Processing for producing hydrocarbon oils from plastic waste

Plastic scrap, especially of polyolefin plastics, is converted to low pour point oils by thermal cracking in the liquid phase followed by catalytic conversion of the vaporous cracking products over an intermediate pore size zeolite such as ZSM-5.

FIELD OF THE INVENTION 
The present invention relates to a process for preparing low boiling 
hydrocarbon oils whih ae useful as the raw material for the production of 
gasoline, from polyolefin plastics as the starting material. 
BACKGROUND OF THE INVENTION 
It is well known that as quantity of plastics manufactured increases in 
recent years, the disposal of the scrap has become a problem. There are 
some plastics for which the technology for recycling has been developed to 
a practical stage for recycling of polyolefin plastics, which are said to 
represent approximately one half of the quantity of thermoplastic resins 
manufactured in Japan, has not yet become satisfactory and effective for 
practical use except on a small scale. Thermal cracking methods, as 
described for example, in U.S. Pat. No. 3,956,414 (Oshima), are 
disadvantageous because considerable amounts of waxy materials are formed, 
and also because carbon is formed and becomes attached to the inner walls 
of the reaction vessels used for the processing. It has therefore not been 
practicable to put these methods to practical use for commonly used 
plastics. 
Proposals for processing scrap plastics have been made in the past. For 
example, U.S. Pat. Nos. 4,108,730 and 4,175,211 (Chen) disclose a process 
for converting polymeric wastes such as rubber tires, plastic ware and 
scrap plastic to more valuable liquid, solid and gaseous hydrocarbon 
products by mixing the waste with a refractory petroleum stream and 
catalytically cracking the mixture. Suitable petroleum streams include 
fractions produced by catalytic cracking, for example, heavy cycle oil 
(HCO). The disadvantage of this method is, however, that it needs to be 
operated in proximity to a catalytic cracker which therefore precludes it 
from being used on a relatively smaller scale close to the source of the 
plastic waste. In addition, relatively large volumes of the petroleum 
stream are necessary for mixing with the scrap. 
U.S. Pat. No. 4,118,281 (Yan) describes a process for converting solid 
wastes including rubbers, plastics and other material to gas, oil and coke 
by slurrying the waste with a petroleum stream, especially the heavy 
recycle fraction from a coker unit and coking the resulting mixture. The 
products of the coking may be used as a catalytic cracker feed to produce 
high yields of gasoline. Although this process is compatible with 
conventional petroleum refining technology it also requires to be carried 
out at the refinery and requires relatively large volumes of the petroleum 
stream to dissolve or slurry the waste before it is coked. It would be 
desirable to eliminate the necessity for using the separate petroleum 
stream for mixing with the waste so as to permit the process to be carried 
out effectively close to the soure of the waste with only the high value 
liquid conversion products being transported off-site. 
SUMMARY OF THE INVENTION 
We have now devised a process for producing high-quality hydrocarbon oils 
of low boiling point and low pour point by more efficiently conducting 
thermal and selective catalytic cracking of polyolefinic plastics by a 
two-stage treatment which does not require the use of separate refinery 
streams for its operation. 
According to the present invention, a process for preparing a hydrocarbon 
oil of low boiling point and low pour point comprises thermally cracking 
molten plastics in the liquid phase and catalytically converting the 
vaporous cracking product by contact with an intermediate pore size 
zeolite.

