METHODS DEVELOPED FOR DETECTING HAZARDOUS ELEMENTS IN PRODUCED GAS

Sherman Chao, Amir Attari
Institute of Gas Technology
Des Plaines, Ill.

The Institute of Gas Technology, Des Plaines, Ill., with financial support from the Gas Research Institute, Chicago, has been developing sampling and analytical methods to detect in natural gas various trace constituents that may pose health, safety, or operational risks.

The constituents of interest include paraffinic and aromatic hydrocarbons, H₂S, organic sulfur compounds, arsenic, mercury, radon, and others.

This effort is an extension of the gas industry's need to maintain the reputation of gas as a source of clean energy and to ensure the adequacy of clean-up processes that remove potentially harmful constituents.

Better sampling and analytical techniques for produced natural gas, similar to those developed by IGT for processed gas, will enhance producers and processors' abilities to monitor undesirable constituents in raw gas streams and improve their clean-up processes.

CONSUMER, WORKER SAFETY

In addition to consumer safety, the gas industry has become concerned about potential exposure of field and production workers to trace constituents from raw natural gas that can concentrate in co-produced liquids and solids during gas production and processing. These constituents may pose an unknown hazard to maintenance crews who handle the materials.

Surveying these trace constituents and Studying their fate among the various streams of a gas-processing plant have become important. A thorough knowledge of the constituents distribution in processed gas, co-produced liquids, and plant wastes can provide information for new and cost-effective clean-up and waste-disposal strategies.

Concern over the presence of trace constituents is not only for their potential environmental and safety implications, but also for their damage to engineering materials. ^{1 2}

Sulfur compounds, carbon dioxide, and excess water can cause accelerated corrosion of transmission, distribution, and storage systems. Other trace and minor constituents of, for example, paraffinic and aromatic hydrocarbons (polynuclear aromatic hydrocarbons - PAHs, and benzenes, toluenes, ethyl-benzenes, and xylenes - BTEX), sulfurs, arsenic, mercury, and naturally occurring radioactive materials (NORM) can also be of engineering and environmental significance.

As a GRI contractor, IGT has developed many sampling and analytical techniques that determine these trace constituents in processed gas. ^{3 4}

These methods include a new gas chromatograph-atomic emission detector (GC-AED) technique for a complete screening of all elements present in trace quantities. Other chromatographic techniques specifically identify trace organic compounds containing, among others, sulfur, nitrogen, phosphorus, and halogen.

A sampling method using cryogenic trapping at -70 C. (-94 F.) and at line pressures up to 12.5 MPa (1,800 psig) has been developed for preservation, preconcentration, and recovery of hydrocarbons heavier than butanes for later analysis in the laboratory (Fig. 1) (12420 bytes).

Many solid sorbents were developed and used to collect in the field such labile species as mercury, arsenic, and radon for subsequent analysis in IGT laboratories (Fig. 2).

Commercial on-line monitors have been selected and used to determine moisture, oxygen, and sulfur concentrations. Development of new monitors for arsenic and mercury in natural gas has also been under way.

The schemes of sampling and analysis that were developed at IGT for pipeline gas can (with modifications) be applied to unprocessed gas as well. A coordinated effort on the part of producers is necessary to formulate a program to survey the state of certain trace constituents in raw gas that may be of interest to them.

TEST METHODS

No standard methods of sampling and analysis exist for the trace constituents of natural gas that form the following discussion.

The methods developed at IGT were modifications of air sampling and analytical methods that are commonly used for air toxic substances. These monitoring methods, when applied to natural gas, present special challenges because gas has a much more complex matrix than the air. As such, natural gas can have more components than air that interfere with measurement of specific trace constituents.

Furthermore, application to the analysis of unprocessed natural gas of IGT methods that were developed for analysis of processed gas may require additional refinements to prevent interfering compounds, such as H₂S, brine mist, etc., from adversely affecting the analytical results.

ARSENIC

Arsenic compounds have been found in a few natural gas fields.

In Southern California, arsenic in gas was indicated by a white deposit containing trialkylarsine sulfides on the interior surfaces of equipment used in the distribution system of Southern California Gas Co.³

Arsenic compounds were originally extracted from natural gas on activated carbon or by contact with concentrated nitric acid. Neither is satisfactory.

Activated carbon is nonspecific and inefficient, causing loss of arsenic due to breakthrough. Nitric acid is adequate but poses a safety hazard to the sampling crew, and recovery of the arsenic is slow. Among the oxidizing solutions, potassium persulfate in a saturated solution is prefered over nitric acid because it is safe for handling and exhibits quantitative recovery of arsenic from the gas phase.

