CA1191806A

CA1191806A - Center ring hydrogenation and hydrocracking of polynuclear aromatic compounds

Info

Publication number: CA1191806A
Application number: CA000407999A
Authority: CA
Inventors: Hugh J. Parkhurst, Jr.; Derk T.A. Huibers
Original assignee: HRI Inc
Current assignee: HRI Inc
Priority date: 1981-07-27
Filing date: 1982-07-23
Publication date: 1985-08-13
Also published as: ZA825157B; GB2104544A; NL8202997A; GB2104544B

Abstract

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 300-900°F
temperature, and 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 800-1300°F temperature, and 500-3000 psig hydrogen partial pressure. The hydrocracking step may use a catalyst comprising cobalt-molybdenum on alumina and 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

D lL81 ~ 6~

CENTER RING HYDROGENATION AND ~YDROCRACKING
OF POLYNUCLEAR AROMATIC COMPOUNDS

_ACKGROUND OF INVENTION

Thls invention pertains to hydrogenatlon of polynuclear aromatic compounds followed by cracking o~ the center rings of such compounds to produce high yields of mononuclear aro-matic compounds and liquid products.

Traditional single step hydrocracking of polynuclear aromatic compounds at elevated temperature conditions favor 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 the successive terminal rings produces only a low value Cl-C4 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 mcre aromatic rings of polynuclear aromatic compounds, such as those found in petroleum residua and coal-derived liquids, effective cracking must ~e preceded by a hydrogenation step. Wiser, et al, in Ind. Eng. Chemistry, Prod. Res. Develo~., 9, 350 (1970), demonstrated the Eeasibility of cen-ter ring hydroge-nation of polynuclear aromatic materials, and obtained a yield of almost 80 W % o~ 9, 10 dihydroanthracene from the hydrogenation of anthracene over nickel tungsten catalyst in an autoclave at ~00C temperature and 1500 psig H2 partial pressure for three hours. However, difficulty occurs in cracking the polynuclear aromatics at the higher teDmperature before dehydrogenation occurs, and Wiser was apparently not successful in demonstrating center ring cracking.
Our experiments made at new conditions have indicated successful center ring catalytic hydrogenation and hydrocrack-ing 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 moleculesu SUMMARY OF INVENTION

The present invention describes an improved process for center-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 % sulfur, to produce desulfurized one-ring and two-ring aromatic products.

The process first reacts the polynuclear aromatic feedstock in a separate catalytic hydrogenation step per-formed at relatively low temperature conditions of 300-900F
to selectively saturate one or more center rings of the feed compound. The resulting hydrogenated compound is then passed to a subsequent hydrocracking step performed at higher temperature of 800-1000F, C~6 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 condit1ons used are selected such that the cracXing reactions which occur proceed at a faster rate than the com-petlng dehydrogenation reactions.

An lmportant 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 bciling point of about 465F, and contains 18 - 40% naphthalenes, 14 - 3~% three-ring aromatics, 7 - 17% four-ring aromatics and 24 - 31% residue, consisting of six-ring and larger aromatics. Many pyrolysis tars contain as much as 5~ sulfur and therefore fail to rneet the EPA fuel standards and cannot be legally burned, however, the S-2 pyrolysis tar contained only 1.3~ sulfur. The hydrogenation step reduces the sulfur concentration to about half that contained in the feed.

The catalytic hydrogenation or hydrotreating step for the preheated feedstoc~ can be accomplished in either a fixed-bed or 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 condition required for the hydrotreating step are within the range of 300-900F
temperature and 1000-1800 psi hydrogen partial pressure.

The polyhydrogenated tar material is next hydrocracked at temperature of 800-1300F and hydrogen partial pressure of SO0-3000 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 crac]cing, it is usually preferable to use thermal cracking so as to avoid plugging of the cracking reactor with coked catalyst ma-terial. ~owever, 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 hydrocracXing, it is usually desirable to fractionate the hydrogenated material into a lower-boiling and a higher-boiling fraction, which are then hydrocracked separately at different optimized conditions.

DESC~IPTION OF DRAr~INGS

Figure 1 is a schematic diagram of a process for hydro-genating and then hydrocracking polynuclear aromatic com-pounds 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 hydrocracXed at different conditions.

Flgure 3 is a graph showing the percentage center-ring cracklng versus terminal-ring cracking achieved in poly-nuclear aromatic compounds.

