In Oil industry, the production of extra-heavy oil or bitumen using the steam-assisted gravity-drainage (SAGD) method needs the generation and injection of huge amount of steam into the reservoir. A similar amount of hot water is produced together with the oil. Considering environmental issues, it is essential that partial, or even all, recycling of the produced water into steam will be required. After circulating in the ground, production water may contain high concentrations of silica. Deposition of silica increases local thermal resistance and can lead to tube failure in boilers. The silica-removal process is an environmental issue owing to the production of sludge and its removal is very expensive in terms of investment and operating costs. Silica can volatilize with the steam in sufficient concentrations to deposit in steam turbines leading to scale formation on boiler surfaces and cooling water systems. In this work, a new correlation is presented to predict silica (SiO2) solubility in steam of boilers as a function of pressure and water silica content. The proposed correlation predicts the solubility of silica (SiO2) in steam for pressures up to 22000 kPa and boiler water silica contents up to 500 mg/kg. The obtained results are compared with the reliable literature data which showed good agreement with absolute deviations varying within ± 4.2%. Current efforts in this investigation pave the way for alleviating the problems associated with the overheating and failure of boiler sections due to scale formation and turbine inefficiencies by arriving at an accurate measure of silica solubility in steam which can be used by the utility personnel for monitoring the operational parameters.
Silicon is the second most abundant element on the surface of the Earth. Many of its compounds are found in natural waters, and some occur quite often as either minor or major constituents of scale [1]. The two prevalent types of siliceous scale are silica (based on silicon dioxide) and silicate (based on the numerous salts of soluble silica). Of the two, the latter is the more complex. Silica occurs extensively in nature, but one of its most common forms is sand [1]. Many oil and gas producing formations are silica sand. Silica in natural waters occurs both as colloidal particles and as highly reactive soluble silica. The soluble silica can react and form an almost infinite series of complex silicates that vary in form with environmental conditions. The Earth's crust is almost entirely (80 to 90%) comprises of silicon compounds: feldspars (silicates), mica (silicates), clays (silicates), quartz (silica), asbestos (silicates), etc. The percolation of water through minerals such as these causes silica to dissolve, and under the right temperature and pressure, the soluble silica is transported, redeposited, or reacted and then redeposited as scale in equipments. There is a significant variation in silica solubility and this makes the problem of silica scale deposition even more complex. The concentration of silica also varies widely in natural waters. Higher concentrations of silica occur in waters where the hardness is low and the alkalinity is high.
