Q4 2009 / How to boost gas sampling reliability in tough environments

In a recent study a number of sampling system surfaces were evaluated for the transport and retention of mercury and sulfur compounds including 316L grade stainless steel, 304 grade stainless steel and functionalized amorphous silicon-coated (SilcoNert™ 2000, Silcotek Corporation, Bellefonte, PA) 316L grade and 304 grade stainless steel. 

These systems are used in stack gas applications, environmental quality testing, refining, oil and gas exploration, and the transport and storage of active compounds.

Sulfur and stainless steel components

Figure 1 shows the results of a comparison in which a gas containing 17ppb of hydrogen sulfide was stored for seven days in both untreated stainless steel sample cylinders, and in SilcoNert™- 2000-treated stainless steel sample cylinders.

Figure 1: Sulfur compounds are stable at 17ppb in Silconert™2000-treated stainless steel containers.

The data shows that a functionalized amorphous silicon, SilcoNert™ 2000-treated system could reliably store ppb levels of an active sulfur-containing compound, like hydrogen sulfide, during transport from the sampling site to the analytical laboratory. On the other hand, the study also showed that the hydrogen sulfide response degraded rapidly in the untreated cylinder. In fact it was totally lost within 24 hours¹.

The study compared SilcoNert™ 2000-treated electro-polished stainless steel tubing (TrueTube™ EPS tubing, surface roughness average (RA): 5-10, O’Brien Corporation, St. Louis, MO), untreated electro-polished stainless steel tubing (TrueTube™ EP tubing, RA 5-10, O’Brien Corporation), and raw commercial grade stainless steel tubing (RA 23-27). It showed that only the SilcoNert™ 2000-treated electro-polished tubing had the inertness necessary for quantitatively transferring low concentrations of sulfur compounds.

How the tests were performed

The tests were performed at room temperature, using a gas flow rate of 40cc/minute¹. Helium containing 0.500 ppm methyl-mercaptan was passed through 100-foot lengths of tubing. The study tracked the amount of time that elapsed before the values for the sulfur content exiting the tubing became stable and could be accurately measured.

The data in Figure 2 shows that SilcoNert™ 2000-treated electro-polished tubing did not adsorb methyl-mercaptan to any measurable extent and delivered a representative sample with minimal delay.

On the other hand, untreated electro-polished tubing, totally adsorbed methyl-mercaptan for 75 minutes. In addition the sulfur gas level did not stabilize for 130 minutes. Conventional 316L seamless tubing totally adsorbed methyl-mercaptan for 90 minutes, and the sulfur gas level did not stabilize for 140 minutes¹.

Mercury concerns

Mercury in hydrocarbon streams and stack gas effluent is of growing concern for environmental monitoring and for equipment service life. Mercury can corrode transfer equipment producing damage and increasing the risk of danger for operators. Aluminum heat exchangers, for example, are quickly damaged on contact with mercury as it forms an amalgam. Other metals as well experience similar changes on contact with mercury. Natural gas streams contain mercury continuously flowing through them. Their gas handling equipment experiences accumulated damage and sometimes catastrophic failure.

Mercury released by coal from electric utilities is an environmental concern. Every year approximately 48 tons of mercury is emitted into the environment through coal-powered power plant stack emissions. The U.S. EPA (United States Environmental Protection Agency) has established mercury monitoring and emissions standards for coal-fired power plants and other point source mercury emitters.

U.S. EPA 40CFR parts 60, 63, 72, and 75 require coal fired power plants to comply with mercury emission standards². Eventually, coal plants will have to install mercury emissions monitoring equipment on nearly 1,300 coal units².

Stack emissions

Stack mercury emissions exist in three forms, elemental mercury (Hg), 2+ oxidation state (Hg⁺⁺), and a form in which mercury is attached to particulate matter. In many stack emission streams, Hg⁺⁺ will react with sulfur compounds, nitrogen, chlorine, and/or oxygen to produce sulfurous, nitrous, chloride and oxide mercury species.

Additionally, elemental and oxidized mercury can be lost to reactions and adsorptions on the inner surfaces of monitoring equipment. The combined effect is inaccurate mercury readings which can result in costly retesting or create broad financial, environmental and regulatory repercussions as a result of non-compliance.

In terms of cost, analytical testing is substantial. Recent studies estimate a per-test sampling cost ranging from $100-$640 for a typical mercury analysis³.

