Process for chemical recycling of post consumption poly(ethylene terephthalate) and apparatus for chemical recycling of post consumption polyethylene terephthalate
View Patent ↗A process for the chemical recycling of post consumption polyethylene terephthalate which includes the steps of performing the depolymerization of polyethylene terephthalate based upon a hydrolysis reaction which causes a rupture in of the ester bonds formed between the polymer precursors of terephthalic acid and ethylene glycol at the time of polymerization to recover terephthalic acid and ethylene glycol. The advantages of the present process are the working conditions at low to moderate pressure, at temperatures of 215 to 450° C., allowing the process to reach the energy level required to achieve the desired polyethylene terephthalate hydrolysis and simultaneously promoting the separation and purification of the terephthalic acid and ethylene glycol formed in this hydrolysis reaction.
1. A continuous process for the chemical treatment of polyethylene terephthalate in the form of flakes granules or scraps to recover terephthalic acid and ethylene glycol which comprises
melting the polyethylene terephthalate at a temperature of 220 to 280° C. to form a molten liquid,
increasing the temperature of the molten, liquid polyethylene terephthalate to a temperature of 300 to 450° C. at ambient pressure, and
depolymerizing the molten liquid polyethylene terephthalate by introducing water to the molten, liquid polyethylene terephthalate, said water being converted to overheated steam, the overheated steam expanding to provide an intimate mixture thereof with the molten polyethylene terephthalate, thereby hydrolyzing the molten polyethylene terephthalate with simultaneous sublimation, whereby terephthalic acid and ethylene glycol are separated from unreacted molten liquid polyethylene terephthalate.
2. The continuous process of claim 1 , wherein the hydrolysis of the molten polyethylene terephthalate produces, as a vapor phase, a mixture of terephthalic acid vapor, ethylene glycol vapor and steam, and said vapor phase is separated from a remaining liquid phase of unreacted molten polyethylene terephthalate by said sublimation.
3. The continuous process of claim 1 , wherein during hydrolysis, the temperature of the unreacted molten liquid polyethylene terephthalate is increased during hydrolysis to 400 to 450° C.
4. The continuous process of claim 2 , wherein the liquid phase is partially discarded and the non-discarded fraction is recycled for further hydrolysis.
5. The continuous process of claim 1 , conducted at a pressure of from ambient pressure to 10 atm.
6. The continuous process of claim 1 , wherein the hydrolysis is facilitated by the addition of caustic soda, caustic potash, soda ash or an alkali.
7. The continuous process of claim 2 , wherein heating is applied to maintain a melted condition of the polyethylene terephthalate as the temperature increases from 215 to 450° C. but said temperature is controlled to prevent pyrolysis of the polyethylene terephthalate, said heating being conducted for a period of 1 to 16 minutes.
8. The continuous process of claim 1 , wherein depolymerization is facilitated by the addition of an organic acid or an inorganic acid.
9. The continuous process of claim 1 , wherein mineral oils or synthetic oils are added to facilitate polyethylene terephthalate depolymerization.
10. The continuous process of claim 1 , wherein nitrogen or carbonic acid are added to the process during the depolymerization step.
11. The continuous process of claim 1 , wherein microwave radiation or radio-frequency energy is used to supply the heat for polyethylene terephthalate depolymerization.