Method for cracking a hydrocarbon feedstock in a steam cracker unit

Abstract

The present invention relates to a process for cracking a hydrocarbon feedstock in a steam cracker unit, comprising the following steps of: feeding a liquid hydrocarbon feedstock to a hydrocracking unit, separating the stream thus hydrocracked in said hydrocracking unit into a high content aromatics stream and a gaseous stream comprising C2-C4 paraffins, hydrogen and methane, separating C2-C4 paraffins from said gaseous stream, feeding said C2-C4 paraffins thus separated to the furnace section of a steam cracker unit.

Claims

1. A process for cracking a hydrocarbon feedstock, the method consisting of the steps of: feeding a liquid hydrocarbon feedstock to a hydrocracking unit, separating the stream thus hydrocracked in said hydrocracking unit into a high content aromatics stream and a gaseous stream comprising C2-C4 paraffins, hydrogen and methane, separating C2-C4 paraffins from said gaseous stream, further separating C2-C4 paraffins into individual streams, each stream predominantly comprising C2 paraffins, C3 paraffins and C4 paraffins, respectively, and feeding each individual stream to a specific furnace section of a steam cracker unit, wherein the process conditions in said hydrocracking unit are a temperature of 300-450 C., a pressure of 300-5000 kPa gauge and a Weight Hourly Space Velocity of 0.1-10 h1; wherein separating said C2-C4 paraffins from said gaseous stream is carried out by solvent extraction; separating said high content aromatics stream into a stream of heavy aromatics and a stream high in mono-aromatics; and separating a C7 to C9 aromatics rich fraction from the stream high in mono-aromatics.

2. A process for cracking a hydrocarbon feedstock, the method consisting of the steps of: feeding a liquid hydrocarbon feedstock to a hydrocracking unit, separating the stream thus hydrocracked in said hydrocracking unit into a high content aromatics stream and a gaseous stream comprising C2-C4 paraffins, hydrogen and methane, separating C2-C4 paraffins from said gaseous stream, further separating C2-C4 paraffins into individual streams, each stream predominantly comprising C2 paraffins, C3 paraffins and C4 paraffins, respectively, and feeding each individual stream to a specific furnace section of a steam cracker unit, wherein the process conditions in said hydrocracking unit are a temperature of 300-450 C., a pressure of 300-5000 kPa gauge and a Weight Hourly Space Velocity of 0.1-10 h1; wherein separating said C2-C4 paraffins from said gaseous stream is carried out by solvent extraction, wherein the reactor of the said hydrocracking unit is chosen from the group of an ebulating bed reactor and a slurry reactor; and separating said high content aromatics stream into a stream of heavy aromatics and a stream high in mono-aromatics; and separating a C7 to C9 aromatics rich fraction from the stream high in mono-aromatics, and converting said C7 to C9 aromatics rich fraction into a benzene rich fraction.

Description

(1) The sole FIGURE provides a schematic flow sheet of an embodiment of the present invention.

EXAMPLES

(2) Feedstock 33, which can include different types of feedstock, for example naphtha 35, kerosene 36, diesel 37, atmospheric gas oil (AGO) 38 originating from tanks 2,3,4,5 respectively, is sent to a hydrocracker unit 17. In hydrocracking unit 17 a feedstock 33 is hydrocracked in the presence of hydrogen. The hydrocracking process results in the formation of a gaseous stream 19 comprising C2-C4 paraffins, hydrogen and methane and a high content aromatics stream 40. The gaseous stream 19 is sent to a separator 12, e.g. cryogenic distillation or solvent extraction, and separated into different streams, i.e. a stream 55 comprising C2-C4 paraffins, a stream 20 comprising hydrogen and methane and a purge stream 18. Stream 20 can be recycled to hydrocracking unit 17.

