Method and apparatus for color removal from PET-flake (shredded PET)
Organic colorants incorporated in colored PET-flake, as being obtained by shredding PET bottles, can be extracted from the PET-flake by extracting the PET-flake with ethylene glycol (EG) at ambient pressure and at the boiling temperature of EG. Pre-treating of PET-flake in EG or other suitable organic compounds prior to extraction enhances the discoloration.
1. Method for pre-processing PET-flakes prior to depolymerization by glycolysis, methanolysis, hydrolysis, saponification, and/or any combination of these PET depolymerization processes, the method comprising:
pretreating the PET flakes by immersing the PET flakes, in a pretreatment vessel, in an liquid organic solvent comprising at least one compound selected from the group consisting of: polyethylene glycol, ethylene glycol, triethylene glycol, and/or tetraethylene glycol, wherein the color of the pretreated PET flakes remains substantially the same;
removing the colored pretreated PET from the pretreatment vessel;
extracting organic colorants from the colored pretreated PET-flakes and simultaneous embrittlement of the pretreated PET-flakes by:
contacting the pretreated PET-flakes with vaporized ethylene glycol in a contacting vessel at an extraction temperature T ext of between 192° C. and 205° C. (192° C.≦T ext ≦205° C.), and
removing used ethylene glycol from the contacting vessel and adding fresh ethylene glycol to the contacting vessel.
2. Method of claim 1 , wherein the pretreated PET-flakes are contacted with the ethylene glycol at ambient pressure.
3. Method of claim 1 , wherein said extracting of organic colorants is stopped if the color extracted PET-flakes show at least one property selected from the group consisting of:
an intrinsic viscosity IV<0.20 dL/g;
a molar mass Mn with 3000 g/mol≦Mn≦8000 g/mol; and
a degree of polymerization Pn<20.
4. Method of claim 1 , further comprising conveying the pre-treated PET-flakes in a PET conveying direction and simultaneously contacting the pre-treated PET-flakes with vaporized ethylene glycol.
5. Method of claim 4 , wherein the pre-treated PET-flakes are contacted with a counter flow of vaporized ethylene glycol being directed against the PET conveying direction.
6. Method of claim 4 , where the contacting vessel comprises a screw conveyor, and the extracting step comprises establishing a counter flow of vaporized ethylene glycol in the screw conveyor.
7. Method of claim 1 , further comprising heating the pre-treated PET-flakes in the contacting vessel by heat transfer from ethylene glycol vapor as it condenses to liquid ethylene glycol.
8. Method of claim 1 , wherein the extracting step occurs at an extraction temperature T ext of between 195° C. and 200° C. (195° C.≦T ext ≦200° C.).
9. Method of claim 8 , wherein the extracting step occurs at an extraction temperature T ext of between 196° C. and 199° C. (196° C.≦T ext ≦199° C.).
10. Method of claim 9 , wherein the extracting step occurs at an extraction temperature T ext of between 197.0° C. and 197.6° C. (197.0° C.≦T ext ≦197.6° C.) at ambient pressure p a (±200 hPa).
11. Method of claim 1 , wherein a temperature T pre of the organic solvent while immersing the untreated PET-flakes is above a glass transition temperature T gl of the PET-flakes and below a melting point temperature T melt of the PET-flake (T gl ≦T pre ≦T melt ).