As shown in the figure below, the two common forms of gas hydrates are known as structures-I and II, which have been investigated with X-ray diffraction methods by von Stackelberg and Müller (1954). They found that the unit cell of structure-I (sI) is a 12Å cube consisting of 46 water molecules which has two types of cavities. The two small cavities are pentagonal dodecahedra (512), whereas the six large cavities are tetradecanhedra (51262) having two opposite hexagonal faces and twelve pentagonal faces. The smaller cavities are almost spherical, whereas the larger cavities of structure-I are slightly oblate. The unit cell of structure-II (sII), which is a 17.3 Å cube with 136 water molecules, also contains two types of cavities. The 16 smaller…show more content… The right combination of temperature and pressure. Hydrate formation is favoured by low temperature and high pressure.
2. A hydrate former must be present. Hydrate formers include: methane, ethane, and carbon dioxide.
3. A sufficient amount of water not too much, not too little
Figure 2.9 gives a visual of the three criteria for hydrate formation. The three are interconnected, and the absence of any one will prevent hydrate formation. Figure 2.9: Simplified Diagram of the Three Criteria for Hydrate Formation.
Low temperature and high pressure favour hydrate formation. The exact temperature and pressure depends upon the composition of the gas. However, hydrates form at temperatures greater than 0oC (32oF); the freezing point of water. The hydrate formation curve defines the temperature and pressure envelope in which the entire subsea hydrocarbons system must operate in at steady state and transient conditions in order to avoid the possibility of hydrate formation. Figure 2.10 shows the hydrate formation curve. Figure 2.10: Hydrate formation and dissociation…show more content… To the left of hydrate formation curve is the region where hydrates are thermodynamically stable and have the potential to form. It was also noted that the stability of hydrates increase with increasing pressure and decreasing temperature.
The sub-cooling of a system as often used when discussing gas hydrates, is defined as the difference between hydrate stability temperature (thermodynamic equilibrium temperature) and the actual operating temperature inside the hydrate region (for instance, seabed temperature) at the same pressure (P. Glenat, SPE, P. Bourg and M-L Bousque, Total S.A, 2013). If the system is operating at 40oF and 3000 psi, the hydrate dissociation temperature is 70oF, the system is experiencing 30oF of sub-cooling.
The sub-cooling of a system without hydrate formation leads to an area between the hydrate formation temperature and the hydrate dissociation temperature, called the metastable region where hydrate is not stable.
Looking at the work done by other researcher, (Estefen et al. 2005) explains the conditions that favour hydrate formation with the solid formation phase diagram in the pressure - temperature plane as shown in figure