- Email: [email protected]
Two-stage pyrolysis of polystyrene: Pyrolysis oil as a source of fuels or benzene, toluene, ethylbenzene, and xylenes
Applied Energy xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Two-stage pyrolysis of polystyrene: Pyrolysis oil as a source of fuels or benzene, toluene, ethylbenzene, and xylenes Ki-Bum Parka, Yong-Seong Jeongb, Begum Guzelciftcia, Joo-Sik Kima,b, a b
⁎
Graduate School of Energy and Environmental System Engineering, 163 Siripdaero, Dongdaemun-Gu, Seoul 02504, Republic of Korea Graduate School of Environmental Engineering, University of Seoul, 163 Siripdaero, Dongdaemun-Gu, Seoul 02504, Republic of Korea
HIGHLIGHTS
two-stage pyrolysis of polystyrene was successfully conducted. • AThecontinuous yield clearly decreased with increasing the auger reactor temperature. • The styrene maximum BTEX and lowest styrene yields were almost the same at 26 wt%. • Oil obtained high temperatures had an enhanced thermal-oxidative stability. • Oil obtained atappeared to be used as a good source of BTEX aromatics. • ARTICLE INFO
ABSTRACT
Keywords: Plastic Two-stage pyrolysis Polystyrene BTEX Fuel
The recycling rate of plastic waste needs to be improved worldwide. In that context, pyrolysis, through which petrochemical feedstock and alternative fuel can be obtained, has received significant attention. In this study, pyrolysis of polystyrene was conducted in a continuous two-stage process that has an auger reactor and a fluidized bed reactor connected in series. The main objective was to produce oils rich in benzene, toluene, ethylbenzene, and xylenes instead of typical polystyrene pyrolysis oils, which contain high amounts of styrene monomers with low thermal-oxidative stability. The effects of different reaction temperatures (in both reactors) and the type of fluidizing medium on the product distribution and composition were investigated. The maximum yield of benzene, toluene, ethylbenzene, and xylenes (26.3 wt%) was obtained at a temperature of 780 °C in the fluidized bed reactor. The oil and styrene yields at 780 °C were 86 and 26 wt%, respectively. To evaluate the fuel properties of the pyrolysis oil, its calorific value, API gravity, viscosity, density, ash content, pour point, flash point, and pH were examined. The results indicate that the pyrolysis oil can be both a good source of benzene, toluene, ethylbenzene, and xylenes and can potentially be used as a substitute source to gasoline or diesel fuels when it is mixed with oils with a low aromatic content.
1. Introduction Since the mid-1900s, plastic use disseminated, and plastics became irreplaceable in modern society. The annual worldwide plastic production reached nearly 350 million metric tons in 2017 and is expected to increase yearly [1]. Plastics are essential in our society, and their use ranges from household goods to industrial appliances, automotive parts, medical equipment, and packaging. Among the various usages of plastic, approximately 40% of the total plastic demand is for packaging, which is problematic due to their short lifespan (less than a year) [2]. With the increasing demand for plastic, there is also a global increase in plastic waste production. In 2015, about 60% of all the plastic ever
⁎
produced (5000 million metric tons) was still discarded or landfilled [2]. Moreover, the import ban policy by the Chinese government accelerated the worldwide plastic waste crisis. Until 2016, China ranked first as importer of plastic, being responsible for over 45% of the total plastic imports worldwide [3]. In this context, appropriate treatment methods for plastic waste need to be developed and adopted. These treatment methods can be categorized as: primary (re-extrusion), secondary (mechanical recycling), tertiary (chemical recycling), and quaternary (energy recovery) [4]. Each treatment method has its own advantages and can be applied according to the constitution of the plastic and local circumstances. The main difference between re-extrusion and mechanical recycling is
Corresponding author at: Graduate School of Environmental Engineering, University of Seoul, 163 Siripdaero, Dongdaemun-Gu, Seoul 02504, Republic of Korea. E-mail address: [email protected] (J.-S. Kim).
https://doi.org/10.1016/j.apenergy.2019.114240 Received 10 June 2019; Received in revised form 19 November 2019; Accepted 23 November 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Ki-Bum Park, et al., Applied Energy, https://doi.org/10.1016/j.apenergy.2019.114240
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
whether the reprocessed material is the same material as the original plastic or not [5]. For example, recycling of polyethylene terephthalate (PET) waste into a PET bottle is re-extrusion, and recycling of PET waste into a fiber is mechanical recycling. Both methods generally require plastics with low impurity to avoid quality deterioration of the recycled products [6]. Furthermore, reprocessing is limited to a few cycles because the mechanical properties of the recycled products deteriorate upon reprocessing [7]. The third recycling option for plastic waste is chemical recycling. A representative of chemical recycling is pyrolysis, which converts plastic waste into smaller hydrocarbons under a high-temperature inert atmosphere. Pyrolysis is a flexible option compared to mechanical recycling because the target products of pyrolysis can be selected by varying the reaction conditions. It has been extensively employed in the thermal decomposition of waste plastics to obtain high energy density products, containing paraffin, olefins and aromatic hydrocarbon [8]. However, not many efforts have completely succeeded in recovering energy and materials by pyrolysis due to the complexity and heterogeneity of the feedstocks [9]. In plastic pyrolysis, various types of reactors have been tested to obtain valuable products. Typical reactors are fixed bed, stirred tank, screw kiln, circulating sphere, fluidized bed, spouted bed, plasma, and microwave-assisted reactors [10]. Among them, fluidized bed, fixed bed and conical spouted bed reactors have been most widely applied [11]. Fixed reactors are simple in design and easy to control operating parameters. Fluidized bed reactors allow operating under high heating rates, isothermal bed and short residence times [12]. The spouted bed reactor is known to avoid defluidization problems by vigorous solid circulation [13]. Only with primary and secondary treatment methods of plastic waste, any country cannot achieve its own recycling rate goal. In particular, sorting plants produce rejected plastic waste which are mixed with inappropriate materials such as paper, glass, and metals and therefore hard to be recycled [14]. That plastic waste can be properly treated by pyrolysis. Polystyrene (PS), one of main commodity plastics, is widely used for many purposes but especially for packaging. However, the recycling rate of PS is only 0.9% in the U.S. [5]. The use of pyrolysis to recycle PS has been extensively investigated. Many researchers have studied the degradation mechanism of PS. Zhou et al. [15], Faravelli et al. [16], and McNeill [17] et al. reported that important reaction mechanisms of the PS pyrolysis were β-scission, unzipping (end-chain β-scission), and intermolecular and intramolecular hydrogen transfer reactions. Among those, the unzipping reaction is considered the main mechanism for the production of styrene monomers. Many researches aimed at a high yield of styrene monomers from PS waste. Artetxe et al. conducted the pyrolysis of PS in a conical spouted bed reactor and found that the styrene yield was strongly influenced by both temperature and gas flow rate [18]. Moqadam et al. performed the pyrolysis of PS using silica-alumina catalysts and could obtain oils containing a high percentage of styrene monomer (> 80%) [19]. Demirbas et al. reported that the yield of styrene increased until 750 °C (65 wt%), and it rapidly decreased after that [20]. Liu et al. investigated the pyrolysis of PS and obtained a styrene monomer yield of 78.7% at 600 °C. Moreover, 99.6% of high purity styrene was obtained by additional vacuum distillation [21]. Karaduman et al. carried out the flash pyrolysis of PS using a free-fall reactor and reported that the oil yield maximized around 750 °C and the styrene yield at 825 °C [22]. Karaduman also investigated the pyrolysis of PS using various organic compounds for enhancing styrene yield. He found in the study that the maximum styrene yield (74 wt%) could be obtained with naphthalene [23]. In a study on the catalytic pyrolysis of PS, solid base BaO was found to be the most efficient to increase the styrene selectivity (84 wt%) [24]. Onwudili et al. investigated the influence of residence time on the styrene yield. They observed that the styrene yield decreased, and the toluene and ethylbenzene yields compensatively increased when the residence time of pyrolysis vapor increased [25]. Achilias et al. demonstrated that the PS synthesized using pyrolysis oil from PS was similar to the original PS. However, the study
revealed that other aromatic compounds in the oil decreased the quality of resynthesized PS compared to the virgin one [26]. If pyrolysis oil from PS cannot be properly used as a feedstock for PS synthesis, its use as a fuel is a possible alternative. However, the pyrolysis oil from PS has a low thermal-oxidative stability, mainly due to the large number of double bonds in the oil. Upon storage in a hot tank during the summer (~50 °C), auto-polymerization can occur and form unwanted styrene dimers, trimers, or high molecular components [27]. To prevent such undesirable auto-polymerization, antioxidants such as 4-tert-butylcatechol should be blended into commercial styrene storage [28]. In the current study, we aim to produce pyrolysis oil from PS with enhanced thermal-oxidative stability without using a catalyst. For that purpose, the styrene content in the oil should be remarkably reduced compared to conventional PS pyrolysis oil. Mertinkat et al. studied the production of such stability-enhanced oils from PS, but with the use of catalysts [29]. To the best of our knowledge, the present study is the first noncatalytic approach to produce oils with a low styrene content for use as fuel or feedstock. For that, a two-stage pyrolysis process was used. It comprises an auger reactor and a fluidized bed reactor in series. Previous works on the two-stage pyrolysis of polyethylene (PE) and polypropylene [30,31] revealed unique characteristics of the pyrolysis process, which yielded a different product spectrum compared to conventional pyrolysis. This different product spectrum was attributed to the action of the auger reactor, which is usually heated to 200–400 °C. In the heated auger reactor, the vibrational states of polymer molecules may be high compared to those of unheated molecules. Consequently, the bond length of all bonds of the heated molecules will slightly increase, making the bond strength of all bonds weak. The fluidized bed reactor, which is located directly after the auger reactor, can exploit these weakened bonds and crack the molecules, which would not occur under conventional pyrolysis. Based on this assumption, we expected to obtain a pyrolysis oil with a different composition compared to the one derived from conventional pyrolysis of PS. In particular, the production of mono-aromatics such as BTEX at the expense of styrene monomer was expected to be enhanced with the help of the two-stage pyrolysis process. Further, the pyrolysis oil from PS appears to be an attractive fuel source because PS, especially expanded polystyrene, can be collected separately from PVC, and because the pyrolysis oil from PS, therefore, will not suffer from chlorine-related problems. Hence, we explored the possibility of the pyrolysis oil from PS as a fuel in the current work. The successful conversion of PS into desirable products such as chemical feedstocks and fuels for energy would contribute not only to the efficient treatment of waste PS, but also to the resource conservation. The variables tested were: temperature of the auger and the fluidized bed reactors, and type of fluidizing medium. 2. Material and methods 2.1. Feed material characteristics A granular virgin PS (Styrolution, PS 147F), with a size of 1–3 mm, was used as the feed material. The proximate analysis of the feed material was conducted according to the ASTM D 3172 standard, and the ultimate analysis was performed using an elemental analyzer (Flash 2000 series, Thermo Scientific). The main characteristics of PS are presented in Table 1. The PS was mainly composed of volatile matter (99.8 wt%) with a high content of carbon (91.9 wt%). To simultaneously examine the degradation behavior and determine the reaction temperature of the two-stage pyrolysis, a thermogravimetric analysis (TGA) was conducted using a thermogravimetric analyzer (TGA Q50, TA Instruments). In the TGA study, the PS (10 mg) was heated to 900 °C under a N2 atmosphere, at heating rates of 10–30 °C/min. Fig. 1 shows the results of the TGA study. The TG and differential thermogravimetry curves show that the main degradation occurred between 300 and 500 °C. Considering that 2
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
The first condenser operates with water as a coolant at 20 °C, and the second one with methanol at −20 °C. The pyrolysis oil is captured in vessels at the bottom of the condensers, and uncondensed vapor goes through an impact separator (IP) and an electrostatic precipitator (EP), which separate the remaining oil fraction from the gas. The gas leaving the EP is recirculated into the fluidized bed reactor by a compressor (N0150ATE, KNF).
