熔融废塑料竖直降膜热解高值回收的基础研究

Pyrolysis is one of the most important ways of recovery and waste-to-energy for waste plastics. One of the most advantages of pyrolysis by vertical falling film is that the production is single and controllable. Taking the four typical waste thermoplastics (PP, PE, PS, PVC) as materials, the flow behaviors and pyrolysis characteristics of molten waste plastics pyrolysis in a vertical falling film reactor are discussed to achieve the high efficiency, high value recovery of waste plastics. Firstly, the pyrolysis processes of molten waste plastics are experimentally studied to obtain the relations between the key parameters such as temperature distribution, the effective heat transfer coefficients and the gas and oil yields and quantity. Then, a VOF-Reaction model fit to simulate molten waste plastics in a falling film reactor is presented and validated by comparing the simulation results to the experiment data. In the model, the interactions between liquid film and gas and plate surface are considered respectively. Using the model, the flow behaviors and reaction characteristics are numerically simulated to predict the morphology and velocity field of liquid film and determine the effect of heating rate and feed rate on the gas and oil yields. Lastly, the influence of impurities such as soil particles on the pyrolysis process is considered, a VOF-Reaction-Lagrange model is presented by coupling the discrete particle model to the VOF method to quantitatively determine the effects of impurities on the pyrolysis process. The result will provide theoretical support for the high value recovery and utilization of waste plastics.

热解是废塑料回收和资源化的重要途径，竖直降膜热解回收具有产品单一可控的优点。本项目以四种典型的热塑性废塑料（PP、 PE、PS、PVC）为研究对象，探索熔融废塑料在竖直降膜板上的流动热解特性，以实现废塑料的高效、高价值回收。首先进行熔融废塑料的竖直降膜热解实验，获得温度、有效传热系数等关键参数与油气产率和品质之间的关系；然后分别考虑热解过程中液膜与油气、降膜板之间的相互作用，建立适合熔融塑料竖直降膜热解模拟计算的VOF-Reaction模型，并将计算结果与实验结果对比进行模型验证，利用模型预测液膜形态和速度分布，确定加热速率、给料速率与油气产率之间的关系。最后考虑渣土等杂质的影响，将离散颗粒模型与VOF模型相耦合，建立适合含杂质颗粒熔融塑料竖直降膜热解计算的VOF-Reaction-Lagrange模型，定量考查杂质颗粒对热解过程的影响。本项目研究成果为废塑料的高值回收利用提供理论基础。

热解作为一种正在逐步发展的固废处理方式，不仅可以回收高热值的油、气产物，对热解油品质升级后可以混用，并能有效控制二次污染。目前工程上所用的固废处理热解器主要处理的对象是废轮胎、生活垃圾和生物质等，并没有热解器专门针对塑料热解而设计，考虑到塑料的热解分为熔化和热分解两个阶段，而熔化后的塑料粘度高而导热系数低，因此需要进行针对性设计，同时热解过程中的传热特性也是设计热解器时的关键基础数据。.本项目自行设计并搭建了降膜式反应实验台，对典型废塑料PE、PP、PS及其混合物进行热解实验，发现其热解油产率随着温度的升高而减小，而不凝气体产率随着温度的升高而增加。550℃时，PE热解油以链烷烃和链烯烃为主，占78wt.%；PP热解油以链烯烃和环烷烃为主，占85wt.%；PS热解油以芳香烃，占92wt.%。随着热解温度的升高，不凝气体成分中的C3、C4和C5含量减小而C1、C2含量增加。PE/PP和PP/PS混合热解后的转化率都低于单独热解的转化率，混合塑料热解时，增加PE、PS的比例能增加热解油的产率，增大PS含量可提高热解油的辛烷值，增大PE含量可提高热解油的热值。.考虑气体与液膜之间的曳力，构建了适合熔融塑料降膜热解的VOF-Reaction模型，并采用此模型，利用OpenFOAM中的interMixingFoam多相流求解器，对熔融塑料的降膜热解过程进行了模拟计算，发现：液膜表面为波状结构，呈现膜、流、滴的综合流型；随着热解温度的升高，液膜长度和平均表观传热系数减小，平均厚度略有增加；随着液膜入口速度的增加，液膜厚度和平均表观传热系数增大；随着反应器倾斜角度的增加，液膜流速变慢，停留时间增长，长度变短，液膜厚度略有增加。.考虑渣土颗粒对降膜热解的影响，构建了VOF-Reaction-Lagrange模型，基于OpenFOAM中MPPICInterFoam多相流求解器，对含渣土的混合塑料降膜热解过程进行了模拟研究。发现：适当的渣土可以强化传热，提高熔融塑料降膜热解的转化率；与不含渣土的熔融塑料降膜热解相比，液膜表面波纹不规则并且液膜长度变短，平均厚度增加，当渣塑比过大时会出现液膜中部先破裂的现象。