Abstract:
This invention relates to a process for producing aromatic divalent carboxylic acid ester and diatomic alcohol by reacting aromatic polyester and super critical monatomic alcohol, and by the process, PET is decomposed for recovering monomer components therefrom, and thus recovered monomer components can be used as the material for reproduction of new PET. 
     In accordance with the present invention, there is provided a process for producing continuously monomer components from aromatic polyester, the process including the steps of: feeding continuously the above aromatic polyester and the above monatomic alcohol into a reactor while the above reactor is kept to be under the super critical condition of the above monatomic alcohol; reacting the above aromatic polyester and the above super critical monatomic alcohol and discharging the resultant reaction products, i.e., aromatic divalent carboxylic acid ester and diatomic alcohol, together with the monatomic alcohol from the reactor; and separating, from the above discharged resultant products, the above aromatic divalent carboxylic acid ester and the above diatomic alcohol and recovering them.

Description:
The present application claims the foreign priority of Japan 11-038396 Feb. 17, 1999 and Japan 11-21448 Jul. 29, 1999. 
     FIELD OF THE INVENTION 
     This invention relates to a process for continuously producing, from aromatic polyester, an aromatic dicarboxylic acid and ester a dihydric alcohol as the monomer components of the aromatic polyester. 
     BACKGROUND OF THE INVENTION 
     As one of typical aromatic polyesters, polyethylene terephthalate is known. The polyethylene terephthalate (PET) is used as the material of bottles containing beverages and other liquids, so-called PET bottles. Most of spent PET bottles are disposed of by incineration or landfill. However, these disposal methods are not preferable from the viewpoints of reuse of resource and environmental protection. So, it is desired to develop recycling technology for recovering spent PET bottles. Necessity for developing such technology is increased with growing consumption of PET bottles. Several methods have been developed in each of which, PET is decomposed for recovering monomer components therefrom, and thus recovered monomer components can be used for reproduction of new PET bottles. 
     Among these conventional methods, typical ones are listed below. 
     (1) Methanolysis method where solvolysis is carried out with liquid methanol; 
     (2) Glycolysis method where solvolysis is carried out with ethylene glycol; 
     (3) Ester interchange method where methyl esterification is carried out after the Glycolysis method (2); and 
     (4) Alkali hydrolysis method where hydrolysis is carried out with alkali solution. 
     Further, in these days, the following two methods are proposed. 
     (A) Method for obtaining terephthalic acid where PET is hydrolized with super critical water; and 
     (B) Method for recovering monomer components where super critical methanol and PET are reacted in a batch type reactor as disclosed in Japanese Patent No. 2807781. 
     However, the above methods have drawbacks respectively. 
     In the method (1) (Methanolysis method), its reaction temperature is so low of about 450 K that its reaction rate is small. Accordingly, catalysts are required to accelerate its reaction, which leads to increased cost. 
     In either of the method (2) (Glycolysis method) and method (3) (Ester interchange method), catalyst is required like the method (1) (Methanolysis method). Further, either of their reaction steps is complicated. 
     In either of method (4) (Alkali hydrolysis method) and method (A) (Method for obtaining terephthalic acid), it is difficult to purify the terephthalic acid obtained by the hydrolysis. Particularly, in the method (A) (Method for obtaining terephthalic acid), the reaction conditions are very severe. Concretely, the reaction temperature is very high (same as or higher than 450 K) and the reaction pressure is also very high (same as or higher than 30 MPa). Further, it is substantially impossible to recover perfectly the monomer components, since one resultant monomer component (ethylene glycol) is decomposed due to catalytic action, which is performed by another resultant monomer component (terephthalic acid) in the solution. 
     In the method (B) (Method for recovering monomer components) disclosed in Japanese Patent No. 2807781, the following process is performed. First, methanol and aromatic polyester are fed in the weight proportion of the same as or higher than 10 mol, preferably 20 to 70 mol of methanol per 1 mol of aromatic dicarboxylic acid in the aromatic polyester. Then, they are reacted under the conditions of temperature of 512.6 to 673 K, preferably 523 to 653 K and of pressure of 3 to 30 MPa, preferably 5 to 25 MPa. But this Japanese Patent does not refer to an industrial process for continuously producing monomer components. In the method of this patent, when the reaction products are recovered after cooling of the reactor, the recovered dimethyl terephthalate is precipitated due to its low solubility to the methanol, and therefore, it takes trouble to discharge this product. Additionally, since the dimethyl terephthalate, on recovering, tends to be mixed with not-yet-decomposed PET, a separation step is required to separate the not-yet-decomposed PET and the produced dimethyl terephthalate. Further, because of low level of recovering efficiency, this method is not suitable for practical use in the field of industry. 
     It is, therefore, the first object of the present invention is to provide a process by which monomer components can be produced continuously from the aromatic polyester with high yield as well as by which the monomer components can be recovered and separated simply under industrially advantageous conditions. 
