SPE-171881-MS

Mercury Removal Units Operation at Front-end Location

Clotilde JUBIN, and Olivier DUCREUX, Axens

Copyright 2014, Society of Petroleum Engineers

This paper was prepared for presentation at the Abu Dhabi International Petroleum Exhibition and Conference held in Abu Dhabi, UAE, 10–13 November 2014.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contentsof the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflectany position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the writtenconsent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations maynot be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

Mercury is present as elemental mercury in natural gas reservoirs and has to be removed in the natural gasprocessing plants to protect health, environment and equipment. The mercury removal options are mainlynon-regenerative products. The adsorption mechanism is a chemical reaction between mercury and thesulfur of the active phase to form non-hazardous and very stable cinnabar phase.

The trend is to implement mercury removal vessels the closest to the production wells to minimizemercury contaminations in natural gas plants. This up-front location implies adsorbents able to removemercury from high operating pressure water saturated natural gas. Metal sulphide active phase adsorbentshave been developed for this purpose.

This paper will present examples of commercial application with existing mercury removal solutionsto illustrate the benefits available from use of the new technology focused on engineered alumina basedproducts. A case study based on performances comparison with different solutions at up-front locationswill be presented.

Reason of Mercury RemovalMercury (Hg) is a natural contaminant found in earth’s crust where its concentration can range from 10to 20 000 ppb. Mercury is thus released into the environment from a variety of natural sources, includingvolcanic, geothermal activities or wildfires; but also from anthropogenic activities. A total of approxi-mately 2000 metric tons of mercury is estimated to be released each year from fossil fuel combustion ormetal production [1]. For instance, natural gas production frequently generates hydrocarbon streamscontaining traces level of mercury [2], especially in Southeast of Asia where Hg concentration can reachup to 300 ppb. Though these levels are rather low, impact on industrial equipment and human health canbe serious [3-4]. For instance, mercury has a strong ability to form amalgams with Al-based alloys usedin LNG (Liquefied Natural Gas) cryogenic exchangers leading to corrosion issues (refer to Figure 1) andpotential industrial disaster like the one encountered in 1973 in Skikda in Algeria [5]. For this reason, amercury limit of 10 ng/Nm3 has been established as specification upstream of liquefaction in LNG plants.

In addition, mercury is harmful for human health and numerous incentives have been issued to controland limit its emission from anthropogenic sources [1]. Suitable personal protective equipment is requiredduring maintenance work. The European Union Scientific Committee on Occupational Exposure Limits

proposes 0.02 mg/m3 as mercury limit in inhaled airduring 8-hours time-weighted average and 0.01mg/l in blood as biological limit values [6].

Hence, mercury removal is an ongoing issue andnatural gas streams are usually decontaminated us-ing guard beds protecting downstream equipment.The mercury is removed in industrial units bystreams circulation through a mercury fixed bedadsorbent. The main available products on the mar-ket are non-regenerative products. These guard bedscan be located at different locations in the naturalgas processing chain: either downstream of dryersor upstream of acid gas removal unit.

Existing Mercury Removal TechnologiesThe most common approach for mercury removal solutions is non-regenerative adsorbents.

For many years operators have been using the reaction between mercury and elemental Sulfur (S) [6]to remove mercury from natural gas, downstream of dryers. The sulfur is deposited on a support, typicallycarbon, and the resulting captive mass is used in a fixed bed reactor.

The typical mercury adsorption mechanism is a chemical reaction between mercury and sulfur of theadsorbent active phase, leading to the most stable solid form of mercury called cinnabar (HgS). Thechemical reaction involve is non-reversible and can be depicted here below:

Mercury is chemically bond with sulfur to form mineral cinnabar (HgS) within the porosity of theadsorbent. Sulfur is either under elemental form (S) or under metal sulphide form (MxSy). Mercury is thusimmobilized in a non-hazardous form and guard beds are designed to decrease traces level of Hg downto 10 ng/Nm3. This process is now established as the industry norm.

However, the activated carbons (elemental sulfur deposited on carbons) suffer from many drawbackssuch as possible sulfur loss during mercury removal operation and are prone to capillary condensation forwet gas [7], in case of location upstream of the drying section. Activated carbon has a very high surfacearea and very small pore size (average pore size � 20 angstrom). This makes it a very effective adsorbentbut also makes it very susceptible to capillary condensation of water and/or C5� compounds. Thisrestricts access of mercury to the sulphur and increases the length of the Mass Transfer Zone in the vessel.In addition, the capillary condensation leads to pressure drop issues.