(3) As mentioned before, stream 55 can be sent directly (not shown) to either a dehydrogenation unit 57 or directly (not shown) to a steam cracker unit 11. However, before sending stream 55 to steam cracker unit 11 it is preferred to carry out a separation on stream 55 first. In separator 56 the C2-C4 paraffins are separated into individual streams 30, 31 and 32. This means that stream 30 predominantly comprises C2 paraffins, stream 31 predominantly comprises C3 paraffins and stream 32 predominantly comprises C4 paraffins. If necessary, further separation of unwanted components or temperature adjustments can made in units 9, 10, 13. The individual streams 21, 27 and 29 will be sent to specific furnace sections of steam cracker unit 11. Although steam cracker unit 11 is shown as one single unit, in the present method it is to be understood that in a preferred embodiment steam cracker unit 11 comprises different furnace sections each dedicated for a specific chemical composition, that is a furnace section for C2, a furnace section for C3 and a furnace section for C4. In a preferred embodiment stream 27 predominantly comprising C3 paraffins and stream 29 predominantly comprising C4 paraffins are sent as stream 54 and stream 23 to dehydrogenation unit 57, respectively. In another embodiment it is possible to separate only C3 and C4 from stream 55 and to send a combined C3 and C4 stream to dehydrogenation unit 57.

(4) In steam cracker unit 11 streams 21, 27 and 29 and a feedstock 58, for example C2 to C4 gases coming from an unit 1, are processed and its reaction products 28 are separated in a separation section 6. A C2-C6 alkanes comprising gas stream 7 is recycled to the steam cracker unit 11. Hydrogen 15 and pygas 14 can be sent to hydrocracking unit 17. The valuable product stream 8 comprising unsaturated hydrocarbons such as lighter alkenes including ethylene, propylene and butadienes is sent to further petrochemical processes. In case heavy hydrocarbons such as carbon black oil, cracked distillate and C9+ hydrocarbons are produced in steam cracker unit 11 these products can optionally be recycled to hydrocracking unit 17 as well.

(5) High content aromatics stream 40 is sent to a separator 16, for example a distillation process, and separated into a stream 41 of heavy aromatics and a stream 43 high in mono-aromatics. Stream 42 which predominantly comprises C7 to C9 aromatics can converted in unit 24 into a benzene rich fraction 59 and a methane rich fraction 44.

(6) The Example disclosed herein makes a distinction between a process (case 1) in which the naphtha is only processed through a steam cracker unit and a process (case 2) wherein the naphtha is sent to a hydrocracking unit, wherein in the gaseous stream thus formed the C2-C4 paraffins is separated and fed to the furnace section of a steam cracker unit. Case 1 is a comparative example and case 2 is an example according to the present invention.

(7) The conditions for the steam cracker are as follows: Ethane and Propane furnaces: Coil Outlet temperature=845 C., Steam-to-oil-ratio=0.37, C4-furnaces: Coil Outlet temperature=820 C., Steam-to-oil-ratio=0.37, Liquid furnaces: Coil Outlet temperature=820 C., Steam-to-oil-ratio=0.37. Regarding the specific conditions for the Hydrocracking unit 17: The modeling was carried out for a hydrocracking reactor operating conditions: mean reactor temperature 510 C., Weight hour space velocity of 1 hr-1 and a reactor pressure of 1379 kPa gauge. The catalyst comprised a mixture of Pt supported on gamma alumina and HZSM-5 with a Si:Al ratio of 100:1.

(8) The composition of the naphtha feed can be found in Table 1.

(9) TABLE-US-00001 TABLE 1 Composition of naphtha as feedstock Naphtha n-Paraffins wt-% 36.7 i-Paraffins wt-% 38.2 Naphthenes wt-% 20.1 Aromatics wt-% 5.0 Density 60F kg/L 0.673 IBP C. 37.8 BP10 C. 45.7 BP30 C. 49.5 BP50 C. 54.7 BP70 C. 64.0 BP90 C. 79.4 FBP C. 103.0
The battery product slate (wt. % of feed) for each of the case 1 and case 2 can be found in Table 2.