     The second object of the present invention is to, in this process, discharge the large amount of reaction products from the reactor during the reaction and to increase the throughout per unit time for improved productivity. By attaining this second object, after reaction is carried out continuously for a certain period and the pressure in the reactor is reduced to the atmospheric pressure, the reaction products can be recovered not in batch operation but in continuous operation. 
     The third object of the present invention is to increase the purity of monomer components as products and to prevent oligomer components from remaining in the monomer components. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a process for continuously producing monomer components from aromatic polyester by reaction of the aromatic polyester and super critical monohydric alcohol to obtain aromatic carboxylic acid ester and diatomic alcohol, the process comprising the steps of: feeding continuously the above aromatic polyester and the above monatomic alcohol into a reactor while the above reactor is kept to be under the super critical condition of the above monatomic alcohol; reacting the above aromatic polyester and the above super critical monatomic alcohol and discharging the resultant reaction products i.e., aromatic divalent carboxylic acid ester and diatomic alcohol, together with the monatomic alcohol from the reactor; and separating, from the above discharged resultant products, the above aromatic divalent carboxylic acid ester and the above diatomic alcohol and recovering them. 
     In the present invention, the aromatic polyester includes not only polyethylene terephthalate (PET) but also polyethylene naphthalate (PEN) and the like. The monatomic alcohol of the present invention includes not only methanol but also ethanol and the like. 
     Since the reaction between the aromatic polyester and monatomic alcohol is one of typical ester exchange reactions, the components are allowed to come to certain equilibrium under the conditions of certain temperature and certain pressure. However, according to the present invention, as the reaction products together with the super critical monatomic alcohol are discharged out of the reactor, the equilibrium is shifted to the side of the reaction products. Hence, the decomposing reaction can be accelerated resulting in that the monomer components can be recovered with high yield. Further, when the reaction products are discharged together with the super critical monatomic alcohol, it is not necessary to cool the reactor after the reaction, which is different from the batch type reaction stated before. Therefore, in spite of low solubility of aromatic divalent carboxylic acid ester to the monatomic alcohol, the acid can be recovered with high yield. 
     On the other hand, under such super critical conditions of high temperature and high pressure, the reactor should be thick-walled and sometimes should be a pressure reactor surrounded by heat medium. Conventionally, in each reactor, there was no way to determine the boundary position formed between the super critical monatomic alcohol phase and the aromatic polyester phase. Precisely, operation was carried out continuously, while the boundary position formed-between these two phases could not be determined. Accordingly, the liquid phase might be often eliminated or introduced into a discharge line, which might, in some cases, led to shutdown of plant. 
     In comparison with this, by feeding the above monatomic alcohol into the above reactor with a constant flow rate and the aromatic polyester with a controlled flow rate while the above boundary position formed between these two phases is determined, the above mentioned undesirable accidents can be prevented. That is to say, the liquid phase will not be eliminated and not be introduced into the discharge line, thus, the shutdown, which would be caused by the liquid phase&#39;s eliminating or its introducing into the discharge line, will not be occurred, whereby the productivity is improved and stable continuous operation can be carried out. 
     The internal part of the reactor is divided into two phases: super critical monatomic alcohol phase, into which the reaction products are dissolved, and not-yet-decomposed polyester phase. The present inventors found that the temperature of the super critical monatomic alcohol phase is lower than that of the not-yet-decomposed polyester phase, particularly that, when the super critical monatomic alcohol is methanol, the temperature of super critical methanol phase is lower than that of not-yet-decomposed polyester phase by about 20° C. 
     That is to say, it is found that, in order to determine the boundary position formed between these two phases by utilizing such temperature difference, the temperature of each phase should be known. Therefore, by detecting the temperature of each phase with the temperature sensor equipped in the reactor, the boundary (interface) position formed between the super critical monatomic alcohol phase and aromatic polyester phase can be determined. Since it is impossible to use a common level meter due to the severe reaction conditions of high temperature and high pressure, it is only way for determining the boundary position to detect the temperature of each phase with the temperature sensor such as a thermocouple. 
     In discharging the above reaction products, the pressure in the above reactor can be reduced and, if required, can be reduced so as to reach the atmospheric. 
     Actually, when each spent PET bottle is recycled for the reuse of resources, metal, ash, resin other than PET, adhesive for attaching labels to the bottle, the labels, and the like, all of which are contained in the PET bottle, must be removed. Then, in order to improve the purity of each monomer component, which is recovered as the product, several treatments are required prior to the reaction step. For example, sorting of different kinds of resins other than the PET, detaching the labels, removing a cap, washing with detergent, ultrasonic cleaning and air separation must be carried out. It is extremely troublesome to perform such pre-treatments. Moreover, these pre-treatments downgrade the productivity and increase the recycle cost. 
