Note: Descriptions are shown in the official language in which they were submitted.
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.
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
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 %.
TABLE 2
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
528-550
Tar Recovered, Gm 18.18 2005
Tachometer not working
Temperature raised to increase reaction rate.
EXAMPLE 2
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
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