Dew Point Measurements in Natural Gas

Andy BentonTechnical Consultant

53rd Technical SeminarABNT – CB-09

Comite Brasileiro de Gases CombustivesNatal – RN – Brazil15th April 2010

Running order:• What is hydrocarbon and water dew point?

• Why is it important to control and measure dew point in natural gas?

• How can HC dew point be measured?1. Manual visual cooled-mirror dewpointmeter,2. Determination of Liqiud to Gas Ratio (PHLC),3. Equation of state method from GC analysis,4. Direct on-line measurement with Automatic,

Optical Condensation Dewpointmeter.

• Water dew point measurement principle

H2O Dew Point

HC Dew Point

-40 -30 -20 -10 0 +100

1

2

3

4

5

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7

8

Wholly gaseousHC liquids

present in

gas

Water and

HC liquids

preset in

gas

Temperature, °C

Pressure

MPa

Cricondentherm

Typical Phase Envelope for Northern Europe natural gas

WHAT IS HYDROCARBON DEW POINT?THEORECTICAL DEFINITION: ISO 14532:2005 (Natural gas – Vocabulary) defines hydrocarbon dew point as “The temperature above which no detectable condensation of hydrocarbons occurs at a specified pressure.”

WORKING DEFINITION: API Manual of Petroleum Measurement Standards Chapter 14.1 – Natural Gas Fluids Measurement: “The temperature at which hydrocarbon condensates first begin to form a visible deposit of droplets on a surface, when the gas is cooled at a constant pressure.”

Water & Hydrocarbon Dew Point Reduction Plant

Well Head

Liquid HC (crude) and

bulk water

Mechanical Separator

Glycol Contactor

10-15MPa

J.T. Valve

�P

= 3 to 5

MPa

Low-Temperature

Separator(LTS)

Condensate

(NGL)

Compressor

Transmission

Pipeline

7 to 10 MPa

Water dew point reduction

Hydrocarbon dew point reduction

After extraction from the gas field, natural gas is processed to remove water and heavy hydrocarbon components, before transmission through the pipeline network. This diagram shows a typical plant for the removal of both excess moisture and heavy hydrocarbons

Seabed Pipeline

Why is it Important to Control and Measure Hydrocarbon and Water Dew Point?

Commercial Issues

• Hydrocarbon and water dew point are gas quality parameters specified in the sales contracts agreed between buyer and seller.

• Typical transmission gas specifications (EASEE-gas):

– Hydrocarbon dew point lower than –2 oC at 27 barg

– Water dew point lower than –8 oC at full pipeline

pressure

1/10/20100.7000.555m3/m3d

1/10/2006- 2-oC at 1 – 70 bar (a)HC DP

- 8-oC at 70 bar (a)H2O DP

1/10/20062.5-mol %CO2

1/10/2010[0.01]*-mol %O2

1/10/20066-mg/m3RSH (as S)

1/10/20065-mg/m3H2S + COS (as S)

1/10/200630-mg/m3Total S

1/10/201015.81[13.60]kWh/m3WI

Recommended

implementation dateMaxMinUnitParameter

1/10/20100.7000.555m3/m3d

1/10/2006- 2-oC at 1 – 70 bar (a)HC DP

1/10/2006–Some exceptions- 8-oC at 70 bar (a)H2O DP

1/10/20062.5-mol %CO2

1/10/2010[0.01]*-mol %O2

1/10/20066-mg/m3RSH (as S)

1/10/20065-mg/m3H2S + COS (as S)

1/10/200630-mg/m3Total S

1/10/201015.81[13.60]kWh/m3WI

Recommended

implementation dateMaxMinUnitParameter

EASEE-gas approved gas quality parameters and values

• EASEE-gas: European Association for the Streamlining of Energy Exchange

• Objectives to harmonise business practices across the industry

• 80 cross border points across Europe

Technical Issues

• Measurement of hydrocarbon and water dew point is important in order to:

– Prevent hydrates and condensates – Solid crystalline and liquid formations that limit pipeline capacity and damage compressors and valves.

– Limit pipeline corrosion – Excess water leads to leaks and fractures.

– Achieve Superheat requirements – Fuel gas specifications for gas turbine power plant.

Hydrate formations

Compressor damage – Liquid impact!

HC Dew Point Applications

Hydrocarbon Dew Point Reduction Processing

Natural Gas Dew Point Applications

Gas quality measurementat custody transfer

HC Dew Point Applications:Trans- and Inter-Continental Supply Contracts –Pipeline Interconnections

• Market de-regulation

• Introduction of competitive practices in gas supply and retailing

European Interconnections

U.S. Natural Gas Pipeline Network, 2009

National Distribution of Natural Gas in Brazil

Underground storage facilities to supply transient demand

HC Dew Point Applications:Gas Trading

HC Dew Point Applications:Gas Turbine Electricity Generation

• Avoid liquids in hot gas path

• Uneven flame temperature and length

• Avoid flashbacks

• Turbine manufacturers stipulate 20 to 30°C of superheat for fuel gas to low NOx plant

• Dew-point measurement enables control of fuel gas heaters

• Potential return on investment through reduced parasitic load

How can Hydrocarbon Dew Point be Measured?

