No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Styrene recovery from polystyrene by flash pyrolysis in a conical spouted bed reactor
Abstract
Continuous pyrolysis of polystyrene has been studied in a conical spouted bed reactor with the main aim of enhancing styrene monomer recovery. Thermal degradation in a thermogravimetric analyser was conducted as a preliminary study in order to apply this information in the pyrolysis in the conical spouted bed reactor. The effects of temperature and gas flow rate in the conical spouted bed reactor on product yield and composition have been determined in the 450-600°C range by using a spouting velocity from 1.25 to 3.5 times the minimum one. Styrene yield is strongly influenced by both temperature and gas flow rate, with the maximum yield being 70.6wt% at 500°C and a gas velocity twice the minimum one.
Copyright © 2015 Elsevier Ltd. All rights reserved.
... In addition to the aromatic compounds, the presence of heavy metals, oligomers and heteroatoms creates complex challenges to overcome in order to achieve the PS circularity (Lange, 2021;Kusenberg et al., 2022;Roosen et al., 2020;Kusenberg et al., 2022;Kusenberg et al., 2022;Perrier and Takolpuckdee, 2005;Mandal, 2013;Mayo, 1943;Mayo, 1968). The proportions and compositions of the resulting products vary with the quality of the feedstock, the type of reactor used and the operating conditions (Anuar Sharuddin et al., 2016;Onwudili et al., 2009;Kaminsky, 2021;Chang, 2023;Al-Salem et al., 2017;Mo et al., 2014;Artetxe et al., 2015). Although important laboratory-scale results are available, there is a paucity of pilot-scale results in the literature, which are essential as they provide the basis for the design of large-scale plants and thus the transition to higher levels of technology readiness. ...
... The highest styrene concentrations in the pyro-oil were found at 500 • C and 6 kg/h (76.0 ± 2.3 wt%), and 550 • C and 9 kg/h (76.1 ± 2.3 wt%). These patterns, which are consistent with findings in the literature (Kaminsky, 2021;Miandad et al., 2016;Park et al., 2020) are comparable to those found in laboratory-scale studies (Artetxe et al., 2015;Bartoli et al., 2015;Liu et al., 2000) and also at TRL-5 scale, as those reported by (Kaminsky, 2021) using a fluidised bed (74.9 wt%). Based on the above, these results serve to demonstrate the suitability of the auger technology for carrying out the pyrolysis of PS waste. ...
... A decrease in styrene concentration is also observed at the highest temperature (600 • C), ascribed to the occurrence of secondary reactions. As a result, the gas yield and the concentration of toluene, benzene and ethylbenzene increase as the degradation of the PS becomes more severe (Onwudili et al., 2009;Artetxe et al., 2015;Park et al., 2020;Ojha and Vinu, 2015;Demirbas, 2004;Adnan and Jan, 2015). ...
This study presents the pyrolysis of polystyrene waste using a single-auger pyrolysis reactor at an industrially relevant scale, i.e. within the fifth technology readiness level (TRL-5). In order to gain a better understanding of the influence of the main variables involved in this technology, a thorough experimental campaign was performed with PS feed rates of 6 and 9 kg/h, covering a temperature range of 450-600 • C. These variables influence not only the distribution of the products, but also the presence of compounds that may affect the suitability of the pyro-oil for re-polymerisation processes. In this sense, gas chromatography (GC), gas chromatography coupled to mass spectrometry (GC/MS) and simulated distillation (SimDist) of the resulting pyro-oils provided valuable insights into styrene purification. The data collected confirms the strength of the auger reactor for thermochemical recycling of polystyrene waste, and contributes to the future development of the plastics circularity on an industrial scale.
... To simulate the reforming of plastic pyrolysis volatiles, the following input streams were considered for the Gibbs-free reactor: i) Pyrolysis oil: This refers to the condensable fraction (liquid) of the product stream obtained in the degradation of the different polymers studied, which is made up of hydrocarbons and oxygenates. The composition of this stream was determined based on previous experimental results obtained in the fast pyrolysis conducted in a CSBR of each specific raw material, namely, high-density polyethylene (HDPE) [51], polypropylene (PP) [52], polystyrene (PS) [53] and polyethylene terephthalate (PET) [54]. ii) Pyrolysis gases: The composition of this non-condensable fraction of the pyrolysis stream was also defined based on previous experimental results of fast pyrolysis of plastics. ...
... Moreover, the yield of waxes was determined using gravimetric techniques. Further details of the procedure and product analysis have been reported elsewhere [51][52][53][54]. ...
... It is to note that the selective formation of waxes and heavy oil in polyolefins fast pyrolysis under mild conditions was widely reported in the literature [55][56][57]. The case of PS is noteworthy, as its thermal degradation is very selective towards the formation of styrene [53,58], whose yield is above 70% under the mentioned conditions. Thus, the yield of incondensable gaseous products is very low and the remaining ones are mainly of aromatic nature. ...
... For the pyrolysis process of waste polystyrene, scientists have investigated a range of reactor designs, such as batch-type, fixed-bed, continuous flow, and pressure reactors [9][10][11][12][13]. These studies focused into how operating factors, such as temperature and pressure, affected product yields during polystyrene pyrolysis [14,15]. Additionally, recent studies have examined at how the composition of plastic blends and solvents affect the yield of the liquid fraction produced by polystyrene pyrolysis [16,17]. ...
... kJ/ mol and E a = 146.1 kJ/mol. The generated free radical (21) can stabilize and give rise to α-methylstyrene (14) with ΔH = -92.2 kJ/mol and E a = 15.6 kJ/mol via TS 19 , as well as isopropylbenzene (22) with ΔH = -36.9 ...
... kJ/mol and 162.1 kJ/mol, respectively. The free radical (23) can further decompose to yield a free radical (24) and α-methylstyrene (14) ( ΔH = 70.3 kJ/mol, E a = 103.8 ...
Context:
Plastic waste pyrolysis offers a potential solution to reduce plastic accumulation, but prioritizing monomer recovery from the process is crucial to effectively address the environmental consequences of plastic accumulation. This study focuses on enhancing the yield of styrene during the pyrolysis of polystyrene by investigating thermal and kinetic data. A comprehensive investigation into the thermal degradation pathways of polystyrene is imperative to overcome the challenges associated with its waste management. The calculated bond dissociation energies reveal that the cleavage of non-terminal carbon-carbon bonds is energetically favorable, resulting in the formation of high molecular weight benzylic radicals. Based on these findings, four pyrolysis pathways are proposed, and the associated thermodynamic and kinetic parameters are determined using the DFT method. The major products identified in this study include styrene, α-methylstyrene, isopropylbenzene, methylbenzene, ethylbenzene, and methane. Furthermore, optimizing the temperature profile of the reactor is shown to enhance the recovery of styrene, thereby contributing to the reduction of plastic waste. This study provides valuable insights into the effective resource recovery from polystyrene waste pyrolysis, emphasizing the significance of managing pyrolysis conditions to achieve maximum yield. By controlling the temperature profile during the pyrolysis process, it is possible to obtain a high yield of styrene, facilitating the efficient recovery of the monomer from waste polystyrene and addressing the environmental concerns associated with plastic accumulation.
Methods:
In this study, all calculations were performed using the B3LYP/6-31G(d) level of theory with the Gaussian 16 program package. The proposed model underwent geometry optimization and frequency calculations. Transition states were optimized using the TS Berny method, and energy profiles along reaction pathways were refined using the QST3 method. The IRC method validated proposed mechanisms and investigated energy profiles. Structural models were visualized using GaussView 6.0.
... The gas release from the plastics during pyrolysis at 10°C/min was monitored online by MS and the results are shown Figure 2b. Since most products from PS are found in oil, only the gases from PE and PP are monitored [44,45]. The temperature range is basically consistent with the pyrolysis weight loss, and the peak temperatures for PE and PP are 481 • C and 463 • C, respectively. ...
... The gas release from the plastics during pyrolysis at 10℃/min was monitored online by MS and the results are shown Figure 2b. Since most products from PS are found in oil, only the gases from PE and PP are monitored [44,45]. The temperature range is basically consistent with the pyrolysis weight loss, and the peak temperatures for PE and PP are 481 °C and 463 °C, respectively. ...
Pyrolysis is a promising technology used to recycle both the energy and chemicals in plastics. Three types of plastics, polyethylene plastic (PE), polypropylene plastic (PP) and polystyrene plastic (PS) were investigated using thermogravimetry–mass spectrometry (TG–MS) and reactive force field molecular dynamics (ReaxFF-MD) simulation. The thermogravimetric analysis showed that all three plastics lost weight during the pyrolysis in one step. The thermal decomposition stability is PS < PP < PE. The activation energies and reaction mechanism function of the three plastics were determined by the Kissinger and CR methods. Meanwhile, the ReaxFF-MD combined with density functional theory (DFT) was used to calculate the kinetics, as well as explore the pyrolysis mechanism. The calculated kinetic results agree well with the experimental methods. The common pyrolysis reaction process follows the dissociation sequence of the polymer to polymeric monomer and, then, to the gas molecules. Based on the bond length between the monomers and the bond dissociation energy for different plastics, the required energy for polymer dissociation is PS < PP < PE, which microscopically explains the macro-activation energy sequence and thermal stability. Moreover, due to the retention of aromatic rings in its monomers, PS almost completely converts into oil.
