Natural gas - Gas chromatographic requirements for hydrocarbon dewpoint calculation (ISO 23874:2006)

ISO 23874:2006 describes the performance requirements for analysis of treated natural gas of transmission or pipeline quality in sufficient detail so that the hydrocarbon dewpoint temperature can be calculated using an appropriate equation of state. ISO 23874:2006 can be applied to gases that have maximum dewpoint temperatures (cricondentherms) between 0 °C and - 50 °C. The pressures at which these maximum dewpoint temperatures are calculated are in the range 2 MPa (20 bar) to 5 MPa (50 bar).
The procedure given in ISO 23874:2006 covers the measurement of hydrocarbons in the range C5 to C12. n-Pentane, which is quantitatively measured using ISO 6974 (all parts), is used as a bridge component and all C6 and higher hydrocarbons are measured relative to n-pentane. Major components are measured using ISO 6974 (all parts) and the ranges of components that can be measured are as defined in ISO 6974-1.

Erdgas - Gaschromatographische Anforderungen für die Berechnung des Taupunktes von Kohlenwasserstoff (ISO 23874:2006)

Diese Internationale Norm beschreibt die Leistungsanforderungen für die Analyse von behandeltem Erdgas in Transport- oder Rohrleitungsqualität so detailliert, dass die Kohlenwasserstoff-Taupunkttemperatur mit einer geeigneten Zustandsgleichung berechnet werden kann. Sie kann auf Gase angewandt werden, die über maximale Taupunkttemperaturen (Krikondentherm Punkte) zwischen 0 °C und −50 °C verfügen. Die Drücke, bei welchen diese maximalen Taupunkttemperaturen berechnet werden, liegen im Bereich zwischen 2 MPa (20 bar) und 5 MPa (50 bar). Hauptbestandteile werden mittels ISO 6974 (alle Teile) gemessen und die Bereiche der Bestandteile, welche gemessen werden können, sind in ISO 6974 1 definiert. Die in dieser Internationalen Norm dargelegte Verfahrensweise deckt die Messung von Kohlenwasserstoffen im Bereich C5 bis C12 ab. n Pentan, welches quantitativ mittels ISO 6974 (alle Teile) gemessen wird, wird als Kombinationskomponente verwendet und alle C6 und höheren Kohlenwasserstoffe werden in Relation zum n Pentan gemessen.

Gaz naturel - Exigences de chromatographie en phase gazeuse pour le calcul du point de rosée hydrocarbures (ISO 23874:2006)

L'ISO 23874:2006 décrit les exigences de performance applicables à l'analyse de gaz naturel traité de qualité transport ou réseau avec suffisamment de détails pour pouvoir calculer le point de rosée hydrocarbures au moyen d'une équation d'état appropriée. Elle peut être appliquée aux gaz dont la température de rosée maximale (cricondentherm) se situe entre 0 °C et −50 °C. Les pressions de calcul de ces points de rosée varient de 2 MPa (20 bar) à 5 MPa (50 bar). Les constituants majeurs sont mesurés suivant l'ISO 6974, les gammes de constituants pouvant être mesurés étant telles que définies dans cette norme (ISO 6974‑1). Le mode opératoire décrit ici couvre le mesurage des hydrocarbures de C5 à C12. Le n-pentane, dont la mesure quantitative s'effectue suivant l'ISO 6974 (toutes parties), sert de composé pont et tous les hydrocarbures en C6 et supérieurs sont mesurés par rapport au n-pentane.

Zemeljski plin - Zahteve plinske kromatografije za izračun rosišča ogljikovodikov (ISO 23874:2006)

Standard ISO 23874:2006 opisuje zahteve glede učinkovitosti za analizo obdelanih zemeljskih plinov za prenos in zemeljskih plinov iz cevovoda dovolj podrobno, da je mogoče izračunati temperaturo rosišča ogljikovodika z ustrezno enačbo stanja. Standard ISO 23874:2006 je mogoče uporabiti za pline z najvišjo temperaturo rosišča (krikodenterma) med 0 °C in –50 °C. Tlaki za izračun teh najvišjih temperatur rosišča so v razponu od 2 MPa (20 barov) do 5 MPa (50 barov).
Postopek iz standarda ISO 23874:2006 zajema merjenje ogljikovodikov v razponu C5 do C12. n-pentan, ki se meri kvantitativno z uporabo standarda ISO 6974 (vsi deli), se uporablja kot povezovalna komponenta, C6 in višji ogljikovodiki pa se merijo glede na n-pentan. Glavne komponente se merijo z uporabo standarda ISO 6974 (vsi deli) in razponi komponent, ki jih je mogoče izmeriti, so takšni, kot je določeno v standardu ISO 6974-1.

