GB2406294A - Mercury removal from natural gas liquids - Google Patents

Abstract

A process of producing a mercury-depleted hydrocarbon stream from a natural gas liquids feedstock by sorption and/or reaction of mercury on a solid is characterised by, upstream of such sorption and/or reaction, removing fine particles from the feedstock in liquid form by contacting with a packed-bed filter (7).

Description

NATURAL GAS LIQUIDS

THIS INVENTION relates to natural gas liquids and in particular to removal of mercury therefrom.

Natural gas liquids (N(3L), a mixture of hydrocarbons C2 up to about Cal widely used as chemical feedstock and fuel, commonly contains small quantities of mercury as inorganic or organic compounds or in elemental form. As well as being environmentally hazardous, mercury may be harmful in process use. For example precious metal catalysed processes are poisoned by mercury; and the presence of even smal] traces of mercury in the feed to cryogenic separation leads to the failure of the aluminium equipment used. It has been proposed to remove mercury by contacting with a sorbent based for example on activated carbon, but long process runs are difficult to achieve and have been the subject of unexplained physical failure of sorbent.

Such failure has been attributed to impurities in the stock fed to the mercury removal, for example one or more of: water, due to steam distillation processing; chloride, especially when the natural gas comes from undersea wells, ant'-corrodants e g amines, added to the NGL to protect transportation/processing vessels.

However, even after removal of such impurities, the process life of mercury removal units may be shorter than desired. We have identified a further class of impurity and devised a mean.:) for removing it.

ACCORL)!NG TO T 1IE VENT101'' a process of producing a mercury-depleted Ir, d, ocarbon stream hom a natural gas liquids feedstocl; he sorption anchor reaction of nervy can a colic is characterised by, upstream of such sorption andlor reaction, removing fine pcrhles fi-on the feed,stock by contacting in liquid form with a packed-bed filter.

if he fine- particles are typically of size at Cast 2, especially at least 5, and up to]0 micr,.'rs or bleater l heir action in the mercury removal process is b rlieved to be mechanical, in that they accumulate on and increasingly block relevant surfaces such the sorbent(s) and fine- pore singe rca>ncile' filters at the inlet of the process. l bus their chemical composition does not apnea; to be critical to mercy; removal The packed bed filter is preferably additional to an:l upstream of such candle filters, that; lir,liiw their deity so tile finest particles. Unlike candle filters, the packed bed filter is leper, r able to)' Lils:lla? ding an] replacing tile packing material The filter packing is sacrificial, but may be regenerated off-line. For continuity of operation a plurality of filters may be connected in parallel with flow switching to permit bed replacement without flow interruption.

The invention provides also a plant for removing mercury from natural gas liquids, comprising in order: a packed bed filter; [optionally] a candle filter; and at least one solid-contact zone removing mercury by sorption.

The plant may include also distillation columns effective to fractionate the product of filtration or of one or more such solid-contact zones. Preferred forms of the plant correspond to the process features described hereinafter.

The packed-bed filter is suitably a deep filter, i e having a length-todiameter ratio much greater than that of a candle filter, for example at least 0.1 especially 0.5 to 10, and typically operated at 5 to 5000 liquid hourly space velocity. Within this range a rate is chosen depending on the concentration of particles to be removed and intended process life of each charge of filter medium. The filter vessel may evidently be large in relation to the other parts of the mercury removal plant; however, this is acceptable in view of its simplicity.

A preferred filter packing comprises particles, referred hereinafter as 'granules', that are much larger than the particles to be removed from the feedstock. The term 'grranule' refers to any method of formation and does not limit to formation by granulation processes. The granules are suitably in the diameter range I to 10, especially 3 to 10, e g 1.5 to 6, mm. The granules preferably have a relatively narrow size-distribution, to limit the extent to which small granules block the flow passages between larger granules. Thus also the granules should be mechanically strong, to limit such blockage by fragments of larger granules and to limit dovstrearn carry-o;er of granules. They are preferably classified or in repetitionproduced shapes, such as compressed cylinders, possibly with enhanced external surface.

