93317472 14 gas dehydration
TRANSCRIPT
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DEHYDRATION
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2The processdehydration
What you should get out of this course.
The purpose of dehydration
Hydrate formation
The composition of the gas
Principles
The different processes
dehydration by glycol absorption dehydration by methanol absorption
dehydration by adsorption
Diagram and location in the process
Operating a dehydration installation Problems encountered
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3The processdehydration
The purpose of dehydration
IntroductionWhen the gas is at reservoir temperature and pressure, it is
generally saturated with water.
Water is responsible for:
most types of corrosion when it is associated
- with acid gases (H2S and CO2)
- or salts (calcium carbonates)
hydrate formation
The gas therefore has to be processed to remove the water it
contains.
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4The processdehydration
The purpose of dehydration
Purpose:
Treat to obtain hydrocarbons which meet the specifications
Optimise recovery of the hydrocarbons
Discharge the non-marketable effluents
Protect persons and installations Facilitate transport in the pipelines (corrosion)
Prevent corrosion problems in the lift gas or injection gas
systems.
Prevent hydrate formation
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5The processdehydration
Hydrate formation
Definition:Crystal structures with a set geometry that require the presence of
water and components present in hydrocarbons, for their formation.
Hydrates are a major problem in the production and transport of natural
gas.
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6The processdehydration
Hydrate formation
How do they form? Presence of l iquid w ater
Example: liquid water released by the gas during a change in the
pressure or temperature conditions
Presence of l ight h ydrocarbons
Only the first four hydrocarbons (methane, ethane, propane, butane)
are likely to form hydrates in presence of liquid water (risk of hydrates
in the presence of CO2or dH2S)
Favourable condit io ns: temperature and p ressure
Hydrate formation conditions: pressure must be sufficiently high and
its temperature sufficiently low.
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7The processdehydration
Hydrate formation
How do they form? Certain factors contribute to hydrate formation
Vortices
Gas velocity
Bends, orifices, changes
High pressure
Self-amplifying effects
Low temperature
Each natural gas has its own specific hydrate formation range, which
depends on:
the density of the gas in relation to air (KATZ method)
its composition and solid-vapour equilibrium factors at P and T forthe most precise methods (CARSON and KATZ method).
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8The processdehydration
Hydrate formation
How do they form?
The area where T < T1, is governed by the hydrate formation
curve.
The area where
T > T1,
is governed by thedew point curve.
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9The processdehydration
Hydrate formation
Why is it a problem? Hydrate formation leads to:
blocking of the pipes and equipment
production shutdown
risks of overpressure in the installations.
water deposition due to condensation in the pipes or free water
from the reservoir may cause large pressure drops with risks of:
"water hammer" effects due to the liquid slugs
erosion
Water is responsible for most types of corrosion when it is
associated with
acid gases (H2S and CO2)
or salts (calcium carbonates).
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10The processdehydration
Hydrate formation
Why is it a problem ?
If a pipe becomes blocked by hydrates, the hydrate
block adheres so strongly to the pipe walls and is so hard that it
cannot be removed by any normal mechanical means.
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11The processdehydration
The gas composition
Commercial gas H2S content: 1.5 to 4 ppm
Total sulphur and contaminants: 50 to 150 mg/Sm3
CO2content: 2 to 3% molar mass
Water dew point: - 15C at 70 bar
Hydrocarbon dew point: - 2C at 70 bar
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12The processdehydration
The gas composition
examples of different natural gas compositions
Composition (% volume)
N2 1.50
H2O 1.00
H2S 15.30
CO2 9.30
C1 68.00
C2 3.00
C3 0.90
C4 0.50
C5 0.20
C6+ 0.30
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13The processdehydration
Principles
This solution consists of moving the hydrate formation curve
outside the facility's operating range. To achieve this, several
solutions are at our disposal.
Displacing the hydrate curve
inhibition by glycol or methanol. Case of uses considered:
inhibition by non-recoverable
methanol (without regeneration)
inhibition by methanol,
regenerated for re-use inhibiting with regenerated
diethylene glycol
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14The processdehydration
Principles
Displacing the operating range Maintain pressure
Increase temperature
reheaters
upstream
heat insulation
for the short pipes
Scope of application
short onshore gas
gathering systems.
heating upstream of the expansion nozzles (in certain cases).
acid gases or gases with non-negligible CO2 content.
Not recommended in the following cases
offshore high heating power
long distances.
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16The processdehydration
Principles
Displacement of the dew point curve
To avoid water condensation
in the dehydrating units
operating range by sufficient
gas dehydration.
Scope of application long distance transport of gas at
commercial specifications.
offshore: large subsea lines
carrying gas containing CO2 (corrosion)
upstream of the cooler units.
Contre-indications
short gas gathering lines.
short offshore inter-platform links.
