Hydrocracking of HDPE and plastic waste

Prévia do material em texto

Hydrocracking and Hydroisomerization of High-Density
Polyethylene and Waste Plastic over Zeolite and
Silica-Alumina-Supported Ni and Ni-Mo Sulfides
Weibing Ding, Jing Liang, and Larry L. Anderson*
Department of Chemical and Fuels Engineering, University of Utah,
Salt Lake City, Utah 84112
Received March 26, 1997X
Conversion of plastic waste into transportation fuels over bifunctional catalysts was systemati-
cally studied. Previous work showed that some acid catalysts were active for degradation of
pure polyolefins, but they were easily deactivated by nitrogen, sulfur, and impurities contained
in actual postconsumer plastic waste. Ni and NiMo sulfides loaded on a hybrid support (HSiAl),
a mixture of HZSM-5 and silica-alumina, were found to be effective for converting both pure
high-density polyethylene and plastic waste to gasoline-range products. Hydrocracking reactions
were carried out mostly at 375 °C, 1000 psig H2 (initial), for a reaction time of 1 h, though the
effects of initial hydrogen pressure and reaction time were also examined. Ni/HSiAl had higher
hydrocracking and hydroisomerization ability than did NiMo/HSiAl. The quality of liquid products
obtained over Ni/HSiAl was comparable to that of a commercial premium gasoline. Moreover,
being resistant to poisoning by N- and S-containing compounds, these catalysts could be
regenerated simply by recalcination and resulfiding.
Introduction
The United States is heavily dependent on liquid
fuels, such as gasoline, diesel, and jet fuel. The present
demand for these fuels far exceeds domestic petroleum
production capacity, and over one-half of them is
imported. Meanwhile, the rapidly increasing use of
plastic products results in a severe waste plastic dis-
posal problem.1-3 Converting waste plastic into liquid
fuels would not only supplement U.S. energy supplies
but could also mitigate environmental disposal prob-
lems.4,5
The hydrocracking process, which basically converts
high-boiling molecules into more desirable lower mo-
lecular weight products with low olefin and high iso/
normal paraffin yields by simultaneous or sequential
hydrogenation and carbon bond breaking, is not only
an important process used in modern oil refineries but
also one of the most promising processes for conversion
of waste plastic. Dual functional catalysts, having both
cracking and hydrogenation-dehydrogenation func-
tions, are used for this process. The cracking function
is realized by an acidic support with a high surface area,
while the hydrogenation component is usually a metal,
oxide, or sulfide of group VIII and/or group VIb. The
acidic supports used in today’s modern petroleum
industry are mostly silica-alumina and various zeo-
lites.6
A polymerization catalyst (TiCl3) and metal-promoted
sulfated zirconia were successfully used for hydrocrack-
ing of pure polyolefins; however, these catalysts had
negative effects on the degradation of commingled
postconsumer plastic. Possibly they were poisoned by
heteroatoms (N and S) and impurities contained in
waste plastic.7-9 Transition metal sulfides are good
candidates for the hydrocracking of waste plastic, since
the hydrodenitrogenation (HDN) and hydrodesulfuriza-
tion (HDS) functions of these materials have been well
recognized for several decades.10
Commercial catalysts usually adopt Ni, Mo, W, Co,
or their combinations as the active metal sulfide com-
ponent, whereas silica-alumina and zeolites, such as
erionite, mordenite, Y, and ZSM-5,6,11-18 were used as
supports. The activity of these catalysts was tested by
measuring the hydrogenation of benzene, the hydroc-
racking and hydroisomerization of n-alkanes, HDN, and
HDS activity.19-25 It was concluded that a flexible
* To whom correspondence should be addressed. Phone/fax: (801)
581-5162. E-mail: larry.anderson@m.cc.utah.edu.
X Abstract published in Advance ACS Abstracts, September 15, 1997.
(1) D’Amico, E.; Roberts, M. Chem. Week 1995, October 4, 32.
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December, 30.
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(4) Ding, W. Ph. D. Dissertation, University of Utah, March, 1997.
(5) Taghiei, M. M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy
Fuels 1994, 8, 1228.
