Applications of lumping kinetics methodology to plastic waste recovery via pyrolysis

Abstract

Plastic waste can present a huge environmental problem for conventional waste management as most plastic is not degradable in the same way as most packaging material. Plastic waste may take hundreds of years for natural degradation, and may then release some toxic chemicals simultaneously. The high cost of collection and separation occurring in the recycling and recovery stages of plastic waste conflicts with low value recyclate, contrasting with metal recycling which is cheaper and gives a much higher value product. Additionally, public opinion links waste plastic with effects on human health and wildlife. A number of technologies are being explored and deployed for the handling of plastic waste. Modelling and simulation to energy and chemicals recovery from the pyrolysis of plastic waste can be one facilitator of valorisation for the disposal of waste plastics, which furnishes a cost-effective method of process design and control of desirable product range and quality. A pyrolysis process operating in the absence or free of oxygen atmosphere permits the recovery of smaller molecular weight hydrocarbon products which can be used as fuels, or preferably chemical feedstock providing additional value. With the prospect of increasing volumes of waste and the increasing imbalance between energy demand and energy resources, modern pyrolysis techniques have recently become attractive for thermochemical conversion to mitigate adverse impacts during recycling and recovery of plastic waste. However, the installation of pyrolysis processes is always confined by location, feedstock resource, secondary pollution due to improper treatment, government policy and other extraneous factors, which leads to the pyrolysis process being costly to install and having a less beneficial social impact and awareness. This thesis is thus aimed at developing unified models to investigate predict the yields of the processes under a range of operational conditions by using the lumping methodology, at investigating the kinetic characteristics of plastic pyrolysis, to contribute a potential engineering solution in plastic waste recovery. To achieve the aim, the thesis is divided into three parts. In the first part of the work, the optimisation of the effect of operational parameters on the yields from thermal conversion of plastic waste was studied. The individual and interaction effects of multiple operational parameters (such as temperature and feed ration of feedstock compositions) of pyrolysis were optimised by using the design of experiments approach during the study of a thermogravimetric analyser (TGA) of plastic waste. The influence of temperature, residence time, particle size, feedstock composition and bed thickness on the conversion process of plastic waste was examined. The work has also been performed using fixed bed pyrolytic reactors (FBPR) to examine the thermal conversion behaviour and yield distribution. Waste polyethylene (PE), polypropylene (PP) and poly(ethylene phthalate) (PET) were chosen as feedstocks for the study. The thermal decomposition of plastic waste and the product distribution were experimentally investigated under inert nitrogen atmosphere over a temperature range from 400℃ to 550℃. The effect of temperature on product yields (liquid, solid and gas) within the batch reactor was discussed. Highly aliphatic nature of the pyrolysis oil and variety of C-C and C=C bonds were identified by using Fourier transform infrared spectroscopy (FTIR) coupled with the changes of wave peaks and wave range between wax (heavy oil) and oil. The experimental results indicated that temperature is a significant process parameter affecting the product distribution and reaction mechanism comparing to other parameters. In the second part of the work, the kinetics of the chemical reactions for the primary pyrolysis of plastic waste were studied. The aim of this part work was to study the primary pyrolysis behaviour of real components of plastic waste, to examine the kinetic characteristics for the primary pyrolysis, and to evaluate the effect of lumping selection on apparent kinetic parameters during models development. A kinetic model comprising of primary and secondary reactions was formulated to describe reaction pathways of primary pyrolysis behaviour and to assess the calculation of kinetic parameters. Different lumping models were developed to explore the suitable description of possible process pathways of the pyrolysis of plastic waste, and also the effect of lumping selection on the estimation of kinetic parameters. Firstly, a three-lump model (plastic lump, volatile lump, and solid residue lump including lower molecular weight polymer, and char-like products) was introduced to predict the kinetics of plastic converted into volatile yield and solid residue; then a four-lump model (plastic, gas, oil, wax) was introduced to ascertain the effect of three phase products (gas, liquid and wax) on lumping strategy and kinetics of plastic primary pyrolysis along with secondary cracking of heavier tar component based on higher conversion of HDPE degradation at higher temperature. A five-lump (feed, gas, liquid, wax and solid char residue) model extended from the four-lump model was proposed to define char residue as an individual lump, three secondary reaction pathways (residue to gas, oil, and wax; wax to gas and oil) were studied to determine the reaction mechanisms of the pyrolysis process of PE, PP and PE/PP mixtures. It was found that the estimation of kinetic parameters of activation energy and pre-exponential factor are affected by lump selection and temperature range. Comparison of the kinetics variation from different reaction pathways for secondary reactions suggested a five-lump model coupled with secondary of wax to oil and gas is the best reaction pathway for the pyrolysis of PE, PP, and PE/PP mixture. The model was validated against the experimental data obtained in the FBPR and showed a good agreement between the model prediction and experimental results. In the third part of this work, a case study of the kinetic characteristic of waste HDPE pyrolysis at different bed thicknesses with a function of temperature was studied via FBPR to examine the pyrolysis kinetic characteristic. Thermal decomposition of HDPE was determined using thermal gravimetric analysis (TGA) under inert nitrogen atmosphere over a temperature range from 400℃ to 550℃. The influence of bed thickness on the products distribution was also examined over a final temperature range of 425-550℃. As a result, it was found that the main thermal decomposition of HDPE samples occurred over a temperature range of 475-510℃ corresponding to a conversion range of 10- 95%. A greater wax fraction was yielded from FBPR with a thin bed at 450℃ due to better heat transfer performance. With the temperature increase to over 500℃, more oil and wax products were generated in FBPR with thick bed, inconsistently, more gases yielded from the thin bed at the same conditions. Based on the experimental results, a discrete lumping model comprising of three primary independent parallel reactions has been developed to describe the yield distribution of gases, oil fractions, wax fractions and solid residue, coupling with secondary cracking reactions of wax fractions into lighter fractions (oil) and gases. The model result was shown to be in good agreement with the experimental data. Additionally, the kinetics of waste HDPE decomposition into gas, oil, and wax fractions were discussed, which are consistent with the measured rate constants.