- Hydrates are ice-like solids that form when:
- a sufficient amount of water is present
- a hydrate former is present
(more on which components are hydrate formers later) - the right combination of temperature and pressure
(hydrate formation is favored by low temperature and high pressure) - Hydrates are notorious for forming at conditions where a solid would not otherwise be expected.
- Water is called the "host" and it forms a hydrogen-bonded lattice; a three-dimensional cage-like structure.
- The hydrate former, called the "guest", enters the lattice and stabilizes it. The stabilized lattice precipitates as a solid.
- The nature of the equilibrium in the hydrate region depends upon the amount of water present:
- A large amount of water means the equilibrium is between water and the hydrate.
- A small amount of water means the equilibrium is between a gas and the hydrate.
- If there is an extreme amount of water, then the hydrate does not form at all, even though the conditions are in the "hydrate" region.
- If the mixture is very lean in water, no hydrate forms even though the conditions are in the "hydrate" region.
- Free-water need NOT be present for a hydrate to form!
It is a commonly held misconception that free water is required
for hydrate formation. This misconception is especially common in
the natural gas business. It is interesting to note that
the latest edition of the
GPSA Engineering Data Book
11th edition, (1998)
has been corrected in this regard.
- Hydrates are basically of three types called Type I, Type II, and Type H. Other types of hydrates are known and proposed, but they are uncommon. The crystal structures of hydrates are three-dimensional and are quite complicated. The links on the hydrates links will take you to web pages showing the crystal structure. Beware! some of these pages show incorrect structures. The crystal structure of the hydrate, be it Typr I, II or H, is not a single polyhedra.
- It is usually smaller molecules form Type I hydrates. Type I hydrate formers include: (1) methane, (2) ethane, (3) carbon dioxide, and (4) hydrogen sulfide
- Type I hydrates are made up of 8 polyhedral cages -- 6 large ones and 2 small. They are made up of 46 water molecules and thus have a theoretical composition of 8X · 46 H2O or X · 5 3/4 H2O, where X is the guest molecule.
- It is usually larger molecules form Type II hydrates. Type II hydrate formers include: (1) propane and (2) isobutane, however, (3) nitrogen, a relatively small molecule, also forms a Type II hydrate.
- Type II hydrates are made up of 24 polyhedral cages -- 8 large ones and 16 small. They are made up of 136 water molecules and thus have a theoretical composition of 24 X · 136 H2O or X · 5 2/3 H2O. If only the large cages are occupied, which is typical, then the theoretical composition is 8 X · 136 H2O or X · 17 H2O.
- Type H hydrates are formed by larger molecules but only in the presence of a smaller molecule, such as methane. Type H hydrates only form in the presence of both the large and small molecules.
- Type H hydrates are made up of six polyhedral cages - 1 large, 3 medium and 2 small. The large molecule occupies the large cage and the small molecule occupies the small and medium cages. They are made up of 34 water molecules and have a theoretical composition of X · 5 Y · 34 H2O where X is the large molecule and Y is the small.
- Type H formers include: (1) 2-methylbutane, (2) methylcyclopentane, (3) methylcyclohexane, and (4) cyclooctane. It bears repeating, Type H hydrates only form if another, small molecule (such as methane) is present.
- Hydrates are non-stoichiometric. A stable hydrate forms without all of the cages being occupied. The degree of saturation (the number occupied cages) is a function of the temperature and the pressure.
- For example, hydrogen sulfide forms a hydrate at 10°C and 290 kPa
(50°F and 42 psia). At these conditions, the large cages are
98.1% full and the small ones only 93.8%.
As a second example, carbon dioxide forms a hydrate at 5°C
and 2230 kPa (41°F and 323 psia).
At this temperature and pressure,
the large cages are 98.6% full and the small ones only 78.2%.
(Calculated with CSMHYD).
- Normal butane is an interesting, special case. On its own it does not form a hydrate, but in a mixture it can enter the hydrate if other hydrate formers are present.
- Molecules larger than n-butane do not form Type I or II hydrates. Some slightly larger molecules do form Type H hydrates.
- Highly soluble gases do not form hydrates, regardless of their size. For example, ammonia and hydrogen chloride do not form hydrates.
- Hydrates can form with either gases or liquids, provided the three criteria given earlier are met. There is a bit of a misconception that liquids cannot form hydrates. This misconception is partially fed by the use of the term "gas hydrates".
- The figure below shows the hydrate loci for several
components in natural gas.
- At temperatures less than the loci and at pressure
greater than the loci (i.e., to the left and above) are
where hydrates will form.
- For example, at 5°C and 1 MPa, hydrogen sulfide,
ethane and propane
form hydrates, whereas carbon dioxide, methane, and isobutane do not.
