PROCESS AND PLANT FOR PREPARING PURIFIED BENZENE COMPOSITION FROM CRUDE HYDROCARBON STREAM CONTAINING BENZENE

Abstract

A process for preparing a purified benzene composition from a crude hydrocarbon stream containing at least 10% by volume of benzene is provided. The process comprises subjecting the crude hydrocarbon stream and a further recycled benzene containing stream to a solvent-based extraction so as to produce a benzene enriched aromatic stream and a benzene depleted non-aromatic stream, subjecting the benzene enriched aromatic stream to a hydrodesulfurization so as to obtain a desulfurized aromatic stream, subjecting the desulfurized aromatic stream to a distillation producing a purified benzene stream and a further benzene containing stream having a benzene concentration of between less than 100% by weight and the azeotropic benzene concentration, and at least partially recycling the further benzene containing stream.

Claims

1. A process for preparing a purified benzene composition from a crude hydrocarbon stream containing at least 10% by volume of benzene comprising the following steps: a) subjecting the crude hydrocarbon stream and a further recycled benzene containing stream to a solvent based extraction so as to produce a benzene enriched aromatic stream and a benzene depleted nonaromatic stream, b) subjecting the benzene enriched aromatic stream to a hydrodesulfurization so as to obtain a desulfurized aromatic stream, c) subjecting the desulfurized aromatic stream to a distillation so as to produce a purified benzene stream and a further benzene containing stream, wherein the further benzene containing stream comprises benzene and at least one of cyclohexane, methylcyclopentane, 2.3 dimethyl pentane and 2.4 dimethyl pentane and has a benzene concentration of between less than 100% by weight and the azeotropic benzene concentration in the azeotrope formed of these compounds as dictated by the vapor-liquid equilibrium of the compounds contained in the further benzene containing stream, and d) at least partially recycling the further benzene.

2. The process in accordance with claim 1, wherein the crude hydrocarbon stream contains, based on 100% by weight, at least 20% by volume of benzene.

3. The process in accordance with claim 1, wherein the crude hydrocarbon stream contains, based on 100% by weight, i) 20 to 60% by volume of paraffins, ii) 10 to 50% by volume of olefins and iii) 5 to 40% by volume of naphthenes.

4. The process in accordance with claim 1, wherein the crude hydrocarbon stream is a C.sub.6-cut or a C.sub.6-C.sub.8-cut from fluid catalytic cracked naphtha or a C.sub.6-cut or a C.sub.6-C.sub.8-cut from a coke oven light oil.

5. The process in accordance with claim 1, wherein the crude hydrocarbon stream is mixed with the recycled further benzene containing stream to obtain a feed stream, which is subjected to the solvent based extraction.

6. The process in accordance with claim 1, wherein the crude hydrocarbon stream and the recycled further benzene containing stream are separately fed to the solvent based extraction.

7. The process in accordance with claim 1, wherein the crude hydrocarbon stream and the recycled further benzene containing stream are subjected to an extractive distillation.

8. The process in accordance with claim 1, wherein the solvent used in the solvent based extraction is selected from a group consisting of sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-ethylsulfolane, N-methyl pyrrolidone, 2-pyrrolidone, N-ethyl pyrrolidone, N-propyl pyrrolidone, N-formyl morpholine, dimethylsulfone, diethylsulfone, methylethylsulfone, dipropylsulfone, dibutylsulfone, tetraethylene glycol, triethylene glycol, dimethylene glycol, ethylene glycol, ethylene carbonate, and propylene carbonate or a combination thereof.

9. The process in accordance with claim 1, wherein the desulfurized aromatic stream is subjected to one or more distillation steps, wherein at least one distillation step is performed with a top divided wall column, wherein one distillation step is performed and the wall of the top divided wall column extends from the upper end of the top divided wall column perpendicular downwards over 20 to 70% of the height of the top divided wall column.

