| Abstract |
The concentration of silica in a geothermal brine increases with temperature. When steam is separated from brine with initial temperature greater than about 250°C, silica-rich deposits accumulate in steam separators and brine pipes. When more steam is separated or the brine is discharged at atmospheric pressure, massive precipitation of colloidal amorphous silica commonly occurs. Aside from increasing separator pressure to avoid super-saturation in respect to amorphous silica, the most common method employed to control silica precipitation is addition of sulfuric acid to the brine to decrease pH, which greatly slows polymerization of dissolved silica and formation of silica deposits. While sulfuric acid is cheap, this method requires large capital investment to avoid corrosion. Brine acidified with sulfuric acid is poorly buffered, whereby acid feed rate must be strictly monitored and controlled, and the vessel wherein acid is added to the brine must be made of a costly, corrosion resistant alloy. There is no such thing as a practical true silica inhibitor. Products promoted as “silica inhibitors” actually inhibit metal silicates, or act to disperse colloidal silica particles. Formation of silica-rich deposits in separators and other equipment at high temperature generally involves precipitation of minute particles of amorphous aluminosilicate composition similar to zeolites or clays. These particles serve as seeds that initiate the growth of amorphous silica particles, which form the deposits. Injecting a silicate inhibitor at the wellhead enables control of such deposits by preventing formation of the aluminosilicate seeds. Organic acids or carbon dioxide present in the condenser vent gas of a geothermal generating unit can be used to decrease brine pH without the danger of corrosion or expensive investments to prevent corrosion. When acetic acid is used, the acidified brine is well buffered by the equilibrium of acetic acid/acetate anion, whereby over-feeding the acid causes a small decrease in pH. Brine acidified by adding carbon dioxide is buffered by the equilibrium with bicarbonate ion in the brine; alkalinity of the brine is preserved and the acidified brine is strongly buffered. |