1. Editorial
Just before Christmas, we received a submitted paper from Carolyn Koh of the University of London, describing some work in the methane water clathrate field. Some time ago, I had seen a brief TV programme on the occurrence of this material beneath the sea and the possibility that it was implicated in the Bermuda Triangle. As it turns out, the implication is rubbish. However, Carolyn and I researched the subject from which it is clear that methane clathrate is THE next energy source, likely to replace coal and oil. My own view is that the unfortunate down side of this development is that it will almost certainly damage the environment beyond repair. Our efforts are below as a feature article entitled The methane water clathrate: treasures of the deep or an ecological timebomb? I hope you find it interesting. Historically, Infrared and other sorts of spectrometer were huge, heavy expensive beasts and then along came the first small instrument – the Perkin Elmer Infracord. Nothing has been the same since. One of the team who developed the Infracord and its successors was Mike Ford, long retired director of research at Perkin Elmer’s UK plant. I have persuaded Mike to write a piece telling us about this revolution in instrumentation. Mike was helped by a wonderful character – Francis Dunston, again a P-E retiree, who has a house and garage full of ancient spectrometers and filing cabinets stuffed with brochures and other memorabilia. I’ve visited Francis and wondered how he had room enough to live; no, not only spectrometers under the bed, they are everywhere – kitchen, passageways, conservatory, the garage(s). I promised last time to produce Article II of "How does FTIR work?" and I have to apologise. Rather than produce a load of old verbiage I have decided to make Article II a sort of tutorial you can run through on your own FTIR. Fortunately, Fabrice Birembaut has volunteered (?) to give me a hand but this type of thing takes a little time. You will see it next Edition – promise!! Following Perkin Elmer’s decision to cease sponsorship of the Journal at Easter –to attract new sponsors and advertisers (if we decide to include advertisements) it is essential that the Journal readership be as large as possible. Could you please urge all your friends and associates to register as readers with us? We really do need numbers – and remember it doesn’t cost anything! List of email addresses - readership It is clear that the list of email addresses is very attractive to
possible sponsors. In the past, Perkin Elmer have had access to the list because they
"owned" the Journal and could have used the list to send you information on
their products. I don’t think they used it very much. However, future sponsors might
well want the list and would want to send you information via email. To be completely fair
to you – please tell us if you wish your address to be withheld from a sponsor or
advertiser. We will erase it from the list which is used by the sponsor, retaining your
name on our subscriber list so we will still email you when we publish a new edition. Louise 2. The methane water clathrate compound – As long ago as the days of Michael Faraday, it has been known that water and methane (and other small hydrocarbons and ring compounds) form clathrate compounds. What is more recent in their history is the discovery that vast amounts of methane hydrate are present in Polar Regions and beneath the deep sea. When water freezes into ice, it forms a three dimensional structure with space between the atoms into which methane can become entrapped. The melting point of the clathrate compound, as it is called is above zero, in fact, well above, the temperature rising if pressure is applied. Now, beneath polar ice caps or layers of frozen tundra, or at great depths in the sea, conditions can easily apply where the methane hydrate compound is stable. The consequence is that if methane is generated through the decay of biological material or more rarely, from volcanic action in these zones, the gas does not diffuse away to the atmosphere, but rather it stays in the water saturated sediments. The methane clathrate remained a laboratory oddity until in the early 1980’s the Russians became aware of just how much of this material was present in Northern Siberia and started to develop methods of extracting it. Methane from this source is now piped to Europe both east and west. Similarly, sources have been exploited in Alaska but the exploitation so far is just the tip of the iceberg (sorry!). Over the last 20 years an enormous amount of work has been done on the physical and structural chemistry of gas clathrate hydrates, on the thermodynamics and phase relationships and this has pointed geologists and geographers in the direction of where to prospect for deposits. It turns out that almost anywhere in the world where there is really deep sea, the compound is at least potentially present. In the Arctic and Antarctic, enormous amounts are found at shallower depths.
