Process for depolymerizing polyethylene terephthalate (PET) by glycolysis with ethylene glycol, and plant for carrying out the same
A process for depolymerizing a polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG) is described. Also described is a plant for carrying out the process. The PET is first mixed in the solid state with EG at a temperature of 60-120° C. to form a heterogeneous mixture with an EG:PET weight ratio of 0.1-3.0, and is then pressed to squeeze out an aliquot of EG. The compressed mixture is fed to a reactor, where the mixture is heated and kept under mixing at a temperature of 170-270° C., with an EG:PET weight ratio of 0.1-4.0, to glycolize the PET and obtain a glycolyzed product containing bis(2-hydroxyethyl) terephthalate and/or oligomers thereof. The process is advantageous in treating waste PET as such and reducing the EG to PET ratio necessary for glycolysis that allows the reaction to be carried out at relatively high temperatures of 230° C. and above to expedite the process.
1 . A process for depolymerizing a polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG), the process comprising:
(a) mixing the PET in a solid state with EG at a temperature of from 60° C. to 120° C. to obtain a heterogeneous mixture (A), and pressing the heterogeneous mixture (A) to squeeze out an aliquot of EG to obtain a heterogeneous mixture (B) with an EG:PET weight ratio R1 of from 0.1 to 3.0; and
(b) feeding said heterogeneous mixture (B) into a blender where the heterogeneous mixture (B) is heated and kept under mixing at a temperature of from 170° C. to 270° C., with an EG:PET weight ratio R2 of from 0.1 to 4.0, to glycolize the PET and obtain a glycolyzed product containing bis(2-hydroxyethyl) terephthalate (BHET) and/or oligomers thereof,
wherein (b) is carried out in an inert atmosphere.
2 . The process according to claim 1 , wherein in (a) the PET in the solid state is mixed with an amount of EG to initially obtain an EG:PET weight ratio R0 of from 0.2 to 4.0.
3 . The process according to claim 1 , wherein (a) is carried out in a screw press.
4 . The process according to claim 3 , wherein the screw press is arranged in an inclined configuration, so that an axis of development of the screw is inclined with respect to a horizontal plane.
5 . The process according to claim 1 , wherein (a) comprises:
(a1) mixing the PET in the solid state with EG at the temperature of from 60° C. to 120° C. to obtain the heterogeneous mixture (A); and
(a2) compressing the heterogeneous mixture (A) to squeeze out the aliquot of EG from the heterogeneous mixture (A) to obtain the heterogeneous mixture (B) with the EG:PET weight ratio R1 of from 0.1 to 3.0 in a screw press.
6 . The process according to claim 1 , wherein said feeding (b) is carried out by passing the heterogeneous mixture (B) through a transfer cell which collects a predetermined quantity of the heterogeneous mixture (B) obtained from (a) and transfers the same to the blender by gravity.
7 . The process according to claim 1 , wherein BHET and/or oligomers thereof are fed to the blender in (b), which are optionally recovered subsequently.
8 . The process according to claim 1 , wherein (b) is carried out in a paddle reactor.
9 . The process according to claim 1 , wherein the PET in the solid state is used in (a) as such.
10 . The process according to claim 1 , wherein the heterogeneous mixture (B) obtained in (a) has a water content equal to or lower than 0.5% by weight, with respect to the PET weight.
11 . The process according to claim 1 , further comprising:
filtering the glycolyzed product obtained from (b).
12 . The process according to claim 1 , further comprising:
(c) subjecting the glycolyzed product obtained from (b) to a further glycolysis to obtain a glycolyzed product with an EG:PET weight ratio R4 of from 0.5 to 10.0.
13 . The process according to claim 12 , wherein the further glycolysis (c) is carried out in a reactor for liquids.
14 . The process according to claim 12 , wherein the further glycolysis (c) is carried out in a plurality of stirred tank reactors arranged in series to form a multistage reactor which operates continuously.
15 . The process according to claim 12 , wherein the glycolyzed product obtained in (c) is subjected to a treatment of removing possible solid foreign materials having a density lower than the glycolyzed product from the surface of the glycolyzed product.
16 . The process according to claim 15 , wherein solid foreign materials removed from the surface of the glycolyzed product are cooled to a temperature of about 90° C.-130° C.
17 . A process for depolymerizing a polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG), the process comprising:
(a) mixing the PET in a solid state with EG at a temperature of from 60° C. to 120° C. to obtain a heterogeneous mixture (A), and pressing the heterogeneous mixture (A) to squeeze out an aliquot of EG to obtain a heterogeneous mixture (B) with an EG:PET weight ratio R1 of from 0.1 to 3.0;
(b) feeding said heterogeneous mixture (B) into a blender where the heterogeneous mixture (B) is heated and kept under mixing at a temperature of from 170° C. to 270° C., with an EG:PET weight ratio R2 of from 0.1 to 4.0, to glycolize the PET and obtain a glycolyzed product containing bis(2-hydroxyethyl) terephthalate (BHET) and/or oligomers thereof, and
(c) subjecting the glycolyzed product obtained from (b) to a further glycolysis to obtain a glycolyzed product with an EG:PET weight ratio R4 of from 0.5 to 10.0,
wherein the further glycolysis (c) is carried out in a plurality of stirred tank reactors arranged in series to form a multistage reactor which operates continuously.
18 . The process according to claim 1 , wherein the glycolyzed product obtained in (b) is subjected to a treatment of removing possible solid foreign materials having a density lower than the glycolyzed product from the surface of the glycolyzed product.
19 . The process according to claim 18 , wherein solid foreign materials removed from the surface of the glycolyzed product are cooled to a temperature of about 90° C.-130° C.