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?

Some wind This is easy! from side fromthe below

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Why is gas/liquid separation a big issue?

It is challenging Movie at real operating conditions!

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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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Gas to Europe

Kvitebjrn Troll A on its way offshore

Production capacities: Troll A Kvitebjrn Kollsnes 120 MSm/d 21 MSm/d 144 MSm/d

Kollsnes

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Separation in Gas processing train1st stage scrubber 1st stage separator 2nd stage scrubber Glycol contactor 3rd stage scrubber

Rich gas (RG)120 110 100 90 80 70 60 50 40 30 20 10 0 -20

Rich Gas Cricondenbar

Testseparator

Pressure [bara]

Calculated dew point curve 2nd stage scrubber outlet Operating point 2nd stage scrubber

EOS = SRK

-10

0 10 Temperature [C]

20

30

Norne

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Basic definitionsGas Load Factor, GLF (often called K-value)

Demisting; removal of remaining liquid; here Axial Flow Cyclones (AFC)

GLF = U gas

gas liq gasMesh Pad for coalescing/demisting

Gas dynamic pressure (often called momentum) gasv Liquid fraction Separation efficiency - each internal - total Liquid entrainment -carry-overvolume liquid volume gasliquidseparatedin scrubber scrubberliquidfeed

Primary separation inlet device; here inlet vane liquid not separated in scrubber

-re-entrainment (liquid separated but teared up again into the gas)

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Souders-Brown constant(Gas Load Factor (GLF), K-value)

Fd = Gdd

GLF

The GLF assumes constant Droplet diameter (varies with pressure and interfacial tension) Drag coefficient Cd (varies with pressure because of different flow regime)

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Important parameters for gas/liquid separationInterfacial 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, no internal research activity was established, but many 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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Test facilities for gas/liquid separationLow pressure test rig 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 Research Centre, Trondheim

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

K-lab large scale test facility K-lab, Krst 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

Offshore and onshore production facilities

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SeparatorsHorizontal 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 designTechnologies Inlet vane Inlet cyclones Spinlet Inlet tangential baffleInlet vane

Purpose Flow distribution (inlet vane) High liquid separation efficiency (Spinlet, inlet cyclones and inlet tangential baffle)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, Wire-dimension, dwire

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Flooding-point behaviourDemisting efficiency Pressure drop [Pa/m]100

Operated mesh Floodingpoint

Efficiency [%]

Dry mesh

GLF [m/s]

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Operational characteristicsPressure drop [Pa/m]100

Demister:

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

Efficiency [%]

High primary separation efficiency Capture small droplets (2-10 microns)

GLF [m/s]

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Primary separation efficiency

Constant liquid load at 0.2 vol%

- 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.

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Demisting- Final polishing of the gas flow

Technologies Axial flow cyclone Vane pack Filters

Purpose Separate remaining liquid Filter

Drainage margin:

P = ( liq gas ) ghdrain hdrain =Vane pack Axial flow cyclone

P g ( liq gas )

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Demisting- Final polishing of the gas flow

Slits for liquid drainage

Double pocket vane pack

Axial flow cyclone

- 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*

- Based on high centrifugal force - Liquid deposits on wall and drains through slits - GLF ~ 1 m/s

*Campbell J.M; Gas conditioning and processing Volume 2, ISBN 0-9703449-1-0

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Droplet separation with cyclone technologyGrade efficiency: Separation efficiency for a given particle size Cut size d50: Particle diameter with 50% separation efficiency Measured droplet sizes over a non flooded mesh pad for natural gas condensation @92 bara: Cut size d50 @92 bara100 Cumulative Volume (%)Grade efficiency

Calculated for a Verlaan cyclone at 92 bar pressure, GLFvessel = 0.15 m/s

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K = 0.063 m/s K = 0.100 m/s K = 0.121 m/s

0 1

10 Particle Diameter (m)

100

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 technology100

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Efficiency [%]

90

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20 bar N2/Exxsol 50 bar N2/Exxsol80

92 bar N2/Exxsol 20 bar Natural gas 50 bar Natural gas 92 bar Natural gas

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Constant cyclone liquid load70 0 1 2 3 4 5 6 7

Superficial Gas Velocity [m/s]

- 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.

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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