JPH08253773A - Method of pyrolyzing plastic and plastic pyrolysis catalyst for use therein - Google Patents

Abstract

PURPOSE: To provide a plastic pyrolysis method which attains a high pyrolysis rate and can yield a lighter distillate oil; and to obtain a solid catalyst for use in the pyrolysis. CONSTITUTION: The method comprises introducing a waste plastic 2 into a pyrolysis tank 1, pyrolyzing the plastic 2, and recovering a light oil 3, the pyrolysis being conducted in the presence of a particulate silica-alumina catalyst dispersed and suspended in the melt 5 of the plastic 2, the catalyst having a content of the sum of SiO2 and Al2 O3 of 90wt.% or higher and an SiO2 /Al2 O3 ratio of 0.05-5 by weight.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for thermally decomposing plastics and a catalyst for thermally decomposing plastics, and more particularly to a method for thermally decomposing waste plastics to recover light oil. The present invention relates to a method for thermally decomposing plastics having a high decomposition rate and capable of obtaining a light distillate oil, and a plastic thermal decomposition catalyst used therefor.

[0002]

2. Description of the Related Art In recent years, with the growing global environmental problems and waste problems, there is a strong demand for waste countermeasures such as non-polluting treatment of wastes and recycling and effective use as resources. , Researches on pollution-free and recycling of waste are actively conducted. The same applies to waste plastics, and various researches on various processing and recycling methods are being conducted in various places.

Recycling methods for waste plastics can be roughly divided into material recycling and thermal recycling.
Thermal recycling is further divided into a method of directly burning waste plastic to recover heat, and a method of once processing and converting it into fuel, and finally recovering it as energy. As a method for converting waste plastics into fuel, attention has been paid to a pyrolysis oilization method capable of recovering highly useful oil as a distillate.

FIG. 14 is an explanatory diagram showing a system flow of a conventional pyrolysis oilification method for converting waste plastic into fuel. In this pyrolysis oilification method, waste plastic 91 which is a material to be treated is supplied to a pyrolysis tank 92 and indirectly heated by a heating pipe 93 to be pyrolyzed to recover a cracked gas 94 and a cracked oil 95. There is also a vis-ble-type thermal decomposition method in which the polymer in the middle of decomposing that remains in the thermal decomposition tank 92 is extracted as so-called vis-bled oil from the bottom and collected. Such a conventional method can adjust the residence time of the plastic in the thermal decomposition tank by changing the withdrawal speed of the visbra oil in the visbra type, but
As a general rule, the decomposition rate of the plastic and the composition of the decomposition product are uniquely determined by the decomposition temperature, the decomposition pressure and the like. That is, the above-mentioned conventional techniques have a problem that the decomposition rate of the plastic cannot be increased and the separated oil, which is a decomposition product, cannot be lightened.

On the other hand, FIG. 15 is an explanatory view showing a process flow of a conventional method for thermally decomposing waste plastics, which uses a catalyst to make distillate oil lighter. In this method, the decomposition products distilled from the thermal decomposition tank 92 are once heated in a heat exchanger 97, and then introduced into a catalyst tank 96 filled with, for example, zeolite as a reforming catalyst to lighten the catalyst in the presence of the catalyst. It is intended.

However, according to this method, the waste plastic 91 to be treated is decomposed in the thermal decomposition tank 92, and is distilled as a decomposition distillate, and then contacted with a catalyst to lighten it. At 92, since the liquid plastic being decomposed is not brought into direct contact with the catalyst, the thermal decomposition reaction of the plastic cannot be promoted. Further, in this method, in addition to the thermal decomposition tank 92, a catalyst tank 9 is newly added.
It is necessary to provide 6, which is not economically advantageous.
That is, the above-mentioned conventional technique has a problem that the rate of plastic thermal decomposition cannot be increased despite the addition of equipment.

