PROCESS FOR SYNTHESISING HYDROCARBONS

Abstract

A process for synthesising hydrocarbons is described comprising the steps of (a) making a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide from a feedstock in a synthesis gas generation unit, (b) removing carbon dioxide to produce a carbon dioxide stream and purified synthesis gas comprising hydrogen and carbon monoxide for synthesis gas in a Fischer-Tropsch hydrocarbon synthesis unit wherein (i) at least a portion of the FT water stream is fed to an electrolysis unit to provide an oxygen stream, which is fed to the synthesis gas generation unit. Carbon dioxide stream recovered from the carbon dioxide removal unit and a portion of the hydrogen stream produced by the electrolysis unit are fed to a reverse water-gas shift unit to produce a carbon monoxide stream, with carbon monoxide stream from the reverse water-gas shift unit fed to the Fischer-Tropsch hydrocarbon synthesis unit.

Claims

1. A process for synthesising hydrocarbons comprising the steps of (a) making a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide from a feedstock in a synthesis gas generation unit, (b) removing carbon dioxide from the synthesis gas in a carbon dioxide removal unit to produce a carbon dioxide stream and purified synthesis gas comprising hydrogen and carbon monoxide, and (c) synthesising a mixture of hydrocarbons from the purified synthesis gas in a Fischer-Tropsch hydrocarbon synthesis unit, with co-production of a FT water stream, wherein (i) at least a portion of the FT water stream is fed to an electrolysis unit to provide an oxygen stream, which is fed to the synthesis gas generation unit, and a hydrogen stream, (ii) at least a portion of the carbon dioxide stream recovered from the carbon dioxide removal unit and a portion of the hydrogen stream produced by the electrolysis unit are fed to a reverse water-gas shift unit to produce a carbon monoxide stream, and (iii) at least a portion of the carbon monoxide stream from the reverse water-gas shift unit is fed to the Fischer-Tropsch hydrocarbon synthesis unit.

2. A process according to claim 1, wherein the feedstock comprises natural gas, associated gas, coal, biomass or municipal solid waste or equivalent containing non-biogenic carbon.

3. A process according to claim 2, wherein the feedstock is natural gas and the synthesis gas generation unit comprises a catalytic partial oxidation unit, a non-catalytic partial oxidation unit or an autothermal reformer.

4. A process according to claim 2, wherein the feedstock is coal, biomass or municipal solid waste or equivalent containing non-biogenic carbon and the synthesis gas generation unit comprises a gasifier, optionally with one or more downstream processing units selected from a partial oxidation unit, a tar reforming unit and purification reactors containing a purification material.

5. A process according to claim 1, wherein the carbon dioxide removal unit comprises a physical wash system or a reactive wash system.

6. A process according to claim 1, wherein the Fischer-Tropsch hydrocarbon synthesis unit comprises a tubular reactor in which catalyst carriers containing a Fischer-Tropsch catalyst are disposed within one or more tubes cooled by a cooling medium.

7. A process according to claim 1, further comprising a step (d) of upgrading the mixture of hydrocarbons synthesised in the Fischer-Tropsch hydrocarbon synthesis unit in a hydrotreating unit to produce hydrocarbon products.

8. A process according to claim 7, wherein the hydrotreating unit comprises one or more vessels containing a catalyst selected from a hydroisomerization catalyst, a hydrogenation catalyst, a hydrodeoxygenation catalyst, and/or a hydrocracking catalyst.

9. A process according to claim 7, wherein a portion of the hydrogen stream from the electrolysis unit is fed to the hydrotreating unit.

10. A process according to claim 1, wherein a water stream produced by the reverse water gas shift unit is fed to the electrolysis unit.

11. A process according to claim 1, wherein a portion of the hydrogen stream from the electrolysis unit is fed to the Fischer-Tropsch hydrocarbon synthesis unit.

12. A process according to claim 1, wherein an oxygen stream provided by the electrolysis unit is used to combust a portion of a feed gas comprising carbon dioxide and hydrogen fed to the reverse water-gas shift unit to raise the temperature of the feed gas.

13. A process according to claim 1, wherein water formed in the reverse water-gas shift unit is fed to the electrolysis unit.

14. A process according to claim 1, wherein a tail gas comprising one or more of methane, ethane, propane, butane and C5-C10 hydrocarbons, is recovered from the Fischer-Tropsch hydrocarbon synthesis unit, and is fed to the synthesis gas generation unit.

