by Cynthia Elmore and Michael Duffy AL
Determination of Sulfur Compounds in Natural Gas and Gaseous Fuels by Gas Chromatography and Pulsed Flame Photometric Detection
The carbonated beverage and petrochemicals industries are just some areas in which the analysis of volatile sulfur-containing compounds in process streams is a critical quality-control step. Failure to quickly detect and accurately assess or quantify these compounds can lead to contami- nated products, damaged equipment, poisoned catalysts and process downtime. This article presents a fast, reliable and robust method for the determination of individual volatile sulfur-containing compounds in gaseous fuels by gas chromatography coupled with pulsed flame photo- metric detection.
Introduction Natural gas and other gaseous fuels contain varying amounts and types
of sulfur compounds, which are odorous and corrosive to equipment, and can inhibit or destroy catalysts used in gas processing. Between 1 and 4 ppmv of sulfur odorant compounds are typically added to natu- ral gas and other liquefied petroleum gases for safety purposes. These compounds may include ethyl mercaptan (EtSH), methyl mercaptan (MeSH) and tetrahydrothiophene (THT). Some odorant compounds can be reactive and may oxidize, forming more stable compounds with lower odor thresholds. Because even trace amounts of sulfur can be destructive, accurate measurement of sulfur compounds is essential to ensure proper gas processing and monitor odorant levels for public safety.
Analysis of sulfur compounds is challenging due to their reactivity and instability during sampling and analysis. Laboratory equipment must be inert, well-conditioned and passivated with a gas containing the sulfur compounds of interest to ensure reliable results. Another challenge can be the separation of some sulfur compounds from their hydrocarbon matrix. Hydrocarbons can produce a quenching effect on sulfur detection as a re- sult of the formation of carbon monoxide (CO). The subsequent reaction of CO with free sulfur (S) atoms in the flame generates carbonyl sulfide (COS). Formation of COS in the flame reduces the amount of available sulfur in the flame and reduces the production of the excited state S2* dimers. The emission from the excited state S2* dimer is measured by a pulsed flame photometric detector. Quenching can be minimized by choosing the right analytical column and by optimizing the chromatographic conditions
AMERICAN LABORATORY 20
to separate the sulfur compounds of interest from the large volume of hydrocarbons present in a gaseous sample.
Several ASTM methods exist for the determination of sulfur in gaseous fuel using gas chromatography with a sulfur selective detector. Although method operating conditions are similar, the primary difference is the type of selective detector used to identify and quantify the sulfur com- pounds. The instruments most commonly used for this analysis are the flame photometric detector (FPD), pulsed flame photometric detector (PFPD) and sulfur selective chemiluminescence detector (SCD).
Instrumentation Data were acquired using the S-PRO Select GC equipped with the
5383 PFPD (Figure 1) (OI Analytical, College Station, Texas). Samples and standards are introduced into the GC column via an automated air-actu- ated six-port valve housed in a heated oven through a low-dead-volume split/splitless volatiles interface. The sample pathway resides inside a heated valve oven and is treated with Sulfinert coating (Restek, Bellefonte, Penn.) to minimize adsorption of sulfur on the system surfaces.
An air/hydrogen mixture at a flow rate that does not produce continu- ous combustion is used by the PFPD. The combustor is filled with a gas
Figure 1 – 5383 PFPD. MARCH 2016
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