Hardly surprisingly, many estimates have been made of the quantities of methane trapped worldwide. They range from what could be present assuming that the compound will always occur where the conditions of temperature and pressure are appropriate, to much more modest estimates based on actual discoveries made so far, but the figures are mind-boggling. The amount of methane in these gas hydrate deposits varies from location to location. For example, it is estimated that there is around 20 trillion m3 of methane in gas hydrate deposits at 300-1200 m under the sea bottom of the Black Sea, while gas hydrate deposits found in the permafrost of the Mackenzie Delta of Canada are estimated to contain around 0.17 trillion m3 of methane. Don’t run away with the idea that this latter amount is small. In just one investigation of the Mackenzie River Delta up in the Northwest Territories in Canada, deposits were found at a depth of about 1 kilometre, the layer being ~100m thick and of many square kilometres in extent. There are many, many deposits of this type. In fact, deposits are likely to exist wherever the mean temperature is below 0ºC. As a result the potential for deposits could be found all over this vast region and not just the MacKenzie Delta.
To put this in perspective, it is likely that the energy trapped in the methane clathrate easily exceeds that available from coal, oil and conventional natural gas put together. Phew! At the moment, exploitation is restricted to land based extraction – either, the pressure is deliberately reduced by drilling and the methane is released or heated water or steam is injected and raises the temperature. So far, the technology for releasing the gas from beneath the sea is either not yet available or is not economically worthwhile. In Polar Regions mud beneath 600 feet of water is of interest but in most temperature climates much greater depths are involved so the extraction problem is far from easy. Assuming people start to explore this resource two environmental consequences are inevitable–
Currently the concentration of methane in the atmosphere is around 2ppm rising by ~0.6% per year. This amounts to about 4 x 1010 tonnes of methane or about 3 x 10-4 of the amount in clathrate deposits. So, any significant release of the entrapped clathrated methane could be disastrous. The amount of CO2 in the atmosphere is around 550ppm, so remembering that methane is 20x more effective than CO2 as a greenhouse absorber, the methane in the atmosphere is currently equivalent to about 40ppm of CO2 – a non trivial contribution – and its rising! Now, there is a further quite natural problem to consider, a problem not related to exploitation of this resource. As the temperature of the atmosphere rises, it is an inevitable consequence that marginally stable deposits particularly those under permafrost will simply evaporate off to atmosphere. Similarly, significant rises in the temperature of the sea could destabilise deposits. This is unlikely to occur at great depths because the well known density inversion in water around 4ºC keeps the temperature at depths remarkably consistent. Further, the heat capacity of the oceans is enormous. However, atmospheric changes can cause temporary flows of warm water e.g. off N. California methane hydrate exists at depths >510m and of course is normally stable. In August 1997, the well-publicised El Nino event caused the water to rise temporarily in temperature in excess of the stability limiting value so presumably some methane was released. Similar events have been monitored in the Gulf of Mexico and elsewhere. There have been sea stories around for centuries describing how ships have been sailing along and then have vanished without trace. No storms, no explosions – just vanished! Some reports have included the sea burning and the ship vanishing. One plausible explanation is the release of methane from the sea bottom. As the gas bubbles rise they reduce the AVERAGE density of the water and hence a ship could simply sink because its displacement x density falls to less than the mass of the ship. If flames derived from electrical equipment, boiler fires etc., are on board the methane could well burn at the sea surface. However, although all this seems plausible, the release of methane may not involve the clathrate and may occur from gas rich oil deposits. Where does the methane already in the atmosphere come from? No, not the release of ‘frozen’ methane, but rather release from more mundane sources, e.g. agriculture, landfills.
Having skimmed over the science let us go back and develop some of the ideas surveyed so far. Physical chemistry of The structure of the compound is based on that of ice. There are various structures of ice i.e. the water phase diagram is complex, various structural forms being present over defined pressure and temperature domains. All have interstitial sites, some of which can be occupied by methane. The methane itself also alters the structure, and hence the phase diagram, by influencing the surfaces between the phases. One typical structure of the clathrate is shown below.
Methane hydrate is a crystalline clathrate compound in
which methane gas molecules are trapped in water cages which form part of a distorted ice
matrix. Each water cage can only be occupied by one methane molecule. Methane hydrate has
a cubic crystal structure consisting of two types of dodecahedral water cages: a small
cage which has twelve pentagonal faces and a large cage having twelve pentagonal and two
hexagonal faces. This leads to the stoichemistry CH4 + 6H2O There is considerable interest in the way that a gas molecule interacts with the host and how it affects the hydrogen-bonded water lattice, and infrared and Raman spectroscopies have been of value. Gas molecules in the hydrate water cages can distort the water lattice as well as modulate the intermolecular vibrational motions of the water lattice. At around 273 K, this crystalline methane hydrate compound is only stable at high pressures in excess of around 3 MPa. Therefore, in situ experimental studies on this compound require specialised high pressure, variable temperature reactors. One way out of this dilemma is to use another molecule which forms a similar clathrate hydrate structure to natural gas, but which does not to require pressurisation. An example of this type of work is given in the submitted paper in Section 2.