... We note that SRF is mainly composed by polyethylene and polypropylene films. But other behaviors of polystyrene pyrolysis have been also reported, such as the discussions on the coating of melted polystyrene on sand particle [44] or on biomass particle [46]. Thus the phase transition characteristics in polystyrene pyrolysis might be still in question in fast pyrolysis condition and further investigations could be suggested in the future. ...
... Additionally, the existence of polystyrene particle above its known boiling point (430 • C) in the simulation still needs to be discussed further. In the thermogravimetric analysis (TGA) of polystyrene [44,[47][48][49][50][51], it had been found that the polystyrene degradation continues up to the temperature near or even higher than 500 • C. For example, the temperature at the maximum degradation rate was 466.5 • C, and the final temperature of pyrolysis was 546.4 • C at the maximum heating rate of 40 • C/min [49]. But a careful interpretation of the TGA data is needed, because the TGA and DTG curves shift to higher temperature with an increased heating rate. ...
Coffee ground is considered one of the promising biomass resources because it is one of the most popular beverages and has higher calorific value than other kinds of biomass. In the previous study on the fast pyrolysis of coffee ground, the viscosity of the pyrolysis oil was found to be very high and it was suggested to mix some amount of alcohol to reduce its viscosity. The quality of the pyrolysis oil also can be improved by mixing synthesis polymer such as polyethylene, polyethylene, or polystyrene as a feedstock. Recently, a tilted-slide reactor for fast pyrolysis was designed with a feeding capacity of 20 kg/h, in which no fluidizing gas is necessary for conveying hot sand. In the previous experiment, the co-pyrolysis of coffee ground and waste polystyrene foam was performed in the tilted-slide reactor. The ratio of waste polystyrene foam in the feedstock mixture was varied from 0% to 100% by weight, and fast pyrolysis was performed near the temperature of 550 °C. In this study, the co-pyrolysis was simulated by a commercial computational fluid dynamics code, and the results were compared with experimental results. A multistep kinetic mechanism was adopted for the pyrolysis of coffee ground and polystyrene. The volatile yields in the simulation approached the pyrolysis oil yields in the experiment by introducing a composite boundary condition to retain the unpyrolyzed polystyrene near the reactor bottom for longer residence time. The elemental composition and moisture contents became similar with measurement when increasing polystyrene blending ratio.
... Ultimately, it can further consume plastic waste and decrease their disposal, especially for plastics that are challenging to recycle as polystyrene [11]. For instance, polystyrene can be converted to styrene by pyrolysis with very high yields (>60 wt%) [12,13], and polyethylene can be converted to petrochemical products [14]. ...
... Due to the confusion about the synergy between biomass and plastics in the literature [11][12][13]21,22], this study clearly describes and visualizes the synergy between the 2 feedstocks, by classification of the products into families and testing the synergy for each family. The objective of this work is to study the co-pyrolysis of certain types of municipal plastic waste as high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), and their mixture (MP); according to their contribution to Europe's plastic wastes (34 wt% PP, 32 wt% LDPE, 21 wt% HDPE, and 13% PS) [23], with beech wood (Fagus sylvatica) (BW) as lignocellulosic forestry residue, for the production of pyrolytic oils of improved quality. ...
This study aimed to investigate the co-pyrolysis of beech wood, with polypropylene, high- and low-density
polyethylene, polystyrene, and their mixture as synergic effects on liquid oil production inside a tubular
reactor under an inert nitrogen atmosphere. The liquid oil recovered was analyzed using gas chromatography
methods. Beech wood to plastic ratio and operating temperature were varied. Results showed that for beech
wood polystyrene mix an increase of aromatic compounds compared to the theoretical value. The oil was mainly
constituted of aromatics of about 68 wt% for the 50-50 mixture. The liquid oil experiences a great reduction in
the oxygen content from 41 wt% (beech wood) to just 8 wt% for the polystyrene-beech wood 50-50 mixture. The
oil also exhibited a high heating value of 37 MJ/kg with high aromatic content making the oil suitable to be
blended with high mass proportions in the gasoline pool. In contrast, a negative synergy was observed between
beech wood and polyolefins leading to an increase in acids and carbohydrates of 7 wt% to 10 wt% for polyethylene
and polypropylene respectively compared with the theoretical value. Regarding the temperature
variation between 450 and 600 ◦C, no remarkable change in oxygenated compounds was observed.
... Char is the solid residue after gases and tar have been generated from a carbonaceous material during devolatilization or pyrolysis. Many researchers observed a low char yield in the gasifier could catalytically improve reforming reactions for H 2 production and may reduce the formation of tar from plastic degradation, due to common plastics which can be almost completely converted into volatiles (63,64). In contrast, a considerable char yield can get in complex plastic wastes or plastic co-gasification with other feedstocks (biomass, coal, etc.) (65,66). ...
Hydrogen is one of the most important feedstocks for the chemical industry, power production, and the decarbonization of other sectors that rely on natural gas. The production of hydrogen from...
... Pyrolysis is the most commonly used approach for chemical recycling of PS [32]. Artetxe et al. [33] achieved about 70 wt% yield of styrene (St) in continuous pyrolysis of PS in a conical spouted bed reactor at 500°C. The operation of thermal pyrolysis is simple but selecting the catalyst is a real challenge. ...
A simple strategy is proposed to offer styrene (St) copolymer with intrinsic and catalyst‐free upcycling capability by incorporating α‐methylstyrene (AMS) units into the copolymer poly(AMS‐co‐St) (PAS). Compared to polystyrene (PS), the PAS is more susceptible to depolymerizing into St and AMS monomers with higher yields at relatively lower temperature. The yield of liquid chemicals of PAS‐4 (26.1 mol% AMS) is up to 93 wt% at 380°C, whereas that PS as control is 78 wt%. The reclaimed liquid chemicals of PAS‐4 contain 58.4 wt% St and 37.7 wt% AMS, while the chemicals of PS contain 81.8 wt% St and 4.1 wt% AMS. The introduction of AMS units in PAS leads to a reduction in the liquid chemicals other than St and AMS monomers. The blends of PS and PAMS do not demonstrate the synergistic effect regarding facilitating the PS depolymerization. In addition, the properties of the new PASs prepared with the reclaimed monomers are comparable with those of the virgin PASs. This work proves the concept that recycling efficiency could be boosted by incorporating liable structural units into the copolymer chains and demonstrates a closed‐loop of PS up‐cycling.
... When the depolymerization reaction type plastic is thermally cracked, the polymer will dissociate, mainly cutting off the chemical bond between the single molecules to generate monomer. This type of plastic mainly includes PS, PMMA, etc., and their pyrolysis has a high monomer recovery rate [30][31][32]. The fracture of chemical bond in the molecule of randomly cracked plastics during pyrolysis is random, and a certain number of molecular compounds combined by carbon atoms and hydrogen atoms will be produced, which results in a wide distribution of products, including gas, liquid hydrocarbon, wax and solid residues [4,[33][34][35]. ...
Plastic pyrolysis technology, as an efficient and stable path for chemical recycling of waste plastics, alleviates current energy pressures and solves the problem of continuous accumulation of waste plastics in the environment. At present, the vast majority of research on plastic pyrolysis is focused on how to improve the yield and quality of liquid fuels, while there is generally little research on the gases generated by plastic pyrolysis. However, gases such as H2, CH4, and light hydrocarbons generated during pyrolysis also have high utilization value, and have very considerable application prospects in chemical, aerospace, and metallurgical fields. In addition, compared with the separation difficulties of liquid products, the treatment of gas products is easier and more conducive to subsequent utilization. This article discusses and analyzes the yield and composition of gases generated by plastic in three different pyrolysis methods: direct pyrolysis, catalytic pyrolysis, and microwave pyrolysis. Compared to traditional direct pyrolysis, catalytic pyrolysis and microwave pyrolysis can treat plastic waste more efficiently and energy-efficient, and have higher gas yields. This article also discusses various factors such as temperature that influence the formation of gas products and their importance. Finally, the challenges faced are proposed, aiming to provide reference and direction for future research on improving the yield of gas generated by plastic pyrolysis.
... This characteristic is especially valuable when dealing with plastic waste, which can be prone to forming clumps that hinder efficient pyrolysis reactions in traditional fixed-bed reactors (Lopez et al., 2010). Prior studies reported the use of this reactor in treating plastic wastes (Orozco et al., 2021), polystyrene (Artetxe et al., 2015), polypropylene and PET (Niksiar, Faramarzi, & Sohrabi, 2015), etc. ...
