Silicon (Si) is a metalloid, the second most abundant element in the earth’s crust. The degradation of rocks results in silicon dioxide found in natural waters

 Silica

Silicon (Si) is a metalloid, the second most abundant element in the earth’s crust. The degradation of rocks results in silicon dioxide found in natural waters. Silicon dioxide, also known as silica, is a chemical compound that is an oxide of silicon with the chemical formula SiO2. Naturally, silica present in water feed in the range of 1-150 ppm. It is existed as silicic acid (H2SiO3) which is a weak acid and dissociated at or below neutral pH. The presence of silicic acid led to form silica colloidal in water at neutral pH. But when pH exceeds the neutral, silicic can be dissociated and form silicate anion (SiO3 2-), which can react with positive ions like calcium, magnesium, iron, manganese and aluminum to form insoluble silicates.

Reactive and unreactive Silica

The chemistry of silica is a complex and somewhat unpredictable subject. The "Total Silica" content of a water is composed of "Reactive Silica" and "Unreactive Silica". Reactive silica (e.g., silicates SiO4) is dissolved silica that is slightly ionized and has not been polymerized into a long chain. Reactive silica, though it has anionic characteristics, is not counted as an anion in terms of balancing a water analysis but it is counted as a part of total TDS. Reactive silica is the form of silica to be used in RO projection programs.

Unreactive silica is polymerized or colloidal silica, acting more like a solid than a dissolved ion. Silica, in the colloidal form, can be removed by a RO but it can cause colloidal fouling of the front-end of a RO. Colloidal silica, with sizes as small as 0.008 micron can be measured empirically by the SDI (Silt Density Index) test, but only that portion that is larger than 0.45 micron or larger. The surface of colloidal silica in contact with water is covered by siloxane bonds (≡Si–O–Si≡) and silanol groups (≡Si–OH). This makes colloidal silica very hydrophilic and capable of forming numerous hydrogen bonds. Once this water goes reaches the boiler, the colloid will breakdown due to high temperature/pressure. Often, colloidal silica is also defined as “anything” that will be retained by a 0.22-micron filter.). Ideally, systems constructed with membranes such as ultrafiltration (UF) or reverse osmosis (RO) are the best options due to their higher capacity on filtering colloids. 

What Problem Can Silica Cause?

Silica is a problem for high pressure boilers. It exhibits enough volatility that high pressure boilers will have silica carry over to the vapor partition. When the steam pressure is reduced in the turbines, the silica will precipitate on the blades as a glassy deposit which reduces efficiency. Both types of silica, colloidal and reactive, can cause this problem as colloidal silica will break down and volatilize under high temperature and pressure.

Silica build-up in HP boiler.

The silica concentration is distributed between the water and vapor phases in a boiler. As the temperature and pressure are increased, more silica becomes present in the steam. In high pressure boilers, therefore, silica is appreciably concentrated in the steam and this is when 'carry-over' may occur, being deposited later, on the super heaters or turbine blades. Silica entering a high-pressure boiler can concentrate very quickly. An impurity entering at a concentration of 2 mgl-1 in a boiler evaporating 1500 tons of water per hour, would collect in the boiler at a rate of 2 tons per month. Figure 4 illustrates the maximum boiler pressure permissible, given the concentration of silica found in the boiler. Should the concentration of silica in the drum become too high, then a 'blowdown' is initiated. This is the removal of contaminated water from the boiler. Adequate control of the blowdown is very important. If it is allowed to occur too often, it becomes expensive and inefficient. The silica monitoring in the boiler drum is one parameter which can be used to control blowdown.

Silica in Steam

The reason: steam selectively “picks up” silica from the boiler water, dissolves it, and carries it to the turbines, where it re deposits. Investigations show that the key to minimizing silica carryover is in keeping the boiler water silica content below certain levels, the concentration depending on operating pressures. SILICA SOLUBILITY Research on the solubility of silica in water and steam shows several important points:

• Steam is a solvent for silica

• For any particular steam density and temperature, there is a definite saturation solubility of silica

• The maximum solubility of silica in steam is a direct function of both the steam density and temperature. As steam temperature or density decreases, the silica solubility also decreases

Silica Carryover

Investigation of the problem of silica carryover in a laboratory experimental boiler revealed two important facts:

With constant pressure and boiler water pH, silica carryover is directly proportional to the amount of silica in the boiler water. This was true over a wide range of boiler water silica concentrations.

The ratio of silica in the steam to silica in the boiler water increases rapidly as boiler pressures increase.

The utility expert determined empirically that 0.02 ppm of silica in the steam is a practical maximum limit for boiler water pH >10. With steam silica contents of 0.02 ppm or less, appreciable turbine deposits would not normally occur.

Corrective Action

Due to silica content in the feed water goes to steam drum where separation of wet steam and water takes place and in this way silica content increased in Boiler drum. The most significant factor in minimizing silica deposits is the maintenance of low silica concentrations in the boiler water. External treatment equipment must be operated carefully to limit the amount of silica entering with the makeup water, and the condensate must be monitored to minimize contamination. After silica enters the boiler water, the usual corrective action is to increase boiler blow down (to decrease the boiler water silica to acceptable levels).

Silica monitoring provides a good indication of the overall steam purity level provided by the drum boiler, and subsequent action is required to avoid deposition of silica on the super heater and turbines. Where deposition takes place in the boiler, it is known as 'hide-out'. It has been found that no deposition occurs if the concentration of silica in the steam does not exceed 20 µgl-1. The same measurement can be carried out on once through boilers, but because there are no separate super heaters, the sample is taken from the superheat

ed steam prior to entering the turbine