As is so often the case in any study of this type, vibrational spectroscopy can be of immense value, but its true worth is only revealed when it is supported with X-ray, D.S.C., neutron scatter and other measurements. The Raman spectrum of methane hydrate (shown below) can tell us about the structure of methane hydrate. From the n1(C-H) symmetric stretching vibration of methane we can find out whether methane hydrate has been formed and also what are the relative amounts of methane in the large and small water cages. Methane trapped in a small water cage has a band at higher frequency than methane trapped in a large water cage.
Perhaps the most significant measurements, because they define the stability parameters for the compound, are those made on pressure/temperature. The isotherms for methane/ice are
from which you can see immediately that the compound is
likely to be found under arctic tundra and in deep seas. The pressure rises with depth at
around 1 atmosphere. (105Ps) every 10m. In tundral deposits, the temperature is
a problem. However cold the atmosphere is and however impermeable the frozen tundra, the
temperature inevitably rises the deeper you go due to heat conducting from the
Earth’s core. Of course, exactly the same happens in alluvial deposits beneath the
sea, so the layer of stable material is inevitably of restricted depth. Put another way,
the pressure and temperature rises linearly with depth but the diagram above shows that
the pressure over the clathrate rises exponentially with temperature. There must always be
a limiting depth. As mentioned above, deep seas are remarkably constant in temperature due
to the well known density temperature inversion near 4ºC. Thus, given enough depth, the
compound should be stable almost anywhere, and it is. It is also worth noting that the DH¼ of the clathrate is high
at 54.2KJ mole-1, compared with ice at 6.008 due no doubt to the phase changes
solid clathrate Methane hydrate as a fuel in vehicles? Assuming that methane becomes a popular source of energy then the possibility of using it in ships or even motor vehicles becomes economically very important. Of course, natural gas is already widely used and has been for many years. In the UK, buses and trucks were fitted with huge bags containing methane from anaerobic biocomposition during World War II and some buses today carry compressed methane as a fuel. The problem with compressed or liquefied methane is the cost of storage and the obvious safety implications. Similar doubts exist if H2 was to be developed extensively as a fuel for road vehicles. It has been suggested that methane hydrate supplied as a slush from pumps might provide an answer, first probably for ships but why not in road vehicles. In the table below, we give some data to explain the attraction.
So the tanks in ships, trucks or cars would have to be larger than they are now by about 5 x. However, in a small car the fuel tank is currently ~50l capacity, but the vehicle can well have an internal capacity of 2500-3000l, so this should pose little problem. Pipeline Plugging We have concentrated so far on the potential of this material to produce energy in the future, but its existence can be a real headache to the petroleum industry. Methane, usually from conventional deep gas fields, is frequently wet and is piped in this condition over long distances. If the pipeline cools down in the winter to low temperatures, the conditions can become just right for the clathrate to form and block the pipe valves or pumps. Although precautions are taken (pipeline heating for example), this is a real problem. The chemical structural work done to date is important in developing methods to inhibit the formation of the blockages and much of the work done using FTIR, X-ray and other structural tools has been driven by urgent needs in this field. Two main types of chemical inhibitors can be used to control methane hydrate formation: traditional thermodynamic inhibitors or as an alternative low-dosage inhibitors. Thermodynamic inhibitors e.g. methanol or ethylene glycol, have been used widely by the gas and oil industries to prevent gas hydrate formation in gas pipelines, however, the costs involved are huge due to the large volumes of inhibitor required. Low-dosage inhibitors, on the other hand are more attractive since as their name suggests, only small concentrations of these chemicals are needed to prevent hydrates from forming. Current research is concerned on optimising these low-dosage inhibitors. Promising candidates are the alkyl acrylamide polymers amongst others (all