GB2104544A - Centre ring hydrogenation and hydrocracking of poly-nuclear aromatic compounds - Google Patents

Abstract

Polynuclear aromatic compounds obtained from petroleum residua and coal liquids and containing three to six aromatic ring compounds are first catalytically hydrogenated and then hydrocracked, either thermally or catalytically, to produce high yields of two-ring and mononuclear aromatic products. Useful hydrogenation reaction conditions are 149-482 DEG C (300-900 DEG F) temperature, and 69-124 bar gauge (1000-1800 psig) hydrogen partial pressure. The hydrogenation catalyst used can be nickel-tungsten on a silica-alumina support. The hydrogenated material is then hydrocracked at conditions of 427-704 DEG C (800-1300 DEG F) temperature, and 34-207 bar gauge (500-3000 psig) hydrogen partial pressure. The hydrocracking step may use a catalyst comprising cobalt- molybdenum on alumina and a space velocity of 3-7 gm/hr/gm catalyst. A preferred feedstock is steam-cracker pyrolysis tar such as produced from steam cracking of ethylene.

Description

SPECIFICATION Centre ring hydrogenation and hydrocracking of polynuclear aromatic compounds This invention relates to hydrogenation of polynuclear aromatic compounds followed by cracking of the centre rings of such compounds to produce high yields of mononuclear aromatic compounds and liquid products.

Traditional single-step hydrocracking of polynuclear aromatic compounds at elevated temperature conditions favours hydrogenation of the terminal rings and their subsequent cracking. Unfortunately, this method wastes valuable feedstock, as it produces only one single aromatic ring molecule per molecule of feed. Further hydrogenation and cracking of the successive terminal rings produces only a low value C1C4 gas product and one molecule of monoaromatic product per molecule of polynuclear aromatic feed. Furthermore, the hydrogen consumption is undesirably high.

In the hydrocracking of one or more aromatic rings of polynuclear aromatic compounds, such as those found in petroleum residua and coal-derived liquids, effective cracking must be preceded by a hydrogenation step. Wiser, et al., in Ind. Eng. Chemistry, Prod. Res. Develop., 9, 350 (1970), demonstrated the feasibility of centre ring hydrogenation of polynuclear aromatic materials, and obtained a yield of almost 80 W % of 9,1 0-dihydroanthracene from the hydrogenation of anthracene over a nickel-tungsten catalyst in an autoclave at 2000C temperature at 103 bar gauge (1500 psig) H2 partial pressure for three hours. However, difficulty occurs in cracking the polynuclear aromatics at the higher temperature before dehydrogenation occurs, and Wiser was apparently not successful in demonstrating centre ring cracking.

Our experiments made under new conditions have indicated successful centre ring catalytic hydrogenation and hydrocracking of four- and six-ring polynuclear aromatic compounds, using a steam pyrolysis tar material as feedstock to produce two-ring and mononuclear aromatic products. By controlling the conditions of the hydrogenation reaction, it has been found possible to predetermine the location and mode of cracking of the polynuclear aromatic molecules to produce desired smaller molecules.

The present invention thus provides a process for hydrogenation and cracking of polynuclear aromatic compounds, comprising: (a) heating a hydrocarbon feedstock comprising polynuclear aromatic compounds with hydrogen and introducing the mixture into a catalytic reaction zone to saturate the centre-ring molecules under mild hydrogenation reaction conditions within the range of 1 49--482 OC (300--9000 F) temperature, and 69-124 bar gauge (1000-1800 psig) hydrogen partial pressure; (b) hydrocracking the hydrogenated compounds in a cracking zone under reaction conditions within the range of 427--7040C (800--1 3000F) temperature, 34-207 bar gauge (500-3000 psig) hydrogen partial pressure;; (c) withdrawing an effluent stream from said cracking zone and passing it to a phase separation step for separation into gaseous and liquid portions; and (d) withdrawing the gaseous portion and a hydrocracked aromatic liquid product.

