Process for producing benzene from a C5-C12 hydrocarbon mixture

Abstract

The invention relates to a process for producing benzene, comprising the steps of: (a) providing a hydrocracking feed stream comprising C5-C12 hydrocarbons, (b) contacting the hydrocracking feed stream in the presence of hydrogen with a hydrocracking catalyst comprising 0.01-1 wt-% hydrogenation metal in relation to the total catalyst weight and a zeolite having a pore size of 5-8 A and a silica (SiO2) to alumina (Al.sub.2O.sub.3) molar ratio of 5-200 under process conditions including a temperature of 425-580? C., a pressure of 300-5000 kPa gauge and a Weight Hourly Space Velocity of 0.1-15 h.sup.?1 to produce a hydrocracking product stream comprising benzene, toluene and C8+ hydrocarbons, (c) separating benzene, toluene and the C8+ hydrocarbons from the hydrocracking product stream and (d) selectively recycling back at least part of the toluene from the separated products of step (c) to be included in the hydrocracking feed stream process for producing benzene from a c5-c12 hydrocarbon mixture

Claims

1. A process for producing benzene, comprising the steps of: (a) providing a hydrocracking feed stream comprising C5-C12 hydrocarbons, (b) contacting the hydrocracking feed stream in the presence of hydrogen with a hydrocracking catalyst comprising 0.01-1 wt-% hydrogenation metal in relation to the total catalyst weight and a zeolite having a pore size of 5-8 ? and a silica (SiO2) to alumina (Al.sub.2O.sub.3) molar ratio of 5-200 under process conditions including a temperature of 425-580? C., a pressure of 300-5000 kPa gauge and a Weight Hourly Space Velocity of 0.1-15 h.sup.?1 to produce a hydrocracking product stream comprising benzene, toluene and C8+ hydrocarbons, (c) separating benzene, toluene and the C8+ hydrocarbons from the hydrocracking product stream, and (d) selectively recycling at least part of the toluene from the separated products of step (c) to the hydrocracking feed stream, wherein the amount of toluene to be recycled to the hydrocracking feed stream is set so that the ratio of [molar amount of benzene+molar amount of ethylbenzene]*[molar amount of xylene] divided by [molar amount of toluene].sup.2 in the hydrocracking feed stream is 0.05 to 0.15.

2. The process according to claim 1, wherein step (a) involves mixing a fresh feed stream and the recycled toluene, wherein the fresh feed stream comprises at least 10 wt % benzene.

3. The process according to claim 2, wherein the fresh feed stream is pyrolysis gasoline, straight run naphtha, light coker naphtha, coke oven light oil or mixtures thereof, optionally after being hydrogenated in a hydrogenation reactor.

4. The process according to claim 1, wherein the hydrocracking feed stream has not been subjected to the step of removing benzene.

5. The process according to claim 1, wherein the hydrocracking feed stream comprises at least 1 wt % benzene.

6. The process according to claim 1, wherein the hydrocracking catalyst comprises less than 0.01 parts tin, less than 0.02 parts lead, and less than 0.01 parts bismuth on the basis of 100 parts by weight of the total catalyst.

7. The process according to claim 1, wherein the amount of toluene to be recycled back to the hydrocracking feed stream is set so that the proportion of benzene in the hydrocracking product stream is 0-5 mol % higher than the proportion of benzene in the hydrocracking feed stream.

8. The process according to claim 1, wherein xylene is separated from the hydrocracking product stream.

9. The process according to claim 1, wherein the zeolite is a ZSM-5 zeolite.

10. The process according to claim 1, wherein the hydrogenation metal is platinum.

11. The process according to claim 1, wherein the hydrocracking catalyst comprises the hydrogenation metal on a zeolite-based support.

