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Chemical Recycling of Waste Plastics
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Chemical Recycling of Waste Plastics

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Materials Chemistry".

Deadline for manuscript submissions: closed (31 December 2022) | Viewed by 27755

Special Issue Editors

Dr. Waheed Afzal

E-Mail Website
Guest Editor
School of Engineering, University of Aberdeen, Aberdeen AB24 3UF, Scotland, UK
Interests: chemical thermodynamics; fluid-phase equilibria; gas solubility; carbon capture and conversion; ionic liquids
Dr. Muhammad Usman Azam

E-Mail Website
Guest Editor
School of Engineering, University of Aberdeen, Aberdeen AB24 3UF, Scotland, UK
Interests: heterogeneous catalysis; waste plastics; life cycle assessment; technoeconomic analysis
Dr. Xiangyang Liu

E-Mail Website
Guest Editor
MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China
Interests: process engineering; computer modeling; thermodynamics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

This Special Issue of Molecules concerns publications of research related to chemical recycling and conversion of waste plastics to petrochemical feedstock, fuels and/or other value-added chemicals. Although the issue aims to bring together research related to catalysis, material synthesis and sciences using various methods for the mitigation of waste plastics, there is also a possibility to include research on the environmental impacts and handling of waste bioplastics. In this context, this issue covers the sustainable routes for the development of catalysts, study of physio-chemical properties and their effect on the activity and selectivity of the synthesized catalysts towards the chemical and biochemical recycling of waste plastics in general. In addition, interdisciplinary works related to the environmental monitoring through life cycle assessment and economic performance utilizing techno-economic analysis with potential applications to analyze and compare different chemical recycling methods are encouraged. While work from academia and research institutes is expected to be published, research findings from existing waste plastic recycling plants at any scale will also be welcomed. However, all the submitted articles should provide the effectiveness of the methodologies over existing solutions, practical applications in terms of lower energy requirement, minimum environmental emissions, productivity, and selectivity of catalysts to generate value-added chemicals and fuels.

Dr. Waheed Afzal
Dr. Muhammad Usman Azam
Dr. Xiangyang Liu
Guest Editors

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Keywords

  • heterogeneous catalysis
  • nanomaterials
  • green chemistry
  • thermoplastics
  • bioplastics
  • waste plastics to fuels
  • chemical recycling of plastics
  • biochemical recycling of plastics
  • feedstock recycling
  • plastics degradation
  • life cycle assessment
  • higher value products
  • cleaner production

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Published Papers (5 papers)

