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Arsenic compounds in natural gas pipeline operations.

Arsenic is a chemical compound found n many forms. Arsenic is the 20th most abundant element in the earth's crust. It is considered a poisonous metalloid. A metalloid is a term used in chemistry to classify chemical elements based upon their general physical and chemical properties.

Metalloids are often in the form of an amphoteric oxide (a metal oxide or hydroxide able to react with both a base and an acid). Other metalloids are Boron (B), Silicon (Si), Germanium (Ge), Antimony (Sb), Tellurium (Te) and Polonium (Po).

The most common oxidation state for arsenic is -3 (arsenide), +3 (arsenates (III) or arsenites, and most organoarsenic compounds) and +5 (arsentate (V) : the most stable inorganic arsenic oxy-compound. Arsenic and its compounds have been used as pesticides, herbicides, insecticides and used in various metal alloys.

The major sources of arsenic from commercial activities are: smelters, coal-fired power plants and pesticides (arsenates have been used to treat lumber).

Arsenic and many of its compounds are potent poisons. The effect of arsenic on living organisms is quite severe, many times leading to death from multi-system organ failure from necrotic cell death. It is evidenced by brick-red colored mucosa due to severe hemorrhage.

The International Agency for Research on Cancer recognizes arsenic and arsenic compounds as Group 1 Carcinogens, and the EU lists arsenic trioxide, arsenic pentoxide and arsenate salts as Category 1 Carcinogens. Arsenic sublimes at 613[degrees]C (1,135[degrees]F) where it transitions from a solid directly to a gaseous form.

Arsine is the simplest form of arsenic, AsH, also referred to as arsenic hydride. It is a flammable, pyrophoric, and highly toxic derivative of arsenic and hydrogen. In its standard state, arsine appears in the form of a colorless, denser than air gas that is soluble in water (200 ml/1) and in many organic solvents as well.

While arsine itself is odorless, owing to its oxidation by air it is possible to smell a slight, garlic-like scent when the compound is present at about 0.5 ppm. This compound is generally regarded as stable since its decomposition into arsenic and hydrogen at room temperature takes place very slowly, unless a temperature of 230[degrees]C (445[degrees]F) or greater is reached.

Arsine can be produced by the reduction of arsenic (III) oxide with zinc and acid.

[Zn.sub.3][As.sub.2] + 6 [H.sup.+] 2 As[H.sub.3] + [Zn.sup.2+] (1)

Typically for heavy hydrides such as arsine they are stable kinetically but not thermodynamically.

2As[H.sub.3] 3[H.sub.2] + 2As (2)

Arsine is oxidized by [O.sub.2] or even in the presence of air.

2 As[H.sub.3] + 3 [O.sub.2] [As.sub.2][0.sub.3] + 3[H.sub.2]0 (3)

Arsine will react violently in the presence of strong oxidizing agents such as potassium permanganate, sodium hypochlorite or nitric acid.

The toxicity of arsine is distinct from the other arsenic compounds. The main route of exposure is by inhalation, although poisoning through the skin is possible. Arsine binds to the hemoglobin of red blood cells, causing them to be destroyed by the body. The first signs of exposure, which can take several hours to become apparent, are headaches, vertigo and nausea, followed by symptoms of hemolytic nausea and neuropathy. In severe cases, the damage to the kidneys can be long-lasting.

Exposure to arsine concentrations of 250 ppm is rapidly fatal: concentrations of 25-30 ppm are fatal for 30 minutes exposure, and concentrations of 10 ppm can be fatal at longer exposure times. In many cases, operators are unaware of its presence and fatalities have been reported due to the presence of arsine. Poisoning symptoms have been reported after exposures for several hours at 3 ppm.

Source Of Arsine In 0il & Gas Operations

In oil and gas operations it is generally recognized that arsenic comes from the geological gas formations. The mechanism of arsenic release is not yet fully understood. It has been reported that if, however, fracturing is accomplished by acidizing, for which 12-15% solution of hydrochloric and hydrofluoric acids in a 4:1 ratio is employed, arsenic can be leached out of surrounding geological formation. This would consequently produce a spectrum of unstable arsenical acids in situ.

All generated arsines will be products of complex reactions occurring during the acidizing processes between the acid and the surrounding mineral formations. The different arsines in question are those of tri-valent arsenic: As[R.sub.x] [H.sub.3-x] and/or of penta-valent arsenic, in which sulfur is also involved: AsR'[sub.x][S.sub.5-x)/2], where: R and R' are [C.sub.1] ... [C.sub.n] hydrocarbon radicals, and x = 1, 2, 3, ... n.

These acids and their mixtures can be formed in the reaction zone and, subsequently, (because of the reducing capacity of hydrogen, and hydrocarbons under pressure and elevated temperatures of the reservoir) could convert to arsine and water. Based on a large literature base, arsine gas is naturally produced in geothermal steam and is believed to be arsine, rather than an organic arsine. As the oil and gas industry moves to deeper, hotter geological formations to produce natural gas, arsine may become more of an issue.

At this time there do not appear to be any federal EPA restrictions on arsines level in natural gas. In natural gas, the presence of arsine can lead to poisoning of palladium and of platinum catalyst in subsequent gas processing. Treatment options are being developed to treat natural gas streams entering gas processing to remove arsine and prevent downstream issues.

