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PROCESS
DESCRIPTION OF GAS DEHYDRATION
The principle of glycol dehydration is contacting a
natural gas
stream with a hygroscopic liquid which
has a greater affinity for the water
vapor than does the gas. Contactor
pressure is subject to economic
evaluation usually influenced by water
removal duty, required water dewpoint,
vessel diameter and wall thickness.
After contacting the gas, the water-rich
glycol is regenerated by heating at
approximately atmospheric pressure to a
temperature high enough to drive off
virtually all the absorbed water. The
regenerated glycol is then cooled and
recirculated back to the contactor.
Triethylene glycol (TEG) is the most commonly used
dehydration liquid and is the assumed
glycol type in this process description.
Diethylene glycol (DEG) is sometimes
used for uniformity when
hydrate
inhibition is required upstream of
dehydration or due to the greater
solubility of salt in DEG. Tetraethylene
glycol (TREG) is more viscous and more
expensive than the other glycols. The
only real advantage is its lower vapour
pressure which reduces absorber vapor
loss. It should only be considered for
rare cases where glycol dehydration will
be employed on a gas whose temperature
exceeds about 50 °C, such as when
extreme ambient conditions prevent
cooling to a lower temperature.
TEG has been applied downstream of
production facilities that use MEG or
DEG as a hydrate inhibitor without
apparently leading to contamination
problems. Methanol used as a hydrate
inhibitor in the feed gas to a glycol
dehydration unit will be absorbed by the
glycol, and according to the GPSA
Engineering Data Book it can pose the
following problems:
- methanol will add additional reboiler heat duty and still vapor load
and therefore increase glycol losses;
- aqueous methanol causes corrosion of carbon steel. Corrosion can thus
occur in the still and reboiler vapor
space;
- high methanol injection rates and
consequent slug carry-over can cause
flooding.
Where there is upstream hydrate
inhibition, credit should be taken for
any favorable reduction in the water
content of the vapor phase. This effect
is less significant at lower
feed temperatures, i.e. equivalent to
about 2 °C reduction in water dewpoint
at 10 °C feed temperature at 9 MPa
pressure and 60 percent by weight MEG in
the aqueous phase.
Adherence to the recommendations in this
DEP can minimize but not eliminate
entrainment and vapor losses of glycol.
Glycol entrainment may lead to the
following downstream problems:
- coalescing and partial condensation in pipelines resulting
in localised corrosion;
- in cryogenic plants, particularly at temperatures below -25
°C, freezing of TEG and plugging of
equipment;
- reduced performance of downstream adsorption plant, e.g.
molecular sieves or silica gel.
Any entrained glycol should be removed
upstream of cryogenic plant in high
efficiency gas/liquid separators to
prevent possible plugging. A range of
lean TEG concentrations can be achieved
with the basic regeneration flow.
a Simple
Diagram of Natural Gas Dehydration Using
TEG
References:
1. Gas Dehydration Field Manual, Maurice Stewart & Ken Arnold
2. Gas Dehydration by TEG and Hydrate
Inhibition Systems, Arthur William
3. Fundamentals of Natural Gas, Arthur J. Kidnay & William R. Parrish |
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