Why is mercury removal critical in the LNG production process?

Mercury removal is critical in the LNG production process primarily because mercury can cause liquid metal embrittlement (LME) of aluminum alloys, which are widely used in cryogenic heat exchangers, particularly plate-fin heat exchangers. Aluminum alloys are favored in LNG plants due to their excellent thermal conductivity and strength-to-weight ratio at cryogenic temperatures. However, even trace amounts of mercury can diffuse into the grain boundaries of the aluminum alloy, weakening its structure and making it susceptible to brittle fracture. This can lead to catastrophic failure of the heat exchanger, resulting in significant equipment damage, plant downtime, and safety hazards. The concentration of mercury in natural gas can vary depending on the source, but even very low levels (parts per billion) can pose a significant risk over the lifespan of the LNG plant. The LME process is accelerated at cryogenic temperatures, making mercury removal even more critical in LNG production compared to other natural gas processing applications. Furthermore, mercury can also amalgamate with other metals in the plant, such as steel, potentially leading to corrosion and equipment damage. Mercury can also contaminate the LNG product, posing environmental concerns and potentially affecting its marketability. Therefore, stringent mercury removal processes are implemented upstream of the liquefaction unit to ensure the integrity and reliability of the cryogenic heat exchangers and the quality of the LNG product.