(6) Bhatia, S. Zeolite Catalytisis: Principles and Applications; CRC
Press Inc.: Boca Raton, FL, 1990.
(7) Ding, W.; Liang, J.; Anderson, L. L. Fuel Process. Technol., in
press.
(8) Venkatesh, K. R.; Hu, J.; Dogan, C.; Tierney, J. W.; Wender, I.
Energy Fuels 1995, 9, 888.
(9) Venkatesh, K. R.; Hu, J.; Wang, W.; Holder, G. D.; Tierney, J.
W.; Wender, I. Energy Fuels 1996, 10, 1163-1170.
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20, 155.
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J. 1977, 75, 165.
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1991, 10, 103.
1219Energy & Fuels 1997, 11, 1219-1224
S0887-0624(97)00051-0 CCC: $14.00 © 1997 American Chemical Society
selectivity combined with high activity of a bifunctional
catalyst depended on the relative strengths of the metal
sulfide and acidic functions.
We have found that HZSM-5 was effective for the
hydrocracking of high-density polyethylene (HDPE) and
plastic waste,7 although the quality of the liquids
produced was far below that of commercial gasoline
possibly because of a lack of sufficient hydrogenation
function. To successfully convert waste plastic into
transportation fuels, a catalyst combining hydrocrack-
ing, hydrogenation, HDN, and HDS abilities is needed.
In this study, a new hybrid support, which is a mixture
of HZSM-5 and silica-alumina, was used as an acidic
component, and NiMo or Ni sulfide was used for
hydrogenation. In this way we attempted to achieve a
better balance between the hydrogenation and acidic
functions and, consequently, produce desired, clean,
gasoline-like products. A commercial hydrocracking
catalyst, KC-2600, obtained from Akzo Nobel Inc., was
also used for this purpose, and the results were com-
pared.
Experimental Section
High-density polyethylene (HDPE, average MW ca. 125 000;
d ) 0.950 g/cm3; in bead form) was purchased from Aldrich
Chemical Co. Commingled postconsumer plastic (CP#2),
obtained from the American Plastics Council, was ground to
-8 mesh. The detailed analyses of CP#2 have been reported
elsewhere.26 Qualitatively, CP#2 consists of mostly high-
density polyethylene with small amounts of polypropylene,
polystyrene, and other polymers as well as some impurities.
The elemental analysis showed that CP#2 contained 0.65 wt
% nitrogen and 0.01 wt % sulfur. A commercial premium
gasoline was collected from a local retail gasoline station on
December 24, 1996 in Salt Lake City, Utah.
KC-2600, a commercial hydrocracking catalyst, was pro-
vided by Akzo Nobel Chemical, Inc. It may contain NiMo/
zeolite and/or NiMo/Al2O3. HZSM-5 and SiO2-Al2O3 (with
13% Al2O3 content) were purchased from United Catalysts Inc.
and Aldrich Chemical Co., respectively. The average pore size
and surface area of the SiO2-Al2O3 were 65 Å and 475 m2/g,
respectively,27 while those of the HZSM-5 were 6.2 Å and
greater than 200 m2/g, respectively.28 The HZSM-5 contained
10-40 wt % of a proprietary binder. The metal salts, nickel-
(II) nitrate hexahydrate and ammonium molybdate tetrahy-
drate, were obtained from Aldrich Chemical Co. and Fluka
Company, respectively.
The hybrid support, namely, HSiAl, was prepared by
cogrinding a mixture of 4 parts by weight of SiO2-Al2O3 with
1 part by weight of HZSM-5. The support particles were
collected undera 100 mesh Tyler standard sieve. Supported
Ni catalysts were prepared by impregnation of Ni(NO3)2‚6H2O
onto the support followed by drying in a vacuum oven at 110
°C for 12 h. The dried samples were then calcined in a muffle
oven at 500 °C in air for 16 h. The resulting catalysts were
stored in vials before sulfidation or used for further prepara-
tion of NiMo catalysts. The procedure for impregnating Mo
metal on the support was the same as the procedure stated
above except that (NH4)6Mo7O24‚4H2O was used as metal
precursor. The Ni metal loading was 2.6%, while Mo loading
was 7.0%, if any.