Owing to the rapid growth of the population in modern society, the generation of wasteis also increasing significantly. This rise in waste generation has severe ecological consequences (Singh, Duan, & Tang, 2020). Traditional methods of waste disposal, such as incineration, dumping, and landfilling, when not managed properly, lead to pollution of air, water, and soil (Ahamed et al., 2020; Azar & Azar, 2016). Problematic waste types encompass municipal solid waste (MSW), agricultural and industrial waste, plastics, e-waste, and biomedical waste. Mismanaged handling of these wastes results in the emission of greenhouse gases (GHGs), contributing to environmental pollution. Despite recognizing the non-renewable nature of fossil fuels, society continues to rely on them for energy, causing global warming and shifts in climate (Raihan & Tuspekova, 2022; Wang & Yan, 2022). Various sectors, including industry, agriculture, mining, and municipalities, contribute to the annual generation of solid waste in the country. It is projected that global waste production will reach 27 billion tonnes annually by 2050. At present, Asia is responsible for one-third of total waste, with China (0 0.49 kg capita21 day21) and India (0.50 0.9 kg capita21 day21) making significant contributions (Kumar & Agrawal,2020). Moreover, the world produces approximately 300 million tonnes of plastic waste each year, of which only 9% is recycled, about 14% is collected for recycling, and the remainder ends up in the oceans annually (Rezania et al., 2019). The persistent nature of plastics poses a global threat, as microplastics (MPs) infiltrate water bodies, polluting rivers and oceans (Hira et al., 2022; Wojnowska-Baryła, Bernat, & Zaborowska, 2022). These MPs originate from larger plastic items that are not properly disposed of, including agricultural plastic films, municipal plastic debris such as bags and bottles, as well as electronic waste (Gangwar & Pathak, 2021). The management of waste from various sources has become paramount. Pyrolysis has emerged as a novel technique to convert waste into solid, liquid, and gaseous products by adjusting the temperature (Bhatnagar, Khatri, Krzywonos, Tolvanen, & Konttinen, 2022). This process enables the transformation of low-energy-density materials into high-energy-density biofuels and valuable chemicals (Zhai et al., 2022). An advantage is the versatility of raw material sources, encompassing industrial and household residues (Al-Mrayat et al., 2022). Agricultural and industrial wastes like crop residues, press mud, synthetic oil, and MSW can be effectively converted to valuable energy sources through pyrolysis, contributing to global sustainability (Cheng et al., 2022). The utilization of agro-industrial waste for biofuel production offersan eco-friendly and renewable alternative to fossil fuels. This shift towards pyrolysis-based energy generation reduces reliance on fossil fuels and mitigates environmental contamination and climate change (Nair, Agrawal, & Verma, 2022). As a result, the aim is to achieve a zero-waste society while promoting sustainable bioenergy production (Sarkar, Butti, & Mohan, 2018). This chapter explores problematic waste types, their ecological risks, current waste management practices, diversification of wastes through pyrolysis, properties of the pyrolyzed products, advancements in pyrolysis technology for waste management, and challenges in handling problematic wastes.
... The collected organic fraction includes substantial compounds dominated by monoaromatic hydrocarbons (MAHs), styrene, phenols, naphthalene, polycyclic aromatics hydrocarbons (PAHs), aliphatic HCs, nitriles, N containing and halogen containing compounds. PVC and PS components in the WEEE sample are potential sources to produce MAHs, PAHs and styrene via pyrolysis process [35]. Moreover, the formation of olefin and alkanes attributed to the thermal decomposition of PP, PE and PVC, respectively [36]. ...
... The recovery of styrene from polystyrene by flash pyrolysis was studied in a conical SBR in the temperature range of 450-600 • C and a variable spouting velocity from 1.25 to 3.5 times the minimum one. The results showed a maximum yield, 70.6 wt%, obtained at 500 • C and a gas velocity twice the minimum one [63]. ...
Recent research advances and technological developments of spouted bed reactors (SBRs) have been discussed in this work. SBR has aroused increasing interest since their invention in 1955 due to its flexibility in processing different feedstocks and the high process yields that can be achieved due to its characteristic fluid dynamics. However, even though highly satisfactory results have been obtained at the laboratory scale for different applications (i.e., drying or thermochemical reactions, among others), their full implementation at an industrial level is still scarce, mainly due to the challenges encountered for their scale-up. In this work, an initial short description of SBR and configurations is followed by a review of the main experimental activities that have been conducted at different scales in the period 2013–2023. Advanced solutions such as multi-unit reactors and the use of rectangular geometries instead of the classical cylindrical ones have arisen as potential areas for further study and development to achieve a reliable implementation of the spouted bed technology at an industrial scale.
... On the other hand, as could be observed in Fig. 4D, the presence of PS generated predominantly larger organic fraction, reaching up to 28.83 wt% at the highest PS percentage. This outcome could be expected since PS consisted of virtually no oxygen content; its pyrolytic oil is mainly consisted of organic fraction [41,42]. Although, the organic fraction decreased regardless of PS percentage in CaO treatments, it was higher than the organic fraction of conventional CG pyrolysis (the organic fractions at PS percentage of 10 w% and 20 wt% were 20.94 wt % and 24.85 wt%, respectively). ...
The high concentration of oxygenated components in biomass pyrolysis oil challenges the conversion of biomass to bio-fuels via fast pyrolysis. Herein, calcium oxide was prepared to convert the oxygenated compounds in the pyrolytic products of coffee-grounds to valuable hydrocarbons. Meanwhile, waste polystyrene foam was co-pyrolyzed with coffee-grounds to promote hydrogen transfer reactions, resulting from scission of the polystyrene radicals. The results show that calcium oxide could effectively reduce the concentration of undesired oxygenated compounds like phenols and fatty acids in pyrolytic products. The effects of calcium oxide and polystyrene were mainly reflected in the production of a deoxygenated organic fraction, rich in aromatics and with a considerable calorific value. An organic fraction with an oxygen content of 13.38 wt% was obtained at a catalyst-to-feedstock ratio of 1:8 and an amount of polystyrene foam of 20 wt%. This fraction, due to its properties and its compositions, is considered for use as potential feedstock in the energy market.
... A low fluidizing flow rate prolongs the contact time of the initial intermediates which intensifies the formation of secondary products (e.g., PAHs) and coke precursors (Soni et al., 2021). In contrast, a higher flow rate of fluidizing gas in the pyrolysis process promotes the heat transfer rate and favorably induces sweeping coke precursors toward outside of the channels of a catalyst (Artetxe et al., 2015;Gayubo et al., 2010). However, an excessively high flow rate of fluidizing gas might lead to some losses in the mass balance, which makes difficult the complete recovery of all the low-molecular-weight compounds from PW pyrolysis (Peng et al., 2022). ...
... Furthermore, this method could consume plastics and limit their disposal, especially those as challenging to recycle as polystyrene [25]. For instance, polystyrene can be converted to styrene via pyrolysis with very high yields (>60 wt.%) [26,27]. ...
This study aims to investigate the catalytic co-pyrolysis of beech wood with polystyrene as a synergic and catalytic effect on liquid oil production. For this purpose, a tubular semi-continuous reactor under an inert nitrogen atmosphere was used. Several zeolite catalysts were modified via incipient wetness impregnation using iron and/or nickel. The liquid oil recovered was analyzed using GC-MS for the identification of the liquid products, and GC-FID was used for their quantification. The effects of catalyst type, beechwood-to-polystyrene ratio, and operating temperature were investigated. The results showed that the Fe/Ni-ZSM-5 catalyst had the best deoxygenation capability. The derived oil was mainly constituted of aromatics of about 92 wt.% for the 1:1 mixture of beechwood and polystyrene, with a remarkably high heating value of around 39 MJ/kg compared to 18 MJ/kg for beechwood-based bio-oil. The liquid oil experienced a great reduction in oxygen content of about 92% for the polystyrene–beechwood 50-50 mixture in comparison to beechwood alone. The catalytic and synergetic effects were more realized for high beechwood percentages as a 75-25 beechwood–polystyrene mix. Regarding the temperature variation between 450 and 600 °C, the catalyst seemed to deactivate faster at higher temperatures, thus constituting a quality reduction in the pyrolytic oil in high-temperature ranges.
... Through thermo-and photochemical strategies, plastics are converted to fuels, [13][14] adhesives, [15][16] fine chemicals, [17] and monomers. [18] With or without catalysts, [19][20][21][22][23][24] PS can be depolymerized to styrene, which can be used for repolymerization and copolymerization. But the monomer is of low value, restricting the scalability of the depolymerizationrepolymerization strategy. ...
Polystyrene (PS) is one of the least recycled large‐volume commodity plastics due to bulkiness of foam products and associated contaminants. PS recycling is also severely hampered by the lack of financial incentive, limited versatility, and poor selectivity of existing methods. To this end, herein we report a thermochemical recycling strategy of “degradation‐upcycling” to synthesize a library of high‐value aromatic chemicals from PS wastes with high versatility and selectivity. Two cascade reactions are selected to first degrade PS to benzene under mild temperatures, followed by the derivatization thereof utilizing a variety of acyl/alkyl and sulfinyl chloride additives. To demonstrate the versatility, nine ketones and sulfides of cosmetic and pharmaceutical relevance were prepared, including propiophenone, benzophenone, and diphenyl sulfide. The approach is also amenable to sophisticated upcycling reaction designs and can produce desired products stepwise. The facile and versatile approach will provide a scalable and profitable methodology for upcycling PS waste into value‐added chemicals.
Plastic waste management is a pressing global problem that requires sustainable solutions to mitigate environmental harm. To this end, pyrolysis offers a practical method for converting waste plastics into valuable...
This work aims to study the catalytic pyrolysis of polystyrene (PS) in order to obtain a styrene-rich liquid appealing for re-polymerisation processes. The process was performed in a fixed-bed reactor using low-cost materials such as ilmenite, olivine, calcium oxide (CaO) and dolomite as catalysts at 600 ºC. A non-catalytic test with sand was also conducted for comparison purposes. The resulting pyro-oil yield was between 89 and 96 wt%, and consisted mainly of styrene monomer, styrene dimer and styrene trimer, as well as other light aromatic compounds depending on the catalyst used. Olivine and CaO were capable to increase the styrene monomer concentration in the pyro-oil by about 10–12 % compared to non-catalytic pyrolysis, reaching up to 74.5 and 71.4 wt%, respectively. Furthermore, these catalysts reduced the concentration of styrene dimer and styrene trimer in the pyro-oil, which could be useful for further re-polymerisation processes. However, the presence of polyaromatics (PAHs) in the pyro-oils obtained from ilmenite, olivine and CaO was also identified, and other families such as mono-aromatics, di-aromatics, BTX, and nitrogen heterocyclic compounds were also increased in all pyro-oils obtained with catalysts compared to those obtained by non-catalytic pyrolysis. In addition, several inorganic species in the pyro-oil were reduced by the addition of the catalysts. This effect was observed after the addition of olivine and CaO, especially in the reduction of S, Mg, Ca and Fe. These results clearly demonstrate the potential of pyrolysis to convert PS waste into valuable building blocks for the production of new plastics.
