PROCESS FOR THE PRODUCTION OF AROMATIC FUEL

Abstract: An aromatic gasoline component is prepared in a multi­ step petroleum refining process starting with a heavy car­ bometallic petroleum fraction which is catalytically cracked to yield a light catalytic cycle oil which, in turn, is mildly hydrogenated to produce a partially saturated bicyclic hydrocarbon fraction which itself is catalytically cracked to yield a monoaromatic hydrocarbon fraction from which is recovered a gasoline product. The bicyclic hydrocarbons are converted to monoaromatics by selective partial satura­ tion and ring scission.

Description: This invention relates to a multistep process for the production of a gasoline boiling range fuel component comprising monoaromatic hydrocarbons. More specifically the process of the invention comprises a process for upgrading a low value fraction from the cracking of carbometallic residual hydrocarbon oil to high octane gasoline. Ashland Oil Inc.'s new heavy oil conversion process (RCC Process) has been described in the literature (Oil and Gas Journal, March 22, 1982, pages 82-91), NPRA paper, AM-84-50 (1984 San Antonio) and in numerous US Patents assigned to Ashland Oil Inc., for example US-A-4,341,624 and US-A-4,332,673. Briefly, the RCC process is designed to crack heavy residual petroleum oils that are contaminated with metals such as vanadium and nickel. The feedstock to the unit will have an initial boiling point above 343°C (650°F), an API gravity of 15 to 25 degrees, a Conradson carbon above 1.0, and a metals content of at least 4 parts per million (ppm). The hot feed is contacted with fluid cracking catalyst in a progressive flow type elongated riser cracking tube and the cracked effluent is recovered and separated. One of the fractions recovered from the main fractionator is a light cycle oil (LCO) boiling in the range 216 to 332°C (430 to 630°F). This fraction is not suitable as a motor fuel component because it contains a high proportion of dual ring (bicyclic) aromatic hydrocarbons, i.e. naphthalene and methyl and ethyl naphthalenes. Generally these dual ring hydrocarbons are present in the LCO fraction in amounts of from 10 to 60% by volume, more usually 20 to 40%. Moreover, because of the refractory nature of LCO it cannot be recycled for further cracking in the RCC process, nor can it be converted in a conventional fluid catalytic cracking (FCC) unit. It is known to upgrade LCO fractions by a separate hydrotreatment followed by recycle through an FCC unit (US-A-3 489 673 and US-A-4 426 476) but nobody has yet specifically addressed the problem of upgrading LCO from an RCC unit. Thus a process is still needed to upgrade such LCO fractions to a high octane aromatic gasoline component. This is achieved in accordance with the present invention by contacting the LCO fraction from an RCC process with hydrogen and a catalyst to preferentially saturate one of the two aromatic rings of the bicyclic aromatic hydrocarbons present in the LCO fraction, and subsequently subjecting the resulting hydrogenated bicyclic fraction to a fluid catayltic cracking (FCC) process thereby to produce a gasoline product comprising monoaromatic (one ring) hydrocarbons. When the initial hydrocarbon feed to the RCC process contains metal compounds such as vanadium and nickel the cracking is preferably carried out in the presence of a metals passivation agent such as an antimony compound, a tin compound or a mixture of antimony and tin compounds. The invention is further described with reference to the accompanying drawing, which is a schematic representation of a preferred mode of the multistep process of this invention. The reduced crude cracking unit (RCCU) employed for the first step of the process of this invention converts a carbometallic hydrocarbon oil feed to a product slate comprising 45 to 55 vol.% gasoline, 16 to 24 vol.% C₄ minus, 10 to 20 vol.% heavy cycle oil and coke, and 15 to 25 vol.% light cycle oil. This latter material contains the dual ring aromatic hydrocarbons to be further treated in the subsequent process steps of this invention. A typical RCC feedstock is a high sulphur untreated reduced crude oil, b.p. 343°C (650°F) plus. Preferably 70 vol.% of the feed boils above 343°C (650°F), the Conradson carbon content is usually > 1.0 wt.%, and the metals content of the feed is usually at least 4 ppm nickel equivalents by weight. A typical RCC feedstock and product analysis is given in Table 1: Referring now to the drawing, the hot reduced crude oil feedstock is passed by line 1 to the bottom of riser reactor 2 where it is mixed with fully regenerated fluid cracking catalyst from line 3. Following conversion in the reactor at temperatures of 482 to 538°C (900 to 1000°F), pressures of from 68.95 to 344.75 kPa (10 to 50 psia) and a vapour residence time of 0.5 to 10 seconds, cracked effluent comprising desired prdoucts and unconverted liquid material is separated from the catalyst in catalyst disengager zone 4. The effluent is passed by line 5 to the main fractionator 6. Spent cracking catalyst contaminated with carbon and metals compounds is passed by line 7 to regeneration zone 8. The catalyst is regenerated by burning with oxygen containing gas from line 9 and the reactivated catalyst is returned to the cracking zone via line 3. As the fluidized catalyst circulates around the RCC cracking unit undergoing repeated phases of cracking and regeneration the metals content (chiefly vanadium and nickel) accumulates to 2,000 to 15,000 ppm nickel equivalents. This metal loading inactivates the zeolite cracking ingredient and fresh makeup catalyst must be added to maintain activity and selectivity. In the main fractionator 6 conditions are controlled to recover by line 10 an RCC gasoline and light ends fraction having a bottom cut point of 204 to 221°C (400 to 430°F) and comprising 45 to 55 vol.