Applied Thermodynamics and Phase
Equilibria:
Research
Ir. M.M. Mooijer-van den
Heuvel, Dr. Ir. C. J. Peters
Subjects on this page are:
What are gas hydrates?
The structure of gas hydrates.
The fields of interest concerning gas hydrates.
Our work on gas hydrates.
Publications.
Hydrates can be formed in systems of water and small molecules. When the small molecules are gaseous at ambient conditions we are speaking of gas hydrates. These small molecules, e.g. methane (CH4), propane (C3H8), carbon dioxide (CO2), nitrogen (N2) but also fluoroform (CHF3) are enclosed in cavities formed by the hydrogen bonded water molecules. The specific conformations of the framework of hydrogen bonded water molecules can exist because of the enclosed molecules in the cavities, i.e., they stabilise the whole structure. Wouldn't the small molecules be present and the conditions be suitable (i.e., temperature < 0 °C) than the framework of hydrogen bonded water molecules would prefer the conformation of ice, which is not able to accommodate other molecules as a guest. Gas hydrates are ice-like inclusion components that are regularly built and can store large amounts of guest molecules. Albeit they look like ice, gas hydrates can exist at temperatures higher than the freezing point of water and elevated pressures, because of the stabilisation by the enclosed guest molecules. When these guest molecules are flammable the gas hydrates can be ignited and you get burning ice.
Figure 1: Burning ice.
The structure of gas hydrates.
The highly regular structure that accommodates molecules might bring to mind zeolite structures. In fact zeolites correspond to gas hydrates in respect of the highly regular structure, the ability to host foreign molecules and the extent of the interaction forces that play a role in accommodating the guest molecules. Distinction between zeolites and gas hydrates lies in the duration the guest molecules are hosted, i.e., zeolites is short term (for the time a reaction takes place generally) and gas hydrates can 'store' the guest molecules for a much longer time. Also gas hydrates consist of cavities between which no exchange of molecules is possible while zeolites contain canal-like structures through which molecules can move freely.
Currently three structures of gas hydrates are known, i.e., structure I (sI), structure II (sII) and structure H (sH), that can be distinguished in the size of the cavities and the ratio of different sizes of cavities. The first two structures, sI and sII, contain both of two types of cavities, i.e., a smaller and a larger one. All three structures have the small type of cavity in common. The large type cavity of sII is slightly larger than that of sI and both structures can be distinguished in the ratio between small and large type cavities. Structure H consists of three types of cavities, two rather small ones and one that is considerably larger than all the other ones. The last aspect enables formation of gas hydrates in systems with significantly large molecules, for instance cyclohexane, neohexane, etc. The size, and less profound the shape, of the molecules present, determines more of less which structure is formed. If a molecule is too large to fit in a cavity the lattice is distorted and the hydrate does not form. On the other hand, when a molecule is too small the interaction between the molecule and the water molecules in the cavity wall is too weak and does not contribute to stabilisation.
Figure 2: Detail of one of the possible crystal configurations of
ice.
(Pictures can be enlarged by clicking on them)
Figure 3: Occupation of the large size cavity of sI by a guest.
Because sizes and shapes of the cavities in the three possible hydrate structures vary considerably, also within the same structure, it is quite well possible that mixtures of components enhance stabilisation of the hydrate structure. This will occur if at least one of the components in the mixture fits into the smaller sized cavities and at least on other molecule fits into the larger type cavity. It should be mentioned that sH always requires presence of a large and bulky molecule to occupy the very large cavity and a so-called help gas, that is responsible for stabilisation of the smaller type cavities and the structure as a whole.
Figure 4: A sI hydrate structure occupied by different guests. For
enough stabilisation of the structure, occupation of a considerable
amount of the available cavities is required.
(Click the photo to enlarge it.)
The fields of interest concerning gas hydrates.
The storage capacity for gas of these structures is considerable; i.e., approximately 150 times the storage capacity of compressed gas in case of natural gas. In 1934 Hammerschmidt discovered that plugging of oil and gas transportation facilities was due to formation of gas hydrates instead of ice. Practical impact of gas hydrates and their ability to store (natural) gas emerged, after being a scientific curiosity for a long time after their discovery in the beginning of the nineteenth century by Sir Humphry Davy. Much research has been performed to prevent from formation of gas hydrates.
Nowadays we have become aware of possibly more advantageous practical impact of gas hydrates, i.e.:
Figure 5: Worldmap with locations of gas hydrate reserves in subsea
sediment on the ocean floor (blue circles) and in permafrost (yellow triangles).
It should be mentioned that in case of the naturally occurring gas hydrate fields there is
a concern with respect to the stability of these at changing conditions. It is suggested
that when the temperature rises, for example due to the greenhouse effect, the hydrates
might become unstable and decompose. A large amount of, mainly, CH4 can be
released into the atmosphere, aggravating the greenhouse effect. Because CH4 is
even a more severe greenhouse gas than CO2. A sort of runaway greenhouse effect
may arise. Lately it was suggested that decomposition of gas hydrate reserves might have
played a part in the end of the ice ages on earth.