The granules may be organic, for example powdered and classified carbon or NGL- inert polymer such as HOPE, PVC, nylon or polyester. Preferably they are inorganic, for example oxides such as alumina, silica, alumina-silica and ceramics generally, suitably as sand, cylinders, rings, honeycomb pieces, foam pieces or spheres. Inorganic bed-particles may be metallic, s' ch as wire mesh, turnings, classified filings, rings or sponge pieces. Iron granules in metal or oxide form, possibly sulfided, are suitable. Each bed may, if desired, comprise a succession of parts differing in filtration characteristics.

Since the granules may take up mercury from the feedstock, though in an amount that may be insignia cant relative to purification of the feedstock, there is potentially a problem in disposal of spent granules. Consequently the above-mentioned metallic (which term includes oxide andlor sulk de) granules are to be preferred because they can be dealt with by mercury handling techniques in use in metal smelting industry. In particular the pre-treatment bed comprises oxides, possibly sulfided, of copper and/or zinc and, when spent, can be delivered to an ore-smelting operator, who will smelt it along with natural ore which commonly contains mercury impurity.

The granule material may be similar to sorbent material. However, since it is not required to provide high mercury removal capacity and is to contact raw hydrocarbon feedstock possibly containing water and amines, it should be formulated for resistance thereto, by decreased content of transition metal as compared with the mercury sorbent. As an example, the packing material may comprise about equal oxides of copper and zinc and over 20% alumina, e g 30-40 % each of copper oxide and zinc oxide, with 20-40% of alumina, in the form of compressed cylinders 5. mm diameter, 3. 5 mm height. To remove any dust formed in the filter bed, a post-filter may be used between the filter vessel and the next following stage.

The packed-bed filter may be used in conjunction with other purifications, especially a preceding subjection to a magnetic field effective to separate andlor agglomerate ef paiticles of iron or iron oxide. Sucl, agglomerated particles are efficiently collected in the Alter. It appears to be sufficient to feed the product of the packed bed to mercury removal direct Of via a coalesces.

The invention is illustrated by the accompanying drawings, in which: Figure] shows pacl;e1-bed contacting according to the invention; Figure 2 shows a combination with a high temperature mercury removal stage followed by a low tempt erasure stage; Figure 3 shows a combination with a low temperature mercury removal stage followed by a high temperature stage, Figure 4 shows integration of tI-e fig. 2 process into a condensate treatment system producing or more fractions possibly differing, in level of mercury depletion, In this specie cation percentage compositions of solids are by sveinht as oxides stable to ambient atmosphere, unless ot'ner\vise stated. The term 'sorbent' includes adsorbent and alsorlent. The tempt 'mercury' includes '.h free metal and compounds thereof.

4 - - The mercury removal sorbent receiving the product of the packed-bed contacting

may be for example:

1. sulfur-impregnated active carbon; 2. sulfur in an active state as metal polysulfide and/or sulfide of metal in one or more of its higher valency states, possibly with elemental sulfur. The active material of the sorbent is preferably based on at least one transition metal from groups ID, IIIA to VIIA and VTIT of the Periodic Table shown in Pure and Applied Chemistry (1971) vol 28, page I l quoted in the European Patents Handbook Part I Chapter 6. 16. 4 ( 1983).

3. solid sorbent comprising an organic sulfur compound and/or a metal capable of amalgamation with mercury. This process preferably comprises the further steps of replacing the spent sorbent and recovering the mercury and, if applicable, amalgam, from it. Such recovery may be carried out by the process operator's organisation or, more conveniently, by a firm specializing in such recovery.

The sulfur compound-containing sorbent is characterized as follows: (i) a high surface area support, suitably 50-500, e g 100-300, magi; (ii) sulfur compound exerting low vapour pressure at the temperature at which the process is to be operated, by virtue of high boiling point or strong physical or chemical sorption.

