GB2365874A

GB2365874A - Purifying hydrocarbons

Info

Publication number: GB2365874A
Application number: GB0115107A
Authority: GB
Inventors: Alwyn Pinto; Alan Edward Pemberton Coates
Original assignee: WCP INTERNAT Ltd
Current assignee: WCP INTERNAT Ltd
Priority date: 2000-06-29
Filing date: 2001-06-21
Publication date: 2002-02-27
Anticipated expiration: 2021-06-21
Also published as: GB2365874B; GB0115107D0

Abstract

A process for purifying a hydrocarbon is characterised by removing mercury by contact at over 90C with a solid sorbent comprising sulfided metal. 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 quoted in the European Patents Handbook Part I Chapter 6.16.4 (1983). The process may be operated operated in conjunction with one or more of:<BR> ```arsenic removal by contacting with lead oxide 20-30% (as PbO) on alumina.;<BR> ```purification at 10-50C by contact with copper oxide/zinc oxide or with activated<BR> ```carbon, which may contain sulfur; and<BR> ```drying to a water dew-point under 4C. In an important application the hydrocarbon feedstock is raw or partly purified natural gas and the process is operated in combination with the stages of pre-cut, debutanisation and condensate splitting.

Description

<Desc/Clms Page number 1> PURIFYING HYDROCARBONS THIS INVENTION relates to a process for purifying hydrocarbons.

Hydrocarbons to be used as fuels or process feedstocks commonly contain 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 small 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 on activated carbon, but high capacity and long process runs are difficult to achieve.

ACCORDING TO THE INVENTION a process for purifying a hydrocarbon is characterised by removing mercury by contact at over 90C with a solid sorbent comprising sulfided metal.

The invention will be illustrated by the accompanying drawings of schematic flowsheets, in which: Figure 1 illustrates a process using a high temperature stage followed by a low temperature stage; Figure 2 illustrates a process using a low temperature stage followed by a high temperature stage; Figure 3 shows integration of the fig. 1 process into a condensate treatment system producing fractions at 2 levels of mercury content; and Figure 4 shows integration of the fig. 2 process into a condensate treatment system all product fractions of which are strongly purified from mercury.

In this specification percentage compositions of solids are by weight as oxides stable to ambient atmosphere, unless otherwise stated. The term 'sorbent' includes adsorbent and absorbent. The term 'mercury' includes the free metal and compounds thereof, whichever may be initially present.

The process preferably comprises the further steps of replacing the spent sorbent and recovering the mercury from it. Such recovery may be carried out by the process operator's organisation or, more conveniently, by a firm specialising in such recovery. Since mercury is a

constituent of some metal ores, and is recovered by the ore-smelter, the spent sorbent, if of appropriate composition, may be passed to such a smelter.

The sorbent especially comprises sulfur in an active state as metal polysulfide and/or sulfide of metal in one or more of its higher valency states. It may comprise elemental sulfur. The active material of the sorbent is preferably based on at least one transition metal from groups IB, IIIA to VIIA and VIII of the Periodic Table shown in Pure and Applied Chemistry (1971) vol 28, page 11 quoted in the European Patents Handbook Part I Chapter 6.16.4 (1983). Two classes of sorbent appear to be especially effective. Supported sorbents typically contain 5-40% of active materials, balance support material. Rich sorbents typically contain 40-70% of active material, balance support material. The active material may comprise other sulfides and/or sulfidable 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 supported sorbent may be for example the sulfided oxide of nickel and/or cobalt (2-10%), or molybdenum (5-20%), or a mixture of these; 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 /a of zinc oxide, with up to 20%, on the copper oxide and zinc oxide, of alumina as support oxide.

The surface area of supported sorbent is typically in the range 100-300 mzg-1 for supported sorbents, such as those supported on alumina. For rich sorbents such as copper-zinc, the surface area is typically in the range 20-250m291.

The conditions of operation are typically: pressure 1-120 bar abs; residence time 2-20 sec for gas or 2-20 min for liquid. temperature 90-300C, especially 150-250C; 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 mercury-rich fresh hydrocarbon to be fed to partly-spent sorbent before contacting fresh sorbent.

The process may be operated in combination with one or more other purification steps. These may include gross-impurity removal, for example fractionation, drying and hydrogenative purifications such as desulfurisation. More particularly the other steps may comprise arsenic

removal, for example by contacting with lead oxide, for example 20-30% (as Pb0) on alumina. In another example the process according to the invention is operated in combination with purification at low temperature (up to 90C, especially 10-50C) by contact with copper oxide/zinc oxide or with activated carbon, which may contain sulfur to promote sorption of mercury.

The feedstock treated by the process can be gaseous or liquid, for example vaporised or vaporisable liquid or vapour/liquid mixture, depending on pressure and temperature of operation. Raw or partly purified natural gas feed to liquefaction is an important example, owing to the effect of mercury on the aluminium heat exchangers of the liquefaction plant. Another important feedstock is ethylene cracker raw material such as ethane, NGL, LPG or naphtha, since separation of the cracked product is cryogenic. Such processes in combination with feed pretreatment according to the invention constitute further aspects of the invention.

Higher hydrocarbons are 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 as 'condensate' or 'natural gas liquids' (NGL) are stabilised and sold for further treatment. They are similar to light crude oil and are distilled in a series of columns to produce various fractions, in particular LPG, light naphtha, heavy naphtha, kerosene, gas oil and fuel oil. The process of the invention can be integrated as described below into the condensate treatment process to remove mercury, if present regularly or occasionally.

