Determination of Total Volatile Sulfur in Gaseous Hydrocarbons and Liquefied Petroleum Gases by Ultraviolet Fluorescence According to ASTM D6667 resp. ASTM D7551 Application Note · multi EA 5100

Introduction

This application note describes different application methods for the fast and reliable determination of widely varying sulfur contents by means of UV fluorescence with a minimum effort for calibration and sample preparation. Gaseous hydrocarbons are valuable source materials for synthesis processes, fuels, energy carriers or auxiliary materials in pharmaceutics and cosmetics. Their quality and composition, especially their sulfur content, play an important role. To use them as propellant gas, they need to be free from sulfur (c < 100 ppb). The same applies to their use in the synthesis of high molecular compounds (e.g., polyethylene), since sulfur compounds may cause catalyst contamination, undesirable byproducts and increased production costs. A reliable sulfur trace detection in such matrices requires precise and safe metering technology, quantitative combustion and highly sensitive detectors.

For the use as combustible or fuel (LPG, CNG), different requirements and standards apply. Here emission limits for sulfur must not be exceeded and the sulfurous odorants added for safety reasons must be at the appropriate level. To guarantee optimum product quality and instant process optimization, the rapid detection of ultra-low and high sulfur contents is required in LPG analysis. LPG analysis confronts the operator with two main challenges: 1) the sample digestion, including introduction and combustion quality and 2) the analysis evaluation, including calibration strategies and quality assurance of results.

Due to widely varying pressures, safe and controlled handling and dosing of LPG into the analysis system is challenging. Samples tend to evaporate too early, affecting analysis quality and operating safety due to deflagration, explosion, soot formation or incomplete combustion. When expanding the LPG to allow a more controlled handling, the samples undergo changes due to adsorption loss, condensation, etc., which clearly influences the analysis quality. Direct LPG analysis out of the liquefied phase followed by stepwise evaporation in controllable safe small volumes is one approach to overcome these challenges. Inert coating (e.g. SilcoNert) of relevant parts thereby supports the detection of the tiniest TS impurities without falsifying component losses. Still, a quantitative sample conversion does not necessarily lead to correct analysis results. These strongly depend on a suitable and correct calibration strategy. In general, the system is calibrated either by just one certified standard using a single point calibration or by doing an error-prone and laborious dilution line for one standard material. The first approach can deliver suitable results when high element contents must be analyzed and the blank need not be considered. For analysis in the trace and ultra-trace range, where there are no suitable calibration gases available, this strategy will deliver results that are false, because they are too low. Here, the use of a dilution line is recommended but requires a dedicated and skilled operator as it bears a high risk of error (changing blank, adsorption loss, cross-contamination, pressure effects, etc.).

The problems discussed here can be easily avoided by applying alternative calibration strategies. These simplify lab work and make it possible to obtain reliable results independent of the operator‘s skills and experience. System calibration can be achieved using a calibration gas with variable injection volumes, as well as calibration kits of liquid standards at different concentrations and constant injection volume. This is possible because the multi EA 5100 provides a quantitative detection. Detection of the SO2 is the same originating from the combustion of a liquid, solid or gaseous sample. Furthermore, the daily factor function helps to ensure the long-term stability of the respective calibration resulting in best results without laborious re-calibration of the system.