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Publication Date
25-Sep-2018
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SLOVENSKI STANDARD
SIST EN ISO 23874:2018
01-december-2018

=HPHOMVNLSOLQ=DKWHYHSOLQVNHNURPDWRJUDILMH]DL]UDþXQURVLãþDRJOMLNRYRGLNRY

,62

Natural gas - Gas chromatographic requirements for hydrocarbon dewpoint calculation

(ISO 23874:2006)
Erdgas - Gaschromatographische Anforderungen für die Berechnung des Taupunktes
von Kohlenwasserstoff (ISO 23874:2006)

Gaz naturel - Exigences relatives à la chromatographie en phase gazeuse pour le calcul

du point de rosée hydrocarbures (ISO 23874:2006)
Ta slovenski standard je istoveten z: EN ISO 23874:2018
ICS:
71.040.50 Fizikalnokemijske analitske Physicochemical methods of
metode analysis
75.060 Zemeljski plin Natural gas
SIST EN ISO 23874:2018 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 23874:2018
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SIST EN ISO 23874:2018
EN ISO 23874
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2018
EUROPÄISCHE NORM
ICS 75.060
English Version
Natural gas - Gas chromatographic requirements for
hydrocarbon dewpoint calculation (ISO 23874:2006)

Gaz naturel - Exigences relatives à la chromatographie Erdgas - Gaschromatographische Anforderungen für

en phase gazeuse pour le calcul du point de rosée die Berechnung des Taupunktes von

hydrocarbures (ISO 23874:2006) Kohlenwasserstoff (ISO 23874:2006)
This European Standard was approved by CEN on 31 August 2018.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this

European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references

concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN

member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by

translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management

Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels

© 2018 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 23874:2018 E

worldwide for CEN national Members.
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SIST EN ISO 23874:2018
EN ISO 23874:2018 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 23874:2018
EN ISO 23874:2018 (E)
European foreword

The text of ISO 23874:2006 has been prepared by Technical Committee ISO/TC 193 "Natural gas” of the

International Organization for Standardization (ISO) and has been taken over as EN ISO 23874:2018 by

Technical Committee CEN/TC 238 “Test gases, test pressures, appliance categories and gas appliance

types” the secretariat of which is held by AFNOR.

This European Standard shall be given the status of a national standard, either by publication of an

identical text or by endorsement, at the latest by March 2019, and conflicting national standards shall

be withdrawn at the latest by March 2019.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CEN shall not be held responsible for identifying any or all such patent rights.

According to the CEN-CENELEC Internal Regulations, the national standards organizations of the

following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,

Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,

France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,

Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and the United Kingdom.
Endorsement notice

The text of ISO 23874:2006 has been approved by CEN as EN ISO 23874:2018 without any modification.

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SIST EN ISO 23874:2018
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SIST EN ISO 23874:2018
INTERNATIONAL ISO
STANDARD 23874
First edition
2006-11-15
Natural gas — Gas chromatographic
requirements for hydrocarbon dewpoint
calculation
Gaz naturel — Exigences relatives à la chromatographie en phase
gazeuse pour le calcul du point de rosée hydrocarbures
Reference number
ISO 23874:2006(E)
ISO 2006
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SIST EN ISO 23874:2018
ISO 23874:2006(E)
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ii © ISO 2006 – All rights reserved
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SIST EN ISO 23874:2018
ISO 23874:2006(E)
Contents Page

Foreword............................................................................................................................................................ iv

1 Scope......................................................................................................................................................1

2 Normative references............................................................................................................................1

3 Principle..................................................................................................................................................2

4 Materials.................................................................................................................................................2

5 Apparatus...............................................................................................................................................2

6 Performance requirements ...................................................................................................................3

