Reactive Crystallization of Terephthalic Acid towards Recycling of Poly(ethylene terephthalate) (Volume 51) (English shop)

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This work presents investigations into the reactive crystallization of terephthalic acid (TA) within the framework of back-to-monomer recycling, which offers a means of overcoming the current limitations of thermomechanical polyethylene terephthalate (PET) recycling. One approach is alkaline hydrolysis, in which PET is selectively depolymerized with sodium hydroxide into the monomers ethylene glycol and disodium terephthalate. Subsequently, TA is precipitated from its disodium salt by acid addition and thus recovered.

To optimize precipitation as a recovery step, various operating parameters were analyzed with regard to their effect on crystal size, filterability and purity of the TA. For this purpose, a parameter screening was first carried out with model solution in batch operation. Here, elevated temperatures and the use of acetic acid in particular led to larger crystals and improved filterability. Despite its lower protonation capacity, a significant yield of TA was also achieved with acetic acid. The investigations were then extended to depolymerized PET from different waste fractions and to the purification potential of the precipitation. In addition to a reduction in discoloration at higher temperature, the isophthalic acid content in the TA crystals was also reduced, which was particularly evident when acetic acid was used. This is especially relevant since isophthalic acid has a decisive influence on the polymer properties.

In the last part of the work, the investigations were transferred to a continuous stirred tank. During precipitation with sulfuric acid, growth of the TA crystals accompanied by improved filterability could thus be achieved. Furthermore, an increase in the pH value induced a significant reduction of the isophthalic acid content. Finally, a process concept for an optimized precipitation step for TA was derived with regard to purity and recovery, taking into account the necessity of a subsequent recrystallization step.

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