Multi-Stage Flash Distillation for Desalination
Multistage stage flash distillation for desalination is carried out in a pressure vessel that is divided into numerous sections that have decreasing pressures and temperatures. The sections each have a set of coils that have seawater pumped through them. These coils serve two purposes: condensing the steam produced by the flash evaporation and preheating the seawater to almost the required temperature for distillation.
The seawater then passes through an inline steam heater, taking it up to a temperature between 70ᴼC and 100ᴼC; the seawater now being considered to be brine. Multi-flash distillation plant engineers will sometimes reduce this temperature to decrease the formation of scale.
The brine now passes into the different stages and, as it is at a higher temperature than the sections, some of it flashes to steam. This is condensed by the seawater coils at the top of the sections, with the condensate dropping down onto the collection trays under the coils.
This continues to the final stage when the brine is pumped to waste and the collected distilled water is delivered to storage tanks. There is a salinometer on the discharge line to the storage tanks that operates a tank isolating valve/dump valve in the event of a high ppm alarm.
A typical multi-stage flash distillation plant is shown below, along with one type of distillation chemical process diagram chart applied to desalination.
1. Wikimedia Commons: Images
2. EBWP: Distillation processes
3. thewatertreatmentplant: desalination methods