1. Manual Visual Dew Point Method –“Dew Scope”

2. Determination of Liquid to Gas Ratio (PHLC)

3. Equation of State Method Using GC Composition Analysis

4. Automatic Optical Condensation Method

Hc Dew Point Measurement Methods

G

a

sP

P

1. Manual Visual Cooled-Mirror Dew-Point Checker

• Bureau of Mines, ASTM/API method

Images courtesy Ametek®

PROS CONS

+ The traditional and widely

used measurement

technique

+ The de facto standard for

hydrocarbon dew point

measurement

+ Low capital investment -

Instrument costs $5,000 to

$10,000

– Periodic spot checking

only

– ‘Subjective’, operator

dependant measurement

of variable sensitive and

repeatability

– Labor intensive, therefore

high running costs

Manual Visual Cooled-Mirror Dew-Point Checker

Gas

Gas Outlet

Liquid Collection

Separator

Cooling Coil

Pressure and flow controls

P

PGas Inlet

Drain

Temperature

Controlled Bath

2. Determination of Liquid to Gas Ratio

PROS CONS

+ A direct measurement method

where contractual specifications

stipulate a liquid to gas ratio

(Potential Hydrocarbon Liquid

Content to ISO 6570)

+ An absolute expression of the

condensate formation potential

of the gas at a certain

temperature and pressure

(grams/cubic metre of gas)

– Impractical for field

application and installation

– High cost ($100,000 plus)

– Not measuring Hc Dew Point,

the parameter most

commonly used to control

this aspect of gas quality

– Averaging analysis technique

leads to slow response to

process changes

Determination of Liquid to Gas Ratio

• GCs provide Calorific Value (BTU)

• Custody transfer is based on BTU and thus there is a large installed base

• Field units are C6+ with a few C9+

• Equations of State (EOS) can give a calculated HC dew point BUT sensitivity of analysis governs accuracy of results

3. Equation of State Method Using GC Analysis

Here is an illustration of how sensitivity of analysis directly effect calculated HcDP estimations.

Why?

ISO 23874 (2006) ‘Natural Gas – GasChromatographic requirements for hydrocarbon dew point calculation’

• “be capable of measuring alkenes up to and including dodecane”,

•“be capable of measuring individual alkenes at a concentration of 0.000 000 1 (0.1 ppm)”

• “be capable of measuring individual alkenes at a concentration of 0.000 000 1 (0.1 ppm)”

•“measure all hydrocarbons in the range C5 to C12”

PROS

+ Potential to combine a number of gas quality/fiscal metering parameters into one analyzer

+ The components contributing to a high dew point level may be identified and so help to determine the reason/source.

+ Possibility to provide a theoretical phase envelope curve

CONS

- Accuracy is dependant on correct use of specialized reference gases

- Indirect method of determining HcDP

- The limit of analysis sensitivity can produce large HcDP errors

- Trained staff required to maintain proper performance

- High initial and operating costs plus the analyzer house, and personnel costs.

EOS Method Using GC Analysis

4. Automatic, Optical Condensation Dewpointmeter

• It is possible to automate the optical condensation measurement principle to give an on-line measurement of hydrocarbon dew point that is reproducible, free from operator bias and drift.

• In Michell’s case, this is achieved with Condumax II Hydrocarbon Dew-Point Analyser, that utilises a patented detection technique known as the Dark Spot Principle.

Cooled Mirror Principle of Operation

Water Dew or IceWater Dew or IceWater Dew or IceWater Dew or Ice

Sample gasSample gasSample gasSample gas

AUTOMATIC, OPTICAL CONDENSATION DEWPOINTMETER

Main difficulties in making such a measurement:

• Natural gas condensate forms an ‘invisible’ low surface tension film

• Fractional condensation

Where is the hydrocarbon dew point?

Liquid to Gas Ratio Curve

• Surface Tension –Water is very High

• Hydrocarbons are very low

HcDP is a different image than a water dew point to a Chilled/Cooled Mirror

• Water dew point sensors are mirrors

• HcDP sensors should be rough

• When water condenses on the mirror by cooling it, the reflected light is diffused.

• HcDP actually forms a reflective surface

So optical detection surfaces are used?

