Hydrocarbon Processing - May 2022 - 17

Circular
Economy
N. MENET AND T. MALLET
Axens, Paris, France
Innovative recycling, driving circularity
while decarbonizing the petrochemical industry
Plastic pollution's environmental challenges, as well as new
government legislation, are having a significant impact on the
plastics industry. Polymer manufacturers are being urged to
experiment and implement alternative production processes-such
as bio-based plastics and recycling-to decarbonize
their products.
The complexity associated with the development of alternative
production processes is amplified by the fact that there
are a variety of polymers, and the formulations generated by
adding additives differ depending on how they are used.
Let us first focus on recycling thermoplastics. Thermoplastic
waste can be either mechanically or chemically recycled.
Presently, chemical recycling only accounts for about 1% of
recycled plastics. However, that proportion is expected to
rise sharply, especially given that this approach enables the
production of polymers that can be reused in a true circular
closed loop, even for the most demanding applications, such
as food or pharmaceutical plastic grades.
This field of chemically recycling plastics is where the authors'
company has applied its long-standing expertise [chemicals
and chemical engineering, catalysis (homogeneous and
heterogeneous), analysis, process engineering and modeling,
pilot and demonstration design, and scaleup to industrial
units] to develop innovative process technologies.
Chemical recycling opens new opportunities to bridge the
gap between the need for additional plastics recycling capacity
and the need for high-quality recycled plastics products. For
example, chemical recycling addresses more difficult waste
plastics feedstock that cannot be valorized through mechanical
recycling, while fully meeting quality requirements and
regulatory objectives. Chemical recycling prevents routing
all plastics waste to a mechanical recycling process. It also enables
the following:
* Valorizes mixed plastics streams in a closed-loop
complex, where sorting and separation stages,
and regeneration processes required for recycling
are too complex.
* Valorizes plastic polluted during the various stages
of its life (from its manufacturing to its arrival at the
recycling plant), including contamination from the
different sorting steps or contamination from contact
with other materials during its use.
* Removes all intentionally added contaminants
to allow for a true closed recycling loop of plastic
wastes (e.g., pigments, dyes, forbidden additives).
* Creates an infinite closed recycling loop of plastics vs.
mechanical recycling, where recycling may be limited
to a certain number of cycles. Those cycles' limitations
are due to the temperature effects associated with
the various stages of the recycling process, ultimately
causing degradation of the recycled raw material and
preventing upcycling (e.g., recycling of waste textile
into food grade packaging such as bottles).
Chemical recycling (also referred to as advanced recycling)
encompasses different processing technologies. Depolymerization
and conversion processes can be used to modify
the chemical structure of the polymer and purify the resulting
product to enable the production of new raw polymers.
Dissolution processes are also being developed to recover additive-free
polymer chains. Some argue that dissolution is an
extension of mechanical recycling, as the chemical structure
of the polymer remains unchanged. However, that process
relies heavily on chemical stages and is often grouped with
chemical recycling.
Depending on the type of polymer waste, some chemical recycling
routes are more appropriate than others. For example,
waste polyethylene terephthalate that cannot be mechanically
recycled will be recycled through a depolymerization process
and is not suitable to conversion or dissolution processes.
The already proven and robust pyrolysis pathway. The
following will focus on the conversion process, with a special
focus on mixed plastic pyrolysis and its associated purification
and decontamination steps. This is a key building block to a
sustainable polyolefin chemical recycling value chain.
Pyrolysis of mixed plastic waste is considered the novel
route to accomplish a true closed-loop recycling process of
polyolefins, adhering to quality requirements and regulatory
objectives. However, this route relies on the ability to properly
purify the pyrolysis oil for reprocessing it in an existing petrochemical
plant. That purification step is not a trivial refining
process, as pyrolysis oil usually combines multiple contaminants
and unstable molecules that-if not removed and stabilized-would
jeopardize the operation of a petrochemical
plant's steam cracker.
Repsol and the authors' company have joined their efforts
to unlock the recovery of plastic waste that would otherwise
remain in landfills or be incinerated. The consortium has developed
and commercialized a proprietary pyrolysis purification
processa
to solve the challenges of purification and decontamination
of pyrolysis oils. This process technology removes
impurities such as silicon, chlorine, diolefins and other metals
from the produced plastics pyrolysis oils, allowing the direct
and undiluted feed to the steam cracker. Proper and reliable
Hydrocarbon Processing | MAY 2022 17
Hydrocarbon Processing - May 2022

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