Page 84 - Plastics News July 2025
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IN THE NEWS
2. Pyrolysis engineer Roger Ruan and other scientists, poly-
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propylene decomposes at 378–456 C, while
The plastics are heated to about 500 C in the low-density polyethylene breaks apart at 437–
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absence of oxygen. The longer molecules break 486 C, and high-density polyethylene at 452–
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into liquid fractions like naphtha and diesel, sol- 489 C. As a result, firms processing mixed plas-
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id cuts like waxes, and lower-molecular-weight tic waste must select a temperature—normally
gases. In most plants, roughly 10% of the prod- over 500 C—at which all the polymers they take
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uct is char, a by-product. in on a given day will break down.
3. Landfill disposal However, temperature affects the composition
of a pyrolysis unit’s output. Pyrolysis yields use-
The char is hauled to the landfill or can be add-
ed to asphalt or concrete. Most plants burn the ful liquids, such as naphtha and diesel. But it also
creates less-desirable waxes that might need to
gases for heat.
be broken down further. And pyrolysis makes
4. Upgrading lighter gases that are typically burned as fuel in
the reactor. High temperatures and long reactor
For the output to be suitable for making new residence times might cut wax output and yield
plastics, adsorbents and hydroprocessing may more naphtha, but they also create gases that
be needed to remove chlorine, nitrogen, and have limited utility.
other pollutants. A hydrocracker, or similar unit,
is sometimes needed to further break down High temperatures can also lead to dehydroge-
large molecules. nation, cyclization, aromatization, and Diels-Al-
der reactions, thereby creating more aromatics.
5. Using waste “For fuels and so on, it’s fine,” Ruan says. “But
sometimes we want naphtha feedstock for new
The naphtha is processed in an ethylene crack- plastics production; we don’t want a lot of aro-
er to create ethylene and propylene, building matics.”
blocks for more polyethylene and polypropyl-
ene. And feeding the wrong plastics into pyrolysis re-
actors creates inefficiency and can contaminate
One challenge of pyrolysis is the variability of the output. PET contains oxygen and tends to
the feedstock. The different polymers that are form carbon dioxide, Ruan says. Polyvinyl chlo-
fed into a pyrolysis reactor break along different ride (PVC) yields chlorinated compounds. Ad-
patterns. In particular, molecules with high de- ditionally, some plastics have a lot of inorganic
grees of branching crack more easily than linear additives, such as carbon black, carbonate, and
ones.
clay. They lead to the formation of char, which
pyrolysis operators must dispose of as solid
According to a review paper by University of
Minnesota Twin Cities bioproduct and biosystem waste.
84 PLASTICS NEWS July 2025
