Natural Gas Dehydration with TEG 1
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Natural Gas Dehydration with
TEG
© 2001 Hyprotech Ltd. - All Rights Reserved.
1.1.9 Natural Gas Dehydration with TEG_4.pdf
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Workshop
At the wellhead, reservoir fluids generally are saturated with water. The
water in the gas can present some problems:
• formation of solid hydrates can plug valves, fittings or pipes
• the presence of water along with H2S or CO2 can cause
corrosion problems
• water can condense in the pipeline causing erosion or
corrosion problems
Generally, a dehydration unit is used in gas plants to meet a pipeline
specification. There are several different processes available for
dehydration: glycols, silica gel, or molecular sieves.
The natural gas industry commonly uses tri-ethylene glycol (TEG) for
gas dehydration where low gas dew point temperatures are required,
such as in the design of offshore platforms in the Arctic or North Sea
regions or for other cryogenic processes.
In this example, the water dewpoint spec for the dry gas is -20°C
(-4°F) at 6200 kPa (900 psia).
Learning Objectives
Once you have completed this section, you will be able to:
• Model a typical TEG dehydration unit
• Determine water dewpoint for a gas.
Prerequisites
Before beginning this section you need to be able to:
• Add streams, operations and columns.
Process Overview
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Column Overview
TEG Contactor
TEG Regenerator
Natural Gas Dehydration with TEG 5
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Building the Simulation
Defining the Simulation Basis
For this case, you will be using the Peng Robinson EOS with the
following components: N2, H2S, CO2, C1, C2, C3, i-C4, n-C4, i-C5, n-C5,
H2O, and TEG.
Starting the Simulation
Adding the feed streams
1. Add a Material stream for the inlet gas with the following values:
In this cell... Enter...
Name Inlet Gas
Temperature 30°C (85°F)
Pressure 6200 kPa (900 psia)
Molar Flow 500 kgmole/h (10 MMSCFD)
Component Mole Fraction
N2 0.0010
H2S 0.0155
CO2 0.0284
C1 0.8989
C2 0.0310
C3 0.0148
i-C4 0.0059
n-C4 0.0030
i-C5 0.0010
n-C5 0.0005
H2O 0.0000
TEG 0.0000
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2. Add a second Material stream for the TEG feed to the TEG
Contactor with the listed values.
The values for the Stream TEG Feed will be updated once the Recycle
operation is installed and has calculated.
Mixer Operation
The composition of the natural gas stream has been provided on a
water-free basis. To ensure water saturation, the gas is mixed with water
prior to entering the Contactor.
Add a Mixer to mix the Inlet Gas and Water to Saturate streams.
In this cell... Enter...
Name TEG Feed
Temperature 50°C (120°F)
Pressure 6200 kPa (900 psia)
LiqVol Flow 0.5 m3/h (2 USGPM)
Component Mass Fraction
H2O 0.01
TEG 0.99
In this cell... Enter
Connections
Name Saturate
Inlets Inlet Gas
Water to Saturate
Outlet Gas + H2O
Parameters
Pressure Assignment Equalize All
Work Sheet
Water to Saturate, Flowrate 0.5 kgmole/h (1.1 lbmole/hr)
Water to Saturate, Composition 100% Water
Water to Saturate, Temperature 30°C (85°F)
Natural Gas Dehydration with TEG 7
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Separator Operation
Any free water carried with the gas is first removed in a separator
operation, FWKO.
Add a Separator and provide the following information:
What is the vapour fraction of the stream Gas+H20? (It
should be less than 1.0 to ensure saturation) _________
In this cell... Enter...
Connections
Name FWKO TK
Feed Gas + H2O
Vapour Outlet Gas to Contactor
Liquid Outlet FWKO
How much water is removed by the Separator?
__________
What is the hydrate temperature of Gas to Contactor?
__________
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Contactor Operation
The TEG Contactor can now be simulated.
Add an Absorber column operation with the following specifications
and Run the column.
Valve Operation
The Rich TEG stream is flashed across the valve, VLV-100. The outlet
pressure will be back calculated.
Add a Valve with the following values:
In this cell... Enter...
