Mercury Removal (2)

Mercury Removal from Hydrocarbon

 

Dissolved elemental mercury (Hg^o) and mercury compound is removed utilized adsorbent. While mercury in solid phase is removed by physical separation.

There are 2 components of adsorbent, i.e.:

(1)    Support, for example zeolite, activated carbon, metal oxide, and alumina.

(2)    Reactive component, for example Ag, KI, CuS, metal sulfide, and thiol.

Materials which can disturb adsorbent are:

-          H₂S

-          water

-          olefin and aromatic hydrocarbon

-          thiol

-          other metal: arsenic

Mercury is volatile. The volatility of mercury limited operating temperature, usually below 100^oC. Ideally operation is conducted at near ambient temperature.

Adsorbent which is utilized for mercury removal can be divided into 2 groups, i.e. non-regenerative adsorbent and regenerative adsorbent.

 

Non-regenerative mercury removal

Advantage of non-regenerative method is:

-          Simple, “Install it and leave it!”

Disadvantage of non-regenerative method are:

-          High installation cost.

-          There will be additional pressure drop if adsorbent has been saturated.

-          Need cost to dispose used adsorbent.

If mercury is detected in effluent and pressure drop through bed is exceeded, adsorbent need to be changed with new one.

Materials which disposed from adsorbent are:

-          Mercury

-          Others (such as benzene)

 

Non-regenerative mercury removal method utilized some options below:

1.       Sulfur

2.       Metal sulfide

3.       Halide

4.       Ion exchange resin

 

1.      Sulfur

Sulfur is dispersed in porous carrier, for example activated carbon. Sulfur impregnated at carbon. Carbon is support component, while sulfur is reactive component.

Sulfur reacts with mercury to form HgS. This reaction occurs fast.

Hg + S à HgS

       This is old method. On 1970s LNG Badak Indonesia utilized this method.

Quality of product depends on:

-          Quality of activated carbon (as support)

-          Method to make sulfur dispersed at carbon

Sulfur must be dispersed properly without any blockage in porous carrier. If not, then:

-          Mercury removal process can’t be optimum.

-          Sulfur is not impregnated well; therefore sulfur can be carried over by gas stream at high temperature.

Sulfur is dissolved in liquid hydrocarbon. Therefore, method of impregnated sulfur at carbon can only be utilized for gas service. Contact between adsorbent and liquid hydrocarbon should be avoided.

Disadvantage of this method are:

-          Used material should be disposed since it can’t be utilized again.

-          In environmental point of view, acceptable mercury disposal is by burning/incineration.

-          Sulfur is dissolved in liquid hydrocarbon, especially aromatic hydrocarbon, therefore there is possibility sulfur will be carry over in product stream.

 

2.      Metal sulfide

Next development is mercury removal utilized inorganic compound/metal. Metal sulfide is dispersed at solid carrier (activated carbon, alumina). Sulfur is impregnated at metal. Reactivity between mercury and metal sulfide is very high.

Hg + M_xS_y à M_xS_y-1 + HgS

      Advantages of this method are:

-          Used adsorbent can be utilized again.

-          Risk of sulfur carry over to product stream (through sublimation or dissolution) is low. 

Disadvantage of this method is:

-          Not fit for “empty” fluid.

This disadvantage can be solved by installing pre-filter at upstream of mercury guard bed. 

Metal sulfide and polysulfide can remove mercury effectively. Common metal utilized are Cu, Zn, and proprietary metal. If required, metal oxide can be added to remove H₂S. If metal is Cu, the reactions are:

CuO + H₂S    à CuS + H₂O

2 CuS + Hg   à HgS + Cu₂S 

Metal sulfide is utilized for gas and liquid hydrocarbon service. This adsorbent is not damaged if contact with liquid water. 

Adsorbent pellet size is circa 0.9 – 4 mm. Pellet with small size can increase efficiency of mercury removal, but also make high pressure drop. 

	Pellet size
	Small	Large
Efficiency of mercury removal	high	low
Pressure drop	high	low

  

3.      Halida

Halide impregnate at activated carbon. This adsorbent is utilized for liquid hydrocarbon.

Hg + I à HgI₂

Liquid water can make this halide wash off from activated carbon and make vessel corroded. 

4.      Ion Exchange Resin

Ion exchange resin is utilized for liquid naphtha service.

 

Regenerative mercury removal

Regenerative adsorbent is like non-regenerative adsorbent. The difference is in mercury removal process utilized regenerative adsorbent, there is thermal regeneration process. Usually regenerative mercury removal is conducted simultaneously with dehydration process or other contaminant removal process.

For example is silver (Ag) which impregnate at molecular sieve. Mercury (from gas or liquid) will form amalgam with silver. At high temperature mercury will be separated from silver, utilize regeneration gas.

Advantages of regenerative method are:

-          No additional pressure drop.

-          Mercury can be recovered as separated stream.

Disadvantage of this method are:

-          Require additional equipment for regeneration process.

-          Gas which is utilized for regeneration probably need additional treatment to remove mercury (secondary mercury removal treatment).

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.