PET Depolymerization: A Novel Process For Plastic Waste Chemical Recycling
PET Depolymerization: A Novel Process For Plastic Waste Chemical Recycling
PET Depolymerization: A Novel Process For Plastic Waste Chemical Recycling
12
PET depolymerization: a novel process for
plastic waste chemical recycling
Maurizio Crippaa,*, Barbara Moricob
a
gr3n SAGL, Lugano, Switzerland; bNextChem srl, L’Aquila, Italy
* Corresponding author. e-mail address: Maurizio.crippa@gr3n-recycling.com
to form ester (COeO) groups, which serve as the Some changes in the chemical composition
chemical links joining multiple PET units can be introduced to develop specific properties
together into long-chain polymers. The overall for various packaging applications and to adapt
reaction can be represented as follows (Fig. 12.2): the polymer to particular production equipment.
As an alternative, a transesterification reac- Usually the modifications are of a chemical na-
tion allows starting from the apparently more ture to facilitate the manipulation of the PET
easily purifiable DMT, rather than from TA, to among different crystalline forms. For example,
produce PET. In this case, methanol is eliminated small concentrations of an appropriate como-
instead of water. nomer such as isophthalic acid (IPA) or
The extent of polymerization reaction, 1,4-cyclohexane-dimethanol reduce the rate of
apparent from the molecular weight of the poly- crystallization and allow the production of
mer, is a function of the polymerization condi- sheets and films of thicker bottles.
tions and significantly affects the properties of Once the polymer is formed, it is very difficult
the resin that is produced. The polymerization re- to be purified, and for this reason the purity of
action product is a molten, viscous mass that can the starting materials is a key factor. The vacuum
be spun directly to fibers or solidified for later distillation processes easily purify the ethylene
processing as a thermoplastic material. glycol, while the TA is purified by repeated
23 million tons of PET from bottles are disposed: fact that PVC and PET are difficult to separate
9 million tons are recycled, and the leftover land- from other plastic wastes.
filled or incinerated. The PET packaging and fi- Another difficulty with mechanical recycling
bers recycled are used to produce new bottles is the presence in plastic wastes of products
(3 million tons) or new fibers (6 million tons). made of the same resin but with different colors,
On a microscopic point of view, the main part which usually impart an undesirable gray color
of produced PET is still landfilled or incinerated to the recycled plastic [3,4].
[2]. If colored PET is collected and sorted together,
Unlike other materials (i.e., aluminum or these numerous colored containers will require
glass), PET is reused and not recycled. Each extra sorting in PET recycling plants. For recy-
time that PET is reused the quality of the poly- clers to sell the colored fraction, the material
mer degrades, inevitably leading to dispersion would have to be tinted in black or gray, but
in the environment, landfill, or combustion for no market currently exists for such a material
energy generation. The PET value chain is not in high quantities [5].
closed: in fact, mechanical recycling is limited In addition, most polymers suffer a certain
by the compatibility between the different types degradation during their use due to the effect
of polymers when mixed, as well as by the fact of a number of factors such as temperature, ul-
that the presence of small amounts of a given traviolet radiation, oxygen, and ozone. This
polymer dispersed in a matrix of a second poly- degradation leads to a progressive reduction in
mer may dramatically change the properties of length and to a partial oxidation of the polymer
the latter, hindering its possible use in conven- chains. Therefore, recycled polymers usually
tional applications. For example, the presence exhibit lower properties and performance than
of low amounts of PVC in recycled PET strongly the virgin material and are useful only for unde-
reduces the commercial value of the latter, due to manding applications.
the possible release of HCl during the PET Recycling plastics without prior separation by
reprocessing. This problem is enhanced by the resin produces a material with mechanical
apparatus. Neutral hydrolysis is usually per- reaction. Such a reaction was, up to now, econom-
formed under pressure (10e40 atm) at tempera- ically unfeasible due to a certain number of tech-
tures in the range 200e280 C. Alkali metal nologic constraints that DEMETO finally solves.