Connections
Name TEG Contactor
No. of Stages 8
Top Stage Feed TEG Feed
Bottom Stage Feed Gas to Contactor
Ovhd Vapour Dry Gas
Bottoms Liquid Rich TEG
Pressures
Top 6190 kPa (897 psia)
Bottom 6200 kPa (900 psia)
In this cell... Enter...
Connections
Inlet Rich TEG
Outlet LP TEG
Natural Gas Dehydration with TEG 9
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Heat Exchanger Operation
Regen Feed is heated to 105°C (220°F) in the lean/rich exchanger, L/R
HEX, before entering the Regenerator
Add a Heat Exchanger with the following values:
In this cell... Enter...
Connections
Name L/R HEX
Tube Side Inlet Regen Bttms
Tube Side Outlet Lean from L/R
Shell Side Inlet LP TEG
Shell Side Outlet Regen Feed
Parameters
Tubeside Delta P 70 kPa (10 psi)
Shellside Delta P 70 kPa (10 psi)
Work Sheet
Regen Feed, Temperature 105°C (220°F)
Regen Feed, Pressure 110 kPa (16 psia)
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Regenerator Operation
The TEG Regenerator is simulated as a Distillation Column. The TEG
Regenerator consists of a condenser, a reboiler and one ideal stage.
1. Add a Distillation Column to the case.
In this cell... Enter...
Connections
Name TEG Regenerator
No. of Stages 1
Feed Regen Feed
Condenser Type Full Reflux
Ovhd Vapour Sour Gas
Bottoms Liquid Regen Bttms
Condenser Energy Cond Q
Reboiler Energy Reb Q
Pressures
Condenser 101 kPa (14 psia)
Condenser Delta P 2 kPa (1 psi)
Reboiler 103 kPa (15 psia)
Specs
First Spec - Tray Temperature
Stage Condenser
Spec Value 102°C (215°F)
Status Active
Second Spec - Tray Temperature
Stage Reboiler
Spec Value 205°C (400°F)
Status Active
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2. Set the Damping Factor (on the Solver page) to Adaptive. This
will result in much faster convergance for this column.
Mixer Operation
TEG is lost in small quantities, so a makeup stream is required to ensure
that the material balance is maintained.
1. Add a Material Stream.
Third Spec - Reflux Ratio
Spec Value 1.0 Molar
Status Estimate
Fourth Spec - Draw Rate
Draw Sour Gas
Spec Value 1 kgmole/h (0.02 MMSCFD)
Status Estimate
In this cell... Enter...
In this cell... Enter...
Connections
Name Makeup TEG
Temperature 15°C (60°F)
Component Mass Fraction
H2O 0.01
TEG 0.99
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2. Add a Mixer with the following information:
Pump Operation
A pump is installed to raise the pressure of the TEG before it enters the
Contactor.
Add a Pump with the following information:
In this cell... Enter...
Connections
Inlets Makeup TEG
Lean from L/R
Outlet TEG to Pump
Parameters
Pressure Assignment Equalize All
Work Sheet
Liquid Vol. Flowrate of TEG to Pump 0.5 m3/h (2 USGPM)
What is the flowrate of Makeup TEG? __________
In this cell... Enter...
Connections
Inlet TEG to Pump
Outlet Pump Out
Energy Pump Q
Work Sheet
Pressure of Pump Out 6275 kPa (910 psia)
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Heat Exchanger
A second heat exchanger is added to cool the TEG returning to the
Contactor.
Add a Heat Exchanger with the following information.
Recycle Operation
The Recycle installs a theoretical block in the process stream. The feed
into the block is termed the calculated recycle stream, and the product
is the assumed recycle stream. The following steps take place during
the convergence process:
• HYSYS uses the conditions of the assumed stream and solves
the Flowsheet up to the calculated stream.
• HYSYS then compares the values of the calculated stream to
those of the assumed stream.
• Based on the difference between the values, HYSYS modifies
the values in the calculated stream and passes the modified
values to the assumed stream.
• The calculation process repeats until the values in the
calculated stream match those in the assumed stream within
specified tolerances.
