Chemical Engineering January 2018 - 52

Engineering Practice
Operating Strategies for
Gas Dehydration Units
The presence of certain contaminants can wreak havoc on gas dehydration systems
and other similar separation processes, but there are design measures that can help
to minimize the issues caused by these substances
Krishnan Madan Mohan and
Suman Pachal
WorleyParsons Engineering Oman
T
he
presence of the
aromatic
compounds benzene,
toluene, ethylbenzene and
p-xylene (BTEX) and acid
gases, such as hydrogen sulfide
(H2S) and carbon dioxide (CO2), in
the wet gas of tri-ethylene glycol
(TEG) gas-dehydration units (Figure
1) can result in numerous operating
problems. These problems can
be minimized by optimizing certain
operating parameters and paying
careful attention to details during
the equipment design stage.
This article presents several ways
to minimize operating and maintenance
problems in TEG gas-dehydration
processes.
In TEG-based natural-gas dehydration
units, most operating
and maintenance problems usually
occur when the circulating glycol
becomes contaminated. The contaminated
glycol has a tendency to
cause foaming and fouling. Foaming
can increase glycol loss and reduce
plant capacity. Foaming can also result
in poor mass transfer between
the gas and the glycol solution and
can affect treated-gas quality. Furthermore,
contaminated glycol will
aggravate fouling in heat exchangers.
Fouling in the exchanger for
the lean and rich glycol streams will
result in poor heat transfer, which
in turn will increase reboiler duty
and affect the purity and quality of
lean glycol, and even potentially
cause pump failure. For longterm
trouble-free operation of glycol
units, it is important to eliminate or
minimize the occurrence of foaming
and fouling.
50
Process basics
Figure 1 depicts a
typical layout of a
TEG-based dehydration
unit. After
the removal of oil
and some condensate
from the
wet gas stream,
it is necessary to
remove most of
the associated
water. The free
water associated
with the extracted
natural gas is removed
by simple
separation methods
at production
stations
or
near
the wellhead. The
equilibrium water
vapor that exists
in the natural
gas is removed
by a gas dehydration
process.
Particle filter
Dry gas
Circulation
pump
Contactor
Air cooler
Lean glycol
particle filter
Rich glycol
Gas
HC skimmer
Inlet
scrubber
Wet gas
Gas
Condensate
Flash
vessel
Activated
carbon filter
Lean/rich
exchanger
Glycol booster pump
FIGURE 1. This schematic shows the layout of a typical gas-dehydration unit
The glycol is used as a dehydrating
agent, since it has high chemical affinity
toward water. Typically, a TEG
unit follows these conventions.
Wet gas from the wet-gas separator
is sent to the bottom of the contactor
in the glycol dehydration unit.
Lean and water-free glycol is fed to
the top of the contactor, where it
countercurrently contacts the wet
gas stream flowing from the bottom
to the top of the contactor. The lean
glycol removes water from the natural
gas by physical absorption and will
flow to the bottom of the contactor.
Upon exiting the contactor, the glycol
stream is referred to as " rich glycol. "
The dehydrated gas leaves from the
top of the contactor through the exchanger
and is routed to the hydrocarbon-gas
dewpointing unit.
The rich glycol from the bottom of
the contactor is routed to the TEG
regeneration unit for initial heating
in the glycol reflux-condenser
tube bundle and passes to the glycol
flash vessel, where hydrocarbon
vapors will be flashed off and
liquid hydrocarbons (HC) will be
skimmed from the glycol. This step
is necessary because the contactor
is operated at high pressure, and
the pressure must be reduced for
adequate separation in the regeneration
process. Due to the composition
of the rich glycol, a vapor
phase with high hydrocarbon content
is formed when the pressure
is lowered. The gas-free rich glycol
is filtered through particulate and
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JANUARY 2018
Lean glycol
Reflux coil
Still
column
Reboiler
Stripper
column
Stripping
gas
Basic process flow scheme
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Chemical Engineering January 2018

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