of low molecular weight). These materials effect the kinetics of hydrate formation. Conclusion How does the methane clathrate influence the future of energy policy and the greenhouse effect. Quite obviously, the availability of relatively pure, sulphur free methane to nations that are energy hungry and available close by and under their own direct control must be very attractive. Obviously too, production of methane this way is incredibly capital intensive but so is nuclear power generation or hydroelectricity. As methane becomes freely available in large quantities from this source, the high thermal yield from burning the gas must start to make wind, solar or wave power generation (where energy yield per dollar is inevitably poor) less attractive. Thus, there is the prospect that as a consequence of this new source of energy the production of CO2 will inexorably rise. However, methane will provide, as it is already doing, a short-term amelioration of this rise by replacing fuels that generate more CO2 per unit thermal output e.g. lignite, the worst offender, coal and oil. Methane can half the emission, thus the ‘dash for gas’ in the UK – the replacement of coal fired power stations by natural gas fuelled units. But remember, once methane has ousted other carbonaceous fuels the output of CO2 is then bound then to rise and rise as more and more energy is required World Wide.. The physical chemistry that entraps CH4 can also clathrate CO2. It has been suggested that this might solve the World’s problem. If the CO2 is removed from the air, compressed and piped to the deep sea it will react with water and freeze as CO2 hydrate, stable at depth at 4ºC. Experiments have been carried out and it is clear that stable CO2 reservoirs could be established at depths of 3600m and probably a lot less. The snag is that to STABILISE not REDUCE the level of CO2 in the air at the moment we will need to ‘lose’ ~1010 tonnes CO2 a year. The cost would be quite overwhelming. Assuming the World accepts that the polluter pays, the Americans have a little problem! To finish on a sombre note – we now know that vast amounts of
methane are there for the having. Burning the stuff is relatively very clean and its
widespread availability must be attractive to politicians. Put another way –
politicians may well wring their hands and the environmental lobby may scream, but it is
hard to believe that this financial bonanza will stay at the bottom of the sea much
longer. Dr Carolyn Koh is Reader in Chemistry at King’s College University of London and an adjunct professor at Cornell University. She has researched in the hydrocarbon hydrate field for some years and has published extensively.
Those of us with long memories are well aware of the revolution the ‘Infracord’ made in infrared practice. When I was flogging away at my PhD research, I used a nearly new Hilger H800 – a monstrous contraption built in the UK. This instrument worked well enough but it was incredibly large and unbelievably heavy and was rather slow. The optical unit was the size and weight of a medium sized coffin (complete with occupant) and the electronics were housed in a floor mounted be-wheeled casing ~1½m high. Around 1959 we (my supervisor, the late Don Powell and I) were loaned an Infracord so that we could "give our opinion" of its merits. Well immediate reactions were negative – small, light, tinny and obviously horrible, but the snag was that it worked as well as the H800. Regardless – we didn’t like it – everyone knew that research grade instruments had to be massive or they weren’t research grade instruments! How wrong we were! Dr. Mike Ford, probably the leading figure in demonstrating that small is beautiful in IR as it is in so much else has written a piece for us on the development of this genuinely epoch making machine. A thought – back in the 1950’s, 60’s and 70’s P-E produced the Infracord (1958), upgraded it in 1959, sat back in 1960 and then offered a UV variant in 1961. To keep their competitors on their toes they improved the design in ’62, ’63 and again in ’64 and upgraded in ’65 and ’66. Clearly, this wasn’t fair so they let the other lot catch their breath for 3 years and then hit the market again and again with new products in the 1970’s. This record seems to me to be more like that of the motor industry. I sense that in the IR field things don't develop like this these days. Could this perhaps be the main reason why the industry has become staid and mature? [See report on IRDG meeting in last edition]. Mike’s fascinating article is below and I recommend it to you all.
