Water vapor crystallization

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

The commercial product is a white, crystalline powder having an active oxygen content of at least 10%. It melts at about 60°C however, if water vapor is free to escape during heating, the crystals do not melt but are converted to the anhydrous peroxoborate. 

Hydrolysis of Dimethyl Terephthalate. Hoechst Celanese and Eormosa Chemical Eibers Corp. produce a polymer-grade terephthahc acid by hydrolysis of high purity dimethyl terephthalate. Hbls-Troisdorf AG hcenses a process with this step (70). Hydrolysis occurs at 260—280°C and 4500—5500 kPa (45—55 atm) in a hydrolysis reactor without catalysis. The overhead methanol and water vapor is separated and the methanol is returned to the dimethyl terephthalate section for reuse. The reactor hquid is crystallized, cycloned, washed, and further cooled. Einahy, the slurry is centrifuged and dried. The product has less than 25 ppm of 4-formylbenzoic acid and very low levels of other impurities. There may be several hundred parts per million of monomethyl terephthalate, which is incompletely hydrolyzed dimethyl terephthalate. 

Forms of Boric Acid. Orthoboric acid, B(OH)3, formula wt, 61.83, crystaUi2es from aqueous solutions as white, waxy plates that are triclinic ia nature sp gi 4, 1.5172. Its normal melting poiat is 170.9°C, however, when heated slowly it loses water to form metaboric acid, HBO2, formula wt, 43.82, which may exist ia one of three crystal modifications. Orthorhombic HBO2-III or a-form d = 1.784 g/mL, mp = 176° C) forms first around 130°C and gradually changes to monoclinic HBO2-II or P-form (d = 2.045 g/mL, mp = 200.9° C). Water-vapor pressures associated with these decompositions foUow. To convert kPa to mm Hg, multiply by 7.5. 

Small changes in impurity content did not affect this rate but the presence of water vapor and changes in partial pressure of oxygen were critical (61,62). Steam and various impurities and binders also affect the oxidation of siUcon carbide (63). Differences have been observed in oxygen adsorption on the different SiC crystal faces (64). 

Sublimation of ice crystals to water vapor under a very high vacuum, about 67 Pa (0.5 mm Hg) or lower, removes the majority of the moisture from the granulated frozen extract particles. Heat input is controlled to assure a maximum product end point temperature below 49°C. Freeze drying takes significantly longer than spray drying and requires a greater capital investment. 

Drying is an operation in which volatile Hquids are separated by vaporization from soHds, slurries, and solutions to yield soHd products. In dehydration, vegetable and animal materials are dried to less than their natural moisture contents, or water of crystallization is removed from hydrates. In freeze drying (lyophilization), wet material is cooled to freeze the Hquid vaporization occurs by sublimation. Gas drying is the separation of condensable vapors from noncondensable gases by cooling, adsorption (qv), or absorption (qv) (see also Adsorption, gas separation). Evaporation (qv) differs from drying in that feed and product are both pumpable fluids. 

Water is constantly evaporated from rivers, lakes, and oceans, and released from vegetation through evapo-transpiration. Water vapor travels through the atmosphere, eventually forming small droplets or ice crystals in clouds. Some particles grow sufficiently... 

Vanadium in the feed poisons the FCC catalyst when it is deposited on the catalyst as coke by vanadyl porphydrine in the feed. During regeneration, this coke is burned off and vanadium is oxidized to a oxidation state. The vanadium oxide (V O ) reacts with water vapor in the regenerator to vanadic acid, HjVO. Vanadic acid is mobile and it destroys zeolite crystal through acid-catalyzed hydrolysis. Vanadic acid formation is related to the steam and oxygen concentration in the regenerator. 

Adsorption of water on salt crystals plays a key role in many atmospheric and environmental processes. Alkah halides in particular play an important role in the first stages of drop growth in clouds. To understand the atomistic details of the wetting and dissolution processes that take place in these crystals, we apphed SPFM to the smdy of the adsorption of water vapor on single crystal surfaces and the role of surface defects, such as steps. 

Water is present in the materials of interest as free water or water of crystallization, or as combined water. The process of dehydration refers to the removal of the water of crystallization, while the removal of combined water is called dehydroxylation because hydroxyl groups in the material are broken down to form water vapor. The dehydroxylation process is very often alternately described as calcination. The drying process used in the present text pertains to both dehydration and dehydroxylation. In the processing of ores for metal extraction, drying essentially implies the removal by evaporation of water which a material holds in it in various forms. 

At a given ambient water vapor pressure (usually the level found in the open atmosphere), the temperature of the material is raised so that the equilibrium water vapor pressure over the hydrated material is higher than the ambient water vapour pressure. Generally, heating up to 400 °C is sufficient to remove all the water of crystallization from materials. This removal of water yields a material which may contain some more strongly bound water. To remove this water, the material requires to be heated to a higher temperature (400-600 °C) so that the equilibrium water vapour pressure exceeds the ambient water vapour pressure. For near-complete removal of the last traces of water, temperatures as high as 1000 °C may be required. In addition to the heat required to raise the temperature of the material, heat is also required for the evaporation of water, which is an endothermic process. The enthalpy of evaporation increases as the water content, and hence the equilibrium water vapor pressure, decreases. 

Sensitivity was found to decrease with increasing temperature (20-50° C) for the gold-plated crystal. Water vapor at 50% relative humidity was not a serious interference. The smallest weight of diisopropyl methylphosphonate detected at 10 g L 1 was approximately 0.5 ng (Kristoff and Guilbault 1983). 

The effect of physical aging on the crystallization state and water vapor sorption behavior of amorphous non-solvated trehalose was studied [91]. It was found that annealing the amorphous substance at temperatures below the glass transition temperature caused nucleation in the sample that served to decrease the onset temperature of crystallization upon subsequent heating. Physical aging caused a decrease in the rate and extent of water vapor adsorption at low relative humidities, but water sorption could serve to remove the effects of physical aging due to a volume expansion that took place in conjunction with the adsorption process. 

The two solutions were also analysed by in situ microscopy. The solution that did not contain mannitol showed a number of different stages of decomposition, as shown in Figure 1.5. The initial solution is a pale red color. The color deepens on heating, and the droplet appears to solidify (Figure 1.5b), followed by a crystallization (Figure 1.5c). Above 100°C, loss of water can be observed as a bubble of water vapor breaks through the surface of the droplet. The final product was a black pellet, confirmed as Co304 by its FTIR spectrum. 

The nature of a surface will depend upon which atoms are exposed. For example, the surface of a crystal with the sodium chloride structure might consist of a mixture of atoms, as on 100 (Fig. 3.34a), or of just one atom type, as on 111 (Fig. 3.34b and 3.34c). However, it must be remembered that no surface is clean and uncontaminated unless it is prepared under very carefully controlled conditions. Absorbed gases, especially water vapor, are invariably present on a surface in air, which leads to changes in chemical and physical properties. 

This process is of special interest if a product has to be frozen more quickly than is possible on belts or in trays A pellet of 2 mm diameter is cooled from 0 °C to -50 °C in approx. 10 s, or at a rate of approx. 300 °C/min. The advantages are minimum freeze concentration, free-flow product, small ice crystals (which are acceptable in this case of small transport distances for energy and water vapor). It is likely that some pellets (those too large or too small) will need to be removed by sieving. 

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