DETAILED DESCRIPTION 
The plastics which may be used in the present process may be selected from 
a wide range of hydrocarbon and oxygenated hydrocarbon plastic resin 
materials although halogenated plastics such as the halogenated vinyl 
polymers e.g. polyvinyl chloride (PVC) and the halo-vinylidene polymers 
such as poly (vinylidene dichloride), should not be used in order to avoid 
catalyst deactivation. The process is of greatest utility with hydrocarbon 
polymers including, especially, polyolefins such as polyethylene, 
polypropylene, polybutene, and polymers and copolymers of these and other 
unsaturated hydrocarbon monomers. Polyvinyl aromatics such as polystyrene 
e.g. foamed polystyrene, and poly (paramethyl-styrene) and copolymers e.g. 
with cross-linking comonomers such as divinylbenzene (DVB) may also be 
recovered by the present process as may oxygenated polymers such as 
polyesters e.g. polyethylene terephthalate (PET), polyacrylates e.g. poly 
(methyl methacrylate), polycarbonates and other such polymers. The 
principal utility of the process will, however, be with polyolefins in 
view of the extent to which they are used at the present. 
Before the plastic scrap is treated by the present process it should be 
shredded or otherwise reduced to a particulate state. Usually, the scrap 
will be in any form of film, sheet, moldings and the like, but preferably 
will be films and sheets used for agricultural or horticultural purposes. 
Separation of non-plastic materials which may accompany the scrap e.g. 
paper, dirt, may be effected by washing and drying or other suitable 
classification techiques. These materials, after shredding or pulverizing 
by appropriate means, are continuously fed to a thermal cracking zone e.g. 
a reaction tank, by means of an extruder while being heated to a softened 
and molten state. 
It is essential to carry out the first stage thermal cracking of the 
plastic in the molten or liquid phase. The temperature in the thermal 
cracking reaction zone at the first stage is typically at least 
360.degree. C. and preferably 390.degree.-500.degree. C., more preferably 
420.degree.-470.degree. C. e.g. 400.degree.-450.degree. C. It is preferred 
to feed the molten plastic into the first stage thermal cracking reacting 
zone in such a way the the level of the molten liquid phase is maintained 
constant, preferably with stirring or other agitation to maintain uniform 
conditions. Pressures in the thermal cracking zone may be atmospheric or 
superatmospheric, as required in order to maintain the liquid phase in the 
vessel at the desired reaction temperature for the cracking to proceed. 
Atmospheric pressure operation is preferred. 
In order to improve heat transfer during the thermal cracking it is 
preferred to employ a particulate, solid, inorganic component in the 
cracking reactor. This is preferably a porous material, preferably with a 
particle size of about 1-10 mm. There are no particular limitations on the 
material used provided that it is essentially free of deformation or 
deterioration in the cracking process. Suitable inorganic materials 
include matural zeolites, bauxite or the residues produced by the removal 
of aluminum from bauxite (sometimes referred to as "red mud"). The solid 
material may be essentially inert to the thermal cracking process or it 
may possess some cracking activity e.g. with natural or synthetic zeolites 
such as faujasite but such acidic cracking activity should be lower than 
that of the zeolite used in the second stage of the process in order to 
ensure that a significant degree of selective catalytic conversion occurs 
in the second stage in the presence of the intermediate pore size zeolite. 
However, since the use of a solid with cracking activity may promote 
conversion in the first stage its use may be regarded as desirable. 
Use of such inorganic particulate material inhibits attachment of carbon to 
the walls of the reaction vessel as well as lowering the boiling point of 
the vaporous cracking products. It has also been found to improve the 
quality and yield of the final hydrocarbon oil from the process. The 
amount of the inorganic particulate material is preferably 5% by weight or 
more of the higher e.g. 100-500 percent of the molten plastic e.g. 200-400 
percent, by weight. 
The vaporous product thus formed in the first-stage thermal cracking 
reaction tank which has a pronounced paraffinic character is then pased to 
the bed filled with the intermediate pore size zeolite for catalytic 
conversion to higher quality products. The yield of the vaporous thermal 
cracking products is typically at least 80 weight percent and in most 
cases above 90 weight percent. 