IGT has sampled arsenic with Calgon HGR carbon and an IGT-developed FECL sorbent.^{3 5} The arsenic absorbed into the HGR sorbent is analyzed with X-ray fluorescence (XRF) for As concentrations in natural gas ranging from 0.05 to 2 ug/cu m.

The FECL, a patented sorbent with 10-25 wt % FeCl₃ on Chromosorb P, is extracted with diluted nitric acid to recover the arsenic and the extract analyzed by spectrochemical method for total arsenic. This latter technique gives a lower detection limit of about 2 ug/cu m and a 100 times wider dynamic range of arsenic detection than the HGR/XRF method.

A prototype organoarsine monitoring instrument has been developed (Fig. 3).³ The instrument is a compact gas chromatograph with a low surface-area gold electrode detector operating at a voltage bias of + 300 mv.

Trimethyl arsine has an elution time of 5 min with a detection limit of about 0.1 ppm (vol) (335 ug As/cu m). With a PHOS25 preconcentrator (an IGT-developed sorbent), measurement of arsenic at a few micrograms/cubic meter concentration level is possible.

More work must be carried out, however, to optimize this analyzer for better sensitivity, reproducibility, and durability. This portable instrument is targeted as an analyzer for monitoring arsenic concentrations before and after cleanup operations.

The results from recent sampling and analysis of a raw natural-gas sample from Abe field, near Roswell, N.M., indicate a total arsenic As concentration of 1,420 ug/cu m.

Speciation by GC-AED analysis indicates the sample contains trithethyl arsine (970 ug As/cu m), dimethylethyl arsine (370 ug As/cu m), and a very small amount of other organoarsines, probably propyldimethyl arsine and methyliethyl arsine.

As would be expected, distribution of various alkyl arsines are in close agreement with their respective vapor pressures; for example, the most abundant arsenic compound is trimethyl arsine which has the highest vapor pressure.

MERCURY

Another co-produced natural gas product is mercury and its alkyl compounds which were detected in Groningen gas (-180 ug/cu m) after having accumulated in NGL-recovery vessels in 1972⁶and again in Algeria because of the extensive damage it caused to the aluminum heat exchangers of the Skikda LNG plant in 1975.^10-13

These incidents have made the gas industry aware of the potential problems mercury could present to gas production and processing operations as well as to consumers' safety.

The most common techniques used to extract mercury from natural gas for measurement are scrubbing the gas through acidic permanganate solution or recovering the mercury on activated charcoal.

Both of these sampling methods are prone to contamination and generally unsuitable for natural-gas analysis.

The preferred sampling method for mercury collection is on supported gold sorbent ^{9 10} with subsequent thermal desorption for mercury determination (Fig. 4). A column packed with 30-60 mesh fused silica coated with 1% Au was used at IGT. Two 6-mm Vycor tubes containing 2.5 cm length of gilded silica beads are placed in series for mercury sampling.

For quantitative recovery of Hg, the maximum capacity of the gilded silica beads in each sorbent tube is about 8 ug, representing an upper collection limit of 500 ug/cu m with 15 min sampling at a flow rate of 1 l./min.

For field applications, IGT normally samples for 15 and 120 min, representing a convenient measurement range of 0.02-500 ug/cu m. A cold-vapor AA (CVAA) technique is currently employed in laboratories to measure mercury thermally desorbed at temperatures of 450-600 C. with a heating rate of 100 C./min.

A portable Jerome 431 Hg analyzer was tested unsuccessfully for direct natural gas analysis.¹¹ A limited number of experiments conducted at IGT, however, demonstrated the feasibility, of fabricating a portable Hg analyzing system consisting of the gilded silica sampling tube, a portable heater, a valving system, and a portable Hg detector, such as gold thin-film detector or CVAA analyzer.

According to the best published information, the highest concentration of mercury in North American natural gases reported is 0.04 ug/cu m, a seemingly low concentration of little consequence.

Recent unpublished information on well-head gas indicates, however, much higher concentrations in a few gas fields. IGT's experience with Algerian natural gas indicated mercury levels up to 50 ug/cu m in wellhead gas and 24 ug/cu m in their pipeline gas.

RADON

Radon-222, a NORM, has been recognized as a health hazard for many years and has been the subject of much study and debate concerning its health effects on the general population.

In 1973, the U.S. Environmental Protection Agency (EPA) surveyed the concentration of radon in natural gas and in recent years has recommended remediation if radon exceeds 4 picoCuries/l. (pCi/l.) in air.

Radon with a half life of 3.8 days can eventually decay and produce lead-210 with a half life of 20 years, thus becoming a more persistent safety hazard. ¹² Radon also dissolves in NGL and can pose an additional problem.¹⁴

For analysis, radon can be collected on two 2 x 3-in. flow-through carbon cartridges.⁴ The gamma radiation of its daughters (Pb-214 and Bi-214) on the charcoal canister is measured with a standard technique (EPA 520/5/87005).¹³ This technique allows the sampling and the measurement of radon in field and laboratory, respectively. This gamma-ray spectroscopy technique also enables detection of other radioactive species in pipeline scales.

