SH/T 0410-1992 Determination of normal paraffin content and carbon number distribution in liquid paraffin and raw materials (gas chromatography)

AI-translated method card — human verification pending. This is not an official translation of the standard. Unclear source values remain unverified; check the original standard and product certificate before use.

Card type: Test method

1. Method number / full name / English name / replacement relationship

• Standard number: SH/T 0410-92
• Chinese full name: Determination of normal paraffin content and carbon number distribution in liquid paraffin and raw materials (gas chromatography)
• English name: not provided in the available source
• Replacement relationship: replaces ZBn E 42 014-89
• Reference adoption: current analytical method UOP 411-75 of the American UOP company, "Determination of normal paraffins by subtraction gas chromatography" (see additional notes)
• Approval/implementation: approved by China Petrochemical Corporation 1992-05-20, implemented on 1992-05-20

2. Scope of application and non-applicability

• This standard specifies the method for determining normal paraffin content and carbon number distribution.
• It is applicable to the determination of normal paraffin content and carbon number distribution in liquid paraffin obtained by molecular sieve or urea dewaxing.
• It is also applicable to the determination of normal paraffin content and carbon number distribution in feedstock oil and dewaxed oil.
• If the sample contains normal olefins, the determination result is the sum of the normal paraffin and normal olefin contents.
• Not applicable: not provided in the available source.

3. Method principle

• The sample is injected into a chromatographic system, which consists of two dual-gas-path gas chromatographs with thermal conductivity cell detectors connected in series. The first chromatograph contains two chromatographic columns, one being a short chromatographic column with no separation effect, and the other being a long partition column with a separation effect. The outlet of the thermal conductivity cell detector of the first unit is connected to the inlet of two identical 5A molecular sieve chromatographic columns of the second chromatograph.
• The total normal paraffin content is determined by the non-distribution method (also called the indirect method): first determine the non-normal hydrocarbon content, then calculate the normal paraffin content. Inject the sample into the short chromatographic column with no separation effect to obtain the whole-sample peak area (all components of the sample are contained in one peak), then the sample enters the 5A molecular sieve column, normal paraffins are subtracted, and the non-normal hydrocarbon peak area is obtained (all components in the non-normal hydrocarbons are contained in one peak). Compare the non-normal hydrocarbon peak area with the whole-sample peak area, calculate the mass percentage of non-normal hydrocarbons by the external standard method, and subtract the mass percentage of non-normal hydrocarbons from one hundred to obtain the total normal paraffin content.
• The carbon number distribution is determined by the distribution method (also called the direct method): inject the sample into the long partition column to obtain the whole-sample chromatogram, then pass the sample through the 5A molecular sieve column to obtain the non-normal hydrocarbon distribution diagram, which has the same non-normal hydrocarbon background as the whole-sample chromatogram, thereby allowing determination of the net normal paraffin peak area on the whole-sample chromatogram.
• By normalization calculation, the relative distribution of each individual normal paraffin is obtained, and by combining it with the total normal paraffin content measured by the non-distribution method, the percentage content of each individual normal paraffin in the sample can be obtained.

4. Reagents and materials (purity / preparation)

• 3.1 normal paraffins (the source text source-text extraction is "field one C18 individual normal paraffins", suspected to be "n-C5~n-C18 individual normal paraffins", requires confirmation against the source standard (source-text extraction)): chromatographic purity.
• 3.2 non-normal hydrocarbons completely free of normal paraffins: pass dewaxed kerosene through a freshly activated 5A molecular sieve adsorption column to remove residual normal paraffins until the effluent, when tested for carbon number distribution according to clause 5.6, shows no residual normal paraffin peaks on the resulting chromatogram.
• 3.3 trimethylchlorosilane: analytical reagent grade.
• 3.4 5A molecular sieve: 30–60 mesh, type 5A, when the partial pressure ratio is 0.1 the n-hexane adsorption capacity is greater than 100 mg/g, stored in a sealed container, protected from moisture, after packing the column pass hydrogen at column temperature for 2 h.
• 3.5 stationary phase: 60–80 mesh acid-washed Chromosorb "P" support, coated with 20% silicone rubber SE30.
• 3.6 101 white silanized support (produced by Shanghai Reagent Factory No. 1).
• 3.7 hydrogen: used as carrier gas.