More recently, there has been proposed a method for pyrolyzing waste plastics into oil by two-step catalytic pyrolysis in the presence of a natural zeolite catalyst. FIG. 13 is an explanatory diagram of an apparatus used in a method for thermally decomposing plastic in which plastic is catalytically pyrolyzed in two stages in the presence of a catalyst.
In the figure, a first reactor 10 having a sample inlet 103
The second reactor 102 filled with the natural zeolite catalyst 111 is connected to the upper part of the first reactor 102.
The upper space part of the above is communicated with the oil trap 104 via the communication channel 109. Further, the upper space portion of the oil trap 104 is communicated with the Teflon bag 105 by a communication flow passage 110 having a cock. Reference numeral 106 is a nitrogen gas for purging, 107 is a thermometer, and 108 is an electric heater.

In the apparatus having such a configuration, waste plastic, for example, 10 g of polyethylene is introduced from the sample inlet 103 into the first reactor 1 together with 10 g of the natural zeolite catalyst, and by the electric heater 108, for example, 430 to 450. It is heated to 0 ° C. and pyrolyzed in the presence of the natural zeolite catalyst 111. The cracked gas flows into the packed bed of the zeolite catalyst 111 in the upper second reactor 102, is lightened, and then flows into the oil trap 104 through the communication channel 109, where it is cooled by, for example, dry ice methanol. It is condensed and condensed, and is recovered as product oil. On the other hand, the gas that does not condense flows into the Teflon bag 105 through the communication channel 110 having a cock and is recovered as a gas.

However, the above-mentioned conventional technique has a problem that although the thermal decomposition rate of plastic is improved to some extent, the two-step treatment must be carried out and the produced oil can hardly be lightened by the one-step treatment.

[0010]

In order to pyrolyze waste plastics more economically, it is necessary to make the decomposition rate faster and to further lighten the decomposition products by the simplest process possible. Become. An object of the present invention is to provide a method for thermally decomposing plastics, which can improve the rate of thermal decomposition of waste plastics and can also reduce the weight of distillate oil, which is a decomposition product.

Another object of the present invention is to provide a plastic thermal decomposition catalyst in which the thermal decomposition activity of plastic is large. Another object of the present invention is to provide a plastic thermal decomposition catalyst having a large separation performance from the residue after thermal decomposition and having a specific surface area and a sufficient pore diameter sufficient to exert the thermal decomposition activity of the plastic. Especially.

[0012]

In order to achieve the above object, the invention claimed in the present application is as follows. (1) In the thermal decomposition method of plastics in which waste plastics are thermally decomposed and recovered as light oil, the total content of silicon dioxide (SiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) is 90% by weight or more, In addition, the aforesaid waste in the presence of the silica-alumina catalyst having a composition ratio (SiO ₂ / Al ₂ O ₃ ) of silicon dioxide (SiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) of 0.05 to 5 on a weight basis. A method for thermally decomposing plastics, which comprises thermally decomposing plastics. (2) The total content of silicon dioxide (SiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) is 90% by weight or more, and the composition of the silicon dioxide (SiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) is the ratio of plastic pyrolysis catalyst, wherein the _{(SiO 2 / Al 2 O 3} ) is 0.05 to 5 by weight. (3) The average particle diameter is 50 to 5000 μm, the specific surface area is 50 to 1500 m ₂ / g, and the average pore diameter is 30 to
The catalyst for plastic thermal decomposition according to (2) above, wherein the catalyst is 100Å.

[0013]

In the present invention, the total content of silicon dioxide (SiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) is 90% by weight or more, and the composition ratio of SiO ₂ and Al ₂ O ₃ (SiO ₂ / Al ₂ O ₃ ) is 0.05-5 by weight, and the thermal decomposition rate of the plastic is improved by thermally decomposing the plastic in the presence of a silica-alumina catalyst.
Moreover, the decomposition products are lightened.

