(636e) Carbon Dioxide Induced Separation of Terephthalic Acid for Polyethylene Terephthalate (PET) Chemical Recycling

Polyethylene terephthalate (PET) chemical recycling by alkaline decomposition consists in reacting PET with a base, most commonly sodium hydroxide, to produce ethylene glycol and the corresponding terephthalate salt; in the case of sodium hydroxide, disodium terephthalate (Na₂TP) is produced. This depolymerization reaction is typically highly selective and it can be carried out under diverse conditions, including high temperatures in aqueous or non-aqueous media; with or without microwave assistance; enzymatically; or even through mechanochemical processes.

Traditionally, the recovery of the original monomer, terephthalic acid (H₂TP), from Na₂TP solutions involves the use of strong mineral acids such as hydrochloric or sulfuric acid via an acid-induced precipitation process. In pursuit of a potentially more sustainable alternative in this acidification step, this study explores replacing mineral acids with gaseous carbon dioxide (CO₂).

In aqueous solutions, Na₂TP dissociates into sodium ions (Na⁺) and terephthalate ions (TP⁻²). Upon introducing CO₂ under closed conditions with adequate mixing, simultaneous phase and chemical equilibrium is achieved. Part of the dissolved CO₂ is hydrated to carbonic acid (H₂CO₃), which further dissociates into bicarbonate (HCO₃^-) and/or carbonate (CO₃^-2) ions along with the release of acidic protons (H⁺). Terephthalic acid then precipitates from a solution containing Na⁺, H⁺, TP⁻², bicarbonate, and carbonate ions. The suspension that is formed is filtrated under pressure separating solid H₂TP from aqueous solutions containing mostly residual Na₂TP and the co-produced NaHCO₃.

Various experimental conditions were tested, revealing that lower Na₂TP concentrations, lower temperatures, and higher pressures enhance the terephthalic acid production from aqueous disodium terephthalate solutions. Both residual sodium hydroxide and ethylene glycol, that could remain from the depolymerization reaction, have little effect on the conversion of Na₂TP to H₂TP up to moderate loadings. Tests with actual depolymerization products as starting material showed similar H₂TP yields compared to those achieved with aqueous solutions of Na₂TP confirming these observations.

The extended UNIQUAC (eUNIQUAC) activity coefficient model was used to correlate the experimental data with good accuracy. Modeling results indicate that terephthalic acid recovery is limited by buffer effects caused by the presence of bicarbonate ions from CO₂ dissolution.

Alternatives to integrate this separation strategy in existing PET alkaline decomposition processes are discussed with emphasis on technoeconomic aspects and environmental impacts.