STUDIES ON PROCESS OPTIMIZATION FOR CATALYTIC GLYCOLYSIS OF
POST-CONSUMER PET WASTES
I. Duque-Ingunza, R. López-Fonseca, B. de Rivas-Martín, J.I. Gutiérrez-Ortiz*
Department of Chemical Engineering,
Faculty of Science and Technology, Universidad del País Vasco/EHU, UPV/EHU, E-48080, Bilbao, Spaine-mail: joseignacio.gutierrez@ehu.es, +34946012683

Abstract
Glycolysis has been the subject of increased interest as a valuable feedstock recycling for poly(ethylene terephthalate).
However, there are no reports on the technical literature that deal with the design and optimization of the global process. In
this study, the global glycolysis process was optimized in terms of recycling of the unconverted residue, recovery of EG, theamount of water required for crystallization and eventual glycolysis of complex PET wastes. The conversion into BHET(monomer) of the non-glycolyzed solid was feasible and could be carried out in the presence or absence of fresh PET. The
yield varied between (63-80%). The monomer could be recovered by extraction with hot water followed by crystallization at4 ºC and the optimized water:BHET ratio was 6.7 mL g-1. The ethylene glycol/water was effectively separated by vacuum
distillation and ethylene glycol of 99.6% v/v purity was recovered. Thus, it could be used in a new glycolysis process. Finally,
the highly active catalytic role of sodium carbonate salt for glycolysis was evident not only for transparent PET wastes but
also for complex wastes. The global glycolysis process was designed and optimized in order to increase the global efficiency
thereby improving the economical viability of the recycling technology.

Keywords: PET wastes, recycling, glycolysis, process optimization, bis(2-hydroxyethyl) terephthalate

1. Introduction
The principle of chemical recycling is to convert highmolecular weight polymers into low molecular weightsubstances via chemical reaction. PET can be
depolymerised into the monomer, bis(hydroxyethyl)
terephthalate (BHET), via glycolysis reaction in excess
ethylene glycol (EG) [1]. The aim of this study was tooptimize the global ecofriendly recycling process of PET
extending our previous studies on the chemicalvalorization of post-consumer PET wastes by glycolysis[2,3], with the specific objectives of minimizing the massand energy requirements of the reaction and separation
processes and reducing the generated solid and liquidwastes.

magnetic resonance. All these techniques revealed thatthe recovered product was highly pure [2].
Afterwards, the global glycolysis process was optimized
in terms of recycling of the unconverted residue, recoveryof EG, the amount of water required for crystallizationand eventual glycolysis of complex PET wastes(Figure 1).

2. Results and Discussion
In this work, glycolysis experiments were carried out in afour-necked double-walled reaction vessel, equipped witha stirrer, a thermometer and a reflux condenser operated
at atmospheric pressure. Sodium carbonate wasselected as transesterification catalyst. The following
operation conditions were used: 196 ºC, PET:EG molar
ratio 1:7.6, PET:Na2 CO3 molar ratio 100:1 and 1 h. The
obtained monomer was separated by means of twoconsecutive steps (extraction with hot water followed bycrystallization), quantified by gel permeation
chromatography (GPC) and characterized by elemental
analysis, differential scanning calorimetry, infrared
spectroscopy, hydroxyl group determination, and nuclear

Figure 1. Scheme of the global recycling process.
When the equilibrium conversion was achieved at 196 ºC(80%) the solid residue obtained after the first filtration
step was composed by the dimer along with small
amounts of oligomers (n<6), as revealed by GPC [3].
This was reasonably expected to be a good raw material
for glycolysis. On one hand, it was found the reaction
rate was higher with increasing amount of partiallyglycolyzed residue in the reactor mixture, thereby
suggesting a higher reactivity of the feed. On the otherhand, the maximum equilibrium conversion decreased(from 80 to 64%) probably due to an accumulation ofimpurities present in the fed residue. These results
regarding the optimized use and exploitation of PET

