Mercury: Detrimental to Midstream
Mercury is universally detrimental to midstream processing plants and can cause failures of cryogenic heat exchangers in gas separation processes including NGL, LNG and LPG. Mercury Removal Units (MRUs) are commonly used to protect process trains but deposition of mercury in AHXs can in some instances occur even if there is a functional MRU in place. Other impacts to process include contamination of glycol dehydration and amine units, accumulation of hazardous wastes in separators and slug catchers and off-spec products. Also, during the regeneration cycle of mol sive dehydrators high mercury spikes can hit process trains or if the regen stream is routed to sales gas can impact downstream users. Process plants have a low tolerance for mercury (10 ng/Nm3) and for this reason there is a growing trend to map mercury through process to understand distribution and speciation in order to protect cryogenic processes.
At the well head, flow line, slug catcher and the initial separators, both particulate HgS and elemental mercury can be present. Elemental mercury is readily stripped from the hydrocarbon phase during the initial gas/liquid separation and subsequent stabilization steps. In the typical case, the elemental mercury in the well head mixture reports to the gas purification steps, and the particulate HgS reports to the fluids (crude/condensate and produced water). If elemental mercury levels are high enough, liquid elemental mercury may be present in the bottom of the inlet separator along with large-particulate sediments.
Elemental mercury in natural gas is soluble in glycol and thus some percentage of mercury is removed by glycol dehydrators. Amine units behave similarly, but to a lesser extent due to the lower solubility of elemental mercury in amines. Mercury removal from the regeneration streams from both of these processes requires special design considerations. Mercury that is not removed in acid gas streams will end up contaminating products made from it like sulfuric acid and elemental sulfur. Waste streams from both processes including MRU medias are hazardous and must be managed as such. The upstream sector has more options under E&P exemptions. However, most midstream plants more likely fall under RCRA, which translates to significantly higher costs for disposal.
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