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。尽管已经努力增加塑料回收，但绝大多数废塑料要么被焚烧，要么被送往垃圾填埋场。在名义上被回收的塑料中，大多数实际上被“降级”为较低等级的产品，这些产品在使用寿命结束时不能进一步回收，仍然被丢弃，因此只会推迟而不是减少对环境的负面影响。这个项目涉及一种很有前途的新工艺，将废塑料转化为石化原料。这一过程包括将废塑料注入加热颗粒的气体流态化床中。传递到塑料中的热量将长链分子裂解成较短的碳氢化合物，然后蒸发并从系统中提取，然后精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，但它的进一步开发、扩大和广泛采用仍然存在重大障碍。虽然人们对裂化和蒸馏过程已有了很好的了解，但所使用的流态化床的内部动力学在很大程度上仍然是未知的。此外，与经典流体不同，流态化颗粒介质的行为并不存在已知的规律，这意味着这些行为及其随关键系统参数的变化无法可靠地预测。因此，系统几何形状、塑料注射入口的位置、加热过程中使用的颗粒的性质以及塑料蒸发对系统动力学的影响等关键参数的具体影响仍不清楚。这反过来又意味着，该工艺的开发和优化是一项缓慢、昂贵和高风险的任务，因为对该系统的任何改变都必须在全尺寸的试验工厂中实际实施，而不能保证成功。该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模拟方法，我们将试图对多组分气态床的动力学行为有一个预测性的了解。这一知识将使我们能够建立将上述关键系统参数与关键床层特性(例如，再循环率、塑料分布、塑料停留时间)以及完整的数值模型相关联的比例定律，从而实现a)当前基于流态床的回收系统的明智和高效的运行和优化，以及b)开发更先进的系统。实验将使用各种方法进行，特别是正电子发射粒子跟踪(PEPT)，它允许在三维中跟踪粒子的运动，即使在大型、密集、不透明的系统中也是如此，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(伯明翰大学是欧洲唯一的PEPT设施)，将促进该技术的广泛使用，包括开发能够对现场全尺寸工业中试工厂进行成像的专门系统。获得的实验数据将用于校准和验证耦合离散元方法和连续介质流体动力学模拟的数值模型，从而准确地再现系统中颗粒和气体成分的运动。这种数值模拟将使我们能够以快速、经济高效和无风险的方式探索系统修改，规避与修改或建造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先者和技术先驱回收技术密切合作，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.