ISO/TR 11150:2007
(Main)Natural gas - Hydrocarbon dew point and hydrocarbon content
Natural gas - Hydrocarbon dew point and hydrocarbon content
ISO/TR 11150:2007 describes the various means of estimating hydrocarbon dew point and hydrocarbon content of natural gas.
Gaz naturel — Point de rosée d'hydrocarbure et teneur en hydrocarbure
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 11150
First edition
2007-12-15
Natural gas — Hydrocarbon dew point
and hydrocarbon content
Gaz naturel — Point de rosée d'hydrocarbure et teneur en hydrocarbure
Reference number
ISO/TR 11150:2007(E)
©
ISO 2007
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ISO/TR 11150:2007(E)
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ISO/TR 11150:2007(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Background . 1
4 Why is hydrocarbon dew point important? . 2
5 What is the definition of hydrocarbon dew point?. 3
6 Specifications. 4
6.1 EASEE-gas European association for the streamlining of energy exchange — Gas . 4
6.2 United Kingdom Health and Safety Executive. 4
7 Hydrocarbon dew point measurement .4
7.1 General. 4
7.2 General sampling. 4
7.3 Hydrocarbon dew point determination/estimation/monitoring. 5
7.4 Physical methods . 5
7.5 Analysis and calculation. 8
7.6 Comparative studies. 10
8 Conclusions . 10
9 Recommendations. 11
Annex A (informative) Dew scope measurements (cold mirror) . 12
Annex B (informative) Chromatographic methods . 16
Annex C (informative) Potential hydrocarbon liquid content . 18
Bibliography . 19
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ISO/TR 11150:2007(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
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International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 11150 was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 1, Analysis
of natural gas.
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ISO/TR 11150:2007(E)
Introduction
With Resolution 6 at its Prague meeting in 2004, ISO/TC 193/SC 1, Analysis of natural gas, decided to publish
a Technical Report on guidance for various International Standards on hydrocarbon dew point and
hydrocarbon content.
The main purpose of this Technical Report is to explain to the wider gas community the complex issues
behind the natural gas property called hydrocarbon dew point on the application of various International
Standards on these subjects.
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TECHNICAL REPORT ISO/TR 11150:2007(E)
Natural gas — Hydrocarbon dew point and hydrocarbon
content
1 Scope
This Technical Report describes the various means of estimating hydrocarbon dew point and hydrocarbon
content of natural gas.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 6327, Gas analysis — Determination of the water dew point of natural gas — Cooled surface
condensation hygrometers
ISO 6570:2001, Natural gas — Determination of potential hydrocarbon liquid content — Gravimetric methods
ISO 6974 (all parts), Natural gas — Determination of composition with defined uncertainty by gas
chromatography
ISO 7504:2001, Gas analysis — Vocabulary
ISO 10715:1997, Natural gas — Sampling guidelines
ISO 14532:2001, Natural gas — Vocabulary
Technical Corrigendum:2002
ISO 23874, Natural gas — Gas chromatographic requirements for hydrocarbon dewpoint calculation
3 Background
Hydrocarbon dew point is often a requirement of gas quality specifications in sales contracts where gas is
traded or crosses international borders. It can also be quoted in health and safety legislation. It is usually
specified as a temperature at a defined pressure or over a range of pressures above which no hydrocarbon
condensation will occur. It may alternatively be expressed as a maximum amount of hydrocarbon liquid which
may condense under specific pressure and temperature conditions.
Under certain conditions, higher hydrocarbons present in natural gas or similar gases may condense and the
condensate formed can cause difficulties in the operation of gas transport and distribution systems. Phase
behaviour in hydrocarbon mixtures such as natural gas is highly non-ideal. More ideal behaviour, such as that
of water in air, or, indeed, in natural gas, gives a dew point temperature which continually increases with
pressure. Retrograde behaviour, which affects hydrocarbon mixtures, produces dew point temperatures which
have a maximum value at an intermediate pressure. Figure 1 shows a typical phase diagram.