Because of problems with capillary condensation, these beds are located at the final stage ofpurification downstream of the molecular sieve dryers even if this location is not the ideal one as only partof the mercury arriving in the plant is removed (mercury is also released into the acid gas, the regenerationgas and the condensed water). Furthermore a pressure drop is introduced into the final stages ofprocessing. Thus, sulfur impregnated carbon can only be used on dry gas.

Axens was pioneer in the development of an alumina-based adsorbent for mercury removal with thelaunching of first CMG product in early 70’s. Intensive R&D leads to improvement of these adsorbentsnow named AxTrap™ 200 Series adsorbents. These adsorbents consist on a finely dispersed active phasefirmly linked to the alumina carrier thanks to a proven manufacturing process.

Other mercury technologies have been launched in the 90’s. These products are called ‘bulk metalsulphides’. These guard bed adsorbents include a metal sulphide active phase and a binder to make theshape of the adsorbent. The main drawback of these adsorbents is linked to this binder addition which is

Figure 1—Picture showing the effect of amalgam formation on alumi-num-based cryogenic heat-exchangers

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responsible of lack mechanical properties during their operation. Figures 2 compare the both metalsulphide active phase technologies.

The main benefit of the alumina carrier is its robustness. Alumina carriers offer very high and stablemechanical resistance and do not generate any dust, even under drastic operating conditions with highoperating pressure and water saturated streams.

Moreover, this technology does not include any binder: there is no risk of fines formation, in case offree water presence,

In addition, there is no risk of loss of sulfur by sublimation or dissolution. All sulfur species aresecurely bound thanks to the metal and this kind of adsorbents can be used also for liquid hydrocarbonspurification.

Some regenerative solutions based on mercury physisorption exist on the market. These adsorbents arelocated in the same vessel as the molecular sieve dedicated to the drying. Nevertheless, part of the mercurycan be desorbed during the regeneration step leading to mercury presence in regeneration gas. Conse-quently, a non-regenerative adsorbent has to be implemented on the regeneration gas. In addition, thisproduct has to be replaced at the same time as the mole sieve for drying which has generally a shorter lifetime than the mercury removal adsorbent itself.

Mercury Removal ChallengesSince the beginning of the 2000’s, the trend is to implement mercury removal vessel the closest as possibleto the production wells in order to avoid mercury accumulation in the process pipes or in the effluents(acid gas from the acid gas removal unit, dryers regeneration gas, condensed water from dryersregeneration gas). At this location the natural gas is at its dew point which implies frequent or permanentliquids entrainments (water and hydrocarbons) in these operating conditions. As a consequence, activatedcarbons cannot be used at this location thus it was a necessity to develop high and stable mechanicalresistance products which are able to remove mercury on water saturated streams and at high operatingpressure.

Inorganic routes including a metal sulphide as active phase have been investigated to ensure activephase stability in such operation. In addition, metal sulphide based on alumina presents a very highmechanical resistance in particular in case of water presence.

One way to avoid operating problems is to optimize the interaction between the alumina carrier and theactive phase deposited within the solid. For instance, pore size distribution can be tuned to avoid capillarycondensation issues and the nature of the active phase and mineral support can be adequately chosen toobtain strongly bound and finely dispersed mercury-reactive nanoparticles.

As enounced, the mercury adsorption reaction on a metal sulphide can be depicted with:

Figure 2—Mercury Removal Technologies with a metal sulphide active phase a) ‘bulk technology’ and b) ‘supported technology’

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The limitation factor of this mechanism is neither the thermodynamic nor the reaction kinetic but themercury diffusion. There are different diffusion types: diffusion of mercury to the adsorbent externalsurface, diffusion of the mercury through the gas film around the adsorbent particle and the mercurydiffusion inside the adsorbent particle as depicted on Figure 3.

The adsorbent particle shape and size can be both tuned to enhance the mercury external diffusion. Thealumina carrier porosity can be tailored to optimize the mercury adsorption efficiency, avoiding capillarycondensation in gas phase and enhancing the access for the mercury to the active sites as modelled onFigures 4. The optimization of the alumina carrier porous profile helps the mercury to diffuse all alongthe adsorbent bead.

The diffusion of the mercury contained in the gas phase can be impacted by the presence of liquids.In case of liquid carry-overs, liquids will fill the porosity and block the access of mercury to the activesites leading to a broadening of the Mass Transfer Zone inside the vessel. The curve b) of Figure 5represents the mercury diffusion across an optimized adsorbent bed in absence of any liquid whereas thecurve a) represents the same but in case of some liquid carry-overs. The saturation capacity of the productis remaining the same but because of diffusion issues, the mercury breakthrough will occur before theexpected life time in case of liquids carry-overs (curve a) of Figure 5). This highlights the crucial impactof diffusion on mercury removal performances and shows that a short life time can be observed even witha capacitive adsorbent.