Table 1 Main characteristics of the feed material. PS Proximate analysisa (wt.%)
Ultimate analysisb (wt.%)
Moisture Volatile matter Fixed carbon Ash
Carbon Hydrogen Nitrogen Sulfur Oxygen
0.10 ± 0.04 99.83 ± 0.04 0.06 ± 0.02 0.01 ± 0.00
Lower heating value (MJ/kg) a b c
91.93 ± 0.06 7.96 ± 0.02 N.D.c N.D.c N.D.c
2.3. Experimental conditions
41.81
An experimental scheme was established to determine the effects of temperature in the auger and in the fluidized bed reactor, and the type of fluidizing medium. Table 2 shows the experimental conditions. Runs 1–3 were conducted to investigate the influence of the auger reactor temperature, which varied from 15 °C when the heater was off (Run 1) to 299 °C (Run 3). In all three runs, the fluidized bed reactor temperature was kept at approximately 510 °C. Runs 3–6 demonstrate the effect of the temperature in the fluidized bed reactor on the pyrolysis product distribution, and composition of gas and oil. The fluidized bed reactor temperature was raised from 514 (Run 3) to 782 °C (Run 6). In the six runs above, product gas was used as the fluidizing medium. In contrast, in Run 7, nitrogen was used as the fluidizing medium. The other reaction conditions were nearly identical in all runs. The amount of PS used for each run was 500 g, and the feeding rate was maintained at approximately 500 g/hr. Quartz sand (1.2 kg) with particle sizes of 0.150–0.425 mm was used as the fluidized bed material. Fluidizing velocity was approximately three times the minimum fluidizing velocity (3 Umf), so it was slightly different according to the reaction temperature of the fluidized bed reactor. The calculated vapor residence time in the fluidized bed reactor was approximately 1 s. Fig. 3 shows the temperature changes in the auger reactor, and in the bubbling and freeboard zones of the fluidized bed reactor during pyrolysis. As it can be observed in Fig. 3, the pyrolysis was conducted at a constant temperature for each reaction part. The other experiments were also performed under nearly isothermal conditions.
As-received basis and ASTM D 3172 method. As-received basis. Not detected.
Fig. 1. Thermogravimetry and differential thermogravimetry curves of the feed material.
the PS was fully degraded at approximately 500 °C, the reaction temperature of the main pyrolysis reactor (fluidized bed reactor) was set at a slightly higher temperature than 500 °C.
2.4. Mass balance and analyses Product fractions were classified into residue from the auger reactor, and oil, gas, and, char from the fluidized bed reactor. The auger reactor residue was the stiffened fraction of the PS formed during the cooling phase after the experiment. The amount of char from the fluidized bed reactor was calculated as the sum of the chars collected by the cyclone separator and the hot filter. The amount of oil was determined as the sum of the oils collected in the vessels of the condensers, the IP, and the EP. The gas yield was calculated through the difference between the amount of PS fed into the system and the sum of all the products collected. The pyrolysis oil was first qualitatively analyzed using a gas chromatography-mass spectroscopy (GC–MS). Subsequently, it was quantitatively analyzed using a gas chromatography-flame ionization detector (GC-FID). The analyses using GC–MS (5975C, Agilent Instruments) and GC-FID (7890A, Agilent Instruments) were performed under identical conditions to match the chromatogram peaks. An HP-5MS column was installed in both instruments, and the carrier gas was helium. The Wiley Registry/NIST Library was used for peak identification, and peaks with high probability (quality exceeding 80%) were identified for the qualitative analysis. The relative response factor, which was calculated using the effective carbon number of each compound, was used to correct a systematic error in the peak area resulting from GC-FID [32]. The product gas was analyzed using a gas chromatography-thermal conductivity detector (GC-TCD) and GC-FID. The GC-TCD (7890A, Agilent Instruments) was equipped with a Carboxen 1000 column, and argon was used as the carrier gas. The GC-FID instrument was equipped with an HP-plot Al2O3/KCl column, and argon was used as the carrier gas.
2.2. Pyrolysis process The two-stage pyrolysis process consists of two reactors (auger and fluidized bed reactor), a char separating system, a quenching system, and a gas regulation system. Fig. 2 shows a detailed diagram of the process. The auger and the fluidized bed reactor are directly connected to each other. Both reactors are made of SUS 310 stainless steel and equipped with their own electric heaters. The inner diameter and length of the auger reactor are 25 and 520 mm, respectively. The inner diameter and height of the fluidized bed reactor are 70 and 560 mm, respectively. In the auger reactor, the feed material is heated to approximately 300 °C, at a temperature in which molecules inside the reactor can exist as a fluid without being severely degraded. Under these conditions, the molecules receive significant thermal energy, elevating their vibrational state, which should increase the bond length of the molecules. Therefore, when the molecules leaving the auger reactor enter the fluidized bed reactor, they can be easily cracked at many places compared to unheated molecules. The fluidized bed reactor is equipped with three thermocouples: two in the bubbling zone and one in the freeboard zone. The reaction temperature of the bubbling zone is determined by the average values of the two thermocouples. The pyrolysis vapor leaving the fluidized bed reactor passes through a char separation system consisting of a cyclone separator and a hot filter, which are designed to capture particulate matter with diameters exceeding 10 and 2 µm, respectively. After the char separation system, the vapor passes through a quenching system consisting of two condensers. 3
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
Fig. 2. Diagram of the two-stage pyrolysis process. Table 2 Experimental conditions of the two-stage pyrolysis of polystyrene. Parameter
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
Auger reactor temperature (°C) Bubbling zone temperature (°C) Freeboard temperature (°C) Fluidizing medium
15.2 505.1 515.1 P.G.a
199.0 509.6 511.1 P.G.a
299.1 513.7 511.7 P.G.a
306.7 628.1 628.6 P.G.a
297.8 697.6 694.4 P.G.a
300.5 782.2 781.9 P.G.a
300.0 694.3 702.3 N2
a
Product gas.
temperature led to a decrease in the oil yield. The difference of oil yield between Runs 3 and 1 was fairly small (0.47 wt%). However, the difference appeared to result from the specific role of the auger reactor. A previous work on the pyrolysis of PE conducted with the same twostage reactor clearly showed a strong decrease in oil yield (11 wt%) with increasing the auger reactor temperature [30]. The large difference in the results can be mainly attributed to the difference between the degradation mechanisms of PS and PE. As mentioned before, the auger reactor appears to increase the vibrational state of molecules inside the reactor, which should allow molecules to be easily cracked at many places and produce mainly light hydrocarbons (for PE, mainly gas). However, in the degradation of PS, the tendency for the unzip reaction seems to be significantly stronger due to the high stability of the reactive intermediate (benzylic radical). Therefore, the dominant product from the fluidized bed reactor was still styrene (a liquid phase). As the temperature of the fluidized bed reactor increased from 514 (Run 3) to 782 °C (Run 6), the oil yield decreased from 98 to 86 wt%: a smooth decrease (3.6 wt%) of oil yield occurred from 514 to 698 °C, whereas a sharp decrease of oil yield (8.8 wt%) occurred from 698 to 782 °C. In particular, the styrene yield was sharply decreased from 69 to 26 wt%, whereas the yield of polyaromatic hydrocarbons was increased due to secondary reactions such as polyaromatic formation reactions. The temperature increase from 698 to 782 °C caused an increase in the gas yield from 4.8 to 13.8 wt%, especially increase in CH4 and light hydrocarbons due to enhanced primary cracking reactions. The results above indicate that a high temperature (in the present work: 780 °C) is needed to obtain an oil with decreased styrene yield. Table 4 shows the system energy balance (Run 5) of the two-stage pyrolysis of PS. In the calculation of energy input for heaters in Table 4, we only included the energy consumption during pyrolysis. Most of the energy
Fig. 3. Temperature change during pyrolysis (Run 6).