7 Sampling.................................................................................................................................................3

8 Analytical procedure.............................................................................................................................4

9 Methods of test ......................................................................................................................................7

10 Uncertainty in composition ..................................................................................................................9

11 Calculation of dewpoint temperature ..................................................................................................9

12 Analytical uncertainty contribution to dewpoint temperature ..........................................................9

Annex A (informative) Typical analytical conditions for C to C analysis ................................................10

5 12

Annex B (informative) Validation of fraction data..........................................................................................12

Annex C (informative) Precision of area ratio ................................................................................................16

Annex D (informative) Recommendations on sample calibration gas introduction ..................................20

Annex E (informative) Calculation of fraction quantities, boiling points and component

uncertainties.........................................................................................................................................22

Bibliography ......................................................................................................................................................26

© ISO 2006 – All rights reserved iii
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SIST EN ISO 23874:2018
ISO 23874:2006(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

non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the

International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

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.

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 23874 was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 1, Analysis of

natural gas.
iv © ISO 2006 – All rights reserved
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SIST EN ISO 23874:2018
INTERNATIONAL STANDARD ISO 23874:2006(E)
Natural gas — Gas chromatographic requirements for
hydrocarbon dewpoint calculation
1 Scope

This International Standard describes the performance requirements for analysis of treated natural gas of

transmission or pipeline quality in sufficient detail so that the hydrocarbon dewpoint temperature can be

calculated using an appropriate equation of state. It can be applied to gases that have maximum dewpoint

temperatures (cricondentherms) between 0 °C and – 50 °C. The pressures at which these maximum dewpoint

temperatures are calculated are in the range 2 MPa (20 bar) to 5 MPa (50 bar). Major components are

measured using ISO 6974 (all parts) and the ranges of components that can be measured are as defined in

ISO 6974-1. The procedure given in this International Standard covers the measurement of hydrocarbons in

the range C to C . n-Pentane, which is quantitatively measured using ISO 6974 (all parts), is used as a

5 12

bridge component and all C and higher hydrocarbons are measured relative to n-pentane.

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 6974-1, Natural gas — Determination of composition with defined uncertainty by gas chromatography —

Part 1: Guidelines for tailored analysis

ISO 6974-2, Natural gas — Determination of composition with defined uncertainty by gas chromatography —

Part 2: Measuring-system characteristics and statistics for processing of data

ISO 6974-3, Natural gas — Determination of composition with defined uncertainty by gas chromatography —

Part 3: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and hydrocarbons up to C using

two packed columns

ISO 6974-4, Natural gas — Determination of composition with defined uncertainty by gas chromatography —

Part 4: Determination of nitrogen, carbon dioxide and C to C and C hydrocarbons for a laboratory and on-

1 5 6+
line measuring system using two columns

ISO 6974-5, Natural gas — Determination of composition with defined uncertainty by gas chromatography —

Part 5: Determination of nitrogen, carbon dioxide and C to C and C hydrocarbons for a laboratory and on-

1 5 6+
line process application using three columns

ISO 6974-6, Natural gas — Determination of composition with defined uncertainty by gas chromatography —

Part 6: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and C to C hydrocarbons using

1 8
three capillary columns
ISO 6975, Natural gas — Extended analysis — Gas-chromatographic method
ISO 10715, Natural gas — Sampling guidelines
© ISO 2006 – All rights reserved 1
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SIST EN ISO 23874:2018
ISO 23874:2006(E)
3 Principle

For hydrocarbon gas mixtures such as natural gas, the phase boundary or dewpoint line distinguishing single-

phase gas from gas-liquid mixtures is a complex function of pressure, temperature and composition. For a

given composition, the pressure at which the dewpoint temperature is at a maximum is intermediate between

those found in transmission and distribution operations. The analysis shall be comprehensive for inert

components and for hydrocarbons up to C . It is not necessary to measure helium, hydrogen, water and

sulfur compounds unless any of these is present at greater than 0,01 mole fraction.

The analysis shall be performed in two parts. Major components (nitrogen, carbon dioxide and hydrocarbons

from C to C ) shall be analysed according to ISO 6974 (all parts). Higher hydrocarbons (C to C ) shall be

1 5 5 12

analysed to satisfy the requirements given in this International Standard. This allows the traceability of

measurements according to ISO 6974 (all parts) to be extended to the higher hydrocarbons.

