Process for producing purified aromatic hydrocarbons from a mixed hydrocarbon feedstream

Abstract

The present invention relates to a process for producing benzene from a mixed hydrocarbon feedstream comprising subjecting C6 cut separated from said mixed hydrocarbon feedstream to aromatization to provide a benzene-rich aromatic stream and recovering the benzene from the benzene-rich aromatic stream.

Claims

1. A process for producing benzene comprising: (a) subjecting a mixed hydrocarbon feedstream to a separation to provide a C6 cut, wherein the mixed hydrocarbon feedstream comprises reformats, and wherein a benzene content of the C6 cut comprises 10-50 wt % benzene; (b) subjecting the C6 cut to aromatization to provide a benzene-rich aromatic stream; and (c) separating and recovering a benzene stream from the benzene-rich aromatic stream without using aromatic extraction.

2. The process according claim 1, wherein in step (c) further comprises subjecting the benzene-rich aromatic stream to hydrocracking to produce a product stream comprising benzene and C1-C4 hydrocarbons.

3. The process according to claim 2, wherein the separating comprises separating the benzene stream from the C1-C4 hydrocarbons by vapor-liquid separation.

4. The process according to claim 1, wherein the aromatization comprises contacting the C6 cut with an aromatization catalyst under aromatization conditions.

5. The process according to claim 4, wherein the aromatization conditions comprise a temperature of 400-600 C., a pressure of 50-1000 kPa gauge, and a Weight Hourly Space Velocity (WHSV) of 0.1-20 h.sup.1.

6. The process according to claim 2, wherein the hydrocracking comprises contacting the benzene-rich aromatic stream in the presence of hydrogen with a hydrocracking catalyst under hydrocracking conditions.

7. The process according to claim 6, wherein the hydrocracking conditions comprise a temperature of 450-580 C., a pressure of 300-5000 kPa gauge and a Weight Hourly Space Velocity (WHSV) of 0.1-20 h.sup.1.

8. The process according to claim 1, wherein the feedstream further comprises hydrocracked gasoline.

9. The process according to claim 2, wherein the process further comprises subjecting the benzene stream to alkylation to produce an alkylated aromatic stream.

10. The process according to claim 9, wherein the alkylation comprises contacting the benzene stream in the presence of ethylene with an alkylation catalyst under alkylation conditions to produce ethylbenzene, wherein said alkylation catalyst comprises beta zeolite, zeolite Y, ZSM-12, MCM-22 or mordenite and wherein said alkylation conditions comprise a temperature of 120-250 C., a pressure of 1000-5000 kPa, a Weight Hourly Space Velocity (WHSV) of 0.5-20 h.sup.1, and a benzene/ethylene molar ratio of 2-10.

11. The process according to claim 9, wherein the alkylation comprises contacting the benzene stream in the presence of propylene with an alkylation catalyst under alkylation conditions to produce cumene, wherein said alkylation catalyst comprises a zeolite selected from the group consisting of beta zeolite, zeolite Y, ZSM-12, MCM-22 and mordenite and wherein said alkylation conditions comprise a temperature of 120-250 C., a pressure of 1000-5000 kPa, a Weight Hourly Space Velocity (WHSV) of 0.5-20 h.sup.1, and a benzene/propylene molar ratio of 3-10.

12. The process according to claim 9, wherein the alkylated aromatic stream is subjected to a separation to provide a monoalkylated aromatic product stream and a stream comprising polyalkylated aromatic product and wherein said polyalkylated aromatic product is recycled to the hydrocracking.

13. The process according to claim 5, wherein the aromatization conditions comprise a temperature of 450-550 C. and a pressure of 75-500 kPa gauge.

14. The process according to claim 10, wherein said alkylation conditions comprise a temperature of 150-230 C., a pressure of 2500-3500 kPa, a Weight Hourly Space Velocity (WHSV) of 1-10 h.sup.1, and a benzene/ethylene molar ratio of 2-8.

15. The process according to claim 11, wherein said alkylation conditions comprise a temperature of 150-230 C., a pressure of 2500-3500 kPa, a Weight Hourly Space Velocity (WHSV) of 1-10 h.sup.1, and a benzene/propylene molar ratio of 5-8.

16. The process according to claim 1, wherein the C6 cut comprises at least 95 wt-% C6 hydrocarbons.

17. The process according to claim 1, wherein the benzene content of the C6 cut comprises 17-50 wt %.

18. The process according to claim 1, wherein the benzene content of the C6 cut comprises 35-50 wt % benzene.

19. The process according to claim 1, wherein separating and recovering the benzene stream from the benzene-rich aromatic stream is done without extractive distillation.