In the present invention, silicon dioxide (hereinafter
Rica and aluminum oxide (hereinafter referred to as alumina)
The total content of (also referred to as) is 90% by weight or more, and S
iO ₂/ Al₂O₃Silica-Alumina with a ratio of 0.05 to 5
A medium (hereinafter, also simply referred to as silica-alumina catalyst) was used.
This accelerates the plastic thermal decomposition reaction, and
It is not always clear why distillate oil is lighter
But not of hydrocarbons derived from plastics on the catalyst
It is thought that cracking has occurred.

In the present invention, the total content of SiO ₂ and Al ₂ O ₃ in the silica-alumina catalyst is 90% by weight.
The above content is preferably 95% by weight or more. If it is less than 90% by weight, a sufficient decomposition promoting effect cannot be obtained. Also, SiO
₂ / Al ₂ O ₃ ratio is 0.05 to 5, preferably 0.1 to 5.
It is 4. If the SiO ₂ / Al ₂ O ₃ ratio is too small or too large, the effect of promoting the thermal decomposition reaction of the plastic is lowered.

In the present invention, the average particle diameter of the silica-alumina catalyst is preferably 50 to 5000 μm, more preferably 300 to 3000 μm. This is because if the average particle size is too small, it becomes difficult to separate it from the residue after the reaction is completed, and if it is too large, the catalytic effect is weakened due to the decrease of the catalyst effective coefficient. The bulk density is 0.38 to 0.52 (g / c
c), preferably 0.40 to 0.47 (g / cc). This is because if the bulk density is too low or too high, the dispersibility in suspension in the plastic melt is reduced. Specific surface area is 50 to 1500 (m ² / g), preferably 50 to 10
It is 00 (m ² / g). If the specific surface area is too small, a sufficient contact area with the plastic cannot be secured,
On the contrary, if it is too large, the manufacturability is lowered. The average pore diameter is 30 to 100Å, preferably 50 to 80Å
Is. This is because if the average pore diameter is too small, the contact with the plastic becomes insufficient, and conversely if it is too large, it causes a problem in production.

In the present invention, the silica-alumina catalyst is preferably used as a granular body, and it is preferable that the plastic is heated and decomposed while being dispersed in the molten liquid of the plastic. This promotes contact between the catalyst and the plastic to be treated, which further improves the rate of plastic thermal decomposition. In the present invention, the silica-alumina catalyst has an average particle diameter of 50 to 5000 μm, a specific surface area of 50 to 1500 (m ² / g), and an average pore diameter of 3
By setting it to 0 to 100Å, the separability of the silica-alumina catalyst from the thermal decomposition residue of the plastic is improved, the reduction of the catalytic action can be prevented, and the contact area with the plastic and the pore size are sufficiently secured. The decomposition activity becomes larger.

In the present invention, by setting the average particle size of the silica-alumina catalyst to 50 to 5000 μm, the separability between the plastic decomposition residue and the catalyst is improved, and further, the reduction of the catalytic effect due to the reduction of the catalyst effective coefficient is prevented. It
In the present invention, the specific surface area of the silica-alumina catalyst is 50
By setting the amount to ˜1500 (m ² / g), the contact area between the plastic melt and the catalyst is sufficiently secured, and the plastic thermal decomposition reaction is promoted.

In the present invention, by setting the average pore diameter of the silica-alumina catalyst to 30 to 100 Å, penetration of the plastic melt into the catalyst pores is promoted and it becomes easy to contact the catalyst, so that the thermal decomposition reaction is promoted. To be done. In the present invention, the amount of the silica-alumina catalyst charged is 0.5 to 20 wt% with respect to the waste plastic which is a substance to be thermally decomposed,
It is preferably 1 to 10 wt%. If the amount of catalyst charged is too small, the increase in the thermal decomposition rate will be insufficient, and if the amount charged is too large, it will be uneconomical. The thermal decomposition temperature of the waste plastic is, for example, 350 to 450 ° C., preferably 380 to 4
It is 00 ° C. If the thermal decomposition temperature is too high, the rate of carbon deposition on the heat transfer surface will be high, and if it is too low, the decomposition rate will be low, making it impossible to achieve a predetermined treatment amount.

