In this chapter you will learn the uses of natural gas, why natural gas is
dehydrated, where dehydration occurs, what triethylene glycol is the dehydration
process and the flow of fluids through a dehydration system.
What is dehydration?
You may think a physical dehydration
where your body has a shortage of water
or it may remind you of dehydrated fruit or meat that has had the water removed.
In natural gas dehydration it is the same idea.
Producers dehydrate gas by removing the water from it.
Natural gas is used for heat, cooking, manufacturing, transportation, and to
generate electricity. However, none of these things would be possible if the
water content of the gas were too high.
Dehydration is used for two primary reasons:
to remove water vapor from natural gas to a level mandated by the purchaser,
and to prevent damage to gas pipelines that result when water accumulates in the system.
Gas dehydration takes place anywhere natural
gas contains water vapor which could cause the quality of the gas to be below
contractual agreements or safety standards.
All three sectors of the oil
and gas industry use dehydration including: upstream, at individual well
locations; midstream, at gathering stations and delivery points; and
downstream, at processing plants.
To take the water out of natural gas
producers use a liquid called triethylene glycol also known as TEG.
When gas comes into contact with the triethylene glycol, the water vapors
entrained in the gas are absorbed in the TEG. In effect, the glycol "soaks up" the water.
The glycol is then removed from the gas stream for processing so that it
can be purified and reused.
There is a lot of equipment needed for the
dehydration process. Natural gas only travels through the inlet scrubber and a contactor tower.
On some sites these two vessels are combined into one unit.
All the other equipment is used to regenerate the glycol.
As the glycol moves through the contactor tower it absorbs the moisture
from the natural gas. At this point it is referred to as either "rich" or "wet" glycol.
Now it must be distilled in order to be reused. The glycol then flows into a
three-phase flash separator which separates the "rich" glycol from the
contaminants, lubrication oil, free water, and condensate. The contaminated liquids
exit and are moved to storage tanks
while the glycol then goes to the reboiler.
The reboilers distillation process removes the water contained in the wet glycol.
Distilling the triethylene glycol in
the reboiler is easily accomplished because of the extreme differences in
boiling points of water and glycol.
Water boils at 212ºF while glycol boils at 550ºF;
However, TEG begins to degrade at 404ºF which is its critical temperature. Due to the degradation
temperature of TEG producers heat the wet glycol between 212ºF and 400ºF.
This allows the water to be boiled off and the glycol to be
distilled to an appropriate level of purity so that it can be reused.
The water flashes off as steam but the glycol remains in the reboiler.
Glycol in this stage is now referred to as "lean" or "dry" glycol.
With the right equipment, the "wet" glycol can be regenerated to more than 99% purity.
With good filtration, this allows producers to reuse it repeatedly around 18-24
months before it needs to be changed. Dehydration units vary in size depending
on gas flow. In a unit this size the flow rate can be a few million cubic feet per day.
A large dehydration system located at a central gathering station, like this one,
can dehydrate hundreds of millions of cubic feet of gas every day.