JP2000072720A - Monomerization of polyethylene terephthalate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To readily enlarge a reactor by advancing a reaction in a homogenous phase since in a conventional method, the reaction is partially or wholly made into two or more phases, the security of the contact of these phases is needed and the enlargement of the reactor is a problem. SOLUTION: This monomerizaton method is a method for thermally depolymerizing a polyethylene terephthalate in a homogenous phase consisting of the polyethylene terephthalate, dimethyl terephthalate and methanol under a pressure at which methanol can exist in a liquid phase. Methanol is additionally added during the advance of the depolymerization.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for depolymerizing polyethylene terephthalate (hereinafter abbreviated as "PET"),
The present invention relates to a method for accelerating a reaction when a monomer is converted into a raw material and a raw material derivative.

[0002]

2. Description of the Related Art When polyester is reused as the same kind of polyester material, there are material recycling and chemical recycling. In material recycling, polyester is softened by heat or the like, and then remolded. Due to the simplicity of the process, recycling costs are low on a one-time basis. However, the physical properties such as moldability are inferior to the original material each time remolding is performed, so that the number of times of recycling is limited, or conversion to a use that can be performed with lower physical properties is inevitable. On the other hand, in chemical recycling, polyester is chemically converted, returned to raw materials, purified, and then resynthesized. For this reason, the system is complicated and recycling is high when viewed only once. However, since the number of times of recycling is dramatically increased and the use of the recycling destination is not limited, the added value is high.

[0003] There are many ways to monomerize polyesters. A typical method is a methanolysis method using high-temperature methanol vapor of DuPont (European Patent No. 0 484 963 A2, US Pat. No. 3,907,868).
issue). In this method, high-temperature methanol vapor is blown from below the molten PET to depolymerize the PET, and ethylene glycol (hereinafter abbreviated as “EG”) and dimethyl terephthalate (hereinafter abbreviated as “DMT”) are monomers. The methanol added as a reaction reagent and a solvent can be recovered from the upper part of the reactor. The feature of this method is that the reaction is completed in one step and no special control is required, so that high efficiency can be obtained under certain conditions. In addition, since the monomer is taken out in the gas phase, there is a feature that continuous processing can be performed even when there are many impurities such as PET mixed with FRP. However, since the reaction is a gas-liquid two-phase reaction, mixing inside the reactor becomes a problem. In particular, it is important to ensure the mixing property during scale-up.

Another method is a methanolysis method using supercritical methanol (US Pat.
148,208, JP-A-9-249597). This method uses molten P
Depolymerization is carried out by contacting ET with methanol in a supercritical state at 239 ° C. and 79 atm or more.
The feature of this method is that the reaction time is very short because methanol is reacted at a high temperature at a density comparable to that of a liquid.
When about tens of mg of PET is treated with methanol at 300 ° C. and 100 atm, the reaction is completed within 10 minutes. However, since it is a reaction between two phases of supercritical methanol and molten PET, mixing inside the reactor becomes a problem. Therefore, when the reaction vessel is simply enlarged and 100 g of PET is treated, the reaction time is several hours or more. Therefore, various methods have been devised, such as melting PET in advance and spraying it into methanol to produce PET fine particles, and introducing the fine particles into the reactor, or arranging a large number of perforated plates to increase the stirring inside the reactor. There is a disadvantage that it is necessary and the device becomes complicated.

Another method is to depolymerize PET using EG, convert it once to bis-2-hydroxyethyl terephthalate (BHET), and then convert it to ester with methanol to obtain EG and DMT. (U.S. Pat. No. 3,257,333). The features of this method are that the first reaction can be performed at almost normal pressure, and that the second reaction can be performed under milder conditions than other methods. However, the two-stage reaction is required, and the first-stage reaction may not be suitable for a continuous process due to a solid-liquid reaction. further,
In the second reaction, the EG unit in BHET is PET
Therefore, there are drawbacks in that a large amount of methanol must be introduced in terms of reaction equilibrium.