A detection limit of 1 pCi/l. Rn can be easily achieved provided that a large volume of gas can be sampled ind the subsequent laboratory analysis performed very shortly after sampling.

The working analytical range is 11,000 pCi/l. with a sampling interval up to 100 min. A certified Pylon Radium source, which emits radon at a constant rate (0.1 pCi/l.), was used to calibrate the measurement system.

The EPA survey of wellhead gas in 1973 reported Rn concentrations as high as 1,450 pCi/l. IGT's survey of processed gas, however, showed a median Rn concentration considerably less than the EPA's safe level of 4 pCi/l.

SULFURS

Hydrogen sulfide and organic sulfur compounds in natural gas, both labile species, are best analyzed, if possible, on location.

Tedlar-lined sampling bags and aluminum and 316 stainless steel sample cylinders with or without interior coatings are often used to collect samples for later laboratory measurement of sulfur species including odorants.

Loss of original sulfur species and formation of other sulfur derivatives, however, can readily occur providing low results that may not represent the actual concentration of sulfur species originally present in natural gas.

For IGT's survey, a Scintrex sulfur analyzer Model OVD-229 from Heath Consultants Inc. was used to monitor the concentration of all sulfur components in natural gas on site.

This instrument is equipped with a gas chromatograph and an electrochemical detector offering an analytical working range of 0.1-10 ppm (vol) for each of eight sulfur components, including H₂S, mercaptans, alkyl sulfides, and thiophane. The typical instrument run time is 20 min. An IGT-certified dimethyl sulfide standard is used for calibration in the field.

HYDROCARBONS

Gas samples are usually analyzed with combined ASTM 1945 and 1946 procedures and GPA methods for major and hydrocarbon components. ^{15 16}

At IGT, GC-flame-ionization detector (GC-FID) was used for extended hydrocarbons analysis with a capillary column (petrocol DH) for more detailed hydrocarbons breakdown up to at least C14

This gas-chromatograph column provides a clean separation of benzene and cyclohexane, improving the accuracy of benzene analysis while reporting the contents of cyclopentane, methylcyclopentane, cyclohexane and methylcyclohexane as well.

This technique enables detection of each hydrocarbon species down to 1 ppm (vol). The cryogenic sampling and on-line cryofocussing techniques have been employed for detection of hydrocarbons heavier than pentane in concentrations as low as 0.001-0.1 ppm (vol) depending on concentration factors.

AROMATICS: BTEX, PAHS

Benzenes, toluenes, ethylbenzenes, and xylenes (BTEX) are among the components listed as hazardous air pollutants in the 1990 Clean Air Act Amendment.¹⁶ They are also classified as toxic under the Occupational Safety & Health Administration (OSHA) and National Institute for Occupational Safety and Health (NIOSH) rules, having a benzene exposure limit of 1 ppm (Vol).

Their determinations can be accomplished easily with the GC-FID technique described previously. For a more sensitive detection, however, a photo ionization detector (PID) can be employed for more specific gas chromatograph analysis. A micro-gas chromatograph technique recently developed by Microsensor Technology Inc. has made possible a quick measurement of BTEX using a TCEP column.

Polynuclear aromatic hydrocarbons (PAHS) are also classified as hazardous substances by the EPA. In natural gas, these are usually determined by analyzing the cryo-condensate of natural gas using the modified EPA 8310 Method.

Sixteen PAHs were monitored at IGT for this program by high performance liquid chromatograph (HPLC) with both the fluorescence and UV detectors. Hydrocarbon components in raw natural gas from a separator were measured recently. Hydrocarbons up to C₁₂, BTEX, C₃-benzenes, and even naphthalenes were found.

The concentrations of aromatic hydrocarbons determined were benzene, 690 ppm (vol); toluene, 370 ppm (vol); ethylbenzene, 3 ppm (vol); total xylenes, 15 ppm (vol); total C₃-benzenes, 5 (vol); and total napthalenes, 2 ppm (Vol).

The results of a co-produced condensate also indicated that the sample contained high concentrations of BTEX and confirmed the finding for gas analysis.

A larger amount of naphthalenes was found in the same condensate. The presence of PAHs was unknown because no separate analysis was carried out for their determination.

OTHER ELEMENTS

AED offers simultaneous detection of elemental contents of every gas chromatograph-eluted compound by a microwave-induced helium plasma operating at about 5,000 C. and a diodearray spectrometer (40-nanometer detection range).

