Method of producing aromatics and light olefins from a hydrocarbon feedstock

Abstract

The present invention relates to method of producing aromatics and light olefins from a hydrocarbon feedstock comprising the steps of: (a) subjecting the hydrocarbon feedstock to a solvent extraction process in a solvent extraction unit; (b) separating from the solvent extracted hydrocarbon feedstock obtained in step (a) a raffinate fraction comprising paraffins and a fraction comprising aromatics and naphtenes; (c) converting said fraction comprising aromatics and naphtenes in a hydrocracking unit and separating into a high content aromatics fraction and a stream high in light paraffins; (d) converting said raffinate fraction in a steam cracking unit into light olefins.

Claims

1. A method of producing aromatics and light olefins from a hydrocarbon feedstock, the method comprising the steps of: (a) subjecting the hydrocarbon feedstock to a solvent extraction process in a solvent extraction unit; (b) separating from the solvent extracted hydrocarbon feedstock obtained in step (a) a raffinate fraction comprising paraffins and a fraction comprising aromatics and naphthenes; (c) hydrocracking said fraction comprising aromatics and naphthenes in a hydrocracking unit and separating reaction products from the hydrocracking into a high content aromatics fraction and a stream high in light paraffins, wherein said hydrocracking unit is a selective hydrocracking and hydro-desulphurization unit suitable for preserving aromatic species whilst simultaneously dehydrogenating a portion of naphthenic species into aromatic species, hydrocracking non-aromatic species, including remaining naphthenic species and paraffinic species, into LPG species and hydro-desulphurising any organo-sulphur species present with the non-aromatic species, wherein the operating conditions are WHSV between 0.5 and 3 hr.sup.1, a H.sub.2:hydrocarbon ratio between 2:1 and 4:1, pressure between 689475 Pa and 2757902 Pa and reactor inlet temperature between 470 and 550 C.; (d) converting said raffinate fraction in a steam cracking unit into light olefins.

2. The method according to claim 1, wherein, before carrying out step (c), the fraction comprising aromatics and naphthenes is further fractionated by a stripping process to increase content of aromatics and naphthenes, wherein said stripping process is based on differences in relative volatility in the presence of said solvent.

3. The method according to claim 1, wherein, before carrying out step (c), the fraction comprising aromatics and naphthenes is further fractionated by recovering the solvent from the solvent extraction process.

4. The method according to claim 2, wherein the materials stripped from the fraction comprising aromatics and naphthenes are returned to the solvent extraction process of step (a).

5. The method according to claim 2, wherein the materials stripped from the fraction comprising aromatics and naphthenes are sent to steam cracking unit of step (d).

6. The method according to claim 1, further comprising recovering a pygas containing byproduct from the steam cracking unit and returning said recovered pygas containing byproduct to said solvent extraction process.

7. The method according to claim 1, further comprising recovering a pygas containing byproduct from the steam cracking unit and feeding said pygas containing byproduct to said hydrocracking unit.

8. The method according to claim 6, further comprising feeding said recovered solvent to the solvent extraction process at a position above inlet points of both recovered pygas containing byproduct and the hydrocarbon feedstock in said solvent extraction unit.

9. The method according to of claim 6, further comprising feeding said hydrocarbon feedstock to the solvent extraction process at a position above an inlet point of recovered pygas containing byproduct in said solvent extraction unit.

10. The method according to claim 1, further comprising recovering an LPG fraction from said hydrocracked high content aromatics fraction and returning said LPG fraction to the steam cracking unit.

11. The method according to claim 1, further comprising recovering the solvent from the raffinate fraction comprising paraffins and returning said solvent recovered into the solvent extraction process.

12. The method according to claim 1, wherein the hydrocarbon feedstock is a member selected from the group consisting of a whole range naphtha having a boiling point lower than 200 C. and kerosene.

13. The method according to claim 12, wherein the hydrocarbon feedstock is the group of whole range naphtha having a boiling point lower than 200 C.