3~

DESCRIPTION OF PREFERRED _ ODIMENTS

As shown in Figure 1, a pyrolysis tar material, such as steam pyrolysis tar obtained from an ethylene steam cracking plant, is provlded at 10, pumped to elevated pressure at 12, and hydrogen is added at 14. The resulting mixture is usually preheated at 15 and then introduced into a downflow fixed bed type hydrogenation or hydrotreating reactor 16.
The reactor contains a particulate hydrogenation catalyst, such as comprising about 6 W % nickel and 19 W ~ tungsten on a silica-alumina support. Reaction conditions are maintained within the broad range of 300-900F temperature and 1000-1800 psig hydrogen partial pressure. Preferred conditions are 350-850F temperature and 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 center ring(s) of the feed molecules with hydrogen.

Following the hydrogenation reactions in reactor 16 to saturate the center ring of the molecules, the hydrogenated material is withdrawn at la. This stream can be passed directly to the hydrocracking step; however, it is preferably first passed to phase separator 20. From 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 cracking reactor 26, which will preferably consist of thermal cracklng, but a catalytic cracking reaction can be used.
Operating conditions for cracking reactor 26 are maintained within the broad range of 800-1300F temperature and 500-3000 psig hydrogen partial pressure. For thermal cracking the preferred conditions are 1000-1300F temperature and c~

1000-2500 psig hydrogen ~artlal pressl~re. For catalytic cracking lower temperatures would be used, preferably 850-950F temperature and 1000-2500 psig hydrogen partial pressure. The space velocity used for catalytic cracking in reactor 26 can be within the range of 3-7 wt/hr/wt catalyst.

Following the cracklng step at 26, -the resulting stream is withdrawn at 28 and passed to phase separation or frac-tionation step 30. Gas stream 32 is removed and can be used as a low-sulfur, fuel-gas product. Liqùid stream 34 is withdrawn as the principal product, and can be fractionated in-to fur~her liquid fractions if desired.

~ n alternative hydrogenation and hydrocracking process for polynuclear aromatic compounds is show~ in Figure 2.
This process is similar to Figure 1 except the hydrotreating step is performed in an upflow ebullated bed type catalytic reactor 36. The effluent stream 38 from the ebullated cata-lyst bed reactor is fractionated at 40 into gas stream 41 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 the higher boiling fractions in the reactor(s). Specifically, stream 42 having lower boiling range of 500-750F is passed with hydrogen at 43 to cracking reactor 46. Liquid stream 4~ having higher boiling range of 700-950F is passed with hydrogen at 45 to cracklng 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 4g are then combined and passed to fractionation step 50, from which i5 withdrawn a desired product gas stream 51, an intermediate boiling range liquid stream 52, and a heavy liquid stream 54.

3()6 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.

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

P~OPERTIES OF EXXON AROMATIC S-2 TAR
. _ API Gravity -4.4 Viscosity, SSU @ 210F 99.4 Flash Point, tPM), F 255 Weight, %
Carbon 91.2 Hydrogen - 6.9 Sulfur 1.14 Conradson Carbon, W % 19.7 Normal Heptane Insoluables W % 27.4 Vacuum Distillation, F
Initial BP 465 5 V % 505 10 V % 517 30 V % 601 50 V ~ 703 70 V % ~58 Final ~P a93 Residue, V % 24 As a first step, 2,000 grams of the steam cracker tar was hydrogentated in a one-gallon capacity stirred autoclave over 200 grams of presulfided catalyst containing 6 W ~ nickel and 19 W % tungsten deposited on a silica-alumina support. The reaction conditions used ~ere 430-550F temperature and 1250-1650 psig hydrogen partial pressure for 7.25 hours, as further shown in Table 2. This hydrogena-tion step increased the hydrogen content of the tar feed material from 6.70 to 7.44 ~l %.

Run X, 207 - ~T-l Catalyst 200 gmRecovered from Presulfied 207-ET-l ~HRI 4001) Tar, Gm 2000 2064 Stirrer, rpm 500 500-1000*
Pressure During Run, pslg1250-16501000 1550 Elapsed Time, Hrs @ 0-2.6 0 4.3 Temperature, F 80-~28 80-570

2.6-5.4 4.3-6.4 42~ 570-550 5.4 6.2 6.4-135 428-528** 550 6.2-13.5 Tar Recovered, Gm 18.18 2005 Tachometer not working Temperature raised to increase reaction rate.