The present invention describes an improved process for centre-ring hydrogenation and hydrocracking of polynuclear aromatic feedstocks containing 3-6 benzene rings, such as aromatic pyrolysis tars which contain aromatic asphaltenes and 1-5 W % sulphur, to produce desulphurized one-ring and two-ring aromatic products.

The process first reacts the polynuclear aromatic feedstock in a separate catalytic hydrogenation step performed at relatively low temperature conditions of 149--4820C (300--9000F) to selectively saturate one or more centre rings of the feed compound. The resulting hydrogenated compound is then passed to a subsequent hydrocracking step performed at a higher temperature of 427--5380C (800-10000F). The cracked product contains increased lower-boiling, two-ring and mononuclear compounds and the process consumes much less hydrogen than conventional hydrocracking methods.

The cracking conditions used are selected such that the cracking reactions which occur proceed at a faster rate than the dehydrogenation reactions.

An important feedstock for the process of this invention is heavy aromatic pyrolysis tar, such as obtained from the production of ethylene from naphtha feeds via steam cracking. Such steam pyrolysis tar, for example, S-2 steam pyrolysis tar produced by Exxon Corporation, is a viscous aromatic liquid from which the monoaromatic fractions have been removed by prior distillation. This typical pyrolysis tar feed material has an initial boiling point of about 241 C (4650 F), and contains 1 8-40% naphthalenes, 1434% three-ring aromatics, 7-1 7% four-ring aromatics and 24-31% residue, consisting of sixring and larger aromatics.Many pyrolysis tars contain as much as 5% sulphur and therefore fail to meet the EPA fuel standards and cannot be legally burned. However, the S-2 pyrolysis tar contains only 1.3% sulphur. The hydrogenation step reduces the sulphur concentration to about half that contained in the feed.

The catalytic hydrogenation or hydrotreating step for the preheated feedstock can be accomplished in either a fixed-bed or an ebullated-bed type reactor. Use of an ebullated catalyst bed reactor is usually preferred because of its capability for catalyst replacement during operation and its relative freedom from plugging. The reaction conditions required for the hydrotreating step are within the range of 1 49-4820C (300--9000F) temperature and 69-124 bar (1000-1800 psi) hydrogen partial pressure.

The polyhydrogenated tar material is next hydrocracked at a temperature of 427-7040C (800--13000F) and a hydrogen partial pressure of 34-207 bar gauge (500-3000 psig), and uses more severe cracking conditions selected so that the cracking reactions proceed faster than the competing dehydrogenation reactions. Although the hydrocracking step can utilize either thermal or catalytic-type cracking, it is usually preferable to use thermal cracking so as to avoid plugging of the cracking reactor with coked catalyst material. However, if a cracking catalyst is desired, a useful catalyst is cobalt-molybdate on alumina support.

Because undesired coking of the highest boiling fraction is a potential problem during hydrocracking, it is usually desirable to fractionate the hydrogenated material into a lower-boiling and a higher-boiling fraction, which are then hydrocracked separately under different optimized conditions.

Reference is now made to the accompanying drawings, in which: Figure 1 is a schematic diagram of a process for hydrogenating and then hydrocracking polynuclear aromatic compounds to produce two-ring and mononuclear aromatic products; Figure 2 shows an alternative process in which the hydrogenated feed material is fractionated and the separate fractions are then hydrocracked under different conditions; and Figure 3 is a graph showing the percentage centre-ring cracking versus terminal-ring cracking achieved in polynuclear aromatic compounds.

As shown in Figure 1, a pyrolysis tar material, such as steam pyrolysis tar obtained from an ethylene steam cracking plant, is provided at 10, pumped to elevated pressure at 12, and hydrogen is added at 14. The resulting mixture is usually preheated at 1 5 and then introduced into a downflow fixed-bed type hydrogenation or hydrotreating reactor 1 6. The reactor contains a particular hydrogenation catalyst, such as comprising about 6 W % nickel and 1 9 W % tungsten on a silicaalumina support. Reaction conditions are maintained within the broad range of 1 49-4820C (300-9000F) temperature and 69-124 bar gauge (1000 1800 psig) hydrogen partial pressure.

