CLATHRATE HYDRATE

April 14, 2012


I certainly enjoy reading about and understanding new technologies.  Those technologies that provide “value added” by their very nature.   I just ran across two “new words” that demonstrate old dogs can learn new tricks and seemingly old technology can be new to the uninitiated—in other words me.  Do you know what a clathrate is?  A clathrate hydrate?  OK, neither did I.  Here we go.

Clathrate hydrate technology was first proposed in 1942 by M.E. Benesh as a method of storing natural gas.   An excellent paper entitled “Gas Hydrate Storage Processes for Natural Gas”, written by R.E. Rogers, Yu Zhong, R. Arunkumar, J.A. Etheridge, L.E. Pearson, J. McCowan and K. Hogncamp give basic details as to how this technology would work in a very practical sense.  All gentlemen teach at Mississippi State University and have spent years working to research and perfect a working prototype used to demonstrate that this can be a viable approach to the problem of storage.  I would like to indicate some of the conclusion derived from that study, as follows:

“Formidable problems (forming hydrates rapidly, collecting and packing hydrates, and reacting interstitial water) to make natural gas storage in gas hydrates an economically viable process are overcome by forming the hydrates from a surfactant solution. In the feasibility study, a non-stirred laboratory test cell could be filled with hydrates in less than 3 hours with a capacity of 156 vol/vol. The important attributes of the laboratory process are incorporated in the design for a proof-of concept scale-up. Simplicity and minimum labor requirements are stressed in the design. The process is designed to store 5,000 scf of natural gas in gas hydrates to be formed from surfactant solutions at 550 psig and 35°F. A finned-tube heat exchanger accommodates latent-heat transfer during hydrate formation and decomposition, but the exchanger also serves to collect by adsorption and symmetrically pack hydrate particles as they form.  The proof-of-concept facility is based on experimental results of the laboratory feasibility study; the facility has been constructed, installed and full-scale tests are proceeding. “

As indicated in the first sentence of the paper—“Gas hydrates are clathrates where guest gas molecules are occluded in a lattice of host water molecules.”  Well and good, but for a “gear-head” like me, what does this mean?  A clathrate hydrate is a very special type of hydrate in which a lattice of water molecules encloses molecules of trapped gas.  This gas could be methane, ethane, syngas, etc etc.  You get the picture.  For our purposes, we will discuss methane only.

 Large amounts of methane, naturally frozen in this form, have been discovered in both permafrost formations and sea beds under the ocean’s floor.  Methane hydrates are believed to form by migration of gas from significant depths along geological faults, followed by precipitation or crystallization, upon contact with rising gas streams of cold sea water.   About 6.4 trillion (that is, 6.4×1012) tons of methane lie at the bottom of the oceans in the form of clathrate hydrate.  Each kilogram of fully occupied hydrate (actually only about 96% occupancy is found) holds about 187 liters of methane (at atmospheric pressure).  

 One significant fact, ice-core methane clathrate records represent a primary source of data for global warming research, along with oxygen and carbon dioxide.   This is one reason why there is research data available on the huge quantities of entrapped methane gas.   As mentioned above, Mr.  M.E. Benesh first proposed using this technique as a method of storing natural gas as early as 1942. At that time, the methodology of doing so was not available, now it very well may be as demonstrated by Mississippi State.  

There are several classifications of clathrates.  The table below will indicate those classifications with a depiction of the lattice structures given above the table:

Since methane clathrates are stable at higher temperatures than LNG, there is a great interest in converting natural gas into clathrates rather than liquefying it prior to transporting by seagoing vessels.  A significant advantage would be the production of natural gas hydrate from natural gas at the terminal.  This would require a much smaller refrigeration plant and less overall energy as compared to the production of LNG.  The only real issue seems to be the rate of production and the economic viability of production.   Both issues are being addressed at this time by Mississippi State University. 

The real benefits would come from incorporating this storage method for locations in which it is impossible to fabricate transmission piping or transmit the gas in an easy fashion other than tanker or truck.  It is something to be aware of and to think about.  At any rate, it is fascinating.  I hope you agree.

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