WO2006125759A1

WO2006125759A1 - Fischer-tropsch plant

Info

Publication number: WO2006125759A1
Application number: PCT/EP2006/062486
Authority: WO
Inventors: Robert Klein Nagelvoort; Jeroen Harrie Moors; Anthony Malcolm Rigby
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2005-05-23
Filing date: 2006-05-22
Publication date: 2006-11-30

Abstract

This invention relates to a Fischer-Tropsch Plant for producing hydrocarbons and a method of the same. A Fischer-Tropsch plant comprises one or more gasifier, one or more Fischer-Tropsch reactor as well as further downstream reactors and separators. Typically a gaseous hydrocarbonaceous feedstock, especially methane, is supplied directly to the gasifier. The Fischer-Tropsch plant of the present invention has a liquefied hydrocarbonaceous feedstock storage tank, especially a liquefied natural gas storage tank(s), to store liquefied hydrocarbonaceous feedstock storage tank, especially a liquefied natural gas, and feed it to a gasifier should the normal supply of methane to the gasifier fail or reduce significantly. Thus the gasifier and all downstream reactors can be kept operating at increased capacity in the event of a restriction in the normal methane supply. In some cases the additional methane supplied by the storage tank can prevent the plant from being shut down.

Description

FISCHER-TROPSCH PLANT

This invention relates to a Fischer-Tropsch plant. The Fischer-Tropsch process can be used for the conversion of one or more hydrocarbonaceous feedstocks into liquid and/or solid hydrocarbons. The feedstock (e.g. natural gas, associated gas and/or coal-bed methane, residual (crude) oil fractions, biomass or coal) is converted in one ore more gasifiers and/or reformers into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then fed into a reactor where it is converted in a single step over a suitable catalyst, usually an iron or cobalt catalyst, into paraffinic compounds ranging from methane to high molecular weight modules comprising up to 200 carbon atoms, or, under particular circumstances, even more. The hydrocarbons formed in the Fischer-Tropsch reactor proceed to a hydroprocessing unit, especially a hydroisomerisation/ hydrocracking unit or a hydrogenation and/or hydroisomerisation unit, and thereafter generally to a distillation unit. Products obtained by the process are especially middle distillates (kerosene, gasoil) , base oils, solvents, detergent feedstocks and waxes.

At the present moment there is a strong interest in the conversion of gaseous feedstocks, especially natural gas, associated gas and/or coal bed methane, i.e. methane containing feedstock, into liquid products

In the event, however, of a restriction in the methane feedstock to the one or more gasifiers and/or reformers, the gasifier(s) and/or reformer (s) and all other downstream units, including the Fischer-Tropsch reactor (s), the hydrocracking/hydroisomerisation/ hydrogenaton/hydroisomerisation unit and the distillation unit, will be affected and may even need to be shut down, causing a loss in production. Moreover, the procedure for start-up of such a plant can, in itself, be time consuming which exacerbates the loss suffered when the plant is shut down. In addition, a shut down and a startup usually results in flowing (burning) hydrocarbons. According to a first aspect of the present invention there is provided a Fischer-Tropsch plant comprising: at least one gasifier, optionally in combination with a reforming unit, adapted to produce a mixture of gases containing carbon monoxide and hydrogen from methane and oxygen; at least one Fischer-Tropsch reactor, connected to and downstream from the gasifier (s), the Fischer-Tropsch reactor (s) adapted to produce C2+ hydrocarbons from carbon monoxide and hydrogen; at least one storage tank to store liquefied natural gas; a regasification unit between the storage tank(s) and the gasifier (s), the regasification unit adapted to convert liquefied natural gas into the gaseous state before it proceeds to the gasifier (s).

