Silica in steam

GPSA Engineering Data Book, Gas Processors Suppliers Association, Vol. II, 10th Ed. 

A boiler s water may have caustic embrittling characteristics. Only a test using a U.S. Bureau of Mines Embrittlement Detector will show whether this is the case. If the water is found to be embrittling, it is advisable to add sodium nitrate inhibitor lest a weak area of the boiler be attacked. 

The amount of sodium nitrate required is specified as a NaN03/NaOH ratio calculated as follows  

Bureau of Mines recommends that the NaNOj/NaOH ratio be as follows  

Ratio = NaNOVNaOH ratio Nitrate = Nitrate as NO , ppmw M alkalinity = M alkalinity as CaCOi ppmw Phosphate = Phosphate as PO4, ppmw 

The American Boiler Manufacturers Association shoots for less than 0.02-0.03 ppm silica in steam by limiting silica in the boiler water. See the section entitled Blowdown Control. 

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NOTE The usual standard for maximum silica in steam used for modern turbines is 0.002 mg/kg. To ensure that this limit is not exceeded, the maximum concentrations of silica in BW is as follows ... 

The Mobility of Silica in Steam. The reactivity of silica and silica-containing materials to steam has been assumed in the literature to explain several phenomena, a few of which are the sintering of silica (35), the aging of amorphous silica alumina cracking catalysts (36) and the formation of ultrastable molecular sieves (37). The basis of all these explanations is the interaction of siliceous materials with water to form mobile, low molecular weight silicon compounds by hydrolysis (38) such as ... 

Silica is known to be volatile in steam (39-40). For instance, the equilibrium concentration of silica in steam at 1 atmosphere from 1200 to 1450 F has been found to range from 0.2 to 0.5 ppm (by weight) showing that the availability of silica in the vapor phase can be substantial under the conditions used in our work. Unfortunately, we cannot predict how much silica will be transferred in our experiments since the rates of either hydrolysis or adsorption on the cerium on alumina additive are unknown. 

Silica. Sihca 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 Httie as 76 pm (3 mils) of deposit can cause measurable loss in turbine efficiency. Severe deposition can also cause imbalance of the turbine and vibration. The solubihty in steam and water is shown in Figure 15, as is a typical steam turbine expansion. Sihca is not a problem except in low pressure turbines unless the concentrations are extraordinarily high. 

The American Boiler Manufacturers Association (ABMA) has established limits for boiler water composition. The limits are set to help assure good quality steam (for example, limiting silica in the steam to 0.02-0.03 ppm). Blowdown is normally based on the most stringent of these limits shown in Table 1. 

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

Sohre, J. S., Causes and Cures for Silica Deposits in Steam Turbines, Hydrocarbon Processing p. 87, Dec. (1972). 

The boiler water pH must be raised to about 9 or over to avoid corrosion, to maintain silica in solution, and to reduce the release of CO2 into the steam. 

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. 

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. 

As a result of the volatility of silica and its ensuing risks, such as the deposition of hard, amorphous, or glassy silicates in superheaters and on turbine blades, various actions are commonly taken in higher pressure boiler plants to limit silica vapor in steam. The maximum permissible silica concentration in steam is generally accepted as 0.02 ppm... 

Boiler water silicas may steam distill and may be present in various forms including silicic acid (H2Si03) and silicate ion (Si032-). They become more volatile at higher boiler pressure, although volatility is decreased slightly with an increase in pH. 

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. 

NOTE It is important to distinguish between water in steam resulting from partial condensation of steam and that resulting from carryover of entrained BW. Even where steam does not contain measurable carryover from mechanical action, it may still contain volatile amines, ammonia, carbon dioxide, oxygen, sulfur gases, and silica. 

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

The binders are silica, lime, slag, or cement. The balls are somewhat dried, if necessary, and then cured in steam autoclaves. During the hydrothermal treatment lime and silica react to form hydrosilicate gels, which act as binders. 

Bartholomew and coworkers32 described deactivation of cobalt catalysts supported on fumed silica and on silica gel. Rapid deactivation was linked with high conversions, and the activity was not recovered by oxidation and re-reduction of the catalysts, indicating that carbon deposition was not responsible for the loss of activity. Based on characterization of catalysts used in the FTS and steam-treated catalysts and supports the authors propose that the deactivation is due to support sintering in steam (loss of surface area and increased pore diameter) as well as loss of cobalt metal surface area. The mechanism of the latter is suggested to be due to the formation of cobalt silicates or encapsulation of the cobalt metal by the collapsing support. 

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