Moisture monitoring in natural

gas

IFEA, On-line analyse 14th September 2011

Torbjørn Vegard Løkken

Kjersti Omdahl Christensen

Statoil Research and Development1

2

Outline

• Introduction

• Definitions

• Measurement technologies

• Field experience

• Conversion dew point/conc.

• Field test K-lab

• Conclusions

3

Introduction

• Why moisture determination?

− Gas hydrate formation

• Trace analysis of water vapour ingases is demanding:

− Water molecules are polarand adsorptive

− Water is omnipresent

− High pressure sampling,possible interferences and/orliquid entrainment

− Corrosion potential Specification

4

Definitions

• Dew point

− the highest temperature, at a

specified pressure, where

water spontaneously can

condense from the gas phase

(typical > 0°C)

• Frost point

− the highest temperature, at a

specified pressure, where ice

can spontaneously precipitate

from the gas phase (< 0°C)

5

Measuring techniques

Chilled mirror

Hygrometers

Electrolytic

hygrometers

Karl Fischer

Mass spectrometry

Capacitor sensors

Spectroscopic methods

Piezoelectric(Quartz Crystal

Microbalance - QCM)

Reaction GC

Direct GC

Gravimetric methods

Fibre Optic/

Refraction index

14

Field experience

• Common challenges:

− Choice of sampling point

− Sample conditioning

− Electronics (capacitor

technology)

− Quality control of water dew

point meters

15

A typical field installation

• Sampling point

− Downstrem glycol contactors

− On export line

• Sample conditioning

− High pressure

• Line pressure

• Intermediate pressure 70 bar = Spec.

− Low pressure – QCM, TDLAS, Rx-GC

− Filtering

• Coalescing filter (Panametrics “glycol filter”)

• Membrane filter

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Ref.: Moisture Control & Measurement

Ltd. (MCM)

Ref.: Chandler engineering, Ametek, Inc.

A typical field installation cont.

• Type of hygrometer

− Capacitor hygrometers still most common in Statoil (HP)

− QCM increasingly used (LP)

• Quality control

− Periodical recalibration/change

of sensor (capacitors)

− Manual check with

• portable (MCM) (LP)

• Chandler chilled mirror (HP)

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Water content of sweet Natural Gas vs. Dew Point

Figure: From GPSA engineering data book Figure: From Campbell’s Gas cond. and proc.

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0

50

100

150

200

250

300

350

-20 -15 -10 -5 0 5 10 15 20

Temperature [ºC]

Wate

r co

nte

nt

[pp

m (

mo

l)]

Measurement (Karl Fischer)

CPA-EoS

GERG-water-EoS

Empirical correlation of Bukacek [17]

Chart of McKetta and Wehe [14])

Experimental data of water content in natural gas at 150 bar

compared to estimates from the empirical correlation of Bukacek

and the chart based method of McKetta and Wehe

CPA-EoS versus older conversion methods

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Phase behaviour

Figure: Phase behaviour of natural gas with traces of water (40 ppm(mole)), NG composition (mole): 85 % C1, 10 % C2, 4 % C3, 0.5 % nC4, 0.5 % iC4.Ref.: Løkken et al., Water content of high pressure natural gas: Data, prediction and experience from field, IGRC conference 2008

0

10

20

30

40

50

60

70

80

90

100

-60 -40 -20 0 20 40

Temperature [ºC]

Pre

ss

ure

[b

ar]

Hydrocarbon dew point

Hydrocarbon buble point

water dew point

frost point

hydrate point

20

Water content of sweet Natural Gas vs. Dew Point

Figure: CPA-EOS model, equilibrium with hydrates (red lines)Ref.: Løkken et al., Water content of high pressure natural gas: Data, prediction and experience from field, IGRC conference 2008

1

10

100

1000

-50 -40 -30 -20 -10 0 10 20 30 40 50

Temperature [°C]

Eq

uilib

riu

m w

ate

r co

nte

nt

of

sw

ee

t n

atu

ral g

as

[pp

m m

ole

]1 bara 10 25 50 75 100 250

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Phase behaviour

Figure: Phase behaviour of natural gas with traces of water (40 ppm(mole)) and TEG (0.5 ppm(mole)), NG composition (mole): 85 % C1, 10 % C2, 4 % C3, 0.5 % nC4, 0.5 % iC4Ref.: Løkken et al., Water content of high pressure natural gas: Data, prediction and experience from field, IGRC conference 2008

