It is a result of the hydrogen bond that water can form hydrates. The hydrogen bond causes the water molecules to align in regular orientations. The presence of certain compounds causes the aligned molecules to stabilize and a solid mixture precipitates.
The water molecules are referred to as the “host” molecules, and the other compounds, which stabilize the crystal, are called the “guest” molecules, the guest molecules are more often referred to as “formers.” The hydrate crystals have complex, three-dimensional structures where the water
molecules form a cage and the guest molecules are entrapped in the cages.
The stabilization resulting from the guest molecule is postulated to be due to van der Waals forces, which is the attraction between molecules that is not because of electrostatic attraction. As described earlier, the hydrogen bond is different from the van der Waals force because it is due to strong electrostatic attraction, although some classify the hydrogen bond as a van der Waals force.
Another interesting thing about gas hydrates is that there is no bonding between the guest and host molecules. The guest molecules are free to rotate inside the cages built up from the host molecules. This rotation has been measured by spectroscopic means. Therefore, these compounds are best described as a solid solution.
The formation of a hydrate requires three conditions:
The exact temperature and pressure depends on the composition of the gas. However, hydrates form at temperatures greater than 0°C (32°F), the freezing point of water.
To prevent hydrate formation, one merely has to eliminate one of the three conditions stated previously. Typically, we cannot remove the hydrate formers from the mixture. In the case of natural gas, the hydrate formers are the desired product. Therefore, we attack hydrates by addressing the other two considerations.
Other phenomena that enhance hydrate formation include:
Hydrate formation is favored in regions where the fluid velocity is high. This makes choke valves particularly susceptible to hydrate formation. First, there is usually a significant temperature drop when natural gas is choked through a valve due to the Joule-Thomson effect. Second, the velocity is high through the narrowing in the valve.
In general terms, a nucleation site is a point where a phase transition is favored, and in this case the formation of a solid from a fluid phase. An example of nucleation is the deep fryer used to make French fries in fast-food restaurants throughout the world. In the fryer, the oil is very hot but does not undergo the full rolling boil because there are no suitable nucleation sites. However, when the potatoes are placed in the oil, it vigorously boils. The French fries provide an excellent nucleation site.
3.Free-Water
No, this is not a contradiction to earlier statements. Free-water is not necessary for hydrate formation, but the presence of free-water certainly enhances hydrate formation.
In addition, the water-gas interface is a good nucleation site for hydrate formation. The items in the previous list enhance the formation of a hydrate, but are not necessary. Only the three conditions given earlier are necessary for hydrate formation.
Another important aspect of hydrate formation is the accumulation of the solid. The hydrate does not necessarily agglomerate in the same location as it is formed. In a pipeline, the hydrate can flow with the fluid phase, especially the liquid. It would tend to accumulate in the same location as the liquid does.
Usually, the accumulations of the hydrates cause the problems. In a multiphase pipeline, the accumulations block line and plug and damage equipment.
Often, pigging is sufficient to remove the hydrate from the pipeline. Pigging is the process of inserting a tool (called a “pig”) into the line. Modern pigs have many functions, but the main one remains pipeline cleaning. The pig fits tightly into the line and scrapes the inside of the pipe. It is transported along the line with the flow of the fluid, and by doing so removes any solids (hydrate, wax, dirt, etc.) from inside the line. The pigging can also be used to remove accumulations of liquids.
However, the pigging must be scheduled such that the accumulations of hydrates do not become problematic. Usually, pigging is not used to clean hydrates from a line. Other measures are more commonly used to deal with hydrates. Another benefit of pigging is the removal of salt, scale, etc., which is important for the proper operation of a pipeline. It also means that potential nucleation sites for hydrate formation are removed.
The water molecules are referred to as the “host” molecules, and the other compounds, which stabilize the crystal, are called the “guest” molecules, the guest molecules are more often referred to as “formers.” The hydrate crystals have complex, three-dimensional structures where the water
molecules form a cage and the guest molecules are entrapped in the cages.
The stabilization resulting from the guest molecule is postulated to be due to van der Waals forces, which is the attraction between molecules that is not because of electrostatic attraction. As described earlier, the hydrogen bond is different from the van der Waals force because it is due to strong electrostatic attraction, although some classify the hydrogen bond as a van der Waals force.
Another interesting thing about gas hydrates is that there is no bonding between the guest and host molecules. The guest molecules are free to rotate inside the cages built up from the host molecules. This rotation has been measured by spectroscopic means. Therefore, these compounds are best described as a solid solution.
The formation of a hydrate requires three conditions:
- The right combination of temperature and pressure. Hydrate formation is favored by low temperature and high pressure.
- A hydrate former must be present. Hydrate formers include methane, ethane, and carbon dioxide.
- A sufficient amount of water – not too much, not too little.
The exact temperature and pressure depends on the composition of the gas. However, hydrates form at temperatures greater than 0°C (32°F), the freezing point of water.
To prevent hydrate formation, one merely has to eliminate one of the three conditions stated previously. Typically, we cannot remove the hydrate formers from the mixture. In the case of natural gas, the hydrate formers are the desired product. Therefore, we attack hydrates by addressing the other two considerations.
Other phenomena that enhance hydrate formation include:
- Turbulence
- High Velocity
Hydrate formation is favored in regions where the fluid velocity is high. This makes choke valves particularly susceptible to hydrate formation. First, there is usually a significant temperature drop when natural gas is choked through a valve due to the Joule-Thomson effect. Second, the velocity is high through the narrowing in the valve.
- Agitation
Mixing in a pipeline, process vessel, heat exchanger, etc. enhances hydrate formation.
2. Nucleation Sites
3.Free-Water
No, this is not a contradiction to earlier statements. Free-water is not necessary for hydrate formation, but the presence of free-water certainly enhances hydrate formation.
In addition, the water-gas interface is a good nucleation site for hydrate formation. The items in the previous list enhance the formation of a hydrate, but are not necessary. Only the three conditions given earlier are necessary for hydrate formation.
Another important aspect of hydrate formation is the accumulation of the solid. The hydrate does not necessarily agglomerate in the same location as it is formed. In a pipeline, the hydrate can flow with the fluid phase, especially the liquid. It would tend to accumulate in the same location as the liquid does.
Usually, the accumulations of the hydrates cause the problems. In a multiphase pipeline, the accumulations block line and plug and damage equipment.
Often, pigging is sufficient to remove the hydrate from the pipeline. Pigging is the process of inserting a tool (called a “pig”) into the line. Modern pigs have many functions, but the main one remains pipeline cleaning. The pig fits tightly into the line and scrapes the inside of the pipe. It is transported along the line with the flow of the fluid, and by doing so removes any solids (hydrate, wax, dirt, etc.) from inside the line. The pigging can also be used to remove accumulations of liquids.
However, the pigging must be scheduled such that the accumulations of hydrates do not become problematic. Usually, pigging is not used to clean hydrates from a line. Other measures are more commonly used to deal with hydrates. Another benefit of pigging is the removal of salt, scale, etc., which is important for the proper operation of a pipeline. It also means that potential nucleation sites for hydrate formation are removed.
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