GLYCOL-DEHYDRATION refinery engineering

1. GLYCOL-DEHYDRATION

2. OUTLINES
• INTRODUCTION
• LIQUID DESICCANT (ABSORBTION)
• GLYCOLS SUITABLE FOR COMMERCIAL APPLICATION
• PURPOSE OF A GLYCOL DEHYDRATION UNIT
• PROCESS - DESCRIPTION
• GLYCOL DEHYDRATION UNIT (DIAGRAM)

3. INTRODUCTION
-Glycol, any of a class of organic compounds belonging to
the alcohol family.
-The organic compound known as glycol that have approximate
properties which meets the commercial application criteria.
-Glycols have a higher boiling point than water and a low vapor
pressure.
-Glycols will, however decompose at elevated temperatures.

4. LIQUID DESICCANT(ABSORBTION)
• A desiccant refers to any substance that has a high affinity for
water and is used as a drying agent.
• A liquid desiccant is simply a liquid that has a high affinity for
water (naturally absorbs moisture from the air) and is used as a
drying agent.
• The liquid desiccant should not chemically react with any of
the natural gas constituents, including carbon dioxide and
sulfur compounds.
• The absorbing liquid should be highly hygroscopic.

5. GLYCOLS SUITABLE FOR COMMERCIAL
APPLICATION
1-Mono ethylene glycol (MEG)
( C2H6O2 )
2-Di ethylene glycol (DEG)
(C4H10O3 )
3-Tri ethylene glycol (TEG)
(C6H14O4 )
4-Tetra ethylene glycol (TREG)
(C8H18O5 )

7. “TEG” is by far the most widely method used in
natural gas dehydration. It exhibits most of the
desirable characteristics that are required to
absorb the water from the wet gas.

8. The main reason for removing water vapor
from natural gas is that
• Hydrates formation
• Corrosion
• decreases the heating value of natural gas

9. PURPOSE OF A GLYCOL DEHYDRATION UNIT
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 (mainly methane hydrates). Glycol
dehydration units depress the hydrate formation point
of the gas through water removal.

10. Hydrates formation

11. Slugging

12. Corrosion

13. Water Content of Natural Gas
Streams
• Water content of untreated natural gases is
normally in the magnitude of a few hundred
pounds of water per million standard cubic
foot of gas (lbm/MMscf);
• while gas pipelines normally require water
content to be in the range of 4-7 lbm/MMscf
and even lower for pipelines in deep water.

14. PROCESS - DESCRIPTION
• Lean, water-free glycol (purity >99%) is fed to the top of an absorber (also known
as a "glycol contactor") 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 (also known as a regenerator).
• 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.

17. PROCESS - DESCRIPTION
• In glycol dehydration of natural gas:
• Absorber:
– Temperature: Typically 25°C to 60°C [1].
– Pressure: Ranges from atmospheric pressure to around 1500 psi (10 MPa) [2].
• Regeneration Tower:
– Temperature: Generally between 200°C to 260°C or higher [4], sometimes
limited to 370 to 390 °F for higher purity [5].
– Pressure: Typically operates at pressures lower than in the absorber, ranging
from atmospheric pressure to slightly above atmospheric pressure [6].
– It's crucial to note that these values are general guidelines, and actual operating
conditions can vary significantly based on the specific equipment design,
glycol type, gas composition, and desired dehydration efficiency.
• 🌐 Sources
• ScienceDirect - Glycol Dehydration Process - an overview
• Kimray - Gas Dehydration System Overview
• IntechOpen - Natural Gas Dehydration
• ScienceDirect - Glycol Dehydration - an overview
• Kimray - 103 Glycol-Gas Dehydration System Questions

19. DEW POINT DEPRESSION
• Water dewpoint is the temperature at any given pressure at which the natural gas is
saturated with water.
• The overall objective of dehydration is to remove a sufficient amount of water from
the natural gas so that the specification for maximum allowable water content in the
treated gas is met.
• The next example shows the difference in dewpoint depression and the amount of
water to be removed per MMscf, if the gas is entering the glycol contactor at 90°F
and at 1,000 psia or at 500 psia. The water content specification in this example is 4
lbm per MMscf for the treated gas.
• Examples of water dewpoint depression for two operating pressures are shown at
90°F.
• Water content at 1,000 psia and 90°F = 45 lbm/MMscf (Fig. 1).
• Dewpoint temperature for 4 lbm/MMscf and 1,000 psia = 18°F (Fig. 1).
• Water to remove = 45 – 4 = 41 lbm/MMscf.
• Dewpoint depression = 90°F – 18°F = 72°F.
• Water content at 500 psia and 90°F = 78 lbm/MMscf.
• Dewpoint temperature for 4 lbm/MMscf and 500 psia = 5°F.
• Water to be removed = 78 – 4 = 74 lbm/MMscf.
• Dewpoint depression = 90°F – 5°F = 85°F.

20. Reasoning
• The higher the dew point, the greater the "relative humidity" or water
content." You have to understand that first. The more water there is in
the gas , the more saturated, the closer the temperature will be to the
gas temperature where the moisture can condense out.
• When the gas is saturated with water, it doesn't need to have a low
temperature to condense. Think of a cool drink on a humid day- the
humidity in the air condenses instantly, Even without ice.
• On a dry day, when the air is not saturated, your cold drink doesn't
have a puddle around it....and unless you load it with ice, you may not
get much condensation.
• Because the glycol removed the humidity from the gas, it is less
saturated, and so it will take a much colder (lower) temperature to
initiate condensation (dew).
• In this case, the lower dew point temperature is the proxy measure for
"how much moisture remains in the gas."