The calcined oxide catalysts were presulfided in a tubular
flow reactor. The temperature profile was programmed as
follows. The catalyst was heated from room temperature to
400 °C in about 20 min with helium flow (100 mL/min) and
remained at that temperature for 1 h. Then gas flow was
switched from helium to a mixture of hydrogen and hydrogen
sulfide (H2:H2S ) 9:1, by volume; 50 mL/min) and maintained
for 2 h followed by stabilization of the catalyst with helium
flow at 400 °C for 1 h. After the catalyst was cooled to room
temperature with helium flow it was transferred to a tubing
reactor for degradation of plastics.
Hydrocracking reactions of HDPE and CP#2 were carried
out in a 27 mL tubing reactor at 375 °C for 0-60 min.
Typically, 2 g of plastic and a calculated amount of presulfided
catalyst were fed into the reactor, which was then closed,
purged with nitrogen, and then pressurized with hydrogen to
the desired initial pressure. The reactor was then immersed
in a preheated fluidized sand bath and reached the desired
reaction temperature within 3-4 min. The mixing of reactants
and catalyst particles was achieved by horizontal shaking of
the reactor at 160 rpm. Detailed reaction procedures and
definitions of yields have been reported elsewhere.7
The gaseous products were collected and analyzed by GC-
FID (HP-5890II) using a column packed with HayeSep Q. The
analyses of liquid products were carried out on a Hewlett-
Packard 5890II gas chromatograph coupled to a Hewlett-
Packard 5971A mass spectrometer (GC/MS). A 30 m DB-5
capillary column was used for the GC-MS analyses. The
gaseous products consisted of mostly C1-C4 hydrocarbons with
some C5 and small amounts of C6 compounds. The liquid
products consisted of mainly C5-C12 compounds with small
amounts of C13+ and C3-C4 compounds.
The boiling point distribution of a commercial gasoline and
liquid products obtained from hydrocracking of CP#2 was
determined by simulated distillation according to ASTM D
2887-89. The analysis was performed on an HP-5890II gas
chromatograph (FID), using a Petrocol B column (length of 6
ft and outside diameter of 1/8 in.).
The amounts of carbon, nitrogen, and hydrogen in the liquid
products obtained from hydrocracking of CP#2 were deter-
mined using a LECO CHN-600 elementary analyzer. Sulfur
was determined with a LECO SC-132 sulfur analyzer. The
detection limits for carbon, hydrogen, nitrogen, and sulfur were
0.01 wt %.
Results and Discussion
Hydrocracking and Hydroisomerization of
HDPE. The effects of catalysts (KC-2600, NiMo/HSiAl,
and Ni/HSiAl) and the effects of different amounts of
catalyst (weight ratio of catalyst-to-feed, 0.20 or 0.40)
on yields and conversion of HDPE are shown in Figure
1. At 375 °C, virtually no thermal hydrocracking
occurred. However, in the presence of 20% KC-2600,
NiMo/HSiAl, or Ni/HSiAl, the conversion of HDPE
reached 64.5%, 65.3%, and 81.7%, respectively. The
liquid yields were higher than the corresponding gas
yields. When the catalyst amount was increased to
40%, more than 99% conversion was obtained over
either NiMo/HSiAl or Ni/HSiAl, and above 90% over
KC-2600. Using more catalyst that the 40% value
resulted in more gases being produced.
When 40% hybrid support (HSiAl containing 20 wt
% HZSM-5 and 80 wt % SiO2-Al2O3) was used as a
catalyst for hydrocracking of HDPE under the same
(20) Vazquez, M. I.; Escardino, A.; Corma, A. Ind. Eng. Chem. Res.
1987, 26, 1495.
(21) Welters, W. J. J.; Vorbeck, G.; Zandbergen, H. W.; de Haan, J.
W.; de Beer, V. H. J.; van Santen, R. A. J. Catal. 1994, 150, 155.
(22) Song, C. S.; Reddy, K. M. Prepr.sAm. Chem. Soc., Div. Pet.
Chem. 1996, 41, 567.
