How Does Dew Point Gas Affect Mercury Adsorbents?
Dew Point Feed Gas & Mercury Removal Units MERCURY ADSORBENTS and “Pore Condensation”
ACTIVATED CARBON (AC) vs. METAL OXIDE (MO & AA)
“Retrograde condensation?” Some metal oxide manufacturers say their product is immune to condensation in its pores, claiming their product is better than activated carbon for treating gases at their dew points. The claim is that their metal oxide based Hg adsorbents have a much larger pore structure that is unaffected by this phenomenon, thus claiming their metal oxide Hg adsorbent will perform better than the carbon based Hg adsorbent. This is just not true. In reality, ALL porous adsorbents are subject to pore condensation.
So why not use the adsorbent with the greatest adsorbing power = the highest surface area?
INTERNAL SURFACE AREA:
Activated Carbon (AC) - 1,000 m2/g (per weight) - 560 m2/cm3 (per volume)
Activated Alumina (AA) - 250 m2/g (per weight) - 225 m2/cm3 (per volume)
Metal Oxide (MO) - 40 m2/g (per weight) - 40 m2/cm3 (per volume)
Activated Carbon has 20X more internal surface area than MO per unit of weight and 14X more surface area per unit of bulk volume. Hg reacts & is trapped only on the surface area of active ingredient (chemical impregnant). The more surface area an adsorbent has, the more capacity it has for contaminant removal, and the faster adsorption rate it will have.
Micropores create high surface area. Metal Oxide granules have low surface area because they have few micropores. Metal Oxides have low mechanical strength, so activated alumina or other inorganic binder is added to increase its crush strength a bit. Activated alumina has micropores, but much fewer than activated carbon has. AA has actually been used to remove C5+ hydrocarbons from natural gas; so obviously AA is not immune to adsorption of hydrocarbons.
HYDROPHOBIC CARBON vs. HYDROPHYLLIC METAL OXIDE
At the Water Dew Point (& below) metal oxide adsorbs water. Hg Atoms must get past water to reach adsorption sites inside MO. Isotherm data shows that carbon-based Hg adsorbent does not adsorb much water at low water concentration & also that AC adsorbs much less water than alumina at high water concentrations. AC is easier to dry out than either AA or MO.
Our adsorption data shows ~ 40% HIGHER Hg capacity for MERSORB® mercury adsorbent when treating pipeline natural gas than when treating air, suggesting that adsorption of hydrocarbons is beneficial to mercury adsorption. Hydrocarbons have a stronger attraction for Hg than water, so water adsorption on MO is probably a bigger drawback than the adsorption of hydrocarbons on either AC or MO. Water lowers the crush strength of MO & AA; but AC keeps its high crush strength when wet.
Dew Point Feed Gas & Mercury Removal Units
MERCURY ADSORBENTS and “Pore Condensation”
Providing Solutions to remove mercury for emission control, corrosion prevention and catalyst protection.
Selective Adsorption Associates, Inc. (SAA) offers over 40 years of experience in Process Engineering of Adsorption Systems for Gas and Liquid Drying, Sulfur Compound Removal, Emission Control, and over 20 years of Process Design experience in Mercury Removal Systems.
CAUTION: Wet activated carbon removes oxygen from air. Thus, an asphyxiation hazard exists inside enclosed spaces containing activated carbon. Use appropriate safety precautions when entering vessels or other enclosed spaces containing activated carbon. Exposure to strong acids and oxidizers can cause formation of toxic H2S gas.
MERSORB® is a trademark of Nucon International, Inc. (NUCON®) for impregnated activated carbons used to remove mercury from gases and liquids. NUSORB® is a trademark of NUCON for its activated carbons. Information presented herein is believed to be accurate and reliable but does not provide any guarantee or warrantee by NUCON® or by Selective Adsorption Associates, Inc. Nothing herein shall be construed as an invitation to use processes covered by patents without proper arrangements with individuals or companies owning those patents.
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