Figure 3—Different kinds of mercury diffusion to active sites

Figure 4—Mercury trapping modelling a) over a standard adsorbent and b) in AxTrap™ 200 Series optimized bead

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Results of a Case Study on Water Saturated Gas StreamA documented Industrial fixed bed column case study will be presented during the conference to show thedifferences between the two existing metal sulphide technologies and to explain why alumina supportedbased adsorbents is the suitable technology for mercury removal at up-front location.

The chosen case concerns a natural gas Floating, Production Storage and Offloading (FPSO) in SouthEast Asia Area, but other similar examples happened elsewhere, in both offshore and onshore plants wherethe MRU is located upstream of the molecular sieves dryers. In this present case, MRU is locateddownstream of a coalesce without any superheater.

Mercury Removal Performances with ‘bulk metal sulphides’The end-user of the project chose first the ‘bulk metal sulphide technology’ with the proposed adsorber’sdesign.

The unit has been designed to handle feed flowrate: 450 MMSCFD, mercury inlet concentration: 500�g/Nm3 , operating temperature: 30°C and operating pressure: 50 bar a.

Since the beginning of the first adsorbent’s load, the mercury outlet specification was not met. Indeed,the mercury outlet concentration was higher than 1 �g/Nm3 after only 2 weeks in operation. In addition,the operators faced pressure drop issues after only 80 days in operation (pressure drop higher than 1 bar).Fifteen batches of different ‘bulk metal sulphides’ were tested and same results were obtained. It appearedthat on water saturated natural gas stream, under high pressure (50 bar), frequent free water and/or liquidentrainment occurred, even if a coalescer is installed. The lack of mercury removal performances and thehigh observed pressure drop have been explained by the ‘bulk metal sulphides’ weakness. In case of freewater upsets, the destruction of the link between the binder and the active phase is possible. Accumulationof fines on the bed was observed and cementation of the product occurred. The adsorbent unloadingoperation was very difficult to perform as a jackhammer was required.

Mercury Removal Performances with ‘supported metal sulphides’Axens adsorbents were loaded in the mercury removal vessel for the first time in October 2011.

For this first load, a high capacitive AxTrap 271 adsorbent was proposed. This product is under beadsshape with a 3 mm average diameter.

The pressure drop was lower than 1 bar specification and was very stable without any fines generation.The 1 �g/Nm3 mercury outlet specification was achieved during 20 days. This has been depicted onFigure 6.

Figure 5—Mercury diffusion profiles a) in case of liquid carry over and b) without liquid carry-over

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The observed performances were improved as the life time was a little bit increased and the pressuredrop met the specification (pressure drop was ca. 0.3 bar). Nevertheless, the mercury removal perfor-mances were lower than expected and this lack of performances was attributed to some liquid carry-overs.

Spent product samples were easily unloaded and the analyzed revealed clearly that only a small partof the active sites have been used.

Axens studied the way to increase the adsorbent lifetime with improving the mercury access to theactive sites. This was possible thanks to 1) modelling tools developed from industrial experience and pilottests and 2) different types of tailored alumina based adsorbents. The investigated ways to improve thediffusion of mercury to active sites is to favour both external and internal diffusions of mercury asdescribed on Figure 3.

As a consequence, Axens decided to select AxTrap 273, an adsorbent with a smaller particle size andwith a higher porous volume in order to improve respectively the extra particles and the intra particlesdiffusion.

Figure 7 represents the porous distribution of two AxTrap 271 and AxTrap 273 adsorbents. Bothadsorbents have an opened porosity without any pores lower than 10 nm (no microporosity). The totalporous volume of AxTrap 273 is higher than the one of AxTrap 271. This is due to:

1. The presence of smallest pores in case of AxTrap 273. AxTrap 273 smallest pores median diameteris in the 10 nm range whereas AxTrap 271 smallest pores median diameter is in the range of 30nm median diameter; and

2. The presence of macropores in case of AxTrap 273 (median diameter higher than 100 nm).

Finally, AxTrap 273 has a higher total porous volume than AxTrap 271 and presents a bimodal porousdistribution (mesopores and macropores) while AxTrap 271 has only a monomodal porous distribution(mesopores). Furthermore, due to a higher total porous volume, AxTrap 273 is less dense than AxTrap271.

At the same time, the way to improve extra particle diffusion was studied. It is important to highlightthat the proposed AxTrapTM products were very stable and generated a low pressure drop in operation (ca.