     For overcoming this problem, in discharging of the reaction products from the reactor, aromatic divalent carboxylic acid ester and monatomic alcohol may be discharged continuously while the alcohol is saturated with this ester. Precisely, in a preferred embodiment, the amount of communicated methanol is determined so that the reaction products; aromatic divalent dimethyl carboxylate (dimethyl terephthalate) and diatomic alcohol (ethylene glycol) are discharged continuously together with the methanol, while the methanol is saturated with all amount of produced dimethyl terephthalate. Residue condensed in the above reactor can be intermittently discharged out of this process system through a discharge line from the bottom of the above reactor. 
     According to this embodiment, spent PET bottles, which have been collected and crushed, can be directly used as the material of this process. That it to say, the collected PET bottles, to each of which the metal, ash, resins other than PET, adhesive for the labels, labels, and the like are contained, can be used as the material as they are without the pre-treatments such as finely sorting, washing, and the like. 
     The discharged reaction products are introduced into a column in which the temperature is lower than that in the reactor, a column in which the pressure is lower than that in the reactor, or a column in which the temperature and pressure are lower than those in the reactor respectively. Then, oligomer components precipitated in the column can be recycled to the reactor. Thus, only monomer components can be directed to a purification step located on the downstream of the column, which ensures to obtain the highly purified monomer components. 
     In order to cool the above discharged products for the crystallization of the aromatic divalent carboxylic acid ester therefrom, heat exchange may be carried out between the reaction products and the above monatomic alcohol to be fed into the above reactor. Then, the above monatomic alcohol is heated while the above reaction products are cooled. Owing to this heat exchange between the discharged reaction products and monatomic alcohol to be fed into the reactor, energy cost can be saved, 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is the flow diagram of the first embodiment according to the present invention; and 
     FIG. 2 is the flow diagram of the second embodiment according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now, the present invention is described more closely, referring to the preferred embodiments of the present invention shown in the accompanying drawings. 
     &lt;The First Embodiment&gt; 
     FIG. 1 illustrates schematically the process flow diagram of the process for producing monomer components in the first embodiment of the present invention. The collected PET material (e.g., PET bottle) is sorted and washed so as to be crushed and the resultant material is fed to the melting tank  1  where the material is melted by heating the tank  1  with the heaters  2 ,  2 . The melted aromatic polyester is fed continuously into the reactor  10  through the means of the metering pump  3   a  consisting a gear pump or the like. In this connection, the aromatic polyester feed line  4  is preferably provided with the heating means or hot insulation means  5   a  such as a heat insulating equipment. 
     Methanol consisting the monatomic alcohol of the present invention is fed from the methanol tank  11  via the methanol feed line  6  through the means of the metering pump  3   b,  into the reactor  10  by means of the nozzle  15 , which is inserted in the reactor  10  so as to reach its bottom. In this case, the methanol is preferably fed with the constant flow rate. The methanol feed line  6  may be also provided with the heating means  5   b  or hot insulation means. 
     The reactor  10  is a pressure reactor, which can be proof against the super critical conditions of methanol (high temperature and high pressure). The reactor  10  is provided with the heaters  20 ,  20 , by which the temperature in the reactor  10  is raised to the temperature being the same as or higher than the super critical temperature of methanol. This temperature in the reactor  10  is 512.6 to 673 K, which is the super critical condition of methanol, and is preferably 523 to 653 K. The pressure in the reactor  10  is controlled so as to be 3 to 30 MPa and be preferably 5 to 25 MPa. On the other hand, when ethanol is used as the monatomic alcohol, the temperature in the reactor is 513.9 to 673 K, which is the super critical condition of ethanol, and is preferably 523 to 653 K. The pressure in the reactor  10  is controlled so as to be 3 to 20 MPa and be preferably 5 to 15 MPa. The pressure control valve  21  controls the pressure in the reactor  10 . 
     In the first embodiment, the aromatic polyester and monatomic alcohol are reacted in the reactor  10  in the weight proportion of 2 mol, preferably equal to or higher than 4 mol of monatomic alcohol per 1 mol of aromatic divalent carboxylic acid in the aromatic polyester. 
     The aromatic polyester and super critical methanol are reacted in the reactor  10  while they are stirred with the stirrer  24  so that the aromatic divalent dimethyl carboxylate and diatomic alcohol can be produced. As stated before, the temperature in the super critical methanol phase F is lower than the temperature of not-yet-decomposed polyester phase L by about 20° C. Then, the temperature difference between the two phases is detected by means of the temperature sensors  22 ,  22  . . . , which are located in the reactor  10  at many levels on its height direction, whereby a boundary position formed between these two phases can be detected by means of the controller  23 . Then, on the basis of the result of this detection, the flow rate of the fed aromatic polyester into the reactor  10  is controlled by, for example, rotating speed of the metering pump  3   b.  Thus, by utilizing the detected temperature in the super critical methanol phase F, the boundary position (interface position) formed between the methanol phase F and the not-yet-decomposed polyester phase L can be controlled. 
