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Gary Barone, Martin Higgins, David Smith, Restek Corporation Performance Coatings Division
Shawn Rowan, Warren J. Gross, O’Brien Corporation
Phil Harris, Harritec Llc.
Many volatile sulfur compounds, including hydrogen sulfide, methyl mercaptan, and ethyl mercaptan, adsorb strongly to
metal surfaces in sampling, storage, and transfer apparatus. In addition to causing inaccurate, falsely low values,
adsorption can prolong analysis cycle times. To determine and compare quantitative losses of active sulfur species, we
sampled, stored, and transferred low ppmv to low ppbv concentrations of active sulfur gases, using control (untreated)
and Sulfinert® treated system components.
Preventing Sulfur Compound Losses During Storage
Figure 1a depicts results from a comparison in which a gas containing 17ppbv of hydrogen sulfide was stored for 7 days in
untreated or in Sulfinert® treated stainless steel sample cylinders. The response ratio for hydrogen sulfide, relative
to a stable reference material, dimethyl sulfide, is steady at approximately 1:1 for at least seven days in Sulfinert®
treated cylinders. The data show a Sulfinert® treated system will reliably store ppb levels of the active
sulfur-containing compound during transport from the sampling site to the analytical laboratory. In contrast, hydrogen
sulfide degraded rapidly in the untreated cylinder, and was lost totally within 24 hours.
In a similar study in which gas containing 18.8ppbv methyl mercaptan was stored for 60 hours in Sulfinert® treated
sample cylinders, recovery of the active sulfur compound was equally high relative to the stable reference material,
dimethyl mercaptan, as shown in Figure 1b.
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Figure 1 Sulfur compounds are stable in Sulfinert® treated stainless steel systems.
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Sample Transfer: Adsorption of Sulfur Compounds to Tubing
Comparison of Sulfinert® treated electropolished stainless steel tubing (TrueTube™ EP tubing, O’Brien Corporation,
St. Louis, MO), untreated electropolished stainless steel tubing (TrueTube™ EPS tubing, O’Brien Corporation), and
raw commercial grade 316L stainless steel tubing showed Sulfinert® treated electropolished tubing has the inertness
necessary for quantitatively transferring low ppmv to low ppbv concentrations of sulfur compounds.
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Figure 2 Sulfinert® treated electropolished seamless stainless steel tubing (red) does not
adsorb methyl mercaptan (500ppbv).
Blue-untreated electropolished tubing;
Violet-raw tubing.
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To determine whether an active sulfur-containing compound in a gas stream would adsorb to active sites on the transfer
tubing surface, we monitored the length of time that elapsed before recovery values for a sulfur compound exiting a
100-foot (30.5-meter) length of tubing were stable and accurate, using helium containing 0.500ppmv methyl mercaptan as
the test material, at a flow rate of 40cc/minute. Figure 2 shows Sulfinert® treated electropolished tubing did not
adsorb methyl mercaptan to any measurable extent, delivering a representative sample with no delay. Untreated
electropolished tubing totally adsorbed methyl mercaptan for more than 75 minutes, and the sulfur gas level exiting
the tubing did not stabilize until approximately 130 minutes. Raw 316L stainless steel tubing totally adsorbed methyl
mercaptan for more than 90 minutes, and the sulfur gas level did not stabilize until approximately 140 minutes.
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Figure 3 Sulfur memory is prolonged in raw commercial grade stainless steel tubing (violet).
Red-Sulfinert® treated electropolished tubing; Blue-untreated electropolished tubing. (500ppbv
methyl mercaptan in helium)
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When adsorption of sulfur-containing compounds is prolonged, as shown in Figure 2, desorption from the surface also is
slow. This “memory” of adsorbed compounds can cause long delays in re-equilibrating a sample pathway for obtaining
accurate values from successive sample streams. In Figure 3, Sulfinert® treated tubing shows the lowest retention
of sulfur compounds, by several orders of magnitude. Samples can be evaluated, with accurate results, with no delay
between them.
Economic Value of an Inert Pathway
The advantages of a Sulfinert® treated sampling and transfer system are twofold: more accurate results and faster
cycle times. Improved accuracy and reliability of data for sulfur mean downstream processes can be more precisely
controlled, with associated cost savings. Shorter cycles translate directly into more samples collected and analyzed
in a given period of time. Savings accrued from shorter cycles can be calculated by looking at typical per-hour costs
of operating processes: a one-hour delay between analyses in an ethylene plant producing 800,000 tons of product per
year can cost $50,000; a styrene plant producing 250,000 tons per year stands to loose $33,000 per hour; even for a
200,000 ton per year anti-freeze grade production process the loss can be $3,600/hour
1.
Conclusion
We obtained accurate data, with no delay between samples, by using Sulfinert® treated electropolished tubing in
the sampling-storage-transport system. In contrast, we obtained significantly less accurate data, even with delays of
more than two hours between samples, by using untreated tubing. Analysts charged with monitoring sulfur levels in
process streams can significantly improve profitability by using Sulfinert® treated system components and
Sulfinert® treated electropolished tubing for transport lines.
REFERENCES:
1 Application of TrueTube™ in Analytical Measurement Cardinal UHP August 2004
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