15. A process according to claim 1, wherein a tail gas comprising one or more of methane, ethane, propane, butane and C5-C10 hydrocarbons, is recovered from the Fischer-Tropsch hydrocarbon synthesis unit, and is subjected to a separate reforming step to form a reformed tail gas containing hydrogen, which is fed to the Fischer-Tropsch hydrocarbon synthesis unit and/or the reverse water-gas shift unit.

16. A process according to claim 1, wherein the hydrocarbon products recovered from the hydrotreating unit are fed to a separation unit to recover C1-C4 gases, a naphtha fraction, at least one kerosene and/or gas oil fraction and a heavy fraction.

17. A system for performing the process according to claim 1 comprising (a) a synthesis gas generation unit for making a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide from a feedstock, (b) a carbon dioxide removal unit coupled to the synthesis gas generation unit for removing carbon dioxide from the synthesis gas to produce a carbon dioxide stream and purified synthesis gas comprising hydrogen and carbon monoxide, and (c) a Fischer-Tropsch hydrocarbon synthesis unit coupled to the carbon dioxide removal unit for synthesising a mixture of hydrocarbons from the purified synthesis gas, with co-production of a FT water stream, wherein (i) an electrolysis unit is coupled to the Fischer-Tropsch hydrocarbon synthesis unit, configured to be fed with at least a portion of the FT water to provide an oxygen stream, which is configured to be fed to the synthesis gas generation unit, and a hydrogen stream, (ii) a reverse water-gas shift unit is coupled to the carbon dioxide removal unit and the electrolysis unit and configured to be fed with at least a portion of the carbon dioxide stream from the carbon dioxide removal unit and a portion of the hydrogen stream produced by the electrolysis unit, to produce a carbon monoxide stream, and (iii) the Fischer-Tropsch hydrocarbon synthesis unit is coupled to the reverse water-gas shift unit to receive at least a portion of the carbon monoxide stream.

18. A system according to claim 17, further comprising (d) a hydrotreating unit coupled to the Fischer-Tropsch hydrocarbon synthesis unit for upgrading the mixture of hydrocarbons to produce hydrocarbon products.

Description

[0050] The invention is illustrated by reference to the accompanying drawing in which: FIG. 1 is a diagrammatic flowsheet of one embodiment of the invention.

[0051] It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, compressors, pumps, vacuum pumps, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, and the like may be required in a commercial plant. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

[0052] In FIG. 1, a municipal solid waste or equivalent feedstock is fed via line 10 to a synthesis gas generation unit 12 comprising a gasifier that is fed with an oxygen gas stream via line 14 produced in an electrolysis unit 16. In the gasifier, the feedstock is reacted at elevated temperature and pressure with oxygen to produce a synthesis gas stream comprising hydrogen, carbon monoxide, carbon dioxide and steam. The synthesis gas generation unit may further comprise separate partial oxidation or tar-reforming units downstream of the gasifier to effect complete conversion of the feedstock to synthesis gas. The synthesis gas generation unit 12 may further comprise heat exchange equipment to cool the synthesis gas to below the dew point and one or more gas-liquid separation vessels to recover condensate from the synthesis gas.

[0053] The synthesis gas is passed from the synthesis gas generation unit 12 at a suitable temperature and pressure via line 18 to a carbon dioxide removal unit 20 operating by means of absorption using a liquid absorbent wash system. The wash system in the carbon dioxide removal unit produces a carbon dioxide stream and a purified synthesis gas stream comprising hydrogen and carbon monoxide. Upstream of the carbon dioxide removal unit, one or more purification steps (not shown) may be used to remove unwanted contaminants, such as carbonyl sulphide, hydrogen cyanide and heavy metals such as mercury from the synthesis gas recovered from the synthesis gas generation unit.

[0054] The carbon dioxide stream is recovered from the carbon dioxide removal unit 20 via line 22, treated if necessary to remove residual contaminants, such as hydrogen sulphide, in a purification unit (not shown) and fed at a suitable temperature and pressure to a reverse water-gas shift unit 24 comprising a vessel containing a suitable transition metal oxide reverse water-gas shift catalyst. The reverse water-gas shift unit is fed with a hydrogen stream via line 26. Where the reverse water-gas shift unit includes a combustion section to preheat the feed gas, an oxygen stream may optionally be provided from the electrolysis unit 16 via a line 41. Carbon dioxide and hydrogen react over the reverse water-gas shift catalyst to produce a product gas stream comprising carbon monoxide and water vapour. The reverse water gas shift unit includes heat exchange apparatus downstream of the reverse water-gas shift reactor that cools the product gas to below the dew point and one or more gas-liquid separators that separate the resulting condensate to provide a carbon monoxide-containing gas stream.