Dr. Mike Ford It was probably in early 1954 that the first inklings of an idea began to form. Some observant people in the company had noticed that many users of the current main Infrared product, the large, research grade model 21, made little use of the range of resolution and scan speeds offered, but settled for one - mostly the same - set of conditions all the time. So why not give up the expensive options and make a much cheaper instrument giving only those conditions? After some tossing about, the idea began to gain acceptance, and to it was added the concept of further reducing the cost by adopting a radically different approach to the construction of the instrument - using parts stamped from sheet metal instead of the traditional castings. There was some scepticism about this approach, and adequate stability for the optics using a sheet steel plate stiffened with a box structure underneath had to be successfully proved. To keep the instrument compact it was decided to use a high aperture, short focal length monochromator which demanded use of aspheric optics to achieve adequate resolution. Although Perkin-Elmer possessed the necessary skills for making aspherics, they had never before been made in the now projected numbers, or for the low cost required, but there was sufficient confidence for the project to be given the 'go ahead' (see footnote). Under the leadership of John Atwood, with Hamilton (Ham) Marshall responsible for the optics and Tom Flynn the electronics, the instrument was well advanced when I joined Perkin-Elmer at Norwalk in the United States in October 1956 (I was involved in other projects for the next six months). In addition to the sheet metal construction, there were many innovative and simplifying features. These included the use of etched foil for the slits and wedge attenuator and a single shaft carrying the recording drum and the scan and slit drive cams, driven by cable and cams to give (roughly) a constant time interval for each resolution increment. In addition, Kurt Opperman designed a new type of source, a ceramic tube with an internal rhodium (later platinum/rhodium) heater, to be simpler than the usual globar or Nernst filament. The first instruments had two scan speeds - 3 and 12 minutes for the full range - and a single slit (resolution) programme The name 'Infracord' was an obvious extension from the existing 'Spectracord' UV spectrometer, but the origin of the model number 137 is probably lost for ever. The introduction at the 1957 Pittsburg meeting was a great success, although it transpired that two other companies, Beckman and Baird had new products aimed at much the same market. - I well recall arguments, and probably bets, between representatives of the three companies as to who would sell the most; history shows a very clear answer! When Perkin-Elmer Ltd was set up in August 1957, it was decided that the Infracord would be the first product, and I returned to England with the full drawings and a part assembled instrument in my luggage. Since there was no habitable space in the company's building, a derelict factory in Beaconsfield, I begged the use of a tiny lab at King's College to do the final assembly and test. Once some space had been converted into something more appropriate, three instruments were built from sets of parts imported from the USA, while UK made components were being procured - import regulations at that time limited us to importing a maximum of 30% of the parts value, which was almost fully absorbed by the aspheric optics and printed circuit board (we were unable to obtain double sided, plated through boards here). The first of the 'fully' UK instruments was shipped in March 1958 and a total of 12 were made at Beaconsfield in that year, while about 140 were made at Norwalk.
Problems & Improvements There were a few hiccups along the way. An early problem to arise concerned the temperature compensation. To save cost, the instrument was not thermostatted like previous models and, to correct for the change of refractive index of the rocksalt prism, a bimetal strip was incorporated in the scanning mechanism. The corrective effect was inclined to stick, resulting in erratic calibration. Fortunately, the problem had already been recognised at Beaconsfield and a solution incorporated before it became so serious at Norwalk that all shipments were stopped - the Americans then accepted the modified design with such alacrity that shipments were resumed within a week! A chance observation when aligning instruments led to the realisation that the best focus in the infrared was different to that at the wavelength of the mercury lamp used for alignment. After the cause of the effect was found, the magnitude of the difference was calculated and a jig made to alter the focus by that amount, giving a marked benefit. Numerous other minor improvements were made, the most obvious being the
increase of the ordinate scale from four inches to six inches and the addition of a
limited range of slit programmes, before major changes required a new model number *- the
first of a long series of progeny of the original 137. The first Progeny Rather surprisingly, the first of the progeny was a UV version, the model 137UV - later the 402 - created by changing from the rocksalt to a fused silica prism, with the thermocouple replaced by tungsten and deuterium sources and the detector by a photomultiplier. In addition, a logarithmic 'wedge' attenuator was used to give an absorbance scale on the recorder. The development was started in the USA, but completed at Beaconsfield, and the instrument shipped from 1961 to 1979, the longest life of any of the 137 series. Diffraction Gratings, not Prisms We all knew that gratings were readily available which would give the instrument much better performance than the rocksalt prism. The problem was that a grating at a given angle reflects not just one wavelength (referred to as first order) but also multiples of that wavelength (second order and so on). To select only the needed order required either an extra prism, for which there wasn't room, or filters. Thus, it was the development of suitable filters which gave us the opportunity to design the next on the 'Family Tree', the first of the filter grating instruments, the 237 (4000 - 625cm-1) and 337 (4000 - 400cm-1). Each of these used two gratings in first order and a mechanical cosecant drive -a geometrical linkage which moves the gratings to give a linear wavenumber scan - in place of the scan cam.
