Process for recovering terephthalic acid from pulverized product of spent polyethylene terephthalate and system for use in such process

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 are 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 step where said ethylene glycol is separated from decomposition reaction slurry of said salt of terephthalic acid and ethylene glycol;
• (3) a dissolution step where said salt of terephthalic acid, which is obtained by separating said ethylene glycol from said slurry, is dissolved into water;
• (4) a neutralization, crystallization step where resulting solution of salt of terephthalic acid is neutralized with acid so that terephthalic acid can be crystallized;
• (5) a solid-liquid separation, washing step where said terephthalic acid crystals are separated from said slurry of terephthalic acid crystals and said terephthalic acid crystals are washed;
• and
• (6) a drying and pulverization step where said washed terephthalic acid crystals are dried and pulverized.

A system recovering terephthalic acid from pulverized products of spent polyethylene terephthalate, said system comprising:

• a horizontal type decomposition reaction chamber;
• stirring means for stirring solvent, alkali, and said pulverized products of spent polyethylene terephthalate, which are charged into said decomposition reaction chamber;
• heating means for heating the same;
• a vacuum chamber, which is communicated with the upper portion of said decomposition reaction chamber;
• and evacuating means, which is connected to said vacuum chamber, and
• said solvent contained in said decomposition reaction chamber being removed through said vacuum chamber, and solid salt of terephthalic acid, which is obtained from said decomposition reaction chamber, being used for recovering said terephthalic acid.

A system recovering terephthalic acid from pulverized products of spent polyethylene terephthalate, said system comprising:

• a decomposition reactor where said pulverized products of spent polyethylene terephthalate are subjected to thermal cracking in the presence of solvent and alkali so that salt of terephthalic acid and ethylene glycol can be obtained;
• and
• an evaporator having a heating surface kept at the temperature equal to or higher than the boiling point of the above solvent;
• and falling means, by which said thermal cracking slurry is fallen downward to said heating surface;
• and
• said solvent being vaporized in said evaporator and recovered for returning to said decomposition reactor.

A process for recovering terephthalic acid from pulverized products of spent polyethylene terephthalate, the process comprising:

• subjecting said pulverized products of spent polyethylene terephthalate to thermal cracking in the presence of alkali so that salt of terephthalic acid and ethylene glycol can be obtained;
• removing said ethylene glycol from thermal cracking slurry so that solid salt of terephthalic acid can be obtained;
• dissolving said solid salt of terephthalic acid into water so that solution of salt of terephthalic acid can be obtained;
• neutralizing said solution of salt of terephthalic acid by adding acid so that terephthalic acid can be crystallized;
• and recovering said terephthalic acid and
• said acid being added in multiple stages, while the amount of added acid is determined so that the pH value of solution in the final stage is adjusted to 2 to 4 and
• terephthalic acid crystals obtained from said final stage being returned to said previous stage for dissolving.

A process for recovering terephthalic acid from pulverized products of spent polyethylene terephthalate, the process comprising:

• subjecting said pulverized products of spent polyethylene terephthalate to thermal cracking in the presence of alkali so that salt of terephthalic acid and ethylene glycol can be obtained;
• removing said ethylene glycol from thermal cracking slurry so that solid salt of terephthalic acid can be obtained;
• dissolving said solid salt of terephthalic acid into water so that solution of salt of terephthalic acid can be obtained;
• neutralizing said solution of salt of terephthalic acid by adding acid so that terephthalic acid can be crystallized;
• and recovering said terephthalic acid and
• said acid being added in multiple stages, while the amount of added acid is determined so that the pH value of solution in the final stage is adjusted to 2 to 4 and
• said solution in said final stage being fed into a classifier for classifying said solution and classified terephthalic acid fine crystals are returned to previous stage together with mother liquor.