Method and device for reducing heavy polycyclic aromatic compounds in hydrocracking units

Abstract

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 portion of the stream present at the level of at least one plate located between the plate for supplying hydrocracked effluent and the plate for withdrawing the distillate fraction which is the heaviest is withdrawn from the fractionation column and at least a portion of said withdrawn stream is recycled to the column directly or after optional liquid separation, and optionally a portion of said withdrawn stream is recycled to the hydrocracking step directly or after optional gas separation.

Claims

1. A process for hydrocracking an oil feed comprising at least 10% by volume of compounds boiling above 340 C., comprising hydrocracking said oil feed to produce a hydrocracked effluent, followed by an optional separation of gases from the hydrocracked effluent, then fractionation of said effluent, separating at least one distillate and a residue, a portion of said residue being recycled to hydrocracking and another portion of the residue being purged, said fractionation comprising a distillation in a column provided with plates, in which column: at least partially vaporized hydrocracked effluent supplies the column over at least one supply plate, said distillate is withdrawn from the level of a withdrawal plate, said residue is evacuated at an evacuation point, and optionally, a stripping gas is injected at an injection point located below the supply plate, in which process a portion of a stream present at the level of at least one plate located between the supply plate and a plate for withdrawing a heaviest distillate fraction is withdrawn from the column, a first portion of said withdrawn stream is recycled to the column, directly or after optional liquid separation, and another portion of the withdrawn stream is recycled to the hydrocracking, directly or after optional separation of gases.

2. The process as claimed in claim 1, in which the withdrawn stream comprises a portion of a stream present at the level of a plate located above the supply plate and at the level of the plate which is closest to the supply plate.

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 The process as claimed in claim 1, in which said withdrawn stream has a concentration of HPNA of less than 500 ppm by weight.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 the entirety of said first portion of said withdrawn stream is recycled directly to the column.

6. The process as claimed in claim 1, in which said first portion of said withdrawn stream is recycled to the column above the supply plate and, when it exists, above the point for injection of the stripping gas.

7. The process as claimed in claim 6, in which said first portion of said withdrawn stream is recycled to the column at the level of the plate closest to said supply plate.

8. The process as claimed in claim 1, in which all or a portion of said withdrawn stream is stripped in an external stripping step using a stripping gas, creating a gaseous effluent and all or a portion of the gaseous effluent separated is recycled to the column above the plate from which the stream has been withdrawn.

9. The process as claimed in claim 8, in which the gaseous effluent is recycled to the column at the level of the plate closest to the plate from which the stream has been withdrawn.

10. The process as claimed in claim 8, in which all or a portion of the liquid effluent separated in said stripping step is recycled directly to the hydrocracking.

11. The process as claimed in claim 8, in which the stripping gas is injected into the fractionation.

12. The process as claimed in claim 11, in which the stripping gas injected into the fractionation is steam.

13. The process as claimed in claim 8, in which the stripping gas injected into the external stripping step is steam.

14. The process as claimed in claim 1, which does not comprise recycling the residue to said fractionation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of the prior art.

(2) FIGS. 2a, and 2b are schematic representations of processes of the invention.

EXAMPLES

Example 1: Prior Art

(3) This example is based on the configuration of FIG. 1. Two samples from an operating industrial unit based on the configuration of FIG. 1 were analysed. The properties are recorded in Table 1 below.

(4) It should be noted that because of the configuration, the streams 15a, 16, 18 and 23 had exactly the same properties.

(5) The fractionation of stream 11 in the column 12 was computer simulated using the PRO/II version 8.3.3 software marketed by SimSci. The physical and analytical properties of the resulting streams were simulated and compared with the physical and analytical properties of actual samples.

(6) The operating conditions for the column used for the simulation are recorded in Table 2 below.

(7) TABLE-US-00001 TABLE 1 Properties of the streams of the layout of FIG. 1 Streams from FIG. 1 Stream number Configuration 11 15a 18 16 Yield % by wt 100 42 39.5 2.5 Quantity of gas % by wt 64.0 10.9 10.9 10.9 oil in stream Density 0.805 0.828 0.828 0.828 HPNA Coronene ppm by wt 209 497 497 497 Dibenzo(e,ghi)per- ppm by wt 33 78 78 78 ylene Naphtho[8,2,1 abc] ppm by wt 81 192 192 192 coronene Ovalene ppm by wt 57 135 135 135 Total HPNA ppm by wt 378 902 902 902 TBP, % by wt Initial boiling point C. 128 200 200 200 10% C. 200 368 368 368 50% C. 326 402 402 402 90% C. 440 477 477 477 Final boiling point C. 524 524 524 524 1: Specific gravity SG = .sub.sample at 20 C./.sub.H20 at 4 C., where is the density expressed in g/cm.sup.3

(8) Starting from the properties of the stream 11 entering the fractionation column (see Table 1), the PRO/II simulation was able to establish the properties of the stream 15 leaving the fractionation column; in particular, the HPNA distribution could be modelled.

(9) TABLE-US-00002 TABLE 2 Operating conditions for the column Operating conditions for fractionation FIG. 1 Pressure, top of column Barg 1.0 Pressure, bottom of column Barg 1.5 Temperature, inlet feed C. 377 Number of theoretical plates 34 Flow rate of stripping steam kg of steam/tonne of feed 17

(10) Based on these results, the configurations of the invention were simulated. The results are disclosed below for each configuration.