The support is preferably silica, for example silica gel or pelleted kieselguhr or ruined silica I he sorbent is preferably dehydrated before applying the sulfur compound to it. The sulfur compound is suitably a thiol, for example an alkylthiol having l-l: carbon atoms, or an arylthiol such as a thiophenol. The sulfur compound may be a thioamide, for example thiourea or thioacetamide, especially if capable of forming a thiol-imide structure. Polythiols may be used.

The metal-containing sorbent may include a support similar to that of the sulfur compound or using metal, as in fluid mixing elements. Metal may be applied to the support by for example vapour-phase deposition, impregnation with pre-formed colloidal metal or impregnation with a solution or suspension of a compound, followed by metal-formation by heating or chemical reduction. Gold and silver appear to be very elective as the metal. they may be impregnated in plating conditions.

Typically the content of gold is in the lower part of the range specified below.

In any of these processes the sorbent may lye present in a fixed random or monolithic bed or a movable e fluidsed bed.

Two classes of sorbent appear to be relevant to processes 2 and 3. The sorbent may be a 'supported sorbent' which typically contain 0.1-40, for example 5-40, % of active materials, balance support material. The metal(poly) sulfide sorbent is a 'rich' sorbent, typically containing 4095, for example 40-70, % of active material, balance support material.

The active material of rich sorbent may comprise other sulfides and/or sulfdable oxides such as zinc oxide The support material may comprise one or more oxides usable as catalyst supports, for example alumina, silica, titania, zirconia and chromia, free or in combinations such as aluminosilicates (e.g. clays or zeolites) and hydraulic cement. The active material of such optional supported sorbent may be for example the sulfided oxide of nickel and/or cobalt (2-10%), or molybdenum (5-20%), or a mixture ofthese, the support oxide is suitably alumina. The active material (calculated as oxides) of rich sorbents is exemplified by 55- 65% of copper oxide CuO with 20-30% of zinc oxide, with up to 20%, on the copper oxide and zinc oxide, of alumina as support oxide.The surface area of rich sorbents is typically in the range 20-250m2g.

The conditions of operation of the mercury removal processes are typically pressure 1-120 bar abs; residence time 2-20 see for gas or 2-20 min for liquid.

temperature 0-300C, especially 10-200C.

T he purification is preferably operated in two or more stages differing in temperature within those ranges, typically 0-50C for the lowtemperature (LT) stage and 80-200C for the high temperature (HT) stage. Preferably the HT stage precedes the LT stage, to effect decomposition of any organo-mercury compounds ahead of final sorption of mercury.

A plurality of reactors may be disposed in parallel with switching valves to permit purification to continue in one reactor while another is being discharged and recharged. Each reactor may be subdivided or duplicated with piping connections for operation on a lead/lag basis, that is, may comprise two series-connected interchangeable independently rechargeable parts, with by-passes and switching valves to permit mercry-rich fresh hydrocarbon to be fed to partly-spent sorbent before contacting fresh sorbent.

The mercury removal preferably comprises also arsenic removal, for example by contacting with lead oxide, for example 20-30 /c (as PbO) oil alumina.

The feedstocl; in contact levity the sorbent can lee gaseous or liquid, for example vaporised or vaporisable liquid or vapourAiquid mixture, depending on pressure and temperature of operation.

UGLY consists of higher hydrocarbons normally present in natural gas obtained from gas wells. These higher hydrocarbons in the gas have to be removed prior to liquefaction of the gas or feeding to long gas pipeline. The separated liquid higher hydrocarbons, known also as 'condensate' are similar to light crude oil and are distilled in a series of columns to produce various fractions, in particular LEG, light naphtha, heavy naphtha, kerosene, gas oil and fuel oil. The pre-treatment can be integrated as described below into the condensate treatment process to remove mercury present regularly or occasionally.

In a particular process mercury removal is operated in combination with the stages of pre-cut, debutanisation and condensate splitting, to produce at least one of those fractions.

substantially free of mercury.

The feedstock may contain enough sulfur compound to load and maintain the solid sorbent from an initial state. If desired, the sorbent may be preloaded, for example by contacting with sulfur compound. If the feedstock has previously been strongly desulfurised, a feed of sulfur compound may be made continuously or intermittently to maintain the sorbent.

