Analytical Requirements for Hydrocarbon Dewpoint Calculation

7/28/2019 Analytical Requirements for Hydrocarbon Dewpoint Calculation

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Analytical requirements for Hydrocarbon Dewpoint calculation.

Chris Cowper, EffecTech Ltd., UK.

1. Introduction.

Hydrocarbon dewpoint is an important characteristic of natural gas in respect of pipeline operation

and critical for fuelling of gas turbines. It is usually specified as a limiting temperature above which

no liquids can separate from the gas. Sometimes this is replaced with a maximum quantity of

condensed liquids specification. In Northern European countries a dewpoint temperature would

typically be not greater than -20C (28

0F) at any pressure. A quantity specification could be not

more than 5 mg of condensate per cubic metre (0.04 gallons/MMSCF) at -20C (28

0F) and any

pressure. The at any pressure phrase recognises that the maximum dewpoint temperature (the

cricondentherm) is found at a pressure intermediate between those used in transmission and those

used in distribution.

The de facto method for dewpoint temperature is the cooled mirror device, in which the temperature

of a mirror surface in contact with the gas at a selected pressure is recorded as the first appearance of

liquid condensate is observed. This may be manual (1) or automatic (2). Hydrocarbon content is

measured by treating a flow of gas to the specified temperature at a selected pressure and measuring

the rate of liquid condensate formation (3). Measurement of gas composition (4) and calculation of

phase properties using an appropriate equation of state (5, 6) gives dewpoint temperature or amount of

condensate at specified conditions or both. This paper reviews the physical and analytical approaches

and gives guidelines for the latter.

2. Physical measurement.

Of the physical methods, it may be assumed that dewpoint temperature is a more rigorous

specification than quantity of condensate. It assumes, after all, zero condensate at the specified

temperature rather than not more than a defined amount. Is this a correct assessment?

Figure 1 shows a cooled mirror device diagrammatically. The cell, of volume V contains gas at

pressure P. The film of condensate is of diameter D and thickness T.

Figure 1. Cooled mirror measurement cell.

condensate film

gas in

gas out

cooled surface

diameter D

thickness T

observation

window

light

cell volume V

EffecTechSpecialists in Gas Measurement


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Assuming that the observed film is 10 mm in diameter, and that its thickness is 500 nm (the

wavelength of light), and that the liquid density is 0.6, then the mass of liquid is

024010

6050056

2

..

mg

Assuming a physical cell volume of 10 ml, containing gas at 28 bar, with a compression factor of

0.9, then the volume of gas is

0003101090

28106

..

m3

which gives a condensate concentration of 76 mg/m3 or 0.6 gallons/MMSCF. This is the amount of

condensate necessary to register a dewpoint temperature and is a considerably greater amount than the

quantity alternative of 5 mg/m3.

To consider the implications of this necessary amount of condensate, we have studied ten natural

gases sampled from different delivery points at inputs to the U.K. National Transmission System.

These gases had dewpoint temperatures ranging from -38.4 to +0.9 0C (-37.1 to +33.60F). The

amounts of condensate which would separate from these gases at different temperatures below their

dewpoints were calculated and are shown in figure 2.

0

30

60

90

120

150

-30-25-20-15-10-50

temp below dewpoint0F

condensategalls/MMSC

F

gas 1 gas 2 gas 3 gas 4 gas 5


Figure 2. Amount of condensate at different temperatures below dewpoint.

The amount of condensate varies widely. Of greater interest is the region closer to the dewpoint

which is shown with expanded scales in figure 3. Also shown in the figure is the line equivalent to

0.6 gallons/MMSCF, or 76 mg/m3. Depending on the gas composition, this value is not achieved until

the mirror reaches a temperature below the theoretical dewpoint varying between 0.3 and 2.50F.

Exceptionally, gas 9 does not deposit the appropriate amount of condensate until 100F below the

theoretical dewpoint. This gas has a dewpoint temperature in the region of -400, and so for normal

operational needs so large an error may not be significant. It may, however, become important where


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dewpoint control is being planned, particularly if the treatment involves a supercooled sidestream

rather than the entire process flow.

If dewpoint temperature is taken to mean the value which is measured with a cooled mirror device,

then these variations are, by definition, both invisible and unimportant. They do, however, represent a

difference between the measured and theoretical values which varies in magnitude between different

gases.