20. The process according to claim 1, wherein separating and recovering the benzene stream from the benzene-rich aromatic stream is done without liquid extraction.

Description

EXAMPLE 1

Cs-exchanged Pt/GeZSM-5 Aromatization Catalyst

(1) This catalyst consists of Pt dispersed on a basic ZSM-5 zeolite containing framework germanium (1% Pt/CsGeZSM-5). This catalyst may be prepared as described in U.S. Pat. No. 7,902,413 or US 2008/0293990 A1.

(2) Accordingly, Ge-ZSM-5 Zeolite may be prepared as follows: Solution #1 is made by diluting 15.84 g of 50 wt % NaOH solution with 131.25 g of deionized (DI) water and subsequently dissolving 7.11 g of germanium dioxide. Solution #2 is made by diluting 3.84 g sodium aluminate solution (23.6 wt % alumina and 19.4 wt sodium oxide) with 153.9 g DI water. Solution #1 is added to 150 g Ludox AS-40 (40 wt % silica in a colloidal state) and vigorously stirred for 10 minutes to obtain a homogeneous mixture. Solution #2 is stirred into this mixture. After 15 minutes of vigorous agitation, 105.42 g of tetra-n-propyl ammonium hydroxide (TPAOH) is added and the mixture is stirred for 60 minutes. Finally, 23.32 g of glacial acetic acid is added to the gel to adjust the pH of the mixture to about 9. This mixture is loaded into a 1 L stainless steel autoclave and heated at 160 C. for 36 hours with stirring. Subsequently, the solids obtained are filtered from the mother liquor and washed with DI water. The solid is calcined at 550 C. for 6 hours in an oven with air flow. The MFI structure of the solid can be confirmed by measuring the powder X-Ray diffraction pattern.

(3) 8 grams of GeZSM-5 prepared as described above are washed with 200 ml of aqueous CsNO.sub.3 (0.5M) then filtered. The filtrate is then rewashed 3 more times with 0.5M CsNO.sub.3 and rinsed with distilled H.sub.2O on the final filtering. The zeolite powder is then calcined for 3 hours at 280 C. in air. Incipient wetness impregnation is carried out by adding drop wise a solution of 0.069 g Pt(NH.sub.2).sub.4(NO.sub.3).sub.2 dissolved in 1.343 g of deionized water to 3.508 grams of the Cs-exchanged Ge ZSM-5. The material is dried for 1 hour in a 110 C. drying oven then calcined at 280 C. for 3 hours. A representative elemental analysis gives 39.92 wt % Si, 0.69 wt % Al, 4.14 wt Ge, 5.03 wt % Cs, and 0.90 wt % Pt. The catalyst powder is typically pressed and sized to 20-40 mesh.

EXAMPLE 2

Aromatization of C6 Heart Cut

(4) The experimental data as provided herein were obtained by modelling the product slates of an aromatization unit fed with reformate C6 heart cut feed. In this aromatization isohexanes (iso-C6) and normal hexanes (n-C6) are transformed into benzene, naphthenic species are dehydrogenated into benzene.

(5) Reaction tests are carried out using a 0.31 inch ID reactor tube containing a catalyst bed comprising 1 to 4.32 cm.sup.3 of the aromatization catalysts as described above in Examples 1 and 2. The bed is diluted to a total of 8 cm.sup.3 with inert silicon carbide to maintain constant length. Liquid n hexane is vaporized and passed over the catalyst bed at temperatures ranging from 500 to 540 C., pressures between 103 kPa absolute (15 psia) and 310 kPa absolute (45 psia), and liquid hourly space velocities ranging from 1 to 8 hr.sup.1. Products are analysed by on-line gas chromatography. Further experiments are carried out in two adiabatic pilot reactors connected in series, where n-hexane or light naphtha is vaporized and passed over a bed containing 80 g of catalyst per reactor, at an inlet temperature of 540 C., outlet temperatures at or above 450 C., and pressures ranging from 241 kPa absolute (35 psia) to 62 kPa absolute (9 psia). For these experiments, products are analysed both by on-line gas chromatography and by off-line analysis of collected liquid samples.