It is not possible to identify all the measured higher hydrocarbons, nor is it possible to obtain a reference gas

mixture that contains more than a few representatives of the higher hydrocarbons. The analytical data are,

therefore, handled with a number of simplifying assumptions.

⎯ Unidentified components are allocated a carbon number or molar mass according to their positions in the

chromatogram with respect to identified n-alkanes.

⎯ Alkanes of carbon number 7 and above are summed by carbon number and treated as fractions for input

to the dewpoint calculation.

⎯ Average boiling points and densities of fractions are calculated from the individual boiling points and

quantities of the components that comprise them; individual-component boiling points are calculated by

[1]
interpolation between the bracketing n-alkanes .

⎯ Sample components are quantified by comparison with n-pentane, which has been measured according

to ISO 6974 (all parts), using relative response factors based on their allocated carbon numbers.

NOTE When using ISO 6974 (all parts), n-pentane can be measured as a direct component that is also present in the

calibration-gas mixture, or as an indirect component using a response factor relative to a different component (for

example, n-butane) in the calibration gas mixture. In either case, the uncertainty on the quantity of n-pentane can be

calculated according to ISO 6974-2.
4 Materials

4.1 Certified-reference gas mixture for major components (CRM1), such as is required for ISO 6974 (all

parts).

Depending upon the working range and the accuracy required, more than one CRM can be needed.

4.2 Certified-reference gas mixture, for higher hydrocarbons (CRM2), containing as a minimum, n-

pentane, n-hexane, benzene, cyclohexane, n-heptane, toluene, methylcyclohexane and n-octane. Ideally, the

CRM2 should also contain n-nonane, n-decane, n-undecane and n-dodecane in methane.

The mole fractions of components in CRM2 shall be chosen to be appropriate for the application, provided

that the mixture is stable and free from the possibility of condensation in transport, storage and in use.

5 Apparatus

5.1 Measurement system for major components, comprised of a sample introduction and transfer unit, a

separation unit, a detection unit, an integrator and a data reduction system, capable of meeting the analytical

requirements described in 6.1.
2 © ISO 2006 – All rights reserved
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SIST EN ISO 23874:2018
ISO 23874:2006(E)
ISO 6974 (all parts) describes equipment suitable for this part of the analysis.

5.2 Measurement system for higher hydrocarbons, comprised of a sample-introduction and transfer

unit, a separation unit, a detection unit, an integrator and a data-reduction system, capable of meeting the

analytical requirements described in 6.2.

Annex A describes a system that has been shown to be suitable. The user is responsible for demonstrating in

each case that his/her chosen system is also suitable.
6 Performance requirements
6.1 Major components

The system for measurement of major components shall have performance as described in ISO 6974 (all

parts).
6.2 Higher hydrocarbons

The system for measurement of higher hydrocarbons shall satisfy the following requirements:

⎯ be capable of measuring alkanes up to and including n-dodecane;

⎯ be capable of measuring individual alkanes at a concentration of 0,000 000 1 mole fraction (0,1 ppm ) or

less;

⎯ be able to distinguish and measure benzene, toluene, cyclohexane and methylcyclohexane as individual

components;

⎯ use a detection system that can, at least in principle, measure all hydrocarbons in the range C to C ;

5 12

⎯ use a detection system that has a predictable response to hydrocarbons based on mass or carbon

content, such that unidentified components of known molar mass or carbon number can be measured

relative to other known components in the sample or in the calibration gas;

NOTE 1 It is most likely that the detection system is a flame ionization detector (FID).

⎯ use a separation procedure such that the boiling points of unidentified components can be calculated by

interpolation between those of known n-alkanes.

NOTE 2 Increasing the column temperature at a constant rate throughout the analysis (linear temperature

programming) allows such interpolation.

NOTE 3 Annex A describes a configuration that has been found to be suitable for the requirements of 6.2.

