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Definition
of Natural Gas Dehydration
the removal of water from natural gas by lowering the dew point
temperature of the
natural gas
Objective:
To prepare natural gas for sale, its undesirable components
(water, H2S and CO2) must be removed.
Most natural gas contains substantial
amounts of water vapor
due to the presence of connate water in
the reservoir rock. At reservoir
pressure and temperature, gas is
saturated with water vapor.
Removal of this water is necessary for sales specifications
or cryogenic gas processing. Primary
concerns in surface facilities are
determining the:
- Water content of the gas.
- Conditions under which hydrates will form.
Liquid water can form hydrates, which
are ice-like solids, that can plug flow
or decrease throughput. Predicting the
operating temperatures and pressures at
which
hydrate form and methods of hydrate
prevention.
Water vapor is the
most common undesirable impurity in gas
streams. Usually, water vapor and
hydrate formation, i.e. solid phase that
may precipitate from the gas when it is
compressed or cooled. Liquid water
accelerates corrosion and ice (or solid
hydrates) can plug valves, fittings, and
even gas lines. To prevent such
difficulties, essentially gas stream,
which is to be transported in
transmission lines, must be dehydrated
as per pipeline specifications.
The processing of natural gas to the
pipeline specifications usually involves
four main processes :
- Oil and condensate removal
- Water removal
- Separation of natural gas liquids
- Sulfur and carbon dioxide removal
Most of the liquid free water associated
with extracted natural gas is removed by
simple separation methods at or near the
wellhead. However, the removal of the
water vapor
requires more complex treatment, which usually involves one of the two
process, either absorption or
adsorption.
In absorption, dehydrating agent (e.g.
glycols) is employed to remove water
vapors and in adsorption, solid
desiccants like alumina, silica gel, and
molecular sieves can be used.
The absorption process has gain wide acceptance because of proven
technology and simplicity in design and
operation.
Dew Point:
The dew
point is the temperature and pressure at
which the first drop of water vapor
condenses into a liquid. It is used as a
means of measuring the water vapor
content of
natural gas. As water vapor
is removed from the gas stream, the dew
point decreases. Keeping the gas stream
above the dew point will prevent
hydrates from forming and
prevent
corrosion from occurring.
Dew point depression is the difference between the
original dew point and the dew point
achieved after some of the water vapor
is removed. It is used to describe the
amount
of water needed to be removed
from the natural gas to establish a
specific water vapor content
PRESENCE OF
WATER IN NATURAL GAS :
Natural gas contains water in 2 forms :
- In liquid form (free water) .
- In vapor form (dissolved)
Water present :
1. At source from reservoir (associated water with gas)
2. As a result of sweetening in aqueous solution.
It is necessary to reduce and control the water content of gas to ensure
safe processing and transmission.
WATER
CONTENT IN NATURAL GAS
Water content is
stated in a number of ways :
1. Mass of water/unit volume lb/MMscf.
2. Dew point Temperature.
3. Concentration, part per million by volume
ppmv.
4. Concentration, part per million by mass
ppmw.
Why Dehydrate?
Dehydration refers to removing water
vapor from a gas to lower the stream’s
dew point. If water vapor is allowed to
remain in the natural gas,
it will:
- Reduce the efficiency and capacity of a pipeline
- Cause corrosion that will eat holes in the pipe or vessels through
which the gas passes Form hydrates or
ice blocks in pipes, valves, or vessels
- Dehydration is required to meet gas sales contracts (dependent upon
ambient temperatures).
Water Content
of Gas:
Liquid
water is removed by gas-liquid and
liquid-liquid separation. The capacity
of a gas stream to hold water vapor is:
A function of the gas composition
Affected by the pressure and temperature
of the gas Reduced as the gas stream is
compressed or cooled When a gas has
absorbed the limit of its water holding
capacity for a specific pressure and
temperature, it is said to be saturated
or at its dew point.
Any additional water added at the saturation point will not
vaporize, but will fall out as free
liquid. If the pressure is increased
and/or the temperature decreased, the
capacity of the gas to hold water will
decrease, and some of the water vapor
will condense and drop out.
Methods of determining the water content of gas include:
- Partial pressure and partial fugacity relationships
- Empirical plots of water content versus P and T Corrections to
the empirical plots above for the
presence of contaminants such as
hydrogen sulfide, carbon dioxide and
nitrogen and Pressure Volume Temperature
(PVT) equations of state.
GAS HYDRATES:
What Are Gas
Hydrates?
Gas hydrates are complex lattice
structures composed of water molecules
in a crystalline structure: Resembles
dirty ice but has voids into which gas
molecules will fit Most common
compounds.
-
Water, methane, and propane
- Water, methane, and ethane
The physical appearance resembles a wet,
slushy snow until they are trapped in a
restriction and exposed to differential
pressure, at which time they become very
solid structures, similar to compacting
snow into a snow ball.
Why Is Hydrate Control Necessary?
Gas hydrates accumulate at restrictions
in flowlines, chokes, valves, and
instrumentation and accumulates into the
liquid collection section of vessels.
Gas hydrates plug and reduce line
capacity, cause physical damage to
chokes and instrumentation, and cause
separation problems.
What Conditions Are Necessary to Promote
Hydrate Formation?
Correct pressure and temperature and
“free water” should be present, so that
the gas is at or below its water dew
point. If “free water” is not present,
hydrates cannot form.
How Do We Prevent or Control Hydrates?
1. Add heat.
2. Lower hydrate formation
temperature with chemical
3. inhibition Dehydrate gas so
water vapor will not condense into “free
water”.
4. Design process to melt
hydrates.
Why
Using Glycols?
Glycols are extremely stable to thermal and chemical
decomposition, readily available at
moderate cost, useful for continuous
operation and are easy to regenerate.
These properties make glycols as obvious
choice as dehydrating agents.
In the liquid state, water molecules are
highly associated because of hydrogen
bonding. The hydroxyl and ether groups
in glycols form similar associations
with water molecules. This liquid –phase
hydrogen bonding with glycols provides
higher affinity for absorption of water
in glycol. Four glycols have been
successfully used to dry natural gas:
ethylene glycol (EG), Diethylene glycol
(DEG), Triethylene glycol (TEG) and
Tetraethylene glycol (TREG).
TEG has gained universal acceptance as the most cost effective
choice because:
- TEG is more easily regenerated to a concentration of 98-99.95% in an
atmospheric stripper because of its high
boiling point and decomposition
temperature.
- Vaporization temperature losses are lower than EG or DEG
- Capital and operating cost are lower
Diethylene glycol is preferred for
applications below about 10oC because of
the high viscosity of TEG in this
temperature range.
for more details, see
Natural Gas Dehydration Part.2
References:
1. Gas Dehydration Field Manual, Maurice Stewart & Ken Arnold
2. Gas Dehydration by TEG and Hydrate
Inhibition Systems, Arthur William
3. Fundamentals of Natural Gas, Arthur J. Kidnay & William R. Parrish |
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