moisture monitoring in natural gas torbjoern vegard loekken

8/12/2019 Moisture Monitoring in Natural Gas Torbjoern Vegard Loekken

1/22

Moisture monitoring in natural

gas

IFEA, On-line analyse 14th September 2011

Torbjrn Vegard Lkken

Kjersti Omdahl Christensen

Statoil Research and Development1


2/22

2

Outline

Introduction

Definitions

Measurement technologies

Field experience

Conversion dew point/conc.

Field test K-lab

Conclusions


3/22

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


4/22

4

Definitions

Dew point

the highest temperature, at a

specified pressure, where

water spontaneously can

condense from the gas phase(typical > 0C)

Frost point

the highest temperature, at aspecified pressure, where ice

can spontaneously precipitate

from the gas phase (< 0C)


5/22

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


6/22

14

Field experience

Common challenges:

Choice of sampling point

Sample conditioning

Electronics (capacitor

technology)

Quality control of water dew

point meters


7/22

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


8/22

16

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)


9/22

17

Water content of sweet Natural Gas vs. Dew Point

Figure: From GPSA engineering data book Figure: From Campbells Gas cond. and proc.


10/22

18

0

50

100

150

200

250

300

350

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

Temperature [C]

Watercontent[ppm

(mol)]

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


11/22

19

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.: Lkken 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 40Temperature [C]

Pres

sure[bar]

Hydrocarbon dew point

Hydrocarbon buble point

water dew point

frost point

hydrate point


12/22

20

Water content of sweet Natural Gas vs. Dew Point

Figure: CPA-EOS model, equilibrium with hydrates (red lines)Ref.: Lkken 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]

Equilibriumwatercontentofsweetnaturalgas

[pp

mmole]

1 bara 10 25 50 75 100 250


13/22

21

Phase behaviour

Figure: Phase behaviour of natural gas with traces of water (40 ppm(mole)) and TEG (0.5ppm(mole)), NG composition (mole): 85 % C1, 10 % C2, 4 % C3, 0.5 % nC4, 0.5 % iC4Ref.: Lkken 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 [C]

Pre

ssure[bar]

Hydrocarbon dew point

Hydrocarbon buble point

aqueous dew point

frost point

hydrate point

TEG freezing point


14/22

22

Field test at K-lab,

Statoil large scale test facility at Krst

A facility for testing of analyticalequipment is built and installed at K-lab

Statpipe rich gas (via Krst

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/orhigh pressure (HP) gas.

Test period: 2009-2010


15/22

23

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

Moisturecon

tent[mol/mol]

Date

QCM Capacitor LP Capacitor HP


16/22

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

Moistureconte

nt[mol/mol]

Date

Capacitor LP QCM QC Capacitor LP, 32C 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

Moistureconte

nt[mol/mol]

Date

Capacitor LP QCM QC Capacitor HP


17/22

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

25


18/22

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 mol/mol

Which frost point at 70 bara corresponds to 30 mol/mol?? Use tools from the suplier (typically based on extrapolated Bukacek IGT

research bulletin #8) to convert 30 mol/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)

26


19/22

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 mol/mol

Magnus formula or Sonntags formula converts30 mol/mol to -52 C frost point.

Offset adjustment of 6 C is necessary

27


20/22

28

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


21/22

29

Conversion between units:

Traces of glycol or other polar compounds complicatesboth conversion and calculation of properties of

existing phases

Campbell/GPSA/Bukacek commonly used conversionat 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


22/22

Moisture monitoring in natural gas

Torbjrn Vegard Lkken

Principal researcher

[email protected], tel: +47 95273028

www.statoil.com

Thank you

30

moisture monitoring in natural gas torbjoern vegard loekken

Documents