Gas Liquid Separation

transcript

Classification: Open

Gas/liquid separation

Course: TPG 4140 Natural Gas

Date: 30.10.2008

Bernt Henning Rusten, StatoilHydro R&D Centre Trondheim

Content

– Where and when is separation required

– Gas/liquid separation theory

– Why gas/liquid separation research

– Laboratory facilities

– Gas/liquid separators and internals

Why is gas/liquid separation a big issue?

This is easy!Some wind

from the side

Some wind

from below

Why is gas/liquid separation a big issue?

It is challenging

at real operating

conditions!

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

Troll A on its way offshore

Gas to Europe

Kollsnes

Production capacities:

Troll A 120 MSm³/dKvitebjørn 21 MSm³/dKollsnes 144 MSm³/d

Kvitebjørn

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]

Calculated dew point curve 2nd stage scrubber outlet

Operating point 2nd stage scrubber

EOS = SRK

Rich Gas Cricondenbar

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

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)

Fd = Gd

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)

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.

Challenges related to insufficient gas/liquid separation

Animation

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

• 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

• 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

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

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

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

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

GLF [m/s]

Dry mesh

Operatedmesh

Demisting efficiency

Flooding-point behaviour

Flooding-point

Operational characteristics

GLF [m/s]

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

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%

Demisting- Final polishing of the gas flow

• Technologies

– Axial flow cyclone

– Vane pack

– Filters

• Purpose

– Separate remaining liquid

gasliqdrain

draingasliq

⇒−=Δ

Drainage margin:

Vane pack Axial flow cyclone

Filter

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

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)

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

1 10 100Particle Diameter (µm)

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

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.

Droplet separation with cyclone technology

0 1 2 3 4 5 6 7

Superficial Gas Velocity [m/s]

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

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

Summary

Efficient gas/liquid separation is essential in the oil and gas value chain.

If it does not work………

Gas Liquid Separation

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