[0055] The carbon monoxide-containing gas stream recovered from the reverse water-gas shift unit 24 may contain unreacted carbon dioxide, in which case the carbon monoxide containing gas may be fed to the carbon dioxide removal unit 20 or, preferably, is fed to a separate carbon dioxide removal unit (not shown) downstream of the one or more gas-liquid separators within the reverse water-gas shift unit 24. An advantage of using a separate carbon dioxide removal unit within the reverse water-gas shift unit is that the carbon dioxide is less likely to contain contaminants and so the carbon dioxide removal unit may be operated differently and/or use a different absorbent at a smaller scale. The carbon dioxide recovered from the carbon monoxide-containing gas stream is recycled to the reverse water-gas shift reactor.

[0056] The output from the reverse water-gas shift unit, including any carbon dioxide removal step, is a carbon monoxide gas stream.

[0057] The carbon monoxide gas stream is recovered from the reverse water-gas shift unit 24 via line 28 and combined with a synthesis gas recovered from the carbon dioxide removal unit 20 via line 30 to form a combined gas mixture in line 32. The combined gas mixture may if desired be treated in a purification unit (not shown) to remove residual contaminants and FT catalyst poisons, such as hydrogen sulphide, downstream of the carbon dioxide removal unit 20 and upstream of a Fischer-Tropsch hydrocarbon synthesis unit 38.

[0058] The combined gas mixture in line 32 may optionally be combined with a hydrogen gas stream provided by line 34 to adjust the hydrogen to carbon monoxide molar ratio, if desired, and the resulting mixture fed via line 36 at a suitable temperature and pressure to the Fischer-Tropsch hydrocarbon synthesis unit 38.

[0059] The Fischer-Tropsch hydrocarbon synthesis unit 38 comprises a tubular reaction vessel containing catalyst carriers containing a cobalt Fischer-Tropsch catalyst disposed in a plurality of tubes within the reactor. The hydrogen and carbon monoxide react over the catalyst to form a mixture of gaseous and liquid hydrocarbons and FT water as a by-product. The mixture of hydrocarbons is processed within the hydrocarbon synthesis unit 38 to separate the FT water from the gaseous and liquid hydrocarbons. The FT water is recovered from the Fischer-Tropsch hydrocarbon synthesis unit 38 and fed via line 40 to the electrolysis unit 16.

[0060] The electrolysis unit 16 comprises one or more electrolysers that convert the FT water 40 into oxygen and hydrogen using electrical energy provided by an electrical energy supply (not shown). Oxygen produced by the electrolysis unit is fed via line 14 to the synthesis gas generation unit 12. If a combustion unit is provided in the reverse water-gas shift unit, oxygen may be provided to it by the electrolysis unit 16 via line 41. Any excess oxygen may be sent to a separate process by an export line (not shown). Hydrogen is recovered from the electrolysis unit 16 via line 42. Hydrogen from line 42 is provided via line 26 to the reverse water-gas shift unit 24. Optionally, a portion of the hydrogen in line 42 may bypass the reverse water gas shift unit 24 and be fed via line 34 directly to the feed gas for the Fischer-Tropsch hydrocarbon synthesis unit 38. Optionally, a portion of the hydrogen from line 42 may be provided via line 56 to the hydrotreating unit 46.

[0061] The Fischer-Tropsch hydrocarbon synthesis unit 38 produces one or more hydrocarbon streams, including but not limited to a molten hydrocarbon wax and/or light hydrocarbon condensate, which is liquid at ambient temperature. One or more of the hydrocarbon products from the Fischer-Tropsch hydrocarbon synthesis unit 38 is fed at a suitable temperature and pressure via line 44 to a hydrotreating unit 46. The hydrotreating unit comprises one or more vessels containing a catalyst, such as a hydroisomerization, hydrogenation, hydrodeoxygenation, and/or hydrocracking catalyst, that converts the hydrocarbon wax or hydrocarbon condensate into one or more valuable hydrocarbon products. The hydrotreating unit is fed with hydrogen. Any source of hydrogen may be used, however, suitably the hydrotreating unit 46 is fed with a portion of the hydrogen produced by the electrolysis unit 16 via line 56. Valuable hydrocarbon products, such as kerosene, are recovered from the hydrotreating unit 46 via line 48.