5. Instruments and equipment

• 4.1 The gas chromatograph shall meet the following conditions:
  • The first unit is equipped with a temperature programming device and can raise the temperature to 300℃; the column temperature of the second unit can be kept constant at 350℃; both chromatographs are dual-gas-path and equipped with thermal conductivity cell detectors; the outlet of the thermal conductivity cell detector of the first unit is connected to the inlet of the molecular sieve chromatographic column of the second unit by a connecting tube, and the temperature of the connecting tube is maintained at 250℃.
  • Chromatographic columns: in the first unit, a short stainless steel packed column with no separation effect, length 30–100 cm, inner diameter 3–4 mm, packed with 101 white silanized support; and a long stainless steel partition column, length 300 cm, inner diameter 3–4 mm, packed with the stationary phase of clause 3.5. The second unit contains two identical 5A molecular sieve stainless steel columns, length 40 cm, inner diameter 3–4 mm.
  • Note: the short chromatographic column required by method UOP 411 is required to have no separation effect, yet a silicone rubber chromatographic column with a separation effect was used. This standard instead uses a 101 white silanized support chromatographic column with no separation effect.
• 4.2 Electronic integrator: 2 unit(s).
• 4.3 Microsyringe: 10 μL.
• 4.4 Reading magnifier: capable of magnifying 10–20 times.

6. Recommended operating conditions

• Carrier gas: hydrogen, flow rate 70 mL/min.
• Vaporization chamber temperature: 275–300℃.
• Detector temperature: 275–300℃.
• Column temperature:
  • For determination of total normal paraffin content, the first chromatograph is isothermal, and according to the properties of the sample, it is selected at 180–240℃.
  • For determination of carbon number distribution, the first chromatograph uses temperature programming, and the heating rate should preferably ensure that the elution time of high-boiling components is 15–25 min (the source text source-text extraction is "15.25min", requires confirmation against the source standard (source-text extraction)).
  • For determination of total normal paraffin content and carbon number distribution, the column temperature of the second chromatograph is 300℃ in both cases.
• Detector bridge current: the detector bridge currents of both chromatographs are approximately 170 mA (the source text source-text extraction is "170nu1", requires confirmation against the source standard (source-text extraction)).

7. Sampling and sample preparation

• 5.1 Preparation of standard sample: according to the sample composition (for an unknown sample, first determine the carbon number distribution according to clause 5.6 to obtain a semi-quantitative result), use individual normal paraffins and non-normal hydrocarbons completely free of normal paraffins to prepare a standard sample close to the sample composition in a certain proportion.
• 5.2 Passivation of 5A molecular sieve:
  • 5A The molecular sieve sometimes has excessive affinity for aromatic hydrocarbons in the sample, causing high results; such molecular sieve should be pretreated before use. One method is to pack the molecular sieve into a chromatographic column, connect one end of the column to the injection port, leave the other end unconnected to the detector and open to atmosphere, adjust the carrier gas flow rate to 20 mL/min, raise the column temperature to 100℃, inject several portions of 10% (V/V) trimethylchlorosilane in carbon tetrachloride solution, each portion 50 μL, after each injection of 50 μL, raise the column temperature by 50℃, so that the molecular sieve column reaches the operating column temperature, thereafter, for each increase of 50℃ in column temperature, inject 5 portions of 50 μL silanization solution, but not in excess, and test with non-normal hydrocarbons completely free of normal paraffins until the non-normal hydrocarbon peak areas before and after the molecular sieve column are equal.
  • Note: the molecular sieve may also be impregnated with trimethylchlorosilane solution, packed into the column immediately after drying, and activated by passing hydrogen at the operating column temperature for 2 h.
  • Another method is to continuously inject several portions (each portion 25 μL) of aromatic hydrocarbons into the molecular sieve column.
  • The method for checking whether the molecular sieve can effectively adsorb normal paraffins is: inject 1 μL n-decane or n-undecane into the molecular sieve column; if the non-normal hydrocarbon impurities deviate significantly from the known values, it indicates that the molecular sieve needs to be replaced.
• 5.3 Adjust the detector bridge current: the detector bridge currents of both chromatographs are approximately 170 mA. Under normal operating conditions, when the signals of the two detectors match, inject non-normal hydrocarbons completely free of normal paraffins; if the peak areas before and after the molecular sieve column are the same, the bridge current will not be changed thereafter.