3. Results and discussion 3.1. Mass and energy balance The mass balance of the two-stage pyrolysis of PS is listed in Table 3. The main product from the two-stage pyrolysis of PS was oil, with yields that ranged from 86 to 99 wt%. The gas yield was very low except when the fluidized bed reactor temperature was very high (782 °C). As shown in Table 3, an increase in the auger reactor 4
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
Table 3 Mass balance of two-stage pyrolysis of polystyrene. Reactor type
Products
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
Auger reactor
Residues
0.00
0.08
0.32
0.36
0.39
0.02
0.38
Fluidized bed reactor
Gases (sum) Hydrogen Methane Ethane Ethene Propene Butenes 1,3-butadiene Other gases Oils (sum) Aliphatics (sum) 1,3-Cyclopentadiene Other aliphatics Aromatics (sum) Mono aromatics (sum) Benzene Toluene Ethylbenzene + Xylene Styrene Cumene α-Methylstyrene Propenylbenzene Indene Dialin Other mono aromatics Poly aromatics (sum) Naphthalene cis-Stilbene 2,4-diphenyl-1-butene (dimer) 1,4-diphenyl-1,3-butadiene 2-phenylnaphthalene 2,4,6-triphenyl-1-hexene (trimer) Other poly aromatic compounds Unknowns Char
1.17 0.08 0.14 0.05 0.54 0.22 0.09 0.01 0.04 98.71 0.01 0.01 0.00 90.48 77.62 0.04 1.58 0.23 73.60 0.10 1.61 0.08 0.15 0.03 0.20 12.86 0.02 0.13 7.80 0.29 1.00 2.21 1.41 8.22 0.12
1.39 0.67 0.32 0.05 0.20 0.07 0.03 0.01 0.04 98.52 0.00 0.00 0.00 90.34 74.45 0.16 1.86 0.27 70.18 0.09 1.41 0.07 0.18 0.03 0.20 15.89 0.03 0.00 9.06 0.33 1.35 1.89 3.23 8.18 0.01
1.41 0.19 0.14 0.05 0.52 0.28 0.00 0.00 0.21 98.23 0.00 0.00 0.00 90.10 74.30 0.32 1.97 0.27 69.76 0.09 1.40 0.07 0.18 0.03 0.21 15.80 0.03 0.00 9.01 0.33 1.34 1.88 3.21 8.13 0.04
1.73 0.14 0.4 0.1 0.65 0.31 0.09 0.03 0.01 97.89 0.02 0.02 0.00 89.45 81.95 0.23 4.56 2.15 68.61 0.24 3.96 0.26 0.68 0.09 1.17 7.50 0.32 0.00 2.13 0.79 1.19 0.63 2.44 8.42 0.02
4.78 0.62 2.11 0.21 1.28 0.38 0.08 0.05 0.05 94.83 0.03 0.03 0.00 87.06 77.11 2.63 9.94 5.42 52.50 0.18 4.07 0.39 1.21 0.05 0.72 9.95 1.45 0.00 1.48 1.80 1.47 0.09 3.66 7.74 0.00
13.84 0.96 7.33 0.75 4.33 0.24 0.01 0.11 0.11 86.01 0.01 0.01 0.00 78.87 56.80 15.55 9.79 0.96 26.09 0.06 1.24 0.13 2.19 0.05 0.74 22.07 5.30 0.43 0.35 2.83 2.03 0.02 11.11 7.13 0.13
5.57 0.16 0.86 0.13 2.75 0.99 0.30 0.30 0.08 94.03 0.10 0.05 0.05 85.32 77.86 1.96 7.31 2.59 59.72 0.18 3.78 0.33 1.10 0.12 0.77 7.46 0.96 0.00 1.32 1.08 1.04 0.03 3.03 8.61 0.02
Total
100
100
100
100
100
100
100
Table 4 System energy balance of Run 5. Energy input PS Heaters Total Energy output Gas Oil Auger reactor residue Char Heat loss Total
Energy (MJ/h)
Ratio (%)
20.90 2.38 23.28
89.79 10.21 100
Energy (MJ/h)
Ratio (%)
1.10 19.29 0.08 0.002 2.80 23.28
4.74 82.89 0.34 0.01 12.02 100
output was concentrated in the oil (~83%). Due to the low gas yield, the energy of product gas among products amounted to only 4.7%. Heat loss was about 12%, which appeared to be attributed mostly to the condensation of the pyrolysis oil, as mentioned by another study [33]. 3.2. Effect of reactor temperature on oil composition
Fig. 4. Oil compositions according to the reactors‘ temperature.
Fig. 4 shows the major compounds of the oil obtained at different reactors' temperatures. The main oil components were styrene, α-methylstyrene, and dimers and trimers of styrene. As the auger reactor temperature increased from 15 to 300 °C, the styrene yield decreased from 73.6 to 69.8 wt%, whereas the BTEX yield
increased from 1.85 to 2.56 wt%. The result shows that the BTEX yield could be increased with the help of the auger reactor. A slight increase in the fluidized bed reactor in Run 3 (9 °C) compared to Run 1 could also contribute to the increase in the BTEX yield. Fig. 5 shows the main degradation mechanism of PS called unzip or end-chain β-scission and 5
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
Fig. 5. Polystyrene pyrolysis: (a) Random-scission and end-chain β-scission mechanisms for styrene formation, (b) Proposed mechanisms for benzene, toluene, and ethylbenzene formation.
possible formation routes for BTEX aromatics. Increasing temperature in the auger reactor likely led to an increased possibility of bond dissociation to form BTEX aromatics, which
was likely attributed to the decreased bond strength at relevant sites. However, the end-chain β-scission, which is the main mechanism for the formation of styrene monomers, appeared to be more dominant 6
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
than other reactions at a fluidized bed reactor temperature of ~510 °C, which produced a large amount of styrene. The result clearly shows that the effect of auger reactor temperature had a limitation and that the inherent degradation mechanism of PS was much more important factor for pyrolysis product distribution. However, the styrene yield was significantly lower with increasing temperatures in the fluidized bed reactor. As the fluidized bed reactor temperature increased from 514 to 782 °C, the styrene yield drastically decreased from 70 to 26 wt %. Liu et al. [21] reported an increase in styrene yield up to 600 °C in the fluidized bed reactor, after which the yield decreased. In contrast, the increased yield of BTEX aromatics with higher temperatures in the fluidized bed reactor was remarkable. The maximum BTEX yield (26.3 wt%) was obtained at 782 °C. The BTEX concentration in the oil (782 °C) amounted to 30.6 wt%. These results are comparable to other studies which aimed at a high BTEX yield (Fig. 6). For a high BTEX yield from PS pyrolysis, catalysts are normally applied. Fig. 6 shows two approaches conducted with catalysts. Achilias et al. [26] applied fluid catalytic cracking (FCC) catalysts, which yielded 19.8 wt% of BTEX. Rehan et al. concluded that synthetic zeolite is better for BTEX production than natural zeolite [34]. In their work, the BTEX yield using synthetic zeolite amounted to 13.1 wt% at the high expense of oil yield. Compared to these studies, the BTEX yield of the current work is very high, without a great expense of oil yield. The third reference [35] in Fig. 6 aimed to produce BTEX aromatics from mixed plastic waste. The authors obtained a BTEX yield of 18.5 wt% using a single fluidized bed reactor. Although the production of BTEX aromatics from mixed plastic waste is an interesting option, it can be compromised by the chlorine in the oil, which is derives from polyvinylchloride (PVC), a polymer always present in mixed plastic waste. The chlorine content in the oil from pyrolysis should be below 10 ppmw to allow for use as a feedstock in oil refineries [12]. In contrast, BTEX production from PS can avoid the chlorine related problem, especially when expanded polystyrene foam (ESP) is pyrolyzed. ESP can be separately collected from houses, at least in Korea, which reduces the possibility of a pyrolysis of EPS together with PVC. Nevertheless, this approach for BTEX production from PS requires a high amount of energy to maintain the high reaction temperatures. Table 3 also shows that the total yield of monoaromatic hydrocarbons (MAH) was maximized at 630 °C and decreased thereafter. The observed trend appeared to be related to the yields of styrene dimers, their derivatives, and trimers. Fig. 7 shows their yields according to the temperature in the
Fig. 6. Comparisons with other works on BTEX and styrene yields.
Fig. 7. Yields of styrene dimer, its derivatives and trimer according to the fluidized bed reactor temperature.
Fig. 8. Formation mechanisms of major styrene dimer derivatives. 7
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
fluidized bed reactor. Fig. 7 shows that yields of 2,4-diphenyl-1-butene (dimer) and 2,4,6triphenyl-1-hexene (trimer) decreased with increasing temperatures. In particular, there was a significant decrease in the dimer yield from 514 to 630 °C, which likely contributed to maintain the high styrene yield (69 wt%) at 630 °C. However, over 600 °C, re-dimerization of styrene monomer was prevalent. Mechanisms (a) and (b) in Fig. 8. which were proposed by Flory and Mayo [36,37], are representative of the re-dimerization. Based on the proposed mechanisms, styrene dimer derivatives, such as 1,4-diphenyl-1,3-butadiene and 2-phenylnaphthalene, can be increased with higher temperatures in the fluidized bed reactor, as demonstrated in the present work.
during the winter, and met the diesel criteria (below 6 °C). In contrast with the pour point, the flash point is important during the summer. The flash point of both oils were the same, with a value of 79 °C, which satisfied the gasoline and diesel standards. The pH of both oils were also the same, at 6.4, which represents a neutral pH status. Pyrolysis oils from the current study, however, consisted mainly of aromatic hydrocarbons. Pyrolysis oils with such high contents of aromatics cannot be direct used as fuels, because in most countries the contents of aromatics and benzene in fuels, for example in gasoline, are limited to 35 and 1 vol%, respectively [39]. However, the pyrolysis oils with a high aromatic content could be used when they are blended with oils with a low aromatic content in order to meet the characteristics of commercial fuels. In conclusion, the pyrolysis oils obtained in the current study almost satisfied the commercial fuels standards, and they appear to be appropriate for use as a substitute source for gasoline and diesel fuels when they are mixed with oils with a low aromatic content.
3.3. Effect of the type of fluidizing medium Run 7 was conducted with nitrogen as the fluidizing medium. Compared to Run 5, where product gas was used, the oil yield from Run 7 was comparable, and the gas yield slightly increased. Regarding individual components, the styrene yield from Run 7 increased by about 7 wt%, whereas the BTEX yield decreased by approximately 6 wt% compared to Run 5. The hydrogen abstraction by phenyl and benzylic radicals to form BTEX aromatics appeared to be hindered by the nitrogen. Due to the dilution effect of nitrogen, reactions with styrene monomers, for example, the dimerization of styrene, appear to have diminished. The contents of dimers, 1,4-diphenyl-1,3-butadiene, 2phenylnaphthalene, and trimers in the pyrolysis oil of Run 7 were clearly reduced compared to those of Run 5. These results indicate that the use of product gas as a fluidizing medium is desirable for the production of BTEX from PS.