0

1

2

3

4

-15-12-9-6-30

temp below dewpoint0F

condensa

tegalls/MMSCF



Figure 3. Amount of condensate at different temperatures below dewpoint.

Another consideration is the effect of measurement pressure. A cooled mirror device is used at a

selected, usually specified, pressure which is assumed to be close to the cricondentherm. If different

gas compositions lead to cricondentherms at different pressures, then an error will be introduced.

Figure 4 shows the dewpoint lines of two gases used in the study.

Figure 4. Effect of dewpoint measurement pressure.

0

200

400

600

800

1000

1200

10 15 20 25 30

Dewpoint temp0

F

Pressurepsi

Measurement ressure

0.6

gas 1 gas 3


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Both gases are measured at 400 psi, and give dewpoint temperatures of 26 0F (gas 1) and 22.50F (gas

3). When the full dewlines are included, however, a different picture emerges. The cricondentherm

for gas 1 is close to 400 psi, and the error from the maximum dewpoint temperature is small. For gas

3, the cricondentherm is at a considerably higher pressure, and the maximum dewpoint temperature is

26.50F. This dewpoint is underestimated by 4

0F. If the measurement pressure were adjusted to be

suitable for gas 3, then the use of that value would give an underestimate for gas 1 of a similar size.

The comprehensive GPA study (7) quotes repeatability and accuracy of the vast majority of

individual Dewscope observers as less than 1.30F and 1.9

0F respectively. There is no reason to doubt

the repeatability claim, but it is difficult to reconcile the accuracy claim with the foregoing.

3. Analysis and calculation.

Gas chromatographic analysis of natural gas for the purpose of hydrocarbon dewpoint calculation

needs to be comprehensive and competent. For the purpose of calculating heating value, density and

Wobbe Index, the analysis can be simplified by, for example, recombining all C 6 and heaviercomponents and measuring then as a single entity (C6+) (8). Their contributions to such properties are

relatively small, and so the group can have properties attributed to it which are sufficiently

representative of the sums of the individual parts. Where more detail is available, for example a

breakdown of higher components by carbon number, there is no need to distinguish between different

isomers at a particular carbon number as their properties are quite uniform.

This type of approach is not suitable when hydrocarbon dewpoint is the aim. Higher hydrocarbons,

although present in diminishing quantities with increasing carbon number, have a disproportional

influence on hydrocarbon dewpoint. Components or groups of components which can be lumped

together or even ignored for the purpose of heating value determination must be measured with lowuncertainty for dewpoint calculation. Assumptions about similar behaviour of isomers cannot be

used. Whereas the heating values of individual C9 isomers are very similar, boiling points and vapour

pressures are not.

To measure higher hydrocarbons in detail, a high efficiency column with a sensitive detector is

needed, capable of handling wide boiling range samples. An analyser capable of temperature

programming, fitted with a wide bore non-polar capillary column and a flame ionisation detector

(FID) represents one of a number of approaches. However, rather than offer prescriptive detail of a

particular configuration, it is preferable to propose performance requirements, which may be

demonstrated (or not) to be satisfied by a number of analytical options.

In this regard, questions to be answered include:

- How far in carbon number should the analysis proceed? It would be theoretically possible,

with concentration sampling techniques, to analyse natural gas to C20, but would this

significantly add to our knowledge?

- What limit of detection is needed? Again, parts per trillion may be possible, but do such trace

quantities influence the hydrocarbon dewpoint temperature?

- How much detail is necessary or indeed possible? How do we account for unidentifiedcomponents?


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- How do we calibrate the system?

Figure 5 shows a chromatogram produced on a laboratory analyser fitted with a wide bore capillary

column and a flame ionisation detector and used with temperature programming. It is a useful

reference point for the above questions.

C5 C7C6 C9C8 C10

benzene

cyclo-C6

me-cyclo-C6

toluene

C11 C12

1 2 4 16

19 ppm

6 ppm

0.8 ppm

0.1 ppm

Figure 5. Chromatogram.

