Technology for a better society
3rd Trondheim Gas Technology Conference
4 June 2014
1
David Berstad, Karl Lindqvist, Petter Nekså, Simon Roussanaly
SINTEF Energy Research
Natural gas sweetening by additive-aided extractive distillation for CO2 freezing point depression
Technology for a better society
• Processing of CO2-rich, sour gas containing 50% CO2 and 1% H2S
• Aspects on H2S removal
• Low-temperature CO2 removal of natural gas
• CO2 removal by extractive distillation
• CO2 fractionation columns operating at temperatures below that of the CO2 freezing point
• Recirculation of heavy hydrocarbons to the low-temperature columns depressing actual CO2 freeze-out temperature
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Outline
Technology for a better society 3
Natural gas composition and product specification
Feed composition Product specification
Temperature 40 °C
Pressure 70 bar Flowrate 590 000 Nm3/hr
15 MSm3/day
Composition
C1 39.28 vol%
C2 3.5 vol%
C3 2.4 vol%
C4 1.8 vol%
C5 1.2 vol%
C6+ 0.2 vol%
CO2 50 vol% 2.5 vol%
H2S 1 vol% 21 ppm
Organic sulphides 0.02 vol%
N2 0.5 vol% ≈0.5 vol%
BTEX 0.1 vol% 0 vol%
Sweetening CO2/H2S-rich raw gas to pipe gas specification
Technology for a better society
Recirculation of heavy hydrocarbons for CO2 freezing point depression
4
Overview of extractive distillation process
CO2 to re-
injection
2.5% CO2
Additive recycle
Gas from BTEX and
H2S pre-removal units
PropaneRaw CO2
Propane
column
CO2
column
Butane
column
Column 1
Column 2
Pentane+
Butane
Technology for a better society
• H2S can in principle be removed prior to CO2 bulk removal ('pre-removal') or from separation products ('post-removal')
• Arguments for pre-removal
• Pre-removal is standard
• No infiltration of H2S into fractionation train, minimises corrosion risk
• Safety measure in case of failure in dehydration unit
• Arguments for post-removal
• Fractionation system needs to be dry in any case, due to low-temperature operation
• Molsieve dehydration to ≈0.1 ppm; Dew point < 100°C; No free water present
• H2S ends up mainly in CO2 to be re-injected and propane product
• Propane sweetening can be carried out with significantly smaller absorption column
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H2S removal
Technology for a better society
CO2 to re-
injection
2.5% CO2
Recycle
Additive
RNG2
BTEX, C6
Drying
BTEX
removal
Propane
Butane
C5+
BTEX bulk
removalAdditive
recovery
Raw CO2
Propane
column
CO2
column
Butane
column
Column 1
Column 2
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H2S product concentrations – post-removal
Natural gas product 21 ppm H2S 0.17 t H2S /d
Propane 1.18% H2S 129.7 t H2S/d
CO2 for injection 15.86% H2S 75.1 t H2S/d
Natural gas feed composition (vol%)
C1 39.28
C2 3.5
C3 2.4
C4 1.8
C5 1.2
C6+ 0.2
CO2 50
H2S 1
Organic sulphides 0.02
N2 0.5
BTEX 0.1
21 ppm H2S in natural gas product. This can be obtained without pre-removal. A normal sweet-gas specification is 4-5 ppm H2S.
Technology for a better society
General recommendations for H2S disposal
Acid Gas Enrichment and Claus
Liquid Redox
Scavengers
10%0% 20% 30% 40% 50% 100%
0.1
1
10
100
H2S fraction
Ton
s o
f su
lph
ur
per
day
Claus Straight- Through
Claus with Split Flow and Pre-
Heat
Claus and
Oxygen
Direct Oxidation
Sweet gas
Propane product
Captured CO2
Figure reproduced from: Linde. Sulphur Process Technology (2012).
