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Natural Gas Dehydration with TEG
Natural Gas Dehydration with TEG 2001 Hyprotech Ltd. - All Rights Reserved. 1.1.9
Natural Gas Dehydration with TEG_4.pdf 1 1 2 Natural Gas Dehydration with TEG 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 -20C (-4F) 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
Column Overview
TEG Contactor
TEG Regenerator
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
Add a Material stream for the inlet gas with the following values:
Name: Inlet Gas
Temperature 30C (85F)
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
Add a second Material stream for the TEG feed to the TEG Contactor with the listed values.
Name : TEG Feed
Temperature 50C (120F)
Pressure 6200 kPa (900 psia)
LiqVol Flow 0.5 m3/h (2 USGPM)
Component Mass Fraction
H2O : 0.01
TEG : 0.99
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.
Name Mixer : Saturate
Inlets :
Inlet Gas
Water to Saturate
Outlet :
Gas + H2O
Design Parameters
Pressure Assignment : Equalize All
Work Sheet Water to Saturate,
Condition Inlet Gas Temperature : 30C (85F)
Condition Flowrate Water to Saturate, : 0.5 kgmole/h (1.1 lbmole/hr)
Composition 100% Water Water to Saturate,
What is the vapour fraction of the stream Gas+H20? (It should be less than 1.0 to ensure saturation) _________
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:
Name : FWKO
Feed : Gas + H2O
Vapour Outlet : Gas to Contactor
Liquid Outlet : Water Out
How much water is removed by the Separator? __________ What is the hydrate temperature of Gas to Contactor? __________
Operation The TEG Contactor can now be simulated.
Add an Absorber column operation with the following specifications and Run the column.
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)
Pressure Bottom 6200 kPa (900 psia)
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:
Inlet : Rich TEG
Outlet : LP TEG
Heat Exchanger Operation
Regen Feed is heated to 105C (220F) in the lean/rich exchanger ( L/R HEX ) before entering the Regenerator. Add a Heat Exchanger with the following values:
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
Design Parameters
Tubeside : Delta P 70 kPa (10 psi)
Shellside : Delta P 70 kPa (10 psi)
Work Sheet Condition
Regen Feed Temperature : 105C (220F) Regen
Feed Pressure : 110 kPa (16 psia)
Regenerator Operation
The TEG Regenerator is simulated as a Distillation Column. The TEG Regenerator consists of a condenser, a reboiler and one ideal stage.
Add a Distillation Column to the case.
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 Dlta P : 103 kPa (15 psia)
Specs
First Spec : Tray Temperature, Stage Condenser, Spec Value 102C (215F) Status Active
Second Spec : Tray Temperature, Stage Reboiler, Spec Value 205C (400F) Status Active
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
Set the Damping Factor (on the Solver page) to Adaptive. This will result in much faster convergence 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. Add a Mixer with the following information:
Add a Material Stream.
Inlets : Makeup TEG and Lean from L/R
Outlet : TEG to Pump Parameters
Makeup TEG Temperature : 15C (60F)
Component Mass Fraction
H2O : 0.01
TEG : 0.99
Parameter 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? __________
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:
Inlet : TEG to Pump
Outlet : Pump Out
Energy : Pump Q
Work Sheet Pressure of Pump Out : 6275 kPa (910 psia)
Heat Exchanger
A second heat exchanger is added to cool the TEG returning to the Contactor.
Add a Heat Exchanger with the following information.
Tube Side Inlet : Pump Out
Tube Side Outlet : TEG to Recycle
Shell Side Inlet : Dry Gas
Shell Side Outlet : Sales Gas
Design Parameters Tube Side : Delta P 70 kPa (10 psi), Shell Side : Delta P 35 kPa (5 psi)
Work Sheet TEG to Recycle Temperature : 50C (120F)
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 the recycle loop converges.
Double click on the Recycle button.
On the Connections tab enter the following information:
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.
Recycle Button The TEG concentration is very high so it is necessary to tighten the tolerances, especially on composition, to ensure accurate solutions.
What is the hydrate temperature of Sales Gas? _________ How does this compare with the hydrate temperature of Gas to Contactor?
Component Splitter
The Component Splitter does not do a flash to separate components. The separation is specified by the user. 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.
Component Splitter Button
Add a Component Splitter with the following values:
Name : Remove TEG
Inlet : Sales Gas
Overhead Outlet : TEG Only
Bottoms Outlet : Water Dewpoint
Energy : Split Q
Design Parameters
Bottoms Pressure 6155 kPa (893 psia)
Overhead Pressure 6155 kPa (893 psia)
Splits TEG Fraction in Overhead
Design Split
Fraction Component TEG in TEG only : 1.00
Work Sheet Condition
Water Dewpoint temperature : -20C (-4F)
TEG Only Temperature : 10C (50F)
Add a Separator to remove the condensed water.
Feed : Water Dewpoint
Vapour Outlet : Gas Out
Liquid Outlet : XS H2O
Adjust Button
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.
Adjusted Variable :
Object : Water Dewpoint
Variable : Temperature
Target Variable
Object : XS H2O
Variable : Master Comp. Mass Flow - H2O
Target Value
Source : User
Specified Target Value : 0.01 kg/h (0.022 lb/hr)
Parameters
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.
Method : Secant
Tolerance : 0.005 kg/h (0.01 lb/hr)
Step Size 5 oC (10 oF)
Exploring with the Simulation Exercise
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 = 70C (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?
