Determination of total volatile sulfur in gaseous hydrocarbons and LPG by ultraviolet fluorescence according to ASTM D6667

ASTM D6667 is an established test method for the determination of total volatile sulphur in gaseous hydrocarbons and liquefied petroleum gases (LPG). This test method is applicable to the analysis of natural, processed, and final product materials containing sulphur in the range of 1 to 100 mg/kg.


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Article Summary

Sulphur could be present in LPG or gases, used for fuel purposes, through the production process as well as component of odorant. In both cases there is a need for measuring the total Sulphur content for regulatory compliance determination.

LPG or gases used as a feedstock for various processes should also be monitored for the sulphur content, as some process catalysts used in petroleum and chemical refining can be poisoned by sulphur bearing materials in these feedstocks.

Sample introduction
A gaseous hydrocarbon or LPG sample is introduced into the ElemeNtS by the automated Accura sampling device. The Accura has a vaporiser to convert the liquified gas (LPG) into a gas. After passing the vaporiser the gas (or vaporised LPG) enters a heated automated gas sampling valve. When the injection pulse is given (from the ElemeNtS software), the valve is switched, and the content of the sample loop transferred to the ElemeNtS with the carrier gas. An additional flow (controlled by a built-in Mass Flow controller) is added to ensure a sufficient flow.

For calibration purposes gases with a known concentration of Sulphur should be used. The calibration gases can be introduced into the ElemeNtS in the same way as samples are introduced, by directly connecting to the Accura.

Measuring principle
The carrier gas coming from the Accura enters the high temperature, dual temperature zone combustion tube where the sulphur components are vaporised and combusted. The released sulphur is oxidised to sulphur dioxide (SO2 ) in an oxygen rich atmosphere.

A stream of inert gas (helium or argon) takes the reaction products, after removal of the produced water vapour, into a reaction chamber. Here the SO2 molecules are converted to excited SO2  by the absorption of energy of a UV lamp and emitting light (fluorescence) while it relaxes to a stable state.

A Photomultiplier tube measures the emitted light signal.

The response signal is integrated to calculate the area. The sulphur concentration of an unknown product is calculated using the linear regression function of the of the concentration of standard mixtures versus integrated area.

The system and methodology of the Antek ElemeNtS total sulphur analyser is thoroughly tested for recovery, response linearity, sample scope and repeatability, to validate its performance according to ASTM D6667.

Two samples were analysed to determine the recovery in different sample streams. One sample is an LPG like sample, containing iso-butane, n-butane and propane mixture (liquid) with sulphur components. The other sample contains propylene with sulphur components.

The linearity of response for the analyser is verified by creating dynamic dilutions of a certified calibration gas (9,99 ppm mol SO2 ). The dilutions are prepared by combining the calibration gas (sulphur compound in Helium) and dilution gas (pure Helium) using two separate mass flow controllers. Concentrations from ~ 10 ppm down to 100 ppb have been created and analysed on the ElemeNtS system. Each dilution level is analysed 10 times (2 series of 5 injections). A Calibration line has been created, with a linearity correlation > 0.9999 (Figure 2).

Detection limit of the ElemeNtS system in combination with the Accura sample injection, is calculated according to ISO 11483. A series of dilutions is prepared as described under “linearity”. An overlay of the injections at low level (< 1 ppm) is depicted in figure 3. The linearity correlation of the diluted calibration gas at this low level is still > 0.999. The calculated LOD according ISO11483 is 0,029 ppm mol (= ~0,03 mg/kg in LPG), which is much better than the desired detection limit (1 mg/kg).

Area (concentration) is the primary measurements in total sulphur analysis. The precision in which it is measured ultimately determines the validity of the generated quantitative data. Area precision require that all parameters (temperatures, pressure, flow, injection) are controlled to exact tolerances. Furthermore, the inertness of the flow path can considerably affect area precision, especially for active Sulphur components at low levels.

Concentration repeatability for the ElemeNtS total sulphur analyser is measured for 10 consecutive runs for two reference samples. Repeatability standard deviation of total sulphur is well within the precision statement of ASTM D6667.

These results demonstrate that the ElemeNtS analyser, in combination with the Accura automatic sample introduction, is a powerful tool for the determination of sulphur in Gaseous Hydrocarbons and Liquefied Petroleum, based on the exceptional calibration linearity, low limit of detection, excellent repeatability and recovery.
The Antek ElemeNtS total Sulphur analyser in combination with the Accura is meeting the ASTM D6667 requirements.

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