10. The process in accordance with claim 1, wherein the desulfurized aromatic stream is subjected to two distillation steps, wherein the desulfurized aromatic stream is subjected to a first distillation step in a first distillation column so as to obtain at the overhead of the first distillation column an off-gas stream containing C.sub.1-C.sub.7-hydrocarbons, hydrogen and hydrogen sulfide and at the bottom of the first distillation column a liquid stream with the remaining compounds, wherein the liquid stream is subjected to a second distillation step in a second distillation column so as to obtain as side stream of the second distillation column the purified benzene stream, at the overhead of the second distillation column the further benzene containing stream and at the bottom of the second distillation column a C.sub.7+-stream.

11. The process in accordance with claim 1, wherein the desulfurized aromatic stream is subjected to two distillation steps, wherein the desulfurized aromatic stream is subjected to a first distillation step in a first column so as to obtain at the overhead of the first distillation column an offgas stream containing C.sub.1-C.sub.7-hydrocarbons, hydrogen and hydrogen sulfide and at the bottom of the first distillation column a liquid stream with the remaining compounds, wherein the liquid stream is subjected to a second distillation step in a second middle divided wall column so as to obtain as first side stream the purified benzene stream, as second side stream a toluene rich stream, at the overhead of the second distillation column the further benzene containing stream and at the bottom of the second distillation column a stream enriched in xylenes.

12. A plant for purifying a crude hydrocarbon stream containing at least 10% by volume of benzene comprising: i) a solvent based extraction unit having at least one inlet line and an outlet line for a benzene enriched aromatic stream, ii) a hydrodesulfurization unit having an inlet line being connected with the outlet line of the solvent based extraction unit, and having an outlet line for a desulfurized aromatic stream, iii) a distillation unit having an inlet line being connected with the outlet line of the hydrodesulfurization unit, an outlet line for a purified benzene stream and an outlet line for a further benzene containing stream, and iv) a recirculation line connecting the outlet line for the further benzene containing stream of the distillation unit with one of the at least one inlet line of the solvent based extraction unit.

13. The plant in accordance with claim 12, wherein the distillation unit comprises only one distillation column having 5 to 100.

14. The plant in accordance with claim 13, wherein the distillation column is a top divided wall column, wherein the wall of the top divided wall column extends from the upper end of the top divided wall column downwards over 20 to 70% the height of the top divided wall column.

15. The plant in accordance with claim 12, wherein the distillation unit comprises two distillation columns, wherein one of the two distillation columns has 5 to 100 theoretical stages, wherein the first of the two distillation columns has an inlet line connected to the outlet line of the hydrodesulfurization unit, has 5 to 30 theoretical stages and has at its bottom an outlet line for a liquid stream, wherein the second of the two distillation columns has an inlet line connected to the outlet line for a liquid stream of the first distillation column, has the outlet line for a purified benzene stream and has the outlet line for a further benzene containing stream.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Specific embodiments in accordance with the present disclosure are subsequently described with reference to the appended drawings and by examples.

[0053] FIG. 1a illustrates a schematic view of a plant for preparing a purified benzene composition from a crude hydrocarbon stream containing benzene in accordance with the prior art.

[0054] FIG. 1b illustrates a more detailed schematic view of the solvent based extraction unit being part of the plant shown in FIG. 1a.

[0055] FIG. 2 illustrates a schematic view of a plant for preparing a purified benzene composition from a crude hydrocarbon stream containing benzene in accordance with a first embodiment of the present disclosure.

[0056] FIG. 3 illustrates a schematic view of a plant for preparing a purified benzene composition from a crude hydrocarbon stream containing benzene in accordance with a second embodiment of the present disclosure.

[0057] FIG. 4 illustrates a schematic view of a plant for preparing a purified benzene composition from a crude hydrocarbon stream containing benzene in accordance with a third embodiment of the present disclosure.

[0058] FIG. 5 illustrates a schematic view of a plant for preparing a purified benzene composition from a crude hydrocarbon stream containing benzene in accordance with a fourth embodiment of the present disclosure.