The recovery and recycling/upcycling of plastics and polymer-based materials is needed in order to reduce plastic waste accumulated over decades. Mechanical recycling processes have made a great contribution to the circularity of plastic materials, contributing to 99% of recycled thermoplastics. Challenges facing this family of processes limit its outreach to 30% of plastic waste. Complementary pathways are needed to increase recycling rates. Chemical processes have the advantage of decomposing plastics into a variety of hydrocarbons that can cover a wide range of applications, such as monomers, lubricants, phase change materials, solvents, BTX (benzene, toluene, xylene), etc. The aim of the present work is to shed light on different chemical recycling pathways, with a special focus on thermochemicals. The study will cover the effects of feedstock, operating conditions, and processes used on the final products. Then, it will attempt to correlate these final products to some petrochemical feedstock being used today on a large scale.
Plastic and mixed plastic waste (PW) has received increased worldwide attention owing to its huge rate of production, high persistency in the environment, and unsustainable waste management practices. Therefore, sustainable PW management and upcycling approaches are imperative to achieve the objectives of the United Nations Sustainable Development Goals. Numerous recent studies have shown the application and feasibility of various PW conversion techniques to produce materials with better economic value. Within this framework, the current
review provides an in-depth analysis of cutting-edge thermochemical technologies such as pyrolysis, gasification, carbonization, and photocatalysis that can be used to value plastic and mixed PW in order to produce energy and industrial chemicals. Additionally, a thorough examination of the environmental impacts of contemporary
PW upcycling techniques and their commercial feasibility through life cycle assessment (LCA) and techno economical assessment are provided in this review. Finally, this review emphasizes the opportunities and challenges accompanying with existing PW upcycling techniques and deliver recommendations for future research
works.
Historically the field of heterogeneous catalysis has focused on the design and optimisation of the catalytic materials. However, as these optimisations start to reach diminishing returns, attention has turned to non-conventional means for improving reaction conditions such as the use of ultrasound, plasma, electromagnetic heating and microwave heating. Microwave-assisted catalysis has been demonstrated to be useful in a wide range of applications including ammonia synthesis, desulfurization and production of chemicals from biomass.
Advances in Microwave-assisted Heterogeneous Catalysis begins with the basics of microwave heating and the role of microwaves in heterogeneous catalysis. It goes on to cover the mechanisms of microwave specific reaction rate enhancement, microwave-assisted synthesis of porous, nonporous and supported metal catalysts, microwave augmented reactor technology and microwave-induced catalysis. The application of microwave-assisted heterogeneous catalysis in various fields of energy conversion, environmental remediation, and bulk and specialty chemicals synthesis are also discussed, making this a great reference for anyone involved in catalysis research.
Catalytic cracking is a promising approach to chemically recycle polyolefins by converting them into smaller hydrocarbons like naphtha, and important precursors of various platform chemicals, such as aromatics. Cracking catalysts, commonly used in the modern refinery and petrochemical industry, are tailored to process gaseous or liquid feedstock. Polyolefins, however, are very large macromolecules that form highly viscous melts at the temperatures required to break their backbone C–C bonds. Therefore, mass transport is expected to limit the performance of traditional cracking catalysts when applied to the conversion of polymers. In this work, we study these effects during the cracking of polypropylene (PP) over catalysts utilized in the fluid catalytic cracking (FCC) process. Thermogravimetric experiments using PP of varying molecular weight (Mw) and catalysts of varying accessibility showed that low Mw model polymers can be cracked below 275 °C, while PP of higher Mw required a 150 °C higher temperature. We propose that this difference is linked to different degrees of mass transport limitations and investigated this at length scales ranging from milli- to nanometers, utilizing in situ optical microscopy and electron microscopy to inspect cut open catalyst–polymer composites. We identified the main cause of transport limitations as the significantly higher melt viscosity of high Mw polymers, which prohibits efficient catalyst–polymer contact. Additionally, the high Mw polymer does not enter the inner pore system of the catalyst particles, severely limiting utilization of the active sites located there. Our results demonstrate that utilizing low Mw polymers can lead to a significant overestimation of catalyst activity, and suggest that polyolefins might need to undergo a viscosity reducing pre-treatment in order to be cracked efficiently.
The method of oxidative upgrade by nitric acid is used to convert polystyrene plastic into a high-value chemical raw material, benzoic acid. The yield can reach nearly 90% at 180 °C within 3 h, while the purity of the product can reach more than 95%.
Polystyrene (PS) is one of the least recycled large‐volume commodity plastics due to bulkiness of foam products and associated contaminants. PS recycling is also severely hampered by the lack of financial incentive, limited versatility, and poor selectivity of existing methods. To this end, herein we report a thermochemical recycling strategy of “degradation‐upcycling” to synthesize a library of high‐value aromatic chemicals from PS wastes with high versatility and selectivity. Two cascade reactions are selected to first degrade PS to benzene under mild temperatures, followed by the derivatization thereof utilizing a variety of acyl/alkyl and sulfinyl chloride additives. To demonstrate the versatility, nine ketones and sulfides of cosmetic and pharmaceutical relevance were prepared, including propiophenone, benzophenone, and diphenyl sulfide. The approach is also amenable to sophisticated upcycling reaction designs and can produce desired products stepwise. The facile and versatile approach will provide a scalable and profitable methodology for upcycling PS waste into value‐added chemicals.
Recycling waste plastics requires the degradation of plastics into small molecules. However, various products are widely distributed using traditional methods of depolymerizing polystyrene (PS) such as catalytic pyrolysis and hydrogenolysis. Here, we creatively report a N-bridged Co, Ni dual-atom (Co-N-Ni) catalyst for the targeted conversion of waste PS plastics to ethylbenzene via a pressurized tandem fixed-bed reactor where hydropyrolysis is coupled with downstream vapor-phase hydrotreatment. The Co-N-Ni catalyst achieves 95 wt % PS conversion with 92 wt % ethylbenzene yield, significantly superior to the corresponding single-atom catalysts, and enables degradation of real PS plastics. Theoretical calculations and experimental results demonstrate that the d-band center of metal atoms is well regulated in the Co-N-Ni catalyst. The Co site activates the C═C bond more easily, while the Ni site spatially optimizes the adsorption configuration of the styrene molecule due to the electronic interaction. This Co-N-Ni catalyst in the tandem reactor also shows excellent durability and provides a new direction for real plastic degradation.
The disposal of single waste plastic is becoming from worst to disaster due to its continuous increasing demand in the market. The majority of single-used plastic ends to landfill after its useful life that leads to several problems including air and water pollution. In the present research, the single-used plastic is converted into liquid fuel by the pyrolysis process. The waste plastic fuel (WPF) is mixed with petroleum diesel in equal volumetric proportion to test it on the engine. To enhance the combustion characteristics of the fuel, oxygenated fuel is added to the blend, and the performance and emissions characteristics of the unmodified engine are tested. The better properties of WPF compared to diesel fuels improve the brake thermal efficiency of the engine by 1% for all loading conditions. However, a slight increase in emissions such as CO and NOX is observed. Further improvement in the BTE is observed by adding a small proportion of n-butanol.KeywordsWPFn-ButanolPerformanceEmissions characteristicsSingle-use plastic
Axial flow compressors/fans are widely used machines in industrial as well as aircraft gas turbine engines. The performance improvement of such machines has improved significantly due to key efforts of researchers and engineers. The hard work and research inputs require for the development of a single compressor which includes years of careful study and investigations. Rotor is a primary element of any axial compressor/fan which imparts kinetic energy to fluid, and so, the designers have emphasized more on the rotor design. Present study incorporates an effort for the development of a low-speed axial flow compressor/fan rotor blade based on NACA 65-series airfoil. The limiting design parameters are selected from the grounded axial fan test rig available in the department lab. The study initiates with one-dimensional flow parameters calculation based on velocity triangle at mean location and is extended for total 11-radial locations from hub to tip. The blade angles are obtained from correlations available in literatures, and the corrected angles are used to set up selected airfoil profile along spanwise locations using commercial modeling tool. Once the airfoils are stacked on one another, a solid blade model is developed. This is further extended to develop entire rotor with calculated number of blades. The data obtained from the present study are validated with the available literature plots and are in good trend. The obtained blade has high pressure variation and total pressure loss close to tip. The blade has higher camber close to hub and lower camber at higher spanwise positions.KeywordsAxial flow compressorSubsonicRotor bladeNACA 65-airfoilBlade plots
The main objective of this study is to enhance the valuable octane booster aromatics benzene, toluene, and ethylbenzene (BTE) and reduction of styrene in the fuel oil obtained from the pyrolysis of equally mixed waste expanded polystyrene (WEPS) and waste high density polyethylene (WHDPE). This research article, demonstrated the pyrolysis of equally mixed (50 wt.%:50 wt.%) waste expanded polystyrene (WEPS) and waste high-density polyethylene (WHDPE) followed by in situ-hydrogenation and aromatization in a small laboratory scale reactor for the production of value added commodity and gasoline octane boosters mainly benzene, toluene, and ethylbenzene (BTE). Three reactor arrangements i.e., liquid phase/L-type, vapour phase/V-type, and multiphase/M-type were used for conducting the catalytic pyrolysis of feedstock using ZSM-5, ammonium powder as a catalyst. The maximum liquid yield of 87.41 wt.% was obtained for thermal pyrolysis at a temperature of 650°C and at a heating rate of 15°C/min. Whereas, the maximum liquid yield of 82.65 wt.% was obtained for liquid phase/L-type catalytic pyrolysis at a temperature of 600°C, and at a heating rate of 15°C/min. On the other side, vapour phase/V-type and multiphase/M-type catalytic pyrolysis produced a maximum liquid yield of 78.62 wt.% and 74.07 wt.%, respectively at a reaction temperature of 550°C and at the same heating rate. The pyrolysis oil obtained from thermal pyrolysis contained lowest BTE content of 10.43 wt.% and highest styrene content of 66.25 wt.%. Whereas, the highest BTE content of 30.16 wt.% and lowest styrene content of 53.32 wt.% was found in pyrolysis oil obtained from multiphase/M-type catalytic pyrolysis. The quantitative measurement of BTE and styrene content in the pyrolysis oil was made by the gas chromatography (GC) in a flame ionization detector (FID) mode.
