One session of the Plastics Recycling Conference was devoted to plastics-to-oil (PTO) technology. Prior to the session, my thoughts had been that the technology was of dubious value - why not just completely burn the plastic for energy? But after the session, I'm beginning to have second thoughts.
PTO is thermal pyrolysis of plastics. The materials are heated in a vacuum or inert atmosphere to about 800 oF (430 oC) and reduced in molecular weight to a liquid that is similar to crude petroleum. Similar, but in fact better, as it is very low in sulfur, thus making it a (very) sweet crude oil, the grade that can command the highest prices. The oil is sold to local refineries who then mix it with their regular petroleum feedstocks for conversion to gasoline/heating oils/... The speakers were all emphatic that building such an operation is fairly easy, as there are no air emissions so permitting is trivial (once you can convince the agencies that there really are no air emissions).
What changed my mind about the usefulness of this technology is that it works best as part of a business model rather than as a stand-alone business. For instance, plastic recyclers receiving a mixed stream of plastics will sort the mixture into various outputs, but will veritably be left with materials at the end that they cannot sort and sell. That material could then be converted into oil.
Another concern that I had had previously about the process was how polymers with heteroatoms were processed - PET or PVC for instance - where the oxygen and the chlorine need to be removed. Without the introduction of hydrogen, the resulting liquid would be unsaturated and unstable, but since the oil is quickly shipped to the refinery, that is not an issue.
One of the speakers also pointed out that this truly is recycling, since the waste plastic is now be brought back - all the way back - to the petroleum refiners, rather than part of the way back to the plastic processors. At the same time, it cannot be ignored that this is most likely a one-time trip, as the refined oil will most likely be burned rather than serve as a chemical/plastic output. Also, the conversion of plastic to oil does nothing to aid public perception of plastic as being a valuable material. Instead, it only reinforces the idea that it is cheap junk (ideas that I will expand on tomorrow).
So as I stated before, I am now convinced that PTO does have some usefulness in certain situations. It is not a universal solution to plastic waste, but should be considered a valuable addition to the other recycling technologies.