The distribution of mercury throughout hydrocarbon processing systems varies and requires significant understanding and planning prior to implementing inspection and maintenance activities. Mercury is present as a contaminant in all geologic hydrocarbons, including oil and gas. Based on available data and experience, levels of mercury present in oil and gas are extremely variable both between and within geographical areas. In natural gas, elemental mercury is the predominant species, although trace amounts may be present in the form of organic complexes. PEI’s Mercury and Chemical Services Group (MCS) has gained valuable experience in the management of mercury across the petroleum industry, including upstream oil and gas operations; gas gathering, processing, and transmission operations; and crude oil refining operations in the Gulf of Mexico (GoM), the southern and western US, Alaska, Canada, the Middle East, and Asia.
The lessons learned apply to the upstream, midstream, and downstream sectors of the oil and gas industry. Experience with assessment and monitoring techniques has generated improvements that enhance and support the effective management of risks associated with occupational exposure, environmental emissions, and process-equipment corrosion. Gas and liquids processing can cause mercury species transformation from one chemical form to another.
One example of this is the mixing or commingling of sour and sweet gas streams where elemental mercury can react with elemental sulfur to form mercuric sulfide. Mercury-contaminated process vessels and piping will begin to desorb mercury at ambient temperatures long after the equipment has been purged and cleaned of hydrocarbons. This phenomenon is multiplied when welding or cutting of equipment is needed for repairs or other modifications, evolving large amounts of mercury vapor from the heated surface area.
Improvements in measurement and monitoring methods for assessing mercury in process streams provide increased confidence in measurement precision and accuracy verified with robust, well-defined numerical data quality performance criteria. The sample collection process is the most difficult and critical aspect of accurately quantifying mercury concentrations in gas phase streams.
Mercury can form amalgams with many metals and alloys commonly used in gas sampling systems, including stainless steel, brass, copper, nickel, chromium, and aluminum. Due to the potential for amalgamation and the tendency for mercury to adsorb and chemisorb to the surface of stainless and carbon steel, the potential for loss of mercury to sample-wetted components is significant.
To minimize this loss, all sampling system components that come into contact with the gas stream should be heated and have their sample-wetted surfaces coated with a silica-type coating or be made of a material that is not reactive with mercury. Also, any pressure reductions (through valves, regulators, reducers, or other fittings) should be designed so as to minimize Joule-Thompson cooling. PEI’s Mak2 sampling systems provide reliable data with low detection limits.
MCS and laboratory alliance partner Eurofins Frontier Global Sciences developed the Modified EPA Method 30B to address the lack of quality assurance/quality control associated with the ISO 6978 and ASTM D6350 methods, which are both based on double amalgamation on gold sorbent traps and analysis with cold vapor atomic florescence spectrometry. Method 30B uses iodated sorbent traps, which are unaffected by hydrogen sulfide and other gas contaminates and that also allow higher mass loading that can provide long-term integrated averages with sampling durations of 20 days or more. Analysis is performed using either modified UOP 938 or EPA Method 1631.
Extensive field experience with the performance of field-portable ambient air mercury vapor analyzers as well as passive and active occupational exposure sampling methods has led to an improved understanding of instrument and method capabilities and limitations. Recent research for a Canadian energy company evaluating chemical and environmental interferences associated with atomic absorption and atomic florescence field-portable mercury vapor analyzers indicate the analyzers are subject to chemical interferences found in hydrocarbon processing environments. These interferences can cause the analyzer to generate inaccurate mercury concentrations.
Mass balance studies, mercury mapping studies, and long-term monitoring programs in refineries, gas processing plants, and gas gathering systems have led to the development of an improved understanding of the dynamics of mercury accumulation in oil and gas processing equipment and facilities.
Mercury in natural gas and process streams can accumulate in production equipment due to adsorption, chemical reaction, dissolution in sludges, and condensation. Condensation of mercury (from the gas phase) occurs whenever the vapor pressure of mercury exceeds the limiting partial pressure for condensation due to changes in temperature and pressure. Precipitation of mercury from the liquid phase can occur whenever the limiting solubility of mercury is reached due to lower temperature.
Ongoing studies of mercury accumulation in steel pipe have led to improved understanding of mercury distribution and accumulation in hydrocarbon processing systems. This has been applied to the development of improved chemical decontamination and waste management techniques used during plant turnarounds, the cleanout of gas processing equipment, and the decontamination of downhole equipment during well intervention operations.
Mercury reacts chemically with iron corrosion products and incorporates into the corrosion scales and possibly the steel grain boundaries. Mercury both adsorbs (reversible bonds) and chemisorbs (irreversible chemical bond) to metallic surfaces. Chemisorption dominates for carbon steel surfaces, while adsorption dominates for stainless steel surfaces. Piping and steel equipment that contacts gas or condensates with measurable concentrations of mercury will contain mercury in proportion to the concentration of mercury in the fluid that contacted the equipment.
Carbon and stainless steel are excellent scavengers of mercury. Both contribute to a lag effect, which can delay the appearance of mercury in downstream processing facilities for months or years. Understanding the nature and distribution of mercury along with depth profiles in carbon and stainless steel process equipment is important to developing effective mercury management and decommissioning plans. Furthermore, understanding the uptake of mercury and mercury compounds to process equipment surfaces can provide valuable information to verify components of a mercury mass flux study.
MCS has developed a unique approach to understanding mercury mass flux, mass loading, and distribution in hydrocarbon processing systems to develop mercury management processes, chemical decontamination solutions, chemistry, and waste minimization/processing plans. As part of that understanding, the company deploys specialized technologies and advanced methods to obtain the information required as part of the mass flux, loading, and distribution models.
Mercury has been an emerging issue with hydrocarbon processors in the US during the last decade. With recent measurements of mercury in US and Canadian shale gas plays this issue will continue to unfold as new shale gas/oil production makes its way into US NGL and LNG plants and refineries. NGL and LNG plants are most at risk since mercury is corrosive to aluminum cryogenic processing systems, but downstream processing is affected as well since mercury poisons precious metal catalysts and complicates turnarounds and shutdowns. The presence of mercury in oil and gas is an important issue for occupational health and safety, environmental stewardship, and process safety management. This production is responsible for the current construction of numerous fractionation plants and the planned construction of many more, all of which are sensitive to low levels of mercury. This underscores the need for US energy companies to understand the risks associated with produced mercury and begin mercury mapping and distribution studies to minimize and mitigate those risks.
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