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ISO/TR 11150:2007(E)
Key
X temperature in degrees centigrade
Y pressure in bar
1 dense phase
2 gas only
3 gas + liquid
4 dewline
5 critical point
6 cricondentherm
7 cricondenbar
Figure 1 — Natural gas phase diagram
In Figure 1 the dew line is the phase boundary. To the right of and above this line only single-phase gas exists.
Within the curve of the dew line, both gas and liquid phases are present, in proportions which are governed by
the pressure and temperature. The closer to the line, the smaller is the proportion of liquid. The
cricondentherm is the point of maximum dew point temperature. Above this temperature only single-phase
gas exists, whatever the pressure. Similarly, at pressures above the cricondenbar, only single phase or dense
phase gas exists, whatever the temperature.
A gas with a dew line similar to that in Figure 1 would be stable single-phase at − 5 °C and 70 bar. If its
pressure is reduced isothermally, it would remain single phase to about 55 bar, at which point it would
encounter the phase boundary and condensate would start to separate. As pressure is further reduced,
varying ratios of gas and condensate will be found until about 15 bar, when it returns to single phase for the
remainder of its depressurization. In fact, once condensate has separated, it is unlikely that it will instantly
return to the gas phase, and so liquids may continue to be present at lower pressures. Another consideration
is that pressure reduction without the addition of heat is isenthalpic rather than isothermal, and
Joule-Thomson cooling will cause the two-phase region to be encountered earlier, unless the gas has been
pre-heated so that the cooling curve stays in the single phase region.
4 Why is hydrocarbon dew point important?
Avoidance of condensate formation is important for pipeline operations. The presence of condensate can
cause problems with filters and with measurement and control instrumentation. There is also an issue with gas
turbines in power plants – significant damage can be caused by the presence of condensate in the burners.
A measured dew point temperature which is lower than the specification limit should give assurance that
condensation will not occur. For certain needs, such as plant design, or if it is known that a certain amount of
condensable material can remain after treatment, knowledge of the quantity of condensable hydrocarbons
produced at specified conditions is useful. This can be physically measured using ISO 6570.
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ISO/TR 11150:2007(E)
Operators require confidence in the measured result. At present, there is little or no information on the
maximum permissible error of measurement, from any type of measurement which is strongly dependent on
the type and nature of the natural gas. The single determined value as measured is accepted as the basis for
decision making. However, for some natural gases, even a small decrease in temperature can result in a large
liquid drop out.
5 What is the definition of hydrocarbon dew point?
The following definitions can be found in various ISO documents, such as in ISO 14532.
2.6.5 Dew points
2.6.5.2 Hydrocarbon dew point
2.6.5.2.1 Hydrocarbon dew point
temperature above which no condensation of hydrocarbons occurs at a specified pressure
NOTE 1 At a given dew point temperature there is a pressure range within which retrograde condensation can
occur.
The cricondentherm defines the maximum temperature at which this condensation can occur.
NOTE 2 The dew point line is the locus of points for pressure and temperature which separates the single phase
gas from the biphasic gas-liquid region.
2.6.5.2.2 Retrograde condensation
phenomenon associated with the non-ideal behaviour of a hydrocarbon mixture in the critical region
wherein, at constant temperature, the vapour phase in contact with the liquid may be condensed by a
decrease in pressure; or at constant pressure, the vapour is condensed by an increase in temperature
NOTE Retrograde condensation of natural gas is the formation of liquid when gas is heated or pressure is
reduced.
2.6.5.2.3 Potential hydrocarbon liquid content (PHLC)
amount of liquid potentially condensable per unit volume of gas at a given temperature and pressure
Or alternatively in ISO 7504.
3.5.2 Dew point
at a specified pressure, the temperature at, or below which, condensation from the gas phase will occur
3.1 Equation of State
mathematical relationship between the state variables (pressure and temperature) of a gas or gas mixture,
and the volume occupied by a given amount of substance
3.5.1 Critical point
single point in pressure-temperature phase diagram at which the composition and properties of the gas
and liquid phases in equilibrium are identical
NOTE 1 The pressure at this point is known as the “critical pressure” and the temperature as the “critical
temperature”.
