JP2001335514A - Method for decomposing polystyrene - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the recovery ratio of styrene monomer by raising the decomposition ratio and suppressing the side reactions of the styrene monomer when polystyrene is decomposed in high-temperature and high-pressure water. SOLUTION: This method is to decompose the polystyrene in the high- temperature and high-pressure water in a reactor. The temperature of the high- temperature and high-pressure water is 250-550 deg.C and the pressure thereof is the saturated steam pressure or above or the critical pressure or above at the temperature. An aqueous solution in which low-molecular compound components produced by the decomposition are dissolved therein is continuously led out to a region under conditions of a lower temperature than that in a decomposition reaction region while continuously feeding the high-temperature and high-pressure water to the reactor. Thereby, the time for the low-molecular compound component to stay in the decomposition reaction region is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decomposing polystyrene using high-temperature and high-pressure water, and more particularly, to a decomposition method for recovering a decomposed product of polystyrene, particularly a styrene monomer in a high yield. It is about.

[0002]

2. Description of the Related Art A technique for pyrolyzing polystyrene in a gas phase has been known for a long time.
A mixture of various compounds such as toluene, ethylbenzene, α-methylstyrene, etc. besides styrene monomer, dimer and trimer. Further, the thermal decomposition of polystyrene in the gas phase has a problem that it takes a long time.

On the other hand, the applicant of the present application has disclosed Japanese Patent Application Laid-Open No. 2000-10
No. 3901 has already disclosed a method for decomposing polystyrene with subcritical or supercritical water. However, in the present invention, polystyrene is only decomposed, and styrene monomer is not recovered from the decomposition product in high yield.

[0004]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to find the optimum reaction conditions for increasing the yield of a styrene monomer when pyrolyzing polystyrene with high-temperature and high-pressure water.

[0005]

SUMMARY OF THE INVENTION The present invention is directed to a reactor comprising:
A method of thermally decomposing polystyrene by contacting high-temperature and high-pressure water with polystyrene, wherein the temperature of the high-temperature and high-pressure water is 250 to 550 ° C., and the pressure is equal to or higher than the saturated vapor pressure or the critical pressure at the temperature, This high temperature
While continuously supplying high-pressure water to the reactor, an aqueous solution in which the low-molecular-weight compound components generated by the decomposition are dissolved is continuously led to a region under a lower temperature condition than the decomposition reaction region, so that the low-molecular-weight compound components are Has a gist in controlling the time during which it stays in the decomposition reaction zone.

[0006] Low molecular compound components such as styrene monomer, dimer, and trimer formed by the decomposition of polystyrene are dissolved in high-temperature and high-pressure water and led to a low-temperature region. Secondary control of styrene monomer can be suppressed by control,
It can be recovered in high yield. Undecomposed polystyrene hardly dissolves in high-temperature, high-pressure water,
Until it is decomposed to a molecular weight that can be dissolved in high-temperature and high-pressure water, it stays in the high-temperature and high-pressure hydrolysis reaction region, so that the decomposition efficiency of the polymer can be improved.

In order to enhance the above effects, it is particularly preferable to control the time (residence time) during which the low molecular weight compound component remains in the decomposition reaction zone to 0.1 to 60 minutes. Secondary reactions such as conversion of the styrene monomer to toluene or the like and repolymerization can be more effectively suppressed. In addition, high temperature
It is preferable to continuously supply not only high-pressure water but also polystyrene in a molten state or a slurry state to the reactor, and it is possible to efficiently and continuously decompose a large amount of polystyrene by improving dispersibility and the like. .

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The decomposition method of the present invention is for recovering styrene monomer in high yield by using high-temperature and high-pressure water as a heat medium when thermally decomposing polystyrene.

First, the present inventors preliminarily examined what decomposition product can be obtained by decomposing polystyrene with high-temperature and high-pressure water in a batch system in which polystyrene and water are charged into a reactor at once. did.

