Method for starting-up a gas to liquid process
The invention relates to a process for starting up a gas-to-liquid process including the production of synthesis gas and a downstream GTL process. The synthesis gas is produced by the use of autothermal reforming (ATR) or catalytic partial oxidation (CPO) and during the starting period the effluent gas from the ATR or CPO is significantly changed to form an off-gas recycle which is fed to the ATR or CPO. When the downstream GTL process is running, the recycle to ATR or CPO is shifted to its off-gas.
1. Method for starting up a gas-to-liquid (GTL) process comprising:
(a) adding steam to a hydrocarbon feedstock to form a mixture of hydrocarbons and steam,
(b) passing said mixture through an autothermal reforming stage (ATR) or catalytic partial oxidation (CPO) stage and withdrawing a stream of hot effluent synthesis gas from the ATR or CPO,
(c) removing water from said hot effluent synthesis gas to form a dehydrated synthesis gas,
(d) removing at least part of the hydrogen from at least a part of said dehydrated synthesis gas to form an off-gas recycle,
(e) directly recycling at least a portion of said off-gas recycle into the hydrocarbon feedstock or into said mixture of hydrocarbons and steam, and
(f) subsequently passing at least part of the dehydrated synthesis gas as feed to downstream GTL process while shifting from recycling said off-gas recycle gas to recycling off-gas from said downstream GTL process.
2. Method according to claim 1 wherein step (f) is conducted when the H2/CO molar ratio of the dehydrated synthesis gas is below 2.3.
3. Method according to claim 1 in which said hydrocarbon feedstock is a gas that has passed through at least one pre-reforming stage.
4. Method according to claim 1 in which said hydrocarbon feedstock or said mixture of hydrocarbons and steam are preheated before being combined with said off-gas recycle.
5. Method according to claim 1 in which the off-gas recycle is introduced to the hydrocarbon feedstock or mixture of hydrocarbons and steam by a recycle-compressor.
6. Method according to claim 5 in which the recycle-compressor is an existing recycle-compressor used for recycling off-gas from said downstream gas-to-liquid (GTL) process.
7. Method according to claim 6 in which the off-gas recycle after passing through the recycle-compressor is subjected to a conversion stage for removal of olefins.
8. Method according to claim 1 in which said step (d) is conducted in a hydrogen separating membrane or Pressure Swing Separation unit (PSA).
9. Method according to claim 1 in which the steam to carbon molar ratio at which the ATR or CPO is operated is 0.2 to 3.0.
10. Method according to claim 1 in which the stream of hot effluent synthesis gas from the ATR or CPO in step (b) is cooled prior to conducting step (c).
11. Method according to claim 10 in which said hot effluent synthesis gas from the ATR or CPO is cooled in one or more waste heat boilers.
12. Method according to claim 10 in which said hot effluent synthesis gas from the ATR or CPO is used as heat exchanging medium in a heat exchange reformer operated in series or in parallel with the ATR or CPO.
13. Method according to claim 1 in which the gas-to-liquid (GTL) process is Fischer-Tropsch synthesis.
14. Method according to claim 2 wherein the H 2 /CO molar ratio of the dehydrated synthesis gas is below 2.2.
15. Method according to claim 14 wherein the H 2 /CO molar ratio of the dehydrated synthesis gas is below 2.1.
16. Method according to claim 7 in which the off-gas recycle after passing through the recycle-compressor is subjected to hydrogenation.
17. Method according to claim 3 in which said at least one pre-reforming stage is preferably an adiabatic pre-reforming stage.
18. Method according to claim 4 in which said hydrocarbon feedstock or said mixture of hydrocarbons and steam are preheated in a fired-heater before being combined with said off-gas recycle.
19. Method for starting up a gas-to-liquid (GTL) process comprising:
(a) adding steam to a hydrocarbon feedstock to form a mixture of hydrocarbons and steam,
(b) passing said mixture through an autothermal reforming stage (ATR) or catalytic partial oxidation (CPO) stage and withdrawing a stream of hot effluent synthesis gas from the ATR or CPO,
(c) removing water from said hot effluent synthesis gas to form a dehydrated synthesis gas,
(d) during start-up, removing at least part of the hydrogen from at least a part of said dehydrated syntheses gas to form an off-gas recycle having a H2/CO molar ratio of the dehydrated synthesis gas below 2.3,
(e) directly recycling at least a portion of said off-gas recycle into the hydrocarbon feedstock or into said mixture of hydrocarbons and steam, and
(f) subsequently passing at least part of the dehydrated synthesis gas as feed to downstream GTL process while shifting from recycling said off-gas recycle gas to recycling off-gas from said downstream GTL process.