Process for depolymerizing a polyester comprising polyethylene terephthalate
A process for depolymerizing a polyester feedstock containing PET by, prior to the step of depolymerization by glycolysis and to the step of purification of the depolymerization effluent, an improved step of conditioning the feedstock in which the polyester feedstock is conditioned in terms of temperature and pressure and then mixed with a diol effluent in a static or dynamic mixer to substantially reduce the viscosity of the feedstock.
1 . A process for depolymerizing a polyester feedstock comprising PET, said process comprising:
a) a conditioning step implementing at least one conditioning section to produce a stream of conditioned feedstock, and a mixing section to produce a mixed stream,
wherein the mixed stream has a viscosity of 10 Pa·s,
wherein said conditioning section is fed at least with said polyester feedstock and is implemented in an extruder and at a temperature of 225 to 275° C.,
wherein said mixing section is at least fed with said stream of conditioned feedstock obtained from the conditioning section and a diol effluent comprising at least 95% by weight of ethylene glycol, and is operated in the extruder, at a temperature of 225 to 275° C., with a residence time of 0.5 second to 20 minutes, and wherein the weight ratio of diol relative to the polyester feedstock is 0.03 to 3.0;
b) a step of depolymerization by glycolysis, fed at least with the mixed stream and optionally with a diol supply, wherein the total amount of diol feeding said step b) is 1 to 20 mol of diol per mole of diester feeding said step b), performed at a temperature of 180 to 400° C., and a residence time of 0.1 to 10 hours;
c) a step of separating out the diol fed at least with the effluent from step b), performed at a temperature of 100 to 250° C., at a pressure below that of step b), and producing a diol effluent and an effluent rich in liquid monomers, wherein said diol separation step is performed in 1 to 5 successive gas-liquid separation sections, wherein each of the gas-liquid separation sections produces a liquid effluent and a gas effluent, wherein the liquid effluent from the preceding section feeding the next section, the liquid effluent obtained from the last gas-liquid separation section constituting the effluent rich in liquid monomers, the gas effluents all being recovered to constitute the diol effluent;
d) a step of separating the effluent rich in liquid monomers obtained from step c) into a heavy impurities effluent and a pre-purified monomers effluent, performed 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 minutes, wherein at least a fraction of the heavy impurities effluent is recycled directly into the reaction section of step b), and
e) a step of decolourizing the pre-purified monomers effluent, performed at a temperature of 100 to 250° C., and at a pressure of 0.1 to 1.0 MPa in the presence of an adsorbent and producing a purified monomers effluent.
2 . The process according to claim 1 , in which said polyester feedstock comprises at least 15% by weight of opaque PET.
3 . The process according to claim 1 , in which said polyester feedstock comprises at least 10% by weight of opaque PET.
4 . The process according to claim 1 , in which said polyester feedstock comprises coloured PET.
5 . The process according to claim 1 , in which said polyester feedstock comprises 0.1% to 10% by weight of pigments.
6 . The process according to claim 1 , in which said polyester feedstock comprises between 0.1% and 5% by weight of pigments.
7 . The process according to claim 1 , in which the conditioning section of step a) is implemented at a temperature of 225 to 250° C.
8 . The process according to claim 1 , in which the conditioning section of step a) is implemented at a temperature of 250 to 275° C.
9 . The process according to claim 1 , in which the conditioning section of step a) is implemented at a temperature of 250° C.
10 . The process according to claim 1 , in which the mixing section is operated at a temperature of 250° C.
11 . The process according to claim 1 , in which the mixing section is implemented with a residence time of 3 seconds to 1 minute.
12 . The process according to claim 1 , in which the diol effluent introduced into the mixing section of step a) is superheated prior to being introduced into the mixing section of step a).
13 . The process according to claim 1 , wherein the weight ratio of the diol relative to the polyester feedstock is 0.05 to 2.0.
14 . The process according to claim 1 , wherein the weight ratio of the diol relative to the polyester feedstock is 0.1 to 1.0.
15 . The process according to claim 1 , wherein the weight ratio of the diol weight relative to the weight of the polyester feedstock is 0.35 to 1.0.
16 . The process according to claim 1 , wherein during depolymerization, which occurs in one or more depolymerization reactors, said one or more reactors are stirred with a stirring power of less than 3000 W/m 3 .
17 . The process according to claim 1 , wherein during depolymerization, which occurs in one or more depolymerization reactors, said one or more reactors are stirred with a stirring power of 500 to 2000 W/m 3 .
18 . The process according to claim 1 , in which the diol effluent introduced into the mixing section of step a) is a fraction of the diol effluent obtained from step c).