[0006]

In the conventional method, two or more phases are involved in a part or the whole of the reaction. Therefore, it is necessary to secure the contact between them, and there is a problem in increasing the size of the reactor. The present invention, by allowing the reaction to proceed in a homogeneous phase,
It is intended to facilitate upsizing of the reactor.

[0007]

The present invention is a method for thermally depolymerizing polyethylene terephthalate in a homogeneous phase comprising polyethylene terephthalate, dimethyl terephthalate, and methanol under a pressure in which methanol can exist in a liquid phase,
Provided is a method for monomerizing polyethylene terephthalate in which methanol is further added during the progress of the depolymerization.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION PET which is the object of the present invention is not particularly limited, but is used as a PET bottle for recycling, a PET film such as a photographic film, a PET tape such as a magnetic tape, and a polyester fiber. And PET sheets used for transparent fibers, cups, trays and transparent packaging. Prior to the monomerization of the present invention, it is preferable that PET is subjected to a pretreatment such as separation from other materials and washing by a usual method to form flakes.

The method for monomerizing PET of the present invention is characterized in that methanol required for the reaction is added stepwise in order to cause the reaction to proceed in a homogeneous phase until the end of the reaction. That is, in the conventional method, two or more phases are formed in part or the whole of the reaction, but in the method of the present invention, the reaction can be allowed to proceed in a uniform phase throughout. Methanol at the start of the reaction,
The composition range of PET and DMT needs to be such that these mixtures are in a homogeneous phase. The mixing ratio for forming a homogeneous phase (mixing three-phase diagram) is obtained by the experiment described later.

FIG. 2 shows a mixed three-phase diagram at 230 ° C. As shown in FIG. 2, in order to form a homogeneous phase, PET is 5
It is understood that a composition requiring 0 to 100% by weight, 20 to 100% by weight of DMT, and 50% by weight or less of methanol exceeding 0 is required. For example, methanol 20% by weight, PET
When 50% by weight, DMT is 30% by weight.
As shown in the hatched area in the figure, the mixture is completely mixed to form a homogeneous phase. However, when methanol is 40% by weight and PET is 50% by weight, DMT is 10% by weight, which is out of the hatched area and undergoes phase separation. As the temperature is increased, the region of the uniform phase is enlarged, and as the temperature is decreased, the region is reduced. From the viewpoint of reaction equilibrium, the smaller the amount of DMT added, the better, and since a certain amount of methanol is required for the reaction,
The lower the left side of the phase diagram, the more preferable the reaction equilibrium.

In the present invention, methanol is added at least twice at the start of the depolymerization reaction and during the reaction. This is because, in order to complete the depolymerization, a large amount of methanol is required in terms of reaction equilibrium, but in order to maintain a homogeneous phase, a large amount of methanol cannot be added at the start of the reaction, as is apparent from FIG. The amount of methanol added at the start of the reaction is determined based on FIG. Further, the amount of methanol added once or twice or more after the start of the reaction, together with the methanol at the start, is 3 to 20 times the weight of PET, preferably 4 to 6 times the weight of PET. So that
Also, regarding the timing of the addition, the reaction temperature, PET
Although it varies depending on the type and amount of the polymerization catalyst remaining therein, it is the time when 5 to 20 minutes or more have elapsed after the start of the reaction. The time required for completing the depolymerization reaction varies depending on the amount of methanol added, the reaction temperature, the type and amount of the polymerization catalyst remaining in PET, and the like, but is usually 30 minutes to 4 hours. Further, when 500 ppm of zinc acetate, which is a known catalyst, is added to PET, the time is 15 minutes to 1 hour.

When a homogeneous phase composed of PET, DMT and methanol is heated to depolymerize PET, three components of DMT, EG and methanol are obtained. That is, the components at the reactor outlet are three components of DMT, EG, and methanol.
These are mixed in any ratio as shown in FIG. Between the phase diagram of FIG. 2 and the phase diagram of FIG. 3, the composition ratio and the physical properties of the PET-PET depolymerized intermediate are continuously changed by the addition of the reaction time and methanol. For example, the complete mixing region of the phase diagram of FIG. 2 expands with time, and after a certain time, complete mixing at an arbitrary ratio. Therefore, the entire reaction can be performed in a homogeneous phase.

