C&EN: What happens in a plastics pyrolysis plant?

Also in this week's C&EN, some pretty cool details of what happens in a plastics pyrolysis plant (article by Alex Tullo): 

In Akron, Ohio, situated amid industrial buildings and auto service centers, is a small oasis of polished steel and nonslip scaffolding. The proprietors of this new facility hope it’s the future of the plastics industry.

The facility, a pyrolysis plant run by Alterra Energy, breaks down plastic waste into a crude oil–like mixture that can be fed into petrochemical plants and transformed back into new plastics...

...In the bunker, the plastics, predominantly polyethylene and polypropylene, are heaped in gray piles randomly sprinkled with colors. Fragments of packaging labels can be seen in the shredded and compacted bits of plastic.

A front-end loader drops the shreds into a hopper, and a conveyor belt carries them to an extruder. The plastic comes out of the extruder with the consistency of saltwater taffy and is sent by pipe to the reactor. The purpose of the extrusion step is to heat the plastic and give it uniform density so it can transfer heat predictably in the pyrolysis reactor.

At this stage, the plastic is mixed with calcium oxide, which scavenges for chlorine from errant PVC. Chlorine can corrode and damage downstream customers’ chemical production equipment if it remains in the pyrolysis oil. DeBenedictis said the calcium oxide reacts with chlorine to form calcium chloride, removing about 80% of the halogen. Customers handle the rest by chemically upgrading the pyrolysis oil themselves.

The pyrolysis reactor is a cylinder about 3 m in diameter and 20 m long. It rotates slowly to mix the contents and is angled downward to allow gravity to keep the fluids flowing. Inside, plastics are heated to about 400–550 °C in the absence of oxygen. This energy breaks down the long polymer chains into smaller hydrocarbons ranging from gaseous 3-carbon propane up through waxes of 30 carbons or more.

The longer-chain hydrocarbon liquids, which compose 70–80% of the plant’s output, are run through a multistage condenser system and then recombined into a final product that is solid at room temperature. One or two trucks, each capable of carrying 22,000 L of the material, depart every day from Akron, headed south to petrochemical customers on the US Gulf Coast.

The rest of the hydrocarbon output is noncondensable gases. The gases supplement the natural gas used to fire the kiln. Another 10% of the total material that exits the reactor is solid char, a mixture of carbon and inorganics such as calcium chloride. This flows into large sacks that hang near the kiln and can be used in asphalt, DeBenedictis said.

I would have not have guessed that the process uses calcium oxide to capture the chlorine, but it makes sense from an economic perspective...