Comparison of methods for direct and indirect measurement of hydrocarbon dewpoint

Andrew Brown, Martin Milton & Gergely VarghaNational Physical Laboratory, Teddington, UK

Richard Mounce & Chris Cowper, EffecTech Ltd, Uttoxeter, UK

Andrew Stokes, Mike Bannister & Andy Benton, Michell Instruments, Cambridge, UK

Dave Lander & Nick Bates, National Grid, Warwick, UK

Andy Ridge, Orbital, Swynnerton, UK

Hydrocarbon dewpoint

Hydrocarbon dewpoint curve

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Mixture wholly in gas phase

Mixture exists in gas and

liquid phases

cricondentherm

maximum temperature at which condensation can occur (at any pressure)

Hydrocarbon dewpoint curves may be: Calculated from gas composition using an equation of state Measured directly using a chilled mirror method

ISO 14532:2005 (Natural gas Vocabulary) definition:

The temperature above which no condensation of hydrocarbons occurs at a specified pressure.

Hydrocarbon Joint Industry ProjectAims and outline

Aims To compare the performance of

GC methods: process v laboratory Dewpoint instruments: manual v automatic Process methods: process GC v automatic chilled mirror

To produce a definitive dataset for use in future studies and to support ISO standardisation activities

Outline Five synthetic natural gas mixtures Seven real natural gases

Collaborative project, but all analyses carried out with no prior knowledge of other results

Measurement techniques:Gas chromatography

GC systems used:

Lab GC 1 Danalyser model 500 (up to C5) with HP 4890 GC (C5+) Lab GC 2 Danalyser model 500 (up to C5) with Varian 3400 GC (C5+) Process GC 3 Danalyser model 700 (all species) Process GC 4 Orbital all in one system based on Siemens Maxum GC

Hydrocarbon dewpoint curve

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Equation of stateGC analysis

Calculated gas composition

Calculated dewcurve

Component % mol/mol Component % mol/molHelium 0.032 2,3-di-me-C4 0.0121

Hydrogen 0.008 2-me-C5 0.0546Oxygen 0.027 3-me-C5 0.0309Nitrogen 6.112 n-C6 0.0327

Carbon dioxide 0.057 Benzene 0.0002Methane 83.507 cyc-C6 0.0171Ethane 5.760 C7 fraction 0.0339

Propane 2.399 Toluene 0.0000i-Butane 0.486 Me-cyC6 0.0056n-Butane 0.891 C8 fraction 0.0022

neo-Pentane 0.009 C9 fraction 0.00026i-Pentane 0.282 C10 fraction 2.6E-06n-Pentane 0.231 C11 fraction ND

2,2-di-me-C4 0.0085 C12 fraction ND

Calibrated using the same traceable, multi-component, reference gas standards

Calibration gas also used to define GC retention times etc.

Gas samples

Use of equations of state

Hydrocarbon dewcurves were calculated using: RKS (Redlich-Kwong-Soave) LRS (London Research Station; variant of RKS with different functional forms for

some constants) PR (Peng Robinson)

All three EoS are cubic

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RKS

LRS

PR

2.0 C5.0 C

Film fully visible

6.0 C

No film visible

5.5 C

Slight mark visible

Measurement techniques:Chilled mirror(manual and automatic)

(diameter of visible mirror approx 1 cm)

7.5 C

No film visible7.0 C

No film visible

6.5 C

No film visible

4.5 C4.0 C3.5 C3.0 C2.5 C

2.0 C

Michell InstrumentsCondumax II automaticchilled mirror

Dark spot optical principle Collimated red light focussed on

conical depression in optical surface - condensate reduces light intensity in dark spot

Chandler model A-2 manual chilled mirror

Manual chilled mirror: Sensitivity

Sensitivity -setting of 275 mV equivalent to manual chilled mirror

Adaptive trigger point correction under development.