In this case, the lean TEG (TEG Feed) stream which was originally
estimated will be replaced with the new calculated lean TEG (TEG to
Recycle) stream and the Contactor and Regenerator will be run until
In this cell... Enter...
Connections
Tube Side Inlet Pump Out
Tube Side Outlet TEG to Recycle
Shell Side Inlet Dry Gas
Shell Side Outlet Sales Gas
Parameters
Tube Side Delta P 70 kPa (10 psi)
Shell Side Delta P 35 kPa (5 psi)
Work Sheet
TEG to Recycle, Temperature 50°C (120°F)
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the recycle loop converges.
1. Double click on the Recycle button. On the Connections tab
enter the following information:
2. Switch to the Tolerance page on the Parameters tab. Complete
the page as shown in the figure below. The tolerances for Flow,
Enthalpy and Composition need to be tightened.
What is the hydrate temperature of Sales Gas?
_________
How does this compare with the hydrate temperature
of Gas to Contactor?
Recycle Button
The TEG concentration is very
high so it is necessary to
tighten the tolerances,
especially on composition, to
ensure accurate solutions.
Natural Gas Dehydration with TEG 15
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Analyzing the Results
One of the criteria used to determine the efficiency of a dehydration
facility is the water dewpoint of the dry gas. This can easily be checked
by finding the temperature at which water will just begin to condense.
First, all traces of TEG must be removed from the stream being tested
because TEG affects the H2O dewpoint. This is accomplished by the
use of a Component Splitter. The resulting stream is then cooled and its
outlet temperature is varied by an Adjust operation to find the point at
which water just forms.
1. Add a Component Splitter with the following values:
Save your case!
The Component Splitter does
not do a flash to separate
components. The separation is
specified by the user.
Component Splitter Button
In this cell... Enter...
Connections
Name Remove TEG
Inlet Sales Gas
Overhead Outlet TEG Only
Bottoms Outlet Water Dewpoint
Energy Split Q
Parameters
Bottoms Pressure 6155 kPa (893 psia)
Overhead Pressure 6155 kPa (893 psia)
Work Sheet
Water Dewpoint temperature -20°C (-4°F)
TEG Only Temperature 10°C (50°F)
Splits
TEG Fraction in Overhead 1.0
All Other Fractions 0.0
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2. Add a Separator to remove the condensed water.
An Adjust operation will vary the temperature of Water Dewpoint until
the dewpoint specification is met for the stream Gas Out.
Add an Adjust operation to manipulate the temperature of the Water
Dewpoint stream until the flow of the XS H2O stream is just greater
than 0, a value of 0.01 kg/h works well here.
The resultant temperature of the Water Dewpoint stream will then be
the dewpoint of that stream.
In this cell... Enter...
Connections
Feed Water Dewpoint
Vapour Outlet Gas Out
Liquid Outlet XS H2O
Adjust Button
In this cell... Enter...
Connections
Adjusted Variable Water Dewpoint
Variable Temperature
Target Variable
Object XS H2O
Variable Comp. Mass Flow - H2O
Target Value
Source User Specified
Target Value 0.01 kg/h (0.022 lb/hr)
Parameters
Method Secant
Tolerance 0.005 kg/h (0.01 lb/hr)
Step Size 5 oC (10 oF)
The tolerance must be small
here as the target value is close
to 0, but an XS H2O flow of 0
means that the dewpoint has
not been reached yet.
Natural Gas Dehydration with TEG 17
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Exploring with the Simulation
Exercise 1
The addition of stripping gas (slipstream from Sales Gas) will enhance
the ability of the Regenerator to remove water from the rich TEG. A Tee
operation is used to split Sales Gas into 2 streams.
• Strip Gas flow = 50 kgmole/h (110 lbmole/hr)
The stream pressure is 6155 kPa which is too high for the Regenerator.
Use a recycle, a cooler and a valve to transfer the flow and composition
of Strip Gas to stream SG to Regen at the following conditions:
• T = 70°C (160 oF)
• P = 110 kPa (15 psia)
SG to Regen enters as a feed to the Regenerator Reboiler. Does the TEG
concentration in Regen Bttms increase?
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