8. Analytical procedure

• 5.5 Determination of total normal paraffin content: under typical chromatographic separation operating conditions, inject 1–2 μL of sample and standard sample each twice into the short chromatographic column; after injection, start both integrators simultaneously; the first records the total peak area of the sample and standard sample, and the second records the peak area of non-normal hydrocarbons. Take the average of the ratio of the non-normal hydrocarbon peak area to the whole-sample peak area from the two determinations of the sample and standard sample, and calculate the total normal paraffin content. A typical chromatogram is shown in Figure 1 (light liquid paraffin sample).
• 5.6 Determination of carbon number distribution: under typical chromatographic separation operating conditions, inject 1–2 μL of sample into the long partition column, start temperature programming and both integrators to record the sample chromatogram. The relative distribution chromatogram of normal paraffins is shown in Figure 2a, and the non-normal hydrocarbon background chromatogram is shown in Figure 2b.
  • Superimpose the two chromatograms, trace the non-normal hydrocarbon background chromatogram (Figure 2b) onto the relative distribution chromatogram (Figure 2a), obtain the non-normal hydrocarbon background of the normal paraffins, and take an average baseline such that the normal paraffin peak area below the average baseline equals the non-normal hydrocarbon peak area above the average baseline. Use a reading magnifier to measure the peak height and half-peak width of the chromatographic peaks above the average baseline, with peak height accurate to 0.5 mm and peak width accurate to 0.1 mm.
  • Note: if the total normal paraffin content in the liquid paraffin is greater than 95% (m/m), the baseline may be used directly as the peak base, and the subtraction method need not be used for measurement.

9. Qualitative and quantitative determination and calculation formulas

• 6.1.1 The total normal paraffin content is calculated according to formula (1):
  • T = 100 − (H × I / J) × 100
  • Where: T— total content of normal paraffins in the sample, % (m/m); H— average value of the ratio of the non-normal hydrocarbon peak area to the whole-sample peak area in the sample; I— concentration of non-normal hydrocarbons in the standard sample, % (m/m); J— average value of the ratio of the non-normal hydrocarbon peak area to the whole-sample peak area of the standard sample.
  • Note: the layout of formula source-text extraction in the source text is incomplete; the above formula has been organized according to the textual description and requires confirmation against the source standard (source-text extraction).
• 6.1.2 The carbon number distribution of normal paraffins is calculated according to formula (2):
  • R = (K × P × W / S) × 100
  • Where: R— mass percentage of a certain individual normal paraffin in the total normal paraffins; K— relative correction factor of each individual normal paraffin, see Appendix A; P— peak height of the individual normal paraffin of a certain carbon number; W— half-peak width of the individual normal paraffin of a certain carbon number; S— sum of the products K×P×W of all individual normal paraffins in the sample.
• 6.1.3 The mass percentage of a normal alkane with a certain carbon number in the whole sample is calculated according to formula (3):
  • u = R × T / 100
  • Where: u— mass percentage of a normal alkane with a certain carbon number in the whole sample, % (m/m); R— percentage of a normal alkane with a certain carbon number in the total content of normal alkanes; T— total content of normal alkanes in the sample, % (m/m).
• 6.2 Report the total content of normal alkanes and the carbon number distribution of the sample as mass percentages, rounded to one digit after the decimal point.
• Appendix A (supplementary document) Relative correction factor table for individual normal alkanes:
  • n-C5: 1.00; n-C10: 1.01; n-C11: 1.03; n-C12: 1.05; n-C13: 1.07; n-C14: 1.09; n-C15: 1.10; n-C16: 1.12
  • n-C17: 1.14; n-C18: 1.15; n-C19: 1.17; n-C20: 1.18; n-C21: 1.19; n-C22: 1.19; n-C23: 1.20; n-C24: 1.21
  • Note: The carbon-number labels are incomplete in the source transcription. Verify the carbon-number correspondences in the table above against the original standard.

10. Precision

Repeatability: For the same operator, the same instrument, and the same sample, the repeatability results shall not exceed the following values:

11. Report

• Report the total content of normal alkanes and the carbon number distribution of the sample as mass percentages, rounded to one digit after the decimal point (see 6.2).
• Other reporting requirements: not provided in the available source.

12. Referenced documents

• No independent “Referenced documents” section is provided in the available source.
• Mentioned in the text: UOP 411-75 “Determination of normal alkanes by subtractive gas chromatography” (referenced and adopted, see additional notes).

13. Source and verification

This page is an AI translation of the available Chinese method card. Human verification is pending. It is not an official translation of the standard. Check the source standard and product certificate before use.

14. Quick reference for questions and answers