12. The process according to claim 2, wherein the fresh feed stream comprises at least 20 wt % benzene.

13. The process according to claim 12, wherein the fresh stream comprises at least 30 wt % benzene.

Description

EXAMPLES

(1) Feed mixtures comprising different compositions of hydrocarbons were subjected to hydrocracking in order to determine the influence of the feed compositions to the product compositions. The experiments were carried out in a 12 mm reactor, wherein the catalyst bed was located in the isothermal zone of the reactor heater. The catalyst used was 2.0 g of Pt deposited on ZSM-5, wherein SiO.sub.2/Al.sub.2O.sub.3=50. The amount of Pt was 0.08 wt % with respect to the total of Pt and ZSM-5. No binder was used in the catalyst.

(2) The feed streams were fed to the reactor. The feed stream enters a vaporizer section prior to the reactor where it is vaporized at 280? C. and mixed with hydrogen gas. The conditions used throughout these experiments were: WHSV=6/hr, pressure was 1379 kPa (200 psig), temperature was 475? C. and the molar ratio H.sub.2/hydrocarbons was 3. The effluent of the reactor was sampled in the gas phase to an online gas chromatograph. Product analyses were carried out once per hour.

(3) In experiment 1, different feed streams (Feed 1-5) obtained by adding toluene to pyrolysis gasoline samples were subjected to hydrocracking. The compositions of the feed streams are given in Table 1. The hydrocracking resulted in hydrocracking product streams (Product 1-5) with compositions according to Table 2. By comparing the results in Table 2 it was observed that when the wt % of toluene in the feed stream rises and the [BX]/T.sup.2 molar value (the ratio of [the product of the molar amounts of (benzene+ethylbenzene) and xylene] to [the molar amount of toluene].sup.2) is reduced, the overall benzene yield is increased during hydrocracking. Benzene loss was almost zero at [BX]/T.sup.2 of 0.16. A benzene gain was obtained at [BX]/T.sup.2 of 0.09.

(4) TABLE-US-00001 TABLE 1 Compositions of hydrocracking feed stream Component Feed #1 Feed #2 Feed #3 Feed #4 Feed #5 Benzene (wt %) 51.64% 50.14% 46.75% 43.28% 39.86% Toluene (wt %) 12.73% 15.64% 21.39% 27.35% 33.13% Ethylbenzene (wt %) 4.19% 4.04% 3.79% 3.51% 3.21% BTX (wt %) 67.35% 68.64% 70.80% 73.10% 75.25% Total Aromatics (wt %) 72.11% 73.20% 75.08% 77.06% 78.86% [BX]/T.sup.2 (molar) 1.03 0.64 0.30 0.16 0.09

(5) TABLE-US-00002 TABLE 2 Compositions of hydrocracking product stream Component Product #1 Product #2 Product #3 Product #4 Product #5 Benzene (wt %) 48.29% 47.25% 45.28% 43.17% 41.28% Toluene (wt %) 18.78% 20.42% 23.47% 26.56% 29.35% Ethylbenzene (wt %) 0.34% 0.35% 0.32% 0.30% 0.26% BTX (wt %) 69.89% 70.86% 72.62% 74.50% 76.23% Total Aromatics (wt %) 70.84% 71.83% 73.64% 75.46% 77.17% [BX]/T.sup.2 (molar) 0.41 0.39 0.34 0.31 0.29 Benzene Purity (%) 99.90% 99.89% 99.88% 99.88% 99.88% Toluene Gain/Loss (%) +47.52% +30.56% +9.72% ?2.89% ?11.41% Toluene Gain/Loss (moles) +0.066 +0.052 +0.023 ?0.009 ?0.041 Benzene Gain/Loss (%) ?6.49% ?5.76% ?3.14% ?0.25% +3.56% Benzene Gain/Loss (moles) ?0.043 ?0.037 ?0.019 ?0.001 +0.018 Benzene purity is defined as [mass of benzene]/[sum of the masses of benzene, 2-methylpentane, 3-methylpentane, hexane, methylcyclopentane and cyclohexane].