The feedstock in contact with the sorbent is preferably substantially water-free, typically having a water dew-point under 4C. It is also preferably substantially free of hydrogen. If hydrogen is to be present, it is conveniently introduced downstream of the particle-removing filter, especially at the inlet of one or both of the sorbers.

The capacity for mercury ol the sorbent per unit volume can be greater than that of the activated carbon. In addition, the volume of sorbent exposed to the feedstock is preferably greater, e g by a factor of 2 to 4, than that of the carbon. Consequently, at a convenient reactor volume, the sorbent need be changed less often than when using carbon alone.

In the processes of the following specific description the pressure is high enough to lamp the NGL liquid for the pre-treatment stages and to effect flow through the sorption and distillation stages.

Referring to figure 1 of the drawings, a pre-treatment system comprises a magnetic agglomerator 1 in which fine particles of iron metal and oxide in raw NGL are brought to a larger particle size. The so-treated NGL is fed at 3 to deep-bed filter vessel 2 having wide- bore top and bOOlTi closures for convenience in chargingldischarging filter medium granules.

The. filter medium granules form bed 4 and consist for example of 1.5 mm cylindrical alumina pellets made ty dry-pelleting or extrusion, or of a combination of 30-40 % each of copper oxide and zinc oxide, svth 20-4010 of alumina, in the form oft compressed cylinders 5.3 mm diameter, 3.5 mm height. Bed 4 collects ferrous agglomerates formed in 1 and also non-magnetic fine particles. Vessel 2 has outlet 5 leading treated NGL to coalesces 6. In coalesces 6 the NGL contacts packed bed 7 from which coalesced droplets of de-emulsified water trickle downwards and drain out at 8. The overhead 9 from coalescer 6 is the feed to mercury removal, suitably as described in figures 2 to 4. In favourable conditions a coalescer may be unnecessary. Further removal of fine particles is effected on candle filters (not shown) at the inlet of the plant described below.

Referring to figure 2 of the drawings, a preferred plant for carrying out mercury removal comprises high temperature (HT) reactor 10 typically in the range 80-200C and low temperature (LT) reactor 20 typically in the range 15-50C. Each reactor includes (not shown) a sorbent charging port and discharge port; and each may be connected for lead/lag operation as described above. Each reactor has respectively feedstock inlet 12, 22 and product outlet 14,24. For reactor 10 inlet 12 is fed with fluid that has entered at 28 from outlet 9 of figure 1, has been warned in feed/effluent heat exchanger 16 and has been brought to reaction temperature in heater 18 heated by steam or combustion gases. Reactor 10 contains a bed of sorbent to be described. For reactor 20 inlet 22 is fed with fluid that has been brought to reaction temperature in water-cooled heat exchanger 26. Reactor 20 contains upper and lower sorbent beds to be described.

In HT reactor 10 and I,T reactor 20 upper bed 32 the sorbent 30 is for example 55- 65 7o of copper oxide CuO with 20-30% of zinc oxide, with up to 20%, on the copper oxide at,] zinc oxide, of alumina as support oxide. In 1,T reactor 20 the lower bed 34 is charged with alumina-supported lead oxide.

The plant functions as follows. Feedstock, typically front gross purification as in the process of figure] or from activated carbon treatment, enters at 28, is warmed in feedleffluent heat exchanger 16, heated further at 18 and fed into IiT reactor 10 at 12. In bed mercury and volatile compounds thereof are immobilized, apparently by conversion to raercuric sulfide under the catalytic effect of the sulOded sorbent. The product of PIT reactor passes through the hot side of heat exchanger 16 and is cooled at 26 to the inlet temperature of LT reactor 20, which it enters at 22. In bed 32 further traces, if any, of mercury are remos ed Ir' bed 34 any traces of arsenic are removed. The product of 1,T reactor passes out at 24 to a user. Figure 4 shows the process of fig.2 as applied to condensate treatment with appropriate minor modifications.