DETAILED DESCRIPTION

[0059] The plant 10 according to the prior art shown in FIGS. 1a and 1b comprises a solvent based extraction unit 12, a hydrodesulfurization unit 14 and a distillation unit 16. The solvent based extraction unit 12 comprises an inlet line 18 for feeding crude hydrocarbon stream into the solvent based extraction unit 12, an outlet line 20 for a benzene enriched aromatic stream and an outlet line 22 for a benzene depleted non-aromatic stream. The outlet line 20 for a benzene enriched aromatic stream is also the inlet line for the hydrodesulfurization unit 14, which further has an inlet line 23 for hydrogen as well as an outlet line 24 for desulfurized aromatic stream. The outlet line 24 for desulfurized aromatic stream is the inlet line for the distillation unit 16, which further has an outlet line 26 for purified benzene stream and an outlet line 28 for off-gas. The distillation unit 16 comprises one distillation column 16′, which has 16 theoretical stages.

[0060] The solvent based extraction unit 12 is in more detail shown in FIG. 1b and comprises a first distillation column 30 and a second distillation column 32. The inlet line 18 for feeding crude hydrocarbon stream is connected with the middle part of the first distillation column 30. Moreover, a solvent (recirculation) line 34 leads into the upper part of the first distillation column 30. During the distillation, a benzene depleted non-aromatic stream is produced as overhead stream and is withdrawn from the first distillation column 30 via the outlet line 22. At the bottom of the first distillation column 30 a mixture of solvent and a benzene enriched aromatic stream is obtained, which is withdrawn from the first distillation column 30 via the line 36 and then fed into the second distillation column 32, in which the solvent is separated from the benzene enriched aromatic stream. While the regenerated solvent is led back from the bottom of the second distillation column 32 via the solvent (recirculation) line 34 into the upper part of the first distillation column 30, the benzene enriched aromatic stream is withdrawn from the overhead of the second distillation column 32 via line 20 and is fed into the hydrodesulfurization unit 14.

[0061] During the operation of the plant 10, the benzene enriched aromatic stream is fed via line 20 and hydrogen is fed via inlet line 23 into the hydrodesulfurization unit 14, in which the sulfur containing compounds as impurities are desulfurized into non-sulfur compounds and hydrogen sulfide. The hydrodesulfurization unit also removes the nitrogen species contained in the benzene enriched aromatic stream so as to ensure that the produced desulfurized aromatic stream fulfils the total nitrogen specification. The produced desulfurized aromatic stream is fed via line 24 into the distillation column 16′. An off-gas stream obtained during the distillation as overhead product and containing low molecular weight hydrocarbons, such as C1-C5-hydrocarbons, in particular methane, ethane, propane, butane, hydrogen (remained after the hydrodesulfurization) and hydrogen sulfide (produced during the hydrodesulfurization), is withdrawn from the distillation column 16′ via outlet line 28, whereas the purified benzene stream obtained as bottom product is withdrawn from the distillation column 16′ via outlet line 26.

[0062] In contrast thereto, the process performed in the plant 10 in accordance with the present disclosure shown in FIGS. 2 to 5 comprises the step of subjecting the desulfurized aromatic stream obtained in the hydrodesulfurization unit 14 to a distillation so as to produce a purified benzene stream and a further benzene containing stream, and the step of at least partially and preferably completely recycling the so obtained further benzene containing stream into the solvent based extraction unit 12. For this purpose, the plant 10 in accordance with the present disclosure shown in FIGS. 2 to 5 comprises a recirculation line 38 for the further benzene containing stream obtained in the distillation unit 16, wherein the recirculation line 34 connects the distillation unit 16 and the solvent based extraction unit 12. Moreover, the distillation unit 16 comprises for this purpose at least one distillation column having a suitable high number of theoretical stages so that the desulfurized aromatic stream obtained in the hydrodesulfurization unit 14 is separated during the distillation into a purified benzene stream and a further benzene containing stream. Preferably, the number of theoretical stages of this distillation column is 5 to 100, more preferably 10 to 80, even more preferably 15 to 60, still more preferably 20 to 60 and most preferably 30 to 60.

