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.
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.
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.
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.
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.
LNG or Sales gas
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.
Hg adsorption with Activated Carbon
Market : mainly Asia, Middle East.
Estimation 200 t/y. Life time 3 - 10 years.
Chemviron mainly (HGR), Selexsorb HG from ALCOA (S on AC), MERSORB, NUSORB, other adsorbents than AC. Puraspec from Synetix, HgSIV molecular sieve of UOP.
For info: AC35 20% I2 ancient CECA product, now SA 1861 only.
Oman, OMIFCO, Fertilizer plant