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Gas dehydration

Wet gas — a mixture of a dry gas and water vapor.

Hydrates- crystalline substances formed by associated molecules of hydrogen and water and having a crystalline structure. Natural gas hydrates look like wet pressed snow turning into ice. Having accumulated in the gas pipeline, they can choke or completely block the pipe and cause damage to the system's operating conditions.

When large gas volumes are transported, dehydration is the most efficient and economical means of preventing the hydrate formation in the trunk pipeline. The existing methods for gas dehydration in the field fall into two main groups: absorption (dehydration by liquid media) and adsorption (dehydration by solid media).

The dehydration is aimed at the depression of the water dew point below the minimal temperature that can be expected in the gas pipeline.

Gas dehydration by liquid media is most widely used in the gas industry.

The liquid sorbents used for the dehydration of natural and petroleum gases should have high solubility in water, be low cost, and should show high corrosion resistance. They must also be chemically neutral towards the gas components and, if regeneration is used, they should be easily regenerated, be of low viscosity, and so on.

Diethylene glycol (DEG), triethylene glycol (TEG) and, to a lower extent, monoethylene glycol (MEG) satisfy most of these requirements.

Advantages of dehydration by means of liquid sorbents:

  • relatively small capital expenditure and operating costs;
  • small pressure differentials in the dehydration system;
  • ability to dehydrate gases containing substances poisoning solid sorbents;
  • continuous processes.

Disadvantages of dehydration by means of liquid sorbents:

  • smaller depression of the dew point (compared to solid sorbents);
  • foaming of glycols when the gas contains lighter hydrocarbons.

Glycol dehydration units are of two types: absorption units and units where glycol is injected into the gas stream.

Advantages of the absorption scheme (glycol concentration is 96-99%):

  • minimal glycol losses.

Disadvantages of the absorption scheme:

  • It is difficult to reach the water dew point of the dehydrated gas.

The economical efficiency of the absorption plants depends heavily on the sorbent looses. To reduce these it is necessary in the first place strictly to maintain the designed temperature regime of the desorber, to separate water vapor from the gas carefully, and to use special additives in order to prevent foaming in the gas/absorbent contact area, if possible.

Advantages of the glycol injection scheme (glycol concentration is 70-80%)- the dew point is depressed as the gas cools down. And not only the gas is dehydrated, but also the condensate which has dropped out of the cooled gas.

Disadvantage of the glycol injection scheme- tangible glycol losses due to its high solubility in the hydrocarbon condensate.

In the gas industry, activated alumina oxide and bauxite with a 50—60% Al2O3 content are widely used as solid sorbents. The absorption capacity of bauxite is 4.0—6.5% of its own mass.

Advantages of the method: low water dew point of the dehydrated gas (down to -65°С), simple sorbent regeneration, small space requirement, simple design, low mounting costs.