The chromatogram has been recorded with changes in detector sensitivity, as indicated by the scaleat the top of the figure. Component separation is not optimised, since the need for low limits of

detection requires a relatively large sample size. In response to the above questions:

- How far to go? Previous work (9) shows that for gases with hydrocarbon dewpoints not

higher than 00C (32

0F), analysis to C12 is sufficient. Analysis to higher carbon numbers will

give more information, but the difference in calculated dewpoint temperature is

insignificantly small. If calculation of higher dewpoints is required, then the analysis needs to

be extended, but such gases are likely to be considerably outside specification limits. With

lower dewpoint temperatures, it may not be necessary to analyse beyond C9, but if a method is

set up to handle gases near the dewpoint limit, then the normal procedure will be to go to C12.

- Limit of detection? Again, previous work suggests 0.1 ppm for individual peaks. As figure 1

shows, this is achievable without the need for concentration sampling. Again, lower limits of

detection give more information, but the difference in calculated property is small.

- How much detail? Figure 1 shows benzene, toluene, cyclohexane and methyl cyclohexane in

addition to the n-alkanes. C5 and C6 alkane isomers, of which there are few, are identified and

handled as individual components. While it is possible to give component names to some of

the alkane peaks in the C7 and C8 range, these should not be regarded as unambiguous

identifications. The presence of C6 and C7 aromatics and cycloalkanes strongly suggests that

higher homologues will also be present, but although the lighter members are identified with

some confidence, the same cannot be said of the heavier ones.


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Thus, while it is an approximation, we treat all measured components emerging after n-C8 up

to and including n-C9 as C9 alkane isomers. An equivalent judgement is applied to C10, C11

and C12 components. We therefore know what these components are as a category C9

hydrocarbons, C10 hydrocarbons, etc., but not as individuals which we could include in an

equation of state calculation. A strategy for coping with this is described in the next section.

- How do we calibrate? Generally the type of analysis shown in figure 5 is quantified by using

the carbon counting property of the flame ionisation detector. A component such as butane or

pentane, which has been measured as part of the major component analysis, is used as a

bridge transferring quantitative information to the higher hydrocarbons. Components are

measured using the ratio of their responses to that of butane or pentane and the ratio of carbon

numbers.

This use of predictable detector responses is an assumption, albeit one which is more often

correct than incorrect. It should be shown to be true for each application and on a regularbasis. This requires either the use of blends of liquid hydrocarbons, which is a difficult and

time consuming procedure, or higher hydrocarbon calibration gases. Traceable calibration

mixtures containing n-alkanes up to C10, benzene, toluene, cyclohexane and methyl

cyclohexane are now available in the UK. These are referenced to blends prepared and

validated by the National Physical Laboratory. It is planned to extend this to cover C11 and

C12.

3.1 Data handling.

Figure 6 expands part of the figure 5 chromatogram to show more detail of the C 9 components.

n-C8

n-C9

126.00C

151.30C

Figure 6. C9 portion of chromatogram.

Approximately 20 peaks are visible between n-C8 and n-C9. While some could be tentatively

identified, all of them cannot. If we assume them to be C9 hydrocarbons, then we can treat them as a

single group or fraction for input to the equation of state. To handle a user-defined fraction of this

type, we need information on the quantity and on the average boiling point and specific gravity of the


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fraction (10). These last two items allow the critical properties to be inferred, which are needed for

the equation of state calculation.

If all the components which constitute the fraction are C 9 hydrocarbons, then we may take their

relative responses as being identical to that of n-C9, which means that we can quantify them either by

reference to n-C9 in a calibration gas, or, using relative response factors, to some lighter component in

the sample (n-C5 or n-C6) which has been measured independently.

The n-alkanes are clearly identified in most natural gases, and have well known boiling points.

Temperature programmed analysis using a non-polar column gives component retention times which

have a linear relationship to their boiling points. By linear interpolation between adjacent n-alkanes,

we can infer the boiling points of the individual isomers, even though they are not identified, as

indicated in figure 6.

We therefore know the quantity and boiling point of each peak, and can calculate the average

boiling point of this group of components according to equation 1.

Eq. 1

where xi is the amount of each peak and B.Pt.i is its calculated boiling point. Specific gravities vary

little between isomers of a particular carbon number, and so the well known specific gravity of the n-

alkane can be applied to the fraction.

Fractions are thus defined for each carbon number group, so that all peaks are accounted for. The

C7 and C8 fractions do not include those components which have been separately identified, i.e.benzene and cyclohexane are not summed with the C7 fraction, and toluene and methyl cyclohexane

are left out of the C8 fraction.

