GB2428598A - Process for removing mercury from gaseous streams - Google Patents

Abstract

A process is described for removing mercury from gases evolved 18 from an adsorbent 14 during its regeneration, comprising passing the gases directly through an absorbent bed 22 comprising a particulate sulphided transition metal compound to remove mercury therefrom. The evolved gases may comprise a regeneration gas consisting of hydrogen, nitrogen, helium or argon. Preferably, the sulphided transition metal compound will consist of one or more sulphided first row transition metals e.g. Mn, Fe, Co, Ni, Cu or Zn, compounds and may be selected from one or more of the hydroxides, carbonates and hydroxy-carbonates. A particularly suitable compound being basic copper carbonate.

Description

1 2428598 Process for removing mercury from gaseous streams This invention

relates to a process for removing mercury from gaseous streams, in particular to a process for removing mercury from gases evolved during the regeneration of mercury- containing adsorbents.

Adsorbents are used in industry to remove water from process streams, such as liquid and gaseous hydrocarbons. Typically such adsorbents are zeolitic materials with a pore diameter in the range 3-5 Angstroms, but may also include other water trapping materials such as alumina or silica gel. Once the adsorbents are saturated with water, the adsorbent may be regenerated by passing a heated dry gas stream through them until the water is removed.

Hydrocarbon, and other industrial process streams also often contain mercury and other volatile contaminants such as arsenic and antimony compounds including arsine [AsH3], trimethylarsine [As(CH3)3] and stibine [SbH3]. Typically the concentration of mercury is from 0.01 to 500 j.ig.Nm3; more usually between lOb 200jig.Nm3. The concentration of the other volatile contaminants is usually less than the mercury content. The mercury and other volatile metal compounds can be trapped with the water on the adsorbents. Trapping the mercury and other volatile contaminants is advantageous as these contaminants are severe pollutants and often are potent catalyst poisons for catalysts used in hydrocarbon processing. Mercury also causes corrosion in process equipment, e.g. aluminium cryogenic heat exchange equipment used in liquefied natural gas processing, the separation of natural gas liquids and nitrogen removal from natural gas.

Zeolitic materials have been used for the collection of water and mercury from liquid or gaseous hydrocarbons such as natural gas or cracked hydrocarbon mixtures. Furthermore, it is likely that adsorbents effective for mercury removal will also be effective for removal of the other volatile contaminants from the process stream and that therefore regeneration may evolve these in addition to mercury from the adsorbent. However little regard has been given to the effluent created when such adsorbents are regenerated. Mercury and the other volatile contaminants released on regeneration of adsorbents poses a problem in terms of waste disposal and a threat to the environment.

US 48925677 describes regenerable adsorbents based on zeolite A containing silver or gold in small amounts to simultaneously remove mercury and water from a fluid. Regeneration of the sieves was accomplished merely by heating the saturated sieves with a hot regeneration gas, preferably at 200-400 C, and there was no consideration for preventing release of the mercury.

US 4874525 describes an arrangement for simultaneously trapping water and mercury using a combination of a silver-containing zeolite X and a silverfree zeolite A. Regeneration of these sieves was again accomplished by passing a hot regeneration gas through the sieves, but in this case the evolved regeneration gas was chilled to 50 C to recover water and the remaining mercury-containing gas stream either burnt as fuel or further processed for the bulk removal of mercury. No indication how the bulk removal of mercury may be performed was given.

Furthermore, this process has the disadvantage that while some separation of mercury from the water is possible, the water effluent will still contain significant amounts of released mercury.

Therefore it is desirable to remove mercury and any other volatile contaminants directly from the evolved gases before recovering the water. However, known mercury absorbents are negatively effected by both heating and free water present in the evolved gases.

Sulphided carbon materials are known to trap mercury under ambient conditions, however the effectiveness of these materials is limited, and heating to temperature >50 C can result in sublimation of the sulphur with undesirable deposition in downstream equipment. Furthermore, due to the high surface area and small pore size of the carbon, it is very sensitive to capillary condensation of water and can only be used on dry gases. It has been shown that as little as 3% loading of water on carbon increases the required mass transfer zone by 12% and allows slippage of 0.02.ig/m3 of mercury.

