Method for producing high-quality feedstock for a steam cracking process

Abstract

The present invention relates to a method for producing high-quality feedstock for a steam cracking process, said method comprising the following steps: i) providing a hydrocarbon feedstock; ii) contacting said hydrocarbon feedstock with a solvent at a dosage effective to remove aromatics and naphthenes from said feedstock forming a refined feedstock and one or more aromatics and naphthenes containing streams; iii) processing said refined feedstock in a steam cracking process.

Claims

1. A method for producing high-quality feedstock for a steam cracking process, said method comprising the steps of: i.) providing an aromatics and naphthenes containing hydrocarbon feedstock; ii.) contacting said hydrocarbon feedstock with furfural at a dosage effective to remove aromatics and naphthenes from said feedstock forming a refined feedstock and one or more aromatics and naphthenes containing streams; iii.) processing said refined feedstock in a steam cracking process; wherein step ii) comprises two sub steps, namely a step iia) comprising separation of aromatics from said hydrocarbon feedstock of step i) thereby forming a naphthenes containing intermediate stream and an aromatics containing stream, and a step iib) comprising separation of naphthenes from said intermediate stream thereby forming a naphthenes containing stream and said refined feedstock; wherein said hydrocarbon feedstock originates from a naphthenic crude oil; wherein step iia) comprises a temperature range of 50 to 125 C. and a solvent dosage within a range of 50 to 450 percent; wherein said hydrocarbon feedstock has a boiling range in a range of 300-550 C.; and wherein the refined feedstock consists of paraffinic vacuum gas oil that does not contain aromatics or naphthenes.

2. The method according to claim 1, wherein step iii) further comprises applying a step of removing traces of solvent from said refined feedstock before processing said refined feedstock in a steam cracking process.

3. The method according to claim 1, further comprising recovering solvent from said one or more aromatics and naphthenes containing streams forming a recovered solvent stream and one or more streams rich in aromatics and naphthenes.

4. The method according to claim 3, wherein said one or more streams rich in aromatics and naphthenes are further processed in refinery process units.

5. A method for producing high-quality feedstock for a steam cracking process, said method consisting of the steps of: i.) providing an aromatics and naphthenes containing hydrocarbon feedstock; ii.) contacting said hydrocarbon feedstock with furfural at a dosage effective to remove aromatics and naphthenes from said feedstock forming a refined feedstock and one or more aromatics and naphthenes containing streams; iii.) processing said refined feedstock in a steam cracking process; wherein step ii) comprises two sub steps, namely a step iia) comprising separation of aromatics from said hydrocarbon feedstock of step i) thereby forming a naphthenes containing intermediate stream and an aromatics containing stream, and a step iib) comprising separation of naphthenes from said intermediate stream thereby forming a naphthenes containing stream and said refined feedstock; wherein said hydrocarbon feedstock originates from a naphthenic crude oil; wherein step iia) comprises a temperature range of 50 to 125 C. and a solvent dosage within a range of 50 to 450 percent; wherein said hydrocarbon feedstock has a boiling range in a range of 300-550 C.; and wherein the refined feedstock consists of paraffinic vacuum gas oil that does not contain aromatics or naphthenes.

Description

(1) FIG. 1 shows an embodiment of the present method for producing high-quality feedstock for a steam cracking process.

(2) FIG. 2 shows another embodiment of the present method for producing high-quality feedstock for a steam cracking process.

(3) FIG. 1 shows a process 10 for producing high-quality feedstock for a steam cracking process. A hydrocarbon feedstock 1 is passed into a solvent extraction unit 5, where it is separated into a bottom stream 2 and a top stream 4. Bottom stream 2 comprises aromatics, naphthenes and solvent, top stream 4 comprises paraffins. Bottom stream 2 is passed into solvent recovery unit 6, where it is separated into a stream 7 rich in aromatics and naphthenes. The solvent 3 thus recovered is recycled to solvent extraction unit 5. Top stream 4 is passed into steam cracking unit 8 for producing a stream 9 comprising olefins and BTX (benzene, toluene and xylenes). FIG. 1 relates to the one-step process, i.e. the simultaneously removal of aromatics and naphthenes from feed 1. In an embodiment the aromatics content of stream 4 (raffinate) is in the range of 0-5% wt and naphthenes in the range of 0-25% wt. The composition of stream 7 (extract) would depend on the composition of the feedstock fed to the solvent extraction unit but basically the part of the feedstock that it's not recovered as raffinate it is recovered as extract.