4. Conclusions Pyrolysis of PS was conducted using a two-stage pyrolysis process that has an auger reactor and a fluidized bed reactor. The main conclusions are as follows: 1) As the temperature in the auger reactor increased, the styrene yield decreased. 2) The maximum yield of benzene, toluene, ethylbenzene, and xylenes was significantly high, with a value of 26.3 wt%, which is higher than the yield obtained using catalysts. 3) The highest yield of benzene, toluene, ethylbenzene, and xylenes did not significantly cost the oil yield (86 wt%), but it demanded high energy input. 4) The product gas is a good fluidizing medium for the production of benzene, toluene, ethylbenzene, and xylenes. 5) The obtained pyrolysis oil can likely be used as a good source of benzene, toluene, ethylbenzene, and xylenes.
3.4. Fuel properties of oil This study proposed the production of BTEX aromatics from PS. The possibility to produce chlorine-free or low-chlorine oils from PS allows for the use of pyrolysis oil as a fuel, which is particularly appropriate for countries where oil refinery facilities are not easily available. In general, when pyrolysis oil is used as a fuel, it must contain the minimum possible amount of chlorine due to the possible formation of dioxins when combusted. It has been reported that the average chlorine content in gasoline is only a few ppmw [38]. Table 5 shows the physical properties of the oils from Runs 5 and 6 along with commercial fuel standards [11]. The calorific value is an important factor when examining fuel properties. The pyrolysis oils from Runs 5 and 6 showed high calorific values, over 40 MJ/kg. They were only slightly lower than the values for gasoline and diesel standards. The American Petroleum Institute (API) gravity of both oils satisfied the gasoline and diesel standards. The densities of the oils at 15 °C were 0.95 and 0.98 g/cm3, which also met the gasoline and diesel standards. Their viscosities were 0.97 (Run 5) and 1.26 (Run 6), within the standard range for gasoline. The ash content of both oils was lower than 0.007 wt%, which was adequate according to the diesel standard. Both oils showed the same pour point (−39 °C), which is important
CRediT authorship contribution statement Ki-Bum Park: Conceptualization, Data curation, Investigation, Methodology, Project administration, Resources, Validation, Writing original draft. Yong-Seong Jeong: Data curation, Investigation, Resources. Begum Guzelciftci: Data curation, Investigation, Resources. Joo-Sik Kim: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Validation, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This research was supported by the Korea Environmental Industry Technology Institute (KEITI) grant (No. 2016000710004) funded by the Ministry of Environment of Korea.
Table 5 Fuel properties of pyrolysis oil from polystyrene. Physical property
Run 5
Run 6
Gasoline [9]
Diesel [9]
Calorific value (MJ/kg) API gravity @ 60 °F Viscosity (mm2/s) Density @ 15 °C (g/cm3) Ash content (wt.%) Pour point (°C) Flash point (°C) pH
40.89 17.74 0.97 0.95 0.007 −39 79 6.4
40.02 13.22 1.26 0.98 0.001 −39 79 6.4
> 42.5 < 55 < 1.17 > 0.78 – – > 42
> 43.0 < 38 1.9–4.1 > 0.81 < 0.01 <6 > 52
References [1] PlasticsEurope. The facts 2018. PlasticsEurope; 2018. [2] Geyer R, Jambeck JR, Law KL. Production, use, and fate of all plastics ever made. Sci Adv 2017;3:e1700782. [3] Brooks AL, Wang S, Jambeck JR. The Chinese import ban and its impact on global plastic waste trade. Sci Adv 2018;4:eaat0131. [4] Al-Salem SM, Lettieri P, Baeyens J. The valorization of plastic solid waste (PSW) by primary to quaternary routes: from re-use to energy and chemicals. Prog Energy
8
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al. Combust Sci 2010;36:103–29. [5] Rahimi A, García JM. Chemical recycling of waste plastics for new materials production. Nat Rev Chem 2017;1:46. [6] Hamad K, Kaseem M, Deri F. Recycling of waste from polymer materials: An overview of the recent works. Polym Degrad Stab 2013;98:2801–12. [7] Marrone M, La Mantia FP. Re-stabilisation of recycled polypropylenes. Polym Recycl 1996;2:17–26. [8] Zhang Y, Duan D, Lei H, Villota E, Ruan R. Jet fuel production from waste plastics via catalytic pyrolysis with activated carbons. Appl Energy 2019;251:113337. [9] Sophonrat N, Sandstrom L, Zaini IN, Yang W. Stepwise pyrolysis of mixed plastics and paper for separation of oxygenate and hydrocarbon condensates. Appl Energy 2018;229:314–25. [10] Lopez G, Artetxe M, Amutio M, Bilbao J, Olazar M. Thermochemical routes for the valorization of waste polyolefinic plastics to produce fuels and chemicals. A review. Renew Sust Energ Rev 2017;73:346–68. [11] Sharuddin SDA, Abnisa F, Daud WMAW, Aroua MK. A review on pyrolysis of plastic wastes. Energy Convers Manag 2016;115:308–26. [12] Jung S-H, Cho M-H, Kang B-S, Kim J-S. Pyrolysis of a fraction of waste polypropylene and polyethylene for the recovery of BTX aromatics using a fluidized bed reactor. Fuel Process Technol 2010;91:277–84. [13] Elordi G, Olazar M, Lopez G, Artetxe M, Bilbao J. Product yields and compositions in the continuous pyrolysis of high-density polyethylene in a conical spouted bed reactor. Ind Eng Chem Res 2011;50:6650–9. [14] Lopez-Urionabarrenechea A, De Marco I, Caballero BM, Adrados A, Laresgoiti MF. Empiric model for the prediction of packaging waste pyrolysis yields. Appl Energy 2012;98:524–32. [15] Zhou J, Qiao Y, Wang W, Leng E, Huang J, Yu Y, et al. Formation of styrene monomer, dimer and trimer in the primary volatiles produced from polystyrene pyrolysis in a wire-mesh reactor. Fuel 2016;182:333–9. [16] Faravelli T, Pinciroli M, Pisano F, Bozzano G, Dente M, Ranzi E. Thermal degradation of polystyrene. J Anal Appl Pyrolysis 2001;60:103–21. [17] McNeill IC, Zulfiqar M, Kousar T. A detailed investigation of the products of the thermal degradation of polystyrene. Polym Degrad Stab 1990;28:131–51. [18] Artetxe M, Lopez G, Amutio M, Barbarias I, Arregi A, Aguado R, et al. Styrene recovery from polystyrene by flash pyrolysis in a conical spouted bed reactor. Waste Manage 2015;45:126–33. [19] Moqadam SI, Mirdrikvand M, Roozbehani B, Kharaghani A, Shishehsaz MR. Polystyrene pyrolysis using silica-alumina catalyst in fluidized bed reactor. Clean Technol Environ Policy 2015;17:1847–60. [20] Demirbas A. Pyrolysis of municipal plastic wastes for recovery of gasoline-range hydrocarbons. J Anal Appl Pyrolysis 2004;72:97–102. [21] Liu Y, Qian J, Wang J. Pyrolysis of polystyrene waste in a fluidized-bed reactor to obtain styrene monomer and gasoline fraction. Fuel Process Technol 2000;63:45–55.
[22] Karaduman A, Şimşek EH, Cicek B, Bilgesü AY. Flash pyrolysis of polystyrene wastes in a free-fall reactor under vacuum. J Anal Appl Pyrolysis 2001;60:179–86. [23] Karaduman A. Pyrolysis of polystyrene plastic wastes with some organic compounds for enhancing styrene yield. Energy Sources 2002;24:667–74. [24] Park JJ, Park K, Kim J-S, Maken S, Song H, Shin H, et al. Characterization of styrene recovery from the pyrolysis of waste expandable polystyrene. Energy Fuels 2003;17:1576–82. [25] Onwudili JA, Insura N, Williams PT. Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: Effects of temperature and residence time. J Anal Appl Pyrolysis 2009;86:293–303. [26] Achilias DS, Kanellopoulou I, Megalokonomos P, Antonakou E, Lappas AA. Chemical recycling of polystyrene by pyrolysis: potential use of the liquid product for the reproduction of polymer. Macromol Mater Eng 2007;292:923–34. [27] Chen C-C, Duh Y-S, Shu C-M. Thermal polymerization of uninhibited styrene investigated by using microcalorimetry. J Hazard Mater 2009;163:1385–90. [28] Pérez-Alonso FJ, Agüí L, Yáñez-Sedeño P, Pingarrón JM. Determination of styrene and styrene additives using cylindrical microelectrodes in acetone. Analyst 2000;125:2006–10. [29] Mertinkat J, Kirsten A, Predel M, Kaminsky W. Cracking catalysts used as fluidized bed material in the Hamburg pyrolysis process. J Anal Appl Pyrolysis 1999;49:87–95. [30] Park K-B, Jeong Y-S, Guzelciftci B, Kim J-S. Characteristics of a new type continuous two-stage pyrolysis of waste polyethylene. Energy 2019;166:343–51. [31] Park K-B, Jeong Y-S, Guzelciftci B, Kim J-S. Activator-assisted pyrolysis of polypropylene. Appl Energy 2019;253:113558. [32] Holm T. Aspects of the mechanism of the flame ionization detector. J Chromatogr A 1999;842:221–7. [33] Boateng AA, Daugaard DE, Goldberg NM, Hicks KB. Bench-scale fluidized-bed pyrolysis of switchgrass for bio-oil production. Ind Eng Chem Res 2007;46:1891–7. [34] Rehan M, Miandad R, Barakat MA, Ismail IMI, Almeelbi T, Gardy J. Effect of zeolite catalysts on pyrolysis liquid oil. Int Biodeterior Biodegradation 2017;119:162–75. [35] Cho M-H, Jung S-H, Kim J-S. Pyrolysis of mixed plastic wastes for the recovery of benzene, toluene, and xylene (BTX) aromatics in a fluidized bed and chlorine removal by applying various additives. Energy Fuels 2009;24:1389–95. [36] Khuong KS, Jones WH, Pryor WA, Houk KN. The mechanism of the self-initiated thermal polymerization of styrene. theoretical solution of a classic problem. J Am Chem Soc 2005;127:1265–77. [37] Kothe T, Fischer H. Formation rate constants of the mayo dimer in the autopolymerization of styrene. J Polym Sci Part A Polym Chem 2001;39:4009–13. [38] Broz J, Grabic R, Kilian J, Lojkasek M, Marklund S, Ocelka T, et al. The Effect of Oils on PAH, PCDD, PCDF and PCB Emissions from a Spark Engine Fueled with Leaded Gasoline. Chemosphere 2000;41:1905–11. [39] Wagner V, Stahl R, Olivares E, Morgan M, Foukes E. EU: Fuels: Diesel and Gasoline. [Online], 2015 (Available from;) http://transportpolicy.net/index.