[0013] The depolymerization reaction of the present invention is carried out under heating, that is, at a temperature of 200 to 250 ° C, preferably 220 to 250 ° C.
C., under pressure where methanol can be present in the liquid phase.
Pure methanol has a critical point at 239 ° C. and does not liquefy at a temperature higher than that, but when mixed with PET and DMT, it can exist as a liquid even at a temperature higher than the critical point of methanol. The pressure is preferably set at a saturated vapor pressure that is increased by sealing the container. This pressure depends on the temperature and is 66 atm at 230 ° C. Below the saturated vapor pressure, methanol cannot exist as a liquid phase due to equilibrium. If the pressure is equal to or higher than the saturated vapor pressure, no gas phase portion exists in the reaction vessel, and the fluid in the reaction vessel becomes incompressible, so that the pressure control becomes complicated. 230 ° C
The mixed three-phase diagram of FIG. 2 is shown in FIG. 2. A similar mixed three-phase diagram is obtained except that when the temperature is increased, the homogeneous phase forming composition region is slightly widened, and when the temperature is decreased, the homogeneous phase forming composition region is slightly narrowed. A phase diagram is obtained, whereby a composition in which the reaction is performed in a homogeneous phase can be obtained.

The depolymerization reaction can be carried out batchwise in one reaction vessel, or alternatively, methanol, PET, D
It can be carried out continuously by flowing a homogeneous phase of MT. In order to treat a large amount of PET depolymerization reaction, it is preferable to carry out the reaction continuously. At the end of the depolymerization reaction, a mixture of methanol, DMT and EG is obtained. These mixtures are prepared in the usual manner, for example by distillation, with methanol,
EG and DMT are separated in this order. If a PET depolymerized intermediate is present, it can be recovered from the bottom of the distillation column during DMT distillation. After removing methanol, PET can also be directly synthesized. If a PET depolymerization intermediate is also present at the end of the reaction, it can be further depolymerized by the same method.

An example of the method for continuously monomerizing polyethylene terephthalate of the present invention will be described with reference to FIG. 1, but the present invention is not limited thereto. PET2 is D
It is dissolved in MT beforehand and charged into the pressurized reaction section A. At the same time, a specific amount of methanol 1 is also charged, and a specific amount of DMT or a mixture 3 of DMT and a PET depolymerized intermediate is also charged. In the reaction section A, PET and methanol are dissolved in DMT to form a homogeneous phase. . When a mixture of DMT and a PET depolymerization intermediate is charged, it is for further depolymerizing an incompletely depolymerized reaction. The pressure inside the reaction vessel consisting of the reaction part A, the reaction part B and the methanol addition part is increased,
Allow methanol to be present in the liquid phase. Where P
ET is depolymerized and converted to lower molecular compounds. The reaction mixture causing the depolymerization reaction moves from the reaction part A to the reaction part B, and from FIG. At the outlet of the reaction section A, DMT and EG due to decomposition are hardly detected. Between the outlet of the reaction part A and the inlet of the reaction part B is partitioned by a perforated plate a and a perforated plate b, and methanol necessary for completing the reaction is added. In the reaction section B, the PET depolymerized methanol (the reaction intermediate reacted in the reaction section A and the reaction section B)
Reaction intermediate B) and D produced in the depolymerization of reaction part B
MT and EG dissolve to form a homogeneous phase. Here, the depolymerization proceeds further, and a mixture of the monomers DMT4, EG5 and methanol 1 is obtained at the outlet of the reaction section B. afterwards,
Each component is separated by a usual method. The perforated plate a
The composition of the depolymerized reaction product is changed from the inlet to the outlet of the reaction section A in accordance with the degree of the progress of the reaction, and methanol is added without disturbing a series of the composition change. This is for enabling the addition.