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Low 1 - Run 1Low 1 - Run 2Low 2 - Run 1Low 2 - Run 2Mid 1 - Run 1Mid 1 - Run 2Mid 2 - Run 1Mid 2 - Run 2High 1 - Run 1High 1 - Run 2

Instrument calibration

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Gravimetric compositionAutomatic chilled mirrorManual chilled mirror

Chilled mirror instruments Calibrated thermometers

/ temperature sensors Instruments validated by

measurement of a gravimetric mixture of 10% n-butane in nitrogen

Synthetic mixtures

Five synthetic Primary Reference Gas Mixtures prepared gravimetrically and verified by NPL Mixtures contained n-isomers up to C10, benzene, toluene and cyclohexane Compositions chosen to encompass the range of hydrocarbon condensation

rates found in gas fields around the British Isles

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Calculated condensation rates

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Calculated hydrocarbon dewcurves

Synthetic mixtures:Results (1)

Phase One - Mixture 'Mid 1'

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Phase One - Mixture 'Mid 1'

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Gravimetric dataLab GC 1

Automatic chilled mirror

Synthetic mixtures: Results (2)

Phase One - Mixture 'Mid 2'

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Phase One - Mixture 'High'

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Phase One - Mixture 'Low 1'

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Phase One - Mixture 'Low 2'

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Gravimetric dataLab GC 1

Automatic chilled mirror

Synthetic mixtures:Results summary

Strong agreement between GC and gravimetric data Average difference at cricondentherm of 0.3C

Good agreement between automatic chilled mirror and gravimetric data Average difference at cricondentherm of 1.9C These differences are within the uncertainty of the measurement

The use of a different equation of state can lead to a difference in calculated dewpoint of more than 2C

Automatic chilled mirror results are always lower than the gravimetric data Closer agreement is observed when comparing the automatic chilled mirror data to

an calculated dewpoint for 70 mg.m-3 of condensate

Synthetic mixtures:Dewpoint at 70 mg.m-3 of condensate

70 mg.m-3 is an empirical estimate of the amount of condensate required for detection by the automatic chilled mirror Corresponds approximately to an automatic chilled mirror sensitivity of 275mV 70 mg.m-3 of condensate equivalent to approx 11ppm n-decane

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Lines = automatic chilled mirror measurementCircles = calculated T for the formation of 70 mg.m-3 of condensate

Low 1 Low 2 Mid 1 Mid 2 High

Average difference between the automatic chilled mirror and 70 mg.m-3 of condensate (calculated from gravimetric data) now 0.7C

(was 1.9C for theoretical dewpoint)

Real natural gases:Outline

Seven samples of real natural gas Sampled (following ISO 10715:2001) into evacuated 50 litre cylinders

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Gas AGas BGas CGas DGas EGas FGas G

Calculated hydrocarbon condensation rates: (from GC analysis data)

Analysis much more challenging than for synthetic mixtures due to the highly complex nature of the gases

Real natural gases:Analysis methods

GC Lab GC 1 [Danalyser 500 + HP 4890 GC] Lab GC 2 [Danalyser 500 + Varian 3400 GC] Process GC 3 [Danalyser 700] Process GC 4 [Orbital all in one system:- Siemens Maxum GC]

Automatic chilled mirror Standard sensitivity setting (275mV)

Manual chilled mirror

Calculations All hydrocarbon dewcurves calculated using RKS equation of state Water dewlines calculated using RKS equation of state with interaction

parameters, Rij = 0.5.

Real natural gases:Gas A results

Highest dewpoint; second highest condensation rate

Manual chilled mirrorAutomatic chilled mirrorWater dewline

Lab GC 1Lab GC 2Process GC 3Process GC 4

Gas A

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Gas A

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Very good agreement between GC methods 1, 2 & 3 Gas A has the highest dewpoint, a high C6 to C7 content and a low C8+ content

easy to analyse Process GC 4 results are lower than lab GCs (but close to manual chilled

mirror) May be due to the automatic allocation of boiling points by the process GC or

choice of equation of state (under investigation)

Real natural gases:Gas C results

Second lowest condensation rate; high aromatic content; highest C8 to C12 content

Manual chilled mirrorAutomatic chilled mirrorWater dewline

Lab GC 1Lab GC 2Process GC 3Process GC 4

Gas C

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Gas C

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Relatively large difference between manual and automatic chilled mirrors Gas has the longest hydrocarbon tail Difference may be due to sampling issues, or other factors related to the operation

of the instrument (cooling rate, sensitivity, etc.) Close agreement between manual chilled mirror and process GC data

Real natural gases:Discussion (1)

GC methods: process GC v laboratory GC

For six out of seven real gases the measured dewpoints are in the order:Lab GC 1 > Lab GC 2 > Process GCs

Order demonstrates sensitivity of instruments to detect higher hydrocarbons The outstanding has a low hydrocarbon dewpoint measurements in this

temperature region are not crucial to legislators

Results for Process GCs show more variability. Due to: Automated assignment of boiling points by process GC instrument? Sampling issues? Sensitivity to equation of state? Under investigation