We have found surprisingly that mercury and any other volatile contaminants may be directly recovered from adsorbent regeneration gases at elevated temperatures using a bed of particulate sulphided transition metal compound. This arises from the surprising discovery that the sulphided transition metal compounds are effective at retaining mercury at elevated temperatures.

Accordingly the invention provides a process for removing mercury from gases evolved from an adsorbent during its regeneration comprising passing the gases directly through an absorbent bed comprising a particulate sulphided transition metal compound to remove mercury therefrom.

In addition to mercury, the evolved gases may comprise volatile metal compounds such as arsine, trialkyl arsines such as trimethyl arsine and stibine. The mercury may be in the form of mercury vapour, organomercuric, or organomercurous compounds. Preferably mercury is in its elemental form and the volatile metal compounds are arsine and stibine. The sulphided transition metal compounds in the process of the present invention may be used to simultaneously remove mercury and volatile reactive arsenic and antimony compounds from the evolved gases.

The temperature and pressure of the evolved gases should preferably be controlled so that any evolved water is maintained in the vapour phase. The pressure of the evolved gases may be in the range 0.1 -200 bar abs, preferably 1-60 bar abs. Hence, a vacuum may be applied to evolve the mercury-containing gases from the adsorbents. However, the gases are preferably evolved from the adsorbents by passing a regeneration gas through the adsorbents, and therefore preferably the evolved gases will comprise a regeneration gas. The regeneration gas may be heated or the adsorbents may be heated by external means, e.g. a steam jacket.

Preferably the gases are evolved from the adsorbents by passing a heated regeneration gas stream through the adsorbent. The regeneration gas stream may be passed co-current or counter current to the normal flow of process fluid. Where an adsorbent bed arrangement has been provided that has resulted in the presence of adsorbed mercury and other volatile contaminants predominantly at one end of the adsorbent bed, the regeneration gas is preferably fed from the opposite end so that the mercury and other contaminants are evolved from the adsorbent as quickly as possible. For example in the arrangement shown in Figure 1 of the aforesaid US 4874525, the regeneration gas is preferably fed co-current to the flow of process fluid in contrast to the teaching therein.

The regeneration gas may be selected from the list consisting of a hydrocarbon, which may be gaseous or a vapourised liquid hydrocarbon, or an inert gas such as nitrogen, helium or argon.

It will be understood that the regeneration gas preferably should be substantially dry and substantially mercury, arsenic and antimony compound free as this will allow the regeneration to proceed to greater extent than the case where they are present. Gaseous C1-C4 hydrocarbons are preferred, particularly natural gas, associated gas, methane, ethane, propane and butane. Preferably the regeneration gas is the dry process fluid from which water and mercury and optionally arsenic and antimony compounds have previously been removed using the adsorbents. Alternatively the un-dried feed gas containing the mercury and any of the other contaminants may be used.

The mercury content of the evolved gases may be in the range of 10-10,000 j.tg/Nm3. If arsenic or antimony compounds are present, they may be in the range 10-5,000.Lg/Nm3. By Nm3, or Normal cubic metre, we mean a cubic metre of evolved gas at standard temperature and pressure.

The adsorbent may be any suitable water-absorbent material. One water absorbent material within the scope of the present invention is silica gel. Another is alumina, preferably a transition alumina such as theta, gamma, delta or alpha alumina. A preferred adsorbent is a molecular sieve such as a zeolite, especially a zeolite with pore diameters in the range 3-15 Angstroms, particularly 3-5 Angstroms. Particularly suitable molecular sieve materials are Zeolite A ion exchanged with Na, K or Ca (zeolite 4A, zeolite 3A and zeolite 5A respectively).