% of the cracking product. The RCC gasoline is olefinic and it has a research octane in the range of 89 to 95. A bottoms fraction boiling above 316 to 343°C, (600 to 650°F) is recovered by line 11 for further processing and recovery. The LCO (light cycle oil) fraction described previously is passed by line 12 to selective hydrotreating vessel 13. The hydrogen treating unit is operated to selectively saturate one ring of dual ring unsaturated aromatic hydrocarbons. By this process from 20 to 80% by weight of the unsaturated aromatic hydrocarbons in the LCO fraction add from 4 to 8 hydrogen atoms to produce a partially saturated bicyclic hydrocarbon fraction. For example, naphthalene gains four hydrogens to yield tetrahydronaphthalene. The hydrotreating or hydrofining process step of the invention is carried out at selected mild conditions designed to achieve partial saturation while avoiding hydrocracking of ring compounds. Preferred operating conditions are as follows: Suitable hydrosaturation catalysts comprise Group VI metal compounds and/or Group VII metal compounds on an alumina base which may be stabilized with silica. Specific examples of suitable metal components of catalysts include molybdenum, nickel and tungsten. Desirable catalyst composites contain 2 to 8 wt.% NiO, 4 to 20 wt.% MoO, 2 to 15% SiO₂, and the balance alumina. The catalyst is placed in one or more fixed beds in vessel 13. The bicyclic aromatic hydrocarbon feed from line 12 is mixed with recycle hydrogen from line 14 and fresh hydrogen introduced through line 15, and the reaction mixture passed downwardly over the catalyst beds in reactor vessel 13. The selectively hydrosaturated effluent passes via line 16 to separator 17. Unreacted hydrogen is recycled by line 14. The fraction recovered from the separator by line 18 is characterized as a naphthene-aromatic fraction. The naphthene-aromatic fraction is passed to the bottom of the riser 19 of a fluid catalytic cracking unit designated generally by reference numeral 20. The naphthene-aromatic fraction can be mixed with additional hydrocarbons to be cracked added by line 21. When a metals passivator is used in the FCC unit it can be added to the cracking feed by line 22. In a preferred embodiment all or a portion of the conventional cracking feed in line 21 is hydrofined prior to cracking. The feed is passed by line 29 and line 12 into saturation hydrogenator 13. Alternatively the cracking feed can be hydrofined in a separate conventional catalytic feed hydrofiner (not shown). Cracking unit 20 is operated in the conventional manner. The naphthene-aromatic fraction is cracked in riser line 19 with regenerated fluid cracking catalyst from line 23. Catalyst is separated from cracked effluent in disengaging zone 24 and the catalyst is passed to regenerator 25. Following regeneration the catlayst is recycled via line 23. Cracked hydrocarbon effluent is passed by line 26 to separation zone 27. The desired aromatic gasoline product fraction is recovered by distillation via line 28. Separation zone 27 is operated in a conventional manner with known devices and equipment, not shown, to recover various products and recycle streams. Suitable fluid catalytic cracking conditions include a temperature ranging from 427 to 704°C (800 to 1300°F), a pressure ranging from 68.95 to 344.75 kPa (10 to 50 psig), and a contact time of less than 0.5 seconds. Preferred FCC conditions include a temperature in the range of 510 to 543°C (950 to 1010°F) and a pressure of 103.43 to 206.86 kPa (15 to 30 psia). Preferred fluid cracking catalysts include activated clays, silica alumina, silica zirconia, etc., but natural and synthetic zeolite types comprising molecular sieves in a matrix having an average particle size ranging from 40 to 100 microns are preferred. Equilibrium catalyst will contain from 1,000 to 3,000 nickel equivalents. The aromatic gasoline fraction cut recovered by line 28 comprises unsubstituted monoaromatics such as benzene, toluene and xylene, but the fraction is characterized by a major proportion of alkyl aromatics having one to four saturated side chains. The side chains have from one to four carbon atoms in the chain. The fraction contains 35 to 55 vol.% monoaromatics with an average octane above 91. In a preferred embodiment the gasoline fraction from line 28 is combined with the gasoline fraction from line 10. Blending of these fractions provides an overall process gasoline recovery of 60 to 70 vol.% based on the carbometallic oil feed to the process. In another preferred embodiment, the cracking step in unit 20 is carried out in the presence of a passivator. When the cracking feed contains metals such as nickel and vanadium, a build-up occurs which not only deactivates the catalyst but catalyses cracking of rings and alkyl groups. Accordingly commercially available passivators such as antimony, tin, and mixtures of antimony and tin are supplied to the cracking unit and/or the catalyst in the known manner. Suitable passivators are disclosed in the following patents: US-A-4,255,207, US-A-4,321,129, and US-A-4,466,884.