(23) van de Ven, L. J. M.; van Oers, E. M.; de Haan, J. W.; de Beer,
V. H. J.; van Santen, R. A. J. Catal. 1996, 161, 819.
(24) Cid, R.; Fierro, J. L. G.; Agudo, A. L. Zeolites 1990, 10, 95.
(25) Kovacheva, P.; Davidova, N.; Novakova, J. Zeolites 1990, 11,
54.
(26) Ding, W.; Liang, J.; Anderson, L. L. Fuel Process. Technol. 1996,
49, 49.
(27) Aldrich Chemical Co.
(28) United Catalysts Inc.
1220 Energy & Fuels, Vol. 11, No. 6, 1997 Ding et al.
reaction conditions stated in Figure 1, 66% conversion
was achieved. The metal function that is present in the
metal-supported catalyst allows facile formation of
olefins, which can then form the carbenium ions, which
in turn undergo the chemistry that is observed. The
hybrid support, on the other hand, does not contain the
metal function, and therefore, the formation of carbe-
nium ions is more difficult in this case. The rates would
be much slower in the latter case, and hence, the
conversion would be lower.
For 20% catalyst loading, the addition of Mo onto Ni/
HSiAl decreased the overall rate of the hydrocracking
reaction of HDPE. This nonsynergistic effect may be
due to a poor distribution of metal sulfides on the
support. Only part of single metal sulfide phases, like
nickel sulfide, would be formed in the HZSM-5 pores,
whereas the MoS2-supported nickel sulfide phase (Ni-
MoS) is unlikely to be formed in the small HZSM-5
pores. Even if some NiMoS is formed on the external
surface of the zeolite particles, its contribution to
hydrocracking is still low because metal sulfide located
on the external surface cannot prevent the deposition
of coke in the channels of HZSM-5 and/or it may block
some pore openings of HZSM-5. Therefore, more coke
and less conversion were obtained over NiMo/HSiAl.
Similar results on zeolite Y-supported Ni and NiMo
sulfides were observed by Welters et al.29
The products obtained from hydrocracking and hy-
droisomerization of HDPE over bifunctional catalysts
are shown in Table 1. Gaseous products consisted of
small amounts of C1 and C2 but large amounts of C3
and C4 with high iso/normal ratio of C4. Liquid products
contained mostly normal and iso paraffins, some aro-
matics and cyclo-paraffins, with little olefins. The ratios
of iso/normal paraffins were 1.43, 2.72, and 4.17 for
liquids obtained from hydrocracking of HDPE over
NiMo/HSiAl, KC-2600, and Ni/HSiAl, respectively.
Liquid products obtained over NiMo/HSiAl contained
more n-paraffins, less aromatics, and less olefins than
those obtained over Ni/HSiAl. This indicated that the
hydrogenation activity of NiMo/HSiAl was higher than
that of Ni/HSiAl, implying that the NiMoS phase was
more active in hydrogenation than nickel sulfide. Simi-
lar observations were also reported by Leglise et al.30
for stabilized HY zeolites as supports for Ni and NiMo
catalysts.
Hydroisomerization contributed to the high ratios of
iso/normal paraffins. HZSM-5 and sulfided Ni/SiO2-
Al2O3 were reported to have hydroisomerization func-
tions.7,31,32 Ni/HSiAl showed higher hydroisomerization
ability than did NiMo/HSiAl and KC-2600, probably
owing to its well-balanced hydrogenation ability and
acidity.
High yield of C3 hydrocarbons (ca. 20%) means
cracking reactions may occur according to a pentacoor-
dinated carbonium ion mechanism.32,33 Disproportion-
ation reactions also occurred during hydrocracking by
a mechanism involving an alkylation process in which
an olefinic intermediate adds on to an adsorbed car-
bonium ion and then cracks to yield a fragment of higher
carbon number and one of lower carbon number than
the reactants.34 Excess butane may be attributed to
disproportionation reactions. Production of excess bu-
tane in n-decane hydrocracking over sulfided Ni/SiO2-
Al2O3 was also reported by Langloiset al.31
The concentration of aromatics and benzene in gaso-
line have been limited by environmental regulations to
less than 25% and 2% respectively, indicating a trend
to reduce aromatics in reformulated gasoline in the near
future.35 Compared with a commercial premium gaso-
line (15.44% n-paraffins, 59.09% iso-paraffins, 1.69%
cyclo-paraffins, 0.94% olefins, and 20.24% aromatics),
liquids obtained from hydrocracking of HDPE over KC-
2600, NiMo/HSiAl, and Ni/HSiAl contained much less
aromatics, 1.41-6.6% (Table 1). A more environmen-
tally acceptable gasoline-like product was obtained over
Ni/HSiAl, which consisted of less aromatics (6.6%) and
(29) Welter, W. J. J.; van der waerden, O. H.; de Beer, V. H. J.; van
Santen, R. A. Ind. Eng. Chem. Res. 1995, 34, 1166.
