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.

Sun Powered Desalination Plants: Sill Workable Ancient Desalination Technology?

The ancient concept seems ingenious, but why doesn’t everyone use the free heat energy from the sun to desalinate seawater into drinking water anymore?

By: Ringo Bones

Believe it or not, the knowledge that salty seawater can be made into safe fresh drinkable water is more that 2,000 years old. Ancient Mediterranean sailors embarking on long seafaring voyages have supplemented their stores of shipboard drinkable fresh water by placing pots of seawater under the sun and trapping the condensed vapor. This very same technique – in an updated scaled-up form – had been tried in some large-scale experimental desalination plants back in the 1960s.

Surprisingly, the concept of using the sun’s free thermal energy to convert salty seawater to potable fresh water can easily work when scaled up to a several thousand-gallon-a-day capacity. Back in the 1960s, the 4,083 inhabitants of Symi, an island near Greece, used to get all of their potable fresh water from a newly constructed experimental solar-distillation unit which can supply about 4,000 gallons a day. It works by tapping the sun’s free thermal energy – i.e. heat – to turn seawater into fresh water by first piping seawater into a flat shallow trough enclosed under a transparent plastic dome. The sun’s heat causes the water to evaporate that re-condenses into chemically pure fresh water on the cooler underside of the dome. This pure salt-free water then trickles down the dome, drips into collecting trough at the edges of the unit and is then collected. The briny residue that’s left behind – which is several times saltier than seawater – is then flushed away back to the sea.

The method is very inexpensive given that the energy source used to desalinate the seawater is virtually free, unlike the more popular reverse osmosis method used today which uses electricity to pressurize seawater up to several thousand pounds per square inch to be squeezed though banks of salt-filtering polymer membranes. But using the sun’s free thermal or heat energy to convert seawater into drinkable fresh water is for all intents and purposes an inefficient and impractical process in most cases because the yield is quite low: at best only 0.13 gallons per square foot of basin area per day. This makes a typical solar thermal desalination plants that can be able to compete the output of a typical modern reverse osmosis desalination plant occupy a prohibitively large real estate for every gallon of fresh water produced.

Reverse Osmosis: Most energy Efficient Desalination Process?

First developed during the heyday of NASA’s Apollo program, is reverse osmosis still the most energy efficient desalination process we have so far?

By: Ringo Bones

Back in the heyday of the Apollo program, reverse osmosis – due to lack of an efficient polymer filtering membrane – can only be able to desalinate or purify human urine into fresh drinkable water. After a few decades of development, polymer chemists had finally been able to develop a reverse osmosis membrane that can actually be able to turn the full-on salinity of sea water into potable fresh drinking water. Not only that, reverse osmosis has since more or less became the most energy efficient way to desalinate sea water for drinking purposes – dethroning its previously most energy-efficient desalination method called low-pressure flash distillation process.

A typical reverse osmosis desalination membrane – usually there are banks of them – turns salt water into fresh water when salty sea water is pressurized through it at 1,000 pounds per square inch. Only the smaller molecules of water can go through the structure of the “filtering fabric” in a typical reverse osmosis membrane while the larger molecules of sodium chloride and other salts are left behind. And what makes a typical reverse osmosis plant more efficient that its predecessors is that the highly pressurized salt water and used briny effluent can be reused to run an electric turbine en route to its release back into the normal prevailing atmospheric pressure.

Despite of energy efficiency figures, it still costs 17 million US dollars annually to run a typical large scale reverse osmosis plant that has the capacity to turn enough sea water to fresh drinking water to supply a typical metropolis – 10 million US dollars of which pays for the yearly electric bill. And compared to other sources of tap water, a reverse osmosis desalinated tap water typically costs around 3.38 US dollars per 1,000 gallons. While a river or lake sourced treated tap water costs around 2 US dollars per 1,000 gallons while subsurface groundwater sourced treated tap water costs around 1 US dollars per 1,000 gallons – something to think about when you decide which water utility company you chose to supply your household needs. 

Renewable Energy Desalination Plants: Viable Solution To A Global Thirst?

Even though humankind has faced water shortage way before the dawn of civilization, will the use of renewable energy resources to power desalination plants provide a viable solution?

By: Ringo Bones

Even though reverse osmosis is currently the most energy efficient and most economically viable scheme we have of turning briny seawater to potable freshwater, it is still energy hunger and the running costs are still very prohibitively expensive to the ones who needed it most in the world’s poorest countries. Will the utilization of renewable energy sources – for example wind and solar – provide a viable solution for meeting the needs to quench the thirst of the planet’s monetarily disadvantaged?

Dr. Corrado Sommariva, president of International Desalination Corporation, says innovations in water desalination that harness renewable energy sources is the only hope for the long-term solution of the desalination industry. Earth-friendly renewable energy sources – like wind turbines and solar photovoltaic cells – had just recently been widely applied in large-scale industrial electricity generation during the first decade of the 21^st Century.

 So far, Earth-friendly renewable energy sources use in the desalination industry is still the exception – not the rule and even most high-pressure reverse osmosis desalination plants still get their electric power from conventional fossil-fueled power plants. Will future trends see more use of renewables in the water desalination industry?