Alternatively, zeolite Y, zeolite X and other faujasite-structure zeolites may be used, e.g. zeolite 13 X. In one embodiment, the adsorbent, e.g. a zeolite, may be doped, throughout or in part, with a mercury, arsenic or antimony collecting material. By mercury, arsenic or antimony collection material' we mean a material that specifically absorbs mercury, and optionally arsenic or antimony compounds under normal process conditions from the process fluid but which is able at least in part to release the absorbed mercury, arsenic or antimony compounds when subject to adsorbent regeneration. Suitable mercury collection materials are elemental gold and silver. Preferably the mercury collecting material is silver. Suitable silver- containing adsorbent materials, and methods for their preparation are described in the aforesaid US 4874525 and US 4892567. For example, the adsorbent is suitably zeolite A, particularly 3A, 4A or 5A, especially 4A containing on its surface 0.001-15% weight of elemental silver.

Alternatively the molecular sieve may be silver-doped zeolite X. The silver may be present in the molecular sieve in an outer shell of thickness less than about 0.1 mm. Notwithstanding this, in the present invention is not necessary for the adsorbents to comprise a mercury collection material. Indeed it may be preferable in some circumstances to use standard adsorbents, e.g. zeolites, without silver or gold present. This may offer the operator considerable cost-savings versus the prior art technology.

The adsorbent may be in the form of a monolith such as a honeycomb or may comprise a bed of particles. Adsorbent particles may typically be in the form of agglomerates, pellets, tablets or extrudates such as spheres, cylinders, rings or other known shaped units with minimum and maximum dimensions preferably in the range 2-25 mm. Furthermore, the adsorberits may be arranged in a layered structure such that the process gas stream passes first through a bed of conventional adsorbent and then through a bed of adsorbent comprising a mercury capture material, e.g. a layer of zeolite A, followed by a layer of silver-doped zeolite X as depicted in the aforesaid US 4874525.

The gases evolved from the adsorbents during regeneration are passed directly, i.e. without a step of water removal, to a bed comprising one or more particulate sulphided transition metal compounds. Preferred transition metal compounds are first row transition metal compounds, more preferably Mn, Fe, Co, Ni, Cu and Zn compounds, most preferably Fe and Cu, particularly Cu compounds. The compounds are preferably selected from one or more of the hydroxides, carbonates and hydroxy-carbonates. A particularly suitable copper compound is basic copper carbonate.

The particulate transition metal compound and any support or binder materials are preferably intimately mixed and in the form of agglomerates, pellets, tablets or extrudates such as spheres, cylinders, rings or other known shaped units with minimum and maximum dimensions preferably in the range 1-25 mm. Agglomerates comprising one or more transition metal compounds and a binder are preferred as these provide the highest porosity and effective surface area for removing mercury and other contaminants from the evolved gases.

Agglomerates may be formed by granulation of mixed powders using methods known to those skilled in the art of absorbent manufacture.

The transition metal compound is sulphided, i.e. one or more of the compounds are at least in part converted to the metal sulphides. Typically at least 5%, preferably at least 15% and more preferably at least 20% of the compounds are present as a transition metal sulphide. The compounds, typically after being formed into shaped units, are preferably pre-suiphided with a suitable sulphur compound. This avoids the problem of having to provide sulphur compounds to the bed in sufficient quantity to form the suiphide compounds necessary to recover mercury and other contaminants from the evolved gases. Sulphiding in-situ may be performed where the process gas contains e.g. hydrogen suiphide, in sufficient quantity to effectively sulphide the transition metal compounds but this is less preferred.

GB-B-1533059 describes the preparation of a pre-suiphided mercury absorbent comprising copper suiphide. The pre-suiphided absorbent is prepared by forming a precursor comprising a copper compound, e.g. an extrudate comprising basic copper carbonate and a refractory cement binder, and then contacting the precursor with a gaseous stream containing a sulphur compound, e.g. hydrogen sulphide, so as to fully sulphide the copper compound. Such absorbents may be used in the present invention to remove mercury and optionally arsenic and antimony compounds from the evolved gases.

However, the particulate suiphided transition metal compound is preferably a copper-zinc- compounds according to US 4871710 that have been pre-sulphided to form copper and zinc sulphides. The presence of zinc, which sulphides more slowly than copper, provides a stable framework for the copper sulphide. Additionally the copper/zinc compounds are preferably agglomerated with alumina using a calcium aluminate binder.

