The commonly available glycols and their uses are:
1. Ethylene glycol—High vapor equilibrium with gas so tend to lose to gas phase in contactor. Use as hydrate inhibitor where it can be recovered from gas by separation at temperatures below 50° F.
2. Diethylene glycol—High vapor pressure leads to high losses in contactor. Low decomposition temperature requires low reconcentrator temperature (315°F to 340°F) and thus cannot get pure enough for most applications.
3. Triethylene glycol—Most common. Reconcentrate at 340°F to 400°F for high purity. At contactor temperatures in excess of 120°F tends to have high vapor losses to gas. Dew point depressions up to
150°F are possible with stripping gas.
4. Tetraethylene glycol—More expensive than triethylene but less losses at high gas contact temperatures. Reconcentrate at 400°F to 430°F.
Almost all field gas dehydration units use triethylene glycol for the reasons indicated. Normally when field personnel refer to “glycol” they mean triethylene glycol and we will use that convention in the remainder of this chapter.