     During the progress of the reaction, the resultant products are dissolved into the super critical methanol phase F. Then, the resultant products together with the super critical methanol, into which the products are dissolved, are introduced to the flash tank  30 , via the discharge line  25 , which is connected from the super critical methanol phase F, through the means of the pressure control valve  21 . On the other hand, the decreased amount of aromatic polyester, which is caused by decomposition and by discharge, is compensated by the aromatic polyester added by means of the metering pump  3   a,  whereby the boundary position formed between the two phases is kept at the same level. 
     Since the reaction between the aromatic polyester and monatomic alcohol is one of typical ester exchange reactions, the components are allowed to come to certain equilibrium under the conditions of certain temperature and certain pressure. However, during the discharge of the reaction products together with the methanol, the equilibrium is shifted to the side of reaction products so that the decomposing reaction can be accelerated. 
     The reaction products and super critical methanol are introduced, through the discharge line  25 , to the flash tank  30  where they are condensed. An extra vapor content, which can not be condensed in the flash tank  30 , is liquefied by cooling water W communicated through the cooling coil  31  and is recovered in the methanol recovery tank  40 . Solid particles of aromatic divalent dimethyl carboxylate as the reaction products are suspended in the above flash tank  30 . If these solid particles were directly moved into the cooling coil  31 , the coil  31  would be blocked. Accordingly, it is preferable that the filter  32  is provided between the flash tank  30  and cooling coil  31  so as to remove the solid particles. 
     A condensate liquid obtained by condensation in the flash tank  30  and a condensate liquid obtained in the methanol recovery tank  40  are, via the recovery line  50 A and  50 B respectively, transferred to a following separation step. In this separation step, the aromatic divalent dimethyl carboxylate and diatomic alcohol are separated from the reaction products, by means of e.g., distillation or crystallization. Then, the aromatic divalent dimethyl carboxylate and diatomic alcohol are recovered. 
     At the terminal stage of this process, the decomposed products, aromatic divalent dimethyl carboxylate and diatomic alcohol, which have been remained in the reactor  10 , are required to be recovered with high yield. Therefore, intermittently or every predetermined hours, the pressure control valve  21  is opened so as to reduce the pressure in the reactor  10  to the atmospheric pressure for the discharge and recovery of the reaction products. 
     &lt;The Second Embodiment&gt; 
     In the above first embodiment, it is often required to carry out the sorting treatment and washing treatment (washing with detergent, ultrasonic cleaning, or the like) for the collected PET material (such as PET bottles). On the other hand, in the second embodiment, as stated below, the collected PET material are crushed as it is and the resultant crushed material can be directly subjected to a decomposing reaction. The crushed PET is heated and melted in the melting tank  1  provided with the heater  2 . Then, the crushed PET is fed, via the aromatic polyester feed line  4 , into the reactor  10  by means of the metering pump  3   a.    
     On the other hand, methanol is derived from the methanol tank  11  by means of the methanol feed pump  3   b  and passed through the methanol feed line  6 . Then, at the heat exchanger  7  on the way, heat exchange is carried out between the methanol and the discharged reaction products so that the temperature of methanol is increased to the reaction temperature. Next, the heated methanol is introduced into the reactor  10  by means of the nozzle  15 , which is inserted in the reactor  10  so as to reach its bottom. Thus, the methanol is always communicated through in this process. 
     In the second embodiment, in the same manner as the first embodiment, the temperature in the reactor  10  is equal to or higher than the critical temperature of methanol, that is 512.6 to 673 K, and is preferably 523 to 653 K The pressure in the reactor  10  is controlled so as to be 3 to 30 MPa and be preferably 5 to 25 MPa by means of the pressure control valve  21 . By the reaction performed between the aromatic polyester and super critical methanol, the aromatic divalent dimethyl carboxylate and diatomic alcohol are produced. 
     In the second embodiment, the reaction materials (aromatic polyester and methanol) in the reactor  10  are reacted in the different weight proportion from that of the first embodiment. Precisely, in the second embodiment, the weight proportion is equal to or higher than 10 mol, preferably equal to or higher than 40 mol of methanol per 1 mol of aromatic divalent carboxylic acid in the aromatic polyester. From the point of equivalency, the weight proportion should be 2 mol of methanol per 1 mol of acid. However, for the smooth discharge of the reaction products out of this process system, the methanol is preferably fed to the reactor in considerably excess of the equivalency. When the weight proportion is equal to or higher than 40 mol of methanol per 1 mol of aromatic divalent carboxylic acid in the aromatic polyester, the almost all amount of reaction products can be discharged in the form of being dissolved into the methanol. 
     During the progress of reaction, the decomposed products are formed and dissolved in the super critical methanol phase. The dissolved products are introduced from the methanol phase, via the discharge line  25 , which is connected from the super critical phase in the upper portion of the reactor  10 , to the oligomer separator  26 , which comprises a packed tower, tower having no packing, or the like. 