(30) Leglise, J.; Janin, A.; Lavalley, J. C.; Cornet, D. J. Catal. 1988,
114, 338.
(31) Langlois, G. E.; Sullivan, R. F.; Egan, C. J. J. Phys. Chem. 1966,
70, 3666.
(32) Abbot, J.; Wojciechowski, W. Ind. Eng. Chem. Prod. Res. Dev.
1985, 24, 501.
(33) Anders, G.; Burkhardt, I.; Illgen, U.; Schulz, I. W.; Scheve, J.
Appl. Catal. 1990, 62, 281.
(34) Holmstrom, A.; Sorrik, E. M. J. Chromatogr. 1970, 53, 95; J.
Appl. Polym. Sci. 1974, 18, 761; J. Polym. Sci., Polym. Symp. 1976,
57, 33.
(35) Bell, A. T.; Manzer, L. E.; Chen, N. Y.; Weekman, V. W.;
Hegedus, L. L.; Pereira, C. J. Chem. Eng. Prog. 1995, February, 26.
Figure 1. Results of degradation of HDPE in a 27 mL tubing
reactor at 375 °C, 1000 psig H2 (initial), for a reaction time of
60 min with the indicated catalysts.
Table 1. Composition of Products Obtained from
Hydrocracking of HDPEa
catalyst KC-2600 NiMo/HSiAl Ni/HSiAl
Carbon Number Distribution, wt %
C1 0.25 0.49 0.54
C2 0.86 1.46 2.44
C3 19.68 23.44 20.96
iso-C4 19.02 10.76 22.90
n-C4 9.40 8.89 7.33
C5 8.75 16.18 10.94
C6 25.07 20.74 21.66
C7 3.72 4.20 3.05
C8 2.07 5.37 4.99
C9 0.71 2.78 3.18
C10 0.31 2.01 1.42
C11 0.12 1.35 0.18
C12 0.04 1.02 0.06
C13+ trace 0.60 trace
Composition of Liquid Products, wt %
n-paraffins 25.37 36.54 16.23
isoparaffins 68.96 52.42 67.66
cyclo-paraffins 2.87 8.09 9.24
olefins 1.38 0.07 0.27
aromatics 1.41 2.89 6.60
a Reaction conditions: 27 mL tubing reactor, 375 °C, 1000 psig
H2 (initial), reaction time of 60 min, catalyst:feed ) 0.4:1.0 by
weight.
Hydrocracking and Hydroisomerization Energy & Fuels, Vol. 11, No. 6, 1997 1221
more isoparaffins (67.66%) than did the commercial
gasoline. The GC-MS profile of liquid products ob-
tained from the hydrocracking of HDPE over Ni/HSiAl
is shown in Figure 2. Among the small amounts of
aromatics produced, no benzene was detected.
Hydrocracking and Hydroisomerization of CP#2.
No catalyst was reported to be active for degradation
of CP#2 (an actual plastic waste sample obtained from
the American plastic Council) at temperatures below
400 °C, since CP#2 contained small amounts of nitrogen
and sulfur as well as impurities that may poison some
solid acids.7 Bifunctional catalysts, KC-2600, NiMo/
HSiAl, and Ni/HSiAl, were very active for converting
CP#2 (Figure 3). At 375 °C, thermal conversion was
minimal, while catalytic effects were significant. More
than 99% conversion was obtained over 40% Ni/HSiAl
and 40% NiMo/HSiAl. Gas yields increased and liquid
yields decreased with increased catalyst loading. An
interesting result was that liquid products obtained over
each catalyst were clean and white or light yellow with
a gasoline-like smell, while the liquids produced from
thermal reactions (under H2) or catalytic reactions over
HZSM-5 and TiCl3 were brown red with a strong
unpleasant smell.36 It is noteworthy that ca. 99%
conversion was also reached with addition of only 20%
Ni/HSiAl, whereas only about 84% conversion was
obtained over 20% NiMo/HSiAl. The reason Ni/HSiAl
is more active in hydrocracking than NiMo/HSiAl was
discussed earlier in this paper.