Thus in the event of a failure or disruption in the methane supply, the storage tank(s) can supplement the supply of methane in the form of natural gas to the gasifier (s) to reduce the likelihood of, or prevent, the plant being shut down. If required, the storage tank(s) can optionally supply all the required methane to the gasifier (s) for a limited period of time. In a preferred embodiment the stored liquefied natural gas is used in combination with a lower production capacity of the Fischer-Tropsch plant. In most cases it is possible to run the Fischer-Tropsch plant at 60-80%, especially 70%, of its maximum production capacity. This can be done by running all equipment at a lower throughput. In this situation there is not yet the requirement to shut down any reactor or other part of the equipment. In the case of a relative small problem with the supply of natural gas, i.e. the amount which is delivered is for instance 80 or 90% of the required amount for full production, it is preferred to use the stored liquefied gas is such a way that the full production is maintained. The same holds for a larger disruption which is expected to last only for a short period of time. In that way maximum production is obtained. In the case of a larger supply problem for a longer period, the preferred method is a combination of reduced capacity of the plant (by running all equipment at a lower throughput but without shutting down any reactors or other equipment) in combination with the use of stored liquefied natural gas.

In the case of a very large supply problem over a long period, e.g. no supply of natural gas at all, the Fischer-Tropsch plant will produce at it lowest production capacity while all natural gas is supplied from the storage tanks.

The synthesis gas for a Fischer-Tropsch plant is usually made by (catalytic) partial oxidation, optionally in combination with reforming, especially steam reforming. Usually a partial oxidation reactor is used and/or an autothermal reforming reactor (i.e. combination of partial oxidation followed by steam reforming) . In case of an emergency these reactors are stopped by shutting down the oxygen supply and the fuel supply. As the result of the emergency stop also the production of steam will be interrupted. This may result in an immediate stop of the whole Fischer-Tropsch complex, which usually results in sending large amounts of hydrocarbons to the flare. In those cases, now, where the emergency stop is due to supply problems of the natural gas, the natural gas storage as provided by the present invention makes it possible to stop the gasification units in a gradual way. Such a controlled stop, in which oxygen and fuel supply are slowly decreased and in which also the steam production - obtained by heating/ evaporising water against the hot synthesis gas, optionally followed by superheating the saturated steam - is gradually decreased, usually takes a few hours, e.g. 1 to 8 hours, especially 1 to 4 hours. This may be combined with a gradual reduction of the pressure in the gasification unit. In this way also the other parts of the plant (air separation plant, Fischer-Tropsch reactors, hydroconversion reactors, steam system, fuel supply utilities etc.) can gradually adapt to the new situation. In such a situation there is hardly any need to send certain streams to the flare. Thus, in a preferred embodiment the methane storage of the present invention is used for the gradual stop of the gasification units (PO-reactors, ATR-reactors) , in e.g. 1 to 8 hours, especially 1 to 4 hours. In this gradual stop the supply of fuel and oxygen and optionally steam is continuously decreased or stepwise decreased, e.g. in 5 to 25 small steps, or both.

In addition to the above, in the case of a shut-down of the gasification units (emergency or gradual shut down) , there is a clear advantage to keep the gasification units at a relatively high temperature. Normal temperature during gasification will be between 800 and 1800 ⁰C, usually between 1000 and 1400 ⁰C in the oxidation zone. The restart of a gasification unit can be done much faster when the unit is kept at a relatively high temperature, e.g. between 500 and 800 ⁰C, especially between 600 and 700 ⁰C. This can be done by burning a relatively small amount of methane, together with oxygen from the oxygen storage. The amount of methane is e.g. between 0.1 and 10% of the design amount fuel for gasification, especially 0.2 to 5%. Thus, in a preferred embodiment of the invention, the gasification unit is kept at a hot standby by burning a small amount of methane together with oxygen from the oxygen storage. In this way the period needed for stable running the gasification units is reduced from 2 or 3 days to several hours, e.g. 2-20 hours, especially 4 to 8 hours. Such a restart will also reduce the amount of hydrocarbons and other fuel streams to be sent to the flare.

Preferably the Fischer-Tropsch plant also comprises an air separation unit for providing oxygen from air.

Preferably the storage tank(s) are capable of supplying sufficient methane to the gasifier(s) for a period up till five day, suitably between 20 minutes and 48 hours, preferably between 30 minutes and 24 hours, more preferably between 1 hour and 4 hours. In this specification the term "liquefied natural gas" also comprises liquefied associated gas and/or liquefied coalbed methane. In this specification the term "methane" comprises almost pure methane streams (e.g. 90 vol% pure methane of the total stream, preferably 95 vol%, more preferably 98 vol% pure) as well as natural gas, associated gas, coalbed methane and/or other methane rich streams, which streams usually consist of at least

50 vol% methane, preferably 70%, more preferably 85 vol% of the total stream. During these periods the storage tanks supply all the required methane for full production of the Fischer-

Tropsch plant.

