Download Glycol Dehydrator Design Manual - 1976 PDF

TitleGlycol Dehydrator Design Manual - 1976
TagsHeat Exchanger Pump Water Gases
File Size6.3 MB
Total Pages36
Document Text Contents
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GLYCOL DEHYDRATOR DESIGN MANUAL
By: C, Richard Sivalls; SIVALLS TANKS, INC.; Odessa, Texas
1976 LAURANCE REID GAS CONDITIONING CONFERENCE


The dehydration of natural gas is defined as the removal of the water that is associated
with the natural gas in the vapor form. It has long been recognized that the dehydration
of natural gases is necessary to ensure efficient operation of gas transmission lines. The
removal of the water vapor prevents the formation of gas hydrates and reduces corrosion
in the pipelines. It also improves the efficiency of the pipelines by reducing liquid
accumulations at low spots in the lines. One of the most popular methods of dehydration
of natural gas now in use is an absorption process employing diethylene or triethylene
glycol as the desiccant. In recent years triethylene glycol has emerged as the most
popular chemical to be used due to its high affinity for the water vapor and other
desirable properties such as non-corrosiveness, ease of regeneration and low chemical
losses.

DESCRIPTION OF PROCESS
For the following description of the process and flow through a typical glycol dehydrator
refer to Figure No. 1. The wet inlet gas stream first enters the unit through an inlet gas
scrubber where any liquid accumulations are removed. A 2-phase or distillate-gas
scrubber is illustrated in Figure No. 1. If any liquid water is in the gas stream, a three-
phase scrubber may be used to discharge the distillate and water from the vessel
separately. The mist eliminator aids in removing any entrained liquid particles from the
wet gas stream leaving the top of the inlet scrubber.

The wet gas then enters the bottom of the glycol-gas contractor and flows upward
through the trays as illustrated countercurrent to the glycol flowing downward through
the column. The gas contacts the glycol on each tray and the glycol absorbs the water
vapor from the gas stream.

The dry gas leaves the top of the contactor vessel through another mist eliminator which
aids in removing any entrained glycol droplets from the gas stream. The gas then flows
down through a vertical glycol cooler, usually fabricated in the form of a concentric pipe
heat exchanger, where the outlet dry gas aids in cooling the hot regenerated glycol before
it enters the contactor. The dry gas then leaves the unit from the bottom of the glycol
cooler.

The dry glycol enters the top of the glycol-gas contactor from the glycol cooler and is
injected onto the top tray. The glycol flows across each tray and down through a
downcomer pipe onto the next tray. The bottom tray downcomer is fitted with a seal pot
to hold a liquid seal on the trays.

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The wet glycol which has now absorbed the water vapor from the gas stream leaves the
bottom of the glycol-gas contactor column, passes through a high pressure glycol filter
which removes any foreign solid particles that may have been picked up from the gas
stream, and enters the power side of the glycol pump. In the glycol pump the wet high
pressure glycol from the contactor column is used to pump the dry regenerated glycol
into the column. The wet glycol stream flows from the glycol pump to the inlet of the
flash separator. The low pressure flash separator allows for the release of the entrained
solution gas which has to be used with the wet glycol to pump the dry glycol into the
contactor. The gas separated in the flash separator leaves the top of the flash separator
vessel and may be used to supplement the fuel gas required for the reboiler. Any excess
vent gas is discharged through a back pressure valve.

The flash separator is equipped with a liquid level control and diaphragm motor valve
which discharges the wet glycol stream through a heat exchange coil in the surge tank to
preheat the wet glycol stream. If the wet glycol stream absorbs any liquid hydrocarbons
in the contactor, it may be desirable to use a three phase flash separator to separate the
glycol from the liquid hydrocarbons before the stream enters the reboiler. Any liquid
hydrocarbons present in the reboiler will cause undue glycol losses from the stripping
still.

The wet glycol stream leaves the heat exchange coil in the surge tank and enters the
stripping still mounted on top of the reboiler at the feed point in the still. The stripping
still is packed with a ceramic Intalox saddle type packing and the glycol flows downward
through the column and enters the reboiler. The wet glycol passing downward through
the still is contacted by hot rising glycol and water vapors passing upward through the
column. The water vapors released in the reboiler and stripped from the glycol in the
stripping still pass upward through the still column through an atmospheric reflux
condenser which provides a partial reflux for the column. The water vapor then leaves
the top of the stripping still column and is released to the atmosphere.

The glycol flows through the reboiler in essentially a horizontal path from the stripping
still column to the opposite end. In the reboiler the glycol is heated to approximately 350
- 400 °F to remove enough water vapor to re-concentrate it to 99.5% by weight or more.
In field dehydration units the reboiler is generally equipped with a direct fired firebox
(reboiler) using a portion of the natural gas stream for fuel. In plant type units the
reboiler may be fitted with a hot oil heated coil or steam coil. A temperature control in
the reboiler operates a fuel gas motor valve to maintain the proper temperature in the
glycol reboiler. The reboiler is also generally equipped with a high temperature safety
overriding temperature controller to shut down the fuel gas system in case the primary
temperature control should malfunction.

In order to provide extra dry glycol, 99% by weight plus, it is usually necessary to add
some dry stripping gas to the reboiler. A valve and small pressure regulator are generally
provided to take a small amount of gas from the fuel gas system and inject it into the

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