GB/T 30519-2024Determination of hydrocarbon types and benzene content in light petroleum distillates and products - Multidimensional gas chromatography method (English PDF)

0 Warning notice

A warning printed before Clause 1 states that the people who use the document should have practical experience of regular laboratory work; that its use may involve hazardous materials, equipment and operations and that it does not point out all the possible safety problems; and that the user is responsible for adopting suitable safety and health measures and for ensuring compliance with the conditions laid down by national regulations.

1 Scope

The document describes a test method that uses multidimensional gas chromatography to determine hydrocarbon types such as olefins and aromatics, and the benzene content, in light petroleum distillates and products.

It applies to the determination of olefins, aromatics and benzene in light petroleum distillates and products with a final boiling point not higher than 215 °C, such as gasoline, gasoline blending components and solvent oils. The measurable concentration ranges are: olefins from 0.5 % to 70 % by volume, corresponding to 0.4 % to 64 % by mass; aromatics from 1 % to 80 % by volume, corresponding to 1.2 % to 92 % by mass; and benzene from 0.2 % to 10 % by volume, corresponding to 0.2 % to 12 % by mass. For samples whose content falls outside those ranges the precision has not been established.

It also applies to gasoline products obtained from unconventional crudes such as shale or oil sands, and to hydrocarbon fuels synthesised from non-petroleum mineral fuels such as Fischer-Tropsch oils, but for these the precision has likewise not been established.

Motor gasoline often contains ether or alcohol oxygenate components, and several oxygenates may be present together. Ethers in the sample elute together with the olefin components and alcohols elute together with the C7 plus aromatic components. For gasoline samples containing oxygenates, the type and content of the oxygenates are determined by another test method, such as NB/SH/T 0663, and the hydrocarbon type results are corrected as necessary following the procedure of Annex A.

The document does not apply to the determination of the content of individual components within the hydrocarbon types, benzene excepted.

3 Terms and definitions

3.1 Saturates, or saturated hydrocarbons: the collective name for the paraffin and naphthene components with carbon numbers from 4 to 12.

3.2 Olefins, or olefin hydrocarbons: chain olefins and cyclic olefins with carbon numbers from 4 to 12. A note excludes cyclic dienes with rings larger than six-membered.

3.3 C7 plus aromatics: the aromatic components in a light petroleum distillate or product other than benzene. A note includes single-ring substituted aromatics, aryl olefins, and cyclic dienes with rings larger than six-membered.

3.4 Olefins trap: the chromatographic column in the analysis system that selectively retains the olefin components out of a mixture of saturates and olefins. A note states that at a given temperature the column traps and releases the olefin components reversibly, which meets the need for repeated use; that at a set temperature it retains the olefin components selectively out of the mixture and lets the saturate components through; and that when the temperature is raised the retained olefin components are released completely.

4 Summary of the method

A measured amount of sample is injected directly into the gas chromatograph. After separation, the hydrocarbon type components enter a flame ionisation detector one after another and are detected, giving the chromatographic peak area of each hydrocarbon type. The retention time of each hydrocarbon type is fixed by means of a reference sample, quantification is by the corrected area normalisation method, and the volume fraction or mass fraction of each hydrocarbon type in the test portion is calculated. The principle of the analysis is shown in Figure 1 and the gas chromatograph with its separation system in Figure 2.

For some light solvent oil products the olefin content need not be determined; in that case the sample does not have to pass through the olefins trap after injection.

5 Interfering substances

5.1 High boiling, high carbon number aliphatic hydrocarbons in the sample, of carbon number 13 and above, may not separate completely from benzene on the column of N,N-bis(alpha-cyanoethyl)formamide, which affects the detection of benzene and of the aromatic components; the final boiling point of the sample should therefore not exceed 215 °C.

5.2 Ethers in a gasoline sample, such as methyl tert-butyl ether, are retained in the olefins trap and elute together with the olefin components, so that the olefin area fraction obtained from the chromatogram includes the share of the ethers. Alcohols such as ethanol and methanol elute within the retention time range of the C7 plus aromatics, so that the C7 plus aromatic area fraction obtained includes the contribution of the alcohols. The content of the ethers or alcohols in the gasoline is determined by a related method such as NB/SH/T 0663 and the result is corrected according to Annex A.

5.3 Small amounts of sulfur and nitrogen compounds in the sample may be irreversibly adsorbed in the olefins trap, which may in the end lower the capacity of the trap and so shorten its service life. Experiments on several kinds of sample have shown no effect on the determination results.