In the case of high-temperature and high-pressure water at 350 ° C. and 30 MPa, the decomposition product of decomposition time 22 minutes contains almost the same amount of styrene monomer, dimer and trimer.
It was found that the amount of other compounds was small, and that when the decomposition time was increased to 60 minutes, the trimer decreased, the dimer did not change much, and the monomer increased.

In the case of high-temperature and high-pressure water at 400 ° C. and 30 MPa, toluene, ethylbenzene and styrene monomer are used as decomposition products for a decomposition time of 33 minutes, respectively.
About 15% by mass, about 15% by mass of a dimer, and less than 10% by mass of ethylbenzene and α-methylstyrene were obtained, and a mixture of various compounds was obtained. When the decomposition time reached 74 minutes under these conditions, toluene, ethylbenzene and ethyltoluene increased, and the amount of styrene monomer decreased to about 5% by mass.

Further, in the case of high-temperature and high-pressure water at 500 ° C. and 30 MPa, the composition of the decomposition product in which the decomposition time is 40 minutes is such that toluene and ethylbenzene account for about 90% by mass in a total amount of about the same amount, and most of the rest It became ethyltoluene and contained almost no styrene monomer, dimer, or trimer.

From the results of these studies, it can be seen that if the temperature of the high-temperature / high-pressure water as the heat medium is high and the time of exposure to the high-temperature / high-pressure water is long, even if the styrene monomer is once formed, the double bond of styrene Is cleaved, toluene, ethylbenzene,
It is considered that conversion to ethyl toluene or the like or secondary reaction such as repolymerization occurs.

Therefore, in the decomposition method of the present invention, the temperature of the high-temperature and high-pressure water used for the decomposition is set to 250 to 550 ° C. If the temperature of the high temperature / high pressure water is too high, the conversion reaction of the styrene monomer cannot be suppressed. On the other hand, if the temperature of the high-temperature and high-pressure water is too low, the decomposition rate of polystyrene itself becomes low, and the amount of the obtained monomer decreases. More preferably, the lower limit of the temperature is 300 ° C and the upper limit is 450 ° C.

The time during which the decomposition product remains in the reactor (residence time: the time during which the decomposition product is exposed to high temperature and high pressure conditions) is 0.1
It is preferably set to 60 minutes. If the residence time is too long, the secondary reaction of the styrene monomer may proceed, and the recovery rate may decrease. A more preferred lower limit of the residence time is 3 minutes. A more preferred upper limit is 30 minutes, and a still more preferred upper limit is 10 minutes.

The pressure of the high-temperature high-pressure water is equal to or higher than a saturated vapor pressure or a critical pressure at which the water can maintain a liquid state. 25MP
a is more preferable, and about 30 MPa is most preferable. The upper limit of the pressure is not particularly limited, but if the pressure is too high, a reactor capable of maintaining a high pressure is required,
40 MPa or less is preferable.

Further, in the decomposition method of the present invention, it is necessary to continuously supply high-temperature and high-pressure water to the reactor. Polystyrene hardly dissolves in high-temperature and high-pressure water, but low-molecular-weight compounds such as styrene monomers generated by decomposition dissolve in high-temperature and high-pressure water. Therefore, by continuously supplying high-temperature and high-pressure water to the reactor, the aqueous solution in which these low-molecular-weight compound components are dissolved can be continuously transferred to the low-temperature region outside the reactor, and the secondary reaction of the styrene monomer is suppressed. can do. By adopting this configuration, the ratio of the styrene monomer contained in the low molecular weight compound component generated by the decomposition becomes extremely high.

Polystyrene is hardly dissolved in high-temperature, high-pressure water under the above conditions, but is dispersed and settled in high-temperature, high-pressure water in a molten state, and an aqueous solution containing a low-molecular-weight compound component is led to a low-temperature region outside the reactor. Even so, it does not accompany. As a result, the undecomposed polystyrene remains in the high-temperature and high-pressure water until it is decomposed into low-molecular-weight compound components that can be dissolved in high-temperature and high-pressure water, so that the decomposition rate of polystyrene is improved. In the present invention, as a result of all these factors, the recovery rate of styrene monomer from polystyrene was able to be increased.

