In the flash evaporation process is when the liquid is preheated and is then subjected to a pressure below its vapor pressure causing boiling or flashing to occur. Sea water is first heated in tubes and is then put in a chamber with a vapor pressure lower than in the heating tubes and the liquid evaporates. The vapors flash off the warm liquid and the salts exit with the remaining water. The process seems inefficient because the evaporation of a small amount causes the temperature to drop dramatically. For example, when only 7.1% of the liquid is evaporated off, the temperature drops 40 degrees, from 100 to 60 degrees Celcius. However, since the design is so simple, it has become a close competitor to multiple-effect evaporation economically. This is true especially in larger plants.
There are several large scale production units currently used throughout the world. In 1974, Kuwait had the largest production of desalted waters of about 240,000 tons per day and almost the same amount under construction. The process used there was primarily flash distillation. In Hong Kong in 1976, trial runs were being conducted on the largest single flash evaporator at 200,000 tons per day capacity.
It is an advantage to perform flash-distillation in several steps. This is called multi-stage flash evaporation, or MSF for short. This is where the preheated liquid passes through a series of stages or chambers with each successive stage at a lower vapor pressure so some of the liquid will flash at each stage.
Flash distillation begins when the salt water enters a bundle of tubes which are located in the vapor space of the preheat chamber. The water then flows into a heater consisting of a bundle of tubes which are heated externally by steam. Here, the water is heated to 100 degrees C, but it does not boil because the pressure is above 1 atm. The hot sea water then enters a flash chamber which is kept under reduced pressure. The vapors which are flashed off are then condesed on the tubes carrying the incoming flow of cold sea water. The distillate and the remaining salt water are then restored to atmospheric pressure by pumps. Condensing the water by heat exchange with the incoming flow in one of the economical advantages of this process.The multiple stage evaporator is able to produce more distillate per unit of heating steam because the flashing occurs in more than one stage since the flashed vapors are used to heat the incoming water. For example, using a four stage evaporator, the temperature of the evaporating salt water is reduced by 10 degrees C and the incoming temperature of the incoming salt water is increased by the same amount. Instead of the remaining salt water being discharged at 60 degrees C after only one evaporator, it exits at 30 degrees C after four units. The amount of heating steam required is only one-fourth of that required for a single unit operation.
Over the years, this process has been modified by some plants. The reason for this is the high costs associated with the required tubes. Attempts have been made to modify the process to reduce the size of the heat-exchange surfaces of the tubes. The resulting process is the vapor-reheat flash-evaporation process, also called direct-contact distillation. The processes are very similar except that the distilled water acts as a direct coolant for the vapors instead of the tubes carrying the cold salt water.
In the vapor-reheat process, the vapors in the flash chambers are condensed on a spray or film of cold distilled water. No metal tube separates the two. As a result, the distilled water gradually heats up and increases in volume. The hot distillate is then passed through a liquid-liquid heat exchanger and transfers the heat to an immiscible oil that it is in direct contact with. The oil then transfers this heat to the incoming sea water. This process has been varied slightly by some by using a soild rock bed rather that oil. It has not yet been determined if this method is more economical than the previous conventional flash evaporation method.