Shell Research

Detection Principle Development Sequence

Dry

Light

Sou

rce

COOLED MIRROR

Shell Research

COOLED MIRROR

H2O Wetted

Light

Sou

rce Detector

� Good sensitivity for water dewpoint

Detection Principle Development Sequence

Shell Research

HC Wetted

Light

Sou

rce Detector

X Totally insensitive for HC dewpoint

COOLED MIRROR

Detection Principle Development Sequence

Shell Research

ETCHED SURFACE

Dry

Light

Sou

rce

Detection Principle Development Sequence

Shell Research

ETCHED SURFACE

HC Wetted

Light

Sou

rce Detector

X Poor sensitivity for HC dewpoint

Detection Principle Development Sequence

Shell Research

ETCHED SURFACE WITH CONICAL DEPRESSION

Dry

Light S

ource Detector

Detection Principle Development Sequence

Shell Research

HC Wetted

Light S

ource Detector

ETCHED SURFACE WITH CONICAL DEPRESSION

Detection Principle Development Sequence

� Excellent sensitivity for HC dew point

Michell DARK SPOT™ detection technique

Dark Spot™ Detector Images

Dry Sensor

Dark Spot™ Detector Images 2

Some Condensate beginning to form

Dark Spot™ Detector Images 3

More Condensate forming –This is the dew point image

Dark Spot™ Detector Images 4

Excess Condensate – This image is below the dew point

Dark Spot™ Detector Images 5

Advanced Condensation – This image is well below the dew point

Typica

l 10 m

inute m

easurement cy

cles

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Tim

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tical S

urfa

ce ('M

irror') T

em

pera

ture

, deg

rees F

Dete

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ign

al C

han

ge, m

V

CONDUMAX II HYDROCARBON DEW-POINT ANALYSER

• High Sensitivity - To better than 5 mg/m3 using DARK SPOT Technique

• Facility to produce an ‘LGR plot’ -‘Sensitivity Calibration’ routine

Confronting the issues:

• Decoupling the sample flow rate/cooling rate inter-relationship - fixed volume sample analysis

CONDUMAX HYDROCARBON DEWPOINTMETER (circa 1985-2004)

CONDUMAX II Combined Hydrocarbon and Water Dew-Point Analyser

• Automatic System

• ‘Dark Spot’ HCDP measurement

• Ceramic Moisture Sensor (optional WDP)

• EEx d ATEX approval

• Verification routines, low maintenance

CONDUMAX II Hydrocarbon Dew-Point Analyser

Main Unit

Remote Interface

Sampling System

Sensor Cell Assembly

Sensor Cell exploded ……

Condumax II

• Self-contained Measuring system

• 0.5 deg C HCDP accuracy

• Touch screen operation

• Modbus RTU communications

• 2 x 4-20 mA output

• Active X controls for plant integration

CONDUMAX II –Measurement range capability

Depression Graph Condumax II Sensor Cells

Pressure with natural gas at 27 barg.

-6.6

-21.7

-34.3

-49

-63-70

-60

-50

-40

-30

-20

-10

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1 2 3 4 5

Tem

pera

ture

, d

eg

ree C

amb Maximum cooling depression

CONDUMAX II – Simplicity….

Condumax II - Installation made simple:

� Wholly hazardous area installation – No safe area requirement

� Direct installation in field locations – No analyser house requirement

� 90-264Vac, 50/60Hz, 200 Watt, is the only utility requirement

� No cooling air or gas required

� Hermetically sealed Peltier cooler - No dry gas purge required

• Close to the sample tap

• Sample tap on top quadrant of pipe

• Heat-traced and insulated sample tubing

• Sun shade over instrument housing, or in-doors

Typical Field Installation

Pros Cons

+ Direct fundamental ‘objective’

measurement with high sensitivity

and repeatability

+ Stand alone operation – Built-in

verification routines

+ No special maintenance staff

requirements or certified reference

cylinder gases

+ Its like having your own skilled

technician taking a API/ASTM

Bureau of Mines reading every 10

minutes, 24/7.

– Significant initial investment but very low operating costs

AUTOMATIC HcDP ANALYZER

Condumax II – Water Dew Point

• Water dew-point measurement

• Michell Ceramic Moisture Sensor

• Line pressure transducer

• Accuracy +/- 1°C WDP

• 206 barg rated

• Sour gas compatible

Michell Ceramic Moisture Sensor

Ceramic Substrate

Conductive Layer

Porous ConductiveLayer

HygroscopicActive Layer

Michell Ceramic Moisture Sensor

0

5

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20

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30

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-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20

Dewpoint, deg. C

Sensor Im

pedance, K o

hm

s

Condumax IIIndustry proven sensor technology

• Michell Ceramic Moisture Sensor

• Measurement range: 0.001 to 30,000 ppmVCalibrated: -100 to +20oC dew point.Response to -130 and up to +30oCdp

• Accuracy: +/-10% of moisture content reading

• 300 barg rated, immune to pressure shock

• Chemically inert, resilient to extreme sour gas (30% H2S)

Condumax IICalibration Integrity

• UKAS accredited calibration laboratory

• Certified calibration traceable to NPL (UK) and NIST (USA)

• Unique cost effective MichellCalibration Exchange Serviceavailable to all users globally

Thanks you for attending

Any questions?

Pls contact ENGEZER

www.engezer.com.br