Mercury has an overall negative impact to petroleum production and processing. While mercury can be measured in almost all petroleum raw materials, measurements of mercury in crude slates globally range from ppt to ppm levels. Concentrations > 50 ppb in crudes and condensates are classified as High Mercury Crudes, which requires Mercury Removal Units (MRUs) to avoid mercury accumulation and product contamination. However, refineries that process Low Mercury Crudes for long periods will also have mercury accumulation in process. This matters as the mercury content of products affects the price that will be paid for them.

The mercury in refineries enters the facility as particulate HgS. Some of this leaves in the desalter wash water as such and for the most part drops out in the oxidation ponds. But the majority of the particulate HgS decomposes to elemental mercury during the high temperatures of the crude distillation furnace. If not addressed, it will condense as liquid elemental mercury in the overhead condensers and contaminate the light products. Since High Mercury Crudes are rarely processed as pure materials, they are often blended with other crudes that contain reactive mercaptans. These mercaptans can react with the elemental mercury in the overhead sections to regenerate very fine particles of HgS that cannot be removed by conventional MRUs, ultimately finding its way into many processing units.

Processed mercury affects worker health and safety, environmental compliance and waste management.  Soluble mercury and elemental mercury in sludges are desorbed during steaming of equipment, which can end up in flare knock out vessels and contribute to atmospheric emissions.  Refinery metal surfaces can have mercury concentrations as high as 100g/m². This mercury is not easily removed and out of service equipment such as valves, piping and heat exchangers should be considered hazardous waste until proven otherwise using XRF surface measurements or vapor burn tests.  Mercury waste streams in refineries are regulated under RCRA and typically cost more than similar waste streams generated in the upstream sector.