Accordingly, the transition metal compounds to be sulphided preferably consist of agglomerates having a size in the range ito 10 mm; a BET surface area of at least 80 m2g1, measured on samples of the agglomerates that have been calcined for 4 hours at 350 DEG C.; a calcined density of not more than 1.5 gcm3; a porosity of not less than 0.6; and comprising (i) compounds of (a) copper, and (b) zinc and aluminum, said compounds being in the form of at least one compound selected from oxides, hydroxides, carbonates and/or basic carbonates; and (ii) calcium aluminate cement binder; said compounds being in such proportions that the copper atoms form 30-97% of the total number of copper, zinc, and aluminum atoms in said agglomerates; said agglomerates having a total copper and zinc compound content such that, after ignition at 900 DEC C., the cupric oxide plus zinc oxide content of the ignited composition is at least 70% by weight; and said binder constituting 5-10% by weight of said agglomerates.

Preferably the agglomerates contain zinc oxide, hydroxide, carbonate and/or basic carbonate, in such proportions that the zinc atoms constitute 10 to 40% of the total number of copper, zinc, and aluminium atoms in said agglomerates, and more preferably the agglomerates contain at least one aluminium compound in addition to said calcium aluminate binder in such proportions that the total number of aluminium atoms in the agglomerates constitutes 5 to 20% of the total number of copper, zinc, and aluminium atoms. A particularly preferred composition comprises > 50%wt sulphided basic copper carbonate. It is preferred that the amount of sulphide of the transition metal present is such that the compounds may be sulphided to achieve a sulphur loading of at least 15% w/w, and particularly at least 20% w/w.

The sulphur compound used to sulphide the transition metal compounds may be one or more sulphur compounds such as hydrogen suiphide, carbonyl suiphide, mercaptans and polysulphides. Hydrogen sulphide is preferred. Pre-sulphiding may be performed by passing the sulphur compound, preferably in a carrier gas at suitable concentration, through the bed of particulate transition metal compounds in a suitable vessel at 0-150 C, preferably 20-100 C for up to 24 hours at pressures in the range 1-200 bar abs.

Using the process of the present invention, the mercury content of the treated gases may be below 0.01.ig.Nm3.

The bed of particulate sulphided transition metal compound may be provided in an axial flow reactor, e.g. of volume between 1 and 10m3, or preferably, where pressure drop through the bed needs to be minimised, in a radial flow reactor. Such reactor configurations are well known to those skilled in the art. If necessary, more than one reactor may be used in parallel or series to maximise mercury absorption and minimise disruption during replacement of saturated absorbent. When the bed of sulphided transition metal compounds becomes saturated with mercury the bed may be discharged and sent to smelters for processing and metal recovery.

The evolved gases contain mercury, optionally arsenic and antimony compounds, and water.

We have found that the vapour pressure of mercury and the other contaminants may be sufficiently high under process conditions to permit their effective recovery.

The temperature of the evolved gases during regeneration may be 60-400 C. Preferably the evolved gases fed to the bed of particulate suiphided transition metal compounds are initially at a temperature in the range 80150 C, more preferably 95-130 C, most preferably 110-125 C.

At these temperature ranges the mercury and any arsenic and antimony is still retained on the sulphided absorbent at high levels and the water evolved is minimised. By using temperature to control the water release during the initial stage of the adsorbent regeneration, the risk of capillary flooding leading to lower absorbent performance is reduced. It will be understood that where a regeneration gas is used, the temperatures of the regeneration gas will be about that of the evolved gases, hence preferably the saturated adsorbent is heated initially with a regeneration gas at a temperature in the range 80-150 C, more preferably 95-130 C and most preferably 110-125 C. The contact time of the evolved gases with the suiphided absorbent is preferably 1-10 seconds. The GHSV is preferably in the range 10,000 to 100,000 h(1. The initial period may proceed until the mercury or other contaminants are recovered from the adsorbent or until the water released with the mercury starts to reduce the effectiveness of the sulphided absorbent, which may be seen from an increase in the mercury content in the suiphided absorbent outlet gases. The temperature of the evolved gases and their flowrate may be used to set the length of the initial period which may be e.g. up to 8 hours, preferably up to 4 hours long. The regeneration process may take up to 72 hour, preferably up to 48 hours, more preferably up to 24 hours to complete.