9
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Two-stage pyrolysis of polystyrene: Pyrolysis oil as a source of fuels or benzene, toluene, ethylbenzene, and xylenes Ki-Bum Parka, Yong-Seong Jeongb, Begum Guzelciftcia, Joo-Sik Kima,b, a b
⁎
Graduate School of Energy and Environmental System Engineering, 163 Siripdaero, Dongdaemun-Gu, Seoul 02504, Republic of Korea Graduate School of Environmental Engineering, University of Seoul, 163 Siripdaero, Dongdaemun-Gu, Seoul 02504, Republic of Korea
HIGHLIGHTS
two-stage pyrolysis of polystyrene was successfully conducted. • AThecontinuous yield clearly decreased with increasing the auger reactor temperature. • The styrene maximum BTEX and lowest styrene yields were almost the same at 26 wt%. • Oil obtained high temperatures had an enhanced thermal-oxidative stability. • Oil obtained atappeared to be used as a good source of BTEX aromatics. • ARTICLE INFO
ABSTRACT
Keywords: Plastic Two-stage pyrolysis Polystyrene BTEX Fuel
The recycling rate of plastic waste needs to be improved worldwide. In that context, pyrolysis, through which petrochemical feedstock and alternative fuel can be obtained, has received significant attention. In this study, pyrolysis of polystyrene was conducted in a continuous two-stage process that has an auger reactor and a fluidized bed reactor connected in series. The main objective was to produce oils rich in benzene, toluene, ethylbenzene, and xylenes instead of typical polystyrene pyrolysis oils, which contain high amounts of styrene monomers with low thermal-oxidative stability. The effects of different reaction temperatures (in both reactors) and the type of fluidizing medium on the product distribution and composition were investigated. The maximum yield of benzene, toluene, ethylbenzene, and xylenes (26.3 wt%) was obtained at a temperature of 780 °C in the fluidized bed reactor. The oil and styrene yields at 780 °C were 86 and 26 wt%, respectively. To evaluate the fuel properties of the pyrolysis oil, its calorific value, API gravity, viscosity, density, ash content, pour point, flash point, and pH were examined. The results indicate that the pyrolysis oil can be both a good source of benzene, toluene, ethylbenzene, and xylenes and can potentially be used as a substitute source to gasoline or diesel fuels when it is mixed with oils with a low aromatic content.
1. Introduction Since the mid-1900s, plastic use disseminated, and plastics became irreplaceable in modern society. The annual worldwide plastic production reached nearly 350 million metric tons in 2017 and is expected to increase yearly [1]. Plastics are essential in our society, and their use ranges from household goods to industrial appliances, automotive parts, medical equipment, and packaging. Among the various usages of plastic, approximately 40% of the total plastic demand is for packaging, which is problematic due to their short lifespan (less than a year) [2]. With the increasing demand for plastic, there is also a global increase in plastic waste production. In 2015, about 60% of all the plastic ever
⁎
produced (5000 million metric tons) was still discarded or landfilled [2]. Moreover, the import ban policy by the Chinese government accelerated the worldwide plastic waste crisis. Until 2016, China ranked first as importer of plastic, being responsible for over 45% of the total plastic imports worldwide [3]. In this context, appropriate treatment methods for plastic waste need to be developed and adopted. These treatment methods can be categorized as: primary (re-extrusion), secondary (mechanical recycling), tertiary (chemical recycling), and quaternary (energy recovery) [4]. Each treatment method has its own advantages and can be applied according to the constitution of the plastic and local circumstances. The main difference between re-extrusion and mechanical recycling is
Corresponding author at: Graduate School of Environmental Engineering, University of Seoul, 163 Siripdaero, Dongdaemun-Gu, Seoul 02504, Republic of Korea. E-mail address: [email protected] (J.-S. Kim).
https://doi.org/10.1016/j.apenergy.2019.114240 Received 10 June 2019; Received in revised form 19 November 2019; Accepted 23 November 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Ki-Bum Park, et al., Applied Energy, https://doi.org/10.1016/j.apenergy.2019.114240
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
whether the reprocessed material is the same material as the original plastic or not [5]. For example, recycling of polyethylene terephthalate (PET) waste into a PET bottle is re-extrusion, and recycling of PET waste into a fiber is mechanical recycling. Both methods generally require plastics with low impurity to avoid quality deterioration of the recycled products [6]. Furthermore, reprocessing is limited to a few cycles because the mechanical properties of the recycled products deteriorate upon reprocessing [7]. The third recycling option for plastic waste is chemical recycling. A representative of chemical recycling is pyrolysis, which converts plastic waste into smaller hydrocarbons under a high-temperature inert atmosphere. Pyrolysis is a flexible option compared to mechanical recycling because the target products of pyrolysis can be selected by varying the reaction conditions. It has been extensively employed in the thermal decomposition of waste plastics to obtain high energy density products, containing paraffin, olefins and aromatic hydrocarbon [8]. However, not many efforts have completely succeeded in recovering energy and materials by pyrolysis due to the complexity and heterogeneity of the feedstocks [9]. In plastic pyrolysis, various types of reactors have been tested to obtain valuable products. Typical reactors are fixed bed, stirred tank, screw kiln, circulating sphere, fluidized bed, spouted bed, plasma, and microwave-assisted reactors [10]. Among them, fluidized bed, fixed bed and conical spouted bed reactors have been most widely applied [11]. Fixed reactors are simple in design and easy to control operating parameters. Fluidized bed reactors allow operating under high heating rates, isothermal bed and short residence times [12]. The spouted bed reactor is known to avoid defluidization problems by vigorous solid circulation [13]. Only with primary and secondary treatment methods of plastic waste, any country cannot achieve its own recycling rate goal. In particular, sorting plants produce rejected plastic waste which are mixed with inappropriate materials such as paper, glass, and metals and therefore hard to be recycled [14]. That plastic waste can be properly treated by pyrolysis. Polystyrene (PS), one of main commodity plastics, is widely used for many purposes but especially for packaging. However, the recycling rate of PS is only 0.9% in the U.S. [5]. The use of pyrolysis to recycle PS has been extensively investigated. Many researchers have studied the degradation mechanism of PS. Zhou et al. [15], Faravelli et al. [16], and McNeill [17] et al. reported that important reaction mechanisms of the PS pyrolysis were β-scission, unzipping (end-chain β-scission), and intermolecular and intramolecular hydrogen transfer reactions. Among those, the unzipping reaction is considered the main mechanism for the production of styrene monomers. Many researches aimed at a high yield of styrene monomers from PS waste. Artetxe et al. conducted the pyrolysis of PS in a conical spouted bed reactor and found that the styrene yield was strongly influenced by both temperature and gas flow rate [18]. Moqadam et al. performed the pyrolysis of PS using silica-alumina catalysts and could obtain oils containing a high percentage of styrene monomer (> 80%) [19]. Demirbas et al. reported that the yield of styrene increased until 750 °C (65 wt%), and it rapidly decreased after that [20]. Liu et al. investigated the pyrolysis of PS and obtained a styrene monomer yield of 78.7% at 600 °C. Moreover, 99.6% of high purity styrene was obtained by additional vacuum distillation [21]. Karaduman et al. carried out the flash pyrolysis of PS using a free-fall reactor and reported that the oil yield maximized around 750 °C and the styrene yield at 825 °C [22]. Karaduman also investigated the pyrolysis of PS using various organic compounds for enhancing styrene yield. He found in the study that the maximum styrene yield (74 wt%) could be obtained with naphthalene [23]. In a study on the catalytic pyrolysis of PS, solid base BaO was found to be the most efficient to increase the styrene selectivity (84 wt%) [24]. Onwudili et al. investigated the influence of residence time on the styrene yield. They observed that the styrene yield decreased, and the toluene and ethylbenzene yields compensatively increased when the residence time of pyrolysis vapor increased [25]. Achilias et al. demonstrated that the PS synthesized using pyrolysis oil from PS was similar to the original PS. However, the study
revealed that other aromatic compounds in the oil decreased the quality of resynthesized PS compared to the virgin one [26]. If pyrolysis oil from PS cannot be properly used as a feedstock for PS synthesis, its use as a fuel is a possible alternative. However, the pyrolysis oil from PS has a low thermal-oxidative stability, mainly due to the large number of double bonds in the oil. Upon storage in a hot tank during the summer (~50 °C), auto-polymerization can occur and form unwanted styrene dimers, trimers, or high molecular components [27]. To prevent such undesirable auto-polymerization, antioxidants such as 4-tert-butylcatechol should be blended into commercial styrene storage [28]. In the current study, we aim to produce pyrolysis oil from PS with enhanced thermal-oxidative stability without using a catalyst. For that purpose, the styrene content in the oil should be remarkably reduced compared to conventional PS pyrolysis oil. Mertinkat et al. studied the production of such stability-enhanced oils from PS, but with the use of catalysts [29]. To the best of our knowledge, the present study is the first noncatalytic approach to produce oils with a low styrene content for use as fuel or feedstock. For that, a two-stage pyrolysis process was used. It comprises an auger reactor and a fluidized bed reactor in series. Previous works on the two-stage pyrolysis of polyethylene (PE) and polypropylene [30,31] revealed unique characteristics of the pyrolysis process, which yielded a different product spectrum compared to conventional pyrolysis. This different product spectrum was attributed to the action of the auger reactor, which is usually heated to 200–400 °C. In the heated auger reactor, the vibrational states of polymer molecules may be high compared to those of unheated molecules. Consequently, the bond length of all bonds of the heated molecules will slightly increase, making the bond strength of all bonds weak. The fluidized bed reactor, which is located directly after the auger reactor, can exploit these weakened bonds and crack the molecules, which would not occur under conventional pyrolysis. Based on this assumption, we expected to obtain a pyrolysis oil with a different composition compared to the one derived from conventional pyrolysis of PS. In particular, the production of mono-aromatics such as BTEX at the expense of styrene monomer was expected to be enhanced with the help of the two-stage pyrolysis process. Further, the pyrolysis oil from PS appears to be an attractive fuel source because PS, especially expanded polystyrene, can be collected separately from PVC, and because the pyrolysis oil from PS, therefore, will not suffer from chlorine-related problems. Hence, we explored the possibility of the pyrolysis oil from PS as a fuel in the current work. The successful conversion of PS into desirable products such as chemical feedstocks and fuels for energy would contribute not only to the efficient treatment of waste PS, but also to the resource conservation. The variables tested were: temperature of the auger and the fluidized bed reactors, and type of fluidizing medium. 2. Material and methods 2.1. Feed material characteristics A granular virgin PS (Styrolution, PS 147F), with a size of 1–3 mm, was used as the feed material. The proximate analysis of the feed material was conducted according to the ASTM D 3172 standard, and the ultimate analysis was performed using an elemental analyzer (Flash 2000 series, Thermo Scientific). The main characteristics of PS are presented in Table 1. The PS was mainly composed of volatile matter (99.8 wt%) with a high content of carbon (91.9 wt%). To simultaneously examine the degradation behavior and determine the reaction temperature of the two-stage pyrolysis, a thermogravimetric analysis (TGA) was conducted using a thermogravimetric analyzer (TGA Q50, TA Instruments). In the TGA study, the PS (10 mg) was heated to 900 °C under a N2 atmosphere, at heating rates of 10–30 °C/min. Fig. 1 shows the results of the TGA study. The TG and differential thermogravimetry curves show that the main degradation occurred between 300 and 500 °C. Considering that 2
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
The first condenser operates with water as a coolant at 20 °C, and the second one with methanol at −20 °C. The pyrolysis oil is captured in vessels at the bottom of the condensers, and uncondensed vapor goes through an impact separator (IP) and an electrostatic precipitator (EP), which separate the remaining oil fraction from the gas. The gas leaving the EP is recirculated into the fluidized bed reactor by a compressor (N0150ATE, KNF).
