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7/29/2019 C-18.C-Gas Dehydration.pdf
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18C-1
The Lease Pumper’s Handbook
Chapter 18
Gas Wells
Section C
GAS DEHYDRATION
Figure 1. An glycol dehydration unit.
C-1. Gas Dehydration.
Gas, as it leaves the separator, will
probably be saturated with distillate and
water vapor. Gas in this condition is
referred to as wet or rich gas. The purpose of
the dehydration unit (Figure 1) is to reduce
the level of the water and distillate
remaining in the gas. After these vapors
have been removed, it is referred to as dry or
lean gas. The field glycol dehydration unithas other popular names such as the stack
pack or thermo pack . As the gas leaves the
dehydrator, it will still contain some
moisture and distillate, but it will have a
lower dewpoint , the temperature at which
the gas will form condensation.
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C-2. Operating the Dehydration Unit.
The dehydrator is part of the separator unit
(Figure 2) and may use either ethylene
glycol or tri-ethylene. In the followingparagraphs, glycol is referred to but the same
principles apply to a system using tri-
ethylene.
Figure 2. A separator unit with
dehydration unit, a knee tub, and the
water and distillate tank battery.
The inlet scrubber. The two-phase inlet
scrubber (Figure 3) is the first vessel in the
dehydration unit that the wet gas enters. The
wet gas is divided into drier gas and liquid.
The gas is diverted into a circular action,
passes up through a stainless steel wire mesh
mist extractor, then flows on toward the
contact tower. The condensate (and water)
that is stripped out of the gas is dumped to
the distillate stock tank.
The contact tower. The contact tower
(Figure 3) is the vessel that is designed to
dry the gas. The gas enters the contact towernear the bottom through a chimney tray. The
gas works its way up through bubble-type
trays filled with ethylene glycol, which has a
natural attraction to water. The glycol
absorbs the water contained in the gas, and
the gas passes up and out the top of the
tower.
Figure 3. Note the inlet scrubber and
contact tower in this view of a
dehydration unit.
As the gas returns to ground level through
the down-comer line, it passes through the
glycol-gas heat exchanger where the
outgoing gas cools the glycol coming into
the contactor. The dry gas leaves the
dehydration unit to be compressed and
measured as it leaves the location.
The glycol pump. Glycol is pumped into
the contact tower by a dual-purpose pump
(such as the Kimray glycol energy exchangepump), which moves glycol up through the
cooler or heat exchanger and into the top of
the contact tower. The glycol trickles down
through the contact trays where it collects
water and condensate and flows to the
bottom of the contact tower.
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The dual-action pumps. The dual-action
glycol pump pulls the wet glycol out of the
bottom of the contact tower through a high-
pressure strainer, where it is pumped along
with a little gas through the bottom sectionof the reboiler.
The heat exchanger surge tank. The wet
glycol is circulated through the heat
exchanger surge tank and into a three-phase
gas, glycol, and condensate separator.
The three-phase gas, glycol, and
condensate separator. As the fluid enters
the end of the three-phase separator, the
fluid stratifies into three layers. The gas
comes to the top and fuels the reboiler and
supplies it with stripper gas.
The second layer formed is the condensate,
which is controlled by an indiscriminate
float. The float directs the fluid into a tank
for distillate and condensate at the tank
battery. The water-laden glycol is directed
toward the reboiler for water removal.
The reboiler. The purpose of the reboiler isto boil off the water that was removed from
the contact tower but leave the glycol. This
is done by raising the temperature to a level
that will cause the water to evaporate but
which is below the boiling point of glycol.
Figure 4. A small reboiler.
The reboiler operates at a temperature of
approximately 350° F. If the reboiler has a
back-up temperature control, this second
control is set 20° higher. Water boils at
212° F, but, if it is under pressure orcontains any contaminants, it requires more
heat. Glycol boils at a temperature higher
than this temperature setting.
The water vapor rises in the stripping still
and condenser. It trickles down an angled
pipe and is collected in a foot tub (Figure 5).
This tube has to be insulated to prevent the
water from freezing in cold weather.
Figure 5. A water collection tub.
The water collects in these short, small
volume tanks, and vacuum trucks come by
periodically to remove the accumulated
water to prevent overflow.
C-3. Tank Batteries for Gas Wells.
Tank batteries for gas wells, as a rule, are
less imposing than tank batteries for oil
wells. Most gas wells will produce distillate
and water. Distillate has such a high API
gravity that these two fluids flash separate
almost instantly. Since they separate usually
in a matter of seconds, no treating vessels
are necessary. This means that there is no
need for heater/treaters, gun barrels, free
water knockouts, or flow splitters.
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There are, however, a host of support
vessels and equipment designed just to take
care of the needs of gas wells. The tank
battery for a gas well (Figure 6) is basically
just like any other tank battery with oneexception. Condensate has a very high
gravity and acts as a penetrating liquid so
that seep leaks that lose a small amount of
liquid may develop unless the fittings are
made up properly. With time, the installation
becomes stained and looks bad. Because the
produced fluids will be a little different from
each well, the vessels to be considered in
new construction are the two- and three-
phase separator, the condensate storage tank,
the water disposal tank, and possibly a
chemical injector.
Figure 6. A gas well tank battery. Note
the two loading lines — one for condensate
and one for water.
The dehydration unit has either one or two
small separators that produce condensate,
according to how much is contained in thegas. The small tank shown in Figure 7 was
set up just to serve the needs of a
dehydration unit because for fire safety, the
tank battery was a long distance away.
Because of the high gravity of the distillate
in the tank battery, the tank vent valve is of considerable importance. Evaporation out
of this vessel can be so great that in the heat
of summer, the evaporation can be several
barrels a day. A higher back pressure may
be required, and if smaller loads or split
loads are sold, income from the distillate
may be dramatically increased. Careful
accounting and measurement will indicate
the lease needs in sales of liquids.
Figure 7. A small tank used to store
condensates.