CP#2 should be easier to convert than HDPE, since
it is a mixture of polyolefins with mostly HDPE, which
was found to be harder to convert among the common
polyolefins.7,37 This is true for thermal reactions.7
However, the conversion of CP#2 was always lower than
that of HDPE over HZSM-5 or TiCl3 under the same
reaction conditions,7 which indicates that nitrogen- and
sulfur-containing compounds may have poisonous ef-
fects. Nevertheless, the conversion of CP#2 reached
99% whereas the conversion of HDPE was only 82%
when 20% sulfided Ni/HSiAl was used. This means that
this bifunctional catalyst is more resistant to nitrogen-
and sulfur-containing compounds and is suitable for
hydrocracking of plastic waste.
Figure 4 shows composition of liquids produced from
the hydrocracking of CP#2 over all three catalysts.
Isoparaffins are the major compounds with the ratio of
iso/normal paraffins decreasing in the order Ni/HSiAl
> KC-2600 > NiMo/HSiAl. Less aromatics and more
n-paraffins were obtained over NiMo/HSiAl than over
Ni/HSiAl. All these trends were the same as those
observed for hydrocracking of HDPE.
Figure 5 shows the results of simulated distillation
of a commercial premium gasoline and liquid products
obtained from hydrocracking of CP#2. The volatility of
plastic waste-derived liquids was close to that of the
commercial premium gasoline. Liquids produced over
KC-2600 were lighter than those obtained over Ni/HSiAl
and NiMo/HSiAl. All liquids contained about 90%
lighter components (bp less than 216 °C).
(36) Ding, W.; Liang, J.; Anderson, L. L. Prepr.sAm. Chem. Soc.,
Div. Pet. Chem., in press.
(37) Anderson, L. L.; Tuntawiroon, W. Prepr.sAm. Chem. Soc., Div.
Fuel Chem. 1993, 38, 816.
Figure 2. GC-MS analyses of liquid products obtained from hydrocracking of HDPE in a 27 mL tubing reactor at 375 °C, 1000
psig H2 (initial), for a reaction time of 1 h with 40% Ni/HSiAl.
Figure 3. Results of degradation of CP#2 in a 27 mL tubing
reactor at 375 °C, 1000 psig H2 (initial), for a reaction time of
60 min with the indicated catalysts.
1222 Energy & Fuels, Vol. 11, No. 6, 1997 Ding et al.
In terms of conversion and oil yield, 20% Ni/HSiAl
was the best catalyst we have found so far. Reaction
time had important effects on conversion and yields
from hydrocracking of CP#2 (Figure 6). After 30 min,
conversion and yields increased linearly with increasing
reaction time, but the increase was not major compared
with reaction times less than 30 min.
The effects of initial hydrogen pressures are shown
in Figure 7. In the range 250-750 psig, yields and
conversion increased only slightly with increasing pres-
sure. Approximately 99% conversion was obtained at
750 psig, whereas 85% conversion was obtained at 250
psig. When the pressure was greater than 750 psig,
yields and conversion were not a function of pressure.
The reason the reactions had to be carried out at higher
pressures of hydrogen (g750 psig) was that the hydro-
genation function associated with the Ni (or NiMo)
sulfide was considerably weaker than that for other
catalysts such as Pt/HY hydroisomerization cata-
lysts,38,39 although the latter may be poisoned by
nitrogen and/or sulfur.