It is reported that a common form of arsenic in natural gas is not simply arsine (ASH) but a group of arsenic compounds called the trialkylarsines (which includes: trimethylarsine (TMA), dimethylethylarsine (DMEA), methyldiethylarsine (MDEA) and triethylarsine) (TEA).

The trialkylarsines are a considerably less reactive group than arsine itself and as such trialkylarsines are more difficult to remove from natural gas. Several methods have been published as means to remove trialkylarsines from gas streams including contacting the natural gas with a solid adsorbent comprised of vanadium pentoxide (VO). Another method describes using a inorganic sorbent comprised of zinc oxide (ZnO) and copper oxide (CuO) on an aluminum oxide substrate. These systems are typically designed in a packed bed system where gas flows through the system until a breakthrough threshold is measured.

Sulfide--Arsenic Compounds

Arsenic is found in low-rank coals where it is concentrated in the sulphidic minerals of coal. Data reports on the form of arsenic in coal are scarce. One group of researchers examined polished blocks of coal using a scanning electron microscope with an energy-dispersive X-ray analyzer (SEM-EDS) and an electron microprobe (EMPA). Arsenic is most commonly found associated with the iron compound pyrite. Pyrite iron compound was the main arsenic form in four U.S. bituminous coals studied, although arsenate and arsenic associated with organic matter were also found.

Iron sulfoarsenide mineral (FeAsS), commonly referred to as arsenopyrite, is the most common ore of arsenic. This iron arsenic compound breaks down thermally to form iron sulfide and arsenic in gaseous state.

FeAsS (700[degrees]C) FeS + As(gas) As (solid)

In sour natural gas systems or in those that contain microbiologically generated hydrogen sulfide (HS) the presence of arsenic sulfide minerals can be found. One form, arsenic trisulfide ([As.sub.2][S.sub.3]) upon heating 'cracks' to produce a gaseous arsenic in the form of [As.sub.4][S.sub.6].

Measuring Arsine

The analytical methods for measuring arsenic in water are well-documented and used in many areas of the environmental testing industry. There exist only few methods and devices available to measure the presence of trialkylarsines in natural gas. Given the extremely poisonous nature of trialklylarsines in natural gas and a growing awareness to its presence will likely drive the commercial development of additional methods and devices to determine its concentration in natural gas.

Qualitatively, arsenic may be detected by precipitation as the yellow arsenous sulfide from hydrochloric acid of 25% or greater concentration. Trace amounts of arsenic are usually determined by conversion to arsine. The latter can be detected by the so-called Marsh test in which arsine is thermally decomposed, forming a black arsenic mirror inside a narrow tube, or by the Gutzeit method in which a test paper impregnated with mercuric chloride darkens when exposed to arsine because of the formation of free mercury.

Removing Arsenic

Up to this point, the removal of arsenic compounds from within gas pipeline systems typically happens when the pipeline undergoes cleaning to remove other types of deposits, most often iron sulfide. Reports show that the levels of arsenic can range from parts per billion levels up to elevated parts per million levels. Waste solids removed that are determined to contain arsenic must follow regulated disposal guidelines in order to ensure proper handling of a hazardous waste material.

Batch-cleaning methods will obviously create a volume of waste liquid and solids that are removed from the pipeline. Continuous "online" treatment methods that remove solids by creating a water-soluble complex result in little or no waste liquids and solids being removed. Given that the continuous online methods use chemical products that chelate the metal ions into water-soluble complexes the metals will travel with the water as a soluble ion. This process is much more passive and occurs over a period of time. Therefore concentrations of metal ions are managed at levels that prevent re-precipitation downstream and at lower soluble concentrations.

Rapid Energy Services has an ongoing research and development program focused on cleaning products designed to remove metal compounds through advanced chelation, forming water-soluble metal complexes. The rate at which this occurs will depend upon the type of metal and its physical form within the pipeline section treated. Where the dominant contaminant is iron sulfide in the mackinawite or troilite form, a noticeable reddish color will occur in treated samples almost immediately. Other forms will form the same reddish color over a slightly longer time interval.

When the contaminant is arsenic in a precipitated form of arsenous sulfide, levels of arsenic measured in physical solids samples removed from treated sections decline over a period of time. This time frame can range from a matter of weeks to months depending on the concentration and nature of the pipeline system treated. The goal is to remove trace amounts of hazardous metals from within gas gathering and transmission systems while minimizing the handling and exposure for workers.

By David O. Trahan, CEO, Rapid Energy Services, LLC, Lafayette, LA
Compound % of Total Arsenic Chemical
 Compounds in Gas Formula

Trimethylarsine (TMA) 60-90% [C.sub.3][H.sub.9]As
Dimethylethylarsine (DMEA) 10-30% [C.sub.4][H.sub.11]As
Methydiethylarsine (MDEA) 5-15% [C.sub.5][H.sub.13]As
Trithylarsine (TEA) 1-5% [C.sub.6][H.sub.15]As

Compound Molecular Boiling Point
 Weight ([degrees]C)

Trimethylarsine (TMA) 120 52
Dimethylethylarsine (DMEA) 134 86
Methydiethylarsine (MDEA) 148 110
Trithylarsine (TEA) 162 139
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Comment:Arsenic compounds in natural gas pipeline operations.
Author:Trahan, David O.
Publication:Pipeline & Gas Journal
Geographic Code:1USA
Date:Mar 1, 2008
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