Global polystyrene (PS) production has been influenced by the lightness and heat resistance this material offers in different applications, such as construction and packaging. However, population growth and the lack of PS recycling lead to a large waste generation, affecting the environment. Pyrolysis has been recognized as an effective recycling method, converting PS waste into valuable products in the chemical industry. The present work addresses a systematic, bibliometric, and statistical analysis of results carried out from 2015 to 2022, making an extensive critique of the most influential operation parameters in the thermo-catalytic pyrolysis of PS and its waste. The systematic study showed that the conversion of PS into a liquid with high aromatic content (84.75% of styrene) can be achieved by pyrolysis. Discussion of PS as fuel is described compared to commercial fuels. In addition, PS favors the production of liquid fuel when subjected to co-pyrolysis with biomass, improving its properties such as viscosity and energy content. A statistical analysis of the data compilation was also discussed, evaluating the influence of temperature, reactor design, and catalysts on product yield.
Pyrolysis has been examined as an attractive alternative to incineration for municipal solid waste (MSW) disposal that allows energy and resource recovery; however, it has seldom been applied independently with the output of pyrolysis products as end products. This review addresses the state-of-the-art of MSW pyrolysis in regards to its technologies and reactors, products and environmental impacts. In this review, first, the influence of important operating parameters such as final temperature, heating rate (HR) and residence time in the reaction zone on the pyrolysis behaviours and products is reviewed; then the pyrolysis technologies and reactors adopted in literatures and scale-up plants are evaluated. Third, the yields and main properties of the pyrolytic products from individual MSW components, refuse-derived fuel (RDF) made from MSW, and MSW are summarised. In the fourth section, in addition to emissions from pyrolysis processes, such as HCl, SO2 and NH3, contaminants in the products, including PCDD/F and heavy metals, are also reviewed, and available measures for improving the environmental impacts of pyrolysis are surveyed. It can be concluded that the single pyrolysis process is an effective waste-to-energy convertor but is not a guaranteed clean solution for MSW disposal. Based on this information, the prospects of applying pyrolysis technologies to dealing with MSW are evaluated and suggested.
Copyright © 2015 Elsevier Ltd. All rights reserved.
In this paper, some clarifications regarding the use of model-fitting methods of kinetic analysis are provided
in response to the lack of plot linearity and dispersion in the activation energy values for the thermal
degradation of polystyrene found in the literature and some results proposing an nth order model as the
most suitable one. In the present work, two model-fitting methods based on the differential and integral
forms of the general kinetic equation are evaluated using both simulated and experimental data, showing
that the differential method is recommended due to its higher discrimination power. Moreover, the
intrinsic limitations of model-fitting methods are highlighted: the use of a limited set of kinetic models
to fit experimental data and the ideal nature of such models. Finally, it is concluded that a chain scission
model is more appropriate than first order.
Thermal decomposition of natural polystyrene, recycled plastics, low density polyethylene, acrylonitrile butadiene styrene, polyenterophthalate of ethylene, and polypropylene has been carried out. Both isothermal and dynamic experiments at different heating rates have been performed in a thermobalance with the objective of determining the kinetic parameters. Also, a first set of experiments, performed in a cylindrical stainless-steel atmospheric pressure reactor, was designed to evaluate the products of the pyrolysis process. The effects of N2 flow rate (200–300 cm3 min−1), initial mass fed to the reactor (15–75 mg), temperature (415–490 °C), and heating rate (5–30 K min−1) were studied. By application of a first order kinetic model, the activation energy for every plastic has been determined. The values of activation energy in isothermic and dynamic regimen are the following: natural polystyrene: 136, and 168 to 286 kJ/mol. Recycled plastic: 250, and 150 to 290 kJ/mol. Low density polyethylene: 285, and 220 to 259 kJ/mol. Acrylonitrile butadiene styrene: 118, and 104 to 251 kJ/mol. Polyenterophthalate of ethylene: 161, and 117 to 255 kJ/mol. Polypropylene: 169, and 153 to 265 kJ/mol.
Plastics have become an indispensable ingredient of human life. They are non-biodegradable polymers of mostly containing carbon, hydrogen, and few other elements such as chlorine, nitrogen etc. Rapid growth of the world population led to increased demand of commodity plastics. High density poly ethylene is one of the largest used commodity plastics due to its vast applications in many fields. Due to its non bio degradability and low life, HDPE contributes significantly to the problem of Municipal Waste Management. To avert environment pollution of HDPE wastes, they must be recycled and recovered. On the other hand, steady depletion of fossil fuel and increased energy demand, motivated the researchers and technologists to search and develop different energy sources. Waste to energy has been a significant way to utilize the waste sustainably, simultaneously add to meet the energy demand. Plastics being petrochemical origin have inherently high calorific value. Thus they can be converted back to useful energy. Many researches have been carried out to convert the waste plastics into liquid fuel by thermal and catalytic pyrolysis and this has led to establishment of a number of successful firms converting waste plastics to liquid fuels. This paper reviews the production and consumption HDPE, different methods of recycling of plastic with special reference to chemical degradation of HDPE to fuel. This also focuses on different factors that affect these degradations, the kinetics and mechanism of this reaction.
With generation of waste plastics increasing, current EU legislation dictates high recovery rates and policy favours waste
management technology choices that occupy a high position on the waste management hierarchy. Pyrolysis is a thermochemical
conversion technology that can be considered a ‘feedstock recycling’ process and may play an increasing role in integrated
waste management systems of the future. The objective of this article is to present a review of current state-of-the-art commercial
pyrolysis processes for the production of liquid transport fuels from waste polyolefins (polyethylenes and polypropylenes).
Current plastic waste generation and management practices are briefly summarised. Waste management infrastructure in Europe
is reliant on landfill, incineration and mechanical recycling, while feedstock recycling plays an insignificant role. Plastic-to-liquid
platforms including delocalised pyrolysis followed by centralised upgrading, stand alone facilities, and integrated waste
management infrastructure concepts are briefly discussed. Commercial operations and their process configurations are compared.
Reactor technology for cracking of plastic waste is presented. Important issues like fuel quality and contamination are also
discussed. Fuel finishing operations and fuel additives required to achieve an engine ready fuel are described in the final
section. Recently published laboratory research in thermal and catalytic pyrolysis and integrated and co-processing studies
are also summarised in this review.
KeywordsLiquefaction–Pyrolysis–Waste–Plastic–Polyolefin–Waste-to-energy–Waste management–Feedstock recycling
Spouted beds are a very interesting class of gas-solid contactors that possess excellent heat transfer and mixing characteristics, while they are particularly suited to process coarse particles. Proper design of such beds requires the prediction of various hydrodynamic characteristics, such as the minimum spouting velocity and maximum spoutable height. Contrary to their typical initial applications, spouted beds have been finding recently more frequent use on the one hand at endothermic processes and on the other hand using much finer particle sizes. In the current work, the hydrodynamic characteristics of a laboratory scale spouted bed of 0.05 m diameter have been investigated via cold flow studies using olivine particles of 3.55-5.00 10−4 m size. Hydrodynamic parameters have been measured at this compact geometry and fine particle size and were compared with common literature correlations. An empirical correlations was derived to predict the fountain height for the studied fine particle spouted bed. Computer simulations have been further used to investigate the heat transfer characteristics of the bed under endothermic reactive conditions, using methane reforming as a case study. Given sufficient external heat supply, a spouted bed operating at a well-mixed regime can efficiently drive even highly endothermic reactions.
The fast pyrolysis of rice husk has been performed in the 400-600 degrees C range in a continuous pyrolysis bench-scale plant equipped with a conical spouted bed reactor (CSBR) with continuous removal of the char. In this paper it is studied the influence of temperature on product yields and products composition (gas, bio-oil and char) as well as the effect over the char properties. Bio-oil yield is very high (70 wt.% at 450 degrees C) due to the high capacity of mass and heat transfer as well as the reduced residence time of the CSBR. Moreover, bio-oil yield decreases slightly with temperature owing to the increase of gas yield, which is very low in the whole range of temperature studied. These results evidence the suitability of CSBR for the fast pyrolysis of rice husk, with the aim of obtaining bio-oil and char. The char can be upgraded by obtaining amorphous silica and activated carbon.