[0016]

EXAMPLES (Experimental example) Methanol for forming a homogeneous phase,
The following experiment was performed to obtain a mixing ratio of PET and DMT (mixing three-phase diagram). In order to determine the initial charging ratio, a mixture of PET, DMT and methanol was confirmed using a transmission type high pressure cell with a quartz window. The cell has PET in advance
And DMT were charged and colored lightly with tar. Methanol was additionally added to this, and it was confirmed whether the whole became uniform. The tar dissolves more in the less polar DMT side, so when the phases separate, the lower layer is strongly colored. In addition, a scale like a ruler was arranged on the back of the quartz window, and it was checked at the same time whether or not the refractive index of the liquid inside was uniform by removing the liquid inside slightly obliquely. When the inside was non-uniform, the scale on the back surface appeared distorted due to the difference in refractive index.

(Example) The same reaction vessel as that shown in FIG. 1 was used. PET and DMT were mixed at a weight ratio of 2: 1 and heated to 230 ° C. to liquefy. This was continuously pumped into the reactor at a flow rate of 150 g / h.
At the same time, methanol was also heated to 230 ° C.
It was continuously fed into the reaction vessel at a weight ratio of 2 and a flow rate of 50 g / h. The reaction vessel is maintained at 66 atm and controlled so that methanol can exist as a liquid phase. The reaction vessel is provided with a perforated plate so that the internal flow is in one direction. Methanol heated to 230 ° C. at a residence time of 2 hours was introduced at a flow rate of 450 g / h so as to be 5 times the weight of PET, and was further retained for 2 hours and continuously taken out of the vessel. The discharged liquid was cooled quickly and stored in a tank. Analysis of this sample showed that DMT was obtained in a yield of 93% of theory and EG was obtained in a yield of 88% of theory. In addition, using a small high-pressure cell with quartz assuming the conditions of the inlet of the reaction section, it was confirmed that the whole was a uniform phase. Further, after maintaining the temperature, methanol was added after 2 hours, and it was confirmed that the mixture was in a homogeneous phase until the end of the reaction.

[0018]

According to the conventional method, two or more phases are required for a part or the whole of the reaction. Therefore, it is necessary to secure the contact between the two phases, which is a problem when the reactor is enlarged. In this case, the reaction proceeds in a uniform phase throughout, so that the size of the reactor can be easily increased.

[Brief description of the drawings]

FIG. 1 shows an example of a method for continuous monomerization of PET according to the present invention.

FIG. 2 shows a mixed three-phase diagram of PET, DMT, and methanol at 230 ° C.

FIG. 3 shows a mixed three-phase diagram of DMT, EG, and methanol at 230 ° C.

[Explanation of symbols]

1 Methanol 2 PET 3 DMT or a mixture of DMT and PET depolymerized intermediate 4 DMT 5 EG

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Takashi Miyake 2-1-1, Shinhama, Arai-machi, Takasago City, Hyogo Prefecture Inside the Takasago Research Laboratory, Mitsubishi Heavy Industries, Ltd. (72) Takeshi Mitsui 1 Wadazakicho, Hyogo-ku, Kobe-shi, Hyogo Prefecture No. 1-1, Mitsubishi Heavy Industries, Ltd. Kobe Shipyard (72) Inventor Kiyonori Nishida 2-8-19, Araimachi Shinhama, Takasago-shi, Hyogo F-term (reference) 4F301 AA25 AD01 AD02 BA21 BE01 BE36 BE50 BF31 CA09 CA43 CA72 CA73 4H006 AA02 AC91 BC10 BC11 BC14 BC37 BD21 BJ50

Claims (2)

[Claims] 1. A method for thermally depolymerizing polyethylene terephthalate in a homogeneous phase comprising polyethylene terephthalate, dimethyl terephthalate, and methanol under a pressure in which methanol can exist in a liquid phase. A method for monomerizing polyethylene terephthalate by adding methanol. 2. The method according to claim 1, wherein the composition of the homogeneous phase at the start of the depolymerization is determined based on a three-phase diagram of a mixture of polyethylene terephthalate, dimethyl terephthalate and methanol. .