In the second stage of the process, the vaporous thermal cracking products 
from the first stge are converted by contact with an acidic, intermediate 
pore size zeolite at an elevated temperature. The intermediate pore size 
zeolites are zeolites which have a structural unit comprised of 
ten-membered oxygen ring systems, as described in J. Catalysis 67, 218-222 
(1981) and Catal. Rev. - Sci. Eng. 28 (2&3) 185-193 (1986). The 
intermediate pore size zeolites are characterized by a Constraint Index of 
1 to 12, as disclosed in U.S. Pat. Nos. 4,016,218 and 4,696,732 to which 
reference is made. These zeolites also and preferably have a 
silica:alumina ratio (structural) of at least 12:1 as described in U.S. 
Pat. No. 4,016,218. 
Examples of this type of zeolite include ZSM-5, ZSM-11, ZSM-12, ZSM-23, 
ZSM-35, ZSM-38 and ZSM-48. ZSM-5 is preferred. ZSM-5 is a crystallize 
zeolite having, in the as-synthesized form, the following lines in the 
X-ray diffraction pattern: 
______________________________________ 
Interplanar Spacing 
Relative Intensity 
______________________________________ 
11.2 .+-. 0.2 S 
10.1 .+-. 0.2 S 
3.86 .+-. 0.08 VS 
3.72 .+-. 0.08 S 
3.66 .+-. 0.05 M 
______________________________________ 
The zeolite is usually used in acid or hydrogen form, generally produced by 
calcining the ammonium-exchanged form of the zeolite. A hydrogenation 
metal component such as platinum, palladium, nickel or another transition 
metal, preferably as Group VIII may be present, either exchanged onto or 
impregnated into the zeolite e.g. in amounts from 0.1-10 weight percent. 
The zeolite is usually used either as it is or after forming e.g. by 
extrusion in any shape having a particle size of about 0.1-10 mm, together 
with a binder such as alumina, silica or silica-alumina. 
The second stage catalytic conversion reaction is typically carried out at 
a temperature usually of at least 200.degree. C. and preferably of 
250.degree.-340.degree. C. Operation at such low temperatures brings about 
not only the desired improvement in the product oil but also inhibits 
undesirable side reactions and other effects. Space velocities are 
typically at least 0.5 WHSV and usually 0.5-2.0 WHSV with values of about 
0.75-1.0 being preferred. Atmospheric pressure operation is preferred 
although higher pressures may be used if desired. The heat requirement for 
the second is readily met by the incoming vapors from the first stage 
thermal cracking and therefore no separate heating is required for the 
second-stage feed or the reactor provided excessive heat losses are 
avoided. 
The use of the zeolite not only enables decreases in temperature to be used 
in continuous operation but also remarkably improves the quality and yield 
of the product. The activity of the catalyst is maintained even after 
repeated use and regeneration. Even regenerated catalysts previously used 
in other reactions e.g. catalytic cracking or catalytic dewaxing, may be 
effectively used in the present process. 
The hydrocarbon oil product has good fluidity characteristics at low 
temperatures i.e. the portion boiling above the gasoline boiling range 
e.g. 165.degree. C.+, has a low pour point. This is indicative of hte 
occurrence not only of cracking reactions over the zeolite but also of 
isomerization reactions. The absence of high molecular weight components 
in the product is also to be noted. In many cases, the hydrocarbon oil 
product contains no substantial amount of hydrocarbons having 22 or more 
carbon atoms and the quantity of non-distillable residua is normally very 
small. 
The product typically contains significant quantities of olefins produced 
by the cracking reactions together with saturates and minor quantities of 
aromatics derived by aromatization of paraffins. The product is, 
notwithstanding the relatively high olefin content, colorless, stable and 
clear. A typical analysis is as follows: 
TABLE 1 
______________________________________ 
Product Ana1ysis 
Wt. Pct. 
______________________________________ 
Saturates 38.4 
Olefins 54.7 
Aromatics 4.5 
RON (clear) 62.5 
______________________________________ 
The gaseous by-products produced by the process under certain temperature 
conditions contain useful C.sub.3 -C.sub.5 components. The liquid yield is 
typically at least 50 weight percent and in most cases over 60 weight 
percent of the plastic material charged, as shown in a typical case below. 