Table 1 Main characteristics of the feed material. PS Proximate analysisa (wt.%)
Ultimate analysisb (wt.%)
Moisture Volatile matter Fixed carbon Ash
Carbon Hydrogen Nitrogen Sulfur Oxygen
0.10 ± 0.04 99.83 ± 0.04 0.06 ± 0.02 0.01 ± 0.00
Lower heating value (MJ/kg) a b c
91.93 ± 0.06 7.96 ± 0.02 N.D.c N.D.c N.D.c
2.3. Experimental conditions
41.81
An experimental scheme was established to determine the effects of temperature in the auger and in the fluidized bed reactor, and the type of fluidizing medium. Table 2 shows the experimental conditions. Runs 1–3 were conducted to investigate the influence of the auger reactor temperature, which varied from 15 °C when the heater was off (Run 1) to 299 °C (Run 3). In all three runs, the fluidized bed reactor temperature was kept at approximately 510 °C. Runs 3–6 demonstrate the effect of the temperature in the fluidized bed reactor on the pyrolysis product distribution, and composition of gas and oil. The fluidized bed reactor temperature was raised from 514 (Run 3) to 782 °C (Run 6). In the six runs above, product gas was used as the fluidizing medium. In contrast, in Run 7, nitrogen was used as the fluidizing medium. The other reaction conditions were nearly identical in all runs. The amount of PS used for each run was 500 g, and the feeding rate was maintained at approximately 500 g/hr. Quartz sand (1.2 kg) with particle sizes of 0.150–0.425 mm was used as the fluidized bed material. Fluidizing velocity was approximately three times the minimum fluidizing velocity (3 Umf), so it was slightly different according to the reaction temperature of the fluidized bed reactor. The calculated vapor residence time in the fluidized bed reactor was approximately 1 s. Fig. 3 shows the temperature changes in the auger reactor, and in the bubbling and freeboard zones of the fluidized bed reactor during pyrolysis. As it can be observed in Fig. 3, the pyrolysis was conducted at a constant temperature for each reaction part. The other experiments were also performed under nearly isothermal conditions.
As-received basis and ASTM D 3172 method. As-received basis. Not detected.
Fig. 1. Thermogravimetry and differential thermogravimetry curves of the feed material.
the PS was fully degraded at approximately 500 °C, the reaction temperature of the main pyrolysis reactor (fluidized bed reactor) was set at a slightly higher temperature than 500 °C.
2.4. Mass balance and analyses Product fractions were classified into residue from the auger reactor, and oil, gas, and, char from the fluidized bed reactor. The auger reactor residue was the stiffened fraction of the PS formed during the cooling phase after the experiment. The amount of char from the fluidized bed reactor was calculated as the sum of the chars collected by the cyclone separator and the hot filter. The amount of oil was determined as the sum of the oils collected in the vessels of the condensers, the IP, and the EP. The gas yield was calculated through the difference between the amount of PS fed into the system and the sum of all the products collected. The pyrolysis oil was first qualitatively analyzed using a gas chromatography-mass spectroscopy (GC–MS). Subsequently, it was quantitatively analyzed using a gas chromatography-flame ionization detector (GC-FID). The analyses using GC–MS (5975C, Agilent Instruments) and GC-FID (7890A, Agilent Instruments) were performed under identical conditions to match the chromatogram peaks. An HP-5MS column was installed in both instruments, and the carrier gas was helium. The Wiley Registry/NIST Library was used for peak identification, and peaks with high probability (quality exceeding 80%) were identified for the qualitative analysis. The relative response factor, which was calculated using the effective carbon number of each compound, was used to correct a systematic error in the peak area resulting from GC-FID [32]. The product gas was analyzed using a gas chromatography-thermal conductivity detector (GC-TCD) and GC-FID. The GC-TCD (7890A, Agilent Instruments) was equipped with a Carboxen 1000 column, and argon was used as the carrier gas. The GC-FID instrument was equipped with an HP-plot Al2O3/KCl column, and argon was used as the carrier gas.
2.2. Pyrolysis process The two-stage pyrolysis process consists of two reactors (auger and fluidized bed reactor), a char separating system, a quenching system, and a gas regulation system. Fig. 2 shows a detailed diagram of the process. The auger and the fluidized bed reactor are directly connected to each other. Both reactors are made of SUS 310 stainless steel and equipped with their own electric heaters. The inner diameter and length of the auger reactor are 25 and 520 mm, respectively. The inner diameter and height of the fluidized bed reactor are 70 and 560 mm, respectively. In the auger reactor, the feed material is heated to approximately 300 °C, at a temperature in which molecules inside the reactor can exist as a fluid without being severely degraded. Under these conditions, the molecules receive significant thermal energy, elevating their vibrational state, which should increase the bond length of the molecules. Therefore, when the molecules leaving the auger reactor enter the fluidized bed reactor, they can be easily cracked at many places compared to unheated molecules. The fluidized bed reactor is equipped with three thermocouples: two in the bubbling zone and one in the freeboard zone. The reaction temperature of the bubbling zone is determined by the average values of the two thermocouples. The pyrolysis vapor leaving the fluidized bed reactor passes through a char separation system consisting of a cyclone separator and a hot filter, which are designed to capture particulate matter with diameters exceeding 10 and 2 µm, respectively. After the char separation system, the vapor passes through a quenching system consisting of two condensers. 3
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
Fig. 2. Diagram of the two-stage pyrolysis process. Table 2 Experimental conditions of the two-stage pyrolysis of polystyrene. Parameter
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
Auger reactor temperature (°C) Bubbling zone temperature (°C) Freeboard temperature (°C) Fluidizing medium
15.2 505.1 515.1 P.G.a
199.0 509.6 511.1 P.G.a
299.1 513.7 511.7 P.G.a
306.7 628.1 628.6 P.G.a
297.8 697.6 694.4 P.G.a
300.5 782.2 781.9 P.G.a
300.0 694.3 702.3 N2
a
Product gas.
temperature led to a decrease in the oil yield. The difference of oil yield between Runs 3 and 1 was fairly small (0.47 wt%). However, the difference appeared to result from the specific role of the auger reactor. A previous work on the pyrolysis of PE conducted with the same twostage reactor clearly showed a strong decrease in oil yield (11 wt%) with increasing the auger reactor temperature [30]. The large difference in the results can be mainly attributed to the difference between the degradation mechanisms of PS and PE. As mentioned before, the auger reactor appears to increase the vibrational state of molecules inside the reactor, which should allow molecules to be easily cracked at many places and produce mainly light hydrocarbons (for PE, mainly gas). However, in the degradation of PS, the tendency for the unzip reaction seems to be significantly stronger due to the high stability of the reactive intermediate (benzylic radical). Therefore, the dominant product from the fluidized bed reactor was still styrene (a liquid phase). As the temperature of the fluidized bed reactor increased from 514 (Run 3) to 782 °C (Run 6), the oil yield decreased from 98 to 86 wt%: a smooth decrease (3.6 wt%) of oil yield occurred from 514 to 698 °C, whereas a sharp decrease of oil yield (8.8 wt%) occurred from 698 to 782 °C. In particular, the styrene yield was sharply decreased from 69 to 26 wt%, whereas the yield of polyaromatic hydrocarbons was increased due to secondary reactions such as polyaromatic formation reactions. The temperature increase from 698 to 782 °C caused an increase in the gas yield from 4.8 to 13.8 wt%, especially increase in CH4 and light hydrocarbons due to enhanced primary cracking reactions. The results above indicate that a high temperature (in the present work: 780 °C) is needed to obtain an oil with decreased styrene yield. Table 4 shows the system energy balance (Run 5) of the two-stage pyrolysis of PS. In the calculation of energy input for heaters in Table 4, we only included the energy consumption during pyrolysis. Most of the energy
Fig. 3. Temperature change during pyrolysis (Run 6).