5.4 No effect on the analysis results has been found from antioxidants, detergents, antistatic additives, lead antiknock additives or manganese antiknock additives in the sample.

5.5 A small amount of dissolved water in the sample does not interfere with the determination; free water, if present, can be removed with anhydrous sodium sulfate or by filtration through filter paper.

6 Apparatus

6.1 Gas chromatograph: the instrument includes at least an injector, a vaporising chamber, a column oven, a flame ionisation detector and a chromatography workstation. Stable control of the carrier gas and detector gas flow rates matters for accurate, reliable and repeatable results, and a system with electronic flow control should be used. The flame ionisation detector meets or betters the requirements of Table 1. Further hardware is needed to carry out the method, including the chromatographic column, the olefins trap, the balancing column, the switching valves and the corresponding temperature control devices.

6.2 Olefins trap: at a given temperature, when the aliphatic hydrocarbons separated by the polar column, that is the mixture of saturates and olefins, pass through it, the trap retains the olefin components completely and lets the saturate components through. The trapping temperature is usually 120 °C to 135 °C. When the temperature is raised the trap releases all the retained olefin components completely; the release temperature is usually 190 °C to 210 °C. The actual temperature setting is fixed according to the particular trap. The performance of the trap can be verified with the system verification sample or the quality control sample. If olefins are found to escape from the trap, the operating conditions should be adjusted, and the trap replaced if that is not enough.

6.3 Balancing column: it neither retains nor adsorbs the hydrocarbon type components and serves only to balance the pressure, so that the baseline stays steady when the valves are switched.

6.4 Switching valves: to operate as the document prescribes, the analysis system includes two two-position six-port valves. The valves may be switched manually or automatically; automatic switching should be used so that the switching time is accurate.

6.5 Temperature control of the system components: the polar separation column, the olefins trap and the switching valves each have an independent temperature control system, and every part in contact with the sample is held at a temperature that prevents the sample from condensing. Table 2 lists typical control temperature ranges for some components. Some components call for isothermal operation, others for a repeatable temperature programme. The temperatures listed in Table 2 are only a typical operating range and may be adjusted to suit the particular polar column or olefins trap; any means of temperature control that meets the requirements of the analysis system may be used.

6.6 Valve switching drive system: where the valves are driven pneumatically, the air pressure supplied to the pneumatic system meets the drive requirement, so that the valves switch quickly.

6.7 Carrier gas purification device: to protect the service life of the olefins trap, besides the molecular sieve and activated carbon purifiers usual in gas chromatography, which remove water and hydrocarbon impurities from the carrier gas, a dedicated deoxygenating purifier is fitted, so that the oxygen in the carrier gas stays below 1 microlitre per litre by volume.

6.8 Chromatographic column, the polar separation column: any column may be used that separates benzene completely from n-dodecane or 1-undecene among the aliphatic hydrocarbons and separates benzene completely from toluene, while leaving a suitable valve switching time. To ensure the separation, the ratio of the retention time of benzene to that of 1-undecene is required to be greater than 1.5 with a resolution greater than 2.0, and the ratio of the retention time of toluene to that of benzene greater than 1.25 with a resolution greater than 1.25. A typical column is prepared with N,N-bis(alpha-cyanoethyl)formamide as the stationary liquid at a coating of 25 % by mass, on acid washed 6201 or Chromosorb P (AW) of 200 micrometres to 300 micrometres as the support, in a passivated stainless steel tube or a stainless steel tube with a deactivated inner wall, 5 m long and 2 mm in internal diameter. Figure 3 is a chromatogram verifying the performance of such a column; a note to that figure records a benzene to 1-undecene retention time ratio of 1.59 and a toluene to benzene retention time ratio of 1.31.

6.9 Recording and data processing unit: a chromatography workstation with the following functions should be used. It can display the acquired chromatogram; it displays the peak area and the area percentage data of the chromatographic peaks; it calculates and applies the correction factors; it can handle noise and ghost peaks; it can carry out the manual integration needed; and it calculates the results by the corrected area normalisation method from the peak areas or area fractions, the corresponding relative mass correction factors and the related parameters.

7 Reagents

7.1 Component reagents, all of analytical grade. A warning states that these compounds are all flammable or toxic and that ingestion, inhalation or absorption through the skin will harm or kill. Thirty reagents are listed: n-pentane, n-hexane, cyclohexane, methylcyclohexane, n-heptane, isooctane, n-octane, n-nonane, n-decane, n-undecane, n-dodecane, 1-pentene, 1-hexene, cyclohexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, benzene, toluene, mixed xylene, ethylbenzene, propylbenzene, 4-ethyltoluene, trimethylbenzene, butylbenzene, tetramethylbenzene, methyl tert-butyl ether and ethanol.