The regasification unit can be powered by, for example, steam and/or electricity. The regasification unit is in general a heat exchange unit, especially an indirect heat exchange unit. These units as such are well known in the fields of liquefaction/transportation/ regasification of natural gas. The heating medium usually is water, air, steam, flue gas streams etc. The regasification unit is an essential part of the invention.

Preferably the storage tank(s) have no more than one user outlet, said one user outlet being to the gasifier(s) via the regasification unit. Preferably the storage tank(s) holds the liquefied natural gas at about ambient pressure, preferably at a pressure of at most 2 bara.

Preferably the storage tank(s) are supplied by a liquefied natural gas plant which is proximate to the Fischer-Tropsch plant. It is observed that the storage tank and the regasification unit are separated units. Optionally a dedicated unit may be provided to condense methane from the normal methane supply. The dedicated unit may utilise condensed nitrogen from the air separation unit to condense the methane.

In a preferred embodiment methane is used which has been produced as a by-product in the Fischer-Tropsch plant. For instance, in the Fischer-Tropsch reactor a few percent methane is formed (based on incoming CO) . Further, in the hydrocracking reactor and the catalytical dewaxing unit methane is formed. The use of "Fischer- Tropsch methane" improves the carbon efficiency of the plant.

The storage tanks for the liquefied natural gas are suitably standard storage tanks for liquefied natural gas. These tanks are commercially available. Suitable tanks are (double wall) stainless steel tanks or concrete tanks provided with inner (thin) stainless steel plates. Regassification is also standard technology, and can for instance be done with steam or water, e.g. sea water. The storage tank is not a separation unit (e.g. a flash vessel) or a distillation unit or an evaporation/ expanding vessel. In the storage unit liquefied natural gas is stored for a certain amount of time until it is needed as feed for the oxidation/reforming units. The liquefied natural gas is then send to the regassifcation unit for conversion into a gaseous product for use in the oxidation/reforming unit.

Processes for the liquefaction of natural gas are well known. In this respect reference is made to for example GB 1,572,899, US 4,504,296, US 4,545,795, US 4,456,459, US 3,203,191, EP 834,046 and WO 97/32172. The natural gas to be liquefied is preferably purified before liquefaction. In most cases there is a need to purify the gaseous hydrocarbon feedstock before use in the gas to liquid plant. Especially sulphur containing compounds need to be removed. Usually an aqueous amine washing unit is used to remove hydrogen sulphide and optionally carbon dioxide. Mercaptans and other organic compounds may be removed by hydrogenation and absorption of the hydrogen sulphide formed, for instance with iron oxide or zinc oxide beds, or by the use of mol sieve units. Carbonyl sulphide may be removed by hydrolysis and absorption of the H2S formed. Also cryogenic techniques may be used to remove sulphur compounds. These cryogenic techniques may even be used to remove nitrogen and/or noble gasses. Guard beds may be used to remove small remaining amounts of sulphur compounds (iron and/or zinc oxide beds) . Larger organic compounds may be removed by adsorption on active carbon beds.

In a preferred embodiment the gaseous hydrocarbon source is purified in the same way and up till the same maximum impurity specification as the natural gas which is (without storage) directly fed to the gasifiers. In a more preferred embodiment it is done at the same time and in the same equipment as the direct natural as stream. Suitably the sulphur content in the stored liquefied natural gas is at most 10 ppmw, preferably at most 5 ppmw, more preferably at most 1 ppmw. According to a second aspect of the present invention there is provided a method of synthesising hydrocarbons from methane, the method comprising: providing a Fischer-Tropsch plant as described herein; feeding methane, at least in part, from the storage tank(s) to the gasifier(s) via the regasification unit; feeding oxygen into the gasifier(s); the gasifier(s) producing a mixture of gases containing carbon monoxide and hydrogen from the methane and the oxygen; feeding the so obtained carbon monoxide and hydrogen into one or more Fischer-Tropsch reactors to synthesise the hydrocarbons. This method is suitably followed by further treatment of the hydrocarbons made. This treatment, beside distillation, comprises hydrotreating, usually hydrogenation (to remove olefins and/or oxygenates) , hydroisomerisaton, hydrocracking and catalytic dewaxing. The products thus obtained n-paraffins (e.g. LDF, HDF, solvents), fuels as naphtha, kero and gasoil, i-paraffins (e.g. drilling fluids, solvents), base oil and waxes.

