c-18.c-gas dehydration.pdf

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.



18C-2

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.



18C-3

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.



18C-4

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.

c-18.c-gas dehydration.pdf

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