METHOD FOR SHUTTING DOWN A FISCHER-TROPSCH REACTOR

Abstract

A method is described for shutting down a Fischer-Tropsch reactor fed with a reactant gas mixture comprising a synthesis gas and a recycle gas recovered from the Fischer-Tropsch reactor in a synthesis loop, said Fischer-Tropsch reactor containing a Fischer-Tropsch catalyst cooled indirectly by a coolant under pressure, comprising the steps of: (a) depressurising the coolant to cool the reactant gas mixture to quench Fischer-Tropsch reactions taking place in the Fischer-Tropsch reactor, (b) stopping the synthesis gas feed to the Fischer-Tropsch reactor, and (c) maintaining circulation of the recycle gas through the Fischer-Tropsch reactor during steps (a) and (b) to remove heat from the Fischer-Tropsch reactor. The method safely facilitates a more rapid return to operating conditions than a full shut-down.

Claims

1. A method for shutting down a Fischer-Tropsch reactor fed with a reactant gas mixture comprising a synthesis gas and a recycle gas recovered from the Fischer-Tropsch reactor in a synthesis loop, said Fischer-Tropsch reactor containing a Fischer-Tropsch catalyst cooled indirectly by a coolant under pressure, comprising the steps of: (a) depressurising the coolant to cool the reactant gas mixture to quench Fischer-Tropsch reactions taking place in the Fischer-Tropsch reactor, (b) stopping the synthesis gas feed to the Fischer Tropsch reactor, and (c) maintaining circulation of the recycle gas through the Fischer-Tropsch reactor during steps (a) and (b) to remove heat from the Fischer-Tropsch reactor.

2. The method according to claim 1, wherein the synthesis gas consists essentially of hydrogen and carbon monoxide with a molar ratio of hydrogen to carbon monoxide in the range of 1.6:1 to 2.5:1, preferably 2.0:1 to 2.2:1.

3. The method according to claim 1, wherein the Fischer-Tropsch reactor, prior to shut down is operated at a pressure in the range 10 to 100 bar abs and a temperature in the range 170 to 350° C.

4. The method according to claim 1, wherein the Fischer-Tropsch reactor comprises a pressure vessel in which the Fischer-Tropsch catalyst is provided as a bed though which coolant-bearing tubes or plates are placed, or in which the catalyst is provided within a plurality of reactor tubes that are bathed in coolant flowing around their outsides.

5. The method according to claim 1, wherein the Fischer-Tropsch catalyst is an iron Fischer-Tropsch catalyst or a cobalt Fischer-Tropsch catalyst.

6. The method according to claim 1, wherein the Fischer-Tropsch catalyst is used in combination with a catalyst carrier suitable for use in a tubular Fischer-Tropsch reactor where the catalyst carrier containing the catalyst is disposed within one or more tubes cooled by the coolant under pressure.

7. The method according to claim 1, wherein the coolant is boiling water under pressure provided by a steam drum coupled to the Fischer-Tropsch reactor.

8. The method according to claim 7, wherein the de-pressurisation step (a) is provided using an orifice and/or a control valve connected to the steam drum.

9. The method according to claim 1, wherein the de-pressurisation rate in step (a) is 2 to 6 bar per minute.

10. The method according to claim 1, wherein the temperature in step (a) is reduced to 150° C. or less.

11. The method according to claim 1, wherein the method is followed directly by a start-up of the process.

12. The method according to claim 11, wherein the method further comprises the steps of: (d1) reducing the pressure of the Fischer-Tropsch reactor and circulating recycle gas, (e1) adjusting the inert gas content of the circulating recycle gas to a minimum value, (f1) re-pressurising the Fischer-Tropsch reactor with synthesis gas feed, (g1) re-pressurising the coolant thereby increasing the Fischer-Tropsch reactor temperature until Fischer-Tropsch reactions begin to occur, and (h1) introducing synthesis gas to the Fischer-Tropsch reactor.

13. The method according to claim 12, wherein in step (d1) the Fischer-Tropsch reactor pressure is reduced to pressure in the range 5-10 bara.

14. The method according to claim 12, wherein in step (e1) the minimum inert gas content of the circulating recycle gas is in the range of 60 to 80% by volume.

15. The method according to claim 12, wherein in step (f1) the pressure is raised to a pressure 60 to 100% of the operating pressure prior to the shutdown.

16. The method according to claim 12, wherein in step (g1), where the coolant is boiling water under pressure, re-pressurisation is achieved by pre-pressurising the steam drum coupled to the Fischer-Tropsch reactor using a steam eductor/jet-pump loop.

17. The method according to claim 1, wherein the method is followed by a full shut-down of the process.

18. The method according to claim 17, wherein the method further comprises the steps of: (d2) stopping the circulation of the recycle gas, (e2) feeding the inert gas feed into the Fischer-Tropsch reactor and loop at a pressure greater than the pressure of the Fischer-Tropsch reactor and loop, and (f2) purging the Fischer-Tropsch reactor and loop of reactant gases using the inert gas feed to displace the recycle gas.