A two-step process has been used for the selective production of light olefins by the thermal cracking of high-density polyethylene. The plastic has been continuously fed into a conical spouted-bed reactor (CSBR) operating at 500 °C, which yields 93 wt % of waxes (C21+) and C12–C21 hydrocarbons. The volatile product stream has been cracked downstream in a multitubular (quartz tubes) reactor in the 800–950 °C range, with short residence times (0.016–0.032 s). A yield of 77 wt % of light olefins (C2–C4) has been obtained by operating at 900 °C in the second step. The maximum yields of ethylene, propylene, and butenes are 40.4, 19.5, and 17.5 wt %, respectively. Given the short residence time of the products in the reactor, the yield of aromatics is only 6.2 wt %. The high light olefin yield is due to the excellent performance of both steps. The CSBR allows maximizing the yield of waxes and avoiding defluidization problems. The operating conditions in the multitubular reactor (low concentration of the compounds in the volatile stream and short volatile residence times) are suitable for minimizing secondary reactions.
A work applied response surface methodology coupled with Box–Behnken design (RSM-BBD) has been developed to enhance styrene recovery from waste polystyrene (WPS) through pyrolysis. The relationship between styrene yield and three selected operating parameters (i.e., temperature, heating rate, and carrier gas flow rate) was investigated. A second order polynomial equation was successfully built to describe the process and predict styrene yield under the study conditions. The factors identified as statistically significant to styrene production were: temperature, with a quadratic effect; heating rate, with a linear effect; carrier gas flow rate, with a quadratic effect; interaction between temperature and carrier gas flow rate; and interaction between heating rate and carrier gas flow rate. The optimum conditions for the current system were determined to be at a temperature range of 470–505 °C, a heating rate of 40 °C/min, and a carrier gas flow rate range of 115–140 mL/min. Under such conditions, 64.52% WPS was recovered as styrene, which was 12% more than the highest reported yield for reactors of similar size. It is concluded that RSM-BBD is an effective approach for yield optimization of styrene recovery from WPS pyrolysis.
Reverse polymerization of waste polystyrene (PS) was exploited to produce styrene and other aromatics through microwave assisted pyrolysis (MAP), due to the benefits associated with microwave (MW) heating. MAP of PS was run by varying MW power and absorber (tire and carbonaceous char from MAP of tire). A clear and low viscosity liquid containing styrene was always collected as the major product, together with low amount of char and gas. Using a MW power of 3 kW and 100 g of PS together with 47.3 g of carbon gave a liquid (yield 86.5%) containing higher amount of single ring aromatic compounds, as evaluated by GC–MS (aromatics C6–C10 93.9%, styrene 66.0%) a char (yield 9.8%) and a gas (yield 3.7%). Improvements in residence time, by using low MW power or a fractionating system directly inserted over the oven and before the collecting system, allowed us to obtain a liquid with low viscosity and density even if the char yield was 10.0%. Liquid was characterized through physical (density, viscosity, high heating value), and chemical tests (elemental analysis, GC–MS and FT-IR).
Pinewood sawdust has been gasified by using steam as gasifying agent in a conical spouted bed reactor operating in continuous regime. A study has been made of the effect of temperature (in the 800–900 °C range), steam/biomass ratio (between 0 and 2) and sawdust particle size (0.3–1, 1–2 and 2–4 mm ranges) on the distribution of products (gas, tar and char) and their composition has been studied. Temperature has a positive effect on the gas composition by increasing H2 content and reducing that of CO. Furthermore, the tar and char yields are reduced as gasification temperature is higher. An increase in steam/biomass ratio from 0 to 1 has a positive effect on hydrogen concentration, char gasification and tar reforming, and a further increase in this parameter from 1 to 2 gives way to only a limited improvement in the results. Sawdust particle diameter plays a minor role in the gasification process in the temperature range studied, which must be attributed to the high heat transfer rate in the bed. Thus, this technology allows operating with coarse sawdust particles without reducing the syngas yield. However, the reduced gas residence time gives way to a high tar yield.
Pyrolysis is one important way to treat polystyrene waste and upcycle it into useful materials. A comparative pyrolysis study of virgin polystyrene (VPS) and two types of commonly used polystyrene products, expanded polystyrene (EPS) and polystyrene container (CPS) was carried out. Various values were found in the thermodynamic study and kinetic study of VPS, EPS, and CPS pyrolysis, suggesting distinct thermal degradation characteristics of these materials. The energy barrier order of the pyrolysis processes was EPS, CPS, VPS, showing activation energy of 230, 219, and 145 kJ mol−1, respectively. The order of amount of heat absorbed was EPS, CPS, VPS, with enthalpy of 224, 213, and 139 kJ mol−1, respectively. The reaction favorability order was EPS, CPS, and VPS with Gibbs free energy of 118, 132, and 210 kJ mol−1, respectively. Thermogravimetric analysis indicated the use of high heating rate would increase the reaction rate and shorten the reaction time. Product evolution profiles showed that VPS and CPS pyrolysis produced mainly aromatics, while EPS pyrolysis produced aromatics at the initial phase of the reaction and aliphatic hydrocarbon at the latter phase. The diverse pyrolysis behaviors of VPS, EPS, and CPS demonstrated that an examination on different polystyrene materials was desired to optimize the pyrolysis conditions and product distribution, and thus benefit the process of valuable materials recovery.
Polymer production and utilization are currently widespread and have greatly improved people's standards of living. However, due to their stable and nonbiodegradable nature, postconsumer polymers pose challenging issues to the environment and ecosystems. Efforts are being made not only to contain the generation of polymer wastes and associated littering but, also, to find ways of utilizing them sustainably. Aside from mechanical recycling, which turns postconsumer polymers into new polymer products, and thermal recycling, which releases the thermal energy contained within waste plastics through combustion, chemical recycling converts waste polymers into feedstock for chemicals/materials/fuels production. This manuscript reviews prior work on a special application of the particular chemical recycling route that converts polymers into carbon-based nanomaterials. These materials feature extraordinary physical and chemical properties with tremendous applications potential. However, their production processes are both resource- and energy-intensive. Yet, by taking advantage of the high carbon content of waste polymers, as well as of their high energy content, a cost-effective, environmentally-friendly, and self-sustaining production of carbon nanomaterials can be achieved.
The influence of the acid and basic properties of a catalyst on the selectivity of polystyrene transformation was studied. The properties of silicaalumina SiO2–Al2O3(45%) dotted with NaOH from 1 to 20 wt% and γ-Al2O3 containing 1 to 8 wt% NaOH or H2SO4 were examined using the test reactions of cumene (acid) and diacetone alcohol (base) transformation. These catalysts were used in polystyrene transformations. It was found that polystyrene decomposition involves thermal and catalytic transformation. In the first case, two main reaction pathways are involved: gradual depolymerization to the monomer (styrene) and pyrolysis leading to the formation of different volatile oligomers (dimers, trimers, tetramers.). The latter species react solely over the catalyst active centers. Linear dimers of styrene activated by Brønsted acid sites undergo decomposition to styrene and ethylbenzene as well as to toluene and α-methylstyrene. Consecutive transformation of the products leads to a simultaneous hydrogen (H+ and H−) production and coke formation. The latter ions are active in hydrogen transfer reactions which causes hydrogenation of styrene to ethylbenzene. In the presence of base catalysts selective styrene formation takes place and transformation leading to coke are slower hence ethylbenzene formation is suppressed. In the presence of acid catalysts linear dimers can also isomerize to cyclic derivatives which after dealkylation give benzene and methylindane. The later product in isomerization and hydrogen transfer reactions forms methylindene and naphthalene. The higher styrene oligomers reactions are catalyzed by both acid and basic sites leading mainly the monomer, i.e. styrene formation.
The steam gasification of high density polyethylene in continuous mode has been carried out in a conical spouted bed reactor. The effect of temperature (in the 800–900 °C range) and steam/plastic mass ratio (between 0 and 2) on the distribution of products (gas and tar) and their composition has been studied. In order to reduce tar formation, two catalysts have been used in situ, namely, olivine and γ-Al2O3. The spouted bed reactor has an excellent performance between 850 and 900 °C, and an increase in the steam/plastic ratio from 1 to 2 only improves slightly both carbon conversion efficiency (to 93.6% with steam/plastic ratio = 2) and hydrogen concentration (61.6%). The use of olivine and γ-Al2O3 instead of sand gives way to a moderate reduction in the tar formation, whose yield is 4.8% with olivine. The syngas obtained has a H2/CO ratio of 2.2, with a low tar content whose composition (monoaromatics, mainly benzene) augurs well for the use of the syngas in DME synthesis.
On the basis of the experimental study of air velocity profiles in a pilot plant scale unit, the validity of the hypotheses of the gas flow model proposed for conical spouted beds, that is to say, flow rate conservation along each streamtube and plug flow in the spout zone, has been proven. A correlation has been obtained for calculation of the local velocity as a function of the contactor geometric factors (angle, inlet diameter) and of the operating conditions (solid density, particle size, relative air velocity). The validity of the gas flow model has been proven in a wide range of operating conditions and a correlation has been obtained for calculation of the dispersion coefficient as a function of the following moduli: bed upper diameter/contactor base diameter, Archimedes number, tangent of the angle, relative air velocity.