Real natural gases:Discussion (2)

Dewpoint instruments: manual chilled mirror v automatic chilled mirror

For all real gases, the measured dewpoints are in the orderAutomatic chilled mirror > Manual chilled mirror

Difference in measured dewpoint ranges from 1.7C (Gas E) to 9C (Gas F) Differences larger than expected

For some gases, closer agreement obtained when using the adaptive trigger method

Replicates retrograde behaviour of calculated dewcurves Method still under development

Automatic chilled mirroradaptive trigger method

Gas B

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Standard (275 mV) sensitivityAdaptive trigger method

Gas C

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Gas F

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Real natural gases:Discussion (3)

Process methods: process GC v automatic chilled mirror

Real gases: Automatic chilled mirror > Process GCs Difference would be more pronounced if the GC data was used to calculate the

dewpoint at 70 mg.m-3 of condensate In complex gas mixtures, GCs may undermeasure, not detect or misidentify

higher hydrocarbons and unexpected components Butthe manual chilled mirror measures dewpoints that are lower than those

measured by the GCs.

Synthetic mixtures: Process GCs > Automatic chilled mirror Simple gas mixtures. Measurement by GC relatively is straightforward GC

results agree very well with the gravimetric data

Real natural gases:Discussion (4)

Role of synthetic gas mixtures

Essential for the calibration of laboratory and process GCs

Role in the calibration of chilled mirror instruments may be limited

Calibration using an n-butane in nitrogen standard is straightforward and inexpensive, but the mixture does not form a typical hydrocarbon film

soltysmZvraznenie

What is the true value of hydrocarbon dewpoint?

What is the true value of hydrocarbon dewpoint?

Methods agree to within a reasonable estimate of their uncertainties None of the methods can be said to definitively measure the true

hydrocarbon dewpoint GCs may not measure trace quantities of higher hydrocarbons, or

unexpected components (e.g. heavy oils, glycols, etc.) Chilled mirror instruments are dependent on detection of a liquid film, and

other operational and instrumental factors Reported hydrocarbon dewpoint should be appended with the method

used and any assumptions made

soltysmZvraznenie

soltysmZvraznenie

Conclusions

Comprehensive study of hydrocarbon dewpoint measurement of synthetic and real natural gases

Close agreement for measurement of synthetic natural gases

Measurement of real natural gases more difficult. Methods agree to within a reasonable estimate of their uncertainties

True value of hydrocarbon dewpoint difficult to define

Data from the project available at:www.npl.co.uk/environment/hydrocarbondewpoint.html

Contributions of project partners

Facilitated collection of real gas samples Owner and operator of Great Britains

national gas transmission system

Laboratory gas chromatography Process gas chromatography

Manual chilled mirror Process gas chromatography

Automatic chilled mirror

Preparation and certification of high-accuracy natural gas standards

What is hydrocarbon dewpoint?

ISO 14532:2005 (Natural gas Vocabulary) definition: The temperature above which no condensation of hydrocarbons occurs

at a specified pressure.

Theoretical dewpoint cannot be measured in practice requires the detection of the first molecule of liquid condensate Discussions on-going within ISO/TC193/SC1 to redefine as a technical

or measurable dewpoint e.g. define the amount of hydrocarbon liquid formed for a given gas at a given temperature and pressure

Alternative approach would be to define a technique-specific correction factor and include suspected error in the uncertainty budget

Hydrocarbon dewpoint often not well defined in legislation Term should be appended with, e.g. the method of determination or a

maximum permissible error

Comparison of methods for direct and indirect measurement of hydrocarbon dewpointHydrocarbon dewpointHydrocarbon Joint Industry ProjectAims and outlineMeasurement techniques:Gas chromatography Use of equations of stateMeasurement techniques:Chilled mirror(manual and automatic)Manual chilled mirror: SensitivityInstrument calibrationSynthetic mixturesSynthetic mixtures:Results (1)Synthetic mixtures: Results (2)Synthetic mixtures:Results summarySynthetic mixtures:Dewpoint at 70 mg.m-3 of condensateReal natural gases:OutlineReal natural gases:Analysis methodsReal natural gases:Gas A resultsReal natural gases:Gas C resultsReal natural gases:Discussion (1)Real natural gases:Discussion (2)Automatic chilled mirroradaptive trigger methodReal natural gases:Discussion (3)Real natural gases:Discussion (4)What is the true value of hydrocarbon dewpoint?ConclusionsContributions of project partnersWhat is hydrocarbon dewpoint?