Silica volatilization

Contamination in this context refers specifically to the debasing of the purity of steam and condensate. Contamination may occur in the presence of unwanted steam volatile materials (such as silica volatiles), minerals carried over with BW, oil and process materials infiltrating the steam-water circuit, or by the results of corrosion processes. 

Silica leakage (DI). Silica volatilization Deposition on boiler surfaces, superheaters, and turbine blades... 

Apart from the risk of silica problems within the boiler section, at pressures above 400 to 500 psig silica volatilization and distillation occurs, resulting in silica deposition in superheaters and on turbine blades. Under these conditions the maximum concentration of silica permitted in steam is 0.02 ppm Si02. 

Volatile silica is seldom a problem at under 400 psig, where only 0.1 to 0.2% of total BW silica may be present as volatiles. It becomes significant at 900 to 950 psig and above, however, when silica volatiles rise sharply to 1.0 to 2.0% or more of total BW silica. 

When silica volatilizes, vapors condense on cooler areas to form a white bloom that can be removed by heat or dilute hydrofluoric acid. Because dilute hydrofluoric acid also attacks the substrate, a mild, careful treatment is required. To minimize volatilization, the temperature should be as low as possible. 

Although silica and silica-bearing materials are very useful in making catalyst supports, they cannot be used for the shift catalyst. This is because the silica volatilizes and migrates from the hotter zone to lower temperature zones downstream. Usually it deposits on the waste heat boiler tubes after the secondary reformer. 

The EPA developed two methods for the radiochemical analysis of uranium in soils, vegetation, ores, and biota, using the equipment described above. The first is a fusion method in which the sample is ashed, the silica volatilized, the sample fused with potassium fluoride and pyrosulphate, a tracer is added, and the uranium extracted with triisooctylamine, purified on an anion exchange column, coprecipitated with lanthanum, filtered, and prepared in a planchet. Individual uranium isotopes are separately quantified by high resolution alpha spectroscopy and the sample concentration calculated using the yield. The second is a nonfusion method in which the sample is ashed, the siUca volatilized, a tracer added, and the uranium extracted with triisooctylamine, stripped with nitric acid, co-precipitated with lanthanum, transferred to a planchet, and analyzed in the same way by high resolution a-spectroscopy (EPA 1984). 

SILICA, VOLATILE - Silica carryover with steam. 

The effect of silica volatility on the oxidation of silica-forming materials has also gone largely unnoticed until recently. Silicon oxidation literature contains only a few comments that there is a slight deficiency of silica formed during thermal oxidation of silicon in water vapor (Deal, 1963 Ligen-za, 1962). This can be attributed to the interest in the short-term oxidation behavior of silicon. 

The pressure dependence, has been demonstrated by Robinson and Smialek (1999) for SiC recession as shown in Figs. 7-21 and 7-22. In addition. Fig. 7-22 shows the temperature and velocity dependence for silica volatility in combustion conditions. Many proposed applications of SiC and Si3N4 involve high pressure and high velocity combustion environments, thus sil-... 

Silica [7631-86-9 (colloidal)-, 112945-52-5 (fumed)]. Purification of silica for high technology applications uses isopiestic vapour distillation from cone volatile acids and is absorbed in high purity water. The impurities remain behind. Preliminary cleaning to remove surface contaminants uses dip etching in HF or a mixture of HCl, H2O2 and deionised water [Phelan and Powell Analyst 109 1299 1984],... 

Regarding the color, we only see a need for colorless ionic liquids in very specific applications (see above). One easy treatment that often reduces coloration quite impressively, especially of imidazolium ionic liquids, is purification by column chromatography/filtration over silica 60. For this purification method, the ionic liquid is dissolved in a volatile solvent such as CFF2C12. Usually, most of the colored impurities stick to the silica, while the ionic liquid is eluted with the solvent. By repetition of the process several times, a seriously colored ionic liquid can be converted into an almost completely colorless material. 

Another example is the determination of pure silica in an impure ignited silica residue. The latter is treated in a platinum crucible with a mixture of sulphuric and hydrofluoric acids the silica is converted into the volatile silicon tetrafluoride ... 

The residue consists of the impurities, and the loss in weight of the crucible gives the amount of pure silica present, provided that the contaminants are in the same form before and after the hydrofluoric acid treatment and are not volatilised in the operation. Although silicon is not the only element that forms a volatile fluoride, it is by far the most abundant and most often encountered element consequently the volatilisation method of separation is generally satisfactory. 

Steam from a field-erected boiler almost always contains significantly less entrained water than the preceding figures because of the greater sophistication of steam separation devices. However, some caution is required because even dry steam from an efficient HP boiler may contain volatilized salts (such as chlorides and silica) through the process of steam stripping. 

If silica (in the form of volatile silicic acid) is present in steam at concentrations greater than 0.02 ppm Si02, turbine deposit problems result. In some cases, volatile silica appears not to significantly affect superheaters but certainly will always form deposits in turbines. Silica appears in different forms, depending on the steam pressure, and affects all turbine surfaces, It is most noticeable on the blades, which eventually results in rotor unbalancing. 

Steam purity indicates the degree of inadvertent BW carryover, typically reflected by an increase in electrical conductivity from sodium and other ions present in the BW TDS. Iron and silica may also carry over, and the silica may be in a volatile form. Steam purity also is affected by the use of neutralizing amines (as an intentional measure). 

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