Simulations of the initial distribution of volatiles from pyrolysis of polystyrene were based on propagation rate constants estimated by thermochemical kinetic procedures. The voluminous database exhibits a disturbing lack of consistency with respect to effects of conversion level, temperature, and reactor type. It therefore remains difficult to assign the true primary distribution of the major products, styrene, 2,4-diphenyl-1-butene (“dimer”), 2,4,6-triphenyl-1-hexene (“trimer”), 1,3-diphenylpropane, and toluene, and its dependence on conditions. Probable perturbations by secondary reactions and selective evaporation are considered. The rate constant for 1,3-hydrogen shift appears much too small to accommodate the commonly proposed “back-biting” mechanism for dimer formation. Dimer more likely arises by addition of benzyl radical to olefinic chain-ends, followed by β-scission, although ambiguities remain in assigning rate constants for the addition and β-scission steps. With this modification, the major products can be successfully associated with decay of the sec-benzylic chain-end radical. In contrast, the minimal formation of allylbenzene, 2,4-diphenyl-1-pentene, and 2,4,6-triphenyl-1-heptene suggests a minimal chain-propagating role for the prim chain-end radical. Compared with polyethylene, the much enhanced “unzipping” to form monomer from polystyrene and the more limited depth of “back-biting” into the chain arise from an enthalpy-driven acceleration of β-scission coupled with a kinetically driven deceleration of intramolecular hydrogen transfer. In contrast, the greater “unzipping” of poly(isobutylene) compared with polyethylene is proposed to result from relief of steric strain.
A specially designed laboratory fluidized-bed reactor apparatus was developed for the pyrolysis of polystyrene (PS) waste in the range 450–700°C with nitrogen as the carrier gas and 20–40 mesh quartz sand as the fluidization medium, operating isothermally at atmospheric pressure. The yield of styrene monomer reached a maximum of 78.7 wt.% at a pyrolysis temperature of 600°C. Some monoaromatics with boiling point lower than 200°C could also be obtained as high-octane gasoline fraction. Styrene monomer with 99.6 wt.% purity was obtained after vacuum distillation of the liquid products, which could be used as the raw material to produce high-quality PS. The attempt to increase the yields of styrene monomer and other monoaromatics by recycling the higher boiling point fraction to the reactor during pyrolysis is also discussed.
Eight samples of polystyrene with various known molecular masses and polydispersity (w/n) close to 1 have been pyrolyzed at 600 and 750°C. The pyrolysis products were identified by means of gas chromatography/mass spectrometry and their relative percentages calculated. We calculate the parameters of the logarithmic correlations between the molecular mass of the pyrolyzed samples and the relative amount of the pyrolysis products. Styrene production increases with the molecular mass of the polymer, when the other products decrease. Benzene formation becomes significant with an increase in degradation temperature. The results for each pyrolysis product are reported.
Previously reported kinetic data on polystyrene thermal degradation are inconsistent, and this may be a potential source of error in modeling the ignition and burning of the polymer composite materials containing polystyrene. To derive formal kinetic model of polystyrene thermal degradation, pyrolysis combustion flow calorimetry (PCFC) has been applied in this work. The heat release rate-temperature dependencies were measured at four heating rates of 0.25, 0.5, 1.0 and 2.0 °C/s under nitrogen flow, and the kinetic parameters were derived by means of the model-free isoconversional method, the peak value method, the method of Kissinger and the model fitting non-linear optimization method. The single-step global reaction model has been demonstrated to have a constant activation energy of 168 kJ/mol in a wide range of conversions. The autocatalytic reaction type has been established by evaluating the dependence of the kinetic function on the conversion derived from the measurement data. Thereby developed kinetic model has been validated against a variety of data sets including PCFC measurements made in this work, published TGA measurements, and isothermal experimental data. The model reproduced the experimental data to a reasonable accuracy for different temperature programs. The nth order reaction model was demonstrated to be unable to predict reaction rates for a range of different heating rates although it could be optimized for a single temperature program. Use of the nth order reaction has been shown to be a reason of obtaining unrealistically high apparent activation energies, reported for polystyrene degradation in the literature. The importance of processing multiple heating rate data to avoid misleading results is highlighted.
The continuous catalytic pyrolysis of high density polyethylene (HDPE) has been carried out in a two-step reaction system involving a pyrolysis conical spouted bed reactor followed by a catalytic fixed bed reactor. The good performance of the conical spouted bed reactor has allowed using a low temperature in thermal cracking (500 °C) without defluidization problems, obtaining a volatile stream with a 90 wt.% overall yield of C12–C20 and waxes in the first step. The effect of the second-step operating conditions on product yields and composition has been studied using a catalyst based on a HZSM-5 (SiO2/Al2O3 = 30) zeolite. The influence of temperature in the 350–550 °C range and space–time in the 0–8 gcat min gHDPE−1 range has been studied. An increase in temperature or space–time gives way to an increase in the yield of light olefins, reaching a value of 62.9 wt.% at 550 °C and 8 gcat min gHDPE−1, with the individual yields of ethylene, propylene and butenes being 10.6, 35.6 and 16.7 wt.%, respectively. Although the yield of single-ring aromatics increases when both variables studied are increased, the maximum yield obtained was lower than 13 wt.%. The yield of waxes (the main product in the first step) is negligible even at low temperatures or spaces-times, which evidences the efficiency of the catalytic step.
The pyrolysis of high-density polyethylene (HDPE) has been carried out in the range between 500 and 700 °C in continuous mode in a pilot-plant unit equipped with a conical spouted bed reactor. The products have been grouped into the lumps of gas (C4–), gasoline (C5–C11), diesel (C12–C20), and waxes (C21+). The product yields and compositions of these fractions were compared both to those previously obtained in the pyrolysis performed in discontinuous mode and to those obtained by other authors in fluidized bed reactors. The results confirm the optimal performance of the conical spouted bed reactor (CSBR) in obtaining high yields of waxes and fuels with low aromatic content, which is explained by the appropriate conditions of the CSBR needed to enhance heat and mass transfer between phases (capacity for coating the sand with plastic) and minimize secondary reactions (short residence time of the volatiles).
This paper describes the hydrodynamic and heat transfer performance of a pilot-plant scale conical spouted bed reactor designed for the pyrolysis of biomass wastes. The spouted bed reactor is the core of a fast pyrolysis pilot plant with continuous biomass feed of up to 25 kg/h, located at the Ikerlan-IK4 facilities.The aim of this paper is to obtain a deeper understanding of the spouted bed reactor performance at pyrolysis temperatures, in order to operate under stable conditions, improve the heat transfer rate in the reactor and minimize energy requirements. The influence of temperature on conical spouted bed hydrodynamics has been studied and wall-to-bed and bed-to-surface heat transfer coefficients have been determined.Highlights► The hydrodynamic performance of a pilot plant (25 kg/h) for biomass pyrolysis has been studied. ► The validity of a previous hydrodynamic correlation obtained at laboratory scale has been analysed. ► Wall-to-bed and bed-to-surface heat transfer rates have been determined. ► Temperature distribution in the bed has been determined for different inlet temperatures.
Local bed voidages have been experimentally determined by means of an optical fiber probe in conical spouted beds of different geometry (angle, inlet diameter) and working under different operating conditions (particle diameter, stagnant bed height, and air velocity). Based on the experimental results, original correlations have been proposed and proven for calculation of bed voidage along the axis of the spout and near the contactor wall. A correlation has been proposed for calculation of bed voidage at any bed position as a function both of bed voidage along the spout axis and at the wall and of the spout radius.
Continuous pyrolysis of poly-(ethylene terephthalate) (PET) was performed in a conical spouted-bed reactor. This reactor is especially suitable for this process, because of its excellent hydrodynamic behavior and its versatility. An optimization of the operating conditions has been performed to avoid particle agglomeration and defluidization, given that these are serious problems that hinder plastic waste pyrolysis. Moreover, the influence of temperature in the pyrolysis product distribution was studied in the range of 500−600 °C. PET pyrolysis led to a high yield of gas fraction, a low amount of liquids, a significant yield of solid fraction, and a solid residue that remained in the reactor coating sand particles. The main products obtained are carbon monoxide, carbon dioxide, benzoic acid, and acetaldehyde.
A study has been carried on the applicability of correlations previously proposed for the design of conical spouted beds, for the treatment of materials of density similar to that of glass, and for the handling of plastic materials (polyethylene, polypropylene, and extruded and expanded polystyrene). The correlations proposed allow for calculation of the geometric factors (angle and design of the base) of conical spouted beds for stable operation, the minimum velocity for spouting and dilute spouting, the operating pressure drop in both regimes, the peak pressure, the bed voidage, and the evolution of this voidage as the gas velocity is increased between the regimes of a spouted bed and a dilute spouted bed.
The degradation of polystyrene was modeled at the mechanistic level using the method of moments to track structurally distinct polymer species. To keep the model size manageable, polymer species were lumped into groups, and within these groups, the necessary polymeric features for capturing the degradation chemistry were tracked. The pyrolysis reactions incorporated into the model included hydrogen abstraction, midchain β-scission, end-chain β-scission, 1,5-hydrogen transfer, 1,3-hydrogen transfer, radical addition, bond fission, radical recombination, and disproportionation. From the evolution of the zeroth, first, and second moments tracked for each dead species, polymer molecular weight distributions were constructed by summing the Schultz (Teymour, F.; Campbell, J. Macromolecules 1994, 27, 2460) and Wesslau (Pladis, P.; Kiparissides, C. Chem. Eng. Sci. 1998, 53 (18), 3315) distributions for the polymer groups. Model results were compared to experimental data collected in our laboratory, where polystyrene samples that differed in the shape and breadth of their initial distributions were pyrolyzed. The model was able to predict the formation of a bimodal distribution during the pyrolysis of polystyrene samples (molecular weight range of 10 000−500 000 g/mol) with narrow unimodal molecular weight distributions (polydispersity index < 1.1). This was accomplished by distinguishing the initial polymer from the polymer formed from midchain β-scission reactions within the model. At high conversions, all of the polystyrene samples investigated evolved to unimodal distributions, and these distributions were best captured by the Schultz distribution.
