Chemical Recycling of Polypropylene

Abstract

There are various types of plastic produced and used today in society. Depending on the plastic type and also the application, different recycling techniques are best suited to recovering as much value (in a economic but mainly environmental sense) as possible. One of the most abundant plastics in our world is polypropylene (PP). This can be a difficult plastic to recycle and as we are pushing for a circular economy we need to find ways to recycle the less pure and more mixed plastic waste streams. Chemical recycling (in this case catalytic pyrolysis) offers an opportunity to break down the polymer chains into either valuable products or to intermediate products that can be further processed by a refinery into valuable products and/or into the building blocks for new plastics. Catalytic pyrolysis in this research looks at heating up a small amount of a polymer in a reactor along with a catalyst material. As the temperature increases the polymer will first melt and then start to react with the catalyst material to produce shorter hydrocarbon chains. If these chains are short enough such that they are in a gaseous form they will diffuse outwards and be carried towards a measurement device (gas chromatography) to enable quantification of the products. The reaction pathway is highly complex as the products can also react with one another to form further products and thus the exact reactor/catalyst conditions can make a significant difference to the products leaving the reactor. The main catalyst material that we studied was a fluid catalytic cracking (FCC) catalyst. This catalyst is used industrially to crack crude oil fractions which in some ways are chemically similar to PP. The ICC research group has vast experience in characterising and studying these catalyst materials so it was a good starting point for our research. The first interesting result we found was the discarded FCC catalyst seemed to outperform the fresh FCC catalyst material in our reaction conditions. We continued our study into trying to understand why this was the case and it seems that the metals (i.e., Fe, Ni, V) deposited on the FCC catalyst particles during the FCC process actually help with the cracking of PP. The location of the deactivating coke species was also much different for the fresh vs discarded FCC catalysts, further suggesting quite a different reaction scheme taking place. Due to the importance of the reaction conditions, in the final chapter of the Thesis there is the outline of the design of a continuous throughput reactor. This reactor vessel allows feeding plastic into the reactor over the course of the reaction and allows for further studies and also more industrially applicable studies. The conclusions of this work is that although there are some promising options for the recycling of plastic, by far the easiest and simplest solution to producing less plastic waste is to produce less plastic in the first place.