The main components required to form gas hydrates are water and a gaseous component that can be enclosed in the cavities of gas hydrates. Two important gases are CH4 and CO2. For many of the above mentioned potential applications of gas hydrates the presence of one or more types of salts is of importance. Previous research showed that the presence of salts elevates the hydrate equilibrium pressures, while generally a lower hydrate equilibrium pressure is preferred. Recently, it was established that some organic components are able to reduce the hydrate equilibrium pressure significantly.
Figure 6: pT-diagram where the influence of different components on
the height
of the hydrate equilibrium pressure is given schematically.
Objective of the project is to investigate influence of certain organic components
(additives) on the phase behaviour of hydrates in systems with H2O + CH4,.
H2O + CO2 or H2O + C3H8.
Retrieval of experimental data on phase equilibria and confirmation of the assumptions on
the structures formed comprise the experimental part of the project. Gathering of the data
on phase behaviour is done using Cailletet equipment.
Important questions with respect to the experimental data are:
Components that can be used as additives to reduce the hydrate equilibrium pressure are limited by their size and shape. It may be expected that best reduction of the hydrate equilibrium is retrieved with cyclic organic components and fluoroalkanes. The additives used in our investigation are the following: tetrahydropyran, cyclobutanone, cyclohexane, methylcyclohexane, tetrafluoromethane and fluoroform.
The experimental data show that the pressure of the hydrate equilibrium H-Lw-V is reduced upon addition of the components given above. Though the extent of the pressure reduction is not equal for the various gases, i.e., CH4, CO2 and C3H8. For the latter two gases the gas hydrate phase behaviour is more complicated than for e.g. CH4. The region of hydrate existence is limited at the high temperature side by the equilibrium H-Lw-Lhc that rises steeply in pressure as a function of temperature. This hydrate equilibrium is also affected upon addition of the cyclic organic components, i.e., it is shifted to higher, or more often, to lower temperatures and weak bends are initiated in this normally steep straight line. These results suggest that both occupation of gas hydrate structures by the additive components as well as mutual solubility of the gas and additive component play an important role in the shifts and other changes of the hydrate equilibria of major importance H-Lw-V and H-Lw-Lhc.
The data on the hydrate equilibria conditions can be correlated and modelled, using a statistical thermodynamic model, proposed by Van der Waals and Platteeuw, as a basis. Retrieval of so called Kihara potential fit parameters for the additives is one of the purposes of the correlation and modelling. Common use in hydrate modelling programmes is that the chemical potentials of the hydrate and aqueous phase are optimised and compared, which implies usage of fit parameters. To elucidate influences of structural and solubility effects on the equilibria, modelling of behaviour of all present phases is of importance.
Mooijer - van den Heuvel, M.M., Peters, C.J., de Swaan Arons, J., Influence of water-insoluble organic components on the gas hydrate equilibrium conditions of methane, Fluid Phase Equilibria, 172, p. 73-91, 2000
Mooijer - van den Heuvel, M.M., Reuvers, M., de Deugd, R.M., Peters, C.J., de Swaan Arons, J., The occurrence of methane hydrate in ternary and quaternary systems of methane, water, certain organics, and sodium chloride, Proceedings on the Third International Conference on Natural Gas Hydrates, Holder, G.D., Bishnoi, P.R. (eds.), Gas Hydrates - Challenges for the Future, Annals of the New York Academy of Sciences, 912, p. 502-514, 2000
Jager, M.D., de Deugd, R.M., Peters, C.J., de Swaan Arons, J., Sloan, E.D. jr., A model for systems with soluble hydrate formers, Proceedings on the Third International Conference on Natural Gas Hydrates, Holder, G.D., Bishnoi, P.R. (eds.), Gas Hydrates - Challenges for the Future, Annals of the New York Academy of Sciences, 912, p. 917-923, 2000
Servio, P., Lagers, F., Peters, C.J., Englezos, P., Gas hydrate phase equilibrium in the system methane-carbon dioxide-neohexane and water, Fluid Phase Equilibria, 158-160, p. 795-800, 1999
Jager, M.D., Deugd, R.M. de, Peters, C.J., Swaan Arons, J. de, Sloan, E.D., Experimental determination and modeling of structure II hydrates in mixtures of methane + water + 1,4-dioxane, Fluid Phase Equilibria, 165, p. 209-223, 1999
Heuvel, M.M. van den, Reuvers, M., Deugd, R. de, Peters, C.J., Swaan Arons, J. de, Methane hydrate formation in the presence of selected organic compounds and sodium chloride: measurements and modelling, Proceedings of the International Conference on Petroleum Phase Behaviour and Fouling, AIChE, Houston, Texas, USA, March, 14-18, p. 261-264, 1999
Roo, J. de, Peters, C.J., Lichtenthaler, R.N., Diepen, G.A.M., Occurrence of
methane hydrate in saturated and unsaturated solutions of sodium chloride in dependence of
temperature and pressure, AIChE Journal, 29, 4, p. 651-657, 1983
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