The polystyrene can be charged to the reactor all at once or continuously. If continuously supplied to the reactor in a heated molten state, slurry state, etc., the dispersibility of polystyrene in high-temperature, high-pressure water will be improved, and the decomposition efficiency will be improved, and long-term continuous decomposition will be possible. In actual operation, this configuration is preferable.

[0020] The residence time is the time from when the high-temperature and high-pressure water is supplied into the reactor to when the aqueous solution in which the low-molecular-weight compound component is dissolved is discharged to the low-temperature region outside the reactor. Outside the reactor, the secondary reaction of the styrene monomer is suppressed by cooling with a cooling means or cooling with decompression to make the temperature lower than the temperature inside the reactor. The residence time can be controlled by the size of the reactor and the supply (flow) rate of high-temperature and high-pressure water. The low molecular compound component refers to styrene monomer, styrene dimer, trimer, toluene, ethylbenzene, ethyltoluene, α-methylstyrene, etc. detected in the preliminary experiment.

In the method of the present invention, in one reactor, the inlet side may be a decomposition reaction zone under high-temperature and high-pressure conditions, and the outlet side of the reactor may be a lower-temperature zone than the decomposition reaction zone. In this case, the time from supply to the reactor to reaching the low temperature region is the residence time. in this case,
The time from the supply to the reactor to the arrival at the low temperature region is the residence time. When a low-temperature region and a hydrolysis reaction region are provided in one reactor, oligomers which cannot be dissolved in low-temperature water due to the setting of conditions are separated from the water, and this oligomer is returned to the hydrolysis reaction region. , And the hydrolysis efficiency is improved.

The ratio of polystyrene to high-temperature / high-pressure water (water ratio) is preferably 0.1 to 200 times by mass of high-temperature / high-pressure water to polystyrene. 20
If it exceeds 0 mass times, the energy cost for obtaining high-temperature and high-pressure water and the energy cost for the separation means when separating low-molecular-weight compound components from water become enormous, and the reactor and dehydration equipment become large-scale. It is not preferable because it has a disadvantage that it must be performed. A more preferred water addition ratio is 1 to 50 times (mass ratio).

The styrene monomer-containing aqueous solution which has been subjected to the decomposition reaction and discharged out of the decomposition reaction region together with the high-temperature and high-pressure water is preferably transferred to a styrene monomer recovery means. The recovery means is not particularly limited, but may be a method in which a low-molecular compound component such as a styrene monomer and the like are separated from water, and distillation is performed to separate the styrene monomer and other low-molecular compounds. As a method of separating the low-molecular compound component, which is a decomposition product, from water, a gravity separation method using a settler or the like, a centrifugal separation method utilizing a specific gravity difference of a cyclone or the like, a distillation method, an extraction method, or the like can be adopted. When performing distillation,
In order to prevent thermal polymerization of the styrene monomer, vacuum distillation is preferably performed.

Next, an example of equipment for carrying out the hydrolysis method of the present invention will be described with reference to FIG. 1 is a reactor. As the reactor 1, a reactor capable of holding water at a high temperature and a high pressure is used. A reactor having a heater may be used.
Further, a configuration in which a plurality of reactors are arranged in parallel may be used.

The polystyrene to be decomposed is heated and melted in the heating tank 2 and then continuously supplied to the reactor 1 by the pump 3. Supply means such as a screw type extruder may be used instead of the pump. Further, polystyrene may be charged into the reactor 1 before the reaction (semi-continuous).
It is also possible to add the pulverized polystyrene to the water tank 4 and supply it to the reactor 1 in a slurry form.