     The oligomer components are separated in the oligomer separator  26  where the temperature is lower than that in the reactor 10 by 5 to 100° C. and preferably by  10  to 40° C.; the pressure is lower than that in the reactor  10  by 0.1 to 4 MPa and preferably by 0.2 to 2 MPa; or the temperature and pressure are lower than those in the reactor by the above mentioned differences respectively. After the pressure of precipitated oligomer components is increased, these components are returned, via the backing line  28  to the reactor  10  by means of the backing pump  27 . Thus, only monomer components can be obtained from the oligomer separator  26 , and introduced, via the heat exchanger  7  and pressure control valve  29 , to the flash tank  30  located at the down stream. 
     The decreased amount of aromatic polyester, which is caused by decomposition, is compensated by the aromatic polyester added by means of the metering pump  3   a,  whereby the level of aromatic polyester can be kept to be constant. Metal, ash, resin other than PET, adhesive for attaching labels to the bottle, the labels, and the like, are contained in the aromatic polyester as the material. During progress of the reaction, the content proportions of the metal, ash and other impurities in the aromatic polyester phase are increased so that the impurities are enriched in the discharged reaction products. Accordingly, in order to keep the purity of product, it is preferable that the enriched residue such as the metal, ash, and the other impurities contained in the reactor  10  are discharged. 
     As stated before, the reaction between the aromatic polyester and monatomic alcohol is one of typical ester exchange reactions, the components are allowed to come to certain equilibrium under the conditions of certain temperature and certain pressure. However, during the discharge of the reaction products together with the methanol, the equilibrium is shifted to the side of reaction products so that the decomposing reaction can be accelerated. 
     In the second embodiment, when a lot of methanol is used, the amount of discharged aromatic divalent dimethyl carboxylate is increased. In this case, however, the temperature of the methanol rises, which increases the energy cost. Accordingly, it is preferable that at the down stream of the oligomer separator  26 , heat exchange is carried out between the reaction products from the oligomer separator  26  and the methanol to be fed to the reactor  10 . After the heat exchange, the reaction products are introduced to the flash tank  30  so as to be condensed. An extra vapor content, which can not be condensed, is passed through the filter  32  so that suspended solid particles of the reaction products are removed, and is liquefied by means of the cooling coil  31  before the recovery in the methanol recovery tank  40 . 
     The reaction products are recovered in the flash tank  30  so that the aromatic divalent dimethyl carboxylate and diatomic alcohol can be recovered with large concentration by means of separating operation (not shown) such as distillation, crystallization, or the like. On the other hand, the methanol recovered in the methanol recovery tank  40  is returned, via the backing line  41 , to the reactor  40 . 
     Now, the effect of the present invention will be explained more closely on the basis of the following examples and comparative examples. 
     EXAMPLE 1 
     According to the First Embodiment 
     In the configuration shown in FIG. 1, an autoclave having the content volume of 5 lit. was used as the reactor (autoclave)  10 . Then, the temperature in the reactor was set to be 603 K. First, 1000 g of melted PET was fed into the reactor and methanol was continuously fed to the autoclave with the flow rate of 1000 g/hr. During this operation, the reaction products were continuously discharged together with super critical methanol from the super critical methanol phase located in the upper portion of the reactor  10 . 
     The boundary position (interface position) formed between the two phases was detected from the indicated values determined on the basis of temperatures measured by three thermocouples located at the lower level, middle level, and upper level, respectively in the reactor  10 . For example, when the temperature in the lower PET liquid phase was 330° C., the temperature in the upper super critical methanol phase should be 310° C. Accordingly, the amount of fed PET is controlled so that the temperature adjacent to the interface position was 310 to 330° C. Thus, the boundary position between the phases could be kept to be at the same level. 
     Under these conditions, reaction was performed for 30 minutes while the reaction products were discharged together with the methanol. After this reaction, the pressure in the reactor  10  was reduced so as to reach the atmospheric pressure and the reaction products were discharged. The reaction products, which were discharged together with the methanol during reaction, and the reaction products, which were discharged after the reaction, were soluble into diethyl ether, which ensures that these two kinds of reaction products did not contain any not-yet-reacted PET. 
     The above reaction products were classified into a solid product, ethylene glycol, and methanol by means of filtering and distillation. Then, the dimethyl terephthalate was quantitatively determined as the monomer component with a gas chromatography. As shown in Table 1, 153 g of solid product was recovered during the reaction and its monomer purity was 95%. After the reaction, the pressure in the reactor  10  was decreased and 534 g of solid product could be additionally recovered and its monomer purity was 76%. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Continuous Type (methanol was fed with the flow rate of 1000 g/hr) 
               
