How Can Clathrate Hydrates Be Used for Water Desalination?
Current Desalination Processes
A real concern for freshwater is growing on the planet. We use it for so many tasks such as basic hydration but also for cleaning and preservation. As we use it, we deplete this resource which is difficult to restock. To prevent a major shortage of it, technology that allows us to retrieve freshwater from saltwater is a key component of our efforts. We can currently heat then distill saltwater or we can use an osmotic filter to remove impurities from the water in a process known as reverse osmosis. Unfortunately, both of these are not commercially viable options. The osmotic filters need to be replaced often, have high energy requirements and also leave much pollution behind. Distilling on a large scale is also a difficult option. The current best rate for distilling per rate of energy is 1000 gallons at 10-12 kilowatt hours. Michael Max, founder of Marine Desalination Systems, says he can beat that with his system: hydrates (64, 66-7).
In the 1960s, the Koppers Company began to experiment with hydrate desalination research using propane as the gas of choice. Later, Barduhn and his colleagues did a general survey of hydration formation, testing compounds and seeing how their decomposition occurred (Bradshaw 14).
Max has studied hydrates since the 1980’s, when he worked for the Navy’s Naval Research Laboratory. They were interested in knowing if hydrates, a combination of ethane (a hydrocarbon gas) and water, were affecting acoustic signals in search of Soviet submarines. In the mid 1990’s, Peter Brewer and Keith Kvenvolden released compressed gases of ethane into a tube of sea water at a deep depth and witnessed hydrate formation (Wolman 65).
How It Works
Essentially, Max has a long column of saltwater that is pressurized. He introduces ethane into the container. Because the volume remains the same and the pressure is increased, the temperature decreases to about freezing point, allowing the ethane and saltwater to react and create hydrate, specifically clathrate which is similar to ice but is flammable because of the hydrocarbons. These hydrates have a cage-like structure to them, which is the water-ice as the bars and the trapped hydrocarbons in the center. Those hydrocarbons cause the hydrate to be less dense than the saltwater, thus it floats to the top. Once the hydrate is removed, the pressure is returned to normal, causing temperatures to rise and letting the hydrocarbon gas to be released and freshwater remaining (Bradshaw 13, Wolman 64, 66).
A Pathway To Easy Water?
As simple as this sounds, it works well but does have a problem. The hydrates that form have gas layers that are thin enough to let the saltwater hold onto it. Once that mix is melted down, saltwater will contaminate the freshwater that was to be harvested. Max has suggested building a longer column that will allow more pure freshwater to float above the mess, for freshwater is less dense than salt water. This is by no means a foolproof solution. Max has also studied if using methane, which would create a thicker and harder-to-cling surface, can be feasible (66). Once this hurdle is solved, this system promises to be less-maintenance than its counterparts. It won’t have adverse effects for the environment because the main by-product is saltwater. Only 5% of saltwater actually is converted, so the returned water is not too chemically different (67). His method should cost about 46 to 52 cents per cubic meter, much less than reverse-osmosis (45 to 92 cents per cubic meter) and thermal purification (110 to 150 cents per cubic meter) (Bradshaw 14, 15). If perfected, then the immediate problem of freshwater will soon be a page for the history books.
Bradshaw, Robert W., Jeffery A. Greathouse, Randall T. Cygan, Blake A. Simmons, Daniel E. Dedrick, and Eric H. Majzoub. Desalination Utilizing Clathrate Hydrates. Tech. no. SAND2007-6565. Alburquergue: Sandia National Laboratories, 2008. Print.
Wolman, David. "Hydrates, Hydrates Everywhere.” Discover Oct. 2004: 62-67. Print.
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