Plastic waste is one of the great environmental challenges of our time. Though efforts have been made to increase plastic recycling, the vast majority of waste plastics are still either incinerated or sent to landfill. Of the fraction of plastics that are notionally recycled, most are actually 'downcycled' into lower-grade products which, at the end of their life, cannot be recycled further and are still discarded, thus simply delaying the negative environmental impact, as opposed to reducing it.
This project concerns a promising new process for converting waste plastics into petrochemical feedstock. The process involves the injection of waste plastics into a gas-fluidised bed of heated particles. Heat transferred to the plastic cracks long-chain molecules into shorter hydrocarbons which are then vaporised and extracted from the system, before being refined into valuable petrochemical products.
While this emergent technology shows considerable potential, there remain significant impediments to its further development, upscaling, and widespread adoption. While the cracking & distillation processes are well-understood, the internal dynamics of the fluidised beds used remain largely unknown. Further, unlike for classical fluids, there exist no known laws governing the behaviours of fluidised granular media, meaning that these behaviours - and their variation with key system parameters - cannot be reliably predicted. Consequently, the specific influences of key parameters such as the system geometry, the positioning of inlets for the injection of plastics, the properties of the particles used in the heating process, and the effect of the vaporisation of plastics on the system's dynamics remain unknown. This, in turn, means that the development and optimisation of the process represents a slow, costly and high-risk task, as any change to the system must be physically implemented in a full-sized pilot plant, with no guarantee of success.
This project aims to directly address these issues. Using cutting-edge experimental imaging techniques and computational modelling methods, we will attempt to gain a predictive understanding of the dynamical behaviours of multi-component gas-fluidised beds. This knowledge will allow us to establish scaling laws relating key system parameters mentioned above to crucial bed properties (e.g. recirculation rate, distribution of plastics, plastic residence time), as well as full numerical models, together enabling a) the informed and efficient operation and optimisation of current fluidised-bed-based recycling systems and b) the development of still more advanced systems.
Experiments will be performed using a variety of methods, notably positron emission particle tracking (PEPT), which allows the motion of particles to be tracked, in 3 dimensions, even within large, dense, opaque systems, with high temporal and spatial resolution - making it ideally suited to the current application. The PI's significant experience with PEPT, and his position at the University of Birmingham, which houses Europe's only PEPT facility, will facilitate extensive use of the technique, including the development of specialised systems capable of imaging full-scale industrial pilot plants in situ.
Experimental data obtained will be used to calibrate and validate numerical models coupling discrete element method and continuum fluid dynamics simulations so as to accurately reproduce the motion of both the particulate and gaseous components of the system. This numerical modelling will allow us to explore system modifications in a rapid, cost-effective and risk-free manner, circumventing the time, expense and risk associated with modifying or building new pilot plants, or sourcing, buying and testing new materials.
We will work closely with leaders in the field and pioneers of the technique, Recycling Technologies, ensuring clear and direct pathways to impact, and thus expedited economic benefits for UK industry.
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