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6 Specifications
Two examples from legislation and gas trading can be given.
6.1 EASEE-gas European association for the streamlining of energy exchange — Gas
Common business practice 2005-001/01 Harmonisation of natural gas quality 3/2/05. Hydrocarbon dew point
shall be maximum of − 2 °C at 1 bar to 70 bar.
NOTE EASEE-gas has not observed any technical constraints in conflict with the proposed harmonized values, and
such values were adopted by 1/10/06. This same published document also made the comment “The need for introducing
a harmonized measuring method has been identified”.
6.2 United Kingdom Health and Safety Executive
A guide to the Gas Safety (Management) Regulations 1996.
Schedule 3 Content and other characteristics of gas.
Regulation 8 Part 1 Requirements under normal conditions.
Hydrocarbon dew point and water dew point shall be at such levels that they do not interfere with the
integrity or operation of pipes or any appliance [within the meaning of regulation 2(1) of the 1994 Regulations]
that a consumer could reasonably be expected to operate.
7 Hydrocarbon dew point measurement
7.1 General
The definition of the theoretical hydrocarbon dew point alone is not well understood by measurement
technicians/scientists/engineers or those who draft sales gas contracts. At the most pedantic level, the dew
point temperature is the minimum value at which no condensate is present, but only homogeneous gas phase.
However this cannot and will never be physically measured as the first molecule will not be detected
because the available methods are not sensitive enough (theoretical hydrocarbon dew point
temperature).
So the method of determination of hydrocarbon dew point is critical, as each approach is based on different
measurement principles, the hydrocarbon dew point can only be estimated with an associated uncertainty,
and the true value cannot be achieved.
7.2 General sampling
Before any instrument or method can be deployed, a sample of the gas to be measured, has to be taken from
the flowing gas in the pipeline and transferred unaltered to the measuring device.
ISO 10715 states:
3 Principles of sampling
3.1 Sampling methods
The main function of sampling is to take an adequate sample that is representative of the gas by direct
sampling such that the sample is drawn from a stream and directly transferred to the analytical method.
Care should be exercised to transfer a representative sample flowing in the pipeline unaltered and in a
homogeneous single gas phase state to the analytical method. In general, turbulent flow is advantageous
in a sampling system and in a gas line to be sampled because turbulence creates a well-mixed gas.
Great care and consideration should be given for a gas near the dew point, as a reduction in line
temperature will cause some condensation to occur, as will rapid reduction of pressure isothermally
(adiabatic expansion causing cooling) resulting in two-phase sample gas flow.
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7.3 Hydrocarbon dew point determination/estimation/monitoring
There are two principal methods: direct determination by physical observation of liquid condensation and
calculation from detailed analysis. A third method measuring a similar/related property of potential
hydrocarbon liquid formation (gravimetric method) is also discussed.
The first technique and apparatus for determining the dew point of gases under pressure was published by
[1]
Deaton and Frost . This manual method was widely adopted as it was the only method available and by
default has become the de facto standard. However, there is currently no published ISO method with any
performance characteristics. There is a method describing how manual cold mirrors operate in the ISO
standard for water dew point. See ISO 6327. That International Standard also describes the procedures
necessary for precautions against hydrocarbon or other liquid formation on the mirror prior to the water dew
point determination (see Annex A for further information).
Similarly, the method for calculation from detailed analysis by gas chromatography is described in ISO 23874.
That International Standard only deals with the chromatographic requirements and not with equation of state
numerical methods. For the benefit of the user this draft is given in Annex B.
There is a published standard for potential hydrocarbon liquid content: ISO 6570. Explanation on the
essentials is given in Annex C.
The real requirement is for an on-line automated method for the determination of the physical gas property of
hydrocarbon dew point. The potential hydrocarbon liquid content method does not readily lend itself to
automation.