7.2 System verification sample: a quantitative mixture of pure hydrocarbon compounds, used to check the reliability of the analysis system, to optimise the operating temperatures and the valve switching time of the system, and to ensure the accuracy of the results. The composition and concentration values of a typical system verification sample are given in Table 3.

7.3 Quality control check sample: used for the routine check of the reliability of the chromatographic system and the separation system and of the trapping capacity of the olefins trap; the analysis of the quality control check sample verifies whether the determination results lie within the precision of the method. The quality control check sample may be made up from hydrocarbon compounds close to the sample to be tested. It is sealed in ampoules, stored at low temperature and kept unchanged during storage. The system verification sample of Table 3 may serve as a quality control check sample. Table 4 gives the typical composition of a system verification sample containing methyl tert-butyl ether and Table 5 that of a system verification sample containing ethanol.

7.4 Gases: compressed air as the combustion supporting gas, of purity not less than 99.9 %, with a warning that it is a high pressure gas; hydrogen as the fuel gas, of purity not less than 99.9 %, with a warning that it is a high pressure gas and highly flammable; both air and hydrogen are purified with molecular sieve and activated carbon purifiers to remove water and hydrocarbons; and the carrier gas, high purity nitrogen or helium, purified as required by 7.6.

7.5 Sample vials: glass vials with a crimp cap or a screw cap lined with a rubber septum faced with polytetrafluoroethylene.

8 Setting up and preparation of the instrument system

8.1 The gas chromatograph and the separation system are integrated as shown in Figure 2. Where a commercial system is used, installation, positioning and system optimisation are settled with the manufacturer.

8.2 Impurities in the carrier gas harm the performance of the chromatographic column and of the olefins trap; a reliable carrier gas purification system as described in 6.7 is fitted so that the system runs properly.

8.3 An actual sample, the system verification sample or the quality control check sample may be used to check how well the polar column separates the aliphatic hydrocarbons from the aromatics and to check the elution times of benzene and of the C7 plus aromatic components, and so to fix the valve switching time. The temperature of the olefins trap is adjusted with the system verification sample or an actual sample until the trapping of olefins and ethers meets the requirement. Typical chromatographic operating conditions are given in Table 6.

9 System verification and standardization

9.1 Reliability check of the instrument system: the system verification sample of 7.2 is used as the test sample and run twice, once through the olefins trap and once bypassing it. The peak area values measured for the C7 plus aromatics in the two runs are compared; if the system is working properly, the difference between the two C7 plus aromatic values does not exceed the repeatability requirement of the method. Otherwise the pipe connections of the instrument system, the six-port valves and the purity of the carrier gas are checked for problems.

9.2 Performance check of the olefins trap: the trap is the most important part of the analysis system of this test method, and a trap that has failed or that falls short of the performance requirement affects the accuracy of the results directly. Its performance may be checked with the system verification sample, the quality control check sample, or an actual sample of high olefin content. Under the set test conditions the trap lets all the saturate components through and traps all the olefin components, as shown in Figure 4. The deviation of the measured results does not exceed the reproducibility requirement at the content level of each component in the system verification or quality control check sample. Otherwise the analysis conditions are adjusted to meet that requirement, and if necessary the trap is replaced.

9.3 Determination of the retention time ranges: the retention time ranges of the saturates, benzene, C7 plus aromatics and olefins may be fixed with the system verification sample or with an actual gasoline sample. Table 7 gives the retention time range of each hydrocarbon type through a 5 m column of N,N-bis(alpha-cyanoethyl)formamide and the olefins trap under the conditions of Table 6; Figure 4 is the chromatogram of the system verification sample of Table 4.

10 Test procedure

10.1 Sample collection and preparation: sampling follows the method of GB/T 4756. If the sample is not analysed immediately after sampling it is sealed and kept in a refrigerator, so that the light components do not evaporate.

10.2 Preparation of the analysis system: after start-up the parameter settings of the analysis system are checked. To purge the system, the instrument is run once through the sample analysis steps with no sample before samples are analysed, so as to drive out the residual impurities from the column and the olefins trap.