Table 2 shows the effects of initial hydrogen pressure
on the composition of liquid products obtained from
hydrocracking of CP#2. Aromatics and olefins de-
creased and cyclo-paraffins increased with increasing
initial hydrogen pressure. Generally, more n-paraffins
and isoparaffins were obtained at higher pressures. This
indicates that hydrogenation of olefins and aromatics
may be favored at higher hydrogen pressures; therefore,
correspondingly more n-paraffins and cyclo-paraffins
were produced. Compared with the composition of a
commercial premium gasoline, the liquid products ob-
tained at 1000 psig H2 contained less n-paraffins, much
more cyclo-paraffins, less isoparaffins, and slightly more
aromatics.
CP#2 contained 0.65% nitrogen and 0.01% sulfur.26
The liquids obtained from the hydrocracking of CP#2
at 1000 psig H2 (initial) over 20% Ni/HSiAl was sub-
jected to elemental analyses; no nitrogen and sulfur was
detected. This reflected that the sulfided Ni/HSiAldid
have HDN function.
Detailed compositional data on the products obtained
from the hydrocracking of CP#2 over 20% Ni/HSiAl
under 1000 psig H2 (initial) are listed in Table 3. The
iso/normal paraffins ratios of C4, C5, C6, C7, C8, C9, and
C11 are 4.61, 1.75, 10.04, 5.98, 12.85, 16.39, and 8.38,
respectively. This means that Ni/HSiAl exhibits high
hydroisomerization ability during hydrocracking of
CP#2. On the other hand, hydrocracking is profound,
since the amount of heavier hydrocarbons (gC13) is very
low (ca. 0.14%), whereas C13+ hydrocarbons remained
(38) Martin, A. A.; Chen, J. K.; John, V. T.; Dadybarjor, D. B. Ind.
Eng. Chem. Res. 1989, 28, 1613.
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20, 283.
Figure 4. Composition of liquid products obtained from
hydrocracking of CP#2 in a 27 mL tubing reactor at 375 °C,
1000 psig H2 (initial), catalyst:feed ) 0.4:1.0 (by weight), at a
reaction time of 60 min.
Figure 5. Boiling point distribution of a commercial premium
gasoline and liquid products obtained from hydrocracking of
CP#2 at 375 °C, 1000 psig H2 (initial), 60 min, with weight
ratio of catalyst-to-feed 0.4:1.0.
Figure 6. Effect of reaction time on hydrocracking of CP#2
over Ni/HSiAl (reaction conditions: 27 mL tubing reactor, 375
°C, 1000 psig H2 (initial), catalyst:feed ) 0.2:1.0 by weight).
Figure 7. Effect of initial hydrogen pressure on hydrocracking
of CP#2 over Ni/HSiAl (reaction condtions: 27 mL tubing
reactor, 375 °C, 60 min, catalyst:feed ) 0.2:1.0 by weight).
Table 2. Effects of Initial Hydrogen Pressures on
Composition of Liquid Products Obtained from
Hydrocracking of CP#2a,b
250 psig 500 psig 750 psig 1000 psig CPGc
n-paraffins 5.03 4.87 5.08 7.49 15.44
isoparaffins 42.83 42.49 48.89 47.15 59.09
cyclo-paraffins 14.75 17.11 18.57 20.57 1.69
olefins 5.00 4.05 1.92 1.70 0.94
aromatics 32.39 31.48 25.54 23.09 20.24
a Reaction conditions are listed in Figure 7. b Numbers in the
table are in wt %. c Commercial premium gasoline.
Hydrocracking and Hydroisomerization Energy & Fuels, Vol. 11, No. 6, 1997 1223
at 11.7% for the products obtained from the hydroc-
racking of CP#2 at 435 °C, 1000 psig H2 (initial), over
2% HZSM-5.7
Reaction temperature had a dramatic effect on con-
version of CP#2. Figure 8 shows the results obtained
from hydrocracking of CP#2 at 350 °C. Compared with
the uncatalyzed reaction, addition of a catalyst improved
conversion significantly. However, catalytic conversion
at 350 °C was much lower than the corresponding
results obtained at 375 °C (Figure 3).
