Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-11-22
2003-06-17
Keys, Rosalynd (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S485000, C562S486000
Reexamination Certificate
active
06580005
ABSTRACT:
DETAILED DESCRIPTION OF THE INVENTION
1. Field of the Invention
This invention relates to a process for technically recovering terephthalic acid as the material of polyethylene terephthalate (PET), from spent PET, which is used as bottles containing beverages and the like and also to a system for use in such process.
2. Background of the Invention
Several methods have been proposed in each of which, PET is decomposed for recovering monomer components therefrom, and thus recovered monomer components can be used as the material of PET. Among these conventional methods, typical ones are listed below.
(1) Methanolysis method where depolymerization is carried out for PET with methanol in vapor phase or liquid phase for producing dimethyl terephthalate;
(2) Glycolysis method where depolymerization is carried out for PET with ethylene glycol for utilizing an intermediate product, i.e., bis hydroxyethyl terephthalate as the, material of polymer; or Ester interchange method where dimethyl terephthalate is obtained from the product of the Glycolysis method, i.e., bis hydroxyethyl terephthalate with methanol; and
(3) Hydrolysis method where hydrolysis is carried out for PET with alkali solution so that the resulting metallic salt terephthalate is neutralized with acid for the crystallization of terephthalic acid.
However, the above methods have drawbacks respectively.
In the method (1) (Methanolysis method), its reaction temperature is so low of about 177° C., which requires a lot of reaction time.
The method (2) (Glycolysis method), like the method (1) (Methanolysis method), also requires a lot of reaction time. Additionally, it is difficult to depolymerize perfectly to recover the monomer components. Further, the intermediate product, i.e., bis hydroxyethyl terephthalate is partly dissolved into the ethylene glycol. Then, it is difficult to separate the dissolved bis hydroxyethyl terephthalate from the ethylene glycol, which results in poor yield.
In the method (2) (Ester interchange method), it is difficult to separate the ethylene glycol from the methanol. Further, since the dimethyl terephthalate is partly dissolved into the methanol, it is also difficult to separate the dissolved dimethyl terephthalate from the methanol.
In the method (3) (Hydrolysis method), it is more difficult to produce the terephthalic acid than the case of bis hydroxyethyl terephthalate and the case of dimethyl terephthalate. Further, the recovered terephthalic acid might be contaminated with various additives contained in the spent PET, which results in a problem.
To cope with the above problems, Japanese Published Unexamined Patent Application No. 286744/97 proposes a method, in which PET is decomposed with alkali in ethylene glycol.
However, the method disclosed in this patent publication has a problem because the terephthalic acid is not sufficiently recovered. Therefore, it is necessary to attain a method for recovering a polymer having high purity. Additionally, since these conventional methods are performed in pilot scales, there is an increased demand for establishment of practical and technical process of this kind.
SUMMARY OF THE INVENTION
It is therefore, the object of the present invention is to provide a technical and practical process of this kind with high recovery of terephthalic acid. The other objects of the present invention will be clear from the following explanation.
In accordance with the first aspect of the present invention, there is provided a process for recovering terephthalic acid from pulverized products of spent polyethylene terephthalate, the process comprising the following steps;
(1) a decomposition reaction step where the pulverized products of spent polyethylene terephthalate arm continuously subjected to decomposition reaction in ethylene glycol in the presence of alkali, which is equimolar or excessmolar relative to the polyethylene terephthalate, so that salt of terephthalic acid and ethylene glycol can be obtained;
(2) a solid-liquid separation, dissolution, impurities-removing step where the ethylene glycol is separated from decomposition reaction slurry of the salt of terephthalic acid and ethylene glycol, and the solid salt of terephthalic acid is dissolved into water and insoluble impurities are removed;
(3) a neutralization, crystallization step, where the resulting solution of salt of terephthalic acid is neutralized with acid so that terephthalic acid can be crystallized;
(4) a solid-liquid separation, washing step where the resulting slurr of terephthalic acid crystals is subjected to solid-liquid separation so that the terephthalic acid crystals can be obtained and washed; and
(5) a drying, pulverization step where the washed terephthalic acid crystals are dried and pulverized.
According to the first aspect of the present invention, the terephthalic acid having high purity can be technically recovered with high efficiency from the pulverized products of spent polyethylene terephthalate.
The alkali comprising mainly sodium carbonate can be used as the above alkali and sulfuric acid can be used as the above acid. Due to the alkali comprising mainly the sodium carbonate, carbon dioxide is generated by the decomposition reaction, thus inlet gas is not required. Further when the sulfuric acid is used as the acid, the sodium sulfate (mirabilite) can be crystallized by the neutralization and recovered.
Water contained in liquid obtained by separation in the step (4) (the solid-liquid separation, washing step) is vaporized so that the sodium sulfate can be crystallized and separated. The resulting steam is condensed for re-using as the water for dissolving the salt of terephthalic acid, whereby the amount of required water can be decreased.
In the step (2) (solid-liquid separation, dissolution, impurities-removing step), by utilizing a crossfeed belt type vacuum filtering dissolver, the separation of the ethylene glycol, the dissolving of the solid salt of terephthalic acid, and the removing of the insoluble impurities can be continuously carried out.
The process in accordance with the first aspect of the present invention might further comprise the following steps:
(6) a return step where the ethylene glycol, which is obtained by the solid-liquid separation in the above step (2) (solid-liquid separation, dissolution, impurities-removing step) is returned to the step (1) (decomposition reaction step); and
(7) a vaporization, crystallization, separation, return step where water contained in separation liquid obtained from the step (4) (the solid-liquid separation, washing step) is vaporized so that the salt can be crystallized and separated, then the reminder, i.e., the ethylene glycol is returned to the step (1) (decomposition reaction step).
By returning the ethylene glycol, the amount of required ethylene glycol can be decreased.
The process in accordance with the first aspect of the present invention might further comprise, between the step (2) (solid-liquid separation, dissolution, impurities-removing step) and step (3) (neutralization, crystallization step), the following step;
(8) soluble impurities-removing step where soluble impurities, which are contained in solution in the above step (2), can be continuously removed with an adsorbent packed adsorber.
As water used for the dissolution in the above step (2), wash water of the adsorber in the step (8) and/or condensate obtained by cooling steam in the step (7) can be re-used for saving water.
Between the above step (1) (decomposition reaction step) and step (2) (solid-liquid separation, dissolution, impurities-removing step), the decomposition reaction slurry can be passed through orifice means. Due to this orifice means, the ethylene glycol can be partly prevented from being passed to the step (2). This means that the slurry having small amount of ethylene glycol is passed to the step (2). Accordingly, load applied to the step (2) can be lowered.
The above orifice means might comprise: a tapered cylinder and a screw conveyor placed in the cylinder. The orifice means of this kind can be easily formed by providi
Funakoshi Nobuyuki
Sakano Kozaburo
Tanaka Kazuho
Yazaki Jinichi
Keys Rosalynd
Koda & Androlia
Tsukishima Kikai Co., Ltd.
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