ISO 23874:2006 pdf download – Natural gas — Gas chromatographic requirements for hydrocarbon dewpoint calculation.
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 7 to C 12 fraction quantities, where measurable, according to 9.3.1. Identify these fractions as the equivalent n-alkanes and calculate the dewpoint on this basis. Now rename the C 7 group as the fraction, FR7, rather than n-C 7 , 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 BP,a , provides the best fit 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 6 fraction and n-C 6 . Even though the C 6 components are likely to be measured individually, the adjusted boiling point of n-C 6 is required for the calculation of the C 7 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.
11 Calculation of dewpoint temperature The composition is described as a mixture of individually measured components, including all those measured using ISO 6974 (all parts), and fractions or groups where components have been summed by carbon number. This combination is entered into a suitable equation of state, in which the dewpoint temperature at one or more defined pressures can be calculated. Alternatively, the cricondenbar or cricondentherm may be calculated directly, or the temperature at which, at a selected pressure, a defined amount of condensate is formed. When entering data for fractions rather than for individual components, it is necessary to include the average boiling points and densities with the quantity. 12 Analytical uncertainty contribution to dewpoint temperature The equation of state has an uncertainty associated with its use; these considerations are outside the scope of this International Standard. However, it is possible to estimate the contribution made by the uncertainty on composition to the dewpoint temperature uncertainty, although this does not account for all the uncertainty of the result. Equations of state use iterative numerical methods, which are not amenable to an analytical approach in the way that Equation (9) is derived from Equation (6). For example, a solution is to use a Monte Carlo method in which a large number of compositions, based on the measured value but adjusted according to the known composition uncertainties, are separately entered into the equation of state and the variation in the calculated dewpoint temperatures is noted. Compositions are created using a random-number generator set up to develop a normal distribution, with, for each component, a mean that is the measured value, and a standard deviation that is the analytical uncertainty. Each composition produces a dewpoint temperature, the mean and standard deviation of these temperature values are calculated. This standard deviation can be taken to be the uncertainty contribution arising from the analytical uncertainty. This does not take into account uncertainties associated with the use of fractions, as described in 8.2.3. The different uncertainty contributions due to the analytical procedure, to the use of fractions and to the equation of state (if known), shall be quoted individually.
ISO 23874:2006 pdf download – Natural gas — Gas chromatographic requirements for hydrocarbon dewpoint calculation