Referring to figure 3 of the drawings, a preferred plant for carrying out the process comprises LT reactor 110 and HT reactor 120. Each reactor includes (not shown) a sorbent charging port and discharge port; and each may be subdivided and connected for lead/lag operation as described above. Each reactor has respectively feedstock inlet 112, 122 and product outlet 114,124. For LT reactor 110 inlet 112 is fed with fluid from a pretreatment as in figure 1 that has entered at 128 at ambient temperature. LT reactor 110 contains an upper and a lower bed of sorbent to be described. For HT reactor 120 inlet 122 is fed with fluid from 114 that has been warmed in feed/effluent heat exchanger 116 and brought to reaction temperature in exchanger 126 heated by steam or combustion gases. Hl reactor 120 contains a sorbent bed to be described.

In LT reactor 110 the upper bed 132 contains pre-sulfided 55-65% of copper oxide CuO and 20-30% of zinc oxide, with up to 20%, on the copper oxide and zinc oxide, of alumina as support oxide, and the lower bed 134 is charged with alumina-supported lead oxide In HT reactor 120 the sorbent 130 is for example the same as in the LT reactor.

The plant functions as follows. Feedstock from 9 in figure I enters at 128, passes into 1,T reactor 110 at 112 and is freed of low-temperature-active mercury in bed 132, and of arsenic compounds in bed 134. The product, containing any residual mercury compounds, is warmed by passing through the cold side of feed/effluent heat exchanger 11.6, then heated at 126 to the inlet temperature of HT reactor 120, which it enters at 122. In bed 130 the mercury and volatile compounds thereof remaining after bed 120 are immobilised, apparently by conversion to mercuric sulfide or amalgam. The product of HT reactor 120 passes out at 124 to a riser Refen ing to fig.4 showing the process of fig.2 as applied to NGL treatment, HT reactor 10 feeds direct to the first condensate distillation column 340 ('precut column') without heat exchanger 16. The overhead of column 340 is fed to debutaniser column 342, in which it is resolved into overhead LPG 344 and bottoms 346.1,PG 344 is passed out to a user accepting single- stage racrcuTy removal. Bottoms 346 is cool and is fed to (first) LT reactor without heist exchanger 26. The product of (first) LT reactor 20 is a light naphtha substantially free of mercury. The bottoms of column 340 is fed to condensate splitter column 35O, In which it is resolved into overhead 359 and bottoms 358. Overhead 352 is cooled in heat exchanger 26 and fed to (second) LT reactor 20, to give a heavy naphtha substantially free of mercury. Bottoms 358 is a fuel oil passed out to a user accepting single-stage mercury removal or to further mercury remove].

The process of fig. 3 can be integrated analogously into a process scheme.

Claims (31)

1. A process of producing a mercury-depleted hydrocarbon stream from a natural gas liquids feedstock by sorption and/or reaction of mercury on a solid, characterized by, upstream of such sorption andfor reaction, removing fine particles from the feedstock in liquid forth by contacting with a packed-bed filter.

2. A process according to claim 1 in which the said fine particles are of size at least 2, especially at least 5, and up to 10 microns or greater.

3. A process according to claim I or claim 2 in which the packed-bed filter is additional to and upstream of candle filters, thus limiting their duty to the finest particles.

4. A process according to any one of the preceding claims in which the packed-bed filter has a length-to-diameter ratio 0.5 to 10.

5. A process according to any one of the preceding claims in which the packed-bed filter comprises granules in the diameter range 3 to lOmm.

6. A process according to claim 5 in which the granules are repetitionproduced shapes.

7. A process according to any one of the preceding claims in which the granules are formed of alumina, silica, alumina-silica and ceramics generally as sand, cylinders, rings, honeycomb pieces, foam pieces or spheres.

8. A process according to any one of claims 1 to 7 in which the granules comprise oxides, possibly sullided, and this material, when spent, is discharged and delivered to an ore smeiting operator for smelting along with natural ore with mercury removal from off-gas.