3. Results and discussion 3.1. Mass and energy balance The mass balance of the two-stage pyrolysis of PS is listed in Table 3. The main product from the two-stage pyrolysis of PS was oil, with yields that ranged from 86 to 99 wt%. The gas yield was very low except when the fluidized bed reactor temperature was very high (782 °C). As shown in Table 3, an increase in the auger reactor 4
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
Table 3 Mass balance of two-stage pyrolysis of polystyrene. Reactor type
Products
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
Auger reactor
Residues
0.00
0.08
0.32
0.36
0.39
0.02
0.38
Fluidized bed reactor
Gases (sum) Hydrogen Methane Ethane Ethene Propene Butenes 1,3-butadiene Other gases Oils (sum) Aliphatics (sum) 1,3-Cyclopentadiene Other aliphatics Aromatics (sum) Mono aromatics (sum) Benzene Toluene Ethylbenzene + Xylene Styrene Cumene α-Methylstyrene Propenylbenzene Indene Dialin Other mono aromatics Poly aromatics (sum) Naphthalene cis-Stilbene 2,4-diphenyl-1-butene (dimer) 1,4-diphenyl-1,3-butadiene 2-phenylnaphthalene 2,4,6-triphenyl-1-hexene (trimer) Other poly aromatic compounds Unknowns Char
1.17 0.08 0.14 0.05 0.54 0.22 0.09 0.01 0.04 98.71 0.01 0.01 0.00 90.48 77.62 0.04 1.58 0.23 73.60 0.10 1.61 0.08 0.15 0.03 0.20 12.86 0.02 0.13 7.80 0.29 1.00 2.21 1.41 8.22 0.12
1.39 0.67 0.32 0.05 0.20 0.07 0.03 0.01 0.04 98.52 0.00 0.00 0.00 90.34 74.45 0.16 1.86 0.27 70.18 0.09 1.41 0.07 0.18 0.03 0.20 15.89 0.03 0.00 9.06 0.33 1.35 1.89 3.23 8.18 0.01
1.41 0.19 0.14 0.05 0.52 0.28 0.00 0.00 0.21 98.23 0.00 0.00 0.00 90.10 74.30 0.32 1.97 0.27 69.76 0.09 1.40 0.07 0.18 0.03 0.21 15.80 0.03 0.00 9.01 0.33 1.34 1.88 3.21 8.13 0.04
1.73 0.14 0.4 0.1 0.65 0.31 0.09 0.03 0.01 97.89 0.02 0.02 0.00 89.45 81.95 0.23 4.56 2.15 68.61 0.24 3.96 0.26 0.68 0.09 1.17 7.50 0.32 0.00 2.13 0.79 1.19 0.63 2.44 8.42 0.02
4.78 0.62 2.11 0.21 1.28 0.38 0.08 0.05 0.05 94.83 0.03 0.03 0.00 87.06 77.11 2.63 9.94 5.42 52.50 0.18 4.07 0.39 1.21 0.05 0.72 9.95 1.45 0.00 1.48 1.80 1.47 0.09 3.66 7.74 0.00
13.84 0.96 7.33 0.75 4.33 0.24 0.01 0.11 0.11 86.01 0.01 0.01 0.00 78.87 56.80 15.55 9.79 0.96 26.09 0.06 1.24 0.13 2.19 0.05 0.74 22.07 5.30 0.43 0.35 2.83 2.03 0.02 11.11 7.13 0.13
5.57 0.16 0.86 0.13 2.75 0.99 0.30 0.30 0.08 94.03 0.10 0.05 0.05 85.32 77.86 1.96 7.31 2.59 59.72 0.18 3.78 0.33 1.10 0.12 0.77 7.46 0.96 0.00 1.32 1.08 1.04 0.03 3.03 8.61 0.02
Total
100
100
100
100
100
100
100
Table 4 System energy balance of Run 5. Energy input PS Heaters Total Energy output Gas Oil Auger reactor residue Char Heat loss Total
Energy (MJ/h)
Ratio (%)
20.90 2.38 23.28
89.79 10.21 100
Energy (MJ/h)
Ratio (%)
1.10 19.29 0.08 0.002 2.80 23.28
4.74 82.89 0.34 0.01 12.02 100
output was concentrated in the oil (~83%). Due to the low gas yield, the energy of product gas among products amounted to only 4.7%. Heat loss was about 12%, which appeared to be attributed mostly to the condensation of the pyrolysis oil, as mentioned by another study [33]. 3.2. Effect of reactor temperature on oil composition
Fig. 4. Oil compositions according to the reactors‘ temperature.
Fig. 4 shows the major compounds of the oil obtained at different reactors' temperatures. The main oil components were styrene, α-methylstyrene, and dimers and trimers of styrene. As the auger reactor temperature increased from 15 to 300 °C, the styrene yield decreased from 73.6 to 69.8 wt%, whereas the BTEX yield
increased from 1.85 to 2.56 wt%. The result shows that the BTEX yield could be increased with the help of the auger reactor. A slight increase in the fluidized bed reactor in Run 3 (9 °C) compared to Run 1 could also contribute to the increase in the BTEX yield. Fig. 5 shows the main degradation mechanism of PS called unzip or end-chain β-scission and 5
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
Fig. 5. Polystyrene pyrolysis: (a) Random-scission and end-chain β-scission mechanisms for styrene formation, (b) Proposed mechanisms for benzene, toluene, and ethylbenzene formation.
possible formation routes for BTEX aromatics. Increasing temperature in the auger reactor likely led to an increased possibility of bond dissociation to form BTEX aromatics, which
was likely attributed to the decreased bond strength at relevant sites. However, the end-chain β-scission, which is the main mechanism for the formation of styrene monomers, appeared to be more dominant 6
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
than other reactions at a fluidized bed reactor temperature of ~510 °C, which produced a large amount of styrene. The result clearly shows that the effect of auger reactor temperature had a limitation and that the inherent degradation mechanism of PS was much more important factor for pyrolysis product distribution. However, the styrene yield was significantly lower with increasing temperatures in the fluidized bed reactor. As the fluidized bed reactor temperature increased from 514 to 782 °C, the styrene yield drastically decreased from 70 to 26 wt %. Liu et al. [21] reported an increase in styrene yield up to 600 °C in the fluidized bed reactor, after which the yield decreased. In contrast, the increased yield of BTEX aromatics with higher temperatures in the fluidized bed reactor was remarkable. The maximum BTEX yield (26.3 wt%) was obtained at 782 °C. The BTEX concentration in the oil (782 °C) amounted to 30.6 wt%. These results are comparable to other studies which aimed at a high BTEX yield (Fig. 6). For a high BTEX yield from PS pyrolysis, catalysts are normally applied. Fig. 6 shows two approaches conducted with catalysts. Achilias et al. [26] applied fluid catalytic cracking (FCC) catalysts, which yielded 19.8 wt% of BTEX. Rehan et al. concluded that synthetic zeolite is better for BTEX production than natural zeolite [34]. In their work, the BTEX yield using synthetic zeolite amounted to 13.1 wt% at the high expense of oil yield. Compared to these studies, the BTEX yield of the current work is very high, without a great expense of oil yield. The third reference [35] in Fig. 6 aimed to produce BTEX aromatics from mixed plastic waste. The authors obtained a BTEX yield of 18.5 wt% using a single fluidized bed reactor. Although the production of BTEX aromatics from mixed plastic waste is an interesting option, it can be compromised by the chlorine in the oil, which is derives from polyvinylchloride (PVC), a polymer always present in mixed plastic waste. The chlorine content in the oil from pyrolysis should be below 10 ppmw to allow for use as a feedstock in oil refineries [12]. In contrast, BTEX production from PS can avoid the chlorine related problem, especially when expanded polystyrene foam (ESP) is pyrolyzed. ESP can be separately collected from houses, at least in Korea, which reduces the possibility of a pyrolysis of EPS together with PVC. Nevertheless, this approach for BTEX production from PS requires a high amount of energy to maintain the high reaction temperatures. Table 3 also shows that the total yield of monoaromatic hydrocarbons (MAH) was maximized at 630 °C and decreased thereafter. The observed trend appeared to be related to the yields of styrene dimers, their derivatives, and trimers. Fig. 7 shows their yields according to the temperature in the
Fig. 6. Comparisons with other works on BTEX and styrene yields.
Fig. 7. Yields of styrene dimer, its derivatives and trimer according to the fluidized bed reactor temperature.
Fig. 8. Formation mechanisms of major styrene dimer derivatives. 7
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al.
fluidized bed reactor. Fig. 7 shows that yields of 2,4-diphenyl-1-butene (dimer) and 2,4,6triphenyl-1-hexene (trimer) decreased with increasing temperatures. In particular, there was a significant decrease in the dimer yield from 514 to 630 °C, which likely contributed to maintain the high styrene yield (69 wt%) at 630 °C. However, over 600 °C, re-dimerization of styrene monomer was prevalent. Mechanisms (a) and (b) in Fig. 8. which were proposed by Flory and Mayo [36,37], are representative of the re-dimerization. Based on the proposed mechanisms, styrene dimer derivatives, such as 1,4-diphenyl-1,3-butadiene and 2-phenylnaphthalene, can be increased with higher temperatures in the fluidized bed reactor, as demonstrated in the present work.
during the winter, and met the diesel criteria (below 6 °C). In contrast with the pour point, the flash point is important during the summer. The flash point of both oils were the same, with a value of 79 °C, which satisfied the gasoline and diesel standards. The pH of both oils were also the same, at 6.4, which represents a neutral pH status. Pyrolysis oils from the current study, however, consisted mainly of aromatic hydrocarbons. Pyrolysis oils with such high contents of aromatics cannot be direct used as fuels, because in most countries the contents of aromatics and benzene in fuels, for example in gasoline, are limited to 35 and 1 vol%, respectively [39]. However, the pyrolysis oils with a high aromatic content could be used when they are blended with oils with a low aromatic content in order to meet the characteristics of commercial fuels. In conclusion, the pyrolysis oils obtained in the current study almost satisfied the commercial fuels standards, and they appear to be appropriate for use as a substitute source for gasoline and diesel fuels when they are mixed with oils with a low aromatic content.
3.3. Effect of the type of fluidizing medium Run 7 was conducted with nitrogen as the fluidizing medium. Compared to Run 5, where product gas was used, the oil yield from Run 7 was comparable, and the gas yield slightly increased. Regarding individual components, the styrene yield from Run 7 increased by about 7 wt%, whereas the BTEX yield decreased by approximately 6 wt% compared to Run 5. The hydrogen abstraction by phenyl and benzylic radicals to form BTEX aromatics appeared to be hindered by the nitrogen. Due to the dilution effect of nitrogen, reactions with styrene monomers, for example, the dimerization of styrene, appear to have diminished. The contents of dimers, 1,4-diphenyl-1,3-butadiene, 2phenylnaphthalene, and trimers in the pyrolysis oil of Run 7 were clearly reduced compared to those of Run 5. These results indicate that the use of product gas as a fluidizing medium is desirable for the production of BTEX from PS.
4. Conclusions Pyrolysis of PS was conducted using a two-stage pyrolysis process that has an auger reactor and a fluidized bed reactor. The main conclusions are as follows: 1) As the temperature in the auger reactor increased, the styrene yield decreased. 2) The maximum yield of benzene, toluene, ethylbenzene, and xylenes was significantly high, with a value of 26.3 wt%, which is higher than the yield obtained using catalysts. 3) The highest yield of benzene, toluene, ethylbenzene, and xylenes did not significantly cost the oil yield (86 wt%), but it demanded high energy input. 4) The product gas is a good fluidizing medium for the production of benzene, toluene, ethylbenzene, and xylenes. 5) The obtained pyrolysis oil can likely be used as a good source of benzene, toluene, ethylbenzene, and xylenes.
