Glycol dehydration is a liquid desiccant system for the removal of water from
natural gas and natural gas liquids. It is the most common and economical means
of water removal from these streams. Glycols typically seen in industry
include triethylene glycol, diethylene glycol, ethylene glycol, and
tetraethylene glycol. TEG is the most commonly used glycol in industry.
Purpose The purpose of a glycol dehydration unit
is to remove water from natural gas and natural gas liquids. When produced from
a reservoir, natural gas usually contains a large amount of water and is
typically completely saturated or at the water dew point. This water can cause
several problems for downstream processes and equipment. At low
temperatures the water can either freeze in piping or, as is more commonly the
case, form hydrates with CO2 and hydrocarbons. Depending on composition,
these hydrates can form at relatively high temperatures plugging equipment and
piping. Glycol dehydration units depress the hydrate formation point of the gas
through water removal. Without dehydration, a free water phase
could also drop out of the natural gas as it is either cooled or the pressure
is lowered through equipment and piping. This free water phase will often contain
some portions of acid gas and can cause corrosion.
For the above two reasons the Gas Processors Association sets out a
pipeline quality specification for gas that the water content should not exceed
7 pounds per million standard cubic feet . Glycol dehydration units must
typically meet this specification at a minimum, although further removal may be
required if additional hydrate formation temperature depression is required, such
as upstream of a cryogenic process or gas plant.
Process description Lean, water-free glycol is fed to the
top of an absorber where it is contacted with the wet natural gas stream. The
glycol removes water from the natural gas by physical absorption and is
carried out the bottom of the column. Upon exiting the absorber the glycol
stream is often referred to as "rich glycol". The dry natural gas leaves the
top of the absorption column and is fed either to a pipeline system or to a gas
plant. Glycol absorbers can be either tray columns or packed columns.
After leaving the absorber, the rich glycol is fed to a flash vessel where
hydrocarbon vapors are removed and any liquid hydrocarbons are skimmed from the
glycol. This step is necessary as the absorber is typically operated at high
pressure and the pressure must be reduced before the regeneration step.
Due to the composition of the rich glycol, a vapor phase having a high
hydrocarbon content will form when the pressure is lowered.
After leaving the flash vessel, the rich glycol is heated in a cross-exchanger
and fed to the stripper. The glycol stripper consists of a column, an
overhead condenser, and a reboiler. The glycol is thermally regenerated to
remove excess water and regain the high glycol purity.
The hot, lean glycol is cooled by cross-exchange with rich glycol entering
the stripper. It is then fed to a lean pump where its pressure is elevated to
that of the glycol absorber. The lean solvent is cooled again with a trim
cooler before being fed back into the absorber. This trim cooler can either be
a cross-exchanger with the dry gas leaving the absorber or an air-cooled
exchanger. Enhanced Stripping Methods
Most glycol units are fairly uniform except for the regeneration step.
Several methods are used to enhance the stripping of the glycol to higher
purities. Since the reboiler temperature is limited to 400F or less to prevent
thermal degradation of the glycol, almost all of the enhanced systems
center on lowering the partial pressure of water in the system to increase
stripping. Common enhanced methods include the use
of stripping gas, the use of a vacuum system, the DRIZO process, which is
similar to the use of stripping gas but uses a recoverable hydrocarbon solvent,
and the Coldfinger process where the vapors in the reboiler are partially
condensed and drawn out separately from the bulk liquid.
References External links
Gas Processors Suppliers Association Website
Hernendez, Hlavinka, and Bullin, "Design Glycol Units for Maximum Efficiency" An
excellent discussion on the design details of glycol units
Comments from the Environmental Protection Agency regarding natural gas
dehydration best-practices Practical oil-field oriented description
of Glycol Dehydration including Operating problems and Glycol care