Method for depolymerising a polyester comprising polyethylene terephthalate by recirculating an oligomer effluent

Process for depolymerizing a polyester feedstock comprising PET, said process comprising:

• a) a conditioning step, fed at least by said polyester feedstock, to produce a conditioned feedstock stream;
• b) a depolymerization step implementing a first reaction section (A) and at least one second reaction section (B), said at least one second reaction section (B) operating at a temperature strictly lower than the temperature of said first reaction section (A), the first reaction section (A) being fed at least by the conditioned feedstock stream and optionally by a first diol makeup, said at least one second reaction section (B) being fed by the effluent of the first reaction section, and by a recycled oligomers effluent and optionally by a second diol makeup, so that the total amount of diol feeding said step b) is adjusted to 1 to 20 moles of diol per mole of diester feeding said step b), said step b) being operated at a temperature of between 18° and 300° C. and with a residence time of between 0.334 and 10 h;
• c) a step for separating the diol fed at least by the effluent of step b), operated at a temperature of between 10° and 250° C., at a pressure lower than that of step b) and producing a diol effluent and a liquid monomer-rich effluent;
• d) a step for separating the liquid monomer-rich effluent from step c) into a heavy impurities effluent and a pre-purified monomers effluent, operated at a temperature of less than or equal to 250° C. and a pressure of less than or equal to 0.001 MPa with a liquid residence time of less than or equal to 10 min,
• e) a step for separating said heavy impurities effluent into two fractions:
• a first fraction (6) of which at least 70% by weight compose the recycled oligomers effluent which feeds step b) and a second fraction (7) which is at least partially discharged from the process;
• f) optionally, a step for decolourizing the pre-purified monomers effluent, operated at a temperature of between 10° and 250° C. and at a pressure of between 0.1 and 1.0 MPa in the presence of an adsorbent, and producing a purified monomers effluent,
• g) optionally, a step for crystallizing the purified monomers effluent, employing at least one solid production section, operated at a temperature between 0 and 100° C. and at a pressure of between 0.00001 and 1.00 MPa, followed by a solid-liquid separation section, producing a decolourized and purified monomer effluent.