PROCESS FOR CONVERSION OF NATURAL GAS TO LIQUID HYDROCARBONS AND A PLANT FOR CARRYING OUT THE PROCESS

Abstract

A process and plant for conversion of a feed hydrocarbon stream to liquid hydrocarbon products in a small scale GTL plant, comprising the use of a cryogenic air separation unit (ASU), optionally together with vacuum pressure swing adsorption (VPSA), an autothermal reformer (ATR) or catalytic partial oxidation (CPO), and pressure swing adsorption (PSA) unit to produce a synthesis gas for downstream Fischer-Tropsch (FT) synthesis for production of liquid hydrocarbons.

Claims

1. A process for the conversion of a feed hydrocarbon stream into liquid hydrocarbons comprising: (a) producing oxygen with a purity of at least 80% vol. in at least a vacuum pressure swing adsorption (VPSA) unit; (b) mixing steam to the feed hydrocarbon stream to form a hydrocarbon-steam stream; (c1) adding the oxygen of step (a) and the hydrocarbon-steam stream of step (b) to an autothermal reformer (ATR), or catalytic partial oxidation (CPO) unit, or (c2) combining the oxygen of step (a) with the resulting stream of step (b) and then adding to an autothermal reformer (ATR), catalytic partial oxidation (CPO) unit; (d) withdrawing from the ATR or CPO a raw synthesis gas which is first dewatered and then split into a first and second raw synthesis gas stream; (e) removing hydrogen from the second raw synthesis gas stream without any water-gas-shift pre-treatment, and withdrawing a hydrogen-rich stream and an off gas stream; (f) converting the first raw synthesis gas stream from step (d) into liquid hydrocarbons through Fischer-Tropsch synthesis and a tail-gas; (g) recycling tail gas from the Fischer-Tropsch synthesis to the feed hydrocarbon stream prior to step (b), to step (b), to step (c1), to step (c2), or combinations thereof.

2. A process according to claim 1, wherein, prior to step (b), the feed hydrocarbon stream is subjected to a desulphurization step.

3. A process according to claim 1, wherein the hydrocarbon-steam stream is pre-reformed in one or more pre-reformers.

4. A process according to claim 1, wherein said producing oxygen with a purity of at least 80% vol. includes cryogenic separation unit.

5. A process according to claim 1, wherein the second raw synthesis gas is fed to a PSA unit to produce said hydrogen-rich stream.

6. Process according to claim 1, wherein the feed hydrocarbon stream is natural gas, associated gas, or combinations thereof.

7. Process according to claim 1, wherein steam is added to the oxygen of step (a).

8. A process for the conversion of a feed hydrocarbon stream into liquid hydrocarbons comprising: (a) producing oxygen with a purity of at least 80% vol. in at least a vacuum pressure swing adsorption (VPSA) unit; (b) mixing steam to the feed hydrocarbon stream to form a hydrocarbon-steam stream; (c1) adding the oxygen of step (a) and the hydrocarbon-steam stream of step (b) to an autothermal reformer (ATR), or catalytic partial oxidation (CPO) unit, or (c2) combining the oxygen of step (a) with the resulting stream of step (b) and then adding to an autothermal reformer (ATR), catalytic partial oxidation (CPO) unit; (d) withdrawing from the ATR or CPO a raw synthesis gas which is first dewatered and then split into a first, major raw synthesis gas stream, and a second minor and second, minor raw gas stream; (e) removing hydrogen from the second raw synthesis gas stream without any water-gas-shift pre-treatment, and withdrawing a hydrogen-rich stream and an off gas stream; (f) converting the first raw synthesis gas stream from step (d) into liquid hydrocarbons through Fischer-Tropsch synthesis and a tail-gas; (g) recycling tail gas from the Fischer-Tropsch synthesis to the feed hydrocarbon stream prior to step (b), to step (b), to step (c1), to step (c2), or combinations thereof.