0. A process according to claim 8 in which the granules comprise oxides, possibly sulfided, of copper andlor zinc.

10. A process according to claim g in which the granules comprise 30-40 % each of copper oxide and zinc oxide, with 20-40% ot alumina, in the form of compressed cylinders 5.3 mm diameter, 3.> norm height.

I] . A process according to any one of the preceding claims in which the feedstock is subjected also to a rr;agnetic field effective to separate andlor agglomerate particles of iron andior iron oxide contained in it.

12. process according to any one of the preceding claims in which the product of the paced fled filter is fad to a coalescer and thence to mercury removal.

13 A process according to any one of the preceding claims i', which the product of the iackedbed Illter and, if used, magnetic field and/or coalesces, is fed to mercury removal is ts c oniai tiring. avid one or more of the sorbents: a. sulfur-impregnated active carbon; b. sulfur in an active state as metal polysulfide andlor sulfide of metal in one or more of its higher valency states, possibly with elemental sulfur; and c. solid sorbent comprising an organic sulfur compound and/or a metal capable of amalgamation with mercury.

14. A process according to any one of the preceding claims in which mercury removal is perated in two or more stages differing in temperature, typically at 0-50C and 80-200C.

15. A process according to claim 13 or claim 14 in which the active material of the sorbent is based on at least one transition metal from groups IB, IIIA to VIIA and VIII of the Periodic Table as hereinbefore defined.

16. A process according to claim 15 using a supported sorbent containing 5-40% of active materials and 95-60% support material.

1 7. A process according to claim 15 using a rich sorbent containing at least 40% of active material and up to 60% support material.

18. A process according to claim 15 or claim 16 in which the active material of the supported sorbent comprises 2-10% of the sulfided oxide of nickel andlor cobalt, or 5-20% of the sulfided oxide of molybdenum, or a mixture of these, and the support oxide is alumina.

1 9. A process according to claim 15 or claim 1 7 in which the active material of the rich sorbent is a sulfided combination of 55-65% of copper oxide CuO with 20-30% of zinc oxide, with up to 20%, on the copper oxide and zinc oxide, of alumina as support oxide.

20. A process according to any one of the preceding claims in which the feedstock is contacted with a sorbent according to claim 18 and at least part of the product of said contacting is contacted with further sorbent according to claim 18.

21. A modification of a process according to any one of the preceding claims in which the sorbent used at 10-SOC is or comprises activated carbon that may contain sulfur.

22. A process according to any one of the preceding claims operated in conjunction with one or more of: arsenic removal by contacting with lead oxide 20-30% (as PbO) on alumina; and preliminarily drying the feedstock to a water dew-point under 4C.

23. A process according to any one of the preceding claims in which the feedstock is NGL, said process comprising also the stages of pre-cut, debutanisation and condensate splitting, to produce at least one of the fractions LPG, light naphtha, heavy naphtha, kerosene and fuel oil, substantially free of mercury.

24. A process according to claim 23 which comprises contacting the whole feedstock in one of the sorbent stages, resolving the resulting product into fractions including LPCi, light naphtha, heavy naphtha and fuel oil and contacting at least the light naphtha and heavy naphtha in the second sorption stage.

25. A process according to claim 23 which comprises subjecting the whole feedstock to both sorption stages and resolving the resulting product into fractions including LPG, light naphtha, heavy naphtha, kerosene, gas oil and fuel oil.

26. A process of purifying hydrocarbons, substantially as described with reference to the

foregoing drawings and specific description.

27. Hydrocarbon substantially free of mercury produced by a process according to any one ofthe preceding claims.

28. A process of producing ethylene by thermal cracking of NGL feedstock and cryogenic separation of the product, characterised by subjecting the feedstock to a mercury removal process according to any one of claims 1 to 25.

29. A plant for removing mercury from natural gas liquids, comprising in order: a packed bed filter; [optionally] a candle filter; and at least one solid-contact zone removing mercury by sorption.

30. A plant according to claim 29 including also distillation columns effective to fractionate the product of filtration of one or more such solid-contact zones.

31. A plant according to claim 29 or claim:3) having features corresponding to the process according to any one of claims I to 26.