Example 2: Configuration 2a

(11) Table 3 below provides the characteristics of the streams 11, 16 and 18 in the configuration 2a obtained from the PRO/II simulation. The operating conditions used for the simulation are recorded in Table 4.

(12) TABLE-US-00003 TABLE 3 Properties of the streams of the layout of FIG. 2a Streams from FIG. 2a Stream number 11 18 16 Configuration inlet liquid recycle purge Yield 100 39.5 2.5 Quantity of gas oil in stream 64.0 14.8 5.1 Density 0.805 0.8275 0.832 HPNA Coronene 209 422 2192 Dibenzo(e,ghi)perylene 33 96 314 Naphtho [8,2,1 abc] coronene 81 114 895 Ovalene 57 70 640 Total HPNA 378 702 4041 TBP, % by wt Initial boiling point 128 79 251 10% 200 362 389 50% 326 401 425 90% 440 476 505 Final boiling point 524 524 524 1: Specific gravity SG = .sub.sample at 20 C./.sub.H20 at 4 C., where is the density expressed in g/cm.sup.3

(13) TABLE-US-00004 TABLE 4 Operating conditions for the column Operating conditions for fractionation FIG. 2a Pressure, top of column Barg 1.0 Pressure, bottom of column Barg 1.5 Temperature, inlet feed C. 377 Number of theoretical plates 34 Flow rate of stripping steam kg of steam/tonne of feed 17

(14) Compared with the configuration of FIG. 1, the configuration 2a can be used to maximize the quantity of HPNA (4041 ppm by weight compared with 902 ppm by weight in configuration 1) in the unconverted fraction (residue) which was purged via the line 16. At the same time, the quantity of HPNA was minimized in the stream which returns to the reaction section via the line 18: 702 ppm by weight (by wt) compared with 902 ppm by weight in configuration 1, which reduced the quantity of HPNA by 22.2%.

(15) In addition, the proportion of heavy HPNA (naphtho [8,2,1 abc] coronene+ovalene) compared with the total quantity of HPNA in the stream 18 returning to the reaction section was much lower for the configuration 2a (26.3%) than for the configuration 1 (36.3%). This indicates that not only was there less total HPNA in the stream returning to the reaction section via the line 18, but also, the proportion of the most refractory and poisonous heavy HPNA (naphtho [8,2,1 abc] coronene+ovalene) was lower.

Example 3: Configuration 2b

(16) Table 5 below provides the characteristics obtained from the PRO/II simulation for the streams 11, 16 and 18 in the configuration 2b. The operating conditions used for the simulation are recorded in Table 6.

(17) TABLE-US-00005 TABLE 5 Properties of the streams of the layout of FIG. 2b Streams from FIG. 2b Stream number 11 18 16 Configuration inlet liquid recycle purge Yield 100 39.5 2.5 Quantity of gas oil in stream 64.0 6.6 5.7 Density 0.805 0.8291 0.8313 HPNA Coronene 209 388 2500 Dibenzo(e,ghi) perylene 33 78 375 Naphtho [8,2,1 abc] coronene 81 122 994 Ovalene 57 80 704 Total HPNA 378 668 4573 TBP, % by wt Initial boiling point 128 217 210 10% 200 384 380 50% 326 401 421 90% 440 476 504 Final boiling point 524 524 524 1: Specific gravity SG = .sub.sample at 20 C./.sub.H20 at 4 C., where is the density expressed in g/cm.sup.3

(18) TABLE-US-00006 TABLE 6 Operating conditions for the column FIG. 2b Operating conditions for fractionation Pressure, top of column Barg 1.0 Pressure, bottom of column Barg 1.5 Temperature, inlet feed C. 377 Number of theoretical plates 34 Flow rate of stripping steam kg of steam/tonne of feed 17 operating conditions for side stripper Pressure, top of column Barg 1.4 Pressure, bottom of column Barg 1.5 Number of theoretical plates 6 Flow rate of stripping steam kg of steam/t of feed 28

(19) Compared with FIG. 1, the configuration 2b can be used to maximize the quantity of HPNA: 4573 ppm by weight compared with 902 ppm by weight for configuration 1 in the unconverted fraction (residue) which was purged via the line 16. At the same time, the quantity of HPNA was minimized in the stream leaving the reaction section via the line 18: 668 ppm by weight compared with 902 ppm by weight for configuration 1, which reduced the quantity of HPNA by 25.9%.

(20) In addition, the proportion of the most refractory and poisonous heavy HPNA (naphtho [8,2,1 abc] coronene+ovalene) compared with the total quantity of HPNA in the stream 18 returning to the reaction section was much lower for configuration 2b (30.3%) than for configuration 1 (36.3%). This indicates that not only was there less total HPNA in the stream returning to the reaction section via the line 18, but also that the proportion of heavy HPNA (naphtho [8,2,1 abc] coronene+ovalene) was lower.

(21) This configuration could also minimize the quantity of gas oil sent to the reaction section via the line 18 because the quantity of gas oil included in the stream which was sent to the reaction section via the line 18 was only 6.6% by weight compared with 10.9% by weight in configuration 1.