TABLE 2 
______________________________________ 
Typica1 Materia1 Balance 
Wt Pct on Feed 
______________________________________ 
Feed Polyethylene 
100 
Products: 
Liquid 62 
Gas 31 
Leve1 Charge 1 
Carbon etc 1 
100 
Fuel Consumed 27 
______________________________________ 
The properties of products produced from two commercial plastics, 
polyethylene(PE) and polystyrene (PS) are shown below in Table 3. 
TABLE 3 
______________________________________ 
Typical Product Properties (Liquid) 
Polyethylene/Polystyrene 
Feed Polyethylene 
90/100 
______________________________________ 
Product: 
Sp. Gr. 0.7498 0.7878 
RVP, kg/cm.sup.2 
0.78 0.45 
(psi) (8.65) (6.4) 
RON 62.5 69.8 
Distillation, .degree.C. 
IBP 30 38 
5% 44 72 
10% 60 89 
20% 84 115 
30% 108 136 
40% 132 157 
50% 159 182 
60% 186 217 
70% 216 257 
80% 245 295 
90% 279 334 
95% 299 355 
EP 316 370 
Res, vol % 1.5 2.0 
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The equipment which may be used for the reactions is shown in the FIGURE. 
The two stage unit 1 comprises a feed supply zone 2, a thermal cracking 
reaction zone 3 with a stirrer 4 mounted on its top. At the bottom of feed 
supply zone 2 the screw feeder 5 is provided which is directed into the 
top of thermal cracking reaction zone 3. A level meter 6 to measure height 
position of the molten feed and a thermometer 7 are inserted inside the 
thermal cracking reaction zone 3. At the bottom of the thermal cracking 
reaction zone 3 is provided a gas burner 10 in the zone jacket for 
supplying heat to the zone and maintaining it at the desired temperature 
for the cracking reactions. 
At the top of thermal cracking reaction zone 3 a catalytic reaction zone 8 
is provided which is filled with a fixed bed of H-ZSM-5 having a particle 
size of about 3 mm into which is also inserted a thermometer 9. 
The thermal cracking reaction zone 3 is maintained at a predetermined inner 
temperature for the thermal cracking reaction, and the catalytic reaction 
zone 8 is maintained at a predetermined temperature in the range typically 
between 250.degree.-350.degree. C. by means of heat carried in by the 
vaporous product and an external heater jacket. 
The polyolefinic plastic placed in the feed supply zone 2 is melted and 
passed into thermal cracking reaction zone 3 through screw feeder 5 and 
subjected to thermal cracking at a predetermined temperature. The vaporous 
product formed by the thermal cracking is then subjected to conversion at 
the predetermined temperature in the course of being passed through the 
catalytic reaction zone 8 to give the desired low molecular products. 
The upper end of the catalytic reaction zone 8 is connected to a cooling 
under 12 equipped with a water-cooled condensor 11. Product storage tanks 
13 and 14 are provided at the end of the cooling tube 12. Thus, in a 
typical case, the product which is converted to a low molecular weight 
components in the catalytic reaction zone 8 is cooled to +11.5.degree. C. 
in the course of passing through the cooling tube 12 and collected in 
storage tanks 13 and 14. 
The results of experiments for producing hydrocarbon oils from polyolefin 
plastics using the above-described equipment are described below. 
(1) Screw Feeder 
A feeder of two-axis screw type was operated at a temperature of 
330.degree. C. and a supply rate of 680-706 g/hr. 
(2) Reaction Zones 
First stage reaction tank 
A tank 560 mm in height, 105 mm in inner diameter and 4.85 l. in volume in 
which the thermal cracking reaction zone is 250 mm in height. This zone 
was filled with 250 g of a particulate natural zeolite produced in 
Kasaoka, Japan (particle size of approximately 0.5 mm) and stirred at 8 
rpm. 
