Classification: Open
Gas/liquid separation
Course: TPG 4140 Natural Gas
Date: 30.10.2008
Bernt Henning Rusten, StatoilHydro R&D Centre Trondheim
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Content
– Where and when is separation required
– Gas/liquid separation theory
– Why gas/liquid separation research
– Laboratory facilities
– Gas/liquid separators and internals
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Why is gas/liquid separation a big issue?
This is easy!Some wind
from the side
Some wind
from below
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Why is gas/liquid separation a big issue?
It is challenging
at real operating
conditions!
Movie
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Natural gas transport network NCS
• Gas is processed offshore. Separation of gas, oil and water. Conditioning of Water, CO2 and H2S if present.
• Rich gas is transported to the onshore terminal in dense phase in pipelines up to 830 km length.
• New pipelines considered for further increase in natural gas production from Norwegian Continental Shelf
• Gas/liquid separation is of great importance for all processes in the oil and gas industry to operate satisfactorily
– Protect process equipment (compressors, pumps and heat exchangers)
– Fulfil product specifications– Essential in future subsea processing
Source: www.gassco.no
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Troll A on its way offshore
Gas to Europe
Kollsnes
Production capacities:
Troll A 120 MSm³/dKvitebjørn 21 MSm³/dKollsnes 144 MSm³/d
Kvitebjørn
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Separation in Gas processing train
Testseparator
1st stage separator
2nd stage scrubber
1st stage scrubber
Glycol contactor
3rd stage scrubber
Rich gas (RG)
0102030405060708090
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
Norne
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Gas dynamic pressure (often called momentum)
ρgasv²
gasliq
gasgasUGLF
ρρρ−
⋅=
Basic definitions
Primary separation inlet device;
here inlet vane
Demisting; removal of remainingliquid;
here Axial Flow Cyclones (AFC)
Mesh Pad for
coalescing/demisting
Gas Load Factor, GLF (often called K-value)
Liquid fraction
Separation efficiency
- each internal
- total
Liquid entrainment liquid not separated in scrubber
-carry-over
-re-entrainment (liquid separated but teared up again into the gas)
gasvolumeliquidvolume
feed liquidscrubber scrubberin separated liquid
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Souders-Brown constant(Gas Load Factor (GLF), K-value)
• The GLF assumes constant– Droplet diameter (varies with pressure and interfacial
tension)– Drag coefficient Cd (varies with pressure because of
different flow regime)
d
GLF
Fd = Gd
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Important parameters for gas/liquid separation
• Interfacial tension decisive for:
– Droplet diameter
– Liquid behaviour
– Re-entrainment (droplets separated into a film ripped back up into droplets)
• Gas and liquid density
• Gas and liquid viscosity
• Gas and liquid loading
• Flow pattern/regimes (CFD modelling used, Computational fluid dynamics)
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Why is there a need for gas/liquid separation research?
Before 2000, Before 2000, no internal research activity was established, but many no internal research activity was established, but many problems were identified in operational units.problems were identified in operational units.
What was wrong?
Vendor design and understanding were based on model fluid data (air/water).
Separation technology performing well with model fluids collapsed when implemented in field at real fluid properties.
More compact technology was used.
New fields with higher pressures started to produce; more difficult separation.
Need for fundamental understanding of phenomena occurring and equipment limitations.
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Challenges related to insufficient gas/liquid separation
Animation
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K-lab large scale test facility K-lab, Kårstø
• In operation since 2004
• Pressure; 1 – 148 bara
• Fluids; hydrocarbon gas and liquid or model liquid
• Test section ID 840 mm, height approx. 6 m
Low pressure test rig Research Centre, Trondheim
• In operation since 2001
• Pressure; 1.8 – 7 bara
• Fluids; air with Exxsol D-60 and/or water/glycol
• Test section ID 400 mm, height 4 m
High pressure test rig Research Centre, Trondheim
• In operation since 2003
• Pressure; 1 – 100 bara
• Fluids; hydrocarbon gas and liquid or Nitrogen with Exxsol D-60
• Test section ID 150 mm, height 4 m
Offshore and onshore production facilities
Test facilities for gas/liquid separation
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Separators• Horizontal separators
– Liquid dominated service
– Oil/water separation
– Better for slug handling
– GLF 0.122 – 0.152 m/s for L/D = 5*