             
          
           
               
                   
                 reaction 
                   
                   
                 amount 
                   
                   
               
               
                   
                 temper- 
                 reaction 
                 reaction 
                 of fed 
                 amount of 
                 monomer 
               
               
                   
                 ature 
                 pressure 
                 time 
                 PET 
                 recovered 
                 purity 
               
               
                 discharge 
                 ° C. 
                 MPa 
                 min. 
                 g 
                 product g 
                 % 
               
               
                   
               
               
                 during 
                 330 
                 8.1 
                 30 
                 1000 
                 153 
                 95 
               
               
                 reaction 
               
               
                 after 
                 330 
                 8.1 
                 30 
                 1000 
                 534 
                 76 
               
               
                 reaction 
                   
                   
                   
                   
                   
               
               
                 total 
                 — 
                 — 
                 — 
                 — 
                 687 
                 — 
               
               
                   
               
             
          
         
       
     
     After the reaction, the pressure in the reactor  10  was reduced and the reaction products were discharged for recovery, and the discharge operation was performed while the pressure in the reactor  10  was intermittently reduced to the atmospheric pressure. Thus, the total amount of recovered solid product was increased to 687 g. Then, the amount of monomer components contained in the residue in the reactor  10  was very small of 40 g. Additionally, deterioration such as carbonization was not occurred in the solid product. Therefore, the decomposition reaction could continue by adding the PET and methanol. 
     EXAMPLE 2 
     According to the First Embodiment 
     The reaction was carried out in the same way as Example 1 except that ethanol was used as the monatomic alcohol in stead of the methanol and except that the pressure in the reactor (autoclave) 10 was 8.0 MPa. 
     The above reaction products were classified into a solid product, ethylene glycol, and ethanol by means of filtering and distillation. Then, dimethyl terephthalate was quantitatively determined as the monomer component with a gas chromatography. 
     The results are shown in Table 2. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Continuous Type (ethanol was fed with the rate of flow of 1000 g/hr) 
               
             
          
           
               
                   
                 reaction 
                   
                   
                 amount 
                   
                   
               
               
                   
                 temper- 
                 reaction 
                 reaction 
                 of fed 
                 amount of 
                 monomer 
               
               
                   
                 ature 
                 pressure 
                 time 
                 PET 
                 recovered 
                 purity 
               
               
                 discharge 
                 ° C. 
                 MPa 
                 min. 
                 g 
                 product g 
                 % 
               
               
                   
               
               
                 during 
                 330 
                 8.0 
                 30 
                 1000 
                 145 
                 91 
               
               
                 reaction 
               
               
                 after 
                 330 
                 8.0 
                 30 
                 1000 
                 508 
                 73 
               
               
                 reaction 
                   
                   
                   
                   
                   
               
               
                 total 
                 — 
                 — 
                 — 
                 — 
                 653 
                 — 
               
               
                   
               
             