In the present invention, examples of the waste plastic include polyethylene (PE) and polypropylene (P
P), polystyrene (PS), or the like, or a mixture thereof, but not particularly limited. In the present invention, in addition to the above-mentioned silica-alumina catalyst, for example, silica containing a trace amount (0.1 to 5 wt%) of aluminum (Al), silica gel or silica-alumina, or a trace amount (0.1 to 5 wt%) of Si is used. Alumina containing (Al ₂
O ₃ ) or the like can also be used.

Next, an example of the method for preparing the silica-alumina catalyst in the present invention will be described. First, Si of a predetermined concentration
Ethyl alcohol solution of (OC ₂ H ₅ ) ₄ and Al (NO ₃ )
₃ · 9H ₂ O were mixed aqueous solution, pH adjusted with HNO _3, for example at pH = 1, and heated to reflux with stirring, for example in the flask. The obtained agar-like substance was aged in a hot water bath and washed with ethyl alcohol to remove unreacted Si (O 2
C ₂ H ₅ ) ₄ is removed, then dried and washed with water. Then, after drying again, it is baked at, for example, 550 ° C. The obtained calcined product is pulverized, and further granulated by, for example, a rotary plate type granulator, so that a granular silica-alumina catalyst can be obtained.

[0022]

Next, the present invention will be described in more detail with reference to examples. FIG. 1 is an explanatory diagram showing a process flow of an embodiment of the method for thermally decomposing plastics according to the present invention. In the figure, an introduction pipe 4 for the waste plastic 2 to be treated, an outflow pipe 8 through which the distillate oil 3 as a decomposition product flows out, and an exhaust pipe 10 through which nonvolatile carbonaceous matter is withdrawn as a decomposition residue 6 are shown. A heating tube 9 is arranged in the provided thermal decomposition tank 1. The waste plastic 2 flows into the thermal decomposition tank 1 through the introduction pipe 4, is heated by the heating pipe 9 to a predetermined temperature, for example 320 ° C., and becomes a melt 5, which is dispersed so that the particulate silica-alumina catalyst 7 is suspended. The mixed state is further heated and thermally decomposed at 400 ° C., for example. The distillate oil 3 which is a decomposition product is recovered to the outside of the system through the outflow pipe 8.

According to the present embodiment, the particulate silica-alumina catalyst for increasing the thermal decomposition rate of plastic is used in the thermal decomposition tank 1.
By directly suspending it in the plastic melt 5 in the interior and performing thermal decomposition while directly contacting it, the thermal decomposition rate of the plastic is increased, and the distillate oil 3 which is a decomposition product is made lighter, for example, gasoline. It is possible to increase the proportion of light oil such as oil and light oil.

2 and 3 are explanatory views showing the process flow of another embodiment of the present invention. The device of FIG. 2 is shown in FIG.
The difference is that the heating pipe 9 is eliminated and the discharge pipe 10 is
A circulation line that connects the return pipe 27 through the valve 26 and returns to the thermal decomposition tank 1 is provided, and a circulation pump 24 is provided in the circulation line.
And that the heat exchanger 25 is provided. According to this embodiment, the plastic melt 5 is circulated to the pyrolysis tank 1 via the discharge pipe 10, the circulation pump 24, the valve 26, the heat exchanger 25 and the return pipe 27, and is heated to a predetermined temperature during this period. In this embodiment, the same effect as that of the above embodiment can be obtained. In the present embodiment, another heating means is used to raise the temperature until the waste plastic 2 becomes a fluid melt.