acetates are typically used as catalysts to pro- DEMETO started in 2010e11 in a lab scale
mote PET hydrolysis. The reaction proceeds experimentation aimed at proof of concept of
more slowly than the acid hydrolysis, several the technology to be applied for PET depolymer-
hours being required to achieve high PET con- ization. The depolymerization reaction, improved
versions. When the hydrolysis is carried out by microwaves, was tested in batch. During the
with steam, the steam acts as a major source of years 2012e14 the process was tested in a contin-
heat for the hydrolysis zone; it stirs up the waste uous manner, and a first arrangement on the mi-
material, accelerating the hydrolysis reaction, crowave reactor was developed. During the
and its partial condensation provides the liquid following 3 years, the project was partially
water necessary for the reaction. funded by the European Commission (EU),
through the project SYMBIOPTIMA: a contin-
uous microwave reactor, having a capacity of
3. DEMETO: a new route for PET 10 kg/h was realized and successfully operated.
chemical depolymerization In the same period, the DEMETO project was
lunched and, in 2017, was granted by the EU
DEMETO technology is based on an interna- more than 9 million euros. The route from lab to
tionally patented technology owned by a DEMETO is shown in Fig. 12.4.
start-up company, Gr3n: it foresees to bring at The DEMETO project is mainly focused on the
the industrial level (through a completely func- construction and management of a pilot plant
tional pilot plant) the usage of microwaves as pro- having a capacity of 60 kg/h of inlet PET. The
cess intensification approach (through an plant is under construction and is installed in
electromagnetic promoting effect) of the well- an industrial site in the center of Italy, inside
known alkaline hydrolysis depolymerization the facilities of NextChem.
3.1 The DEMETO technology thermal heating, provides some advantages such
as rapid heating with high specificity without
DEMETO is based on a chemical route for contact with material and shorter reaction times.
PET recycling and, particularly, on the alkaline The reaction is carried out at high pressure
hydrolysis of PET. The depolymerization reac- and high temperature, and it has been demon-
tion at the base of the technology is as follows strated that the conversion of PET is complete.
(Fig. 12.5): Results from lab scale and prepilot scale tests
According to the stoichiometry of the reac- have been compared with data coming from
tion, 1 mol of PET repeating unit produces literature [7]: some authors refer to processes car-
1 mol of the intermediate sodium terephthalate ried out at high temperature (200e250 C) and
and 1 mol of ethylene glycol. Sodium tere- pressure (1.4e2 MPa). The use of microwave
phthalate is converted in 1 mol of TA. Sodium irradiation for PET depolymerization by alkaline
hydroxide is added in excess with respect to hydrolysis provides a significant reduction in re-
the stoichiometric value, and hydrochloric acid action time (less than 30 min) compared with the
is added to keep the pH of the solution low conventional heating methods. Depolymeriza-
enough to have the precipitation of the insoluble tion of PET waste was complete and TA yield
TA. Sodium chloride is obtained as by-product reached more than 98%.
of the overall process. DEMETO technology claims a reaction time of
The alkaline depolymerization reaction is 10 min to complete the depolymerization of PET
assisted by microwave radiation: heating by mi- flakes even if the process is carried out at lower
crowave irradiation, compared with conventional pressure and temperature with respect to similar
literature data. The inlet stream to the DEMETO is easy removed after the depolymerization of
depolymerization process is PET flakes, powder, the PET. PVC, if exposed to severe conditions
or fibers. As an example, the characteristics of in terms of pressure and temperature, can release
the flakes are illustrated in the following table chlorine or hydrochloric acid. Another concern is
(Table 12.2). related to the different melting points of polyole-
The bigger concern related to PET chemical fine/PVC and PET: the purification process can
recycling is the presence of contaminants that be affected by the presence of other polymers
can be found in the waste mixed with the flakes. during the cooling of the process stream in
Contaminants derive from the PET production which they can solidify and cause fouling and
and manufacturing processes and can be divided obstruction of equipment.
