Hg removal from
Natural Gas
Preliminary information : Hg analysis
The analysis of Hg in Gases is very sophisticated as very low Hg contents have to be
analyzed. The mercury content in the feed gas may be up to several thousands of
ng/Nm3 while the standard outlet specifications are 10 µg/Nm3.
It is important to be aware that the Hg content of the surrounding environment can be
higher than in the fluid to be analyzed.
Another important point especially for the sampling is the characteristics of Hg to be
adsorbed on all kind of metal. This is why the Natural Gas well where Hg has been
detected should be followed up regularly. The Hg content may vary depending on the
“saturation” of the equipment. At the start up of the well a certain Hg content can be
analyzed which may increase once all the surfaces of the tubes, heat exchangers,
vessels etc.. are saturated with Hg.
Preliminary information : Hg analysis
The analysis method consists in adsorbing Hg on gold deposed on a quartz support. A
sampling line in Teflon feeds two sampling probes with a defined gas flow rate. The first
probe will trap all the Hg while the second serves to make sure that no Hg broke through
the first probe.
The probes are then heated up to 600-800°C and the Hg is removed by Argon and
analyzed by atomic fluorescence.
Caution: the important point of this analysis is to be sure to measure the good content (to
be sure of the “zero” value). Only the fact to leave the probes with the gold to air and in
contact with a metal can lead to false the analysis.
Hg removal : why?
Natural Gas and crude oil can contain Mercury.
Known wells for gas / oil containing mercury are in North Africa (e.g.
Algeria), Middle East (e.g. Egypt) and Asia (e.g. Malaysia, Indonesia).
In LNG units as well as in cracked gas drying units mercury has to be
removed in order not to harm equipment made of copper or aluminum.
This equipment is used to cool down the gases to very low temperatures
to allow their separation.
Hg removal : how?
1) Chemical reactions
Mercury reacts with sulfur creating HgS. The sulfur is usually deposed on an adsorbent. The
adsorbent can be activated alumina, activated carbon or metal sulfides.
For activated carbon hydrocarbon or water condensation should be excluded or taken into account
for the design.
2) “Physical” adsorption
“Physical” adsorption on a silver exchanged molecular sieve (e.g. HgSIV).
3) Process arrangements
A possible solution for eg Steam Crackers could be to purge regularly the mercury condensed in
the heat exchangers. Such a solution has been seen in a Steam Cracker of ARKEMA in Carling,
France.
Hg removal : solution proposed by CECA
Our product: ACTICARBONE SA1861 M in pellets of 3 mm diameter,
sulfur impregnated (drums of 80 Kg)
Influence of different operating parameters:
- gas composition (water saturation, heavy hydrocarbons, initial Hg
content usually below 300 µg Hg/Nm3)
- important for design: contact time and total asorption quantity (life time)
- Specification: 10 ng Hg Nm3 = 0.01 µg Hg/Nm3
Raw
Acid
Natural
Gas
Gas
Removal
Drying /
Purification
Molecular
sieves
LNG or Sales gas
Gas
Ethane
Fractionation
Plant
Propane
Butane
gasoline
Hg
Removal
SA1861 M
The activated carbon bed is very
often installed behind the molecular
sieves to prevent problems with
water condensation.
Be careful: the Hg removal should
be done at a very early stage in the
process to prevent pollution of the
complete unit.
Market
Hg adsorption with Activated Carbon
Market : mainly Asia, Middle East.
Estimation 200 t/y. Life time 3 - 10 years.
Competition :
Chemviron mainly (HGR), Selexsorb HG from ALCOA (S on AC),
MERSORB, NUSORB, other adsorbents than AC. Puraspec from
Synetix, HgSIV molecular sieve of UOP.
CECA versus competition
SA 1861 M: a high performing product
Hg removal : SA1861 compared to other carbons
100
AC35 20% I2
Hg removal efficiency (%)
90
SA1861
AC1
80
AC 2
70
AC3
AC 4
60
50
40
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Time on stream (h)
For info: AC35 20% I2 ancient CECA product, now SA 1861 only.
References
ARKEMA, Jarrie
Nafoora, Libya
Oman, OMIFCO, Fertilizer plant
…