How Desalination Works

There are many different techniques for desalination. Among the main ones are the multistage flash (MSF) process and reverse osmosis. Countries that do not have an abundance of fresh water are prime candidates for the use of this technology. Quatar's installations include the Ras Abu Fontas B power and desalination plant, which cost $1 billion to build. Dubai in the United Arab Emirates (UAE) has a 60-million-gal/day desalination plant at Jebel Ali. The plant's eight MSF units are part of a cogeneration power facility. The fresh water would have been at least as much incentive in the Middle East as the increased total thermal efficiency. The desalination equipment in both cases was supplied by Weir Westgarth (WW).

WW designed the MSF process. The principle of the system is simple: Water and steam in a closed system can be made to boil at temperatures lower than at standard temperature and pressure by reduction of the system pressure. MSF plants contain a series of closed chambers as many as 20 each held at a lower pressure than the preceding one.

Heated salt water is passed through the overall system. Some of the salt water in each chamber vaporizes into steam. Moisture droplet separators remove salt water droplets. The steam condenses to fresh water when faced with cold tubes and is collected for storage. The last chamber's brine is quite cool, and it is, in fact, used as the coolant fluid. Then it starts to...