One of the great environmental problems of our time is waste plastics, and there are apparently huge volumes floating around in the oceans of the world. These would generally get there by people throwing them away, so in principle this problem is solved if we can stop that irresponsible attitude. I can already hear the, “Good luck with that,” response. Serious fines for offenders would help, as would more frequent proper rubbish disposal bins. But this raises the question, what should we do with waste plastics?
The first answer is it is unlikely there is a single answer because there are such a variety of plastics. Some, like polyester or polyethylene, can be reasonably easily recycled for low specification uses, but the problem here is there is a limit to how many plastic buckets, etc, can be sold. Technically, quite a high level of recycling can be achieved. Quite a while ago, during the first oil crisis, a client asked me to devise a means of recycling mixed coloured polyethylene so I devised a process that recovered a powder that could be used to make almost anything that virgin polyethylene could make, except maybe clear: there was always a slight beige colour from residual dyes etc that could not be got out, at least in a one-cycle process. Polyethylene degrades – you will all have seen it go brittle from sunlight. This shortens the chains and oxidizes parts and I was proud of this process because it got rid of all the degradation and short-chain material.
A pilot plant was built, then the process was abandoned. The reason was the oil prices tumbled, and there was no way the process could make money, particularly since big multinationals appeared to be dumping polyethylene into New Zealand. Some manufacturers loved this, and were able to export all sorts of plastic things, at least for a while. Part of the reason the process would have lost money, of course, was that despite getting the raw material rather cheaply, the yield at the end was lower because of the loss of the degradation products, but the killer was getting rid of the degradation products. They could be burnt for process heat, but that would need a specially designed burner, and there would still be the pigment remains to be disposed of. Good idea, but could not compete with the oil industry.
Another possible process is pyrolysis. This came to my attention when I recently saw a paper in the latest copy of “Energy and Fuels” put out by the American Chemical Society. Polyethylene gives a mix of oil, gas and carbonaceous solid, but you can get almost 80% in the form of oil that could be directly used as a diesel fuel after distillation. There appear to be a fraction that boils too high for the diesel range, and gets waxy, but those who have recalled a recent post by me will see that it would do well in the heavier marine heavy fuel oil. The resultant oil has a mix of linear alkanes and terminal alkenes, and the fragmentation is such that the double bond prefers the smaller fragment. There is also some miscellaneous stuff resulting from the oxidative degradation. Polypropylene, however, showed a lot more oxygen, with a range of alcohols, esters and also acids in addition to highly branched hydrocarbons, however, almost 20% was the single compound 2,4-dimethyl-1-heptene. It would manage with the light ends as petrol, and the heavier ends contributing to diesel.
Polystyrene gave what corresponds more to a heavy oil, although 40% was actually styrene, which could be used to make more polystyrene. Importantly, the cetane rating for the oil from polyethylene was 73; for polypropylene, 61. Polystyrene oil was unsuitable for diesel, but if hydrogenated, the lower boiling cut would make a high octane petrol. The average pump diesel fuel has a cetane rating of about 50, and the higher the rating, the faster the engines can go, so pyrolysis of waste polyethylene and waste polypropylene will make an excellent diesel fuel, with the heavy ends going towards shipping. However, the heavy ends of polystyrene would have to be dumped because they contain fluorinated material, presumably a consequence of additives, and you certainly do not want an exhaust stream rich in hydrogen fluoride. And here is the curse that plagues anything involving recycling: too many companies put in additives that will be impossible to remove, and which either prevent proper recycling or will have consequences that are at best highly unpleasant, while they offer no option for dealing with them.
How do we separate these plastics out? Fragment them, and stir in water. Polyethylene and polypropylene are the two plastics that float. Foam, of course, has to be omitted. So, will this end up being done? My guess is, not in the immediate future. In terms of economics, it cannot beat the entrenched oil industry, unless governments decide that cleaning up the environment is worth the effort.