          
         
       
     
     As shown in Table 2, 145 g of solid product was recovered during the reaction and its monomer purity was 91%. After the reaction, the pressure in the reactor  10  was reduced and 508 g of solid product could be recovered again and its monomer purity is 73%. 
     Comparative Example 1 
     For comparison with Examples 1 and 2, experiments were performed with known batch type reactions. In each experiment, first, 50 to 1000 g of PET and 650 to 700 g of methanol were fed into the reactor (autoclave) having the content volume of 5 lit. Then, the reactions were carried out at the reaction temperature of 300 to 330° C. and the pressure of 8.1 MPa for the reaction time of 15 to 120 minutes. After each reaction, the reactor  10  was cooled and opened so as to recover the reaction products. 
     In each experiment, the amount of recovered reaction products was measured. Then, the effects of variation in reaction time on the amount of recovered products and on the monomer purity were checked. Further, the effect of variation in amount of fed PET and the effect of variation in reaction temperature on the amount of recovered products were also checked. The results are shown in Table 3. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Batch Type (amount of fed methanol was 650 to 700 g) 
               
             
          
           
               
                   
                 reaction 
                   
                   
                 amount 
                   
                   
               
               
                 Experi- 
                 temper- 
                 reaction 
                 reaction 
                 of fed 
                 amount of 
                 monomer 
               
               
                 ment 
                 ature 
                 pressure 
                 time 
                 PET 
                 recovered 
                 purity 
               
               
                 No. 
                 ° C. 
                 MPa 
                 min. 
                 g 
                 product g 
                 % 
               
               
                   
               
             
          
           
               
                 1 
                 300 
                 8.1 
                 15 
                 500 
                 48 
                 9 
               
               
                 2 
                 300 
                 8.1 
                 30 
                 500 
                 161 
                 51 
               
               
                 3 
                 300 
                 8.1 
                 60 
                 500 
                 252 
                 49 
               
               
                 4 
                 300 
                 8.1 
                 120 
                 500 
                 208 
                 50 
               
               
                 5 
                 300 
                 8.1 
                 30 
                 1000 
                 168 
                 37 
               
               
                 6 
                 330 
                 8.1 
                 30 
                 500 
                 266 
                 46 
               
               
                   
               
             
          
         
       
     
     As shown in Table 3, by the increase of reaction time, once it exceeded 30 minutes, the amount of recovered products and monomer purity were not improved greatly. Further, both of variation in amount of fed PET and variation in reaction temperature did not give large effect on the amount of recovered products. 
     Comparing with the continuous type in Example 1, in each experiment, the amount of recovered product was decreased by about or more than 50%, and its monomer purity was decreased by about or more than 25%. Additionally, it took few hours to increase the temperature so as to reach the reaction temperature and it took also few hours to cool the reactor  10  for opening the reactor. Therefore, in this comparative example, the PET decomposing process was carried out with extremely low efficiency. 
     Reference Example 1 
     In the configuration shown in FIG. 1, an autoclave having the content volume of 5 lit. was used as the reactor (autoclave). Then, the temperature in the autoclave was set to be 603 K. First, 1000 g of melted PET was fed into the autoclave. Then, methanol was continuously fed to the autoclave with the flow rate of 1000 g/hr. During this operation, the reaction products were continuously discharged together with the super critical methanol from the upper portion of reactor  10  so that the pressure in the reactor  10  is 4.0 MPa. 
     The reaction was carried out for 30 minutes in the same manner as Example 1. Then, after the reaction, the pressure in the reactor  10  was reduced so as to reach the atmospheric pressure for the discharge of reaction products. The results are shown in Table 4. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Continuous Type (methanol was fed with the flow rate of 1000 g/hr) 
               
             
          
           
               
                   
                 reaction 
                   
                   
                 amount 
                   
                   
               
               
                   
                 temper- 
                 reaction 
                 reaction 
                 of fed 
                 amount of 
                 monomer 
               
               
                   
                 ature 
                 pressure 
                 time 
                 PET 
                 recovered 
                 purity 
               
               
                 discharge 
                 ° C. 
                 MPa 
                 min. 
                 g 
                 product g 
                 % 
               
               
                   
               
               
                 during 
                 330 
                 4.0 
                 30 
                 1000 
                 224 
                 73 
               
               
                 reaction 
               
               
                 after 
                 330 
                 4.0 
                 30 
                 1000 
                 314 
                 78 
               
               
                 reaction 
                   
                   
                   
                   
                   
               
               
                 total 
                 — 
                 — 
                 — 
                 — 
                 538 
                 — 
               