Original aspects in the design of conical contactors used in spouting and jet spouting regimes have been studied. The experimental results correspond to different solids: glass spheres in the 0.001-0.008-m particle diameter range, polystyrene, vegetable seeds, gravel, and Teflon, which have been used in contactors with different geometries (several angles and inlet diameters). The conditioning factors for instability of the spouted bed, which limit the maximum and minimum spoutable bed height, have been analyzed. A correlation has been proposed for the calculation of the minimum spoutable bed height as a function of the geometric factors of the contactor-particle system and density of the solid. From the bed hydrodynamics, correlations have been proposed to dimension the contactor as a function of the volume and properties of the solid to be treated in spouting and jet spouting regimes.
The operation regimes of spouting and of jet spouting have been delimited in the bed expansion in conical contactors. Both contact regimes, but in particular the original regime of jet spouting, have a characteristic hydrodynamic behavior different from that of the conventional spouted bed, and they combine the characteristics of high velocity of gas and solid, typical of transport beds, with the cyclic movement typical of the spouted bed. Its interest is centered on the handling of particles of large diameter, with adherent solids and with size distribution. The ranges of the geometric factors of the contactor-particle system and of the gas velocity for stable operation in both regimes have been determined by experiments in a pilot plant using different solids. The operative ranges of both regimes in conical contactor are compared with the conventional gas-solid contact regimes. The limitations of the few correlations in the literature for design of conical spouted beds and the nonvalidity of these conventional correlations proposed for cylindrical spouted beds have been proven. Consequently, original hydrodynamic correlations for spouting and jet spouting corresponding to conical contactors have been proposed for the calculation of the minimum velocity in stable operational conditions.
Catalytic and thermal degradation of waste expandable polystyrene (WEPS) have been studied in a semi-batch reactor with continuous flow of nitrogen to achieve greater oil yield and maximize styrene monomer recovery. Effect of temperature, nature of catalyst and its size, reaction time of catalytic pyrolysis, and effect of repyrolysis have been also investigated. Higher reaction temperature favors the oil yield and also decreases the reaction time with maximum styrene selectivity (76.31 wt %) at 450 °C. Among the catalysts studied, solid base BaO is found to be the most efficient and increases the styrene selectivity (84.29 wt %) significantly at a reaction temperature of 350 °C in comparison to thermal and acid catalytic degradation. Modified catalyst Fe−A/Al (A = basic material) also shows better activity than Fe2O3 or Fe/Al. Increase in the size of the catalyst and repyrolysis decrease the oil yield, and styrene production also decreases on repyrolysis.
The pyrolysis of waste tires in continuous mode has been studied in a pilot plant provided with a conical spouted bed reactor, in the 425-600 degrees C range, by feeding two types of tire materials with different contents of natural and synthetic rubber. The properties of the pyrolysis products have been characterized using different techniques for both the volatile and solid fraction. The liquid has been analyzed by means of GC/MS, GCxGC and simulated distillation, and the residual carbon black by elemental analysis and surface characterization. Neither temperature nor tire composition has a significant effect on the yields of gas, liquid and residual carbon black fractions, but they considerably affect product properties. The contents of natural rubber and styrene-butadiene rubber in file tire have a significant effect on the liquid composition (contents of aromatics and interesting raw materials, Such as styrene and limonene). The carbon black obtained at 600 degrees C has a high surface area and is suitable for active carbon production.
The continuous pyrolysis of waste tires under vacuum conditions (25 and 50 kPa) has been studied in a pilot plant equipped with a conical spouted bed reactor and operating with continuous feed at 425 and 500 degrees C. The effects of vacuum on product distribution and properties have been studied. The main effect of vacuum is an increase in the diesel fraction yield of the liquid product. A remarkable yield of isoprene has been obtained operating under vacuum, reaching yields of higher than 7 wt % Moreover, a positive effect on the quality of the residual carbon black has been observed, given that a decrease in pore blockage gives way to higher surface areas of the carbon blacks obtained. The results show that vacuum operation does not limit the good perspectives for waste tire valorization by pyrolysis in a conical spouted bed, and energy requirements for heating the inert gas and the condensation section are significantly reduced.
A conical spouted bed reactor (CSBR) has been used for the kinetic study of polystyrene pyrolysis in the 723-823 K range and the results have been compared with those obtained by thermogravimetry (TGA) and in a microreactor (MR) of very high sample heating rate. The comparison proves the advantages of the gas-solid contact of this new reactor for the kinetic study of pyrolysis of plastics at high temperature, which stem from the high heat transfer rate between gas and solid and from the fact that particle agglomeration is avoided. It has been proven that, in the temperature range required for maximizing the yield of styrene, heat and mass transfer limitations within the particle are important when polystyrene particles of 1 mm, are fed into the reactor. A high yield of styrene is obtained (64.5 wt.%) in the 723-773 K range. (C) 2002 Elsevier Science B.V. All rights reserved.
Thermal pyrolysis for upgrading plastic wastes is one of the better methods for recycling plastics in terms of its perspectives for industrial implementation. The conical spouted bed reactor proposed in this paper may be a solution to the problems arising in fluidized beds handling sticky solids, as particle agglomeration phenomena, which can cause defluidization. In order to avoid defluidization, experiments have been carried out in batch mode in the temperature range of 450-600degreesC. A good performance of the reactor is proven under the conditions of maximum particle stickiness.
Pyrolysis is considered as possible technique to thermally convert waste plastics into chemicals and energy. Literature on experimental findings is extensive, although experiments are mostly performed in a dynamic heating mode, using thermogravimetric analysis (TGA) and at low values of the heating rate (mostly below 30 K/min). The present research differs from literature through the application of far higher heating rates, up to 120 K/min. The use of these dynamic results to define the reaction kinetics necessitates the selection of an appropriate reaction mechanism, and 21 models have been proposed in literature considering the rate limiting step being diffusion, nucleation or the reaction itself.The current research studied the cracking of PET and PS by TGA at different heating rates (temperature ramps). Results were used to check the validity of the proposed mechanisms. Several conclusions are drawn: (i) to obtain fair results, the heating ramp should exceed a minimum value, calculated at 30 K/min for PET and 80 K/min for PS; (ii) application of the majority of the models to experimental findings demonstrated that they do not meet fundamental kinetic considerations and are questionable in their use; and (iii) simple models, with reaction order 1 or 2, provide similar results of the reaction activation energy.A further comparison with literature data for dynamic and isothermal experiments confirms the validity of these selected models. Since TGA results are obtained on a limited amount of sample, with results being a strong function of the applied heating rate, the authors believe that isothermal experiments, preferably on a large scale both towards equipment and/or sample size, are to be preferred.
The potential use of the liquid product obtained from the pyrolysis of polystyrene as a raw material for the reproduction of the polymer was investigated in this study. Catalytic and non‐catalytic pyrolysis experiments were carried out in a fixed bed reactor using either model polymer or commercial waste products as the feedstock. The liquid fraction produced from all the pyrolysis experiments consisted mainly of the styrene monomer and this was subjected to re‐polymerization without any further purification, in a DSC with AIBN initiator. It was found that the pyrolysis oil fraction could be re‐polymerized to again produce polymer. However, aromatic compounds included in this fraction may act as chain transfer agents, resulting in alterations in the shape of the reaction rate curve and lowering significantly the average molecular weight and the T g of the polymer produced.
magnified image
Feedstock recycling of polymers is an attractive option to recycle the increasing amount of plastic wastes and respond to restrictions for waste disposal in a lot of countries. Different processes are investigated such as degradation of plastics to monomers, pyrolysis into monomers and oil, gasification into syngas. Pyrolysis of mixed plastic wastes and elastomers is a cost-effective process to recover feedstocks for the petrochemical industry. The Hamburg process, using an indirectly heated fluidised bed, can be varied to produce mainly monomers, aliphatic hydrocarbons, or aromatics. At temperatures of 450 °C, poly(methyl methacrylate) (PMMA) is depolymerised to more than 98% of the monomer. The influence of fillers on the monomer yield has been studied. Polystyrene as feed gives up to 75% of styrene and 10% of oligomers. First demonstration plants are running for feedstock recycling of PMMA in a fluidised bed.
The pyrolysis of poly-(methyl methacrylate) (PMMA) has been studied in a pyrolysis plant provided with a conical spouted bed reactor. This reactor is an interesting technology for the pyrolysis of waste plastics due to its excellent hydrodynamic behaviour and its high heat transfer and versatility. A previous kinetic study was carried out in thermobalance, in which the degradation of this polymer was observed to begin at low temperatures, 553 K. Consequently, the activation energy is low compared to other plastics. The influence of temperature on pyrolysis product distribution in the conical spouted bed reactor has been studied in the 673–823 K range. The products obtained at low temperatures are mainly the monomers of the polymer used for the study methyl methacrylate (MMA) and ethyl acrylate (EA). When the pyrolysis temperature is increased, the yield of monomers is lower due to the higher severity of secondary reactions, and there is a significant increase in the yield of gases. The ma