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Research

35 pages, 6445 KiB  
Article
Process Simulation and Life Cycle Assessment of Waste Plastics: A Comparison of Pyrolysis and Hydrocracking
by Muhammad Usman Azam, Akshay Vete and Waheed Afzal
Molecules 2022, 27(22), 8084; https://doi.org/10.3390/molecules27228084 - 21 Nov 2022
Cited by 15 | Viewed by 5552
Abstract
Pyrolysis and hydrocracking of plastic waste can produce valuable products with manageable effects on the environment as compared to landfilling and incineration. This research focused on the process simulation and life cycle assessment of the pyrolysis and hydrocracking of high-density polyethylene. Aspen Plus [...] Read more.
Pyrolysis and hydrocracking of plastic waste can produce valuable products with manageable effects on the environment as compared to landfilling and incineration. This research focused on the process simulation and life cycle assessment of the pyrolysis and hydrocracking of high-density polyethylene. Aspen Plus was used as the simulator and the Peng-Robinson thermodynamic model was employed as a fluid package. Additionally, sensitivity analysis was conducted in order to optimize product distribution. Based on the simulation, the hydrocracking process produced value-added fuels, i.e., gasoline and natural gas. In contrast, pyrolysis generated a significant quantity of pyrolysis oil with a high number of cyclo-compounds and char, which are the least important to be utilized as fuels. Moreover, in the later part of the study, life cycle assessment (LCA) was adopted in order to investigate and quantify their impact upon the environment using simulation inventory data, which facilitates finding a sustainable process. Simapro was used as a tool for LCA of the processes and materials used. The results demonstrate that hydrocracking is a better process in terms of environmental impact in 10 out of the 11 impact categories. Overall, the present study proposed a promising comparison based on energy demands, product distribution, and potential environmental impacts, which will help to improve plastic waste management. Full article
(This article belongs to the Special Issue Chemical Recycling of Waste Plastics)
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17 pages, 2792 KiB  
Article
Production and Analysis of the Physicochemical Properties of the Pyrolytic Oil Obtained from Pyrolysis of Different Thermoplastics and Plastic Mixtures
by Paul Palmay, Carla Haro, Iván Huacho, Diego Barzallo and Joan Carles Bruno
Molecules 2022, 27(10), 3287; https://doi.org/10.3390/molecules27103287 - 20 May 2022
Cited by 9 | Viewed by 3816
Abstract
The constant search for the proper management of non-degradable waste in conjunction with the circular economy makes the thermal pyrolysis of plastics an important technique for obtaining products with industrial interest. The present study aims to produce pyrolytic oil from thermoplastics and their [...] Read more.
The constant search for the proper management of non-degradable waste in conjunction with the circular economy makes the thermal pyrolysis of plastics an important technique for obtaining products with industrial interest. The present study aims to produce pyrolytic oil from thermoplastics and their different mixtures in order to determine the best performance between these and different mixtures, as well as to characterize the liquid fraction obtained to analyze its use based on said properties. This was carried out in a batch type reactor at a temperature of 400 °C for both individual plastics and their mixtures, from which the yields of the different fractions are obtained. The liquid fraction of interest is characterized by gas chromatography and its properties are characterized by ASTM standards. The product of the pyrolysis of mixtures of 75% polystyrene and 25% polypropylene presents a yield of 82%, being the highest, with a viscosity of 1.12 cSt and a calorific power of 42.5 MJ/kg, which has a composition of compounds of carbon chains ranging between C6 and C20, for which it is proposed as a good additive agent to conventional fuels for industrial use. Full article
(This article belongs to the Special Issue Chemical Recycling of Waste Plastics)
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12 pages, 2195 KiB  
Article
Conversion of Polypropylene Waste into Value-Added Products: A Greener Approach
by Jan Nisar, Maria Aziz, Afzal Shah, Iltaf Shah and Munawar Iqbal
Molecules 2022, 27(9), 3015; https://doi.org/10.3390/molecules27093015 - 7 May 2022
Cited by 10 | Viewed by 3247
Abstract
Plastic has made our lives comfortable as a result of its widespread use in today’s world due to its low cost, longevity, adaptability, light weight and hardness; however, at the same time, it has made our lives miserable due to its non-biodegradable nature, [...] Read more.