'Flowchart' Recorder The concept behind the Beaconsfield development of the 'Flowchart' (sprocket driven chart) recorder was to remove the dependence upon the operator correctly aligning the pre-printed chart on the recorder drum and eliminate the sensitivity to chart stretch or shrinkage. In addition it provided the ability to perform abscissa scale expansion. Since the wavenumber accuracy of the instrument depended on the alignment of the printed grid with the perforations which drove the chart, it was essential to find a chart printer who could meet the necessary, very tight tolerance; such a supplier was finally found in Germany. It proved very successful and was applied to all later models, starting with the 157(the last of the prism instruments, made until 1977) and 257, essentially the 137 and 237 with the new recorder.
'Star' Cam At first sight, this might seem a trivial innovation, but it was, in fact, very significant. In order to further extend the range of the 257 it was necessary to use one grating in both first and second orders and a second grating in first order only. While this would have been possible with the existing scan drive, it would have been complicated and would have extended the total scan time. By using a three-lobe scan cam in the shape of a three-pointed star, the full range could be covered in a single rotation of the cam shaft. A first, rough calculation of the cam shape indicated slopes so steep that many believed it could not possibly work - but it did!- and the first outcome was the 457 with a range of 4000 to 250cm-1. In operation the cam rotation was continuous, with the chart uncoupled and pen deadened while tracking the cam return zones and with simultaneous filter and grating change. Later, restricted range, lower cost versions followed; the 357 (to 400cm-1) and the 157G (to 625cm-1). The addition of ordinate expansion - by putting gears between the wedge attenuator and the pen drive - and more user friendly controls were the next step, approximately replacing the 1,3and 457s with the 1,3 and 577s. (newly developed filters allowed the range of the 577 to be extended to 200 cm-1). Solid State Electronics Up to 1976 the original valve (vacuum tube) signal handling electronics had remained almost unchanged since they had continued to function reliably, but they were increasingly 'old fashioned' and bulky and were finally replaced with a 'modern' solid state system. The outcomes were the 2,3 and 597s, with ranges to 600,400 and 200cm-1 respectively, plus the 197, a simpler, lower cost version of the 297. Microprocessor In the models 2, 3 and 598, introduced in 1979, a microprocessor controlled the scanning parameters (not the signal handling) but, more importantly, it provided full external computer compatibility. These instruments were replaced only a year later by the first of the low cost RATIO RECORDING models. The 681, 2 and 3, as before, had spectral ranges to 600, 400 and 200cm-1 and, in addition, there was the 684, which incorporated pre-sample chopping to prevent the error in ordinate accuracy arising from the infrared radiation emitted by hot or warm samples. Ratio Recording Prior to the 680 series, all instrument versions had used the optical null principle, in which the reference, or background, beam is attenuated to match the attenuation caused by the sample, with the recording pen coupled to the variable attenuator. With ratio recording, sample (S) and reference (R) signals are measured separately and the ratio S/R plotted by the recorder. This gives many advantages over optical null, too numerous to detail here. In the 680 series the signal from the thermocouple was digitised and all further processing was digital in a second microprocessor. Quite a complex calculation was performed in order to minimise the problem that, in a fast scan, the background energy could change significantly between 'sample' and 'reference' in the same chopping cycle. In addition, users were not accustomed to having the recorder become 'noisy' in regions of high background absorption, rather than going 'dead' as in optical null instruments. To overcome this objection the option of 'pseudo optical null' was provided, which increased the time constant in such regions, but without any lag in the written position on the chart. The Last of the Progeny Although many of the features of the original 137 continued to be used, the 780 series (similar to the 680s with added features) must be considered to be the last of the true progeny, as later models reverted to a cast base rather than the sheet metal construction, a change made economic by advances in manufacturing methods. Over the lifetime of the series, from the introduction of the 137 in 1957 to the final shipment of 780s in 1987, over 12,000 instruments were produced. Later, all the grating (dispersive) instruments were replaced by Fourier Transform models, although they are still preferred for some specialist applications. But that is another story.
Acknowledgements Footnote REF: M. Ford, Int.J.Vibr.Spec., [www.ijvs.com] 6, 1, 4 (2002)
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CH4.6H2O.
The amount of methane incorporated in a methane hydrate compound varies depending on the
method of preparation, but can be of the order of 90% of the water cages being occupied by
methane gas. The quantity of methane that can be stored in this methane hydrate material
is vast, with as much as 180 volumes of methane per volume of the methane hydrate
compound. 
