Mercury Removal
It is not unusual for gas streams to contain mercury. Some gas streams have been reported to have over 100 micrograms/m3. The mercury can attack aluminum in the plate fin heat exchangers used in most modern cryogenic plants. In order for the attack to occur, the mercury must be present as a free liquid. This situation cannot occur above –40F. Technically, mercury containing feedstocks can be handled without aluminum corrosion. Mercury containing equipment which is kept at low temperature can be decontaminated by carrying out a cold, then warm purge with bone dry gas. However, this is not a practical method to be assured that mercury attack does not occur.
NoDoC cost model simulate the cost of the system in which the mercury in the feed gas removed with a mercury removal bed. In this model the bed uses a sulfur based trapping material which reacts with the mercury to form cinnabar (HgS) on the bed. The trapping material is carried on activated carbon, zeolite or alumina. The trapping bed is usually located downstream of thedehydration. In this location, the gas is free of entrained liquids and water. Locating the bed in other locations is very dependent on the material used as recommended by the vendor. Figure 1 shows an example mercury removal bed. NoDoC model considers the mercury beds to remove the mercury to 0.001 micrograms/Nm3.
Each vendor has criteria for sizing beds for their material but some rules of thumb are that the bed should be sized for a superficial flow velocity of about 50 ft/min and a residence time of 10 seconds. With the rather small mass of mercury which is typically removed, the beds can last many years between change outs.
NoDoC models the following adsorbants and absorbants for mercury removal from gas:
Calgon
Solid adsorbents can remove mercury from gas to produce residuals in the range of 0.01–0.001 μg/Nm3. Calgon sulfur impregnated HGR® (4 x 10 mesh) and HGR®-P (4 mm dia.) carbons are used for mercury removal and indicate designs removing mercury down to very low levels. Removal of both inorganic and organic mercury is achieved. By first drying the gas the degree of mercury removal increases. The sulfur impregnate reacts with the mercury to produce a mercury sulfide that is fixed in the carbon microstructure.
Merespec
MereSpec fixed bed absorbents are used for removal of traces, elemental and organic, of mercury from hydrocarbon liquids and gases. The absorbents have been shown to be capable of providing the outlet mercury concentration normally specified for LNG production and are in service in several European locations including an offshore oil/gas production platform. Merespec is a trademark of Synetix.
Desorex
Activated carbon provides only a limited storage capacity for the strictly physical adsorption of mercury. Desorex HGD2S and HGD4S from Donau Carbon can be employed to bind mercury through the process of chemical adsorption involving oxidation and adsorption in the form of stable compound or fixation in metallic form as an amalgam. Many reference installations of these Desorex products for the purification of natural gases to levels as low as 10 ng/m3 of mercury have been realized over a long period of time.
HgSIV
HgSIV is another adsorbents which are molecular sieves coated with elemental silver. Mercury in the gas is trapped by amalgamation with the silver. The adsorbent also serves the dual function of dehydrating the gas. HgSIV is regenerated thermally, just like molecular sieves for dehydration. This material can be added as a layer to existing molecular sieve dryers58. However, one must take care to appropriately handle the regeneration gas in this case, as it will contain mercury.
CMG 275 and 273
Another mercury trapping material labeled CMG 275 was developed by IFP and Procatalyse (today Axens) which is sulfur supported onto mesoporous alumina. The advantage of mesoporous alumina based product is its resistance to capillary condensation59. The larger pore size of this material, compared to carbon based trapping materials permits utilization under near dew point conditions. In another material from Axens labeled CMG 273 the trapping component is anchored on the alumina carrier making it completely insoluble in liquid hydrocarbons and water. The material has been subjected at gas plant sites to both DEA and liquid hydrocarbon carry-over with no active phase leaching. This same material has been used to eliminate elemental mercury from LPG and full range condensates.
