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Selecting the best column for Speciation of Sulfur Compounds in Natural Gas

6 Apr 2011

Last Pittcon conference, a poster was presented based on some recent  work done by Rick Morehead from our applications laboratory in Restek Bellefonte.

The focus was sulfur speciation in natural gas, Sulfur gases were tested up to tetrahydrothiopene.

Figure 1: sulfur gases in natural gas matrix are well separated on thick-film Rtx-1

Natural gas is a complex mixture of low molecular weight hydrocarbons, inert gases and other impurities including a variety of sulfur containing compounds. Raw natural gas containing significant amounts of hydrogen sulfide and other organic sulfurs is typically processed to remove these compounds. Odorants are added to the final product to meet safety regulations. Testing of natural gas in its raw and refined states requires the use of chromatography systems that supply sufficient resolution of the hydrocarbons normally found in natural gas from any of the sulfur contaminants or odorants.

Wall coated open tubular (WCOT) columns coated with methyl silicone stationary phases have been successfully used for the separation of low molecular weight hydrocarbons. Analyzing low molecular weight sulfur compounds can also be performed on the thick-film Rtx-1 columns, see figure 1.  Here a PFPD selective detector was used. This detector has a hydrocarbon as well as a sulfur channel, so we can see where the sulfurs elute relative to the hydrocarbons. Peak shape and sensitivity for sulfur gases is  highly influenced by the inertness of the column and proper deactivation is required.  Heating this column to higher temperatures also elutes higher sulfurs very nicely as a stable baseline. The thick-film Rtx-1 is one of the best column for sulfur speciation in natural gas.

Figure 2: Sulfur gases on Rt-U-BOND. Note the symmetry and position relative to ethane and propane

Besides the Rtx-1, we also looked at the application of porous polymers for volatile and heavier sulfur compounds.  The separation mechanism for PLOT columns is significantly different than that observed for WCOT columns. The unique selectivity of different porous polymers can influence relative elution order of sulfur containing compounds versus low molecular weight hydrocarbons. A comparison illustrating elution order patterns and relative inertness of PLOT columns versus WCOT columns for sulfur analysis is shown.

Very interesting results were obtained for the Rt-U-BOND porous polymer. This adsorbent is highly inert resulting in near perfect H2S and COS peaks, see figure 2.  Also the position of both peaks is very interesting as they elute far away from propane and ethane.  On the Rt-U-BOND porous polymer, the propylene elutes together with propane and ethylene elutes before the ethane.

This selectivity together with the unique inertness, makes this column  a very good candidate for a single column solution to measure trace sulfur gases in ethylene AND propylene.