into various categories, as follows. Metals: In general, recycled PET includes
PVC/polyolefine: Polymers other than PET various metal catalyst residues that can be used
can be present as contaminants in the flakes. for the PET manufacturing process, as summa-
They mainly derive from bottle caps and rings rized in Table 12.3 [9]. These metal ions, which
that are generally made of PP or PE. Regarding promote transesterification and polycondensa-
PVC, there are PVC bottles that resemble PET tion reactions, lead to chemical heterogeneity of
bottles. PVC is used to produce safety seals on the recycled PET that affects the melt rheological
PET bottles (such as mouthwash bottles) that behavior.
need to be removed before PET reuse/recycle. Colorants: these components are added to the
PVC is also used as liners inside bottle caps polymers to give a color to the PET final product.
and closures. While this is no longer being They can be used in form of dyes or pigment. To
used in the United States, occasional bottles produce colored packaging, colorants in low
with PVC liners will occasionally appear. It is concentrations (usually less than 500 ppm) are
also possible to find PVC derived from labels generally added at either the resin or the
wrapped around the PET bottles. Polyolefines container manufacturing stage. Colorants can
are easily removed by flotation from the inlet be organic as well as inorganic type. The
streams, and if the removal process is not perfect Table 12.4 gives a list of some colorants usually
the chemical inertness at the alkaline conditions used for PET.
must be dried to reduce the contaminants Brine can be used in a chloro-alkali system to
entrainment. The solid is then treated with hy- produce hydrochloric acid and sodium hydrox-
drochloric acid to produce TA, which is very ide, which are required by the depolymerization
insoluble in water, and then can be easily sepa- and purification process. In this way the process
rated and recovered. The next crystallization sec- can be considered self-consistent. From 1 kg/h
tion allows one to obtain pure TA with the of PET entering the reactor, about 0.6 kg/h of so-
correct size distribution (the same of the commer- dium chloride is generated.
cial TA). Ethylene glycol, which comes from the The brine is fed to the electrolyzer, where it re-
liquid recovered from the first solid separation, acts producing caustic soda, chlorine, and
is purified by distillation. The distillation is car- hydrogen according to the reaction:
ried out in two steps, under vacuum to avoid
the glycol deterioration. A part of contaminants 1 1
NaCl þ H2 O/ Cl2 þ H2 þ NaOH
(mainly colorants) are recovered and wasted 2 2
from the bottom of the second column. Finally, hydrogen is burnt with chlorine in a
From 1 kg/h entering the reactive unit, it is dedicated oven to produce hydrochloric acid
possible to produce about 0.8 kg/h of pure TA through the reaction:
and 0.3 kg/h of ethylene glycol.
After monomers recovery and purification, a 1 1
Cl2 þ H2 /HCl
stream mainly composed by brine is recovered. 2 2
UAB “NEOGROUP”
No. Parameter specification Testing method (standard)
5
0
Fe <0.1 ppm
Ni <0.1 ppm
Al <0.10 ppm
Br <30 ppm
Cr <1.0 ppm
F <1.0 ppm
Hg <0.2 ppm
TOC (total organic carbon) <10 ppm
Total heavy metals <0.30 ppm as Pb
Total suspended solid 99% light transmittance as referred to distilled water
Membranes designed for the production of chlorine and caustic soda through electrolysis of NaCl pure brine are
composite thin polymer films reinforced with a fabric made of Teflon. Membranes have sulfonate and carboxylate
polymer layers with surface modifications to enhance gas release from the membrane. To give optimum
performance, membranes require a pure brine having impurities in full accordance with the table.
that can act as poison for the electrolytic mem- sustainable process that allows the complete re-
brane cell. An activated carbon filter and a covery of PET from fibers or packaging.
chelating resin tower are used for this purpose. DEMETO will contribute to the demonstration
of a new route for the recovery of monomers
and the production of new virgin “PET” having
4. Conclusions and future trends the same characteristics of the original. The pilot
plant will demonstrate the concept at the base of
The chemical recycle of PET is a very impor- DEMETO and will constitute the base for the
tant challenge for the future. Nowadays a big scale-up of the process on industrial scale. Data
effort is dedicated to the development of a coming from pilot plant experimental phase
References
[1] GreenBlu, Chemical Recycling. Making Fiber-To-Fiber
Further reading
Recycling a Reality for Polyester Textiles. [1] https://www.demeto.eu.