The apparatus of FIG. 3 is different from that of FIG. 1 in that, as a heating source for the pyrolysis tank 1, a heating furnace 28 having a burner 29 for heating the bottom of the pyrolysis tank 1 instead of the heating pipe 9. Is provided, instead of the discharge pipe 10, a discharge pipe 31 for extracting the decomposition residue 6 upward is provided, and further, the thermal decomposition tank 1 is provided with a stirrer 30. According to this embodiment, the waste plastic 2 introduced into the thermal decomposition tank 1 is heated by the heating furnace 28, stirred and mixed by the stirrer 30 and thermally decomposed.
In this example, as in the above example, the thermal decomposition rate of the plastic is increased, and the distillate 3 which is a decomposition product is used.
Can be made lighter to increase the proportion of light oil such as gasoline and light oil. Further, since the stirring and mixing are promoted by the stirrer 30, the catalytic activity is further exerted.

FIG. 4 is an explanatory view showing a system flow of a method for thermally decomposing plastics showing another embodiment of the present invention. In this method, waste plastics are pyrolyzed in a continuous manner, and the waste plastics 18 mixed with a particulate silica-alumina catalyst 19 at a predetermined mixing ratio are introduced into a thermal decomposition tank 11 by a constant amount supply device 12. Further, it is further heated in the presence of the silica-alumina catalyst 19 which is heated to a predetermined temperature in the heating tube 17, for example 320 ° C., melted, dispersed and suspended, and thermally decomposed at 380 ° C., for example. The decomposition product is distilled from the distilling pipe 13, cooled by the cooling device 14, and then continuously recovered as the cracked gas 20 and the cracked oil 21.
On the other hand, the molten residue is extracted through the discharge pipe 15, the silica-alumina catalyst is separated as the used catalyst 23 by the catalyst separator 16, and then recovered as the extracted oil 22.

According to the present embodiment, in addition to the effects of the above embodiment, light plastics can be continuously pyrolyzed to continuously collect light oil. Next, specific examples of the present invention will be described. Example 1 Using the apparatus of FIG. 1, silica (SiO ₂ ) on a weight basis:
50%, alumina _{(Al 2 O 3): 43.2} %, ferric oxide _{(Fe 2 O 3): 3.6} %, quicklime (CaO): 1.1
%, Other 2.1%, average particle size: 1.0 m
m, bulk density: 0.45 (g / cc), specific surface area: 100
0 (m ² / g), average pore diameter: 80 Å silica-alumina catalyst was added at 2.5 wt% to the plastic to be treated, decomposition temperature: 380 ° C., decomposition pressure: atmospheric pressure, polypropylene which is a waste plastic When 5 kg of (PP) was decomposed by a batch method, the decomposition was almost completed 60 minutes after the start of the reaction. The results are shown in Fig. 5. In FIG. 5, the distillate oil begins to distill at about 30 minutes after the reaction starts, and almost all distillate distills at about 60 minutes after the reaction starts.
It can be seen that the decomposition is almost complete. The carbon number distribution in the obtained distillate oil is shown in FIG. From FIG. 6, it can be seen that the distillate oil obtained in this example has a peak at 7 carbon atoms and is mostly a light oil having 11 or less carbon atoms.

Comparative Example 1 A similar plastic was subjected to a thermal decomposition treatment in the same manner as in Example 1 except that the silica-alumina catalyst was not used. As a result, about 600 to 700 was reached by the completion of the thermal decomposition reaction.
It took a minute. The results are shown in Fig. 7. It can be seen from FIG. 7 that the decomposition products started to distill 20 minutes after the reaction started, but it took about 600 to 700 minutes until the decomposition reaction was almost completed. The carbon number distribution of the decomposition product obtained at this time is shown in FIG. From FIG. 8, it can be seen that the distillate obtained when pyrolyzed without using a silica-alumina catalyst has peaks at carbon numbers 9, 14, 18 and 23 and has a broad carbon number distribution.

Example 2 The same plastic pyrolysis was carried out as in Example 1 except that the SiO ₂ / Al ₂ O ₃ ratio of the silica-alumina catalyst was changed to 0.1. Pyrolysis was almost complete in minutes. Example 3 The same thermal decomposition of plastic was carried out as in Example 1 except that the silica / alumina catalyst had a SiO ₂ / Al ₂ O ₃ ratio of 4.0. The disassembly is almost complete.

