Do We Need a Dust Filter Downstream of a Mercury Removal Unit?
There should not be any dust and larger particles coming out of a bed of MERSORB® mercury adsorbent. Dust or fine particles coming out of an adsorbent bed happens under two specific situations. There can be dust or fine particles in the adsorbent as it goes into the vessel (this should not be the case when using MERSORB® mercury adsorbent pellets). Or there can be dust/fines created in the vessel during operation.
The most likely cause of dust creation in the MRU vessel during operation is inlet gas impingement on the pellets present on top of the bed. Gas impingement causes the pellets to be moved abound and rub against each other. This pellet against pellet abrasion rubs off pellet surface material. The way to eliminate pellet-pellet abrasion is to avoid high localized inlet gas velocities by using a well-designed inlet gas flow distributor. Proper inlet Distributor Design is the subject of a separate FAQ.
Dust/fines can also be formed by the pellets braking up due to high stress and weakening (like molecular sieve pellets in hydrothermal operation or where there is liquid water carryover and freeze/thaw conditions exits). This is not of any concern in a non-regenerative design, since none of those conditions exists and MERSORB® mercury adsorbent pellets are strong.
Only when there is a problem with the bed support will adsorbent particles end up in the outlet gas. This is much more of a concern in a thermally regenerated vessel where the bed support I-beams, etc., undergo dimension changes as the temperature cycles. In this design, if the bed support is properly designed and properly installed (and visually inspected right before adsorbent installation to confirm that everything is still ok after shipping/erection) then there is zero chance for this to happen.
Operators should not see any dust (other than very small amount that may be made during the MERSORB® mercury adsorbent installation where the pellets may rub a little against each other as they fall into the vessel). This very small amount of dust most likely will stay in the vessel. We do not envision any mechanism that would cause dust formation during operation.
In a typical MRU Design, the bed of MERSORB® mercury adsorbent pellets sits at constant temperature and pressure and the flow is always in the same downward direction. Even if there is some variation in flow rate and pressure, that would not be enough to cause pellet movement - which is the mechanism that creates dust.
In mercury removal vessels where the gas flow is upwards, there is a possibility that if the gas flow rate is allowed to exceed the design flow rate, that the adsorbent pellets can be fluidized. If the constant gas feed flow rate exceeds the mass flow rate of the original design, or if operating pressure is significantly reduced vs. design pressure, then the gas flow velocity may exceed the design value and the adsorbent pellets can be fluidized. If the vessel is depressurized upwards at a high rate of pressure change, then the pellets can be fluidized.
If fluidization occurs, the pellets rub against each other, creating dust. Fluidization can be avoided by always depressurizing in the downwards direction through the vessel. If you anticipate significant increases in mass flow of gas, or if you wish to operate at significantly lower gas pressure, please contact your Application Engineer for MERSORB® mercury adsorbents to help you evaluate your options.
If there is a compressor or turbo expander directly downstream of the MRU, most operators feel more comfortable in having a small particle filter downstream of the MRU vessel and upstream of the rotating equipment.
We do not have specific recommendations for selecting or designing particle filters. Your best option is to review your concerns with a filter manufacturer and see if they have a low pressure drop filter to filter out any particles of a specific size range that you are concerned with. Through the years in natural gas plants, some people have used what are called "witches hat" conical screen strainers sitting in the pipe, but they are not effective for many reasons.
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.
AntiMercure® is a Registered Trademark of Selective Adsorption Associates, Inc. (SAA) for its Process for removing mercury from water; AntiMercure™ is a trademark of SAA for its pre-Engineered Systems for removing mercury from water. All Rights Reserved.