[seetniex]

Can anybody help with my problem.

I need to vaporize 50 kg of each plastic, and I need to find out how much energy is needed to do that.
I can't find any useful information to solve this problem.
I know that formulas are - Q(1)=λ x m; Q(2)= L x m. (if wrong correct me)
So i tried to solve it and for me is like - Q(1) = λ x m = 670 kJ/kg x 50 kg = 33500kJ ->9.305kW/h
Q(2) = L x m = 104.62 kcal/kg x 50 kg = 5231 kcal -> 6.08 kw/h

Not sure if λ and L is right...
Anyone?

This is needed for a pyrolysis reactor.

[Enthalpy]

Plastics decompose partly before they evaporate, that's why you don't find data about the vaporization enthalpy. I have heavy user's books about plastics, but never seen this data.

I suggest to take the enthalpy from resembling molecules:
- A straight alkane long enough (~ C₂₀) for both polyethylene, a branched one for polypropylene
- Maybe stilbene for polystyrene
and scale the enthalpy per repeated pattern, or per mass unit.

[seetniex]

okay...

Well, im really not that good of a chemist...
...not any kind of a chemist.

But I really have to find out how much energy is needed to vaporize the plastics. Its for my final work at university. - Synthetic fuel pyrolysis reactor. so I need to find out the energy amont needed to decide what kind of heater to use.

[Enthalpy]

www.nist.gov/data/PDFfiles/jpcrd628.pdf

Eicosane C₂₀H₄₂ 102kJ per mol of 282g, so take 362kJ/kg for low and high density polyethylene.

n-Tetradecane C₁₄H₂₀ ~67kJ/mol but the more branched 2,4,6-trimethyl-undecane 53kJ/mol, so take 30-40% less for polypropylene than for polyethylene.

Stilbene C₁₄H₁₂ 67kJ/mol (cis) and 80kJ/mol (trans) for 180g, so take 408kJ/kg for polystyrene.

Though, I can't believe that the gas molecules you get resemble the solid polymer! They won't by widely pyrolysed yet, but already shortened a lot when gaseous, and this absorbs additional heat - I'd say, more than vaporization. And anyway, the desired pyrolysis to C₁₀ or C₂₀ will absorb more heat than the initial "vaporization".

By the way, fuel production by pyrolysis of waste plastic is widely studied and published, with data available on the Internet.

Is polystyrene alone any useful for fuel production, or only diluted in much polyethylene? Last time I heated some, it made soot mainly; in addition, its C/H ratio doesn't fit valuable fuels. Or do you add hydrogen, methane...?

[Enthalpy]

http://cdn.intechopen.com/pdfs/32566/InTech-Pyrolysis_of_waste_polystyrene_and_high_density_polyethylene.pdf

http://ir.canterbury.ac.nz/handle/10092/4303

http://library.certh.gr/libfiles/PDF/LAP-PAPYR-3680-CHEMICAL-in-MACROMOL-MATS-ENG-V-292-ISS-8-PP-923-934-Y-2007.pdf

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During and after pyrolysis, you better don't breathe the gas nor the vapours, especially with polystyrene.

[Corribus]

To the OP,
Not all LDPEs (to use an example) are created equal.  Thermal properties depend heavily on molecular weight, tacticity, degree of branching, crystallinity, and so forth.  There isn't going to be a simple formula you can use to come up with a generally relevant value.  It'll probably have to be measured experimentally on a case-by-case basis, and as Enthalpy said, at the temperatures required for vaporization you're going to access competitive decomposition pathways.

[Enthalpy]

Molecular weight changes a lot the melting point for instance, but does it change the heat of "vaporization" (including some cracking) per mass unit? I expect the influence is small.

From existing papers, polystyrene is the common plastic in waste after polyolefins. It gives much liquid styrene upon heating to moderate temperature, so the required heat per monomer mole for this particular reaction would only be the inverse of polymerization, which is certainly known.

Though, styrene is a bad fuel, and I suppose it will contain big amounts of carcinogenic aromatic compounds. If much hydrogen is available - for instance by full pyrolysis of a part of the polystyrene or polyolefins - you might hydrogenate the liquid to get ethylcyclohexane and the like, which would be acceptable fuels.

Papers consider instead to recycle the styrene into plastics. Obscure to me: how expensive it is to get decently pure monomers from the pyrolysis mushy, and how this cost compares with cleaner paths.

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If this can fit in your study, I wish you check the pyrolysis of waste tyres. The tetramer, pentamer and trimer of isoprene would make excellent fuels after hydrogenation (phytane, farnesane) - for rockets, Diesel engines, jet engines, and as transformer oil, lubricants... If the oligomer result from pyrolysis, fine. If not, take isoprene as an intermediate. Superior transformer oil and rocket fuel have more value than a bad heating fuel, making economic recycling more credible.

Isoprene itself is cheap, but if obtaining the proper oligomers (head-to-tail) directly by pyrolysis, it avoids a delicate operation, and the price ratio between waste tyres and good transformer oil should afford moderate yields. Maybe hydrogenation can be conducted during the pyrolysis and reduce the proportion of by-products like limonene.

The competitor to waste tyres in that particular transformation would be hevea and gutta-percha.

http://www.chemicalforums.com/index.php?topic=56069.msg202845#msg202845
(I've put many mistakes there, some are noted within the thread, more should remain, caution).