Comparative Example 2 The same thermal decomposition of plastic was carried out in the same manner as in Example 1 except that the SiO ₂ / Al ₂ O ₃ ratio of the silica-alumina catalyst was set to 30 or more, and 1000 minutes after the start of the reaction. However, the thermal decomposition did not end. Comparative Example 3 The SiO ₂ / Al ₂ O ₃ ratio of the silica-alumina catalyst was 0.01.
The same plastic thermal decomposition was carried out in the same manner as in Example 1 except that, but the thermal decomposition did not end even after about 1000 minutes had elapsed after the start of the reaction.

The results of Examples 2 to 3 and Comparative Examples 2 to 3 are shown in FIG. 9 together with the results of Comparative Example 1 (non-catalyst example). In FIG. 9, it can be seen that in Examples 2 and 3, the distillation rate of the distillate oil was remarkably improved as compared with Comparative Example 1, and the decomposition reaction of the plastic was promoted. On the other hand, in Comparative Examples 2 and 3, no plastic decomposition promoting effect was observed. From the above, the SiO ₂ / Al ₂ O ₃ ratio is 0.1
In the range of up to 4.0, a remarkable effect of promoting plastic decomposition is observed, and the SiO ₂ / Al ₂ O ₃ ratio is 30 or more or 0.0 or more.
It can be seen that when it is 1 or less, the decomposition promoting effect is not obtained, and the decomposition rate is smaller than that in the case of no catalyst.

The carbon number distributions of the distillate oils obtained in Examples 2-3 and Comparative Examples 2-3 are shown in FIG. In FIG. 10, it can be seen that in Examples 2 and 3 of the present application, no peak of the carbon number was found near the carbon numbers of 14, 16 and 18, and almost all the light oils had 11 or less carbon atoms. On the other hand, in Comparative Examples 2 and 3, there was almost no difference from the distillate oil in Comparative Example 1 (see FIG. 8) that was thermally decomposed in the non-catalyst state, and the lightening effect of the distillate oil was not obtained.

Example 4 The plastic was thermally decomposed in the same manner as in Example 1 except that the decomposition temperature was 450 ° C., the treated waste plastic was polyethylene, and the treated plastic amount was 10 g. Almost all of the distillate oil was distilled out in about 15 minutes, and the decomposition reaction was almost completed.

Comparative Example 4 Instead of the silica-alumina catalyst, natural zeolite (SiO 2
₂ : 62.9%, AlO ₂ : 8.2%, Fe ₂ O ₃ : 1.5
%, MgO: 0.4%, CaO: 1.4%, Na ₂ O:
2.6%, K ₂ O: 1.8%, H ₂ O: 14.8%, and other 4.9%) were calcined in the air at 500 ° C. for 3 hours for a zeolite catalyst (average particle diameter: 3.0 mm, Bulk specific gravity: 0.76
g / cc, specific surface area: 31.3 m ² / g) and the same polyethylene was thermally decomposed in the same manner as in Example 4 except that the catalyst addition amount was 100 wt%. Almost all of the distillate oil was distilled out in 30 minutes, and the decomposition reaction was almost completed.

The results of the thermal decomposition reaction of Example 4 and Comparative Example 4 are shown in FIG. In FIG. 11, it can be seen that in the case of Example 4, almost all the distillate was distilled off about 15 minutes after the reaction was started, and the decomposition was almost completed. On the other hand, in the case of Comparative Example 4, it takes about 30 minutes until the decomposition reaction is almost completed, which shows that the thermal decomposition rate is slower than that of Example 4. That is, it can be seen that the silica-alumina catalyst of the present invention has an extremely high plastic thermal decomposition activity as compared with the natural zeolite catalyst.

