One of the main challenges when treating industrial water in evaporators is the prevention of scale. Scaling is major reason for performance degradation and shutdowns.
In order to minimize waste, many industrial water treatment evaporators operate at high recoveries as a result of which the concentration of salts increases and reaches saturation level. The salts then precipitate.
Scaling of carbonate salts (especially calcium carbonate, CaCO3) is a common phenomenon in evaporators. CaCO3 is highly insoluble, and even a small presence of calcium and carbonate in the feed water might create scale.
Stripping is a process that prevents scaling of carbonate-based salts. Stripping is performed upstream of the evaporator, and removes carbonate from the feed water. Consequently, the calcium has no counter ion with which to precipitate, and scale will not be formed.
The process is divided into two steps:
- Shifting the carbonate to carbon dioxide (CO2).
- Removing CO2 from the water in a stripping tower.
The presence of the carbonate species (carbonate, bicarbonate and CO2) in the carbonate system depends on the pH. At high pH, the majority of the species appears as carbonate; at low pH the majority appears as CO2. Lowering the pH will shift Carbonate to CO2.