[0062] In further embodiments, the process may be enhanced as follows;

[0063] 1. The reverse water-gas shift unit 24 produces water as a by-product. The water, or a portion of it, may be fed from the reverse water-gas shift unit 24 via line 52 to the electrolysis unit to supplement the FT water. The FT water may also be supplemented with a supplemental water feed via line 54 if necessary.

[0064] 2. The Fischer-Tropsch hydrocarbon synthesis unit 38 produces gaseous hydrocarbons as part of the hydrocarbon mixture. A portion of the gaseous hydrocarbons may be recovered from the Fischer-Tropsch hydrocarbon synthesis unit 38 and fed via line 58 as a FT tail gas back to the synthesis gas generation unit 12 where it may be used as fuel, and/or steam reformed and/or subjected to partial oxidation to form a hydrogen/carbon monoxide-containing gas stream for use in the process, or combined with the feedstock. Alternatively, or in addition, a portion of the FT tail gas may be fed directly to the reverse water-gas shift unit 24 or subjected to a step of adiabatic steam reforming (pre-reforming) to convert higher hydrocarbons to methane, and the resulting pre-reformed gas mixture fed to the reverse water-gas shift unit 24.

[0065] 3. A cryogenic air separation unit (ASU), not shown, may be used to generate supplemental oxygen that is fed to the synthesis gas generation unit via line 60.

[0066] The invention will be further described by reference to the following calculated example of a flowsheet according to FIG. 1 in which, additionally, O.sub.2 from the electrolysis unit 16 was fed to a combustion section of a reverse water-gas shift reactor and a portion of the hydrogen from line 42 was fed to the hydrotreating unit 46 via line 56 The flowsheet was based on 1000 kmol/h of syngas from syngas generation unit 12 and the final FT production, expressed as “CH.sub.2” was based on CO content to provide a final comparison.

TABLE-US-00001 Stream 14 18 22 26 28 30 32 36 Molar Flowrate Water kmol/h - - - - - - - - Hydrogen kmol/h - 404 - 1573 851 404 1255 1255 Carbon Monoxide kmol/h - 351 - - 241 351 592 592 Carbon Dioxide kmol/h - 245 241 - - 4 4 4 Oxygen kmol/h 420 - - - - - - - FT Product (as moles of CH.sub.2) kmol/h - - - - - - - - Total kmol/h 420 1000 241 1573 1092 759 1851 1851

TABLE-US-00002 Stream 40 41 42 44 48 52 54 56 Molar Flowrate Water kmol/h 564 - - - - 723 319 - Hydrogen kmol/h - - 1605 - - - - 31 Carbon Monoxide kmol/h - - - - - - - - Carbon Dioxide kmol/h - - - - - - - - Oxygen kmol/h - 241 - - - - - - FT Product (as moles of CH.sub.2) kmol/h - - - 507 507 - - - Total kmol/h 564 241 1605 507 507 723 319 31

[0067] A comparative example without the reverse water-gas shift unit 24 coupled to the electrolysis unit 16 was also modelled on the same basis. The results were as follows;

TABLE-US-00003 Stream 14 18 22 30 34 36 Molar Flowrate Water kmol/h - - - - - - Hydrogen kmol/h - 404 - 404 340 744 Carbon Monoxide kmol/h - 351 - 351 - 351 Carbon Dioxide kmol/h - 245 241 4 - 4 Oxygen kmol/h 179 - - - - - FT Product (as moles of CH.sub.2) kmol/h - - - - - - Total kmol/h 179 1000 241 759 340 1099

TABLE-US-00004 Stream 40 42 44 48 54 56 60 Molar Flowrate Water kmol/h 335 - - - 24 - - Hydrogen kmol/h - 359 - - - 19 - Carbon Monoxide kmol/h - - - - - - - Carbon Dioxide kmol/h - - - - - - - Oxygen kmol/h - - - - - - 241 FT Product (as moles of CH.sub.2) kmol/h - - 301 301 - - - Total kmol/h 335 359 301 301 24 19 241

[0068] The FT product in this case at 301 kmol/h is 41% lower than the case containing the reverse water-gas shift unit.