7 Sampling

Carry out representative sampling in such a way that the sample represents the gas, particularly the higher

hydrocarbons, at the time of sampling. Sampling and sample transfer shall be performed in accordance with

ISO 10715.
1) ppm is a deprecated unit.
© ISO 2006 – All rights reserved 3
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SIST EN ISO 23874:2018
ISO 23874:2006(E)
8 Analytical procedure
8.1 Major components
Follow the procedure specified in ISO 6974 (all parts).
8.2 Higher hydrocarbons

The analytical procedure consists of the following steps. All of these steps shall be carried out when the

method is first introduced, and some of them when changes in equipment cause historical measurement data

to be no longer valid. For normal use, steps may be left out provided that the procedures and equipment

remain under statistical control.
8.2.1 Step 1 — Precision

Since quantitative information is derived by comparing the response of unknown components with that of n-

pentane, which has been measured according to ISO 6974 (all parts), the precision information of interest is

that of the ratio of component responses to that of n-pentane.

⎯ Perform a number of repeat analyses on a typical natural gas and, for each analysis, calculate the ratio of

the area of each peak to the area of n-pentane. For each peak, calculate the mean and the standard

deviation of the ratios from all analyses.

⎯ Where component data are to be summed into fractions or groups (see 9.3), calculate the ratio of the total

peak area for that fraction or group to the area of n-pentane. For each fraction, calculate the mean and

the standard deviation of the ratios.

⎯ Convert both sets of mean and standard deviation data into natural logarithms, plot the natural logarithm

of the standard deviation, ln s, as a function of natural logarithm of the area ratio, ln R , and perform a

first-order regression analysis on the data. Annex C shows a worked example.

⎯ Calculate the 95 % confidence limits for the regression line and plot these on the same graph. Select

values of a and b in Equation (1) by trial and error such that
ln s = a + b × ln R (1)

results in a straight line that closely approximates to the upper 95 % confidence line (see Annex C). This

line now defines the precision of measurement, as given in Equation (2):
s = exp(a + b × ln R ) (2)

⎯ This standard deviation is used as the standard uncertainty for each peak or fraction.

8.2.2 Step 2 — Relative response factors

When using a flame ionization detector, relative response factors, F , are claimed to be proportional to

carbon number. Under most circumstances this is true, but variations from ideal operating conditions can

cause this assumption to be false, and so it is necessary that it be checked. This is all the more necessary for

other types of detector. The procedure below uses the known composition of CRM2 to check the relative

response factors. The most likely outcome is that the factors are directly related to carbon number, but the

uncertainty with which this is validated shall be taken into account in the overall uncertainty calculation. Since

n-pentane is the reference component, the response factor is conveniently expressed as a relative carbon

response factor, F , to that of n-pentane.
RR,C

⎯ Perform a number (not less than 5) of repeat analyses using CRM2. Measure the mean peak area

response for each component and the equivalent standard deviations.
4 © ISO 2006 – All rights reserved
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SIST EN ISO 23874:2018
ISO 23874:2006(E)

⎯ Calculate the relative carbon response factor for each component, i, as given in Equation (3):

5××Ac
i k,nC5
F = (3)
RR,Ci
NA××c
nC5 k,i
where
A is the average peak area of component i;
A is the average peak area of n-C5;
nC5
c is the known concentration of component i;
k,i
c is the known concentration of n-C5;
k,nC5
N is the carbon number of component i;
5 is the carbon number of n-C5 (n-pentane).
Equation (3) can also be expressed using area ratios, as given in Equation (4):
5××Rc
A,i:Cnn5 k,C5
F = (4)
RR,Ci
Nc×
k,i

where R is the average peak area ratio of component i to n-C . F values should be very close

A,i:nC5 5 RR,C
to 1.

⎯ Calculate the average relative carbon response factor for each component, and the standard deviation of

this value. The uncertainty of the F uses these standard deviation values and the uncertainties of the

RR,C

composition data for CRM2. The overall uncertainty of the F is calculated from the averages of the

RR,C
individual component values.
8.2.3 Step 3 — Validation of fraction data

Inert gases and individual hydrocarbons up to hexane can be measured unambiguously and used in the

calculation software. Beyond this, n-alkanes up to C , benzene, toluene, cyclohexane and

methylcyclohexane can be identified, but other hydrocarbons, in general, cannot be clearly identified and

measured because of the large amount of overlap that occurs between isomers and the lack of reliable

retention data. Even if individual isomers can be identified, it is unlikely that their critical properties, which are

necessary for the calculation, are tabulated in the software.