Claims: A process for the production of a high octane gasoline component from a carbometallic heavy residual oil feedstock, which comprises the sequential steps of: (a) catalytically cracking the heavy carbometallic residual oil feedstock in a riser cracking zone in the presence of fluid cracking catalyst; (b) recovering by distillation from the cracked product of step (a) a light cycle oil (LCO) fraction boiling in the range 216 to 332°C (430 to 630°F) and containing from 10 to 60% by volume of dual ring unsaturated aromatic hydrocarbons; (c) contacting the LCO fraction under mixed phase conditions in a saturation hydrogenation zone with a nickel-containing hydrogenation catalyst under selective mild conditions of temperature, pressure, space velocity and hydrogen circulation rate whereby from 20 to 80% by weight of the unsaturated aromatic hydrocarbons in the LCO fraction add hydrogen to one of the rings to produce a partially saturated bicyclic hydrocarbon fraction; (d) subjecting the partially saturated bicyclic hydrocarbon fraction to fluid catalytic cracking in a riser cracking zone under short contact time cracking conditions in the presence of zeolite fluid cracking catalyst and in the absence of added hdyrogen whereby one of the rings of the bicyclic hydrocarbon cracks to yield a mono-ring aromatic hydrocarbon and thereby to produce a mono-ring aromatic hydrocarbon containing fraction; and (e) recovering from said mono-ring aromatic hydrocarbon containing fraction produced in step (d) a gasoline component having an average octane rating at least 91 and containing from 35 to 55 % by volume of mono-ring aromatic hydrocarbons. A process according to claim 1, in which the carbometallic heavy residual oil feed is a reduced crude oil containing at least 70% by volume of 343°C (650°F) plus material, a Conradson carbon content of at least 1.0% by weight, and a metals content of at least 4 ppm nickel equivalents by weight. A process according to claim 1 or 2, in which the cracking conditions in step (a) comprise a temperature in the range of 482 to 538°C (900 to 1000°F), a pressure in the range 68.95 to 344.75 kPa (10 to 50 psia) and a vapour residence time in the riser of 0.5 to 10 seconds. A process according to claim 1, 2 or 3, in which a major proportion of the cracked product of step (a) is gasoline. A process according to claim 4, wherein at the end of the process that gasoline and the gasoline component product recovered in step (e) are combined to provide a single gasoline product, whereby the total gasoline recovery is in the range 60-70 vol.% based on the carbometallic oil feed. A process according to any one of claims 1 to 5, in which the light cycle oil fraction recovered in step (b) contains 20 to 40 % by weight of naphthalenes. A process according to any one of claims 1 to 6, in which the hydrogenation catalyst used in step (c) comprises nickel oxide, nickel molybdate or nickel tungstate, or a mixture of two or more thereof, supported on alumina. A process according to any one of claims 1 to 7, in which the mild hydrogenation conditions in step (c) include a temperature in the range 316 to 399°C (600 to 750°F), a pressure in the range 4.1 to 10.3 MPa (600 to 1,500 psia), a space velocity in the range 0.5 to 3.0, and a hydrogen circulation rate of 0.18 to 0.71 m³ per litre of feed (1,000 to 4,000 ft³ per bbl). A process according to any one of claims 1 to 8, in which the fluid catalytic cracking conditions in step (a) include a temperature in the range 510 to 543°C (950 to 1010°F), and a pressure in the range 103.43 to 206.86 kPa (15 to 30 psia). A process according to any one of claims 1 to 9, in which the fluid cracking catalyst employed in step (d) comprises a zeolite supported on a matrix having a metals content of 1,000 to 3,000 ppm nickel equivalents at catalyst equilibrium operating conditions and containing a compound of tin or antimony or a mixture thereof as a passivator.

Inventors: HETTINGER, WILLIAM P. JR., YANCEY, ROBERT E. JR.
Applicants: ASHLAND OIL, INC.
Application year: 1985
Main classification: C10G69
IPC codes: C10G11, C10G69, C10G45, C10G47, C10G67