Wednesday, November 28, 2012

Flash Distillation: The Most Energy Efficient Desalination Method That Was?

Before the advent of the discovery of an efficient polymer based membrane for reverse osmosis, was flash distillation the most energy efficient method of desalinating seawater during its heyday?

By: Ringo Bones

Back in the 1950s, when polymer-based membrane for use in an energy efficient reverse osmosis desalination plant use were still decades away, a way of converting seawater to potable freshwater called flash distillation was deemed the most energy efficient method of desalination at the time. But what makes Flash Distillation Desalination Plants so energy efficient compared to say merely distilling seawater at normal atmospheric pressure?

The boiling point of water – at 212 degrees Fahrenheit or 100 degrees Celsius – is largely determined by the prevailing atmospheric pressure of 1 atmosphere – or 14.7 pounds per square inch or 760mm of Hg at sea level.  At about 60,000 feet above sea level, where the prevailing atmospheric pressure is only 2 percent that at sea level, water now boils at human body temperature of 98 degrees Fahrenheit or 37 degrees Celsius – thus this is why we need pressure suits / space suits when we ascent at higher altitudes – and this is the working principle behind the flash distillation desalination system.

When superheated water enters a chamber at reduced pressure, the water flashes almost instantaneously into steam, this is the basis of flash distillation where seawater first enters the system in a pipe which forms coils as it passes through successive evaporating chambers. The pipe carries the water past a heating furnace where it is superheated (heated above boiling point without boiling it) to 250 degrees Fahrenheit. As the superheated seawater flows into and through the reduced pressure evaporators, each of the chambers is filled with steam. A steady inflow of seawater keep the coils cool and the resulting steam condenses on them and drips into drains that leads to storage tanks and since salt is not carried into the steam, the resulting condensation is fresh water while a briny residue many times saltier than the seawater is drained away.

Back in 1958, the city of Freeport in the US state of Texas was selected by the US government as the site to build an experimental flash distillation desalination plant to solve the chronic thirst of what then the city’s 11,800 inhabitants. Freeport got the priority because even the water obtained from the local artesian wells was deemed too salty for long-term consumption even though it is several times less salty than the seawater taken from the Gulf Coast.

Back then, it cost 1.2 million US dollars to build, the experimental Freeport Flash Distillation Plant uses extremely low pressures to cause water to boil, or “flash” almost instantaneously while leaving salt behind. And as a bonus, less energy is required - in the form of heating oil or natural gas – to convert the incoming seawater into steam. The method proved so efficient that Freeport’s first flash distillation desalination plant’s first batch of fresh water output produced had too little salt in it that the residents complained that what came out of their taps tasted too flat – almost akin to triply-distilled water used in a typical chemistry lab. To remedy the situation, the distilled water had to be mixed with the slightly “briny” water from Freeport’s local artesian wells so that some of its “taste” could be restored.

Back in the late 1950s, even the experts predict that within 20 years, flash distillation desalination plants located at critical spots will be producing up to 500 million gallons worth of potable freshwater a day, enough to supply even the largest cities. Well, this was way before reverse osmosis went industrial and there was even a nuclear fission powered flash distillation desalination plant being planned to supply the city of New York with low-cost freshwater during the critical summer months. How times have changed indeed.