Second stage reaction tower 
A tower 300 mm in height, 76 mm in inner diameter and 1.36 l. in volume was 
filled with 613 g of ZSM-5 in the acid (H) form. 
(3) Plastic Feed 
Urban polyethylene film waste was collected and pulverized to a size of 
approximately 5 mm. The feed was placed in the feed supply zone 2 and 
melted in the screw feeder 5 and passed to the thermal cracking reaction 
zone 3. The vaporous product generated by thermal cracking was passed to 
the catalytic reaction zone 8 in which catalytic conversion was carried 
out respectively at the temperatures shown in Table 4 below. 
TABLE 4 
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Temperature 
Second stage 
Amount 
Yield Yield of Average 
Proportion 
First stage 
catalytic 
of Feed 
of hydrocarbon 
Specific 
State of the 
Range of 
molecular 
of C.sub.5 
-C.sub.14 
cracking 
conversion 
supplied 
product 
oil weight of 
hydrocarbon 
carbon 
weight 
components 
(.degree.C.) 
(.degree.C.) 
(g/h) 
(wt. %) 
(wt. %) 
product 
oil product 
numbers 
(Mn) (wt. 
__________________________________________________________________________ 
%) 
430.sup.(1) 
-- 680 100 94.0 0.773 
Wax at +20.degree. C. 
C.sub.5 -C.sub.38 
197.3 39.6 
430 270 680 45.3 83.9 0.742 
Liquid at -20.degree. C. 
C.sub.5 -C.sub.18 
119.0 96.8 
430 285 680 65.1 84.1 0.750 
" C.sub.5 -C.sub.18 
127.1 91.1 
430 295 680 93.1 90.4 0.751 
" C.sub.5 -C.sub.20 
116.4 84.4 
430 310 680 100 88.1 0.752 
" C.sub.5 -C.sub.19 
119.9 84.6 
430 320 680 100 79.6 0.754 
" C.sub.5 -C.sub.19 
110.8 90.0 
430 345 680 100 60.4 0.762 
" C.sub.5 -C.sub. 
103.9 100 
450.sup.(2) 
-- 706 100 91.1 0.776 
Wax at +20.degree. C. 
C.sub.5 -C.sub.39 
210.0 36.2 
450 257 706 34.0 77.4 0.742 
Liquid at -20.degree. C. 
C.sub.5 -C.sub.17 
117.8 97.6 
450 300 706 83.9 85.4 0.742 
" C.sub.5 -C.sub.19 
109.5 89.5 
450 310 706 100 89.1 0.752 
" C.sub.5 -C.sub.21 
129.8 73.3 
450 328 706 100 81.9 0.752 
" C.sub.5 -C.sub.20 
107.6 89.3 
450 350 706 100 65.0 0.767 
" C.sub.5 -C.sub.15 
108.4 98.8 
__________________________________________________________________________ 
Notes: 
.sup.(1), (2) Comparative examples without the second stage catalytic 
cracking. 
Results of the analysis of gas i.e. the thermal cracking products other 
than hydrocarbon oil, from the second stage, using a temperature of 
430.degree. C. in the first stage cracking tank and a temperature of 
310.degree. C. in the second stage catalytic conversion tower are given in 
Table 5 below (total gas component is taken as 100%): 
TABLE 5 
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Off-gas Composition 
Percent 
______________________________________ 
H.sub.2 
7.0 
CH.sub.4 
8.0 
C.sub.2 H.sub.4 
4.5 
C.sub.2 H.sub.6 
7.6 
C.sub.3 H.sub.8 
5.6 
C.sub.3 H.sub.6 
19.9 
i-C.sub.4 H.sub.10 
1.1 
n-C.sub.4 H.sub.10 
9.8 
i-C.sub.4 H.sub.8 
24.5 
i-C.sub.5 H.sub.12 
0.5 
n-C.sub.5 H.sub.12 
11.5 
______________________________________ 
No attachment of carbon to the inner walls of the reaction vessel occurred 
for a long period of time.