Mercury (Hg) can be present in feed gas in wide range of concentrations. It is important to remove mercury from the feed gas to avoid mercury attack on the brazed aluminum heat exchangers. Aluminum material is highly reactive with mercury and can be corroded quickly resulting heat exchangers failure and potentially can pose environmental and safety hazards. In addition, mercury will also cause poisoning precious metal catalyst if it presents in petrochemical process. This post outlines the method on mercury removal in oil and gas processing facilities.
Mercury presents in most natural gas field in concentration ranging from less than 10 ppb to more than 1 ppm as elemental (metallic), organic, and inorganic compound. Due to its strong affinity for higher molecular weight fractions, mercury predominantly concentrates within the liquid phase rather than the gas stream.
Mercury Removal Methods
Most contemporary mercury removal systems utilize downflow fixed-bed configurations designed to reduce mercury concentrations to below 0.01 µg/Nm3. Process gas enters the top of the adsorption tower and flows downward through the bed, where the mercury is captured before the treated stream exits the bottom for downstream processing or sales. These systems deploy either non-regenerative sorbents or regenerative adsorbents.
Mercury Removal using Nonregenerative Sorbents
In non-regenerative mercury removal processes, mercury reacts irreversibly with sulfur on the sorbent surface to form a stable mercuric sulfide compound (HgS, or cinnabar). This process typically utilizes sulfur-impregnated activated carbon, where mercury is securely fixed within the carbon bed’s microporous structure.
The reaction of elemental sulfur with mercury is as follows:
Hg + S -> HgS
The advantage of nongenerative mercury removal process:
- The process is simple because no regeneration equipment is required
However, there are several drawbacks of this method:
- Sulfur-impregnated carbon is limited to dry gas applications because moisture is preferentially adsorbed, blocking mercury access to active sulfur sites
- sulfur can be lost through sublimation or dissolution in hydrocarbon liquids.
- Mercury-laden bed presents severe disposal challenges. Managing the spent solid requires high-temperature vacuum distillation to recover the mercury, and the waste matrix may be further contaminated with other hazardous substances, such as benzene and cyanides
To address these limitations, advanced non-regenerative adsorbents utilizing transition metal oxides and sulfides have been developed. These robust materials safely handle wet gas environments without loss of efficacy, offering a more stable and versatile alternative to traditional sulfur-impregnated media.
The reaction of metal sulfide with mercury is as follows:
Hg + 2 MeS -> HgS + Me2S
The system design consideration for mercury removal using nonregenerative sorbents is typically as follows:
- Bed adsorption = 15-20%-wt mercury
- Operating pressure = 300-1100 psig
- Operating temperature = up to 175oF
- Gas contact time = 20 seconds
- Maximum velocity = 35 ft/s
Mercury Removal using Regenerative Adsorbents
Regenerative mercury removal utilizes silver-impregnated molecular sieves to adsorb elemental mercury within a multi-tower configuration, allowing continuous operation while one or more beds undergo regeneration. The mercury-saturated bed is regenerated using a hot gas stream—typically at 288°C (550°F). The desorbed mercury concentrates within the condensed water phase of the regeneration gas separator. Consequently, preventing liquid water from entering and contaminating the upstream beds is critical to maintaining mercury removal efficiency and ensuring a long, reliable adsorbent lifespan.
The advantage of mercury removal using regenerative adsorbent:
- The accumulation of mercury in the adsorbent is avoided
The disadvantage of mercury removal using regenerative adsorbent:
- Despite the regeneration process, safe disposal of the recovered mercury remains a significant challenge.
- trace amounts of mercury may remain in the effluent regeneration gas, requiring careful downstream monitoring.
Choice of Mercury Removal in Oil and Gas Processing Facilities
There are several possible options of mercury removal in natural gas processing facilities:
- Installation nonregenerative mercury removal sorbents at the gas plant inlet before amine unit. Based on this source, mercury removal units are implemented as close as possible to the production wells in order to minimize mercury contamination in the effluents and along the natural gas processing chain
- Installation nonregenerative mercury removal sorbents downstream of Acid Gas Removal Unit (AGRU) before molecular sieve unit.
- Add silver-impregnated mercury sieve section to the molecular sieve beds.
- Installation nonregenerative mercury removal bed or a silver-impregnated molecular sieve bed after the molecular sieve unit
Each option poses several advantages and disadvantages.
There are several commercial mercury removal vendors/providers. This post will explain some of them.
Mercury Removal using Mixed Metal Sulfides by CECO Peerless (Nonregenerative Process)
CECO Peerless offers mercury removal unit in three-stage process.
- First stage comprises of natural gas flowing into the inlet coalescer for bulk removal of liquid followed by fine removal of liquid in coalescing section. This ensures dry gas flows into mercury removal bed.
- Second stage is adsorption of elemental mercury by proprietary media adsorbent (mixed metal sulfides).
- Third stage is removal of any solid dust using After Filter.
Mercury Removal using Axen’s AxtrapTM
Axens’ AxTrap™ Mercury Series offer a complete portfolio of adsorbents for different level of Mercury contamination and gas conditions, for a wide range of applications: dry gas, wet gas (high operating pressure) and liquid phase operation. The adsorbents physical properties are designed to minimize diffusion issues and maximize mercury trapping capacity.
Non-Regenerative and Regenerative Adsorption by PALL
PALL offers mercury removal using non-regenerative process and regenerative process. Figure below shows the simplified process flow diagram.
Mercury Removal using Sulfur Impregraned Carbon by Calgon Carbon
One of mercury removal from Carlgon Carbon is Calgon Cargon HGR activated carbon which is sulfur impregnated carbon. Mercury capacity on HGR activated carbon can be as high as 30%-weight.
HGR carbon system can handle gas stream with temperature up to 160oF and 99% RH. The performance of mercury removal is affected by temperature and relative humidity. HGR systems treating gas streams above 122oF and 50% RH will result in treated gas mercury concentrations of less than 0.1 µg/Nm3. System treating gas streams below 122oF and 50% RH will result in less than 0.01 µg/Nm3 mercury. Figures below show effect of temperature and moisture on mercury removal.
I hope you find this post useful.
References:
- Mokhatab, Saeid et al. (2015). Handbook of Natural Gas Transmission and Processing – Principles and Practices Third Edition. Elsevier Inc
- Stewart, Maurice I. (2014). Surface Production Operations – Design of Gas Handling Systems and Facilities 3rd Edition. Elsevier Inc
- https://www.cecoenviro.com/wp-content/uploads/2024/03/GPT-Mercury-Removal-Unit-Final0420.pdf
- https://blog.axens.net/how-to-remove-mercury-from-extracted-natural-gas
- https://www.pall.com/en/oil-gas/midstream/midstream-merc-rem-unit-mru.html
- https://www.calgoncarbon.com/app/uploads/HGR_MercuryRemoval_1pg_web_4x10.pdf