If desired during the initial period, heat exchange and water recovery apparatus may be provided downstream of the suiphided absorbent to cool the evolved gases and recover any water evolved with the mercury and other volatile contaminants.

Once the mercury and other volatile contaminants have been evolved the temperature of the adsorbent may, if desired, be increased to release the remaining water at a faster rate.

However to prevent volatilisation of mercury from the suiphided transition metal absorbent, and to prevent possible condensation of liquid water in the pores of the suiphided transition metal absorbent, it is desirable to divert the evolved gases through heat exchange apparatus to cool the evolved gases to below the dew point of water and recover the water. If the remaining water depleted gases still contain mercury at unacceptable levels, the cooled gases may be passed though a bed of mercury absorbent.

For example, following an initial period of regeneration at low temperature as described above wherein the evolved gases are fed directly (without water removal) to the mercury absorbent, once the evolved gases are diverted to the heat exchange and water recovery apparatus, the adsorbent may be subjected to a period of regeneration at >150 C, e.g. 200-400 C in order to drive off the water and any remaining mercury or other contaminants. During this latter period, preferably the evolved gases are cooled to below 60 C in one or more heat exchangers and the cooled gases fed to a separator to recover water. The cooled waterdepleted gases may then be fed from the separator to a bed of mercury absorbent. The mercury absorbent in this case may be the same or different from that used for direct absorption from the evolved gases.

However, preferably the absorbent is the same, i.e. preferably the mercury absorbent is particulate sulphided copper and zinc compounds and more particularly the same particulate bed disposed in the vessel to which the evolved gases were previously fed directly.

Thus in the present invention, the operator may by-pass the water recovery stages of the prior art for part of the regeneration. This has the advantage that the mercury in the wastewater will be reduced. For example, prior art methods may lead to mercury contents in was the condensed wastewater =2ppm. Using the present invention levels of <2ppm may be obtained.

The invention is further described by reference to the Figure in which; Figure 1 is a flowsheet of one embodiment according to the present invention in which evolved gases are initially fed directly to a bed of mercury absorbent.

In Figure 1 a regeneration gas stream of substantially dry natural gas at 120 C, 56 bar abs, is fed via line 10 to the top of vessel 12 in which a fixed particulate bed 14 of 3-5 Angstrom zeolite molecular sieve containing adsorbed mercury is disposed. The regeneration gas flows through the bed from the top heating the molecular sieve and releasing mercury and any arsenic and antimony compounds. The evolved gases leave the bottom of vessel 12 via line 16. A line 18 taken from line 16 feeds the evolved gases directly, without intervening process steps, to a vessel 20 containing a fixed bed of particulate copper-zinc suiphide absorbent 22. Mercury and any arsenic and antimony compounds are absorbed from the hot gases as they pass through the bed.

The mercury depleted gases leave the absorbent vessel 20 via line 24 and may be taken from line 24 via line 25 and be sold or subjected to further processing. If water is present in sufficient amount, the mercury depleted gases may be fed via line 24 to heat exchanger 26 where they are cooled below the dew point of water, e.g. to 40 C. The resulting mixture is fed via line 28 from heat exchanger 26 to separator 30, where the liquid water is recovered from the bottom via line 32, and the cooled, de- watered gases is recovered from the top via line 34.

These gases may likewise be sold or subjected to further processing.

When substantially all the mercury and any arsenic and antimony compounds have been recovered or when the concentration of water in the evolved gases increases to a point at which the effectiveness of the mercury absorbent is reduced, the flow of evolved gases is diverted entirely from line 18 via a line 36 to heat exchanger 26 and separator 30 to recover water. Furthermore once the flow of evolved gas has been diverted, the temperature of the regeneration gas may be increased to >150 C, preferably 200-400 C to increase the rate of water evolution. At this time, if mercury is present in the cooled gases leaving the separator in line 34, they may be diverted via line 38 to line 18 feeding the bed of suiphided absorbent 22.

