Polystyrene PS Pyrolysis

Neat polystyrene feedstocks will depolymerize in a pyrolysis process to give predominantly styrene monomer-a liquid fuel with good energy content. 

The optimum pyrolysis temperature is 395°C to give a recovery ratio of 0.97 (i.e. 1000 kg polystyrene will yield 970 L liquid monomer) and 5 to 10% char residue. Fuel made from polystyrene feedstock will be high in aromatic character and have an energy content of 50 MJ/kg and a pour point of —67°C. However the flash point is only 26°C and the cetane rating only 12.6. The fuel needs to be blended with polyolefin-derived diesel or regular diesel in order to upgrade the flash point and cetane rating to within specification. 

Polystyrene has somewhat lower thermal stability than the polyolefins and its pyrolysis can be undertaken between 380 and 420°C without producing gases or leaving significant 

The pyrolysis of PET by Sakata [3] has been found surprisingly to yield no liquid products. It is widely known that compounds that undergo sublimation, such as tereph-thalic acid and benzoic acid, are produced by the thermal decomposition of PET and this causes problems in plastic pyrolysis plants. Interestingly Yoshioka et al. [4] found that the addition of calcium hydroxide (slaked lime) gives high selectivity for benzene formation without producing sublimation compounds such as terephthalic acid. The yield of benzene is around 35 wt% at 700°C and a 10.0 calcium hydroxide/PET molar ratio. 

PVC is not recommended as a feedstock material for pyrolysis. The reasons for this being that it contains about 57% chlorine by weight which will affect diesel quality and can produce chlorinated hydrocarbons, and also because it thermally decomposes to hydrochloric acid that is very corrosive and toxic. 

---