Wednesday 25 Apr 2018

:: Multi-stage Flash Distillation

In this process, seawater is heated in a heat exchanger known as the brine heater. This is done by condensing steam on the surface of pipes passing through the heat exchanger, which leads to the heating of seawater in these pipes. Heated seawater flows through the brine heater to another container called distiller (in its first phase), where there is low pressure, which makes water boil immediately.
The sudden introduction of water in the phase makes it boil fast and evaporate suddenly. Only a small part of this water is transformed into water steam, based on the level of pressure in the phase.
Distillation continues until water starts to cool down releasing the necessary evaporation temperature until it reaches boiling point.
The idea of distilling water in a low pressure receptacle is not new. It has been used for more than a century. In the 1950’s, a desalination unit was established where a series of phases were set at increasingly low pressures. In this unit, feed water passes from one phase to the other and boils repeatedly without adding thermal energy. Each desalination unit may include 4 to 40 phases.
As for the steam which is generated through the flash system, it is transformed into potable fresh water upon condensation at the surface of the exchange heaters pipes which go through the phase. The pipes are in this instance cooled by incoming feed water going towards the brine heater. This, in turn. heats feed water and therefore reduces the amount of thermal energy needed to heat feed water in the brine heater.
Desalination plants using flash distillation have been commercially established since the 1950’s. Generally, the units had a capacity of 4,000 to 30,000 cubic meters of water per day (1 to 8 million gallons per day). The units are operated under feed water temperatures (after the brine heater) ranging between 90 to 120 degrees Centigrade, corresponding to 194 to 248 degrees Fahrenheit.
Among the factors that have a direct impact on the thermal efficiency of the plant in this system is the difference in temperature between the brine heater and the coolest part in the plant. Operating the plant at a temperature exceeding 120 degrees centigrade in order to increase its efficiency may do the job, but it could also cause scale formation and corrosion on the metal surfaces.