[0063] By performing the distillation in step c) so that the desulfurized aromatic stream obtained in the hydrodesulfurization of step b) is separated into a purified benzene stream and a further benzene containing stream, the at least one of cyclohexane, methylcyclopentane, 2,3 dimethyl pentane and 2,4 dimethyl pentane contained as impurities in the further benzene containing stream, is reliably and nearly completely separated from the purified benzene stream, thus leading to a benzene stream having a very high purity of for instance at least 99.8% by volume. Since the further benzene containing stream is recycled at least partially and preferably completely to the solvent based extraction of step a), none or, if at all, only very minor amounts of the benzene contained in the further benzene containing stream is lost. This is due to the fact that the further benzene containing stream is broken in the solvent based extraction step a) to a benzene enriched aromatic stream and a benzene depleted nonaromatic stream, i.e. the benzene and other aromatics contained in the further benzene containing stream are separated from the at least one of cyclohexane, methylcyclopentane, 2,3 dimethyl pentane and 2,4 dimethyl pentane contained as impurities or other impurities in form of olefins, paraffins, naphthenes and other non-aromatic compounds. By subjecting the benzene enriched aromatic stream after the solvent based extraction to a hydrodesulfurization so as to obtain a desulfurized aromatic stream and then by subjecting the desulfurized aromatic stream to a distillation so as to produce a purified benzene stream and a further benzene containing stream, the benzene recycled via the further benzene containing stream from the distillation to the solvent based extraction is indeed recovered. All in all, the process in accordance with the present disclosure allows to prepare from a crude hydrocarbon stream containing benzene reliably and with high yield a purified benzene composition with a high benzene concentration of for instance at least 99.8% by volume and the desired at most minimal amount of sulfur compound impurities of for example at most 1 ppmw, qualified as total sulfur, and total nitrogen content of less than 1 ppmw. In particular, these advantageous effects are even obtained, when a C.sub.6-cut or a C.sub.6-C.sub.8-cut of FCC gasoline or a C.sub.6-cut or a C.sub.6-C.sub.8-cut of COLO is used as feedstock of the process. In addition, these advantageous effects are obtained even in the case of a very long operation time including start-of-run to end-of-run operating temperature envelope of catalyst, in the case of heavier than expected feedstock being fed to the FCC, RFCC or COLO or in the case of slippage of C.sub.6-cyclic olefins and/or C.sub.7-isoolefins into the aromatic stream obtained in the solvent based extraction step.

[0064] In the embodiment shown in FIG. 2, the distillation unit 16 comprises one distillation column 16′ preferably having 5 to 100 theoretical stages, wherein the distillation is preferably performed at a pressure of 0.1 to 0.2 MPa, such as 0.15 MPa, and at a temperature 90 to 100° C., such as 95° C. This embodiment is particularly suitable when using as feedstock a C.sub.6-cut from fluid catalytic cracked naphtha or a coke oven light oil.

[0065] The embodiment shown in FIG. 3 differs from that shown in FIG. 2 in that the distillation unit 16 comprises a divided wall column 17 and more specifically a top divided wall column 17. Preferably, the wall of the top divided wall column 17 extends from the upper end of the top divided wall column 17 perpendicular downwards over 20 to 70% and preferably over 20 to 50% of the height of the top divided wall column. In this embodiment, at the left side of the overhead of divided wall column 17 an off-gas stream containing low molecular weight hydrocarbons, such as methane, ethane, propane, butane, hydrogen and hydrogen sulfide is obtained, whereas at the right side of the overhead of the divided wall column 17 the further benzene containing stream is obtained and at the bottom of the divided wall column 17 the purified benzene stream is obtained. Preferably, the top divided wall column 17 has 5 to 100 theoretical stages, wherein the distillation is preferably performed at a pressure of 0.1 to 0.2 MPa, such as 0.15 MPa, and at a temperature 90 to 100° C., such as 95° C. The use of the divided wall column 17 has the advantages that the divided wall distillation column 17 requires a reduced height for obtaining the same separation result compared to a distillation column being no divided wall column and that it needs in operation less energy than a traditional distillation column without dividing wall. This embodiment is in particular suitable, when using as a crude hydrocarbon stream a C.sub.6-cut from fluid catalytic cracked naphtha or a C.sub.6-cut from a coke oven light oil.

