Upload
others
View
12
Download
0
Embed Size (px)
Citation preview
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
2
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
0102030405060708090
100110120
-10 -5 0 5 10 15 20 25Temperature [°C]
Pres
sure
[bar
a]
Individual dew points
3
Phase envelope of a typical natural gas
0
20
40
60
80
100
120
-50 -40 -30 -20 -10 0 10 20 30 40
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
4
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
5
Success factors
• Gas sampling and conditioning
• Chromatographic gas analysis
• Characterisation of C7+ components
• Thermodynamic models
Dew points for real gases: Success factors
6
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!
7
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)
0
10
20
30
40
50
60
70
80
90
100
110
120
-35 -25 -15 -5 5 15 25 35
Temperature [°C]
Pre
ssur
e [b
ar]
GC-analysis up to C5 (C6+ fraction)
GC-analysis up to C6 (C7+ fraction)
GC-analysis up to C9
8
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
0
10
20
30
40
50
60
70
80
90
100
110
120
-15 -10 -5 0 5 10 15 20 25 30 35
Temperature [ºC]
Pres
sure
[bar
a]
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)
9
• 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
0
10
20
30
40
50
60
70
80
90
100
110
120
-15 -10 -5 0 5 10 15 20 25 30 35
Temperature [ºC]
Pres
sure
[bar
a]
C7+ as n-alkanes/SRK
PNA characterisation/SRK
C7+ as pseudo-components/SRK
C7+ as pseudo-components/PR
Experimental
10
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
102030405060708090
100110120
-20 -10 0 10 20 30Temperature [°C]
Pres
sure
[bar
a]
Calculated dew point curve 2nd stage scrubber outlet
Operating point 2nd stage scrubber
EOS = SRK
Rich Gas Cricondenbar
11
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
30
40
50
60
70
80
90
100
110
-20 -15 -10 -5 0 5 10 15 20 25Temperature [°C]
Pres
sure
[bar
a]
Calculated dew point curve 2nd stage scrubber outlet
Measured dew points
Scrubber operating point
CCB prediction 6 bartoo low
EOS = SRK
12
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
13
Thank you for your attention!
14
Back-up slides
15
Gas in
Gas out
Cooling media inCooling media in
Cooling media outCooling media out
Mirror
Chilled mirror optical apparatus from
Chandler Engineering
0102030405060708090
100110120
-10 -5 0 5 10 15 20 25Temperature [°C]
Pres
sure
[bar
a]
Individual dew points
Dew points for real gases: Experimental measurements
16
Dew point predictions: Pure components (ethane)• Agreement between experimental dew points and predictions with SRK and PR EoS
Dew points of ethane
10
15
20
25
30
35
40
45
50
-20 -15 -10 -5 0 5 10 15 20 25 30 35
Temperature [°C]
Pres
sure
[bar
]
Simulated SRK EoS (Pro/II)
Simulated PR EoS (Pro/II)
Experimental measurements(Rotvoll)
17
• 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
0
10
20
30
40
50
60
70
80
90
100
110
-30 -25 -20 -15Temperature [°C]
Pres
sure
[bar
a] PR
SRK
PC-SAFT
Gerg
Lab
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 -
18
SNG6 & SNG7 & SNG8 - Effect of PNA
0
10
20
30
40
50
60
70
80
90
100
110
-20 -15 -10 -5 0 5 10 15 20Temperature [°C]
Pres
sure
[bar
a]
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
19
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