(4) FIG. 2 shows a process 20 for producing high-quality feedstock for a steam cracking process. A hydrocarbon feedstock 21 is passed into a first solvent extraction unit 15, where it is separated into a bottom stream 12 and a top stream 11. Bottom stream 12 comprises aromatics and solvent, top stream 11 comprises naphthenes and paraffins. Bottom stream 12 is passed into first solvent recovery unit 16, where it is separated into a stream 17 rich in aromatics. The solvent 13 thus recovered is recycled to first solvent extraction unit 15. Top stream 11 is passed into a second solvent extraction unit 23 where it is separated into a bottom stream 22 and a top stream 27. Bottom stream 22 comprises naphthenes and solvent, top stream 27 comprises paraffins. Bottom stream 22 is passed into second solvent recovery unit 24, where it is separated into a stream 25 rich in naphthenes. The solvent 26 thus recovered is recycled to second solvent extraction unit 23. Top stream 27 is passed into steam cracking unit 18 for producing a stream 19 comprising olefins and BTX (benzene, toluene and xylenes). The naphthenic compounds containing stream 25 can be sent to several process units, such as steam cracker furnaces, steam cracker quench system and sold as naphthenic lube stock. In an embodiment the aromatics content of stream 27 (steam cracker feedstock) is in the range of 0-2% wt and naphthenes in the range of 0-10% wt. For stream 25 (naphthenic-rich stream) the aromatics content is in the range of 0-10% wt, naphthenes in the range of 50-100% wt, paraffins in the range of 0-40% wt. For stream 17 (aromatic-rich stream) the aromatics content is in the range of 60-100% wt, naphthenes in the range of 0-40% wt, paraffins in the range of 0-20%. For stream 11 (feed to second solvent-extraction process) the aromatics content is in the range of 0-25% wt, naphthenes in the range of 10-50% wt, paraffins in the range of 40-100%.

(5) FIG. 2 relates to the two-step process, i.e. a step comprising separation of aromatics from the hydrocarbon feedstock thereby forming an intermediate stream and a step comprising separation of naphthenes from the intermediate stream. The inventors assume that the purity of the paraffinic stream originated in FIG. 2 is higher than the one created in FIG. 1.

(6) In addition, the embodiment shown in FIG. 2 comprises two separate solvent recovery units, namely first solvent recovery unit 16 and second solvent recovery unit 24. However, in a preferred embodiment (not shown) these solvent recovery units could be combined into a single unit.

(7) In addition the processing scheme according to FIG. 2 allows for the independent production of paraffins and naphthenes. Cracking conditions in steam cracker furnaces could be tuned for optimal yields for each stream. This is not possible when sending paraffins together with naphthenes to the furnaces.

(8) The present inventors found that in the process as shown in FIG. 1 the lighter the molecules, i.e. the composition of top stream 4, the better the steam cracker yields will be. In the process as shown in FIG. 2, naphthenes could be used as quenching material that would act as hydrogen donor and minimize the condensation reactions happening in this part of the steam cracking process. Moreover, since the temperature of this stream will be in the order of 150-200 C., it can constitute a comparable or even better quenching material than the typically used aromatic-rich streams.

(9) Thus, the apparatus used in the present method can comprise a single extraction zone or multiple extraction zones equipped with shed rows or other stationary devices to encourage contacting, orifice mixers, or efficient stirring devices, such as mechanical agitators, jets of restricted internal diameter, turbo mixers and the like. The operation may be conducted as a batch wise or as a continuous-type operation with the latter operation being preferred. A particularly preferred operational configuration comprises continuous countercurrent extraction. It is important to note that the equipment employed in the operation of the extraction process is not critical to the overall efficiency of the extraction and can comprise rotating disc contactors, centrifugal contactors, countercurrent packed bed extraction columns, countercurrent tray contactors and the like.