Technology for a better society
H2S pre-removal
Stripper
Absorber
Sweetened
gas
H2S+CO2
Natural gas from BTEX
removal section
Drying unit
To low-
temperature
columns
Feed Sweet gas Propane product CO2 product
CO2
[mol%]
H2S
[ppmv]
H2S
[ppmv]
H2S
[ppmv]
H2S
[kg/hr]
H2S
[ppmv]
No removal 50 10 000 21 158 000 3130 11 800
MDEA 43 100 0.29 1600 30 130
MDEA + scavenger 43 10 0.26 200 3.7 16
MDEA + scavenger 43 1 0.25 65 1.2 5
MDEA + scavenger 43 0.1 0.25 51 0.93 4
Selective MDEA absorption
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Base layout
• 2 CO2 fractionators
• CO2 /HHC fractionator
• C3/C4+ fractionator
• C4/C5+ fractionator
Also investigated: configurations with fewer hydrocarbon fractionators for simpler process
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CO2 separation by low-temperature extractive distillation
CO2 to re-
injection
2.5% CO2
Additive recycle
Gas from BTEX and
H2S pre-removal units
PropaneRaw CO2
Propane
column
CO2
column
Butane
column
Column 1
Column 2
Pentane+
Butane
Technology for a better society 10
Temperature profile of column 1 – no additive recirculation
-80
-70
-60
-50
-40
-30
-20
-10
0
10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Tem
pe
ratu
re [
°C]
Stage
Tray temperature
Freeze-out temperature
Freeze-out pinch ∆T = 3.0°C
Technology for a better society 11
Temperature profile of column 2 – additive ratio = 0.35% (mol additive / mol feed)
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Tem
pe
ratu
re [
°C]
Stage
AR=0.0035
AR=0.0035
Technology for a better society 12
Temperature profile of column 2 – additive ratio = 0.88% (mol additive / mol feed)
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Tem
pe
ratu
re [
°C]
Stage
AR=0.0088
AR=0.0088
Technology for a better society 13
Temperature profile of column 2 – additive ratio = 1.8% (mol additive / mol feed)
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Tem
pe
ratu
re [
°C]
Stage
AR=0.018
AR=0.018
Technology for a better society 14
Temperature profile of column 2 – additive ratio = 3.5% (mol additive / mol feed)
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Tem
pe
ratu
re [
°C]
Stage
AR=0.035
AR=0.035
Freeze-out pinch ∆T = 3.6°C
Technology for a better society 15
Temperature profile of column 2 – additive ratio = 3.5% (mol additive / mol feed)
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Tem
pe
ratu
re [
°C]
Stage
AR=0.035
AR=0.035
AR=0.035
Freeze-out pinch ∆T = 3.6°C
Vapour-phase CO2 freeze-out temperature
Technology for a better society
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Tem
pe
ratu
re [
°C]
Stage
AR=0.035; no.1
Liquid phase freeze-out temperature
AR=0.035; No. 2
Liquid phase freeze-out temperature
AR=0.035; No. 3
Liquid phase freeze-out temperature
AR=0.035; No. 4
Liquid phase freeze-out temperature
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Effect of varying the recirculated additive composition
Composition of recycled additive stream [mol%] No. C1 C2 C3 C4 (n+i) C5 (n+i) C6 CO2
1 0.0 0.0 0.6 49.4 47.5 2.5 0
2 0.0 0.0 2.3 21.4 72.1 4.1 0
3 0.0 0.0 0.0 72.5 26.2 1.3 0
4 0 0.1 36.1 36.7 24.7 1.3 1.0
Column 2 temperature profile
Technology for a better society
Possible to obtain pipe gas quality (2.5%) in a single-column configuration when the additive recirculation ratio is about 9.5%
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Single-stage CO2 separation column
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Tem
pe
ratu
re [
°C]
Stage
NO ADDITIVE
AR=0.01
AR=0.02
AR=0.04
AR=0.08
AR=0.095
AR=0.095 (Fr. P)
Freeze-out pinch ∆T = 3.0°C
Technology for a better society
• The minimum temperature difference between tray temperature and CO2 freeze-out temperature occurs in different locations in the configuration with two CO2 separator columns
• Column 1: Pinch in condenser Risk of cold spot and freeze-out on exchanger surface
• Column 2: Pinch in tray. Required safety margin (∆Tmin) depending on axial heat and mass transfer and degree of equilibrium/non-equilibrium conditions
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Freeze-out pinch considerations
CO2 to re-
injection
2.5% CO2
Recycle
Additive
RNG2
BTEX, C6
Drying
BTEX
removal
Propane
Butane
C5+
BTEX bulk
removalAdditive
recovery
Raw CO2
Propane
column
CO2
column
Butane
column
Column 1
Column 2
-80
-70
-60
-50
-40
-30
-20
-10
0
10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Tem
pe
ratu
re [
°C]
Stage
Tray temperature
Freeze-out temperature
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Tem
pe
ratu
re [
°C]
Stage
AR=0.035
AR=0.035
Technology for a better society
• Pipeline quality natural gas can be produced from gas with high initial CO2 concentration – 50% in the investigated case – by low-temperature distillation
• The pipeline quality can be obtained with one or two fractionation columns for CO2 removal
• With two columns, no additive recirculation is in principle required for the first column if specified correctly
• The second column needs approximately 0.035 mol additive per mol feed recirculation to obtain > 3°C freeze-out pinch temperature margin
• A single-column configuration requires about 0.095 mol additive per mol feed recirculation in order to keep pinch temperature margin > 3°C
• The number of fractionators for C2, C3, C4 etc. depends on the overall product specifications targeted in the process
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Conclusions and further work
Technology for a better society
• HHC fractionation columns are likely to be required. In this regard, the CO2 removal process may be considered to be a sub-process within, and an added functionality to, the NGL extraction and fractionation plant
• H2S removal can in principle and under certain conditions be carried out post CO2 removal, as the low-temperature system must be operated in dry conditions with no free-water formation
• H2S pre-removal, however, will reduce the risk of corrosion in case of dehydration malfunctioning, and is recommended
• The freeze-out estimations are based on tool embedded in Aspen HYSYS. Theoretical freeze-out calculations must therefore be verified by experimental measurements
• Detailed mass and heat transfer simulations are needed to determine required temperature margin for safe operation avoiding freeze-out in the low-temperature CO2 separation columns
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Conclusions and further work
Technology for a better society
This publication is based on the results from the research project “A Green Sea”, performed under the Petromaks program. The authors acknowledge the partners: Statoil, Total, Gassco, Petrobras and the Research Council of Norway (200455/S60) for their support.
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Acknowledgements
