Desalination Equipment Information
Desalination equipment is used to remove salts and other minerals from water or soils, typically for drinking water, oilfield, and process separation applications. Desalination equipment uses three basic technologies. Thermal desalination systems change water to a vapor or a solid, physically separate the new phase from the remaining salt solution, and cause the separated water to revert to a liquid. Separation desalination systems physically separate components during movement in response to an externally-applied gradient such as a membrane. Chemical desalination systems use chemical processes, but typically in conjunction with membranes or distillation. Product specifications for desalination equipment includes materials of construction, temperature range, pressure range, and the use of corrosion-resistant linings. There are two major commercial processes for desalination equipment: distillation and membrane. Distillation processes use multi-stage flash (MSF), multi-effect evaporation (MEE), or vapor compression units. Membrane processes use reverse osmosis (RO) or electrodialysis (ED) units. MSF is a thermal distillation process for the evaporation and condensation of water. MEE is similar to MSF, but condenses the vapor from each phase into the next successive stage; each stage then operators at a successively lower pressure. There are two types of vapor compression techniques: thermal vapor compression (TVC) and mechanical vapor compression (MVC). ED distillation equipment sends saline water in separate, parallel channels so that an applied voltage causes cations and anions to migrate in opposite directions. Desalination equipment includes various configurations of multi-effect evaporators and reverse osmosis (RO) systems for treating seawater. Multi-effect evaporators produce distilled water and reduce the volume of oil field water, typically by concentration. For seawater desalination, waste heat from an engine or gas turbine is often used. Because multi-effect evaporators can distill most oilfield brine to potable water, the use of waste heat makes their application cost-effective with from four to six effects. Compared to reverse osmosis systems and other types of desalination equipment, the treatment of brackish water is also more effective with multi-effect evaporators because of the water’s unpredictable total dissolved solids (TDS) composition and concentration. For seawater desalination, reverse osmosis (RO) is generally more cost-effective than multi-effect evaporation (MEE) or other desalination equipment. RO systems are also used with cooling tower make-up water, food process separations, and other predictable process separations. For seawater desalination, most RO systems require pressures from 700 to 900 pounds per square inch (psig) in order to overcome hydraulic resistance and osmotic pressure. The recovery of a significant amount of potable water (permeate) is possible, but RO membranes are susceptible to fouling. Typically, ultra filtration and multi-media filters are used, and periodic membrane flushing is performed. RO systems are not recommended for the treatment of oilfield water, however.