Preferred conditions are 1 77-4540C (350--8500F) temperature and 76-117 bar gauge (1100-1700 psig) hydrogen partial pressure. The space velocity used should be within the range of 0.5-5.0 Vf/hr/Vc, to effectively saturate the centre ring(s) of the feed molecules with hydrogen.

Following the hydrogenation reaction in reactor 16 to saturate the centre ring of the molecules, the hydrogenated material is withdrawn at 1 8. This stream can be passed directly to the hydrocracking step; however, it is preferably first passed to a phase separator 20. From the separator 20, a gas stream is withdrawn at 22 and passed to further processing steps as desired, such as for recovery of hydrogen for recycle and reuse at stream 14. The liquid portion is withdrawn at 24 and passed with hydrogen at 25 to a cracking reactor 26, which will preferably consist of thermal cracking, but a catalytic cracking reaction can be used.Operating conditions for the cracking reactor 26 are maintained without the broad range of 427-7040C (800--13000F) temperature and 34-207 bar gauge (500-3000 psig) hydrogen partial pressure. For thermal cracking the preferred conditions are 538-7040C (1000-13000F) temperature and 69-172 bar gauge (1000-2500 psig) hydrogen partial pressure.

For catalytic cracking lower temperatures would be used, preferably 454-51 00C (850--9500F) temperature and 69-172 bar gauge (1000-2500 psig) hydrogen partial pressure. The space velocity used for catalytic cracking in the reactor 26 can be within the range of 3-7 wt/hr/wt catalyst.

Following the cracking step at 26, the resulting stream is withdrawn at 28 and passed to a phase separation of fractionation step 30. A gas stream 32 is removed and can be used as a low-sulphur, fuelgas product. A liquid stream 34 is withdrawn as the principal product, and can be fractionated into further liquid fractions if desired.

An alternative hydrogenation and hydrocracking process for polynuclear aromatic compounds is shown in Figure 2. This process is similar to Figure 1 except that the hydrotreating step is performed in an upflow ebullated-bed type catalytic reactor 36. The effluent stream 38 from the ebullated catalyst bed reactor is fractionated at 40 into a gas stream 41 and at least two separate liquid fractions having different boiling ranges. These fractions are then passed to separate cracking steps in which the reaction conditions are selected so as to minimize or avoid coking of the higher boiling fractions in the reactor(s). Specifically, stream 42 having a lower boiling range of 260-3990C (500--7500F) is passed with hydrogen at 43 to a cracking reactor 46.Liquid stream 44 having a higher boiling range of 371-5100C (700-9500F) is passed with hydrogen at 45 to a cracking reactor 48. Although these cracking reactors are shown for downflow type operation, they could be operated as upflow reactors which is particularly desirable if a cracking catalyst is used. The resulting cracked product streams 47 and 49 are then combined and passed to a fractionation step 50, from which is withdrawn a desired product gas stream 51, an intermediate boiling range liquid stream 52, and a heavy liquid stream 54.

This invention will be better understood by reference to the following Examples of hydrogenation and hydrocracking operations, which should not be regarded as limiting the scope of the invention.

EXAMPLE 1 A stream cracker tar feed material, supplied by Exxon Corporation and designated S-2 tar, had characteristics as listed in Table 1.

TABLE 1 Properties of Exxon Aromatic S-2 Tar API Gravity -4.4 Viscosity, SSU at 2100F (990C) 99.4 Flash Point (PM), F( C) 255 (124) Weight, % Carbon 91.2 Hydrogen 6.9 Sulphur 1.14 Conradson Carbon, W % 19.7 Normal Heptane Insolubles, W % 27.4 Vacuum Distillation, F( C) Initial BP 465 (241) 5 V % 505 (265) 10V% 517(269) 30V% 601 (316) 50 V % 703 (373) 70 V % 858 (459) Final BP 893 (478) Residue, V % 24 As a first step, 2,000 grams of the steam cracker tar was hydrogenated in a one-gallon (3.8 liters) capacity stirred autoclave over 200 grams of presulphided catalyst containing 6 W % nickel and 19 W % tungsten deposited on a silica-alumina support. The reaction conditions used were 221-288 C (430-5500F) temperature and 86-114 bar gauge (1250-1650 psig) hydrogen partial pressure for 7.25 hours, as further shown in Table 2. This hydrogenation step increased the hydrogen content of the tar feed material from 6.70 to 7.44 W %.