Components that are measured but unidentified are summed into fractions or carbon number groups. Thus all

components eluting after n-C up to and including n-C , with the exceptions of benzene and cyclohexane, are

6 7

summed as the C fraction (FR7). The average boiling point and specific gravity of this summed group are

calculated (see 9.3.2) and used by the software to calculate appropriate critical properties for the fraction. Two

checks are made at this stage to ensure that the fraction data are as close an approximation as possible and

to estimate the uncertainties involved. These are
⎯ to compare attributed and “true” boiling points,
⎯ to show that linear interpolation is valid.
© ISO 2006 – All rights reserved 5
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SIST EN ISO 23874:2018
ISO 23874:2006(E)
8.2.3.1 Boiling-point comparison

The n-alkanes are easily distinguished in the separation and their properties are well known. They are,

therefore, used as the basis for checking boiling point data. From an existing and comprehensive analysis,

calculate the C to C fraction quantities, where measurable, according to 9.3.1. Identify these fractions as

7 12

the equivalent n-alkanes and calculate the dewpoint on this basis. Now rename the C group as the fraction,

FR7, rather than n-C , and give it a boiling point and specific gravity that are those of the n-alkane.

Recalculate the dewpoint.

This fraction dewpoint is likely to be slightly different from the n-alkane dewpoint. Adjust the boiling point value

until the dewpoint agrees with that from the n-alkane. This adjusted boiling point, T , provides the best fit

BP,a

when comparing the fraction property with that of the n-alkane. Repeat this substitution for each

n-alkane individually, and note the adjusted boiling point in each case. Include the C fraction and n-C . Even

6 6

though the C components are likely to be measured individually, the adjusted boiling point of n-C is required

6 6
for the calculation of the C fraction properties in 9.3.2.

Having defined these adjusted boiling points (which can, in some cases, coincide with the “true” values), now

define all carbon-number groups as fractions with these adjusted boiling points in each case. Recalculate the

dewpoint. The difference between this last dewpoint value and that found when all groups are treated as

n-alkanes is taken as the uncertainty value.
NOTE Annex B shows an example of such calculations.
8.2.3.2 Interpolation

Fraction boiling points are calculated on the assumption that the boiling points of individual unidentified

components can be calculated by linear interpolation between the values for the bracketing n-alkanes. This is

checked by using known data for n-alkanes. The boiling point of n-C is calculated by interpolation between

the known values (adjusted boiling points) for n-C and n-C and compared with the adjusted boiling point for

7 9

n-C found in 8.2.3.1. The calculated boiling point of the n-alkane is calculated as given in Equation (5):

tt−×T −T
( ) ( )
R,nn-CxxR, -C−+1 BP,a,n-Cx 1 BP,a,n-Cx−1
TT=+ (5)
BP,cal,nn-CxxBP,a, -C−1
tt−
R,nn-Cxx+−1 R, -C 1
where
t is the retention time of component i in the sample;
R,i
T is the adjusted boiling point of n-alkane i in the sample.
BP,a,n-Ci

The same is applied to other n-alkanes where interpolation is possible. For each n-alkane, the difference

between the calculated boiling point and the adjusted boiling point from 8.2.3.1 is recorded. The boiling point

uncertainty arising from interpolation is calculated as the mean of the absolute values of the differences

between the calculated and adjusted boiling points for n-C , n-C , n-C and n-C .
8 9 10 11

The sensitivity of the calculated dewpoint to these boiling point uncertainties is now determined. The analytical

data used in 8.2.3.1 are used again but with fraction boiling-point data based on the adjusted boiling points for

n-alkanes calculated as in 9.3.2. The dewpoint temperature is calculated from this analysis. Each fraction

boiling point is then incremented by the boiling-point uncertainty calculated above and the dewpoint

recalculated. The difference between the two dewpoint values is the uncertainty associated with interpolation.

NOTE Annex B shows an example of such calculations.
8.2.4 Step 4 — Sample introduction
Introduce the sample in accordance with 9.1.
6 © ISO 2006 – All rights reserved
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SIST EN ISO 23874:2018
ISO 23874
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