The input data for the equation of state thus consists of a mixture of individual components

(nitrogen, carbon dioxide, alkanes from C1 to C6, benzene, toluene, cyclohexane and methyl

cyclohexane), and carbon number fractions (FR7, FR8, FR9, FR10, FR11 and FR12).

3.2 Quantity calculation and uncertainty.

If the molar concentration of, for example, n-C5 has been established by a separate major component

analysis, and it is also measured without interference in the minor component analysis, as in figure 5,

it can be used as an internal standard for measurement of other minor components, using equation 2.

5

55

nC

CniCni

iA

RRFxAx

, Eq. 2

Where xi is the amount of unknown component i,

xn-C5 is the known amount of n-C5

Ai and An-C5 are the areas of component i and of n-C 5 respectively, and

RRFi,n-C5 is the response factor for component i relative to n-C 5.

i

ii

x

Pt.B.xPt.B.Fraction


8/13

The uncertainty of the measurement, Uxi, can then be derived from the relative uncertainties of the

contributing terms, summed in quadrature, as in equation 3.

22

2

2

2

2

2

2

5

5

5

5

5

5

Cni

Cni

Cn

Cn

Cn

i

Cn

i

i

i

RRFRRFu

xxu

A

A

A

Au

xxu

,

, Eq. 3

These individual uncertainty contributions are described below.

3.2.1 Quantity of n-C5.

This is derived from a major component analysis of the gas. In our case, this is a UKAS accredited

method (11), following ISO 6143 Gas analysis Comparison method (12). The best measurement

uncertainty for n-C5 is1.4% relative, with a coverage factor (k) =2.

3.2.2 Precision of peak area ratio measurement.

Gases with individual minor component concentrations ranging from 0.03 to 230 parts per million

were analysed repeatedly, and the ratios of peak areas relative to that of n-C5 were measured and the

standard deviations of these ratios derived. These are plotted against concentration, using logarithmic

axes, in figure 7.

-14.0

-12.0

-10.0

-8.0

-6.0

-4.0

-9.0 -7.0 -5.0 -3.0 -1.0 1.0

ln(area ratio)

ln(stddevratio)

Individuals Groups Regression

Upper 95% Lower 95% Limit

Figure 7. Precision of peak area ratios.

In figure 7 both individual peaks and component groups (FR7, FR8, etc.) are plotted. Regression

analysis shows that the relationship is described by

ln(standard deviation) = -7.6189 + 0.4619 ln(peak area ratio) (Eq. 4)


9/13

The upper and lower 95% confidence limits for the regression line are calculated and also shown in

figure 7. These limits show little curvature, and so the upper limit, which is the criterion which

should be used, can be closely approximated by a straight line which has the equation

ln(standard deviation) = -5.9 + 0.47 ln(peak area ratio) (Eq. 5)

3.2.3 Relative response factor uncertainty.

As mentioned above, the flame ionisation detector is deemed to be a carbon counter: without such a

belief a huge number of analyses would not be possible. This observation and the development of the

effective carbon number concept followed hard on the heels of the introduction of the FID, which

means that many of the references which are still used (13-15) are forty years old. The most recent

(16) is from 1985. Without tempting fate by challenging these authorities, we thought it worth

checking this assumption because by doing so we could make an estimate of the uncertainty of such

factors. None of the earlier references mention uncertainty, which is, of course, a relatively recentconsideration in analytical chemistry. Without an estimate of uncertainty on relative response factors,

we cannot complete the uncertainty budget shown in equation 3.

Liquid hydrocarbon mixtures were prepared gravimetrically, containing n-alkanes from C 5 to C12, i-

alkanes 2-me-C5, 2,3-di-me-C5, 2,5-di-me-C6 and 2,3,4-tri-me-C5, aromatics benzene and toluene, and

cycloalkanes cyclohexane and me-cyclohexane. These were analysed on the same column using the

same conditions as for the gas analyses. The mixtures were injected directly on column, using a

splitless temperature programmed vaporiser so that there was minimum chance of discrimination.

The liquid mixtures were diluted successively so as to cover three orders of magnitude. All analyses

were repeated at least three times.