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17The processdehydration
Principles
Displacement of the dew point curve
Advantages
no water condensation
no corrosion when there is H2S and/or CO2
good reliability
dew points obtained at commercial sales standards (-15 / -20C at
70 bar).
Disadvantages
relatively complex to install (investment)
safety problems if bare flame glycol reboiler used.
footprint not negligible (offshore)
continuous monitoring preferable.
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18The processdehydration
The different processes
dehydration by glycol absorption
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19The processdehydration
The different processes
dehydration by glycol absorption (cont'd)
Principle:
Absorption section
- The glycol absorbs water
- The gas circulates from
bottom to top- the regenerated glycol
is injected at the top
of the absorber
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20The processdehydration
The different processes
dehydration by glycol absorption (cont'd) Principle:
Regeneration section
- water-laden glycol is
drawn off from the flash drum
- series of filters
- glycol flows downthrough the column
- exits the column towards
the reboiler for regeneration
- the water vapour exits
the distillation column inthe reverse direction
- the concentrated glycol
exits the reboiler via
a weir
Vapeur d'eau
Condenseur
de tte
Rebouilleur
Still columnBrleur
Glycol humide
StrippingFuel gas
Gazsec
Colonnede
stripping
Filtre
Stockage
Gazoline
Sparateur gazolineglycol
Glycol sec
Pompe glycol
Glycol
Gazde flash
Vapeur d'eau
Condenseur
de tte
Rebouilleur
Still columnBrleur
Glycol humide
StrippingFuel gas
Gazsec
Colonnede
stripping
Filtre
Stockage
Gazoline
Sparateur gazolineglycol
Glycol sec
Pompe glycol
Glycol
Gazde flash
arie
OW:
te parmi
rni
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21The processdehydration
The different processes
dehydration by glycol absorption (cont'd) Performances
most commonly used process
dew point -15 to -20 C at 70 bars
use of TEG preferred (Triethylene glycol)
Scope of application
protection of treatment units by cooling protection of collection systems when there is no salt water
ingress or when there are WKOs at the well head.
protection on medium distance pipes.
subsea wells when there is no salt water ingress.
upstream of long-distance gas lines
protection of downstream lines
upstream of the turboexpander
presence of CO2--> corrosion
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22The processdehydration
The different processes
dehydration by glycol absorption (cont'd)Not recommended in the following cases :
long lines subject to corrosion, sea lines,
long pipes with many low points (there is a danger of the glycol
being unevenly distributed over the whole of the facility).
production of salt water (contamination by salts from the DEG at
regeneration).
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23The processdehydration
The different processes
dehydration by methanol absorption
Inhibition by methanol (not recovered)
Scope of application:
- small installations
- seasonal injection
- small quantity of gas- subsea wells
- short lines
- stand-alone installation
- commissioning after testing
Not recommended in the following cases:
- long lines
- prohibitive quantity to be injected
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24The processdehydration
The different processes
dehydration by methanol absorption (cont'd)
Inhibition by methanol (regenerated)
Scope of application:
- developments with subsea wells
- long distances
Not recommended in the following cases:- lines which are impossible to repair
- prohibitive quantity to be injected
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25The processdehydration
The different processes
dehydration by adsorption
property of certain solids (= desiccants) to fix certain molecules on
their surface.
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26The processdehydration
The different processes
dehydration by adsorption
The main desiccants are:
Alumina: Good activity but becomes deteriorated by absorbing the
heavy hydrocarbons which are not eliminated by heating.
Silicagels: These are highly active amorphous substances, which are
easy to regenerate and which adsorb the heavy hydrocarbons to a
lesser degree. They are sensitive to liquid water.
Molecular sieves:These consist of zeolite crystals
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27The processdehydration
The different processes
dehydration by adsorptionDifferences between the main desiccants:
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28The processdehydration
The different processes
dehydration by adsorption (molecular sieve)
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29The processdehydration
The different processes
Advantages and disadvantages of the various processes
Inhibition by glycol with regeneration
Advantages:
- low glycol consumption in simple regeneration (little
vaporisation in the gas) )
- no pollution problem (water eliminated during the vapour
phase).
- safe storage (low volatility product).
Disadvantages:
- presence of liquid in the transport facility (injection flow rate
higher than that of the methanol)- corrosion if H2S or CO2 present
- difficulties (or impossibility) to regenerate if salt water present
- gas does not meet the specifications
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30The processdehydration
The different processes
Advantages and disadvantages of the different processes Inhibition by methanol (not recovered)
Advantages:
- simple to install
- low investments
- small equipment size
- good reliability
Disadvantages:
- creation of a two-phase flow
- corrosion if H2S or CO2 present
- high operating costs
- methanol supply?
- storage (safety)
- gas does not meet the commercial standards with respect
to water content.