(10) TABLE-US-00002 TABLE 2 battery product slate (wt. % of feed) Feed: naphtha CASE 1 CASE 2 BATTERY LIMIT PRODUCT SLATE SC FHC + SC H2 0.9 0.5 CH4 17.9 18.3 ETHYLENE 35.1 50.1 PROPYLENE 19.3 12.8 BUTADIENE 5.3 2.0 ISO-BUTENE 3.3 0.3 BENZENE 9.1 8.3 TX CUT 3.4 7.0 STYRENE 0.9 0.1 OTHER C7-C8 0.4 0.0 C9 RESIN FEED 0.7 0.0 CD 1.5 0.2 CBO 2.0 0.3 % HIGH VALUE CHEMICALS 76.9 80.5

(11) From Table 2 one can see that treating the naphtha in a hydrocracking unit according to the present method (case 2) causes an increase of total BTX (benzene, toluene plus xylenes. Accordingly. the results disclosed in Table 2 show a considerable increase in BTX from case 1 (comparative example) to case 2 (according to the present invention). CD means cracked distillate and CBO means carbon black oil, respectively. Table 2 further illustrates that the yield of high value chemicals (ethylene+propylene+butadiene+iso-butene, benzene, TX cut, Styrene and other C7-C8) is significantly higher when naphtha is processed according to the present invention (case 2) than can be achieved by conventional processing means (case 1).

(12) Because in the hydrocracking unit (case 2) the heavier paraffins are all reduced to lighter components such as C2-C4 paraffins, the production of ethylene increases from 35 to 50% by pre-treating the naphtha in the hydrocracking unit. Thus case 2 provides a significantly higher ethylene yield than case 1.

(13) Table 2 also shows that the production of heavier products (C9Resin feed, cracked distillate and carbon black oil) is reduced by pre-treating the naphtha in the hydrocracking unit (case 2). This means that according to the present method the formation of heavy unwanted byproducts can be reduced to a minimum.

(14) Another example shows the influence of the operating temperature of a hydrocracking unit on the product slate. The catalyst mixture is a physical mixture of 2 gram ZSM-5 and 2 gram Pt on alumina catalyst, wherein 0.4-0.8 mm SiC chips have been incorporated in the catalyst bed to ensure good approximation to plug flow and reduce axial/radial temperature differences. Olefins6 naphtha was used as feedstock (see Table 3).

(15) TABLE-US-00003 TABLE 3 composition of feed stock Carbon Naph- i- n- number thenes Paraffins Paraffins Olefins Aromatics Total 3 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 4 <0.01 0.22 0.51 0.92 <0.01 1.65 5 3.99 14.63 17.77 1.08 <0.01 37.47 6 8.34 13.40 10.75 0.07 2.74 35.31 7 4.77 5.00 3.12 <0.01 0.93 13.82 8 2.56 2.18 1.61 <0.01 0.83 7.18 9 1.36 1.27 0.73 <0.01 0.34 3.70 10 <0.01 0.28 0.53 <0.01 <0.01 0.81 11 <0.01 <0.01 0.06 <0.01 <0.01 0.06 Total 21.03 36.98 35.08 2.07 4.85 100.00

(16) In this example the temperature was varied between 425 C. and 500 C. (WHSV=1, H:HC=3, 200 psig). The effluent composition is shown in Table 4.

(17) TABLE-US-00004 TABLE 4 effluent composition 500 C. 475 C. 450 C. 425 C. Methane 12.43 7.45 4.42 2.28 LPG (C2-C4) 72.66 78.50 80.29 78.10 n-Paraffins (C5+) 0.08 0.24 0.58 1.22 i-Paraffins (C5+) 0.02 0.06 0.24 1.03 Olefins <0.01 <0.01 <0.01 <0.01 Naphthenes <0.01 0.04 0.43 0.43 Aromatics 14.81 12.23 9.98 9.98 Total 100 100 100 100

(18) The data from Table 4 show that higher temperatures result in high methane (low value by-product) yields.