Flash Vacuum Pyrolysis of Low Density Polyethylene in a Free-Fall Reactor

Abstract

Polyethylene (PE) is the principal component of postconsumer plastic wastes. It is important to develop chemical techniques to decompose PE with a view to recycling and production of compounds that are valuable as fuel or industrial raw materials. The most common reactors in polymer pyrolysis are fluidized-bed types. Free-fall reactors do not suffer the disadvantages of fluidized beds related to inert gas employment. This flash pyrolysis study has shown that low density polyethylene (LDPE) can be continuously degraded in a free-fall reactor under vacuum to give a product quality superior to other methods. The total conversion (i.e., the sum of liquid and gas yield) is 43% at 875°C when the feed is 150–75 μm LDPE particles. Over 99% of the gaseous product is C1 to C4, the ethylene monomer exceeding 64%. The bulk of the liquid product is paraffinic; 96% of which is below C40, while over 55% is C12–C20. As is known, C12–C20 hydrocarbons are essential raw materials for the production of fatty acids, fatty alcohols, and detergents. Reduction in operation temperature down to 750°C causes the total conversion to fall from 40% to 25%. This is accompanied by a slight increase (from about 64% to 67%) in the ethylene monomer yield. Thus higher temperatures should be preferred. On the other hand, indications are that lowering the particle size down to −75 μm favorably influences the total conversion, with a gain of about 20%.