[0066] The plant shown in FIG. 3 is also in particular suitable, when using as a crude hydrocarbon stream a C.sub.6-C.sub.8-cut from fluid catalytic cracked naphtha or a C.sub.6-C.sub.8-cut from a coke oven light oil. In this case, at the bottom of the divided wall distillation column 17 not a purified benzene stream (i.e. not a C.sub.6-stream) is obtained, but a purified C.sub.6-C.sub.8-aromatics stream. This C.sub.6-C.sub.8-aromatics stream can then be easily processed, for instance in a conventional distillation column, so as to separate the C.sub.6-C.sub.8-aromatics stream into a purified benzene stream and one or more other aromatics streams.

[0067] In the embodiment shown in FIG. 4, the distillation unit 16 comprises two distillation columns 40, 42. The first distillation column 40 preferably has 5 to 30 theoretical stages, whereas the second distillation column 42 has preferably 5 to 100 theoretical stages. While the distillation in the first distillation column 40 is preferably performed at a pressure of 0.1 to 0.2 MPa, such as 0.15 MPa, and at a temperature 90 to 100° C., such as 95° C., the distillation in the second distillation column 42 is preferably performed at a pressure of 0.2 to 0.3 MPa, such as 0.25 MPa, and at a temperature 90 to 140° C. During the distillation performed in the first distillation column 40 at the overhead of the first distillation column an off-gas stream containing low molecular weight hydrocarbons, i.e. C.sub.1-C.sub.4-hydrocarbons, such as methane, ethane, propane and butane, hydrogen and hydrogen sulfide is obtained and withdrawn via outlet line 28, whereas at the bottom of the first distillation column 40 a liquid with the remaining compounds stream is obtained. This liquid stream is fed via liquid line 43 into the second distillation column 42 and distilled in the second distillation column 42 so as to obtain as side stream of the second distillation column 42 the purified benzene stream, which is withdrawn via outlet line 26, at the overhead of the second distillation column 42 the further benzene containing stream, which is recycled to the solvent based extraction unit 12 via the recirculation line 38, and at the bottom of the second distillation column 42 a C.sub.7+-stream, which is withdrawn via outlet line 44. This embodiment is in particular suitable, when using as a crude hydrocarbon stream a C.sub.6-C.sub.8-cut from fluid catalytic cracked naphtha or a coke oven light oil.

[0068] The embodiment shown in FIG. 5 differs from that shown in FIG. 4 in that the distillation unit 16 comprises as second distillation column 42 a divided wall column and more specifically a middle divided wall column. Preferably, the wall of the middle divided wall column 42 extends, seen from the bottom to the overhead of the second distillation column, from a point being located at 10 to 30% of the distance from the bottom to the overhead of the second distillation column to a point being located at 70 to 90% of the distance from the bottom to the overhead of the second distillation column. The first distillation column 40 preferably has 5 to 30 theoretical stages, whereas the second distillation column 42 has preferably 5 to 100 theoretical stages. While the distillation in the first distillation column 40 is preferably performed at a pressure of 0.1 to 0.2 MPa, such as 0.15 MPa, and at a temperature 90 to 100° C. such as 95° C., the distillation in the second distillation column 42 is preferably performed at a pressure of 0.2 to 0.3 MPa, such as 0.25 MPa, and at a temperature 90 to 160° C. During the distillation performed in the first distillation column 40 at the overhead of the first distillation column an off-gas stream containing low molecular weight hydrocarbons. i.e. C.sub.1-C.sub.4-hydrocarbons, such as methane, ethane, propane and butane, hydrogen and hydrogen sulfide is obtained and withdrawn via outlet line 28, whereas at the bottom of the first distillation column 40 a liquid with the remaining compounds stream is obtained. This liquid stream is fed via liquid line 43 into the second distillation column 42 and is distilled in the second distillation column 42 so as to obtain as a first side stream of the second distillation column 42 the purified benzene stream, which is withdrawn via outlet line 26, as a second side stream of the second distillation column 42 a toluene rich fraction, which is withdrawn via outlet line 46, at the overhead of the second distillation column 42 the further benzene containing stream, which is recycled to the solvent based extraction unit 12 via the recirculation line 38, and at the bottom of the second distillation column 42 a stream rich in mixed xylenes, which is withdrawn via outlet line 48. This embodiment is in particular suitable, when using as a crude hydrocarbon stream a C.sub.6-C.sub.8-cut from fluid catalytic cracked naphtha or a coke oven light oil.