EXAMPLES

(10) Following the processing scheme shown in FIG. 1 different types of steam cracker feedstocks have been produced starting from vacuum gasoil (VGO) depending on the degree of separation in the solvent extraction process. Starting from Arab Light VGO (properties shown in Table 1) six different feedstocks could be generated:

(11) VGO1: Full VGO

(12) VGO2: Dearomatized VGO (not containing aromatics)

(13) VGO3: Paraffinic VGO (not containing aromatics or naphthenes)

(14) VGO4: Feed containing all paraffins and 20% of naphthenes present in VGO

(15) VGO5: Feed containing all paraffins and all mono-ring naphthenes present in VGO

(16) VGO6: Feed containing all paraffins and 20% of lighter naphthenes present in VGO

(17) TABLE-US-00001 TABLE 1 Arab Light VGO composition % wt. Paraffins 31.4 Total Naphthenes 12.6 Mononaphthenes 9.8 Aromatics 56.0
Apart from full VGO (identified as VGO1) and solvent-extracted VGOs (identified as VGO2-VGO6), two unconverted oil streams (UCO1 and UCO2) were also provided as feedstocks for comparative examples.

(18) Table 2 shows that the main difference between these two streams lies on their different hydrogen content (UCO1=14.3% wt. and UCO2=13.7% wt). Hydrotreatment/hydrocracking is the conventional way to enable the processing of vacuum distillates in a steam cracker.

(19) TABLE-US-00002 TABLE 2 Unconverted oil properties Property UCO1 UCO2 H-content wt. % 14.3 13.7 Specific gravity kg/kg 0.83 0.84 IBP C. 342 342 10% C. 367 367 30% C. 402 402 50% C. 429 429 70% C. 461 461 90% C. 516 516 FBP C. 579 579

(20) Once-through steam cracker yields (in % wt. for all different feeds) are presented in Table 3. These yields have been estimated using Spyro software using the following characteristics: feed rate=30 ton/h, Coil Outlet Temperature (COT)=775 C. Steam/Oil ratio=0.75 w/w, Coil Outlet Pressure (COP)=1.7 bara.

(21) TABLE-US-00003 TABLE 3 Once-through steam cracker yields in % wt. VGO1 VGO2 VGO3 VGO4 VGO5 VGO6 UCO1 UCO2 H.sub.2 0.3 0.4 0.4 0.4 0.4 0.4 0.4 0.3 CH.sub.4 5.8 7.3 7.0 7.1 7.3 7.0 7.0 7.1 Ethylene 17.8 26.7 29.2 28.6 27.3 28.6 26.3 24.6 Ethane 2.3 4.0 4.3 4.2 4.1 4.3 3.9 3.6 Propylene 12.6 18.7 19.5 19.3 19.1 19.3 17.7 16.9 Butadiene 4.2 7.1 7.3 7.2 7.2 7.2 6.8 6.2 Isobutene 1.8 2.4 2.1 2.2 2.4 2.2 1.9 1.9 Benzene 1.9 3.3 2.6 2.8 3.3 2.8 3.1 2.5 Toluene 1.1 1.7 1.1 1.3 1.5 1.3 1.9 1.3 Xylene 0.6 0.5 0.3 0.3 0.4 0.4 0.9 0.5 C9+ 37.7 4.6 2.9 3.3 3.2 3.3 9.0 16.0 Propylene + 30.4 45.4 48.7 47.9 46.4 47.9 44.0 41.5 Ethylene

(22) In the above examples VGO2 is a completely aromatic-depleted raffinate but with all naphthenes, VGO3 is a completely aromatic and naphthenic-depleted raffinate and VGO4-6 show the effect of still having some naphthenes in the feed to the steam cracker: VGO4=8% wt naphthenes; VGO5=17% wt naphthenes; VGO6=3% wt naphthenes). The present inventors found that the raffinate composition is partially determined by the efficiency of the solvent extraction process and the economic trade-offs: higher temperatures and higher solvent/oil ratios will lower the aromatics and naphthenes content but the higher the energy consumption. These ranges of aromatic and naphthenic content are different than those shown by the discussed prior art references, i.e. Nouri et al. (Arom=19% and Naph=28%) and GB 1 248 814 (raffinate contains all paraffins & cycloparaffins of the gas oil and extract contains 71% of the aromatics contained in the gas oil so a considerable amount of aromatics are still in raffinate).

(23) From Table 3, it can be seen that dearomatization of VGO (VGO2) enhances the propylene and ethylene yields by 50% compared to processing full VGO (VGO1) while reducing the production of C9-plus components by 88%. Subsequent removal of all naphthenes (VGO3) provides a further increase of propylene and ethylene yields to 48.7% (3.3% more than VGO2) reducing even more the C9-plus production.

(24) All solvent-extracted VGOs show better ethylene and propylene yields than unconverted oils with the advantage of not requiring hydrogen or capital-intensive hydroprocessing units.