Plastic has made our lives comfortable as a result of its widespread use in today’s world due to its low cost, longevity, adaptability, light weight and hardness; however, at the same time, it has made our lives miserable due to its non-biodegradable nature, which has resulted in environmental pollution. Therefore, the focus of this research work was on an environmentally friendly process. This research work investigated the decomposition of polypropylene waste using florisil as the catalyst in a salt bath over a temperature range of 350–430 °C. A maximum oil yield of 57.41% was recovered at 410 °C and a 40 min reaction time. The oil collected from the decomposition of polypropylene waste was examined using gas chromatography-mass spectrometry (GC-MS). The kinetic parameters of the reaction process were calculated from thermogravimetric data at temperature program rates of 3, 12, 20 and 30 °C·min−1 using the Ozawa–Flynn–Wall (OFW) and Kissinger–Akahira–Sunnose (KAS) equations. The activation energy (Ea) and pre-exponential factor (A) for the thermo-catalytic degradation of polypropylene waste were observed in the range of 102.74–173.08 kJ·mol−1 and 7.1 × 108–9.3 × 1011 min−1 for the OFW method and 99.77–166.28 kJ·mol−1 and 1.1 × 108–5.3 × 1011 min−1 for the KAS method at a percent conversion (α) of 0.1 to 0.9, respectively. Moreover, the fuel properties of the oil were assessed and matched with the ASTM values of diesel, gasoline and kerosene oil. The oil was found to have a close resemblance to the commercial fuel. Therefore, it was concluded that utilizing florisil as the catalyst for the decomposition of waste polypropylene not only lowered the activation energy of the pyrolysis reaction but also upgraded the quantity and quality of the oil. Full article
(This article belongs to the Special Issue Chemical Recycling of Waste Plastics)
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26 pages, 4345 KiB  
Article
The Cradle-to-Cradle Life Cycle Assessment of Polyethylene terephthalate: Environmental Perspective
by Muhammad Tamoor, Nadia A. Samak, Maohua Yang and Jianmin Xing
Molecules 2022, 27(5), 1599; https://doi.org/10.3390/molecules27051599 - 28 Feb 2022
Cited by 22 | Viewed by 10178
Abstract
Over the last several years, the number of concepts and technologies enabling the production of environmentally friendly products (including materials, consumables, and services) has expanded. One of these ways is cradle-to-cradle (C2C) certifiedTM. Life cycle assessment (LCA) technique is used to [...] Read more.
Over the last several years, the number of concepts and technologies enabling the production of environmentally friendly products (including materials, consumables, and services) has expanded. One of these ways is cradle-to-cradle (C2C) certifiedTM. Life cycle assessment (LCA) technique is used to highlight the advantages of C2C and recycling as a method for reducing plastic pollution and fossil depletion by indicating the research limitations and gaps from an environmental perspective. Also, it estimates the resources requirements and focuses on sound products and processes. The C2C life cycle measurements for petroleum-based poly (ethylene terephthalate) (PET) bottles, with an emphasis on different end-of-life options for recycling, were taken for mainland China, in brief. It is considered that the product is manufactured through the extraction of crude oil into ethylene glycol and terephthalic acid. The CML analysis method was used in the LCIA for the selected midpoint impact categories. LCA of the product has shown a drastic aftermath in terms of environmental impacts and energy use. But the estimation of these consequences is always dependent on the system and boundary conditions that were evaluated throughout the study. The impacts that burden the environment are with the extraction of raw material, resin, and final product production. Minor influences occurred due to the waste recycling process. This suggests that waste degradation is the key process to reduce the environmental impacts of the production systems. Lowering a product’s environmental impact can be accomplished in a number of ways, including reducing the amount of materials used or choosing materials with a minimal environmental impact during manufacture processes. Full article
(This article belongs to the Special Issue Chemical Recycling of Waste Plastics)
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16 pages, 1544 KiB  
Article
Entrained Flow Gasification of Polypropylene Pyrolysis Oil
by Fredrik Weiland, Muhammad Saad Qureshi, Jonas Wennebro, Christian Lindfors, Taina Ohra-aho, Hoda Shafaghat and Ann-Christine Johansson
Molecules 2021, 26(23), 7317; https://doi.org/10.3390/molecules26237317 - 2 Dec 2021
Cited by 15 | Viewed by 2932
Abstract
Petrochemical products could be produced from circular feedstock, such as waste plastics. Most plants that utilize syngas in their production are today equipped with entrained flow gasifiers, as this type of gasifier generates the highest syngas quality. However, feeding of circular feedstocks to [...] Read more.
Petrochemical products could be produced from circular feedstock, such as waste plastics. Most plants that utilize syngas in their production are today equipped with entrained flow gasifiers, as this type of gasifier generates the highest syngas quality. However, feeding of circular feedstocks to an entrained flow gasifier can be problematic. Therefore, in this work, a two-step process was studied, in which polypropylene was pre-treated by pyrolysis to produce a liquid intermediate that was easily fed to the gasifier. The products from both pyrolysis and gasification were thoroughly characterized. Moreover, the product yields from the individual steps, as well as from the entire process chain, are reported. It was estimated that the yields of CO and H2 from the two-step process were at least 0.95 and 0.06 kg per kg of polypropylene, respectively, assuming that the pyrolysis liquid and wax can be combined as feedstock to an entrained flow gasifier. On an energy basis, the energy content of CO and H2 in the produced syngas corresponded to approximately 40% of the energy content of the polypropylene raw material. This is, however, expected to be significantly improved on a larger scale where losses are proportionally smaller. Full article
(This article belongs to the Special Issue Chemical Recycling of Waste Plastics)
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