On the other hand, water is brought into a high pressure state by a pump 5 from a water tank 4, brought into a high temperature state by a heater 6, and is continuously supplied to the reactor 1. In the example of FIG.
Immediately upstream of the reactor 1, polystyrene and high-temperature / high-pressure water are combined and then introduced into the reactor 1, but they may be introduced through separate supply paths. The control of the residence time can be controlled by the supply speed of high-temperature and high-pressure water.

The contact between the polystyrene and the high-temperature and high-pressure water in the reactor 1 causes a decomposition reaction of the polystyrene. High-temperature, high-pressure water is supplied to the reactor 1 at a speed corresponding to the feed rate.
Move from below to above. Oligomer is first generated by the decomposition of polystyrene, and the oligomer having a molecular weight that is soluble in high-temperature and high-pressure water is dissolved in high-temperature and high-pressure water and decomposed into styrene monomer while moving upward in the reactor. When the aqueous solution containing the styrene monomer and the low-molecular compound component such as toluene is discharged from the top of the reactor 1, the retention of the low-molecular compound component ends. The water discharged from the reactor 1 is cooled to, for example, about 100 ° C. by the cooling means 8 or by reducing the pressure to the atmospheric pressure or the like by the pressure regulating valve 7 without the cooling means 8.
Secondary reactions of the styrene monomer no longer occur.
In addition, a filter or the like may be attached to the outlet of the reactor 1 in order to prevent the polystyrene in a molten and dispersed state from being taken out of the reactor 1 accompanying high-temperature and high-pressure water.

The aqueous solution containing the low-molecular-weight compound component discharged from the top of the reactor 1 is reduced in pressure to, for example, the atmospheric pressure by the pressure regulating valve 7, and is appropriately cooled through the cooling means 8, so that the low-molecular-weight compound component is removed. It is sent to the water separation device 9. The separation device 9 includes a gravity separation method using a settler or the like,
Various devices capable of performing liquid-liquid separation using centrifugation, distillation, extraction methods and the like utilizing the specific gravity difference of a cyclone or the like are preferable. It is preferable that the separated water is returned to the water tank 4 through the flow path 10 and is circulated.

The low molecular weight compound component separated from water is sent to the styrene monomer recovery means 11. In the illustrated example, the styrene monomer recovery means 11 is a distillation column 12. Since the low molecular weight compound component may contain a styrene monomer and other low molecular weight compounds (toluene, ethylbenzene, etc.), a plurality of distillation columns may be arranged to separate various components.

The low molecular weight compound component may contain an oligomer soluble in high-temperature and high-pressure water. Oligomers are those having a higher molecular weight than styrene trimers, but have lower solubility in high-temperature and high-pressure water than styrene trimers. Therefore, by setting the temperature and pressure conditions of the low molecular compound component-containing aqueous solution discharged from the reactor 1 to conditions under which only the oligomer can be precipitated,
The oligomer can be separated by a solid-liquid separation means (not shown) upstream of the water separation means 9. The separated oligomer is supplied to the reactor 1
It is preferable to return to the vicinity of the central portion. Since the oligomer discharged in a solution state from the reactor 1 has already been partially decomposed, it is introduced near the center of the reactor 1 to shorten the residence time and suppress the secondary reaction of the styrene monomer. can do.

As described above, the equipment that can be used in carrying out the method of the present invention has been described with reference to FIG. 1. However, the present invention can be implemented in equipment modified without departing from the spirit of the present invention. Needless to say.

[0032]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which do not limit the present invention.

EXPERIMENTAL EXAMPLE 1 Decomposition experiments of polystyrene were carried out by the batch method and the method of the present invention. The decomposition target is a commercially available polystyrene having a number average molecular weight (Mn) of about tens of thousands. In the batch method, the decomposition time was set to 33 minutes as a condition under which the polystyrene decomposes almost 100%. A reactor equipped with a heater was charged with 1 g of the above polystyrene and 10 g of water at a time, and was heated at 350 ° C.
After 30 MPa, the decomposition reaction was performed for 33 minutes. After the completion of the reaction, the mixture was cooled and returned to normal pressure. No polystyrene remained in the reactor, and it was confirmed that all of the polystyrene had been decomposed. The composition of the reaction product was analyzed by gas chromatography. The results are shown in Table 1.

