Method and device for reducing heavy polycyclic aromatic compounds in hydrocracking units
The invention concerns a process and a facility for reducing the concentration of heavy polycyclic aromatic compounds (HPNA) in the recycle loop of hydrocracking units, which comprises a fractionation column. In accordance with this process, a stream is withdrawn from the fractionation column at the level of at least one plate located between the supply plate and the plate for withdrawing the heaviest distillate fraction; the stream is stripped in an external stripping step by a stripping gas, in the presence of a portion of the residue. The separated gaseous effluent is recycled to the column, advantageously as a stripping gas, and the liquid fraction is recycled to the hydrocracking step; a residue is purged in the stripping step.
1. A process for hydrocracking an oil feed comprising at least 10% by volume of compounds boiling above 340° C., comprising a hydrocracking step, optionally followed by a separation of the gases from the hydrocracked effluent, then a step for fractionation of said effluent, which separates at least one distillate and a residue, a portion of said residue being recycled to the hydrocracking step, said fractionation step comprising a distillation in a column provided with plates, in which column:
said at least partially vaporized effluent supplies the column over a supply plate,
said distillate is withdrawn from the level of a withdrawal plate,
said residue is evacuated at an evacuation point,
and a stripping gas is injected at an injection point located below the supply plate, in which process
a portion of the stream present at the level of at least one plate located between the supply plate and the plate for withdrawing the heaviest distillate fraction is withdrawn from the column,
all or a portion of said withdrawn stream is stripped in an external stripping step by a stripping gas, in the presence of a portion of the residue evacuated from the column, and a gaseous effluent and a liquid fraction are obtained,
all or a portion of the separated gaseous effluent is recycled to the column,
all or a portion of said liquid fraction is removed at the level of a plate located between a plate introducing withdrawn steam and a plate introducing portion of the residue evacuated from the column, and recycled to the hydrocracking step,
and a residue is purged in the stripping step.
2. The process as claimed in claim 1 , in which a portion of the stream present at the level of a plate close to the supply plate and located above it is withdrawn from the column.
3. The process as claimed in claim 1 , in which said withdrawn stream has a concentration of HPNA of less than 500 ppm by weight.
4. The process as claimed in claim 1 , in which said withdrawn stream has a proportion of at least 70% by weight of unconverted hydrocarbons.
5. The process as claimed in claim 1 , in which all or a portion of the gaseous effluent is recycled to the column below the supply plate.
6. The process as claimed in claim 1 , in which the gaseous effluent is recycled to the column as the stripping gas.
7. The process as claimed in claim 1 , in which all or a portion of the liquid effluent separated in said stripping step is recycled directly to the hydrocracking step.
8. The process as claimed in claim 1 , in which the stripping gas injected into the external stripping step and optionally into the fractionation step is steam.
9. The process as claimed in claim 1 , in which a portion of the residue evacuated from the column is recycled to the hydrocracking step.
10. The process according to claim 1 , wherein introduction of the portion of the residue evacuated from the column in the external stripping step is done at a plate located below a plate introducing said withdrawn stream.
11. The process according to claim 1 , wherein a portion of the stream present at the level of at least one plate located below the supply plate and above the plate withdrawing the heaviest distillate fraction is withdrawn from the column.
12. The process according to claim 1 , wherein all of said withdrawn stream is stripped in an external stripping step by a stripping gas, all of the separated gaseous effluent is recycled to the column, and of said liquid fraction is removed at the level of a plate located between a plate introducing withdrawn steam and a plate introducing portion of the residue evacuated from the column, and recycled to the hydrocracking step.
13. The process according to claim 1 , wherein a portion of the stream present at the level of the plate which is closest to the supply plate is withdrawn from the column.
14. The process as claimed in claim 1 , in which said withdrawn stream has a concentration of HPNA of less than 350 ppm by weight.
15. The process as claimed in claim 1 , in which said withdrawn stream has a proportion of at least 80% by weight of unconverted hydrocarbons.
16. The process as claimed in claim 1 , in which all of the gaseous effluent is recycled to the column below the supply plate.