Standard compounds containing 0, N, As, Hg, and others were selected and used to calibrate the instrument and to optimize the sensitivity of each elemental detection by changing instrumental parameters, such as the makeup gas flow rate and the type of reagent gas used for the AED detector.

The GC-AED method offers a sensitive and reliable means to screen elements that make-up the trace compounds recovered from natural gas. It is particularly useful for screening those trace elements which cannot be easily detected by any other method.

Two standard gas chromatograph columns, DB-5 and DB-WAX, are used for determination of nonpolar and polar compounds, respectively.

This technique can be employed for quantitation of compounds containing those specific elements, especially nitrogen and oxygen-containing species and organometallics.

On-line cryofocussing can enhance the instrument's sensitivity for heteroatom hydrocarbons. To supplement the qualitative elemental determination by GC-AED method, the HGR sorbent used to collect arsenic species was analyzed by XRF for elements with atomic weights heavier than 12.

This HGR-XRF technique was evaluated with an IGT-prepared multi-element gas standard and was found to be capable of efficiently collecting the compounds of As, Se, Sn, and Ge.

REFERENCES

Runge, P.S., "How Gas Quality Changes Affect The Different Sectors of The Industry: Implications for Costs and Markets," Preprint, IGT Gas Quality Symposium, 1991.
Liss, W.E., Thrasher, W.H., Steinmetz, G.F., Chowdiah, P., and Attari, A., "Variability of Natural Gas Composition in Select Major Metropolitan Areas of The United States," GRI Final Report-92/0123, March 1992.
Chao, S.S., and Attari, A., "Characterization and Measurement of Natural Gas Trace Constituents, Volume 1: Arsenic," Final report, GRI Contract No. 5089-253-1832, Dec. 1993.
Chao, S.S., and Attari, A., "Characterization and Measurement of Natural Gas Trace Constituents, Volume II: Natural Gas Survey," Final report, GRI Contract No. 5089253-1832, December 1993.
Chao, S.S., and Attari, A., "Speciation and Measurement of Arsenic Compounds in Natural Gas," presented at the 1992 National AIChE Meeting, New Orleans, March 1992.
Achterberg, A., and Zaanen, J.J., "Mercury Trace in the Natural Gas from the Gronningen Field," Chemisch "Weekblad, Jan. 1972.
Morrison, J., "NAM recovers mercury produced with Dutch natural gas," OGJ Apr. 17, 1972, p. 73.
Bodle, W.W., Attari, A., and Serauskas, R., "Considerations for Mercury in LNG Operations," 6th International Conference on Liquified Natural Gas, 1980.
Braman, R.S., and Johnson, D.L., "Selective Absorption Tubes and Emission Technique for Determination of Ambient Forms of Mercury in Air," Environmental Science & Technology, 996.
Schroeder, W.H., "Sampling and Analysis of Mercury and its Compounds in the Atmosphere Environmental Science Technology, Vol. 10, No. 7, 1982.
McNerney, R.T., "Analysis of Mercury using a Gold Film Detector," American Laboratory, Vol. 64, June 1983.
Cahill, R.A., and Coleman, D.D., "An Overview of the Significance of Radon in Natural Gas." Gas Quality and Energy Measurement Symposium (IGT), June 12-13, 1989.
"Environmental Radon Measurements by Gamma Spectroscopy," EG&G Ortec, Radon Monitoring Note, January 1988.
Kraemer, T.F., "Geological Factors Governing Radon Concentration in Natural Gas," IGT Gas Quality Symposium, 1991.
"Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography," GPA standard method 2261, GPA.
"Extended Analysis for Natural Gas and Similar Gaseous Mixtures by Temperature Programmed Gas Chromatography," GPA standard method 2288, GPA.
Federal Register, Vol. 58, No. 11 (Jan. 1993).

THE AUTHORS

Sherman S. Chao is associate director for chemical research services for the Institute of Gas Technology, Des Plaines, Ill. For 3 years before joining IGT in 1980, he was associated with the central corporate analytical service department of Nalco Chemical Co., Chicago, following a year in a post-doctoral appointment at the University of Missouri. Chao holds a BS (1970) in chemistry from Tam-Kang College and MS (1974) and PhD (1977) degrees in analytical chemistry from Loyola University (Chicago). He is a member of ACS, ASTM, and Society for Applied Spectroscopy.

Amir Attari is a consultant to IGT's chemical research services. He joined IGT in 1957 as an assistant chemist, became an assistant supervisor in 1958, and supervisor in chemical analysis services in 1965. In 1973, he was appointed manager of a new department of chemical and instrumental activities and in 1989 was made director. Attari retired from IGT in August 1994. Attari holds a BS in chemistry from Adams State College of Colorado and an MS in analytical chemistry from Loyola University (Chicago). He is a member of ACS and GPA.