A hydrocracXing operation was next performed on the hydrogenated tar material from Example 1 using a 30-cc volume, down1Ow reactor filled with 20 cc of a standard cobaLt-molydenum hydrodesulfurization catalyst (HRI-3830) which was treated with Ba (OH)2 to reduce coking. Reaction conditions used were about 950F average temperature and 1000-1455 psig hydrogen partial pressure; flow rate was 15 cc/hr for space velocity of 4.46-4.8 wt.feed/hr/wt.
catalyst, as shown in Table 3.

TA~LE 3 CONDITIONS FOR CRACKING HYDROTRE~TED TAR

Run ~o. 214-45 214-~6 Run Time, Hours 1.03 1.53 Hydrogen Pressure, psig 1000 1455 Reactor Average Temperature, F 951 950 L,iquid Feed Rate, Gm/Hour81.5 81.8 Weight Catalyst, Gm 18.3 17.1 Space Velocity, Gm Feed/Hour/Gm Catalyst4.46 4.80 H2 FLow Rate, SCF/Hour 2.73 3.70 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 correlates the percent carbon in the aromatic structure with the weight percent conversion achieved.

Center ring cracking requires only one center ring to be hydrogenated, leaving the bulk of the carbon atoms in the aromatic structure. Terminal rina cracking requires on average the hydrogenation of two rings. This defines the range of carbon in aromatic structure for 100~ conversion, by either center 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 con-verted to naphthalenes and anthracenes, of which an esti-mated 80% occurred via center-ring crac~ing. In addition, 52% of the four-ring compounds (boiling above 750F) were converted to anthracenes, naphthalenes and benzenes, of which an estimated 45% occurred via center ring cracking.

The liquid fractions being below 550F were increased from about 18% 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~, sulfur in the 950F - fraction was reduced to below about 0.70 W ~.

v~

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

Although we have disclosed certain preferred embodiments of our invention, it is recognized that various modifications can be made thereto, all within the spirit and scope of the invention, which is defined by the following claimsO

Claims

We claim:

l. A process for hydrogenation and cracking of poly-nuclear aromatic compounds, comprising:

(a) heating the hydrocarbon feedstock with hydrogen and introducing the mixture into a catalytic reaction zone to saturate the center-ring molecules under mild hydrogenation reaction conditions within the range of 300-900°F temperature, and 1000-1800 psig hydrogen partial pressure;

(b) hydrocracking the hydrogenated compounds in a cracking zone at reaction conditions within the range of 800-1300°F temperature, 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. The process of Claim 1, wherein step (a) utilizes an upflow ebullated-bed, type catalytic reactor.
3. The process of Claim l, 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. The process of Claim 1, wherein step (b) utilizes a hydrocracking catalyst, and the operating conditions are maintained within the range of 800-1000°F temperature and 1000-2500 psig hydrogen partial pressure, and space velocity of 3-7 gm feed/hr/gm catalyst.
5. The process of Claim 1, wherein the liquid product from step (e) is fractionated to produce light and heavy fraction liquid streams.
6. The process of Claim 1, wherein the effluent from hydrogenation step (a) is fractionated into at least two fractions, each fraction is passed to a separate cracking step operated at 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 poly-nuclear aromatic compounds, comprising:

(a) heating the hydrocarbon feedstock with hydrogen and introducing the mixture into a catalytic ebullated bed reaction zone to saturate the center-ring molecules under mild hydrogenation reaction conditions within the range of 300-900°F temperature, and 1000-1650 psig hydroqen partial pressure;

(b) phase separating the hydrogenated stream to provide a gas stream and a liquid stream;

(c) thermal hydrocracking the hydrogenated liquid stream in a cracking zone at reaction conditions within the range of 900-1300°F temperature, and 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 poly-nuclear aromatic compounds, comprising:

(a) heating the hydrocarbon feedstock with hydrogen and introducing the mixture into a catalytic ebullated bed reaction zone to saturate the center-ring molecules under mild hydrogenation reaction conditions within the range of 100-600°F temperature, and 1000-1650 psig hydrogen partial pressure;

(b) fractionating the hydrogenated stream to provide a gas stream and a liquid stream;

(c) catalytic hydrocracking the hydrogenated liquid streams in separate cracking zones at reaction conditions within the range of 850-950°F temperature, 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.