Figure 6—Mercury removal performances with AxTrap 271

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0.3 bar with 3 mm average diameter beads). As a consequence, some pressure drop margins existed andthe replacement with AxTrap 273 in small beads has been investigated.

Thanks to our modeling tools, the predicted performances have been simulated with the real operatingconditions (see Figure 8). The results of these simulations shows a sharper mass transfer zone (between1 and 2.5 meters of the bed’s height), associated with a longer saturation zone (between 0 and 1 meter)and thus, a better use of the mercury removal bed with AxTrap 273. Loading of AxTrap 273 1.4-2.8 mmadsorbent seemed to be a very good way to improve the mercury removal performances of the unit, tooptimize the dynamic adsorption capacity.

Figure 7—Comparison of Porous Distributions of AxTrap 271 and AxTrap 273

Figure 8—Modeling of mercury adsorption profiles versus bed height with a) AxTrap 271 and b) AxTrap 273 small beads

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Finally, AxTrap 273 1.4-2.8 mm was loaded in the unit and the life time was very significantlyincreased: the mercury outlet concentration was on specification during more than 160 days (Figure 9)instead of 20 days with AxTrap 271 (Figure 6).

This product has been replaced many times and the better run reached 220 days in operation (Refer toFigure 10) after minimizing the liquid’s upsets. This shows that the mercury removal performancesprediction can be difficult to foresee, due to the difficulty to know the real amount of liquids presentsupstream of the Mercury Removal Units in a “front-end” configuration. Nevertheless, the mercuryremoval performances have been notably enhanced, after the simulation study with AxTrap 273 solution.

Figure 9—Improvement of mercury removal performances with AxTrap 273

Figure 10—Improvement of mercury removal performances with AxTrap 273 small beads

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The observed pressure drop versus feed flow rate diagram is presented on Figure 11.The observed pressure drop has been below the specification and very stable since the first day in

operation, as shown on Figure 12.Thanks to this very robust adsorbent and its diffusion transfer properties optimization, the unit

performances have been highly increased fom 14 days with ‘bulk metal sulphides’ technology to 220 dayswith ‘alumina supported metal sulphides’ technology. In addition, the pressure drop issues have beensolved.

Figure 11—Monitored pressure drop versus feed flowrate with AxTrap 273 small beads

Figure 12—Flowrate normalised pressure drop with AxTrap 273 small beads

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ConclusionThe mercury removal vessels are now located in a “front-end” location in the natural gas treatment chainin order to minimize mercury contamination along the natural gas processing chain. As a consequence,operating conditions are more drastic with higher operating pressure and water over-saturated streams.Thus, very stable and liquid resistant adsorbents are required. Furthermore, presence of free water leadsto diffusion issues with a broadening mass transfer zone and therefore a premature mercury breakthrough.

In these conditions, Alumina supported metal sulphides allow to radically improve mercury removalperformances compared to other technologies thanks to 1) the higher mechanical resistance and 2) bothtailored adsorbent particle and porosity. The external and the internal diffusion of mercury to active sitesdiffusion can be improved with respectively, an optimized particle size and can be made easier with anopened porosity. Pore size distribution has to be tuned to avoid capillary condensation issues.

This kind of adsorbents is very robust due to the alumina carrier, a very well-known product for dryingapplication. In addition, the interaction between the carrier and the active phase is optimized, leading tovery stable products.

This paper shows clearly that it is not efficient to have only very capacitive adsorbents in these highpressure and water saturated conditions. These materials have to be very robust and to be designed suchas to maximize the active sites accessibility for optimizing the mercury dynamic adsorption capacity. Inparallel, the development of very powerful modeling tools help a lot to manage an optimization study andto improve the performances of mercury removal units.

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du pétrole et du gaz 4 août 2011 15 pages3. Bensefa-Colas, L.; Andujar, P.; Descatha, A. (2011) Mercury poisoning. Revue de Médecine

Interne, 32, 416–244. Wilhelm S. M. (1999) Avoiding exposure to mercury during inspection and maintenance oper-

ations in oil and gas processing. Process Safety Progress, 18, 178–885. Kehal, M; Mennour, A; Reinert, L; Fuzellier, H (2004) Heavy metals in water of the Skikda Bay.

Environmental Technol., 25, 1059–656. XX Pirrone, N., and Mahaffey, K. R., “Dynamics of Mercury Pollution on Regional and Global

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7. McNamara, J.D. and Wagner, M.J. Process effects on activated carbon performance and analyticalmethods used for low levels mercury removal in natural gas applications. Gas. Sep. Purif., 1996, 10, pp. 137–140.

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