TABLE 2 Operating Conditions for S-2 Tar Hydrogenation Run No. 207 - ET-1 ET-2 Catalyst 200 gm Recovered from Presulphied (HRI 4001) 207-ET-1 Tar, Gm 2000 2064 Stirrer, rpm 500 5001000* Pressure During Run, psig 1250-1 650 1000-1 550 (bar gauge) (86-114) (69-107) Elapsed Time, Hrs. at 0 2.6 0-4.3 Temperature, F ( C) 80-428 (27-220) 80-570 (27-299) 2.6-5.4 4.3-6.4 428 (220) 570-550 (299-288) 5.4-6.2 6.4-135 428-528 ** 550 (288) (220-276) 6.2-13.5 528550o (276-288) Tar Recovered, Gm 18.18 2005 * Tachometer not working.

** Temperature raised to increase reaction rate.

EXAMPLE 2 A hydrocracking operation was next performed on the hydrogenated tar material from Example 1 using a 30 cc volume, downflow reactor filled with 20 cc of a standard cobalt-molybdenum hydrodesulphurization catalyst (HRI-3830) which was treated with Ba(OH)2 to reduce coking. The reaction conditions used were about 5100C (9500F) average temperature and 69-100 bar gauge (1000--1 455 psig) hydrogen partial pressure; the flow rate was 1 5 cc/hr for a space velocity of 4.46-4.8 wt. feed/hr/wt. catalyst. as shown in Table 3.

TABLE 3 Conditions for Cracking Hydrotreated Tar Run No. 214-45 214-46 Run Time. Hours 1.03 1.53 Hydrogen Pressure, psig (bar gauge) 1000(60) 1455 (100) Reactor Average Temperature, F ( C) 951(511) 950 (510) Liquid Feed Rate. Gm,'Hour 81.5 81.8 Weight Catalyst. Gm 18.3 17.1 Space Velocity Gm Feed/Hour/Gm Catalyst 4.46 4.80 H2 Flow Rate, SCF/Hour (m /h) 2.73 (77.3) 3.70 (104.8) From the analysis of the individual product fractions, the percent carbon in the aromatic structure can be obtained. Also, the size of the product fractions and the percent conversions can be estimated.

Figure 3 is a graph of the weight percent conversion of feed achieved against % CA (average) in the product fraction (% CA = the percent carbon in the aromatic structure).

Centre ring cracking requires only one centre ring to be hydrogenated, leaving the bulk of the carbon atoms in the aromatic structure. Terminal ring cracking requires on average the hydrogenation of two rings. This defines the range of carbon in the aromatic structure for 100% conversion, by either centre ring cracking or terminal ring cracking.

The experimental points in Figure 3 showed that on a normalized basis, 86% of the six-ring aromatics were converted to napthalenes and anthracenes, of which an estimated 80% occurred via centre-ring cracking. In addition, 52% of the four-ring compounds (boiling above 399"C or 7500 C) were converted to anthracenes, naphthalenes and benzenes, of which an estimated 45% occurred via centre ring cracking. The liquid fractions boiling below 2880C (5500 F) were increased from about 1 8% to 41 % of the original feed, indicating conversion of polynuclear aromatics with four or more condensed rings to two-ring compounds. The gas produced was 6-12%, the sulphur in the 51 00C (9500 F) - fraction was reduced to below about 0.70 W %.

Only terminal ring cracking was observed in the case of three-ring aromatic feeds, where conversion was limited to 30% centre ring cracking of three-ring aromatics, which may well be feasible at higher temperatures.

