12thealchemist
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Destructive distillation of polypropylene
Some years ago I collected together a large quantity of waste polypropylene (PP) plastic in the form of yoghurt pots for the purpose of pyrolysis. I
had discovered a book section[1] which detailed the products of thermal decomposition of polymers. This stated that the products of thermal
decomposition of polypropylene were "...pentane (24 percent), 2 methyl-1-pentene (15 percent), and 2–4 dimethyl-1-hep-tene (19 percent)." It was on
this basis that I planned to pyrolyse a quantity of polypropylene followed by fractionation for these high-value products. It seems, however, from my
own experiments that the result under my conditions more closely resembled crude oil.
Earlier this month I finally got around to shredding about a third of them into small enough pieces to fit through the neck of a bottle.
I bought a cheap 500 mL stainless-steel water bottle and filled it as full as I could with chopped PP, stoppered it with a sparkling wine cork and set
up for distillation with a gas burner. Once the charge of plastic had been exhausted, the cooled steel bottle appeared empty without any char,
fortunately. I then refilled it, and repeated several times until I had destructively distilled all 660 g of chopped plastic. I found that cork was a
poor choice of stopper for the high temperatures involved, and I turned to a sacrificial rubber bung instead. After about half a dozen runs (the whole
lot), the rubber stopper was in no fit state for anything, and was thrown away.
My yield of pyrolysis oil was 650 mL, with a density of 0.701 g/mL at about 22°C (est.).
This oil was then simply distilled, and crudely fractionated into six fractions:
rt 2 L (est.)
35-85°C 50 mL
85-135°C 50 mL
135-155°C 70 mL
150-250°C (est.) 400 mL
250-300°C (est.) 40 mL
300+°C (est.) 40 g
My thermometer does not go above 150°C, hence estimated temperatures based on vegetable oil viscosity of the penultimate fraction. The fractions were
dried over calcium chloride, with the higher boiling ones taking much longer to dry than the lightest. The final result was five crystal-clear
liquids, darkening to olive-oil coloured for the 250-300°C fraction. The final "fraction" was the residue left in the flask, and is the colour of
black treacle with a consistency of hard vaseline.
The first fraction to boil out of solution was not condensed by an ice-water-cooled double-surface Leibig condenser, I concluded that this was
petroleum gas, comprising (presumably) ethylene, propane, propylene, butane, and butenes. I was not expecting this, so I cannot confirm or deny if
alkenes were present in this gas.
Based on these results, I would propose that the pyrolysis oil of polypropylene serves a remarkably cheap and safe alternative to crude oil for
educational purposes, and contains a full range of crude oil fractions, without carcinogenic aromatics like benzene. This later assertion is based on
the C:H ratio in the feedstock plastic compared to the C:H ratio of aromatic compounds. A considerable amount of hydrogen would have to be released
for this class of compound to be formed, which is unlikely under the conditions.
[1] C. L. Beyler and M. M. Hirschler, Chapter 7, SPE Handbook of Fire Protection Engineering, 2001, pp. 110–131.
Attachment: Beyler, Hirschler - 2001 - Thermal Decomposition of Polymers(2).pdf (165kB)
This file has been downloaded 469 times
My set-up for pyrolysis:
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hissingnoise
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I thought yogurt pots were made of polystyrene (vinylbenzene).
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12thealchemist
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In my case, they're polypropylene:
I've done destructive distillation of polystyrene before, and that produces a completely different product with a very different and distinctive
smell. The pyrolysis oil described here has a petroleum odour, similar to fuel (fuel meaning petrol, diesel, kerosene, etc.)
[Edited on 26-8-2019 by 12thealchemist]
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draculic acid69
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Excellent work.if the resulting oil you obtained is similar to that list of products once the pentane is distilled off and the resulting alkenes
reduced to alkanes you should have a mixture quite similar to petrol which when done on a large scale would prove quite profitable.a video I seen on
YouTube of a bench top model pyrolysis unit that spits out a petroleum substitute was using a polypropylene/polyethylene 90% 10%polycarbonate mix.
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12thealchemist
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The resulting oil appears to be completely different to the list of products given. Almost no pentane was collected, and 60% of the oil boiled at a
temperature above that of the highest-boiling component of the list. As I described, the composition of the oil I obtained was closer to that of crude
oil than that of petrol.
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Ubya
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pyrolisis produces a big range of molecules, a long chain like PP can break forming different chain length molecules, i would be surprised if the
products were only 3 and with pretty short chains, maybe with higher temperature or a catalyst you can get smaller chains, but in general you'll never
be able to get just one or a few molecules from the pyrolisis of a big polymer.
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unionised
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Quote: Originally posted by 12thealchemist  |
contains a full range of crude oil fractions, without carcinogenic aromatics like benzene. This later assertion is based on the C:H ratio in the
feedstock plastic compared to the C:H ratio of aromatic compounds. A considerable amount of hydrogen would have to be released for this class of
compound to be formed, which is unlikely under the conditions.
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Not really.
Two observations- you produced coke/ char.
If you can get carbon, you can get aromatics.
Also, you can "lose" hydrogen by breaking long carbon chains.#
I'm not saying this sort of reaction
C14 H30 --> C6H6 + 4 C2H6
is common, just that it's possible.
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draculic acid69
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He said there was no char leftover in the vessel after pyrolysis.
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