Condensing behaviour of natural gases
Version:
Author
1.0
Markus Wolf
Contents
1 2 3 4 Phase behaviour of natural gases.................................................................................................... 2 Phase behaviour calculated from gas composition.......................................................................... 2 How to read the diagrams ................................................................................................................ 4 References ....................................................................................................................................... 5 Version Bearbeiter
1.0
Markus Wolf
Art der Änderung
Status
Freigabe / Datum
29.05.2015
1
Phase behaviour of natural gases
Pressure
Natural gases are multi-component mixtures. Unlike pure components natural gases cover a region
where the gaseous phase co-exists with the liquid phase. This phase region is enclosed by the bubble
point and dew point curves. The two curves intersect at the critical point. The maximum temperature at
which condensation occurs is called
the cricondentherm. Experience has
confirmed that the cricondentherm is in
Cricondenbar
Retrograde condensation
a pressure range of 20 to 30 bar for
traded natural gases. Line 1 describes
how the cricondentherm changes as
liquid drop-out increases. It connects
Critical point
the points where the largest liquid
Dew point curve
drop-out volumes are to be expected.
Bubble point
The region enclosed by line 1 and the
curve
dew point curve is the region where
Liquid
retrograde condensation occurs; it
Gas + liquid
depends directly on gas quality. The
word retrograde is of Latin origin and
1
means moving backward. Retrograde
Cricondentherm
behaviour means transition from the
Constant-condensate
volume curves
gas phase (or liquid phase) to partial
(quality curves)
Gas
condensation (or partial vaporisation)
and back to the initial phase at an
isothermal change in pressure or an
isobaric change in temperature. This
Temperature
property is particularly pronounced for
mixtures boiling over a wide temperature range such as natural gas [1].
In practical operations the most interesting aspect is the transition from the single-phase gas region to the
two-phase gas-liquid region. Phase transition occurs along the dew point curve which describes the
change in the hydrocarbon dew point (HCDP) as a function of pressure and temperature. The position of
the dew point curve is determined by the hydrocarbon with the highest bubble point; often, it is only
contained in traces in the natural gas. The method or instrument used for measuring the HCDP has a
major influence on the results obtained as detection and determination limits differ significantly. It may not
be possible to determine the exact position of the dew point curve [1]. The curves connecting points for
which liquid drop-out remains constant are also called quality curves.
2
Phase behaviour calculated from gas composition
The method calculates the phase behaviour of a natural gas on the basis of an equation of state, gas
composition and substance data from a database. The laboratory at Open Grid Europe (OGE) uses the
Peng-Robinson equation for this purpose. The quality of the results obtained is greatly dependent on the
quality of the gas composition and substance data used as input data. The condensation behaviour of a
natural gas can only be calculated if the composition of the gas is known accurately in both qualitative
and quantitative terms. The analysing method selected is gas chromatography which allows the complex
compositions encountered in natural gases to be handled. The level of resolution depends on the
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situation to be examined. For example, a C6 analysis is sufficient where the superior calorific value is to
be determined for billing purposes in custody transfer metering. C6 analyses are provided by process gas
chromatographs (PGCs) installed in the gas network. In Germany, PGCs must be approved by PTB, the
Federal Institute of Physics and Metrology. C6 analyses are obtained directly from the gas phase. The
hydrocarbon fractions through to C5 are determined individually; hexanes and higher hydrocarbons are
summarised in a C6+ fraction.
The OGE laboratory makes a C10 analysis where information about gas quality is required. This
information can also be obtained directly from the gas phase. The hydrocarbon fractions through to C9
are determined individually; decanes and higher hydrocarbons are summarised in a C10+ fraction.
But C6 and C10 analyses are not sufficient where a realistic prediction of phase behaviour and liquid
drop-out is needed. A higher resolution is then required. For this purpose the OGE laboratory developed
the C40 analysis for which the higher hydrocarbons are enriched on an adsorber. Approx. 0.5 m³ of
sample gas are sent through a
stainless steel tube filled with the
adsorbent. In the process the higher
hydrocarbons collect on the adsorber
and can then be eluted with a solvent
in the laboratory. The eluate is
analysed using gas chromatography.
The method determines components
from C10 to C40. The detection limits
for the individual components are in the
one- to two-digit ppb range. The
method can also be employed to detect
traces of oil carried into the gas stream
during compression for transportation.
The C10 and C40 analyses are combined to form the extended analysis which is the basis for calculating
phase behaviour and liquid drop-out.