Optionally under normal operating conditions the methane is fed into the storage tank(s) from the normal methane source.

Typically when the normal methane source is below 70%, especially below 60% of its capacity and the methane supply pressure falls below the normal operating pressure, the storage tank(s) feed methane to the gasifier (s) .

The gasifier (s) to be used in the present invention are suitably partial oxidation gasifier(s), well known in the art. Optionally catalytic partial oxidation gasifier may be used. The gasifier (s) may be used in combination with a reforming unit, for example an autothermal reforming unit or a corrective steam reforming unit.

In use, a small proportion, for example up to 5%, of the methane from the normal methane source may be diverted to the storage tank(s) for storage until the storage tank is full.

Thus the invention also provides a method of synthesising hydrocarbons from methane, the method comprising: providing a Fischer-Tropsch plant as described herein; wherein methane is supplied to the gasifier (s), optionally in combination with a reforming unit, and to the storage tank(s); feeding oxygen into the gasifier (s) /reforming unit; producing a mixture of gases containing carbon monoxide and hydrogen from the methane and the oxygen; feeding the so obtained carbon monoxide and hydrogen into one or more Fischer-Tropsch reactors to produce the hydrocarbons . The storage tank(s) may also be filled with methane when the Fischer-Tropsch reactor and hence the gasifier (s) are not in use, for example, during regeneration of a catalyst.

Typically the Fischer-Tropsch plant comprises a hydrocracking unit connectable to and downstream of the Fischer-Tropsch reactor.

A hydrogenation unit may be provided between the Fischer-Tropsch reactor and the hydrocracking unit.

The Fischer-Tropsch synthesis is well known to those skilled in the art and involves synthesis of hydrocarbons from a gaseous mixture of hydrogen and carbon monoxide, by contacting that mixture at reaction conditions with a Fischer-Tropsch catalyst.

Products of the Fischer-Tropsch synthesis may range from methane to heavy paraffinic waxes. Preferably, the production of methane is minimised and a substantial portion of the hydrocarbons produced have a carbon chain length of a least 5 carbon atoms. Preferably, the amount of C5+ hydrocarbons is at least 60% by weight of the total product, more preferably, at least 70% by weight, even more preferably, at least 80% by weight, most preferably at least 85% by weight. Reaction products which are liquid phase under reaction conditions may be separated and removed using suitable means, such as one or more filters. Internal or external filters, or a combination of both, may be employed. Gas phase products such as light hydrocarbons and water may be removed using suitable means known to the person skilled in the art.

Fischer-Tropsch catalysts are known in the art, and frequently comprise, as the catalytically active component, a metal from Group VIII of the periodic table. (References herein to the Periodic Table relate to the previous IUPAC version of the Periodic Table of Elements such as that described in the 68th Edition of the Handbook of Chemistry and Physics (CPC Press) ) . Particular catalytically active metals include ruthenium, iron, cobalt and nickel. Cobalt is a preferred catalytically active metal. Typically, the catalysts comprise a catalyst carrier. The catalyst carrier is preferably porous, such as a porous inorganic refractory oxide, more preferably alumina, silica, titania, zirconia or mixtures thereof. The optimum amount of catalytically active metal present on the carrier depends inter alia on the specific catalytically active metal. Typically, the amount of cobalt present in the catalyst may range from 1 to 100 parts by weight per 100 parts by weight of carrier material, preferably from 10 to 50 parts by weight per 100 parts by weight of carrier material.