Response factors were calculated relative to n-C6, on the basis of moles of carbon. The results are

shown in figures 8 to 11. Among the alkanes, the greater variability for n-C5 is almost certainly due

to its volatility. It is very difficult to keep stable mixtures containing n-C5 for more than a short

period of time. Other than that, alkanes show RRF values which are close to 1 over three orders of

magnitude.

0.90

0.95

1.00

1.05

1.10

0.01 0.1 1 10 100

micrograms

area/C

relton-C6

n-C5 n-C7 n-C8 n-C9

Figure 8. Relative response factors for n-alkanes


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0.90

0.95

1.00

1.05

1.10

0.01 0.1 1 10 100

micrograms

area/C

relton-C6

n-C10 n-C11 n-C12

Figure 9. Relative response factors for n-alkanes

0.90

0.95

1.00

1.05

1.10

0.01 0.1 1 10 100micrograms

area/C

relton-C6

i-C6 i-C7 i-C8(1) i-C8(2)

Figure 10. Relative response factors for i-alkanes

0.95

1.00

1.05

1.10

1.15

0.01 0.1 1 10 100

micrograms

area/Cre

lton-C6

Benzene T oluene cyclo-C6 me-cy-C6

Figure 11. Relative response factors for aromatics/cycloalkanes

. Figure 11 is more difficult to explain. Analysis of the original mixture gave a RRF for benzene

which was 15% higher than the theoretical carbon-based value, and for toluene 10% higher. This


11/13

anomaly was visible at the first stage of dilution, though to a reduced degree. Further dilution gave

values close to the theoretical value of 1. The presence of this anomaly emphasises the need to check

assumptions about detector response and not just to take it on trust.

We have no explanation for this behaviour. The entire experiment was repeated, with new mixtures

prepared and analysed, with the same result. Fortunately, when analysing these components in gases,

we are measuring fractions of micrograms, which is far below the levels at which this anomalous

behaviour is seen. There appear to be no implications for gas analysis, but there may be when

analysing liquids such as condensates.

For alkanes over the full range tested, and for aromatics and cycloalkanes over the range 28 ng to

1.7 g, the response factor per mole of carbon relative to n-C 6 may be taken as 1.000, with a standarduncertainty of 0.012 or 1.2% relative. Combining this with the gravimetric uncertainty of mixture

preparation gives a combined uncertainty of 1.3% relative, or an expanded uncertainty (k=2) of 2.6%

relative. Note that while these values may offer a useful indication to other users, they are only

strictly valid for the instrument on which these measurements were made, under the conditions in useat that time.

4. Dewpoint temperature uncertainty.

From the foregoing, we can derive an overall uncertainty for the concentration of each component

or fraction in a gas sample following equation 3. These uncertainties then need to be converted to an

uncertainty of dewpoint temperature. The equation of state calculation which derives phase data from

composition uses iterative numerical methods. It is not based on a relatively simple algorithm which

would be amenable to an analytical solution, such as is used to create equation 3 from equation 2.

The approach therefore is to use a Monte Carlo technique, creating a large number of randomisedcompositions, and solving the dewpoint temperature for each. This methodology is consistent with

the GUM (17).

Typically, 1000 compositions may be postulated. For each component, a random number generator

creates 1000 values which are normally distributed, with a mean of the analysed value and a standard

deviation of the overall uncertainty. A dewpoint temperature is calculated for each composition, and

the mean and standard deviation of these 1000 values are taken. The standard deviation then

represents the uncertainty of calculated dewpoint temperature.

For the configuration described here, the composition contribution to the uncertainty of dewpoint

temperature for a range of natural gases is around 0.30C (0.60F). There is another source of

uncertainty related to the properties which are attributed to the fraction data, and to the interpolation

whereby the boiling points of unidentified peaks are calculated (clause 3.1). These uncertainty

sources do not influence composition, and so cannot be included with the previously described

calculations. These latter influences are estimated to create an uncertainty in dewpoint of similar

magnitude. Conservative combination of these sources means that the uncertainty of calculated

dewpoint due to the analytical processes should be better than 10C (1.8

0F).

There are unquantified uncertainties associated with the choice of equation of state. Use of the

Peng-Robinson equation gives lower dewpoint temperatures than the Redlich-Kwong-Soave

calculation by 2.5 to 30

C (4.5 to 5.50

F). Where comparative analysis and calculation approaches arerequired, it is essential that the same equation and the same interaction parameters are used.