In a particular process according to the invention the mercury removal is operated in combination with 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, each substantially free of mercury. Further, the invention provides a process of producing ethylene by thermal cracking of hydrocarbon feedstock and cryogenic separation of the product, characterised by subjecting the feedstock to mercury removal as herein described..

The feedstock may contain enough sulfur to sulfide the solid sorbent from an initial oxide state and maintain its sulfide content. If desired, the sorbent may be pre-sulfided, for example by contacting with HZS or ammonium sulfide or sulfur-containing feedstock or an easily decomposed sulfur compound such as carbon disulfide, dimethyl sulfide or dimethyl disulfide. If the feedstock has previously been strongly desulfurised, a feed of sulfur compound may be made continuously or intermittently to maintain the sorbent in an adequately sulfided state.

The feedstock is preferably substantially water-free, typically having a water dew-point under 4C.

The capacity for mercury of the sulfided sorbent per unit volume is greater than that of the activated carbon. In addition, the volume of the sulfided 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.

Referring to figure 1 of the drawings, a preferred plant for carrying out the process comprises high temperature (HT) reactor 10 and low temperature (LT)reactor 20. 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, has been warmed 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 the sorbent (30) is for example nickel molybdate, cobalt molybdate or copper oxide-zinc oxide-alumina, each having been pre-sulfided. In LT reactor 20 the upper bed (32) contains sulfided copper oxide/zinc oxide/alumina and the lower bed 34 is charged with alumina-supported lead oxide.

The plant functions as follows. Feedstock, typically from gross purification or from activated carbon treatment, entering at 28 is warmed in feed/effluent heat exchanger 16, heated further heated at 18 and fed into HT reactor 10 at 12. In bed 30 mercury and volatile compounds thereof are `immobilised, apparently by conversion to mercuric sulfide under the catalytic effect of the sulfided sorbent. The product of HT reactor 10 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 removed. In bed 34 any traces of arsenic are removed. The product of LT reactor 20 passes out at 24 to a user. Figure 3 shows the process of fig. l as applied to condensate treatment with appropriate minor modifications.

Referring to figure 2 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 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. HT reactor 120 contains a sorbent bed to be described.

In LT reactor 110 the upper bed (132) contains sulfided copper oxide/zinc oxide/alumina and the lower bed 134 is charged with alumina-supported lead oxide. In HT reactor 120 the sorbent (130) is for example nickel molybdate, cobalt molybdate or copper oxide-zinc oxide-alumina, each having been pre-sulfided.

The plant functions as follows. Feedstock, typically from gross sulfur removal or activated carbon treatment, entering at 128 passes into LT 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 116, 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 under he catalytic effect of the sulfided sorbent. The product of HT reactor 120 passes out at 124 to a user.

Referring to fig.3 showing the process of fig. l as applied to condensate 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. LPG 344 is passed out to a user accepting single-stage mercury removal. Bottoms 346 is cool and is fed to (first) LT reactor 20 without heat 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 350, in which it is resolved into overhead 352 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. Both LPG 344 and fuel oil 358 are products of HT mercury removal by the process of the invention.

Referring to fig.4 showing the process of fig.2. as applied to condensate treatment, the product of LT reactor 110 is fed via heater 126 to HT reactor 120 without heat exchangers 116 and 118 since the product of HT reactor 120 is required to be hot as feed to precut column 440. The overhead of column 440 is fed to debutaniser column 442, in which it resolved into overhead LPG 444 and light naphtha 446. The bottoms of column 442 is fed to condensate splitter column 450, in which it is resolved into overhead heavy naphtha 452. middle fractions kerosene 454 and gas oil 456, and bottoms fuel oil 458. A11 the products of the process of fig.4 have undergone 2-stage mercury removal.

Claims

CLAIMS 1. A process for purifying a hydrocarbon characterised by removing mercury by contact at over 90C with a solid sorbent comprising sulfided metal.
2. A process according to claim 1 comprising the fiuther steps of replacing the spent sorbent and recovering the mercury from it.
3. A process according to claim 1 or claim 2 in which the sorbent comprises sulfur in an active state as metal polysulfide and/or sulfide of metal in one of its higher valency states.
4. A process according to any one of the preceding claims 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 shown in Pure and Applied Chemistry (197l ) vol 28, page 11 quoted in the European Patents Handbook Part I Chapter 6.16.4 (I983).
5. A process according to claim 4 using a supported sorbent containing 5-40% of active materials, balance support material.
6. A process according to claim 4 using a rich sorbent containing 40-70% of active material, balance support material.
7. A process according to claim 4 or claim 5 in which the active material of the supported sorbent is the sulfided oxide of nickel and/or cobalt (2-10%), or molybdenum (5-20%), or a mixture of these and the support oxide is alumina.
8. A process according to claim 4 or claim 6 in which (calculated as oxides) the active material of the rich sorbent is 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.
9. A process according to any one of the preceding claims in which the conditions of operation are: pressure 1-l20 bar abs; residence time 2-20 sec for gas or 2-20 min for liquid. temperature 90-300, especially 150-250C;
10. 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 Pb0) on alumina.; purification at 10-50C by contact with copper oxide/zinc oxide or with activated carbon that may contain sulfur; and drying to a water dew-point under 4C.
11.A process according to any one of the preceding claims in which the hydrocarbon feedstock is raw or partly purified natural gas.

<Desc/Clms Page number 7>
12.A process according to claim 11 in combination with 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, each substantially free of mercury.
13.A process of producing ethylene by thermal cracking of hydrocarbon 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 10..
14.A process of purifying hydrocarbons, substantially as described with reference to the foregoing drawings and specific description.