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013

wastes gave valuable information about the way of
reintroducing unconverted PET into the reaction process.
In all the cases a purge would be necessary in order toavoid the accumulation of impurities in the global system.
The filtrate obtained from the BHET crystallization mainlycontained unreacted EG and water. Since a high yield toBHET requires an excess of EG, it is of great importanceto recover EG and re-use it for glycolysis. The EG wasseparated by vacuum distillation. Hence, ethylene glycolof 99.6% v/v purity was recovered and fed again in thereactor. The global cycle (glycolysis, extraction, anddistillation) was repeated eleven times. It was found thatconversion was virtually constant, around 80%. Although
this observation could lead to assume that recovered EGis as reactive as fresh EG, it has to be pointed out thatthese results may be affected to an increased catalyst
concentration during the consecutive reactions. In thissense, some additional experiments to clarify the intrinsic
reactivity of recovered EG were performed. Theseexperiments were focused on an eventually thermal
degradation of the glycol during extended time onreaction at high temperatures. It was observed that thereaction rate was negatively affected when aged EG wasused. However, in view of the yield reached after 1.5 h all
the reactions carried out showed the same value(equilibrium conversion), probably owing to the excess ofglycol used in all these experiments. These results
therefore evidenced the recyclability of the EG in theglycolysis reaction.
The Na content in different streams of the global process
was measured by ICP-AES. It must be pointed out thatthe presence of organic compounds in the sample
interfered in the analysis and did not allow an accuratequantification of the sodium concentration. Therefore, theobtained values of sodium concentration were taken as aqualitative approximation. It was found that the catalyst
used in the reaction was mainly present in the filtratefrom extraction and crystallization and in the EG solution
after distillation.
Next, experiments were focused on the BHET/H2O
separation process, more specifically on thecrystallization step. The scope was to minimize thequantity of water required for the complete extraction of
the obtained BHET. The degree of recovery by extraction
notably increased up to a H2O:BHET ratio close to
6.7 mL g-1. Above this value it remained constant (99%),
while the global degree of recovery also increased up tothis value but decreased for higher quantities of water.
As the last step of this study, the glycolysis of complex(highly colored and multi-layered) PET wastes wasperformed. This task is of great relevance given theincreased presence of this type of wastes in recycling
plants. These fractions are not appropriate for amechanical recycling and up to now their management
can be only focused on incineration or landfill. The scopeof this study was to evaluate the affinity of complex PET
wastes for glycolysis in order to extend the versatility ofthe proposed recycling process. A set of glycolysis runs

was carried out with these complex wastes. Within
experimental error, yields were between 70 and 80%,very close to the yield obtained with clear PET.
Apparently the coloring compounds did not play any
chemical role in the process and the recovered monomer
was highly pure as determined by various analyticaltechniques (GPC, DSC and FTIR). However, they tended
to discolor the glycolyzed products and a further
purification process of the BHET would be necessary toextract the pigments.

3. Conclusions
Glycolysis can be described as a moleculardepolymerization process by transesterification between
PET ester groups and a diol (ethylene glycol) to obtainthe monomer bis(2-hydroxyehtyl) terephthalate (BHET).
The monomer was separated by means of twoconsecutive steps, extraction with hot water followed bycrystallization.
The global glycolysis process was optimized in terms ofrecycling of the unconverted residue, recovery of EG, theamount of water required for crystallization and eventual
glycolysis of complex PET wastes.
The recirculation of the solid residue from extraction, theseparation of the ethylene glycol and water mixture fromthe crystallization and the subsequent refeeding of theorganic reactant in the depolymerization reaction, theminimization of the water amount used to recover all themonomer obtained in the glycolysis reaction and thesuitability of the chemical recycling of PET of a more
complex nature, are operational aspects whichsignificantly increased the global efficiency and therefore
improve the economical viability of the recycling process.

Acknowledgements
The financial support for this work provided by theSpanish Ministerio de Ciencia e Innovación (CTQ200806868-C02) and Gobierno Vasco (S-PE12UN023) isgratefully acknowledged. Also the supply of postconsumer waste samples of PET by ECOEMBES andthe human and technical support from SGIker(UPV/EHU) are also gratefully acknowledged.

References[1] SR. Shukla and KS. Kulkarni. Appl. Polym. Sci. 85(2001) 1765-1770.
[2] R. López-Fonseca, I. Duque-Ingunza, B. de Rivas, S.
Arnaiz and JI. Gutiérrez-Ortiz. Polym. Degrad. Stab. 95(2010) 1022-1028.
[3] R. López-Fonseca, I. Duque-Ingunza, B. de Rivas, L.
Flores-Giraldo and JI. Gutiérrez-Ortiz. Chem. Eng. J. 168(2011) 312-320.

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013