Claims (10)

1. A process for hydrogenation and cracking of polynuclear aromatic compounds, comprising: (a) heating a hydrocarbon feedstock comprising polynuclear aromatic compounds with hydrogen and introducing the mixture into a catalytic reaction zone to saturate the centre-ring molecules under mild hydrogenation reaction conditions within the range of 149-4820C (300-9000F) temperature, and 69-124 bar gauge (1000-1800 psig) hydrogen partial pressure; (b) hydrocracking the hydrogenated compounds in a cracking zone under reaction conditions within the range of 427-7040C (800--13000F) temperature, 34-207 bar gauge (500-3000 psig) hydrogen partial pressure;; (c) withdrawing an effluent stream from said cracking zone and passing it to a phase separation step for separation into gaseous and liquid portions; and (d) withdrawing the gaseous portion and a hydrocracked aromatic liquid product.

2. A process as claimed in Claim 1, wherein step (a) utilizes an upflow ebullated-bed type catalytic reactor.

3. A process as claimed in Claim 1 or 2, wherein a phase separation step is provided between steps (a) and (b), a gaseous stream is removed from said phase separation step for separate processing, and the remaining liquid stream is passed to step (b) for the hydrocracking reaction.

4. A process as claimed in any of Claims 1 to 3, wherein step (b) utilizes a hydrocracking catalyst, and the operating conditions are maintained within the range of 427-5380C (800-10000F) temperature and 69-1 72 bar gauge (1000-2500 psig) hydrogen partial pressure, and space velocity of 3-7 gm feed/hr/gm catalyst.

5. A process as claimed in any of Claims 1 to 4, wherein the liquid product from step (e) is fractionated to produce light and heavy fraction liquid streams.

6. A process as claimed in any of Claims 1 to 5, wherein the effluent from the hydrogenation step (a) is fractionated into at least two fractions, each fraction is passed to a separate cracking step operated under different conditions and the cracked product streams are combined and fractionated to produce a gas stream, an intermediate boiling range liquid stream, and a heavy liquid stream.

7. A process for hydrogenation and cracking of polynuclear aromatic compounds, comprising: (a) heating the hydrocarbon feedstock with hydrogen and introducing the mixture into a catalytic ebullated bed reaction zone to saturate the centre-ring molecules under mild hydrogenation reaction conditions within the range of 1 49-4820C (300--9000F) temperature, and 69-114 bar gauge (1000-1 650 psig) hydrogen partial pressure; (b) phase separating the hydrogenated stream to provide a gas stream and a liquid stream;; (c) thermally hydrocracking the hydrogenated liquid stream in a cracking zone under reaction conditions within the range of 482-7040C (900--1 3000 F) temperature, and 69-1 72 bar gauge (1000-2500 psig) pressure; (d) withdrawing an effluent stream from said cracking zone and passing it to a phase separation step for separation into gaseous and liquid portions; (e) withdrawing the gaseous portion; and (f) withdrawing a hydrocracked aromatic liquid product and fractionating said stream to produce lower boiling and higher boiling aromatic streams.

8. A process for hydrogenation and cracking of polynuclear aromatic compounds, comprising: (a) heating the hydrocarbon feedstock with hydrogen and introducing the mixture into a catalytic ebullated bed reaction zone to saturate the centre-ring molecules under mild hydrogenation reaction conditions within the range of 38-31 60C (1 006000 F) temperature, and 69-114 bar gauge (1000-1650 psig) hydrogen partial pressure; (b) fractionating the hydrogenated stream to provide a gas stream and a liquid stream;; (c) catalytically hydrocracking the hydrogenated liquid streams in separate cracking zones under reaction conditions within the range of 454-51 00C (850--9500F) temperature, 69-172 bar gauge (1000-2500 psig) hydrogen partial pressure, and space velocity of 3-7 cc/hr/gm catalyst; (d) withdrawing the effluent streams from said separate cracking zones, combining the streams together and passing the combined stream to a phase separation step for separation into gaseous and liquid streams; (e) withdrawing the gaseous portion; and (f) withdrawing the hydrocracked aromatic liquid product streams.

9. A process as claimed in Claim 1, substantially as hereinbefore described with reference to any of the Examples and/or the accompanying drawings.

10. A hydrocracked, aromatic liquid product produced by a process as claimed in any of Claims 1 to 9.