The 6% Mo superaustenitic alloys have been used extensively in seawater handling applications. The properties that allow these alloys to be used in seawater make them well suited for use in construction of equipment for the production of potable water by desalination of brackish or seawater. N08367 alloy has been specified and used in a variety of reverse osmosis and multistage flash distillation applications. Several brine concentrators have also been built using the UNS N08367 alloy. Data on the performance and use ofN08367 alloy in desalination testing and service are presented.
Potable flesh water is a necessity for the maintenance of life and of civilization. A shortage of potable water exists in many areas around the world. Manufacture of potable water via desalination of brackish or seawater is one way of alleviating this shortage. Several technologies for desalination exist. In all of these fresh water is produced from brackish or seawater. Major commercially available desalination processes include the thermal technologies of Multi-Stage Flash (MSF) and Multiple-Effect Distillation (MED) and the membrane-based technologies of Electrodialysis (ED) and Reverse Osmosis (RO). This technology has become particularly valuable in the Middle East, North Africa, the Caribbean, Florida and Southern California, where the lack of fresh water severely limited development. survey [ll indicated that the 1998 total capacity of installed desalination plants worldwide was 22.7 million m3/d (6 billion gallons/day) of which about 85 percent was still in operation. Desalination equipment is now used in over 100 countries, with 10 countries having about 75 percent of the capacity. Saudi Arabia is first with about 24 percent of the world's capacity. The United States is second, with about 16 percent of world capacity. Most of the world's installed capacity consists of the MSF and RO equipment. These two processes make up about 86 percent of the total capacity.
All desalination processes use saline water and energy to produce flesh water and a waste stream of brine. Figure 1 shows a schematic of the desalination process.
About half of the world's desalinated water is produced using heat to distill flesh water from seawater. In the distillation process, salt water is heated to the boiling point - producing water vapor that is condensed as flesh water. To do this economically, the pressure of the water being boiled in the desalination plant is reduced to lower its boiling point. To reduce further the energy needed for vaporization, the distillation process usually uses multiple boiling in successive vessels, each operating at a lower temperature and pressure. Figure 2 [21 shows a schematic of an MSF distillation plant.
Membranes are used in the electrodialysis (ED) and reverse osmosis (RO) desalination processes. ED uses an electrical potential to transport salts through a membrane, leaving fresh water behind. RO is a pressure-driven process, which drives fresh water through a membrane, leaving more concentrated brine behind. Figure 3 [2] shows a schematic of an RO desalination plant.
All desalination processes produce a stream of brine waste. This brine waste must be disposed in an environmentally appropriate manner. The major solute in the concentrate stream is salt but the effects of corrosion products and chemicals added for scale control, etc., must be considered.
Materials used in desalination must resist environments that include brackish or seawater, potable water and brine discharge. Recent progress in desalination has involved an increase in equipment reliability. This improvement is partially attributable to the use of better materia
