Process for the depolymerization of a polyester comprising opaque polyethylene terephthalate
A subject-matter of the invention is a process for the depolymerization of a polyester feedstock comprising opaque PET, the said process comprising at least the stages of conditioning, of depolymerization and of separation of the diol and of separation of the liquid effluent rich in monomers, followed by a decolouration stage.
1. A process comprising depolymerization of a polyester feedstock comprising opaque PET, the said feedstock comprising from 0.1% to 10% by weight of pigment, the said process comprising at least:
a) conditioning the polyester feedstock;
b) depolymerization by glycolysis of the effluent from a) with diol, carried out at a temperature of between 200 and 400° C., with from 1 to 20 mol of diol per mole of diester in the said polyester feedstock and a residence time of the polyester of between 0.1 and 5 h, converting the PET into BHET monomer and BHET oligomers;
c) separation of the diol from the effluent from b), carried out at a temperature of between 100 and 250° C., at a lower pressure than that of b) and producing a diol effluent and a liquid effluent rich in monomers, carried out in 1 to 5 successive gas/liquid separation sections, the liquid effluent from a preceding section feeding a subsequent section, all of the gas effluents being condensed to form the diol effluent, a liquid effluent resulting from the final gas/liquid separation section comprising the liquid effluent rich in monomers;
d) separation of the liquid effluent rich in monomers resulting from c) into a heavy impurities effluent and a prepurified monomers effluent carried out at a temperature of less than 250° C. and a pressure of less than 0.001 MPa with a liquid residence time of less than 10 min; and
e) decoloration of the prepurified monomers effluent, carried out at a temperature of between 100 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.
2. The process according to claim 1 , in which the said polyester feedstock comprises at least 10% by weight of opaque PET.
3. The process according to claim 1 , in which a) is carried out at a temperature of between 225 and 275° C.
4. The process according to claim 1 , in which the said stage a) comprises an extrusion section.
5. The process according to claim 1 , in which the said polyester feedstock is brought into contact with at least a fraction of the diol effluent resulting from c) in a).
6. The process according to claim 1 , in which b) is carried out in the presence of a solid adsorbent.
7. The process according to claim 1 , in which b) is carried out in the presence of a heterogeneous catalyst comprising at least 50% by weight, with respect to the total weight of the catalyst, of a solid solution composed of at least one spinel of formula Z x Al 2 O (3+x) in which x is between 0 (limit excluded) and 1 and Z is Co, Fe, Mg, Mn, Ti or Zn, and comprising at most 50% by weight of alumina and of oxide of the element Z.
8. The process according to claim 1 , in which a fraction of the diol effluent resulting from c) is recycled to b).
9. The process according to claim 1 , in which d) is carried out at a pressure of less than 0.0005 MPa.
10. The process according to claim 1 , in which d) is carried out with a liquid residence time of less than 1 min.
11. The process according to claim 1 , in which a fraction of the said heavy impurities effluent is recycled to conditioning in a) and/or to the depolymerization b).
12. The process according to claim 11 , in which a fraction of the diol effluent resulting from c) is mixed with the heavy impurities effluent resulting from d).
13. The process according to claim 1 , in which the prepurified monomer effluent resulting from d) is sent to a gas/liquid separation section, which separation is carried out at a temperature of between 100 and 250° C. and at a pressure of between 0.00001 and 0.1 MPa.
14. The process according to claim 1 , in which the purified monomer effluent feeds a polymerization producing PET.
15. The process according to claim 1 , wherein the feedstock comprises 0.25 to 10% by weight of pigment.