3.4. Fuel properties of oil This study proposed the production of BTEX aromatics from PS. The possibility to produce chlorine-free or low-chlorine oils from PS allows for the use of pyrolysis oil as a fuel, which is particularly appropriate for countries where oil refinery facilities are not easily available. In general, when pyrolysis oil is used as a fuel, it must contain the minimum possible amount of chlorine due to the possible formation of dioxins when combusted. It has been reported that the average chlorine content in gasoline is only a few ppmw [38]. Table 5 shows the physical properties of the oils from Runs 5 and 6 along with commercial fuel standards [11]. The calorific value is an important factor when examining fuel properties. The pyrolysis oils from Runs 5 and 6 showed high calorific values, over 40 MJ/kg. They were only slightly lower than the values for gasoline and diesel standards. The American Petroleum Institute (API) gravity of both oils satisfied the gasoline and diesel standards. The densities of the oils at 15 °C were 0.95 and 0.98 g/cm3, which also met the gasoline and diesel standards. Their viscosities were 0.97 (Run 5) and 1.26 (Run 6), within the standard range for gasoline. The ash content of both oils was lower than 0.007 wt%, which was adequate according to the diesel standard. Both oils showed the same pour point (−39 °C), which is important
CRediT authorship contribution statement Ki-Bum Park: Conceptualization, Data curation, Investigation, Methodology, Project administration, Resources, Validation, Writing original draft. Yong-Seong Jeong: Data curation, Investigation, Resources. Begum Guzelciftci: Data curation, Investigation, Resources. Joo-Sik Kim: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Validation, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This research was supported by the Korea Environmental Industry Technology Institute (KEITI) grant (No. 2016000710004) funded by the Ministry of Environment of Korea.
Table 5 Fuel properties of pyrolysis oil from polystyrene. Physical property
Run 5
Run 6
Gasoline [9]
Diesel [9]
Calorific value (MJ/kg) API gravity @ 60 °F Viscosity (mm2/s) Density @ 15 °C (g/cm3) Ash content (wt.%) Pour point (°C) Flash point (°C) pH
40.89 17.74 0.97 0.95 0.007 −39 79 6.4
40.02 13.22 1.26 0.98 0.001 −39 79 6.4
> 42.5 < 55 < 1.17 > 0.78 – – > 42
> 43.0 < 38 1.9–4.1 > 0.81 < 0.01 <6 > 52
References [1] PlasticsEurope. The facts 2018. PlasticsEurope; 2018. [2] Geyer R, Jambeck JR, Law KL. Production, use, and fate of all plastics ever made. Sci Adv 2017;3:e1700782. [3] Brooks AL, Wang S, Jambeck JR. The Chinese import ban and its impact on global plastic waste trade. Sci Adv 2018;4:eaat0131. [4] Al-Salem SM, Lettieri P, Baeyens J. The valorization of plastic solid waste (PSW) by primary to quaternary routes: from re-use to energy and chemicals. Prog Energy
8
Applied Energy xxx (xxxx) xxxx
K.-B. Park, et al. Combust Sci 2010;36:103–29. [5] Rahimi A, García JM. Chemical recycling of waste plastics for new materials production. Nat Rev Chem 2017;1:46. [6] Hamad K, Kaseem M, Deri F. Recycling of waste from polymer materials: An overview of the recent works. Polym Degrad Stab 2013;98:2801–12. [7] Marrone M, La Mantia FP. Re-stabilisation of recycled polypropylenes. Polym Recycl 1996;2:17–26. [8] Zhang Y, Duan D, Lei H, Villota E, Ruan R. Jet fuel production from waste plastics via catalytic pyrolysis with activated carbons. Appl Energy 2019;251:113337. [9] Sophonrat N, Sandstrom L, Zaini IN, Yang W. Stepwise pyrolysis of mixed plastics and paper for separation of oxygenate and hydrocarbon condensates. Appl Energy 2018;229:314–25. [10] Lopez G, Artetxe M, Amutio M, Bilbao J, Olazar M. Thermochemical routes for the valorization of waste polyolefinic plastics to produce fuels and chemicals. A review. Renew Sust Energ Rev 2017;73:346–68. [11] Sharuddin SDA, Abnisa F, Daud WMAW, Aroua MK. A review on pyrolysis of plastic wastes. Energy Convers Manag 2016;115:308–26. [12] Jung S-H, Cho M-H, Kang B-S, Kim J-S. Pyrolysis of a fraction of waste polypropylene and polyethylene for the recovery of BTX aromatics using a fluidized bed reactor. Fuel Process Technol 2010;91:277–84. [13] Elordi G, Olazar M, Lopez G, Artetxe M, Bilbao J. Product yields and compositions in the continuous pyrolysis of high-density polyethylene in a conical spouted bed reactor. Ind Eng Chem Res 2011;50:6650–9. [14] Lopez-Urionabarrenechea A, De Marco I, Caballero BM, Adrados A, Laresgoiti MF. Empiric model for the prediction of packaging waste pyrolysis yields. Appl Energy 2012;98:524–32. [15] Zhou J, Qiao Y, Wang W, Leng E, Huang J, Yu Y, et al. Formation of styrene monomer, dimer and trimer in the primary volatiles produced from polystyrene pyrolysis in a wire-mesh reactor. Fuel 2016;182:333–9. [16] Faravelli T, Pinciroli M, Pisano F, Bozzano G, Dente M, Ranzi E. Thermal degradation of polystyrene. J Anal Appl Pyrolysis 2001;60:103–21. [17] McNeill IC, Zulfiqar M, Kousar T. A detailed investigation of the products of the thermal degradation of polystyrene. Polym Degrad Stab 1990;28:131–51. [18] Artetxe M, Lopez G, Amutio M, Barbarias I, Arregi A, Aguado R, et al. Styrene recovery from polystyrene by flash pyrolysis in a conical spouted bed reactor. Waste Manage 2015;45:126–33. [19] Moqadam SI, Mirdrikvand M, Roozbehani B, Kharaghani A, Shishehsaz MR. Polystyrene pyrolysis using silica-alumina catalyst in fluidized bed reactor. Clean Technol Environ Policy 2015;17:1847–60. [20] Demirbas A. Pyrolysis of municipal plastic wastes for recovery of gasoline-range hydrocarbons. J Anal Appl Pyrolysis 2004;72:97–102. [21] Liu Y, Qian J, Wang J. Pyrolysis of polystyrene waste in a fluidized-bed reactor to obtain styrene monomer and gasoline fraction. Fuel Process Technol 2000;63:45–55.
[22] Karaduman A, Şimşek EH, Cicek B, Bilgesü AY. Flash pyrolysis of polystyrene wastes in a free-fall reactor under vacuum. J Anal Appl Pyrolysis 2001;60:179–86. [23] Karaduman A. Pyrolysis of polystyrene plastic wastes with some organic compounds for enhancing styrene yield. Energy Sources 2002;24:667–74. [24] Park JJ, Park K, Kim J-S, Maken S, Song H, Shin H, et al. Characterization of styrene recovery from the pyrolysis of waste expandable polystyrene. Energy Fuels 2003;17:1576–82. [25] Onwudili JA, Insura N, Williams PT. Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: Effects of temperature and residence time. J Anal Appl Pyrolysis 2009;86:293–303. [26] Achilias DS, Kanellopoulou I, Megalokonomos P, Antonakou E, Lappas AA. Chemical recycling of polystyrene by pyrolysis: potential use of the liquid product for the reproduction of polymer. Macromol Mater Eng 2007;292:923–34. [27] Chen C-C, Duh Y-S, Shu C-M. Thermal polymerization of uninhibited styrene investigated by using microcalorimetry. J Hazard Mater 2009;163:1385–90. [28] Pérez-Alonso FJ, Agüí L, Yáñez-Sedeño P, Pingarrón JM. Determination of styrene and styrene additives using cylindrical microelectrodes in acetone. Analyst 2000;125:2006–10. [29] Mertinkat J, Kirsten A, Predel M, Kaminsky W. Cracking catalysts used as fluidized bed material in the Hamburg pyrolysis process. J Anal Appl Pyrolysis 1999;49:87–95. [30] Park K-B, Jeong Y-S, Guzelciftci B, Kim J-S. Characteristics of a new type continuous two-stage pyrolysis of waste polyethylene. Energy 2019;166:343–51. [31] Park K-B, Jeong Y-S, Guzelciftci B, Kim J-S. Activator-assisted pyrolysis of polypropylene. Appl Energy 2019;253:113558. [32] Holm T. Aspects of the mechanism of the flame ionization detector. J Chromatogr A 1999;842:221–7. [33] Boateng AA, Daugaard DE, Goldberg NM, Hicks KB. Bench-scale fluidized-bed pyrolysis of switchgrass for bio-oil production. Ind Eng Chem Res 2007;46:1891–7. [34] Rehan M, Miandad R, Barakat MA, Ismail IMI, Almeelbi T, Gardy J. Effect of zeolite catalysts on pyrolysis liquid oil. Int Biodeterior Biodegradation 2017;119:162–75. [35] Cho M-H, Jung S-H, Kim J-S. Pyrolysis of mixed plastic wastes for the recovery of benzene, toluene, and xylene (BTX) aromatics in a fluidized bed and chlorine removal by applying various additives. Energy Fuels 2009;24:1389–95. [36] Khuong KS, Jones WH, Pryor WA, Houk KN. The mechanism of the self-initiated thermal polymerization of styrene. theoretical solution of a classic problem. J Am Chem Soc 2005;127:1265–77. [37] Kothe T, Fischer H. Formation rate constants of the mayo dimer in the autopolymerization of styrene. J Polym Sci Part A Polym Chem 2001;39:4009–13. [38] Broz J, Grabic R, Kilian J, Lojkasek M, Marklund S, Ocelka T, et al. The Effect of Oils on PAH, PCDD, PCDF and PCB Emissions from a Spark Engine Fueled with Leaded Gasoline. Chemosphere 2000;41:1905–11. [39] Wagner V, Stahl R, Olivares E, Morgan M, Foukes E. EU: Fuels: Diesel and Gasoline. [Online], 2015 (Available from;) http://transportpolicy.net/index.
9