12/13

5. Conclusions.

- Direct measurement of hydrocarbon dewpoint underestimates the true value because of two

factors; the need to form a film of liquid thick enough to be visible and the fact that different

gases have cricondentherms at different pressures, not necessarily the chosen measurement

pressure. The size of the error is not related to the actual dewpoint temperature.

- Detailed analysis and calculation can give, among other properties, dewpoint temperature at

any pressure, the dewline, the amount and composition of condensate formed at any chosen

conditions, and the cricondentherm. There is also the advantage that when a change in

property is observed, the analytical data shows why it has occurred.

- A carefully selected and properly controlled analytical procedure can generate data which can

contribute an uncertainty of not more than 10C (1.80F) to a calculated dewpoint temperature.

- The analytical method must be capable of analysing components to C12, and have a limit of

detection of not less than 0.1 ppm. Sample handling must be paid careful attention, to ensure

that the analysed sample is not changed by component loss or enrichment. A competent

calibration procedure is necessary for accurate quantitative data.

6. References.

1. Deaton, W.M. and E.M. Frost, Jr., Bureau of Mines Apparatus for Determining the Dew

Point under Pressure, U.S.B.M. R.I. 3399, May 1938.

2. Bannell, J.L.K., A.G. Dixon and T.P. Davies, The Monitoring of Hydrocarbon Dewpoint,

Gas Quality, edited by G.J. van Rossum, Elsevier, April 1986, pp. 263-271.

3. ISO 6570:2001 Natural gas Determination of potential hydrocarbon liquid content

Gravimetric methods. International Standards Organisation, Geneva.

4. ISO/TC 193/SC 1/WG 14 CD 23874 - Natural gas Gas chromatographic requirements for

hydrocarbon dewpoint calculation. International Standards Organisation, Geneva.

5. Soave, G., Equilibrium Constants for a Modified Redlich-Kwong Equation of State, Chem.

Eng. Sci., 27, (1972), pp.1197-1203.

6. Peng, D.Y. and D.B. Robinson, A New Two-constant Equation of State, Ind. Eng. Chem.Fundam., 15, (1976), pp.59-64.

7. Warner, H.R. Jr. et al., Hydrocarbon Dewpoint Determination of Lean Natural Gases, GasProcessors Association.

8. ISO 6974:2001 Natural gas Determination of composition with defined uncertainty by gas

chromatography. International Standards Organisation, Geneva.

9. Cowper, C.J., Natural gas hydrocarbon dewpoint; comparison of measurement andcalculation methods, 2

ndGas Analysis Symposium and Exhibition, Maastricht, (2002).


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10. Kesler, M.G. & B.I. Lee, Improve Prediction of Enthalpy of Fractions, Hydrocarbon

Processing, (March 197, pp.153-158.

11. Squire, G.D., Natural gas calibration mixtures Getting it right in an accredited calibration

laboratory, Natural Gas Quality Conference, Loughborough, (2002).

12. ISO 6143:2001 Gas analysis Determination of composition and checking of calibration gas

mixtures Comparison methods. International Standards Organisation, Geneva.

13. Sternberg, J.C., W.S. Gallaway and D.T.L. Jones, The mechanism of response of flame

ionisation detectors, in Gas Chromatography, edited by Brenner, N., J.E. Callen and M.D.

Weiss, Academic Press, New York, (1962), pp. 231-267.

14. Ettre, L.S., Relative molar response of hydrocarbons on the ionisation detectors, in Gas

Chromatography, edited by Brenner, N., J.E. Callen and M.D. Weiss, Academic Press, NewYork, (1962), pp. 307-327.

15. Kaiser, R., Gas Phase Chromatography, Volume 3. Butterworths, Washington, (1963), pp.

99-103.

16. Scanlon, J.T. and D.E. Willis, Calculation of flame ionisation detector relative response

factors using the effective carbon number concept, J. Chromatogr. Sci., 23, (1985), pp.333-

340.

17. Guide to the Expression of Uncertainty in Measurement, BIPM/ IEC/ IFCC/ ISO/ IUPAC/IUPAP/ OIML, International Standards Organisation, Geneva, (1995).

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Analytical Requirements for Hydrocarbon Dewpoint Calculation

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