7.4 Physical methods
7.4.1 General
Natural gas is passed through a cell at a defined pressure – typically in the range 25 bar to 35 bar. The cell
contains a reflecting surface on to which light is directed and which can be observed. The surface is cooled
and its temperature at the first appearance of liquid is recorded. This is regarded as the dew point temperature
at the pressure of measurement. The operation can be controlled manually, in which case the appearance of
liquid on the surface is usually detected by eye, or automatically, in which case the detection is by means of a
photocell.
7.4.2 Physical methods — Advantages
⎯ Condensation of liquid is clearly demonstrated.
⎯ It is the de facto standard method, and experienced operators can achieve good agreement when using it.
⎯ The manual equipment is portable, so sampling should not be a problem. The cooling can be done by
expansion of carbon dioxide or air, avoiding the need for electrical power. Of the physical methods, it may
be assumed that dew point temperature is a more rigorous specification than quantity of condensate
(PHCL). It assumes, after all, “zero” condensate at the specified temperature rather than “not more than a
defined amount”.
⎯ The automatic equipment conforms to safety requirements for installation on process plant.
7.4.3 Physical methods — Disadvantages
⎯ A small power source is needed to illuminate the cooled surface with the manual equipment, but it is
regularly used under a permit to work scheme.
⎯ A defined amount of liquid needs to condense before it becomes visible. This will occur at a temperature
below the theoretical dew point temperature (the first molecule) and the extent of the difference varies
between gases from different sources. The effect may be significant. The rate at which liquid condensate
forms as a function of the temperature below the theoretical dew point depends upon the composition of
the gas. Figures 2 to 4 show the dew lines and iso-vapour fraction lines for three gases. The iso-vapour
3 3 3
fraction lines correspond to condensate quantities of 50 mg/m , 100 mg/m and 200 mg/m in each case.
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Key
X temperature in degrees centigrade
Y pressure in bar
1 dewline
3
2 50 mg/m
3
3 100 mg/m
3
4 200 mg/m
Figure 2 — Gas 1 condensation behaviour
Key
X temperature in degrees centigrade
Y pressure in bar
1 dewline
3
2 50 mg/m
3
3 100 mg/m
3
4 200 mg/m
Figure 3 — Gas 2 condensation behaviour
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Key
X temperature in degrees centigrade
Y pressure in bar
1 dewline
3
2 50 mg/m
3
3 100 mg/m
3
4 200 mg/m
Figure 4 — Gas 3 condensation behaviour
3
Gas 1 generates condensate at a rapid rate below the dew point temperature: 50 mg/m is found at a
temperature of 0,2 °C below the dew point. Gas 2, which has a higher dew point value, is also reasonably
3
prompt, at 0,6 °C; 50 mg/m is a reasonable estimate of the amount of condensate necessary to form a visible
3
deposit on a cold mirror. The quantity will vary with cell design. Gas 3 is exceptional, generating 50 mg/m at
7,6 °C below the dew point temperature.
The behaviour of gas 3 is unusual, but it is a genuine natural gas, accepted into the transmission system. It
may be that the increasing use of a larger number of smaller supplies will lead to more such gases being
delivered.
The point of maximum dew point temperature (the cricondentherm) is found at different pressures with
different gases. Physical dew point measurement occurs at a selected pressure (default in Europe is 27,6 bar
gauge or 400 psi). This default pressure may not reflect the actual cricondentherm as gas composition varies.
It is not practicable to make a series of measurements at different pressures, and so significant bias may be
present. Figure 5 shows dew lines for gases 2 and 3, as above, and also for gas 4, compared to a typical
measurement pressure of 28 bar. The cricondentherm for gas 2 is at around 30 bar, and so the difference
between the maximum dew point temperature and that at 28 bar is only 0,1 °C.
Gas 3 has a cricondentherm at around 20 bar, some way away from the measurement pressure, which means
that the measured dew point would be underestimated by 1 °C from the maximum. The cricondentherm for
gas 4 occurs at about 47 bar, which means that the measured value would be 2,4 °C below the maximum.