Don’t forget about moisture

Moisture hold-up in samples and transfer equipment must also be considered because mercury as a polar molecule inhibits and adsorbs organic and inorganic materials. In the presence of halogens, adsorbed moisture facilitates rapid corrosion. Sampling systems need to address moisture hold-up and drying times that accompany restart of a system after any upset.

Figures 3 and 4 graphically display wet-up and dry-down curves for different tubing substrates and treatment⁴. Table I summarizes the amount of time required to stabilize a moisture level, wet-up and to remove moisture to an equilibrium level, dry down level. These graphs clearly show that the surface modified through electro-polishing and application of a modified amorphous silicon layer performs the best. (TABLE 1)

The impact of mercury adsorption

304 stainless steel (UNS S30400) and SilcoNert™ 2000-treated surfaces were compared under static conditions to determine the impact of mercury surface adsorption. Here’s how.

Four, one-gallon stainless steel sample cylinders (1,800 psi DOT-rated, Swagelok Corp., Solon, OH) were used in the study. Two sample cylinders were treated with a functionalized amorphous silicon surface (provided by Silcotek Corporation, Bellefonte, PA).

All sample cylinders were filled with a target concentration of 5 ug/m³ Hg standard. NIST traceable, internal mercury gas standards used in the study were supplied by Spectra Gasses Inc. Alpha, NJ. The calibrated mercury standards were injected into the sample cylinders and the samples were stored at a nominal room temperature of 70°F.

The samples were then tested at day 0, day seven, day 19, and day 50. The samples were tested by direct interface gas sampling connected to an atomic absorption (AA) detector. The sample’s pathway, regulator, and tubing were all treated with functionalized amorphous silicon to ensure a consistent sample pathway and experimental isolation of the sample cylinder test pieces.

In the final reel . . .

seven days. The total 50-day sample loss was 10% (Table II).

Figure 5: Average mercury response comparison of stainless steel versus a functionalized silicon surface.

The 304SS cylinders showed an initial mercury sample loss of 42% at day seven with no sample stabilization during the 50-day test period. The total sample loss after 50 days was 82%.

Using the right material really matters

All in all, these tests revealed that material composition is a key factor in boosting analytical system accuracy in mercury, sulfur, and moisture containing streams. They showed that SilcoNert™ 2000-coated surfaces exhibited 70% less mercury loss than bare 304SS surfaces. The studies on the holding and transfer of sulfur-containing streams also demonstrated dramatic improvements when amorphous silicon-coated stainless steel components were used. Finally, moisture transfer and equilibration times can also be reduced by using amorphous silicon coatings to improve the inertness of stainless steel components.

References

1. Barone, G., Smith, D., Higgins, M., Rowan, S., Gross, W., Harris, P., “Impact of Sampling and Transfer Component Surface Roughness and Composition on the Analysis of Low-Level Sulfur and Mercury Containing Streams”, Restek Corp., O’Brien Corp., Haritec LLC, ISA Symposium, October 1995.

2. Environmental Protection Agency, “Standards of Performance for New and Existing Stationary Sources: Electric Utility Steam Generating Units”, 40 CFR Parts 60, 63, 72, and 75 [OAR-2002-0056; FRL-] Rin2060-AJ65.

3. Serne, J.C., “An Overview and Comparison of Available Mercury Emission Test Methods for Boilers”, Symposium on Air Quality Measurement; Methods and Technology 2005; San Francisco, CA; Air & Waste Management Association. Paper no. 439, pg 9.

4. Harris, P., “Relative Response Time of TrueTube™ when Measuring Moisture Content in a Sample Stream”, Haritec Scientific & Engineering Support, May 2004.

About the Authors

Gary Barone – Gary Barone is a Business Manager with SilcoTek Corporation. He is responsible for overseeing marketing, research and manufacturing.

He received a B.S. in Chemical Engineering from The Pennsylvania State University in 1990. He is a member of the Society of Automotive Engineers and the National Association of Corrosion Engineers.

Marty Higgins – Marty Higgins is a Business Development Specialist with SilcoTek Corporation. He is responsible for product marketing, technical support and new market support of silicon coatings.

He received a B.S. in Mechanical Engineering from Lehigh University in 1982.  He is a member of the American Society of Mechanical Engineers, Society of Automotive Engineers, Specialty Equipment Market Association, and the National Association of Corrosion Engineers.