Summary This paper details the operating implications of handling and processingnatural gas containing mercury, methods for detecting mercury, suggestedsampling procedures, and related field experiences.
Introduction Mercury occurs naturally in trace quantities in air and natural gas. Although difficult to generalize, mercury concentrations in air typicallyrange between 1 ng/m and 10 µg/m (1 ppmby volume ~10,000 µg/m). Some authors have reportedthat natural gas typically contains mercury concentrations between 1 and 200µg/m, but concentrations are probably best evaluated on aproducing-formation basis.
The implication of the effects of mercury in natural gas was not reporteduntil 1973, when a catastrophic failure of aluminum heat exchangers occurred atthe Skikda liquefied natural gas plant in Algeria. Investigationsdetermined that mercury corrosion caused the failure and that the mercurylikely came from an accidental source, such as test instruments used in plantand field startup.
After the Skikda failure, a study of the Groningen field in Holland revealedsimilar corrosion in the gas-gathering system. CO2 was initiallythought to be the cause, but later investigationspinpointed mercury, with concentrations ranging from 0.001 to as high as 180µg/m.
Phannestiel et al. state that most if not all of themercury in natural gas is in the elemental form and that no natural gasprocessing plant problems are suspected to have been caused by organic orinorganic mercury compounds. These statements would imply that elementalmercury is the probable cause of mercury corrosion problems. Although theconcentration of mercury in a given natural gas may be considered extremelylow, Audeh oberves that "its effect is cumulative as itamalgamates."
Amalgamation is the formation of an alloy. To date, the most seriousproblems reported by the industry owing to mercury corrosion have been theresult of mercury forming an alloy with aluminum. Copper has also presentedsome problems. Saunders et al. observe that" brazed aluminum plate-fin heat exchangers are the predominant choice forcryogenic service. Aluminum is used due to its brazeability, excellentmechanical properties at cold temperatures, and superior heat transfercharacteristics." They further state that mercury can damage the aluminum usedin these exchangers and must be completely removed to nondetectable levels inupstream equipment. Unfortunately, complete removal is not economical. Butfortunately, the design of systems capable of minimizing operating problemsassociated with mercury in natural gas is currently being emphasized.
Operating Implications of Mercury in Natural Gas
The problems associated with mercury can begin at each producing well. Theinvestment required and the remote nature of wellsites prohibit mercury removalat these locations. Thus, mercury is introduced into the wellbore and gatheringsystems simply by gas production.
Mercury can also be introduced into gathering systems accidentally. Testinstruments incorporating mercury and mercury-type bellows meters are oftenfound on wellsites and in gas-gathering systems.
As mercury enters gathering-system pipelines, the mercury content of thenatural gas is reduced because of chemisorption onto steel pipe walls. Leeper suggests the following reactions as the driving force behindthis reduction:H2S+Fe2O3?FeO+S+H2O . . .(1)
and Hg+S HgS. . . . (2)
Trace quantities of H2S are the catalyst for the reaction ofmercury with iron oxide fiom the pipe. The mercury sulfide prccipitates and isadsorbed onto the pipe wall. Grotewold et al. report thatfor one 68-mile [110-km] pipeline, mercury content decreased from about 50 to20 µg/m. This reduction is influenced by pipe-wall roughness andadhesive forces. Similar reductions are also experienced on pipe and vesselwalls in facilities and plants.
Other factors influence mercury distribution in flowing streams. Mercurycondenses into the liquid phases of hydrocarbons and gas-treating chemicalsbecause of its greater solubility with higher-molecular-weight streams. Mostfacilities and processing plants that handle natural gas base have some form ofseparation, sweetening, and/or dehydration equipment. The amount of mercuryreduction resulting from contact with higher-molecular-weight solutions isdifficult to define owing to the wide range of hydrocarbon saturations andtreating conditions. Grotewold et al. report that some 50 to 60% of theinlet mercury accumulates at the bottom of the glycol absorber and that 15to20% is separated in scrubbers. This leaves 20 to 35% of the mercury enteringthe plant or facility to carry over into downstream processing equipment and/ortransportation pipelines.
We have experieuced similar results at the Painter complexnitrogen-rejection-unit/natural-gas-liquid (NRU/NGL) recovery plant. Themercury concentration in the vapors off the N2 rejection column is three timeslower than that encountered on the methane liquid stream.
Corrosion Mechanism of Mercury and Aluminum Elemental mercury forms an amalgam with the surface layer of the metal itcontacts. With aluminum, the amalgam is much weaker than the metal itself andis often referred to as an embrittlement. To initiate aluminum corrosion, thetightly adhering aluminum oxide layer on the surface of the aluminum must beremoved. The mercury/aluminum amalgam process removes this oxide layer. Thelayer can be removed chemically or mechanically and is catalyzed by thepresence of an aqueous electrolyte. Phannenstiel et al.state that in their test studies, "in no instances did test results indicatethat mercury in contact with aluminum surfaces could produce gross corrosionunless condensed water was present." The mercury/aluminum amalgamationgenerally does not occur as a direct chemical reaction because the base-metalaluminum is usually protected by the oxide film. The aqueous corrosion cellforms aluminum hydroxide and gaseous hydrogen through the followingreactions:Al+Hg?AlHg . . . (3)
and 2AlHg+6H2O?2Al(OH)3+3H2+2Hg. . . .(4)
These reactions leave the previously amalgamated mercury free to formadditional amalgam with the base metal in a continuous corrosion process.
Observations of the physical nature of the corrosion indicate that theamalgamation/corrosion is preceded by small half-moon intrusions that form acontinuous network as edges of the intrusions join together. The formation ofaluminum hydroxide can be identified by a grayish-white "whisker" appearance. Rapid pitting is common because mercury tends to collect in localizedareas.