Effects of Used Catalysts. To test the effects of
used catalysts (after reactions) on hydrocracking of
HDPE and CP#2, the used catalysts were recalcinated
and resulfided. The results of reactions over used
catalysts are listed in Table 4. It is clear that conversion
and yields obtained over used catalysts are nearly the
same as those obtained over fresh catalysts. This
suggests that Ni/HSiAl, NiMo/HSiAl, and KC-2600 can
be regenerated simply by recalcination and resulfiding,
showing possible feasibility for commercial purposes.
Conclusions
Bifunctional catalysts, sulfided Ni/HSiAl, NiMo/
HSiAl, and KC-2600, were very active for hydrocracking
of HDPE and especially plastic waste at 375 °C. The
three catalysts were resistant to nitrogen, sulfur, and
impurities contained in plastic waste, and they may be
regenerated simply by recalcination and resulfiding,
indicating potential commercial use for hydrocracking
of plastic waste to produce transportation fuels in a
single-stage process.
The hydrocracking ability of these catalysts was
profound, since products obtained were mostly lighter
hydrocarbons (eC13). The hydroisomerization ability of
these catalysts was also significant and decreased in the
order Ni/HSiAl > KC-2600 > NiMo/HSiAl. The higher
hydrocracking and hydroisomerization ability of Ni/
HSiAl may be due to its metal sulfide-acid balance. No
synergetic effects were observed when Mo was used as
a promoter for Ni/HSiAl. Probably MoS2-supported
nickel sulfide (NiMoS) could not form in the small
HZSM-5 channels.
The liquid products obtained from hydrocracking of
HDPE over Ni/HSiAl and KC-2600 had better quality
than did a commercial premium gasoline, i.e., contained
more isoparaffins and less aromatics.
Acknowledgment. We gratefully acknowledge the
funding support from the U.S. Department of Energy
through the Consortium for Fossil Fuel Liquefaction
Science and the University of Utah.
EF970051Q
Table 3. Carbon Number Distribution of Products
Obtained from Hydrocracking of CP#2 over Ni/HSiAla
n-paraffins isoparaffins cyclo-paraffins olefins aromatics
C1 0.54
C2 2.87
C3 8.09 0.37
C4 3.42 15.75
C5 2.82 4.94 0.44
C6 1.34 13.46 0.21 0.44 1.55
C7 0.60 3.59 4.72 0.35
C8 0.47 6.04 4.02 0.14 2.07
C9 0.18 2.95 2.84 3.66
C10 1.48 0.82 0.17 3.53
C11 0.08 0.67 2.44
C12 0.28 1.72
C13+ trace trace trace trace 0.14
a Reaction conditions: 375 °C, 60 min, 1000 psig H2 (initial),
catalyst:feed ) 0.2:1.0 by weight.
Figure 8. Results of degradation of CP#2 in a 27 mL tubing
reactor at 350 °C, 1000 psig H2 (initial), for a reaction time of
60 min with the indicated catalysts.
Table 4. Effects of Used Catalysta on Hydrocracking of
HDPE and CP#2b
catalyst
gas yield,
wt %
oil yield,
wt %
conversion,
wt %
HDPE Feed
fresh KC-2600, 40% 57.2 32.8 90.0
used KC-2600, 40% 56.2 34.6 90.8
fresh NiMo/HSiAl, 40% 54.1 45.2 99.3
used NiMo/HSiAl, 40% 51.4 43.8 95.2
fresh Ni/HSiAl, 40% 57.6 42.0 99.6
used Ni/HSAl, 40% 56.1 43.4 99.5
CP#2 Feed
fresh KC-2600, 40% 48.1 42.0 90.1
used KC-2600, 40% 44.0 44.9 88.9
fresh NiMo/HSiAl, 40% 52.0 47.1 99.1
used NiMo/HSiAl, 40% 47.9 46.5 94.4
fresh Ni/HSiAl, 20% 34.4 64.8 99.2
used Ni/HSiAl, 20% 35.6 64.0 99.6
fresh Ni/HSiAl, 40% 51.8 47.7 99.5
used Ni/HSiAl, 40% 51.3 48.4 99.7
a Used catalysts: recalcinated and resulfided catalysts. b Reac-
tion conditions: 375 °C, 1000 psig H2 (initial), for a reaction time
of 60 min.
1224 Energy & Fuels, Vol. 11, No. 6, 1997 Ding et al.