• Vertical separators
– Gas dominated service (scrubber)
– Gas/liquid separation
– GLF 0.10 – 0.30 m/s depending on internals used and operating conditions
Tordis SSBI (Subsea separation Boosting and Injection)
*Campbell J.M; Gas conditioning and processing Volume 2, ISBN 0-9703449-1-0
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Inline separators
• StatoilHydro inline technology
– Static swirl element
– Separation chamber
– Bulk separation
• Advantages
– Small footprint
– Installed as a part of the piping system
– Low installation cost compared to conventional solution
– Ideal for debottlenecking
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Scrubber inlet design
• Technologies
– Inlet vane
– Inlet cyclones
– Spinlet
– Inlet tangential baffle
• Purpose
– Flow distribution (inlet vane)
– High liquid separation efficiency(Spinlet, inlet cyclones and inlettangential baffle)
Inlet vane
Spinlet Inlet Cyclones
Inlet tangential baffle
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Meshpad- Coalescence of small droplets
• Structures
– Layered
– Rolled
– Random
• Material
– Metals: SS, alumina, copper, titanium
– Polymers: PP, PE,co-knit(multifilament glas fibers)
• Porosity, ε• Wire-dimension, dwire
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GLF [m/s]
Pres
sure
dro
p[P
a/m
]
Dry mesh
Operatedmesh
Effic
iency
[%]
100
Demisting efficiency
Flooding-point behaviour
Flooding-point
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Operational characteristics
GLF [m/s]
Pres
sure
dro
p[P
a/m
]
Effic
iency
[%]
100
Demister:
• High primary separation efficiency
• Capture small droplets (2-10 microns)
Pre-conditioner:
• Lower primary separation efficiency
• Act as an agglomerator, coalesces smaller droplets to larger ones
• Conditions the secondary demister elements, i.e vanes or cyclones
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Primary separation efficiency
- Separation efficiency decreases with increased pressure
- K-value is not a correct way to scale
Figure is taken from: Austrheim, T. Experimental Characterization of High-Pressure Natural Gas Scrubbers, University of Bergen 2006, ISBN 82-308-0248-3, Ph.D Thesis.
Constant liquid load at 0.2 vol%
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Demisting- Final polishing of the gas flow
• Technologies
– Axial flow cyclone
– Vane pack
– Filters
• Purpose
– Separate remaining liquid
)(
)(
gasliqdrain
draingasliq
gPh
ghP
ρρ
ρρ
−Δ
=
⇒−=Δ
Drainage margin:
Vane pack Axial flow cyclone
Filter
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Demisting- Final polishing of the gas flow
Double pocket vane pack Axial flow cyclone
- Based on high centrifugal force
- Liquid deposits on wall and drains through slits
- GLF ~ 1 m/s
- Vertical and horizontal flow vane pack
- Based on change in flow direction
- Liquid deposits on wall and drains through slits
- GLF (vertical flow) ~ 0.12 m/s*
- GLF (horizontal flow) ~ 0.20 – 0.30 m/s*
Slits for liquid drainage
*Campbell J.M; Gas conditioning and processing Volume 2, ISBN 0-9703449-1-0
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Droplet separation with cyclone technology
Measured droplet sizes over a non flooded mesh pad for natural gas condensation @92 bara:
1 10 100Particle Diameter (µm)
0
50
100
Cum
ulat
ive
Vol
ume
(%)
K = 0.063 m/sK = 0.100 m/sK = 0.121 m/s
1 10 100Particle Diameter (µm)
0
50
100
Cum
ulat
ive
Vol
ume
(%)
K = 0.063 m/sK = 0.100 m/sK = 0.121 m/s
Cut size d50 @92 bara
Grade efficiency:
Separation efficiency for a given particle size
Cut size d50:
Particle diameter with 50% separation efficiency
Gra
de e
ffici
ency
Calculated for a Verlaan cyclone at 92 bar pressure, GLFvessel = 0.15 m/s
Figures are taken from: Austrheim, T. Experimental Characterization of High-Pressure Natural Gas Scrubbers, University of Bergen 2006, ISBN 82-308-0248-3, Ph.D Thesis.
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Droplet separation with cyclone technology
70
75
80
85
90
95
100
0 1 2 3 4 5 6 7
Superficial Gas Velocity [m/s]
Eff
icie
ncy
[%]
20 bar N2/Exxsol50 bar N2/Exxsol92 bar N2/Exxsol20 bar Natural gas50 bar Natural gas92 bar Natural gas
- Separation efficiency decreases with increased velocity and centrifugal force
- Re-entrainment of separated liquid is critical
Figure is taken from: Austrheim, T. Experimental Characterization of High-Pressure Natural Gas Scrubbers, University of Bergen 2006, ISBN 82-308-0248-3, Ph.D Thesis.
Constant cyclone liquid load
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Separation efficiency is a balance between conflicting mechanisms
Testing at real operating conditions shows that re-entrainment is the dominant mechanism
Experimental facilities with real fluid systems at high pressure is crucial to get the correct answers
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Summary
Efficient gas/liquid separation is essential in the oil and gas value chain.
If it does not work………