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31The processdehydration
The different processes
Advantages and disadvantages of the different processes
Inhibition by methanol with regeneration
Advantages:
- good reliability
- no water discharge
Disadvantages:
- presence of liquid in the lines
- corrosion if H2S / CO2 present
- loss of methanol (50%)
- complex to install
- gas does not meet specifications
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34The processdehydration
Representation and location in the process
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35The processdehydration
Representation and location in the process
Location
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36The processdehydration
Representation and location in the process
Location (Example: Girassol)
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37The processdehydration
Representation and location in the process
Criticality
If the dehydration unit (TEG) shuts down, the methanol injection
is automatically opened at the column outlet.
If methanol injection is impossible, the following must be stopped:
gas-lift gas injection
which generates a loss of production
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38The processdehydration
Operating an installation
Absorption section
Parameters governing absorption Concentration of the regenerated glycol
The glycol's purity level depends on:
- The bathtemperature in the reboiler.
The higher the temperature, the more water is released by the
TEG.The limit is set at 204C because the TEG deteriorates above
215 C.
- The operating pressureof the distillation column
Operating below atmospheric pressure generates higher
concentrations at equivalent temperatures.
- The useof a dry gas stripping column.
With the stripping column, a level of 99.9% can be reached
(
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39The processdehydration
Operating an installation
Absorption section
Parameters governing absorption
Gas temperature in the absorber
The dew point at the top of the absorber depends on the
temperature there. A reduction in the gas temperature at the inlet
to the unit reduces the dew point at the outlet.
Glycol circulation rate
- The minimum glycol circulation rate for a good glycol-gas
contact is approximately 15 litres per kg of water to be
removed from the gas.
- Average flow rate of 25 l/kg of water to be removed, for a
conventional installation..
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40The processdehydration
Operating an installation
Absorption section Normal operation
Downgraded operation
Dehydration column by
passed MeOh pump operating
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41The processdehydration
Operating an installation
Regeneration section
Regeneration makes use of the distillation principle by heating
the glycol - water solution in a reboiler whose energy is normally
supplied either by a fire tube, or by electric heating elements
The temperature of the glycol bath in the reboiler must be
maintained at 204C, for example, for the TEG.
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42The processdehydration
Operating an installation
Recirculation system section
Pumps
Pumps are used to circulate the glycol through the regeneration
system
Filtration
The solid particles are stopped by the filters, which prevents them
being drawn into and deposited in the regeneration equipment by
the glycol.
The hydrocarbons present in the glycol are removed with an
activated charcoal filter which prevents foaming problems,
generally due to the presence of corrosion inhibitors, solid
particles, etc. in the crude. pH neutralisation equipment
A chemical injection unit is used to neutralise the pH of the glycol,
which must be maintained at 6 -7 to prevent foaming.
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43The processdehydration
Problems encountered
Operating problems in the regeneration section
Glycol oxidation
The oxygen, which penetrates into the system through the
atmospheric storage tanks and pump seals, can oxidise the glycol
and form corrosive acids.
The use of a gas atmosphere is recommended in the storage
tanks
Thermal breakdown
An excessive temperature in the reboiler can break down the
glycol and form corrosive products (the TEG decomposition
temperature is 215C). Local overheating may be caused by salt or bitumen deposits on
the fire tubes or heating tubes.
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44The processdehydration
Problems encountered
Operating problems in the regeneration section
Controlling the pH
The acidity of the glycol is due to the two points mentioned above
and to the presence of acid compounds in the gas to be treated
(H2S, CO2) which increase the equipment corrosion rate.
The glycol must be maintained at a level of pH = 7 - 8 by injecting
a pH neutraliser
Deposits
Good filtration and activated charcoal treatment prevents the solid
particles and bituminous hydrocarbons from being deposited.
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45The processdehydration
Problems encountered
Operating problems in the regeneration section Foaming
Foaming may increase the glycol losses and reduce the capacity
of the equipment.
he causes of foaming are related to the presence of the following
in the glyco:
- liquid hydrocarbons,- corrosion inhibitors,
- salt,
- fine particles in suspension.
Presence of condensates The liquid hydrocarbons cause the glycol to foam.
They can be eliminated in the flash drum and in the activated
charcoal filters.
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46The processdehydration
Problems encountered
Operating problems in the regeneration section
Salt contamination
The salt deposits increase the equipment corrosion rate, and
reduce the heating tube heat transfers.
This salt is transported by a fine water vapour mist, which can be
trapped by demister at the separator.
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Problems encountered
Operating problems in the regeneration section
Glycol losses The glycol losses increase the operating costs of this type of unit. They
can be caused by:
- Vaporisation
These losses can be limited by sufficiently cooling the gas
upstream of the absorber.- Entrainment
The high points in the column are generally equipped with
internal systems (separator, demister, coalescer) designed to
prevent the glycol being mechanically entrained through the
system.- Mechanical leaks
Mechanical leaks can be reduced by keeping the pumps,
valves and other equipment on the lines correctly maintained