These calculations assume that the theoretical dew point is instantly recognized; in fact, as we have seen
from Figure 4, gas 3 will be subject to a larger error due to its slow rate of condensate generation. Figure 5
also shows that choice of a measurement pressure more suited to gas 3 or gas 4 would produce larger errors
for the others.
It is possible (but relatively unlikely) that the condensed phase may be water or other liquid (glycols, methanol,
etc.) rather than hydrocarbons. While experienced manual cold mirror operators claim they can identify the
type of condensate, inexperienced operators may incorrectly identify other liquids as hydrocarbons.
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ISO/TR 11150:2007(E)
Key
X temperature in degrees centigrade
Y pressure in bar
1 gas 4
2 gas 2
3 gas 3
4 28 bar
Figure 5 — Effect of measurement pressure
7.5 Analysis and calculation
7.5.1 General
Equation of state software can calculate phase properties, including hydrocarbon dew point, from a detailed
analysis. The analysis usually requires equipment and conditions that are laboratory-based, although there is
increasing interest and an increasing use of on-line process analysers to perform the task. The analysis must
be detailed, measuring major components and also, importantly, trace quantities of higher hydrocarbons.
Simplifications such as measuring all C and higher hydrocarbons as a single “pseudo-component”, which is
6
satisfactory for calorific value determination, are not acceptable. It is not possible to use a single analytical
system, as is the case for calorific value applications; the detail required for higher hydrocarbons needs a
different analytical system from that used for major components. It is possible to use a single instrument, but it
needs to contain different analytical sub-systems.
The approaches taken for laboratory and process use are sufficiently different such that they are considered
separately.
7.5.2 Laboratory analysis
7.5.2.1 General
The sample must come to the analyser. The gas may be analysed directly, in which case the combination of
sample size and column dimensions must be well adapted to allow the increasingly small quantities of
increasingly higher-boiling hydrocarbons to be measured with sufficient precision. Alternatively, the higher
hydrocarbons can be pre-concentrated in an adsorption tube and/or cold trap to allow larger quantities to be
analysed. Lower boiling components are not amenable to pre-concentration and so the method must be well
defined to ensure that the intended components are quantitatively pre-concentrated.
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ISO/TR 11150:2007(E)
7.5.2.2 Laboratory analysis — Advantages
⎯ Gas chromatography using capillary columns, temperature programming and a flame ionization detector
(FID) is well established for the range of components, and has sufficient sensitivity to be applied to sub-
parts per million levels.
⎯ Pre-concentration techniques allow measurement of parts per billion levels.
⎯ From a comprehensive analysis, equation of state software allows multiple calculations. The dew point at
a selected pressure, or a range of dew points, or just the cricondentherm can be calculated as required.
Alternatively, it is possible to calculate the amount of condensate at particular conditions in the two-phase
region, and the properties and composition of the condensate and of the gas with which it is in equilibrium
can be found.
7.5.2.3 Laboratory analysis — Disadvantages
⎯ The quality of the result is totally reliant on the quality of the sample. Great care must be taken to ensure
that the sample is representative, with no components lost in whole or in part, and no cross-contamination
from previous use.
⎯ Quantitative data is usually calculated using the assumption that the FID is a carbon counter.
Components are then quantified relative to a component, such as butane or pentane, which has been
measured as part of a major component analysis. This assumption may not be true, depending on the
operating parameters of the analyser, and should be checked. Certified gas mixtures containing typical
components of interest are not widely available (although that is improving) and expensive.
⎯ There are clear differences in the result when using the same analytical data with different equations of
state. Figure 6 shows the differences for gases 2, 3 and 4 when using the same analytical data with the
Redlich-Kwong-Soave (RKS) and the Peng-Robinson (PR) equations of state. For results to be compared,
the same equation must be used. There is little information as to which equation should be preferred.
Current work in ISO/TC 193 should improve knowledge in this area as will the GERG studies.
⎯ As with any laboratory measurement, there is an inevitable delay between sampling and reporting.
Key
X temperature in degrees centigrade
Y pressure in bar
1 RKS
2 PR
Figure 6 — Different equations of state
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