Qa3 often talk and present on the topic of mercury loss to various process systems across a gas processing facility. One key process is the dehydration stage which employs molecular sieve media to dry the gas.
This media also has the secondary effect of removing mercury from the gas via adsorption (rather than chemical reaction). This is typically between 10 - 30% of the mercury. When this occurs, during the mol sieve regeneration cycle, mercury will be driven off the media resulting in highly concentrated regeneration gas. It is very important to consider the fate of this regeneration gas whether that be flaring or fed back into the process.
On a recent study into mercury removal from gas during the regeneration stage on a gas processing facility, Qa3 observed ~ 85% loss of the mercury to the mol sieve media. This removal was unusually high. To investigate further, samples of the media were taken from the inlet and outlet of the mol sieve unit. It was found that the media at the inlet contained ~ 100 ppm mercury but the outlet samples contained 0.9 ppm mercury.
Upon further investigation, it was found that H2S in the gas was forming a sulphide on the surface of the media. This sulphide was reacting with mercury and forming mercuric sulphide (HgS), which was not breaking down during the regeneration cycle. In this way, mercury was accumulating in the unit.
From this study, Qa3 have highlighted that if your mol sieve is being fed by a high mercury, high H2S gas consideration must be given to the potential for exposure to highly contaminated media upon changeout.