Mercury Removal (3)

Factors Influencing Mercury Removal

 

Particle Size Effect

Size of adsorbent particle has effect on performance and pressure drop. For same weight, the smaller particle size, the smaller its surface area, therefore the performance is better.

In contrast with pressure drop, for same weight, the smaller particle size, the greater number of adsorbent (the larger adsorbent surface area), the greater gas friction with adsorbent, therefore the higher pressure drop. 

Gas velocity effect

Below is result of test which conducted  by one of mercury adsorbent vendor.

Table 2. Effect of Residence Time and Gas Velocity to Mercury Removal Efficiency

Residence time (s)	Mercury removal efficiency (%)
	3 ft/min	6 ft/min
1,7	43	58
3,3	80	89
5,0	91	100

From table above, we can conclude that:

-          the longer residence time of gas in bed, the higher efficiency of mercury removal,

-          the higher gas velocity, the higher efficiency of mercury removal

 

Temperature Effect

Generally, the higher temperature, the faster reaction occurs. But at high temperature, the impregnated sulfur will be:

-          vaporized at inert condition, or

-          oxidized if contact with air

Therefore adsorbent performance will be decreased.
 

Mercury Content Measurement

Main problem in mercury content measurement is mercury amount is decreased because some of mercury will be sticked at sample line and container wall. To overcome this issue and to obtain accurate result, purging is conducted.

Currently mercury in gas is usually measured by atomic fluorescence spectrometry (AFS). Gas is flowing to gold trapping tube to dense mercury. For gas with high pressure, gas pressure is decreased utilize pressure regulator. To avoid condensation due to pressure drop, electrical heating is utilized. Then mercury is desorbed from tube by heating (up to 800^oC), utilize carrier gas flowing to analyzer.

To measure mercury content in liquid, same method is utilized with modification, i.e. formerly change liquid to gas.

 

Interaction between Mercury and Metal Surface 

Mercury can stick at steel surface of pipe and vessel with concentration of 2 – 10 g/m². Until now, mechanism of how mercury can be adsorbed by steel is not known. So far, the postulate is mercury reacts with grain boundary of element or compound in steel.

Chemical properties of steel surface which contain mercury is different with chemical properties of steel-with-no-mercury. 

Pipeline

Cooling and compression process can produce liquid mercury which settles in pipeline. This is known when pigging is conducted.

The question is: Does liquid mercury which settles in pipeline accelerate galvanic corrosion? 

Tank in Ship

Contamination can occur:

-          at bottom of tank due to sludge.

-          at exterior of nonmetallic surface layer (scale, inorganic material, sometimes at coating).

-          mercury is possible “in” or “on” steel surface.

Mercury which is at steel surface can make inhalation problem for worker when pipe is welded or cutted.

Mercury interacted with:

-          aluminum: causing LME (liquid metal embrittlement)

-          steel

-          copper: causing crack

Mercury will affect to:

-          equipment integrity

-          health and safety of worker

-          product quality

-          environment

Mercury and Carbon Steel

The good news is:

-          mercury doesn’t accelerate corrosion

-          no significant galvanic effect

-          no detrimental impact to stress corrosion cracking (SCC)

Mercury can be adsorbed by metal. When gas flow from well to plant through pipeline, some mercury is stuck at pipeline wall. Pipeline wall has capacity to adsorb mercury. When saturated, there will be no additional mercury stuck at pipeline wall. The impact is at that time mercury content in gas which comes to plant will increase than previous. Table below shows estimated lag time of mercury at pipeline.

Table 3. Lag Time of Mercury at Pipeline

Surface area (m2)	Gas flowrate (MMSCFD)	Mercury flowrate (g/h)	Surface area capaacity (gram mercury/m2)	Time to come to shore / station (months)
200.000	40 - 50	20 - 40	1	9
			2	18
			5	46
			10	93

 

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.