The catalytically active metal may be present in the catalyst together with one or more metal promoters or co- catalysts. The promoters may be present as metals or as the metal oxide, depending upon the particular promoter concerned. Suitable promoters include oxides of metals from Groups HA, IHB, IVB, VB, VIB and/or VIIB of the Periodic Table, oxides of the lanthanides and/or the actinides. Preferably, the catalyst comprises at least one of an element in Group IVB, VB and/or VIIB of the Periodic Table, in particular titanium, zirconium, manganese and/or vanadium. As an alternative or in addition to the metal oxide promoter, the catalyst may comprise a metal promoter selected from Groups VIIB and/or VIII of the Periodic Table. Preferred metal promoters include rhenium, platinum and palladium.

A most suitable catalyst comprises cobalt as the catalytically active metal and zirconium as a promoter. Another most suitable catalyst comprises cobalt as the catalytically active metal and manganese and/or vanadium as a promoter.

The promoter, if present in the catalyst, is typically present in an amount of from 0.1 to 60 parts by weight per 100 parts by weight of carrier material. It will however be appreciated that the optimum amount of promoter may vary for the respective elements which act as promoter. The Fischer-Tropsch synthesis is preferably carried out at a temperature in the range from 125 to 350 ⁰C, more preferably 175 to 275 ⁰C, most preferably 200 to 260 °C. The pressure preferably ranges from 5 to 150 bar abs., more preferably from 5 to 80 bar abs . Hydrogen and carbon monoxide (synthesis gas) is typically fed to the reactor at a molar ratio in the range from 0.4 to 2.5. Preferably, the hydrogen to carbon monoxide molar ratio is in the range from 1.0 to 2.5.

The Fischer-Tropsch synthesis can be carried out in a slurry phase regime or an ebullating bed regime, wherein the catalyst particles are kept in suspension by an upward superficial gas and/or liquid velocity. Another regime for carrying out the Fischer-Tropsch reaction is a fixed bed regime, especially a trickle flow regime. A very suitable reactor is a multitubular fixed bed reactor. An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawing, in which:

Fig. 1 shows the interconnections between various reactors in a Fischer-Tropsch Plant in accordance with the present invention.

An air separation unit (ASU) 10 receives air from the atmosphere and separates the oxygen out by distillation. Downstream of the ASU 10 is a gasifier 12 which receives the oxygen. A methane supply 13 is connected to gasifier 12. The gasifier 12 produces a mixture of carbon monoxide and hydrogen from the oxygen and methane.

The methane supply 13 is also connected to a liquefied natural gas (LNG) storage tank 20 and can supply methane thereto which is condensed by a condenser (not shown) . The LNG storage tank 20 is in turn connected to a methane regasifier 22 which can reconvert LNG to gaseous methane. The methane regasifier 22 is connected to the gasifier 12. Thus methane can be supplied to the gasifier 12 either from the methane supply 13 directly or via the LNG storage tank 20 and methane regasifier 22 or a combination thereof.

In a preferred embodiment the storage tank 20 is connected to a dedicated LNG production plant (not shown) and receives LNG therefrom. This obviates the requirement to provide a condenser at the storage tank 20. Downstream of the gasifier 12, is a known configuration of a Fischer-Tropsch plant. The gasifier 12 supplies synthesis gas to a Fischer-Tropsch (FT) reactor 14. Hydrocarbons are formed in the FT reactor 14 which are processed further downstream in a hydrocracking reactor 16 and separated in a distillation reactor 18. Under optimal operating conditions the methane supply 13 supplies methane to the gasifier 12 directly. A small amount of methane, preferably up to 5% of the total methane supplied to the plant, may be diverted to the LNG storage tank 20 for storage until the LNG storage tank 20 is full.

In the event of a significant restriction or even stoppage in supply of methane from the methane supply 13, methane from the LNG storage tank 20 can be gasified in the methane regasifier 22 and fed to the gasifier 12, thereby providing sufficient methane for the gasifier 12 to operate. This can reduce the likelihood or prevent the gasifier 12 and all other downstream units being affected, especially shut down.

Thus the present invention can increase the production of a plant or even maintain plant production in situations where previously a shut-down may have occurred.