10.3 About 0.1 microlitre of a representative test portion is injected into the prepared gas chromatographic system; the sample travels as follows. a) It passes through the polar separation column, on which the aliphatic hydrocarbons and the aromatic components separate completely. b) The mixture of saturates and olefins separated by the polar column enters the olefins trap; the olefin components are selectively retained there while the saturates pass through the trap and enter the detector, as shown in Figure 4. Before benzene leaves the polar column, six-port valve 3B is switched so that the trap leaves the carrier gas path and is sealed, and the benzene separated on the polar column passes through the balancing column and enters the detector, as shown in Figure 5 a). c) Once benzene has finished eluting, the other six-port valve 3A is switched, so that the C7 plus aromatics, C7 included, are backflushed out of the polar column and enter the detector, as shown in Figure 5 b). d) While the C7 plus aromatics are being backflushed, the temperature of the olefins trap begins to be raised; once the C7 plus aromatic components have eluted completely, valve 3B is switched again so that the trap re-enters the carrier gas path, and the olefins desorb from the trap and enter the detector, as shown in Figure 5 c). The chromatogram obtained is processed by the workstation and the corresponding analysis software, and the mass fraction or volume fraction of each component is calculated. Figure 6 shows the chromatogram of a typical gasoline analysis. For a solvent oil sample where the olefins need not be analysed, only steps a) and c) are carried out; the typical chromatogram obtained is shown in Figure 7.

11 Quality control

To confirm the reliability of the analysis system, the system verification sample of 7.2 or the quality control check sample of 7.3 is analysed every two weeks. The difference between the determination result and the reference value of the system verification or quality control sample is to be smaller than the reproducibility requirement; otherwise the source of the error is identified and the necessary correction made. An actual sample may serve as the quality control sample, but the preparation of the quality control sample is carried out with reference to GB/T 15000.

12 Calculation and report

12.1.1 The integration of the chromatogram by the integrator or the workstation is checked, to confirm that every chromatographic peak has been integrated sensibly; where it has not, the baseline is corrected with the manual integration function of the workstation and the peak integrated again. A note states that, because the benzene content of gasoline is low, unsound baseline cutting and integration have a large effect on the analysis result.

12.1.2 Relative mass correction factors: the values and the calculation of the relative mass correction factor of each hydrocarbon type are given in Annex B. For gasoline products meeting the distillation requirements of GB 17930, the values of the relative mass correction factors of the hydrocarbon types with respect to the C7 plus aromatics, calculated from the distribution of the components of different carbon number in each hydrocarbon type and from their response on the flame ionisation detector, are given in Table 8. For samples of other distillates, the mass correction factor with respect to the C7 plus aromatics may also be obtained experimentally from a standard sample using formula (1). The legend of formula (1) names the relative mass correction factor; the mass fraction of the C7 plus aromatics in the standard sample, as a percentage; the peak area fraction of the C7 plus aromatics measured chromatographically in the standard sample; the mass fraction of the saturates, olefins or benzene in the standard sample; and the peak area fraction of the saturates, olefins or benzene measured chromatographically in the standard sample.

12.1.3 The mass fraction of the saturates, olefins, C7 plus aromatics and benzene in the test portion is calculated from formula (2). The legend names the mass fraction of component i in the test portion as a percentage, the relative mass correction factor of component i, and the peak area fraction of component i measured chromatographically.

12.1.4 The volume fraction of the saturates, olefins, C7 plus aromatics and benzene in the test portion is calculated from formula (3). The legend names the volume fraction of component i in the test portion as a percentage, the relative mass correction factor of component i, the peak area fraction of component i measured chromatographically, and the weighted relative density of the saturates, olefins and C7 plus aromatics together with the relative density of benzene, as given in 12.1.5.

12.1.5 The values and the calculation of the weighted relative density of each hydrocarbon type are given in Annex C. For gasoline products meeting the distillation requirements of GB 17930, the values of Table 9 may be used; for gasoline of a particular distillation range the calculation method is given in Annex C. Where a weighted relative density other than the value of Table 9 is used, the value used is stated under other circumstances in the report.

12.1.6 Where the gasoline contains ethers or alcohols, the content of each oxygenate is first determined by NB/SH/T 0663, and the content of each hydrocarbon type is then calculated by the method of Annex A.

12.2 Report: the volume fraction, or mass fraction, of the saturates, olefins and aromatics in the test portion is reported to the nearest 0.1 %, the aromatic content being the sum of the C7 plus aromatic content and the benzene content. The volume fraction, or mass fraction, of benzene is reported to the nearest 0.01 %. Other circumstances are also reported.

