (I d 200 °C) causes the transformation of initial primary product, jS-SeCL into a-SeCLi. Thus crystalUzation from the gas phase obeys Ostwald s rule. a-SeCU is also formed... 

Volatilisation of alkali compounds like KOH or KCI at temperatures above 600 to 700 C causes deposition, plugging and corrosion problems by desublimation during cooling down in the piping downstream. 

The K and the residual half of the chlorine can also be leached from the straw char with hot water in the same way. If K and Cl are not completely removed prior to the high temperature gasification step, they are routed into the hot fuel gas in the form of volatile tCCl and other volatile alkali salts. These salt vapours can be removed in a specially desiped desublimation step in combination with the recovery of the sensible heat from the fuel gas. 

The purification of the raw furoic acid is best carried out by sublimation as the triple point pressure of furoic acid is high (10.3 torr) and as the impurities (polymers of ftirfuryl alcohol) are essentially nonvolatile. Thus, passing a hot carrier gas over the raw furoic acid selectively vaporizes the desired compound while leaving the nonvolatile impurities behind. If, by a conservative estimate, the vapor pressure of furoic acid is 100 times greater than the vapor pressure of the polymeric impurities, then the quantity of furoic acid vaporized by a hot carrier gas exceeds the quantity of vaporized polymer by a factor of 100 as the rate of vaporization is proportional to the vapor pressure. Hence, if in the initial raw furoic acid the ratio of furoic acid to polymer were 100 1, then in the carrier gas the ratio of furoic acid to polymer would be 10 to 1, so that desublimation yields an enormously purified product, and contrary to recrystallization from a solution, where huge losses are incurred, this effect is obtained at essentially no loss at all as the desublimation temperature can be chosen so low that practically no furoic acid is retained in the carrier gas. This is shown graphically in Figure 82. 

Figure 82. Sublimation and Desublimation of Furoic Acid in the Phase Diagram.

The triple point of furoic acid is at 10.3 torr and 133 C. A sublimation at 124 C corresponding to a furoic acid vapor pressure of 5.5 torr, and a desublimation at 36 C corresponding to a furoic acid vapor pressure of 0.001 torr, result in a relative loss of only 0.001/5.5 = 0.00018 (0.018 %). 

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. 

For the conditions shown in Figure 83, the sublimation rate is 26.7 kg of furoic acid per 100 kg of combustion gas. The customary design value for the desublimation chamber is 3.33 m of chamber volume per kg/h of desublimate [75]. 

Figure 83. Plant for the Purification of Furoic Acid by Sublimation and Desublimation.

Periodic operation at higher temperatures to desublime the deposited ammonium bisulfate. If possible, flow can be stopped to the convection section to raise temperatures. Alternately, duct burners could be installed for periodic operation. 

Furthermore, it was implicitly assumed that the air above the ice is exactly saturated with water vapor at 0°C. At significant supersaturation of the air, water can condense on the ice (beginning in crevices). The air may also be much drier, i.e., its relative humidity can be well below 100%. In that case, water will be removed from the ice by desublimation (especially when a wind is blowing). 

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. 

Desublimation. Sometimes a solid phase can be obtained directly by desublimation, as for example the separation of phtalic anhydride by the oxidation of ortho-xylene. 

Sublimation is the transfer of a substance from the solid to the gaseous state without formation of an intermediate liquid phase, usually at a relatively high vacuum. Major applications have been in the removal of a volatile component from an essentially nonvolatile one separation of sulfur from impurities, purification of benzoic acid, and freeze drying of foods, for example. The reverse process, desublimation (16), is also practiced, for example in the recovery of phthalic anhydride from reactor effluent. The most common application of sublimation in everyday life is the use of dry ice as a refrigerant for storing ice cream, vegetables and other perishables. The sublimed gas, unlike water, does not puddle and spoil the frozen materials. 

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