Organic Mercury Removal
Removal of all forms of organic mercury compounds from natural gases and liquids requires firstly the conversion of the compounds to elemental metallic mercury followed by trapping materials to remove the metallic mercury formed. This requires in the first stage some hydrogen for the organo-mercury hydrogenolysis with a suitable catalyst. The first stage catalyst such as MEP 841 also traps arsenic and lead impurities in the feed. The two stage impurities removal process is called RAM and is available from Axens.
NoDoC cost model simulate the cost of the system in which the mercury in the feed gas removed with a mercury removal bed. In this model the bed uses a sulfur based trapping material which reacts with the mercury to form cinnabar (HgS) on the bed. The trapping material is carried on activated carbon, zeolite or alumina. The trapping bed is usually located downstream of thedehydration. In this location, the gas is free of entrained liquids and water. Locating the bed in other locations is very dependent on the material used as recommended by the vendor. Figure 1 shows an example mercury removal bed. NoDoC model considers the mercury beds to remove the mercury to 0.001 micrograms/Nm3.
Each vendor has criteria for sizing beds for their material but some rules of thumb are that the bed should be sized for a superficial flow velocity of about 50 ft/min and a residence time of 10 seconds. With the rather small mass of mercury which is typically removed, the beds can last many years between change outs.
NoDoC models the following adsorbants and absorbants for mercury removal from gas:
Calgon
Solid adsorbents can remove mercury from gas to produce residuals in the range of 0.01–0.001 μg/Nm3. Calgon sulfur impregnated HGR® (4 x 10 mesh) and HGR®-P (4 mm dia.) carbons are used for mercury removal and indicate designs removing mercury down to very low levels. Removal of both inorganic and organic mercury is achieved. By first drying the gas the degree of mercury removal increases. The sulfur impregnate reacts with the mercury to produce a mercury sulfide that is fixed in the carbon microstructure.
Merespec
MereSpec fixed bed absorbents are used for removal of traces, elemental and organic, of mercury from hydrocarbon liquids and gases. The absorbents have been shown to be capable of providing the outlet mercury concentration normally specified for LNG production and are in service in several European locations including an offshore oil/gas production platform. Merespec is a trademark of Synetix.
Desorex
Activated carbon provides only a limited storage capacity for the strictly physical adsorption of mercury. Desorex HGD2S and HGD4S from Donau Carbon can be employed to bind mercury through the process of chemical adsorption involving oxidation and adsorption in the form of stable compound or fixation in metallic form as an amalgam. Many reference installations of these Desorex products for the purification of natural gases to levels as low as 10 ng/m3 of mercury have been realized over a long period of time.
HgSIV
HgSIV is another adsorbents which are molecular sieves coated with elemental silver. Mercury in the gas is trapped by amalgamation with the silver. The adsorbent also serves the dual function of dehydrating the gas. HgSIV is regenerated thermally, just like molecular sieves for dehydration. This material can be added as a layer to existing molecular sieve dryers58. However, one must take care to appropriately handle the regeneration gas in this case, as it will contain mercury.
CMG 275 and 273
Another mercury trapping material labeled CMG 275 was developed by IFP and Procatalyse (today Axens) which is sulfur supported onto mesoporous alumina. The advantage of mesoporous alumina based product is its resistance to capillary condensation59. The larger pore size of this material, compared to carbon based trapping materials permits utilization under near dew point conditions. In another material from Axens labeled CMG 273 the trapping component is anchored on the alumina carrier making it completely insoluble in liquid hydrocarbons and water. The material has been subjected at gas plant sites to both DEA and liquid hydrocarbon carry-over with no active phase leaching. This same material has been used to eliminate elemental mercury from LPG and full range condensates.
Organic Mercury Removal
Removal of all forms of organic mercury compounds from natural gases and liquids requires firstly the conversion of the compounds to elemental metallic mercury followed by trapping materials to remove the metallic mercury formed. This requires in the first stage some hydrogen for the organo-mercury hydrogenolysis with a suitable catalyst. The first stage catalyst such as MEP 841 also traps arsenic and lead impurities in the feed. The two stage impurities removal process is called RAM and is available from Axens.