9. Plant for producing liquid hydrocarbons from a feed hydrocarbon stream comprising: feed means for feeding a gaseous hydrocarbon stream, a reforming section, communicating with said feed means, and producing synthesis gas, said reforming section comprising a cryogenic air separation unit (ASU) for providing oxygen with purity of at least 80% volume, an autothermal reformer (ATR) or catalytic partial oxidation unit (CPO) for producing a raw synthesis gas, means for splitting the raw synthesis gas into a first and second raw synthesis gas, a pressure swing adsorption (PSA) unit for hydrogen removal from the second raw synthesis gas; a Fischer-Trospch synthesis section for converting the synthesis gas into liquid hydrocarbons, said Fischer-Trospch synthesis section including an upgrading section comprising a tail gas discharge outlet, means for passing the first raw synthesis gas to the Fischer-Tropsch synthesis section, and means for recycling the Fischer-Tropsch tail gas to the feed hydrocarbon stream, to the ATR or CPO, or both.

10. Plant according to claim 9, further comprising a desulphurization unit for removal of sulphur compounds from the feed hydrocarbon stream.

11. Plant according to claim 9, further comprising means for adding steam to: the feed hydrocarbon stream to form a hydrocarbon-steam stream, the oxygen with purity of at least 80% volume, or both.

12. Plant according to claim 9, further comprising one or more pre-reformers, preferably adiabatic pre-reformers, for removal of higher hydrocarbons (C2+) from the hydrocarbon-steam stream.

13. Plant according to claim 9, wherein the plant further comprises using Vacuum Pressure Swing Adsorption (VPSA) for the provision of the oxygen with purity of at least 80% vol.

14. Plant according to claim 9, further comprising means for using the hydrogen removed in the PSA unit in the upgrading section of the Fischer-Tropsch section, or in the desulphurization unit, or both.

15. Plant according to claim 9, further comprising upstream said means for splitting the raw synthesis gas: one or more heat exchangers for cooling the raw synthesis gas, and a process condensate separator to remove water from the thus cooled raw synthesis gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The sole figure shows a schematic representation of a process and plant for small scale GTL according to the present invention with tail gas recycle from the Fischer-Tropsch synthesis to autothermal reformer and/or to the desulfurization unit as well as recycle of hydrogen-rich stream from the PSA unit to the desulfurization unit.

DETAILED DESCRIPTION OF THE INVENTION

[0047] In the figure, a schematic process and plant 1 for producing about 3000 BPD of liquid hydrocarbon product is shown. A feed hydrocarbon stream such as natural gas 2 is passed through a desulfurization unit 30 suitably arranged as a hydrogenator followed by an absorption unit (not shown). To the natural gas 2 or to the desulfurization unit 30, more specifically to the hydrogenator therein, a hydrogen-rich stream 3 from the PSA-unit downstream is added. The desulfurized stream 4 is then mixed with steam and pre-reformed in one or more pre-reformers (not shown) before entering an autothermal reformer (ATR) 40 under the addition of oxygen 5. A raw synthesis gas 6 is withdrawn from the ATR, cooled in heat exchangers and air cooler (not shown) before passing to a water removal unit 50 such as a process condensate separator. A large portion of water is removed from this unit and the raw synthesis gas 6, now dewatered, is split into a first raw synthesis gas 7 which represents the major portion and second raw synthesis gas 8 which represents the minor portion of the raw synthesis gas 6. The first raw synthesis gas 7 is then converted to liquid hydrocarbon product by Fischer-Tropsch synthesis which includes an upgrading section (not shown) from which tail gas 9 is recycled to the hydrocarbon feed 2 (not shown), to the desulfurization unit as shown here, to the pre-reformers (not shown), or to the autothermal reformer 40, e.g. by adding tail gas 9 to the hydrocarbon stream entering the autothermal reformer. The second raw synthesis gas 8 is passed through a Pressure Swing Adsorption (PSA) unit 60 out of which a PSA off-gas 10 is produced and used as fuel as well as a hydrogen-rich stream 11 which can be diverted as hydrogen-product stream 12 due to its high purity, e.g. 99.9% hydrogen. A hydrogen recycle stream 3 is used in the hydrogenator of desulfurization unit 30 and suitably also in downstream hydrocracking units of the upgrading section of the Fischer-Tropsch synthesis (not shown).