Post on 25-Mar-2020
Thermodynamics in Gas Processing –Phase Envelope Predictions and Process Design
Efstathios (Stathis) Skouras, Principal Researcher
Gas Processing, StatoilHydro Research Centre Trondheim
1st Trondheim Gas Technology Conference, 21-22 October 2009
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Presentation outline
• Importance of correct phase envelope predictions
• Hydrocarbon dew points for real gases
– Success factors
– Predictions with thermodynamic models
• Phase envelope predictions: Impact on process design
• Conclusions
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Pres
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Individual dew points
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Phase envelope of a typical natural gas
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Temperature/°C
Crit P
EOS = SRK Penelou
Dew point lineDew point line
GasGas
Two Two phasesphases
Bubble point lineBubble point line
CricondenbarCricondenbar
LiquidLiquid
CricondenthermCricondentherm
Dense phaseDense phase
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Importance of correct phase envelope predictions• In offshore processing the cricondenbar
specification must be fulfilled to avoid condensation in the pipelines
• Rich gas transported to onshore terminals in dense phase in pipelines up to 830 km long
• In onshore processing the cricondenthermspecification must be fulfilled to achieve desired gas quality for the sales gas
Correct phase envelope predictions
•• Efficient operation/separation
• Optimise pipeline capacity
• Reduce design margins
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Success factors
• Gas sampling and conditioning
• Chromatographic gas analysis
• Characterisation of C7+ components
• Thermodynamic models
Dew points for real gases: Success factors
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Success factors: Gas sampling and conditioning
Sampling at places where the gas is at:
• High temperature (one phase area)
• High pressure (in dense phase)
1st stage separator
2nd stage scrubber
1st stage scrubber
Glycol contactor
3rd stage scrubber
Rich gas (RG)
Right place!
Wrong place!
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Success factors: Chromatographic gas analysis• How detailed should the gas analysis be?
• Enough with GC-analysis with a C6+, C7+ fraction?
• Does it really matter?
Example: Rich gas with C6+ fraction of 0.38 mol%
• Deviations up to 20°C at the cricondentherm
• Deviations up to 10 bar at the cricondenbar
• The leaner the gas, the more effect the heavy ends have in the dew point
• Detection limit for lean gases 0.1 ppm for the heavy ends (ISO 23874)
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Temperature [°C]
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GC-analysis up to C5 (C6+ fraction)
GC-analysis up to C6 (C7+ fraction)
GC-analysis up to C9
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Success factors: Characterisation of C7+ components
• Is it enough to characterise C7+ components as n-alkanes?
• Is it important to distinguish between paraffinic (P), napthenic (N) and aromatic (A) components?
• Shall we characterise them as pseudo-components (C7*, C8*, etc)?
• How should we assign physical properties (mol. weight, density) and model parameters (Tc, Pc, ω) to the pseudo-components?
• Does it really matter?
Effect of characterisation of C7+ components
• Deviations up to 15°C at the cricondentherm
• Deviations up to 7-8 bar at the cricondenbar
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C7+ components as n-alkanes
PNA characterisation
C7+ components as pseudo-components
C7+ as pseudo-components
• Mol.weight and density by detailed GC-analysis
• Tc, Pc and ω from generalised correlations as function of mol.weight and density *
* Tc, Pc from Riazi-Daubert (1987) and ω from Kesler-Lee (1976)
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• Characterisation with PNA analysis gives better results than assuming C7+ as n-alkanes
• Characterisation with pseudo-components (utilising correct mol.weight and density and generalised correlations for Tc, Pc and ω) provides the correct shape of the phase envelope
• Cricondenbar underestimated. No model can predict both the cricondenbar and the cricondentherm
• Experimental dew point measurements still required to decide the correct phase envelope
Dew points of real gases: Results for a rich gas
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C7+ as n-alkanes/SRK
PNA characterisation/SRK
C7+ as pseudo-components/SRK
C7+ as pseudo-components/PR
Experimental
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Phase envelope predictions: Impact on process design
Testseparator
1st stage separator
2nd stage scrubber
1st stage scrubber
Glycol contactor
3rd stage scrubber
Rich gas (RG)
• The rich gas (RG) has to meet a specification of 105 barg spec
• The quality of the gas is decided at the 2nd stage scrubber (operating point: 40 barg, 20°C)
• The model predicts a cricondenbar of 100 barg
• Design margin of 5 bar to cricondenbar spec0
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Calculated dew point curve 2nd stage scrubber outlet
Operating point 2nd stage scrubber
EOS = SRK
Rich Gas Cricondenbar
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Phase envelope predictions: Impact on process design (cont.)
Testseparator
• Experimental measurements show give a cricondenbar of 106 barg. Gas is off spec!
• Design margin of 5 bar not sufficient. Higher design margin needed (10 bar)
• Improve accuracy of the model in order to reduce the design margin
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Calculated dew point curve 2nd stage scrubber outlet
Measured dew points
Scrubber operating point
CCB prediction 6 bartoo low
EOS = SRK
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Conclusions• Focus on sample chain is decisive (sample taking,
conditioning and GC-analysis)
• Characterisation of the heavy ends (C7+) is crucial
• The thermodynamic models are not capable to model sufficiently the whole phase envelope
• The models underpredict the cricondenbar
• Experimental dew point measurements are still needed to verify the model predictions
• Focus on thermodynamic models in order to achieve good process designs, reduce design margins and ensure product quality
Source: www.statoilhydro.com
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Thank you for your attention!
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Back-up slides
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Gas in
Gas out
Cooling media inCooling media in
Cooling media outCooling media out
Mirror
Chilled mirror optical apparatus from
Chandler Engineering
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Individual dew points
Dew points for real gases: Experimental measurements
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Dew point predictions: Pure components (ethane)• Agreement between experimental dew points and predictions with SRK and PR EoS
Dew points of ethane
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Simulated SRK EoS (Pro/II)
Simulated PR EoS (Pro/II)
Experimental measurements(Rotvoll)
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• Both cricondenbar and cricondentherm is under-predicted
• Deviations up to 5°C in cricondentherm and 15 bar in cricondenbar
• Good accordance at low pressure
• Shape of the experimental dew point line is different from predicted by the model
SNG1
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SRK
PC-SAFT
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Dew point predictions: Simple synthetic gases (up to C5)
Component SNG1
Methane 93.505
Ethane 2.972
Propane 1.008
i-Butane 1.050
n-Butane 1.465
n-Pentane -
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SNG6 & SNG7 & SNG8 - Effect of PNA
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ParaffinLab ParaffinNaphteneLab NaphteneAromaticLab Aromatic
• Cricondenbar still under-predicted, but cricondentherm over-predicted
• Aromatic and naphtene compounds give significant steeper dew point line than paraffins
• PNA characterization important for phase envelope prediction
Synthetic gases with a selected C7 component
Component SNG7 SNG8 SNG9
Methane 93.121 93.176 83.940
Ethane 3.048 3.064 10.016
Propane 0.994 1.014 4.109
i-Butane 1.032 1.027 0.601
n-Butane 1.510 1.521 1.031
i-Pentane - - -
n-Pentane - - -
n-Hexane - - -
Benzene (A) 0.295 - -
N-Heptane (P) - 0.198 -
Cyclo-Hexane (N) - - 0.302
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Sales gas
Onshore processing
Rich gas
Offshore processing
Industry
Household
Power plant
Other
Terminal
Market
Cricondenbar spec
Cricondentherm spec
Correct phase envelope predictions
•• Efficient operation/separation
• Optimise pipeline capacity
• Reduce design margins
Importance of correct phase envelope predictions