Page 170 - CW E-Magazine (18-6-2024)

Special Report

Catalysing change: Defossilising the chemical industry

Part 3: Plastic waste & CO as feedstock
2
The Royal Society mised product performance, as well as or glycolysis. These reactions occur
the introduction of virgin product and under accessible temperatures and are
Plastic waste as a feedstock for additives, which can raise sustainability selective for monomer production.
chemicals concerns. Purification can allow for the However, these reactions will not
whole polymer to be recovered without work with PE, PP, polystyrene or other
Types of plastic waste additives and impurities, using a solu- commodity hydrocarbon plastics.
lastics are a type of polymer tion to separate the polymer chains.
which are composed of thou- This could be thought of as a variant of Plastic waste availability
Psands to millions of chemically ‘back-to-polymer’ recycling method. Approximately 300-400 mt of plas-
bonded ‘monomer’ units. These mono- The main products undergoing purifi- tic is produced annually. This range is
mers are often either basic chemicals cation are PE, PP, PVC and poly- an indication of varying standards and
or produced from them. Among the styrene. However, solvent-based purifi- types of reporting, such as whether tex-
various types of polymers, polyethy- cation recycling is not operating at tiles are included, and difficulties with
lene (PE), polypropylene (PP) and poly- any meaningful scale. The recycling verification. Plastic production has
ethylene terephthalate (PET) dominate rates of PP and polystyrene are very rapidly increased in recent decades
global plastic waste streams. PE is low and PVC is hardly recycled at all. and will continue to do so for the next
produced from ethene, PP is produced several decades under a business-
from propene, and PET is produced Chemical recycling as-usual scenario, potentially reaching
from ethene and p-Xylene. Chemical recycling converts waste 1 Gt by 2050. As there are a wide range
plastic into monomers, other hydrocar- of uses for polymers, there are also
Plastic waste to chemicals processes bons or chemicals. There are numerous many sources of waste for potential to
Plastic waste needs to be separated types of chemical recycling technolo- use as a feedstock, including clothing,
from the wider waste stream for recyc- gies and processes, which are suited plastic products, pipes and plastic pack-
ling. Mechanical recycling applies to different polymer types and are at aging. Globally, approximately 9% of
forces or heat to reprocess the polymer different stages of maturity. all plastic waste is currently recycled.
‘back-to-polymer’ into a new product. It is important to note that actual re-
Chemical recycling applies biochemical It is possible to use heat or catalytic cycling rates vary widely between and
processes to chemically breakdown the reactions to convert waste plastics into within countries and there is uncertainty
plastic, potentially back into monomers, simple monomers, oils and gases. This around exact recycling figures and
in which case it can also be known as can be done via hydrothermal treatment reporting.
‘back-to-monomer’ recycling. Chemical or gasification into syngas, pyrolysis,
recycling differs from mechanical and enzymatic polymer recycling. This Without policy intervention, low
recycling in that it creates a feedstock can be a challenging process for some collection rates may significantly
for chemical production. polymer types, such as polyethylene or impact the viability of plastic waste as
polypropylene, since it requires very a carbon source for chemical produc-
Mechanical recycling high temperatures and is unselective, tion, particularly in countries with low
Mechanical recycling breaks waste meaning it does not cleanly yield back recovery and recycling rates.
plastics down into flakes or pellets the monomer, such as ethene or
through physical processes including propene. Plastic waste is a well-recognised
shredding, pressing and melting. source of environmental pollution, with
Mechanical recycling can be energy The chemistry of other chemical approximately 80-mt of global plastic
efficient, avoiding the need to break recycling routes depends on the poly- waste being ‘mismanaged’ – not stored
down to primary chemicals. How- mer under consideration. For poly- in secure landfills, recycled or incine-
ever, not all products can be mechani- esters, such as PET and polylactic acid rated. Of that, 19-mt is leaked into the
cally recycled. There are also limits to (PLA), it involves processes such as environment, of which 13-mt enters
mechanical recycling, such as compro- solvolysis, hydrolysis, alcoholysis terrestrial environments, 6-mt enters

170 Chemical Weekly June 18, 2024

Contents Index to Advertisers Index to Products Advertised