In most cases a Fischer-Tropsch plant will also comprise a hydrogen production facility. The hydrogen may be used for e.g. correcting the hydrogen/CO ratio of the synthesis gas for use in the Fischer-Tropsch reactors, for hydrogenation, hydroisomerisation, hydrocracking and hydrofinishing reactions, as well as for catalytical dewaxing. Further cases are the activation and/or regeneration of the catalysts, especially the Fischer- Tropsch catalysts and for desulfurisation of e.g. condensates. The hydrogen is usually made by means of steam methane reforming. In the same way as above has been described for the production of synthesis gas, the present application also covers the use of stored liquefied natural gas for the production of hydrogen from methane via a steam methane reforming process, where applicable, the same preferred embodiments apply as described above.

Claims

C L A I M S

1. A Fischer-Tropsch plant comprising: at least one gasifier, optionally in combination with a reforming unit, adapted to produce a mixture of gases containing carbon monoxide and hydrogen from methane and oxygen; at least one Fischer-Tropsch reactor, connected to and downstream from the gasifier (s), the Fischer-Tropsch reactor adapted to produce C2+ hydrocarbons from carbon monoxide and hydrogen; at least one storage tank to store liquefied natural gas; a regasification unit downstream of the storage tank(s) and upstream of the gasifier (s), the regasification unit adapted to convert liquefied natural gas into the gaseous state before it proceeds to the gasifier (s) .

2. A Fischer-Tropsch plant as claimed in claim 1, wherein the storage tank(s) are capable of supplying sufficient methane to the gasifier (s) for between 20 minutes and 48 hours, preferably between 30 minutes and 24 hours, more preferably between 1 hour and 4 hours.

3. A Fischer-Tropsch plant as claimed in claim 1 or claim 2, wherein the storage tank(s) have no more than one user outlet, said one user outlet to the gasifier (s) via the regasification unit.

4. A Fischer-Tropsch plant as claimed in any preceding claim, wherein the storage tank(s) are adapted to hold the liquefied natural gas at a pressure of at least 2 bara .

5. A Fischer-Tropsch plant as claimed in any preceding claim comprising an air separation unit for providing oxygen from air.

6. A method of synthesising hydrocarbons from methane, the method comprising: providing a Fischer-Tropsch plant as claimed in any of claims 1 to 5; feeding methane, at least in part, from the storage tank(s) to the gasifier(s), optionally in combination with a reforming unit, via the regasification unit; feeding oxygen into the gasifier (s) /reforming unit; the gasifier (s) producing a mixture of gases containing carbon monoxide and hydrogen from the methane and the oxygen; feeding the so obtained carbon monoxide and hydrogen into one or more Fischer-Tropsch reactors to synthesise the hydrocarbons, the method optionally followed by hydrotreating the Fischer-Tropsch hydrocarbons into, especially, n-paraffins, fuels as naphtha, kero and gasoil, i-paraffins, base oil and waxes.

7. A method as claimed in claim 6, wherein the storage tank(s) are supplied with liquefied natural gas by a liquefied natural gas production plant proximate to the Fischer-Tropsch plant.

8. A method as claimed in claim 6, wherein a dedicated unit condenses methane from a normal methane supply, preferably wherein the dedicated unit utilises, in part at least, condensed nitrogen from an air separation unit to condense the methane.

9. A method of synthesising hydrocarbons from methane, the method comprising: providing a Fischer-Tropsch plant as claimed in any one of claims 1 to 5; wherein methane is supplied to the gasifier(s) and to the storage tank(s); feeding oxygen into the gasifier (s) ; producing a mixture of gases containing carbon monoxide and hydrogen from the methane and the oxygen; feeding the so obtained carbon monoxide and hydrogen into one or more Fischer-Tropsch reactors to produce the hydrocarbons .

10. A hydrocarbon synthesised by a method as claimed in claim 9 or claim 10, optionally after hydroprocessing, preferably hydrogenation, hydroisomerisation and/or hydrocracking, preferably wherein the hydrocarbon is a fuel, preferably naphtha, kero or gasoil, a waxy raffinate or a base oil.