14. The method according to claim 1, wherein, before carrying out step (c), the fraction comprising aromatics and naphthenes is further fractionated by a stripping process to increase a content of aromatics and naphthenes, said stripping process resulting in a solvent stream rich in aromatics and naphthenes and a stream comprising stripped light paraffins, in which said stripping process is based on differences in relative volatility in the presence of said solvent.

15. The method according to claim 14, wherein, before carrying out step (c), the solvent stream rich in aromatics and naphthenes is further fractionated by recovering the solvent resulting in a solvent rich stream and an extract comprising aromatics and naphthenes, and returning said solvent rich stream to said solvent extraction process of step (a), in which said fractionation is based on differences in boiling points.

16. The method according to claim 14, wherein the stream comprising stripped light paraffins is returned to the solvent extraction process of step (a).

17. The method according to claim 1, wherein there is no hydrodesulphurization ahead of the extraction process.

18. A method of producing aromatics and light olefins from a hydrocarbon feedstock, the method comprising the steps of: (a) subjecting the hydrocarbon feedstock to a solvent extraction process in a solvent extraction unit; (b) separating from the solvent extracted hydrocarbon feedstock obtained in step (a) a raffinate fraction comprising paraffins and a fraction comprising aromatics and naphthenes; (c) hydrocracking said fraction comprising aromatics and naphthenes in a hydrocracking unit and separating the reaction products into a high content aromatics fraction and a stream high in light paraffins, wherein said hydrocracking unit is a selective hydrocracking and hydro-desulphurization unit suitable for preserving aromatic species whilst simultaneously dehydrogenating a portion of naphthenic species into aromatic species, hydrocracking non-aromatic species, including remaining naphthenic species and paraffinic species, into LPG species and hydro-desulphurising any organo-sulphur species present with the non-aromatic species, wherein the operating conditions are WHSV between 0.5 and 3 hr.sup.1, a H.sub.2:hydrocarbon ratio between 2:1 and 4:1, pressure between 689475 Pa and 2757902 Pa and reactor inlet temperature of 550 C.; (d) converting said raffinate fraction in a steam cracking unit into light olefins.

Description

(1) The sole FIGURE is an embodiment of the present method of producing aromatics and light olefins from a hydrocarbon feedstock.

EXAMPLE

(2) The process scheme can be found in the sole FIGURE. Naphtha as feedstock 11 is sent to a solvent extraction unit 2 and separated into a bottom stream 15 comprising aromatics, naphthenes, light paraffins and solvent and a raffinate 24 comprising normal and iso paraffins and solvent. To minimize solvent losses the solvent may be separated from the raffinate 24 by washing the raffinate stream 24 with water (not shown) which produces an essentially solvent free raffinate stream and a water stream containing some solvent. The latter stream may then be distilled to evaporate the water, and the raffinate stream 24 is sent to steam cracker unit 1. The solvent thus recovered can be combined with solvent rich stream 17. Stream 24 may thus be purified to recover any solvent and produce an essentially solvent free raffinate stream to be sent to steam cracker unit 1.

(3) The bottom stream 15 is sent to a stripper column 3 and separated into a solvent stream 16 rich in aromatics and naphthenes and a stream 12 comprising stripped light paraffins. Stream 12 can be returned to solvent extraction unit 2 or can be sent as stream 22 to steam cracker unit 1. The stream 7, i.e. a combination of raffinate 24 and stream 22, comprising normal and iso paraffins, is sent to steam cracker unit 1.

(4) Solvent stream 16 rich in aromatics and naphthenes is sent to a distillation column 4 and separated into an extract 13 comprising aromatics and naphthenes and a solvent rich stream 17, which stream 17 is returned to the solvent extraction unit 2. Extract 13 is further treated in a hydrocracking unit 5. The stream 18 thus hydrocracked is sent to a separator 6, e.g. a distillation column 6. The top stream 21 from distillation column 6 is sent as a stream 10 to the steam cracker unit 1. In addition, it is also possible to send top stream 21 to a dehydrogenation unit 25. Top stream 21 will contain a mixture of LPG species, unused hydrogen, methane and any H2S made via the hydrodesulphurization process in hydrocracking unit 5. The LPG species may be sent either to steam cracker 1 unit or to a dehydrogenation unit 25 after suitable processing to remove the hydrogen, methane and H2S. Suitable methods for carrying out this separation include cryogenic distillation. The bottom stream 14 from the distillation column 6 can be used for further processes. In the steam cracking unit 1 a pygas containing stream 23 is preferably sent to the solvent extraction unit 2. In another embodiment pygas containing stream 26 is sent to the hydrocracking unit 5.