Fig. 1 Hydrate Loci for Several Components Found in Natural Gas
- The extension to mixtures is not obvious from this diagram.
Other methods should be used for estimating the
hydrate forming conditions for mixtures.
- Conventional wisdom has been that hydrogen is too small
to form a hydrate.
- Recent work by Mao, W.L. et al., Science, 297,
2247-2249, (2002) indicate that hydrogen can indeed form a hydrate.
- The formation of a hydrogen hydrate was observed to take
place at relatively high pressures.
- Another unusual feature of the hydrogen hydrate is that there
is multiple occupancy of the wate cages. Hydrogen forms a Type II
hydrate with two hydrogen molecuiles in the small cages and four
in the large.
- The formation of hydrates can be inhibited in a manner similar to de-icing. Note, they do not prevent the formation of hydrates, they inhibit it. That is, for a given pressure, they reduce the temperature at which the hydrate will form.
- The mere presence of an inhibitor does not assure that a hydrate will not form. There has to be a sufficient amount of inhibitor present.
- Common inhibitors include: (1) alcohols, (2) glycols, and (3) ionic salts (including common salt).
- There are three ways to combat the formation of hydrates:
- The use of inhibitors, particularly methanol
- The application of heat
- Dehydration - removing enough of the water from the stream such that a hydrate will not form.
- In the natural gas industry, methanol is usually the method of choice. High pressure pumps are used to inject methanol into process lines and equipment.
- Methanol costs required to combat hydrates in typical
gas fields may be tens of millions of dollars annually
- The Hammerschmidt equation provides a method for rapidly
estimating the temperature depression in the hydrate formation
due to the presence of an inhibitor:
dT = 2335W/(100M - MW) dT = temperature depression, deg F W = weight per cent inhibitor M = molar mass (molecular weight) of inhibitor (g/mol) = 32.043 for methanolFor example, a 25 wt% solution of methanol will effect the following temperature depression:dT = 2335(25)/[(100)(32.043) - (32.043)(25)] = 24.3 deg F = 13.5 deg CTherefore you would expect a hydrate to form at a temperature about 24 Fahrenheit degrees (or 13.5 Celsius degrees) less than in pure water. (Note: this is a temperature difference, so you do not merely convert 24.3°F to -4.8°C) - From the diagram shown earlier, at 1.5 MPa
the ethane hydrate forms at
about 10°C.
From the Hammerschmidt equation the hydrate formation
temperature at 1.5 MPa in a 25 wt% solution of methanol
would be about -3.5°C.
- The Hammerschmidt equation is limited to inhibitor concentrations of about 25 wt%. Errors on the order of 5 to 10% can be expected when using this equation.
- The following are the properties of the methane hydrate:
- density: 913 kg/m³
- molar mass (molecular weight): 17.74 kg/kmol
- methane concentration: 14.1 mole per cent
- this means there are 141 molecules of methane per 859 molecules of water in the methane hydrate
- From this information we can determine the volume of gas in
the methane hydrate.
- From the density, 1 m³ of hydrate has a mass of 913 kg.
- Converting this to moles 913/17.74 = 51.45 kmole of hydrate, of which
7.257 kmoles are methane.
- The ideal gas law can be used to
calculate the volume of gas when expanded to standard conditions
(15°C and 1 atm or 101.325 kPa)
- V = nRT/P = (7.257)(8.314)(15+273)/101.325 =
171.5 m³[std]
- V = nRT/P = (7.257)(8.314)(15+273)/101.325 =
171.5 m³[std]
- Therefore 1 m³ of hydrate contains about 170 m³[std] of
methane gas.
- For those who prefer American Engineering Units, this converts to
1 ft³
of hydrate contains 170 SCF of gas - not a difficult conversion.
And 1 ft³ of hydrate weighs about 57.0 lb.
- By comparison, 1 m³ of liquid methane (at its boiling point
111.7 K or -161.5°C) contains
26.33 kmol, which converts to 622 m³ of gas at standard conditions.
- Alternatively, 1 m³ compressed methane at 7 MPa and 300 K (27°C)
(1015 psia and 80°F)
contains 3.15 kmol or 74.4 m³[std] of methane gas.
- To store 25,000 m³[std] (0.88 MMSCF) of methane requires
about 150 m³ (5300 ft³) of hydrates. This compares with
40 m³ (1400 ft³) of liquefied methane or 335 m³
(11,900 ft³) of compressed methane.
- Methane properties from: Wagner, W. and K.M. de Reuck,
Methane Thermodynamic
Tables of the Fluid State - 13, Blackwell Science, London, (1996).