0

10

20

30

40

50

60

70

80

90

100

-60 -40 -20 0 20 40

Temperature [ºC]

Pre

ss

ure

[b

ar]

Hydrocarbon dew point

Hydrocarbon buble point

aqueous dew point

frost point

hydrate point

TEG freezing point

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Field test at K-lab,

Statoil large scale test facility at Kårstø

• A facility for testing of analytical

equipment is built and installed at K-lab

− Statpipe rich gas (via Kårstø

processing plant)

− gas from closed gas loop at K-lab

(water, MEG)

• Several moisture analysers are set up for

comparison at low pressure (LP) and/or

high pressure (HP) gas.

• Test period: 2009-2010

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Field test of moisture analysers

• Drifting behaviour

• Stability and regularity

• Sensitivity over time

• Speed of response

• Accuracy

• Sensitivity on contamination

• High versus low pressure

New capacitor probes

0

10

20

30

40

50

21-Jun 26-Jun 1-Jul 6-Jul 11-Jul 16-Jul 21-Jul 26-Jul

Mo

istu

re c

on

ten

t [µ

mo

l/m

ol]

Date

QCM Capacitor LP Capacitor HP

24

Drifting of capacitors

0

30

60

90

120

150

3-Jun 5-Jun 7-Jun 9-Jun 11-Jun 13-Jun 15-Jun 17-Jun

Mo

istu

re c

on

ten

t [µ

mo

l/m

ol]

Date

Capacitor LP QCM QC Capacitor LP, 32°C offset Capacitor HP

0

30

60

90

120

150

3-Jun 5-Jun 7-Jun 9-Jun 11-Jun 13-Jun 15-Jun 17-Jun

Mo

istu

re c

on

ten

t [µ

mo

l/m

ol]

Date

Capacitor LP QCM QC Capacitor HP

Main conclusion from field test

• Weekly quality control is necessary, even more frequent if

irregularities/water peaks

• Capacitor hygrometers were prone to drifting – offset adjustments

necessary.

− Offset adjustments only valid close to the reference

concentration/frost point

• Set-up and sampling system was not suited for comparison of response

times

• No clear conclusions regarding influence of glycol were made

• All moisture monitoring should be performed at low pressure for increased

accuracy

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QC of capacitor hygrometers - HP

• Example at high pressure monitoring

− Capacitor hygrometer reads -25 °C at 70 bara = FROST POINT

− Spot check with portable hygrometer gives 30 µmol/mol

− Which frost point at 70 bara corresponds to 30 µmol/mol??

− Use tools from the suplier (typically based on extrapolated Bukacek – IGT

research bulletin #8) to convert 30 µmol/mol to a frost point at 70 bara:

-20 °C

− Offset adjustment of 5 °C is necessary

− Calculation with CPA-model: offset adjustment of 8,5 °C !!!(CPA-model availability: DTU, CERE)

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QC of capacitor hygrometers - LP

• Example at low pressure – preferable!

− Capacitor reads -58 °C at atmospheric pressure = FROST POINT

− Spot check with portable hygrometer gives 30 µmol/mol

− Magnus formula or Sonntags formula converts

30 µmol/mol to -52 °C frost point.

− Offset adjustment of 6 °C is necessary

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• Quality control system absolutely necessary for reliable

measurements

− monitoring at atmospheric pressure provides higher

accuracy and easier QC

− hygrometers with built-in QC is preferable

− irregular moisture concentration demands more

frequent QC

Conclusions

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• Conversion between units:

− Traces of glycol or other polar compounds complicates

both conversion and calculation of properties of

existing phases

− Campbell/GPSA/Bukacek commonly used conversion

at high pressure. Inaccurate at low moisture conc.

− Several accurate methods available for conversion at

atmospheric pressure (Magnus and Sonntag formula)

− Statoil uses the CPA-model for calculations on water,

and other polar compounds, in natural gases

Conclusions

Moisture monitoring in natural gas

Torbjørn Vegard Løkken

Principal researcher

tvl@statoil.com, tel: +47 95273028

www.statoil.com

Thank you

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