(6) The experimental data as provided herein were obtained by modelling the product slates of an aromatization unit fed with reformate C6 heart cut feed. In this example three cases are considered, i.e. a low, medium and high benzene concentration in the c6 heart cut feed. One-pass experiments allow the estimation of what conversions would be obtained in a complete process using partial recycle of unconverted hexanes; the predicted conversions are shown in Table 1:

(7) TABLE-US-00001 TABLE 1 Conversions for C6 hydrocarbons obtained in aromatization experiment described above. % Benzene 0 iso-C6 25 n-C6 75 cyclo-C6 100

(8) A C6 cut from reformate may vary in composition. Roughly, the benzene content varies between 10-50 wt % with the remainder being mainly paraffins, of which iso-paraffins are much more dominant. The naphthene content (mainly cyclohexane cyclo-C6) typically is below 10 wt % since refinery reformers dehydrogenate the naphthenic species almost completely.

(9) TABLE-US-00002 TABLE 2 Three feed scenarios modelled in this example C6 heart cut feed composition (wt %) LOW MEDIUM HIGH Benzene 17 35 50 iso-C6 56 44 34 n-C6 22 17 13 cyclo-C6 5 4 3 Sum 100 100 100

(10) Based on the examples explained above and given the obtained conversions explained in table 1 the following product slates are modelled for the three feed scenario's described in table 2.

(11) The tables below indicate the estimated effluent composition of an aromatization unit with Cs-exchanged Pt/GeZSM-5 aromatization catalyst.

(12) TABLE-US-00003 TABLE 3 LNA effluent composition in aromatization unit with using Cs-exchanged Pt/GeZSM-5 aromatization catalyst as described in Example 1. All numbers in wt %. LNA effluent composition LOW MEDIUM HIGH Benzene 46 58 70 iso-C6 42 33 25 n-C6 6 4 3 cyclo-C6 0 0 0 Hydrogen & Light gases 6.5 4.8 1.4 (C1-C4)

(13) Accordingly, it was found that n-C6, cyclo-C6 and the iso-C6 comprised in the C6 heart cut are converted into benzene when using a Cs-exchanged Pt/GeZSM-5 aromatization catalyst.

EXAMPLE 3

Aromatic Alkylation

(14) The benzene rich effluent stream from the aromatization unit is subsequently subjected to an alkylation unit where an olefins source is added to alkylate the benzene to ethylbenzene and/or cumene. The alkylation process step is modelled based on literature data published in Laredo et al. (2009) Applied Catalysis A: General 363, 11-18. Accordingly, the benzene rich effluent stream from the aromatization unit is subjected to an alkylation unit loaded with zeolite Beta (220 C., 3100-4800 kPa and benzene-to-olefin ratio of 2), where EB and cumene are produced with approximately 50% conversion (Laredo et al. (2009) loc. cit).

(15) The major advantage of boosting the benzene concentration by means of an aromatization unit is the fact that less tonne of C6 heart cut feedstock is required to produce one tonne of cumene/ethylbenzene. Secondly when using an aromatization unit containing the Cs-exchanged Pt/GeZSM-5 aromatization catalysts then the benzene concentration is even higher resulting in a further decrease of required C6 heart cut feedstock per tonne of cumene/ethylbenzene. This is illustrated by the numbers in the table below.

(16) TABLE-US-00004 Case LOW MEDIUM HIGH 1 Required feedstock/tonne of cumene 3.8 1.9 1.3 without aromatization unit (tonne C6 feed/tonne cumene) 2 Required feedstock/tonne of cumene 1.4 1.1 0.9 with aromatization unit with Cs-exchanged Pt/GeZSM-5 catalyst (tonne C6 feed/tonne cumene)

(17) The LOW, MEDIUM and HIGH cases refer to the benzene concentration in the C6 heart cut feed.

(18) Comparing case 1 and 2 illustrates the fact that processing the C6 heart cut feed in an aromatization unit results in a lower overall feed consumption per tonne of cumene.

(19) In the context of the present invention, it was further surprisingly found that the aromatization catalysts considered have an improved carbon number preservation. This can be exploited to increase the benzene concentration in a C6 heart cut from reformate since this C6 cut contains a significant amount of paraffins (iso+normal). By feeding this C6 cut to an aromatization unit using a Cs-exchanged Pt/GeZSM-5 aromatization catalyst, the benzene concentration can be increased significantly. The main benefit of the Cs-exchanged Pt/GeZSM-5 aromatization catalyst compared to other known aromatization catalysts is the fact that the Cs-exchanged Pt/GeZSM-5 catalysts also aromatizes iso-paraffins, iso-hexanes to benzene in our case.