[0069] Subsequently, the present disclosure is described with reference to illustrative, but not limiting examples.

EXAMPLES

Example

[0070] With the software ASPEN HYSYS a process has been simulated, which is performed in a plant as shown in FIG. 3 using a C.sub.6-cut of fluid catalytic cracked naphtha as crude hydrocarbon stream. A ring loss of 0.4 wt %, which is above the design value of 0.1 wt %, was assumed. Moreover, a benzene recovery of more than 99% has been assumed during the solvent based extraction step and a benzene purity of more than 99.9 wt % has been set as targets.

[0071] The composition of the desulfurized aromatic stream, the composition of the further benzene containing stream as well as the temperature conditions, the pressure conditions and other parameters during the distillation in the top divided wall distillation column 17 are shown in the below tables.

Comparative Example

[0072] A respective process performed in a plant as shown in FIG. 1 has been simulated using the same C.sub.6-cut of fluid catalytic cracked naphtha as crude hydrocarbon stream.

[0073] The composition of the desulfurized aromatic stream as well as the temperature conditions, the pressure conditions and other parameters during the distillation in the distillation column 16′ are shown in the below tables.

TABLE-US-00001 TABLE 1 Composition, temperature and pressure of the desulfurized aromatic stream Comparative Example Example Pressure (bara) 20.5 20.5 Temperature (° C.) 150 150 Compound kg/h kg/h Benzene 9604.2 10230.3 Cyclohexane 45.2 48.6 Methylcyclopentane 0.1 0.1 Methane 0.0 0.0 Carbon-Dioxide 0.0 0.0 Ammonia 0.1 0.1 Propane 0.0 0.0 Isobutane 0.2 0.2 n-Butane 4.9 4.9 Thiophene 0.0 0.0 Toluene 3.2 3.2 Hydrogen 1.5 1.5 Hydrogen-Sulfide 2.8 2.8

[0074] It is to be noted that in Example, the feed is higher because it includes the recycled further benzene containing stream.

TABLE-US-00002 TABLE 2 Parameters of the distillation column used for the distillation of step c) and product characteristics Comparative Example Example No. of Theoretical stages 16 7 (left side of wall), 7 (right side of wall), 13 non-walled section Column Top Pressure (bara) 1.5 1.5 (left side of wall), 1.5 (right side of wall) Column Bottom Pressure (bara) 1.66 1.7 Column Top Tray Temperature 91.8 92.22 (left wall)/ (° C.) 92.96 (right wall) Column Bottom Tray 96.85 97.9 Temperature (° C.) Overhead Cond Process Side 40 40 (left condenser)/ Cooled Temperature (° C.) 60 (right condenser) Cond Pressure Drop (bara) 0.5 0.5 (left condenser)/ 0.3 (right condenser) Reboiler Duty (Kcal/h) 20451.3 2706005.7 Feed location 6.sup.th stage 3 stage on left side from top of wall Benzene purity (wt %) 99.49% 99.80% Column Bottoms Product flow 9631 9616 (kg/h)

TABLE-US-00003 TABLE 3 Composition, temperature and pressure of the further benzene containing stream obtained by the distillation of step c) of the Example. Pressure (bara) 1.2 Temperature (° C.) 60 Compound kg/h Benzene 593.2 Cyclohexane 32.1 Methylcyclopentane 0.1 Methane 0.0 Carbon-Dioxide 0.0 Ammonia 0.0 Propane 0.0 Isobutane 0.0 n-Butane 0.0 Thiophene 0.0 Toluene 0.0 Hydrogen 0.0 Hydrogen-Sulfide 0.00