In the method of the present invention, 1 g of polystyrene was charged into an experimental reactor similar to that shown in FIG. The feed rate of water at 350 ° C. and 30 MPa was adjusted so that the residence time was 5 minutes, and was continuously supplied to the reactor. An aqueous solution containing the low-molecular compound was led out from the top of the tower, and the pressure was reduced and cooled. This experiment was performed for 60 minutes. After the end of the experiment, no polystyrene was found in the reactor, confirming that all polystyrene had been decomposed.
The decomposition products were analyzed by gas chromatography, and the results are shown in Table 1. The yields (% by mass) in Table 1 are the ratios of various components to the mass of the charged polystyrene.

[0035]

[Table 1]

As apparent from Table 1, in the system of the present invention, the reaction product contained as much as 65% of the styrene monomer, whereas in the case of the batch system, the styrene monomer was as small as less than 26%.
The superiority of the method of the present invention was confirmed. This is because, in the method of the present invention, the styrene monomer generated by the decomposition is quickly led out of the reactor, and is less likely to undergo secondary reactions such as conversion to toluene and the like and repolymerization. In addition, since the undecomposed polystyrene stays in the reactor until it is decomposed, as a result of combining them, the recovery rate of the styrene monomer was improved as compared with the batch type.

[0037]

According to the present invention, an optimum reaction condition / method capable of suppressing the conversion reaction of styrene monomer when decomposing polystyrene has been found. Therefore, styrene can be recovered in high yield without conversion to toluene, ethylbenzene, or the like. Also,
Since a high temperature and high pressure condition in which the polystyrene was not dissolved was selected and only the aqueous solution was continuously discharged, the undecomposed polystyrene stayed in the decomposition reaction zone, so that the decomposition rate could be improved. Therefore, it was possible to achieve both an improvement in the decomposition rate of polystyrene and an improvement in the recovery rate of the styrene monomer.

The method of the present invention is useful as an industrial recycling method for polystyrene, and is useful for producing polystyrene containers, films, tableware, housing for electric appliances, various furniture, cushioning materials, and other various molded products. It is useful for processing of generated defective products or waste plastics after they are distributed to the market as products.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view of equipment for carrying out a decomposition method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reactor 2 Heating tank for polystyrene 3 Pump for supplying polystyrene 4 Water tank 5 Pump for supplying water 6 Heater 7 Pressure control valve 8, 8 'Cooler 9 Separation device 10 Flow path 11 Recovery device 12 Distillation tower

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4F301 AA15 CA09 CA24 CA43 CA62 CA72 CA73 4H006 AA02 AC91 BB31 BC10 BC11 BC19 BD10

Claims (3)

[Claims] 1. A method for thermally decomposing polystyrene by bringing high-temperature / high-pressure water into contact with polystyrene in a reactor, wherein the temperature of the high-temperature / high-pressure water is from 250 to 550.
℃, the pressure is equal to or higher than the saturated vapor pressure or the critical pressure at the temperature. While continuously supplying the high-temperature and high-pressure water to the reactor, the aqueous solution in which the low-molecular-weight compound component generated by the decomposition is dissolved is subjected to the decomposition reaction zone. A method for decomposing polystyrene, comprising controlling the time during which a low-molecular-weight compound component stays in a decomposition reaction zone by continuously leading out to a zone under a lower temperature condition. 2. The method for decomposing polystyrene according to claim 1, wherein the time during which the low molecular compound component remains in the decomposition reaction zone is controlled to 0.1 to 60 minutes. 3. The method according to claim 1, wherein the polystyrene is continuously supplied to the reactor in a molten state or a slurry state.
Or the decomposition method according to 2.