⛽ Hydrocarbon composition of LPG (propane, butane) — gas chromatography

Petrochemistry PN-EN 27941

Overview

The standard specifies a gas chromatographic method for the quantitative determination of the hydrocarbons in liquefied petroleum gas (LPG), excluding constituents at concentrations below 0.1 % (m/m). It applies to the analysis of propane, butane and their commercial mixtures, which may contain saturated and unsaturated C2, C3, C4 and C5 hydrocarbons; it does not apply to on-line chromatography. PN-EN 27941:2015-12 adopts EN 27941:1993 = ISO 7941:1988 (the description follows the preview of ISO 7941:1988).

The constituents are identified by passing a reference mixture or pure hydrocarbons through the column, or by comparing the relative retention times with typical chromatograms; the concentrations are calculated from the peak areas and correction factors by the method of internal normalisation. For a thermal conductivity detector the area correction factors are given in the tables of the standard (as guidance — laboratories with experience should establish their own), and for a FID the factor Fi is calculated from the numbers of carbon and hydrogen atoms of the constituent. Under the conditions of the standard the following pairs are not separated: air–methane, ethane–ethene (columns a, b, c) and butane–isobutene (columns a, b).

In Polish interlaboratory studies of LPG (the Oil and Gas Institute, 2011) the hydrocarbon composition was determined according to PN-ISO 7941:1993 + Ap1; it was recommended that the way the LPG is injected into the chromatograph (in the liquid or in the gaseous form) should be recorded, because it affects how representative the test portion is — the standard indicates liquid injection as the preferred method and gas injection as less desirable.

Method principle

The physical separation of the constituents by gas chromatography on a packed column: for commercial propane 8 m of packing with di-n-butyl maleate + 3 m with ββ′-oxydipropionitrile, for commercial butane 8 m with di-n-butyl maleate, alternatively 6 m with sebaconitrile (1,8-dicyanooctane); the support acid-washed Chromosorb P, fraction 180–250 µm, a loading of 25 g of phase to 75 g of support (columns a, b) or 20 g to 80 g (c); a tube of 2–5 mm internal diameter. Carrier gas: hydrogen (with special safety precautions), helium or nitrogen; oven temperature 40 ± 1 °C (columns a, b) or 20 ± 1 °C (c). The resolution required between propane and propene (commercial propane) and between propene and isobutane (commercial butane): R ≥ 1.5. A TCD (thermistor 40–50 °C, filament 100–150 °C) or a FID (100–150 °C) detecting 0.1 % of any separated constituent. The mass fraction of a constituent: xi = Ki·Ai/Σ(Ki·Ai) × 100 (TCD) or xi = Fi·Ai/Σ(Fi·Ai) × 100 (FID), where Fi = (12.01·nCi + 1.008·nHi) × 0.8265/(12.01·nCi); the mole fraction from the numbers of carbon atoms.

Applications

• Quality control of LPG (autogas) according to PN-EN 589 — the hydrocarbon composition and the parameters calculated from it (the motor octane number from the proportions of the constituents)
• Commercial settlement of propane, butane and propane-butane mixtures
• Interlaboratory and proficiency testing of gaseous fuel laboratories
• Control of the gas separation processes in refineries and terminals

Key parameters

Standard

Standard number

PN-EN 27941:2015-12

Title (PL)

Techniczny propan i butan — Analiza metodą chromatografii gazowej

Title (EN)

Commercial propane and butane — Analysis by gas chromatography

Step-by-step procedure

1. Sampling

   Take the LPG sample according to ISO 4257.

2. Preparation of the column

   Dissolve 25 g (or 20 g for sebaconitrile) of the phase in the solvent, coat 75 g (80 g) of the support, evaporate the solvent (a rotary evaporator), sieve out the 180–250 µm fraction and pack the column reproducibly (vacuum at the outlet, tapping or vibration). Condition it at 40 °C for 5 h with a flow of carrier gas and the detector disconnected.

   ⏱ Time: 5 h • 🌡 Temperature: 40 °C

3. Setting of the instrument

   A liquid injector at 40 ± 5 °C (the liquid valve at ambient temperature) or a gas injector with a heated loop (for example 70 °C); the oven at 40 ± 1 °C (a, b) or 20 ± 1 °C (c); choose the flow so as to obtain a resolution of ≥ 1.5; the detector: thermistor 40–50 °C, filament or FID 100–150 °C; the chart speed such that the first peak is ≥ 3 mm at half height.

4. Calibration

   Qualitatively: pass a reference mixture or pure hydrocarbons through the column, or compare with typical chromatograms and relative retentions (figures 2–4, table 1). Quantitatively: TCD — internal normalisation with the correction factors of tables 2 and 3 (as guidance; use your own with sufficient experience); FID — Fi from the numbers of C and H atoms (table 4). Check the linearity of the response with reference mixtures of various concentrations or with pure gases at various partial pressures.

5. Liquid injection (preferred)

   Set the sample container upright with the outlet valve downwards and connect it through the metal filter to the injector with transparent, earthed, pressure-resistant tubing; downstream of the purge system use a reducing valve, so as to avoid evaporation. Open the valve until the tubing fills with liquid, inject 0.5–1 µl and close the valve.

6. Gas injection (less desirable)

   Variant 1: fill the 2 ml container with liquid, close it after about 10 min, release about 20 % of the contents, evaporate the rest completely into an empty vessel at a pressure slightly above atmospheric, mix and inject through the loop. Variant 2: the standard container upright with the vapour-space tube at the top, a Drechsel bottle with a 6 mm column of water at the outlet of the loop; purge the loop with vapour from the lower valve at a rate of 2 bubbles/s (do not open it too far — the lighter hydrocarbons evaporate faster), for about 10 loop volumes, bring the loop to atmospheric pressure, inject and disconnect the bottle.

7. Evaluation of the chromatogram

   Identify the constituents (a reference mixture or relative retentions); remember the pairs that are not separated: air–methane, ethane–ethene, butane–isobutene. The peak areas: from the recorder — the height × the width at half height with a correction for the attenuation, or from the integrator/computer.

8. Calculation

   The mass fraction xi = Ki·Ai/Σ(Ki·Ai) × 100 (TCD) or Fi·Ai/Σ(Fi·Ai) × 100 (FID); the mole fraction from the areas and the numbers of carbon atoms. Round to a tenth of the reproducibility (table 5).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• LPG is under pressure and extremely flammable — take and inject it in a ventilated room free of sources of ignition, with earthed tubing and the containers secured upright
• Hydrogen as the carrier gas — check the tightness of the lines carefully, especially in the oven; a hydrogen sensor
• The solvents for coating the phases (pentane, dichloromethane, toluene) — a fume hood, no naked flame

Frequently asked questions