(5) The experimental data as provided herein were obtained by flowsheet modeling in Aspen Plus. The steam cracking kinetics were taken into account rigorously (software for steam cracker product slate calculations).

(6) Applied steam cracker furnace conditions:

(7) ethane and propane furnaces: COT (Coil Outlet temperature)=845 C. and steam-to-oil-ratio=0.37, C4-furnaces and liquid furnaces: Coil Outlet temperature=820 C. and Steam-to-oil-ratio=0.37. The dearomatization unit was modeled as a splitter into 2 streams, one stream containing all the aromatic and naphthenic components and the other stream containing all the normal- and iso-paraffinic components.

(8) For the gasoline hydrocracking, a reaction scheme has been used that is based on experimental data.

(9) As feedstock 11 naphtha was used (see Table 1)

(10) TABLE-US-00001 TABLE 1 characteristics of naphtha Naphtha n-Paraffins wt-% 36.3 i-Paraffins wt-% 27.4 Naphthenes wt-% 24.1 Aromatics wt-% 12.3 Density 60 F. kg/L 0.728 IBP C. 20 BP50 C. 120.0 FBP C. 178

(11) The specific distribution of n-paraffins, 1-paraffins, naphthenes and aromatics can be found in Table 2.

(12) TABLE-US-00002 TABLE 2 C-# N-paraffins I-paraffins Naphthenes Aromatics Sum 5 6.6 3.1 0.4 0.0 10.1 6 7.7 5.9 2.2 0.5 16.3 j 7 7.3 5.6 3.3 1.8 18 8 6.6 6.7 3.8 3.7 20.8 9 5.7 6.6 6.3 4.4 23 10 3.3 7.2 0.0 1.0 11.5 Sum 37.2 35.1 16.0 11.4

(13) The Example disclosed herein makes a distinction between a process (case 1) in which the naphtha is processed through a steam cracker unit and the present process (case 2) according to the sole FIGURE. Case 1 is a comparative example. Case 2 is an example according to the present invention.

(14) The battery product slate (wt. % of feed) can be found in Table 3.

(15) From Table 3 one can see that sending the naphthenic part of the naphtha to a hydrocracking unit causes BTX (benzene, toluene, xylenes) to increase. The present inventors assume that the hydrocracking unit converts naphthenics into aromatics.

(16) The present inventors also found that by preventing naphthenes and aromatics to be steam-cracked, the production of heavier products (C9Resin feed, Cracked Distillate and Carbon Black Oil) is reduced from 8.5% to 2.6%.

(17) Moreover, the methane production decreases, probably because the naphthenics are not being steam cracked but sent to the hydrocracking unit.

(18) In addition, the overall high value chemicals content increases from 75.3% to 80.5%.

(19) On the basis of the comparison between case 1 (comparative example) and case 2 (example according to the present invention) one can conclude that according to the present method the yield of high value products from typical naphtha can be maximized by preventing naphthenes and aromatics from generating heavier products in the steam cracker.

(20) TABLE-US-00003 TABLE 3 Feed: naphtha CASE 1 CASE 2 BATTERY LIMIT PRODUCT SLATE SC DEARO + SC + GHC H2 0.8 1.3 CH4 15.3 13.2 ETHYLENE 33.6 32.8 PROPYLENE 17.3 14.9 BUTADIENE 5.1 4.1 1-BUTENE 1.8 1.6 ISO-BUTENE 2.1 1.7 BENZENE 7.2 8.9 TX CUT 5.8 18.7 STYRENE 1.2 0.0 OTHER C7-C8 1.3 0.0 C9 RESIN FEED 2.2 0.4 CD 1.9 1.0 CBO 4.4 1.2 % HIGH VALUE CHEMICALS 75.3 80.5