Mercury Removal (1)

Mercury content in reservoir is varied, start from ppt(w) until ppm (w), depends on its geology. When the reservoir is produced, mercury (in elemental or compound form) in reservoir, will be distributed into gas, oil/condensate, and produced water phase, then flowing to surface. Table below describe mercury content in gas and condensate.

Table 1. Mercury content in gas and condensate

Location	Gas (mg/Sm3)	Condensate (mg/kg) or ppb(w)
Europe	100 – 150	-
South America	50 - 120	50 – 100
South East Asia	100 - 700	400 - 1200
Africa	80 - 100	500 - 1000

When temperature drops, mercury gas maybe condenses. Mercury can condense in cooler, separator, and heat exchanger.

 

Mercury Properties

Elemental mercury Hg^o properties are below:

-          Solubility in water                  : 0.064 mg/L  (64 mg/L)
-          Saturated consentration in air : 14 mg/m³ (14,000 mg/m³) at 20^oC
-          Melting point                          : -38.9^oC
-          Boiling point                            : 356.6^oC
-          Density                                     : 13.5 g/mL
-          Mass weight                            : 200.59

Mercury solubility in hydrocarbon is usually higher than solubility in water.

There are some types of mercury:

1.       Elemental mercury, Hg^o

Hg^o is dissolved in crude oil and hydrocarbon gas. Its solubility depends on temperature. If temperature decreases, condensation will occur, and Hg^o  will be precipitated.

2.       Organomercury (R-Hg or R-Hg-R)

Organomercury is dissolved in crude oil and condensate. Its solubility is higher than Hg^o. The most “famous” organomercury is dimethyl mercury (CH₃-Hg-CH₃).

3.       Inorganic mercury

For example HgCl₂, its solubility in water is high, while solubility in crude oil and condensate is low.

4.       Complex mercury

Organic ligan, i.e. organic acid, EDTA, porphyrin, and thiol.

5.       Suspended mercury compound

For example HgS and HgSe. Both are solids and not dissolved in fluid.

Hg^o, R-Hg, and R-Hg-R can be adsorbed into solid (iron oxide, iron sulfide, sand, wax, etc).
Mercury which common in gas is Hg^o, while common in condensate is organomercury.

 

Mercury Hazard

At ambient temperature, phase of Hg^o is liquid. Mercury is commonly utilized for thermometer, manometer, barometer, electric switch, lamp, pharmacy and cosmetic product. Mercury container in those instruments will not cause health problem, except mercury vapor is released to air. WHO determined that mercury is one from top ten chemical which should be aware regarding health problem.

Permissible exposure limit (PEL) of mercury is:

-          OSHA  : 100 mg/m³ for worker with activity duration of 8 hours.

-          NIOSH :   50 mg/m³ for worker with activity duration exceed 10 hours.

-          ACGIH :   25 mg/m³ for worker with activity duration exceed 8 hours.

PEL determined by ACGIH is tighter, compared with OSHA and NIOSH.

There was no report regarding effect of mercury in gas to equipment/plant, until 1973 when catastrophic incident occurred in LNG Plant in Algeria. From investigation, it was found that corrosion in aluminum heat exchanger by mercury is cause of the incident.

Source:

- Interaction of Mercury with Metal Surfaces, Johnson Matthey Catalysts, 2009.

- Carnell and Willis, Mercury Removal from Liquid Hydrocarbons, Johnson Matthey Catalysts, 2005.

- NUCON, MERSORB®Mercury Adsorbents, Design and Performance Characteristics, Bulletin 11B28– 2010.

- https://www.osha.gov/Publications/mercuryexposure_fluorescentbulbs_factsheet.pdf

- Abu El Ela, I.S. Mahgoub, M.H. Nabawi, and Abdel Azim, Mercury Monitoring and Removal at Gas Processing Facilities: Case Study of Salam Gas Plant, Society of Petroleum Engineer (SPE), 2008.