               
                   
               
             
          
         
       
     
     As shown in Table 4, 224 g of products was recovered during the reaction and its monomer purity is 73%, and after the reaction, 314 g of products was recovered and its monomer purity was 78%. Comparing with Example 1, the total amount 538 g of recovered products was smaller than that of Example 1 by 149 g, further, the monomer purity of the recovered solid products was decreased by about 22%. According to this reference example, it was ensured that such decrease was caused by the low pressure of 4.0 MPa in the reactor  10 . 
     Reference Example 2 
     The reaction was performed in the configuration shown in FIG. 1, but in this reference example, any temperature detecting means is not provided. An autoclave having the content volume of 5 lit. was used as the reactor (autoclave)  10 . First, 1600 g of PET was fed into the reactor. Then, methanol was continuously fed into the autoclave with the flow rate of 1000 g/hr. During this operation, the super critical methanol and decomposed product were continuously discharged from the upper portion of the reactor  10 . When about 30 minutes passed since the reaction was started, not-yet-reacted PET was attached and accumulated on the internal surface of the upper flange (cover) of the reactor  10  so that the stirrer could not rotate, whereby the operation could not continue. 
     EXAMPLE 3 
     In the configuration shown in FIG. 2, an autoclave having the content volume of 5 lit. was used as the reactor (autoclave)  10 . The temperature in this autoclave was set to be 603 K First, 1000 g of collected PET flakes, which included 10 ppm of ash and 60 ppm of label material, was melted and fed into the reactor. Then, methanol was fed continuously to the reactor. During this operation, the pressure in the reactor  10  was reduced to 8.1 MPa, and the discharge from the methanol phase located at the upper portion of the reactor  10  was performed continuously. 
     PET was always fed into the reactor  10  so that the level of the PET phase could be kept to be constant. Further, every 24 hours, PET and impurities contained in the reactor  10  were discharged, via the discharge line from the bottom of the reactor, so as to prevent impurities from contaminating into the discharged decomposed product. The reaction products discharged from the reactor  10  were introduced into the oligomer separator  26  which was kept to be at the temperature of 573 K and at the pressure of 7.0 MPa. In the separator  26 , the oligomer components were precipitated and separated from the introduced reaction products, and were recovered from the bottom of the separator for recycling to the reactor  10 . 
     On the other hand, the reaction products were discharged from the oligomer separator  26 . Next, heat exchange was carried out at the heat exchanger  7  between the reaction products and the methanol to be fed. After that, the reaction products were recovered in the flash tank  30 . 42 mol of methanol was communicated per 1 mol of decomposed product (dimethyl terephthalate). The reaction products recovered from the flash tank  30  were separated and refined by this communicated methanol. Thus, 1270 g of dimethyl terephthalate and 406 g of ethylene glycol were obtained per 1 hour. According to the analysis of this dimethyl terephthalate, purity was equal to or more than 99% and the ash content was equal to or smaller than 1 ppm. 
     Comparative Example 2 
     An autoclave having the content volume of 5 lit. was used as the reactor (autoclave)  10 . The temperature in this autoclave was set to be 603 K. The collected PET flakes being similar to that of Example 3 were used as the material. That is to say, the material contained 10 ppm of ash and 60 ppm of label material. First, 1000 g of collected PET flakes were melted and fed into the reactor. Then, methanol was fed continuously into the reactor. During this operation, the pressure in the reactor  10  was reduced to 8.1 MPa, and the continuous discharge operation from the methanol phase located at the upper portion of the reactor  10  was performed. 
     PET was always fed into the reactor  10  so that the level of the PET phase could be kept to be constant. The reaction products, which had been discharged from the reactor  10 , were recovered in the flash tank. 10 mol of methanol was communicated per 1 mol of decomposed product (dimethyl terephthalate). The reaction products recovered from the flash tank  30  were separated and refined by this communicated methanol. Thus, 306 g of dimethyl terephthalate and 98 g of ethylene glycol were obtained per 1 hour. According to the analysis of this dimethyl terephthalate, the oligomer components were contained and its monomer purity was 95 % and the ash content was 40 ppm. Additionally, in this comparative example, the temperature of the methanol to be fed to the reactor  10  was risen. Hence, the power consumption was increased so as to be larger by 20% than that of the case where heat exchange was carried out. 
     As stated above, according to the present invention, the recovery, separation, and the like of monomer components can be simply carried out under industrially advantageous conditions, and the monomer components can be continuously produced from aromatic polyester with high efficiency.