100
In the example shown, the cricondentherms of
the calculated dew point curves differ by up to
24 Kelvin depending on which analysis is
employed. Using the extended analysis, those
curves for constant liquid drop-out were
calculated which are congruent with the dew
point curves calculated on the basis of the C6
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C6 analysis
80
C10 analysis
Extended analysis
70
60
Constant-condensate
volume curves
(calculated on the basis
of standard/extended
analyses)
40
30
T = 3 K
50
T = 21 K
The analysis used (C6, C10 or extended) is an
important factor influencing the position of a
calculated dew point curve.
90
Pressure in bar
When comparing calculated condensate
volumes and condensate volumes determined
gravimetrically to DIN EN ISO 6570 it was
found that an extended analysis is
indispensable for a realistic prediction of the
condensation behaviour of natural gases.
20
10
1,500 mg/m³
10 mg/m³
0
-50
-45
-40
-35
-30
-25
-20
Temperature in °C
-15
-10
-5
0
analysis and C10 analysis respectively. When predicting phase behaviour on the basis of a C10 analysis
liquid would start to drop out, in the example shown, at a pressure of 27 bar and a temperature of -7 °C;
in the case of a C6 analysis liquid would not form until a temperature as low as -28 °C is reached. But
realistically a liquid drop-out volume of 10 and 1,500 mg/m³ respectively would have to be expected at
these p,T conditions.
The calculation method based on gas chromatography provides information about condensation
behaviour, i.e. phase envelope, cricondentherm, cricondenbar including drop-out volumes over the entire
pressure and temperature range.
3
How to read the diagrams
For better explanation a blue point in the diagrams marks the liquid drop-out volume at a pressure of
30 bar and a temperature of -10 °C.
2,600
100
2,400
90
2,200
Liquid drop-out in mg/m³
Pressure in bar
80
70
60
50
Dew point curve
40
30
1 mol ppm (approx. 5 mg/m³)
20
2,000
1,800
-30 °C
1,600
1,400
1,200
-25 °C
1,000
800
600
10
-15 °C
-10 °C
200
~ 50
0
0
-50
-40
-30
-20
Temperature in °C
> -5 °C
-20 °C
400
10 mol ppm (approx. 50 mg/m³)
-10
0
0
10
20
30
40
50
60
70
80
90
100
Pressure in bar
The diagram on the left shows that the point is in the two-phase region, i.e. in the region where the gas
and liquid phases coexist. For the sample gas considered a condensate volume of approx. 50 mg per
normal cubic metre of natural gas drops out at the conditions specified. This point is also shown in the
diagram on the right. The diagram plots liquid drop-out as a function of pressure and temperature.
In practical operations at transfer points, HCDP measuring instruments are increasingly used for HCDP
online measurement in addition to calculation. The instruments are usually operated at factory settings.
The detection limit is between 5 and 50 mg condensate per normal cubic metre of natural gas [2 to 8].
This means that the relevant measurement points are in the region delimited by the quality curves for 1
mol ppm (approx. 5 mg/m³) and 10 mol ppm (approx. 50 mg/m³).
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4
References
[1]
Oellrich, L.; Engler, T.; Kaesler, H.; Nixdorf, J.: Untersuchung zum retrograden
Kondensationsverhalten einiger europäischer Erdgase, GWF-Gas/Erdgas, 137 (1996) No. 1, 1-6.
[2]
Wolf, M.; Anderbrügge, T.; Kaesler, H.: Erfahrungsbericht – Bestimmung des potentiellen
Kohlenwasserstoff-Kondensatgehalts in Erdgasen, GWF-Gas/ Erdgas, (150) 2009, 186-193.
[3]
ISO/TR 12148:2009; Natural gas - Calibration of chilled mirror type
instruments for hydrocarbon dew point (liquid formation).
[4]
Wolf, M.; Kaesler, H.: Hydrocarbon Dew Point Measurement of Natural Gas – Field Report
Gas2009, Rotterdam, Netherlands, 11 to 13 March 2009.
[5]
Wolf, M.: Installation, calibration and validation guidelines for online hydrocarbon dew point
analyzers – GERG PC1 / Project 1.64 / Phase 1, The European Gas Technology Conference
2011 (EGATEC 2011), Copenhagen, Denmark, 12 to 13 May 2011.
[6]
Panneman, H.J.: A traceable calibration procedure for hydrocarbon dew point
meters, AGA Operations Conference, Chicago, 2005.
[7]
Van Wesenbeck, J.M.M.; Panneman, H.J.: Eine rückführbare Kalibriermethode
für Kohlenwasserstofftaupunkt-Messgeräte, GWF – Gas Erdgas, (147) 2006, 27-34.
[8]
Technical report: GERG 1.64 Project / Phase 1 “Installation, calibration and
validation guidelines for online hydrocarbon dew point analyzers”, December, 2010.
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