13 Precision

13.1 The precision of the document was obtained from a cooperative trial by ten laboratories on twenty-four samples. It was established by statistical analysis of the interlaboratory results in accordance with GB/T 6683.1. The reliability of a test result is judged according to 13.2 to 13.4 at the 95 % confidence level.

13.2 Repeatability: the difference between two test results obtained by the same operator, in the same laboratory, with the same apparatus, by the same test method, on successive determinations of the same test portion, is not to exceed the values listed in Table 10 or Table 11.

13.3 Reproducibility: the difference between two single, independent results obtained by different operators, in different laboratories, with different apparatus, by the same test method, on determinations of the same test portion, is not to exceed the values listed in Table 10 or Table 11.

13.4 Precision for solvent oils: the precision of the aromatics method for solvent oils is estimated according to Table 10.

Annexes Annexes

Annex A, normative, gives the correction of the result when oxygenates are present in the gasoline sample. Its introduction records that oxygenates such as ethers or alcohols are often added to finished gasoline to improve the octane number and the emissions; that when such a gasoline is analysed by this method the ethers elute together with the olefin components and the alcohols together with the C7 plus aromatic components; and that the response of the oxygenates is therefore subtracted from the corresponding hydrocarbon components, the correction giving the true hydrocarbon type contents of the gasoline. As required by GB 17930, the oxygenate content of gasoline is determined by the method NB/SH/T 0663. From the mass fraction of each oxygenate so determined and the apparent peak areas of the hydrocarbon components measured during this test method, the mass fraction or volume fraction of each hydrocarbon type in the gasoline is obtained according to A.2 to A.5.

A.2.1 Where the test portion contains ethers, the olefin peak area fraction is corrected by formula (A.1), whose legend names the corrected area fraction of the olefin chromatographic peak in the test portion as a percentage, the apparent olefin peak area fraction measured chromatographically as a percentage, the mass fraction of the i-th ether as a percentage, and the relative mass correction factor of that ether, taken from Table A.1.

A.2.2 Where the test portion contains alcohols, the C7 plus aromatic peak area fraction is corrected by formula (A.2), whose legend names the corrected area fraction of the C7 plus aromatic chromatographic peak as a percentage, the apparent C7 plus aromatic peak area fraction measured chromatographically as a percentage, the mass fraction of the j-th alcohol as a percentage, and the relative mass correction factor of that alcohol, taken from Table A.1.

Table A.1 lists the relative correction factors of the oxygenates typical of gasoline: methyl tert-butyl ether, ethyl tert-butyl ether, tert-amyl methyl ether, ethanol, methanol, isopropanol and tert-butanol.

Annex B, informative, gives the calculation of the relative mass correction factor of each hydrocarbon type, and Annex C, informative, the calculation of the weighted relative density of each hydrocarbon type. The bodies of Annexes B and C could not be read from the source and their content is not reproduced here.

Note on the tables that could not be read Note on the tables that could not be read

Table 1 fixes the performance requirements of the flame ionisation detector, giving a typical value for the noise in amperes, the drift in amperes per hour, the detection limit for n-hexane in grams per second, and the linear range. The powers of ten in that table lost their exponents in the extraction, so its figures are not reproduced here.

Tables 3, 4 and 5 fix the composition of the typical system verification samples, respectively the plain one, the one containing methyl tert-butyl ether and the one containing ethanol, listing for each hydrocarbon type the individual components, their mass fraction and the total for the type. In all three tables the mass fractions of trimethylbenzene, butylbenzene and tetramethylbenzene are printed detached from their component names, so the pairing of those rows cannot be relied on and the figures of these tables are not reproduced here.

Table 10 fixes, for the saturates, olefins, aromatics, benzene and C7 plus aromatics, the repeatability, the reproducibility and the volume fraction range over which they hold, with a note that the variable in the expressions is the mean volume fraction of the component. The repeatability and reproducibility are printed as powers of the mean volume fraction and lost their exponents in the extraction, so its figures are not reproduced here.

Table 11 fixes the repeatability and the reproducibility, both as volume fractions, at typical content levels of olefins, aromatics and benzene. Its content levels and its precision values are printed detached from the component names, so the pairing cannot be relied on and its figures are not reproduced here.

Table 8 gives the average mass correction factor of each hydrocarbon type and Table 9 the weighted relative density of each hydrocarbon type at 20 °C in grams per millilitre; both pair four hydrocarbon types with four values and both were read without difficulty, but the values are not restated here.