Advanced Imaging and Numerical Modelling of Segregation and Transport of Plastics in Fluidised Beds: Toward a Circular Economy for Plastics

塑料垃圾是我们这个时代最大的环境挑战之一。尽管人们已经努力增加塑料的回收利用，但绝大多数废塑料仍然被焚烧或送到垃圾填埋场。在那些名义上被回收的塑料中，大多数实际上被“降级回收”成低级产品，这些产品在使用寿命结束时，无法进一步回收，仍然被丢弃，因此只是推迟了对环境的负面影响，而不是减少它。这个项目涉及一个有前途的将废塑料转化为石化原料的新工艺。该工艺包括将废塑料注入加热颗粒的气相流化床。传递到塑料的热量将长链分子裂解成较短的碳氢化合物，然后从系统中蒸发和提取，然后提炼成有价值的石化产品。虽然这项新兴技术显示出相当大的潜力，但它的进一步发展、升级和广泛采用仍然存在重大障碍。虽然裂化和蒸馏过程被很好地理解，但所使用的流化床的内部动力学在很大程度上仍然未知。此外，与经典流体不同，流化颗粒介质的行为没有已知的规律，这意味着这些行为以及它们随关键系统参数的变化不能可靠地预测。因此，诸如系统几何形状、塑料注射入口位置、加热过程中使用的颗粒特性以及塑料汽化对系统动力学的影响等关键参数的具体影响仍然未知。反过来，这意味着流程的开发和优化是一项缓慢、昂贵和高风险的任务，因为对系统的任何更改都必须在一个全尺寸的试验工厂中实际实施，无法保证成功。这个项目旨在直接解决这些问题。利用尖端的实验成像技术和计算建模方法，我们将尝试获得对多组分气流化床动力学行为的预测性理解。这些知识将使我们能够建立上述关键系统参数与关键床属性（例如再循环速率，塑料分布，塑料停留时间）相关的缩放规律，以及完整的数值模型，共同实现a)当前基于流化床的回收系统的明智和有效的操作和优化；b)开发更先进的系统。实验将使用多种方法进行，特别是正电子发射粒子跟踪（PEPT），它允许在三维空间中跟踪粒子的运动，甚至在大型，密集，不透明的系统中，具有高时间和空间分辨率-使其非常适合当前的应用。PI在PEPT方面的丰富经验，以及他在伯明翰大学的职位，该大学拥有欧洲唯一的PEPT设施，将促进该技术的广泛使用，包括能够在原位成像全尺寸工业中试工厂的专业系统的开发。获得的实验数据将用于校准和验证耦合离散元法和连续流体动力学模拟的数值模型，以便准确地再现系统中颗粒和气体组分的运动。这种数值模拟将使我们能够以一种快速、具有成本效益和无风险的方式探索系统修改，避免与修改或建立新的试验工厂或采购、购买和测试新材料相关的时间、费用和风险。我们将与该领域的领导者和技术，回收技术的先驱密切合作，确保明确和直接的影响途径，从而加快英国工业的经济效益。

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.