The cooled gases leaving the bed of sulphided absorbent 22 via line 24 may be taken via line and burnt as fuel.

The flow of regeneration gases through the apparatus is controlled by valves (not shown) which may be automatic and may be programmed to open or close based upon the water content of the gas in line 18 or the mercury content of the gas in line 24. Alternatively the valves may be controlled by other methods known in the art.

In a regeneration process operated not inaccordance with the present invention, during an eight hour regeneration of standard molecular sieves, the peak measured evolved gas temperature was 247.5 C and the highest level of mercury was 64,000 ng/Nm3. The water evolved during regeneration was condensed and found to contain 2ppm mercury.

The present invention also offers a means to reduce the vessel size required for mercury recovery compared to that required for recovering mercury directly from the process gas, because the flowrate of regeneration gases is considerably lower than that of the process gas.

Reduction in vessel size is advantageous for cost reasons but also particularly in saving on space and weight, which are particularly important for off-shore applications such as natural gas processing. Furthermore by recovering at least a portion of the mercury directly from the regeneration gases, the mercury content in recovered water will be reduced, thereby providing environmental and waste disposal savings.

Claims (20)

Claims.

1. A process for removing mercury from gases evolved from an adsorbent during its regeneration comprising passing the gases directly through an absorbent bed comprising a particulate suiphided transition metal compound to remove mercury therefrom.

2. A process according to claim 1 wherein the evolved gases further comprise volatile arsenic or antimony compounds which are also removed from the evolved gases by the particulate suiphided transition metal compound.

3. A process according to claim 1 or claim 2 wherein the evolved gases comprise a regeneration gas.

4. A process according to claim 3 wherein the regeneration gas is selected from the list consisting of hydrocarbon, nitrogen, helium or argon.

5. A process according to any one of claims 1 to 4 wherein the evolved gases are at a temperature in the range 80-150 C.

6. A process according to any one of claims 1 to 5 wherein the adsorbent comprises a silica gel or alumina.

7. A process according to any one of claims 1 to 5 wherein the adsorbent comprises a zeolite molecular sieve.

8. A process according to claim 7 wherein the zeolite has a pore diameter of 3-15 angstroms.

9. A process according to claim 7 or claim 8 wherein the zeolite is based on zeolite A or zeolite X.

10. A process according to any one of claims 1 to 9 wherein the adsorbent comprises a mercury collecting material.

11. A process according to claim 10 wherein the mercury collecting material is silver.

12. A process according to any one of claims I to 11 wherein the suiphided transition metal compounds consist of one or more sulphided Mn, Fe, Co, Ni, Cu and Zn compounds.

13. A process according to any one of claims 1 to 12 wherein the transition metal compounds are selected from one or more of the hydroxides, carbonates and hydroxy- carbonates.

14. A process according to claim 12 or claim 13 wherein the suiphided transition metal compounds comprise >50% wt sulphided basic copper carbonate.

15. A process according to any one of claims 1 to 14 wherein the amount of sulphide of the transition metal present is such that the absorbent has a sulphur loading of at least 15%wt.

16. A process according to any one of claims 1 to 15 wherein the gases leaving the bed of particulate sulphided transition metal compounds are fed through heat exchange means to cool said gases to below the dew point of water and thence through a separator to separate water from said cooled gases.

17. A process according to claims ito 15 wherein after a period of mercury removal, said evolved gases are diverted through heat exchange means to cool said gases to below the dew point of water and thence through a separator to separate water from the cooled gases.

18. A process according to claim 17 wherein said cooled gases are subsequently passed through a bed of mercury absorbent.

19. A process according to claim 18 wherein the mercury absorbent comprises a particulate sul phided transition metal compound.

20. A process according to any one of claims i7to 19 wherein once the evolved gases have been diverted through the heat exchange means, the temperature of the adsorbent is increased.