Silica, solubility in steam

Figure 1. Relationships between boiler pressure, boiler water silica content, and silica solubility in steam.

Silica limits are based on the avoidance of silicate scale deposition and the limitation of silica solubility in steam. 

Silica limits are based on the avoidance of silicate scale deposition and the limitation of silica solubility in steam. The BW silica levels in this table have been set to achieve a level of 0.002 mg/kg in the steam, the normal level specified for modem steam turbines. 

Silica reinforced rubber, 22 703 Silica sheets, 22 383-385 Silica-silane system, 22 377-378 Silica sol-gel fiber processing, 23 80 Silica sols, 22 383, 473-474 applications of, 22 394 modification of, 22 393-394 preparation of, 22 392-393 properties of, 22 391-392 purification of, 22 393 Silica, solubility in steam, 23 212-213 Silica-supported activated manganese dioxide, 76 568... 

Silica. Silica is not actually a corrodent of turbines. However, it can deposit on and cause blocking of turbine passages, thus reducing turbine capacity and efficiency. As litde as 76 ]Am (3 mils) of deposit can cause measurable loss in turbine efficiency. Severe deposition can also cause imbalance of the turbine and vibration. The solubility in steam and water is shown in Figure 15, as is a typical steam turbine expansion. Sflica is not a problem except in low pressure turbines unless the concentrations are extraordinarily high. 

Treatment chemicals should preferably be fed to the FW tank to minimize sludge deposits in the coils. Hydroxide alkalinity in ppm (mg/1) CaC03 must be maintained at a sufficient concentration to keep silica soluble and avoid complex silicate deposits. These precautions are necessary because scale-control internal treatment chemicals usually are not employed to assist in the prevention of such deposits in coil-type steam generators. 

Fig. 13. Quartz ai>d amorphous silica solubility vs. temperature along the vapour saturation curve. The dashed lines show the silica concentration in water initially in equilibrium with quartz during adiabatic boiling to 100 C and subsequent cooling. The increase in aqueous silica concentrations during boiling is the consequence of steam formation. Amorphous silica saturation (shown by the dots) is attained at 188 C in the case of the 300 C aquifer water, but at 94 C in the case of the 200 C aquircr water. It was assumed that the pH of the water is not raised sufficiently during boiling to cause significant ionization of the aqueous silica. If some ionization had occurred, amorphous silica saturation would be reached at lower temperatures than those indicated in Fig. 13.

Solubility. An important aspect of silica chemistry concerns the silica— water system. The interaction of the various forms of silica with water has geological significance and is applied in steam-power engineering where the volatilization of silica and its deposition on turbine blades may occur (see Power GENERATION), in the production of synthetic quartz crystals by hydrothermal processes (qv), and in the preparation of commercially important soluble silicates, colloidal silica, and silica gel. 

Figure 1 from GPS A shows how the silica content of boiler water affects the silica content of steam. For example at l,600psia, 100 ppm silica in the boiler water causes 0.9 ppm silica in the steam. If this steam, were expanded to lOOpsia, following the steam line down (the saturated and superheated curves converge by the time lOOpsia is reached), the solubility of silica decreases to 0.1 ppm. Therefore the difference (0.8 ppm) would tend to deposit on turbine blades. 

Kennedy and associates developed data against which subsequent work was often compared (25, 130, 131). In 1954, extensive studies were made by Wyart and Sabatier (132), who measured the solubility of quartz, tridymite, cristobalite, and vitreous silica in both water and steam phases at up to 480 bars and 470 C. Increased solubility of quartz with pressure had earlier been examined up to 600 C by Morey (133), and more recently by Heitmann (30) Anderson and Burnham (134) examined solubility in water and salt and alkali solutions up to 900 C and 6 kilo-bars. Solubility was only slightly reduced by the presence of salt, but increased in direct proportion to the base added. The solubility of quartz under hydrothermal conditions is described in three papers by Kitahara (135) with special attention to supercritical conditions up to 500°C and 900 bars. The heat of solution of quartz calculated from solubility data was 7.8 kcal moIe. In 1965, Heitmann (30) surveyed the solubility of silica in water and stream and assembled data based on more than 1000 experiments up to 650 C and 300 kg cm on the basis of which a complete solubility diagram was constructed. Verifying earlier work, the solubility of silica is shown to increase with increasing density of steam or water, and reach a maximum near the critical point of water. The thermodynamic properties and solubility of quartz up to 600°C and 5 kilobars pressure are being summarized by Hegel-son(136). 

Patented proposals have been made to heat sodium chloride with phosphoric acid (A. Delhaye) zinc or lead pyrophosphate (L. J. F. Margueritte) or ferric phosphate (A. R. Arrott). The resulting soluble sodium phosphate is decomposed by boiling with lime to form sodium hydroxide, which, if needed, can be converted into carbonate by a current of carbon dioxide. These methods are quite impracticable. In 1809, J. L. Gay Lussac and L. J. Thenard proposed to make soda by the action of steam on a mixture of sodium chloride and silica If these two compounds are melted together there is very little action, for the salt volatilizes before anything but a superficial combination takes place, and the action of salt in the glazing of pottery is probably made possible by the aq. vapour in the furnace gases. The sodium silicate formed by the joint action of sodium and... 

Potassium Phosphates.—Basic or neutral phosphates (Rhenania phosphates) may be made directly from rock phosphate by mixing it with potassium chloride, some form of carbon and soda, and heating to over 1000° C. in an electric furnace.2 One of these products has the composition Ca2KNaP208, with some silica. Such products may contain 23 to 31 per cent, of soluble phosphoric acid. Potassium superphosphate has been made by mixing potassium sulphate and calcium carbonate with concentrated phosphoric acid in a lead-lined vessel.3 The CaS04.2Ha0 is separated and the filter-pressed solution is evaporated to dryness by steam heat. The residue may be treated with more phosphoric acid and again evaporated. The reaction is expressed by the equation... 

Finely ground phosphate rock (Ca3(P04)2) is occasionally added to fertilizer formulations as a diluent or filler. However, because phosphate rock has only a very limited water solubility its action as a phosphate nutrient is small and very slow. Hence, this ingredient is not allowed to be included in the % P2O5 analysis appearing on fertilizer packaging. Before use as a filler, phosphate rock is normally defluorinated by heating with silica and steam to decrease the risk of soil contamination by fluoride (Chap. 10). 

The solubility of silica in supercritical water was first studied in the laboratory in 1879. It has some far-reaching influences in industry. During the production of high-pressure steam, silica that is present in the water is dissolved. The silica enters the (almost) closed-loop steam side partly through... 

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