19. The method according to claim 18, wherein in step (e2) the inert gas feed is a nitrogen gas feed.

20. The method according to claim 18, wherein in step (e2), the inert gas is supplied from a pressurised tank dedicated for the method.

21. The method according to claim 1, wherein the method is activated manually or operated autonomously using a computerised plant control system and/or plant safety-instrumented system.

22. The method according to claim 21, wherein the computerised plant control system and/or plant safety-instrumented system additionally instructs upstream and/or downstream operations to shut-down or enter a safe-operating state.

Description

[0051] The invention is further described by reference to the drawings in which:

[0052] FIG. 1 is a depiction of one embodiment of a system to which the method of the present invention may be applied.

[0053] It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as feedstock drums, pumps, vacuum pumps, compressors, gas recycling compressors, 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. Provision of such ancillary equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

[0054] In FIG. 1 a synthesis gas generation unit 10 produces a purified synthesis gas mixture consisting of hydrogen and carbon monoxide at elevated temperature and pressure. The synthesis gas is fed from the synthesis gas generation unit 10 via line 12 and combined with a recycle stream in line 14 to produce a reactant gas mixture which is fed via line 16 to a Fischer-Tropsch reactor 18 containing a plurality of Fischer-Tropsch catalyst-containing reaction tubes 20. The Fischer-Tropsch catalyst may be contained within a plurality of catalyst carriers within each of the reaction tubes. The tubes 20 are cooled by boiling water under pressure provided to the reactor via line 22, supplied by a steam drum 24. Steam is recovered from the Fischer-Tropsch reactor 18 via line 26 and returned to the steam drum 24. The steam drum is fed with a stream of boiler feed water (not shown) and steam is recovered from the steam drum via line 28. Hydrocarbons are synthesis by reaction of the hydrogen and carbon monoxide over the Fischer-Tropsch catalyst. A product mixture is recovered from the Fischer-Tropsch reactor 18 via line 30 and fed to a first gas-liquid separator 32 where a liquid wax product is separated from product and unreacted gases and recovered via line 34 for optional further processing. The gaseous product and unreacted gases are fed from the first gas-liquid separator 32 via line 36 to one or more heat exchangers 38 where it is cooled to condense a mixture of co-produced water and condensable hydrocarbon products. The cooled mixture formed in the one or more heat exchangers 38 is fed via line 40 to a second gas-liquid separator 42, where the condensed water and hydrocarbons are separated and recovered via line 44 for further processing. An unreacted gas mixture comprising hydrogen, carbon monoxide and possibly carbon dioxide and/or non-condensable hydrocarbons is recovered from the second gas-liquid separator 42 via line 46 and compressed in a circulating compressor 48 to form the recycle gas stream 14. A purge line 50 is taken from the unreacted gas mixture line 46 upstream of the compressor 48.

[0055] Optionally, a pressure vessel 52 containing high-pressure nitrogen at a pressure greater than the compressed recycle gas in line 14 and synthesis gas feed in line 12 may be connected to the recycle gas in line 14 via a feed line 54 downstream of the compressor 48 for use in case of emergency. In addition, it may be advantageous to have a branch line from line 54 directly to the reactant gas feed line 16 near the inlet of the Fischer-Tropsch reactor 18 to help to purge reactants from the reactor as quickly as possible.

[0056] In order to operate the partial shut-down method, a valve 56 in the steam line 28 is opened to de-pressurise the steam drum 24 coolant fed to the Fischer-Tropsch reactor 18 via line 22. At the same time a valve 58 in the synthesis gas feed line 12 is closed to stop the flow of synthesis gas to the reaction tubes 20. The de-pressurisation of the steam drum reduces the temperature of the coolant, which has the effect of quenching the reactions taking place in the reaction tubes 20. The circulating compressor 48 is operated to maintain a flow of gas through the cooled reaction tubes 20 in the Fischer-Tropsch reactor 18.

[0057] In order to re-start the process a valve 60 on the purge line 50 may be opened. Nitrogen, e.g. from a local low-pressure nitrogen source (not shown) may be used to adjust the inert gas content of the circulating gas to a level that avoids the reaction tubes 20 in the Fischer-Tropsch reactor 18 from overheating when the synthesis gas feed is restarted. Valve 58 is then re-opened to allow pressurisation of the reaction tubes 20 in the Fischer-Tropsch reactor 18. Once the desired pressure has been reached, the valve 58 is closed. Valve 56 is then closed to allow re-pressurisation of the steam drum 24 and coolant fed to the reactor via line 22. This has the effect of bringing the Fischer-Tropsch reactor 18 and reaction tubes 20 up to a temperature where the Fischer-Tropsch reactions re-commence. Once the desired temperature has been reached, valve 58 is re-opened to feed synthesis gas via line 12 to the reaction tubes 20 in the Fischer-Tropsch reactor 18.

[0058] In the event that the system is required to be fully shut-down, for example in case of emergency, then the circulating compressor 48 is stopped and a valve 62 in the high-pressure nitrogen feed line 54 opened to admit nitrogen to the reaction tubes 20 of the Fischer-Tropsch reactor 18. Valve 60 in the purge line 50 is opened to enable the inert gas to flow through the entire system.