The carbon number distributions in the distillate oils obtained in Example 4 and Comparative Example 4 are shown in FIG. From FIG. 12, the fourth embodiment
It can be seen that the distillate obtained in the above step has a peak at a carbon number of 7, and most of it is a light oil having a carbon number of 10 or less. On the other hand, the carbon number distribution of the distillate oil of Comparative Example 4 using the zeolite catalyst was widely distributed from 5 to 45, and almost no effect on the lightening of the distillate oil was observed.
That is, it is understood that the silica-alumina catalyst of the present invention has a significantly higher distillate oil lightening activity than the natural zeolite catalyst.

[0037]

According to the invention described in claim 1 of the present application, by thermally decomposing waste plastic in the presence of a silica-alumina catalyst having a specific composition, the decomposition rate of the plastic is improved and distillate oil Lightening can be achieved. According to the invention of claim 2 of the present application, the main components of the plastic thermal decomposition catalyst are silica and alumina, and further, SiO ₂ / Al ₂ O.
_By setting the ₃ ratio to 0.05 to 5, a catalyst having a high plastic decomposition activity can be obtained.

According to the invention of claim 3 of the present application, it is possible to obtain a catalyst for thermal decomposition of plastics, which has good separability from decomposition residues, avoids deterioration of plastic decomposition activity, and has a sufficient contact area and contact volume. .

[Brief description of drawings]

FIG. 1 is a system flow chart showing an embodiment of the present invention.

[Fig. 2]

[Fig. 3]

FIG. 4 is a system flow diagram showing another embodiment of the present invention.

[Fig. 5]

[FIG. 6]

[FIG. 7]

FIG. 8

FIG. 9

FIG. 10:

FIG. 11

FIG. 12 is a diagram showing the results of Examples and Comparative Examples of the present invention.

FIG. 13

FIG. 14

FIG. 15 is a diagram showing a conventional technique.

[Explanation of symbols]

1 ... Thermal decomposition tank, 2 ... Waste plastic, 3 ... Distilled oil, 4 ...
Introducing pipe, 5 ... Plastic melt, 6 ... Decomposition residue, 7 ...
Silica-alumina catalyst, 8 ... Distillation tube, 9 ... Heating tube, 10 ...
Discharge pipe, 11 ... Pyrolysis tank, 12 ... Constant amount supply device, 13 ...
Distillation pipe, 14 ... Cooling device, 15 ... Discharge pipe, 16 ... Catalyst separator, 17 ... Heating pipe, 18 ... Waste plastic, 19 ... Silica-alumina catalyst, 20 ... Decomposition gas, 21 ... Decomposition oil, 2
2 ... Extracted oil, 23 ... Used catalyst, 24 ... Circulation pump,
25 ... Heat exchanger, 26 ... Valve, 27 ... Return pipe, 28 ...
Heating furnace, 29 ... Burner, 30 ... Stirrer.

Claims (3)

[Claims] 1. A method of thermally decomposing plastics, which comprises thermally decomposing waste plastics and recovering them as light oil, comprising the steps of silicon dioxide (SiO ₂ ) and aluminum oxide (Al).
The total content of ₂ O ₃ ) is 90% by weight or more, and the silicon dioxide (SiO ₂ ) and aluminum oxide (Al ₂ O
₃ ) and the composition ratio (SiO ₂ / Al ₂ O ₃ ) is 0.
A method for thermally decomposing plastics, comprising thermally decomposing the waste plastic in the presence of a silica-alumina catalyst of No. 05-5. 2. The total content of silicon dioxide (SiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) is 90% by weight or more, and the silicon dioxide (SiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) are combined. Composition ratio (SiO ₂ / Al ₂ O ₃ )
Is 0.05 to 5 on a weight basis, a catalyst for thermal decomposition of plastics. 3. The thermal decomposition of plastics according to claim 2, wherein the average particle diameter is 50 to 5000 μm, the specific surface area is 50 to 1500 m ² / g, and the average pore diameter is 30 to 100 Å. catalyst.