TDS And Hardness Removal
Technical Profile
The objective of this process is to remove calcium, magnesium, silica and other dissolved solids and minerals prior to steam generation.
Boiler operation requires very high quality water. Evaporation is an alternative to conventional water treatment for boiler water preparation. Calcium and magnesium must be removed to prevent buildup of scale on tube walls. Above 600 psig (41 barg), silica (SIO2) starts to volatilize and deposit on downstream equipment and in production formations. Chlorides are very soluble in water and will carryover with steam and corrode downstream equipment creating chloride pits. Oxygen and carbon dioxide must be removed as well since they will combine with water to corrode downstream equipment. Total dissolved solids (TDS) must be removed to avoid extremely high blowdown rates. TDS must be controlled in the boiler below saturation limits that would promote precipitation. Boiler operating pressure in the 1500 to 2500 psig range requires removal of TDS to 50 mg/l or less.
Falling film, vertical tube, vapor compression evaporators have been proven in SAGD operations. This equipment has the highest efficiency of all evaporators. Deoiled produced water enters the evaporator sump after passing through a heat exchanger that raises its temperature to the boiling point and a deaerator that strips oxygen and carbon dioxide. In the sump, produced water combines with circulating brine where it is pumped to the top of the evaporator and travels down the inside walls of individual vertical tubes as a thin film. As the film falls down the tube a small portion is evaporated and the rest falls into the sump to be recirculated.
A compressor pulls the vapor through a mist eliminator and discharges compressed vapor around the outside of the heat transfer tubes where it gives up heat and condenses as distilled water. Distilled water is transferred through a heat exchanger to preheat feed water going to the evaporator. A small amount of brine is removed as blowdown to control solids in the evaporator.
The use of an evaporator generally replaces warm lime softening, ion exchange and membrane processes for boiler water treatment. Hardness, silica and dissolved solids are removed through evaporator blowdown.
